Single-Site Cobalt-Catalyst Ligated with Pyridylimine-Functionalized Metal-Organic Frameworks for Arene and Benzylic Borylation

Article abstract of DOI:10.1021/acs.inorgchem.0c00747

We report a highly active single-site heterogeneous cobalt-catalyst based on a porous and robust pyridylimine-functionalized metal-organic frameworks (pyrim-MOF) for chemoselective borylation of arene and benzylic C-H bonds. The pyrim-MOF having UiO-68 topology, constructed from zirconium-cluster secondary building units and pyridylimine-functionalized dicarboxylate bridging linkers, was metalated with CoCl2 followed by treatment of NaEt3BH to give the cobalt-functionalized MOF-catalyst (pyrim-MOF-Co). Pyrim-MOF-Co has a broad substrate scope, allowing the C-H borylation of halogen-, alkoxy-, alkyl-substituted arenes as well as heterocyclic ring systems using B2pin2 or HBpin (pin = pinacolate) as the borylating agent to afford the corresponding arene- or alkyl-boronate esters in good yields. Pyrim-MOF-Co gave a turnover number (TON) of up to 2500 and could be recycled and reused at least 9 times. Pyrim-MOF-Co was also significantly more robust and active than its homogeneous control, highlighting the beneficial effect of active-site isolation within the MOF framework that prevents intermolecular decomposition. The experimental and computational studies suggested (pyrim?-)CoI(THF) as the active catalytic species within the MOF, which undergoes a mechanistic pathway of oxidative addition, turnover limiting σ-bond metathesis, followed by reductive elimination to afford the boronate ester.

Full text of DOI:10.1021/acs.inorgchem.0c00747

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Article
Single-Site Cobalt-Catalyst Ligated with Pyridylimine-Functionalized
Metal−Organic Frameworks for Arene and Benzylic Borylation
Rajashree Newar, Wahida Begum, Neha Antil, Sakshi Shukla, Ajay Kumar, Naved Akhtar, Balendra,
and Kuntal Manna*
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ABSTRACT: We report a highly active single-site heterogeneous cobalt-catalyst based on a porous and robust pyridylimine-
functionalized metal−organic frameworks (pyrim-MOF) for chemoselective borylation of arene and benzylic C−H bonds. The
pyrim-MOF having UiO-68 topology, constructed from zirconium-cluster secondary building units and pyridylimine-functionalized
dicarboxylate bridging linkers, was metalated with CoCl2 followed by treatment of NaEt3BH to give the cobalt-functionalized MOF-
catalyst (pyrim-MOF-Co). Pyrim-MOF-Co has a broad substrate scope, allowing the C−H borylation of halogen-, alkoxy-, alkyl-
substituted arenes as well as heterocyclic ring systems using B2pin2 or HBpin (pin = pinacolate) as the borylating agent to afford the
corresponding arene- or alkyl-boronate esters in good yields. Pyrim-MOF-Co gave a turnover number (TON) of up to 2500 and
could be recycled and reused at least 9 times. Pyrim-MOF-Co was also significantly more robust and active than its homogeneous
control, highlighting the beneficial effect of active-site isolation within the MOF framework that prevents intermolecular
decomposition. The experimental and computational studies suggested (pyrim•−)CoI(THF) as the active catalytic species within the
MOF, which undergoes a mechanistic pathway of oxidative addition, turnover limiting σ-bond metathesis, followed by reductive
elimination to afford the boronate ester.
