Creation and stabilisation of tuneable open metal sites in thiocyanato-bridged heterometallic coordination polymers to be used as heterogeneous catalysts

Article abstract of DOI:10.1039/c9dt03679b

A series of thiocyanato-bridged heterometallic coordination polymers with a 3D reticular network have been synthesised by the reaction of [Pt^IV(SCN)₆]^2- with M^II ions to form {M^II[Pt^IV(SCN)₆]}_n and {[M^II(CH₃OH)₂][Pt^IV(SCN)₆]}_n (M^II = Mn^II, Fe^II, Co^II, Ni^II or Cu^II) in water and methanol, respectively. Single-crystal X-ray analyses revealed the absence of open metal sites in {M^II[Pt^IV(SCN)₆]}_ns and the formation of potential open metal sites at the M^II ions of {[M^II(CH₃OH)₂][Pt^IV(SCN)₆]}_ns by the coordination of methanol. One of the two coordinating methanol molecules in {[Co^II(CH₃OH)₂][Pt^IV(SCN)₆]}_n was replaced with pyridine to stabilise the open metal sites, because the methanol molecules are too labile to maintain open metal sites in water. The heterogeneous catalysis of coordination polymers with and without open metal sites was examined for organophosphate hydrolysis and photocatalytic water oxidation to clarify the requisites for heterogeneous catalysts.

Full text of DOI:10.1039/c9dt03679b

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Creation and stabilisation of tuneable open metal
sites in thiocyanato-bridged heterometallic
coordination polymers to be used as
heterogeneous catalysts†
Cite this: Dalton Trans., 2019, 48,
17063
a,b
a
a,c
a
Hiroyasu Tabe,
Masaaki Matsushima, Rika Tanaka and Yusuke Yamada
*
A series of thiocyanato-bridged heterometallic coordination polymers with a 3D reticular network
IV
]2− with M ions to form {M [Pt (SCN)
II
II
IV
have been synthesised by the reaction of [Pt (SCN)
6
]} and
6 n
II
IV
II
II
II
II
II
II
{
[M (CH
3
OH)
2
][Pt (SCN)
6
]}
n
(M = Mn , Fe , Co , Ni or Cu ) in water and methanol, respectively. Single-
II
IV
6 n
crystal X-ray analyses revealed the absence of open metal sites in {M [Pt (SCN) ]} s and the formation of
II
II
IV
potential open metal sites at the M ions of {[M (CH
3
OH)
2
][Pt (SCN)
6
]}
n
s by the coordination of methanol.
II
IV
One of the two coordinating methanol molecules in {[Co (CH
3
OH)
2
][Pt (SCN)
6
]}
n
was replaced with pyri-
Received 14th September 2019,
Accepted 22nd October 2019
dine to stabilise the open metal sites, because the methanol molecules are too labile to maintain open
metal sites in water. The heterogeneous catalysis of coordination polymers with and without open metal
sites was examined for organophosphate hydrolysis and photocatalytic water oxidation to clarify the
requisites for heterogeneous catalysts.
DOI: 10.1039/c9dt03679b
rsc.li/dalton
nate to two different metal ions selectively at each end, and
electronic interaction takes place between the different metal
Introduction
2
3–26
Heterogeneous catalysts are essential functional materials to ions through conjugation.
−
realise an environmentally benign and sustainable society.
In this context, the cyano ligand (CN ) is used to synthesise
However, the development of conventional heterogeneous cata- heterometallic coordination polymers acting as heterogeneous
2
7–29
lysts made of metal oxides and metals is an arduous task, catalysts for various reactions.
because structural and electronic fine tuning of their active polymers are composed of
Cyano-bridged coordination
hexacyanometallate anion
a
C
n−
N
C
N
sites is far from straightforward. Recently, coordination poly- ([M (CN) ] ) and a metal ion (M ) to form a cubic M –CN–M
mers composed of metal ions and organic bridging ligands framework, where the number of M ions is often larger than
emerged as a new class of heterogeneous catalysts with highly that of hexacyanometallate anions for charge compensation.
