Dehydrogenation of Dimethylamine–Borane Catalyzed by Half-Sandwich Ir and Rh Complexes: Mechanism and the Role of Cp* Noninnocence

Abstract

Half-sandwich Cp*RhIII complexes (Cp* = η5-1,2,3,4,5-pentamethylcyclopentadienyl) supported by 2,2′-bipyridine or 4,4′-di-tert-butyl-2,2′-bipyridine catalyze dehydrogenation of dimethylamine–borane (Me2NH·BH3) to produce H2 and dimethylamino–borane dimer (Me2NBH2)2 with turnovers of 2200. The IrIII analogues, on the other hand, display dramatically poorer catalytic activity. Mechanistic inferences drawn from stoichiometric reactions and DFT calculations suggest noninnocent involvement of the Cp* moiety as a proton shuttle.

Dehydropolymerization of H3B·NMeH2 Using a [Rh(DPEphos)]+ Catalyst: The Promoting Effect of NMeH2

“…32 Added [H 2 B(NMeH 2 ) 2 ][BAr F 4 ] (10 equiv) significantly slows catalysis, now taking 24 h for completion to produce very short polymer ( M n = 2 800 g/mol, Đ = 2.3). This argues against its role in productive catalysis, in contrast with other systems, 10,29,46 in particular the [Rh(Xantphos- i Pr)] + system, where it promotes catalysis. 10 At low relative concentrations, H 3 B·THF presumably acts to titrate out NMeH 2 , while we propose that excess [H 2 B(NMeH 2 ) 2 ] + acts to poison catalysis, possibly sequestering NMeH 2 via N–H···NMeH 2 hydrogen bonding, as noted for related bis(phosphine)boronium salts.…”

The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst

“…We have recently 28,31 The role of boronium in productive turnover has also been probed in pincer-like [Rh(Xantphos-i Pr)] + systems, B, 30 and Pt-based systems related to E, 28 Scheme 2, as well as others. 29 We propose a similar set of fundamental processes operates here, in which amine-induced B-H bond cleavage gives a metal hydride that is then reprotonated. Two realistic scenarios presented themselves for this process: (a) the boronium route as described, or, (b) one that invoked the formation of ammonium (Scheme 6 Fig.…”

“…Depending on the catalyst system a number of different routes have been proposed to operate for this, that all invoke σ-amine-borane complexes 18,19 as early intermediates (Scheme 2): (i) step-wise, or concerted, inner sphere; 17,[20][21][22] (ii) ligand-assisted cooperation; [23][24][25][26] and (iii) hydride transfer to form a boronium 27 cation that reprotonates the transiently formed hydride. [28][29][30][31] The initial reports of dehydropolymerisation of H 3 B•NMeH 2 used the neutral pincer catalyst Ir(POCOP)H 2 7,8,16 [A, POCOP = κ 3 -C 6 H 3 -2,6-(OP t Bu 2 ) 2 ], Scheme 3. A number of closely related pincer-based systems have since been used to catalyse amine-borane dehydrocoupling.…”

Single-Electron Redox Chemistry on the [Cp*Rh] Platform Enabled by a Nitrated Bipyridyl Ligand

“…We have also found that [Cp*Rh] complexes bearing bidentate diphosphine [ 25 ] or hybrid phosphine-quinoline ligands [ 26 ] are not capable of similar catalysis. Thus, although the role of supporting bidentate ligand structure in formation of [Cp*H] intermediates is not yet clear, it is of high interest considering the new reactivity manifolds that may be accessible with [Cp*H] complexes [ 27 , 28 ].…”