Water oxidation by mononuclear ruthenium complexes with TPA-based ligands

Article abstract of DOI:10.1021/ic200050g

The synthesis, characterization, and water oxidation activity of mononuclear ruthenium complexes with tris(2-pyridylmethyl)amine (TPA), tris(6-methyl-2-pyridylmethyl)amine (Me₃TPA), and a new pentadentate ligand N,N-bis(2-pyridinylmethyl)-2,2′-bipyridine-6-methanamine (DPA-Bpy) have been described. The electrochemical properties of these mononuclear Ru complexes have been investigated by both experimental and computational methods. Using Ce^IV as oxidant, stoichiometric oxidation of water by [Ru(TPA)(H₂O)₂]²⁺ was observed, while Ru(Me₃TPA)(H₂O)₂]²⁺ has much less activity for water oxidation. Compared to [Ru(TPA)(H₂O) ₂]²⁺ and [Ru(Me₃TPA)(H₂O) ₂]²⁺, [Ru(DPA-Bpy)(H₂O)]²⁺ exhibited 20 times higher activity for water oxidation. This study demonstrates a new type of ligand scaffold to support water oxidation by mononuclear Ru complexes.

Full text of DOI:10.1021/ic200050g

ARTICLE
pubs.acs.org/IC
Water Oxidation by Mononuclear Ruthenium Complexes with
TPA-Based Ligands
†
‡
†
†
Bhasker Radaram, Jeffrey A. Ivie, Wangkheimayum Marjit Singh, Rafal M. Grudzien,
§
,†
,†
Joseph H. Reibenspies, Charles Edwin Webster,* and Xuan Zhao*
†
Department of Chemistry, The University of Memphis, Memphis, Tennessee 38152, United States
‡
Department of Chemistry, Physics, and Astronomy, Georgia College & State University, Milledgeville, Georgia 31061, United States
§
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
S Supporting Information
b
ABSTRACT: The synthesis, characterization, and water oxidation activity of mononuclear
ruthenium complexes with tris(2-pyridylmethyl)amine (TPA), tris(6-methyl-2-pyridyl-
methyl)amine (Me TPA), and a new pentadentate ligand N,N-bis(2-pyridinylmethyl)-
3
0
2
,2 -bipyridine-6-methanamine (DPA-Bpy) have been described. The electrochemical
properties of these mononuclear Ru complexes have been investigated by both experi-
mental and computational methods. Using Ce as oxidant, stoichiometric oxidation of
water by [Ru(TPA)(H O) ] was observed, while Ru(Me TPA)(H O) ] has much less activity for water oxidation. Compared
to [Ru(TPA)(H O) ] and [Ru(Me TPA)(H O) ] , [Ru(DPA-Bpy)(H O)] exhibited 20 times higher activity for water
IV
2+
2+
2
2
3
2
2
2
+
2+
2+
2
2
3
2
2
2
oxidation. This study demonstrates a new type of ligand scaffold to support water oxidation by mononuclear Ru complexes.
9
’
INTRODUCTION
RudO species has been reported. Since TPA is a tetradentate
ligand, it provides a scaffold to design novel types of penta- and
hexadentate ligands and offers the opportunity to include a
second-coordination sphere environment to tune the activity
of metal complexes. We are interested in exploring the activity of
water oxidation by high-valent RudO complexes based on TPA
ligand. Here, we describe the synthesis, characterization, and
water oxidation activity of mononuclear Ru complexes with TPA
and its derivatives (Figure 1).
Oxidation of water to molecular oxygen is a reaction of vital
1
importance to sustain life on earth. Using the energy from
sunlight, photosynthesis splits water into oxygen and extracts elec-
trons from water to reduce carbon dioxide into carbohydrates.
Design of water oxidation catalysts represents a critical step in
artificial photosynthesis for conversion and storage of sunlight
into high-energy chemicals such as H . Synthetic structural and
functional models of the oxygen-evolving center (OEC) in
2
3
2
Photosystem II (PSII) have offered important insight into the
4
water oxidation process catalyzed by the OEC. It has been
’ EXPERIMENTAL SECTION
generally recognized that high-valent metalꢀoxo species, such as
Materials and Synthesis. All chemicals were purchased from
Sigma-Aldrich except noted. The reagent N-bromosuccinimide (NBS)
was purified by recrystallization from boiling water before use. Other
reagents such as 2-(aminomethyl)pyridine, 2,6-lutidine, benzoyl per-
oxide, potassium phthalimide, RuCl 3H O, (NH ) Ce(NO ) , and
MndO, RudO, and CodO, are involved in splitting of the water
5
molecule to produce molecular dioxygen. Ru complexes have
played an important role in developing water oxidation catalysts.
4+
The dinuclear cis,cis-[(bpy) (H O)RuORu(H O)(bpy) ]
2
2
2
2
3
3
2
4 2
3 6
complex known as “blue dimer” was the first molecular complex
0
5e
6-methyl-2,2 -dipyridyl were used as received without further purifica-
discovered to catalytically oxidize water to dioxygen. Over the
past few years, a number of other types of dinuclear and mono-
nuclear Ru complexes capable of oxidizing water to molecular
18
tion. Milli-Q H
obtained from Cambridge Isotope Inc. Syntheses of tris(2-pyridyl-
methyl)amine (TPA), tris(6-methyl-2-pyridylmethyl)amine (Me TPA),
and 6-(bromomethyl)-2,2 -bipyridine were carried out following litera-
2 2
O (18.2 MΩ) was used in all experiments. H O was
6
3
dioxygen have been designed, synthesized, and characterized.
0
Tetradentate ligand tris(2-pyridylmethyl)amine (TPA) and
its derivatives have been used extensively in modeling the active
sites of mono- and dinuclear nonheme metalloproteins involved
in dioxygen activation, such as Fe-, Cu-, and Ni-containing
10
ture methods. [Ru(TPA)(H
2
O)
2
](PF
6
)
2
(1a) was synthesized ac-
9
cording to the literature method.
Synthesis of N,N-Bis(2-pyridinylmethyl)-2,2 -bipyridine-
0
6
-methanamine (DPA-Bpy). To a solution of 6-(bromomethyl)-
7
metalloproteins. Ru complexes with TPA ligand have also
0
2,2 -bipyridine (0.66 g, 2.65 mmol) in acetonitrile (50 mL) was added
equiv of di-(2-picolyl)amine (0.52 g, 2.65 mmol) at room temperature.
8
received much attention. Formation of high-valent RudO species
1
9
has been reported for ruthenium complexes with TPA ligand.
