Hydrogallation of alkynes with H-GaCl2: Formation of organoelement dichlorogallium compounds potentially applicable as chelating lewis acids

Article abstract of DOI:10.1021/ic700767a

Treatment of trimethylsilylethynylbenzenes C₆H _6-x(C≡C-SiMe₃)_x (x = 1-3) with the hydridodichlorogallium compound H-GaCl₂ afforded, almost quantitatively, the alkenylphenyl compounds C₆H

Full text of DOI:10.1021/ic700767a

Inorg. Chem. 2008, 47, 4463-4470
Hydrogallation of Alkynes with H-GaCl2: Formation of Organoelement
Dichlorogallium Compounds Potentially Applicable as Chelating Lewis
Acids
Werner Uhl* and Michael Claesener
Institut für Anorganische and Analytische Chemie der UniVersität Münster, Corrensstrasse 30,
D-48149 Münster, Germany
Received April 23, 2007
Treatment of trimethylsilylethynylbenzenes C6H6-x(Ct C-SiMe3)x (x ) 1–3) with the hydridodichlorogallium compound
H-GaCl2 afforded, almost quantitatively, the alkenylphenyl compounds C6H6-x[C(H)dC(SiMe3)-GaCl2]x [x ) 1
(6), 2 (7), and 3 (8)] by hydrogallation. Only compound 6 was readily soluble in n-hexane; it formed dimers via
Ga-Cl bridges. The bisalkenyl compound 7 was only sparingly soluble; its molecular structure consisted of a
singular dimeric formula unit with a cyclophane-type constitution and two bridging Ga2Cl2 heterocycles. The overall
structure may be described by a molecular box formed by a large macrocycle comprising 22 Ga, C, and Cl atoms.
Compound 8 proved to be insoluble in hydrocarbon solvents. Its molecular structure could not be detected. Extraction
of the solid raw products of 7 and 8 with diethyl ether yielded small quantities of the ether adducts
C6H6-x[C(H)dC(SiMe3)-GaCl2(OEt2)]x (x ) 2, 3) [7(OEt2)2 and 8(OEt2)3], both of which are monomeric because
of the coordinative saturation of their gallium atoms. The tetraalkyne 1,2,4,5-tetrakis(trimethylsilylethynyl)benzene
gave a different reaction course. Complete hydrogallation resulted in the release of 2 equiv of GaCl3, and neighboring
alkenyl groups of the product 9 were connected by GaCl bridges to form seven-membered heterocycles and an
overall tricyclic compound. Compound 9 was characterized as a diethyl ether adduct.
Introduction
minum, a novel class of carbaalanes possessing clusters of
carbon and aluminum atoms and a delocalized bonding
situation, that is, (AlMe)8(CCH2C6H5)5H (1, Scheme 1).2 The
spontaneous release of trialkylaluminum was also observed
upon treatment of some alkyl-aryl-ethynes with dialkyla-
luminum hydrides; however, the course of these reactions
could not be clarified completely until now. In contrast, the
corresponding reactions of trimethylsilylethynes afforded the
expected dialkylaluminumalkenyl compounds (2, Scheme 1)
with the aluminum atoms exclusively bonded to those carbon
atoms of the CdC double bonds that also are attached to
the trimethylsilyl groups.3 A cis arrangement of Al and H
across the CdC double bonds resulted in the first step of all
reactions. A rearrangement to form the trans products could
Hydroalumination is a powerful method for the reduction
of organic compounds containing homo- or heteronuclear
double or triple bonds.1 The mechanisms of these reactions
and the structures of the organoaluminum products remained
unknown in most cases because usual workup included the
hydrolysis of the intermediates with isolation of the respec-
tive hydrocarbons only. In some recent investigations, we
found that the courses of these reactions and the compositions
of the intermediate organoelement compounds are much
more interesting than may be derived from textbook knowl-
edge and that fast secondary reactions often yielded unprec-
edented products. Hydroalumination of dialkylaluminum
alkynides gave, by the spontaneous release of trialkylalu-
(2) (a) Uhl, W.; Breher, F. Angew. Chem. 1999, 111, 1578 Angew. Chem.,
Int. Ed. 1999, 38, 1477. (b) Uhl, W.; Breher, F.; Lützen, A.; Saak, W.
Angew. Chem. 2000, 112, 414 Angew. Chem., Int. Ed. 2000, 39, 406.
(c) Uhl, W.; Breher, F.; Grunenberg, J.; Lützen, A.; Saak, W.
Organometallics 2000, 19, 4536. (d) Uhl, W.; Breher, F.; Mbonimana,
A.; Gauss, J.; Haase, D.; Lützen, A.; Saak, W. Eur. J. Inorg. Chem.
2001, 3059.
* To whom correspondence should be addressed. E-mail: uhlw@uni-
muenster.de.
(1) (a) Winterfeldt, E. Synthesis 1975, 10, 617. (b) Marek, J.; Normant,
J.-F. Chem. ReV. 1996, 96, 3341. (c) Eisch, J. J. In ComprehensiVe
Organic Chemistry; Frost, B. M., Flemming, J., Eds.; Pergamon:
Oxford, U.K., 1991; Vol. 8, p 773.
(3) (a) Uhl, W.; Breher, F. J. Organomet. Chem. 2000, 608, 54. (b) Uhl,
W.; Matar, M. J. Organomet. Chem. 2002, 664, 110.
