Twofold hydrogallation of C≡C triple bonds by H-GaCl2 - Synthesis of chelating Lewis acids and their application in adduct formation

Article abstract of DOI:10.1021/ic701928s

The twofold addition of Ga-H to C≡C triple bonds (hydrogallation) did not succeed by treatment of alkynes with an excess of dialkylgallium hydrides. In contrast, double hydrogallation was easily achieved by the reactions of H-GaCl₂ with trimethylsilyl-substituted alkynes [trimethylsilylphenylethyne and 1,4-bis(trimethylsilylethynyl)benzene] in appropriate stoichiometric ratios. The monoalkyne yielded the compound [H ₅C₆-CH₂-C(SiMe₃)(GaCl ₂)₂]₂, 1, which was only sparingly soluble in n-hexane. Crystal structure determination revealed a dimeric formula unit possessing two parallel Ga₂Cl₂ heterocycles in the solid state. Treatment of the dialkyne with four equivalents of the hydride gave a colorless precipitate, which was completely insoluble in hydrocarbons. Addition of diethyl ether to both products afforded soluble and monomelic etherates 1(OEt₂)₂ and 2 in which the two or four gallium atoms, respectively, were coordinated by ether molecules. Preliminary experiments with simple Lewis bases showed the principle capability of 1 to act as an effective chelating Lewis acid. Adducts of the type [H₅C₆-CH ₂-C(SiMe₃)(GaCl₂)(GaClX)(μ-Cl)]^- (4-6) resulted upon treatment with halide ions (X = Cl, Br, I).

Full text of DOI:10.1021/ic701928s

Inorg. Chem. 2008, 47, 729−735
Twofold Hydrogallation of CtC Triple Bonds by H−GaCl2sSynthesis of
Chelating Lewis Acids and Their Application in Adduct Formation
Werner Uhl* and Michael Claesener
Institut fu¨r Anorganische und Analytische Chemie der UniVersita¨t Mu¨nster, Corrensstraâe 30,
D-48149 Mu¨nster, Germany
Received September 28, 2007
The twofold addition of Ga
an excess of dialkylgallium hydrides. In contrast, double hydrogallation was easily achieved by the reactions of
GaCl2 with trimethylsilyl-substituted alkynes [trimethylsilylphenylethyne and 1,4-bis(trimethylsilylethynyl)benzene]
in appropriate stoichiometric ratios. The monoalkyne yielded the compound [H5C6 CH2 C(SiMe3)(GaCl2)2]2, 1, which
−H to CtC triple bonds (hydrogallation) did not succeed by treatment of alkynes with
H−
−
−
was only sparingly soluble in n-hexane. Crystal structure determination revealed a dimeric formula unit possessing
two parallel Ga2Cl2 heterocycles in the solid state. Treatment of the dialkyne with four equivalents of the hydride
gave a colorless precipitate, which was completely insoluble in hydrocarbons. Addition of diethyl ether to both
products afforded soluble and monomeric etherates 1(OEt2)2 and 2 in which the two or four gallium atoms, respectively,
were coordinated by ether molecules. Preliminary experiments with simple Lewis bases showed the principle capability
of 1 to act as an effective chelating Lewis acid. Adducts of the type [H5C6
(4 6) resulted upon treatment with halide ions (X Cl, Br, I).
−CH2−C(SiMe3)(GaCl2)(GaClX)(µ
-Cl)]-
−
)
Introduction
real capability to act as a chelating Lewis acid was tested in
rare cases only. An example is the methylene-bridged
dialuminum compound R2Al-CH2-AlR2 which is stabilized
against dismutation processes by very bulky bis(trimethyl-
silyl)methyl groups.2 It forms persistent and ether-soluble
complexes by treatment with hydride, nitrate, nitrite, azide,
acetate anions, etc. (examples are given in Scheme 1).3
Chelating Lewis acids possessing more than one acceptor
function in a single molecule form an interesting class of
compounds which has found increasing attention in current
research. Owing to their particular functionality they may
be applicable in a broad variety of chemical processes such
as catalysis, phase transfer, or anion recognition. Compounds
containing coordinatively unsaturated aluminum, gallium, or
indium atoms seem to be particularly suitable for the
formation of those polyacceptors; however, their access is
rather limited, and relatively inflexible organic backbones
such as aromatic rings were often employed for their
generation.1 Furthermore, these compounds usually do not
have more than two group 13 atoms in one molecule. Their
Addition of Al-H or Ga-H bonds of easily accessible
dialkylelement hydrides to the CtC triple bonds of poly-
alkynes (hydroalumination or hydrogallation) may be suitable
to open a facile route for the generation of a broad variety
of chelating Lewis acids. However, in contrast to general
textbook knowledge, these reactions proved to be rather
complicated in many cases, and very interesting and un-
precedented products were obtained by secondary reactions.
For instance, carbaalanes possessing clusters of aluminum
and carbon atoms and a delocalized bonding situation
resulted from the hydroalumination of aluminum alkynides
* Author to whom correspondence should be addressed. E-mail:
uhlw@uni-muenster.de.
(1) Selected publications: (a) Kaul, F. A. R.; Tschinkl, M.; Gabbai, F. P.
J. Organomet. Chem. 1997, 539, 187. (b) Saied, O.; Simard, M.;
Wuest, J. D. Inorg. Chem. 1998, 37, 2620. (c) Saied, O.; Simard, M.;
Wuest, J. D. Organometallics 1998, 17, 1128. (d) Thiyagarajan, B.;
Jordan, R. F.; Young, V. G. Organometallics 1998, 17, 281. (e) Piers,
W. E.; Irvine, G. J.; Williams, V. C. Eur. J. Inorg. Chem. 2000, 2131.
(f) Cottone, A., III; Scott, M. J. Organometallics 2002, 21, 3610. (g)
Melaimi, M.; Gabbai, F. P. J. Am. Chem. Soc. 2005, 127, 9680. (h)
Schulte, M.; Gabbai, F. P. Can. J. Chem. 2002, 80, 1308. (i) Schulte,
M.; Gabbai, F. P. Chem. Eur. J. 2002, 8, 3802. (j) Melaimi, M.;
Gabbai, F. P. AdV. Organomet. Chem. 2005, 53, 61.
