Magnetic Properties of High-Nuclearity Fe_<i>x</i>-oxo (<i>x</i> = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach

Abstract

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [Fe₇O₃(O₂CPh)₉(mda)₃(H₂O)] (<b>1</b>), [Fe₂₂O₁₄(OH)₃(O₂CMe)₂₁(mda)₆](ClO₄)₂ (<b>2</b>), and [Fe₂₄O₁₅(OH)₄(OEt)(O₂CMe)₂₁(mda)₇](ClO₄)₂ (<b>3</b>), where mdaH₂ is <i>N</i>-methyldiethanolamine. <b>1</b> was prepared from the reaction of [Fe₃O(O₂CPh)₆(H₂O)₃](NO₃) with mdaH₂ in a 1:2 ratio in MeCN, whereas <b>2</b> and <b>3</b> were prepared from the reaction of FeCl₃/NaO₂CMe/mdaH₂ in a 2:∼13:2 ratio and FeCl₃/NaO₂CMe/mdaH₂/pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of <b>1</b> consists of a central octahedral Fe^III ion held within a nonplanar Fe₆ loop by three μ₃-O^2- and three μ₂-RO^- arms from the three mda^2- chelates. The cores of the cations of <b>2</b> and <b>3</b> consist of an <b>A</b>:<b>B</b>:<b>A</b> three-layer topology, in which a central Fe₆ (<b>2</b>) or Fe₈ (<b>3</b>) layer <b>B</b> is sandwiched between two Fe₈ layers <b>A</b>. The <b>A</b> layers structurally resemble <b>1</b> with the additional Fe added at the center to retain virtual <i>C</i>₃ symmetry. The central Fe₆ layer <b>B</b> of <b>2</b> consists of a {Fe₄(μ₄-O)₂(μ₃-OH)₂}⁶⁺ cubane with an Fe on either side attached to cubane O^2- ions, whereas that of <b>3</b> has the same cubane but with an {Fe₃(μ₃-O)(μ-OH)} unit attached on one side and a single Fe on the other. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling in all complexes leading to ground-state spins of <i>S</i> = ⁵/₂ for <b>1</b> and <i>S</i> = 0 for <b>2</b> and <b>3</b>. All Fe₂ pairwise exchange parameters (<i>J</i>_<i>ij</i>) for <b>1</b>-<b>3</b> were estimated by two independent methods: density functional theory (DFT) calculations using broken symmetry methods and a magnetostructural correlation previously developed for high-nuclearity Fe^III/O complexes. The two approaches gave satisfyingly similar <i>J</i>_<i>ij</i> values, and the latter allowed rationalization of the experimental ground states by identification of the spin frustration effects operative and the resultant relative spin vector alignments at each Fe^III ion.