Structure and conformational motion of seven-coordinate diorganotin(IV) complexes derived from salen and salan type ligands

Article abstract of DOI:10.1016/j.jorganchem.2009.08.016

Reaction of R₂SnCl₂ (R = Me, nBu, Ph) and the potassium salts of salenN₃H₃ (N,N′-bis(salicylidene)diethylenetriamine) and saleanN₃H₅ (N,N′-bis(o-hydroxybenzyl)diethylenetriamine) provided d

Full text of DOI:10.1016/j.jorganchem.2009.08.016

Journal of Organometallic Chemistry 694 (2009) 3965–3972
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Structure and conformational motion of seven-coordinate diorganotin(IV)
complexes derived from salen and salan type ligands
b
Rolando Luna-García a, Berenice M. Damián-Murillo a, Victor Barba a, Herbert Höpfl a, , Hiram I. Beltrán ,
*
Luis S. Zamudio-Rivera c
a Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, 62209 Cuernavaca, Mexico
b Departamento de Ciencias Naturales, DCNI, Universidad Autónoma Metropolitana, Cuajimalpa, Pedro Antonio de los Santos 84, 11850 México D.F., Mexico
c Laboratorio de Síntesis Química y Electroquímica, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152, Col. San Bartolo Atepehuacan, México D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Reaction of R2SnCl2 (R = Me, nBu, Ph) and the potassium salts of salenN3H3 (N,N0-bis(salicylidene)dieth-
ylenetriamine) and saleanN3H5 (N,N0-bis(o-hydroxybenzyl)diethylenetriamine) provided diorganotin(IV)
complexes of the composition [Me2Sn(salenN3H)]ꢀsolvate (solvate = 2.5H2O, MeOH or DMSO), [nBu2-
Sn(salenN3H)]ꢀH2O, [Ph2Sn(salenN3H)]ꢀ2EtOH and [Me2Sn(saleanN3H3)]ꢀ2.5H2O. In all compounds the
tin atoms are seven-coordinate and have pentagonal–bipyramidal coordination environments, in which
the organic substituents attached to the tin atoms occupy the axial positions. This occurs both in solution
and the solid state; however, in solution the molecules are involved in conformational equilibria that
require the presence of intermediates, in which the N ? Sn bonds are dissociated. Although the [sale-
anN3H3]2ꢁ ligand is more flexible and basic, a very similar complexing behavior to that of [salenN3H]2ꢁ
has been found, and there is evidence that it is even a weaker ligand. Both ligands show the tendency to
adopt a curved conformation within the complex, thus indicating that the dynamic process resembles the
flapping of butterfly wings. However, the folding is reduced with increasing steric bulk of the organic
substitutents attached to the tin atoms. The six-membered heterocyclic rings in the [R2Sn(salenN3H)]
derivatives have envelope conformation, while those in [Me2Sn(saleanN3H3)] have distorted boat-confor-
mation. Thus, small changes in the hybridization and basicity of the nitrogen atoms cause significant
differences of the stability and the dynamic behavior of the resulting molecules.
Article history:
Received 14 July 2009
Received in revised form 11 August 2009
Accepted 11 August 2009
Available online 15 August 2009
Keywords:
Salen and salan
Tin
Hypercoordination
Conformational equilibrium
X-ray crystallography
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
tetradentate ONNO ligands, e.g. with the salen ligands [11] shown
in Scheme 1a, compounds having a mer–mer configuration are
In former research work diorganotin(IV) moieties have been
brought to reaction with a series of ligands having ONO [1,2] and
ONNO [3] donor systems. Generally, the resulting complexes are
monomeric or dimeric with coordination geometries close to
trigonal–bipyramidal, square–pyramidal, octahedral and pentago-
nal–bipyramidal polyhedra. Some of them have received special
interest in view of possible applications as biocides [4], anti-tumor
agents [5], materials with non-linear optical properties [6], fluores-
cence probes for DNA traces [7] and catalysts [8]. More recently,
also di- and trinuclear diorganotin(IV) complexes with macrocyclic
structures have been reported [3l,9].
So far, only few diorganotin complexes with ONNNO ligands are
known [1l,10], which is somewhat surprising, since such multiden-
tate ligands can give compounds having configurations and
nuclearities different from those known for complexes with lower
coordination numbers. This has been shown for molecules bearing
formed. By contrast, with the reduced and also more flexible salan
ligands given in Scheme 1b, compounds having a fac–fac configura-
tion are obtained [3k]. Furthermore, as outlined in Scheme 2, either
mononuclear [3] or dinuclear diorganotin derivatives may be
prepared [3m,12].
The combination of salicylaldehyde and diethylenetriamine
(DETA) gives a pentadentate ligand that is ideal for complexation
studies with diorganotin(IV). So far, this ligand is almost unex-
plored [13] and a search of the Cambridge Structural Database
(CSD) [14] yielded mainly structures of metal complexes derived
from the tripodal tris(((salicylidene)amino)ethyl)amine analogue.
When salen ligands are reduced at the imine functions, the
structurally related, but more flexible and basic salan derivatives
are obtained [15].
With the latter statements in mind, in this contribution we
report on the preparation, structural characterization and confor-
mational motion of four diorganotin(IV) complexes (1–4), which
have been obtained from the ONNNO-type salen and salan ligands
shown in Scheme 3. All compounds crystallized in form of solvates,
* Corresponding author. Tel./fax: +52 777 329 79 97.
