1
difficult to redissolve in common organic solvents. The H NMR
spectrum of 2 reveals a single aryl-proton resonance at d = 7.16,
which is shifted to higher magnetic field strengths relative to the
corresponding two signals of 8 (resonating at d = 7.73 and d =
7.67, respectively; all measurements in CDCl3) due to the presence
of the paratropic dehydroannulene core in the former compound.
Subjecting 2 to a solution of [(bpy)2Ru(phenanthroline-5,6-dio-
ne)]2+(PF62)2 1012 in acetonitrile in the presence of trifluoroacetic
acid furnished the desired dinuclear RuII complex 1 as a dark red
hygroscopic solid in 21% yield.†
For an evaluation of the electrochemical and photophysical
properties of 1, the mono- and dinuclear [(bpy)2RuII(dipyr-
idophenazine)] complexes 11 and 12 were prepared for comparison
using methods similar to those depicted in Scheme 1.†
Fig. 1 Electronic absorption spectra of 1 (—), 11 (Ã) and 12 (…) in
acetonitrile at 25 °C.
suggesting that the intramolecular quenching process in 1 is
accelerated by the presence of the bridging dehydroannulene. The
nature of this process is the focus of ongoing investigations.
In this work, we have described the synthesis of the dinuclear
ruthenium complex of the first rigid dehydroannulene with
exotopically-fused metal binding sites via an acid-catalysed one-
pot deprotection–condensation sequence. The central all-carbon
core is clearly reflected in the electronic and electrochemical
properties of the novel complex, rendering it a good electron
acceptor. Work is currently under way to further explore the
synthetic potential of 2, to chemically differentiate between the two
coordination sites and to further elucidate the photophysical
properties of 1.
This work was supported by the Engineering and Physical
Sciences Research Council UK (Grant No. GR/N09503). We wish
to acknowledge a UCL Provost studentship to S. O. and Dr Walter
Amrein and Oliver Scheidegger (ETH Zürich) for recording
MALDI-TOF mass spectra. We are grateful to Dr Andy Beeby and
Dr Simon FitzGerald for providing luminescence data.
The electrochemical properties of 1, 11 and 12 were investigated
by cyclic voltammetry.† The redox potential of the RuII/RuIII
couple at +1.25 V (CH3CN, vs. SCE, Fc+/0 at 0.31 V) and the
reductions of the ancillary bpy-ligands (around 21.47 V) are
largely invariant of the nature of the dipyridophenazine ligands in
all three complexes. Differences occur, however, in the reduction
potential directly associated with the acetylenic heterocycles.
While the dipyridophenazine ligand in mononuclear 11 is reduced
at 20.89 V, the first and second reductions of the dehydroannulene
complex 1 occur more readily at 20.72 V and 20.87 V,
respectively. The butadiynyl-linked dinuclear complex 12 also
features two dipyridophenazine reductions, but at slightly more
negative potentials (20.79 V and 20.91 V). It is thus clear that the
electron affinity of the dehydroannulene in 1 is significantly higher
than that of non-cyclic alkynyl dipyridophenazines. In comparison,
the reductions in 1 proceed at a potential similar to that required for
the first reduction of fullerene C60 (CH2Cl2, 1.02 V vs. Fc+/0),
which is known as a good electron acceptor.13
The UV/Vis absorption spectra of 1, 11 and 12 are typical for
RuII polypyridine complexes in the sense that they are dominated
by ligand-centred p? p* transitions around 300 nm and by MLCT
bands in the region beyond 400 nm (Fig. 1).14 There are, however,
remarkable differences between the spectrum of 1 and those of 11
and 12. For example, the spectrum of 1 shows a distinct absorption
maximum at 369 nm, which we assign to transitions located mainly
on the quinoxalinodehydro[12]annulene subunit. In addition, the
MLCT bands of 1 are bathochromically shifted relative to those of
11 and 12 and are split into two maxima at 433 and 459 nm,
respectively, perhaps indicative of electronic interactions between
the phenanthroline moieties and the dehydroannulene framework.
Preliminary luminescence measurements show that the emission
maximum of 1 at 772 nm is shifted bathochromically compared to
that of 11 and 12 by 14 and 6 nm, respectively. The excited state
lifetime of 1 is with 28 ns (at rt in CH3CN) significantly shorter than
that of the “linear” model complexes 11 (88 ns) and 12 (52 ns)
Notes and references
‡ All compounds have been fully characterised by 1H, 13C NMR, mass
spectrometry and microanalysis.
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