The base di-isopropyl ethyl amine (0.69 mL, 3.98 mmol) was added
2+
For example, mononuclear [Ru(TPA)(H O) ] (1a) can achi-
eve redox states from Ru to Ru through a proton-coupled
electron transfer process. Oxygenation of hydrocarbons by
2
2
II
VI
Received: January 8, 2011
Published: October 14, 2011
r 2011 American Chemical Society
10564
dx.doi.org/10.1021/ic200050g
|Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
ARTICLE
CN,
1
concentrated solution of the complex in acetonitrile. H NMR (CD
00 MHz): δ 4.90 (t, 2H, axial bipyridyl methylene, H6), 5.17 (d, 4H,
equatorial pyridyl methylene, H1), 6.93 (t, J = 13.12 Hz, 2H, py-H4,),
.15 (d, J = 7.7 Hz, 1H, bpy-H7), 7.17 (dt, J = 11.88, 5.45 Hz, 2H, py-
H5), 7.37 (d, J =7.91 Hz, 2H, Py-H2), 7.54 (t, J =7.91 Hz, 1H, bpy-H8),
3
5
7
7
8
8
.59 (dt, J = 7.91 Hz, 2H, Py-H3), 7.78 (dt, J = 14.09 Hz, 1H, bpy-H12),
.14 (dt, J = 8.21 Hz, 1H, bpy-H9), 8.17 (d, J = 7.8 Hz, 1H, bpy-H11),
.40 (d, J = 8.15 Hz, 1H, bpy-H10), 9.46 (d, J = 5.67 Hz, 1H, bpy-H13).
+
ꢀ +
ESI-MS: m/z 504.13 (calcd m/z for [M ꢀ Cl ] 503.97). Anal. Calcd
for C24 Ru:C,46.16;H,4.84;N,11.21.Found:C,46.35;H,4.75;
N, 11.23. Absorption maxima (CH Cl ) (λmax, nm): 384 (sh), 430, 516.
Ru(DPA-Bpy)Cl](PF ) 3H O. To a cold aqueous solution of 3a
2 5 4
H30Cl N O
2
2
[
6
3
2
Figure 1. Mononuclear Ru complexes.
was added dropwise a saturated solution of NH
mixture was filtered, washed with cold water, and dried to yield
Ru(DPA-Bpy)Cl](PF ) 3H O as a dark red solid (0.30 g, 73%).
4 6
PF . The reaction
[
dropwise at 0 ꢀC. The reaction mixture was stirred at room temperature
for 5 h, and the reaction progress was monitored by TLC. Upon
completion of the reaction, water (50 mL) was added. The product
was extracted into ethyl acetate (3 ꢁ 50 mL) and washed with brine (2 ꢁ
6
3
2
+
+
+
ESI-MS: m/z 504.24 (calcd m/z for [M ꢀ Cl] 503.97). Anal. Calcd
for C23 26ClF PRu: C, 39.35; H, 3.73; N, 9.98. Found: C, 39.48; H,
.38; N, 9.90.
[Ru(DPA-Bpy)(H
Bpy)Cl](PF ) 3H O (0.085 g, 0.13 mmol) in 10 mL of H
2
dropwise a solution of AgPF (0.050 g, 0.20 mmol) in 2 mL of H O
H
6 5 3
N O
3
30 mL). The combined organic layers were dried over anhydrous
2
O)](PF )
6
2
(3b). To a solution of [Ru(DPA-
O was added
Na
2
SO
4
, filtered, and evaporated under reduced pressure to afford the
6
2
2
3
crude compound. Recrystallization in hexane yielded a white solid (0.8 g,
6
1
81%). H NMR (CDCl
3
, 500 MHz): δ 3.88 (s, 4H, pyridyl methylene,
under an argon atmosphere. The reaction mixture was refluxed over-
night, and the precipitate was filtered through Celite. After removal of
solvent, the residue was dissolved in a minimum amount of methanol,
H1), δ 3.90 (s, 2H, bipyridyl methylene, H6), 7.07 (dt, J = 5.76 Hz, 2H,
py-H4), 7.21 (dt, J = 8.4 Hz, 1H, bpy-H12), 7.46 (d, J = 8.0 Hz, 1H, bpy-
H7), 7.59 (d, J = 6.5 Hz, 2H, py-H2), 7.60 (d, J = 6.5 Hz, 2H, py-H3),
washed with diethyl ether, and dried under reduced pressure to yield a
1
7
8
.71 (dt, J = 7.5 Hz, 1H, bpy-H8), 7.73 (dt, J = 7.5 Hz, 1H, bpy-H11),
.17 (d, J = 7.5 Hz, 1H, bpy-H9), 8.36 (d, J = 8.0 Hz, 1H, bpy-H10), 8.46
reddish solid (0.068 g, 68%). H NMR (CD
3
CN, 500 MHz): δ 4.93 (t,
4 H, axial bipyridyl methylene-H6), 5.07 (d, 2 H, equatorial pyridyl
methylene-H1), 7.06 (t, J = 6.6 Hz, 2H, py-H4), 7.28 (dd, J = 8.0 Hz, 1H,
bpy-H7), 7.30 (dd, J = 5.3 Hz, 2H, py-H5), 7.51 (d, J = 7.64 Hz, 2H, py-
H2), 7.75 (dt, J = 7.81 Hz, 2H, py-H3), 7.79 (dt, J = 8.1 Hz, 1H, bpy-H8),
7.86 (dt, J = 6.5 Hz, 1H, bpy-H12), 8.23 (dt, J = 7.8 Hz, 1H, bpy-H11),
8.25 (d, J = 7.8 Hz, 1H, bpy-H9), 8.52 (d, J = 7.82 Hz, 1H, bpy-H10),
(d, J=5 Hz, 2H, py-H5), 8.58(d, J =5.0Hz, 1H, bpy-H13). Anal. Calcdfor
C H N : C, 75.18; H, 5.76; N, 19.06. Found: C, 75.01; H, 5.85; N, 18.89.
2
3 21 5
[
Ru(TPA)(H O) ](CF SO ) H O (1b). To a refluxed solution of
2 2 3 3 2 2
3
RuCl
3
3H
2
O (0.20 g, 0.764 mmol) in ethanol (30 mL) was added TPA
0.222 g, 0.764 mmol). After refluxing for 10 min, 20 mL of an aqueous
solution of AgCF SO (0.588 g, 2.294 mmol) was added. The mixture
3
(
+
+
9.22 (d, J =5.42 Hz, 1H, bpy-H13). ESI-MS: m/z 613.7 (calcd m/z for
3
3
+
was then refluxed for 24 h, and the precipitate was removed by filtration.
After removal of solvent, the product was dissolved in a minimum
amount of ethanol and filtered. After washing with ether and drying
[M ꢀ H
35.58; H, 2.99; N, 9.02. Found: C, 35.83; H, 2.95; N, 8.90. Absorption
maxima (CH Cl ) (λmax, nm): 372 (sh), 410, 496.
