Assembly of ethylzincate compounds into supramolecular structures

Article abstract of DOI:10.1016/j.jorganchem.2010.12.001

Sodium and potassium triethylzincate were isolated by Wanklyn in 1858, and are corner stones in the history of organometallic chemistry. Crystallisation of organozincates from neat dialkylzinc, in the absence of a coordinating solvent, can be expected to result in assembly of supramolecular structures, rather than formation of discrete molecules in the crystalline state. This inspired us to reinvestigate Wanklyn's classical compounds. Crystallisation of sodium triethylzincate from benzene led to metallation of benzene and the formation of diethylphenylzincate anions. The compound is a two-dimensional network where Na⁺ ions link the zincate anions by coordination to both ethyl- and phenyl groups. We have also, accidently, isolated crystals of the two-dimensional coordination network [K₂(ZnEt₂) ₄O]_n, displaying a rare oxo-centred core with an octahedral oxide ion surrounded by four zinc atoms and two potassium atoms.

Full text of DOI:10.1016/j.jorganchem.2010.12.001

Journal of Organometallic Chemistry 696 (2011) 2269e2273
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Assembly of ethylzincate compounds into supramolecular structures
,
Anna Hedström a, Anders Lennartson b
*
a Department of Chemistry, University of Gothenburg, SE-412 96 Gothenburg, Sweden
b Department of Physics and Chemistry, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history:
Sodium and potassium triethylzincate were isolated by Wanklyn in 1858, and are corner stones in the
history of organometallic chemistry. Crystallisation of organozincates from neat dialkylzinc, in the
absence of a coordinating solvent, can be expected to result in assembly of supramolecular structures,
rather than formation of discrete molecules in the crystalline state. This inspired us to reinvestigate
Wanklyn’s classical compounds. Crystallisation of sodium triethylzincate from benzene led to metallation
of benzene and the formation of diethylphenylzincate anions. The compound is a two-dimensional
network where Naþ ions link the zincate anions by coordination to both ethyl- and phenyl groups. We
have also, accidently, isolated crystals of the two-dimensional coordination network [K2(ZnEt2)4O]n,
displaying a rare oxo-centred core with an octahedral oxide ion surrounded by four zinc atoms and two
potassium atoms.
Received 31 August 2010
Received in revised form
24 November 2010
Accepted 1 December 2010
Keywords:
Organozinc
Zincate
Supramolecular chemistry
Metallation
Ó 2010 Elsevier B.V. All rights reserved.
Diethylzinc
1. Introduction
[ZnEt3] and K[ZnEt3], are found in the Cambridge Structural Database
[11]. We recently reinvestigated Wanklyn’s synthesis, and found that
The discovery of diethylzinc and dimethylzinc by Edward Frank-
land in 1848 [1e4] marks the birth of modern organometallic
chemistry. Diethylzinc and dimethylzinc were not only the first
organozinc compounds to be prepared, but were also the first
alkylmetal compounds, and the first main group organometallic
compounds. The isolation of organozinc compounds promoted
research on organometallic derivatives of other metals, and ten years
later J. Alfred Wanklyn found that diethylzinc reacts with metallic
sodium forming a white crystalline and highly reactive compound
[5,6]. Wanklyn described his new compound as a “double compound
between sodium-ethyl and zinc-ethyl” but it is now known to be
sodium triethylzincate. The target for Wanklyn’s synthetic attempts,
ethylsodium, was first prepared by Schlenk and Holtz in 1917 [7].
Wanklyn noted that zincates were useful in the synthesis of
carboxylic acids, since Na[ZnEt3] react with carbon dioxide to give
propionic acid [5,8]. Since the late 1970’s, zincates have attracted
more and more attention in organic synthesis, and are today exten-
sively used in conjugate addition to enones, halogen-zinc exchange
reactions, nucleophilic additions to ketones and ring-opening of
epoxides [9]. Even though Wanklyn’s synthetic approach has been
successfully used to crystallise other zincates [10], still today, no
crystal structures of Wanklyn’s historically important compounds, Na
heating metallic sodium in an excess of diethylzinc led to a violent
reaction with evolution of gas and deposition of metallic zinc.
