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

Abstract

Crystals of a (μ-oxo)(μ-formato)diiron(III) complex [Fe 2(μ-O)(TPA) 2(O 2CH)](ClO 4) 3 ( 1) formed from a methanolic solution of Fe(ClO 4) 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 [Fe 2(μ-O)(TPA) 2(O 2CCH 3)](ClO 4) 3 ( 2), and its formula weight was corroborated by fast atom bombardment mass spectrometry. Complex 1 crystallized in the monoclinic space group P2 1 (No. 4) with a = 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.