Theoretical Investigation into the Mechanism of Reductive Elimination from Bimetallic Palladium Complexes

Reductive elimination of C-Cl and C-C bonds from binuclear organopalladium complexes containing Pd-Pd bonds with overall formal oxidation state +III are explored by density functional theory for dichloromethane and acetonitrile solvent environments. An X-ray crystallographically authenticated neutral complex, [(L-C,N)ClPd(mu-O2CMe)](2) (L = benzo[h]quinolinyl) (I), is examined for C-Cl coupling, and the proposed cation, [(L-C,N)PhPd1(mu-O2CMe)(2)Pd-2 (L-C, N)](+) (II), examined for C-C coupling together with (L-C, N)PhPd1(mu-O2CMe)(2)(PdCl)-Cl-2(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd-2 to form Ills predicted to be favored. Cation II undergoes Ph-C coupling at Pd-1 with concomitant Pd-1-Pd-2 lengthening and shortening of the Pd-1-O bond trans to the carbon atom of L; natural bond orbital analysis indicates that reductive coupling from II involves depopulation of the d(x2-y2) orbital of Pd-1 and population of the d(z2) orbitals of Pd-1 and Pd-2 as the Pd-Pd bond lengthens. Calculations for the symmetrical dichloro complex I indicate that a similar dissociative pathway for C-Cl coupling is competitive with a direct (nondissociative) pathway in acetonitrile, but the direct pathway is favored in dichloromethane. In contrast to the dissociative mechanism, direct coupling for I involves population of the d(x2-y2) orbital of Pd-1 with Pd-1-O-1 lengthening, significantly less population occurs for the d(z2) orbital of Pd-1 than for the dissociative pathway, and de at Pd2 is only marginally populated resulting in an intermediate that is formally a Pd-1(I)-Pd-2(III) species, (L-CI-N,Cl)Pd-1(mu-O2CMe)(PdCl)-Cl-2(O2CMe)(L-C,N) that releases chloride from Pd-2 with loss of Pd(I)-Pd(III) bonding to form a Pd (II) species. A similar process is formulated for the less competitive direct pathway for C-C coupling from III, in this case involving decreased population of the d(z2) orbital of Pd-2 and strengthening of the Pd(I)-Pd(III) interaction in the analogous intermediate with eta(2)-coordination at Pd-1 by L-Ph-N, C-1-C-2.