Synthesis and reactivity of an isolable cobalt(I) complex containing a β-diketiminate-based acyclic tetradentate ligand

Article abstract of DOI:10.1021/ic201780c

A model for cobalamin was synthesized using a new monoanionic tetradentate nitrogen donor ligand; 2-(4- tolyl)-1,3-bis(2-isopropylpyridyl)propenediimine (Tol- BDI^(2-pp)2H) (1), which utilizes isopropylpyridines as pendant arms on a β-diketiminate (BDI) backbone. During the synthesis of 1, the rearrangement product, Tol-BDI^(2-pp)(4-pp)H (2) was observed. Metalation of 1 with zinc iodide and cobalt chloride yielded the corresponding Tol-BDI^(2-pp)2ZnI (3) and Tol-BDI^(2-pp)2CoCl (4) complexes. The redox properties of 4 in comparison to cobalamin were examined through electrochemical studies. Electrochemical and bulk reduction of complex 4 gave a diamagnetic cobalt(I) complex, Tol-BDI^(2-pp)2Co (5). Reactivity of 5 toward C-X bonds was investigated using methyl iodide and 1-iodo-2-(trimethylsilyl)acetylene, yielding Tol-BDI^(2-pp)2Co(CH ₃)I and Tol-BDI^(2-pp)2Co(C₂Si(CH ₃)₃)I respectively. Synthesis and characterization details for these complexes, including the crystal structure of 3, are reported.

Full text of DOI:10.1021/ic201780c

Article
pubs.acs.org/IC
Synthesis and Reactivity of an Isolable Cobalt(I) Complex Containing
a β-Diketiminate-Based Acyclic Tetradentate Ligand
Elodie E. Marlier, Bridget A. Ulrich, and Kristopher McNeill*,†
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States
S
* Supporting Information
ABSTRACT: A model for cobalamin was synthesized using a
new monoanionic tetradentate nitrogen donor ligand; 2-(4-
tolyl)-1,3-bis(2-isopropylpyridyl)propenediimine (Tol-
BDI(2‑pp)2H) (1), which utilizes isopropylpyridines as pendant
arms on a β-diketiminate (BDI) backbone. During the
synthesis of 1, the rearrangement product, Tol-BDI(2‑pp)(4-pp)
H
(2) was observed. Metalation of 1 with zinc iodide and cobalt
chloride yielded the corresponding Tol-BDI(2‑pp)2ZnI (3) and
Tol-BDI(2‑pp)2CoCl (4) complexes. The redox properties of 4 in comparison to cobalamin were examined through
electrochemical studies. Electrochemical and bulk reduction of complex 4 gave a diamagnetic cobalt(I) complex, Tol-
BDI(2‑pp)2Co (5). Reactivity of 5 toward C-X bonds was investigated using methyl iodide and 1-iodo-2-(trimethylsilyl)acetylene,
yielding Tol-BDI(2‑pp)2Co(CH3)I and Tol-BDI(2‑pp)2Co(C2Si(CH3)3)I respectively. Synthesis and characterization details for
these complexes, including the crystal structure of 3, are reported.
Chart 1. Tetradentate Monoanionic Ligands with the Ligand
Reported Here Shown Boxed
INTRODUCTION
■
Cobalamin complexes are cofactors that play a critical role in a
variety of enzymes, including B12-dependent isomerases, methyl
transferases, and reductive dehalogenases.1 To study the
mechanism of these reactions, researchers have long employed
structurally simpler model complexes, such as dimethylglyox-
ime- and porphyrin-containing complexes.2 While such model
complexes have proven useful in modeling the chemistry of
cobalamin, there is room for improvement. In particular, the
porphyrin and glyoxime-based systems mimic the tetraaza
coordination environment of the cobalamin corrin, but do not
match its monoanionic charge.
pendant trialkylamine donors and by Zatka et al. with pendant
quinoline donors to create a variety of metal complexes.6
Closely related examples include an unsymmetrical N4−BDI
complex reported by Chen and co-workers and a BDI ligand
with two pendant ether donors reported by Chisholm and co-
workers.7 The initial attempts to prepare similar ligands with
methylpyridine pendant arms prompted the discovery of a new
route to pyridylpyrroles due to intramolecular cyclization of the
ligand.8 The cyclization is thought to occur through a
nucleophilic attack by the carbon atom α to the pyridine on
the neighboring ketone (or imine). To avoid the cyclization, we
followed the lead of Zatka et al., and employed quinolines as
pendant arms for the second generation ligand.9 Subsequent
study revealed that a ligand equipped with quinoline pendant
arms was too rigid to accommodate the different oxidation
states of cobalt.10 This prompted the design of a more flexible
ligand.
