Tuning the [L2Rh⋯H3B·NR3]+ interaction using phosphine bite angle. Demonstration by the catalytic formation of polyaminoboranes

Abstract

Efficient catalysts for the dehydrocoupling or dehydropolymerisation of H(3)B·NMe(x)H((3-x)) (x = 1, 2) have been developed by variation of the P-Rh-P angle in {Rh(Ph(2)P(CH(2))(n)PPh(2))}(+) fragments (n = 2-5).

Experimental and Theoretical Studies of the Potential Interconversion of the Amine–Borane iPr2NH·BH(C6F5)2 and the Aminoborane iPr2N=B(C6F5)2 Involving Hydrogen Loss and Uptake

“…[63] We therefore investigated the employment of the same system with 1, in which elimination of H 2 might be expected to be feasible and possibly entropically more favourable than elimination of pentafluorobenzene. Using a 10-mol-% loading of 1,3-ditert-butyl-4,5-dihydroimidizol-2-ylidene (3), we observed complete consumption of the adduct by 11 B NMR spectroscopy over 24 h. This was accompanied by the growth of a new peak at δ = 33.2 ppm, which appeared as a doublet in the proton-coupled spectrum ( 1 J BH = 129 Hz). The product of this reaction was identified as iPr 2 N=BH(C 6 F 5 ) (4), produced via elimination of pentafluorobenzene, which was also detected in the 1 H NMR spectrum as a complex multiplet centred at 5.7 ppm when the reaction was repeated in C 6 D 6 (Scheme 7).…”

“…Initially, dehydrogenation of 1 was attempted in toluene solution at 50°C, with no conversion of 1 apparent over 24 h. Heating an identical solution to 100°C for 70 h was then attempted. Under these conditions the adduct was shown by 11 B and 19 F NMR analysis to be unstable, partially dehydrogenating to produce the aminoborane 2, in low yield (23 %), along with other unidentified boron-containing products which proved to be inseparable from the desired product (Scheme 6). Attempts to prepare the aminoborane under milder conditions via the use of transition metal catalysts were unsuccessful, with no dehydrogenative reaction prevailing at 20°C with [Rh(μ-Cl)(cod)] 2 (cod = 1,5-cyclooctadiene), colloidal Rh, [10,74] "Cp 2 Ti", [23] IrH 2 POCOP (POCOP = κ 3 -1,3-(PtBu 2 ) 2 C 6 H 3 ) [19,75] or the Fagnou catalyst system (KOtBu and RuCl 2 L 2 [L = iPr 2 PCH 2 CH 2 NH 2 ]).…”

“…[26] In addition, photoactivated catalysts of the first-row transition metals have also recently been reported. [27][28][29] Mechanistic insights into metal-catalysed dehydrocoupling processes have also been provided through various studies demonstrating the coordination of amine-and aminoboranes to metal centres, [11,[30][31][32][33][34][35][36][37][38][39] with the isolation of species implicated in the catalytic cycle. [40] Transition-metal-catalysed dehydrocoupling of amineborane adducts generally affords aminoboranes and borazines, with the products defined by both the choice of substituents on the adduct and the catalyst.…”

“…[10] Warming to 20°C and removal of the solvent gave the product as a white solid in 92 % yield and high purity, with additional purification possible by recrystallisation from toluene/hexanes. Analysis of the product by 11 B NMR spectroscopy in CDCl 3 showed a single peak at δ = -14.8 ppm, which split into a doublet on proton coupling ( 1 J BH = 101 Hz). These data are consistent with a tetra-coordinate boron centre with a single hydrogen substituent.…”

“…[4][5][6][7][8][9][10] More recently, the range of metal complexes reported to be capable of catalytically dehydrogenating amine-boranes has been significantly expanded. For example, homogeneous systems based on Rh, [11] Ru, [12,13] Re, [14] Ni [15,16] and Ir [17][18][19] are now known, along with those based on Group 2 and 4 metals, [20][21][22][23][24][25] and systems based on p-block metals. [26] In addition, photoactivated catalysts of the first-row transition metals have also recently been reported.…”