Catalyst displacement assay: A supramolecular approach for the design of smart latent catalysts for pollutant monitoring and removal

Article abstract of DOI:10.1039/c6sc05584b

Latent catalysts can be tuned to function smartly by assigning a sensing threshold using the displacement approach for targeted analytes. Three cyano-bridged bimetallic complexes were synthesized as smart latent catalysts through the supramolecular assembly of different metallic donors [Fe^II(CN)₆]^4-, [Fe^II(tBubpy)(CN)₄]^2-, and Fe^II(tBubpy)₂(CN)₂ with a metallic acceptor [Cu^II(dien)]²⁺. The investigation of both their thermodynamic and kinetic properties on binding with toxic pollutants provided insight into their smart off-on catalytic capabilities, enabling us to establish a threshold-controlled catalytic system for the degradation of pollutants such as cyanide and oxalate. With these smart latent catalysts, a new catalyst displacement assay (CDA) was demonstrated and applied in a real wastewater treatment process to degrade cyanide pollutants in both domestic (level I, untreated) and industrial wastewater samples collected in Hong Kong, China. The smart system was adjusted to be able to initiate the catalytic oxidation of cyanide at a threshold concentration of 20 μM (the World Health Organization's suggested maximum allowable level for cyanide in wastewater) to the less harmful cyanate under ambient conditions.

Full text of DOI:10.1039/c6sc05584b

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DOI: 10.1039/C6SC05584B
Catalyst Displacement Assay: A Supramolecular Approach for the Design of Smart
Latent Catalysts for Pollutant Monitoring and Removal
Cheuk-Fai Chow,*[a] Pui-Yu Ho,[b] Wing-Leung Wong,*[b] Yu-Jing Lu,[c] Qian Tang[a] and
Cheng-Bin Gong,*[a]
a Department of Science and Environmental Studies, The Education University of Hong Kong,
10 Lo Ping Road, Tai Po Hong Kong SAR, China and College of Chemistry and Chemical
Engineering, Southwest University, Chong Qing, China
b Centre for Education in Environmental Sustainability, The Education University of Hong
Kong, 10 Lo Ping Road, Tai Po, Hong Kong SAR, China.
c Institute of Natural Medicine and Green Chemistry, School of Chemical Engineering and
Light Industry, Guangdong University of Technology, Guangzhou 510006, P.R. China.
E-mail: cfchow@ied.edu.hk; Fax: (+852) 29487676; Tel: (+852) 29487671
Abstract:
Latent catalysts can be tuned to function smartly by assigning a sensing threshold using the
displacement approach for targeted analytes. Three cyano-bridged bimetallic complexes
were synthesized as “smart” latent catalysts by the supramolecular assembly of different
metallic donors ([FeII(CN)6]4−, [FeII(tBubpy)(CN)4]2−, and FeII(tBubpy)2(CN)2 with a
metallic acceptor [CuII(dien)]2+. The investigation of both their thermodynamic and kinetic
properties on binding with toxic pollutants provided insight into their smart off-on catalytic
capabilities, enabling us to establish a threshold-controlled catalytic system for the
degradation of pollutants such as cyanide and oxalate. With these smart latent catalysts, a
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DOI: 10.1039/C6SC05584B
new catalyst displacement assay (CDA) was demonstrated and applied in a real wastewater
treatment process to degrade cyanide pollutants in both domestic (level I, untreated) and
industrial wastewater samples collected in Hong Kong, China. The smart system was
smartly adjusted to able to initiate the catalytic oxidation of cyanide at a threshold
concentration of 20 μM (the World Health Organization’s suggested maximum allowable
level for cyanide in wastewater) to the less harmful cyanate under ambient conditions.
Introduction
The development of “smart” catalysts for process monitoring and reaction control is highly
important in modern chemistry. In particular, a smart catalyst that is able to execute a specific
task under certain conditions would be very attractive. Different from traditional catalysts that
initiate a reaction once the reactants are present, this type of catalyst would provide extra
benefits for certain industrial processes such as contaminant removal during water treatment.
