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

Abstract

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 [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A 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 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 (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 approximately 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 D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 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.

Crystal structure of bis(acetato-κO)bis(pyridine-2-carboxamide oxime-κ2N,N′)cadmium ethanol disolvate

“…H, Me, Ph etc. ), are remarkable sources of homo-and heterometallic complexes with novel structures and interesting physical properties (Miyasaka et al, 2003;Stamatatos et al, 2007). A logical extension of such studies is the investigation of the coordination chemistry of analogous organic molecules in which the non-donor R group is replaced by a donor group such as pyridine, cyano etc.…”

Relieving frustration: The case of antiferromagnetic Mn3molecular triangles

“…Thus, angledependent EPR measurements were first performed so that the crystal could be aligned in situ (see [13] for details) for temperature and frequency-dependent measurements with the DC magnetic field applied approximately parallel to the average easy-axis direction, i.e., ~16° away from the local easy-axis (z-axis) of each molecule. As will be seen below, better sample alignment was achieved in the high-field magnet.…”

“…The triangular [Mn 3 -(μ 3 -oxo)] motif seen at the core of the molecule in Fig. 1 has been extensively studied in this regard [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. For a long time, it was believed that this topology would result exclusively in antiferromagnetic (AF) coupling and, hence, to a low-spin ground state.…”

Review. Solvothermal Processes: Definition, Key Factors Governing the Involved Chemical Reactions and New Trends

“…To prepare hybrid materials, the comparison of conventional heating and microwave heating has shown that the crystalline quality and the yield of the product are obviously better with a shortened reaction time [63].…”