Experimental and computational studies on the reactivity and binding mode of thiophene with N-heterocyclic carbene iridium complexes

Abstract

The reactivity of thiophene (T), 2-methylthiophene (2-MeT), and benzothiophene (BT) with [Ir(cod)(IPr)(L)]BF complexes (L = acetone (1), pyridine (2) or dimethylphenylphosphine (3); IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) in the presence of molecular hydrogen has been investigated. Under these conditions the 1,5-cyclooctadiene ligand is hydrogenated to cyclooctane, which renders an unsaturated species (Ir-IPr-L) able to coordinate the thiophene moiety. The coordination mode of T, 2-MeT, and BT depends on the nature of the substrate and the ligand trans to the IPr (L). The reaction of 1 with T and 2-MeT leads to dissociation of the acetone ligand to afford the complexes [Ir(H)(IPr)(η--T)]BF (4) and [Ir(H)(IPr)(η--2-MeT)(η-S-2-MeT)]BF (5), respectively, but no stable complex is observed on reaction with BT. Analogously to 1, complex 2 does not give a stable complex on reaction with BT, while reaction with 2-MeT yields complex 5 again. Conversely, reaction with T affords a mixture of complexes, [Ir(H)(IPr)(η--T)(Py)]BF (6) and [Ir(H)(IPr)(η-S-T)(Py)]BF (6′), both featuring a coordinated pyridine ligand. The reaction of 3 with T, 2-MeT, and BT yields in all cases η-S complexes, namely [Ir(H)(IPr)(η-S-T)(PPhMe)]BF (7), [Ir(H)(IPr)(η-S-2-MeT)(PPhMe)]BF (8) in equilibrium with [Ir(H)(IPr)(η-S-2-MeT)(PPhMe)]BF (8′), and [Ir(H)(IPr)(η-S-BT)(PPhMe)]BF (9). Finally, DFT calculations were employed to rationalize the coordination modes of T, 2-MeT, and BT, as well as the tendency of these complexes to undergo hydrogenation instead of hydrogenolysis of the thiophene moiety under catalytic conditions.