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

Abstract

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–coupling strategy permits the controlled synthesis of a luminescent Y-shaped heterometallic assembly, in which efficient energy transfer to the terminus occurs.

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The covalent linkage of metal complexes to generate multimetallic assemblies is an important theme within contemporary coordination chemistry. Such products may offer potential as photochemical molecular devices, (e.g. in solar energy conversion, electroluminescence and information storage), and in the fundamental study of photoinduced energy- and electron-transfer processes.¹ The preparation of heterometallic assemblies generally requires prior synthesis of multitopic ligands, followed by stepwise 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 subsequent 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 –C≡C– units.⁵

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 carried out sequentially, allowing a Y-shaped tetrameric complex to be prepared containing three different types of metal environment.

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). The ³MLCT excited state energies of [Ir(ppy)₂(phbpy)]⁺, Ir, and [Ru(bpy)₂(phbpy)]²⁺, Ru, lie at approximately 16 300 and 16 000 cm⁻¹ respectively (phbpy = 4-phenylbipyridine), whilst we have also shown during this study that [Ir(F₂ppy)₂(phbpy)]⁺, Ir^F4, {F₂ppyH = 2-(2,4-difluoro-phenyl)-pyridine}, emits at higher energy (18 500 cm⁻¹) (Fig. 1), such that the structure (Ir^F4)₂–Ir–Ru was an attractive target.

Scheme 1

The boronate-substituted ligand bpy-ϕ-Bneo {4-(4-neopentylglycolatoboronphenyl)-2,2′-bipyridine} was synthesised by Pd-catalysed cross-coupling of 4-(4-bromophenyl)-2,2′-bipyridine (bpy-ϕ-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-ϕ-Bneo at room temperature. The boronate ester hydrolyses during work-up to give the desired boronic acid-substituted complex, [Ru(bpy)₂{bpy-ϕ-B(OH)₂}](PF₆)₂, Ru–B. The bromo-substituted complex [Ir(ppy)₂(bpy-ϕ-Br)]PF₆, Ir–Br, was prepared by cleavage of the chloro-bridged dimer [Ir(ppy)₂μ-Cl]₂ with bpy-ϕ-Br, using well-established methodology.⁷ The fluorinated complex [Ir(F₂ppy)₂{bpy-ϕ-B(OH)₂}]PF₆, Ir^F4–B, was similarly accessible by reaction of [Ir(F₂ppy)₂μ–Cl]₂ with bpy-ϕ-Bneo.

Cross-coupling of Ru–B with Ir–Br proceeded readily in DMSO solution at 80 °C, 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 diastereoisomers), was confirmed by ¹H-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 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 4′-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 desired dibrominated complex Ir^Br2–Ru in quantitative yield. Analysis of the coupling patterns in the ¹H-¹H COSY spectrum confirmed the C4′ 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 Ir^F4–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, (Ir^F4)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 components, 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 Ir^F4 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; τ = 1.6 µs under the same conditions.

Fig. 1 Absorption (solid line, left), excitation spectrum (dashed line; λ_em = 629 nm) and corrected emission spectrum (solid line, right, λ_ex = 360 nm) of (Ir^F4)₂–Ir–Ru, in CH₃CN at 295 K. The corrected emission spectra of the individual constituent building blocks are also shown.

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 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.

In conclusion, this first report of the use of the Suzuki cross-coupling reaction to link different polypyridyl metal complexes 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.

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. Börge, O. Körge 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.
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 Ru→Ir energy transfer and weak Ir-based emission in a dpp-bridged system: see ref. 2. Partial Ir→Ru 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.

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Footnote

† 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/

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