Synthesis and structural characterization of the first completely alkyl-substituted Ga-Sb Lewis acid-base adductsf

Article abstract of DOI:10.1039/a908969a

The simple Lewis acid-base adducts (n-Bu)₃Ga(SbR₃) (R = Et 1, n-Pr 2, i-Pr 3, n-Bu 4) and (r-Bu),Ga(SbR₃) (R = Et 5, n-Pr 6, i-Pr 7, r-Bu 8) were prepared by combination of H-Bu₃Ga or n-Bu₃Ga, respectively, with the corresponding trialkylstibanes in a 1:1 molar ratio. 1-8 were fully characterized by multinuclear NMR (¹H and ¹³C) and mass spectroscopy. In addition, the solid state structures of 5 and 7 were determined by single crystal X-ray diffraction studies. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a908969a

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Synthesis and structural characterization of the first completely
alkyl-substituted Ga–Sb Lewis acid–base adducts†
Stephan Schulz* and Martin Nieger
Institut für Anorganische Chemie der Universität Bonn, Gerhard-Domagk-Str. 1,
D-53121 Bonn, Germany. E-mail: stephan@ac4010se.chemie.uni-bonn.de
Received 11th November 1999, Accepted 21st January 2000
The simple Lewis acid–base adducts (n-Bu)3Ga(SbR3) (R = Et 1, n-Pr 2, i-Pr 3, t-Bu 4) and (t-Bu)3Ga(SbR3) (R = Et
5, n-Pr 6, i-Pr 7, t-Bu 8) were prepared by combination of n-Bu3Ga or t-Bu3Ga, respectively, with the corresponding
trialkylstibanes in a 1:1 molar ratio. 1–8 were fully characterized by multinuclear NMR (1H and 13C) and mass
spectroscopy. In addition, the solid state structures of 5 and 7 were determined by single crystal X-ray diffraction
studies.
Introduction
Results and discussion
The bulk of our recent research has focused on the preparation
and structural characterization of compounds containing
Group 13–Group 15 elements, which are of current interest due
to their potential to serve as single source precursors for the
preparation of the corresponding semiconducting materials by
MOCVD technology. Primarily, we were interested in the syn-
thesis of Group 13 antimonides. Our investigations have led to a
simple synthetic route for the preparation of nanocrystalline
GaSb material,1 as well as to several metallo-organic AlSb2 and
GaSb3 compounds, which were obtained as simple Lewis acid–
base adducts or heterocycles containing σ-bonds between Al/
Ga and Sb. The potential of [R2AlSb(SiMe3)2]2 (R = Et, i-Bu)
to produce AlSb thin films by HV-MOCVD (high vacuum
metal-organic chemical vapor deposition) at temperatures
between 325–550 ЊC has previously been demonstrated.4 How-
ever, films so-prepared showed a temperature-dependent in-
corporation of Si, obviously resulting from the silyl groups.
Therefore, we became interested in the synthesis of precursor
compounds containing only alkyl substituents.
We focused on the preparation of simple Lewis acid–base
adducts of the type R3Ga(SbRЈ3) (R, RЈ = alkyl). While
adducts of Al, Ga and In compounds with amines and phos-
phines are well known, investigations concerning the synthesis
of Group 13–Sb adducts are rare and only a few examples have
been structurally characterized: X3B[Sb(SiMe3)3],5 R3Al[Sb-
(SiMe3)3],2b R2AlCl[Sb(SiMe3)3],2b R3Ga[Sb(SiMe3)3],6 and
R3In[Sb(SiMe3)3].6a Almost fifty years ago, Coates investigated
the synthesis of Me3Ga(EMe3) adducts (E = N, P, As, Sb, Bi)
and found their stability to decrease from NMe3 to SbMe3,
while BiMe3 did not react.7 To the best of our knowledge, no
investigations concerning the structural characterization of all-
alkyl-substituted Ga–Sb adducts were performed. Wells et al.
have reported two structurally characterized Ga–Sb adducts,
both containing silyl substituents bound to the Sb center.6 The
only example of completely alkyl-substituted Ga–Sb com-
pound was described by Cowley et al. They prepared [Me2-
GaSb(t-Bu)2]3, a six-membered heterocycle, and demonstrated
its potential to give GaSb films under MOCVD conditions.8
Herein, we report the synthesis and solid state structures of
several Lewis acid–base adducts.
