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Magnesium Bis(tetrahydridogallate(III)) Structure and Reaction with tert-Butyl Alcohol.

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Metal Hydrides
DOI: 10.1002/anie.200600906
Magnesium Bis(tetrahydridogallate(III)):
Structure and Reaction with tert-Butyl Alcohol**
Michael Veith,* Markus Burkhart, and Volker Huch
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 described by Wiberg and Bauer more than 50 years ago,[1] but
was first crystallographically characterized in molecular form
as a tetrahydrofuran (THF) adduct by N5th et al. in 1995.[2] To
date, only two representatives have been described for
gallium, namely the adduct (BH3·GaH3)8[3, 4] and the amine
complex [(pmdeta)ZnCl(GaH4)] (pmdeta = pentamethyldiethylenetetramine).[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
4 LiH þ GaCl3 ƒƒƒ!LiGaH
4 þ 3 LiCl
78 C
formulated in Equation (2). For the corresponding tetrahydridoaluminates, it has been known for some time that the
MgBr2 þ 2 LiGaH4 ƒƒ!MgðGaH
4 Þ2 4 OEt2 ð1Þ þ 2 LiBr
counterion and donor solvent have a pivotal influence on
product formation.[6] Analogously, in the case of the tetrahydridogallates, adept choice of anion and reaction conditions
leads not only to Mg(GaH4)2·4 OEt2 [1; Eq. (2)], but also to
MgBr(GaH4)·4 OEt2 [2; Eq. (3)]. If chloride, rather than
MgBr2 þ LiGaH4 ƒƒ!MgBrðGaH
4 Þ 4 OEt2 ð2Þ þ LiBr
[*] Prof. Dr. M. Veith, M. Burkhart, Dr. V. Huch
Anorganische Chemie
Universit<t des Saarlandes
Postfach 15 11 50, 66041 Saarbr=cken (Germany)
Fax: (+ 49) 681-302-3995
Prof. Dr. M. Veith
Leibniz-Institut f=r Neue Materialien
Im Stadtwald, Geb.10.2.2, 66123 Saarbr=cken (Germany)
[**] This work was supported by the DFG within the framework of the
SFB 277.
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 crystallizes from the reaction solution, even when only one
equivalent of LiGaH4 is used.
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
could be determined. According to 1H and 13C NMR spectroscopy, 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: ñ(Ga-H) =
1863, 1693 cm1; 2: ñ(Ga-H) = 1833, 1524 cm1), 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
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 [']
and angles [8]: MgO1 2.102(2), MgO2 2.103(3), GaH1 1.6(1),
GaH2 1.6(1), GaH3 1.5(2), GaH4 1.5(2), MgH1 1.9(3), Mg···Ga
3.558(1); Mg-H1-Ga 177.0(5), O1-Mg-O2 90.45(1), O1-Mg-O2’
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 connected 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 environment of the magnesium atom. The MgO distances are
typical for bonds between oxygen donors and magnesium
atoms of coordination number 6.[9] The GaH bonds in
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 tetrahydridogallate and only the second molecular tetrahydridogallate, in
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 5544 –5546
Figure 2. Molecular structure of 2. Carbon-bound hydrogen atoms are
omitted for clarity. Thermal ellipsoids are set at 50 % probability.
Selected bond lengths ['] and angles [8]: MgO1 2.102(8), MgO2
2.109(9), GaH1 1.5(1), GaH2 1.4(2), GaH3 1.4(2), GaH4 1.4(2),
MgH1 2.0(1), MgBr 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 1 and
[(pmdeta)ZnCl(GaH4)] have very similar bond lengths, and
the M-H-Ga (M = Mg, Zn) angle is almost linear in both cases
(1: 177.0(5)8; [(pmdeta)ZnCl(GaH4)]: 176.6(2)8). Compound 2 has a relatively long GaBr bond,[9] which must be
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[GaH4n(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 formation of these products could be explained by a Lewis acid–
base dissociation of the hypothetical “Mg[GaH2(OtBu)2]2”
intermediate [Eq. (5)].
