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

Abstract

The dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir( i Pr-PNHP)} fragment [ i Pr-PNHP = κ3-(CH2CH2P i Pr2)2NH] is reported. Neutral Ir( i Pr-PNHP)H3 or Ir( i Pr-PNHP)H2Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F2C6H4 solution. In contrast, addition of [NMeH3][BArF 4] (ArF = 3,5-(CF3)2C6H3) to Ir( i Pr-PNHP)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( i Pr-PNHP)(H)2(H2)][BArF 4]; σ-amine-borane [Ir( i Pr-PNHP)(H)2(H3B·NMe3)][BArF 4]; and [Ir( i Pr-PNHP)(H)2(NMeH2)][BArF 4]. 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( i Pr-PNMeP)(H)2(H3B·NMe3)][BArF 4] 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( i Pr-NHP)(H)2(H2)][BArF 4], 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( i Pr-PNHP)H3, with a coproduct of [NMeH3]+/[H2B(NMeH2)2]+. While reprotonation of Ir( i Pr-PNHP)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.

Dehydropolymerization of Amine−Boranes using Bis(imino)pyridine Rhodium Pre‐Catalysis: σ‐Amine−Borane Complexes, Nanoparticles, and Low Residual‐Metal BN−Polymers that can be Chemically Repurposed

“…Of relevance to the catalytic manifold (see below) is the isolation, as dark brown crystals, of the methylamine adduct [Rh(L1)(NMeH Initial catalytic studies on the dehydropolymerization of H 3 B • NMeH 2 focused in using σ-amineÀ borane complex [2]OTf as a pre-catalyst in 1,2-F 2 C 6 H 4 solvent, as used previously for other cationic dehydropolymerization systems. [27,29,30,45] Catalyst loading was 1 mol % and the nominal concentration of H 3 B • NMeH 2 was 0.446 M (~50 mg), although its poor solubility in 1,2-F 2 C 6 H 4 meant that the reaction was, in fact, a slurry, with a limiting concentration of ~0.223 M, as reported previously. [30] H 2 release was measured at 25 °C eudiometrically, as a proxy for the formation of "real" monomer aminoÀ borane, H 2 B=NMeH.…”

Mechanistic insights into H3B·NMeH2 dehydrogenation by Co-based complexes: a DFT perspective

“…5 Moreover, these polyaminoboranes ([H 2 BNRH] n ) with alternating B-N backbones are potentially exciting new materials for application as piezoelectric materials 6,7 and may serve as precursors for boron-based preceramics 8,9 and few-layer h-BN. 10 Transition-metal-catalyzed dehydropolymerization routes have been achieved (Scheme 1b) by the exploitation of 4d or 5d metals (like Zr, 11,12 Ru, [13][14][15][16] Rh, [17][18][19][20][21][22][23][24][25] Pd, 26 Re, 18 Os, 27 Ir, [28][29][30][31] or Pt 32 ) as well as through much more cost-effective and earthabundant 3d elements (like Ti, 33,34 Cr, 35 Mn, 36 Fe [37][38][39][40][41][42] or Co [43][44][45][46][47][48][49] ). The catalysis of these processes by transition metals offers potential for both fine kinetic control and goo...…”

“…Transition-metal-catalyzed dehydropolymerization routes have been achieved (Scheme 1b) by the exploitation of 4d or 5d metals (like Zr, 11,12 Ru, 13–16 Rh, 17–25 Pd, 26 Re, 18 Os, 27 Ir, 28–31 or Pt 32 ) as well as through much more cost-effective and earth-abundant 3d elements (like Ti, 33,34 Cr, 35 Mn, 36 Fe 37–42 or Co 43–49 ). The catalysis of these processes by transition metals offers potential for both fine kinetic control and good product distributions.…”

Rhodium(I) PNN Pincer Complexes with Proton‐responsive Ligands: Synthesis, Characterisation, and Catalytic Dehydrocoupling of Amine Boranes

“…Of note, the structurally related neutral PN(H)P Iridium(III) trihydride complex is a poor catalyst that can be activated by transformation into a cationic species that dehydrogenates the amine borane via an inner‐sphere mechanism (Figure 1a). [10] …”

“…[9] Of note, the structurally related neutral PN(H)P Iridium(III) trihydride complex is a poor catalyst that can be activated by transformation into a cationic species that dehydrogenates the amine borane via an inner-sphere mechanism (Figure 1a). [10] This observation of significant differences when changing the metal using the identical pincer ligand prompted us to investigate the influence of other potentially cooperative phosphine-based ligands in combination with group 9 metals. Based on previous work on a Co system bearing a pyridinepyrazole PNN(H) ligand, that was initially reported by Caulton and co-workers, [11] for catalytic transfer semihydrogenation of alkynes using H 3 B•NH 3 as the hydride source, [12] we now present the coordination chemistry of this ligand at Rh I along with a study of selected complexes for the dehydrocoupling of amine boranes (Figure 1b).…”