Metal and Ligand Effects on Bonding in Group 6 Complexes of Redox-Active Amidodiphenoxides

Group 6 complexes M­(ONO)₂ (M = Cr, Mo, W; ONO = bis­(2-oxy-3,5-di-tert-butylphenyl)­amide) are prepared by the reaction of divalent metal halide precursors with Pb­(ONO^Q)₂. Analogous complexes containing the 2,4,6,8-tetra-tert-butyl-1,9-dioxophenoxazinate ligand (DOPO) are prepared by protonolysis of chromocene with H­(DOPO^Q) or by reaction of Pb­(DOPO^Q)₂ with M₂Br₄(CO)₈ (M = Mo, W). The molybdenum and tungsten complexes are symmetrical, octahedral compounds for which spectroscopic data are consistent with M­(VI) complexes with fully reduced [L^Cat]^3– ligands. Quantitative analysis of the intraligand bond lengths, by comparison with literature standards, allows calculation of metrical oxidation states (MOS) for the ONO ligands. The MOS values of the tungsten and molybdenum complexes indicate that π donation from the ligand is weak and that differences between the ONO and DOPO ligands are small. In both the solid state and in solution, Cr­(DOPO)₂ is paramagnetic with localized quinone and semiquinone ligands bound to Cr­(III). The geometry and electronic structure of Cr­(ONO)₂ differ in the solid state and in solution, as determined by crystallography, magnetic measurements, and Cr K-edge X-ray absorption spectroscopy. In solution, the structure resembles that of the DOPO analogue. In contrast, solid Cr­(ONO)₂ is a singlet, and X-ray absorption near-edge spectroscopy indicates that the chromium is significantly more oxidized in the solid state than in solution. An electronic description compounds to that of the tungsten and molybdenum analogues, but with considerably more charge transfer from the ligand to chromium via π donation, is in agreement with the experimental observations.