A more important shortcoming of the glyoxime- and
porphyrin-based model compounds is the difficulty in preparing
and isolating cobalt(I) complexes. While examples of Co(I)
have been prepared in these systems, the complexity in
synthesizing these compounds has hindered their use in further
studies.3 Researchers have opted instead to generate the Co(I)
oxidation state in situ. This is unfortunate from the standpoint
of studying reaction mechanisms, given that the Co(I)
oxidation state can often play a central role. For example,
Co(I) is the critical oxidation state in cobalamin-mediated
dechlorination reactions.4 In recent years, isolable cobalt(I)
complexes have been supported by a variety of nitrogen donor
ligands such as N2P2, bidentate BDI, and iminopyridine
pincers.5 With this in mind, we set out to prepare a ligand
system that would allow for the isolation of a Co(I) complex
that effectively models cobalamin.
Our approach to preparing a potentially improved structural
model for the corrin has focused on using a β-diketiminate
ligand with two pendant donor arms (Chart 1). This strategy
has previously been employed by Roesky and co-workers with
Received: August 16, 2011
Published: February 9, 2012
© 2012 American Chemical Society
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1
In this work, we report the synthesis and characterization of a
new monoanionic tetradentate nitrogen donor ligand; 2-(4-
tolyl)-1,3-bis(2-isopropylpyridyl)propenediimine (Tol-
BDI(2‑pp)2H) (1), which employs isopropylpyridines as pendant
arms on a β-diketiminate backbone (Chart 1). Learning from
prior work, greater flexibility was introduced by switching from
an sp2-hybridized carbon to an sp3-hybridized carbon between
the pyridyl group and the BDI backbone. Furthermore, gem-
dimethyl substitution on the carbon atom α to the pyridine
prevents pyrrole formation through intramolecular cyclization.
Compound 1 was used to prepare zinc iodide (3) and cobalt
chloride (4) complexes. Most notably, we report the synthesis
and characterization of an isolable cobalt(I) complex, Tol-
BDI(2‑pp)2Co (5) from the reduction of 4 with sodium amalgam.
Lastly, the ability of compound 5 to model cobalamin was
examined by exploring its reactivity toward carbon−halogen
bonds. Substrates investigated were methyl iodide (6) and 1-
iodo-2-(trimethylsilyl)acetylene (7). In addition, we report a
ligand rearrangement product; Tol-BDI(2‑pp)(4‑pp)H (2), which
is formed during the synthesis of 1.
Compound 2 was fully characterized by H NMR and 13C
NMR spectroscopy, H/1H COSY, and ESI-HRMS. The H
NMR spectrum of 2 does not contain a singlet for the proton α
to the imine as is seen for 1 (Figure 2). Rather, it has two
doublets (7.82 and 7.53 ppm) that share a 2.5 Hz coupling
constant, confirmed by 1H/1H COSY. Furthermore, the
isopropylpyridyl methyls give rise to two separate resonances
(1.70 and 1.63 ppm), each integrating to 6H against the tolyl
methyl proton signal (2.31 ppm, 3H). The assignment of a 4-
pyridyl group comes from the two second-order doublets each
integrating to 2H at 8.52 and 7.36 ppm.
1
1
Metal Complexes Synthesis. Metalation of compound 1
was achieved by deprotonation of the ligand with potassium
bis(trimethylsilyl)amide, followed by the addition of the desired
metal halide salt (Scheme 2). Zinc (3) and cobalt (4)
complexes were synthesized in 29% and 98% yield using zinc
iodide and cobalt chloride, respectively. We speculate that the
large difference in isolated yield is due in part to the superior
ability of compound 3 to form a zinc dimer, [Tol-
BDI(2‑pp)ZnI]2 where the ligand has been partially hydrolyzed.
The dimer was isolated from preparation of 3 in low
nonquantified yields as a crystalline solid. Crystal structure
and important crystallographic data of the zinc dimer are
reported in the Supporting Information, Figure 1S, Tables 1S
and 2S.
RESULTS AND DISCUSSION
■
Ligand Synthesis. Compound 1 was prepared through the
condensation of 2-(4-tolyl)-malondialdehyde and 2-amino-2-
(2-pyridyl) propane (Scheme 1). The latter starting material
Compound 3 was characterized by H NMR, 13C NMR
spectroscopy, ESI-HRMS, UV−vis spectroscopy, cyclic voltam-
metry, and X-ray crystallography. The H NMR spectrum is
1
Scheme 1. Synthetic Route to Tol-BDI(2‑pp)2H, 1
1
similar to that of compound 1, apart from the disappearance of
the amine proton and a downfield shift of the proton α to the
imine which is now located at 8.16 ppm (Figure 3). A 1:4 ratio
is still observed between the tolyl methyl (2.33 ppm) and the
isopropylpyridyl methyl resonances (1.73 ppm). The UV−vis
spectrum of 3 in CH2Cl2 showed three bands at 263, 297, and
393 nm which were also observed in the UV−vis absorbance
spectrum of 1 in tetrahydrofuran (THF) at 210, 262, 279
(shoulder), and 344 nm (Supporting Information, Figure 2S).
