Stable Divalent Triarylstannates R3SnLi Derived from 2-[(Dimethylamino)methyl]phenyllithium and SnCl2 or R2Sn

Article abstract of DOI:10.1021/om5012215

The triarylstannate lithium compound [{κ¹-C-(2-Me₂NCH₂C₆H₄)₂}{κ²-C, N-(2-Me₂NCH₂C₆H₄)}SnLi(THF)₂] (3a) was obtained in 63% yield from the reaction of 2-Me₂NCH₂C₆H₄Li (1) with SnCl₂ in THF. Quantitative formation of 3a was also observed on reacting 1 with [κ²-C, N-(2-Me₂NCH₂C₆H₄)₂Sn] (2). Removal of one THF molecule occurred when 3a was kept in vacuo; this process afforded [{κ¹-C-(2-Me₂NCH₂C₆H₄)}{κ²-C, N-(2-Me₂NCH₂C₆H₄)₂}SnLi(THF)₂] (3b). Also, THF-free (2-Me₂NCH₂C₆H₄)₃SnLi (3c) is likewise accessible. In the solid state (X-ray) both 3a and 3b are monomeric; notable structural features are the Sn-Li distances (2.860(6) and 2.72(2) ?, respectively) and the distortion of the tetrahedral geometry at Sn in the direction of a trigonal pyramidal one. In 3a, one of the 2-Me₂NCH₂C₆H₄ anions is C-bonded to Sn, while N-coordination occurs to Li; in 3b, two of the three 2-Me₂NCH₂C₆H₄ anions are κ²-C,N bonded. ¹H, ¹³C, ¹¹⁹Sn, and ⁷Li NMR spectroscopic studies of the species 3 in toluene showed that both 3a and 3b are monomeric in solution; at temperatures below 253 K, via both the ¹¹⁹Sn and ⁷Li NMR spectra (toluene-d₈), the ¹¹⁹Sn-⁷Li coupling (289 Hz) is nicely resolved. The exchange of aryl groupings (vide infra) as well as between coordinated and free-NMe₂ substituents remains fast on the NMR time scale at 183 K. 2D ¹H-¹H EXSY spectroscopy confirmed that the 2-Me₂NCH₂C₆H₄ groups present in stannate 3b and bisaryltin(II) 2 undergo chemical exchange. The 2D ⁷Li-⁷Li EXSY spectrum points to the occurrence of chemical exchange of the lithium atoms of stannate 3b and aryllithium 1 (Chemical Equation).

Full text of DOI:10.1021/om5012215

Article
pubs.acs.org/Organometallics
Stable Divalent Triarylstannates R3SnLi Derived from
2‑[(Dimethylamino)methyl]phenyllithium and SnCl2 or R2Sn
Johann T. B. H. Jastrzebski, Mayra van Klaveren, and Gerard van Koten*
Organic Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, 3584 CG
Utrecht, The Netherlands
S
* Supporting Information
ABSTRACT: The triarylstannate lithium compound [{κ1-C-
(2-Me2NCH2C6H4)2}{κ2-C, N-(2-Me2NCH2C6H4)}SnLi-
(THF)2] (3a) was obtained in 63% yield from the reaction
of 2-Me2NCH2C6H4Li (1) with SnCl2 in THF. Quantitative
formation of 3a was also observed on reacting 1 with [κ2-C, N-
(2-Me2NCH2C6H4)2Sn] (2). Removal of one THF molecule
occurred when 3a was kept in vacuo; this process afforded
[{κ1-C-(2-Me2NCH2C6H4)}{κ2-C, N-(2-Me2NCH2C6H4)2}-
SnLi(THF)2] (3b). Also, THF-free (2-Me2NCH2C6H4)3SnLi
(3c) is likewise accessible. In the solid state (X-ray) both 3a
and 3b are monomeric; notable structural features are the Sn−
Li distances (2.860(6) and 2.72(2) Å, respectively) and the
distortion of the tetrahedral geometry at Sn in the direction of a trigonal pyramidal one. In 3a, one of the 2-Me2NCH2C6H4
anions is C-bonded to Sn, while N-coordination occurs to Li; in 3b, two of the three 2-Me2NCH2C6H4 anions are κ2-C,N
1
7
bonded. H, 13C, 119Sn, and Li NMR spectroscopic studies of the species 3 in toluene showed that both 3a and 3b are
monomeric in solution; at temperatures below 253 K, via both the 119Sn and Li NMR spectra (toluene-d8), the 119Sn−7Li
7
coupling (289 Hz) is nicely resolved. The exchange of aryl groupings (vide infra) as well as between coordinated and free-NMe2
substituents remains fast on the NMR time scale at 183 K. 2D 1H−1H EXSY spectroscopy confirmed that the 2-Me2NCH2C6H4
groups present in stannate 3b and bisaryltin(II) 2 undergo chemical exchange. The 2D Li−7Li EXSY spectrum points to the
7
occurrence of chemical exchange of the lithium atoms of stannate 3b and aryllithium 1.
Scheme 1
INTRODUCTION
■
Stannyl metal reagents of the type “R3SnM” (M = Li, Na, K)
have been known for many years and have an important role in
modern organic chemistry.1 In general, the R-grouping of the
stannyl reagent is activated to react with organic electrophiles
such as ketones, epoxides, or acid chlorides.2
In 1972 Lappert and co-workers reported the use of
triorganostannate alkali metal complexes, [R3Sn]M, as ligand
transfer reagents for the synthesis of [Zr(η5-C5H5)2(Cl)-
triorganostannane with an organoalkali metal compound (ii)11
or reaction of a diorganotin compound with an organoalkali
metal derivative (iv).13,14
It is obvious that the latter route is restricted to
diorganotin(II) compounds that are stabilized toward oligome-
rization (typically through Sn−Sn bond formation) either by
intramolecular coordination,15 by groups providing steric
constraints near the tin center,16 or by multiatom bonding of
the organic groups to tin, as, for example, in Cp2Sn.17
In related “ate” chemistry, the formally monoanionic, κ2-C,N-
chelating [2-Me2NCH2C6H4]− ligand has proven to be a
3
(SnPh3)]2 and [Yb{Sn(CH2But)3}2(THF)].4 The structural
features of the triorganostannate anions [SnR3]− (R = Ph or
CH2But) in the respective transfer reagents were studied in
more detail by elucidating the structures of both [Li(SnPh3)-
(pmdeta)]5 and [K{Sn(CH2But)3}(η6-C6H5Me)3],6 respec-
tively. These stannate compounds have Sn(II)−alkali metal
close contacts as a common structural feature (Li−Sn 2.871(7)
Å and K−Sn 3.548(3) Å).
