Single Metal-Organic Cage Decorated with an Ir(III) Complex for CO2Photoreduction

Dongxu Cui, Xinlong Wang,* and Zhongmin Su

Research Article

Single Metal−Organic Cage Decorated with an Ir(III) Complex for

CO Photoreduction

Xiangjuan Qi, Ronglin Zhong, Mengmeng Chen, Chunyi Sun,* Siqi You, Jianxia Gu, Guogang Shan,*

‡

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sı Supporting Information

ABSTRACT: CO photoreduction is a promising avenue to alleviate climate

change and energy shortage, and highly active and selective photocatalysts have

been pursued. Discrete metal−organic cages (MOCs) with tunable structures and

dispersion not only render integration of multiple functional moieties but also

facilitate the accessibility of catalytic sites, yet the studies of MOCs on CO₂

reduction are still underexplored. Herein, a single molecular cage of the Ir(III)

III

complex-decorated Zr-MOC (Ir -MOC-NH ) is proposed for CO photo-

III

reduction. Ir -MOC-NH shows high reactivity and selectivity in converting CO

into CO under visible light. The selectivity is of 99.5% and the turnover frequency

−

III

reaches ∼120 h which is 3.4-fold higher than that of bulk Ir -MOC-NH and

III

two orders of magnitude higher than that of the classical metal−organic framework counterpart (Ir -Uio-67-NH ). The apparent

quantum yield is up to 6.71% that ranks the highest among the values reported for crystalline porous materials. Moreover,

aggregation-induced deactivation of the Ir(III) complex is restrained after incorporating into MOC-NH . The density functional

theory calculations and dedicated experiments including cyclic voltammetry, mass spectrometry and in situ IR show that the Ir(III)

complex is the catalytic center, and −NH in the framework plays the synergetic role in the stabilization of the transition state and

CO adducts.

KEYWORDS: metal−organic cage, metal complex, amino, photocatalysis, CO reduction, synergetic eﬀect

. INTRODUCTION

dispersion of MOCs will render them more accessible catalytic

4,34−40

active centers.

Among them, Zr-based MOCs could be

Sunlight-driven CO reduction to fuels and chemical stocks

represents one of the most promising approaches to alleviate

the shortage of fossil fuels and climate change.

the appealing one for their excellent stability in polar solvents,

9,36,41−54

−9

water with broad pH ranges (2.0−10.0).

The isolated

The

cage of Zr-MOCs in methanol has been conﬁrmed by Choe et

al. Molecular metal complexes with various redox states have

achievement of eﬃcient conversion requires the linking of

CO₂ﬁxation, light-absorbing, and catalytic components.

Crystalline metal−organic frameworks (MOFs) have garnered

exhibited substantial superiorities in photocatalytic CO

reduction such as high reactivity and selectivity, while

tremendous attention on CO reduction, not only for their

these complexes often lose their activity gradually in

photocatalysis, mainly due to decomposition or formation of

aggregation. Combination of Zr-MOCs and metal complexes

porous channels providing the residence for CO molecules

but also for their tunable crystal structure which allows

hierarchical integration of multiple functional moieties.

The integration of photosensitizers and catalytic centers could

facilitate multielectron transfer in photocatalysis, and the

incorporation of hydrogen interaction sites may assist the

0−18

may amend such defects and is expected to construct eﬃcient

16−18,56−58

and robust CO photoreduction catalysts.

Choi and

co-workers found that the photocatalytic durability of

9−21

Re (CO) (BPYDC)(Cl) to CO reduction is prolonged after

anchoring of CO and intermediated species.

Moreover,

being grafted into the Zr-based MOC (ReTC-MOP) and the

reactivity of ReTC-MOP is 19.6- and 29.3-folds higher than

the values of ReTC-MOFs(nano) and ReTC-MOFs(micro).

the well-deﬁned structures can help understand the structure−

activity relationship. However, their generally bulky nature

encumbers the diﬀusion of substrates and intermediates

through inner pores, which may aﬀect the utilization eﬃciency

of active sites of MOFs.

porous metal−organic materials as photocatalysts might be a

promising avenue to relieve this issue.

2−29

Received: April 30, 2021

Revised: May 21, 2021

Published: June 4, 2021

Exploring low-dimensional

Discrete metal−organic cages (MOCs) with permanent

porosity and modiﬁable structures might be alternative

0−33

candidates.

