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.