Probing intermetallic coupling in dinuclear N-heterocyclic carbene ruthenium(II) complexes

A series of bimetallic N-heterocyclic carbene (NHC) ruthenium(II) complexes were synthesized, which comprise two [RuCl₂(cymene)(NHC)] units that are interlinked via the NHC nitrogens by alkyl chains of different length. Electrochemical characterization revealed two mutually dependent oxidation processes for the complex with a methylene linker, indicating moderate intramolecular electronic coupling of the two metal centers (class II system). The degree of coupling decreases rapidly upon increasing the number of CH₂ units in the linker and provides essentially decoupled class I species when propylene or butylene linkers are used. Electrochemical analyses combined with structural investigations suggest a through-bond electronic coupling. Replacement of the alkyl linker with a p-phenylene group afforded cyclometalated complexes, which were considerably less stable. The electronic coupling in the methylene-linked complex and the relatively robust NHC–ruthenium bond may provide access to species that are switchable on the molecular scale