E-mail address: hhopfl@ciq.uaem.mx (H. Höpfl).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.08.016

3966
R. Luna-García et al. / Journal of Organometallic Chemistry 694 (2009) 3965–3972
d
13C = 0.0 ppm) and SnMe4 (external, d119Sn = 0.0 ppm). COSY and
(a)
(b)
HETCOR experiments have been carried out in order to completely
assign the 1H and 13C spectra. IR spectra have been recorded on a
Bruker Vector 22 FT spectrophotometer. Mass spectra were
obtained on HP 5989A (EI) and Jeol JMS 700 (FAB+) equipments.
Elemental analyses have been carried out on an Elementar Vario
ELIII instrument. For these analyses samples of compounds 3ꢀ2EtOH
and 4ꢀ2.5H2O have been treated previously in an Abderhalden
equipment.
R
R'
N
R'
N
R'
R'
H
N
Sn
R
H
Sn
R
O
O
O
N
O
R
trans
cis
mer-mer
fac-fac
R = Me, nBu, Ph
R' = H, (R')2=1,2-C4H8
R = Me, nBu, Ph
R' = H, (R')2=1,2-C4H4
2.2. Preparative part
Diethylenetriamine (DETA), 2-hydroxybenzaldehyde (salicylal-
dehyde), sodium borohydride, dimethyltin dichloride, di-n-butyl-
tin dichloride and diphenyltin dichloride were commercial
starting materials and have been used without further purification.
N,N0-bis(salicylidene)diethylenetriamine (salenN3H3) and N,N0-
bis(o-hydroxybenzyl)diethylenetriamine (saleanN3H5) have been
prepared as described in the literature [13,15].
Scheme 1. For six-coordinate diorganotin complexes containing salen and salan
type ligands two different coordination modes have been observed: (a) mer–mer
and (b) fac–fac [3k].
(a)
(b)
R'
R
N
N
O
N
2.3. Preparation of the diorganotin complexes
Sn
O
2.3.1. [Me2Sn(salenN3H)] (1)
R
For the preparation of compound 1, salenN3H3 (0.277 g,
0.89 mmol) and two equivalents of potassium hydroxide (0.100 g,
1.78 mmol) were dissolved in 15 mL of a 2:1 solvent mixture of
ethanol and water, whereupon dimethyltin dichloride (0.212 g,
0.96 mmol) previously dissolved in water (5 mL) was added. A pale
yellow solid precipitated that was filtered and washed with small
amounts of ethanol. Crystals suitable for X-ray crystallography
could be grown from ethanol–water, methanol and DMSO solu-
tions to give three different solvates of the composition 1ꢀ2.5H2O,
1ꢀMeOH and 1ꢀDMSO. Yield: 0.314 g (77%). Mp: 246–248 °C (for
1ꢀ2.5H2O).
R'
N
N
N
N
O
O
O
O
R
R
R
R
Sn
Sn
N
(C@N) (s) cmꢁ1
.
N
~
IR (KBr):
v
= 3200
v
(N–H) (m), 1639
v
1H NMR (400 MHz, MeOH-d4, 20 °C, TMS): d 8.18 (s, 2H, H7),
7.24 (dd, 2H, H5), 7.16 (d, 2H, H3), 6.88 (d, 2H, H6), 6.63 (dd, 2H,
H4), 3.96 (dd, 2H, H8a), 3.52 (d, 2H, H8b), 3.19 (d, 2H, H9b), 2.66
R'
Scheme 2. Pentadentate ONNNO-type ligands can give either (a) mononuclear or
(b) dinuclear tin complexes [3m,12].
2
(dd, 2H, H9a), 0.86 (s, 3H, JSn–H = 115 Hz, Sn–Me), 0.15 (s, 3H,
2JSn–H = 104 Hz, Sn–Me0).
13C NMR (50 MHz, MeOH-d4, 20 °C, TMS): d 167.3 (C1, C7),
134.5 (C5), 133.8 (C3), 123.5 (C4), 122.2 (C2), 115.9 (C6), 59.9
(C8), 50.9 (C9) ppm.
and in the case of compound 1 solvates of three different
compositions could be obtained: [Me2Sn(salenN3H)]ꢀsolvate
with solvate = 2.5H2O (1ꢀ2.5H2O), MeOH (1ꢀMeOH) and DMSO
(1ꢀDMSO), [nBu2Sn(salenN3H)]ꢀH2O (2ꢀH2O), [Ph2Sn(salenN3H)]ꢀ
2EtOH (3ꢀ2EtOH) and [Me2Sn(saleanN3H3)]ꢀ2.5H2O (4ꢀ2.5H2O).
The main objectives of this study were (i) to determine, if mononu-
clear or dinuclear compounds are obtained, (ii) to study the role of
the central amino group in the hypercoordination of the tin atom
to reach seven-coordinated geometries, and (iii) to compare the
coordination patterns of the more rigid salen and the more flexible
salan ligands [3]. The crystal structures of 1ꢀ2.5H2O and 4ꢀ2.5H2O
contain infinite chains of spirocyclic water hexamers. Apparently,
the hydrogen-bond forming functions in these tin complexes
(NH, O) have the proper spatial distribution to stabilize extended
water clusters, which might be reminiscent of the solvation sphere
in solution. These features have been reported in a preliminary
communication [16].
119Sn NMR (74.5 MHz, MeOH-d4, 20 °C, SnMe4): d ꢁ505 (s) ppm.
