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-NH 2 )H(dppe)(NH 3 )} 2 ][Cl] 2 (1), whose molecular structure has been solved by X-ray methods. The related cationic complexes [{Ir(μ-NH 2 )H(dppp 4) (dppp = bis(diphenylphosphane)propane; dppb = bis(diphenylphosphane)butane) are only accessible from the reactions of NH 3 (g) with the cationic starting materials [Ir(cod)(dppp)]- [BF 4 ] and [Ir(cod)(dppb)][BF 4 ], 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-NH 2 )H(dppe)(NH 3 )} 2 ][BF 4 ] 2 (2) directed the reactivity toward dppm (dppm = bis(diphenylphosphane)methane), generating the triply bridged amido complex [{Ir 2 (μ-NH 2 ) 2 (μ-dppm)H 2 (dppe) 2 ]-[BF 4 ] 2 (5). DFT calculations show that the reaction between [IrCl(cod)(dmpe)] (dmpe = bis(dimethylphosphane)ethane) and NH 3 to yield complex [{Ir(μ-NH 2 )H(dmpe)(NH 3 )} 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.

In this work, a computational study at the DFT level is carried out to determine the reaction mechanism for the N−H bond activation of ammonia by dinuclear [{M(μ-OMe)(cod)} 2 ] complexes (M = Ir, Rh) to yield amido species [{M(μ-NH 2 )(cod)} 2 ] reported experimentally by Mena et al. (Angew. Chem., Int. Ed. 2011, 50, 11735−11738). A stepwise mechanism is proposed for the replacement of μ-OMe bridging ligands considering associative or dissociative approaches for NH 3 coordination to the metal. Reaction pathways for the homolytic and heterolytic N−H σ-bond cleavage of ammonia, such as oxidative addition through M III species or hydrogen transfer to the ligand, are investigated. The energetically preferred mechanism involves the participation of both metallic centers through the formation of and intermediate bearing M 1 -NH 3 and M 2 -OMe moieties followed by heterolytic hydrogen transfer of the amino ligand to the methoxo ligand. A bonding analysis on the metallacycle [M 2 X 2 ] core (M = Ir, Rh; X = μ-OMe, μ-NH 2 ) is performed, showing that the amido bridging complex is stabilized due to the presence of metal−metal bonding interactions. ■ INTRODUCTIONN−H bond activation of ammonia promoted by late transition metal complexes is a relevant phenomenon in modern chemistry. 1 In this context, understanding the reaction mechanism and the factors that control N−H activation of ammonia by late metal complexes is of key importance in order to develop compounds that could be active in the catalytic functionalization of ammonia. 2 In general, electrophilic early metal complexes readily interact with ammonia, favoring N−H activation processes that generate parent amido and imido complexes, 3 and very recently it has been found that metal-free compounds based on main elements (P, 4 Si, 5 Ge, 6 C, 7 Sn 8 ) readily activate the N−H bond of ammonia in an homolytic fashion (see Scheme 1a), yielding the corresponding oxidative addition products. 9 However, the use of late metal complexes in the N−H activation of ammonia still remains challenging because the strength of the N−H bond (∼104 ± 2 kcal mol −1 ) makes its activation by metal centers very difficult to achieve and also because late metal compounds usually form stable ammine Werner-like adducts with NH 3 . 10 Far from this behavior, earlier studies of N−H oxidative addition of ammonia to iridium complexes pioneered by Milstein et al. demonstrated that the structure of a given complex, that is, the nature of the auxiliary ligands and oxidation state of the metal, dictates its reactivity toward ammonia. 11 In this way, the Ir I complex [Ir(PEt 3 ) 2 (CH 2  CH 2 ) 2 ][Cl] undergoes the N−H oxidative addition of ammonia, affording amido-bridged Ir III complexes [{Ir-(PEt 3 ) 2 (μ-NH 2 )(H)(NH 3 )} 2 [Cl] 2 , 12 while the use of the analogous complex [Ir(P i Pr 3 ) 2 (CH 2 CH 2 ) 2 ][Cl] favors C− H activation. 13 Similar results were reported by Braun et al. with the diiridium system [Ir(PiPr 3 ) 2 (CH 2 CH 2 )(4-C 5 NF 4 )],

Abstract:The treatment of [{Ir(μ-Cl)(cod)} 2 ] (cod = 1,5-cyclooctadiene) with 2,5-norbornadiene (nbd) in the presence of triethylphosphane under an atmosphere of ammonia afforded the cation complex [Ir(κC,κC-C 21 H 24 )(NH 3 ) 3 (PEt 3 )]Cl (1a) and complex [Ir(κC,κC-C 21 H 24 )Cl(NH 3 ) 2 (PEt 3 )] (1b), the structures of which include a novel seven-membered iridacycle formed by three half-coupled nbd molecules. The formation of the metallacycle has been found to occur through several steps. First, the nbd displaces the cod in [{Ir(μ-Cl)(cod)} 2 ] to give the Ir III chlorobridged complex [{Ir(κC,κC-C 14 H 16 )(μ-Cl)(η 4 -C 7 H 8 )} 2 ] (2), which bears a η 4 -C=C-coordinated nbd and a five-membered iridacycle formed by two half-coupled nbd moieties. Further reac-