The role of pi-pi, stacking interactions in square planar palladium complexes : combined quantum mechanics/molecular mechanics QM/MM studies

Density functional (DFT) studies and hybrid QM/MM-DFT calculations demonstrate the importance of pi-pi stacking interactions in determining the structural features of two exemplary d(8) palladium complexes, PdBr(p-NCC6H4)({S}-MeO-Biphep), 1, and PdBr(C6F5)-({S}-MeO-Biphep), 2. Despite the superficial similarity of the two compounds, the former shows marked distortions from square planar geometry, while the latter exhibits an almost ideal structure. Attractive pi-pi stacking interactions between two pairs of P-phenyl 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 additional stacking interaction between one P-phenyl ring and the pentafluorophenyl a-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 pi-pi interactions distinctly alters the coordination geometry. These results reveal a previously unrecognized role for pi-pi stacking interactions in the stabilization of structural features in transition metal compounds. This suggests pi-pi stacking interactions as a potential new design principle in tailoring coordination compounds.