Anion stability in stannylium, oxonium, and silylium salts of the weakly coordinating anion [C6F4-1,2-{B(C6F5)2} 2(μ-OCH3)]-

Article abstract of DOI:10.1021/om010750x

Reaction of [Ph₃C]+[C₆F₄-1,2-({B(C₆F₅ )₂}₂(μ-OCH₃)]^-, 1, with Bu₃SnH gives the solvated stannylium ion [Bu₃Sn(arene)]⁺[C₆F₄-l,2-{B(C₆ F₅)₂}₂(μ-OCH₃)]-, 2, as a light brown oil (δ ¹¹⁹Sn = 434 ppm). This material is thermodynamically stable toward transfer of the chelated OMe^- anion to "Bu₃Sn⁺" as evidenced by the reaction of free diborane C₆F₄-1,2-[B(C₆F₅)₂]₂ with 2 equiv of Bu₃SnOMe. This reaction produces the stannyloxonium complex [(Bu₃Sn)₂OCH₃]⁺[C₆F₄ -1,2-{B(C₆F₅)₂}₂(μ-OCH ₃)]^-, 3 (δ ¹⁹⁹Sn = 277 ppm), which is stable at -60 °C. Upon warming, the cation in 3 undergoes decomposition to unidentified products, while the anion remains intact. Ion pair 2 reacts rapidly with ethereal HCl in CH₂Cl₂ to generate Bu₃SnCl and the oxonium acid [(Et₂O)₂H]⁺[C₆F₄-1,2-{B(C ₆F₅)₂}₂(μ-OCH₃)]^- , 4, isolated in 76% yield as a white, crystalline solid. Acid 4 is thermally stable in solution and was characterized crystallographically. The C₂B₂OMe core of the anion in 4 deviates from planarity due to intermolecular interactions in the crystal, in contrast to the structures found in other ion pairs with this anion. Reaction of 2 with anhydrous HCl gives the unsymmetrical MeOH adduct of C₆F₄-1,2-[B(C₆F₅)₂]₂ , 5, which was separately synthesized by direct reaction of methanol with the free diborane. The anion [C₆F₄-l,2-{B(C₆F₅)₂} ₂(μ-OCH₃)]^- is not stable in the presence of the triethylsilylium ion, generated from 1 and Et₃SiH.

Full text of DOI:10.1021/om010750x

340
Organometallics 2002, 21, 340-345
An ion Sta bility in Sta n n yliu m , Oxon iu m , a n d Silyliu m
Sa lts of th e Wea k ly Coor d in a tin g An ion
[C6F 4-1,2-{B(C6F 5)2}2(µ-OCH3)]-
Lee D. Henderson,† Warren E. Piers,*,†,1 Geoffry J . Irvine,† and
Robert McDonald‡
Department of Chemistry, University of Calgary, 2500 University Drive N.W., Calgary,
Alberta, T2N 1N4 Canada, and X-Ray Structure Laboratory, Department of Chemistry,
University of Alberta, Edmonton, Alberta T6G 2G2 Canada
Received August 14, 2001
Reaction of [Ph3C]+[C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-, 1, with Bu3SnH gives the solvated
stannylium ion [Bu3Sn(arene)]+[C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-, 2, as a light brown oil (δ
119Sn ) 434 ppm). This material is thermodynamically stable toward transfer of the chelated
OMe- anion to “Bu3Sn+” as evidenced by the reaction of free diborane C6F4-1,2-[B(C6F5)2]2
with 2 equiv of Bu3SnOMe. This reaction produces the stannyloxonium complex [(Bu3-
Sn)2OCH3]+[C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-, 3 (δ 119Sn ) 277 ppm), which is stable at -60
°C. Upon warming, the cation in 3 undergoes decomposition to unidentified products, while
the anion remains intact. Ion pair 2 reacts rapidly with ethereal HCl in CH2Cl2 to generate
Bu3SnCl and the oxonium acid [(Et2O)2H]+[C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-, 4, isolated in
76% yield as a white, crystalline solid. Acid 4 is thermally stable in solution and was
characterized crystallographically. The C2B2OMe core of the anion in 4 deviates from
planarity due to intermolecular interactions in the crystal, in contrast to the structures found
in other ion pairs with this anion. Reaction of 2 with anhydrous HCl gives the unsymmetrical
MeOH adduct of C6F4-1,2-[B(C6F5)2]2, 5, which was separately synthesized by direct reaction
of methanol with the free diborane. The anion [C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]- is not stable
in the presence of the triethylsilylium ion, generated from 1 and Et3SiH.
