Switching on the properties of single-molecule magnetism in triangular Manganese(III) complexes

The reaction between oxide-centered, triangular [(Mn3O)-O-III(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, [(Mn3O)-O-III(O2CR)(3)(mpko)(3)](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [Mn-3(III)(mu(3)-O)](7+) triangular core, but with each Mn-2 edge now bridged by an eta(1): eta(1): mu-RCO2- and an eta(1): eta(1): eta(1): mu-mpko(-) group. The tridentate binding mode of the latter causes a buckling of the formerly planar [Mn-3(III)(A(3)-O)](7+) core, resulting in a relative twisting of the three Mn-III octahedra and the central O2- ion now lying similar to 0.3 angstrom 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 (chi ''(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 chi ''(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 center dot 3CH(2)Cl(2) displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below similar to 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 center dot 3CH(2)Cl(2) have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B-4(0) = -3 similar to 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.