NMR and X-ray studies of 2,6-bis(alkylimino)phenol Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2007.05.029

A series of model Schiff bases - 2,6-bis(alkylimino)phenol derivatives (Me, c-Pr, c-Bu, c-Pen and Ph) - has been studied by the solution and solid state (c-Pr and Ph derivatives) NMR methods. All compounds under investigations exhibit tautomeric equilibri

Full text of DOI:10.1016/j.molstruc.2007.05.029

Available online at www.sciencedirect.com
Journal of Molecular Structure 844–845 (2007) 94–101
www.elsevier.com/locate/molstruc
NMR and X-ray studies of 2,6-bis(alkylimino)phenol Schiff bases
a,
a
b
b,
*
*
´
Wojciech Schilf , Bohdan Kamienski , Anna Szady-Chełmieniecka , Eugeniusz Grech
,
c
c,
*
Anna Makal , Krzysztof Woz´niak
a
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland
Department of Inorganic and Analytical Chemistry, University of Technology, 71-065 Szczecin, Al. Piastow 42, Poland
b
c
Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warszawa, Poland
Received 13 February 2007; received in revised form 16 May 2007; accepted 19 May 2007
Available online 8 June 2007
Abstract
A series of model Schiff bases – 2,6-bis(alkylimino)phenol derivatives (Me, c-Pr, c-Bu, c-Pen and Ph) – has been studied by the solu-
tion and solid state (c-Pr and Ph derivatives) NMR methods. All compounds under investigations exhibit tautomeric equilibria in which
proton is distributed between two imine sites. For the whole series, the strongest intramolecular H-bond is formed by the methyl deriv-
ative. The cycloaliphatic derivatives form weaker H-bonds. Their strength is proportional to the size of the aliphatic ring. The principal
components of 15N chemical shift anisotropy (CSA) tensor have been measured in the solid state for the cyclopropyl derivative. The first
two principal components of the CSA tensor change in the same way with respects to hydrogen bond formation as the isotropic nitrogen
chemical shift values. A relationship between the CSA principal components and structure of tautomers present in the system has been
discussed. In chloroform solutions we also measured the coalescence temperatures to estimate the reaction rate and activation energy.
For the c-Pr Schiff-base derivative, the X-ray structure has been established. This compound crystallizes in the monoclinic P21 space
group. There are two independent molecules in the crystal lattice. The geometry of the two moieties is very similar with the exception of
some subtle changes in the intramolecular hydrogen bond and its vicinity. Although the molecules seem to be related by a glide plane, the
symmetry is non-crystallographic and there is no indication of the existence of such a plane in the intensities of the reflections. We present
details of the hydrogen bonds in both moieties.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Schiff bases; X-ray analysis; NMR studies
1. Introduction
been applied for structural investigation of Schiff bases,
some examples are published in: optical spectroscopy [5–
7], X-ray diffraction [4,8–11], hetero nuclear NMR mea-
surements in the liquid and in the solid state [12–17], as well
as theoretical DFT calculation [18]. The Schiff bases
derived from 2-hydroxy-1,3-benzenedicarboxaldehyde are
even more interesting from a structural point of view since
they have two equivalent basic sites and only one hydrogen
bond donor. The photo- and thermochromic properties of
those compounds have been investigated by Metelitsa et al.
on a number of differently substituted 2-hydroxy-1,3-dial-
dehyde derivatives [19]. The dynamic and kinetic aspects
of internal rotation of the OH group in aromatic amines
Schiff bases are very interesting compounds because of
their model character and practical applications[1,2]. Some
of the Schiff bases, containing the hydroxyl group(s) at the
ortho position(s), can form intramolecular hydrogen
bonds. Such H-bonds have tautomeric character. The
properties of such H-bonds and the correlation between
structure and the proton position in the hydrogen bridge
were reviewed and discussed by Filarowski [3] and Domin-
iak et al. [4]. Many instrumental analytical methods have
*
and
2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde
Corresponding authors. Tel./fax: +48 22 8222892 (K. Woz´niak).
E-mail address: kwozniak@chem.uw.edu.pl (K. Woz´niak).
derivatives have been considered by Dziembowska et al.
0022-2860/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.05.029

W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
95
[20]. On the basis of the line-shape analysis of proton sig-
nals they calculated the reaction rates and energy of activa-
tion for three aromatic amine derivatives.
