Synthetic and structural studies of copper, gold and palladium heterocyclic carbene complexes

Chapter 1. Introduction 1. General Properties of N-Heterocyclic Carbenes (NHCs) N-heterocyclic carbenes (NHCs) have attracted significant interest over the past decade as versatile and prolific ligands in catalysis, triggered mainly by the starting use of NHC complexes in catalysis by Herrmann and co-workers,1 and the preparation of Grubbs‘ second-generation and related catalysts.2-5 Back in 1993, transition metal heterocarbenes were believed to exhibit bonding properties similar to those of trialkylphosphines and alkylphosphinates.6 Six years later, Nolan et al. reported that, based on structural and thermochemical studies, NHCs, with the exception of the sterically demanding (adamantyl) carbene, generally behave as better donors than the best phosphines donor ligands.7 In numerous instances, simple substitution reaction routes involving replacement of phosphines by NHC ligands improved not only the catalytic activity but also thermal stability of the resulting organometallic complexes. This is presumably due to the more powerful σ-donating ability of NHCs than the closely related phosphine ligands, forming stronger bonds to transition metals and thereby also leading to electron-rich metal centers.8-11 As phosphine mimics, NHCs avoid the drawbacks of phosphine ligands such as air sensitivity, high toxicity and thermal instability. In addition, it is relatively easy to modify the structural and electronic components of the NHC manifold to bring a range of desirable traits to the NHC-stabilized compounds. As a result of the rapid and extensive development in NHCs work, many review articles on NHCs have been published, 12-18 and NHCs continue to be a hot research topic in organometallics, as evident from over 100 NHC-related publications appearing in JACS alone in the two-year period of 2008-2009.19 However, most of these studies were focused on Pd-, Ni-, Ru- and Rh-NHC complexes. NHC complexes of copper and gold have been relatively overlooked in spite of the wide use of copper and gold in catalysis, medicine etc.20-31 Preparations of copper and gold NHC complexes have been reported for decades.15 For example, Arduengo and co-worker isolated the first copper carbene complex in 1993.32 Study of gold NHC complexes started even earlier. In 1974, Lappert‘s group reported the generation of ionic complexes [Au(NHC)2]X (X = anion) from electron-rich olefins.33 In the same year, Fehlhammer‘s group also claimed the formation of Au(I)- and Au(III)-NHC complexes through the spontaneous cyclization of isocyanide ligands. 34 In 1989, Au(I)-NHC complexes were isolated by Burini et al. through the reaction of AuCl(PPh3) with lithiated benzylimidazoles, followed by protonation.35 However, copper and gold NHC complexes did not attract significant research interest in the past. With fruitful advancement in the study of transition-metal NHC complexes, the value of copper and gold NHC complexes is being increasingly appreciated by current researchers.17,36-47 The research interest centers on the structural and bonding curiosities of σ-dominant carbene moiety on the electron-rich and soft late-metals that usually require an intricate balance of σ and π ligands.48,49 Another key focus is on the application of copper and gold NHC complexes in catalysis and medicine. 50 Recent developments in the chemistry of copper and gold NHC complexes are summarized herein. 17,36-43 1.1 Copper(I) N-Heterocyclic Carbene Complexes 1.1.1 General Synthetic Methods for Cu(I)-NHC Complexes Four methods are usually applied in the synthesis of Cu(I)-NHC complexes15 (Scheme 1.1): (1) Reaction of free carbenes with suitable copper sources. 51 In this method, imidazolium salts are deprotonated by a strong base e.g. NaOtBu, KOtBu, or KH to produce free NHC ligands which are further used to react with copper sources e.g. Cu(I) halide to obtain Cu(I)-NHC complexes in dry THF or acetonitrile. (2) Transmetalation from relevant NHC complexes.52 In this method, Ag(I)-NHCs are often used as carbene transfer-agents to prepare Cu(I)-NHC complexes because the Cu-NHC bond is stronger than the Ag-NHC bond.53 (3) Alkylation of azolylcuprates.54 In this method, Cu(I)-NHC complexes are obtained from alkylation of thiazolyl or imidazolyl-cuprates complexes formed from the reaction of Cu(I) sources with lithiated azoles. (4) Direct reaction of