pH-controlled crystal growth of copper/gemini surfactant complexes with bipyridine groups

Abstract

The pH-controlled crystal growth of two complexes with different coordination modes, derived from gemini surfactant molecules with a bipyridyl spacer (12Bpy) and metal copper ions (Cu²⁺) is presented in this work. Such crystalline forms obtained in appropriate pH ranges exhibit dissimilar morphologies, colors and crystalline structures. Under weak acidic conditions with a slightly higher pH (>4.3), blue grain crystals, made from dihydroxo-bridged binuclear complexes with a square pyramidal coordination mode, are formed, whereas under slightly stronger acidic conditions (pH <3.8 in this work), green crystals with mononuclear complexes with a distorted trigonal bipyramidal geometry are readily fabricated, and meanwhile the blue crystals are completely inhibited. In particular, these two crystals concomitantly existed in an intermediate pH range of 3.8–4.3. We suggest a fivefold coordinated Cu(II)/12Bpy complex with a 1 : 1 metal–ligand ratio and three hydrated water ligands and a pH-controlled crystal growth mechanism on the basis of UV-vis spectra and density functional theory (DFT) calculations. Our findings indicate that pH adjustment is a straightforward and efficient way to control the crystal growth, having potential application in the preparation of smart and multifunctional materials.