The earliest report of a triorganostannate, Me3SnNa, dates
back to 1922.7 Its synthesis involved the reaction of Me3SnBr
with metallic sodium in liquid ammonia.8 Later, various
alternate routes have proven successful; see Scheme 1. These
protocols involve either the reduction of a triorganotin halide
(i)9 or a hexaorganoditin compound (iii)10,11 with an alkali
metal reagent. Alternate reactions involve reduction of either a
Special Issue: Mike Lappert Memorial Issue
Received: December 4, 2014
Published: April 1, 2015
© 2015 American Chemical Society
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valuable building block for the stabilization of neutral and ionic
organocuprates,18 organoaurates,19 and organozincates.20 In the
latter series of “ate” compounds, Rm−nMnLi, the 2-
Me2NCH2C6H4 anion (abbreviated as R), shows a versatile
Cipso−metal bonding behavior, while the amine ligand,
depending on the nature of the solvent, stabilizes either the
solvent-separated ion pair or the close contact pair state of the
“ate” compound. In the present study, we report on the direct
synthesis and structural characterization in the solid state of the
arylstannate(II) compound [{κ1-C-(2-Me2NCH2C6H4)2}{κ2-
C,N-(2-Me2NCH2C6H4)}SnLi(THF)2] from the reaction of
the corresponding organolithium compound with tin dichlor-
ide. Additional intramolecular Li−N coordination with
replacement of coordinated THF is observed, affording [{κ1-
C-(2-Me2NCH2C6H4)}{κ2-C,N-(2-Me2NCH2C6H4)2}SnLi-
(THF)]. Interestingly, NMR studies showed that these ortho-
chelated arylstannates exist in equilibrium with the neutral
bis(aryl)tin(II) and aryllithium species (see iv in Scheme 1).
3a consists of a tris(aryl)stannate with the tin center in a formal
2+ oxidation state; this anionic unit is linked via a direct Li−Sn
interaction to the lithium cation. This latter cation is further
coordinated by two THF molecules (Figure 1). Two of the
Figure 1. Molecular geometries of 3a and 3b as found in the solid
state. Some selected bond distances (A) and angles (deg) are
presented in Table 1.
RESULTS AND DISCUSSION
three 2-Me2NCH2C6H4 anions are exclusively σ-C−Sn bonded
to the tin(II) center, leaving their respective 2-Me2NCH2
groupings as free, noncoordinating units. The third 2-
Me2NCH2C6H4 anion is bridge bonded, i.e., σ-C−Sn bonded
to the formal tin(II) center, while its 2-Me2NCH2 substituent is
intramolecularly coordinated to the Li center. The observed
Sn−Li bond (2.860(6) Å) is slightly larger than the sum of the
covalent radii (2.75 Å), but is in the range of that observed for
the few known tris(organo)stannate(II) lithium compounds
having a direct Sn−Li interaction, e.g., 2.882(7) Å in
Ph3SnLi(PMDTA) (4),5 2.876(14) Å in But3SnLi(THF)3
(5),23 and 2.809(6) Å in [Ar*Ph2SnLi]2 (Ar* is C6H3-2,6-
Trip2; Trip is C6H2-2,4,6-Pri3) (6).14
■
Synthesis Aspects. The reaction of the diaryltin(II)
compound (2-Me2NCH2C6H4)2Sn (2) with one equivalent of
2-Me2NCH2C6H4Li (1) in THF as solvent (see Scheme 2)
Scheme 2. Synthesis of Tris(aryl)stannates 3
The lithium centers in the respective tris(organo)stannates
have similar, slightly distorted tetrahedral geometries, while the
coordination geometries at the tin centers can be best described
as being trigonal pyramidal, i.e., distorted from tetrahedral, with
the three carbon atoms in the basal plane and the lithium atom
approximately at the apex. For example, in Ph3SnLi(PMDTA)
afforded (after workup; see Experimental Section) the pure bis-
THF stannate adduct (2-Me2NCH2C6H4)3SnLi(THF)2 (3a) as
a yellow, crystalline solid. The reaction of three equivalents of
2-Me2NCH2C6H4Li (1) with one equivalent of SnCl2 in THF
involves a more direct route toward 3a (see Scheme 2). It is
obvious that during the latter reaction 2 is likely formed as an
intermediate.
Table 1. Selected Bond Distances and Angles for 3a and 3b
3a
3b
Distances (Å)
2.860(6)
Li−Sn
2.72(2)
2.24(1)
2.206(9)
2.24(1)
2.15(2)
1.92(2)
2.13(2)
C(1)−Sn
C(2)−Sn
C(3)−Sn
N(1)−Li
O(1)−Li
O(2)−Li
2.249(3)
2.218(3)
2.221(3)
2.117(7)
1.949(6)
1.956(7)
One of the THF molecules in 3a is observed to be weakly
bound. When pure 3a is kept in vacuo for 1 h, the crystalline
material converts into a pale yellow, amorphous powder. The
1H NMR spectrum (in toluene-d8) of this powder confirmed
the formation of (2-Me2NCH2C6H4)3SnLi(THF) (3b), i.e., the
loss of one THF molecule. Pure crystalline 3b can be obtained
by recrystallization from n-hexane (see Experimental Section).
