Controlling alkyne dimerization and trimerization with ruthenium(ii) arene isocyanide catalysts

Ruthenium(ii) arene complexes represent a renowned platform to develop effective catalysts for a variety of organic transformations, including C-C bonding processes. On the other hand, isocyanides are overlooked ligands in the design of transition metal catalysts. Herein a panel of ruthenium(ii) arene isocyanide complexes were found to be versatile catalytic precursors for the dimerization/trimerization of aryl alkynes in aqueous medium. Fifteen compounds of general formula [RuX2(CNR)(eta 6-arene)] (X = Cl, I; R = alkyl or aryl; arene = C6H6, p-cymene, C6Me6) were prepared from the corresponding halido-bridged Ru dimers and the selected isocyanide according to optimized procedures, including examples with the simplest alkyl isocyanide (MeNC) and arene (C6H6). Next, two acetylide complexes of the type [RuCl(CCPh)(CNR)(eta 6-C6Me6)] were obtained by reaction of the corresponding dichlorido complexes with phenylacetylene and NaOH. In addition, a protocol for the thermally promoted p-cymene/MeCN substitution was optimized, giving access to hexacoordinate complexes with isocyanide and acetonitrile ligands, [RuCl2(MeCN)3(CNR)] (two examples). The Ru(ii) compounds, fourteen of which are unprecedented, were characterized by CHNS analyses, IR and NMR spectroscopy and X-ray diffraction in eight cases. The catalytic activity of the complexes was assessed, highlighting the role of the solvent, base, Ru loading, energy source and, more importantly, isocyanide/arene ligands to control the selectivity between dimerization and trimerization of phenylacetylene. Pointing to a sustainable process, a catalytic protocol involving Na2CO3 as a base, water as a solvent and a low Ru loading (1%) was applied for the dimerization/trimerization of a range of terminal alkynes, with [RuCl2(CNCy)(eta 6-p-cymene)] emerging as the best performing pre-catalyst. Combined IR, NMR and MS data were instrumental in the elucidation of the reactivity of the isocyanide-arene complexes with PhCCH/Na2CO3 and the formulation of a possible mechanism of pre-catalyst activation.