Boronic acid-substituted metal complexes: Versatile building blocks for the synthesis of multimetallic assemblies

Article abstract of DOI:10.1039/b414929g

Polypyridyl complexes of Ru(n) and Ir(III) incorporating a boronic acid substituent undergo cross-coupling with bromosubstituted complexes, and a sequential coupling-bromination-coupling strategy permits the controlled synthesis of a luminescent Y-shaped heterometallic assembly, in which efficient energy transfer to the terminus occurs.

Full text of DOI:10.1039/b414929g

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Boronic acid-substituted metal complexes: versatile building blocks for
the synthesis of multimetallic assemblies{
Kathryn J. Arm and J. A. Gareth Williams*
Received (in Cambridge, UK) 28th September 2004, Accepted 20th October 2004
First published as an Advance Article on the web 8th December 2004
DOI: 10.1039/b414929g
carried out sequentially, allowing a Y-shaped tetrameric complex
to be prepared containing three different types of metal
environment.
Polypyridyl complexes of Ru(II) and Ir(III) incorporating a
boronic acid substituent undergo cross-coupling with bromo-
substituted complexes, and a sequential coupling–bromination–
The complexes and the overall synthetic strategy employed are
shown in Scheme 1. The choice of component building blocks was
determined by the desire to obtain a well-defined gradient of
excited state energy levels, in such a way that excitation of any site
within the assembly would be followed by a ‘‘funnelling’’ of the
energy to the terminus of the structure, (the base of the Y-shape).
coupling strategy permits the controlled synthesis of
a
luminescent Y-shaped heterometallic assembly, in which
efficient energy transfer to the terminus occurs.
The covalent linkage of metal complexes to generate multimetallic
assemblies is an important theme within contemporary coordina-
tion chemistry. Such products may offer potential as photo-
chemical molecular devices, (e.g. in solar energy conversion,
electroluminescence and information storage), and in the funda-
mental study of photoinduced energy- and electron-transfer
processes.1 The preparation of heterometallic assemblies generally
requires prior synthesis of multitopic ligands, followed by step-
wise metal complexation. Although this strategy has led to a
diverse range of complexes, it is limited by a need for ligands
that can complex reliably in a stepwise manner and, frequently,
by incomplete control over the structure and composition of
the final assembly.1 Moreover, the presence of such bridging
ligands often leads to reduced performance of the constituent
units, in terms of quantum efficiency and lifetime, compared to
simpler mononuclear complexes {e.g. 2,3-bis(2-pyridyl)-pyrazine
(dpp) versus bpy}.2 Two elegant approaches that seek to
circumvent these problems have appeared recently. Hanan et al.
and Juris et al. introduce a new binding site onto the back of
a suitably functionalised, pre-coordinated ligand, allowing sub-
sequent coordination to a second metal ion,3,4 whilst Tor et al.
have demonstrated that pre-formed complexes of Ru(II) and
Os(II), one incorporating a bromo substituent in one of its
ligands and the other an ethynyl group, can undergo Pd-
catalysed cross-coupling to give dimers or trimers linked
(necessarily) by –CMC– units.5
3
The MLCT excited state energies of [Ir(ppy)2(phbpy)]+, Ir, and
[Ru(bpy)2(phbpy)]2+, Ru, lie at approximately 16 300 and 16 000
cm21 respectively (phbpy 5 4-phenylbipyridine), whilst we have
also shown during this study that [Ir(F2ppy)2(phbpy)]+, IrF4,
{F2ppyH 5 2-(2,4-difluoro-phenyl)-pyridine}, emits at higher
energy (18 500 cm21) (Fig. 1), such that the structure (IrF4)2–Ir–
Ru was an attractive target.
