'Switching On' the Properties of Single-Molecule Magnetism in Triangular Manganese(III) Complexes

摘要

The reaction between oxide-centered, triangular [Mn~(III)_3O(O_2CR)_6(py)_3](ClO_4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [Mn~(III)_3O(O_2CR)_3(mpko)3](ClO_4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [Mn~(III)_3(μ_3-O)]~(7+) triangular core, but with each Mn_2 edge now bridged by an η~1:η~1:μ-RCO_2~- and an η~1:η~1:η~1:μ-mpko~- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [Mn~(III)_3(μ_3-O)]~(7+) core, resulting in a relative twisting of the three Mn~(III) octahedra and the central O~(2-) ion now lying ～0.3 A above the Mn_3 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 D 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 D 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 (U_(eff)) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4·3CH_2Cl_2 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 M_s = ±6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4·3CH_2Cl_2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm~(-1), B_4~0 = -3 x 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.