Synthesis and study of triorganostannyl esters of 3-,4- and 3,5-pyridinylimino substituted aminobenzoic acids: Crystal structures of dimorphs of aqua-trimethyltin 3-pyridinyliminobenzoate

Article abstract of DOI:10.1016/j.molstruc.2009.11.038

The synthesis and the structural investigation of triorganostannyl esters of the 3- and 4-[1-pyridin-2-yl-methylidene]- and 3,5-bis-[1-pyridin-2-yl-methylidene]-benzoic acids are reported with methyl, n-butyl, cyclohexyl, phenyl and benzyl substituents on tin. The organic carboxylates may be considered as iminopyridines emerging from the condensation of the corresponding aminobenzoic acid with pyridine-2-carboxaldehyde. In this respect it is interesting to investigate their physicochemical properties since their coordination to metal centers will affect both the photophysical properties of the metal and the conformation and intermolecular interactions of the ligands. Therefore, the structure of the above triorganotin compounds is studied and discussed in relation to those of the unsubstituted benzoates as well as of the free ligands. X-ray crystallography and a coalescence of spectroscopic methods applied both in the solid state and in solution have been used in this effort.

Full text of DOI:10.1016/j.molstruc.2009.11.038

Journal of Molecular Structure 965 (2010) 56–64
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and study of triorganostannyl esters of 3-,4- and 3,5-pyridinylimino
substituted aminobenzoic acids: Crystal structures of dimorphs of
aqua-trimethyltin 3-pyridinyliminobenzoate
c,
d
Dimitris Tzimopoulos a, Agnieszka Czapik b, Maria Gdaniec b, , Thomas Bakas , Anvarhousein A. Isab ,
*
*
Anastasia-Catherine Varvogli a, Pericles D. Akrivos a,
*
a Department of Chemistry, Aristotle University of Thessaloniki, P.O.B. 135, 541 24 Thessaloniki, Greece
b Faculty of Chemistry, Adam Mickiewicz University, 60-780 Poznan´, Poland
c Department of Chemistry, University of Ioannina, P.O.B. 1186, 451 10 Ioannina, Greece
d Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2009
Received in revised form 30 October 2009
Accepted 17 November 2009
Available online 20 November 2009
The synthesis and the structural investigation of triorganostannyl esters of the 3- and 4-[1-pyridin-2-yl-
methylidene]- and 3,5-bis-[1-pyridin-2-yl-methylidene]-benzoic acids are reported with methyl, n-butyl,
cyclohexyl, phenyl and benzyl substituents on tin. The organic carboxylates may be considered as imino-
pyridines emerging from the condensation of the corresponding aminobenzoic acid with pyridine-2-car-
boxaldehyde. In this respect it is interesting to investigate their physicochemical properties since their
coordination to metal centers will affect both the photophysical properties of the metal and the confor-
mation and intermolecular interactions of the ligands. Therefore, the structure of the above triorganotin
compounds is studied and discussed in relation to those of the unsubstituted benzoates as well as of the
free ligands. X-ray crystallography and a coalescence of spectroscopic methods applied both in the solid
state and in solution have been used in this effort.
Keywords:
Tin
Triorganotin carboxylates
X-ray diffraction
Mössbauer spectroscopy
NMR spectroscopy
MOPAC computations
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
pyridine-2-carboxaldehyde is favoring the study of substitutions
at this site and this is the starting point for our present study
Metal complexes of
optical properties and in this respect they have received much and
detailed interest [1]. Substituted -diimines offer the presence of
a
-diimines reveal interesting electronic and
where we introduce a triorganotin residue to the carboxylic group.
Triorganotin carboxylates are not a new group of compounds and
in fact they have found a wide variety of applications including
their action as pesticides, antifouling agents or cytostatic drugs
[6], although their high toxicity has prohibited or halted their
extensive use. Coordination of carboxylate to metals in general
and to tin in particular offers the possibility of studying the varia-
tions of the coordination mode which include monodentate, che-
late (symmetric or more often asymmetric) or the more subtle
bridging which may produce oligomeric or polymeric compounds
[7]. The energetic differences between the above modes of coordi-
nation are usually small and therefore it is not uncommon to ob-
serve more than one co-existing in the same crystal therefore
giving rise to more complicated supramolecular structures.
a
more than one coordination site which may produce complex or
unique structures and produce oligo or polymetallic systems with
varying electronic and optical properties [2]. Vast numbers of coor-
dination compounds are reported having in the coordination envi-
ronment 2,20-bipyridine, 9,10-phenanthroline or the condensation
products of amine-bearing molecules with diacetyl, benzil and
analogous compounds. Diimines produced from amino substituted
benzoic acids have also been studied in this respect and to our
knowledge have been used as the free acids to coordinate late tran-
sition metals in heteroleptic coordination compounds [3–5]
through their diimine site.
Tuning the electronic effects of coordinated diimines has been
attempted by many and is usually accomplished through substitu-
tion reactions at the ligand backbone. The carboxylate moiety in
the Schiff bases synthesized from aminobenzoic acids and
Our current interest lies in the synthesis and study of the coor-
dinating ability of triorganotin esters of aminobenzoate derivatives
towards main group or transition metals. In this respect we are
engaged in the synthesis of the pyridin-2-yl-methylidene substi-
tuted compounds which may act as iminopyridine donors to metal
centers [8,9] and may prove to alter the metal site through changes
in the organotin environment [10,11]. Concerning the tin environ-
* Corresponding author. Fax: +30 2310 997738.
E-mail address: akrivos@chem.auth.gr (P.D. Akrivos).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.11.038

D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
57
ment the previous knowledge of the coordination behavior of the
HO
O
unsubstituted benzoates is valuable in correlating spectroscopic
information with structure in the cases where the latter was not
possible to be directly solved [12]. Semi-empirical computations
are also used as an aid in the investigation of the electronic struc-
ture of the organic ligands and of the triorganotin derivatives and
they ere found to be in general agreement with the known struc-
tures and observed spectroscopic data. Moreover, we describe
the crystal structure of trimethyltin 3-[1-pyridin-2-yl-methyli-
dene]-aminobenzoate hydrate in the two polymorphs it was iso-
lated as well as those of two ligands, namely 3- and 4-[1-
pyridin-2-yl-methylidene]-aminobenzoic acid. The substituted
aminobenzoic acids utilized in the present study are presented in
Fig. 2 and the general reaction scheme in summarized in Fig. 1.