Characterization Methods. UVꢀvis absorption spectra were
measured in 18.2 MΩ Milli-Q H O at room temperature using a HP-
2 6 23 12 5 2
O - -PF ] 613.0). Anal. Calcd for C23H F N OP Ru: C,
2
2
1
under vacuum, a dark green solid was obtained. Yield: 0.19 g (33%). H
NMR (270 MHz, CD
3
CN): δ 4.56 (s, 2H, CH
2
(axial)), 4.87ꢀ5.09
2
1
(ABq, JAB =15.47 Hz, 4H, CH (equatorial), 7.04(d, J = 7.91 Hz, 1H,
8452A diode array spectrometer. H NMR spectroscopy was conducted
on a Joel 270 MHz or a Varian DirectDrive 500 MHz spectrometer.
Cyclic voltammetric measurements were performed in 0.1 M Brittonꢀ
Robinson buffer using a glassy carbon electrode, a platinum wire counter
electrode, and a Ag/AgCl reference electrode. When a redox wave was
not clear in cyclic voltammetry, square wave voltammetry was used to
determine its redox potential. All oxygen evolution measurements were
2
Pyr-H3(axial)), 7.23 (t, J = 6.54 Hz, Pyr-H5 (axial)), 7.31 (t, 6.6 Hz, 2H,
Pyr-H5 (equatorial)), 7.47 (d, J = 7.91 Hz, 2H, Pyr-H3 (equatorial)),
7
.59 (t, J = 15.57 Hz, 1H, Pyr-H4 (axial)), 7.80 (t, J = 15.57 Hz, 2H, Pyr-
H4 (equatorial), 8.7(d, J = 4.96 Hz, 2H, Pyr-H6 (equatorial)), 8.97 (d,
J = 5.09 Hz, 1H, Pyr H6 (axial)). Anal. Calcd for C20 Ru:
23 6 4 9 2
H F N O S
C, 32.35; H, 3.12; N, 7.54. Found: C, 32.66; H, 3.00; N, 7.62. Absorption
maxima (λmax, nm): 338, 404, 604.
performed using a calibrated O
Ce(IV) solution (0.33 M) was rapidly added to a stirred 4.0 mM complex
solution in a 10 mL round-bottom flask, and O evolution was recorded
over time. Analysis of dioxygen in a reaction headspace was conducted
using a HP G1800A GCD system gas chromatography with an electron
2
electrode (YSI 5300A). Generally,
1
[
Ru(Me TPA)(H O) ](CF SO ) (2). Yield: 40%. H NMR (270
3
2
2
3
3 2
MHz, CD
3
CN): δ 2.92 (S, 9H, methyl), 4.25 (s, 6H, ꢀCH
2
ꢀ), 7.25 (d,
2
J = 7.02 Hz, 1H, Pyr-H3 (axial)), 7.27 (d, J = 7.02 Hz, 2H, Pyr-H3
equatorial)), 7.74 (d, J = 7.02 Hz, 1H Pyr-H4 (axial)), 7.76 (d, J =7.01
Hz, 2H, Pyr-H4 (equatorial)), 8.34 (t, J =7.02 Hz, 3H, Pyr-H5). Anal.
Calcd for C23 Ru: C, 35.98; H, 3.68; N, 7.30. Found: C,
6.41; H, 3.52; N, 7.40. Absorption maxima (λmax, nm): 610 (br).
(
ionization detector. GC-MS monitoring of the formation of CO
and O
2
2
28 6 4 8 2
H F N O S
by complexes 1b, 2, and 3b (0.2 mM) was conducted in the presence of
IV
3
3
Ce (0.66 M) in 1 M HNO . Elemental analyses were conducted by
[
RuCl(DPA-Bpy)]Cl 3H O CH OH (3a 3H O CH OH). To a
Atlantic Microlab, Inc., Atlanta, GA.
3
2
3
3
3
2
3
3
refluxed solution of RuCl
3
3H
2
O (0.17 g, 0.65 mmol) in 80 mL of
X-ray Structure Determination. Crystals of 3a were obtained
from slow diffusion of diethyl ether into an acetonitrile solution contain-
ing 3a at room temperature. A Leica MZ 75 microscope was used to
identify a suitable crystal with very well-defined faces with dimensions
(max, intermediate, and min) 0.1 mm ꢁ 0.1 mm ꢁ 0.01 mm. The crystal
mounted on a nylon loop was then placed in a cold nitrogen stream
maintained at 110 K. A BRUKER D8-GADDS X-ray (three-circle)
diffractometer was employed for crystal screening, unit cell determina-
tion, and data collection. The goniometer was controlled using the
3
ethanol was added dropwise a solution of DPA-Bpy (0.24 g, 0.65 mmol)
in 10 mL of ethanol over a period of 25 min under an argon atmosphere.
The reaction mixture was refluxed for 12 h. Crude reaction mixture was
filtered to remove inorganic material present in solution. The filtrate was
evaporated under reduced pressure and purified by dissolving in a
minimum amount of methanol and then washed with 20 mL of diethyl
ether to get the purified product as a precipitate, which was dried under
reduced pressure to afford dark red solid (0.30 g, 73%). Crystals suitable
for X-ray crystallography were grown by slow diffusion of ether into a
1
1
FRAMBO software suite. The detector was set at 6.0 cm from the
1
0565
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
Table 1. Experimental and Computed Redox Potentials of 1, 2, and 3 in BrittonꢀRobinson Buffer at pH 7, E1/2, V vs NHE
ARTICLE
III/II
III/II c
IV/III
IV/III c
V/IV
V/IV c
VI/V
VI/V c
Ru
(Ru
)
Ru
(Ru
)
Ru
(Ru
)
Ru
(Ru
)
a
1
1
2
3
3
a
0.42
ꢀ0.12
0.72
0.30
0.90
0.55
NA
0.85
b
0.42
0.26
0.60
0.89
0.73
0.62
0.84
1.20
0.97
1.00
NA
1.20
1.36
NA
0.00
0.26
0.24
0.56
0.63
0.83
b
b
b
1.75
NA
a
b
c
3
From ref 9. In 0.1 M HNO . Computed redox potential.
crystal sample (MWPC Hi-Star Detector, 512 ꢁ 512 pixel). X-ray
radiation employed was generated from a Cu sealed X-ray tube (Cu
Kα = 1.54184 Å with a potential of 40 kV and a current of 40 mA) fitted
with a graphite monochromator in the parallel mode (175 mm collimator
with 0.5 mm monocapillary optics).
ruthenium], and nondefault SCF convergence for geometry optimiza-
ꢀ6
tions (10 ). All PBE calculations were carried out using Gaussian 03.
