The Two Faces of Tetramethylcyclam in Iron Chemistry:
Distinct Fe–O–M Complexes Derived from [Fe<sup>IV</sup>(O<sub><i>anti</i>/<i>syn</i></sub>)(TMC)]<sup>2+</sup> Isomers
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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<sup>IV</sup>(O<sub><i>anti</i></sub>)(NCCH<sub>3</sub>)](OTf)<sub>2</sub>, where <i>anti</i> indicates that the O atom is located on the face opposite all four
methyl groups, and the recently reported <i>syn</i> isomer
[(TMC)Fe<sup>IV</sup>(O<sub><i>syn</i></sub>)(OTf)](OTf).
The ability to access two isomers of [(TMC)Fe<sup>IV</sup>(O<sub><i>anti</i>/<i>syn</i></sub>)] raises the fundamental
question of how ligand topology can affect the properties of the metal
center. Previously, we have reported the formation of [(CH<sub>3</sub>CN)(TMC)Fe<sup>III</sup>–O<sub><i>anti</i></sub>–Cr<sup>III</sup>(OTf)<sub>4</sub>(NCCH<sub>3</sub>)]
(<b>1</b>) by inner-sphere electron transfer between Cr(OTf)<sub>2</sub> and [(TMC)Fe<sup>IV</sup>(O<sub><i>anti</i></sub>)(NCCH<sub>3</sub>)](OTf)<sub>2</sub>. Herein we demonstrate
that a new species <b>2</b> is generated from the reaction between
Cr(OTf)<sub>2</sub> and [(TMC)Fe<sup>IV</sup>(O<sub><i>syn</i></sub>)(NCCH<sub>3</sub>)](OTf)<sub>2</sub>, which is formulated
as [(TMC)Fe<sup>III</sup>–O<sub><i>syn</i></sub>–Cr<sup>III</sup>(OTf)<sub>4</sub>(NCCH<sub>3</sub>)] 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>1</b> (11-unit pre-edge area and 1.81 Å
Fe–O distance) but more closely resemble the values reported
for [(TMC)Fe<sup>III</sup>–O<sub><i>syn</i></sub>–Sc<sup>III</sup>(OTf)<sub>4</sub>(NCCH<sub>3</sub>)] (<b>3</b>, 32-unit pre-edge area and 1.75 Å Fe–O distance).
This comparison suggests that <b>2</b> has a square pyramidal
iron center like <b>3</b>, rather than the six-coordinate center
deduced for <b>1</b>. Density functional theory calculations
further validate the structures for <b>1</b> and <b>2</b>. The influence of the distinct TMC topologies on the coordination
geometries is further confirmed by the crystal structures of [(Cl)(TMC)Fe<sup>III</sup>–O<sub><i>anti</i></sub>–Fe<sup>III</sup>Cl<sub>3</sub>] (<b>4</b><sub><b>Cl</b></sub>) and [(TMC)Fe<sup>III</sup>–O<sub><i>syn</i></sub>–Fe<sup>III</sup>Cl<sub>3</sub>](OTf) (<b>5</b>). Complexes <b>1</b>–<b>5</b> thus constitute a set of complexes
that shed light on ligand topology effects on the coordination chemistry
of the oxoiron moiety