Traditionally, chemists have focused on catalyst use to increase rates, yields, and/or the
stereo-selectivity of reactions.[1] However, it is possible that catalysts could be made “smarter”
with the incorporation of extra control parameters, such as introducing an analyte-selective
receptor which is also an inhibitor of the catalyst, so that the catalyst can only function when
a specific analyte exists under certain conditions. This sort of smart catalyst is rarely found in
the literature, although it could be rationally designed to associate with an inhibitor and be
temporarily deactivated, but readily re-activated in the presence of a specific trigger or an
initiator to start the targeted reaction under given conditions such as concentration,
temperature, or pH. Recently, several catalytic systems demonstrated a portion of this “smart
off-on” function for controlling targeted reactions, which led to very interesting reaction
outcomes.[2] A switch-based negative feedback loop demonstrated with a zinc(II) ion
coordination-coupled deprotonation of a hydrazine is a typical example of smart control
system, in which a particular threshold of zinc(II) causes the releasing of certain
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concentration of protons to the environment to trigger a cascade of reactions.[2DjO] IB: 10a.s10e3d9/Co6nSC05584B
these ideas, we conjectured that a further advancement of the “off-on” approach would be the
introduction of a threshold-controlled function into the catalysts. The advancement
demonstrated in this study overcomes the limitations of simple “catalyst-inhibitor” systems,
arising from their simple off-on designs: i.e., they lack a mechanism to control the initiation
threshold, so that the catalytic reaction will ensue in the presence of small amounts of
initiator without having reached a specific initiator threshold.
The catalyst displacement assay (CDA) is a useful protocol for developing smart latent
catalytic systems, particularly in chemical or waste treatment applications, because
chemical toxicity depends on dosage. For example, cyanide (CN−) is well known for its
high toxicity and has been identified as one of the most serious threats toward the
environment and human life. Nevertheless, it is widely used in gold mining, electroplating,
and the production of synthetic fibers.[3] Considering its industrial importance as well as its
adverse effects, the World Health Organization has suggested maximum allowable levels of
cyanide in wastewater, fresh water (class III), and drinking water of 0.5, 0.2, and 0.05
mg∙L−1, respectively.[4] The elimination of cyanide is therefore only sensible when its levels
exceed these thresholds. The removal of cyanide in aqueous solution by oxidation has been
well studied with oxidants in presence of various catalysts such as TiO2,[5] Cu(I)/Cu(II)
transition metal complexes[6] and Fe(II)-Cu(II) bimetallic complex[18a]. Nonetheless, these
systems are not “smart” enough to begin functioning under a specific set of conditions. In
recent years, the molecular design and synthesis of chemosensing systems specifically for
cyanide recognition or determination have been reported.[7a–d] Some of these molecular
systems are indicator displacement assays (IDA), which show sensing properties with good
selectivity and signal changes upon binding with CN− in solution.[7e–j] IDA involves the
specific analyte competition, which causes the displacement of the indicator from the
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receptor and gives off-on signals. The advantage of an IDA system is that the seleDcOtiIv: 1i0t.y103a9n/Cd6SC05584B
sensitivity can be adjusted by tuning the thermodynamics of the ensemble. A number of
IDA sensing systems[8–11] are well-established for the determination of anions,[8a–b,9a–b,10a–
e,11b-c]
neutral organic molecules,[11d-e] zwitterions,[8c–d,9c–d,11a] and other molecules;[8e]
however, there are no examples of IDA systems showing smart properties that can effect a
catalytic reaction under pre-set conditions.
On the basis of IDA systems, we attempted to establish a CDA system based on
supramolecular donor-acceptor complexes which would feature a smart system that could
selectively recognize a target analyte (a pollutant like CN-), after which a pre-set catalytic
reaction would be automatically performed under given conditions. With the new CDA
system design, the catalyst is controllably activated when the analyte level reaches a certain
threshold to start a catalytic reaction. In this study, we demonstrate that a threshold and the
off/on control parameter can be rationally adjusted by understanding the thermodynamic and
kinetic properties of cleavage processes in the donor-acceptor complexes. Both
thermodynamic and kinetic characteristics are the crucial factors in deriving a CDA system
that could generate a number of advantages for chemical processes—including the reduced or
minimized use of reagents, costly catalysts, energy, and manpower—which cannot be
provided by traditional chemical reactions or catalytic systems.
Experimental Section
Materials and general procedures. 4,4′-Di-tert-butyl-2,2′- bipyridine (tBubpy),
diethylenetriamine (dien), K4[Fe(CN)6], potassium oxalate, and potassium cyanide were
obtained from Aldrich. Complexes K2[FeII(tBubpy)(CN)4],[12] FeII(tBubpy)2(CN)2,[13] and
CuII(dien)Cl2[14] were prepared according to reported methods.
