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

Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide results in the formation of [L-H-2]Br(2)which, upon salt metathesis with NH4PF6, readily yields the bis(triazolium) salt [L-H-2](PF6)(2)with non-coordinating counterions. [L-H-2](PF6)(2) and Ag2O react in a 1 : 1 ratio to yield a binuclear Ag-I-tetracarbene complex of the composition [(L)(2)Ag-2](PF6)(2) which undergoes a facile transmetalation reaction with [Cu(SMe2)Br] to deliver the corresponding Cu-I-NHC complex [(L)(2)Cu-2](PF6)(2). In contrast, the [L-H-2]Br-2 reacts with [Ir(Cp*)Cl-2](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 beta-alkylation of a wide range of secondary alcohols using primary alcohols as alkylating partners via a borrowing hydrogen strategy.