Capped-tetrahedrally coordinated Fe(II) and Co(II) complexes using a click -derived tripodal ligand: Geometric and electronic structures

Article abstract of DOI:10.1021/ic300392e

Figure Persented: The 'Click'-derived tripodal ligand tris[(1-benzyl-1H-1, 2,3-triazole-4-yl)methyl]amine, tbta, was used to synthesize the complexes [Fe(tbta)Cl]BF₄, 1, and [Co(tbta)Cl]BF₄, 2. Both complexes were characterized by ¹H NMR spectroscopy and elemental analysis. Single-crystal X-ray structural determination of 2 shows a 4 + 1 coordination around the cobalt(II) center with a rather long bond between Co(II) and the central amine nitrogen atom of tbta. Such a coordination geometry is best described as capped tetrahedral. 1 and 2 are thus the first examples of pseudotetrahedral coordinated Fe(II) and Co(II) complexes with tbta. A combination of SQUID susceptometry, EPR spectroscopy, Moessbauer spectroscopy, and DFT calculations was used to elucidate the electronic structures of these complexes and determine the spin state of the metal center. Comparisons are made between the complexes presented here with related complexes of other ligands such as tris(2-pyridylmethyl)amine, tmpa, hydrotris(pyrazolyl) borate, Tp, and tris(2-(1-pyrazolyl)methyl)amine, amtp. 1 and 2 were tested as precatalysts for the homopolymerization of ethylene, and both complexes delivered distinctly different products in this reaction. Blind catalyst runs were carried out with the metal salts to prove the importance of the tripodal ligand for product formation.

Full text of DOI:10.1021/ic300392e

Article
pubs.acs.org/IC
Capped-Tetrahedrally Coordinated Fe(II) and Co(II) Complexes Using
a “Click”-Derived Tripodal Ligand: Geometric and Electronic
Structures
David Schweinfurth,† Serhiy Demeshko,∥ Marat M. Khusniyarov,§ Sebastian Dechert,∥
Venkatanarayana Gurram,⊥ Michael R. Buchmeiser,⊥ Franc Meyer,∥ and Biprajit Sarkar*,†,‡
†Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70550, Stuttgart, Germany
̈
̈
‡Institut fur Chemie und Biochemie, Anorganische Chemie, Freie Universitat Berlin, Fabeckstrasse 34-36, D-14195, Berlin, Germany
̈
̈
§Department Chemie und Pharmazie, Friedrich-Alexander Universitat Erlangen-Nurnberg, Egerlandstrasse 1, D-91058, Erlangen,
̈
̈
Germany
∥Institut fur Anorganische Chemie, Georg-August Universitat Gottingen, Tammanstrasse 4, D-37077, Gottingen, Germany
⊥Institut fur Polymerchemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70550, Stuttgart, Germany
̈
̈
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The ‘Click’-derived tripodal ligand tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine, tbta, was used to
1
synthesize the complexes [Fe(tbta)Cl]BF4, 1, and [Co(tbta)Cl]BF4, 2. Both complexes were characterized by H NMR
spectroscopy and elemental analysis. Single-crystal X-ray structural determination of 2 shows a 4 + 1 coordination around the
cobalt(II) center with a rather long bond between Co(II) and the central amine nitrogen atom of tbta. Such a coordination
geometry is best described as capped tetrahedral. 1 and 2 are thus the first examples of pseudotetrahedral coordinated Fe(II) and
Co(II) complexes with tbta. A combination of SQUID susceptometry, EPR spectroscopy, Mossbauer spectroscopy, and DFT
̈
calculations was used to elucidate the electronic structures of these complexes and determine the spin state of the metal center.
Comparisons are made between the complexes presented here with related complexes of other ligands such as tris(2-
pyridylmethyl)amine, tmpa, hydrotris(pyrazolyl) borate, Tp, and tris(2-(1-pyrazolyl)methyl)amine, amtp. 1 and 2 were tested as
precatalysts for the homopolymerization of ethylene, and both complexes delivered distinctly different products in this reaction.
Blind catalyst runs were carried out with the metal salts to prove the importance of the tripodal ligand for product formation.
