cis-Dioxo- and cis-(hydroxo)oxo-Mo(V) complexes stabilized by intramolecular hydrogen-bonding.

The reactions of Tp(iPr)Mo(VI)O(2)Cl with salicylanilides and NEt(3) produce cis-Tp(iPr)Mo(VI)O(2)(2-OC(6)H(4)CONHR) (Tp(iPr) = hydrotris(3-isopropylpyrazol-1-yl)borate, R = Ph, 4-C(6)H(4)Cl, 4-C(6)H(4)OMe). The N-methyl complex, Tp(iPr)MoO(2){2-OC(6)H(4)CON(Me)Ph}, is similarly prepared. Reduction of the amido complexes by cobaltocene produces green, EPR-active compounds, [CoCp(2)][Tp(iPr)Mo(V)O(2)(2-OC(6)H(4)CONHR)], that exhibit strong, low energy, ν(MoO(2)) IR bands at ∼ 895 and 790 cm(-1) (cf. ∼ 935 and 900 cm(-1) for the Mo(VI) analogues). The X-ray structures of all seven complexes have been determined. In each case, the Mo center exhibits a distorted octahedral coordination geometry defined by mutually cis oxo and phenolate ligands and a tridentate fac-Tp(iPr) ligand. The Mo(V) anions exhibit greater Mo═O distances (av. 1.738 Å vs 1.695 Å) and O═Mo═O angles (av. 112.4° vs 102.9°) than their Mo(VI) counterparts, indicative of the presence of a three-center (MoO(2)), π* semioccupied molecular orbital in these d(1) complexes. The amido Mo(VI) and Mo(V) complexes exhibit an intramolecular hydrogen-bond between the NH and O(phenolate) atoms. Protonation of [CoCp(2)][Tp(iPr)Mo(V)O(2)(2-OC(6)H(4)CONHR)] by lutidinium tetrafluoroborate is quantitative and produces EPR-active, cis-(hydroxo)oxo-Mo(V) complexes, Tp(iPr)Mo(V)O(OH)(2-OC(6)H(4)CONHR), related to the low pH Mo(V) forms of sulfite oxidase.