10.1021/ic700767a CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 11, 2008 4463
Published on Web 08/01/2007

Uhl and Claesener
1).6,7 Only 1,4-di(tert-butylethynyl)benzene and dialkylgal-
lium hydrides bearing relatively small substituents (R ) Et,
n-Pr) yielded the stable addition products without condensa-
tion.6,7 In these cases, the low steric shielding allowed for
strong intermolecular interactions via the gallium atoms and
the negatively charged carbon atoms of the ethenyl groups,
which resulted in the formation of one-dimensional coordi-
nation polymers and prevented the substituent exchange
processes. Dialkylgallium alkynides, R2Ga-Ct C-R′, gave
condensation reactions which afforded heteroadamantane
type cages (5).8 Although they are analogues of the above-
mentioned carbaalanes and were generated on similar routes,
the bonding situation of both types of cages is completely
different with localized Ga-C bonds in the heteroadamantane
systems. Trimethylsilylethynylbenzenes gave stable addition
products with few exceptions only, for which, however, the
reaction courses could not be clarified at all.9 The gallium
atoms attacked exclusively the carbon atoms in ꢀ-position
to the phenyl rings, which also are attached to the trimeth-
ylsilyl groups. Spontaneous cis/trans isomerization occurred
in most reactions, and the products of the cis addition were
obtained for the sterically most shielded di(tert-butyl)gallium
hydride only.9
Scheme 1
Here, we report on hydrogallation reactions with the halide
H-GaCl2. This compound constitutes a very interesting
starting compound and is available on a facile route by a
procedure optimized in the group of Schmidbaur several
years ago.10 They also conducted a first hydrogallation
reaction with an alkene derivative.11 Investigations into the
reactivity of H-GaCl2 against trimethylsilylethynes seemed
to be rather interesting because of the three most important
reasons. (i) The products are expected to contain up to three
GaCl2 groups and, because of the enhanced Lewis-acidity
compared to dialkylgallium groups, should form very effec-
tive chelating Lewis acids. (ii) In the absence of donor
molecules, they should give interesting oligomers with the
formation of macrocycles or cages via Ga-Cl bridges. (iii)
These GaCl2 derivatives seem to be very important starting
compounds for application in secondary reactions, that is,
alkylation reactions by salt elimination.
be detected upon heating of one compound only [2(trans)],
while in other cases, decomposition occurred.3 Another
unprecedented and fascinating product resulted from the
double hydroalumination of di(tert-butyl)butadiyne with
di(tert-butyl)aluminum hydride. C-H bond activation and
the chelating coordination of a hydride ion yielded a
remarkable compound containing a butadienyl cation as the
organic backbone (3, Scheme 1).4
In some recent investigations, we found facile access to a
great variety of di(alkyl)gallium hydrides, which with the
exception of dimethylgallium hydride are stable enough to
be handled in boiling n-hexane without decomposition.5,6
Hydrogallation reactions with alkynes involving these hy-
drides proved to be slower and more selective than hydroalu-
mination reactions, which may depend on the lower polarity
of the Ga-H bonds. Systematic investigations gave a concise
insight into the course of these reactions. Ethynes bearing
tert-butyl and phenyl substituents usually gave condensation
reactions with the release of trialkylgallium and the formation
of [3,3]- and [3,3,3]cyclophane-type molecules (4, Scheme
(7) (a) Uhl, W.; Breher, F.; Haddadpour, S. Organometallics 2005, 24,
2210. (b) Uhl, W.; Haddadpour, S.; Matar, M. Organometallics 2006,
25, 159.
(8) Uhl, W.; Cuypers, L.; Neumüller, B.; Weller, F. Organometallics 2002,
21, 2365.
(9) Uhl, W.; Bock, H. R.; Breher, F.; Claesener, M.; Haddadpour, S.;
Jasper, B.; Hepp, A. Organometallics 2007, 26, 2363.
(10) Nogai, S.; Schmidbaur, H. Inorg. Chem. 2002, 41, 4770.
(11) (a) Ohshita, J.; Schmidbaur, H. J. Organomet. Chem. 1993, 453, 7
For further contributions to hydrogallation reactions, see: (b) Schmid-
baur, H.; Klein, H. F. Angew. Chem. 1966, 78, 306. (c) Schmidbaur,
H.; Klein, H. F. Chem. Ber. 1967, 100, 1129. (d) Schumann, H.;
Hartmann, U.; Wassermann, W. Polyhedron 1990, 9, 353. (e) Johnsen,
E.; Downs, A. J.; Goode, M. J.; Greene, T. M.; Himmel, H. J.; Müller,
M.; Parsons, S.; Pulham, C. R. Inorg. Chem. 2001, 40, 4755. (f)
Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5419. (g) Atwood,
J. L.; Bott, S. G.; Jones, C.; Raston, C. L. Inorg. Chem. 1991, 30,
4868. (h) Henderson, M. J.; Kennard, C. H. L.; Raston, C. L.; Smith,
G. J. Chem. Soc., Chem. Commun. 1990, 1203. (i) Uhl, W.; Molter,
J.; Neumüller, B. J. Organomet. Chem. 2001, 634, 193. (j) Jennings,
J. R.; Wade, K. J. Chem. Soc. A 1967, 1222.
(4) Uhl, W.; Grunenberg, J.; Hepp, A.; Matar, M.; Vinogradov, A. Angew.
Chem. 2006, 118, 4465 Angew. Chem., Int. Ed. 2006, 45, 4358.
(5) (a) Uhl, W.; Cuypers, L.; Graupner, R.; Molter, J.; Vester, A.;
Neumüller, B. Z. Anorg. Allg. Chem. 2001, 627, 607. (b) Uhl, W.;
Cuypers, L.; Geiseler, G.; Harms, K.; Massa, W. Z. Anorg. Allg. Chem.
2002, 628, 1001. (c) Baxter, P. L.; Downs, A. J.; Goode, M. J.; Rankin,
D. W. H.; Robertson, H. E. J. Chem. Soc., Dalton Trans. 1990, 2873.
(6) Uhl, W.; Claesener, M.; Haddadpour, S.; Jasper, B.; Hepp, A. Dalton
Trans. 2007, 4, 417.