MS (EI): m/z (%) 459 (M, 2), 444 (M–CH3, 28), 325 (M–CH3–
C7H5NO, 100), 295 (C10H11N2OSn, 50), 253 (C8H7NOSn, 16), 225
(C7H5OSn, 21), 134 (CH2Sn, 22).
Anal. Calc. for C20H25N3O2Snꢀ2.5H2O (503.16 g/mol): C, 47.74;
H, 6.01; N, 8.35. Found: C, 47.62; H, 6.28; N, 8.26%.
2.3.2. [nBu2Sn(salenN3H)] (2)
For the preparation of compound 2, salenN3H3 (0.088 g,
0.28 mmol) and two equivalents of potassium hydroxide (0.031 g,
0.56 mmol) were dissolved in ethanol (18 mL), whereupon di-n-
butyltin dichloride (0.094 g, 0.31 mmol) previously dissolved in
ethanol (2 mL) was added. A pale yellow solid precipitated that
was filtered and washed with small amounts of ethanol. Crystals
of the composition 2ꢀH2O, which were suitable for X-ray crystallog-
raphy, could be grown from ethanol. Yield: 0.130 g (85%). Mp:
191–193 °C.
~
2. Experimental
IR (KBr):
v
= 3259
v
(N–H) (w), 1634
v
(C@N) (s) cmꢁ1
.
1H NMR (400 MHz, MeOH-d4, 20 °C, TMS): d 8.23 (s, 2H, H7), 7.21
2.1. Instrumental
(dd, 2H, H5), 7.12 (d, 2H, H3), 6.87 (d, 2H, H6), 6.58 (dd, 2H, H4), 3.73
(br, s, 4H, H8), 2.96 (br, s, 4H, H9), 1.78 (m, 2H, Sn–H
a
), 1.55 (m, 2H,
), 1.31 (m,
NMR studies were carried out on Varian Gemini 200 and Varian
Sn–Ha0), 1.42 (m, 4H, Sn–Hb, Sn–Hb0), 1.10 (m, 2H, Sn–H
2H, Sn–Hc0), 0.98 (t, 3H, Sn–Hd), 0.65 (t, 3H, Sn–Hd0).
c
Inova 400 instruments. Standards were TMS (internal, d1H = 0.00,

R. Luna-García et al. / Journal of Organometallic Chemistry 694 (2009) 3965–3972
3967
13C NMR (100 MHz, MeOH-d4, 20 °C, TMS): d 169.5 (C7), 168.7
age [17c–d]. Non hydrogen atoms were refined anisotropically,
while C–H hydrogen atoms were placed in geometrically calculated
positions using a riding model. O–H and N–H hydrogen atoms have
been localized by difference Fourier maps and refined fixing the
bond lengths to 0.84 and 0.86 Å, respectively; the isotropic temper-
ature factors have been constrained to a value 1.5 times that of the
corresponding oxygen/nitrogen atoms. Solvent molecules were
present in the crystal lattices of all compounds, of which the EtOH
molecules in 3ꢀ2EtOH were disordered over two positions. The
complex molecules in 1ꢀDMSO are localized on crystallographic
mirror planes within the crystal lattice. In compound 2ꢀH2O one
of the n-butyl groups attached to the tin atoms is disordered over
two positions (occ = 0.62 and 0.38), while compound 3ꢀ2EtOH pre-
sents twofold disorder of the NCH2CH2NCH2CH2N fragment
(occ = 0.34 and 0.66). DFIX, SAME, SIMU and DELU instructions have
been used for a proper refinement of the disordered moieties.
(C1), 135.7 (C3), 134.8 (C5), 123.6 (C4), 122.1 (C2), 116.2 (C6),
60.5 (C8), 50.7 (C9), 29.5 (Sn–Ca), 28.7 and 28.4 (Sn–Cc
, Sn–Cc0),
27.6 (Sn–Cb), 14.1 (Sn–Cd).
119Sn NMR (74.5 MHz, MeOH-d4, 20 °C, SnMe4): d ꢁ503 (s) ppm.
MS (EI): m/z (%) 543 (M, 1), 486 (M-nBu, 89), 428 (M-nBuH-nBu,
30), 295 (C10H11N2OSn, 100), 253 (C8H7NOSn, 41), 225 (C7H5OSn,
43), 134 (CH2Sn, 31).
Anal. Calc. for C26H37N3O2SnꢀH2O (560.32 g/mol): C, 55.73; H,
7.02; N, 7.50. Found: C, 55.43; H, 6.95; N, 7.48%.
2.3.3. [Ph2Sn(salenN3H)] (3)
For the preparation of compound 3, salenN3H3 (0.307 g,
0.98 mmol) and two equivalents of triethylamine (0.200 g,
1.97 mmol) were dissolved in ethanol (15 mL), whereupon diphe-
nyltin dichloride (0.340 g, 0.98 mmol) previously dissolved in etha-
nol (15 mL) was added. A pale yellow crystalline solid precipitated
that was filtered and washed with small amounts of ethanol. Crys-
tals of the composition 3ꢀ2EtOH suitable for X-ray crystallography
could be grown from ethanol. Yield: 0.540 g (94%). Mp: 209–211 °C.
~
3. Results and discussion
IR (KBr):
v
= 3244
v
(N–H) (w), 1630
v
(C@N) (s) cmꢁ1
.
3.1. Preparation and structural characterization of compounds 1–3
1H NMR (400 MHz, DMSO-d6, 20 °C, TMS): d 8.53 (s, 2H, H7),
7.48 (m, 2H, Sn–Horto), 7.13 (m, 9H, H3, H5, Sn–Hmeta, Sn–Hpara),
6.44 (m, 4H, m, H4, H6), 4.37 (dd, 2H, H8a), 3.61 (d, 2H, 8b), 2.75
(d, 2H, H9b), 2.26 (dd, 2H, H9a) ppm.