On cooling to ambient temperature, crystals of Na2[Zn2Et4(m-H)2]
were obtained [12]. Although hydridoalkylzincates [13e17] and
related compounds [18] have gained considerable interest, this was
the first synthetic route to well-defined hydridodialkylzincates. We
continued the studies of Wanklyn’s compounds, and in this paper we
describe the crystal structures of the supramolecular networks
[NaZnEt2Ph]n and [K2(ZnEt2)4O]n.
2. Results and discussion
We have repeated Wanklyn’s original synthesis several times in
order to determine the crystal structure of the product. We were,
however, unable to obtain single-crystals of X-ray quality of Wank-
lyn’s compound Na[ZnEt3] through crystallisation from neat dieth-
ylzinc; neither cooling solutions to ꢀ40 ꢁC or ꢀ80 ꢁC, nor layering
solutions with hexane was successful. Even crystals having the visual
appearance of single-crystals, were found to be multi-component
congeries, resisting all attempts of indexing the reflections. Recrys-
tallisation from a coordinating solvent, such as tetrahydrofuran,
would most probably give crystals with solvent molecules coordi-
nated to Naþ [19], and was therefore not considered. Na[ZnEt3] is
insoluble in alkane solvents, but has been reported to be soluble
in benzene [20], and recrystallisation of Wanklyn’s compound from
benzene was attempted. It was found that crystallisation of Na
* Corresponding author.
E-mail address: lennartson@ifk.sdu.dk (A. Lennartson).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.12.001

2270
A. Hedström, A. Lennartson / Journal of Organometallic Chemistry 696 (2011) 2269e2273
[ZnEt3] from benzene led to metallation of benzene by the strongly
basic zincate (Scheme 1), and crystals of the coordination network
[NaZnEt2Ph]n,1 (Fig.1), were isolated. Compound 1 is built up by Naþ
and [ZnEt2Ph]ꢀ ions, and there seems to be only two previous
structures of zincate-like species with ethyl ligands in the Cambridge
Structural Database. These two structures are very different from 1,
since they display functionalised R-groups and additional neutral
ligands [21,22]. Both of these two compounds were, like 1, obtained
by metallation of aromatic substrates. Other recent studies of zin-
cation of aromatic compounds by zincate species include the zin-
cation of toluene [23] and anisol [24]. Deprotonative metallation
(and polymetallation) byate compounds was recently reviewed [25].
Recrystallisation from other aromatic solvents was not attempted.
The species present in solution may be difficult to assign, since zin-
cates show highly dynamic behaviours in solution, even at low
temperatures, and the NMR-spectra are therefore frequently difficult
interpret [26e28]. The 1H NMR spectrum of 1 in THF-d8 shows a set
of broad signals, indicating a dynamic behaviour in solution.
The zincate anion in 1 displays trigonal planar coordination
geometry, as expected (for bond angles and distances, see Table 1).
Each [ZnEt2Ph]ꢀ anion coordinates three Naþ ions, and each Naþ
ion, consequently, coordinates three zincate ions. The coordination
of the zincate ions to Naþ occur both through the ethyl groups and
through the phenyl groups; each Naþ ion coordinates three ethyl
groups and two phenyl groups. Within the asymmetric unit, Na1
Fig. 1. Molecular structure of 1 displaying the crystallographic numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level, and hydrogen atoms
have been omitted for clarity.
This results in a three-dimensional network structure comparable
to that of di(2-methylallyl)zinc. Lithium tetramethylzincate gives
rise to a three-dimensional network structure, with tetrahedral
coordination geometries around both lithium and zinc atoms [36].
Here, the network is constructed from alkali metal-carbon bonds
and zinc-carbon bonds like in 1. Among the structures most similar
to 1 is potassium tri(cyclopentadienyl)zincate which forms a two-
dimensional layered structure that was recently published by
Carmona and co-workers [19]. The two zincates potassium trineo-
pentylzincate benzene solvate and sodium trineopentylzincate, on
the other hand, does not give rise to supramolecular assemblies [10].
