Reactivity of Phosphorus Donors

Abstract

A greatly expanded database has led to a revised set of E B and C B basicity parameters for the analysis of the reactivity of phosphorus donors with the ECW model. Reactivity and spectral shift analyses are reported for 79 acceptors. General patterns for the reactivity of phosphorus donors emerge, for example, π-back-bond acceptor tendencies are shown to decrease as the σ basicity increases. Steric effects are encountered with a few acceptors, and a procedure is offered that uses an extension of ECW to quantitatively confirm steric repulsions. Very different interpretations of reactivity result from ECW compared to those from analyses that use one-parameter electronic basicities, χ, and cone angles, θ. When donor orders reverse with acceptor softness (covalency), one-parameter electronic basicity scales cannot accommodate the reversals. It will be shown that the cone angle is used inappropriately to compensate for changing softness in the χ and θ correlations. Attempting to compensate for softness using θ leads to an exaggeration of the steric contributions and to unprecedented interpretations of reactivity trends when the parameters for different acceptors are compared. The meaning of the correlations is questioned. Donors for which θ does not correlate to E B and C B parallel those that require extra parameters, e.g., E ar, to obtain good fits with the QALE procedures. It is also shown that the extra QALE terms provide parameters that enable one to correlate systems which demonstrate nonlinear free-energy behavior, clouding even further the meaning of QALE correlations.

Development of a Ligand Knowledge Base, Part 1: Computational Descriptors for Phosphorus Donor Ligands

“…In addition, a number of groups have sought to estimate the quantitative contributions of steric and s-/p-electronic effects by fitting regression models to various experimentally observed linear free energy relation- www.chemeurj.org ships and to explore their use in quantitative structure-property relationships. [2,3,11,21,22] The availability of a range of experimental data makes phosphorus donor ligands attractive for a proof-of-concept study such as this. We report below the design of calculated phosphorus(iii) ligand descriptors and their statistical analysis, and describe potential applications in mapping chemical space as well as the interpretation and prediction of experimental data.…”

Wide bite angle diphosphine rhodium complexes: Synthesis, structure, and catalytic 1,4-addition of arylboronic acids to enones

“…Consequently, a number of parameters are used to assist in the classification of phosphine properties most pertinent to their role in catalysis. Tolman cone angles [1] and molecular mechanics are used to quantify phosphine sterics [2,3] and various scales have been forwarded to describe phosphine electronics [4][5][6][7][8][9][10][11]. More recently, widespread application of bidentate P-donor ligands resulted in the introduction of a bite angle (P-M-P angle, b) [12] parameter that is applied to predict coordination modes.…”

Ligand effects upon deuterium exchange in arenes mediated by [Ir(PR3)2(cod)]+.BF4−

“…Alternatives include the solvatochromic parameters, E and C, 11 the enthalpy of protonation (DH HP ) of the ligand, 12 and ligand pK a . Giering and co-workers 13 have found that both pK a and w are reasonable measures of ligand s-donicity, although pK a is limited in that the interaction between a soft phosphorus and a proton cannot be entirely predictive of that between a phosphorus and a soft metal centre.…”