Bioinspired Catalytic Reduction of Aqueous Perchlorate by One Single-Metal Site with High Stability against Oxidative Deactivation

Research Article

Bioinspired Catalytic Reduction of Aqueous Perchlorate by One

Single-Metal Site with High Stability against Oxidative Deactivation

Changxu Ren and Jinyong Liu*

Cite This: ACS Catal. 2021, 11, 6715 −6725

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sı Supporting Information

−

ABSTRACT: Reduction of perchlorate (ClO ) with an active

and stable catalyst is of great importance for environmental,

energy, and space technologies. However, after the rate-limiting

−

oxygen atom transfer (OAT) from inert ClO , the much more

−

reactive ClO_x(x ≤ 3) intermediates can cause catalyst

deactivation. The previous Re−Pd/C catalyst contained a

[

Re (O)(hoz) ] site (Hhoz = 2-(2′-hydroxyphenyl)-2-oxazoline)

− −

and readily reduced ClO , but ClO intermediates led to rapid

formation and hydrolysis of [Re (O) (hoz) ] . While microbes

VII

use delicate enzymatic machinery to survive the oxidative stress

−

during ClO₄reduction, a synthetic catalyst needs a straightfor-

ward self-protective design. In this work, we introduced a methyl

group on the ligand oxazoline moiety and achieved a substantial

enhancement of catalyst stability without sacriﬁcing the performance of ClO₄reduction. A suite of kinetics measurement, X-ray

−

photoelectron spectroscopy characterization, reaction modeling, stopped-ﬂow photospectrometry, and H NMR monitoring

revealed the underlying mechanism. The most critical and unexpected eﬀect of the methyl group is the deceleration (for 2 orders of

magnitude) of OAT from ClO₃⁻to [Re (O)(Mehoz) ] . However, the rate of OAT with ClO was not aﬀected. The methyl group

−

VII

also slowed down the hydrolysis of [Re (O) (Mehoz) ] and allowed the introduction of methoxy onto the phenolate moiety to

further accelerate ClO₄⁻reduction. With 1 atm H at 20 °C, the Re−Pd/C catalyst used [Re (O)(MehozOMe) ] as the only

−

reaction site to reduce multiple spikes of 10 mM ClO₄into Cl without decomposition. This work showcases the signiﬁcant eﬀect

−

of simple ligand modiﬁcation in improving catalyst stability for high-performance ClO₄reduction.

KEYWORDS: perchlorate reduction, catalyst stability, rhenium, ligand modiﬁcation, oxygen atom transfer, hydrolysis,

oxidative deactivation

INTRODUCTION

reductase, which contains a molybdopterin-coordinated Mo

■

−

cofactor as the active site. A series of Fe−S clusters, heme

Over the past two decades, perchlorate (ClO ) has

increasingly drawn public attention as a pervasive and

persistent water pollutant due to the improper disposal of

complexes, and electron shuttle compounds enable the

6−18

electron transfer and redox cycling of the Mo cofactor.

−

−4

The further reduction of ClO₂in the same pathway will

energetic materials and natural atmospheric formation.

−

generate highly reactive HClO/ClO (pK

7.5), which can

Because excess exposure to ClO₄can cause thyroid

irreversibly inactivate the enzyme by reacting with both the

malfunction, many states in the U.S. have set the limits for

−

−1

3,6

metal factors and the protein. Thus, a second enzyme,

^C^l^O4 at 0.8−18 μg L in drinking water. The interest in

−

chlorite dismutase, uses a heme factor to convert ClO

into

^C^l^O4 has been further stimulated by the recent discovery of

−

7−10

11,12

innocuous Cl and O .

However, ClO can still be released

from approximately 1 out of 100 reaction cycles. Another

defense mechanism against ClO is realized by a methionine-

rich periplasmic protein. The oxidized sulfoxide product is

^C^l^O4 on Mars,

moon, and meteorites,

which

collectively imply its wide distribution throughout (and

−

1,13

perhaps beyond) the solar system.

Thus, the reduction

and utilization of ClO₄⁻have considerable importance for

human’s extraterrestrial exploration by removing chemical

hazard, improving soil habitability, providing life support, and

Received: December 2, 2020

Revised: May 10, 2021

Published: May 24, 2021

fueling vehicle operations.

−

While ClO₄is highly inert under ambient conditions,

microorganisms have developed delicate enzymatic machinery

−

for ClO₄reduction (Figure 1a). The sequential 2e reduction

of ClO₄to ClO₃⁻and ClO₂⁻are fulﬁlled by (per)chlorate

−

2021 American Chemical Society

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ACS Catal. 2021, 11, 6715−6725

and hypochlorite scavenging; (b) simpli ﬁ ed bioinspired design of Re −Pd/C; and (c) mechanistic challenge of the previous catalyst.

Research Article

Figure 1. (a) Overall microbial reduction of ClO₄⁻including electron harvest from organic donors, (per)chlorate reduction, chlorite dismutation,

Figure 2. (a) Synthesis of HL_N₋_Oligands and Re (O)(L ) Cl complexes; (b) comparison of H NMR spectra for 2a and 2b with signals of

N−O

−

−1

the two 4-methyloxazoline moieties highlighted in blue; (c) degradation proﬁle of 1 mM ClO₄by Re−Pd/C catalysts (0.5 g L , 5 wt % Re and 5

wt % Pd, pH 3.0, 1 atm H , 20 °C); and (d) chlorine balance using the Re−Pd/C prepared from 2b.

regenerated by a methionine sulfoxide reductase that uses a

system. Therefore, a novel “self-defense” mechanism for the Re

site against oxidative deactivation is of great interest to both

catalytic science and engineering. In other words, can we

develop a single-metal site that is both active and stable

without a multicomponent protection mechanism? If yes, how

simple is the design of such a metal site?

