Homo and heterometallic assemblies from lewis acidic and basic metallic precursors 6

Chapter Six Conclusion and Future Work 179 Chapter Six: Conclusion and Future Work Chapter Six Conclusion and Future Work 6.1 Conclusion Three types of complexes have been studied in this work. They are the pincer [PCP] Pd(II) complexes, Ag/Mn heterometallic clusters bridged by diphosphine and phosphido ligands as well as the heterometallic oxalato-bridged complexes. The objective of this work was to develop convenient synthetic methodologies, understand the formation pathways and decipher the structures of these complexes. (See Chapter One, Section 1.5) The results have been presented and discussed in Chapters Two, Three, Four and Five. It is important to point out that the complexes described in this thesis are different in nature. The pincer [PCP] complexes described in Chapter Two and Chapter Three are organometallic complexes in which the Pd-C bond plays an important role in their chemistry. The Ag/Mn heterometallic clusters reported in Chapter Four, on the other hand, are carbonyl clusters with metal-metal bonds that are bridged by the phosphido and bisphosphine ligands. The oxalato complexes reported in Chapter Five are polymeric networks containing the paramagnetic metal centres [Mn(II) and Cr(III)] and the main group elements (K, Ba and In), which can be decomposed by thermolysis to give metal oxides. Their descriptions in the previous chapters highlighted the diversity of the homo- and heterometallic assemblies. 180 Chapter Six: Conclusion and Future Work Three different approaches are highlighted to represent the homo- and heterometallic formation. Complex 2.1 was prepared by a C-H activation of a diphosphine Pd(II) complex. Other pincer [PCP] Pd(II) complexes were obtained via ligand displacement reactions. The Ag/Mn heterometallic clusters 4.3a and 4.3b were obtained from condensation reactions between 4.1144 and the Ag(I) diphosphine complexes 4.2a and 4.2b.147, 148 Complexes 4.4a and 4.4b were synthesised by decarbonylation reactions. Polymers 5.1161c and 5.2162 were prepared according to the literature procedures. Complex 5.1 and 5.3 were obtained by cation exchange reactions between K3[Cr(C2O4)3] · 3H2O172 with MnCl2 (5.1) and InCl3 (5.3). The tetrabutylammonium salt of 5.1 was obtained upon a cation exchange reaction using [(n-C4H9)4N]Cl. Complex 5.2 was obtained from a reduction of BaCrO4 with BaC2O4 and oxalic acid. The mixed-metal oxide Mn1.5Cr1.5O4 was obtained by heating complex 5.1 at 500ºC in its solid state. The use of pincer complexes suggested that different ligands, spacers and metals can be introduced into the bimetallic core. The phosphido bridging ligand is another ligand that connects different metals together with or without a metalmetal link. The oxalate ligand is proven to be a useful spacer to link the metals together to give network-type molecular materials. 6.2 Suggestion for Future Work This thesis has described the synthetic methodologies and structures of three types of homo- and heterometallic complexes. The chemistry described can be extended further to cover as given below. 181 Chapter Six: Conclusion and Future Work 6.2.1 Suggestion for Future Work on the Pincer Complexes It has been shown that the homometallic dinuclear pincer [PCP] Pd(II) complexes could be prepared. However, isolation of the heterometallic pincer [PCP] pincer complexes remains a challenge,134 as discussed in Section 3.2.3. This will be a potential direction for future work since the heterometallic complexes generated from a pincer Pd(II) or Pt(II) complex are rare. Study on such complexes is important to gain a better understanding on the mechanism involved in a bi-component catalytic system where a pincer complex is one of the catalytic components. An example shown in Scheme 6-1 is the tandem alkane dehydrogenation-olefin metathesis catalysed by a pincer [PCP] Ir complex (alkane dehydrogenation) and a Mo complex (olefin metathesis).192 There is no evidence showing that a heterometallic pincer [PCP] Ir/Mo complex is formed in the reaction. However such complex is important since it could be the inhibited catalyst28 or an active catalyst that leads to a different product due to the cooperative effect between the metals.3 Therefore it will be interesting to prepare a heterometallic pincer [PCP] complex and test its catalytic activity in the same reaction. 