Assessing the stereoelectronic properties of pyrrolyl phosphines and related ligands. The quantitative analysis of ligand effects (QALE)

Abstract

By application of the QALE model (quantitative analysis of ligand effects) to the standard reduction potentials (E° values) and the standard enthalpies of reduction (ΔH°) of the η-Cp(CO)(PZ₃)Fe(COMe)^+/0 couple (PZ₃ = a phosphorus(III) ligand), and ν_CO for η-Cp(CO)(PZ₃)Fe(COMe)⁰, we have determined that a minimum of four parameters are necessary to describe the stereoelectronic properties of the set of ligands PPh_i(Pyr)_3 − i (Pyr = pyrrolyl) and P(NC₄H₈)₃ (NC₄H₈ = pyrrolidinyl). These parameters are χ_d, θ, E_ar and the π acidity parameter, π_p. The values of these parameters were determined by linear regression analysis of a set of QALE equations. The coefficients of these equations were based on the analyses of data for PR₃, PPh_iR_3 − i, P(p-XC₆H₄)₃, P(OR)₃, and P(O-p-XC₆H₄)₃. The parameters for P(Pyr)₃ are χ_d = 31.9 ± 0.7, θ = 145 ± 3, E_ar = 3.3 ± 0.2 and π_p = 1.9 ± 0.2; and for P(NC₄H₈)₃ the parameters are χ_d = −1.2 ± 1.4, θ = 145 ± 5, E_ar = −0.6 ± 0.4 and π_p = 0.9 ± 0.3. P(Pyr)₃ is a poor σ donor that possesses an E_ar parameter comparable to P(p-XC₆H₄)₃ and a π acidity that is about two thirds that of P(OR)₃ and half that of P(O-p-XC₆H₄)₃. On the other hand, P(NC₄H₈)₃, which is one of the strongest σ donor phosphorus(III) ligands, is a weak π acid with a value for E_ar that is statistically indistinguishable from zero. PPh_i(Pyr)_3 − i and P(NC₄H₈)₃ appear to be isosteric to P(p-XC₆H₄)₃.