The unique discrete structure and excellent

2021 American Chemical Society

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Figure 1. Schematic illustration on Ir -MOC-NH , bridging ligands, and trinuclear zirconium clusters.

Research Article

III

III

Figure 2. (a) PXRD patterns of Ir -MOC-NH . (b) IR spectra of Ir -MOC-NH , H -bpdc-NH , ZrT-3, and H Ir-ppyc. (c) UV−vis diﬀuse

III

reﬂection spectra of H Ir-ppyc, MOC-NH , and Ir -MOC-NH in the solid state. (d) ESI-MS analysis.

However, the photocatalytic activity of a single MOC to CO

which is the highest among the reported crystalline porous

III

reduction is still unexplored.

In this work, the metal complex of Ir-ppyc was chosen to

photocatalysts. As for durability, bulk Ir -MOC-NH could be

recycled three times while the Ir(III) complex shows a

complete loss of catalytic activity after 1.5 h. The density

functional theory (DFT) calculations and dedicated experi-

ments disclose the details of the synergetic eﬀect between the

framework and Ir(III) complex.

be incorporated into Zr-MOC-NH and via a mix-and-match

III

strategy, single crystals of Ir -MOC-NH were prepared. In

MeOH solution, the existence of a single Ir -MOC-NH cage

III

is determined by electrospray ionization mass spectrometry

(

ESI-MS). Under visible-light irradiation, CO is generated by

. RESULTS AND DISCUSSION

the single molecular cage with a selectivity of 99.5% and a

turnover frequency of ∼120 h which is 3.4 times as much as

that of bulk Ir -MOC-NH and two orders of magnitude

higher than that of the classical MOF counterpart Ir -Uio-67-

NH . The apparent quantum yield (AQY) is up to 6.71%,

−

.1. Synthesis and Characterizations. Single crystals of

III

Ir -MOC-NH were synthesized by a mix-and-match strat-

III

egy from a mixture of zirconocene dichloride, 2-amino[1,1′-

biphenyl]-4,4′-dicarboxylic acid (H -bpdc-NH ), and H Ir-

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Research Article

ppyc (ppyc = 6-(4-carboxy-phenyl)-nicotinic acid) (Figure 1 )

scattering (DLS). The DLS results show that the size

distribution of the cage mainly occupies 5.5 nm (Figure S4).

This range is slightly larger than the size measured by the

detailed synthesis procedures are provided in the Supporting

III

Information). The structure of Ir -MOC-NH was determined

by single-crystal X-ray diﬀraction (XRD). The result revealed

crystal data (3 nm), which may be due to the solvation eﬀect.

III

that Ir -MOC-NH crystallizes in the cubic Fm-3m space

These demonstrate that Ir -MOC-NH maintains a mono-

group (Table S1 ). Four Cp Zr μ -O(μ -OH) as secondary

dispersed state in MeOH.

building units (SBUs) coordinating to six linkers of Ir-ppyc

and bpdc-NH form a discrete V E -type cage (Figure 1). The

The UV−vis diﬀuse reﬂection spectra of H Ir-ppyc, MOC-

III

NH , and Ir -MOC-NH in the solid state were recorded

Cp Zr μ -O(μ -OH) SBUs occupy vertexes (V) while the

ligands serve as edges (E). In this coordination conﬁguration,

(Figure 2c). H Ir-ppyc shows an absorption peak near 300 nm

which may belong to π−π* absorption and the two absorption

bands at 390 nm and 450−550 nm which may originate from

metal-to-ligand charge transfer (MLCT) singlet and triplet

the distance between −NH and Ir is 6.5 Å. Such conﬁguration

facilitates the −NH participating in the ﬁxation of CO and

the intermediates during the catalysis. Due to the high

symmetry of the structure, the distribution of Ir-ppyc and

absorptions, respectively. MOC-NH can absorb light from

III

300 to 800 nm. The UV−vis diﬀuse reﬂection spectrum of Ir -

bpdc-NH ligands in the structure is disordered. The content

MOC-NH has broad absorption peaks at 300−450 nm and a

of Ir in the structure was measured by inductively coupled

plasma-optical emission spectrometry (ICP-OES) analysis and

characteristic absorption band of H Ir-ppyc at 450−550 nm

which may correspond to MLCT triplet absorptions. The

III

the results showed the molar ratio of Ir/Zr to be 1:24.19.