These transitions are likely ligand-based, as they resemble the
transitions observed in the absorbance spectrum of 1. X-ray
quality crystals were grown from slow vapor diffusion of diethyl
ether into a solution of 3 in CH2Cl2 (Figure 4). The geometry
of compound 3 can be described as distorted square pyramidal
with a τ value of 0.32.12 We have previously reported two
similar BDI zinc complexes which use quinolines for pendant
arms.9,10 The structures for these complexes are very similar
with Zn−N bonds matching within 0.03 Å. Important
crystallographic data for compound 3 can be found in the
Supporting Information, Tables 1S and 2S.
was synthesized in four steps from published procedures.11 The
condensation to 1 was conducted in a toluene/chloroform
solvent mixture heated to reflux for one week. This yielded a
mixture of 1 and the intermediate monoimine, in which only
one aldehyde had undergone reaction. Compound 1 was
purified by flash chromatography and was isolated in 67% yield.
1
Compound 1 was characterized by H NMR and 13C NMR
spectroscopy, electrospray ionization high-resolution mass
spectrometry (ESI-HRMS), and UV−vis spectroscopy. The
1H NMR spectrum shows a singlet at 11.6 ppm for the amine
proton and seven resonances in the aromatic region including a
singlet at 7.73 ppm belonging to the proton α to the imine on
the β-diketiminate backbone (Figure 1). In the methyl region, a
1:4 ratio is observed between the tolyl methyl (2.31 ppm) and
the isopropylpyridyl methyl resonances (1.70 ppm). This
indicates that the two isopropylpyridine arms of compound 1
are symmetric on the NMR time scale.
Interestingly, a rearrangement is observed during the
synthesis of 1 when the reaction concentration is above
approximately 0.07 M. The ligand rearrangement product, 2,
only differs from 1, in that one pendant arm contains a 4-
pyridyl group instead of a 2-pyridyl group. Compound 2 was
isolated from the flash chromatography purification of 1 in 14%
yield.
1
Compound 4 was characterized by H NMR spectroscopy,
ESI-HRMS, and UV−vis spectroscopy. The 1H NMR spectrum
of 4 exhibits broadened resonances, which are expected for a
paramagnetic cobalt(II) complex (Supporting Information,
Figure 3S). The elemental composition of 4 (456. 1715 m/z)
was checked by ESI-HRMS with a polyethylene glycol exact
mass internal standard. The UV−vis spectrum of 4 showed four
transitions at 211, 256, 287, and 355 nm (Figure 5). All four of
these transitions are assigned as ligand-based transitions. The
solution magnetic moment of 4 was found to be μeff = 3.64 μB
by Evans’ method. This is slightly smaller than the spin-only
magnetic moment for an S = 3/2 state (μS.O. = 3.88 μB), which
leads to the assignment of a high-spin Co(II) center. While the
geometry and coordination mode of 4 remains unknown, 4-
coordinate and 5-coordinate cobalt(II) complexes with four
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1
Figure 1. H NMR spectrum of Tol-BDI(2‑pp)2H, 1, in CDCl3.
Figure 2. 1H NMR spectrum of rearranged ligand, 2 (Tol-
BDI(2‑pp)(4‑pp)H), in CDCl3.
1
Figure 3. H NMR spectrum of Tol-BDI(2‑pp)2ZnI, 3, in CD2Cl2.
Scheme 2. Synthetic Routes to Tol-BDI(2‑pp)2 Zn and Co
while two resonances at 2.29 and 1.52 ppm can be observed in
the methyl region belonging to the tolyl methyl and the pyridyl
methyl groups (Figure 6). Compound 5 has a unique teal color.
The UV−vis spectrum of 5 is similar to 4. In addition to the
four ligand-based transitions at 210, 257, 270, and 318 nm, a
distinctive transition is observed at 610 nm (Figure 5). This
transition is associated with a molar absorptivity of 5000 M−1
cm−1. Transitions with molar absorptivities of similar
magnitude have been observed for other square planar
cobalt(I) complexes.14
Complexes, 3−7 (L = Tol-BDI(2‑pp)2
)
There is some ambiguity of the oxidation state of the
complex 5, specifically whether it is truly a neutral ligand bound
to Co(I) or a reduced ligand radical anion bound to Co(II).
Such ligand radical cobalt complexes have been reported with a
variety of ligands.15 In this case, we do not have a crystal
structure to make a definitive assignment, but we favor the
Co(I) assignment based on the nucleophilicity demonstrated
by the complex (vide infra).
nitrogen donors have been shown to support a variety of
geometries and spin states.13
Reduction of compound 4 using sodium amalgam in THF
yields the diamagnetic, cobalt(I) complex, Tol-BDI(2‑pp)2Co(I)
1
(5). Compound 5 was characterized by H NMR, 13C NMR
spectroscopy, UV−vis spectroscopy, and electron-impact high-
resolution mass spectrometry (EI-HRMS). The 1H NMR
spectrum of 5 shows seven resonances in the aromatic region,
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1
Figure 6. H NMR spectrum of Tol-BDI(2‑pp)2Co, 5, in C6D6.
