Microwave-assisted synthesis of cyclopentadienone iron tricarbonyl complexes: molecular structures of [{η 4 -C 4 R 2 C(O)C 4 H 8 }Fe(CO) 3 ] (R = Ph, 2,4-F 2 C 6 H 3 , 4-MeOC 6 H 4 ) and attempts to prepare Fe(II) hydroxycyclopentadienyl–hydride complexes

Microwave irradiation of 1,6-diynes, RC≡C(CH 2 ) 4 C≡CR, with Fe(CO) 5 in dimethylether leads to the facile and clean formation of cyclopentadienone complexes [{η 4 -C 4 R 2 C(O)C 4 H 8 }Fe(CO) 3 ] in good yields resulting from a [2 + 2 + 1] cycloaddition. The molecular structures of three examples (R = Ph, 2,4-F 2 C 6 H 3 , 4-MeOC 6 H 4 ) have been obtained. The addition of HBF 4 leads to the clean and reversible formation of cationic hydroxycyclopentadienyl complexes [{η 5 -C 4 R 2 C(OH)C 4 H 8 }Fe(CO) 3 ][BF 4 ]. 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 1 H NMR spectroscopy. Reasons for the differing behaviour of [{η 4 -C 4 (SiMe 3 ) 2 C(O)C 4 H 8 }Fe(CO) 3 ] and the related aryl-functionalised derivatives are considered. Graphical Abstract Microwave irradiation of 1,6-diynes, RC≡C(CH 2 ) 4 C≡CR, with Fe(CO) 5 gives cyclopentadienone complexes [{η 4 -C 4 R 2 C(O)C 4 H 8 }Fe(CO) 3 ], the molecular structures of three (R = Ph, 2,4-F 2 C 6 H 3 , 4-MeOC 6 H 4 ) being carried out. Sequential addition of hydroxide and acid was carried out in an attempt to prepare hydroxycyclopentadienyl–hydride complexes, and while largely unsuccessful, in one case a Shvo-type complex with a bridging hydride was suggested by 1 H NMR spectroscopy.