Unusual anions [LAI(SH)(S)]- and [LAI(S)2] 2- stabilized by weakly coordinating imidazolium cations. Synthesis of LAI(SSiMe2)2O (L = HC[C(Me)N(Ar)]2, Ar = 2,6-iPr2C6H<

Article abstract of DOI:10.1021/ic050693q

Deprotonation of an Al-SH moiety has been achieved easily by using N-heterocyclic carbene as the base. Monomeric mono- and bis-imidazolium salts [C_tH⁺][LAl(SH)(S)]^- ([C_tH ⁺] = N,N′-bis-tert-butylimida

Full text of DOI:10.1021/ic050693q

Inorg. Chem. 2005, 44, 5556−5558
-
-
Unusual Anions [LAl(SH)(S)] and [LAl(S)2]2 Stabilized by Weakly
Coordinating Imidazolium Cations. Synthesis of LAl(SSiMe2)2O (L
HC[C(Me)N(Ar)]2, Ar 2,6-iPr2C6H3)
)
)
Vojtech Jancik and Herbert W. Roesky*
Institut fu¨r Anorganische Chemie der UniVersita¨t, Tammannstrasse 4, 37077 Go¨ttingen, Germany
Received May 4, 2005
Deprotonation of an Al
−
SH moiety has been achieved easily by
by a bulky carborane anion [CB11H6Br6]-,1a whereas the
second group can be demonstrated by numerous examples
of polyoxometalates.1k-q Recently, we have reported the
successful preparation of mono- and dilithium salts ([{LAl-
(SH)[SLi(thf)2]}2] (1) and [{LAl[(SLi)2(thf)3]}2]‚2THF (2))
containing relatively short S-Li bonds.2 Easy access of these
compounds prompted us to examine the stability of the
corresponding anions [LAl(SH)(S)]- and [LAl(S)2]2- by
weakly coordinating cations. These species are of interest
as precursors for further reactions because of the high
nucleophilicity and weak Lewis basicity of the sulfur atom.
N-Heterocyclic carbenes were used for deprotonation of the
SH moieties due to their strong Lewis basicity.3 Finally, it
is well-known that imidazolium cations stabilize a wide
range of anions (e.g., BPh4 , PF6 , Cd(SCN)3 ).1q,4 The
equimolar reaction of [LAl(SH)2] (3) and N,N′-bis-tert-
butylimidazol-2-ylidene5 (N-heterocyclic carbene) in THF
resulted in the formation of the expected monoimidazolium
salt [CtH+][LAl(SH)(S)]- (4) ([CtH+] ) N,N′-bis-tert-bu-
tylimidazolium(1+)) in a quantitative yield (section S2 in
the Supporting Information). Surprisingly, the reaction
between 3 and N,N′-bis-tert-butylimidazol-2-ylidene in a
1:2 molar ratio did not give the expected [CtH+]2[LAl(S)2]2-
but led to a mixture of 4 and free carbene. Double de-
protonation was successfully achieved by using 2 equiv of
N,N′-bismesitylimidazol-2-ylidene,6 giving quantitatively
[CmH+]2[LAl(S)2]2- ([CmH+] ) N,N′-bismesitylimidazolium-
(1+)) (5) (section S3 in the Supporting Information).
using N-heterocyclic carbene as the base. Monomeric mono- and
bis-imidazolium salts [CtH+][LAl(SH)(S)]- ([CtH+]
)
N,N
′
-bis-tert-
butylimidazolium), [CmH+][LAl(SH)(S)]-, and [CmH+]2[LAl(S)2]2
-
([CmH+]
) N,N′-bismesitylimidazolium), containing unusual anions
[LAl(SH)(S)]- and [LAl(S)2]2-, have been synthesized in nearly
quantitative yields. Furthermore, [CmH+]2[LAl(S)2]2 has been
-
successfully used for the preparation of LAl(SSiMe2)2O containing
-
the [O(Me2SiS)2]2 ligand.
The field of weakly coordinating ions has developed as
an important area of chemistry over the last 2 decades.1 These
ionic species can be divided into two groups; the first one
contains ion pairs featuring mostly unique cations,1a-j while
the second one uses stable cations (e.g., R4N+,1k-m R4P+,1n-p
or imidazolium(1+);1q R ) alkyl, aryl) to stabilize unusual
anions.1k-q A representation of the first group can be
demonstrated by the nearly planar [iPr3Si]+ cation stabilized
-
-
-
* To whom correspondence should be addressed. E-mail: hroesky@
gwdg.de. Fax: (+49) 551-39-3373.
