Cobalt-Catalyzed C(sp2)-H borylation with an air-stable, readily prepared terpyridine cobalt(II) Bis(acetate) precatalyst

Article abstract of DOI:10.1021/acs.organomet.6b00630

A bench-stable, 4-aryl-substituted terpyridine supported, high-spin cobalt(II) bis(acetate) complex, (ArTpy)Co- (OAc)2 (ArTpy = 4′-(4-N,N′-dimethylaminophenyl)-2,2′:6′,2″- Terpyridine), is active for the C(sp2)-H borylation of arenes and heteroarenes with B2Pin2 (Pin = pinacolato). Optimization of the catalytic borylation reaction revealed improved performance in the presence of LiOMe and turnover numbers of up to 100 have been observed using all air-stable components. EPR specstroscopy identified formation of inactive cobalt species, promoted by excess HBPin. A high-spin cobalt(II) bis[(diacetoxy)- pinacolatoborate-κ3O,O,O] compound has been isolated and characterized by X-ray diffraction and is the result of catalyst deactivation.

Full text of DOI:10.1021/acs.organomet.6b00630

Article
pubs.acs.org/Organometallics
Cobalt-Catalyzed C(sp2)−H Borylation with an Air-Stable, Readily
Prepared Terpyridine Cobalt(II) Bis(acetate) Precatalyst
Nadia G. Leonard, Mate J. Bezdek, and Paul J. Chirik*
́
́
́
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: A bench-stable, 4-aryl-substituted terpyridine
supported, high-spin cobalt(II) bis(acetate) complex, (ArTpy)Co-
(OAc)2 (ArTpy = 4′-(4-N,N′-dimethylaminophenyl)-2,2′:6′,2″-
terpyridine), is active for the C(sp2)−H borylation of arenes and
heteroarenes with B2Pin2 (Pin = pinacolato). Optimization of the
catalytic borylation reaction revealed improved performance in
the presence of LiOMe and turnover numbers of up to 100 have
been observed using all air-stable components. EPR specstro-
scopy identified formation of inactive cobalt species, promoted by
excess HBPin. A high-spin cobalt(II) bis[(diacetoxy)-
pinacolatoborate−κ3O,O,O] compound has been isolated and
characterized by X-ray diffraction and is the result of catalyst deactivation.
Mankad and co-workers have extended this design to
INTRODUCTION
■
heterobimetallic variants that are catalytically active upon
exposure to UV irradiation.19 Other examples of iron borylation
catalysts include monomeric bis(diphosphine) complexes20 and
Fe2O3 nanoparticles.21 Advances have also been made in nickel
catalysis where combination of the appropriate metal precursor
with phosphine or N-heterocyclic carbene ligands generates
active C−H borylation catalysts.22 Additionally, metal-free
variants have also recently been reported.23 While these
breakthroughs are significant, these catalysts have yet to reach
the synthetic utility, ease of handling, and broad scope that is
associated with state-of-the-art iridium complexes.
Four-coordinate cobalt(I) alkyl and hydride complexes
bearing electron-donating tridentate bis(phosphine)pyridine
(PNP)-type pincers promote the two-electron oxidative
addition of H−H, B−H, Si−H, and C−H bonds.24 These
cobalt(I) alkyl precursors are also among the most active first-
row transition metal catalysts for C(sp2)−H borylation of
arenes and heteroarenes, operating through a Co(I)−Co(III)
redox cycle with C−H activation by a cobalt(I)−boryl
intermediate.25,26 Inspired by these findings, other tridentate,
meridionally coordinating pincers including bis(imino)-
pyridines,25 various phosphine(diimines), triphosphine, and
amine(bis)phosphine have also been explored for catalytic
borylation activity. In certain cases, catalyst deactivation
pathways through formation of mixed-valent bridging cobalt
hydrides have been identified.27
The ubiquity of carbon−hydrogen bonds in organic molecules
makes them attractive targets from which to build molecular
complexity.1 Among the many transition metal-catalyzed C−H
functionalization methods, arene and heteroarene borylation2,3
is among the most powerful and widely utilized due to its high
sterically driven site selectivity, complementary reactivity to
traditional aromatic substitution chemistry, and versatility of
the boronate ester products,4,5 particularly as building blocks in
the construction of C−C and C−N bonds.6,7
Among contemporary catalysts for C−H arene borylation,
those generated from the treatment of [Ir(COE)2Cl]2 or
[Ir(COD)OMe]2 (COE = cyclooctene; COD = 1,5-cyclo-
octadiene) with bipyridine or related ligands8 are the most
widely used and well-studied due to their high activity, ease of
use, commercial availability, and broad functional group
tolerance. The versatility of these catalysts has enabled
optimization through high throughput experimentation.9
Other precious metal catalysts have been reported including
other iridium variants10 as well as rhodium11 and platinum12,13
and heterogeneous Rh14 and Ir15 examples. In some instances,
improved performance, particularly site selectivity, is ob-
served.16
Earth-abundant transition metals such as iron, cobalt, and
nickel are attractive for C−H borylation catalysis given their
relatively low cost and potential environmental and toxicity
advantages. Perhaps more importantly, these metals by virtue of
their smaller atomic radii and different electronic structures,
particularly a relatively high density of electronic states, offer
the opportunity for new reaction chemistry distinct from
existing precious metal catalysts.17 In 1995, Hartwig and co-
workers reported stoichiometric C−H borylation with a UV-
activated Fp-type iron complex [Fp = (η5-C5H5)Fe(CO)2].18
For base metal catalysts to attain the widespread use of
iridium and other precious metal catalysts, improvements in
Special Issue: Hydrocarbon Chemistry: Activation and Beyond
Received: August 3, 2016
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of (ArTpy)Co(OAc)2
a
b
For ligand synthesis see ref 35. Synthesis adopted from ref 37.
