Insertion reactions of CO2, OCS, and CS2 into the Sn-N bonds of (Me2N)2Sn: NMR and X-ray structural characterization of the products

Article abstract of DOI:10.1016/j.ica.2011.05.036

The interactions of the heteroallenes CO₂, OCS, and CS ₂ with (Me₂N)₂Sn have been investigated. These CX₂ species insert into the Sn-N bonds under mild conditions to provide products bis-(N,N-dimethylcarbamato)tin(II), [(Me₂NCO ₂)₂Sn]₂, bis-(N,N-dimethylthiocarbamato)tin(II) , [Me₂NC(O)S]₂Sn and bis-(N,N-dimethyldithiocarbamato) tin(II), (Me₂NCS₂)₂Sn. These molecules have been fully characterized by traditional spectroscopic methods as well as by X-ray crystallography. The [Me₂NC(O)S]₂Sn product is the first example of a structurally characterized Sn(II) thiocarbamate. The solid-state structures of the final products vary depending on the heteroallene inserted. The CO₂-inserted product is dimeric in the solid-state, with both bridging and chelating carbamate ligands. These dimers form a chain-like network via intermolecular Sn...O interactions. The monomeric thiocarbamate also shows a chain-like extended structure, through both Sn...O and Sn...S interactions, while the dithiocarbamate product has no significant intermolecular contacts.

Full text of DOI:10.1016/j.ica.2011.05.036

Inorganica Chimica Acta 376 (2011) 73–79
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Inorganica Chimica Acta
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Insertion reactions of CO2, OCS, and CS2 into the Sn–N bonds of (Me2N)2Sn: NMR
and X-ray structural characterization of the products
Constantine A. Stewart a, Diane A. Dickie b, Yongjun Tang b, Richard A. Kemp a,b,
⇑
a Advanced Materials Laboratory, Sandia National Laboratories, Albuquerque, NM 87106, USA
b Department of Chemistry, University of New Mexico, Albuquerque, NM 87131, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The interactions of the heteroallenes CO2, OCS, and CS2 with (Me2N)2Sn have been investigated. These CX2
species insert into the Sn–N bonds under mild conditions to provide products bis-(N,N-dimethylcarb-
amato)tin(II), [(Me2NCO2)2Sn]2, bis-(N,N-dimethylthiocarbamato)tin(II), [Me2NC(O)S]2Sn and bis-(N,N-
dimethyldithiocarbamato)tin(II), (Me2NCS2)2Sn. These molecules have been fully characterized by
traditional spectroscopic methods as well as by X-ray crystallography. The [Me2NC(O)S]2Sn product is
the first example of a structurally characterized Sn(II) thiocarbamate. The solid-state structures of the
final products vary depending on the heteroallene inserted. The CO2-inserted product is dimeric in the
solid-state, with both bridging and chelating carbamate ligands. These dimers form a chain-like network
via intermolecular Snꢀ ꢀ ꢀO interactions. The monomeric thiocarbamate also shows a chain-like extended
structure, through both Snꢀ ꢀ ꢀO and Snꢀ ꢀ ꢀS interactions, while the dithiocarbamate product has no
significant intermolecular contacts.
Received 9 March 2011
Received in revised form 27 May 2011
Accepted 30 May 2011
Available online 6 June 2011
Keywords:
Tin(II) complexes
CO2 insertion
Carbamate
Thiocarbamate
Dithiocarbamate
X-ray structure
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
making the atoms nucleophilic, while the tin center has both a
lone pair of electrons and an empty p-orbital, thus making it
The reactivity of small, unsaturated molecules such as carbon
dioxide with metal amides has been an interest in many research
groups [1], including our own [2], over the last several years. While
some might propose that the use of CO2 as a carbon source in the
process of preparing industrially important chemicals such as iso-
cyanates and/or carbodiimides would partially alleviate the effects
of this greenhouse gas on the environment, the reality is that the
quantities involved in chemicals production are miniscule when
compared to the daily output of CO2 in the combustion of fossil
fuels. However, a fundamental understanding of how to make
the thermodynamically stable CO2 molecule more reactive in a cat-
alytic cycle could lead to its use as a component in a transportation
fuel [3]. In this case, the quantities of CO2 recycled would be sub-
stantial and could offset the release of CO2 by the combustion of
additional fossil fuels.
Carbon dioxide can be considered to have two sites of reactivity.
The presence of the two electronegative oxygen atoms causes the
carbon to be electron-deficient and prone to nucleophilic attack,
while the electron-rich oxygen atoms can act as Lewis bases and
are thus susceptible to attack by electrophiles. Similarly, a divalent
group 14 metal amide can also be considered to have several
sites for reactivity – the nitrogen atoms have lone pairs of electrons
amphoteric.
