Synthesis, Structural, Magnetic, Optical, and Electronic Studies of a Novel Honeycomb Kagome Polyoxometalate-Based Copper(II) Complex

Abstract

A novel hybrid pentaphosphomolybdate copper complex C_7.5H_36.3Cu_1.3Mo₅N₃O_31.4P₂, abbreviated as (H₂MP)_1.5[Cu_1.3P₂Mo₅], featuring a honeycomb Kagome (HK) lattice, is synthesized in aqueous solution. Single-crystal X-ray diffraction analysis reveals structural defects associated with the nonstoichiometric occupancy of copper ions and water molecules. The compound crystallizes in the monoclinic space group P2₁/c, with the asymmetric unit comprising two distinct Cu(II) ions (one occupying the Wyckoff position 4e and the other 2d, with an occupancy ratio of ≈60%), along with one and a half 2-methylpiperazinium cations, a unique Strandberg-type [P₂Mo₅O₂₃]⁵⁻ anion, 4.2 coordinated water molecules, and 3.48 lattice water molecules. The negative Curie–Weiss temperature, derived from the high temperature magnetic susceptibility data, indicates the presence of antiferromagnetic interaction between Cu²⁺ moments. Additionally, the observed power law behavior and magnetization data collapse are indicative of a random singlet state, suggesting the presence of a quantum spin liquid ground state. Electronic structure calculations show that the conduction band is primarily derived from Mo(4d) states while the valence band arises from a strong hybridization between N(2p), Cu(3d), and O(2p) orbitals. The experimental bandgap, estimated using Kubelka–Munk theory, reveals three prominent optical transitions in the ultraviolet-visible region at 2.6, 2.96, and 3.24 eV.