absorption intensity of Ir -MOC-NH ranging from 550 to

III

Therefore, the formula of Ir -MOC-NH was determined to

800 nm was higher than that of H Ir-ppyc. In solution, the

III

be [Cp Zr μ -O(μ -OH) ] (bpdc-NH ) (Ir-ppyc) ·Cl .

single Ir -MOC-NH cage showed apparent absorption peaks

5.5

0.5

Compared with the reported ReTC-MOP which has 1−4 Re

metal complexes on the zirconium cage, in our structure, only

one Ir complex is detected by single-crystal XRD and ICP-MS,

which may be owing to the feed ratio of Ir complexes and the

in the visible-light range similar to that in the solid state

III

(Figure S5). Ir -MOC-NH may have good response to visible

light in photocatalysis.

The emission spectrum of Ir -MOC-NH exhibits a broad

III

bpdc-NH ligand and the steric hindrance of Ir complexes. The

band peaking at 537 nm which is speculated to be derived from

deﬁnite structure makes it possible to clearly study its catalytic

mechanism, allowing us to deeply understand the root reason

of the eﬃcient catalysis. As control, single-linker MOC-NH₂

was prepared by zirconocene dichloride and H -bpdc-NH .

MLCT of Ir-ppyc, since a similar emission band is observed in

the emission spectrum of pure Ir-ppyc (Figure S6). The

III

diﬀerence is that the emission intensity of Ir -MOC-NH is

more than six times lower than that of Ir-ppyc, which implies

III

The structure of MOC-NH is isomorphic with Ir -MOC-

that the photogenerated electron and hole separation is more

III

NH (Figure S2).

The high phase purity and crystallinity of Ir -MOC-NH is

eﬃcient on Ir -MOC-NH than that on the Ir-ppyc complex.

III

Transient photocurrent further illustrates the superiority of

III

conﬁrmed by the good consistence between its as-synthesized

charge separation on Ir -MOC-NH . The current density of

III

−2

and simulated powder XRD (PXRD) patterns (Figure 2a). The

Ir -MOC-NH is 0.14 μ A cm which is 1.5-fold higher than

that of Ir-ppyc (Figure S7 ). As the excited Ir complex is

III

incorporation of two ligands in Ir -MOC-NH is illuminated

by an IR spectrum (Figure 2b) which presents the peak at

quenched by the sacriﬁcial electron donor, triethanolamine

−

113 cm attributing to the C−N stretching mode and the

(TEA), the quenching rate constant (k ) is determined by the

−

III

peak at 1600 cm corresponding to the N−H bending mode

Stern−Volmer plot. For Ir -MOC-NH , k is 5.79 × 10 while

14,43

of the −NH group.

Thermogravimetric analysis (TGA) of

for Ir-ppyc this value decreases to 3.53 × 10 (Figure S8 ).

III

Ir -MOC-NH and MOC-NH shows signiﬁcant weight loss

These results indicated that, comparing with Ir-ppyc, the

III

of before 200 °C which might be ascribed to the loss of

encapsulated solvents. A gentler weight loss process appears

between 200 and 480 °C. After this, the weight loss aggravates

signi ﬁ cantly, indicating that the skeleton is decomposed

photoexcited state of Ir -MOC-NH is more easily quenched

by TEA during the photocatalytic CO reduction reaction.

III

The cyclic voltammetry (CV) curves of H Ir-ppyc and Ir -

MOC-NH were tested in an Ar and CO atmosphere (Figure

Figure S3).

S10 ). Under Ar, the H Ir-ppyc complex exhibits one reduction

Inspired by the potential dispersivity of Zr-MOCs in organic

wave of the terpyridine ligand at −1.23 V vs saturated calomel

1,42

III

solvent,

the dispersivity of Ir -MOC-NH in MeOH was

electrode (SCE). Under a CO atmosphere, a catalytic

III

studied. After immersing Ir -MOC-NH in MeOH, a golden

current peak is observed at the reduction wave (−1.23 V vs

yellow clear solution was gradually formed. ESI-MS analysis

was performed to investigate the species in solution. The ESI-

MS spectra (Figure 2d) demonstrate the presence of two kinds

of single tetrahedral cage in solution. Mass to charge ratio (m/

SCE) with signiﬁcantly increased current compared to that

III

under Ar. The catalytic current peak of Ir -MOC-NH slightly

shifts to −1.17 V vs SCE. It is also worth noting that the CO

III

reduction peak current intensity of Ir -MOC-NH is much

z) values at 824.13, 1029.67, and 1371.24 should belong to one

greater than that of H Ir-ppyc, suggesting that the catalytic

III

Ir -MOC-NH cage (simpliﬁed as [1]) which loses ﬁve

activity of Ir -MOC-NH may be higher than H Ir-ppyc.

