Unexpected formation of gallium-gallium single bonds by irradiation of the hydride [(Me3C)2GaH]3

Article abstract of DOI:10.1039/b103409j

Di(tert-butyl)gallium hydride 1 dismutates partially in solution forming tri(tert-butyl)gallium 2 and the sesquihydride [(Me₃C)₂GaH]₂[H₂GaCMe ₃]₂ (3). The loss of tert-butyl radicals upon irradiation of this mixture with day light or an UV lamp gave the hexagallium compound (Me₃CGaGaCMe₃)₂(μ-H)₂[μ-H ₂Ga(CMe₃)₂]₂ (4), which possesses two Ga-Ga single bonds. These Ga₂ groups are bridged by two hydrogen atoms to give a six-membered Ga₄H₂ heterocycle. Couples of opposite gallium atoms of this heterocycle are bridged via Ga-H-Ga 3c-2e bonds by two H₂Ga(CMe₃)₂ ligands, which are situated above and below the molecular plane. Compound 4 may be described as a hypho-hexagallane(14) cluster compound.

Full text of DOI:10.1039/b103409j

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Unexpected formation of gallium–gallium single bonds by
irradiation of the hydride [(Me C) GaH]
3
2
3
Werner Uhl,* Lars Cuypers, Gertraud Geiseler, Klaus Harms and Bernhard Neumüller
Fachbereich Chemie der Philipps Universität Marburg, Hans-Meerwein-Straße,
D-35032 Marburg, Germany. E-mail: uhl@chemie.uni-marburg.de
Received 17th April 2001, Accepted 11th June 2001
First published as an Advance Article on the web 31st July 2001
Di(tert-butyl)gallium hydride 1 dismutates partially in solution forming tri(tert-butyl)gallium 2 and the sesquihydride
[
(Me C) GaH] [H GaCMe ] (3). The loss of tert-butyl radicals upon irradiation of this mixture with day light or an
3 2 2 2 3 2
UV lamp gave the hexagallium compound (Me CGaGaCMe ) (µ-H) [µ-H Ga(CMe ) ] (4), which possesses two
3
3
2
2
2
3 2 2
Ga–Ga single bonds. These Ga groups are bridged by two hydrogen atoms to give a six-membered Ga H
2
4
2
heterocycle. Couples of opposite gallium atoms of this heterocycle are bridged via Ga–H–Ga 3c–2e bonds by two
H Ga(CMe ) ligands, which are situated above and below the molecular plane. Compound 4 may be described as a
2
3 2
hypho-hexagallane(14) cluster compound.
Ϫ1
Introduction
at 1638 cm in the IR spectrum verified the occurrence of
bridging hydrido ligands. Compound 4 is quite stable and
decomposes at 158 ЊC forming elemental gallium.
The compound [(Me Si) CH] Ga–Ga[CH(SiMe ) ] containing
a Ga–Ga single bond was the first stable organoelement digal-
lane(4) derivative reported in the literature. It possesses a sur-
3
2
2
3 2 2
1
prisingly high thermal stability and was obtained by our group
about 12 years ago when we treated the dioxane adduct of
2
3
Ga Br4 with the bulky alkyllithium compound LiCH(SiMe ) .
2
3 2
During the last decade the synthesis of such digallanes(4)
developed into a broad field of activities, and many derivatives
were synthesized by similar methods or by the reduction of
4
diaryl- or dialkyl-gallium halides with alkali metals. However,
these compounds merit particular interest not only because of
their singular structures, but also owing to their fascinating
chemical reactivity, which led to the observation of five differ-
ent types of reaction and the isolation of many unprecedented
(
1)
5
products. Here we report on the unexpected formation of a
Ga–Ga single bond by the homolytic cleavage of Ga–C bonds.
Results and discussion
Recently, we published the synthesis and crystal structure of
6
di(tert-butyl)gallium hydride 1. In contrast to other, dimeric
7,8
dialkylgallium hydrides compound 1 forms a trimer in the
solid state via three 3c–2e Ga–H–Ga bonds. However, upon
dissolution in non co-ordinating organic solvents dismutation
occurred, and the characteristic resonances of tri(tert-butyl)-
gallium 2 and the sesquihydride [(Me C) GaH] [H GaCMe ]
The constitution of compound 4 was elucidated by a crystal
structure determination (Fig. 1). A singular product had been
formed which contains Ga–H and two Ga–Ga single bonds.
