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 Fe4 single-molecule magnets. Herein, we describe the synthesis, crystal and molecular structure and magnetic properties of a novel Fe4 derivatives with R′ = phenyl, obtained by site-specific ligand substitution on [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane). The compound, which has the lowest molecular symmetry among all Fe4 derivatives so far reported (C1), 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 ) × 10 - 5 cm - 1 (from high frequency and X-band EPR). The thermal-activation parameters for magnetic moment reversal are Ueff/kB = 15.7(2) K and τ0 = 3.5(5) × 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 MS = ±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 ∼2.5 × 104 s (∼7 h).