−

counter ions (Cl ) [1] and continuous loss of one hydrogen

1-H] and two hydrogen [1-2H] . The peaks at m/z values

of 918.41 and 1223.08 correspond to [0] and [0-H] ,

respectively (where [0] represents the MOC-NH₂cage),

which may be owing to the loss of four counter ions (Cl ) and

2.2. Photocatalytic CO Reduction. The photocatalytic

[

reactions were ﬁrst conducted under model conditions using

acetonitrile (MeCN) as solvent and triethylamine (TEA) as

III

the sacriﬁcial electron donor agent. Under this condition, Ir -

−

MOC-NH is in bulk which is beneﬁcial to study its recycling

sequential loss of hydrogen from MOC-NH . These results are

performance. As control, a homogeneous system with H Ir-

ppyc as the catalyst is also carried out under similar conditions.

Under visible-light irradiation, CO is detected as the main gas

consistent with the results from single crystal structure analysis

III

and ICP. The size of Ir -MOC-NH in MeOH with a

concentration of 0.5 mg mL⁻was analyzed by dynamic light

product and H is the byproduct in both heterogeneous and

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ACS Catal. 2021, 11, 7241−7248

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homogeneous counterpart (Figure 3 a). Such catalytic superi-

Research Article

III

homogeneous systems, while no liquid phase product was

detected. In the ﬁrst 15 min, the rate of CO generation in the

heterogeneous system is 2.4 times as much as that of the

dimers. After catalysis, IR and ESI-MS spectra of Ir -MOC-

NH₂were collected to illuminate the structural stability.

Comparing the IR spectrum of Ir -MOC-NH before

catalysis, we found that the characteristic peaks (Figure S12)

are maintained after catalysis, indicating that the basic

III

composition of the catalyst is unchanged. The ESI-MS (Figure

III

S13 ) of Ir -MOC-NH after catalysis shows the characteristic

III

peak of Ir -MOC-NH at m/z = 823.96 and a new signal at

m/z = 879.17. The new peak corresponds to the solvated

structure after the leaving of Cl. A similar species was observed

by Ishitani and co-workers and is proved to be the active

species to the catalysis. These results indicate that the skeleton

III

of Ir -MOC-NH is maintained after catalysis.

To determine the signiﬁcance of the components in the

photocatalytic system, a series of reference experiments were

carried out and the results are summarized in Table 1. The

control experiment with bare MOC-NH as the catalyst shows

no gas detectable product. In the system containing MOC-

III

NH and H Ir-ppyc with a molar ratio (1:3) same as Ir -

MOC-NH , TONs of CO reach 11.85 which are equal to that

of bare H Ir-ppyc (Table 1). These results indicate that the

III

catalytic center in Ir -MOC-NH should be Ir-ppyc, and the

enhancement of catalytic reactivity of Ir -MOC-NH may

III

Figure 3. (a) TONs of CO and H with bulk Ir -MOC-NH or the

Ir(III) complex as the photocatalyst, separately. (b) Recycling

result from the synergetic eﬀect of the skeleton of the cage and

III

experiments over bulk Ir -MOC-NH . (c) GC−MS spectra of the

the Ir(III) complex. The nature of the MOC-NH framework

III

gaseous phase from bulk Ir -MOC-NH as the photocatalyst purged

as a photoinduced electron source might also play a positive

role in reductive reactivity. The control experiment using

with ¹³CO . (d) CO-TON and H -TON in 0.5 h over diﬀerent

photocatalysts.

