Conversion between doubly and triply carboxylate bridged Bis(ethylzinc) Complexes and Formation of the (μ-Oxo)tetrazinc Carboxylate [Zn 4O(ArTolCO2)6]

Article abstract of DOI:10.1021/om5000503

Ethylzinc 2,6-bis(p-tolyl)benzoate converts between two forms in solution. Through NMR spectroscopic techniques and X-ray crystallography, the species in equilibrium were identified as [Zn₂(Ar^TolCO ₂)₂(Et)₂(THF)₂] (1), [Zn ₂(Ar^TolCO₂)₃(Et)(THF)] (2), and diethylzinc (Ar^Tol = 2,6-bis(p-tolyl)phenyl). The equilibrium provides a model for understanding the speciation between doubly and triply m-terphenylcarboxylate bridged diiron(II) and mononuclear iron(II) complexes. Evidence is presented for the occurrence of coordinatively unsaturated trigonal zinc species in solution. Both 1 and 2 decompose in air to form the T-symmetric oxozinc carboxylate [Zn₄O(Ar^TolCO₂) ₆] (3).

Full text of DOI:10.1021/om5000503

Article
pubs.acs.org/Organometallics
Open Access on 03/05/2015
Conversion between Doubly and Triply Carboxylate Bridged
Bis(ethylzinc) Complexes and Formation of the (μ-Oxo)tetrazinc
Carboxylate [Zn4O(ArTolCO2)6]
Mikael A. Minier and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ABSTRACT: Ethylzinc 2,6-bis(p-tolyl)benzoate converts between two forms in solution. Through NMR spectroscopic
techniques and X-ray crystallography, the species in equilibrium were identified as [Zn2(ArTolCO2)2(Et)2(THF)2] (1),
[Zn2(ArTolCO2)3(Et)(THF)] (2), and diethylzinc (ArTol = 2,6-bis(p-tolyl)phenyl). The equilibrium provides a model for
understanding the speciation between doubly and triply m-terphenylcarboxylate bridged diiron(II) and mononuclear iron(II)
complexes. Evidence is presented for the occurrence of coordinatively unsaturated trigonal zinc species in solution. Both 1 and 2
decompose in air to form the T-symmetric oxozinc carboxylate [Zn4O(ArTolCO2)6] (3).
he steric bulk of m-terphenylcarboxylates facilitates the
techniques can be employed to probe solution structures. An
Tassembly of biomimetic diiron(II) complexes that act as excellent candidate for such a study is the ethylzinc m-
terphenylcarboxylate [Zn2(ArTolCO2)2(Et)2(THF)2] (1),
models for the active site of soluble methane monooxyge-
nase.1,2 These complexes share the general formula
which we discovered during pursuit of a dizinc analogue of
the biomimetic diiron(II) carboxylate [Fe2(PIM)(ArTolCO2)2]
[Fe2(RCO2)4(L)1−2], where R is an m-terphenyl group and L
(8) (see Figure S12 in the Supporting Information for the
structure of PIM2−).5 NMR spectroscopic experiments revealed
the presence of a dynamic equilibrium between doubly bridged
is a neutral donor (Scheme 1). Within this diiron family,
doubly, triply, and quadruply carboxylate bridged compounds
have been isolated as solids, but the role, if any, of the neutral
ligand L in modulating the number of bridging carboxylates
remains unclear. In particular, the reaction of
[Fe2(ArTolCO2)4(THF)2] (4; ArTol = 2,6-bis(p-tolyl)phenyl)
with N,N-dimethylethylenediamine (N,N-Me2en) produces a
mixture of doubly bridged [Fe2(ArTolCO2)4(N,N-Me2en)2] (5),
triply bridged [Fe2(ArTolCO2)4(N,N-Me2en)] (6), and [Fe-
(ArTolCO2)2(N,N-Me2en)2] (7), all characterized in the solid
1
and the triply carboxylate bridged complex
[Zn2(ArTolCO2)3(Et)(THF)] (2), which forms together with
diethylzinc (Et2Zn) in solution (Scheme 1). This equilibrium
may be relevant to related diiron carboxylate complexes such as
4−7. Compound 1 can lose its coordinated THF molecules,
and evidence for a THF-free species having trigonal-planar zinc
centers is also presented here. We further describe the oxozinc
m-terphenylcarboxylate complex [Zn4O(ArTolCO2)6] (3),
formed in air by decomposition of 1 or 2, which has an
interesting structure of T symmetry constructed by alignment
of Th and Td local symmetries.
