Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η4-C4R2C(O)C4H8}Fe(CO)3] (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) and attempts to prepare Fe(II) hydroxycyclopentadienyl-hydride complexes

Microwave irradiation of 1,6-diynes, RCae<inverted exclamation>C(CH2)(4)Cae<inverted exclamation>CR, with Fe(CO)(5) in dimethylether leads to the facile and clean formation of cyclopentadienone complexes [{eta(4)-C4R2C(O)C4H8}Fe(CO)(3)] in good yields resulting from a [2 + 2 + 1] cycloaddition. The molecular structures of three examples (R = Ph, 2,4-F2C6H3, 4-MeOC6H4) have been obtained. The addition of HBF4 leads to the clean and reversible formation of cationic hydroxycyclopentadienyl complexes [{eta(5)-C4R2C(OH)C4H8}Fe(CO)(3)][BF4]. Sequential addition of hydroxide and acid has also been carried out in an attempt to prepare hydroxycyclopentadienyl-hydride complexes. These were largely unsuccessful but in one case a Shvo-type complex with a bridging hydride was detected by H-1 NMR spectroscopy. Reasons for the differing behaviour of [{eta(4)-C-4(SiMe3)(2)C(O)C4H8}Fe(CO)(3)] and the related aryl-functionalised derivatives are considered.