Synthesis and structures of solvent-separated lithium zincates of the type [Li(tmeda)2]+[Me3-nZn{CH(SiMe 3)Ph}n]- (n = 1-3, tmeda = 1,2-Bis(dimethylamino)ethane)

Article abstract of DOI:10.1021/om971092c

The reaction of methyllithium with the tmeda complex of ((trimethylsilyl)benzyl)zinc chloride in the presence of 1,2-bis(dimethylamino)ethane (tmeda) in 2:1 molar ratio quantitatively yields the heteroleptic zincate [Li(tmeda)2]⁺[Me_3-n

Full text of DOI:10.1021/om971092c

1438
Organometallics 1998, 17, 1438-1441
Notes
Syn th esis a n d Str u ctu r es of Solven t-Sep a r a ted Lith iu m
Zin ca tes of th e Typ e
[Li(tm ed a )2]+[Me3-n Zn {CH(SiMe3)P h }n ]- (n ) 1-3, tm ed a
) 1,2-Bis(d im eth yla m in o)eth a n e)
Matthias Westerhausen,*,† Michael Wieneke,† Werner Ponikwar,†
Heinrich No¨th,† and Wolfgang Schwarz‡
Institut fu¨r Anorganische Chemie der Ludwig-Maximilians-Universita¨t Mu¨nchen, Meiserstr. 1,
D-80333 Mu¨nchen, Germany, and Institut fu¨r Anorganische Chemie der Universita¨t,
Pfaffenwaldring 55, D-70550 Stuttgart, Germany
Received December 12, 1997
Summary: The reaction of methyllithium with the tmeda
complex of ((trimethylsilyl)benzyl)zinc chloride in the
presence of 1,2-bis(dimethylamino)ethane (tmeda) in 2:1
molar ratio quantitatively yields the heteroleptic zincate
[Li(tmeda)2]+[Me3-nZn{CH(SiMe3}Ph]n]- (n ) 1, 1).
Under similar conditions, MeLi and ((trimethylsilyl)-
benzyl)lithium react with bis((trimethylsilyl)benzyl)zinc
to form the zincates with n ) 2 (2) and 3 (3), respectively.
The molecular structures of 1 and 2 show shorter Zn-C
bond lengths to the methyl group than to the benzylic
substituent.
(CH2Ph)3]2 into the corresponding neutral dibenzyl
derivatives of magnesium and zinc.6 In contrast to this
finding, the addition of a tetradentate amine base to an
equimolar mixture of dimethylmagnesium and dimeth-
ylcadmium yielded the solvent-separated trimethylcad-
mate and MeMg[14N4]+.7
If the zinc-bonded groups contain donor atoms, in-
tramolecularly coordinating ligands and contact ion
pairs of zincates were observed.8,9 In contrast to these
observations, the zincates of sodium and potassium do
not need to be stabilized by neutral coligands. Thus,
Purdy and George10 were able to isolate solvent-free
sodium and potassium tris((trimethylsilyl)methyl)zin-
cate as well as the heteroleptic potassium phenylbis-
((trimethylsilyl)methyl)zincate.
The presence of chelating Lewis bases such as tmeda
results in the formation of solvent-separated lithium
organyl zincates as can be seen from the metathesis
reaction of ZnCl2 with 2 equiv of (tmeda)LiCH(SiMe3)Ph11
yielding [(tmeda)2Li] [{Ph(Me3Si)CH}2ZnCl], which is
insoluble in nonpolar solvents. On adding toluene, LiCl
precipitates and the tmeda complex of homoleptic bis-
[phenyl(trimethylsilyl)methyl]zinc is formed.12 The in-
termediate heteroleptic ClZn(tmeda)CH(SiMe3)Ph can
be isolated by reacting equimolar amounts of (tmeda)-
LiCH(SiMe3)Ph and ZnCl2.13 The reaction of this
compound with 2 equiv of methyllithium yields the
The first structurally investigated zincates were
solvent-free dilithium tetramethylzincate1 and potas-
sium trimethylzincate.2 Whereas Li2[ZnMe4] exists
without complexation of the lithium cations by neutral
Lewis bases (coligands), lithium tris[bis(trimethylsilyl)-
methyl]zincate3 only forms in the presence of neutral
coligands, thus forming ion-separated ion pairs. In the
absence of Lewis bases, the dissociation into alkyl-
lithium and dialkylzinc occurs. Similar results were
obtained for calcium bis{tris[bis(trimethylsilyl)amino]-
zincate},4 which is isolable in good yield if the calcium
dication is surrounded by neutral coligands such as 1,2-
dimethoxyethane. The structure of this zincate anion
with the countercation [(12-crown-4)2Na]+ has been
published by Dehnicke and co-workers.5 Due to elec-
trostatic repulsion of the three negatively charged amide
substituents, long Zn-N bond lengths of 1.97 Å were
observed. On the other hand, the chelating tmeda base
leads to the decomposition of the zincate [(thf)6Mg][Zn-
(6) Rijnberg, E.; J astrzebski, J . T. B. H.; Boersma, J .; Kooijman, H.;
Spek, A. L.; van Koten, G. J . Organomet. Chem. 1997, 541, 181.
