Unexpected assembly of a (μ-oxo) (μ-formato) diiron(III) complex from an aerobic methanolic solution of Fe (III) and the TPA ligand

Crystals of a (μ-oxo) (μ-formato) diiron(III) complex [Fe₂(μ-O) (TPA)₂( O₂CH)] (C1O₄)₃ (1) formed from a methanolic solution of Fe(ClO₄)₃ and the tetradentate tripodal ligand tris(2-pyridylmethyl) amine (TPA) in methanol when left standing for days under aerobic conditions, despite the fact that no formate ion had been added to the solution. Complex 1 was found to have UV-Vis and ¹H NMR properties very similar to those of [Fe₂( μ-O) (TPA) ₂ (O₂CCH₃)] (ClO₄)₃ (2), and its formula weight was corroborated by fast atom bombardment mass spectrometry. Complex 1 crystallized in the monoclinic space group P2₁ ( No. 4) with α = 11.693(3), b = 10.414(4), c = 18,144(6) Å, and β = 93.98(3)°. The structure was determined at - 84°C from 4464 out of a total of 6753 reflections with R = 0.058 and R_w = 0.068. The crystal structure confirmed the presence of a (μ-oxo) ( μ-carboxylato)diiron(III) unit with distinct iron sites as found for 2. The formate bridge was shown to derive from the aerobic oxidation of methanol: 1 was not formed in the absence of air, and 2 was formed from the reaction carried out in ethanol solvent. Manometric measurements showed that the amount of dioxygen consumed was approximately stoichiometric to the combined amounts of formate found in 1 (80%) and formaldehyde (20%) found in the reaction mixture.