Cerium(IV)-driven water oxidation catalyzed by a manganese(V)-nitrido complex

Article abstract of DOI:10.1002/anie.201500507

The study of manganese complexes as water-oxidation catalysts (WOCs) is of great interest because they can serve as models for the oxygen-evolving complex of photosystem II. In most of the reported Mn-based WOCs, manganese exists in the oxidation states III or IV, and the catalysts generally give low turnovers, especially with one-electron oxidants such as Ce^IV. Now, a different class of Mn-based catalysts, namely manganese(V)-nitrido complexes, were explored. The complex [Mn^V(N)(CN)₄]^2- turned out to be an active homogeneous WOC using (NH₄)₂[Ce(NO₃)₆] as the terminal oxidant, with a turnover number of higher than 180 and a maximum turnover frequency of 6 min^-1. The study suggests that active WOCs may be constructed based on the Mn^V(N) platform. High turnover: The study of manganese complexes as catalysts for the oxidation of water is of great interest as they can serve as models for the oxygen-evolving complex of photosystem II. The manganese(V)-nitrido complex [Mn(N)(CN)₄]^2- was now shown to catalyze the oxidation of water by cerium(IV) with a turnover number (TON) of higher than 180.

Full text of DOI:10.1002/anie.201500507

Angewandte
Chemie
DOI: 10.1002/anie.201500507
Water Oxidation
Cerium(IV)-Driven Water Oxidation Catalyzed by
a Manganese(V)–Nitrido Complex**
Li Ma, Qian Wang, Wai-Lun Man, Hoi-Ki Kwong, Chi-Chiu Ko, and Tai-Chu Lau*
Abstract: The study of manganese complexes as water-
oxidation catalysts (WOCs) is of great interest because they
can serve as models for the oxygen-evolving complex of
photosystem II. In most of the reported Mn-based WOCs,
manganese exists in the oxidation states III or IV, and the
catalysts generally give low turnovers, especially with one-
catalyst for the oxidation of alkanes, alkenes, and alcohols
[
15,16]
using H O or CAN as the oxidant.
Based on experi-
2
2
mental results and DFT calculations we proposed that the
VII
active intermediate is
a
Mn
nitrido oxo species,
VII
2ꢀ
[Mn (N)(O)(CN) ] , which is generated through O-atom
4
V
transfer from H O to Mn , or by proton-coupled electron
2
2
IV
V
IV
V
IV
electron oxidants such as Ce . Now, a different class of Mn-
transfer between Mn and Ce (Mn -OH2 + 2Ce !
VII III + [16]
based catalysts, namely manganese(V)–nitrido complexes,
Mn =O + 2Ce + 2H ). However, when 2 was dissolved
V
2ꢀ
were explored. The complex [Mn (N)(CN) ] turned out to
in H O/CH CN without any organic substrate, O evolution
4
2
3
2
be an active homogeneous WOC using (NH ) [Ce(NO ) ] as
was readily observed upon the addition of CAN. Most water-
oxidation experiments were carried out using the water-
soluble complex 1 as the catalyst, which is predominantly
present as the 5-coordinate [Mn(N)(CN)4] ion in water
(Supporting Information), and it gave a TON that was around
4
2
3 6
the terminal oxidant, with a turnover number of higher than
ꢀ1
1
80 and a maximum turnover frequency of 6 min . The study
2
ꢀ
suggests that active WOCs may be constructed based on the
V
Mn (N) platform.
2
0% higher than that of 2 (Table 1, entries 6 and 8).
T
years.
he search for efficient catalysts for the oxidation of water
Table 1 shows the TONs for the oxidation of water by
[
17]
has been an important challenge for chemists in recent
catalyst 1 and 2 under various conditions.
The evolved
[1–3]
Among the various transition-metal-based water-
oxygen was detected by GC-TCD (TCD = thermal conduc-
oxidation catalysts (WOCs), those based on manganese have
received special attention because they can function as
models for the catalytic Mn CaO center in the oxygen-
V
[a]
Table 1: Water oxidation by Mn (N)/CAN under various conditions.
4
5
[
b]
[4–6]
Entry
Cat./Conc.
O [mmol]
2
TON
evolving complex (OEC) of photosystem II (PSII).
A
[
c]
variety of dimanganese and tetramanganese complexes were
1
2
3
4
5
6
7
8
9
1/20 mm
1/20 mm
1/10 mm
1/5 mm
0.96
0.96
0.56
0.48
0.34
0.52
0.73
0.42
0.44
6
6
7
[
7,8]
[d]
designed as models of the OEC in recent years.
However,
few manganese complexes are efficient WOCs, especially
3
+
IV [9–13]
12
42
130
183
105
110
with one-electron oxidants such as [Ru(bpy)3] or Ce .