borylation in the presence of C(sp2)−H bonds is far less
1. INTRODUCTION
common.7 The regioselectivity of the nondirected borylation
Organoboranes are versatile synthetic intermediates for
of C(sp3)−H bonds strongly favors at primary C−H bonds,
making the functionalization of secondary C−H bonds
challenging. Recently, cobalt- and nickel-catalyzed borylation
of secondary benzylic C−H bonds has been reported.8a,7,8b
carbon−carbon and carbon−heteroatom bond formations.1
Among many methods, transition-metal-catalyzed borylation of
C−H bonds is the most efficient and atom economic for direct
formation of organoboron compounds.2 Although significant
progress has been made using precious-metal-catalyzed C−H
borylation,3 the development of catalysts with cheap and
nontoxic earth-abundant metals is of greater interest. Recently,
iron-,4 cobalt-,5 and nickel-based4a,6 catalysts have been
developed for C−H borylation. Transition-metal catalysts
generally favor C(sp2)−H sites due to the kinetic preference of
oxidative addition. Therefore, chemoselective C(sp3)−H
Received: March 12, 2020
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Figure 1. (a) Synthesis of pyrim-MOF and its metalation with CoCl2 to afford pyrim-MOF-CoCl2(THF). (b) PXRD pattern simulated from the
CIF file of UiO-68-MOF (black),15,16 freshly prepared pyrim-MOF (red), pyrim-MOF-CoCl2 (blue), pyrim-MOF-Co after the treatment with
NaEt3BH (pink), and pyrim-MOF-Co recovered from C−H borylation of o-xylene (green). (c) TEM image of pyrim-MOF-CoCl2. (d) Nitrogen
sorption isotherms (77 K) of pyrim-MOF and pyrim-MOF-CoCl2.
However, significant challenges remain for its practical
applicability such as chemoselectivity, low catalyst loading,
recyclability of the catalysts, and ease of purification of the
products. In addition, the lack of a report of chemoselective
borylation of secondary or tertiary C(sp3)−H bonds in the
presence of C(sp2)−H and primary C(sp3)−H bonds requires
further catalyst development for these challenging trans-
formations.
recovery, high stability, less leaching, as well as those offered by
their homogeneous counterparts such as homogeneity of active
sites, reproducibility, and selectivity.12 Herein, we report a
robust and recyclable single-site cobalt-catalyst ligated with
redox noninnocent pyridylimine-functionalized metal−organic
frameworks for arene and benzylic C−H bonds. The MOF-
catalyst is also enabled to catalyze unprecedented nondirected
and chemoselective borylation of the secondary or tertiary
C(sp3)−H bond in the presence of C(sp2)−H and primary
C(sp3)−H bonds.
We are interested to develop robust and recyclable first-row
transition-metal catalysts having open metal sites for C−H
borylation of sterically congested C−H bonds using bidentate
Schiff base ligands due to their ease of synthesis and redox
noninnocent properties. Schiff bases, synthesized from amino
and carbonyl compounds, are an important class of chelating
ligands in coordination chemistry, and their base-metal
complexes have been widely used as catalysts in many
important organic transformations such as epoxidation,
hydrogenation, hydroelementation, and aldol reactions.9
Bidentate redox-active Schiff base ligands such as pyridylimine
and salicylaldimine can influence the breaking and making of
chemical bonds in a catalytic process by tuning the electronic
properties of the metal and also allow multielectron trans-
formations. However, their metal complexes readily undergo
decomposition due to the weak ligand−metal bonding as well
as deleterious intermolecular interaction.10 Robust base metal
catalysts with such bidentate Schiff base ligands have been
afforded via active-site isolation in porous solid supports
without relying on bulky substituents.11 Unlike traditional
supported catalysts, metal−organic framework-based single-site
solid catalysts combine both the advantages offered by
heterogeneous catalysts such as facile catalyst separation and
2. RESULTS AND DISCUSSION
2.1. Synthesis and Characterization of Pyridylimine-
Functionalized MOF and Its Postsynthetic Metalation
with Cobalt. The MOF-cobalt-catalyst was prepared by the
metalation of pyridylimine-functionalized zirconium-MOF
(pyrim-MOF) with CoCl2 followed by treatment of NaEt3BH.
