Syntheses and structural characterization of a monomeric tin(II) diamide and a novel chlorotin(II) amide trimer

Article abstract of DOI:10.1021/ic049154w

We have found that addition of a 2:1 ratio of the bulky ligand Li[N(mesityl)(SiMe₃)] to SnCl₂ yields Sn[N(mesityl) (SiMe₃)]₂, which is found by X-ray crystallography to be monomeric in the solid state. Interestingly, the solid state structure of the stannylene exhibits a minipocket caused by the parallel arrangement of the phenyl rings. A 1:1 ratio of ligand to SnCl₂ affords the new chlorotin amide {Sn(μ-Cl)[N(mesityl)(SiMe₃)]}₃, which adopts a unique trimeric structure in the solid state. The chairlike (Sn-Cl)₃ backbone shown here has not been seen previously in Sn-halide chemistry.

Full text of DOI:10.1021/ic049154w

Inorg. Chem. 2004, 43, 7239−7242
Syntheses and Structural Characterization of a Monomeric Tin(II)
Diamide and a Novel Chlorotin(II) Amide Trimer
Yongjun Tang,† Ana M. Felix,† Lev N. Zakharov,‡ Arnold L. Rheingold,‡ and Richard A. Kemp*,†,§
Department of Chemistry, UniVersity of New Mexico, Albuquerque, New Mexico 87131,
Department of Chemistry and Biochemistry, UniVersity of California, San Diego,
La Jolla, California 92093, and AdVanced Materials Laboratory, Sandia National Laboratories,
Albuquerque, New Mexico 87106
Received June 29, 2004
We have found that addition of a 2:1 ratio of the bulky ligand Li[N(mesityl)(SiMe3)] to SnCl2 yields Sn[N(mesityl)-
(SiMe3)]2, which is found by X-ray crystallography to be monomeric in the solid state. Interestingly, the solid state
structure of the stannylene exhibits a “minipocket” caused by the parallel arrangement of the phenyl rings. A 1:1
ratio of ligand to SnCl2 affords the new chlorotin amide
{Sn(µ-Cl)[N(mesityl)(SiMe3)]}3, which adopts a unique
trimeric structure in the solid state. The chairlike (Sn−Cl)3 backbone shown here has not been seen previously in
Sn halide chemistry.
−
Introduction
prepare this compound, we also discovered the first example
of a trimeric µ-chlorotin(II) amide compound, {Sn(µ-Cl)-
[N(mesityl)(SiMe3)]}3, 2, which differed structurally in the
solid state from a binuclear µ-chlorotin(II) amide reported
earlier by Lappert.7
The coordination chemistry of low-valent main group
metal amide compounds has been of great interest in recent
years, with potential uses as varied as precursors for chemical
vapor deposition to the chemical fixation of carbon dioxide.1-6
Of special interest to us are oxo-transfer reactions in which
CO2 undergoes metathesis reactions with other materials to
form new products. Recently, this includes the elegant work
of Sita on the generation of trimethylsilylisocyanates from
CO2 and Sn and Ge diamides.6 Via processes such as these,
one can hope to discover new chemical transformations to
yield valuable organics from relatively useless feedstocks.
Our laboratory has recently been interested in the synthesis
and characterization of novel main group amide compounds
and their insertion chemistries with carbon dioxide. We here
describe our initial efforts to prepare and characterize the
complex Sn[N(mesityl)(SiMe3)]2, 1. During our efforts to
Experimental Section
General Considerations. Due to the air-sensitive nature of
reactants and products, all manipulations were carried out in an
Ar-filled glovebox or by using standard Schlenk techniques.8
Anhydrous solvents were purchased from Aldrich and stored in
the glovebox over 4 Å molecular sieves. n-Butyllithium (2.5 M in
hexanes) (CAUTION: alkyllithium reagents are pyrophoric and are
very reactive toward moisture and as such should be handled in an
inert atmosphere), Celite filter agent, 2,4,6-trimethylaniline, trim-
ethylsilyl chloride, and anhydrous tin(II) chloride were purchased
from Aldrich and used without further purification. Benzene-d6 was
obtained from Aldrich and dried and stored over 4 Å molecular
sieves. 1H, 13C, and 119Sn spectra were obtained on a Bruker AMX
250 spectrometer using C6D6 as a solvent. UV-vis spectra were
recorded on a Cary 3E UV-vis model spectrophotometer. Elemen-
tal analyses for carbon, hydrogen, and nitrogen were measured in
sealed volatile sample pans using a Perkin-Elmer series II CHNS/O
2400 analyzer.
