Cobalt-bridged secondary building units in a titanium metal-organic framework catalyze cascade reduction of N-heteroarenes

Article abstract of DOI:10.1039/C8SC04610G

We report here a novel Ti₃-BPDC metal-organic framework (MOF) constructed from biphenyl-4,4′-dicarboxylate (BPDC) linkers and Ti₃(OH)₂ secondary building units (SBUs) with permanent porosity and large 1D channels. Ti-OH groups from neighboring SBUs point toward each other with an O-O distance of 2 ?, and upon deprotonation, act as the first bidentate SBU-based ligands to support Co^II-hydride species for effective cascade reduction of N-heteroarenes (such as pyridines and quinolines) via sequential dearomative hydroboration and hydrogenation, affording piperidine and 1,2,3,4-tetrahydroquinoline derivatives with excellent activity (turnover number ～ 1980) and chemoselectivity.

Full text of DOI:10.1039/C8SC04610G

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Cobalt-bridged secondary building units in
a titanium metal–organic framework catalyze
cascade reduction of N-heteroarenes†
Cite this: DOI: 10.1039/c8sc04610g
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
a
a
ab
a
Xuanyu Feng, ‡ Yang Song, ‡ Justin S. Chen, Zhe Li, Emily Y. Chen,
a
b
ab
Michael Kaufmann, Cheng Wang
and Wenbin Lin
*
0
We report here a novel Ti
3
–BPDC metal–organic framework (MOF) constructed from biphenyl-4,4 -
dicarboxylate (BPDC) linkers and Ti
3
(OH) secondary building units (SBUs) with permanent porosity
2
and large 1D channels. Ti–OH groups from neighboring SBUs point toward each other with an O–O
distance of 2 A˚ , and upon deprotonation, act as the first bidentate SBU-based ligands to support
Received 16th October 2018
Accepted 18th December 2018
II
Co -hydride species for effective cascade reduction of N-heteroarenes (such as pyridines and quinolines)
DOI: 10.1039/c8sc04610g
via sequential dearomative hydroboration and hydrogenation, affording piperidine and 1,2,3,4-
tetrahydroquinoline derivatives with excellent activity (turnover number ꢀ 1980) and chemoselectivity.
rsc.li/chemical-science
IV
Ti –OH groups in the neighboring SBUs of Ti
3
–BPDC point at
Introduction
˚
each other with a 2 A distance and act as excellent bidentate
II
The development and functionalization of Ti-oxo materials is ligands to complex Co -hydride species to provide a sterically
a fertile research area due to their high crust abundance, low open and electronically-rich Earth-abundant metal catalyst for
toxicity, excellent chemical and thermal stability, and unique the cascade reduction of N-heteroarenes to piperidine and
photophysical properties. The introduction of permanent 1,2,3,4-tetrahydroquinoline derivatives (Fig. 1).
porosity into bulk TiO
2
allows efficient utilization of all surface
functionalizable sites to ensure sustainable resource utiliza-
1
–3
tion.
For example, surface modications of mesoporous
titania have generated a series of Ti–OH anchored metal species
4–6
displaying impressive catalytic or photocatalytic activities.
Ti metal–organic frameworks (MOFs) provide a highly
tunable platform to realize molecular-level design and
construction of single-crystalline and porous Ti-oxo materials
7–10
that can be further functionalized for catalytic applications.
However, only a few Ti-carboxylate MOFs have been synthesized
and characterized since the report of the rst Ti-carboxylate
MOF (MIL-125) with Ti O (OH) secondary building units
8
8
4
11–17
(SBUs) in 2009 (Table S1, ESI†).
Ti MOFs with permanent
porosity, large channels, and sterically open functionable sites
are even rarer.
