Electronic Control of the Protonation Rates of Fe鈥揊e Bonds

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• 作者：Au拧ra Jablonskyt臈 ; Lee R. Webster ; Trevor R. Simmons ; Joseph A. Wright ; Christopher J. Pickett
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：September 17, 2014
• 年：2014
• 卷：136
• 期：37
• 页码：13038-13044
• 全文大小：392K
• ISSN：1520-5126

文摘

Protonation at metal鈥搈etal bonds is of fundamental interest in the context of the function of the active sites of hydrogenases and nitrogenases. In diiron dithiolate complexes bearing carbonyl and electron-donating ligands, the metal鈥搈etal bond is the highest occupied molecular orbital (HOMO) with a 鈥渂ent鈥?geometry. Here we show that the experimentally measured rates of protonation (k_H) of this bond and the energy of the HOMO as measured by the oxidation potential of the complexes (E_1/2^ox) correlate in a linear free energy relationship: ln k_H = ((F(c 鈥?尾E_1/2^ox))/(RT)), where c is a constant and 尾 is the dimensionless Br酶nsted coefficient. The value of 尾 of 0.68 is indicative of a strong dependence upon energy of the HOMO: measured rates of protonation vary over 6 orders of magnitude for a change in E_1/2^ox of ca. 0.55 V (ca. 11 orders of magnitude/V). This relationship allows prediction of protonation rates of systems that are either too fast to measure experimentally or that possess additional protonation sites. It is further suggested that the nature of the bridgehead in the dithiolate ligand can exert a stereoelectronic influence: bulky substituents destabilize the HOMO, thereby increasing the rate of protonation.