(2) (a) Layh, M.; Uhl, W. Polyhedron 1990, 9, 277. (b) Uhl, W.; Layh,
M. J. Organomet. Chem. 1991, 415, 181.
(3) (a) Uhl, W.; Layh, M. Z. Anorg. Allg. Chem. 1994, 620, 856. (b) Uhl,
W.; Koch, M.; Heckel, M.; Hiller, W.; Karsch, H. H. Z. Anorg. Allg.
Chem. 1994, 620, 1427. (c) Uhl, W.; Hannemann, F.; Saak, W.;
Wartchow, R. Eur. J. Inorg. Chem. 1998, 921. (d) Uhl, W.;
Hannemann, F. J. Organomet. Chem. 1999, 579, 18. (e) Uhl, W.; Koch,
M.; Vester, A. Z. Anorg. Allg. Chem. 1993, 619, 359.
10.1021/ic701928s CCC: $40.75
Published on Web 12/15/2007
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 2, 2008 729

Uhl and Claesener
Scheme 1
These compounds are of particular interest owing to several
reasons: The chlorine atoms may be replaced by alkyl or
aryl groups upon treatment with organolithium derivatives;
the electronegative halogen atoms enhance the Lewis acidity
of the gallium atoms; and they may exhibit interesting
structural motifs by oligomerization via Ga-Cl bridges. So
far twofold hydroalumination or hydrogallation reactions
were only observed for aluminum or gallium alkynides under
relatively drastic reaction conditions. We report here on
attempts to realize double hydrogallation with other alkynes
too.
Experimental Section
All procedures were carried out under an atmosphere of purified
argon in dried solvents (n-hexane over LiAlH4, toluene, diethyl
ether, and THF over Na/benzophenone). H-GaCl2 was obtained
according to a literature procedure.10 H-GaCl2 is only sparingly
soluble in n-hexane, and high dilution is necessary to obtain
homogeneous mixtures. The commercially available alkynes (Al-
drich) trimethylsilylphenylethyne and 1,4-bis(trimethylsilylethynyl)-
benzene were dried over molecular sieves. Only the most intensive
peaks of the mass spectrum of 1 were given; the complete isotopic
patterns are in accordance with the calculated ones. The assignment
of the NMR spectra is based on HMBC, HSQC, ROESY, and
DEPT135 data.
Reaction of H-GaCl2 with Trimethylsilylphenylethyne in a
Molar Ratio of 2 to 1: Synthesis of 1. H-GaCl2 (1.16 g, 8.19
mmol) was suspended in 90 mL of n-hexane and treated with neat
trimethylsilylphenylethyne (0.70 g, 0.79 mL, 4.02 mmol) at room
temperature. The mixture was heated under reflux for 15 h. A
colorless solid of 1 precipitated, which was filtered off and washed
with 10 mL of n-hexane. Owing to its high purity, further
purification of the product was unnecessary; recrystallization
succeeded from toluene. Yield: 1.68 g (91%). Treatment with
diethyl ether yielded quantitatively the etherate 1(OEt2)2. It was
recrystallized from diethyl ether (20/+4 °C).
by the release of trialkylaluminum derivatives.4 The corre-
sponding reactions of gallium alkynides afforded heteroada-
mantane structures with localized Ga-C bonds.5 Di(tert-
butyl)butadiyne and different dialkylaluminum hydrides
yielded singular persistent carbocationic species by C-H
bond activation and chelating coordination of the released
hydride ion by two aluminum atoms.6 tert-Butylethynylben-
zenes gave cyclophane-type molecules with up to three
bridging aluminum or gallium atoms by condensation. These
secondary reactions were prevented by very bulky substit-
uents attached to the aluminum or gallium atoms of the
hydrides and unexpectedly also by application of very small
groups.7 Trimethylsilyl residues in the terminal position of
the ethynes favored formation of the expected simple addition
products, R2E-(Me3Si)CdC(H)-Aryl (E ) Al, Ga). The
gallium or aluminum atoms attacked selectively those carbon
atoms which bear the trimethylsilyl groups.8 Depending on
the steric shielding and thermal conditions cis/trans-isomer-
ization processes occurred which finally yielded the ther-
modynamically favored products with aluminum or gallium
and the hydrogen atoms on different sides of the resulting
CdC double bonds. In a recent publication we reported on
hydrogallation reactions involving dichlorogallium hydride
H-GaCl2.9 Monoaddition products, Aryl-(H)CdC(SiMe3)-
GaCl2, having exclusively a trans configuration were isolated.
Characterization of 1. Mp (argon, closed capillary): 204 °C.
MS (EI, 70 eV): m/z 439 (3%), 441 (7%), 443 (8%), 445 (5%)
M+ of the monomer - Me; 419 (5%), 421 (14%), 423 (13%), 425
(5%) M+ of the monomer - Cl. 1H NMR (C6D6, saturated solution,
low concentration, 400 MHz): δ 7.28 (4 H, o-H of phenyl), 7.05
(4 H, m-H of phenyl), 6.98 (2 H, p-H of phenyl), 3.18 (4 H, s,
CH2), 0.20 (18 H, s, SiMe3). 1H NMR (THF-d8, 400 MHz): δ 7.57
3
(4) (a) Uhl, W.; Breher, F. Angew. Chem. 1999, 111, 1578; Angew. Chem.,
Int. Ed. Engl. 1999, 38, 1477. (b) Uhl, W.; Breher, F.; Lu¨tzen, A.;
Saak, W. Angew. Chem. 2000, 112, 414; Angew. Chem., Int. Ed. Engl.
2000, 39, 406. (c) Uhl, W.; Breher, F.; Grunenberg, J.; Lu¨tzen, A.;
Saak, W. Organometallics 2000, 19, 4536. (d) Uhl, W.; Breher, F.;
Mbonimana, A.; Gauss, J.; Haase, D.; Lu¨tzen, A.; Saak, W. Eur. J.
Inorg. Chem. 2001, 3059.
(5) Uhl, W.; Cuypers, L.; Neumu¨ller, B.; Weller, F. Organometallics 2002,
21, 2365.