These Ga couples are connected by two bridging hydrogen
3
2
2
2
3 2
2
1
13
(
3) were detected beside the signals of 1 in the H and C NMR
atoms via 3c–2e bonds to give a six-membered Ga H hetero-
4
2
6
spectra. This dismutation reaction is reversible, and 1 could be
isolated from solution by cooling and crystallization. However,
storing such solutions in day light at room temperature led
to slow decomposition and the formation of only one new
product, (4, (Me CGaGaCMe ) (µ-H) [µ-H Ga(CMe ) ] ). A
cycle in a twist conformation (torsion angles starting with
H1–Ga2–Ga2Ј–H1Ј counter clockwise 54.5Њ, Ϫ19.5Њ, Ϫ26.3Њ,
56.5Њ, Ϫ26.3Њ, Ϫ19.5Њ). The heterocycle is bridged by two
Ϫ
[H Ga(CMe ) ] groups, each hydrogen atom of which is
2
3 2
attached to one gallium atom of the ring. These anions are
situated above and below the molecular plane and co-ordinate
to opposite gallium atoms. The structure of the molecular
center may be described alternatively as a distorted octahedron
of six gallium atoms, two edges of which are Ga–Ga single
bonds, while six further edges are bridged by hydrogen atoms.
Four edges remain unoccupied. The point group of the whole
3
3
2
2
2
3 2 2
faster reaction was observed on irradiation with an UV lamp
366 nm) in n-hexane as solvent, which led to the complete
consumption of the mixture of 1, 2 and 3 over a period of
4 hours, eqn. (1). The new component was almost insoluble
under these conditions and precipitated as a colourless solid in
(
2
1
8
3% yield. The H NMR spectrum of 4 showed two resonances
of chemically different tert-butyl groups in a 1 to 1 intensity
ratio and two resonances for hydrogen atoms attached to gal-
lium at δ 3.53 and 2.73 in a 1 to 2 ratio. A very broad absorption
molecule is D . All gallium atoms are co-ordinatively saturated
and have co-ordination numbers of four with distorted tetra-
hedral co-ordination spheres. The Ga–Ga single bonds are
2
2
398
J. Chem. Soc., Dalton Trans., 2001, 2398–2400
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b103409j

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was stored over molecular sieves. Di(tert-butyl)gallium hydride
6
1
was obtained according to the literature procedure.
Synthesis
Compound 4. Di(tert-butyl)gallium hydride 1 (1.622 g, 8.78
mmol) was dissolved in 25 mL of n-hexane. The solution was
irradiated with an UV lamp (366 nm, 6 W) through the glass
wall of the reaction flask for 24 h. A colourless solid precipi-
tated, which was filtered off and was washed with 20 mL of
n-hexane. Spectroscopically pure 4 remained. Owing to its low
solubility in non co-ordinating organic solvents, only small
quantities of 4 may be recrystallized from cyclopentane or penta-
fluorobenzene for further purification. Yield: 1.066 g (83%). Mp
(
argon, sealed capillary): 158 ЊC (decomp.) (Found: Ga, 47.5%;
C H Ga requires Ga, 47.1%). δ (C D , 300 MHz) 3.53 (s,
32
78
6
H
6
6
2
H, H1), 2.73 (s, 4 H, H31/H32), 1.35 and 1.28 (each s, 36 H,
CMe ). δ (C D , 75.5 MHz) 32.6 and 32.0 (CMe ); GaC not
3
C
6
6
3
Ϫ1
detected. ν
˜
max/cm (CsBr, paraffin) 1638br,vs νGaH; 1460vs,
Fig. 1 Molecular structure of 4. The thermal ellipsoids are drawn at
the 40% probability level. Methyl groups are omitted. Important bond
lengths (pm) and angles (Њ): Ga1–Ga1Ј 263.8(2), Ga2–Ga2Ј 264.2(2),
Ga1–C1 201.7(8), Ga2–C2 201.0(8), Ga3–C3 200(1), Ga3–C4 196.9(9);
Ga1Ј–H1–Ga2 140(2), Ga1–Ga1Ј–H1 98(2), Ga2Ј–Ga2–H1 101(3),
Ga1–H31–Ga3 152(2), Ga2–H32–Ga3 156(2); the atoms Ga1Ј and
Ga2Ј were generated by Ϫx ϩ 1, y, Ϫz ϩ 3/2.