Pyrim-MOF was constructed from Zr-cluster nodes and
pyridylimine-functionalized dicarboxylate bridging linkers
(pyrim-TPDC) to afford a UiO-type framework of
Zr6O4(OH)4(pyrim-TPDC)6.13 The dicarboxylic acid linker
(H2pyrim-TPDC) was synthesized by the condensation
between amino-triphenyldicarboxylic acid (NH2-TPDC) and
2-pyridinecarboxaldehyde in 56% yield. The solvothermal
reaction between ZrCl4 and H2 pyrim-TPDC in the presence
of benzoic acid in DMF at 80 °C afforded pyrim-MOF as an
off-white solid (Figure 1a). Alternatively, pyrim-MOF was also
prepared by the treatment of 2-pyridinecarboxaldehyde and
amino-functionalized UiO-68-MOF in DMF.14 The crystal-
linity and the structure of pyrim-MOF were established by the
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similarity of its PXRD pattern with that of UiO-68-MOF
(Figure 1b).15 The presence of the pyridylimie moiety in the
MOF was confirmed by 1H NMR of the digested pyrim-MOF,
which made evident the presence of the ∼100% intact
pyridylimine moiety within the MOF (Figure S1, SI). The
nitrogen sorption experiment indicated that pyrim-MOF is
highly porous with a BET surface area of 2042 m2/g and pore
size of 2.5 nm.
Table 1. Pyrim-MOF-Co-Catalyzed Borylation of Arene C−
a
H Bonds
The metalation of pyrim-MOF with anhydrous CoCl2 in
THF at room temperature gave Co-functionalized pyrim-MOF
(pyrim-MOF-CoCl2) as a green solid. The DFT calculation
suggested a five-coordinate cobalt(II) species of (pyrim)-
CoCl2(THF) within the MOF in the ground state (Figures
S17 and S22, SI). The liberation of 1 equiv of THF with
1
respect to Co was also detected in the H NMR spectrum
upon heating a mixture of (pyrim)CoCl2(THF) and D2O in
CDCl3 (Figure S8, SI). The crystallinity of the MOF was
maintained after metalation as made evident by the similarity
of the PXRD pattern with that of pyrim-MOF (Figure 1b).
Inductively coupled plasma optical emission spectroscopy
(ICP-OES) of the digested metalated MOF showed the Co-
loading of 41% with respect to the pyridylimine moiety. The
particles of pyrim-MOF-CoCl2 are of an octahedral shape and
have a diameter of 0.8−1.0 μm as shown by the TEM image
(Figure 1c). TEM-EDX analysis of pyrim-MOF-CoCl2
exhibited the presence of Zr, Co, and Cl atoms within MOF
(Figure S5b, SI). Pyrim-MOF-CoCl2 has a BET surface area of
1450 m2/g and pore size of 2.1 nm. The decrease in the surface
area and pore size of pyrim-MOF after metalation is attributed
to the partial filling of the void space of MOF with the cobalt
chloride moiety (Figure 1d).
2.2. Pyrim-MOF-Co-Catalyzed Borylation of Arene C−
H Bonds. Upon the treatment of NaEt3BH in THF, pyrim-
MOF-Co material became an active catalyst for the
dehydrogenative borylation of C−H bonds of various activated
as well as unactivated arenes and heterocyclic compounds
using B2pin2 or HBpin (pin = pinacolate) as the borylating
agent (Table 1).17 In the optimized conditions, the catalytic
borylation reactions were performed at 110 °C either in neat
arene substrates or using heptane as a solvent. With 0.5 mol %
of Co-loading, pyrim-MOF-Co gave complete conversions in 2
days and afforded arylboronate esters from toluene and xylenes
in 73−94% isolated yields (Table 1, entries 2−6). Importantly,
a turnover number (TON) of 2500 was obtained for the
borylation of o-xylene. Notably, pure boronate ester products
were obtained by simply removing the catalyst via
centrifugation followed by removal of the volatiles. Pyrim-
MOF-Co-catalyzed borylation reactions have a broad substrate
scope with good regioselectivity allowing the C−H borylation
of halogen- and alkoxy-substituted arenes (Table 1, entries 8−
13) as well as heterocyclic ring systems (Table 1, entries 14−
18). However, pyrim-MOF-Co displayed a higher TOF for
electron-rich arenes having electron-donating substituents
compared to that for arenes bearing electron-withdrawing
groups. The borylation of heterocycles such as indole, 2,3-
benzofuran, 2-methylindole, and quinoline proceeded
smoothly under the reaction conditions to give the
corresponding monoborylated products in good yields
(Table 1, entries 15−18). The C−H borylation occurred at
the least sterically hindered position of the unactivated arenes
(Table 1, entries 2−14) and at the 2-position of heteroarenes
such as indole and benzofuran (Table 1, entries 15 and 16). In
the case of borylation of N,N-dimethylaniline, N4,N4,N4′,N4′-
a
Reaction conditions: 0.5 mol % Co, B2pin2 (0.134 mmol), arene
b
(0.28 mmol), 2.0 mL of heptane, 110 °C, 48 h. Isolated yield in the
c
parentheses, regioselectivity was determined by GC−MS. Reaction
d
e
was performed at 85 °C. Neat arene was used. 1.0 mol % Co, 20 h.