(6) (a) Uhl, W.; Grunenberg, J.; Hepp, A.; Matar, M.; Vinogradov, A.
Angew. Chem. 2006, 118, 4465; Angew. Chem., Int. Ed. 2006, 45,
4358. (b) Uhl, W.; Vinogradov, A.; Grimme, S. J. Am. Chem. Soc.
2007, 129, 11259.
(7) (a) Uhl, W.; Claesener, M.; Haddadpour, S.; Jasper, B.; Hepp, A.
Dalton Trans. 2007, 4, 417. (b) Uhl, W.; Breher, F.; Haddadpour, S.
Organometallics 2005, 24, 2210. (c) Uhl, W.; Haddadpour, S.; Matar,
M. Organometallics 2006, 25, 159. (d) Uhl, W.; Hepp, A.; Matar,
M.; Vinogradov, A. Eur. J. Inorg. Chem. 2007, 4133. (e) Uhl, W.;
Matar, M. Z. Anorg. Allg. Chem. 2005, 631, 1177.
(8) (a) Uhl, W.; Breher, F. J. Organomet. Chem. 2000, 608, 54. (b) Uhl,
W.; Matar, M. J. Organomet. Chem. 2002, 664, 110. (c) Uhl, W.;
Bock, H. R.; Breher, F.; Claesener, M.; Haddadpour, S.; Jasper, B.;
Hepp, A. Organometallics 2007, 26, 2363.
(4 H, d, JHH ) 7.6 Hz, o-H of phenyl), 7.21 (4 H, m, m-H of
phenyl), 7.14 (1 H, m, p-H of phenyl), 3.51 (4 H, s, CH2), 0.17 (18
H, s, SiMe3). 13C NMR (C6D6, saturated solution, low concentration,
100 MHz): δ 130.2 (m-C of phenyl), 129.2 (o-C of phenyl), 128.1
(p-C of phenyl), 36.1 (CH2), 1.4 (SiMe3); carbon atoms attached
Ga and Si and the ipso-C atoms of the phenyl rings were not
observed. 13C NMR (THF-d8, 100 MHz): δ 144.3 (ipso-C of
phenyl), 131.2 (o-C of phenyl), 129.0 (m-C of phenyl), 127.3 (p-C
of phenyl), 36.7 (CH2), 2.3 (SiMe3). 29Si NMR (THF-d8, 79.5
MHz): δ 2.0. IR (paraffin, CsBr plates, cm-1): 1599 m, 1584 s,
1557 m, 1493 m (phenyl); 1465 vs (paraffin); 1408 m δ(CH); 1377
vs (paraffin); 1302 w, 1265 m, 1256 s δ(CH3); 1209 w, 1175 m,
1153 w, 1123 w, 1080 s, 1028 m, 1003 w, 984 vw, 974 vw ν(CC),
(phenyl); 947 m, 933 m, 912 s, 839 vs, 762 vs, 752 s F(CH3(Si));
729 vs (paraffin); 704 vs, 689 m, 667 s νas(SiC); 627 w νs(SiC);
561 s, 522 w, 424 m ν(GaC), ν(GaCl), ν(Ga2Cl2). Anal. Calcd for
(10) (a) Nogai, S.; Schmidbaur, H. Inorg. Chem. 2002, 41, 4770. (b)
Ohshita, J.; Schmidbaur, H. J. Organomet. Chem. 1993, 453, 7.
(9) Uhl, W.; Claesener, M. Inorg. Chem. 2007, 46, in press.
730 Inorganic Chemistry, Vol. 47, No. 2, 2008

Synthesis of Chelating Lewis Acids
C22H32Si2Cl8Ga4 (915.1): C, 28.9; H, 3.5; Cl, 31.0; Ga, 30.5.
Found: C, 28.7; H, 3.7; Cl, 30.7; Ga, 30.4.
1339 vw, 1322 w, 1308 w, 1265 m, 1246 s δ(CH3); 1169 m, 1109
w, 1080 w, 1057 w, 1030 w, 1008 m, 993 m ν(CC), ν(CO), ν-
(CN); 945 w, 926 w, 910 m, 895 m, 883 w, 839 vs, 772 m, 750 vs,
737 m F(CH3(Si)); 698 s, 685 w, 654 m νas(SiC); 623 w, 617 w
νs(SiC); 556 m, 529 w, 437 m ν(GaC), ν(GaCl), ν(Ga2Cl). Anal.
Calcd for C27H52NSiCl5Ga2 (735.5): C, 44.1; H, 7.1; C, 24.1; Ga,
19.0. Found: C, 44.1; H, 7.1; Cl, 24.2; Ga, 18.8.
Characterization of 1(OEt2)2. Mp (argon, closed capillary):
75 °C. 1H NMR (C6D6, 400 MHz): δ 7.59 (2 H, d, 3JHH ) 7.6 Hz,
o-H of phenyl), 7.09 (2 H, pseudo-t, 3JHH ) 7.6 Hz, m-H of phenyl),
3
6.98 (1 H, pseudo-t, JHH ) 7.6 Hz, p-H of phenyl), 3.62 (8 H, q,
3
3JHH ) 6.8 Hz, OCH2), 3.59 (2 H, s, CH2), 0.88 (12 H, t, JHH
)
6.8 Hz, CH3 of ether), 0.37 (9 H, s, SiMe3). 13C NMR (C6D6, 100
MHz): δ 143.5 (ipso-C of phenyl), 129.8 (o-C of phenyl), 129.0
(m-C of phenyl), 127.1 (p-C of phenyl), 67.5 (OCH2), 36.2 (CH2-
Ph); 2.0 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 1.8. IR (paraffin,
CsBr plates, cm-1): 1600 w, 1582 w (phenyl); 1463 vs, 1377 s
(paraffin); 1325 m, 1310 m, 1287 m, 1263 m, 1251 m δ(CH3);
1223 w, 1184 w, 1149 m, 1120 w, 1090 m, 1022 m ν(CC), (phenyl);
937 w, 918 w, 889 w, 841 m F(CH3(Si)); 721 m (paraffin); 657 w
νas(SiC); 623 vw νs(SiC); 526 m, 457 m ν(GaC), ν(GaCl), ν(GaO).