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DOI: 10.1039/C6SC05584B
Physical measurements and instrumentation. Electrospray ionization mass spectrometry
(ESI-MS) was performed using an AB SCIEX API 2000 LC/MS/MS system. Elemental
analyses were conducted using a Vario EL CHN analyzer. Infrared spectra in the 500–4000
cm−1 range using KBr pellets were recorded using a Perkin Elmer Model Frontier FTIR
spectrometer, and UV-vis spectra were measured on a Cary 50 ultraviolet-visible
spectrophotometer. Dissolved organic carbon was recorded using a Shimadzu TOC-L CSH
High-Sensitivity Total organic Analyzer. The emission spectra were recorded using a Horiba
FluoroMax-4 spectrofluorimetric with a 5 nm slit width and a 0.5 s integration time.
Broad-band UV irradiation was provide by 500W Hg(Xe) light source (Model 66485) with
a digital power supply system (Model 69911) by Newport Inc.
UV-vis spectroscopic titrations. The analytical grade DMF used in the UV-vis
spectroscopic titrations was purified by distillation. The titrations of K4[FeII(CN)6] (3.3 
10-4 M) by CuII(dien)Cl2 (0 to 2  10-5 M) were conducted in HEPES buffer at pH 7.4.
While the titrations of K2[FeII(tBubpy)(CN)4] (5  10-5 M) by CuII(dien)Cl2 (0 to 2  10-4 M)
and FeII(tBubpy)2(CN)2 (5  10-5 M) by CuII(dien)Cl2 (0 to 1  10-4 M) were carried out in a
mixture of aqueous HEPES buffer (1.50 mL, pH 7.4) with DMF(1.50 mL). UV–vis
spectroscopic titrations of CuII(dien)Cl2 (5  10-4 M) with oxalate (0 to 1  10-3 M) were
carried out in aqueous phosphate buffer pH 4. All measurements were taken after standing
for 12 h to ensure their equilibria were reached. Binding constants (log K) and formation
energies (ΔG/kJ mol−1) were analyzed according to the Benesi–Hildebrand equations[15-16]
from the UV-vis spectroscopic titrations.
Kinetic measurements: Cleavage of cyano-bridges between the FeII and CuII metal
centers of 1–3 by cyanide. The experimental procedure for the rate constant measurements
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was performed according to previous studies.[17] Except complex 1 in pH 7.4 HEPES buffer,
all reactions were performed in DMF/pH 7.4 HEPES buffer (1:1 v/v). By mixing a known
amount of cyanide solution (2.5 × 10−3 to 1.25 × 10−2 M) with the test solutions at 25 °C
containing 2.5 × 10−4 M complex, the changes in the absorbance due to the addition of
cyanide were measured at different time intervals. All kinetics measurements were conducted
under ambient conditions.
DOI: 10.1039/C6SC05584B
Oxidation of cyanide to cyanate with complexes 1–3 as catalysts. All experiments were
conducted in a 40 mL boiling tube in the absence of light, and the test solutions (15.0 mL)
were stirred during the experiments. The concentration of H2O2 was 6.53 mM, and the
concentrations of complexes 1, 2, and 3 were 0.1, 0.1, and 0.2 mM, respectively. Different
initial concentrations of cyanide (0 to 1000 μM) were used in the studies. The concentrations
of CN− and NCO− in the test solutions were measured at regular intervals using previously
reported analytical methods.[18a]
Photocatalytic degradation of oxalate to CO2 by complex 1 as catalyst. All experiments
were conducted in a 50.0 mL volumetric flask with a 500 W Hg(Xe) ultraviolet–visible lamp
(Newport) irradiation source. The experimental setup was completely shielded from
surrounding light. The distance between the lamp and test solution was about 25 cm. A water
bath was placed between the lamp and test solution to absorb heat generated by the UV
irradiation. Prior to irradiation, the pH of the test solution was adjusted to 1.5 with HCl/KCl
buffer. Generally, during the photocatalytic experiments, a test solution (50.0 mL) containing
complex 1 (6.25  10−4 M) and H2O2 (0.4 M) was irradiated while the oxalate concentration
was varied from 6.25  10−4 to 3.125  10−2 M. The dissolved organic carbon (DOC) in the
system was determined at regular intervals to evaluate catalytic efficiency.[18b]
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DOI: 10.1039/C6SC05584B
Synthesis and characterization
[FeII(CN)6]–[CuII(dien)(H2O)]2 (1). The complex was synthesized by modification of the
reported method.[19] A mixture of K4[FeII(CN)6] (1.00 g, 2.37 mmol) and CuII(dien)Cl2 (0.56
g, 2.37 mmol) was stirred in a water/methanol mixture (1:1 v/v, 25 mL) at room
temperature overnight. The green precipitate obtained by filtration was washed with
deionized (DI) water and acetone, and then air-dried. Yield: 0.54 g (84.5%). IR (KBr): C N

=
2039, 2047, 2103 cm−1. ESI-MS (MeOH, +ve mode): m/z 544.8
{[Fe(CN)6][CuII(dien)]2}H+ (mass = 545.0 g mol−1; charge = +1); elemental analysis calcd
(%) for C14Cu2FeH26N124H2O (1): C 27.23; H 5.55; N 27.22; found: C 27.29; H 5.52; N
26.95.