Herein we report the rational synthesis of a novel porous Ti-
0
carboxylate MOF, Ti
ylate) with unprecedented Ti
3
–BPDC (BPDC ¼ biphenyl-4,4 -dicarbox-
3
(OH)
2
SBUs. Two terminal
a
th
Department of Chemistry, University of Chicago, 929 E. 57 St., Chicago, Illinois
6
0637, USA. E-mail: wenbinlin@uchicago.edu
b
College of Chemistry and Chemical Engineering, iCHEM, State Key Laboratory of
Physical Chemistry of Solid Surface, Xiamen University, Xiamen 361005, PR China
Electronic supplementary information (ESI) available. CCDC 1859034. For ESI
and crystallographic data in CIF or other electronic format see DOI:
Fig. 1 (a) Surface modification of mesoporous TiO
tional materials. (b) Select Ti-MOF SBU structures (MIL-125, PCN-22,
MOF-901, and DGIST-1). (c) Schematic illustration of novel Ti (OH)
SBUs in Ti –BPDC and their metalation to generate anchored Co -H
species for cascade reduction of N-heteroarenes.
2
generates func-
†
3
2
II
1
0.1039/c8sc04610g
3
‡
X. F. and Y. S. contributed equally to this work.
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1
3 3 4
H NMR analysis of digested Ti –BPDC (in D PO /HF) showed
Results and discussion
Synthesis and postsynthetic metalation of Ti –BPDC
3
the presence of 1.38 equiv. of HOAc w.r.t. H BPDC (Fig. S6,
2
ESI†), affording a formula of [Ti (BPDC) (OH) ](OAc) . Powder
3
3
2
4
To prevent the formation of amorphous TiO
2
, we used X-ray diffraction (PXRD) showed that bulk phase and single
i
18
Ti
source for MOF growth. Colorless rhombic Ti
were obtained in 31% yield via a solvothermal reaction between carboxylate to Ti , Ti
6
O
6
(O Pr)
6
(abz)
6
(abz ¼ 4-aminobenzoate) cluster as the Ti crystals of Ti
3
–BPDC have identical structure (Fig. 2b). Due to
12
–BPDC crystals relatively stable Ti
3
-cluster SBUs and strong coordination of
3
IV
ꢁ
3
–BPDC was thermally stable up to 274 C
i
Ti O (O Pr) (abz) and H BPDC in N,N-dimethylformamide at (Fig. S7, ESI†) and maintained its crystalline structure in a range
6
6
6
6
2
ꢁ
1
20 C for 3 days with acetic acid (HOAc) as modulator. Single- of organic solvents and bases (Fig. S15, ESI†). Ti
3
–BPDC exhibits
crystal X-ray diffraction (SXRD) revealed that Ti –BPDC crystal- a type I isotherm with a Brunauer–Emmett–Teller (BET) surface
3
2
ꢂ1
ꢀ
area of 636 m g and a pore size of 11.8 A˚ . To the best of our
SBUs, with the central Ti knowledge, Ti –BPDC is the rst Ti-MOF based on the biphe-
atom in octahedral coordination with six carboxylate groups nyldicarboxylate ligand, which provides a potential for further
lizes in the R3m space group. Every three Ti atoms are linked by
carboxylate groups to form Ti (OH)
3
2
3
20,21
and each of the two terminal Ti atoms in tetrahedral coordi- functionalization.
nation with three carboxylate groups and one OH group.
The unique structure of Ti–BPDC, particularly the proximity
Ti (OH) SBUs are connected by six BPDC linkers to afford a 3D of the neighboring Ti –OH groups, inspired us to use the SBUs
framework of dia topology with 1D triangle channel of 11 A in
to support Earth-abundant metal catalysts. Although indi-
length running along the c axis (Fig. 2a). The void space was vidual SBUs of MOFs have been previously used as structural
calculated to be 71.9% by PLATON. Diffuse reectance infrared and functional mimics of metal oxide catalyst supports,
Fourier transform spectroscopy (DRIFTS) revealed the presence there has been no study of using neighboring SBUs as bidentate
of nO–H stretching band at 3678 cm for the terminal Ti –OH.
IV
3
2
22
˚
23–28
ꢂ1
IV
19
ligands to support Earth-abundant metal catalysts. We posited
that chelation by neighboring SBUs affords several advantages
ꢂ
including stronger coordination from two or more M–O
groups, more electron-rich active metal sites, and more steri-
cally open coordination environment around catalytic centers.