The boronate-substituted ligand bpy-w-Bneo {4-(4-neopentyl-
glycolatoboronphenyl)-2,29-bipyridine} was synthesised by Pd-
catalysed cross-coupling of 4-(4-bromophenyl)-2,29-bipyridine
(bpy-w-Br) with bis(neopentylglycolato)diboron. A heteroleptic
trisbipyridyl ruthenium(II) complex incorporating one boronic
acid-substituted bpy ligand was then prepared by treating
[Ru(bpy)2Cl2] with AgBF4 in acetone, to generate
[Ru(bpy)2(Me2CO)2]2+, followed by reaction with bpy-w-Bneo at
room temperature. The boronate ester hydrolyses during work-up
to give the desired boronic acid-substituted complex,
[Ru(bpy)2{bpy-w-B(OH)2}](PF6)2, Ru–B. The bromo-substituted
complex [Ir(ppy)2(bpy-w-Br)]PF6, Ir–Br, was prepared by cleavage
of the chloro-bridged dimer [Ir(ppy)2m-Cl]2 with bpy-w-Br, using
well-established methodology.7 The fluorinated complex
[Ir(F2ppy)2{bpy-w-B(OH)2}]PF6, IrF4–B, was similarly accessible
by reaction of [Ir(F2ppy)2m–Cl]2 with bpy-w-Bneo.
Cross-coupling of Ru–B with Ir–Br proceeded readily in DMSO
solution at 80 uC, using Pd(PPh3)4 as the catalyst (3–6 mol%), in
the presence of Na2CO3 (3 equiv.). The dimetallic product Ir–Ru
was isolated as its hexafluorophosphate salt, by precipitation from
KPF6(aq). The reaction proceeds remarkably cleanly; typically, the
main side-product after washing the crude product with water is a
small amount of the deboronated starting material,
[Ru(bpy)2(phbpy)]2+, which is readily removed by chromatography
on silica, along with any remaining traces of starting materials or
phosphine. The structure of the dimer (isolated as a mixture of
We have been investigating the synthesis and reactivity of
boronic acid-substituted polypyridyl complexes, a hitherto scarcely
explored class of compound.6 In this contribution, we describe
for the first time how such complexes can be used as building
blocks in the controlled synthesis of heterometallic assemblies
by means of Suzuki-type cross-couplings. Moreover, by exploiting
the ease with which a bis-cyclometallated iridium complex can
undergo electrophilic bromination, we show that cross-coupling
reactions of a single complex with different partners can be
1
diastereoisomers), was confirmed by H-1H COSY and NOESY
NMR spectroscopy and by high-resolution electrospray mass
spectrometry.{
In order to allow subsequent elaboration of this dimetallic
complex to give the Y-shaped tetramer by a second cross-coupling
{ Electronic supplementary information (ESI) available: details of
synthetic procedures and characterisation data for key compounds; UV-
vis data. See http://www.rsc.org/suppdata/cc/b4/b414929g/
*j.a.g.williams@durham.ac.uk
230 | Chem. Commun., 2005, 230–232
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Scheme 1
desired dibrominated complex IrBr2–Ru in quantitative yield.
1
Analysis of the coupling patterns in the H-1H COSY spectrum
confirmed the C49 positions in the two ppy ligands as the exclusive
sites of bromination.
The dimer, now primed for further reaction, was then cross-
coupled with two equivalents of IrF4–B, under the same conditions
as those used for the first cross-coupling reaction above. Again, the
coupling partners reacted smoothly, to give the tetrameric product,
(IrF4)2–Ir–Ru in 40% yield.{
The UV-visible absorption spectrum of the tetramer displays
strong absorption bands up to 500 nm. Compared with the
sum of the absorption spectra of the four individual com-
ponents, the molar absorptivity in the 320–400 nm region is
enhanced, attributable to the more extended conjugation in the
product in comparison to the building blocks.{ Upon excitation
at any wavelength within the range 300–500 nm, a single
emission band is observed in acetonitrile solution at room
temperature, centred at 629 nm, very close to that displayed by
Ru (626 nm; Fig. 1). No emission bands are observed at higher
energy, in the region where the mononuclear units IrF4 and Ir emit
(Fig. 1). The excitation spectrum registered at the emission
maximum matches the profile of the absorption spectrum, and
the luminescence quantum yield of 0.12 (degassed CH3CN, 295 K)
is independent of excitation wavelength. The emission decay
follows monoexponential kinetics; t 5 1.6 ms under the same
conditions.