[FeII(tBubpy)(CN)4]–[CuII(dien)Cl]2 (2). The complex was synthesized following the
reported method.[18] A mixture of K2[Fe(tBubpy)(CN)4] (0.115 g, 0.2 mmol) and
CuII(dien)Cl2 (0.05 g, 0.2 mmol) was stirred in a water/methanol mixture (1:1 v/v, 5 mL) at
room temperature overnight. The brown precipitate obtained upon centrifugation was
washed with DI water and acetone, and then air-dried. Yield: 0.069 g (83.1%). IR (KBr):
C N = 2059, 2084, 2103, 2114 cm−1. ESI-MS (MeCN, +ve mode): m/z 380.1

{Fe(tBubpy)(CN)4[CuII(dien)]2}2+ (mass = 760.2 g mol−1; charge = +2); elemental analysis
calcd (%) for C30Cl2Cu2FeH50N12 (2): C 43.27; H 6.05; N 20.19; found: C 43.22; H 6.10; N
20.10.
{[FeII(tBubpy)2(CN)2]2–[CuII(dien)]}Cl2 (3). A mixture of FeII(tBubpy)2(CN)2 (0.10 g,
0.16 mmol) and CuII(dien)Cl2 (0.038 g, 0.16 mmol) was stirred in methanol (25 mL) at
room temperature overnight. The reddish brown solid obtained after evaporation was
washed with chloroform, DI water, and acetone, and then air-dried. Yield: 0.0745 g
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−1
DOI: 10.1039/C6SC05584B
(61.1%). IR (KBr): C N = 2068, 2082 cm . ESI-MS (MeOH, +ve mode): m/z 726.9

{[Fe(tBubpy)2(CN)2]2CuII(dien)}2+ (mass = 1454.70 g mol−1; charge = +2); elemental
analysis calcd (%) for C80Cl2Cu1Fe2H109N15CH3C(O)CH3 (3): C 62.81; H 7.62; N 11.94;
found: C 63.52; H 7.62; N 11.73.
Results and Discussion
Syntheses of bimetallic complexes 1–3. Metallic donors (MD) coordinated with a bridging
−
−
−
ligand such as the commonly used CN, NCS, or NCO ligands are able to bond with a
metallic acceptor (MA) to form a desirable bimetallic complex of the form MD–bridge–MA.
Through this supramolecular assembly, bimetallic complexes can be designed to achieve
desired functionalities which are not offered by the individual mono-metallic precursors
(MD or MA). We studied a series of bimetallic complexes containing a cyano-bridge, MD–
C≡N–MA, where MD = ReI, FeII, RuII, or OsII; and MA = NiII, CuII, PtII, or LnIII.[11]
Interestingly, the strength of the bridging bond between the metal centers is adjustable and
depends on the combination of MD and MA. Therefore, by regulating the two metal centers
connected by the cyano-bridge, a bimetallic complex with specific functionality can be
tailored. In this study, MD, functioning as an inhibitor, was introduced into a catalyst MA to
generate a CDA system with an “off-on” function in which a catalytic oxidation reaction
could be triggered when a certain concentration of cyanide ion was present in the solution.
The influence of the thermodynamic and kinetic properties of the cyano-bridges of the
bimetallic complexes on the control of the off-on and threshold operation of the catalyst was
then illustrated comprehensively.
