The Reactivity of Zirconium Hydrides with Transition Metal Carbonyls

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*).