Organotin(IV) derivatives of some O,C,O-chelating ligands. Part 2

Article abstract of DOI:10.1021/om700576n

A new O,C,O-chelating ligand [2,6-C₆H₃(CH ₂OEt)₂]^- and its organotin(IV) derivatives, LSnPh₃, LSnPh₂-Cl, and LSnPhCl₂ (4-6), were prepared and structurally characterized in solution as well as in the solid state by NMR and XRD techniques. The structure of these compounds was compared to those of analogous compounds containing ligands of the type [2,6-C ₆H₃(CH₂OR)₂]^-, where R = Me, i-Pr, and t-Bu, on the basis of various NMR spectral parameters, mainly on ^πJ(¹H-¹¹⁹Sn) long-range couplings, solid-state structure determinations, and theoretical calculations at the B3LYP/LANL2DZ level. All of these methods showed that the structure is similar in all three phases (solid, solution, and in silico), but that it dramatically depends on the R substituent in the O,C,O-pincer ligand.

Full text of DOI:10.1021/om700576n

6312
Organometallics 2007, 26, 6312-6319
Organotin(IV) Derivatives of Some
O,C,O-Chelating Ligands. Part 2
Libor Dosta´l,† Roman Jambor,† Alesˇ Ru˚zˇicˇka,*,† Ivana C´ısarˇova´,‡ Jaroslav Holecˇek,†
Monique Biesemans,§ Rudolph Willem,§ Frank De Proft,| and Paul Geerlings|
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, UniVersity of
Pardubice, na´m. Cˇ s. legi´ı 565, CZ-532 10, Pardubice, Czech Republic, Charles UniVersity in Prague,
Faculty of Science, HlaVoVa 2030, 128 40 Praha 2, Czech Republic, and High Resolution NMR Centre
(HNMR), Department of Materials and Chemistry (MACH), Faculty of Engineering, and Eenheid
Algemene Chemie (ALGC), Department of Chemistry (DSCH), Faculty of Sciences,
Vrije UniVersiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium
ReceiVed June 12, 2007
A new O,C,O-chelating ligand [2,6-C6H3(CH2OEt)2]- and its organotin(IV) derivatives, LSnPh3, LSnPh2-
Cl, and LSnPhCl2 (4-6), were prepared and structurally characterized in solution as well as in the solid
state by NMR and XRD techniques. The structure of these compounds was compared to those of analogous
compounds containing ligands of the type [2,6-C6H3(CH2OR)2]-, where R ) Me, i-Pr, and t-Bu, on the
n
basis of various NMR spectral parameters, mainly on J(1H-119Sn) long-range couplings, solid-state
structure determinations, and theoretical calculations at the B3LYP/LANL2DZ level. All of these methods
showed that the structure is similar in all three phases (solid, solution, and in silico), but that it dramatically
depends on the R substituent in the O,C,O-pincer ligand.
polyhedral shapes depend on the degree of donor-acceptor
Sn-O interactions, going from very weak to medium-strong.
This interaction increases with increasing Lewis acidity of the
tin atom, but decreases with increasing steric demands of the R
substituents in the O,C,O-chelating ligands. In the compounds
investigated, the Sn-O distances (2.475-2.966 Å) are sub-
stantially smaller than the sum of the van der Waals radii of
oxygen and tin (3.70 Å),4 but larger than the sum of the covalent
radii (2.066 Å).4 The carbon and/or chlorine atoms occupy the
axial positions in the tin coordination sphere. The donor O-atoms
coordinate to the tin atom with a O-Sn-O cis configuration
resulting in a pseudofacial O,C,O-coordination mode of the
“pincer” ligands. This cis configuration is retained, even in
diorganotin compounds 3, 9, and 12, in contrast to related
diorganotin dichlorides containing another O,C,O-pincer ligand,
the 5-t-Bu-1,3-C6H2[P(O)(OEt)2]2 one, in which the oxygen
atoms coordinate to the tin atom in a trans configuration.5
Solution NMR spectra indicate similar structural arrangements
around tin in solution and in the solid state, but direct data for
the estimation of the strength of the Sn-O interactions in
solution are missing so far. The measurement of long-range
coupling constants appears to be a possibility to obtain this
information.
Introduction
The use of chelating, especially “pincer”, ligands for the
stabilization of coordinative metal centers in organometallic
compounds is well documented.1 These ligands have been
applied to a class of organotin compounds as well.2 Recently,
we reported the synthesis of three monoanionic O,C,O-pincer
ligands and their organotin derivatives.3a We investigated their
solid-state structures, using XRD and CP/MAS NMR, as well
as the solution-state structures of 11 compounds, including 1-3
and 7-12 (Scheme 1), using multinuclear NMR techniques. The
structures of the 11 compounds mentioned were found to be
similar in solution and in the solid state, with the tin atom
varying from a tetrahedral 4-coordinate to an octahedral
6-coordinate configuration upon going from tetra- to monoor-
ganotin compounds.3a The deviations from the ideal coordination
Dedicated to Assoc. Prof. Dr. Jarom´ır Plesˇek on the occasion of his
80th birthday in recognition of his outstanding contributions to the area of
borane and carbaborane cluster chemistry.
* Corresponding author. Fax: + 420 46 603 7068. E-mail: ales.ruzicka@
upce.cz.
† University of Pardubice.
‡ Charles University in Prague.
§ Department of Materials and Chemistry, Vrije Universiteit Brussel.
| Department of Chemistry, Vrije Universiteit Brussel.