When a solution of 3b (or 3a) in n-hexane is evaporated at 50
°C to dryness at reduced pressure, a yellow, viscous material is
N(2)−Li
Angles (deg)
C(1)−Sn−Li
C(2)−Sn−Li
C(3)−Sn−Li
C(1)−Sn−C(2)
C(1)−Sn−C(3)
C(2)−Sn−C(3)
Sn−Li−N(1)
Sn−Li−O(1)
Sn−Li−O(2)
N(1)−Li−O(1)
N(1)−Li−O(2)
O(1)−Li−O(2)
79.9(1)
128.5(1)
136.2(1)
94.1(1)
94.9(1)
95.1(1)
94.7(2)
115.8(2)
116.2(2)
112.9(3)
108.5(3)
108.1(3)
89.0(5)
101.3(4)
158.8(5)
93.7(4)
99.2(5)
97.6(4)
98.2(7)
120.2(8)
92.2(6)
113.6(9)
106.3(8)
125.3(9)
1
obtained, which according to its H NMR spectrum (toluene-
d8) is devoid of any THF (see Experimental Section). This
material we ascribe to [(2-Me2NCH2C6H4)3SnLi] (3c).
Compounds 3a, 3b, and 3c were all thereafter fully
1
7
characterized by H, 13C, Li, and 119Sn NMR spectroscopy
(see Experimental Section and Supporting Information).
Structural Features of 3a and 3b in the Solid State.
The molecular geometries of both 3a and 3b in the solid state
were established by X-ray crystal structure determinations (see
data deposited at the CCDC).21,22 The core structural motif of
Sn−Li−N(2)
N(2) -Li−O(1)
N(1)−Li−N(2)
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1
7
Table 2. Relevant H, 13C, 119Sn, and Li NMR Data of 2 and 3a−c at 298 K in Toluene-d8
1H
13C
compound
H6
NCH2
NMe2
C1
CH2
NMe2
119Sn
7Li
a
c
b
d
f
2
7.88
7.82
7.87
7.37
7.95
3.52
3.68
3.67
3.63
3.58
2.14
2.03
1.97
2.11
1.67
169.7
161.8
162.1
163.5
162.8
68.1
68.3
68.4
68.3
67.9
45.9
45.6
45.5
45.6
44.2
143.4
−158.1
−157.9
−157.2
−78.2
3a
3b
0.80
0.78
0.34
1.53
e
g
h
i
3b
j
3c
a
b
c
d
e
f
1J(117, 119Sn−13C) 393, 411 Hz. w1/2 22 Hz. 1J(117, 119Sn−13C) 220 Hz, w1/2 35 Hz. w1/2 460 Hz. 1J(117, 119Sn−13C) 231 Hz. w1/2 470 Hz; see
g
h
Figure 2 for the 119Sn and 7Li NMR spectra at 233 K. In THF-d8. At 298 K broad signal, at 248 K, sharp signal, average 1J(117, 119Sn−13C) 276 Hz.
i
j
w1/2 111 Hz. w1/2 725 Hz.
(4) and But3SnLi(THF)3 (5) the (average) C−Sn−C bond
angles around Sn are 96.1(2)° (4) and 102.9(5)° (5), while the
respective (average) C−Sn−Li angles amount to 120.7(2)° (4)
and 115.3(4)° (5).
With respect to complex 3a, a similar such distortion toward
trigonal pyramidal geometry at tin is observed, with an average
C−Sn−C angle of 94.6(1)°. This value points to a low s-
character of the Sn−C bond. However, due to the intra-
molecular N−Li coordination of one of the dimethylamino
substituents (i.e., acting as a monoanionic κ2-C,N chelate), the
lithium atom is pulled away from an idealized apical position.
This is reflected by one very acute C−Sn−Li bond angle
(79.9(1)°) that seems compensated by two larger C−Sn−Li
bond angles (128.5(1)° and 136.2(1)°, respectively).
The structural motif of 3b in the solid state shows large
similarities with that of 3a (see Figure 1). The main difference
between these THF solvates is the replacement of one
coordinating THF molecule in 3a by an intramolecular
coordinating 2-dimethylamino substituent of a second aryl
grouping in 3b. Most likely as a consequence of the presence of
two intramolecular coordinating dimethylamino groups, the
Sn−Li distance in 3b is significantly shorter (Sn−Li 2.72(2) Å)
than that found in 3a. Also for 3b, a distortion toward a trigonal
pyramidal coordination geometry at tin is observed (the
average of the C−Sn−C angles is 96.9(4)°). Two of the C−
Sn−Li angles are rather acute (C(1)− and C(2)−Sn−Li
amount to 89.0(5)° and 101.3(4)°, respectively), whereas the
C(3)−Sn−Li bond angle has opened up to 158.8(5)° in the
direction of an almost linear C−Sn−Li arrangement with Cipso
of the monodentate κ1-C-bonded aryl formal anion.
range of 343 K down to 203 K, the 1H and 13C NMR spectra of
3a and 3b are virtually unchanged. At 203 K, all resonances
start to broaden, and extremely broad lines result at 183 K (the
lowest temperature studied). Thus, even at this low temper-
ature limit, the exchange processes, which involve the exchange
of aryl groupings (vide infra) as well as between coordinated
and free-NMe2 substituents, are still fast on the NMR time
scale. More conclusive evidence concerning the structural
1
features of 3a and 3b could be obtained from their H, 13C,
7
119Sn, and Li NMR spectra (toluene-d8; at RT). The 119Sn
chemical shift values of both 3a (−158.1 ppm) and 3b (−157.9
ppm) are considerably upfield shifted, by about 300 ppm, as
compared to the neutral bis(aryl)tin(II) compound 2 (+143.4
ppm). Note that for the Ph3Sn anion δ 119Sn is −98.4 ppm,9
whereas for Ph2Ar*SnLi a δ119Sn of −117 ppm14 has been
reported. The 119Sn resonances of 3a and 3b are rather broad
(w1/2 = 460 and 470 Hz, respectively) and unresolved, a
1
7
situation that could be the result of unresolved J Li−119Sn
coupling. To our surprise, this appears to contrast the case for
7
3b; at temperatures below 253 K, in both the 119Sn and Li
NMR spectra (toluene-d8), this coupling is nicely resolved
indeed (see Figure 2) and has a measured value of 289 Hz. This
Structural Features of 3a, 3b, and 3c in Toluene-d8.