Mo cofactor. Therefore, the rapid and robust microbial

reduction of ClO₄⁻is achieved through the cooperation of all

components and functions.

Signiﬁcant eﬀorts have been taken to design synthetic metal

−

catalysts that mimic biochemical principles to reduce ClO

and other oxyanions.

24−30

In particular, we have developed a

In this contribution, we report on the discovery and

elucidation of a surprising and advantageous structure-stability

bioinspired Re−Pd/C catalyst, in which the single-site

−

^f^e^a^t^u^r^e^.^Wⁱ^t^h^o^u^t^l^o^w^e^rⁱⁿ^g^t^h^e^r^a^t^e^o^f^C^l^O4 reduction, the

[

Re (O)(hoz) ] complex (Hhoz = 2-(2′-hydroxyphenyl)-2-

introduction of a methyl group to the original hoz ligand

oxazoline), Pd nanoparticles with H gas, and the porous

substantially slowed down (1) the oxidation of the Re site by

carbon (Figure 1b) mimic the three essential components of

−

VII

^C^l^Ox intermediates and (2) the hydrolysis of the Re site.

The “evolved” catalyst exhibited high stability against oxidative

deactivation and thus signiﬁcantly enhanced the performance

the enzyme system, the Mo cofactor, the electron transfer

chain, and the protein support. While homogeneous metal

4,29,30

catalysts are moisture-sensitive

or require special

−

in ClO₄reduction. This simple ligand modiﬁcation provided

electron donors such as hydrazine, ferrocene, sulﬁde, and

4−26

eﬀective protection of the reactive site, demonstrating a novel

strategy to design catalysts with a single-metal site against

deactivation by reaction intermediates.

phosphine,

the heterogeneous Re−Pd/C platform enables

−

rapid reduction of aqueous ClO₄by 1 atm H at 20 °C.

However, upon rapid oxidation by the highly reactive ClO

intermediates, accumulated [Re (O) (hoz) ] in the Re

cycle is subject to irreversible hydrolysis (Figure 1c). Because

simplicity is essential for both bioinspired design and practical

application, the multicomponent defense mechanism used by

microbes cannot be readily mimicked in a synthetic catalyst

−

VII

V/VII

RESULTS AND DISCUSSION

■

Catalyst Preparation and Basic Performance. To

interrogate the eﬀects of ligand structure modiﬁcation on

catalyst performance, we synthesized new N,O-bidentate

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ACS Catal. 2021, 11, 6715−6725

Research Article

Figure 3. (a) Degradation proﬁle of 10 mM ClO₄⁻by Re−Pd/C catalysts (0.5 g L , 5 wt % Re and 5 wt % Pd, pH 3.0, 1 atm H , 20 °C); (b)

−1

illustration of the two-step stability test and the control experiment; (c−e) results of the stability test showing the eﬀect of the L

common legend is in panel d); and (f) stability challenge of catalyst 2b′ with ﬁve spikes of 10 mM ClO .

structure (the

N−O

−

ligands (HL_N₋_O) derived from the original Hhoz ligand (HL1,

immobilized by hydrophobic and electrostatic interactions.

−

Figure 2a). The oxazolinylphenolate structure is naturally

We observed >95% of Cl in all three precursors released in

water, indicating a near-complete immobilization and

occurring in microbial siderophores. The HL_N₋_Oligands

were synthesized via one-step construction of the oxazoline

−

activation of Re for the reduction of aqueous ClO . All

−

ring from speciﬁc benzonitriles and amino alcohols. The

Re−Pd/C catalysts exhibited a rapid reduction of 1 mM ClO

Re (O)(L ) Cl complexes were prepared with our estab-

by 1 atm H at 20 °C (Figure 2c). The −Me substitution on

N−O 2

−

lished method and used as the precursor for Re−Pd/C

catalysts. Detailed procedures are described in Experimental

Section of the Supporting Information.

oxazoline did not alter the rate of ClO reduction (1a′ vs

−

2a′). The −OMe substitution provided a 68% faster ClO

reduction (2b′ vs 2a′). As the oxygen atom transfer (OAT)

−

To enhance the reactivity of Re with ClO , we ﬁrst

from ClO to Re decreases the electron density of Re, an

introduced an electron-donating −OMe to the phenol moiety

electron-rich ligand can promote this process. The chlorine

−

of Hhoz (L2). However, the synthesis of Re (O)(L2) Cl (1b)

balance was closed by ClO and Cl (Figure 2d). The initial

0.13 mM Cl was released from the precursor 2b (0.134 mM

was added). More than 99.99% of 1 mM ClO was

−

yielded a mixture of two products with poor solubility in most

solvents, preventing further isolation and characterization. To

−

increase the solubility and restrict the isomerization, we

introduced a methyl group on the oxazoline moiety (L3 and

L4) by switching the amino alcohol building block from

completely reduced to Cl within 1 h. The turnover number

(TON) on each Re site was 7.5 for the reduction of ClO₄⁻(or

30 if all four oxygen atoms were abstracted by the same OAT

mechanism). The absence of a hydrogen-bonding motif (e.g.,

glycinol to L-alaninol. The corresponding Re (O)(L ) Cl

N−O 2

products (2a and 2b) showed good solubilities in dichloro-

amino acid residues in enzymes

or the secondary

coordination sphere in an artiﬁcial Fe complex ) for the

methane and chloroform. Comparison of H NMR spectra

with the previously characterized 2a (Figure 2b, δ 4.0−5.5 for

metal-bound oxyanion required a slightly acidic environment

(e.g., pH 3.0) to enable OAT.

