Crystal structures of organometallic compounds of lithium and magnesium containing the bulky ligands C(SiMe3)2(SiMe2X) X=Me, Ph, NMe2, or C5H4N-2

Article abstract of DOI:10.1016/S0022-328X(01)01032-4

The complex [Li(TMEDA){C(SiMe₃)₂SiMe₂NMe₂}] (1) (TMEDA=N,N,N′,N′-tetramethylethane-1,2,-diamine) was found to crystallise with an internally coordinated structure like that of [Li(THF)₂{C(SiMe₃)₂SiMe₂NMe ₂}] (THF=tetrahydrfuran). In contrast, the compound with Ph in place of NMe₂ crystallised as a dialkyllithate [Li(TMEDA)₂][Li{C(SiMe₃)₂(SiMe ₂Ph)}₂] (4). The reaction of 4 with MgBr₂ gave the doubly bromide-bridged lithium-magnesium complex [Li(TMEDA)(μ-Br)₂Mg{C(SiMe₃)₂(SiMe ₂Ph)}(THF)] (6), and that of [Li(THF){C(SiMe₃)₂(SiMe₂C₅H ₄N-2)}] gave the singly bridged compound [Li(THF)₃(μ-Br)MgBr{C(SiMe₃)₂(SiMe ₂C₅H₄N-2)] (8). The Grignard reagents [Mg{C(SiMe₃)₃}I(OEt₂)]₂ (10) and [Mg{C(SiMe₃)₂ (SiMe₂Ph)}I(OEt₂)]₂ (11) were obtained from the reactions between (Me₃Si)₃CI and (Me₂Ph)(Me₃Si)CI, respectively, with magnesium metal and shown to have halide-bridged structures. The unsymmetrical dialkylmagnesium [MgBu{C(SiMe₃)₂(SiMe₂NMe₂)}(THF)] (13), was prepared from a mixture of LiBu, 1 and [MgBr₂ (OEt₂)₂].

Full text of DOI:10.1016/S0022-328X(01)01032-4

Journal of Organometallic Chemistry 631 (2001) 76–86
www.elsevier.com/locate/jorganchem
Crystal structures of organometallic compounds of lithium and
magnesium containing the bulky ligands C(SiMe3)2(SiMe2X)
X=Me, Ph, NMe2, or C5H4N-2
Salih S. Al-Juaid, Anthony G. Avent, Colin Eaborn *, Salima M. El-Hamruni,
Simon A. Hawkes, Michael S. Hill, Martijn Hopman, Peter B. Hitchcock,
J. David Smith *
School of Chemistry, Physics and En6ironmental Science, Uni6ersity of Sussex, Brighton BN1 9QJ, UK
Received 13 March 2001; received in revised form 4 April 2001; accepted 4 April 2001
Abstract
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
The complex [Li(TMEDA){C(SiMe3)2SiMe2NMe2}] (1) (TMEDA=N,N,N%,N%-tetramethylethane-1,2,-diamine) was found to
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
crystallise with an internally coordinated structure like that of [Li(THF)2{C(SiMe3)2SiMe2NMe2}] (THF=tetrahydrfuran). In
contrast, the compound with Ph in place of NMe2 crystallised as a dialkyllithate [Li(TMEDA)2][Li{C(SiMe3)2(SiMe2Ph)}2] (4).
The reaction of
4
with MgBr2 gave the doubly bromide-bridged lithium–magnesium complex [Li(TMEDA)(m-
Br)2Mg{C(SiMe3)2(SiMe2Ph)}(THF)] (6), and that of [L¸i(¹T¹H¹F¹){¹C¹(S¹iM¹¹e3¹)2¹(S¹iM¹¹e2¹C¹5H¹4ºN-2)}] gave the singly bridged compound
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Li(THF)3(m-Br)MgBr{C(SiMe3)2(SiMe2C5H4N-2)] (8). The Grignard reagents [Mg{C(SiMe3)3}I(OEt2)]2 (10) and [Mg{C(SiMe3)2
(SiMe2Ph)}I(OEt2)]2 (11) were obtained from the reactions between (Me3Si)3CI and (Me2Ph)(Me3Si)CI, respectively, with
magnesium
metal
and
shown
to
have
halide-bridged
structures.
The
unsymmetrical
dialkylmagnesium
¸¹¹¹¹¹¹¹¹¹¹¹¹¹º
[MgBu{C(SiMe3)2(SiMe2NMe2)}(THF)] (13), was prepared from a mixture of LiBu, 1 and [MgBr2(OEt2)2]. © 2001 Elsevier
Science B.V. All rights reserved.
Keywords: Lithium; Magnesium; Bulky ligands
1. Introduction
range of elements [1]. In many cases they are made in
ether solution and used without isolation, but the
lithium and magnesium compounds themselves show a
variety of structural types. In this paper we describe the
X-ray structures of seven such compounds and com-
pare them with those of previously described related
species.
Organolithium and to a lesser extent organomagne-
sium compounds containing bulky tris(triorganosi-
lyl)methyl groups are starting materials for the
preparation of organometallic compounds of a wide
* Corresponding authors. Tel.: +44-1273-678-124; fax: +44-1273-677-196 (J.D.S.).
E-mail address: j.d.smith@sussex.ac.uk (J.D. Smith).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01032-4

S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
77
(SiMe3)2}·PMDTA [6]) and the SiꢀC1 bonds (1.803(4)
2. Results and discussion
,
,
A in 1 and 1.806(4) A in 2) are significantly shorter (cf.
