Triarylamine-based porous coordination polymers performing both hydrogen atom transfer and photoredox catalysis for regioselective α-amino C(sp3)-H arylation

Article abstract of DOI:10.1039/d1sc00828e

Direct functionalization of C(sp³)-H bonds in a predictable, selective and recyclable manner has become a central challenge in modern organic chemistry. Through incorporating different triarylamine-containing ligands into one coordination polymer, we present herein a heterogeneous approach to the combination of hydrogen atom transfer (HAT) and photoredox catalysis for regioselective C-H arylation of benzylamines. The different molecular sizes and coordination modes of the ligands, tricarboxytriphenylamine (H₃TCA) and tris(4-(pyridinyl)phenyl)amine (NPy₃), in one coordination polymer consolidate the triarylamine (Ar₃N) moiety into a special structural intermediate, which enhances the chemical and thermal stability of the polymers and diminishes structural relaxation during the catalytic process. The inherent redox potentials of Ar₃N moieties prohibit thein situformed Ar₃N˙⁺to earn an electron from C(sp³)-H nucleophiles, but allow the abstraction of a hydrogen atom from C(sp³)-H nucleophiles, enabling the formation of the C(sp³)˙ radical and the cross-coupling reaction to proceed at the most electron-rich sites with excellent regioselectivity. The new heterogeneous photoredox HAT approach skips several interactions between transient species during the typical synergistic SET/HAT cycles, demonstrating a promising redox-economical and reagent-economical heterogeneous platform that has not been reported for α-amino C-H arylation to form benzylamine derivatives. Control experiments based on monoligand coordination polymers suggested that the mixed-ligand approach improved the photochemical and photophysical properties, providing important insight into rational design and optimization of recyclable photocatalysts for rapid access to complex bioactive molecules and late-stage functionalized pharmaceuticals.

Full text of DOI:10.1039/d1sc00828e

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Triarylamine-based porous coordination polymers
performing both hydrogen atom transfer and
photoredox catalysis for regioselective a-amino
C(sp3)–H arylation†
Cite this: Chem. Sci., 2021, 12, 8512
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have been paid for by the Royal Society
of Chemistry
*
*
Hanning Li, Yang Yang, Xu Jing,
Cheng He
and Chunying Duan
Direct functionalization of C(sp3)–H bonds in a predictable, selective and recyclable manner has become
central challenge in modern organic chemistry. Through incorporating different triarylamine-
a
containing ligands into one coordination polymer, we present herein a heterogeneous approach to the
combination of hydrogen atom transfer (HAT) and photoredox catalysis for regioselective C–H arylation
of benzylamines. The different molecular sizes and coordination modes of the ligands,
tricarboxytriphenylamine (H3TCA) and tris(4-(pyridinyl)phenyl)amine (NPy3), in one coordination polymer
consolidate the triarylamine (Ar3N) moiety into a special structural intermediate, which enhances the
chemical and thermal stability of the polymers and diminishes structural relaxation during the catalytic
process. The inherent redox potentials of Ar3N moieties prohibit the in situ formed Ar3Nc+ to earn an
electron from C(sp3)–H nucleophiles, but allow the abstraction of a hydrogen atom from C(sp3)–H
nucleophiles, enabling the formation of the C(sp3)c radical and the cross-coupling reaction to proceed at
the most electron-rich sites with excellent regioselectivity. The new heterogeneous photoredox HAT
approach skips several interactions between transient species during the typical synergistic SET/HAT
cycles, demonstrating a promising redox-economical and reagent-economical heterogeneous platform
that has not been reported for a-amino C–H arylation to form benzylamine derivatives. Control
experiments based on monoligand coordination polymers suggested that the mixed-ligand approach
improved the photochemical and photophysical properties, providing important insight into rational
design and optimization of recyclable photocatalysts for rapid access to complex bioactive molecules
and late-stage functionalized pharmaceuticals.
Received 10th February 2021
Accepted 11th May 2021
DOI: 10.1039/d1sc00828e
rsc.li/chemical-science
hydridic C–H bonds in the presence of other weaker, polarity-
mismatched ones. However, besides the limits of photo-
catalysts capable of performing single electron transfer and
hydrogen atom transfer (SET/HAT) synergistic catalysis in
a homogeneous fashion,13,14 developing efficient heterogeneous
photocatalytic systems15,16 to expand the real applications in
organic syntheses and the ne chemicals industry remains
largely unexplored, partly due to the difficulties in controlling
the kinetics of the multistep electron transfer or hydrogen atom
transfer catalytic cycles in a heterogeneous fashion.17
Porous coordination polymers with ordered photoactive
organic ligands and inorganic nodes are well-known for their
promising catalytic performance.18–22 The regular distribution
of photocatalytic sites in conned environments benets the
xation and stabilization of the active radicals formed under
light-irradiation to overcome the restrictions in a homogeneous
system.23–25 Taking advantage of Ar3N-based coordination
polymers for photocatalysis with excellent photocatalytic
performance and recyclability26–28 and the tertiary amine radical
cation (i.e., quinuclidine radical) capable of engendering
Introduction
The use of C(sp3)–H bonds, which are ubiquitous in organic
molecules, as latent nucleophile equivalents for cross-coupling
reactions has the potential to substantially streamline synthetic
efforts in organic chemistry.1,2 Visible-light photocatalysis has
led to a paradigm shi in organic synthesis that encompasses
the activation of small molecules,3–7 fostering the discovery of
promising tools to activate aliphatic C–H bonds; however,
functionalization of C(sp3)–H bonds in a predictable, selective,
and efficient manner remains a central challenge.8,9 Hydrogen
atom transfer (HAT),10–12 when involved in photocatalysis, can
activate organic substrates without the limitation of redox
potentials, allowing kinetically controlled functionalization of
State Key Laboratory of Fine Chemicals, Zhang Dayu College of Chemistry, Dalian
University of Technology, 116024, P. R. China. E-mail: xjing@dlut.edu.cn; cyduan@
dlut.edu.cn
† Electronic supplementary information (ESI) available. CCDC 1970513, 1970515,
1970516 and 1970538. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d1sc00828e
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a selective HAT at the most electron-rich site of C(sp3)–H
nucleophiles,29 we envisioned that incorporating triarylamine
(Ar3N) moieties into one coordination polymer would combine
HAT and photoredox catalysis for regioselective C(sp3)–H
functionalization at electron-rich C–H sites.