catalyst preparation and handling of these air- and moisture
sensitive organometallic compounds are important consider-
ations for catalyst design. A common strategy to improve air
stability of earth-abundant metal catalysis has been to generate
the active catalyst in situ by treatment of a stable metal dihalide
precursor with a strong reducing agent such as NaHBEt327,28 or
Grignard reagent.29 However, the selectivity and activity of the
in situ activated complexes are often inferior to those of
catalytic reactions performed with the isolated reduced
complexes, likely a consequence of the strong reducing agent
and incomplete formation of the catalytically active cobalt
compound.28c
Cobalt(II) carboxylates are air-stable, commercially available,
and one of the most inexpensive sources of cobalt. Our group
has shown that addition of the appropriate supporting ligand
generates highly active and selective catalysts for alkene
hydroboration,30 alkene hydrosilylation,31,32 and C(sp3)−H
benzylic borylation.33 Nagashima and co-workers have also
demonstrated alkene hydrosilylation using this approach.34 In
each case, the borane or silane substrate delivers the active
catalyst avoiding the use of strongly reducing and pyrophoric
activators and demonstrates a convenient strategy for activation
of first row transition metal catalysts.
The handling, air stability, and cost benefits of the cobalt
carboxylate source prompted efforts to also improve the cost,
handling, and accessibility of the supporting ligand for C(sp2)−
H borylation catalysts. The presence of alkyl-substituted
phosphines in the PNP family of pincers renders the ligands
air-sensitive. The 4-substituted terpyridine derivative, ArTpy,
was targeted due to its air stability and simple and scalable
synthesis from commercially available precursors.35 Structural
similarity to pyridine diimine (PDI) ligands, chelates known to
support active cobalt C(sp2)−H borylation catalysts,25 as well
as demonstrated accessibility of terpyridine cobalt acetate
coordination compounds36 prompted our investigation into
this ligand class. Here we describe (ArTpy)Co(OAc)2, its
evaluation as a C(sp2)−H borylation catalyst with B2Pin2, and
identification of important catalyst deactivation pathways.
The planarity and open coordination geometry imparted by
the 4-substituted terpyridine ligands prompted synthesis of the
6,6″-dimethyl variant with the goal of increasing steric
protection of the cobalt. The desired terpyridine, ArTpyMe,
was isolated as a yellow solid in 40% yield using the
condensation−cyclization procedure adapted for the synthesis
of 4,6,4″,6″-tetramethyl 4′-Ph-terpy.37The desired cobalt bis-
(acetate) complex, (ArTpyMe)Co(OAc)2, was obtained as a tan
solid in 94% isolated yield.
Both (ArTpy)Co(OAc)2 and (ArTpyMe)Co(OAc)2 are high-
spin S = 3/2 cobalt(II) complexes. Each exhibits a solid-state
magnetic moment of 4.4 μB at 23 °C. As such, the chloroform-
1
d H NMR spectra of (ArTpy)Co(OAc)2 and (ArTpyMe)Co-
(OAc)2, respectively, display paramagnetically shifted reso-
nances. In general, the peaks were sharp and well-resolved and
the number of resonances observed are most consistent with
C2v symmetric complexes. However, the methyl groups of the
acetate ligands were the exception, appearing as broad singlets
at ∼45 ppm in both cases, an assignment confirmed through
synthesis of (ArTpy)Co(OAc)2-d6 (Figure S2). The acetate
resonances support rapid κ1−κ2 interconversion of the [OAc]
groups on the NMR time scale, a phenomenon previously
reported by Constable and co-workers in terpyridine cobalt
acetate derivatives.38
The (ArTpy)CoCl2 and (ArTpy)CoCH2SiMe3 complexes
were also synthesized and evaluated as catalysts to benchmark
for the activity of the corresponding cobalt(II) bis(acetate).
Stirring ArTpy and anhydrous CoCl2 in THF afforded
(ArTpy)CoCl2 as a red-brown solid in 90% yield. Addition of
2 equiv of (trimethylsilyl)methyllithium to a THF slurry of
(ArTpy)CoCl2 resulted in formation of four-coordinate
(ArTpy)CoCH2SiMe3 isolated as a deep red-purple solid in
82% yield. Crystals suitable for X-ray diffraction were obtained
via slow diffusion of pentane in a concentrated toluene solution
at −35 °C (Figure 1). An idealized planar geometry was
observed with N(1)−C(5), C(5)−C(6), and N(2)−C(6)
distances of 1.377(2), 1.447(3), and 1.365(2) Å, respectively.