Metal complexes of tmpa have been used in the context of
bioinorganic research as well as for studying electron
transfer.20−24 The ligand Tp has been used for a variety of
purposes by coordination and bioinorganic chemists, including
activation of small molecules by its metal complexes.25−28
Various metal complexes of the ligand amtp have been reported
in the literature.29−33 The modular synthetic route offered by
the “Click” method helps in easy modification of the steric and
electronic properties of such ligands and makes a library of
INTRODUCTION
■
The Cu(I)-catalyzed cycloaddition reaction between azides and
alkynes has been widely used by synthetic chemists in recent
years.1−3 The resulting 1,2,3-triazole-containing compounds are
becoming increasingly popular ligands in coordination
chemistry.4 The bidentate and tridendate forms of such ligands
have been compared to the extensively used 2,2′-bipyridine and
2,2′,6′,2′′-terpyridine ligands.5−17 The potentially tetradentate
ligand tris[(1-benzyl-1H-1,2,3-triazole-4-yl)methyl]amine,
tbta,18,19 is analogous to tris(2-pyridylmethyl)amine, tmpa,
hydrotris(pyrazolyl) borate, Tp, and tris(2-(1-pyrazolyl)-
methyl)amine, amtp (Scheme 1).
Received: February 21, 2012
Published: June 25, 2012
© 2012 American Chemical Society
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Scheme 1. Analogy between tbta and Related Ligands
ligands easily accessible.18,19 We made use of this protocol and
recently reported on 6-fold-coordinated Co(II) complexes with
tbta that showed coordination ambivalence and spin cross-
over.34 There have also been reports on Cu(I) complexes of
tbta.35,36
In the following we present the synthesis of the first examples
of pseudotetrahedral-coordinated iron and cobalt complexes
[Fe(tbta)Cl]BF4, 1, and [Co(tbta)Cl]BF4, 2, with the tripodal
1
ligand tbta. Characterization of compounds 1 and 2 by H
NMR spectroscopy, elemental analyses, X-ray crystallography,
SQUID susceptometry, EPR spectroscopy, Mossbauer spec-
̈
troscopy, and DFT calculations will be presented. These results
will be used to elucidate the geometric and electronic structures
of the complexes. Results on the use of 1 and 2 as precatalysts
for homopolymerization of ethylene will also be presented.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Complexes and
Crystal Structures of tbta and 2. The ligand tbta was
synthesized by following methods reported in the literature.37
We were able to crystallize tbta for single-crystal diffraction
studies. tbta crystallizes in the P21/n space group (Figure S1,
Supporting Information). The bond lengths inside the 1,2,3-
triazole rings are very similar to those observed for other
ligands containing such a ring.15,16
Figure 1. (a) C−H···Cl interactions in 2 in the solid state. (b) π−π
interaction in 2 in the solid state. (c) ORTEP plot of 2 drawn at 50%
ellipsoid probability; hydrogen atoms and counterions are omitted for
clarity.
Access to 1 and 2 was achieved using a mixture of dichloride
and tetrafluoroborate salts of the respective metals and tbta in
CH3CN (see Experimental Section). Synthesis of 1 required
the use of an inert atmosphere possibly because of the
sensitivity of the Fe(II) centers in the salts and such
compounds toward oxidation and hydration. In contrast,
complex 2 could be synthesized under air.