1
7
The relevant H, 13C, 119Sn, and Li NMR spectroscopic data
for 2 and 3a−c are presented in Table 2.
In terms of solution behavior, a single resonance pattern is
observed for the three 2-Me2NCH2C6H4 groupings in both the
1H and 13C NMR spectra of solutions of 3a and 3b, respectively
(toluene-d8, at RT; see Supporting Information). This indicates
that in toluene all three organic moieties are equivalent on the
NMR time scale, whereas in the solid state there is a clear
difference between these three groups with one κ2-C,N- and
two κ1-C-bonded. The observation of an average
1J(117,119Sn−13C) on Cipso (220 Hz: 3a and 231 Hz: 3b)
indicates that aryl exchange involves an intramolecular process
with retention of the Sn−C interaction. Furthermore, the
observation of a single methyl resonance for the NMe2 groups
indicates that fast exchange occurs between coordinated and
free-NMe2 functionalities. Likewise, the singlet resonance
observed for the CH2 protons points to the occurrence of
processes involving a rapid averaging of the local geometry
about Sn on the NMR time scale. Notably, in the temperature
7
Figure 2. 119Sn and Li NMR spectra of 3b in toluene-d8 at 233 K.
value is close to the value reported for But3SnLi(THF)3 (287
Hz)23 but considerably smaller than that reported for
Ph3SnLi(PMDTA) (412 Hz).5 The observation of 1J is
conclusive evidence that in solution a bonding interaction
between lithium and tin is present in 3b, and most likely also in
3a; the multiplicity of the respective 119Sn and 7Li resonances is
further conclusive evidence for a monomeric solution structure.
As mentioned before, we obtained 3c as a mixture of
products, of which we ascribe the main component (>75%) to
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be THF-free “R3SnLi”, of which relevant 1H, 13C, 119Sn, and 7Li
NMR data are summarized in Table 2. The origin of some of
the other minor components in the mixture is discussed below.
Its 1H and 13C NMR spectra show single resonance patterns at
RT and above, indicating that the three 2-Me2NCH2C6H4
anions in 3c are equivalent. The observed singlet NMe2-methyl
proton resonance (δ = 1.67 ppm) is notable and is considerably
upfield shifted as compared to those in THF-solvated 3a and
3b. It is known24 that coordination of the NMe2 grouping to a
metal center causes such an upfield shift. In the case of 3c, this
upfield shift indicates that all three nitrogen substituents are
involved in N−Li coordination in either an intra- or
intermolecular sense. Furthermore, its respective δ119Sn
Scheme 3. Equilibria Involving THF-Solvated and THF-Free
Tris(aryl)stannates 3 in Toluene
limiting situation, a 119Sn chemical shift in the order as
observed for 3a in THF-d8, i.e., as for R3SnLi(THF)3. At high
temperatures, however, the equilibrium lies completely to the
right-hand side with a limiting 119Sn chemical shift value as
observed for THF-free 3c.
A further and somewhat remarkable observation was made
when a sample of 3b (toluene-d8) was reacted with H2O. It
appeared that a mixture of (2-Me2NCH2C6H4)3SnH (6), (2-
Me2NCH2C6H4)2Sn (2), and the arene Me2NCH2C6H5 in
approximately a 2.5:1:1 molar ratio was formed. The triaryltin
7
(−78.2 ppm) and δ Li (1.53 ppm) are significantly different
from those observed for 3a and 3b (Table 2). In particular, the
latter data suggest that the structure of 3c in toluene solution
must be significantly different from that of 3a or 3b.
1
hydride could be unequivocally identified by its H NMR
spectrum (see Experimental Section) with a telltale Sn−H
Nature of 3a, 3b, and 3c in THF-d8. In contrast to the
temperature dependence of the NMR spectra of 3a and 3b in
toluene-d8, no significant changes are observed in similar
spectra of solutions of both 3a and 3b in THF-d8 over the
whole temperature range studied (298 down to 193 K). The 1H
and 13C NMR spectra of 3b show a single resonance pattern for
the aryl moieties. An exception is the temperature dependence
of the 13Cipso resonance that is rather broad at RT, but at 248 K
becomes a sharp line with a well-resolved average
1J(117,119Sn−13C) coupling (276 Hz). In contrast to the
temperature-dependent 119Sn chemical shifts for 3a and 3b in
toluene-d8, their respective spectra in THF-d8 are rather
temperature independent (see Table 2) in the temperature
range from 298 K (−157.2 ppm) down to 193 K (−160.4
ppm). This might suggest that in neat THF, 3a and 3b may
exist as tris-solvated species, R3SnLi(THF)3, i.e., as complexes
lacking any intramolecular N−Sn coordination.
1
resonance at δH = 6.83 ppm displaying typical J(117,119Sn−1H)
coupling values of 2368 and 2469 Hz, respectively.
Furthermore, the H(6) resonance at δH = 7.45 ppm shows a
2
typical J(117,119Sn−1H) value (52 Hz). The proton-coupled
119Sn spectrum of this mixture clearly indicated the presence of
2 (single line at δSn = 143.5 ppm), while the 119Sn resonance of
(2-Me2NCH2C6H4)3SnH (6) is present at δSn = −213 ppm
1
with as expected a large J(119Sn−1H) value of 2469 Hz, which
is on the same order of magnitude as observed for Ph3SnH.28
The formation of 2 and Me2NCH2C6H5 upon reaction of 3a
or 3b with H2O might be explained by the existence of an
equilibrium between the tris(aryl)stannate, neutral bis(aryl)tin-
(II) compound 2 and the aryllithium-THF species 1, a situation
as suggested in Scheme 4. Interestingly, it is the latter reaction
(to the left-hand side) that we applied for the direct synthesis of
3, as shown in Scheme 2.