MgðGaH4 Þ2 4 OEt2 þ 2 n tBuOH !
‘‘Mg½GaH4n ðOtBuÞn 2 ’’ þ 2 n H2 þ 4 OEt2
‘‘Mg½GaH2 ðOtBuÞ2 2 ’’ !
=2 ½ðtBuOÞGaH2 2 þ 1=2 fMgðOtBuÞ½GaH2 ðOtBuÞ2 g2 ð3Þ
The known gallane [(tBuO)GaH2]2[10] can be separated
from 3 by sublimation, and 3 can subsequently be recrystallized from toluene. The 1H 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 (ñ(Ga-H) = 1842 cm1). This band is notably
shifted in comparison to that of [(tBuO)GaH2]2 (ñ(Ga-H) =
Angew. Chem. Int. Ed. 2006, 45, 5544 –5546
Figure 3. Molecular structure of 3. Carbon-bound hydrogen atoms are
omitted for clarity. Thermal ellipsoids are set at 50 % probability.
Selected bond lengths ['] and angles [8]: MgO1 1.965(3), MgO2
1.987(3), MgO3 1.959(3), MgO3’ 1.964(3), GaO1 1.925(3), GaO2
1.909(3), GaH1 1.58(9), GaH2 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).
1906 cm1),[10] reflecting the different partial charges in the (mOtBu)2GaH2 units common to both compounds. The GaO
bonds are shorter than the MgO bonds, in agreement with
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.
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.
1: A precooled, freshly prepared solution of LiGaH4[12] (0.81 g,
10.0 mmol) in OEt2 (30 mL) was added dropwise to a solution of
MgBr2·4 OEt2[13] (2.4 g, 5.00 mmol) in OEt2 (30 mL) over 20 min at
78 8C, and a colorless suspension formed. The reaction was stirred
for 30 min at 78 8C. Subsequently, the cooling was removed, and the
solution was slowly brought to approximately 0 8C. As soon as the
precipitate completely dissolved, the reaction solution was cooled to
30 8C and kept at this temperature for crystallization over 24 h.
Yield: 0.98 g colorless crystals, 42 %. Elemental analysis (%) calcd for
C16H48Ga2MgO4 (468.31 g mol1): Ga 29.78, Mg 5.19; found: Ga 29.25,
Mg 5.07. M.p.: 5 8C. IR (nujol): ñ(Ga-H) = 1863 (br, w), 1693 cm1 (br,
w). 1H NMR (200.13 MHz, C6D6, 25 8C): d = 1.05 (t, 24 H, 3J(1H,1H) =
6.8 Hz, OCH2CH3), 3.32 (q, 16 H, 3J(1H,1H) = 6.8 Hz, OCH2CH3),
3.47 ppm (br s, 8 H, GaH). 13C{1H} NMR (50.3 MHz, C6D6, 25 8C): d =
65.9, 15.0 ppm.
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 g mol1):
Br 18.84, Ga 14.70, Mg 5.12; found: Br 17.52, Ga 14.15, Mg 4.93. M.p.:
8 8C. IR (solid): ñ(Ga-H) = 1833 (br, w), 1524 cm1 (br, w).
3: A solution of 1 (2.34 g, 5.0 mmol) in OEt2 (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 78 8C. 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
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
solvent under reduced pressure, fractional sublimation was carried
out. First, [(tBuO)GaH2]2 sublimed at 35 8C, followed by 3 at 130 8C,
at 105 mbar. Yield: 0.423 g, 27 %. By dissolving the product in
toluene and cooling to 20 8C, crystals of composition 3·toluene
formed. Elemental analysis (%) calcd for C24H58Ga2Mg2O6
(630.78 g mol1): C 51.51, H 9.20; found: C 49.71, H 8.57. 1H NMR
(200.13 MHz, C6D6, 25 8C): d = 1.35 (s, 36 H, O(CH3)3), 1.39 (s, 18 H,
O(CH3)3), 5.5 ppm (br s, 4 H, GaH). IR (nujol): ñ(Ga-H) = 1842 cm1
(br, m).