2
5
All M06 calculations were carried out using Gaussian 09. The basis sets
2
6
utilized were the LANL08, which is a triple-ζ-modified version of the
original HayꢀWadt basis set with an ECP for ruthenium, and 6-311G-
28
2
7
Data Reduction, Structure Solution, and Refinement. In-
tegrated intensity information for each reflection was obtained by
(d) basis set for all other atoms. The density fitting approximation
for the fitting of the Coulomb potential was used for all PBE calculations;
auxiliary density-fitting basis functions were generated automatically (by
the procedure implemented in Gaussian 03) for the specified AO basis
set. The Hessian was computed on gas-phase-optimized geometries, and
standard statistical mechanical relationships were used to determine the
change in Gibbs free energy in the gas phase, ΔGgas. The solvation free
12
reduction of the data frames with the program SAINT. The integration
method employed a three-dimensional profiling algorithm, and all data
were corrected for Lorentz and polarization factors as well as for crystal
decay effects. Finally, the data was merged and scaled to produce a
suitable data set. The absorption correction program SADABS was
13
comp
29
employed to correct the data for absorption effects.
energies, ΔG
solv
, were calculated using two different methods, C-PCM
3
0
Systematic reflection conditions and statistical tests for the data
suggested the space group P1. A solution was obtained readily using
and SMD. The C-PCM method was used to compute solvation free
energies via the self-consistent reaction field (SCRF) keyword in
14
29b
SHELXTL (SHELXS). All non-hydrogen atoms were refined with
anisotropic thermal parameters. The hydrogen atoms bound to carbon
were placed in idealized positions [CꢀH = 0.96 Å, Uiso(H) = 1.2 ꢁ
Gaussian 03. The solvent cavity was built using the universal force
3
1
field (UFF) radii from the UFF force field, as recommended by Roy
1
6
et al. for this computational scheme. In Gaussian 03 solvent calcula-
tions, the SCFVAC keyword was used and solvent parameters consistent
with water were used to model implicit solvation by the solvent. All other
default solvent parameters for water were used. The SMD method was
used to compute solvation free energies via the self-consistent reaction
field (SCRF) keyword for M06 in Gaussian 09. The SMD solvation
model was used with the default parameters consistent with water as the
solvent. The results from the PBE functional with C-PCM solvation free
energies gave a superior linear correlation with the experimental values;
therefore, the results from the M06 functional with SMD solvation free
energies are not discussed further and are given in the Supporting
Information (Figure S1).
U (C)]. The structure was refined (weighted least-squares refinement
iso
2
13
on F ) to convergence. 5X-seed was employed for final data presenta-
1
tion and structure plots.
Density Functional Theory (DFT) Calculations and Com-
puted Redox Potentials. The redox potentials were calculated using
16
a method similar to the one devised by Roy et al. The computed free
comp
energy of solution, ΔGsoln , is calculated from the free energy change in
redox
the gas phase of the redox couple, ΔGgas , the solvation free energy
comp
change between the oxidized (ΔGsolv (ox)) and the reduced species
comp
solv
(
ΔG
(red)), and the number of protons, lost from the complex to
solution (n) (eq 1). In order to account for the loss of a proton to solvent
exp
+
water, the experimental value for the solvation of a proton (ΔGsolv (H ) =
ꢀ
1
17
ꢀ
265.9 kcal mol , the value suggested by Truhlar ) and the gas-phase
exp
gas
+
ꢀ1 18
’
RESULTS AND DISCUSSION
Gibbs free energy of a proton (ΔG (H ) = ꢀ6.28 kcal mol
)
were
used.
Synthesis and Structure. Complex 1a was synthesized
comp
redox, comp
comp
solv
comp
solv
ΔGsoln ¼ ΔGgas
þ ΔG
ðredÞ ꢀ ΔG
ðoxÞ
following literature method from reaction of bis-μ-chloro
II
exp
solv
þ
exp
þ
Ru dimer, [RuCl(TPA)] (PF ) , with AgPF under refluxing
2
6 2
6
ꢀ
n½ΔG ðH Þ þ ΔG ðH Þꢂ
ð1Þ
gas
8,9
conditions. However, attempts to synthesize the tris(6-methyl-
2-pyridylmethyl)amine (Me TPA) analog of the bis-μ-chloro
Ru dimer resulted in a low yield. We prepared mononuclear
comp
soln
ΔG
and the absolute value of the standard hydrogen electrode (SHE,
3
II
II
also called the normal hydrogen electrode, NHE) in water (ꢀ4.44 ( 0.02
19
V) are used to determine the standard one-electron redox potential,
Ru complexes [Ru(TPA)(H
2 2 2 2
O) ](OTf) H O (1b) and
3
ꢀ
,comp
ꢀ1 ꢀ1
E
soln , where F is the Faraday constant, 23.06 kcal mol
V
(eq 2).
[Ru(Me TPA)(H O) ](OTf) (2) from refluxing an ethanol
3
2
2
2
comp
soln
solution containing RuCl
3
3H
2
O, AgOTf, and the correspond-
TPA).
Bipyridine (bpy) and its derivatives have been used widely in
3
,
comp
ΔG
ꢀ
E
¼
ꢀ 4:44 V
ð2Þ
ing ligand (TPA and Me
3
soln
kcal
ꢀ
23:06
mol V
5e,32
3
ruthenium catalysts for water oxidation.
Since TPA is a
tetradentate ligand, both complexes 1 and 2 have two coordi-
The computed oxidation potentials the ruthenium species in water are
shown in Table 1 with results referenced to the absolute value of the
standard hydrogen electrode (SHE) in water.
nated H O molecules. In order to investigate the chelating effects
2
of polydentate ligands on the stability of metal complexes toward
water oxidation, we introduced a bpy group into TPA ligand
to produce a new type of pentadentate ligand, N,N-bis(2-
2
0
Theoretical calculations were carried out using Gaussian 03 and
2
1
22
Gaussian 09. Density functional theory PBE [PBE exchange
23
24
0
functional and correlation functional ] and M06 functionals with the
default pruned fine grids for energies (75, 302), default pruned course
grids for gradients and Hessians (35, 110) [neither grid is pruned for
pyridinylmethyl)-2,2 -bipyridine-6-methanamine (DPA-Bpy).
DPA-Bpy was synthesized from reaction of di-2-picolylamine
0
10a
and 6-(bromomethyl)-2,2 -bipyridine (Scheme 1). Reaction
1
0566
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
ARTICLE
of RuCl 3H O with DPA-Bpy in refluxing ethanol solution led
are shown in Table S3, Supporting Information. The two trans
pyridine groups and the bipyridine group caused distortion of the
Ru center from octahedral geometry, having — N(5)ꢀRu(1)ꢀ
N(4) = 164.1(5)ꢀ and — N(1)ꢀRu(1)ꢀN(3) = 164.5(5)ꢀ, res-
pectively. The RuꢀN and RuꢀCl distances are similar to those
3
3
2
to formation of a water-soluble complex [RuCl(DPA-Bpy)]Cl
ꢀ
(
3a). The coordinated Cl in 3a could be substituted by a H O
2
molecule to yield 3b from refluxing an aqueous solution of 3a in
the presence of AgPF . Alternatively, complex 3b can be
6
8,33
prepared from reaction of 1 equiv of AgPF with [RuCl(DPA-
reported in the literature.
