Synthesis and Reactivity toward H2 of (h5- C5Me5)Rh(III) Complexes with Bulky Aminopyridinate Ligands

Abstract

Electrophilic, cationic Rh(III) complexes of composition [(η5-C5Me5)Rh(Ap)]+, (1+), were prepared by reaction of [(η5-C5Me5)RhCl2]2 and LiAp (Ap = aminopyridinate ligand) followed by chloride abstraction with NaBArF (BArF = B[3,5-(CF3)2C6H3]4). Reactions of cations 1+ with different Lewis bases (e.g., NH3, 4-dimethylaminopyridine, or CNXyl) led in general to monoadducts 1·L+ (L = Lewis base; Xyl = 2,6-Me2C6H3), but carbon monoxide provided carbonyl–carbamoyl complexes 1·(CO)2+ as a result of metal coordination and formal insertion of CO into the Rh–Namido bond of complexes 1+. Arguably, the most relevant observation reported in this study stemmed from the reactions of complexes 1+ with H2. 1H NMR analyses of the reactions demonstrated a H2-catalyzed isomerization of the aminopyridinate ligand in cations 1+ from the ordinary κ2-N,N′ coordination to a very uncommon, formally tridentate κ-N,η3 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 °C with excess H2. The transformations resulted in the hydrogenation of the C5Me5 and Ap ligands with concurrent reduction to Rh(I) and yielded complexes [(η4-C5Me5H)Rh(η6-ApH)]+, (2+), in which the pyridinic xylyl substituent is η6-bonded to the rhodium(I) center. New compounds reported were characterized by microanalysis and NMR spectroscopy. Representative complexes were additionally investigated by X-ray crystallography.