Coordination Geometry and Structure–Property Relationships in Alkaline-Earth β-Diketonate Complexes with 2-Methylimidazole

High Resolution Image Download MS PowerPoint Slide 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 (Mg 2+, Ca 2+, Sr 2+, Ba 2+ ) 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 (H 2 O)] − unit with capped trigonal prismatic CaO 7 geometry, whereas Sr– and Ba–TTA form eight-coordinate [M(TTA) 4 ] 2 – complexes with distorted square-prismatic MO 8 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.