Cobalt complexes of the chelating dicarboxylate ligand esp : A paddlewheel-type dimer and a heptanuclear coordination cluster

Article abstract of DOI:10.1039/c8dt03030h

The coordination chemistry of Co(ii) with the chelating dicarboxylate ligand esp (esp = α,α,α′,α′-tetramethyl-1,3-benzenedipropionate) is explored. We report here the bimetallic paddlewheel-type dimer, Co₂(esp)₂(EtOH)₂ (1), and a bowl-shaped, heptanuclear coordination cluster, Co₇(OH)₄(Hesp)₂(esp)₄(MeCN)₂·4MeCN (2). Crystal structures of both complexes are reported as well as their magnetic properties, which indicate antiferromagnetic interactions among the Co(ii) ions.

Full text of DOI:10.1039/c8dt03030h

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Cobalt Complexes of the Chelating Dicarboxylate Ligand “esp”: A
Paddlewheel-Type Dimer and a Heptanuclear Coordination
Cluster
Received 00th January 20xx,
Accepted 00th January 20xx
Ryan J. Pakula and John F. Berry*
DOI: 10.1039/x0xx00000x
www.rsc.org/
The coordination chemistry of Co(II) with the chelating dicarboxylate ligand esp (esp = α,α,α’,α’-tetramethyl-1,3-
benzenedipropionate) is explored. We report here the bimetallic paddlewheel-type dimer, Co2(esp)2(EtOH)2 (1), and a
bowl-shaped, heptanuclear coordination cluster, Co7(OH)4(Hesp)2(esp)4(MeCN)2·4MeCN (2). Crystal structures of both
complexes are reported as well as their magnetic properties, which indicate antiferromagnetic interactions among the
Co(II) ions.
Introduction
The bimetallic paddlewheel-type structure (I, Chart 1) is a
ubiquitous structural motif for Rh2(II,II) carboxylates (with over
5,000 entries in SciFinder!).1 In comparison, relatively few
bimetallic Co(II) carboxylate compounds adopt this structure
type; molecular examples have been known since the 1970s,2
as well as more recent examples of metal-organic frameworks
(MOF) built upon the Co2-paddlewheel unit.3 The molecular
examples that are known are generally supported by a limited
set of bulky N-donor heterocycles, such as quinoline, that
serve as ligands in the axial sites, whereas the MOF examples
can contain smaller axial ligands. Based on these examples, we
hypothesized that we could use the chelate effect to generate
Co2 paddlewheel-type complexes that could potentially bind
and activate small molecules at the axial sites. A similar idea
has been used in the chemistry of Rh2 carboxylates, where the
Chart
1
Comparison of the dirhodium tetracarboxylate
) with the Co2(esp)2 structure (II).
structure (
I
chelating dicarboxylate ligand “esp” (esp
= α,α,α’,α’-
tetramethyl-1,3-benzenedipropionate) has been found to be
critical for the expansion of intermolecular C–H amination
catalyzed by Rh2 complexes.4 Furthermore, the esp ligand has
been shown to support more stable and typically more soluble
systems than monodentate carboxylates.5
Results and Discussion
Synthesis and Structural Studies
In line with these goals, we report here our preparation of
the first Co2 paddlewheel-type structure supported by the esp
ligand (II) and also a surprising and topologically unique
heptanuclear Co7 cluster.
The Co2(esp)2 paddlewheel unit was successfully prepared
as its ethanol adduct, Co2(esp)2(EtOH)2 (1), in the reaction
between CoCO3 and H2esp in refluxing EtOH (Eq. 1):
2 CoCO3 — x H2O, EtOH
2 H2esp
Co2(esp)2(EtOH)2 + H2O + CO2
1
Eq. 1
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Black crystals suitable for X-ray diffraction are formed Co(II) ions around the periphery of the molecule. Four of the
directly from the reaction mixture; under polarized light, the six peripheral Co(II) ions are six-coordinate, and two are four-
black solid is revealed to show every color of the rainbow, coordinate. The result is a bowl-shaped molecule with a single
depending on the angle of the crystals. The crystals grow in the Co(II) ion at the center of the bowl with six Co(esp) units
ꢂ
around the rim. All of the six-coordinate Co ions are easily
There are two unconnected half units of the Co2(esp)2 identified as having a +2 oxidation state in this structure due to
charge balance considerations, reflected in the Co–L bond
triclinic space group ꢀꢁ as twins with a 180° rotation about b*.
