A New Spin on Cyclooctatetraene (COT) Redox Activity: Low-Spin Iron(I) Complexes That Exhibit Antiferromagnetic Coupling to a Singly Reduced η<sup>4</sup>‑COT Ligand

Formally zerovalent (κ<sup>3</sup>-phosphine)­Fe­(η<sup>4</sup>-COT) complexes supported by either Triphos (PhP­(CH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>) or Triphos* (H<sub>3</sub>CC­(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>) have been prepared following chelate addition to (COT)<sub>2</sub>Fe (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(η<sup>4</sup>-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.