(7) Tang, H.; Parvez, M.; Richey, H. G. Organometallics 1996, 15,
5281.
(8) Westerhausen, M.; Rademacher, B.; Schwarz, W.; Henkel, S. Z.
Naturforsch. 1994, 49b, 199.
(9) Rijnberg, E.; J astrzebski, J . T. B. H.; Boersma, J .; Kooijman, H.;
Veldman, N.; Spek, A. L.; van Koten, G. Organometallics 1997, 16,
2239.
(10) Purdy, A. P.; George, C. F. Organometallics 1992, 11, 1955.
(11) Zarges, W.; Marsch, M.; Harms, K.; Koch, W.; Frenking, G.;
Boche, G. Chem. Ber. 1991, 124, 543.
(12) Westerhausen, M.; Wieneke, M.; Rademacher, B. B.; Schwarz,
W. Chem. Ber./ Rec. 1997, 130, 1499.
† Institut fu¨r Anorganische Chemie der Ludwig-Maximilians-Uni-
versita¨t Mu¨nchen.
‡ Institut fu¨r Anorganische Chemie der Universita¨t.
(1) Weiss, E.; Wolfrum, R. Chem. Ber. 1968, 101, 35.
(2) Schleyer, P. v. R.; Schade, C. Adv. Organomet. Chem. 1987, 27,
169.
(3) Westerhausen, M.; Rademacher, B.; Schwarz, W. Z. Anorg. Allg.
Chem. 1993, 619, 675.
(4) Westerhausen, M. Z. Anorg. Allg. Chem. 1992, 618, 131.
(5) Putzer, M. A.; Neumu¨ller, B.; Dehnicke, K. Z. Anorg. Allg. Chem.
1997, 623, 539.
(13) Westerhausen, M.; Wieneke, M.; Schwarz, W. J . Organomet.
Chem. 1996, 522, 137.
S0276-7333(97)01092-3 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 03/12/1998

Notes
Organometallics, Vol. 17, No. 7, 1998 1439
Sch em e 1
zincate [Li(tmeda)2] [Me2ZnCH(SiMe3)Ph] (1). The ho-
moleptic complex (tmeda)Zn[CH(SiMe3)Ph]2 reacts with
methyllithium/tmeda or (tmeda)LiCH(SiMe3)Ph to form
[Li(tmeda)2] [MeZn{CH(SiMe3)Ph}2] (2) and [Li(tme-
da)2] [Zn{CH(SiMe3)Ph}3] (3), respectively (Scheme 1).
The thermal stability of the new zincates of the type
[Me3-nZn{CH(SiMe3}Ph]n]- increases with the number
n and the steric shielding of the central ZnC3 moiety.
Compound 1 already decomposes at 35 °C, whereas 3
melts at 202 °C. The sensitivity toward moisture and
air decreases with increasing n due to increasing steric
shielding of the ZnC3 moiety.
The solvent-separated zincate anions of 1 and 2
contain trigonal-planar-coordinated zinc atoms. The
molecular structure of the anion [Me2ZnCH(SiMe3)Ph]-
1 and the numbering scheme are shown in Figure 1.
Figure 2 shows the anion [MeZn{CH(SiMe3)Ph}2]- of
2. The cation {(tmeda)2Li}+ displays no unusual struc-
tural features and, therefore, it is not depicted. The
Li-N bond lengths (2.082(6)-2.158(6) Å) are normal.
F igu r e 1. Molecular structure and numbering scheme of
the zincate anion [Me2ZnCH(SiMe3)Ph]- of 1. The ellipsoids
represent a probability of 50%. Selected bond lengths (Å):
Zn-C1 2.113(4), Zn-C2 2.005(5), Zn-C3 2.011(5), C1-Si11
1.824(4), C1-C11 1.470(6), C1-H1 96(4), Si11-C111 1.874-
(5), Si11-C112 1.864(5), Si11-C113 1.877(4). Bond angles
(deg): C1-Zn-C2 113.8(2), C1-Zn-C3 119.3(2), C2-Zn-
C3 126.8(3), Zn-C1-Si11 107.0(2), Zn-C1-C11 109.6(3),
Si11-C1-C11 120.8(3).