The simple Lewis acid–base adducts (n-Bu)3Ga(SbR3) (R = Et
1, n-Pr 2, i-Pr 3, t-Bu 4) and (t-Bu)3Ga(SbR3) (R = Et 5, n-Pr
6, i-Pr 7, t-Bu 8) were prepared by reaction of n-Bu3Ga and
t-Bu3Ga and the corresponding trialkylstibanes in a 1:1 molar
ratio.
1
The H NMR spectra show resonances due to the substitu-
ents bound to the Ga center shifted to lower field. Comparable
results were observed in analogous adducts, e.g. R3Al(SbRЈ3),9
Me3Al(PR3),10 R3Ga(PRЈ3) (R = Me, Et),11 and Me3In-
(NR3).12 α-H and α-C shifts, as well as ∆(H), ∆(C), ∆H–H and
∆
values, for n-Bu3Ga, t-Bu3Ga and the adducts 1–8 are
C–C
summarized in Table 1.
The shifts to lower field of the α-H–Ga and α-C–Ga reson-
ances observed for the adducts do not show a uniform devel-
opment between the two adduct groups. While the resonances
due to the n-Bu3Ga adducts (1–4) show the biggest lowfield
shift (and therefore the biggest ∆(H) and ∆(C) values) with the
electronically strongest base, t-Bu3Sb, those due to the t-Bu3Ga
adducts (5–8) show the biggest lowfield shift with the elec-
tronically weakest base, Et3Sb. The spectra of 8 show reson-
ances at exactly the same shift as was found for the starting
materials, indicating this sterically overcrowded adduct to be
fully dissociated in solution. Obviously, the absolute degree of
shift extension of α-H–Ga or α-C–Ga related to the pure tri-
alkyl is not useful for determination of the dative Ga–Sb bond
strength. Comparable results were found for Al–P adducts.10 In
the case of the t-Bu3Ga adducts 5–8, a comparison of the internal
13C shift [∆C–C = δ(β-C)trialkylgallium Ϫ δ(α-C)trialkylgallium] of the
adduct and the pure trialkylgallium compound seems to be
more useful for a qualitative determination of bond strength
within Ga–Sb adducts. The largest value is observed for 5, while
8 shows exactly the same value as t-Bu3Ga. Electronically, 5
should be the weakest adduct due to the low basicity of
the trialkylstibane (Et3Sb) in this compound. However, with
sterically hindered Lewis acids like t-Bu3Ga, steric repulsion
between the ligands becomes the dominating factor influencing
the adduct strength. The melting points, which generally
decrease within this group (5: 106; 6: 56; 7: 64; 8: 37 ЊC), also
agree with this expectation. The ∆C–C values of 1–4 are higher
than the ∆C–C value of pure n-Bu3Ga, but they are almost within
the same range and are independent from the steric size of the
antimony substituents.
† Electronic supplementary information (ESI) available: experimental
data for adducts 2–4 and 6–8. See http://www.rsc.org/suppdata/dt/a9/
a908969a/
Due to the extreme sensitivity of 1–8 in solution towards air
and moisture, cryoscopic molecular weight measurements did
DOI: 10.1039/a908969a
J. Chem. Soc., Dalton Trans., 2000, 639–642
This journal is © The Royal Society of Chemistry 2000
639

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1
Table 1 Selected H and 13C NMR shifts, ∆(H), ∆(C) values and internal shifts ∆H–H and ∆C–C of the pure trialkylgallium precursors and the Ga
component within the adducts 1–8 in C6D6
δ1Ha
δ13Cb
∆(H)c
∆(C)d
∆
∆
C–C
e
f
Compound
H–H
n-Bu3Ga
0.61
0.73
0.66
0.74
0.80
19.3
16.1
16.9
16.6
16.4
—
—
Ϫ3.2
Ϫ2.4
Ϫ2.7
Ϫ2.9
0.71
0.77
—
0.76
0.71
9.2
12.7
11.9
12.3
12.7
(n-Bu)3Ga(SbEt3) 1
(n-Bu)3Ga[Sb(n-Pr)3] 2
(n-Bu)3Ga[Sb(i-Pr)3] 3
(n-Bu)3Ga[Sb(t-Bu)3] 4
0.12
0.05
0.13
0.19
t-Bu3Ga
1.16
1.32
1.32
1.23
1.16
31.5
26.9
27.3
30.0
31.5
—
—
Ϫ4.6
Ϫ4.2
Ϫ1.5
0
—
—
—
—
—
0.4
6.7
6.3
2.1
0.4
(t-Bu)3Ga(SbEt3) 5
(t-Bu)3Ga[Sb(n-Pr)3] 6
(t-Bu)3Ga[Sb(i-Pr)3] 7
(t-Bu)3Ga[Sb(t-Bu)3] 8
0.16
0.16
0.07
0
e
n-Bu3Ga: a δ1H(α-H); b δ13C(α-C); c ∆(H) = δ(α-H)adduct Ϫ δ(α-H)trialkylgallium
;
d ∆(C) = δ(α-C)adduct Ϫ δ(α-C)trialkylgallium
;
∆H–H = δ(β-H)trialkylgallium Ϫ
f
δ(α-H)trialkylgallium
;
∆
C–C = δ(β-C)trialkylgallium Ϫ δ(α-C)trialkylgallium
.