(1) For example, see: (a) Reed, C. A. Acc. Chem. Res. 1998, 31, 325-
332. (b) Reed, C. A. Acc. Chem. Res. 1998, 31, 133-139. (c) Krossing,
I.; Rabe, I. J. Am. Chem. Soc. 2004, 126, 7571-7577. (d) Krossing,
I.; Rabe, I. Angew. Chem. 2004, 116, 2116-2142; Angew. Chem.,
Int. Ed. 2004, 43, 2066-2090. (e) Krossing, I.; Reisinger, A. Angew.
Chem. 2003, 115, 5903-5906; Angew. Chem., Int. Ed. 2003, 42,
5725-5728. (f) Strauss, S. H. Chem. ReV. 1993, 93, 927-942. (g)
Hurlburt, P. K.; Rack, J. J.; Luck, J. S.; Dec, S. F.; Webb, J. D.;
Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc. 1994, 116, 10004-
10014. (h) Van Seggen, D. M.; Hurlburt, P. K.; Anderson, O. P.;
Strauss, S. H. J. Am. Chem. Soc. 1992, 114, 10995-10997. (i) Atwood,
J. L.; Steed J. W. In Supramolecular Chemistry of Anions; Bianchi,
A., Bowman-James, K., Garcia-Espana, E., Eds.; VCH: New York,
1997; pp 147-215. (j) Stasch, A.; Roesky, H. W.; Noltemeyer, M.;
Schmidt, H.-G. Inorg. Chem. 2005, 44, in press. (k) Long, D.-L.;
Ko¨gerler, P.; Cronin, L. Angew. Chem. 2004, 116, 1853-1856; Angew.
Chem., Int. Ed. 2004, 43, 1817-1820. (l) Vasylyev, M.; Neumann,
R. J. Am. Chem. Soc. 2004, 126, 884-890. (m) Xu, L.; Lu, M.; Xu,
B.; Wei, Y.; Peng, Z.; Powell, D. R. Angew. Chem. 2002, 114, 4303-
4306; Angew. Chem., Int. Ed. 2002, 41, 4129-4132. (n) Bhattacharyya,
R.; Biswas, S.; Armstrong, J.; Holt, E. M. Inorg. Chem. 1989, 28,
4297-4300. (o) Kang, H.; Liu, S.; Shaikh, S. N.; Nicholson, T.;
Zubieta, J. Inorg. Chem. 1989, 28, 920-933. (p) Day, V. W.;
Klemperer, W. G.; Maltbie, D. J. J. Am. Chem. Soc. 1987, 109, 2991-
3002. (q) Sun, H.; Harms, K.; Sundermeyer, J. J. Am. Chem. Soc.
2004, 126, 9550-9551.
(2) Jancik, V.; Roesky, H. W.; Neculai, D.; Neculai, A. M.; Herbst-Irmer,
R. Angew. Chem. 2004, 116, 6318-6322; Angew. Chem., Int. Ed.
2004, 43, 6192-6196.
(3) Magill, A. M.; Cavell, K. J.; Yates, B. F. J. Am. Chem. Soc. 2004,
126, 8717-8724.
(4) (a) Chen, W.; Liu, F.; You, X. J. Solid State Chem. 2002, 167, 119-
125. (b) Liu, F.; Chen, W.; You, X. J. Chem. Crystallogr. 2002, 32,
27-31. (c) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem.
Soc. 1991, 113, 361-363. (d) Niehues, M.; Kehr, G.; Erker, G.;
Wibbeling, B.; Fro¨hlich, R.; Blacque, O.; Berke, H. J. Organomet.
Chem. 2002, 663, 192-203.
(5) Arduengo, A. J., III; Bock, H.; Chen, H.; Denk, M.; Dixon, D. A.;
Green, J. C.; Herrmann, W. A.; Jones, N. L.; Wagner, M.; West, R.
J. Am. Chem. Soc. 1994, 116, 6641-6649.
(6) Arduengo, A. J., III; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1992, 114, 5530-5534.
