PALLADIUM (II) COMPLEXES BEARING SULFUR FUNCTIONALIZED n HETEROCYCLIC CARBENE LIGANDS

PALLADIUM(II) COMPLEXES BEARING SULFUR-FUNCTIONALIZED N-HETEROCYCLIC CARBENE LIGANDS Tang Haoyun (B.Sc., FuDan UNIVERSITY) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF SCIENCE DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2012 Thesis Declaration The work in this thesis is the original work of Tang Haoyun, performed independently under the supervision of Associate Professor Huynh Han Vinh, (in the Carbene & Organometallic Synthesis Laboratory aboratory), Chemistry Department, National University of Singapore, between August 2010 and July 2012. The content of the thesis has been partly published in: 1) Publication 1 (Dalton Transactions, 2011 011, 35, 7262.) Tang Haoyun Name Signature Date I Acknowledgements First of all, I would like to thank my supervisor, Associate Professor Huynh Han Vinh, for all the guidance, patience and inspiration he gifted me. I am grateful to the efforts he has put into training me and making me a better person. The education I have received from him will benefit my whole life. I am grateful to my lab mates, Dr. Yuan Dan, Dr. Lee Chen-Shiang, Teng Qiao qiao, Jan Christopher Bernhammer and Haresh S/O Sivaram for their company and encouragement during this period of time. Special thanks to my friend Guo Shuai, for all the company and help he gifted me. I am appreciated to the technical staff at Nuclear Magnetic Resonance, Mass Spectrometry, X-ray Diffraction and Elemental Analysis laboratories in our department for their technical support. Thanks to Prof. Koh Lip Lin and Ms Tan Geok Kheng for their professional help in solving all the molecular structures. Thanks to Mdm Han Yan Hui for her help and support in all the NMR experiment. I would like to thank my family, for all the help and support in these years. I am grateful to my mountain bike, for its support in trips with thousands of miles. Last but not least, I would like to present my gratitude to NUS for providing the research scholarship. II Table of contents List of Tables IV List of Figures V List of Schemes VI Chart 1. Compounds synthesized in this work Chapter 1. Introduction VII 1 1.1 Definition of carbenes 1 1.2 Introduction to N-Heterocyclic Carbenes 1 1.3 Objective of this thesis 3 Chapter 2. Palladium(II) complexes with CSC-pincer type NHC ligands 5 2.1 Pd(II) CSC pseudo-pincer benzimidazolin-2-ylidene complexe 5 2.2 Pd(II) CSC-pincer imidazolin-2-ylidene complexes 10 2.3 Pd(II) CSC pseudo-pincer 4,5-dichloroimidazolin-2-ylidene complexes 13 Chapter 3. Donating ability of NHC ligands and conductivity of pincer-type complexes 19 3.1 Donating ability of two mono-NHC ligands 19 3.2 Donating ability of pincer-type ligands 22 3.3 Conductivity of pincer-type complexes 26 II Chapter 4. Sulfonate-functionalized NHC Palladium( Palladium(II II)) complex and catalytic studies in Suzuki-Miyaura coupling reaction 4.1 Synthesis of sulfonate-functionalized NHC Pd(II) complex 30 30 III 4.2 Catalytic studies of complex 22 in Suzuki-Miyaura coupling reaction 31 Chapter 5. Summary and conclusion 35 Chapter 6. Experimental Section 37 Appendix (Selected crystallographic data) 49 References 50 IV List of Tables 14 Table 3.1 Selected 1H and 13C NMR data in ppm for complexes 9-14 24 Table 3.2 Conductivity test for pincer type complexes 28 Table 3.3 Conductivity test for pincer type complexes after anion exchange 29 Table 4.1 Effect of the solvent on the Suzuki-Miyaura cross-coupling reactions catalyzed by 22 31 Table 4.2 Suzuki-Miyaura coupling of aryl bromides with phenyl boronic acid catalyzed by complex 22 32 Table 4.3 Suzuki-Miyaura coupling reactionsa catalyzed by complex 22 33 V List of Figures Figure 1.1 Electronic structure of carbenes 1 Figure 1.2 Three major types of NHCs 2 Figure 2.1 Variable temperature 1H NMR experiment for complex 3 7 Figure 2.2 Molecular structure of 3 8 Figure 2.3 Experimental X-ray diffraction pattern (a) and calculated pattern (b) for 3 9 Figure 2.4 Variable temperature 1H NMR experiment for complex 4 11 Figure 2.5 Molecular structure of 4 12 Figure 2.6 Variable temperature 1H NMR experiment for complex 6 15 Figure 2.7 Molecular structure of 6 16 Figure 2.8 Experimental X-ray diffraction pattern (a) and calculated pattern (b) for 6 17 Figure 3.1 Donor abilities of common NHCs on the 13C NMR scale 19 Figure 3.2 Molecular structure of 7 21 Figure 3.3 Donating abilities of pincer-type NHCs on 13C NMR scale 25 Figure 3.4 Previously synthesized pincer-type complexes used for conductivity test 27 VI List of Schemes Scheme 1.1 Synthesis of the first free NHC by Arduengo 1 Scheme 1.2 Two important synthetic routes for NHC complexes (X = anion) 2 Scheme 2.1 Synthesis of complexes 1 and 2 5 Scheme 2.2 Synthesis of thioether-bridged dibenzimidazolium salts D⋅2HBr 6 Scheme 2.3 Synthesis of thioether-bridged diimidazolium salts D⋅2HBr 10 Scheme 2.4 Synthesis of complex 5 13 Scheme 2.5 Synthesis of thioether-bridged diimidazolium salts G⋅2HBr 14 Scheme 3.1 Synthesis of complexes 7 and 8 20 Scheme 3.2 Synthesis of complexes 9 to 14 23 Scheme 3.3 Ligand disproportionation of Tetra carbene complexes 24 Scheme 4.1 Synthesis of salt M 30 Scheme 4.2 Synthesis of complex 22 31 VII Chart 1. Compounds synthesized in this work N N Br N N Br S N N N Br 2Br N N B . 2HBr A S N N Br 2Br N D . 2HBr C O3 S Cl N Cl N Br Cl N Cl N S N Cl Cl N N Cl Cl N S 2Br Ph E N Br N N N N Pd N Br Cl N N Cl N Ph G . 2HBr F S N S N M Cl N Cl N Br Br N Cl N Cl Pd Pd Br S N Ph Ph Br 3 4 6 VIII N N Br Pd Br N Cl N Cl N N Br Pd Br 7 N Cl X S N N Br Br N R Cl 8 X X N Pd Br L Pd L Br N R N R 9: X = H, R = isopropyl 10: X = Cl, R = benzyl 13: X = H, R = mesityl 14: X = H, R = benzyl N Br Br Pd Br Pd L L Br N R 11 : R = isopropyl 12 : R = benzyl N N L= L= S N X N N N KO 3S N Br Pd Br Br Pd N Br SO3 K N 22 IX List of Abbreviation Anal. Calc. Ar br n Bu Bn cf. d dd DMF DMSO δ e.g. Equiv ESI et al. etc. FAB h I i.e. J m M Me Mes min MS m/z NMR Ph i Pr RT s t THF Analysis Calculated Aryl Broad n-Butyl benzyl compare (Latin confer) doublet (NMR) / day doublet of doublet (NMR) Dimethylformamide Dimethylsulfoxide NMR chemical shift in ppm for example (Latin exempli gratia) Equivalent(s) Electrospray Ionisation and others (Latin et alii) and so on (Latin et cetera) Fast Atom Bombardment Hour Inductive effect that is (Latin id est) coupling constant multiplet (NMR) Mesomeric effect Methyl 2,4,6-trimethylphenyl Minute Mass Spectrometry mass to charge ratio Nuclear Magnetic Resonance Phenyl Isopropyl room temperature singlet triplet Tetrahydrofuran X Chapter 1. Introduction 1.1 Definition of carbenes There has been a rapid growth of interest in carbenes since the isolation of the first free carbene by Arduengo (Scheme1.1).1 NaH/THF catalyst DMSO N N - NaCl - H2 Cl N N Scheme 1. 1.11 Synthesis of the first free NHC by Arduengo. Carbenes are electrically neutral divalent carbon atoms with six valance electrons.2 Carbenes can exist in either singlet or triplet state.3 The two non-bonding electrons in singlet carbene occupy the σ orbital with an anti-parallel spin orientation, and the pπ orbital is empty. On the contrary, both σ and pπ orbitals of carbene in triplet state are occupied by its two non-bonding electrons with a parallel spin orientation (Figure 1.1).4 pπ σ pπ σ singlet triplet Figure 1.1 Electronic structure of carbenes. 1.2 Introduction to N-Heterocyclic Carbenes N-heterocyclic carbenes (NHCs) are a type of singlet carbenes that have the carbene carbon integrated in a 1 nitrogen-containing heterocycle. There are three major types of NHCs, namely benzimidazolin-2-ylidene, imidazolin-2-ylidene, and imidazolidin-2-ylidene (Figure 1.2). R N R N R N N R N R N R A) benzimidazolin-2-ylidene (A B) imidazolin-2-ylidene (B imidazolidin-2-ylidene ( C) Figure 1.2 Three major types of NHCs. NHCs are usually viewed as strong σ-donating ligands with little or negligible π back-bonding.5,6 Compared to free carbenes, much more effort has been focused on metal-NHC complexes, because of their wide application in catalysis. Two important methods that have been employed to synthesize metal-NHC complexes in this study are (a) reactions of azolium salts with suitable metal precursors;7,8 and (b) the Ag-NHC transfer method (Scheme 1.2).9,10 R N [M] R N [M] R N [M] N R [M'] [M] M = Ag method b N X R method a N R M = Ag method b R N [M'] N R Scheme 1. 1.22 Two important synthetic routes for NHC complexes (X = anion). Electronic and steric properties of NHCs can be easily tuned by changing of the N-substituents and the backbone, which helps to enlarge the diversity of NHC chemistry.11,12 Donor-functionalized NHCs are 2 potentially polydentate ligands, which can give rise to complexes with enhanced stability through ligand chelation.13 NHCs with N, O or P donors are more extensively investigated, those with sulfur donors are relatively rare.14,15 Although some work on the coordination chemistry of NHCs with alkyl thioether groups has been done, most of the previous work focused on chelating ligands, leaving pincer-type ligands unexplored.16 Previously investigated pincer ligands contain a rather rigid structure, which is intended to afford better stability.9 On the other hand, a more delicate balance between lability and stability may exist in ligands with more flexible backbone,17 which may be beneficial to certain types of catalytic reactions.18,19 CSC-pincer type NHCs with S donor from a thioether-bridge will be discussed in this work. 1.3 Objective of this thesis According to our previous work, formation of pincer versus pseudo pincer in Pd(II) complexes with pincer-type NHC ligands is affected by the electron donating abilities of the latter.20 Carbenes with stronger donating ability favor pincer formation even with the presence of halide ions. On the contrary, less electron donating carbenes prefer to form neutral pseudo-pincer complexes. The electron donating ability of our previously synthesized pincer-type NHC ligands were evaluated by using the 13C NMR based methodology established by our group, and the results will be discussed in chapter 3.21 More donating carbene ligands will have a downfield chemical shifts for the carbenoid signal of iPr2-bimy in the 13C NMR spectra.Previous study shows that there is difference in catalytic property between pincer and pseudo-pincer complexes.22,23 As most catalytic reactions are conducted in solution, determination of their identity in solution is critical to these pincer-type complexes.24,25 X-ray diffraction study on single crystals can only determine the molecular structure in the solid state. Conductivity measurement has been used for structure determination of metal complexes since Alfred Werner.26 Much work has been done to establish the molecular complexity by doing conductivity.27 In this situation, this methodology, which is based on conductivity measurement, was used to distinguish pincer from pseudo-pincer complexes in solution, and the result will be discussed in chapter 3. NHC palladium complexes have been widely used in Suzuki-Miyaura coupling reaction because of their 3 superior performance compared to phosphanes.28,29,30 Investigation on sulfonate-functionalized NHCs is a rarely explored field.31,32 There have been only a few reports on the synthesis of sulfonate-NHC complexes in recent years.33,34 The free sulfonate moiety can improve the solubility of metal-NHC complexes, making the sulfonate-functionalized NHC-metal complex a promising catalyst for homogenous catalysis in water.36,37 The synthesis of a sulfonated-functionalized Pd(II) NHC complex and its catalytic study in Suzuki-Miyaura coupling reaction are described in chapter 4. 