Reaction between tBuMgCl and SnCl4, illustrating the solvent-dependent predominant formation of Cl(tBu2Sn) nCl (THF, n = 1; Toluene, n = 2; Hexane, n = 3) and the subsequent wavelength-selective photochemical transformation of n = 3 → 2 → 1

Article abstract of DOI:10.1021/om200548s

The addition of THF solutions of ^tBuMgCl to SnCl₄ in varying solvents (S) results in a dramatic change of the predominant product obtained Cl(^tBu₂Sn)_nCl (n = 1-3). For S = THF the product obtained was ^tBu₂SnCl₂, S = toluene, benzene yields Cl(^tBu₂Sn)₂Cl, while for S = hexane the tristannane Cl(^tBu₂Sn)₃Cl was isolated in 45% yield. The last compound is photochemically sensitive when irradiated at 350 nm to form Cl(^tBu₂Sn)₂Cl. Irradiation of this latter material with a 254 nm lamp results in the formation of ^tBu₂SnCl₂.

Full text of DOI:10.1021/om200548s

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pubs.acs.org/Organometallics
Reaction between tBuMgCl and SnCl4, Illustrating the
Solvent-Dependent Predominant Formation of Cl(tBu2Sn)nCl (THF,
n = 1; Toluene, n = 2; Hexane, n = 3) and the Subsequent
Wavelength-Selective Photochemical Transformation of n = 3 f 2 f 1
Hemant K. Sharma, Alma Miramontes, Alejandro J. Metta-Maga~na, and Keith H. Pannell*
Department of Chemistry, The University of Texas at El Paso, El Paso, Texas 79968, United States
S Supporting Information
b
t
ABSTRACT: The addition of THF solutions of BuMgCl to SnCl4 in
varying solvents (S) results in a dramatic change of the predominant product
obtained Cl(tBu2Sn)nCl (n = 1ꢀ3). For S = THF the product obtained was
tBu2SnCl2, S = toluene, benzene yields Cl(tBu2Sn)2Cl, while for S =
hexane the tristannane Cl(tBu2Sn)3Cl was isolated in 45% yield. The last
compound is photochemically sensitive when irradiated at 350 nm to form
Cl(tBu2Sn)2Cl. Irradiation of this latter material with a 254 nm lamp results
in the formation of tBu2SnCl2.
atenated group 14 compounds exhibit a σꢀσ* delocaliza-
tion along the elementꢀelement backbone which imparts
Furthermore, there is some suggestion that increasing the solvent
polarity can enhance SnꢀSn bond formation; however, in these
reports the reaction temperatures varied considerably.12b Our
interest in the transition-metal-catalyzed dehydrogenative for-
mation of SnꢀSn bonds13 has led to a need for simple difunc-
tional oligostannane precursors, e.g. HtBu2SnꢀSntBu2H; thus,
we have reinvestigated the Uhlig synthesis for the production of
2. We now report that a simple solvent change not only
dramatically improves upon the 5% yields but can completely
alter the product formation.
C
interesting electronic and optical properties with progressive red
shifts in the UV/vis absorption maximum for increasing chain
lengths.1ꢀ4 Silicon chains of 6ꢀ18 atoms5 and germanium
analogues containing 4ꢀ6 atoms6 have been systematically
designed, whereas the related tin chains have received less
attention due to the lability of SnꢀC and SnꢀSn bonds
(which can often lead to scrambling reactions) and a lack of
simple synthetic procedures for SnꢀSn bond formation. Salt-
elimination reactions have been used to synthesize chains up to 6
tin atoms;7,8 however, these reactions give side products due to
cleavage of the SnꢀSn bond, resulting in purification problems.
Thermal treatment of monostannanes readily yield distannanes
along with oligostannanes,9a and additionally ring-opening treat-
ment of octa-tert-butylcyclotetrastannane (1) with I2 results in
the formation of 1,4-diiodoocta-tert-butyltetrastannane.9b
Theroleofsolventsinsalt-eliminationreactionshasbeenstudied
a great deal,14 and we recently noted that for the chemistry of the
transition-metal salt [(η5-Me3SiC5H4)Fe(CO)(PPh3)]ꢀNa+
changing from THF to hexane had a dramatic impact upon solution
ion pairing and reaction product outcomes.15 Replacing THF as the
solvent for SnCl4 with toluene in the Uhlig procedure resulted in a
t
dramatic suppression of the formation of Bu2SnCl2 (3) and
The Sita group designed an interesting but multistep
methodology,10 and more relevant to this report, the Uhlig
group reported the synthesis of 1,2-dichlorotetra-tert-butyldis-
increased the isolated yield of 1,2-dichlorotetra-tert-butyldistannane
(2) to >25%. There are still significant amounts of 1 (∼30%)
formed; however, this material is very insoluble and precipitates out
of solution, so that no separation problems are encountered. The
119Sn NMR of the soluble crude reaction products is depicted in
Figure 1a and illustrates the clean nature of the solution containing
only 3 (55.2 ppm) and 2 (110.5 ppm).
Replacement of toluene with benzene results in the crude
product distribution noted in Figure 1b, illustrating two new low-
intensity signals at 114.9 ppm and ꢀ0.8 ppm in addition to the
major and minor signals at 110.5 and 55.2 ppm attributed to 2 and
3, respectively. This polar solvent clearly results in a further increase
t
tannane (2) from the reaction between BuMgCl and SnCl4
using THF as solvent (eq 1). The overall yield was 5% based
upon starting SnCl4.11
THF
f
t
tBuMgCl þ SnCl4
cyclo-ð Bu2SnÞ4
1
t
t
þ Clꢀ Bu2SnꢀSntBu2ꢀCl þ Bu2SnCl2
ð1Þ
2
3
The limited formation of distannanes R3SnꢀSnR3 from
t
t
reactions between R3SnCl and BuLi or BuMgCl has been
previously observed and tentatively explained via metalꢀhalogen
exchange reactions involving transient R3SnLi compounds.12
Received: July 5, 2011
Published: August 05, 2011
r
2011 American Chemical Society
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|

