The Role of p-p Stacking Interactions in Square Planar Palladium Complexes. Combined Quantum Mechanics/Molecular Mechanics QM/MM Studies

D. functional (DFT) studies and hybrid QM/MM-DFT calcns. demonstrate the importance of p-p stacking interactions in detg. the structural features of two exemplary d8 palladium complexes, PdBr(p-NCC6H4)({S}-MeO-Biphep), 1, and PdBr(C6F5)({S}-MeO-Biphep), 2. Despite the superficial similarity of the two compds., the former shows marked distortions from square planar geometry, while the latter exhibits an almost ideal structure. Attractive p-p stacking interactions between two pairs of P-Ph rings and the arene backbone of the MeO-Biphep are the main origin of the distortion in complex 1. The planar structure of complex 2 is preferred as a consequence of an addnl. stacking interaction between one P-Ph ring and the pentafluorophenyl s-ligand. The artificial introduction of an analogous stacking interaction in complex 1 reestablishes an ideal square planar geometry, thus demonstrating that switching on/off specific p-p interactions distinctly alters the coordination geometry. These results reveal a previously unrecognized role for p-p stacking interactions in the stabilization of structural features in transition metal compds. This suggests p-p stacking interactions as a potential new design principle in tailoring coordination compds. [on SciFinder (R)]