4 II Chapter 2. Palladium( Palladium(II II)) complexes with CSC-pincer type NHC ligands 1) and its "quasi-pincer" analogue (22) were synthesized in our The first CSC-pincer-type Pd(II) complex (1 previous work (Scheme 2.1), which indicated that the hemilability of this thioether-bridged di-NHC ligand determined pincer versus pseudo-pincer formation was influenced by the presence of bromide anions.20 As discussed in chapter 1, pincer versus pseudo-pincer formation was also affected by the donating ability of the NHC ligand. To further support our theory, a stronger donating benzimidazolin-2-ylidene derived CSC pincer type NHC ligand and its corresponding Pd(II) complex were synthesized. S N N 2X N N Ph Ph X = NO3 X = Br Pd(OAc)2 DMSO, 80 °C Pd(OAc)2 , KBr DMSO, 80 °C S N N Ph N N N Pd N Pd Br S Br N Ph Ph 1 Br NO3 N Ph 2 Scheme 2. 2.11 Synthesis of complexes 1 and 2. 2.1 Pd(II) CSC pseudo-pincer benzimidazolin-2-ylidene complexe Sulfur-bridged dibenzimidazolium salt B ⋅ 2HBr. The preparation of thioether-bridged dibenzimidazolium salt as precursor to CSC-pincer type Pd(II) complex is depicted in Scheme 2.2. Reaction of 1-isopropyl-benzimidazole with neat 1,2-dibromoethane afforded salt A in a moderate yield of 78%. This type of salt is a suitable precursor for other compounds bearing different functional groups, because it 5 contains a good leaving group. Compared to 1-isopropyl-benzimidazole, the 1H NMR spectrum for salt A shows a downfield chemical shift at 11.26 ppm, characteristic for the NCHN proton in benzimidazolium salts. The spectrum also shows two triplets at 5.25 ppm and 4.10 ppm with a coupling constant of 3J(H,H) = 5.7 Hz assignable to the two inequivalent methylene groups of the bromoethylene N-substituent. The spectrum also shows a doublet at 1.83 ppm and a septet at 5.00 ppm, which are characteristic chemical shifts for the isopropyl group. The formation of salt A is also supported by a base peak in the ESI mass spectrum at m/z = 269 for the monocation [M – Br]+. N N BrCH 2CH2 Br N 85 C N Br Br Na2 S 9H 2O N CH3 CN, RT N A (78%) S 2Br N N B 2HBr (90%) Scheme 2. 2.22 Synthesis of thioether-bridged dibenzimidazolium salts B⋅2HBr. Two equivalent of A can subsequently undergo nucleophilic substitution with Na2S to afford the thioether-bridged salt B ⋅ 2HBr, which was isolated as an off-white powder in90% yield. Compared to its precursor A, the 1H NMR signals for B⋅2HBr do not change significantly upon thioether-formation. Only an upfield shift from 4.10 ppm to 3.61 ppm was observed for the BrCH2 methylene group. Formation of B⋅2HBr was supported by a base peak in the ESI mass spectrum at m/z = 489 for the monocation [M – Br]+. B-κ2C)] (33) was synthesized in a typical Pd(II) CSC pseudo-pincer complex 3. 3.The complex trans-[PdBr2(B method.12 The precursor salt B ⋅ 2HBr reacts with Ag2O to afford a silver-carbene complex, which was directly added to [PdBr2(CH3CN)2], affording the Pd(II) CSC-pincer complex 3. Complex 3 is soluble in DCM, chloroform, CH3CN, MeOH, DMSO, and DMF, but insoluble in non-polar solvents such as ether, toluene and hexane. Compared to its precursor B ⋅ 2HBr, the disappearance of the NCHN signal in the 1H NMR spectrum supports the successful deprotonation. The methylene protons of the bridging thioether 6 afford two broad peaks at 5.13 ppm and 3.92 ppm at room temperature in CDCl3, indicating a certain degree of rotational freedom in line with a pendant sulfur function. Palladation of B ⋅2HBr is further supported by the 13C NMR signal for the carbene carbon resonance found at 181.9 ppm, indicating a trans arrangement of the two benzimidazolin-2-ylidene moieties. Variable temperature 1H NMR experiment was conducted in order to resolve the two broad signals, and the spectra are depicted in Figure 2.1. When the temperature was decreased to 223 K, the signals sharpened and split into a range of multiplets in line with a reduced movement of the thioether-bridge. Palladation of B⋅2HBr is further supported by the 13C NMR signal for the carbene carbon found at 181.9 ppm, which is slight downfield than that of complex 1. The formation of complex 3 is also supported by a base peak in the ESI mass spectrum at m/z = 593 assigned to the monocation [M – Br]+ fragment. Figure 2.1 Variable temperature 1H NMR experiment for complex 3. The identity of 3 was finally confirmed by X-ray diffraction analysis on single crystals obtained from slow diffusion of a concentrated chloroform solution. The molecular structure is depicted in Figure 2.2. 7 Figure 2.2 Molecular structure of 3 showing 50% probability ellipsoids; solvent molecules, hydrogen atoms and disorder are omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1-C1 2.029(5), Pd1-C15 2.030(5), Pd1-Br2 2.4320(9), Pd1-Br1 2.4414(8), N1-C1 1.345(6), N2-C1 1.346(6), N3-C15 1.342(6), N4-C15 1.352(7); C1-Pd1-C15 170.5(2), C1-Pd1-Br2 89.08(15), C15-Pd1-Br2 90.08(17), C1-Pd1-Br1 90.87(15), C15-Pd1-Br1 90.47(17),Br2-Pd1-Br1 176.89(3), N1-C1-N2 106.7(4), N3-C15-N4 106.4(4); PdC2Br2/NHC dihedral angle 79.0(1)°, 86.4(1)°; inter-NHC angle 11.3(2)°. The molecular structure of complex 3 obtained from a single crystal was unambiguously identified to be a pseudo-pincer complex. Unlike complex 1, the two carbene donors in 3 are trans to each other. Pd(II) center forms a square planar coordination sphere, surrounded by two carbene donors and two bromido ligands. The sulfur atom points away from the coordination plane, which is similar to that of complex 1. The inter-NHC angle [11.3(2)°] is quite smaller than that of complex 1 [80°]. The dihedral angles between the NHC plane and the [PdC2SBr] coordination plane are 79.0(1)° and 86.4(1)°, which are similar to that of complex 1.10 The Pd-Br bond lengths of 2.4320(9) and 2.4414(8) Å are shorter, as expected, than those in the cis-configured benzimidazole based system, since they do not bear any trans influence of the carbene donors.10 8 Figure 2.3 Experimental X-ray diffraction pattern (a) and calculated pattern (b) for 3. Molecular structure from one single crystal is inadequate to conclude that complex 3 adopts pseudo-pincer solely. Further evidence is also needed to determine the identity for the bulk of complex 3. X-ray powder diffraction analysis was performed on crystalline material of complex 3 to confirm whether the pseudo-pincer structure determined on a single crystal is representative of the bulk. However, due to small scale of complex, the background signal is very strong. Despite of that, the major peaks provided by powder diffraction are similar to the calculated pattern (Figure 2.3), indicating that complex 3 probably forms a trans-dibromido-dicarbene pseudo-pincer complex. The pseudo-pincer formation of 3 indicates that the donating ability of B may be not strong enough to form a pincer. A CSC pincer-type NHC ligand with stronger donating ability and its corresponding complex were synthesized. 9 2.2 Pd(II) CSC-pincer imidazolin-2-ylidene complexes Sulfur-bridged diimidazolium salt D ⋅ 2HBr 2HBr.. Thioether-bridged diimidazolium salts as precursor to CSC-pincer complex were synthesized in the route depicted in Scheme 2.3. Reaction of isopropylimidazole with neat 1,2-dibromoethane afforded salt C with a moderate yield of 72%. The 1H NMR spectrum for salt C shows a downfield chemical shift at 10.33 ppm, characteristic for the NCHN proton in imidazolium salts. The spectrum also shows two triplets at 4.89 ppm and 3.91 ppm with a coupling constant of 3J(H,H) = 5.67 Hz assignable to the two inequivalent methylene groups on the bromoethylene substituent. The isopropyl group shows a doublet at 1.59 ppm and a septet at 4.78 ppm. N N BrCH2 CH 2 Br N 85 C N Br Br Na 2S 9H2 O N CH 3CN, RT N C (72%) S 2Br N N D . 2HBr (82%) Scheme 2. 2.33 Synthesis of thioether-bridged diimidazolium salts D⋅2HBr. The reaction of two equivalent of C with Na2S afforded the thioether-bridged salt D ⋅ 2HBr,, which was isolated as an off-white and hydroscopic powder in 82% yield. The signal for the NCH2 methylene groups overlaps with that for the NCH isopropyl methine groupfrom 4.76 to 4.67 ppm. An upfield shift (δ = 0.55 ppm) was also observed for the SCH2 methylene groups. The methyl group is only slightly affected by the thioether-formation. The formation of D ⋅ 2HBr was supported by a base peak in the ESI mass spectrum at m/z = 387 arising from the monocation [M – Br]+. Pd(II) CSC pincer complex 4. Direct transfer of the silver-carbene complex to [PdBr2(CH3CN) 2] afforded the Pd(II) CSC-pincer type complex 4. Complex 4 is soluble in DCM, chloroform, CH3CN, MeOH, DMSO, and DMF, but insoluble in less-polar solvents such as THF, ether, toluene and hexane. 