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

Abstract

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)(η⁶-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)(η⁶-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)(η⁶-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.