In the present work we extend heteronuclear NMR and
X-ray investigations on several model Schiff-base deriva-
tives. We also support NMR studies by X-ray structural
investigations of one of the studied compounds. Unfortu-
nately we were unable to get single crystals of the quality
suitable for single crystal X-ray investigations for the other
compounds belonging to the series.
.
H
X
.
4-methyl-2,6-bis-(alkylimino)-phenol
X= Me, c-Pr, c-Bu, c-Pen
4-methyl-2,6-bis-(phenylimino)-phenol
X-Ph
O
N
8
1
X
7
N
2
3
6
5
4
CH3
Fig. 1. Definition of compounds studied.
mated Mo Ka radiation (k = 0.71073 A, 50.0 kV,
40.0 mA) at liquid nitrogen temperature (100 K). The big,
dark orange crystal was mounted on the glass fiber with sil-
icon grease and positioned at 62 mm from the KM4CCD
camera. 400 frames were collected at 1.5ꢁ intervals with
counting time of 35 s. The multi-scan absorption correction
was applied to the collected dataset. Data reduction and
analysis were carried out with the Kuma Diffraction
programs.
The structure was solved by direct methods using the
SHELXS-97 [21] program and refinement was carried out
with SHELXL-97 [22]. The refinement was based on F2
for all reflections except those with negative intensities.
Weighted R factors wR and all goodness-of-fit S values
were based on F2, whereas conventional R factors were
based on the amplitudes, with F set to zero for negative
F2. The F 20 > 2rðF 20Þ criterion was applied only for R fac-
tors calculation and was not relevant to the choice of reflec-
tions for the refinement. The R factors based on F2 are for
both structures about twice as large as those based on F.
The positions of hydrogen atoms were found exactly from
the difference electron density map and isotropic tempera-
ture displacement parameters were refined for each of
them. Scattering factors were taken from Tables 4.2.6.8
and 6.1.1.4 from the International Crystallographic Tables
Vol.C [23].
2. Experimental
2.1. Synthesis
All compounds under investigations have been synthe-
sized by condensation of 2-hydroxy-5-methyl-1,3-benzene-
dicarboxaldehyde with appropriate amine in methanol
solution. The 1:1 mixtures of the aldehyde and amine were
refluxed for about 4 h and then evaporated. The com-
pounds crystallized from methanol solution.
2.2. NMR
All NMR spectra were measured using a Bruker DRX
500 Avance spectrometer equipped with triple resonance
TBI 5 mm inverse probehead. The chemical shifts assign-
ments were made by analysis of standard 1D and 2D
COSY, HGSQC and GHMBC spectra. The proton and
carbon chemical shifts are presented with respect to inter-
nal TMS, the nitrogen spectra are referred to external
nitromethane as a standard. The variable temperature
experiments were performed using a BTO 2000 Variable
Temperature Unit. The temperature scale was calibrated
using a pure methanol sample as a standard. The solid state
spectra were run using a 4 mm CPMAS Bruker probehead.
The typical acquisition parameters for carbon CPMAS
spectra were: spectral width 31 kHz, acquisition time
20 ms, contact time 2 ms and spin rate 9–12 kHz. To distin-
guish the CHn and quaternary atoms short contact time
experiments were applied. For such experiments the con-
tact time was 40 ls. Typical spectral conditions for the
nitrogen solid state spectra were: spectral width 28 kHz,
acquisition time 40 ms, contact time 5 ms and spin rate
6–12 kHz. For all solid state measurements the relaxation
delay was in the range 10–120 s depending on the relaxa-
tion properties of the sample. Originally the CPMAS spec-
tra were referred to an external glycine sample and then
chemical shift values were recalculated to TMS and nitro-
methane scale for 13C and 15N spectra, respectively.
Crystallographic data (excluding structure factors) for
the structure reported in this work have been deposited
with the Cambridge Crystallographic Data Centre and
allocated the deposition number: CCDC 636043. Copies
of the data can be obtained free of charge on application
to CCDC, 12 Union Road, Cambridge CB2 1EW, UK
(fax: Int. code +1 223 336 033; e-mail: deposit@ccdc.cam.
ac.uk).
The details of the data collection and structure refine-
ment parameters are given in Table 1.