Figure 4. ORTEP diagram of Tol-BDI(2‑pp)2ZnI, 3, with thermal
ellipsoids drawn at the 50% probability level. Hydrogen atoms have
been omitted for clarity.
Figure 7. Cyclic voltammograms of 4 at various scan rates (50, 200,
500 mV/s; 0.1 M [nBu4N]PF6 in THF). Currents were multiplied by
(scan rate)−1/2
.
cyclic voltammogram of 4 is complex, a quasi-reversible
electron transfer wave was observed at −1.55 V with a peak-
to-peak separation of 300 mV (vs Fc/Fc+) when scanning at 50
mV (Figure 7). This separation increases to 600 mV, and the
peak potential shifts slightly cathodically to −1.56 V at the
faster rate of 500 mV/s. The reduction/oxidation wave
centered at −1.55 V is assigned to the CoI/II couple. This is
slightly lower than the CoI/II couple observed for cobalamin,
indicating that 5 is a slightly stronger reductant than
cob(I)alamin.2h,16
Reactivity of 5. In addition to being a strong reductant,
nucleophilicity of the cobalt(I) center is believed to be
important to its reactivity. The reaction of cobalt(I) with
methyl iodide has previously been used to measure the
nucleophilicity of the metal center.17 For this reason, complex
5 was treated with methyl iodide, which produced an
immediate color change from teal to green upon mixing. The
product of this reaction was isolated and found to be the
cobalt(III) methyl iodide complex, 6, which was characterized
Figure 5. Electronic absorption spectrum of complexes 4−7.
Cyclic Voltammetry. Considering its CoI/II couple of
−1.38 V (vs Fc/Fc+), cobalamin is a strong reductant, capable
of reducing difficult substrates, such as chlorinated organ-
ics.2h,16 To investigate the reducing power of complex 5, cyclic
voltammetry experiments in 0.1 M [nBu4N]PF6 THF solution
were performed on both 3 and 4 with the aim of finding the
CoI/II couple. Full cyclic voltammograms of 3 and 4 can be
found in the Supporting Information, Figure 4S. Although the
1
by H NMR and 13C NMR spectroscopy, ESI-HRMS, and
1
UV−vis spectroscopy. The H NMR spectrum of 6 has the
expected seven resonances in the aromatic region while three
resonances in a 1:4:1 ratio are present in the alkyl region
(Figure 8). While the resonances at 2.37 ppm and 1.85 ppm
could be identified as belonging to the methyl of the tolyl group
and the methyl groups on the pendant arms, respectively, the
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composed of a β-diketiminate backbone with isopropylpyridine
pendant arms. It is both flexible enough to support the three
important oxidation states of Co (I, II, III) and stable with
respect to decomposition to pyridylpyrrole products. An
isolable Co(I) complex of Tol-BDI(2‑pp)2 was prepared and
was found to be comparable to cobalamin(I) both in terms of
its Co(I)/Co(II) redox couple and in its reactions as a
nucleophile.
EXPERIMENTAL SECTION
■
General Considerations. Experiments were conducted under
anhydrous and anaerobic (dinitrogen) conditions using a glovebox or
Schlenk line at the exception of the ligand synthesis which was
conducted in air. Reagents with the exception of 2-amino-2-(2-pyridyl)
propane were purchased commercially and used without further
purification. 2-Amino-2-(2-pyridyl) propane was synthesized in four
steps from published procedures.11 Solvents were purchased
commercially and dried under standard procedures.20 UV−vis
absorbance data was collected with an OceanOptics CHEMUSB2
spectrophotometer using a quartz cuvette. Infrared (IR) spectra were
collected with a Midac M series spectrometer. 1H NMR and 2D NMR
spectra were acquired on Varian Inova instruments at 300 and 500
MHz.13C shifts were established from 2D HMQC (Heteronuclear
Multiple Quantum Correlation) and HMBC (Heteronuclear Multiple
Bond Correlation) NMR experiments. The assignment of the carbon
shifts can be found in the Supporting Information, Table 3S. 13C NMR
spectrum for the 13CH3I experiment was acquired on a Varian Inova
Figure 8. 1H NMR spectrum of Tol-BDI(2‑pp)2Co(CH3)I, 6, in
CD2Cl2.
resonance at 1.74 ppm was tentatively assigned as the methyl
bound to the cobalt center. This assignment was verified
through the reaction of 5 with CD3I and observation of the
2
2
methyl resonance by H NMR spectroscopy. The H NMR
spectrum showed two resonances: one at 2.17 ppm belonging
to residual CD3I and one at 1.74 ppm, verifying the shift of the
cobalt-bound methyl. The shift of the methyl is notably
downfield compared to those of methyl groups bound to
cobalamin and cobaloxime, which have been reported at −0.12
ppm in D2O and 0.82 ppm in CDCl3, respectively.18 For the
assignment of the carbon resonance of the methyl cobalt, it was
necessary to use 13C-labeled methyl iodide, as the carbon shift
could not be unambiguously assigned with unlabeled material.
