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).