The Imidazole Role in Strontium β-Diketonate Complexes Formation

A selection of new strontium β-diketonate derivatives (imH2)2[Sr2(β-dike)6] [where imH = imidazole and β-dike = tfac (tfacH = 1,1,1-trifluoro-2,4-pentanedione), tfbz (tfbzH = 1,1,1-trifluoro-4-phenyl-2,4-butanedione), or hfac (hfacH = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione)], [Sr2(tfac)4(Meim)2(H2O)2], (MeimH)2[Sr(β-dike)4] (where Meim = 1-methylimidazole and β-dike = tfbz or hfac), [Sr2(thd)4(imH)2(EtOH)], and [Sr2(thd)4(Meim)2(EtOH)] (where thdH = 2,2,6,6-tetramethyl-3,5-heptanedione) have been synthesized and fully characterized. (imH2)2[Sr2(β-dike)6] and (MeimH)2[Sr(β-dike)4] are di- and mononuclear Sr anionic complexes, respectively, while [Sr2(tfac)4(Meim)2(H2O)2], [Sr2(thd)4(imH)2(EtOH)], and [Sr2(thd)4(Meim)2(EtOH)] are neutral dinuclear molecular derivatives. The derivative (imH2)2[Sr2(hfac)6] slowly decomposes in solution under aerobic conditions, giving (imH2)2[Sr(H2O)2(tfa)3](tfa) (tfaH = trifluoroacetic acid), which is an ionic compound containing polynuclear anionic chains composed of Sr(H2O)2(tfa)3 units. When a deficiency of imH is employed, the thdH proligand forms not only the dinuclear derivative [Sr2(thd)4(imH)2(EtOH)] but also an additional product with the formula [Sr(thd)2(H2O)2(EtOH)], in which the Sr atom is seven-coordinated. A complete solid-state characterization has been accomplished by comparing X-ray and solid-state 13C NMR data. Elucidation of the H-bond interaction between the heterocyclic rings and metal complexes by cross-polarization magic-angle-spinning 15N NMR is also reported.