Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington 99352, USA.
J Am Chem Soc. 2011 Sep 21;133(37):14604-13. doi: 10.1021/ja202830w. Epub 2011 Aug 29.
The free energies interconnecting nine tungsten complexes have been determined from chemical equilibria and electrochemical data in MeCN solution (T = 22 °C). Homolytic W-H bond dissociation free energies are 59.3(3) kcal mol(-1) for CpW(CO)(2)(IMes)H and 59(1) kcal mol(-1) for the dihydride CpW(CO)(2)(IMes)(H)(2) (where IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), indicating that the bonds are the same within experimental uncertainty for the neutral hydride and the cationic dihydride. For the radical cation, CpW(CO)(2)(IMes)H, W-H bond homolysis to generate the 16-electron cation CpW(CO)(2)(IMes) is followed by MeCN uptake, with free energies for these steps being 51(1) and -16.9(5) kcal mol(-1), respectively. Based on these two steps, the free energy change for the net conversion of CpW(CO)(2)(IMes)H to CpW(CO)(2)(IMes)(MeCN) in MeCN is 34(1) kcal mol(-1), indicating a much lower bond strength for the 17-electron radical cation of the metal hydride compared to the 18-electron hydride or dihydride. The pK(a) of CpW(CO)(2)(IMes)H in MeCN was determined to be 31.9(1), significantly higher than the 26.6 reported for the related phosphine complex, CpW(CO)(2)(PMe(3))H. This difference is attributed to the electron donor strength of IMes greatly exceeding that of PMe(3). The pK(a) values for CpW(CO)(2)(IMes)H and CpW(CO)(2)(IMes)(H)(2) were determined to be 6.3(5) and 6.3(8), much closer to the pK(a) values reported for the PMe(3) analogues. The free energy of hydride abstraction from CpW(CO)(2)(IMes)H is 74(1) kcal mol(-1), and the resultant CpW(CO)(2)(IMes) cation is significantly stabilized by binding MeCN to form CpW(CO)(2)(IMes)(MeCN), giving an effective hydride donor ability of 57(1) kcal mol(-1) in MeCN. Electrochemical oxidation of CpW(CO)(2)(IMes) is fully reversible at all observed scan rates in cyclic voltammetry experiments (E° = -1.65 V vs Cp(2)Fe(+/0) in MeCN), whereas CpW(CO)(2)(IMes)H is reversibly oxidized (E° = -0.13(3) V) only at high scan rates (800 V s(-1)). For CpW(CO)(2)(IMes)(MeCN), high-pressure NMR experiments provide an estimate of ΔG° = 10.3(4) kcal mol(-1) for the displacement of MeCN by H(2) to give CpW(CO)(2)(IMes)(H)(2).
已从 MeCN 溶液中的化学平衡和电化学数据确定了九个钨配合物的自由能。CpW(CO)(2)(IMes)H 的均裂 W-H 键离解自由能为 59.3(3) kcal mol(-1)，CpW(CO)(2)(IMes)(H)(2)](+) 的二氢化物为 59(1) kcal mol(-1)(其中 IMes = 1,3-双(2,4,6-三甲基苯基)咪唑-2-亚基)，这表明在中性氢化物和阳离子二氢化物中，键相同，实验不确定度范围内。对于自由基阳离子CpW(CO)(2)(IMes)H，W-H 键均裂生成 16 电子阳离子CpW(CO)(2)(IMes)，随后是 MeCN 的摄取，这两步的自由能分别为 51(1)和-16.9(5) kcal mol(-1)。基于这两个步骤，CpW(CO)(2)(IMes)H向CpW(CO)(2)(IMes)(MeCN)在 MeCN 中的净转化的自由能变化为 34(1) kcal mol(-1)，表明与 18 电子氢化物或二氢化物相比，金属氢化物的 17 电子自由基阳离子的键强度低得多。CpW(CO)(2)(IMes)H 在 MeCN 中的 pK(a)值确定为 31.9(1)，明显高于相关膦配合物 CpW(CO)(2)(PMe(3))H 的 26.6。这种差异归因于 IMes 的电子给体强度大大超过 PMe(3)。CpW(CO)(2)(IMes)H和CpW(CO)(2)(IMes)(H)(2)的 pK(a)值分别确定为 6.3(5)和 6.3(8)，与 PMe(3)类似物报道的 pK(a)值更接近。从 CpW(CO)(2)(IMes)H 中吸氢的自由能为 74(1) kcal mol(-1)，形成的CpW(CO)(2)(IMes)阳离子通过与 MeCN 结合形成CpW(CO)(2)(IMes)(MeCN)而得到显著稳定，在 MeCN 中提供 57(1) kcal mol(-1)的有效氢供体能力。CpW(CO)(2)(IMes)在循环伏安实验中的所有观察到的扫描速率下都完全可逆地电化学氧化(E°=-1.65 V 相对于 Cp(2)Fe(+/0)在 MeCN 中)，而 CpW(CO)(2)(IMes)H 仅在高扫描速率(800 V s(-1))下可逆氧化(E°=-0.13(3) V)。对于CpW(CO)(2)(IMes)(MeCN)，高压 NMR 实验提供了 ΔG°=10.3(4) kcal mol(-1)的估计值，用于通过 H(2)取代 MeCN 生成CpW(CO)(2)(IMes)(H)(2)。
