Cationic and formally zwitterionic rhodium(I) and iridium(I) derivatives of a P,N-substituted indene: A comparative synthetic, structural, and catalytic investigation

A family of neutral, cationic, and formally zwitterionic Rh(I) and Ir(I) complexes featuring 1-(PPr2)-Pr-i-2-NMe2-indene (3a), 3-(PPr2)-Pr-i-2-NMe2-indene (3b), or 3-(PPr2)-Pr-i-2-NMe2-indenide (3c) ligands have been prepared and structurally characterized. Whereas treatment of 3a with 0.5 equiv of [(COD)RhCl](2) afforded (COD)RhCl(kappa(1)-P,N-3a) (4a) in 96% isolated yield, [(COD)M(kappa(2)-P,N-3a)]X-+(-) ([5a]+PF6-, M = Rh; [6a]+SO3CF3-, M = Ir) complexes were obtained in 14% and 41% isolated yield from 3a and the appropriate in-situ-prepared [(COD)M(THF)(2)]X-+(-) precursor. The isomeric complexes [(COD)M(kappa(2)-P,N-3b)]X-+(-) ([5b]X-+(-), M = Rh; [6b]X-+(-), M = Ir) were prepared using a similar protocol employing NEt3, with isolated yields ranging from 37% to 94% depending on the identity of M and X. The zwitterionic complexes (COD)M(kappa(2)-P,N-3c) (5c, M = Rh; 6c, M = Ir) were prepared in 89% and 93% isolated yield, either upon treatment of [5b]X-+(-) or [6b]X-+(-) with NaN(SiMe3)(2) or via addition of 0.5 equiv of [(COD)MCl](2) to [3c]Li. The Rh(I) complexes (CO)(Cl)Rh(kappa(2)-P,N-3b) (7) and [(CO)(2)Rh(kappa(2)-P,N-3b)]+PF6- (8) were prepared in 43% and 66% isolated yield. Single-crystal X-ray diffraction data were obtained for 4a center dot CH2Cl2, [5b]+BF4-, 5c, [6b]+SO3CF3-, [6b]+PF6-, 6c, and 7, the analysis of which revealed an effectively isostructural relationship among the metal coordination environments in [5b]X-+(-), [6b]X-+(-), 5c, and 6c. Despite the structural similarities that are apparent within the cation/zwitterion pairs [5b](+)X(-)5c and [6b]X-+(-)/6c, reactivity studies involving these complexes highlighted important distinctions within and between these pairs of complexes. While both the kappa(2)-P,N-3b and the COD ligands in [5b]+BF4- were displaced upon exposure to excess PMePh2, under similar conditions only the COD ligand in 5c underwent substitution, affording (PMePh2)(2)Rh(kappa(2)-P,N-3c) (9) in 66% isolated yield. Temperature-dependent H-1 NMR line shape changes observed for 9 were rationalized in terms of the operation of at least one dynamic process in this complex (Delta G(298) approximate to 14 kcal/mol). Further reactivity differences were observed when [5b]+PF6-, [6b]+PF6-, 5c, and 6c were employed as catalysts for the hydrogenation and hydrosilylation of alkenes. While [6b]+PF6- proved to be the most effective styrene hydrogenation catalyst in CH2Cl2 or THF, similarly high conversion to ethylbenzene was achieved by use of 5c as a catalyst in benzene, a solvent in which [5b]+PF6- and [6b]+PF6- are not soluble. The catalytic utility of 5c and 6c was demonstrated futher in hydrosilylation reactions employing triethylsilane and styrene, wherein these zwitterionic complexes were observed to outperform their cationic relatives. While nearly quantitative yields were obtained by use of either 5c (toluene or THF) or 6c (1,2-dichloroethane or THF) as a catalyst, these complexes were found to exhibit remarkably high, but divergent, selectivity for E-1-triethylsilyl-2-phenylethene (10b) and 1-triethylsilyl-2-phenylethane (10a), respectively. By comparison, 10a and 10b were produced in a 1.3:1 ratio in reactions employing Crabtree's catalyst ([(COD)Ir(PCy3)(Py)]+PF6-, COD = eta(4)-1,5-cyclooctadiene, Cy = cyclohexyl, Py = pyridine; 1) under similar conditions. When catalytic activity/selectivity and solvent compatibility are considered, the results of these catalytic studies suggest that zwitterionic species such as 5c and 6c represent an effective class of catalyst complexes for the addition of E-H bonds to unsaturated substrates, whose properties are in some cases complementary to those of more traditional Rh(I) and Ir(I) [(COD)M(kappa(2)-P,N)]X-+(-) salts.