5556 Inorganic Chemistry, Vol. 44, No. 16, 2005
10.1021/ic050693q CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/08/2005

COMMUNICATION
Scheme 1. Synthesis of Imidazolium Salts 4-6
Compound 5 is labile and decomposes to [CmH+]-
[LAl(SH)(S)]- (6) in the presence of trace amounts of
moisture both in solution and in the solid state even at low
temperature. The latter can be prepared by the direct reaction
of 3 with 1 equiv of N,N′-bismesitylimidazol-2-ylidene
(section S4 in the Supporting Information; Scheme 1). The
1H NMR spectra of 5 and 6 are very similar and indicate
substitution on the Al center with the absence of SH protons
for both species, although this condition is not fulfilled for
6 (one Al-S moiety and one Al-SH moiety). This can be
explained by fast proton migration. Further measurements
revealed that this process is fast even at low temperature
and starts immediately after mixing 3 with small amounts
of the carbene. In the presence of air, the pale yellow
solutions of these three salts turned to blue-green, indicating
their decomposition. In the case of 4, the intense blue-green
color appeared even if degassed and dry solvents were used.
We were not able to obtain any 1H NMR data of satisfactory
quality for 4, even at low temperature and 500 MHz. In the
salts 5 and 6, the color remained for 3 days when the flasks
were stored at -30 °C but disappeared within 1 h at ambient
temperature. The high reactivity of the salts 4-6 did not
allow their characterization by IR spectra and elemental
analysis. Thus, the purities of compounds 5 and 6 were
Figure 1. Molecular structure of 4‚THF (50% probability ellipsoids).
Hydrogen atoms (except the S-H and imidazole ring protons), the second
position of the disordered tBu group, and the THF molecule are omitted
for clarity. The cation containing N(3A) and N(4A) is stabilized by another
[LAl(SH)(S-)] anion, which is not depicted. Selected bond lengths [Å] and
angles [deg]: Al(1)-N(1), 1.946(2); Al(1)-N(2), 1.921(2); Al(1)-S(1),
2.290(1); Al(1)-S(2), 2.116(1); S(1)-H(1), 1.17(3); S(2)‚‚‚H(30A), 2.54;
S(2)‚‚‚H(31AA), 2.76; N(1)-Al(1)-N(2), 95.1(1); S(1)-Al(1)-S(2), 115.0-
(1); Al(1)-S(1)-H(1), 96(1).
1
determined by H NMR spectroscopy, whereas the purity
Figure 2. Molecular structure of 6‚THF (50% probability ellipsoids).
Hydrogen atoms (except the S-H and imidazole ring protons) and the
solvent molecule are omitted for clarity. The cation containing N(3A) and
N(4A) is stabilized by another [LAl(SH)(S-)] anion, which is not depicted.
Selected bond lengths [Å] and angles [deg]: Al(1)-N(1), 1.938(2);
Al(1)-N(2), 1.930(2); Al(1)-S(2), 2.115(1); Al(1)-S(1), 2.279(1);
S(1)-H(1), 1.25(2); S(2)‚‚‚H(30A), 2.46; S(2)‚‚‚H(31AA), 2.63;
N(1)-Al(1)-N(2), 95.7(1); S(1)-Al(1)-S(2), 117.8(1); Al(1)-S(2)-H(2),
93(1).
of compound 4 was confirmed by mounting 10 different
crystals from several preparations and measuring their cell
parameters. For all of the crystals, the cell parameters were
nearly identical. Pale yellow crystals of compounds 4 and 6
were obtained by slow crystallization of their saturated THF
solutions at -32 °C. Compound 4 crystallizes in the ortho-
rhombic space group P212121 and 6 in the monoclinic space
group P21/c (section S6 in the Supporting Information). Both
4 and 6 contain an ion pair and one solvating THF molecule
in the asymmetric unit (Figures 1 and 2). The deprotonation
of the S-H moiety resulted in the formation of a “naked”
Al-S center and shows a significant influence on the geom-
etry of the N2AlS2 core. The Al-S- bond lengths are 2.116
Å for 4 and 2.115 Å for 6 and represent the shortest Al-S
bonds described so far (compared with 2.159-2.483 Å for
known covalent Al-S bonds).7 The remaining Al-S(H)
bond lengths are 2.290 (4) and 2.279 Å (6), respectively,
and significantly longer than those in 3 (2.217 and 2.223
Å). The negative charge on the sulfur in both cases is reduced
by two close contacts with hydrogen atoms of the two
independent imidazolium cations. In the case of 6, the first
cation coordinates to the sulfur with the hydrogen atom on
C(30) (MesN-CH-NMes; S‚‚‚H ) 2.46 Å) and the second
cation with the hydrogen atom on C(31) (MesN-CH-CH-
NMes; 2.63 Å). In compound 4, the bonding mode is the
Inorganic Chemistry, Vol. 44, No. 16, 2005 5557

COMMUNICATION
Scheme 2. Synthesis of Compound 7
same like that of 6 (tBuN-CH-NtBu; S‚‚‚H ) 2.54 Å;
tBuN-CH-CH-NtBu; 2.78 Å). The weak S‚‚‚H interac-
tions are depicted in Figures 1 and 2. The originally deformed
tetrahedral core N2AlS2 in LAl(SH)28 is further deformed in
4 and 6 with smaller N-Al-N angles (97.3° in 3, 95.1° in
4, and 95.7° in 6) and wider S-Al-S angles (105.4° in 3,
115.0° in 4, and 117.8° in 6).
