Single open sites on FeIIions stabilized by coupled metal ions in CN-deficient prussian blue analogues for high catalytic activity in the hydrolysis of organophosphates

the Hydrolysis of Organophosphates

Article

Single Open Sites on Fe Ions Stabilized by Coupled Metal Ions in

CN-Deﬁcient Prussian Blue Analogues for High Catalytic Activity in

Mari Yamane, Hiroyasu Tabe, Masami Kawakami, Hisashi Tanaka, Tohru Kawamoto,

and Yusuke Yamada*

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c02528

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sı Supporting Information

ABSTRACT: CN-deﬁcient Prussian blue analogues (PBAs),

III

[

M (H O) ] [Fe (CN) (NH )] (M = Cu , Co , or Ga ),

2 x y 5 3

were synthesized and examined as a new class of heterogeneous

catalysts for hydrolytic decomposition of organophosphates often

used as pesticides. The active species of the CN-deﬁcient PBAs

were mainly C-bound Fe ions with only single open sites

generated by liberation of the NH ligand during the catalytic

reactions. [Cu (H O)

]

[Fe (CN) (NH )] showed higher

8/3 3/2

catalytic activity than [Co (H O)

]

[Fe (CN) (NH )] and

8/3 3/2

III

[

Ga (H O)][Fe (CN) (NH )], although N-bound Cu species

has been reported as less active than Co and Ga species in

conventional PBAs. IR measurements of a series of the CN-

2 5 3

III

deﬁcient PBAs after the catalytic reactions clariﬁed that a part of the NH ligands remained on [Co (H O)

] [Fe (CN) (NH )]

8/3 3/2 5 3

III

and that hydrogen phosphate formed as a product strongly adsorbed on the Fe ions of [Ga (H O)][Fe (CN) (NH )]. Hydrogen

phosphate also adsorbed, but weakly, on the Fe ions of [Cu (H O)

] [Fe (CN) (NH )]. These results suggest that

8/3 3/2 5 3

heterogeneous catalysis of the Fe ions with single open sites were tuned by the M ions through metal−metal interaction.

INTRODUCTION

as H O because the number of M ions in a PBA is larger than

■

n−

that of [M (CN) ] anions to maintain charge balance,

indicating that M ions have extra ligands to satisfy the

octahedral coordination structure (Figure 1a). The catalytic

properties of M^Nions can be modulated by the electronic

interaction between M^Nand M^Cions, resulting in the

enhancement of heterogeneous catalysis. The average number

of open sites formed on M ions can be theoretically calculated

under the consideration of the valence of [M (CN) ] and

M . However, the exact number of open sites formed on each

M ion has yet to be precisely controlled.

Utilization of defective coordination polymers as heteroge-

neous catalysts attracts much attention because active species

with open sites can be intentionally created, resulting in

showing unique catalysis contrast to metals or metal oxides.

In the defective coordination polymers, active species are metal

ions with open sites usually formed by the liberation of extra

ligands bound to the metal ions.

of open sites in a coordination polymer composed of

ruthenium(II) ion (Ru ) and 1,3,5-benzenetricarboxylate

trianion was regulated by adding isophthalate or pyridine-

−5

−23

For example, the number

n−

,5-dicarboxylate dianion as a second ligand, because a certain_I_I

We report herein intentional creation of an open site on the

3−

amount of extra ligand such as chloride coordinated to the Ru

C-bound Fe ions in PBAs by using [Fe (CN) L] (L = NH₃

2,23

4−

ions.

The extra ligands are liberated to provide open sites

or H O) instead of [Fe (CN) ] as a building unit (Figure

on the Ru ions acting as catalytic active species. The average

b). The resulting CN-deﬁcient PBAs possess exactly single

number of open sites on the Ru ions can be controlled by

changing the ratio of tri- and dianion ligands.

Another example is Prussian blue analogues (PBAs)

composed of a hexacyanometallate anion ([M (CN) ] )

open sites on the Fe ions. The catalysis of a series of CN-

deﬁcient PBAs composed of [Fe (CN) L]_a_n_d_v_a_r_i_o_u_s_MN

3−

n−

Received: August 24, 2020

and a metal ion (M ) to form a M −CN−M frame-

4−27

work.

Catalysis of the PBAs has been studied for various

reactions including photocatalytic water oxidation and

28−47

hydrolysis of organophosphates often used as pesticides.

Usually, open sites acting as catalytic active sites are selectively

formed on the M ions by liberation of the extra ligands such

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 1. Partial structures of (a) [M (H O) ] [Fe (CN) ] (M Fe) and (b) [M (H O) ] [Fe (CN) (NH )] (M Fe-NH ) complexes. (c) The

hydrolysis of p-nitrophenyl phosphate (p-NPP).

III

ions (Cu , Co , or Ga ) are examined for organophosphate

milled using a mortar to obtain ﬁne powder of

[

Cu (H O) ] {[Fe (CN) (NH )] [Fe (CN) ]1−n^}^c^o^m^p^l^e^x^e^s^.

Synthesis of [Co (H O) ] [Fe (CN) ] (CoFe) and

2 3 2 6

III II

hydrolysis, as Fe ions have been reported to show high

2 x y 5 3 n 6

activity for the hydrolysis reaction (Figure 1c). The

correlation between catalytic behavior and adsorbed species

on the surfaces of the CN-deﬁcient PBAs composed of various

M ions is scrutinized.