4464 Inorganic Chemistry, Vol. 47, No. 11, 2008

Organoelement Dichlorogallium Compounds
C6D6, 400 MHz, only two resonances could be identified unam-
biguously): δ 7.58 (phenyl?), 0.30 (18 H, s, SiMe3). 1H NMR (THF-
d8, 400 MHz): δ 7.83 (2 H, s, 3JSi-H ) 10.7 Hz, CdC-H), 7.51 (4
H, s, phenyl), 0.25 (18 H, s, SiMe3). 13C NMR (THF-d8, 100 MHz):
δ 155.7 [CdC(Si)-Ga], 148.3 [CdC(Si)-Ga], 141.3 (ipso-C of
phenyl), 128.4 (ortho-C of phenyl), 0.0 (SiMe3). 29Si NMR (THF-
d8, 79.5 MHz): δ 0.0. IR (paraffin, CsBr plates, cm-1): 1578 s,
1557 s, 1462 vs phenyl, ν(C)C); 1455 vs paraffin; 1402 m δ(CH3);
1377 s paraffin; 1306 m, 1254 vs δ(CH3); 1209 w, 1155 w, 1115 w,
961 w ν(CC), phenyl; 901 s, 864 s, 841 s, 812 s, 791 m, 750 s
F(CH3(Si)); 719 s paraffin; 696 m, 667 w νas(SiC); 646 w, 621 w
νs(SiC); 584 w, 540 w, 517 w, 507 w ν(GaC); 467 w, 438 w, 424 w
ν(GaCl), ν(Ga2Cl2). Anal. Calcd for C32H48Si4Cl8Ga4 ·1/2C6H14
(1150.6): Ga, 24.2; Cl, 24.6, C, 36.5; H, 4.82. Found: Ga, 24.0;
Cl, 24.6; C, 36.3; H, 4.95.
Experimental Section
All procedures were carried out under an atmosphere of purified
argon in dried solvents (n-hexane over LiAlH4, diethyl ether and
THF over Na/benzophenone). H-GaCl2, 1,3,5-tris(trimethylsilyl-
ethynyl)benzene, and 1,2,4,5-tetrakis(trimethylsilylethynyl)benzene
were obtained according to literature procedures.9,10,12 H-GaCl2
is only sparingly soluble in n-hexane, and high dilution is necessary
to obtain homogeneous mixtures. The commercially available
alkynes (Aldrich), trimethylsilylethynylbenzene, and 1,4-bis(trim-
ethylsilylethynyl)benzene were employed as purchased. Only the
most intensive peaks of the mass spectra 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 Trimethylsilylethynylbenzene.
Synthesis of 6. H-GaCl2 (1.05 g, 7.41 mmol, small excess) dissolved
in 80 mL of n-hexane was treated with a solution of trimethylsilyl-
ethynylbenzene (1.15 g, 6.61 mmol) in 20 mL of n-hexane. The
mixture was heated under reflux for 15 h. Small quantities of a colorless
solid precipitated, which may result from a double hydrogallation, and
were filtered off. The filtrate was concentrated and cooled to 4 °C to
obtain colorless crystals of compound 6. Yield: 1.72 g (82%). Mp
(argon, closed capillary): 75 °C (dec). MS (EI, 70 eV): m/z 314 (28%),
316 (38%), 318 (17%) (M+ of the monomeric formula unit); 299
(77%), 301 (100%), 303 (43%) (1/2M+ - CH3). 1H NMR (C6D6, 400
MHz): δ 7.84 (2 H, s, 3JH-Si ≈ 10 Hz, CdC-H), 7.24 (4 H, pseudo-
d, ortho-H of phenyl), 7.05 (4 H, pseudo-t, meta-H of phenyl), 6.99
(2 H, pseudo-t, para-H of phenyl), 0.21 (18 H, s, SiMe3). 13C NMR
(C6D6, 100 MHz): δ 157.7 [CdC(Si)-Ga], 148.7 [CdC(Si)-Ga],
142.2 (ipso-C of phenyl), 129.3 (meta- and para-C of phenyl), 127.1
(ortho-C of phenyl), 0.2 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ -1.3.
IR (paraffin, CsBr plates, cm-1): 1950 w, 1873 w, 1811 w, 1771 w,
1580 s, 1553 vs, 1489 vs phenyl, ν(CdC); 1456 vs paraffin; 1404 m
δ(CH3); 1377 vs paraffin; 1323 m, 1248 vs δ(CH3); 1206 w, 1175 w,
1157 w, 1117 w, 1076 s, 1028 s, 986 w ν(CC), phenyl; 932 vs, 901
vs, 837 vs, 754 vs, 743 vs F(CH3(Si)); 723 m paraffin; 700 vs νas(SiC);
621 s νs(SiC); 583 m, 532 m ν(GaC); 482 vs, 419 vs ν(GaCl),
ν(Ga2Cl2). Anal. Calcd for C22H30Si2Cl4Ga2 (631.9): Ga, 22.1; Cl, 22.4;
C, 41.8; H, 4.79. Found: Ga, 22.1; Cl, 22.3; C, 41.4; H, 4.94.
Reaction of H-GaCl2 with 1,4-Bis(trimethylsilylethynyl)-
benzene. Synthesis of 7. A solution of 1,4-bis(trimethylsilylethy-
nyl)benzene (0.39 g, 1.44 mmol) in 50 mL of n-hexane was added
dropwise (3 h) to a cooled (-78 °C) solution of H-GaCl2 (0.43 g,
3.03 mmol, small excess) in 300 mL of n-hexane. The mixture
was warmed to room temperature and heated under reflux for 14 h.
Small quantities of a colorless precipitate were filtered off. The
product crystallized upon cooling of the reaction mixture to 4 °C.
Yield: 0.55 g (66%); the crystals of 7 enclosed n-hexane (half a
molecule to one molecule per formula unit of the dimer). The
colorless product is soluble only in very dilute n-hexane or benzene
solutions; however, it is readily soluble in THF. Solutions in THF
were applied for the spectroscopic characterization. Adducts should
be formed under these conditions. The solubility in diethyl ether is
relatively low. Small quantities of 7(Et2O)2 crystallized from those
solutions after filtration and cooling to 4 °C.
