Preparation and characterization of substituted pentaarylcyclopentadienyliron dicarbonyl bromide complexes containing methyl or fluorine substituents on the arene rings

The bromopentaarylcyclopentadienes C5(C6H5)4(Ar′)(Br) (Ar′=3,5-C6H3Me2, 1a3; Ar′=2,4,6-C6H2Me3, 1a5; Ar′=3,5-C6H3F2, 1a7; Ar′=2,6-C6H3F2, 1a8), and C5(3,5-C6H3Me2)4(Ar′)(Br) (Ar′=3-C5H4Me, 1b2; 3,5-C5H3Me2, 1b3; 3,6-C5H3Me2, 1b4; 2,4,6-C5H2Me3, 1b5; Ar′=3-C6H4F, 1b6; Ar′=3,5-C6H3F2, 1b7; Ar′=2,6-C6H3F2, 1b8) containing either methyl groups or fluorine atoms on the Ar′ rings were reacted with Fe(CO)5. At 20 °C, the reaction quantitatively produces the corresponding radicals [C5(C6H5)4(Ar′)]radical dot and [C5(3,5-C6H3Me2)4(Ar′)]radical dot which were isolated and characterized by ESR. A bromide-containing iron derivative, identified by far FTIR was also formed in this reaction. Above 50 °C, a second reaction took place yielding the corresponding bromo iron complexes {C5(C6H5)4(Ar′)}Fe(CO)2Br (Ar′=3,5-C6H3Me2, 3a3, Ar′=3,5-C6H3F2, 1a7) and {C5(3,5-C6H3Me2)4(Ar′)}Fe(CO)2Br (Ar′=3-C5H4Me, 3b2; 3,5-C5H3Me2, 3b3; 3,6-C5H3Me2, 3b4; Ar′=3-C6H4F, 3b6; Ar′=3,5-C6H3F2, 3b7). In the case of the bromopentaarylcyclopentadienes 1a5, 1a8, 1b5, and 1b8, the presence of two methyl groups or two fluorine atoms on the ortho and ortho′ position of Ar′ prevents the complexation of the iron atom onto the substituted cyclopentadienyl radicals. A discussion of the mechanism of the reaction, supported by ESR, cyclic voltammetry, and far-FTIR spectroscopy, is also reported.