R3E+ carborane salts of the heavier group 14 elements: E = Ge, Sn, and Pb

Article abstract of DOI:10.1021/om100662r

The synthetic and structural chemistry of catalytically useful trialkylsilylium salts with weakly coordinating carborane anions, R ₃Si(CHB₁₁X₁₁), has been extended to the heavier group 14 elements. Et₃Ge(CHB₁₁H₅Br ₅) was prepared from Et₃GeH and trityl CHB ₁₁H₅Br₆^-. Its X-ray crystal structure shows ion-like character very similar to its Si congener. The heavier element analogues, Et₃E(CHB₁₁H₅Br₆) (E = Sn, Pb), were prepared by chloride ion abstraction from Et₃SnCl and Et₃PbCl, respectively, using Et₃Si(CHB ₁₁H₅Br₆). Their crystal structures differ from those of the four-coordinate lighter elements by adopting five-coordinate trigonal-bipyramidal stereochemistries, reflecting the periodic table transition of these elements from semi-metals to metals. The carborane anions are weak, bridging, axial ligands connecting trigonal Et₃E⁺ cation-like moieties in polymeric chain structures. When the less coordinating CHB₁₁Cl₁₁^- anion is used, excess germane competes with the carborane anion for coordination to Ge and a salt of the new hydride-bridged cation [Et₃Ge-H-GeEt₃]⁺ can be isolated. It has a distinctive αGeHGe IR band at ca. 1740 cm^-1 analogous to the 1875 cm^-1 band of the [Et₃Si-H-Si-Et ₃]⁺ cation. Comparable chemistry is not observed for the Sn and Pb congeners.

Full text of DOI:10.1021/om100662r

4066 Organometallics 2010, 29, 4066–4070
DOI: 10.1021/om100662r
R3Eþ Carborane Salts of the Heavier Group 14 Elements:
E = Ge, Sn, and Pb
James H. Wright II, Gabriel W. Mueck, Fook S. Tham, and Christopher A. Reed*
Department of Chemistry, University of California, Riverside, California 92521-0403
Received July 14, 2010
The synthetic and structural chemistry of catalytically useful trialkylsilylium salts with weakly coordi-
nating carborane anions, R3Si(CHB11X11), has been extended to the heavier group 14 elements.
Et3Ge(CHB11H5Br5) was prepared from Et3GeH and trityl CHB11H5Br6-. Its X-ray crystal struc-
ture shows ion-like character very similar to its Si congener. The heavier element analogues,
Et3E(CHB11H5Br6) (E = Sn, Pb), were prepared by chloride ion abstraction from Et3SnCl and
Et3PbCl, respectively, using Et3Si(CHB11H5Br6). Their crystal structures differ from those of the
four-coordinate lighter elements by adopting five-coordinate trigonal-bipyramidal stereochemistries,
reflecting the periodic table transition of these elements from semi-metals to metals. The carborane
anions are weak, bridging, axial ligands connecting trigonal Et3Eþ cation-like moieties in polymeric
-
chain structures. When the less coordinating CHB11Cl11 anion is used, excess germane competes
with the carborane anion for coordination to Ge and a salt of the new hydride-bridged cation
[Et3Ge-H-GeEt3]þ can be isolated. It has a distinctive νGeHGe IR band at ca. 1740 cm-1
analogous to the 1875 cm-1 band of the [Et3Si-H-Si-Et3]þ cation. Comparable chemistry is not
observed for the Sn and Pb congeners.
