Water oxidation catalysed by iron complex of N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane. Spectroscopy of iron-oxo intermediates and density functional theory calculations

Article abstract of DOI:10.1039/c5sc01680k

The macrocyclic [Fe^III(L1)Cl₂]⁺ (1, L1 = N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane) complex is an active catalyst for the oxidation of water to oxygen using [NH₄]₂[Ce^IV(NO₃)₆] (CAN), NaIO₄, or Oxone as the oxidant. The mechanism of 1-catalysed water oxidation was examined by spectroscopic methods and by ¹⁸O-labelling experiments, revealing that Fe^IVO and/or Fe^VO species are likely to be involved in the reaction. The redox behaviour of 1 and these high-valent FeO species of L1 has been examined by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the cyclic voltammograms of 1 at different pH show a pH-dependent reversible couple (E_1/2 = +0.46 V vs. SCE at pH 1) and an irreversible anodic wave (E_pa = +1.18 V vs. SCE at pH 1) assigned to the Fe^III/Fe^II couple and the Fe^III to Fe^IV oxidation, respectively. DFT calculations showed that the E value of the half reaction involving [Fe^V(L1)(O)(OH)]²⁺/[Fe^IV(L1)(O)(OH₂)]²⁺ is +1.42 V vs. SCE at pH 1. Using CAN as the oxidant at pH 1, the formation of an Fe^IVO reaction intermediate was suggested by ESI-MS and UV-vis absorption spectroscopic measurements, and the rate of oxygen evolution was linearly dependent on the concentrations of both 1 and CAN. Using NaIO₄ or Oxone as the oxidant at pH 1, the rate of oxygen evolution was linearly dependent on the concentration of 1, and a reactive Fe^VO species with formula [Fe^V(L1)(O)₂]⁺ generated by oxidation with NaIO₄ or Oxone was suggested by ESI-MS measurements. DFT calculations revealed that [Fe^V(L1)(O)₂]⁺ is capable of oxidizing water to oxygen with a reaction barrier of 15.7 kcal mol^-1.

Full text of DOI:10.1039/c5sc01680k

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Water oxidation catalysed by iron complex of N,N -
dimethyl-2,11-diaza[3,3](2,6)pyridinophane.
Spectroscopy of iron–oxo intermediates and
density functional theory calculations†
Cite this: DOI: 10.1039/c5sc01680k
a
a
a
a
Wai-Pong To, Toby Wai-Shan Chow, Chun-Wai Tse, Xiangguo Guan,*
a
ab
Jie-Sheng Huang and Chi-Ming Che*
III
+
0
The macrocyclic [Fe (L1)Cl
2
]
(1, L1 ¼ N,N -dimethyl-2,11-diaza[3,3](2,6)pyridinophane) complex is an
IV
active catalyst for the oxidation of water to oxygen using [NH
4
]
2
[Ce (NO
3
)
6
] (CAN), NaIO
4
, or Oxone as
the oxidant. The mechanism of 1-catalysed water oxidation was examined by spectroscopic methods
18
IV
V
and by O-labelling experiments, revealing that Fe ]O and/or Fe ]O species are likely to be involved
in the reaction. The redox behaviour of 1 and these high-valent Fe]O species of L1 has been examined
by both cyclic voltammetry and density functional theory (DFT) calculations. In aqueous solutions, the
cyclic voltammograms of 1 at different pH show a pH-dependent reversible couple (E1/2 ¼ +0.46 V vs.
III
II
SCE at pH 1) and an irreversible anodic wave (Epa ¼ +1.18 V vs. SCE at pH 1) assigned to the Fe /Fe
III
IV
couple and the Fe to Fe oxidation, respectively. DFT calculations showed that the E value of the half
V
2+
IV
2+
2
reaction involving [Fe (L1)(O)(OH)] /[Fe (L1)(O)(OH )] is +1.42 V vs. SCE at pH 1. Using CAN as the
IV
oxidant at pH 1, the formation of an Fe ]O reaction intermediate was suggested by ESI-MS and UV-vis
absorption spectroscopic measurements, and the rate of oxygen evolution was linearly dependent on
the concentrations of both 1 and CAN. Using NaIO
4
or Oxone as the oxidant at pH 1, the rate of oxygen
Received 8th May 2015
Accepted 23rd June 2015
V
evolution was linearly dependent on the concentration of 1, and a reactive Fe ]O species with formula
V
+
[
Fe (L1)(O)
2
]
generated by oxidation with NaIO
4
or Oxone was suggested by ESI-MS measurements.
DOI: 10.1039/c5sc01680k
V
+
DFT calculations revealed that [Fe (L1)(O)
2
]
is capable of oxidizing water to oxygen with a reaction
ꢀ
1
www.rsc.org/chemicalscience
barrier of 15.7 kcal mol .
metals such as manganese, iron and cobalt bearing chelating N
and/or O ligands, have been reported for water oxidation.
Introduction
2
Water oxidation is an energetically uphill reaction (E ¼ +1.23 V Mechanistic studies revealed that the availability of labile
vs. NHE at pH 0) involving the simultaneous removal of four coordination site(s) for the formation of metal–oxo species is
electrons and four protons from two water molecules to give one crucial for a metal complex to be used to catalyse water oxida-
1
2
oxygen molecule. Due to its importance, there have been tion. To support the formation of reactive/oxidizing metal–oxo
tremendous efforts dedicated to the design of metal catalysts for species, ligands that bind strongly to metal ions and are resis-
2
water oxidation over the past decades. It is envisioned that tant to oxidation are highly desirable. In this regard, macrocy-
mechanistic insights into the fundamental steps of this clic N-donor ligands are appealing, as highly oxidizing metal–
important reaction can be obtained by using structurally oxo complexes of both ruthenium and iron have been isolated
dened molecular catalysts.
and structurally characterized by using macrocyclic tertiary
In the literature, many examples of molecular catalysts, amine ligands such as 14-TMC (1,4,8,11-tetramethyl-1,4,8,11-
3
particularly polypyridyl ruthenium(II) and organometallic iri- tetraazacyclotetradecane).
dium(III) complexes, and also complexes of 1 -row transition
st
Because of the high earth abundance of iron and the recent
impressive advances made in the isolation and characterization
of non-heme iron–oxo complexes, notably by Nam, Que, and co-
workers, there has been increasing interest in the develop-
ment of new iron-based oxidation chemistry, including the
a
Department of Chemistry and State Key Laboratory of Synthetic Chemistry, The
4,5
University of Hong Kong, Pokfulam Road, Hong Kong, China. E-mail: xgguan@hku.
hk; cmche@hku.hk
2
b
HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, China
oxidation of water. Since the rst report by Bernhard, Collins,
6
a
†
Electronic supplementary information (ESI) available: Experimental section, and co-workers in 2010, several types of water oxidation
Table S1, Fig. S1–S34, Scheme S1 and computational details. See DOI:
reactions catalysed by mononuclear iron complexes have been
1
0.1039/c5sc01680k
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reported, including chemical oxidation with CAN (cerium
IV
6
ammonium nitrate [NH ] [Ce (NO ) ]) or NaIO , photochem-
4
2
3 6
4
8
6c,e,7
ical oxidation,
electrocatalytic oxidation, and photo-
electrochemical oxidation. Dinuclear iron complexes that can
9
10
catalyse water oxidation with CAN or NaIO
4
,
or can oxidize
11
water to the hydroxyl radical, are also documented.
Mononuclear iron catalysts for water oxidation are sup-
6
a,7,8b
ported by tetraanionic tetraamide ligands (TAML)
or
8a
monoanionic pentadentate N
5
ligands, or bear neutral
6b–g,8c
Me2
chelating N ligands,
Fig. 1), which allow the formation of mononuclear metal
complexes with two cis labile sites (the abovementioned 14-TMC
ligand forms trans complexes such as trans-[Fe(14-TMC)(OTf)
such as mcp,
Pytacn, tpa, and bqen
(
2
]
which was found to be unreactive for catalytic water oxidation Fig. 1 Examples of N ligands in iron complexes used as water oxida-
II
V
with CAN or NaIO
NaIO ’ system reported by Costas, Lloret-Fillol, and co-workers
formed oxygen with a turnover number (TON) of up to >1000 at
4
(ref. 6b)). Notably, the ‘[Fe (mcp)(OTf)
2
] + tion catalysts. The corresponding proposed Fe ]O species are indi-
ꢀ
cated. X in II stands for solvent, OH or another ligand.
