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

Zhou, A.; Prakash, J.; Rohde, G. T.; Klein, J. E. M. N.; Kleespies, S. T.; Draksharapu, A.; Fan, R.; Guo, Y.; Cramer, C. J.; Que, L., Jr.
Inorg. Chem. 2017, 56, 518 (doi:10.1021/acs.inorgchem.6b02417)

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)FeIV(Oanti)(NCCH3)](OTf)2, where anti indicates that the O-atom is located on the face opposite to all four methyl groups, and the recently reported syn isomer [(TMC)FeIV(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)FeIV(Oanti/syn)] raises the fundamental question how ligand topology can affect the properties of the metal center. Previously we have reported the formation of [(CH3CN)(TMC)FeIII–Oanti–CrIII(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)FeIV(Oanti)(NCCH3)](OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)FeIV(Osyn)(NCCH3)](OTf)2, which is formulated as [(TMC)FeIII–Osyn–CrIII(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)FeIII–Osyn–ScIII(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 6-coordinate center deduced for 1. Density functional theory (DFT) 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)FeIII–Oanti–FeIIICl3] (4Cl) and [(TMC)FeIII–Osyn–FeIIICl3](OTf) (5). Complexes 15 thus constitute a set of complexes that shed light into ligand topology effects on the coordination chemistry of the oxoiron moiety.