Synthesis, spectroscopic characterization and in vitro antimicrobial activity of diorganotin(IV) dichloride adducts with [1,2,4]triazolo-[1,5-a]pyrimidine and 5,7-dimethyl-[1,2,4]triazolo-[1,5-a]pyrimidine

Article abstract of DOI:10.1016/j.jorganchem.2005.10.007

The heterocyclic ligands [1,2,4]triazolo-[1,5-a]pyrimidine (tp) and 5,7-dimethyl-[1,2,4]triazolo-[1,5-a]pyrimidine (dmtp), react with diorganotin dichlorides giving the addition compounds Me₂SnCl ₂(tp)₂, Et₂SnCl₂(tp)₂, Me₂SnCl₂(dmtp)₂, Et₂SnCl ₂(dmtp)₂, Bu₂SnCl₂(dmtp), Ph ₂SnCl₂(dmtp). The organotin:ligand stoichiometry goes from 1:2 to 1:1 by increasing the steric hindrance of the organic groups bound to tin. The compounds have been characterized by means of infrared, ¹¹⁹Sn M?ssbauer and ¹H AND ¹³C NMR spectroscopy. The ligands presumably coordinate to tin classically through the nitrogen atom at the position 3. The 1:1 complexes adopt trigonal bipyramidal structures, with the organic groups on the equatorial plane and the ligand in the apical position. All-trans octahedral structures are inferred for the 1:2 complexes, except for Et₂SnCl₂(tp)₂, characterized by a skew-trapezoidal structure. ¹¹⁹Sn M?ssbauer measurements, at room temperature, in concomitance with DFT calculations, performed on isomeric structures of R₂SnCl₂(tp) ₂ (R = Me, Et), allowed us to conclude that the all-trans octahedral coordination induces self-assembly in the solid state, possibly accomplished through π-π stacking interactions among the planar ligands coordinated to the organotin(IV) compound, while the skew-trapezoidal structure attributed to Et₂SnCl₂(tp)₂, induces the formation of monomeric adducts in the solid state. In vitro antimicrobial tests showed that [n-Bu₂SnCl₂(dmtp)] has interesting properties as anti Gram-positive and antibiofilm agent.

Full text of DOI:10.1016/j.jorganchem.2005.10.007

Journal of Organometallic Chemistry 691 (2006) 693–701
www.elsevier.com/locate/jorganchem
Synthesis, spectroscopic characterization and in vitro
antimicrobial activity of diorganotin(IV) dichloride adducts
with [1,2,4]triazolo-[1,5-a]pyrimidine and 5,7-dimethyl-[1,2,4]triazolo-
[1,5-a]pyrimidine
a
b
a
a
M. Assunta Girasolo , Domenico Schillaci , Clelia Di Salvo , Giampaolo Barone ,
a
a,
*
Arturo Silvestri , Giuseppe Ruisi
a
`
Dipartimento di Chimica Inorganica e Analitica Stanislao Cannizzaro, Universita di Palermo, Viale delle Scienze, Edificio 17, I-90128 Palermo, Italy
b
`
Dipartimento di Chimica e Tecnologia Farmaceutiche, Universita di Palermo, via Archirafi 32, I-90123 Palermo, Italy
Received 27 July 2005; received in revised form 4 October 2005; accepted 4 October 2005
Available online 9 November 2005
Abstract
The heterocyclic ligands [1,2,4]triazolo-[1,5-a]pyrimidine (tp) and 5,7-dimethyl-[1,2,4]triazolo-[1,5-a]pyrimidine (dmtp), react with
diorganotin dichlorides giving the addition compounds Me2SnCl2(tp)2, Et2SnCl2(tp)2, Me2SnCl2(dmtp)2, Et2SnCl2(dmtp)2,
Bu2SnCl2(dmtp), Ph2SnCl2(dmtp). The organotin:ligand stoichiometry goes from 1:2 to 1:1 by increasing the steric hindrance of the
organic groups bound to tin. The compounds have been characterized by means of infrared, 119Sn Mo¨ssbauer and 1H AND 13C
NMR spectroscopy.
The ligands presumably coordinate to tin classically through the nitrogen atom at the position 3. The 1:1 complexes adopt trigonal
bipyramidal structures, with the organic groups on the equatorial plane and the ligand in the apical position. All-trans octahedral struc-
tures are inferred for the 1:2 complexes, except for Et2SnCl2(tp)2, characterized by a skew-trapezoidal structure.
119Sn Mo¨ssbauer measurements, at room temperature, in concomitance with DFT calculations, performed on isomeric structures of
R2SnCl2(tp)2 (R = Me, Et), allowed us to conclude that the all-trans octahedral coordination induces self-assembly in the solid state,
possibly accomplished through p–p stacking interactions among the planar ligands coordinated to the organotin(IV) compound, while
the skew-trapezoidal structure attributed to Et2SnCl2(tp)2, induces the formation of monomeric adducts in the solid state.
In vitro antimicrobial tests showed that [n-Bu2SnCl2(dmtp)] has interesting properties as anti Gram-positive and antibiofilm agent.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Triazolopyrimidine; Diorganotin(IV); Mo¨ssbauer; DFT calculations; Antimicrobial activity
1. Introduction
it differs for the presence of a pyrimidine nitrogen atom in a
bridgehead position and the consequent disappearance of
the acidic hydrogen atom of the five-membered ring [1].