Introduction
short of trigonal planarity. Some might call this ionic. While
not quite free silylium ions, these ion-like species behave like
silylium ions. They are electrophiles par excellence, and
research emphasis on silylium ions has now shifted from
debate over their existence to exploitation of their reactivity.7
For reasons of increasing size and metallic character, it is
expected that R3Eþ ions will become progressively less “free”
when descending group 14. The closest approach to trialkyl
R3Eþ cations (E = Ge, Sn, Pb) has been made with tri-
n-butyl- and trimethyl cationic moieties coupled with per-
fluorinated tetraphenylborate8 or permethylated carborane
Of all group-related elements in the periodic table, those of
group 14 have perhaps the greatest opportunity to show
diverse behavior. C is a nonmetal, Si and Ge are semi-metals,
and Sn and Pb are metals. Trigonal-planar trialkyl R3Eþ
cations are stabilized in R3Cþ carbocations by hyperconju-
gation and can be put in a bottle and characterized by X-ray
crystallography when weakly coordinating carboranes,
-
CHB11R5X6 (Figure 1), are used as counterions.1 On the
other hand, three-coordinate trialkylsilylium ions, R3Siþ,
are unknown in condensed phases because hyperconjugative
stabilization is weaker and silicon is both larger and more
electropositive than carbon.2 It is difficult to prevent coordi-
nation of a solvent molecule to Si, and there is no anion
sufficiently weakly coordinating to yield a truly free R3Siþ
ion, unless of course the R group is switched to a bulky
aryl substituent.3 Nevertheless, “ion-like” trialkylsilyl species
R3SiδþYδ- are well known with weakly coordinating
anions,2,4,5 and the closest approach to date uses a fluori-
nated carborane anion in Me3Si(EtCB11F11).6 The sum of
the C-Si-C angles in the Me3Si moiety is 354.4°, only ca. 6°
-
CB11Me12 anions.9,10 The solid-state structures with per-
methylated carborane anions have been deduced from X-ray,
EXAFS, and DFT calculations to favor five-coordination
via significant coordination of methyl groups from the
alkane-like anion. The tendency of these heavier R3Eþ
cations to behave as metals and expand their coordination
number overrides any vestige of nonmetallic sp2 behavior. It
is hard to imagine the existence of a three-coordinate trialkyl
cation with Ge, Sn, or Pb in a condensed phase, although
free, or nearly free, cations have been well established with
trialkylsilyl11 and bulky aryl12 substituents. However, these
*To whom correspondence should be addressed. E-mail: chris.reed@
ucr.edu.
(7) Reed, C. A. Acc. Chem. Res. 2010, 43, 121–128.
(8) Lambert, J. B.; Kuhlmann, B. J. Chem. Soc., Chem. Commun.
1992, 931–932.
(9) Zharov, I.; King, B. T.; Havlas, Z.; Pardi, A.; Michl, J. J. Am.
Chem. Soc. 2000, 122, 10253–10254.
(10) Zharov, I.; Weng, T.-C.; Orendt, A. M.; Barich, D. H.; Penner-
Hahn, J.; Grant, D. M.; Havlas, Z.; Michl, J. J. Am. Chem. Soc. 2004,
126, 12033–12046.
(1) Kato, T.; Reed, C. A. Angew. Chem., Int. Ed. 2004, 43, 2908–2911.
(2) Reed, C. A. Acc. Chem. Res. 1998, 31, 325–332.
(3) Kim, K.-C.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.;
Lin, L.; Lambert, J. B. Science 2002, 297, 825–827.
(4) Lambert, J. B.; Kania, L.; Zhang, S. Chem. Rev. 1995, 95, 1191–
1201.
(5) Mueller, T. Adv. Organomet. Chem. 2005, 53, 155–215.
(6) Kuppers, T.; Bernhardt, E.; Eujen, R.; Willner, H.; Lehmann,
C. W. Angew. Chem., Int. Ed. 2007, 46, 6346–6349.
(11) Lee, V. Y.; Sekiguchi, A. Acc. Chem. Res. 2007, 4, 410–419.
(12) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.; Kuhlmann, B.
J. Am. Chem. Soc. 1999, 121, 5001–5008.
r
pubs.acs.org/Organometallics
Published on Web 08/30/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 18, 2010 4067
the group. Thus, as it does with Et3SiH, trityl ion readily
abstracts hydride from Et3GeH at room temperature (eq 1)
to yield triphenylmethane and the Et3Geþ moiety. A non-
donor, weakly basic solvent such as o-dichlorobenzene must
be used to prevent solvent coordination to the Ge center.