Bimetallic complexes 1–3 shown in Scheme 1 were synthesized with the metallic acceptor
CuII(dien)Cl2 and the latent catalyst donors K4[Fe(CN)6], K2[FeII(tBubpy)(CN)4], and
Fe(tBubpy)2(CN)2. All three bimetallic complexes were isolated as air-stable solids and
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were characterized by IR spectroscopy, electrospray ionization mass spectrometryD(OEI:S10I.-10M39S/C)6,SC05584B
and elementary CHN analysis. The three cyano-ferrous complexes, K4[FeII(CN)6],
K2[FeII(tBubpy)(CN)4], and FeII(tBubpy)2(CN)2, were designed systematically as the MD
species because they are able to participate in coordinating cyano bridging with other metal
complexes to form bimetallic complexes, whereas [CuII(dien)]Cl2 was selected as the model
acceptor MA because it can catalyze the oxidation of organic/inorganic pollutants such as
oxalate and cyanide ions. We expected the strengths of the cyano-bridges of the three
bimetallic complexes to be different due to the variation of the charge densities among the
ferrous MD from −4, −2, to 0. The cyano bridge activities of the bimetallic complexes 1–3
significantly impact their thermodynamics and kinetics, which are considered as the
determinant for the threshold of a smart catalytic system.
The formation of cyano-bridges between the MD and MA units in the bimetallic complexes
was confirmed by IR spectroscopy, as shown in Table 1. In general, all the νC≡N stretching
frequencies observed in 1–3 are red shifted with respect to their precursor cyano-ferrous
complexes, and the degrees of the shift vary by 2, 29, and 60 cm−1, respectively. These
changes can be attributed to the electron density differences among the MD complexes,
FeII(tBubpy)2(CN)2, [FeII(tBubpy)(CN)4]2−, and [FeII(CN)6]4−.
In the bimetallic complex syntheses, 1:1 molar ratios of the FeII and CuII starting materials
were used in all cases. Interestingly, however, the compositions of supramolecular structures
1–3 contained FeII/CuII ratios of 1:2, 1:2, and 2:1, as verified by ESI-MS, spectroscopic
titration studies, and CHN elemental analysis. As shown in Figure 1, 1–3 show peaks at m/z
=
544.8,
380.1,
and
726.9,
representing
{[FeII(CN)6]–[CuII(dien)]2}H+,
{[FeII(tBubpy)(CN)4]–[CuII(dien)]2}2+,
and
{[FeII(tBubpy)2(CN)2]2–[CuII(dien)]}2+,
respectively. The elemental analysis results for 1–3 also matched the proposed structures. In
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DOI: 10.1039/C6SC05584B
addition, the binding isotherms (Figures S1–S3, Supporting Information (SI)) obtained by
the titration of CuII(dien)Cl2 solution with K4[FeII(CN)6], K2[FeII(tBubpy)(CN)4], and
FeII(tBubpy)2(CN)2, respectively, show that the bimetallic complexes 1–3 formed in the
solution in the above FeII/CuII ratios.[15]
Thermodynamic and kinetic investigations of the CDA system. In the design of a CDA,
a catalyst should first be bound temporarily to an inhibitor. CuII complexes have been well
studied as active catalysts for the oxidation of cyanide using H2O2 as the oxidant.[18a,20]
However, the catalytic activity is inhibited when the CuII-centers are bridged to FeII-centers
with a ligand (C≡N) in the form of bimetallic complexes.[18a] The Cu-catalyst at this stage is
de-activated as it is bridging with its metallic counterpart. When a competitive analyte, i.e.,
the reactant, is introduced into the system, it causes the displacement of the catalyst from
the inhibitor, thus freeing it.[18a] At this stage, the Cu-catalyst is activated to execute its
catalytic function under the given conditions. For the present CDA system ([(L)x(CN)yFeII––
C≡N––CuII(dien)], Scheme 2), the study of the coordination with cyano-bridges between
the FeII and CuII metal centers allows us to understand the thermodynamic and kinetic
properties of these bimetallic systems. These investigations were conducted using cyanide
as the model analytes.
