Crystal structure of [1,1′′′-bis(pyrimidin-2-yl)-4,4′:2′,2′′:4′′,4′′′-quaterpyridine-1,1′′′-diium-κ2 N 1′,N 1′′]bis[2-(pyridin-2-yl)phenyl-κ2 N,C 1]iridium(III) tris(hexafluoridophosphate) acetonitrile trisolvate

In the title compound, the Ir3+ cation is coordinated by two C atoms and four N atoms in a slightly distorted octahedral geometry. The asymmetric unit consists of one complex trication, three hexafluoridophosphate anions and three acetonitrile solvent molecules.

However, such compounds containing ligands with pyridinium substituents are scarce, and the only ones reported to our knowledge are the complex salts [Ir III (C-N) 2 (Me 2 qpy 2+ )] [PF 6 ] 3 (L-L = ppy or benzo[h]quinoline) (Ahmad et al., 2014). We report here a related new compound and what appears to be the first X-ray crystal structure determination of an iridium complex containing a qpy-based ligand. ISSN 2056-9890

Structural commentary
The molecular structure ( Fig. 1) of the complex cation in [Ir III (ppy) 2 {(2-pym) 2 qpy 2+ }][PF 6 ] 3 Á3CH 3 CN (I) is as indicated by 1 H NMR spectroscopy, with a slightly distorted octahedral coordination geometry. The bite angle of the qpy-based ligand is 76.6 (2) , while those of the ppy ligands are slightly larger at 80.1 (6) and 80.8 (5) . As for other related complexes (Ladouceur et al., 2010;Zhao et al., 2010;Constable et al., 2013;Schneider et al., 2014), the strong trans effects of a -bonded phenyl ring (Coe & Glenwright, 2000) causes these units to adopt a cis orientation, so that the pyridyl rings of the ppy ligands are oriented trans. The structural trans effect of the phenyl rings is shown by the ca 0.08 Å lengthening of the Ir-N(qpy) distances [average = 2.132 (10) Å ] with respect to the Ir-N(ppy) ones [average = 2.05 (6) Å ]. The Ir-C distances (Table 1) are shorter still, with an average value of 2.01 (14) Å . All of the geometric parameters around the Ir 3+ cation are similar to those reported for related structures.

Figure 2
Crystal packing diagram, viewed approximately along the b axis, showing the alignment of the qpy fragments. The H atoms, PF 6 À anions and acetonitrile solvent molecules have been removed for clarity.

Figure 3
Crystal packing diagram, viewed approximately along the a axis, showing the antiparallel alignment of the molecular dipoles (represented by arrows for the extreme left and right complexes). The H atoms, PF 6 À anions and acetonitrile solvent molecules have been removed for clarity.

Supramolecular features
The unit cell contains four complex cations with their qpy units aligned approximately parallel (Fig. 2). There may be a weak -stacking interaction between a 2-pym ring and one of the rings of the bpy fragment in an adjacent complex, with a centroid-to-centroid distance of 3.854 (8) Å and a dihedral angle of 9.8 (6) . Ru II complexes of (2-pym) 2 qpy 2+ and related ligands show interesting non-linear optical (NLO) properties (Coe et al. 2005). In this context, crystal packing arrangements are of great importance because macroscopic polarity is necessary for the existence of bulk quadratic NLO effects. The space group Cc adopted by (I) is non-centrosymmetric, potentially affording a polar material that could display such NLO properties. However, the overall orientation of the dipoles formed by the electron-donating Ir III (ppy) 2 units and the accepting (2-pym) 2 qpy 2+ ligands is antiparallel (Fig. 3). Therefore, significant bulk quadratic NLO behaviour is not expected for this particular crystal form.
Single crystals (amber plates) suitable for X-ray diffraction studies were grown by slow diffusion of diethyl ether vapour into an acetonitrile solution at room temperature.