Et3GeH þ ½Ph3Cþꢀ½CHB11H5Br6 ꢀ f Ph3CH
-
þ Et3GeðCHB11H5Br6Þ
ð1Þ
The analogous reactions with Et3SnH and Et3PbH are
experimentally problematic. Color changes with Et3SnH
suggest complex redox chemistry is occurring rather than
hydride abstraction, and the low thermal stability of lead
hydrides18 makes room-temperature reactions impossible.
Instead, the preferred method is to use a silyl carborane as a
chloride rather than hydride acceptor (eqs 2 and 3):
Figure 1. Schematic representation of the icosahedral CB11-based
-
carborane anions used in this work: CHB11H5Br6 (red = H,
green=Br) and CHB11Cl11- (red, green=Cl).
Et3SnCl þ Et3SiðCHB11H5Br6Þ f Et3SiCl
substituents are less useful than simple alkyls for reactivity
and catalytic studies.
þ Et3SnðCHB11H5Br6Þ
ð2Þ
ð3Þ
We have become interested in developing the systematic
chemistry of trialkylgermyl, -stannyl, and -plumbyl cation-
like species with halogenated carborane anions in order to
make comparisons to their silicon congeners. R3Si(carborane)
species are effective new catalysts for the room-temperature
polymerization of cyclo-hexachlorotriphosphazene in solution13
and the hydrodefluorination of fluorocarbons.14 In the latter
case, carborane anions are the only anions sufficiently inert
to survive the extremely electrophilic conditions.
Herein, we report the synthesis, isolation, and X-ray crystal
structures of the heavier group 14 element Et3E(carborane)
compounds (E = Ge, Sn, and Pb) using halogenated carbo-
rane anions (Figure 1). Ethyl substituents on the central
element E and the hexabrominated carborane anion CHB11-
H5Br6- were initially chosen for these studies so that direct
structural comparisons could made to the previously X-ray
characterized silyl analogue Et3Si(CHB11H5Br6).15 When
coupled with the less basic16 undecachlorinated carborane
anion, CHB11Cl11-, the Ge species is vulnerable to forma-
tion of the dimeric hydride-bridged [R3Ge-H-GeR3]þ
cation in the presence of excess R3GeH. This unusual μ-hydride
is analogous to the [R3Si-H-SiR3]þ[CHB11Cl11]- salts that
were discovered when silyl cation formation is performed in
the presence of excess silane and the very weakest coordinat-
ing anions.17 Attempts to prepare the corresponding hydride-
bridged cations with Sn failed.
Et3PbCl þ Et3SiðCHB11H5Br6Þ f Et3SiCl
þ Et3PbðCHB11H5Br6Þ
All three Et3E(CHB11H5Br6) compounds (E=Ge, Sn, Pb)
were isolated as colorless solids in reasonable unoptimized
yield (45-80%). As shown in the Supporting Information
Figure S7, IR spectroscopy identifies the νE-C peak of the
Et3Eþ moiety at 531, 510, and 451 cm-1 for E=Ge, Sn, and Pb,
respectively. The corresponding peak for νSiC is 570 cm-1
.
Only the germyl carborane had sufficient solubility in non-
donor solvents for characterization by NMR spectroscopy,
and the 1H results in o-dichlorobenzene are as expected. Ge
does not have a suitable NMR nucleus to explore the relation-
ship between downfield chemical shift and the degree of
germylium ion character that was so useful in Si chemistry.
Thus, crystals for X-ray analysis were sought in each case
and were typically grown directly from reaction mixtures
by layering o-dichlorobenzene solutions with hexanes. Also
included is a fourth structure: that of the Et3Snþ cation coup-
led with the somewhat less coordinating undecachloro carbor-
ane anion. This compound was prepared in the same manner
as eq 3 using Et3Si(CHB11Cl11) as the halide-abstracting
agent.
