Neutral and ionic multiple hydrogen bonded moieties in crystal structure of a one tripodal Schiff base

Article abstract of DOI:10.1016/j.molstruc.2003.11.040

Structural and spectroscopic properties of solid tris(3-formyl-5-methyl-2- salicylidenamino-ethyl)amine have been investigated. This is a compound containing two moieties with triple Schiff base fragments. One of the moieties has all three neutral intramolecular O-H?N hydrogen bonds, whereas there are only ionic H-bonds in the other moiety. Both moieties form cavities with the amine N atom lone electron pairs inside strongly shielded by three aliphatic wings. The H-bond formed in the Schiff fragments influence the other structural parameters of the nearest aromatic parts of the moieties. ¹³C and ¹⁵N CP/MAS NMR results (chemical shifts) allow to identify two pairs of Schiff and amino nitrogen nuclei thus supporting formation of the two types of moieties in the crystal lattice.

Full text of DOI:10.1016/j.molstruc.2003.11.040

Journal of Molecular Structure 691 (2004) 133–139
www.elsevier.com/locate/molstruc
Neutral and ionic multiple hydrogen bonded moieties in crystal structure
of a one tripodal Schiff base
a
Beata Kołodziej , Paulina M. Dominiak , Anna Ko s´ cielecka , Wojciech Schilf ,
b
b
c
a
Eugeniusz Grech , Krzysztof Wo z´ niak
b,
*
a
Technical University of Szczecin, ul. Piast o´ w 42, Szczecin 71-065, Poland
Department of Chemistry, University of Warsaw, ul. Pasteura 1, Warszawa 02-093, Poland
b
c
Polish Academy of Sciences, Institute of Organic Chemistry, 01-224 Warszawa, Kasprzaka 44/52, Poland
Received 22 April 2002; revised 11 October 2003; accepted 19 November 2003
Abstract
Structural and spectroscopic properties of solid tris(3-formyl-5-methyl-2-salicylidenamino-ethyl)amine have been investigated. This is a
compound containing two moieties with triple Schiff base fragments. One of the moieties has all three neutral intramolecular O–H· · ·N
hydrogen bonds, whereas there are only ionic H-bonds in the other moiety. Both moieties form cavities with the amine N atom lone electron
pairs inside strongly shielded by three aliphatic wings. The H-bond formed in the Schifffragments influence the other structural parameters of
1
3
15
the nearest aromatic parts of the moieties. C and N CP/MAS NMR results (chemical shifts) allow to identify two pairs of Schiff and amino
nitrogen nuclei thus supporting formation of the two types of moieties in the crystal lattice.
q 2003 Elsevier B.V. All rights reserved.
Keywords: X-ray diffraction; Schiff bases; Hydrogen bond; Neutral; Ionic
1
. Introduction
Since Ref. [13], more and more Schiff bases with ionic
[
2
O · · ·H–N ] hydrogen bond in the solid state have been
þ
published. Polydente Schiff bases have focused much
attention due to their uses in complexation of lanthanoids
Imines, sCyN–R, and their derivatives, containing an
aryl or alkyl group at nitrogen or carbon, form a group of
compounds known as Schiff bases [1]. Schiff bases and their
derivatives are subject to ca. 500 publications per year. They
are so popular owing to a wide range of their applications in
organic synthesis [2–6]. A special case of Schiff bases is
salicylideneanilines. They—and related Schiff bases with
the intramolecular O–H· · ·N hydrogen bond—have
received considerable attention due to their interesting
photophysical properties and potential applications as
organic materials for reversible optical data storage devices
[14,15], as contrasts agents in NMR imaging [16], because
of their photochemical properties [17] and electrochemical
applications [18].
In this paper, we would like to present the structure of a
multiple Schiff base tris(3-formyl-5-methyl-2-salicylidena-
mino-ethyl)amine (hereafter abbreviated NMSB £ IMSB,
where NMSB stands for the neutral H-bonded Schiff base
moiety and IMSB stands for the ionic one). This is a
tripodal Schiff base ligand. It consists from two moieties.
According to the CSD searches, this is the first case of a
multiple Schiff base being a complex of two moieties
containing separately neutral and ionic hydrogen bonds
[
7–10]. These interesting properties result from coupling of
intramolecular H-bonding with p-electron density of the
aromatic ring.