It is noted that organic dyes with Ar3N have never been used
as HAT catalysts in a homogeneous fashion, partly due to the
heavy structural relaxation30 from Ar3Nc+ to Ar3NH+ limiting the
kinetics of H-atom exchange in solution. To avoid the potential
structural relaxation during the HAT process, tertiary amine
HAT catalysts, such as quinuclidine,29,31,32 were always con-
strained in a robust sp3 conformation. Alternatively, the effi-
ciency of the photocatalytic conversions using Ar3N-based
chromophores is dependent on the planar conformation of
the N-center in Ar3N chromophores essentially. Our approach
thus focused on the modication of the N-center conformation
in the Ar3N-based coordination polymers to balance the effi-
ciency of photosensitization with that of HAT catalysis. We
envisioned that incorporating both tricarboxytriphenylamine
(H3TCA)33 and tris(4-(pyridinyl)phenyl)amine (NPy3)34 into one
coordination polymer would provide the possibility of opti-
mizing the conformation of Ar3N moieties into a special inter-
mediate structure that mixed the characteristics of both N(sp3)
and N(sp2), decreasing the potential structural relaxation
during the HAT process. Meanwhile, electron-rich C–H bonds
adjacent to amine functionalities could serve as useful precur-
sors to form the C-centered radical C(sp3)c via a HAT event when
interacting with the aforementioned catalytically generated
amine radical cation.35,36
Fig. 1 (a) The mixed-ligand strategy for the construction of coordi-
nation polymers showing the structures of the two ligands mentioned
and the measured oxidative potentials (vs. Ag/AgCl) (in parentheses) of
the coordination polymers. (b) Schematic illustration of the hetero-
geneous SET/HAT photocatalytic approach in which the in situ formed
oxidized Ar3Nc+ in mixed-ligand Cd-MIX acts as a HAT catalyst for
regioselective C(sp3)–H arylation.
DMSO-d6/DCl gave a 2 : 1 molar ratio of TCA3ꢀ and NPy3 ligands
in the structure (Fig. S16†). The similar powder X-ray diffraction
(PXRD) patterns based on the single-crystal and as-synthesized
Cd-MIX indicated that the bulk Cd-MIX consisted of a pure
phase (Fig. S17†). Changing the molar ratio of the two ligands
does not affect the structure of Cd-MIX, but affects its purity and
yield. Single-crystal analysis revealed that Cd-MIX crystallized in
the C2/c space group, and the asymmetric unit of Cd-MIX
comprised three independent Cd ions, one NPy3, and two
TCA3ꢀ ligands. Both Cd1 and Cd3 were six-coordinated by one N
atom from NPy3, four O atoms from carboxylates, and one O
atom from H2O molecules (Fig. 2a). However, Cd2 was seven-
coordinated by one N atom from NPy3, four O atoms from two
bidentate carboxylates, and two O atoms from bridged carbox-
ylates. The carboxylates connected Cd1 and Cd2 together to
form tetranuclear clusters. The propeller-like TCA3ꢀ ligands
coordinated to the clusters and Cd3 to generate a 3D coordi-
The proposed catalytic strategy was evaluated by the activa-
tion of an N-benzylic C(sp3)–H bond, as benzylic amines are
important pharmaceutical structural motifs that are difficult to
access via simple substrates. The mechanistic details are out-
lined in Fig. 1. Irradiation of polymers containing Ar3N moieties
produces a strongly reductive Ar3N* (Ered ¼ ꢀ2.04 V, vs. Ag/AgCl)
that initially undergoes SET with an electron-decient radical
red
1/2
precursor, 1,4-dicyanobenzene (1,4-DCB, E
¼ ꢀ1.61 V, vs.
SCE),37 to afford the corresponding anionic radical (sub1cꢀ) and
the oxidized photosensitizer Ar3Nc+. The latter selectively
abstracts a H atom from the substrate that contains a C(sp3)–H
bond with a lower C–H bond dissociation energy (BDE) to form
C(sp3)c, as the inherent moderate oxidization potential of Ar3Nc+
prohibits the direct oxidation of substrates via SET. The direct
coupling of the two generated organic radicals predominates
the desired product at the most electron-rich site of the C–H
nucleophile and completes the SET/HAT catalytic cycles. This
redox-economical heterogeneous platform skipped several
interactions between transient species during the reactions,
allowing rapid access to complex bioactive molecules and late-
stage functionalized pharmaceuticals in a recyclable manner.
