Reversible Humidity-Induced Vapochromism of a Cu(II) Tetramine Complex Driven by Iodide Access

Vapochromic metal complexes have attracted considerable interest as functional materials for sensing and chromic applications. Herein, we report a humidity-responsive Cu(II) vapochromic complex based on reversible iodide coordination. A series of Cu(II) tetramine complexes with 1R,2R-cyclohexanediamine (chxn), including [Cu^II(chxn)₂(H₂O)]Br₂ (1) and [Cu^II(chxn)₂(H₂O)₂]I₂ (2), were synthesized and characterized by single-crystal x-ray diffraction. Complex 2 adopts a discrete hydrated structure, unlike the previously reported chain-type [Cu^II(chxn)₂I]I. Whereas the chloride and bromide analogues exhibited little or no color change upon humidity variation, complex 2 displayed rapid and reversible vapochromism, changing from bluish purple to bluish green under low humidity or vacuum. Powder x-ray diffraction and IR spectroscopy revealed reversible structural changes associated with water adsorption and desorption. UV-Vis spectroscopy showed a pronounced red shift of the d-d transition band upon dehydration. Electron spin resonance measurements combined with density functional theory calculations indicate that dehydration promotes axial coordination of iodide ions to Cu(II), yielding a [Cu^II(chxn)₂I₂] species in the solid state. These findings demonstrate a new vapochromic mechanism based on reversible iodide coordination at Cu(II) centers, providing a promising strategy for designing humidity-responsive chromic materials.