a
Table 1. Selected Bond Lengths of 2 (in Angstroms)
Co−Cl
Co−N1
Co−N11
Co−N21
Co−N31
Cl···H12A
Cl···H22A
Triazole1···Phenyl2
Triazole3···Phenyl1
2.2638(8)
2.350(3)
2.037(3)
2.026(3)
2.035(3)
2.7194(9)
2.7322(9)
3.679
Crystals of 2 suitable for single-crystal diffraction studies
were obtained by slow diffusion of diethyl ether into an
acetonitrile solution at ambient temperatures. Complex 2
crystallizes in the orthorhombic Pnn2 space group. The cobalt
center in 2 is coordinated by one nitrogen atom from each of
the three triazole rings of tbta (Figure 1 and Table 1); the
distances are as follows: Co−N11, 2.037(3) Å; Co−N21,
2.026(3) Å; Co−N31, 2.035(3) Å. Additionally, there is a
chloride ligand with a Co−Cl distance of 2.264(1) Å. Finally,
there is a relatively long bond between the cobalt center and
the central amine nitrogen of tbta with a Co−N1 distance of
2.350(3) Å. The angles between the triazole nitrogen atoms
and cobalt are in the range of 113−114°, those between the
triazole nitrogen atoms, cobalt, and chloride are in the range
104−106° (Table S3, Supporting Information). The cobalt
center is 0.525(1) Å above the plane containing the three
triazole nitrogen-donor atoms. All of the above data point to a 4
+ 1 coordination at the cobalt center, and hence, the
coordination geometry around the cobalt center is best
described as capped tetrahedral.
3.698
a
−
Triazole2 is involved in binding the BF4 anion.
For comparison, the literature reported Co(II) complex
[Co(tmpa)Cl]+ with tmpa is better described with a
coordination geometry of trigonal bipyramidal.38−40 The Co−
N distances to the pyridine rings of tmpa in that complex are
slightly longer than the Co−N(triazole) distances observed in
the case of 2. The main difference, however, is the Co−
N(amine) distance, which in the present case is 2.350(3) Å. For
[Co(tmpa)Cl]+ the corresponding distance is 2.184(6) Å.40
Thus, the central amine nitrogen binds much more strongly to
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the Co(II) center in [Co(tmpa)Cl]+ as compared to 2. Another
ligand that is similar to tbta and tmpa is amtp (Scheme 1), and
five-coordinated complexes of the form [Co(amtp)Cl]+ have
also been reported with that ligand.29−33 In one such complex
with R = H in amtp the Co(II) center is coordinated in a
pseudotetrahedral fashion as in the case of 2.41 The distances
between Co(II) and the pyrazol N donors are slightly shorter
than the corresponding Co−N(triazole) distances in 2. The
Co−N(amine) distance in the case of [Co(amtp)Cl]+ is
2.393(4) Å and hence comparable to 2. Thus, in 2 and
[Co(amtp)Cl]+ the coordination around the metal center is
comparable. However, for [Co(tmpa)Cl]+ the ligand tmpa is
coordinated to the Co(II) center in a more compact manner.
The ligand Tp (Scheme 1) in complex [Co(Tp)Cl] displays
Co−N(pyrazol) distances that are comparable to [Co(amtp)-
Cl]+ and the corresponding Co−N(triazole) distances in
2.25,26,42,43 The coordination around the Co(II) center in
[Co(Tp)Cl] has thus also been described as pseudotetrahedral
in the literature.43 A pseudotetrahedral coordination similar to
that in 2 has also recently been observed for a 5-fold-
coordinated Co(II) complex with a tris(1,2,3-triazole) motif
built on an azatriquinacene platform.44
is similar to that of the Co(II) center in 2 for which we have
structural evidence from single-crystal X-ray diffraction studies.
Magnetic and Spectroscopic Investigation and DFT
Calculations. SQUID susceptometric measurements were
carried out on 1 and 2 in order to investigate their magnetic
properties. 1 and 2 show magnetic moments at 300 K that
correspond to S = 2 and 3/2 ground states, respectively (Figure
2). Similar values have been reported in the literature for the
Figure 2. Plot of μeff vs T for 1 (top) and 2 (bottom) at 0.5 T; solid
lines represent calculated curve fits.
The presence of three nitrogen atoms within a five-
membered ring makes the C−H group of 1,2,3-triazoles highly
acidic.45 In the case of 2, the C−H groups of triazoles and the
chlorides bound to cobalt are involved in extended hydrogen
bonding in the solid state as shown in Figure 1a. Relevant
distances are Cl−H12A= 2.719(1) Å and Cl−H22A = 2.732(1)
Å. There are further π−π stackings between the triazole rings
and phenyl rings of an adjacent molecule (Figure 1b and Table
S2, Supporting Information). Additionally, one triazole ring
from each molecule is also involved in a C−H···F interaction
corresponding five-coordinate Fe(II) and Co(II) complexes
with tmpa and amtp.29−33,40 Magnetic moments for both
compounds remain constant on cooling until about 20 K.