While there are many reports in the literature of insertion of
CO2 into transition metal-amide bonds there are far fewer for the
CO2 insertion into the main group metal analogs. In addition to
our interest in the reactivity of CO2 with main group metal amides,
we also wanted to investigate a series of reactions using the related
heteroallenes OCS and CS2, as their reactivity can often be dramat-
ically different [4]. In order to gain insight into the fundamental
reaction details, pathways and mechanisms of small molecules
with metal amides, and to develop new synthetic strategies to pro-
duce useful compounds we have chosen to study a simple divalent
metal amide – bis(dimethylamido)tin(II), (Me2N)2Sn (1). Herein,
we report on the reactivity of (Me2N)2Sn with CO2, OCS, and CS2.
This work serves as a complement to our recently published report
on the interactions of these heteroallenes with silyl-substituted
tin(II) amides [4].
These reactions, as will be discussed shortly, produced the
inserted products bis-(N,N-dimethyldicarbamato)tin(II), [(Me2-
NCO2)2Sn]2 (2), bis-(N,N-dimethylthiocarbamato)tin(II), [Me2-
NC(O)S]2Sn (3) and bis-(N,N-dimethyldithiocarbamato)tin(II),
(Me2NCS2)2Sn (4). The synthesis of 4 has been reported previously
by Perry and Geanangel [5], as has the preparation and crystal
structure of the related bis-(N,N-diethyldithiocarbamato)tin(II)
species (Et2NCS2)2Sn (5) [6]. However, both prior compounds were
prepared by different synthetic routes involving the reactions of
SnCl2 and ammonium dithiocarbamate salts [5,6]. We report the
⇑
Corresponding author at: Department of Chemistry, University of New Mexico,
Albuquerque, NM 87131, USA. Tel.: +1 505 272 7609; fax: +1 505 272 7336.
E-mail address: rakemp@unm.edu (R.A. Kemp).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.05.036

74
C.A. Stewart et al. / Inorganica Chimica Acta 376 (2011) 73–79
Table 1
direct reaction of (Me2N)2Sn with these heteroallenes to prepare
the desired products and discuss some of the unusual bonding
modes in the crystallographically characterized products using
each of these related heteroallenes.
Crystallographic data and refinement parameters for 2, 3, and 4.
2
3
4
Empirical
formula
Formula
weight
Crystal size
(mm)
Crystal
system
Space group
T (K)
C
12H24N4O8Sn2
C6H12N2O2S2Sn
C6H12N2S4Sn
589.74
327.03
359.11
2. Experimental section
0.35 ꢂ 0.16 ꢂ 0.10 0.45 ꢂ 0.10 ꢂ 0.07 0.40 ꢂ 0.36 ꢂ 0.10
2.1. General consideration
monoclinic
monoclinic
monoclinic
All manipulations were carried out in an Ar-filled glove box or
by using standard Schlenk techniques [7]. Anhydrous solvents
were purchased from Aldrich or Fisher Scientific Company, after
which they were stored in the glove box over 4 Å molecular sieves
prior to use. 1H, 13C{1H} and 119Sn{1H} NMR spectra were obtained
on a Bruker AMX 250 spectrometer. The 1H and 13C{1H} spectra
were referenced to residual solvent downfield of SiMe4 while
119Sn{1H} spectra were referenced to external SnEt4. IR spectra
were recorded on a Bruker Vector 22 MIR spectrometer. Melting
points were obtained under Ar. Elemental analyses were per-
formed at Columbia Analytical Services, Inc., 3860 S. Palo Verde
Road, Tucson, Arizona 85714. While compounds 2, 3, and 4 were
always handled under Ar, they each possess sufficient stability to
be briefly handled in the atmosphere. Although there are no obvi-
ous visual changes in color or texture upon exposure to air in the
solid state over the course of >24 h, changes in the NMR and IR
spectral properties of 2, 3, and 4 suggest that these molecules do
not tolerate prolonged exposure to the atmosphere.