X-ray Structures of Et3E(CHB11H5Br6) for E = Ge, Sn,
and Pb. True to the trend in the periodic table that makes the
covalent radius of Ge relatively close to Si (filling of the 3d
orbitals) and the radius of Sn relatively close to that of Pb
(filling of the 4f orbitals, i.e., the lanthanide contraction), the
structure of Et3Ge(CHB11H5Br6) is very similar to its Si
congener, while that of Sn is very similar to Pb. As shown in
Figure 2, Et3Ge(CHB11H5Br6) has a flattened tetrahedral,
four-coordinate geometry and is molecularly discrete. On the
other hand, the Sn and Pb analogues have five-coordinate
trigonal-bipyramidal structures with bridging carborane
anions, giving rise to polymeric structures. This explains
their relative solubilities. Only the molecularly discrete Si
and Ge derivatives have significant solubility in chlorocar-
bon solvents. The five-coordinate structures of the heavier
elements have stereochemistries that are related to bis-alkene
Results and Discussion
Synthesis of R3E(CHB11H5Br6) for E=Ge, Sn, and Pb. Of
the routes available to synthesize ion-like R3Eþ species for
the heavier group 14 elements, electrophilic removal of
hydride from R3EH using an analogous lighter element
cation (eq 1) is one of the most reliable. This method exploits
the generally decreasing E-H bond strength going down
(13) Zhang, Y.; Huynh, K.; Manners, I.; Reed, C. A. Chem. Commun.
2008, 494–496.
(14) Douvris, C.; Ozerov, O. V. Science 2008, 321, 1188–1190.
(15) Xie, Z.; Manning, J.; Reed, W. R.; Mathur, R.; Boyd, P. D. W.;
Benesi, A.; Reed, C. A. J. Am. Chem. Soc. 1996, 118, 2922–2928.
(16) Stoyanov, E. S.; Kim, K.-C.; Reed, C. A. J. Am. Chem. Soc.
2006, 128, 8500–8508.
(17) Hoffmann, S. P.; Kato, T.; Tham, F. S.; Reed, C. A. Chem.
(18) Becker, W. E.; Cook, S. E. J. Am. Chem. Soc. 1960, 82, 6264–
Commun. 2006, 767–769.
6265.

4068 Organometallics, Vol. 29, No. 18, 2010
Wright et al.
Figure 2. X-ray structure of Et3Ge(CHB11H5Br6) (H atoms
omitted for clarity).
Figure 4. X-ray structure of Et3Pb(CHB11H5Br6) (H atoms
omitted for clarity).
-
interactions from Br atoms of the CHB11H5Br6 anion,
giving rise to trigonal-bipyramidal coordination. The axial
interactions to the carborane anions are relatively weak and
suggest distinctly ion-like structures: [Et3Eδþ][CHB11H5Br6δ-].
The two axial Sn-Br distances of 2.965 and 3.074 A are ca.
0.5 A longer than that in Ph3SnBr (2.491 A),24 and the Pb-Br
distances of 3.117 and 3.158 A in Et3Pb(CHB11H5Br6) are
ca. 0.3 A longer than in a typical covalent Pb-Br bond such
as that in Ph3PbBr (2.852 A).25 As expected, these long bonds
are nevertheless considerably shorter than the sum of the van
der Waals radii (4.02 and 3.87 A for Sn and Pb, respectively).
The Pb-coordinated B-Br bonds of the carborane anion
(1.973 and 1.963 A) are marginally elongated compared to
the noncoordinated B-Br bonds (1.943-1.954 A). This
distortion of the anion is not significantly different from
that in the Sn structure, where the coordinated B-Br dis-
tances are 1.975 and 1.966 A and the noncoordinated ones
are 1.940-1.947 A.
Figure 3. X-ray structure of Et3Sn(CHB11H5Br6) (H atoms
omitted for clarity).
complexes of trialkyl Sn and Ge cations19 and base-stabilized
triaryl tin cations.20,21
Like the Si analogue, the Ge structure is not fully ionic.
Nevertheless, the Et3Geþ moiety is clearly only weakly coordi-
nated to the carborane anion and shows a close approach
toward trigonal planarity. The coordinated B-Br bond
(1.994 A) of the anion is detectably elongated relative to
the range of uncoordinated B-Br bonds (1.937-1.955 A).
The sum of the C-Ge-Canglesis351.9°,ca.8° shy of planarity.
One additional X-ray structure has been determined in
connection to this work, that of Et3Sn(CHB11Cl11). As
expected, when the central atom is a metal, it also forms a
carborane anion-bridged polymeric structure with trigonal-
bipyramidal coordination at Sn, but there is one difference.