N
N
N
N
HO
O
N
N
10
N
HO
O
12
N
11
Fig. 2. Representation of the ligands synthesized and used in the present study.
2. Experimental
carried out with CrysAlis CCD [13] and data reduction with CrysAlis
RED [13]; SHELX-97 programs [14] were used to solve and refine
the structure. Molecular graphics were generated with ORTEP-III
for Windows [15] and Mercury 1.4 software [16]. CCDC 739745–
739748 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crys-
tallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033).
Mössbauer spectra were obtained on powder samples at 80 K,
using a constant acceleration spectrometer and a 10-mCi calcium
stannate source kept at room temperature. Spectrometer calibra-
tion was effected using a metallic iron foil. Isomer shifts are
reported relative to CaSnO3, assuming that they are the same as
the BaSnO3 shifts.
2.1. Materials and measurements
The triorganotin chlorides and the aminobenzoic acids were ob-
tained from ACROS and were used without any further purification.
The solvents used were of reagent grade and were not subjected to
any further drying process prior to their use. Elemental analyses
for C, H and N were performed on a Perkin-Elmer 240B elemental
analyzer. Infrared spectra were recorded in KBr pellets on a Perkin-
Elmer Spectrum One FTIR spectrometer with a resolution of 2 cmꢀ1
following the collection of 16 scans over the range 4000–360 cmꢀ1
.
NMR for 1H and 13C were measured at 300 and 75 MHz, respec-
tively, in CDCl3 solutions on a Bruker 300 spectrometer using
TMS as internal standard. Electronic excitation spectra were re-
corded in 1 ꢁ 10ꢀ4 M solutions in methanol and dichloromethane
in 1 cm cuvettes on a Perkin-Elmer Spectrum One spectrometer
and were processed with the Peakfit program at the Aristotle Uni-
versity central computing facility. Fluorescence measurements
were carried out on the previous solutions or in the solid state in
KBr pellets using a Hitachi F-7000 fluorescence spectrometer. The
2.2. Ligand synthesis
The general scheme for the synthesis of the triorganotin com-
pounds is shown in Fig. 1. In general, ‘‘route a” was followed. How-
ever, for comparison reasons and especially for correlating
spectroscopic data the imines themselves were synthesized, 10
and 11 according to literature method [3] with slight variations,
which include the use of a single solvent (ethanol) and the avoid-
ance of use of formic acid as catalyst. For 12 the following reaction
crystal structures were determined using a Kuma KM4CCD
geometry diffractometer. Data collection and cell refinement were
j
route a
O
O
O
O
K+
i
ii
procedure was adopted. To
a suspension of of the acid
H
N
NH2
(10.0 mmols) in methanol (10.0 mL) a solution of 2-pyridinecar-
boxaldehyde in methanol (5.0 mL) was added under stirring at
room temperature. After the completion of the addition the mix-
ture was refluxed for 1.5 h and then the grayish-yellow precipitate
was collected by filtration, washed successively with cold ethanol,
diethylether and finally dried in air.
N
1 - 3
i, iii
4 - 6
route b
iii
O
O
O
ii
2.3. Synthesis and characterization of the compounds
R3Sn
O
NH2
N
R3Sn
N
The imine salts of the parent acids were synthesized in the form
of their potassium salts following the general route described be-
low. 10.0 mmol of the appropriate potassium aminobenzoate were
dissolved in methanol (10.0 mL) under stirring at room tempera-
ture. When dissolution was completed there was added pyridine-
2-aldehyde (10.0 mmols) dissolved in methanol (5.0 mL) and the
reaction was continued for a further 1.5 h under refluxing condi-
tions. The solid product emerging (yellow for 5, brownish-yellow
for 4 and greyish for 6) was filtrated through a Buchner funnel
and washed with ethanol and diethylether before drying in air.
The triorganotin derivatives studied were prepared from the
above salts without their prior isolation. At the end of the above
7 - 9
a, Me
b, Bu
c, Cy
d, Ph
e, Bz
R=
i: KOH/MeOH
ii: pyridine-2-aldehyde/MeOH
iii: R3SnCl/toluene
Fig. 1. General reaction scheme followed. The numbering of compounds corre-
sponds to 3- (4, 7), 4- (5, 8) and 3,5- (6, 9) substituted benzoic acids, respectively.

58
D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
procedure there were added to the mixture under continuous stir-
ring 10.0 mmol of the appropriate triorganotin chloride in toluene
(20.0 mL). The solution became clear instantly and was left to react
under reflux for 5–6 h during which KCl deposited gradually on the
vessel walls. The solution was then filtered while hot in order to re-
move the remaining KCl, the solvent was removed at the rotary
evaporator and the solid (R = Me, Cy) or oily product (R = Du, Ph,
Bz) isolated was dried under vacuum. The products obtained were
dissolved in the minimum amount of hot hexane-dichloromethane
(10:1) and the precipitate formed upon cooling was filtrated and
recrystallized from hot toluene–hexane (5:1).
needed in order to verify the coordination mode present in every
case. In the IR spectrum the symmetric and asymmetric carboxyl-
ate stretching modes provide the widely accepted criterion of their
spacing,
Dm. Generally Dm
differences less than 240 cmꢀ1 are
accounting for bridging or chelating bonding modes while ones
above 260 cmꢀ1 for monodentate, [17]. In borderline cases and in
absence of crystallographic evidence only speculations can be
made, i.e. assumption o asymmetric bridging [18]. In addition,
individual peak positions have been sporadically used as indication
for the presence of specific coordination modes, i.e. the existence of
a carboxylate band in the proximity of 1630 cmꢀ1 is related to the
asymmetric bridging of the carboxylate [19].