10 In addition, compared to its precursor D ⋅ 2HBr, the disappearance of the NCHN signal in the 1H NMR spectrum characteristic for D ⋅ 2HBr supports the successful metalation. The chemical shifts in the 1H NMR spectrum are broad at room temperature, due to the fluxionality of the ethylene bridges. Variable temperature 1H NMR experiment was conducted in order to resolve the broad signals, and the spectra are depicted in Figure 2.4. The 1H NMR spectrum in CDCl3 at 243K shows that the two protons on SCH2 methylene group are diastereotopic, resonating a doublet at 4.46 ppm and a triplet at 2.99 ppm, indicating that the thioether-bridge is fixed at low temperature. The chemical shift for the NCH2 methylene groups remains broad despite the low temperature. Palladation of D⋅2HBr is further supported by the 13 C NMR signal for the carbene carbon found at 166.1 ppm. The successful metalation is also supported by a base peak in the ESI mass spectrum at m/z = 493 assigned to the monocation [M – Br]+. Figure 2.4 Variable temperature 1H NMR experiment for complex 4. The identity of 4 as a pincer complex was finally confirmed by X-ray diffraction analysis on single crystals obtained from slow diffusion of a concentrated chloroform solution. The molecular structure is depicted in 11 Figure 2.5. The cationic CSC pincer complex 4 contains a square planar Pd(II) center coordinated by two NHC moieties, one thioether and one bromido ligand. The two carbene donors are trans to each other as a result of the pincer formation. The inter-NHC angle is 10.8(1)°, which is close to complex 3. The dihedral angles between the NHC planes and the [PdC2SBr] coordination plane are 53.54(9)° and 55.15(9)°, respectively, which deviate substantially from the ideal 90° due to the rigid pincer formationn. The complex-cation is charge-balanced by a free bromide counter-anion. Figure 2.5 Molecular structure of 4 showing 50% probability ellipsoids; solvent and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1-C9 2.024(3), Pd1-C1 2.027(3), Pd1-S1 2.2907(8), Pd1-Br1 2.4304(5), N1-C1 1.343(4), N2-C1 1.349(4), N3-C9 1.351(4), N4-C9 1.348(4); C9-Pd1-C1 175.45(12), C9-Pd1-S1 90.32(9), C1-Pd1-S1 89.34(9), C9-Pd1-Br1 89.76(9), C1-Pd1-Br1 90.08(9), S1-Pd1-Br1 173.67(2), N1-C1-N2 105.0(3), N4-C9-N3 104.8(3); PdC2BrS/NHC dihedral angle 53.54(9)°, 55.15(9)°; inter-NHC angle 10.8(1)°. The formation of pincer for complex 4 confirmed our theory that a complex with strong electron donating ligand prefers to form pincer. To further support our theory, another strong electron donating 12 imidazolin-2-ylidene derived CSC pincer-type Pd(II) complex was synthesized (Scheme 2.4). Complex 5 was also identified to be a pincer. 2Br N N Ph S N N i) Ag 2O, MeOH N ii) PdBr2 (CH3 CN) 2, CH 3CN N S Pd Br Ph Ph N Br N Ph 5 Scheme 2. 2.44 Synthesis of complex 5. To further prove the concept that pincer-type complex with weak electron donating ligand would prefer to form pseudo-pincer, synthesis of 4,5-dichloroimidazolin-2-ylidene derived CSC pincer-type NHC ligand and its corresponding Pd(II) complex were carried out in the route depicted by Scheme 2.5. 2.3 Pd(II) CSC pseudo-pincer 4,5-dichloroimidazolin-2-ylidene complexes Sulfur-bridged 4,5-dichlorodiimidazolium salt G ⋅ 2HBr 2HBr.. The successful strategy for the synthesis of B⋅2HBr, however, failed in the preparation of its 4,5-dichloroimidazolium-analogue G⋅2HBr. 1-benzyl-4,5-dichloroimidazole is too electron deficient to be alkylated by 1,2-dibromoethane to afford “X” (Scheme 2.2). Another route was conducted to synthesize salt G ⋅ 2HBr. The thioether-bridge was installed before quaternization with benzyl bromide. 4,5-dichloroimidazole first reacted with 1,2-dibromoethane to afford 1-bromoethyl-4,5-dichloroimidazole E. The 1H NMR spectrum of E shows two triplets at 4.25 ppm and 3.51 ppm which can be assigned for the two inequivalent methylene groups. E was then treated with Na2S to afford an off-white powder, the sulfur-bridged diimidazole F. Compared to its precursor E, the 1 H NMR spectrum for F did not change significantly upon thioether-formation. Only a slight upfield shift was observed for the SCH2 methylene group, from 3.51 ppm to 2.72 ppm. Reaction of F with benzyl bromide finally led to the target compound G ⋅ 2HBr. The 13 successful alkylation was supported by 1H NMR spectrum, which shows one singlet at 9.15 ppm characteristic for the NCHN proton. Two two triplets at 4.39 ppm and 2.88 ppm are assigned to the two inequivalent methylene groups on the thioether-bridge. A base peak in the ESI mass spectrum at m/z = 621 for the monocation [M – Br]+ also corroborates the formation of G⋅2HBr. Cl H N Cl N K2 CO3, PhCH 2Br CH3 CN N Cl N Br Na 2S 9 H 2O CH 3CN 80 °C, 48 h N Cl N BrCH2 CH2 Br CH 3CN Cl N Cl N X Br "X" Cl N Cl N S N Cl N Cl PhCH 2Br CH 3 CN Cl N Cl N Ph E (58%) Br Ph Ph K2 CO3 BrCH 2CH 2Br CH3 CN, RT Cl Cl F (75%) S 2Br N Cl N Cl Ph G . 2HBr (78%) Scheme 2. 2.55 Synthesis of thioether-bridged diimidazolium salts G⋅2HBr. Pd(II) CSC pseudo-pincer complex 6. Mixing G ⋅ 2HBr with PdBr2 prior to the addition of Ag2O, G-κ2C)] (6) in 72% yield. Complex 6 is well soluble in would afford the complex trans-[PdBr2(G chlorinated solvents, CH3CN, acetone, DMF, DMSO and MeOH, but insoluble in THF, hexane, toluene and ether. The 1H NMR spectrum for complex 6 at room temperature also shows two poorly resolved broad signals in the range of 7 ppm to 2 ppm. These two broad chemical shifts could be attributed to fluxionality of the dangling thioether-moiety as a result of pseudo-pincer formation. When the temperature is decreased to 223 K, the broad signals become narrow and split into a range of multiplets in line with a reduced movement of the thioether-bridge. Figure 2.6 shows the variable temperature 1H NMR experiment for complex 6. The signals for NCN, NCH2 and CH2S could not be detected due to 14 fluxionality in the 13 C NMR spectrum. The formation of complex 6 is also supported by a base peak in the ESI mass spectrum at m/z = 727 assignable to the mobocation [M – Br]+. Figure 2.6 Variable temperature 1H NMR experiment for complex 6. The identity of 6 was finally confirmed by X-ray diffraction analysis on single crystals obtained by slow evaporation of a concentrated CH2Cl2 solution. The molecular structure of complex 6 was depicted in Figure 2.7, showing that the sulfur moiety was uncoordinated. Unlike the cis-configured pseudo pincer complex 1, the two carbene donors in 6 are trans to each other. Two bromido ligands with the two carbene ligands complete the square planar coordination sphere around Pd(II). Similar to complex 1, the sulfur atom points away from the coordination plane. The inter-NHC angle is 38.2(3)°, which is smaller than that of complex 1 [80°]. The dihedral angles between the NHC plane and the [PdC2SBr] coordination plane are 72.3(2)° and 69.5(2)° respectively. The Pd-Br bond lengths of 2.4165(11) and 2.4243(10) Å are expectedly shorter than those in the cis-configured benzimidazole based system, since they do not experience any trans influence of the carbene donors.10 15 Figure 2.7 Molecular structure of 6 showing 50% probability ellipsoids; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1-C1 2.027(7), Pd1-C13 2.030(7), Pd1-Br2 2.4165(11), Pd1-Br1 2.4243(10), N1-C1 1.349(9), N2-C1 1.366(9), N3-C13 1.325(9), N4-C13 1.357(10); C1-Pd1-C13 171.7(3), C1-Pd1-Br2 89.7(2), C13-Pd1-Br2 90.2(2), C1-Pd1-Br1 91.2(2), C13-Pd1-Br1 89.3(2), Br2-Pd1-Br1 177.46(4), N1-C1-N2 106.4(6), N3-C13-N4 106.2(6); PdC2Br2/NHC dihedral angle 72.3(2)°, 69.5(2)°; inter-NHC angle 38.2 (3)°. 16 Figure 2.8 Experimental X-ray diffraction pattern (a) and calculated pattern (b) for 6. More evidence is needed to draw the conclusion that complex 6 is purely a pseudo pincer in the solid state. In order to confirm that the pseudo-pincer structure determined on a single crystal is representative of the bulk, X-ray power diffraction analysis was performed on crystalline material of complex 6. Indeed, the powder pattern obtained agrees well with the calculated pattern (Figure 2.8), indicating that 6 preferably forms a trans-dibromido-dicarbene pseudo-pincer complex. These results suggest that the donor strength of the NHC moiety in CSC-type ligands indeed influences the 17 coordination mode. Complexes with strong electron donating ligands would form pincer; on the contrary, complexes with weak electron donating ligands prefer to be pseudo-pincer. 18 Chapter 3. Donating ability of NHC ligands and conductivity of pincer-type complexes 3.1 Donating ability of two mono-NHC ligands As presented in our previous work, the donating ability of ligands can be evaluated by a NHC probe. We also determined the donating ability sequence for some common NHCs, which are depicted in figure 3.1.5 Figure 3.1 Donor abilities of common NHCs on the 13C NMR scale. However, we noticed that the difference between the two ligands iPr2-bimy and Bn2-bimy is 2.3 ppm on 13C NMR scale, which is quite a significant difference.38 Furthermore, in the case of iPr2-bimy, there are two i Pr2-bimy ligands in the desired complex. The accuracy of the measuring may be affected by the presence of two iPr2-bimy in the homo-bis(carbene) complex. In order to inspect the measuring, complexes 7 and 8 were synthesized in the route depicted by Scheme 3.1. 19 Cl N N + 1/2 Cl N R Br N Pd Br Br Pd N Br Br N H. HBr: R = isopropyl I. HBr: R = benzyl 1.1 Ag 2O CH 2Cl2 Cl N - AgBr Cl N R Br Pd Br N N 7 : R = isopropyl 8 : R = benzyl Scheme 3. 