This Article describes the measurement of long-range cou-
pling constants in the series of organotin compounds 1-12
(1) (a) Chemistry of HyperValent Compounds; Akiba, K., Ed.; Wiley
VCH: Weinheim, Germany, 1999; and references therein. (b) Jastrzebski,
J. T. B. H.; Boersma, J.; Esch, P. M.; van Koten, G. Organometallics 1991,
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10.1021/om700576n CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/06/2007

Organotin(IV) DeriVatiVes of O,C,O-Chelating Ligands
Organometallics, Vol. 26, No. 25, 2007 6313
Scheme 1
and 7 show slightly increasing negative chemical shifts and
increasing couplings upon decreasing bulkiness of the substituent
on oxygen, indicating an increasing (but weak) interaction
between tin and oxygen, confirmed by the Sn-O distances
decreasing concertedly in the solid state (Table S11). The
4J(1H-119Sn) coupling constants of the benzylic protons increase
in the triphenyltin series with increasing size of the substituent
on oxygen. This can be ascribed either to the increasing
electronegativity of the oxygen atom bound to these benzylic
protons (see Table S7, Supporting Information, where the O
atomic charge appears highest for R ) t-Bu) or to the
contribution of an increasing interaction with tin, resulting in a
(Scheme 1) and attempts to bring them in relationship to Sn-O
bond lengths, as determined by XRD techniques and obtained
from DFT calculation. The synthesis and X-ray structure of
organotin(IV) derivatives (4-6) (Scheme 1) based on the new
O,C,O-chelating ligand [2,6-C6H3(CH2OEt)2]- are presented as
well.
3
second coupling pathway over three bonds with J(1H-119Sn)
coupling constant of opposite sign.
The low- and room-temperature spectra of these compounds
Results and Discussion
are very similar. However, compound 1 has a slightly smaller
Solution Study. In the previous study,3 the solution 119Sn
4
benzylic J(1H-119Sn) coupling constant (4.7 as compared to
1
chemical shifts and J(13C-119Sn) coupling constants of 9 of
5.6 Hz), in line with the proposal of the contribution of a second
pathway with coupling constant of opposite sign logically
increasing at lower temperature as the OfSn interaction
becomes stronger.
the 12 compounds (1-3, 7-12) indicated similar structural
arrangements in solution and in the solid state, but no estimation
of the strength of their Sn-O interaction in solution was
available. It was believed that measuring long-range J(1H-
n
The 119Sn NMR spectra of the chlorodiphenyltin compounds
2, 5, 8, and 11 show a more pronounced shift to higher field of
80-100 ppm, when compared to triphenyltin chloride, charac-
teristic for coordination expansion by one ligand, as confirmed
119Sn) coupling constants, from 1D6 and/or 2D7 ge-1H-119Sn
J-HMQC spectra, could offer the possibility to obtain this
information. The nominal 4J(1H-119Sn) coupling constants for
the benzylic CH2 protons are reported in Table 1, together with
some relevant 119Sn chemical shifts and 1J(13C-119/117Sn)
coupling constants of the new compounds, other previously
studied ones,3 and a reference compound without pincer ligand,8
reported for comparison. The triphenyltin derivatives, when
compared to the reference compound, display a chemical shift
to lower frequency by some 25-35 ppm, due to shielding from
neighboring substituents and/or a weak interaction between the
tin atom and the oxygen atoms.8,9 The smallest upfield shift in
1
by a J(13C-119Sn) coupling increase of about 100 Hz. This
coordination expansion, by interaction of the tin atom with
alternatively one of the ligand oxygen atoms, is due to the
increased Lewis acidity of the tin atom upon substitution of
one phenyl substituent by a chlorine atom. The t-Bu group in
11 does not prevent this interaction, confirming the higher Lewis
acidity of tin in the triaryltin chlorides than in the tetraaryl
compounds. At room temperature, the same relative trend in
chemical shift as in the previous series 1, 4, 7, and 10 reveals
again an increasing interaction with decreasing size of the
119Sn chemical shift and smallest increase in J(13C-119/117Sn)
1
coupling constant with respect to tetraphenyltin are observed
for compound 10, with the bulky t-butyl-group on the oxygen
atom. Because solid-state data of 10 (see below) show the
absence of interaction between the tin atom and the oxygen
atoms, the increased shielding in chemical shift of 10 with
respect to the reference can be ascribed solely to the steric bulk
of the t-butyl groups on the oxygen atoms of the pincer. The
119Sn NMR spectra of the other triphenyltin derivatives 1, 4,
4
oxygen substituent. The benzylic J(1H-119Sn) couplings all
have about the same value, to be interpreted as the result of
several changing parameters with opposing but mutually balanc-
ing effects. First, a coupling increase is expected as a conse-
quence of the presence of the electronegative chlorine atom on
the tin atom. Second, the additional coordination between tin
3
and oxygen creates a second scalar J coupling pathway of
4
opposite sign to the J through the benzene ring. Third, the
additional coordination can generate a stronger steric repulsion
with bulky groups. At low temperature, the 119Sn nuclei are
more shielded (Table 2), and the benzylic protons resonances
decoalesce into two singlets for compounds 2 and 5, as the result
of the equilibrium in which either of the oxygen atom
coordinates the tin atom, slowing down. For the t-butyl
derivative, 11, and the i-propyl one, 8, decoalescence is not yet
(5) (a) Jurkschat, K.; Pieper, N.; Seemeyer, S.; Schu¨rmann, M.; Biese-
mans, M.; Verbruggen, I.; Willem, R. Organometallics 2001, 20, 868. (b)
van Koten, G.; Noltes, J. G. J. Am. Chem. Soc. 1976, 98, 5393. (c) Jambor,
R.; C´ısarˇova´, I.; Ru˚zˇicˇka, A.; Holecˇek, J. Acta Crystallogr. 2001, C57, 373.
(d) Nath, M.; Yadav, R.; Eng, G.; Musingarimi, P. Appl. Organomet. Chem.
1999, 13, 29 and references therein. (e) Crowe, A. J. In Metal Complexes
in Cancer Chemotherapy; Keppler, B. K., Ed.; VCH: Weinheim, Germany,
1993; pp 369-379. (f) Gielen, M.; Lelieveld, P.; de Vos, D.; Willem, R.