Characterization of 7(OEt2)2. Mp (argon, closed capillary):
decomposition above 155 °C by formation of black particles, no
1
melting until 320 °C. H NMR (C6D6, 400 MHz): δ 7.86 (2 H, s,
3JSi-H ) 10.7 Hz, CdC-H), 7.57 (4 H, s, phenyl), 3.44 (8 H, q,
3
3JH-H ) 7.0 Hz, OCH2), 0.75 (12 H, t, JH-H ) 7.0 Hz, CH3 of
ether), 0.44 (18 H, s, SiMe3). 13C NMR (C6D6, 100 MHz): δ 154.6
[CdC(Si)-Ga], 148.1 [CdC(Si)-Ga], 141.3 (ipso-C of phenyl),
128.3 (ortho-C of phenyl), 67.8 (OCH2), 13.6 (CH3 of ether), 0.0
(SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 1.0. IR (paraffin, CsBr
plates, cm-1): 1578 s, 1556 m, 1503 vw, 1462 s phenyl, ν(CdC);
1454 s paraffin; 1402 w δ(CH3); 1375 m paraffin; 1338 vw, 1326
vw, 1290 w, 1266 w, 1254 w δ(CH3); 1188 w, 1148 sh, 1024 vs
ν(CC), ν(CO), phenyl; 918 w, 885 w, 837 w, 754 w F(CH3(Si));
719 w paraffin; 696 w νas(SiC); 621 w νs(SiC); 567 w, 561 w,
527 w, 517 m, 503 w ν(GaC), ν(GaO); 422 w ν(GaCl).
Reaction of H-GaCl2 with 1,3,5-Tris(trimethylsilylethynyl)-
benzene. Synthesis of 8. A solution of 1,3,5-tris(trimethylsilyl-
ethynyl)benzene (0.99 g, 2.70 mmol) in 25 mL of n-hexane was
treated with H-GaCl2 (1.38 g, 9.74 mmol, small excess) in 60 mL
of n-hexane. The mixture was heated under reflux for 14 h. The
colorless product precipitated. It was filtered off and washed with
10 mL of n-hexane. Yield: 1.65 g (77%). Compound 8 is insoluble
in noncoordinating solvents, but it dissolved completely in THF.
Solutions in THF were applied for its NMR spectroscopic charac-
terization. Adducts with THF coordinated to the gallium atoms
similar to compound 8(OEt2)3 may exist under these conditions.
The solubility in diethyl ether is relatively poor. Small quantities
of 8(OEt2)3 crystallized from these solutions after filtration and
cooling to 4 °C.
Characterization of 8. Mp (argon, closed capillary): decomposi-
tion above 230 °C. MS (EI, 70 eV): m/z 687 (0.7%), 689 (0.9%),
691 (0.8%) (M+ - SiMe3 - 2CH3). 1H NMR (THF-d8, 400 MHz):
δ 7.89 (3 H, s, 3JSi-H ) 10.4 Hz, CdC-H), 7.61 (3 H, s, phenyl),
0.27 (27 H, s, SiMe3). 13C NMR (THF-d8, 100 MHz): δ 155.7
[CdC(Si)-Ga], 148.4 [CdC(Si)-Ga], 141.9 (ipso-C of phenyl),
127.7 (ortho-C of phenyl), 0.1 (SiMe3). 29Si NMR (THF-d8, 79.5
MHz): δ 0.0. IR (paraffin, CsBr plates, cm-1): 1589 m, 1566 vs,
1464 vs phenyl, ν(CdC); 1454 vs paraffin; 1423 m, 1402 m δ(CH3);
1377 s paraffin; 1341 w, 1304 w, 1268 s, 1250 vs δ(CH3); 1159 w
ν(CC), phenyl; 916 vs, 903 vs, 887 vs, 876 vs, 857 vs, 833 vs,
752 s F(CH3(Si)); 718 s paraffin; 692 m, 669 s νas(SiC); 635 m,
611 m νs(SiC); 565 m, 501 m ν(GaC); 424 vs ν(GaCl). Anal. Calcd
for C21H33Si3Cl6Ga3 (791.6 of the monomeric formula unit): Ga,
26.4; Cl, 26.9; C, 31.9; H, 4.20. Found: Ga, 27.0; Cl, 27.3; C, 30.9;
H, 4.08.
Characterization of 7. Mp (argon, closed capillary): decomposi-
tion above 155 °C by formation of a black solid. MS (EI, 70 eV):
m/z 515 (0.05%), 517 (0.2%), 519 (0.1%), 521 (0.1%) (M+ of the
monomeric formula unit - Cl), 441 (4%), 443 (10%), 445 (10%),
447 (7%) (M+ - HCl - SiMe3). 1H NMR (very dilute solution in
Characterization of 8(OEt2)3. Mp (argon, closed capillary):
1
decomposition above 140 °C. H NMR (C6D6, 400 MHz): δ 8.07
(12) Berris, B. C.; Hovakeemian, G. H.; Lai, Y.-H.; Mestdagh, H.;
3
Vollhardt, K. P. C. J. Am. Chem. Soc. 1985, 107, 5670.