3.11 Synthesis of complexes 7 and 8. H⋅HBr was prepared according to a literature method.39 Complex 7 was synthesized with a well-established method, involving H ⋅ HBr, 0.6 equiv of Ag2O and 0.5 equiv of [PdBr2(iPr2-bimy)]2, affording complex 7 (scheme 3.2).40 The reaction proceeded straightforwardly, and complex 7 was isolated in 82% yield by simple filtration to remove AgBr. Complex 7 is soluble in DCM, chloroform, DMF, DMSO, acetone, acetonitrile and methanol, but insoluble in toluene, diethyl ether and hexane. The absence of the downfield signal characteristic for H ⋅ HBr at 11.67 ppm indicates successful deprotonation of the precursor salt. A singlet at 5.98 ppm is assigned to the NCH protons of the methylene group adjacent to the nitrogen atoms on H. A septet centered at 5.83 ppm is assigned to the NCH protons of the isopropyl substituents on iPr2-bimy.. The methyl groups on iPr2-bimy show a doublet at 1.58 ppm. As expected, two carbene signals are detected in the 13 C NMR spectrum. The assignment of the carbene peak is confirmed by HMBC experiment. The relatively downfield peak at 176.1 ppm is due to the carbene carbon of iPr2-bimy, while the one at 175.0 ppm is assigned to that of H. Compared with previous NHC analogues, 13C NMR resonance for H is one of the most upfield, suggesting that its electron donating ability is one of the poorest, due to the two chlorine atoms on its imidazole backbone. The formation of 7 is also supported by a base peak at m/z = 705 in the ESI mass spectrum assignable to the monocation [M – Br]+. The identity of 7 as a hetero-bis(carbene) complex was finally confirmed by X-ray diffraction analysis on 20 single crystals obtained from slow diffusion of a concentrated DCM solution. The molecular structure is depicted in Figure 3.4. It shows that Pd(II) center is coordinated by one iPr2-bimy, H and two bromido ligands. As expected, the two carbene donors adopts a trans-configuration. The dihedral angles between the NHC planes and the [PdC2Br2] coordination plane are 64.6(1)° and 74.0(1)° for i Pr2-bimy and H, respectively. The Pd−Ccarbene bond with iPr2-bimy [2.017(4)Å] is within the normal range compared to other NHC analogues.5 The bond between Pd(II) and H is slightly shorter than the Pd−Ccarbene bond with iPr2-bimy. Figure 3.2 Molecular structure of 7 showing 50% probability ellipsoids; solvent and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Pd1-C1 2.017(4), Pd1-C14 2.040(4), Pd1-Br1 2.4226(6), Pd1-Br2 2.4356(6), N1-C1 1.351(5), N2-C1 1.341(5), N3-C14 1.352(5), N4-C14 1.352(5); C1-Pd1-C14 176.92(16), C1-Pd1-Br1 87.82(11), C14-Pd1-Br1 90.39(11), C1-Pd1-Br2 88.87(11), C14-Pd1-Br2 92.93(11), Br1-Pd1-Br2 176.67(2), N2-C1-N1 108.1(3), N4-C14-N3 105.0(3). Salt I ⋅ HBr was synthesized according to a similar reported method.41 Complex 8 was synthesized in analogy to complex 7. Complex 8 is soluble in DCM, chloroform, DMF, DMSO, acetone, acetonitrile and methanol, but insoluble in toluene, diethyl ether and hexane. The absence of the downfield signal characteristic for I ⋅ HBr at 11.56 ppm indicates successful deprotonation of the precursor salt. A singlet at 21 5.92 ppm is assigned to the NCH2 methylene protons on I. One septet at 6.08 ppm is assigned to the NCH protons of the isopropyl substituents on I, and another septet at 5.98 ppm is assigned to that of iPr2-bimy. The methyl groups on iPr2-bimy show two doublets at 1.82 ppm and 1.56 ppm, assignable for iPr2-bimy and I, respectively. As expected, two carbene signals are detected in the i 13 C NMR spectrum. The relatively downfield peak at 177.3 ppm is due to the carbene carbon of Pr2-bimy, while the one at 173.2 ppm is assigned to that of I. Compared with H, 13C NMR resonance for I is downfield by 1.2 ppm, suggesting that the electron donating ability of isopropyl group is stronger than that of benzyl group by 1.2 ppm on 13C NMR scale. Our previous study shows that changing two benzyl groups to two isopropyl groups will lead to a downfield shift of 2.3 ppm. In this study, changing one benzyl group to isopropyl group causes a downfield shift of 1.2 ppm, which is almost half of 2.3 ppm. The result in this study agree well with our previous work. The formation of 8 was also supported by a base peak at m/z = 659 in the ESI mass spectrum assignable to the monocation [M – Br]+. 3.2 Donating ability of pincer-type ligands The donating abilities of ligands play an important role in influencing the chemical properties and reactivities of their metal complexes.42 According to our previous work, the electron donating ability of NHC ligands is also a main factor affecting the formation of pseudo-pincer versus pincer complexes.12 It is thus of great interest to determine the donor strengths of these pincer-type carbene ligands with the methodology established by our group.5 Hence, a series of Pd(II) dinuclear-tetracarbene complexes 9 to 14 一 {[Pd2Br2L(iPr2-bimy)2] (L= B, D, G, J, K, L )}} was synthesized according to the method in our previous B, D, G, J, K, work to evaluate the electron donating ability of those pincer-type ligands. Ligand precursors (B L) were treated with 1.2 equiv of Ag2O and 1.0 equiv of [PdBr2(iPr2-bimy)]2 in DCM or chloroform to afford complexes 9 to 14 (Scheme 3.2). 一 J, K and L are synthesized by my labmate Dr. Yuan Dan, hence no discussion on their synthesis is included in this dissertation. 22 Scheme 3. 3.22 Synthesis of complexes 9 to 14 14. X X S N X N R N Br Br Pd Br Pd L Br L X X N S N X N R X Pd N R X N R Br Br Ligand disproportionation + S N Br Br N R N Pd Pd Br L L Br N N R S Br L Pd L Br N Pd N R Br Br N R N L= N ene complexes Scheme 3.3 Ligand disproportionation of Tetra carb carbene Complexes 9 to 14 were isolated in moderate to good yields by a simple filtration, which removes AgBr. All of the complexes are soluble in DCM, chloroform, DMF, DMSO, acetone, acetonitrile and methanol, but insoluble in toluene, diethyl ether and hexane. However, due to ligand disproportionation, pure complexes 二 could not be obtained. Selected chemical shifts for complexes 9 to 14 are listed in Table 3.1. 二 Good elemental analysis result could not be obtained due to ligands disproportionation. 23 Complexes 9 10 11 12 13 14 NCH2 4.64 5.00 5.09 5.17 4.84 4.76 SCH2 3.31 3.64 3.64 3.67 3.51 3.39 CH3-iPr2-bimy 1.82 1.90, 1.66 1.88-1.84 1.84, 1.66 1.79, 1.42 1.88-1.67 NCH-iPr2-bimy 6.24-6.11 6.21, 5.93 6.34-6.08 6.23, 6.01 6.02, 5.47 6.21, 6.01 NCN-L 169.1 164.6 182.2 184.5 172.9 171.0 NCN-iPr2-bimy 179.9 176.4 179.4 178.5 178.2 179.0 14 Table 3.1 Selected 1H and 13C NMR data in ppm for complexes 9-14 14. All 1H NMR spectra for complexes 9 to 12 show one triplet assignable to the NCH2 groups on the thioether bridges in the range of 5.17 ppm to 4.64 ppm. The spectra also show one triplet, which is assigned to the SCH2 groups on the thioether bridges, in the range of 3.67 ppm to 3.31 ppm. For complex 10 10, 11 11, 12 12, 13 and 14 14, two doublets or overlap-caused multiplet are found in the range of 1.90 ppm to 1.42 ppm, which are assigned to the CH3 groups on iPr2-bimy. Complex 7 only shows one doublet for the methyl groups on i Pr2-bimy. Spectra for complexes 10 10, 12 12, 13 and 14 show two mutiplets assigned to the NCH groups on i Pr2-bimy. Due to overlap, complexes 9 and 11 only afford one mutiplet which is assignable to the NCH groups on iPr2-bimy. As expected, two carbene signals are found in the 13C NMR spectrum for each complex. The signals for carbene carbons on B, D, G, J, K, and L fall in the range of 184.5 ppm to 164.6 ppm. NHC ligands derived from benzimidazolin-2-ylidenes show more downfield signals (complexes 10 and 12 12), and ligands derived from imidazolin-2-ylidenes show more upfield resonance (complex 9, 11 11, 13 and 14 14). The signals for carbene carbons on iPr2-bimy appear in a relatively narrow range, from 179.9 ppm to 176.4 ppm. Carbene carbon for iPr2-bimy in 9 is the most downfield one at 179.9 ppm and iPr2-bimy in 10 is the most upfield one at 176.4 ppm. The formations of all the complexes are further supported by a base peak in their corresponding ESI mass spectra. According to our previous work, a more downfield carbene resonance for iPr2-bimy indicates stronger donating co-ligands.5 With the results discussed above, the donating ability of the pincer-type carbene 24 ligands is in the order D (179.9 ppm) > B (179.4 ppm) > L (179.0 ppm) > J (178.5 ppm) > K (178.2 ppm) > G (176.4 ppm) (Figure 3.1). This observation is reasonable and can be explained by the inductive effect of different substituents. For example, compared to L, there are four chlorine atoms on the backbone of G. The stronger –I effect caused by chlorine atoms leads to an upfield shift of the resonance for the carbene carbon of iPr2-bimy, in this case, from 179.0 ppm to 176.4 ppm. D L S N N N N N N S K N N N Mes Ph N S N N N Mes Ph N N N N S N N Ph B S N J Ph Cl N Cl N S Ph N Cl N Cl Ph G Figure 3.3 Donating abilities of pincer-type NHCs on 13C NMR scale. Our previous work has shown that electron donating ability will affect the formation of pincer versus pseudo-pincer. However, in this study, complex 3 disobey this principle established early. In our previous work, the pincer-type complex corresponding to L was a pincer.12 According to the 13Ccarbene resonance, B is a stronger σ-donating ligand than L, indicating that complex 3 should have even stronger tendency to be a pincer. However, as discussed in chapter 2, the corresponding pincer-type complex for B, complex 3 was identified to be a pseudo-pincer by both X-ray diffraction analysis from single crystals and X-ray powder diffraction analysis from crystalline material. Furthermore, the comparison betwee B and J, whose corresponding pincer-type complex is also identified to be a pseudo-pincer in our previous work, may indicate that these different substituents on the benzimidazolium backbone may not induce effect large enough to change the formation of pseudo-pincer versus pincer. However, taking aside ligand B, the 25 remaining ligands D, G, J, K and L comply with the rule discovered in our previous work. 3.3 Conductivity of pincer-type complexes As discussed in chapter 1, X-ray diffraction study can only determine the molecular structure in solid state. As most catalytic reactions are conducted in solution, a method to determine the structure in solution is necessary. Considering that pseudo pincer complex is a neutral molecule while pincer complex is ionic, remarkable difference in conductivity between pincer and pseudo pincer in solution is expected. Furthermore, the free bromide anion in a pincer complex could be exchanged for another anion with greater conductivity, which could lead to a greater increase of conductivity of a pincer complex. On the contrary, the two coordinated bromido ligands in a pseudo pincer complex should not undergo exchange with other anions easily. Based on this, a new method to determine the identity of pincer-type complex is established. 