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

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.