We have recently reported on the synthesis, characteriza-
tion and antimicrobial activity of organotin(IV) derivatives
of oxo-substituted triazolopyrimidines [3]. Polymeric com-
pounds were obtained, where exocyclic oxygen atoms are
involved in the coordination. [1,2,4]triazolo-[1,5-a]pyrimi-
dine (tp) and its derivative 5,7-dimethyl-[1,2,4]triazolo-[1,5-
a]pyrimidine (dmtp) (see Fig. 1) may also give polymeric
compounds through N(1), N(3) coordination [1], but they
The interaction of metal ions with [1,2,4]-triazolo[1,5-
a]pyrimidine derivatives has been widely investigated
[1,2], the interest mainly arising from their biological prop-
erties and from the circumstance that triazolopyrimidine
ring has a chemical structure similar to purine, from which
*
Corresponding author. Tel.: +39 091 489369; fax: +39 091 427584.
E-mail address: gruisi@unipa.it (G. Ruisi).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.10.007

694
M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
Infrared spectra (nujol and hexachlorobutadiene mulls,
CsI windows) were recorded with an FT-IR spectrometer
Perkin–Elmer Spectrum One. Selected bands are reported
in Table 1.
The Mo¨ssbauer (nuclear c resonance) spectrometers, the
related instrumentation and data reduction procedures
were as previously described [6]. A 10-mCi Ca 119SnO3
source (RITVERC GmbH, St. Petersburg, Russia) was
employed. The isomer shifts (d) are relative to room tem-
perature Ca119SnO3. Methanol solution spectra were
recorded on 0.1 M samples, frozen by immersion in liquid
nitrogen. The 119Sn Mo¨ssbauer parameters are reported in
Table 2.
Fig. 1. The ligands employed and the atom numbering scheme.
normally act as monodentate ligands, the preferred binding
position being N(3) [1]. A few compounds have also been
described, in which there is not a direct bond between the
metal atom and the triazolopyrimidine derivative. Among
these, the tin(IV) compound [H2dmtp]2[SnCl6] [4], where
the [Hdmtp]+ cation is protonated at N(3). Continuing
our research in the field, we report here the results of infra-
red, 119Sn Mo¨ssbauer, 1H and 13C NMR spectroscopic
studies on diorganotin(IV) dichloride adducts of tp and
dmtp, see Fig. 1, and of antimicrobial activity tests on both
fungi (Candida albicans, Candida tropicalis) and bacteria
(Escherichia coli, Pseudomonas aeruginosa, Staphylococcus
aureus).
1H and 13C NMR spectra in deuterated methanol,
reported in Tables 4 and 5, were recorded with a Bruker
AC 250 E instrument operating at 250 and 63 MHz
respectively.
2.2. Synthesis of the adducts
The adducts were obtained by reacting the diorgano-
tin(IV) dichlorides with the ligands as detailed below.
Dimethyl- and diethyltin derivatives: the organotin
dichloride (2 mmol) and the ligand (4 mmol) were mixed
in diethyl ether (20 mL). The ligand is sparingly soluble
in such conditions. The suspension was stirred at room
temperature for 24 h, then the residue collected and dried
under vacuum.
[Me2SnCl2(tp)2] (1) m.p. 140–142 °C. Anal. Calc. for
C12H14Cl2N8Sn (459.91 g molꢀ1): C, 31.34; H, 3.07; N,
24.36; Cl, 15.42. Found: C, 31.32; H, 2.94; N, 24.02; Cl,
15.49%.
2. Experimental
2.1. Materials and methods
The products employed in the present study were pur-
chased from C. Erba (Milan, Italy), Sigma–Aldrich (Milan,
Italy), and Merck KGaA (Germany), and used without
further purification except methanol, which was distilled
over magnesium.
Carbon, hydrogen and nitrogen assays were carried out
with a Vario EL III elemental analyzer (Elementar GmbH,
Germany); chlorine was determined by potentiometric
titration with standard silver nitrate after combustion in
pure oxygen according to Scho¨niger [5].
[Et2SnCl2(tp)2] (2) m.p. 108–110 °C. Anal. Calc. for
C14H18Cl2N8Sn (487.97 g molꢀ1): C, 34.46; H, 3.72; N,
22.96; Cl, 14.53. Found: C, 34.34; H, 3.28; N, 22.96; Cl,
14.79%.
Table 1
Selected IR bands (cmꢀ1) for diorganotin(IV) chloride adducts
Compound
tp
mtp
mpy
m(Sn–Cl)
m(Sn–C)
1621m
1533m
1515m
1539s
1521s
1508s
1540m
1520s
1547vs
1549s
a
1
2
Me2SnCl2(tp)2
1616vs
1619vs
255s
Et2SnCl2(tp)2
dmtp
Me2SnCl2(dmtp)2
Et2SnCl2(dmtp)2
n-Bu2SnCl2(dmtp)
Ph2SnCl2(dmtp)b
322s
251s
533w(asym)
497w(sym)
1637s
1632s
1618m
1632m
1622s
1634s
1621s
1635m
1622m
3
4
5
6
259s
573mw
1552vs
1550vs
1550s
231m
329s
270s
287m
267s
Abbreviations: vs, very strong; s, strong; m, medium; w, weak.
a
796s = q(CH3).
b
1431s = m(C@C); 997mw = ring vibration; 731vs, 695s = C–H out of plane [30].

