Synthesis and crystal structures of rhodium acetate paddle-wheel complexes with anchor group-functionalised and hydrogen bond-supported axial ligands

We report the synthesis and X-ray structures of four Rh-2(OAc)(4)(L-Ax)(2) (OAc=acetate, CH3COO-) paddle-wheel complexes (C1-C4) with methylthio-modified axial ligands L-Ax derived from benzamidine (L1), anilinopyrimidine (L2) or isothiourea (L3, L4) that are capable of forming N-H center dot center dot center dot O hydrogen bonds to the equatorially bridging acetate ligands. This was done with the aim to suppress dissociation of the axial ligands and to make the complexes amenable to single-molecule conductance measurements in a scanning tunneling microscope break-junction (STM-BJ) setup. The characteristic spectroscopic features (NMR, IR, vis), crystal structures, the hydrogen-bonding motifs and a DFT-based screening of their frontier molecular orbitals are presented. Our calculations suggest that hydrogen-bonding stabilises axial ligand binding by 15 kJ/mol to 30 kJ/mol and that the HOMO of the rhodium paddle-wheels is closer to the Au work function than the LUMO, so that the rhodium paddle-wheels are expected to constitute hole conductors.