3. Results and discussion
The results of the carbon and nitrogen chemical shift
measurements are collected in Table 2. According to our
previous experience in hydrogen bond structure estimation,
one can expect that the best parameters for this task could
be nitrogen chemical shifts and carbon chemical shifts of
atoms at the C-1 position (Fig. 1).
2.3. X-ray data collection and structure refinement
The structure of the c-Pr Schiff base (see Figs. 1 and 2)
was determined in a single-crystal X-ray diffraction exper-
iment. The measurement was performed on a Kuma
KM4CCD j-axis diffractometer with graphite-monochro-
Unfortunately, the carbon chemical shifts of C-1 atoms
are practically useless in the discussed group of

96
W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
moleculeI
moleculeII
H9B
C9
H9A
H10A
H10B
H9D
H9E
H10E
C9A
C10
a
b
H10D
b
c
a
C10A
H8
c
N1
C8
H1OD
H1O
C1
N1A
H13D
H12D
C8A
H7D
O1A
H11D
H8D
O1
C6
C7
C13A
C12A
H13E
H7
H14D
H11
C11
C2
C2A
C6A
H12
C7A
C1A
C11A
C14A
H14E
N2A
C
C12
C3
C
N2
H
N
O
H13A
H14B
H3
H14A
C14
H
N
O
C5
C4A
C4
H3D
C13
H13B
C3A
C5A
H5D
H5
C15A
H15D
C15
H15A
H15E
H15F
H15B
H15C
Fig. 2. Molecules I and II of the c-Pr Schiff-base in ORTEP representation. Thermal displacement ellipsoids are drawn at the 50% probability level.
Table 1
Data and refinement statistics
the exchange limit. This approach can provide some useful
information on the dynamics of the exchange but no infor-
Empirical formula
Formula weight
C15H18N2O
242.31
Monoclinic, P21
mation about the proton position in the intramolecular
hydrogen bridge.
Crystal system, space group
Unit cell dimensions
The first approximation of the proton position in the H-
bridge can be achieved by examination ofthe nitrogen chem-
ical shifts. When the values of this parameter are in the range
from –70 to –100 ppm, this indicates that the molecule can
exist as the OH structure with a relatively weak hydrogen
bond to the nitrogen atom. The Schiff bases with the proton
transferred to the nitrogen site are characterized by nitrogen
chemical shifts close to –200 ppm (the lowest observed value
is about –240 ppm). The nitrogen chemical shift by itself is
not a very precise measure of the proton position. A much
better approximation can be achieved by investigations of
the difference between chemical shifts of the pure (no H-
bond at all) OH compounds and the ones from the H-
bonded structure. Generally, it is very difficult to find a good
reference compound for the non-bonded structure. It is
much easier in the discussed group of compounds, because
in the same molecules we have both the bonded and non-
bonded structural Schiff molecular fragments. For the exact
estimation of the proton position in intramolecular hydro-
gen bonds, it is necessary to consider the existence and
strength of the interactions with the solvent. In chloroform
solution such interactions (weak hydrogen bonds) do exist
and can cause measurable effects of an upfield shift of the
values of the nitrogen chemical shift. Those effects are in
the range of a few ppms and they depend on the basicity of
the investigated nitrogen site.
˚
a (A)
7.0253(11)
16.957(3)
11.4895(19)
90
103.769(14)
90
1329.4(4)
4, 1.211
0.077
520
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
Volume (A )
Z, calculated density (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
h Range for data collection (ꢁ)
Limiting indices
2.99–27.00
ꢀ8 6 h 6 8, ꢀ21 6 k 6 21,
ꢀ14 6 l 6 14
11999/5543 [0.0189]
Reflections collected/unique
[R(int)]
Completeness to h = 27.00
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
99.90%
Semi-empirical from equivalents
0.997 and 0.987
Full-matrix least-squares on F2
5543/1/469
0.981
R1 = 0.0326, wR2 = 0.0845
R1 = 0.0467, wR2 = 0.0888
compounds. Using this parameter, one can only state that
hydrogen bonding does exist but the proton position in the
H-bond bridge remains undetermined. The other carbon
chemical shifts do not provide useful structural informa-
tion either. From those data we can distinguish both the
bonded and non-bonded parts of the molecules but we can-
not estimate the proton position. The proton signals of all
investigated compounds at room temperature in chloro-
form are broadened indicating dynamic processes occur-
ring in the molecules. Lowering the temperature can shift
the exchange rate to coalescence and, finally, to slow down
The best approximation of the structure without inter-
molecular H-bonds are solid state spectra of reference com-
pounds where the intermolecular hydrogen bonds with
solvent cannot take place. For the aliphatic derivatives,
two nitrogen chemical shift values in the solid state for imi-
nes without hydrogen bonds are available: ꢀ47.9 ppm for
the cyclopropyl derivative and ꢀ47.2 ppm for the cyclopen-
tyl compound (the average value ꢀ47.55 ppm).