The zincate anion of 1 shows a rather strong distor-
tion of the trigonal-planar coordination sphere at the
zinc atom. The Zn-C bond lengths to the methyl groups
are approximately 0.1 Å shorter than to the (trimeth-
ylsilyl)benzyl substituent. Furthermore, the largest
bond angle (126.8(3)°) is observed for C(2)-Zn-C(3)
between the methyl groups. The negative charge lo-
cated mainly on the benzyl ligand leads to a shortening
of the C(1)-Si(11) bond length (1.824(4) Å) compared
to the Si(11)-C(11m) distances (mean value 1.872 Å).
The angle C(11)-C(1)-Si(11) is widened to a value of
120.8(3)°. With these structural features, the Zn-CH-
(SiMe3)Ph moiety of the zincate 1 displays structural
parameters between those of the corresponding lithium
salt, where the lithium atom bonds side-on to the benzyl
fragment,11 and the tmeda complex (tmeda)Zn[CH-
(SiMe3)Ph]2.12 In a highly simplified view, this anion
can be regarded as coordination of a benzyl anion to a
molecule of dimethylzinc.
The zincate anion 2 shows an even shorter Zn-C(3)Me
bond length of 1.989(8) Å. The Zn-C distances to the
methyl and to the (trimethylsilyl)benzyl substituents
differ by approximately 0.1 Å. The smallest CZnC bond
angle is observed between the sterically demanding
benzyl groups. Figure 2 shows that the hydrogen atoms
are oriented in such a manner as to minimize the
intramolecular steric strain.
A dependence between the bond length and the
opposite bond angle was already observed for complexes
with a zinc atom with a coordination number four13,14
(14) Westerhausen, M.; Rademacher, B.; Schwarz, W. J . Organomet.
Chem. 1992, 427, 275.

1440 Organometallics, Vol. 17, No. 7, 1998
Notes
Ta ble 1. Cr ysta llogr a p h ic Da ta of 1 a n d 2 a s Well
a s Deta ils of Str u ctu r e Solu tion a n d Refin em en t
P r oced u r es
1
2
formula
fw
C
24H53LiN4SiZn
C33H65LiN4Si2Zn
646.38
498.10
T, K
193
193
space group16
P212121 (no. 19)
9.5739(3)
13.4655(6)
24.3814(9)
3143.2(2)
4
1.053
0.835
1088
Pbca (No. 61)
28.071(4)
14.569(3)
19.631(3)
8029(3)
8
1.070
0.696
2816
a, Å
b, Å
c, Å
V, Å3
Z
d
calcd, g cm3
µ, mm-1
F(000)
scan range (deg)
no. of measd data
no. of unique data
no. of params
wR2a (all data)
R1a (all data)
R indicesa with I > 2σ(I)
data
1.7 e θ e 29.6
1.9 e θ e 26.0
18 251
7870
6506 (Rint ) 0.045) 7870
347
0.107
0.084
455
0.173
0.169
F igu r e 2. Molecular structure and numbering scheme of
the zincate anion [MeZn{CH(SiMe3)Ph}2]- of 2. The el-
lipsoids represent a probability of 50%. Selected bond
lengths (Å): Zn-C1 2.089(6), Zn-C2 2.084(5), Zn-C3
1.989(8), C1-Si11 1.828(7), C1-C11 1.476(8), C1-H1 87-
(5), C2-Si21 1.838(6), C2-C21 1.488(8), C2-H2 85(5).
Bond angles (deg): C1-Zn-C2 115.2(3), C1-Zn-C3 125.1-
(3), C2-Zn-C3 119.7(3), Zn-C1-Si11 106.9(3), Zn-C1-
C11 110.6(4), Si11-C1-C11 118.4(5), Zn-C2-Si21 106.2-
(3), Zn-C2-C21 107.7(4), Si21-C2-C21 122.5(4).
3939
0.073
0.041
1.173
3351
0.118
0.065
1.110
wR2
R1
goof sbon F2
residual dens, e Å-3
0.251; -0.178
0.342; -0.291
a
Definition of the R values: R1 ) (∑||Fo| - Fc||)/∑|Fo|; wR2 )
{∑[w(Fo - Fc2)2]/∑[w(Fo2)2]}1/2 with w-1 ) σ2(Fo2) + (aP)2. s )
2
b
{∑[w(Fo - Fc2)2]/(No - Np)}1/2
.
