FULL PAPER
DOI: 10.1002/ejic.200600772
Catechol-Bearing Dipyrazinylpyridine Complexes of Ruthenium(II)
Fahad A. Al-mutlaq,
[a]
Pierre G. Potvin,*
[a]
Athanassios I. Philippopoulos,
[b]
and
Polycarpos Falaras
[b]
Keywords: Ruthenium / Tridentate ligands / Photosensitizers / Electron transfer / Energy conversion
The new tridentate ligand 4-(3,4-methylenedioxyphenyl)-
2,6-dipyrazinylpyridine (2) was prepared in good yield in a
one-pot reaction. The Ru
II
complexes [Ru(2)
2
](PF
6
)
2
and
[Ru(1)(2)](PF
6
)
2
[1 is 2,6-dipyrazinyl-4-(4-tolyl)pyridine] were
prepared in good yields and tested as photosensitizers
against [Ru(1)
2
](PF
6
)
2
and [Ru(ttpy)
2
](PF
6
)
2
[ttpy is 4'-(4-
tolyl)-2,2':6',2''-terpyridine]. The photosensitization ability
follows the order [Ru(1)
2
](PF
6
)
2
[Ru(1)(2)](PF
6
)
2
[Ru(2)
2
]-
(PF
6
)
2
[Ru(ttpy)
2
](PF
6
)
2
, which is explainable in terms of
mechanism and driving force. Hydrolysis of the methylene
Introduction
Ru
II
complexes of polypyridine-type ligands continue to
enjoy great interest as photosensitizers,
[1,2]
owing to strong
absorption of visible light, favourable redox potentials, ap-
preciable excited state lifetimes and the ability of the excited
states to transfer an electron to acceptor molecules or elec-
trodes.
[3–5]
We have a long-standing interest in discovering
non-stereogenic ligands that promote these properties. The
present work arises from a desire to incorporate three
promising phenomena into the design of a new ligand.
Firstly, we devised sometime ago a new tridentate ligand
family with promising features. The first member of the di-
pyrazinylpyridine (dpp) family of ligands, the 4-p-tolyl de-
rivative 1 (Scheme 1), was prepared in a remarkable one-
step reaction of commercial materials, in excellent yield and
without purification.
[6]
We have since used this procedure
to prepare other dpp examples,
[7–9]
and extended it to terpy-
ridine synthesis, although those reactions were slower.
[9,10]
Besides their easier syntheses, the homoleptic Ru
II
complex
[Ru(1)
2
]
2+
revealed an additional advantage of dpp ligands
over other tridentates: a longer excited-state lifetime, τ
(18 ns at room temperature),
[6]
compared to that of the
analogous terpyridine complex [Ru(ttpy)
2
]
2+
[0.95 ns,
[11]
ttpy is 4'-(4-tolyl)-2,2':6',2''-terpyridine], a result which can
be attributed to the electron-withdrawing effect of the ad-
ditional nitrogen atoms in stabilizing the ligand π* orbital
relative to metal-centred orbitals. At the same time, its ab-
[a] Department of Chemistry, York University,
Toronto, ON, Canada M3J 1P3
E-mail: pgpotvin@yorku.ca
[b] Institute of Physical Chemistry, NCSR “Demokritos”,
Aghia Paraskevi, 15310 Attiki, Greece
Eur. J. Inorg. Chem. 2007, 2121–2128 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2121
acetal function affords the catechol-appended complexes
[Ru(3)
2
](PF
6
)
2
and [Ru(1)(3)](PF
6
)
2
[3 is 4-(3,4-dihydroxy-
phenyl)-2,6-dipyrazinylpyridine] in excellent yields. These
insoluble, paramagnetic precipitates when tested as photo-
sensitizers in homogeneous solution and in one case ad-
sorbed on titania are able to generate photocurrents in a pho-
tovoltaic cell, albeit more weakly compared to the so-called
N719 dye.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
sorption (498 nm) and emission (667 nm) were only of
slightly lower energy (cf. 490 nm
[12]
and 640 nm,
[11]
respec-
tively, for the ttpy analogue), and the shift is the direction
favouring the more efficient harvesting of visible light. The
Ru
III/II
oxidation potential was comparatively very high
(1.62 V vs. SCE; cf. 1.25 V
[13]
for the ttpy analogue), again
because of the additional nitrogens. This enabled the opera-
tion of an electrostatically advantaged reductive quenching
mechanism in photosensitization experiments using other
dpp complexes.
[9]
Scheme 1.
Secondly, we recently demonstrated the significant bene-
fit conferred by peripheral carboxylate groups in establish-
ing a binding site for a cationic electron acceptor during
oxidative quenching in organic solvents.
[14]
We wished to