Syntheses and crystal structures of some lithium di- And tri-(thiolato)[tris(trimethylsilyl)methyl]aluminates

Article abstract of DOI:10.1039/b002489i

The first organodi- and organotri-(thiolato)aluminates have been prepared and structurally characterised. They have been obtained by the reactions of [Li(thf)j{AlH3[C(SiMe3)3]}]21 (thf = tetrahydrofuran) with disulfides R2S2 (R = Me, Et or Ph) or thiols RSH (R = Pr, Bu' or Ph). The structure of the methyl compound [Li(thf)r {Al[C(SiMe3)3](SMe)3}] 2 has been determined by X-ray crystallography and shown to comprise lithium organotri(thiolato)aluminates each containing a folded four-membered LiS2Al ring. NMR spectroscopy indicates that the Et, Pr1 and Ph derivatives have similar structures in solution. Treatment of compound 1 with a three-fold excess of Bu'SH led to the replacement of only two of the available Al-H bonds and formation of [Li(thf){AlH[C(SiMe3)3](SBu')2}] 6, which contains an almost planar four-membered LiS2Al ring. The derivatives [Li(tmen)2][Al{C(SiMe3)3}(SR)3] (R = Bu' or Ph) have been made by treatment of [Li(tmen)2][AlH3{C(SiMe3)3}] 9 (tmen = N,N,N,N-tetramethylethane-l.Z-diamine) with RSH,: and the Bu' derivative has been shown to crystallise in a lattice containing separated [Li(tmen)2] cations and [Al{C(SiMe3)3}(SBu')3] anions. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b002489i

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Syntheses and crystal structures of some lithium di- and
tri-(thiolato)[tris(trimethylsilyl)methyl]aluminates
Wu-Yong Chen, Colin Eaborn,* Ian B. Gorrell, Peter B. Hitchcock and J. David Smith*
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton,
UK BN1 9QJ. E-mail: C.Eaborn@sussex.ac.uk; J.D.Smith@sussex.ac.uk
Received 29th March 2000, Accepted 31st May 2000
Published on the Web 22nd June 2000
The first organodi- and organotri-(thiolato)aluminates have been prepared and structurally characterised. They
have been obtained by the reactions of [Li(thf)2{AlH3[C(SiMe3)3]}]2 1 (thf = tetrahydrofuran) with disulfides
R2S2 (R = Me, Et or Ph) or thiols RSH (R = Pri, But or Ph). The structure of the methyl compound [Li(thf)2-
{Al[C(SiMe3)3](SMe)3}] 2 has been determined by X-ray crystallography and shown to comprise lithium
organotri(thiolato)aluminates each containing a folded four-membered LiS2Al ring. NMR spectroscopy
indicates that the Et, Pri and Ph derivatives have similar structures in solution. Treatment of compound 1 with
a three-fold excess of ButSH led to the replacement of only two of the available Al–H bonds and formation of
[Li(thf){AlH[C(SiMe3)3](SBut)2}] 6, which contains an almost planar four-membered LiS2Al ring. The derivatives
[Li(tmen)2][Al{C(SiMe3)3}(SR)3] (R = But or Ph) have been made by treatment of [Li(tmen)2][AlH3{C(SiMe3)3}] 9
(tmen = N,N,NЈ,NЈ-tetramethylethane-1,2-diamine) with RSH, and the But derivative has been shown to crystallise
in a lattice containing separated [Li(tmen)2] cations and [Al{C(SiMe3)3}(SBut)3] anions.
Lithium aluminium hydride and related compounds obtained
by replacement of one or more hydrogen atoms by alkoxy or
alkyl groups are widely used as reducing agents in organic syn-
thesis. Some of the most important applications are in the
reduction of carbonyl compounds; the intermediates are
thought to be alkoxoaluminates of the form LiAlHn(OR)4 Ϫ n
(n = 0–3),1 but it is not easy to isolate and characterise them.
With this background in mind we recently reported the syn-
thesis and structures of a series of lithium trialkoxoaluminates
[Li(thf)n{Al[C(SiMe3)3](OR)3}] (n = 1, R = Me, Pri, But, CH2Ph
or CHPh2; n = 2, R = Et or Ph; n = 4, R = Me), obtained by
reaction of [Li(thf)2{AlH3[C(SiMe3)3]}]2 1,2 with alcohols,
were made from [Li(tmen)2][AlH3{C(SiMe3)3}] 98 and RSH.