26 N N Ph S Pd Br Br S N N Pd Br N Mes N Ph S Ph Br Br N N N Ph Ph N Pd Br Ph 17 S Br N Ni N Br N N N N Pt Br N Br Ph S N 18 N N 16 Pt N N N Mes Br 15 N S N Br Ph 19 Ph Ph 20 S N Pt N Br N Mes Mes Br 21 Figure 3.4 Previously synthesized pincer-type complexes used for conductivity test. Initially, non-coordinated solvents such as DCM and chloroform were used for the conductivity study. However, only a limited number of complexes could dissolve in DCM and chloroform. Some complexes do not have good solubility even in more polar solvents such as nitromethane and methanol. In order to conduct the experiments with all our pincer-type complexes, DMF was finally used as solvent. Complexes 三 synthesized in our previous work (complex 15 to 2143, figure 3.4) were also included in the conductivity experiment. The preliminary results are depicted in Table 3.2. 三 19 were synthesized by my labmate Dr. Yuan, hence no discussion on their synthesis is included in this dissertation. Complexes 15 15-19 27 Table 3.2 Conductivity test for pincer type complexesa. Pincer complex 4 15 16 18 Conductivity in DCM/μS −b 21 Pseudo pincer complex 3 6 17 19 −b 20.4 33.2 −b Conductivity in DCM/μS 3.8 1.3 7.5 −b Conductivity in DMF/μS 233.0 108.3 151.8 232.0 192.8 Conductivity in DMF/μS 10.9 4.4 33.4 28.9 −b 20 10.9 Blank 0.03 1.56 a All results are average of two runs. b Conductivity could not be tested due to insolubility. According to the results of conductivity test, the pincer-type complexes are sorted into two groups. In DMF, the group made up by pincer complexes 4, 15 15, 16 16, 18 and 21 shows much higher conductivity than the other group composed by supposedly pseudo pincer complexes 3, 6, 17 17, 19 and 20 20. The difference in conductivity between these two groups is significant, which supports our first hypothesis that a ionic pincer complex will show better conductivity. Based on the results presented by Table 3.3, further experiment on exchanging the anion was carried out. Pincer and pseudo pincer complexes were stirred with 10 equiv of NaBF4 in DMF overnight. Then solvent was removed under vacuum, followed by washing with large amounts of deionized water. The BF4– anion was used due to its better conductivity compared to Br–. Table 3.3 summarizes the results. 28 Table 3.3 Conductivity test for pincer type complexes after anion exchangea. Pincer complex 4 15 16 18 Conductivity in DCM/μS −b 21 Pseudo pincer complex 3 6 17 19 −b 29.8 37.5 −b Conductivity in DCM/μS 4.1 2.3 7.6 −b Conductivity in DMF/μS 254.0 180.8 204.6 243.0 263.0 Conductivity in DMF/μS 14.0 5.7 35.3 42.7 −b 20 37.8 Blank 0.03 1.56 a b All results are average of two runs. Conductivity could not be tested due to insolubility. Table 3.3 further shows the difference between pincer complexes and pseudo complexes. After anion exchange with excess NaBF4, conductivity of the pincer complexes generally increases greatly. On the contrary, and as expected, the conductivity for pseudo complexes was only improved slightly, because the bromido ligands in pseudo-pincer complexes could not be exchanged easily. It is noteworthy that when the solvent was changed from DCM to DMF, the conductivity for pincer complexes increases largely, while the increase for pseudo pincer complexes is much smaller. 29 II Chapter 4. Sulfonate-functionalized NHC Palladium( Palladium(II II)) complex and catalytic studies in Suzuki-Miyaura coupling reaction 4.1 Synthesis of sulfonate-functionalized NHC Pd(II) complex The synthesis of M is depicted in Scheme 4.1. 1-benzylbenzimidazole was refluxed with 1,3-propane sultone to afford salt M in a yield of 97%. The 1H NMR spectrum of salt M shows a downfield signal at 9.91 ppm, which is characteristic for the NCHN proton in benzimidazolium salts. The spectrum also shows two triplets and one multiplet at 4.66 ppm, 2.50 ppm and 2.22 ppm, respectively, which are assignable to the three inequivalent methylene groups of the sulfopropyl N-substituents. The PhCH2 methylene group gives rise to a singlet at 5.74 ppm. The formation of salt M is also supported by a base peak in the ESI mass spectrum at m/z = 329 arising from the monoanion [M – H]–. O 3S N O S + N O O Toluene Reflux, 12 h N N Scheme 4. 4.11 Synthesis of salt M. Complex 22 was synthesized according to Scheme 4.2. Stirring 2 equiv of salt M with 10 equiv of KBr and 2 equiv of Pd(OAc)2 at 90 ºC in DMSO afforded complex 22 in a yield of 93%. Complex 22 is soluble in water, CH3CN, MeOH, DMSO, and DMF, but insoluble in solvents such as CH2Cl2, CHCl3, THF, ether, toluene and hexane. Compared to its salt precursor M, the disappearance of the NCHN signal in the 1H NMR spectrum supports the successful deprotonation. A singlet at 6.18 ppm was observed for the PhCH2 methylene group. Unfortunately, the carbene carbon resonance in the 13C NMR spectrum was not detected. 30 The formation of complex 22 is further supported by a base peak in ESI mass spectrum at m/z = 1308 assigned to the monocation [M + K]+. SO3K O3 S N N DMSO + 10 KBr + 2 Pd(OAc)2 2 N 90 C N Br Br Pd Br Pd N Br N KO3 S 22 Scheme 4. 4.22 Synthesis of complex 22. 4.2 Catalytic studies of complex 22 in Suzuki-Miyaura coupling reaction In a preliminary study, the dimeric complex 22 was tested for its catalytic activity in the Suzuki-Miyaura reaction. The coupling of activated 4-bromobenzaldehyde with phenylboronic acid with 0.5 mol % catalyst loading (1 mol % Pd) and a reaction time of 6 h at ambient temperature was chosen as a standard test reaction. Table 4.1 Effect of the solvent on the Suzuki-Miyaura cross-coupling reactionsa catalyzed by 22 31 O Br H O Cat 22 + B(OH)2 H 6h b Entry Solvent Yield [%] 1 H2O >99 2 CH3CN 93 3 CH3CN/H2O (1:1 in volume) >99 4 Toluene 97 a Reaction conditions: 1 mmol of aryl halide; 1.2 mmol of phenylboronic acid; 1.5 mmol of K2CO3; 0.5 mol% of catalyst loading (1 mol % Pd); 1 mL of solvent; ambient temperature. b Yields were determined by 1 H NMR spectroscopy for an average of two runs. Solvents screening was carried out first. The results summarized in Table 4.1 shows that complex 22 gives near quantitative yields in all selected solvents (entries 1 to 4). Entry 1 is especially noteworthy, since efficient coupling reactions in aqueous media usually require higher temperatures,44,45 microwave heating,46 or higher catalysts loading.47 In addition, the use of acetonitrile resulted in slightly lower yield (entry 2). Encouraged by these results, the coupling of other substrates with phenylboronic acid in pure water was investigated (Table 4.2). Similar to 4-bromobenzaldehyde, the coupling of 4-bromoacetophenone proceeded smoothly at ambient temperature, giving a quantitative yield (entry 1). However, the reaction of less activated 4-chlorobenzaldehyde, 4-bromotoluene, 2-bromotoluene and 3-bromoanisole, proceeded only at elevated temperature (80 ºC), and in lower yields (entries 4 to 7). The addition of [N(n-Bu)4]Br leads to a substantial improvement and the less activated substrates can be coupled in near quantitative yield (entries 8, 9, 11, and 12). Coupling of 4-bromobenzaldehyde with 4-biphenylboronic acid and 3-nitrophenylboronic acid are also investigated, giving moderate to good yields. Table 4.2 Suzuki-Miyaura coupling of aryl bromides with phenyl boronic acida catalyzed by complex 22 22. 32 Cat 22 X + R B(OH)2 R Entry Aryl halide Time [h] Yield [%]b 1 4-bromoacetophnone 6 >99 2 3-bromoanisole 6 0 3 4-chlorobenzaldehyde 6 0 c 4 4-chlorobenzaldehyde 21 14 5c 4-bromotoluene 21 32.5 6c 2-bromotoluene 21 93 7c 3-bromoanisole 21 13 c,d 8 3-bromoanisole 21 >99 c,d 9 2-bromoanisole 21 >99 10c,d 4-chlorobenzaldehyde 21 77 c,d 11 4-bromotoluene 21 >99 12c,d 2-bromotoluene 21 >99 e 13 4-Bromobenzaldehyde 6 >99 14f 4-Bromobenzaldehyde 6 77 a Reaction conditions: 1 mmol of aryl halide; 1.2 mmol of phenyl boronic acid; 1.5 mmol of K2CO3; 0.5 mol% of catalyst loading (1 mol % Pd); 1 mL of H2O. b Yields were determined by 1H NMR spectroscopy for an average of two runs. c Reaction was conducted at 80 ºC. d 1.5 equiv of TBAB is added. e 4-biphenylboronic acid was used instead of phenyl boronic acid and reaction was conducted at ambient temperaturee. f 3-nitrophenylboronic acid was used instead of phenyl boronic acid and reaction was conducted at ambient temperature. In a separate study, the influence of the catalyst loading on the Suzuki-Miyaura coupling reaction catalyzed by complex 22 was investigated as well (Table 4.3). It was observed that complex 22 could give quantitative yield with catalyst loading down to 5 × 10-6 mol%, and the reaction time extended to 96 h. However, further decrease of catalyst loading to 5 × 10-7 mol% resulted in a dramatic decrease of yield to 29%, although a relatively higher TON (based on Pd) of 29,000,000 was achieved. Table 4.3 Suzuki-Miyaura coupling reactionsa catalyzed by complex 22 22. O Br B(OH)2 H Entry 1 2 3 4 5 6 7 O Cat 22 + TBAB, H2 O [Cat 22 22] [mol%] 5 × 10-1 5 × 10-2 5 × 10-3 5 × 10-4 5 × 10-5 5 × 10-6 5 × 10-7 t [h] 21 21 21 96 96 96 96 Temp [ºC] 80 80 80 80 80 80 80 H Yield [%] >99 >99 >99 >99 >99 >99 29 b TON 100 1000 10,000 100,000 1,000,000 10,000,000 29,000,000 33 a Reaction conditions: 1 mmol of aryl halide; 1.2 mmol of phenylboronic acid; 1.5 mmol of K2CO3; 1.5 mmol of TBAB; 1 mL of H2O. b Yields were determined by 1H NMR spectroscopy for an average of two runs. Because of its good solubility in water, complex 22 can be easily separated from the organic starting materials and product in Suzuki-Miyaura reaction. Based on this, recovery of the catalyst is also investigated.48 The Suzuki-Miyaura coupling of 4-bromobenzaldehyde with phenylboronic acid was conducted in water. After coupling reaction, DCM was used to remove the coupling product and starting materials from the reaction mixture, followed by addition of a fresh batch of substrate for the next run. Catalyst 22 could keep active for 3 successive catalytic runs, each with quantitative yield. However, the catalyst only afforded 30% yield in the 4th run, indicating catalyst decomposition. 