A Mixed-Valence Polyoxovanadate(III,IV) Cluster with a Calixarene Cap Exhibiting Ferromagnetic V(III)−V(IV) Interactions

A series of compounds (cat)[V6O6(OCH3)8(calix)(CH3OH)] was obtained under anaerobic
conditions and solvothermal reaction of VOSO4 with p-tert-butylcalix[4]arene (calix) in methanol using
different types of bases (Et4NOH, NH4OH, pyridine, Et3N). All compounds contain the same polyoxo(alkoxo)hexavanadate anion [V6O6(OCH3)8(calix)(CH3OH)]- (1) exhibiting a mixed valence {VIIIVIV5O19} core with
the so-called Lindqvist structure coordinated to a calix[4]arene macrocycle and cocrystallizing with the
conjugated acid of the base (cat = Et4N+, NH4+, pyridinium, Et3NH+) involved in the synthesis process.
The structures have been fully established from X-ray diffraction on single crystals and the mixed valence
state has been confirmed by bond valence sum calculations. The magnetic behavior of all compounds are
the same because of the polyalkoxohexavanadate anion [V6O6(OCH3)8(calix)(CH3OH)]- (1) and have been
interpreted by DFT calculations. Thus the V(III)···V(IV) interactions are found to be weakly ferromagnetic
(<5.5 cm-1) while the V(IV)···V(IV) are antiferromagnetic (−17.6; −67.6 cm-1). The set of the coupling
exchange parameters allows a good agreement with the magnetic experimental data.