Reaction of H-GaCl2 with 1,4-Bis(trimethylsilylethynyl)-
benzene in a Molar Ratio of 4 to 1: Synthesis of 2. 1,4-Bis-
(trimethylsilylethynyl)benzene (0.503 mg, 1.86 mmol) dissolved
in 30 mL of n-hexane was added to a suspension of H-GaCl2
(1.058 g, 7.47 mmol) in 60 mL of n-hexane at room temperature.
The mixture was heated under reflux for 16 h. A colorless solid
precipitated, which was filtered off and washed with 10 mL of
n-hexane. Diethyl ether (30 mL) was added. The clear solution
obtained after filtration was concentrated in vacuum and cooled to
4 °C to yield colorless crystals of the product 2. Yield: 0.832 g
(39%). 1H NMR (C6D6, 400 MHz): δ 7.53 (4 H, s, C6H4), 3.55 (4
H, s, CH2C6H4), 3.48 (16 H, q, 3JHH ) 7.2 Hz, OCH2), 0.91 (24 H,
Characterization of Compound 5. Mp (argon, closed capil-
lary): 134 °C. 1H NMR (C6D6, 400 MHz): δ 7.71 (2 H, m, o-H of
3
phenyl), 7.14 (2 H, t, JHH ) 7.6 Hz, m-H of phenyl), 7.02 (1 H,
t, 3JHH ) 7.6 Hz, p-H of phenyl), 3.98 (2 H, s, CH2), 2.60 (8 H, t,
NCH2), 1.24 (8 H, m, NCH2CH2), 1.19 (8 H, m, NCH2CH2CH2),
3
0.94 (12 H, t, JHH ) 7.2 Hz, NCH2CH2CH2CH3,), 0.46 (9 H, s,
SiMe3). 13C NMR (C6D6, 100 MHz): δ 144.5 (ipso-C of phenyl),
129.7 (o-C of phenyl), 129.0 (m-C of phenyl), 126.6 (p-C of
phenyl), 58.8 (NCH2), 47.8 (CGa2Si), 37.8 (CH2C6H5), 24.0
(NCH2CH2), 19.9 (NCH2CH2CH2), 13.9 (NCH2CH2CH2CH3), 1.8
(SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 1.6. IR (paraffin, CsBr
plates, cm-1): 1601 m, 1582 m, 1555 m, 1497 m (phenyl); 1454
vs, 1447 vs (paraffin); 1402 w δ(CH3); 1377 vs (paraffin); 1337
vw, 1319 w, 1308 w, 1258 m, 1244 m δ(CH3); 1211 vw, 1169 m,
1155 m, 1120 w, 1078 w, 1057 w, 1048 w, 1005 w, 991 m, 945 w
ν(CC), ν(CO), ν(CN); 925 w, 907 w, 897 w, 881 vw, 841 m, br,
772 w, 750 m F(CH3(Si)); 721 m (paraffin); 702 m, 687 w, 654 m
νas(SiC); 617 w νs(SiC); 588 vw, 556 m, 527 w, 469 vw, br, 438
m ν(GaC), ν(GaCl), ν(Ga2Cl), ν(GaBr). Anal. Calcd for C27H52-
NSiCl4Ga2Br (779.9): C, 41.6; H, 6.7; Cl, 18.2; Ga, 17.9; Br, 10.2.
Found: C, 42,0; H, 6.8; Cl, 18.3; Ga, 17.9.
3
t, JHH ) 7.2 Hz, CH3 of ether), 0.34 (18 H, s, SiMe3). 13C NMR
Characterization of Compound 6. Mp (argon, closed capil-
lary): 124 °C. 1H NMR (C6D6, 400 MHz, 350 K): δ 7.65 (2 H, d,
(C6D6, 100 MHz): δ 142 (ipso-C of phenyl), 129.9 (o-C of phenyl),
67.7 (OCH2), 42.5 (CGa2Si), 35.7 (CH2C6H4), 14.3 (CH3 of ether),
1.9 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 1.6. IR (paraffin,
CsBr plates, cm-1): 1578 s (phenyl); 1460 vs (paraffin); 1404 w
δCH; 1375 m (paraffin); 1304 vw, 1248 w δ(CH3); 1186 w, 1146
m, 1113 s, br, 1094 m ν(CC), ν(CO); 934 w, 887 w, 845 w, 814
m F(CH3(Si)); 721 m (paraffin); 662 vw νas(SiC); 621 vw νs(SiC);
588 w, 478 s, 448 m ν(GaC), ν(GaCl), ν(GaO). Anal. Calcd for
C32H66O4Si2Cl8Ga4 (1133.6): C, 33.9; H, 5.9; Cl, 25.0; Ga, 24.6.
Found: C, 35.0; H, 6.0; Cl, 24.5; Ga, 24.0.
General Procedure for the Synthesis of Adducts 4-6. A
solution of the digallium compound 1 in toluene [4, 0.287 g (0.63
mmol, based on monomeric 1) in 40 mL of toluene; 5, 0.418 g,
(0.91 mmol) in 45 mL of toluene; 6, 0.190 g (0.42 mmol) in 50
mL of toluene] was added to the corresponding tetrabutylammonium
halide [4 (X ) Cl), 0.174 g (0.63 mmol); 5 (X ) Br), 0.294 g
(0.91 mmol); 6 (X ) I), 0.153 g (0.42 mmol)] at room temperature.
After stirring for 16 h the reaction mixture was filtered, and the
filtrate was concentrated in vacuum. Crystals of compounds 4-6
were obtained upon cooling to 4 °C. Yields: 0.300 g (65%) of 4,
0.325 g (46%) of 5, 0.160 g (47%) of 6.
