Crystal structure of (2,2′-bipyridyl)[2,6-bis(1-butyl-1H-benzimidazol-2-yl)pyridine]chloridoiridium(III) trifluoromethanesulfonate

The title complex, [Ir(C27H29N5)(C10H8N2)Cl]2+·2CF3SO3 −, was synthesized via the reaction of 2,6-bis(N-butylbenzimidazol-2′-yl)pyridine (bubzimpy) and 2,2′-bipyridine (bipy) with sodium hexachloroiridate(III) and precipitated by adding aqueous sodium trifluoromethanesulfonate solution. The compound was characterized using single-crystal X-ray diffraction, FT–IR, cyclic voltammetry/rotating disc electrode polarography, fluorescence spectrometry, high resolution mass spectrometry, CHN elemental analysis and 1H NMR.


Structural commentary
The cationic complex of the title salt is composed of one molecule each of bipy and bubzimpy, and a chloride ion coordinating to the iridium(III) atom, with charge balance provided by two crystallographically independent trifluoromethanesulfonate ions (Fig. 1). The bond lengths and angles are comparable to similar complexes (Yutaka et al., 2005), though the torsion angles show distinct differences. The bond angles involving Ir range from 79.55 (12) (N6-Ir-N7) to 178.09 (13) (N3-Ir-N7), with the bond lengths between 1.992 (3) Å (Ir-N3) and 2.3510 (9) Å (Ir-Cl). The Ir complex with 2,6-bis(N-methylbenzimidazol-2 0 -yl)pyridine (mebzimpy) and bipy synthesized by Yutaka et al. (2005) is closely related to the title complex. Selected bond lengths, bond angles and torsion angles from their complex are compared with those of the title complex in Table 1. The torsion angle N1-C7-C8-N3 [À6.6 (5) ] for one of the benzimidazoles indicate that the benzimidazole is further removed from coplanarity with the central pyridine plane than it is in the mebzimpy analogue. Meanwhile, the two halves of the coordinating bipy molecule are slightly more rotated vs one another than in the mebzimpy analogue, as indicated by the N6-C32-C33-N7 torsion angle of 7.3 (5) . The dihedral angle between the mean planes of the bubzimpy and bipy ligands is 89.32 (6) . The r.m.s. angular deviation from ideal octahedral rectangularity, defined as 0.312[AE( i À 90) 2 ] 1/2 where i are the twelve cis-angles in the pseudo-octahedron (Popovitch et al., 2012), is 8.8 (8)% for the title complex, which is comparable to the value of 7.9 (7)% in the analogous N-methylated complex. One of the two trifluoromethane-sulfonate anions in the title complex is disordered over two orientations around the C-S bond with an occupancy ratio of 0.582 (6):0.418 (6).

Supramolecular features
The molecules stack in the crystal so that the benzimidazole ring systems of neighbouring molecules are parallel to each other, enablinginteractions to occur. The centroidcentroid distances and the slippages of the slippedstacking interactions are given in Table 2. The shortest interplanar distance is 3.337 (6) Å with the twostacked benzene rings slipped by 2.033 (8) Å . These interactions link the molecules into a staircase structure along [011] as shown in Figs. 2 and 3. The slippedstacking arrangement (Fig. 3) suggests that isomorphous replacement of iridium(III) molecules by non-luminescent/non-quenching analogues could lead to the formation of a superantenna system (Mikhalyova et al., 2015). The title complex with two trifluoromethanesulfonate counter-anions. Displacement ellipsoids are drawn at the 50% probability level. H atoms are rendered as spheres of arbitrary radius. Only one component of the disordered trifluoromethanesulfonate anion is shown. Table 1 Comparison of selected bond lengths, bond angles and torsion angles (Å , ).
(bipy)(mebzimpy)chloridoiridium(III)-(PF 6 ) 2 (Yutaka et al., 2005) (12) Torsion Angles Atom labels correspond to atoms of the title complex, analogous relationships reported by Yutaka et al. (2005) were compared. balance the complex charge and display C-HÁ Á ÁO and C-HÁ Á ÁF hydrogen bonds (Table 3). These interactions involve the O and F atoms from the anions interacting with the CH units from bipy as well as the pyridine ring of bubzimpy. An intermolecular C-HÁ Á ÁCl interaction is also observed between the coordinating chloride ion and the benzimidazole ring of bubzimpy on the neighboring complex (Table 3). Although this interaction is weaker than the prominent C-HÁ Á ÁO interactions, it contributes to the overall orientation of the packing in the crystal.

