Porphyrin Grafting on a Mercapto-Equipped Zr(IV)-Carboxylate Framework Enhances Photocatalytic Hydrogen Production

Framework Enhances Photocatalytic Hydrogen Production

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Porphyrin Grafting on a Mercapto-Equipped Zr(IV)-Carboxylate

Yingxue Diao, Nanfeng Xu, Mu-Qing Li, Xunjin Zhu,* and Zhengtao Xu*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01744

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sı Supporting Information

ABSTRACT: We employ facile aromatic nucleophilic substitution

between the mercapto (−SH) and arylﬂuoro (Ar−F) groups to

achieve extensive and robust cross-linking of a coordination host by

porphyrin guests that also serve the purpose of versatile

postsynthetic functionalization. For this, a tritopic linker with

three trident-like thiol-ﬂanked carboxyl units are reacted with

ZrOCl ·8H O to aﬀord a two-dimensional (3,6-connected) net.

The wide aperture of the porous framework solid, together with its

stability in both air and boiling water, facilitates the entry of bulky

metalloporphyrin guests and the subsequent property studies. On

the porphyrin side, four pentaﬂuorophenyl (C F −) groups oﬀer

multiple ﬂuoro groups to facilitate their replacement by the thiol groups from the host net. The inserted metalloporphyrin bridges

impart to the metal−organic framework (MOF) host stable and recyclable activities for photocatalytic hydrogen production. We also

disclose an improvement in synthetic methodology, in which BBr is used to simultaneously cleave the ester and benzyl thioether

groups to more eﬃciently access thiol-equipped carboxylic acid building block.

INTRODUCTION

The versatile mercapto (−SH) function plays an increasingly

important role in the development of advanced framework

materials for heavy metal removal,

electronic/magnetic properties.

(e.g., from air oxidation) across the linker molecules to

promote framework stability. The seemingly paradoxical pair

of stability and reactivity is therefore organically united in the

Zr(IV) framework built upon the thiol-ﬂanked carboxyl units.

This study explores the trident motif in the new trigonal

■

−5

6−10

catalysis,

and

1−17

The recent synthetic

breakthrough in widening the scope of thiol-equipped

multitopic carboxylate linker molecules opens further

possibilities for the construction and functionalization of

metal−organic frameworks (MOFs). Speciﬁcally, the benzylth-

planar building block H TTA-6SH (Figure 1 ), while aiming for

iol-based methodology allows for systematic variation of the

thiol arrays/arrangements (e.g., in terms of number and

geometric conﬁguration) on the aromatic carboxylic backbone.

The thiol-carboxyl building blocks become especially powerful

in connection with the chemically hard, highly modular/

persistent Zr(IV)-based nodal units, e.g., in the form of the

Zr O (OH) cluster as ﬁrst uncovered by Lillerud et al. in the

original UiO series of coordination solids; the soft thiol

groups refrain from bonding to the Zr(IV) centers and, thus,

remain free-standing as accessible anchors for postsynthetic

modiﬁcation. Notably, even when it is ﬂanked by the thiol

groups (e.g., one on each side to give a symmetrical trident-like

motif), the carboxyl donor continues to bind to Zr(IV) to form

the prototypal MOF grid.

Figure 1. Structures of building block H TTA-6SH featuring the

trident motif of thiol-ﬂanked carboxyl units, and the porphyrin guests

MTFPP with C F − units (for easy F-substitution by the thiol groups

on the host net).

Received: June 13, 2020

The versatile reactivity of the thiol-equipped MOF solids

being generally underexplored, the newly accessed thiol-

ﬂanked carboxyl trident motif merits even more attention.

For example, the trident motifs serve to surround the

Zr O (OH) cluster with a dense array of thiol groups,

which provide steric protection as well as disulﬁde S−S bridges

XXXX American Chemical Society

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Article

a MOF host that proﬀers the highly nucleophilic thiol

functions for conveniently anchoring and integrating the

electronically and photochemically active metalloporphyrin

guests (Figure 1). Together several design elements can be

identiﬁed here. The long tris(biphenyl)triazine-based back-

168.03, 142.93, 140.09, 137.35, 137.07, 135.08, 134.00, 129.42,

29.17, 128.57, 127.60, 127.35, 126.56, 37.79 ppm.

