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The stereoselective synthesis of a highly luminescent neutral Ir(III) complex comprising two bidentate chiral, cyclometalating phenylpyridine derivatives, and one acetylacetonate as ligands is described. The final complex and some intermediates were characterized by X-ray structural analysis, NMR-, CD-, and CPL-spectroscopy.
Current interest in stereoselective synthesis and in emissive metal complexes has motivated the synthesis of chiral luminescent materials. Before, we reported the fabrication of C3-symmetric complexes of Ru(II) and Ir(III), and C2-symmetric species of Pt(II), with chiral enantiopure bipyridine- or cyclometalating phenylpyridine-derivatives. 1 Two types of chiral pinene-phenylpyridine ligands have been synthesized as precursors for the tripod ligands, having the pinene moiety annellated in 4,5-position (HLI) or in 5,6-position (HLII) (Scheme 1).2 As reported recently, HLI yields with IrCl3·3H2O a HEXOL-type tetranuclear species Ir4Cl6LI6, which shows an interesting stereochemistry.3 In the present communication, we report on the fabrication of a dinuclear complex Ir2Cl2LII4 and derivatives thereof. Of special interest is the stereoselective synthesis of a mononuclear molecular complex Ir(LII)2(acac), which has interesting properties, especially with regard to its luminescence behavior.
As shown in Scheme 2, the reaction of iridium trichloride hydrate with enantiopure 2-phenyl-5,6-pinenepyridine derivative (−)-HLII, in a refluxing mixture of 2-ethoxyethanol and water, leads to the formation of a red precipitate. Flash chromatography on a short silica column gives complex 1 [IrLII2(μ-Cl)]2 as a red powder. Reaction of complex 1 with 2,4-pentanedione (Hacac) and sodium carbonate in refluxing 2-ethoxylethanol under inert gas atmosphere, followed by chromatography, gives the mononuclear molecular complex 2, Ir(LII)2(acac). Attempts to crystallize complex 1 from various solvents resulted finally in the formation of a solvolysis product of 1, namely Ir(LII)2(CH3CN)Cl. The molecular structure of the latter was determined by X-ray crystallography. Crystals of 2 were obtained from CH2Cl2/n-hexane.
Figure 1 illustrates the aromatic region of the 1H NMR spectra of complexes 1 and 2. These spectra, as well as the 13C NMR spectra, show the presence of two sets of magnetically non-equivalent cyclometalating ligands in complex 1, while one set of magnetically equivalent cyclometalating ligands in complex 2. Comparison of the NMR spectra of dissolved crystals from complex 1 and the powder obtained from chromatography, an additional signal attributed to CH3CN appears. Therefore, the μ-chloro bridge was broken by CH3CN during the process of crystallization, yielding the acetonitrile coordinated complexes shown in Figure 2.
The ORTEP plots4 in Figure 2 show that there are two independent mononuclear molecules of the complex per asymmetric unit. It shows that the formation of complex 1 is completely regioselective, i.e. only C,C-cis-N,N-trans isomers are obtained. Metal atoms Ir1 and Ir2 have Λ- and Δ-configuration, respectively. It has already been reported that in the formation of [Ir(phpy)2(μ-Cl)]2, structural models suggest that interligand steric interactions in the meso form favor formation of the racemate.5 In the formation of complex 1, the two diastereoisomers [Λ-Ir(LII)2(μ-Cl)]2 and [Δ-Ir(LII)2(μ-Cl)]2, which can be designated as pseudoenantiomers, occur in a 1:1 ratio, causing the appearance of the two sets of magnetically non-equivalent cyclometalating ligands in the NMR spectra.
The X-ray structure6 in Figure 3 shows clearly that in complex 2, the metal center Ir1 atom has Δ-configuration and thus it explains the difference between the NMR spectra of complex 1 and 2. During the reaction of complex 1 with 2,4-pentanedione (Hacac), only Δ-Ir(LII)2(acac) is stereoselectively formed.
Neither the chiral didentate ligand nor complex 1 show detectible CD activities in the range 250–600 nm, whereas upon formation of the mononuclear IrLII2(acac), a strong Cotton effect is observed in the 250–400 nm region. The absence of signals in the CD spectrum of complex 1 is undoubtedly due to the opposite configuration of the two Ir-centers. Figure 4 shows the CD spectrum of complex 2 in CH2Cl2 at RT.
Complex 2 exhibits intense emission spectra similar to the achiral analogous,7 displaying a maxima around 507 nm and a shoulder around 542 nm. The luminescence quantum yield, ϕ, was determined using the following equation:
where the subscript r and x denote reference and sample, respectively; A is the absorbance at the exciting wavelength, I is the intensity of the excitation light at the same wavelength, n is the refractive index of the solution (n=1.343 in CH3CN and n=1.424 in CH2Cl2), and D is the measured integrated luminescence intensity.
The luminescence quantum yield of complex 2 was determined to be 6.1% by reference to the [Ru(bpy)3]2+ complex (absolute quantum yield: 6.2%8) in deoxygenated CH2Cl2 and CH3CN solutions at concentrations of 1.3×10−5 and 3.6–6.7×10−6 M at 295 K, respectively. The luminescence quantum yield was determined at excitation wavelengths at which (i) the Lambert-Beer law is obeyed and (ii) the absorption of the reference closely matches that of the sample.
The circularly polarized luminescence (ΔI) and total luminescence (I) spectra measured for complex 2 in CH2Cl2 solution (at 295 K) are shown in Figure 5. The degree of circularly polarized luminescence is given by the luminescence dissymmetry ratio:
where IL and IR refer, respectively, to the intensity of left and right circularly polarized emissions. The total luminescence of complex 2 is remarkably high, while as is usual for most chiral organic chromophores and transition metal complexes, 9 the glum obtained for complex 2 is small: +0.0019±0.0004 as determined at the maximum emission wavelength. Although the glum values are very small (a value equal to ~0.002 corresponding to light that is only 0.2% circularly polarized) and the signals here are weak,1 they are measurable.
We thank the Swiss National Science Foundation for financially supported to this work. G.M. thanks the National Institute of Health Minority Biomedical Research Support (2 S06 GM008192-27) and Research Corporation Cottrell Science Award (CC6624) for their financial support, and Ms. Nicole M. Kosareff for her help in the spectroscopic measurements.
Supporting Information: Experimental details and spectral properties of complexes 1 and 2.
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