,
,
2.1. The dimethylamino compound 1
1.830(5)
A
in
3
and av. 1.853(12)
A
in
Li(THF)C(SiMe2Ph)3 [7].
A
similar effect was
The TMEDA derivative of LiC(SiMe3)2(SiMe2NMe2)
was prepared in order to compare the complexing
power of TMEDA with that of: (a) THF as shown in
the previously reported complex 2 [2]; and (b) the
NMe2 substituent in the ligand. A 1:1 mixture of
TMEDA and 2 in hexane gave a high yield of 1, the
THF having been displaced by TMEDA. The crystals
of the product 1 contained two independent molecular
species with barely significant differences in bond
lengths and angles. One of these is shown in Fig. 1 and
molecular parameters are given in Table 1. The molecu-
lar structures of 1 and 2 are similar. The NꢀLiꢀN bite
angle of the TMEDA in 1 (83.0(3)°) is narrower than
the OꢀLiꢀO angle (95.7(3)°) in 2 and there are minor
differences in bond angles at Li, but the other bond
lengths and angles in 1 are not significantly different
from the corresponding parameters in 2. The differ-
ences between the LiꢀN bond lengths in 1 are not
significant, indicating that the bonds to the NMe2
noted in the structures of the compounds
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
Li(THF)2C(SiMe3)2(SiMe2OMe) (LiꢀC 2.304(11) and
,
SiꢀC 1.792(6) A) [8] and [LiC(SiMe3)(SiMe2OMe)2]2
,
(LiꢀC 2.256(9) and SiꢀC 1.808(3) A) [9]. In all cases,
coordination of NMe2 or OMe results in a significant
weakening of the LiꢀC bond, and the carbanionic
charge is more effectively delocalised into the bonds
between carbon and the silicon bearing the nitrogen or
oxygen substituents than into the other CꢀSi bonds.
2.2. The ate complex 4
The compound LiC(SiMe3)3 crystallises from THF as
an ate complex [Li(THF)4][Li{C(SiMe3)3}2] [10]. If
TMEDA is added to the THF solution, the THF is
displaced from the coordination sphere of the lithium
and the compound [Li(TMEDA)2][Li{C(SiMe3)3}2] (5)
crystallises out [11,12]. Crystals suitable for a determi-
nation of the structure of the species separating from
THF solutions of LiC(SiMe3)2(SiMe2Ph) could not be
obtained but better crystals formed when the organo-
lithium compound was prepared in Et2O [3]. These
were shown to contain molecular species 3, in which the
lithium is linked by bonds to carbon and oxygen, and
interacts weakly with the ipso-carbon atom of the
phenyl group. There are similar interactions in
[Li(THF)C(SiMe2Ph)3] [7] and [{LiCH(SiMe2Ph)2}2] [3].
Treatment of a THF solution of LiC(SiMe3)2(SiMe2Ph)
with one equivalent of TMEDA gave a white solid 4,
and elemental analysis and NMR spectra showed that
the crystalline compound contained no THF. Although
the initial batch of crystals was not of sufficient quality
for an X-ray diffraction study, a second batch, obtained
during several months from a concentrated solution in
benzene, proved to be satisfactory. The crystals were
groups of the TMEDA and the ligand are of similar
¸¹¹º
strength. Steric effects associated with the
have been discussed elsewhere [2].
ring
LiNSiC
,
The LiꢀC bond lengths in both 1 (2.315(8) A) and 2
,
(2.287(9) A) are significantly longer than those in other
,
monomeric organolithium compounds (cf. 2.096(10) A
,
3 [3], 2.13(16) A
in Li(OEt2)C(SiMe3)2(SiMe2Ph)
in LiPh·PMDTA (PMDTA=N,N,N%,N¦,N¦-penta-
t
,
methyldiethylenetriamine) [4], 2.12 A in LiC6H2Bu3-
2,4,6·TMPN (TMPN=N,N,N%N%-tetramethylpropane-
,
1,2-diamine) [5], and 2.13(5)
A
in Li{CH-
shown
to
consist
of
the
ate
complex
[Li(TMEDA)2][Li{C(SiMe3)2(SiMe2Ph)}2] (4). The
cation, which lies on a two-fold rotation axis, is not
discussed further; the anion, shown in Fig. 2, lies on an
inversion centre, so the CꢀLiꢀC skeleton is linear.
Although diorganolithates were postulated by Wittig
50 years ago [13], very few have been isolated in the
solid state and structurally characterised [10,12,14–16].
As in the case of previously reported examples, the high
angle diffraction from 4 was weak, but the structural
data are free from crystallographic disorder and can be
compared with those from related compounds. The
,
LiꢀC bond length (2.191(6) A) is longer than those in
Fig. 1. Molecular structure of one of the independent molecules of
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
monomeric organolithium compounds, but similar to
,
that (2.213(5) A) in the dialkyllithate 5. The mean inner
[Li(TMEDA)C{(SiMe3)2(SiMe2NMe2)}] (1).

