Phosphorescent mono- and diiridium(III) complexes cyclometalated by fluorenyl- or phenyl-pyridino ligands with bulky substituents, as prospective OLED dopants

The structures of four phosphorescent monoiridium and chloro-, cyanato- or oxamidato-bridged diiridium complexes with a potential for OLED applications are described.


Chemical context
Over the two decades since the pioneering work of Baldo et al. (1998), cyclometalated Ir III complexes have been developed as emitters (phosphorescent dopants) for organic light-emitting diodes (OLEDs) or light-emitting electrochemical cells (LECs) (Li et al., 2018;Adeloye, 2019). These complexes are structurally and synthetically versatile. A large contribution of the Ir orbitals to the excited state results in efficient spin-orbit coupling, allowing in principle the harvesting of all the electrogenerated excitons and a 100% internal quantum efficiency of electro-phosphorescence. Because the luminescent properties are metal-ligand based, colour fine-tuning can be achieved by choosing the ligands (of which phenylpyridine, ppy, is by far the most important) and substituents to systematically vary the electronic and steric properties, or by incorporation of ancillary ligands. Originally the research focused on monoiridium species, because the di-iridium complexes then known (mostly bis--chloro bridged) proved generally poor emitters because of their electron-withdrawing bridges; for example [Ir(ppy) 2 Cl] 2 has a quantum yield 80 times lower than fac-Ir(ppy) 3 (King et al., 1985).

Structural commentary
The mononuclear complex (I) (Fig. 1) crystallized as a pentane monosolvate, like its isomer (Ia), which had the methoxy substituent in the 4-rather than 5-position of the pyridine ring (M'hamedi et al., 2012), and shows a rather similar molecular geometry. Both structures show extensive disorder; however, while in (Ia) it is confined to the n-hexyl chains and the solvent, in (I) the disorder (between two half-occupied positions) involves one ligand entirely, most of another one (except the pyridine ring) and both n-hexyl chains of the third ligand, as well as the pentane molecule (Fig. 2). The Ir coordination in (I) is distorted fac-octahedral, with each Ir-N bond in a trans orientation to an Ir-C bond, confirming the earlier assessment from NMR spectra (M'hamedi et al., 2012). The mean distances Ir-N = 2.13 (1) and Ir-C = 2.02 (1) Å are slightly longer than those in (Ia) (2.119 and 2.006 Å , respectively) and similar to those in the unsubstituted Ir(ppy) 3 , both in its trigonal (Breu et al., 2005) and tetragonal polymorphs (Berger et al., 2010;Takayasu et al., 2013;Wang et al., 2013).
Near-parallel alignment of pyridyl rings coordinated to different Ir atoms [interplanar angles Py1/Py = 13.4 (2) , Py2/Py4 = 6.3 (3) , see Fig. 3], may seem propitious to intramolecularstacking, which is important for optoelectronic properties in these phases. However, with the parallel slip Disorder in the structure of (I); H atoms are omitted. All disordered fragments have 50% occupancies The molecular structure of (II), showing 30% probability displacement ellipsoids and the notation of pyridyl rings. The disorder and all H atoms are omitted.
Complex (III) (Fig. 5) crystallized as a pentane monosolvate in a centrosymmetric monoclinic structure with two molecules per asymmetric unit. Like (II), (III) crystallized as a racemate; the molecule has no crystallographic symmetry but an approximate local twofold symmetry relating the iridium centres, which have the same configurations. Thus, molecules with ÁÁ or ÃÃ configurations are equally present, whereas the previously reported analogues of (III) with non-substituted (IIIa) or 4-fluorinated (IIIb) pyridyl rings (M'hamedi et al., 2012) gave non-solvated chiral crystals (space group P2 1 ), which were isomorphous and contained one independent molecule each. The precision of structure (III) is limited, due to massive disorder, generally weak diffraction intensities and, possibly, incommensurate modulation along the a* direction, as indicated by 'streaky' reflection peaks. The intramolecular IrÁ Á ÁIr distances in (III), 3.410 (1) and 3.432 (1) Å , are similar to those in (IIIa) [3.402 (1) Å ] and IIIb [3.425 (1) Å ] and ca 0.3 Å shorter than in chloro-bridged complexes. Similarly to (II) and especially to (IIc), in each independent molecule one pair of pyridyl rings shows substantialstacking [interplanar Py/Py angles of 11.4 (3) and 10.7 (4) , slips of 1.90 Å , mean interplanar separations of 3.25 Å ], the other only a fringe overlap [Py/Py angles = 13.9 (3) and 19.5 (4) , slips of 2.88 and 2.83 Å ]. In this structure, the n-hexyl chains also show extensive disorder (Fig. 6), which could be only imperfectly resolved. These chains surround well-defined voids containing disordered solvent, which was masked. The electron density maps, and the shape and size of the voids suggest the solvent to be pentane rather than chlorobenzene.
In fact, for such diiridium complexes with monoatomic bridges, both metal centres have to adopt the same chirality: the meso diastereomer with opposite (ÁÃ) configurations of the two iridium centres would have sterically impossible short intramolecular contacts between the cyclometalating ligands. With longer bridges, however, both the ÁÁ/ÃÃ and meso isomers are sterically possible -and in some cases have been isolated and structurally characterized (Congrave et al., 2017). Thus, the molecule of (IV) (Fig. 7) is crystallographically centrosymmetric, i.e. meso. As in complexes (II) and (III), the Ir atoms in (IV) have a distorted octahedral coordination with the pyridine N atoms in trans positions to each other. The The molecular structure of (III), showing 30% probability displacement ellipsoids for non-H atoms. The disorder and all H atoms are omitted for clarity.