The structures of the methanethiolato derivative 2, the mono-
hydroaluminate 6, and the benzene solvate of 7 were deter-
mined. As far as we are aware, no structural data on lithium
thioaluminates have previously been reported but the com-
pounds [Li(OEt2)2M(SC6H2Pri3-2,4,6)4] (M = Ga or In)9 and
[RbAlMe2{SC6H3(C6H2Pri3-2,4,6)2-2,6}]10 have been described.
Experimental
Air and moisture were excluded as far as possible from all reac-
tions by the use of Schlenk techniques, flame-dried glassware
and Ar as blanket gas. NMR spectra from samples dissolved in
C6D6 (thf-d8 for 7 and 8) unless otherwise stated were recorded
at 300.1 (1H), 75.4 (13C), 194.5 (7Li), 130.4 (27Al) and 99.4 MHz
(29Si); chemical shifts are given relative to SiMe4 (H, C and Si),
aqueous LiCl or Al(NO3)3. IR spectra were recorded as Nujol
mulls. The compounds 12 and 98 were prepared as described
previously. Microanalyses were by Medac Ltd; the C values
for some compounds were low, possibly owing to difficulties
associated with combustion of organoaluminium compounds.
aldehydes and ketones.3,4 The introduction of the bulky organic
group greatly facilitates the isolation of stable species which
may be structurally similar to the intermediates in the reduction
process.
Syntheses
[Li(thf)2{Al[C(SiMe3)3](SMe)3}] 2. A solution of Me2S2
(0.11 g, 1.17 mmol) in toluene (10 cm3) was added to a solution
of 1 (0.32 g, 0.77 mmol as monomer) in toluene (20 cm3) and
the mixture stirred for 3 h. Gas was evolved. Most of the
solvent was removed under vacuum and the remaining solution
kept at room temperature for several hours. The colourless
moisture-sensitive blocks which separated were washed with
toluene to give 2 (0.20 g, 47%), mp 166–168 ЊC (Found: C, 45.4;
H, 9.3. C21H52AlLiO2S3Si3 requires C, 45.8; H, 9.5%); νmax/cmϪ1
1311w, 1260m, 1247 (sh), 1048s, 958m, 848m and 798m; δH 0.70
(27 H, s, Me3Si),1.27 (8 H, m, thf), 2.21 (9 H, s, MeS) and 3.38
(8 H, m, thf); δC 6.6 (Me3Si), 10.1 (MeS), 25.3 and 62.3 (thf);
δLi 0.47; δAl 153, ∆ν1/2 3.3 kHz; δSi Ϫ1.51.
Since thiolato groups are much softer than alkoxo groups
and since the tendency of sulfur to adopt a more pyramidal
coordination than oxygen should have a significant effect on the
configuration of LiS2Al rings we considered it worthwhile to
extend our previous work by studying the reactions of the
organotrihydroaluminate 1 with diorganodisulfides or thiols.
We expected that these would give a class of compounds that
have not previously been studied, viz. organotri(thiolato)-
aluminates, related to the intermediates in the reduction of
disulfides and thiocarbonyls by LiAlH4.5–7 Good yields of the
organotri(thiolato)aluminates [Li(thf)2{Al[C(SiMe3)3](SR)3}]
with R = Me 2; Et 3; Pri 4 or Ph 5 were obtained, but the
reaction of compound 1 with ButSH gave only the dithiolato
derivative [Li(thf){AlH[C(SiMe3)3](SBut)2}] 6. The ionic com-
pounds [Li(tmen)2][Al{C(SiMe3)3}(SR)3] (R = But 7 or Ph 8)
[Li(thf)2{Al[C(SiMe3)3](SEt)3}] 3. Liquid Et2S2 (0.16 cm3,
1.30 mmol) was slowly added to a stirred solution of 1 (0.35 g,
0.85 mmol) in toluene (20 cm3) at room temperature. Gas was
DOI: 10.1039/b002489i
J. Chem. Soc., Dalton Trans., 2000, 2313–2317
This journal is © The Royal Society of Chemistry 2000
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evolved. After 16 h the solvent was removed under vacuum and
the sticky residue crystallised from light petroleum (bp 40–
60 ЊC) to give 3 (0.25 g, 50%), mp 96 ЊC (Found: C, 45.0; H, 9.5.
