72
COMPLEXATION AND DECOMPOSITION OF BENZENEDIAZONIUM ION
stituents decrease the complexation, i.e. the intensity of
[crown ether–PhN2]+ . These bulky groups reduce the
flexibility of crown ethers to allow more optimized
geometries of the electrostatic bonds. In addition, the
electron-withdrawing power of the benzene group(s) weak-
ens the electron-donor ability of the oxygen atoms.3 It was
observed recently9 that the host–guest complexation
between an acyclic polyether and the benzenediazonium ion
in the gas phase, in contrast to the effects in solution, does
not increase with increasing number of oxygen atoms in the
polyether chain. Correspondingly, and in contrast with the
strength of the complexation in solution (see below), the
values of the relative abundances of the complexes
[18-crown-6–PhN2]+ and [21-crown-7–PhN2]+ , 13% and
12% in Table 1, respectively, were closely similar.
In agreement with Zollinger et al.’s results,6 but in
contrast with the effects of complexation on the thermody-
namic and kinetic stabilities (K and k2/k1 values, see below),
the complexation with 18-crown-6 was recently5 found to
produce, in all solvents studied, the maximum hypso-
Figure 2. Effect of the number of oxygen atoms of unsubstituted
and benzo-(mono- or di-) and dicyclohexano-substituted crown
ethers on the thermodynamic stability of the complexed benzene-
diazonium ion in 1,2-dichloroethane
chromic shift ⌬ in the UV spectrum. Zollinger et al.
max
noted that no conclusions regarding the kinetic or thermody-
namic stability of the host–guest complexation of
arenediazonium ion with crown ethers can be drawn from
the spectroscopic data. Leaving aside the benzo-substituted
crown ethers, since they themselves exhibit strong UV
18-crown-6 and 21-crown-7 is the inability of the latter to
assume the planar D3d conformation, which is well docu-
mented for 18-crown-6 and its derivatives. It seems
reasonable to assume that one of the CH2—O—CH2 units in
21-crown-7 will turn upwards and away from the mean
plane of the other oxygen atoms, or else inwards over the
benzene ring, as shown in the modified insertion-type
complex structure 3 (Figure 1)8, 21 with -base–-acid
interactions. Both modified IC structures will present to the
diazonium group a cavity of essentially the same size as that
found in 18-crown-6, but the extra oxygen donor atom of
21-crown-7 can now interact with other electrophilic center
to provide additional stabilization to the overall complex.
Correspondingly, the larger crown ethers can be suggested
to be capable in solution of wrapping around the cation to
form a three-dimensional cavity with all oxygen atoms
coordinated to the cation.3 The relatively large K values
found for the complexation between 24- and 30-membered
crown ethers and benzenediazonium ion in this work (see
Fig. 2) are in good accord with this suggestion.
With the presence of benzene or cyclohexano groups in
crown ethers, there is a small reduction in the K value. Our
results for 18-membered crown ethers in Table 3 and Fig. 2
show that the effect of benzo and cyclohexano substituents
on the thermodynamic stability of crown ether-complexed
benzenediazonium ion in solution is relatively small, but the
following order is suggested: 18-crown-6≈dicyclohexano-
18-crown-6>benzo-18-crown-6>dibenzo-18-crown-6. The
effect of these groups on the K value may be attributed to a
combination of ligand bulkiness leading to the isolation of
the cation from the solvent molecules, which strengthens
the thermodynamic stability of a complex, and the electron-
withdrawing power of the benzene group(s), which weakens
the electron-donor ability of the oxygen atoms. The
absorption, the results in Table 3 suggest for ⌬ of the
max
crown ether–PhN2+ system in 1,2-dichloroethane the order
18-crown-6 ≈ dicyclohexano-18-crown-6 > 15-crown-5 >
21-crown-7≈dicyclohexano-24-crown-8ӷ12-crown-4.
Excluding 12-crown-4 (⌬max =2 nm), the changes of
13–19 nm in ⌬
in 1,2-dichloroethane show that the
max
maximum hypsochromic shift is not sensitive to the ring
size of crown ether. For the acyclic polyether–PhN2+ system
the corresponding hypsochromic shift in 1,2-dichloroethane
is smallest for the smallest complexing agents but independ-
ent of the host (⌬max =13±1 nm) when the polyether chain
is long enough to wrap fully around the diazonium group
(see structure 2, Figure 1).9
The K values in Table 3 and Figure 2 show that the
thermodynamic stability of benzenediazonium ion com-
plexed with crown ether in solution varies markedly with
the ring size of the host but relatively little with the benzo
or cyclohexano substituent(s) of the host molecule. The K
values of the IC complexes containing six or more oxygen
atoms in the host are clearly larger than those of CT
complexes of hosts containing five or less oxygen atoms. It
might be assumed that the cavity of 21-crown-7, with its
diameter2 of 0·34–0·43 nm is slightly too large for the
diazonium group, which has a cylindrical diameter of about
0·24 nm. However, the greater ring flexibility, compared
with 18-crown-6 with its cavity diameter2 of 0·26–0·32 nm,
may allow for the relief of steric interactions between the
macrocyclic ring and the 2-hydrogens or other 2-sub-
stituents18 in the arenediazonium cation. On the basis of
their examination of C–P–K molecular structures, Beadle et
al.21 suggested that an important difference between
© 1997 by John Wiley & Sons, Ltd.
JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 67–75 (1997)
Kuokkanen, Toivo
