A (bpy)2Ru-coordinated dehydro[12]annulene with exotopically fused diimine binding sites

Article abstract of DOI:10.1039/b313788k

Synthesis and electronic properties of a dinuclear (bpy)₂Ru ^II polypyridyl complex are described in which the bridging ligand consists of two dipyridophenazines fused to a formally antiaromatic dehydro[12]annulene and where the elect

Full text of DOI:10.1039/b313788k

A (bpy)2Ru-coordinated dehydro[12]annulene with exotopically fused
diimine binding sites†
Sascha Otta and Rüdiger Faust*ab
a Christopher Ingold Laboratories, Department of Chemistry, University College London, 20 Gordon Street,
London, UK WC1H 0AJ
b Institute of Chemistry, Faculty of Physical Sciences, University of Kassel, Heinrich-Plett-Str. 40, 34132
Kassel, Germany. E-mail: r.faust@uni-kassel.de; Fax: (+49)561 8044752
Received (in Cambridge, UK) 31st October 2003, Accepted 16th December 2003
First published as an Advance Article on the web 23rd January 2004
Synthesis and electronic properties of a dinuclear (bpy)2RuII
polypyridyl complex are described in which the bridging ligand
consists of two dipyridophenazines fused to a formally anti-
aromatic dehydro[12]annulene and where the electronic and
electrochemical properties of the complex are markedly influ-
enced by the cyclic all-carbon core.
can therefore be conducted in a single operation. Furthermore, the
voluminous protecting groups in 2 should aid in avoiding solubility
problems that are often encountered in the construction of large
planar systems.5,9
1,2-Diethynyl-4,5-dinitrobenzene 4 was prepared from diiodoar-
ene 310 under cross-coupling conditions developed by Sonogashira
et al. (Scheme 1).11 Reduction of 4 with tin in HCl–ethanol
produced the unstable diamine 5 which was immediately Boc-
protected to furnish 6. The stepwise, controlled assembly of 2 was
rendered possible by a desymmetrisation of 6, achieved under
kinetic control with TBAF in THF. Intermolecular oxidative
acetylene homocoupling of 7 furnished butadiyne 8 in 80% yield.
Formation of 2 was concluded by a second, exhaustive proto-
desilylation to afford 9 and by its subsequent cyclisation via
intramolecular oxidative acetylene coupling.‡
The linearity and rigidity of acetylene moieties render them
attractive inter-connectors for polypyridine metal binding sites, as
these features allow excellent control over the distance and hence
over the interaction between coordinated metal centres.1 In
addition, by virtue of their extensive p-orbital network, acetylene
substituents on polypyridine ligands contribute to metal-to-ligand
interactions and offer additional through-bond communication
pathways.2 In recognition of these properties, poly(phenyl)acety-
lenes have been widely used for the construction of linear rod-like
polypyridine scaffolds.1,2 In contrast, cyclic acetylene structures of
the dehydroannulene type have been less well explored as inter-
connectors for diimine binding sites, although the first examples
have emerged over the last five years.3–5 The acetylenic cores of
these prototypes, however, are neither rigorously rigid nor planar
and hence fail to maximise possible metal-to-ligand and metal–
metal interactions. We therefore set out to explore a dipyr-
idophenazine-fused dehydro[12]annulene as a scaffold for coor-
dinating metal fragments as we envisioned that the formally
antiaromatic electron count of the cyclic core could facilitate
