Phosphorus proton affinity

Kovacevic, B. and Maksic, Z.B. (2006) High basicity of phosphorus-proton affinity of tris-(tetramethylguanidylphosphine) and tris(hexamethyltriaminophosphazenyl)phosphine by DFT calculations. Chemical Communications, 1524-1526. 

As seen in Table V, there is a clear dependence of the equilibrium position on the basicity of amines, excluding triethylamine. However, it is necessary to take into account not only the proton affinity of the amine, but also the ability of the amine to form a dative bond with a boron atom. The equilibrium position also depends on the structure of the phosphorus-... 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

FIGURE 24. Correlation between core ionization energy and proton affinity for arsenic and phosphorus compounds. Reprinted with permission from Reference 90. Copyright (1979) American Chemical Society... 

Although organic derivatives of the type [MR4]+X are well known for M = P, As and Sb (sec page 390), only phosphorus gives the hydrogen-containing prototype, PH4, and this does not form any very stable compounds. As noted in Section 13-4, the proton affinity of PH3 is substantially less than that of NH3. The PH4 ion is tetrahedral45 with P—H = 1.414 A as... 

The highlight of synthetic studies with bridged phosphorus compounds concerns additions to (139). Cyclobutadiene gives an adduct, readily photo-isomerized to (140). Addition of diethyl azodicarboxylate to (141) gives the quinquevalent phosphorus derivative (142), which is readily transformed into (143). Gas-phase proton affinities have been reported " for (141) and related cyclic phosphites. 

Calculations have revealed that the proazaphosphatrane bases are approximately equal in thermodynamic basicity to the Schwesinger P2 phosphazene bases (Table 2.14). The RHF/ 6-3IG calculations of proton affinities indicate that phosphazene (Z = NH) and phosphorus ylide (phosphorane, Z = CH2) counterparts (146 and 145) are stronger than the parent Verkade superbase 139. Higher basicity is associated with the higher degree of delocalization of the positive charge in the protonated iminophosphoranes and phosphorus ylides as a result of the more electropositive character of phosphorus atom [67]. On the other hand, phosphoms oxides 147 and 150 are weaker than 139, but still above the superbasicity borderline. The strained polycycles 138, 149 and 148, as representatives of superbases... 

As can be seen, the diphenylguanidine and presumably all of the alkyl amines are strong enough bases to protons to form the N-H bond in preference to the N-Ag. Summation of meq for each quantity gives the total for meq taken. The greater stability of the P-Ag bond and lower proton affinity of the phosphine phosphorus for hydrogen gives a more complex picture for the phosphines. The... 

In summary, it appears that the formation of silver complexes is a function of proton affinity and ultimately of 2a values for both phosphorus and nitrogen compounds. However, the stability of the complex depends on the extent of the substitution. The utility of silver ion for titration of compounds of the phosphine class is potentially much more useful than for the amines. It is probable that silver titrations can be used to differentiate P and N atoms either in mixtures or in a single molecule. 

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