6
Bpy)](PF ). The prepared compounds were characterized by
1D and 2D NMR experiments were carried out to characterize
all complexes in solution state, and the assignments of each
resonance were accomplished based on their integration, multi-
plicity, and symmetry (see the Experimental Section and Sup-
porting Information Figures S2ꢀS5). As shown in Figure S3,
6
elemental analysis, UVꢀvis, and 1D and 2D NMR (see the
Supporting Information).
A single-crystal of complex 3a was obtained from slow
diffusion of diethyl ether into an acetonitrile solution containing
1
3
a at room temperature. The X-ray crystal structure of the
Supporting Information, the H NMR spectrum of complex 3a
cationic form of 3a is shown in Figure 2, with the crystallographic
parameters listed in Table S2, Supporting Information. As shown
in Figure 2, the Ru center adopts a pseudooctahedral geometry
with five positions occupied by the pentadentate ligand, DPA-
Bpy, and the final position by Cl , which lies in a position cis to
the tertiary amine group and the two pyridine moieties which are
shows the two pyridine groups have the same NMR features,
indicating the presence of C symmetry for 3a in solution with a
s
plane containing the tertial amine and the bipyridine group. Such
structural feature of 3a in solution is consistent with its solid state
X-ray structure. Table S1, Supporting Information, lists the
comparison of each resonance in free ligand, DPA-Bpy, and its
Ru complexes 3a and 3b. Compared to free DPA-Bpy, coordina-
tion of Ru caused the downfield shifts of all three methylene
groups and H11ꢀ13 (Figure S3, Supporting Information). An
upfield shift was clearly observed for H2, H5, H7, and H9 in both
ꢀ
trans to each other. The selected bond distances and angles of 3a
Scheme 1. Synthesis of DPA-Bpy Ligand and Complexes 3a
and 3b
3a and 3b.
Spectroscopic and Redox Properties. The UVꢀvis spectra
of 1b and 2 in water solution are displayed in Figure 3a, which
showed slight changes in MLCT bands from the Ru(dπ) f
py(pπ*) transition, indicating the TPA and Me TPA ligands
3
8
have similar π-acceptor character. As seen from Figure 3b,
substitution of Cl by a H O molecule shifts the absorption
ꢀ
2
spectrum of 3a in CH Cl from 430 and 516 nm to 410 and
2
2
4
96 nm, respectively, in 3b.
Previous studies have shown that compound 1a at pH 7
exhibits a sequence of three redox events centered at 0.42,
III/II
0
Ru
.72, and 0.90 V (vs NHE), corresponding to the Ru
,
IV/III
V/IV
9
, and Ru
couples, respectively. The electrochemical
properties of complexes 1b, 2, and 3 were investigated in aqueous
solution at pH 7 at room temperature. The cyclic voltammogram
III/II
of 1b at pH 7 exhibits a reversible Ru
couple at 0.42 V (vs
NHE), the same as that reported for 1a (Figure 4a). The barely
IV/III
visible Ru
couple was determined to be 0.73 V from square
V/IV VI/V
wave voltammetry. The Ru
and Ru
couples appear at
0
.97 and 1.20 V, respectively, similar to that previously observed
9
for 1a. Compared to 1b, the CV of 2 showed much weaker redox
signals (Figure 4b). The potentials for the redox state changes of
II
VI
2
from Ru to Ru were determined by square wave voltam-
metry (Table 1 and Figure 4c). At pH 7, complex 3b exhibits two
reversible redox waves at 0.60 and 0.84 V, which were assigned to
III/II
IV/III
Ru
and Ru
, respectively. At pH 1 (0.1 M HNO ), the
3
III/II IV/III
Figure 2. X-ray structure of the cationic moiety of 3a.
corresponding redox potentials for the Ru
and Ru
Figure 3. UVꢀvis spectra of (a) 1b (solid line) and 2 (dotted line) in H O and (b) 3a (dotted line) and 3b (solid line) in CH Cl .
2
2
2
1
0567
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
ARTICLE
Figure 4. Cyclic voltammograms of (a) 1b and (b) 2 and (c) square wave voltammogram of 2 in 100 mM BrittonꢀRobinson buffer at pH 7: scan rate,
00 mV/s; working electrode, glassy carbon; counter electrode, Pt wire; reference electrode, Ag/AgCl.
1
3
Figure 5. (a) Cyclic voltammograms of 3b (1 mM) in 100 mM KPi buffer at pH 7 (solid line) and 0.1 M HNO (dotted line). (b) Comparison of CV in
the absence (dotted line) and presence (solid line) of 3b (1 mM). Scan rate, 100 mV/s; working electrode, glassy carbon; counter electrode, Pt wire;
reference electrode, Ag/AgCl.
Figure 6. Pourbaix diagram for complex 3b in aqueous Brittonꢀ
Robinson buffer (E1/2 vs Ag/AgCl).
Figure 7. UVꢀvis spectra of 3b at different oxidation states:
II
2+
III
2+
IV
2+
[
Ru ꢀOH
2
]
(solid line), [Ru ꢀOH] (dotted line), and [Ru dO]
IV
(dashed line) generated from stoichiometric oxidation with Ce in
couples appear at 0.89 and 1.22 V, respectively (Figure 5a).
0.1 M HNO .
3
V/IV
Another shoulder assignable to the Ru
couple appeared at
1
(
.72 V before the catalytic wave of water oxidation at pH 1
Figure 5b). The Pourbaix diagram for complex 3b in the region
of pH 1ꢀ14 is shown in Figure 6, with redox state changes from
350 nm peak to a weak band at 430 nm, corresponding to
formation of Ru dO species.
IV
Introduction of the CH
to reduce the redox potential of its metal centers because of the
electron-donating ability of the CH group. However, previous
studies have shown that the metal complexes with α-substituted
Me TPA ligand have higher redox potentials than those with
TPA ligand.
effect of the 6-CH
the bond length of the MꢀNpy bond and result in a much weaker
group on the pyridine ring is expected
3
II
V
III/II
IV/III
Ru to Ru . The Ru
couple is ꢀ54 mV/pH, and the Ru
couple is ꢀ61 mV/pH, consistent with a proton-coupled elec-
tron transfer process which enables formation of a high-valent
RudO species. The pK of coordinated H O in 3b was deter-
3
a
2
3
7a,34
mined to be 11.8 from pH titration in the range of pH 7ꢀ13
The unexpected observation is due to the steric
substituent on pyridine, which could increase
(
Figure S6, Supporting Information), consistent with that ob-
3
served the Pourbaix diagram of complex 3b. The UVꢀvis spectral
7
a,34
changesof 3bat different oxidationstatesarereported inFigure7.
electron donor for 6-CH
3
pyridine.
