Sorensen-Stowell and Hengge
TABLE 4. ∆δP Values from 31P NMR of Mixtures of
16O-18O Nonbridge-Labeled Phosphate Monoester
Dianions in D2Oa
It is emphasized that the present study is not an at-
tempt to calculate bond orders from isotope shifts. Ra-
ther, the empirical relationship between bond order and
∆δP is used to examine whether the R moiety imparts
significant differences in P-OR bond order in aqueous
solution, as are implied by the computational results and
seen in X-ray structures. The comparative values (bond
order differences implied by differences in ∆δP, or their
absence) are the focus of this investigation. Using bond
orders calculated from Pauling’s rule and the X-ray data
in Table 1, the magnitudes of ∆δP for phenyl and p-
nitrophenyl phosphates should decrease by ∼3.6 and
∼5.4 ppb, respectively, relative to ∆δP for methyl phos-
phate.
The difference in ∆δP between the alkyl and the aryl
esters is in the opposite direction expected if the bridging
bonds of the aryl esters are longer and weaker than those
of the alkyl esters. The larger ∆δP values for phenyl and
p-nitrophenyl phosphate may be due to greater anhar-
monicity in the aryl esters relative to the alkyl esters.
However, the magnitude of ∆δP is the same, within
experimental error, for the cyclohexylammonium and the
tetrabutylammonium salts of phenyl phosphate and
p-nitrophenyl phosphate. This indicates that the P-OR
ester bond is essentially unchanged in these two esters,
despite the ∼3 unit difference in pKa between these two
aryl ester groups, evidence that the basicity of the ester
group does not significantly affect the bridging bond order
in aqueous solution. The ∆δP for disodium phenyl phos-
phate of 16.7 ( 1.1 ppb lies just outside the experimental
uncertainty of the values for other salts of phenyl
phosphate and those of p-nitrophenyl phosphate.
We measured ∆δP values with three different counter-
ions since X-ray structures of phosphate esters with
different counterions have shown some variability in
bridging P-OR bond lengths. X-ray structures reported
after the original study5 cited in Table 1 report an
essentially identical P-OR bond length for pentakis-
(imidazole)copper(II) phenyl phosphate dianion28 as for
the potassium salt. However, a P-OR bond of 1.626 Å is
found in disodium methyl phosphate,29 in contrast to the
earlier finding of 1.597 Å for the diammonium salt.9 In
our work, no significant differences were observed in ∆δP
due to differences in the identity of the counterions.
Clearly, then, the variations in bridging bond lengths as
a function of ester group pKa and counterions that occur
in X-ray structures are not seen in the aqueous 31P NMR
studies reported here. The differences seen in the X-ray
structures are likely due to effects manifested in the
ester
salt
Na2
TBA2
CHA2
TBA2
CHA2
Na2
TBA2
CHA2
Na2
TBA2
CHA2
Na2
∆δP (ppb)
methyl-O-P18O3
23.6 ( 0.1
24.9 ( 0.8
20.1 ( 1.2
23.7 ( 0.4
23.2 ( 0.1
23.8 ( 0.4
23.8 ( 0.4
23.6 ( 0.1
23.5 ( 0.5
23.5 ( 0.3
23.3 ( 0.2
23.3 ( 0.4
22.8 ( 0.2
23.5 ( 0.1
23.5 ( 0.2
20.7 ( 0.4
2-
2-
methyl-O-P18O3
2-
ethyl-O-P18O3
2-
ethyl-O-P18O3
2-
2-
2-
phenethyl-O-P18O3
phenethyl-O-P18O3
phenethyl-O-P18O3
2-
2-
2-
propargyl-O-P18O3
propargyl-O-P18O3
propargyl-O-P18O3
2-
phenyl-O-P18O3
2-
phenyl-O-P18O3
2-
phenyl-O-P18O3
TBA2
CHA2
Na2
2-
2-
2-
p-nitrophenyl-O-P18O3
p-nitrophenyl-O-P18O3
p-nitrophenyl-O-P18O3
TBA2
a TBA)tetrabutylammonium, CHA)cyclohexylammonium.
crystal packing environments, including differences in
interactions of the phosphoryl group within the crystals
of the solids, versus the solvated ions in solution.
We also measured the 13C-31P coupling constants for
methyl, phenyl, and p-nitrophenyl phosphates. This two-
bond coupling constant reflects the P-O-C interaction,
not solely the P-O bond. Previously published data show
that two-bond phosphorus-carbon coupling constants
depend on the hybridization of the carbon atom, with sp2
carbons exhibiting slightly higher coupling constants
than sp3 carbon atoms.30,31 We obtained similar results;
the P-O-C coupling constant is 4.8 Hz in methyl
phosphate, 6.0 Hz in phenyl phosphate, and 5.8 Hz in
p-nitrophenyl phosphate. The differences between these
coupling constants for alkyl versus aryl phosphate esters
may be explained by the aid of Jameson’s s-character
theory.32 This theory holds that as a phosphorus atom is
bound to more electronegative atoms, or groups of atoms,
the coupling constant becomes increasingly more positive.
Since hybridization makes the aryl ester group slightly
more electronegative than the alkyl ester group, the
coupling constants for aryl phosphate esters are predicted
to be slightly larger. The close similarity of the two-bond
coupling constants for the two aryl esters further sug-
gests that no significant difference in the P-O bond order
exists between these two esters in aqueous solution.
To assess whether the identity of the ester group
affects the nonbridge P-O bond orders, these isotope
shifts were measured as well and are shown in Table 4.
This table includes propargyl phosphate, which is absent
from Table 3 because we were not able to synthesize
bridge-18O-propargyl phosphate.
(19) Lowe, G.; Sproat, B. S. J. Chem. Soc., Chem. Commun. 1978,
565-566.
(20) Cohn, M.; Hu, A. J. Am. Chem. Soc. 1980, 102, 913-916.
(21) Lowe, G.; Potter, B. V. L.; Sproat, B. S. J. Chem. Soc., Chem.
Commun. 1979, 733-735.
The data in Table 4 demonstrate that none of the esters
has significantly different nonbridge P-O bonds from one
another, and none is significantly different from the
formal P-O bond order of 1.33. Nor, at least in water,
does the identity of the counterion result in a consequen-
tial difference in bond order. The magnitudes of the
nonbridge P-O isotope shifts for phenyl phosphate and
pNPP are the same as those for the other esters, despite
(22) Cohn, M.; Hu, A. Proc. Natl. Acad. Sci. 1978, 75, 200-203.
(23) Arnold, J. R. P.; Lowe, G. J. Chem. Soc., Chem. Commun. 1986,
1487-1486.
(24) Baraniak, J.; Frey, P. A. J. Am. Chem. Soc. 1988, 110, 4059-
4060.
(25) Frey, P. A.; Sammons, D. Science 1985, 228, 541-545.
(26) Iyengar, R.; Eckstein, F.; Frey, P. A. J. Am. Chem. Soc. 1984,
106, 8309-8310.
(27) Pauling, L. In The nature of the chemical bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960; pp 255-260.
(28) Glowiak, T.; Wnek, I. Acta Crystallogr. 1985, C41, 324-327.
(29) Klooster, W. T.; Craven, B. M. Acta Crystallogr. 1992, C48, 19-
22.
(30) Ernst, L. Org. Magn. Reson. 1977, 9, 35-43.
(31) Szalontai, G. Org. Magn. Reson. 1977, 10, 63-69.
(32) Jameson, C. J. J. Am. Chem. Soc. 1969, 91, 6232-6234.
4808 J. Org. Chem., Vol. 70, No. 12, 2005