M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
695
Table 2
119Sn Mo¨ssbauer parameters of diorganotin(IV) chlorides and their tp and dmtp adductsa
Compound
db (mm sꢀ1
)
DEc (mm sꢀ1
)
Cd (mm sꢀ1
)
Area (mm sꢀ1
)
1
Me2SnCl2(tp)2
at 294 K
MeOH sol., 0.1 M
Et2SnCl2(tp)2
MeOH sol., 0.1 M
Me2SnCl2(dmtp)2
MeOH sol., 0.1 M
Et2SnCl2(dmtp)2
MeOH sol., 0.1 M
n-Bu2SnCl2(dmtp)
MeOH sol., 0.1 M
Ph2SnCl2(dmtp)
MeOH sol., 0.1 M
Me2SnCl2, MeOH sol., 0.1 M
Et2SnCl2, MeOH sol., 0.1 M
n-Bu2SnCl2, MeOH sol., 0.1 M
Ph2SnCl2, MeOH sol., 0.1 M
1.41
1.38
1.38
1.51
1.56
1.44
1.41
1.59
1.55
1.45
1.54
1.26
1.28
1.41
1.55
1.55
1.32
4.16
4.11
4.10
3.45
4.02
4.00
3.91
4.02
3.91
3.24
4.01
2.89
3.65
3.92
4.07
3.90
3.67
0.86
0.84
0.82
0.80
0.82
0.81
0.82
0.77
0.79
0.79
0.84
0.84
0.84
0.82
0.85
0.87
0.93
0.0780
0.0075
2
3
4
5
6
a
In the solid state and at liquid-nitrogen temperature, unless otherwise specified.
Isomer shift with respect to room-temperature CaSnO3.
Nuclear quadrupole splitting.
b
c
d
Full width at half-height of the resonant peaks.
[Me2SnCl2(dmtp)2] (3) m.p. 94–96 °C. Anal. Calc. for
energy minima found, after a preliminary conformational
analysis at the same level of theory, on the five different
trial structures deriving from cis or trans disposition of
the Cl2, R2 and (tp)2 ligands. Geometrical parameters
and relative energy values are reported in Table 3.
C16H22Cl2N8Sn (516.02 g molꢀ1): C, 37.24; H, 4.30; N,
21.72; Cl, 13.74. Found: C, 37.20; H, 4.00; N, 21.54; Cl,
13.76%.
[Et2SnCl2(dmtp)2] (4) m.p. 85–87 °C. Anal. Calc. for
C18H26Cl2N8Sn (544.08 g molꢀ1): C, 39.74; H, 4.82; N,
20.60; Cl, 13.03. Found: C, 39.90; H, 4.50; N, 20.90; Cl,
13.38%.
[n-Bu2SnCl2(dmtp)] (5): di-n-butyltin dichloride (2 mmol)
and dmtp (2 mmol) were mixed in diethyl ether (20 mL); the
resulting clear solution was stirred at room temperature
for 24 h, then evaporated to dryness under vacuum. m.p.
69–70 °C. Anal. Calc. for C15H26Cl2N4Sn (452.02 g molꢀ1):
C, 39.86; H, 5.80; N, 12.40; Cl, 15.69. Found: C, 40.10; H,
5.94; N, 12.30; Cl, 16.04%.
The 119Sn Mo¨ssbauer nuclear quadrupole splitting (DE)
of the organotin(IV) complexes (Table 3) was calculated as
recently reported [9], by the calibration formula
DEcalcd = 0.93V 0.58 mm sꢀ1
,
where V ¼ V zz½1 þ ꢁ
1
3
V xxꢀV yy 2 1=2
ð
Þ ꢂ and where Vxx, Vyy, Vzz, following the condi-
tion |Vzz| P |Vyy| P |Vxx|, are the eigenvalues of the diago-
nalized electric field gradient tensor, obtained at the same
level of theory [9].
V zz
2.4. Antimicrobial activity
[Ph2SnCl2(dmtp)] (6): diphenyltin dichloride (2 mmol)
and dmtp (2 mmol) were mixed in diethyl ether (20 mL);
the resulting clear solution was stirred at room temperature
for two days: a white precipitate separated, which was col-
lected and dried under vacuum. m.p. 112–114 °C. Anal.
Calc. for C19H18Cl2N4Sn (492.00 g molꢀ1): C, 46.39; H,
3.69; N, 11.39; Cl, 14.41. Found: C, 46.45; H, 3.49; N,
11.65; Cl, 14.24%.
2.4.1. Microorganisms
The microbial strains used were E. coli ATCC 25922, P.
aeruginosa ATCC 9027, S. aureus ATCC 25923, Staphylo-
coccus epidermidis DSM 3269, C. albicans ATCC 10231
and C. tropicalis ATCC 13803.
2.4.2. Antibacterial activity
2.3. Quantum chemical calculations
The determination of the minimum inhibitory concen-
tration (MIC) by broth dilution micro-method was per-
formed as reported [3]. For comparative purposes and
quality control of the method, we tested the antibiotic
amikacin.
