Bis(allyl)zinc revisited: Sigma versus Pi bonding of allyl coordination

Article abstract of DOI:10.1021/ja303480a

The reinvestigation of two allyl zinc compounds, parent bis(allyl)zinc [Zn(C₃H₅)₂] (1) and 2-methallyl chloro zinc [Zn(C₄H₇)Cl] (2), revealed two new coordination modes in the solid state for the allyl ligand, viz cis- and trans-μ₂- η¹:η¹. These results call for modification of the conventional interpretation of zinc-allyl interactions. Computational results indicate that the classical η³-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc-olefin interaction in the dimer of [Zn(η¹-C₃H₅)(OC(C ₃H₅)Ph₂)] was found in the monoinsertion product of 1 with benzophenone.

Full text of DOI:10.1021/ja303480a

Article
pubs.acs.org/JACS
Bis(allyl)zinc Revisited: Sigma versus Pi Bonding of Allyl Coordination
Crispin Lichtenberg,† Julien Engel,‡ Thomas P. Spaniol,† Ulli Englert,† Gerhard Raabe,‡ and Jun Okuda*,†
‡
†Institute of Inorganic Chemistry and Institute of Organic Chemistry, RWTH Aachen, Landoltweg 1, 52056 Aachen, Germany
S
* Supporting Information
ABSTRACT: The reinvestigation of two allyl zinc com-
pounds, parent bis(allyl)zinc [Zn(C3H5)2] (1) and 2-methallyl
chloro zinc [Zn(C4H7)Cl] (2), revealed two new coordination
modes in the solid state for the allyl ligand, viz cis- and trans-
μ2-η1:η1. These results call for modification of the conventional
interpretation of zinc−allyl interactions. Computational results
indicate that the classical η3-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc−olefin
interaction in the dimer of [Zn(η1-C3H5)(OC(C3H5)Ph2)] was found in the monoinsertion product of 1 with benzophenone.
and possible π-type bonding (E).8 We report here our findings
on allyl zinc compounds, which disagree with earlier
interpretations of experimental data. Our data suggest that
purely π-type interactions in allyl zinc complexes are not
favored. Rather, two σ-type bonding modes of allyl ligands, cis-
B and trans-B, have been found as new bonding modes for the
Lewis acidic divalent zinc centers.
1. INTRODUCTION
Understanding the interactions of delocalized π-electron
systems with metal centers is of fundamental importance in
organometallic chemistry.1 The allyl group represents the
simplest delocalized π-electron system, and allyl metal
complexes are therefore ideal model systems. Six fundamentally
different coordination modes of the allyl ligand have been
structurally authenticated for main group and group 12 metals
2. RESULTS AND DISCUSSION
(A−F; Scheme 1).2,3 Two of them show σ-type carbon metal
The parent allyl zinc compound, bis(allyl)zinc (1), was first
reported in 19659a and has been used as a nucleophilic allyl
Scheme 1. Coordination Modes of the Allyl Ligand Involving
transfer reagent ever since.10 Its properties and dynamics in
solution were studied by Lehmkuhl and co-workers.9 The allyl
ligands in 1 are bonded to the metal center in an η1 fashion in
donor solvents at low temperature, but they exhibit fluxional
behavior in solution at ambient temperature. On the basis of
solid state CP MAS 13C NMR measurements, it was suggested
that 1 shows η3 bound allyl ligands exclusively, i.e. π-type
carbon−metal interactions (C, Scheme 1).8c However, the
detailed solid state structure of 1 remained unknown for almost
50 years.