4
6
b
IV
pH 2. Oxoiron(IV) (Fe ]O) intermediates supported by
V
2+
Me2
6b,f
6e
2 2
of alkenes by H O via a reactive [Os (L1)(O)(OH)] inter-
mediate, and [Pd (L1)(Me) ], which reacts with oxygen to
generate [Pd (L1)(Me) (OOc)] followed by protonation to give
Pytacn and bqen, generated from reaction of the corre-
1
6c
II
sponding iron catalysts with CAN in aqueous solution, were
detected by UV-vis spectroscopy and electrospray-ionization
mass spectrometry (ESI-MS); it was proposed that
)] may be involved in O–O bond for-
mation, and density functional theory (DFT) calculations on
the Fe – Pytacn system favoured Fe ]O reactive inter-
mediates. A recent report demonstrated the involvement of
O]Fe –O–Ce species in the water oxidation with CAN cata-
lysed by [Fe (mcp)(OTf) ]. In most cases, oxoiron(V) (Fe ]O)
2
III
2
IV
+
16b
[
Pd (L1)(Me)
2
(OOH)] .
Hydrogen peroxide disproportion-
2
+
IV
2+
ation catalysed by [Mn(L1)(H O) ]
and electrochemical
[
Fe (bqen)(O)(OH
2
2
2
0
2+
0
6e
oxidation of water catalysed by [Mn(L1 )(H O) ] (L1 ¼ the
2
2
t
17
II Me2
IV
N- Bu counterpart of L1) have been reported as well.
III
+
12b
2
In the present work, we report the use of [Fe (L1)Cl ] (1) for
water oxidation with Oxone, as well as CAN and NaIO
mild conditions, together with mechanistic studies by means of
high-resolution ESI-MS, UV-vis absorption spectroscopy, O-
labelling experiments, kinetic studies, cyclic voltammetry, EPR
analysis, and DFT calculations. During the course of this study,
Sun and co-workers communicated their ndings on water
IV
IV
4
, under
II
13
V
2
1
8
species are proposed to be the active intermediates directly
responsible for O–O bond formation in water oxidation reac-
6a,b,f,7,8a
tions,
as revealed by DFT calculations on water oxidation
III
12a,d
II Me2
12e,f
with CAN catalysed by the Fe –TAML,
Fe – Pytacn,
II
2+ 6g
II
12f
V
2
oxidation with CAN catalysed by [Fe (L1)(MeCN) ] . Detailed
and Fe –mep systems. Several non-heme Fe ]O species,
mechanistic studies on water oxidation catalysed by Fe–L1
systems have not been reported in the literature. Also, Oxone
has not been used as an oxidant in previously reported water
which were generated by oxidation with peracids, H O or
BuOOH in organic solvents, have been reported in the
2
2
t
1
4
V
ꢀ
14a,j
literature. Notable examples include [Fe (TAML)(O)] ,
6,10,11,13
V Me2
2+ 14d
V
3+
oxidation reactions catalysed by iron complexes,
despite
[
Fe ( Pytacn)(O)(OH)] , and [Fe (L)(O)(S)] (L ¼ tpa, mep;
14c
V
ꢀ
literature reports on the manganese-catalysed oxidation of
S ¼ H
2
O or MeCN). Recently, [Fe (TAML)(O)] (I, Fig. 1)
2c,d
III
ꢀ
water with Oxone. The experimental studies and DFT calcu-
generated by the photochemical reaction of [Fe (TAML)(H
2
O)]
’ in 50% MeCN–borate buffer
mixture, was reported to be an active intermediate in the
V
II
2+
lations in the present work point to the generation of an Fe ]O
with ‘[Ru (bipy)
3
]
2 2 8
+ Na S O
species responsible for O–O bond formation in water oxidation
catalysed by Fe–L1 systems.
III
ꢀ
7
[
Fe (TAML)(H O)] -catalysed photochemical water oxidation.
The proposed Fe ]O intermediates with neutral chelating N
ligands (II, Fig. 1) remain elusive for water oxidation reactions.
N,N -Dimethyl-2,11-diaza[3,3](2,6)pyridinophane (L1, Fig. 1),
a neutral macrocyclic N ligand, is well known to form iron
complexes in the cis-conguration, including [Fe (L1)Cl
2
V
Results
0
15a
Water oxidation catalysed by complex 1
At the outset, a series of iron complexes bearing N , N and N
5
4
III
+
15b
2
] (1).
3
4
III
0 0 00
In 2010, we reported that [Fe (L1)Cl
cient catalyst for the cis-dihydroxylation of alkenes using Oxone terpyridine, L1, and 2,2 :6 ,2 :6 ,2 :6 ,2 -quinquepyridine)
potassium peroxymonosulfate, 2KHSO $KHSO $K SO ) as the were screened for catalytic water oxidation with CAN as oxidant
2 4 4
][FeCl ] (1$FeCl ) is an effi- ligands (e.g. 1,4,7-trimethyl-1,4,7-triazacyclononane, 2,2 :6 :2 -
0
0
00 00 000 000 0000
(
5
4
2
4
V
+
oxidant, and the generation of an [Fe (L1)(O) ] intermediate in under different reaction conditions (Table S1 in the ESI†). We
2
III
the reaction was inferred from high resolution ESI-MS analysis found that [Fe (L1)Cl ][FeCl ] (1$FeCl ) exhibited the best
2
4
4
16a
and DFT calculations.
This prompted us to examine the performance, affording oxygen with TONs of up to 41 and 32 in
catalytic activity of 1 toward water oxidation. It is noted that 0.1 M HNO and in pure water, respectively, aer 30 minutes
3
ligand L1 has also been employed for developing the oxidation with CAN (840 equiv.) as oxidant (Table 1, entries 1d, 1e). With
4 3
chemistry of other transition metal complexes, including NaIO as oxidant, the reaction in 0.1 M HNO afforded oxygen
III
2+
[
Os (L1)(OH)(OH
2
)] , which can catalyse the cis-dihydroxylation with TON of 12, whereas oxygen evolved with TON of only 3
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from the reaction in pure water (Table 1, entries 2a and 2b). Kinetic studies
With Oxone as oxidant, under the conditions of 100 mM 1 and
The time courses of 1-catalysed water oxidation with CAN in 0.1
M HNO using different concentrations of CAN (12.5–125 mM)
8
8
4 mM Oxone in 0.1 M HNO , oxygen was produced with TON of
9 in 30 minutes (Table 1, entry 3a); lowering the concentration
3
3
and 1 (6.25–25 mM) were examined; the plots of oxygen evolu-
tion (measured by GC) over time are depicted in Fig. S1 and S2
in the ESI.† A linear dependence of the initial rate of oxygen
evolution on both concentration of CAN and concentration of 1
was observed, as depicted in Fig. 2.
of 1 to 12.5 mM increased the TON to 113 (Table 1, entry 3b), a
value higher than those obtained using CAN or NaIO as oxidant
TONs of 93 and 44 respectively, entries 1f and 2c in Table 1).
Control experiments using [Et N][FeCl ] instead of 1$FeCl as
4
(
4
4
4
the catalyst did not produce detectable amounts of oxygen for
the reactions with CAN and NaIO , and afforded oxygen with
TON of only 1.2 for the reaction with Oxone, revealing that 1 is
the key species accounting for the catalytic activity of 1$FeCl4.
For 1-catalysed water oxidation with NaIO , the time course
4
4
(0–60 min) plots of oxygen evolution at different concentrations
of NaIO (12.5–125 mM) and 1 (6.25–25 mM) in 0.1 M HNO are
4
3
shown in Fig. S3 and S4,† respectively. In this case, the initial
rate of oxygen evolution showed a linear dependence on the
With 1$ClO
lysed by 1$FeCl
mM CAN as oxidant, 11 vs. 12 with 84 mM NaIO
4
as catalyst, similar oxygen evolution to that cata-
was observed (e.g. TON of O : 40 vs. 41 with 84
as oxidant, and
1 vs. 89 with 84 mM Oxone as oxidant for reactions using
00 mM catalyst in 0.1 M HNO ).
The origin of the oxygen gas produced from the reaction
mixture of 1-catalysed water oxidation was investigated by using
4
2
concentration of 1 but not on the concentration of NaIO
Fig. S5†).
4
(see
4
9
1
We then examined the time courses of oxygen evolution
during water oxidation with Oxone (37.5–125 mM) catalysed by
3
1
(6.25–25 mM) in 0.1 M HNO (Fig. S6 and S7†). Again, a linear
3
correlation between the initial rate of oxygen evolution and the
concentration of 1 was observed, whereas the initial rate was not
signicantly dependent on Oxone concentration (Fig. S8†),
which is analogous to the ndings obtained with NaIO as
4
oxidant.