Three diﬀerent types of point defect sites that are potential

catalytic active sites can be formed in the PBAs. First, the

major defect sites are generated by the lack of [Fe (CN) ]

or [Fe (CN) (NH )] moiety because the number of M

ions are larger than that of an Fe moiety. At the defect sites,

M ions with ligation of aqua ligands can act as active species.

The second minor defect sites could be formed by the lack of

M ions, where the CN ligand acts as a monodentate ligand.

The catalytic activity of this type of defect site can be estimated

from homogeneous catalysis of [Fe (CN) ]

Fe (CN) (NH )] complexes. The third type of defect

sites can be intrinsically formed only for the complexes

containing [Fe (CN) (NH )] after liberation of NH₃

ligands. Here, we mainly focus on catalysis examination of

the third type of defect sites.

[

Ga (H O) ] [Fe (CN) ] (GaFe). CoFe and GaFe were synthe-

2 3/2 4/3 6

sized according to the literature procedure with a slight

modiﬁcation. An aqueous solution of K [Fe (CN) ] (0.10 M, 8

mL) was slowly added to an aqueous solution of cobalt(II) nitrate or

gallium(III) nitrate (0.10 M, 16 mL) with vigorous stirring for 17.5 h.

The formed brown precipitates were collected by centrifugation and

washed with distilled water a few times. The precipitates were dried in

vacuo for 12 h. The dried precipitates were milled using a mortar to

obtain a ﬁne powder of CoFe and GaFe.

4−

3−

Synthesis of [Co (H O) ] [Fe (CN) (NH )] (CoFe-NH ) and

2 8/3 3/2 5 3 3

III II

[

Ga (H O)][Fe (CN) (NH )] (GaFe-NH ). An aqueous solution of

2 5 3 3

Na [Fe (CN) (NH )] (0.10 M, 8.0 mL) was slowly added to an

−

aqueous solution of cobalt(II) nitrate or gallium(III) nitrate (0.10 M,

2 mL) with vigorous stirring for 17.5 h. The formed brown

4−

and

precipitates were collected by centrifugation and washed with distilled

water a few times. The precipitates were dried in vacuo for 12 h. The

dried precipitates were milled using a mortar to obtain ﬁne powder of

3−

[

3−

CoFe-NH

and GaFe-NH .

3 3

Synthesis of [Cu (H O)

]

{[Fe (CN) (H O)] [Fe (CN) -

5 2 3/4 5

8/3 3/2

(

NH₃)]₁_/₄} (CuFe-H O). Sodium pentacyanoaquaammineferrate(II)

Na [Fe (CN) (H O)]) was synthesized according to the reported

3 5 2

procedure with a slight modiﬁcation. Na [Fe (CN) (NH )] (25

EXPERIMENTAL SECTION

mM) in a deaerated 4-(2-hydroxyethyl)-1-piperazineethanesulfonate

HEPES) buﬀer solution (pH 7.2, 100 mM, 40 mL) containing 0.87

M sodium chloride was allowed to stand for 15 min under N

■

(

Materials. Potassium hexacyanoferrate(II) trihydrate, sodium

pentacyanoammineferrate(II) hydrate, copper(II) nitrate trihydrate,

cobalt(II) nitrate hexahydrate, gallium(III) nitrate hydrate, disodium

p-nitrophenyl phosphate hexahydrate, p-nitrophenol, concentrated

hydrochloric acid, concentrated nitric acid, sodium chloride, disodium

hydrogen phosphate, and sodium hydroxide were purchased from

FUJIFILM-Wako Pure Chemical Industries Corporation. Potassium

hexacyanoferrate(II) trihydrate and 4-(2-hydroxyethyl)-1-piperazinee-

thanesulfonic acid (HEPES) were obtained from Sigma-Aldrich Co.,

LLC. All chemicals were used without further puriﬁcation. Puriﬁed

water was provided by a water puriﬁcation system, Advantec

RFD210TA, where the electronic conductance was 18.2 MΩ cm.

3−

atmosphere with vigorous stirring. The ratio of [Fe (CN) (H O)]

3−

and [Fe (CN) (NH )] in the resulting solution was 3:1, as

determined by IR absorption using the attenuated total reﬂectance

3−

(

ATR) technique; however, [Fe (CN) (H O)] cannot be further

5 2

puriﬁed by either recrystallization or chromatography due to its

3−

instability. Then, the solution containing [Fe (CN) (H O)] was

slowly added to an aqueous solution of copper(II) nitrate (0.3 M, 7.5

mL) with vigorous stirring for 17.5 h. The formed brown precipitates

were collected by centrifugation and washed with distilled water a few

times. The precipitates were dried in vacuo for 12 h. The dried

precipitates were milled using a mortar to obtain ﬁne powder of

Synthesis of [Cu (H O) ] {[Fe (CN) (NH )] [Fe (CN) ] }

6 1−n

Complexes (n = 0, 0.50, 0.83 or 1). An aqueous solution (3.0

x y

CuFe-H O.

mL) containing sodium pentacyanoammineferrate(II)

Physical Measurements. Ultraviolet−visible (UV−vis) absorp-

tion spectra were recorded on a JASCO V-770 spectrometer. Infrared

(IR) spectra were obtained on a JASCO FT/IR-6200 spectrometer

with an ATR unit using a diamond window. The amounts of ammine

ligand and ammonium ion in the PBAs were calculated according to a

literature procedure with slight modiﬁcations. Powder X-ray

diﬀraction (PXRD) patterns were recorded on a Shimadzu XD-3A.