Carbon Metal Interactions of σ-Type (A: η1; B: μ2-η1:η1 (cis
and trans Isomer Possible)), π-Type (C: η3; D: μ2-η3:η3), or
σ- and Possibly π-Type (E: μ2-η1:η2; F: μ2-η1:η3)
We obtained results from a single crystal X-ray analysis of 1
(Figure 1) which contradict the above-mentioned interpreta-
tion of solid state NMR data. Compound 1 crystallizes in the
tetragonal space group I41/a with Z = 4 and an asymmetric unit
consisting of only three non-hydrogen atoms. The zinc atom
occupies a special position of symmetry −4 with a tetrahedral
coordination geometry (C1−Zn1−C1′(′), 103.86(12)° and
112.35(6)°). All Zn−C distances of 2.171(2) Å are equivalent
by symmetry. They are elongated compared to corresponding
values in zinc complexes with η1 bonded allylic ligands
(1.969(8)−2.031(12) Å).8a,b The elongated Zn−C bonds in
1 may also correlate with weaker intermolecular bonds,
affecting the volatility of this compound: the 3D polymer 1
can be sublimed at 5 × 10−2 mbar and temperatures as low as
35 °C (see Experimental Section).9a,c Significant bonding
interactions (A, B), two exhibit π-type carbon metal bonding
(C, D), and two show possibly both types of carbon metal
interactions (E, F). A precise discrimination is crucial for the
understanding of the reactivity of such compounds. In
palladium complexes, for instance, the allyl group reacts as a
nucleophile when bonded in σ fashion (type A, B) but as an
electrophile when bonded in π fashion (type C; Scheme
1).1f−h,4,5 Recently, interactions of divalent, electropositive
metals with allyl ligands have attracted some attention.2b,c,f,6
For alkaline earth metal compounds, each element favors either
σ-type interactions (Be, Mg) or π-type interactions (Ca, Sr,
Ba).7 In contrast, allyl zinc complexes have been reported to
show a broader scope of coordination modes with examples of
purely σ-type bonding (A), π-type bonding (C), as well as σ
Received: April 11, 2012
Published: May 16, 2012
© 2012 American Chemical Society
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Figure 3. Lewis drawing (a) and calculated structure (b) of the bis(η1-
allyl)zinc trimer optimized at the MP2/aug-cc-pVDZ level of theory.
Figure 1. Cut-out of the solid state structure of bis(allyl)zinc (1): (a)
three-dimensional network of 1; (b) trans μ2-η1:η1 coordination mode
of the allyl ligand. Displacement ellipsoids are shown at the 50%
probability level. Both symmetrically independent carbon atoms of the
ligand occupy general positions; this implies 2-fold disorder for C2.
Only one of the two split positions for C2 is shown. Hydrogen atoms
are omitted for clarity. Bond lengths (Å) and angles (deg): Zn1−C1,
2.171(2); C1−C2, 1.423(4); C2−C1′, 1.333(5); C1−Zn1−C1′,
103.86(12); C1−Zn1−C1″, 112.36(8); C1−C2−C1′, 130.6(3).
Primed atoms are related by symmetry code [3/2 − x, 3/2 − y, 1/2− z].
in bis(η1-allyl)zinc. The metal−carbon bonds are strongly
polarized toward the carbon atom (≈85%) and involve almost
exclusively the 2s- and 2p-orbitals of carbon and the 4s- and 4p-
orbitals of the zinc atom. This is different from the case for
bis(allyl)nickel, where the partly filled 3d-valence orbitals
participate to a significant degree in metal−carbon bonding,
resulting in an η3-coordination of the metal atom.16 Both the
first and the second steps of the formation of bis(η1-allyl)zinc
from Zn2+ and two allyl anions are highly exergonic. Addition
of the first allyl anion to Zn2+ is associated with a reaction
energy of −444.7 kcal·mol−1, while addition of the second allyl
anion yields further −206.3 kcal·mol−1 (MP2/aug-cc-pVDZ).
The trimer of bis(allyl)zinc was chosen as a model system to
study the intermolecular interactions between bis(allyl)zinc
units. In contrast to the high reaction energies mentioned
above, trimerization results in a reaction energy of only −42.3
kcal·mol−1 (MP2/aug-cc-pVDZ), corresponding to −14.1
kcal·mol−1 per molecular unit (Figure 3). Intramolecular
zinc−allyl interactions in the trimer are purely σ in nature.
