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

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posted on 22.12.2014, 00:00 by Tufan K. Mukhopadhyay, Marco Flores, Russell K. Feller, Brian L. Scott, R. Dean Taylor, Moshe Paz-Pasternak, Neil J. Henson, Francisca N. Rein, Nathan C. Smythe, Ryan J. Trovitch, John C. Gordon
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.