Synthesis of 2,4,6-Tris{4-[3,5-bis(mercapto)-4-carboxy-

phenyl]phenyl}-1,3,5-triazine (H TTA-6SH). Molecule MA5

(281 mg, 0.20 mmol), anhydrous AlCl₃(640 mg, 4.80 mmol),

anhydrous DCM (17.0 mL), and dry toluene (2.0 mL) were mixed in

bone of H TTA-6SH is intended to extend the pore aperture

a reaction tube in a N -ﬁlled glovebox. After the reaction tube was

in the prospective MOF solid for entering the large porphyrin

guests. The porphyrin guests (ZnTFPP, NiTFPP, and

FeTFPP), on the other hand, are equipped with multiple

ﬂuoro groups, to facilitate their replacements by the thiol

groups from the host net, and to help forge sulﬁde (−S−)

crosslinks throughout the coordination host. The resulted

thioether (−S−) links can also enhance stability as well as

electronic interaction between the MOF scaﬀold and the

porphyrin fragments. In the long run, it is our hope that the

easily tunable amount and types of inserted porphyrin guests

will help optimize the photochemical and catalytic eﬃcacy in

the solid state (e.g., to minimize concentration quenching of

taken out, the tube was connected to a N manifold and stirred at

room temperature for 2 h. Afterward, 10% HCl (aq, 10 mL, bubbled

with N for 5 min beforehand) was added to the mixture, followed by

stirring at room temperature for 2 h. The yellow precipitate formed

was collected by suction ﬁltration and washed by 10% HCl (aq),

distilled water, and then DCM and dried under vacuum to aﬀord

H TTA as a brown solid (147 mg, 85% yield based on MA5). H

NMR (400 MHz, DMF-d ): δ = 8.91−8.94 (d, 6H, J = 8.4 Hz), 8.02−

.05 (d, 6H, J = 8.4 Hz), 7.85 (s, 6H) ppm. C NMR (100 MHz,

DMF-d ): δ = 171.97, 169.02, 143.76, 142.07, 137.42, 136.52, 130.39,

129.82, 128.38, 126.70 ppm. ESI-MS m/z (%): calcd, 860.0 (100%)

−

for [(M-H) ]; found, 860.1 (100%) (M − H) ; see also Figure S5 .

Alternative Synthesis Route to H TTA-6SH by Using BBr .

Under nitrogen protection, molecule M5 (281 mg, 0.20 mmol) was

dissolved in anhydrous DCM (3.0 mL) in a 50 mL Schlenk tube

the chromophore ). We now describe the robust two-

dimensional ZrTTA-6SH network assembled from the

H TTA-6SH molecule and ZrOCl ·8H O, and the postsyn-

charged with a stirring bar and cooled by an ice bath. A BBr solution

(

1.0 M, 9.0 mL in DCM) was then dropwise added over a period of

0 min. The resulted mixture was further stirred at room temperature

thetic porphyrin anchoring that enhances its performance in

photocatalytic hydrogen production from visible light.

for 12 h, after which distilled water was added to the reaction mixture.

The resulted yellow precipitate was ﬁltered, washed extensively by

distilled water and DCM, and suction-dried on the ﬁlter paper to

EXPERIMENTAL SECTION

■

aﬀord H TTA-6SH as a yellow solid (164 mg, 95% yield based on

The general procedures for synthesis and characterization of ZrTTA-

SH samples (including details of structure simulation and reﬁne-

M5). The FT-IR spectra of products from two synthesis routes were

compared, as shown in Figure S2 .

Preparation of Polycrystalline Framework Solid ZrTTA-6SH.

ment) are included in the Supporting Information (SI).

Synthesis of 2,4,6-Tris{4-[3,5-bis(benzylthio)-4-methoxy-

carbonylphenyl]phenyl}-1,3,5-triazine (M5). Molecule S1 (2.00

g, 4.36 mmol), bis(pinacolato)diboron (1.107 g, 4.36 mmol),

ZrOCl ·8H O (90 mg, 0.28 mmol) and formic acid (720 mg, 88%,

3.77 mmol) were ultrasonically dissolved in DEF (N,N-diethylfor-

mamide, 5.0 mL). The solution was mixed with H TTA-6SH (38 mg,

PdCl (PPh ) (77 mg, 0.11 mmol), and anhydrous potassium acetate

0.044 mmol) and 1,2-ethanedithiol (170 mg, 1.81 mmol) in a 25 mL

Pyrex glass ampule. The ampule was then ﬂame-sealed and heated in

an oven at 120 °C for 48 h, followed by programmed cooling to room

temperature for 12 h. The pale yellow powder formed was collected

by ﬁltration (ﬁlter membrane, 0.22 μm), then washed by DMF (N,N-

dimethylformamide, 3 mL × 10) and acetone (3 mL × 10), and dried

under nitrogen ﬂow for weight measurement (63 mg, yield 85% based

(

941 mg, 9.59 mmol) were loaded in a 100 mL two-neck round-

bottom ﬂask and dried under vacuum for 10 min. The ﬂask was then

connected to a nitrogen atmosphere. Anhydrous 1,4-dioxane (30 mL,

bubbled with nitrogen for 10 min beforehand) was added to the ﬂask.