structure per asymmetric unit, with an inversion center
bisecting each Co–Co vector. The Co–Co distances are distances in Table S3. Additionally, the protonation states of
the esp ligands are clear: all the dianionic esp2– ligands are
2.7245(6) and 2.7595(7) Å, quite long in comparison to
either bridging or chelating, while the monoanionic Hesp–
compounds containing genuine Co–Co bonds. Co–Co distances
ligands form hydrogen bonding interactions to a neighboring
carboxylate group with O···O distances of 2.57 – 2.60 Å. The
bowl shape of the structure is further supported by weak
intramolecular hydrogen bonds between two of the μ3-OH
groups and bridging esp ligands (the O···O separations are 3.10
in neutral, Co-Co bonded compounds range from 2.27 – 2.39
Å;1a however, longer Co–Co bonds of 2.80 and 2.74 Å are
known for the [Co2(CN)10]6– and [Co2(CNCH3)10]4+ ions,
respectively.6 If anything, we may at most anticipate only
σ-type Co–Co interactions to be possible in the compound.
Notable is the “squished” appearance of the dimer (Fig. 1). – 3.19 Å).
Lacking a strong Co–Co bonding interaction, the two cobalt
square pyramids in 1 are not coaxial (the Co-Co-OEtOH angles
a
are 162, 163, and 170°), in contrast to the near-linear angles
seen in Rh2(O2CR)4(L)2 species. One of the two paddlewheel
units has ordered ethanol axial ligands while the other has
ethanol ligands that are disordered over two positions. The
ethanol molecules in both cases engage in hydrogen bonding
to a carboxylate oxygen on a neighboring Co2(esp)2 unit, with
O···O distances ranging from 2.72 – 2.76 Å.
b
Fig. 1 Crystal structure of
1 with displacement ellipsoids drawn
Fig. 2 X-ray structure of 2, with thermal ellipsoids drawn at the
at the 50% probability level. The non-alcoholic hydrogen atoms
have been omitted for clarity. Co1–Co1A: 2.7245(6) Å,
Co1–O(EtOH): 2.022(2) Å, Co1–O(esp): 2.008(2)–2.106(2) Å.
Distances for the other symmetry independent molecule are
given in the supporting information.
50% probability level. In (a), for clarity, the hydrogen atoms
have been omitted, except those on the carboxylate and
hydroxide groups. Additionally, the esp ligands have been
truncated to only include the carboxylate and α carbon atoms,
the latter of which are represented as dashed ellipsoids. A
further truncated representation is shown in (b), highlighting
the Co7(OH)4 core, plus the acetonitrile nitrogen atoms.
Interestingly, incomplete reaction of CoCO3 and H2esp
followed by recrystallization from acetonitrile in air yielded
crystals of a structurally complex heptanuclear Co7 cluster with
Compound
2 belongs to a fairly extensive class of Co7
the formula Co7(OH)4(Hesp)2(esp)4(MeCN)2 · 4 MeCN (
2
ion bound to four μ3-OH ligands and two neutral MeCN ligands
in a cis-CoO4N2 geometry. Each of the four μ3-OH ligands
bridges an additional two Co(II) ions to the central Co(II) ion,
and the six [esp]2–/[Hesp]– ligands chelate and bridge the six
2) (Fig.
2). The structure of consists of a central, six-coordinate Co(II)
coordination cluster compounds, mostly containing Co(II)
ions,7 though some are Co(II)/Co(III) mixed-valent species.8 The
known Co7 cluster compounds are exclusively supported by
mixed N,O-donor ligands at the molecular periphery, and they
typically support a central [Co(OH)6]4– unit;
2 is the only Co7
2 | J. Name., 2012, 00, 1-3
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cluster supported exclusively by carboxylates, and it possesses
a central [Co(OH)4(CH3CN)2]2– unit. Moreover, all previously
reported Co7 clusters have a D3d symmetrical, planar, disc-like
topology in which the central Co ion is coordinated in a trans
geometry (Chart 2A); the cis geometry of the central Co atom
in 2 gives rise to its unique C2-symmetric bowl-like structure
(Chart 2B).