To understand the thermodynamic properties of the CDA, the Gibbs free energy changes
(G°) between MD and MA of 1–3 were found and calculated as −30.4, −25.3, and −15.1
kJ∙mol−1, respectively (Table 2). The ΔG° value of the adduct between CuII(dien)2+ and CN−
(−37.9 kJ∙mol−1) is more stable relative to those of bimetallic systems 1–3. By comparing
these four ΔG° values, we predicted that when CN− is introduced into the bimetallic systems
(1, 2, or 3), it would displace the CuII(dien)2+ unit from its ferrous partner by breaking the
cyano-bridges, because the formed [CN–CuII(dien)]+ adduct is more stable than the
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precursor. Figures 2a–c show the evidence of breaking the cyano-bridges of complexes 1–3
by showing the restoration of the spectroscopic properties of [FeII(CN)6]4−,
[FeII(tBubpy)(CN)4]2−, and FeII(tBubpy)2(CN)2 upon the addition of cyanide to 1–3,
respectively. Most importantly, through cleaving the cyano-bridges upon the addition of
cyanide to 1–3, the corresponding displaced CuII-complex [CuII(dien)(CN)]+ (m/z 192.0
[M]+) was found and identified by ESI-MS spectroscopic analysis (Figure S4a).
We believe that the displacement of the CuII-complex (through bridge cleavage by the target
analyte) from the bimetallic systems is not only controlled by the abovementioned
thermodynamic factors, but also that the different displacement kinetics of the systems
could provide “smart” deactivation-activation functions which would enable us to establish
a CDA system. To prove this concept, kinetic studies were conducted on the rates of
cyano-bridge cleavage between the FeII and CuII metal centers in the bimetallic systems by
cyanide. Figure 3 shows that the rate constants for cyano-bridge cleavage in 1–3 by cyanide
were determined to be 18.8, 32.0, and 58.3 M−1∙s−1, respectively. The results indicate that
complex 3 is the fastest to release the Cu-catalyst for the oxidation reaction. The rates of
cyanate formation with 1–3 were found increased with respect to their decreasing G0 (1–3
= −30.4, −25.3 and −15.1 kJ∙mol−1, respectively) and also in line with the rate constants for
cyano-bridge cleavage of 1–3 by cyanide ion (18.8, 32.0, and 58.3 M−1∙s−1 Figure 3). The
rate constant of cyanate formation from the oxidation of cyanide with H2O2 as the oxidant
under the catalytic action of CuII was reported as 249.5 M−1∙s−1 (Figure S5).[21] The results
reveal that the cleavage of the cyano-bridge between the metallic donor and acceptor to
release the CuII complex (Scheme 2: 1st step of the overall CDA process) is the rate
determining step, whereas the 2nd step of the process, the catalytic oxidation of cyanide, is
faster.
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Threshold-controlled catalytic properties of the CDA system.
For the investigation of the smart threshold-controlled properties of the CDA, the initial test
solutions containing cyanide, H2O2, and 1, 2 or 3 in a molar ratio of 10:65:1 in buffered
aqueous DMF solution were prepared for the analyses. The concentrations of CN− and
NCO− in the test solutions were measured under ambient conditions.[17,20d] Figure 4a shows
that all the complexes could catalyze the oxidation of cyanide to cyanate quantitatively
within 150 min with H2O2 as the oxidant. Control experiments in the absence of the
complexes or H2O2 revealed that no cyanide was oxidized to cyanate throughout the period.
On the other hand, in the reaction using CuII(dien)Cl2 instead of the bimetallic complexes
under the same conditions, cyanide was quantitatively oxidized to cyanate. This confirmed
that the CuII-complex is the active catalyst in the oxidation reaction.
From the thermodynamics consideration, all these complexes of 1–3 are able to release the
inhibited Cu-catalyst in the presence of cyanide as the ΔG° value (−37.9 kJ∙mol−1) of
CuII(dien)2+ and CN− adduct is much lower than the formation of [(L)x(CN)yFeII––C≡N––
CuII(dien)] (Table 2). However, Figures 4b–d show the most interesting findings that
complexes 1–3 do not exhibit their catalytic properties, even though the reactions are
considered thermodynamically feasible, if the cyanide concentration does not reach certain
thresholds. However, by taking into account their kinetic properties, the initial cyanide
concentration needs to reach thresholds of 0.2, 0.1, and 0.02 mM for 1, 2, and 3, respectively,
to break their cyano-bridges and release [CN-CuII(dien)]+ to catalyze cyanide oxidation. In
addition, we observed obvious plateaus from Figures 4b–d when the oxidation reaction of
CN− proceeding for a certain period of time that the product of NCO− accumulated in the
reaction environment. The plateaus may indicate that the catalytic system is being
slow-down and/or an inhibition effect from the negative feedback loop of the product
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−
−
generated.[2j] A proposed mechanism for the catalytic oxidation of CN to NCO by the
DOI: 10.1039/C6SC05584B
latent catalyst is shown in Scheme 2.