In Metal-Based Antitumor Drugs; Gielen, M., Ed.; Freund Publishing
House: Tel Aviv, Israel, 1992; Vol. 2, pp 29-54.
(6) (a) Willem, R.; Bouhdid, A.; Meddour, A.; Camacho-Camacho, C.;
Mercier, F.; Biesemans, M.; Ribot, F.; Sanchez, C.; Tiekink, E. R. T.
Organometallics 1997, 16, 4377. (b) Willem, R.; Biesemans, M.; Jaumier,
P.; Jousseaume, B. J. Organomet. Chem. 1999, 572, 233. (c) Martins, J.
C.; Biesemans, M.; Willem, R. Prog. Nucl. Magn. Reson. Spectrosc. 2000,
36, 271.
(7) (a) Biesemans, M.; Martins, J. C.; Willem, R.; Lycˇka, A.; Ru˚zˇicˇka,
A.; Holecˇek, J. Magn. Reson. Chem. 2002, 40, 65. (b) Biesemans, M.;
Martins, J. C.; Jurkschat, K.; Pieper, N.; Seemeyer, S.; Willem, R. Magn.
Reson. Chem. 2004, 42, 776.
(8) (a) Holecˇek, J.; Na´dvorn´ık, M.; Handl´ırˇ, K.; Lycˇka, A. J. Organomet.
Chem. 1983, 241, 177. (b) Holecˇek, J.; Na´dvorn´ık, M.; Handl´ıˇr, K.; Lycˇka,
A. Collect. Czech. Chem. Commun. 1990, 55, 1193.
n
complete at -80 °C.3 At this temperature, the J(1H-119Sn)
coupling constants for the two benzylic protons of 2 (9.8 and
10.5 Hz) and 5 (12.6 and 13.1 Hz) are both higher than at room
temperature, being higher for the latter ethyl than the former
methyl derivative, in contrast to room temperature. These data
match the expected strengthened coordination at lower temper-
ature, the slightly higher value of the ethoxy group being
compatible with its higher inductive electron-releasing effect
than the methoxy group, as supported by the more negative
charge of oxygen in the ethoxy versus the methoxy compound
(see Table S7, Supporting Information). Thus, the coordination
being stronger but the interaction ring configuration being also
likely more rigid at lower temperature, the relative weight of
both possibly antagonistic effects is difficult to assess.
(9) (a) Holecˇek, J.; Na´dvorn´ık, M.; Handl´ıˇr, K.; Lycˇka, A. Z. Chem. 1990,
30, 265. (b) Lycˇka, A.; Holecˇek, J.; Schneider, B.; Straka, J. J. Organomet.
Chem. 1990, 389, 29. (c) Pejchal, V.; Holecˇek, J.; Lycˇka, A. Sci. UniV.
Pap. Pardubice 1996, A2, 35; Chem. Abstr. 1997, 126, 317445.

6314 Organometallics, Vol. 26, No. 25, 2007
Dosta´l et al.
1
Table 1. Selected 119Sn and H NMR Data for Compounds 1-12 in CDCl3 Solution at Room Temperature (Chemical Shift in
ppm, Coupling Constants in Hz)a
SnPh3
SnPh2Cl
SnPhCl2
2,6 substituent
δ
119Sn [1J(13C-119Sn)] δ1H [4J(1H-119Sn)]
δ
119Sn [1J(13C-119Sn)] δ1H [4J(1H-119Sn)]
δ
119Sn [1J(13C-119Sn)] δ1H [4J(1H-119Sn)]
CH2OMe (1-3)
CH2OEt (4-6)
CH2OiPr (7-9)
CH2OtBu (10-12)
no subst.
-164 [571/543]
-160 [560/540]
-155 [542/527]
-156 [536/521]
-128 [531]
4.18 [5.6]
4.22 [5.9]
4.30 [7.1]
4.24 [8.0]
-145 [730/734]
-141 [712/737]
-136 [684/738]
-122 [714/721]
-45 [614]
4.51 [7.6]
4.57 [7.1]
4.67 [7.2]
4.60 [7.2]
-210 [997/1027]
-198 [975/1010]
-178 [931/996]
-149 [872/946]
-32 [786]
4.68 [10.9]
4.71 [10.5]
4.73 [9.9]
4.71 [9.1]
a The first 1J(13C-119Sn) coupling constant refers to the tin-bound ipso carbon atom of the O,C,O-pincer ligand, while the second refers to the tin-bound
carbon of the phenyl groups.
1
Table 2. Selected 119Sn and H NMR Data for the Chlorodiphenyltin and Dichlorophenyltin Compounds in CD2Cl2 Solution at
193 K (Chemical Shift in ppm, Coupling Constants in Hz)
SnPh2Cl
δ1Ha [nJ(1H-119Sn)]
SnPhCl2
δ1Hb [nJ(1H-119Sn)]
2,6 substituent
δ
119Sn
δ
119Sn
CH2OMe (2,3)
CH2OEt (5,6)
CH2OiPr (8,9)
CH2OtBu (11,12)
-155
-154
-163
-144
4.57 [9.8], 4.23 [10.5]
4.60 [12.6], 4.27 [13.1]
4.72, 4.41 (broadened)
4.55 (broadened)
-228
-213
-198
-168
4.86 [12.4], 4.30 [8.7]
4.85 [10.7], 4.48 [6.0]
4.85, 4.44 (broadened)
4.89, 4.34 (broad)
a Singlet. b AX pattern: 2J(H-H) ) 11-12 Hz.
