Enhancement of CO insertion into a Pd-C bond in a Pd-Co heterodinuclear complex

A series of Pd-containing heterodinuclear methyl complexes, (dppe)(CH3)Pd-M L, (ML, MoCp(CO)(3), WCp(CO)(3), Co(CO)(4); dppe = 1,2-bis(diphenylphosphin o)ethane), have been prepared by the metathetical reactions of Pd(CH3)(NO3) (dppe) with Na+[MLn](-), and the complexes were characterized by spectrosco pic methods and/or X-ray structure analysis. Related Pt-containing dinuclea r complexes were similarly prepared and characterized. The rate of CO inser tion into a Pd-CH3 or Pt-CH3 bond was investigated using these complexes. T he Pd-Co complex (dppe)(CH3)Pd-Co(CO)4 shows a high activity of CO insertio n, giving (dppe)(CH3CO)Pd-Co(CO)(4), and the initial rate is ca. 80 times h igher than those of the analogous complex Pd(CH3)Cl(dppe). Whereas slow ins ertion of CO is observed in the Pt-Co complex (dppe)(CH3)Pt-Co(CO)(4), no r eaction takes place for Pt(CH3)Cl(dppe). It is revealed by using (CO)-C-13 that CO in the Co(CO)(4) fragment preferentially inserts into the PdCH3 bon d over CO in the gas phase. The carbonyl insertion reaction in the Pd-Co he terodinuclear complex has been theoretically studied using B3LYP hybrid den sity functional theory to clarify its reaction mechanism and the electronic factors controlling the reaction. The calculations for a model complex, (H 2PCH2CH2PH2)Pd(CH3)-Co(CO)(4), have shown that the most favorable reaction path consists of methyl migration from the Pd to the Co atom, carbonyl inse rtion reaction on the Co atom, CO coordination to the Co atom, and acetyl m igration from the Co atom to the Pd atom, which is in accord with the exper imental results. The electron donation from the Pd d orbital to the bridgin g CO pi* orbitals plays an important role in stabilizing the intermediates and transition states.