Unusual Hexanuclear Fe(II, III) Pyrazole–Pyrazolate–Pivalate Formed upon η⁵,π-Bonding of Bridging Pyrazolate Anion to Potassium Cation

The reaction of aqueous iron(II) sulfate with potassium pivalate in MeCN followed by removal of the solvent and dissolution of the residue in Et3N and hexane gives a brown solution and a white precipitate. The reaction of the mother liquor with excess 3,5-dimethylpyrazole (HDmpz) afforded the hexanuclear complex {[Fe3(µ3-O)(µ-OOCtBu)4(OOCtBu)2(HDmpz)(µ,η5-Dmpz)(µ-K)]2 ∙ 2HDmpz} (I). The complex was characterized by elemental analysis, IR spectroscopy, and X-ray diffraction (CIF file CCDC no. 1956015). Complex I is formed upon deprotonation of HDmpz, which was observed for the first time in the presence of iron carboxylates. Deprotonation gives the bridging pyrazolate anion, which assembles two trinuclear Fe3(µ3-O)(µ-OOCtBu)4(OOCtBu)2(HDmpz) moieties into a hexanuclear cluster though unusual η5,π-coordination with the potassium cation.