Synthesis, Electrochemistry, and Single-Molecule Conductance of Bimetallic 2,3,5,6-Tetra(pyridine-2-yl)pyrazine-Based Complexes

The ligands 4′-(4-(methylthio)­phenyl)-2,2′:6′,2″-terpyridine (L1), 4′-((4-(methylthio)­phenyl)­ethynyl)- 2,2′:6′,2″-terpyridine (L2), and bis­(tridentate) bridging ligand 2,3,5,6-tetra­(pyridine-2-yl)­pyrazine (tpp) were used to prepare the complexes [Ru­(L1)2]­[PF6]2 ([1]­[PF6]2, [Ru­(L2)2]­[PF6]2 ([2]­[PF6]2), [{(L1)­Ru}­(μ-tpp)­{Ru­(L1)}]­[PF6]4 ([3]­[PF6]4), and [{(L2)­Ru}­(μ-tpp)­{Ru­(L2)}]­[PF6]4 ([4]­[PF6]4). Crystallographically determined structures give S···S distances of up to 32.0 Å in [4]4+. On the basis of electrochemical estimates, the highest occupied molecular orbitals of these complexes fall between −5.55 and −5.85 eV, close to the work function of clean gold (5.1–5.3 eV). The decay of conductance with molecular length across this series of molecules is approximately exponential, giving rise to a decay constant (pseudo β-value) of 1.5 nm–1, falling between decay factors for oligoynes and oligophenylenes. The results are consistent with a tunnelling mechanism for the single-molecule conductance behavior.