Magnesium bis(tetrahydridogallate(III)): Structure and reaction with tert-butyl alcohol

Article abstract of DOI:10.1002/anie.200600906

(Figure Presented) Unknown terrain: Mg(GaH₄)₂· 4 OEt₂ (see molecular structure; blue Ga, green Mg, red O, gray C, white H) can be regarded as a parent compound for the essentially unexplored group of alkaline-earth tetrahydridogallates. Reaction of Mg-(GaH ₄)₂·4 OEt₂ with tert-butyl alcohol leads to {Mg(OtBu)[GaH₂(OtBu)₂]}₂.

Full text of DOI:10.1002/anie.200600906

Communications
Metal Hydrides
bromide, is selected as the counterion, inseparable product
mixtures are formed. Direct hydrogenation of GaCl3 with an
excess of MgH2 in THF and OEt2 does not lead to isolable
products either. However, if the bromide counterion is
replaced by iodide, the doubly substituted product 1 crystal-
lizes from the reaction solution, even when only one
equivalent of LiGaH4 is used.
DOI: 10.1002/anie.200600906
Magnesium Bis(tetrahydridogallate(III)):
Structure and Reaction with tert-Butyl Alcohol**
The two new compounds 1 and 2 are extremely unstable
and decompose within a few minutes at room temperature;
therefore, only the metal and halogen contents of 1 and 2
Michael Veith,* Markus Burkhart, and Volker Huch
1
For many years, adducts of the hydrogen compounds of
Group 13 elements with other metal hydrides have received
considerable attention, because they are considered as
important fundamental compounds of these elements, and
are also used as reducing agents and hydrogen-transfer agents
in inorganic and organic chemistry. Apart from classical
representatives such as MM’H4 (M = Li, Na, K, Rb, Cs; M’ =
B, Al, Ga, In), there are few examples of compounds of the
heavier Group 13 elements, in particular, with electropositive
elements other than the alkali metals that provide reliable
structure information. Mg(AlH4)2 was prepared and de-
scribed by Wiberg and Bauer more than 50 years ago,[1] but
was first crystallographically characterized in molecular form
as a tetrahydrofuran (THF) adduct by Nöth et al. in 1995.[2] To
could be determined. According to H and 13C NMR spec-
troscopy, the ratio of OEt2 molecules to GaH4 units is 2:1 in 1
and 4:1 in 2. In the IR spectra of 1 and 2, two broad
vibrational bands attributed to terminal and bridging hydride
species occur in the Ga–H stretching region (1: n˜(Ga-H) =
1863, 1693 cmÀ1; 2: n˜(Ga-H) = 1833, 1524cm À1), in agreement
with the structural analysis (see below).
The molecular structure of 1, determined by X-ray
diffraction on a single crystal, is shown in Figure 1.[7,8] In the
date, only two representatives have been described for
[3,4]
gallium, namely the adduct (BH3·GaH3)8
and the amine
complex [(pmdeta)ZnCl(GaH4)] (pmdeta = pentamethyldie-
thylenetetramine).[5] Herein, we describe the synthesis and
complete structure determination of a diethyl ether (OEt2)
adduct of Mg(GaH4)2, and through its reaction with tert-butyl
alcohol, give a first glimpse of its reactivity.
Mg(GaH4)2 is synthesized by the reaction of freshly
prepared LiGaH4 [Eq. (1)] with MgBr2 in OEt2 as solvent, as
OEt2
4LiH þ GaCl3
LiGaH þ 3 LiCl
ð1Þ
ƒƒƒ!
4
À78 ꢀ
C
Figure 1. Molecular structure of 1. Carbon-bound hydrogen atoms and
minor disorder components of the ethyl groups are omitted for clarity.
Thermal ellipsoids are set at 50% probability. Selected bond lengths [Š]
formulated in Equation (2). For the corresponding tetrahy-
dridoaluminates, it has been known for some time that the
À
À
À
and angles [8]: Mg O1 2.102(2), Mg O2 2.103(3), Ga H1 1.6(1),
OEt2
À
À
À
À
Ga H2 1.6(1), Ga H3 1.5(2), Ga H4 1.5(2), Mg H1 1.9(3), Mg···Ga
3.558(1); Mg-H1-Ga 177.0(5), O1-Mg-O2 90.45(1), O1-Mg-O2’
89.55(1).
