The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_{<i>anti</i>/<i>syn</i>})(TMC)]²⁺ Isomers

Abstract

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(O_anti)(NCCH₃)](OTf)₂, 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(O_syn)(OTf)](OTf). The ability to access two isomers of [(TMC)Fe^IV(O_anti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH₃CN)(TMC)Fe^III-O_anti-Cr^III(OTf)₄(NCCH₃)] (1) by inner-sphere electron transfer between Cr(OTf)₂ and [(TMC)Fe^IV(O_anti)(NCCH₃)](OTf)₂. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)₂ and [(TMC)Fe^IV(O_syn)(NCCH₃)](OTf)₂, which is formulated as [(TMC)Fe^III-O_syn-Cr^III(OTf)₄(NCCH₃)] 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 1 (11-unit pre-edge area and 1.81 Å Fe-O distance) but more closely resemble the values reported for [(TMC)Fe^III-O_syn-Sc^III(OTf)₄(NCCH₃)] (3, 32-unit pre-edge area and 1.75 Å Fe-O distance). This comparison suggests that 2 has a square pyramidal iron center like 3, rather than the six-coordinate center deduced for 1. Density functional theory calculations further validate the structures for 1 and 2. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)(TMC)Fe^III-O_anti-Fe^IIICl₃] (4_Cl) and [(TMC)Fe^III-O_syn-Fe^IIICl₃](OTf) (5). Complexes 1-5 thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety.