Trinuclear oxo-centered “basic iron carboxylates” are well-known coordination compounds containing an [Fe₃O]⁺⁷ core. They may be considered partial hydrolysis products of Fe⁺³(aq) en route to larger iron oxide mineral phases. Basic iron carboxylates of fatty acids (e.g. oleic acid) are precursors to superparamagnetic iron oxide nanoparticles (SPION). We have prepared the model compound [Fe₃O(TFB)₆(H₂O)₃]ClO₄ (1, TFB = 4,4,4-trifluorobutyrate) to investigate the early stages of cluster growth in SPION using a combination ¹⁹F-NMR and high resolution MS (ESI-Orbitrap and MALDI-TOF). The metal to ligand ratio used in the synthesis of 1 is shown to affect iron cluster speciation. Under conditions that mimic SPION formation (excess ligand) a complex mixture is obtained that contains the undecanuclear cluster Fe₁₁O₆(OH)₆(TFB)₁₅ (2). Solutions of 1 in acetonitrile are indefinitely stable, but addition of one equivalent of exogenous base (sodium bicarbonate or triethylamine) induces the formation of 2 according to the equation ¹¹/₃ [Fe₃O(TFB)₆]⁺ + ¹¹/₃ OH^- + ¹⁴/₃H₂O → Fe₁₁O₆(OH)₆(TFB)₁₅ + 7 H(TFB). NMR and MS data show that each iron in 1 has a labile coordination site that may be occupied by water or other Lewis bases. Deprotonation of a coordinated water appears to initiate the conversion of 1 to 2. When a stoichiometric amount of base is employed 2 is formed cleanly, however in the presence of excess base, 2 degrades into a heterogeneous mixture of higher molecular weight species. Progress towards the synthesis of an undecanuclear iron(III) cluster of oleic acid, and the relevance of this species to the formation of SPION will be presented.

William Kerber

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