f
g
h
GC yield. HBpin was used as the borylating reagent. 72 h.
tetramethyl-[1,1′-biphenyl]-4,4′-diamine was formed as the
major product presumably due to pyrim-MOF-Co-catalyzed
tandem C−H borylation to give N,N-dimethyl-4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)aniline and its subsequent
homocoupling (Table 1, entry 20).
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Figure 2. (a) Plots of GC conversion (%) vs time (day) for C−H borylation of toluene using pyrim-MOF-Co (0.2 mol % Co), pyrim-UiO-66-
MOF-Co (0.2 mol % Co), and [Ph(pyrim)(PhCO2Me)2]Co (0.2 mol % Co) as catalysts under identical conditions. (b) Plot for the GC yield (%)
of arene boronate ester at various runs in the recycle and reuse of pyrim-MOF-Co (0.5 mol % Co) for borylation of o-xylene.
2.3. Effect of Pore Sizes and Comparison Studies with
Homogeneous Control. The effect of pore sizes of MOF in
borylation reactions was further studied by comparing the
catalytic activity of pyrim-MOF-Co with pyrim-UiO-66-Co
MOF having similar UiO-topology but smaller pore sizes
(Figure 2a). Pyrim-UiO-66-Co MOF having UiO-66 topology
was synthesized via postsynthetic modification of UiO-66-
NH2-MOF with pyridinecarboxaldehyde,18 followed by metal-
ation with CoCl2 to afford a bluish-green crystalline solid.
Under identical reaction conditions, pyrim-MOF-Co was
almost 5 times more active than pyrim-UiO-66@Co in the
C−H borylation of toluene, indicating that the catalysis
facilitated by pyrim-Co sites was both on the surface and inside
the pores (Figure S15, SI). Interestingly, pyrim-MOF-Co was
also significantly more active than its homogeneous control
([Ph(pyrim)(PhCO2Me)2]Co) (Figure S12, SI). The homo-
geneous control was prepared by the condensation reaction
between dimethyl-2′-amino-[1,1′:4′,1″-terphenyl]-4,4″-dicar-
boxylate and pyridinecarboxaldehyde followed by metalation
with Co precursor. Under identical reaction conditions and
Co-loading (0.2 mol %), the homogeneous control was
inactive in the borylation of toluene, giving only 2%
conversion; in contrast, the borylation proceeded smoothly
by pyrim-MOF-Co until full conversion as indicated by the
time-dependent GC-conversion plot (Figure 2a). This result
demonstrates that a highly active and sterically open
pyridylimine-Co species can be afforded within the MOF
frameworks via site isolation, which cannot be stabilized in
solution by the analogous pyridylimine or Ph(pyrim)-
(PhCO2Me)2 ligand due to the lack of steric protection
around the Co-center for shutting down the intermolecular
decomposition pathways.
reused at least 9 times without significant loss of catalytic
activity (Figure 2b). The leaching of Co and Zr in the
supernatant for the first run was only 0.05% and 1.9%,
respectively, and that for the fourth run was 0.1% and 3.01%,
respectively (Table S2, SI). The PXRD pattern of pyrim-MOF-
Co after the borylation reaction remained unchanged,
indicating the robustness of the MOF frameworks under the
catalytic conditions (Figure 1b).
