Break-Junction Studies on Paddle-Wheel Complexes with Four Lateral Anchor Groups: Small Influence of Metal-Metal Bond Order

Abstract

This contribution investigates the role of the metal-metal bond in paddle-wheel complexes for molecular conductance. To these ends, we compare two pairs of Mo and Rh paddle-wheel complexes M2(LN)4 and M2(LSMe)4 (M = Rh, Mo), each with four lateral 4-pyridyl- (LN-) or 4-(methylthio)- (LSMe-) functionalized benzamidinate ligands mutually disposed at 90° angles. These complexes represent bond orders of 4 (Mo) and 1 (Rh), while offering metal-based highest occupied frontier orbitals of the same δ symmetry. The structural features of the complexes were established by X-ray diffraction on single crystals. Molecular conductance measurements were performed with the aid of a scanning-tunnelling microscopy break-junction setup and revealed that the Mo complexes surpass their Rh congeners. Decoration of the paddle-wheel complexes with four laterally disposed anchor groups results in four different possible modes of molecule attachment to the Au electrodes with two, three or even four anchoring points. The conductances of possible junction geometries were assessed quantum chemically with the DFT+Σ approach using Au slab electrodes. Calculated variations by about one order of magnitude for the different anchoring geometries can explain the rather broad conductance distributions observed in our experiments. Further transport calculations considered two-point molecule attachment to sharp or blunt nanoelectrodes for mutual cis and trans dispositions of the anchor groups. Our computational results indicate that the better performance of the Mo complexes originates from superior conjugation between the Mo2 δ-binding and the ligand π orbitals as compared to the Rh2 δ* orbital, rather than from the larger metal-metal bond order. Upon increasing the bias voltage, we observed a new conductance feature associated with a nearly 100 times higher G value, which we ascribe tentatively to oxidation of the molecule inside the junction