These values are consistent with the analogous parent
terpyridine cobalt alkyl complex.39,40
Evaluation of Cobalt Precatalysts. The four terpyridine
cobalt complexes prepared were evaluated for the catalytic
borylation of toluene using B2Pin2. Standard conditions
employed neat arene with 5 mol % of precatalyst in the
presence or absence of an additive at 80 °C for 24 h. A 15:1
ratio of arene to B2Pin2 ensured homogeneity of both the
catalyst and B2Pin2 substrate during catalysis, though the
reaction still proceeds using a 5:1 excess of arene. Attempts to
RESULTS AND DISCUSSION
■
Preparation and Characterization of Cobalt Terpyr-
idine Complexes. Addition of ArTpy35 to a toluene solution
of anhydrous Co(OAc)2 afforded (ArTpy)Co(OAc)2 as an air-
stable orange solid in 93% yield. Notably, (ArTpy)Co(OAc)2
was routinely prepared open to air on the benchtop on a large
(>10 g) scale (Scheme 1).
B
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with excess HBPin resulted in formation of the Co(III)
complex, trans-(iPrPNP)Co(H)2(BPin), a known resting state
during C(sp2)−H borylation.26 With the terpyridine com-
pounds shown in Table 1, monitoring reactions with HBPin
produced no evidence for a diamagnetic, octahedral Co(III)
complex, and the inverse relationship between added HBPin
and yield of borylated product suggests enabling of deactivation
pathways.27 Attempts to borylate toluene by in situ activation of
(ArTpy)CoCl2 with NaHBEt3, LiOMe, or HBPin were
unsuccessful. This is likely due to either incomplete activation
of the precatalyst, as in the case of HBPin and LiOMe, or rapid
formation of a catalytically inactive species, as in the case of
NaHBEt3. These experiments again highlight the utility of the
air-stable cobalt bis(carboxylate) precursor which easily forms
the catalytically active species following treatment with B2Pin2.
The effect of lithium methoxide is less straightforward. In
fact, LiOMe may have multiple roles during catalysis. The
product inhibition observed in the presence of HBPin is slowed
in the presence of LiOMe. A similar effect was observed with
addition of other alkoxide salts; however, LiOMe was selected
as the optimal base (Table S1). This effect could be the result
of formation of a methoxide-borohydride adduct that effectively
sequesters excess HBPin from the reaction mixture and slows
deactivation pathways. In a control experiment, 1 equiv of
LiOMe was added directly to HBPin at room temperature
resulting in the formation of an intractable white precipitate.
Attempts to detect the methoxide-borohydride adduct in situ
by 11B NMR spectroscopy, however, have been unsuccessful.
An activating effect of the LiOMe on the diboron reagent itself,
akin to the role of alkoxide salts in the copper catalyzed
diboration of aldehydes and ketones41,42 or organocatalytic
boron conjugate addition to α,β-unsaturated carbonyl com-
pounds,43 may also be operative. For (ArTpy)Co(OAc)2-
promoted toluene borylation, optimal yield of the aryl boronate
ester was observed with a 1:1 ratio of B2Pin2 and LiOMe.
Figure 1. Solid-state molecular structure of (ArTpy)CoCH2SiMe3 at
30% probability ellipsoids. Hydrogen atoms omitted for clarity.
run the reaction with a 1:1 stoichiometry of arene to B2Pin2 at
1.0 M concentration in pentane resulted in significantly slower
turnover with less than 15% yield of desired borylated product
observed. The results of the additive experiments are reported
in Table 1.
The air-stable precursor, (ArTpy)Co(OAc)2, exhibited
superior performance in all trials. In each experiment, a
diagnostic color change from orange to deep purple was
observed within 5 min of addition of B2Pin2 at room
temperature, indicating catalyst activation. Evaluation of
additives such as HBPin, LiOMe, and NaHBEt3 established
two trends: (i) Excess HBPin in the catalytic reaction had
deleterious effects on product yield. (ii) Addition of LiOMe to
the reaction mixture improved yield of the desired borylated
product for (ArTpy)Co(OAc)2. Previous studies from our group
have demonstrated that treatment of (iPrPNP)CoCH2SiMe3
Table 1. Results of the Catalytic C(sp2)−H Borylation of Toluene with B2Pin2 Using Cobalt(II) Terpyridine Complexes
a
Reaction conditions: toluene (5.7 mmol), B2Pin2 (0.38 mmol), catalyst (0.019 mmol, 5 mol %), 80 °C. Ar = 4-NMe2-Ph. Percent yields based on
GC-FID using cyclooctane as an internal standard.