* To whom correspondence should be addressed. E-mail: rakemp@
unm.edu. Phone: (505) 277-0510. Fax: (505) 277-2609.
† University of New Mexico.
‡ University of California, San Diego.
§ Sandia National Laboratories.
(1) Caudle, M. T.; Kampf, J. W. Inorg. Chem. 1999, 38, 5474.
(2) McCowan, C. S.; Groy, T. L.; Caudle, M. T. Inorg. Chem. 2002, 41,
1120.
(3) Caudle, M. T.; Benedict, J. B.; Mobley, C. K.; Straessler, N. A.; Groy,
T. L. Inorg. Chem. 2002, 41, 3183.
(4) Sebestl, J. L.; Nadasdi, T. T.; Heeg, M. J.; Winter, C. H. Inorg. Chem.
1998, 37, 1289.
(5) Dell’Amico, D. B.; Calderazzo, F.; Labella, L.; Marchetti, F.;
Pampaloni, G. Chem. ReV. 2003, 103, 3857.
(6) Sita, L. R.; Babcock, J. R.; Xi, R. J. Am. Chem. Soc. 1996, 118, 10912.
Sn[N(mesityl)(SiMe3)]2 (1). (Me3Si)(Mesityl)NH9 (5.00 g, 24.1
mmol) was dissolved in 50 mL of diethyl ether and cooled to -20
(7) Chorley, R. W.; Hitchcock, P. B.; Jolly, B. S.; Lappert, M. F.; Lawless,
G. A. J. Chem. Soc., Chem. Commun. 1991, 1302.
(8) Shriver, D. F. The Manipulation of Air-SensitiVe Compounds; McGraw-
Hill: New York, 1986.
(9) Murugavel, R.; Chandrasekhar, V.; Voigt, A.; Roesky, H. W.; Schmidt,
H. G.; Noltemeyer, M. Organometallics 1995, 14, 5298.
10.1021/ic049154w CCC: $27.50
Published on Web 10/05/2004
© 2004 American Chemical Society
Inorganic Chemistry, Vol. 43, No. 22, 2004 7239

Tang et al.
Scheme 1. Synthetic Routes to 1 and 2
Table 1. Crystallographic Data and Parameters for Complexes 1 and 2
°C. n-Butyllithium (9.64 mL, 24.1 mmol, 2.5 M in hexane) was
slowly added to the diethyl ether solution. The solution was allowed
to warm to room temperature and stir for 5 h. This solution was
then added dropwise to a stirred suspension of 2.29 g (12.05 mmol)
of anhydrous SnCl2 in 20 mL of diethyl ether. After stirring for 12
h, the volatiles were then removed in vacuo, and the crude product
was taken up in hexane and filtered through Celite held on a glass
frit. After concentrating the filtrate, 4.99 g of pure 1 was obtained
in 78% yield as orange-red crystals after storage at -20 °C for
several days. 1H NMR (250 MHz, d6-benzene): δ 0.53(s), 2.37(s),
2.53(s), 6.82(s). 13C NMR (250 MHz, d6-benzene): δ 4.95, 20.86,
21.56, 128.64, 130.53, 131.88, 147.13. 119Sn (93.275 MHz, Me4-
Sn): δ 473. Anal. Calcd for C24H40N2Si2Sn (531.45): C, 54.24;
H, 7.59; N, 5.27%. Found: C, 53.62; H, 7.45; N, 5.12%.