To demonstrate the double deprotonation of 5, it was
reacted with O(Me2SiCl)2 in a 1:1 molar ratio. Nearly
quantitative conversion of 5 to the Al-S-Si moiety contain-
ing LAl(SSiMe2)2O (7) was observed (Scheme 2; section
S5 in the Supporting Information). Furthermore, the N,N′-
bismesitylimidazolium chloride can be easily separated from
the products after extraction with THF. It should be
mentioned that our earlier attempt to use the lithium salts 1
and 22 failed and only a mixture of products was isolated
from the reaction with O(Me2SiCl)2. Furthermore, only one
example with the Al-S-Si framework (Me2Al(µ-SSiPh3)2-
AlMe2) that contains the sulfur atom in coordination number
three has been reported so far.9 There are no reports on the
preparation of such a moiety from Al-S precursors. Com-
pound 7 was obtained as a white microcrystalline solid
sensitive toward moisture, and its composition and molecular
structure were determined by multinuclear NMR spectros-
copy and mass spectrometry (MS) and unambiguously by
X-ray structural analysis. The electron impact MS spectrum
of 7 shows the molecular ion peak at m/z 640 (90%),
demonstrating the high thermal stability of the Al(SSi)2O
six-membered ring. Recrystallization of 7 from a toluene/
hexane mixture at -32 °C resulted in X-ray quality monoc-
rystals. Compound 7 crystallizes in the monoclinic space
group P21/n with one molecule in the asymmetric unit
(Figure 3; section S6 in the Supporting Information). The
Al(SSi)2O ring possesses a deformed boat conformation, and
the S(1)-Al-S(2) plane is almost perpendicular to the
N(1)-Al(1)-N(2) one (89.2°). The Al-S bond lengths are
Figure 3. Molecular structure of 7 (50% probability ellipsoids). All
hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles
[deg]: Al(1)-N(1), 1.908(1); Al(1)-N(2) 1.900(1); Al(1)-S(1), 2.234-
(1); Al(1)-S(2), 2.223(1); S(1)-Si(1), 2.128(1); S(2)-Si(2), 2.138(1);
O(1)-Si(1), 1.641(1); O(1)-Si(2), 1.639(1); N(1)-Al(1)-N(2), 96.6(1);
S(1)-Al(1)-S(2), 113.1(1); Al(1)-S(1)-Si(1), 106.0(1); Al(1)-S(2)-
Si(2), 106.6(1); S(1)-Si(1)-O(1), 111.3(1); S(2)-Si(2)-O(1), 110.3(1);
Si(1)-O(1)-Si(2), 135.9(1).
2.222 and 2.234 Å, slightly longer than those in 3 and
significantly longer than the corresponding ones in
LAl(µ-S)2TiCp2 (2.197 and 2.208 Å)2 but shorter than those
in LAl(µ-S)2AlL (2.237 and 2.245 Å).10 The Si-S bond
lengths (2.128 and 2.138 Å) are within the range published
for other compounds (1.949-2.288 Å).11 As expected, the
formation of the stable Al(SSi)2O six-membered ring resulted
in a more obtuse S-Al-S angle [113.1°, compared to those
of 3 (105.4°)3 and LAl(µ-S)2TiCp2 (102.5°)],2 which confirms
lower ring strain than that in LAl(µ-S)2TiCp2.
In summary, the imidazolium cation is an ideal system
for the stabilization of unusual anions. [CxH+][LAl(SH)(S)]-
and [CmH+]2[LAl(S)2]2- are promising precursors for further
reactions and are, together with their oxidation products, the
subject of our ongoing research.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Che-
mischen Industrie and is dedicated to Professor Narayan S.
Hosmane.
Supporting Information Available: All experimental details
(S1-S5); crystal data collection, structure solution, and refinement
details for compounds 4, 6, and 7 (S6) (pdf); and X-ray structural
information in CIF format for 4, 6, and 7. This material is available
free of charge via the Internet at http://pubs.acs.org.
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IC050693Q
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5558 Inorganic Chemistry, Vol. 44, No. 16, 2005