Figure 6
Disorder in the structure of (III), showing occupancies. H atoms are omitted.

Figure 4
Disorder in the structure of (II), showing occupancies. H atoms are omitted.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.
In structure (I), the entire cyclometalating ligand N3^C37, ligand N1^C1 except the pyridyl ring and the methyl atom C67, and n-hexyl substituents (except atoms C75 and C76) in the ligand N2^C19 are disordered equally between two positions (one of them labelled B). In the pentane solvent molecule, all atoms except C(1S) are also disordered between two positions (alternative carbon positions are primed).
In structure (III), the ethyl group at C100 is disordered between positions C101-C102 and C125-C126 and the butyl group at C104 between positions C105-C106-C107-C108 and C10A-C10B-C10C-C10D, with occupancies 2/3 and 1/ 3, respectively, in both cases. A methylene group is disordered between positions C80 0 and C80B with occupancies 0.6 and 0.4 and a dimethylene fragment is disordered between positions C204-C205 and C20B-C20C with occupancies 0.7 and 0.3, respectively. The propyl chain at C211 is disordered between positions C212-C213-C214 and C21B-C21C-C21D, and the butyl chain at C216 is disordered between positions C217-C218-C219-C220 and C21E-C21F-C21G-C21H with equal (0.5) occupancies in both cases. The structure contains enclosed solvent-accessible voids of 374 Å 3 (four per unit cell), occupied by disordered solvent, which could not be refined at atomic resolution and was masked using the OLEX2 SMTBX solvent-masking procedure based on Rees et al. (2005). The diffuse electron density in the voids being too low for chlorobenzene, the integral of 70 e per void can be interpreted as two pentane molecules (42 e each).
In structures (I)-(III), all H atoms were permitted to ride in geometrically idealized positions with C-H = 0.95, 0.99 and 0.98 Å for aromatic, methylene and methyl C atoms, respectively.
In structure (IV), aromatic H atoms were permitted to ride in geometrically idealized positions with C-H = 0.95 Å . Methyl groups were ascribed idealized geometry (C-H = 0.98 Å ) and were permitted to rotate around the C-C bonds (with a common refined U iso for all H atoms of each group), except the C48H 3 group, which was treated as ideally disordered between two opposite orientations. For the latter and aromatic H atoms, U iso (H) = 1.2U eq (C). The C31F 3 group is disordered (by rotation about the C-C bond) between orientations A and B with occupancies of 0.774 (5) and 0.226 (5), respectively, while the C30F 3 group is disordered by a similar rotation and tilt of the C-C bond, between orientations A and B with occupancies of 0.586 (15) and 0.414 (15), respectively. The chlorobenzene molecule in a general position has the chlorine atom disordered equally between positions Cl1 and Cl2. The void around the inversion centre (0, 0, 0) with the solvent-accessible volume of 204 Å 3 is shared by ca 0.15 of a chlorobenzene and 0.20 of a pentane molecule, refined at atomic resolution; the occupancies are in agreement with the integral electron density of 33.4 e per void, as estimated by OLEX2 SMTBX. The benzene ring has crystallographic inversion symmetry: the Cl atom is disordered between two positions related by this inversion. The central pentane atom C61 lies at the inversion centre, the adjacent atom is disordered equally between two positions with the terminal atom ordered. The pentane H atoms were not located. A strong peak of residual electron density (4.2 e Å À3 ) near the Ir1 atom can be interpreted as an alternative position, Ir1, of this atom, due to a whole-molecule disordered by a 12 rotation around its (crystallographic) inversion centre. Refinement of the occupancies of Ir1 and Ir2 (assuming equal ADPs) converged at 0.9817 (6) and 0.0183 (6), respectively, with a Ir1Á Á ÁIr2 distance of 0.62 Å .