mixture solvent of MeCN/H O (V:V = 4:1) shows an obvious

increase in H production and the selectivity of CO decrease to

III

ority of bulk Ir -MOC-NH is maintained during the next 45

2.3%. This result indicates that water may be a favorable

min, and the total TON (turnover number) is 20.26 and

selectivity reaches 96.3%. The CO-TON of [Ir-ppy] within 1 h

in Ishitani’s work is about 12.5. When using the catalytic

condition of our system, the CO-TON of [Ir-ppy] is 3.74

Table 1). The CO-TON of bulk Ir -MOC-NH is 5.4-folds

proton donor to H production (Table 1). The absence of a

suitable proton source may be one reason for the high CO

selectivity under the MeCN condition. Although increasing the

amount of water elevates the production of H , the production

III

(

of CO is still dominant. In order to gain a deep understanding,

theoretical calculations were implemented (Figure S14 ). The

calculation results show that the activation energy barrier of

Table 1. TONs of CO and H during the Photocatalytic

Reactions for Diﬀerent Conditions

CO reduction on the Ir complex is 15.1 kcal/mol, which is

photocatalyst

H₂

lower than that (22.8 kcal/mol) of H O. Therefore, the

favorable reaction energy barrier of CO reduction over H

TON

production may be the main reason for the high selectivity of

CO. The control experiment without TEA shows no detectable

reduction product, demonstrating that TEA could be an

important electron donor in photocatalysis.

Ir-ppy

3.74

12.56

undetected

11.85

undetected

7.76

undetected

1.54

2.53

undetected

2.33

undetected

2.97

H Ir-ppyc

MOC-NH₂

MOC-NH + H Ir-ppyc

Ir -MOC-NH

Ir -MOC-NH₂

To probe the origin of generated CO, isotope tracer analysis

III

over CO was carried out. The gas product is analyzed by gas

III

chromatography-mass spectrometry (GC−MS). The peak at

m/z = 29 is observed (Figure 3c) rather than m/z = 28,

III

undetected

The photocatalytic reactions were performed for 5 h with 6 mL of a

:1 (V/V) MeCN/TEA solution in 10 mL reactors. TON is deﬁned

as the amount of evolved product per catalytic site. In the dark. The

photocatalytic reactions were performed for 5 h with 6 mL of a 4:1:1

V/V) MeCN/H O/TEA solution in 10 mL reactors. The

photocatalytic reactions were performed for 5 h with 6 mL MeCN

solution in 10 mL reactors.

verifying that CO originates from CO reduction rather than

c d

the decomposition of any other organic species in the

photocatalytic reaction. In addition, the participation of CO₂

in the reaction is further conﬁrmed by performing the reaction

(

in the dark (Table 1), where no CO was detected (Table 1).

III

The excellent dispersion of Ir -MOC-NH in MeOH

provides an appealing platform to study the reactivity of a

III

single Ir -MOC-NH cage toward CO photoreduction. The

higher than that of Ishitani’s work under the same condition.

catalytic reaction was performed in the mixed solvent of

More signiﬁcantly, homogeneous H Ir-ppyc almost has no

catalytic activity after 1.5 h (Figure S11 ), while bulk Ir -MOC-

MeOH and MeCN (V:V = 1:2) with TEA as the sacriﬁcial

III

electron donor agent. Before the catalysis, the dispersion of

III

NH can be recycled (Figure 3b). Even after three cycles, it

Ir -MOC-NH under catalytic conditions (V_M_e_C_N/V _T _E _A

still maintains 64% catalytic activity of the ﬁrst round. The

improved durability probably results from the anchor of Ir-

ppyc into the MOC preventing the formation of deactivated Ir

V _M _e _O _H = 2:1:1) is studied via DLS measurement (Figure

III

S15 ). The results present that mainly Ir -MOC-NH cages

have a size distribution of 5.5 nm. The TEM (transmission

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Research Article

−

Figure 4. (a) Catalytic mechanism of photocatalytic CO to CO. (b) Gibbs energy proﬁle (kcal mol ) of the CO reduction step catalyzed by

III

H Ir-ppyc and Ir -MOC-NH . (c) Important geometrical parameters in the Ir(III) complex (2a) and the cage (2b).

