A New Spin on Cyclooctatetraene (COT) Redox Activity: Low-Spin Iron(1) Complexes That Exhibit Antiferromagnetic Coupling to a Singly Reduced eta(4)-COT Ligand

摘要

abstract: Formally zerovalent (κ[superscript 3]-phosphine)Fe(η[superscript 4]-COT) complexes supported by either Triphos (PhP(CH[subscript 2]CH[subscript 2]PPh[subscript 2])[subscript 2]) or Triphos* (H[subscript 3]CC(CH[subscript 2]PPh[subscript 2])[subscript 3]) have been prepared following chelate addition to (COT)[subscript 2]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[superscript 4]-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.