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The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnII3(μ3-O)]7+ triangular core, but with each Mn2 edge now bridged by an η¹/η¹:μ-RCO2- and an η¹:η¹:η¹:μ-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(μ3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying ~0.3 Å above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S= 6 ground states, and gave the following 0 and g values: -0.34 cm-1 and 1.92 for 4, -0.34 cm-1 and 1.93 for 5, and -0.35 cm-1 and 1.99 for 6, where 0 is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (χ′M ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the χ′M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4·3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below ~0.3 K, indicating only ground-state QTM between the lowest- lying Ms = ±6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4·3CH2Cl2 have provided precise spin Hamiltonian parameters, giving 0 = -0.3 cm-1, B40 = -3 · 10-5 cm-1, and g = 2.00. The spectra also suggest a significant transverse anisotropy of E ≥ 0.015 cm-1. The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM. [ABSTRACT FROM AUTHOR]

Reaction of [Fe 3 O(O 2 C t Bu) 6 (H 2 O) 3 ](NO 3) with mdaH 2 (N -methyl-diethanolamine) gave [Fe 7 O 3 (O 2 C t Bu) 9 (mda) 3 (H 2 O) 3 ] containing an FeIII inside a non-planar FeIII 6 loop. Its exchange couplings were obtained using DFT and magnetostructural correlation methods, and used to rationalize the S = 5/ 2 ground state by identifying spin frustration effects, and also as input values to fit the experimental data. [Display omitted] The synthesis, structure, and magnetic properties are reported for [Fe 7 O 3 (O 2 C t Bu) 9 (mda) 3 (H 2 O) 3 ] (1), where mdaH 2 is N -methyldiethanolamine. 1 was prepared from the reaction of [Fe 3 O(O 2 C t Bu) 6 (H 2 O) 3 ](NO 3) with mdaH 2 in a 1:∼3 ratio in MeCN. The core of 1 consists of a central octahedral FeIII ion held within a non-planar Fe 6 loop by three μ 3 -O2- and three μ 2 -RO- arms from the three mda2- chelates. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling, leading to a ground state spin of S = 5/ 2. The ground state was confirmed by a fit of magnetization data collected in the 0.1–7.0 T and 1.8–10.0 K ranges. The four Fe 2 pairwise exchange parameters (J 1 - J 4) were estimated by independent methods: theoretical calculations using either broken symmetry energy differences (−46.3, −16.2, −3.9, and − 28.1 cm−1, respectively) or Green's function approximation methods (−41.4, −14.8, −13.2, and − 24.7 cm−1), and a magnetostructural correlation (MSC) previously developed for high nuclearity FeIII/O complexes (−39.5, −13.8, −6.7, and − 23.5 cm−1). Additionally, the J 1 - J 4 obtained from the MSC and theoretical methods were used with the program PHI to both simulate χ M T vs T as well as to serve as reasonable input values to fit the experimental data (−41.0, −11.4, −5.0, and − 27.3 cm−1). Analysis of the J ij led to identification of the spin frustration effects operative and the resultant spin vector alignments at each FeIII ion, thus allowing for the rationalization of the experimental ground state. [ABSTRACT FROM AUTHOR]

Abstract: We report single crystal high-frequency electron paramagnetic resonance studies of two recently discovered half-integer Fe7 complexes: [Fe7O3(O2CBu t )9(mda)3(H2O)3] (1) and [Fe7O4(O2CPh)11(dmem)2] (2). The obtained spectra confirm spin S =5/2 ground states for both complexes. On the basis of detailed frequency and field orientation dependent studies, we find that complex 1 is a single-molecule magnet (uniaxial zero-field-splitting parameter, D <0) while complex 2 is not (D >0). The EPR linewidth for complex 1 is considerably narrower in comparison to spectra obtained for other single-molecule magnets, suggesting the possibility of reduced decoherence from nuclear spins (56Fe has no nuclear moment). [Copyright &y& Elsevier]