Intermolecular interactions are realized by coordination of the
olefinic moieties of the allyl ligands to zinc centers of the
neighboring [Zn(C3H5)2] molecules. The trimer shows two
different types of bis(allyl)zinc units (I vs II and III; cf. Figure
3). In addition to the two intramolecular Zn−C σ bonds, the
zinc atom in I undergoes intermolecular interactions with two
CC units (belonging to the allyl ligands of II and III). It
adopts a distorted tetrahedral coordination geometry with
angles around Zn(I) ranging from 99.6° to 122.6°. The zinc
atoms in II and III show only one intermolecular interaction
each: they are coordinated by the olefinic functionalities of the
allyl ligands in I. A strongly distorted trigonal planar
coordination geometry was assigned to Zn(II) and Zn(III)
(angle sums around Zn: 358°).17 Covalent Zn−C bonds in the
trimer are elongated compared to corresponding bonds in
monomeric bis(allyl)zinc. This effect is more pronounced for
allyl ligands that are involved in intermolecular interactions
(2.006−2.057 Å) than for “dangling” allyl ligands (1.985 Å,
1.990 Å). Accordingly, CC bonds which coordinate to zinc
centers are elongated compared to noncoordinating olefinic
groups (1.375 Å vs 1.358 Å). Zn−Colefin bond lengths range
from 2.393 Å to 2.591 Å. As in the case of the monomer
(Figure 2), Zn−C σ-bonding is accomplished by approximate
sp hybrids. These bonds are strongly polarized to the carbon
atoms (∼89%), with average natural charges of about −1.0 at
the carbons and +1.3 at the metal atoms. The second order
interactions between Zn1 and C2 were ruled out.11 Thus, the
allyl ligands of 1 bind to the metal centers in a bridging trans
μ2-η1:η1 fashion (B, Scheme 1; Figure 1b), resulting in a three-
dimensional network.12 This is the first example of an allyl zinc
compound showing a type B coordination mode.13 In contrast
to earlier interpretations, the metal−carbon bonds in 1 in the
solid state are purely σ in nature without any significant π-type
contributions.
To further explore the nature of zinc−allyl interactions,
quantum-chemical ab initio calculations were performed on two
model systems: (a) monomeric bis(allyl)zinc (Figure 2) and
Figure 2. Structure of bis(η1-allyl)zinc optimized at the MP2/aug-cc-
pVDZ level of theory.
(b) a cluster of three monomers (Figure 3). The structure of
bis(allyl)zinc obtained at the MP2/aug-cc-pVDZ level of theory
exhibits two η1 bonded allyl ligands (Figure 2). The optimized
structure has an approximate C2-symmetry with a Zn−C bond
length of 1.958 Å, which falls into the range of the
corresponding values obtained for dialkylzinc compounds by
rotational spectroscopy14 and electron diffraction (1.930(2)−
1.974(3) Å).15 The C−Zn−C angle amounts to 175.7°, and an
NBO analysis shows that the Zn atom binds to the alkyl groups
via sp-type hybrids. The average distances between the zinc
atom and the doubly bonded carbon atoms are 2.806 and 3.707
Å, respectively. According to the NBO analysis, there are no
bonding interactions between the allyl π-bonds and the metal
atom. Moreover, within the framework of the NBO analysis,
neither the metal 3d-like orbitals nor those of higher principal
quantum number contribute significantly to the Zn−C σ-bonds
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interaction energies between the bonding orbitals of the allylic
CC bonds and lone pair type orbitals at the metal atom
(Lp*) range from −36.3 kcal·mol−1 (Zn(I)···CC(II,III)) to
−27.0 kcal·mol−1 (Zn(II)···CC(I)). In contrast, interactions
of the antibonding MOs of these CC bonds and occupied
metal orbitals can be neglected; that is, there is no significant
d(Zn) → π*(CC) back-donation.
104.11(2)−117.66(8)°). Each chlorine atom and each 2-
methylallyl ligand bridge two zinc centers. Notably, the 2-
methylallyl ligand adopts a cis μ2-η1:η1 coordination mode. This
is the first example of an allyl zinc complex showing a type B
bonding mode of cis configuration. Thus, as in the case of
bis(allyl)zinc (1), Zn−C interactions in compound 2 are also
exclusively of σ-type. Taken together, the results obtained for 1
and 2 represent a rare case of two closely related compounds to
exhibit a cis and a trans configuration of a type B coordination
mode.19 The Zn−C bond lengths in 2 are all very similar,
ranging from 2.071(2) Å to 2.143(2) Å, which is slightly longer
than the corresponding values reported for zinc compounds
with allylic ligands in η1-coordination modes.8a,b C−C distances
in the allylic part of the 2-methylallyl ligand (1.380(3)−
1.421(3) Å) are between those expected for C−C single and
double bonds. All these parameters are in good agreement with
the μ2-η1:η1-coordination mode.
Our computational results concur with the experimental
findings mentioned above in that Zn−C σ-bonding is
energetically strongly favored over Zn···C π-bonding. η2-
Coordination of the metal atom to the localized double
bonds of the allyl ligands was observed as a secondary effect.
Further calculations at the HF level were carried out for model
compounds involving coordination modes such as C and D (cf.