(
Fig. 1). Such compounds as this one provide a framework
Most of the Schiff bases in the crystalline state have
neutral O–H· · ·N hydrogen bond with the proton localized
at the oxygen atom. In some cases, the proton position is
disordered over two sites—at the N and O atoms [11,12].
from which polimetalic metal complexes with controlled
topology and composition can be prepared. Their three
arms form a cavity, which contains lone electron pair of the
bridging N atom. Each arm has two potential binding
sites: an imine nitrogen and a phenolic oxygen. Thus,
two limiting forms of hydrogen bonding are possible:
*
Corresponding author. Tel./fax: þ48-22-822-2892.
E-mail address: kwozniak@chem.uw.edu.pl (K. Wo z´ niak).
0022-2860/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2003.11.040

134
B. Kołodziej et al. / Journal of Molecular Structure 691 (2004) 133–139
Fig. 1. Numbering of atoms in two independent moieties of solid tris(3-formyl-5-methyl-2-salicylidenamino-ethyl)amine (NMSB £ IMSB). Hydrogens
omitted for clarity with exception of those participating in H-bonding in the Schiff base fragments. The labelling scheme for NMR results is given in Fig. 3.
1
The C solid state spectra were referenced to methylene
3
enolimino form (neutral hydrogen bonding): O–H· · ·N and
2
þ
ketoamino one (ionic form): O · · ·H–N [19]. The
different hydrogen bond types are accompanied by
different p-electron distributions within the moieties
which may have also some intermolecular consequences.
NMSB £ IMSB is a very good model compound to
study not only intramolecular electronic and structural
carbon signal of glycine ðd
¼ 43:3 ppmÞ Additionally,
glycine
in order to distinguish protonated and unprotonated carbon
atoms, the short contact cross-polarization spectra [21] were
1
5
recorded (contact time 40 ms). The N chemical shifts in
solution were measured by GHSQC or GHMBC methods.
X-ray diffraction. X-ray measurements were made on a
KUMA4CCD diffractometer using monochromated Mo Ka
radiation in the v scan mode. A monocrystal was positioned
at 65 mm from the KM4CCD camera. Ca. 1200 frames were
measured at 1.08 intervals with a counting time of 30 s. Data
reduction and analysis were carried out with the Kuma
Diffraction (Wrocław) programs. One standard frame was
monitored for every 25 frames collected and none showed a
significant decrease in intensity over the data collection
time. The data were corrected for the Lorentz and
polarization effects, and the structure was solved by direct
methods [22] and refined using SHELXL [23]. The refinement
(
conformational) effects connected with a given type of
hydrogen bond formed in the multiple Schiff bases, but this
is also an excellent system to investigate the role of
weak intermolecular interactions between slightly different
multiple Schiff base neutral and ionic moieties.
2
. Experimental
Synthesis. A novel tripodal Schiff base ligand NMSB £
IMSB has been prepared by condensation of 2-hydroxy-5-
methyl-isophthaldehyde with tris(2-aminoethyl)amine,
molar ratio 3:1 [20] in acetonitrile, m.p. 177–182 8C.
NMR. The all NMR spectra were collected on Bruker
Avance 500 spectrometer operating at 500.13, 125.75 and
2
was based on F for all reflections except those with very
2
negative F : Weighted R factors vR and all goodnesses-of-
2
fit S-values are based on F : Conventional, R factors are
2
based on F with F set to zero for negative F : The criterion
2
1
13
15
2
5
0.70 MHz frequencies for H, C and N nuclei,
F . 2sðF Þ was used only for calculating R factors and is
respectively. The CPMAS spectra were run on natural
abundance samples using 4 mm Bruker probehead. The
not relevant to the choice of reflections for the refinement. R
factors based on F are about twice as large as those based
2
1
5
typical parameters for N CPMAS spectra were: spectral
width 28 kHz, acquisition time 40 ms, spin rate 6–12 kHz,
contact time for spin lock 5 ms, relaxation delay 10–120 s
depending on relaxation properties of the compounds.
on F; and R factors based on all data are even larger.
Hydrogen atom positional and thermal parameters were also
refined. Scattering factors and absorption coefficients were
taken from Tables 6.1.1.4 and 4.2.4.2 in Ref. [24].
