The crystal structure of bis{3,5-difluoro-2-[4-(2,4,6-trimethylphenyl)pyridin-2-yl]phenyl}(picolinato)iridium(III) and its 4-tert-butylpyridin-2-yl analogue

The crystal structures of two blue-emitting iridium(III) cyclometallates were determined and related to the photophysical properties of the complexes.


Structural commentary
The molecular structure of complexes 1 and 2 have been confirmed by X-ray crystallography and displacement ellipsoid diagrams are shown in Figs. 1 and 2. Both complexes exhibit the same distorted octahedral geometry with two bidentate phenylpyridine ligands (coordinated through the pyridine N atom and a phenyl C atom) and one bidentate pyridine-2-carboxylate ligand, also known as 2-picolinate (coordinated through the pyridine N atom and a carboxylate O atom). The iridium-bound N atoms of the phenylpyridine ligands are trans to each other, while the phenyl C atoms bound to iridium are cis. The two ligated atoms in the picolinate ligand are then trans to the phenyl C atoms of the phenylpyridine ligand. The trans effect of the phenyl groups is most clearly seen when comparing Ir-N bond lengths. Thus, the Ir-N(picolinate) bond is on average 0.1 Å longer than the Ir-N(phenylpyridine) bond due to its trans disposition to the phenyl group. Although there is no such intramolecular comparison available for the picolinate Ir-O bond (which is also trans to a phenyl group), we note that the Ir-O bond length in both these molecules is 2.16 Å . This compares with the shorter value of 2.09 Å reported in the Cambridge Structural Database (CSD; Version 5.39, May 2018, with three updates; Groom et al., 2016) for an Ir-O bond, again illustrating the trans effect of the phenyl group in these molecules. The C-Ir-N 'bite' angle for the phenylpyridine ligand averages 80.8 (4) for these complexes, while the N-Ir-O angle for the picolinate ligand is somewhat smaller at 76.7 (2) . The phenyl and pyridine rings in each phenylpyridine ligand are slightly twisted with respect to each other across the C-C bond linking the two rings. The dihedral angle between the best planes for the two rings is in the range 6-10 in these molecules. A feature of special interest in 1 is the dihedral angle between the plane of the pyridine ring and that of the attached mesityl group, e.g. between the N1/C7-C11 ring and the C12-C17 ring. These values are 67.0 (1) and 78.7 (1) for the two mesityl-phenylpyridine ligands in 1. The presence of two ortho-methyl groups on the mesitylene (e.g. C18 and C20) presumably causes this large twist of the mesityl ring out of the plane of the attached pyridine ring. This possibility has been proposed (Kozhevnikov et al., 2013) as an explanation for the blue emission of 1 since it minimizes the -conjugation between the mesityl and pyridine rings which would otherwise lead to red-shifted emission. Our results confirm the structural basis for this proposal.

Supramolecular features
Neither complex forms any significant supramolecular interactions with neighboring molecules.

Database survey
The molecular structure of 2 has been reported previously (Laskar et al., 2006; CSD refcode CEHGOM); however, the structure reported below is a new polymorph as a solvate. A search of the CSD (Groom et al., 2016) provided no additional crystal structures related to 1 or 2.

Synthesis and crystallization
The title compounds were prepared as described previously (Kozhevnikov et al., 2013). Diffraction-quality crystals of 1 were obtained by slow evaporation using methanol as solvent, while 2 utilized a 1:2 (v/v) methyl ethyl ketone-hexane mixture as solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. All H atoms were placed in calculated positions and refined as riding atoms; for aryl H atoms, C-H = 0.95 Å and U iso (H) = 1.2U eq (C), and for methyl H atoms, C-H = 0.98 Å and U iso (H) = 1.5U eq (C). Both specimens used for this study contained badly disordered solvent of crystallization. Specimens for compound 1 tended to lose solvent during mounting. The initial structure solution appeared to be either a methanol trisolvate or a methanol/ water disolvate, where all three molecules were concatenated through hydrogen bonds to the O2 hydrogen-bond acceptor. Only one methanol solvent molecule was clearly indicated, but it had considerable translational displacements toward a putative compositionally disordered methanol/water site. The last member of this chain was likely a methanol, but it was pathologically disordered. The SQUEEZE routine (Spek, 2015) from PLATON (Spek, 2009) was applied to this disordered solvent region since in least-squares no reasonable disorder model could be achieved. Void spaces centered at (0, 0, 0) and (0, 0.5, 0.5) totaling 727 Å 3 were found to contain an electron count of 177. This electron count would correspond to approximately ten methanol molecules per unit cell. The specimen for compound 2 did not appear to lose solvent during mounting. The initial structure solution found the expected compound and a region near an inversion center composed of unknown solvent. The peaks in the difference Fourier map did not provide any reasonable solvent molecule (or molecules) after numerous attempts. The SQUEEZE Table 1 Experimental details.

Bis{3,5-difluoro-2-[4-(2,4,6-trimethylphenyl)pyridin-2-yl]phenyl-κ 2 N,C 1 }(picolinato-κ 2 N,O)iridium(III) (1)
Crystal data  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.

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.