1
6
18
18
2
a mixture of H O and H2 O (v/v ¼ 1 : 1; the H O used had 97
2
1
8
atom% O) as the medium and by analysing the gaseous
product(s) using GC-MS. With CAN as oxidant, the gaseous
products obtained in the rst 5 minutes were a mixture of
1
6
16 18 18
2 2
O / O O/ O with a ratio of 30.0 : 49.6 : 20.4, which is close
to the theoretical value of 26.5 : 50.0 : 23.5 (ref. 6b) expected for
High-resolution ESI-MS analysis
oxygen gas derived from water. In the case of NaIO as oxidant,
4
1
8
18
similar O-labelling studies using H O were not performed,
because the oxygen atoms of IO undergo rapid exchange with
those of water. However, the exchange of oxygen atoms
between Oxone and water is sufficiently slow for useful O-
2
In our previous work, analysis of a reaction mixture of [Fe(L1)
ꢀ
+
4
Br2] and Oxone (8 equiv.) in MeCN–H O (5 : 1 v/v) by high-
2
1
8
resolution ESI-MS revealed a cluster peak at m/z 356.0981,
1
8
V
+
16a
attributed to [Fe (L1)(O)
the reaction intermediates in the oxidation reactions of 1 with
CAN and NaIO in aqueous solution, we performed ESI-MS
2
] . In this work, in order to detect
1
9
labelling studies. Water oxidation with Oxone (84 mM) in a
1
8
1
: 1 mixture of H
2
O and 0.2 M HNO
ratio of 68.9 : 29.9 : 1.2 in the rst 5 minutes.
This ratio falls within the range of (49–91.9) : (7.6–39) : (0.51–
2) reported for water oxidation with Oxone catalysed by a
3
gave oxygen with a
4
1
6
16 18 18
2 2
O / O O/ O
measurements on the corresponding reaction mixtures.
CAN as oxidant. Before examination of the reaction inter-
mediates in 1-catalysed water oxidation with CAN, high reso-
lution ESI-MS was employed to analyse an aqueous solution of
1
III
IV
19
Mn (O) Mn complex.
2
1, and revealed a major cluster peak at m/z 358.0906, attributed
a
Table 1 Catalytic water oxidation by 1 under different reaction conditions
b
c
Entry
Oxidant
[Catalyst] (mM)
[Oxidant] (mM)
Medium
TON of O
2
OE (%)
1
1
1
1
1
1
1
2
2
2
2
3
3
a
b
c
d
e
f
g
a
b
c
d
a
b
CAN
CAN
CAN
CAN
CAN
CAN
CAN
NaIO
NaIO
NaIO
NaIO
100
100
100
100
100
12.5
12.5
100
100
12.5
12.5
100
12.5
10
20
42
84
84
125
125
84
0.1 M HNO
0.1 M HNO
0.1 M HNO
0.1 M HNO
3
3
3
3
17
19
31
41
32
93
81
12
3
44
3
89
113
68
38
30
20
15
4
3
3
1
1
2
H O
0.1 M HNO
3
3
3
H
2
O
4
4
4
4
0.1 M HNO
84
H
2
O
125
125
84
0.1 M HNO
H
2
O
0.1
21
2
Oxone
Oxone
0.1 M HNO
0.1 M HNO
3
125
3
a
b
All reactions were performed under argon atmosphere and in 0.1 M HNO
3
or water at room temperature for 30 min. Determined by GC
c
ꢀ
integration. Based on the stoichiometric ratio (1 : 4) of O
formed/(no. of moles of oxidant used/n)] ꢁ 100%, where n ¼ 4 for CAN and n ¼ 2 for NaIO
2
formed per oxidant in 4e oxidation; oxidant efficiency (OE) ¼ [no. of moles of O
2
4
and Oxone.
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Fig. 3 Cluster peaks at m/z 357.0992 and 402.0854 observed during
high-resolution ESI-MS analysis of a reaction mixture of 1 and CAN
IV
+
(
200 equiv.) in H
2
O. Simulated isotopic patterns for (A) [Fe (L1)(O)(OH)]
IV
+
and (B) [Fe (L1)(O)(NO
3
)] are shown.
4
NaIO as oxidant. ESI-MS analysis of a reaction mixture of 1
and NaIO4 (800 equiv.) in 0.1 M HNO3 during the rst 30
seconds of the reaction revealed new species at m/z 356.0944
(
Fig. 4) and 373.0940 (Fig. S20†), which can be respectively
V
+
III
+
assigned to [Fe (L1)(O)
induced dissociation spectra of the two species are depicted in
Fig. S21 and S22.† When the reaction of 1 with NaIO (800
. (B) Plot of initial rate equiv.) was conducted in H O under similar conditions, a new
2
evolution against different [1] (6.25–25.0 mM) at fixed [CAN] (125
2
] and [Fe (L1)(OOc)(OH)] . Collision-
2
Fig. 2 (A) Plot of initial rate of O evolution against different [CAN]
4
(
12.5–125 mM) at fixed [1] (12.5 mM) in 0.1 M HNO
3
2
of O
3
mM) in 0.1 M HNO .
III
+
2
to [Fe (L1)(OH) ] (calcd m/z 358.1092) based on the isotopic
1
8
distribution, collision-induced dissociation, and O-labelling
Fig. S9–S12†). We then performed high-resolution ESI-MS
analysis of a reaction mixture of 1 in H O aer addition of CAN
(
2
(200 equiv.) for 30 seconds; the spectrum showed three new
cluster peaks at m/z 357.0992, 402.0854 and 448.0815
IV
+
(
Fig.
S13B†),
assignable
to
[Fe (L1)(O)(OH)] ,
] , respectively. The ESI-
MS spectrum for the reaction in 0.1 M HNO (Fig. S13C†)
resembles that obtained for the reaction in H O. The isotopic
IV
+
III
+
3 3 2
[Fe (L1)(O)(NO )] , and [Fe (L1)(NO )
3
2
distributions of the three new cluster peaks are depicted in
Fig. 3 and in Fig. S14† (for their collision-induced dissociation,
see Fig. S15 and S16†). The new cluster peaks at m/z 357.0992
and 402.0854 shied to m/z 361.1107 and 404.0927, respec-
tively, when the reaction of 1 with CAN (200 equiv.) was con-
1
8
16
ducted in H
2
O (instead of H
2
O); the corresponding isotopic Fig. 4 Cluster peak at m/z 356.0944 observed during high-resolution
(800 equiv.) in
patterns and collision-induced dissociation spectrum obtained ESI-MS analysis of the reaction mixture of 1 with NaIO
in the O-labelling study are shown in Fig. S17–S19.†
4
III
+
1
8
3 2
0.1 M HNO . Simulated isotopic patterns for (A) [Fe (L1)(OH) ] and (B)
V
+
[
2
Fe (L1)(O) ] are shown.
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V
+
peak at m/z 356.0937 assignable to [Fe (L1)(O)
2
] was formed
within 10 seconds (Fig. S23†). By carrying out the same reaction
1
8
16
in H2 O instead of H2 O, a shi of the m/z 356.0937 signal to
m/z 360.1042 was observed (Fig. S24†).
UV-vis absorption spectroscopy
The reaction of 1 (1.5 mM) with 5 equiv. of CAN in 0.1 M HNO3
at room temperature was monitored by UV-vis absorption
spectroscopy. This reaction immediately (within 10 seconds)
generated a new species with lmax 830 nm and a shoulder near
540 nm (Fig. 5). The new species decayed rapidly with a half-life
of 107 seconds (Fig. 5, inset).
4
For the reaction of 1 (1.5 mM) with 5 equiv. of NaIO or
Oxone in 0.1 M HNO , UV-vis measurements revealed different
phenomena from that observed for the ‘1 + CAN’ system. Upon
3
Fig. 6 X-band EPR spectra (at 6 K) of 1$ClO
line), and a reaction mixture of 1$ClO (1 mM) and NaIO
M HNO (red line).
4
in 0.1 M HNO
3
(black
4
4
(10 mM) in 0.1
treatment of 1 with NaIO , a new species with l
830 nm and a
max
4
3
shoulder near 540 nm was gradually formed over 10 minutes
(
Fig. S25†) and decayed much more slowly with a half-life of ꢂ40
minutes (see the inset of Fig. S26†), in contrast to the CAN
counterparts (Fig. 5). In the case of Oxone, the new band at
ꢂ830 nm was barely discernible in the UV-vis spectrum of the
reaction mixture, being much weaker than that observed for the
reaction with NaIO4.
(red line). In this EPR spectrum, there are two prominent
signals, one with gavg ¼ 5.54 and the other with g z 2.