t-Bu3Ga: a δ1H(β-H); b δ13C(α-C); c ∆(H) = δ(β-H)adduct Ϫ δ(β-H)trialkylgallium
;
f
d ∆(C) = δ(α-C)adduct Ϫ δ(α-C)trialkylgallium
; ∆C–C = δ(β-C)trialkylgallium Ϫ δ(α-C)trialkylgallium.
not give reliable values. Therefore, we were not able to deter-
mine in detail the degree of dissociation in solution, but we
believe all the adducts to be extensively dissociated in solution.
Beachley and Maloney investigated the adduct formation of
several trialkylgallium compounds with phosphanes and found
most of them to be extensively dissociated in solution.13 Due to
the weaker basicity of stibanes compared to phosphanes,14 1–8
should also dissociate in solution. However, in the absence of
solvent, we believe 1–8 to be “real” adducts for the following
reasons:
(1) The combination of the weak acid t-Bu3Ga with the
stibanes yields solids in each case, indicating adduct formation.
The solid state structures of two such compounds were
determined.
(2) n-Bu3Ga is the stronger Lewis acid compared to the more
sterically demanding t-Bu3Ga and should give stronger adducts.
(3) n-Bu3Ga forms a solid adduct with the most sterically
demanding stibane t-Bu3Sb.
(2) Steric repulsion between the ligands, which should yield
weaker adducts, is greater in Sb(SiMe3)3 compared to Et3Sb.
Therefore, the Ga–Sb bond length in 5 should be shorter
than that in Et3Ga[Sb(SiMe3)3]. On the other hand, t-Bu3Ga is
less acidic than Et3Ga, which should lead to an elongated Ga–
Sb distance. Obviously, both effects compensate for each other,
resulting in very similar bond lengths. Compared to (t-Bu)3-
Ga[Sb(SiMe3)3], 5 and 7 are stronger adducts, due to less steric
repulsion between the ligands, hence the shorter Ga–Sb bond
lengths. On steric grounds 8 should show the longest Ga–Sb
bond distance, but we were not able to obtain suitable crystals
for a single crystal X-ray structure determination.
MOCVD studies are currently underway in our laboratories,
using the t-Bu3Ga adducts in an attempt to demonstrate their
potential for producing GaSb thin films.15
Experimental
Mass spectra of 1–8 show only the respective starting Ga and
Sb trialkyls, indicating dissociation of the adducts in the gas
phase. However, the solid adducts can be sublimed without
decomposition at 60–80 ЊC at 10Ϫ2 mbar.
General considerations
All manipulations were performed in a glovebox under a N2
atmosphere or by standard Schlenk techniques. Pentane was
carefully dried over sodium–potassium alloy under dry N2.
n-Bu3Ga and t-Bu3Ga,16 as well as Et3Sb, n-Pr3Sb and i-Pr3Sb,17
were prepared according to literature methods. t-Bu3Sb
was isolated from a standard salt elimination reaction between
t-BuLi and SbCl3 at Ϫ100 ЊC. A Bruker AMX 300 spectrometer
was used for NMR spectroscopy. 1H and 13C{1H} spectra were
referenced to internal C6D5H (δ1H 7.154, δ13C 128.0). Mass
spectra were recorded on a VG Masslab 12-250 spectrometer in
electron ionization mode at 20 eV. Melting points were observed
in sealed capillaries and were not corrected.