6
N
1
–10
N
tuneable active sites.
The active sites of the coordination The excess M ions have an octahedral structure by the coordi-
polymers are mostly metal ions with a labile ligand such as nation of water molecules, which are easily liberated to gene-
1
1–15
30–38
solvent molecules.
The structural and electronic properties rate open metal sites, in a reaction solvent.
However, the
N
of the active sites can be tuned by making suitable choices of number of water molecules coordinating to each M ion is not
bridging ligands and metal ions in the coordination the same, indicating that the structures of active sites are not
1
6–22
polymers.
The choice of an asymmetric and conjugated completely tuned. Active sites with uniform structures could
bridging ligand is promising, because the ligand can coordi- be achieved by utilising an extra ligand, which can stabilise
open metal sites, other than water. The coordination of such
N
an extra ligand may require large spaces around M ions.
a
Graduate School of Engineering, Osaka City University, Sugimoto, Sumiyoshi,
Osaka 558-8585, Japan. E-mail: ymd@a-chem.eng.osaka-cu.ac.jp
−
Thiocyanate ligand (SCN ) is a promising ligand to obtain
b
heterometallic coordination polymers with larger coordination
The OCU Advanced Research Institute for Natural Science and Technology
N
spaces around M ions. The S and N atoms of the thiocyanate
(
OCARINA), Osaka City University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan
c
Analytical Centre of Osaka City University, Sugimoto, Sumiyoshi, Osaka 558-8585,
ligand are more selectively bound to soft and hard metal ions,
S
N
Japan
respectively, to form an M –SCN–M framework based on the
†
Electronic supplementary information (ESI) available: Experimental procedure;
39–43
hard–soft acid–base (HSAB) principle.
The long distance
thermogravimetric and differential thermal analyses (TG/DTA, Fig. S1 and S3);
crystal structures (Fig. S2); XRD patterns (Fig. S4) and crystal data (Table S1).
CCDC 1949922, 1949923 and 1949925. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/C9DT03679B
S
N
C
between the M and M ions compared to that of the M and
ions in cyano-bridged heterometallic coordination poly-
mers allows the coordination of large extra ligands to M ions.
N
M
N
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ing a corresponding metal salt (see the ESI†). The formed pre-
cipitates were collected by centrifugation and dried in vacuo.
II
IV
Among the coordination polymers, {Mn [Pt (SCN)
6 n
]} formed
large single crystals suitable for a single-crystal X-ray structure
II
IV
analysis. The crystal structure of {Mn [Pt (SCN) ]} clearly
6
n
showed a non-interpenetrated, three-dimensional framework
in the triclinic P ˉ1 space group, with octahedral coordination of
IV
II
II
Pt and Mn ions (Fig. 2 and Table S1†). Each Mn ion is fully
IV
2−
coordinated by six N atoms of the neighbouring [Pt (SCN)
moieties. The observed bond angles of Pt–S–C (104.0(2)–
05.1(2)°) and the bond lengths of Pt–S (2.380(18)–2.3840(17)
Å) are within the typical range of those reported for
6
]
1
IV
63
2 6
K [Pt (SCN) ] (105.6(2)–105.8(2)° and 2.380(1)–2.39(1) Å).
The Mn–N bond lengths (2.207(6)–2.220(6) Å) are comparable
to the typical Mn–N bond lengths (2.225(2)–2.229(6) Å)
6
4
reported for Na
4 6
[Mn(NCS) ]. Similar powder X-ray diffraction
II
IV
II
II
II
II
(
XRD) patterns for {M [Pt (SCN) ]} s (M = Mn , Fe , Co ,
6
n
II
II
Ni or Cu ) assure their identical structure (Fig. 3), in which
no potential open metal sites formed in the M ions as evi-
II
Fig. 1 Synthesis conditions for the tuning of open metal sites of thio-
IV
denced by thermogravimetric analyses (vide infra, Fig. S1†).
cyanato-bridged heterometallic coordination polymers containing Pt
II
and Co ions as the S- and N-bound metal ions, respectively.