Ti –BPDC was deprotonated with 5 equiv. of LiCH Si(CH )
3 3
3
2
and then metalated with 1 equiv. of CoCl in THF to afford Ti –
2
3
BPDC–CoCl as the purple solid. Both the carboxylate groups
and linkers remained intact during the lithiation and metal-
ation as evidenced by unchanged carbonyl stretching vibrations
in the IR spectra. The disappearance of terminal OH stretching
IV
band (Fig. 2c) indicated complete deprotonation of Ti –H
groups. Inductively coupled plasma-mass spectrometry (ICP-
MS) analysis of digested Ti –BPDC–CoCl revealed the presence
3
of 0.92 ꢃ 0.04 Co per Ti SBU, indicating nearly complete
3
metalation of all Ti–OH pairs. Crystallinity maintained aer
metalation as evidenced in the PXRD patterns, and SXRD of Ti
BPDC–CoCl displayed a nearly identical unit cell parameter to
Ti –BPDC. Unfortunately, aer numerous trials, the coordina-
3
–
3
tion environment of Co could not be established through SXRD
due to the highly disordered nature of coordinated cobalt
atoms. Instead, extended X-ray absorption ne structure
(EXAFS) spectroscopy and density functional theory (DFT) were
used to determine the Co coordination environment in Ti3–
BPDC–CoCl. DFT calculation at the B3LYP level of theory
ꢂ
converged at a distorted tetrahedral [(Ti–O )
2
CoCl(THF)] coor-
dination with two oxo groups from the deprotonation of Ti–OH
ꢂ
˚
(
Co–O distances of 1.89 and 1.95 A), one chloride ion (Co–Cl
THF
˚
distance of 2.06 A˚ ).
distance of 2.33 A), and one THF (Co–O
The calculated model tted well to the experimental EXAFS data
Ti
of Ti –BPDC–CoCl (Fig. 2d, Table S5, ESI†), which gave Co–(O )
Fig. 2 (a) Structure and SBU arrangement of Ti
conversion to Ti –BPDC–CoH. (b) PXRD patterns of Ti
Ti –BPDC–CoCl (navy), and Ti –BPDC–CoH (violet) and after-reac-
tion (orange) along with simulated PXRD based on the SXRD structure
3
–BPDC and its
3
˚
3
3
–BPDC (red), distances of 1.86 and 1.93 A, respectively, with a Co–Cl distance
THF
3
3
of 2.25 A and a Co–O
Treatment of Ti –BPDC–CoCl with 10 equiv. of NaBEt
toluene generated Ti –BPDC–CoH as a dark blue solid through
Cl/H exchange. MOF crystallinity was maintained based on
similar PXRD patterns before and aer NaBEt H treatment
˚
distance of 2.03 A˚ .
3
3
H in
(
black). (c) DRIFT spectrum of Ti
3
–BPDC showing stretching vibration
ꢂ1
3
of Ti–OH at 3678 cm from Ti
3
(OH) SBUs. (d, e) EXAFS spectra (black
2
circles) and fits (grey solid line) in R-space at the Co K-edge adsorption
of (d) Ti –BPDC–CoCl and (e) Ti –BPDC–CoH.