imidazolium salts with copper base.55,56 In this method, reactions of imidazolium halide with Cu2O or CuOAc or copper powder give Cu(I)-NHC complexes. The acidity of the imidazolium moiety determines the ease of deprotonation of the C-proton by copper base. Scheme 1.1 Preparation methods for Cu(I)-NHC complexes According to the survey of Lin et al., over 60% of Cu(I)-NHC complexes in the literature were synthesized from free carbenes and only 22% of Cu(I)-NHC complexes were synthesized by the Ag-carbene transfer route.15 Although there are few reports comparing these two methods, Meyer et al. observed that the Ag-carbene transfer route could give a higher yield than the free carbene method.57 The third and fourth methods for the preparation of Cu(I)-NHC complexes are rarely used. 1.1.2 Structure and Reactivity of Cu(I)-NHC Complexes There are generally three types of copper NHC complexes: monocarbene [(NHC)CuX] and [(NHC)CuL]X, dicarbene [(NHC)2Cu]X (X = anion) and di-, tri- and multinuclear copper(I) NHC complexes. 1.1.2.1 Monocarbene Cu(I)-NHC Complexes [(NHC)CuX] and [(NHC)CuL]X For [(NHC)CuX]-type complexes, X can be a halide or other coordinating anion. Among them, [(NHC)CuX] (X = halide) complexes are most important. Nolan and co-workers prepared a series of [(NHC)CuX] (X = halide) complexes through the free carbene method (examples are given in Fig. 1.1) with either unsaturated or saturated NHCs. 58 Further studies indicated that [(NHC)CuX] (X = halide) complexes function not only as catalysts or catalyst precursors but also as starting materials for synthesis of [(NHC)CuX] (X ≠ halide) or cationic [(NHC)Cu(L)]+ species. Fig. 1.1 Structures of [(NHC)CuCl] complexes For example, [(NHC)Cu(OtBu)] complexes (I-8), which can be obtained from the reaction of [(NHC)CuCl] (I-7) with NaOtBu59, are known to be the active species in many transformations to yield [(NHC)CuX] (X ≠ halide) or cationic [(NHC)Cu(L)]+ species, as outlined in Scheme 1.2. Subsequent reaction of complexes I-8 with triethoxysilane in the presence of excess 3-hexyne yielded [(NHC)Cu(vinyl)] complexes (I-9) as the first hydrocupration product.59,60 Complexes I-8 also react with triethoxysilane to form hydride copper NHCs (I-10) which are powerful catalysts for the hydrosilylation of ketones.61 [(NHC)CuCl] I-7 t NaO Bu H (NHC)Cu (EtO)3SiH I-10 [(NHC)Cu(CF3)] I-13 CF e3 iM S [(NHC)CuOtBu] I-8 NHC Cu (EtO)3SiH 3-hexyne O Ph O NHC Cu O O Ph I-9 Cp Li B(pin)2 Ph NHC Cu Ph I-11 NHC Cu B O O NHC CO2, -CO Cu B(pin)2, - OB(pin)2 O B(pin) I-15 I-12 Ph NHC Cu B(pin) Ph I-17 I-14 Mes ([NHC)CuOAc] I-20 CO2 O H Mes HO AlMe3 NHC Cu H B(pin) B(pin) Mes [(NHC)CuMe] O I-16 I-19 O B(pin) = NHC Cu Ph B(pin) I-18 pin = pinacolate = 2,3-dimethyl-2,3-butanediolate B O Scheme 1.2 Reactions of [(NHC)CuOtBu] complexes Complex I-11, obtained from the reaction of the Cu(I)-NHC complexes with dibenzoylmethanoate (α,β-diketonate), is an efficient catalyst for the three-component coupling of electrophilic alkenes, aldehydes and silane. 62 Complex I-12, [(NHC)CuX] (X = cyclopentadienyl), could be prepared by reacting a [CuCl(NHC)] or [(NHC)Cu(OtBu)]-type species with cyclopentadienyl lithium.63 X-ray structures show an η5-type bonding mode for the cyclopentadienyl ligand in I-12. Although saturated NHC complex [(SIPr)Cu(OtBu)] (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol- 2-ylidene) showed high catalytic activity in fluorination reaction, its unsaturated NHC analogues [(IPr)Cu(OtBu)] 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) recorded (IPr low = activity. Through the study of complex I-13, Vicic et al. found out the possible reason was side reactions occurring on the unsaturated NHC ring backbone. 64 The carbene boryl copper complexes I-14 were formed from the reaction of I-8 with B(pin)2 (pin = pinacolate = 2,3-dimethyl-2,3-butandiolate). I-14 can promote the reduction of CO2 to CO effectively through the formation of I-1565 as well as activate the diboration of alkenes to form complexes I-16.66 Furthermore, it can serve as the intermediate for the catalysis of hydroboration of aryl-substituted alkenes promoted by the corresponding copper NHC complexes. Compound I-17 was formed through the insertion of alkene to the Cu-boron bond and it can convert to complex I-18 through hydrogen elimination and re-insertion.67,68 The mononuclear copper(I) alkyl complex I-19, [(IPr)Cu(Me)], was obtained from the reaction of I-20 with