Before we further discuss the kinetics, we note that the

28,35

H’s on the oxazoline ring and δ 1.7 for H’s of the methyl

group) conﬁrmed the exclusive yield of N,N -trans Re (O)-

) Cl. Single crystallography of 2b (Figure S1 , not

heterogeneous nature of Re−Pd/C catalysts has been

N−O 2

8,31

reﬁned due to high-level disorders) also conﬁrmed the N,N-

conﬁrmed in previous studies.

The direct use of Re

−

trans conﬁguration. For comparison, the original L1 led to the

complex 1a or 2b did not show ClO₄reduction in the pure

formation of both N,N-cis and N,N-trans Re (O)(hoz) Cl

aqueous medium (Figure S2) for two reasons: (1) the

(

1a). Previous studies have conﬁrmed that N,N-cis isomers are

solubility of Re(O)(L) Cl in water is low and (2) the

−

much less active than the N,N-trans counterparts for ClO

Re(O)(L) Cl precursor cannot spontaneously release the

−

reduction. Therefore, the use of the minimally steric −Me on

the oxazoline moiety mimics the biochemical replacement of

glycine with alanine and ensures the exclusive formation of the

desirable N,N-trans Re complexes.

chloro ligand to allow the coordination with ClO . The

hydrophobic and electrostatic interactions between the Re

complex and carbon surface provide unique advantages for the

−

heterogeneous system. When cationic 1a and 2a (the Cl in

1a and 2a was removed by AgOTf) were dissolved in

To evaluate the activity and stability of Re−Pd/C catalysts

(

labeled 1a′, 2a′, and 2b′), we immobilized the corresponding

acetonitrile for homogeneous LiClO reduction by alkyl

sulﬁdes, 2a was quickly inhibited by the Cl from ClO ₄⁻

−

Re (O)(L ) Cl precursors (1a, 2a, and 2b) in Pd/C with

an aqueous deposition method. Brieﬂy, the powders of the

Re complex and Pd/C were sonicated and stirred in water

suspension under 1 atm H . The transiently dissolved Re

N−O 2

reduction, whereas the activity of 1a sustained longer (Figure

S3 ). Product inhibition by Cl was also observed from a

−

24,25

homogeneous Fe catalyst.

In stark comparison, 1a′ and 2a′

complex in water was adsorbed into porous carbon and

showed almost identical activity (Figure 2c). The immobilized

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ACS Catal. 2021, 11, 6715−6725

ACS Catalysis

Research Article

Scheme 1. Proposed Transformation of Molecular Re Species at the Carbon−Water Interface

Re complexes interact more strongly with the carbon surface

>97 and >99.98% reduction were achieved by 2b′ within 2 h

−

than with aqueous Cl . During the aqueous deposition process,

and 4 h, respectively. The initial turnover frequency (TOF )

−

95% of Cl in Re (O)(L ) Cl precursors were expelled

calculated from the reduction of the ﬁrst 5% of 10 mM ClO

N−O 2

−

−1

into the aqueous phase. Compared to the high sensitivity of

was 109 h (or 436 h for abstracting all four oxygen atoms,

see the Supporting Information for the method of TOF₀

calculation).

−

[

Re (O)(L ) ] with Cl in the acetonitrile solution, the

N−O 2

.13 mM Re in Re−Pd/C catalysts was only partially inhibited

−

even in the presence of 10−50 mM Cl (vide infra).

Catalyst Stability against Oxidative Deactivation.

While the higher activity of 2b′ than 1a′ was expected, to

The high stability of catalyst 2b′ was further veriﬁed by

−

reusing it for reducing ﬁve spikes of 10 mM ClO (Figure 3f).

−

Each ClO₄spike was completely reduced using 24 h before

our surprise, 2b′ exhibited high stability against oxidative

the next spike. The kinetics of the ﬁfth spike was almost the

−

deactivation. For the reduction of 1 mM ClO , the apparent

same as the control experiment, where freshly prepared 2b′

−

ﬁrst-order rate constant by 2b′ was 68% higher than that by 1a’

reduced 10 mM ClO in the presence of 40 mM NaCl

−

(

Figure 2c). However, the rate constants for 10 mM ClO

(mimicking the four earlier ClO₄spikes). Hence, the gradual

TON = 75 for ClO₄⁻or 300 for abstracting all four oxygen

decrease in catalyst activity was solely attributed to the

−

atoms) by 2a′ and 2b′ were 8.6- and 9.7-fold higher than that

by 1a′, respectively (Figure 3a). This signiﬁcant diﬀerence in

reducing concentrated ClO₄⁻implies that the Re site in 2a′

and 2b′ is much more stable than that in 1a’.

signiﬁcant buildup of Cl in the water. This high stability

−

makes 2b′ the most active catalyst for aqueous ClO

reduction among all chemical reduction systems reported to

34,36−44

date (Tables S1 and S2).

As elucidated in previous studies, the OAT from ClO

−

We systematically assessed the catalyst stability by

conducting two sets of kinetic experiments (Figure 3b). In

VII

oxidizes [Re (O)(L ) ] into [Re (O) (L ) ] (steps i

N−O 2

8,45

the ﬁrst set, each Re−Pd/C catalyst was used to reduce 10 mM

and ii in Scheme 1).