The mixture was stirred at 90 °C for 12 h. After cooling to room

temperature, K PO aqueous solution (2.3 M, 5.0 mL bubbled with

nitrogen for 10 min beforehand) was added, followed by the addition

of 2,4,6-tris(4-bromophenyl)-1,3,5-triazine (tBPT, 693 mg, 1.27

mmol). The mixture was stirred at 90 °C for 12 h. After cooling to

room temperature, the mixture was poured into water (400 mL) and

extracted with DCM (3 × 100 mL). The combined organic phases

were washed by distilled water (3 × 200 mL). The organic phase was

^t^h^eⁿ^d^rⁱ^e^d^o^v^e^r^aⁿ^h^y^d^r^o^u^s^M^g^S^O4 and evaporated by a rotary

evaporator. The solid residue obtained was puriﬁed by ﬂash column

chromatography to aﬀord M5 as a white solid (1.190 g, 65% yield

on H TTA-6SH). The above powder was denoted as as-made

ZrTTA-6SH.

Activation of ZrTTA-6SH. In a nitrogen-ﬁlled glovebox, an as-

made ZrTTA-6SH sample (∼50 mg) and acetonitrile (6 mL) were

added to an 8 mL glass vial with a screw cap (PTFE/Silicone Septa).

After the mixture was kept still for 24 h at room temperature, the

supernatant was replaced by fresh acetonitrile (6 mL). After three

solvent changes, the precipitate was vacuum-dried and denoted as

activated ZrTTA-6SH. Elemental analysis found [C (29.60%), H

(3.711%), N (2.60%)], a ﬁtting formula can be determined to be

Zr O (OH) (HCOO) (OH) (C H N O S ) (H O) (mw

based on 2,4,6-tris(4-bromophenyl)-1,3,5-triazine). H NMR (300

MHz, CDCl ): δ = 8.74−8.77 (d, 6H, J = 8.4 Hz), 7.37−7.40 (d, 6H,

2.2

42 24

1.6

J = 8.4 Hz), 7.22−7.31 (m, 36H), 4.12 (s, 12H), 4.01 (s, 9H) ppm.

2688.3), which gives a calculated proﬁle as [C (31.01%), H

(3.73%), N (2.50%), S (11.45%), Zr (20.36%)]. The as-made

ZrTTA-6SH sample was also characterized by TGA (Figure S6),

which indicated a ﬁnal residual weight fraction of 27.8% (ZrO₂),

equivalent to a Zr content of 20.56%, matching the ﬁtting formula.

C NMR (75 MHz, CDCl ): δ = 171.38, 168.07, 143.22, 141.51,

41.40, 137.19, 135.82, 133.32, 131.89, 129.57, 129.37, 128.65,

27.51, 127.45, 52.74, 40.90 ppm.

Synthesis of 2,4,6-Tris{4-[3,5-bis(benzylthio)-4-

carboxyphenyl]phenyl}-1,3,5-triazine (MA5). Molecule M5

The linker/Zr cluster ratio thus determined points to some linker

(

578 mg, 0.40 mmol) was dissolved in tetrahydrofuran (7.0 mL) in

deﬁciency (about 1.6 TTA-6SH instead of 2.0 for the full occupancy).

−

a 100 mL two-neck round-bottom ﬂask, followed by addition of a

The capping sites can be occupied by the formate (HCOO ) and

−

KOH solution (8.9 M, 7.0 mL in MeOH/H O, v/v = 1:1). The

HO /H O species, but notice their ratios here cannot be pinpointed

mixture was stirred at 70 °C for 24 h. After cooling to room

temperature, 10% HCl (aq) was added to the resulting mixture to

attain pH < 2. A yellow precipitate that formed was ﬁltered, washed

extensively by distilled water, and suction-dried on the ﬁlter paper to

by the current data as their small weight fractions do not impact

signiﬁcantly the elemental and TGA results.

