Slow quantum relaxation in a tetrairon(III) single-molecule magnet

Monoethers of pentaerythritol, R'O-CH2C(CH2OH)(3), are convenient site- specific ligands for the design and preparation of functionalized Fe-4 single-molecule magnets. Herein, we describe the synthesis, crystal and molecular structure and magnetic properties of a novel Fe-4 derivatives with R' = phenyl, obtained by site-specific ligand substitution on [Fe-4(OMe)(6)(dpm)(6)] (Hdpm = dipivaloylmethane). The compound, which has the lowest molecular symmetry among all Fe-4 derivatives so far reported (C-1), retains the same S = 5 ground spin state as the parent compound, but show an enhanced easy-axis anisotropy with D = -0.433(2) cm (1), E = 0.014(2) cm (1) and B-4(0) = 1.5(1) x 10(-5) cm(-1) (from high frequency and X-band EPR). The thermal-activation parameters for magnetic moment reversal are U-eff/k(B) = 15.7(2) K and tau(0) = 3.5(5) x 10 (8) s (from AC susceptometry). Micro-SQUID measurements on single crystals show that below about 0.2 K the spin dynamics is dominated by quantum tunneling within the M-S = +/- 5 ground doublet. In spite of the low molecular symmetry, which is generally believed to enhance tunneling effects, the relaxation time in the purely quantum regime is as long as similar to 2.5 x 10(4) s (similar to 7 h)