The salenN3H3 ligand shown in Scheme 3 has been prepared as
described in the literature [13]. Reaction with dimethyl-, di-n-bu-
tyl- and diphenyltin dichloride in the presence of two equivalents
of base (KOH or NEt3) gave products in yields of 77%, 85% and 94%,
which were characterized by elemental analysis, mass spectrome-
try, spectroscopic methods (FTIR, 1H, 13C and 119Sn NMR) and X-ray
crystallography.
13C NMR (50 MHz, DMSO-d6, 20 °C, TMS): d 169.1 (C7), 167.7
(C1), 149.6 (Sn–Cipso), 133.2 (Sn–Corto), 134.3 (C3), 133.0 (C5),
127.0 (Sn–Cmeta), 127.5 (Sn–Cpara), 122.3 (C4), 119.2 (C2), 112.9
(C6), 57.0 (C8), 46.7 (C9) ppm.
119Sn NMR (74.5 MHz, DMSO-d6, 20 °C, SnMe4): d ꢁ640 (s) ppm.
MS (FAB+): m/z (%) 584 (M+H, 64), 505 (M-Ph, 8), 449 (M–
C8H8NO, 27).
The successful formation of the tin compounds (1–3) was indi-
cated by the mass spectra, which gave peaks characteristic for
monomeric species at m/z = 459 for 1, 542 for 2 and 584 for 3.
In the FTIR spectra of 1–3, the bands for the imine and N–H
stretching vibrations are well-separated from the remaining bands
and their shifts in comparison to the starting ligand permit to
establish that all three nitrogen atoms of the ligand are involved
in N ? Sn coordinative bonds. The spectral changes for the imine
functions of the tin compounds are small but significant
~
Anal. Calc. for C30H29N3O2Sn (582.28 g/mol): C, 61.88; H, 5.02;
N, 7.21. Found: C, 61.85; H, 5.12; N, 7.23.
2.3.4. [Me2Sn(saleanN3H3)] (4)
For the preparation of compound 4, saleanN3H5 (0.100 g,
0.32 mmol) and two equivalents of potassium hydroxide (0.036 g,
0.64 mmol) were dissolved in 15 mL of a 2:1 solvent mixture of
ethanol and water, whereupon dimethyltin dichloride (0.075 g,
0.34 mmol) previously dissolved in water (5 mL) was added. Upon
cooling to ꢁ20 °C a pale yellow crystalline solid precipitated that
was filtered and washed with small amounts of ethanol. These
crystals were suitable for X-ray crystallography. Yield: 0.050 g
(34%). Mp: 207–209 °C.
(
D
v
= +7, +2 and ꢁ2 cmꢁ1 for 1, 2 and 3, respectively), thus indicat-
ing a weak coordination [3g–h]. Furthermore, while the FTIR spec-
trum of the ligand shows only a very broad band for the N–H
stretching vibration at 3301 cmꢁ1, this band is well-defined for
complexes 1–3, having wavenumbers of 3200 for 1, 3259 for 2
and 3244 cmꢁ1 for 3 [3k,13b].
The 1H NMR data for compounds 1–3 indicate that the coordi-
native bond with the central nitrogen atom of the ligand is rela-
tively weak, since signal broadening is observed for the hydrogen
atoms belonging to the two NCH2CH2N groups. While for com-
pounds 1 and 3 an ABCD pattern typical for ethylene groups
embedded in a rigid chelate ring can be identified, for the di-n-
butyltin derivative 2 only two broad signals are detected in the ali-
phatic region. In order to examine the dynamic process occurring
in solution, a variable temperature NMR study has been performed
for this complex in the temperature range of ꢁ80 to +50 °C in
CD3OD. At ꢁ80 °C four coupled signals for the expected ABCD pat-
tern are observed, which converge to two signals at ꢁ10 °C.
According to the Eyring equation [18], the free activation enthalpy
for this process is 12.3 kcal molꢁ1. In agreement with these results
and according to previously described reaction sequences for five-
coordinate diorganotin complexes with ONO ligands [19], the dy-
namic process must involve at least three steps: (i) rupture of
the N(H) ? Sn coordinative bond, (ii) inversion of the central nitro-
gen atom, and (iii) recoordination of the NH group to the tin atom
(Scheme 4). Dreiding models show that in the present case the rup-
ture of all three N ? Sn bonds is required to allow for a strain-re-
duced inversion of the central nitrogen atom.
~
IR (KBr):
v
= 3286
v
(N–H) (m), 3222
v
(N–H) (m) cmꢁ1
.
1H NMR (200 MHz, DMSO-d6, 20 °C, TMS): d 6.99 (m, 2H, H3),
6.94 (m, 2H, H5), 6.56 (d, 2H, H6), 6.38 (dd, 2H, H4), 3.42 (s, 2H,
2
H7), 2.51 (br, s, 4H, H8, H9), 0.35 (s, 6H, JSn–H = 106 Hz, Sn–H
a,
Sn–Ha0).
119Sn NMR (74.5 MHz, DMSO-d6, 20 °C, SnMe4): d ꢁ450 (s) ppm.
MS (FAB+): m/z (%) 925 (2 M+H, 3), 464 (M+H, 100), 448 (M–
CH3, 43), 327 (M+H–CH3–C7H8NO, 46).