34 Chapter 5. Summary and conclusion Four main parts included in this dissertation are (a) synthesis of a series of CSC-pincer type palladium(II) complexes; (b) evaluating electron donating strength of CSC-pincer type NHC ligands; (c) establishing of a new methodology to determine the identity of pincer-type complex in solution; and (d) synthesis of a sulfonate-functionalized NHC palladium(II) complex and its catalytic application in Suzuki-Miyaura coupling reaction. Chapter 2 describes the synthesis and characterization of the precursor salts of CSC-pincer type ligands and B-κ2C)] (3) and pincer their corresponding palladium(II) complexes. Pseudo-pincer complex trans-[PdBr2(B D-κ3CSC)]Br (4) were synthesized by direct transfer of their corresponding complex trans-[PdBr(D G-κ2C)] (6) was silver-carbene complexes to [PdBr2(CH3CN)2]. Pseudo-pincer complex trans-[PdBr2(G synthesized by treating the ligand precursor with PdBr2 followed by deprotonation with Ag2O. Chapter 3 details with the synthesis and characterization of hetero-bis(carbene) complexes (77 and 8) and dinuclear-tetracarbene complexes (99 to 14 14). One-pot reaction of ligand precursors with [PdBr2(iPr2-bimy)]2 and Ag2O afforded the corresponding dinuclear-tetracarbene and hetero-bis(carbene) complexes. The electron donating abilities of CSC-pincer type ligands are measured by a methodology established by our group. A new methodology to determine the identity of pincer-type complex by conductivity study established as well. Chapter 4 describes the synthesis of sulfonate-functionalized NHC Pd(II) complex 22 and its catalytic activity in the Suzuki-Miyaura coupling reaction. Complex 22 was synthesized by reaction of its ligand precursor M with Pd(OAc)2. Complex 22 was proven to be an excellent catalyst for Suzuki-Miyaura coupling reaction in water. Furthermore, a preliminary study about the recycling of the catalyst is also included. Most of the new compounds synthesized in this work have been characterized by 1H NMR, 13 C NMR 35 spectroscopies and ESI mass spectrometry. Some of them are further characterized by X-ray diffraction analysis and Elemental analysis. Future investigations may involve extending the structural diversity of pincer-type NHC ligands as well as pincer-type NHC-metal complexes. Expanding the scope of sulfonate-functionalized NHC ligands and sulfonate-functinalized NHC-metal complexes may also be included in further work. Thorough catalytic study with more substrates in the Suzuki-Miyaura coupling reaction may also be investigated in the future work. 36 Chapter 6. Experimental Section 1-isopropyl-3-(2-bromoethyl)-benzimidazolium bromide (A) A mixture of 1-isopropyl-1H-benzimidazole (641 mg, 4.0 mmol) and 1,2-dibromoethane (4 mL) was heated at 85 ºC overnight. All the volatiles were N Br N Br removed in vacuo and acetone (20 mL) was added to dissolve the residue. The resulting suspension was filtered over Celite. Removing acetone in the filtrate gave the product as a white solid (1086 mg, 3.1 mmol, 78%). 1H NMR (500 MHz, CDCl3): δ 11.26 (s, 1 H, NCHN), 7.93 −7.91 (m, 1 H, Ar −H), 7.78 −7.76 (m, 1 H, Ar −H), 7.66 −7.63 (m, 2 H, Ar −H), 5.25 (t, 3 J(H,H) = 5.7 Hz, 2 H, NCH2), 5.00 (m, 1 H, CH3CH), 4.10 (t, 3J(H,H) = 5.7 Hz, 2 H, CH2Br), 1.83 (d, 3 J(H,H)= 4.17 Hz, 6 H, CH3). 13 C{1H} NMR (125.76 MHz, CDCl3): 142.4 (s, NCHN), 132.6, 131.0, 128.0, 127.8, 114.5, 114.0 (s, Ar−C), 52.7 (s, NCH2), 49.4 (s, CHCH3), 30.9 (s, CH2Br), 22.9 (s, CH3). MS (ESI): m/z = 269 [M – Br]+. B⋅2HBr A mixture of salt A (696 mg, 2 mmol) and Na2S·9H2O (240 mg, 1 mmol) N N S 2Br N was stirred in CH3CN at ambient temperature for 48 h. All the volatiles N were removed in vacuo and the resulting solid was washed with acetone (3 × 10 mL). CH2Cl2 (10 mL) was added to the residue, and the suspension was filtered over Celite. Removal of the solvent from the filtrate gave the product as a white solid (512 mg, 0.9 mmol, 90%). 1H NMR (300 MHz, CDCl3): δ 11.25 (s, 2 H, NCHN), 7.83 − 7.80 (m, 2 H, Ar − H), 7.73 − 7.70 (m, 2 H, Ar−H), 7.62−7.58 (m, 4 H, Ar−H), 5.15 (t, 3J(H,H) = 8.1 Hz, 4 H, NCH2), 4.94 (m, 2 H, CH3CH), 3.61 (t, 3 J(H,H) = 8.1 Hz, 4 H, CH2S), 1.88 (d, 3J(H,H)= 6.75 Hz, 12 H, CH3). 13 C{1H} NMR (75.47 MHz, CDCl3): 143.2 (s, NCHN), 132.2, 131.2, 127.7, 127.4, 114.1, 113.8 (s, Ar−C), 52.6 (s, NCH2), 47.4 (s, 37 CHCH3), 29.9 (s, CH2Br), 22.7 (s, CH3). MS (ESI): m/z = 489 [M – Br]+. isopropyl -3-(2-bromoethyl)-imidazolium bromide (C) 11-isopropyl isopropyl-3-(2-bromoethyl)-imidazolium A mixture of 1-isopropylimidazole (441 mg, 4.0 mmol) and 1,2-dibromoethane (4 mL) was heated at 85 ℃ overnight. All the volatiles were removed in vacuo and acetone (20 mL) N N was added to dissolve the residue. The resulting suspension was filtered over Celite. Br Removing acetone in the filtrate gave the product as a white solid (858 mg, 2.9 mmol, 72%). Br 1 H NMR (300 MHz, CDCl3): δ 10.33 (s, 1 H, NCHN), 7.88 (s, 1 H, NCH), 7.56 (s, 1 H, NCH), 4.89 (t, 3 J(H,H) = 5.60 Hz, 2 H, NCH2), 4.78 (m, 1 H, CH 3CH), 3.91 (t, 3J(H,H) = 5.67 Hz, 2 H, CH 2Br), 1.59 (d, 3 J(H,H) = 6.72 Hz, 6 H, CH3 ). 13C{1H} NMR (125.76 MHz, CDCl3): 136.6 (s, NCHN), 123.9, 120.5 (s, Ar−C), 54.1 (s, NCH2), 51.6 (s, CHCH3), 31.4 (s, CH2Br), 23.7 (s, CH3). D⋅2HBr A mixture of salt C (596 mg, 2 mmol) and Na2S·9H2O (240 mg, 1 mmol) was stirred N S N 2Br N in CH3CN at ambient temperature for 48 h. All the volatiles were removed in vacuo N and the resulting solid was washed with acetone (3 × 10 mL). CHCl3 (10 mL) was added to the residue and the suspension was filtered over Celite. Removal of the solvent from the filtrate gave the product as a white solid (384 mg, 0.82 mmol, 82%). 1H NMR (500 MHz, CDCl3): δ 10.46 (s, 2 H, NCHN), 8.35 (s, 2 H, NCH), 7.38 (s, 2 H, NCH), 4.76 − 4.67 (m, 6 H, NCH2 + CH3CH), 3.36 (t, 3 J(H,H) = 7.55 Hz, 4 H, SCH2), 1.61 (d, 3J(H,H)= 6.30 Hz, 12 H, CH3). 13 C{1H} NMR ( 125.76 MHz, CDCl3): 136.6 (s, NCHN), 124.7, 120.0 (s, Ar −C), 54.0 (s, NCH2), 49.9 (s, CHCH3), 31.9 (s, CH2Br), 23.8 (s, CH3). MS (ESI): m/z = 387 [M – Br]+. 1-(2-bromoethyl)-4,5-dichloroimidazole (E) 38 Cl N Cl N Br 4,5-dichloro-1H-imidazole (2.74 g, 20 mmol) and K2CO3 (4.14 g, 30 mmol) were stirred in acetonitrile (20 mL) for 0.5 h. 1,2--dibromoethane (6 mL) was added to the mixture and stirring was continued at ambient temperature overnight. H2O (100 mL) was added to the reaction mixture and the aqueous phase was extracted with CH2Cl2 (4 × 50 mL). The organic phases were combined and dried over Na2SO4. After the solvent was removed under reduced pressure, the solid residue was dissolved in diethyl ether (20 mL). The resulting suspension was filtered over Celite, and the solvent of the filtrate was removed in vacuo to obtain E as an off-white solid (2.83 g, 11.6 mmol, 58%). 1H NMR (300 MHz, CDCl 3): δ 7.44 (s, 1H, NCHN), 4.25 (t, 3J(H,H) = 6.2 Hz, 2H, NCH2), 3.51 (t, 3 J(H,H) = 6.2 Hz, 2H, CH2Br). 13 C{1H} NMR (75.47 MHz, CDCl3): 135.5 (s, NCHN), 126.5, 113.0 (s, CCl), 47.7 (s, NCH2), 29.8 (s, CH2Br). MS (ESI): m/z = 245 [M + H]+. Sulfur-bridged 4,5-dichloroimidazole (F) E (2.44 g, 10 mmol) and Na2S·9H2O (1.2 g, 5 mmol) were stirred for 2 days in Cl N Cl N S N N Cl Cl acetonitrile (20 mL) at 80 ℃. H2O (100 mL) was added to the reaction mixture and the aqueous phase was extracted with CH2Cl2 (4 × 50 mL). The organic phases were combined, dried over Na2SO4, and the solvent was removed under reduced pressure. The resulting solid was washed with diethyl ether (4 × 20 mL) to give F as a light yellow solid (1.35 g, 3.8 mmol, 75%). 1H NMR (500 MHz, CDCl3): δ 7.48 (s, 2H, NCHN), 4.06 (t, 3J(H,H) = 6.7 Hz, 4H, NCH2), 2.72 (t, 3 J(H,H) = 6.7 Hz, 4H, CH2S). 13C{1H} NMR (125.77 MHz, CDCl3): 135.4 (s, NCHN), 126.9, 113.5 (s, CCl), 46.7 (s, NCH2), 32.5 (s, CH2S). MS (ESI): m/z = 361 [M + H]+. G⋅2HBr Cl N S F (1.08 g, 3 mmol) and benzyl bromide (14.4 mL, 12 mmol) were stirred for 2 N Cl days in acetonitrile (20 mL) under reflux. All the volatiles were removed under 2Br Cl N N Cl 39 reduced pressure, and the residue was washed with ethyl acetate (4 × 20 mL) to obtain G ⋅ 2HBr as a light brown solid (1.43 g, 2.0 mmol, 68%). 1H NMR (500 MHz, D2O): δ 9.15 (s, 2H, NCHN), 7.41 (s, 10H, Ar−H), 5.43 (s, 4H, PhCH2), 4.42 (t, 3J(H,H) = 5.8 Hz, 4H, NCH2), 2.91 (t, 3J(H,H) = 5.8 Hz, 4H, SCH2). 13 C{1H} NMR (125.77 MHz, D2O): 131.8 (s, NCN), 129.7, 129.5, 129.2, 128.9, 120.1, 119.9 (s, Ar−C and Cl−C, 52.2 (s, CH2Ph), 47.8 (s, NCH2), 30.1 (s, SCH2). MS (ESI): m/z = 621 [M – Br]+. Salt M 1-benzylbenzimidazole (1.458 g, 7 mmol) was refluxed with 1,3-propane sultone (10 mL) in O3 S toluene for overnight. After removing the volatiles in vacuo, the remaining solid was washed with EA (4 × 20 mL) to give a white solid. Yield: 2.243 g, 6.79 mmol, 97%. 1H NMR (500 N N MHz, DMSO-d6): δ 9.91 (s, 1 H, NCHN), 8.11 (d, 1 H, Ar–H), 7.90 (d, 1 H, Ar–H), 7.67–7.60 (m, 2 H, Ar–H), 7.49 (d, 2 H, Ar–H), 7.40–7.35 (m, 2 H, Ar–H), 5.74 (s, 2 H, CH2Ph), 5.03 (t, 3 J(H,H) = 6.9 Hz, 2 H, NCH2), 2.50 (m, 2 H, CH2SO3), 2.22 (m, 2 H, CH2CH2SO3). 13C{1H} NMR (125.76 MHz, DMSO–d6): 142.7 (s, NCHN), 134.0, 131.4, 130.9, 129.0, 128.6, 128.2, 126.6, 113.9, 113.8 (s, Ar–C), 49.9 (s, NCH2Ph), 47.5 (s, NCH2), 45.8 (s, CH2SO3), 25.3 (s, CH2CH2SO3). MS (ESI): m/z = 329 [M – H]–. κ2C)] (3) trans-[PdBr2(B(B-κ A mixture of salt B ⋅2HBr (568 mg, 1 mmol) and Ag 2O (254 mg, 1.1 mmol) S Br N Pd N Br N N was stirred in chloroform (5 mL) at ambient temperature for 6 h shielded from light. The resulting Ag-carbene complex was then directly transferred into a solution of [PdBr2(CH3CN)2] in CH3CN, which was in turn prepared in situ by heating PdBr2 (264 mg, 1 mmol) in CH3CN (5 mL) at 70 ºC for 6 h. Immediate precipitation of AgBr was observed. The reaction mixture was stirred at ambient temperature overnight. All the volatiles were removed in vacuo 40 and DCM (20 mL) was added to the residue. The resulting suspension was filtered over Celite and the solvent of the filtrate was removed under vacuum to afford the product as a yellow solid (437 mg, 0.65 mmol, 65%). Slow evaporation of a concentrated chloroform solution afforded yellow crystals. 1H NMR (500 MHz, CDCl3): 7.62−7.57 (m, 2 H, Ar−H), 7.45 (d, 2 H, Ar−H), 7.32−7.25 (m, 4 H, Ar−H), 5.70 (m, 2 H, CH3CH), 5.13 (br, 4 H, NCH2), 3.92 (br, 4 H, SCH2), 1.89 (d, 3J(H,H)= 6.90 Hz, 12 H, CH3). 