A
B
Co
O
Co
O
Co
Co
O
O
Co
O
L
O
O
Co
Co
Co
Co
Co
L
O
O
O
Co
Co
Co
Co
Fig. 3 Plot of χ·T vs T for
using the model described in the text.
1
; the red line is a best fit to the data
Central trans geometry
Central cis geometry
Symmetric discꢀlike structure
Unsymmetric bowlꢀlike structure
Magnetic susceptibility data were also measured from 2 to
300 K for the heptacobalt complex 2, and a plot of χ·T vs T is
given in Fig. 4. At room temperature, χ·T approaches a value of
21 emu·K·mol-1, well above the expected spin-only value for
seven non-interacting Co(II) ions (13.125 emu·K·mol-1). The χ·T
value drops precipitously as the temperature is lowered below
100 K, consistent with an overall antiferromagnetic interaction
within the cluster. For a system this complicated, it is
impossible to attempt to model the χ·T data taking into
account all of the spin centers, their (anisotropic) g tensors,
their zero-field splitting, and their exchange couplings without
grossly overparameterizing the model. We have therefore
Chart
heptacobalt clusters.
2 Comparison of trans and cis geometries for
Magnetic Studies
Magnetic properties of molecular Co compounds are of
significant interest.9 Magnetic susceptibility data for
1 were
measured from 2 to 300 K, and a plot of χ·T vs T is given in
Fig. 3. The room temperature χ·T value of ~4.4 emu·K·mol-1 is
slightly larger than expected for two non-interacting high-spin
Co(II) centers (spin only χ·T = 3.75 emu·K·mol-1), suggesting
average g values greater than 2, as expected for d7 Co(II)
centers. The value of χ·T decreases monotonically with
decreasing temperature, approaching a value of 0 emu·K·mol-1
at 2 K. There is a small bump in the data below 20 K that could
not be modeled, and we suggest it is due to an impurity in the
sample. As shown in Fig. 3, the data between 15 and 300 K
were fitted to a model taking into account both the zero-field
splitting (D) of the two Co(II) ions and the antiferromagnetic
exchange interaction (J) between them. This model yields
adopted
a
simplified model in order to obtain
a
phenomenological fit to the data that yields chemically useful
information.
g
avg = 2.57, D = –55 cm-1, and J = –13.2 cm-1, indicating a large
zero-field splitting and antiferromagnetic coupling. Notably,
we do not find an acceptable fit of the data with D > 0,
suggesting that D is negative. The D value is similar to that
reported
by
Boča
and
coworkers
for
Co2(benzoate)4(quinoline)2 (gavg = 2.39, |D| = 55 cm-1), who
used a similar model to fit their magnetic data.2g We find an
order of magnitude larger intramolecular exchange coupling (J)
in
1
than in the benzoate analog (–13.2 cm-1 vs –1.5 cm-1), in
agreement with the fact that our data tend towards
0 emu·K·mol-1 at low temperatures and the benzoate data
remain at ~0.25 emu·K·mol-1 at the lowest recorded
temperature. It is currently unclear why exchange coupling in
Co2(esp)2(EtOH)2 is so much larger than that in the benzoate
analog, though we note that the Co–Co distance in the esp
compound is shorter by 0.05 – 0.1 Å, perhaps enabling a direct
orbital overlap mechanism for spin coupling.
Fig. 4 Plot of χ·T vs T for 2. The solid line is a theoretical fit to
the data based on the model described in the text.
This model takes into consideration the unusual geometry
of the complex and makes a number of simplifying
assumptions. First, anisotropy of the Co(II) ions is ignored, and
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all Co(II) ions are assigned to the same (isotropic) g parameter.
Second, considering the closer distance between the central
Co atom and two pairs of outer Co atoms, we assume that the
spin of the central Co sets the spins of the outer pairs such
that each pair aligns as an effective S = 3 unit. Thus, we have
effectively a central S = 3/2 Co(II) unit (Co1) with two outer
identical S = 3/2 Co(II) units (Co2 and Co3) and two identical
S = 3 Co2(II,II) units (sites 4 and 5, Chart 3). Treating this as a
system of five spins is a much more tractable problem than
attempting to treat seven separate spins.