(

Na [Fe (CN) (NH )], 0−0.10 M) and potassium hexacyanoferrate-

II) (K [Fe (CN) ], 0−0.10 M) was slowly added to an aqueous

solution of copper(II) nitrate (0.13 M, 4.5 mL) with vigorous stirring

for 17.5 h. The formed brown precipitates were collected by

centrifugation and washed with distilled water a few times. The

precipitates were dried in vacuo for 12 h. The dried precipitates were

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 2. Infrared spectra of (a) [Cu (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CuFe-NH , solid line) and [Cu (H O) ] [Fe (CN) ] (CuFe, broken

3/2

line), (b) [Co (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CoFe-NH , solid line) and [Co (H O) ] [Fe (CN) ] (CoFe, broken line), and (c)

Ga (H O)][Fe (CN) (NH )] (GaFe-NH , solid line) and [Ga (H O)₃_/₂]₄_/₃[Fe (CN) ] (GaFe, broken line). The magniﬁed view of the δ

region (1000−1500 cm ) is shown in the inset.

3/2

III

[

HNH

−

Incident X-ray radiation was produced by an Fe X-ray tube operating

at 40 kV and 15 mA with Fe−Kα radiation (λ = 1.94 Å). The scan

rate was 1° min⁻from 2θ = 20−60°. The atomic ratio of PBAs was

determined using a Shimadzu EDX-730 X-ray ﬂuorescence

spectrometer. Inductively coupled plasma optical emission spectros-

copy (ICP-OES) analyses were performed on a Shimadzu ICPE-9810.

Prior to the analyses, PBAs (ca. 1 mg) were dispersed in a mixed

solution of nitric acid and sulfuric acid (v/v = 1/1, 1.0 mL). The

dispersion was ultrasonicated for several minutes to dissolve the PBAs.

The obtained clear solutions were then diluted by adding a certain

amount of 2 M nitric acid and puriﬁed water to obtain the sample

solutions in the optimum concentration range for ICP-OES analyses.

Inductively coupled plasma mass spectrometry (ICP-MS) analyses

were performed on a PerkinElmer NexION 300D. Prior to the

analyses, PBAs (5.0 mg) were dispersed in a mixed solution of nitric

acid and hydrochloric acid (v/v = 1/1, 6.0 mL). The dispersion was

heated using a PerkinElmer Multiwave 3000 system (1200 W) for 10

min followed by holding it at the temperature for 15 min to dissolve

the PBAs. The obtained clear solutions were then diluted by adding a

certain amount of 2% nitric acid and puriﬁed water to obtain the

sample solutions in the optimum concentration range for ICP-MS

analyses. Scanning electron microscope (SEM) images of the PBAs

were obtained using a JEOL JSM-6500F operated at 15 kV. X-ray

photoelectron spectroscopy (XPS) analyses were performed using a

Shimadzu ESCA-3400HSE. An incident radiation was Mg−Kα X-ray

(1253.6 eV) at 200 W. The samples were mounted on a stage with a

double-sided carbon tape. The binding energy of each element was

corrected by the C 1s peak (284.6 eV) from the carbon tape.

Nitrogen (N ) adsorption−desorption isotherms at −196 °C were

obtained with a MicrotracBEL Belsorp-mini II. Weighed samples

(∼100 mg) were used for adsorption analysis after pretreatment at

150 °C for 1 h in vacuo. The samples were exposed to N within a

relative pressure range from 0.01 to 101.3 kPa. Adsorbed amount of

N was calculated from the pressure change in a cell after reaching

equilibrium at −196 °C. The total surface area was calculated from

the Brunauer−Emmett−Teller (BET) plot. The sizes of the

micropores and mesopores were calculated by the microporous

(MP) method and the Barrett−Joyner−Halenda (BJH) method,

respectively.

Catalysis Evaluation for the Hydrolysis of Organophos-

phates. A typical procedure for catalysis measurements is as follows.

A HEPES buﬀer solution (pH 6.0 or 8.3, 100 mM, 0.75 mL)

containing disodium p-nitrophenyl phosphate (p-NPP, 25 mM) and a

catalyst (0.063 mmol of Fe) in a sealed microtube was shaken at 900

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

by the additional coordination to M^Nions. Also, the

rpm at 50 °C using an a Block Bath Shaker (MyBL-100S, As One,

Japan). An aliquot (10 μL) of the reaction mixture periodically

sampled was diluted with HEPES buﬀer solution (pH 8.3, 100 mM,

wavenumbers of ν_C_Nsimilar to those of corresponding CuFe,

−1

CoFe, and GaFe appeared at 2096, 2084, and 2117 cm ,

,490 μL) and analyzed by a UV−vis spectrophotometer. The

respectively, supported the Fe −CN−M formation without

conversion ratio of p-NPP at a certain reaction time was determined

by the absorbance change at 400 nm ascribed to the formed p-

ligand isomerization (Figure 2). The wavenumber of ν of

GaFe-NH (2106 cm ) lower than that of GaFe (2117 cm )

by 11 cm resulted from a partial oxidation of Fe ions in GaFe

as reported previously, indicating that the valence of Fe ions

−

−1

nitrophenolate ion (p-NP, ε = 1.57 × 10 M cm ). Recycling

performance was evaluated by adding a buﬀer solution containing p-

NPP to a catalyst taken out from the reaction solution by

centrifugation.