Equilibria Involving THF-Solvated and THF-Free Tris-
(aryl)stannates 3. Irrespective of the batch of crystallization,
Scheme 4. Addition of H2O to a Sample of 3b in toluene-d8
1
7
the H, 119Sn, and Li NMR spectra of toluene solutions of 3a
and 3b always showed the presence of some other molecules/
1
materials in quite minor amounts (<2%). In the H NMR
spectra, there is invariably observed additional H(6)-resonance
patterns, one at δ = 7.82 ppm (which we ascribe to the
presence of minor amounts of 2) and another at δ = 8.15 ppm.
This latter resonance occurs at the identical chemical shift value
of the corresponding organolithium compound 1. In apolar
solvents and in the presence of only small amounts of THF,
this organolithium compound exists as a THF-solvated dimer
[2-Me2NCH2C6H4Li(THF)n]2.25−27 Moreover, the 119Sn NMR
spectra of 3a and 3b showed the presence of minor amounts of
Both stannate 3 and aryllithium 1 are highly sensitive to
water but will almost certainly react at different rates.
Obviously, reaction of 1 is causing this equilibrium to shift to
the right side, which explains the formation of equivalent
amounts of 2 and the free arene Me2NCH2C6H5. The product
distribution (2-Me2NCH2C6H4)3SnH (6) versus (2-
Me2NCH2C6H4)2Sn is then dependent on (i) the rate
constants of the equilibrium and (ii) the kinetics of the
hydrolysis of the aryllithium species, leading to Me2NCH2C6H5
versus the kinetics of the conversion of (2-
M e 2 N C H 2 C 6 H 4 ) 3 S n L i ( T H F ) l e a d i n g t o ( 2 -
Me2NCH2C6H4)3SnH.
2D EXSY spectroscopy would be an excellent technique to
study the existence of the equilibrium as shown in Scheme 4 in
more detail. Unfortunately, the concentration of the
components at the right side in the equilibrium mixture is
too low to obtain reliable EXSY spectra. Therefore, we
prepared a sample in which the concentration of 2 was
enhanced by adding pure 2 to a solution of 3b. Similarly,
7
2, while the Li NMR spectra always showed an additional
resonance of low intensity at 2.3 ppm, which is very close to the
known chemical shift value of pure [2-Me2NCH2C6H4Li-
(THF)n]2 dissolved in toluene.25,26
Combining these latter observations, the data of temper-
ature-dependent NMR data for 3a−c and, last but not least, the
knowledge that even in the solid state the THF molecules are
rather weakly bonded leads us to propose the existence of the
sequence of equilibria as shown in Scheme 3.
Such a series of events would explain the large temperature-
dependent 119Sn chemical shifts as observed (toluene-d8) for 3a
and 3b. At 343 K, these values are δSn = −138.1 and −121.9
ppm, respectively, while at 193 K these values rise to −172.3
and −172.2 ppm, respectively. At low temperature, this
equilibrium appears to lie completely to the left with, in the
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Article
1
Figure 3. 2D H−1H EXSY spectra of solutions of pure 3b to which either pure R2Sn, 2, or pure RLi, 1, has been added.
another sample was prepared in which the concentration of the
aryllithium component was enhanced by adding pure 2-
Me2NCH2C6H4Li to a solution of 3b. Each sample was then
studied in detail by 2D 1H−1H EXSY spectroscopy, running the
spectra at various temperatures and with various mixing times
(see Experimental Section). The spectra of the first mixture
confirmed that the 2-Me2NCH2C6H4 groups present in 3b and
2 undergo chemical exchange; see Figure 3a. Similarly, the 2D
EXSY spectra of the second mixture showed clearly that the 2-
Me2 NCH2 C6 H4 groups present in 3b and [2-
Me2NCH2C6H4Li(THF)n]2, 1, are also chemically exchanging;
see Figure 3b. This latter Li−Li exchange phenomenon was
moreover corroborated by the observation of strong cross-
exist in solution as solvent-separated ion pairs. Another type of
stannates has a direct metal to tin bond as in, for example
But3SnLi(THF)3,23 Ph3SnLi(PMDTA),5 (ButCH2)3SnK-
(toluene)3,6 and [Ph2Ar*SnLi]2 (Ar* = C6H3-2,6-Trip2; Trip
= C6H2-2,4,6-Pri).14 They can be considered as close contact
ion pairs.
It is interesting to note that the coordination geometries of
the tin center in the latter stannates, e.g., Ph3SnLi(PMDTA)5
23
(4) and But3SnLi(THF)3 (5) (Li−Sn distances: 4 2.817(7)
and 5 2.876(14) Å), show similar distortions toward a trigonal
pyramidal one (with the lone pair at the apex). This distortion
of geometry has been explained in terms of enhanced p-
character in the tin−carbon bond, caused by the differences in
energy between the s and p orbitals.30 As a consequence, the
lone pair in R3Sn anions and the tin−lithium bond in R3SnLi
species possess an enhanced s-character, i.e., the Sn hybrid-
ization that is more [p3+s] than sp3 with the Sn s orbital being
very important in the Sn−Li interaction. The resulting reduced
s-character and consequently enhanced p-character in the Sn−
C bonds are also reflected in the observation of a relatively
7
peaks in the 2D Li−7Li EXSY spectrum, pointing to the
occurrence of chemical exchange of the lithium atoms of 3b
and 1 (see Experimental Section and Supporting Information).
These spectra, clearly displaying aryl exchange on the NMR
time scale, confirm that in solutions of 3a and 3b an
equilibrium exists between the stannate and the neutral tin(II)
compound and the aryllithum species as suggested in Scheme 4.