1
5
Originally, the N CPMAS spectra were referred to solid
glycine and then the chemical shifts were recalculated to
Crystal data for NMSB £ IMSB. Empirical formula ¼
C H N O , formula weight ¼ 389:77, temp. ¼
2
2
24 2.67
4
˚
nitromethane scale ðd
¼ 2347:6 ppmÞ: Typical para-
293(2) K, wavelength (A) ¼ 0.71069, crystal system ¼
glycine
1
3
ꢀ
meters for C CPMAS spectra were: spectral width 31 kHz,
acquisition time 20 ms, contact time 2 ms, spin rate 12 kHz.
rhombohedral, space group ¼ R3 unit cell dimensions:
˚
˚
˚
a ¼ 14:680ð2Þ A,
b ¼ 14:680ð2Þ A,
c ¼ 49:751ð4Þ A,

B. Kołodziej et al. / Journal of Molecular Structure 691 (2004) 133–139
135
3
˚
˚
This distance for the neutral H-bonds is equal ca. 2.556 A,
˚
whereas for the ionic H-bonds is 2.562 A. Also, the neutral
a ¼ 608; b ¼ 608; g ¼ 1208; volume (A ) ¼ 9286(3), Z ¼
3
1
8; density calc. (Mg/m ) ¼ 1.255, absorption coefficient
2
1
3
(
mm ) ¼ 0.087, Fð000Þ ¼ 3720; crystal size (mm ) ¼
H-bonds are more linear with the hydrogen angle equal to
ca. 1548 compared to ca. 1458 for the other case. Small
differences in hydrogen· · ·acceptor (H· · ·A) distances are
due to the errors associated with H atom parameters.
The differences in H-bonds—as the ones described above—
are hardly seen on qualitative differential maps of electron
density calculated in the planes of hydrogen bonds as it is
shown in Fig. 2.
0
.15 £ 0.30 £ 0.25, theta range for data collection
(
8) ¼ 3.28–22.008, index ranges; 215 # h # 15; 215 #
k # 15; 252 # l # 52; reflections collected ¼ 35,641,
independent reflections ¼ 2534 ðRðintÞ ¼ 0:0395Þ; refine-
2
ment method ¼ full-matrix least-squares on F ; data/
restraints/parameters ¼ 2534/0/356, goodness-of-fit on
2
F ¼ 1:153; final R indices ðI . 2sðIÞÞ; R1 ¼ 0:054;
wR2 ¼ 0:142; R indices (all data); R1 ¼ 0:069; wR2 ¼
These differences in the quality of H-bonds formed is
then transmitted into the more distant fragments of the
moieties. The most spectacular differences in the values of
structural parameters for the both moieties are shown in
Fig. 3.
0
:155; extinction coefficient ¼ 0.0023(15), largest diff. peak
˚
2
3
and hole (eA ) ¼ 0.18 and 20.14.
Crystallographic data (excluding structural factors) for
the structure reported in this paper have been deposited with
the Cambridge Crystallographic Data Center and allocated
the deposition number: CCDC 176825. Copies of the data
can be obtained free of charge on application to CCDC,
When the geometry of NMSB is compared to the
geometry of IMSB, the largest differences in bond lengths
˚
equivalent in the both moieties are for: CO (20.027 A),
˚ ˚ ˚
1
2 Union Road, Cambridge CB2 1EW, UK (Fax: Int code
CaldO (0.021 A), C5C6 (0.025 A), C4C3 (20.010 A) and
˚
þ 44(1223)336-033; E-mail: deposit@ccdc.cam.ac.uk).
CyN (0.010 A). These differences—some of them topo-
logically quite far from the hydrogen bonding site—result
from the redistribution of electron density in the system
imposed by the change of proton position in the H-
bonding. Of course, when the proton is at the nitrogen
3
. Results and discussion
atom the C O bond becomes shorter and it acquires partly
ar
X-ray data. NMSB £ IMSB crystallises in a general
ꢀ
double character. This means that increasing amount of
electron density in the C O bond must be accompanied
position in the rhombohedral R3 space group with two
symmetry unrelated moieties (two different third parts of the
tripodal tris(3-formyl-5-methyl-2-salicylidenamino-ethyl)-
amine—see Fig. 1) in the independent part of the unit
cell. This structure can also be refined in the pseudotriclinic
ar
by a decrease in other bonds (mainly the nearest
neighbouring one, C5C6, and also quite distant CaldO—
see Fig. 3). Another consequences of different H-bonds
are differences in the valence angles—in particular the
ipso angles of the oxygen atoms (ca. 117.9(3)8 for the
ionic and ca. 118.7(3)8 for neutral H-bonding). The key
angles at the Schiff nitrogen atoms differ for the both
bonds by ca. 38. For the other angles, the differences are
rather small and non-significant.
