posted on 2014-10-06, 00:00authored byLeila
G. Ranis, Kalpani Werellapatha, Nicholas J. Pietrini, Bruce A. Bunker, Seth N. Brown
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(ONOQ)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(DOPOQ) or by reaction of Pb(DOPOQ)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 [LCat]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.