(3 H, s, JSi-H ) 10.5 Hz, CdC-H), 7.79 (3 H, s, phenyl), 3.59
Inorganic Chemistry, Vol. 47, No. 11, 2008 4465

Uhl and Claesener
Table 1. Crystallographic Data for 6–9
6
7
7
7(OEt2)2
8(OEt2)3
9
empirical formula
cryst syst
space group
a (pm)
b (pm)
c (pm)
C28H44Cl4Ga2Si2 C35H55Cl8Ga4Si4 C35H55Cl8Ga4Si4 C24H44Cl4Ga2O2Si2 C33H63Cl6Ga3O3Si3 C34H62Cl2Ga2O2Si4
monoclinic
C2/c
monoclinic
P21/c
triclinic
P1
monoclinic
P21/n
orthorhombic
Pbca
monoclinic
P21/c
¯
961.2(1)
1582.5(2)
2263.8(2)
90
92.645(2)
90
1774.7(1)
1059.99(9)
2986.6(2)
90
96.181(2)
90
1063.2(2)
1554.6(3)
1736.8(4)
81.12(3)
83.96(3)
70.71(3)
2.6725(9)
2
1028.4(1)
1537.4(2)
1088.1(1)
90
94.62(1)
90
2514.2(10)
1355.9(5)
2825.9(11)
90
90
90
1373.0(3)
1034.5(2)
1689.7(4)
90
112.335(4)
90
R (deg)
ꢀ (deg)
γ (deg)
V (nm3)
Z
3.4398(6)
4
5.5855(8)
4
1.7148(4)
2
9.63(1)
8
2219.9(9)
2
temp (K)
153(2)
1.387
153(2)
1.368
153(2)
1.430
153(2)
1.360
153(2)
1.398
153(2)
1.235
Dcalcd (g cm-3
)
independent data
observed data
params
5316
4335
167
1.963
0.0360
0.0880
+444/-347
17 181
10 936
471
15 853
12 169
473
3990
3193
180
1.971
0.0376
0.0949
+935/-410
11 669
7883
506
6441
4946
218
1.468
0.0400
0.1017
+751/-424
µ (mm-1
)
2.399
2.507
2.102
R1a
0.0545
0.1575
+1394/-384
0.0372
0.0966
+735/-530
0.0510
0.1026
+586/-300
wR2 (all data)b
residual density (e nm-3
)
a Observation criterion: I > 2σ(I). R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2
.
2
2
2
3
3
(12 H, q, JH-H ) 7.2 Hz, OCH2), 0.84 (18 H, t, JH-H ) 7.2 Hz,
CH3 of ether), 0.44 (27 H, s, SiMe3). 13C NMR (C6D6, 100 MHz):
δ 155.7 [CdC(Si)-Ga], 148.2 [CdC(Si)-Ga], 141.6 (ipso-C of
phenyl), 127.5 (ortho-C of phenyl), 67.9 (OCH2), 13.8 (CH3 of
ether), 0.1 (SiMe3). 29Si NMR (C6D6, 79.5 MHz): δ 1.1. IR (paraffin,
CsBr plates, cm-1): 1577 sh, 1570 sh, 1557 vs, br, 1472 sh, 1462
vs phenyl, ν(CdC); 1454 vs paraffin; 1447 vs, 1425 vs, 1402 s
δ(CH3); 1377 vs paraffin; 1339 m, 1323 m, 1306 m, 1292 m, 1244 s
δ(CH3); 1190 s, 1148 s, 1119 m, 1090 s, 1030 m, 989 sh ν(CC),
ν(CO), phenyl; 930 m, 889 sh, 879 m, 845 s, 814 s, 785 m, 764 s,
741 s F(CH3(Si)); 719 s paraffin; 698 sh, 671 m νas(SiC); 635 w,
604 w νs(SiC); 575 m, 560 w, 492 m, br, 467 m, br ν(GaC), ν(GaO);
437 s, 422 m ν(GaCl).
to 4 °C. Crystals of 7 grew from the reaction mixture at 4 °C
(monoclinic form) or at room temperature upon slow concentration
(triclinic form). Crystals of the etherates 7(OEt2)2, 8(OEt2)3, and
9 were generated by the treatment of the corresponding solid
reaction products with diethyl ether. After filtration, the crystals
were obtained upon cooling of the filtrate to 4 °C. The crystal-
lographic data were collected with a Bruker APEX diffractometer
with graphite-monochromated Mo KR radiation. 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-Plus13 and refined
with the SHELXL-9713 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 6 crystallized with one molecule
of n-hexane per formula unit of the dimer; the gallium compound
was located on a crystallographic 2-fold rotation axis, and the
solvent molecule was on a crystallographic center of symmetry.
Compound 7 was isolated in a triclinic and a monoclinic form.
The triclinic cell included half of a molecule of n-hexane (located
on a crystallographic center of symmetry) per formula unit of
dimeric 7. The monoclinic form had a solvent molecule located on
a general position. It showed some disorder and was refined with
occupation factors of the carbon and hydrogen atoms of 0.5. The
molecules of 7(OEt2)2 and 9 reside on crystallographic centers of
symmetry. One methyl group of an ether ligand was disordered in
both cases. Two ether ligands of 8(OEt2)3 were disordered (at OE1
and OE5); the carbon atoms were refined on split positions.
Reaction of H-GaCl2 with 1,2,4,5-Tetrakis(trimethylsilyl-
ethynyl)benzene. Synthesis of 9. A solution of H-GaCl2 (0.830
g, 5.86 mmol) in 50 mL of n-hexane was treated with a solution of
the tetraalkyne, 1,2,4,5-tetrakis(trimethylsilylethynyl)benzene (0.608
g, 1.32 mmol), in 50 mL of the same solvent at room temperature.
The suspension adopted a red color, and small quantities of a
colorless solid precipitated. Subsequently, the mixture was heated
under reflux for 14 h. The product precipitated and was isolated
by filtration. It was washed with 10 mL of n-hexane and treated
with 50 mL of diethyl ether to yield a reddish suspension, which
after filtration, concentration, and cooling to -45 °C yielded a
colorless solid of the ether adduct 9. Yield: 0.500 g (46%). Mp
(argon, closed capillary): decomposition above 220 °C. MS (EI,
70 eV): m/z 787 (4%), 789 (5%), 791 (1%) (M+ - Cl). H NMR
(C6D6, 400 MHz): δ 8.02 (4 H, s, JH-Si ) 11.3 Hz, CdC-H),
1
3
3
7.68 (2 H, s, phenyl), 3.28 (8 H, q, JH-H ) 7.6 Hz; OCH2), 0.65
(12 H, t, 3JH-H ) 7.6 Hz, CH3 of ether), 0.43 (36 H, s, SiMe3). 13
C
NMR (C6D6, 100 MHz):
δ 162.3 [CdC(Si)-Ga], 152.4
[CdC(Si)-Ga], 140.0 (ipso-C of phenyl), 138.8 (C-H of phenyl),
66.5 (OCH2), 13.7 (CH3 of ether), -0.2 (SiMe3). 29Si NMR (C6D6,
79.5 MHz): δ -3.2. IR (paraffin, CsBr plates, cm-1): 1650 s, 1575 s,
1543 s phenyl, ν(CdC); 1456 vs, 1377 vs paraffin; 1315 w, 1285 w,
1242 m δ(CH3); 1190 w, 1150 w, 1123 w, 1092 w, 1038 m, 995 w
ν(CC), ν(CO), phenyl; 938 w, 918 m, 897 w, 878 w, 839 s, 773 m
vs F(CH3(Si)); 719 m paraffin; 687 m νas(SiC); 651 vw, 621 m
νs(SiC); 567 m, 527 vw, 505 w, 470 m, 436 m ν(GaCl), ν(GaC),
ν(GaO). Anal. Calcd for C34H62O2Si4Cl2Ga2 (825.5): Ga, 16.9; Cl,
8.6; C, 49.5; H, 7.6. Found: Ga, 17.5; Cl, 8.9; C, 48.9; H, 7.4.