The 119Sn NMR chemical shifts of the dichlorophenyltin
compounds 3, 6, 9, and 12 shift to lower frequencies over 120-
180 ppm, when compared to the reference Ph2SnCl2 compound,
being intermediate shifts for expansion by one and two
coordination numbers. The 1J(13C-119/117Sn) couplings increase
by 100-220 Hz relative to the reference that confirms this. The
Ph4Sn11 analogue, the largest deviation from ideal tetrahedral
shape being found for C(11)-Sn-C(41) 118.63° (116.98° for
1 and 111.2° in Ph4Sn).10 The Sn-C bond distances are almost
identical to the corresponding bond distances in 1. The strength
of the Sn-O interaction is most dramatically different between
1 and 10. The values of both Sn-O distances (4.538 and 4.799
Å, respectively) indicate a total absence of interaction for 10
(the sum of the van der Waals radii of oxygen and tin is 3.70
Å), which is in striking contrast with 1 where the Sn-O
distances (2.908 and 2.966 Å, respectively)3 do suggest a weak
but effective interaction. This result shows again that the strength
of the Sn-O bond is strongly influenced by the steric hindrance
developed by the substituents on the oxygen atoms of the O,C,O-
chelating ligand.3a-c,h
The triorganotin compound 5 is a good example of a [3+2]-
coordination with a C3ClOSn-distorted trigonal bipyramid
(Figure 1). The central tin atom together with the carbon atoms
form an equatorial plane (the sum of the angles for the C-Sn-C
girdle is 351.86°), while the chlorine and oxygen atoms are in
axial positions (the value of the bonding angle O(1)-Sn-Cl-
(1) is 179.03(3)°). The value of the Sn-O(1) bond length
(2.454(1) Å) is smaller than the sum of the van der Waals radii
of oxygen and tin (3.70 Å), but larger than the sum of the
covalent radii (2.066 Å), thus indicating the presence of a
medium-strong Sn-O interaction in this compound. The second
oxygen atom is out of the tin coordination sphere or nearly so
(the value of Sn-O(2) bond length is 3.473(1) Å). The
mentioned parameters together with the O(1)-Sn-O(2) angle
of 113.63(4)° demonstrate a different shape of the tin coordina-
tion polyhedron in 5 as compared to 2.
1
difference between the J(13C-119/117Sn) couplings of the ipso
carbon atom of the O,C,O-pincer ligand and the unsubstituted
phenyl group on the tin atom is highest in this series, the
strongest s character being concentrated in the tin hybrid atomic
orbitals involving tin-carbon bonds. The low-temperature
spectra of 3, 6, 9, and 12 display a decoalescence of the benzylic
proton singlet into two doublets of an AX pattern, only
compatible with a C2 structure in which both oxygen atoms are
coordinated to the tin atom, the two benzylic protons of each
of the two homotopic alkoxy substituents being mutually
diastereotopic. With this C2 6-coordinate complex in equilibrium
with the open 5-coordinate structure, internal rotation about the
C-C bond and the association-dissociation exchange of the
two Sn-O bonds become fast on the NMR time scale at room
temperature and the chirality of the molecule expressed at low
temperature averages out and results in symmetry equivalent
protons. This structure is confirmed by X-ray diffraction data
and theoretical calculations (see below). The relative differences
in chemical shift and coupling constants reflect the mean Sn-O
distance, increasing from compound 3 to 12. The nJ(1H-119Sn)
1
coupling constants follow perfectly the J(13C-119/117Sn) cou-
pling sequence. At low temperature, the cyclic structure causes
the diastereotopic benzylic protons to have different 3J couplings
because they have different dihedral angles, which makes
irrelevant comparison with room-temperature values.
The diorganotin compound 6 is a characteristic example of a
[5+1]-coordination where the distorted C2Cl2OSn-trigonal bi-
pyramid is attacked by an oxygen donor atom (Figure 2). The
equatorial plane is formed by one chlorine (Cl(2)) and two
carbon atoms (the sum of bonding angles for the C2ClSn girdle
is 348.1). The second chlorine atom and the oxygen atom occupy
the axial positions, in agreement with the value of the bonding
angle O(1)-Sn-Cl(1) of 167.65(3)°. The values of both Sn-O
bond lengths (Sn-O(1) 2.447(1) Å and Sn-O(2) 2.864(1) Å)
indicate the presence of both medium-strong (Sn-O(1)) and
weak interactions (Sn-O(2)) in 6. However, this weak Sn-O
Solid-State Structures. The X-ray structures of compounds
5 and 6 were determined. Their crystallographic data are
summarized in Table S10 and selected bond distances and angles
in Table S11, where these parameters are gathered with
computed values (see below) and experimental data published
earlier, for the sake of comparison.3 The geometry of the
tetraorganotin compound 10 (Scheme 1)10 is nearly perfectly
tetrahedral, with an average C-Sn-C angle of 109.42° (to be
compared with 109.22° for 1) and 109.46° for the unsubstituted
(10) Dosta´l, L.; Jambor, R.; Ru˚zˇicˇka, A.; Jira´sko, R.; C´ısaˇrova´, I.;
Holecˇek, J. J. Organomet. Chem. 2006, 691, 35.
(11) Belsky, V. K.; Simonenko, A. A.; Reikhsfeld, V. O.; Saratov, I. E.
J. Organomet. Chem. 1983, 244, 125.

Organotin(IV) DeriVatiVes of O,C,O-Chelating Ligands
Organometallics, Vol. 26, No. 25, 2007 6315
where no crystal structures were available, starting structures
were generated in silico. These structures are given in the
Supporting Information (S1). It has been shown that this level
of theory is capable of qualitatively describing intramolecular
interactions in organotin compounds, including compounds
containing pincer ligands.14
Table S11 lists geometrical parameters for all of the com-
pounds considered in this work. For the L-SnPh3 compounds
1 (Figure 3), 4, 7, and 10, it can be deduced from the different
C-Sn-C bond angles that the geometry of these compounds
can be described as a distorted tetrahedron, the average of these
angles amounting to 113.1° (R ) Me), 113.0° (R ) Et), 113.0°
(R ) i-Pr), and 113.4° (R ) t-Bu). The difference between the
two Sn-O bond distances largely increases when going from
R ) Me to R ) i-Pr, but decreases again for t-Bu, the shortest
Sn-O distance decreasing from 2.911 Å for R ) Me to 2.664
Å for R ) i-Pr, indicating an apparent increase of the Lewis
acidity of the Sn atom throughout this series. However, while
the two Sn-O distances are very different for 4 (R ) Et) and
7 (R ) i-Pr), they are rather similar for 1 (R ) Me), in good
agreement with experimental X-ray diffraction data for 1 and
10, not available for 4 and 7. In all three cases, the local
geometry around the Sn atom could also be described as a
distorted trigonal bipyramid, even though the different relevant
angles deviate substantially from the ideal trigonal bipyramid
angles. For the t-Bu compound 10, however, the two Sn-O
bond distances are substantially longer than 3 Å, but shorter
than the sum of the van der Waals radii, 3.70 Å, indicating a
weak contact in strong contrast with the experimental values.
