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

Abstract

Monoethers of pentaerythritol, R′O–CH 2C(CH 2OH) 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] (H dpm = 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 ) × 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 τ 0 = 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 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 ∼2.5 × 10 4 s (∼7 h).