Heat capacity of a cubane-tetramer, [Ni(OCH3) (SAL) (CH3OH)]4, between 0.4 AND 288 K: spin-spin interactions and tunnel-splitting of the internal rotation of methyl-groups2

The heat capacity of a cubane-tetramer, tetra-μ₃-methoxy-tetrakis salicylaldehydato (methanol) nickel (II)] (abbreviated as Ni(OCH₃) (SAL) (CH₃OH)]₄), has been measured from 0.4 to 288 K. A broad peak centered at 0.85 K was observed. Subtraction of the lattice heat capacity from the observed one brought about another broad peak centered around 13.5 K. The magnetic anomaly is accounted for by a nearest-neighbor intracluster ferromagnetic exchange interaction characterized by $\text{J}\text{k}\text{= 3.8 K}$ and a single-ion zero-field splitting due to biaxial crystalline anisotropy$\text{(}\text{D}\text{K}\text{= 3.8 K}$ and$\text{E}\text{k}\text{= 1.9 K).}$ The tunnel-splitting of the ground vibrational-rotational state is δ = 2.4 J mol^?1, which corresponds to a potential barrier of V₀ = 2.3 kJ mol^?1 when a three-fold symmetry of a methyl-rotator is assumed. Comparison of the heat capacities between the present compound and a similar cubane-tetramer, Ni(OCH₃) (acac) (CH₃OH)]₄, indicates that among eight methyl-groups the four belonging to methoxy-ligands are concerned with the tunnel-splitting at low temperatures.