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[Ru(bpy) 3 ] 2+ analogues containing a N-heterocyclic carbene ligand

A synthetic procedure is described that provides access to [Ru(bpy) 3 ] 2+ analogues in which one bpy ligand is replaced by a C,N -bidentate coordinating carbene-benzimidazole ligand (bpy = 2,2’-bipyridine). These new complexes were prepared by first installing the chelating carbene ligand onto a Ru(cymene) platform and subsequent ligand substitution using bpy or terpy (terpy = 2:2’,6’:2’’-terpyridine). The carbene ligand significantly affects the optical properties of the complex and lowers the ruthenium(II) oxidation potential substantially. Such modifications may be advantageous for the development of new classes of photosensitizer materials. Coordination of a chelating carbene to ruthenium followed by substitution of the ancillary ligands by bi- or terpyridine provides access to carbene complexes which are analogous to 2+ and which feature electrochemical and spectroscopic properties that are attractive for application in domains such as photosensitation.


Introduction
Ruthenium polypyridine complexes like [Ru(bpy) 3 ] 2+ (bpy = 2,2'-bipyridine) and its derivatives have been extensively studied as active components in a variety of applications including supramolecular assembly, (photoinduced) electron-transfer reactions, and photochemistry. 1 In particular, this class of compounds show excellent properties as photosensitizers in dye-sensitized solar cells (DSSCs) and have thus been pivotal for advancing light-to-energy-conversion technology. 2 While substantial modifications in the polypyridine substitution pattern has been successfully applied as a methodology for improving the metal-centered redox and excited state properties, 3 replacement of one or several pyridine ligand sites by N-heterocyclic carbenes (NHCs) has received surprisingly little attention. Tanaka and coworkers used mono-quaternized bpy and terpy analogues (terpy = 2:2',6':2''-terpyridine) to induce a mono-pyridylidene coordination mode of these ligands (I and II, Fig. 1), 4 and Son et al. developed a procedure for the synthesis of homoleptic ruthenium(II) complexes containing C,N-bidentate carbenepyridine and C,N,C-tridentate carbene-pyridine-carbene ligands, respectively (III and IV, Fig. 1). 5 More recently, Hahn and coworkers succeeded in the preparation of bis(cyclometalated) systems of type V. 6 The scarce appearance of NHC analogues of ruthenium polypyridine complexes is remarkable when considering that the strong donor properties of NHCs 7 favorably affect the redox potential of coordinated metal centers, 8 thus inducing useful properties for the fabrication of functional materials. 9 Specifically, the stronger donor ability is expected to facilitate charge separation as required in solar cell active sites. Here, we report on a convenient approach towards NHC ruthenium complexes comprising different ancillary polypyridine ligands such as bpy or terpy. This methodology is based on arene substitution from widely studied and readily accessible complexes of type [RuCl(arene)(NHC)L] + , 10 and affords heteroleptic complexes comprising a single (chelating) NHC ligand. Due to the coordination of the carbene ligand, the photophysical and electrochemical properties are significantly altered as compared to the parent [Ru(bpy) 3 ] 2+ system, rendering these complexes potentially useful active sites for photovoltaic applications.

Results and discussion
Synthesis of complexes. Our initial attempts to prepare complexes of type [Ru(bpy) 2 (C^N)] 2+ (where C^N represents a C,N-bidentate carbene-imidazole ligand) focused on using Ru(bpy) 2 Cl 2 as starting material. 11 However, substitution of the chlorides by the carbene ligand either via the free carbene route or via transmetallation protocols 12 from the corresponding silver carbene intermediate failed. As a consequence, the reaction sequence for installing the ligands was changed and the carbene was coordinated prior to the pyridine ligands. Complexes 2 were thus obtained via a silver carbene intermediate, generated in situ by adding Ag 2 O to the carbene ligand precursor 1, and subsequent transmetallation with [RuCl 2 (cymene)] 2 via standard methods (Scheme 1). 12

