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

Abstract

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.

Spectroscopic and Structural Elucidation of Uranium Dioxophenoxazine Complexes

“…4 Applying that model to 1 gave an MOS value of −1.25 (19), which is consistent with other reports describing the quinone form of the ligand, including Pb(DOPO q ) 2 (MOS = −1.06 (15)). 4 Accordingly, the assignment of a hexavalent uranyl with two monoanionic DOPO q ligands is well supported by crystallographic and spectroscopic data.…”

“…These parameters are on the order of those observed for Pb(DOPO q ) 2 , which has C−O distances of 1.297(4) and 1.246(4) Å and C−N distances of 1.347(4) and 1.316(4) Å. 4 Similarly, Minkin reports values for Mn(DOPO q ) 2 through Zn(DOPO q ) 2 with C−O values that range from 1.266(2) to 1.294(5) Å and C−N distances from 1.315(1) to 1.333(5) Å. 5 These structural parameters measured for the ligands in 1 were used to determine the metrical oxidation states (MOS) as established by Brown and coworkers for the DOPO ligand, which is useful due to the sensitivity of this method for the degree of π bonding in a molecule.…”

“…Previously, treating UI 3 (THF) 4 with the related iminoquinone ligand, R iq, resulted in electron transfer from the uranium to the ligand, forming the uranium(IV) iminosemiquinone product, ( R isq)-UI 3 (THF) 2 (R = dipp), as confirmed by magnetometry experiments. 10 Given the similarities between the iminoquinone and the dioxophenoxazine frameworks, it is hypothesized that the analogous uranium oxidation event will occur for the dioxophenoxazine complexes as well.…”

“…Low-valent derivatives of the DOPO ligand were also attempted by metalation with trivalent UI 3 (THF) 4 . Previously, treating UI 3 (THF) 4 with the related iminoquinone ligand, R iq, resulted in electron transfer from the uranium to the ligand, forming the uranium(IV) iminosemiquinone product, ( R isq)-UI 3 (THF) 2 (R = dipp), as confirmed by magnetometry experiments.…”

14. Solute–solvent interactions mediated by main group element(lone-pair)···π(aryl) interactions

“…bis(diphenylpentane-2,4-diiminato)lead(II) toluene solvate [28], is coordinated by four nitrogen atoms, but there is a large void above the lead(II) atom (Figure 14.7a), which is occupied by the toluene molecule [d = 3.66 Å and θ = 8.6°]. A relatively high coordination number is also found in (11), bis(2,4,6,8-tetra-t-butyl-1-oxo-1H-phenoxazin-9-olato)lead(II) benzene di-solvate [29], where the lead(II) is in a N 2 O 4 donor set. However, once again, there is a large void to accommodate one of the two benzene molecules co-crystallised with the lead compound via a Pb(II)···π(benzene) interaction [d = 3.77 Å and θ = 17.9°] (Figure 14.7b).…”