3
In tr od u ction
of the chelating diborane C6F4-1,2-[B(C6F5)2]2 with
trityl methoxy ether as shown in Scheme 1. Compound
1 is a highly crystalline, convenient reagent for generat-
ing a number of ion pairs via hydride abstraction
protocols.4 For example, reaction of 1 with Bu3SnH
occurs rapidly with loss of Ph3CH to provide the solvated
stannylium ion pair [Bu3Sn(arene)]+[C6F4-1,2-{B(C6F5)2}2-
(µ-OCH3)]-, 2, quantitatively as a light brown liquid
clathrate-like oil5 when the reaction is done in benzene
or toluene. Attempts to purify this species via removal
of solvent in vacuo or via recrystallization procedures
using more polar solvents (CD2Cl2 in particular) failed
to generate conveniently workable solid samples of 2.
However, when generated in deuterated arene solvents,
the oil is separable and can be studied directly using
multinuclear NMR spectroscopy.
Samples were prepared by placing an insert filled
with the oil into a 5 mm NMR tube containing either
d6-benzene or d8-toluene as a lock solvent. 19F NMR
spectroscopy at room temperature showed only the five
signals characteristic of the symmetrical µ-methoxy
anion for 2, indicating that the oil is at least 95% pure
as generated in this fashion (aside from any occluded
Previously, we have reported the synthesis of the
weakly coordinating anions [C6F4-1,2-{B(C6F5)2}2(µ-
OR)]- (R ) CH3, C6F5) as their trityl salts and a
preliminary account of their behavior as counteranions
for zirconocenium cations.2 We found them to be both
kinetically and thermodynamically stable toward trans-
fer of the µ-OR group to zirconium, suggesting that these
anions might find application in the stabilization of
other highly electrophilic species. Herein we report the
generation of stannylium, oxonium, and silylium ions
in the presence of the [C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-
anion. We chose the µ-methoxide anion for two reasons.
First, its trityl salt crystallizes beautifully and is more
easily isolated analytically pure than other OR deriva-
tives, and second, the methoxide group provides a
1
convenient H NMR handle, allowing for easy spectro-
scopic monitoring of the chemistry described.
Resu lts a n d Discu ssion
The trityl salt of the µ-methoxy anion [C6F4-1,2-{B-
(C6F5)2}2(µ-OCH3)]- (1) is readily prepared via reaction
† University of Calgary.
(3) Williams, V. C.; Piers, W. E.; Clegg, W.; Collins, S.; Marder, T.
B. J . Am. Chem. Soc. 1999, 121, 3244.
‡ University of Alberta.
(1) Phone: 403-220-5746. Fax: 403-289-9488. E-mail: wpiers@
ucalgary.ca. S. Robert Blair Professor of Chemistry 2000-2005. E. W.
R. Steacie Memorial Fellow, 2001-2003.
(2) Williams, V. C.; Irvine, G. J .; Piers, W. E.; Li, Z.; Collins, S.;
Clegg, W.; Elsegood, M. R. J .; Marder, T. B. Organometallics 2000,
19, 1619.
(4) Corey, J . Y. J . Am. Chem. Soc. 1975, 97, 3237.
(5) The oils formed in these and related reactions are of uncertain
stoichiometry and structure but behave as though they were liquid
clathrates. For a more detailed discussion on this topic, see: Lambert,
J . B.; Zhao, Y.; Wu., H.; Tse, W. C.; Kuhlmann, B. J . Am. Chem. Soc.
1999, 121, 5001.
10.1021/om010750x CCC: $22.00 © 2002 American Chemical Society
Publication on Web 12/21/2001

Anion Stability in Stannylium Salts
Organometallics, Vol. 21, No. 2, 2002 341
Sch em e 1
solvent and Ph3CH byproduct). 11B NMR spectroscopy
was less informative in this regard, exhibiting a broad
signal at 4.8 ppm. The 119Sn NMR spectrum of oil 2 was
recorded at -60 °C using samples generated at room
temperature in d8-toluene. A sharp signal at 434 ppm
was observed and is attributed to the [R3Sn(arene)]+
cation, ligated by toluene in an undefined mode of
bonding.6 This chemical shift is almost exactly the same
as what we observe for the related species [Bu3Sn-
(arene)]+[B(C6F5)4]- when the spectrum is recorded
under the same conditions.7 A signal of lower intensity,
representing no more than 10% of the tin in the sample,
is also observed at 262 ppm under these conditions for
both anions. Previously, Lambert and co-workers have
reported a chemical shift of 263 ppm for this stannylium
moiety,8 but their spectrum was recorded at room
temperature. When samples of 2 or [Bu3Sn(arene)]+[B-
