Magnetic Properties of High-Nuclearity Fe x -oxo ( x = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach.

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [Fe 7 O 3 (O 2 CPh) 9 (mda) 3 (H 2 O)] ( 1 ), [Fe 22 O 14 (OH) 3 (O 2 CMe) 21 (mda) 6 ](ClO 4 ) 2 ( 2 ), and [Fe 24 O 15 (OH) 4 (OEt)(O 2 CMe) 21 (mda) 7 ](ClO 4 ) 2 ( 3 ), where mdaH 2 is N -methyldiethanolamine. 1 was prepared from the reaction of [Fe 3 O(O 2 CPh) 6 (H 2 O) 3 ](NO 3 ) with mdaH 2 in a 1:2 ratio in MeCN, whereas 2 and 3 were prepared from the reaction of FeCl 3 /NaO 2 CMe/mdaH 2 in a 2:∼13:2 ratio and FeCl 3 /NaO 2 CMe/mdaH 2 /pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of 1 consists of a central octahedral Fe III ion held within a nonplanar Fe 6 loop by three μ 3 -O 2- and three μ 2 -RO - arms from the three mda 2- chelates. The cores of the cations of 2 and 3 consist of an A : B : A three-layer topology, in which a central Fe 6 ( 2 ) or Fe 8 ( 3 ) layer B is sandwiched between two Fe 8 layers A . The A layers structurally resemble 1 with the additional Fe added at the center to retain virtual C 3 symmetry. The central Fe 6 layer B of 2 consists of a {Fe 4 (μ 4 -O) 2 (μ 3 -OH) 2 } 6+ cubane with an Fe on either side attached to cubane O 2- ions, whereas that of 3 has the same cubane but with an {Fe 3 (μ 3 -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 S = 5 / 2 for 1 and S = 0 for 2 and 3 . All Fe 2 pairwise exchange parameters ( J ij ) for 1 - 3 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 J ij 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.