Bifunctional activation of amine-boranes by the W/Pd bimetallic analogs of "frustrated Lewis pairs"

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3-iPr-PNRP Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

The dehydropolymerization of H₃B·NMeH₂ to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir(ⁱPr-PN^HP)} fragment [ⁱPr-PN^HP = κ³-(CH₂CH₂PⁱPr₂)₂NH] is reported. Neutral Ir(ⁱPr-PN^HP)H₃ or Ir(ⁱPr-PN^HP)H₂Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F₂C₆H₄ solution. In contrast, addition of [NMeH₃][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃) to Ir(ⁱPr-PN^HP)H₃ immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(N-methylaminoborane) with M_n ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir(ⁱPr-PN^HP)(H)₂(H₂)][BAr^F₄]; σ-amine-borane [Ir(ⁱPr-PN^HP)(H)₂(H₃B·NMe₃)][BAr^F₄]; and [Ir(ⁱPr-PN^HP)(H)₂(NMeH₂)][BAr^F₄]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal–ligand cooperativity, and this is supported experimentally by N–Me substituted [Ir(ⁱPr-PN^MeP)(H)₂(H₃B·NMe₃)][BAr^F₄] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H₃B·NMeH₂ with the N–H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir(ⁱPr-N^HP)(H)₂(H₂)][BAr^F₄], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH₂ (via B–N bond cleavage), results in the formation of essentially inactive Ir(ⁱPr-PN^HP)H₃, with a coproduct of [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺. While reprotonation of Ir(ⁱPr-PN^HP)H₃ results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺/catalyst in insoluble polyaminoborane. The role of [NMeH₃]⁺/[H₂B(NMeH₂)]⁺ as chain control agents is also noted.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H]

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)₃H] and Lewis acids (LA = Ph₃C⁺, B(C₆F₅)₃) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)₃H⋯LA] and unusual dearomatization of the Ph₃C⁺ cation upon hydride transfer.

Cooperative approaches in catalytic hydrogenation and dehydrogenation

Metal-ligand cooperativity (MLC) is an established strategy for developing effective hydrogenation and dehydrogenation catalysts. Metal-metal cooperativity (MMC) in bimetallic complexes is not as well understood, and to date has had limited implementation in (de)hydrogenation. Herein we use (de)hydrogenation processes as a platform to examine modes of cooperativity, with a particular focus on catalytic mechanisms. We investigate how lessons learnt from the extensive development of metal-ligand cooperative catalysts can aid the ongoing development of metal-metal cooperative catalysts.

Amine-boranes reactions promoted by lanthanide(II) ions

The catalytic activity in amine-borane dehydrogenation is shown for the first time for Ln(II) species using complexes [{(p-tBu-C₆H₄)₂CH}₂M·L] (M = Yb, Sm, L = (DME)₂, TMEDA). The protonation of M(II)-C bonds with HNR¹R²BH₃ affords amidoborane complexes [M(NR¹R²BH₃)₂L], which under excess HNMe₂BH₃ transform to [NMe₂BH₂NMe₂BH₃]^- derivatives, both serving as the dehydrocoupling intermediates.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H₂ can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

Metal-ligand cooperative κ1-N-pyrazolate Cp*RhIII-catalysts for dehydrogenation of dimethylamine-borane at room temperature

3,5-Dimethylpyrazole (Pz*H) in well-defined Cp*RhIII (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) complexes, or as an additive to [Cp*RhCl2]2 enhances catalytic activity in the dehydrogenation of dimethylamine-borane (DMAB) at room-temperature. Mechanistic studies indicate that the Lewis acidic RhIII-centre and dangling N-atom of the Pz* fragment operate cooperatively in accepting a hydride and proton from DMAB, respectively, leading directly to dimethylamino-borane and a RhIII-H complex. The rate limiting step involves protonation of the RhIII-H by the proximal NH fragment of the Pz*H moiety.