MgBr2 þ 2 LiGaH4
MgðGaH Þ Á 4OEt ð1Þ þ 2 LiBr
ð2Þ
ƒƒ!
4
2
2
counterion and donor solvent have a pivotal influence on
product formation.[6] Analogously, in the case of the tetrahy-
dridogallates, adept choice of anion and reaction conditions
leads not only to Mg(GaH4)2·4OEt2 [1; Eq. (2)], but also to
MgBr(GaH4)·4OEt2 [2; Eq. (3)]. If chloride, rather than
case of 2, only a mixed crystal of approximately 80% 1 and
20% 2 could be isolated. Nevertheless, the molecular
structure of 2 was determined, as shown in Figure 2.[7]
Molecule 1 is centrosymmetric, with the magnesium atom at
the inversion center. The tetrahedral GaH4 units are con-
nected to the central magnesium atom, in a trans orientation,
through nearly linear m2-hydrogen bridges. The four oxygen
atoms of the OEt2 molecules form a nearly square rhombus
and complete the distorted octahedral coordination environ-
OEt2
ƒƒ!
MgBr2 þ LiGaH4
MgBrðGaH Þ Á 4OEt ð2Þ þ LiBr
ð3Þ
4
2
[*] Prof. Dr. M. Veith, M. Burkhart, Dr. V. Huch
Anorganische Chemie
Universität des Saarlandes
Postfach 15 11 50, 66041 Saarbrücken (Germany)
Fax: (+49)681-302-3995
À
ment of the magnesium atom. The Mg O distances are
typical for bonds between oxygen donors and magnesium
E-mail: veith@inm-gmbh.de
atoms of coordination number 6.[9] The Ga H bonds in
À
Prof. Dr. M. Veith
Leibniz-Institut für Neue Materialien
ImStadtwald, Geb.10.2.2, 66123 Saarbrücken (Germany)
general are shorter for the terminal hydrogen atoms than
for the bridging hydrogen atom (Figure 1). Compound 1 is the
first structurally characterized alkaline-earth tetrahydrido-
gallate and only the second molecular tetrahydridogallate, in
[**] This work was supported by the DFG within the framework of the
SFB 277.
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Chemie
Figure 3. Molecular structure of 3. Carbon-bound hydrogen atoms are
omitted for clarity. Thermal ellipsoids are set at 50% probability.
Figure 2. Molecular structure of 2. Carbon-bound hydrogen atoms are
omitted for clarity. Thermal ellipsoids are set at 50% probability.
À
À
Selected bond lengths [Š] and angles [8]: Mg O1 1.965(3), Mg O2
À
À
À
À
1.987(3), Mg O3 1.959(3), Mg O3’ 1.964(3), Ga O1 1.925(3), Ga O2
À
À
Selected bond lengths [Š] and angles [8]: Mg O1 2.102(8), Mg O2
À
À
1.909(3), Ga H1 1.58(9), Ga H2 1.56(8), Mg···Mg’ 2.922(2), Mg···Ga
2.947(2); Mg’-Mg-Ga 175.91(6), O1-Mg-O2 80.0(1), O3-Mg-O3’
83.7(1), O1-Ga-O2 83.0(1), Mg-O1-Ga 98.5(1), Mg-O2-Ga 98.3(1),
Mg-O3-Mg’ 96.3(1).