In fact, the coordination of large extra ligands such as terpyri-
dine, ethylenediamine and N,N,N′,N′-tetramethylethylene-
N
diamine to the M ions of thiocyanato-bridged coordination
4
4–62
polymers has been reported.
A careful choice of extra
ligands would result in structural fine tuning of the open
metal sites formed during catalytic reactions.
We report herein the forced creation of potential open
metal sites in thiocyanato-bridged heterometallic coordination
polymers by using methanol, which is more donating solvent
II
IV
II
6 n
than water, for synthesis. A series of {M [Pt (SCN) ]} s (M =
II
II
II
II
II
Mn , Fe , Co , Ni or Cu ) synthesised in water possess a
II
coordinatively saturated structure at M ions, on the other
hand, potential open metal sites were created in
II
IV
{
[M (CH
The methanol molecules in {[Co (CH OH) ] [Pt (SCN) ]}
6 n
3 2 2 6 n
OH) ] [Pt (SCN) ]} s synthesised in methanol (Fig. 1).
II
IV
3
2 2
were partially replaced with the more basic pyridine for the
stabilisation of open metal sites during catalytic reactions. The
catalytic activity of a series of thiocyanato-bridged heterometal-
lic coordination polymers was examined for organophosphate
hydrolysis and photocatalytic water oxidation to clarify the
requisites for heterogeneous catalysts.
Results and discussion
Crystal structure of the thiocyanato-bridged heterometallic
coordination polymers without potential open metal sites
II
IV
Fig. 2 Crystal structures of {Mn [Pt (SCN)
6 n
]} (CCDC 1949922†). (a)
The Oak Ridge thermal ellipsoid plot (ORTEP) of the asymmetric unit
A series of thiocyanato-bridged heterometallic coordination
II
IV
II
IV
II
IV
containing one Mn ion and one Pt ion. Ellipsoids are drawn at the 50%
probability level. (b and c) The three-dimensional network structure
viewed along the b and c axis for (b) and (c), respectively. Atoms are
coloured as follows: Mn, purple; Pt, white; C, grey; N, blue; S, yellow.
polymers including {Mn [Pt (SCN) ]} , {Fe [Pt (SCN) ]} ,
6
n
6 n
II
IV
II
IV
II
IV
{
Co [Pt (SCN)
6 n 6 n 6 n
]} , {Ni [Pt (SCN) ]} and {Cu [Pt (SCN) ]}
were synthesised by mixing an aqueous solution of potassium
IV
hexathiocyanatoplatinate(IV) (K [Pt (SCN) ]) and that contain- A unit cell is represented by a parallelepiped.
2
6
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Fig. 3 (a)
Simulated
Mn [Pt (SCN) ]} and (b–f) observed powder X-ray diffraction patterns
of {M [Pt (SCN)
powder
X-ray
diffraction
pattern
of
II
IV
{
6 n
II
IV
II
IV
II
IV
6 n 6 n 6 n
s [(b) {Mn [Pt (SCN) , (c) {Fe [Pt (SCN) , (d)
]}
, (e) {Ni [Pt (SCN)
simulated pattern was calculated from the single-crystal data.
]} ]}
II
IV
II
IV
II
IV
{
Co [Pt (SCN)
6
]}
n
6
]}
n
and (f) {Cu [Pt (SCN) ]} ]. The
6 n
II
IV
Fig. 4 Crystal structures of {[Co (CH
3
OH)
2
][Pt (SCN)
6
]}
n
(CCDC
Crystal structure of the thiocyanato-bridged heterometallic
coordination polymers with potential open metal sites
1
949923†). (a) The Oak Ridge thermal ellipsoid plot (ORTEP) of the
asymmetric unit. Ellipsoids are drawn at the 50% probability level. (b and
c) The three-dimensional network structure viewed along the b and c
axis for (b) and (c), respectively. Atoms are coloured as follows: Co, pink;
Pt, white; C, grey; N, blue; O, red; S, yellow. A unit cell is represented by
a parallelepiped. Hydrogen atoms are omitted for clarity.
