Cis-dioxo-and cis-(hydroxo)oxo-Mo(V) complexes stabilized by intramolecular hydrogen-bonding

The reactions of TpiPrMoVIO2Cl with salicylanilides and NEt3 produce cis-TpiPrMoVIO2(2-OC6H4CONHR) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, R = Ph, 4-C6H4Cl, 4-C6H4OMe). The N-methyl complex, TpiPrMoO2{2-OC6H4CON(Me)Ph}, is similarly prepared. Reduction of the amido complexes by cobaltocene produces green, EPR-active compounds, [CoCp2][TpiPrMoVO2(2-OC6H4CONHR)], that exhibit strong, low energy, ν(MoO2) 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-TpiPr 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 (MoO2), π* semioccupied molecular orbital in these d1 complexes. The amido Mo(VI) and Mo(V) complexes exhibit an intramolecular hydrogen-bond between the NH and Ophenolate atoms. Protonation of [CoCp2][TpiPrMoVO2(2-OC6H4CONHR)] by lutidinium tetrafluoroborate is quantitative and produces EPR-active, cis-(hydroxo)oxo-Mo(V) complexes, TpiPrMoVO(OH)(2-OC6H4CONHR), related to the low pH Mo(V) forms of sulfite oxidase.