1
small J(13C−7Li) coupling in the 13C NMR spectrum of 3a
CONCLUSIONS
(vide infra). Comparison of the structural features of 4 and 5,
which can be considered as parent compounds for stannates 3a
and 3b,31 reveals that the intramolecular N−Sn coordination
considerably affects the mutual orientation of the Sn−Li vector
versus the C3-manifold interaction but has little influence on
the nature of the Sn−Li (e.g., for 3a 2.860(5) Å) interaction.
The variation in the C−Sn−Li bond angle of the κ1-C-bonded
aryl grouping (136.2(13)° in 3a and 158.8(5)° in 3b) is
another indication for the high s-character of the Sn−Li
interaction.
■
In this study we have demonstrated that, in addition to the
synthetic methods i−iii (Scheme 1), all involving reduction of
formally Sn(IV) organometallics, triaryltin lithium compounds
can be directly synthesized through reaction of the correspond-
ing, pure aryllithium compound with SnCl2. An obvious
prerequisite for the success of this route is that the intermediate
bisaryltin(II) species is stable against oligomerization through
(irreversible) Sn−Sn bond formation. In the case of the present
synthesis of stannates (2-Me2NCH2C6H4)3SnLi(THF)3 3, it is
the formation of bis({κ2-C,N-(2-Me2NCH2C6H4)2}Sn (2) as
intermediate along the synthesis route. This diaryltin
compound is effectively stabilized by intramolecular Sn−N
coordination (cf. refs 15a and 15e). Consequently, stannate 3 is
likewise nicely available via route iv (Scheme 1), in which 2 is
reacted with corresponding 2-Me2NCH2C6H4Li in an apolar
solvent. Alternate stabilization can also be achieved when at
least one of the aryl groups contains bulky ortho-substituents, as
is the case for the stannate lithium compounds
Ar*3−nPhnSnLi.14
Removal of the last THF molecule from 3b provides 3c,
which according to NMR spectral data seems to lack a Sn−Li
interaction. A likely possibility for a structure of this material is
provided in Figure 4, which is inspired by the structural features
of the zincate {κ2-C,N-(2-Me2NCH2C6H4)}4ZnLi2 (7; see ref
20 for the structural data in the solid state, in particular the
Two types of stannates are known that have distinctly
different structural features both in the solid state and in
solution. One type consists of separate formal R3Sn anions and
a countercation stabilized by coordination through a crown
ether or polydentate ligand, e.g., [Ph3Sn][K(18-Crown-6)],9,29
[Ph3Sn][Ba(18-Crown-6)(HMPA)2],11 [Cp3Sn][Mg-
(THF)6],13 and [(9-fluorenyl)3Sn][Li(THF)4].13 These species
Figure 4. Schematic representation of the structure in the solid state of
the zincate [κ2-C,N-(2-Me2NCH2C6H4)]4ZnLi2 (7) and the proposed
limiting structure of 3c.
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Cipso−Li distances).33 In the latter compound, each of the Li
cations is four-coordinate, and this is provided by both Li−N
coordination and via interaction with the π-electron density on
(aryl)Cipso of the Cipso−Zn bond. Application of this finding
provides a limiting structure for 3c in which the electron
density of the Sn orbital with high s-character is directed away
from the Sn−Li vector, consistent with the absence of any
117,119Sn−7Li coupling for 3c in toluene solution. In fact, it is
suggested that interaction of the π-electron density on either
(aryl)Cipso center with Li compensates for the lack of
coordinating (THF) solvent molecules as present in 3a and
3b, cf. Scheme 3.
Hz), 128.5 (C(3)), 126.4 (C(5)), 125.5 (C(4)), 68.3 (CH2N), 68.2
(THF), 45.6 (N(CH3)2), 25.6 (THF). 119Sn NMR (149.215 MHz,
7
toluene-d6, 298 K): δ (in ppm) −158.1 (w1/2 460 Hz). Li NMR
1
(155.376 MHz toluene-d6, 298 K): δ (in ppm) 0.80 (w1/2 16 Hz). H
NMR (400.093 MHz, THF-d8, 248 K): δ (in ppm) 7.23 (d, 3H,
H(6)), 7.19 (d, 3H, H(3)), 6.81 (t, 3H, H(4)), 6.57 (t, 3H, H(5)),
3.63 (s, 6H, CH2N), 3.55 (m, 8H, THF), 2.07 (s, 18H, N(CH3)2),
1.71 (m, 8H, THF). 13C NMR (100.614 MHz, THF-d8, 248 K): δ (in
ppm) 165.0 (C(1), 1J(117, 119Sn−13C) 275 Hz), 147.3 (C(2)
2J(117, 119Sn−13C) 37 Hz), 139.9 (C(6)), 126.6 (C(3)), 125.2
(C(5)), 124.6 (C(4)), 67.4 (CH2N), 68.3 (THF), 46.1 (N(CH3)2),
26.4 (THF). 119Sn NMR (149.215 MHz, THF-d8, 248 K): δ (in ppm)
−159.4 (w1/2 18 Hz). 7Li NMR (155.376 MHz, THF-d8, 248 K): δ (in
ppm) −0.10 (w1/2 4 Hz). Anal. Calcd for C35H52LiN3O2Sn: C 62.51,
H 7.79, N 6.25. Found: C 62.37, H 7.78, N 6.21.
EXPERIMENTAL SECTION
(2-Me2NCH2C6H4)3SnLi(THF) (3b). Crystalline 3a (1.3 g, 2 mmol)
■
was kept in vacuo (10−2 mmHg) at room temperature for 1 h, after
All experiments were carried out under a dry, oxygen-free nitrogen
atmosphere using standard Schlenk techniques. Solvents were dried
and distilled prior to use. The starting material 2-[(dimethylamino)-
methyl]phenyllithium (1) was prepared according to a literature
procedure.25 Bis{2-[(dimethylamino)methyl]phenyl}tin (2) was pre-
pared according to a modified (vide infra) procedure reported
earlier.15a Crystallographic data and refinement of the crystal structures
of 3a and 3b were deposited at the Cambridge Crystallographic
Database earlier.21,22 1H, 13C, 119Sn, and 7Li NMR spectra were
recorded on 400 MHz Varian VNMRS400 and Agilent MRF400
spectrometers at ambient temperature unless otherwise stated.
