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 m H) and 3,5-bis-(5-(methylthio)-pyridin-2-yl)-phenyl hexanoate (L p H) 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/G 0) and Pt m -3.3 log-(G/G 0), in contrast to the usual conductance trend (para > meta) for purely organic π-electron systems.