Modiﬁcation of ZrTTA-6SH by MTFPP. MTFPP (M = Zn ,

Ni , FeCl ) (20 mg, ∼0.02 mmol, the preparation of MTFPP

molecules was included in SI), ZrTTA-6SH (30 mg, containing

∼0.107 mmol S), DMF (1.0 mL), and triethylamine (50 μL, 0.36

mmol) were loaded into a Pyrex glass tube. The tube was then sealed

under vacuum, with the reagents being frozen in liquid nitrogen. After

aﬀord MA5 as a brown solid (550 mg, 98% yield based on M5). H

NMR (300 MHz, DMSO-d ): δ = 8.80−8.82 (d, 6H, J = 8.4 Hz),

.78−7.81 (d, 6H, J = 8.4 Hz), 7.53 (s, 6H), 7.24−7.38 (m, 30H),

.34 (s, 12H) ppm. C NMR (75 MHz, DMSO-d ): δ = 170.75,

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returning to room temperature, the mixture was ultrasonicated for 30

min and heated at 100 °C in an oven for 24 h (Figure S14). The solid

was collected by centrifugation and then washed by DMF (1.0 mL ×

) and THF thoroughly until the supernatant became totally

colorless. The above vacuum-dried sample was denoted as ZrTTA-

SH-MTFPP [ZrTTA-6SH-ZnTFPP (purplish red powder), ZrTTA-

SH-NiTFPP (red powder), ZrTTA-6SH-FeTFPP (green powder)].

Photocatalytic H Production Measurement of ZrTTA-6SH

Samples. The photocatalytic hydrogen evolution experiments were

performed in a quartz vial reactor (20 mL) sealed with a rubber

septum, i.e., a gas-closed system, at ambient temperature and pressure.

Initially, the sample powder (1.0 mg) was stirred and suspended in

aqueous triethanolamine (TEOA) solution (5.0 mL, water/TEOA, v/

v = 9:1). Then, a H PtCl aqueous solution (10 μL, 15.4 mM;

equivalent to 0.03 mg of Pt, 3 wt % of the catalyst) was injected (for

the subsequent in situ photoreduction deposition of Pt particles as

cocatalyst). The resulted suspension was purged with argon gas for 20

min to ensure anaerobic conditions, and then it was placed 15 cm

away from the lamp (CEL-HXF300). After 1.0 h of irradiation, the

released gas (400 μL) was syringed from the headspace of the reactor

and analyzed by a gas chromatograph (Shimadzu, GC-2014, Japan,

with ultrapure Ar as a carrier gas) equipped with a TDX-01 (5 Å

molecular sieve column) and a thermal conductivity detector (TCD).

The total content of photocatalytic H₂evolution was calculated

according to the standard curve. Continuous stirring was applied to

the reactor for uniform dispersion and irradiation of the photocatalyst

particles.

Figure 2. Powder X-ray diﬀraction patterns (Cu Kα, λ= 1.5418 Å):

(a) calculated from the structure model of ZrTTA-6SH (with

modiﬁcation of crystalline size (20 nm × 20 nm × 2 nm) and

preferred orientation (March-Dollase, 100, 3); (b) measured from an

as-made ZrTTA-6SH sample; (c) measured from an active ZrTTA-

6SH sample; (d) measured from a boiling-water-treated ZrTTA-6SH

sample.

RESULTS AND DISCUSSION

■

Improved Linker Synthesis. The synthesis was based on

the recently reported method using benzyl mercaptan as a

masked thiol agent (e.g., in the form of the starting molecule

S1, Figure S1 ). The diﬃculty in synthesizing thiol-equipped

organic linkers had long been a hinderance in their deployment

in MOF chemistry, while the synthesis of H TTA-6SH further

showcases the wide scope of this method for accessing

carboxyl-thiol linker building blocks. As shown in Figure S1,

using a one-pot Suzuki−Miyaura protocol, S1 was borylated

via bis(pinacolato)diboron and coupled with 2,4,6-tris(4-

bromophenyl)-1,3,5-triazine to establish the trigonal M5. M5

can be converted into ligand H TTA-6SH by the AlCl -based

procedure, i.e., ﬁrst hydrolyzing the ester groups in M5 and

then removing the benzyl groups by AlCl₃(Figure S1 ).

Interestingly, the two transformations can be more eﬃciently

accomplished by using BBr to simultaneously remove both

the methyl and benzyl groups from M5 and to directly aﬀord

ligand H TTA-6SH in one pot. BBr was previously used to

Figure 3. Single layer (a) and packing structure (b) of ZrTTA-6SH.

Zr coordination polyhedra are displayed in green. N, S, O, and C

atoms are displayed as blue, yellow, red, and gray beads, respectively.

cleave a benzylthio group ortho to a sulfonamino unit; as the

benzylthio groups of M5 are also ortho to the carboxyl unit, we

suspect that the ease of benzyl removal in both cases may be

assisted by the anchoring of the chemically hard center of BBr₃

onto the terminal oxygen atoms of the sulfonamino or the

carboxyl unit, which consequently facilitates the attack on the

adjacent benzylthio groups. In practice, the one-pot BBr₃

protocol simpliﬁes the synthesis procedure, increases the

yield (up to 95%), and therefore facilitates the exploitation of

novel thiol-functionalized ligands.

