N–H Activation of Ammonia by [{M(μ-OMe)(cod)}₂] (M = Ir, Rh) Complexes: A DFT Study

Abstract

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 )],

Rhodium-catalyzed regioselective addition of the ortho C–H bond in aromatic amides to the C–C double bond in α,β-unsaturated γ-lactones and dihydrofurans

“…The coordination of the quinoline nitrogen in the aromatic amide 1 to give a Rh( i )X species, followed by the oxidative addition of an N–H bond, gives complex A . 15 The insertion of 5a into the H–Rh bond in A gives complex B , which, after the elimination of HX, affords the carbene complex C . 16 The migratory insertion of the ortho C–H bond to a carbene moiety in the complex C through the oxidative addition of the ortho C–H bond followed by α-hydride migration gives D , 17 , 18 which undergoes reductive elimination followed by protonation to give the final product with regeneration of the Rh( i ) species.…”

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

“…Theoretical studies on transition metal complexes have also rationalized experimental data using trends in bond energies, formation of stable intermediates, and analysis of orbital interactions . Recently, it has been shown that diiridium organometallic complexes may cleave heterolytically the N–H bond assisted by metal–metal bonding interactions . Reaction pathways for hydroamination of olefins catalyzed by transition metal complexes have been determined by Poli and Leitner .…”

“…23 Recently, it has been shown that diiridium organometallic complexes may cleave heterolytically the N−H bond assisted by metal−metal bonding interactions. 24 Reaction pathways for hydroamination of olefins catalyzed by transition metal complexes have been determined by Poli 25 and Leitner. 26 Also, mechanistic details of the metal-free N−H bond activation of ammonia have been unraveled by Radosevich, 3a pointing out the participation of other ammonia molecules, resulting in an entropy-controlled electrophilic mechanism.…”

Rh(i)- and Rh(ii)-catalyzed C–H alkylation of benzylamines with alkenes and its application in flow chemistry

“…The oxidative addition of an N–H bond to a Rh center provides Rh( iii ) or Rh( iv ) 9 species A (see ESI for details † ) (the evidence of Rh( iv ) formation was that the solution turned to a dark purple colour). 10 In the next step, the insertion of an alkene into the Rh–H bond gives intermediate B , in which the NH hydrogen is transferred to the β position, as evidenced by deuterium labelling experiments ( Scheme 3a ). Two possible pathways are proposed from intermediate B .…”