Expanding the Coordination Chemistry of Decavanadate through π-Hole Interactions with Transition-Metal Cyclen Complexes: Electronic Features and Dye Adsorption.

The hybrid decavanadate with macrocycle cyclen (1,4,7,10-tetraazacyclododecane) complexes with the formula [Ni(cyclen)(H₂O)₂]₂[H₂V₁₀O₂₈]·2H₂O (1) was prepared as an ionic pair, while [{Cu(cyclen)}₂(H₂V₁₀O₂₈)]·9H₂O (2) and [{Zn(cyclen)}₃(V₁₀O₂₈)]·4H₂O (3) were obtained as discrete molecular entities. The structures of 2 and 3 revealed a new coordination mode for the decavanadate anion, involving a triply bridging oxygen (-μ₃-OB) and {TM(cyclen)}²⁺, where TM = Cu(II) or Zn(II). Electrostatic Surface Potential analysis showed pronounced π-holes in the {Cu(cyclen)}²⁺ and {Zn(cyclen)}²⁺ fragments, whereas {Ni(cyclen)}²⁺ lacks this feature due to stronger metal-cyclen σ-bonding. The Independent Gradient Model based on Hirshfeld partition analysis of {TM(cyclen)}/decavanadate interfaces demonstrated that intramolecular noncovalent interactions play a key role in structural stability. The low Intrinsic Bond Strength Index for the Cu-OB bond (0.087) suggests a weak, semicoordinative interaction with approximately half the strength of the Zn-OB coordination bond (0.169). The adsorption of methylene blue was characterized as a surface phenomenon. The bleaching efficiency, 2 > 1 > 3, was determined by the distribution of their asymmetric surface charge and particle size. These hybrid compounds provide a valuable model for understanding and advancing decavanadate coordination chemistry and for the application of polyoxometalates to remove prevalent contaminants from wastewater.