Reversible Solvatomagnetic Effect in Novel Tetranuclear Cubane-Type Ni4 Complexes and Magnetostructural Correlations for the [Ni43‑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 (ST = 4) to diamagnetic (ST = 0). Likewise the (irreversible) loss of lattice solvent in [Ni4L4(H2O)4]·4C3H6O (2·4C3H6O) to give 2·2C3H6O changes the ground state from ST = 4 to ST = 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 {Ni43-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.