2
˚
nation polymer with channel openings of 8.6 ꢁ 13.0 A (Fig. 2c).
The NPy3 ligand coordinated with three different Cd ions and
was positioned in a conformation consolidated by the TCA3ꢀ
-
linked Cd nodes (Fig. 2b). Obvious p/p stacking interactions
between the two ligands were also found to stabilize the mixed-
ligand coordination polymer with the interplanar separation
˚
reaching 3.62 A (Fig. S4†).
Results and discussion
Detailed structural analysis revealed that the included angles
around the central nitrogen atom N2 of the NPy3 ligands in Cd-
MIX were 121.3, 123.4, and 115.0ꢂ. The structural constraints
The solvothermal reaction of tricarboxytriphenylamine (H3TCA,
0.05 mmol) and tris(4-(pyridinyl)phenyl)amine (NPy3, 0.05
mmol) with Cd(NO3)2$4H2O (0.25 mmol) gave crystals of Cd-
MIX in a yield of approximately 50%. 1H NMR of Cd-MIX in
˚
consolidated the central N2 atom which was positioned 0.36 A
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Fig. 2 (a) Coordination environments of Cd ions with related TCA3ꢀ
and NPy3 ligands in Cd-MIX. (b) The structural constraints of NPy3
ligands consolidated by the coordination of TCA3ꢀ with Cd ions in Cd-
MIX. (c) Structure of Cd-MIX showing the 3D framework with channel
openings of 8.6 ꢁ 13.0 A˚2. (d) The conformations of the NPy3 ligand in
Cd-MIX (top) and Cd-NPy3 (bottom). Hydrogen atoms and solvent
molecules are omitted for clarity. Cyan, Cd; red, O; blue, N; gray, C;
green, Cl.
Fig. 3 (a) Solid-state UV-vis spectra of Cd-MIX, Cd-TCA, and Cd-
NPy3. (b) Cyclic voltammetry of Cd-MIX in the solid state and amines
examined in solution. Fluorescence decay curves (c) and EPR spectra
(d) of Cd-MIX suspension with and without 1a, 1c, and 1,4-dicyano-
benzene, respectively.
polymers resulted in an obvious redshi of the absorption
band, providing the possibility of photocatalytic conversion
under visible light irradiation.43 Solid-state cyclic voltammetry
revealed that Cd-MIX exhibited simultaneous two oxidation
couples at 0.56 V and 0.76 V (vs. Ag/AgCl), corresponding to the
oxidation of ligands TCA3ꢀ and NPy3, respectively (Fig. 3b). The
inherent moderate oxidization potential of Ar3N in the coordi-
nation polymer prohibited the oxidized state of Cd-MIX from
earning an electron from the C(sp3)–H nucleophiles, i.e., N-
phenylpyrrolidine (1a) (Fig. 3b).44
above the plane dened by the other three N atoms (Fig. S10†).
The conformation of NPy3 thus was xed as a special structural
intermediate that mixed both N(sp3) and N(sp2) characteristics
of the central N atom, which is benecial for the transformation
from Ar3Nc+ to Ar3NH+ to abstract a hydrogen atom at the most
electron-rich site of C(sp3)–H nucleophiles. The average
distance between the central N2 atom and NPy3 ligand-bridged
˚
Cd ions (Cd/N2 distance ca. 10.7 A) in Cd-MIX was approxi-
˚
mately 0.2 A shorter than that of the monoligand coordination
When excited at 420 nm, the suspension of Cd-MIX in
acetonitrile solution exhibited a broad emission band at 508 nm
(Fig. S24†). In combination with the free energy change (E0–
polymer Cd-NPy3, which is assembled from only the NPy3 ligand
(Fig. 2d), further conrming a distorted conformation in the
mixed-ligand coordination polymer.38,39
0
45,46
)
between the ground and excited states, which was ob-
Moreover, the d10 structure of the Cd centers as well as the
variable bridging mode between the two kinds of ligands and
the metal nodes brought some exible elements, allowing the
inherent structural relaxation to occur in the solid state in
a controllable way during the hydrogen atom transfer process.
The free volume in the desolvated coordination polymer Cd-
MIX is approximately 43.5%, as determined by PLATON.40 Dye
uptake experiments performed by soaking Cd-MIX crystals in
a methylene blue solution exhibited 8.0% uptake based on
crystal weight. The uniform distribution of methylene blue
throughout the crystals suggested that these dye molecules
penetrated deeply into these channels (Fig. S22 and S23†),
revealing the porosity of Cd-MIX for guest accommodation.41
The solid-state ultraviolet-visible (UV-vis) absorption spec-
trum of Cd-MIX exhibited broader absorption that spanned the
visible light range than those of the monoligand coordination
polymers Cd-TCA,42 which is assembled from ligand H3TCA,
and Cd-NPy3, respectively (Fig. 3a). Similar to that in the re-
ported mixed-ligand coordination polymers, the stacking p/p
interactions between ligands and the distorting coordination
geometry of the metal centers in the mixed-ligand coordination
tained from the absorption and emission spectra of the
triarylamine-based coordination polymer Cd-MIX, the excited
state potentials were calculated to be ꢀ2.04 and ꢀ1.84 V (vs. Ag/
AgCl) for TCA3ꢀ and NPy3 (Fig. S19†), respectively. These
potentials demonstrated that the excited state of Cd-MIX was
able to reduce the radical precursor 1,4-dicyanobenzene
(ꢀ1.61 V, vs. SCE)37 to form the anionic radical sub1cꢀ and
cation radical Ar3Nc+.
