Synthesis and Reactivity of Cationic Bis(N-Heterocyclic Dicarbene) Ruthenium(II) Complexes

Four ruthenium(II) N-heterocyclic dicarbene complexes of general formula [Ru(MeIm-(CH2)n-ImMe)2(RCN)2](PF6)2 (1, n=1 and R=Me; 2, n=1 and R=Ph; 3, n=2 and R=Me; 4, n=2 and R=Ph) have been synthesised via transmetalation of the dicarbene ligand from a dicarbene silver complex, using [RuCl2(COD)]x or [RuCl2(p-cymene)]2 as ruthenium precursors. The relative position of the coordinated nitrile molecules (cis or trans) depends on the group linking the two carbene moieties: the dicarbene with the methylene bridge affords the trans isomer, while the cis complex is obtained with the ethylene bridge. The lability of coordinated nitriles towards entering nucleophiles, in particular cyclohexyl- and t-butylisocyanides, has been carefully checked and the corresponding substitution complexes [Ru(MeIm-(CH2)n-ImMe)2(RNC)2](PF6)2 (6, n=1 and R=Cy; 7, n=2 and R=Cy; 8, n=2 and R=tBu) synthesised. The trans complexes are more inert than the cis ones and benzonitrile is a better leaving group than acetonitrile, and these characteristics allow a proper tuning of the coordination set and of the stereo-electronic properties of these dicarbene ruthenium complexes. The X-ray crystal structure of complexes 1 and 7 has been also determined.