3
3JHH ) 7.6 Hz, o-H of phenyl), 7.14 (2 H, t, JHH ) 7.6 Hz, m-H
of phenyl), 7.02 (1 H, t, 3JHH ) 7.6 Hz, p-H of phenyl), 3.94 (2 H,
m, CH2C6H5), 2.66 (8 H, t, NCH2), 1.23 (8 H, m, NCH2CH2CH2),
1.23 (8 H, m, NCH2CH2), 0.90 (12 H, t, NCH2CH2CH2CH3), 0.41
(9 H, s, SiMe3). 13C NMR (C6D6, 100 MHz, 298 K): δ 144.6 (br,
ipso-C of phenyl), 129.5 (br, o-C of phenyl), 129.0 (br, m-C of
phenyl), 126.6 (p-C of phenyl), 58.6 (NCH2), 47.3 (CGa2Si), 37.4
(CH2C6H5), 23.8 (NCH2CH2), 19.8 (NCH2CH2CH2), 13.9 (NCH2-
CH2CH2CH3), 1.7 (SiMe3). IR (paraffin, CsBr plates, cm-1): 1599
m, 1580 m, 1555 m, 1493 m (phenyl); 1466 vs (paraffin); 1402 w
δ(CH3); 1377 vs (paraffin); 1366 m, 1306 w, 1260 m, 1242 s δ-
(CH3); 1204 vw, 1169 s, 1109 w, 1078 w, 1057 w, 1030 m, 1005
w, 982 m ν(CC), ν(CO), ν(CN); 941 m, 926 w, 912 w, 895 m, 885
w, 839 vs, 754 vs F(CH3(Si)); 727 s (paraffin); 706 s, 684 s, 652
m νas(SiC); 621 w, 617 w νs(SiC); 551 s, 523 m, 463 m, 436 s
ν(GaC), ν(GaCl), ν(Ga2Cl). Anal. Calcd for C27H52NSiCl4Ga2I
(826.9): C, 39.2; H, 6.3; Cl, 17.1; Ga, 16.9; I, 15.3. Found: C,
39.1; H, 6.3; Cl, 17.5; Ga, 16.5; I, 14.7.
Crystal Structure Determinations. Single crystals of all
compounds were obtained by cooling solutions (1, 4, and 6 in
toluene; 1(OEt2)2 and 2 in diethyl ether) to +4 °C. Crystallographic
data were collected with a Bruker APEX diffractometer with
graphite-monochromated Mo KR radiation and a Bruker Smart 6000
diffractometer with Cu KR radiation (4). The crystals were coated
with a perfluoropolyether, picked up with a glass fiber, and
immediately mounted in the cooled nitrogen stream of the diffrac-
tometer. The crystallographic data and details of the final R values
are provided in Table 1. All structures were solved by direct
methods using the program system SHELXTL PLUS11 and refined
Characterization of Compound 4. Mp (argon, closed capil-
lary): 142 °C. 1H NMR (C6D6, 400 MHz): δ 7.68 (2 H, d, 3JHH
)
7.6 Hz, o-H of phenyl), 7.13 (2 H, t, 3JHH ) 7.6 Hz, m-H of phenyl),
3
7.01 (1 H, t, JHH ) 7.6 Hz, p-H of phenyl), 4.00 (2 H, s, CH2),
3
2.56 (8 H, t, JHH ) 7.6 Hz, NCH2), 1.21 (8 H, m, NCH2CH2),
1.17 (8 H, m, NCH2CH2CH2), 0.92 (12 H, t, 3JHH ) 7.2 Hz, NCH2-
CH2CH2CH3), 0.44 (9 H, s, SiMe3). 13C NMR (C6D6, 100 MHz):
δ 144.6 (ipso-C of phenyl), 129.5 (o-C of phenyl), 129.0 (m-C of
phenyl), 126.5 (p-C of phenyl), 58.8 (NCH2), 47.4 (CGa2Si), 37.4
(CH2-C6H5), 23.9 (NCH2CH2), 19.9 (NCH2CH2CH2), 13.9 (NCH2-
CH2CH2CH3), 1.7 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 1.2.
IR (paraffin, CsBr plates, cm-1): 1599 m, 1582 w, 1555 w, 1492
m (phenyl); 1454 vs (paraffin); 1400 w δ(CH3); 1379 s (paraffin);
(11) (a) SHELXTL-Plus, REL. 4.1; Siemens Analytical X-RAY Instruments
Inc.: Madison, WI, 1990. (b) Sheldrick, G. M. SHELXL-97, Program
for the Refinement of Structures; Universita¨t Go¨ttingen: Go¨ttingen,
Germany, 1997.
Inorganic Chemistry, Vol. 47, No. 2, 2008 731

Uhl and Claesener
Table 1. Crystallographic Data for 1, 1(OEt2)2, 2, 4, and 6
1‚toluene
1(OEt2)2
2
4
6
empirical formula
cryst syst
space group
a (pm)
b (pm)
c (pm)
C29H40Cl8Ga4Si2
triclinic
P-1
C19H36Cl4Ga2O2Si
triclinic
P1c
C32H66Cl8Ga4O4Si2
monoclinic
P21/c
1503.5(5)
995.5(3)
1645.5(5)
90
92.985(5)
90
2.460(1)
2
C27H52Cl5Ga2NSi
monoclinic
P21/c
1274.37(6)
1140.17(6)
2593.7(1)
90
92.378(3)
90
3.7653(3)
4
C27H52Cl4Ga2INSi
monoclinic
P21
1069.6(3)
1254.9(3)
1399.9(3)
90
95.701(5)
90
d
859.8(2)
1355.2(3)
1724.1(4)
101.698(4)
93.413(4)
91.305(4)
1.9624(8)
2
798.2(1)
913.4(1)
1002.2(1)
99.349(2)
104.236(2)
104.348(2)
0.6664(2)
1
R (deg)
â (deg)
γ (deg)
V (nm3)
Z
1.8697(8)
2
temp (K)
D
153(2)
1.705
153(2)
1.510
153(2)
1.531
153(2)
1.297
153(2)
1.469
calcd (g cm-3
)
independent data
obsd data
11 234
6693
6790
6054
7134
4900
6943
5394
7293
5744
params
395
3.343
0.0606
0.1534
260
2.480
0.0423
0.1036
233
2.682
0.0386
0.0977
+959/-467
332
5.437
0.0446
0.1245
+901/-623
368
2.601
0.0820
0.2363
+1174/-1487
µ (mm-1
)
R1a
wR2 (all data)b
residual density (e nm-3
)
+1378/-835
+912/-698
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo |. b wR2 ) [∑[w(Fo2 - Fc )2]/∑[w(Fo )2]]1/2
.
c Flack parameter: 0.01(1). d Flack parameter
2
2
0.04(3).
with the SHELXL-9711 program via full-matrix least-squares
calculations based on F2. All non-hydrogen atoms were refined with
anisotropic displacement parameters; hydrogen atoms were calcu-
lated on ideal positions and allowed to ride on the bonded atom
with U ) 1.2Ueq(C). Compound 1 crystallized with one molecule
of toluene per formula unit of the dimer. 2 is located on a
crystallographic center of symmetry. The tetrabutylammonium ion
of 6 is strongly disordered; the disorder was partially resolved by
applying split positions for some carbon atoms.
