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

Abstract

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(μ-O₂CMe)]₂ (L = benzo[h]quinolinyl) (I), is examined for C–Cl coupling, and the proposed cation, [(L-C,N)PhPd¹(μ-O₂CMe)₂Pd²(L-C,N)]⁺ (II), examined for C–C coupling together with (L-C,N)PhPd¹(μ-O₂CMe)₂Pd²Cl(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd² to form II is predicted to be favored. Cation II undergoes Ph–C coupling at Pd¹ with concomitant Pd¹–Pd² lengthening and shortening of the Pd¹–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 Pd¹ and population of the d_z² orbitals of Pd¹ and Pd² 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 Pd¹ with Pd¹–O¹ lengthening, significantly less population occurs for the d_z² orbital of Pd¹ than for the dissociative pathway, and d_z² at Pd² is only marginally populated resulting in an intermediate that is formally a Pd¹(I)-Pd²(III) species, (L-Cl-N,Cl)Pd¹(μ-O₂CMe)Pd²Cl(O₂CMe)(L-C,N) that releases chloride from Pd² 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 Pd² and strengthening of the Pd(I)–Pd(III) interaction in the analogous intermediate with η²-coordination at Pd¹ by L-Ph-N, C¹-C².