How To Insulate a Reactive Site from a Perfluoroalkyl Group:  Photoelectron Spectroscopy, Calorimetric, and Computational Studies of Long-Range Electronic Effects in Fluorous Phosphines P((CH2) m (CF2)7CF3)3

This study advances strategy and design in catalysts and reagents for fluorous and supercritical CO2 chemistry by defining the structural requirements for insulating a typical active site from a perfluoroalkyl segment. The vertical ionization potentials of the phosphines P((CH2) m Rf8)3 (m = 2 (2) to 5 (5)) are measured by photoelectron spectroscopy, and the enthalpies of protonation by calorimetry (CF3SO3H, CF3C6H5). They undergo progressively more facile (energetically) ionization and protonation (P(CH2CH3)3 > 5 > 4 ≈ P(CH3)3 > 3 > 2), as expected from inductive effects. Equilibrations of trans-Rh(CO)(Cl)(L)2 complexes (L = 2, 3) establish analogous Lewis basicities. Density functional theory is used to calculate the structures, energies, ionization potentials, and gas-phase proton affinities (PA) of the model phosphines P((CH2) m CF3)3 (2‘−9‘). The ionization potentials of 2‘−5‘ are in good agreement with those of 2−5, and together with PA values and analyses of homodesmotic relationships are used to address the title question. Between 8 and 10 methylene groups are needed to effectively insulate a perfluoroalkyl segment from a phosphorus lone pair, depending upon the criterion employed. Computations also show that the first carbon of a perfluoroalkyl segment exhibits a much greater inductive effect than the second, and that ionization potentials of nonfluorinated phosphines P((CH2) m CH3)3 reach a limit at approximately nine carbons (m = 8).