13 C{1H} NMR (125.76 MHz, CDCl3): 181.9 (s, NCHN), 136.5, 133.2, 123.8, 123.4, 113.4, 113.2, 110.7 (s, Ar−C), 55.0 (s, NCH2), 47.9 (s, CHCH3), 31.4 (s, CH2S), 22.5 (s, CH3). Calc. for C24H30Br2N4PdS: C, 42.84; H, 4.49; N, 8.33. Found: C, 42.70; H, 4.58; N, 7.31% (better result could not be obtained despite repeated purification and analysis). MS (ESI): m/24= 593 [M – Br]+. κ3CSC)] (4) trans-[PdBr2(B(B-κ A mixture of D ⋅ 2HBr (468 mg, 1 mmol) and Ag 2O (254 mg, 1.1 mmol) was stirred N N S Pd Br Br N N in chloroform (5 mL) at ambient temperature for 6 h shielded from light. The resulting Ag-carbene complex was then directly transferred into a solution of [PdBr2(CH3CN)2] in CH3CN, which was prepared in situ by heating PdBr2 (264 mg, 1 mmol) in CH3CN (5 mL) at 70 ºC for 6 h. Immediate precipitation of AgBr was observed. The reaction mixture was stirred at ambient temperature overnight. All the volatiles were removed in vacuo and DCM (20 mL) was added to the residue. The resulting suspension was filtered over Celite and the solvent of the filtrate was removed under vacuum to afford the product as a yellow solid (412 mg, 0.72 mmol, 72%). 1H NMR (500 MHz, 243 K, CDCl3): 7.19 (s, 2 H, NCH), 7.02 (s, 2 H, NCH), 5.38 (m, 2H, CH3CH), 4.74 (s, br, 4 H, NCH2), 4.46 (d, 3J(H,H) = 13.85 Hz, 2 H, SCHH), 2.99 (t, 3J(H,H) = 10.05 Hz, 2 H, SCHH), 1.57 (d, 3J(H,H)= 6.3 Hz, 6 H, CH3), 1.47 (d, 3J(H,H)= 6.35 Hz, 6 H, CH3). 13C{ 1H} NMR (125.76 MHz, CDCl3): 166.1 (s, NCN), 123.4, 118.1 (s, Ar−C), 53.6 (s, NCH2), 37.3 (s, CH2S), 24.3 (s, CH3). Calc. for C16H26Br2N4PdS: C, 33.56; H, 4.58; N, 9.78. Found: C, 30.98; H, 4.56; N, 8.15% (better 41 result could not be obtained despite repeated purification and analysis). MS (ESI): m/z = 493 [M – Br]+. κ2C)] (6) trans-[PdBr2(G(G-κ A mixture of G ⋅ 2HBr (702 mg, 1 mmol) and PdBr2 (266 mg, 1 mmol) was Cl S N Br N Cl Pd Cl N Br N Cl stirred at 80 °C in acetonitrile (20 mL) overnight. Ag2O (348 mg, 1.5 mmol) was added to the reaction mixture, and the resulting mixture was heated under reflux for another 12 hours shielded from light. The suspension was filtered over Celite, and the solvent of the filtrate was removed under vacuum. The product was purified by column chromatography (eluent: CH2Cl2 : MeOH = 4 : 1) followed by recrystallization in CH2Cl2. Analytically pure compound were obtained as light greenish yellow crystals (580 mg, 0.72 mmol, 72%). 1H NMR (300 MHz, CDCl3): δ 7.41 (s, 4H, Ar − H), 7.27 (s, 6H, Ar − H), 5.52 (s, br, 4H, PhCH2), 4.91 (s, br, 4H, NCH2). The CH2S signal was not detected due to fluxionality. 13C{1H} NMR (75.47 MHz, CDCl3): 135.4, 129.6, 129.1, 128.9, 118.3 (s, Ar−C and CCl), 53.9 (s, CH2Ph). The NCN, NCH2 and CH2S signals could not be detected due to fluxionality. Anal. Calc. for C24H22Br2Cl2N4PdS: C, 35.65; H, 2.99; N, 6.93. Found: C, 35.94; H, 2.86; N, 7.15%. MS (ESI): m/z = 727 [M – Br]+. Complex 7 Salt I ⋅ HBr (79.2mg, 0.2 mmol), Ag2O (28 mg, 0.12 mmol), and [PdBr2(iPr − bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) and Cl Cl N Br N stirred at ambient temperature overnight shielded from light. The resulting N Pd Br N suspension was filtered over Celite, and the filtrate was dried in vacuo. Slow evaporation at ambient temperature of a concentrated DCM solution yielded light yellow needle crystals suitable for X−ray diffraction studies (128 mg, 0.16 mmol, 82%). 1H NMR (300 MHz, CDCl3): δ 7.69−7.66 (t, 4H, Ar−H), 7.50−7.48 (dd, 2 H, Ar−H), 7.46−7.43 (t, 4H, Ar−H), 7.38−7.35 (t, 2H, 42 Ar−H), 5.98 (s, 4 H, CH2Ph), 5.83 (m, 3J(H,H) = 7.08 Hz, 2 H, CH(CH3)2), 1.58 (d, 3J(H,H) = 7.08 Hz, 12 H, K), 135.8, 134.1, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 176.1 (s, NCN− iPr2 −bimy), 175.0 (s, NCN−K 129.4, 128.7, 128.5, 122.7, 118.2, 113.2 (s, Ar − C and CH), 54.5 (s, CH(CH3)2), 54.1 (s, CH2Ph), 21.4 (s, CH(CH3)2). Anal. Calcd for C30H32Br2Cl2N4Pd: C, 45.86; H, 4.10; N, 7.13. Found: C, 45.51; H, 4.33; N, 7.51%. MS (ESI): m/z = 705 [M − Br]+. Complex 8 Salt H ⋅ HBr (70.0mg, 0.2 mmol), Ag2O (28 mg, 0.12 mmol), and [PdBr2(iPr − bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) and Cl N Br Pd N stirred at ambient temperature overnight shielded from light. The resulting Cl N Br N suspension was filtered over Celite, and the filtrate was dried in vacuo. Slow evaporation at ambient temperature of a concentrated DCM solution yielded light yellow needle crystals suitable for X−ray diffraction studies (125 mg, 0.17 mmol, 85%). 1H NMR (500 MHz, CDCl3): δ 7.66 (d, 2 H, Ar−H), 7.56 (d, 1 H, Ar−H), 7.50 (d, 1 H, Ar−H), 7.41 (t, 2 H, Ar−H), 7.34 (t, 2 H, Ar−H), 7.20−7.15 (m, 2 H, Ar−H), 6.10−6.05 (m, 1 H, CH(CH3)2), 6.02−5.93 (m, 2 H, CH(CH3)2), 5.92 (s, 4 H, CH2Ph), 1.82 (d, 3J(H,H) = 7.6 Hz, 12 H, CH3), 1.56 (d, 3J(H,H) = 7.55 Hz, 6 H, CH3). 13C{1H} NMR H), 135.5, 134.2, 134.1, 129.3, 128.6, (125.76 MHz, CDCl3): δ 177.3 (s, NCN− iPr2 −bimy), 173.2 (s, NCN−H 128.5, 128.4, 122.7, 118.8, 116.1, 113.2, 113.2 (s, Ar−C), 57.4 (s, CH(CH3)2), 54.5 (s, CH(CH3)2), 54.5 (s, CH(CH3)2), 54.1 (s, CH2Ph), 22.1 (s, CH(CH3)2), 21.7 (s, CH(CH3)2), 21.2 (s, CH(CH3)2). MS (ESI): m/z = 659 [M − Br]+. 43 Complex 9 D ⋅ 2HBr (47 mg, 0.1 mmol), Ag2O (28 mg, 0.12 mmol), and S N N Br Br N Pd L Br Pd Br L [PdBr2(iPr−bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) and N stirred at ambient temperature overnight shielded from light. The resulting suspension was filtered over Celite. Removing CH2Cl2 from the filtrate gave N L= the product as a white solid (104 mg, 0. 084mmol, 84%). 1H NMR (300 N MHz, CDCl3): δ 7.58−7.54 (m, 4 H, Ar−H), 7.21−7.16 (m, 6 H, Ar−H), 6.87 D), (s, 2 H, Ar−H), 6.24−6.11 (m, 4 H, CH(CH3)2 − iPr2 −bimy), 5.55 (m, 3J(H,H) = 6.6 Hz, 2 H, CH(CH3)2 −D 4.65 (m, 3J(H,H) = 6.42 Hz, 4 H, NCH2), 3.31 (t, 3J(H,H) = 6.24 Hz, 4 H, SCH2), 1.82 (d, 3J(H,H) = 6.90 Hz, 24 H, CH3), 1.55 (d, 12 H, 3J(H,H) = 6.57 Hz, CH3). 13 C{1H} NMR (75.47 MHz, CDCl3): δ 179.9 (s, D), 134.2, 123.0, 122.6, 117.0, 113.2, 113.1 (s, Ar−C), 54.7 (s, CH(CH3)2), NCN−iPr2−bimy), 169.1 (s, NCN−D 54.3 (s, CH(CH3)2), 53.1 (s, CH(CH3)2), 52.1 (s, CH(CH3)2), 33.1 (s, SCH2), 23.8 (s, CH(CH3)2), 21.8 (s, CH(CH3)2), 21.7 (s, CH(CH3)2). Calc. for C42H62Br4N8Pd2S: C, 40.57; H, 5.03; N, 9.01. Found: C, 40.63; H, 4.89; N, 8.76%. MS (ESI): m/z = 1163 [M − Br]+. Complex 10 G ⋅ 2HBr (70 mg, 0.1 mmol), Ag2O (28 mg, 0.12 mmol), and Cl Cl Cl N Br Br N Ph S N Pd Br L Pd L Br Cl [PdBr2(iPr2−bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) Ph and stirred at ambient temperature overnight shielded from light. The N resulting suspension was filtered over Celite, and removing CH2Cl2 from L= N the filtrate gave the product as a white solid (136 mg, 0.092 mmol, 92%). N 1 H NMR (300 MHz, CDCl3): δ 7.68−7.66 (m, 4 H, Ar−H), 7.60−7.51 (m, 4 H, Ar − H), 7.45 − 7.34 (m, 8 H, Ar − H), 7.21 − 7.18 (m, 4 H, Ar − H), 6.21 (m, 3J(H,H) = 7.08 Hz, 2 H, CH(CH3)2), 5.96 − 5.86 (m, 6 H, CH(CH3)2 + PhCH2), 4.96 (t, 3J(H,H) = 7.95 Hz, 4 H, NCH2), 3.60 (t, 44 3 J(H,H) = 7.80 Hz, 4 H, SCH2), 1.86 (d, 3J(H,H) = 6.90 Hz, 12 H, CH3), 1.62 (d, 3J(H,H) = 6.9 Hz, 12 H, G), 135.3, 134.1, CH3). 13C{1H} NMR (75.47 MHz, CDCl3): δ 176.4 (s, NCN− iPr2 −bimy), 174.6 (s, NCN−G 134.0, 129.3, 128.7, 128.5, 122.7, 117.9, 117.6, 113.3, 113.2 (s, Ar − C), 54.8 (s, CH(CH3)2), 54.4 (s, CH(CH3)2), 54.0 (s, PhCH2 ), 49.8 (s, NCH2),32.4 (s, SCH2), 21.6 (s, CH(CH3)2), 21.3 (s, CH(CH3)2). Anal. Calcd for C48H58Br4Cl4N8Pd2S: C, 39.67; H, 4.02; N, 7.71. Found: C, 40.50; H, 3.63; N, 6.59% (better result could not be obtained due to ligands disproportionation). MS (ESI): m/z = 1397 [M − Br]+. Complex 11 B ⋅ 2HBr (57mg, 0.1 mmol), Ag2O (28 mg, 0.12 mmol), and S N N Br Br Pd Br [PdBr2(iPr − bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) N N Br L L and stirred at ambient temperature overnight shielded from light. The Pd resulting suspension was filtered over Celite, and removing CH2Cl2 from the L= N filtrate gave the product as a white solid (117 mg, 0. 087mmol, 87%). 1H N NMR (300 MHz, CDCl3): δ 7.60−7.51 (m, 8 H, Ar−H), 7.27−7.20 (m, 8 H, Ar − H), 6.34 − 6.08 (m, 6 H, CH(CH3)2), 5.09 (t, 3J(H,H) = 8.25 Hz, 4 H, NCH2), 3.64 (t, 3J(H,H) = 8.10 Hz, 4 H, SCH2), 1.88 − 1.84 (m, 36 H, CH3). 13 C{1H} NMR (75.47 MHz, B), 179.4 (s, NCN− iPr2 −bimy), 136.0, 134.3, 134.2, 133.1, 127.7, 123.6, 123.2, CDCl3): δ 182.2 (s, NCN−B 122.7, 113.4, 113.2, 113.0, 111.4 (s, Ar − C), 54.9 (s, CH(CH3)2), 54.8 (s, CH(CH3)2), 54.5 (s, CH(CH3)2), 48.6 (s, NCH2), 32.3 (s, SCH2), 22.0 (s, CH(CH3)2), 21.8 (s, CH(CH3)2), 21.8 (s, CH(CH3)2). Calc. for C50H66Br4N8Pd2S: C, 44.69; H, 4.95; N, 8.34. Found: C, 44.38; H, 4.99; N, 8.43%. MS (ESI): m/z = 1263 [M − Br]+. S N N Ph Br Br Pd Br L Complex 12 N L N Pd Br Ph J ⋅ 2HBr (66.5mg, 0.1 mmol), Ag2O (28 mg, 0.12 mmol), and 45 N L= N [PdBr2(iPr−bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) and stirred at ambient temperature overnight shielded from light. The resulting suspension was filtered over Celite, and removing CH2Cl2 from the filtrate gave the product as a white solid (130 mg, 0.09 mmol, 90%). 1H NMR (300 MHz, CDCl3): δ 7.66 (d, 4 H, Ar−H), 7.59−7.51 (m, 6 H, Ar−H), 7.42−7.30 (dd, 6 H, Ar−H), 7.23−7.11, (m, 10 H, Ar−H), 6.23 (m, 3 J(H,H) = 7.26 Hz, 2 H, CH(CH3)2), 6.13 (s, 4 H, PhCH2), 6.01 (m, 3J(H,H) = 7.08 Hz, 2 H, CH(CH3)2), 5.17 (t, 3J(H,H) = 7.64 Hz, 4 H, NCH2), 3.68 (t, 3J(H,H) = 7.80 Hz, 4 H, SCH2), 1.84 (d, 3J(H,H) = 6.90 Hz, 12 H, CH3), 1.66 (d, 3J(H,H) = 7.05 Hz, 12 H, CH3). 13 C{1H} NMR (75.47 MHz, CDCl3): δ 184.5 (s, H), 178.5 (s, NCN− iPr2 −bimy), 136.2, 135.3, 135.0, 134.2, 129.5, 128.7, 128.6, 124.0, 123.8, 122.7, NCN−H 113.4, 113.3, 111.9, 111.2 (s, Ar − C), 54.9 (s, CH(CH3)2), 54.5 (s, CH(CH3)2), 53.4 (s, CH2Ph), 48.9 (s, NCH2), 32.8 (s, SCH2), 21.8 (s, CH(CH3)2), 21.6 (s, CH(CH3)2). Calc. for C58H66Br4N8Pd2S: C, 46.47; H, 5.06; N, 8.03. Found: C, 48.38; H, 5.42; N, 8.33% (better result could not be obtained due to ligands disproportionation). MS (ESI): m/z = 1359 [M − Br]+. Complex 13 K ⋅ 2HBr (62 mg, 0.1 mmol), Ag2O (28 mg, 0.12 mmol), and S N Mes N Br Br N Pd Br L Pd L Br N Mes [PdBr2(iPr − bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) and stirred at ambient temperature overnight shielded from light. The resulting suspension was filtered over Celite, and removing CH2Cl2 from the N L= N filtrate gave the product as a white solid (124 mg, 0.089 mmol, 89%). 1H NMR (500 MHz, CDCl3): δ 7.61−7.59 (m, 1 H, Ar−H), 7.52 (d, 2 H, Ar−H), 7.42 (d, 2 H, Ar−H), 7.37 (s, 1 H, Ar−H), 7.16−7.12 (m, 4 H, Ar−H), 7.04 (s, 4 H, Ar−H), 6.85 (s, 2 H, Ar−H), 6.02 (m, 3J(H,H) = 7.55 Hz, 2 H, CH(CH3)2), 5.47 (m, 3J(H,H) = 6.95 Hz, 2 H, CH(CH3)2), 4.86 (t, 3 J(H,H) = 7.35 Hz, 4 H, NCH2), 3.53 (t, 3J(H,H) = 7.35 Hz, 4 H, SCH2), 2.41 (s, 6 H, p −Me), 2.27 (s, 6 H, o−Me), 2.16 (s, 6 H, o−Me), 1.80 (d, 3J(H,H) = 6.90 Hz, 12 H, CH3), 1.44 (d, 3J(H,H) =7.20 Hz, 12 H, CH3). 