W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
97
Table 2
Carbon and nitrogen chemical shifts of compounds under investigation
Me
160.0
c-Pr
c-Pr solid state
c-Bu
c-Pen
Ph
Ph solid state
160.4a
118.0
b
C1
C2
C3
C4
C5
C6
C7
C8
157.5
119.1
133.2
128.1
128.8
123.3
162.8
154.7
ꢀ4.2
4.4
157.2
119.4
133.4
126.9
130.1
123.6
163.7
152.6
ꢀ4.2
3.3
160.4
118.3
134.1
127.3
130.1
123.4
161.9
154.1
ꢀ5.1
4.0
159.9
118.3
133.9
127.4
130.3
123.7
162.4
154.5
ꢀ5.4
3.6
159.7
119.4
136.2
128.6
131.0
123.6
162.4
156.0
ꢀ4.2
5.2
118.6
134.3
127.5
130.0
123.1
165.9
158.5
ꢀ4.5
b
b
124.1
160.4a
154.8
ꢀ6.1
Dd = dC2 ꢀ dC6
Dd = dC3 ꢀ dC5
Dd = dC7 ꢀ dC8
N–H
4.3
7.4
ꢀ103.8
ꢀ72.7
ꢀ31.1
ꢀ56.2
8.1
11.1
7.8
7.9
6.4
5.6
ꢀ74.8
ꢀ53.3
ꢀ21.5
ꢀ27.2
20.7
ꢀ73.3
ꢀ47.9
ꢀ25.4
ꢀ25.7
21.3
ꢀ86.6
ꢀ50.2
ꢀ24.4
ꢀ39.0
20.3
ꢀ83.4
ꢀ47.2
ꢀ36.2
ꢀ35.8
20.5
ꢀ86.6
ꢀ61.8
ꢀ24.8
ꢀ32.9
20.3
ꢀ92.1
ꢀN@
–
Dd = dNH ꢀ d
DdR = dNH ꢀ ꢀdN@
–
–
ꢀN=REF
CH3
C10
19.4
40.6
42.1
60.7
69.5
42.6
9.9
63.0
30.5
72.6
34.6
C20
C30
10.3
8.9
15.6
24.6
d
d
ꢀN = REFꢀ reference nitrogen chemical shift for Schiff base without intermolecular H-bond with solvent. For aliphatic compounds
ꢀN = REF = ꢀ47.6 ppm (the average value for aliphatic derivative in the solid state where intermolecular H-bonds with solvent does not exist). For the
aromatic derivative it is the value obtained for Ph–CH=N–Ph in DMSO solution; dꢀN = REF = ꢀ53.7 ppm (Annual Reports on NMR Spectroscopy, Vol.
11B).
a
Overlapping signals.
b
Unresolved pattern of overlapping signals from three aromatic rings.
For the aromatic X=Ph derivative, this procedure
cannot be applied since the signal of the nitrogen atom
without H-bond was missing in the nitrogen-15 solid
state spectrum. This was probably due to a severe broad-
ening of this signal or very inconvenient relaxation prop-
erties of this compound. In this case, it was necessary to
use the model compounds for the structure without H-
bond. The best approximation seems to be the phe-
nyl[1-phenylmethylidene]-amine in DMSO solution
concerted action of both intra and intermolecular hydro-
gen bond interactions with solvent moieties. The effects
of intermolecular hydrogen bonding can be removed by
application of second parameter DdR = dNH ꢀ d ꢀ N=REF
.