Electronic Structure Calculations
The electronic structure of
1 was examined using the
geometry of the crystal structure. Consistent with the
magnetometry data, we find the compound to be best
modeled as containing a pair of antiferromagnetically-coupled
high-spin Co(II) ions. The magnetic orbitals are shown in
Figure 4, and there are three types of interactions observed.
The bottom set of orbitals is of π symmetry with respect to the
Co–Co vector, whereas the middle set of orbitals resembles
canted
d
orbitals pointed in such a way as to only yield a π-
2
z
Co
type Co–Co interaction. The top set of orbitals is of δ
symmetry and, as expected, has the weakest interaction. The
antiferromagnetic interactions within the complexes yield
calculated J values ranging from –16 cm-1 to –22 cm-1, which
are quite comparable to the experimentally determined value
of –13 cm-1.
Co
O
4
J3
J2
J3
J2
J1
O
L
Co
Co
2
1
3
Co
L
O
J1
O
J3
J3
Co
Co
5
3
1
=
2
5
=
= S = 3/2
S = 3
=
4
=
Chart
3 Simplified model for the magnetic data of 2,
highlighting the antiferromagnetic interaction between the
purple spin centers (modeled as Co2 units with S = 3) and the
green spin centers (each corresponding to an individual S = 3/2
Co(II) ion).
At 2 K, the value of χ·T for
2
drops to ~3.3 emu·K·mol-1,
which allows us to make an educated guess about the nature
of the ground spin state. If all seven Co(II) ions were
ferromagnetically coupled, the resulting S = 21/2 ground state
would have a spin-only low-temperature limit of χ·T
½S(S+1) = 60.375 emu·K·mol-1, which is much larger than the
observed value. If we have single central Co(II) ion
=
Fig 4 Calculated corresponding magnetic orbitals of 1 of both α
(left) and β (right) spin. The numbers in the middle are the
calculated overlap integral values, S.
a
antiferromagnetically coupled to the outer six Co(II) ions, the
resulting S = 15/2 ground state yields a spin-only low-
temperature limit of χ·T = 35.625 emu·K·mol-1, which is also
too large. The most reasonable spin coupling model is one in
which three Co(II) ions align together ferromagnetically and
couple against the other four Co(II) ions antiferromagnetically,
yielding a ground state of S = 3/2 (χ·T = 1.875 emu·K·mol-1 at
the low-temperature limit). This value compares best with the
Conclusions
The chelating dicarboxylate ligand esp is used to support a
dimeric Co2(II,II) paddlewheel-type structure bearing axial
ethanol
Co7(OH)4(Hesp)2(esp)4(MeCN)2. In the dimer, the relatively
2.76 support an
intramolecular magnetic exchange interaction with
ligands
and
a
heptanuclear
complex
observed value of 3.3 emu·K·mol-1. We therefore propose that short Co–Co distances of 2.72
–
Å
the three individual Co(II) ions in our model (spins 1, 2, and 3
in Chart 3) couple ferromagnetically with each other and
antiferromagnetically with spins 4 and 5, the S = 3 Co2 units.
Thus, of the three distinct J values describing the exchange
coupling in the complex outlined in Chart 2, we expect J1 and J3
to be negative (antiferromagnetic) and J2 to be positive
(ferromagnetic). The best fit of this model to the experimental
data (Fig. 4) yields g = 2.34, J1 = +1.4 cm-1, J2 = +6.4 cm-1, and
J3 = –5.8 cm-1. With this model, the anticipated signs of J are
correct, except for that of J1, but this value is considerably
smaller than the other two J values, which will dominate the
magnetic properties.
J = –13 cm-1, which is an order of magnitude larger than in the
corresponding benzoate complex that has longer Co–Co
distances. In the Co7 complex, the magnetic properties suggest
an S = 3/2 ground state due to antiferromagnetic alignment of
four of the Co(II) ions against the other three. We propose a
possible model for the spin alignment within the structure.