state (Scheme 1).3
A
19F NMR spectroscopic study of a
fluorinated analogue of 4 revealed interconversion between
doubly and quadruply bridged forms,4 for which triply bridged
intermediates were proposed.1,3,4 The speciation of these
diiron(II) complexes in solution during the oxidation of
tethered substrates remains unclear, however, and is important
for understanding their reactivity.
One approach to studying the solution dynamics of m-
terphenylcarboxylate-bridged diiron(II) complexes is through
parallel work on the redox-stable dizinc(II) analogues. With the
use of diamagnetic zinc complexes, NMR spectroscopic
Ethylzinc carboxylate 1 was prepared by addition of 1 equiv
of Et2Zn to a solution of ArTolCO2H in THF. X-ray diffraction
quality crystals of 1 formed directly from the reaction mixture,
and the solid-state structure was determined (Figure 1). In 1,
Received: January 16, 2014
Published: March 5, 2014
© 2014 American Chemical Society
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Scheme 1. (A) Diiron ArTolCO2 Complexes and (B)
Equilibrium among 1, 2, and Et2Zn
−
on alkylzinc carboxylates dates back to the 1960s, and in 1974 it
was demonstrated that the simple ethylzinc carboxylates of
acetate, trifluoroacetate, and benzoate have increased reactivity
toward methanol in comparison to Et2Zn itself.10 Although
aggregation in solution was described, structures of the simple
ethylzinc carboxylates with acetate and benzoate reported in
recent years display large variations in nuclearity and overall
structure. Ethylzinc acetate exists as [Zn5(CH3CO2)6(Et)4] (9)
in C6H6 or toluene and as [Zn2(CH3CO2)2(Et)2] (10) in
THF.7,8 Additional solution studies revealed that these two
motifs convert upon heating in an appropriate solvent.
Ethylzinc benzoate also has two forms. From noncoordinating
solvents,
a hexazinc barrel-shaped compound,
[Zn6(PhCO2)6(Et)6] (11),6 is obtained with a carboxylate-to-
ethyl ratio of 1:1. From THF, a dizinc complex,
[Zn2(PhCO2)3(Et)(THF)] (12),9 crystallizes, with a ratio of
3:1. One report of an ethylzinc m-terphenyl carboxylate,
[Zn2(ArMesCO2)2(Et)2] (13),11 is particularly interesting
because it features two trigonal zinc centers (ArMes = 2,6-
bis(mesityl)phenyl). The structure of 1 differs from that of 13
in that the latter has an eight-membered ring in a boat
conformation (Figure 1), trigonal-planar zinc, and a Zn···Zn
1
distance of 3.58 Å. The H NMR spectrum of 13 in C6D6
supports the persistence of this structure in solution.11
A
summary of the carboxylate to ethyl ratios of the simple
ethylzinc carboxylates is presented in Table S2 (Supporting
Information).
When crystals of 1 are dissolved in C6D6 or toluene-d8, more
than one species is observed in the 1H NMR spectrum (Figure
2 and the Supporting Information). In the aromatic region, two
Figure 1. X-ray crystal structure of 1 (top) with thermal ellipsoids at
50% probability and the chair and boat conformations of 1 and 13
(bottom), respectively. Hydrogen atoms are omitted for clarity.
1
Figure 2. Upfield region of the H NMR spectrum (500 MHz) of 1
dissolved in C6D6. Labels A−C are used to distinguish between the
three species present in equilibrium. The ethyl resonances at 3.26 and
1.11 ppm correspond to a minor zinc ethoxide impurity.
the two carboxylates bridge in a μ-1,3 mode, creating an eight-
membered ring with a Zn···Zn distance of 4.06 Å. The ring
adopts a chair conformation, with the ethyl groups and the
THF molecules trans to each other across the ring, as required
by a crystallographic inversion center in the middle (Figure 1).