AlMe3 and it was reported to react with substrates possessing N-H, O-H, and acidic C-H bonds to form neutral type complexes [(IPr)Cu(X)] (X = anilido, phenoxide, ethoxide, phenylacetylide, or N-pyrrolyl).69 Such transformations could be integral to the development of catalytic cycles when metal-mediated bond-forming reactions are accessible with these and related copper systems. The results also indicated that the acidity of the X-H bond could be a key factor, albeit not likely the sole factor, for kinetic accessibility to these reactions. Based on these results, Gunnoe et al. predicted that inert bonds eg. arenes and alkanes might be activated by more electrophilic copper NHC complexes. In subsequent studies, Gunnoe et al. claimed that Cu(I) NHC anilido complexes were more active than Ru(II) anilido complexes in the traditional SN2 transformation of bromoethane.70 In addition, complex I-19 could react with α-borobenzyl alcohol to from complex I-16. To improve the stability and catalytic selectivity of copper NHC complexes, a series of mixed donor carbene complexes were studied. Examples are given in Fig. 1.2. Compound I-21 was prepared by the treatment of a pyridyl-functionalized imidazolium salt with Cu2O and crystallized as a monomer with the copper center taking up a T-shaped geometry.55 The Cu-C and Cu-N distances are 1.880(6) and 2.454(5) Å respectively. Unlike carbonyl or phosphine ligands, NHCs were initially known to be non-bridging ligands. This misconception was shattered when the dinuclear copper complex [Cu2I2(PCP)] (PCP = (SP-4)-[-1,3-Bis[(R)-1- ((S)-2-diphenylphosphino-P-ferrocenyl)ethyl]imidazol-2-ylidene]) (I-22) emerged.56 Complex I-22, with a tridentate PCP ligand based on a ferrocene scaffold, was prepared via the free carbene method with 92% yield while an alternative method involving the direct reaction of CuOAc with the imidazolium salt [PCPH]I achieved only 54% yield. Fig. 1.2 Structures of copper NHC complexes with donor side-arm(s) With μ-X (X = H, O, Cl, I), it is easy to form copper monocarbene dinuclear compounds in solid state. (Fig. 1.3) For example, the first N,S-heterocyclic carbene (NSHC) copper complex I-23 was prepared via the alkylation method in 1994.71 (Scheme 1.3) The X-ray structure of I-23 indicates that the copper atom bonds to the carbene carbon of the thiazolylidene ligand and the two bridging chloride atoms in trigonal planar state. Scheme 1.3 Preparation of copper(I)-NSHC complexes Fig. 1.3 Structures of copper NHC complexes with μ-X (X = H, O or I) As shown in Scheme 1.2, the hydride bridged complex I-10 was obtained by the reaction of [Cu(NHC)(OtBu)] with triethoxysilane.59 The hydride species are powerful catalysts for the hydrosilylation of ketones and the conjugate reduction of α,β-unsaturated cyclic enone and ester.61,72 In practical use, the hydride can be prepared by one-pot reaction of [CuX(NHC)]-type (X = halide) complexes with NaOtBu or KOtBu in the presence of silane.60 NHC ligands were found to stabilize the Cu(I)-hydride species in lower nuclearity compared with the phosphine ligands. Complex I-24, with two bridging OtBu groups, was prepared by the same procedure as that for the mononuclear compound I-9.64 (Scheme 1.2) The sterical bulk of N-substituents in [Cu(OtBu)(NHC)] complex determines the dinuclear or mononuclear formation. Dinuclear complexes I-25 and I-26 were all prepared via the free carbene route.73,74 In complex I-25, the Cu2I2 core is bridged by a dicarbene ligand with short Cu .Cu (2.663(1) Å) distance and the average Cu-Ccarbene bond length is 1.923 Å. The structure of I-26 consisting of a Cu3I3 core coordinated by three NHCs, could be viewed as an adduct of [Cu(NHC)I] and a dinuclear [Cu(NHC)I]2 molecule, resulting in weak copper-copper interactions (Cu…Cu 2.635 and 2.658 Å respectively). 1.1.2.2 Dicarbene Cu(I)-NHC Complexes [Cu(NHC)2]X Fig. 1.4 describes some examples for dicarbene copper complexes. For azolium salts or copper sources with weak coordinating anions such as BF4- or PF6-, the free carbene method usually yields cationic [Cu(NHC) 2]+ species, instead of neutral [CuX(NHC)] complexes. 