We carried out DFT
group
IV
Addition of 1 equiv of Ce to 3b in 0.1 M HNO resulted in a
calculations to investigate the effects of introducing a CH
3
3
bleaching of the MLCT bands and the appearance of a broad
peak at 350 nm. Further addition of 1 equiv of Ce shifted the
on the pyridine ring of TPA ligand on the redox potential of Ru
complexes. The computed structures of Ru dO species for
IV
IV
1
0568
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
ARTICLE
IV
Figure 8. DFT-optimized structures for dicationic Ru dO species of (a) 1 and (b) 2.
Supporting Information). For complex 2, we observed less than
0% of O formation compared to that of 1. This result could
1
2
possibly be due to the steric effect of the CH group. To test the
3
water oxidation activity of 1 under neutral conditions, cyclic
voltammetry of 1b was performed in 100 mM sodium phosphate
buffer at pH 7. As shown in Figure S8b, Supporting Information,
the presence of 1b in buffer resulted in a strong irreversible
anodic current at potential higher than 1.5 V (vs NHE) due to the
electrocatalytic oxidation of water to O . The observation of
2
water oxidation at a potential higher than 1.5 V indicated that a
high-valent Ru(V)dO or higher oxidation state may be respon-
sible for water oxidation. As shown in Figure 10a, the water
oxidation activity of 3b was determined with a turnover number
of 24. Under the same conditions, complexes 1a and 2 have much
less activity in water oxidation. Therefore, there is a significant
improvement in the water oxidation activity by introducing a bpy
group in complex 1. The activity of these complexes is relatively low
in terms of turnover number and turnover frequency, in an order
Figure 9. Linear relationship between the computed and the experi-
mental redox potential for complexes 1 (ꢁ) and 2 (+).
2+ 36
similar to that reported for mononuclear [Ru(bpy) (H O) ] .
Decomposition of Ru catalysts under acidic conditions in Ce
2
2
2
IV
1
and 2 showed similar effects, with 2 having longer RuꢀN
eq
solution has been noted, and a variety of pathways have been
proposed to account for catalyst deactivation. Recently, Llobet
and co-workers reported water oxidation by a series of tetra-
nuclear Ru complexes and demonstrated that decomposition of
bond distances (2.17 Å) than that of 1 (2.11 Å) (Figure 8). As
shown in Table 1, DFT calculations predict that the Ru
V/IV
couple for 2 has a potential of ∼80 mV higher than that of 1. The
V/IV
observed redox potentials for the Ru
couple of 1 and 2 remain
methylene groups of tetranucleating ligands to CO could be one
nearly unchanged, suggesting that the α-substituted Me TPA
2
3
3
7
possible pathway for deactivation of Ru catalysts. Indeed, we
ligand has little effect on the redox potential of the Ru centers
also observed formation of CO during the water oxidation pro-
(Table 1). The trend in the computed redox potentials closely
2
cess of 1ꢀ3; this result is probably due to the presence of methylene
groups in TPA ligand. In order to determine whether complexes
1ꢀ3 act as real catalysts or just precursors for water oxidation,
matches the trend observed in the experimental values redox
potentials (Figure 9), suggesting DFT calculations can be used to
predict the redox behavior of metal complexes and thus provid-
ing important guidelines for future studies.
we monitored the simultaneous formation of O
and CO
formation for
formation in the presence
. Under the same conditions,
formation at 1.5 times that of
formation (Figure S9a, Supporting Information). However,
the rate for CO formation of complex 2 is 1.7 times that of O
2
using
2
2
Water Oxidation Studies. The oxygen evolution activity of com-
plexes 1ꢀ3 was examined following literature methods using
GC-MS. As shown in Figure 10b, the rate of O
complex 3b is 12 times of that of CO
2
2
IV
IV
+
III
IV
Ce as oxidant (2H O + 4Ce f O + 4H + 4Ce ), and the
of excess Ce in 1 M HNO
complex 1a displays a rate of O
CO
3
2
2
3
5
evolved oxygen was monitored using a calibrated O electrode.
Addition of excess Ce solution to 4.0 mM 1a (3 mL) solution
2
2
IV
2
resulted in rapid production of O (Figure S7a, Supporting
2
2
Information). The evolved gas was confirmed to be O by GC-
formation (Figure S9b, Supporting Information); this result may
explain the much lower water oxidation activity of 2 when
compared to 1 and 3. These results clearly demonstrated oxidation
of water by complexes 1ꢀ3, and simultaneous oxidation of ligand
2
MS, and the amount of O produced was calculated to be ∼12
2
μmol, corresponding to formation of 1 equiv of O . When 97%
2
18
H2 O was used as a solvent, the appearance of a main peak at
m/z = 36 was observed, suggesting formation of O with H O as
18
18
2
2
to CO contributes to catalyst decomposition.
2
the source of O (Figure S7b, Supporting Information). Similar
Oxidation of water by mononuclear Ru complexes has been
2
5
g,35,36,38
to that of 1a, water oxidation by complex 1b was investigated
demonstrated by a family of Ru complexes.
The in-
V
following the same procedure and the amount of O produced
volvement of a high-valent RudO species, presumably Ru dO,
2
was determined to be 1 O /Ru, suggesting that the triflate count-
has been proposed as an active intermediate for oxidation of
2
II
IV
erion has little effect on the water oxidation activity (Figure S8a,
water. Oxidation of Ru to Ru by the PCET process and its
1
0569
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
ARTICLE
IV
Figure 10. (a) Oxygen evolution vs time after addition of 0.33 M Ce to 3 mL of 0.2 mM complexes 1a (dotted line), 2 (dashed line), and 3b (solid
line) in 1 M HNO
Ce (0.33 M) in 1 M HNO .
3
. (b) GC-MS monitoring of the formation of O
2
and CO
2
during the catalytic oxidation of water by 3b (0.2 mM) in the presence of
IV
3.
Scheme 2. Proposed Mechanism for Water Oxidation by
Mononuclear Ru Complex
format; complete list for refs 20 and 21. This material is available
free of charge via the Internet at http://pubs.acs.org.
’
AUTHOR INFORMATION
Corresponding Author
*
Email: cewebstr@memphis.edu; xzhao1@memphis.edu.
