Synthesis, crystal structure and photophysical properties of chlorido[2-(2′,6′-difluoro-2,3′-bipyridin-6-yl-κN 1)-6-(pyridin-2-yloxy-κN)phenyl-κC 1]platinum(II)

In the crystal of the title compound, which shows blue–green emission, the molecules are connected via C—H⋯Cl/F, halogen⋯π and weak π–π stacking interactions.


Chemical context
The C,N-chelating 2,3 0 -bipyridine-based transition-metal compounds have attracted much interest because of their wide applications as biological labels, photosensitizers in water reduction, sensors and organic light-emitting diodes (OLEDs) (Zaen et al., 2019a,b). Especially, highly efficient phosphorescent metal complexes containing Ir III and Pt II can be synthesized by using 2,3 0 -bipyridine as ligand, which feature a high triplet-state energy (Lee et al., 2018). In terms of the efficiency and stability of OLEDs, tetradentate ligand-based Pt II complexes are known to be very good candidates as triplet emitters (Wang & Wang, 2019). The design of tetradentate ligands is focused on making appropriate coordination modes in order to form five or six-membered metallacycles. To achieve blue emission in Pt-based triplet emitters, two strategies have been employed as follows: (i) incorporation of a high-triplet-energy moiety into the ligand framework; (ii) the breakage of -conjugation in the ligand to increase the energy gap (Fleetham et al., 2016;Kang et al., 2020). With these in mind, we have recently synthesized 2 0 ,6 0 -difluoro-6-[3-(pyridin-2-yloxy)phenyl]-2,3 0 -bipyridine as a ligand with high triplet energy . By using this ligand, we have ISSN 2056-9890 synthesized its coordination metal complex containing Pt II and determined its crystal structure: herein, we report the structural and photophysical characteristics of chlorido[2 0 ,6 0 -difluoro-6-[3-(pyridin-2-yloxy)phenyl]-2,3 0 -bipyridine-3 N,C,N 0 ]platinum(II).

Structural commentary
The asymmetric unit of the title compound, Pt(C 21 H 12 F 2 N 3 O)Cl, contains two crystallographically independent molecules (A and B denote the Pt1-and Pt2containing molecules, respectively), which adopt similar conformations ( Fig. 1 and Table 1). The coordination sphere of the Pt II atoms in both molecules is a distorted square-planar geometry, with the respective coordination sites occupied by one C and two N atoms from the 2 0 ,6 0 -difluoro-6-[3-(pyridin-2yloxy)phenyl]-2,3 0 -bipyridine ligand together with a chloride anion. The average length [1.949 (4) Å ] of the Pt-C bonds is slightly shorter than that [2.042 (3) Å ] of the Pt-N bonds because of back bonding between the metal and the anionic C atom of the ligand. The Cl1 and Cl2 atoms deviate from the mean plane consisting of the Pt and coordinated N/C atoms [r.m.s. deviations = 0.013 (1) (A) and 0.017 (1) Å (B)] with deviations of 0.700 (6) Å for A and 0.720 (6) Å for B.
In each molecule, there are intramolecular C-HÁ Á ÁCl/F interactions, contributing to the stabilization of the molecular structure (Table 2 and black dashed lines in Fig. 1). Moreover, an intramolecular ClÁ Á Á interaction [Cl1Á Á ÁCg4 = 3.4537 (19) Å , Cl2Á Á ÁCg8 = 3.455 (2) Å ; green dashed lines in Fig. 1; Cg4 and Cg8 are the centroids of the N3/C17-C21 and N6/C38-C42 rings, respectively] between the coordinated chloride ion and the pyridine ring with fluorine substituents are also observed. Molecules A and B are interlinked by a C-HÁ Á ÁCl interaction (Table 2 and yellow dashed line in Fig. 1). In the 6-phenyl-2,3 0 -bipyridine system in both molecules, the phenylpyridine moieties are approximately coplanar with the dihedral angles between the pyridine ring and the attached phenyl rings being 10.01 (11) for A and 9.64 (11) for B. However, the terminal difluoro-pyridine ring is tilted by 46.08 (9) for A and 46.96 (8) for B with respect to phenylpyridine ring plane. This distortion may be caused by the intramolecular ClÁ Á Á interaction described above. The pyridine ring of the pyridine-2-yloxy group is slightly tilted by 22.09 (13) for A and 19.70 (13) for B relative to the phenylpyridine ring plane.

Figure 1
The molecular structure of the title compound, showing the atomnumbering scheme and displacement ellipsoids at the 50% probability level. Black and green dashed lines represent intramolecular C-HÁ Á ÁCl/ F and ClÁ Á Á interactions, respectively, and yellow dashed line represents intermolecular C-HÁ Á ÁCl interaction. H atoms not involved in intra-and intermolecular interactions are not shown for clarity.

Luminescent property
The bright blueish-green emission of the title compound in solution is dominated by phosphorescence as supported by an excited-state lifetime of more than 1 ms. Emission maxima appear at 517 and 544 nm at room temperature, as shown in Fig. 4. The emission observed in the title compound is attributable to an intra-ligand charge transfer (ILCT) transition mixed with a metal-to-ligand charge-transfer (MLCT) transition based on previous reports (Wang & Wang, 2019). Contrary to our expectations, the title compound shows green emission. It may be that the chloride ion bound directly to the platinum ion causes this effect because 2 0 ,6 0 -difluoro-2,3 0 -bipyridine (dfpypy)-based platinum complexes without chloride ions often exhibit blue emission at room temperature. The photoluminescence quantum efficiency of the title compound was estimated to be $0.2-0.3 (Fig. 4, inset). Such efficiency is enough to use the title compound as the emitting material in organic light-emitting diode (OLED) applications.

Figure 2
The two-dimensional supramolecular network formed through intermolecular C-HÁ Á ÁCl/F hydrogen bonds (yellow dashed lines), FÁ Á Á (red dashed lines) andstacking (black dashed lines) interactions between aromatic rings of inversion-related B molecules. For clarity, H atoms not involved in the intermolecular interactions have been omitted. Colour codes: violet = platinum, plum = chloride, green = fluorine, red = oxygen, blue = nitrogen, grey = carbon and white = hydrogen.
title ligand: to the best of our knowledge, this is the first crystal structure reported for a platinum complex with the title ligand.

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.