Hydrogen elimination from a hydroxycyclopentadienyl ruthenium(II) hydride: study of hydrogen activation in a ligand-metal bifunctional hydrogenation catalyst.
Hydrogen elimination from a hydroxycyclopentadienyl ruthenium(II) hydride: study of hydrogen activation in a ligand-metal bifunctional hydrogenation catalyst.
复制标题
DOI：
10.1021/ja710851z
复制
发表时间：
2005-02
影响因子：
15
通讯作者：
C. Casey;Jeffrey B. Johnson;S. Singer;Q. Cui
中科院分区：
文献类型：
--
作者：
C. Casey;Jeffrey B. Johnson;S. Singer;Q. Cui
在高温下，在甲苯中，[2，5-Ph（2）-3，4-Tol（2）（eta（5）-C（4）COH）]Ru（CO）（2）H（3）在PPh（3）的存在下发生氢消除，生成钌膦络合物[2，5-Ph（2）-3，4-Tol（2）-（eta（4）-C（4）CO）]Ru（PPh（3）（CO）（2）（6）。在没有醇的情况下，RuH/OD交换的缺乏，Ru的一级速率定律和膦的零级速率定律，以及动力学氘同位素效应都指向一种机制，该机制涉及不可逆地形成瞬态二氢钌络合物B，H（2）损失得到不饱和钌络合物A，以及PPh（3）捕获得到6。DFT计算表明，涉及从CpOH基团直接转移氢形成B的机制具有太高的势垒而不能考虑。DFT计算还表明，3的醇或CpOH基团可以为B的形成提供低能量途径。PGSE NMR测量确定3在甲苯中是氢键二聚体，并且一级动力学表明3的两个分子也参与氢转移形成B的过渡态，这是限速步骤。在乙醇的存在下，从3的氢损失加速和RuD/OH交换发生250倍的速度比在它的情况下。计算表明，形成二氢络合物的过渡态是在酸性CpOH和3的无规RuH之间存在一个乙醇桥，乙醇促进了质子转移，加速了二氢络合物B的可逆形成.在EtOH存在下，限速步骤转移到从B损失氢。
At high temperatures in toluene, [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COH)]Ru(CO)(2)H (3) undergoes hydrogen elimination in the presence of PPh(3) to produce the ruthenium phosphine complex [2,5-Ph(2)-3,4-Tol(2)-(eta(4)-C(4)CO)]Ru(PPh(3))(CO)(2) (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H(2) to give unsaturated ruthenium complex A, and trapping by PPh(3) to give 6. DFT calculations showed that a mechanism involving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered. DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene, and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from 3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen from B.