DFT calculations, with full optimization of the geome-
try, were performed by the hybrid B3LYP method and
using the double-zeta valence plus polarization (DZVP)
all electron basis set [7], on the ligand [1,2,4]triazolo-[1,5-
a]pyrimidine (tp), as well as on the two isomeric complexes
R2SnCl2(tp)2, R = Me and Et, shown in Fig. 3, by using
the GAUSSIAN98 [8] program package. The structures of
the two isomers selected were corresponding to the lowest
2.4.3. Biofilms susceptibility testing,
methylthiazotetrazolium (MTT) method
The tests were performed as previously reported [3] on
S. aureus ATCC 25923 [10] biofilms employing drug

696
M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
Table 3
˚
Geometrical parameters (bond distances, in A, and angles, in degrees), their relative energy and calculated nuclear quadrupole splitting values [8],
calculated at DFT level (see text) for the compounds shown in Fig. 3
tp
Me2SnCl2(tp)2
A
Me2SnCl2(tp)2
B
Et2SnCl2(tp)2
A
Et2SnCl2(tp)2
B
Sn–N(3)
–
–
–
–
–
–
3.008
3.028
2.157
2.161
2.452
2.440
1.336
1.356
1.337
1.399
1.345
1.323
1.423
1.374
1.357
1.359
135.5
106.1
97.2
2.470
2.470
2.179
2.166
2.548
2.630
1.330
1.360
1.343
1.393
1.340
1.323
1.423
1.374
1.358
1.363
179.5
168.6
176.6
95.5
2.992
3.120
2.179
2.182
2.452
2.450
1.337
1.356
1.338
1.399
1.345
1.322
1.423
1.374
1.357
1.359
136.4
103.5
97.4
2.423
2.605
2.201
2.190
2.553
2.645
1.331
1.360
1.342
1.393
1.340
1.323
1.423
1.374
1.358
1.363
171.1
167.4
168.3
87.2
Sn–N(30)
Sn–C
Sn–C0
Sn–Cl
Sn–Cl0
N(1)–C(2)
C(2)–N(3)
N(3)–C(3)
C(3)–N(8)
C(3)–N(4)
N(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–N(8)
N(1)–N(8)
C–Sn–C0
1.340
1.353
1.335
1.405
1.348
1.322
1.425
1.374
1.357
1.358
–
–
–
–
–
–
–
N(3)–Sn–N(30)
Cl–Sn–Cl0
N(3)–Sn–Cl
N(3)–Sn–Cl0
N(30)–Sn–Cl
N(30)–Sn–Cl0
N(1)–C(2)–N(3)
C(2)–N(3)–C(3)
N(3)–C(3)–N(8)
C(3)–N(4)–C(5)
N(4)–C(5)–C(6)
C(5)–C(6)–C(7)
C(6)–C(7)–N(8)
C(7)–N(8)–C(3)
Energy (kJ/mol)
78.7
78.8
175.4
175.2
78.1
84.4
95.6
84.4
175.5
177.6
80.2
81.5
105.5
85.9
117.1
103.0
108.5
116.4
124.4
118.6
116.9
122.1
–
116.6
103.2
108.3
116.3
124.3
118.6
116.9
122.0
0.0
115.7
104.2
107.4
116.1
124.2
118.6
116.9
121.7
21.3
116.6
103.2
108.3
116.3
124.2
118.6
116.9
122.0
0.0
115.7
104.1
107.5
116.1
124.2
118.6
116.9
121.7
22.4
DEcalcd (mm sꢀ1
)
–
2.82
4.15
2.89
4.10
concentrations, in Mueller–Hinton broth, ranging from
100 to 6.2 lg/mL, the MIC of n-Bu2SnCl2(dmtp) (5).
Amphotericin B was used for comparative purpose and
quality control of the method.
Biological activity data are reported in Table 6 and in
Fig. 4.
2.4.4. Biofilms susceptibility testing, crystal violet method
S. aureus ATCC 25923 was cultivated, diluted and the
wells were washed as for the MTT method. The plates were
air dried at 37 °C and to each well, 100 lL of Mueller–
Hinton broth, supplemented with the same concentration
of n-Bu2SnCl2(dmtp) used for MTT method, were added.
The plates were incubated at 37 °C for 24 h; after this incu-
bation time the medium was removed, the plates were air
dried and each well was filled with crystal violet solution
(0.1%) for 15–20 min. After this time the plate was washed
three times with water and the crystal violet was dissolved
adding 150–200 lL of ethanol. The plate was read at
570 nm using a microplate reader. Inhibition percentages
at several concentrations of n-Bu2SnCl2(dmtp) were
obtained by comparing the optical density of control wells
with that of sample wells.
3. Results and discussion
3.1. Solid state structures of the adducts
Me2SnCl2 and Et2SnCl2 react with tp and dmtp in
diethyl ether to give addition compounds with 1:2 organo-
tin(IV):ligand stoichiometry, while 1:1 adducts were
obtained by reaction of n-Bu2SnCl2 and Ph2SnCl2 with
dmtp.
Monodentate binding through the endocyclic nitrogen
atom N(3) (see Fig. 1) is recognized as the main coordina-
tion behaviour of tp and dmtp [1]. It has been observed that
the coordination of metal ions through N(3) results in a
shift of mtp and mpy bands, assigned to triazolopyrimidine
and pyrimidine ring mode vibrations, respectively, usually
towards higher frequencies [11–18]. The shifts observed in
the spectra of our complexes, reported in Table 1, are
consistent with N(3) coordination of the organotin(IV)
2.4.5. Antifungal activity
The determination of MICs by broth dilution was
effected as previously reported [3].

M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
697
moiety. The 1:2 complexes Me2SnCl2(tp)2, Me2SnCl2-
(dmtp)2 and Et2SnCl2(dmtp)2 show high 119Sn Mo¨ssbauer
quadrupole splitting values, characteristic of octahedral
trans-R2 structures, while the DE value observed for
Et2SnCl2(tp)2 points out that this compound assumes
a lower symmetry structure (see Table 2). In the infrared
spectra of Me2SnCl2(tp)2, Me2SnCl2(dmtp)2 and Et2SnCl2-
(dmtp)2 only one m(Sn–Cl) absorption is observed, at 255,
259 and 231 cmꢀ1, respectively, indicative of a trans-Cl2
arrangement. The resulting all-trans configuration (struc-
ture I in Fig. 2) is generally adopted by octahedral
complexes of transition metals with triazolopyrimidine
ligands [1]. A medium intensity band at 573 cmꢀ1 in the
spectrum of Me2SnCl2(dmtp)2, can be attributed to the
Fig. 2. Proposed structures for R2SnL2 and R2SnL complexes: (I)
R = Me; L = tp, dmtp; R = Et; L = dmtp. (II) L = tp. (III) R = n-Bu,
Ph; L = dmtp.