Anal. Calc. for C20H29N3O2Sn (462.17 g/mol): C, 51.97; H, 6.32;
N, 9.09. Found: C, 52.44; H, 6.24; N, 9.31%.
2.4. X-ray crystallography
X-ray diffraction studies were carried out on a BRUKER-AXS
APEX diffractometer with a CCD area detector (kMoK = 0.71073 Å,
a
monochromator: graphite). Frames were collected at T = 100 K
(1ꢀ2.5H2O, 1ꢀMeOH, 2ꢀH2O, 3ꢀ2EtOH) and 293 K (1ꢀDMSO,
4ꢀ2.5H2O) via
x- and h-rotation (D/x = 0.3°) at 10 s per frame
[17a]. The measured intensities were reduced to F2 and corrected
for absorption with SADABS [17b]. Structure solution, refinement
and data output were carried out with the SHELXTL-NT program pack-

3968
R. Luna-García et al. / Journal of Organometallic Chemistry 694 (2009) 3965–3972
H
N
H
N
H
N
H
N
NaBH4
EtOH
N
N
OH
HO
OH
HO
1. KOH (2 eq)
2. Me2SnCl2
EtOH/H2O
-20 oC
EtOH/H2O
20 oC
1. KOH (2 eq)
or NEt3 (2 eq)
2. R2SnCl2
9Me
8
9 R
H
N
H
N
8
7 H
H
7
3
2
6
3
N
O
N
O
N
2
6
N
O
Sn
Sn
4
4
O
1
1
5
5
R
Me
1 R = Me
2 R =
4
n
Bu
3 R = Ph
Scheme 3. Reaction sequence for the preparation of compounds 1–4 and atom numbering for the assignment of the NMR spectra.
That a significant population of the molecules dissolved in
to the tin atoms, X-ray diffraction analyses have been carried out
(Table 1). For 1 crystals suitable for X-ray diffraction could be
grown from three different solvents: (i) a solvent mixture of etha-
nol and water, (ii) methanol and (iii) DMSO, giving solid-state
structures having the composition 1ꢀ2.5H2O, 1ꢀMeOH and 1ꢀDMSO.
Despite the variation of the crystal structures for these solvates,
the molecular structures are similar (Table 2, vide infra). For com-
pounds 2 and 3 crystals were obtained from water and ethanol,
respectively, to give solid-state structures having the composition
2ꢀH2O and 3ꢀ2EtOH. The crystal lattice of 1ꢀ2.5H2O contains an
interesting water cluster, which has been described already in a
preliminary communication [16]. Selected geometric parameters
for compounds 1–3 are listed in Table 2 and the molecular struc-
tures are shown in Fig. 1.
As already shown by NMR spectroscopy, the tin atoms are se-
ven-coordinate in all three complexes and adopt distorted pentag-
onal–bipyramidal coordination geometries, in which the
pentadentate ONNNO ligands occupy equatorial and the organic
substituents axial sites. The Sn–C bond lengths vary from
2.108(4)–2.142(4) Å and are in the range expected for seven-coor-
dinate diorganotin(IV) species [10]. The Sn–O bond lengths range
from 2.145(2) to 2.194(3) Å and are within the range observed
for a series of R2Sn(salen) and R2Sn(salan) complexes, 2.073(3)–
2.237(2) Å [3k]. In contrast, the Sn–N bond lengths are significantly
longer, 2.445(3)–2.649(7) Å vs. 2.266(2)–2.431(4) Å. Since the
nitrogen atoms in compounds 1–2 have different hybridizations,
the N ? Sn bond lengths can vary due to this change [23]. Indeed,
for compounds 1 and 2 the Sn–Nimine bonds are shorter than the
Sn–Namine bonds (Table 2). However, compound 3 shows a differ-
ent behavior: in this case the Sn–Namine bond is intermediate,
2.461(4) Å, between the Sn–Nimine bond lengths, 2.450(4) and
2.543(4) Å, which can be explained with the enhanced steric strain
of the organic substituents attached to the tin atoms. Fig. 1 shows
that the variation in the steric bulk of the organic substituents at-
tached to the tin atoms has an important influence on the confor-
mation of the molecular structures. In compounds 1 and 2, which
have aliphatic substituents, the ligand adopts a butterfly conforma-
tion, while in compound 3 with the more voluminous phenyl
groups the ligand has an almost planar arrangement. The differ-
ence in the geometric organization of the ligand molecules influ-
ences both the bond and torsion angles within the ligand and
around the tin atom. In the case of the bond angles, mayor differ-
ences occur for the C–Sn–C bond angles, 172.1(2)–174.1(2)° for 1
and 172.0(2)° for 2 vs. 175.5(2)° for 3, the O–Sn–O bond angles,
76.2(1)–80.3(1)° for 1 and 77.1(1)° for 2 vs. 74.4(1)° for 3, and
CD3OD is seven-coordinate can be seen from both the 13C and
119Sn NMR spectra. In comparison to the ligand the chemical shifts
of the signals for the imine and NCH2 carbon atoms are significantly
different after complex formation: dC=N = 160.9 ppm and dNCH2
=
49.9 and 59.8 ppm for salenN3H3 vs. dC=N = 167.3 ppm and
dNCH2 = 50.9 and 59.9 ppm for 1, dC=N = 169.5 ppm and dNCH2 = 50.7
and 60.5 ppm for 2, and dC=N = 169.1 ppm and dNCH2 = 46.7 and
57.0 ppm for 3. The chemical shift displacements observed in the
119Sn NMR spectra, d = ꢁ505 for 1, d = ꢁ503 for 2 and d = ꢁ640 ppm
for 3, are typical for seven-coordinate tin complexes [10,20].