{Sn(µ-Cl)[N(mesityl)(SiMe3)]}3 (2). (Me3Si)(Mesityl)NH9 (5.0
g, 24.1 mmol) was dissolved in 50 mL of diethyl ether and cooled
to -20 °C. n-Butyllithium (9.64 mL, 24.1 mmol, 2.5 M in hexane)
was slowly added to the diethyl ether solution. The solution was
allowed to warm to RT and stir for 5 h. The resulting solution was
then added to a stirred suspension of 4.57 g (24.1 mmol) of
anhydrous SnCl2 in 75 mL of diethyl ether cooled to 0 °C. After
gradual warming to room temperature and stirring for 12 h, the
volatiles were then removed in vacuo, and the crude product was
taken up in a toluene/hexane mixture (1:5) and filtered through
Celite held on a glass frit. After concentrating the filtrate in vacuo,
6.08 g of pure 2 was obtained in 70% yield as yellow crystals after
storage at -20 °C for 2 days. 1H NMR (250 MHz, d6-benzene): δ
0.26(s), 2.11(s), 2.30(s), 6.79(s). 13C NMR (250 MHz, d6-
benzene): δ 4.00, 20.75, 21.37, 129.55, 132.63, 136.61, 143.24.
119Sn (93.275 MHz, Me4Sn): δ 67. Anal. Calcd for C36H60Cl3N3-
Si3Sn3 (1081.56): C, 39.98; H, 5.59; N, 3.88%. Found: C, 39.59;
H, 5.57; N, 3.88%.
1
2
empirical formula
fw
C24H40N2Si2Sn
531.45
C36H60Cl3N3Si3Sn3
1081.56
T, K
213
100
cryst size (mm3)
cryst syst
space group
a, Å
0.40 × 0.35 × 0.20
0.30 × 0.27 × 0.23
monoclinic
P21/c
17.3324(11)
13.6799(9)
19.6416(12)
90
90.5220(10)
90
4656.9(5)
4
triclinic
P1h
11.4745(9)
14.5719(12)
17.1003(13)
100.2770(10)
95.159(2)
90.1720(10)
2801.5(14)
4
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
calcd density, g/cm3
µ(Mo KR), mm-1
indep reflns
R1, wR2 (all data)
R1, wR2 [I > 2σ(I)]
GOF on F2
1.260
1.009
1.543
1.873
12 594 [Rint ) 0.0260]
0.0580, 0.1250
0.0458, 0.1166
1.021
10 568 [Rint ) 0.0228]
0.0305, 0.0614
0.0252, 0.0592
1.047
Results and Discussion
We have recently been investigating the reactions of main
group amides or diamides with CO2. In particular, we are
interested in examining ligands attached to Sn that might
prove useful to generate carbamates or isocyanates after the
reaction with CO2. Sita has shown previously that silyl-
containing Sn(II) diamides react with CO2, albeit somewhat
slowly, to liberate isocyanates.6 We were interested in
determining whether different silyl-containing ligands at-
tached to Sn might accelerate this insertion reaction. During
the process of synthesizing a number of these modified
ligands, we prepared Sn(II) complexes using the -N(mesi-
tyl)(SiMe3) ligand. While the 2:1 ligand/Sn(II) ratio gave
us the expected stannylene, the 1:1 ligand/Sn(II) afforded a
new chlorotin amide species which adopts a unique trimeric
structure in the solid state. The chairlike (Sn-Cl)3 backbone
reported here has not been seen previously in Sn-halide
chemistry.