78
S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
Table 1
,
Selected bond lengths (A) and bond angles (°) for 1, 4, 6, 8, 10, 11 and 13
Bond lengths
Bond angles
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
[Li(TMEDA)C{(SiMe3)2(SiMe2NMe2)}] (1) a
LiꢀN (tmen) b
LiꢀC
2.234(8)
2.315(8)
2.195(8)
1.790(4)
1.803(4)
1.829(4)
1.463(6)
1.885(5)
LiꢀCꢀSi3
83.9(2)
107.8(2)
87.8(2)
80.1(3)
83.0(3)
123.7(4)
111.6(3)
CꢀSi3ꢀN1
Si3ꢀN1ꢀLi
N1ꢀLiꢀC1
N2ꢀLiꢀN3
N1ꢀLiꢀN2
N1ꢀLiꢀN3
LiꢀN
NꢀSi
SiꢀC1
SiꢀC(exo) b
NꢀC b
SiꢀMe b
[Li(TMEDA)2][Li{C(SiMe3)2(SiMe2Ph)}2] (4)
LiꢀC
2.191(6)
1.833(6)
1.882(7)
1.904(7)
CꢀLiꢀC
180
SiꢀC1 b
SiꢀMe b
SiꢀPh b
LiꢀCꢀSi
100.7(3), 106.6(3), 104.1(3)
114.5(3)
103.8(4)
SiꢀCꢀSi b
MeꢀSiꢀMe b
[Li(TMEDA)(m-Br)2Mg{C(SiMe3)2(SiMe2Ph)}(THF)] (6)
LiꢀN b
LiꢀBr b
MgꢀBr b
MgꢀO
2.06(2)
LiꢀBrꢀMg b
BrꢀMgꢀBr
BrꢀLiꢀBr
82.2(3)
97.25(9)
98.4(5)
89.2(6)
120.0(2), 118.5(2)
96.2(2). 98.9(2)
120.9(3)
114.4(4), 110.6(4), 111.4(4)
104.4(4)
2.507(13)
2.530(3)
2.056(5)
2.186(8)
1.859(7)
1.900(8)
1.875(8)
NꢀLiꢀN
MgꢀC
BrꢀMgꢀC
BrꢀMgꢀO
CꢀMgꢀO
C1ꢀSi b
SiꢀPh
SiꢀMe b
SiꢀCꢀSi
Me(Ph)ꢀSiꢀMe b
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
Li(THF)3(m-Br)MgBr{C(SiMe3)2(SiMe2C5H4N)} (8) c
LiꢀO1
LiꢀO2
LiꢀO3
LiꢀBr
1.887(11)
1.925(13)
1.933(12)
2.505(10)
2.535(2)
2.488(2)
2.219(5)
2.122(4)
1.856(5)
1.904(5)
1.887(5)
LiꢀBrꢀMg
BrꢀMgꢀBr
NꢀMgꢀBr1
NꢀMgꢀC
BrꢀLiꢀO
129.2(2)
105.12(6)
100.69(12)
94.18(16)
105.9(5), 116.2(6), 112.2(5)
105.0(5), 110.1(6), 107.1(5)
96.6(2)
108.0(2)
116.3(3)
MgꢀBr1
MgꢀBr2
MgꢀC
MgꢀN
C1ꢀSi b
SiꢀC4
OꢀLiꢀO
MgꢀC1ꢀSi1
C1ꢀSi1ꢀC4
Si1ꢀC4ꢀN
C4ꢀNꢀMg
SiꢀC1ꢀSi b
MeꢀSiꢀMe
112.6(3)
112.4(2)
103.9(3)–106.3(2)
SiꢀMe b
[Mg(OEt2){C(SiMe3)3}I]2 (10) and [Mg(OEt2){C(SiMe3)2(SiMe2Ph)}I]2 (11)
10
11
10
11
Bond lengths
MgꢀC
Bond angles
MgꢀIꢀMg
IꢀMgꢀI
2.146(7)
2.849(3)
2.860(3)
2.080(6)
1.869(7)
1.880(9)
2.181(3)
2.814(1)
2.842(2)
2.059(3)
1.870(3)
1.882(4)
90.01(7)
89.99(7)
119.2(2) b
90.34(4)
89.66(4)
117.90(10), 123.00(9)
MgꢀI
MgꢀI%
IꢀMgꢀC
MgꢀO
IꢀMgꢀO
101.9(2), 100.2(2) 100.04(8), 100.99(8)
C1ꢀSi b
SiꢀMe b
SiꢀCꢀSi
110.0(4) b
109.1(2), 110.5(2), 113.5(2)
101.6(2)–106.9(2)
MeꢀSiꢀMe
102.2(4)–106.2(4)
¸¹¹¹¹¹¹¹¹¹¹¹¹º
[MgBu{C(SiMe3)2(SiMe2NMe2)}·THF] (13)
MgꢀC(Bu)
MgꢀC1
MgꢀO
MgꢀN
SiꢀN
Si1ꢀC1
Si2ꢀC1
Si3ꢀC1
SiꢀMe b
NꢀC b
2.130(3)
2.241(2)
2.069(2)
2.203(2)
1.829(2)
1.831(2)
1.851(2)
1.847(2)
1.877(3)
1.474(3)
BuꢀMgꢀO
101.44(13)
107.63(8)
80.41(8)
118.97(11)
131.60(12)
114.69(9)
88.75(8)
103.24(10)
NꢀMgꢀO
C1ꢀMgꢀN
C(Bu)ꢀMgꢀN
C(Bu)ꢀMgꢀC1
C1ꢀMgꢀO
MgꢀNꢀSi
NꢀSiꢀC1
MgꢀC1ꢀSi1
SiꢀCꢀSi
87.53(9)
117.60(12), 114.21(12),
114.49(12)
MeꢀSiꢀMe
102.07(15)–106.08(18)
a The data are for one of the two independent molecules in the asymmetric unit. Those for the other molecule differ insignificantly.
b Average value with e.s.d.s for individual measurements, none of which differs significantly from the mean.
c In 8: NꢀMgꢀBr2 104.01(13); CꢀMgꢀBr2 128.70(14); CꢀMgꢀBr1 118.13(13); MgꢀC1ꢀSi2 116.5(2); MgꢀC1ꢀSi3 105.3(2).