The yellow or brownish-yellow products isolated in the case of
the methyl derivatives were further dissolved in hot toluene and
upon addition of petroleum ether the corresponding crystals used
for X-ray determination were deposited for compound 7a. In the
case of the butyl and benzyl derivatives it proved necessary to
use molecular sieves in order to avoid turpidity of the initial mix-
ture and undesired by-products. For the compounds of the 3,5-
disubstituted benzoic acid, the synthesis of the desired products
was carried out after in the same manner but the refluxing periods
had to be expanded to 3 and 8 h, respectively, for the two steps de-
scribed previously. In the case of compound 7a, upon standing at
room temperature a light yellow solid deposited within minutes
and was isolated and submitted for structural analysis which
proved that it was actually the hydrate of 7a. Upon further stand-
ing for a few days a second batch of crystals was deposited and
these were also collected and analyzed in the same way, proving
to be another polymorph of the same compound. Both structures
are reported and they are denoted as polymorphs B and A of com-
pound 7aꢂH2O, respectively. Selected crystallographic data are in-
cluded in Table 5.
It should be noted at this point that the same products can be
obtained by the interaction of the appropriate triporganostannyl
esters of the aminobenzoates, prepared and isolated as described
elsewhere [12] with a slight excess of pyridine-2-aldehyde in
refluxing methanol for a period of 2 h.
The complexes were characterized spectrophotometrically
(FTIR and UV–vis) and by elemental analysis; (theoretical values
are shown in brackets):
10: C, 68.87% (69.02%); H, 4.73% (4.46%); N, 12.34% (12.38%); 11:
C, 68.42% (69.02%); H, 4.81% (4.46%); N, 12.06% (12.38%); 12: C,
68.56% (69.08%); H, 4.54% (4.27%); N, 16.71% (16.96%); 7a: C,
49.07% (49.40%); H, 4.67% (4.66%); N, 6.99% (7.20%); 8a: C, 48.79%
(49.40%); H, 4.70% (4.66%); N, 6.79% (7.20%); 9a: C, 53.35%
(53.58%); H, 4.50% (4.66%); N, 11.77% (11.36%); 7b: C, 57.52%
(58.27%); H, 7.16% (7.04%); N, 5.19% (5.44%); 8b: C, 57.32%
(58.27%); H, 7.18% (7.04%); N, 5.05% (5.44%); 9b: C, 59.63%
(60.11%); H, 6.97% (6.51%); N, 8.75% (9.05%); 7c: C, 62.38%
(62.75%); H, 7.30% (7.13%); N, 3.99% (4.72%); 8c: C, 62.60%
(62.75%); H, 7.49% (7.13%); N, 4.12% (4.72%); 9c: C, 64.01%
(63.71%); H, 6.74% (6.65%); N, 7.86% (8.03%); 7d: C, 64.86%
(64.73%); H,4.55% (4.21%); N, 4.38% (4.87%); 8d: C, 64.55%
(64.73%); H,4.50% (4.21%); N, 4.50% (4.87%); 9d: C, 65.32%
(65.40%); H,4.23% (4.15%); N, 8.33% (8.25%); 7e: C, 65.78%
(66.15%); H, 5.08% (4.90%); N, 4.43% (4.54%); 8e: C, 65.92%
(66.15%); H, 4.94% (4.90%); N, 4.68% (4.54%); 9e: C, 66.08%
(66.59%); H, 4.79% (4.75%); N, 7.63% (7.77%).
The relevant vibrational information for the ligands (10–12),
their potassium salts (4–6) and the triorganotin derivatives (7–9)
are reported in Table 1. The Dm discussed above are also computed
and tabulated in order to aid the interpretation of the carboxylate
bonding mode.
The imine part of the ligands does not appear to take part in the
coordination to tin because the corresponding bands do not sift
from their position in the free ligands. Notable exceptions are 7b,
7c and 9a. In the first two the strengthening of the C@N bond
may be attributed to some twisting of the two rings of the ligand
therefore precluding participation of the specific bond in any ex-
tended aromatic system. In the latter case the lower wavenumber
of the bond stretching relative to the ligand may indicate a partic-
ipation of the imine N atom in an interaction which may be either
hydrogen-bond formation or coordination to tin. Regarding the
coordination mode of the carboxylate moiety the trimethyltin
derivatives 7a and 8a are proposed to be polymeric with carboxyl-
ate bridging adjacent tin centers while most of the rest compounds
appear to have monodentate carboxylate. There exist a number of
compounds, namely 7e, 8e, 9a and 9c for which no clear conclusion
may be drawn; the compounds may be of polymeric nature like the
trimethyltin derivatives discussed previously.
The above assignments for the asymmetric carboxylate and the
imino C@N bond stretchings are aided by the known spectra of the
corresponding unsubstituted benzoates [10,11,18,20] and by their
correlation with the spectra of the studied compounds. Further
support is provided by computations carried out on single mole-
cules which were performed using the PM6 Hamiltonian [21] as
implemented in the MOPAC2009 program [22]. Simple semi-
empirical computations are not expected to result the accurate
vibrational eigenvalues and there is always need for scaling in
Table 1
Relevant vibrational spectral data. Band maxima are given in wavenumbers.
Compound
m
as(COO)
m
s(COO)
D
m
m
(C@N)
m(Sn–O)
m(Sn–C)
10
4
1703
1557
1594
1578
1585
1587
1597
1701
1544
1599
1600
1604
1600
1598
1704
1563
1589
1587
1586
1587
1596
1293
1395
1374
1332
1328
1333
1335
1272
1403
1367
1338
1315
1331
1366
1294
1390
1338
1331
1326
1331
1332
410
162
220
246
257
254
232
429
141
232
262
289
269
232
410
173
251
256
250
256
267
1628
1629
1627
1645
1643
1632
1630
1629
1623
1625
1630
1637
1633
1627
1635
1633
1621
1636
1638
1636
1633
–
–
–
–
7a
7b
7c
7d
7e
11
5
8a
8b
8c
8d
8e
12
6
417
407
410
407
410
–
553
517
492
446
452
–
–
–
439
407
418
406
406
–
551
510
493
447
452
–
3. Results and discussion
–
–
3.1. Vibrational spectra
9a
9b
9c
9d
9e
407
407
406
407
406
550
526
482
448
452
The structure adopted by organotin carboxylates is subtle since
the coordination of the carboxylate ion may be monodentate,
bridging or chelate. A compilation of spectroscopic evidence is

D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
59
1.6
1.4
1.2
1
order to approach the experimental observations. However the
correct ordering of the normal coordinate analysis cannot be over-
ruled especially when it consistently determines higher frequen-
cies for the C@O relative to the neighboring C@C and C@N bonds.
