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 complex Zr(η⁵-C₅H₅)BH₄[N(SiMe ₂CH₂PPr₂ⁱ)₂] disproportionates to give two diamagnetic species: the bis-(tetrahydroborate) complex Zr(η⁵-C₅H₅)(BH₄) ₂[N(SiMe₂CH₂PPr₂ⁱ) ₂] and the dicarbonyl derivative Zr(η⁵-C₅H₅)(CO)₂[N(SiMe ₂CH₂PPr₂ⁱ)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)(HCOPPr ₂ⁱCH₂SiMe₂)N(SiMe₂CH ₂PPr₂ⁱBH₃) 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.