Experimental and Theoretical Studies of Isomeric Metal (N^C^N)Cl Coordination Complexes (Metal = Pt, Pd) with Multiple Conductance Pathways in Single-Molecule Junctions

The present work provides insight into the effect of connectivity within isomeric 3,5-bis(pyridin-2-yl)phenyl (N^C^N) platinum and palladium complexes on their electron transmission properties within gold|molecule|gold junctions. The ligands 3,5-bis(4-(methylthio)pyridin-2-yl)phenyl hexanoate (L^mH) and 3,5-bis(5-(methylthio)pyridin-2-yl)phenyl hexanoate (L^pH) were synthesized and coordinated with either PtCl or PdCl to form complexes Pt^m, Pt^p, Pd^m and Pd^p. X-ray photoelectron spectroscopy (XPS) measurements evaluated the contacting modes of the molecules in the junctions. A combination of scanning tunneling microscopy-break junction (STM-BJ) measurements and density functional theory (DFT) calculations demonstrate that for the single-molecule S···S contacted junctions metal coordination enhanced the conductance compared with the free ligands. Notably, the higher degree of orbital mixing between the metal center and the ligand π-orbitals in the metal complexes plays a greater role than quantum interference to the extent that the complexes that incorporate ligands substituted with thiomethyl groups in meta positions relative to the pyridine-benzene linkages have a higher conductance than their para-analogs, e.g., Pt^p −3.8 log(G/G₀) and Pt^m −3.3 log(G/G₀), in contrast to the usual conductance trend (para > meta) for purely organic π-electron systems.