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

• Published in: Journal of the American Chemical Society 2007-08, Vol.129 (30), p.9484-9499
• Main Authors: Stamatatos, Theocharis C, Foguet-Albiol, Dolos, Lee, Sheng-Chiang, Stoumpos, Constantinos C, Raptopoulou, Catherine P, Terzis, Aris, Wernsdorfer, Wolfgang, Hill, Stephen O, Perlepes, Spyros P, Christou, George
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja072194p

The reaction between oxide-centered, triangular [MnIII 3O(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, [MnIII 3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1−3, the cations of 4−6 contain an [MnIII 3(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 [MnIII 3(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 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 4·3CH2Cl2 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 M s = ±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, B 4 0 = −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.