Electrochemistry of complex 1
In order to avoid interference arising from the electrochemical
ꢀ ꢀ
4 4 4
anion in 1$FeCl , we used the ClO salt
reactions of the FeCl
of 1 and examined its electrochemical properties in 0.1 M HNO
EPR analysis
3
at pH 1 and in buffered solutions at various pH by means of
cyclic voltammetry and rotating disk voltammetry using glassy
carbon as the working electrode. The cyclic voltammograms of
EPR spectroscopy was employed to examine the reaction
mixture of 1-catalysed water oxidation, using NaIO oxidant as
4
III
an example. Complex 1 is a high-spin Fe complex with S ¼ 5/
15b
1$ClO at pH 1–6 are depicted in Fig. 7A and S27,† and show a
4
2;
4
however, the X-band EPR spectrum of 1$ClO in 0.1 M
reversible couple I at E1/2 +0.46 V (pH 1) to +0.12 V (pH 6) vs.
SCE, a small irreversible oxidation wave II at Epa +1.18 V (pH 1)
to +0.86 V (pH 6) vs. SCE, and the onset of a catalytic oxidation
wave at +1.4 V (pH 1) to ca. +1.1 V (pH 6) vs. SCE. Both E1/2 of the
reversible couple I and Epa of the irreversible wave II shi
cathodically by ꢂ67 mV per pH unit upon increasing the pH
3
HNO recorded at 6 K did not show an appreciable signal (Fig. 6,
black line), presumably due to quick relaxation. The X-band
EPR spectrum of the reaction mixture obtained by mixing
4 4 3
1$ClO with NaIO (10 equiv.) in 0.1 M HNO at room temper-
ature followed by immediate cooling to 6 K is depicted in Fig. 6
(Fig. 7B and S28†). The magnitude of the irreversible wave II was
found to be sensitive to the electrode surface, but could be
reproducibly observed during both the cyclic voltammetric scans
and the rotating disk voltammetric experiments. The linear scan
4
voltammograms of 1$ClO at pH 1, 3, and 5 obtained from
rotating disk voltammetric measurements are depicted in Fig. 7C
and S29,† and that recorded at pH 1 at various rotation rates is
shown in Fig. S30.† For example, at pH 3, a small but distinct
current was recorded at ca. +1.05 V vs. SCE (Fig. 7C), which could
be correlated to the small irreversible wave II in the corre-
sponding cyclic voltammogram (Fig. 7A). Notably, the catalytic
oxidation wave, presumably due to water oxidation, can be
observed at pH 1 to 6 (Fig. 7A and S27†). The limiting current (i
L
)
of the redox process corresponding to the reversible couple I
1
/2
increased linearly with the square root of the rotation rate (u ).
1
/2
2
A plot of i
0.999 (Fig. S31†). This is indicative of a totally mass-transfer-
limited condition at the electrode surface. From the slope of the
Levich plot, the diffusion coefficient of 1$ClO under the exper-
imental conditions was calculated to be 0.48 ꢁ 10 cm s
L
against u (Levich plot) shows a straight line with R
¼
Fig. 5 UV-vis absorption spectral changes for the reaction of 1 (1.5
mM) with CAN (7.5 mM) in 0.1 HNO at room temperature. Inset: time
3
course of the decay of the absorption band at 830 nm.
4
ꢀ5
2
ꢀ1
.
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Density functional theory calculations
Electrochemical potentials of iron–oxo complexes of L1. The
electrochemical potentials of iron–oxo complexes
Fe(L1)(O)(X)] (X ¼ H O, HO , O ) in different oxidation
n+
ꢀ
2ꢀ
III
[
2
II
states, together with that of the Fe /Fe couple of 1, were
estimated using DFT calculations. To avoid direct calculation
of the proton free energy and systematic errors, we computed
the redox potentials through isodesmic reactions using the
electrochemical proton-coupled electron transfer (PECT) reac-
VI
2+
tions of the cis-(dioxo)ruthenium(VI) complex [Ru (L2)(O) ]
2
0
0
(
L2 ¼ N,N,N ,N -tetramethyl-3,6-dimethyl-3,6-diazaoctane-1,8-
20
diamine) as references. The isodesmic reactions are based on
the assumption that both the proton free energy and the
V
IV
systematic errors for each pair of redox reactions (Fe /Fe vs.
V
IV
IV
III
IV
III
III
II
III
II
Ru /Ru , Fe /Fe vs. Ru /Ru , and Fe /Fe vs. Ru /Ru ) are
comparable and can be cancelled out in the calculations of
DDG1, DDG2, and DDG3 depicted in reactions (1a)–(3a) and
(1b)–(3b).
ꢀ
ꢁ2þ
DG1a
þ H þ e ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
V
þ
Fe ðL1ÞðOÞðOHÞ
h
ꢃi2þ
IV
Fe ðL1ÞðOÞðOH
(1a)
(1b)
2
DG1b
V
2þ
þ
½
Ru ðL2ÞðOÞðOHÞꢃ þ H þ e ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
h
i
ꢄ
IV
Ru ðL2ÞðOÞðOH
2þ
DDG1 ¼ DG1b ꢀ DG1a
2
h
i
ꢅ
ꢄ
2þ
þ
DG2a
þ H þ e ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
IV
Fe L1ÞðOÞðOH2
h
i
ꢄ
III
Fe ðL1ÞðOHÞðOH
2þ
(2a)
2
h
i
ꢅ
ꢄ
2þ
þ
DG2b
þ H þ e ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
IV
Ru L2ÞðOÞðOH2
III
h
i
ꢄ
2þ
Ru ðL2ÞðOHÞðOH
DDG2 ¼ DG2b ꢀ DG2a (2b)
2
h
i
ꢅ
ꢄ
III
Fe L1ÞðOHÞðOH2
2þ
þ
DG3a
þ H þ e ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
h
i2þ
II
Fe ðL1ÞðOH Þ
(3a)
2
2
h
i
ꢅ
ꢄ
2þ
þ
DG3b
þ H þ e ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
III
Ru L2ÞðOHÞðOH
2
h
i
II
2þ
Ru ðL2ÞðOH Þ
DDG3 ¼ DG3b ꢀ DG3a (3b)
2
2
From the DFT computed DDG1, DDG2 and DDG3 values, and
V
IV
IV
III
the experimental redox potentials of the Ru /Ru , Ru /Ru
III
II
20
V
IV
and Ru /Ru couples, the redox potentials of the Fe /Fe ,
Fe /Fe and Fe /Fe couples can be calculated as:
IV
III
III
II
V
IV
V
IV
Fig. 7 (A) Solid line: cyclic voltammogram of 1$ClO
4
in 0.1 M acetate
E(Fe /Fe ) ¼ E(Ru /Ru ) + DDG1
buffer at pH 3; dotted line: buffer background; working electrode:
ꢀ1
glassy carbon; scan rate: 100 mV s . (B) Redox potentials (E1/2 of
reversible couple I) of 1$ClO in solutions of various pH. (C) Linear scan
voltammogram of 1$ClO in 0.1 M acetate buffer at pH 3. Working
electrode: rotating glassy carbon disk; rotation rate: 100 rpm; scan
IV
III
IV
III
E(Fe /Fe ) ¼ E(Ru /Ru ) + DDG2
4
4
III
II
III
II
E(Fe /Fe ) ¼ E(Ru /Ru ) + DDG3
ꢀ1
rate: 5 mV s (inset: magnified region at approximately 1.0 V).
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V
IV
IV
III
V
2+
where E(Ru /Ru ) ¼ +0.72 V, E(Ru /Ru ) ¼ +0.63 V, and where pK
a
([Ru (L2)(O)(OH)] ) ¼ 1.8 according to the experi-
III
II
20
E(Ru /Ru ) ¼ +0.26 V at pH 1 according to the experimental mental study.
20
V
IV
IV

ndings.
