What Determines Catalyst Functionality in Molecular Water Oxidation? Dependence on Ligands and Metal Nuclearity in Cobalt Clusters
详细信息 查看全文
- 作者：Paul F. Smith ; Christopher Kaplan ; John E. Sheats ; David M. Robinson ; Nicholas S. McCool ; Nicholas Mezle ; G. Charles Dismukes
- 刊名：Inorganic Chemistry
- 出版年：2014
- 出版时间：February 17, 2014
- 年：2014
- 卷：53
- 期：4
- 页码：2113-2121
- 全文大小：476K
- ISSN：1520-510X
文摘
The metal-oxo M4O4 鈥渃ubane鈥?topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt鈥搊xo clusters including the Co4O4 cubanes, Co4O4(OAc)4(py)4 and [Co4O4(OAc)2(bpy)4]2+, using the common Ru(bpy)32+/S2O82- photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s鈥?, respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither 鈥渋ncomplete cubane鈥?trimers [Co3O(OH)3(OAc)2(bpy)3]2+ and [Co3O(OH)2(OAc)3(py)5]2+ nor 鈥渉alf cubane鈥?dimers [Co2(OH)2(OAc)3(bpy)2]+ and [Co2(OH)2(OAc)3(py)4]+ were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O4]5+ core. The origin of catalytic activity by Co4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.