A Caged Neutral 17-Valence-Electron Iron(I) Radical [Fe(CO)₂(Cl)(P((CH₂)₁₀)₃P)]^•: Synthetic, Structural, Spectroscopic, Redox, and Computational Studies

UV irradiation of yellow CH₂Cl₂ solutions of <i>trans</i>-Fe(CO)₃(P((CH₂)₁₀)₃P) (<b>2a</b>) and PMe₃ (10 equiv) gives, in addition to the previously reported dibridgehead diphosphine P((CH₂)₁₀)₃P (46%), a green paramagnetic complex that crystallography shows to be the trigonal-bipyramidal iron(I) radical <i>trans</i>-[Fe(CO)₂(Cl)(P((CH₂)₁₀)₃P)]^• (<b>1a</b>^•; 31% after workup). This is a rare example of an isolable species of the formula [Fe(CO)_4-<i>n</i>(L)_<i>n</i>(X)]^• (<i>n</i> = 0-3, L = two-electron-donor ligand; X = one-electron-donor ligand). Analogous precursors with longer P(CH₂)_<i>n</i>P segments (<i>n</i> = 12, 14, 16, 18) give only the demetalated diphosphines, and a rationale is proposed. The magnetic susceptibility of <b>1a</b>^•, assayed by Evans' method and SQUID measurements, indicates a spin (<i>S</i>) of ¹/₂. Cyclic voltammetry shows that <b>1a</b>^• undergoes a partially reversible one-electron oxidation, but no facile reduction. The UV-visible, EPR, and ⁵⁷Fe Mössbauer spectra are analyzed in detail. Complex <b>2a</b> is similarly studied, and, despite the extra valence electron, exhibits a comparable oxidation potential (Δ<i>E</i>_1/2 ≤ 0.04 V). The crystal structure shows a cage conformation, solvation level, disorder motif, and unit cell parameters essentially identical to those of <b>1a</b>^•. DFT calculations provide much insight regarding the structural, redox, and spectroscopic properties.