pentachlorophenol-
amine complexes also illustrate the salt-cocrystal
continuum.14a The bond length of the phenol C-O group
was used to determine the ionic state of the complex. The
C-O distance is 1.36 Å for the neutral phenol, but is
shorter (1.26 Å) when the phenolic anion is formed. The
1:1 crystalline complex formed between pentachlorophenol
and 3-cyanopyridine, a weak base (¢pKa)-3.9), was found
to have a C-O bond distance of about 1.3 Å.14a Clearly
proton transfer does not occur and that complex is a cocrystal.
As stronger bases are used as the amine partner in the
complexes (¢pKa values increase), the C-O distances
become shorter. At the other end of the continuum, a
complex with the strong base 1,8-bis(dimethylaminomethyl)-
naphthalene (¢pKa ) 6.9) is a salt (C-O 1.26 Å).
Additionally, NâââO (between the amine and phenol), O-H
(phenol), and NâââH (amine) bond lengths also corroborate
the conclusions made with C-O bond lengths. A plot
correlating C-O bond length and ¢pKa is shown in Figure
3, demonstrating that a cocrystal is formed at one end of the
continuum, and a salt is formed at the other end of the
pentachlorophenol-amine complexes also illustrate the salt-cocrystalcontinuum.14a The bond length of the phenol C-O groupwas used to determine the ionic state of the complex. TheC-O distance is 1.36 Å for the neutral phenol, but isshorter (1.26 Å) when the phenolic anion is formed. The1:1 crystalline complex formed between pentachlorophenoland 3-cyanopyridine, a weak base (¢pKa)-3.9), was foundto have a C-O bond distance of about 1.3 Å.14a Clearlyproton transfer does not occur and that complex is a cocrystal.As stronger bases are used as the amine partner in thecomplexes (¢pKa values increase), the C-O distancesbecome shorter. At the other end of the continuum, acomplex with the strong base 1,8-bis(dimethylaminomethyl)-naphthalene (¢pKa ) 6.9) is a salt (C-O 1.26 Å).Additionally, NâââO (between the amine and phenol), O-H(phenol), and NâââH (amine) bond lengths also corroboratethe conclusions made with C-O bond lengths. A plotcorrelating C-O bond length and ¢pKa is shown in Figure3, demonstrating that a cocrystal is formed at one end of thecontinuum, and a salt is formed at the other end of the
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