‘‘skew-trapezoidal’’ structure, which can be regarded as a
distorted cis-Cl2, cis-N(3)2, trans-R2 octahedral structure
with Sn–C bonds pointing between the couple of ligands
with the largest L–Sn–L angle (structure II in Fig. 2).
To rationalize these structural differences, DFT calcula-
tions were performed on R2SnCl2(tp)2 compounds
(R = Me, Et), and the two lowest energy structures found,
i.e., the trans-R2 isomers with cis-Cl2 and trans-Cl2, are
shown in Fig. 3A and B, respectively. Their relevant geo-
metrical parameters, relative energy and calculated DE val-
ues are reported in Table 3.
The results obtained point out that the structure of the
cis-Cl2 isomer, in vacuo, is more than 20 kJ/mol (see Table
3) more stable than the one corresponding to the trans-Cl2
isomer, for both Me2SnCl2(tp)2 and Et2SnCl2(tp)2 com-
pounds. On the other hand, the comparison of the calcu-
lated and experimental nuclear quadrupole splitting
values (Tables 2 and 3) would indicate that the preferred
geometry is Me2SnCl2(tp)2 _B, i.e., the all-trans geometry,
m
asym(Sn–C2); the analogous band expected in the spectrum
of Me2SnCl2(tp)2 is probably masked by a ligand absorp-
tion at the same frequency.
A different structure must be adopted by the diethyl-
tin(IV) derivative, Et2SnCl2(tp)2. Mo¨ssbauer and infrared
data clearly indicate that the C–Sn–C fragment is not lin-
ear. Following the Parish approach [19], which assumes
that the quadrupole splitting of diorganotin(IV) derivatives
is set up just by the Alk2Sn unit, employing the pqs
[Alk]oct = ꢀ1.03 mm sꢀ1 [20], a C–Sn–C angle of 141°
was estimated. The infrared spectrum shows two weak
bands at 533 and 497 cmꢀ1 which can be attributed to
m
asym(Sn–C2) and msym(Sn–C2) respectively, as expected
for a bent C–Sn–C arrangement. Two strong absorptions,
at 322 and 251 cmꢀ1, are probably associated to stretching
vibrations m(Sn–Cl). These data are consistent with a
Fig. 3. Calculated structures of two possible R2SnCl2 adducts with two tp ligands (R = Me, Et), obtained after full geometry optimization at DFT level
(see text). The structures considered present a trans-dialkyl geometry, with a cis-Cl2 (A) or trans-Cl2 (B) coordination.

698
M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
and Et2SnCl2(tp)2 _A, i.e., the trans-diethyl, cis-Cl2 geome-
try, (see also Fig. 2 for their schematic representation).
In fact, concerning the structure of the compound
Et2SnCl2(tp)2, there is a better agreement between the
experimental DE, 3.45 mm sꢀ1 (see Table 2) and the calcu-
lated value relative to the most stable structure,
Et2SnCl2(tp)2 _A, 2.89 mm sꢀ1, compared to that calculated
for Et2SnCl2(tp)2 _B, 4.10 mm sꢀ1 (see Table 3). It is note-
worthy the good agreement between the calculated value
of the C–Sn–C angle of Et2SnCl2(tp)2 _A, 136.4° (Table
3), and the one evaluated by the point charge formalism,
141°, for the same structural environment (see above).
Concerning the structure of the compound Me2Sn-
Cl2(tp)2, on the other hand, the preferred structure would
not be the most energetically stable in vacuo, i.e.,
The Mo¨ssbauer spectra of the dimethyl and diethyl-
tin(IV) adducts of dmtp, compounds 3 and 4, are charac-
terized by quadrupole splitting values which are very
close to that of Me2SnCl2(tp)2; on this base, we can
hypothesize they have the same structure, probably stabi-
lized by p–p stacking, also considering that the presence
of the methyl groups in positions 5 and 7 (see Fig. 1) would
favour the stacking interactions by electron releasing effects
on the aromatic system of the ligand.
As already mentioned, n-Bu2SnCl2 and Ph2SnCl2, prob-
ably due to the bulkiness of the organic groups, give 1:1
addition compounds with dmtp. As a consequence, exclud-
ing a polydentate coordination, tin is presumably five-coor-
dinated. Assuming a trigonal bipyramidal configuration,
the presence of two m(Sn–Cl) absorptions is indicative of
a cis-Cl2 configuration in both the complexes. It is not pos-
sible to assign Sn–C vibrations, because these are masked
by the ligandÕs absorption bands. On the other hand, it is
well-known that, according to the Bent rule [24], organic
groups bound to tin normally occupy equatorial positions.
Accordingly, the complexes should have the structure III in
Fig. 2, with the organic groups and a chlorine atom in the
equatorial plane while a chlorine atom and the dmtp mol-
ecule occupy the axial positions.
Me2SnCl2(tp)2A, because
a much better agreement
between the experimental and calculated DE is found for
Me2SnCl2(tp)2 _B, i.e., 4.16 and 4.15 mm sꢀ1, respectively
(see Tables 2 and 3).
The preference for the all-trans structure can be ratio-
nalized in terms of self-assembly through p–p stacking. In
fact, we may hypothesize that strong p–p stacking interac-
tions are present in the solid phase among the
Me2SnCl2(tp)2 species, through the planar ligands. Analo-
gous interactions have been observed also for metal com-
plexes with similar ligands [21,22]. This intermolecular
interaction should be favoured by the almost coplanar dis-
position of the two coordinated tp ligands, as observed in
Me2SnCl2(tp)2 _B, compared to the almost perpendicular
disposition of the two planar ligands in Me2SnCl2(tp)2 _A.
On the other hand, the almost perpendicular disposition
of the two planar ligands in Et2SnCl2(tp)2 _A (see Fig. 3)
would hinder the self-assembly process through p–p stacks.
