Structural and Theoretical Study of Copper(II)-5-fluoro Uracil Acetate Coordination Compounds: Single-Crystal to Single-Crystal Transformation as Possible Humidity Sensor

This paper describes the synthesis and characterization of seven different copper(II) coordination compounds, as well as the formation of a protonated ligand involving all compounds from the same reaction. Their synthesis required hydrothermal conditions, causing the partial in situ transformation of 5-fluoro uracil-1-acetic acid (5-FUA) into an oxalate ion (ox), as well as the protonation of the 4,4′-bipyridine (bipy) ligand through a catalytic process resulting from the presence of Cu(II) within the reaction. These initial conditions allowed obtaining the new coordination compounds [Cu(2)(5-FUA)(2)(ox)(bipy)](n)·2n H(2)O (CP2), [Cu(5-FUA)(2)(H(2)O)(bipy)](n)·2n H(2)O (CP3), as well as the ionic pair [(H(2)bipy)(+2) 2NO(3)(−)] (1). The mother liquor evolved rapidly at room temperature and atmospheric pressure, due to the change in concentration of the initial reagents and the presence of the new chemical species generated in the reaction process, yielding CPs [Cu(5-FUA)(2)(bipy)](n)·3.5n H(2)O, [Cu(3)(ox)(3)(bipy)(4)](n) and [Cu(ox)(bipy)](n). The molecular compound [Cu(5-FUA)(2)(H(2)O)(4)]·4H(2)O (more thermodynamically stable) ended up in the mother liquor after filtration at longer reaction times at 25 °C and 1 atm., cohabiting in the medium with the other crystalline solids in different proportions. In addition, the evaporation of H(2)O caused the single-crystal to single-crystal transformation (SCSC) of [Cu(5-FUA)(2)(H(2)O)(bipy)](n)·2n H(2)O (CP3) into [Cu(5-FUA)(2)(bipy)](n)·2n H(2)O (CP4). A theoretical study was performed to analyze the thermodynamic stability of the phases. The observed SCSC transformation also involved a perceptible color change, highlighting this compound as a possible water sensor.