The coordination polymers were then synthesised by mixing a
IV
methanol solution of K
2
II
[Pt (SCN)
6
] and that containing an
II
II
II
II
II
II
M
salt (M = Mn , Fe , Co , Ni or Cu ) to create potential
II
open metal sites in M ions by the coordination of methanol.
Single crystals suitable for X-ray structure analysis were
obtained from the reaction solutions containing M = Mn ,
Fe and Co . Single crystal X-ray analysis for the crystal contain-
ing Co ions indicated that the chemical formula of the crystal
II
II
II
IV
{
[Co (CH OH) ][Pt (SCN) ]} as shown in Fig. S2 and
3 2 6 n
II
II
Table S1.† Although single crystals suitable for X-ray structure
analysis were hardly obtained by the reaction of K [Pt (SCN) ]
with nickel(II) nitrate or copper(II) nitrate, the XRD patterns of
the resulting precipitates showed remarkable changes from
those of the corresponding {M [Pt (SCN) ]} s without metha-
nol coordination (Fig. 5). Also, thermogravimetric and differ-
ential thermal analyses (TG/DTA) of the precipitates showed
endothermic weight losses (5% and 4%) below 200 °C
II
IV
2
6
II
IV
is {[Co (CH OH) ][Pt (SCN) ]} , clearly indicating that potential
3
2
6 n
open metal sites are forcedly created by the coordination of
II
methanol molecules to Co ions.
II
IV
6
n
II
IV
IV
{
[Co (CH OH) ][Pt (SCN) ]} possessing one Pt ion and
3 2 6 n
II
two Co ions crystallographically inequivalent in the asym-
metric unit has a three-dimensional framework in the mono-
IV
clinic C2/c space group (Fig. 4 and Table S1†). The Pt ion has
II
II
(
Fig. S3†), suggesting the methanol coordination to Ni or Cu
an octahedral geometry coordinated by six sulphur atoms of
thiocyanate ligands. The observed bond lengths of Pt–S (2.372
ions. The weight losses that were smaller than expected from
the composition of {[M (CH OH) ][Pt (SCN) ]} (ca. 10%)
resulted from the partial removal of methanol molecules
during the drying treatment before sampling due to the weak
coordination of methanol.
II
IV
3
2
6 n
(
(
15)–2.382(16) Å) and the bond angle of Pt–S–C (102.6(2)–105.4
2)°) are comparable to those of K [Pt (SCN) ] (vide supra).
2 6
IV
63
Each Co ion has a similar octahedral geometry composed of
four equatorial N atoms from thiocyanate ligands and two
axial O atoms from a pair of coordinating methanol molecules.
An exceptional example is the coordination polymer precipi-
IV
tated from the methanol solution of K
nese(II) nitrate. The XRD pattern of the precipitates was the
same as that obtained for {Mn [Pt (SCN) ]} synthesised in
water. This result indicates that a labile but more basic extra
ligand is necessary for the creation of potential open metal
2
[Pt (SCN)
6
] and manga-
IV
2−
Four N atoms of [Pt (SCN)
6
]
described as N1–N4 coordinate
II
to the neighbouring Co ions, where the Co–N bond lengths
II
IV
6
n
(
2.087(4)–2.120(5) Å) lie within the typical Co–N bond lengths
II
4−
65
of [Co (NCS)
6
]
(2.107(2)–2.139(2) Å). However, two N atoms
II
described as N5 and N6 do not coordinate to the Co ions but
form N⋯H–O hydrogen bonds with the coordinated methanol
molecules (N⋯O, 2.747 and 2.763 Å).
II
sites of Mn ions.