THF. Complete dissolution occurred also in diethyl ether.
The resulting etherate [1(OEt2)2] (eq 1) is soluble in benzene.
The successful double hydrogallation of the CtC triple
bonds was easily established by a singlet at δ ) 3.51 in the
1H NMR spectrum characteristic of a CH2 group and the
missing signals of alkene protons at a low field (δ > 6).
Similarly indicative were the data of the 13C NMR spectra
which had the resonance of the CH2 group at δ ) 36.7.
Results and Discussion
Hydrogallation Reactions. To date all experiments failed
to achieve twofold hydrogallation of alkynes by application
of an excess of different dialkylgallium hydrides under
standard conditions in boiling n-hexane over several hours.
As mentioned before, we recently reported the reaction of
H-GaCl2 with trimethylsilyl-substituted alkynes, which
yielded the corresponding alkenes in high yields.9 Only the
monoalkyne Me3Si-CtC-C6H5 gave an addition product
which was easily soluble in hydrocarbons. However, an only
sparingly soluble byproduct precipitated in small quantities,
which dissolved readily in diethyl ether. NMR spectroscopic
characterization of the resulting etherate indicated the pos-
sible occurrence of twofold hydrogallation as a minor side
reaction, which is in accordance with the application of a
small excess of the hydride in all reactions. Thus, we treated
the monoalkyne trimethylsilylphenylethyne with the hydride
in a 1 to 2 molar ratio and isolated the colorless product of
the twofold hydrogallation (1) in an almost quantitative yield
(eq 1). A schematic drawing of the solid-state structure of 1
is depicted in eq 1. It formed dimeric formula units via Ga-
Cl bridges. 1 precipitated from the reaction mixture and was
obtained by filtration directly without further purification in
high purity. It is only sparingly soluble in hydrocarbons such
as n-hexane or benzene, and NMR characterization of the
pure compound could only be conducted with some difficul-
ties in saturated benzene solutions having a relatively low
concentration of 1. The most accurate data were obtained in
Reaction of the dialkyne 1,4-bis(trimethylsilylethynyl)-
benzene with four equivalents of H-GaCl2 afforded a
colorless precipitate, which proved to be completely insoluble
in hydrocarbon solvents. The solid dissolved partially upon
treatment with diethyl ether. However, decomposition oc-
curred by formation of a black precipitate, which probably
contained elemental gallium. Filtration and cooling of the
filtrate to +4 °C gave colorless crystals of the etherate 2 in
39% yield (eq 2). A gallium mirror was formed by
decomposition in some cases during the purification process.
732 Inorganic Chemistry, Vol. 47, No. 2, 2008

Synthesis of Chelating Lewis Acids
Compound 2 is soluble in benzene. It resulted from the
twofold hydrogallation of both triple bonds of the starting
dialkyne and has four dichlorogallium groups in one
molecule. All gallium atoms are coordinated by a diethyl
ether ligand. The spectroscopic findings are quite similar to
those of the etherate 1(OEt2)2.
Figure 1. Molecular structure and numbering scheme of 1; the thermal
ellipsoids are drawn at the 40% probability level; hydrogen atoms are
omitted for clarity. Important bond lengths [pm] and angles [deg]: Ga1-
C1 195.2(5), Ga2-C1 194.8(5), Ga3-C2 195.4(5), Ga4-C2 196.6(5),
Ga1-Cl1 214.4(1), Ga1-Cl13 234.4(1), Ga1-Cl31 231.2(1), Ga2-Cl2
212.2(2), Ga2-Cl24 233.7(1), Ga2-Cl42 231.7(1), Ga3-Cl3 213.8(2),
Ga3-Cl13 234.5(1), Ga3-Cl31 231.8(1), Ga4-Cl4 213.2(2), Ga4-Cl24
234.1(1), Ga4-Cl42 233.0(1), C1-Si1 192.2(5), Si2-C2 191.4(5), Ga1-
C1-Ga2 112.1(2), Ga3-C2-Ga4 111.8(2), Cl13-Ga1-Cl31 87.42(5),
Cl24-Ga2-Cl42 88.59(4), Cl13-Ga3-Cl31 87.24(5), Cl24-Ga4-Cl42
88.20(5), Ga1-Cl13-Ga3 91.12(4), Ga1-Cl31-Ga3 92.60(5), Ga2-
Cl24-Ga4 91.14(4), Ga2-Cl42-Ga4 91.93(4).
At least the second hydrogallation step of the reaction
described in eq 1 is reversible. We verified this in an NMR
experiment when we mixed equimolar quantities of the
hydrogallation product 1 and the starting alkyne Me3Si-
CtC-C6H5 in benzene (eq 3). Quantitative formation of
the alkenyl compound 3 occurred without NMR evidence
of any byproduct within 16 h at room temperature. Com-
pound 3 was described in a former publication.9 It was
obtained almost quantitatively by treatment of the starting
monoalkyne with one equivalent of H-GaCl2 and may be
considered as a reasonable intermediate in the course of the
formation of 1.
free compound 1 possesses an unprecedented structure
(Figure 1). By twofold hydrogallation two dichlorogallium
groups were introduced per formula unit of the starting
monoalkyne, and both gallium atoms are geminally attached
to the same carbon atom. In accordance with the charge
separation of the ethynyl moiety and the expected monogal-
lium intermediate9 both gallium atoms attacked exclusively
that carbon atom which was attached to the trimethylsilyl
group. The second carbon atom of the triple bond in the R
position to the phenyl ring was transferred to a CH2 group.