The excellent agreement between the experimental and calcu-
lated values is especially noticeable for 1, while discrepancies
observed for 10 appear mostly in all distances and bond angles
involving either oxygen atom O(1) or O(2). While this is most
likely related, at least in part, to specific packing effects and
geometry distortions arising from the bulky t-Bu group in the
crystalline state, it can likewise be suggested that in silico, where
in principle gas-phase structures are reflected and crystal packing
effects should be absent, very weak potential wells related to
attractive OfSn interactions still can exist and contribute to
the overall stability of compound 10. It thus appears that these
kinds of calculations have a good predictive value for the
geometrical parameters in the crystalline state in structural
situations where the substituents on the coordinating oxygen
atoms of the pincer ligand are not too sterically demanding. It
should be stated that the absence of any supramolecular structure
resulting from intermolecular aggregation largely contributes
to this conclusion. Globally, among the four tetraaryl compounds
1, 4, 7, and 10, the one with R ) Me, 1, can be considered as
having a 4-coordinate tin atom, with a weak symmetrical
extension to 6-coordination, so that 1 can be considered as a
compound with symmetrical “4 + 2”-coordination. The ones
with R ) Et and i-Pr, 4 and 7, respectively, have one stronger
and one weaker coordination expansion, when compared to 1,
which make them to be considered as compounds with non-
symmetrical “4 + 2”-coordination, or alternatively “4 + 1”-
coordination if the weaker interaction corresponding to a contact
distance between Sn and O of 3.3 Å or even more is neglected.
Compound 10 would then have, in these pictures, either a weak
symmetric “4 + 2”-coordination, both Sn-O contacts around
3.3 Å being in silico of the same order in 10 as in 4 and 7, or
basically a standard 4-coordination.
Figure 1. General view (ORTEP) of a molecule showing 50%
probability displacement ellipsoids and the atom numbering scheme
for 5. The hydrogen atoms are omitted for clarity.
Figure 2. General view (ORTEP) of a molecule showing 50%
probability displacement ellipsoids and the atom numbering scheme
for 6. Only one molecule from the crystal lattice of 2 without
hydrogen atoms has been chosen for clarity.
interaction causes the deformation of the equatorial plane, where
the bonding angles (107.6°, 108.9°, and 131.6°) are significantly
different from those in an ideal trigonal bipyramid (120°). The
value of the bonding angle of O(1)-Sn-O(2) ) 118.51(4)°
shows that both donor oxygen atoms are mutually in cis position,
this value being comparable to those found in the other
diorganotin compounds 3, 9, and 12.
Theoretical Calculations. All geometries were optimized at
the B3LYP12/LANL2DZ13 level starting from the experimental
crystal structural parameters of the compounds. In those cases
(12) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) Stevens, P. J.; Delvin, F.
J.; Chablaoski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623.
(13) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 270.
(14) (a) Buntine, M. A.; Hall, V. J.; Kosovel, F. J.; Tiekink, E. R. T. J.
Phys. Chem. A 1998, 102, 2472. (b) Ryner, M.; Finne, A.; Albertsson, A.-
C.; Kricheldorf, H. R. Macromolecules 2001, 34, 7281. (c) Obora, Y.;
Nakanishi, M.; Tokunaga, M.; Tsuji, Y. J. Org. Chem. 2002, 67, 5835. (d)
Hu, Y.-H.; Su, M.-D. J. Phys. Chem. A 2003, 107, 4130. (e) Peveling, K.;
Henn, M.; Lo¨w, C.; Mehring, M.; Schu¨rmann, M.; Costisella, B.; Jurkschat,
K. Organometallics 2004, 23, 1501. (f) Fischer, J.; Schu¨rmann, M.; Mehring,
M.; Zachwieja, U.; Jurkschat, K. Organometallics 2006, 25, 2886.
Also, in the case of the L-SnPh2Cl compounds 2, 5, 8, and
11, there is a considerable difference between the two Sn-O
bond distances. From the data, it can be seen that these structures

6316 Organometallics, Vol. 26, No. 25, 2007
Dosta´l et al.
Figure 3. Computed molecular models of the methyl derivative of the L-SnPh3 (1), L-SnPh2Cl (2), L-SnPhCl2 (3), and L-SnCl3
compounds studied in this work.