Scheme 1). The results of these calculations are not discussed
here in detail, but suffice it to mention that the model
compounds collapse to structures featured by Zn−C σ-bonding
when all structural constraints are lifted in the optimization
process (see Experimental Section).
In retrospect, earlier misinterpretations regarding the
coordination modes in 1 and 2 in the solid state can be
rationalized, as the formerly postulated η3 and the newly
detected μ2-η1:η1 bonding mode are indistinguishable by means
of solid state CP MAS 13C NMR analysis.20
Apart from bis(allyl)zinc (1), (2-methylallyl)zinc chloride
(2) has also previously been studied by solid state CP MAS 13
C
NMR analysis. On the basis of this data, 2 was reported to
exhibit an η3-bonding mode in the solid state.8c In light of our
new data on 1, we set out to reinvestigate the solid state
structure of 2. Single crystal X-ray analysis revealed that 2
crystallizes in the monoclinic space group C2/c with Z = 4. It
arranges as a cyclic hexamer in the solid state with one formula
unit of 2 corresponding to a monomeric unit (Figure 4).18
Reaction of bis(allyl)zinc (1) with 1 equiv of benzophenone
led to the selective formation of the monoinsertion product
[Zn(C3H5)(OC(C3H5)Ph2)] (3). Allyl alkoxide 3 offers the
possibility to investigate the bonding mode of the allyl ligand at
a zinc center in the presence of an anionic ligand with olefin
and aryl groups as additional donors. NMR spectroscopic
analysis of THF solutions of 3 revealed an η1-coordination of
the allyl ligand at ambient temperature. In contrast, allyl ligands
without substituents in the α- and γ-positions usually show
fluxional behavior in solution at ambient temperature, when
bonded to zinc centers, as shown for 1, [Zn(CH2C(CH3)-
CH2)2],9d and 2 (see the Experimental Section). In compound
3, the bulky, strong σ-donor (OC(C3H5)Ph2)− significantly
slows down the dynamic process which the allyl ligands
undergo in solution. The solid state structure of compound 3
was investigated by means of single crystal X-ray analysis. 3
crystallizes in the monoclinic space group C2/c with Z = 4. It
arranges as a dimer in the solid state with two Zn and two O
atoms forming an essentially planar, rhomboidal unit (Figure
5). This arrangement causes severe distortions of the
tetrahedral coordination geometry around the Zn centers (C/
O−Zn1−C/O, 82.2(7)−135.2(1)°). The allyl ligand in 3
exhibits localized C−C bonds and coordinates to the zinc
center in an η1 fashion. The Zn1−C1 distance compares well to
corresponding values in other zinc complexes bearing η1 bound
allylic ligands.8a,b The most important point concerning the
molecular structure of 3 centers around the question which
ligand occupies the fourth coordination site of the zinc atom.
Hypothetically, an η3-bonding mode of the allyl ligand would
have increased the coordination number of the zinc center to
four. However, coordination of the olefinic moiety of the
(OC(C3H5)Ph2)− ligand is realized instead. This is only the
second structurally authenticated example of a zinc atom
interacting with a nonconjugated CC double bond.21 In
compound 3, a rare case of a Zn−Colefin interaction is preferred
over an η3-coordination mode of the allyl ligand to increase the
coordination number of the metal center to four. Zn−Colefin
distances of 2.619(4) Å and 2.741(4) Å are found in 3, which
compares well to values in [Zn(C(Me)CH2)2]∞.22
Figure 4. Molecular structure of (2-methylallyl)zinc chloride (2): (a)
top view on cyclic hexameric arrangement; (b) side view on
monomeric unit. Displacement ellipsoids are shown at the 50%
probability level. Hydrogen atoms and a molecule of Et2O which is
found per hexamer and does not interact with 2 are omitted for clarity.
Three monomeric units of 2 are crystallographically independent.
Selected bond lengths (Å) and angles (deg): Zn1−C1, 2.071(2);
Zn1−C11, 2.103(2); Zn2−C3, 2.141(2); Zn2−C5, 2.071(2); Zn1−
Cl2, 2.3597(5); Zn2−Cl2, 2.3127(6); C1−C2, 1.419(3); C2−C3,
1.381(3); C2−C4, 1.507(3); C1−C2−C3, 124.3(2). Primed atoms are
related by symmetry code [−x, −y, −z].
