The Imidazole Role in Strontium β-Diketonate Complexes Formation

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