Pincer and chelate N-heterocyclic carbene complexes of iridium, palladium and rhodium: Synthetic routes, dynamics, catalysis, and counter-ion effects

Four topics are discussed: the synthesis of chelating bis-N-heterocyclic carbene (NHC) complexes of iridium, palladium, and rhodium, the catalytic activity of iridium and rhodium complexes, the dynamics of palladium pincer complexes, and counter-ion effects.; Chapters 2, 3, and 4 describe the synthesis and transfer-hydrogenation activity of chelating bis-N-heterocyclic carbene complexes of iridium and rhodium. The chelating bis-N-heterocyclic carbene ligands are robust ligands and, once bound to the metal, exhibit remarkable thermal, air, and moisture stability, which are very useful features for homogenous catalysis. These precatalysts were screened for transfer hydrogenation activity of ketones, aldehydes, enolizable aldehydes, alpha,beta-unsaturated compounds, and imines with either potassium hydroxide or alkali carbonates as base. The iridium complexes presented higher activity for the transfer hydrogenation of ketones when compared to the rhodium ones. The aldehydes could also be reduced more slowly using a mild carbonate base and no iridium complex. Chelating bis-imidazole and bis-triazole complexes are prepared and their activities compared. Imidazole and 1,2,4-triazole compounds had comparable activities for the transfer hydrogenation reduction of aldehydes. Variation of the wingtip R-groups is also explored with the neopentyl wingtip group showing the highest activity. A mechanism for transfer hydrogenation of these precatalysts is proposed; catalyst reuse and selectivity is explored.; Chapter 5 describes the synthesis and reactivity differences of rhodium(I) and rhodium(III) chelating bis-carbene complexes. A possible reason that may account for these reactivity differences, which also apply to the iridium complexes described in chapters 2--4, is also presented.; Chapter 6 describes the synthesis of a variety of CNC palladium(II) pincer complexes with different counter-ions and their fluxional behavior. The conformer interchange in these complexes was studied by variable temperature proton NMR spectroscopy. In collaboration with Prof. Odile Eisenstein's computational chemistry group at the Universite Montpellier, France, two different mechanisms are proposed for the conformer interchange of closely related complexes.; Finally in the appendix, the remarkable role of the counter-ion in switching the kinetic product in CN chelating ligands from normal C-2 binding on the imidazole ring to abnormal C-5 binding on the imidazole ring upon metalation with IrHs(PPh3)2 is described. Certain counter-ions favor the formation of the normal product and other counter-ions favor the formation of the abnormal product. DFT calculations, which were carried out by Prof. Odile Eisenstein's computational chemistry group, may reveal why different products were seen when the counter-ion was varied.