The calculated redox potentials for various Fe /Fe , Fe /
III
II
IV
III
III
III
II
n+
We calculated the Fe /Fe and Fe /Fe redox potentials at Fe and Fe /Fe couples, including those of [Fe(L1)(O)(X)]
ꢀ
2ꢀ
different pH by using three commonly used density functionals (X ¼ H
2
O, HO , O ), along with the calculated pK
a
values, are
(
(
DFs): BPW91 (pure-GGA), B3LYP (hybrid-GGA), and M06L depicted in Scheme 1, revealing the following features: (1) the
V
2+
V
+
meta-GGA). A correlation between the experimental redox pK
potentials of Fe /Fe and Fe /Fe and those calculated using (2) the calculated E1/2 of [Fe (L1)(O)(OH)] /[Fe (L1)(O)(OH )]
2
these DFs is depicted in Fig. 8 (note of caution: calculated redox (+1.42 V vs. SCE at pH 1) is comparable to the E1/2 of Ce /Ce
potentials are thermodynamic values). It is evident that BPW91 (+1.38 V vs. SCE) and close to the calculated E
a
of the [Fe (L1)(O)(OH)] /[Fe (L1)(O)
2
] equilibrium is 3.0;
III
II
IV
III
V
2+
IV
2+
IV
III
values of
/2
Pytacn or mep
1
V
2+
IV
2+
Me2
and B3LYP led to marked-to-severe (up to ca. +0.57 V) over- [Fe (L)(O)(OH)] /[Fe (L)(O)(OH )] with L ¼
2
III
II
12f
estimation of the E1/2 of Fe /Fe , although B3LYP showed good (+1.70 to +1.73 V vs. SHE, i.e. +1.46 to +1.49 V vs. SCE); (3) the
IV
2+
III
2+
performance in the prediction of the electrochemical potential calculated E1/2 of [Fe (L1)(O)(OH
of Fe /Fe . Only M06L gave good estimations of the electro- (+1.25 V vs. SCE at pH 1) matches the experimental Epa value of
2
)] /[Fe (L1)(OH)(OH
2
)]
IV
III
III
II
IV
III
IV
III
chemical potentials of both Fe /Fe and Fe /Fe at different Fe /Fe (Epa ¼ 1.18 V vs. SCE at pH 1, see Discussion section);
III
II
pH with a linear t (E(M06L) ¼ E(expt) + 0.06, R ¼ 0.998, SD ¼ (4) the calculated E1/2 of the Fe /Fe couple is +0.49 V at pH 1,
III
II
0
.03) between the calculated redox potentials and the experi- which matches the experimental E1/2 value for Fe /Fe (+0.46 V vs.
mental data. Hence, M06L was chosen for the subsequent DFT SCE at pH 1, see Discussion section) well; (5) the calculated E of
1
/2
V
III
calculations in this work. In the literature, the M06L functional the Fe /Fe couple (E1/2 ¼ +1.34 V vs. SCE at pH 1) coincides with
has been reported to show good performance in modelling the catalytic oxidation wave at +1.4 V observed in the cyclic vol-
2
1
12a,c
water oxidation by ruthenium and iron
For the calculation of the pK values, as an example, the pK
of [Fe (L1)(O)(OH)] was calculated based on the following pair [Fe (L1)(O)(OH
of isodesmic reactions (reactions (4a) and (4b)). at pH 1.
Electronic properties of [Fe (L1)(O)
complexes.
tammogram of 1 at pH 1. It is noted that there is just 170 mV
difference in the calculated E1/2 values of [Fe (L1)(O)(OH)] /
V
2+
a
a
V
2+
IV
2+
IV
2+
III
2+
2
)] and [Fe (L1)(O)(OH
2
)] /[Fe (L1)(OH)(OH
2
)]
V
+
ꢀ
ꢁ2þ
ꢀ
ꢁ
ꢁ
2
] . The ground state of
[Fe (L1)(O) ] is a quartet state with three unpaired electrons;
the lowest-lying excited state is a sextet state which is 2.8 kcal
mol higher in energy than the ground state. A quartet ground
DG4a
þ
V
V
þ Hþ
þ Hþ
Fe ðL1ÞðOÞðOHÞ ꢂꢂꢂꢂ ꢂꢂ ! Fe ðL1ÞðOÞ
(4a)
V
+
2
2
ꢀ
þ
V
2þ
DG4b
V
ꢀ1
Ru ðL2ÞðOÞ
½
Ru ðL2ÞðOÞðOHÞꢃ ꢂꢂꢂꢂꢂꢂ ꢂꢂ !
2
V
state for an Fe ]O species has previously been predicted by
DDG4 ¼ DG4b ꢀ DG4a
(4b)
V Me2
2+ 12f,14d
DFT calculations for [Fe ( Pytacn)(O)(OH)] .
computed Fe]O distance in [Fe (L1)(O)
The
V
+
2
] in the quartet state
Fe ) is 1.614 A, which is shorter than that in the sextet state
Fe , 1.695 A) due to the antibonding character of the d1 and
d2 orbitals (Fig. 9, right). Similar Fe]O distances were reported
4
V
˚
(
(
V
2+
V
2+
6
V
pK
a
([Fe (L1)(O)(OH)] ) ¼ pK
a
([Ru (L2)(O)(OH)] ) ꢀ DDG4/
˚
2
.303RT
V
+
˚
for [Fe (L1)(O) ] (1.613–1.638 A), computed at the B3LYP/6-
2
V Me2
3
1(G) (lanl2dz) level in our previous work, and for [Fe ( Py-
2
+
12f
V
ꢀ
˚
tacn)(O)(OH)]
1.60 A˚ ).1 The spin density plot for [Fe (L1)(O)
(1.63 A)
and low-spin [Fe (TAML)(O)]
4a
V
+
(
2
] calculated in
this work is shown in Fig. 9 (le). Apart from the spin density on
Fig.
8 Correlation between experimental and calculated redox
III
II
IV
III
potentials. The E(expt) values for the Fe /Fe and Fe /Fe couples
were taken from Fig. 7 and S28† (see Discussion section). Note of Scheme 1 Calculated reduction potentials for Fe /Fe , Fe /Fe and
caution: E(expt) values for Fe /Fe are the Epa values of the irreversible Fe /Fe , together with calculated pK
V
IV
IV
III
IV
III
III
II
a
values, based on the M06L
wave II.
functional.
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work. Fig. 11 depicts the computed potential energy surfaces
and key structure parameters of the stationary points. The
4
6
oxidation of water starts from the reactant complex RC1/ RC1,
4
V 6
V
formed between Fe / Fe and the substrate (water molecule),
4
6
which proceeds to the transition state TS1/ TS1 in which an
O/O bond and an O/H bond are formed. The ground state is
4
ꢀ1
6
RC1, which is 2.8 kcal mol lower in energy than RC1. The
‡
ꢀ1
free energy barrier (DG ) for the quartet state is 15.7 kcal mol
lower than that for the sextet state (18.9 kcal mol ). Both
activation barriers are comparable to those for the similar
oxidation of water by [Fe (TAML)(O)] (20.0 kcal mol , esti-
mated from experimental data) and [Fe ( Pytacn)(O)(OH)]
,
ꢀ
1
V
ꢀ
ꢀ1
12a
V Me2
2+
ꢀ
1
12f
(18.8 kcal mol
obtained from DFT calculations). As the
V
+
Fig. 9 Spin density plot (left) and MO diagram (right) of [Fe (L1)(O)
2
] . reactions proceed through the transition state, there is a reversal
6
of the energy levels of the sextet and quartet states, with INT1
ꢀ
1
4
being 6.4 kcal mol more stable than the quartet state INT1.
Fe of 2.3, each oxo group has considerable spin density of 0.27. The location of the minimum energy crossing point (MECP)
The MO diagram of [Fe (L1)(O) ] (Fig. 9, right) reveals a d
conguration in accordance with the assignment of the Fe(V) developed by Harvey and co-workers. The energy of the MECP is
oxidation state.
Electronic properties of [Fe (L1)(O)(OH)] . The ground state spin state reversion can easily occur, leading to the more stable
of [Fe (L1)(O)(OH)] is a quintet state with four unpaired elec- complex INT1.
trons, and the triplet state is 16.0 kcal mol higher in energy.
The computed Fe]O distance of [Fe (L1)(O)(OH)] ( Fe ) in [Fe (L1)(O)
the ground state is 1.632 A, very close to the experimental Fe]O
V
+
3
between the sextet and quartet states was found using the code
2
22
ꢀ
1
4
only 0.9 kcal mol higher than that of INT1, suggesting that
IV
+
IV
+
6
ꢀ
1
6
The INT1 complex can subsequently be oxidized by
IV
+
5
IV
V
+
4
V
2
]
( Fe ) to give the superoxo complex
III
+
7
5
4
V
˚
[Fe (L1)(OOc)(OH)] ( INT2/ INT2). Meanwhile, Fe is reduced
IV
2+
5a
IV
+
5
IV
distance in [Fe (N4Py)(O)] (1.639(5) A˚ ). Fig. 10 (le) shows the
spin density plot calculated in this work for [Fe (L1)(O)(OH)] . include exchange of an oxygen molecule by a water molecule
Fe has the largest spin density of 3.1; the oxo and hydroxyl and the release of
to [Fe (L1)(O)(OH)] ( Fe ). The subsequent reaction steps
IV
+
3
1
O / O (reaction (5)). The search for the
2 2
groups have spin densities of 0.59 and 0.18, respectively. The transition state has not been successful. A barrier of 8.8 kcal
IV
+
4
ꢀ1
IV
ꢀ
MO diagram of [Fe (L1)(O)(OH)] (Fig. 10, right) reveals a d
conguration, which is consistent with the assignment of the reported by Crammer and co-workers. Aer release of O
mol
2
for the release of O from [TAML–Fe –OOc] was
12a
2
,
II
+
5
II
Fe(IV) oxidation state.