Introduction of Lewis base ligands may give an even richer variety
of structures. For instance, di-tert-butylzinc form discrete linear
molecules in the crystalline state, but upon coordination of 1,2-di
(4-pyridyl)ethane, it forms a one-dimensional polymer that gives
rise to a two-dimensional nanofabric [37].
Crystals of compound 2, [K2(ZnEt2)4O]n (Table 2), were acci-
dently isolated in low yield in an attempt to prepare K[ZnEt3] from
metallic potassium and diethylzinc. Compound 2 is built up from
a [K2(ZnEt2)4O] tecton (Fig. 4) that has not been characterised
previously in the solid state.
The most striking feature of this unit is the central oxide ion
surrounded by a square planar array of four zinc atoms with two Kþ
ions completing an octahedral coordination geometry. A search in
the Cambridge Structural Database reveals that oxygen atoms
forms a bond to C9, the a-carbon atom in one of the ethyl groups, to
C1, the ipso-carbon and to the two m-carbon atoms C2 and C6. The
other face of the phenyl ring is coordinated to Na1(x, 1/2 ꢀ y, 1/
2 þ z), in a similar h3 fashion. The other
a-carbon atom, C7, is
coordinated to Na1(2-x, 1/2 þ y, 1/2 ꢀ z). As a result, compound 1 is
a coordination network forming infinite layers in the bc-plane
(Fig. 2). There are no directed interactions, such as CH/
interactions, between these layers (Fig. 3).
The supramolecular aspect of organometallic chemistry is an
p [29,30] or
pep
important field, which has been thoroughly treated in a book by
Haiduc and Edelmann [31]. There are only a limited number of
crystal structures of Lewis base free organozinc compounds in the
Cambridge Structural Database, and these structures show large
variations from a supramolecular point of view. Dicyclopentadie-
nylzinc, for example, gives rise to polymeric zigzag chains, which are
interconnected through CH/
network [32]. Bis(2,3,4,5-tetramethylcyclopentadienyl)zinc forms
similar chains, but there is no evidence of CH/ -interactions, and no
p-interactions into a three-dimensional
p
network is formed [33]. In di(2-methylallyl)zinc, a three-dimen-
sional supramolecular network is obtained through coordination of
four methylallyl groups to each zinc; two through primarily cova-
lent ZneC
s-bonds, and two additional methylallyl ligands from
adjacent molecules coordinate to the zinc atom through their
p-
systems [34]. This structure is more similar to 1, since it is entirely
based on metal-carbon bonds rather than weak hydrogen bonds.
Still it is different, since the carbon atoms bearing the negative
charge coordinate to one zinc atom only, and supramolecular link-
ing is caused by a second functionality of the ligand in contrast to 1.
In rubidium tetra(ethynyl)zincate, four ethynyl-groups radiate
linearly from a tetrahedral zinc atom [35]. Each ethynyl ligand is
Table 1
Selected bond distances (Å) and angles (ꢁ) for 1.
Zn1eC1
Zn1eC7
Zn1eC9
Na1eC1
Na1eC2
Na1eC6
Na1eC7
Na1eC9
2.075(2)
2.028(2)
2.043(2)
2.654(2)
3.331(2)
3.055(2)
2.717(2)
2.861(2)
2.633(2)
2.780(2)
2.955(2)
119.90(7)
118.14(7)
121.90(7)
surrounded by three Rbþ ions coordinated to the ethynyl
p-systems.
Na1ieC1
Na1ieC2
Na1ieC6
C1eZn1eC7
C1eZn1eC9
C7eZn1eC9
Scheme 1. Metallation of benzene by Na[ZnEt3]
Symmetry code: (i) x, 1/2 ꢀ y, 1/2 þ z.

A. Hedström, A. Lennartson / Journal of Organometallic Chemistry 696 (2011) 2269e2273
2271
Table 2
Selected bond distances (Å) and angles (ꢁ) for 2.