Using 13CH3I, the carbon resonance for the cobalt methyl was
observed at −4.67 ppm. The UV−vis spectrum of 6 has a total
of four transitions (Figure 5). These ligand-based transitions
are similar to those in complexes 4 and 5.
2
instrument at 75 MHz. H NMR spectrum was acquired on a Varian
Inova instrument at 300 MHz. Mass spectrometry (MS) data were
acquired on a Bruker BioTOF ESI-MS under positive mode.
Polyethylene glycol (PEG) and polypropylene glycol (PPG) standards
were used for ESI-HRMS. Solution magnetic susceptibilities were
determined by the Evans' method.21 A Finnigan MAT 95 double-
focusing mass spectrometer with BE-geometry was used to collect EI-
HRMS with sample introduction using a solids probe. Perfluoroker-
osene (PFK) was used as an internal standard for EI-HRMS.
Tol-BDI(2‑pp)2H (1). Under ambient atmosphere, 2-amino-2-(2-
pyridyl) propane (0.200 g, 1.47 mmol) was dissolved in toluene (22
mL) in a round-bottom flask. 2-(4-Tolyl)-malondialdehyde (0.107 g,
0.66 mmol) was dissolved in CHCl3 (7 mL) and was added to the
solution of 2-amino-2-(2-pyridyl)propane. A concentration of 0.070 M
was found to be optimal to prevent the formation of compound 2. The
reaction was heated to reflux with a bath temperature of 125 °C for
one week. The reaction was cooled to room temperature, and solvent
was removed using a rotoevaporator. A silica column (99:1
CH2Cl2:Et3N eluent mixture) was used to separate compound 1
from the monocondensed intermediate, 2-(4-tolyl)-1-(2-
isopropylpyridyl)propeneimine-3-aldehyde. A brown oil was isolated
and dried under vacuum (0.176 g, 67%). UV (THF) λmax, nm (ε, L
In addition to methyl iodide, complex 5 reacts with 1-iodo-2-
(trimethylsilyl)acetylene to form complex 7 which was
1
characterized by H NMR and 13C NMR spectroscopy, ESI-
1
HRMS, IR and UV−vis spectroscopy. The H NMR spectrum
revealed five resonances in the aromatic region, integrating to a
total of 14 protons and three resonances in the alkyl region in a
1:4:3 ratio (Supporting Information, Figure 5S). The methyl
resonances from the trimethylsilyl group are shifted downfield
from the starting material from 0.17 ppm to −0.24 ppm.
Similarly, the IR spectrum shows a shift from 2099 cm−1 to
2046 cm−1 from starting material to complex 7 for the carbon−
carbon triple bond stretching mode. In addition, a band at 840
cm−1 was observed and assigned to the C(methyl)-Si stretch.
These values are similar to those reported for a [(PP3)Co(H)-
(CCSiMe3]BPh4 complex where bands at 2023 cm−1 and
851 cm−1 were reported for the CC and SiMe3 groups.19 In
addition to having similar ligand-based transitions as the
previously mentioned compounds, the UV−vis spectrum of 7
also has peaks at 425 and 447 nm. Through its reactivity toward
methyl iodide and 1-iodo-2-(trimethylsilyl)acetylene, complex
5 has been shown to be nucleophilic and thus similar to
cobalamin in this regard.
1
mol−1 cm−1): 210, 262 (9200), 279 sh., 344 (4700). H NMR (500
MHz, CDCl3): δ ppm 11.62 (1 H, br. s, HN); 8.57 (2 H, d, J 4.0, pyr.
CH); 7.73 (2 H, s, CH); 7.60 (2 H, ddd, J 2.0, 8.0 and 9.0, pyr. CH);
7.49 (2 H, d, J 8.0, pyr. CH); 7.12 (2 H, m, pyr. CH); 7.10 (2 H, m,
tolyl CH); 7.10 (2 H, m, tolyl CH); 2.31 (3 H, s, tolyl CH3); 1.70 (12
H, s, CH3). 13C NMR (125 MHz, CDCl3): δ ppm 167.3, 150.3, 149.5,
138.7, 136.7, 134.1, 129.3, 125.5, 121.7, 120.4, 105.7, 61.7, 29.4, 21.0.
ESI-TOF HRMS (MeOH) with PEG-400−600 m/z: [M + H]+ calcd.
for C26H31N4, 399.2543; found, 399.2538; 1.25 ppm error.
Tol-BDI(2‑pp)(4‑pp)H (2). Compound 2 was synthesized as a side
product of the synthesis of compound 1. Under high reaction
concentration (approximately 0.1 M), compound 2 was observed.
