Structural and aromatic aspects of tautomeric equilibrium in hydroxy aryl Schiff bases

Article abstract of DOI:10.1002/poc.1403

The synthesis and X-ray measurements of four Schiff bases were carried out at 100K. The HOMA and HOSE aromaticity indices wer estimated on the basis of the experimental data. The aromaticity of the phenyl ring and the chelate chain was analysed. A comparison of the aromaticity of naphthalene and phenyl derivatives of hydroxy aryl Schiff bases is presented. The balance between the aromaticity of adjacent rings of the naphthalene fragment and its effect on proton transfer is defined. Research on the interrelations between aromaticity and the intramolecular proton transfer in hydroxy aryl Schiff bases is shown. Copyright

Full text of DOI:10.1002/poc.1403

Research Article
Received: 21 March 2008,
Revised: 6 May 2008,
Accepted: 7 May 2008,
Published online in Wiley InterScience: 11 June 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1403
Structural and aromatic aspects of tautomeric
equilibrium in hydroxy aryl Schiff basesy
a
a
a
b
*
A. Filarowski , A. Kochel , M. Kluba and F. S. Kamounah
The synthesis and X-ray measurements of four Schiff bases were carried out at 100 K. The HOMA and HOSE aromaticity
indices were estimated on the basis of the experimental data. The aromaticity of the phenyl ring and the chelate chain
was analysed. A comparison of the aromaticity of naphthalene and phenyl derivatives of hydroxy aryl Schiff bases is
presented. The balance between the aromaticity of adjacent rings of the naphthalene fragment and its effect on
proton transfer is defined. Research on the interrelations between aromaticity and the intramolecular proton transfer
in hydroxy aryl Schiff bases is shown. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: intramolecular hydrogen bonding; crown ether; proton transfer; Schiff base; aromaticity; HOMA and HOSE
indices
Schiff bases (with quasiaromatic formation) compared with
INTRODUCTION
o-hydroxy aryl Mannich bases (without quasiaromatic formation)
and complexes of phenols with amines[27] definitely corroborates
the existence of quasiaromatic formation and its uniqueness. The
specificity of o-hydroxy aryl Schiff bases is also exposed through
the phenyl ring’s aromaticity, which decreases upon proton
transfer,[7,28] similar to the complexes of phenols with amines.[29]
This paper reports on studies of the interrelation between the
Intramolecular quasiaromatic hydrogen bonding[1] and the
aromaticity notion are studied in this paper.[2–12] Lately these
phenomena have aroused great interest and difference of
opinion.[13–27] Gilli et al.[13] introduced a contemporary definition
of a resonance-assisted hydrogen bond (RAHB) and conducted
an investigation of quasiaromatic bonding on the basis of both
VB theory and crystallographic studies.[13,14] According to
Reference [15], RAHB is found in the intramolecular as well as
in the intermolecular hydrogen bond. This confirmation
corroborates the opinion suggested in Reference [16]. This
phenomenon rests upon conjugation through the hydrogen
bridge and the chelate chain,[17] which leads to smoothing of the
acid-basic equilibrium. The existence of p-electronic coupling in
the chelate chain influencing the X—H bond was presented by
Grabowski[18] by means of quantum mechanical calculations. It
should be underlined that coupling through the hydrogen bridge
was not predicted in that paper. However, some divergences
were found in References [20,21], where a significant negative
charge on the protono-donor and the protono-acceptor was
determined. Recently, the existence of RAHB was questioned by
Elguero and Del Bene,[22,23] who showed that a quasiaromatic
—
—
hydrogen bridge, the chelate ring (O—C C—C N) and
—
—
aromatic formations. In a previous paper[7] we investigated the
change in aromaticity in o-hydroxy phenyl Schiff bases
depending on tautomeric equilibrium (Scheme 1). In this paper
we widened the range of our research, completing it with
naphthalene derivatives and comparing them with phenyl
derivatives.
EXPERIMENTAL AND COMPUTATIONAL
The synthesis of the hydroxy aryl Schiff bases from stoichiometric
mixtures of ketones and anilines in methanol was performed
[30]
according to Reference
The intensity data were collected at 100 K using a Kuma
.