can generally be described as a distorted trigonal bipyramid,
the chlorine and the alkoxy substituents taking the axial
positions. The ipso-carbon-Sn-Cl angle averages to about 106°
(103.1° in the case of the t-Bu compounds), thus showing a
deviation of around 16° from the angle in a perfect trigonal
bipyramid toward the tetrahedral geometry. The angles between
the axial compounds range from 160.4° (for R ) i-Pr) to 172.1°
(for R ) Et). Finally, the interaction of the second alkoxy group
with the Sn atom is generally smaller but not negligible in the
case of R ) Me, which is in line with the considerable deviations
of the bond angles around the tin atom with respect to the ideal
trigonal bipyramid. Again, the agreement between experimental
and calculated data is excellent for 2, with R ) Me, but less
for 5. Table S11 shows globally that the calculated bond lengths
and angles for 8 and 11 have a satisfactory predictive value,
because they are in global good agreement with the correspond-
ing geometrical parameters of compounds 2 and 5 for which
the agreement between experimental X-ray diffraction data and
computed values is likewise satisfactory. While the non-
symmetry in the Sn-O contacts is of variable degree, with a
distance difference of at least ca. 0.35 Å (R ) Me, 2 and R )
i-Pr, 8), and even greater for R ) Et, 5 (ca. 0.6-0.7 Å), or
dramatically more for R ) t-Bu, 11, this non-symmetry reflects
the higher degree of stability of the “3 + 2”-coordination
characteristic for triorganotin monohalides undergoing a rather
strong coordination expansion by any oxophilic ligand, where
the three organic groups occupy the equatorial and the nucleo-
philic or electronegative substituents occupy the axial, mutually
trans positions of the usual distorted trigonal bipyramidal
configuration of the metal atom. This explains why the
symmetrical “4 + 2”-coordination is not favored, despite the
second intramolecular oxygen containing ligand. Note neverthe-
less that the contact between the second oxygen containing
ligand and the tin, of the order of 2.9-3.1 Å, remains by far
lower than the sum of the van der Waals radii of tin and oxygen,
so that a weaker, non-symmetrical “4 + 2”-coordination can
also be considered for 2, 5, and 8.

Organotin(IV) DeriVatiVes of O,C,O-Chelating Ligands
Organometallics, Vol. 26, No. 25, 2007 6317
Next, for the geometrical parameters of the L-SnPhCl2
compounds 3, 6, 9, and 12, in the case of R ) Et, 6, and R )
i-Pr, 9, the asymmetry in the Sn-O bond distances is large,
whereas for the Me and t-Bu compounds, these are equal. This
duality is remarkably self-consistent in the X-ray data and the
theoretical calculations, confirming the high predictive value
of the latter in this series of compounds. This can be ascribed
to the Lewis acidity factor being so dominant that computational
deviations related to steric hindrance are faded away. All of
the structures can be considered as heavily distorted octahedra.
In all four compounds 3, 6, 9, and 12 of Table S11, the two
ipso-carbon-Sn-Cl angles deviate from 90° toward the tetra-
hedral angle. Because, however, in all cases the Sn-O contacts
appear shorter than 3 Å, compounds 3, 6, 9, and even 12, with
R ) t-Bu, can be considered to be of a stable and strong “4 +
2” type.
coupling pathways displaying opposite signs, and involving an
actual 4J “organic” H-C-C-C-Sn coupling pathway through
3
only C-Sn and C-C bonds, and another J “coordinative”
H-C-O-Sn pathway, the latter being itself the average of two
mostly non-symmetry equivalent ones and involving coordina-
tive Sn-O bonds of increasing averaged strength as the number
of chlorine atoms increases.
Details of an investigation using density functional theory
based descriptors,15 aiming at gaining further insight into such
trends in the oxygen-tin bond distances, are presented in the
Supporting Information (S2), together with tin atomic charges
in the various compounds.
From this analysis, it could be concluded that the combination
of moieties with the highest Lewis acidity for the tin atom and
basicity for the oxygen atom yields the compound with the
shortest O-Sn distance (Table S11), that is, the trichlorotin
compound with R ) i-Pr.
Finally, for the L-SnCl3 compounds, for which, apart from
R ) Me, no X-ray diffraction data are available, the two Sn-O
distances are virtually equal for R ) Me, whereas the asymmetry
in these distances increases upon increasing alkyl group size.
These compounds can again be considered to be heavily
distorted octahedra with a typical distorted “3 + 3” 6-coordina-
tion of the type R3SnCl3 with a fac configuration for the three
chlorine atoms, as evidenced by the fact that none of the Cl-
Sn-Cl bonds deviates by more than 15° from the ideal 90°
one, one of the Cl-Sn-Cl angles for a mer configuration being
180°. Again, the comparison between the calculated and the
experimental X-ray data for R ) Me shows the good predictive
value of the calculations. Because globally the corresponding
values for R ) Et and R ) i-Pr are rather similar, it can be
concluded that the corresponding in silico structures should
properly predict the actual structure of these compounds. For
R ) t-Bu, no optimum could be found in the calculation, a
dissociation being obtained during the optimization, explaining
why no computational data are provided for this case.
Conclusion
This work proposes an integrated approach for structural
investigations of organotin molecules of the type LSnPh(3-n)Cln,
where L represents an aromatic O,C,O-pincer ligand with two
oxygen donors potentially able to interact with the tin atom
toward coordination expansion of the tin atom. The combined
use of X-ray diffraction in the crystalline state, NMR measure-
ments of heteronuclear long-range 1H-119Sn coupling constants
involving dual coupling pathways in solution, and structural
parameter determinations of isolated molecules in silico provides
complementary information on these compounds in three
aggregation states. For a constant number of chlorine atoms on
the tin atom, the strength of the interaction of the Sn-O contacts
appears to be extremely sensitive to the substituent on the
oxygen atoms, which influences in particular the symmetry
equivalence or not of these two Sn-O contacts. For the
tetraaryltin compounds, the coordination at tin turns out to vary
mainly from the weak “4 + 2”- to pure 4-coordination state
when going from Me to t-Bu substitution on the potentially
coordination oxygen atoms. For the monochlorotriorganotins,
the general trend is mainly to a “3 + 2”-coordination of a
distorted trigonal bipyramid with triequatorial arrangement of
the organic ligands, with, however, strong differences between
the two O-Sn coordinative interactions, allowing an alternative
weak “4 + 2”-coordination for some of the compounds. The
dichlorodiorganotins have clear-cut, strong 6-coordination in
which the two organic aryl ligands have a configuration lying
slightly closer to the cis than the theoretically ideal trans one,
confirming the major coordination distortion induced by the
O,C,O-pincer ligand. The trichloromonoorganotins have likewise
clear-cut, strong 6-coordination with the three chlorine atoms
displaying the most pincer ligand strain releasing fac configu-
ration.
