The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe-O-M Complexes Derived from [Fe(O)(TMC)] Isomers.

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(O)(NCCH), 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(O)(OTf). The ability to access two isomers of [(TMC)Fe(O)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CHCN)(TMC)Fe-O-Cr(OTf)(NCCH)] (1) by inner-sphere electron transfer between Cr(OTf) and (TMC)Fe(O)(NCCH). Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf) and (TMC)Fe(O)(NCCH), which is formulated as [(TMC)Fe-O-Cr(OTf)(NCCH)] 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-O-Sc(OTf)(NCCH)] (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-O-FeCl] (4) and (TMC)Fe-O-FeCl (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.