II
II
IV
Stability of potential open metal sites in {[Co (CH OH) ]
[Pt (SCN) ]}
3
2
Single crystals of {[Fe (CH OH) ][Pt (SCN) ]} were also
3
2
6 n
IV
IV
6
n
fabricated in a methanol solution containing K
and
{
2
[Pt (SCN)
structure
to that
6
]
II
IV
iron(II)
chloride.
[Fe (CH OH) ][Pt (SCN) ]}
6 n
The
is
crystal
similar
of {[Co (CH OH) ][Pt (SCN) ]} was characterised by TG/DTA to
of examine the behaviour of the coordinating methanol (Fig. 6).
3
2
6 n
II
IV
3
2
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II
IV
6 n
]}
are assignable to the oxidation of SCN−
{
Co [Pt (SCN)
ligand.
The stability of {[Co (CH
II
IV
3
OH)
evaluated by soaking {[Co (CH
2
][Pt (SCN)
6
]}
n
in water was
]} in distilled
II
IV
3
OH)
2
][Pt (SCN)
6
n
water for 60 min at room temperature. The XRD pattern of
II
IV
{
[Co (CH
3 2 6 n
OH) ][Pt (SCN) ]} after water treatment clearly indi-
II
IV
cated the conversion to {Co [Pt (SCN)
6 n
]} (Fig. 7). This conver-
sion indicates that the liberation of coordinating methanol
molecules, which are labile as predicted from TG/DTA, results
in the loss of forcedly created potential open metal sites from
II
IV
{
[Co (CH OH) ][Pt (SCN) ]} . On the other hand, the soaking
3 2 6 n
II
IV
of {Co [Pt (SCN)
tion of {[Co (CH OH) ][Pt (SCN) ]} , suggesting that the
6 n
]} in methanol resulted in the nonforma-
II
IV
3
2 6 n
structural change is irreversible. The irreversible structural
change caused by water treatment was also observed for
Fig. 5 Powder X-ray diffraction patterns of thiocyanato-bridged het-
erometallic coordination polymers fabricated in methanol [Simulated
II
IV
{
[Fe (CH
ments (Fig. S4†).
The open metal sites at the Co ions of {[Co (CH
3 2 6 n
OH) ][Pt (SCN) ]} as evidenced by XRD measure-
II
IV
II
IV
patterns of (a) {Mn [Pt (SCN)
6
]}
]}
and (e) {[Cu (CH
n
, (b) {[Fe (CH
3
OH)
2
][Pt (SCN)
of
]}
6
]}
n
and
(d)
II
IV
II
II
(
{
c)
{[Co (CH
3
OH)
2
][Pt (SCN)
6
n
:
observed
patterns
3
2
OH) ]
II
IV
II
IV
IV
[Ni (CH
3
OH)
2
][Pt (SCN) ]}
6
n
3
OH)
2
][Pt (SCN)
6
n
]. The
[Pt (SCN) ]} can be stabilised in water by the partial replace-
6 n
simulated patterns were calculated from the corresponding single
crystal data.
ment of the methanol molecules with more inert ligands.
Hence, a thiocyanato-bridged heterometallic coordination
polymer was synthesised in a methanol solution containing
IV
K [Pt (SCN) ], cobalt nitrate and pyridine. The amount of
II
IV
2
6
6 n
The TG/DTA curve of {Co [Pt (SCN) ]} indicated no signifi-
pyridine was adjusted to one or two equivalents of the
cant weight loss (<1%) under 275 °C, on the other hand, a
II
II
IV
Co
{
ion to obtain {[Co (py)(CH
3
OH)][Pt (SCN)
6
]}
n
and
weight loss of ∼4% assignable to methanol desorption was
II
IV
[Co (py) ][Pt (SCN) ]} , respectively. The number of pyridine
II
IV
2
6 n
3 2 6 n
observed for {[Co (CH OH) ][Pt (SCN) ]} at the same temp-
II
IV
molecules contained in {[Co (py)(CH
3
OH)][Pt (SCN)
was confirmed by the weight losses in
TG/DTA (Fig. 6c and d). The endothermic weight losses of 16%
6 n
]} and
erature range. The weight loss that was smaller than expected
from the chemical formula (10%) resulted from the partial
removal of methanol during drying treatment before sampling,
II
IV
{
2 6 n
[Co (py) ][Pt (SCN) ]}
II
IV
and 23% below 230 °C for [Co (py)(CH
[Co (py) ][Pt (SCN) ]} , respectively, agreed with the total
3
OH)][Pt (SCN)
6 n
]} and
II
indicating that the interaction between Co ion and methanol
II
IV
{
2 6 n
is not strong. Exothermic weight losses starting from ca.
