An alkylzinc bromide and a lithium alkyldibromozincate containing tris(organosilyl)methyl groups

Article abstract of DOI:10.1016/j.jorganchem.2004.02.018

The lithium reagent (3) reacts with a molar equivalent of anhydrous zinc bromide to give the dimeric compound (2a), in which zinc is four-coordinate. The product from a similar reaction with Li{C(SiMe₃)₂(SiMe₂NPhMe)} is the lithium zincate [Li(THF)₂(μ-Br)₂Zn{C(SiMe₃) ₂(SiMe₂NPhMe)}] (4), in which the zinc is only three-coordinate. The crystal structures of 2a and 4 have been determined.

Full text of DOI:10.1016/j.jorganchem.2004.02.018

Journal of Organometallic Chemistry 689 (2004) 1718–1722
www.elsevier.com/locate/jorganchem
An alkylzinc bromide and a lithium alkyldibromozincate containing
tris(organosilyl)methyl groups
a
Davood Azarifar , Martyn P. Coles , Salima M. El-Hamruni , Colin Eaborn
b
c
b,1
,
b
Peter B. Hitchcock , J. David Smith
b,
*
a
Department of Chemistry, Bu-Ali Sina University, Postal Code 65174, Hamedan, Iran
b
c
Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, UK
Department of Chemistry, Faculty of Science, Al-Fatih University, PO Box 13203, Tripoli, Libya
Received 16 January 2004; accepted 17 February 2004
Abstract
The lithium reagent Li(THF)2{C(SiMe3)2(SiMe2NMe2)} (3) reacts with a molar equivalent of anhydrous zinc bromide to give the
dimeric compound [Zn( -Br){C(SiMe ) (SiMe NMe )}] (2a), in which zinc is four-coordinate. The product from a similar reaction
µ
3 2
2
2
2
with Li{C(SiMe3)2(SiMe2NPhMe)} is the lithium zincate [Li(THF)2(l-Br)2Zn{C(SiMe3)2(SiMe2NPhMe)}] (4), in which the zinc is
only three-coordinate. The crystal structures of 2a and 4 have been determined.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Organozinc; Organosilyl; Crystal structures
1. Introduction
Me3Si SiMe3
Me3Si
SiMe3
M
Me2
N
N
Br
Br
Zn
Zn
SiMe2
Me2Si
Br
Br
Methyl- and ethylzinc iodides, prepared by Frankland
in 1849 [1], played an important part in the development
of organometallic chemistry and the concept of valency
[2]. A number of organozinc halides have been structur-
ally characterised as complexes with oxygen or nitrogen
donors [3], but structural data on unsolvated organozinc
halides are sparse. The compounds [ZnClSiPhMe2)3}]2
[4], [ZnCl (OEt2){CCl2CF3}]2 [5], [ZnCl{(CH2)3 N-
SiMe2
M
Me2Si
N
N
Me3Si SiMe3
Me2
Me3Si SiMe3
1
2a M = Zn 2b M = Co
Me
Ph
N
THF
SiMe2
SiMe3
Br
Mg
Br
Br
Br
(THF)2 Li
(THF)2 Li
Zn
[ZnBr{C(SiMe ) SiMe (C H N-2)}]
(1) [7]
2
Me2}]2 [6] and
3 2
2
5
4
Me3Si
C(SiMe3)3
form halide-bridged dimers. The crystalline iodides ZnIEt
[8] and ZnICH2CH2CN [9] have polymeric structures.
We surmised that organozinc halides bearing bulky
organic groups might be precursors to catalysts for ring
opening polymerisation of lactide, particularly since
their molecular architecture, and hence the access of
substrates, can be controlled by variations in substitu-
ents within the bulky group [10].
4
8
We
therefore
made
the
[Zn( -Br)C(SiMe ) (SiMe NMe )]
(2a) from the lithium
2 2
compound
µ
3 2
2
Li(THF) {C(SiMe ) (SiMe NMe )}
reagent
(3) [11] and
2
3 2
2
2
a molar equivalent of zinc bromide. An attempt to make
the analogous compound containing the new ligand
C(SiMe3)2(SiMe2NPhMe) in the same way led to the
isolation of the lithium organodibromozincate
[Li(THF)2(l-Br)2Zn{C(SiMe3)2(SiMe2NPhMe)}] (4).
Although the compounds [Li(OEt2)(l-X)2ZnL] (X ¼ Cl
(5a) or I (5b), L ¼ 2,6-Pri2C6H3N@C(Me)CH@C(Me)-
* Corresponding author. Tel.: +44-1273-678124; fax: +44-1273-
677196.