’
ACKNOWLEDGMENT
We thank the Department of Chemistry at The University
of Memphis for support. We thank the NSF, CHE 0851880
C.E.W.), for the support of J.A.I. The authors also acknowledge
(
computational hardware and software support from the Uni-
versity of Memphis High Performance Computing Facility and
CROMIUM (Computational Research on Materials Institute at
the University of Memphis). We thank Dr. Theodore Burkey for
helpful discussions. ESI-MS measurements were done with the
help of Dr. Daniel Baker and Dr. Trucchi Pham. The Thermo-
Electron LCQ Advantage liquid chromatograph mass spectrom-
eter and the Varian DirectDrive 500 MHz NMR spectrometer
at the Department of Chemistry at The University of Memphis
were purchased from funding provided by the National Science
Foundation (MRI, CHE-0619682 and CRIF, CHE-0443627).
The X-ray diffractometers, small angle scattering instrumen-
tation, and crystallographic computing systems in the X-ray
Diffraction Laboratory at the Department of Chemistry, Texas A
V
further oxidation to Ru dO may trigger oxidation of water to
III
produce a peroxo-bound intermediate (Ru ꢀOꢀOꢀH), which
can be oxidized with release of a proton and molecular O2
(Scheme 2).
’
SUMMARY
We have prepared and characterized new types of mono-
nuclear Ru complexes with tripodal TPA tetradentate-ligand and
DPA-Bpy pentadentate-ligand architectures. Compared to Ru com-
plexes with the tetradentate TPA ligand, Ru complexes with the
new pentadentate DPA-Bpy ligand have significantly increased
stability and catalytic activity for water oxidation. While TPA
ligand and its derivatives have been widely used in designing metal
&
M University, were purchased with funds provided by the
National Science Foundation (CHE-9807975, CHE-0079822, and
CHE-0215838). We also thank the reviewers for valuable comments.
complexes for O activation, our studies have demonstrated that
2
Ru complexes with TPA, TPA derivatives, and DPA-Bpy pro-
’
REFERENCES
(1) Eisenberg, R.; Gray, H. B. Inorg. Chem. 2008, 47, 1697–1699.
(2) (a) Barber, J. Biochem. Soc. Trans. 2006, 34, 619–631. (b) Renger,
duce O from water oxidation, demonstrating the versatile reac-
2
tivities of TPA and DPA-Bpy ligand architectures when bound
39
with different metal ions. Further mechanistic studies of water
2
+
2+
G. Biochim. Biophys. Acta 2001, 1503, 210–228.
(3) (a) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006,
oxidation by [Ru(TPA)(H O) ] , [Ru(Me TPA)(H O) ] , and
2
2
3
2
2
2+
[
Ru(DPA-Bpy)(H O)] complexes (1, 2, and 3, respectively)
2
1
4
03, 15729–15735. (b) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005,
4, 6828–6840.
and further modifications of TPA and DPA-Bpy ligands to im-
prove the water oxidation activity of their metal complexes are
currently in progress.
(
4) (a) Mukhopadhyay, S.; Mandal, S. K.; Bhaduri, S.; Armstrong,
W. H. Chem. Rev. 2004, 104, 3981–4026. (b) Dau, H.; Limberg, C.;
Reier, T.; Risch, M.; Roggan, S.; Strasser, P. ChemCatChem 2010, 2,
7
24–761.
5) (a) Yagi, M.; Narita, K. J. Am. Chem. Soc. 2004, 126, 8084–8085.
b) Poulsen, A. K.; Rompel, A.; McKenzie, C. J. Angew. Chem., Int. Ed.
’
ASSOCIATED CONTENT
(
(
S
Supporting Information. 1D and 2D NMR spectra,
b
2005, 44, 6916–6920. (c) Limburg, J.; Vrettos, J. S.; Liable-Sands,
L. M.; Rheingold, A. L.; Crabtree, R. H.; Brudvig, G. W. Science 1999,
283, 1524–1527. (d) Brimblecombe, R.; Swiegers, G. F.; Dismukes,
further results for calculation of redox potentials with DFT,
oxygen evolution figures, and crystallographic data in CIF
1
0570
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571

Inorganic Chemistry
ARTICLE
G. C.; Spiccia, L. Angew. Chem., Int. Ed. 2008, 47, 7335–7338. (e)
Gersten, S. W.; Samuels, G. J.; Meyer, T. J. J. Am. Chem. Soc. 1982,
(23) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996,
77, 3865–3868. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev.
Lett. 1997, 78, 1396.
104, 4029–4030. (f) Gilbert, J. A.; Eggleston, D. S.; Murphy, W. R., Jr.;
Geselowitz, D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. J. Am.
Chem. Soc. 1985, 107, 3855–3864. (g) Zong, R.; Thummel, R. P. J. Am.
Chem. Soc. 2005, 127, 12802–12803. (h) Geletii, Y. V.; Botar, B.;
Kogerler, P.; Hillesheim, D. A.; Musaev, D. G.; Hill, C. L. Angew. Chem.,
Int. Ed. 2008, 47, 3896–3899. (i) Kanan, M. W.; Nocera, D. G. Science
(24) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215–241.
(25) A reviewer asked for a comparison to the computed results from
the M06 functional.
(26) Roy, L. E.; Hay, P. J.; Martin, R. L. J. Chem. Theory Comput.
2008, 4, 1029–1031.
2
008, 321, 1072–1075. (j) McEvoy, J. P.; Brudvig, G. W. Phys. Chem.
(27) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650–654.
Chem. Phys. 2004, 6, 4754–4763. (k) Pecoraro, V. L.; Baldwin, M. J.;
Caudle, M. T.; Hsieh, W.-Y.; Law, N. A. Pure Appl. Chem. 1998, 70,
(28) (a) Dunlap, B. I. J. Chem. Phys. 1983, 78, 3140–3142. (b) Dunlap,
B. I. J. Mol. Struct.: Theochem 2000, 529, 37–40. (c) Dunlap, B. I.; Connolly,
J. W. D.; Sabin, J. R. J. Chem. Phys. 1979, 71, 3396–3402. (d) Dunlap, B. I.;
Connolly, J. W. D.; Sabin, J. R. J. Chem. Phys. 1979, 71, 4993–4999.
(29) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102,
1995–2001. (b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput.
Chem. 2003, 24, 669–681.
925–929. (l) Limburg, J.; Szalai, V. A.; Brudvig, G. W. J. Chem. Soc.,
Dalton Trans. 1999, 1353–1362. (m) Betley, T. A.; Wu, Q.; Van Voorhis,
T.; Nocera, D. G. Inorg. Chem. 2008, 47, 1849–1861. (n) McEvoy, J. P.;
Brudvig, G. W. Chem. Rev. 2006, 106, 4455–4483. (o) Hull, J. F.;
Balcells, D.; Blakemore, J. D.; Incarvito, C. D.; Eisenstein, O.; Brudvig,
G. W.; Crabtree, R. H. J. Am. Chem. Soc. 2009, 131, 8730–8731.
(6) (a) Muckerman, J. T.; Polyansky, D. E.; Wada, T.; Tanaka, K.;
(30) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B
Fujita, E. Inorg. Chem. 2008, 47, 1787–1802. (b) Hurst, J. K.; Cape, J. L.;
Clark, A. E.; Das, S.; Qin, C. Inorg. Chem. 2008, 47, 1753–1764.
