Novel carbonyl iron–bismuth clusters – synthesis, structure, CO2 insertion and potential as molecular precursors for BiFeO3

The reaction of Fe2(CO)9 with Bi(OSiMe2tBu)3 gave soluble [(CO)4FeBi(OSiMe2tBu)]2 (1) in moderate yield whereas in case of Bi(OtBu)3 used as starting material both [(CO)4FeBi(OtBu)]n (2) and the bismuth–iron cluster [(CO)3FeBi3(OtBu)4{OCO(OtBu)}]2 (3) were isolated. The latter forms upon insertion of CO2, released during reaction of diiron nonacarbonyl with bismuth tert-butoxide, into a Bi–OtBu bond. The compounds were characterized by IR and 1H NMR spectroscopy as well as thermogravimetric analyses. Additionally, the molecular structures of compounds 1 and 3 were elucidated by single crystal X-ray diffraction. The core structure of [(CO)4FeBi(OSiMe2tBu)]2 (1) is build up by a four-membered Bi2Fe2 ring whereas [(CO)3FeBi3(OtBu)4{OCO(OtBu)}]2 (3) is composed of two tetrahedral FeBi3 cluster cores that dimerise via bridging –OCO(OtBu) ligands. Analysis of the TGA residues by PXRD revealed that compound 2 is the best precursor for multiferroic BiFeO3 among the compounds studied here, although Bi25FeO39 was detected as minor impurity.