C24H58AlLiO2S3Si3 requires C, 48.6; H, 9.8%); νmax/cmϪ1 1261s,
1179w, 1076m, 1035s, 972m, 919w, 861m and 801m; δH 0.66
(27 H, s, SiMe3), 1.23 (8 H, m, thf), 1.33 (9 H, t, CH2CH3), 2.85
(6 H, q, CH2CH3) and 3.42 (8 H, m, thf); δC 6.6 (SiMe3), 20.0
(CH2CH3), 22.3 (CH2CH3), 25.2 and 68.6 (thf); δLi 0.56; δAl 136,
∆ν1/2 4.6 kHz; δSi Ϫ3.4.
[Li(thf)2{Al[C(SiMe3)3](SPri)3}] 4. A solution of PriSH (0.11
g, 1.46 mmol) in toluene (10 cm3) was added to a stirred solu-
tion of 1 (0.20 g, 0.49 mmol) in toluene (15 cm3). Gas evolution
was rapid. The mixture was kept at room temperature for 3 h,
then solvent was removed under vacuum and the residue
recrystallised from methylcyclohexane to give colourless
moisture-sensitive crystals of 4 (0.24 g, 78%), mp 115–117 ЊC
(Found: C, 51.0; H, 10.1. C27H64AlLiO2S3Si3 requires C, 51.0;
H, 10.1%); νmax/cmϪ1 1257s, 1150m, 1046s, 857vs, 666m and
632m; δH 0.69 (27 H, s, Me3Si), 1.31 (8 H, m, thf), 1.47 [18 H, d,
CH(CH3)2], 3.49 (8 H, m, thf) and 3.61 [6 H, sept, CH(CH3)2];
δC 6.8 (SiMe3), 25.3 (CH2), 29.7 [CH(CH3)2], 32.9 [CH(CH3)2]
and 68.5 (OCH2); δLi Ϫ0.11; δAl 125, ∆ν1/2 4.1 kHz; δSi Ϫ3.55.
Fig. 1 Molecular structure of [Li(thf)2{Al[C(SiMe3)3](SMe)3}] 2.
from the lower layer to leave a white solid, which was recrystal-
lised from warm toluene to give colourless crystals of 8 (0.30 g,
52%), mp 152 ЊC (Found: C, 56.0; H, 8.9; N, 6.4. C40H74AlLiN4-
S3Si3 requires C, 58.2; H, 9.0; N, 6.8%); νmax/cmϪ1 1578m,
1289m, 1245s, 1183w, 1159m, 1129m, 1095m, 1066w, 1031s,
1016 (sh), 947s, 850vs, 786 (sh), 742s, 698m, 661s and 629m;
δH(C6D6) 0.63 (27 H, s, SiMe3), 1.83 (32 H, s br, tmen), 7.03
(9 H, m, m- and p-H), 7.80 (6 H, m, o-H); δH(thf-d8) 0.35
(SiMe3), 2.15 and 2.31 (displaced tmen), 6.77 (m- and p-H), 7.08
(o-H); δC 7.4 (SiMe3), 46.2 and 58.9 (tmen), 123.2 (p-C), 127.3
(o-C), 136.4 (m-C) and 141.7 (ipso-C); δLi Ϫ0.58; δAl 137, ∆ν1/2
1.8 kHz; δSi Ϫ4.2.