2
complex [Fe (L1)(OH )(OH)] ( Fe ) with a quintet state ground
Mechanism of water oxidation based on [Fe (L1)(O) ] . Both state is formed; this complex can further comproportionate
V
+
2
4
V
6
V
the quartet (S ¼ 3/2, Fe ) and sextet (S ¼ 5/2, Fe ) states of
V
+
[
Fe (L1)(O) ] were considered in the mechanism studies. The
2
V
+
doublet state (S ¼ 1/2) of [Fe (L1)(O)
2
] was found to be the most
ꢀ
1
energetically unfavourable (16.4 kcal mol higher in energy
than the quartet ground state) and was not considered in this
ꢀ1
Fig. 11 Potential energy surfaces (in kcal mol ), key structural
ꢀ
1
parameters (in A˚ and degrees) and relative stabilities (in kcal mol ) of
ꢀ1
Fig. 10 Spin density plot (left) and MO diagram (right) of the stationary points. The energy of the MECP point (13.4 kcal mol ) is
IV
+
[
Fe (L1)(O)(OH)] .
the average energy of the quartet and sextet states in water.
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IV
+
5
IV
with
[
[Fe (L1)(O)(OH)]
( Fe )
to
regenerate
two approximately 1.4 times (TON ¼ 56) and 2.9 times (TON ¼ 35)
the amounts formed in the 1-catalysed reactions, respectively.
Changing the catalyst/oxidant ratio from 1 : 840 to 1 : 10 000 led
III
+
6
III
Fe (L1)(OH) ] ( Fe ) molecules.
2
III
+
II
+
[
2 2 2
Fe (L1)(OOc)(OH)] + OH / [Fe (L1)(OH )(OH)] + O
to an increase in the TON for oxygen formation from 41 to 93 for
II
(5)
‘
1 + CAN’, from 56 to 177 for ‘[Fe (mcp)Cl
4 for ‘1 + NaIO ’, and from 35 to 114 for ‘[Fe (mcp)Cl
NaIO ’. These results showed that [Fe (mcp)Cl
than 1 in catalysing water oxidation.
Iron oxide nanoparticles, which can be generated from
2
] + CAN’, from 12 to
II
4
4
2
] +
2
] is more active
II
4
Discussion
6
c
The main objective of this work is to gain insight into the simple iron salts or iron complexes at high pH, have been
6c,24
mechanism of water oxidation by iron complexes of tetra- shown to be efficient catalysts for water oxidation.
dentate macrocyclic ligands with a cis non-planar coordination catalysed water oxidation with CAN, NaIO , or Oxone in 0.1 M
geometry. The macrocyclic N
ligand L1 (which is well known to HNO (pH 1), the possibility that the observed catalytic activity
form iron complexes in the cis conguration ) was chosen, as of 1 is due to Fe nanoparticles generated in situ could be
our previous work had suggested the involvement of in situ excluded, as in acidic solution either formation of Fe
generated high-valent Fe]O species of L1, including nanoparticles would be prevented or any Fe nanoparticles
] , in the cis-dihydroxylation of alkenes with Oxone formed would be converted to Fe (aq) ions. It has been repor-
For the 1-
4
4
3
15b
2 3
O
2 3
O
2 3
O
V
+
3+
[
Fe (L1)(O)
catalysed by [Fe (L1)Cl
proposed to be the reactive intermediates in iron-catalysed with CAN at low pH.
2
III
+
16a
V
3+
]
(1),
and Fe ]O species were ted that Fe (aq) ions are not active in catalysing water oxidation
2
6
c
6a,b
water oxidation reactions reported in the literature. Also, it is
envisaged that the strong donor strength of L1 would stabilize water oxidation catalysts oen revealed strong catalytic currents
iron–oxo species against demetallation in solution. due to water oxidation occurring at potentials corresponding to
During optimization of the reaction conditions, it was noted the generation of oxidizing Ru–oxo complexes. In general,
that although the turnover number of oxygen increased with electrochemical oxidation of Ru–OH to Ru]O species would
oxidant concentration, the oxidant efficiency (dened in note c be kinetically slow as a result of deprotonation of Ru–OH prior
Electrochemical studies of ruthenium-based molecular
25
2
2
of Table 1) decreased from 68% to 20% (Table 1, entries 1a–1d). to oxidation. Hence the working electrode is well documented
Such a signicant decrease in oxidant efficiency is attributed to to play a crucial role in the electrochemical reversibility of the
partial decomposition/deactivation of the catalyst under Ru]O/Ru–OH
couple(s). Glassy carbon or pyrolytic carbon is
strongly oxidizing reaction conditions. Similar ndings were commonly used as a working electrode for electrochemical
observed when NaIO or Oxone was used as oxidant. studies of Ru]O complexes in aqueous solutions. While there
Unlike the ‘[Fe(bqen)(OTf) ] + CAN’ system (which can have been numerous reports on the electrochemistry of Ru]O/
2
23
4
2
oxidize water to O
.1 M HNO solution), the ‘1 + CAN’ system produced oxygen knowledge, related studies on the electrochemical oxidation of
with comparable TONs in water and in 0.1 M HNO
and 1e in Table 1), consistent with the similar ESI mass spectra
2 2
in non-buffered aqueous solution but not in Ru–OH couples in aqueous solution, to the best of our
6e
0
3
26
(entries 1d Fe–OH to Fe]O species in aqueous solution are sparse.
2
3
The cyclic voltammetric and rotating disk voltammetric
obtained under the two conditions (Fig. S13†). This reects the measurements of 1 at various pH provide useful information on
remarkable stability of 1 toward ligand dissociation under the electrochemical potentials of the iron–oxo species of L1. As
acidic conditions, like other iron complexes such as seen in Fig. 7B and S28,† the plots of the redox potentials
II
6b
II
2+
[
Fe (mcp)(OTf) ] and [Fe (L1)(MeCN) ] (ref. 6g) used as against pH (from 1 to 6) for both couple I and wave II show
2
2
water oxidation catalysts at pH 1. The 1-catalysed water oxida- negative slopes, and the redox potential decreases by ꢂ67 mV
tion was sensitive to the oxidant, with the TON of oxygen per pH unit, lending support to the two oxidation processes
evolved under the same conditions (in 0.1 M HNO
the order Oxone > CAN > NaIO
(113 vs. 93 vs. 44, see Table 1, These ndings, together with the experimental data obtained
entries 1f, 2c and 3b). According to the
2
ratio from rotating disk voltammetry (Fig. 7C and S29†) and the
) following which are due to one electron and one proton transfer reactions.
3
4
1
6
16 18 18
O
2
/ O O/ O
1
8
(
68.9 : 29.9 : 1.2) obtained in the O-labelling studies, the results of the DFT calculations (Fig. 8 and Scheme 1), support
oxygen evolved in the ‘1 + Oxone’ system originated not only the assignment of couple I to the [Fe (L1)(OH)(OH
from Oxone but also from water (see the possible pathways of [Fe (L1)(OH
III
2+
2
)] /
II
2+
2 2
) ]
redox couple and the assignment of wave II to
III
2+
IV
2+
oxygen evolution, along with the corresponding calculated the oxidation of [Fe (L1)(OH)(OH
2
)] to [Fe (L1)(O)(OH
2
)] .
ratios, depicted in Scheme S1†), indicating the Despite the low scan rate of 5 mV s for the rotating disk vol-
involvement of water oxidation in the reaction.