C
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a
Table 2. Cobalt-Catalyzed Borylation of Arenes and Heteroarenes
a
Reaction conditions: arene (5.7 mmol), B2Pin2 (0.38 mmol), LiOMe (0.38 mmol), catalyst (0.019 mmol, 5 mol %), 80 °C. Reported numbers are
isolated yields unless otherwise noted. The value in parentheses is percent conversion of B2Pin2 determined by GC analysis using cyclooctane as an
internal standard. Product ratios were determined by NMR analysis. Values under the percent conversions are selectivities. Reported numbers are
yield based on H NMR. Reaction run for 36 h. Reaction performed at 1 mol % loading of catalyst.
b
c
d
1
Substrate Scope and Optimization of Reaction
Conditions. To further evaluate catalyst performance and
the generality of the optimized catalytic conditions, (ArTpy)-
Co(OAc)2 was studied as a catalytic precursor for the
borylation of other substituted arenes (Table 2). Both
electron-rich and -poor arenes were evaluated. The borylation
of benzene and toluene proceeded smoothly, yielding 80 and
84% yield of isolated products 1a and 1b, respectively, after 24
h at 80 °C. Electron-poor arenes such as fluorobenzene proved
more reactive than electron-rich substrates such as anisole and
N,N-dimethylaniline, likely a result of the increased acidity of
the C−H bonds, analogous to the established trends in
iridium-2 and cobalt-catalyzed25 borylation. A selection of
heterocycles were also studied with (ArTpy)Co(OAc)2.
Borylation of 2-methylfuran, benzofuran, and N-methylindole
proceeded readily, resulting in >98, 88, and 60% yield of
isolated products 1i, 1j, and 1k, respectively. The borylation of
2-methylfuran was also achieved at reduced catalyst loading (1
mol %) in 36 h without any loss in yield of desired borylated
product, 1i.
Stoichiometric Experiments and EPR Spectroscopic
Studies. Stoichiometric experiments were conducted to gain
insight into the activation mode of (ArTpy)Co(OAc)2 and to
determine the nature of the active species responsible for
C(sp2)−H borylation. Addition of 2 equiv of B2Pin2 to a slurry
of (ArTpy)Co(OAc)2 in toluene-d8 at room temperature and
monitoring the progress of the reaction by 1H NMR
spectroscopy resulted in disappearance of the paramagnetic
1
resonances with no concomitant formation of an H NMR
observable compound. Only resonances for remaining B2Pin2
were detected at short reaction times. Likewise, addition of 1
equiv of B2Pin2 to (ArTpy)CoCH2SiMe3 resulted in liberation
of PinB-CH2SiMe3 with no 1H NMR observable cobalt
compounds. Attempts to recrystallize the 1H NMR silent
cobalt compounds from the reaction mixture formed
immediately after B2Pin2 addition have been unsuccessful. As
discussed below, a catalytically inactive cobalt product can be
crystallized over longer time periods.
Use of an odd-electron cobalt(II) precursor suggested that X-
band EPR spectroscopy may be useful for determining the fate
of the cobalt compounds during catalysis. Addition of 2 equiv
of B2Pin2 to a toluene slurry of (ArTpy)Co(OAc)2 and stirring
for 5 min at ambient temperature produced a dark purple
solution indicating precatalyst activation. The X-band EPR
spectrum of this sample collected in toluene glass at 10 K
exhibited two new signals. One signal corresponds to a
rhombic, S = 3/2, high-spin Co(II) compound (A) while the
These results are encouraging and establish (ArTpy)Co-
(OAc)2 as a practical and readily accessed precursor for
C(sp2)−H borylation. The robustness of the reaction
components, commercial availability of all reagents involved,
and in situ activation of the precatalyst by B2Pin2 obviate the
need for specialized reaction setups involving UV irradi-
ation17,18 or use of pyrophoric activating agents.26
D
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other species, (B), appears as an isotropic signal with giso ≈ 2
(Figure 2). Importantly, the same mixture of products was also
Figure 2. X-band EPR spectrum of (ArTpy)Co(OAc)2 and 2 equiv of
B2Pin2. The spectrum was recorded in toluene glass at 10 K.
Collection parameters for experimental spectrum: microwave
frequency = 9.380 GHz, power = 2.0 mW, modulation amplitude =
1 mT/100. Simulation parameters for complex A: gz = 2.66, gy = 2.04,
gx = 1.90, gstrain = (0.05, 0.17, 0.19), Ax = 225 MHz, Ay = 1 MHz, Az =
1 MHz, Astrain = (58, 0, 0). Simulation parameters for complex B: gz =
2.01, gy = 1.97, gx = 1.99, gstrain = (0.02, 0.06, 0.02).
Figure 3. Solid-state molecular structure of Co[PinB(OAc)2]2 at 30%
probability ellipsoids. Hydrogen atoms omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
Co[PinB(OAc)2]2
Co1−O1
Co1−O5
Co1−O6
O1−B1
O3−B1
O4−B1
2.0486 (12)
2.1002 (12)
2.0698 (12)
1.466 (2)
1.506 (2)
1.505 (2)
180.0
observed by EPR spectroscopy following the addition of 2
equiv of HBPin to a stirring slurry of (ArTpy)Co(OAc)2 in
toluene at ambient temperature. A color change was observed
immediately upon addition of HBPin, suggesting that these
products may be the result of HBPin catalyst deactivation.