(C6F5)4]- are allowed to warm to room temperature, the
signal at 434 is broadened almost into the baseline,
while the signal of lesser overall intensity at 262
remains. As the sample is cooled again, the broad signal
centered around 434 ppm sharpens, probably as ex-
change between free and bound toluene slows. The peak
at 262 remains sharp throughout these changes, but is
a minor component of the sample. We are unsure as to
what gives rise to this signal, but it appears in the range
expected for some species [Bu3Sn(L)]+[A]-.9 Since the
appearance of the signal for this species is temperature
independent, it is not likely to be in equilibrium with
complex 2 or [Bu3Sn(arene)]+[B(C6F5)4]- and likely
arises due to an impurity in the solvent.7,10 Further-
more, the chemical shift of 434 for toluene-stabilized
[Bu3Sn(arene)]+ is more in line with observed chemical
shifts for the ions [Bu3Sn(arene)]+[B(3,5-(CF3)2C6H3)4]-
(356 ppm at -20 °C in CD2Cl2),11 [Bu3Sn(arene)]+[HB-
(C6F5)3]- (360 ppm),12 and [Bu3Sn]+[CB11Me12]- (454.3
ppm, cyclohexane).13
Stannylium ion pair 2 is thermally stable as a liquid
clathrate-like oil in arene media or in CD2Cl2 solution
for long periods of time at room temperature and for at
least 2 h at temperatures as high as 60 °C. This stability
appears to be thermodynamic in nature, since the
diborane C6F4-1,2-[B(C6F5)2]2 is capable of abstracting
a methoxide group from Bu3SnOMe,14 resulting in clean
formation of the [C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]- anion
as judged by 19F NMR spectroscopy (Scheme 1). The
stannyl ether reagent effectively competes for the stan-
nylium cation generated, and so clean overall reactions
require the use of 2 equiv of Bu3SnOMe to generate the
ion pair we have assigned as the stannyloxonium ion
3. Moderately stable oils of 3 can thus be generated by
treating C6F4-1,2-[B(C6F5)2]2 with 2 equiv of Bu3SnOMe
in toluene at -78 °C. This species exhibits a 119Sn NMR
chemical shift of 277.0 ppm at -60 °C. To our knowl-
edge, stannyloxonium ions of this type have not been
studied in detail; however, related silyloxonium cations
of general formula [(R3Si)nOR′3-n]+[B(ArF)4]- (n ) 1-3,
ArF ) 3,5-(CF3)2C6H3,15 C6F515b) have been generated
and studied in solution by 29Si and 13C NMR spectro-
scopy. The closest silicon analogue of 3 is the silyloxo-
nium ion pair [(Me3Si)2OEt]+[B(3,5-(CF3)2C6H3)4]-, which
exhibits a 29Si chemical shift of 59.0 ppm in CD2Cl2 at
-70 °C.15a Using an empirically derived correlation
between 29Si and 119Sn chemical shifts,16
a
119Sn reso-
nance at ≈261 ppm would be predicted for the analogous
(6) Arshadi, M.; J ohnels, D.; Edlund, U. Chem. Commun. 1996,
1279.
(7) Blackwell, J . M.; Piers, W. E.; Parvez, M. J . Am. Chem. Soc., in
press.
(10) Separate experiments show that it is not the water adduct of
[Bu3Sn]+.7
(11) Kira, M.; Oyamada, T.; Sakurai, H. J . Organomet. Chem. 1994,
471, C4.
(12) Lambert, J . B.; Kuhlmann, B. J . Chem. Soc., Chem. Commun.
1992, 931.
(13) Zharov, I.; King, B. T.; Havlas, Z.; Pardi, A.; Michl, J . J . Am.
Chem. Soc. 2000, 122, 10253.
(14) Alleston, D. L.; Davies, A. G. J . Chem. Soc. 1962, 2050.
(15) (a) Kira, M.; Hino, T.; Sakurai, H. J . Am. Chem. Soc. 1992,
114, 6697. (b) Olah, G. A.; Li, X.; Wang, Q.; Rasul, G.; Prakash, G. K.
J . Am. Chem. Soc. 1995, 117, 8962.
(8) Lambert, J . B.; Zhao, Y.; Wu, H. J . Org. Chem. 1999, 64, 2729.
(9) (a) To our knowledge, no systematic study of the chemical shift
trends in solvated stannylium ions has been published. However, we
have observed that stannylium ions of general formula [Bu3Sn-
(aldehyde)]+[A]- give rise to 119Sn signals in the 280-310 range, with
more basic aldehydes giving more upfield-shifted resonances. Shifts
to higher field for higher coordination numbers has been noted in
neutral organotin compounds,9b and a detailed study on the 29Si NMR
chemical shifts of solvated silylium ions has indicated a similar trend.9c
(b) Nadvornik, M.; Holecek, J .; Handlir, K. J . Organomet. Chem. 1984,
275, 43. (c) Arshadi, M.; J ohnels, D.; Edlund, U.; Ottosson C.-H.;
Cremer, D. J . Am. Chem. Soc. 1996, 118, 5120.
(16) δSn ) 5.2δSi - 46: see ref 6 and: (a) Mitchell, T. N. J .
Organomet. Chem. 1983, 255, 279. (b) Watkinson, P. J .; Mackay, K.