Monomeric units form six membered rings which adopt slightly
distorted chair conformations and are linked via zinc atoms in a
spiro fashion. The six zinc atoms of a hexamer are located in
one plane (largest deviation: 0.0998(2) Å). The chlorine atoms
are found on top of and below this plane in an alternating
manner. The same holds for the methyl groups of the 2-
methylallyl ligands. The zinc atoms in 2 show a slightly
distorted tetrahedral coordination geometry, each interacting
with two chlorine and two allyl ligands (C/Cl−Zn−C/Cl,
Coordination modes of the allyl ligand in organozinc
compounds may be compared to those in the related allyl
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intermolecular mechanism for the allyl exchange of [Zn-
(C3H5)2] (1) in THF.9f Our results now offer an explanation
for these experimental observations: as the η3-coordination
mode of the allyl ligand should be disfavored in allylzinc
compounds, the allyl exchange probably proceeds in an
intermolecular fashion. For this type of exchange process, we
suggest a transition state involving a μ2-η1:η1-coordination
mode resembling the solid state structure of 1. A transition
state with two (μ2-η1:η1-C3H5)− groups would be symmetry
forbidden.25
3. CONCLUSION
Allyl zinc compounds exhibit zinc−allyl bonding modes with σ-
type contributions in the solid state, in solution, and in the gas
phase. Notably, no indications for π-type zinc−allyl interactions
have been detected. Instead, two new bonding modes of the
allyl ligand in the coordination chemistry of zinc (cis and trans
μ2-η1:η1) have been authenticated. This suggests modifications
of earlier interpretations8c,9f and revises the conventional
picture of interactions of zinc centers with small π-electron
systems. Our results could contribute to the understanding of
the differences in reactivity, chemoselectivity, and stereo-
selectivity between allyl zinc compounds and allylation reagents
based on other Lewis-acidic metals such as Li, Mg, Al, or Cu.
Figure 5. Lewis drawing (a) and molecular structure (b) of the dimer
of [Zn(η1-C3H5)(OC(C3H5)Ph2)] (3). Displacement ellipsoids are
shown at the 50% probability level. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Zn1−C1,
1.976(4); Zn1−C6, 2.741(4); Zn1−C7, 2.619(4); Zn1−O1, 1.975(2);
C1−C2, 1.485(7); C2−C3, 1.280(8); C4−O1, 1.429(5); C4−C5,
1.544(6); C5−C6, 1.502(6); C6−C7, 1.325(6); C1−Zn1−center-
(C6−C7), 107.0(1); C1−Zn1−O1, 134.9(1); C1−Zn1−O1′,
135.2(1); center(C6−C7)−Zn1−O1, 82.2(7); center(C6−C7)−
Zn1−O1′, 101.0(8); O1−Zn1−O1′, 82.7(1); Zn1−O1−Zn1′,
97.3(1); C1−C2−C3, 129.9(5); C5−C6−C7, 124.9(4). Primed
atoms are related by symmetry code [−x, −y, −z].
4. EXPERIMENTAL SECTION
General Considerations. All operations were carried out under
argon using standard Schlenk-line and glovebox techniques. Starting
materials were purchased from ABCR Chemicals or Sigma Aldrich and
purified following standard laboratory procedures. Nondeuterated
solvents were purified using an MB SPS-800 solvent purification
system. THF-d8 was distilled from sodium benzophenone ketyl. All
NMR spectra were recorded at ambient temperature using a Bruker
compounds [KR], [CaR2], [ScR2]+, and [GaR2]+ (R =
(C3H5)−, (C3H3(SiMe3)2)−). For such compounds containing
group 1−3 metals, η3 bound allyl groups are found
predominantly.6d,e,f,23 In contrast, no η3-coordination of the
allyl ligand to a zinc or a gallium center has been structurally
authenticated to date.24 These differences are most likely due to
a significant stabilization of the η3-bonding mode of the allyl
ligand by πallyl → dmetal interactions and the strongly
electropositive nature of group 1−3 metals.
The insights we gained from the coordination chemistry of
allyl zinc compounds also contribute to a better description of
the dynamic solution behavior of these complexes. The allyl
ligands of allylzinc (halide) compounds undergo rapid
exchange in solution at ambient temperature. An intramolecular
mechanism involving an η3 bonded allyl ligand and an
intermolecular mechanism without allyl ligands in an η3-
coordination mode has been discussed in the literature for this
exchange (Scheme 2).8c,9f Although an intramolecular reaction
can be expected to proceed faster, kinetic studies revealed an
1
Avance II 400 MHz spectrometer. The chemical shifts of H and 13C
NMR spectra were referenced internally using the residual solvent
resonances and are reported relative to the chemical shift of
1
tetramethylsilane. The resonances in H and 13C NMR spectra were
assigned on the basis of two-dimensional NMR experiments (COSY,
HMQC, HMBC).