2009, 113, 6378–6396.
(31) Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard, W. A., III;
Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024–10035.
(32) Brudvig, G. W. Philos. Trans. R. Soc. London, Ser. B: Biol. Sci.
2008, 363, 1211–1219.
(c) Deng, Z. P.; Tseng, H. W.; Zong, R. F.; Wang, D.; Thummel, R.
Inorg. Chem. 2008, 47, 1835–1848. (d) Liu, F.; Concepcion, J. J.; Jurss,
J. W.; Cardolaccia, T.; Templeton, J. L.; Meyer, T. J. Inorg. Chem. 2008,
4
7, 1727–1752. (e) Mola, J.; Mas-Marza, E.; Sala, X.; Romero, I.;
Rodriguez, M.; Vinas, C.; Parella, T.; Llobet, A. Angew. Chem., Int. Ed.
008, 47, 5830–5832. (f) Concepcion, J. J.; Jurss, J. W.; Norris, M. R.;
Chen, Z.; Templeton, J. L.; Meyer, T. J. Inorg. Chem. 2010,
9, 1277–1279. (g) Duan, L. L.; Fischer, A.; Xu, Y. H.; Sun, L. C.
(33) Rodríguez, M.; Romero, I.; Llobet, A.; Deronzier, A.; Biner, M.;
Parella, T.; Stoeckli-Evans, H. Inorg. Chem. 2001, 40, 4150–4156.
(34) (a) Benhamou, L.; Lachkar, M.; Mandon, D.; Welter, R. Dalton
Trans. 2008, 6996–7003. (b) Goodson, P. A.; Oki, A. R.; Glerup, J.;
Hodgson, D. J. J. Am. Chem. Soc. 1990, 112, 6248–6254. (c) Hayashi, Y.;
Kayatani, T.; Sugimoto, H.; Suzuki, M.; Inomata, K.; Uehara, A.; Mizutani, Y.;
Kitagawa, T.; Maeda, Y. J. Am. Chem. Soc. 1995, 117, 11220–11229.
(35) Concepcion, J. J.; Jurss, J. W.; Templeton, J. L.; Meyer, T. J.
J. Am. Chem. Soc. 2008, 130, 16462–16463.
(36) Sala, X.; Ertem, M. Z.; Vigara, L.; Todorova, T. K.; Chen, W.;
Rocha, R. C.; Aquilante, F.; Cramer, C. J.; Gagliardi, L.; Llobet, A. Angew.
Chem., Int. Ed. 2010, 49, 7745–7747.
(37) Franc ꢀa s, L.; Sala, X.; Escudero-Ad ꢀa n, E.; Benet-Buchholz, J.;
Escriche, L. s.; Llobet, A. Inorg. Chem. 2011, 50, 2771–2781.
(38) Wasylenko, D. J.; Ganesamoorthy, C.; Henderson, M. A.;
Koivisto, B. D.; Osthoff, H. D.; Berlinguette, C. P. J. Am. Chem. Soc.
2010, 132, 16094–16106.
2
4
J. Am. Chem. Soc. 2009, 131, 10397–10399. (h) Yoshida, M.; Masaoka,
S.; Sakai, K. Chem. Lett. 2009, 38, 702–703.
(7) (a) Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.; Appelman,
E. H.; Que, L., Jr. J. Am. Chem. Soc. 1997, 119, 4197–4205. (b) Nagao,
H.; Komeda, N.; Mukaida, M.; Suzuki, M.; Tanaka, K. Inorg. Chem. 1996,
3
Que, L., Jr.; Dong, Y. Acc. Chem. Res. 1996, 29, 190–196. (e) Jacobson,
R. R.; Tyeklar, Z.; Farooq, A.; Karlin, K. D.; Liu, S.; Zubieta, J. J. Am.
Chem. Soc. 1988, 110, 3690–3692.
5, 6809–6815. (c) Suzuki, M. Acc. Chem. Res. 2007, 40, 609–617. (d)
(
8) Kojima, T.; Amano, T.; Ishii, Y.; Ohba, M.; Okaue, Y.; Matsuda,
Y. Inorg. Chem. 1998, 37, 4076–4085.
9) Hirai, Y.; Kojima, T.; Mizutani, Y.; Shiota, Y.; Yoshizawa, K.;
Fukuzumi, S. Angew. Chem., Int. Ed. 2008, 47, 5772–5776.
10) (a) Tyeklar, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.;
Zubieta, J.; Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677–2689.
b) Gultneh, Y.; Yisgedu, T. B.; Tesema, Y. T.; Butcher, R. J. Inorg. Chem.
003, 42, 1857–1867. (c) Murashima, T.; Tsukiyama, S.; Fujii, S.;
Hayata, K.; Sakai, H.; Miyazawa, T.; Yamada, T. Org. Biomol. Chem.
005, 3, 4060–4064.
11) APEX2, Program for Data Collection on Area Detectors; BRU-
KER AXS Inc.: Madison, WI, 2007.
12) SAINT, Program for Data Reduction from Area Detectors;
BRUKER AXS Inc.: Madison, WI, 2007.
13) Sheldrick, G. M. SADABS, Program for Absorption Correction of
Area Detector Frames; BRUKER AXS Inc.: Madison, WI, 2003.
(
(39) After submission of this article, a report by Fillol, Costas, and
coworkers was published on an analogous FeTPA complex that has high
activity for water oxidation, see: Fillol, J. L.; Codol ꢁa , Z.; Garcia-Bosch, I.;
G ꢀo mez, L.; Pla, J. J.; Costas, M. Nature Chem. 2011, 3, 807–813.
(
(
2
2
(
(
(
(
(
(
14) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112–122.
15) Barbour, L. J. J. Supramol. Chem. 2003, 1, 189–191.
16) Roy, L. E.; Jakubikova, E.; Guthrie, M. G.; Batista, E. R. J. Phys.
Chem. A 2009, 113, 6745–6750.
17) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2006,
10, 16066–16081.
18) Moser, A.; Range, K.; York, D. M. J. Phys. Chem. B 2010,
14, 13911–13921.
(
1
1
(
(
(
19) Trasatti, S. Pure Appl. Chem. 1986, 58, 955–966.
20) Frisch, M. J., et al. Gaussian 03, revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.
21) Frisch, M. J., et al. Gaussian 09, revision A.02; Gaussian, Inc.:
Wallingford, CT, 2009.
22) Parr, R. G.; Yang, W. P. Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(
(
1
0571
dx.doi.org/10.1021/ic200050g |Inorg. Chem. 2011, 50, 10564–10571