The anion bridges through the 7,9 positions rather than the
more common 7,12 positions. The reason probably lies in
the predominance of crystal-packing forces over site basic-
ity, as seen in the X-ray structure of the carborane acid
H(CHB11Cl11).26 Although the 12 position is calculated to be
more basic than the 7-11 positions, the difference is small
and the solid-state structure of the acid shows protonation
via Cl atoms in the 7-11 positions.
ꢀ
ꢀ
ꢀ
The Ge-Br distance of 2.535(2) A is ca. 0.21 A longer than
the covalent Ge-Br bond in Ph3GeBr (2.317 A)22 but very
ꢀ
much shorter than the sum of the van der Waals radii (3.85 A).
The average Ge-C bond length is 1.943 A. In the corre-
sponding Si compound,23 the sum of the C-Si-C angles is
ꢀ
349.0° and the Si-Br bond lengthening is ca. 0.23 A. The close
parallel between Si and Ge is striking.
There is also a close parallel between Sn and Pb in their
polymeric Et3E(CHB11H5Br6) structures (Figures 3,4). Indeed,
they are isomorphous and crystallize in the same space
group. The sum of the C-E-C angles is very close to 360°
in both cases, but this should not be taken as an indication
of Et3Eþ ionicity because it reflects nearly equivalent axial
The undecachloro carborane anion is less basic16 and less
coordinating17 than the hexabromo analogue, so a somewhat
more ionic complex might be expected. The average Sn-C
bond length of 2.136(2) A in the undecachloro compound
is nominally shorter than that in the hexabromo complex
(2.140(2) A), but this is barely statistically significant. The
average axial Sn-Cl distance of 2.991 A is 0.45 A longer than
(19) Yang, Y.; Panisch, R.; Bolte, M.; Meuller, T. Organometallics
2008, 27, 4847–4853.
(20) Sarazin, Y.; Coles, S. J.; Hughes, D. L.; Hursthouse, M. B.;
Bochmann, M. Eur. J. Inorg. Chem. 2006, 3211–3220.
ꢁ ꢀ
ꢀ
ꢀ ꢀ ꢁ
sarova, I.; Holecek, J.;
(21) Kasna, B.; Jambor, R.; Dostal, L.; Cı
´
´
ꢁ
Stı
br, B. Organometallics 2006, 25, 5139–5144.
(24) Preut, H.; Huber, F. Acta Crystallogr. 1979, B35, 744.
(25) Preut, H.; Huber, F. Z. Anorg. Allg. Chem. 1977, 435, 234.
(26) Stoyanov, E. S.; Hoffmann, S. P.; Juhasz, M.; Reed, C. A. J. Am.
Chem. Soc. 2006, 128, 3160–3161.
(22) Preut, H.; Huber, F. Acta Crystallogr. 1979, B35, 83–86.
(23) Xie, Z.; Bau, R.; Benesi, A.; Reed, C. A. Organometallics 1995,
14, 3933–3941.

Article
Organometallics, Vol. 29, No. 18, 2010 4069
prevented our exploration of the corresponding reaction
with Et3PbH.
Conclusion
The successful synthesis and isolation of Et3E(carborane)
species for E=Ge, Sn, and Pb adds to the available repertory
of cation-like R3Eþ Lewis acids of group 14 elements that are
coupled with extremely inert carborane counterions. As illus-
trated by the example of the reaction of Et3Ge(CHB11Cl11)
with Et3GeH to form the [Et3Ge-H-GeEt3]þ cation, this
opens the way to an exploration of their reactivity with weak
bases L to form higher coordinate cations of the type [R3EL]þ.
Of further interest is the possibility that these cation-like
Lewis acids will allow systematic studies of R3Eþ cations in
important catalytic roles such as the polymerization of phos-
phazenes13 and the activation of C-F bonds.14
Figure 5. IR spectra of [Et3SiHSiEt3][CHB11Cl11] (black) and
[Et3GeHGeEt3][CHB11Cl11] (blue).
that in Me3SnCl27 and 0.50 A longer than in Ph3SnCl,28 but
on a percentage basis, these extensions are not particularly
different from those in the hexabromo structure. Overall,
these data indicate that in this particular structural motif it is
difficult to use X-ray structural data to measure relative
binding tendencies of carborane anions to R3Eþ cations.
