Guest-adjusted encapsulation and thermal studies of non-porous mononuclear Cu(ii) coordination complexes through electrostatic interactions induced by fluorine substitution

Abstract

Flexible host frameworks of mononuclear coordination complexes were designed and encapsulated for several organic guest molecules through electrostatic interactions in their crystals, in which the unique guest recognitions and the uniform cavities were induced by fluorination of the coordination complexes. These unusual pseudopolymorphic cocrystal states of the fully fluorinated coordination complex, bis[bis(2,3,4,5,6-pentafluorobenzoyl)methanido]copper(II) (1), achieved several guest encapsulations, e.g., three benzenes; four toluenes; one, two, and four p-xylenes; one and two m-xylenes; one o-xylene; two mesitylenes; one durene; one and two anisoles; and two dimethoxybenzenes. Based on X-ray crystallographic studies of these crystals, the axial positions on the metal and the surrounding spaces by the pentafluorophenyl groups produced the flexible cavity, and the guest molecules were recognized by the cooperative effects of metal⋯π and arene–perfluoroarene interactions. However, the guest recognition of partially fluorinated coordination complexes, in which the pentafluorophenyl groups of 1 were replaced by 2,3,5,6-tetrafluorophenyl (2), 2,4,6-trifluorophenyl (3a), 2,6-difluorophenyl (4a), and 4-fluorophenyl (5) groups, clearly depended on the fluorination numbers, e.g., the number of encapsulated benzene molecules was three (1) > two (2 and 3a) > zero (4a and 5). No encapsulation was observed with any guest molecules for 4a, although cavity spaces similar to 1, 2, and 3a were present around the axial position on the metal as the result of steric hindrance of the ortho-substituted fluorines. Thermal studies of the corresponding guest releases also indicated that the number of fluorinations amplified the host–guest interactions. The cocrystals exhibit reversible guest encapsulation and release processes with the same and different guest molecules, which can be clearly monitored by color change and thermogravimetric observations. We now report the synthesis of fully and partially fluorinated β-diketonato Cu²⁺ complexes and their unique guest encapsulation phenomena depending on the position and the number of fluorine substitutions using X-ray crystallography and thermal analysis. These results prove that simple mononuclear complexes become host materials due to fluorination effects.