2
and for zinc bis(thiolates).15 This concept that a length-
ening of a Zn-X distance widens the opposite Y-Zn-Y
angle at the zinc center nearly regardless of the size of
the ligands seems to be valid, in general, as also shown
for the methylbis(2,2,4,4,6,6-hexamethyl-2,4,6-trisila-
cyclohexyl)zincate anion.8 However, the Zn-C dis-
tances differ by only 0.05 Å. This observation allows
the conclusion that a better charge delocalization within
the carbanionic ligand leads to a lengthening of the
corresponding Zn-C bond length.
at 0 °C to a solution of 0.70 g of ClZn(tmeda)CH(SiMe3)Ph (1.84
mmol) in 25 mL of diethyl ether. After 1 h, an additional 1.15
mL of the MeLi solution was added together with 0.28 mL of
tmeda (1.84 mmol). After removal of the precipitated LiCl,
the solution was reduced to 10 mL. At -20 °C, 0.87 g of
colorless crystalline 1 (1.75 mmol, 97%) precipitated. Decom-
position occurs at temperatures g35 °C.
1H NMR (-40 °C): δ -1.21 (Zn-Me), 1.46 (CH), -0.07
(SiMe3), 7.23-5.85 (Ph). 7Li{1H} NMR: δ 2.14. 13C{1H} NMR
(-40 °C): δ -6.70 (Zn-Me, br), 39.01 (CH), 2.42 (SiMe3),
157.55 (i-C), 127.16 (o-C), 125.08 (m-C), 114.19 (p_C). 29Si-
{1H} NMR (-40 °C): δ -6.37. MS (70 eV, sample 323 K, R )
CH(SiMe3)Ph): 390 (ZnR2, 70), 326 (35), 303 (90), 242 (RZnMe,
100), 227 (ZnR, 40), 116 (tmeda, 100), 94 (ZnMe2, 90).
[Li(tm ed a )2][MeZn {CH(SiMe3)P h }2] (2). Methyllithium
(2.15 mL of a 1.6 M solution in diethyl ether) was added at 0
°C to a solution of 1.75 g of (tmeda)Zn[CH(SiMe3)Ph]2 (3.44
mmol) and 0.52 mL of tmeda (3.44 mmol) in 40 mL of diethyl
ether. After 12 h, the clear solution was separated from the
LiCl precipitate and was reduced in vacuo to approximately
15-20 mL. At -30 °C, 1.24 g of colorless crystalline 2 (1.92
mmol, 56%) was separated. Mp: 75 °C (dec).
1H NMR data show a low-field shift of the methyl as
well as methine proton signals on going from 1 to 2.
For 3, a further low-field shift is observed for the R-CH
fragments. Due to the chiral R-carbon atom, two or
three sets of signals were obtained for 2 and 3, respec-
tively. The 13C{1H} NMR spectra display the opposite
trend for the carbon atoms bonded to the zinc center.
The methyl groups cause resonances at high field at δ
) -6.70 for 1 and at δ ) -7.95 and -8.53 for the two
7
diastereomers of 2. A Li{1H} chemical shift of 2.4 is
characteristic for the cation [Li(tmeda)2]+. These zin-
cates are more reactive than the homoleptic dialkylzinc
and the corresponding alkyllithium.
1H NMR (2 diastereomers): δ -1.06, -1.10 (Zn-Me), 1.52,
1.48 (CH), -0.10, -0.14 (SiMe3), 6.78-6.20 (Ph). 7Li{1H}
NMR: δ 2.28. 13C{1H} NMR: δ -7.95, -8.53 (Zn-Me), 35.00,
34.57 (CH), 2.31, 2.20 (SiMe3), 156.43 (i-C), 127.30 (o-C),
126.55, 126.41 (m-C), 115.75 (p-C). 29Si{1H} NMR: δ -0.95,
-1.02. MS (70 eV, sample 360 K, R ) CH(SiMe3)Ph): 390
(ZnR2, 58), 374 (ZnR2 - Me, 11), 242 (RZnMe, 12), 227 (ZnR,
45), 163 (R, 38), 73 (SiMe3, 75), 58 (SiMe2, 100).