182 Chapter Six: Conclusion and Future Work 2M1 2M1H R R olefin metathesis M2 R 2M1H 2M1 R R R + H2C=CH H 3C-CH M1 O P tBu2 Ir(L) O P tBu2 L = C 2H 4, H2 M2 Pri PR or R' Ir(L) PR N (H 3C)(F 3C) 2CO (H 3C)(F 3C)2CO R = tBu; R' = H R = iPr; R' = OMe i Pr Mo CHC(CH 3)2Ph L = H and/or H4 Scheme 6-1: Tandem alkane dehydrogenation-olefin metathesis catalysed by a pincer [PCP] Ir and a Mo complex.192 A coordination polymer containing Ag/Pd metal-metal bonds was recently synthesised by Braunstein and co-workers from a phosphanyl iminolate Pd(II) complex and AgOTf (Scheme 6-2).193 The Pd(II) complex behaves as a metalloligand that is able to coordinate to the Ag(I) centre via the N atom of the phosphanyl iminolate ligand and the Pd-C(aryl) bond. This character is related to the earlier pincer [NCN] Pd/Ag complexes 1.24 and 1.25 (See Section 1.2.1) which contain a Pt(II) to Ag(I) bond.45 The [p-tolylNYNR]- ligands (Y = CH, N; R = organic groups) in complexes 1.24 and 1.25 play an important role in stabilising the M-M bond. This type of complexes may serve as models to study the bonding and electronic property of the Pd/Ag bonded complexes such as the phosphanyl iminolate Pd/Ag coordination polymer.193 Similar complexes 6.1 can be prepared for this purpose (Scheme 6-3). 183 Chapter Six: Conclusion and Future Work N Me2 Ph2 P Ph2P AgOTf Pd N THF O N Me2 Pd Me2 O Ag N Me2 n Scheme 6-2: A Ag/Pd bonded coordination polymer synthesised from a phosphanyl iminolate Pd(II) complex and AgOTf.193 X PR2 M X (1/n)[Ag(p-tolylNYNR)]n N Ag PR2 R' Y M N p-tol PR2 PR2 M = Pd, Pt; X = halogen Y = CH, R = Me, Et, iPr, p-tol Y = N; R = Me , Et, iPr 6.1 PR2 M PR2 H PR2 PR2 C M or PR2 M PR2 Scheme 6-3: A proposed synthetic pathway of complex 6.1. It is also possible to use the Pd(II) or Pt(II) [p-tolylNYNR] complex 6.2 as a metalloligand to the cationic metal complexes (Scheme 6-4). The vast majority of metalloligands uses their ligand donor atoms as the donor-coordination sites.194 Complex 6.2 however could use the metal (Pd or Pt) as one of the donor atoms. 184 Chapter Six: Conclusion and Future Work N PR2 M R' Y N [M'Ln]+[X]- p-tol PR2 LnM' PR2 N R' Y M N p-tol PR2 [X]- or LnM' PR2 M N R' Y [X] N p-tol PR2 M = Pd, Pt 6.2 6.3 6.4 Scheme 6-4: Coordination of the metalloligand 6.2 to the cationic metal complexes. It is currently unknown whether the formation of the Ag-Pt bond (in complexes 1.24 and 1.25)45 and Ag-Pd bond (in the coordination polymer)193 is reversible. A reversible M-M bond is important in a dynamic metal catalyst which can simultaneously support substrate coordination and product elimination.5b Future investigation should be focused on the understanding and design of such dynamic M-M bonding. ESI-MS study of pincer complexes is relatively unexplored.88 The present study described in Chapter Two can be extended to other compounds such as the pincer [SCS] complexes, which were studied recently for their catalytic activity.195 A proposed synthesis of pincer [SCS] Pd(II) complex 6.5 is shown in Scheme 6-5. The dithiol ligand 6.6 can be prepared from commercially available organothiol compounds and alkyl halide [Scheme 6-5(b)]. 185 Chapter Six: Conclusion and Future Work (CH2)5 R S (H2C)5 X Pd Pd SR X Pd X SR X Pd (a) X X X S R SR RS SR RS (CH2)5 6.5 R = C6H5, C5H4N; X = Cl, Br HSR or KSR X (CH2)5 X - HX or KX RS (b) (CH2)5 SR 6.6 Scheme 6-5: A proposed synthetic pathway of the pincer [SCS] Pd(II) complex 6.5. The study could be extended to unsymmetrical and chiral pincer complexes. The former complexes are catalysts that contain ligands with different carbon backbones or donor atoms that often show hemilabile behaviour196 The latter is potential catalysts for asymmetric synthesis.197 Study of the cyclometallation reaction of the nonsymmetrical and chiral pincer ligands using ESI-MS is not straightforward, as the hemilabile ligands in nonsymmetrical pincer complexes may dissociate in