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Oxidative Addition of the N–H Bond of Ammonia to Iridium Bis(phosphane) Complexes: A Combined Experimental and Theoretical Study

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posted on 2016-03-02, 00:00 authored by M. Pilar Betoré, Miguel A. Casado, Pilar García-Orduña, Fernando J. Lahoz, Víctor Polo, Luis A. Oro
The chloro complex [IrCl­(cod)­(dppe)] (dppe = bis­(diphenyl­phosphane)­ethane; cod = 1,5-cyclooctadiene) was found to react with gaseous ammonia, affording the amido-bridged diiridium complex [{Ir­(μ-NH2)­H­(dppe)­(NH3)}2]­[Cl]2 (1), whose molecular structure has been solved by X-ray methods. The related cationic complexes [{Ir­(μ-NH2)­H­(dppp)­(NH3)}2]­[BF4]2 (3) and [{Ir­(μ-NH2)­H­(dppb)­(NH3)}2]­[BF4]2 (4) (dppp = bis­(diphenylphosphane)­propane; dppb = bis­(diphenylphosphane)­butane) are only accessible from the reactions of NH3(g) with the cationic starting materials [Ir­(cod)­(dppp)]­[BF4] and [Ir­(cod)­(dppb)]­[BF4], respectively. The formation of these species comes from an oxidative addition of an N–H bond of ammonia to the metals. The main structural difference between complex 2 and 3/4 relies on the relative stereochemistry of both ammonia and hydrido ligands; their cisoidal disposition in dppe complex [{Ir­(μ-NH2)­H­(dppe)­(NH3)}2]­[BF4]2 (2) directed the reactivity toward dppm (dppm = bis­(diphenylphosphane)­methane), generating the triply bridged amido complex [{Ir2(μ-NH2)2(μ-dppm)­H2(dppe)2]­[BF4]2 (5). DFT calculations show that the reaction between [IrCl­(cod)­(dmpe)] (dmpe = bis­(dimethylphosphane)­ethane) and NH3 to yield complex [{Ir­(μ-NH2)­H­(dmpe)­(NH3)}2]­[Cl]2 comprises four steps: (i) formation of the cationic complex, (ii) replacement of the cod ligand by ammonia molecules, (iii) oxidative addition of the N–H bond to the metal, and (iv) dimerization of the resulting Ir­(III) intermediate, step (iii) being the rate-determining one. The calculations reveal that the N–H bond activation takes place heterolytically through an ammonia-assisted stepwise pathway, instead of a concerted homolytic N–H bond cleavage and hydrido formation through a classical three-center transition structure.

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