UMCM-309a, which is based upon the smaller tritopic linker

1,3,5-(4-carboxylphenyl)benzene. Speciﬁcally, the structure

of ZrTTA-6SH can be modeled on a hexagonal cell, with the

optimized parameters of a = b = 27.20 Å, c = 7.26 Å. The

Pawley reﬁnement produced a close match between the peak

positions (R = 4.63%, R = 3.13%) (Figure S13). The

Preparation and Characterization of MOF Solids. A

reliability of the structure model was also checked by

comparing the calculated PXRD patterns with the experimental

ones (as shown in Figure 2). The results conﬁrmed the

formation of eclipsed, hexagonal stacking of the MOF layers,

which was built on planar tripod linkers to form the rhombus-

tiled, binodal 3,6-connected kgd net. The particle size of

ZrTTA-6SH was found by SEM imaging in a range of 200−

400 nm (Figure S8), while transmission electron microscopy

solvothermal reaction of H TTA-6SH and ZrOCl ·8H O in

DEF (N,N-diethylformamide), with formic acid as the

modulator and 1,2-ethanedithiol to minimize linker oxidation,

yielded a pale yellow, crystalline powder of the framework

ZrTTA-6SH (nonemissive under UV light). Powder X-ray

diﬀraction studies (Figure 2) indicate that ZrTTA-6SH

crystallizes into a kgd net (Figure 3), being isoreticular to

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Figure 4. (a) Kubelka−Munk plots of a series of ZrTTA-6SH. (b) Powder X-ray diﬀraction patterns (Cu Kα, λ = 1.5418 Å) of modiﬁed ZrTTA-

SH samples. (c) Photocatalytic performance of a series of ZrTTA-6SH samples. The photocatalytic irradiation systems were purged by Ar every 3

h. (d) Schematic diagram illustrating the proposed mechanism of charge separation and hydrogen production upon irradiation with visible light.

TEM) and selected area electron diﬀraction (SAED) images

Figure S9) conﬁrmed the hexagonal geometry and layered

host framework (e.g., against the corrosive NaF aqueous

solutions). Further cross-linking studies using polyﬂuoro

aromatics with more advanced functions, however, are still

needed to impart more diverse properties to thiol-equipped

porous solids. In this connection, we set our eyes on the

porphyrin guests not only for the versatile chemical and

photophysical properties of the porphyrin core but also for the

multiple (i.e., 20 in total) ﬂuoro groups on its four

pentaﬂuorophenyl arms. To incorporate the bulky π-con-

jugated planar metalloporphyrin guests of MTFPP (each

molecule is about 1.30 nm wide), the large pore aperture

(about 1.8 nm wide; see Figure S15 for an illustration) of the

ZrTTA-6SH framework is helpful.

morphology. The SEM and TEM also reveal very thin platelets

(

single layers. The elemental analysis results on an activated

ZrTTA-6SH solid can be ﬁtted with the formula Zr O (OH) -

20−30 nm thick), corresponding to the thickness of 30−40

(

HCOO) (OH) (TTA-6SH) (H O) (mw 2688, TTA-

2.2 5 1.6 2 25

SH = C H N O S ), indicating one-ﬁfth of the 3-connected

42 24 3 6 6

linkers are missing in the kgd net (see also Figures S6 and S7

for the TGA plot and FT-IR spectra).

N sorption (77 K) of activated ZrTTA-6SH features a

typical type-I isotherm (Figure S10), revealing a BET surface

area of 265 m /g. QSDFT analysis on the sorption data

showed an average pore size value of 1.26 nm and a micropore

The reaction of the metalloporphyrin guests MTFPP (M =

volume of 0.082 cm/g (Figure S11 ). Meanwhile, CO sorption

Zn , Ni , and FeCl ) with the MOF host ZrTTA-6SH was

conducted using DMF as the solvent and triethylamine as base

under anaerobic conditions. After the metalloporphyrin

treatment (24 h at 100 °C), the colors of the MOF solids

became darker (ZrTTA-6SH-ZnTFPP is purplish red, ZrTTA-

6SH-NiTFPP is red, and ZrTTA-6SH-FeTFPP is green);

accordingly, MTFPP modiﬁed solids exhibit strong absorption

in the longer wavelength region, with relatively well-deﬁned

absorption edges identiﬁable for ZrTTA-6SH-ZnTFPP and

ZrTTA-6SH-NiTFPP (corresponding band gaps: 1.75 eV;

Figure 4a). The absorption edge for ZrTTA-6SH-FeTFPP,

however, is less distinct, with a slowly decreasing absorption

slope extending to as low as 1.2 eV. By comparison, the

(

195 K) and Langmuir linear ﬁtting provided a type-I isotherm

plot as well and a surface area of 360 m /g (Figure S12 ).