Scheme 1
The formation of complexes 2 was evidenced by spectroscopic and mass spectrometric analyses. The 1 H NMR spectra of both complexes revealed a diagnostic AB doublet for the bridging CH 2 group between 5 and 6 ppm ( 2 J HH = 16.8 and 16. 5 Hz for 2a and 2b, respectively). Diastereotopicity of the methylene protons indicated chelation of the carbene ligand. The carbene carbon appeared at δ C 171 and 171.3 (2a and 2b, respectively) in the 13 C NMR spectra. An X-ray diffraction analysis of the PF 6 -salt of 2a confirmed the structure surmised from analyses in solution (Fig. 2). Subsequent displacement of the arene ligand from complexes of type Ru(arene)(phenylpyridine)X with bpy was reported to occur in MeCN at room temperature. 13 While complexes 2 were exceedingly stable under these conditions, smooth substitution of cymene was observed upon heating complex 2a in DMSO in the presence of bpy and AgPF 6 , indicated by a diagnostic color change of the reaction mixture from bright orange to dark red. Benzene displacement from complex 2b required slightly less harsh conditions (150 °C, 15 h) than the dissociation of cymene (170 °C, 20 h), however, the latter proceeded cleaner and its arene precursor complex 2 is available in higher yields (71% vs 40%). Therefore the cymene rather than benzene dissociation was the preferred pathway for the synthesis of complex 3 as a [Ru(bpy) 3 ] 2+ analogue comprising a carbene-benzimidazole rather than a third bpy ligand. Formation of 3 was indicated by the absence of the characteristic arene signals in the 1 H NMR spectrum. Instead, 16 mostly well-resolved signals in equal ratios appeared in the aromatic region, supporting the coordination of two bipyridine ligands. In addition, the diastereotopic isopropyl CH 3 (2), Ru1-Investigation of the arene displacement reaction revealed that the presence of a silver salt was pivotal.
In the absence of AgPF 6 , ill-defined product mixtures were obtained along with significant portions of decomposition products. When using MeCN instead of DMSO as solvent under otherwise identical reaction conditions, no bpy coordination was observed and complex 3 did not form in detectable quantities. The reaction proceeded smoothly, however, if the imidazolylidene is replaced by a less basic benzimidazolylidene ligand 14 as in complex 4, yielding the [Ru(bpy) 3 ] 2+ analogue 5 in good yields (Scheme 2). In solution, complex 5 displays features that are strongly related to those of complex 3, e.g., an AB doublet at δ H 6.01 and 5.38 ( 2 J HH = 18.0 Hz) for the benzimidazole-benzimidazolylidene methylene bridge. The 13 C NMR resonance for the ruthenium-bound carbene carbon was located at δ C

This downfield shift as compared to the imidazolylidene analogue 3 is in agreement with
literature data and has been attributed to a poor overlap of the arene π orbital network with the NCN amidinylidene π orbitals. 15

Scheme 2
Replacing bpy by terpy produced, after stirring for 20 h at 130 °C, complex 7 as a dark purple complex (Scheme 3). At higher temperature, oxidation of the methylene group and concomitant cleavage of the N carbene -C bond takes place as a side reaction and complex 6 featuring a monodentate carbene and a N,O-bidentate chelating benzimidazole-carboxylate ligand was obtained instead along with various undefined products (Fig. 4). Noteworthy, the carbene ligand in complex 6 is an unusual 16 protic NHC ligand, i.e. a true tautomer of N-alkylated imidazole.  only at wavelengths beyond 760 nm. Such broadening in the visible area may be particularly attractive for better photon harvesting in light-to-energy conversion devices such as solar cells.

Ru
Electrochemical analysis of the carbene-modified ruthenium polypyridine complexes 3, 5, and 7 were performed using cyclic and differential pulse voltammetry. The bpy-containing complexes 3 and 5 display a fully reversible oxidation at E 1/2 = +1.07 and +1.16 V vs SCE, respectively (ΔE < 80 mV; Table 1). In addition, two reversible reductions were apparent, which were better resolved in complex 5 (E 1/2 -1.33 and -1.56) than in complex 3 (Fig. 5 The slightly higher oxidation potential in 5 as compared to complex 3 is in agreement with benzimidazolylidene being a weaker donor ligand than imidazolylidene. The former has been previously compared to saturated imidazolinylidene ligands, reflecting the poor orbital overlap between the benzene fragment and the carbenoid NCN portion of benzimidazolylidenes. These electrochemical measurements suggest that the energy of ruthenium-centered HOMO can be successfully decreased by the introduction of a NHC ligand, while the p-type properties of the complexes are essentially unaltered, preserving the bpy ligands as electron acceptor sites.  Complex 7 revealed two reversible oxidations at +0.73 and + 1.08 V, which may be rationalized by an equilibrium situation between the octahedral complex 7 and the penta-coordinate dicationic species 8a in MeCN solution. Tentatively, the lower oxidation may thus be assigned to the monocationic complex 7 and the redox process at +1.08 V to Ru II /Ru III oxidation in complex 8a.