[Zn(η1-C3H5)2] (1). 1 can be prepared according to the literature9a,c
or following an alternative route: ZnBr2 (1.380 g, 6.13 mmol) was
suspended in diethyl ether (20 mL). Allyl potassium (982 mg, 12.25
mmol) was slowly added at ambient temperature to give a suspension.
The liquid phase was separated by centrifugation and decantation after
30 min. The residue was extracted with diethyl ether (20 mL). All
volatiles were removed from the combined liquid phases under
reduced pressure to give a yellow solid which was dried in vacuo for
3.5 h. Yield: 774 mg (525 mmol), 86%. NMR data was consistent with
the literature. Anal. Calcd for C6H10Zn (147.55 g/mol): Zn, 44.33.
Found: Zn, 44.06. A test for halides was negative. The crude product
can be further purified by sublimation. Single crystals were obtained by
sublimation at 35 °C under a dynamic vacuum of 5 × 10−2 mbar.
[Zn(C4H7)Cl(Et2O)0.3]·2(Et2O)0.3. The compound was synthesized
following a procedure slightly modified from that in the literature.9f No
analytical data for 2 was reported in the literature. A solution of
Zn(CH2C(CH3)CH2)2 (131 mg, 0.75 mmol) in diethyl ether (1.0
mL) was added to a solution of ZnCl2 (102 mg, 0.75 mmol) in diethyl
ether (2.0 mL). All volatiles were removed from the reaction mixture
under reduced pressure to give a pale yellow solid which was washed
with pentane (3 × 1 mL) and dried in vacuo. Substoichiometric
amounts of diethyl ether could not be removed by recrystallization or
prolonged drying in vacuo.26 Yield: 240 mg (0.60 mmol), 81%. Single
crystals were obtained by cooling a saturated solution of 2 in diethyl
ether/pentane (1:1) to −30 °C.
Scheme 2. (a) Intramolecular Allyl Exchange Mechanism via
the η3 Transition State; (b) Intermolecular Allyl Exchange
Mechanism via the μ2-η1:η1 Transition State
1H NMR (400.1 MHz, THF-d8). δ = 1.11 (t, 3JHH = 7.0 Hz, 0.3 × 6H,
O(CH2CH3)2), 1.71 (s, 3H, CH2C(CH3)CH2), 2.71 (br s, 4H,
3
CH2C(CH3)CH2), 3.38 (q, JHH = 7.0 Hz, 0.3 × 4H, O(CH2CH3)2)
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ppm. 13C NMR (100.6 MHz, THF-d8): δ = 15.79 (s, O(CH2CH3)2),
26.41 (s, CH2C(CH3)CH2), 66.42 (s, O(CH2CH3)2), 116.20 (br s,
CH2C(CH3)CH2), 152.46 (s, CH2C(CH3)CH2) ppm. Anal. Calcd for
C4H7OClZn·(C4H10O)0.3 (178.17 g/mol): Zn, 36.70. Found: Zn,
36.75.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic information files (cif) for compounds 1, 2, and
3. This material is available free of charge via the Internet at
http://pubs.acs.org.
[Zn(C3H5)(OC(C3H5)Ph2)] (3). A solution of benzophenone (32
mg, 0.18 mmol) in diethyl ether (1 mL) was added to a solution of 1
(26 mg, 0.18 mmol) in diethyl ether (1 mL). All volatiles were
removed from the reaction mixture under reduced pressure to give a
colorless solid which was washed with pentane (2 × 1 mL) and dried
in vacuo. Yield: 48 mg (0.15 mmol), 83%. Single crystals were
obtained by cooling a saturated solution of 3 in diethyl ether/pentane
(1:1) to −30 °C.
AUTHOR INFORMATION
■
Corresponding Author
jun.okuda@ac.rwth-aachen.de
Notes
The authors declare no competing financial interest.