S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
79
7
,
SiꢀC bond length (1.833(6) A) is similar to that in 3 and
would be expected to give two signals in the Li spec-
trum, but only one sharp peak was detected. However,
as the Li signal from the lithate anion [Li-
{C(SiMe3)3}2]− is broad and difficult to detect [12], it
would not be surprising if the only clear signal from 4
was that from the cation at l 0.54 [3]. The alternative
explanation that there is rapid exchange of lithium
between the cation and dialkyllithate cannot be ruled
out, but it seems unlikely in view of the considerable
chemical reorganisation required (Eq. (1)).
in other tris(organosilyl)methyl–lithium derivatives,
reflecting the delocalisation of anionic charge, but this
delocalisation does not affect the lengths of the outer
,
SiꢀC bonds (mean 1.882(7) A), which are similar to
,
those (1.875(2) A) in SiMe4 [17]. The wide SiꢀCꢀSi and
narrow MeꢀSiꢀMe angles are normal [18].
The NMR spectra of 4 at room temperature show
sharp peaks assigned to SiMe3, SiMe2 and TMEDA
groups. Samples containing an excess of TMEDA gave
separate signals for complexed and free TMEDA but
chemical exchange was detected by polarisation transfer
experiments. We showed previously [12] that the
changes in the NMR spectra from solutions of
LiC(SiMe3)3 in THF between 288 and 318 K could be
understood in terms of chemical exchange between the
ate complex [Li(THF)4][Li{C(SiMe3)3}2] and molecular
species Li(THF)nC(SiMe3)3, as shown in Eq. (1) [L=
THF, R=C(SiMe3)3].
The formation of an ate rather than a molecular
complex upon treatment of 3 with TMEDA reflects the
greater basicity towards Li of the amine than of the
ether ligand, as shown also in the formation of 1 from
2. Possible Li···Ph interactions are insufficiently strong
to compete with the strongly basic TMEDA and the
phenyl groups point away from the central lithium
atom. In the presence of only the more weakly basic
diethyl ether, however, Li···Ph interactions are suffi-
ciently strong to stabilise the molecular species 3. In the
case of LiC(SiMe3)3, both THF and TMEDA are suffi-
ciently basic to give species with the ate structure, and
(weak) Li···Me interactions are seen only in the absence
of donor solvents [19]. In contrast, the NMe2 group in
1 and 2 is able to compete successfully with TMEDA
for space in the coordination sphere of lithium, and
formation of molecular species is favoured over that of
ate complexes.
The presently available structural data on LiL2-
{C(SiMe3)2(SiMe2X)} show that molecular species are
formed, i.e. the equilibrium of Eq. (1) lies to the left,
when X has high and L has low coordinating power.
Ate complexes are formed when X has low and L has
high coordinating power. (There are fewer data for
sodium analogues but it appears that the stronger
interaction of phenyl with sodium than with lithium
gives a molecular species in the presence of TMEDA.
Introduction of a second phenyl group does not, how-
ever, lead to displacement of TMEDA, and
[Na(TMEDA){C(SiMe3)(SiMe2Ph)2}] has a molecular
structure in which one phenyl group interacts with
sodium and the other hangs free [3].)
(1)
We therefore sought evidence for the presence of simi-
lar species in the case of 4. As samples in toluene-d8
were cooled, the NMR spectra remained sharp until the
temperature was 220 K, then the signals attributed to
SiMe3 and SiMe2 protons each split into two of equal
intensity (DG‡=42 kJ mol−1 at 202 K). This suggests
that there is a single species 4 in solution, since ex-
change between species of the kind envisaged for
LiC(SiMe3)3 would be likely to result in changes in the
NMR spectra at temperatures higher than 220 K. The
observed splitting probably arises from restricted rota-
tion about CꢀSiMe2Ph bonds at low temperatures. It is
likely but not certain that the predominant species in
solution is the ate complex present in the crystal. This
2.3. The lithium magnesates 6 and 8
The use of the lithium compound 3 to attach the
C(SiMe3)2(SiMe2Ph) group to silicon [20] and cadmium
[21] has been described previously. The TMEDA
derivative 4 was employed similarly for the synthesis of
the lithium organodibromomagnesate 6, obtained pure
but in poor yield from reaction with MgBr2 in THF.
Fig. 2. Structure of the anion of [Li(TMEDA)2][Li{C(SiMe3)2-
(SiMe2Ph)}2] (4).

80
S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
Fig. 3. Molecular structure of [Li(TMEDA)(m-Br)2Mg{C(SiMe3)2(SiMe2Ph)}(THF)] (6).
