Dipartimento di Chimica, Università della Calabria, I-87030, Arcavacata di Rende, Italy.
J Am Chem Soc. 2010 Mar 31;132(12):4178-90. doi: 10.1021/ja908453k.
The mechanistic details of the hydrogenation of molecular oxygen by the 18e amino-hydride CpIrH(TsDPEN) (1H(H)) complex to give CpIr(TsDPEN-H) (1) and 1 equiv of H(2)O were investigated by means of hybrid density functional calculations (B3LYP). To comprehensively describe the overall catalytic cycle of the hydrogenation of dioxygen using H(2) catalyzed by the Ir complex 1, the potential energy surfaces for the hydrogenation process of both the catalyst 1 and the corresponding unsaturated iridium(III) amine cation (1H) were explored at the same level of theory. The results of our computations, in agreement with experimental findings, confirm that the addition of H(2) to the 16e diamido complexes 1 is favorable but is slow and is accelerated by the presence of Bronsted acids, such as HOTf, which convert 1 into the corresponding amine cation 1H. By deprotonation of the subsequently hydrogenated 1H(H(2)) complex the amine hydride catalyst 1H(H) is generated, which is able to reduce molecular oxygen. Calculations corroborate that the O(2) reduction goes through formation of an intermediate iridium hydroperoxo complex that reacts with 1H(H) to eliminate water, restore 1, and restart the catalytic cycle. From the outcomes of our computational analysis it results that the slow step of the overall O(2) hydrogenation process is the O(2) insertion into the Ir-H bond, and the highest calculated barrier along this pathway to give the hydroperoxo product shows a good agreement with the experimentally estimated value. As a consequence, unreacted 1H(H) approaches 1H(OOH) to give 1H(OH) and water according to the experimentally observed second-order kinetics with respect to [1H(H)]. Calculations were carried out to explore the possibility that H(2)O(2) is released from the hydroperoxo intermediate together with catalyst 1, and the subsequent water elimination reaction occurs by reduction of produced H(2)O(2) with 1H(H) to regenerate catalyst 1. Preliminary results concerning the O(2) reduction in acidic conditions show that the reaction proceeds by intermediate production of H(2)O(2), which reacts with 1H(H) to eliminate water, restore 1H, and restart the catalytic cycle. The energetics of the process appear to be definitely more favorable with respect the analogous pathways in neutral conditions.
我们运用杂化密度泛函理论（B3LYP）研究了 18e 氨基氢化物 CpIrH(TsDPEN)（1H(H)）复合物将分子氧氢化生成 CpIr(TsDPEN-H)（1）和 1 当量 H2O 的反应机理。为了全面描述 Ir 配合物 1 催化 H2 氢化氧气的整个催化循环，我们在相同理论水平上探索了催化剂 1 和相应的不饱和铱（III）胺阳离子(1H)的氢化过程的势能面。我们的计算结果与实验结果一致，证实 H2 与 16e 二酰胺配合物 1 的加成是有利的，但速度较慢，且受到布朗斯特酸（如 HOTf）的加速，这些酸将 1 转化为相应的胺阳离子1H。随后氢化的1H(H2))配合物的去质子化生成胺氢化物催化剂 1H(H)，它能够还原分子氧。计算结果证实，O2 的还原通过形成中间铱过氧配合物进行，该配合物与 1H(H)反应消除水，恢复 1，并重新启动催化循环。从我们的计算分析结果可知，整个 O2 氢化过程的缓慢步骤是 O2 插入 Ir-H 键，并且沿着该途径生成过氧产物的最高计算势垒与实验估计值吻合良好。因此，未反应的 1H(H)根据实验观察到的与[1H(H)]的二级动力学接近 1H(OOH)，生成 1H(OH)和水。我们进行了计算以探索过氧中间物与催化剂 1 一起释放 H2O2 的可能性，随后通过用 1H(H)还原产生的 H2O2 发生后续的水消除反应，从而再生催化剂 1。关于酸性条件下 O2 还原的初步结果表明，该反应通过 H2O2 的中间产物生成进行，该产物与 1H(H)反应消除水，恢复1H，并重新启动催化循环。与中性条件下的类似途径相比，该过程的能量学似乎明显更有利。