M. J . Organomet. Chem. 1984, 275, 39.

342 Organometallics, Vol. 21, No. 2, 2002
Henderson et al.
tin compound, i.e., [(Me3Sn)2OEt]+[B(3,5-(CF3)2C6H3)4]-,
in good agreement with what we observe for the closely
related stannyloxonium ion pair 3.
While stable at -60 °C for long periods, upon warm-
ing samples of 3, signals for other, unidentified tin-
containing species begin to appear over the course of a
few hours. However, these transformations are exclu-
sively associated with the cationic portion of 3; by 19F
NMR spectroscopy, the integrity of the anion is retained.
The related silyloxonium species discussed above were
also found to be quite thermally unstable.15 Despite the
complications encountered in the reactions of diborane
C6F4-1,2-[B(C6F5)2]2 and Bu3SnOMe, it is clear that ion
pair formation to the µ-methoxide stannylium species
is thermodynamically preferred over back-transfer of
OMe- to the cationic tin center, making the [C6F4-1,2-
{B(C6F5)2}2(µ-OCH3)]- anion suitable for the stabiliza-
tion of stannylium ions.
The µ-OMe anion is also appropriate for use with
oxonium acids. Ether-solvated proton salts of weakly
coordinating fluorinated aryl borate anions have proven
to be effective reagents for the generation of electrophilic
transition metal cations.17 A complementary reagent
incorporating the [C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]- anion
can be obtained in high yield by treating the solvated
stannylium salt 2 with an ether solution of HCl.18 Loss
of Bu3SnCl is accompanied by production of [(Et2O)2H]+-
[C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-, 4, as an isolable white
solid in 76% yield. Again, 19F NMR spectroscopy was
informative for showing that the anion remains intact,
giving the five sharp signals characteristic of the sym-
F igu r e 1. ORTEP diagram of 4: (A) the [(Et2O)2H]+
cation; (B) the [C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]- anion. Only
the ipso carbons of the -C6F5 rings are shown for clarity.
Selected bond distances (Å): O(1)-H(1), 1.21(7); O(2)-H(1),
1.18(7); B(1)-C(11), 1.605(7); B(2)-C(12), 1.579(7); B(1)-
O(10), 1.541(6); B(2)-O(10), 1.563(5); O(1)-C(10), 1.455-
(5); C(11)-C(12), 1.398(6). Selected bond angles (deg):
O(1)-H(1)-O(2), 178.3; B(1)-O(10)-B(2), 112.7(3); B(1)-
O(10)-C(10), 121.1(3); B(2)-O(10)-C(10), 124.2(3); B(1)-
C(11)-C(12), 110.3(4); B(2)-C(12)-C(11), 113.3(4); C(11)-
B(1)-O(10), 98.7(3); C(12)-B(2)-O(10), 98.4(3).
1
metrical µ-methoxide anion. In the H NMR spectrum,
signals for the coordinating ether molecules are ob-
served, along with a downfield-shifted signal at 16.4
ppm for the solvated proton. Similar shifts are observed
for Brookhart’s acid ([(Et2O)2H]+[B(3,5-(CF3)2C6H3)4]-,
16.7 ppm, CD2Cl219) and J utzi’s acid ([(Et2O)2H]+[B-
(C6F5)4]-, 15.50 ppm, CD2Cl217b). Interestingly, the oxo-
nium ion in 4 exhibits no tendency to protonate the
anion, an observation probably reflecting the kinetic
stability of ether-solvated protons (vide infra). Extended
heating of solutions of 4 results eventually in production
of detectable amounts of C6F5H, but, in contrast to
Brookhart’s acid, this species is quite thermally robust
in solution and the solid state.
The solid state structure of 4 was determined, con-
firming its formulation. ORTEP diagrams of the cation
and anion are shown in Figure 1A and B, respectively,
along with selected metrical data. Further details can
be found in the Supporting Information. In the cation,
the structural parameters are unremarkable and simi-
lar to other determinations of this cation. The anion,
however, differs in structure from other [C6F4-1,2-{B-
(C6F5)2}2(µ-OR)]- anions we have structurally charac-
terized, which invariably contain essentially planar
C2B2OR molecular cores. In 4, the µ-methoxide anion
adopts a structure in which the dihedral angle between
the planes defined by C(11)-C(12)-B(1)-B(2) and
O(10)-C(10)-B(1)-B(2) is 35.0(3)°. Thus, while O(10)
is only slightly pyramidalized (the sum of the angles
about O(10) is 358.0(5)°), the methoxide is tilted out of
the chelating pocket of the diborane significantly. This
results in a closing of the B(1)-O(1)-B(2) angle to
112.7(2)° from the 117.72(12)° observed in the structure
of the trityl salt of this anion,2 in which the six atoms
in question are coplanar. The reasons for this distortion
from planarity are likely packing forces in the crystal.