Below 20 K a slight dip in the magnetic moment is observed
that is likely due to zero-field splitting. Magnetic parameters
were determined using a fitting procedure to the appropriate
spin Hamiltonian for zero-field splitting and Zeeman
interaction and including a term for temperature-independent
paramagnetism (TIP). The best simulation parameters are g =
2.02, |D| = 5.0 cm−1, and TIP = 3.7 × 10−4 cm3 mol−1 for 1 and
g = 2.17, |D| = 4.6 cm−1, and TIP = 3.2 × 10−4 cm3 mol−1 for 2.
Magnetic data thus suggest the presence of high-spin (HS)
Fe(II) and Co(II) centers in 1 and 2, respectively.
−
with the BF4 counterions.
Despite several attempts, we were not able to obtain suitable
single crystals for X-ray structure determination of 1. Hence, to
obtain an idea about the local symmetry around the metal
Mossbauer spectroscopy is known to be an extremely useful
̈
1
centers, H NMR spectroscopy was performed on both 1 and
tool for determination of the oxidation and spin states of iron
centers, and EPR spectroscopy is known to be helpful for
investigating the spin states of cobalt centers. Thus, in order to
further probe the oxidation and spin states of 1 and 2,
1
2. H NMR data is consistent with the highly paramagnetic
nature of the Fe(II) and Co(II) centers in 1 and 2 with the
proton signals displaying large paramagnetic shifts and
broadening (Experimental Section). This broadening results
in the loss of the multiplicities that would otherwise be
Mossbauer and EPR spectroscopies were, respectively, used.
̈
The 57Fe Mossbauer spectrum of 1 (Figure 3) shows a single
̈
1
quadrupole doublet with an isomer shift δ = 1.02 mm·s−1 and a
quadrupole splitting ΔEQ = 3.14 mm·s−1, which corroborates
that the iron ion is in the HS d6 configuration. The X-band EPR
spectrum of 2, recorded at 110 K, shows a broad signal with a
virtually axial g splitting. The spectrum could be simulated
using g values of 4.33, 4.20, and 2.17 (Figure 3). The spectrum
and g values are typical for a HS Co(II) center, where in the X
expected for certain signals of the tbta ligand. The H NMR
spectra of both 1 and 2 shows six proton resonances, suggesting
that the complexes possess a 3-fold symmetry with respect to
the tbta ligand (three signals from the phenyl rings and one
each from the benzyl−methylene, triazole C−H, and the
methylene groups attached to the central amine atom).
Exchange between the triazole(benzyl) arms averages their
environments in solution on the NMR time scale. Such a
phenomenon has recently been reported for similar Co(II)
complexes of the tmpa ligand.46 The paramagnetic shifts of the
signals seem to correlate with their distances from the
paramagnetic metal centers. Thus, the resonances for the
aromatic protons of the benzyl ring as well as the methylene
protons of the benzyl groups are largely in the expected region
(Experimental Section). However, the signals corresponding to
the C−H group of the triazole rings and the methylene groups
attached to the central amine nitrogen atoms are highly
paramagnetically shifted. The appearance of the same number
of signals in the 1H NMR spectrum of 1 and 2 shows the 3-fold
symmetry of the tbta ligand in both complexes and gives
confidence that the coordination around the Fe(II) center in 1
band usually transitions corresponding to the mS
=
1/2
manifold are observed.
In order to corroborate the experimental results, DFT
calculations were used to determine spin densities at the metal
centers in 1 and 2. DFT calculations were carried out on 1 and
2 at the B3LYP level using the ORCA 2.8 package (Supporting
Information).47 Lowdin population analysis provides spin
̈
densities of 3.68 and 2.65 for the Fe and Co centers in 1 and
2, respectively (Figure 3). DFT results thus support all
experimental data and confirm that the spin density is
predominantly located at the metal centers in these HS Fe(II)
and HS Co(II) complexes.