Then it appears that the strongest intramolecular H-bond
is formed by the methyl derivative. The cycloaliphatic
derivatives form weaker H-bonds the strength of which is
proportional to the size of the aliphatic ring. The hydrogen
bond strength of the aromatic derivatives is studied using
the cyclopropyl and cyclobutyl derivatives. This means that
the cyclopropyl ring does not force the proton to transfer
from the oxygen atom to the nitrogen, which is in contrast
to the linear aliphatic chains or larger aliphatic rings (cyc-
lobutyl or cyclopentyl).
d
ꢀN=REF = ꢀ53.7 ppm. The results of such calculations
are collected in Table 3.
The effects of hydrogen bond formation on the nitrogen
chemical shifts are presented by two parameters
Dd = dNH ꢀ d
and DdR = dNH ꢀ d ꢀ N=REF. The first
ꢀN@
one is a simple difference between the nitrogen chemical
shifts of bonded and non-bonded atoms and it suggests
that in all compounds the intramolecular H-bonds are very
similar. However, the observed effects are generated by
Table 4 contains a comparison of the principal compo-
nents of the chemical shift anisotropy (CSA) tensor of com-
pounds containing intramolecular hydrogen bonds. For all,
except one, presented compounds, the principal compo-
Table 3
The principal components of CSA tensor
c-Pr OH
i-Pr
c-Pr H-bond
Me H-bond
Me H-bond
Me NH
Me NH
dIso
ꢀ47.9
204
ꢀ62.6
ꢀ73.3
165
ꢀ85.4
198.1
ꢀ90.4
188.0
ꢀ235.3
ꢀ104.1
ꢀ246.2
ꢀ355.6
ꢀ237.4
ꢀ105.1
ꢀ253.2
ꢀ354.0
d
d
d
11 = dt
22 = dr
33 = d^
139.5
ꢀ11.0
ꢀ12
ꢀ62
ꢀ81.6
ꢀ86.6
ꢀ335
ꢀ316.4
ꢀ324
ꢀ372.7
ꢀ372.5
c-Pr OH, cyclopropyl derivative in the OH form without H-bond; i-Pr, isopropyl naphtyl derivative in OH form with very weak H-bond [14]; c-Pr H-bond,
cyclopropyl derivative in OH form with H-bond; Me H-bond, N-methyl derivative in OH form with H-bond [14]; Me NH, N-methyl derivative in NH
form [14].

98
W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
Table 4
approximate character of our calculations based on the
coalescence temperature measurement. The second source
of difference could be error in temperature measurement
in both series of experiments. The precise temperature mea-
surements can be crucial in standard experiments using dif-
ferent probe head and temperature control units. However
one can state that activation energies for all the aliphatic
compounds are very close and slightly lower than those cal-
culated for the aromatic compound.
Activation energies and exchange rates of investigated compounds
calculated from proton NMR data
Me
c-Pr
c-Bu
c-Pen
Ph
H7–H8
DdH [Hz]
kco [1/s]
Tco [K]
DG+
235
521
275
52.8
221
490
274
52.8
260
577
280
53.6
255
566
290
55.7
187
415
293
57.2
H4–H6
DdH [Hz]
kco [1/s]
Tco [K]
DG+
310
688
279
53.4
282
626
279
53.2
365
810
287
54.2
335
743
295
56.2
348
772
303
57.7
3.1. Structural results
The structural picture of H-bond interactions in a model
c-Pr Schiff base is quite a complex one. The c-Pr Schiff base
crystallizes in the monoclinic P21 space group. There are
two independent molecules in the crystallographic asym-
metric part of the unit cell and all the atoms occupy general
positions. The geometry of the two moieties is very similar,
some subtle changes are present only in the case of the
intramolecular hydrogen bond and its vicinity. Otherwise,
the molecules seem to be related by a glide plane, although
the symmetry is non-crystallographic and there is no indi-
cation of the existence of such a plane in the intensities
of the reflections.
nents dt and dr change in the same way as the isotropic
nitrogen chemical shift values.
The third component d^, does not follow this relation.
First of all, the amplitude of this component is much smal-
ler then the other ones and, hence, it is much less sensitive
to structural differences. This component describes the elec-
tron density along the direction perpendicular to the nitro-
gen atom environment. This means that the hydrogen bond
formation does not affect the electron density distribution
in this direction. The only compound which behaves differ-
ently (c-Pr) has the methyl substituent on the Schiff carbon
atom. The methyl group in this position causes some steric
effects promoting the OH structure and can disturb elec-
tron distribution at the nitrogen atom neighborhood. Gen-
erally, one can say, that the principal components dt and dr
are far more sensitive to H-bond formation compared with
the isotropic value.