Fig. 4, and the most important bond lengths and angles
are given in Table 1. The MgꢀBr2, MgꢀC and MgꢀN
distances are not significantly different from those
The crystal contains molecular species (Fig. 3) and the
structure is very similar to that of the previously re-
ported [Li(THF)2(m-Br)2Mg{C(SiMe3)3}(THF)] (7) [22],
which appears to be the only analogous bromide-
bridged lithium–organomagnesium compound previ-
ously reported. The bond lengths and angles in 6 are
more precisely determined than those in 7, but there is
no significant difference between the corresponding
parameters in the two species. The wide endocyclic
angles at lithium and magnesium and narrow angles at
bromine reflect lower inner electron repulsion between
Li and Mg than between Br and Br, and the wide
exocyclic CꢀMgꢀO angle can be attributed to repulsion
between the large organic group and THF. The LiꢀN
,
(2.487(3), 2.189(9) and 2.097(9) A, respectively) in the
Grignard reagent 9. The wide C1ꢀMgꢀBr angles, and
¸¹¹¹º
the configuration of the MgCSiCN ring are very similar
in the two compounds. The MgꢀBr1 bond length is
significantly greater than that of MgꢀBr2 but it is
similar to that of the MgꢀBr bonds in the doubly-
bridged compounds 6 and 7. The LiꢀBr distance is
similar to those in 6, even though the LiꢀBrꢀMg and
BrꢀMgꢀBr angles are wider. The isolation of 8 confirms
that the coordinating power of Br attached to Mg is
similar to that of THF, since quite small changes in the
coordination of the magnesium cause the disruption of
the four-membered LiBr2Mg rings found in 6 and 7.
,
bond lengths (2.06(2) A) are similar to those in the
,
cation of 4 (2.125(9) A) but significantly shorter than
those in 1, probably because there is less crowding
round the lithium atom. The MgꢀC and MgꢀO bond
lengths are in the usual range [23,24].
2.4. The Grignard reagents 10 and 11
The Grignard reagent (Me3Si)3CMgCl was made
some time ago by reaction between the anthracene
derivative Mg(C14H10)(THF)3 and (Me3Si)3CCl [27].
The corresponding bromide was obtained from
(Me3Si)3CBr and magnesium metal and found to react
as a ligand transfer reagent with benzyl chloride and
HgCl2 [28]. In neither case was the Grignard reagent
isolated. When the organomagnesium bromide was
made in THF a crystalline compound could be isolated,
but this was shown to be a Grignard reagent–MgBr2
complex [(Me3Si)3CMg(m-Br)3Mg(THF)3] (12) [29]. We
have now found that the iodides RI [R=C(SiMe3)3 or
C(SiMe2Ph)3] react with activated magnesium metal to
give the Grignard reagents RMgI, 10 and 11, which
readily crystallise as diethyl ether complexes. The io-
dides are conveniently obtained by treatment of the
lithium compounds LiR with 1,2-diiodoethane; this ap-
The lithium magnesate 8 was obtained from the
reaction
between
the
lithium
reagent
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
Li(THF){C(SiMe3)2(SiMe2C5H4N-2)} and [MgBr2-
(OEt2)2] [25]. It was previously reported to have the
composition Li(THF)2MgBr2C(SiMe3)2(SiMe2C5H4N-
2) [26], but an X-ray structure determination has shown
that in the crystalline compound there is a third
molecule of THF and a single bridging bromine be-
tween Li and Mg. The molecular structure is shown in

S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
81
pears to be a general method for synthesis of iodides RI
[R=C(SiMe3)n(SiMe2X)3−n] from the corresponding
RH (see, e.g. Ref. [30]).
(SiMe3)2(SiMe2X)}I]2 (X=Me, CHꢁCH2 or OMe) have
been reported [31,32]. The MgꢀC bond in 11 is slightly
longer and the MgꢀI bonds are slightly shorter than the
corresponding bonds in 10, and the Mg2I2 ring is
slightly less symmetrical than that in 10. Taken together
these data suggest that there is greater crowding in 11
from interactions between the tris(organosilyl)methyl
group and diethyl ether. The other bond lengths and
angles in 10 are not significantly different from the
corresponding values in 11, and within each molecule
no individual value differs significantly from the aver-
age values given in Table 1. The MgꢀC bond lengths
The structures of 10 and 11 are shown in Figs. 5 and
6, respectively, and selected bond lengths and angles are
given in Table 1. The compounds crystallise as cen-
trosymmetrical halide-bridged dimers. Despite the im-
portance of Grignard reagents in organic synthesis, the
structures of only a few dimeric organomagnesium
halides have been reported [23]. They are found when
the Lewis basicity of the solvent is similar to that of the
halide. Mononuclear structures [MgRXLn] are obtained
in the presence of stronger donors L, and more com-
plex halide bridged structures when the basicity of the
donor is low or the amount available is insufficient. As
far as we are aware, no other dimeric organomagne-
sium iodide, has been structurally characterised previ-
ously, though some ytterbium analogues [Yb{C-
,
,
(2.146(7) A in 10 and 2.181(3) A in 11) are similar to
,
those in [Mg{CH(SiMe3)2}Cl(OEt2)] (2.131(8) A) [33],
,
6, 12 and other compounds (2.094(11)–2.18 A (no e.s.d.
given)) containing (m-Br)2 [34–37] or (m-Cl)2 bridges
[38]. The MgꢀO bonds in 10 and 11 are also similar to
,
those in 6 and related compounds (2.012(4)–2.09(2) A).
Fig. 4. Molecular structure of Li(THF)3(m-Br)MgBr{C(SiMe3)2(SiMe2C5H4N)} (8).
Fig. 5. Molecular structure of [Mg(OEt2){C(SiMe3)3}I]2 (10).

82
S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
Fig. 6. Molecular structure of [Mg(OEt2){C(SiMe3)2(SiMe2Ph)}I]2 (11).
The structure is shown in Fig. 7 and some molecular
parameters are given in Table 1. The MgꢀC(Bu) bond
The Mg2I2 ring, like the Mg2Br2 ring in dimeric organo-
magnesium bromides, is almost square, reflecting the
greater transannular inner-shell repulsion between the
Mg atoms in 10 and 11 than between Li and Mg in 6.