X-ray Crystallography. X-ray quality crystals of 1 and 2 were
grown from either hexane or a mixture of toluene and hexane. X-ray
diffraction intensities were collected using λ(Mo KR) ) 0.71073
Å radiation on Bruker P4/CCD (1) and a Smart Apex CCD (2)
diffractometers at 223 and 100 K, respectively. Crystallographic
data and details of the X-ray studies are given in Table 1. As all of
the compounds are extremely air-sensitive, the crystals were always
handled under inert atmosphere. They were covered with oil and
immediately transferred to the N2 stream of the diffractometer prior
to data collection. SADABS absorption corrections were applied
in both cases (Tmin/Tmax ) 0.729 (1), and 0.862 (2)). Non-hydrogen
atoms were refined with anisotropic displacement coefficients. The
H atoms were found from the difference F-maps and refined with
isotropic thermal parameters. All software and sources of scattering
factors are contained in the SHELXTL (5.10) program package
(G. Sheldrick, Bruker XRD, Madison, WI).10
The ligand precursor Li[N(mesityl)(SiMe3)]9 was generated
in situ by the reaction of (Me3Si)(mesityl)NH with n-BuLi,
followed by the treatment with 0.5 equiv of SnCl2 to yield
1
1 (Scheme 1). The H NMR spectrum of 1 in d6-benzene
consisted of four single resonances, indicating retention of
the basic ligand structure. The -SiMe3 resonance is seen at
δ 0.53 ppm, the two distinct methyl resonances from the
mesityl group are at δ 2.37 (2,6-positions) and 2.53 ppm
(4-position), and the two protons on the mesityl ring are
found at δ 6.82 ppm. The 13C NMR spectrum of 1 was
consistent with the 1H NMR data. The -SiMe3 resonance is
found at δ 4.95 ppm, the two different methyl peaks from
(10) (a) Sheldrick G. M. SADABS (2.01), Bruker/Siemens Area Detector
Absorption Correction Program; Bruker AXS: Madison, WI, 1998.
(b) Sheldrick, G. M. SHELXTL: Program package for Structure
Solution and Refinement; Bruker AXS: Madison, WI, 1997.
7240 Inorganic Chemistry, Vol. 43, No. 22, 2004

Monomeric Tin(II) Diamide and Chlorotin(II) Amide Trimer
Figure 2. Thermal ellipsoid plot of 2 shown at the 30% probability level.
Selected bond lengths (Å) and bond angles (deg): Sn(1)-N(1) 2.0890(19),
Sn(1)-Cl(1) 2.6844(6), Sn(1)-Cl(3) 2.6868(6), Sn(2)-N(2) 2.0703(18),
Sn(2)-Cl(2) 2.5789(6), Sn(2)-Cl(1) 2.6201(6), Sn(3)-N(3) 2.0864(19),
Si(1)-N(1) 1.744(2), Si(2)-N(2) 1.746(2), Si(3)-N(3) 1.744(2); N(1)-
Sn(1)-Cl(1) 92.56(5), N(1)-Sn(1)-Cl(3) 96.20(5), Cl(1)-Sn(1)-Cl(3)
84.99(2), N(2)-Sn(2)-Cl(1) 98.35(5), N(2)-Sn(2)-Cl(2) 92.83(5),
Cl(1)-Sn(2)-Cl(2) 91.33(2), N(3)-Sn(3)-Cl(2) 93.27(5), N(3)-Sn(3)-
Cl(3) 96.43(5), Cl(2)-Sn(3)-Cl(3) 83.32(2), Sn(1)-Cl(1)-Sn(2) 114.16(2),
Sn(2)-Cl(2)-Sn(3) 116.96(2), Sn(1)-Cl(3)-Sn(3) 113.99(2).