1
KM4CCD diffractometer and graphite-monochromated MoKa
˚
(0.71073 A) radiation generated by an X-ray tube operating at
50 kV and 35 mA. The images were indexed, integrated and
formation fails to change the coupling constant J(XY), which is
an indicator of hydrogen bond strength.[24] It is noteworthy that
the discussion of the existence of p-electronic coupling via a
hydrogen bond is not new. Shygorin,[16] who was a pioneer in
discovering significant strengthening of hydrogen bonding
under quasiaromatic assistance (saturation of a chain closed
*
Correspondence to: A. Filarowski, Faculty of Chemistry, University of Wrocław,
F. Joliot-Curie 14, 50-383 Wrocław, Poland.
E-mail: afil@wchuwr.chem.uni.wroc.pl
by
a hydrogen bridge with p-electrons), explained this
phenomenon by p-electronic conjugation in the chain as well
as via a hydrogen bridge. With respect to p-electronic coupling
through the hydrogen bridge, its existence was questioned by
Luckiy,[25,26] who supported the opinion about hydrogen
bonding assistance by p-electronic conjugation in a chelate
chain, but rejected p-electronic assistance via a hydrogen bridge
(using semi-empirical calculations and measurement of ioniz-
ation potential). One should note that the visible decrease in the
n(XH) band’s integral intensity in IR spectra of o-hydroxy aryl
a
b
y
A. Filarowski, A. Kochel, M. Kluba
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383
Wrocław, Poland
F. S. Kamounah
Department of Science, Systems and Models, Roskilde University, P.O. Box 260,
DK-4000 Roskilde, Denmark
This paper is dedicated to Professor T.M. Krygowski on the occasion of his
70th birthday.
J. Phys. Org. Chem. 2008, 21 939–944
Copyright ß 2008 John Wiley & Sons, Ltd.

A. FILAROWSKI ET AL.
Scheme 1. A scheme of tautomeric equilibrium in o-hydroxy aryl Schiff bases
scaled using the KUMA data reduction package.[31] The data
were corrected for Lorentzian and polarization effects. Absorp-
tion correction was omitted. The structure was solved by
direct methods using SHELXS97[32] and refined by the full-
matrix least-squares method on all F2 data (SHELXL97).[33]
Non-hydrogen atoms were refined with anisotropic thermal
parameters; hydrogen atoms were included from Dr maps and
refined isotropically. CCDC-679628 for a-phenyl-4-(50-chloro-
20-hydroxybenzalamino) benzene (I), CCDC-679627 for N-(40-
benzo-15-crown-5)-3,5-dichloro-2-hydroxyphenylaldimine (II),
CCDC-679626 for 2-(1-phenylamino-ethyl)-naphthalene-1-ol (III)
and 679625 for 1-[(40-cyanophenylimino)methyl]-2-naphthol (IV)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK: fax:
(þ44) 1123-336-033; e-mail: deposit@ccdc.cam.ac.uk.
taken into account due to the absence of p-electronic coupling
via a hydrogen bridge (as stated in References [22,23,25,26]).
RESULTS AND DISCUSSION
The molecular structures with atom labelling are presented in
Fig. 1.
Structural aspect of tautomeric equilibrium
The structure of the hydroxy aryl Schiff bases in the OH form
dominates over the HN form in the Cambridge Crystallographic
Database.[37] The described phenyl and naphthalene derivatives
of the hydroxy aryl Schiff bases embrace three tautomeric forms:
the OH, the HN and the LBHB (low-barrier hydrogen bonds)
forms. The structures of compounds I and II show prevalence of
the OH form. This phenomenon is supported by both the location
of the hydrogen atom close to the nitrogen atom in the hydrogen
The HOMA and HOSE aromaticity indices were calculated by
the following formulae:[34,35]
—
bridge and the lengths of the C—O and C N bonds (the
—
n
X
ꢀ
ꢁ
1
2
—
extended C—O bond and the reduced C N bond) which
—
HOMA ¼ 1 ꢀ
ai Ropt ꢀ Ri
(1)
correspond to the lengths of the bonds of the OH form.[38] The
prevalence of the OH form is conditioned by the weakened
basicity of the nitrogen atom resulting from the electro-
n-withdrawing activity of the phenyl ring. This phenomenon
takes place despite the strong acidity of the hydroxyl group due
to the chlorine substituents in the phenol moiety. We observed
the HN form for these substituents in the N-alkyl derivatives of the
o-hydroxy aryl Schiff bases.[39] The structure of compound III
reveals the prevalence of the LBHB form, and equalization of
the bonds in the chelate chain also supports this fact. One
should underline that there are very few structures of such
type of o-hydroxy aryl Schiff bases in the Cambridge Crystal-
lographic Database[37] and they were obtained only for
the o-hydroxy aryl ketimines.[39–42] The next compound
(1-[(40cyanophenylimino)methyl]-2-naphthol) presents the HN
form in the solid state, which is unexpected: a compound without
cyanic substitution (1-[(phenylimino)methyl]-2-naphthol) takes
the OH form,[43–45] whereas a cyanic substituent must weaken the
nitrogen atom basicity on account of the –M mesomeric and –I
inductive effects. The paradoxical observation of the proton
transfer is explained by the coupling of two resonances (rather
reinforced due to the molecule flattening) in the chelate chain
n
i¼1
where n is the number of bonds, a an empirical constant and Ropt
and Ri the optimal and individual bond lengths taken from
Reference [36].