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in yield of 2at the expense of 3, but thetwonew119Sn resonances at
114.9 and ꢀ0.8 ppm signify the formation of a new product.
Decreasing the polarity of the solvent even further by use of
hexane clarified the situation and resulted in the crude NMR
spectrum illustrated in Figure 1c, exhibiting only the resonances
associated with the minor product observed using benzene. Pur-
ification of the crude material depicted in Figure 1c via recrystalliza-
tion from hexanes resulted in the isolation of 1,3-dichlorohexa-tert-
butyltristannane (4) in 45% isolated yield (eq 2).16 No 2 and only
trace amounts of 3 were formed in this reaction. The new results
clearly demonstrate that reducing the solvent polarity enhances
SnꢀSn bond formation, a result directly opposite to that tentatively
suggested by an earlier report noted above.12b
Tristannane 4 was characterized by elemental analysis, NMR
spectroscopy, and single crystal X-ray diffraction.17 The 119Sn NMR
spectrum of 4 exhibited the two signals noted in Figure 1c at 114.9
1
and ꢀ0.8 ppm assigned to the terminal (119Snꢀ119/117Sn, J =
1514/1472 Hz) and central tin atoms, respectively. The low-field
signal has a chemical shift value comparable to the tin signal
1
observed for 2 at 110.5 ppm.11 The H NMR spectrum of 4
exhibited two singlets at 1.36 and 1.44 ppm due to the methyl
t
protons of the Bu groups of the terminal and internal tins,
respectively. In the 13C NMR spectrum two distinct reso-
nances were observed at 31.7 and 34.8 ppm for the methyl
carbons of the tert-butyl groups attached to the terminal and
internal tins and resonances at 35.6 and 39.1 ppm were
assigned to ipso carbons of the tert-butyl groups.
hexane
t
tBuMgCl þ SnCl4 f cyclo-ð Bu2SnÞ4
The structure of 4 is illustrated in Figure 2. Selected bond
lengths and angles are given in Table 1.
þ Clꢀ Bu2SnꢀSntBu2ꢀSntBu2ꢀCl
ð2Þ
t
4
In the unit cell the two terminal tin atoms are related by a C2
axis of rotation and all tin atoms exhibit distorted-tetrahedral
geometry with the SnꢀSnꢀSn angle being the largest. The
SnꢀSn distance of 2.8681(6) Å is slightly longer than the SnꢀSn
distance of 2.829(1) Å observed in 211 but significantly shorter
than the SnꢀSn bond distance of 2.966(3)Å in octa-tert-butyl-
tristannane, tBu3SnꢀSntBu2ꢀSntBu3, due to the substitution of
t
two Bu groups by chlorine atoms.18 Overall the SnꢀSn bond
Figure 1. 119Sn NMR spectra of the crude reaction mixture in (a)
toluene, (b) benzene, and (c) hexane.
distance is longer than the literature median value of 2.805
Å,7,19,20 and the SnꢀCl bond distance of 2.4005(8)Å is also
slightly longer than SnꢀCl distance of 2.393(1) Å observed in
2.11 The two chlorine atoms are oriented in a gauche conforma-