In all our previous investigations of H-bonding in Schiff
bases at room temperature, we always observed a proton
exchange process – very fast on the NMR time scale. In
contrast to this, in the proton spectra of 4-methyl-2,6-bis-
(alkylimino)-phenol derivatives, a severe dynamic broaden-
ing was observed. Previously, the dynamic of proton
exchange in Schiff bases was investigated in a few arylimino
derivatives. Dziembowska et al. [20], using line-shape anal-
ysis found that activation energy for those compounds is in
range from 47.2 to 49.5 kJ/mol and only slightly depends
on the substituent in the phenyl ring.
The general structure of the molecule I and II is pre-
sented in the Fig. 2.
The central aromatic ring is planar and shows only
slight bond alternation, with the C1–C2 bond significantly
˚
˚
longer (1.410 A vs. 1.39 A on the average). There is an
intramolecular hydrogen bond between the hydroxyl group
and one of the Schiff-base nitrogen atoms, denoted further
as N1, and therefore a pseudo-six membered ring is
formed. The N1 and the Schiff-base C7 atoms are located
in the plane of the main aromatic ring. The cyclopropyl
substituent connected with the N1 atom appears in the
trans position with respect to the Schiff-base carbon atom.
In the case of the other cyclopropyl substituent, it is located
in the cis position with respect to the C11 Schiff-base car-
bon atom. The substituent is slightly distorted with respect
to the main aromatic ring, so that C14 lies in the plane
defined by the main aromatic ring, whereas the C12 atom,
connecting the cyclopropyl moiety with the rest of the mol-
ecule, is well out of that plane.
In the present work, we measure the coalescence temper-
ature to estimate the reaction rate and activation energy.
The results of these measurements are collected in Table
4. For all compounds the reaction rates and DG+ values
were calculated for two temperatures at which coalescence
was observed for two pairs of proton signals: H7–H8 and
H4–H6, respectively. The results of such calculations show
that the activation energies calculated on the basis of the
coalescence temperature of protons H4–H6 are slightly
higher then those obtained from the H7–H8 data. The dif-
ferences are on an acceptable low level. The most interest-
ing conclusion from comparison of our and published data
is a systematic difference in the activation energies calcu-
lated by both methods. This may be a consequence of the
The main differences between the molecules are mani-
fested in the geometry of the intramolecular hydrogen
bonds. The H-bond is responsible for many of the proper-
ties of the Schiff-base and the crystal structure gives us
means to analyze the two different kinds of H-bonds in
the same structure and similar crystal environment. Addi-
tionally, the geometrical information from that particular
structure is valuable because there are no intermolecular
interactions in the structure, in which the atoms constitut-
ing the H-bonds might have been involved.
In the case of the molecule I, the H1O hydrogen posi-
tion is better defined. The H1O atom is located closer
to the O1 oxygen and in the plane formed by the
other members of the pseudo-six membered ring (i.e.

W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
99
Fig. 3. c-Pr Schiff base – difference electron density maps prepared in O1–C1–C2–C7–N1 plane with the H-bond hydrogen included for molecule I (a) and
molecule II (b) or excluded for molecule I (c) and molecule II (d) during the calculation of the map coefficients.