"
#
n1
n2
X
X
ꢀ
ꢁ
ꢀ
ꢁ
2
2
HOSE ¼ 301:15
R0r ꢀ Rs0 kr0 þ
R0r0 ꢀ Rd0 kr00
(2)
r¼1
r¼1
where R0r and R0r0 are the lengths of the p bonds in the real
molecule and n1 and n2 are the numbers of the single and double
bonds, respectively. The reference single and double bond
lengths Rs0 and Rd0 were taken from Reference [36]. The force
constants were calculated as kr ¼ a þ bRr. The constants a and b
were taken from Reference [36]. The higher the HOMA
aromaticity index with respect to the more aromatic formation,
the more delocalized the p-electrons of the formation. The HOSE
index means the energy required to stabilize an aromatic
formation up to the standard state, in our case the energy
required to feedback a chelate formation to an enol-imine state
—
—
(O—C C—C N).
—
It is necessary to note that in the calculations of the HOMA and
HOSE aromaticity indices, the OH and H. . .N bonds were not
—
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 939–944

HYDROXY ARYL SCHIFF BASES
Figure 1. An ORTEPIII diagram showing the molecular structure and the atom labelling scheme of a-phenyl-4-(50-chloro-20-hydroxybenzalamino)-
benzene (I), N-(40-benzo-15-crown-5)-3,5-dichloro-2-hydroxyphenyl-aldimine (II), 2-(1-phenylamino-ethyl)-naphthalene-1-ol (III) and 1-[(40-
cyanophenylimino)methyl]-2-naphthol (IV). The displacement ellipsoids of the non-H atoms are shown at the 50% probability level. The
intramolecular hydrogen bond is shown as a broken line
and the cyanic group (Scheme 2), as well as the influence of
˚
crystal packing and short contacts (d(H5O1) ¼ 2.57(2) A,
˚
˚
d(H13N2) ¼ 2.55(2) A, d(H17O1) ¼ 2.50(2) A), leading to the
domination of the HN form in compound IV.
Evaluating the state of tautomeric equilibrium in the
compounds, a transition from the OH form to the HN form is
observed according to the sequence I ! II ! III ! IV. This
sequence is obtained from the changes in the lengths of
the C—O and ON bonds. The reduction in the C—O bond
˚
˚
according to the sequence (d(C—O) ¼ 1.353(2) A (I) > 1.333(8) A
˚
˚
(II) > 1.312(2) A (III) > 1.274(2) A (IV), Table 1) indicates the
increase in the quinonoid form. This sequence is also consistent
with the changes in hydrogen bridge length: the strengthening
of the hydrogen bond from the OH form to the transition state
˚
˚
˚
form (d(ON) ¼ 2.516(2) A (I) > 2.493(6) A (II) ffi 2.496(2) A (III),
Table 1) and the proton transfer of the HN form (d(ON) ¼ 2.555(2)
˚
A (IV), Table 1). However, this trend is not in full agreement with
—
—
—
˚
the sequence for the
˚
C
N
˚
bond (d(C N) ¼ 1.281(6)
—
A
Scheme 2. A scheme of resonance coupling between cyanic group and
´
chelate chain in 1-[(4-cyanophenylimino)methyl]-2-naphthol
˚
(II) < 1.301(2) A (I) < 1.318(2) A (III) < 1.330(2) A (IV), Table 1)
—
owing to the elongated C N bond for compound I. Such a
—
‘discrepancy’ is grounded on a mesomeric effect of the phenyl
ring attached to the C7 atom of compound I (Fig. 1), which brings
—
be the C—O bond. This fact is consistent with the inference for
the hydrogen bonding complexes of phenols with pyridines.[46]
Another common feature of the studied hydroxy aryl ketimines (I,
II, III) and the hydroxy naphthalene aldimine (IV) is the N-phenyl
about elongation of the C N bond. The aforesaid observations
—
state that the most reliable characteristic in the evaluation of
tautomeric equilibrium in the hydroxy aryl Schiff bases appears to
J. Phys. Org. Chem. 2008, 21 939–944
Copyright ß 2008 John Wiley & Sons, Ltd.