1
9
5
377vs paraffin; 1363sh, 1316m, 1188w, 1161s δCH ; 1009s,
3
69m, 939m, 836m, 808s δC C; 719m, 695s δGaH; 602m, 556w,
3
48w, 521w νGaC; 437w, 385w δC C. λmax/nm (n-hexane)
3
Ϫ1
Ϫ1
(
ε/M cm ): 210 (14000), 260 (1600) and 300 (sh, 500).
Crystal structure determination of 4
1
,4
Single crystals of 4 were obtained on cooling a pentafluoro-
benzene solution to 8 ЊC. The crystallographic data were
longer (263.8 pm) than usually observed for digallane(4) or
9
digallate(5) derivatives (<255 pm), which may be caused by the
collected with a STOE IPDS diffractometer system. C H Ga ,
co-ordinative saturation of the gallium atoms, electrostatic
repulsion and some steric stress in the cage. The Ga1Ј–H1–Ga2
bridge of the six-membered heterocycle is not symmetric [Ga2–
H1 143(8) pm, Ga1Ј–H1 167(8) pm]. A similar argument holds
for the Ga–H–Ga bridges to the H GaR ligands, which have
alternating short and long Ga–H distances to the atoms of the
Ga–Ga bond [165(6) and 176(5) pm] or to the atom Ga3 [164(5)
and 175(5) pm]. The shortest Ga–H distance is in the range of
the longer ones correspond to nor-
mal values of Ga–H–Ga bridges. The Ga ؒ ؒ ؒ Ga distances of
these bridges are quite different (292.1 pm, Ga1 ؒ ؒ ؒ Ga2Ј, and
31 pm on average for Ga1 ؒ ؒ ؒ Ga3 and Ga2 ؒ ؒ ؒ Ga3). Smaller
ones (261 and 258 pm, respectively) were observed for the
32 78
6
monoclinic, a = 1441.26(8) pm, b = 1732.8(2) pm, c = 1835.9(1)
Ϫ30
3
pm, β = 101.582(7)Њ, U = 4491.6(6) × 10
m , T = 193(2) K,
12
Ϫ3
space group C2/c (no. 15 ), Z = 4, D
= 1.303 g cm ,
calcd
Ϫ1
µ(Mo-Kα) = 3.564 mm , T
= 0.332, T max = 0.509, numerical
min
2
2
absorption correction, crystal dimensions 0.39 × 0.24 × 0.21
mm, 2θ range 4.0 ≤ 2θ ≤ 51.8Њ, index ranges Ϫ17 ≤ h ≤ 17,
Ϫ21 ≤ k ≤ 21, Ϫ22 ≤ l ≤ 22, 4192 unique, number of parameters
6–8,10
2
58. The structure was solved by direct methods and refined
terminal Ga–H bonds,
13
6–8
with the program SHELXL-97 by a full-matrix least-squares
method based on F . R1 (2269 reflections I > 2 σ(I )) = 0.0576;
2
wR2 (all data) = 0.1624,3 max./min. residual electron
3
0
Ϫ3
density = 0.875/Ϫ1.379 × 10 e m . The molecules of 4 are
located on a twofold crystallographic rotation axis perpen-
dicular to the Ga–Ga single bonds. The hydrogen atoms of
the methyl groups were refined on ideal positions by employing
the riding model while those attached to gallium were located
in a Fourier map and refined freely. Two tert-butyl groups
7
2
8
compounds Me Ga(µ-H) GaMe and Ga H , however, both
2
2
2
6
derivatives possess two hydrogen bridges between their gallium
atoms. As expected, the Ga–C distances to the Ga() atoms of
the Ga–Ga single bonds (201.4 pm on average) are longer than
those of the H GaR group (198.4 pm on average).
The mechanism of the unexpected formation of 4 is not clear.
The homolytic cleavage of Ga–C bonds of the starting com-
pound (Me C) GaH (1) may be initiated by irradiation, and the
radical intermediates may dimerize via Ga–Ga bond formation.
