The Reactivity of Zirconium Hydrides with Transition Metal Carbonyls

Citation

Barger, Paul Theron (1983) The Reactivity of Zirconium Hydrides with Transition Metal Carbonyls. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/qvev-5987. https://resolver.caltech.edu/CaltechTHESIS:09262018-130043347

Abstract

The reactivity of bis(pentamethylcyclopentadienyl) zirconium hydride complexes with a variety of Group VIII transition metal carbonyls has been investigated. These reactions are observed to follow two distinct pathways; one involving reductive loss of the zirconium hydrides as H₂, the other proceeding by. hydride transfer to the carbon atom of a carbonyl to afford CO reduction. Treatment of CpM(CO)₂ (M=Co, Rh, RuH) with Cp₂*ZrH₂ or [Cp₂*ZrN₂]₂N₂(Cp=C₅H₅, Cp*=C₅Me₅) give the "early" and "late" metal dimers, CpM(CO)₂ZrCp₂*: with elimination of H₂ or N₂. The X-ray crystal structure of CpCo(CO)₂ZrCp₂*; is reported and shows that this molecule contains a Co-Zr single bond bridged by a conventionally bound μ-CO and a four-electron donating μ-η¹, η¹ CO. The reactions of Cp₂*ZrHX (X=F, Cl) with these carbonyls proceed by the second pathway to give oxycarbene complexes, Cp(CO)M=CHO-Zr(X)Cp₂* (M=Co, Rh). These compounds demonstrate that the zirconium hydride reduction of a Group VIII metal carbonyl is reversible; an equilibrium is observed between the carbene complexes and the starting metal dicarbonyl and ziconium hydride. Treatment of CpM(CO)(PMe₃)H or CpM(CO)₂CH₃ (M=Fe, Ru) with Cp₂*ZrH₂, in the presence of PMe₃, affords Cp(PMe₃)₂M-CH₂O-Zr(H)Cp₂* or Cp₂*Zr(OCH=CH₂)H. The mechanisms of the transformation are proposed to involve initial formation of an iron or ruthenium oxycarbene intermediate which undergoes migratory insertion into the metal hydride or alkyl bond followed by phosphine trapping or β-elimination to give the observed products.

Several zirconium oxycarbene complexes have been prepared by the reduction of the corresponding zirconium carbonyl by Cp₂*ZrH₂. These molecules represent some of the first isolable examples of Group IV metal to carbon multiple bonding. The X-ray crystal structure of Cp₂(PMe₃)Zr=CHO-Zr(I)Cp₂* • C₆H₆ is reported. Treatment of Cp₂(CO)Zr=CHO-Zr(H)Cp₂* with MeI or Cp₂(PMe₃)Zr=CHO-Zr(I)Cp₂* with CO gives a new product, the structure of which has been shown by X-ray diffraction to be Cp₂*ZrOCH=C(Zr(I)Cp₂*)O. The mechanism for this transformation has been shown to involve an intramolecular coupling of carbene and carbonyl ligands on a zirconium center to give a zirconium ketene intermediate, which rearranges to the observed product. In the presence of pyridine the ketene intermediate can be trapped to give the isolable Cp₂(pyr)Zr(O=C=CHO-Zr(H)Cp₂*).

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