Compound 2 was isolated using a silica column (99:1 CH2Cl2:Et3N
eluent mixture). A brown oil was isolated and dried under vacuum
(0.050 g, 14%). 1H NMR (500 MHz, CDCl3): δ ppm 11.54 (1 H, br.
s, HN); 8.57 (1 H, d, J 3.5, pyr. CH); 8.52 (2 H, d, J 6.0, pyr. CH);
7.82 (1 H, d, J 2.5, CH); 7.61 (1 H, t, J 8.0, pyr. CH); 7.53 (1 H, d, J
2.5, CH); 7.42 (1 H, d, J 8.0, pyr. CH); 7.36 (2 H, d, J 6.0, pyr. CH);
7.15 (1 H, dd, J 5.0 and 7.0, pyr. CH); 7.09 (4 H, br s, tolyl CH); 2.31
(3 H, s, tolyl CH3); 1.70 (6 H, s, CH3); 1.63 (6 H, s, CH3). 13C NMR
CONCLUSION
■
A monoanionic, tetraaza ligand was prepared in an effort to
model the monoanionic corrin ligand in cobalamin. The ligand
reported here, Tol-BDI(2‑pp)2H, is a third-generation ligand that
is the product of an iterative design process. The ligand is
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(125 MHz, CDCl3): δ ppm 166.5, 158.2, 153.6, 149.8, 148.7, 147.8,
138.6, 136.5, 134.5, 129.1, 125.3, 121.6, 121.3, 119.8, 105.6, 60.6, 59.7,
29.7, 29.1, 20.9. ESI-TOF HRMS (MeOH) with PPG-425 m/z: [M +
H]+ calcd. for C26H31N4, 399.2543; found, 399.2544; 0.25 ppm error.
Tol-BDI(2‑pp)2ZnI (3). Under inert atmosphere, compound 1 (0.028
g, 0.07 mmol) was dissolved in toluene (10 mL). A solution of
potassium bis(trimethylsilyl)amide was prepared by dissolving KN-
(SiMe3)2 (0.016 g, 0.08 mmol) into toluene (5 mL). Upon addition of
KN(SiMe3)2, a slight color change from brown to reddish brown was
observed. After 10 min of stirring, a solution of ZnI2 (0.026 g, 0.08
mmol) was prepared in the dark by dissolving ZnI2 in CH3CN (5 mL)
and was added dropwise to the deprotonated ligand solution. The
solution became orange when left stirring overnight. The solvent was
removed under vacuum. The residue was extracted into minimal
CH3CN and was placed in the freezer overnight. The solids
precipitated and were dried under vacuum (0.012 g, 29%). X-ray
quality crystals were grown from vapor diffusion of diethyl ether into a
solution of 3 in CH2Cl2. UV (CH2Cl2) λmax, nm (ε, L mol−1 cm−1):
CH); 8.11 (2 H, dd, J 1.0 and 8.0, pyr. CH); 7.84 (2 H, s, CH); 7.71
(2 H, dd, J 1.0 and 7.0, pyr. CH); 7.65 (2 H, dd, J 1.0 and 8.0, pyr.
CH); 7.29 (2 H, d, J 8.5, tolyl CH); 7.24 (2 H, d, J 8.0, tolyl CH); 2.37
(3 H, s, tolyl CH3); 1.85 (12 H, s, CH3); 1.74 (3 H, s, CoCH3). The
1H NMR shift of the methyl bound to the cobalt was verified by
2
reacting compound 5 with CD3I. H NMR (300 MHz, CH3CN): δ
ppm 1.74 (s, CoCD3). 13C NMR (125 MHz, CD2Cl2): δ ppm 173.1,
151.9, 151.1, 139.4, 137.9, 135.9, 130.0, 126.8, 124.9, 122.2, 112.8,
75.5, 31.7, 29.4, 21.2. The resonance for the methyl bound to the
cobalt was not observed when using CH3I; however, the use of 13CH3I
1
allowed for its observation. H NMR (500 MHz, CD3CN): δ ppm
1.76 (d, JCH 141, Co13CH3). 13C NMR (125 MHz, CD3CN): δ ppm
−4.7. ESI-TOF HRMS (CH3CN) with PPG-425 m/z: [M − I]+ calcd.
for C27H32N4Co, 471.1959; found, 471.1957; 0.42 ppm error.
Tol-BDI(2‑pp)2Co(C2Si(CH3)3)I (7). Under inert atmosphere, com-
pound 5 (0.010 g, 0.02 mmol) was dissolved in toluene (7 mL). While
stirring, a solution of 1-iodo-2-(trimethylsilyl)acetylene (10 μL, 0.07
mmol) in toluene (3 mL) was added dropwise to the solution of
compound 5. The solution changed color immediately to red brown.