[Li(thf)2{Al[C(SiMe3)3](SPh)3}] 5. A solution of diphenyl
disulfide (0.32 g, 1.46 mmol) in toluene (5 cm3) was added
slowly to a stirred solution of 1 (0.40 g, 0.97 mmol) in toluene
(20 cm3) at room temperature. Gas was evolved. After 20 h a
white solid was filtered off and the filtrate was reduced to 5 cm3
then cooled to Ϫ30 ЊC to give colourless crystals of 5 (0.16 g,
23%), mp 133 ЊC (Found: C, 56.8; H, 7.9. C36H58AlLiO2S3Si3
requires C, 58.7; H, 7.9%); νmax/cmϪ1 1937w, 1858w, 1798w,
1731w, 1582s, 1344w, 1261s, 1173w, 1088s, 1025s, 800m and
732w; δH 0.63 (27 H, s, SiMe3), 1.16 and 3.23 (8 H, m, thf), 6.88
(3 H, m, p-H), 6.98 (6 H, m, m-H), 7.86 (6 H, d, o-H); δC 6.9
(Me), 25.2, 68.5 (thf), 125.1 (p-C), 128.5 (m-C), 134.6 (o-C) and
137.9 (ipso-C); δLi Ϫ0.51; δAl 136, ∆ν1/2 6.2 kHz; δSi Ϫ3.4. When
PhSH was used in this reaction, instead of Ph2S2, the product
was contaminated with LiSPh.
Crystallography
Data were collected on a CAD4 diffractometer by use of
Mo-Kα radiation (λ = 0.71073 Å) and details are given in
Table 1. All non-hydrogen atoms were anisotropic and H atoms
were included in riding mode. In compound 2, the methyl
groups attached to sulfur were fixed at idealised geometry but
the torsion angle defining the H atom positions was refined. In
6, hydride H atoms were freely refined isotropically. In 8, the
C(19) But group had methyl groups disordered equally over two
orientations.
[Li(thf){AlH[C(SiMe3)3](SBut)2}] 6. This was made in the
same way as 4 but from ButSH (2.4 mmol) and 1 (0.8 mmol)
and obtained from heptane as colourless moisture-sensitive
needles (0.35 g, 85%), mp 132.5–133.5 ЊC (Found: C, 50.8; H,
10.5. C22H54AlLiOS2Si3 requires C, 51.1; H, 10.5%); νmax/cmϪ1
1809 (Al–H), 1387s, 1257s, 1164m, 1033m, 916m, 862s and
780m; δH 0.64 (27 H, s, Me3Si), 1.11 (4 H, m, thf), 1.58 (18 H,
s, But) and 3.31 (4 H, m, thf); δC 6.4 (SiMe3), 25.2 (CH2), 36.8
[(CH3)3C], 44.9 [(CH3)3C] and 68.7 (OCH2); δLi 0.93; δAl 140,
∆ν1/2 3.4 kHz; δSi Ϫ1.3.
CCDC reference number 186/2009.
See http://www.rsc.org/suppdata/dt/b0/b002489i/ for crystal-
lographic files in .cif format.
Discussion
The organotri(thiolato)aluminates 2–5
Reaction of 1 with the organic disulfides R2S2 (R = Me, Et or
Ph) or propanethiol gave dihydrogen and the lithium organo-
tri(thiolato)aluminates
2–5
[Li(thf)2Al{C(SiMe3)3}(SR)3]
[Li(tmen)2][Al{C(SiMe3)3}(SBut)3] 7. Dry tmen (0.20 cm3,
1.32 mmol) was added to a stirred solution of 1 (0.27 g, 0.65
mmol) in toluene (20 cm3) at room temperature. After 16 h,
ButSH (0.23 cm3, 2.04 mmol) was added and gas was evolved.
The mixture was stirred for 22 h and the solvent then removed.
The residue was crystallised from warm benzene to give 7 as
colourless crystals (0.20 g, 40%), mp 176 ЊC. νmax/cmϪ1 1357s,
1288s, 1252s, 1158s, 1127m, 1094m, 1065m, 1029s, 1013s, 945s,
860s br, 804 (sh), 787 (sh), 665s, 610m, 578m; δH(thf-d8) 0.29
(27 H, s, SiMe3), 1.57 (27 H, s, But), 2.15 and 2.31 (32 H, s, free
tmen displaced by thf solvent); δC 8.4 (SiMe3), 37.8 (CMe3),
44.4 (CMe3), 46.2 and 58.9 (tmen); δLi Ϫ0.65; δAl 127, ∆ν1/2 600
Hz; δSi Ϫ4.6.