−

is +2 in GaFe-NH . Also, ammine ligand coordination in

CuFe-NH , CoFe-NH , and GaFe-NH was conﬁrmed by the

−

RESULTS AND DISCUSSION

peaks at 1344, 1269, and 1270 cm , respectively, assigned to

H−N−H bending vibration (δ ), which was not observed

■

Structure of [M (H O) ] [Fe (CN) (NH )] (M Fe-NH )

HNH

x y

−1

III

for CuFe, CoFe, and GaFe. The peaks at around 1400 cm

observed for CuFe-NH , CoFe-NH , and GaFe-NH can be

Complexes. Cu , Co , and Ga ions were chosen as M ions

of the CN-deﬁcient PBAs because of diﬀerent valence and

^a^s^sⁱ^gⁿ^e^d^t^o^δHNH of ammonium ion (NH ) trapped in the

catalytic behavior of each meal ion as reported previously. Cu

ion is a divalent ion with low activity for the hydrolysis of p-

interstitial sites of PBAs.^N^H4 could be formed by partial

replacement of NH to H O ligands during synthesis in an

nitrophenyl phosphate (p-NPP); Co ion is also divalent but

III

aqueous solution (vide infra).

catalytically active, and Ga is trivalent and catalytically

Measurements of nitrogen (N ) adsorption−desorption

active. [Cu (H O)

]

[Fe (CN) (NH )] (CuFe-NH ),

8/3 3/2

isotherms of PBAs were performed to investigate their porous

structures (Figure S4 ). The total surface areas were calculated

from the Brunauer−Emmett−Teller (BET) method (Table 1).

[

Co (H O)₈_/₃] [Fe (CN) (NH )] (CoFe-NH ), and

Ga (H O)][Fe (CN) (NH )] (GaFe-NH ) were obtained

by the reaction of Na [Fe (CN) (NH )] with Cu , Co , and

Ga ions, respectively. Similarly, [Cu (H O) ] [Fe (CN) ]

CuFe), [Co (H O) ] [Fe (CN) ] (CoFe), and

Ga (H O)

the reaction of K [Fe (CN) ] with the corresponding metal

3/2

III

3 2

Table 1. Total Surface Areas Obtained by the Brunauer−

Emmett−Teller (BET) Method and Pore Diameters

Obtained by the Microporous (MP) Method and the

Barrett−Joyner−Halenda (BJH) Method of a Series of

(

[

2 3 2 6

III

] [Fe (CN) ] (GaFe) were synthesized by

3/2 4/3 6

ions. The scanning electron microscope (SEM) images showed

that the particles size of each PBA was 50 μm at the largest,

and most of the particles were less than 20 μm (Figure S1). No

shape-controlled particles were observed even at smaller

particles. Insigniﬁcant contamination of Na and K ions in

the PBAs was assured by inductively coupled plasma optical

emission spectroscopy (ICP-OES) and X-ray ﬂuorescence

spectroscopy, where the molar ratios of Na/Fe and K/Fe were

lower than 0.0035 and 0.026, respectively (Figure S2 and

Table S1).

PBAs Calculated from Their N Adsorption-Desorption

Isotherms

BET surface area,

micropore

diameter, nm

mesopore

diameter, nm

−1

PBA

m g

CuFe-NH₃

CuFe

101

0.7

0.6

0.8

7.0

10.2

4.6

5.3

10.8

3.2

CuFe-H O

CoFe-NH₃

CoFe

GaFe-NH₃

GaFe

284

179

393

X-ray photoelectron spectroscopy (XPS) measurements of

CuFe-NH and CuFe were performed for the energy regions

3.5

of Cu 2p, Fe 2p, O 1s, and N 1s (Figure S3). The values for

binding energy of Cu 2p₃_/₂of CuFe-NH and CuFe were

33.8 and 933.2 eV, respectively, which are close to the typical

Homogeneous distribution of micropores (0.6−0.8 nm)

formed by the cubic lattice structures of PBAs were evidenced

by the microporous (MP) method. Type IV isotherms

observed for a series of PBAs suggested the presence of

mesopores in the sizes of 3−11 nm as determined by the

Barrett−Joyner−Halenda (BJH) method. The mesopores in

this size range are formed by the gaps among PBA particles, as

values for the Cu species (933.5 eV). The binding energies

of Fe 2p₃_/₂peaks were found at 708.7 and 709.1 eV for CuFe-

NH and CuFe, respectively, which are also comparable to the

typical binding energy of Fe species (709.1 eV). These

results suggest that the involvement of NH ligands hardly

aﬀected the oxidation states of Cu and Fe ions. The binding

energy of O 1s in CuFe-NH was 532.5 eV, which is virtually

reported previously.

the same to that of CuFe (532.2 eV). The binding energy of N

The powder X-ray diﬀraction (PXRD) patterns obtained for

s in CuFe-NH (397.9 eV) was also similar to that in CuFe

M Fe-NH complexes were assignable to a cubic structure

(

398.3 eV) even in the presence of NH ligands because the

(Figure 3a, c, e). The cell parameters obtained for M = Cu,

−

peaks for N 1s in CN and NH ligands (397.7 and 399.0 eV,

Co, and Ga were a = 10.00, 9.94, and 10.04 Å, respectively.

