The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [FeIV(Oanti/syn)(TMC)]2+ Isomers

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• 作者：Ang Zhou ; Jai Prakash ; Gregory T. Rohde ; Johannes E. M. N. Klein ; Scott T. Kleespies ; Apparao Draksharapu ; Ruixi Fan ; Yisong GuoChristopher J. Cramer ; Lawrence Que Jr.
• 刊名：Inorganic Chemistry
• 出版年：2017
• 出版时间：January 3, 2017
• 年：2017
• 卷：56
• 期：1
• 页码：518-527
• 全文大小：539K
• ISSN：1520-510X

文摘

Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)Fe^IV(Ob>antib>)(NCCHb>3b>)](OTf)b>2b>, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)Fe^IV(Ob>synb>)(OTf)](OTf). The ability to access two isomers of [(TMC)Fe^IV(Ob>anti/synb>)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CHb>3b>CN)(TMC)Fe^III–Ob>antib>–Cr^III(OTf)b>4b>(NCCHb>3b>)] (<b>1b>) by inner-sphere electron transfer between Cr(OTf)b>2b> and [(TMC)Fe^IV(Ob>antib>)(NCCHb>3b>)](OTf)b>2b>. Herein we demonstrate that a new species <b>2b> is generated from the reaction between Cr(OTf)b>2b> and [(TMC)Fe^IV(Ob>synb>)(NCCHb>3b>)](OTf)b>2b>, which is formulated as [(TMC)Fe^III–Ob>synb>–Cr^III(OTf)b>4b>(NCCHb>3b>)] based on its characterization by UV–vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry. Its pre-edge area (30 units) and Fe–O distance (1.77 Å) determined by X-ray absorption spectroscopy are distinctly different from those of <b>1b> (11-unit pre-edge area and 1.81 Å Fe–O distance) but more closely resemble the values reported for [(TMC)Fe^III–Ob>synb>–Sc^III(OTf)b>4b>(NCCHb>3b>)] (<b>3b>, 32-unit pre-edge area and 1.75 Å Fe–O distance). This comparison suggests that <b>2b> has a square pyramidal iron center like <b>3b>, rather than the six-coordinate center deduced for <b>1b>. Density functional theory calculations further validate the structures for <b>1b> and <b>2b>. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)(TMC)Fe^III–Ob>antib>–Fe^IIIClb>3b>] (<b>4b>b><b>Clb>b>) and [(TMC)Fe^III–Ob>synb>–Fe^IIIClb>3b>](OTf) (<b>5b>). Complexes <b>1b>–<b>5b> thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety.