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Iridium Complexes of the Conformationally Rigid IBioxMe4 Ligand: Hydride Complexes and Dehydrogenation of Cyclooctene

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journal contribution
posted on 14.09.2015 by Simone A. Hauser, Ralf Tonner, Adrian B. Chaplin
A method for accessing the formally 14 VE iridium­(III) hydride fragment {Ir­(IBioxMe4)2(H)2}+ (2), containing the conformationally rigid NHC ligand IBioxMe4, is reported. Hydrogenation of trans-[Ir­(IBioxMe4)2(COE)­Cl] (1) in the presence of excess Na­[BArF4] leads to the formation of dimeric [{Ir­(IBioxMe4)2(H)2}2Cl]­[BArF4] (3), which is structurally fluxional in solution and acts as a reservoir of monomeric 2 in the presence of excess halogen ion abstractor. Stable dihydride complexes trans-[Ir­(IBioxMe4)2(2,2′-bipyridine)­(H)2]­[BArF4] (4) and [Ir­(IBioxMe4)3(H)2]­[BArF4] (5) were subsequently isolated through in situ trapping of 2 using 2,2′-bipyridine and IBioxMe4, respectively, and fully characterized. Using mixtures of 3 and Na­[BArF4] as a latent source of 2, the reactive monomeric fragment’s reactivity was explored with excess ethylene and cyclooctene, and trans-[Ir­(IBioxMe4)2(C2H4)2]­[BArF4] (6) and cis-[Ir­(IBioxMe4)2(COD)]­[BArF4] (7) were isolated, respectively, through sacrificial hydrogenation of the alkenes. Complex 6 is notable for the adoption of a very unusual orthogonal arrangement of the trans-ethylene ligands in the solid state, which has been analyzed computationally using energy and charge decomposition (EDA-NOCV). The formation of 7 via transfer dehydrogenation of COE highlights the ability to partner IBioxMe4 with reactive metal centers capable of C–H bond activation, without intramolecular activation. Reaction of 7 with CO slowly formed trans-[Ir­(IBioxMe4)2(CO)2]­[BArF4] (8), but the equivalent reaction with bis-ethylene 6 was an order of magnitude faster, quantifying the strong coordination of COD in 7.