(R = Me 2, Et 3, Pri 4 or Ph 5) analogous to the trialkoxo-
aluminates described previously.3,4 The reductive cleavage of
disulfides by organoaluminium hydrides to give thiolates is
known11 though (PhCH2)2S2 and Ph2S2 were reported not to
react with [Me3NAlH3]2.12 A few reactions between thiols
and aluminium hydrides have been described,13,14 but thiol-
atoaluminium compounds are usually prepared from triorgano-
aluminium compounds.15,16 In the reactions described in this
work the Al–C bond is apparently protected by the bulky alkyl
group attached to aluminium.
The structure of 2 (Fig. 1) was confirmed by an X-ray diffrac-
tion study and bond lengths and angles are given in Table 2.
The structure is analogous to that of [Li(thf)2Al{C(SiMe3)3}-
(OEt)3] 104 but with a LiS2Al rather than a LiO2Al ring. The
bridging Al–S bonds [mean 2.310(2) Å] are slightly shorter than
those in other thiolato compounds (2.35–2.41 Å)15,16 and the
terminal Al–S bond length [2.239(2) Å] is similar to those in
the only other previously reported four-co-ordinate thiolato-
aluminium compounds, viz. [Al(SPh)(µ-Se)(NMe3)]2 [2.243(2)
[Li(tmen)2][Al{C(SiMe3)3}(SPh)3] 8. PhSH (0.22 cm3, 2.14
mmol) was slowly added to a stirred solution of 9 (0.35 g, 0.70
mmol) in toluene (20 cm3) at room temperature. Gas was
evolved rapidly. After 14 h two layers were present. The upper
layer was removed by cannula and solvent was pumped away
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Table 1 Crystal data and structural determinations for compounds 2, 6 and 7
[Li(thf)2{Al[C(SiMe3)3](SMe)3}]
2
[Li(thf){AlH[C(SiMe3)3](SBut)2}]
6
[Li(tmen)2][Al{C(SiMe3)3}(SBut)3]ؒ
2.5C6H6 7ؒ2.5C6H6
Chemical formula
Formula weight
C21H52AlLiO2S3Si3
551.0
C22H54AlLiOS2Si3
517.0
C34H86AlLiN4S3Si3ؒ2.5C6H6
960.7
T/K
Crystal system
Space group
a/Å
b/Å
c/Å
173(2)
173(2)
173(2)
Monoclinic
P21/c (no. 14)
16.891(6)
9.826(4)
19.456(11)
98.45(4)
Monoclinic
P21/c (no. 14)
21.737(4)
14.610(3)
23.040(4)
115.15(1)
6623(2)
Monoclinic
C2/c (no. 15)
34.256(7)
11.863(3)
30.519(4)
99.88(2)
β/Њ
U/Å3
3194(3)
12218(4)
Z
4
0.39
8
0.31
8
0.23
µ/mm–1
R1, wR2 [I > 2σ(I)]
(all data)
Measured/indep. rflns./Rint
Rflns. with I > 2σ(I)
0.071, 0.203
0.092, 0.225
4589/4426/0.073
3385
0.063, 0.157
0.107, 0.204
11948/11632/0.038
7701
0.086, 0.166
0.191, 0.211
8619/8468/0.082
4068
Table 2 Bond lengths (Å) and angles (Њ) in [Li(thf)2{Al[C(SiMe3)3](SMe)3}] 2, [Li(thf){AlH[C(SiMe3)3](SBut)2}] 6 and the anion [Al{C(SiMe3)3}-
(SBut)3]Ϫ of 7
2a
6b
Anion of 7c
Al–C
2.008(5)
2.305(2), 2.314(2)
2.239(2)
2.026(4)
2.090(7)
d
Al–Sbd
2.328(2)e
Al–St
2.291(3)e
Al ؒ ؒ ؒ Li
Li–S
3.233(10)
2.462(10), 2.483(11)
1.891(5)
1.876(5)
1.822(6)
3.032(9)
2.361(10), 2.389(10)
1.885(5)
1.881(5)
1.866(6)
Si–C1e
Si–Mee
S–Ce
1.886(7)
1.881(8)
1.849(8)
C1–Al–St
C1–Al–Sb
St–Al–Sb,t
Sb–Al–Sb
S–Li–S
115.1(2)
107.8(2), 108.7(2), 106.2(2)
113.3(2)e
107.77(9)e
98.04(8)
108.97(14)e
111.27(11), 112.58(12), 110.07(11)
101.05(7)
98.4(3)
89.7(3)
102.8(2)
C–St–Al
121.0(2), 116.8(3), 121.4(3)
C–Sb–Al
Al–S–Li
107.8(2), 106.9(2)
85.1(3), 84.9(2)
114.9(3), 117.9(3)
110.2(2)
105.2(2)
113.5(2)
110.6(2), 117.7(2)
80.6(3), 80.0(2)
108.3(3), 101.1(3)
110.5(2)
104.8(2)
113.8(2)
Li–Sb–C
Si–C–Sie
109.3(3)
