Catalytic Water Oxidation by Mononuclear Ru Complexes with an
Anionic Ancillary Ligand
Lianpeng Tong,
†
A. Ken Inge,
‡
Lele Duan,
†
Lei Wang,
†
Xiaodong Zou,
‡
and Licheng Sun*
,†,§
†
Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm,
Sweden
‡
Berzelii Centre EXSELENT on Porous Materials and Inorganic and Structural Chemistry, Department of Materials and
Environmental Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
§
DUT-KTH Joint Education and Research Center on Molecular Devices, State Key Laboratory of Fine Chemicals, Dalian University
of Technology (DUT), 116012, Dalian, China
* S Supporting Information
ABSTRACT: Mononuclear Ru-based water oxidation catalysts containing
anionic ancillary ligands have shown promising catalytic efficiency and
intriguing properties. However, their insolubility in water restricts a detailed
mechanism investigation. In order to overcome this disadvantage, complexes
[Ru
II
(bpc)(bpy)OH
2
]
+
(1
+
, bpc = 2,2′-bipyridine-6-carboxylate, bpy = 2,2′-
bipyridine) and [Ru
II
(bpc)(pic)
3
]
+
(2
+
, pic = 4-picoline) were prepared and
fully characterized, which features an anionic tridentate ligand and has enough
solubility for spectroscopic study in water. Using Ce
IV
as an electron acceptor,
both complexes are able to catalyze O
2
-evolving reaction with an impressive
rate constant. On the basis of the electrochemical and kinetic studies, a water
nucleophilic attack pathway was proposed as the dominant catalytic cycle of the catalytic water oxidation by 1
+
, within which
several intermediates were detected by MS. Meanwhile, an auxiliary pathway that is related to the concentration of Ce
IV
was also
revealed. The effect of anionic ligand regarding catalytic water oxidation was discussed explicitly in comparison with previously
reported mononuclear Ru catalysts carrying neutral tridentate ligands, for example, 2,2′:6′,2″-terpyridine (tpy). When 2
+
was
oxidized to the trivalent state, one of its picoline ligands dissociated from the Ru center. The rate constant of picoline dissociation
was evaluated from time-resolved UV-vis spectra.
■
INTRODUCTION
Oxidation of water to molecular oxygen (2H
2
O → O
2
+ 4H
+
+
4e
-
) is a vital reaction in either naturally occurring or an
envisaged artificial photosynthesis that converts solar energy to
chemical energy.
1,2
In the biological world, water oxidation is
catalyzed by the oxygen-evolving complex (OEC) of Photo-
system II (PSII).
3
There has been a long-standing effort for
chemists to develop practically applicable catalysts that can
duplicate the function of OEC in the context of solar energy
utilization.
4,5
Over the past few years, emergence of single-site
Ru complexes that are capable of catalyzing O
2
evolution from
water has shed new light in this field.
6,7
Their well-defined
chemical properties and tailorable ligands allow for profound
mechanism investigation and feasibility of systematic structure
design. Furthermore, research on Ru-based molecular water
oxidation catalysts (WOC) has accumulated valuable experi-
ence for development of first-row transition-metal-based
WOCs.
8
Most of the reported mononuclear Ru WOCs carry a
polypyridyl ancillary ligand, and their coordinative matrices can
be roughly categorized into the following motives: [Ru(N
3
)-
(N
2
)L], [Ru(N
3
)(N
1
)
2
L], and [Ru(N
4
)(N
1
)
2
](N
n
= n-dentate
nitrogen-heterocyclic ligand and L = monodentate non-
nitrogen ligand, water or halogen typically).
9-18
Mechanistic
studies based on these competent Ru WOCs have established
the following fundamental facts: (i) the catalytic cycle of water
oxidation is an intricate process that contains multiple steps;
(ii) high-valent ruthenium intermediates, such as [Ru
IV
O]
and [Ru
V
O], are involved in the cycle; and (iii) the critical
step of O-O bond formation can undergo diverse pathways
that are relevant to both structures of WOCs and conditions of
catalytic reaction.
15,19-23
Experimental results also demon-
strated that properties of ancillary ligands including electronic
parameters, flexibility, and orientation influence the activity of
mononuclear Ru WOCs significantly.
10,15,16,24
However, there
is not enough proof yet to elucidate correlations between the
performance, mechanism, and ligand environments of WOCs.
Our research group has synthesized and characterized several
series of Ru
II
molecular WOCs featuring anionic donors
(carboxylic and phenonate group specifically) as ancillary
ligands.
25-30
Recent examples of our work are [Ru
II
(pdc)-
(pic)
3
] and [Ru
II
(hqc)(pic)
3
] (Scheme 1, H
2
pdc = 2,6-
pyridinedicarboxylic acid, H
2
hqc = 8-hydroxyquinoline-2-
carboxylic acid, and pic = 4-picoline), upon which a
combination of experimental and theoretical investigations
Received: November 8, 2012
Published: February 14, 2013
Article
pubs.acs.org/IC
© 2013 American Chemical Society 2505 dx.doi.org/10.1021/ic302446h | Inorg. Chem. 2013, 52, 2505-2518