Single crystals of 5 and 7 suitable for X-ray crystallographic
study were obtained from solutions in pentane at Ϫ30 ЊC. In 5
and 7, the Ga and Sb centers reside in distorted tetrahedral
environments with their ligands adopting a staggered conform-
ation relative to one another. The mean Ga–C (5: 2.044; 7:
2.042 Å) and Sb–C bond lengths (5: 2.151; 7: 2.184 Å), as
well as the mean C–Ga–C (5: 116.4; 7: 115.9Њ) and C–Sb–C
bond angles (5: 97.6; 7: 100.2Њ), are within the expected
range. The C–Ga–Sb angles range from 100.57(12) to
101.64(12)Њ in 5 and from 99.07(5) to 104.48(5)Њ in 7. The C–
Sb–Ga angles in 5 vary from 118.95(13) to 120.84(14)Њ, while 7
shows a much greater variation, from 111.26(4) to 122.51(4)Њ.
Smaller ranges were observed in Et3Ga[Sb(SiMe3)3] [Si–Sb–Ga
114.50(22)–116.10(24)Њ] and (t-Bu)3Ga[Sb(SiMe3)3] [Si–Sb–Ga
114.68(7)–118.53(6)Њ]. However, the analogous Al–Sb adduct
(t-Bu)3Al[Sb(i-Pr)3] shows comparable C–Sb–Al bond angles
[112.20(4)–121.98(4)].9 The Ga–Sb distances [5: 2.8479(5); 7:
2.9618(2) Å] clearly display the influence of steric bulk on
the bond lengths. i-Pr3Sb is more sterically demanding than
Et3Sb, leading to an elongated Ga–Sb bond distance due to an
increased steric repulsion between the ligands. The Ga–Sb bond
distances determined in Et3Ga[Sb(SiMe3)3] [2.846(5) Å] and
(t-Bu)3Ga[Sb(SiMe3)3] [3.027(2) Å] span a wide range of almost
18 pm. 5 and 7 fit into this “bond distance window” very well.
Compared to Et3Ga[Sb(SiMe3)3], the steric pressure within 5 is
almost the same. The basicity of Et3Sb should be greater than
that of Sb(SiMe3)3 for the following reasons:
General synthesis of R3Ga(SbR3) adducts
Pure R3Ga (2 mmol) and R3Sb (2 mmol) were combined in the
glovebox. 5–8 were obtained as white solids, the other adducts
stayed liquid. At Ϫ30 ЊC 3 and 4 also solidified. 5–8 were
crystallized in almost quantitative yield from pentane (5–10
mL) at Ϫ30 ЊC. Experimental data are given for one n-Bu3Ga
and one t-Bu3Ga adduct only. Data for the other six adducts
have been deposited as electronic supplementary information
(ESI).
(n-Bu)3Ga(SbEt3) (1). Elemental analysis (C18H42GaSb,
M = 450.0 g molϪ1), found (calc.): C, 47.82 (48.04); H, 9.32
1
(9.41). H NMR (300 MHz, C6D5H, 25 ЊC): δ = 0.73 (m, 2H,
3
GaCH2CH2CH2CH3), 1.02 (t, JHH = 7.2 Hz, 3H, GaCH2CH2-
CH2CH3), 1.10 (m, 3H, SbCH2CH3), 1.23 (m, 2H, SbCH2CH3),
1.50 (m, 3JHH = 7.1 Hz, 2H, GaCH2CH2CH2CH3), 1.67 (m, 2H,
GaCH2CH2CH2CH3). 13C{1H} NMR (80 MHz, C6D5H,
25 ЊC): δ = 5.4 (SbCH2CH3), 11.4 (SbCH2CH3), 14.5 (GaCH2-
(1) The SiMe3 group is a π-acceptor, which leads to reduced
electron density at the Sb atom.
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Table 2 Crystallographic data for (t-Bu)3Ga(SbEt3) (5) and (t-Bu)3-
Ga[Sb(i-Pr)3] (7)
5
7
Chemical formula
Formula weight
C18H42GaSb
449.99
C21H48GaSb
492.06
Crystal system
Space group
Crystal dimensions/mm
a/Å
b/Å
c/Å
Monoclinic
P21/c (no. 14)
0.35 × 0.25 × 0.20
14.1615(3)
9.5735(2)
16.0291(4)
90.905(2)
2172.88(8)
4
Monoclinic
P21/c (no. 14)
0.60 × 0.55 × 0.50
13.6128(2)
9.0089(2)
20.0307(2)
91.750(1)
2455.35(7)
4
β/Њ
V/Å3
Z
µ/mmϪ1
2.477
2.198
Temperature/K
Dcalcd/g cmϪ3
Reflections collected
Non-equivalent reflections
Rint
123(2)
1.376
40096
5368
0.052
0.041, 0.114
123(2)
1.331
31295
5802
0.039
0.020, 0.049
R1,a wR2b
Fig. 2 ORTEP diagram (50% probability ellipsoids) showing the solid-
a For I > 2σ(I). b For all data.
