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) 22H2V10O28·2H2O (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.