Reduction of Carbon Monoxide by Tandem Electron Transfer and Migratory Insertion of a Masked Zirconium(IV) Hydride. Formation of a Zirconium Formyl−Ylide Complex

Abstract

In the presence of carbon monoxide, the Zr(III) tetrahydroborate complex Zr(η5-C5H5)BH4[N(SiMe2CH2PPri2)2] disproportionates to give two diamagnetic species: the bis(tetrahydroborate) complex Zr(η5-C5H5)(BH4)2[N(SiMe2CH2PPri2)2] and the dicarbonyl derivative Zr(η5-C5H5)(CO)2[N(SiMe2CH2PPri2)2]. The bis(tetrahydroborate) undergoes a further slow reaction with CO to generate a zirconium−formyl complex in which the formyl unit is stabilized by phosphine coordination; in other words, electron transfer in tandem with hydride migratory insertion have been used to reduce CO. The structure of the formyl−ylide species Zr(η5-C5H5)(BH4)(HCOPPri2CH2SiMe2)N(SiMe2CH2PPri2BH3) was determined by both solution (NMR spectroscopy) and solid-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 bond that subsequently undergoes migratory insertion with CO to generate the formyl unit.