The reaction of 10 with SiMe3Cl at room tempera-
ture was incomplete after 1 h and C(SiMe3)4 was ob-
tained in low (23%) yield. The reaction with
LiC(SiMe3)3 for 1 h gave Mg{C(SiMe3)3}2 (13%) and
CH(SiMe3)3 (29%) as well as unchanged LiC(SiMe3)3
(32%) and 10 (26%). The low reaction rates can be
attributed to steric hindrance. The compound 11 ap-
peared to be stable for more than one year in the solid
,
length (2.130(3) A) is similar to that of the MgꢀEt
,
bonds in 14 (2.142(3), 2.129(3) A) and almost as short
as the bonds in compounds in which the magnesium is
,
two-coordinate (2.116(2) A in [Mg{C(SiMe3)3}2] [40,41]
,
and 2.126(6) A in [Mg{CH2CMe3}2] [42]). The MgꢀC1
,
bond length (2.241(2) A) is significantly longer than the
MgꢀBu bond, longer also than those in the Grignard
reagents 10 and 11, but shorter than those in the
¸¹¹º
three-centre bonds of 14. Within the MCSiN rings, the
MgꢀC bond in 13 is shorter than the LiꢀC bond in 1
1
,
,
(2.315(8) A), but the SiꢀC (1.831(2) A) and SiꢀN
state in the absence of air and moisture, but H-NMR
signals from a sample in C6D6 rapidly broadened and
,
(1.829(2) A) bonds are longer in 13 (cf. 1.803(4) and
,
1.790(4) A, respectively, in 1), reflecting the greater
the
spectra
showed
that
decomposition
to
transfer of charge from lithium than from magnesium
to the carbanionic centre. In 1, 13 and other com-
pounds containing the same chelate ring [2], the C1ꢀSi1
bonds are significantly shorter than the other two C1ꢀSi
bonds, indicating that the SiMe2NMe2 is more effective
than the SiMe3 group in accommodating the carban-
ionic charge. The wide C1ꢀMgꢀC12 angle is character-
istic of dialkylmagnesium complexes with ether and
amine donors, as discussed elsewhere [23].
CD(SiMe3)2(SiMe2Ph) was complete in 5 h. It is likely
that free-radical processes are involved.
2.5. The dialkylmagnesium 13
In an attempt to make a dialkylmagnesium from 2
and [MgBr2(OEt2)2], a mixture of 2 and butyl–lithium
was inadvertently used. The product was the unsym-
¸¹¹¹¹¹¹¹¹¹¹¹¹¹º
metrical
dialkyl
[MgBu{C(SiMe3)2(SiMe2NMe2)}-
(THF)], seemingly obtained because the LiBu reacts
much faster than the sterically hindered 2. We have not
studied the conditions for the formation of this com-
pound in detail, nor explored its chemistry, but we give
details of its structure because there are very few data
in the literature for unsymmetrical compounds of this
3. Experimental
Air and moisture were excluded as far as possible
from all reactions by the use of Schlenk techniques,
flame-dried glassware and Ar as blanket gas. NMR
spectra from samples in C6D6 were recorded at 300.1
(1H), 75.4 (13C), 99.4 (29Si) and 116.6 MHz (7Li); chem-
ical shifts are given relative to SiMe4 and aqueous LiCl.
At the time most of the work was done we did not have
type. The closest analogues appear to be the metallacy-
¸¹¹¹¹¹¹¹¹º
cles [MgEt{m-(CH2)3NMeR}]2 (R=Me (14a) or cyclo-
hexyl (14b)), which dimerise through electron deficient
dialkylaminopropyl bridges [39].

S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
83
Fig. 7. Molecular structure of [MgBu{C(SiMe3)2(SiMe2NMe2)}·THF] (13).
access to satisfactory C, H, and N analyses for air- and
moisture-sensitive compounds, so had to rely on clean
NMR spectra, consistent with the structures determined
by X-ray crystallography, as evidence that the com-
pounds were obtained pure.
The clear yellow mixture was stirred for 1 h at −
80 °C, allowed to warm to r.t., and stirred for a further
hour. The solvent was removed under vacuum and the
residue extracted with warm (40 °C) hexanes (4×5
cm3). The extract was reduced to 5 cm3 and kept at
5 °C overnight to give colourless crystals of 6 (0.11 g,
22%). l (H): 0.42 (18H, s, SiMe3), 0.64 (6H, s, SiMe2),
1.41 and 3.57 (4H, s, THF), 1.5–2.2 (16H, m,
TMEDA), 7.63–7.77 (5H, m, Ph). l (Li): 1.98.
