Targeted construction of azido-bridged Ni-4 complexes with decisive effect of mu-1,3-azide torsion on the spin ground state

Abstract

Highly preorganized pyrazolate-based dinickel(II) systems are shown to constitute suitable building blocks for the targeted assembly of azido-bridged Ni-4 complexes with rectangular arrangement of the metal ions. A set of such complexes has been prepared and structurally characterized. mu(-1), 1-Azide binding within the bimetallic sub-units is controlled by the chosen topology of the pyrazolate ligand scaffold and gives rise to the anticipated ferromagnetic intradimer coupling. Overall magnetic properties of the Ni-4 array, however, are mainly determined by the Ni-NNN-Ni torsion of the interdimer mu-1,3-azido linkages. According to the crystallographic results, these torsion angles vary over a wide range, and partial disorder of the mu-1,3-azide bridge in one of the compounds indicates high structural flexibility even in the solid state. Two of the compounds represent rare examples of molecular complexes with a Ni-NNN-Ni torsion angle of almost exactly 90 degrees. The resulting magnetic ground state (neglecting zero-field splitting) is either S = 0 or S = 4 depending on the Ni-NNN-Ni torsion, and in one case a drastic change is observed upon extrusion of lattice solvent.