Distinct Thermodynamics for the Formation and Cleavage of N−H Bonds in Aniline and Ammonia. Directly-Observed Reductive Elimination of Ammonia from an Isolated Amido Hydride Complex

Abstract

The reactions of aryl and alkylamines with the (PCP)Ir fragment (PCP = 1,3-di-tert-butylphosphinobenzene) were studied to determine the reactivities and stabilities of amine and amido hydride complexes relative to C-H activation products. Reaction of aniline with the (PCP)Ir unit generated from (PCP)IrH2 and norbornene resulted in the N-H oxidative addition product (PhNH)(H)Ir(PCP) (1a). In contrast, reaction of this fragment with ammonia gave the ammonia complex (NH3)Ir(PCP) (2). The amido hydride complex that would be formed by oxidative addition of ammonia, (PCP)Ir(NH2)(H) (1b), was generated independently by deprotonation of the ammonia complex (NH3)Ir(H)(Cl)(PCP) (3) with KN(SiMe3)2 at low temperature. This amido hydride complex underwent reductive elimination at room temperature to form the ammonia complex 2. Addition of CO to anilide complex 1a gave (PCP)Ir(PhNH)(H)(CO) (4a). Addition of CNtBu to terminal amido complex 1b formed (PCP)Ir(NH2)(H)(CNtBu) (4b), the first structurally characterized iridium amido hydride. Complexes 4a and 4b underwent reductive elimination of aniline and ammonia; parent amido complex 4b reacted faster than anilide 4a. These observations suggest distinct thermodynamics for the formation and cleavage of N-H bonds in aniline and ammonia. Complexes 1a, 2, 4a, and 4b were characterized by single-crystal X-ray diffraction methods.

Density Functional Theoretical Study on the N-H Bond Activation by Ir Pincer Complexes

“…The Ir-N bond length in 1 was 2.07 Å, which was comparable to the values that were found in the X-ray structure 1 and the related pincer Ir(III) complexes. 3 The Ir-N bond length in 2 was 2.01 Å, which was also comparable to the value that was found in the related pincer Ir(III) amido hydride complexes. In 1', the Ir-N bond was lengthened to 2.37 Å.…”

A Computational Study of Rhodium Pincer Complexes with Classical and Nonclassical Hydride Centres as Catalysts for the Hydroamination of Ethylene with Ammonia

“…[12] However, the replacement of H 2 in A by NH 3 yields a complex, which, in principle, could undergo s-bond-metathesis to form a metal amide and a new nonclassical hydride. This H 2 molecule can be displaced by ethylene, followed by C À N bond formation.…”

“…[11] The nonclassically bound H 2 moiety is a good leaving group, that is, for the introduction of ethylene or ammonia. We chose iridium [12] and rhodium [13] compounds of type A with synthetically available NCN and PCP pincer ligands 2,5-bis(dimethylaminomethyl)benzene and 2,5-bis(dimethylphosphanylmethyl)benzene, respectively, for the investigation of key hydroamination reaction steps.…”

“…While attempting to synthesise trans-or cis-1, one might instead arrive at 10 as the product, though the energy differences are low enough to be reversed by solvent effects. As with 2, competitive pathways resulting from either the addition of NH 3 or C 2 H 4 to 10 exist and, accordingly, the analogous routes lead to 5 by Path III: Addition of NH 3 (11), s-bond metathesis (12), and H 2 -elimination (5) and Path IV: addition of C 2 H 4 (13), insertion into the RhÀH bond (14), addition of NH 3 (15), and s-bond metathesis with concomitant elimination of ethane (5). The loss of hydrogen is the first step in Paths I to IV.…”

Germanium(iii) corrole complex: reactivity and mechanistic studies of visible-light promoted N–H bond activations

“…Most previously reported N-H bond activation processes involve mononuclear direct two-electron concerted oxidative addition reactions of the N-H bond to a low-valent metal center, 3b external base assisted N-H bond cleavage, 25 or metal-ligand cooperative effects.…”