46 C), 139.1, 137.3, 136.4, C{1H} NMR (75.47 MHz, CDCl3): δ 178.2 (s, NCN− iPr2 −bimy), 172.9 (s, NCN−C 13 134.3, 134.1, 129.4, 123.1, 123.0, 122.4, 113.1, 112.8 (s, Ar− C), 54.6 (s, CH(CH3)2), 53.7 (s, CH(CH3)2), 52.5 (s, NCH2 ), 41.6 (s, SCH2), 33.0 (s, CH(CH3)2), 21.7 (s, p −Me), 21.2 (s, o −Me), 20.3 (s, o −Me). Calc. for C54H70Br4N8Pd2S: C, 46.47; H, 5.06; N, 8.03. Found: C, 46.08; H, 4.46; N, 7.71%. MS (ESI): m/z = 1315 [M − Br]. Complex 14 L ⋅ 2HBr (56 mg, 0.1 mmol), Ag2O (28 mg, 0.12 mmol), and S N N Ph N Br Br Pd Br L Pd L Br [PdBr2(iPr−bimy)]2 (93 mg, 0.1 mmol) were suspended in DCM (15 mL) N Ph and stirred at ambient temperature overnight shielded from light. The resulting suspension was filtered over Celite, and removing CH2Cl2 from N L= the filtrate gave the product as a white solid (111 mg, 0.083 mmol, 83%). N 1 H NMR (300 MHz, CDCl3): δ 7.53 (d, 12 H, Ar − H), 7.37 (d, 4 H, Ar−H), 7.18 (d, 6 H, Ar−H), 6.21 (m, 2 H, CH(CH3)2), 6.01 (m, 2 H, CH(CH3)2), 5.74 (s, 4 H, PhCH2), 4.76 (t, 3J(H,H) = 7.05 Hz, 4 H, NCH2), 3.39 (t, 3J(H,H) = 7.05 Hz, 4 H, SCH2), 1.86 − 1.67 (m, 24 H, CH3). C{1H} NMR (75.47 MHz, CDCl3): δ 179.0 (s, NCN− iPr2 −bimy), 171.0 (s, NCN−JJ), 136.7, 134.2, 129.4, 13 129.2, 128.7, 123.2, 122.6, 121.1, 113.2, 112.8 (s, Ar−C), 55.1 (s, CH(CH3)2), 54.6 (s, CH(CH3)2), 54.3 (s, NCH2), 52.2 (s, PhCH2 ), 33.2 (s, SCH2), 21.7 (s, CH(CH3)2), 21.6 (s, CH(CH3)2). MS (ESI): m/z = 1259 [M − Br]+. Complex 22 SO 3K A mixture of salt M (99 mg, 0.3 mmol), Pd(OAc)2 (67 mg, 0.3 mmol), and KBr (179 mg, 1.5 mmol) in DMSO (7 mL) was stirred N Br N Pd Br Br Pd 47 N Br N KO3 S at 90 °C for 24 h. The reaction mixture was filtered over Celite, and the solvent of the filtrate was removed by in vacuo. The resulting residue was suspended in MeOH (30 mL) and then was filtered over Celite. Removing the solvent of the filtrate in vacuo afforded the product as an orange solid (180 mg, 0.14mmol, 93%). 1H NMR (500 MHz, DMSO-d6): 7.77 (d, 2 H, Ar-H), 7.65 (d, 4 H, Ar-H), 7.36-7.31 (m, 8 H, Ar-H), 7.16 (d, 4 H, Ar-H), 6.18 (s, 4 H, PhCH2), 3.39 (s, 4 H, NCH2), 3.16 (s, br, 4 H, CCH2C). 13 C{1H} NMR (125.76 MHz, DMSO-d6): 136.6, 136.3, 135.6, 129.6, 129.4, 129.0, 124.4, 124.1, 112.5, 111.9 (s, Ar − C), 54.1 (s, PhCH2), 49.9 (s, NCH2), 48.7 (s, CH2SO3), 26.0 (s, CH2CH2SO3), carbene signal not detected. MS (ESI): m/z = 1308 [M + K]+. Miyaura cross-couplings General procedure for the SuzukiSuzuki-Miyaura cross-couplings. In a typical run, a reaction tube was charged with a mixture of aryl halide (1.0 mmol for monohalides, 0.5 mmol for dihalides), K2CO3 (1.4 mmol), phenylboronic acid (1.4 mmol), precatalyst and solvent (1 mL). The reaction was stirred at elevated temperature. After the desired reaction time, the mixture was cooled to the ambient temperature, and DCM (10 mL) was added. The organic layer was then washed with water (6 × 8 mL) and dried over Na2SO4. The solvent was allowed to evaporate and the residue was analyzed by 1 H NMR spectroscopy. The yields were calculated based on the comparsion of the signals of starting materials and products. X-ray Diffraction Studies X-ray data were collected with a Bruker AXS SMART APEX diffractometer, using Mo Kα radiation with the SMART suite of Programs.49 Data were processed and corrected for Lorentz and polarization effects with SAINT,50 and for absorption effect with SADABS.51 Structural solution and refinement were carried out with the SHELXTL suite of programs.52 The structure was solved by direct methods to locate the heavy atoms, followed by difference maps for the light, non-hydrogen atoms. All hydrogen atoms were put at calculated positions. All non-hydrogen atoms were generally given anisotropic displacement parameters in 48 the final model. A summary of the most important crystallographic data is given in the appendix and the CIF files are given in the CD attached. 49 References Appendix (Selected crystallographic data data)) Formula Formula weight Crystal size [mm] Temperature [K] Crystal system Space group a [Å] b [Å] c [Å] α [°] β [°] γ [°] V [Å3] Z Dc [g⋅cm-3] μ [mm-1] θ range [°] Reflection collected Independent reflections Max., min. transmission Final R indices [I > 2σ(I)] R indices (all data) Goodness-of-fit on F2 Peak/hole [e⋅Å-3] 3 C25 H31 Br2Cl3N4PdS 792.17 0.54 × 0.53 × 0.10 223(2) Monoclinic P2(1)/n 16.3698(10) 11.8129(8) 17.3367(11) 90 115.4390(10) 90 3027.4(3) 4 1.738 3.609 1.43−27.50 20738 6945 (Rint = 0.0520) 0.7142, 0.2461 R1 = 0.0542, wR2 = 0.1479 R1 = 0.0820, wR2 = 0.1618 1.024 1.753/−1.192 4 C17.5H29.5Br2Cl4.5N4OPdS 769.76 0.28 × 0.20 × 0.08 293(2) triclinic P1 6.3099(9) 14.1270(19) 16.220(2) 96.392(3) 99.448(3) 99.937(2) 1390.3(3) 2 1.839 4.067 1.09−27.50 17350 6363 (Rint = 0.0325) 0.5630, 0.4042 R1 = 0.0349, wR2 = 0.0887 R1 = 0.0409, wR2 = 0.0916 1.051 1.762/−0.724 6 C24H22Br2Cl4N4PdS 806.54 0.56 × 0.26 × 0.20 100(2) monoclinic Cc 13.1950(12) 11.2449(10) 19.3922(17) 90 109.592(2) 90 2710.8(4) 4 1.976 4.128 2.23−27.50 9595 5610 (Rint = 0.0297) 0.6963, 0.5262 R1 = 0.0476, wR2 = 0.1251 R1 = 0.0527, wR2 = 0.1285 1.049 1.229/−1.456 7 C30 H32 Br2 Cl2 N4 Pd 785.72 0.44 × 0.16 × 0.08 223(2) orthorhombic Pbca 17.5301(15) 9.7933(8) 37.441(3) 90 90 90 6427.7(9) 8 1.624 3.256 1.09−27.50 42970 7383 (Rint = 0.0703) 0.7807, 0.3284 R1 = 0.0442, wR2 = 0.1056 R1 = 0.0674, wR2 = 0.1196 1.031 1.110/−0.610 50 References References [1] A. 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Madison, WI, 2000 2000. 53 [...]... the carbene carbon integrated in a 1 nitrogen-containing heterocycle There are three major types of NHCs, namely benzimidazolin-2-ylidene, imidazolin-2-ylidene, and imidazolidin-2-ylidene (Figure 1.2) R N R N R N N R N R N R A) benzimidazolin-2-ylidene (A B) imidazolin-2-ylidene (B imidazolidin-2-ylidene ( C) Figure 1.2 Three major types of NHCs NHCs are usually viewed as strong σ-donating ligands. .. trans-dibromido-dicarbene pseudo-pincer complex These results suggest that the donor strength of the NHC moiety in CSC-type ligands indeed influences the 17 coordination mode Complexes with strong electron donating ligands would form pincer; on the contrary, complexes with weak electron donating ligands prefer to be pseudo-pincer 18 Chapter 3 Donating ability of NHC ligands and conductivity of pincer-type... explained by the inductive effect of different substituents For example, compared to L, there are four chlorine atoms on the backbone of G The stronger –I effect caused by chlorine atoms leads to an upfield shift of the resonance for the carbene carbon of iPr2-bimy, in this case, from 179.0 ppm to 176.4 ppm D L S N N N N N N S K N N N Mes Ph N S N N N Mes Ph N N N N S N N Ph B S N J Ph Cl N Cl N S Ph N. .. with an anti-parallel spin orientation, and the pπ orbital is empty On the contrary, both σ and pπ orbitals of carbene in triplet state are occupied by its two non-bonding electrons with a parallel spin orientation (Figure 1.1).4 pπ σ pπ σ singlet triplet Figure 1.1 Electronic structure of carbenes 1.2 Introduction to N- Heterocyclic Carbenes N- heterocyclic carbenes (NHCs) are a type of singlet carbenes... growth of interest in carbenes since the isolation of the first free carbene by Arduengo (Scheme1.1).1 NaH/THF catalyst DMSO N N - NaCl - H2 Cl N N Scheme 1 1.11 Synthesis of the first free NHC by Arduengo Carbenes are electrically neutral divalent carbon atoms with six valance electrons.2 Carbenes can exist in either singlet or triplet state.3 The two non-bonding electrons in singlet carbene occupy the... According to our previous work, formation of pincer versus pseudo pincer in Pd(II) complexes with pincer-type NHC ligands is affected by the electron donating abilities of the latter.20 Carbenes with stronger donating ability favor pincer formation even with the presence of halide ions On the contrary, less electron donating carbenes prefer to form neutral pseudo-pincer complexes The electron donating... [M] N R [M'] [M] M = Ag method b N X R method a N R M = Ag method b R N [M'] N R Scheme 1 1.22 Two important synthetic routes for NHC complexes (X = anion) Electronic and steric properties of NHCs can be easily tuned by changing of the N- substituents and the backbone, which helps to enlarge the diversity of NHC chemistry.11,12 Donor -functionalized NHCs are 2 potentially polydentate ligands, which can... overlap, complexes 9 and 11 only afford one mutiplet which is assignable to the NCH groups on iPr2-bimy As expected, two carbene signals are found in the 13C NMR spectrum for each complex The signals for carbene carbons on B, D, G, J, K, and L fall in the range of 184.5 ppm to 164.6 ppm NHC ligands derived from benzimidazolin-2-ylidenes show more downfield signals (complexes 10 and 12 12), and ligands derived... versus pseudo-pincer formation was influenced by the presence of bromide anions.20 As discussed in chapter 1, pincer versus pseudo-pincer formation was also affected by the donating ability of the NHC ligand To further support our theory, a stronger donating benzimidazolin-2-ylidene derived CSC pincer type NHC ligand and its corresponding Pd(II) complex were synthesized S N N 2X N N Ph Ph X = NO3 X = Br... 3.2 Donating ability of pincer-type ligands The donating abilities of ligands play an important role in influencing the chemical properties and reactivities of their metal complexes. 42 According to our previous work, the electron donating ability of NHC ligands is also a main factor affecting the formation of pseudo-pincer versus pincer complexes. 12 It is thus of great interest to determine the donor ... Cl N Cl N Br Cl N Cl N S N Cl Cl N N Cl Cl N S 2Br Ph E N Br N N N N Pd N Br Cl N N Cl N Ph G 2HBr F S N S N M Cl N Cl N Br Br N Cl N Cl Pd Pd Br S N Ph Ph Br VIII N N Br Pd Br N Cl N Cl N N... chlorine atoms leads to an upfield shift of the resonance for the carbene carbon of iPr2-bimy, in this case, from 179.0 ppm to 176.4 ppm D L S N N N N N N S K N N N Mes Ph N S N N N Mes Ph N N N N... carbenes 1.2 Introduction to N- Heterocyclic Carbenes N- heterocyclic carbenes (NHCs) are a type of singlet carbenes that have the carbene carbon integrated in a nitrogen-containing heterocycle