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Coordination Geometry and Structure-Property Relationships in Alkaline-Earth β‑Diketonate Complexes with
José Manuel Bravo-Arredondo1, Anayeli Carrasco-Ruiz2, José Andrés Reyes-Avendaño3
1Benemérita Universidad Autónoma de Puebla, Instituto de Ciencias, Centro de Investigación en Dispositivos Semiconductores, Edificio 105 C, Boulevard 14 Sur y Av. San Claudio, Col. San Manuel, Puebla, Puebla C. P. 72570, México.
Abstract:
A series of alkaline-earth β-diketonate complexes derived from 2-thenoyltrifluoroacetone (TTA) and 2-methylimidazole was synthesized to study the influence of the metal ion (Mg2+, Ca2+, Sr2+, Ba2+) on coordination geometry, lattice organization, and thermal behavior. Single-crystal X-ray diffraction reveals that Ca-TTA crystallizes as a seven-coordinate [Ca-(TTA)3(H2O)]- unit with capped trigonal prismatic CaO7 geometry, whereas Sr- and Ba-TTA form eight-coordinate [M-(TTA)4]2 - complexes with distorted square-prismatic MO8 environments. The systematic increase in average M-O bond lengths (Ca-O 2.38 Å; Ba-O 2.74 Å) and unit-cell volume reflects expansion of the coordination sphere with increasing ionic radius. In contrast, Mg-TTA yields only microcrystalline material unsuitable for full structural resolution; spectroscopic and powder diffraction data suggest formation of a magnesium β-diketonate framework but without definitive crystallographic assignment. Infrared spectroscopy confirms deprotonation of TTA and consistent bidentate coordination across the series. A pronounced O-H stretching band is observed for Mg- and Ca-TTA, consistent with coordinated water identified crystallographically for Ca-TTA, while N-H bands in the 3150-3100 cm-1 region confirm incorporation of protonated 2-methylimidazolium counterions in all complexes. Thermogravimetric analysis reveals metal-dependent decomposition behavior, where Ca- and Sr-TTA exhibit staged degradation between 180 and 300 °C, Ba-TTA displays more clearly resolved stepwise decomposition at elevated temperatures, and Mg-TTA undergoes substantial early mass loss below 110 °C, indicating the removal of weakly bound, labile components or occluded species. Final residues (23-26%) are consistent with formation of metal-rich inorganic phases, likely oxides and/or carbonates. These results demonstrate how variation in alkaline-earth ionic radius modulates coordination number, lattice stabilization, crystallinity, and thermal response in TTA-based complexes.
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