Other Characterization
The complex salt (I) shows a relatively weak, broad visible absorption band at max = 562 nm (" = 1,800 M À1 dm 3 ) in acetonitrile. Based on the results of time-dependent density functional theory (TD-DFT) calculations on the related complex [Ir III (ppy) 2 (Me 2 qpy 2+ )] 3+ (Peers, 2012), this absorption is attributable to d!* metal-to-ligand charge-transfer (MLCT) transitions directed towards the qpy-based ligand, with significant contributions by the ppy ligands to the donor orbitals introducing also ligand-to-ligand CT character. Below 500 nm, absorption increases steadily into the UV region, with another maximum at 378 nm (" = 16,600 M À1 dm 3 ), and a shoulder at ca 410 nm. By way of contrast, the lowest energy band for [Ir III (ppy) 2 (Me 2 qpy 2+ )][PF 6 ] 3 appears at max = 531 nm (" = 1,200 M À1 dm 3 ) in acetonitrile (Peers, 2012). The substantial red-shift of this band on moving to (I) is due to the enhanced electron-accepting ability of the N-(2-pyrimidyl)pyridinium groups. The higher intensity for (I) is a consequence of extended -conjugation involving the 2-pym rings.
Cyclic voltammetric studies on (I) reveal an irreversible oxidation process at E pa = 1.43 V vs Ag-AgCl {acetonitrile, 0.1 M [N(n-Bu 4 )]PF 6 , 2 mm Pt disc working electrode, 100 mV s À1 , ferrocene/ferrocenium standard at 0.44 V (ÁE p = 70-90 mV)}. The reductive region shows a reversible wave at E 1/2 = À0.29 V (ÁE p = 80 mV), followed by an irreversible process with E pc = À0.79 V. Based on the relative peak currents, the reversible wave is assigned as a two-electron process involving reduction of both pyridinium units.    (Peers, 2012). The irreversible oxidation wave corresponds with removing an electron from the HOMO comprising the Ir and ppy ligands. The first and second reductions involve adding electrons to the LUMO based on the (2-pym) 2 qpy 2+ ligand. The oxidation occurs at the same E pa value for (I) and its methylated analogue, [Ir III (ppy) 2 (Me 2 qpy 2+ )][PF 6 ] 3 , but the first two reductions appear as overlapping reversible waves at E 1/2 = À0.62 V (ÁE p = 70 mV) and E 1/2 = À0.73 V (ÁE p = 60 mV) in the latter compound. These waves can be resolved by using differential pulse voltammetry (potential increment = 2 mV, amplitude = 50 mV, pulse width = 0.01 s). The anodic shift in the reduction waves is consistent with the qpy-based ligand being more electron-deficient, and therefore easier to reduce, in (I). The lack of splitting of these waves in (I) indicates that electronic communication between the pyridyl radicals is diminished with respect to its methyl analogue. Interestingly, for the related compound [Ru II (bpy) 2 {(2pym) 2 qpy 2+ }][PF 6 ] 4 , the first two reductions are irreversible under the same conditions using a glassy carbon working electrode (Coe et al., 2011).

Refinement
The structure was solved by direct methods. The two rings of one of the ppy ligands are indistinguishable by bond lengths, and the presented structure gives the lowest R factors. Crystal twinning is present. There is a pseudo-twofold axis that manifests itself as high correlation between parameters during refinement. The non-hydrogen atoms were refined anisotropically, but a rigid bond restraint (RIGU in SHELX) was applied for atoms with pseudo-symmetry-related counterparts. H atoms were included in calculated positions with C-H bond lengths of 0.95 (CH), 0.99 (CH 2 ) and 0.98 (CH 3 ) Å ; U ĩso (H) values were fixed at 1.2U eq (C) except for CH 3 where it was 1.5U eq (C). Crystal data, data collection and structure refinement details are given in Table 2.
Absolute structure parameter: 0.384 (7) 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. Refined as a 2-component inversion twin.