2.5. Pyrim-MOF-Co-Catalyzed Borylation of Benzylic
C−H Bonds. Pyrim-MOF-Co also catalyzes regioselective
borylation of secondary or tertiary C(sp3)−H bonds at the
benzylic position over arene groups. Heating the mixture of
pyrim-MOF-Co (0.5 mol % Co) and B2pin2 in neat alkylarenes
at 110 °C gave the corresponding benzylic boronate esters in
100% conversion with good selectivity. Due to significant loss
of yields of benzylic boronates during isolation (Table S1, SI),
the crude products were directly oxidized, and the final
products were isolated as the corresponding benzyl alcohols in
high yields. The borylation of mesitylene occurred selectively
at the benzylic position presumably due to the steric factor
(Table 2, entry 1). 2,4,6-Trimethylbenzyl alcohol was afforded
after oxidation of the corresponding boronate ester in 82%
yield, and no aryl boronate ester was detected. In the case of
borylation of ethylbenzene, 1-ethyl-4-methoxybenzene, 1-
ethyl-4-chlorobenzene, and cumene, good selectivities of
borylation were also observed in favor of benzylic C−H
bonds over aryl C−H bonds, affording high yields of the
corresponding benzyl alcohols (Table 2, entries 2−5).
Interestingly, in the case of p-cymene, the regioselectivity of
borylation strongly favored at tertiary C−H bonds over aryl
C(sp2)−H or primary C(sp3)−H bonds, and 2-(p-tolyl)-
propan-2-ol was obtained in 89% yield (Table 2, entry 6).
2.6. Stabilization of Catalytically Active (Pyrim•−)-
CoI(THF) Species within MOF. We then investigated the
mechanism of the pyrim-MOF-Co-catalyzed C−H borylation
of arenes by the characterization of the active cobalt species,
kinetics, and computational studies. The reaction between
pyrim-MOF-CoCl2(THF) and NaEt3BH in THF resulted in a
black MOF-material and 1 equiv of H2 with respect to cobalt.
In a related bis(imino)pyridine (ArPDI) system, the treatment
of 2 equiv of NaEt3BH with (ArPDI)CoCl2 produced the Co-
2.4. Heterogeneity Test. We have performed several
experiments to test the heterogeneous nature of the pyrim-
MOF-Co-catalyst. The rate of borylation was unchanged in the
presence of metallic mercury, ruling out the role of any in situ
generated Co-nanoparticles or leached Co-particles during the
reactions (Figure S10, SI). In addition, no further conversion
was detected after the removal of the MOF-catalyst during the
course of the borylation reaction (Figure S9, SI). Furthermore,
at 0.5 mol % Co-loading, pyrim-MOF-Co was recovered and
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of pyrim-MOF-Co(THF) is close to that of pyrim-MOF-
CoCl2(THF) and also significantly higher (10−12 eV) than
that of metallic cobalt. The XANES analysis indicated a higher
actual oxidation state for the Co-center within pyrim-MOF-
Co(THF) than formally assigned zero, and we thus infer that
the oxidation state of cobalt in pyrim-MOF-Co(THF) is +1.
Natural bond orbital (NBO) analysis also showed the
distribution of high positive charge (+0.78) on the Co atom
and high negative charge (−0.82) on the pyridylimine moiety
(Figure 3b and Table S6). These electronic properties of
pyrim-MOF-Co(THF) are similar to those of other low-valent
transition-metal complexes with redox noninnocent ligand-
s.17a,20 The experimental and computational data thus
suggested a (pyrim•−)CoI(THF) ground state in the MOF,
which is likely the catalytic species for C−H borylation
reactions.
Table 2. Pyrim-MOF-Co-Catalyzed Benzylic C−H
a
Borylation
2.7. Investigation of the Mechanism of MOF-Co-
Catalyzed C−H Borylation. The reaction pathway was
further characterized by kinetic studies. The empirical rate law
was determined by the investigations of instantaneous rate
dependence on product concentration for the first 3−15%
conversion of o-xylene. The concentration of the boronate
ester was monitored by GC-MS throughout the catalytic
conversion. The average initial substrate conversion rates
increase linearly with initial concentrations of o-xylene and
cobalt, indicating a first-order rate dependence on both
substrate and catalyst concentrations (Figure 3c). However,
the initial rate of the C−H borylation of o-xylene showed a
zeroth-order dependence on B2pin2 concentration (Figure 3d).
