Probing the potential of N-heterocyclic carbenes in molecular electronics: redox-active metal centers interlinked by a rigid ditopic carbene ligand

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Abstract
Bimetallic homonuclear iron(II) and ruthenium(II)N-heterocyclic carbene complexes have been synthesized and crystallographically analyzed. As a spacer ligand for interconnecting the two redox-active metal centers, a ditopic carbene ligand has been used that comprises two carbene sites annelated to benzene. Detailed electrochemical and spectroelectrochemical analyses of the bimetallic systems revealed that despite the potentially π-delocalized nature of the ditopic ligand, the iron centers are only moderately coupled. In the ruthenium complexes, the intermetallic interactions are very weak and the centers are electrochemically nearly independent. A model is proposed for rationalizing these observations which is based on (i) relatively weak charge delocalization in the spacer ligand and (ii) on electrostatic factors governing the metal–carbene bond.