K1eC1
3.169(3)
3.169(2)
3.123(3)
3.123(2)
2.814(2)
3.341(3)
3.341(3)
3.173(3)
3.173(3)
2.707(2)
2.001(2)
2.022(2)
2.076(4)
2.0763(4)
2.0831(4)
2.0830(4)
2.006(2)
2.025(2)
138.3(1)
109.86(9)
111.74(9)
136.5(1)
112.51(9)
110.89(9)
180.0
K1eC1ii
K1eC7
K1eC7ii
K1eO1
K2eC3
K2eC3ii
K2eC5
K2eC5ii
K2eO1
Zn1eC1
Zn1eC3
Zn1eO1
Zn1iieO1
Zn2eO1
Zn2iieO1
Zn2eC5
Zn2eC7
Fig. 2. Compound 1 forms a two-dimensional coordination network in the solid state.
The zincate anions are linked via ethyl- and phenyl groups by the Naþ counter ions into
layers in the bc-plane. All hydrogen atoms have been omitted for clarity.
C1eZn1eC3
C1eZn1eO1
C3eZn1eO1
C5eZn2eeC7
C5eZn2eO1
C7eZn2eO1
K1eO1eK2
Zn1eO1eK1
Zn1eO1eK2
Zn1iieO1eZn1
Zn1eO1eZn2
Zn2eO1eK1
Zn2eO1eK2
Zn1eO1eZn2ii
Zn2iieO1eZn2
surrounded by six metal atoms is not a common motif except in
molybdenum, vanadium, tungsten, tantalum and niobium chem-
istry; there are only six structures involving zinc atoms. One complex
displays aLi4Zn2(m6-O)corewithtwozincatoms inacis-arrangement
[38] and one complex a fac-Li3Zn3(m6-O) core [39]. There are three
complexes displaying double-cubane cores comprising K2Zn4(m6-O)
and Cs2Zn4(m6-O) motifs [40]. These are quite dissimilar to 2, having
the two alkali metal ions in a cis-configuration, for example. The
compound showing most similarities with 2 is [Ba2Zn4(Me)2(O)
(PSiMe3)4(thf)6] (Fig. 5), isolated and characterised by Westerhausen
and co-workers [41]. The two compounds share the M2Zn4(m6-O)
core with the two M atoms in a trans-arrangement, but apart from
this core, the two structures are quite different. In 2, the four zinc
atoms are chemically equivalent, displaying distorted trigonal planar
geometries. The zinc atoms coordinate two ethyl groups and the
central oxygen atom. In [Ba2Zn4(Me)2(O)(PSiMe3)4(thf)6], on the
other hand, there are two types of zinc atoms. One type is three-
coordinated with a T-shaped coordination geometry. These two zinc
atoms coordinates two phosphide ligands and the central oxygen
atom, while the other two zinc atoms are four-coordinated and
coordinate two phosphide ligands, the central oxygen atom and one
bridging methyl group. Another difference is the Ba2þ ions which
88.77(6)
91.23(6)
177.5(1)
94.20(2)
89.08(6)
90.92(6)
85.76(2)
178.2(1)
Symmetry code: (ii) 2 ꢀ x, y, 1/2 ꢀ z.
coordinate THF molecules. Since 2 was crystallised from neat dieth-
ylzinc, no coordinating solvent is available for the two Kþ ions, and
a coordination network is formed rather than discrete molecules.