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A. FILAROWSKI ET AL.
Figure 2. Scatter plot of the HOMA(ph) and HOMA(A) aromaticity
indices versus d(OH) bond length. Open and filled circles correspond
to the phenyl (data taken from Reference [7]) and naphthalene derivatives
of hydroxy Schiff bases, respectively.
fragment. However, this fragment behaves differently for
different compounds. In hydroxy naphthalene aldimine, this
ring lies in the plane of the naphthalene fragment in view of the
strong p-conjugation which, in turn, is disrupted in the hydroxy
aryl ketimines because of the steric effect of the methyl group (II,
III) or the phenyl ring (I).
Aromatic aspect of tautomeric equilibrium
This section presents the dependences of aromaticity on
tautomeric equilibrium. Figure 2 features two correlations: the
phenyl ring’s aromaticity versus the hydroxyl bond length
(HOMA(ph) ¼ f(d(OH)) and the
A naphthalene ring’s (ring
adjacent to the chelate chain, Scheme 1) aromaticity versus
the hydroxyl bond length (HOMA(A) ¼ f(d(OH)). It is important
that the length of the hydroxyl bond can be used as an indicator
of the degree of proton transfer in a hydrogen bridge and an
estimation of the tautomeric equilibrium. The observed
correlations are linear in approximation both for the phenyl ring
and the A ring’s aromaticity. However, a significant difference is
seen between the behaviour of the A ring’s aromaticity of the
naphthalene derivatives and the aromaticity of the phenyl ring.
Under the prevalence of the OH form of the phenyl ring, the
aromaticity is larger than that of the A ring of the naphthalene
derivatives (DHOMA ꢁ 0.25). This phenomenon is well grounded
considering the smaller aromaticity of the naphthalene rings
(HOMA ffi 0.8)[36] with respect to the aromaticity of the benzene
ring (HOMA ffi 1).[36] However, a much larger difference is seen for
the HN form (DHOMA ꢁ 0.45), where the HOMA index is ꢂ0.3 for
the naphthalene derivatives and ꢂ0.75 for the phenyl ones. This
reveals a larger loss of the A ring’s aromaticity compared with the
phenyl ring’s aromaticity under the proton transfer process.
Assuming aromaticity as a criterion of the degree of proton
transfer (HOMA(ph) ¼ f(d(OH)), we can confirm that the HN form
is easier to reach in the naphthalene derivatives than in the
phenyl ones. This can be explained by the HOMA(A) ¼ f(HO-
HOSE(ph)) and HOMA(B) ¼ f(HOSE(ch)) dependences (Fig. 3),
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HYDROXY ARYL SCHIFF BASES
Figure 3. Scatter plot of the HOMA(A) and HOMA(B) aromaticity indices
versus the HOSE(ch) index. Black circles and open squares correspond to
the A and B naphthalene rings, respectively
Figure 4. Scatter plot of the HOMA(ph) and HOMA(A) aromaticity
indices versus the HOSE(ch) index. Open and black circles correspond
to the phenyl (data taken from Reference [7]) and naphthalene derivatives
of hydroxy Schiff bases, respectively
which establish that the proton transfer process (the increasing
HOSE(ch) value[7]) brings about a weakening of the A ring’s
aromaticity on the one hand and an increase in the B ring’s
aromaticity on the other. The above comparison of the
phenyl and naphthalene derivatives shows that the aromaticity
balance between the A and B rings enhances the destabilization
(according to the HOSE index) of the chelate chain, namely
the OH form, and facilitates the proton transfer process.