ꢀ
P1 space group, also with two different moieties in the
1
independent part of the unit cell. The labelling of atoms in
the asymmetric unit without redundant hydrogen atoms
(
but including hydrogens participating in the H-bonds) is
shown in Fig. 1. The compound consists from two tripodal
moieties each containing three equivalent side arms in the
independent part of the unit cell. The arms of the moieties
are arranged in such a manner that a three-fold axis passes
through the bridging N atom. These bridged N-atoms are
pyramidal and their lone pairs are pointing towards the
cavity formed by the three Schiff arms of the compound.
The both moieties look like being related by a centre of
symmetry, but, in fact, they are not symmetry related. The
only reason why they are not symmetry related is a
significant difference in H-bonds formed in these two
independent moieties. In one of the moieties (IMSB)
hydrogen atoms in the Schiff fragments are localised at
the nitrogens (see Fig. 1 and Table 1), whereas in the other
one (NMSB) they are localised at the oxygen atoms. As one
can see from Table 1, these two limiting cases of H-bonding
differ significantly. Firstly, the neutral H-bond seems to be
slightly stronger than the ionic one because it is character-
ised by shorter donor· · ·acceptor distance (N· · ·O).
It is interesting why these two moieties differ that
dramatically as far as the properties of hydrogen bonding
are concerned. To analyse this problem, one can take a look
at 3D arrangement of molecules in the crystal lattice. The
packing of the NMSB £ IMSB molecules in the crystal
lattice is shown in Fig. 4. We want to stress that the aromatic
fragments of the neighbouring moieties are arranged in such
a way as to allow p· · ·p interactions between them (with the
˚
shortest C · · ·C as short as 3.354 A). These interactions of
ar
ar
the aromatic fragments of both moieties can be a driving
force for changes in hydrogen bonding. Structural differ-
ences between neutral and ionic moieties clearly show quite
a strong coupling of electron density of the aromatic
fragments and the nearest H-bonding. This means that
changes in concentration of electron density should easily
be transmitted from one point to another one. By
differentiation of H-bonds in the both moieties, the aromatic
fragments are becoming slightly differently charged which
adds an additional small electrostatic component to Ph· · ·Ph
interactions.
1
Authors would like to thank the PT Referee for pinpointing the higher
symmetry solution.

136
B. Kołodziej et al. / Journal of Molecular Structure 691 (2004) 133–139
Table 1
Structural parameters of H-bonds and the shortest intermolecular contacts
H-bond donor-H· · ·acceptor symmetry
D–H
H· · ·A
D· · ·A
/(D· · ·H· · ·A)
N2–H2N· · ·O1
1.14(4)
1.10(4)
0.97(4)
1.04(4)
1.00(4)
0.97(4)
1.07(4)
1.54(4)
1.54(4)
2.58(4)
2.62(4)
2.68(4)
2.82(4)
2.48(4)
2.562(3)
2.556(4)
3.372(4)
3.316(4)
3.335(4)
3.760(4)
3.367(4)
3.354(4)
145(3)
154(3)
138(3)
124(3)
123(3)
166(3)
140(3)
N4· · ·H3O–O3
C1–H1A· · ·O4 1 þ X 2 Y; 1 þ X; 2Z
C1–H1B· · ·O4 Y; 2X þ Y; 2Z
C2–H2A· · ·O1 1 2 X; Z 2 Y; 2Z
C3–H3· · ·O1 Y 2 X; 1 2 X; Z
C12–H12A· · ·O2 2=3 þ X 2 Y; 1=3 þ X; 1=3 2 Y
C10· · ·C20
˚
Distances in (A), and angles in (8).
There are also weaker C–H· · ·O hydrogen bonds
Table 1)—mainly to the aldehyde oxygen atoms. They
the CDCl solution is quite different comparing to the solid
3
(
state. In the chloroform solution at the room temperature
fast—on NMR time scale—tautomeric exchange was
observed (Table 2). The position of this equilibrium can
be estimated by examination of two spectral parameters: the
nitrogen chemical shift of N2 atom (together with one bond
proton–nitrogen coupling constant, when available) and
chemical shift of carbon atom C5. The value of
dN2 ¼ 299:6 ppm indicates that in these conditions the
hydrogen bonding protons are located at the oxygen atoms
and form relatively strong hydrogen bonds with the imine
nitrogens [25,26]. For the non-H-bonded OH structure,
the nitrogen chemical shift of the imine N atom is about
are different for the two moieties and represent the weakest
intermolecular interactions.