E-mail address: j.d.smith@sussex.ac.uk (J.D. Smith).
1
Died 22.02.04. This paper is dedicated to his memory.
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.02.018

D. Azarifar et al. / Journal of Organometallic Chemistry 689 (2004) 1718–1722
1719
NC6H3Pri2-2,6) have been reported [12], compound 4 is,
as far as we are aware, the first containing the LiBr2Zn
core to be structurally characterised.
the white precipitate filtered off, and solvents removed
from the filtrate under vacuum. The oily residue was
distilled at 88–90 °C/0.02 mmHg to give
(Me3Si)2{(PhMeN)Me2Si}CH (6) (8.0 g, 60%). Anal.
Calc. for C16H33NSi3: C, 59.44; H, 10.22; N, 4.33.
1
2. Experimental
Found: C, 59.27; H, 10.40; N, 4.05%. H NMR (C6D6):
d 0.08 (18H, s, SiMe3), 0.16 (1H, s, CH), 0.32 (6H, s,
3
Air and moisture were excluded as far as possible by
the use of Schlenk techniques, flame-dried glassware and
a nitrogen-filled drybox. NMR spectra from samples in
C6D6 were recorded at 300.1 (1H), 75.4 (13C), 194.5
(7Li), 50.7 (15N) or 99.4 Hz (29Si), with signals from
quaternary 13C, 29Si and 15N enhanced by polarisation
transfer. Chemical shifts are relative to SiMe4 for H, C
and Si; LiCl in D2O for Li, and MeNO2 for N. EI mass
spectra were obtained at 70 eV; except where indicated
for M2 (2a), indicated for M2 (2a), m=z values are for
79Br and 64Zn.
SiMe2), 2.64 (3H, s, NMe), 6.78 (1H, tt, JHH ¼ 7:3 Hz
and 1 Hz, p-H), 6.85 (2H, m, m-H), 7.15 (2H, m, o-H).
13C NMR: d 3.2 (1JCH ¼ 118 Hz, 1JSiC ¼ 51 Hz, SiMe3),
1
3.7 (1JCH ¼ 119 Hz, JSiC ¼ 58 Hz, SiMe2), 4.8
(1JCH ¼ 97 Hz, 1JSiC ¼ 36:5 and 43.5 Hz, 3JCH ¼ 3:5 Hz,
CSi3), 35.4 (1JCH ¼ 135:0 Hz, NMe), 118.1
(1JCH ¼ 154:6 Hz, o-C), 118.7 (1JCH ¼ 160:2 Hz, p-C),
128.8 (1JCH ¼ 156:4 Hz, m-C), 150.9 (i-C). 15N NMR:
)328.8. 29Si NMR: d ꢀ 0:9 (SiMe3), 6.6 (SiMe2). MS:
m=z 323 (50, M), 308 (50, M–Me), 217 (100, M–NMePh),
203 (30, (Me3Si)2CHSiMeH), 164 (20, Me2SiNMePh),
129 (40, Me2Si@CHSiMe2), 73 (40, SiMe3).
2.1.
(2a)
[ZnBr{C(SiMe3)2(SiMe2NMe2)}]2
2.3. Reaction of (Me3Si)2{(PhMeN)Me2Si}CH with
LiMe
A solution of LiMe (5.2 mmol) in Et2O (3.2 cm3) was
added slowly to a stirred solution of (Me3Si)2(Me2NSi-
Me2)CH [11] (1.35 g, 5.2 mmol) in THF (40 cm3) at room
temperature. The mixture was stirred for 5 h, then added
dropwise to a stirred solution of ZnBr2 (1.16 g, 5.2 mmol)
in THF (20 cm3) at )110 °C. The mixture was allowed to
warm to room temperature and the solvents were re-
moved under vacuum to leave a solid that was extracted
with n-hexane. The extract was filtered and the solvent
pumped from the filtrate to leave a white air- and mois-
ture-sensitive solid that was recrystallised from heptane at
0 °C to give crystals of 2a, m.p. 112–114 °C, suitable for an
X-ray study. Anal. Calc. for C22H60Br2N2Si6Zn2: C,
32.55; H, 7.45; N, 3.45. Found: 32.47; H, 8.65; N, 3.86%.