Each Kþ ion coordinates six ethyl groups, four within the
[K2(ZnEt2)4O] tecton, and two ethyl groups from two different adja-
cent tectons. K1 forms bonds to C1, C7, C1(2 ꢀ x, y, 1/2 ꢀ z) and C7
(2 ꢀ x, y,1/2 ꢀ z) within the tecton and two additional bonds to C3(1/
2 þ x, ꢀ1/2 þ y, z) and C3(3/2 ꢀ x, ꢀ1/2 þ y, 1/2 ꢀ z). K2 is chemically
equivalent to K1 and forms bonds to C3, C5, C3(2 ꢀ x, y, 1/2 ꢀ z), C5
(2 ꢀ x, y,1/2 ꢀ z), C7(ꢀ1/2 þ x, ꢀ1/2 þ y, z) and C7(5/2 ꢀ x, 1/2 þ y,1/
2 ꢀ z). This means that that there are two types of ethyl groups; the
negative charge on a-carbon atoms C3 and C7 is shared between one
Fig. 3. The layers in 1 are stacked along the crystallographic b-axis, without any
indications of directed interactions between the layers. All hydrogen atoms have been
omitted for clarity.
Fig. 4. Molecular structure of the tecton in 2 displaying the crystallographic
numbering scheme. Displacement ellipsoids are drawn at the 50% probability level,
and all hydrogen atoms have been omitted for clarity.

2272
A. Hedström, A. Lennartson / Journal of Organometallic Chemistry 696 (2011) 2269e2273
3.1. General
thf
SiMe3
thf
thf
Me3Si
K
O
K
Et
All experiments were performed under nitrogen atmosphere
using Schlenk technique. All glass equipment was dried at 130 ꢁC over
night. Benzene was distilled from sodium/benzophenone shortly
prior to use; hexane was distilled from sodium/benzophenone/tet-
raglyme shortly prior to use. Diethylzinc (Aldrich), 15% diethylzinc
solution in hexane (Acros Organics), sodium (E. Merck) and potas-
sium (Aldrich) were used as received. NMR spectra were recorded on
a Bruker Advance III 400 MHz spectrometer.
P
Zn
P
P
Ba
Et
Et
Et
Zn
Me
Zn
Zn
Zn
O
Zn
Me
Et
Zn
Zn
Et
Et
P
Ba
thf
Et
SiMe3
Me3Si
thf
thf
3.2. [Na Zn(Et)2Ph] (1)
Fig. 5. Comparison between the tecton in 2 (left) and [Ba2Zn4(Me)2(O)(PSiMe3)4(thf)6]
(right) with the common features highlighted.
Diethylzinc solution (50 ml, 15% in hexane, 60 mmol) was
evaporated until 1/3 of the solution remained. Sodium (0.8 g,
33 mmol) was added in small pieces under stirring. The mixture was
heated until the sodium started to melt and the mixture was stirred
over night. The solution was withdrawn, the grey solid residue was
washed with hexane (2 ꢂ 5 ml) and extracted with benzene (10 ml).
The mixture was centrifuged (2 min, 2500 rpm) and the resulting
clear, orange/brown solution was placed at 4 ꢁC. After a few days, 1
had crystallised in colourless, irregular-shaped crystals. Yield: 2.3 g
zinc atom and two Kþ ions, while the negative charge on
a-carbon
atomsC1andC5issharedbetweenonezincatomandoneKþ ion. The
linking of [K2(ZnEt2)4O] units results in a two-dimensional network
structure composed of infinite layers stacked parallel to the c-axis
(Fig. 6). Bothfacesof these layers aredominatedbyhydrophobicethyl
groups, and there are no indications of directed interactions between
these layers. The 1H NMR spectrum in THF-d8 shows only one set of
signals from the ethyl groups.
(30%). 1H NMR (400.12 MHz, THF-d8):
7.04 (s, Ph), 6.91 (s, Ph), 1.39 (s, CH3), 0.00 (s, CH2). 13C NMR
(100.62 MHz, THF-d8):
d
¼ 8.05 (s, Ph), 7.87 (s, Ph),
The most probable source of the central oxide ion in 2 is water
rather than dioxygen, which would more likely give rise to alkoxide
complexes. We have previously reported an octanuclear ethyl(oxo)
zinc complex with a cubane core, which forms upon addition of
water to a solution of diethylzinc and pyridine in hexane [42], and
a zincphenoxide complex obtained by exposure of a solution of
diphenylzinc in diethyl ether to air [43]. Due to the potential
explosion hazard, we have not attempted to deliberately add water
in any form to the reaction mixture of potassium in neat diethylzinc.
