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 )(NCCH 3 )](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 [(CH 3 CN)(TMC)Fe III –O anti –Cr III (OTf) 4 (NCCH 3 )] ( 1 ) by inner-sphere electron transfer between Cr(OTf) 2 and [(TMC)Fe IV (O anti )(NCCH 3 )](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 )(NCCH 3 )](OTf) 2, which is formulated as [(TMC)Fe III –O syn –Cr III (OTf) 4 (NCCH 3 )] 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 (NCCH 3 )] ( 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 III Cl 3 ] ( 4 Cl ) and [(TMC)Fe III –O syn –Fe III Cl 3 ](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.