C2/c
213(2)
P21/c
C2/c
223(2)
7.8608(6)
7.4397(5)
20.1075(14)
90
105.755(4)
90
1131.75(14)
4
223(2)
8.9253(4)
12.1265(5)
12.2492(6)
90
109.596(2)
90
1248.98(10)
4
a (Å)
b (Å)
c (Å)
11.6271(9)
13.2951(10)
14.0241(11)
90
111.7980(10)
90
2012.9(3)
4
1.946
a
(°)
b (°)
c
(°)
V (Å3)
Z
Calculated
1.919
1.910
density (g/
cm3)
Absorption
coefficient
l
2.526
2.599
2.675
(mmꢃ1
)
Measured
7503
24768
8958
reflections
Independent
reflections
(Rint
R1, wR2 (all
data)
2354 (0.0196)
4312 (0.0176)
2965 (0.0265)
)
0.0228, 0.0539
0.0216, 0.0532
1.119
0.0216, 0.0663
0.0172, 0.0529
1.324
0.0337, 0.0729
0.0250, 0.0663
1.041
2.2. X-ray crystallography
R1, wR2
[I > 2
r(I)]
GOF on F2
Single crystals of 2, 3, and 4 were coated with oil and mounted
on a CryoLoop™ that had previously been attached using epoxy to
a metallic pin. All measurements were made on a Bruker X8 Apex2
CCD-based X-ray diffractometer equipped with an Oxford Cryo-
stream 700 low temperature device and normal focus Mo-target
X-ray tube (k = 0.71073 A) operated at 1500 W power (50 kV,
30 mA).
plates and allowed to dry, cmꢃ1) 2359 (s), 2341 (s), 1651 (vw),
1539 (m), 1471 (m), 1384 (vs), 1259 (s), 1019 (s), 793 (w), 668
(m), 648 (m), 548 (s).
Data collection and processing were done using the Bruker APEX
2
software suite [8]. Structure solution was done by direct methods
with the SHELX suite of software [9]. Non-hydrogen atoms were
refined anisotropically, while hydrogen atoms were placed in
geometrically calculated positions using a riding model. X-ray data
are summarized in Table 1.
2.5. [Me2NC(O)S]2Sn (3)
Under argon, 1 (1.062 g; 5.1 mmol) was dissolved in hexanes
(4 g) and added to a heavy walled glass reactor tube fitted with a
74 cc stainless steel bomb. The pale yellow hexanes solution of 1
was cooled to ꢃ78 °C and the system evacuated to remove argon.
The 74 cc bomb was charged with carbonyl sulfide to a pressure
of 60 psig (ꢁ5 atm) and then condensed into the heavy walled
glass reactor. Another 60 psig was charged to the 74 cc bomb
and condensed into heavy walled glass reactor tube and a final
22 psig of OCS was charged to the bomb and condensed into the
tube. The cumulative charges of OCS amounted to 31 mmol. Once
all of the OCS was condensed into the heavy walled glass reactor
tube at ꢃ78 °C the system pressure read <0 psig. The reaction
was allowed to slowly warm to room temperature while stirring
over a period of 12 h. During that time, the pressure of the system
was ꢁ12 psig. The reaction vessel was then vented; the crude
product was collected by filtration and washed with three portions
of pentane (ꢁ25 mL) to give 1.385 g of a white solid (crude
yield = 83%). This white solid was recrystallized from hexanes to
2.3. (Me2N)2Sn (1)
(Me2N)2Sn was synthesized according to the literature methods
[10]. 1H NMR (C6D6) d 2.80 (lit. in C6H6 d 2.75 ppm) and melting
point confirmed the product (m.p. = 95–96 °C; lit. m.p. =
91–93 °C); 119Sn{1H} (C6D6) d 120 ppm.
2.4. [(Me2NCO2)2Sn]2 (2)
Under argon, 1 (0.459 g; 2.2 mmol) was dissolved in hexanes
(ꢁ5 g). Carbon dioxide was bubbled through the pale yellow solu-
tion at room temperature for approximately 10 min. After several
minutes the reaction turned cloudy. The crude product was
collected by filtration and washed with 3 ꢂ 25 mL portions of
pentane to give 0.629 g of a white solid. This white solid was
recrystallized from warm toluene to give 2 as clear, colorless
give
3 as white crystals. m.p. = 154–156 °C. Anal. Calc. for
(C6H12N2O2S2Sn): C, 22.04; H, 3.70; N, 8.57. Found: C, 21.65; H,
3.42; N, 8.32%. 1H NMR (CDCl3) d 2.92 (s, 3H, CH3), 3.05 (s, 3H,
CH3) ppm. 13C{1H} NMR (CDCl3) d 35.7 (CH3), 39.6 (CH3), 181.4
(O–C–S) ppm. 119Sn{1H} (CDCl3) d ꢃ351 ppm. IR (Nujol mull, KBr
plates, cmꢃ1) 2066 (w), 547 (m), 1530 (m), 1180 (vw), 965 (m),
1018 (s), 800 (vw), 699 (m).