Powder X-ray diﬀraction (PXRD) of ZrTTA-6SH also

indicated no degradation of crystallionity when the activated

sample was exposed in air, or when it was boiled in water for

2 h (Figure 2, pattern d).

Postsynthetic Porphyrin Insertion. The facile substitu-

tion of aromatic ﬂuorides by thiol nucleophiles has been well

documented. In our earlier work, the simple pentaﬂuoro-

benzaldehyde (C F CHO) guests were used to cross-link the

mercapto side groups in the MOF solid of ZrOMTP (NU-

100-type net) to enhance the stability of the coordination

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pristine ZrTTA-6SH (light yellow in color) exhibits an optical

band gap of about 2.74 eV (Figure 4a), being over 1.0 eV

higher than the metalloporphyrin modiﬁed systems. The

absorbance spectra of MTFPP-modiﬁed ZrTTA-6SH samples

of the H on Pt to produce hydrogen gas (as shown in Figure

4d).

The photocatalytic H generation reaction was set up under

visible light irradiation, in which diﬀerent ZrTTA-6SH samples

act as heterogeneous photocatalysts, TEOA acts as a sacriﬁcial

electron donor, and Pt acts as a cocatalyst to enhance the

hydrogen-evolution capability. Initially, we tested the pristine

ZrTTA-6SH as photocatalyst, which exhibited very little H₂

evolution, as shown in Figure 4c. Impressively, after the

grafting of the ZnTFPP guests, the amount of H evolution

was found to be 110 μmol/g/h and the generation rate was

constant for 9 h (i.e., 3 cycles, driven by visible light), which is

better than commercial Pt/Black TiO

Figure S16) reveal more clearly the characteristic Soret and Q

bands (located around 400 and 550 nm, respectively) from the

MTFPP moieties. The incorporation of the large MTFPP

guest molecules into the MOF matrix is facilitated by the large

pore width and the layered morphology (as revealed by the

above SEM and TEM images): for example, the latter allows

the exfoliation into many thin platelets to maximize contacts

between the MTFPP guest molecules and the MOF phase.

The expandable interlayer spacing of 2D MOF sheets has also

Pure TiO

and

39−42

been exploited by Choe for facile pillar replacement, and

more recently by Wang, Yuan, and Zhou to anchor porphyrin

ligands across the nanosheets of a 2D Zr-carboxylate solid as

some reported MOF systems.

Similarly, ZrTTA-6SH-

NiTFPP and ZrTTA-6SH-FeTFPP samples also exhibit better

catalytic activity (71 and 17 μmol/g/h, respectively) than

pristine ZrTTA-6SH (Figure 4c). The activities thus observed

of the three systems formally correlate with the amount of the

MTFPP guests anchored into the ZrTTA-6SH host: i.e., 175,

90, and 62 μmol/g for ZrTTA-6SH-ZnTFPP, ZrTTA-6SH-

NiTFPP, and ZrTTA-6SH-FeTFPP, respectively. Heteroge-

neous photocatalytic processes are, however, complex,

involving various intertwined steps of excitation and energy/

accessible photosensitizers for artemisinin production.

After the MTFPP modiﬁcation, the crystalline lattice of the

ZrTTA-6SH host remained unchanged, as indicated by the

PXRD patterns (Figure 4b). The formation of thioether links

between the MTFPP guests and the ZrTTA-6SH scaﬀold is

consistent with the depletion of thiol groups (the weakening of

−

the S−H stretch at 2570 cm in FT-IR, Figure S17), and with

the emergence of higher binding energy peaks in the S 2p XPS

spectra (see Figure S18). ICP-OES (Table S1) results also

revealed the successful incorporation of the MTFPP guests:

electron transfer; it therefore remains to be seen how the H

production activity relates to the concentration and electronic

conﬁguration of the metalloporphyrin moieties grafted. As for

each individual site of MTFPP as an active center, the turnover

frequency (TOF) was correspondingly determined to be 0.63,

0.79, and 0.28 h , which is moderate compared with other

heterogeneous photocatalytic MOFs.

of the ZrTTA-6SH solids were collected by centrifugation after

the photoirradiation and their crystalline structure of ZrTTA-

SH framework was maintained, as conﬁrmed by PXRD

the ratios of the Zn/Ni/Fe ions over the Zr cluster were

determined by ICP-OES to be 0.460, 0.236, and 0.164,

respectively. Semiquantitative EDX results (Figure S19) also

conﬁrm the decreasing order of ZnTFPP, NiTFPP, and

FeTFPP for the amounts loaded. The loading possibly depends

on the aggregation and solvation of the porphyrin guests in

solution: e.g., the ﬂat, square planar Ni²allows stronger

porphyrin stacking (relative to the octahedral, axially

−

43−45

More than 95 wt %

(

Figure S20).