additional charge transfer processes. The planarity of the targeted
novel ligand system is ensured by obvious rotational restrictions
and has been demonstrated crystallographically for more simple
dibenzodehydro[12]annulene systems.6,7 We report here the syn-
thesis and an initial investigation of the electronic properties of the
dinuclear Ru(II) complex 1, the first representative of this class of
compounds.
The functionalised benzodehydroannulene 2 was isolated as a
yellow, hygroscopic solid that, due to extensive aggregation, was
Our previous experience in the construction of ruthenium
complexes with acetylenic polyimine ligands, and our difficulties
in separating dehydroannulenes with varying ring sizes,8 prompted
us to pursue a synthetic approach in which dehydroannulene
formation precedes metal complexation. Hence we designed the
tetra-Boc-protected tetraaminodibenzooctadehydro[12]annulene 2
(Scheme 1) as a key building block. Compound 2 offers the
advantage that both amine-deprotection and the subsequent im-
plementation of the diimine binding site onto the core of 2 through
condensation of the intermediate tetraamine with suitably function-
alised [1,10]phenanthroline-5,6-diones are both acid-catalysed and
Scheme 1 Reagents and conditions: i, (iPr)3SiC·CH, [Pd(PPh3)2Cl2] (5
mol%), CuI (2 mol%), Et3N, 80 °C, 3 h, 70%; ii, Sn, HCl (conc.)–EtOH, 70
°C, 15 min, 57%; iii, Boc2O, (iPr)2EtN, THF, 3 d, D, 94%; iv, Bu4NF, THF,
0 °C, 30 min, 50% (73% based on recovered starting material); v,
Cu(OAc)2, CuCl, pyridine, MeOH, 60 °C, 4 h, 80% for 8, 72% for 2; vi,
Bu4NF, THF, rt, 90 min, 96%; vii, 10, TFA, CH3CN, 70 °C, 24 h, 21%.
† Electronic supplementary information (ESI) available: Analytical data for
the ruthenium complexes. See http://www.rsc.org/suppdata/cc/b3/
b313788k/
388
C h e m . C o m m u n . , 2 0 0 4 , 3 8 8 – 3 8 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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.
1 A. Harriman and R. Ziessel, Chem. Commun., 1996, 1707; A. Harriman
and R. Ziessel, Coord. Comm. Rev., 1998, 331; A. Khatyr and R.
Ziessel, J. Org. Chem., 2000, 7814.
2 R. Ziessel, M. Hissler, A. El-ghayoury and R. Ziessel, Coord. Chem.
Rev., 1998, 178–180, 1251.
3 C. Grave, D. Lentz, A. Schäfer, P. Samorì, J. P. Rabe, P. Franke and A.
D. Schlüter, J. Am. Chem. Soc., 2003, 125, 6907; O. Henze, D. Lentz, A.
Schäfer, P. Franke and A. D. Schlüter, Chem. Eur. J., 2002, 357; C.
Grave and A. D. Schlüter, Eur. J. Org. Chem., 2002, 3075.
4 P. N. W. Baxter, Chem. Eur. J., 2003, 9, 2531; P. N. W. Baxter, Chem.
Eur. J., 2002, 8, 5250; P. N. W. Baxter, J. Org. Chem., 2001, 66,
4170.
5 M. Schmittel and H. Ammon, Synlett, 1999, 750; M. Schmittel and A.
Ganz, Synlett, 1997, 710.
6 F. Sondheimer, Acc. Chem. Res., 1972, 5, 81.
7 U. H. F. Bunz and V. Enkelmann, Chem. Eur. J., 1999, 5, 263.
8 R. Faust and S. Ott, J. Chem. Soc., Dalton Trans., 2002, 1946.
9 E. Ishow, A. Gourdon and J.-P. Launay, Chem. Commun., 1998,
1909.
10 J. Arotsky, R. Butler and A. C. Darby, J. Chem. Soc., 1970, 1480.
11 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975,
4467.
12 C. A. Goss and H. D. Abruna, Inorg. Chem., 1985, 24, 4263.
13 D. Dubois, G. Moninot, W. Kutner, M. T. Jones and K. M. Kadish, J.
Phys. Chem., 1992, 96, 7137.
14 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. v.
Zelewsky, Coord. Chem. Rev., 1988, 84, 85.
C h e m . C o m m u n . , 2 0 0 4 , 3 8 8 – 3 8 9
389