The a values were comparable to those of corresponding

respectively, according to the literature) are severely over-

lapped.

M Fe complexes without open sites on Fe ions, where a =

The bridging structures of Fe −CN−M in M Fe-NH₃

complexes were conﬁrmed by infrared (IR) spectroscopy. The

CN-stretching bands (ν_C_N) of CuFe-NH , CoFe-NH , and

9.95, 10.10, and 10.02 Å for M = Cu, Co, and Ga, respectively

(Figure 4b, d, f). Broad peaks observed for M Fe-NH₃

complexes, especially for GaFe-NH , compared with those of

−

GaFe-NH₃appeared at 2094, 2084, and 2106 cm ,

M Fe complexes resulted from the low crystallinity due to

local disorder originated from partial deﬁciencies of CN

ligands of M Fe-NH complexes, or from small crystallite

−

respectively, which were shifted from that of

−1

K [Fe (CN) (NH )] (2038 cm ) in the higher wavenumber

region (Figure 2). Such higher wavenumber shift evidenced

even with maintaining high crystallinity. The high wavenumber

shift of ν_C_Nin IR spectra and the presence of microporous

structures evidenced by the N₂adsorption−desorption

the formation of Fe −CN−M structures, as the electron

−

density of an antibonding orbital of CN ligand was reduced

Inorg. Chem. XXXX, XXX, XXX−XXX

and their precursors for the hydrolysis of p -nitrophenyl

Article

phosphate (p-NPP) was examined at 50 °C in a 4-(2-

hydroxyethyl)-1-piperazineethanesulfonate (HEPES) buﬀer

solution (pH 8.3, 100 mM, 0.75 mL) containing metal

complexes (0.063 mmol of Fe) and p-NPP (25 mM). The

yield of the hydrolysis product, p-nitrophenolate ion (p-NP),

was determined by characteristic absorbance change at 400 nm

(

[

Figure S5 ). No catalytic activity was observed for

4−

3−

Fe (CN) ] and [Fe (CN) (NH )] dissolved in buﬀer

solutions (Figure 4). The IR spectrum of an aqueous solution

3−

containing [Fe (CN) (NH )] and hydrogen phosphate

−

indicates the absence of a δ

peak at 1255 cm , suggesting

HNH

that the open Fe sites formed during the reaction (Figure S6).

On the other hand, the presence of the peaks at 1078 and 989

−

cm assignable to the PO stretching vibration (ν ) suggests

the coordination of hydrogen phosphate to the Fe ions.

Thus, tuning of electronic properties of Fe ions by M ions

through CN ligands is essential to suppress the product

55,56

−

inhibition by hydrogen phosphate.

The higher conversion ratios and initial reaction rates for 1 h

(

v ) were observed for the reaction solutions containing CoFe

−

−1 −1

28% for 24 h and 0.3 × 10 mol L h ) and GaFe (26% for

4 h and 0.3 × 10 mol L h ) compared with that

containing CuFe (5% for 24 h and 0.1 × 10 mol L h ),

indicating that the N-bound Co and Ga ions act as the

−

−1

−

−1 −1

III

Figure 3. Powder X-ray diﬀraction (PXRD) patterns of (a)

active species; however, Cu ions are inactive (Figure 4a).

[

Cu (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CuFe-NH ), (b)

I I

CuFe-NH showed the higher catalytic activity compared with

C u ( H ₂O ) ₂] ₃_/₂[ F e ( C N ) ₆

]

( C u F e ) , ( c )

Co (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CoFe-NH ), (d)

CuFe. The conversion ratio and v value observed for CuFe-

III

−3

−1

Co (H O) ] [Fe (CN) ] (CoFe), (e) [Ga (H O)]-

NH were 58% for 24 h and 2.0 × 10 mol L h ,

respectively, suggesting that open Fe sites were successfully

3/2

I I

F e⁽^C^N⁾5 ( N H ) ]⁽^G^a^F^e^-^N^H3 ) , a n d ( f )

III

Ga (H O)3/2^] [Fe (CN) ] (GaFe).

4/3

created during the catalytic reaction. The conversion ratios and

v values observed for CoFe-NH (44% for 24 h and 0.4 ×

−3

−1 −1

isotherms indicate the formation of M Fe-NH complexes

10 mol L h ) and GaFe-NH (53% for 24 h and 1.0 ×

−

−1 −1

having cubic lattice structures.

10 mol L h ) were higher than those for CoFe and GaFe,

Catalysis of [M (H O) ] [Fe (CN) (NH )] Complexes

suggesting that the Fe ions with single open sites contribute

x y

for the Hydrolysis of p-Nitrophenyl Phosphate. The

to p-NPP hydrolysis (Figure 4b). However, the conversion

ratios and v values for CoFe-NH and GaFe-NH were lower

than those of CuFe-NH . These results imply that the catalytic

catalytic activity of M Fe-NH complexes, M Fe complexes,

Figure 4. Time proﬁles of the p-nitrophenolate ion (p-NP) formation in a HEPES buﬀer solution (pH 6.0, 100 mM, 0.75 mL, 50 °C) containing

disodium p-nitrophenyl phosphate (p-NPP, 25 mM) in the absence and presence of (a) [Cu (H O) ] [Fe (CN) ] (CuFe),

III

4−

[

Co (H O) ] [Fe (CN) ] (CoFe), [Ga (H O) ]₄_/₃[Fe (CN) ] (GaFe), and hexacyanoferrate ion ([Fe (CN) ] ), or (b)

Cu (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CuFe-NH ), [Co (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CoFe-NH ), [Ga (H O)][Fe (CN) (NH )] (GaFe-

3/2

III

3−

NH ), and pentacyanoammineferrate ion ([Fe (CN) (NH )] ).