1,4-Dicyanobenzene is able to quench the emission of the
coordination polymer Cd-MIX in an acetonitrile suspension
with a Stern–Volmer constant47 of about 1.0 ꢁ 103 Mꢀ1
,
demonstrating a SET from the excited state of Cd-MIX to the
radical precursor 1,4-dicyanobenzene (Fig. S24†). No obvious
emission variation could be detected when C(sp3)–H nucleo-
philes N-phenylpyrrolidine (1a) and N-benzylpyrrolidine (1c)
were added (Fig. S27 and S28†). Time-dependent emission
decay experiments estimated a signicant decrease of the
emission lifetime of Cd-MIX from 1.40 to 1.15 ns when 1,4-
dicyanobenzene was added, but no lifetime decrease could be
found when N-phenylpyrrolidine (1a) and N-benzylpyrrolidine
8514 | Chem. Sci., 2021, 12, 8512–8520
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(1c) were present (Fig. 3c). The IR spectrum of solid-state Cd- in the dark, which suggested that both the light source and Cd-
MIX immersed in a 1,4-dicyanobenzene solution revealed a CN MIX were indispensable (Tables S4 and S5,† entries 2 and 14).
stretching vibration at 2233 cmꢀ1 (Fig. S29†), suggesting the Cd salt, or the simple mixing of Cd salt and ligands (H3TCA and
direct absorption of 1,4-dicyanobenzene within the pores of Cd- NPy3), resulted in negative activity under identical conditions
MIX. Direct irradiation of the solid-state Cd-MIX adsorbed with (Tables S4 and S5,† entries 3–5), verifying the high photo-
1,4-dicyanobenzene led to a radical signal with a g value of catalytic activity of Cd-MIX for the C–H arylation and that the
approximately 2.004 only aer a minute (Fig. 3d), whereas heavy structural relaxation from Ar3Nc+ to Ar3NH+ in a homo-
irradiation of Cd-MIX itself did not cause any obvious radical geneous fashion indeed limited the kinetics of H-atom
signals under identical conditions in electron paramagnetic exchange. With the decrease of NaOAc content, the conver-
resonance experiments. We infer that the excited state of Cd- sion of the reaction sharply declined, whereas the replacement
MIX cannot oxidize 1a or 1c but is able to reduce the substrate of NaOAc by K2HPO4 (with a similar pKb in aqueous solution)
1,4-dicyanobenzene to generate the aromatic radical for further hardly affected the conversion (Tables S4 and S5,† entries 12
coupling reactions.
and 13). We estimate that an appropriate weak base environ-
The photocatalytic experiment was initiated with N-phenyl- ment is important for neutralizing the acidic Ar3NH+ or HCN
pyrrolidine (1a) and 1,4-dicyanobenzene as starting substrates. produced in the reaction system to obtain an efficient reaction.
With a 23 W household uorescent lamp as the light source, the
Meanwhile, when triphenylamine was used as the photo-
photocatalytic transformation of an amine gave an a-amino catalyst with 1a or 1c as coupling substrates, almost no products
cyanobenzene product (1b) with a conversion of 90% upon were obtained because the free rotation of the three arms in the
0.5 mol% (based on the Ar3N moiety in the mixed-ligand coor- free triphenylamine molecules consumes the energy at the
dination polymer) loading of Cd-MIX at room temperature for excited state and diminishes the photosensitization efficiency
12 h (Table 2). In the reaction of N-benzylpyrrolidine (1c) with (Tables S4 and S5,† entries 11), in addition to the heavy struc-
1,4-dicyanobenzene, arylation at the N-benzylic position gave tural relaxation from Ar3Nc+ to Ar3NH+ limiting the kinetics of
a coupling product (1d) with a conversion of 93% and regiose- H-atom exchange in solution. As the oxidative state of Ar3N
lectivity >96% under the standard reaction conditions, and moieties in Cd-MIX cannot oxidize the substrates 1a and 1c via
almost no arylation products at the cyclic N-methylene position the SET process, and the BDE of Ar3N+–H, i.e., the quinuclidine
were detected (Table 1). Meanwhile, no additional activation of radical cation (N+–H 100 kcal molꢀ1),49 is larger than that of the
products 1b and 1d was detected, as only electron-decient N-benzylic C–H bond (85 kcal molꢀ1),50 we deduced that the
benzonitriles could be activated in this photocatalytic regioselectivity at the N-benzylic position was well consistent
system.48 In addition, no further arylation of the resulting with the idea of a mechanistic switchover from SET to HAT
amine products could be observed, and benzylic amine catalysis.44 Deuterium labelling experiments were conducted by
substrates were added in twice the amount to ensure the treating N-((phenyl-D5)methyl-D2)pyrrolidine (1cD) and 1c with
selectivity.44 Control experiments exhibited negligible conver- 1,4-dicyanobenzene in two different vessels or in one vessel
sion in the absence of Cd-MIX, while no products were detected under optimal conditions (Fig. 4a), resulting in kH/kD values of
3.26 and 2.78, respectively. The kinetic isotope effect indicated
that the C–H cleavage was indeed a hydrogen atom transfer
process and might occur at the rate-determining step of the
reaction.51
Table 1 Photocatalytic a-amino C–H arylation with different catalysts
under standard conditionsa
Entry
Photocatalysts
Conversion (%)
1
2
Cd-MIX
None
98 (93b)
No reaction
3
4
5
6
7
8
9
10
Cd(NO3)2$4H2O
H3TCA + NPy3
H3TCA + NPy3 + Cd(NO3)2$4H2O
Cd-TCA
No reaction
27
32
16
49
19
48
53
Cd-NPy3
Cd-TCA + NPy3
Cd-NPy3 + H3TCA
Cd-TCA + Cd-NPy3
Fig. 4 (a) Kinetics curves of the C–H arylation using N-benzylpyrro-
lidine or N-((phenyl-D5)methyl-D2)pyrrolidine with 1,4-dicyano-
benzene under optimal conditions. (b) The turnover number in scale-
up experiments for C–H arylation of N-benzylpyrrolidine with 1,4-
dicyanobenzene (0.64 g) within 50 h under optimal conditions.