Recrystallization of the product mixture from slow diffusion
of pentane into a concentrated toluene solution at −35 °C
resulted in the formation of large yellow crystals suitable for X-
ray diffraction after 1 week, identifying the high-spin compound
as the bis[(diacetoxy)pinacolatoborate-κ3O,O,O] complex, Co-
[PinB(OAc)2]2 (Figure 3). The solid-state structure of
Co[PinB(OAc)2]2 established an idealized octahedral environ-
ment around the cobalt with borates bound to cobalt through a
pinacolato oxygen at the apical positions. Selected bond
distances and angles are presented in Table 3. Side-on bound
borates of this type have been observed with copper44 and
ruthenium45 complexes but to our knowledge not for cobalt.
The benzene-d6 1H NMR spectrum of the isolated crystals
exhibited two peaks at 15.41 and −25.37 ppm, corresponding
to the pinacolato and acetoxy methyls, respectively.
O1′−Co1−O1
O6−Co1−O5
87.56 (5)
The toluene glass X-band EPR spectrum recorded at 10K of
isolated Co[PinB(OAc)2]2 cleanly exhibits only the rhombic
signal, confirming its assignment as species A (Figure 4). The
effective g values of approximately geffz ≈ 5.9, geffy ≈ 2.6, and geff
x
Figure 4. X-band EPR spectrum of isolated Co[PinB(OAc)2]2. The
spectrum was recorded in toluene glass at 10 K. Collection parameters
for experimental spectrum: microwave frequency = 9.378 GHz, power
= 6.32 mW, modulation amplitude = 1 mT/100. Simulation
≈ 1.8 are consistent with values typical for an S = 3/2 cobalt(II)
complex with large anisotropic zero-field splitting (D ≫ hυ ≈
0.3 cm−1 at X-band) and rhombicity, E/D ≈ 0.22. The
experimental spectrum of the rhombic signal was simulated
with g values of gz = 2.66, gy = 2.04, and gx = 1.90, E/D = 0.22
(red line, Figure 4). The observation of highly anisotropic
signals and very large 59Co (I = 7/2, 100% natural abundance)
hyperfine coupling interaction on gz clearly supports a cobalt-
based radical and suggests that a classical description of a high
spin, d7 ion is appropriate.
parameters for complex A: gz = 2.66, gy = 2.04, gx = 1.99, gstrain
(0.01, 0.17, 0.19), Az = 225 MHz, Ay = 1 MHz, Ax = 1 MHz, Astrain
(58, 0, 0).
=
=
Isolation of S = 3/2 Co[PinB(OAc)2]2 raises the question of
the fate of the terpyridine ligand following formation of this
E
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Scheme 2. Stoichiometric Experiment Resulting in Identification of Co[PinB(OAc)2]2 as a Product of the Reaction of HBPin
with (ArTpy)Co(OAc)2
Figure 5. (a) X-band EPR spectrum of (ArTpy)2Co recorded in toluene glass at 10 K. Collection and simulation parameters: microwave frequency =
9.3799 GHz, power = 2.0 mW, modulation amplitude = 1 mT/100, gz = 2.01, gy = 1.97, gx = 1.99, gstrain = (0.02, 0.06, 0.02). (b) X-band EPR
spectrum of (ArTpy)2Co recorded in toluene at 298 K. Collection and simulation parameters: microwave frequency = 9.377829 GHz, power = 2.0
mW, modulation amplitude = 1 mT/100, gz = 2.44, gy = 2.21, gx = 2.29, gstrain = (2.27, 0.19, 1.71).
compound under catalytic conditions. Mass balance suggests
that (ArTpy)2Co could be the other cobalt product (Scheme 2).
Experiments with the methyl-substituted terpyridine complex,
(ArTpyMe)Co(OAc)2 support this hypothesis. Addition of 2
equiv of HBPin to (ArTpyMe)Co(OAc)2 at room temperature in
toluene produced only the signal for the S = 3/2 Co[PinB-
(OAc)2]2 product in the EPR spectrum collected at 10 K in
toluene glass (Figure S9), consistent with the notion that 6,6′-
dimethylation of the terpyridine prevents formation of
bis(terpyridine) cobalt complexes. Analyzing the reaction
DFT calculations and have determined a S = 3/2 ground
state.47a This is also in agreement with the related bis(chelate)
cobalt complex with a bis(imino)pyridine chelate,
(iPrAPDI)2Co, which has an experimentally determined S =
3/2 ground state.50 With B, the resonance observed in the 10 K
toluene glass EPR spectrum does not exhibit g values consistent
with an S = 3/2 spin state assignment, and an S = 1/2
description seems more appropriate. However, recording the
spectrum at ambient temperature (298 K) produced a
broadened and shifted spectrum, more consistent with the S
= 3/2 assignment (Figure 5b).
1
mixture by H NMR spectroscopy established formation of
free ArTpyMe and resonances for Co[PinB(OAc)2]2.
Both Co[PinB(OAc)2]2 and (ArTpy)2Co were independently
evaluated for C−H borylation activity. Heating crystals of
Co[PinB(OAc)2]2 (5 mol % loading) and a 15:1 excess of
toluene to B2Pin2 at 80 °C for 24 h resulted in <5%
consumption of the B2Pin2 as determined by GC analysis.
