Reversible Solvatomagnetic Effect in Novel Tetranuclear Cubane-Type Ni₄ Complexes and Magnetostructural Correlations for the [Ni₄(μ₃‑O)₄] Core

A new family of tetranuclear nickel cube complexes [Ni₄L₄(solv)₄] (1, solv = MeOH; 2, solv = H₂O; H₂L = pyrazole-based tridentate {ONO} ligand) has been studied in detail, in particular by X-ray diffraction and superconducting quantum interference device (SQUID) magnetometry. Different solvates 1·H₂O, 2·4C₃H₆O, 2·CH₂Cl₂, and 2·H₂O were obtained in crystalline form. Only small structural variations were found for the Ni–O–Ni angles of the [Ni₄O₄] cores of those compounds, but these slight variations have dramatic consequences for the magnetic properties. [Ni₄L₄(MeOH)₄]·H₂O (1·H₂O) and [Ni₄L₄(H₂O)₄]·H₂O (2·H₂O) can be reversibly interconverted in the solid state by exposure to the respective solvent, MeOH or H₂O, and this goes along with a switching of the spin ground state from magnetic (S_T = 4) to diamagnetic (S_T = 0). Likewise the (irreversible) loss of lattice solvent in [Ni₄L₄(H₂O)₄]·4C₃H₆O (2·4C₃H₆O) to give 2·2C₃H₆O changes the ground state from S_T = 4 to S_T = 0. In view of these dramatic solvatomagnetic effects for the present [Ni₄L₄(solv)₄] complexes, which occur upon extrusion of lattice solvent or facile exchange of coordinated solvent molecules while keeping the robust [Ni₄O₄] core intact, a note of care is issued: whenever magnetic data are obtained for powdered material or for crystals that easily loose lattice solvent molecules, the magnetic properties may not necessarily reflect the situation observed in the corresponding single crystal diffraction study. Finally, a thorough analysis of the present series of complexes as well as other {Ni₄(μ₃-OR)₄} cubes reported in the literature confirms that a correlation between the (Ni–O–Ni)_av bond angle and J in [Ni₄O₄] cubane complexes does indeed exist.