In conclusion, two new supramolecular zincate structures have
been prepared, one of which displays a rare m6-oxo-centred zincate
tecton. The determination of the crystal structure of Wanklyn’s
original compounds, however, still remains a challenge.
d
¼ 142 (Ph), 141 (Ph), 129 (Ph), 126 (Ph), 125
(Ph), 123 (Ph), 15.6 (CH3), 6.95 (CH2).
3.3. [K2(ZnEt2)4O] (2)
A small piece of potassium (c. 50 mg) was added to diethylzinc
(2 ml) in a Schlenk tube. A white precipitate formed immediately.
The mixture was stirred at ambient temperature over night, and
placed at ꢀ35 ꢁC. A fewcolourless needles were isolated afterapprox.
3 months. 1H NMR (400.12 MHz, THF-d8):
d
¼ 1.09 (m, CH3), ꢀ0.09
(q, CH2). 13C NMR (100.62 MHz, THF-d8):
d
¼ 13.41 (CH3), 4.29 (CH2).
3.4. X-ray crystallography
3. Experimental
The crystals of 1 and 2 were carefully selected using microscope
at low-temperature [44]. Data were recorded at 100 K using
Caution! Diethylzinc and alkylzincates are highly pyrophoric,
ignite instantaneously upon contact with air, and react explosively
with water. Sodium and potassium should be handled with care.
Table 3
Crystal and refinement data for compounds 1e3.
Parameter
1
2
Empirical formula
M
T(K)
NaZnC10H15
223.58
100(2)
K2Zn4C16H40
588.16
100(2)
O
Crystal system
Space group
a/Å
b/Å
c/Å
Monoclinic
P21/c
8.3949(17)
13.298(3)
9.408(2)
90
92.947(9)
90
1048.9(4)
4
1.416
2.326
0.3 ꢂ 0.2 ꢂ 0.2
2.65e25.50
6446
Monoclinic
C2/c
9.3390(18)
13.845(3)
18.098(4)
90
91.063(10)
90
2339.6(8)
4
1.670
4.408
0.3 ꢂ 0.15 ꢂ 0.15
3.71e25.00
6958
ꢁ
ꢁ
a
/
b
/
ꢁ
/
g
V/Å3
Z
Dc/g cmꢀ3
m
(Mo-K
Crystal size/mm3
Range/ꢁ
a
)/mmꢀ1
q
Reflections collected
Unique reflections, Rint
No. of parameters
1873
111
0.0253/0.0629
0.0281/0.0639
1840
110
0.0264/0.0631
0.0309/0.0644
Final R1(F)a(I > 2
s
(I))/wR2(F2)b
Fig. 6. Like 1, compound 2 gives rise to a two-dimensional supramolecular network
structure. This figure displays four vertical layers viewed along the b-axis; there are no
directed interactions between the layers. All hydrogen atoms have been omitted for
clarity.
R1a/wR2(F2)b (all data)
a
R1(F) ¼ S(jjFojꢀjFcjj)/SjFoj.
b
½
wR2(F2) ¼ {S[w(Fo2eFc2)2]/S[w(F2o)2]}
.

A. Hedström, A. Lennartson / Journal of Organometallic Chemistry 696 (2011) 2269e2273
2273
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a Rigaku R-AXIS IIc area detector with graphite monochromated
Mo-K radiation (
¼ 0.71073 Å) from a Rigaku RU-H3R rotating
a
l
anode, operating at 50 kV and 90 mA. 90 oscillation photos with
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matrix least squares calculations on F2 using the SHELXL-97
program [47]. All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were included in calculated positions and
refined using a riding model. Structures were drawn using ORTEP3
[48] for Windows and Pluton [49,50]. SIR-92, SHELXL-97, Pluton
and ORTEP3 were enclosed in the WinGX-software package [51].
Crystal and refinement data are given in Table 3.
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Appendix A. Supplementary material
CCDC 790585 (1) and 790586 (2) contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Appendix. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.12.001.
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