Conducting an identical procedure with 5 mol % of (ArTpy)2Co
resulted in <5% yield of borylated arene. The inactivity of both
Co[PinB(OAc)2]2 and ArTpy2Co for catalysis is indicative that
these complexes are a result of a deactivation pathway
promoted by HBPin during the course of catalysis.
To gain experimental evidence for the formation of
(ArTpy)2Co, an independent synthesis of (ArTpy)2Co was
attempted. Bis(terpyridine) metal complexes are well-known
and exhibit rich electrochemistry.46 Specifically for cobalt, the
tri-, di-, and monocationic variants have been reported and their
electronic structures extensively studied.47 Reduction of
48
[(ArTpy)2Co][PF6]2 with 2 equiv of 1% Na/Hg in THF
generated a deep purple, NMR-silent product. The X-band
EPR spectrum recorded in toluene glass at 10 K exhibits an
apparent isotropic signal with giso ≈ 2 (Figure 5a), identical to
that observed for species B resulting from treatment of
(ArTpy)2Co(OAc)2 with HBPin or B2Pin2 (Figure 2, sim B).
Neutral bis(terpyridine) cobalt complexes have previously been
observed electrochemically but not isolated.49 Wieghardt and
co-workers have extensively investigated the electronic
structure of the neutral bis(terpyridine) cobalt complex via
CONCLUDING REMARKS
■
An air-stable, readily prepared, and scalable, high-spin cobalt-
(II) terpyridine bis(acetate) complex has been prepared and
characterized. When treated with B2Pin2 and in the presence of
LiOMe, this complex is active for the C(sp2)−H borylation of
F
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Synthesis of Cobalt Complexes. Benchtop, Scaled-Up Syn-
thesis of (ArTpy)Co(OAc)2. A 250 mL round-bottomed flask was
charged with 7.5 g (21.28 mmol) of (ArTpy) in 100 mL of toluene.
Then, 6.07 g (21.28 mmol) of Co(OAc)2 was added to the solution,
instantly turning the solution color dark green. Stirring for 3 h resulted
in the formation of a bright orange precipitate. Collection of the
precipitate on a fritted glass filter and washing with pentane afforded
the product as an orange solid. The solid was then heated (50 °C)
under vacuum for 72 h to remove water before use in catalysis. 10.5 g
(93%) were collected after drying. IR (ATR) νmax 1586.2, 1538.1,
1438.1, 1422.6, 1363.5, 1214.2, 1168.3, 1020.3, 818.8, 790.2, 729.1,
658.7, 513.1 cm−1. Anal. Calcd for C27H26CoN4O4: C, 61.25; H, 4.95;
N, 10.58. Found: C, 60.96; H, 5.12; N, 9.99. Magnetic susceptibility
(Guoy balance, 295 K) μeff = 4.4 μB.
arenes and heteroarenes. While the activity of the terpyridine
compounds is less than that of PNP-cobalt based catalysts, their
ease of synthesis and handling coupled with their generation
from inexpensive reagents make them attractive for C−H
borylation even if higher loadings are required. In situ
monitoring of the catalytic reaction by EPR spectroscopy
coupled with recrystallization of the cobalt products following
turnover identified both bis(chelate) cobalt and Co[PinB-
(OAc)2]2 as deactivation products.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were carried out using standard high-vacuum line, Schlenk, or
cannula techniques or in an M. Braun inert atmosphere drybox
containing an atmosphere of purified nitrogen. The M. Braun drybox
was equipped with a cold well designed for freezing samples in liquid
nitrogen. Solvents for air- and moisture-sensitive manipulations were
dried and deoxygenated using literature procedures.51 Deuterated
solvents for NMR spectroscopy were distilled from sodium metal
under an atmosphere of argon and stored over 4 Å molecular sieves.
4′-(4-N,N′-Dimethylaminophenyl)-2,2′:6′,2″-terpyridine (ArTpy)37
Preparation of (ArTpy)CoCl2. In a nitrogen atmosphere glovebox, a
20 mL scintillation vial was charged with 0.410 g (1.163 mmol) of
ArTpy in 10 mL of THF. Then, 0.151 g (1.163 mmol) of anhydrous
cobalt dichloride was added to the solution, gradually turning the color
to a brown suspension. After stirring for 8 h, the precipitate was
collected on a fritted glass filter and washed with pentane to afford
0.507 g (90%) of red/brown solid. Anal. Calcd for C23H20Cl2CoN4: C,
57.28; H, 4.18; N, 11.62. Found: C, 57.23; H, 3.90; N, 11.71. Magnetic
susceptibility (Guoy balance, 295 K) μeff = 4.0 μB.