weight of methanol and pyridine molecules in each co-
II
IV
3
3 2 6 n
00 °C observed for both {[Co (CH OH) ][Pt (SCN) ]} and
ordination polymer (17% and 23%, respectively). Similar XRD
II
patterns assure identical structures of {[Co (py)
x
(CH
3
OH)2−x
]
IV
[
Pt (SCN) ]} s (x = 0, 1 or 2) (Fig. 8). Insignificant changes in
6 n
II
IV
the XRD patterns of [Co (py)(CH
3 6 n
OH)][Pt (SCN) ]} after water
treatment indicated that potential open metal sites remained.
Fig. 6 Thermogravimetric analyses (TGA, solid lines) and differential
II
IV
thermal analyses (DTA, dotted lines) of (a) {Co [Pt (SCN)
6 n
(b)
]}
and (d)
in an air flow. The insets of (a) and (b) are the
II
IV
II
IV
{
{
[Co (CH
[Co (py)
3
OH)
2
IV
][Pt (SCN)
6
]}
n
(c) {[Co (py)(CH
3
OH)][Pt (SCN)
6
]}
n
Fig. 7 Powder
X-ray
diffraction
(XRD)
patterns
][Pt (SCN)
of
]}
(a)
after
II
II
IV
II
IV
2
][Pt (SCN)
6
]}
n
{[Co (CH
3
OH)
2
][Pt (SCN)
6
]}
n
,
(b) {[Co (CH
3
OH)
2
6
n
II
IV
expanded views below 250 °C.
being soaked in distilled water and (c) {Co [Pt (SCN)
6
]}
n
.
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phenolate ion (p-NP, λmax = 400 nm) was periodically moni-
tored by UV-visible spectroscopy (Fig. 9b). The highest reaction
−
5
−1
−1
rate of 9.9
×
10
mol
][Pt (SCN) ]}
that of {[Co [Pt (SCN) ]} (9.0 × 10 mol L
L
h
was observed for
II
IV
{
[Co (CH
3
OH)
2
6 n
but was almost comparable to
II
IV
−5
−1 −1
h
), resulting
6
n
II
IV
from an irreversible structural change to {Co [Pt (SCN)
6 n
]} in
the aqueous reaction medium (Fig. 9c). The active sites of
II
IV
II
{
Co [Pt (SCN) ]} would be surface-exposed Co ions tem-
6 n
porally generated by the coordination of p-NPP. The lowest
−
5
−1
−1
reaction rate (5.8 × 10
mol L
h ) was observed for
II
IV
{
[Co (py) ][Pt (SCN) ]} , indicating that the occupation of
2
6 n
open metal sites by the pyridine molecules inhibited the
II
coordination of p-NPP to Co ions. The reaction rate observed
II
IV
−5
−1 −1
for {[Co (CH OH)(py)][Pt (SCN) ]} (7.4 × 10 mol L
h )
3
6 n
II
IV
II
was slightly higher than that for {[Co (py)
2 6 n
][Pt (SCN) ]} ,
Fig. 8 (a) Powder X-ray diffraction (XRD) patterns of (a) {[Co (py)
IV
II
IV
(
CH
3
OH)][Pt (SCN)
6
]}
n
, (b) {[Co (py)(CH
3
OH)][Pt (SCN)
6
]}
2
, (d) {[Co (py) ]
n
after being because the methanol molecules are liberated to generate
II
IV
II
II
IV
soaked in distilled water, (c), {[Co (py)
2
][Pt (SCN)
6
]}
n
open metal sites in {[Co (CH OH)(py)][Pt (SCN) ]} acting as
3
6 n
IV
II
[
Pt (SCN)
6 n
]} after being soaked in distilled water and (e) {[Co (py)
the active sites during the reactions. The lower catalytic activity
IV
(
CH OH)][Pt (SCN) ]} .
