Synthesis and Reactivity toward H2 of (η5-C5Me5)Rh(III) Complexes with Bulky Aminopyridinate Ligands

Electrophilic, cationic Rh(III) complexes of composition [(eta(5)-C5Me5)Rh(Ap)](+), (1(+)), were prepared by reaction of [(eta(5)-C5Me5)RhCl2](2) and LiAp (Ap = aminopyridinate ligand) followed by chloride abstraction with NaBArF (BArF = B[3,5-(CF3)(2)C6H3](4)). Reactions of cations 1(+) with different Lewis bases (e.g., NH3, 4-dimethylaminopyridine, or CNXyl) led in general to monoadducts 1 center dot L+ (L = Lewis base; Xyl = 2,6-Me2C6H3), but carbon monoxide provided carbonyl-carbamoyl complexes 1 center dot(CO)(2)(+) as a result of metal coordination and formal insertion of CO into the Rh-N-amido bond of complexes 1(+). Arguably, the most relevant observation reported in this study stemmed from the reactions of complexes 1(+) with H-2. H-1 NMR analyses of the reactions demonstrated a H-2-catalyzed isomerization of the aminopyridinate ligand in cations 1(+) from the ordinary kappa(2)-N,N' coordination to a very uncommon, formally tridentate kappa-N,eta(3) pseudoallyl bonding mode (complexes 3(+)) following benzylic C-H activation within the xylyl substituent of the pyridinic ring of the aminopyridinate ligand. The isomerization entailed in addition H-H and N-H bond activation and mimicked previous findings with the analogous iridium complexes. However, in dissimilarity with iridium, rhodium complexes 1(+) reacted stoichiometrically at 20 degrees C with excess H-2. The transformations resulted in the hydrogenation of the C5Me5 and Ap ligands with concurrent reduction to Rh(I) and yielded complexes [(eta(4)-C5Me5H)Rh(eta(6)-APH)](+), (2(+)), in which the pyridinic xylyl substituent is eta(6)-bonded to the rhodium(I) center. New compounds reported were characterized by microanalysis and NMR spectroscopy. Representative completes were additionally investigated by X-ray crystallography.