Inspection of a packing diagram reveals that the anions
appear to pair up in the crystal via interactions between
an ortho fluorine and a µ-methoxide C-H bond (Figure
2). No such motif is found in the trityl salt of this anion,
suggesting that these close contacts are responsible for
the methoxide’s deviation from the chelation plane.
Another result of this is that the two C6F5 rings pointing
away from the µ-OMe group (beginning with C(21) and
C(51)) are roughly parallel to one another, while the
other two are splayed out to the sides, a distinctly
different arrangement from that found in the structure
of 1 and the related µ-OC6F5 derivative.2
(17) (a) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992,
11, 3920. (b) J utzi, P.; Mu¨ller, C.; Stammler, A.; Stammler, H.-G.
Organometallics 2000, 19, 1442. (c) White, P. A.; Calabrese, J .;
Theopold, K. H. Organometallics 1996, 15, 5473.
(18) Reed, C. A.; Fackler, N. L. P.; Kim, K.; Stasko, D.; Evans, D.
R. J . Am. Chem. Soc. 1999, 121, 6314.
As mentioned, the isolability of 4 reflects the kinetic
stability of the diethyl ether solvated proton. When
(19) Tempel, D. J .; J ohnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J . Am. Chem. Soc. 2000, 122, 6686.

Anion Stability in Stannylium Salts
Organometallics, Vol. 21, No. 2, 2002 343
F igu r e 2. Partial packing diagram of 4, illustrating the
ion-ion contacts responsible for the deviation of the
µ-methoxide from the C(11)-C(12)-B(1)-B(2) plane.
Sch em e 2
F igu r e 3. 19F NMR spectra of methanol adduct 5 derived
from reaction of C6F4-1,2-[B(C6F5)2]2 with 1 equiv of MeOH
(top) and the reaction of 2 with anhydrous HCl (bottom).
through its separate synthesis from C6F4-1,2-[B(C6F5)2]2
and 1 equiv of MeOH. In the 19F NMR spectrum, 10
separate resonances are observed for 5, suggesting an
unsymmetrical structure where the methanol interacts
only with one of the two boron centers. The OH proton
for the bound methanol appears at 6.52 ppm, shifted
downfield from that of free methanol in C6D6. Attempts
to isolate this species revealed it to be a viscous oil which
did not readily solidify, so characterization was limited
to solution spectroscopy.
Sch em e 3
As can be seen in Figure 3, the 19F NMR spectra of 5
generated via these two methods are almost identical.
The broadness of some of the signals and the impurities
present in the baseline of the spectra are due to the
presence of small amounts of a bis-methanol adduct, 6,
where the second equivalent of MeOH is bound via
hydrogen bonding to the first methanol molecule. Ad-
dition of 2 equiv of MeOH to C6F4-1,2-[B(C6F5)2]2 gives
this bis-methanol adduct directly (Scheme 3). This type
of second sphere adduct has precedent in the alcohol20
and water21 binding chemistry of the monofunctional
analogue of C6F4-1,2-[B(C6F5)2]2, i.e., B(C6F5)3, but is
somewhat surprising in light of the presence of a second
highly electrophilic boron center. Nonetheless, the 19F
spectroscopic data for 6, where a different set of 10
resonances is observed, conclusively show that the
asymmetry is retained in the bis-adduct. A 19F-19F
COSY experiment allows for assignment of the ortho,
meta, and para fluorines of each B(C6F5)2 moiety. The
protonation of 2 is carried out in the absence of ether
with anhydrous HCl gas, the resulting product, in
addition to Bu3SnCl, is the methanol adduct of the
diborane, 5 (Scheme 3). Thus, when there are no
stronger bases present, the [C6F4-1,2-{B(C6F5)2}2(µ-
OCH3)]- anion is basic enough to be protonated. Even
when 2 is treated with HCl (anhydrous) in the presence
of mesitylene, formation of adduct 5 is preferred over
protonation of the relatively electron-rich arene. This
contrasts with the behavior of the [B(C6F5)4]- anion.17
The identity of the adduct 5 was further confirmed
(20) (a) Bergquist, C.; Bridgewater, B. M.; Harlan, C. J .; Norton, J .
R.; Friesner, R. A.; Parkin, G. J . Am. Chem. Soc. 2000, 122, 10581. (b)
Siedle, A. R.; Elmo, L.; Lamanna, W. M.; Stillwater, L. U.S. Patent
5,296,433, 1994.
(21) (a) Doerrer, L. H.; Green, M. L. H. J . Chem. Soc., Dalton Trans.
1999, 4325. (b) Kalamarides, H. A.; Iyer, S.; Lipian, J .; Rhodes, L. F.;
Day, C. Organometallics 2000, 19, 3983. (c) Beringhelli, T.; Maggioni,
D.; D’Alfonso, G. Organometallics 2001, in press.

