Coordination Effects on Electron Distributions for Rhodium Complexes of the Redox-Active Bis(3,5-di-tert-butyl-2-phenolate)amide Ligand

Abstract

New rhodium complexes of bis(3,5-di-tert-butyl-2-phenol)amine ([ONO(cat)]H(3)) were synthesized, and their electronic properties were investigated. These compounds were prepared by combining [ONO(q)]K and [(cod)Rh(μ-Cl)](2) in the presence of an auxiliary donor ligand to yield complexes of the type [ONO]RhL(n) (n = 3, L = py (1); n = 2, L = PMe(3) (2a), L = PMe(2)Ph (2b), PMePh(2) (2c), PPh(3) (2d)). Single-crystal X-ray diffraction studies on [ONO]Rh(py)(3) (1) revealed a six-coordinate, octahedral rhodium complex. In the case of [ONO]Rh(PMe(3))(2) (2a), X-ray diffraction showed a five-coordinate, distorted square-pyramidal coordination environment around the rhodium center. While 1 is static on the NMR time scale, complexes 2a-d are fluxional, displaying both rapid isomerization of the square-pyramidal structure and exchange of coordinated and free phosphine ligands. UV-vis spectroscopy shows stark electronic differences between 1 and 2a-d. Whereas 1 displays a strong absorbance at 380 nm with a much weaker band at 585 nm in the absorption spectrum, complexes 2a-d display an intense (ε > 10(4) M(-1) cm(-1)), low-energy absorption band in the region 580-640 nm; however, in the cases of 2a and 2b, the addition of excess phosphine resulted in changes to the UV-vis spectrum indicating the formation of six-coordinate adducts [ONO]Rh(PMe(3))(3) (3a) and [ONO]Rh(PMe(2)Ph)(3) (3b), respectively. The experimental and DFT computational data for the six-coordinate complexes 1, 3a, and 3b are consistent with their formulation as classical, d(6), pseudo-octahedral, coordination complexes. In the five-coordinate complexes 2a-2d, π-bonding between the rhodium center and the [ONO] ligand leads to a high degree of covalency and metal-ligand electron distributions that are not accurately described by formal oxidation state assignments.