For the various compounds considered in this work, it can
be assumed that the strength of the oxygen-tin interactions
observed can be inversely correlated to the observed bond
lengths between these atom pairs. As can be seen from the data
in Table S11, the majority of Sn-O distances are substantially
shorter than the sum of the van der Waals radii of the Sn and
O atoms, but also significantly longer than the sum of their
covalent radii. These interactions between the nucleophilic
oxygen atom and the electrophilic tin atom thus vary from weak
to moderately strong. The O-Sn bond distances decrease upon
increasing number of chlorine atoms on the central tin atom.
Moreover, the two oxygen-tin bond distances tend to equalize
upon increasing the number of chlorine atoms, in agreement
with the experimentally measured distances, meaning that the
degree of 6-coordination increases upon increasing the number
of chlorine atoms. For the SnPhCl2 and the SnCl3 compounds,
6-coordination is clear-cut. In general, when going from the
methyl to the t-butyl-substituted compounds, the difference
between the oxygen-tin bond distances increases. When
focusing on the smaller of these two distances, the lower values
are observed systematically for the i-propyl compounds, the only
exception being the triorganotin chloride series. These general
trends match also pretty well the NMR data in solution, even
though it appears difficult to give an accurate interpretation of
For the assessment of the increasing strength of the donor-
acceptor O-Sn interactions, the combined use of X-ray dif-
fraction for the crystalline state, NMR long-range coupling
constants in solution, and ab initio calculations for simulating
the gas phase turned out to be powerful in providing concurrent
data evidencing that these interactions do not significantly
depend on the aggregation state, but, rather, are highly sensitive
to both the substitution pattern on tin and even more, in part
unexpectedly, to the substitution pattern of the coordinating
oxygen atoms.
4
the nominal J(119Sn-1H) long-range couplings, because the
scalar coupling values observed reflect necessarily the average
of the differences in Sn-O interactions observed in the
crystalline state and in silico, and, more importantly, because
these long-range coupling constants raise from two different
(15) Geerlings, P.; De Proft, F.; Langenaeker, W. Chem. ReV. 2003, 103,
1793.

6318 Organometallics, Vol. 26, No. 25, 2007
Dosta´l et al.
of PhSnCl3. Yield: 2.7 g (79%), mp 98-100 °C. Anal. Calcd for
C18H22Cl2O2Sn (MW 459.97): C, 47.00; H, 4.82; Cl, 15.42.
Found: C, 46.95; H, 5.21; Cl, 15.63. MW 460. MS: m/z 483, 3%,
Experimental Section
General Methods. All solvents were dried by standard proce-
dures and distilled prior to use. All reactions were carried out
under purified argon atmosphere using standard Schlenk tech-
niques.
1
[M + Na]+; m/z 425, 100% [M - Cl]. H NMR (CDCl3, 360.13
MHz): δ (ppm) 0.79 (t, 6H, CH3), 3.50 (q, 4H, OCH2), 4.73 (s,
4H, CH2O), 7.20-7.85 (complex pattern, 8H, SnPhCl2, SnC6H3).
13C NMR (CDCl3, 90.14 MHz): δ (ppm) 13.6 (CH3), 66.3 (OCH2),
70.4 (CH2), SnC6H3, 133.4 (C(1), 1J(119Sn, 13C ) 975.0 Hz),
1H and 119Sn NMR spectra were recorded for long-range coupling
constant measurements in CDCl3 and CD2Cl2 at 303 and 193 K on
a Bruker AMX500 instrument, operating at frequencies of 500.13
1
126.2, 130.0, 144.9; SnPh, 142.2 (C′(1), J(119Sn, 13C ) 1009.9
1
and 186.47 MHz for H and 119Sn nuclei, respectively. Routine
Hz), 128.6, 130.4, 134.7. 119Sn NMR (CDCl3, 134.27 MHz): δ
(ppm) -197.
characterizations were performed on a Bruker AMX 360 instrument,
operating at frequencies of 360.13, 90.14, and 134.27 MHz for 1H,
Crystallography. Crystals suitable for X-ray structure determi-
nation were obtained by vapor diffusion of n-hexane into the 5%
dichloromethane solutions of compounds 5, 6. Data for colorless
crystals were collected at 150(2) K on a Nonius KappaCCD
diffractometer using Mo KR radiation (λ ) 0.71073 Å), with a
graphite monochromator. The structures were solved by direct
methods (SIR92) and refined on F2 by full-matrix least-squares
technique (SHELXL97). Hydrogen atoms were recalculated into
idealized positions (riding model) and assigned temperature factors
Hiso(H) ) 1.2Ueq (pivot atom) or 1.5Ueq for the methyl moiety.
Absorption corrections were carried out using multiscan procedure
(PLATON, SORTAV). From the last cycle of refinement of both
structures it follows that (∆/δ)max < 0.002. Crystallographic data
for individual structures are summarized in Table S10. The
experimental standard deviations for all compounds are within
intervals: bond type Sn-C 0.0015,0.0030 ; Sn-Cl 0.0004,0.0009 ;
Sn-O 0.0012,0.0020) Å; bond angle around Sn 0.01,0.11 °;
torsion angles C-C-C-O 0.1-0.3 °.
Crystallographic data for structural analysis have been deposited
with the Cambridge Crystallographic Data Centre, CCDC no.
604290 for 5 and 604291 for 6. Copies of this information may be
obtained free of charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EY, UK (fax, +44-1223-336033; e-mail,
deposit@ccdc.cam.ac.uk; or website http://www.ccdc.cam.ac.uk).