344 Organometallics, Vol. 21, No. 2, 2002
m,p values found are 12.0 and 8.0 ppm, consistent with
Henderson et al.
purchased from Sigma-Aldrich and used as received or purified
according to standard procedures.
∆
the presence of a neutral three-coordinate and a neutral
four-coordinate boron center;22 furthermore, the chemi-
cal shifts of 40 and 4.6 ppm for the two 11B nuclei
support this assignment.23 Two methyl and two OH
P r epar ation of [Bu 3Sn ]+[C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]-,
2. To an orange suspension of trityl salt 1 (30 mg, 0.027 mmol)
in d8-toluene (0.4 mL) was added Bu3SnH (8.0 µL, 0.030 mmol),
generating a light brown colored liquid oil that settled to the
bottom of the reaction vial. This material was placed into a 3
mm glass tube which was inserted into a 5 mm NMR tube
containing d8-toluene as a reference and locking solvent. 1H
NMR: 3.54 (s, 3H, µ-OCH3); 1.96 (m, 12H, Bu3Sn); 1.08 (m,
6H, Bu3Sn); 0.72 (m, 9H, Bu3Sn). 19F NMR: -131.3 (8F,
o-C6F5); -136.2 (2F, -C6F4); -159.2 (4F, p-C6F5); -162.9 (2F,
-C6F4); -165.2 (8F, m-C6F5). 13C{1H} NMR: 60.4 (µ-OCH3),
29.2, 27.4, 13.5, 11.8 (Bu3Sn). 11B NMR: 4.8 (br s). 119Sn NMR
(-60 °C): 434.9 ppm.
P r ep a r a tion of [(Bu 3Sn )2OMe]+[C6F 4-1,2-{B(C6F 5)2}2(µ-
OCH3)]-, 3. Diborane C6F4-1,2-[B(C6F5)2]2 (47 mg, 0.056 mmol)
was dissolved in d8-toluene (0.4 mL) and placed in a 5 mm
NMR tube. The solution was cooled to -78 °C, and a solution
of Bu3SnOMe (36 mg, 0.112 mmol) in d8-toluene (0.2 mL) was
added via syringe. The resulting liquid oil settled to the bottom
of the NMR tube, and the sample was subjected to NMR
spectroscopic analysis. 1H NMR: 3.88 (s, 3H, µ-OCH3); 3.10
(s, 3H, Sn2OCH3); 1.38 (m, 12H, Bu3Sn-); 1.16 (m, 24H,
Bu3Sn-); 0.85 (m, 18H, Bu3Sn-). 19F NMR: -131.1 (8F,
o-C6F5); -136.0 (2F, -C6F4); -159.2 (4F, p-C6F5); -162.9 (2F,
-C6F4); -165.1 (8F, m-C6F5). 13C{1H} NMR: 56.8 (µ-OCH3),
28.3, 27.7, 20.2, 13.8 (Bu3Sn). 119Sn (-60 °C): 277.0.
1
resonances are observed in the H NMR spectrum for
the inner and outer bound MeOH ligands. This material
also failed to solidify upon attempted workup, and loss
of varying amounts of the coordinated alcohol is ob-
served upon exposure to vacuum. The fact that the
second equivalent of MeOH binds in this fashion, as
opposed to forming a symmetrical bis-adduct via bond-
ing to the second borane center, attests to the steric
blocking of this second boron upon pyramidalization of
the first borane with adduct formation.
Attempts to generate arene-stabilized silylium ions24
using this counteranion were less successful. For ex-
ample, reaction of 1 with 1 equiv of Et3SiH resulted in
production of Ph3CH, but did not completely consume
the full equivalent of 1. In addition at least three
methyl-containing products resulted, including a major
species with a 10-resonance 19F NMR spectral pattern
reminiscent of that observed for adduct 5. Addition of a
further 0.5 equiv of Et3SiH resulted in consumption of
the remaining amount of 1 and production of detectable
amounts of the free diborane. We speculate that back-
transfer of OMe- to the silylium ion produces Et3SiOMe,
which then forms unstable silyloxonium ions, which
degenerate into a mixture of products. Given the
complex nature of these reactions and the apparent
unsuitability of this weakly coordinating anion for use
with “R3Si+”, we abandoned further investigations.