Figure 1. Thermal ellipsoid plot for one of the two independent molecules
of 1 shown at the 50% probability level. Selected bond lengths (Å) and
bond angles (deg) for the two independent molecules: Sn(1)-N(2) 2.060(3)
(2.057(3)), Sn(1)-N(1) 2.058(3) (2.057(3)), Si(1)-N(1) 1.743(3) (1.742(3)),
Si(2)-N(2) 1.740(3) (1.744(3)); N(1)-Sn(1)-N(2) 105.8(1) (105.6(1)),
Sn(1)-N(1)-Si(1) 114.9(1) (114.9(1)), Sn(1)-N(1)-C(1) 126.1(2) (125.7(2)),
Si(1)-N(1)-C(1) 118.6(2) (118.5(2)), Sn(1)-N(2)-Si(2) 114.8(1) (115.6(1)),
Sn(1)-N(2)-C(13) 125.2(2) (126.0(2)), Si(2)-N(2)-C(13) 119.1(2)
(117.8(2)).
isopropylphenyl)]2, a significantly more sterically crowded
analogue prepared previously by Sita, a molecule in which
the rings are twisted away from each other.11
the mesityl group are at δ 20.86 (4-position) and 21.56 ppm
(2,6-positions), and the four aromatic carbons are found at
δ 128.64 (3,5-positions), 130.54 (4-position), 131.88 (2,6-
positions), and 147.13 ppm (1-position). The single resonance
at 473 ppm in the 119Sn NMR spectrum of 1 indicated that
a monomeric structure was likely. This value was similar to
the value of 501 ppm obtained by Sita for Sn[N(SiMe2Ph)2]2,
a monomer whose existence was established by X-ray
diffraction.11 Since the nuclearity of 1 could not be deter-
mined with certainty via NMR, it was necessary to determine
this by single-crystal X-ray diffraction analysis.
Crystals of 1 suitable for X-ray structural analysis were
grown from a hexane solution by cooling to -20 °C for
several days. There are two symmetrically independent
molecules in the crystal of 1. As can be seen in Figure 1,
the overall structure is monomeric, similar to the structure
of Sn[N(SiMe3)2]2 reported previously by Lappert.12 The Sn
atom in 1 resides at the bottom of a pocket formed by the
two phenyl rings of the mesityl groups which are essentially
parallel to each other (in both independent molecules the
two rings are approximately 1.5° removed from coplanarity).
The distance between the two parallel phenyl ring centers is
approximately 3.60 Å. While the rings are not absolutely
concentric, they do provide a pocket that should be accessible
by other atoms and molecules. Interestingly, in the solid state
1 adopts a planar zigzag conformation in the Si(1)-N(1)-
Sn-N(2)-Si(2) backbone. Furthermore, due to the pseudo-
C2 axis that is present in this conformation, the two-aryl
groups are essentially identical. It is also interesting to note
that the structure of 1, with parallel aromatic rings, is quite
different from the solid state structure of Sn[N(SiMe3)(2,6-
Compound 2 was prepared in 70% yield in a manner
similar to that of 1; however, in this case 1 equiv of SnCl2
was used. Yellow crystals of 2 were obtained by recrystal-
lization from a toluene/hexane solution at -20 °C. This
compound is air-sensitive in solution, yet is relatively stable
1
to air in solid state. The H NMR spectrum in d6-benzene
again consisted of four peaks, which are shifted slightly to
higher chemical field when compared with the corresponding
peaks in compound 1. The -SiMe3 resonance for 2 is seen
at δ 0.26 ppm, the two distinct methyl resonances from the
mesityl group are at δ 2.11 (4-position) and 2.30 ppm (2,6-
positions), and the two protons on the mesityl ring are found
at δ 6.79 ppm. In the 13C NMR spectra, the -SiMe3
resonance is found at δ 4.00 ppm, the two different methyl
peaks from the mesityl group are at δ 20.75 (4-position) and
21.37 ppm (2,6-positions), and the four aromatic carbons are
found at δ 129.55 (3,5-positions), 132.63 (4-position), 136.61
(2,6-positions), and 143.24 ppm (1-position). Again, both
1H NMR and 13C NMR data indicate retention of the basic
ligand structure. The 119Sn NMR spectrum of 2, however,
was a single peak at 67 ppm, at significantly higher field
than found in 1. This value is at even higher field than the
119Sn shifts of 181 and 193 ppm seen earlier by Lappert with
the binuclear µ-chlorotin(II) amides.7 This indicated to us
that the structure of 2 may well be different from that seen
in the solid state with the dimeric species. To examine the
exact structure, we studied the structure of 2 using single-
crystal X-ray diffraction analysis (Table 1).