tammetry, the magnitude of the current recorded for the
Considering the excellent catalytic ability of [Fe (mcp)(OTf)
1
6
16 18 18
ꢀ1
2 2
O / O O/ O
II
IV
2+
III
2+
2
]
[Fe (L1)(O)(OH )] /[Fe (L1)(OH)(OH )] oxidation wave is not
2 2
III
2+
for water oxidation, as reported by Costas, Lloret-Fillol and co- the same as that recorded for the [Fe (L1)(OH)(OH
workers, we compared the activity of catalysts [Fe (mcp)Cl
and [Fe (L1)Cl ] (1) for water oxidation. Under our reaction an Fe ]O species involves the deprotonation of an Fe–OH
)] /
2
6
b
II
6b
II
2+
III
[Fe (L1)(OH
2
)
2
]
redox process. As the oxidation of Fe –OH to
2
]
III
+
IV
2
conditions with catalyst/oxidant ¼ 1 : 840, the amounts of unit, the electrochemical oxidation would be kinetically
II
oxygen formed in [Fe (mcp)Cl
2
]-catalysed water oxidation in slow and highly sensitive to the electrode surface. Indeed, the
0.1
M
HNO
3
with CAN and NaIO
4
as oxidants were electrochemical oxidation of Ru–OH
2
/Ru–OH to a Ru]O
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species is well documented to be signicantly affected by the intensity of the cluster peak at m/z 357.0992, attributed to
27
IV
+
nature/surface of the working electrode. At pH 1 to 6, the [Fe (L1)(O)(OH)] , at different reaction times. The ion count of
difference between the observed redox potentials for wave II this signal decreased with reaction time, as depicted in
IV
III
III
II
(
Fe /Fe ) and couple I (Fe /Fe ) is about 700 mV. For a Fig. S32,† which correlates with the decay of the absorption
IV
ruthenium complex (also with cis vacant sites) supported by band at lmax 830 nm assigned to the Fe ]O species in the UV-
the mcp ligand,
potentials of the [Ru (mcp)(O)(OH
and [Ru (mcp)(OH)(OH
50 mV, which is 150 mV smaller than that between the iron [Fe (mcp)(O)(H
analogues and can be attributed to the stronger p-bond formed [Fe (mcp)(OTf) ] and CAN in aqueous solution), which could
in the Ru ]O moiety. There is a previous report on the obser- persist under catalytic conditions with a half-life of 2.4 hours,
vation of a pH-dependent reversible couple assigned as a proton- the Fe ]O species of L1 decayed with a half-life of 107 seconds
coupled electron transfer Fe /Fe couple for [Fe (N4Py)(O)]
N4Py ¼ N,N-bis(2-pyridylmethyl)-bis(2-pyridyl)methylamine) that either it is taking part in a further reaction, or it is
in buffered aqueous solution (pH 1.5–4) with an E1/2 value of decomposing rapidly.
0.41 V vs. SCE at pH 4 (the E1/2 value anodically shied upon Given the instant generation of an Fe ]O species upon
decreasing the pH with a shi of 55 mV per pH unit). However, oxidation of 1 with CAN in aqueous solution, as supported by
the Fe /Fe couple was not reported for [Fe (N4py)(O)] . The ESI-MS and UV-vis analysis, together with the absence of an
E1/2 value of +0.41 V for the [Fe (N4py)(O)] /[Fe (N4py)(OH)]
couple in aqueous solution at pH 4 is unexpectedly low. Should suggested to be involved in water oxidation by the ‘1 + CAN’
28
IV
the difference between the redox vis spectra shown in Fig. 5. Fe ]O species of chelating N-donor
IV
2+
III
2+
2
)] /[Ru (mcp)(OH)(OH
2
)]
ligands have been reported to be reasonably stable, with half-
III
2+
II
2+
30
2 2 2
)] /[Ru (mcp)(OH ) ]
couples is lives generally ranging from 2 to 60 hours. Unlike the
IV
II
2+
5
2
O)]
species (generated in situ from
2
IV
6b
IV
IV
III
IV
2+
under these conditions (Fig. 5). Such a fast decay may suggest
(
IV
+
29
III
II
IV
2+
IV
2+
III
2+
IV
induction period for oxygen evolution, an Fe ]O species is
III
II
this be the case, the corresponding Fe /Fe couple would have system. Kinetic studies revealed a linear dependence of the
to occur at a potential less than 0.0 V vs. SCE based on the DFT initial rate of oxygen evolution on the concentrations of both
calculations in this work. Our DFT calculations using the M06L CAN and 1 (Fig. 2; increasing [CAN] to 125 mM lowered the pH
functional gave redox potentials of +1.21 V vs. SCE for the to ꢂ0.6, and such pH variation alone accounted for ꢂ18% of the
IV
2+
III
2+
[
Fe (N4py)(O)] /[Fe (N4py)(OH)] couple and +0.3 V vs. SCE rate increase depicted in Fig. 2A), which suggests that a key
III
2+
II
2+
for the [Fe (N4py)(OH)] /[Fe (N4py)(OH )] couple at pH 1. step of the reaction involves an iron species, presumably
The observation of a catalytic wave for 1 (Fig. 7A and S27†) [Fe (L1)(O)(OH)] , and one equivalent of CAN. One of the
beyond the irreversible wave II (assigned to Fe /Fe ) at, for possibilities is the oxidation of [Fe (L1)(O)(OH)] by CAN to give
example, Epa +1.18 V vs. SCE (pH 1), which is comparable to the Fe ]O species, such as [Fe (L1)(O)(OH)] or [Fe (L1)(O)
2
IV
+
IV
III
IV
+
V
V
2+
V
+
2
] ,
IV
+
DFT-calculated potential of +1.25 V vs. SCE (pH 1) for the and another possibility is the reaction of [Fe (L1)(O)(OH)] with
IV
2+
III
2+
IV
IV
[
Fe (L1)(O)(OH
2
)] /[Fe (L1)(OH)(OH
2
)] couple, corroborates CAN to form an O]Fe –O–Ce species similar to that recently
II
13
the nding that 1 can catalyse water oxidation through iron–oxo reported for the ‘[Fe (mcp)(OTf)
2
] + CAN’ system, although
Fe ]O and O]Fe –O–Ce species have not been clearly
ESI-MS analysis of a solution of 1 in water (Fig. S9–S12†), detected in ESI-MS analysis of the reaction mixture of 1 with
IV
V
IV
IV
species at oxidation states beyond Fe .
which revealed
a
major cluster peak assigned to CAN in H O.
2
III
+
ꢀ
[
Fe (L1)(OH) ] , suggests rapid exchange of the Cl ligands of 1
For water oxidation by the ‘1 + NaIO ’ system, the new signals
4
2
with solvent. The new signals, generated upon addition of CAN, at m/z 356.0944 (major) and 373.0940 (minor), revealed by ESI-
at m/z 357.0992, 402.0854, and 448.0815 can be attributed to MS analysis of a mixture of 1 and NaIO
4
in 0.1 M HNO
3
(Fig. 4
] (calcd m/z
(calcd m/z 402.0865, Fig. 3) and 356.0936) and [Fe (L1)(OOc)(OH)] (calcd m/z 373.0963),
IV
+
V
+
[
[
[
Fe (L1)(O)(OH)]
(calcd
m/z
357.1014,
Fig.
3), and S20–S22†), could be assigned to [Fe (L1)(O)
2
IV
+
III
+
Fe (L1)(O)(NO
3
)]
III
+
V
+
Fe (L1)(NO
3
)
2
] (calcd m/z 448.0794, Fig. S14†), respectively, respectively. The assignment of [Fe (L1)(O)
2
] is supported by
1
8
based on their m/z values and isotopic patterns, together with
the O-labelling experiments which revealed new signals with 356.0944 to m/z 360.1042 attributable to [Fe (L1)( O) ] (calcd
m/z 361.1107 and 404.0927 attributable to [Fe (L1)( O)( OH)]
(
O-labelling studies, revealing a shi of the signal at m/z
1
8
V
18
+
2
IV
18
18
+
18
m/z 360.1021) upon changing the reaction medium to H2
O
IV
18
16
+
ꢀ
calcd m/z 361.1099, Fig. S17†) and [Fe (L1)( O)(N O
3
)] (calcd (Fig. S24†). Since IO
4
can undergo rapid oxygen exchange with
water, the oxo ligands of [Fe (L1)( O)
IV
18
16
+
18
V
18
+
m/z 404.0907, Fig. S19†), respectively. Species [Fe (L1)( O)( OH)]
2
]
could come from
IV
16
18
+
18
18
ꢀ
and [Fe (L1)( O)( OH)] (calcd m/z 359.1057) were not detec-
H
2
O directly and/or from O-incorporated IO
4
. Changing
ted in the experiment, indicating that the oxygen atoms of the the reaction medium from 0.1 M HNO
3
to pure water reduced
IV
+
V
+
hydroxide and oxo ligands in [Fe (L1)(O)(OH)] come from the intensity of the signal assigned to [Fe (L1)(O)
water and not from CAN. observation is in line with the smaller TON of oxygen produced
In the UV-vis absorption spectra of the reaction mixture of 1 in pure water than in 0.1 M HNO (entries 2b vs. 2a, and 2d vs.