crystals in 97% yield. m.p. = 129–130 °C. Anal. Calc. for (C6H12
-
N2O4Sn): C, 24.44; H, 4.11; N, 4.50. Found: C, 24.12; H, 4.03; N,
9.64%. 1H NMR (CDCl3) d 2.88 (s, 3H, CH3) ppm. 13C{1H} NMR
(CDCl3) d 36.3 (s, CH3), 164.5 (s, O–C–O) ppm. 119Sn{1H} (CDCl3) d
ꢃ613 ppm. IR (deposited 2 from a solution of hexanes onto KBr

C.A. Stewart et al. / Inorganica Chimica Acta 376 (2011) 73–79
75
2.6. (Me2NCS2)2Sn (4)
Under argon, 1 (0.285 g; 1.4 mmol) was dissolved in ether
(10 g). To this stirring solution at room temperature was added
CS2 (0.35 mL; 5.8 mmol) drop-wise via syringe. An immediate,
exothermic reaction occurred to produce a yellow precipitate.
The volatiles were removed under vacuum to give a pale yellow
powder (0.385 g, crude yield = 77%, m.p. = 244–246 °C dec.). The
crude product was recrystallized from hot dimethyl sulfoxide to
give pale yellow crystals of 4. m.p. = 245 °C; lit. 185 °C dec. [5].
Anal. Calc. for (C6H12N2S4Sn): C, 20.07; H, 3.37; N, 7.80. Found: C,
20.06; H, 3.20; N, 7.80%. 1H NMR (CDCl3) d 3.38 (s, CH3) ppm; (in
C6D6) d 2.57 (s, CH3) ppm; 13C{1H} NMR (CDCl3) d 42.7 (CH3),
201.4 (S–C–S) ppm. 119Sn{1H} (CDCl3) d ꢃ556 ppm. IR (Nujol mull,
KBr plates, cmꢃ1) 1520 (m), 1244 (m), 1148 (m), 1018 (m), 965 (m),
722 (m), 572 (m).
Fig. 1. Two types of bonding observed for Me2NCE2 with Sn(II); type a – terminal
bidentate (chelating) and type b – bridging bidentate.
and single crystal X-ray diffraction were all used to characterize
the products from these reactions. X-ray diffraction results were
particularly useful in studying the solid-state structure of the prod-
ucts and revealed some peculiar features in the intermolecular
interactions, discussed in more detail below.
The geometry and bonding of the Me2NCE2 ligands in these
carbamato compounds can be described as either type a or type
b, as seen in Fig. 1 [2a]. In type a bonding the Me2NCE2 ligand is
3. Results and discussion
The reactions of 1 with CO2, OCS, and CS2 were straightforward
and involved insertion of the heteroallene into the Sn–N bonds to
provide [(Me2NCO2)2Sn]2 2, [Me2NC(O)S]2Sn 3, and (Me2NCS2)2Sn
4, respectively (Scheme 1). The reactions with CO2 and CS2 took
place under exceedingly mild conditions at room temperature.
The reaction with OCS was accomplished by condensing the gas
into the reactor at low temperature (ꢃ78 °C) and allowing the
reaction to warm to room temperature. The yields for these reac-
tions to produce analytically pure products ranged from 77% for
4 to 97% for 2. Elemental analysis, NMR and IR spectroscopies,
terminal bidentate (chelating), while in type
b bonding the
Me2NCE2 ligand is bridging bidentate. As will be seen, we have
observed both types a and b bonding modes in 2 while only type
b bonding is present in complexes 3 and 4 (vide infra).
For the preparation of 2, CO2 was bubbled through a hexanes
solution of compound 1 at atmospheric pressure and room
Scheme 1.

76
C.A. Stewart et al. / Inorganica Chimica Acta 376 (2011) 73–79
Fig. 2. Molecular structure of 2 (50% ellipsoids). For clarity, hydrogen atoms are not shown and the dimethylamino groups are shown as sticks. Close interaction between
neighboring molecules: Sn(1)ꢀ ꢀ ꢀO(1), 2.8499(18) Å. Selected bond lengths (Å) and angles (°): Sn(1)–O(1) 2.3320(17), Sn(1)–O(3) 2.4323(15), Sn(1)–O(2) 2.3352(18),
Sn(1)–O(4) 2.1152(16); O(1)–Sn(1)–O(3) 137.92(6), O(2)–Sn(1)–O(4) 86.18(7).