¸¹¹¹¹¹¹¹¹¹¹¹¹¹¹¹º
3.1. [Li(TMEDA)C(SiMe3)2SiMe3NMe2] (1)
Dry TMEDA (0.085 g, 0.73 mmol) was added drop-
wise to a solution of 2 [2] (0.29 g, 0.70 mmol) in
hexanes (10 cm3) at room temperature (r.t.). The clear
mixture was reduced in volume to 5 cm3 and kept at
−30 °C to give colourless crystals of 1 (0.20 g, 74%). l
(H): 0.44 (6H, s, SiMe2), 0.45 (18H, s, SiMe3), 2.2 (6H,
3.4. Li(THF)3(v-Br)Mg¸B¹r{¹C¹(S¹i¹M¹e¹3)¹2(¹S¹iM¹e¹2¹C¹5H¹4ºN-2)}
(8)
1
7
The isolation, H-, and Li-NMR spectra were re-
ported previously. l (C): 1.3 (CSi3), 3.3 (SiMe3), 4.7
(SiMe2), 7.2 and 25.4 (THF), 123.8, 129.1, 133.4, 150.1,
170.2 (C5H4N).
s, NMe2), 1.85, (16H, m, TMEDA). l (C): 3.9 (SiMe2),
1
3.9 (detected by the INEPT pulse sequence, q, JCLi
=
11.6 Hz, CSi3), 8.9 (s, SiMe3), 40.6 (s, NMe2), 46.4 and
57.3 (TMEDA). l (Si): −10.2 (SiMe3), 5.5 (SiMe2). l
(Li): 0.23 (Dw1/2 2 Hz).
3.5. [Mg{C(SiMe3)3}I(OEt2)]2 (10)
3.2. [Li(TMEDA)2][Li{C(SiMe3)2(SiMe2Ph)}2] (4)
Magnesium turnings (0.70 g, 29.2 mmol) were stirred
overnight under Ar in a flame-dried Schlenk vessel to
break up the metal surface and a solution of (Me3Si)3CI
(2.0 g, 5.6 mmol) in Et2O (30 cm3) was added. The
mixture was stirred for 30 min, then the excess of
magnesium was filtered off and the filtrate concentrated
to 15 cm3 and kept at −30 °C to give colourless
crystals of 10, which were recrystallised from toluene
(2.26 g, 89%). l H (toluene-d8): 0.38–0.53 (27H, broad,
SiMe3), 0.77 (6H, t) and 3.57 (4H, broad, Et). l (C): 7.1
(SiMe3), 3.6 and 12.8 (Et). m/z: 382 (2%, [M]−Et2O),
367 (21, [M]−Me−Et2O), 217 [100, CH(SiMe3)3−
Me], 201 (60), 185 (10), 129 (60).
The preparation of this compound has been de-
scribed previously [3]. Crystals suitable for an X-ray
structural determination were obtained from a concen-
trated solution in benzene that had been kept at r.t. for
several months.
3.3. Li(TMEDA)(v-Br)2Mg{C(SiMe3)2-
(SiMe2Ph)}(THF) (6)
A sample of [MgBr2(OEt2)2] (0.31 g, 0.95 mmol)
(made from Mg turnings and 1,2-dibromoethane) [43]
was heated in a Schlenk tube under vacuum to remove
Et2O and the resulting solid was dissolved in THF (5
cm3) then cooled to −80 °C. A solution of complex 4
(0.50 g, 0.6 mmol) in THF (5 cm3) was added slowly.
Chlorotrimethylsilane (0.25 cm3, 2.0 mmol) was
added from a syringe to a stirred solution of 10 (2.0
mmol) in Et2O (12 cm3) at r.t.. After 30 min the solvent
was removed from the mixture, the residue extracted

Table 2
Crystal data and structure refinement parameters for the compounds studied
1
4·2C6H6
6
8
10·C6H5CH3
35H82I2Mg2O2Si6
11
13
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
C17H46LiN3Si3
383.8
C54H102Li2N4Si6
989.8
C25H53Br2LiMgN2OSi3
673.0
C26H52Br2LiMgNO3Si3
702.0
C
C38H78I2Mg2O2Si6
1038.0
C19H47MgNOSi3
414.2
1006.0
/
173(2)
Triclinic
173(2)
Monoclinic
C2/c no. 15
173(2)
Triclinic
173(2)
Orthorhombic
P212121 no. 19
173(2)
Orthorhombic
Pbca no 61
173(2)
Monoclinic
P21/n no 14
173(2)
Triclinic
(
(
(
P1 no. 2
P1 no. 2
P1 no. 2
,
a (A)
9.415(1)
15.202(1)
17.712(2)
90.03(1)
92.74(1)
91.31(1)
2531.5(4)
4
15.188(7)
19.117(9)
22.205(9)
90
91.19(3)
90
6446(5)
4
0.16
11.375(3)
11.439(6)
14.094(3)
95.04(3)
96.87(2)
103.37(3)
1758.7(11)
2
9.7103(2)
13.1308(5)
28.1309(19)
90
90
90
3586.8(2)
4
13.564(3)
24.903(7)
15.047(5)
90
90
90
5083(3)
4
1.43
15.238(5)
11.122(3)
16.412(11)
90
112.07(3)
90
2578(2)
2
1.41
9.108(2)
,
b (A)
9.7455(12)
15.8928(12)
98.971(9)
96.576(14)
105.393(15)
1325.2(4)
2
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
v (mm−1
)
0.19
2.44
2.40
0.21
Final R indices [I\2|(I)]
R1=0.051,
wR2=0.125
R1=0.082,
wR2=0.161
6176/6176
R1=0.095,
wR2=0.207
R1=0.172,
wR2=0.241
5906/5662
[Rint=0.054]
3136
R1=0.069, wR2=0.125 R1=0.047, wR2=0.089 R1=0.055,
R1=0.029,
wR2=0.074
R1=0.038,
wR2=0.081
4698/4524,
[Rint=0.032]
3888
R1=0.055,
wR2=0.134
R1=0.083,
wR2=0.151
6346/6346
631
wR2=0.139
R1=0.155, wR2=0.159 R1=0.064, wR2=0.096 R1=0.074,
wR2=0.156
)
R indices (all data)
Reflections measured/independent
Reflections with [I\|(I)]
6190/6190
11298/5988,
[Rint=0.043]
4978
3099/3099
7
8
4515
3368
2360
4564

S.S. Al-Juaid et al. / Journal of Organometallic Chemistry 631 (2001) 76–86
85
with benzene, and the extract filtered. The solvent was
removed from the filtrate to give a sticky solid, which
was shown by 1H-NMR spectroscopy to consist of
C(SiMe3)4 (23%), CH(SiMe3)3 (44%), and unchanged 10
(32%).