Further details about the crystal structure, including selected
bond distances and angles, are provided in the Supporting
Information (Table S4).
Compound 1 is a member of the well-known class of
alkylzinc carboxylates, interest in which has increased in recent
years because of their use as starting materials for oxozinc
carboxylates and in polymerization catalysis.6−9 Original work
sets of peaks from the 2,6-bis(p-tolyl)phenyl groups are
present, but the signals are not fully resolved. Resolution is
obtained for the two tolyl methyl groups (2.08 and 1.97 ppm)
and two ethyl groups (−0.28/1.28 and 0.17/1.23 ppm for the
CH2/CH3 resonances, respectively). The upfield CH2 reso-
nances suggest that both ethyl groups are bound to zinc. Large,
broad THF resonances occur at 3.49 and 1.37 ppm. Another
set of broad resonances, appearing at 2.80 and 1.14 ppm, is also
assigned as THF. Because a chemical shift of 2.80 ppm is
unusual for the OCH2 resonance of THF, 1 was prepared from
THF-d8 and its 1H NMR spectrum was acquired. The
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disappearance of peaks at 3.49, 2.80, 1.37, and 1.14 ppm
confirmed the THF assignments (Figure S2, Supporting
Information). Heating the mixture in toluene-d8 to 100 °C
results in coalescence to a single species (Figure S3, Supporting
Information), which suggests that the compounds exchange
ligands, are in equilibrium, or both.
The species with methyl group resonances at 2.08 and 1.97
ppm are hereby referred to as A and B, respectively. Adding
THF has an effect on the speciation of A and B (Figure S4,
Supporting Information). Resonances corresponding to B and
the ethyl group at −0.28/1.28 ppm decrease in intensity with
addition of increasing amounts of THF and are most likely the
same species. The THF resonances at 2.80/1.14 ppm also
disappear, presumably due to increased exchange with excess
THF. The methyl resonance of species A increases and shifts
from 2.08 to 2.14 ppm upon addition of 50 equiv of THF, and
the broad Et2Zn ethyl group CH2 resonance shifts from 0.17 to
0.27 ppm. The changes upon addition of THF suggest a shift in
equilibrium from B toward A.
radius of 6.38 Å is consistent with the formulation of 1 (RX‑ray
6.47 Å), supporting the assignment of A as 1.
=
The equilibrium described above shifts upon removal of
Et2Zn from solution under vacuum. When crystals of 1 are
dissolved in toluene, stripped to dryness, and redissolved in
1
deuterated solvent, the H NMR spectrum in C6D6 reveals the
presence of species B, for which the THF and ethyl group
resonances can now be confidently assigned. The carboxylate/
ethyl/THF ratio in B is 3:1:1. This ratio is the same for 12
(Table S2, Supporting Information) and supports the
formulation of B as [Zn2(ArTolCO2)3(Et)(THF)] (2). Prep-
aration of 2 in bulk quantities and its subsequent crystal growth
by slow diffusion of pentane into a benzene solution of the
compound produces colorless plates suitable for X-ray
diffraction. The structure confirms the assigned formula (Figure
4). The molecule is a triply m-terphenylcarboxylate bridged
To gain a deeper insight into the nature of the species
present, DOSY spectra were recorded at different concen-
trations of THF. In C6D6 or toluene-d8 and with no extra THF
added, three species are observed: m-terphenyl carboxylate
complexes A and B and another having an ethyl group, C
(Figure 3 and Figure S9 (Supporting Information)). Species C
Figure 4. Structure of a single isomer of 2 with 50% thermal ellipsoids.
Hydrogen and disordered atoms are omitted for clarity.
dizinc compound with a Zn···Zn distance of 3.32 Å. This value
is smaller than that in 1, as expected following the addition of a
third bridging carboxylate. A pseudo-C3 symmetry axis exists
along the Zn−Zn vector, which, in conjunction with the lack of
a collinear improper axis of rotation, produces Λ and Δ
isomers, both of which occur in the crystal structure (Figure
S13, Supporting Information). The DOSY NMR spectrum of 2
was also obtained, and the calculated hydrodynamic radius of
7.81 Å is larger than RX‑ray (7.20 Å) but still consistent with
retention of this structure in solution. With the formula of the
species in solution now in hand, a balanced equation (eq 1)
describing the solution equilibrium can be written.