O1–C1–C2–C7–N1), with the H1O–O1–C1–C2 torsion
defined. In the molecule I, the H1OD hydrogen is
located significantly further from the O1D oxygen and
out of the O1D–C1D–C2D–C7D–N1D plane, with the
H1OD–O1D–C1D–C2D torsion angle ꢀ8.44(10)ꢁ. The
angle 0.78(11)ꢁ. The isotropic thermal displacement fac-
2
˚
tor of the H1O atom has a value of 0.031(5) A which
shows, that the position of the hydrogen atom is well
Table 5
Selected bond lengths and valence angles in the vicinity of intramolecular H-bond
˚
Bonds (A)
Molecule I
Molecule II
Angles (ꢁ)
Molecule I
Molecule II
H1O–O1
O1–C1
C1–C6
C1–C2
C2–C3
C2–C7
C7–H7
C7–N1
N1–H1O
N1–C8
N1–O1
0.878(20)
1.349(5)
1.398(11)
1.410(6)
1.397(5)
1.448(11)
0.928(21)
1.273(6)
1.795(22)
1.42(1)
0.983(33)
1.352(5)
1.394(11)
1.410(6)
1.390(5)
1.448(11)
1.041(23)
1.275(6)
1.729(36)
1.418(10)
2.590(3)
N1–H1O–O1
H1O–O1–C1
O1–C1–C2
O1–C1–C6
C1–C2–C7
C1–C2–C3
C2–C7–H7
C2–C7–N1
C7–N1–C8
C7–N1–H1O
149.86(205)
107.31(139)
121.08(16)
119.17(16)
121.03(19)
119.32(17)
114.39(129)
122.39(18)
118.14(16)
98.10(69)
144.08(312)
108.66(197)
121.22(16)
119.09(16)
121.16(19)
119.37(17)
118.24(119)
122.01(17)
119.00(16)
100.87(112)
2.592(3)

100
W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
as well as without it look rather similar and there is
a
no strong indication that a proton transfer form oxygen
to nitrogen might take place in the molecule II. Never-
theless, the differences in the location and behavior of
the H atoms in the two H-bonds influence also the
geometry of the adjacent part of the molecule. A com-
parison of the more important geometrical parameters
is presented in Table 5.
The packing of the c-Pr molecules in the crystallo-
graphic unit cell reveals a nice illustration of the non-crys-
tallographic symmetry in the structure. It is shown in
Fig. 4. Both in the case of the view along the X and Z crys-
tallographic directions the molecules I and II seem to be
related by a plane – an a glide plane. Closer analysis of
the distances, however reveals that the translation needed
to transform molecule I into II is not 1/2 of the a crystal-
lographic unit.
b
c
b
The molecules I and II form layers perpendicular to the
crystallographic Y direction each consisting of molecule I
or II. The central aromatic rings in a single layer are ori-
ented parallel to one another so that the hydrogens from
the cyclopropyl moiety are oriented toward the central aro-
matic ring of the next molecule of the same kind. Short
contacts appear also between the carbons from the cyclo-
propyl moieties and the hydrogens from the aromatic ring
that belongs to the molecule of the different kind.
In conclusion we have presented structural (for c-Pr)
and NMR spectroscopic properties for a series (Me, c-Pr,
c-Bu, c-Pen, Ph) of model Schiff bases – 2,6-bis(alkylimino)
phenol derivatives. It appears that the strongest intra-
molecular H-bond is formed by the methyl derivative.
The cycloaliphatic derivatives form weaker H-bonds with
strength proportional to the size of the aliphatic ring. For
the cyclopropyl derivative we were able to measure a slow
rotated solid state spectrum and calculate the principal
components of chemical shift anisotropy tensor (CSA).
We found out that the first two principal components of
the chemical shift anisotropy (CSA) tensor change in the
same way as the isotropic nitrogen chemical shift values.
We also measured the coalescence temperature to estimate
the reaction rate and activation energy.
a
c
c
For the c-Pr Schiff-base derivative, the X-ray structure
has been established. It appears that there are two indepen-
dent molecules in the crystal lattice. The geometry of the
two moieties is very similar with the exception of some sub-
tle changes in the intramolecular hydrogen bond and its
vicinity. Although the molecules seem to be related by a
glide plane, the symmetry is non-crystallographic and there
is no indication of the existence of such a plane in the inten-
sities of the reflections. We present details of the hydrogen
bonds in both moieties.
b
a
Fig. 4. Crystal packing for the c-Pr Schiff-base along the X-axis (a), Y-axis
(b) and Z-axis (c).
isotropic thermal displacement factor of the H1OD atom
has a value of 0.087(5) A , nearly three times higher than
2
˚
the same parameter measured for the H1O atom and
twice as high as the parameters for the other H atoms
in the structure. This indicates a significant amount of
thermal motion or possibly the partial transfer of the
proton to the N1D atom. However, the difference elec-
tron density maps (Fig. 3) prepared for both molecules
with omission of the H atoms involved in the H-bond
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.
2007.05.029.

W. Schilf et al. / Journal of Molecular Structure 844–845 (2007) 94–101
101
´
[12] W. Schilf, B. Kamienski, A. Szady-Chelmieniecka, E. Grech, Solid
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