We think the low value for H is in error since none of the
NMR spectra shows any evidence for impurities. 1H
NMR: d 0.17 (6H, s, SiMe2), 0.41 (18H, s, SiMe3), 2.18 (6
H, s, NMe2). 13C NMR: d 3.0 (SiMe2), 7.3 (SiMe3), 14.8
(CSi3), 41.6 (NMe2). 29Si NMR: d ꢀ 4:5 (SiMe3), 22.3
(SiMe2). m=z: 812 (0.5, C22H7690Br81BrN2Si664Zn66Zn, M2),
797 (0.5, M2–Me), 724 (5, M2–SiMe4), 659 (1, M2–
Me3SiBr), 581 (10, M2–Me–(Me3Si)2CSiMe2), 499 (10,
possibly M2–MeBr–(Me3Si)2CSiMe2), 388 (10, M–Me),
372 (20, M–Me–CH4), 310 (5, M–Me–Br), 230 (10,
Me2Si@C(SiMe2NMe2)SiMe2), 219 (30, MH–CH2@
NCH2), 201 (100, Me2Si@C(SiMe3)SiMe2), 129 (15,
Me2Si@CHSiMe2), 73 (60, SiMe3).
A solution of LiMe (1.76 mmol) in Et2O (1.1 cm3)
was added dropwise to a stirred solution of 6 (0.50 g,
1.54 mmol) in THF (15 cm3) at room temperature. The
mixture was stirred overnight, then solvent was slowly
removed under vacuum. No crystals were obtained. All
solvent was removed to leave a pale yellow oil, shown by
NMR spectroscopy to be a 1:2 mixture of unchanged 6
and a new compound assumed to be Li(THF)2C(Si-
Me3)2(SiMe2NMePh) (7). 1H NMR (C6D6): d 0.42
(18H, s, SiMe3), 0.58 (6H, s, SiMe2), 1.31 (8H, m, THF),
2.73 (3H, s, NMe), 3.46 (8H, m, THF), 6.5–7.3 (m, Ph).
13C NMR: d 6.85 (SiMe2), 7.55 (SiMe3), 25.3 (THF),
34.9 (NMe), 68.3 (THF), 115.3(o-C), 117.5 (p-C), 129.9
(m-C) 152.7 (i-C).
2.4. Preparation of [Li(THF)2(l-Br)2Zn{C(Si-
Me3)2(SiMe2NPhMe)}] (4)
A solution of LiMe (2.5 cm3, 4.0 mmol) in diethyl
ether was added slowly to a stirred solution of 6 (1.15 g,
3.56 mmol) in THF (40 cm3) at room temperature. The
mixture was stirred overnight, then added to a stirred
solution of ZnBr2 (0.80 g, 3.56 mmol) in THF (15 cm3) at
)90 °C. This mixture was then allowed to warm slowly to
room temperature and solvents removed to leave an or-
ange solid, which was extracted with hexane (2 ꢁ 20
cm3). The volume of the combined extract was reduced
to 5 cm3 and the solution was kept at )30 °C for 2 days to
give colourless crystals of 4 (1.84 g. 74%), m.p. 245–249
°C. Anal. Calc. for C24H48Br2LiNO2Si3Zn: C, 41.26; H,
2.2. Preparation of (Me3Si)2{(PhMeN)Me2Si}CH (6)
N-methylaniline (9.0 cm3, 83 mmol) was added
dropwise to a stirred solution of (Me3Si)2(Me2BrSi)CH
[13] (12.8 g, 43 mmol) in hexane (120 cm3) at )78 °C.
The mixture was allowed to warm to room temperature,
then stirred for 2 h. The volume was reduced to 15 cm3,
1
6.87; N, 2.00. Found: C, 41.09; H, 6.93; N, 1.92%. H
NMR (C6D6): d 0.55 (18H, s, SiMe3), 0.73 (6H, s,
SiMe2), 1.30 (8H, m, THF), 2.86 (3H, s, NMe), 3.49 (8H,

1720
D. Azarifar et al. / Journal of Organometallic Chemistry 689 (2004) 1718–1722
Table 1
Crystal data for compounds 2a and 4
2a
4
Chemical formula
Formula weight
T (K)
C22H60Br2N2Si6Zn2 C24H48Br2LiNO2Si3Zn
811.8
173(2)
699.03
173(2)
Crystal system
Space group
Triclinic
P1 (no. 2)
9.1945(2)
9.2103(2)
13.5042(4)
75.060(1)
77.882(1)
60.704(1)
958.84(4)
1
Triclinic
P1 (no. 2)
13.6053(1)
8.9953(1)
29.0818(3)
87.306(1)
84.259(1)
69.225(1)
3310.8(1)
4
ꢀ
a (A)
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
3
ꢀ
U (A )
Z
[ZnBrC(SiMe ) (SiMe NMe )]
(2a)
2
Fig. 1. The molecular structure of
with 50% thermal ellipsoids.