CSi3), 9.4 (CH2), 14.6 (CH3), 25.1 (THF), 32.6 and 33.8
(CH2), 40.4 (NMe2), 69.3 (THF). l (Si): −7.6 (SiMe3),
15.1 (SiMe2).
3.8. Crystallography
A solution of [Li(THF)4][Li{C(SiMe3)3}2] (0.75 g,
0.98 mmol) in Et2O (29 cm3) was added to a stirred
solution of 10 (2.0 mmol) in Et2O (12cm3). After 30 min
the solvent was removed from the mixture, the residue
extracted with toluene, and the extract filtered. The
sticky solid obtained after removal of the toluene from
the extract was shown by NMR spectroscopy to con-
tain Mg{C(SiMe3)3}2 (13%), CH(SiMe3)3 (29%), as well
as unidentified products, unchanged [Li(THF)4]-
[Li{C(SiMe3)3}2] and 10.
Data for 1, 4, 6, 10, 11 and 13 were recorded on a
CAD4 and for 8 on a Kappa CCD diffractometer by
,
use of Mo–Ka radiation (u=0.71073 A). Further de-
tails are given in Table 2. Structural analysis was by
direct methods (SHELXS-86 or SHELXS-97) and refine-
ment by full least-squares on all data (SHELXL-93 or
SHELXL-97). Non-H atoms were anisotropic and H
atoms were included in riding mode. In 4, there are two
molecules of occluded benzene in the asymmetric unit;
the high R factors result from the weak high angle
diffraction. In 10 there is a molecule of toluene disor-
dered across an inversion centre and the H atoms in
this were omitted.
3.6. [Mg{C(SiMe3)2(SiMe2Ph)}I(OEt2)]2 (11)
Magnesium turnings (0.24 g, 10 mmol) were acti-
vated as above, then Et2O (20 cm3) and a crystal of
iodine were added. A solution of (Me3Si)2(PhMe2Si)CI
[32] (2.1 g., 5.0 mmol) in Et2O (20 cm3) was added
dropwise and the mixture stirred for 1 h. The precipi-
tated solids were filtered off and extracted with toluene
(2×10 cm3) at 50 °C and the extract left at r.t. to give
colourless crystals of 11 (0.69 g, 31%). The Et2O was
pumped from the filtrate to leave a sticky solid, which
was shaken with hexanes (20 cm3) and washed with
heptane at −50 °C to leave a white powder that
4. Supplementary material
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Center, CCDC nos. 162000–162006 for com-
pounds 1, 4·2C6H6, 6, 8, 10·C6H5CH3, 11 and 13,
respectively. Copies of this information may be ob-
tained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
1
showed the same H-NMR spectrum as the crystals. l
(H): 0.48 (18H, s, SiMe3), 0.69 (6H, s, SiMe2), 0.74 (6H,
t) and 3.41 (4H, q, Et2O), 7.25 (3H, m, m- and p-H),
7.91 (2H, d, o-H). l (C): 2.7 (CSi3), 3.4 (SiMe3), 7.9
(SiMe2), 15.3 and 65.8 (Et2O), 125.6, 129.3, 135.8 and
142.5 (Ph). l (Si): −9.6 (SiMe2), −6. 0 (SiMe3). m/z:
429 (20%, [M]−Me−THF), 279 [100, CH(SiMe3)2-
(SiMe2Ph)−Me].
Acknowledgements
We thank the EPSRC and the EU for financial
support and the Governments of Saudi Arabia and
Libya, respectively, for granting study leave to S.S.Al-J.
and S.M.El-H.
¸¹¹¹¹¹¹¹¹¹¹¹¹¹º
3.7. [MgBu{C(SiMe3)2(SiMe2NMe2)}(THF)] (13)
A solution of LiBu (8.7 mmol) in hexanes (3.5 cm3)
was added dropwise to
a
stirred solution of
(Me3Si)2(Me2NMe2Si)CCl (1.86 g, 6.31 mmol) [2] in
THF (40 cm3) at −78 °C. The mixture was stirred for
2 h and then added dropwise to a stirred solution of
[MgBr2(OEt2)2] (0.79 g, 2.40 mmol) in THF (20 cm3) at
−120 °C. The mixture was allowed to warm to r.t. and
the solvent was pumped away to leave a sticky brown
residue, which was extracted with hexanes. The extract
was concentrated to 15 cm3, then kept at −30 °C to
give yellow crystals of 13 (0.95 g, 95% based on
MgBr2), m.p. 147–154 °C. l (H): −0.13 (2H, t,
CH2Mg), 0.29 (6H, s, SiMe2), 0.41 (18H, s, SiMe3),
1.19–1.23 (7H, m, THF and CH3), 1.66 and 1.76 (2H,
m, CH2), 2.09 (6H, s, NMe2), 3.42 (4H, m, THF). l
(C): 3.5 (SiMe2), 8.1 (SiMe3), 8.7 (1JCSi=41.5 Hz,
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