electron microscopy) image reveals that the average size of

of Zr O (OH) (Ir-ppyc)₀_.₄₈₂(bpdc-NH )

was prepared via

2 5.518

III

Ir -MOC-NH is about 5.7 nm (Figure S16), which is good

a similar mix-and-match strategy of the cage. To eliminate

the inﬂuence of MeOH on photocatalytic performance, the

photocatalysis was separately carried out in MeCN and

MeOH/MeCN solution with TEA as the sacriﬁcial electron

agreement with the DLS results. These results demonstrate

III

that Ir -MOC-NH can still maintain a monodispersed state

under the catalytic condition. Under visible-light irradiation for

0 min, the TON and TOF (turnover frequency) of CO

donor agent. The results show that the photocatalytic

III

generation over single Ir -MOC-NH reach up to 59.0 and

performance of Ir -Uio-67-NH in MeOH/MeCN is similar

−

∼

120 h , respectively, which are 3.4-fold higher than that of

to that in pure MeCN (Figure S17 ), which illustrates that the

III

bulk Ir -MOC-NH (Figure 3d). It is noteworthy that the

added MeOH does not aﬀect the photocatalytic performance.

III

TON of H for a single Ir -MOC-NH cage was almost like

that of bulk Ir -MOC-NH (Figure 3d), giving the selectivity

The TON of Ir -Uio-67-NH in CO generation is 0.64 which

III

is two orders of magnitude lower than single Ir -MOC-NH

of CO up to 99.5%. Such high selectivity is quite rare in the

(Figure 3d) under MeOH/MeCN conditions. These results

reported materials toward CO photoreduction. The AQY

well conﬁrm the superiority of the single molecular cage in

could evaluate the utilization eﬃciency of one material to the

irradiated light in photocatalysis. The AQY of a single Ir -

photocatalytic CO reduction.

III

DFT calculations are employed to illuminate the catalytic

III

MOC-NH cage is estimated at a wavelength of 420 nm and

site of Ir -MOC-NH in CO reduction. Calculation results

calculated based on eq S1 presented in the Supporting

Information and the value is up to 6.71% (Table S2 ). This

value is the highest among the ones reported for crystalline

porous materials and semiconductors (Table S2 ). These

results undoubtedly reﬂect the superiority of the single

show that the lowest unoccupied molecular orbital (LUMO) is

distributed in the Ir-ppyc complex and the triplet spin density

is mainly located on Ir. Combining the results of DFT

calculations with that of CV and ESI-MS after the reaction, we

III

conjecture that the Ir on Ir -MOC-NH could be the active

molecular cage in photocatalytic CO reduction.

In order to further clarify the prominence of the single

site for CO reduction and the catalytic route is similar to that

III

of the bare Ir complex ([Ir(tpy)(ppy)Cl] ), involving the

III

molecular cage over MOFs in CO photoreduction, H Ir-ppyc

departure of Cl from the Ir complex and production of a

was incorporated into the classical Zr-based MOF, Uio-67-

pentacoordinate structure for CO reduction. Therefore, the

III

NH . Crystalline Ir -Uio-67-NH with the molecular formula

mechanism of photocatalytic CO reduction over Ir -MOC-

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https://pubs.acs.org/doi/10.1021/acscatal.1c01974.

Research Article

NH is shown in Figure 4a which is referred from the reported

3. CONCLUSIONS

In summary, metal complex Ir-ppyc was incorporated into Zr-

mechanism of the [Ir(tpy)(ppy)Cl]^c^o^m^p^l^e^x^oⁿ^C^O2

photoreduction. First, formation of the one-electron-reduced

species by reductive quenching of TEA and generation of

hydride species are key steps in the photocatalysis. Then,

under irradiation, the hydride species acts as a photoacid to

III

MOC-NH and formed Ir -MOC-NH . The single molecular

III

cage of Ir -MOC-NH was obtained in MeOH as conﬁrmed

by ESI-MS analysis and TEM. Under visible-light irradiation,

III

the single Ir -MOC-NH cage can convert CO into CO with

−

form pentacoordinate species involved in the photocatalytic

a selectivity of 99.5% and a TOF of ∼120 h which is 3.4

times as much as that of bulk Ir -MOC-NH and two orders

III

reduction of CO . The pentacoordinate Ir complex (C-trans

structure) is an active species for the reduction of CO₂.

