Redox and guest tunable spin-crossover properties in a polymeric polyoxometalate

Abstract

A bifunctionalized polyoxometalate (POM), [V₆O₁₉(C₁₆H₁₅N₆O)₂]²⁻, which contains a redox active hexavanadate moiety covalently linked to two tridentate 2,6-bis(pyrazol-1-yl)pyridine (1-bpp) ligands, has been prepared and characterized. Reaction of this hybrid molecule with Fe(II) or Zn(II) ions produces crystalline neutral 1D networks of formula Fe[V₆O₁₉(C₁₆H₁₅N₆O)₂]·solv (2) and Zn[V₆O₁₉(C₁₆H₁₅N₆O)₂]·solv (3) (solv = solvent molecules). Magnetic properties of 2 show an abrupt spin-crossover (SCO) with the temperature, which can be induced by light irradiation at 10 K (Light-Induced Excited Spin-State Trapping, LIESST effect). Interestingly, this porous and flexible structure enables reversible exchange of solvents in 2, which allows tuning the temperature of the thermal SCO. In 2 and 3, the hexavanadate unit can be reduced by electrochemical or chemical means in a reversible way. Chemical reduction and reoxidation by a postsynthetic method is accompanied by the insertion in the structure of the reductant and oxidant molecules (cobaltocene and tribromide, respectively), which provokes drastic changes in the spin state of Fe(II). This leads to an elegant switching multifunctional material in which SCO properties of the Fe(II) complexes coexist with the redox properties of the POM and can be tuned by a variety of stimuli such as temperature, light, solvent exchange or electron transfer. During the reduction process, 3 undergoes a single-crystal-to-single-crystal one-electron reduction, which confirms the presence of cobaltocenium cations by single crystal X-ray diffraction.