A Vanadium (VO2+) Metal–Organic Framework: Selective Vapor Adsorption, Magnetic Properties, and Use as a Precursor for a Polyoxovanadate

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 PO32– unit of CEP connects three VIV 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][VV12VIV2O38(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.