Conclusions
Novel ruthenium(II) complexes comprising an N-heterocyclic carbene and ancillary polypyridine ligands were successfully prepared from the corresponding Ru(NHC)(arene) precursors via arene displacement. Provided reaction times and conditions are carefully adjusted depending on the starting material and the polypyridine used, the arene substitution reaction may be of considerable scope as a variety of carbene ruthenium precursors are known and readily accessible. This methodology provides direct access to a diversity of [Ru(bpy) 3 ] 2+ surrogates with modified photochemical and electronic properties, which provide new prospects in DNA intercalation and scission. In addition, such systems may constitute a base for the development of a next generation of active dyes for solar cells, since NHCmodified ruthenium polypridine complexes display beneficial optical and electrochemical properties.
Their application potential in photochemical light-to-energy conversion will be the subject of further investigation in our laboratories.

Experimental Section
General. General procedure for synthesis of arene containing complexes (B): The ligand precursor (2.1 eq) and Ag 2 O (1.6 equiv.) were vigorously stirred in CH 2 Cl 2 (10 mL). After 1 h, the mixture was filtered over Celite and washed with CH 2 Cl 2 (20 mL). The combined filtrates were added dropwise to a solution of CH 2 Cl 2 (10 mL) containing [Ru(arene)Cl 2 ] 2 (1 equiv.) and stirred for 2 h. The mixture was subsequently filtered over Celite and washed with CH 2 Cl 2 (20 mL) then concentrated to about 10 mL.
The product precipitated upon addition of Et 2 O (100 mL) and was collected and dried. An analytically pure sample was obtained after purification by column chromatography (SiO 2 ; CH 3 CN/H 2 O 9:1).
Sodium chloride (2 g, large excess) was added to the eluted product solution, then solvents were evaporated and the product was extracted with CH 2 Cl 2 and dried.

General procedure for the synthesis of NHC-polypyridine containing complexes (C):
The starting ruthenium complex (1 equiv.) and 2,2'-bipyridine (2 equiv.) or 2,2':6,2"-terpyridine (1 equiv.) were stirred in DMSO (5 mL) then AgPF 6 (2.1 equiv.) in DMSO (2 mL) was added dropwise and the reaction mixture was heated. After cooling a precipitate formed, which was eliminated by centrifugation. CH 2 Cl 2 (20 ml) was added to the supernatant DMSO solution. After addition of Et 2 O (200 mL), a residue formed, which was collected and purified by column chromatography (neutral Al 2 O 3 ; CH 3 CN/MeOH 9:1). The red fraction was collected and solvents were evaporated and the product was extracted from the residue with CH 2 Cl 2 and dried.

Synthesis of 3:
According to general procedure C, starting from 2a (280 mg, 0.5 mmol), AgPF 6 (263 mg, 1.05 mmol) and 2,2'-bipyridine ( Crystal structure determination of complexes 3, 5, and 6. Intensity data for crystals of 3 and 5 were collected at 130 K for 3 and at 173 K for 5 on a Stoe Mark II-Imaging Plate Diffractometer System using Mο-Kα graphite monochromated radiation (λ = 0.71073 Å). Nominal crystal to detector distances were 120 mm (3) and 130 mm (5). Data for 6 were collected using an Oxford Diffraction SuperNova A diffractometer fitted with an Atlas detector using Cu-Kα radiation (λ = 1.54184 Å). A complete dataset was collected, assuming that the Friedel pairs are not equivalent.
The structures were solved by direct methods using the program SHELXS-97. 23 Non-hydrogen atoms were refined anisotropically using weighted full-matrix least-squares on F 2 . The hydrogen atoms were included in calculated positions and treated as riding atoms using SHELXL-97 default parameters. A numerical absorption correction was applied for 3 using STOE X-Red & X-Shape. Semi-empirical absorption correction was applied for structure 5 using MULscanABS as implemented in PLATON03. 24 An analytical absorption correction based on the shape of the crystal of 6 was performed. 25 The unit cell of crystals of 3 contains two crystallographically independent complex molecules. The independent PF 6 counterions were disordered (occupancies 0.52/0.48 and 0.81/0.19, respectively). The disorder was resolved by applying restraints to the minor positions using the SAME instruction in SHELXL. The major positions were refined anisotropically, while the minor positions were refined isotropically. In complex 5, one of the two PF 6 anions was disordered over two positions (occupancies 0.5/0.5). In the final cycles of refinement their anisotropic displacement parameters were made mutually equal using the EADP instruction in SHELXL. Further crystallographic details are compiled in the Supporting Information. CCDC 794590 (3), 794591 (5), and 794592 (6) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Supporting Information Available:
Crystallographic data for complexes 3, 5, and 6 in CIF format.
This material is available free of charge via the Internet at http://pubs.acs.org.