The 1H and 13C NMR spectra reveal also that the organic groups
attached to the tin atoms in compounds 1 and 2 have different
chemical environments, since two different sets of signals are
observed. This excludes the presence of horizontal mirror planes
within the molecules and allows the proposal of a curved butterfly
conformation generating a concave environment opposite to the
N–H hydrogen. For compound 1 the unresolved 2J(1H–117/119Sn)
coupling constants could be measured (115 and 104 Hz), which
allowed to estimate the C–Sn–C bond angle in solution. According
to Lockhart’s equation [21] a bond angle of 182° was calculated
[22], which is somewhat larger than the values extracted from
the solid-state structures (vide infra): 172.1(2)–174.1(2)°.
In order to determine whether compounds 1–3 have cis- or
trans-configuration regarding the organic substituents attached
R
R
H
N
N
N
N
Sn bond
rupture
O
O
O
H
Sn
Sn
R
N
O
N
N
R
nitrogen
inversion
R
R
N
N
N
Sn bond
O
O
O
Sn
R
Sn
N
H
O
formation
N
N
N
H
R
Scheme 4. Proposed dynamic conformational equilibrium for compounds 1–3 in
solution.

R. Luna-García et al. / Journal of Organometallic Chemistry 694 (2009) 3965–3972
3969
Table 1
Crystallographic data for compounds 1ꢀ2.5H2O, 1ꢀMeOH, 1ꢀDMSO, 2ꢀH2O, 3ꢀ2EtOH and 4ꢀ2.5H2O.
Crystal dataa
1ꢀ2.5H2Of
1ꢀMeOHf
1ꢀDMSOf
2ꢀH2O
3ꢀ2EtOH
4ꢀ2.5H2Of
Formula
MW (g molꢁ1
Space group
T (K)
C20H25N3O2Snꢀ2.5H2O
C20H25N3O2SnꢀMeOH
C20H25N3O2SnꢀDMSO
C26H37N3O2SnꢀH2O
560.29
P21/c
C30H29N3O2Snꢀ2EtOH
C20H29N3O2Snꢀ2.5H2O
)
503.16
P42bc
490.16
P212121
100
536.25
Pnma
293
674.39
P21/c
507.19
P42bc
100
100
100
293
a (Å)
b (Å)
c (Å)
20.3573(11)
20.3573(11)
10.4631(9)
90
12.1196(8)
12.5833(8)
13.8292(9)
90
20.362(2)
15.581(2)
7.5321(9)
90
9.2724(2)
18.687(2)
15.064(2)
90
11.8667(9)
14.5986(12)
17.7800(14)
90
20.8481(12)
20.8481(12)
10.4021(9)
90
a
(°)
b (°)
90
90
4336.1(5)
8
1.21
1.542
0.033
0.083
90
90
2109.0(2)
4
1.24
1.544
0.029
0.064
90
90
2389.6(5)
4
1.18
1.491
0.032
0.079
90.939(2)
90
2609.9(6)
4
1.01
1.426
107.584(1)
90
2936.2(4)
4
0.92
1.526
90
90
4521.2(5)
8
1.16
1.490
0.043
0.091
c
(°)
V (Å3)
Z
l
(mmꢁ1
)
qcalcd (g cmꢁ3
)
Rb,c
0.031
0.082
0.048
0.120
d,e
Rw
a
b
c
d
e
f
kMo
Fo > 4
= 0.71073 Å.
a
K
r
(Fo).
P
P
R = ||Fo| ꢁ |Fc||/ |Fo|.
All data.
P
P
Rw = [ w(F2o ꢁ F2c )2/ w(Fo2)2]
.
½
See also Ref. [16].
Table 2
Selected bond lengths [Å], bond angles [°], torsion angles [°] and centroidꢀꢀꢀcentroid distances for compounds 1ꢀ2.5H2O, 1ꢀMeOH, 1ꢀDMSO, 2ꢀH2O, 3ꢀ2EtOH and 4ꢀ2.5H2O.