solution. Complexes with different chirality are also not distinguishable in their mass spectra. Nevertheless, this study is important as it proves that ESI-MS can be used as a tool to study aggregation and fragmentation of complex chemical compounds. Examples of the nonsymmetrical and chiral pincer complexes that are suitable as a starting point for this study are shown in Figure 6-1. The pincer ligands are different in term of their donor atoms [Figure 61, (a)],196a carbon backbones [Figure 6-1, (b)]196c or stereogenic centres. The chirality can be either at the carbon [Figure 6-1, (c)] 198 or the phosphorus [Figure 6-1, (d)]199 atoms. These complexes are proposed for future study because their 186 Chapter Six: Conclusion and Future Work chemical structures are similar to complex 2.1. This similarity allows a direct comparison of the experimental results with those of complex 2.1. SPh Pd PPh2 Cl Pd PPh2 Cl PPh2 (b) (a) Ph PPh2 Pd Cl PPh2 (c) P t Bu Pd Cl P Ph t Bu (d) Figure 6-1: Examples of the nonsymmetrical [(a) and (b)] and chiral [(c) and (d)] pincer complexes.196, 198, 199 A preliminary catalytic study of complexes 2.1 and 2.16a showed that the complexes are catalytically active. It was mentioned in Section 2.3 that further research has to be carried out to identify the true identity of the active catalysts, as the complexes 2.1 and 2.16a are only catalytically active under harsh conditions. It is debatable if the catalytic process is homogenous or heterogeneous in nature.118 Apart from that, it is important to search for other reactions that can be catalysed the pincer [PCP] complexes. A proposed reaction shown here is the 187 Chapter Six: Conclusion and Future Work hydroamination reaction of alkynes. This reaction was previously catalysed by a pincer [PPP] complex (Scheme 6-6).200 The electron-rich metal centre in the pincer [PCP] complex such as 2.16a might help to stabilise the olefin intermediate (Int) better than the pincer [PPP] complex and thus facilitates the formation of the cyclic imine. PhP Pd PPh2 PhP NH2 PPh2 PhP PPh2 PPh2 NH2 Pd NH2 H PPh2 H Int Pd PPh2 PPh2 PhP Pd N H2 PPh2H H3C N H3C PPh2 PhP Pd CH2 PPh2 N PhP PPh2 PPh2 PPh2 ±H PhP N H ±H H3C Pd PPh2 H2C Pd N H Scheme 6-6: Hydroamination of alkynes catalysed by a pincer [PPP] Pd(II) complex.200 6.2.2 Suggestion for Future Work on the Ag/Mn Heterometallic Clusters It was mentioned in Section 4.3 that the Au(I) analogue of complex 4.4b was stable toward CO and PPh3 but could be destroyed by the PPhMe2 ligand. However the details of the study were not reported.145c Therefore, a study on the 188 Chapter Six: Conclusion and Future Work chemical reactivity of complexes 4.4a, 4.4b and AuMn2(CO)7(μ-PPh2)(μ-P-P) (PP = dppm; P-P = dppf) is required if the yields of these products can be improved to obtain more details of their chemical reactivity. Preliminary electrochemical study of complexes 4.3a and 4.3b suggested that both clusters could have decomposed upon oxidation as a precipitate was observed in the electrochemical cell (Table 6-1). All of the waves are irreversible with no observation of a reduction wave. It is possible that the radical cation generated initially could catalyse the decomposition. The data suggest that the Ag/Mn bonds in complexes 4.3a and 4.3b are weak. The identity of the precipitates is currently unknown. Characterisation of the solids formed is critical in order to understand the decomposition pathway. The structures of the clusters need to be modified to enhance their stability for electrochemical study. Table 6-1: Preliminary electrochemical data of clusters 4.3a and 4.3b a Cluster Supporting Electrolyte Irreversible Oxidation Waves (vs. Fc0/+) 4.3a [N(C4H9)4]PF6 4.3a [N(C4H9)4][B(C6F5)4] (Tetrakis) 4.3b [N(C4H9)4]PF6 4.3b [N(C4H9)4][B(C6F5)4] (Tetrakis) 528.5 mV 220.0 mV 471.30 mV 1.327 V 817.0 mV 1.283 V 557.35 mV 655.65 mV 1.378 V 849.50 mV a Condition: Cyclic voltammetry was performed on 10.0 ml of 0.1M [N(C4H9)4]PF6 or 0.05 M tetrakis and mM cluster in dichloromethane. The cyclic voltammagrams were obtained for both oxidation and reduction of the cluster (0.0 to approx. +2.0 V) at scan rates ranging from 100 