These considerations speak in favour of a larger efficiency of
the resonance assistance between the chelate chain and the
aromatic formation adjacent to the chelate chain. As for
the hydroxy aryl Schiff bases it is reasonable to divide the
p-electronic resonances into two: the first being in the chelate
chain and the second between p-conjugation in the chelate chain
and an aromatic formation (phenyl, naphthalene formation and
so on).
aromaticity of the HN form is approximately stable, whereas the
phenyl ring’s aromaticity tends to decrease. The explanation for
this phenomenon lies in the re-distribution of the rate of the
canonical structures for the HN tautomeric form. The growing
prevalence of the keto-amine canonical structure causes a
weakening of the
A naphthalene ring’s aromaticity (this
confirmation is well substantiated for the phenolic complexes
in papers by Krygowski[2,29]), while the re-distribution between
the zwitter-ionic canonical structures and the keto-amine
canonical structures hardly changes the chelate chain’s aroma-
ticity. This reasoning gives proof of the rational use of canonical
structures in the description of the proton transfer tautomeric
form in the hydroxy aryl Schiff bases (Fig. 5).
The difference is also characteristic of the HOMA(ph) ¼ f(HO-
HOSE(ch)) and the HOMA(A) ¼ f(HOSE(ch)) correlations (Fig. 4).
This figure features two correlations for the phenyl and
naphthalene derivatives. Values of the HOMA and HOSE indices
for particular tautomeric forms are different for the phenyl and
naphthalene derivatives, respectively. For phenyl derivatives the
HOSE(ph) index is within the range of ꢁ30–40 kJ/mol for the OH
form and ꢁ70–100 kJ/mol for the HN form; the HOMA(ph) index
is found within the range of ꢁ1–0.9 for the OH form and ꢁ0.8–0.7
for the HN form. For the naphthalene derivatives, these values are
different: the HOSE(A) index is within the range of ꢁ20–60 kJ/mol
for the OH form and ꢁ80–140 kJ/mol for the HN form; the
HOMA(A) index is found to be within the range of ꢁ0.8–0.5 for
the OH form and ꢁ0.5–0.2 for the HN form. Based on these data
we confirm that the HOMA and HOSE aromaticity indices can be
applied in estimating the prevalence of a particular tautomeric
form for a particular aromatic formation (phenyl or naphthalene
etc).
The most interesting is the nonlinear HOMA(ph), HOMA(A) ¼
f(HOMA(ch)) dependence, which demonstrates that a transition
from the OH tautomeric form to the HN tautomeric form calls
forth a decrease in the phenyl ring’s aromaticity and an increase
in the chelate chain’s aromaticity. Noticeably, the chelate chain’s
Figure 5. Scatter plot of the HOMA(ph) and HOMA(A) aromaticity indices
versus the HOMA(ch) index. Black circles and open squares correspond to
the phenyl and naphthalene hydroxy Schiff bases, respectively
J. Phys. Org. Chem. 2008, 21 939–944
Copyright ß 2008 John Wiley & Sons, Ltd.
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A. FILAROWSKI ET AL.
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This paper scrutinized crystallographic structural data of four
hydroxy aryl Schiff bases in three different tautomeric forms. The
crystallographic structure of the o-hydroxy aryl Schiff base in
combination with crown ether was solved for the first time.
Compounds of this type can serve as light-optical switches,[47]
elements of electronic memory[48] and biologically active
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forms. It was found that the CO bond is the most reliable
characteristic for defining tautomeric equilibrium.
We accomplished an analysis of aromaticity with the assistance
of the HOMA and HOSE aromaticity indices of the phenyl and
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shown that the intramolecular proton transfer process in the
hydroxy naphthalene Schiff bases brings
a simultaneous
decrease in the A ring’s (adjacent to the chelate chain) aromaticity
and an increase in the aromaticity of the B ring (distant from the
chelate chain). Moreover, it was found that the aromatic system,
strongly coupled with the chelate formation, plays an important
role in describing tautomeric equilibrium.
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in describing tautomeric equilibrium in Schiff bases were
elucidated. The aromaticity balance was studied: the increase
in p-electron delocalization in the chelate chain during
the decrease in p-electron delocalization in the adjacent aromatic
ring and the increase in p-electron delocalization in the distant
aromatic ring under the shift of tautomeric equilibrium into the
direction of the proton transfer form.
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