a
Catalyst (0.5 mol%, based on the Ar3N moiety, Cd salt is based on the
molar ratio to Ar3N in Cd-MIX), 1,4-DCB (1.0 mmol), NaOAc (1.0 mmol),
amine 1c (2.0 mmol), and 4.0 mL of DMSO for 10 h. Conversions were
determined by GC-MS. b Isolated yield.
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Table 2 Photocatalytic a-amino C–H arylation with selected amines
and benzonitriles
Compared to typical direct HAT photocatalysis in which the
activated photocatalyst acts as a HAT catalyst to abstract a H
atom from a substrate,12 our approach, in which the in situ
formed oxidized Ar3Nc+ acts as a HAT catalyst, possibly avoids
the use of a strong oxidizer to abstract the H-atom from the
substrate and diminishes the direct oxidization of the substrate
to form a byproduct. It is noted that the H-atom abstraction
agents in previously reported SET/HAT synergistic photo-
catalysis have always been transient radical species generated
either through electron transfer or energy transfer by photo-
catalysts.52 Our approach thus simplied the transformation
steps by using the oxidized state of the photosensitizer itself for
the H-atom abstraction, representing a new HAT strategy
parallel to direct HAT catalysis in which the excited state of the
photocatalyst behaves as
abstraction.53
a
HAT catalyst for H-atom
Besides the advantages of fewer constituents and decreased
reaction steps, the new heterogeneous photoredox-mediated
HAT approach exhibits recyclability of the heterogeneous pho-
tocatalyst.54 We adopted a model uidized bed reactor55 for C–H
arylation of amine compounds to investigate the recycling
properties of Cd-MIX: aer one round of reaction, the reaction
solution was removed, and then fresh reaction solution con-
taining substrates was added for another round of reaction. No
signicant decrease in conversion was observed aer three
cycles (Fig. S36 and S37†). The similar powder X-ray diffraction
patterns of the synthesized and recovered Cd-MIX conrmed
that the polymer structure was maintained (Fig. S40†). By add-
ing excess substrates, 5.0 mmol of 1,4-dicyanobenzene and
10.0 mmol of N-benzylpyrrolidine (1c), the loading of 0.1 mol%
Cd-MIX (based on the Ar3N moiety) gave a TON value of about
900 per mole of catalyst aer 50 hours of irradiation. The total
pseudo-rst order kinetics behavior (rate constant k ¼ 0.048 ꢃ
0.004 hꢀ1) demonstrated that the catalytic efficiency was
maintained aer 50 hours of constant irradiation (Fig. 4b and
S39†). There was no obvious variation in catalytic efficiency
when the size of Cd-MIX was reduced from 200 to 1.0 mm by
mechanical grinding. We infer that the surface area of the
catalyst is not a crucial factor for the arylation reaction
(Fig. S41†). Therefore, Cd-MIX is a real heterogeneous photo-
catalyst that allows the catalytic reaction to occur in the chan-
nels of the coordination polymer.
a
Reaction conditions: catalyst (0.5 mol%, based on the Ar3N moiety),
1,4-DCB (1.0 mmol), NaOAc (1.0 mmol), amine (2.0 mmol), and
4.0 mL of DMAC for 12 h. Catalyst (1.0 mol%, based on the Ar3N
moiety), arenes (0.5 mmol), NaOAc (0.5 mmol), amine (1.0 mmol),
b
c
and 2.0 mL of DMAC for 12 h. DMSO for 10 h. *0.2 mmol scale.
Conversions were determined by 1H NMR. The regioselectivity was
determined by GC-MS.
To further explore the potential factors affecting the new
direct HAT strategy, monoligand coordination polymers Cd-
TCA and Cd-NPy3 were used as reference photocatalysts (Fig. 5).
Cd-NPy3 has a 2D layered structure, while Cd-TCA is a 3D
framework consolidated by pentanuclear Cd-clusters with
As functional benzylic amines are important building blocks
in pharmaceuticals,56 scope exploration began with a change in
amines (Table 2). Piperidines and morpholines provided
desirable results (88% and 83%). Acyclic amines were converted
with moderate conversions (74% and 68%). Meanwhile, amines
with benzyl groups resulted in good conversions to N-benzylic
arylation products with high regioselectivity. Several electron-
decient arenes or heteroaromatic benzonitriles as coupling
substrates could also undergo coupling with amines (1a and 1c),
forming the corresponding products when 1.0 mol% Cd-MIX
was used. The results indicated that Cd-MIX as a new hetero-
geneous photoredox HAT catalyst can achieve efficient conver-
sion via photocatalysis and excellent regioselectivity via HAT
catalysis, which has not been reported for regioselective func-
tionalization of C(sp3)–H bonds.
