Ruthenium Azocarboxamide Half-Sandwich Complexes: Influence of the Coordination Mode on the Electronic Structure and Activity in Base-Free Transfer Hydrogenation Catalysis

Azocarboxamides were used as chelating ligands in ruthenium half-sandwich complexes. The synthesis and characterization of two new complexes with an unprecedented coordination motif are presented together with an in-depth investigation of two recently published complexes. Three different coordination modes of the ligands were realized, as evident by NMR spectroscopy and single-crystal X-ray diffraction. The use of base during the synthesis leads to a coordination of a deprotonated ligand, while the introduction of additional donor atoms results in a noncoordinated amide group. The first systematic experimental (cyclic voltammetry and UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) investigation of the electronic structure of metal complexes bearing this redox-active ligand class is presented, revealing redox processes with ligand contribution. The absorption spectra and electrochemistry are mainly determined by the protonation state of the ligand. While complexes 2[PF6], 3[PF6], and 4[PF6] with neutral azocarboxamides show similar electronic spectra and cyclovoltammograms, the incorporation of a deprotonated monoanionic ligand in complex 1 leads to significant changes of these properties. In contrast, the catalytic activity in the base-free transfer hydrogenation reaction is mainly dependent on the coordination of the amide group, with only minor effects of the protonation state. While complexes 3[PF6] and 4[PF6], with an uncoordinated amide group, are inactive without the addition of base, complexes 1 and 2[PF6], with a metal-bound amide group, show activity under base-free conditions. The impact of the position of the amide group together with the detection of metal hydride species in H-1 NMR spectroscopy suggests the operation of metal ligand bifunctional catalysis to take place when no base is added.