3
1H NMR (400.1 MHz, THF-d8). δ = 0.72 (d, JHH = 8.8 Hz, 2H,
3
Zn−CH2−CHCH2), 3.15 (d, JHH = 7.1 Hz, 2H, C−CH2−CH
ACKNOWLEDGMENTS
■
2
3
CH2), 4.11 (dd, JHH = 2.9 Hz, JHH = 9.8 Hz, 1H, Zn−CH2−CH
CHcisHtrans), 4.23 (ddm, 2JHH = 2.9 Hz, 3JHH = 16.8 Hz, 1H, Zn−CH2−
C.L. is grateful for a Kekule
the Fonds der Chemischen Industrie.
́
scholarship generously provided by
3
CHCHcisHtrans), 4.92 (dm, JHH = 17.0 Hz, 1H, C−CH2−CH
3
CHcisHtrans), 4.95 (dm, JHH = 17.0 Hz, 1H, C−CH2−CH
3
3
3
CHcisHtrans), 5.50 (ddt, JHH = 8.8 Hz, JHH = 9.8 Hz, JHH = 16.8
REFERENCES
■
Hz, 1H, Zn−CH2−CHCH2), 5.67 (ddt, 3JHH = 7.1 Hz, 3JHH = 10.2
(1) (a) Schlosser, M.; Desponds, O.; Lehmann, R.; Moret, E.;
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3
3
Hz, JHH = 17.0 Hz, 1H, C−CH2−CHCH2), 7.14 (tm, JHH = 7.3
Hz, 2H, p-Ph), 7.22−7.26 (m, 4H, m-Ph), 7.40−7.43 (m, 4H, o-Ph)
ppm. 13C NMR (100.6 MHz, THF-d8): δ = 20.61 (s, Zn−CH2−CH
CH2), 48.70 (s, C−CH2−CHCH2), 80.81 (s, C−CH2−CHCH2),
103.38 (s, Zn−CH2−CHCH2), 119.48 (s, C−CH2−CHCH2),
127.08 (s, p-Ph), 127.82 (s, o-Ph), 128.54 (s, m-Ph), 137.86 (s, C−
CH2−CHCH2), 144.26 (s, Zn−CH2−CHCH2), 150.20 (s, ipso-
Ph) ppm. Anal. Calcd for C19H20OZn (329.77 g/mol): Zn, 19.83.
Found: Zn, 19.97.
Single Crystal X-ray Crystallography. Diffraction measurements
were performed on a Bruker AXS diffractometer equipped with an
Incoatec microsource and an APEX area detector using Mo Kα
radiation. Data reductions were performed with the Bruker SAINT
software. Structure solution (SIR-92) and refinement (SHELXL-97 as
implemented in the WinGX program27 system) for 2 and 3 were
unexceptional. 1 crystallizes as a merohedral twin as indicated by a
very low ⟨E2 − 1⟩ value of ca. 0.6.
Theoretical Calculations. All quantum-chemical calculations were
carried out with the Gaussian09 suite of quantum-chemical routines28
running on the facilities of the Computing and Communication
Center of the RWTH Aachen University. All model compounds under
consideration were preoptimized at the Hartree−Fock level (HF) with
the split valence 3-21G29 set of contracted Gaussian functions. Since
this basis set contains neither polarization nor diffuse functions for
some of the elements contained in the molecules under study, we
considered the resulting structures merely as starting points for further
optimization at higher levels of theory. Thus, additional HF
calculations were performed by employing the correlation-consistent
aug-cc-pVDZ basis set,30 which is also doubly split in the valence
orbitals but which also has polarization and diffuse functions for all the
elements of our model compounds. Final geometry optimizations for
the model compounds were performed including correlation energy
with Møller−Plesset perturbation theory to the second order (MP2).31
Attempts have also been made to optimize structures of Zn(C3H5)2
monomers featured by metal−π interactions (e.g., in an η3 fashion) at
the MP2/aug-cc-pVDZ level of ab initio theory without previous
optimization at the HF/3-21G level. Initial optimizations were
performed under the constraint that all four distances between the
terminal carbon atoms of the two allyl moieties and the Zn atom are
equal. However, normal coordinate analyses revealed that the resulting
structures were not local minima but saddle points of higher order.
These structures collapsed, resulting in the structure shown in Figure 2
when the constraints were lifted. To arrive at a better understanding of
bonding between the metal atom and the allyl ligands, we also
performed natural bond orbital analyses (NBO)32 for the optimized
structures, employing the NBO method as implemented in the
Gaussian09 program.
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