In our case, both the free acids and their idealized triaryl and trial-
kyltin derivatives were studied by means of tight convergence cri-
teria, i.e. setting the SCF convergence to 1.0 ꢁ 10ꢀ12 and the
gradient norm to 0.01, respectively, for the geometry optimization
which preceded the FORCE calculation.
Initially there is observed a coincidence of the computed and
the crystallographically obtained molecular geometries for the li-
gands 10 and 11. Both compounds are predicted to be non-planar
due to a steric hindrance between the methylidene and benzene H
atoms. Furthermore the carboxylate and imino region geometries
are predicted reasonably close to the experimentally obtained ones
although the computations assume an idealized gas-phase mole-
cule with no intermolecular interactions and packing forces. C–O
bond lengths are computed to be 1.209 and 1.380 Å (1.209 and
1.318 Å from X-ray data, vide infra) for compound 10 and 1.211
and 1.383 Å (1.212 and 1.329 Å X-ray) for compound 11. The C-N
bond lengths of the imino site are computed to be 1.428 and
1.294 Å (1.424 and 1.264 Å X-ray) for 10 while for 11 the values
are 1.423 and 1.290 Å (1.415 and 1.273 Å X-ray), respectively.
The computations, in line with the intuitive assignment predict
the C@O vibration to occur at the highest wavenumber with a very
high consistency and a scaling factor of 0.932 while the imino C@N
bond is predicted to follow. In the latter case however the scaling
factor needed to be applied is 0.918. The vibrations computed ap-
pear to be strongly coupled to the stretches of the neighboring aro-
matic system. In both cases the ring C@C bonds are predicted to lie
at lower wavenumbers.
0.8
0.6
0.4
0.2
0
200
300
400
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
3.2. Visible absorption spectra
The parent ligands and the triorganotin compounds synthesized
reveal good solubility in a variety of organic solvents which made
it possible to record their spectra in dichloromethane and metha-
nol. The spectra of the ligands (Fig. 3) show some general similar-
ities and some discrepancies which are due to the different
solvating ability of the two solvents used. There is an apparent
existence of at least five overlapping bands in every case, the posi-
tion of which depends obviously on the solvent and on the ligand
studied. This type of solvatochromism is expected in analogous
aromatic systems. The analysis of the overlapping bands was car-
ried out using the Peakfit program [23] within the AFS system of
the Aristotle University of Thessaloniki [24]. The results of this
analysis provided evidence for the occurrence of six bands in the
recorded region most of which indicate, on grounds of both inten-
sity and band half-width, a significant difference between the
ground and excited states. Based on the good approximation of
the molecular geometry obtained by the semi-empirical computa-
tions applied we proceeded with the investigation of the possible
electronic excitations occurring within these molecules and we
considered the single electron excitations involving the four higher
occupied and the four lower occupied molecular orbitals, an ap-
proach incorporated into the MOPAC2009 package.
200
300
400
Fig. 3. Electronic excitation spectra of the ligands 10 (above) and 11 (bottom) in
dichloromethane. Individual excitations are Gaussian type curves and appear in
ordinary line width while the total spectrum is represented by a bold line.
pyridine, ring and the LUMO + 1 extends over the benzoic and
the carboxylic group. In general the LUMOs appear more delocal-
ized in 10 indicating a less pronounced charge transfer character
bands in its spectra relative to 11.
Analogous computations were carried out for the methyl and
phenyl derivatives and although there is only limited value in their
outcome due to the assumption of only s and p orbitals for the
main group metals, their findings reveal limited contribution by
the organotin fragment to the above considered frontier MOs.
There appears to be only one occupied molecular orbital with Sn
character, a fact that is supported by the striking similarity of the
organotin compound electronic excitation spectra with the ones
of the free ligands.
The frontier molecular orbitals for the two ligands are presented
in Fig. 4 and 5. Among them there are orbitals localized mainly on
the pyridine ring or bearing imino or carboxylate character as well,
whereas there exist, especially in the case of compound 10,
delocalized ones. Besides the expected differences, there are some
similarities in the orbital constitution of the two ligands since the
HOMO ꢀ 2 orbital is largely located in the benzoic ring, the
HOMO ꢀ 3 in the pyridine ring, while the HOMO ꢀ 1 orbital ex-
tends over the pyridine ring and the neighboring imine bond in
both compounds.The LUMO + 2 orbital is mainly located on the
3.3. Solution NMR data
The study of 1H and 13C NMR spectra in solution and especially
the coupling constants of the carbon atoms directly attached to the
tin center may serve as an indication of the coordination environ-
ment of tin in solution. Since the compounds are soluble in CDCl3
solvolysis is not expected to occur and therefore the spectroscopic
information is correlated with the solid state structure of the com-
pounds. In Table 2 there are summarized the most relevant data

60
D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
The NMR data indicate that in solution there is no participation
of the imino nitrogen atom in the coordination to tin since there
are no appreciable changes in the imine proton and carbon reso-
nances between the free ligands and the organotin compounds.
The carboxylate coordination is apparent from the slight but
detectable downfield shift of the carboxylate carbon atom [25].