Fig. 5 (a) Conversions of the C–H arylation of 1c using Cd-MIX, Cd-
TCA, and Cd-NPy3 as catalysts and the excited state potentials of the
three coordination polymers under optimal conditions. (b) Conver-
sions of the C–H arylation of 1c using Cd-MIX, Cd-TCA, and Cd-NPy3
as catalysts and the regioselectivity of N-benzyl arylation products
under optimal conditions.
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2
˚
channel openings of 7.7 ꢁ 8.1 A (Fig. S6†). PXRD analyses which is benecial for the synergistic SET/HAT catalysis to
indicated that the bulk samples of Cd-TCA and Cd-NPy3 con- proceed under visible light irradiation.
sisted of a pure phase (Fig. S18†). Dye adsorption experiments
Transient photocurrent/time tests revealed that Cd-MIX
on the two monoligand coordination polymers revealed that the produces a higher photocurrent response than Cd-TCA and Cd-
layered Cd-NPy3 has a poor adsorption effect, while the porous NPy3 (Fig. 6a), indicating that Cd-MIX has a higher electron–hole
Cd-TCA has comparable adsorption ability to Cd-MIX (e–h) separation efficiency. As photoinduced excitons are active
(Fig. S22†). Control experiments demonstrated that conversions only aer effective charge separation,59 the higher photocurrent
of 20% and 35% with 1a as the substrate, or 16% and 49% with response of Cd-MIX indicates the higher photocatalytic activity.60
1c as the substrate were obtained using Cd-TCA and Cd-NPy3 as Electrochemical impedance spectroscopy plots showed that Cd-
catalysts, respectively (Tables S4 and S5,† entries 6 and 7). The MIX has lower transport resistance than Cd-TCA and Cd-NPy3
higher catalytic efficiency of Cd-NPy3 than that of Cd-TCA (Fig. 6b). The faster electron transfer rate in the mixed-ligand
demonstrated that the different porous behaviors of the coor- polymer Cd-MIX would be another factor endowing higher
dination polymers were not the important factors dominating catalytic efficiency.21,61 We concluded that the mixed-ligand
the catalytic behavior directly.
approach rst improved the photochemical and photophysical
When one monoligand polymer was mixed with another free properties over two monoligand polymers, and then improved
ligand as a photocatalyst, such as Cd-TCA with NPy3 or Cd-NPy3 the photocatalytic efficiency of the C–H arylation reaction, which
with H3TCA, the conversions of the reaction were not ideal under provided insights into the rational design and optimization of
the same conditions (Tables S4 and S5,† entries 8 and 9). The recyclable photocatalysts for rapid access to complex bioactive
photocatalytic efficiency of the mixture of Cd-TCA and Cd-NPy3 molecules and late-stage functionalized pharmaceuticals.
was still lower than that of the mixed-ligand Cd-MIX (Tables S4
1H NMR of the desolvated Cd-MIX immersed in a methanol
and S5,† entries 10). Solid-state cyclic voltammetry of Cd-TCA solution of 1a and 1c revealed that Cd-MIX could adsorb these
(0.53 V) and Cd-NPy3 (0.75 V) (vs. Ag/AgCl) (Fig. S20 and S21†) substrates within its pores (in DMSO-d6/DCl, Fig. S30a and
revealed that the oxidation potentials of both were less positive 31a†). The IR spectra of Cd-MIX adsorbed with 1a and 1c
than those of 1a and 1c (Fig. 3b), and the excited state potentials showed characteristic signals at 2973–2869 cmꢀ1 and 2961–
of Cd-TCA (ꢀ2.42 V) and Cd-NPy3 (ꢀ1.94 V) were both sufficient 2732 cmꢀ1, demonstrating the adsorption and activation of the
to reduce 1,4-dicyanobenzene (Fig. 5a). We concluded that it is substrates within the pores of Cd-MIX (Fig. S30b and 31b†). The
not the redox potential but the special structural intermediate crystal structures of the regular crystals of 1a- and 1c-impreg-
that mixed both N(sp3) and N(sp2) characteristics of the central nated Cd-MIX (denoted as 1a@Cd-MIX and 1c@Cd-MIX)
nitrogen atom, which decreased the structural relaxation from provided activation information for elucidating the mechanism
Ar3Nc+ to Ar3NH+ in the mixed-ligand coordination polymer Cd- details.28,62
MIX, dominates the differentiation of catalytic performance of
As shown in Table S1,† the uptake of substrates 1a or 1c did
the aforementioned coordination polymers, which is consistent not inuence the main structure of the framework, and the cell
well with the idea of a mechanistic switchover from SET to HAT dimensions and pores were retained. In 1a@Cd-MIX, the
catalysis. The proton-coupled electron transfer mechanism,57,58 in amines were located close to the NPy3 moiety; they were
which a proton and an electron are exchanged in a concerted
elementary step, which would require a deprotonation step of 1a
and 1c ahead of the oxidation event, was precluded.
