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

Abstract

AbstractWe report the synthesis and X‐ray structures of four Rh2(OAc)4(LAx)2 (OAc=acetate, CH3COO−) paddle‐wheel complexes (C1−C4) with methylthio‐modified axial ligands LAx derived from benzamidine (L1), anilinopyrimidine (L2) or isothiourea (L3, L4) that are capable of forming N−H⋅⋅⋅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.