Received: March 8, 2006
Published online: July 21, 2006
Keywords: gallium · hydrides · magnesium ·
structure elucidation
Crystallographic Data Centre via
G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, UniversitUt G5ttingen, 1997.
N. Metzler, H. N5th, M. Schmidt, A. Treitl, Z. Naturforsch. B
1994, 49, 1448 – 1451.
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b) M. Veith, S. Faber, H. Wolfanger, Chem. Ber. 1996, 129, 381 –
W. A. Holleman, E. Wiberg, Lehrbuch der anorganischen
Chemie, 101st ed., W. de Gruyter, Berlin, 1995, pp. 127.
a) A. E. Shirk, D. F. Shriver, Inorg. Synth. 1977, 17, 45 – 47;
b) A. E. Finholt, A. C. Bond, H. I. Schlesinger, J. Am. Chem.
Soc. 1947, 69, 1199 – 1203.
W. M. Schubert, B. S. Rabinovitch, N. R. Larson, V. A. Sims, J.
Am. Chem. Soc. 1952, 74, 4590 – 4592.
[1] E. Wiberg, R. Bauer, Z. Naturforsch. B 1950, 5, 397 – 398.
[2] H. N5th, 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.
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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 P) at 200(2) K. 1: colorless rectangular crystals, 0.5 Q
0.4 Q 0.4 mm3, C16H48Ga2MgO4, orthorhombic, space group
Pbca, a = 13.523(3), b = 12.419(2), c = 16.395(3) P, V =
2753.4(9) P3, Z = 4, 1calcd = 1.130 g cm3, 2qmax = 48.208, 16 180
reflections, 2128 independent, solution by direct methods,
refinement (against Fo2) with SHELXL-97,[8] all non-hydrogen
atoms refined anisotropically, data/parameters: 2128/118, R1 =
0.0517 (I > 2s), wR2 = 0.1308, GooF = 1.063, residual electron
density: 0.515/0.672 e P3. The ethyl groups are disordered,
such that there are two positions for each of the methylene
groups. 2/4 Q 1: colorless rectangular crystals, 0.3 Q 0.2 Q 0.2 mm3,
C16H47.2Br0.2Ga1.8MgO4, orthorhombic, space group Pbca, a =
13.515(3), b = 12.418(2), c = 16.303(3) P, V = 2736.1(9) P3, Z =
4, 1calcd = 1.140 g cm3, 2qmax = 48.08, 15 920 reflections, 2071
independent, solution by direct methods, refinement (against
Fo2) with SHELXL-97,[8] all non-hydrogen atoms refined anisotropically, data/parameters: 2071/133, R1 = 0.0632 (I > 2s),
wR2 = 0.1583, GooF = 1.11, residual electron density: 0.451/
1.007 e P3. 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
20 % 2 (that is, H3GaHMgBr/BrMgHGaH3). 3·toluene:
colorless crystals, 0.8 Q 0.5 Q 0.4 mm3, C31H66Ga2Mg2O6, monoclinic, space group P21/n, a = 9.297(2), b = 14.004(3), c =
16.324(3) P, b = 103.49(3)8, V = 2066.7(7) P3, Z = 2, 1calcd =
1.240 g cm3, 2qmax = 48.288, 12 617 reflections, 3209 independent,
solution by direct methods, refinement (against Fo2) 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: 0.921/0.481 e P3. CCDC 601597
(1), CCDC 601598 (2/4 Q 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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structure, butyl, reaction, tetrahydridogallate, magnesium, tert, bis, alcohol, iii
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