Polyoxometalate-Supported Copper(I)-Pyrazole Complex: Unusual Stability, Geometrical Isomers, Organic Transformation,...

We have described the synthesis and characterization of a polyoxometalate (POM)-supported copper(I)-pyrazole complex, [Cu^I(C₁₅H₁₂N₂)₂] [PW₁₂O₄₀{Cu^I(C₁₅H₁₂N₂)₂}₂]·CH₃OH (1). There are three Cu(I)-pyrazole coordination complexes in compound 1, out of which two are supported by the {PW₁₂O₄₀}^3- Keggin POM by coordinate covalent bonds from the POM surface through oxygen donors to the Cu(I) centers of two Cu(I) complexes and one remains uncoordinated to the POM surface, acting as a cationic complex species in the crystals of 1. The POM-coordinated Cu(I) complexes have a T-shaped geometry, and the uncoordinated Cu(I) complex is a linear one. During the solvothermal synthesis of compound 1, remarkably, the associated 1,5-diphenylpyrazole ligand is formed from cinnamaldehyde phenylhydrazone through oxidative cyclization at the cost of Cu(II) reduction to Cu(I), and then, these two (copper(I) and pyrazole ligand) form the coordination complex. Compound 1 undergoes desolvation on heating the single crystals of compound 1 at 55 °C in the aerial atmosphere with the formation of the desolvated compound [Cu^I(C₁₅H₁₂N₂)₂][PW₁₂O₄₀{Cu^I(C₁₅H₁₂N₂)₂}₂] (2). Interestingly, when an aqueous suspension of compound 1 is bubbled with O₂ gas at room temperature, it undergoes solid-to-solid transformation, resulting in the formation of the compound [Cu^I(C₁₅H₁₂N₂)₂]₃[PW₁₂O₄₀] (3). Compounds 1, 2, and 3 have been characterized by routine spectral analyses (including cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) studies) and unambiguously by single-crystal X-ray crystallography. We have performed density functional theory (DFT) calculations on compound 1 to understand the rationale of its unusual stability toward oxidation.