The Development and Study of new Chelating Ligands with Combinations of Pyridinylidene Amide (PYA) and remote-NHC Donors as Supporting Ligands for Homogeneous Metal Catalysts

Abstract

This thesis describes the design and syntheses of metal complexes that contain chelating ligands with different combinations of pyridinylidene amide (PYA), amidate, and remote-N-heterocyclic carbene (rNHC) or a mesoionic-remote-N-heterocyclic carbene (m-rNHC) donor functions. Although ligands with amidate donors are relatively well known, metal complexes of ligands containing the other donors are relatively unexplored. All these groups exhibit strong donating properties. It was expected that suitable metal complexes of these new ligands could show interesting catalytic activity in reactions such as transfer hydrogenation. In Chapter 1, an overview of the chemistry and existing synthetic routes to a range of different pyridine derived ligands are given. The coordination chemistry and applications of these classes of ligands are briefly discussed and the motivation for this work is outlined at the end. In Chapter 2, the syntheses of two new palladium(II) NC’N-tridentate pincer complexes ([Pd(L)(LPPy{Me}3)]OTf), L= Cl, OAc) that features both rNHC and two PYA donors are described. The tricationic pro-ligand [H2LPPy{Me}3][OTf]3, bearing two pendant alkylated 4-pyridyl arms at the 3,5-positions of the head group pyridine was metallated with Pd(OAc)2 or [PdCl2(PhCN)2] by heating with NaOAc in situ. The two complexes were stable in air and moisture, but sparingly soluble in common solvents and this precluded further studies. The smaller related bidentate pro-ligands [LmPy{Me}2]OTf and [LPPy{Me}2]OTf, were synthesised and on metallation by heating in the presence of acetate afforded [RhCl(LmPy{Me}2)Cp*]OTf and [RhCl(LPPy{Me}2)Cp*]OTf. X-ray crystallography and NMR studies of these complexes confirmed coordination occurred through PYA and rNHC donors. In Chapters 3 and 4, the syntheses of the sets of pro-ligands [HLp1-6{Bz}]Cl and [HLm1-6{Bz}]Clis described. The compounds within each set differ in that they have different substituents (e.g. Ph, Me, OMe, Naphthyl, Cl and NO2) in the para-position of the aryl ring attached to the insipient amidate nitrogen donor. On metallation, these pro-ligands coordinate as bidentate ligands with amidate and rNHC donors (in the case of [HLm1-6{Bz}]Cl) or amidate and m-rNHC donors (in the case of [HLp1-6{Bz}]Cl). Metallation of the pro-ligands using [IrCp*Cl2]2as the metal substrate gives the sets of neutral complexes [IrCl(Lm1-6{Bz})Cp*] and [IrCl(Lp1-6{Bz})Cp*]. The corresponding cationic complexes [Ir(Lx1-6{Bz})(L)Cp*]OTf(L= DMSO, CO, Py, x= m, p) were prepared by Ag+ promoted exchange of the chloride ligands in [IrCl(Lx1-6{Bz})Cp*] (x= m, p) with DMSO, CO or pyridine. Comparison of the molecular structures of both the neutral and cationic forms of all the complexes obtained via single-crystal XRD suggests that the complexes with the rNHC donors ([IrCl(Lm1-6{Bz})Cp*], [Ir(Lm1-6{Bz})(L)Cp*]OTf(L= DMSO, CO, Py) all exhibit a similar pattern of C–C bond lengths within the pyridinium ring of the rNHC donor with some 2,5-diene character. In contrast, the complexes with m-rNHC donors ([IrCl(Lp1-6{Bz})Cp*], [Ir(Lp1-6{Bz})(L)Cp*]OTf(L= DMSO, CO) the C–C bond lengths in the corresponding rings are nearly equal suggesting a more pronounced aromatic character in these cases. On treatment of the pro-ligands [HLm1-2{Bz}]Cl with RuCl3.xH2Othe complexes [RuCl(Lm2,3{Bz})(PPh3)2(CO)]Cl were obtained. Unlike the iridium complexes formed with the same pro-ligands, the donor groups in these complexes were the carbon of a rNHC and the oxygen of the unchanged carboxamide group. The catalytic activities of the complexes [Ir(Lx1-6{Bz})(L)Cp*]OTf(L = DMSO, x= m,p) and [RuCl(Lm2,3{Bz})(PPh3)2(CO)]Cl were tested in both transfer hydrogenation and dehydrogenation. The results of these studies are described in Chapter 5. Notably, it was found that the iridium mesoionic-rNHC-containing complexes with methoxy substituents were the most active in the dehydrogenation of benzyl alcohol to benzaldehyde, achieving 99.9% conversion under 12 h. In transfer hydrogenation using isopropanol as the hydrogen source, the same mesoionic-rNHC-containing iridium complexes with OMe substituents were also the best iridium catalysts. Complete conversion of benzaldehyde to benzyl alcohol with 2 mol% catalyst loading occurred in 8 min. However, the ruthenium complexes displayed exceptionally good transfer hydrogenation catalytic activity reaching 99.9% conversion within3 min at 1 mol% catalyst loading, with TOF 1700 h-1. It was proposed that the hemilabilenature of the O-bound carboxamide group, the ancillary ligands and trans-influence alongside the donor strength of the rNHC group were important factors for the high activity displayed by the Ru(II) complexes. Finally, the thesis conclusion is outlined in this chapter.