mV/sec-1000mV/sec. 189 Chapter Six: Conclusion and Future Work A feature of complexes 4.3 and 4.4 is that both the complexes contain a AgMn2 core in which the metals are bonded to each other. The metal core is further supported by the carbonyl, phosphine and the phosphido ligands. The carbonyl ligands can be easily removed upon heating201 whereas the P-C (phenyl rings) bonds can be cleaved (See Section 1.3).The Phosphine202 and phosphido203 ligands have been proposed as possible source of phosphorus. The results described in Chapter Four suggested that the complexes are relatively unstable. Therefore it is possible to use complexes 4.3 and 4.4 as molecular precursors to Ag/Mn phosphides. Silver phosphide is interesting because the compound behaves as a semiconductor204 while manganese phosphide has been studied for its magnetic property.205 Hence it would be interesting to study the property of a phosphide compound containing both silver and manganese. It will be an advantage if the compound can be generated through a molecular precursor route, since the elements required for the final mixed metal phosphide are present in the precursor. 6.2.3 Suggestion for Future Work on the Oxalato Complexes Study of thermal property of complex 5.1 highlighted its potential as a precursor to the mixed-metal oxide Mn1.5Cr1.5O4. It is possible to optimise the experimental conditions of the decomposition reaction by temperature control, reaction time control and use of surfactants to have a better control over on the particle size and shape.173b, 173d, 206 As mentioned in Chapter Five, The composition of metal oxides formed is depending on the structures and the ratio of metals in the molecular precursors. Therefore, a study is needed in order to understand the relationship between the molecular precursors and the materials 190 Chapter Six: Conclusion and Future Work formed upon thermolysis. This can be achieved by using a molecular precursor with a metal ratio similar to that of the desired metal oxide.207 However preparation of a suitable precursor is the first challenge to overcome. Synthesis of the In/Cr oxalato complex has not yet succeeded; an optimised synthetic strategy is needed in order to achieve this objective. Therefore, it is suggests herein to carry out more studies in order to optimise the experimental conditions. A proposed strategy adopted from the synthesis of Ba/Cr oxalato complex162 is shown in Equations 6-1 and 6-2. Acid hydrolysis in the presence of different metal ions could result in different mixed metal and mixed ligand system (Equation 6-3). It is also possible to replace the K(I) ions in complex 5.3 with other metal cations. InCl3 + H2CrO4 → In2(CrO4)3 + HCl In2(CrO4)3 + In2(C2O4)3 → In4(CrO4)3(C2O4)3 Equation 6-1 Equation 6-2 In4(CrO4)3(C2O4)3 + HX’ + KX’ → KIn3(CrO4)3(C2O4)2 + InX’3 + H2C2O4 Equation 6-3 Apart from the oxalato complexes, other complexes containing ligands that can undergo decomposition upon thermolysis208 such as 2-ethylhexanoato [Figure 6-2, (a)],208b citrato [Figure 6-2, (b)],208c squarato [Figure 6-2, (c)]208d and trifluoroacetato [Figure 6-2, (d)]208e ligands have also been studied. Synthesis of the heterometallic molecular precursors containing the above mentioned and the related ligands, as well as subsequent study of their thermal behaviour would be 191 Chapter Six: Conclusion and Future Work interesting. Design of new ligands and study of their complexes as single source precursors is also a promising direction of further investigation. C4H9 O CH C O C2H5 H O HO O O O O (b) (a) O O O O CH CF3 O O (c) O (d) Figure 6-2: Examples of the ligands that readily decompose upon heating.208 192 [...]