The above preliminary tests therefore showcase the

coordinated Zn ), stacking that hinders the individual guest

from entering the pores, while the polar, ionic Fe(III)-Cl unit

in FeTFPP may induce even greater aggregation and solvation

that further hamper diﬀusion. The quantitative diﬀerence

between the ICP and EDX data can be partly due to surface

enrichment; namely, EDX only detects elements near the

sample surfaceand it may be easier for the S atoms closer to

the surface to anchor the MTFPP guests. The ICP-OES

results, on the other hand, are more representative of the bulk

sample. Incidentally, a longer time of treatment and higher

guest concentrations did not increase the loading; perhaps, the

inserted guest molecules into the narrow openings hamper the

entry of additional guests (see Figure S15 for an illustration on

the occupation of the channel by an MTFPP molecule).

Hydrogen Production Catalysis Studies. Metallopor-

phyrin compounds have been widely studied for their versatile

photophysical properties, e.g., strong absorbance in the UV−

visible region, photosensitizing capabilities, and ﬂexible

functionalization via peripheral substitution or inner metal

complexations. Porphyrin-equipped MOF materials are much

researched as heterogeneous catalysts (e.g., photocatalysts),

because of the large accessible surface area, potentially easy

photocatalytic activity imparted by the anchoring of the

metalloporphyrin guests. Also notable are the recyclability of

the metalloporphyrin-modiﬁed MOF materials and their intact

crystalline structures after multiple cycles of photocatalytic

reactions. In this light, the strong C−S−C (thioether) covalent

bonds between the framework and the metalloporphyrin

(

MTFPP) guests serve as a robust anchor that prevents guest

leaching and ensures long-standing activity of catalysis. In

future studies, it is of interest to probe how the triazine core,

with its electron-withdrawing and polar features, impacts the

optoelectronic properties, and whether it may facilitate the

diﬀusion of the hydrophilic reagents (e.g., aqueous triethanol-

amine, TEOA) into the channels. It would also be instructive

to investigate the roles of the multiple sulfur groups in the

photocatalytic process, e.g., how they interact with and impact

the catalytically active Pt species.

SUMMARY

■

Taken together, the accommodation and the covalent grafting

of the metalloporphyrin guests onto the porous framework of

ZrTTA-6SH have been demonstrated by the resultant

22,24−35

separation, and good recyclability.

In photocatalytic H₂

enhancement of photocatalytic activity for H production.

production, for instance, a general mechanism for charge

separation and catalysis proceeds as follows. Photonic

excitation of the metalloporphyrin groups creates electrons

and holes. The sacriﬁcial electron donor (e.g., triethanolamine)

quenches the hole subsequently, while the photoexcited

electron migrates to cocatalyst Pt species, leading to reduction

The robust catalytic performance is reﬂected in the persistent

activity as well as the intact crystallinity of the powder sample

after multiple cycles of catalytic tests. The photoactive, large π-

conjugated metalloporphyrin bridges conveniently built into

the MOF matrix represent another step forward in our

longstanding eﬀorts to both structurally and functionally

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

K. S.; Xu, Z. Dense thiol arrays for metal-organic frameworks: boiling

Article

integrate the two worlds of coordination and covalent

frameworks. On the level of molecular design, the trigonal

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planar geometry of the H TTA-6SH molecule, with its

extended arms, helps to generate the two-dimensional

ZrTTA-6SH coordination framework with large pore width

to facilitate guest encapsulation. The 2D, lamellar ZrTTA-6SH

net also facilitates exfoliation for future studies on solution

dispersion, functionalization, and processing. Such future

studies would clearly beneﬁt from the framework stability

and reactivity arising from the trident donor motif, in which

the carboxyl unit is abutted by the mercapto groups in a

symmetrical and appealing form.

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ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01744.

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AUTHOR INFORMATION

■

Corresponding Authors

Zhengtao Xu − Department of Chemistry, City University of

Hong Kong, Kowloon, P. R. China; orcid.org/0000-0002-

9) Wong, Y.-L.; Diao, Y.; He, J.; Zeller, M.; Xu, Z. A Thiol-

Functionalized UiO-67-Type Porous Single Crystal: Filling in the

Synthetic Gap. Inorg. Chem. 2019, 58, 1462−1468.