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 5. Infrared (IR) spectra of (a) [Cu (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CuFe-NH ), (b) [Co (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CoFe-NH ),

III

and (c) [Ga (H O)][Fe (CN) (NH )] (GaFe-NH ) before (broken lines) and after (solid lines) the catalytic hydrolysis of disodium p-

nitrophenyl phosphate (p-NPP, 25 mM) in a HEPES buﬀer solution (pH 6.0, 100 mM, 0.75 mL, 50 °C) for 24 h. Magniﬁed views of the δ

ν _P _O region (1000 −1500 cm ) are shown in the insets.

and

HNH

−

Figure 6. Schematic drawing of the diﬀerence of adsorbed species on [M (H O) ] [Fe (CN) (NH )] (M Fe-NH ) complexes (M = (a) Cu ,

III

(b) Co , or (c) Ga ).

properties of open Fe sites of CuFe-NH , CoFe-NH , and

respectively (Figure S7). The conversion ratio at the second

run was lower than that at the ﬁrst run. No further decrease in

conversion ratios were observed at the third and fourth runs. A

possible reason for the deceleration was structural deformation

GaFe-NH were not the same.

Repetitive catalysis experiments using CuFe-NH indicated

that the conversion ratios (24 h) obtained for the reactions

were 42%, 13%, 12%, and 11% at the ﬁrst to fourth runs,

of CuFe-NH ; however, virtually the same PXRD patterns of

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 7. Time proﬁles of the p-nitrophenolate ion (p-NP) formation in a HEPES buﬀer solution (100 mM, 0.75 mL, 50 °C) containing disodium

p-nitrophenyl phosphate (p-NPP, 25 mM) in the absence and presence of [Cu (H O)₈_/₃]₃_/₂[Fe (CN) (NH )] (CuFe-NH ),

[

Cu (H O)₈_/₃]₃_/₂{[Fe (CN) (H O)] [Fe (CN) (NH )] } (CuFe-H O), and [Cu (H O) ] [Fe (CN) ] (CuFe) at (a) pH 6.0 and (b)

2 5 2 3/4 5 3 1/4 2 2 3 2 6

pH 8.3.

CuFe-NH before and after the catalytic reaction strongly

at 2066 cm⁻after the reaction is attributed to a contaminant,

support no degradation of CuFe-NH (Figure S8 ). Thus,

as reported previously. The Fe ions with enhanced Lewis

acidity by Co and Ga ions favor interaction with hydrogen

phosphate and/or ammonia, which decelerated the catalytic

reactions (Figure 6). Thus, the metal ions coupled with Fe

ions with single open sites in CN-deﬁcient PBAs precisely

III

product inhibition of the Fe ions would be a reason for the

deceleration.

Adsorbed Species on Open Sites of Fe Ions in

[

M (H O) ] [Fe (CN) (NH )] Complexes After the Cata-

x y

lytic Reactions. The liberation of NH ligands from CuFe-

regulated the catalysis and adsorption properties of the Fe

NH during the catalytic reaction was conﬁrmed by ex situ IR

ions with single open sites.

Lability of NH₃Compared with That of H O at

Diﬀerent pH Values. Open Fe sites formed during the

catalytic reactions are active sites; thus, liberation of NH₃

ligands is important to achieve high catalytic activity. Lability

spectroscopy. The IR measurement of CuFe-NH after the

catalytic reaction clariﬁed that the strength of the most

characteristic peak at 1348 cm⁻ascribed to δ_H_N_Hdecreased by

∼

85% compared with that of fresh CuFe-NH , suggesting that

open Fe sites successfully formed during the catalytic reaction

of NH ligand in CuFe-NH₃was conﬁrmed by catalysis

−1

(Figure 5a). Instead, the peaks at 1014 cm and around 989

comparison with CuFe-H O which mainly possesses H O as

assignable to ν_P_Oappeared for CuFe-NH after the catalytic

reaction, indicating the adsorption of hydrogen phosphate.

an extra ligand instead of NH . CuFe-H O was prepared by

55,56

mixing an aqueous solution of Na [Fe (CN) (H O)], which

3 5 2

The adsorbed hydrogen phosphate on CuFe-NH and CuFe

was prepared from Na [Fe (CN) (NH )] according to a

3 5 3

after the catalytic reactions was more directly quantiﬁed by

inductively coupled plasma mass spectrometry (ICP-MS). The

reported procedure, with that of copper(II) nitrate.

Insigniﬁcant contamination of Na ions was conﬁrmed by

molar ratio of P and Fe (P/Fe) in CuFe-NH after the catalytic

reaction was 0.12, which was six times higher than P/Fe in

the ICP-OES analysis, in which the molar ratio of Na/Fe was

0.021 (Table S1 ). The IR spectra of CuFe-H O indicated that

CuFe (0.021, Table S2 ). These results suggest that hydrogen

75% of NH was replaced with H O to provide

phosphate formed by the p-NPP hydrolysis preferably bound

[Cu (H O) ] {[Fe (CN) (H O)] [Fe (CN) (NH )]₁_/₄}

2 8/3 3/2 5 2 3/4 5 3

to the open Fe sites in CuFe-NH (Figure 6).