... adopted from the synthesis of Ba/Cr oxalato complex 162 is shown in Equations 6- 1 and 6- 2 Acid hydrolysis in the presence of different metal ions could result in different mixed metal and mixed ligand system (Equation 6- 3) It is also possible to replace the K(I) ions in complex 5.3 with other metal cations 2 InCl3 + 3 H2CrO4 → In2(CrO4)3 + 6 HCl In2(CrO4)3 + In2(C2O4)3 → In4(CrO4)3(C2O4)3 Equation 6- 1 Equation... Equation 6- 1 Equation 6- 2 In4(CrO4)3(C2O4)3 + 2 HX’ + KX’ → KIn3(CrO4)3(C2O4)2 + InX’3 + H2C2O4 Equation 6- 3 Apart from the oxalato complexes, other complexes containing ligands that can undergo decomposition upon thermolysis208 such as 2-ethylhexanoato [Figure 6- 2, (a)],208b citrato [Figure 6- 2, (b)],208c squarato [Figure 6- 2, (c)]208d and trifluoroacetato [Figure 6- 2, (d)]208e ligands have also been... 6- 2, (d)]208e ligands have also been studied Synthesis of the heterometallic molecular precursors containing the above mentioned and the related ligands, as well as subsequent study of their thermal behaviour would be 191 Chapter Six: Conclusion and Future Work interesting Design of new ligands and study of their complexes as single source precursors is also a promising direction of further investigation... stability for electrochemical study Table 6- 1: Preliminary electrochemical data of clusters 4.3a and 4.3b a Cluster Supporting Electrolyte Irreversible Oxidation Waves (vs Fc0/+) 4.3a [N(C4H9)4]PF6 4.3a [N(C4H9)4][B(C6F5)4] (Tetrakis) 4.3b [N(C4H9)4]PF6 4.3b [N(C4H9)4][B(C6F5)4] (Tetrakis) 528.5 mV 220.0 mV 471.30 mV 1.327 V 817.0 mV 1.283 V 557.35 mV 65 5 .65 mV 1.378 V 849.50 mV a Condition: Cyclic... over on the particle size and shape.173b, 173d, 2 06 As mentioned in Chapter Five, The composition of metal oxides formed is depending on the structures and the ratio of metals in the molecular precursors Therefore, a study is needed in order to understand the relationship between the molecular precursors and the materials 190 Chapter Six: Conclusion and Future Work formed upon thermolysis This can be... voltammetry was performed on 10.0 ml of 0.1M [N(C4H9)4]PF6 or 0.05 M tetrakis and 1 mM cluster in dichloromethane The cyclic voltammagrams were obtained for both oxidation and reduction of the cluster (0.0 to approx +2.0 V) at scan rates ranging from 100 mV/sec-1000mV/sec 189 Chapter Six: Conclusion and Future Work A feature of complexes 4.3 and 4.4 is that both the complexes contain a AgMn2 core in... phosphine and the phosphido ligands The carbonyl ligands can be easily removed upon heating201 whereas the P-C (phenyl rings) bonds can be cleaved (See Section 1.3).The Phosphine202 and phosphido203 ligands have been proposed as possible source of phosphorus The results described in Chapter Four suggested that the complexes are relatively unstable Therefore it is possible to use complexes 4.3 and 4.4...Chapter Six: Conclusion and Future Work chemical reactivity of complexes 4.4a, 4.4b and AuMn2(CO)7(μ-PPh2)(μ-P-P) (PP = dppm; P-P = dppf) is required if the yields of these products can be improved to obtain more details of their chemical reactivity Preliminary electrochemical study of complexes 4.3a and 4.3b suggested that both clusters could have decomposed upon... precursor 6. 2.3 Suggestion for Future Work on the Oxalato Complexes Study of thermal property of complex 5.1 highlighted its potential as a precursor to the mixed-metal oxide Mn1.5Cr1.5O4 It is possible to optimise the experimental conditions of the decomposition reaction by temperature control, reaction time control and use of surfactants to have a better control over on the particle size and shape.173b,... electrochemical cell (Table 6- 1) All of the waves are irreversible with no observation of a reduction wave It is possible that the radical cation generated initially could catalyse the decomposition The data suggest that the Ag/Mn bonds in complexes 4.3a and 4.3b are weak The identity of the precipitates is currently unknown Characterisation of the solids formed is critical in order to understand the decomposition . diversity of the homo- and heterometallic assemblies. Chapter Six: Conclusion and Future Work 181 Three different approaches are highlighted to represent the homo- and heterometallic formation complex 6. 5 is shown in Scheme 6- 5. The dithiol ligand 6. 6 can be prepared from commercially available organothiol compounds and alkyl halide [Scheme 6- 5(b)]. Chapter Six: Conclusion and Future. [Figure 6- 2, (a)], 208b citrato [Figure 6- 2, (b)], 208c squarato [Figure 6- 2, (c)] 208d and trifluoroacetato [Figure 6- 2, (d)] 208e ligands have also been studied. Synthesis of the heterometallic