3
3
3
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(Fig. 2b). 96.5% of chloride was exchanged by hydride as ring-closing pathways, catalytic hydrogenation of pyridines
determined by X-ray Fluorescence analysis (Fig. S19, ESI†). X-ray provides an atom-/step-efficient strategy to construct piperi-
33
photoelectron spectroscopy (XPS) and X-ray absorption near- dines. However, there has been only a few examples of
34
35
edge spectroscopy (XANES) were used to determine the oxida- hydrogenation of pyridines using homogeneous Rh and Ir,
3
6
37
38
39
tion states of Ti and Co centers in Ti
3
–BPDC–CoH. The Co heterogeneous Rh, Pd, and Co, and metal-free systems, as
40,41
centers exhibited strong 2p3/2 and 2p1/2 peaks at 781.1 and well as the semi-reduction of pyridines,
likely due to strong
7
8
96.8 eV along with 2p3/2 and 2p1/2 shake-up peaks at 785.9 and poisoning effect of pyridines on the catalytic sites. Inspired by
II
02.6 eV, consistent with Co species (Fig. 3a). Ti centers our previous success in hydroboration and hydrogenation
23,42
showed strong 2p3/2 and 2p1/2 peaks at 458.3 and 464.1 eV, reactions catalyzed by SBU-supported Co-hydrides,
which are characteristic of Ti species (Fig. 3b). The Co pre- envisaged a one-pot cascade reduction pathway to prepare
we
IV
edge features of Ti –BPDC–CoCl and Ti –BPDC–CoH are iden- piperidines from pyridines using Ti –BPDC–CoH. Pinacolbor-
3
3
3
II
tical to that of CoCl (Fig. 3c), conrming the Co oxidation ane (HBpin) was added to the reactions to rst dearomatize
2
state before and aer hydride activation. Similar Ti pre-edge N-heteroarenes via hydroboration, followed by the hydrogena-
features were observed for Ti
CoH (Fig. 3d), indicating the identical Ti oxidation states. intermediates.
Treatment of Ti –BPDC–CoH with excess formic acid generated At 0.2 mol% Ti
.96 ꢃ 0.04 equiv. of H
3 3
–BPDC–CoCl and Ti –BPDC– tion of the remaining unsaturated bonds of the hydroborated
3
3
–BPDC–CoH, treatment of 3-picoline with
ꢁ
0
2
w.r.t. Co, indicating the presence of Co– 1.05 equiv. of HBpin in n-octane under 20 bar H
2
at 100 C for
H species. EXAFS tting on Ti –BPDC–CoH showed that Co 22 h gave 3-methylpiperidine in 97% yield. No obvious change
3
ꢂ
Ti
centers adopt (Ti–O ) Co bridging mode with Co–(O ) was observed in the MOF's PXRD pattern aer catalysis
2
˚
˚
distances of 1.93 A and 1.96 A (Fig. 2e) which are similar to (Fig. 2b), indicating the stability of the MOF framework under
those of Ti –BPDC–CoCl. Due to the unique site-isolation effect reaction condition. Changing n-octane to coordinating solvents
within MOF structure,
oxo-based chelating ligands cannot be prepared in homoge- yields (Table S8, ESI†), suggesting the inhibition effect from
3
29,30
such monomeric Co–H species with (e.g., THF) or solvent-free condition led to a dramatic drop in
neous systems due to their tendency to oligomerize.
coordination of the solvent or pyridine substrate. No piperidine
product was observed in the absence of MOF, HBpin, or H
(Table S9, ESI†). An unprecedentedly high turnover number
TON) of 1980 was achieved at 0.05 mol% catalyst loading in 3
2
Ti–BPDC–CoH catalyzed cascade reduction of pyridines
(
3
Ti –BPDC–CoH displayed high catalytic activities in the cascade
days. At 0.2–0.5 mol% of Co loading, a wide range of pyridines
with electron donating or withdrawing groups on 3-, 4-, or 5-
positions of the pyridine rings, were all reduced to piperidines
in high yields (73–100%). This cascade reduction reaction also
exhibits good functional group tolerance and outstanding
product selectivity. Alkoxy, ester, dialkyl amide, and silyl groups
all remained intact during the reduction process. For pyridine
substrates containing aromatic rings, e.g., 3/4-phenylpridines
and 4-benzylpyridines, the pyridyl rings were selectively
reduced without affecting the phenyl rings (Table 1).