Electrochemistry
The redox chemistry of the Ir III complex was studied using cyclic voltammetry (CV) and rotating disc electrode (RDE) polarography, which were performed at 298 K on 0.3 mM Ir complex in acetonitrile with 0.1 M tetrabutylammonium hexafluoridophosphate (TBAPF 6 ) as the supporting electrolyte, at scan rates ranging from 50 to 800 mV s À1 for CV, and 1200 and 2400 rpm for the RDE. Experiments were run on a BASi-Epsilon instrument using a three-electrode cell, a nonaqueous reference electrode (APE) (Pavlishchuk & Addison, 2000) and a 3 mm diameter Pt disc working electrode. No welldefined anodic process is observed below +1400 mV, indicating that the oxidative potential for the Ir complex is higher than the potential window available in our experiments. The cathodic electrochemistry is not straightforward; however, there are three reductive processes with cathodic peak potentials of À1211, À1472 and À1719 mV. Similar results have been reported for the mebzimpy complex (Yutaka et al.,  A perspective view (from 150 Å , inverse stereo stick-structure) along the c-axis direction, with the bis(benzimidazolyl)pyridine-Ir planes oriented horizontally and rendered in purple, versus the other atoms (pale green). The slipped stacks form a 'staircase'; in the N-methyl analogue (Yutaka et al., 2005), the corresponding array appears as an alternating 'stepping stone' pattern.

Figure 3
Similarly to Fig. 2, a view (inverse stereo stick-structure) along the a-axis direction, showing the bis(benzimidazolyl)pyridines (purple) and the other atoms (pale green).
2005). In the RDE polarogram, a reductive wave was seen at E 1/2 = À1042AE5 mV, from which the diffusion coefficient of the molecule is estimated to be D = 9.0Â10 À6 cm 2 s À1 in MeCN, corresponding to a D value of 3.3Â10 À8 g cm s À2 , consistent with a one-electron transfer.

UV-Vis and Fluorimetry
The photochemical and photophysical properties of iridium(III) complexes have been studied extensively in the last few decades in order to better understand their potential for applications in areas like solar energy and electroluminescence (EL) devices (Nazeeruddin et al., 2003). The optical absorption spectrum of the title complex is displayed in Fig. 4. In such mixed-ligand complexes, ligand -* transition bands typically overlap; however, the ligand -* bands for bipy and bubzimpy in our complex were well-resolved at 315 and 352 nm, respectively, similarly to those observed by Yutaka et al. (2005). As has often been observed in compounds of this type (Yutaka et al., 2005), there is a strong emission in the yellow region of the spectrum with the intensity peaking at 542 nm (Fig. 5). The excitation profile is dominated by an absorption maximizing at 302 nm, corresponding closely to the bipy -* transition at 315 nm.

Database survey
Crystal structures of complexes containing bubzimpy as a ligand exist in the literature. This ligand chelates well to other transition metals, such as ruthenium (Yu et al., 2012), copper , gadolinium, lanthanum (Drew et al., 2004) and manganese . Hijazi et al. (2010) reported a platinum complex with a ligand similar to bubzimpy, 2,6-di(N-hexylbenzimidazol-2 0 -yl)pyridine. Similarly, Mathew & Sun (2010) showed a variety of 2,6-bis(Nalkylbenzimidazol-2 0 -y)pyridine platinum(II) complexes with one coordinating chloride as in our iridium complex. These platinum complexes involved variation of the alkyl chain on the benzimidazole ligand, as well as varied counter-ions, such as PF 6 À , ClO 4 À , and BF 4 À .

Figure 5
Emission spectrum of the title Ir(III) complex (0.8 mM) in non-purged acetonitrile at ambient temperature, excited at 295 nm. The ordinate unit is arbitrary.

Figure 4
UV-Vis spectrum of the title complex (10 mM) in acetonitrile. heated on a steam bath for 4 h, after which the reddish brown solid was filtered off and washed with ether and chloroform (Fig. 6). This resulting trichlorido-intermediate [0.057 g, 78 mmol; FAB-LSIMS MS: calculated (m+) m/z 721.110, found 721.135] was then dissolved in hot ethylene glycol (10 mL) with 2,2 0 -bipyridine (0.015 g, 94 mmol) and stirred at 433 K for 18 h (Fig. 7). The resulting iridium complex was precipitated by addition of aqueous sodium trifluoromethanesulfonate and then filtered off and washed with ether and chloroform. The crude product was purified via a two month diffusion of toluene into a methylene chloride solution, yielding orange crystals. M.p. > 523 K;

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. H atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H bond lengths of 0.95, 0.99 and 0.98 Å for aromatic CH, aliphatic CH 2 and CH 3 groups, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. U iso (H) values were set to a multiple of U eq (C) with 1.5 for CH 3 and 1.2 for CH and CH 2 units.
One of the two trifluoromethanesulfonate anions was refined as disordered over two orientations [occupancy ratio 0.582 (6):0.418 (6)]. The two components were restrained to have geometries similar to that of the non-disordered anion (SAME with esd 0.02 Å ), and the disordered atoms were subjected to a rigid-bond restraint (RIGU with esd 0.001 Å 2 ). Step 1: Reaction of bubzimpy with hexachloridoiridate(III) in a 1:1 ratio.

(2,2′-Bipyridyl)[2,6-bis(1-butyl-1H-benzimidazol-2-yl)pyridine]chloridoiridium(III) trifluoromethanesulfonate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 3.37 e Å −3 Δρ min = −1.91 e Å −3 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. Refinement. One of the two triflate anions is disordered with two alternative orientations. The two moieties were restrained to geometries similar to that of the not disordered anion, and disordered atoms were subjected to a rigid bond restraint (RIGU in Shelxl). Reflections 0 0 1 and -1 1 0 were affected by the beam stop and were omitted from the refinement.