l (mmꢀ1
)
3.54
0.033, 0.083
0.038, 0.087
3.28
3
2
2
2
R1wR2I > 2rðIÞ
0.070, 0.180
0.085, 0.194
36761/12056/0.052
All data
Measured/independent 8575/3420/0.039
reflections/R
ðintÞ
Hz) are characteristic of compounds containing the
Si3CH core [14]. The reaction between 6 and methyl-
lithium at room temperature gave a mixture of
unchanged 6 and what appeared to be the lithium com-
pound Li(THF)2{C(SiMe3)2(SiMe2NPhMe)} (7). With
a longer reaction time the proportion of 7 decreased,
suggesting that it was being removed by an as yet un-
identified side reaction. Attempts to obtain crystals of 7
suitable for an X-ray structure determination were not
successful, but its formation was confirmed by the iso-
lation in reasonable yield and characterisation of the
lithium zincate 4, after treatment of the mixture ob-
tained from (Me3Si)2{(PhMeN)Me2Si}CH (1 mol) and
LiMe (1 mol) with zinc bromide (1 mol).
Reflections with
I > 2rðIÞ
3098
9824
m, THF), 7.15 (1H, m, p-H), 7.24 (2H, m, m-H), 7.43
(2H, dd, o-H). 13C NMR: d 6.3 (SiMe3), 7.2 (SiMe2), 11.1
(CSi3), 25.3 (THF), 35.0 (NMe), 68.5 (THF), 114.4 (o-C),
117.1 (p-C), 131.6 (m-C), 152.6 (i-C). 29Si NMR: d ꢀ 4:0
7
(SiMe3), 6.3 (SiMe2). Li NMR: d 0.05. No change was
observed in the 1H NMR spectrum when 4 was heated in
either hexane or THF for 2 h.
2.5. Crystallography
The bromide 2a is isomorphous with the previously
described cobalt compound 2b. The dimeric molecules,
shown in Fig. 1, lie on a centre of symmetry so that the
Data for 2a and 4 were collected on a Kappa CCD
diffractometer and full matrix least squares refinement
was by use of SHELX-97 programs. In 4, the asymmetric
unit contains two independent molecules. One is well-
defined. The other has an alternative lower occupancy
(0.17) set of positions for one of the bridging bromides
and nearly all the atoms of the alkyl group. In the dis-
ordered molecule the lower occupancy sites were left
isotropic and in both positions the phenyl group was
assumed to be a rigid body. The value of Uiso for the
lower occupancy position of C11 was fixed. Further
details are given in Table 1.
Table 2
Selected
ꢀ
(A)
bond
lengths
and
angles
(°)
in
(2a) and [Li(THF)2(l-Br)2Zn{C(Si-
[ZnBrC(SiMe3)2(SiMe2NMe2)]2
Me3)2(SiMe2NPhMe)}] (4)
2aa
4b;c
Zn–C
Zn–N
2.045(3)
2.152(2)
2.014(5)
Zn–Br or Br1
Zn–Br0 or Br2
Si–Cd
2.4614(4)
2.4963(4)
1.868(3)
2.4028(9)
2.4406(9)
1.875(6)
1.876(8)
100.77(3)
Si–Med
1.875(4)
Br–Zn–Br0 or Br1–Zn–Br2
Zn–Br–Zn
Br–Zn–C1
93.577(14)
86.423(14)
131.09(8),
127.06(8)
3. Results and discussion
131.04(15),
125.27(15)
The zinc compound 2a was made without difficulty
from the lithium reagent 3. The new ligand precursor
(Me3Si)2{(PhMeN)Me2Si}CH (6) was made from
(Me3Si)2(Me2BrSi)CH and two equivalents of N-
methylaniline. It was purified by distillation and fully
characterised by chemical analysis and NMR spectros-
a Br–Zn–N 110.03(7)°, 106.42(7)°; N–Zn–C 85.70(10)°.
b For the undisordered molecule, values for the disordered molecules
do not differ significantly.
c Br1–Li–Br2 94.9(4)°; Li–Br1–Zn 81.6(3)°; Li–Br2–Zn 80.6(3)°.
d Average values, with e.s.d.’s of individual measurements in pa-
rentheses.