Ethylene Homopolymerization. Fe(II) and Co(II)
complexes containing nitrogen-rich ligands and dissociable
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Figure 3. (a) 57Fe Mossbauer spectrum of 1 recorded at 7 K. (b) DFT-calculated spin density distribution of 1. (c) X-band EPR spectrum of 2 at 110
̈
K recorded as a powder. (d) DFT-calculated spin density distribution of 2.
halides are known to be effective precatalysts for homopolyme-
rization of ethylene.48 The presence of a tripodal platform
around the metal center with a dissociable chloride ligand
attached to the metal center prompted us to test the activity of
both 1 and 2 as precatalysts for ethylene homopolymerization
in the presence of MAO as a cocatalyst. Both 1 and 2 showed a
very moderate activity of ≤20 kgpolymer/molcat·h as precatalysts
for homopolymerization of ethylene in the presence of MAO as
cocatalyst (Table 2 and Supporting Information).
catalyst used at 80 °C produced a linear polymer. The
corresponding 13C NMR spectra are enclosed in the Supporting
Information. In order to verify the effect of the ligand tbta on
the observed catalytic activity blind catalytic runs were carried
out using the corresponding metal salts without the ligand tbta.
Reactions carried out using either FeCl2 or CoCl2 under
otherwise identical conditions did not yield any significant
amount of polyethylene neither at T = 50 nor 80 °C. Using a
catalyst:MAO ratio of 1:2000, activities were ≤0.3 kgpolymer
/
molcat·h throughout. These results clearly demonstrate the
requirement of the ligand tbta for achieving catalytic activity in
ethylene polymerization.
Table 2. Homopolymerization of Ethylene Using Co (2) and
a
Fe (1) Catalysts
1 and 2 showed no activity in the homopolymerization of
styrene, cyclopentene, or cis-cyclooctene (Supporting Informa-
tion).
b
c
no.
catalyst
cat:MAO
T (°C)
activity
Tm [°C]
1
2
3
4
2 (2.7 μmol)
2 (2.7 μmol)
1 (2.8 μmol)
1 (2.8 μmol)
1:2000
1:2000
1:2000
1:2000
50
80
50
80
20
18
10
18
132
136
130
131
CONCLUSIONS
■
a
In summary, we presented here the first examples of
pseudotetrahedral HS Fe(II) and Co(II) complexes containing
the tripodal tbta ligand. The complexes display a 3-fold
Polymerizations were carried out in 250 mL of toluene for 1 h at 4
b
c
bar ethylene pressure. Activity = kgpolymer/ molcat·h. Measured by
DSC.
1
symmetry in solution as has been observed with H NMR
Analyses of the resulting polymers by 13C NMR spectroscopy
(Supporting Information) interestingly showed that the Fe-
containing precatalyst 1 produced only linear polymers,
whereas the Co-containing catalyst 2 produced slightly
branched polymers (20 branches/1000 ethylenes). By increas-
ing the polymerization temperature from 50 to 80 °C, the
activity of the Co catalyst increased slightly; however, all
resulting polymers were very poorly solubile in chlorinated
solvents (1,1,2,2-tetrachloroethane and 1,2,4-trichlorobenzene),
thereby impeding the recording of high-quality NMR spectra.
Furthermore, no relevant GPC data could be recorded. 13C
NMR measurements again suggest a branched structure (20
branches/1000 ethylenes) for PE obtained by action of the Co
catalyst (Supporting Information).49−51 Interestingly, the Fe
spectroscopy. Structural parameters obtained for the Co(II)
complex show that the coordination geometry around the metal
center is similar to reported 5-fold-coordinated Co(II)
complexes with the related amtp ligand. A combination of
structural, magnetic, spectroscopic data and DFT calculations
unequivocally establishes the coordination, oxidation, and spin
states of these molecules. Both complexes show moderate
activities as precatalysts in ethylene homopolymerization.