(Figure S10 ). The ν_C_Npeak of CuFe-H O that appeared at

−

Adsorbed species on the surfaces of CoFe-NH and GaFe-

2094 cm , which is comparable to those of CuFe-NH and

−

NH after the catalytic reactions were also investigated by IR

CuFe (2094 and 2096 cm , respectively), supports the

spectroscopy. The IR spectrum of CoFe-NH₃after the

formation of bridging structures of Fe −CN−M . However,

slightly broader PXRD peaks assignable to a cubic structure

(Figure S11 ) and smaller total surface areas calculated from the

catalytic reaction showed that the δ

and ν_P_Opeaks

HNH

−

appeared at 1262 and 1038 cm , respectively (Figure 5b);

however, no peak assignable to ν _P _O was observed for CoFe

after the catalytic reaction (Figure S9a). Thus, limited

liberation of NH₃ligand in CoFe-NH₃suppressed the

interaction with substrates, resulting in low catalytic activity.

N adsorption−desorption measurements (Table 1) compared

with those of CuFe-NH may suggest the low crystallinity of

CuFe-H O.

The catalytic hydrolysis of p-NPP (25 mM) was examined at

50 °C in a HEPES buﬀer solution (pH 8.3, 100 mM, 0.75 mL)

containing CuFe-NH or CuFe-H O (0.063 mmol of Fe).

The IR spectrum of GaFe-NH after the catalytic reaction

−

showing a strong ν_P_Opeak which appeared at 1037 cm

indicated the adsorption of a hydrogen phosphate (Figure 5c).

On the other hand, the much weaker ν_P_Opeak observed for

GaFe after the catalytic reaction indicated that hydrogen

CuFe-NH₃and CuFe-H O showed very similar reaction

proﬁles in the reaction solutions of both pH 6.0 and 8.3

(Figure 7). These results suggest that open Fe ions acting as

III

phosphate mainly adsorbed on the Fe not Ga ions of GaFe-

NH (Figure S9b ). The ν peak of GaFe-NH that appeared

catalytic active species were successfully created on CuFe-NH₃

and CuFe-H O irrespective of their crystallinity and solution

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

−1

pH. The smaller BET surface area of CuFe-H O (32 m g )

CONCLUSION

■

CN-deﬁcient Prussian blue analogues (PBAs),

compared with those of CuFe-NH and CuFe (77 and 101 m

g , respectively) may result from its lower crystallinity (Table

). The lower crystallinity resulted in increasing the numbers

−

[M (H O) ] [Fe (CN) (NH )] (M Fe-NH , M = Cu ,

x y

III

Co , or Ga ), were synthesized to examine C-bound Fe

−

ions with single open sites as precisely controlled active species

of H O ligands on the Cu ions and free N terminals of CN

−

4−

for the hydrolysis of p-nitrophenyl phosphate. Liberation of

ligands. The N terminals of CN ligands of [Fe (CN) ] are

NH from the Fe ions during the catalytic reactions resulted

catalytically inactive (Figure 4), and the IR spectrum of CuFe-

H O indicates that the number of nonbridging CN ligand is

negligible (Figure S10 ), thus, increasing the number of H O

ligands on Cu ions should be carefully considered. However,

in the formation of exactly one single open site on each Fe

−

ion, diﬀerent from conventional PBAs. The high catalytic

activity of the Fe ions with open sites coupled with

catalytically inert Cu ions as the M ions clearly indicated

the direct involvement of the Fe ions as the active species in

the catalytic behavior of CuFe-H O was quite similar to that of

CuFe-NH , indicating that increasing the number of H O

the catalytic hydrolysis. Then, heterogeneous catalysis of the

CN-deﬁcient PBAs employing Co and Ga ions as the M^N

III

ligands has less inﬂuence on the catalysis.

ions, which are catalytically active species in conventional

Concentration Eﬀect of Fe ions with Single Open

PBAs, instead of Cu ions was examined. However, no catalysis

Sites on Catalysis. The concentration of Fe ions with single

enhancement was observed for the CN-deﬁcient PBAs

open sites was precisely tuned by concomitant use of

III

containing Co and Ga ions, because Lewis acidity of the

3−

4−

[

(

Fe (CN) (NH )] and [Fe (CN) ] as building blocks.

Fe ions enhanced by the M ions favors interaction with

hydrogen phosphate and/or ammine ligand. These results

suggest that heterogeneous catalysis of metal ions with

precisely controlled open sites can be tuned by interaction

between coupled metal ions in CN-deﬁcient PBAs, leading to

the rational design of active sites for highly sophisticated

heterogeneous catalysis.

Cu (H O) ] {[Fe (CN) (NH )] [Fe (CN) ] } complexes

x y

6 1−n

n = 0, 0.50, 0.83, or 1) were synthesized by mixing an aqueous

solution containing the various ratios of [Fe (CN) (NH )]

and [Fe (CN) ] and that containing copper(II) nitrate.

3−

4−

Similar PXRD patterns of [Cu (H O) ] {[Fe (CN) (NH )] -

x y

[

Fe (CN) ]₁₋_n} complexes are ascribed to the cubic structure

The Supporting Information is available free of charge at

Figure S12 ).

ASSOCIATED CONTENT

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02528.