1
1
copy. The low values of JCH (97 Hz) and JSiC (37–43

D. Azarifar et al. / Journal of Organometallic Chemistry 689 (2004) 1718–1722
1721
Zn2Br2 ring is planar. As the Zn–Br and Zn–Br0 bond
lengths are significantly different, the dimer is not dis-
posed quite symmetrically about the C2N2 plane per-
pendicular to that of the central ring. Bond lengths and
angles are given in Table 2. The Zn–C bond lengths in
(sum of angles at Zn 357°). The structure may be
compared with that of the magnesate 8, in which there
is an additional molecule of THF attached to the metal
centre [16]. The THF is absent in 4, probably because
the NMePh group is larger than Me and zinc is both
smaller and less acidic than magnesium. The weakening
of the Zn–N bond is associated with a strengthening of
the bridging Zn–Br bonds, which are shorter to the
ꢀ
2a and in the pyridyl derivative 1 [2.037(4) A] are not
significantly different, but the Zn–N bond in 2a [2.152(2)
ꢀ
ꢀ
A] is longer than that in 1 [Zn–N 2.077(4) A], reflecting
the weaker basicity of the dimethylamido nitrogen and
possibly greater strain in a four-membered than in a
five-membered ring. The coordination at zinc is that of
an extremely distorted tetrahedron with the Br2ZnC
system nearly planar (sum of angles at Zn 352°). The
central ring may be compared with that in the anion
ꢀ
three-coordinate zinc in 4 [2.4028(9) and 2.4406(9) A]
than those to the four-coordinate zinc in 1 [Zn–Br
ꢀ
2.4601(6) and 2.5398(7) A] or 2a [2.4614(4) and
ꢀ
2.4963(4) A]. The angles within the MBr2Zn rings
(M ¼ Li or Zn) in 2a and 4 (Table 2) reflect transan-
nular inner shell repulsions, e.g. the Br–Zn–Br angle is
narrower and the M–Br–Zn angle is wider when M is
Zn. Although no other structure containing a LiBr2Zn
ring appears to be available for comparison, three-
coordinate zinc has been found in a number of com-
pounds containing bulky ligands [17].
Zn2Br26ꢀ, in which the Zn–Br bond length is 2.501(3) A
ꢀ
but the Br–Zn–Br angles (108–112°) are close to the
tetrahedral value [15]. It is noteworthy that although the
length of the Zn–C bond in 2a is similar to that of
ꢀ
the Co–C bond [2.065(8) A] in 2b, the Zn–N bond is
ꢀ
significantly longer than the Co–N bond [2.087(7) A],
showing that zinc(II) is a weaker Lewis acid than
cobalt(II) towards the NMe2 ligand.
We reported previously that the lithium organocad-
mates Li(THF)CdX2C(SiMe3)3 (X ¼ Cl or Br), obtained
from the reaction between LiC(SiMe3)3 and the halides
CdX2, decomposed on heating to give lithium halides
and organocadmiun halides [16]. No similar reaction
was detected when the zincate 4 was heated under reflux
in either hexane or THF. No lithium halide was pre-
cipitated and no change was detected in the NMR
spectrum.
Although we have not yet examined the potential of
2a and 4 as ring opening polymerisation catalysts, their
isolation and characterisation suggest that by making
compounds of this type with a series of substituents at
nitrogen it may be possible to build derivatives having a
range of sterically constrained zinc coordination sites at
which transformations or polymerisations of small
molecules may be envisaged.
The mass spectrum of 2a showed several weak peaks
having patterns corresponding to the Zn2Br2 fragment,
indicating that the species observed in the crystal is
preserved in the gas phase.
The structure of the lithium alkyldibromozincate 4 is
shown in Fig. 2. The LiBr2Zn ring is folded with an
angle of 17° along the Brꢂ ꢂ ꢂBr vector, with the Zn
pulled towards the organosilicon fragment bearing the
ꢀ
NMePh group. However, the Znꢂ ꢂ ꢂN distance (3.76 A)
is considerably greater than those in 1 or 2a, indicating
that the N-methylanilido group is too weakly basic to
coordinate to the metal centre. The zinc is only three-
coordinate, as in [(PhMe2Si)3CZnCl)]2 [4], and the
bonds to bromine and carbon are almost coplanar
4. Supplementary material
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic
Data Centre CCDC nos. 228791 and 228792 for com-
pounds 2a and 4, respectively. Copies of this informa-
tion may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(Fax: +44-1223-336033; email: deposit@ccdc.cam.ac.uk
or http://www.ccdc.cam.ac.uk.
Acknowledgements
We thank the EPSRC and the Governments of Libya
and Iran for financial support, and Drs A. Abdul-Sada
and A.G. Avent for help with mass spectrometry and
NMR spectroscopy, respectively.
Fig. 2. The structure of the ordered molecule of 4 with 20% thermal
ellipsoids.

1722
D. Azarifar et al. / Journal of Organometallic Chemistry 689 (2004) 1718–1722
[8] P.T.Moseley,H.M.M.Shearer,J.Chem.Soc.,DaltonTrans.(1973)64.
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