The basicity of phosphines

Abstract

The basicities of the triarylphosphines P(4-XC6H4)3 (X = Cl, F, H, CH3, CH3O, (CH3)2N), P(3-CH3C6H4)3, and P(2-CH3C6H7)3 as well as the trialkylphosphines P(t-Bu)3 and PCy3 have been measured by the nitromethane titration method. The range of basicity available by aryl substitution is very large, being pKa = 8.65 for X = (CH3)2N to 1.03 for X = Cl. The most basic phosphine is P(t-Bu)3 whose pKa = 11.40. The measured basicities correlate well with σp, [Formula: see text], and ν as well as with the lone pair ionisation potentials of the triarylphosphines. Generally the 1H, 31P, and 13C nmr spectral parameters of the free and protonated phosphines do not correlate well with pKa.

The synthesis, X-ray structure, and substitution lability of chloro(2,3,7,8,12,13,17,18-octaethylporphinato)(triphenylphosphine)rhodium(III)

“….C1-] in dichloromethane. Stronger binding of PnBu3 relative to triarylphosphines has been reported for the isoelectronic, d 6 Ru(1I)OEP and Ru(1I)TPP complexes (17) and was accounted for by the higher basicity of the trialkylphosphines (25).…”

Preparation and characterization of some ruthenium(II) porphyrins containing tertiary phosphine axial ligands, including the crystal structure of (octaethylporphinato)bis(triphenylphosphine)ruthenium(II)

“…The bis(tri-rz-butylphosphine) complexes of the TPP and OEP systems at M gave spectra of the six-coordinate species only ( Table 1, and Table 1 in ref. 4, respectively) implying much smaller dissociation constants M); the much higher basicity of the trialkylphosphines compared to triphenylphospine (>7 pK, units (27)) readily accounts for this difference, but steric factors could be as important. Within either porphyrin system, the less basic PPh, compared to P(p-CH30C6H4),, by -2 pK, units (27), accounts for larger K values with the PPh, system, although the factors differ consid-' erably (-2.5 and -35 for the OEP and TPP systems, respectively).…”

“…4, respectively) implying much smaller dissociation constants M); the much higher basicity of the trialkylphosphines compared to triphenylphospine (>7 pK, units (27)) readily accounts for this difference, but steric factors could be as important. Within either porphyrin system, the less basic PPh, compared to P(p-CH30C6H4),, by -2 pK, units (27), accounts for larger K values with the PPh, system, although the factors differ consid-' erably (-2.5 and -35 for the OEP and TPP systems, respectively). The difference in porphyrin basicities is one factor 1 (OEP, pK, -6; TPP, pK, -3 (28)), and while normally within a related series of porphyrins (such as TPP derivatives containing p-substituents on the phenyl rings), axial ligand binding is enhanced with decreasing porphyrin basicity (28,29), this has not been established for porphyrins as diverse as OEP and TPP, where contacts with the phenyl or ethyl groups on TPP and OEP, respectively, may also be important.…”

2-Thioanisyldichlorophosphine, new starting material for the preparation of multidentate phosphine ligands: syntheses and characterization of derivatives of 2-anisyl- and 2-thioanisyldichlorophosphines

“…Simple titrations to measure pK a values of phosphines are difficult because the tertiary phosphines are typically insoluble in water. Nonaqueous titrimetry [22,24], infrared technique [25], ultraviolet photoelectron spectra [26] and NMR studies [27][28][29] are the examples from the methods that have been used in estimating the basicities of phosphines.…”

“…It has been postulated that there is no general correlation between basicity and 31 P-NMR chemical shifts of the phosphine ligands [29,35]. Instead, the basicity of phosphines has been shown to be directly related to the 31 P-NMR chemical shifts of corresponding phosphine oxides [36].…”