¹H and ¹⁹F NMR Investigation of the Reaction of B(C₆F₅)₃ with Water in Toluene Solution

Abstract

Titrations of B(C6F5)3 (1) with water, in toluene-d 8 solution, monitored by 19F and 1H NMR at 196 K, showed first the formation of the adduct [(C6F5)3B(OH2)] (2) and then its stepwise transformation into the two aqua species [(C6F5)3B(OH2)]·H2O (3) and [(C6F5)3B(OH2)]·2H2O (4) containing, respectively, one or two water molecules hydrogen-bonded to the protons of the B-bound water molecule. The NMR data show that in each titration step only two species were present in significant concentration: 1 and 2 up to 1 equiv, 2 and 3 between 1 and 2 equiv, 3 and 4 between 2 and 3 equiv. Above 3 equiv the solutions rapidly attained saturation and phase separation occurred (although there was evidence of interaction of 4 with more water molecules). Titrations at room temperature indicated an analogous stepwise course. Variable-temperature experiments demonstrated water exchange between the different aqua species and between the different water sites in the adducts 3 and 4 (“internal” or B-bound and “external” or H-bound). The rate of these processes increased with the amount of water bonded to B(C6F5)3. The exchange of B-bound water among the different B(C6F5)3 molecules (resulting in the 1 ⇔ 2 interconversion) caused the averaging of the 19F resonances of 1 and 2, above 273 K. Band shape analysis in the temperature range 235−312 K provided the kinetic constants, whose dependence on the concentration revealed a dissociative mechanism (ΔH ⧧ 67(2) kJ mol-1, ΔS ⧧ 58(7) J mol-1 K-1). For the adduct [(C6F5)3B(OH2)]·H2O (3), four different dynamic processes have been recognized: (i) the exchange of H-bound water among different [(C6F5)3B(OH2)] adducts (the 2 ⇔ 3 exchange) or (ii) among different [(C6F5)3B(OH2)].H2O adducts (the 3 ⇔ 4 exchange), (iii) the exchange between H-bound and B-bound water, (iv) the hopping of H-bound water between the two protons of B-bound water. This process was so fast that an averaged signal for the protons of internal water was observed even at 187 K. The rate of the process (i) increased with the concentration of 2, so that separate 19F and 1H signals for 2 and 3 were observed only in very dilute solutions at the lowest temperatures. Linear plots of the kinetic constants (estimated from 1H NMR spectra in the near fast exchange region, temperature range 188−214 K) vs the concentration of 2 allowed the estimation of the constant for the dissociative pathway (4 orders of magnitude faster than for the exchange of B-bound water) and for the bimolecular pathway [ΔH ⧧ 30(2) kJ mol-1, ΔS ⧧ 3(10) J mol-1 K-1]. Process (ii) was too fast on the NMR time scale to allow any kinetic investigation. Process (iii) caused the parallel broadening of both the 1H signals of 3 at T > 225 K, with a rate quite close to that of the dissociative exchange of water among different B(C6F5)3 molecules. The activation parameters (ΔH ⧧ 55(2) kJ mol-1, ΔS ⧧ 7(3) J mol-1 K-1, temperature range 233−273 K) allowed no discrimination between the exchange of an entire water molecule and the mere exchange of protons. Even small amounts...

Syntheses, Structure, and Reactivity of Amino(azido)stibanes

“…[22] The coordination sphere around the antimony(III) can be best understood as a strongly distorted bisphenoidal arrangement with the axial unit along N2-Sb-N1Ј [N1-Sb1-C1 91.15(9), N1-Sb1-N2 73.35(6), C1-Sb1-N2 85.42(9), N1-Sb1-N1Ј 71.12(6), C1-Sb1-N1Ј 86.82(9), N2-Sb1-N1Ј 143.41(6)°] when the weak van der Waals interactions are not considered. -(X = CN, OH), [29][30][31] [Y-B(C 6 F 5 ) 3 ] -(OH), [32][33][34][35][36] [N{CN·B(…”

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…8,28 This cumulative body of evidence has resulted in a high degree of acceptance for this mechanism in the literature, but the specific nature of the adducts I, or encounter complexes of type II, remain experimentally opaque. 22 29 Si NMR spectroscopy at 233 K. Noting, however, that their procedures did not appear to involve careful drying of the borane before use, it seemed likely that the peak assigned to this adduct is due in fact to Et3SiOSiEt3, formed rapidly when wet borane 35,36 is treated with silane via rapid dehydrogenative silation of water. 37,38 We confirmed this postulate by recording the 29 Si NMR spectrum of an authentic sample of Et3SiOSiEt3.…”

A Catalyst Platform for Unique Cationic (Co)Polymerization in Aqueous Emulsion

“…Styrene, [8] cyclopentadiene, [9] isoprene, [10] and even acrolein [11] polymerizations were performed in aqueous suspension and/or emulsion for the first time using as ingular Lewis acid, B(C 6 F 5 ) 3 , which does not decompose in water. [12] Despite obvious progresses in terms of reaction rates,t he molar masses of synthesized polymers were still not higher than 3000 gmol À1 , alimit value above which interfacial polymerization ceases. [1] M(C 6 F 5 ) 3 (M = Al, Ga) [13] or more complex aromatic boranes (C 6 F 4 -1,2[B(C 6 F 5 ) 2 ], [14] C 6 F 4 -1,2[B(C 12 F 8 ) 2 ]) [15] successfully polymerized isobutylene [13][14][15] and styrene [15] into high-molarmass polymers (M n = 10-100 kg mol À1 ).…”