Experimental
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General Procedures
All reagents were obtained from commercial sources and Instrumentation
used as received, except as noted. All work was performed Mass spectral data were collected using
a
Bruker
with oven-dried glassware under a dry, nitrogen atmosphere ULTRAFLEX III MALDI-TOF mass spectrometer equipped with a
using standard Schlenk technique or an inert-atmosphere SmartBeam laser. IR spectra were collected on a Bruker Tensor
glovebox, except as noted. Ethanol was dried over Mg(OEt)2 27 FTIR spectrometer using the ATR technique. Elemental
and distilled under nitrogen immediately prior to use.
analyses were measured at Midwest Microlab, LLC in
The dinitrile precursor to H2esp (“esp-diCN”) and the Indianapolis, Indiana, USA.
dicarboxylic acid H2esp were synthesized as originally reported
by DuBois and coworkers4 but with important modifications Crystallography
previously described,10 and with additional modifications
Crystallographic data were measured at the Molecular
described in the supporting information to improve yield and Structure Laboratory of the Chemistry Department of the
purity.
University of Wisconsin–Madison. Crystals were selected
under oil under ambient conditions and attached to the tip of
a MiTeGen MicroMount. Each crystal was mounted in a stream
of cold nitrogen at 100(1) K and centered in the X-ray beam
Synthetic Procedures
Co2(esp)2(EtOH)2 (1): A 25 mL Schlenk flask was charged with a using a video camera. The crystal evaluation and data
stirbar, CoCO3 · x H2O (0.07 g, 43-47% Co by weight), and H2esp collection were performed on a Bruker Quazar SMART APEX-II
(0.14 g). The flask was evacuated and backfilled with nitrogen diffractometer with Cu Kα (λ = 1.54178 Å) radiation for
1 or
three times. Freshly distilled EtOH (25 mL) was added, and the Mo Kα (λ = 0.71073 Å) radiation for 2. The data were collected
flask was fitted with a condenser. The suspension was heated using a routine to survey an entire sphere of reciprocal space
with gentle stirring at 90 °C for 24 hours. Within the first ten and indexed by the SMART program.11 The structures were
minutes, a purple mother liquor quickly formed, and the solid solved using the Olex2 program12 via direct methods or charge
slowly converted from a pink powder to black crystals over the flipping and refined by iterative cycles of least-squares
24 hour period. The reaction mixture was cooled to room refinement on F2 followed by difference Fourier synthesis.13 All
temperature and filtered, and the solid was washed with room H atoms were included in the final structure factor calculation
temperature EtOH. After air-drying, shiny black crystals were at idealized positions and allowed to ride on the neighboring
collected (0.15 g, 79%, assuming 47% Co by weight in atoms with relative isotropic displacement coefficients, except
CoCO3 · x H2O). The crystals are polychromatic under polarized as noted.
light, displaying pink, yellow, blue, and orange among other
Crystals of 1 and 2 were both found to be twinned. For the
less prominent colors. Analysis calc’d for Co2C36H52O10: resulting data sets, all possible combinations of domains and
C, 56.69 %; H, 6.87 %. Found: C, 56.03(5) %; H, 6.77(2) %. MS inclusion or exclusion of overlapping reflections were used to
(MALDI-TOF) m/z calc’d for Co2C36H52O10 [M-2(EtOH)]+: 670.1; generate .hkl files using either HKLF4 or HKLF5 format. The
found 670.4. IR (ATR, cm-1): 1592.2 s, 1480.5 m, 1416.3 s, HKLF4 or HKLF5 file that yielded the smallest uncertainties in
1375.9 m, 1358.2 m, 1046.5 m, 925.9 m, 907.2 m, 879.2 m, bond distances was used for the final refinement.
707.9 s, 613.7 m. 1H NMR (CD3OD): δ = 14.05 (br s), 7.59 (br s),
4.75 (br s), 2.34 (d), -0.24 (t), -20.68 (br s).
When selecting a crystal of 1 for diffraction, only crystals
having non-merohedral twinning could be found; the two twin
domains are related by a 180° rotation about b*. The HKLF5
Co7(OH)4(Hesp)2(esp)4(MeCN)2 · 4 MeCN (2): A 100 mL Schlenk file generated based on component one and excluding
flask was charged with a stirbar, CoCO3 · x H2O (0.62 g, 43-47% overlapping reflections was used for the final refinement.
