Transition metal complexes of a bis(carbene) ligand featuring 1,2,4-triazolin-5-ylidene donors: structural diversity and catalytic applications

Abstract

Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide results in the formation of [L–H₂]Br₂ which, upon salt metathesis with NH₄PF₆, readily yields the bis(triazolium) salt [L–H₂](PF₆)₂ with non-coordinating counterions. [L–H₂](PF₆)₂ and Ag₂O react in a 1 : 1 ratio to yield a binuclear Ag^I–tetracarbene complex of the composition [(L)₂Ag₂](PF₆)₂ which undergoes a facile transmetalation reaction with [Cu(SMe₂)Br] to deliver the corresponding Cu^I–NHC complex [(L)₂Cu₂](PF₆)₂. In contrast, the [L–H₂]Br₂ reacts with [Ir(Cp*)Cl₂]₂ to generate a doubly C–H activated Ir^III–NHC complex 5. Similarly, the triazolinylidene donor supported diorthometalated Ru^II-complex 6 is also obtained. Complexes 5 and 6 represent the first examples of a stable diorthometalated binuclear Ir^III/Ru^II-complex supported by 1,2,4-triazolin-5-ylidene donors. The synthesized Ir^III–NHC complex 5 is found to be more effective than its Ru^II-analogue (6) for the reduction of a range of alkenes/alkynes via the transfer hydrogenation strategy. Conversely, Ru^II-complex 6 is identified as an efficient catalyst (0.01 mol% loading) for the β-alkylation of a wide range of secondary alcohols using primary alcohols as alkylating partners via a borrowing hydrogen strategy.