103.8(4)
114.7(3)
Me–Si–Mee
C–Si–Mee
LiS1S2/S1S2Al fold
17
1
a Mean Li–O 1.903(11); O2–Li–S2 119.3(5), O1–Li–S2 110.1(5), O2–Li–S3 116.7(5), O1–Li–S3 109.4(5), O2–Li–O1 110.0(5). b Values for molecule 1
only: Li–O 1.856(10); O–Li–S1 133.0(6), O–Li–S2 125.6(5); Al–H 1.65(5); C1–Al–H 117(2), S1–Al–H 111(2), S2–Al–H 108(2). c Li–N mean
2.097(15); N–Li–N 90.4(6), 116.6(7), 119.4(7), 91.3(6), 119.4(7), 122.6(7). d Sb in bridge, St terminal. e Mean value. The precision of individual
measurements, none of which differs significantly from the mean, is indicated in parentheses.
Å]12 and [Al(SC6H2Pri3-2,4,6)3(thf)] [2.227(2) Å].13 The five-co-
2 compared with that at the oxygen of the terminal OEt group
ordinate aluminium compound [AlH(SCH2CH2NEt2)2] has a
mean Al–S bond length of 2.275(1) Å.14 The Li–S bond lengths
in 2 [mean 2.473(11) Å] are at the upper end of the usual range
(2.34–2.49 Å).9,17 The compound is therefore best regarded
as a lithium alkyl(methanethiolato)aluminate and the related
compounds 3–5 can be assumed from spectroscopic evidence
to have similar structures. Tetra(thiolato)aluminates do not
appear to have been characterised [no details have been given
about LiAl(SC6H2Pri3-2,4,6)4],13 but the gallium analogues con-
taining the species [Ga(SR)4]Ϫ are known.18 The mean Ga–S
bond lengths (2.264(3) Å for R = Et, 2.2678(6) Å for R = Pri and
2.257(3) Å for R = Ph) are, as expected in view of the similar
covalent radii of Al and Ga,13 close to the Al–S bond lengths in
2. The four-membered AlS2Li ring in 2 is folded by 17Њ about the
S ؒ ؒ ؒ S axis [cf. a fold angle of 14Њ in [Li(thf){Al[C(SiMe3)3]-
(OBut)3}] 11 and an almost planar ring in 10] and the bonds at
sulfur are more strongly pyramidalised [sum of angles 307.8 at
S(2) and 309.7Њ at S(3)] than those at oxygen [mean 348Њ in 10
and 345Њ in 11]; cf. also the narrower angle at S(1) [102.8(2)Њ] in
in 10 [130.1(8)Њ].4 The methyl groups attached to sulfur are
pushed to one side of the AlS2Li ring and the bulky C(SiMe3)3
group is located on the other side, but there is still room for the
co-ordination of two thf molecules at lithium, as found in 10.4
The Li–O bond distances are normal. The angles at the metal
centres in 2 and 10 show that the terminal SMe group in 2 lies
further over the ring than does the OEt group in 10, and that
the adjacent thf is pushed further away [mean S–Li–O(2) =
118.0(5), mean S–Li–O(1) = 109.8(5)Њ]. In contrast, in 10 it is
the other thf molecule, i.e. that adjacent to the bulky organic
group, that is pushed away from the ring. As far as we are aware,
2 is the first organotri(thiolato)aluminate to be structurally
characterised. It is noteworthy that the analogous methoxo
compound contained only one thf molecule per lithium, was
insoluble in hydrocarbons, and is probably polymeric.4
The NMR spectra of 2 and 3 in toluene-d8 at room temper-
ature show only one set of signals attributable to SR groups,
showing that bridging and terminal thiolato groups exchange
rapidly on the NMR timescale. There was no separation of
J. Chem. Soc., Dalton Trans., 2000, 2313–2317
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Fig. 2 Molecular structure of [Li(thf){AlH[C(SiMe3)3](SBut)2}] 6.
