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

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.The authors acknowledge the support by the Ministry of Higher Education, Saudi Arabia, in establishment of the Centre of Research Excellence in Petroleum Refining & Petrochemicals at KFUPM (KACST-funded project ART-32-68). The support of the KFUPM under the KACST funded project (ART-32-68) and the KFUPM−University of Zaragoza research agreement are also highly appreciated. This work was further supported by the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (CONSOLIDER INGENIO CSD2009-0050, CTQ2011-27593, and CTQ2012-35665 projects) and the Diputación General de Aragón (DGA/FSE-E07).Peer Reviewe