In summary, we have prepared and characterized the
stannylium ion pair 2, incorporating the weakly coor-
dinating [C6F4-1,2-{B(C6F5)2}2(µ-OCH3)]- anion, as a
liquid clathrate-like oil. This species may be protonated
with ethereal HCl to give the oxonium acid 4 as a stable,
usable Bronsted acid reagent for organometallic syn-
thesis. Protonation of 2 in the absence of diethyl ether
results in protonation of the anion, to give the unsym-
metrical methanol adduct 5, itself a potential Bronsted
acid activator for use in protonolysis reactions.20b
P r ep a r a tion of [(Et2O)2H]+[C6F 4-1,2-{B(C6F 5)2}2(µ-OC-
H3)]-, 4. Stannylium ion pair 2 was generated in situ by
loading 1 (50 mg, 0.045 mmol) into a two-necked 25 mL flask
equipped with frit assembly, dissolving in CH2Cl2 (5 mL), and
treating with nBu3SnH (12.1 µL, 0.045 mmol). After stirring
for 20 min, HCl/Et2O (22.5 µL of a 2 M solution, 0.045 mmol)
was added to the pale yellow solution and the reaction stirred
for 8 h. The solvent was removed in vacuo, leaving an oily
product, which was washed with hexanes (3 × 5 mL) to remove
byproducts. The resulting beige solid was recrystallized from
CH2Cl2 (2 mL) layered with hexanes (4 mL) at -40 °C to afford
1
a beige crystalline product (35 mg, 76.1%). H NMR (CD2Cl2):
16.44 (br s, 1H, H(OEt2)2); 4.08 (q, 8H, J H-H ) 6.80 Hz, OCH2);
3.65 (s, 3H, -µ-OCH3); 1.43 (t, 12H, OCH2CH3). 19F NMR (CD2-
Cl2): -132.3 (8F, o-C6F5), -137.6 (2F, -C6F4), -160.1 (4F,
p-C6F5), -163.6 (2F, -C6F4), -166.0 (8F, p-C6F5). 13C{1H} NMR
(CD2Cl2): 69.0 (-OCH3), 57.3 (-OCH2CH3), 14.5 (-OCH2CH3).
11B NMR (CD2Cl2): δ 5.8 (br s). Anal. Calcd for C39H24B2F24O3:
C, 46.0; H, 2.4. Found: C, 45.43; H, 2.57.
Exp er im en ta l Section
Gen er a t ion of 5 via R ea ct ion of 2 w it h An h yd r ou s
HCl. Stannylium ion pair 2 was generated in situ by loading
1 (12 mg, 0.011 mmol) into a flame-sealable NMR tube,
suspending in d8-toluene (0.8 mL), and treating with Bu3SnH
(3 µL, 0.011 mmol). The resulting sample was cooled to -78
°C, and a 10-fold excess of anhydrous HCl gas (0.110 mmol)
was condensed into the tube. The NMR tube was flame sealed
and allowed to warm to room temperature. Stannylium ion
pair 2 was consumed immediately, generating the methanol
adduct 5 and Bu3SnCl as a byproduct. 1H NMR: 6.52 (br s,
1H, MeOH); 3.29 (s, 3H, CH3OH); additional resonances for
the Ph3CH and Bu3SnCl byproducts also present. 19F NMR:
-125.4 (1F, -C6F4); -128.9 (4F, o-C6F5); -132.6 (4F, o-C6F5);
-137.6 (1F, -C6F4); -141.0 (1F, -C6F4); -143.6 (2F, p-C6F5);
-148.7 (2F, p-C6F5); -153.6 (1F, -C6F4); -159.9 (4F, m-C6F5);
-160.9 (4F, m-C6F5).
Gen er a l P r oced u r es. All manipulations of air- and mois-
ture-sensitive materials were undertaken using standard
vacuum and Schlenk techniques or in a glovebox under an
atmosphere of nitrogen. All solvents were dried and purified
by passing through suitable drying agents (alumina and Q5).25
NMR spectra were recorded in C7D8 unless otherwise noted.
Data are given in ppm relative to solvent signals for 1H and
13C spectra or relative to external standards for 119Sn (SnMe4,
0.0 ppm), 11B NMR (BF3‚OEt2, 0.0 ppm), and 19F NMR (CFCl3
at 0.0 ppm) experiments. Elemental analyses were performed
by Mrs. Dorothy Fox in the microanalytical laboratory of the
Department of Chemistry at the University of Calgary. The
compounds C6F4-1,2-[B(C6F5)2]2,3 1,2 and Bu3SnOMe14 were all
prepared via literature procedures; other materials were
Gen er a tion of 5 via Rea ction of C6F 4-1,2-[B(C6F 5)2]2
w ith Meth a n ol. Diborane C6F4-1,2-[B(C6F5)2]2 (10 mg, 0.012
mmol) was loaded into a 5 mm NMR tube and dissolved in
d6-benzene (0.6 mL). Dry and degassed methanol (0.5 µL, 0.012
mmol) was injected into this solution via a gastight syringe,
affording the methanol adduct 5 in a quantitative yield by
(22) Horton, A. D.; de With, J . Organometallics 1997, 16, 5424.
(23) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 2.
(24) (a) Reed, C. A. Acc. Chem. Res. 1998, 31, 325. (b) Lambert, J .
B.; Zhang, S.; Stern, C. L.; Huffman, J . C. Science 1993, 260, 1917.
See also ref 9c for further leading references on this topic.