temperature (see Scheme 1, reaction 1). The reaction was slightly
exothermic and produced very small, white crystals of 2. Com-
pound 2 was fully characterized by 1H, 13C{1H}, 119Sn{1H}, and IR
spectroscopies, as well as single crystal X-ray diffraction. The 1H
NMR spectrum exhibited one resonance for the methyl groups at
d 2.88 ppm. The 13C{1H} NMR spectrum displayed the expected
two singlets, one at d 36.3 (–CH3 groups) and another at d
164.5 ppm (Me2NCO2– carbons). The NMR results suggested a sim-
ple insertion of CO2 into the Sn–N bond to form the symmetrically
coordinated carbamato group. The 13C{1H} resonance for the carb-
amato carbons in 2 is similar to that reported by Calderazzo and
co-workers at 22 °C for the Al2(O2CNiPr2)6 carbamato carbons (d
163.5 ppm in CDCl3), which has both bridging and terminal carb-
amato ligands [11]. However, at ꢃ55 °C they observed two peaks
for the carbamato resonances (d 165.4 O2CNterminal; d 158.6
O2CNbridging). Based on their ambient and low temperature NMR
observations, we knew that we could not rule out 2 being dimeric
and possessing both bridging and terminal carbamato ligands. The
IR spectrum of 2 (film on KBr plates) exhibited absorbances below
1600 cmꢃ1, which is suggestive of a compound containing both
bridging and terminal bidentate carbamate groups according to
Calderazzo and co-workers [11]
us [4], and two homoleptic Sn(IV) carbamates [(R2NCO2)4Sn,
R = Et, iPr] are also known [13]. Examination of the structure shows
that the bidentate carbamate groups occupy both bridging and ter-
minal roles. For the bridging carbamate ligand the O(3)–C(4)–O(4)
bond angle is 121.8(2)°, and as would be expected this angle is
slightly greater than the terminal O(2)–C(1)–O(1) bond angles of
118.9(2)°. The two C–O bond distances of the terminal bidentate
carbamate ligands are the same length within experimental error
–
C(1)–O(2) 1.280(3) Å and C(1)–O(1) 1.272(3) Å, suggesting
near-complete charge delocalization. The C–O bond lengths for
the bridging bidentate carbamate ligand exhibit both short and
long C–O bond distances, with a shorter C(4)–O(3) bond length
of 1.263(3) Å and a longer one, C(4)–O(4), of 1.295(3) Å. As with
Sn(IV) carbamates, the shorter C–O bonds are also the ones that
are associated with the longer Sn–O bonds [13]. We note that the
g
2 chelation mode exhibited by the terminal ligand is significantly
less common in both carbamates and carboxylates relative to
bridging modes when bound to main group metals (M). This find-
ing has been previously attributed to the reluctance of a highly
strained, four-membered MO2C ring to form in the presence of
the two electron-rich oxygen atoms of the chelating ligand
[1e,2a]. Thus, the tendency is for the ligand to reduce electron den-
sity at the metal by forming bridges to multiple metals, resulting in
a more stable binding mode. Substitution of one or both of the
highly donating oxygen atoms with less electron-rich atoms such
as sulfur render the chelating mode much more favorable.
A single crystal X-ray diffraction study revealed that in fact 2
exists as a dimer in the solid-state, and that both type a and b
carbamato ligands are bound to the tin centers as shown in
Fig. 2. Based on our search of the literature and the Cambridge
Structural Database (CSD, version 5.32) [12] this is the first struc-
turally characterized homoleptic Sn(II) carbamate to be reported.
One other heteroleptic Sn(II) carbamate was recently reported by
The local geometric environment around the two Sn atoms in 2
can best be described as a highly distorted trigonal bipyramidal
(tbp) structure (vide infra). In addition, compound 2 exhibits close
Fig. 3. Molecular structure of 3 (50% ellipsoids). For clarity, hydrogen atoms are not shown and the dimethylamino groups are shown as sticks. Close interactions between
neighboring molecules: Sn(1)ꢀ ꢀ ꢀS(2), 3.3900(5) Å and Sn(1)ꢀ ꢀ ꢀO(2), 2.9706(11) Å. Selected bond lengths (Å) and angles (°): Sn(1)–O(2) 2.4231(12), Sn(1)–S(1) 2.7650(5),
Sn(1)–O(1) 2.2196(11), Sn(1)–S(2) 2.6070(5); S(1)–Sn(1)–O(2) 134.46(3), S(2)–Sn(1)–O(1) 89.54(3).

