Modelling the Magnetic Behaviour of Square-Pyramidal Co-5(II) Aggregates: Tuning SMM Behaviour through Variations in the Ligand Shell

Three new mu(4)-bridged Co-5(II) clusters with similar core motifs have been synthesised with the use of N-tert-butyldiethanolamine (tbdeaH(2)) and pivalic acid (piv): [Co-5(II)(mu(4)-N-3)(tbdea)(2)-(mu-piv)(4)(piv)(CH3CN)(2)]center dot CH3CN (1), [Co-5(II)(mu(4)-Cl)(Cl)(tbdea)(2)(mu-piv)(4)-(pivH)(2)] (2) and [Co-5(II)(mu(4)-N-3)(Cl)-(tbdea)(2)(mu-piv)(4)(pivH)(2)] (3). Magnetic measurements were performed for all three compounds. It was found that while the chloride-bridged cluster 2 does not show all out-of-phase signal, which excludes single-molecule magnet (SMM) behaviour, the azide-bridged compounds 1 and 3 show out-of-phase signals as well as frequency dependence of the ac susceptibility, as expected for SMMs. We confirmed that 1 is a SMM with zero-field quantum tunnelling of the magnetisation at 1.8 K. Compound 3 is likely a SMM with a blocking temperature well below 1.8 K. We established a physical model to fit the chi T versus T and M versus B curves of the three compounds to reproduce the observed SMM trend. The analysis showed that small changes in the ligand shell modify not only the magnitude of exchange constants, but also affect the J and g matrices in a non-trivial way.