So far, in most of the reported Mn-based WOCs, the Mn
center exists in oxidation states III or IV. In search for
more active catalysts, we explored a different class of Mn
catalysts, namely manganese(V)–nitrido complexes. We
report herein that the manganese(V)–nitrido compounds
1/1 mm
1/0.5 mm
1/0.5 mm
2/0.5 mm
2/0.5 mm
[e]
[f]
[
a] Reaction conditions: [CAN]=125 mm, T=258C, t=30 min. For 1,
solvent was 8 mL 0.1m HNO . For 2, the catalyst was dissolved in
[
N(CH ) ] Na[Mn(N)(CN) ] (1) and (PPh ) [Mn(N)(CN) ]
3 4 2 5 4 2 4
3
(
2) are highly active homogeneous WOCs when (NH ) [Ce-
0.02 mL CH CN and added to 7.98 mL 0.1m HNO . [b] TON=(mol of
3 3
4
2
(
NO ) ] (CAN) is used as the terminal oxidant, with
O after subtracting the blank)/(mol of catalyst). 0.04 mmol of O was
2 2
3
6
produced in the absence of catalyst. O was determined by GC-TCD.
a turnover number (TON) of higher than 180.
The six-coordinate complex 1 and the square-pyramidal
complex 2 used in this work were prepared by Wieghardt and
2
Each value is the average of at least three replicates. Error ꢁ5%. [c] 93%
of CAN (116 mm) remained after the catalysis, as determined by
iodometric titration. [d] Pure water was used as solvent. [e] The catalyst
was slowly added by syringe pump over 40 min. [f] 250 mm of CAN was
used.
[14]
co-workers. We recently reported that 2 is a highly efficient
[*] L. Ma, Q. Wang, Dr. W.-L. Man, Dr. H.-K. Kwong, Dr. C.-C. Ko,
Prof. T.-C. Lau
Department of Biology and Chemistry and Institute of Molecular
Functional Materials, City University of Hong Kong
Tat Chee Avenue, Kowloon Tong, Hong Kong (China)
E-mail: bhtclau@cityu.edu.hk
tivity detector). A TON of 6 was observed when 1 and CAN
were used in concentrations of 20 mm and 125 mm, respec-
tively. The TON increased upon lowering the concentration of
the catalyst, and a TON of 130 could be achieved using 0.5 mm
[
**] The work described in this paper was supported by Hong Kong
University Grants Committee (AoE/P-03-08), the Research Grants
Council of Hong Kong (CityU 101612), and the Shenzhen Science
and Technology Research Grant (JCYJ20120613115247045).
ꢀ
1
of 1, with a maximum turnover frequency (TOF) of 6 min
Figure S1b). The TON further increased to 183 when
a solution of 1 in 0.1m HNO was slowly added to CAN
(
3
using a syringe pump. The amount of O that evolved also
increased with the concentration of CAN (Figure S1a). When
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500507.
2
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1
8
18
VI/V
the reaction was carried out in H O (98% O-labeled), the
60 mV), which we assigned to the Mn
couple. This couple
2
1
8
evolved oxygen was 97% O-labeled, as monitored by GC-
MS.
The oxygen evolution from the 1/CAN system was also
continuously monitored by using a Clark-type electrode
is independent of the pH value in the range of 1 to 7 (Figure 2,
and Figures S3–S8 in the Supporting Information). On further
scanning to a more positive potential, a catalytic current is
VII
observed, which is consistent with further oxidation to a Mn
(
Figure 1). The amount of O2 increased with [1] and the
species that oxidizes water on the CV time scale.
initial rates depended linearly on [1], with a first-order rate
constant of (1.18 ꢁ 0.04) ꢀ 10ꢀ s .
2
ꢀ1
Figure 2. Cyclic voltammogram of 1 in 0.1m HNO . Scan
3
ꢀ1
rate=50 mVs . SCE=saturated calomel electrode.
VII
Although attempts to detect an active Mn intermediate
from 1/CAN in water by ESI/MS were unsuccessful, we have
VII
2ꢀ
previously been able to observe [Mn (N)(O)(CN)4] from
[
16]
the reaction of 2 with H O in CF CH OH by ESI/MS.
2
2
3
2
We also monitored the reaction of 1 with CAN by UV-Vis
spectrophotometry. The reaction of 1 with 1 equiv of CAN
immediately generated a species with lmax at 435 nm, which
gradually decayed with t1/2 of approximately 30 s at 258C
Figure 1. a) Plots of O evolution versus time for water oxidation
catalyzed by 1 at various concentrations using 125 mm Ce as oxidant
VI
ꢀ
2
(Figure S9a). This species was assigned as [Mn (N)(CN) ]
4
IV
VI
ꢀ
(
or [Mn (N)(CN) (OH )] ). On the other hand, when
4 2
in 0.1m HNO . b) Plot of initial rate of O evolution versus the
3
2
2
equiv of CAN were added, a species with lmax at 409 nm
concentration of Mn ([Mn]).
was immediately observed (Figure S9b), which may be
VII
2ꢀ
[
Mn (N)(O)(CN) ] . This species also decayed with a t
4 1/2
In the catalytic oxidation of water, metal complexes may
function only as precatalysts that are converted to metal
of around 30 s.