Organic sulﬁde (in the homogeneous

−

ClO for 24 h, so that even the least active 1a′ could complete

system) or Pd-activated H (in the heterogeneous system)

removes one oxo group and thus reduces Re back to Re^V

VII

the reduction (Figure 3a). To probe whether the Re sites were

−

still active, we added another spike of 1 mM ClO₄to the used

(step iii). If the oxidation is faster than the reduction, the

VII

catalysts and measured the reduction kinetics. Because the

accumulated [Re (O) (L ) ] is subject to hydrolytic

N−O 2

−

VII

−

generation of 10 mM Cl can slightly inhibit the Re−Pd/C

decomposition into Re O and [H₂L_N₋_O

Because ClO₄is much more inert than ClO_xintermediates,

we attributed the catalyst deactivation to the highly reactive

]

(step iv).

catalyst by competing with ClO₄⁻for the reactive site, in the

second set of control experiments, we measured the reduction

−

of 1 mM ClO₄by freshly prepared Re−Pd/C catalysts in the

ClO , ClO , and ClO intermediates. When the initial

3 2

−

presence of 10 mM NaCl (mimicking the generation of Cl

concentration of ClO was elevated, the concentrations o₋f

−

from 10 mM ClO ). The signiﬁcant disparity between the

ClO_xintermediates from the ﬁrst-order reduction of ClO₄

two experiments for 1a’ (Figure 3c) suggests a major

were also proportionally elevated, thus imposing higher

oxidative “stress” to the Re sites. To conﬁrm this mechanistic

deactivation of Re sites during the reduction of 10 mM

−

ClO . In stark contrast, 2a′ and 2b′ showed a negligible

insight, we further challenged Re−Pd/C catalysts by directly

−

diﬀerence between the two experiments (Figure 3d,e). Hence,

exposing them to ClO (Figure 4a). The one-time addition of

the reduction of 10 mM ClO₄⁻by 2a′ and 2b′ did not lead to

0.5−1.0 mM ClO imposed substantially higher oxidative

−

a noticeable activity loss. For a single batch of 10 mM ClO ,

stress than the gradual generation (accompanied with rapid

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ACS Catal. 2021, 11, 6715−6725

Research Article

−

Figure 4. (a) Illustration of the two-step stability test using ClO and the control experiment; (b−d) results of the stability tests (the common

legend is shown in panels c or d, 0.5 g L⁻of 5 wt % Re and 5 wt % Pd, pH 3.0, 1 atm H , 20 °C); and (e−p) Re 4f XPS spectra of the three

catalysts under various conditions. The Re 4f₇_/₂peaks are highlighted with dotted lines.

degradation) of ClO₃⁻from the pseudo-ﬁrst-order decay of 10

cause diﬀerent catalytic activities by 1a′ and 2a′ (Figure 2c).

−

VII

mM ClO . As shown in Figure 4c,d, the one-time addition of

The detection of Re is caused by the trace amount of O

−

.5 mM ClO₃did not deactivate catalyst 2a′ and 2b′ in the

during the sample preparation and transfer (see Supporting

Information , Experimental Section). After the reduction of 10

−

following reduction of 1 mM ClO . When ClO was

−

increased to 1 mM, we observed deactivation to a limited

mM ClO , two new Re 4f peaks at 40.7 eV and 41.6 eV

7/2

extent. In stark comparison, 1a′ was severely deactivated by 0.5

showed up in 1a′ (Figure 4f). To understand these two

species, we prepared the ReO −Pd/C catalyst from KReO

−

and 1 mM ClO (Figure 4b).

−

We used X-ray photoelectron spectroscopy (XPS) to probe

the evolution of Re speciation in the Re−Pd/C catalysts after

use under various conditions. The fresh catalyst 1a′ contained

with the same aqueous deposition method. The ClO

reduction activity of this catalyst (Figure 5a) was 150- and

VII

−

257-fold lower than 1a′ and 2b′, respectively. The Re O is

6−48

three Re species with 4f₇binding energies of 42.8 eV, 44.0

reductively immobilized into low-valent ReO species.

eV, and 45.2 eV (Figure 4e), which are consistent with

The XPS spectrum of ReO −Pd/C contained a minor peak for

III

VII

III

reported Re , Re , and Re , respectively. The detection of

Re (42.6 eV) and two major peaks at 40.8 eV and 41.7 eV

III

Re was attributed to the reduction of the oxo group in

(Figure 5b), which have been identiﬁed as two diﬀerent Re

47,49

[

Re (O)(L ) ] by Pd-activated H (heterogeneous) or by

structures.

Thus, the two new peaks found in Figure 4f

N−O 2

VII

−

PPh (homogeneous). Interestingly, 2a′ and 2b′ contained

conﬁrmed the decomposition of active Re sites into Re O

III

little Re (Figure 4i,m), probably due to the steric eﬀect of

and the subsequent reduction into Re . The two Re species

took 42% of the total immobilized Re (Table S3). The XPS

methyl groups in L3 and L4. However, this diﬀerence did not

719

ACS Catal. 2021, 11, 6715−6725

the y-axis in panel a starts from C /C = 0.9.

