Interaction of Thioether Groups at the Open Coordination Sites of Palladium(II) and Platinum(II) Complexes Probed by Luminescence Spectroscopy at Variable Pressure

The crystal structures of [PtCl2(ttcn)], [PdCl2(ttcn)], [Pt(ethylenediamine)(ttcn)](PF6)2, and [Pd(ethylenediamine)(ttcn)](PF6)2 (ttcn = 1,4,7-trithiacyclononane) show short apical metal---S(ttcn) distances, qualitatively indicating an interaction. Luminescence spectroscopy was used to study these crystalline complexes at room temperature and variable hydrostatic pressure. The luminescence band maximum of [PdCl2(ttcn)] shows a pressure-induced blue shift of +6 cm−1/kbar, while the platinum(II) compounds show a red shift of approximately −20 cm−1/kbar. This difference is rationalized in terms of a competition between blue shifts due to pressure-induced metal−ligand bond shortening in the equatorial plane and increasing out-of-plane distance of the metal center, and a red shift due to the approach of the apical sulfur donor to the metal center. Density functional theory (DFT) calculations indicate d-d luminescence transitions and a different nature of the highest occupied molecular orbital (HOMO) for [PdCl2(ttcn)] than for [PtCl2(ttcn)], while the lowest unoccupied molecular orbitals (LUMOs) are identical in character. This electronic structure difference is used to rationalize the different pressure effects.