Crystal Structure Determinations. Single crystals of compound
6 were obtained from the reaction mixture in n-hexane upon cooling
Results and Discussion
Because of the low solubility of the starting compound
H-GaCl2 in hydrocarbon solvents, the reactions with the
corresponding alkynes were conducted in very dilute n-
hexane solutions (eq 1). The reaction of the monoalkyne,
trimethylsilylethynylbenzene, afforded a clear solution of the
(13) (a) SHELXTL-Plus, release 4.1; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1990. (b) Sheldrick, G. M. SHELXL-97, Program
for the Refinement of Structures; Universität Göttingen: Göttingen,
Germany, 1997.
4466 Inorganic Chemistry, Vol. 47, No. 11, 2008

Organoelement Dichlorogallium Compounds
Scheme 2. Schematic Drawings of Compounds 6, 7, 7(OEt2)2, and
8(OEt2)3
chemical shifts of δ 7.9 on average. The phenyl protons
(ortho-protons in the case of the monoalkenyl derivative 6)
show a systematic shift to a lower field with the increasing
number of alkenyl groups attached to the aromatic ring from
δ 7.24 of 6 to δ 7.89 of 8. The 13C NMR resonances of the
ethenyl carbon atoms occur at about δ 155 (attached to the
aromatic rings) and 148 (attached to Ga and Si). They show
a relatively strong difference from the corresponding chemi-
cal shifts of the addition products of dialkylgallium hydrides
to silylalkynes, which contain coordinatively unsaturated,
tricoordinated gallium atoms (δ 149 and 170, respectively).9
NMR evidence for the particular Z configuration came from
the coupling constants 3JSi-H across the CdC double bonds,
which with about 10 Hz on average are in accordance with
a coupling between atoms in a cis arrangement.
A different and unexpected reaction course was observed
for the hydrogallation of the tetraalkyne 1,2,4,5-tetrakis-
(trimethylsilylethynyl)benzene with H-GaCl2 (eq 2). A
colorless precipitate was formed under conditions similar to
those described before. It dissolved completely upon treat-
ment with diethyl ether. Cooling of the solution afforded
product in n-hexane. The colorless, crystalline compound 6
was isolated from the reaction mixture by concentration and
cooling to 4 °C in 82% yield. In contrast, the product of the
reaction with the bisalkyne, 1,4-bis(trimethylsilylethynyl)-
benzene, was only sparingly soluble in n-hexane, and very
dilute solutions were required to completely dissolve the
colorless compound 7. It possesses a dimeric formula unit
in the solid state with a Ga-H group added to each Ct C
triple bond of the starting compound. 1,3,5-Tris(trimethyl-
silylethynyl)benzene yielded a colorless solid (8) that was
completely insoluble in n-hexane and was isolated by
filtration of the reaction mixture in a yield of about 80%.
The hexane solutions obtained from the syntheses of 8 gave
small quantities of a colorless solid upon cooling; the NMR
spectrum of which exhibited broad resonances of unknown
products and did not show any resonance of aromatic protons.
Partial dissolution occurred upon treatment of 7 and 8 with
a few milliliters of diethyl ether. Filtration and cooling of
the filtrates yielded relatively small quantities of the mon-
omeric etherates 7(OEt2)2 and 8(OEt2)3, in which each
gallium atom was coordinated by an oxygen atom of an ether
molecule. Complete dissolution of the solids occurred in THF
only. Schematic drawings of the molecular structures of all
isolated and characterized compounds are depicted in Scheme
2.
1
colorless crystals of the product (9) in 46% yield. The H
NMR spectrum verified the formation of CdC double bonds
by the characteristic resonances of the vinylic hydrogen
atoms (δ 8.02). The integration ratio showed the presence
of only two molecules of ether per formula unit of the
product. A schematic drawing of the molecular structure is
shown in eq 2. A novel tricyclic compound was formed by
an intramolecular condensation reaction probably accompa-
nied by the release of gallium trichloride. Seven-membered
heterocycles resulted, in which the gallium atoms bridged
the ꢀ-carbon atoms of the CdC double bonds. Further
spectroscopic findings are similar to those discussed before,
but the 13C NMR resonance of the alkenyl carbon atoms
attached to gallium and silicon (δ 162.3) is strongly shifted
to a lower field compared to the corresponding signals of
3
the GaCl2 compounds. Particularly, the JSi-H coupling
constant across the CdC double bond (11.3 Hz) verified the
cis arrangement of hydrogen and silicon and the spontaneous
cis/trans isomerization as discussed before.