2
] ; this
3
with CAN (Fig. 5), the new band at l
830 nm is tentatively 2c, Table 1).
max
IV
V
+
assigned to an Fe ]O species of L1, with reference to the
The DFT calculations on [Fe (L1)(O)
2
] in this work and on
IV
V Me2
2+
12f,14d
characteristic bands of Fe ]O species (ranging from 700– [Fe ( Pytacn)(O)(OH)] in the literature
revealed that the
30
V
8
50 nm) reported in the literature. This assignment is in line Fe ]O species supported by these neutral chelating N ligands
IV
+
with the detection of species formulated as [Fe (L1)(O)(OH)]
all adopt a quartet ground state (S ¼ 3/2), different from the
IV
+
V
ꢀ
and [Fe (L1)(O)(NO
3
)] by ESI-MS. We further examined the doublet ground state (S ¼ 1/2) of [Fe (TAML)(O)] complexes,
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which bear tetraanionic tetraamide ligands and have been
7,14a,j
detected by EPR spectroscopy.
To further test the validity of
the DFT method used in this work, we performed DFT calcu-
V
ꢀ
lations on [Fe (TAML)(O)] using the M06L functional, which
revealed a doublet ground state for this species, consistent with
12a,14a
previous DFT calculations reported in the literature.
To the
V
best of our knowledge, no Fe ]O species with S ¼ 3/2 ground
state has been characterized by EPR spectroscopy. The X-band
EPR spectrum of the reaction mixture of 1 with NaIO
4
in 0.1 M
HNO (Fig. 6) is dominated by two signals corresponding to S ¼
3
5
/2 and S ¼ 1/2 states; the former could be attributed to a high-
spin Fe(III) species whereas the latter probably arose from
decomposed NaIO
, as a similar S ¼ 1/2 signal was observed in
the X-band EPR spectrum of a solution of NaIO in 0.1 M HNO
4
4
3
recorded at 7 K (Fig. S33†). EPR signals of S ¼ 3/2 Fe complexes
are rather close to, or considerably overlap with, those of S ¼ 5/2
Scheme 2 Proposed pathways for 1-catalysed water oxidation with
31
ones. The prominent broad S ¼ 5/2 signal in Fig. 6 would CAN or NaIO4.
therefore render it difficult to clearly conrm the presence of
V
+
the [Fe (L1)(O) ] species by EPR since the signal of this S ¼ 3/2
2
V
Fe ]O species, which is likely to have a low concentration,
molecules, assisted by hydrogen-bond interactions, leading to
could easily be masked by the S ¼ 5/2 signal.
the formation of an O–O bond, analogous to the O–O bond
V
+
V
32
2
Generation of [Fe (L1)(O) ] from the oxidation of 1 with
formation in water oxidation by Mn ]O, but the possibility of
V
+
16a
IV
6e
IV
IV
13
NaIO
both NaIO
can directly oxidize Fe(III) to Fe(V). UV-vis spectroscopy revealed
4
resembles the generation of [Fe (L1)(O)
2
] from Oxone;
the involvement of Fe ]O or O]Fe –O–Ce species in the
O–O bond formation could not be excluded, considering, for
example, the small difference between the DFT-calculated redox
4
and Oxone are typically two-electron oxidants and
IV
2+
III
2+
that the reaction of 1 with NaIO or Oxone is different from the
4
2 2
potentials of +1.25 V for [Fe (L1)(O)(OH )] /[Fe (L1)(OH)(OH )]
V
2+
IV
2+
reaction of 1 with CAN, as exemplied by the immediate
and +1.42 V for [Fe (L1)(O)(OH)] /[Fe (L1)(O)(OH )] at pH 1.
2
formation of a new band at l
Fe ]O species in the ‘1 + CAN’ system (Fig. 5) but gradual
formation of a much weaker band at lmax 830 nm over 10
830 nm attributable to an
max
The resulting iron(III)–peroxo intermediate in Scheme 2 is subse-
IV
quently oxidized by the sacricial oxidant (CAN, NaIO , or Oxone)
4
III
+
to give [Fe (L1)(OOc)(OH)] (DFT-calculated potential for
Fe (L1)(OOc)(OH)] /[Fe (L1)(OOH)(OH)] : +0.64 V vs. SCE), the
III
+
III
+
4
minutes in the ‘1 + NaIO ’ system (Fig. S25†). Also, for both the
[
‘
1 + NaIO ’ and ‘1 + Oxone’ systems, kinetic studies revealed that
4
presence of which is supported by ESI-MS analysis (Fig. S20†).
the initial rate of oxygen evolution showed a linear dependence
on the concentration of 1 but was relatively insensitive to the
concentration of NaIO4 (Fig. S5†) or Oxone (Fig. S8†). This
supports exclusion of the possibility of an “oxo–oxo coupling”
reaction between two Fe]O species.
Extrusion of oxygen is achieved upon the substitution of
III
+
II
+
[
Fe (L1)(OOc)(OH)] with water; the resulting [Fe (L1)(OH)(OH
2
)]
is oxidized by the sacricial oxidant (CAN, NaIO , or Oxone) to
4
III
+
regenerate the catalyst [Fe (L1)(OH) ] (DFT-calculated poten-
tial for [Fe (L1)(OH) ] /[Fe (L1)(OH)(OH )] : +0.26 V vs. SCE).
2
III
+
II
+
2
2
On the basis of the above ndings, a mechanism involving
IV
V
Fe ]O and/or Fe ]O intermediate(s) for the 1-catalysed water
oxidation with CAN or NaIO
is proposed as depicted in Scheme 2. Conclusions
4
For the corresponding reaction with Oxone, a mechanism similar
A mononuclear iron(III) complex bearing a macrocyclic N dia-
4
to that of the reaction with NaIO could be proposed. The iron–
4
IV
+
zapyridinophane ligand catalyses the oxidation of water to
oxygen with NaIO or Oxone, as well as CAN, as the oxidant in
4
oxo species depicted in Scheme 2 are [Fe (L1)(O)(OH)] and
Fe (L1)(O) ] based on ESI-MS analysis; their protonated forms
2
V
+
[
[
IV
2+
V
2+
acidic and/or neutral aqueous media. Studies using kinetic
measurements, high-resolution ESI-MS, UV-vis absorption
Fe (L1)(O)(OH
2
)] and [Fe (L1)(O)(OH)] could be involved as
values shown in Scheme 1),
well (note also the computed pK
but are not included in Scheme 2. Similar to [Fe (L1)(O)
a
1
8
V
+
spectroscopy, O-labelling experiments, cyclic voltammetry,
and DFT calculations lend support to the possible involvement
2
] ,
Fe (L1)(O)(OH)] also adopts a quartet ground state, with the
V
2+
[
IV
+
ꢀ
1
of high-valent iron(IV)–oxo species such as [Fe (L1)(O)(OH)] and/
doublet state being ꢂ15 kcal mol higher in energy, according
V
+
V
or iron(V)–oxo species such as [Fe (L1)(O) ] , or their protonated
2
forms [Fe (L1)(O)(OH )] and/or [Fe (L1)(O)(OH)] , in the 1-cat-
2
to DFT calculations. The possible involvement of the Fe ]O
IV
2+
V
2+
reactive species in the reaction is supported by the reasonably
V
+
alysed water oxidation reactions.
low reaction barrier for the oxidation of water by [Fe (L1)(O) ]
2
ꢀ1
of 15.7 kcal mol obtained by the DFT calculations (Fig. 11). For
Fe (L1)(O)(OH)] , the reaction barrier for O–O bond formation
V
2+
[
Acknowledgements
ꢀ1
with water was calculated to be 21.0 kcal mol (Fig. S34†), which
ꢀ
1
V
+
is 5.3 kcal mol higher than that calculated for [Fe (L1)(O)
2
] . This work was supported by the Hong Kong Research Grants
V
The Fe ]O species is suggested to be attacked by water Council (HKU 700813), the National Key Basic Research
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Program of China (No. 2013CB834802), the University Grants
Committee of the Hong Kong Special Administrative Region
2013, 52, 1789; (d) W. A. Hoffert, M. T. Mock, A. M. Appel
and J. Y. Yang, Eur. J. Inorg. Chem., 2013, 3846; (e)
D. Hong, S. Mandal, Y. Yamada, Y.-M. Lee, W. Nam,
A. Llobet and S. Fukuzumi, Inorg. Chem., 2013, 52, 9522; (f)
Z. Codol `a , I. Garcia-Bosch, F. Acuna-Par ´e s, I. Prat,
J. M. Luis, M. Costas and J. Lloret-Fillol, Chem. – Eur. J.,
(AoE/P-03/08), and the CAS-Croucher Foundation Funding
Scheme for Joint Laboratories. We thank Prof. Hung Kay Lee for
assistance with EPR measurements.
2
013, 19, 8042; (g) B. Zhang, F. Li, F. Yu, H. Cui, X. Zhou,
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