Chemical shifts (δ) are given in ppm relative to Me4Si as an internal
standard (1H and 13C), to Me4Sn in C6D6 (119Sn), or to LiCl (7Li) in
D2O (1 M) as an external standard. Coupling constants are in Hz.
1
which a pale yellow, amorphous powder was obtained. The H NMR
spectrum of this material showed the loss of one THF molecule. The
crude material was recrystallized from n-hexane at −30 °C. The yellow
crystalline material was isolated by decantation, washed with 5 mL of
pentane, and dried in vacuo to afford pure 3b (0.7 g, 58% yield).
1H NMR (400.093 MHz, toluene-d8, 298 K): δ (in ppm) 7.87 (d,
3H, H(6)), 7.25 (d, 3H, H(3)), 7.10 (t, 3H, H(4)), 7.01 (t, 3H, H(5)),
3.67 (s, 6H, CH2N), 3.35 (m, 8H, THF), 1.97 (s, 18H, N(CH3)2),
1.36 (m, 8H, THF). 13C NMR (100.614 MHz, toluene-d8, 298 K): δ
(in ppm) 162.1 (C(1), 1J(117, 119Sn−13C) 231 Hz), 145.8 (C(2)
2J(117, 119Sn−13C) 37 Hz), 140.6 (C(6), 2J(117, 119Sn−13C) 16 Hz),
128.8 (C(3)), 126.5 (C(5)), 125.6 (C(4)), 68.4 (CH2N), 68.3 (THF),
45.5 (N(CH3)2), 25.5 (THF). 119Sn NMR (149.215 MHz, toluene-d8,
1
Complete assignments in H and 13C NMR spectra could be made
7
298 K): δ (in ppm) −157.9 (w1/2 470 Hz). Li NMR (155.376 MHz
from COSY and HMQC spectra.
Elemental analyses were obtained from Kolbe Mikroanalytisches
toluene-d8, 298 K): δ (in ppm) 0.78 (w1/2 16 Hz). Anal. Calcd for
C31H44LiN3OSn: C 62.02, H 7.39, N 7.00. Found: C 61.88, H 7.48, N
7.06.
Laboratorium, Mulheim a.d. Ruhr, Germany.
̈
(2-Me2NCH2C6H4)2Sn (2). To a solution of 2.80 g (20 mmol) of 2-
[(dimethylamino)methyl]phenyllithium in 20 mL of THF was added a
solution of 1.90 g (10 mmol) of SnCl2 in 15 mL of THF during 5 min,
which resulted in the formation of a clear yellow solution. This mixture
was stirred for 30 min, after which all volatiles were removed in vacuo.
The remaining yellow solid material was extracted with 75 mL of warm
(50 °C) n-hexane. The hexane extract was cooled to −30 °C for 12 h.
The pale yellow crystalline material was isolated by decantation,
washed with cold pentane, and dried in vacuo to afford pure 2 (2.5 g,
65% yield).
(2-Me2NCH2C6H4)3SnLi (3c). Compound 3b (500 mg, 0.83
mmol) was dissolved in 10 mL of n-hexane. The solvent was removed
in vacuo at 323 K, yielding a yellow, very viscous material. To this
material again 10 mL of n-hexane was added, affording a clear yellow
solution, and evaporated in vacuo at 323 K. According to its 1H NMR
spectrum the product (3c) did not contain any THF but contained
some contaminations, of which (2-Me2NCH2C6H4)2Sn and
Me2NCH2C6H5 could be identified. Due to its extreme solubility,
purification by recrystallization appeared to be impossible.
1H NMR (400.093 MHz, toluene-d8, 298 K): δ (in ppm) 7.95 (d,
3H, H(6)), 7.09 (m, 9H, H(3), H(4), and H(5)), 3.58 (s, 6H, CH2N),
1.67 (s, 18H, N(CH3)2). 13C NMR (100.614 MHz, toluene-d8, 298
K): δ (in ppm) 162.8 (C(1)), 147.6 (C(2)), 143.5 (C(6)), 130.3,
126.9 (C(3), C(4), and C(5)), 67.9 (CH2N), 44.2 (N(CH3)2). 119Sn
NMR (149.215 MHz, toluene-d8, 298 K): δ (in ppm) −78.2 (w1/2 725
1H NMR (400.093 MHz, benzene-d6, 298 K): δ (in ppm) 7.88 (d,
1H, H(6)), 7.25 (t, 1H, H(5)), 7.15 (t, 1H, H(4)), 7.03 (d, 1H, H(3)),
3.52 (s, 2H, CH2N), 2.14 (s, 6H, N(CH3)2). 13C NMR (100.614
MHz, benzene-d6, 298 K): δ (in ppm) 169.7 (C(1), 1J(117, 119Sn−13C)
2
393, 411 Hz), 146.4 (C(2) J(117, 119Sn−13C) 10 Hz), 136.6 (C(6)
2J(117, 119Sn−13C) 29 Hz), 126.7 (C(3)), 126.4 (C(5)), 126.0 (C(4),
7
Hz). Li NMR (155.376 MHz, toluene-d8, 298 K): δ (in ppm) 1.53.
2
68.1 (CH2N), 45.9 (N(CH3)2, J(117, 119Sn−13C) 32 Hz). 119Sn NMR
Alternative Synthesis of 3a. Pure bis{2-[(dimethylamino)-
methyl]phenyl}tin, 2 (770 mg, 2 mmol), and 280 mg (2 mmol) of
2-[(dimethylamino)methyl]phenyllithium (1) were mixed and dis-
(149.215 MHz, benzene-d6, 298 K): δ (in ppm) 143.4 (w1/2 22 Hz).
