A New Spin on Cyclooctatetraene (COT) Redox Activity: Low-Spin Iron(I) Complexes That Exhibit Antiferromagnetic Coupling to a Singly Reduced η⁴‑COT Ligand

Formally
zerovalent (κ3-phosphine)­Fe­(η4-COT)
complexes supported by either Triphos (PhP­(CH2CH2PPh2)2) or Triphos* (H3CC­(CH2PPh2)3) have been prepared
following chelate addition to (COT)2Fe (COT = 1,3,5,7-cyclooctatetraene)
and by reduction of the respective dibromide complexes in the presence
of excess COT. The solid-state structure of each complex was determined
by single-crystal X-ray diffraction, and close inspection of the metrical
parameters revealed significant COT ligand reduction, independent
of the coordination geometry about iron. While the neutral and dianionic
forms of the redox-active COT ligand have historically received a
great deal of attention, a dearth of information regarding the often-evoked
radical monoanion form of this ligand prompted the full electronic
structure investigation of these complexes using a range of techniques.
Comparison of the Mössbauer spectroscopic data collected for
both (Triphos)­Fe­(η4-COT) complexes with data obtained
for two appropriate reference compounds indicated that they possess
a low-spin Fe­(I) center that is antiferromagnetically coupled to a
COT radical monoanion. Further evidence for this electronic structure
determination by EPR spectroscopy and cyclic voltammetry is presented.
A comparison of the solid-state metrical parameters determined in
this study to those of related first-row transition-metal complexes
has provided insight into the electronic structure analysis of related
organometallic complexes.