The observed increase in the TON with a decreasing
concentration of the catalyst (Table 1) suggests that the Mn
catalyst is deactivated through a bimolecular pathway. Nitrido
[
18]
oxides as the real catalysts. In order to determine whether
2
ꢀ
the catalytic oxidation by [Mn(N)(CN)4] is homogeneous or
heterogeneous, dynamic light scattering (DLS) experiments
were performed to detect any particles formed during the
oxidation of water. As shown in Figure S2 (Supporting
Information), no particles were detected after 30 min upon
[
20]
[21]
complexes such as those of ruthenium and osmium are
known to undergo an N···N coupling reaction, that is,
n
nꢀ3
2M ꢂN!2M + N . In order to detect any stoichiometric
2
production of N , we used a relatively high concentration of
2
adding 1 (30 or 100 mm in 0.1m HNO ) to CAN (125 mm) in
1 (2.5 mm) and treated it with 125 mm of CAN in 0.1m HNO3
3
0
.1m HNO , which suggests that the catalysis is homogeneous.
(8 mL). Under these conditions, we could detect N in 85%
3
2
The DLS instrument we used can detect MnO particles down
to a concentration of 0.1 mm (Figure S2d). Hence for 100 mm
of 1, we would be able to detect particles even if only 0.1% of
yield (8.5 mmol), together with O (23.4 mmol) using GC-
2
2
1
5
TCD. When we used N-labelled 2, the N that evolved was
2
1
5
exclusively N labeled (Figure S13), thus confirming the
1
has decomposed to MnO . Moreover, although MnO in
N···N coupling of a Mn(N) species formed between 1 and
x
x
[19]
VII
2ꢀ
various forms are known to be WOCs, TONs (mol of O2/
mol of Mn) are in general smaller than 1 per hour at room
temperature, whereas 1 can reach a TON of 180 within
CAN, presumably cis-[Mn (N)(O)(CN) ] . The product of
4
this coupling reaction could be isolated as a pale orange
precipitate upon addition of 2 equiv of CAN to an aqueous
solution of 1. The solid is insoluble in water and common
organic solvents. Elemental analysis is consistent with the
formula Mn(CN) ·2H O (3). The IR spectroscopic analysis
3
0 min. Based on these results, we conclude that 1 functions
as homogeneous, molecular catalyst for the oxidation of water
by CAN.
3
2
ꢀ1
The cyclic voltammogram of 1 in 0.1m HNO showed
showed a single n(CN) band at 2183 cm . The observed
magnetic moment of meff = 4.91 BM (Gouy method) is con-
3
a reversible electron transfer at 1.18 V versus SCE (DE =
2
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Angewandte
Chemie
III
sistent with high-spin Mn (S = 2). These results suggest that
In
conclusion,
we
have
demonstrated
that
III
V
2ꢀ
compound 3 is probably a 3D cyano-bridged Mn coordina-
[Mn (N)(CN) ] is an active molecular water-oxidation
4
tion polymer.
catalyst using CAN as the terminal oxidant, with a TON of
Compound 3 readily redissolved upon adding CAN in
higher than 180, which is more than 10 times higher than
II/III
0
.1m HNO , and oxygen evolved, which was monitored by
reported Mn
catalysts and various forms of MnO . We
x
3
a Clark electrode (Figure S14). However, it is a much less
efficient catalyst than 1, with a maximum TON of 2.4.
Based on our experimental results, Scheme 1 shows the
proposed water oxidation and catalyst deactivation reactions
anticipate that a new class of robust Mn-based catalysts for
the oxidation of water may be constructed based on Mn (N)
V
platform, for example by using bulky multidentate ancillary
ligands to prevent N···N coupling and ligand dissociation, and/
[
22]
V
2ꢀ
V
that occur upon adding 1 to CAN. [Mn (N)(CN) ] (which
or by entrapping the Mn (N) catalyst into a metal–organic
4
[12]
framework.
Received: January 19, 2015
Published online: && &&, &&&&
Keywords: cerium · homogeneous catalysis ·
.
manganese complexes · nitrides · oxidation
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2ꢀ
[
VII
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[
[
by UV-Vis spectrophotometry shows the gradual appearance
ꢀ
of signals that are characteristic for MnO (Figure S15). In
4
IV
IV
the absence of excess Ce , the Mn (O) species may
III
decompose to a [Mn (CN) ] polymer. According to this
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should be the N···N coupling of the active Mn (N)(O)
species, and the coupling product eventually decomposes to
3
n
VII
[
ꢀ
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[
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3
to give MnO or a cyano-bridged oligomer as the real catalyst,
x
they are stable for days in these solutions. The stock solutions
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the catalytic water oxidation is usually complete within 20 min,
3
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Angewandte
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Communications
Water Oxidation
L. Ma, Q. Wang, W.-L. Man, H.-K. Kwong,
C.-C. Ko, T.-C. Lau*
&&&& — &&&&
Cerium(IV)-Driven Water Oxidation
Catalyzed by a Manganese(V)–Nitrido
Complex
2
ꢀ
High turnover: The study of manganese
complexes as catalysts for the oxidation
of water is of great interest as they can
serve as models for the oxygen-evolving
complex of photosystem II. The manga-
nese(V)–nitrido complex [Mn(N)(CN)4]
was now shown to catalyze the oxidation
of water by cerium(IV) with a turnover
number (TON) of higher than 180.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!