Figure 3. DOSY NMR spectrum (400 MHz) of 1 dissolved in
toluene-d8. Species A−C and THF are indicated by colored lines.
has a calculated hydrodynamic radius (RH) of 3.31 Å, which is
close to, but larger than, the X-ray-determined radius (RX‑ray) of
Et2Zn (3.14 Å), calculated from the crystal structure of Et2Zn.12
Because we expect Et2Zn to coordinate THF under these
conditions, an RH value larger than RX‑ray is reasonable. Isolated
Et2Zn in C6D6 resonates at 0.10 and 1.08 ppm. The slight
deviation of the resonances in our system from the values for
Et2Zn, 0.17 and 1.23 ppm, may indicate a conversion to the
THF adduct or an exchange process. As the amount of THF is
increased, the diffusion coefficient of C decreases (Table S1 and
Figures S9 and S10, Supporting Information). This observation
implies that Et2Zn is in equilibrium with larger species in
solution, such as A and/or B. The possibility that Et2Zn self-
associates into oligomers cannot be ruled out. Because Et2Zn is
on the same side of the equilibrium as B (see the following
paragraphs), and the proportion of B decreases with increasing
amounts of THF, there must be an ethyl group associated with
A. Thus, the observed ethyl group resonance of C is a mixture
of species A and Et2Zn. As A becomes the dominant species in
solution with increasing THF, the calculated hydrodynamic
To further test the presence of this equilibrium in solution as
well as its reversibility, Et2Zn and THF were added to isolated 2
in an attempt to produce 1. Addition of THF to a sample of 2
in C6D6 does not afford any 1, and the only observable change
in the 1H NMR spectrum is a broadening of the THF
resonance of 2 owing to exchange with free THF (Figure S8,
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Supporting Information). When Et2Zn is added to a solution of
2 in C6D6, however, 1 is clearly detectable along with a THF
OCH2 resonance at 3.30 ppm (Figure 5 and Figure S8). This
temperature. The reason for this difference may be a higher
energy barrier associated with formation and disassembly of 9,
owing to its tightly packed pentazinc cluster core. Thus,
modification of the carboxylate R group in ethylzinc
carboxylates may change the kinetics of structure interconver-
sion as well as the nuclearity of the corresponding complex.
Understanding these simple zinc compounds can provide
insight into the manner by which carboxylates can be used
effectively for producing well-defined metal complexes.
Upon exposure to air, compounds 1 and 2 decompose into
the oxozinc carboxylate [Zn4O(ArTolCO2)6] (3). Colorless
crystals of 3 were obtained by allowing an NMR spectroscopic
sample of 1/2 in C6D6 to slowly react with air over the course
of 1 week. Compound 3 crystallizes in the rhombohedral space
group R3 with one-sixth of a molecule in the asymmetric unit.
̅
The six m-terphenyl wings align with Th symmetry (Figure 6).
Figure 5. Upfield region of the 1H NMR spectrum (500 MHz) of 2 in
C6D6 upon addition of Et2Zn. The red arrows indicate the presence of
1. The peaks between 0.8 and 1.0 ppm and an underlying multiplet at
1.19−1.23 ppm correspond to hexane and methylcyclopentane
impurities in the Et2Zn solution.
experiment proves that extra THF is not required for the
conversion of 2 to 1. Integration of the methyl and THF OCH2
resonances in 2 reveals that 2 retains all its bound THF. On the
basis of the stoichiometry of the balanced equation between 1
and 2, only two-thirds of a THF molecule is available per
molecule of 1. It therefore appears that THF-free 1,
[Zn2(ArTolCO2)2(Et)2] (1′), may exist in solution. In
combination with DOSY experiments that confirm the
dinuclearity of these species in solution and the crystal
structure of 13, 1′ most likely contains two coordinatively
unsaturated trigonal-planar zinc centers. Generation of such
potentially reactive zinc centers suggests that zinc m-
terphenylcarboxylates might be good catalysts. Addition of
both THF and Et2Zn to a sample of 2 in C6D6 restores the 1H
NMR spectrum to that of a mixture of 1 and 2 similar to that
obtained upon dissolution of crystals of 1 in C6D6 or toluene-d8
(Figure S8). Again, THF is not required for the conversion of 2
to 1, although it promotes the conversion between 1′ and 1.