Research Article

Figure 5. (a) Reduction of 1 mM ClO₄⁻by Re−Pd/C catalysts prepared from KReO with and without L4 and (b + c) Re 4f XPS spectra of the

two as-prepared catalysts. Reaction conditions: 0.5 g L⁻of 5 wt % Re and 5 wt % Pd, molar ratio of L4:Re = 2:1, pH 3.0, 1 atm H , 20 °C. Note

Figure 6. Time-dependent H NMR (600 MHz) spectra for the hydrolysis of 2B (10 mM) in 5/95 (v/v) D O/CD CN at 20 °C. The resonance

indicated by the blue arrow was used for quantitation shown in Figure 7a.

spectra after reducing 0.5 and 1.0 mM ClO₃⁻also showed the

two Re species, taking 35% and 56% of the total Re,

and Table S3). A small increase in Re species was observed

−

after the reaction with 1 mM ClO₃(Figure 4l,p and Table

respectively (Figure 4g,h and Table S3). Therefore, direct

S3 ). Thus, XPS characterization results are consistent with the

kinetic data shown in Figure 4b−d, providing spectroscopic

evidence for the high stability of Re sites in 2a′ and 2b’. We

exposure to ClO₃⁻caused more severe damage to the active Re

−

sites than treating concentrated ClO . Moreover, the

−

VII

−

did not further test ClO or ClO as the more challenging

decomposition of Re sites into Re O₄and free L

is a

N−O

VII

−

substrates because their side reactions in acidic media may

permanent deactivation because the use of Re O and L4

complicate the analysis and understanding. Moreover, the

(

the ligand for 2b) did not yield a higher activity than ReO −

−

stepwise reduction from ClO is less likely to accumulate

Pd/C (Figure 5a). The Re speciation was also similar to

−

^C^l^O2 or ClO in high concentrations.

ReO −Pd/C but very diﬀerent from 2b′ (Figures 5c vs 5b and

Mechanisms for the Enhanced Catalyst Stability.

Based on Scheme 1, we developed a quasi-steady-state

equation to model the rate of removing Re(L_N₋_O)₂sites

from the catalytic cycle into decomposition (eq 1, detailed

derivation steps provided in Text S1)

vs 4m−p). This aspect also shows that during the catalyst

preparation and normal catalysis, the Re(L ) complex

N−O 2

remained intact rather than undergoing “dissociation and

reassembly” of Re metal and free L_N₋_Oligands on the carbon

surface.

In stark contrast, the surface Re speciation in catalysts 2a′

d[6]

nK k k

1 2 4

and 2b′ did not show a meaningful increase in Re species after

= −

[XO ][Re]

reducing either 10 mM ClO₄⁻or 0.5 mM ClO₃⁻(Figure 4i−p

k₃

(1)

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VII

Figure 7. (a) Time proﬁles of H NMR measured hydrolytic decomposition of [Re (O) (L ) ] in 5/95 (v/v) D O/CD CN at 20 °C and (b)

N−O

time-dependent UV−vis absorption spectra for 1a → 1A upon stopped-ﬂow mixing of 0.5 mM 1a and 25 mM LiClO at 1:1 (v/v). Both species

were dissolved in anhydrous CH CN. The inset shows the change in absorption after subtracting the initial spectrum (t = 0 s) from all spectra and

c−f) dependence of the initial rate constant (k ) on the concentrations of ClO , ClO , and PyO (for Re oxidation) and of DMS (for Re

−

VII

(

reduction).

The apparent decomposition rate of the active Re(L

)

The rate constants of hydrolysis for 2A and 2B were

N−O 2

site is dependent on three reactions: the formation (K k ),

merely 20% and 33% of that for 1A (Figure 7a). Thus, the

VII

reduction (k ), and hydrolysis (k ) of [Re (O) (L ) ]

−Me substitution on oxazoline signiﬁcantly slowed down the

hydrolysis. The faster decomposition of 2B⁺than 2A is

probably attributed to the increased electron density at the

N−O 2

(

6). Because the Re−Pd/C catalysts 1a′ and 2b′ showed

surprisingly diﬀerent stabilities, we expected that the rate

constants of one or more steps were signiﬁcantly altered by the

minor modiﬁcations of the ligand with −Me on the oxazoline

or −OMe on the phenolate.

phenolate O, which facilitates the protonation of L

and

N−O

54,55

detachment from the metal.

Although a slower hydrolytic

step iv can, to some extent, help preserve 6 in the catalytic

cycle, the modestly diﬀerent rates of hydrolysis for the three

structures may not be fully responsible for the markedly

diﬀerent catalyst stability (Figures 3c−e and 4b−d).

Generation of [Re (O) (L ) ] Via OAT Oxidations.

We measured the kinetics of the formation of 6 from the

corresponding 4 (dissolved in acetonitrile) via OAT from

ClO , ClO , and pyridine N-oxide (PyO). In anhydrous

acetonitrile, 1a and 2a exhibit very similar dark green color,

whereas 2b shows a dark brown color (Figure S6). Because all

VII

Hydrolytic Decomposition of [Re (O) (L ) ] . We

N−O 2

^ﬁ^r^s^t^h^y^p^o^t^h^e^sⁱ^z^e^d^t^h^a^t^t^h^e^r^a^t^e^o^f^h^y^d^r^o^l^y^sⁱ^s^f^o^r⁶⁽^k4⁾

determines the lifetime of Re(L_N₋_O)₂sites. A slow step iv

could preserve 6 in the catalytic cycle (Scheme 1). Due to the

diﬃculty of directly probing and modeling molecular trans-

formations at the heterogeneous water−carbon interface, we

VII

N−O 2

investigated the hydrolysis of various 6 (1A , 2A , and 2B ) in

−

organic solutions by H NMR (Figures 6, S4, and S5). The

solutions of 6 were prepared by sequentially treating the

Re (O)(L ) Cl precursor 3 (1a, 2a, and 2b) with 1

N−O 2

three Re complexes have the same N,N-trans coordination

structure, the absorption red shift of 2b is attributed to the

equivalent of AgOTf and 0.25 equivalent of LiClO in a dry

box. In anhydrous CD CN, 6 was stable for at least 3 days. The

extended conjugation by −OMe substitution on the phenolate

H NMR spectra showed symmetry for the two L_N₋_Oligands.

