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 demonstr ate the importance of pi-pi stacking interactions in determining the struct ural features of two exemplary d(8) palladium complexes, PdBr(p-NCC6H4)({S} -MeO-Biphep), 1, and PdBr(C6F5)-({S}-MeO-Biphep), 2. Despite the superficia l similarity of the two compounds, the former shows marked distortions from square planar geometry, while the latter exhibits an almost ideal structur e. Attractive pi-pi stacking interactions between two pairs of P-phenyl rin gs and the arene backbone of the MeO-Biphep are the main origin of the dist ortion in complex 1. The planar structure of complex 2 is preferred as a co nsequence of an additional stacking interaction between one P-phenyl ring a nd the pentafluorophenyl a-ligand. The artificial introduction of an analog ous stacking interaction in complex 1 reestablishes an ideal square planar geometry, thus demonstrating that switching on/off specific pi-pi interacti ons distinctly alters the coordination geometry. These results reveal a pre viously unrecognized role for pi-pi stacking interactions in the stabilizat ion of structural features in transition metal compounds. This suggests pi- pi stacking interactions as a potential new design principle in tailoring c oordination compounds.