3
6 n
II
IV
of {[Co (CH
3
OH)(py)][Pt (SCN)
6
]}
n
compared to those of
II
IV
II
IV
{
[Co (CH OH) ][Pt (SCN) ]} and {[Co [Pt (SCN) ]} resulted
3
2
6
n
6 n
The inert Co–pyridine bonds suppressed the structural change from the strong basicity of pyridine reducing the Lewis acidity
II
of the coordination polymers.
of Co ions, which is important for water polarisation to accel-
−
erate OH formation.
II
IV
Catalysis of {[Co (py)
x
(CH
3
OH)2−x][Pt (SCN)
6
]}
n
(x = 0, 1 or 2)
II
IV
II
IV
Catalysis of {[Co (py) (CH OH) ][Pt (SCN) ]} (x = 0, 1 or 2)
x 3 2−x 6 n
and {Co [Pt (SCN) ]} for photocatalytic water oxidation
and {Co [Pt (SCN) ]} for organophosphate hydrolysis
6
n
II
IV
6
n
The catalytic activity of thiocyanato-bridged heterometallic
coordination polymers containing N-bound Co ions for orga-
nophosphate hydrolysis was measured by dispersing the
coordination polymers (2.5 mg) in a 4-(2-hydroxyethyl)-1-piper-
azineethanesulfonate (HEPES) buffer solution (100 mM, pH
II
The activity of thiocyanato-bridged heterometallic coordi-
nation polymers was also examined for photocatalytic water
8
.2, 750 μL) containing p-nitrophenyl phosphate disodium salt
3
1
(
p-NPP, 25 mM) at 60 °C (Fig. 9a). The formation of p-nitro-
Fig. 9 (a) Chemical equation of the catalytic hydrolysis of p-nitrophenyl
phosphate. (b) UV-vis spectral change during the hydrolysis of
p-nitrophenyl phosphate disodium salt (p-NPP, 25 mM) in HEPES
Fig. 10 (a) Overall catalytic cycle of photocatalytic water oxidation
II
2−
buffer (100 mM, pH 8.2, 750 μL) in the presence of {[Co (CH
3
OH)
2
]
under visible-light irradiation.
Ru(2,2’-bipyridine)
evolution under visible-light irradiation of
solution (2.0 mL, 50 mM, pH 8.0) containing [Ru(bpy)
S
2
O
8
3
: persulphate ion; [Ru(bpy) ]:
IV
[
Pt (SCN)
6
]}
n
(2.5 mg) at 60 °C. The peak at 315 nm assigned to p-NPP
3
, WOC: water oxidation catalyst. (b) Time courses of
phosphate buffer
]SO (1.0 mM),
decayed with the growth of the peak at 400 nm showing the formation
of p-nitrophenolate ion (p-NP). Each spectrum was taken at a certain
reaction time. (c) Time profiles of the p-NP formation in the presence or
O
2
a
3
4
IV
II
Na
2
II
S
2
O
8
IV
(5.0 mM) in the presence of {[Co (CH
3
OH)
2 6 n
][Pt (SCN) ]} ,
II
IV
II
IV
II
II
IV
II
absence of {[Co (CH
3
OH)
2
][Pt (SCN)
6
]}
n
IV
, {Co [Pt (SCN)
6
]}
n
, {[Co (py)
{Co [Pt (SCN)
6
]}
n
,
{[Co (py)(CH
3
OH)][Pt (SCN)
6
]}
n
and {[Co (py) ]
2
IV
II
IV
(
CH
3
OH)][Pt (SCN)
6
]}
n
and {[Co (py)
2
] [Pt (SCN)
6
]}
n
.