Besides, a significant enhancement of reaction rate was
observed upon the addition of 10−30 equiv of THF with
respect to Co, indicating the potential coordination of THF to
the catalytic intermediates in the favorable pathway (Table S3,
SI).
a
Reaction conditions: (i) borylation; 0.5 mol % Co, B2pin2 (0.134
Based on the spectroscopic and kinetic studies, two plausible
pathways are proposed (Figure S16, SI). According to A-
pathway, the oxidative addition of B2pin2 to cobalt of
(pyrim)Co(THF) would give (pyrim)Co(Bpin)2[INT-1(A)],
which undergoes turnover limiting σ-bond metathesis with the
arene C−H bond to generate (pyrim)Co(H)(Bpin)[INT-
2(A)] and arylboronate. Subsequent coordination of THF to
the cobalt of INT-2(A) followed by reductive elimination
would lead to the regeneration of the active state of the Co-
catalyst and completes the catalytic cycle. Alternatively, the
rate-limiting oxidative addition of the arene C−H bond
followed by σ-bond metathesis with B2pin2 would give INT-
3(B), which undergoes reductive elimination to regenerate the
Co-catalyst and arylboronate (B-pathway, Figure S16, SI).
2.8. Computational Study and Catalytic Cycle. To
further probe the mechanism of the borylation reaction, we
performed DFT calculations, and the calculated energy profile
of both pathways using the DFT method (B3LYP) and a basic
set of 6-311G (d,p) on Gaussian 09 software suit as shown in
Figure 4. In A-pathway, the catalytic cycle commences with
(pyrim)Co(THF) and B2pin2, which undergoes oxidative
addition via TS-1(A), requiring free energy of 6.5 kcal/mol.
This step is exergonic and gives INT-1(A). Then, the σ-bond
metathesis of Co−Bpin and arene C−H bond transforms INT-
1(A) to INT-2(A) via TS-2(A), requiring an activation free
energy of 20 kcal/mol. Subsequently, the coordination of THF
leads to the formation of INT-3(A), which is 5.3 kcal/mol
lower in energy than INT-2(A). Then, INT-3(A) undergoes
reductive elimination to give (pyrim)Co(THF) and HBpin via
mmol), neat arene, 110 °C, 2 days, 100% conversion; (ii) oxidation; 2
b
M NaOH, 30% H2O2, room temperature, 2 h. The ratio of benzyl-
and aryl-boronate ester in the crude product obtained after borylation
reaction as determined by GC-MS is mentioned in the parentheses.
c
Heptane was used as the solvent.
dinitrogen complex (ArPDI)Co(N2)2 via reductive elimination
of H2 from the (ArPDI)Co(H)2 intermediate.19 We thus infer
that the reaction of NaEt3BH with the (pyrim)CoCl2(THF)
moiety within MOF generates the putative (pyrim)Co(H)2
intermediate, which undergoes rapid reductive elimination to
furnish the reduced cobalt species, (pyrim)Co. However,
unlike the analogous ArPDI-system, no characteristic band of
NN was observed in the infrared spectrum ruling out the
coordination of dinitrogen to Co-centers in pyrim-MOF-Co
(Figure S6a, SI). The DFT optimized structure revealed a Co
intermediate of (pyrim)Co(THF) containing a pyridylimine
and a THF molecule as ligands in the ground state, which is 33
and 17.1 kcal/mol lower in energy than (pyrim)Co and
(pyrim)Co(H)2, respectively (Figure S22, SI). The presence of
1 equiv of THF with respect to Co within (pyrim)Co(THF)
1
was also established by H NMR spectroscopy. The pyrim-
MOF-Co suspension in THF showed a broad UV−vis peak at
670 nm, indicating a strong metal-to-ligand charge-transfer
(MLCT) transition in this low-valent cobalt species (Figure
S7b, SI). The X-ray absorption near edge structure (XANES)
analysis of pyrim-MOF-Co(THF) showed that the position of
its Co K-pre-edge is 3 eV lower than that of (pyrim)-
CoCl2(THF) (Figure 3a). In addition, the Co K-edge position
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Figure 3. (a) XANES spectra of (pyrim)CoCl2(THF) and pyrim-MOF-Co(THF). (b) Atomic charge distribution of (pyrim)Co(THF) as
calculated by NBO population analysis; atoms bearing positive and negative charges are denoted by green and red, respectively. (c) Kinetic plots of
initial rates (d[product]/dt) for arene C−H borylation of o-xylene versus initial concentrations of cobalt and o-xylene for the first 12 h, showing the
first-order dependence on both components. (d) Plot of initial rate (d[product]/dt) versus [B2pin2]initial for first 12 h (<10% conversion) in neat o-
xylene showing the independence of initial rates on the B2pin2 concentrations. (e) Proposed catalytic cycle based on kinetic and computational
studies.
