Synthesis and Mossbauer spectroscopic studies of chemically oxidized ferrocenyl(phenyl)phosphines

The electrochemical potentials of Fc(3-x)PPh(x), (1-3, x = 0-2) and (FcPPh)(n) (4) indicate that iodine should oxidize ferrocenyl(phenyl)phosphines. The molar conductivity of solutions of 1-3 increases sharply when the solutions are titrated with iodine, leveling off after the addition of >2 equiv of oxidant, consistent with formation of 1:1 electrolytes. Diamagnetic salts 6-9 are observed upon addition of a benzene solution of iodine to a benzene solution of 1-4 at ambient temperature in ratios of I-2/metallocene ranging from 1:1 to 2:1. Well-resolved H-1 and P-31 NMR spectra are obtained for 6-8. Absorptions assigned to the I-3(-) anion dominate the UV-vis spectrum of 6-8, whereas characteristic absorptions for [Fc][I-3] are absent. Mossbauer spectra of 7-9 reveal isomer shifts consistent with low-spin iron(II) in ferrocene derivatives rather than those in ferricenium ions. Small amounts of low-spin Fe-III appear to be present in 6. Taken together, the results suggest that 6-9 are iodophosphonium salts and not ferricenium salts. Diferrocenyl(phenyl)phosphine oxide (5) reacts with iodine to produce a diamagnetic, dark solid 10. Low-spin Fe-II is observed at 77 and 293 K in the Mossbauer spectra of 10 with no evidence for oxidation of Fe-II to Fe-III. Compound 10 is proposed to be a neutral complex between 5 and I-2. Reactions between 5 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinon (DDQ) yield [Fc(2)P(=O)][DDQ](2) (11). Mossbauer spectroscopy of 11 indicates the presence of a mixture of low-spin Fe-II and low-spin Fe-III at 77 K, suggesting that some electron transfer occurs from 5 to DDQ. The fraction of low-spin Fe-III increases at room temperature.