À
À
À
À
2.109(9), Ga H1 1.5(1), Ga H2 1.4(2), Ga H3 1.4(2), Ga H4 1.4(2),
À
À
Mg H1 2.0(1), Mg Br 2.87(2), Mg···Ga 3.539(4); Mg-H1-Ga 179.2(5),
O1-Mg-O2 89.66(5), O1-Mg-O2’ 90.34(5).
addition to [(pmdeta)ZnCl(GaH4)].[5] Both
[(pmdeta)ZnCl(GaH4)] have very similar bond lengths, and
1
and
1906 cmÀ1),[10] reflecting the different partial charges in the (m-
À
the M-H-Ga (M = Mg, Zn) angle is almost linear in both cases
OtBu)2GaH2 units common to both compounds. The Ga O
À
(1: 177.0(5)8; [(pmdeta)ZnCl(GaH4)]: 176.6(2)8). Com-
bonds are shorter than the Mg O bonds, in agreement with
[9]
À
pound 2 has a relatively long Ga Br bond, which must be
the smaller ionic radius of four-coordinate Ga3+[11] and the
greater steric demands on the magnesium atom. The fact that
the hydride ligands in 3 exclusively occupy terminal positions
underlines the distinctiveness of the hydride bridges in the
starting compound 1.
interpreted with caution, however, because of the small
fraction of 2 in the mixed crystal with 1. The Mg···Ga distance
in 2 is slightly smaller than in 1, since the bromide ligand
evidently withdraws electron density from the gallium atom.
An aluminum compound analogous to 2 with THF ligands
instead of the OEt2 molecules has a similar structure.[2]
When 1 is treated with tert-butyl alcohol (tBuOH),
hydrogen gas is formed, and the hydride ligands should be
replaced by OtBu groups, as formulated in Equation (4).
However, none of the plausible substitution products “Mg-
[GaH4Àn(OtBu)n]2” were successfully isolated (regardless of
the amount of tBuOH used); rather, {Mg(OtBu)[GaH2-
(OtBu)2]}2 (3) and [(tBuO)GaH2]2 are obtained. The forma-
tion of these products could be explained by a Lewis acid–
base dissociation of the hypothetical “Mg[GaH2(OtBu)2]2”
intermediate [Eq. (5)].
Experimental Section
Owing to the extraordinary sensitivity of the compounds towards
hydrolysis, all reactions were carried out in a modified Stock vacuum
apparatus under nitrogen as inert gas. The solvents were dried over
sodium wire and used immediately after distillation. The NMR
spectra were recorded on an AC-200-F NMR spectrometer (Bruker),
and the IR spectra were recorded on an FTS 165 IR spectrometer
(BioRad). LiH and GaCl3 were purchased from Aldrich and used
without further purification.
[12]
1: A precooled, freshly prepared solution of LiGaH4 (0.81 g,
10.0 mmol) in OEt2 (30 mL) was added dropwise to a solution of
MgBr2·4OEt2 (2.4g, 5.00 mmol) in OEt 2 (30 mL) over 20 min at
[13]
À788C, and a colorless suspension formed. The reaction was stirred
for 30 min at À788C. Subsequently, the cooling was removed, and the
solution was slowly brought to approximately 08C. As soon as the
precipitate completely dissolved, the reaction solution was cooled to
À308C and kept at this temperature for crystallization over 24h.
Yield: 0.98 g colorless crystals, 42%. Elemental analysis (%) calcd for
C16H48Ga2MgO4 (468.31 gmolÀ1): Ga 29.78, Mg 5.19; found: Ga 29.25,
Mg 5.07. M.p.: 58C. IR (nujol): n˜(Ga-H) = 1863 (br, w), 1693 cmÀ1 (br,
MgðGaH4Þ2 Á 4OEt 2 þ 2 n tBuOH !
ð4Þ
‘‘Mg½GaH4ÀnðOtBuÞnŠ2’’ þ 2 n H2 þ 4OEt 2
‘‘Mg½GaH2ðOtBuÞ2Š2’’ !
ð5Þ
1
1= ½ðtBuOÞGaH2Š2 þ = fMgðOtBuÞ½GaH2ðOtBuÞ2Šg2 ð3Þ
2
2
[10]
The known gallane [(tBuO)GaH2]2 can be separated
3
w). 1H NMR (200.13 MHz, C6D6, 258C): d = 1.05 (t, 24H, J(1H,1H) =
from 3 by sublimation, and 3 can subsequently be recrystal-
6.8 Hz, OCH2CH3), 3.32 (q, 16H, 3J(1H,1H) = 6.8 Hz, OCH2CH3),
3.47 ppm (br s, 8H, GaH). 13C{1H} NMR (50.3 MHz, C6D6, 258C): d =
65.9, 15.0 ppm.