Funding information
Funding for this research was provided by: Deanship of Scientific Research at Prince Sattam Bin Abdulaziz University (grant No. 11187/01/2019 to Ahmed M'hamedi).

Special details
Experimental. The data collection nominally covered full sphere of reciprocal space, by a combination of 4 runs of 600 and 1 run of 50 narrow-frame ω-scans (scan width 0.3° ω, 40s exposure), every run at a different φ and/or 2θ angle. Crystal to detector distance 4.84 cm. 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. Refinement. Intense disorder of n-hexyl chains, overlapping with the pentane molecule of crystallisation which has the occupancy of 0.3.

Special details
Experimental. Data were collected in shutterless mode. Full sphere of reciprocal space was nominally covered by 4 runs of 340 narrow-frame ω-scans (scan width 0.5°, 30s exposure), every run at a different φ angle. Two runs of 358 φ-scans (scan width 1°, 3s exposure) were used for scaling overflowing intensities. Crystal to detector distance 3.49 cm. 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.

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Acta Cryst. (2020). E76, 392-399 Refinement. A strong (4.2 eÅ -3 ) peak of electron density near the Ir atom was interpreted as an alternative position of the same atom with the occupancy of 0.0183 (6), corresponding to a rotation of the whole molecule by ca. 12 °. The alternative positions of light atoms cannot be resolved. The C(30)F 3 group is disordered by rotation and tilt between orientations A and B with occupancies 0.586 (15) and 0.414 (15), the C(31)F 3 group is disordered by rotation alone [with the C(31) ordered] between orientations A and B with occupancies 0.776 (5) and 0.224 (5), respectively. The opposite F atoms were refined with identical ADP. The PhCl molecule [C(50) to C(55)] has the Cl atom disordered between positions Cl(1) and Cl(2) with equal occupancies, the former is sterically incompatible with its inversion equivalent. The void of 204 Å 3 around the inversion centre (0, 0, 0) is shared by disordered PhCl and pentane molecules, with the occupancies tentatively estimated as 0.15 and 0.2, respectively. Methyl group C(48)H 3 was refined as ideally disordered, other methyl groups as rigid bodies rotating around C-C bonds, with a common refined U for three H atoms. Other H atoms: riding model.