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Au(I)/(III)-NHC Complexes Unlike Cu(I)-NHC complexes with several coordination modes (two-, three- and four-coordination modes), Au(I)-NHC complexes only exhibit two-coordination mode and Au(III)-NHC complexes exhibit 22 four-coordination mode Au(I)-NHC complexes are discussed below, according to the three categories: monocarbene [AuX(NHC)] and [Au(NHC)L]X, dicarbene [(NHC)2 Au]X, and di- and multinuclear gold. .. et al.131 Fig 1.10 Structures of dinuclear Au(I)-NHC complexes Fig 1.11 descripts the examples for di- and multinuclear Au(I)-NHC complexes The tridentate polycarbene complex II-21 was synthesized through Ag -carbene transfer by Meyer and co-workers 57 In 2008, Hahn et al reported the gold( I) complex (II-22) with cyclic tetracarbene ligands 132 28 Reaction of the dicarbene gold containing pyridyl side... oxidation of benzyl alcohol to benzaldehyde 104, methoxycyclization of 1,6-enynes44 , etc In addition, gold NHC complexes also have wide application in medicine 24,27,37,112,129,152 1.3 Solvento Complexes of Palladium( I)/(II) Complexes with or without NHC 1.3.1 Pd(II) Complexes with Ferrocene-derivatized NHC Palladium complexes have been well studied due to their wide use in synthetic organic chemistry and. .. catalytic activity of a series of cationic gold( I) NHC complexes (Scheme 1.19) This reaction allows a strict comparison of both the counteranion and the neutral ligand bound to the gold center in addition to the NHC 150 The combination of [(IPr)AuCl]/AgBF4 exhibits high activity towards a variety of allylic esters In contrast to the high catalytic activity of acetonitrile based Au(I) NHC complexes, their... 1.2.3 Catalytic Activity of Au(I)/(III)-NHC Complexes Although simple gold salts like AuCl or NaAuCl4 are known to catalyze many organic reactions, spontaneous reduction of Au(I) or Au(III) to inactive metal occurs in the absence of a stabilizing ligand This highlights the importance of the stabilizing phosphine and NHC ligands in this field.41 Therefore the application of gold NHC complexes in homogeneous... 1.920(8) and 1.932(9) Ǻ respectively, which is consistent with those of other reported copper carbene complexes. 83 The two imidazolylidene rings coordinated to the middle copper 13 atom are twisted by 79.64o relative to each other Scheme 1.4 Preparation of compound I-34 Copper complexes of oligo- or polycarbene ligands are rare Several examples are given in Fig 1.6 Gade and co-workers studied copper(I) complexes. .. II-23.133 Most of these Au-Ag heteronuclear polymer show Au(I) Au(I)/Ag(I) interactions and exhibit luminescence properties Fig 1.11 Structures of di- and multinuclear Au(I)-NHC complexes 1.2.2.4 Au(III)-NHC Complexes Reports on Au(III)-NHC complexes are scarce Fig 1.12 depicts some examples of Au(III)-NHC complexes The [(NHC)AuBr3]-type complexes, II-24, were prepared by oxidative addition of elemental... reaction,106 conjugate addition of carbonyl aziridination of aliphatic alkenes,107 oxidative carbonylation of amino 108 etc In addition, Cu(I)-NHC systems may find industrial applications, such as in the reduction of CO2 to CO65,109, hydrogen storage110 and other medical applications111 1.2 Gold( I)/(III) N -Heterocyclic Carbene Complexes 1.2.1 General Synthetic Methods for Au(I)/(III)-NHC Complexes Five strategies... carbenoid carbon atom and a pyridine ring of the same ligand as well as an acetonitrile molecule The four Cu-Ccarbene bonds fall in the range of 1.902(5)–1.935(4) Ǻ and the distance of Cu Cu is 2.852(1) Ǻ in I-34 However, the Cu…Cu distance shortens to 2.587 Ǻ in I-35 reflecting the formation of a covalent Cu-Cu bond (the sum of covalent radii of two copper atoms is 2.64 Ǻ) Cu-Ccarbene bond lengths... NHC complexes 1.2.2.1 Monocarbene Au(I)-NHC Complexes [AuX(NHC)] and [Au(NHC)L]X Although Ag-NHC transfer is the powerful method for Au(I)-NHC complexes preparation, neutral [AuX(NHC)] (X = Br or I) complexes cannot be prepared via the direct reaction of [AuCl(SMe2)] with [AgX(NHC)] (X = Br or I) due to the formation of [AuCl(NHC)] instead of [AuBr(NHC)] or [AuI(NHC)] species Examples of [AuCl(NHC)] complexes . period of 2008-2009. 19 However, most of these studies were focused on Pd-, Ni-, Ru- and Rh-NHC complexes. NHC complexes of copper and gold have been relatively overlooked in spite of the. electron-rich and soft late-metals that usually require an intricate balance of σ 3 and π ligands. 48,49 Another key focus is on the application of copper and gold NHC complexes in catalysis and. the chemistry of copper and gold NHC complexes are summarized herein. 17,36-43 1.1 Copper(I) N -Heterocyclic Carbene Complexes 1.1.1 General Synthetic Methods for Cu(I)-NHC Complexes Four