Axial Ligand Coordination to the C–H Amination Catalyst Rh2(esp)2: A Structural and Spectroscopic Study

The compound Rh2(esp)2 (esp = α,α,α′,α′-tetramethyl-1,3-benzenediproponoate) is the most generally effective catalyst for nitrenoid amination of C–H bonds. However, much of its fundamental coordination chemistry is unknown. In this work, we study the effects of axial ligand coordination to the catalyst Rh2(esp)2. We report here crystal structures, cyclic voltammetry, UV–vis, IR, Raman, and 1H NMR spectra for the complexes Rh2(esp)2L2 where L = pyridine, 3-picoline, 2,6-lutidine, acetonitrile, and methanol. The compounds all show well-defined π* → σ* electronic transitions in the 16500 to 20500 cm–1 range, and Rh–Rh stretching vibrations in the range from 304 to 322 cm–1. Taking these data into account we find that the strength of axial ligand binding to Rh2(esp)2 increases in the series CH3OH ∼ 2,6-lutidine < CH3CN < 3-methylpyridine ∼ pyridine. Quasi-reversible Rh2 4+/5+ redox waves are only obtained when either acetonitrile or no axial ligand is present. In the presence of pyridines, irreversible oxidation waves are observed, suggesting that these ligands destabilize the Rh2 complex under oxidative conditions.