The solution became green after stirring for an hour. It was filtered
and dried under vacuum. The residue was washed heavily with hexanes
to remove starting material and was dried under vacuum (0.013 g,
86%). UV (THF) λmax, nm (ε, L mol−1 cm−1): 205 (3900), 301
(13000), 367 (3200), 425 (2800), 447 (2800). IR °υmax/cm−1: 2046
1
263, 297, 393 (8500). H NMR (500 MHz, CD2Cl2): δ ppm 8.77 (2
H, d, J 5.0, pyr. CH); 8.16 (2 H, s, CH); 7.97 (2 H, dt, J 1.5 and 8.5,
pyr. CH); 7.67 (2 H, d, J 8.5, pyr. CH); 7.46 (2 H, t, J 6.0, pyr. CH);
7.22 (2 H, d, J 8.0, tolyl CH); 7.12 (2 H, d, J 8.0, tolyl CH); 2.33 (3 H,
s, tolyl CH3); 1.73 (12 H, s, CH3). 13C NMR (125 MHz, CD2Cl2): δ
ppm 166.9, 156.8, 147.3, 141.5, 139.6, 133.2, 129.5, 125.9, 123.4,
123.1, 104.6, 61.6, 31.1, 21.2. ESI-TOF HRMS (CH3CN) with PEG-
400−600 m/z: [M − I]+ calcd. for C26H29N4Zn, 461.1684; found,
461.1680; 0.87 ppm error.
1
(CC), 840 (Si-CH3) (Nujol Mull). H NMR (500 MHz, CD2Cl2):
δ ppm 8.96 (2 H, br. d, pyr. CH); 7.97 (2 H, t, J 7.0, pyr. CH); 7.49 (2
H, t, J 6.0, pyr. CH); 7.45 (2 H, d, J 7.0, pyr. CH); 7.30 (2 H, s, CH);
7.28 (2 H, d, J 7.5, tolyl CH); 7.19 (2 H, d, J 7.5, tolyl CH); 2.35 (3 H,
s, tolyl CH3); 1.87 (12 H, s, CH3), −0.23 (9 H, s, Si(CH3)3). 13C
NMR (125 MHz, CD2Cl2): δ ppm 173.0, 152.2, 152.0, 139.0, 138.8,
134.5, 129.8, 126.0, 124.2, 121.7, 118.8, 108.7, 89.6, 75.8, 33.4, 22.1,
1.2. ESI-TOF HRMS (CH3CN) with PEG-400−600 m/z: [M − I]+
calcd. for C31H38N4CoSi, 553.2198; found, 553.2205; 1.27 ppm error.
Electrochemistry. Cyclic voltammetry experiments were per-
formed inside a glovebox with a CHInstruments Model 600D
potentiostat/galvanostat. A single-chamber cell was set up with a
glassy carbon working electrode (3 mm diameter), a Pt wire as the
auxiliary electrode, and a reference electrode consisting of a silver wire
in 10 mM AgNO3 solution (0.1 M [nBu4N]PF6 in CH3CN). The
measurements were taken in a 0.1 M [nBu4N]PF6 THF solution. The
background cyclic voltammograms of the electrolyte solution were
taken prior to the addition of the sample. All measurements were
calibrated to an internal ferrocene standard.
Tol-BDI(2‑pp)2CoCl (4). Under inert atmosphere, compound 1 (0.389
g, 0.98 mmol) was dissolved in toluene (50 mL). A solution of
potassium bis(trimethylsilyl)amide was prepared by dissolving KN-
(SiMe3)2 (0.187 g, 0.94 mmol) into toluene (25 mL). Upon addition
of KN(SiMe3)2, a slight color change from brown to reddish brown
was observed. After 10 min of stirring, a solution of CoCl2 (0.129 g,
0.99 mmol) was prepared by dissolving it in CH3CN (25 mL) and was
added dropwise to the deprotonated ligand solution. No visible change
of color was observed. The reaction was left stirring overnight. The
solvent was removed on a Schlenk line, and the residue was extracted
using toluene, THF, and CH3CN. The extraction solutions were
filtered and evaporated to dryness. The residues were combined and
dried under vacuum (0.473 g, 98%). UV (THF) λmax, nm (ε, L mol−1
cm−1): 211 (21000), 256 (12000), 287 (8700), 355 (5300). 1H NMR
(300 MHz, CD3CN): δ ppm 15.67, 12.24, 11.44, 11.12, 7.98, 7.19,
5.44, 2.32, 1.63, −2.87. Evans’ method (CDCl3/CHCl3, 300 MHz):
μeff = 3.64 μB. ESI-TOF HRMS (CH3CN) with PEG-400−600 m/z:
[M − Cl]+ calcd. for C26H29N4Co, 456.1724; found, 456.1715; 1.97
ppm error.
X-ray Crystallography. A single crystal of 3 was placed onto the
tip of a 0.1 mm diameter glass capillary and mounted on a Bruker
SMART Platform CCD diffractometer for data collection at 173(2) K.
