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

The reaction of Fe₂(CO)₉ with Bi(OSiMe₂^tBu)₃ gave soluble (CO)₄FeBi(OSiMe₂^tBu)]₂ (1) in moderate yield whereas in case of Bi(O^tBu)₃ used as starting material both (CO)₄FeBi(O^tBu)]_n (2) and the bismuth-iron cluster (CO)₃FeBi₃(O^tBu)₄{OCO(O^tBu)}]₂ (3) were isolated. The latter forms upon insertion of CO₂, released during reaction of diiron nonacarbonyl with bismuth tert-butoxide, into a Bi-O^tBu 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(OSiMe₂^tBu)]₂ (1) is build up by a four-membered Bi₂Fe₂ ring whereas (CO)₃FeBi₃(O^tBu)₄{OCO(O^tBu)}]₂ (3) is composed of two tetrahedral FeBi₃ cluster cores that dimerise via bridging -OCO(O^tBu) 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.