Our hypothesis is supported by the circumstance that
Me2SnCl2(tp)2 exhibits Mo¨ssbauer spectrum at room tem-
perature (see Table 2) characterized by a relatively large
area, compared to that of the liquid-nitrogen temperature
spectrum. Assuming a linear trend of the function ln(A)
vs. T, a value of ca. 0.01 is found for dln(A)/dT, wich is
typical of polymeric R2Sn(IV) compounds [23]. On the
other hand, no room temperature 119Sn Mo¨ssbauer effect
was occurring for Et2SnCl2(tp)2.
The 119Sn Mo¨ssbauer parameters are consistent with
the proposed structure: the C–Sn–C angle estimated by
the Parish approach [19] is 126° for n-Bu2SnCl2
(dmtp) ([Alk]tbe = ꢀ1.13 mm sꢀ1 [25]) and 129° for
Ph2SnCl2(dmtp) ([Ph]tbe = ꢀ0.98 mm sꢀ1 [25]). Point-
charge model calculations [26] do not allow us to definitely
establish if the ligand occupy an axial (cis-Cl2 configura-
tion) or equatorial (trans-Cl2 configuration) position, as
the contributions of nitrogen and chlorine atoms to the
electrical field gradient on 119Sn nucleus are quite similar.
However, the presence of two m(Sn–Cl) absorptions allows
us to exclude the trans-Cl2 configuration.
3.2. Methanol solution studies
The adducts were tested for their in vitro antimicrobial
activity; the nature of the complexes in solution phase was
investigated by means of 1H and 13C NMR (Tables 4 and 5)
Table 4
1H NMR spectra of tp and dmtp and their diorganotin(IV) chloride adductsa
H-2
H-5
H-6
H-7
CH3
R–Sn
|2J(1H–119Sn)|
tp
8.58
8.58
–
8.91
8.91
–
7.35
7.35
–
9.23
9.23
–
1
2
Me2SnCl2(tp)2
Me2SnCl2
1.11
1.21
1.18
94.7
94.0
94.6
b
Et2SnCl2(tp)2
dmtp
Me2SnCl2(dmtp)2
Et2SnCl2(dmtp)2
Bu2SnCl2(dmtp)b
Ph2SnCl2(dmtp)
8.58
8.47
8.47
8.47
8.47
8.48
8.91
7.36
7.12
7.13
7.12
7.13
7.12
9.23
–
–
–
–
–
–
–
–
–
–
2.65, 2.81
2.65, 2.81
2.65, 2.81
2.65, 2.81
2.66, 2.80
3
4
5
6
1.12
94.4
0.92–1.81
7.40–8.06
a
In CD3OD; d in ppm from internal TMS; coupling constants in MHz.
Ref. [31].
b

M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
699
Table 5
13C NMR spectra of the complexes 2–6 and related compoundsa
C-2
C-3a
C-5
C-6
C-7
CH3
R–Sn
tp
157.17
157.14
155.92
156.09
156.5
156.37
112.40
112.42
138.54
138.48
–
–
2
Et2SnCl2(tp)2
25.01(a), 10.23(b)
1J(Sn–C) = 682.7
2J(Sn–C) = 50.7
dmtp
Et2SnCl2(dmtp)2
155.62
155.42
156.01
155.91
167.37
167.29
112.48
112.44
149.30
149.19
16.81, 24.74
16.76, 24.70
4
5
24.79(a), 10.16(b)
1J(Sn–C) = 683.1
2J(Sn–C) = 50.7
1J(Sn–C) = 619.5
2J(Sn–C) = 40.9
3J(Sn–C) = 111.3
1J(Sn–C) = 619.6
2J(Sn–C) = 40.9
3J(Sn–C) = 111.9
2J(Sn–C) = 65.4
3J(Sn–C) = 100.7
4J(Sn–C) = 19.9
Bu2SnCl2(dmtp)
Bu2SnCl2
155.35
–
155.80
–
167.39
–
112.59
–
149.23
–
16.87, 24.82
–
30.98(a), 28.40(b),
27.05(c), 13.93(d)
30.87(a), 28.42(b),
27.09(c), 13.94(d)
6
Ph2SnCl2(dmtp)
154.96
155.50
167.58
112.74
149.26
16.84, 24.81
145.11(ipso), 136.14(ortho),
129.71(meta), 131.07(para)
a
In CD3OD; d in ppm from internal TMS; nJ(Sn–C) = |nJ(119Sn–13C)|, MHz.
and 119Sn Mo¨ssbauer (Table 2) spectroscopy in CD3OD and
CH3OH, respectively. Bothtechniques would suggest that the
adducts dissociate in methanol. In fact, the 1H and 13C chem-
ical shifts of the ligand atoms in CD3OD are practically iden-
tical for the complexes and the free ligands, the differences
being within the limits of the measurement error. It is, how-
ever, worth mentioning that the lack of any shift in 1H and
13C ligand signals was already observed in stable metal com-
plexes of dmtp and analogous ligands [11].
An accurate further characterization of the solution
phases of 5 and 6, e.g., by potentiometric titrations, seems
to be necessary to explain their biological effects.
3.3. Antimicrobial activity
3.3.1. MIC, anti-biofilm and antifungal results
The compounds were screened for their in vitro antimi-
crobial activity on a group of representative Gram-positive,
Gram-negative bacteria and two human pathogen fungal
strains.
The antimicrobial activities of substances, expressed as
MIC are reported in Table 6.
