Reactions of Anilines and Benzamides with a 14-Electron Iridium(I) Bis(phosphinite) Complex:  N−H Oxidative Addition versus Lewis Base Coordination

Abstract

Anilines react with (POCOP)Ir(C 6 H 5 )(H), 12, (POCOP = 2,6-(OPtBu 2 ) 2 C 6 H 3 ) to yield equilibrium mixtures of 12, the Ir(I) σ-complexes (POCOP)Ir(NH 2 Ar), 13, and the Ir(III) oxidative addition adducts (POCOP)Ir(H)(NHAr), 14. Quantitative studies of these equilibria for a series of anilines were carried out. Anilines possessing electron-withdrawing groups favor the Ir(III) oxidative addition adduct over the Ir(I) sigma complex. Low temperature studies using p-chloroaniline show that the Ir(I) σ-complex is the kinetic product of reaction and is likely the precursor to the Ir(III) oxidative addition adduct. Reductive elimination of complexes 14 in the presence of ethylene led to the corresponding anilines and the ethylene complex (POCOP)Ir(C 2 H 4 ). Kinetic analysis of these reactions for 14e,f,g bearing electron-withdrawing aryl groups (Ar-= p-CF 3 C 6 H 4 -, C 6 F 5 -, 3,5-bis (CF 3 )C 6 H 3 -) shows the rate is independent of ethylene concentration. The ΔG ‡ values for these reductive eliminations fall in the range of 21-22 kcal/mol. X-Ray analysis establishes 14f (Ar-= C 6 F 5 -) as a square pyramidal complex with the hydride occupying the apical site. Reaction of 12 with benzamides 21a,b yields quantitatively the Ir(III) oxidative addition adducts, (POCOP)Ir(H) (NHC(O)Ar), 22. X-Ray analysis of 22b (Ar-= C 6 F 5 -) shows significant interaction of the carbonyl oxygen with Ir in the site trans to hydride. The barrier to reductive elimination of 22a, 29 kcal/mol, is substantially higher than for complexes 14e,f,g.

Reactivity of Pincer Complexes Toward Carbon Monoxide

“…Besides the studies carried out by Goldman and Hartwig in 2003 [37] with the anilide pincer complex (PCP)Ir(H)(NHPh) (91) (see Scheme 2.42), Brookhart and coworkers also reported [41] similar results by changing the phosphino PCP ligand for their phosphinite POCOP analogues. Thus, complexes (99a-b) were obtained by successive reaction of complex (93c) with NaO t Bu, the corresponding aniline, C 6 F 5 NH 2 (99a) or 4-CF 3 C 6 H 4 NH 2 (99b) and CO (Scheme 2.45).…”

“…20 Sacco and coworkers [18] also made an important contribution to the chemistry of rhodium carbonyl pincer complexes. Thus, reactions of the Rh derivative (PNP)Rh(I) (41) were performed with carbon monoxide to yield the corresponding carbonyl complex (42) (Scheme 2.22).…”

Addition of N–H bonds to iridium: Synthesis and characterization of N–Ir–H complexes and the observation that an iridium N-bonded indole ring becomes activated for Michael addition to alkynes

“…Examples of catalytic amination based on oxidative addition of a N-H bond followed by an insertion-reductive elimination sequence have been reported. More recent attention to N-H activation has increased [13][14][15][16][17][18][19][20][21].…”

A catalytic synthesis of dialkylamines from alkylamines using neopentyl-substituted PNP pincer–iridium complex

“…We also recently applied the robustness of pincer ligated iridium complex toward catalytic hydrogenation of CO 2 to form potassium formate with outstanding activity [22]. On the other hand, some recent reports on phosphine-based pincer-iridium complex showed a series of unique oxidative addition reactions to cleave C-H bond of arenes [23][24][25][26][27], alkenes [28], and ketones [29], N-H bond of amines [30][31][32], and O-H bond of water [33]. All of these reactions may involve a formation of T-shape, three-coordinate iridium complex as an intermediate.…”