A vanadium (VO²⁺ ) metal–organic framework: selective vapor adsorption, magnetic properties, and use as a precursor for a polyoxovanadate

Abstract

The reaction of VOSO₄ with 2-carboxyethylphosphonic acid (H₂-CEP) in presence of piperazine (PIP) produces a 3D inorganic–organic hybrid framework, {(H₂PIP)_0.5[VO(CEP)]·H₂O} (1) with bidirectional channels occupied by the H₂PIP cations and H₂O molecules. The PO₃^2– unit of CEP connects three V^IV centers to generate a 1D ladder, which is further linked to four such ladders by the CEP linkers to form a 3D hybrid framework. The dehydrated framework, {(H₂PIP)_0.5[VO(CEP)]} (1′) shows selective and gated adsorption behavior with H₂O but not with methanol and ethanol. Very interestingly, when 1 is treated with an aqueous solution of LiNO₃/NaNO₃, the framework breaks down and results in a new polyoxovanadate (POV) cluster, [H₅(H₂PIP)₃][V^V₁₂V^IV₂O₃₈(PO₄)]·8H₂O (2) at pH ≈ 2.1. The cluster has been characterized by single-crystal X-ray diffraction, 31P NMR, EPR, and magnetic studies. The temperature-dependent magnetic susceptibility measurement suggests antiferromagnetic ordering in 1 with TN ≈ 3.8 K.