Reactivity studies of phenylcalcium iodide towards THF yielding phenyl-free cage compounds – Crystal structures of [{(thf)Ca(O–CHCH2)2}4 · CaO · CaI2] and [(CaO)4 · 4 (thf)3CaI2]

THF proved to be the favoured solvent for the direct synthesis of phenylcalcium iodide. However, ether degradation is the major side reaction in this solvent. Zinc-mediated degradation of THF leads to the formation of the cage compound [{(thf)Ca(O–CHCH2)2}4 · CaO · CaI2] (1) besides the zincate [(thf)6Ca] [ZnEt2Ph2]. The structure of 1 shows an oxygen-centered Ca6 octahedron with iodine atoms bound to two opposite corners. Heating of phenylcalcium iodide in THF solution for several hours yields [(CaO)4 · 4 (thf)3CaI2] (2) with a central Ca4O4 heterocubane unit. NMR monitoring of the reaction shows that the phenyl groups are converted to benzene due to α-deprotonation of THF.

The post-Grignard reagent phenylcalcium iodide can best be prepared in THF. However, far-reaching degradation reactions include α-deprotonation of THF, formation of the vinylalcoholate [{(thf)Ca(O–CHCH2)2}4 · CaO · CaI2] and finally calcium oxide iodide [(CaO)4 · 4 (thf)3CaI2] (see picture: Ca brown, I blue, O red; H and C omitted and only the O atoms of the THF ligands are shown).Figure optionsDownload as PowerPoint slide