Fig. 3 The anion of [Li(tmen)2][Al{C(SiMe3)3}(SBut)3] 7.
signals at temperatures down to Ϫ90 ЊC. In contrast, separate
signals can be observed below room temperature from samples
of the alkoxo derivatives e.g. 10.4 The faster exchange of
thiolato than of alkoxo groups is consistent with weaker bridge
bonds in the sulfur derivatives.
85Њ in 2) and wider S–Li–S and S–Al–S angles (98.4 and 101.0,
cf. 89.7 and 98.0Њ, respectively, in 2) and closer metal centres.
The shortest Li ؒ ؒ ؒ Me contact (3.05 Å) in 6 is to C16; this is
longer than the shortest Li ؒ ؒ ؒ Me contacts in 11 (2.60 Å) or
[LiC(SiMe3)3]2 (2.47 Å),20 and probably does not imply any
significant agostic interaction.
The organodi(thiolato)aluminate 6
Attempts to make the analogous isopropyl compound [Li-
(thf){Al[C(SiMe3)3]H(SPri)2}] from 1 and two equivalents of
PriSH gave only complex mixtures which appeared from NMR
data to include the trithiolato derivative, showing that the iso-
propyl group is not large enough to prevent access of thiol to
the third Al–H bond in the starting hydride.
After the characterisation of the methyl derivative 2 we
decided to make the corresponding tert-butyl compound
in order to compare it with the already reported alkoxo
analogue [Li(thf)Al{C(SiMe3)3}(OBut)3] 11.3 In particular we
wished to discover what effect the greater tendency to
pyramidalisation at sulfur would have on the congestion at
the Li and Al centres. The reaction of 1 with a three-fold
excess of tert-butanethiol at room temperature gave dihydro-
gen and a solution from which colourless needles were iso-
lated. NMR and IR spectroscopic data from these needles
indicated that only two of the available Al–H bonds had been
cleaved to give the organodi(thiolato)aluminate 6. The third
Al–H bond appeared to be stable and well protected from
attack by the excess of thiol.
The asymmetric unit in the crystalline compound, the first
organodi(thiolato)aluminate to be structurally characterised,
consists of two independent lithium aluminate molecules, one
of which is shown in Fig. 2, but, as the differences in bond
lengths and angles between them are insignificant, values for
molecule 1 only are given in Table 2. The thf and C(SiMe3)3
groups lie on one side of the central LiS2Al ring and the hydride
and SBut groups on the other. A comparison between the bond
lengths and angles in 2 and those in 6 shows that the Al–C
bonds are the same, within experimental uncertainty, and the
Al–S bonds are only slightly longer in 6. The Li–S bonds are,
however, significantly shorter in 6, suggesting that the lithium
and aluminium fragments are more firmly held together than
those in 2. The shorter bonds in 6 may reflect the fact that there
is only one thf rather than two, so that the coordination at Li is
almost planar rather than tetrahedral (sum of angles 357Њ in
one independent molecule and 359.7Њ in the other). The But
groups are bent towards the Li to give narrow Li–S–C (105Њ)
and wide Al–S–C angles (114Њ). The steric effect of the large But
group is shown in the C–S distances [1.861(5) and 1.872(6) Å],
which are significantly longer than those in 2 [1.816(6)–
1.834(7) Å]. A similar effect is found in the compounds [R2Al-
NRЈ2]2, in which N–C bonds become longer as the size of RЈ
increases.19 The tighter endocyclic bonding together with the
steric hindrance provided by the large But group accounts for
the difficulty in the displacement of the final hydrogen from
aluminium to give a tri(thiolato) derivative analogous to 2. The
LiS2Al ring is almost planar (fold angle along the S ؒ ؒ ؒ S axis
only 1Њ), a consequence, presumably, of repulsion between the
tert-butyl groups in the bridging thiolato ligands. It is more
distorted from square, with narrower Li–S–Al angles (80Њ, cf.
