Crystal structure and luminescent properties of bis[2,6-dimethyl-3-(pyridin-2-yl-κN)pyridin-4-yl-κC 4](2,2,6,6-tetramethylheptane-3,5-dionato-κ2 O,O′)iridium(III) ethyl acetate monosolvate

The IrIII atom in the solvated title complex adopts a distorted octahedral geometry coordinated by two C,N-chelating 2,6-dimethyl-3-(pyridin-2-yl)pyridin-4-yl ligands and one O,O′-chelating 2,2,6,6-tetramethylheptane-3,5-dionate ligand. The title compound shows bright blue–green emission in solution at room temperature.


Chemical context
Bipyridine-based iridium(III) complexes have recently attracted much attention because of their applications in organic light-emitting diodes (OLEDs) (Kim et al., 2018a;Reddy et al., 2016). In particular, fluorinated-or alkoxofunctionalized bipyridine ligands have attracted increasing interest in materials research fields because of their large energy differences (T 1 !S 0 ) between the triplet (T 1 ) excited states and singlet ground states . This large triplet energy makes them useful and effective ligands for the design of blue phosphorescent metal complexes. Interestingly, Ir III complexes bearing either methoxy or isopropoxy substituents in C-coordinating pyridine show blue emission at room temperature, although these substituents act as electrondonating groups Kim et al., 2018b). This could be due to their large triplet energy (T 1 = 2.70-2.82 eV). Compared with alkoxy substituents, the methyl group has been regarded as essentially the same substituent because of its electron-donating nature. However, an Ir III complex based on methyl-substituted bipyridine as a main ligand emits strong green phosphorescence emission at room temperature . This fact prompted us to investigate the structure of a new Ir III compound possessing methyl-substituted bipyridine ligands because the emission of the Ir III complex is dependent on both the main ligand and the structural diversity of the metal complex. Herein, we describe the results of our investigation of the crystal structure, thermal and luminescent properties of the title solvated Ir III complex possessing methyl-substituted bipyridine, which was motivated by its potential application for OLEDs. ISSN 2056-9890

Supramolecular features
In the extended structure, pairs of inversion-related Ir III complexes are linked by C-HÁ Á Á interactions (Table 2, yellow dashed lines in Figs. 2 and 3) between H9 with Cg2 and H33A with Cg1 (Cg1 and Cg2 are the centroids of the N1/C6-C10 and N4/C13-C17 rings, respectively), leading to the formation of a dimeric structure. The Ir III complex molecules and the ethyl acetate solvent molecules are also connected by a C-HÁ Á Á interactions (Table 2, green dashed lines in Fig. 2) between C38A and Cg2. No further intermolecular interactions between the dimeric structures could be identified (Fig. 3).

Figure 1
The molecular structure of the title compound, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as circles of arbitrary radii. Yellow dashed lines represent intramolecular C-HÁ Á ÁO hydrogen bonds. The ethyl acetate solvent molecule is not shown for clarity.

Thermal and luminescence properties
As shown in Fig. 4, the title complex has a high thermal stability. The decomposition temperature, which is defined as a 5% loss of weight, of more than 573 K is high enough to allow deposition of molecules under reduced pressure without any degradation (Lee et al., 2017). Thermogravimetric analysis of the title complex revealed that it was thermally stable up to 553 K. During the first stage, a significant weight loss (10%) occurred at approximately 423 K, a phenomenon that may be attributed to the loss of a subset of absorbed solvent molecules as supported by crystal structure. Subsequently, a small weight loss of ca 5% was observed at approximately 593 K. This suggests that the complex possesses sufficient thermal stability to sublime under reduced pressure without degradation. However, it may be noted that the decomposition temperature of the title complex is lower than that of its heteroleptic analog , bis(2 0 ,6 0 -dimethoxy-4-methyl-2,3 0 -bipyridinato-N,C 4 )Ir(acetylacetonate) (617 K). This may be due to the methyl substituents of the main bipyridine ligand. The title compound displays bright bluish-green emission in solution at room temperature, as shown in Fig. 5. Emission maxima were observed at 503 nm; this wavelength is blueshifted by approximately 10 nm from the 511 nm emission peak of mer-tris(2 0 ,6 0 -dimethyl-2,3 0 -bipyridinato-2 N,C 4 )iridium(III) . Moreover, a broad and featureless emission band at 298 K was observed, indicating that this emission can be ascribed to a metal-to-ligand charge The crystal structure of the title compound: C-HÁ Á Á interactions are shown as green and yellow dashed lines. The C atoms of ethyl acetate solvent molecules represent are shown in green and H atoms not involved in intermolecular interactions have been omitted for clarity.

Figure 2
The dimeric structure of the title compound caused by C-HÁ Á Á interactions between the Ir III complexes (yellow dashed lines). Green dashed lines represent C-HÁ Á Á interactions between ethyl acetate solvent molecules and the Ir III complex. transfer (MLCT) transition . However, a structured emission band with max = 491 nm was observed at 77 K. This emission mainly originates from the ligandcentered (LC, 3 -*) transition based on a previous report (Lee et al., 2015). The triplet energy (E T ) of the title complex was estimated to be 2.52 eV using the emission spectrum at 77 K. The quantum efficiency (È PL ) of the title complex was estimated using FIrpic, bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium(III), as a standard (È PL = 0.6) to be 0.4. The high thermal stability and good quantum efficiency of the title complex makes it a potentially useful triplet emitter for applications in OLEDs.

Synthesis and crystallization
All experiments were performed under a dry N 2 atmosphere using standard Schlenk techniques. All solvents were freshly distilled over appropriate drying reagents prior to use. All starting materials were commercially purchased and used without further purification. The 1 H NMR spectrum was recorded on a Bruker Avance 400 MHz spectrometer. The thermogravimetric spectrum was recorded on a Perkin-Elmer TGA-7 under a nitrogen environment at a heating rate of 10 K min À1 over a range of 298-973 K. The Ir III dimer, [(Me 2 pypy) 2 Ir(-Cl)] 2 , and the title compound were synthesized according to previous reports . The Ir III dimer, [(Me 2 pypy) 2 Ir(-Cl)] 2 , (0.15 g, 0.126 mmol), sodium carbonate (0.13 g, 1.26 mmol), and 2,2,6,6-tetramethylheptane-3,5-dione (0.066 ml, 0.32 mmol) were dissolved in THF/MeOH (1:1, 10 ml). The reaction mixture was stirred overnight at ambient temperature. All volatile components were removed under reduced pressure. The mixture was poured into EtOAc (50 ml), and then washed with water (3 Â 50 ml) to remove excess sodium carbonate. Silica gel column purification with EtOAc and hexane gave a yellow powder in 60% yield. Yellow plates were recrystallized from ethyl acetate/hexane solution at low temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3 TGA curve of the title compound.

Figure 5
Emission spectra of the title compound at 298 K and 77 K. geometrically and refined using a riding model, with d(C-H) = 0.95 Å , U iso (H) = 1.2U eq (C) for Csp 2 H atoms, and 0.98 Å , U iso (H) = 1.5U eq (C) for methyl protons. program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2010); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and publCIF (Westrip, 2010). Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.