6 n
[Pt (SCN) ]} (1.0 mg).
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Dalton Transactions
oxidation. The catalytic reaction was performed by photoirra-
diation (λ > 400 nm) of a phosphate buffer solution (2.0 mL,
Notes and references
5
0 mM, pH 8.0) containing the coordination polymer (1.0 mg),
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2
+
3
tris(2,2′-bipyridine)ruthenium ion ([Ru(bpy) ] , 1.0 mM) and
persulphate ion (S O , 5.0 mM) as a photosensitiser and a
sacrificial electron acceptor, respectively (Fig. 10a).
2
−
2
8
3
5
The
amounts of O evolved observed for the reaction solution con-
2
3 L. Jiao, Y. Wang, H. L. Jiang and Q. Xu, Adv. Mater., 2018,
30, 1703663.
II
IV
II
IV
taining {Co [Pt (SCN) ]} and {Co (CH OH) [Pt (SCN) ]}
6 n
6
n
3
2
were almost comparable (2.6 and 2.7 μmol for 10 min, respect-
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II
ively) because of the structural change of {[Co (CH
3
OH)
2
]
IV
II
IV
[
Pt (SCN) ]} to {Co [Pt (SCN) ]} in the aqueous reaction
6
n
6 n
II
IV
medium (Fig. 10b). {[Co (py)(CH
3
OH)][Pt (SCN)
showed a lower catalytic activity for
the photocatalytic water oxidation (ca. 1 μmol for 10 min) com-
6
]}
n
and
II
IV
{
[Co (py)
2
] [Pt (SCN)
6
]}
n
II
IV
pared to {[Co (CH
3
OH)
2
][Pt (SCN)
6
]}
n
. The coordination of
the pyridine molecules resulted in the decrease of oxidation
II
IV
potential of the Co ions in {[Co (py)(CH OH)][Pt (SCN) ]}
6 n
3
II
IV
2 6 n
and {[Co (py) ][Pt (SCN) ]} . These results indicate that not
only structure but also the electronic properties of the active
sites, Co ions, are important for heterogeneous catalysis.
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II
8
H. Yamashita, K. Mori, Y. Kuwahara, T. Kamegawa,
M. Wen, P. Verma and M. Che, Chem. Soc. Rev., 2018, 47,
8
072–8096.
Conclusion
9
Y. B. Huang, J. Liang, X. S. Wang and R. Cao, Chem. Soc.
Rev., 2017, 46, 126–157.
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A series of thiocyanato-bridged heterometallic coordination
polymers without and with potential open metal sites,
1
II
IV
II
IV
{
6 n 3 2 6 n
M [Pt (SCN) ]} and {[M (CH OH) ][Pt (SCN) ]} (M = Mn,
Co, Fe, Ni or Cu), were successfully synthesised and character-
ised by single crystal X-ray structure analysis. The stability of
the structure with potential open metal sites was enhanced by
the partial replacement of the coordinating methanol with the
more basic pyridine although the pyridine coordination
reduced the catalytic activity for both hydrolysis and oxidation
reactions due to the electronic effect on the active sites. These
results indicate that the coordination of an extra-ligand is
effective for the intentional creation of open metal sites,
however, the electronic tuning effect should be carefully con-
sidered for catalysis enhancement. The investigation of the
ligands suitable for stabilizing a coordinatively unsaturated
structure without loss of catalytic activity will be reported in
future communications.
8
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Acknowledgements
This work was supported by Innovative Science and 19 W. Q. Sun, J. Tong, H. L. Lu, T. T. Ma, H. W. Ma and
Technology Initiative for Security, ATLA, Japan, to Y. Y. (No. S. Y. Yu, Chem. – Asian J., 2018, 13, 1108–1113.
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to H. T. (No. JP17H07020 and JP19K15591); and Koyanagi
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17068 | Dalton Trans., 2019, 48, 17063–17069
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Dalton Trans., 2019, 48, 17063–17069 | 17069