Information on the tin environment was deduced from
|2J(119Sn–1H)| and |1J(119Sn–13C)| coupling constants, such
as from 119Sn Mo¨ssbauer quadrupole splitting values
measured in frozen solutions, which correlate with the
C–Sn–C bond angles [27,19]. The quadrupole splittings
(Table 2) and the coupling constants (Tables 4 and 5)
measured for the complexes are very similar to those of
the parent diorganotin(IV) chlorides; 119Sn Mo¨ssbauer
data suggest trans-R2 octahedral structures for the diorg-
anotin(IV) moieties while 13C NMR data relative to 2, 4
and 5 are compatible with lower coordination numbers
or symmetry being the calculated [27] C–Sn–C bond angle
137° for 2 and 4 and 131° for 5. These differences may be
related to the freezing process needed for the Mo¨ssbauer
measurements which may be associated to crystallization
processes. Anyhow, the close similarity of the parameters
relative to the organotins and to the complexes does not
support the hypothesis of interaction, in solution, between
the organotin(IV) dichlorides and tp and dmtp molecules.
The correspondence of the spectroscopic signals of the
complexes with those of the parent ligands and organotin
moieties, would explain the observed biological activity of
compounds 1–4, (see below). However, the analogous
results obtained for compounds 5 and 6 do not agree with
the interesting antimicrobial properties for compounds 5
and, to a lesser extent, 6 (see Section 3.3), for which some
cooperative effect is developed in such systems, in contrast
with the apparent dissociation of the complexes in
solution.
The ligands tp and dmtp resulted inactive at screening
concentration of 100 lg/mL, but their complexation with
organotin led to derivatives with a better antimicrobial
activity. However, the activity of the complexes did not
always result better than that of the parent organotin com-
pounds. In particular, Et2SnCl2 and its derivatives
Et2SnCl2(tp)2 and Et2SnCl2(dmtp)2 showed about the same
activity against Gram negative E. coli, P. aeruginosa and
Gram positive S. aureus with MIC values ranging from
25 to 50 lg/mL and none of them resulted active against
S. epidermidis. Concerning n-Bu2SnCl2(dmtp), we observed
an improvement of activity against E. coli and S. aureus
with MIC value of 6.2 lg/mL in both cases. A good activ-
ity (MIC equal to 3.1 lg/mL) against S. epidermidis is
exhibited by Ph2SnCl2(dmtp).
Most compounds resulted inactive as antifungal at
screening concentration of 100 lg/mL or showed a weak
activity, in fact the MIC values ranged from 25 to
100 lg/mL (see Table 6).
Considering the important role played by staphylococci
in biofilm-associated infections [28], we also decided to test
the derivative n-Bu2SnCl2(dmtp) (5), which showed a good
activity on S. aureus, against preformed biofilms of the
same strain. We obtained inhibition percentages ranging

700
M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
Table 6
Antimicrobial activity in vitro, MIC values in lg/mL
Compound
E. coli
ATCC 25922
P. aeruginosa
ATCC 9027
S. aureus
ATCC 25923
S. epidermidis
DSM 3269
C. albicans
ATCC 10231
C. tropicalis
ATCC 13803
Me2SnCl2
>100
>100
>100
25
25
50
100
6.2
50
100
>100
>100
10
100
>100
>100
25
25
50
>100
>100
>100
>100
>100
>100
5
100
>100
>100
25
50
50
50
6.2
25
>100
>100
>100
>100
>100
>100
6.2
6.2
25
3.1
>100
>100
>100
>100
>100
>100
100
>100
50
50
>100
>100
>100
>100
>100
>100
100
>100
50
25
Me2SnCl2(tp)2
Me2SnCl2(dmtp)2
Et2SnCl2
Et2SnCl2(tp)2
Et2SnCl2(dmtp)2
Bu2SnCl2
Bu2SnCl2(dmtp)
Ph2SnCl2
Ph2SnCl2(dmtp)
tp
25
>100
>100
1
>100
>100
0.62
–
>100
>100
–
>100
>100
–
dmtp
Amikacin
Amphotericin B
–
–
–
0.15
0.31
from 76.3% to 17.2% or 45.5% to 16.1% at concentrations
ranging from 100 to 6.2 lg/mL (Fig. 4) using, respectively,
MTT or crystal violet method for staining biofilms grown
in microtiter plates.
on the surface of important indwelling medical devices
(catheters and other plastic foreign bodies) and are difficult
to treat and persistent infections. Antibiofilm activity
should then be considered an important factor in evaluat-
ing compounds as candidates for further development
stages in antimicrobial research [29]; for this reason we
think that 5 might be a potential lead compound in the dis-
covery of new anti Gram-positive and antibiofilm agents.
This discrepancy in the observed data, with reference to
the method used to staining biofilms, is due to the different
information derived from these methods; in fact MTT is a
respiratory indicator and in this way we can detect killing
activity of 5, whereas crystal-violet staining, as an indicator
of total attached biomass, shows removing activity of com-
pound. Compound 5 has a better killing effect on the staph-
ylococcal biofilm rather than removing the biofilm itself.
Biofilms of staphylococci represent an intrinsic form of
resistance to antibiotics, they are causes of colonization
Acknowledgement
`
This work was supported by Universita di Palermo,
Italy.
References
´
´
[1] J.M. Salas, M.A. Romero, M.P. Sanchez, M. Quiros, Coord. Chem.
Rev. 193–195 (1999) 1119.
[2] J.G. Haasnot, Coord. Chem. Rev. 200–202 (2000) 131.
[3] M.A. Girasolo, C. Di Salvo, D. Schillaci, G. Barone, A. Silvestri, G.
Ruisi, J. Organomet. Chem. 690 (2005) 4773.
[4] J.M. Salas, A. Rahmani, M.A. Romero, A.D. Rae, A.C. Willis,
E.R.T. Tiekink, Z. Kristallogr. 213 (1998) 302.