The data collection was carried out using Mo Kα radiation (graphite
monochromator). The data intensity was corrected for absorption and
decay (SADABS). Final cell constants were obtained from least-
squares fits of all measured reflections. The structure was solved using
SHELXS-97 and refined using SHELXL-97.22 A direct-methods
solution was calculated which provided most non-hydrogen atoms
from the E-map. Full-matrix least-squares/difference Fourier cycles
were performed to locate the remaining non-hydrogen atoms. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were placed in ideal positions and
refined as riding atoms with relative isotropic displacement parameters.
Crystallographic data for complex 3 is summarized in Supporting
Information, Table 1S and 2S.
Tol-BDI(2‑pp)2Co (5). Under inert atmosphere, compound 4 (0.069 g,
0.14 mmol) was dissolved in THF (30 mL). Sodium amalgam was
created by adding elemental mercury (1.093 g, 5.45 mmol) to sodium
metal (0.013 g, 0.57 mmol). The solution of compound 4 was added
to the sodium amalgam and was left stirring overnight. The brown
solution became teal. The THF solution was filtered, and filtrate was
dried under vacuum. The residue was extracted using toluene, and the
extraction solution was dried under vacuum (0.046 g, 72%). UV
(THF) λmax, nm (ε, L mol−1 cm−1): 210, 257 (10000), 270 sh., 318
(5900), 610 (5000). 1H NMR (500 MHz, d8-THF): δ ppm 9.02 (2 H,
d, J 3.5, pyr. CH); 7.77 (2 H, s, CH); 7.56 (2 H, t, J 7.0, pyr. CH);
7.11 (2 H, d, J 6.5, tolyl CH); 7.02 (2 H, d, J 6.5, tolyl CH); 6.72 (2 H,
d, J 7.0, pyr. CH); 6.57 (2 H, t, J 8.0, pyr. CH); 2.29 (3 H, s, tolyl
CH3); 1.52 (12 H, s, CH3). 13C NMR (125 MHz, d8-THF): δ ppm
173.4, 150.5, 149.5, 142.4, 136.7, 132.7, 129.3, 125.5, 121.7, 120.4,
112.1, 71.3, 29.4, 21.0. EI-HRMS (70 eV) with PFK m/z: [M]+ calcd.
for C26H29N4Co, 456.1724; found, 456.1700; 5.26 ppm error.
Tol-BDI(2‑pp)2Co(CH3)I (6). Under inert atmosphere, compound 5
(0.010 g, 0.02 mmol) was dissolved in THF (5 mL). While stirring,
methyl iodide (13 μL, 0.02 mmol) was added dropwise. The solution
changed color immediately to green. The solution was stirred for 1 h
before being filtered and dried under vacuum (0.012 g, 94%). UV
(THF) λmax, nm (ε, L mol−1 cm−1): 212, 236, 304 (9800), 418 (3600).
1H NMR (500 MHz, CD2Cl2): δ ppm 8.73 (2 H, dd, J 1.0 and 5.5, pyr.
ASSOCIATED CONTENT
■
S
* Supporting Information
ORTEP diagram for zinc dimer, crystallographic information
for zinc dimer and 3, UV−vis of 1 and 3, cyclic voltammograms
1
of 3 and 4, H spectrum of 7, 13C assignments for 1−7, and
CIF file for 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
2084
dx.doi.org/10.1021/ic201780c | Inorg. Chem. 2012, 51, 2079−2085

Inorganic Chemistry
Article
(10) Marlier, E. E.; Sadowsky, D.; Cramer, C. J.; McNeill, K. Inorg.
Chim. Acta 2011, 369, 173−179.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kris.mcneill@env.ethz.ch.
(11) (a) Hadzovic, A.; Song, D.; MacLaughlin, C. M.; Morris, R. H.
Organometallics 2007, 26, 5987−5999. (b) Song, D.; Morris, R. H.
Organometallics 2004, 23, 4406−4413.
Present Address
(12) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.
†Department of Environmental Sciences, ETH Zurich, Switzer-
land.
(13) (a) Carabineiro, S. A.; Silva, L. C.; Gomes, P. T.; Pereira, L. C.
J.; Veiros, L. F.; Pascu, S. I.; Duarte, M. T.; Namorado, S.; Henriques,
R. T. Inorg. Chem. 2007, 46, 6880−6890. (b) Carabineiro, S. A.;
Bellabarba, R. M.; Gomes, P. T.; Pascu, S. I.; Veiros, L. F.; Freire, C.;
Pereira, L. C. J.; Henriques, R. T.; Oliveira, M. C.; Warren, J. E. Inorg.
Chem. 2008, 47, 8896−8911.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the National Science Foundation
(CHE-0809575 and 0952054) and by the University of
Minnesota through an Undergraduate Research Opportunities
Program (UROP) grant to B.A.U. The authors acknowledge
Dr. Victor Young, Jr., and Lindsay Hinkle from the X-ray
Crystallographic Laboratory and Deanna Miller for solving the
crystal structure of the zinc dimer as part of CHEM 5755. We
also thank Dr. Letitia Yao for helpful NMR discussions.
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