Computations. All geometries were optimized at the B3LYP12/
LANL2DZ13 level starting from the experimental crystal structures
of the compounds using the Gaussian 03 program.17
1
13C, and 119Sn nuclei, respectively. H chemical shifts were refer-
enced to the residual solvent peak and converted to the standard
TMS scale by adding 5.32 and 7.24 ppm, for CD2Cl2 and CDCl3,
respectively. For 119Sn nuclei, external referencing was used with
¥ ) 37.290665 MHz.16 1D ge-1H-119Sn HMQC experiments6 and
2D ge-1H-119Sn J-HMQC spectra were recorded as previously
described.7
In the mass spectrometry, the positive-ion electrospray ionization
ESI mass spectra were measured on an Esquire3000 ion trap
analyzer (Bruker Daltonics, Bremen, Germany) in the range m/z
100-1000, and negative-ion ESI mass spectra were measured on
the Platform quadrupole analyzer (Micromass, UK) in the range
m/z 15-600. The ion trap was tuned to give an optimum response
for m/z 500. The samples were dissolved in acetonitrile and analyzed
by direct infusion at a flow rate of 1-3 µL/min.
Synthetic Procedures. 1,3-Bis(ethoxymethyl)benzenes, 1-3,
7-12, were prepared according to literature procedures.3a
[2,6-Bis(ethoxymethyl)phenyl](triphenyl)tin (4). An equimolar
amount of 4 mL of n-BuLi (1.60 M, 6.4 mmol) was added dropwise
at ambient temperature to a solution of 1,3-bis(ethoxymethyl)-
benzene (1.23 g, 6.34 mmol) in n-hexane (25 mL); the resulting
yellow solution was stirred for an additional 2 h. The resulting
solution was added dropwise to a suspension of Ph3SnCl (2.44 g,
6.34 mmol) in 30 mL of n-hexane at room temperature, followed
by stirring for another 12 h. The resulting solid was filtered and
washed with 15 mL of n-hexane; the filtrate was concentrated to
15 mL. Crystallization at -10 °C and filtration afforded 4 as a
white solid. Yield: 2.4 g (70%), mp 105-108 °C. Anal. Calcd for
C30H32O2Sn (MW 543.28): C, 66.23; H, 5.94. Found: C, 66.52;
H, 5.81. MW 544. MS: m/z 567, 100%, [M + Na]+; 583, 56%,
[M + K]+. 1H NMR (CDCl3, 360.13 MHz): δ (ppm) 0.75 (t, 6H,
CH3), 2.87 (q, 4H, OCH2), 4.23 (s, 4H, CH2O), 7.31-7.69 (complex
pattern, 18H, SnPh3, SnC6H3). 13C NMR (CDCl3, 90.14 MHz): δ
(ppm) 14.4 (CH3), 65.2 (OCH2), 74.1 (CH2), SnC6H3, 136.8 (C(1),
1J(119Sn,13C ) 560.0 Hz), 128.0, 128.9, 147.1; SnPh, 142.1 (C′(1),
1J(119Sn,13C ) 539.8 Hz), 128.0, 128.2, 136.8. 119Sn NMR (CDCl3,
134.27 MHz): δ (ppm) -159.
Acknowledgment. The Pardubice group would like to thank
the Grant Agency of the Czech Republic (grant no. 203/07/
0468) and the Ministry of Education of the Czech Republic (VZ
0021627501) for financial support. The Brussels group wishes
to acknowledge the Fund for Scientific Research Flanders
(Belgium) (FWO (grants G.0016.02, G.0469.06 (R.W., M.B.)),
the Research Council (Onderzoeksraad) of the Vrije Universiteit
Brussel (Concerted Research Action, grant GOA31), and the
Free University of Brussels (VUB) for continuous support to
their research group.
[2,6-Bis(ethoxymethyl)phenyl](chlorodiphenyl)tin (5). Proce-
dure analogous to 4: 1.15 g, 5.92 mmol of 1,3-bis(ethoxymethyl)-
benzene, 3.7 mL, 5.92 mmol of n-BuLi 1.60 M, 2.04 g, 5.92 mmol
of Ph2SnCl2. Yield: 2.0 g (68%), mp 105-107 °C. Anal. Calcd
for C24H27ClO2Sn (MW 501.62): C, 57.47; H, 5.43; Cl, 7.07.
Found: C, 57.56; H, 5.92; Cl, 7.25. MW 502. MS: m/z 467, 100%,
[M - Cl]+. 1H NMR (CDCl3, 360.13 MHz): δ (ppm) 0.72 (t, 6H,
CH3), 3.21 (q, 4H, OCH2), 4.59 (s, 4H, CH2O), 7.24-7.74 (complex
pattern, 13H, SnPh2Cl, SnC6H3). 13C NMR (CDCl3, 90.14 MHz):
δ (ppm) 13.9 (CH3), 66.0 (OCH2), 72.3 (CH2), SnC6H3, 135.5 (C(1),
1J(119Sn, 13C ) 712.1 Hz), 127.1, 129.0, 146.4; SnPh, 142.7 (C′-
Supporting Information Available: The in silico generated
starting geometries for the geometry optimizations, further insights
(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A.
G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;
Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian
03, revision B.03; Gaussian, Inc.: Wallingford, CT, 2004.
(1), J(119Sn, 13C ) 736.8 Hz), 128.3, 129.0, 135.5. 119Sn NMR
(CDCl3, 134.27 MHz): δ (ppm) -140.
[2,6-Bis(ethoxymethyl)phenyl](dichlorophenyl)tin (6). Proce-
dure analogous to 4: 1.45 g, 7.47 mmol of 1,3-bis(ethoxymethyl)-
benzene, 4.7 mL, 7.52 mmol of n-BuLi 1.60 M, 2.22 g, 7.47 mmol
1
(16) Multinuclear NMR; Mason, J., Ed.; Plenum Press: New York and
London, 1987; Appendix p 623.

Organotin(IV) DeriVatiVes of O,C,O-Chelating Ligands
Organometallics, Vol. 26, No. 25, 2007 6319
into the observed trends in Sn-O distances, and crystallographic
data for 5 and 6 (CIF), as well as the experimental and computed
values of selected geometrical parameters of L-SnPh3, L-SnPh2-
Cl, L-SnPhCl2, and L-SnCl3. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM700576N