1
lized from toluene. The H NMR spectrum of 3 in solution
reveals the presence of two different types of tert-butyl
groups. According to an X-ray structure analysis,[7,8] the
centrosymmetric molecule 3 consists of one Mg2O2 and two
GaMgO2 four-membered rings; the OtBu groups are bridging
and the hydride ligands are terminal (Figure 3). The
Ga···Mg···Mg···Ga axis is nearly linear (Ga-Mg-Mg
175.91(6)8). Owing to the absence of bridging hydride ligands,
only one band is observed in the Ga-H stretching region of the
IR spectrum of 3 (n˜(Ga-H) = 1842 cmÀ1). This band is notably
shifted in comparison to that of [(tBuO)GaH2]2 (n˜(Ga-H) =
2: The synthesis was carried out analogously to that of 1, with
LiGaH4 (0.40 g, 5.00 mmol) in OEt2 (30 mL). Yield: 0.55 g, 23%.
Elemental analysis (%) calcd for C16H44BrGaMgO4 (474.46 gmolÀ1):
Br 18.84, Ga 14.70, Mg 5.12; found: Br 17.52, Ga 14.15, Mg 4.93. M.p.:
88C. IR (solid): n˜(Ga-H) = 1833 (br, w), 1524cm À1 (br, w).
3: A solution of 1 (2.34g, 5.0 mmol) in OEt 2 (30 mL) was treated
dropwise with tert-butyl alcohol (1.82 mL, 1.41 g, 19.0 mmol) in OEt2
(5 mL) over 30 min at À788C. Vigorous gas generation ensued. The
temperature was slowly raised to room temperature with stirring, and
the stirring was subsequently continued for 6 h. After removal of the
Angew. Chem. Int. Ed. 2006, 45, 5544 –5546
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5545

Communications
solvent under reduced pressure, fractional sublimation was carried
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[8] G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, Universitꢀt Göttingen, 1997.
[9] N. Metzler, H. Nöth, M. Schmidt, A. Treitl, Z. Naturforsch. B
1994, 49, 1448 – 1451.
out. First, [(tBuO)GaH2]2 sublimed at 358C, followed by 3 at 1308C,
at 10À5 mbar. Yield: 0.423 g, 27%. By dissolving the product in
toluene and cooling to À208C, crystals of composition 3·toluene
formed. Elemental analysis (%) calcd for C24H58Ga2Mg2O6
1
(630.78 gmolÀ1): C 51.51, H 9.20; found: C 49.71, H 8.57. H NMR
(200.13 MHz, C6D6, 258C): d = 1.35 (s, 36H, O(CH3)3), 1.39 (s, 18H,
O(CH3)3), 5.5 ppm (br s, 4H, GaH). IR (nujol): n˜(Ga-H) = 1842 cmÀ1
(br, m).
[10] a) H. Nöth, H. Suchy, Z. Anorg. Allg. Chem. 1968, 358, 4 4 – 66;
b) M. Veith, S. Faber, H. Wolfanger, Chem. Ber. 1996, 129, 381 –
384.
[11] W. A. Holleman, E. Wiberg, Lehrbuch der anorganischen
Chemie, 101st ed., W. de Gruyter, Berlin, 1995, pp. 127.
[12] a) A. E. Shirk, D. F. Shriver, Inorg. Synth. 1977, 17, 4 5 – 4 7;
b) A. E. Finholt, A. C. Bond, H. I. Schlesinger, J. Am. Chem.
Soc. 1947, 69, 1199 – 1203.
Received: March 8, 2006
Published online: July 21, 2006
Keywords: gallium· hydrides · magnesium·
.
[13] W. M. Schubert, B. S. Rabinovitch, N. R. Larson, V. A. Sims, J.
Am. Chem. Soc. 1952, 74, 4590 – 4592.
structure elucidation
[1] E. Wiberg, R. Bauer, Z. Naturforsch. B 1950, 5, 397 – 398.