52
Preparation of (ArTpy)CoCH2Si(CH3)3. In a nitrogen atmosphere
glovebox, a 20 mL scintillation vial was charged with 0.215 g (0.446
mmol) of (ArTpy)CoCl2 in 10 mL of THF. The reaction vial was
chilled to −35 °C for 15 min. In a separate vial, 0.084 g (0.892 mmol)
of LiCH2Si(CH3)3 was dissolved in 5 mL of THF and chilled to −35
°C for 15 min. The solution of LiCH2Si(CH3)3 was added to the
dichloride suspension and allowed to stir and warm up to room
temperature, during which time the solution color turned deep red/
purple. The solution was stirred for an additional hour, and the
volatiles were removed in vacuo. The solid was then redissolved in
toluene and filtered through a pad of Celite on a fritted glass filter. The
volatiles were again removed to afford 0.183 g (82%) of deep red/
purple solid. Crystals suitable for X-ray diffraction were obtained by
recrystallization from a toluene/pentane solution at −35 °C. Anal.
Calcd for C27H31CoN4Si: C, 65.04; H, 6.27; N, 11.24. Found: C,
65.04; H, 5.79; 11.09. 1H NMR (500 MHz, C6D6, 23 °C, δ) 12.30 (s,
2H, 6,6″ C−H), 8.68 (app t, JHH = 7.1 Hz, 2H, 4,4″ C−H), 8.02 (app
t, JHH = 5.8 Hz, 2H, 3,3″ C−H), 7.96 (d, JHH = 8.1 Hz, 2H, 5,5″ C−
H), 7.55 (m, 4H, 3′,5′ C−H and 2,2″ C−H), 6.60 (d, JHH = 8.1 Hz,
2H, 2′,6′ C−H), 2.44 (s, 6H, NMe2), 1.13 (s, 2H, CH2SiMe3), −0.08
(s, 9H, CH2SiMe3) ppm. 13C {1H} NMR (126 MHz, C6D6, 23 °C, δ)
186.45, 162.2, 156.9, 156.6, 148.2, 129.7, 129.4, 125.8, 125.1, 124.0,
112.1, 4.0, 3.5.
and Co(OAc)2-d6 were prepared following literature procedures.
LiOCH3 was used as purchased from Sigma-Aldrich.
1H NMR spectra were recorded on Bruker AVANCE 300 and
Bruker AVANCE 500 spectrometers operating at 300.13 and 500.62
MHz, respectively. 13C NMR spectra were recorded on a Bruker
1
AVANCE 500 operating at 125.89 MHz. All H and 13C chemical
1
shifts are reported relative to SiMe4 using the H (residual) and 13C
chemical shifts of the solvent as a secondary standard. 31P NMR
spectra were collected on a Bruker 500 AVANCE spectrometer
operating at 202.40 MHz and were referenced to 85% H3PO4 as an
external standard. 19F NMR spectra were collected on a Bruker 300
AVANCE spectrometer operating at 282.23 MHz and were referenced
to CFCl3 as an external standard. 11B NMR spectra were collected on a
Bruker 300 AVANCE spectrometer operating at 96.251 MHz and
were referenced to BF3·OEt2 as an external standard. Elemental
analyses were performed at Robertson Microlit Laboratories, Inc., in
Ledgewood, NJ. Gouy magnetic susceptibility balance measurements
were performed with a Johnson Matthey instrument that was
calibrated with HgCo(SCN)4.
GC analyses were performed using Shimadzu GC-2010 gas
chromatograph equipped with a Shimadzu AOC-20s autosampler
and a Shimadzu SHRXI-5MS capillary column (15 m × 250 μm). The
instrument was set to an injection volume of 1 μL, inlet split ratio of
20:1, and inlet detector temperatures of 250 and 275 °C, respectively.
UHP-grade S3 helium was used as carrier gas with a flow rate of 1.82
mL/min. The temperature program used for all the analyses is as
follows: 60 °C, 1 min; 15 °C/min to 250 °C, 2 min. GC yields were
determined by integration of the desired product peaks using
cyclooctane as an internal standard.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox, transferred to a nylon loop and then
quickly transferred to the goniometer head of a Bruker D8 APEX3
Venture diffractometer equipped with a molybdenum X-ray tube (λ =
0.71073 Å) and a Cu X-ray tube (λ = 1.54178 Å). Preliminary data
revealed the crystal system. The data collection strategy was optimized
for completeness and redundancy using the Bruker COSMO software
suite. The space group was identified, and the data were processed
using the Bruker SAINT+ program and corrected for absorption using
SADABS. The structures were solved using direct methods (SHELXS)
completed by subsequent Fourier synthesis and refined by full-matrix
least-squares procedures. Gouy magnetic susceptibility balance
measurements were performed with a Johnson Matthey instrument
that was calibrated with HgCo(SCN)4. Continuous wave EPR spectra
were recorded at room temperature on an X-band Bruker EMXPlus
spectrometer equipped with an EMX standard resonator and a Bruker
PremiumX microwave bridge. The spectra were simulated using
EasySpin for MATLAB.53
Preparation of ArTpyMe. Synthesis was achieved through a
modification of the literature procedure for 4′-aryl-substituted
2,2′:6′,2″-terpyridines.37 To 40 mL of ethanol in a 100 mL round-
bottomed flask was added 5.00 g (37.0 mmol) of 2-acetyl-6-
methylpyridine, 2.76 g (18.5 mmol) of 4-(dimethylamino)-
benzaldehyde, 2.08 g (37.0 mmol) potassium hydroxide, and 6.43
mL (28% wt) of ammonium hydroxide. We allowed the reaction to
reflux for 4 h, during which time a yellow precipitate formed. We
cooled the reaction mixture and collected yellow solid via vacuum
filtration on a fritted glass filter. Washing with cold methanol afforded
2.8 g (40%) of ArTpyMe as a bright yellow solid. IR (powder) νmax
1612.8, 1572.1, 1525.8, 1459.9, 1388.2, 1358.9, 794.1, 762.8, 736.0,
1
656.4, 560.9 cm−1. H NMR (500 MHz, CDCl3, 23 °C, δ) 8.70 (s,
2H), 8.43 (d, JHH = 7.5 Hz, 2H), 7.86 (d, JHH = 9 Hz, 2H), 7.74 (dd,
JHH = 7.5, 8 Hz, 2H), 7.18 (d, JHH = 7.5 Hz, 2H), 6.84 (d, JHH = 9 Hz,
2H), 3.04 (s, 6H, N(CH3)2), 2.68 (s, 6H, ArCH3) ppm. 13C {1H}
NMR (126 MHz, CDCl3, 23 °C) 157.75, 156.04, 155.84, 150.99,
149.90, 137.01, 128.07, 125.98, 123.15, 118.41, 117.58, 112.35, 40.42
(NMe2), 24.72 (ArMe) ppm.
