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)nImMe}2(RCN)2][PF 6]2 (1: n = 1, R = Me; 2: n = 1, R = Ph; 3: n = 2, R = Me; 4: n = 2, R = Ph; MeIm = 3-methylimidazol-2-yliden-1-yl) have been synthesised by transmetalation of the dicarbene ligand from a dicarbene silver complex with [RuCl2(COD)]x (COD = 1,5-cyclooctadiene) or [RuCl 2(p-cymene)]2 as ruthenium precursors. The relative position of the coordinated nitrile molecules (cis or trans) depends on the group that links the two carbene moieties: the dicarbene with the methylene bridge affords the trans isomer, whereas the cis complex is obtained with the ethylene bridge. The lability of coordinated nitriles towards entering nucleophiles, in particular, cyclohexyl and tert-butyl isocyanide, has been carefully checked, and the corresponding substitution complexes [Ru{MeIm(CH 2)nImMe}2(RNC)2][PF 6]2 (6: n = 1, R = Cy; 7: n = 2, R = Cy; 8: n = 2, R = tBu) have been synthesised. The trans complexes are more inert than the cis ones, and benzonitrile is a better leaving group than acetonitrile; these characteristics allow a proper tuning of the coordination set and the stereoelectronic properties of these dicarbene ruthenium complexes. The X-ray crystal structures of 1 and 7 have also been determined. The transfer of a di-N-Heterocyclic carbene (diNHC) ligand from silver complexes to ruthenium(II) derivatives {[RuCl2(COD)]x or [RuCl2(p-cymene)] 2 (COD = 1,5-cyclooctadiene)} affords cationic cis- or trans-bis(diNHC)ruthenium(II) complexes, depending on the nature of the linker between the two imidazol-2-ylidene rings.