The conversion between solvated species of 1,
[Zn2(ArTolCO2)2(Et)2(THF)0−2], probably occurs through
simple association and dissociation of THF molecules.
However, the mechanism of conversion of 1 to 2 is still
unclear and more than one pathway may be involved. The
demonstrated conversion between doubly and triply carbox-
ylate bridged species 1 and 2 depends on the amount of THF
present. The diiron complexes 5−7 represent analogues of 1, 2,
and Et2Zn, respectively, where the ethyl groups are replaced by
Figure 6. (top) Stereographic projections of Td (red) and Th (blue)
symmetry and their superposition which preserves the elements of T
symmetry (purple). (bottom) Td core structure of 3 with the ArTol
groups removed for clarity (left) and space-filling diagram of 3
showing the Th shell created by the ArTol groups (right). Disordered
atoms are removed for clarity.
The Zn4O core has Td symmetry, however, and the alignment
of Th and Td symmetries limits the symmetry of 3 to T. This
concept can be visualized with stereographic projections shown
in Figure 6. The superposition of Th and Td symmetries to
produce T symmetry in a molecule is interesting. None of the
60 T-symmetric molecules contained in the CSDSymmetry
database13 provide similar examples. The two enantiomers in
the crystal lattice are related by the crystallographic inversion
center. Variations of the orientation of the tolyl groups show
that the symmetry in the solid state is only pseudo-T-
1
symmetric. In the H NMR spectrum of 3 in CD2Cl2, the
tolyl CH3 resonance is observed at 1.43 ppm, which is 1 ppm
upfield from that of the free carboxylate. The large shift is
attributed to positioning of the methyl groups above a
neighboring m-terphenyl benzene ring and is consistent with
the crystal structure. Because a single set of resonances is
observed in the NMR spectrum, the molecule must have T
symmetry on average in solution at 25 °C.
This report reveals that that two formulations of ethylzinc
2,6-bis(p-tolyl)benzoate, 1 and 2, readily convert in solution.
This behavior differs from that of the ethylzinc acetate
complexes, which require heat to convert them between 9
and 10. With the knowledge of the solution equilibrium among
ArTolCO2 and THF is replaced by N,N-Me2en. Thus, the
−
equilibrium among 1, 2, and Et2Zn provides insight into why
iron complexes 5−7 could be isolated from the same reaction
mixture. These equilibria are crucial when considering
oxidation reactions supported by diiron complexes in solution,
because a species different from that observed in the solid-state
structure may be responsible for the reactivity.
Unlike the interconversion between ethylzinc carboxylate
compounds 9 and 10, which occurs at elevated temperatures,8
interconversion between 1 and 2 occurs readily at room
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1, 2, and Et2Zn, we can now propose conversion of doubly and
triply carboxylate bridged diiron(II) and mononuclear iron(II)
complexes, such as those observed in discrete solid-state
complexes 5−7. Furthermore, the use of sterically demanding
m-terphenyl substituents provides access to coordinatively
unsaturated zinc centers, offering a possible strategy for use
in zinc-catalyzed reactions.
AUTHOR INFORMATION
Corresponding Author
*E-mail for S.J.L.: lippard@mit.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation for a Graduate
Research Fellowship to M.A.M. under Grant No. (1122374)
and the National Institute of General Medical Sciences for a
grant (R01-GM032114 to S.J.L.) for financial support.
EXPERIMENTAL SECTION
■
General Considerations. Diethylzinc (1 M in heptane or
hexanes) was purchased from Aldrich and used as received. A solution
of Et2Zn in toluene-d8 for NMR spectroscopic experiments was
prepared by adding toluene-d8 to Et2Zn in hexanes and distilling off
the hexanes. Solvents were saturated with argon, passed through two
columns of activated alumina, and stored over activated 3 or 4 Å
molecular sieves. The compound 2,6-bis(p-tolyl)benzoic acid
(ArTolCO2H) was prepared by a literature procedure.14 All
manipulations of compounds 1 and 2 were performed under a
nitrogen atmosphere in an MBraun drybox. IR spectra were obtained
on a ThermoNicolet Avatar 360 spectrometer using the OMNIC
software. Details about NMR spectroscopy and X-ray data collection
and refinement are provided in the Supporting Information.