C.A. Stewart et al. / Inorganica Chimica Acta 376 (2011) 73–79
77
Snꢀ ꢀ ꢀO interactions of 2.8499(18) Å between the Sn(1) and O(1)
atoms of neighboring molecules to give an extended chain-like
polymeric network in the solid state.
der Waals radii sums (Snvan der Waals = 2.17 Å; Svan der Waals = 1.80 Å;
O
van der Waals = 1.52 Å). These close Snꢀ ꢀ ꢀS intermolecular distances
in 3 suggest an extended network in the solid state, similar to that
found in 2. However, even with this extended network, in contrast
to the indium thiocarbamates [14,15] 3 is surprisingly soluble in
hexanes and chloroform.
The reaction of 1 with OCS was conducted under slightly differ-
ent conditions. In this case 1 was dissolved in hexanes, placed in a
heavy walled glass vessel, cooled to ꢃ78 °C, evacuated, and the
reaction vessel charged with an excess of OCS (ꢁ31 mmol).
Work-up of the reaction and purification of the crude product by
crystallization from hexanes gave compound 3 in very good iso-
lated yield (83%). The 1H NMR spectrum of 3 exhibited singlets at
d 2.92 and 3.05 ppm, suggesting the existence of nonequivalent
methyl groups on the thiocarbamate ligand [Me0(Me)NC(O)S]–.
The 13C{1H} NMR spectrum showed three resonances at d 35.7,
39.6, and 181.4 ppm for the non-equivalent methyl groups and
the thiocarbamate carbon, respectively. The 13C{1H} shift of the
carbon of the [Me2NC(O)S]– group for 3 is similar to other
The last reaction in this series is between 1 and CS2 to give
(Me2NCS2)2Sn (4). We noted that 4 has been reported previously
by Perry and Geanangel [5], although prepared in 45% yield via
the reaction of ammonium dithiocarbamate salts with SnCl2. We
prepared 4 by reacting of a solution of 1 in diethyl ether with a
slight excess of CS2. The reaction was immediate and exothermic,
and produced a yellow precipitate isolated in 77% yield. The prod-
uct was largely insoluble in common solvents (e.g., CH2Cl2, THF,
toluene, ether, hexanes, or benzene) and was purified by recrystal-
lization from hot dimethyl sulfoxide. Compound 4 was only
slightly soluble in CDCl3 and C6D6, which made obtaining the
13C{1H} NMR spectrum challenging. It is interesting to note that
Perry and Geanangel reported two singlets in the 1H NMR spec-
trum (in C6H6) at d 3.04 and 2.90 ppm, which they postulated were
due to restricted rotation about the C–N bond. However, we
observed only a single resonance at d 3.38 ppm for 4 in the proton
NMR spectrum (CDCl3). Their reported observation of the two
resonances for the methyl groups prompted us to reevaluate the
1H spectra of 4 in C6D6 to see if the two singlets were due to solvent
or polarity effects. Therefore, we prepared a sample of 4 in C6D6
and again observed only a single resonance in the 1H NMR spec-
trum, although this time it was found at d 2.57 ppm. As mentioned
previously, 4 was only sparing soluble in most solvents, which pre-
sented some difficulty when trying to obtain the 13C{1H} NMR
spectrum. The methyl groups on Me2NCS2 group exhibited a single
resonance at d 42.7 ppm. However, only a very weak signal for the
carbon of the dithiocarbamate could be observed at d 201.4 ppm,
as listed in Table 2. This resonance at d 201.4 ppm for the dithiocar-
bamate carbon of Me2NCS2– is comparable to that reported by van
Gaal and co-workers for (Et2NCS2)2Sn (5) at d 199.9 ppm in CDCl3
[20].
Comparisons of the NMR spectra of 2, 3, and 4 deserve added
discussion. As can be seen from Table 2, there is a smooth down-
field shift for the 1H and 13C resonances with the increase in the
number of sulfur atoms in the complexes. In particular, the carbon
resonance in the Me2NCE2– group becomes more deshielded as
sulfur atoms replace oxygen atoms in the carbamate group. This
downfield shift tracks well with the 13C shifts of the carbon in
CO2, OCS, and CS2 (d 124.2, 153.8, and 192.6 ppm, respectively).
However, there appears to be no corresponding order to the chem-
ical shifts in the 119Sn NMR spectra for compounds 2, 3, and 4.
The structure of 4 was verified by single crystal X-ray diffraction
and is shown in Fig 4. Compound 4 is unique in this series in that
there are no extended intermolecular interactions in the solid-state
between the Sn and S atoms. In this case the shortest intermolecular
tin–sulfur distance is 4.059 Å, which is outside the sum of the van
homoleptic main-group thiocarbamates
– e.g., [Et2NC(O)S]3In
(d 182.4 ppm [14]) and [iPr2NC(O)S]3In (d 181.4 ppm [15]). Both
of these indium-containing complexes were prepared via the
metathesis reaction of stoichiometric quantities of InCl3 and the
lithium dialkylthiocarbamate salts and not by the direct insertion
of OCS. As well, these complexes were described as being only
sparingly soluble in most organic solvents.
