Enhancement of CO Insertion into a Pd−C Bond in a Pd−Co Heterodinuclear Complex

A series of Pd-containing heterodinuclear methyl complexes, (dppe)(CH 3 )Pd−ML n (ML n = MoCp(CO) 3, WCp(CO) 3, Co(CO) 4; dppe = 1,2-bis(diphenylphosphino)ethane), have been prepared by the metathetical reactions of Pd(CH 3 )(NO 3 )(dppe) with Na + [ML n ] -, and the complexes were characterized by spectroscopic methods and/or X-ray structure analysis. Related Pt-containing dinuclear complexes were similarly prepared and characterized. The rate of CO insertion into a Pd−CH 3 or Pt−CH 3 bond was investigated using these complexes. The Pd−Co complex (dppe)(CH 3 )Pd−Co(CO) 4 shows a high activity of CO insertion, giving (dppe)(CH 3 CO)Pd−Co(CO) 4, and the initial rate is ca. 80 times higher than those of the analogous complex Pd(CH 3 )Cl(dppe). Whereas slow insertion of CO is observed in the Pt−Co complex (dppe)(CH 3 )Pt−Co(CO) 4, no reaction takes place for Pt(CH 3 )Cl(dppe). It is revealed by using 13 CO that CO in the Co(CO) 4 fragment preferentially inserts into the Pd−CH 3 bond over CO in the gas phase. The carbonyl insertion reaction in the Pd−Co heterodinuclear complex has been theoretically studied using B3LYP hybrid density functional theory to clarify its reaction mechanism and the electronic factors controlling the reaction. The calculations for a model complex, (H 2 PCH 2 CH 2 PH 2 )Pd(CH 3 )−Co(CO) 4, have shown that the most favorable reaction path consists of methyl migration from the Pd to the Co atom, carbonyl insertion reaction on the Co atom, CO coordination to the Co atom, and acetyl migration from the Co atom to the Pd atom, which is in accord with the experimental results. The electron donation from the Pd d orbital to the bridging CO π* orbitals plays an important role in stabilizing the intermediates and transition states.