A metal alkynyl molecular wire with PN ligands: Synthesis, isomerization, physical properties and single-molecule conductance

Abstract

As new candidates for organometallic molecular wires, we designed a ruthenium trans-di(alkynyl) molecular wire 1H, trans-Ru(PN)2(C≡CC≡C-H)2, with the two mixed phosphine-pyridine supporting ligands (PN: 2-diphenylphosphinomethyl-6-methylpyridine). While two stereo-isomers with the syn and anti configurations were possible, the syn isomers of triisopropylsilylbutadiynyl precursors syn-1TIPS and syn-1H were prepared in direct and stereo-selective manners. Electrochemical analysis of syn-1TIPS revealed a virtually reversible redox wave with the potential cathodically shifted compared to that of the bis(diphosphine) analogue 2TMS, trans-Ru(dppe)2(C≡CC≡C-SiMe3)2. The density functional theoretical (DFT) study supports the similarity of the electronic structures of 1 and 2 with the HOMOs being close to the Fermi energy level of gold electrodes. STM-break junction measurements of Au-syn-1-Au (Au: gold electrode) revealed high single-molecule conductance (1.9 × 10–2G0), which is comparable to that of Au-2-Au. Calculations on the basis of the DFT-non equilibrium Green’s function method also support the experimental results. Notably, the molecular junction Au- syn-1-Au can be formed from syn-1H through C-H activation and formation of Au-C bonds without any pre-functionalization in contrast to the bis(diphosphine) analogue 2H, which requires terminal gold functionalization.