56,57

VII

moiety.

We performed oxidation of 4 with excess oxidants

Based on the reported crystal structures of similar dioxo Re

5,51

under pseudo-ﬁrst-order conditions and monitored the fast

kinetics with stopped-ﬂow spectrophotometry. After the

addition of oxidants, all solutions turned into red color due

to the increased absorption of 380−700 nm (Figure 7b). The

maximum increase in absorbance from 4 to 6 (λ = 440 nm

for 1a → 1A and 2a → 2A and 510 nm for 2b → 2B )

were determined by subtracting the initial absorption spectra

of 4 (Figures 7b inset and S7). The representative time proﬁles

for the absorbance at λmax are shown in Figure S7d. The initial

complexes,

we postulate a C -symmetric cis-dioxo

VII

conﬁguration for [Re (O) (L

study. Although the reported crystal structure of [Re (O)-

H O)(L ) ] (4) has the H O trans to the oxo group, it

2 N−O 2 2

) ] structures in this

N−O

(

is reasonable to assume that during the rapid catalytic cycles,

max

−

both H O and ClO_xcoordinate with the Re center from the

equatorial site cis to the oxo. To normalize the peak

intensities of the time-dependent spectra, we used 3,4,5-

trichloropyridine as an internal standard (δ = 8.6 ppm). The

addition of 5% (v/v) D O initiated the ﬁrst-order hydrolysis of

rate constant (k ) from such proﬁles at varied concentrations

of ClO , ClO , and PyO are shown in Figure 7c−e. All data

4 3

sets showed the gradual saturation of kinetics as the oxidant

−

VII

−

(Figure 7a) directly into Re O and double-protonated

free ligand [H₂L_N₋_O] . Other intermediate structures, such as

VII

the previously reported Re (O) (L_N₋_O), were not observed.

concentrations increased.

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Research Article

To model the oxidation of 4 to 6 (steps i and ii in Scheme

) measured by the stopped-ﬂow experiment, we developed

the nonsteady-state eq 2 (detailed derivation steps provided in

Text S2)

oxide (PyO) as a highly reactive surrogate (entries 3, 6, and 9

in Table 1). For the OAT from PyO to Re , the k for 1a

−

(10.43 M ) was 1 order of magnitude higher than for 2a

(1.107 M ). Therefore, the introduction of −Me on the

oxazoline moiety of L not only causes a steric hindrance for

Re to coordinate with the oxygen donor (e.g., PyO) but also

slows down the OAT from the coordinated donor (e.g., ClO

−

nK k [XO ][Re]

N−O

d[6]

1 + K [XO ]

(2)

−

Nonlinear-least-squares ﬁtting of the initial rate data to eq 2

provided the constants K and k for individual oxidants and

and PyO) to Re . The comparison between 2a and 2b shows

that the −OMe on the phenolate moiety can both decrease K

−

Re complexes (Table 1). The equilibrium constants for step i

and increase k for ClO and PyO but not for ClO (entries

2 3 4

−6 vs 7−9 in Table 1).

Notably, the proﬁles for the oxidation from 4 to 6 by ClO ₃⁻

Table 1. Best Fitted Rate Constants from the Data in Figure

7c−e with eq 2

contained two phases with diﬀerent reaction rates (Figure S8).

The fast oxidation in the ﬁrst phase took only about 0.2 s for

oxygen donor

(XO)

K k

1 2

−1

K₁(M⁻1₎

a⁺(from L1)

k₂(s−1₎

−1₎

1a , 15 s for 2a , and 12 s for 2b . The much slower oxidation

entry

in the second phase took >30 s for 1a , >300 s for 2a , and

150 s for 2b . Both phases exhibited ﬁrst-order kinetics. A

ClO₄⁻

ClO₃⁻

PyO

13.2 ± 2.1

19.7 ± 1.8

191.0 ± 7.2

0.032 ± 0.004

12.75 ± 0.670

10.43 ± 0.306

0.4

251.2

1992.1

detailed analysis of the time-evolved absorption spectra found

the formation of Re (O)(L ) Cl (the precursor 3 in

N−O 2

Scheme 1), which is responsible for the relatively slow reaction

a⁺(from L3)

−

in the second phase. The reduction of highly reactive ClO

−

^C^l^O4

9.2 ± 0.4

33.2 ± 8.3

17.3 ± 0.4

0.031 ± 0.001

0.109 ± 0.018

1.107 ± 0.009

0.3

3.6

19.2

−

ClO₃⁻

PyO

rapidly generates Cl , which inhibited the reaction by

competing for the coordination site on Re (cf. Figure S3).