The most relevant information however is extracted from the
chemical shift and the coupling constant of the
a-carbon atom of
the organic substituent and, when possible, from the same observ-
ables of the proton of the above carbon atom. The values obtained
for the alkyl and benzyl derivatives range well within the limits set
for four-coordinate tin since they do not exceed the high limit va-
lue 390 Hz [26–28] except in the case of the trimethyl derivatives
7a and 8a which, however, are not very far from it. The crystal
structure determination of 7a has revealed the coordination of a
water molecule to tin (vide infra) and it is unlikely that the water
has been removed during the solution in CDCl3 for the spectra
recording whereas it was retained throughout the recrystallization
procedure which involved treatment in boiling toluene. Note
should be taken at this point that it was not possible to observe
the coupling in the case of all triphenyl derivatives. For these com-
pounds however the corresponding carbon atom signal was ob-
served at the field strength proposed for tetrahedral tin
environment, i.e. around 138 ppm [29]. In the case of 7d the ob-
served 1J(119Sn–13C) coupling constant of 650 is again within the
limits 550–670 Hz established in literature.[18] In the methyl
and benzyl derivatives it was possible to observe the 2J(119Sn–1H)
couplings which range between 57 and 71 Hz, being consistently
higher for the benzyl derivatives. For this coupling it has been
documented that values around 80 Hz indicate octahedral tin,
[30] in which case our findings support a coordination number
not exceeding five.
Fig. 4. Frontier molecular orbitals for compound 10. The HOMOs are on the left and
the LUMOs on the right side, while in every case the orbitals are drawn in increasing
energy order.
3.4. Mössbauer measurements
The Mössbauer spectra of the triorganostannyl esters of carbox-
ylates are more subtle in their interpretation as the range over
which the quadrupole splittings are observed may be shifted due
to large electronegativity differences between the donor atoms
and tin therefore calling again for speculation. In general higher
quadrupole splittings,
D, correspond to higher coordination
Table 2
Relevant 1H and 13C NMR data in CDCl3 solution. Chemical shifts (ppm) are downfield
from internal TMS standard and the 1J(119Sn–13C) and 2J(119Sn–1H) coupling constants
(Hz), where observed, following in parentheses.
a
Compound
Ccarb
Cim
C
Him
H
–
a
a
10
7a
7b
7c
7d
7e
11
8a
8b
8c
8d
8e
12
9a
9b
9c
9d
9e
167.80
171.06
171.09
170.85
172.44
172.30
166.90
171.21
171.59
170.70
172.30
171.49
166.60
170.37
169.62
170.64
172.05
171.33
161.90
161.17
161.14
161.14
161.41
160.99
162.30
161.80
161.65
161.47
161.98
161.92
162.40
161.68
160.70
161.71
161.92
161.56
–
8.63
8.66
8.66
8.65
8.62
8.63
8.61
8.58
8.59
8.58
8.56
8.60
8.70
8.69
8.71
8.70
8.67
8.64
ꢀ2.35 (395)
16.55 (369)
33.89 (355)
138.26 (650)
24.43 (310)
–
ꢀ2.32 (395)
16.55 (355)
33.53 (342)
138.37
24.19 (319)
–
ꢀ2.32 (388)
16.01 (362)
34.04 (337)
138.21
0.65 (57.6)
1.36 mb
1.97–2.05 mb
–
2.68 (68.3)
–
0.64 (57.2)
b
1.36 m
1.91–2.01 mb
–
2.68 (70.4)
–
0.64 (57.2)
1.35 mb
1.97–2.01 mb
–
Fig. 5. Frontier molecular orbitals for compound 11 drawn as the corresponding
ones of 10 in Fig. 4.
24.12 (328)
2.68 (68.4)
a
The subscripts denote carboxylate (carb), imino (im) and the atom of the alkyl
or aryl group directly attached to the tin center ( ), respectively.
Non-resolved multiplet incorporating more than one types of proton.
a
with respect to their aid in the confirmation of the geometry
adopted by the compounds in solution.
b

D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
61
numbers, the accepted ranges being 2.0–2.4, 2.6–3.0 and around
3.5 mm sꢀ1 for coordination numbers four, five and six, respec-
tively, [31]. Chelate ligands are expected to affect the above ranges
since they contribute significantly to the asymmetry of the tin
environment [32]. Besides providing grounds for polymeric net-
work formation. In the case of high coordination numbers one
may not exclude the participation of the amino substituent to
the formation of the tin coordination sphere, a fact that has gener-
ally have received little attention. The isomer shift, d, and the ratio
The twisted conformation can be rationalized as a relieve of a steric
hindrance between the ꢀN@CHꢀ group H atom and the neighbor-
ing H atoms of the benzene ring at the cost of the conjugation en-
ergy of the imino group with the benzene ring. A survey of the
Cambridge Crystallographic Database (CSD; [35]) shows 10 crystal
structures with 14 (2-pyridylmethylene) benzeneamine fragments
that possess H atoms at the
a- positions of the benzene ring. The
twist angle around the C–N bond between the benzene ring and
the imino group ranges from 11° to 43° with the intramolecular
Hꢂ ꢂ ꢂH contact from 2.03 Å (i.e. nearly equal to the sum of van der
Waals radii of H atoms [36]) to the maximum value of 2.49 Å. In
the reported crystal structures the twist angle is relatively large,
46.1° and 37.7° for 10 and 11, respectively, but as shown by the
polymorphism of 10, it is more a result of crystal packing forces
than a molecular property. In all molecules with the (2-pyridylm-
ethylene) benzeneamine fragment deposited in the CSD and in
molecules 10 and 11 reported here the imino and pyridine N atoms
are transoidaly oriented with the twist angle of the pyridine unit
relative to the imino group being small and not exceeding 17°.