This contrasts with the data on discrete molecular structures
like Et3Si(carborane), where the degree of pyramidalization
at Si is an excellent guide to silylium ion character.2 Never-
theless, the following reactivity studies confirm that the
undecachloro anion is less coordinating than the hexabromo
analogue.
Experimental Section
Reactions were carried out in an inert atmosphere glovebox
(O2, H2O < 1 ppm). Solvents were dried following literature
procedures,30 distilled under Ar, and stored over 4 A molecular
sieves in the glovebox. [Ph3C][CHB11H5Br6], [Ph3C][CHB11Cl11],
Et3Si(CHB11H5Br6), and Et3Si(CHB11Cl11) were prepared by
literature methods.31 Triethyltin chloride (98%) was purchased
from Strem, triethylgermane (98%) from Aldrich, and triethyl-
lead chloride from Pfaltz and Bauer and used as received.
Caution: Trialkyl derivatives of group 14 elements are highly
toxic and should be handled with great care.
Et3Ge(CHB11H5Br6). In a 12 mL vial equipped with a magne-
tic stir bar, [Ph3C][CHB11H5Br6] (122 mg, 0.142 mmol) was
dissolved in dry o-dichlorobenzene (1 mL). To this orange
solution was added Et3GeH (0.1 mL, 0.6 mmol) with stirring.
The solution turned clear within seconds. Within 10 min, a white
precipitate had formed and hexane was added to facilitate
additional crystallization. The product was filtered off and
collected on a medium frit (89 mg, 81%). Single crystals were
grown by careful layering of the reaction mixture with n-hexane.
1H NMR (ODCB-d4, 300 MHz, 25 °C): 2.71 (s, CH carborane),
1.75-1.67 (q, CH2), 1.16-1.10 ppm (t, CH3). Anal. Calcd for
C7H21B11Br6Ge: C, 15.80; H, 3.63. Found: C, 13.69; H, 2.48.
FT-IR [ATR]: see Supporting Information.
[(Et3Ge)2H][CHB11Cl11]. Et3GeH (0.5 mL, 3.09 mmol) was
added to a stirring suspension of [Ph3C][CHB11Cl11] (308.5 mg,
0.403 mmol) in o-dichlorobenzene (1 mL). The mixture became
clear after a few seconds and was allowed to stir for an addi-
tional 10 min. n-Hexane was added while stirring to give a white
precipitate, which was filtered off (0.23 g, 68%). FT-IR [ATR]
spectrum: see Figure 5. Upon heating at 130 °C under vacuum
for 3 h, the diagnostic νGeHGe band at 1740 cm-1 disappeared,
giving Et3Ge(CHB11H5Br6); see Supporting Information Figure S3.
Anal. Calcd for C13H32B11Cl11Ge2: C, 18.53; H, 3.83. Found: C,
18.71; H, 3.40.
Et3Sn(CHB11H5Br6). Et3Si(CHB11H5Br6) was prepared from
[Ph3C][CHB11H5Br6] (135 mg, 0.157 mmol) and excess Et3SiH
in o-dichlorobenzene (3 mL). Hexanes (0.5 mL) was added to
ensure complete product precipitation. The product was filtered
off and resuspended in o-dichlorobenzene, and Et3SnCl (52 mg,
0.21 mmol) was added dropwise. n-Hexane (0.5 mL) was care-
fully layered onto the reaction mixture, and small, colorless
X-ray quality crystals of Et3Sn(CHB11H5Br6) grew over a week
(58 mg, 45%). FT-IR [ATR]: Supporting Information Figure S4.
Anal. Calcd for C7H21B11Br6Sn: C, 10.19; H, 2.57. Found: C,
10.16; H, 2.37.