[5] W. Scho¨niger, Mikrochim. Acta (1955) 123.
[6] A. Barbieri, G. Ruisi, A. Silvestri, A.M. Giuliani, A. Barbieri, G.
Spina, F. Pieralli, F. Del Giallo, J. Chem. Soc., Dalton Trans. (1995)
467, and refs. therein.
[7] (a) N. Godbout, D.R. Salahub, J. Andzelm, E. Wimmer, Can. J.
Chem. 70 (1992) 560;
(b) C. Sosa, J. Andzelm, B.C. Elkin, E. Wimmer, K.D. Dobbs, D.A.
Dixon, J. Phys. Chem. 96 (1992) 6630.
[8] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb,
J.R. Cheeseman, V.G. Zakrzewski, J.A. Montgomery Jr., R.E.
Stratmann, J.C. Burant, S. Dapprich, J.M. Millam, A.D. Daniels,
K.N. Kudin, M.C. Strain, O. Farkas, J. Tomasi, V. Barone, M.
Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G.A. Petersson, P.Y. Ayala, Q. Cui, K. Morokuma,
D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.
Cioslowski, J.V. Ortiz, A.G. Baboul, B.B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R.L. Martin,
D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara,
M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,
J.L. Andres, C. Gonzalez, M. Head-Gordon, E.S. Replogle, J.A.
Fig. 4. In vitro inhibition percentages on S. aureus ATCC 25923 biofilm
obtained for n-Bu2SnCl2(dmtp) with MTT and crystal-violet methods.
Values are the average of at least three independent determinations. The
coefficient of variation was less than 15%.

M.A. Girasolo et al. / Journal of Organometallic Chemistry 691 (2006) 693–701
701
Pople, GAUSSIAN98, Revision A.8, Gaussian, Inc., Pittsburgh, PA,
1998.
[9] G. Barone, A. Silvestri, G. Ruisi, G. La Manna, Chem. Eur. J. 11
(2005) 6185.
[10] C. Heilmann, C. Gerke, F. Pendreau-Remington, F. Go¨tz, Infect.
Immun. 64 (1996) 277.
[19] R.V. Parish, Structure and bonding in tin compounds, in: G.J. Long
(Ed.), Mo¨ssbauer Spectroscopy Applied to Inorganic Chemistry,
Plenum Press, New York, 1984, pp. 527–575 (Chapter 16).
[20] M.G. Clark, A.G. Maddock, R.H. Platt, J. Chem. Soc., Dalton
Trans. (1972) 281.
´
´
[21] S. Orihuela, M.P. Sanchez, M. Quiros, J. Molina, R. Faure, J. Mol.
[11] E. Szłik, A. Grodzicki, L. Pazderski, J. Sitkowski, Polish J. Chem. 72
(1998) 55.
Struct. 415 (1997) 285.
[22] J.M. Salas, J.A.R. Navarro, M.A. Romero, M. Quiro´s, An. Quim.
Int. Ed. 93 (1997) 55.
[23] R. Barbieri, F. Huber, L. Pellerito, G. Ruisi, A. Silvestri, Tin-119m
Mo¨ssbauer Studies on Tin Compounds, in: P.J. Smith (Ed.),
Chemistry of Tin, Chapman and Hall, London, 1998, p. 496 (Chapter
14).
[12] I. Łakomska, E. Szłyk, J. Sitkowski, L. Kozerski, J. Wietrzyk, M.
Pełczynska, A. Nasulewicz, A. Opolski, J. Inorg. Biochem. 98 (2004) 167.
[13] E. Szłyk, L. Pazderski, I. Łakomska, A. Surdykowski, T. Glowiak, J.
Sitkowski, L. Kozerski, Polyhedron 21 (2002) 343.
[14] E. Szłyk, A. Grodzicki, L. Pazderski, E. Bednarek, B. Kamienski,
Polyhedron 19 (2000) 965.
[24] H.A. Bent, Chem. Rev. 61 (1961) 275.
[15] E. Szłyk, I. Łakomska, A. Surdykowski, T. Glowiak, L. Pazderski, J.
Sitkowski, L. Kozerski, Inorg. Chim. Acta 333 (2002) 93.
[16] J.M. Salas, M.A. Romero, A. Rahmani, R. Faure, G. Alvarez de
Cienfuegos, E.R.T. Tiekink, J. Inorg. Biochem. 64 (1996)
259.
[25] G.M. Bancroft, V.G. Kumar Das, T.K. Sham, M.G. Clark, J. Chem.
Soc., Dalton Trans. (1976) 643.
[26] G.M. Bancroft, R.H. Platt, Adv. Inorg. Chem. Radiochem. 15 (1972)
59.
[27] T.P. Lockart, W.F. Manders, J.J. Zuckerman, J. Am. Chem. Soc. 107
(1985) 4546.
[17] E. Szłik, A. Grodzicki, L. Pazderski, A. Wojtczak, J. Chatłas, G.
´
Wrzeszcz, J. Sitkowski, B. Kamienski, J. Chem. Soc., Dalton Trans.
(1998) 901.
[28] F. Go¨tz, Mol. Microbiol. 43 (2002) 1367.
[29] S.J. Projan, P.J. Youngman, Curr. Opin. Microbiol. 5 (2002) 463.
[30] R.C. Poller, J. Nucl. Chem. 24 (1962) 593.
[31] F. Caruso, M. Giomini, A.M. Giuliani, E. Rivarola, J. Organomet.
Chem. 506 (1996) 67.
[18] A.H. Velders, L. Pazderski, F. Ugozzoli, M. Biagini-Cingi, A.M.
Manotti-Lanfredi, J.G. Haasnot, J. Reedijk, Inorg. Chim. Acta 273
(1998) 259.