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Figure 4. DFT-calculated reaction energy profile of pyrim-MOF-Co-catalyzed C−H borylation of arenes. See the SI for computational details.
TS-3(A), and this step is endergonic by 13.3 kcal/mol (Figure
4). In the case of B-pathway, the oxidative addition of the
arene C−H bond is disfavored by 6 kcal/mol compared to that
of A-pathway. Furthermore, the subsequent σ-bond metathesis
of the Co−H and B−B bond is associated with a step barrier of
29.5 kcal/mol, whereas the reductive elimination requires 10.5
kcal/mol. Therefore, the DFT-calculated energy profile of
pyrim-MOF-Co(THF)-catalyzed C−H borylation reveals A-
pathway as energetically favorable as compared to the B-
pathway, and TS-2(A) is the turnover limiting step (Figure
3e). The DFT calculation identifies TS-2(A) as a four-member
cyclic transition state involving σ-bond metathesis of Co−Bpin
and the arene C−H bond (Figure S21). The first-order
dependence on the concentrations of both cobalt and arene in
the kinetic studies further supports the involvement of the
arene C−H bond in the turnover limiting step. The proposed
catalytic cycle (A-pathway) is thus in good agreement with our
theoretical and kinetic studies (Figure 3e).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00747.
Synthesis and characterization of pyridylimine linkers,
MOF, metalated MOF, and molecular analogue;
procedures for catalytic reactions; and details for
kinetics, GC analysis, and DFT calculations (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Kuntal Manna − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India; orcid.org/
0000-0002-2924-0353; Email: kmanna@chemistry.iitd.ac.in
Authors
Rajashree Newar − Department of Chemistry, Indian Institute
of Technology Delhi, New Delhi 110016, India
Wahida Begum − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Neha Antil − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Sakshi Shukla − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Ajay Kumar − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Naved Akhtar − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Balendra − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
3. CONCLUSIONS
In conclusion, we have developed a highly active, robust, and
recyclable cobalt-catalyst for arene and benzylic C−H
borylation. This heterogeneous single-site cobalt-catalyst was
prepared by metalation of easily synthesized pyridylimine-
functionalized zirconium MOF with cobalt precursor. The
MOF-Co-catalyst was significantly more active than its
homogeneous control, highlighting the beneficial effect of
active-site isolation within the MOF framework that prevents
intermolecular decomposition. The experimental and compu-
tational studies showed that MOF stabilize highly reactive and
solution-inaccessible (pyrim•−)CoI(THF) as the active cata-
lytic species via site isolation. The proposed catalytic cycle of
the borylation reaction has also been evaluated based on
spectroscopic, kinetic, and computational studies. This work
thus highlights the importance of MOF-ligated base-metal
catalysis for the sustainable synthesis of fine chemicals.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00747
Notes
The authors declare no competing financial interest.
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Article
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ACKNOWLEDGMENTS
■
This research is supported by the Science and Engineering
Research Board (SERB), India (project ECR/2017/001931).
R.N., S.S., and N.A. acknowledge CSIR, and W.B. acknowl-
edges UGC for financial support. We thank Prof. Sreedevi
Upadhyayula for the BET analysis. The authors acknowledge
the Central Research Facility, IIT Delhi, for instrument
facilities.
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