Exp er im en ta l Section
The general working conditions and facilities are given
elsewhere.4 The starting materials (tmeda)LiCH(SiMe3)Ph,11
ClZn(tmeda)CH(SiMe3)Ph,13 and (tmeda)Zn[CH(SiMe3)Ph]2
[Li(tm ed a )2][Zn {CH(SiMe3)P h }3] (3). Anhydrous zinc
chloride (0.88 g, 6.4 mmol) was added in small portions to an
ice-cooled solution of 5.53 g of (tmeda)LiCH(SiMe3)Ph (19.3
mmol) in 40 mL of diethyl ether. After complete addition, the
solution was heated at reflux for 2 h. After removal of
precipitated LiCl, the solution was cooled to -20 °C. Crystal-
line colorless 3 (3.41 g, 4.3 mmol, 67%) was collected. Mp: 202
°C.
12
were prepared by literature procedures. IR spectra were
recorded on Nujol suspensions between CsBr windows. The
NMR data were collected on tetrahydrofuran-d8 solutions at
30 °C if not otherwise stated.
[Li(tm ed a )2][Me2Zn CH(SiMe3)P h ] (1). Methyllithium
(1.15 mL of a 1.6 M solution in diethyl ether) was added slowly
1H NMR (3 overlapping sets of signals): δ 1.76, 1.71, 1.66
(CH), -0.15, -0.18, -0.36 (SiMe3), 6.80-6.30 (Ph). 7Li{1H}
NMR: δ 2.39. 13C{1H} NMR: δ 34.34, 33.38 (CH), 2.63, 2.41,
(15) Bochmann, M.; Bwembya, G. C.; Grinter, R.; Powell, A. K.;
Webb, K. J .; Hursthouse, M. B.; Abdul Malik, K. M.; Mazid, M. A.
Inorg. Chem. 1994, 33, 2290.

Notes
Organometallics, Vol. 17, No. 7, 1998 1441
1.86 (SiMe3), 155.42, 154.82, 154.54 (i-C), 127.99, 127.85,
127.73 (o-C), 127.33, 127.30 (m-C), 117.05, 116.74 (p-C).
29Si{1H} NMR: δ -4.32, -4.43, -4.46. MS (70 eV, sample
243 K, R ) CH(SiMe3)Ph): 390 (ZnR2, 100), 227 (ZnR, 88),
163 (R, 98), 135 (90), 116 (tmeda, 70), 73 (SiMe3, 66).
X-r a y Cr ysta llogr a p h y. The single crystals were covered
with Nujol, sealed in thin-walled capillaries, and mounted on
a four-circle diffractometer with graphite-monochromated Mo
KR radiation. For 1, a Siemens SMART-CCD area detector
was used to monitor the reflections. Crystallographic param-
eters and details of data collection performed at -80 °C are
summarized in Table 1.
All structures were solved by direct methods with the
software package SHELXTL Plus17 and refined with the
program SHELXL-93.18 Neutral atom scattering factors were
taken from Cromer and Mann,19 and for the hydrogen atoms,
they were taken from from Stewart et al.20 The non-hydrogen
atoms were refined anisotropically.
The H atoms of the cations [Li(tmeda)2]+ and of the
trimethylsilyl substituents of 1 and 2 were refined with a
riding model under restriction of ideal symmetry at the
corresponding carbon atom, however, the C-H bond lengths
were refined groupwise. The hydrogen atoms at the zinc-
bonded carbon atoms were refined isotropically. For 1, the
hydrogen atoms of the phenyl substituent were refined iso-
tropically, whereas for 2, the positional parameters of the
phenyl H atoms were refined but the U values were restricted
to the 1.2 times the size of the corresponding C atom.
Ack n ow led gm en t. This research was supported by
the Deutsche Forschungsgemeinschaft, Bonn, and the
Fonds der Chemischen Industrie, Frankfurt/Main. We
also thank Dr. J . Opitz and Mr. F. M. Bender of the
University of Stuttgart for the performance of the mass
spectra.
Su p p or tin g In for m a tion Ava ila ble: For compounds 1
and 2, IR data, tables of positional coordinates, bond distances,
bond angles, fractional parameters of all H atoms, and thermal
parameters of the non-H atoms, and stereoscopic views of the
molecular structures and unit cells (20 pages). Ordering
information is given on any current masthead page.
(16) International Tables for Crystallography, 2nd ed.; Hahn, T., Ed.;
D. Reidel: Dordrecht, 1984; Vol. A.
(17) SHELXTL Plus, PC version; Siemens Analytical X-ray Instru-
ments, Inc.: Madison, WI, 1980.
(18) Sheldrick, G. M. SHELXL-93; Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1993.
(19) Cromer, D. T.; Mann, J . B. Acta Crystallogr. 1968, 24, 321.
(20) Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J . Chem. Phys.
1965, 42, 3175.
OM971092C