Tertiary phosphine induced migratory carbonyl insertion in cyclopentadienyl complexes of iron(II)

Cyclopentadienyldicarbonylmethyliron, [CpFe(CO)2Me] (1), undergoes migratory carbonyl insertion under the influence of isosteric phosphine ligands P(4-FC6H4)3 and P(4-MeC6H4)3. The products of the reaction, [CpFe(CO)(COMe)P(4-FC6H4)3] (2a) and [CpFe(CO)(COMe)P(4-MeC6H4)3] (2b), were characterised by X-ray crystallography. In both structures, the iron atom adopts a pseudo octahedral coordination geometry. Fe–P bond distances are the same at 2.1932(8) Å in 2a and 2b, respectively. Thus, contrary to what was expected, X-ray data could not be used to quantitatively differentiate between the two phosphine ligands in 2a and 2b. Therefore, additional spectroscopic techniques such as IR and NMR were employed. Similarly, the Fe–C bond lengths of the carbonyl (Fe–CO) and acetyl (Fe–COMe) are 1.748(3) and 1.955(3) in 2a, and 1.744(3) and 1.951(3) Å in 2b, respectively. The migratory carbonyl insertion was studied by NMR, IR, and UV–vis spectroscopies to determine the mechanism and the rate law. Results from NMR spectroscopy show that the formation of the product is accompanied by oxidation of the corresponding phosphine ligand. An increase in the reactivity of migratory carbonyl insertion for P(4-MeC6H4)3 was observed when the solvent was changed from CH2Cl2 to MeCN. The kinetic data showed that P(4-MeC6H4)3 reacts faster than P(4-FC6H4)3.