The catalytic mechanism is further studied by DFT

of magnitude higher than that of the classical MOF

III

counterpart, Ir -Uio-67-NH . The AQY is up to 6.71%

calculations. Given that the activation of CO is one of the

ranking among the highest value of reported crystalline porous

III

key elementary steps, we calculated and made comparison of

the Gibbs energy change between the reaction catalyzed by

photocatalysts. As for durability, bulk Ir -MOC-NH

could be

recycled by three times, while the Ir(III) complex shows a

complete loss of catalytic activity after 1.5 h. The DFT

calculations together with CV, MS, and in situ IR reveal that Ir-

ppyc is the catalytic center, and the framework of the cage

plays a positive role in stabilizing the transition state and the

III

H Ir-ppyc and that by Ir -MOC-NH , and the results are

shown in Figure 4. Previous investigation suggests that the

Gibbs reaction energy (ΔG°) of the bare Ir-ppy complex is

−

‡

3.2 kcal mol and the Gibbs activation barrier (ΔG° ) is 15.3

−

‡

CO adduct in photocatalysis, leading into the decrease of

kcal mol . The ΔG° and ΔG° values catalyzed by the H Ir-

Gibbs reaction energy and the activation barrier. The study

illuminates the potential of the single molecular cages as next

generation photocatalysts for eﬃcient solar fuel conversion.

ppyc complex are well reproduced. Interestingly, ΔG° of this

III

−1

step catalyzed by Ir -MOC-NH decreases to 3.4 kcal mol

‡

−1

and ΔG° declines to 12.7 kcal mol , suggesting that the CO

reduction elementary step catalyzed by Ir -MOC-NH is less

III

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

endergonic and the barrier is lower than that of H Ir-ppyc.

This is consistent with experimental results that the reaction

III

rate catalyzed by Ir -MOC-NH is faster and the yield is

higher than that by H Ir-ppyc.

To disclose the origin of the higher catalytic reactivity of

Experimental procedures, characterization, crystal data,

photocatalytic measurements, and computational details

III

Ir -MOC-NH , we made detailed analysis on the important

(PDF )

geometry and interaction energy. Results show that the

interaction energy between CO and the Ir(III) complex

Crystallographic information ﬁle (CIF)

−

(

2a) is −55.0 kcal mol , which is less negative than that

−

(−73.8 kcal mol ) in the cage (2b). It indicates that the

AUTHOR INFORMATION

Corresponding Authors

■

stabilization energy of CO is stronger in 2b than that in 2a.

Correspondingly, the hydrogen bond between O of CO and

Chunyi Sun − Key Lab of Polyoxometalate Science of Ministry

of Education, Northeast Normal University, Changchun

130024, China; Email: suncy009@nenu.edu.cn

Guogang Shan − Key Lab of Polyoxometalate Science of

Ministry of Education, Northeast Normal University,

Changchun 130024, China; orcid.org/0000-0002-1050-

219X ; Email: shangg187@nenu.edu.cn

Xinlong Wang − Key Lab of Polyoxometalate Science of

Ministry of Education, Northeast Normal University,

Changchun 130024, China; orcid.org/0000-0002-5758-

6351 ; Email: wangxl824@nenu.edu.cn

the H of NH in the neighboring ligand was found and the

III

distance is 1.846 Å. The angle of O−C−O in Ir -MOC-NH

is 129.43° which is smaller than the value of the bare Ir(III)

complex (133.7°). Therefore, it suggests that the hydrogen

bond between CO₂and NH₂in the neighboring ligand

stabilizes the transition state and the important CO adduct.

Therefore, it is concluded that the enhancement of catalytic

III

reactivity of Ir -MOC-NH arises from the synergetic eﬀect of

the framework of the cage and the Ir(III) complex.

To further explain the mechanism of CO reduction, in situ

IR were performed under an Ar and CO atmosphere (Figure

Authors

S18 ). Under an Ar atmosphere, the vibration peak of NH is

−

61,62

Xiangjuan Qi − Key Lab of Polyoxometalate Science of

Ministry of Education, Northeast Normal University,

Changchun 130024, China

Ronglin Zhong − Laboratory of Theoretical and

Computational Chemistry, Institute of Theoretical Chemistry,

College of Chemistry, Jilin University, Changchun 130023,

China

Mengmeng Chen − Key Lab of Polyoxometalate Science of

Ministry of Education, Northeast Normal University,

Changchun 130024, China

Siqi You − Key Lab of Polyoxometalate Science of Ministry of

Education, Northeast Normal University, Changchun

130024, China

observed to be located at 3500−3650 cm .