As is shown in Figure 2, the crystal structure indicates
that the molecule adopts a chairlike configuration in the solid
state. Two of the -[N(mesityl)(SiMe3)] groups lie on one
side of the six membered ring (up), while the third group is
located on the opposite side of the ring (down). This can be
(11) Babcock, J. R.; Liable-Sands, L.; Rheingold, A. L.; Sita, L. R.
Organometallics 1999, 18, 4437.
(12) Fjeldberg, T.; Hope, H.; Lappert, M. F.; Power, P. P.; Thorne, A. J.
J. Chem. Soc., Chem. Commun. 1983, 639.
Inorganic Chemistry, Vol. 43, No. 22, 2004 7241

Tang et al.
ascribed to the sterically crowded nature of the ligands in 2
as well as the presence of the stereochemically active lone
pairs on the Sn atoms. The tin-chlorine bonds range from
2.5789(6) to 2.6868 (6) Å in length. These values fall within
the range observed for the sterically stabilized dimeric trans-
{Sn(µ-Cl)[N(SiMe3)2]}2 (Sn-Cl 2.5998(1) Å and Sn-Cl′
steric requirements for mesityl are much less than for the
-[C(SiMe3)3] group.
Summarizing, we have examined the reaction of the bulky
ligand -N[(mesityl)(SiMe3)] with both 2:1 and 1:1 molar
ratios of SnCl2. These reactions produce a monomeric
stannylene and a structurally unique trimeric tin amide. We
have synthesized and characterized the first example of a
trimeric µ-Cl tin(II) amide, {[Sn(µ-Cl)[N(mesityl)(SiMe3)]}3,
2, whose crystal structure indicates that the molecule adopts
a chairlike conformation in the solid state. The structure of
the monomeric tin amide Sn[N(mesityl)(SiMe3)]2, 1, reveals
it to be a highly symmetric molecule with the phenyl groups
virtually parallel to each other. Insertion reactions of carbon
dioxide into these tin(II) complexes are currently in progress
in our laboratory.
2.741(1) Å) and cis-{Sn(µ-Cl)[NCMe2(CH2)3CMe2]}2
(Sn-Cl 2.704(1) Å and Sn-Cl′ 2.786(1) Å).7 As such,
they are not particularly informative with respect to the nature
of any steric interactions. Further, the tin-chlorine-tin
angles (Sn(2)-Cl(1)-Sn(1) 114.16(2)°, Sn(2)-Cl(2)-
Sn(3) 116.96(2)°, and Sn(3)-Cl(3)-Sn(1) 113.99(2)°) are
much larger than those in trans-{Sn(µ-Cl){N(SiMe3)2}]2 and
cis-{Sn(µ-Cl)[NCMe2(CH2)3CMe2]}2 (Sn-Cl-Sn′ 98.67(4)°,
and Sn-Cl-Sn′ 100.91(4)°).7 While the exact reason is not
known for the existence of this new trimeric chairlike
structure for 2, there may be clues in the structure of a related
lead compound, {Pb[C(SiMe3)3]Cl}3, in which the six-
membered ring adopts a distorted boat form rather than a
chair form.13 In this molecule, the -[C(SiMe3)3] groups are
so sterically crowded that the boat configuration allows them
to get as far apart as possible. It is clear that in our case the
Acknowledgment. We thank the National Science Foun-
dation (Grant CHE-0213165) for financial support of this
work.
Supporting Information Available: X-ray crystal data in CIF
format for 1 and 2. UV-vis spectra of 1, 2, and the free ligand
HN[(mesityl)(SiMe3)]. This material is available free of charge via
the Internet at http://pubs.acs.org.
(13) Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; So¨zerli, S. E. Organome-
tallics 1997, 16, 5653.
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7242 Inorganic Chemistry, Vol. 43, No. 22, 2004