_b+ ₍_f_r_o_m_L₄₎

In the homogeneous system, the Re−Cl binding in 2a is much

stronger than in 1a . Spectroscopic evidence and reasoning are

ClO₄⁻

ClO₃⁻

PyO

9.8 ± 0.9

20.1 ± 4.6

12.1 ± 0.6

0.030 ± 0.002

0.219 ± 0.029

2.859 ± 0.092

0.3

4.4

34.6

provided in Figures S9 −S11 . Such two-phase kinetics was less

−

pronounced using the much more inert ClO and was not

observed from the reaction with PyO.

VII

Reduction of [Re (O) (L ) ] . Because it is challenging

N−O 2

(

^K1^,^f^o^r^lⁱ^g^aⁿ^d^e^x^c^h^aⁿ^g^e^f^r^o^m^t^h^e^s^o^l^v^eⁿ^t^t^o^t^h^e^o^x^y^g^eⁿ^d^oⁿ^o^r⁾

to measure redox transformations of Re in the three-phase

^d^e^p^eⁿ^d^oⁿ^b^o^t^h^t^h^e^o^xⁱ^d^aⁿ^t^aⁿ^d^LN−O structure. Among the

^t^h^r^e^e^o^xⁱ^d^aⁿ^t^s^,^C^l^O4 showed the lowest K . This result is

consistent with the well-known weak coordinating capability of

ClO . The substantially lower K for PyO by 2a and 2b

system of H +Pd/C in water, we measured homogeneous

−

reduction of 6 to 4 with dimethyl sulﬁde (DMS) as the

reductant. In a sequential mixing ﬂow circuit, the two solutions

containing 4 and PyO (equal molar concentration in

anhydrous acetonitrile) were ﬁrst mixed for a preset aging

time to ensure complete oxidation of 4 into 6. The reduction

was initiated by the subsequent mixing with excess DMS. The

dependence of the reaction rate constants on DMS

concentration is shown in Figure 7f. The second-order rate

constants were obtained as the slopes of the linear ﬁttings. The

−

−1

(

17.3 and 12.1 M ) than by 1a (191 M ) is attributed to

the steric hindrance by the methyl group on the ligands. The

K₁for ClO₄with 1a (13.2 M ) is also slightly higher than

that with 2a and 2b (9.2 and 9.8 M ).

−

−1

For step ii, the three complexes showed very similar k for

the OAT from coordinated ClO₄⁻to Re (0.030−0.032 s ).

However, to our surprise, the OAT from coordinated ClO to

(

−1

−

−1 −1

−1

reduction of 2A (842.6 M s ) was 82% slower than that of

a (12.75 s ) was 2 orders of magnitude faster than to 2a

−1 −1

−

1A (4778 M s ). This is probably caused by the steric

0.109 s ). For the reaction with ClO , the overall second-

−1

repulsion between the ligand methyl group and DMS. We

order rate constants (K k ) for 2a and 2b (3.6 and 4.4 M

−

−1 −1

note that in the Re−Pd/C catalyst, Pd-activated hydrogen is

much less steric demanding than DMS. However, similar to the

s ) were 70- and 57-fold smaller than for 1a (251.2 M s ,

entries 2, 5, and 8 in Table 1). On the basis of eq 1, the

substantially slower formation of 6 is beneﬁcial for its stability.

We expected similar trends for the even more reactive ClO

and ClO substrates, but experimental attempts were not

successful due to (1) the low solubility of NaClO in pure or

water-mixed acetonitrile and (2) uncontrolled reactivity of

HClO/ClO with common organics. Hence, we used pyridine

orders of magnitude diﬀerent reactivity of oxidative OAT

−

^f^r^o^m^c^o^o^r^dⁱⁿ^a^t^e^d^C^l^O3 to Re (Table 1, k of entry 2 vs 5), it

−

is also possible that the −Me substitution decreased the

VII

intrinsic reactivity of reductive OAT from Re to sulﬁde. The

comparison between 2A⁺and 2B⁺indicates the electron-

donating eﬀect of the −OMe substitution. The reduction of

−

Table 2. Rate Constants of All Steps Shown in Scheme 1 and Estimated Decomposition Rate Constants for eq 1

4K k (ClO ⁻) (M⁻¹s⁻¹

)

3K k (ClO ⁻) (M⁻¹s⁻¹

)

k₃(M⁻¹s⁻¹

)

k₄(M⁻¹s⁻¹

)

k_d_e_c(ClO ⁻) (M⁻¹s⁻¹

)

k_d_e_c(ClO ⁻) (M⁻¹s⁻¹

)

Re precursor

3.9 × 10⁻

7.9 × 10⁻

1.3 × 10⁻

1.3 × 10

−7

6.0 × 10

1.0 × 10

9.6 × 10

−5

a (from L2)

a (from L3)

b (from L4)

1.6

1.2

753.6

10.8

13.2

4778

842.6

171.9

−7

−6

1.1 × 10

−7

−6

8.8 × 10

Measurement of K in acetonitrile solution involved ligand exchange with CH CN rather than with H O. The reductant was DMS and might

have steric hindrance with the ligand methyl groups in 2a and 2b. Second-order rate constants derived from the ﬁrst-order k for hydrolysis

obs

shown in Figure 7a. The molar concentration of D O in the 5:95 (v/v) D O/CD CN mixture is approximately 2.8 M.

722

ACS Catal. 2021, 11, 6715−6725

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https://pubs.acs.org/doi/10.1021/acscatal.0c05276.

Research Article

the more electron-rich 2B is slower than that of 2A by 80%

modiﬁcation with a methyl group signiﬁcantly enhanced th₋e

stability of the Re site without sacriﬁcing the rate of ClO₄

reduction.