Considering the difference in spectral properties of Cd-MIX,
Cd-TCA, and Cd-NPy3 (Fig. 3a), a comparison of the photo-
catalytic efficiency of the three catalysts was conducted under
LED irradiation with three single wavelengths (365 nm, 395 nm,
and 420 nm). As expected, when irradiated by a 365 nm LED
with 1a or 1c as the substrate, all three catalysts were able to
achieve complete transformation of 1,4-dicyanobenzene (Tables
S4 and S5,† entries 15), but byproducts were present owing to
excessive activation by the strong irradiation (Fig. S42 and
S43†). When irradiated by a 395 nm (or 420 nm) LED with 1a or
1c as the substrate, Cd-MIX gave high conversions (>95%) based
on 1,4-dicyanobenzene, but the catalytic conversions by Cd-TCA
and Cd-NPy3 were both lower than those of Cd-MIX (Tables S4
and S5,† entries 16 and 17). When using a household uores-
cent lamp that closely matches natural light as the light source,
the gap in the catalytic efficiency of the three photocatalysts
gradually widened (Tables S4 and S5,† entries 1, 6, and 7). We
infer that the mixed-ligand approach was of great signicance
to extend the absorption spectra of the catalysts, in addition to
Fig. 6 Photocurrent tests (a) and EIS plots (b) of Cd-MIX, Cd-TCA, and
Cd-NPy3 under the same conditions, showing the superiority of Cd-
MIX over the others. The interaction patterns between the embedded
substrates (1c and 1a) and active sites in the channels of 1c@Cd-MIX (c)
the favourable photosensitivity and the HAT performance, and 1a@Cd-MIX (d).
© 2021 The Author(s). Published by the Royal Society of Chemistry
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inserted between the layers of molecular sheets and interacted used without further purication. H3TCA and NPy3 were
with the phenyl rings of NPy3 via p/p stacking interactions synthesized using previously reported procedures.27,63 Cd-TCA
with an interplanar separation of approximately 3.75 A (Fig. 6d was synthesized according to a published procedure.42
˚
and S12†). The a-position of 1a was positioned near the
Elemental analyses of C, N, and H were performed on a Vario
central N atom of NPy3, reecting that the well-positioned NPy3 EL III elemental analyzer. Powder X-ray diffraction (PXRD) tests
ligands in Cd-MIX should be the active sites for the HAT event. were performed on a SmartLab 9kW X-ray diffractometer
When the trapped substrates contained an N-benzyl group (1c), instrument with a Cu sealed tube. Fourier transform infrared
the embedded amine molecules were adjusted into a suitable spectroscopy (IR) spectra were recorded using KBr pellets on
1
conformation to allow the phenyl rings of NPy3 and 1c to stack a JASCO FT/IR-430 instrument. H NMR and 13C NMR spectra
˚
with an interplanar separation of 3.66 A (Fig. 6c and S13†). In were recorded on a Varian INOVA 400 M spectrometer. Solution
addition, there is also a weak interaction between the phenyl and solid-state UV-vis spectra were obtained on TU-1900 and
rings of 1c and the TCA3ꢀ ligand, and the vertical p/p stacking Hitachi U-4100 spectrophotometers. Thermogravimetric anal-
distance is approximately 3.72 A (Fig. S13†). Such double p/p yses (TGA) were carried out at a ramp rate of 10 ꢂC minꢀ1 up to
˚
stacking interactions provided a driving force for the amine to 800 ꢂC under a nitrogen ow with a Mettler-Toledo TGA/
approach the active sites. Substrate 1c can be docked near the SDTA851 instrument.
central N atom of NPy3 by the C–H/N interactions between N-
Scanning electron microscopy (SEM) images were taken
alkyl and central N as well as between N-benzyl and central N using a HITACHI UHR FE-SEM SU8200 microscope. Fluores-
˚
˚
(ca. 3.87 A and 3.40 A), which facilitates the in situ formed cence spectra were collected on an EDINBURGH F900 instru-
oxidized NPy3c+ to abstract a H atom from the benzyl C–H bond ment. GC analyses were performed on an Agilent Technologies
of 1c. These results support our hypothesis that the strong 6890 N GC system. Electron paramagnetic resonance (EPR)
interaction between the amines and photocatalyst is an experiments were performed on a Bruker E500 instrument, and
important factor in promoting catalytic conversion. The inter- the intensity was recorded at 100 K under an Ar atmosphere.
actions between the amines and active sites in the channels of Cyclic voltammograms (CV) and electrochemical impedance
the polymer accelerate the capture of the substrate amines and spectroscopy (EIS) plots were measured on a ZAHNER ENNIUM
lock the amines around the catalytic sites,24 facilitating the electrochemical workstation. Transient photocurrent/time
generation of amine radicals through the activation of N-alkyl measurements were performed on
a CHI 650E electro-
and N-benzyl C–H bonds by the HAT process.
chemical workstation. Confocal laser scanning microscopy
micrographs were collected using an Olympus Fluoview FV1000
instrument.