When CO is

blown in, the vibration intensity of the peak increases

signiﬁcantly. Meanwhile, a new vibration peak appears at

−

650−3740 cm which may result from the hydrogen bond

interaction between CO and NH . As the light is turned on,

the intensity of two peaks gradually decreases. These results

indicate the synergistic catalytic eﬀect of −NH toward CO

reduction. This conclusion is consistent with that of theoretical

calculations. In addition, the vibration peaks of the

intermediate product of CO reduction are observed. The

−

peaks at 1697, 1415, and 1257 cm are attributed to the C

O stretch vibration, the C−O stretch vibration, and OH

deformation vibration in the carbon dioxide intermediate

Jianxia Gu − Key Lab of Polyoxometalate Science of Ministry

of Education, Northeast Normal University, Changchun

130024, China

(

COOH*). The intensity of these peaks gradually increased

with the prolonged illumination time.

246

ACS Catal. 2021, 11, 7241−7248

ACS Catalysis

Reduction with Rhodium-based Catalysts in Solution and Hetero-

Research Article

Dongxu Cui − Key Lab of Polyoxometalate Science of Ministry

genized within Metal −organic Frameworks. ChemSusChem 2015, 8,

of Education, Northeast Normal University, Changchun

(

03−608.

30024, China

12) Kajiwara, T.; Fujii, M.; Tsujimoto, M.; Kobayashi, K.; Higuchi,

Zhongmin Su − Jilin Provincial Science and Technology

Innovation Center of Optical Materials and Chemistry,

School of Chemistry and Environmental Engineering,

Changchun University of Science and Technology, Changchun

M.; Tanaka, K.; Kitagawa, A. S. Photochemical Reduction of Low

Concentrations of CO in a Porous Coordination Polymer with a

Ruthenium(II)−CO Complex. Angew. Chem., Int. Ed. 2016, 55,

(

697−2700.

30022, China; orcid.org/0000-0002-3342-1966

13) Choi, K. M.; Kim, D.; Rungtaweevoranit, B.; Trickett, C. A.;

Barmanbek, J. T.; Alshammari, A. S.; Yang, P.; Yaghi, O. M. Plasmon-

^E ⁿ ^h ^a ⁿ ^c ^e ^d ^P ^h ^o ^t ^o ^c ^a ^t ^a ^l ^y ^t ⁱ ^c ^C ^O 2 Conversion within Metal −Organic

Frameworks under Visible Light. J. Am. Chem. Soc. 2017, 139, 356−

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.1c01974

362.

Author Contributions

‡

(14) Ryu, U. J.; Kim, S. J.; Lim, H. K.; Kim, H.; Choi, K. M.; Kang, J.

K. Synergistic Interaction of Re Complex and Amine Functionalized

Multiple Ligands in Metal −organic Frameworks for Conversion of

Carbon Dioxide. Sci. Rep. 2017, 7, 612.

X.Q. and R.Z. contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

(15) Liao, W. M.; Zhang, J. H.; Wang, Z.; Yin, S. Y.; Pan, M.; Wang,

ACKNOWLEDGMENTS

H. P.; Su, C. Y. Post-Synthetic Exchange (PSE) of UiO-67

Frameworks with Ru/Rh Half-sandwich Units for Visible-light-driven

H Evolution and CO Reduction. J. Mater. Chem. A 2018, 6, 11337−

■

We sincerely thank Prof. T Duan of the Changchun Institute of

Applied Chemistry Chinese Academy of Sciences for the ICP-

OES experiments. This work was supported by the NSFC of

China (No. 21971032, 21801038, 21771035); Jilin Provincial

Department of Science and Technology (No.

0190103018JH); and Fundamental Research Funds for the

Central Universities (No. 2412019FZ007).

1345.

(16) Luo, Y. H.; Dong, L. Z.; Liu, J.; Li, S. L.; Lan, Y. Q. From

molecular metal complex to metal −organic framework: The CO

Reduction Photocatalysts with Clear and Tunable Structure. Coord.

Chem. Rev. 2019, 390, 86−126.

(17) Yamazaki, Y.; Takeda, H.; Ishitani, O. Photocatalytic Reduction

of CO Using Metal Complexes. J. Photochem. Photobiol. C 2015, 25,

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