(2-Me2NCH2C6H4)3SnLi(THF)2 (3a). To a solution of 2.15 g (15
mmol) of 2-[(dimethylamino)methyl]phenyllithium in 25 mL of THF
was added a solution of 0.95 g (5 mmol) of SnCl2 in 15 mL of THF
during 5 min. The resulting clear yellow solution was stirred for 30
min, after which all volatiles were removed in vacuo. The remaining
yellow solid material was extracted two times with 30 mL of benzene
(removal of LiCl). The combined benzene extracts were evaporated at
reduced pressure, affording a pale yellow, waxy solid. This crude
product was recrystallized from n-hexane containing 5% of THF at
−30 °C. The yellow crystalline material was isolated by decantation,
washed with 10 mL of pentane, and dried for 5 min in vacuo to afford
pure 3a (2.1 g, 63% yield).
1
solved in 5 mL of THF. The H NMR spectrum of this solution
showed the immediate and exclusive formation of 3. The THF was
removed in vacuo at room temperature, and the remaining yellow solid
material was recrystallized from 10 mL of n-hexane containing 5% of
THF. The yellow crystalline material was isolated by decantation,
washed with 5 mL of pentane, and dried in vacuo, affording 800 mg
(59% yield) of pure 3a.
Hydrolysis of 3a. Pure 3a (30 mg) was dissolved in 0.5 mL of
1
benzene-d6 in an NMR tube. The H NMR spectrum of this solution
showed that it contained pure 3a. To this solution was added 10 μL of
degassed H2O, and the reaction mixture was vigorously shaken for 1
min, after which the tube contained a colorless solution and a few
microdroplets of H2O. The 1H NMR spectrum of this solution showed
the presence of (2-Me2NCH2C6H4)3SnH (4) (vide infra), (2-
Me2NCH2C6H4)2Sn (2), and Me2NCH2C6H5 in a 2.5:1:1 molar
ratio. The presence of (2-Me2NCH2C6H4)2Sn in the reaction mixture
was confirmed by the observation of a signal at 146.5 ppm in its 119Sn
1H NMR (400.093 MHz, toluene-d8, 298 K): δ (in ppm) 7.82 (d,
3H, H(6)), 7.31 (d, 3H, H(3)), 7.10 (t, 3H, H(4)), 6.99 (t, 3H, H(5)),
3.68 (s, 6H, CH2N), 3.42 (m, 8H, THF), 2.03 (s, 18H, N(CH3)2),
1.41 (m, 8H, THF). 13C NMR (100.614 MHz, toluene-d8, 298 K): δ
1
(in ppm) 161.8 (C(1), J(117, 119Sn−13C) 220 Hz, w1/2 35 Hz), 145.9
2
2
(C(2) J(117, 119Sn−13C) 39 Hz), 140.1 (C(6), J(117, 119Sn−13C) 15
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NMR spectrum. In an independent experiment in which 10 μL of H2O
was added to 0.5 mL of a solution of pure 2, it was shown that (2-
Me2NCH2C6H4)2Sn is inert toward hydrolysis for at least a few hours.
Data Assigned to (2-Me2NCH2C6H4)3SnH, 6. 1H NMR (400.093
MHz, benzene-d6, 298 K): δ (in ppm) 7.45 (d, 3H, H(6),
2J(117,119Sn−1H 52 Hz), 7.08 (m, 9H, H(3), H(4), and H(5)), 6.83
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■
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(s, 1 H, SnH, J(117.119Sn−1H) 2368, 2469 Hz), 3.33 (s, 6H, CH2N),
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1
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1
7
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7
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ASSOCIATED CONTENT
■
S
* Supporting Information
(15) (a) Angermund, K.; Jonas, K.; Kruger, C.; Latten, J. L.; Tsay, Y.-
̈
1
7
The relevant H, 13C, 119Sn, and Li NMR spectroscopic data
for 2 and 3a−c are presented as well as a Mercury
representation of the molecular structures of 3a and 3b. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
(16) (a) Grutzmacher, H.; Pritzkow, H.; Edelmann, F. T.
Organometallics 1991, 10, 23−25. (b) Tajima, T.; Takeda, N.;
Sasamori, T.; Tokitoh, N. Organometallics 2006, 25, 3552−3553.
(c) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P.
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Fettinger, J. C.; Power, P. P. Z. Anorg. Allg. Chem. 2006, 632, 1005−
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125, 7520−7521. (f) Eichler, B. E.; Power, P. P. Inorg. Chem. 2000, 39,
5444−5449.
*E-mail: g.vankoten@uu.nl.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
(17) Atwood, J. L.; Hunter, W. E.; Cowley, A. H.; Jones, R. A.;
Stewart, C. A. J. Chem. Soc., Chem. Commun. 1981, 925−927.
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Jastrzebski, J. T. B. H.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc.
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ACKNOWLEDGMENTS
■
Professor A. L. Spek and Dr. H. Kooijman (Utrecht University)
are thanked for the X-ray crystallographic data. Mrs. M. E.
Bruin (MSc) and Dr. D. M. Grove are thanked for helpful
advice and experimental support.
(19) van Koten, G.; Jastrzebski, J. T. B. H.; Niemann, N. C.; Stam, C.
H. J. Am. Chem. Soc. 1984, 106, 1880−1881.
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DEDICATION
■
Dedicated to Professor Mike Lappert in honor of his excellent
and inspiring contributions to the field of organometallic
chemistry.
2606
DOI: 10.1021/om5012215
Organometallics 2015, 34, 2600−2607

Organometallics
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7
based on variable-temperature Li and 13C NMR data, a temperature-
dependent equilibrium exists between this compound and a dimer in
which two Li(THF)2 cationic moieties are bridging via Ge−Li bonds
between two (2-Me2NC6H4)3Ge anionic groups. Also in the Si−Si
33
linked dimer [(2-Me2NCH2C6H4)2SiLi(THF)2]2 the two (2-
Me2NCH2C6H4)2SiLi(THF)2 units show large structural similarities
to that observed in 3a.
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2607
DOI: 10.1021/om5012215
Organometallics 2015, 34, 2600−2607