Clearly, the monomeric product Me C(H)Ga–Ga(H)CMe is
not shielded enough to be stable and isolable. It gives a dimer
via two Ga–H–Ga 3c–2e bonds and is further stabilized by two
bridging [(Me C) ] GaH groups. Interestingly, tri(tert-butyl)-
gallane gave the same product (4) upon irradiation. Owing to its
particular constitution and number of electrons compound 4
may be assigned to the hypho-class of cluster compounds. The
corresponding boron hydride B H is known from quantum-
chemical calculations only. One of the calculated minimum
structures is quite similar, although not identical to that of
2
2
(
C2, C3) showed disorder; their methyl groups were refined on
split positions (occupation factors 0.74/0.26 and 0.55/0.45,
respectively).
CCDC reference number 161949.
See http://www.rsc.org/suppdata/dt/b1/b103409j/ for crys-
tallographic data in CIF or other electronic format.
3
2
3
3
3
2 2
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for generous financial
support.
6
14
11
References
4
, and has a six-membered B H2 heterocycle in the mole-
4
cular center with neighboring boron atoms bridged by BH4
1 W. Uhl, M. Layh and T. Hildenbrand, J. Organomet. Chem., 1989,
364, 289.
ligands, instead of opposite ones as in 4. Thus, the synthesis
and characterization of 4 may mark the starting point of a new
type of cluster chemistry with the heavier elements of the third
main-group.
2 R. W. H. Small and I. J. Worrall, Acta Crystallogr., Sect. B, 1982, 38,
2
1
50; J. C. Beamish, R. W. H. Small and I. J. Worrall, Inorg. Chem.,
979, 18, 220.
3
4
P. J. Davidson, D. H. Harris and M. F. Lappert, J. Chem. Soc.,
Dalton Trans., 1976, 2268.
Alkyl- and aryl-compounds only; excluding cluster derivatives:
R. D. Schluter, A. H. Cowley, D. A. Atwood, R. A. Jones, M. R.
Bond and C. J. Carrano, J. Am. Chem. Soc., 1993, 115, 2070; X. He,
R. A. Bartlett, M. M. Olmstead, K. Ruhlandt-Senge, B. E. Sturgeon
Experimental
All procedures were carried out under purified argon. n-Hexane
and cyclopentane were dried over LiAlH , pentafluorobenzene
4
J. Chem. Soc., Dalton Trans., 2001, 2398–2400
2399

View Article Online
and P. P. Power, Angew. Chem., 1993, 105, 761; X. He, R. A. Bartlett,
M. M. Olmstead, K. Ruhlandt-Senge, B. E. Sturgeon and P. P.
Power, Angew. Chem., Int. Ed. Engl., 1993, 32, 717; R. J.
Wehmschulte and P. P. Power, Angew. Chem., 1998, 110, 3344; R. J.
Wehmschulte and P. P. Power, Angew. Chem., Int. Ed., 1998, 37,
S. Konietzny, H. Fleischer, S. Parsons and C. R. Pulham, J. Chem.
Soc., Dalton Trans., 2001, 304.
8 C. R. Pulham, A. J. Downs, M. J. Goode, D. W. H. Rankin and H. E.
Robertson, J. Am. Chem. Soc., 1991, 113, 5149; HGa(BH ) : A. J.
4
2
Downs, T. M. Greene, L. A. Harman, P. F. Souter, P. T. Brain, C. R.
Pulham, D. W. H. Rankin, H. E. Robertson, M. Hofmann and P.
von Ragué Schleyer, Inorg. Chem., 1995, 34, 1799; H Ga(BH ): A. J.
3
152; X.-W. Li, P. Wei, B. C. Beck, Y. Xie, H. F. Schaefer III, J. Su
and G. H. Robinson, Chem. Commun., 2000, 453; A. K. Saxena,
H. Zhang, J. A. Maguire, N. S. Hosmane and A. H. Cowley, Angew.
Chem., 1995, 107, 378; A. K. Saxena, H. Zhang, J. A. Maguire, N. S.
Hosmane and A. H. Cowley, Angew. Chem., Int. Ed. Engl., 1995, 34,
2
4
Downs, S. Parsons, C. R. Pulham and P. F. Souter, Angew. Chem.,
1997, 109, 910; A. J. Downs, S. Parsons, C. R. Pulham and P. F.
