NMR Spectra of Some CF3-Substituted Triphenylphosphines. Evidence for “Through-Space” P–F Spin–Spin Coupling

In the ortho-substituted compounds (o-CF3Φ)3P, (o-CF3Φ)2PΦ, and (o-CF3Φ)PΦ2, JPF is 55.00 ± 0.17 Hz, 52.82 ± 0.12 Hz, and 53.41 ± 0.20 Hz, respectively. In contrast, we could not observe any phosphorus–fluorine coupling in the compounds (m-CF3Φ)3P and (p-CF3Φ)3P. These results, together with the very short phosphorus–fluorine distance in the ortho-substituted compounds, strongly suggest that the P–F coupling results from a “through-space” mechanism in the ortho-substituted compounds. This hypothesis is supported by the strong positive temperature dependence found for JPF, which is similar to that found for JFF in compounds in which the “through-space” mechanism is believed to make a major contribution to the fluorine–fluorine coupling constant.