reduction of pyridines to piperidine derivatives. As one of the
most important building blocks in drugs and bioactive alka-
31,32
loids,
the synthesis of piperidines has drawn signicant
interest in the past few decades. Compared to traditional
Several lines of evidences support the cascade process
(
Tables 1, S8, Fig. S30†). First, 2-picoline and 2,6-lutidine
showed dramatic activity decrease compared to 3-picoline and
,5-lutidine, respectively, likely due to the blocking of Co coor-
3
dination by the methyl group(s) to inhibit the hydroboration
step. Second, we detected 25% hydroborated 3-picoline by Ti3–
BPDC–CoH in the absence of H , which demonstrates the Co
2
centers could catalyze the hydroboration step. Third, isolated
41
hydroborated pyridines were quantitatively converted to
piperidine at 0.5 mol% of Ti –BPDC–CoH under 20 bar H
3
2
,
while no piperidine was observed without the MOF catalyst.
Cascade reduction was thus initiated by dearomative hydro-
boration of pyridines followed by hydrogenation of the
remaining unsaturated bonds (Fig. 4a). Both steps are catalyzed
Fig. 3 (a) Co 2p XPS spectra of Ti
3
–BPDC–CoH and the fitted result by the Co–H species.
indicates the Co oxidation state after NaBEt H treatment. (b) Ti 2p
XPS spectra of Ti –BPDC–CoH and the fitted result indicates the Ti
oxidation state after NaBEt H treatment. (c) Co pre-edge XANES
features of Ti –BPDC–CoCl (navy) and Ti –BPDC–CoH (violet) in
comparison to that of CoCl (black). (d) Ti pre-edge XANES features of
Ti –BPDC–CoCl (navy) and Ti –BPDC–CoH (violet).
2+
3
3
Ti –BPDC–CoH was recovered and reused at least 6 times
4+
3
without signicant decrease in yields (93–100%, Fig. S32, ESI†).
ICP-MS showed minimal leaching of Co and Ti (0.4% and 0.6%,
respectively). Hot ltration experiment ruled out the possibility
of leached Co species contributing to the cascade reduction
3
3
3
2
3
3
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a
a
3
Table 1 Ti –BPDC–CoH catalyzed cascade reduction of pyridines
3
Table 2 Ti –BPDC–CoH catalyzed selective reduction of quinolines
a
0
3
.50 mmol quinoline, 0.525 mmol HBpin, 20 bar H
2
, 0.2–0.5 mol% of
ꢁ
Ti
mesitylene as internal standard. 1.05 equiv. HBpin, 50 bar H
equiv. HBpin, 20 bar H
–BPDC–CoH, 100–140 C, 22 h; yield determined by GC-MS using
b
c
2
.
3
2
.
a
0
3
.50 mmol pyridine, 0.525 mmol HBpin, 20 bar H
2
, 0.2–0.5 mol% of Ti–BPDC–CoH catalyzed cascade reduction of quinolines
ꢁ
Ti
–BPDC–CoH, 100–140 C, 22 h; yield determined by GC-MS using
b
c
We then applied this cascade protocol to dearomatize other N-
mesitylene as internal standard. Reaction time: 40 h. 1.05 equiv.
d
e
HBpin, 35 bar H
bar H
bond was hydrogenated.
2
.
3 equiv. HBpin, 50 bar H
2
.
3 equiv. HBpin, 35 heteroarenes. Semi-hydrogenation of quinolines is the most
f
2
.
3-((Trimethylsilyl)ethynyl)pyridine as substrate, the triple
straightforward and convenient way to synthesize 1,2,3,4-tetra-
hydroquinolines (1,2,3,4-4HQLs), which have broad applica-
43
tions in pharmaceuticals and agrochemicals. Although several
catalysts have been reported for such semi-hydrogenation
44–46
reactions,
challenges still exist in terms of product selec-
tivity and functional group tolerance. Ti –BPDC–CoH catalyzed
3
semi-hydrogenation of quinolines to generate 1,2,3,4-4HQLs
with excellent selectivity and functional group tolerance via the
cascade reduction process. At 0.2 mol% catalyst loading and
ꢁ
1
00 C, 1,2,3,4-4HQL was generated in >99% yield with <1% of
,6,7,8-4HQL byproduct. Quinolines with methyl groups on
5
different positions (3-,4-,6-,7-) and with different functional
groups (Cl, OMe, COOMe) as well as quinoxaline were all semi-
hydrogenated to 1,2,3,4-4HQLs in >84% yields and good selec-
tivities (Table 2). No conversion was detected for 2,6-dime-
thylquinoline due to the inhibition of substrate coordination to
the Co center.
