Chemistry Reference
In-Depth Information
obtained pKa-values were compared with the corresponding values of RNH
2
amines. The
pKa values of the synthesized phosphazenes in acetonitrile ranged from 14.6 to 26.8 pKa
units [10]. The UV-Visible spectrophotometric titration method was used to establish the
relative basicity of phenyl-substituted phosphazenes (P1, P3 and P4 bases), and to extend
the ion pair basicity scale for tetrahydrofuran (THF) medium. These measurements gave a
continuous basicity scale in THF ranging from 2.6 (2-MeO-pyridine) to 26.6 (2-Cl-
C
6
H
4
P
4
(pyrr)phosphazene) in pKa units, that is for 24 orders of magnitude and containing
58 compounds (pyridines, anilines, amines, guanidines, amidines, phosphazenes) [11]. The
gas phase basicity (GB) values were determined for 19 strong bases, among them such well
known bases as BEMP (1071.2 KJ/mol), Verkade
s methyl-substituted base (1083.8 kJ/
¼
¼
mol), Et-N
P(NMe
2
)
2
-N
P(NMe
2
)
3
(Et-P2 phosphazene, 1106.9 KJ/mol) and t-Bu-
¼
N
P(NMe
2
)
3
(t-Bu-P1 phosphazene, 1058.0 KJ/mol). The first experimental GB values
were determined for P2 phosphazenes and an important region of the gas phase basicity
scale is now covered with organic bases. The GB values for several superbases were
calculated using density functional theory at the B3LYP/6-311
G
**
level [12].
A new unique principle for creating novel nonionic superbases has been reported
(Figure 5.3). It is based on attachment of tetraalkylguanidino, 1,3-dimethylimidazolidine-
2-imino or bis(tetraalkylguanidino)carbimino groups to the phosphorus atom of the
iminophosphorane group using tetramethylguanidine or the easily available 1,3-dimethy-
limidazolidine-2-imine. Their base strengths are established in THF solution by means of
spectrophotometric titration and compared with reference superbases designed specially
for this study, P2- and P4-iminophosphoranes. The gas phase basicities of several guanidine
and N
0
,N
0
,N
0
,N
0
-tetramethylguanidino(tmg) substituted phosphazenes, and of their cyclic
analogues, have been calculated and the crystal structures of (tmg)
3
P
þ
N-
t
Bu and
¼
N-
t
Bu HBF
4
determined. The enormous basicity-increasing effect of this princi-
ple is experimentally verified for the tetramethylguanidino groups in the THF medium, and
the basicity increase when moving from (dma)
3
P
(tmg)
3
P
¼
N-
t
Bu (pKa
N-
t
Bu
¼
¼
18.9) to (tmg)
3
P
¼
(pKa
29.1) is 10 orders of magnitude [13].
The gas phase basicities and pKa values of tris(phosphazeno) substituted azacalix[3](2,6)
pyridine in acetonitrile and some related compounds were examined by the density
functional theory (DFT) computational method. It was shown that the hexakis(phospha-
zeno) derivative of azacalx[3](2,6)pyridine is a hyperstrong neutral base, as evidenced by
the absolute proton affinity of 314.6 kcal/mol and pKa (MeCN) of 37.3 units. It is a
consequence of the very strong bifurcated hydrogen bond (32 kcal/mol) and substantial
cationic resonance effect [14].
A combination of phosphazene base concept and the disubstituted 1,8-naphthalene
spacer was shown and a new bisphosphazene 1,8-bis(hexamethyltriaminophosphazenyl)
naphthalene (HMPN) represents the most basic representative of this class of
¼
proton
sponge
274 kcal/mol and
the measured pK
BH
þ
(MeCN) 29.9. HMPN is by nearly 12 orders of magnitude more basic
than Alder
, as evidenced by the theoretically estimated proton affinity PA
¼
s classical 1,8-bis(dimethylamino)naphthalene (DMAN). The new bispho-
sphazene, HMPN, has been prepared and fully characterized. The spatial structure of
HMPN and its conjugate acid have been determined by X-ray technique and theoretical
DFT calculations. It is found that monoprotonated HMPN has an unsymmetrical intramo-
lecular hydrogen bridge. This cooperative proton chelating effect renders the bispho-
sphazene more basic than P1 phosphazene bases. The density functional calculations are in