Conclusions
In summary, we have reported a mixed-ligand approach to the
construction of dual functional coordination polymers which
enables the arylation of a-amino C(sp3)–H bonds in a predictable,
Preparation of coordination polymers
Synthesis of Cd-MIX. A mixture of H3TCA (19.0 mg, 0.05
regioselective and recyclable manner. The different molecular mmol), NPy3 (24.0 mg, 0.05 mmol), and Cd(NO3)2$4H2O
sizes and coordination modes of the two Ar3N-containing ligands (77.0 mg, 0.25 mmol) was dissolved in a mixed solvent of DMF,
consolidate and optimize the Ar3N moieties into special struc- acetonitrile, and H2O in an autoclave. The resulting mixture was
tural intermediates, facilitating the oxidized Ar3Nc+ to abstract kept in an oven at 110 ꢂC for 72 h. Aer cooling the autoclave to
a hydrogen atom from C(sp3)–H nucleophiles, which is not room temperature, bright yellow lamellar crystals were ob-
capable of proceeding on a homogeneous triarylamine-based tained. Yield: 50%. Prolonging the reaction time or changing
photocatalyst. The new direct SET/HAT strategy endows the the molar ratio between the two ligands will not affect the
electron transfers via a synergistic effect and skips several inter- formation of the structure, but will affect the purity and yield of
actions between transient species during the typical synergistic the crystals. Anal. calcd for C78H57Cd3N7O15: C, 56.11; H,
SET/HAT cycles, demonstrating a promising redox-economical 3.44; N, 5.87%. Found: C, 56.10; H, 3.54; N, 5.81%. IR/cmꢀ1
and reagent-economical heterogeneous photocatalytic platform (KBr): 3062 (w), 3037 (w), 1594 (s), 1523 (br, m), 1390 (s), 1315
that has not been reported for a-amino C–H arylation to form (m), 1174 (m), 813 (w), 782 (m).
benzylamine derivatives. The mixed-ligand approach improves
Syntheses of 1a@Cd-MIX and 1c@Cd-MIX. Cd-MIX was rst
the photochemical and photophysical properties over two mon- soaked in a methanol solution (48 h) to remove guest molecules
oligand coordination polymers, providing deep insights into the and then fully dried in a vacuum oven (100 ꢂC, 1 h). 1a@Cd-MIX
rational design and optimization of recyclable photocatalysts for and 1c@Cd-MIX were obtained by soaking the activated Cd-MIX
rapid access to complex bioactive molecules and late-stage in a methanol solution containing 1a and 1c (10.0 mmol) for
functionalized pharmaceuticals.
48 h, respectively.
Synthesis of Cd-NPy3. A mixture of NPy3 (12.0 mg, 0.025
mmol) and CdCl2$2.5H2O (29.0 mg, 0.13 mmol) was dissolved in
a mixed solvent of DMF, acetonitrile, and H2O in an autoclave.
The resulting mixture was kept in an oven at 100 ꢂC for 72 h. Aer
Experimental section
Materials and measurements
Unless stated otherwise, all chemicals and solvents were of cooling the autoclave to room temperature, pale yellow block-
reagent grade quality obtained from commercial sources and shaped crystals of Cd-NPy3 were obtained. Yield: 56%.
8518 | Chem. Sci., 2021, 12, 8512–8520
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Single crystal X-ray crystallography
General procedure for the consecutive photocatalytic C–H
arylation. A glass tube was lled with 1,4-dicyanobenzene (5.0
mmol), photocatalyst (0.1 mol%, based on the Ar3N moiety),
NaOAc (5.0 mmol), amine (10.0 mmol), and solvent (20.0 mL).
Then the reaction mixture was cooled to ꢀ78 ꢂC, degassed and
backlled with N2 three times. The resulting mixture was stirred
and irradiated with a 23 W uorescent lamp at room tempera-
ture for 50 or 60 h.
Data of Cd-MIX, 1a@Cd-MIX, 1c@Cd-MIX, and Cd-NPy3 were
measured on a Bruker SMART APEX CCD diffractometer with Mo-
˚
Ka radiation (l ¼ 0.71073 A) using the SMART and SAINT
programs.64,65 The crystal structures were solved by direct methods
and then rened by full-matrix least-squares renements on F2
using SHELXL-97 soware. And an absorption correction was
performed using the SADABS program.66 The remaining atoms
were found from successive full-matrix least-squares renements
on F2 and Fourier syntheses. Non-H atoms were rened with Author contributions
anisotropic displacement parameters. H atoms in the backbones
C. Duan conceived the project, designed the experiments and
were xed geometrically at calculated distances and allowed to
ride on the parent non-hydrogen atoms, whereas some of the
disordered solvent molecules were not treated during the struc-
tural renements. To help the stability of the renement for the
impregnated related substrate molecules, the bond distances
between several atoms were xed, and thermal parameters on
adjacent atoms in two molecules were restrained to be similar.
The SQUEEZE program was carried out for Cd-MIX, 1a@Cd-MIX,
and 1c@Cd-MIX. The crystallographic data for Cd-MIX, 1a@Cd-
MIX, 1c@Cd-MIX, and Cd-NPy3 are summarized in Table S1.†
supervised the work. H. Li and X. Jing carried out the main
experiments, collected and interpreted the data. Y. Yang and C.
He solved and rened the X-ray single crystal structures. All
authors discussed the results and commented on the
manuscript.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Catalysis details
This work was supported by the National Natural Science
Before catalysis experiments, solvent exchange experiments of Foundation of China (21890381 and 21971030).
catalysts were carried out to activate the catalysts. First, fresh
catalysts were immersed in a methanol solution for 48 h and
then fully dried out in a vacuum oven (373 K) for 10 h to obtain
Notes and references
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8520 | Chem. Sci., 2021, 12, 8512–8520
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