Solid-state NMR. The extensive NMR study of
tris (3-formyl-5-methyl-2-salicylidenamino-ethyl)amine
indicates that the molecular structure of this compound in
2
60 ppm and for the form with hydrogen completely
Fig. 2. Examples of hydrogen bonding in selected Schiff fragments:
þ
2
a) ionic hydrogen bond: [O(1) · · ·H(1n)–N(2) ] in IMSB, (b) neutral
(
hydrogen bond: O(7)–H(3o)· · ·N(6) in NMSB. Key hydrogens participat-
ing in the H-bonding were suppressed during refinement. Only atoms
related to the interacting molecular fragment are labelled.
Fig. 3. Selected most pronounced differences in the average values of bond
˚
lengths (A) for the neutral and ionic moieties: (kNMSBl–kIMSBl) and the
labelling scheme for the NMR results.

B. Kołodziej et al. / Journal of Molecular Structure 691 (2004) 133–139
137
Fig. 4. 3D-packing of molecules: (a) projection towards the (001) plane, (b) columns of tripodal moieties.
transferred to the N atom, the nitrogen signal is upfield
shifted to ca. 2240 ppm [26]. Lowering of the temperature
causes the increase in hydrogen bond strength (upfield shift
of imine nitrogen atom signal to 2119.4 ppm and
The observed value 163.7 ppm supports the conclusion
drown on the base of nitrogen NMR data but only at the
qualitative level.
1
3
15
In the C and N CP/MAS spectra different situation
was observed. In all solid-state spectra, the superposition of
two sets of signals was observed. This means that in this
1
JNH ¼ 23:3 Hz).
1
3
The analysis of C chemical shift of C5 atom leads to the
some conclusion. For the structure with proton located on
oxygen atom this parameter is closed to 155 ppm, whereas
for NH tautomer is downfield shifted to about 180 ppm [26].
1
5
phase two different moieties are present. The analysis of
N
NMR data (Fig. 5 and Table 2) leads to the conclusion that
these two moieties are differentiated by proton position in

138
B. Kołodziej et al. / Journal of Molecular Structure 691 (2004) 133–139
Table 2
NMR chemical shifts (ppm) of particular nuclei in the solution and solid-state
C
1
C
2
C
3
C
4
C
5
C
6
C
7
C
8
C
9
C
10
C
11
N
1
N
2
a
a
CDCl
3
303 K
55.3
56.6
57.8,
55.9
165.9
167.0,
165.1
119.2
119.2,
119.5
163.7
171.5,
164.9
124.1
123.8,
122.8
132.0
132.5,
128.9
126.3
125.5,
124.8
138.1
139.8,
138.7
19.9
20.7,
19.8
189.2
188.3,
187.1
2351.0
2351.0,
2352.3
299.6
2153.3,
2103.5
Solid state
55.9,
53.8
For the labelling scheme see Fig. 3.
1
a
At 223 K, dN2 ¼ 2119:4 ppm; J ¼ 23:3 Hz; dN1 ¼ 2351:8 ppm.
NH
the intramolecular hydrogen bridge. For the first moiety
dN2 ¼ 2103:5 ppm: This signal was assign to the OH
tautomer H-bonded to the imine nitrogen atom (similarly as
in the chloroform solution) and the second signal to the NH
tautomer H bonded to the O atom.
strongly shielded by three aliphatic wings. The H-bond
formed in the Schiff fragments influence the other structural
1
3
parameters of the nearest aromatic parts of the moieties.
C
1
5
and N CP/MAS NMR results (chemical shifts) allow to
identify two pairs of Schiff and amino nitrogen nuclei
conforming formation of the two types of moieties.
The nitrogen chemical shifts of the amine-type N-
atoms, d ; are very close to each other and they are
N1
almost insensitive to such small structure differences—
this spectral parameter does not provide any valuable
information. The conclusion drawn from the d data can
Acknowledgements
NS
1
3
be proved by analysis of C chemical shifts of C5
atoms. In the range typical for this atom we found two
signals, the low field signal d ¼ 171:5 ppm which could
be assign to the NH tautomer and the second signal,
d ¼ 164:9 ppm, coming from the OH moiety. Unfortu-
nately, sensitivity of other carbon atom signals, on proton
position, is too low to allow to draw definite conclusions.
In conclusion, we want to stress that the solid tris
The X-ray measurements were undertaken in the
Crystallographic Unit of the Physical Chemistry Lab. at
the Chemistry Department of the University of Warsaw.
References
[
[
1] J. March, Advanced Organic Chemistry, Wiley, New York, 1992.
