Water-soluble crystalline germanium complex with lower critical solution temperature transition

Compounds with lower critical solution temperatures reported to date exhibit liquid- or gel-phase transitions. Although crystalline phases with lower critical solution temperatures have potential applicability in various domains, such phases have not yet been reported. Here, we report the synthesis of a novel crystalline germanium complex (C15H30N2O3Ge) with an unusual solubility behavior in water it precipitates when heated and dissolves upon cooling. This complex (0.4 M in water) is soluble below a lower critical solution temperature of 50 °C, but becomes insoluble, forming rod-shaped crystals as the temperature increases. To the best of the authors' knowledge, this is the first report of the synthesis of a crystalline complex with a lower critical solution temperature. X-ray structural analysis revealed the presence of a β-amino alcohol ligand, viz., 1,1′-[(2-hydroxy-2-methylpropylamino)propylazanediyl]bis(2-methylpropan-2-ol), and germanium dioxide in a 1:1 ratio in this synthesized compound. To elucidate the temperature-responsive behavior, temperature- and concentration-dependent nuclear magnetic resonance spectroscopy analyses were performed. A self-assembly mechanism is proposed based on these data.