Intriguingly, the Fe(II) complex produces only linear polymers,
whereas the Co(II) complex produces slightly branched
polymers. The importance of the ligand tbta has been verified
by performing blind catalytic runs with ligand-free salts which
show negligible catalytic activity. In view of the modular
synthesis afforded by the “Click“ method, it will be intriguing to
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data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_requests/cif.
see if other substituents on the tripodal ligands would deliver
better polymerization catalysts. In addition, the present
complexes can also be envisaged for small molecule activation.
Current work in our laboratories is directed in both directions.
ASSOCIATED CONTENT
■
S
* Supporting Information
EXPERIMENTAL SECTION
Crystallographic, DFT details, and polymerization experiments.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
Materials and General Methods. The ligand tbta was
synthesized according to a published procedure.37 All solvents used
during the metalation reaction were dried and distilled under argon
and degassed by common techniques before use. Other chemicals and
solvents were reagent grade and used as received. HPLC-grade
solvents were used for spectroscopic studies. EPR spectra in the X
band were recorded with a Bruker System EMX. Elemental analyses
were performed with a Perkin-Elmer Analyzer 240.
AUTHOR INFORMATION
■
Corresponding Author
*Email: biprajit.sarkar@fu-berlin.de.
Notes
Magnetic Susceptibility Measurements. Temperature-depend-
ent magnetic susceptibility measurements were carried out with a
Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with a
5 T magnet in the range from 295 to 2.0 K at a magnetic field of 0.5 T.
The powdered sample was contained in a gel bucket and fixed in a
nonmagnetic sample holder. Each raw data file for the measured
magnetic moment was corrected for the diamagnetic contribution of
the sample holder and gel bucket. The molar susceptibility data were
corrected for the diamagnetic contribution.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are indebted to the Fonds der Chemischen Industrie (FCI)
for financial support of this project (Chemiefondsstipendium
for D. S. and Liebig-Fellowship for M.M.K). Dr. I. Hartenbach
and Dr. S. Strobel are kindly acknowledged for crystal structure
determination of 2. We are grateful to the bw GRID for access
to computer clusters. Special thanks to Dr. D. Wang for
running some of the homopolymerization and NMR experi-
ments.
Magnetic parameters were determined using a fitting procedure to
̂
the spin Hamiltonian for zero-field splitting and Zeeman interaction H
= D[Sz −1/3S(S + 1)]+ gμBS·B.
2
̂
̂
Temperature-independent paramagnetism (TIP) was included
according to χcalcd = χ + TIP. Simulation of the experimental magnetic
data with a full-matrix diagonalization of exchange coupling and
Zeeman splitting was performed with the julX program (E. Bill, Max-
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̈
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[Co(TBTA)Cl](BF4), 2. CoCl2·6H2O (112 mg, 0.47 mmol), Co-
(BF4)2·6H2O (160 mg, 0.47 mmol), and TBTA (500 mg, 0.94 mmol)
were dissolved in CH3CN (30 mL). The solution was stirred at room
temperature for 2 h. Then diethyl ether (50 mL) was added, and the
purple product was precipitated. It was filtered, washed with ether, and
isolated in 75% yield (502 mg). Single crystals suitable for X-ray
diffraction were grown by slow diffusion of diethyl ether into an
acetonitrile solution. Anal. Calcd for C30H30BClCoF4N10: C, 50.62; H,
4.25; N, 19.68. Found C, 50.35; H, 4.39; N, 19.46. 1H NMR (CD3CN,
δ/ppm): 112.63 (s), 26.42 (s), 11.06 (s), 6.58 (s), 6.27 (s), 4.90 (s).
X-ray Crystallography. Suitable crystals for X-ray analysis of all
compounds were obtained as described above. Intensity data were
collected at 100(2) K on a Kappa CCD diffractometer (graphite
monochromated Mo Kα radiation, λ = 0.71073 Å) for 2 and at 133(2)
K on a Stoe IPDS II (graphite-monochromated Mo Kα radiation, λ =
0.71073 Å) for tbta. Crystallographic and experimental details for the
structures are summarized in Table S1, Supporting Information.
Structures were solved by direct methods (SHELXS-97) and refined
by full-matrix least-squares procedures (based on F2, SHELXL-97).52
CCDC 804879 and 804876 contain the cif files for this manuscript. All
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