■

The hydrolysis of p-NPP was examined in a HEPES buﬀer

solution (pH 6.0, 100 mM, 0.75 mL) containing

[Cu (H O) ] {[Fe (CN) (NH )] [Fe (CN) ] } complexes

2 x y 5 3 n 6 1−n

(

0.063 mmol of Fe) and p-NPP (25 mM). The conversion

Scanning electron microscope (SEM) images, X-ray

ﬂuorescence spectra, X-ray photoelectron spectra (XPS),

ratios in 24 h and v values for [Cu (H O) ] {[Fe (CN) -

x y

−3

(

NH )] [Fe (CN) ] } complexes (22% and 0.4 × 10 mol

3 n 6 1−n

nitrogen (N ) adsorption−desorption isotherms, infra-

−

−1

−3

−1 −1

for n = 0.50, and 34% and 1.2 × 10 mol L

for n

red (IR) spectra, ultraviolet−visible (UV−vis) spectra,

powder X-ray diﬀraction (PXRD) patterns, reaction

proﬁles, and elemental analyses data (PDF)

0.83) were higher than those for CuFe (n = 0) but lower

than those for CuFe-NH (n = 1), suggesting that increasing

the number of Fe ions with single open sites in the PBAs is

important to achieve high catalytic activity (Figure 8 ).

AUTHOR INFORMATION

Corresponding Author

■

Yusuke Yamada − Department of Applied Chemistry and

Bioengineering, Graduate School of Engineering and Research

Center of Artiﬁcial Photosynthesis (ReCAP), Osaka City

University, Osaka 558-8585, Japan; orcid.org/0000-0001-

259-5255 ; Email: ymd@a-chem.eng.osaka-cu.ac.jp

Authors

Mari Yamane − Department of Applied Chemistry and

Bioengineering, Graduate School of Engineering, Osaka City

University, Osaka 558-8585, Japan

Hiroyasu Tabe − Department of Applied Chemistry and

Bioengineering, Graduate School of Engineering and Research

Center of Artiﬁcial Photosynthesis (ReCAP), Osaka City

University, Osaka 558-8585, Japan; orcid.org/0000-0003-

028-7371

Masami Kawakami − Nanomaterials Research Institute,

National Institute of Advanced Industrial Science and

Technology (AIST), Tsukuba 305-8565, Japan

Hisashi Tanaka − Nanomaterials Research Institute, National

Figure 8. Time proﬁles of the p-nitrophenolate ion (p-NP) formation

in a HEPES buﬀer solution (pH 6.0, 100 mM, 0.75 mL, 50 °C)

containing disodium p-nitrophenyl phosphate (p-NPP, 25 mM) in the

Institute of Advanced Industrial Science and Technology

(

AIST), Tsukuba 305-8565, Japan

presence of [Cu (H O) ] {[Fe (CN) (NH )] [Fe (CN) ]₁₋_n} com-

Tohru Kawamoto − Nanomaterials Research Institute, National

Institute of Advanced Industrial Science and Technology

plexes (n = 0, 0.50, 0.83, or 1).

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

AIST), Tsukuba 305-8565, Japan; orcid.org/0000-0002-

Article

Porosity Generation and Desalination Performance in Metal-Organic

Frameworks. Chem. Sci. 2018, 9, 3508−3516.

984-2980

(13) Kirchon, A.; Feng, L.; Drake, H. F.; Joseph, E. A.; Zhou, H. C.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02528

From Fundamentals to Applications: A Toolbox for Robust and

Multifunctional MOF Materials. Chem. Soc. Rev. 2018, 47, 8611−

638.

Author Contributions

(14) Dhakshinamoorthy, A.; Li, Z. H.; Garcia, H. Catalysis and

The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

Photocatalysis by Metal Organic Frameworks. Chem. Soc. Rev. 2018,

(

7, 8134−8172.

15) DeStefano, M. R.; Islamoglu, T.; Garibay, S. J.; Hupp, T. H.;

Farha, O. K. Room-Temperature Synthesis of UiO-66 and Thermal

Modulation of Densities of Defect Sites. Chem. Mater. 2017, 29,

Funding

This work was supported by the Innovative Science and

Technology Initiative for Security (ATLA, Japan) to Y.Y.

357−1361.

(16) Gutov, O. V.; Hevia, M. G.; Escudero-Adan

, E. C.; Shafir, A.

(

JPJ161000157 and JPJ191047001), JSPS KAKENHI to Y.Y.

JP16H02268) and to H.T. (JP19K15591 and JP20H05110),

Metal-Organic Framework (MOF) Defects Under Control: Insights

into the Missing Linker Sites and Their Implication in the Reactivity

of Zirconium-Based Frameworks. Inorg. Chem. 2015, 54, 8396 −8400.

the Koyanagi Foundation, and the Paloma Environmental

Technology Development Foundation.

(17) Fang, Z. L.; Bueken, B.; De Vos, D. E.; Fischer, R. A. Defect-

Engineered Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2015,

4, 7234−7254.

18) Wu, H.; Chua, Y. S.; Krungleviciute, V.; Tyagi, M.; Chen, P.;

Notes

(

The authors declare no competing ﬁnancial interest.

Yildirim, T.; Zhou, W. Unusual and Highly Tunable Missing-Linker

Defects in Zirconium Metal −Organic Framework UiO-66 and Their

Important Effects on Gas Adsorption. J. Am. Chem. Soc. 2013, 135,

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