C6H6-x(Ct C-SiMe3)x + xH-
GaCl2 f C6H6-x[C(H)dC(SiMe3)-GaCl2]x
6 (x ) 1), 7 (x ) 2), 8 (x ) 3) (1)
(2)
The spectroscopic characterization of compounds 7 and 8
was conducted essentially with THF solutions, and the results
are similar to those obtained for solutions of the etherates in
1
C6D6. The integration ratios of the H NMR spectra are in
accordance with the structures obtained by crystal structure
determinations (see below), which verify the geminal ar-
rangement of Ga and Si atoms at one carbon atom of the
CdC double bonds and the trans arrangement of Ga and H
in the ethenyl groups (Z configuration). The vinyl protons
Molecular structures were determined of the ether-free
addition products of the mono- and bisalkyne (6 and 7) and
1
in the H NMR spectra resonate with relatively constant
Inorganic Chemistry, Vol. 47, No. 11, 2008 4467

Uhl and Claesener
additional terminal chlorine atom is attached to each gallium
atom. As expected the Ga-Cl distances differ strongly
between the terminal and the bridging situation (214.6 vs
233.9 pm on average). The CdC double bond lengths
correspond to standard values (133.8 pm). The gallium atoms
exclusively attacked those carbon atoms which also bear the
trimethylsilyl groups. This high regioselectivity may be
caused by two effects. Because of the electronegativity
difference between silicon and carbon, a small partial
negative charge is induced at these carbon atoms, which then
preferably may be attacked by the positively charged gallium
atoms. Furthermore, that position allows for an effective
hyperconjugation with the SiMe3 group and an optimum
stabilization of the negative charge, which in the products
are enhanced at that position by the bonding to the more
electropositive gallium atoms. Hydroalumination3 and hy-
drogallation reactions9 of trimethylsilylalkynes with dialkyl-
element hydrides show a comparable selectivity. The gallium
and hydrogen atoms attached to the CdC double bonds adopt
a trans arrangement and are located on different sides of
the alkenyl groups. A cis arrangement may be expected as
the first step of the insertion into the Ga-H bonds, as usually
observed for the hydrogallation products obtained with tert-
butyl(phenyl)ethynes.6,7 A fast cis/trans rearrangement may
occur here which is in accordance with similar reactions of
dialkylgallium hydrides with trimethylsilylethynes.9 The
mechanism of the rearrangement process is not clear yet. It
may depend on C-H bond activation, as recently observed
for hydroalumination products (see Introduction). Further
experimental work is necessary for a better understanding
and is presently being conducted in our group. Two different
crystal systems were observed for the crystals of compound
7 (triclinic and monoclinic). The structural parameters of the
organogallium components are quite similar in both forms,
but they differ with respect to the location of the enclosed
n-hexane molecules. The triclinic form has one molecule of
the solvent n-hexane per two formula units of the dimer.
The solvent molecules are localized on a crystallographic
center of symmetry between the trimethylsilyl groups of two
different tetragallium molecules and are close to two
Ga-Cl-Ga bridges. The monoclinic form has the n-hexane
molecules above and below both Ga2Cl2 moieties of one
formula unit and close to the sides of the bridging benzene
rings. Thus, when these gallium compounds are considered
Figure 1. Molecular structure and numbering scheme of 6; the thermal
ellipsoids are drawn at the 40% probability level. Hydrogen atoms, with
the exception of vinylic hydrogen atoms, are omitted for clarity. Important
bond lengths (pm) and angles (deg): Ga1-C1 ) 194.1(2), Ga1-Cl1 )
235.25(6), Ga1-Cl1′ ) 233.31(5), Ga1-Cl2 ) 214.96(6), C1-C2 )
133.6(3), Cl1-Ga1-Cl1′ ) 87.59(2), Ga1-Cl1-Ga1′ ) 91.39(2),
C2-C1-Si11 ) 122.0(2), C2-C1-Ga1 ) 118.5(1), C1-C2-C21 )
127.3(2). Ga1′ and Cl1′ generated by -x + 2, y, -z + 0.5.
Figure 2. Molecular structure and numbering scheme of 7; the thermal
ellipsoids are drawn at the 40% probability level. Hydrogen atoms, with
the exception of vinylic hydrogen atoms, are omitted for clarity. Important
bond lengths (pm) and angles (deg) for the monoclinic form (the structural
parameters of the triclinic system are almost identical): Ga1-C1 ) 193.1(4),
Ga1-Cl1 ) 213.4(1), Ga1-Cl11 ) 234.7(1), Ga1-Cl31 ) 233.9(1),
C1-C11 ) 134.2(5), Ga2-C2 ) 193.5(4), Ga2-Cl2 ) 214.5(1), Ga2-Cl21
) 232.9(1), Ga2-Cl41 ) 235.3(1), C2-C21 ) 133.7(5), Ga3-C3 )
194.0(4), Ga3-Cl3 ) 215.4(1), Ga3-Cl31 ) 232.1(1), Ga3-Cl11 )
234.0(1), C3-C31 ) 133.5(5), Ga4-C4 ) 193.1(4), Ga4-Cl4 ) 214.0(1),
Ga4-Cl41 ) 233.2(1), Ga4-Cl21 ) 234.5(1), C4-C41 ) 133.8(5),
C-C-Si ) 120.5 (av), C-C-Ga ) 118.8 (av), CdC-C ) 128.7 (av).
(14) (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) Neumüller, 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.
of the etherates of the bis- and trisalkenyl derivatives
[7(OEt2)2 and 8(OEt2)3]. The solvent-free compounds give
dimers via Ga-Cl bridges (Figures 1 and 2) and by the
formation of Ga2Cl2 heterocycles as usually observed for
organogallium halides.14 However, that dimerization afforded
an unprecedented nice molecular structure for compound 7.