tion with a torsion angle of 107°. No inter- or intramolecular
interactions between Sn and Cl atoms were observed.
The new tristannane 4 is stable at ambient temperature in the
solid state; however, it is very susceptible to photochemical
irradiation in solution. A typical irradiation using a 350 W
medium-pressure Hg lamp (wavelength output range
700ꢀ200 nm) to irradiate a benzene solution in a quartz tube
was monitored by 119Sn NMR and illustrated a clean removal of 4
along with concomitant formation of 2, which finally also
disappeared with final formation of 3 (Figure A in the Supporting
Information). Independent photolysis of 2 under the same
conditions led, as expected, to the formation of 3 (eq 3).
hν
t
Clꢀ Bu2SnꢀSntBu2ꢀSntBu2ꢀCl f
Figure 2. Molecular structure of ClꢀSntBu2ꢀSntBu2ꢀSntBu2ꢀCl.
hν
t
t
Clꢀ Bu2SnꢀSntBu2ꢀCl f Clꢀ Bu2SnꢀCl
ð3Þ
The ellipsoids are drawn at the 35% probability level.
Table 1. Select Bond Lengths (Å), Angles (deg), and Torsion Angles (deg) for ClꢀSntBu2ꢀSntBu2ꢀSntBu2ꢀCla
Sn(1)ꢀSn(2)
Sn(2)ꢀSn(1)#1
Sn(1)ꢀCl(1)
Sn(1)ꢀC(1)
2.8681(6)
2.8681(6)
2.4005(8)
2.223(2)
Sn(1)ꢀC(5)
Sn(2)ꢀC(9)
Sn(2)ꢀC(9)#1
2.202(2)
2.225(2)
2.225(2)
Cl(1)ꢀSn(1)ꢀC(5)
Cl(1)ꢀSn(1)ꢀC(1)
Cl(1)ꢀSn(1)ꢀSn(2)
C(1)ꢀSn(1)ꢀSn(2)
C(5)ꢀSn(1)ꢀSn(2)
C(5)ꢀSn(1)ꢀC(1)
100.47(7)
99.64(7)
C(9)#1ꢀSn(2)ꢀC(9)
C(9)#1ꢀSn(2)ꢀSn(1)
C(9)ꢀSn(2)ꢀSn(1)
Sn(1)ꢀSn(2)ꢀSn(1)#1
113.66(14)
105.89(7)
107.47(7)
116.71(2)
106.408(18)
119.35(7)
113.50(6)
114.07(9)
Cl(1)ꢀSn(1)ꢀSn(2)ꢀSn(1)#1
ꢀ57.204(19)
a Symmetry transformations used to generate equivalent atoms: (#1) ꢀx, y, ꢀz + 1/2.
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COMMUNICATION
of the 285 nm band of 4 (10ꢀ5 M/L) during irradiation (Figure B),
and a CIF file giving crystallographic data for 4. This material is
available free of charge via the Internet at http://pubs.acs.org.
’ ACKNOWLEDGMENT
This work was supported by the Welch Foundation, Houston,
TX (Grant AH-0546), and the NIH-MARC U-STAR program.
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THF than in methanol (Figure B in the Supporting In-
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secondary rationale for the absence of 4 in the THF reaction
products; however, in a single experiment we could not observe
t
the formation of 4 when the reaction between BuMgCl and
SnCl4 (in only THF) was performed in the dark. Studies on the
photochemistry of Cl(tBu2Sn)nCl and related compounds, in-
cluding trapping of the expected stannylene, [tBu2Sn], are
continuing.
’ ASSOCIATED CONTENT
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b
cedures, figures detailing the photolysis of 4 in a quartz tube
monitored by 119Sn NMR spectra (Figure A) and disappearance
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