Coordinative saturation of the metal atoms occurred by
dimerization via Ga-Cl bridges and formation of two
neighboring and almost ideally coplanar Ga2Cl2 heterocycles.
Each gallium is further attached to a carbon and a terminal
chlorine atom. Expectedly the Ga-Cl distances differ
considerably with the different bonding situations and adopt
values of 213.4 (terminal Ga-Cl) versus 233.1 pm (bridging
Ga-Cl-Ga). Both are in the ranges usually observed.12
Interestingly a centrosymmetric molecule did not form.
Instead, the molecular structure approaches the point group
(12) (a) Atwood, D. A.; Cowley, A. H.; Jones, R. A.; Mardones, M. A.;
Atwood, J. L.; Bott, S. G. J. Coord. Chem. 1992, 25, 233. (b) Power,
M. B.; Cleaver, W. M.; Apblett, A. W.; Barron, A. R.; Ziller, J. W.
Polyhedron 1992, 11, 477. (c) Schaller, F.; Schwarz, W.; Hausen, H.-
D.; Klinkhammer, K. W.; Weidlein, J. Z. Anorg. Allg. Chem. 1997,
623, 1455. (d) Crittendon, R. C.; Li, X.-W.; Su, J.; Robinson, G. H.
Organometallics 1997, 16, 2443. (e) Beachley, O. T.; Hallock, R. B.;
Zhang, H. M.; Atwood, J. L. Organometallics 1985, 4, 1675. (f)
Carmalt, C. J.; Mileham, J. D.; White, A. J. P.; Williams, D. J.; Steed,
J. W. Inorg. Chem. 2001, 40, 6035. (g) Lustig, C.; Mitzel, N. W. Z.
Naturforsch., B: Chem. Sci. 2004, 59, 140. (h) Su, J.; Li, X.-W.;
Robinson, G. H. Chem. Commun. 1998, 2015. (i) Gillan, E. G.; Bott,
S. G.; Barron, A. R. Chem. Mater 1997, 9, 796. (j) Neumu¨ller, B.;
Gahlmann, F. Chem. Ber. 1993, 126, 1579. (k) Uhl, W.; Cuypers, L.;
Geiseler, G.; Harms, K.; Massa, W. Z. Anorg. Allg. Chem. 2002, 628,
1001. (l) Petrie, M. A.; Power, P. P.; Rasika Dias, H. V.; Ruhlandt-
Senge, K.; Waggoner, K. M.; Wehmschulte, R. J. Organometallics
1993, 12, 1086.
The three hydrogallation products 1, 1(OEt2)2, and 2 were
characterized by crystal structure determinations. The ether-
Inorganic Chemistry, Vol. 47, No. 2, 2008 733

Uhl and Claesener
of this class of hydrogallation products to act as bifunctional
acceptors we treated 1 in some preliminary, simple experi-
ments with tetrabutylammonium halides (X ) Cl, Br, I)
which have the considerable advantage of being soluble in
noncoordinating solvents such as toluene. The reactions
proceeded at room temperature overnight (eq 4). Suspensions
were obtained in all cases, and small quantities of an
unknown solid were removed by filtration. Colorless crystals
of the three adducts (4, X ) Cl; 5, X ) Br; 6, X ) I) were
obtained in yields between 65% and 45% upon concentration
and cooling of the filtrates to +4 °C. A liquid phase separated
in some cases, which showed NMR parameters identical to
those of the corresponding solid products and may be
1
described as an ionic liquid. The H NMR spectra of the
chloride adduct 4 exhibited singlets of the trimethylsilyl and
CH2 groups, and the integration ratios gave the correct
intensities. In contrast, the iodine product showed compli-
cated NMR spectra with several signals in the range of the
CH2 and SiMe3 groups, indicating the occurrence of several
different species in solution. Heating to temperatures above
330 K was required to get the expected simple pattern of
one singlet for each group. These observations verify a
hindered exchange process in solution. In the solid state (see
below) the iodine atom does not occupy the bridging position
between both gallium atoms but is terminally arranged at
one metal atom only. This particular molecular structure has
a chiral gallium atom and, hence, should result in diaste-
reotopic CH2 protons with two different resonances in the
1H NMR spectrum showing the characteristic shape of an
AB spectrum. Although that particular pattern was not clearly
resolved in the spectrum, it seems to be clear that the splitting
is at least partially caused by this unsymmetric situation. The
simple spectrum obtained at elevated temperature may be
caused by a fast exchange process with iodine rapidly
changing between terminal and bridging positions. The
bromine compound 5 showed a similar splitting of resonances
only when a solution in toluene was cooled to 280 K. Very
broad signals were obtained at lower temperatures (below
240 K). Thus, a dynamic behavior occurred also in that case,
but owing to the smaller size and molar mass of bromine
compared to iodine the activation barrier seems to be lower
for the exchange of chlorine and bromine positions.
Figure 2. Molecular structure and numbering scheme of 1(OEt2)2; the
thermal ellipsoids are drawn at the 40% probability level; hydrogen atoms
and ethyl groups of the ether molecules are omitted for clarity. Important
bond lengths [pm] and angles [deg]: Ga1-C1 197.0(4), Ga1-Cl11 218.6-
(1), Ga1-Cl12 219.1(1), Ga2-C1 200.2(4), Ga2-Cl21 219.9(1), Ga2-
Cl22 217.1(1), C1-Si1 189.6(4), Ga1-C1-Ga2 103.5(2), Cl11-Ga1-
Cl12 106.35(4), Cl21-Ga2-Cl22 104.32(5), C1-C2-C3 119.9(3).
Figure 3. Molecular structure and numbering scheme of 2; the thermal
ellipsoids are drawn at the 40% probability level; ethyl groups of the ether
ligands and hydrogen atoms are omitted for clarity. Important bond lengths
[pm] and angles [deg]: Ga1-C1 197.2(3), Ga1-Cl11 219.45(9), Ga1-
Cl12 218.8(1), Ga2-C1 197.6(3), Ga2-Cl21 218.47(9), Ga2-Cl22 219.4-
(1), Ga1-C1-Ga2 114.2(1), Cl11-Ga1-Cl12 103.62(4), Cl21-Ga2-Cl22
107.46(4), C1-C2-C3 118.7(2).
