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

## TL;DR

This paper explores new ways decavanadate interacts with metal complexes and shows how these structures can help remove pollutants from water.

## Contribution

The study reveals a new coordination mode for decavanadate and demonstrates its application in dye adsorption.

## Key findings

- Structures 2 and 3 show a new coordination mode for decavanadate involving triply bridging oxygen.
- π-holes in {Cu(cyclen)}²⁺ and {Zn(cyclen)}²⁺ contribute to structural stability through noncovalent interactions.
- Compound 2 shows the highest methylene blue bleaching efficiency due to surface charge and particle size.

## Abstract

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.