Controlling Coordination Geometries: Ru–Carbene Complexes with Tetra-NHC Ligands

The synthesis of
the first ruthenium­(II) complexes bearing open-chain
tetra-N-heterocyclic carbene (tetra-NHC) ligands
via in situ transmetalation is described. The ruthenium complexes
show differing coordination geometries depending on the length of
the alkyl linker. Replacement of the two cis acetonitrile
ligands in the methylene bridged ruthenium complex by nucleophiles
also influences the coordination geometry. Both structural motifs
were evaluated in transfer hydrogenation (TH) of acetophenone, and
in particular the sawhorse-type coordinated system exhibits remarkable
activity with turnover frequencies of more than 100 000 h–1.