C2V with either trimethylsilyl or benzyl groups on the same
sides of the average molecular plane.
The Ga-Cl-Ga bridges were opened upon formation of
the monomeric etherates 1(OEt2)2 and 2 by dissociation and
coordination of each gallium atom by an ether molecule. The
molecular structures of both compounds are depicted in
Figures 2 and 3. Their structural parameters are quite similar.
The Ga-Cl distances (218.9 pm on average) correspond to
the values expected for terminal Ga-Cl bonds, and the
Ga-O distances (201.5 pm) are in the expected ranges of
dative Ga-O interactions.
Application of Compound 1 as a Chelating Lewis Acid.
The hypothetical, monomeric formula unit of compound 1
possesses two coordinatively unsaturated, tricoordinate gal-
lium atoms. Hence, it should be able to act as a dipodal
chelating Lewis acid upon treatment with suitable donor
atoms or molecules. In order to test the principle capability
Regrettably we did not succeed in growing single crystals
of the bromine compound 5. Crystal structures were deter-
mined for the chlorine and iodine products 4 and 6. The most
important anionic parts are depicted in Figures 4 and 5, while
we abstain from a pictorial presentation of the tetrabutylam-
734 Inorganic Chemistry, Vol. 47, No. 2, 2008

Synthesis of Chelating Lewis Acids
pound 6 showed a severe disorder of the tetrabutylammonium
ion. The relatively high displacement parameters observed
even for the heavier atoms of the anionic part may be further
caused by some intrinsic statistical disorder of the positions
of the halogen atoms. However, we were not able to find
any reasonable split position. An essential result is the
terminal arrangement of the iodine atom and occupation of
the bridging position between both chelating gallium atoms
by chlorine. The Ga-Cl and Ga-I distances are in the
expected ranges (223.4 and 244.0 pm for terminal and
bridging Ga-Cl bonds,12 249.2 pm for terminal Ga-I
bonds13). The angles Ga-Cl-Ga are much more acute (74°
on average) in adducts 4 and 6 than in the Ga2Cl2 hetero-
cycles of the dimeric compound 1 (92° on average). We
suppose that the preferred chelating coordination of the
chlorine atom compared to iodine does not depend on the
different sizes with the larger iodine atom not fitting into
the bite of the gallium atoms. Instead, that particular situation
may be favored by the higher strength of the Ga-Cl
interactions. These results show impressively the potential
applicability of these gallium halides (1 and 2) as effective
chelating Lewis acids. Preliminary experiments showed
further that they are interesting starting compounds for
generation of the corresponding alkyl derivatives, which are
not accessible by direct hydrogallation with dialkylgallium
hydrides and act as multiacceptors as well. Thus, these
compounds are promising reagents for comprehensive future
investigations.
Figure 4. Molecular structure and numbering scheme of the anion of 4;
the thermal ellipsoids are drawn at the 40% probability level; hydrogen
atoms are omitted for clarity. Important bond lengths [pm] and angles [deg]:
Ga1-C1 198.5(3), Ga1-Cl1 236.83(8), Ga1-Cl11 217.45(9), Ga1-Cl12
217.89(8), Ga2-C1 197.2(3), Ga2-Cl1 243.11(9), Ga2-Cl21 217.46(9),
Ga2-Cl22 218.08(8), Ga1-Cl1-Ga2 74.02(2).
Acknowledgment. We are grateful to the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for generous financial support.
Supporting Information Available: X-ray crystallographic files
in CIF format for compounds [C6H5-CH2-C(SiMe3)(GaCl2)2]2 (1),
C6H5-CH2-C(SiMe3)(GaCl2OEt2)2 [1(OEt2)2], C6H4[CH2-C(SiMe3)
(GaCl2OEt2)2]2 (2), [C6H5-CH2-C(SiMe3)(GaCl2)2(µ-Cl)]-[NnBu4]+
(4), and [C6H5-CH2-C(SiMe3)(GaCl2)(GaClI)(µ-Cl)]-[NnBu4]+
(6). This material is available free of charge via the Internet at http://
pubs.acs.org. Further details of the crystal structure determinations
are also available from the Cambridge Crystallographic Data Center
on quoting the depository numbers CCDC-662291 (1), -662292 [1-
(OEt2)2], -662293 (2), -662294 (4), and -662295 (6).
Figure 5. Molecular structure and numbering scheme of the anion of 6;
the thermal ellipsoids are drawn at the 40% probability level; hydrogen
atoms are omitted for clarity. Important bond lengths [pm] and angles [deg]:
Ga1-C1 196(1), Ga2-C1 196(1), Ga1-Cl22 244.7(3), Ga2-Cl22 243.3-
(3), Ga1-Cl11 223.1(3), Ga1-Cl12 223.4(3), Ga2-Cl21 223.7(4), Ga2-
I3 249.2(2), Ga1-Cl22-Ga2 73.07(8).
monium counterions. Compound 4 has five chlorine atoms
bonded to gallium; one of these (Cl1) is coordinated in a
chelating manner by both metal atoms. A four-membered,
nonplanar heterocycle results which consists of one carbon,
one chlorine, and two gallium atoms. In accordance with
the different bonding situation the Ga-Cl bond lengths differ
with the shorter ones observed for the terminal Ga-Cl bonds
(217.7 versus 240.0 pm). The molecular structure of com-
IC701928S
(13) Selected references only: (a) Aldridge, S.; Jones, C.; Junk, P. C.;
Richards, A. F.; Waugh, W. J. Organomet. Chem. 2003, 665, 127.
(b) Kehrwald, M.; Kostler, W.; Rodig, A.; Linti, G.; Blank, T.; Wiberg,
N. Organometallics 2001, 20, 860. (c) Wells, R. L.; Aubuchon, S.
R.; Lube, M. S.; White, P. S. Main Group Chem. 1995, 1, 81. (d)
Uhl, W.; El-Hamdan, A.; Pro¨tt, M.; Spuhler, P.; Frenking, G. Dalton
Trans. 2003, 1360.
Inorganic Chemistry, Vol. 47, No. 2, 2008 735