In both crystalline forms of 10, the molecules join via a typical
carboxylic acid–pyridine R22ð8Þ synthon consisting of a strong O–
Hꢂ ꢂ ꢂN and weak C–Hꢂ ꢂ ꢂO interactions [37,38] but there is an inter-
esting difference in the structure of the hydrogen bonded assem-
blies: in the C2/c form molecules assemble into discrete, nearly
planar, centrosymmetric dimers whereas in the P21/c form they ar-
range into helices along the twofold screw axis. The latter organi-
zation leads to a more efficient packing as shown by the crystal
densities: 1.38 and 1.42 g cmꢀ3 for C2/c and P21/c polymorphs,
respectively. Also in compound 11 the best hydrogen-bond donor,
the carboxylic group, interacts with the best hydrogen-bond accep-
tor, the pyridine N atom, forming a 1D polymeric structure, but in
this case a weaker C–Hꢂ ꢂ ꢂO interaction is not present.
q = D/d have been used to determine the coordination number of
tin and of the asymmetry in it, being directly or indirectly related
to the coordinating mode of the ligands present in the tin coordi-
nation sphere [33]. The trimethyl derivatives (7a, 8a and 9a) and
a tributyl one (7b) are at the borderline of being considered five-
coordinated, a fact supported by the infrared spectra and the struc-
ture determination for one of them. The single tricyclohexyl deriv-
ative studied (8c) is certainly tetrahedral with monodentate
carboxylate. Tetrahedral environment can also be assigned to the
tribenzyl derivatives 8e and 9e, the tributyl ones of 4- and 3,5-
substituted benzoates (8b, 9b) and to the triphenyl compound
8d. The occurrence of more than a signal for these compounds sug-
gests a varying degree of secondary interaction from the second
carboxylate oxygen in an intermolecular chelating or intramolecu-
lar bridging fashion. The triphenyl ester of the disubstituted acid,
9d, resembles the methyl esters and is expected to be polymeric
in nature. The interesting finding is the existence of high coordina-
tion number for the phenyl and benzyl esters of the 3- substituted
ligand, where there appears to exist some coordination of the pyr-
idine or imine nitrogen to tin (Table 3).
3.5. Crystallographic information
For compound 7aꢂH2O, which is an aqua-trimethyltin ester of
The ORTEP representation of the free acids 10 and 11, belonging
to the P21/c space group, is shown in Fig. 6. The crystal structure
for the other monoclinic form of 10, in the C2/c space group, has
been reported recently [34]. Whereas in the structures reported
here the pyridinylimino benzoic acids are in non-planar conforma-
tions, in the C2/c polymorph of 10 the benzene fragment is only
slightly twisted relative to the remaining part of the molecule.
the acid 10, two conformational polymorphs, designated as
a and
b forms, were isolated, both belonging to the monoclinic space
group P21/n. Whereas in both structures the tin environment is
similar, the benzoate ligands strongly differ in conformation The
tin atoms are in a distorted trigonal bipyramidal environment with
the apical positions occupied by one carboxylate oxygen atom and
one water molecule whereas methyl groups are located in the
Table 3
Mössbauer spectroscopic data. The line half-widths, the quadrupole splittings and the
isomer shifts are reported in mm sꢀ1
.
Compound
d
D
q
(D
/d)
C1/2
7a
1.23
1.59
1.36
1.88
1.24
0.61
1.37
0.65
1.33
1.23
1.60
1.47
1.16
1.02
1.39
1.29
0.78
1.30
1.42
1.49
0.98
1.37
1.04
1.49
3.05
3.23
3.34
3.38
2.63
2.26
1.92
3.49
3.50
2.87
2.94
2.68
2.51
1.70
3.34
2.48
1.53
3.39
2.72
3.54
2.53
2.72
2.57
2.95
2.48
2.03
2.46
1.80
2.12
3.70
1.40
5.37
2.63
2.33
1.84
1.82
2.16
1.67
2.40
1.92
1.96
2.61
1.92
2.38
2.58
1.99
2.47
1.98
0.84
0.68
1.08
0.70
0.90
0.90
0.90
0.90
0.86
0.84
0.94
0.84
0.90
0.90
0.88
0.88
0.88
0.84
0.90
0.86
1.04
0.80
1.02
1.00
7b
7d
7e
8a
8b
8c
8d
8e
9a
9b
9d
9e
Fig. 6. ORTEP repesentations of the monomers of the free acids 10 (a) and 11 (b).

62
D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
equatorial plane. The Sn atom deviates 0.16 Å out of this plane in
the direction of the carboxylate O atom.
The carboxylate group is clearly monodentate to the tin center
and the Sn–O bond lengths of 2.166 and 2.187 Å correlate nicely
with the Sn–OH2 distances, with the sum of the two Sn–O dis-
tances in both molecules equal within the experimental error.
The coordination geometry around the Sn atom is very similar to
that observed in other aqua-trimethyltin carboxylates [39–44].
The details of molecular geometry for 7aꢂH2O are given in Table
4. In the polymorphs of 7aꢂH2O the pirydyliminobenzoate ligand
shows different orientation of the pirydylimine fragment as well
as different orientation of the carboxylate ester group as illustrated
in Fig. 7. As both polymorphs were isolated from the same crystal-
lization batch, with the polymorph
polymorph b appearing only later on, the form b should be consid-
ered as slightly more stable than . Additional support is provided
by crystal densities of the both forms (Table 5) showing a more
dense packing for the b form of 7aꢂH2O than for the form. In
a precipitating first and the
a
a
the two crystal structures water molecules donate hydrogen bonds
to the pyridine N atom and the carboxylate non-coordinating O
atom to form a two-dimensional polymeric structure. The imine
N atom which generally should be a good acceptor for hydrogen
bonding is not easily accessible to this interaction as in the flat-
tened ligand conformation the electron pair occupying a sp2 orbital
of the imine nitrogen atom is significantly shielded by the pyridine
and benzene hydrogen atoms.
Semi-empirical calculations are not expected to enlighten the
structural conformations of the triorganotin compounds studied
since they do not treat adequately hypervalent compounds of tin
or any other main group metalloid. In view of the experimentally
obtained structure of 7a hydrate we attempted a series of calcula-
tions in which the approach of a water molecule to the tin center
was considered. In the process it was found obligatory to force
the perpendicularity of the three Sn–C bonds relative to the Sn–O
of the carboxylate site. When the optimization was carried out in
this way, it was possible to obtain final structures with general
similarity to the crystallographically obtained one as can be seen
in Fig. 8, except for a weak Sn–OH2 interaction with the computed
Sn–O distance converging at around 2.80 Å while the observed one
is around 2.40 Å.
Fig. 7. Molecular structure of 7aꢂH2O in (a) polymorph
a and (b) polymorph .
by the easy isolation of both conformers by slightly varying the
crystallization procedure.