2] S. Patai, The Chemistry of Carbon–Nitrogen Double Bond, Wiley,
New York, 1970, p. 276.
(
3-formyl-5-methyl-2-salicylidenamino-ethyl)amine is the
first example of a Schiff base containing two moieties in
the independent part of the unit cell: one which has all three
intramolecular O–H· · ·N hydrogen bonds neutral and the
other one which has only ionic H-bonds. Both moieties form
cavities with the amine N atom lone electron pairs inside
[
3] K. Harada, in: S. Patai (Ed.), The Chemistry of Carbon–Nitrogen
Double Bond, Wiley, New York, 1970, p. 266.
[4] H. Tanaka, H. Dhimane, H. Fujita, Y. Ikemoto, S. Torii, Tetrahedron
Lett. 29 (1988) 3811.
[
[
5] M.J. Brown, Heterocycles 29 (1989) 2225.
6] C. Wittig, H.D. Frommeld, B. Suchanek, Angew. Chem. Int. Ed. Engl.
2
(1963) 683.
7] J. Bromilow, R.T.C. Brownlee, V.O. Lopez, R.W. Taft, J. Org. Chem.
4 (1979) 4766.
[
4
[
[
8] L. Feringa, W.F. Jager, B. Lange, Tetrahedron 49 (1993) 8267.
9] E. Hadjoudis, Molecular Engineering 5 (1995) 301–307. and
literature cited herein.
[
[
10] E. Hadjoudis, Studies in organic chemistry, in: H. Durr, H. Bouas-
Laurent (Eds.), Photochromism, vol. 40, Elsevier, Amsterdam, 1990,
(
Chapter 17).
11] K. Wozniak, H. He, J. Klinowski, W. Jones, T. Dziembowska, E.
Grech, J. Chem. Soc., Faraday Trans. 91 (1995) 77–85.
[
[
12] T. Dziembowska, Pol. J. Chem. 72 (2) (1998) 193–209.
13] T.M. Krygowski, K. Wozniak, R. Anulewicz, D. Pawlak, W.
Kolodziejski, E. Grech, A. Szady, J. Phys. Chem. 101 (1997)
9
399–9404.
[
14] S.J. Archibald, A.J. Blake, M. Schroder, R.E.P. Winpenny, J. Chem.
Soc., Chem. Commun. (1994) 1669.
[
[
15] M. Kanesato, T. Yokoyama, Anal. Sci. 16 (2000) 335–336.
16] S. Liu, L-W. Yang, S.J. Rettig, C. Orvig, Inorg. Chem. 32 (1993)
2
773.
17] N. Sabbatini, M. Guardigli, J.-M. Lehn, Coord. Chem. Rev. 123
1993) 201.
[
(
1
5
Fig. 5. N CP/MAS NMR spectrum of solid tris(3-formyl-5-methyl-2-
[18] N. Aslan (Yilmaz), A. Kenar, O. Atakol, E. Kilic, Anal. Sci. 17 (2001)
1269–1272.
salicyliden-amino-ethyl)amine (NMSB £ IMSB).

B. Kołodziej et al. / Journal of Molecular Structure 691 (2004) 133–139
139
[
[
19] P. Dominiak, E. Grech’, G. Barr, S. Teat, P.R. Mallinson, K. Wozniak,
Chem. Eur. J. 9/4 (2003) 963–970.
[23] G.M. Sheldrick, SHELXL97. Program for the Refinement of Crystal
Structures (1997). University of Gottingen, Germany.
20] F. Avecilla, R. Bastida, A. de Blas, D.E. Fenton, A. Macias, A.
Rodriguez, T. Rodriguez-Blas, S. Garcia-Granda, R. Corzo-Suarez,
J. Chem. Soc. Dalton Trans. (1997) 409.
[24] A.J.C. Wilson (Eds.), International Tables for Crystallography, vol. C,
Kluwer, Dordrecht, 1992.
[25] W. Schilf, B. Kamie n´ ski, T. Dziembowska, Z. Rozwadowski, A.
Szady-Chelmieniecka, J. Mol. Struct. 552 (2000) 33.
[
[
21] X. Wu, S.T. Burns, K.W. Zilm, J. Magn. Reson., Ser. A 111
(1994) 29.
[26] B. Kamie n´ ski, W. Schilf, T. Dziembowska, Z. Rozwadowski, A.
Szady-Chelmieniecka, Solid State NMR 16 (2000) 285.
22] G.M. Sheldrick, Acta Crystallogr. A46 (1990) 467.