Oxidative addition of the N−H bond of ammonia to iridium bis(phosphane) complexes: A combined experimental and theoretical study

Abstract

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.