Second-coordination-sphere and electronic effects enhance iridium(III)-catalyzed homogeneous hydrogenation of carbon dioxide in water near ambient temperature and pressure
Second-coordination-sphere and electronic effects enhance iridium(III)-catalyzed homogeneous hydrogenation of carbon dioxide in water near ambient temperature and pressure
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DOI：
10.1039/c2ee21888g
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发表时间：
2012-06
影响因子：
32.5
通讯作者：
Wanhui Wang;Jonathan F. Hull;J. Muckerman;E. Fujita;Y. Himeda
中科院分区：
文献类型：
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作者：
Wanhui Wang;Jonathan F. Hull;J. Muckerman;E. Fujita;Y. Himeda
设计并合成了一系列新型的水溶性Ir(III)催化剂，以确定催化剂配体在CO2均相催化加氢反应中的电子效应和二配位球效应对活化CO2的作用。报道了[Cp*Ir(6，6‘-R2-bpy)(OH2)]SO4(bpy=2，2’-联吡啶，R=OH)在常温常压下的高催化加氢活性。通过合成R=H，Me，OMe和OH取代的配体和催化剂，我们改变了配体取代基的Hammett参数与催化氢化速率之间的良好相关性，清楚地说明了电子效应的重要性。值得注意的是，当取代基从2，2‘-联吡啶的4，4’位移动到6，6‘位时，可以一致地观察到额外的速率增强。结合密度泛函理论计算和核磁共振实验表明，这些效应的根源在于H_2的侧碱辅助异解，它显著降低了过渡态的能量。这些研究对于阐明具有优异催化活性的二氧化碳加氢新设计原则具有重要意义。
A new series of water soluble Ir(III) catalysts has been designed and synthesized to determine the catalyst ligand's role in activating CO2 through electronic and second-coordination-sphere effects for the homogeneous catalytic hydrogenation of CO2. We report high catalytic hydrogenation activity of [Cp*Ir(6,6′-R2-bpy)(OH2)]SO4 (bpy = 2,2′-bipyridine, R = OH) at ambient temperatures and pressures. Good correlation between the ligand substituents' Hammett parameters, which we varied by synthesizing ligands and catalysts substituted with R = H, Me, OMe and OH, and catalytic hydrogenation rates clearly illustrates the importance of electronic effects. Remarkably, additional rate enhancements are consistently observed when substituents are moved from 4,4′ positions to 6,6′ positions on 2,2′-bipyridine. Combined DFT calculations and NMR experiments suggest that the origin of these effects lies in the pendent base-aided heterolysis of H2, which significantly lowers the transition state energy. These studies are significant in elucidating new design principles for CO2 hydrogenation that lead to superior catalytic activity.