(25) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
1
NMR spectroscopy. H and 19F NMR spectra were essentially

Anion Stability in Stannylium Salts
Organometallics, Vol. 21, No. 2, 2002 345
Ta ble 1. Su m m a r y of Da ta Collection a n d
Refin em en t Deta ils for 4
identical to those found for 5 generated as described above
(see also Figure 3). 13C{1H} NMR: 57.2 (CH3OH). 11B NMR:
40.3.
formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C39H24B2F24O3
1018.20
triclinic
P1h (#2)
11.8694(17)
12.8153(18)
13.527(2)
76.633(3)
82.751(3)
78.299(3)
1953.5(5)
2
1.731
0.185
-80
0.46 × 0.29 × 0.04
0.9918-0.9936
ω scans (0.2°)
52.82
7283
3099
Gen er a tion of Bis-m eth a n ol Ad d u ct 6. Diborane C6F4-
1,2-[B(C6F5)2]2 (10 mg, 0.012 mmol) was loaded into a 5 mm
NMR tube and dissolved in d6-benzene (0.6 mL). Dry and
degassed methanol (1.0 µL, 0.024 mmol) was injected into this
solution via a gastight syringe to afford the bis-methanol
1
adduct 6 in quantitative yield by NMR. H NMR: 6.49 (br s,
1H, inner MeOH); 4.70 (br s, 1H, outer MeOH); 3.26 (s, 3H,
inner CH3OH); 2.25 (s, 3H, outer CH3OH). 19F NMR: -132.7
(4F, o-C6F5); -133.7 (1F, -C6F4); -133.9 (4F, o-C6F5); -139.7
(1F, -C6F4); -148.8 (2F, p-C6F5); -154.0 (2F, p-C6F5); -155.2
(1F, C6F4); -156.9 (1F, -C6F4); -160.9 (4F, m-C6F5); -161.9
(4F, m-C6F5). 13C{1H} NMR: 57.2 (inner CH3OH), 53.0 (outer
CH3OH). 11B NMR: δ 39.8 (s); 4.6 (br).
Z
d
calc, mg m-3
µ, mm-1
T, °C
cryst dimens, mm3
rel transmn factors
scan type
X-r a y Cr yst a llogr a p h y for [(E t2O)2H ]+[C6F 4-1,2-{B-
(C6F 5)2}2(µ-OCH3)]-, 4. Colorless crystals of 4 were obtained
from layering of hexanes onto a dichloromethane solution of
the compound. Data were collected on a Bruker PLATFORM/
SMART 1000 CCD diffractometer26 using Mo KR radiation at
-80 °C. Unit cell parameters were obtained from a least-
squares refinement of the setting angles of 2984 reflections
from the data collection. The space group was determined to
be P1h (No. 2). The data were corrected for absorption through
use of Gaussian integration (indexing of crystal faces). See
Table 1 for a summary of crystal data and X-ray data collection
information. The structure of 4 was solved using direct
methods (SHELXS-8627), and refinement was completed using
the program SHELXL-93.28 Hydrogen atoms were assigned
positions based on the geometries of their attached carbon
atoms and were given thermal parameters 20% greater than
those of the attached carbons except for H1, which was located
from a difference Fourier map; its positional and thermal
parameters were allowed to refine freely. Within one of the
diethyl ether molecules of the [(Et2O)2H]+ ion, severely elon-
gated thermal ellipsoids indicated that one of the methylene
carbons and both carbons of the other ethyl group were
positionally disordered; these atoms were split into two posi-
tions each (C5A, C5B for the methylene, C7A, C8A and C7B,
2θ (max), deg
no. of unique reflns
no. of reflns with I > 2σI
no. of variables
R1
615
0.0653
wR2
0.1373
gof
0.932
-0.273-0.382
residual density, e/Å3
C8B for the ethyl) that were refined with equal occupancy
factors (50% each). The final model for 4 refined to values of
R1(F) ) 0.0653 (for 3099 data with Fo g 2σ(Fo2)) and wR2(F2)
2
) 0.1373 (for all 7283 independent data).
Ack n ow led gm en t. Funding for this work came
from the Natural Sciences and Engineering Research
Council of Canada in the form of a Research Grant (to
W.E.P.), an E. W. R. Steacie Fellowship (2001-2003) (to
W.E.P.), and Scholarship support (PGSA) to L.D.H.
Su p p or tin g In for m a tion Ava ila ble: Full listings of
crystallographic data, atomic parameters, hydrogen param-
eters, atomic coordinates, and complete bond distances, angles,
and torsion angles for 4. This material is available free of
charge via the Internet at http://pubs.acs.org.
(26) Programs for diffractometer operation, data collection, data
reduction, and absorption correction were those supplied by Bruker.
(27) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(28) Sheldrick, G. M. SHELXL-93. Program for crystal structure
determination; University of Go¨ttingen: Germany, 1993.
OM010750X