3.6. Emission data
However, applying the above approximation to the conforma-
tions as they appear in the two polymorphs of the compound iso-
lated they proved to be of approximately equal stability as
reflected in their calculated heat of formations which were com-
puted to be ꢀ62.242 and ꢀ62.612 kJ, respectively, a fact supported
Emission spectra of the pyridinylimino benzoic acids were re-
corded both in the solid state and in methanol solution following
an excitation at 324 nm. In the solid state the three compounds re-
veal emission at almost at identical wavelengths, with small inten-
sities 12 presenting the lower and 11 the higher one. This is due to
the polymeric nature of the compounds as the X-ray structure
determination revealed. In methanol solution however, where
the corresponding monomers are the predominant species, the ex-
cited states differ significantly and the emissions recorded show
maxima at 456 nm for 12, 430 nm for 10 and 390 nm for 11, while
the relative intensities are two orders of magnitude larger than in
the solid state and increasing in the given order.
Table 4
Selected bond lengths (Å) and angles (°) for 7aꢂH2O in both polymorphic forms.
Bond/angle
7aꢂH2O polymorph
a
7aꢂH2O polymorph b
Sn–O
2.1877(17)
2.3773(19)
2.111(3)
2.1660(15)
2.3974(19)
2.106(3)
Sn–OH2
Sn–Ca
2.114(3)
2.099(2)
Preliminary measurements for the organostannyl esters in
methanol, following excitation at 324 nm reveal the fact that
although the 3-substituted derivatives emit at longer wavelengths,
the relative emission intensity is reversed in their favor with re-
spect to their 4-substituted counterparts, while the substitution
at tin does not affect significantly the emission maximum. The ob-
served emission, on the other hand is by an order of magnitude less
than the one recorded for the free ligands.
2.123(3)
1.290(3)
1.234(3)
2.107(3)
1.287(3)
1.232(3)
92.03(8)
95.10(10)
95.65(10)
85.79(8)
84.59(11)
86.70(12)
177.32(7)
124.5(2)
O–C
O0–Cb
O–Sn–C
95.91(10)
90.95(10)
95.69(10)
87.39(10)
83.47(10)
86.37(10)
174.38(7)
123.8(2)
H2O–Sn–C
H2O–Sn–O
O–C–O0
4. Conclusion
a
The carbon atoms bonded to tin are not identified but all observed values are
listed.
b
Superscript used to define the non-bonding oxygen in the triorganotin
compounds.
Triorganostannyl esters of pyridinyliminobenzoates can be
synthesized by two routes giving organotin compounds with a

D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
63
Table 5
Crystallographic details for the studied compounds.
10
11
7aꢂH2O polymorph
a
7aꢂH2O polymorph b
CCDC
739745
739746
739747
739748
Formula
C13H10N2O2
C13H10N2O2
C16H20N2O3Sn
C16H20N2O3Sn
Wavelength
Mo K
226.23
a
Mo K
226.23
a
Mo K
407.03
a
Mo Ka
407.03
Mr (g molꢀ1
)
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
P21/n
Temperature [K]
293
100
130
130
a [Å]
b [Å]
c [Å]
b [°]
V [Å3]
Z
10.8012(5)
7.4645(4)
13.1830(5)
92.461(4)
1061.91(9)
4
3.7588(1)
18.2354(4)
15.3571(5)
92.098(3)
1051.92(5)
4
6.5874(3)
23.9532(19)
11.425(3)
95.957(11)
1793.0(4)
4
11.3719(3)
8.3578(2)
18.3187(6)
90.881(3)
1740.88(8)
4
qcald (g cmꢀ3
)
1.415
0.098
1.428
0.099
1.508
1.437
1.553
1.480
l
(VoK ) (mmꢀ1
)
a
Measured reflns
8259
12650
17404
18518
Independent reflns
Observed reflns
2159
1338
2153
1949
3657
3098
3766
3021
Refined parameters
R1 (R1 all data)
158
159
207
209
0.0339 (0.0687)
0.0785 (0.0938)
0.15, ꢀ0.18
0.0324 (0.0357)
0.0806 (0.0829)
0.28, ꢀ0.19
0.0219 (0.0296)
0.0505 (0.0534)
0.84, ꢀ0.36
0.0201 (0.0339)
0.0434 (0.0524)
0.97, ꢀ0.50
wR2 (wR2 all data)
Max. and min. [e Åꢀ3
]
[6] I. Omae, Appl. Organometal. Chem. 17 (2003) 81–105. and references therein.
[7] E.R.T. Tiekink, Trends Organomet. Chem. 1 (1995) 71–116.
[8] S. Chowdhury, G.K. Patra, M.G.B. Drew, N. Chattopadhyay, D. Datta, J. Chem.
Soc., Dalton Trans. (2000) 235–237.
[9] E.C. Riesgo, Y.Z. Hu, R.P. Thummel, D.V. Scaltrito, G.J. Meyer, Inorg. Chem. 40
(2001) 3413–3422.
[10] D.R. McMillin, K.M. McNett, Chem. Rev. 98 (1998) 1201–1220.
[11] S.D. Ittel, L.K. Johnson, M. Brookhart, Chem. Rev. 100 (2000) 1169–1204.
[12] D. Tzimopoulos, M. Gdaniec, T. Bakas, P.D. Akrivos, J. Coord. Chem. 62 (2009)
1218–1231.
[13] Oxford Diffraction, Crysalis CCD and RED, ver. 1.171.31, Oxford Diffraction,
Abingdon, Oxfordshire, UK, 2006.
[14] G.M. Sheldrick, Acta Crystallogr. A64 (2008) 112–122.
[15] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
Fig. 8. Computed structure for 7a hydrate by PM6. Structures drawn using the Jmol
program [45].
[16] C.F. Macrae, P.R. Edgington, P. McCabe, E. Pidcock, G.P. Shields, R. Taylor, M.
Towler, J. van de Streek, J. Appl. Cryst. 39 (2006) 453–457.