1ꢀ2.5H2Oa
1ꢀMeOHa
1ꢀDMSOa,b
2ꢀH2Oc
3ꢀ2EtOHc
4ꢀ2.5H2Oa
Bond lengths
Sn–O1
Sn–O2
Sn–N1
Sn–N2
Sn–N3
Sn–C19
Sn–C20/23/25
2.194(3)
2.176(3)
2.448(4)
2.488(4)
2.463(4)
2.125(4)
2.108(4)
2.160(2)
2.171(2)
2.445(3)
2.558(3)
2.509(3)
2.120(4)
2.119(3)
2.150(2)
2.150(2)
2.509(3)
2.552(4)
2.509(3)
2.121(4)
2.106(5)
2.169(2)
2.145(2)
2.535(2)
2.602(2)
2.496(2)
2.139(6)
2.130(2)
2.156(3)
2.156(3)
2.450(4)
2.461(4)
2.543(4)
2.142(4)
2.142(4)
2.156(4)
2.150(4)
2.524(5)
2.591(6)
2.649(7)
2.124(5)
2.116(6)
Bond angles
O1–Sn–O2/O10
O1–Sn–N1
O1–Sn–C19
O1–Sn–C20/C23/C25
O2–Sn–N3
O2–Sn–C19
O2–Sn–C20/23/25
N1–Sn–N2
N2–Sn–N3
76.2(1)
75.5(1)
91.0(2)
90.8(1)
71.9(1)
96.6(1)
89.3(1)
69.4(1)
68.0(1)
89.7(2)
85.6(1)
174.1(2)
132.9(3)
120.1(3)
76.7(9)
74.8(1)
95.9(1)
90.4(1)
74.4(1)
93.7(1)
89.1(1)
67.9(1)
66.9(1)
90.0(1)
84.2(1)
173.6(2)
130.4(2)
132.0(2)
80.3(1)
73.3(1)
95.4(1)
90.7(1)
73.3(1)
95.4(1)
90.7(1)
67.0(1)
67.0(1)
88.9(2)
83.2(2)
172.1(2)
126.8(2)
126.8(2)
77.1(1)
74.7(1)
96.0(3)
89.8(1)
75.1(1)
95.7(3)
90.1(1)
66.4(1)
67.1(1)
87.6(3)
85.0(1)
172.0(2)
135.2(2)
132.5(2)
74.4(1)
76.3(1)
91.1(2)
93.4(2)
74.0(1)
91.7(1)
90.1(1)
68.3(3)
67.1(3)
86.8(3)
89.3(3)
175.5(2)
137.6(3)
142.0(3)
81.6(2)
77.9(2)
93.4(3)
90.2(2)
73.2(2)
98.4(2)
88.3(2)
67.6(2)
64.8(2)
92.7(3)
80.7(2)
172.8(3)
129.4(4)
118.8(4)
N2–Sn–C19
N2–Sn–C20/23/25
C19–Sn–C20/23/25
Sn–O1–C1
Sn–O2–C14
Torsion angles
N1–C8–C9–N2
N2–C10–C11–N3
Sn–O1–C1–C2
Sn–O2–C14–C13
ꢁ58.0(5)
55.0(5)
ꢁ54.9(4)
56.5(4)
ꢁ56.8(4)
56.8(4)
ꢁ56.6(3)
58.0(3)
56(3)/ꢁ54(2)
ꢁ58(2)/58(1)
60.9(8)
ꢁ58.5(8)
56.0(8)
ꢁ76.0(6)
ꢁ39.2(7)
ꢁ36.4(5)
ꢁ50.0(4)
ꢁ36.2(4)
23.0(7)
11.0(7)
59.4(5)
42.5(5)
50.0(4)
41.4(4)
Others
Centroidꢀꢀꢀcentroidd
6.67
7.19
6.76
7.43
7.83
6.60
a
b
c
See also Ref. [16].
Molecule has crystallographic mirror symmetry.
Molecule presents disorder (see Section 2); where possible, average values are given.
Centroid 1 corresponds to phenyl ring C1–C6 and centroid 2 corresponds to phenyl ring C13–C18 (C10–C60 in the case of 1ꢀDMSO).
d
the range of the C–Sn–X (X = O, N) bond angles, 83.2(2)–96.6(1)°
for 1 and 85.0(1)–96.0(3)° for 2 vs. 86.8(3)–93.4(2)° for 3. This
structural comparison indicates that the tin atoms in compound
3 deviate less from the ideal pentagonal–bipyramidal coordination
geometry.
The six-membered {C3NOSn} heterocyclic rings in compounds 1
and 2 have envelope conformations, in which the tin atoms occupy
the vertex. The five-membered {C2N2Sn} rings have also approxi-
mate envelope conformations, but in this case the N(H)–CH2 meth-
ylene moieties occupy the vertex. The degree of ligand folding in

3970
R. Luna-García et al. / Journal of Organometallic Chemistry 694 (2009) 3965–3972
Fig. 1. Perspective views of the molecular structures of compounds 1–4. Ellipsoids are shown at the 30% probability level.
compounds 1–3 can be evaluated threefold: first, comparing the
SnOCC torsion angles (ꢁ41.9(7)/50.6(5)° for 1, ꢁ36.2(4)/41.4(4)°
for 2 and 23.0(7)/11.0(7)° for 3) [24], second, comparing the cen-
troidꢀꢀꢀcentroid distances between the aromatic rings of the salicy-
lidene moieties (6.67–7.19 Å for 1, 7.43 Å for 2 and 7.83 Å for 3),
and third, comparing the angle formed between the tin atom and
the centroids of the aromatic rings of the salicylidene moieties
(97.8–105.9° for 1, 110.0° for 2 and 115.9° for 3). All three param-
eters show clearly that the ligand is folded to a higher degree in
complex 1 (R = Me), followed by compounds 2 and 3, in agreement
with the increase of the steric bulk of the organic groups attached
to the tin atoms. However, it should be noticed that the molecular
conformations present some flexibility, since the above-described
geometric parameters vary significantly between the three solvate
structures 1ꢀ2.5H2O, 1ꢀMeOH and 1ꢀDMSO. A comparison of the
NCCN torsion angles for the five-membered heterocycles shows
that the variations are relatively small in this case (Table 2).
There are two further interesting details regarding the overall
molecular conformations of 1–3. The ligand distortion from planar-
ity is asymmetric in all three cases, as it can be seen from the dif-
ferences in the absolute values of the SnOCC torsion angles
compounds 1 and 2, but in alternate directions for compound 3
(Fig. 1).
