1H and 19F NMR investigation of the reaction of B(C6F5)3 with water in toluene solution
1H and 19F NMR investigation of the reaction of B(C6F5)3 with water in toluene solution
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DOI：
10.1021/om010610n
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发表时间：
2001-11-12
期刊：
影响因子：
2.8
通讯作者：
D'Alfonso, G
中科院分区：
文献类型：
--
作者：
Beringhelli, T;Maggioni, D;D'Alfonso, G
在甲苯-d（8）溶液中，用水滴定B（C6 F5）（3）（1），在196 K下通过F-19和H-1 NMR监测，显示首先形成加合物[（C6 F5）（3）B（OH 2）]（2），然后逐步转化为两种水相物质[（C6 F5）（3）B（OH 2）]·H2O（3）和[（C6 F5）（3）B（OH 2）]。2 H（2）O（4），分别含有一个或两个与B-结合水分子的质子氢键合的水分子。NMR数据显示，在每个滴定步骤中，仅两种物质以显著浓度存在：I和2高达1当量，2和3在1和2当量之间，3和4在2和3当量之间。高于3当量，溶液迅速达到饱和并发生相分离（尽管有证据表明4与更多的水分子相互作用）。室温下的滴定表明类似的逐步过程。变温实验证明了不同水生物种之间以及加合物3和4中不同水位点之间的水交换（“内部”或B结合和“外部”或H结合）。这些过程的速率随着与B（C6 F5）（3）结合的水的量而增加。不同B（C_6F_5）（3）分子之间的B-结合水交换（导致1 - 2相互转换）导致1和2的19 F共振在273 K以上平均化。在235-312 K温度范围内的带形分析提供了动力学常数，其对浓度的依赖性揭示了解离机制（Δ H（double dagger）67（2）kJ mol（-1），Δ S（double dagger）58（7）J mol（-1）K-1）。对于加合物[（C6 F5）（3）B（OH 2）]·H2O（3），已经认识到四种不同的动力学过程：（i）不同[（C6 F5）（3）B（OH 2）]加合物之间的H-结合水交换（2 - 3交换）或（ii）不同[（C6 F5）（3）B（OH 2）]H2O加合物之间（3 - 4交换），（iii）H-结合水和B-结合水之间的交换，（iv）H-结合水在B-结合水的两个质子之间的跳跃。这个过程是如此之快，以至于即使在187 K下也能观察到内部水质子的平均信号。过程（i）的速率随着2的浓度增加而增加，使得仅在最低温度下的非常稀的溶液中观察到2和3的单独的F-19和H-1信号。动力学常数的线性图（根据近快交换区的H-1 NMR谱估计，温度范围188-214 K）与2的浓度的关系允许估计解离途径的常数（比B结合水的交换快4个数量级）和双分子途径[Δ H（double dagger）30（2）kJ mol（-1），DeltaS（double dagger）3（10）J mol（-1）K-1]。过程（ii）在NMR时间尺度上太快，无法进行任何动力学研究。过程（iii）在T > 225 K时引起3的两个H-1信号的平行增宽，其速率与不同B（C6 F5）（3）分子间水的解离交换速率相当接近。（Δ H（double dagger）55（2）kJ mol（-1），Δ S（double dagger）7（3）J mol（-1）K-1，温度范围233-273 K）不能区分整个水分子的交换和仅仅质子的交换。即使少量的4加速过程（iii），由于发生两个快得多的过程：3 - 4交换和内部和外部水的质子之间的交换4。关于三水合物4的任何类型的水流动性的研究被质子交换过程的发生阻止（如此之快以至于甚至在188 K下加宽内部和外部水的信号），这可能受到4的B-键合水分子的质子的酸性解离的支持。
Titrations of B(C6F5)(3) (1) with water, in toluene-d(8) solution, monitored by F-19 and H-1 NMR at 196 K, showed first the formation of the adduct [(C6F5)(3)B(OH2)] (2) and then its stepwise transformation into the two aqua species [(C6F5)(3)B(OH2)].H2O (3) and [(C6F5)(3)B(OH2)]. 2H(2)O (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: I 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 (DeltaH(double dagger) 67(2) kJ mol(-1), DeltaS(double dagger) 58(7) J mol(-1) K-1). For the adduct [(C6F5)(3)B(OH2)].H2O (3), four different dynamic processes have been recognized: (i) the exchange of H-bound water among different [(C6F5)(3)B(OH2)] adducts (the 2 3 exchange) or (ii) among different [(C6F5)(3)B(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 F-19 and H-1 signals for 2 and 3 were observed only in very dilute solutions at the lowest temperatures. Linear plots of the kinetic constants (estimated from H-1 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 [DeltaH(double dagger) 30(2) kJ mol(-1), DeltaS(double dagger) 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 H-1 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 (DeltaH(double dagger) 55(2) kJ mol(-1), DeltaS(double dagger) 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 of 4 accelerated process (iii), due to the occurrence of two much faster processes: the 3 - 4 exchange and the exchange between the protons of internal and external water in 4. The study of any kind of water mobility concerning the trihydrate 4 was prevented by the occurrence of proton exchange processes (so fast as to broaden the signals of internal and external water even at 188 K), possibly favored by the acidic dissociation of the protons of the B-bonded water molecule of 4.