The ionic organotri(thiolato)aluminate 7
The Li–S bond is significantly weaker than the Li–O bond so
sulfur ligands are less able than oxygen ligands to compete with
the commonly used bases having oxygen or nitrogen donor
atoms. We therefore considered that the organotri(thiolato)-
aluminates would be more likely to give solvent-separated ion
pairs than the anlaogous alkoxo derivatives. Our attempts to
obtain solvent-separated lithium trialkoxoorganoaluminates
had been unsuccessful,4 but when the ionic compound 9 was
treated with a three-fold excess of ButSH, the resulting solid,
after crystallisation from benzene, showed no absorption in the
Al–H stretching region of the IR spectrum and the NMR spec-
tra also indicated that all three hydrogens had been displaced
from aluminium to give a tri(thiolato)aluminate 7. The impli-
cation is that the separated ions found in crystalline 9 remain
separated in solution during attack by butanethiol molecules.
There is thus room for reaction at all three Al–H bonds and the
reaction is not inhibited, after only two have been attacked, by
the formation of a tightly bound ring analogous to that in com-
pound 6. It was not possible to obtain C and H analyses for 7
but no impurities were detected by NMR spectroscopy. The
structure was confirmed by an X-ray study, which revealed the
presence in the crystal of separated cations and anions. The
cation has been found in a number of structures and is not
discussed further. Data for the anion are given in Table 2 and
the structure is shown in Fig. 3. The Al–C bond lengths are
significantly longer than those in 2 and 6, and the Al–S bonds
slightly shorter, though they are not as short as the terminal
Al–S bond in 2. The S–Al–S angles are only slightly wider and
the C–Al–S angles slightly narrower than the tetrahedral value.
Other bond lengths and angles are in the usual range but
the S–But bonds [1.842(8)–1.858(7) Å] are again long. The
Al–S–But angles (110–121Њ) in 6 and 7 are much larger than the
Al–S–Me angles in 2 (102–107Њ), showing that the flexible pyr-
amidalisation at sulfur is able to accommodate alkyl groups of
various sizes. An analogous organotri(amido)aluminate was
isolated from the reaction between compound 1 and an excess
of aniline.8
2316
J. Chem. Soc., Dalton Trans., 2000, 2313–2317

View Article Online
8 S. S. Al-Juaid, C. Eaborn, I. B. Gorrell, S. A. Hawkes, P. B.
In all three compounds the Si–C1 bond lengths are similar, as
are the Si–Me bond lengths. The Si–C1 bonds are also of simi-
lar length to the Si–Me bond lengths, as in other compounds
containing the C(SiMe3)3 group attached to aluminium.21
Hitchcock and J. D. Smith, J. Chem. Soc., Dalton Trans., 1998, 2411.
9 F. Pauer and P. P. Power, in Lithium Chemistry: A Theoretical and
Experimental Overview, eds. A. M. Sapse and P. v. R. Schleyer,
Wiley, New York, 1995, ch. 9, pp. 295–392.
10 M. Niemeyer and P. P. Power, Inorg. Chim. Acta, 1997, 263, 201.
11 K. Maruoka, T. Miyazaki, M. Ando, Y. Matsumura, S. Sakane,
K. Hattori and H. Yamamoto, J. Am. Chem. Soc., 1983, 105, 2831.
12 W. J. Grigsby, C. L. Raston, V.-A. Tolhurst, B. W. Skelton and
A. H. White, J. Chem. Soc., Dalton Trans., 1998, 2547.
13 R. J. Wehmschulte, K. Ruhlandt-Senge and P. P. Power, Inorg.
Chem., 1995, 34, 2593.
14 C. Jones, F. C. Lee, G. A. Koutsantonis, M. G. Gardiner and
C. L. Raston, J. Chem. Soc., Dalton Trans., 1996, 829.
15 J. P. Oliver and R. Kumar, Polyhedron, 1990, 9, 409; J. P. Oliver,
J. Organomet. Chem., 1995, 500, 269.
Acknowledgements
We thank the EPSRC for financial support, the Chinese
Education Commission for the award of a scholarship to
W.-Y. C., and Chengdu University of Science and Technology
for granting him sabbatical leave.
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