REDUCTION OF CARBON-MONOXIDE BY TANDEM ELECTRON-TRANSFER AND MIGRATORY INSERTION OF A MASKED ZIRCONIUM(IV) HYDRIDE - FORMATION OF A ZIRCONIUM FORMYL-YLIDE COMPLEX

In the presence of carbon monoxide, the Zr(III) tetrahydroborate compl ex Zr(eta(5)-C5H5)BH4[N(SiMe(2)CH(2)PPr(2)(i))(2)] disproportionates t o give two diamagnetic species: the bis-(tetrahydroborate) complex (et a(5)-C5H5)(BH4)(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)] and the dicarbonyl der ivative (eta(5)-C5H5)(CO)(2)[N(SiMe(2)CH(2)PPr(2)(i))(2)]. The bis(tet rahydroborate) undergoes a further slow reaction with CO to generate a zirconium-formyl complex in which the formyl unit is stabilized by ph osphine coordination; in other words, electron transfer in tandem with hydride migratory insertion have been used to reduce CO. The structur e of the formyl-ylide species PPr(2)(i)CH(2)SiMe(2))N(SiM(2)CH(2)PPr(2 )(i)BH(3)) was determined by both solution (NMR spectroscopy) and soli d-state (X-ray crystallography) methods. Not only does one end of the tridentate amido-diphosphine ligand bind to the formyl carbon but the other phosphine donor is coordinated to a BH3 molecule. Presumably, it is this latter interaction that initially frees up a reactive Zr-H bo nd that subsequently undergoes migratory insertion with CO to generate the formyl unit.