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 prepn. of functionalized Fe4 single-mol. magnets. Herein, we describe the synthesis, crystal and mol. structure and magnetic properties of a novel Fe4 deriv. with R' = Ph, obtained by site-specific ligand substitution on [Fe4(OMe)6(dpm)6] (Hdpm = dipivaloylmethane). The compd., which has the lowest mol. symmetry among all Fe4 derivs. so far reported (C1), retains the same S = 5 ground spin state as the parent compd., but shows an enhanced easy-axis anisotropy with D = -0.433(2) cm-1, E = 0.014(2) cm-1 and B40 = +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 mol. symmetry, which is generally believed to enhance tunneling effects, the relaxation time in the purely quantum regime is as long as ∼2.5×10^4 s (∼7 h).