Co by weight), and H2esp (1.39 g). The flask was evacuated and During refinement, there was positional disorder in the
backfilled with nitrogen three times. Freshly distilled EtOH ethanol molecule on one of the two half-dimers in the
(50 mL) was added and the flask was fitted with a condenser. asymmetric unit, the two components of which refined to a
The suspension was heated with gentle stirring at 90 °C for 64:36 occupancy ratio. Additionally, the DFIX restraint was
4 hours, during which time a purple mother liquor quickly used to set the O-H distances to ~0.87 Å, and the DANG
formed and a black solid formed, though some purple solid restraint was used, on C···H and Co···H distances, to set the
remained. The reaction mixture was cooled to room angles to those typically adopted by ethanol molecules.
temperature and filtered, and the solid was washed with room
When selecting a crystal of 2 for diffraction, only crystals
temperature EtOH. After air-drying, the shiny black solid was having non-merohedral twinning could be found; the two twin
dissolved in MeCN, the resulting mixture was filtered, and the domains are related by a 180° rotation about c*. The HKLF5
purple filtrate was allowed to concentrate in air to yield purple file generated based on component one and excluding
plate-shaped crystals suitable for X-ray diffraction (0.37 g, overlapping reflections was used for the final refinement.
22%, assuming 47% Co by weight in CoCO3 · x H2O). Analysis
Crystallographic data have been deposited with the
calc’d for Co7C96H126O28: C, 53.87 %; H, 5.93 %. Found: Cambridge Crystallographic Data Center under CCDC 1857930-
53.51(3) %; H, 5.88(5) %. IR (ATR, cm-1): 2961 w, 2923 w, 2872 1857931.
w, 1680 w, 1583 s, 1476 m, 1413 m, 1375 m, 1361 m, 1265 w,
749 w, 707 m, 648 w, 614 m.
Magnetic Susceptibility Measurements
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avoid contamination from metallic materials. Crystalline
samples were placed in a glass vial with a glass ball bearing and
ground into powders using a vortex mixer. The powdered
samples were weighed and placed inside of plastic capsules in
plastic
drinking
straws.
Variable-temperature
DC
magnetization was measured using a Quantum Design MPMS
3 SQUID magnetometer over a temperature range of 2 – 300 K
at an applied magnetic field of 1000 G (0.1 T). Magnetization
data were converted to magnetic susceptibility via ꢃ ≈ ꢄ/ꢅ
.
The data were fitted using the PHI software program.14
Variable-frequency, variable-field AC magnetic susceptibility
measurements were also made on
2 at 1.8 K at frequencies
from 10 to 1000 Hz and applied fields ranging from 0 to 1 T. An
out-of-phase signal is detected at modest applied fields
(maximum signal at ~2300 G), but no maxima were observed
in the plots of χ
ʺ
vs log(freq.). Diamagnetic corrections15 for
each sample were calculated using the equation
ꢄꢉ
ꢃꢆꢇꢈ ≈ −
× 10ꢊꢋꢌꢍꢎ/ꢍꢏꢐ
2
and were applied to the experimental data along with a
correction for the intrinsic diamagnetism of the sample holder.
Magnetic susceptibility data were additionally corrected for
temperature independent paramagnetism.
Electronic Structure Calculations
Calculations were performed using the ORCA program, version
4.0.0.2.16 The B3LYP functional17 was used with Def2-TZVP
bases on all atoms.18 The broken symmetry formalism was
used to model the electronic structure of
1 with 3 α and 3 β
weakly interacting electrons. The interactions were analyzed
using the corresponding orbital transformation.19 Three
methods were used for the prediction of J: The method of
Ginsburg20 and Noodleman21 yields J = –22 cm-1, as does the
method of Yamaguchi,22 while the method from Bencini and
Gatteschi23 yields J = –16 cm-1.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the U.S. Department of Energy for support through
Grant DE-SC0016442.
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Reaction of cobalt carbonate with α,α,α’,α’-tetramethyl-1,3-
benzenedipropionic acid (H2esp) in ethanol forms the paddlewheel-
type dimer Co2(esp)2(EtOH)2, which hydrolyzes in acetonitrile
solution to give a bowl-shaped heptacobalt coordination cluster
complex; both compounds display antiferromagnetic interactions
between high-spin Co(II) ions.
8 | J. Name., 2012, 00, 1-3
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