Formation of the [Et3Ge-H-GeEt3]þ Cation. When the
synthesis of Et3Ge(CHB11Cl11) is carried out according to
eq 1 in the presence of excess Et3GeH, the germane competes
with the carborane anion for coordination to Ge and forms
the [Et3Ge-H-GeEt3]þ cation (eq 4):
Et3GeðCHB11Cl11Þ þ Et3GeH
-
f ½Et3Ge- H- GeEt3þꢀ½CHB11Cl11
ꢀ
ð4Þ
The reaction is reversible. Indeed, the easiest way to
prepare Et3Ge(CHB11Cl11) is to remove Et3GeH from the
“dimer” product by heating under vacuum. The formation
reaction only proceeds with the undecachloro carborane
anion; the hexabromo anion is too strongly coordinated.
This chemistry exactly parallels that of Si.17
The μ-hydrido product is readily characterized by IR
spectroscopy, where a broad, distinctively shaped peak assig-
ned to νasGeHGe is observed at ca. 1740 cm-1. This absorp-
tion is similar to that observed in the corresponding [Et3Si-
H-SiEt3]þ cation at 1875 cm-1 (Figure 5) and at generally
higher frequencies in related hydride-bridged carbocations.29
The analogous Sn hydride-bridged species does not form
when [Ph3Cþ][CHB11Cl11-] is treated with excess Et3SnH
(or n-Bu3SnH) in o-dichlorobenzene. There was no evi-
dence in the IR spectrum for a νasSnHSn peak expected at
ca. 1650 cm-1. The lack of stannane adduct formation may
be related to intrinsic Lewis basicity differences or to the
higher lattice energy of the low-solubility polymeric Et3Sn-
(CHB11Cl11). The low thermal stability of lead hydrides18
(27) Lefferts, J. L.; Molloy, K. C.; Hossain, M. B.; van der Helm, D.;
Zuckerman, J. J. J. Organomet. Chem. 1982, 240, 349.
(28) Tse, J. S.; Lee, F. L.; Gabe, E. J. Acta Crystallogr. 1986, C42,
1876–1878.
(29) Sorensen, T. S. In Stable Carbocation Chemistry; Surya Prakash,
G. K.; Schleyer, P. v. R., Eds.; Wiley: New York, 1997; Chapter 3.
(30) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press Ltd.: Sydney, 1980.
(31) See Supporting Information in ref 7.

4070 Organometallics, Vol. 29, No. 18, 2010
Wright et al.
Et3Pb(CHB11H5Br6). This was prepared in the same manner
as Et3Sn(CHB11H5Br6) using Et3PbCl (40.6 mg, 0.123 mmol)
and Et3Si(CHB11H5Br6) (91.0 mg, 0.124 mmol) in o-dichloro-
benzene (3 mL) and stirring for 24 h. X-ray quality crystals were
grown by careful layering of n-hexane (78.2 mg, 70%). FT-IR
[ATR]: Supporting Information Figure S5. Anal. Calcd for
[Et3Pb][CHB11H5Br6]: C, 9.20; H, 2.32. Found: C, 9.52; H, 2.56.
mixture stirred for 24 h. Crystals of an o-dichlorobenzene
solvate suitable for X-ray analysis were grown by careful layer-
ing of the reaction mixture with n-hexane (5.3 mg, 40%). Anal.
Calcd for Et3Sn(CB11Cl11): C, 11.51; H, 2.21. Found: C, 10.98;
H, 2.10. FT-IR [ATR]: Supporting Information Figure S6.
Acknowledgment. We thank Prof. Evgenii Stoyanov for
assistance with IR spectra and NSF for support (CHE-
0841428).
Et3Sn(CHB11Cl11) C6H4Cl2. Et3Si(CHB11Cl11) was prepa-
3
red from [Ph3C][CHB11Cl11] (53.4 mg, 0.07 mmol) and Et3SiH
in o-dichlorobenzene (3 mL), adding n-hexane (0.5 mL) after 10
min to precipitate the product. Longer stirring times sometimes
led to the formation of waxy solids. Et3Si(CHB11Cl11) was
filtered off (11.7 mg, 0.018 mmol) and resuspended in o-dichloro-
benzene. Three drops of Et3SnCl was added and the reaction
Supporting Information Available: IR and NMR spectra and
X-ray data. This material is available free of charge via the
Internet at http://pubs.acs.org.