408-4951; Email: zhengtao@cityu.edu.hk

(

10) Liu, D.-C.; Ouyang, T.; Xiao, R.; Liu, W.-J.; Zhong, D.-C.; Xu,

Xunjin Zhu − Department of Chemistry and Institute of

Advanced Materials, Hong Kong Baptist University, Kowloon

Tong, P. R. China; orcid.org/0000-0001-7929-6964;

Email: xjzhu@hkbu.edu.hk

Z.; Lu, T.-B. Anchoring Co Ions into a Thiol-Laced Metal-Organic

Framework for Efficient Visible-Light-Driven Conversion of CO into

CO. ChemSusChem 2019, 12, 2166−2170.

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disulfhydrylbenzene-1,4-dicarboxylate): A Microporous Metal-Organ-

ic Framework with Infinite (-Mn-S-)_∞Chains and High Intrinsic

Charge Mobility. J. Am. Chem. Soc. 2013 , 135 , 8185 −8188.

Authors

Yingxue Diao − Department of Chemistry and Department of

Materials Science and Engineering, City University of Hong

Kong, Kowloon, P. R. China

Nanfeng Xu − Department of Chemistry and Institute of

Advanced Materials, Hong Kong Baptist University, Kowloon

Tong, P. R. China; orcid.org/0000-0001-9469-1743

Mu-Qing Li − Department of Chemistry, City University of

Hong Kong, Kowloon, P. R. China; Department of Materials

Science and Engineering, Southern University of Science and

Technology, Shenzhen 518055, Guangdong, P. R. China

12) Cui, J.; Xu, Z. An electroactive porous network from covalent

metal-dithiolene links. Chem. Commun. 2014, 50, 3986−3988.

13) Sun, L.; Hendon, C. H.; Minier, M. A.; Walsh, A.; Dinca, M.

Million-Fold Electrical Conductivity Enhancement in Fe (DEBDC)

versus Mn (DEBDC) (E = S, O). J. Am. Chem. Soc. 2015, 137, 6164−

6167.

(14) Huang, X.; Sheng, P.; Tu, Z.; Zhang, F.; Wang, J.; Geng, H.;

Zou, Y.; Di, C.-a.; Yi, Y.; Sun, Y.; Xu, W.; Zhu, D. A two-dimensional

π −d conjugated coordination polymer with extremely high electrical

conductivity and ambipolar transport behaviour. Nat. Commun. 2015,

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c01744

15) Huang, J.; He, Y.; Yao, M.-S.; He, J.; Xu, G.; Zeller, M.; Xu, Z.

, 7408.

A semiconducting gyroidal metal-sulfur framework for chemiresistive

Notes

sensing. J. Mater. Chem. A 2017, 5, 16139−16143.

The authors declare no competing ﬁnancial interest.

(16) Huang, X.; Li, H.; Tu, Z.; Liu, L.; Wu, X.; Chen, J.; Liang, Y.;

Zou, Y.; Yi, Y.; Sun, J.; Xu, W.; Zhu, D. Highly Conducting Neutral

Coordination Polymer with Infinite Two-Dimensional Silver −Sulfur

Networks. J. Am. Chem. Soc. 2018, 140, 15153−15156.

ACKNOWLEDGMENTS

■

This work is supported by a GRF grant of the Research Grants

Council of Hong Kong SAR (Project 11303519). Prof. Xunjin

Zhu is thankful for the ﬁnancial support from Hong Kong

Baptist University (FRG2-17-18-068), the Interinstitutional

Collaborative Research Scheme (RC-ICRS-18-19-01A), and

the Interdisciplinary Research Matching Scheme (RC-IRMS/

(

17) Dong, R.; Zhang, Z.; Tranca, D. C.; Zhou, S.; Wang, M.; Adler,

P.; Liao, Z.; Liu, F.; Sun, Y.; Shi, W.; Zhang, Z.; Zschech, E.;

Mannsfeld, S. C. B.; Felser, C.; Feng, X. A coronene-based

semiconducting two-dimensional metal-organic framework with

ferromagnetic behavior. Nat. Commun. 2018, 9, 2637.

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6/17/02CHEM).

C.; Bordiga, S.; Lillerud, K. P. A New Zirconium Inorganic Building

Brick Forming Metal Organic Frameworks with Exceptional Stability.

J. Am. Chem. Soc. 2008, 130, 13850−13851.

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