Characterization of a Tricationic Trigonal Bipyramidal Iron(IV) Cyanide Complex, with a Very High Reduction Potential, and Its Iron(II) and Iron(III) Congeners

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• 作者：Jason England ; Erik R. Farquhar ; Yisong Guo ; Matthew A. Cranswick ; Kallol Ray ; Eckard Mu虉nck ; Lawrence Que ; Jr.
• 刊名：Inorganic Chemistry
• 出版年：2011
• 出版时间：April 4, 2011
• 年：2011
• 卷：50
• 期：7
• 页码：2885-2896
• 全文大小：1061K
• 年卷期：v.50,no.7(April 4, 2011)
• ISSN：1520-510X

文摘

Currently, there are only a handful of synthetic S = 2 oxoiron(IV) complexes. These serve as models for the high-spin (S = 2) oxoiron(IV) species that have been postulated, and confirmed in several cases, as key intermediates in the catalytic cycles of a variety of nonheme oxygen activating enzymes. The trigonal bipyramidal complex [Fe^IV(O)(TMG₃tren)]²⁺ (1) was both the first S = 2 oxoiron(IV) model complex to be generated in high yield and the first to be crystallographically characterized. In this study, we demonstrate that the TMG₃tren ligand is also capable of supporting a tricationic cyanoiron(IV) unit, [Fe^IV(CN)(TMG₃tren)]³⁺ (4). This complex was generated by electrolytic oxidation of the high-spin (S = 2) iron(II) complex [Fe^II(CN)(TMG₃tren)]⁺ (2), via the S = 5/2 complex [Fe^III(CN)(TMG₃tren)]²⁺ (3), the progress of which was conveniently monitored by using UV鈭抳is spectroscopy to follow the growth of bathochromically shifting ligand-to-metal charge transfer (LMCT) bands. A combination of X-ray absorption spectroscopy (XAS), M枚ssbauer and NMR spectroscopies was used to establish that 4 has a S = 0 iron(IV) center. Consistent with its diamagnetic iron(IV) ground state, extended X-ray absorption fine structure (EXAFS) analysis of 4 indicated a significant contraction of the iron-donor atom bond lengths, relative to those of the crystallographically characterized complexes 2 and 3. Notably, 4 has an Fe^IV/III reduction potential of 1.4 V vs Fc^+/o, the highest value yet observed for a monoiron complex. The relatively high stability of 4 (t_1/2 in CD₃CN solution containing 0.1 M KPF₆ at 25 掳C 鈮?15 min), as reflected by its high-yield accumulation via slow bulk electrolysis and amenability to ¹³C NMR at 鈭?0 掳C, highlights the ability of the sterically protecting, highly basic peralkylguanidyl donors of the TMG₃tren ligand to support highly charged high-valent complexes.