A cagelike structure resulted, which in some respects
resembles the one of cyclophane-type molecules and has two
bridging Ga2Cl2 moieties almost perpendicularly arranged
to the parallel organic backbones. In both compounds, an
4468 Inorganic Chemistry, Vol. 47, No. 11, 2008

Organoelement Dichlorogallium Compounds
Figure 3. Molecular structure and numbering scheme of 7(OEt2)2; the
thermal ellipsoids are drawn at the 40% probability level. Ethyl groups of
the ether ligands (O1) and hydrogen atoms are omitted for clarity. Important
bond lengths (pm) and angles (deg): Ga1-C1 ) 195.1(3), Ga1-O1 )
199.0(2), Ga1-Cl1 ) 218.81(8), Ga1-Cl2 ) 217.67(8), C1-C2 )
134.1(3), C2-C1-Si1 ) 118.3(2), C2-C1-Ga1 ) 125.4(2), C1-C2-C21
) 130.3(2).
Figure 5. Molecular structure and numbering scheme of 9; the thermal
ellipsoids are drawn at the 40% probability level. Ethyl groups of the ether
ligands and hydrogen atoms, with the exception of the vinylic hydrogen
atoms, are omitted for clarity. Important bond lengths (pm) and angles
(deg): Ga1-C12 ) 194.0(2), Ga1-C22 ) 194.0(2), Ga1-O1 ) 202.6(2),
Ga1-Cl1 ) 220.80(7), C11-C12 ) 134.6(3), C21-C22 ) 134.5(2),
C12-Ga1-C22 ) 113.45(9), Ga1-C12-C11 ) 115.7(2), Ga1-C22-C21
) 116.4(2), C12-C11-C1 ) 131.8(2), C22-C21-C2 ) 131.9(2).
Symmetry equivalent atoms were generated by -x, -y + 2, -z.
ment with the GaCl2 residues oriented to the same side of
the molecule. With this particular orientation, they are nicely
preorganized to potentially act as a chelating Lewis acid, if
the diethyl ether molecules could be replaced by suitable
multiple donors. The bond lengths are quite similar to those
of the ether-free molecules and do not require a more detailed
discussion. The Ga-O distances (198.5 pm on average) are
in the expected range of donor-acceptor complexes.
The molecular structure of compound 9 is different from
those of 6–8. The tetraalkyne starting compound was success-
fully transformed into the corresponding tetraalkenyl derivative;
however, the product did not possess GaCl2 moieties as in the
cases before. Instead intramolecular condensation reactions gave
seven-membered heterocycles, in which Ga-Cl groups bridge
two alkenyl groups (Figure 5). Each gallium atom is further
coordinated by an ether molecule, which prevents polymeri-
zation by intermolecular Ga-Cl bridges. We did not observe a
similar intramolecular process before. Instead condensation
reactions by the release of trialkylgallium derivatives proceeded
on intermolecular pathways in all cases and gave exclusively
cyclophane-type molecules or heteroadamantane cages by
oligomerization. A singular tricyclic, nonplanar molecular
structure resulted for 9 which contains two GaC6 heterocycles
in addition to the inner benzene ring. The heterocycles deviate
considerably from planarity. Their structures may be described
by a boat conformation. The inner carbon atoms C1, C2, C3,
C11, and C21 and the symmetry equivalent atoms are almost
ideally in a plane with a maximum deviation of only 3.6 pm
(C1). The normals of this plane and of the planes formed by
the atoms C11, C12, Ga1, and Si12 or C21, C22, Ga1, and
Si22 include angles of 35.2 and 52.1°, respectively. Finally,
the gallium atoms are 67.8 pm above the plane formed by the
alkenyl carbon atoms. All other structural parameters are quite
similar to those of the other compounds described before and
do not require a detailed discussion.
Figure 4. Molecular structure and numbering scheme of 8(OEt2)3; the
thermal ellipsoids are drawn at the 40% probability level. Ethyl groups of
the ether ligands (OE1, OE3, OE5) and hydrogen atoms are omitted for
clarity. Important bond lengths (pm) and angles (deg): Ga1-C12 )
195.0(3), Ga1-OE1 ) 198.8(3), Ga1-Cl11 ) 216.4(1), Ga1-Cl12 )
218.6(1), C11-C12 ) 133.1(4), Ga3-C32 ) 194.1(3), Ga3-OE3 )
196.8(2), Ga3-Cl21 ) 217.0(1), Ga3-Cl22 ) 219.3(1), C31-C32 )
134.4(4), Ga5-C52 ) 194.4(3), Ga5-OE5 ) 199.4(2), Ga5-Cl51 )
215.8(1), Ga5-Cl52 ) 218.6(1), C51-C52 ) 133.5(4), C-C-Si ) 118.1
(av), C-C-Ga ) 124.0 (av), CdC-C ) 130.9 (av).
to be cyclophane-type molecules, they cover the open faces
of the resulting macrocycles.
Upon treatment with diethyl ether, the Ga-Cl bridges of
the dimeric formula units are cleaved and monomeric
fragments result, in which the gallium atoms are coordina-
tively saturated by interactions with the ether oxygen atoms.
Two etherates were characterized by crystal structure deter-
minations, which had two and three alkenyl groups (Figures
3 and 4). The molecules of 7(OEt2)2 reside on crystal-
lographic centers of symmetry. Thus, the GaCl2 groups are
in an ideal trans arrangement on opposite sides of the central
phenyl ring. In contrast, the alkenylgallium groups of the
molecular structure of 8(OEt2)3 adopt a chalicelike arrange-
Inorganic Chemistry, Vol. 47, No. 11, 2008 4469

Uhl and Claesener
C6H2[(CHdC-SiMe3)2GaCl(OEt2)]2 (9). 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-642134 (6), -642135 (7, monoclinic), -642136 (7,
triclinic), -642137 [7(OEt2)2], -642138 [8(OEt2)3], and -642139
(9).
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 the compounds [C6H5-C(H)dC(SiMe3)-GaCl2]2
(6), [C6H4{C(H)dC(SiMe3)-GaCl2}2]2 (7; triclinic and mono-
clinic),
C6H4[C(H)dC(SiMe3)-GaCl2(OEt2)]2
[7(OEt2)2],
IC700767A
C6H3[C(H)dC(SiMe3)-GaCl2(OEt2)]3
[8(OEt2)3], and
4470 Inorganic Chemistry, Vol. 47, No. 11, 2008