3.2. Preparation and structural characterization of compound 4
As already mentioned, the strength of the N ? Sn coordinative
bonds and the configuration of the tin complexes can be modified
using the reduced form of the salen ligands. In order to evaluate
these parameters, the potassium salt of saleanN3H5, which has
been prepared from salenN3H3 by reduction with NaBH4 [15],
was reacted with dimethyltin, di-n-butyltin and diphenyltin
dichloride. However, only in the first case a monomeric product
could be separated and characterized, while for the reactions with
nBu2Sn and Ph2Sn unsoluble product mixtures involving probably
oligo- and polymeric compounds were obtained. Complex 4 was
isolated as hydrate of the composition 4ꢀ2.5H2O and characterized
by elemental analysis, mass spectrometry, spectroscopic methods
(FTIR, 1H, and 119Sn NMR) and X-ray crystallography.
The FAB+ mass spectrum showed a peak characteristic for a
monomeric species at m/z = 464, but interestingly there was also
a peak, albeit of low intensity, corresponding to a dimeric species
(m/z = 925). In the FTIR spectrum there are two bands for N–H
outlined above:
D = 8.8(7)° for 1, 5.2(4)° for 2 and 12.0(7)° for 3.
This may be a consequence of intermolecular interactions within
the crystal lattice. Furthermore, it should be noticed that the fold-
ing of the salicylidene groups occurs in the same directions for
stretching vibrations with wavenumbers of 3286 and 3222 cmꢁ1
,
which correspond to the benzylamino and the central amino
group [3k,13b]. The 1H NMR spectrum of compound 4 showed

R. Luna-García et al. / Journal of Organometallic Chemistry 694 (2009) 3965–3972
3971
significantly broadened signals for the NCH2 groups at d = 2.51 and
3.51 ppm, and for the methyl substituents attached to the tin atoms
at d = 0.35 ppm. The 119Sn NMR signal, d = ꢁ450 ppm, is low-field
shifted when compared to 1, d = ꢁ505 ppm, thus probably indicat-
ing a weaker N ? Sn coordination. The unresolved 2J(1H–117/119Sn)
coupling constant of 106 Hz gave a calculated C–Sn–C bond angle of
174° in solution [21], which is in agreement with the bond angle ex-
tracted from the solid-state structure (vide infra): 172.8(3)°.
Due to the higher flexibility of the reduced five-coordinate li-
gand one might expect that the configuration of the complex is
cis instead of trans, as it occurred in the case of the diorgano-
tin(salan) complexes reported earlier (Scheme 1b) [3k]. Appar-
ently, in the present case the trans-configuration is more
favorable as it is demonstrated by the X-ray crystallographic study.
The crystallographic data for compound 4ꢀ2.5H2O and selected
geometric parameters have been summarized in Tables 1 and 2.
The molecular structure of this complex is shown in Fig. 1.
The NMR spectroscopic studies have shown that the complexes
are involved in conformational equilibria in solution, which re-
quires the dissociation of all three N ? Sn bonds. For those diorga-
notin compounds, for which curved conformations have been
observed in the solid state, the dynamic process resembles the
flapping of butterfly wings.
Acknowledgment
The authors thank Instituto Mexicano del Petróleo (Project No.
IMP-D.00178) for financial support.
Appendix A. Supplementary material
CCDC 273788, 273789, 273790, 273791, 736978 and 736979
contain the supplementary crystallographic data for compounds
1ꢀ2.5H2O, 1ꢀMeOH, 1ꢀDMSO, 4ꢀ2.5H2O, 2ꢀH2O and 3ꢀ2EtOH. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.08.016.
Albeit saleanN3H32ꢁ is more flexible than salenN3H2ꢁ, the over-
all conformation of compound 4 is very similar to its salen ana-
logue 1. A comparison of the Sn–C, Sn–O and Sn–N bond lengths
for 1 and 4 shows that there are significant differences only for
the Sn–N bonds. This is not unexpected due to the sp2 ? sp3
hybridization change for two of the three nitrogen atoms
(2.467(4) vs. 2.524(5) Å for Sn–N1, 2.494(4) vs. 2.649(7) Å for
Sn–N3), but interestingly, the central N ? Sn coordinative bond in-
creases also, from 2.532(4) to 2.591(6) Å. The increase of the
N3 ? Sn bond length is larger than it might be expected from
the sp2 ? sp3 hybridization change (approximately 0.1 Å) [23], so
that additional intra- or intermolecular strain might occur in this
part of the molecule. These results show that the reduced salen li-
gand does not necessarily coordinate stronger to the tin atom than
the unreduced species, although sp3-hybridized nitrogen atoms
use to be stronger Lewis bases than those with sp2-hybridization.
Including, a comparison of the bond angles around the tin atom
shows that the deviations from the values for a regular pentagonal
bipyramid are larger in compound 4. Some representative values
for the X–Sn–X bond angles are 81.6(2)° for O–Sn–O, 72.3(2) and
77.9(2)° for O–Sn–N, and 64.8(2) and 67.6(2)° for N–Sn–N. The
C–Sn–C angle is 172.8(3)° and the C–Sn–X (X = O, N) bond angles
range from 77.6(3) to 98.4(2)°. In part, the variations in comparison
to compound 1 (Table 2) can be attributed to the conformational
differences of the {C3NOSn} heterocyclic rings, which have now
distorted boat-conformations. The SnOCC torsion angles are larger
for compound 4, ꢁ76.0(6)/56.0(8) vs. ꢁ41.9(7)/50.6(5)° for 1, and,
accordingly, the centroidꢀꢀꢀcentroid distances between the aro-
matic rings of the salicylidene moieties are smaller, 6.60 vs.
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2
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ˇ
O, 1ꢀMeOH and 1ꢀDMSO) average
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values are given.