Fig. 4 (a) Proposed cascade reduction pathway. (b) Shape selectivity Conclusions
of the cascade reduction of 3-(m/p/o-toly)pyridine.
We have synthesized a novel single-crystalline Ti-carboxylate
MOF with permanent porosity and large 1D channels based
reactivity (Fig. S34, ESI†). Additionally, neither Co nanoparticles on unique Ti (OH) SBUs and BPDC linkers. Each pair of closely
3
2
IV
nor NaBEt
3
H afforded any product in the cascade reduction of spaced Ti –OH groups from neighboring SBUs are deproto-
II
3
3
-picoline (Table S10, ESI†). Notably, different regioisomers of nated and then chelate to Co centers. Such SBU-supported
-tolylpyridines showed remarkable shape selectivity in Ti
II
3
–
Co -hydride species is highly active for selective cascade
BPDC–CoH catalyzed cascade reduction (Fig. 4b). Under the reduction of N-heterocyclic rings of pyridines and quinolines:
same conditions, 3-(o-tolyl)pyridine and 3-(m-tolyl)pyridine heteroarenes rst undergo dearomative hydroboration followed
were completely reduced to piperidines, while most of 3-(p-tolyl) by hydrogenation of the remaining unsaturated bonds to afford
pyridine remained unreacted, likely caused by unfavorable synthetically useful piperidines and 1,2,3,4-tetrahydroquino-
steric repulsion between the methyl group of Co-coordinated lines with excellent activity and chemoselectivity. This work
3-(p-tolyl)pyridine and BPDC ligands from far side of the expands the applications of MOFs in developing single-site
channel wall (Fig. 4b) to inhibit the hydroboration step.
solid catalysts by using neighboring SBUs to support Earth-
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abundant metal complexes and highlights the great potential of
MOF catalysts in ne chemical synthesis.
Y. Tsutsui, V. Briois, N. Steunou, G. Maurin, T. Uemura
and C. Serre, Nat. Commun., 2018, 9, 1660.
17 Y. Keum, S. Park, Y.-P. Chen and J. Park, Angew. Chem.,
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18 K. Hong and H. Chun, Inorg. Chem., 2013, 52, 9705–9707.
There are no conicts to declare.
19 N.-Y. Topsøe, J. Catal., 1991, 128, 499–511.
20 D. Sun, Y. Gao, J. Fu, X. Zeng, Z. Chen and Z. Li, Chem.
Commun., 2015, 51, 2645–2648.
Acknowledgements
21 D. Kim, D. R. Whang and S. Y. Park, J. Am. Chem. Soc., 2016,
138, 8698–8701.
This work was supported by NSF (CHE-1464941). We thank Dr
Yuanyuan Zhu, Dr Zekai Lin, and Vlad Kamysbayev for experi- 22 J. Liu, L. Chen, H. Cui, J. Zhang, L. Zhang and C.-Y. Su, Chem.
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supported by the Materials Research Collaborative Access Team 23 K. Manna, P. Ji, Z. Lin, F. X. Greene, A. Urban, N. C. Thacker
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Science User Facility operated for the U.S. DOE Office of Science 24 P. Ji, Y. Song, T. Drake, S. S. Veroneau, Z. Lin, X. Pan and
by ANL, was supported by the U.S. DOE under Contract No. DE- W. Lin, J. Am. Chem. Soc., 2018, 140, 433–440.
AC02-06CH11357. Z. Li acknowledges nancial support from 25 I. S. Kim, Z. Li, J. Zheng, A. E. Platero-Prats,
(
the China Scholarship Council and the National Science
Foundation of China (21671162).
A. Mavrandonakis, S. Pellizzeri, M. Ferrandon, A. Vjunov,
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