Spectroscopic and Theoretical Investigation of a Complex with an [O═FeIV–O–FeIV═O] Core Related to Methane Monooxygenase Intermediate Q

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• 作者：Sebastian A. Stoian ; Genqiang Xue ; Emile L. Bominaar ; Lawrence Que ; Jr. ; Eckard Münck
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：January 29, 2014
• 年：2014
• 卷：136
• 期：4
• 页码：1545-1558
• 全文大小：556K
• ISSN：1520-5126

文摘

Previous efforts to model the diiron(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron(IV) TPA complex, 2, with an O=Fe^IV鈥揙鈥揊e^IV鈥揙H core that has two ferromagnetically coupled S_loc = 1 sites. Addition of base to 2 at 鈭?5 掳C elicits its conjugate base 6 with a novel O鈺怓e^IV鈥揙鈥揊e^IV鈺怬 core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using M枚ssbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (S_loc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral O鈺怓e鈥揊e鈺怬 angle of 180掳. M枚ssbauer and EPR analyses show that 6b is a diiron(IV) complex with ferromagnetically coupled S_loc = 1 and S_loc = 2 sites to give total spin S_t = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral O鈺怓e鈥揊e鈺怬 angle of 6b is 90掳. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe^IV鈺怬 complexes. Complex 6 represents an intriguing addition to the handful of diiron(IV) complexes that have been characterized.