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

A novel hybrid pentaphosphomolybdate copper complex C 7.5 H 36.3 Cu 1.3 Mo 5 N 3 O 31.4 P 2 , abbreviated as (H 2 MP) 1.5 [Cu 1.3 P 2 Mo 5 ], 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 P 2 1 /c, with the asymmetric unit comprising two distinct Cu(II) ions (one occupying the Wyckoff position 4 e and the other 2 d , with an occupancy ratio of ≈60%), along with one and a half 2‐methylpiperazinium cations, a unique Strandberg‐type [P 2 Mo 5 O 23 ] 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 Cu 2+ 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(4 d ) states while the valence band arises from a strong hybridization between N(2 p ), Cu(3 d ), and O(2 p ) 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.