Kinetic studies on the reaction of PhSâ with [Fe4S4Cl4]2â (pKa = 18.8) in the presence of the weak acid [NH2(CH2)3CH2]+ in MeCN showed that the mechanism involves initial, rate-limiting, binding of PhSâ to the cluster, followed by protonation of the cluster core (presumably a µ3-S), then dissociation of chloride. This is different to the sequence of elementary reactions established with the stronger acid, [NHEt3]+ (pKa = 18.5), where protonation precedes binding of the thiol. Quantitative comparison of these two systems reveals that the literature value of pKa = 19.6 for [NH2(CH2)3CH2]+ is inconsistent with our kinetic results and that pKa = 21.5 is more appropriate, both in this and other systems. The kinetic data show that the rate of protonation of the cluster core falls in the range 2 Ã 105 â¤Â k â¤Â 4.8 Ã 106 dm3 molâ1 sâ1, for a thermodynamically favourable reaction. The reasons why this protonation is 104â105 times slower than the diffusion-controlled limit are discussed.
在MeCN中,对PhS与[Fe4S4Cl4]2(pKa = 18.8)的反应进行的动力学研究表明,反应机理包括初始的、速率限制的PhS与簇的结合,然后是簇核(可能是μ3-S)的质子化,最后是
氯的解离。这与用强酸[NHEt3]+(pKa = 18.5)建立的基本反应顺序不同,在强酸中,质子化先于
硫醇的结合。对这两个系统的定量比较表明,文献中[NH2(
CH2)3 ]+的pKa = 19.6与我们的动力学结果不一致,pKa = 21.5更合适,在此系统和其他系统中都是如此。动力学数据显示,簇核的质子化速率在2×105≫ k≫ 4.8×106 dm3 mol-1 s-1的范围内,这是一个热力学上有利的反应。本文讨论了这种质子化比扩散控制的极限慢104-105倍的原因。