171.9 vs 842.6 M⁻s ).

−1

(

VII

Overall Stability of [Re (O) (L ) ] . The rate con-

N−O 2

stants obtained above allow a quantitative comparison of the

decomposition rates for 6 with eq 1. Table 2 shows the

apparent rate constants, k = nK k k /k , when the substrate

Unlike the biological system that uses delicate enzymati₋c

machinery to work against the oxidative stress from ClO_x

intermediates, a bioinspired catalyst system relies on simplicity

and self-sustainability. The self-protection mechanism empow-

dec

−

is ClO₄(n = 4) and ClO₃(n = 3). For each Re complex,

−

k_d_e_c_,_C_l_O₄is 1−3 orders of magnitude lower than k

ers the catalyst to treat concentrated ClO₄solutions using a

dec,ClO3

indicating that the oxidative stress mainly comes from the

highly reactive ClO_x⁻intermediates. The methyl group on the

ligand oxazoline moiety slows down the oxidation (K k ) by

ClO₃for 2 orders of magnitude, while the rate of oxidation by

ClO₄was not signiﬁcantly changed (see Table 1 for

single metal center, thus greatly simplifying the catalyst design

and preparation. We anticipate the mechanistic insights and

design strategy to beneﬁt the development of a wider scope of

catalytic systems, where the deactivation by reaction

intermediates is limiting the overall TON and catalyst life.

−

uncertainties of the model-ﬁt values). We postulate that the

rate constants of the reduction step (k ) measured using Me S

could be impacted by the steric repulsion. When Pd-activated

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

■

H is the reductant, although the magnitude of rate constants

are expected to be lower than using Me S, it is possible that k

for 2a reaches the same order-of-magnitude as that for 1a (i.e.,

about ﬁvefold higher). In such a case, k_d_e_cfor 2a and 2b could

be further lowered for about ﬁvefold, resulting in much higher

catalyst stability than 1a. Although 2b exhibited a slower

reduction and faster hydrolysis than 2a due to the electronic

eﬀect from −OMe substitution, 2b′ already showed high

stability after treating ﬁve spikes of 10 mM ClO₄⁻(Figure 3f).

Therefore, the methyl group on the ligand oxazoline moiety

played a critical role in protecting the Re site from

Experimental section for Re complex synthesis and

characterization, heterogeneous perchlorate reduction,

water sample analyses, TOF calculation, XPS character-

ization, kinetic measurements with H NMR and

stopped ﬂow spectrophotometry; additional data for

reaction kinetics and XPS characterization; comparison

with other perchlorate reduction systems; and derivation

of kinetic eqs 1 and 2 (PDF )

decomposition. It even allowed −OMe substitution for a

−

limited enhancement of the apparent rate of aqueous ClO

AUTHOR INFORMATION

■

reduction (2b′ vs 2a′ in Figures 2c and 3a). Due to the

introduction of the methyl group, the relatively limited

detrimental eﬀects from the −OMe substitution, such as

Corresponding Author

Jinyong Liu − Department of Chemical and Environmental

Engineering, University of California, Riverside, California

4% faster hydrolysis (k ) and 80% slower reduction (k , 2b vs

92521, United States; orcid.org/0000-0003-1473-5377;

a in Table 2), did not cause catalyst decomposition during

the reduction of 10 mM ClO₄or 0.5 mM ClO . Although

−

Email: jinyongl@ucr.edu, jinyong.liu101@gmail.com

technical challenges prevented us from measuring K and k for

Author

the reactions with ClO₂⁻and ClO , we expect the trends to be

−

Changxu Ren − Department of Chemical and Environmental

Engineering, University of California, Riverside, California

92521, United States; orcid.org/0000-0002-1109-794X

similar to the observations using ClO and PyO. Moreover,

the generation of ClO₂⁻and ClO will not cause signiﬁcant

−

deactivation of 2b. The OAT reduction of ClO (i.e., the

−

generation of ClO ) has already been slowed down for 2

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.0c05276

orders of magnitude by the ligand methyl group (Table 1).

CONCLUSIONS

Notes

■

The authors declare no competing ﬁnancial interest.

When a bioinspired catalyst system can reduce the highly inert

−

ClO , the oxidative deactivation by the much more reactive

−

ACKNOWLEDGMENTS

ClO_xintermediates can be a major challenge to the reactive

■

metal sites. Our results show that a simple ligand modiﬁcation

provided multiple beneﬁts to the catalyst development. The

methyl group on the oxazoline moiety led to an exclusive

formation of N,N-trans Re (O)(L ) Cl precursors and

protected the Re(L_N₋_O)₂site from decomposition after

Financial support was provided by the UCR faculty research

startup grant, the National Science Foundation (CHE-

1709719), and the Strategic Environmental Research and

Development Program (ER19-1228). Dr. Ich Tran is acknowl-

edged for assistance in XPS characterization performed at the

UC Irvine Materials Research Institute (IMRI) using

instrumentation funded in part by the National Science

Foundation Major Research Instrumentation Program under

grant CHE-1338173.

N−O 2

−

treating multiple spikes of concentrated ClO (10 mM or 1

−

g L ). In comparison to the original L

ligand, the added

N−O

−

methyl group decelerated the OAT from ClO₃to Re (L

for 2 orders of magnitude and the hydrolysis of Re (L

for several folds. Since the rate of OAT from ClO to

)

N−O 2

VII

)

N−O 2

−

Re (L ) was not impacted, the apparent rate for aqueous

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