Fig. 1 Absorption (solid line, left), excitation spectrum (dashed line;
lem 5 629 nm) and corrected emission spectrum (solid line, right,
lex 5 360 nm) of (IrF4)2–Ir–Ru, in CH3CN at 295 K. The corrected
emission spectra of the individual constituent building blocks are also
shown.
reaction, halogen substituents must first be introduced. Coudret
et al. have shown that cyclometallation of 2-phenylpyridine (ppy)
to Ru(II), as in [Ru(bpy)2(ppy)]+, activates the 49-position of the
phenyl ring to electrophilic bromination by NBS under mild
conditions, leading to the mono-bromo functionalised complex,
upon which further chemistry can be carried out.8 A similar
activation was anticipated for the dimer Ir–Ru, which incorporates
two such cyclometallated ppy units, albeit coordinated to Ir(III)
rather than to Ru(II). Treatment of Ir–Ru with NBS (2.2 equiv.) in
acetonitrile at room temperature for 18 h led specifically to the
These observations provide conclusive evidence that excitation
into either of the Ir components is followed by rapid energy
transfer to the Ru centre, in line with the ordering of the energy
levels of the three constituent units.9 Thus, the tetramer behaves as
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a light-harvesting device in which the absorbed energy is
channelled efficiently into a single component. A detailed study
of the photophysical properties of the assemblies and individual
building blocks is in progress.
Notes and references
1 V. Balzani, A. Juris, M. Venturi, S. Campagna and S. Serroni, Chem.
Rev., 1996, 96, 759; V. Balzani, S. Campagna, G. Denti, A. Juris,
S. Serroni and M. Venturi, Acc. Chem. Res., 1998, 31, 26.
2 S. Serroni, A. Juris, S. Campagna, M. Venturi, G. Denti and V. Balzani,
J. Am. Chem. Soc., 1994, 116, 9086.
3 K. O. Johansson, J. A. Lotoski, C. C. Tong and G. S. Hanan, Chem.
Commun., 2000, 819; Y.-Q. Fang, M. I. J. Polson and G. S. Hanan,
Inorg. Chem., 2003, 42, 5.
4 A. Bo¨rge, O. Ko¨rge and A. Juris, New J. Chem., 2001, 25, 191.
5 P. J. Connors, Jr., D. Tzalis, A. L. Dunnick and Y. Tor, Inorg. Chem.,
1998, 37, 1121; Y. Tor, Synlett., 2002, 7, 1043.
6 C. J. Aspley and J. A. G. Williams, New J. Chem., 2001, 25, 1136.
7 K. A. King and R. J. Watts, J. Am. Chem. Soc., 1987, 109, 1589. The
complex Ir–Br has previously been cross-coupled with benzene
diboronic acid: E. A. Plummer, J. W. Hofstraat and L. De Cola,
Dalton Trans., 2003, 2080.
In conclusion, this first report of the use of the Suzuki
cross-coupling reaction to link different polypyridyl metal com-
plexes under mild conditions demonstrates the power of new
boronic acid-substituted complexes as versatile building blocks
in the construction of large, multimetallic assemblies. No pre-
formed bridging ligands are required, the linking units in the
final products are simple aryl groups, and the excited state
properties may be predicted on the basis of simple model building
blocks.
We thank the EPSRC for a studentship to K. J. A.
and the EPSRC National Mass Spectrometry Service Centre,
Swansea.
8 C. Coudret, S. Fraysse and J.-P. Launay, Chem. Commun., 1998, 663;
S. Fraysse, C. Coudret and J.-P. Launay, J. Am. Chem. Soc., 2003, 125,
5880.
9 This contrasts with RuAIr energy transfer and weak Ir-based emission
in a dpp-bridged system: see ref. 2. Partial IrARu energy transfer was
observed in a weakly emissive hexazatriphenylene-bridged dimer:
I. Ortmans, P. Didier and A. Kirsch-De Mesmaeker, Inorg. Chem.,
1995, 34, 3695.
Kathryn J. Arm and J. A. Gareth Williams*
Department of Chemistry, University of Durham, South Road, Durham,
UK DH1 3LE. E-mail: j.a.g.williams@durham.ac.uk;
Fax: + 44 (0)191 384 4737
232 | Chem. Commun., 2005, 230–232
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