Synthesis and characterisation of a Ni4 single-molecule magnet with S4 symmetry

Abstract

[Ni₄Cl₄(HL)₄] (1) {H₂LHN(CH₂CH₂OH)₂} has S₄ symmetry and crystallises in the tetragonal space groupI4₁/a. Two exchange couplings are observed between the four Ni(II) centres, with J1 = 7.29 cm⁻¹ and J2 = −2.08 cm⁻¹, leading to an S = 4 ground state. The Ni₄ complex shows the onset of frequency dependent signals in the out-of-phase ac susceptibility below 3 K. In single-crystal measurements carried out using a micro-SQUID, hysteresis loops are observed below 0.5 K, confirming that 1 shows slow relaxation of magnetisation. The loops are temperature dependent but only weakly sweep rate dependent due to the presence of small intermolecular interactions, which hinder quantum tunnelling. This exchange bias between Ni₄ molecules is also seen in high-frequency high-field EPR measurements, which give the parameters D = −0.75 cm⁻¹, B₄° = −6.7 × 10⁻⁵ cm⁻¹ and g_z = 2.275.