Dipole fluctuation and structural phase transition in hydrogen-bonding molecular assemblies of mononuclear CuII complexes with polar fluorobenzoate ligands

A series of mononuclear Cu-II complexes, [Cu-II(4-FBA)(2)(py)(2)(H2O)] (1), [Cu-II(3-FBA)(2)(py)(2)(H2O)] (2), and (Cu-II(3,4-F(2)BA)(2)(py)(2)(H2O)] (3), where 4-FBA = 4-fluorobenzoate, 3-FBA = 3-fluorobenzoate, 3,4-F(2)BA = 3,4-difluorobenzoate, and py = pyridine, respectively, was synthesized and the complexes crystallographically identified. All the Cu-II complex crystals share a one-dimensional O-H center dot center dot center dot O hydrogen-bonding chain substructure, although the mutual alignment of fluorinated benzoate (F(x)BA) ligands exhibits subtle differences among the various compounds, i.e., F(x)BA ligands align in an antiparallel fashion in crystals 1 and 3, while 3-FBA ligands in crystal 2 are interdigitated with a tilt along the a axis. Reversible phase transitions were found upon heating at 170.7, 171.3, and 267.5 K for crystals 1, 2, and 3, respectively; all crystals showed approximately 3% expansion and shrinkage of the intermolecular O-H center dot center dot center dot O hydrogen bond distances associated with the thermally activated orientational fluctuations of the F(x)BA ligands in crystals 1 and 3. The increase in dielectric constant with increasing temperature, at 240 K, activated molecular fluctuation in the 3,4-F(2)BA ligands in crystal 3. Heat capacity measurements indicated that both the expansion and shrinkage of hydrogen bonds, and the molecular fluctuation in 3,4-F(2)BA ligands, contributed to phase transition, and the latter caused dipole fluctuation, resulting in a dielectric anomaly in crystal 3.

Automated summary

A series of mononuclear Cu(II) complexes were synthesized and crystallographically identified. These complexes share a one-dimensional O-H···O hydrogen-bonding chain substructure in their crystal structures, with differences in the alignment of the fluorinated benzoate ligands leading to reversible phase transitions upon heating. The expansion and shrinkage of hydrogen bonds, as well as molecular fluctuations in the fluorinated benzoate ligands, contributed to the phase transition phenomena observed.