state structure and atom-numbering scheme for 7; selected bond
lengths (Å) and angles (Њ): Ga(1)–Sb(1) 2.96178(19), Ga(1)–C(10)
2.0392(15), Ga(1)–C1(4) 2.0429(16), Ga(1)–C(18) 2.0450(16), Sb(1)–
C(1) 2.1866(15), Sb(1)–C(4) 2.1805(15), Sb(1)–C(7) 2.1843(17); C(10)–
Ga(1)–C(14) 115.74(7), C(10)–Ga(1)–C(18) 115.70(7), C(14)–Ga(1)–
C(18) 116.20(7), C(1)–Sb(1)–C(4) 98.29(6), C(1)–Sb(1)–C(7) 100.96(6),
C(4)–Sb(1)–C(7) 101.33(7), C(10)–Ga(1)–Sb(1) 104.48(5), C(14)–
Ga(1)–Sb(1) 99.07(5), C(18)–Ga(1)–Sb(1) 102.05(5), C(1)–Sb(1)–Ga(1)
111.26(4), C(4)–Sb(1)–Ga(1) 122.51(4), C(7)–Sb(1)–Ga(1) 118.74(5).
7, including selected bond lengths and angles. Data were
collected on a Nonius Kappa-CCD diffractometer. Absorp-
tion corrections were applied. The structures were solved by
direct methods (SHELXS-86)18 and refined by full-matrix
least-squares on F2. All non-hydrogen atoms were refined
anisotropically and hydrogen atoms by a riding model
(SHELXL-97).19
CCDC reference number 186/1817.
See http://www.rsc.org/suppdata/dt/a9/a908969a/ for crystal-
lographic files in .cif format.
Acknowledgements
Stephan Schulz gratefully thanks the DFG for a Fellow-
ship award. Financial support was given by the Fonds der
Chemischen Industrie, DFG and Professor E. Niecke,
Universität Bonn.
Fig. 1 ORTEP diagram (50% probability ellipsoids) showing the
staggered conformation and atom-numbering scheme for 5; selected
bond lengths (Å) and angles (Њ): Ga(1)–Sb(1) 2.8479(5), Ga(1)–
C(1) 2.058(4), Ga(1)–C(5) 2.035(4), Ga(1)–C(9) 2.039(4), Sb(1)–
C(13) 2.156(4), Sb(1)–C(15) 2.150(4), Sb(1)–C(17) 2.148(5); C(1)–
Ga(1)–C(5) 116.71(17), C(1)–Ga(1)–C(9) 115.88(17), C(5)–Ga(1)–
C(9) 116.72(17), C(13)–Sb(1)–C(15) 97.32(18), C(13)–Sb(1)–C(17)
97.91(19), C(15)–Sb(1)–C(17) 97.55(19), C(1)–Ga(1)–Sb(1) 101.64(12),
C(5)–Ga(1)–Sb(1) 100.83(12), C(9)–Ga(1)–Sb(1) 100.57(12), C(13)–
Sb(1)–Ga(1) 120.84(14), C(15)–Sb(1)–Ga(1) 118.95(13), C(17)–Sb(1)–
Ga(1) 119.26(13).
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M = 450.0 g molϪ1), found (calc.): C, 47.91 (48.04); H, 9.34
(9.41). Mp: 106 ЊC. 1H NMR (300 MHz, C6D5H, 25 ЊC):
δ = 1.06 (m, 3H, SbCH2CH3), 1.28 (m, 2H, SbCH2CH3), 1.32 (s,
9H, t-Bu). 13C{1H} NMR (80 MHz, C6D5H, 25 ЊC): δ = 5.9
(SbCH2CH3), 11.1 (SbCH2CH3), 26.9 (GaC(CH3)3), 33.7
(GaC(CH3)3). EI-MS (m/z, %) 240 (55) [t-Bu3Gaϩ], 208 (25)
[Et3Sbϩ], 57 (100) [t-Buϩ].
X-Ray structure solution and refinement
Crystallographic data are summarized in Table 2. Figs. 1 and
2 show ORTEP diagrams of the solid state structures of 5 and
J. Chem. Soc., Dalton Trans., 2000, 639–642
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Paper a908969a
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J. Chem. Soc., Dalton Trans., 2000, 639–642