To further demonstrate the scope of the CDA as a general and smart catalytic system,
another substrate, oxalate, was also investigated. The ΔG° value (−39.0 kJ∙mol−1) of the
adduct between CuII(dien)2+ and HOOC-COO− was found to be larger than those of the
systems 1–3 (−30.4, −25.3, and −15.1 kJ∙mol−1, respectively, Table 2) (Figure S6). Because
of this thermodynamic trigger, an off-on control is introduced into the CDA: Figure S7 in
the SI shows that, in the presence of 1 under UV-vis irradiation at room temperature, the
degradation of the oxalate (3.125 × 10−2 M) to CO2 is boosted rapidly in the first 150 min
and gradually increases afterwards. In addition, the kinetic properties of the systems impart
smart properties. Figure S7 also reveals that even though complex 1 could
thermodynamically undergo cyano-bridge cleavage in the presence of oxalate to release the
CuII unit (Figure S4b) for the catalytic oxidation of oxalate into CO2, it does not start the
CDA reaction. For example, complex 1 requires that the initial oxalate concentration
reaches 1.25 × 10−3 M (the threshold) to cleave the cyano-bridge, release the CuII unit, and
catalyze oxalate oxidation. A proposed mechanism for the catalytic oxidation of oxalate to
CO2 by the latent catalyst is shown in Scheme 2. The CDA system is therefore being smart,
playing two consecutive roles of sensing a target pollutant and executing a preset chemical
process at a specific concentration level of pollutants.
Degradation of cyanide in real wastewater samples by Catalyst Displacement Assay.
After carefully verifying the basic and crucial factors (thermodynamic and kinetic
properties) of the CDA, complex 3 was used to test its applicability and “smart”
performance in real wastewater treatment. Both domestic and industrial wastewater samples
(level I, untreated sewage from a residential area system and sewage from an industrial zone,
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respectively) were collected in Hong Kong, China. The samples were filtered through 0.45
μm pore-size membrane filters (Pall Corporation) to remove insoluble substances before
examination. The World Health Organization suggests a maximum allowable level of
cyanide in wastewater of 20 μM (0.5 mg∙L−1).[4] The wastewater samples were spiked with
10, 20 or 30 μM cyanide and their degradation/oxidation was analyzed at room temperature
with complex 3 in the presence of H2O2. (Based on the results described above, complex 3
was able to initiate the oxidation of cyanide at 20 μM.) In the presence of 3, when the
cyanide content of the samples reached 20 μM, the oxidation of cyanide to cyanate was
initiated (Table 3; Figure S8). However, it is important to note that while the cyanide
content of the samples set as 10 μM (i.e., less than the threshold concentration), no cyanide
was oxidized to cyanate throughout the period. Control experiments in the absence of the
complex revealed that no cyanide was oxidized. These results indicate that the
smart-functioning CDA concept is applicable, even in the presence of organic/inorganic
matters in actual domestic and industrial wastewater samples.
Conclusion
Three bimetallic complexes were synthesized using a supramolecular approach with
different metallic donors, [FeII(CN)6]4−, [FeII(tBubpy)(CN)4]2−, and FeII(tBubpy)2(CN)2, to
bridge with a metallic acceptor, [CuII(dien)]2+, through a cyano-bridge as latent catalysts.
The study of their thermodynamic and kinetic properties provided insight on how to
establish a smart off-on sensing and threshold-controlled latent catalytic system for
degradation of toxic pollutants. With this “smart” deactivation-activation function for
controlling targeted reactions, the catalyst displacement assay (CDA) system could be used
for on-site applications such as wastewater treatment with a pre-set threshold for the
elimination of cyanide from reservoirs.
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Acknowledgements
The work described in this paper was funded by a grant from The Education University of
Hong Kong (Project No. R3444, R4175 and R4201), and grants from the Research Grants
Council of Hong Kong SAR, China (ECS No. 800312 and GRF 18300415).
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Scheme 1. Bimetallic complexes 1–3 used as the CDA systems
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Scheme 2. Proposed mechanism for the overall CDA process. The release of the active
catalyst is triggered by cyanide/oxalate in solution, which then catalyzes the oxidation of
free cyanide/oxalate to the products cyanate/carbon dioxide, respectively. The development
of the colored solution from the Fe(CN)2(L)4 complex indicates the reaction has taken place.