Preparation of (ArTpyMe)Co(OAc)2. A 20 mL scintillation vial was
charged with 0.250 g (0.657 mmol) of ArTpyMe in 10 mL of THF.
Then, 0.116 g (0.657 mmol) of anhydrous Co(OAc)2 was added to
the yellow solution. Stirring for 24 h at room temperature resulted in
the formation of an orange precipitate. Removal of the volatiles in
G
DOI: 10.1021/acs.organomet.6b00630
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
vacuo and washing with pentane afforded 0.350 g (96%) of tan solid.
IR (ATR) νmax 1586.9, 1539.0, 1437.5, 1420.9, 1362.7, 1215.6, 1022.4,
819.0, 789.9, 730.1, 558.7 cm−1. Anal. Calcd for C29H30CoN4O4: C,
62.48; H, 5.42; N, 10.05. Found: C, 62.23; H, 5.30; N, 10.16. Magnetic
susceptibility (Guoy balance, 295 K) μeff = 4.4 μB.
ACKNOWLEDGMENTS
■
N.G.L. thanks the National Science Foundation (NSF) for a
Graduate Research Fellowship (DGE-1148900). We also thank
Dr. B. A. Schaefer and Dr. John Eng (Princeton) for assistance
with EPR spectroscopy.
Isolation of Co[PinB(OAc)2]2. A 20 mL scintillation vial was charged
with 0.516 g of (ArTpy)Co(OAc)2 and 15 mL of toluene. Then, 2
equiv of HBPin was added to the stirring slurry, during which time the
solution color turned dark purple and eventually to a bright emerald
green. The reaction was allowed to stir at room temperature for 24 h.
The contents of the vial were then filtered through a pad of Celite on a
fritted glass filter. The solution was concentrated and layered with
pentane. Recrystallization at −35 °C for several weeks produced large
yellow crystals suitable for X-ray diffraction. Anal. Calcd for
C20H36B2CoO12: C, 43.75; H, 6.61. Found: C, 43.59; H, 6.74.
Preparation of ArTpy2Co. A 20 mL scintillation vial was charged
with 0.060 g (0.124 mmol) of (ArTpy)CoCl2 and 0.034 g (0.124
mmol) of ArTpy in 10 mL of THF to form a slurry that was chilled to
−35 °C for 30 min. Then, 0.132 mmol of 1 M NaHBEt3 solution was
added dropwise to the stirring slurry. The reaction was vigorously
stirred at room temperature for 2 h during which time the color
changed from deep orange/brown to a dark brown color. The
resulting mixture was filtered through a pad of Celite on a fritted glass
filter, and the volatiles were removed in vacuo. The solid was then
redissolved in toluene and filtered through a pad of Celite on a fritted
glass filter. The volatiles were again removed to afford 0.032 g (63%
yield) of dark brown solid.
General Procedure for Catalytic Arene Borylation. Glovebox
Procedure. In a nitrogen-filled glovebox, a 20 mL scintillation vial was
charged with a magnetic stir bar, 0.010 g (0.019 mmol) of cobalt
catalyst, and 5.7 mmol of arene. Then, 0.096 g (0.38 mmol) of B2Pin2
and 0.014 g (0.38 mmol) of LiOMe were added to the vial. The vial
was capped and sealed with electrical tape, brought out of the
glovebox, and placed into an 80 °C oil bath for 24 h. The reaction was
then quenched by exposure to air and the volatiles removed either in
vacuo or by vacuum distillation. The crude product was then dissolved
in CDCl3, passed through a plug of silica gel in a Pasteur pipet, and
analyzed by 1H and 13C NMR spectroscopy without additional
purification.
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