The diversity of the self-assembled structures of a Cu complex via hydrogen bonding, including a square planar unit, is reported. The Cu complex forms two self-assembled structures by hydrogen bonding, depending on the acidity of the recrystallization conditions. A linear chain structure can be produced under acidic conditions, and a three-dimensional network structure is observed under basic conditions. The linear chain structure is constructed from intermolecular sharing of a hydrogen atom between two 2-pyridinemethanolate units, with an O⋯O distance of 2.412(1) Å and an O–H–O bond angle of 170(3)°. The linear chain structure exhibits relatively strong magnetic coupling (J = −9.91(2) cm−1) via hydrogen bonding between Cu atoms; this coupling was also confirmed by electron spin resonance experiments. Thermochromic behavior was also observed for the complex.

•A new six-coordinated Ni complex was synthesised and characterised.•The complex showed thermochromism in the solid state.•The complex showed structural change depends on the temperature.•The thermochromism was assessed by absorption spectrum and TD-DFT calculation.•Structural change at various temperatures would cause the thermochromism.The Ni(II) complex [Ni(diethylenetriamine)(2,6-bis(hydroxymethyl)pyridine)]·PF6·CF3COO·H2O is described here. A single crystal X-ray crystallographic study revealed that the Ni(II) in the complex has a six-coordinated octahedral structure. The complex shows thermochromic behaviour in the solid state. X-ray crystallographic studies at various temperatures suggested that the thermochromism is caused by anisotropic structural change. The relationship between the structural change and the colour change was confirmed by absorption spectrum and TD-DFT calculation.Thermochromism in a six-coordinated Ni complex was assessed by comparing structural changes and colour changes at various temperatures.

The diversity of the self-assembled structures of a Cu complex via hydrogen bonding, including a square planar unit, is reported. The Cu complex forms two self-assembled structures by hydrogen bonding, depending on the acidity of the recrystallization conditions. A linear chain structure can be produced under acidic conditions, and a three-dimensional network structure is observed under basic conditions. The linear chain structure is constructed from intermolecular sharing of a hydrogen atom between two 2-pyridinemethanolate units, with an O⋯O distance of 2.412(1) Å and an O–H–O bond angle of 170(3)°. The linear chain structure exhibits relatively strong magnetic coupling (J = −9.91(2) cm−1) via hydrogen bonding between Cu atoms; this coupling was also confirmed by electron spin resonance experiments. Thermochromic behavior was also observed for the complex.