[17] G.K. Sandu, S.P. Verma, L.S. Moore, R.V. Parish, J. Organomet. Chem. 321 (1987)
15–25.
[18] F.P. Pruchnik, M. Banbula, Z. Ciunik, M. Latocha, B. Skop, T. Wilczok, Inorg.
Chim. Acta 356 (2003) 62–68.
[19] T.S. Basu Baul, R. Wandondor, E. Rivarola, C. Pettinari, A. Linden, J. Organomet.
Chem. 690 (2005) 1413–1421.
[20] Z. Rzaczynska, R. Mrozek, J. Lenik, M. Sikorska, T. Glowiak, J. Chem. Crystallogr.
30 (2000) 519–524.
[21] J.J.P. Stewart, J. Mol. Mod. 13 (2007) 1173–1213.
[22] MOPAC2009, James J.P. Stewart, Stewart Computational Chemistry, Colorado
Springs, CO, USA, 2008. Available from: <http://OpenMOPAC.net>.
[23] Systat Software Inc. Peakfit program version 4.11.
[24] AFS client-server computational environment of the Aristotle University of
Thessaloniki.
potential diimine coordinating site. The ligands form supramolec-
ular structures in the solid state with extensive intermolecular
hydrogen bonding and readily coordinate to triorganotin sites
through the carboxylate group. Solid state interactions are also
present in the organotin carboxylates giving rise to a variety of
local and overall structures and the occurrence of more than one
local tin environment as IR and Mössbauer data indicate. Crystal
structure determination for a methyl derivative determined, be-
sides its existence as a hydrate, the presence of two discrete poly-
morphs differing in the relative orientation of the carboxylate
towards the imino group. Inspection and analysis of the UV–vis
spectra of the ligands and the organotin compounds indicate the
occurrence of bands with appreciable charge transfer character
which may facilitate the use of the compounds as metalloligands
towards transition metals with possible tuning of the final hetero-
dimetallic system’s photophysical and photochemical properties.
[25] M. Nath, R. Yadav, Bull. Chem. Soc. Jpn. 71 (1998) 1355–1362.
ˇ
ˇ
[26] J. Holecek, M. Nadvornlk, K. Handllr, A. Lycka, J. Organomet. Chem. 241 (1983)
177–184.
ˇ
ˇ
[27] M. Nadvornik, J. Holecek, K. Handllr, A. Lycka, J. Organomet. Chem. 275 (1984)
43–51.
ˇ
ˇ
[28] A. Lycka, J. Holecek, M. Nadvornlk, K. Hadler, J. Organomet. Chem. 280 (1985)
323–329.
ˇ
ˇ
[29] A. Lycka, M. Nadvornlk, K. Handllr, J. Holecek, Collect. Czech. Chem. Commun.
49 (1984) 2903–2911.
[30] H.C. Clark, V.K. Jain, R.C. Mebrotra, B.P. Singh, G. Srivastava, T. Birchall, J.
Organomet. Chem. 279 (1985) 385–394.
Appendix A. Supplementary data
[31] R. Willem, A. Bouhdid, B. Mahieu, L. Ghys, M. Biesemans, E.R.T. Tiekink, D. de
Vos, M. Gielen, J. Organomet. Chem. 531 (1997) 151–158.
[32] L.C.M. Costa, J.R.daS. Maia, G.M. de Lima, J.D. Ardisson, Solid State Commun.
137 (2006) 376–380.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2009.11.038.
[33] G. Eng, X. Song, A. Zapata, A.C. de Dios, L. Casabianca, R.D. Pike, J. Organomet.
Chem. 692 (2007) 1398–1404. and references therein.
[34] Q. Wang, Y.-Y. Sun, D.-Q. Wang, Acta Crystallogr. T63 (2007) o4597.
[35] F.H. Allen, Acta Crystallogr. b58 (2002) 380–388.
[36] A. Bondi, J. Phys. Chem. 68 (1964) 441–451.
[37] T.R. Shattock, K.K. Arora, P. Vishweshwar, M.J. Zaworotko, Cryst. Growth Des. 8
(2008) 4533–4545.
[38] J.A. Bis, P. Vishweshwar, D. Weyna, M.J. Zaworotko, Mol. Pharm. 4 (2007) 401–
416.
References
[1] A. Vlcek Jr., Coord. Chem. Rev. 230 (2002) 225–242. and references therein.
[2] R.S. Herrick, C.J. Ziegler, H. Bohan, M. Corey, M. Eskander, J. Giguere, N.
McMicken, I. Wrona, J. Organomet. Chem. 687 (2003) 178–184. and references
therein.
[3] B.P. Buffin, E.B. Fonger, A. Kundu, Inorg. Chim. Acta 355 (2003) 340–346.
[4] B.P. Buffin, P.J. Squattrito, A.O. Ojewole, Inorg. Chem. Commun. 7 (2004) 14–17.
[5] R. Garcia-Rodriguez, D. Miguel, Dalton Trans. (2006) 1218–1225.
[39] C. Ma, J. Li, R. Zhang, D. Wang, Inorg. Chim. Acta 359 (2006) 2407–2416.

64
D. Tzimopoulos et al. / Journal of Molecular Structure 965 (2010) 56–64
[40] F. Ahmad, S. Ali, M. Parvez, A. Munir, M. Mazhar, K.M. Khan, T.A. Shah,
Heteroat. Chem. 13 (2002) 638–649.
[41] M. Parvez, S. Ali, M. Mazhar, M.H. Bhatti, M.N. Khokhar, Acta Crystallogr. C55
(1999) 1280–1282.
[43] H.-Y. Wang, H.-D. Yin, D.-Q. Wang, Acta Crystallogr. E63 (2007) o1226–o1227.
[44] T.I. Ovsetsina, N.G. Furmanova, E.V. Chuprunov, V.I. Shcherbakov,
Kristallografiya (Russ.) 38 (1993) 71–76.
[45] Jmol: an open-source Java viewer for chemical structures in 3D. Available
from: <http://www.jmol.org/>.
[42] C. Ma, J. Li, R. Zhang, D. Wang, J. Organomet. Chem. 691 (2006) 1713–1721.