REFERENCES
■
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Synthesis. [Zn2(ArTolCO2)2(Et)2(THF)2] (1). ArTolCO2H (50.8 mg,
168 μmol) was dissolved in 1 mL of THF. Diethylzinc (170 μL, 1 M in
heptane) was injected into the reaction mixture with stirring. After 20
s, stirring was stopped and the solution was allowed to sit overnight,
forming colorless crystals of 1. The solution was decanted, and the
crystals were washed twice with 1 mL of pentane before drying under
vacuum to yield 67.1 mg (71.7 μmol, 85.4%) of 1. NMR: compound 1
does not exist as the only species in solution; see text for discussion. IR
(KBr): 3055, 3025, 2980, 2924, 2881, 2850, 2808, 1917, 1597, 1515,
1453, 1410, 1381, 1111, 1037, 986, 883, 842, 819, 802, 784, 768, 734,
703, 605, 583, 544, 517 cm−1. Anal. Calcd for C54H60O6Zn2: C, 69.31;
H, 6.46. Found: C, 68.88; H, 6.32. Decomposes above 136 °C (turns
yellow-brown).
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Williams, C. K. Organometallics 2011, 30, 2223−2229.
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Lewinski, J. Inorg. Chem. 2012, 51, 7410−7414.
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Jones, A. C.; Steiner, A. Angew. Chem., Int. Ed. 2011, 50, 11685−
11687.
[Zn2(ArTolCO2)2(Et)2(THF-d8)2] (1·THF-d8). The same procedure was
used for the synthesis of 1, except that THF-d8 was used as the solvent.
Yield: 46.2 mg (48.5 μmol, 57.8%).
(13) Yao, J. W.; Cole, J. C.; Pidcock, E.; Allen, F. H.; Howard, J. A.
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5960.
[Zn2(ArTolCO2)3(Et)(THF)] (2). A sample of 1 (48.0 mg, 51.3 μmol)
was dissolved in 2.5 mL of toluene, and the solvent was removed
under vacuum. The crude material was dissolved in 800 μL of benzene,
and pentane was allowed to slowly diffuse into the solution slowly.
After 5 days, colorless plates of 2 (31.5 mg, 81.2%) were obtained. 1H
NMR (C6D6): δ 7.40 (d, 3JHH = 8.1 Hz, 12H), 7.27 (d, 3JHH = 7.7 Hz,
6H), 7.12 (t, 3JHH = 7.7 Hz, 3H), 7.02 (d, 3JHH = 7.7 Hz, 12H), 2.80 (t,
3JHH = 6.4 Hz, 4H), 1.97 (s, 18H), 1.28 (t, 3JHH = 8.1 Hz, 3H), 1.13 (t,
3
3JHH = 6.6 Hz, 4H), −0.28 (q, JHH = 8.1 Hz, 2H). 13C{1H} NMR
(C6D6): δ 178.2, 140.8, 139.3, 136.7, 136.1, 129.7, 129.4, 128.8, 128.6,
69.55, 25.0, 21.0, 13.1, −2.4. IR (KBr): 3054, 3022, 2920, 2849, 2807,
1727, 1604, 1544, 1515, 1454, 1408, 1385, 1151, 1109, 1033, 1028,
845, 817, 801, 789, 767, 733, 706, 585, 540 cm−1. Anal. Calcd for
C69H64O7Zn2: C, 72.95; H, 5.68. Found: C, 73.33; H, 5.96; N, 0.05.
Decomposes above 246 °C (turns yellow-brown).
Reactivity. Exposure of 1 and 2 to Air. An NMR solution of 1
dissolved in C6D6 was allowed to slowly react with air through a plastic
cap over 1 week. Colorless prisms of [Zn4O(ArTolCO2)6] (3) were
obtained. NMR was used to confirm that the crystals correspond to a
single species in solution. No further attempts were made to isolate the
compound in bulk quantities.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, text, and CIF files giving experimental details,
NMR spectra, and X-ray crystallographic data for 1−3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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