The reason for the two distinct, non-equivalent methyl group
resonances for 3 in both the 1H and 13C{1H} NMR spectra were also
supported by the single crystal X-ray diffraction results, as shown
in Fig. 3. In contrast to 2, the thiocarbamate ligands of 3 chelate
rather than bridge, and 3 exists as a monomer. A search of the
CSD [12] showed that compound 3 appears to be the first structur-
ally-characterized thiocarbamate Sn(II) complex, although Sn(IV)
complexes of thiocarbamates based on pyrrole or morpholine
made via routes other than OCS insertion are known and have been
structurally characterized [16].
The geometric environment of the Sn atom in compound 3 can
best be described as a highly distorted tbp structure with a stereo-
active lone pair of electrons occupying an equatorial position at the
formally Sn(II) center. This distorted tbp environment is expected
based on the discussions present in reviews by Heard [17] and
Tiekink [18] about dithiocarbamate complexes. The stereoactive
lone pair of electrons forces the O(2)–Sn(1)–S(1) bond angle to
distort to 134.46(3)°, significantly wider than in other main group
thiocarbamates (116–120°) [14–16,19]. Comparing the bond dis-
tances of the four atoms attached to Sn(1) – (S(1), S(2), O(1) and
O(2) – the atoms S(1) and O(2) can be assigned the axial positions
due to their longer Sn–O bonds, Sn(1)–S(1) 2.7650(5) Å and Sn(1)–
O(2) 2.4231(12) Å. The comparatively shorter Sn(1)–S(2) and
Sn(1)–O(1) bond distances of 2.6070(5) and 2.2196(11) Å, respec-
tively, places the S(2) and O(1) atoms in the equatorial positions.
For related main group thiocarbamates, the M–S and M–O
bonds range in length from 2.44–2.77 and 2.24–2.81 Å, respec-
tively. This assignment of the longer bonds to the axial positions
and shorter ones to the equatorial positions is consistent with
descriptions of the bonding in tin(II) dithiocarbamates [17,18] as
well as an indium thiocarbamate [14]. The axial and equatorial
assignments for these four atoms are also manifest in their com-
paratively shorter C(1)–S(1)ax and C(2)–O(2)ax bonds (1.7369(17)
and 1.2623(19) Å, respectively) versus the somewhat longer
C(2)–S(2)eq and C(1)–O(1)eq bonds (1.7552(17) and 1.2815(18) Å,
respectively). As with the M–O and M–S bonds, these bonds are
on the edge of the range of comparable main group thiocarbamates
(C–S = 1.73–1.78 Å; C–O = 1.22–1.27 Å).
Table 2
1H, 13C{1H} and 119Sn{1H} NMR chemical shifts for 2, 3, and 4.
Compound
1Ha
13C{1H}b
119Sn{1H}c
2
2.88 (CH3)
36.3 (CH3)
164.5 (O–C–O)
35.7 (CH3)
39.6 (CH3)
181.4 (O–C–O)
42.7 (CH3)
ꢃ613
3
4
2.92 (CH3)
3.05 (CH3)
ꢃ351
Close inspection of the packing of compound 3 shows that
the nearest interactions between adjacent molecules are Sn(1)–
S(2), 3.3900(5) Å and Sn(1)–O(2), 2.9706(11) Å. These distances are
3.38 (CH3)
ꢃ556
201.4 (O–C–O)
a
b
c
(CDCl3; Ref. Me4Si), ppm.
(CDCl3; Ref. Me4Si), ppm.
(CDCl3; Ref. Et4Sn), ppm.
all outside the sum of the covalent radii of the atoms (Sncovalent radius
=
1.39 Å; S = 1.05 Å; Ocovalent radius = 0.66 Å), but within the van
covalent radius

78
C.A. Stewart et al. / Inorganica Chimica Acta 376 (2011) 73–79
4. Conclusions
The reaction of (Me2N)2Sn 1 with CO2, OCS, and CS2 all provide
insertion of the CE2 moiety into the Sn–N bonds to give 2, 3, and 4,
respectively. The localized geometry at the tin center of all
of these complexes can best be described as a distorted tbp
structure with the tin center containing a stereoactive lone pair
of electrons. The major difference among these compounds is that
[(Me2NCO2)2Sn]2 2 is a dimer while complexes 3 and 4 are mono-
mers in the solid-state. Compound 2 exhibits both bridging and
terminal carbamate ligands and is somewhat surprisingly the first
structurally characterized Sn(II) dialkylcarbamate. Because of the
close intermolecular interactions between neighboring Sn and O
atoms in the solid state, 2 can be described as a loosely-coordi-
nated polymer. The reaction of 1 with OCS yielded [Me2NC(O)S]2Sn
3. In this case complex 3 is a monomer, and is also the first exam-
ple of a structurally characterized thiocarbamate Sn(II) complex.