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Figure 1. ESI-MS spectra of the isotopic distribution of (a) complex 1, (b) complex 2, and
(c) complex 3: and (inset) simulations based on their molecular formulas. All the
experiments were conducted in DI water/methanol.
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Figure 2. UV-vis spectroscopic titration curves of (a) 1, (b) 2, and (c) 3 with CN−. Addition
of cyanide anions to 1–3 restored the characteristic spectroscopic properties of [FeII(CN)6]4−,
[FeII(tBubpy)(CN)4]2−, and FeII(tBubpy)2(CN)2, respectively. All the titrations were examined
in aqueous DMF (1:1 v/v, 1.50 mL aqueous HEPES buffer at pH 7.4 + 1.50 mL DMF) at 298
K with complex concentrations of 1  10−4 M and CN− concentrations from 0 to 2.0  10−4 M.
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Figure 3. Kinetic plots of the apparent association rate constant kobs (s−1) versus cyanide
concentration. The rate constant values were calculated from the slopes of the curves (y = mx;
1 = 18.8 M−1s−1; 2 = 32.0 M−1s−1; and 3 = 58.3 M−1s−1).
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Figure 4. (a) Formation of cyanate from the oxidation of cyanide (1 mM) in the pDreOsI:e1n0.c10e39o/Cf61SC05584B
(), 2 (■), and 3 (▲) against time. The formation of cyanate in the absence of H2O2 (●) or
catalyst (▼) concentration. All the experiments were performed with a 10:65:1 molar ratio of
cyanide, H2O2, and the “CuII(dien)2+” of 1, 2, or 3 at room temperature under ambient
atmosphere. Formation of cyanate in the presence of (b) 1, (c) 2, and (d) 3 with respect to
different initial concentrations of cyanide versus time. All the experiments were performed
with a 65:2 molar ratio of H2O2 (6.53 mM) and the “CuII(dien)2+” of 1, 2, or 3 at room
temperature under ambient atmosphere. Error bar is the mean value of three independent
experiments.
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Table 1. IR spectroscopic study of the cyano-stretching frequencies (νC≡N) of comDpOleI:x10e.1s0319–/C36SC05584B
and their precursor complexes
Entry Complexes
νC≡N (cm–1)
1
2
3
4
5
6
[FeII(CN)6]–[CuII(dien)]2 (1)
2039, 2047, 2103
2059, 2084, 2103, 2114
2068, 2082
[FeII(tBubpy)(CN)4]–[CuII(dien)Cl]2 (2)
{[FeII(tBubpy)2(CN)2]2–[CuII(dien)]}Cl2 (3)
K4[FeII(CN)6]
2030, 2043
K2[FeII(tBubpy)(CN)4]
2053, 2067, 2085
2080
FeII(tBubpy)2(CN)2
Table 2. Binding constants (log K) and Gibbs free energy changes (G0) for the
complexation of cyanide anion and different FeII species to the [CuII(dien)]2+ complex
log K a
a
G0
Acceptor
Donor
/ kJmol-1
-39.0
1
2
3
4
5
[CuII(dien)Cl2]
Oxalate (HOOC-COO-)
6.84
6.64b
5.32
4.44
2.65
[CuII(dien)(ClO4)](ClO4) CN-
-37.9b
-30.4
[CuII(dien)Cl2]
[CuII(dien) Cl2]
[CuII(dien)Cl2]
K4[FeII(CN)6]
K2[FeII(tBubpy)(CN)4]
FeII(tBubpy)2(CN)2
-25.3
-15.1
[a]Binding strengths were measured by UV spectroscopic titration and calculated using the
Benesi-Hildebrand equations. Except entry 1, which was performed in aqueous phosphate
buffer at pH 4, all reactions were performed in aqueous DMF (1:1 v/v, 1.50 mL aqueous
HEPES buffer at pH 7.4 + 1.50 mL DMF) at 298 K. [b]log K and G0 data are cited from
reference [11b and 18a].
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Table 3 Results of cyanide degradation by 3 in domestic (level I, untreated) and industrial
wastewater samples
Wastewater samples
Cyanide added
Cyanate generated
Conversion after 4 h
(μM)
10
(μM)
0
(%)
0
Domestic
(level I, untreated)
20
6.1
12.9
0
40.4
42.9
0
30
10
Industrial
20
6.6
14.8
33.1
49.4
30
29