[2] H. Nöth, M. Schmidt, A. Treitl, Chem. Ber. 1995, 128, 999 – 1006.
[3] A. J. Downs, S. Parsons, C. R. Pulham, P. F. Souter, Angew.
Chem. 1997, 109, 910 – 911; Angew. Chem. Int. Ed. Engl. 1997,
36, 890 – 891.
[4] A. J. Downs, T. M. Greene, E. Johnsen, P. T. Brain, C. A.
Morrison, S. Parsons, C. R. Pulham, D. W. H. Rankin, K.
Aarset, I. M. Mills, E. M. Page, D. A. Rice, Inorg. Chem. 2001,
40, 3484 – 3497.
[5] G. A. Koutsantonis, F. C. Lee, C. L. Raston, J. Chem. Soc. Chem.
Commun. 1994, 1975 – 1976.
[6] E. C. Ashby, R. D. Schwarz, B. D. James, Inorg. Chem. 1970, 9,
325 – 332.
[7] All X-ray structural analyses were carried out on a Stoe IPDS
diffractometer (Darmstadt), with MoKa radiation (l =
0.71073 Š) at 200(2) K. 1: colorless rectangular crystals, 0.5 ”
3
0.4” 0.4mm , C16H48Ga2MgO4, orthorhombic, space group
Pbca, a = 13.523(3), b = 12.419(2), c = 16.395(3) Š, V=
2753.4(9) Š3, Z = 4, 1calcd = 1.130 gcmÀ3, 2qmax = 48.208, 16180
reflections, 2128 independent, solution by direct methods,
refinement (against Fo ) with SHELXL-97,[8] all non-hydrogen
2
atoms refined anisotropically, data/parameters: 2128/118, R1 =
0.0517 (I > 2s), wR2 = 0.1308, GooF = 1.063, residual electron
density: 0.515/À0.672 eŠÀ3. The ethyl groups are disordered,
such that there are two positions for each of the methylene
groups. 2/4” 1: colorless rectangular crystals, 0.3 ” 0.2 ” 0.2 mm3,
C16H47.2Br0.2Ga1.8MgO4, orthorhombic, space group Pbca, a =
13.515(3), b = 12.418(2), c = 16.303(3) Š, V= 2736.1(9) Š3, Z =
4, 1calcd = 1.140 gcmÀ3, 2qmax = 48.08, 15920 reflections, 2071
independent, solution by direct methods, refinement (against
Fo ) with SHELXL-97,[8] all non-hydrogen atoms refined aniso-
2
tropically, data/parameters: 2071/133, R1 = 0.0632 (I > 2s),
wR2 = 0.1583, GooF = 1.11, residual electron density: 0.451/
À1.007 eŠÀ3. Starting with the atomic positions of 1, a difference
Fourier analysis revealed additional electron density between
the gallium and magnesium atoms, which was assigned to a
bromine atom. Thus, the crystal is a mixed crystal of 1 and 2, in
which 1 is inversion-symmetrically substituted by approximately
À À
À
À
À À
20% 2 (that is, H3Ga H Mg Br/Br Mg H GaH3). 3·toluene:
colorless crystals, 0.8 ” 0.5 ” 0.4mm 3, C31H66Ga2Mg2O6, mono-
clinic, space group P21/n, a = 9.297(2), b = 14.004(3), c =
16.324(3) Š, b = 103.49(3)8, V= 2066.7(7) Š3, Z = 2, 1calcd
=
1.240 gcmÀ3, 2qmax = 48.288, 12617 reflections, 3209 independent,
2
solution by direct methods, refinement (against Fo ) with
SHELXL-97,[8] all non-hydrogen atoms refined anisotropically,
data/parameters: 3209/276, R1 = 0.057 (I > 2s), wR2 = 0.158,
residual electron density: 0.921/À0.481 eŠÀ3. CCDC 601597
(1), CCDC 601598 (2/4” 1), and CCDC 601599 (3·toluene)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
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Angew. Chem. Int. Ed. 2006, 45, 5544 –5546