However, in the solid-state the structure for 3 exhibits some
extended intermolecular interactions between the Snꢀ ꢀ ꢀO and
Snꢀ ꢀ ꢀS atoms of neighboring molecules, forming a coordination
polymer similar to 2. Lastly, compound 4, prepared from 1 and
CS2, is also monomeric in the solid state; however, there are no
close interactions between tin and any neighboring S atoms.
Fig. 4. Molecular structure of 4 (50% ellipsoids). For clarity, hydrogen atoms are not
shown. Selected bond lengths (Å) and angles (°): Sn(1)–S(1) 2.7514(4), Sn(1)–S(2)
2.5722(4); S(1)–Sn(1)–S(2), 88.630(12), S(1)–C(1)–N(1) 121.60(9), S(2)–C(1)–N(1)
120.34(9), S(1)–C(1)–S(2) 118.05(7).
der Waals’ radii of Sn and S (Snvan der Waals = 2.17 Å; Svan der Waals
=
1.80 Å). While Sn(IV) thiocarbamate complexes are ubiquitous and
have been recently reviewed [17,18], there are only two dithiocarba-
mate Sn(II) structures reported in the literature that have similar
structures to 4. These two structures are both of (Et2NCS2)2Sn (5), re-
ported by Potenza and Mastropaolo in 1973 [6a], and by Hoskins
et al. in 1976 [6b]. Potenza and Mastropaolo describe 5 as consisting
of essentially discrete monomeric units; however, they do point out
that the Sn and S atoms of neighboring molecules are at a distance of
3.849 Å, just within the sum of the van der Waals radii (3.97 Å) for
these two atoms (Fig. 5). For structural comparisons between 4
and 5 we chose to use the X-ray metrical data from Hoskins et al.
due to the higher quality of the structure as seen by the lower
R-factor value.
Acknowledgments
The geometry around the Sn atom in 4, as found in compound 3,
is best described as distorted tbp in nature. The S(1)ax–Sn(1)–S(1)ax
bond angle in 4 (142.26(2)°) is slightly larger than that found in 5
(139.64°). The longer Sn(1)–S(1)ax bonds for 4 are associated with
the shorter C(1)–S(1) bonds (1.7092(12) Å), while the shorter
Sn(1)–S(2)ax bonds are attached to S(2) bound to the longer
C(1)–S(2) bonds (1.7349(12) Å). In compound 4 the S(2)eq–Sn(1)–
S(2)eq angle is 101.306(18)°, larger than the Seq–Sn–Seq angle found
in 5 (96.05°). The Sn(1)–S(1)ax bond in 4 is 2.7514(4) Å, while in 5
the two Sn–Sax bond lengths are 2.768 and 2.882 Å. It is notewor-
thy that both Sn–S axial bonds in 4 are shorter than those axial
bonds in 5. Recall there are no Sn interactions with other atoms
of neighboring molecules in 4. This may be the reason for the
slightly longer Sn–Sax bond lengths in 5 as compared to 4. The Sn
atom in 5 is receiving electron density from a sulfur atom of a
neighboring molecule, likely causing a slight lengthening of its
internal Sn–Sax bond distances.
This work was financially supported by the National Science
Foundation (Grant CHE09-11110) and by the Laboratory Directed
Research and Development (LDRD) program at Sandia National
Laboratories (LDRD 14938 and 151300). The Bruker X-ray diffrac-
tometer was purchased via a National Science Foundation CRIF:MU
award to the University of New Mexico (CHE-0443580). Sandia is a
multiprogram laboratory operated by Sandia Corporation, a Lock-
heed Martin Company, for the United States Department of Energy
under Contract No. DE-AC04-94AL85000.
Appendix A. Supplementary material
CCDC 806650, 806651, and 806652 contain the supplementary
crystallographic data for compounds (2), (3), and (4), respectively.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
Fig. 5. Molecular structure of 5 (50% ellipsoids). For clarity, hydrogen atoms are not shown and diethylamino groups are shown as sticks. Close interactions between
neighboring molecules: Sn(1)ꢀ ꢀ ꢀS(3)0 3.849 Å.

C.A. Stewart et al. / Inorganica Chimica Acta 376 (2011) 73–79
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