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.