STRUCTURAL CHANGES ASSOCIATED WITH ELECTRON-TRANSFER REACTIONS. ONE VS TWO-STEP REACTIONS IN THE OXIDATION OF W(ETA 5-C5(CH3)5)(CH3)4
STRUCTURAL CHANGES ASSOCIATED WITH ELECTRON-TRANSFER REACTIONS. ONE VS TWO-STEP REACTIONS IN THE OXIDATION OF W(ETA 5-C5(CH3)5)(CH3)4
复制标题
与电子转移反应相关的结构变化。
DOI：
10.1021/ja00152a019
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发表时间：
1995
影响因子：
15
通讯作者：
Dennis H. Evans
中科院分区：
文献类型：
--
作者：
S. Lerke;Dennis H. Evans
与Liu等人(J.Am)的原始解释一致化学。SoC。1987,109,4282-4291)，证明了标题化合物在二氯甲烷中的电化学氧化是通过两步过程进行的，在该过程中，正方锥体中性反应物(SP)被氧化成具有类似结构的阳离子(SP+)，然后异构化成优选的三角双锥体阳离子(TBP+)。结合室温慢循环伏安法和低温快速扫描实验，测定了SP+转化为TBP+的热力学和动力学参数(AH0=-6.05kcal/mol；AS0=-3.97kcal-1K-1；AG*298=9.5kcal/mol)。在相同的溶剂中，SP/SP+对钴镉钴对的可逆式电势为+1.20V，SP直接一步氧化为TBP+的可逆式电势为0.99V，即直接氧化比两步反应所需的能量少5kcal/mol。反应实际上是通过两步途径进行的，这一事实可以用直接氧化的一个很大的势垒来解释，计算得出的势垒大约大于12千卡/摩尔。考虑到来自外部和内部重组能量的预期贡献，认为这个巨大的障碍是合理的。
In accord with the original interpretation of Liu et al.(J. Am. Chem. Soc. 1987, 109, 4282—4291), it has been demonstrated that electrochemical oxidation of the title compound in methylene chloride proceeds by a two-step process by which the square-pyramidal neutral reactant (SP) is oxidized to a cation of similar structure (SP+) followed by isomerization to the preferred trigonal-bipyramidal cation (TBP+). A combination of slow cyclic voltammetry near room temperature and fast scan experiments at low temperature has allowed determination of the thermodynamic and kinetic parameters for the conversion of SP+ toTBP+(AH0=—6.05 kcal/mol; AS0=—3.97 cal mol-1 K-1; AG* 298= 9.5 kcal/mol). The reversible formal potential for the SP/SP+ couple is+ 1.20 V vs cobaltocenium/cobaltocene in the same solvent and the reversible formal potentialfor the direct one-step oxidation of SP to TBP+ is 0.99 V, ie, the direct oxidation requires 5 kcal/mol less energy than thetwo-step reaction. The fact that the reaction actually proceeds via the two-step pathway is explained by a large barrier to the direct oxidation, calculated to be greater than about 12 kcal/mol. It is argued that this large barrier is reasonable in view of the expected contributionfrom the outer and inner reorganization energies.