Souter, Angew. Chem., Int. Ed. Engl., 1997, 36, 890; C. R. Pulham,
P. T. Brain, A. J. Downs, D. W. H. Rankin and H. E. Robertson,
J. Chem. Soc., Chem. Commun., 1990, 177.
3
32; A. H. Cowley, A. Decken and C. A. Olazábal, J. Organomet.
Chem., 1996, 524, 271; N. Wiberg, K. Amelunxen, H. Nöth, H.
Schwenk, W. Kaim, A. Klein and T. Scheiring, Angew. Chem., 1997,
9 W. Uhl and T. Spies, Z. Anorg. Allg. Chem., 2000, 626, 1059.
10 Some recent examples: P. T. Brain, H. E. Brown, A. J. Downs, T. M.
Greene, E. Johnsen, S. Parsons, D. W. H. Rankin, B. A. Smart and
C. Y. Tang, J. Chem. Soc., Dalton Trans., 1998, 3685; E. Johnsen,
A. J. Downs, T. M. Greene, P. F. Souter, K. Aarset, E. M. Page, D. A.
Rice, A. N. Richardson, P. T. Brain, D. W. H. Rankin and C. R.
Pulham, Inorg. Chem., 2000, 39, 719; B. Luo, V. G. Young, Jr. and
W. L. Gladfelter, Inorg. Chem., 2000, 39, 1705; J. F. Janik, R. L. Wells,
V. G. Young, Jr., A. L. Rheingold and I. A. Guzei, J. Am. Chem.
Soc., 1998, 120, 532.
11 M. L. McKee, J. Phys. Chem., 1990, 94, 435; H. Horn, R. Ahlrichs
and C. Kölmel, Chem. Phys. Lett., 1988, 150, 263; M. Bühl, P. von
Ragué Schleyer and M. L. McKee, Heteroatom Chem., 1991, 2, 499;
E. Fontain, Heteroatom Chem., 1994, 5, 61.
1
09, 1258; N. Wiberg, K. Amelunxen, H. Nöth, H. Schwenk, W.
Kaim, A. Klein and T. Scheiring, Angew. Chem., Int. Ed. Engl., 1997,
6, 1213; G. Linti and W. Köstler, Angew. Chem., 1996, 108, 593; G.
3
Linti and W. Köstler, Angew. Chem., Int. Ed. Engl., 1996, 35, 550;
X.-W. Li, W. T. Pennington and G. H. Robinson, J. Am. Chem. Soc.,
1
995, 117, 7578; X.-W. Li, Y. Xie, P. R. Schreiner, K. D. Gripper,
R. C. Crittendon, C. F. Campana, H. F. Schaefer and G. H.
Robinson, Organometallics, 1996, 15, 3798; G. H. Robinson, Chem.
Commun., 2000, 2175; B. Twamley and P. P. Power, Angew. Chem.,
2
2
000, 112, 3643; B. Twamley and P. P. Power, Angew. Chem., Int. Ed.,
000, 39, 3500.
5
W. Uhl, Coord. Chem. Rev., 1997, 163, 1; W. Uhl, Chem. Soc. Rev.,
000, 29, 259; W. Uhl, L. Cuypers, K. Schüler, T. Spies, C.
2
Strohmann and K. Lehmen, Z. Anorg. Allg. Chem., 2000, 626, 1526.
W. Uhl, L. Cuypers, R. Graupner, J. Molter, A. Vester and B.
Neumüller, Z. Anorg. Allg. Chem., 2001, 627, 607.
12 T. Hahn (Editor), International Tables for Crystallography, Space-
Group Symmetry, Kluwer Academic Publishers, Dordrecht, Boston,
London, 1989, vol. A.
13 SHELXTL-Plus, Release. 4.1; Siemens Analytical X-RAY
Instruments Inc., Madison, WI, 1990; G. M. Sheldrick, SHELXL-
97, Program for the Refinement of Structures, Universität
Göttingen, 1997.
6
7
P. L. Baxter, A. J. Downs, M. J. Goode, D. W. H. Rankin and H. E.
Robertson, J. Chem. Soc., Dalton Trans., 1990, 2873; P. L. Baxter,
A. J. Downs, M. J. Goode, D. W. H. Rankin and H. E. Robertson,
J. Chem. Soc., Chem. Commun., 1986, 805; Me Ga(µ-H) BH :
2
2
2
2
400
J. Chem. Soc., Dalton Trans., 2001, 2398–2400