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

New rhodium complexes of bis­(3,5-di-tert-butyl-2-phenol)­amine ([ONOcat]­H3) were synthesized, and their electronic properties were investigated. These compounds were prepared by combining [ONOq]K and [(cod)­Rh­(μ-Cl)]2 in the presence of an auxiliary donor ligand to yield complexes of the type [ONO]­RhLn (n = 3, L = py (1); n = 2, L = PMe3 (2a), L = PMe2Ph (2b), PMePh2 (2c), PPh3 (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(PMe3)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 > 104 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(PMe3)3 (3a) and [ONO]­Rh­(PMe2Ph)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, d6, 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.