Crystal structure of fac-{5-[(hexylazaniumyl)methyl]-2-(pyridin-2-yl)phenyl-κ2 N,C 1}bis[2-(pyridin-2-yl)phenyl-κ2 N,C 1]iridium(III) chloride

The asymmetric unit of the title compound, fac-[Ir(C11H8N)2(C18H24N2)]Cl or fac-[Ir(ppy)2(Hppy-NC6)]Cl, contains two [Ir(ppy)2(ppy-NC6)](H+) cations, two Cl− anions and a disordered solvent. In each complex molecule, the IrIII ion is coordinated by two C,N-bidentate 2-(pyridin-2-yl)phenyl ligands and one C,N-bidentate N-[4-(pyridin-2-yl)benzyl]hexan-1-aminium ligand, leading to a distorted fac-octahedral coordination environment. In the crystal, the molecules are linked by N—H⋯Cl, C—H⋯π and π–π interactions, forming a three-dimensional supramolecular structure.


Chemical context
Luminescent iridium complexes have attracted a significant amount of interest over the past decades as they have been shown to possess potential for use in a number of applications such as in organic-light emitting devices (OLED), cellular imaging and photoredox catalysis (You, 2013;You & Nam, 2012;Kö nig, 2017;Caporale & Massi, 2018). The beneficial photophysical properties of these complexes, which are at the core of their potential utilization, arise both from the properties of the Ir 3+ ion and its coordination environment. The large spin-orbit coupling constant of iridium ensures efficient involvement of triplet excited states in the photophysical properties, which results in luminescent lifetimes in the microsecond regime (Ladouceur, S. & Zysman-Colman, 2013;Zanoni et al., 2015;Thorp-Greenwood et al., 2012). This is significantly longer than for fluorescence from organic fluorophores, a benefit for imaging applications, yet also much shorter than phosphorescence lifetimes of organic phosphors, which is important for OLED applications. The NC cyclometalating ligands such as 2-phenylpyridine usually used to chelate the iridium center provide strong ligand fields, which result in lifting of the unfilled metal-based orbitals above the * orbitals of the ligands, thus eliminating metal-centered ISSN 2056-9890 transitions from the photophysical properties (You & Nam, 2012). Thus, the usual electronic transitions present in the photochemistry of luminescent iridium complexes have charge-transfer characteristics such as metal-to-ligand charge transfer (MLCT) or ligand-to-ligand charger transfer (LLCT).
Luminescent iridium complexes can be divided into several distinct classes, one of which is tris-cyclometalated complexes. These complexes contain three cyclometalating NC ligands such 2-phenylpyridine (ppy) and the prototypical example of this structural class is [Ir(ppy) 3 ] (You & Nam, 2012). These complexes usually exhibit good photophysical properties. However, their use in cellular imaging is limited as they do not seem to be very readily taken up by cells (Ferná ndez-Moreira et al., 2010;Steunenberg et al., 2012;Ho et al., 2012). It has been noted that this problem can be alleviated by introducing protonatable groups into their structures, which helps them to become positively charged and thus be better taken up by cells (Kando et al., 2015). We have recently reported two simple derivatives of the prototypical structure mentioned above, which contain an aminoalkyl side chain on one of the ppy ligands (Sansee et al., 2016). The complexes differ only in the length of the alkyl chain, one being butyl while the other one is dodecyl. Both complexes are capable of staining live cells in fluorescence microscopy experiments. Furthermore, the complexes also exhibit ratiometric response to pH, which depends on their structure and is attributed to changes in their aggregation status. Several further analogues of these complexes are currently being investigated in order to obtain more detailed knowledge of the relationship between the structure of these compounds and their photophysical properties. The complex reported herein is one of these further compounds studied for this purpose.

Structural commentary
The asymmetric unit of the title compound contains two [Ir(ppy) 2 (Hppy-NC 6 )] + cations, two Cl À anions and disordered solvent molecules. In each complex molecule, the Ir III ion is coordinated by two C,N-bidentate ppy ligands and one C,N-bidentate Hppy-NC 6 ligand, leading to a distorted facoctahedral coordination environment as shown in Fig. 1. The Ir-C and Ir-N bond lengths in the title compound range from 2.010 (6) to 2.036 (5) Å and 2.105 (5) to 2.144 (4) Å , respectively, whereas the bond angles in the [IrN 3 C 3 ] octahedral core vary from 79.1 (2) to 172.1 (2) . These structural features are typical of related iridium(III) complexes containing C,N-donor set ligands (Steunenberg et al., 2012). The current molecule is isostructural with the butyl equivalent and displays similar packing and voids (see Refinement section) in the solid state. Full details of this structure have been published by Sansee et al. (2016).

Photophysical properties
The photophysical properties of the title compound have also been investigated in dichloromethane solution and the results can be seen in Fig. 4, which shows normalized absorption and emission spectra. The spectra exhibit the expected features, which are analogous to those of the parent complex and the complexes previously reported by our group. The absorption spectra can be roughly divided into three portions. The first portion lies between 250 and 320 nm and is mainly attributed to ligand-based to * transitions. The second portion of this spectrum lies between 320 and 430 nm and is attributed to spin-allowed singlet metal-to-ligand charge transfer ( 1 MLCT) transition. Finally, the tail of the spectrum extending from 430 nm beyond 500 nm is attributed to spin-forbidden triplet metal-to-ligand charge transfer ( 3 MLCT). The emission spectrum exhibits a single unstructured peak centered at The photophysical properties of the title compound. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A perspective view of the title compound, showing the intermolecular N-HÁ Á ÁCl hydrogen bonds (dotted lines) between the two independent molecules.

Figure 3
A perspective view showing the parallel fourfold phenyl embrace in the title compound. 520 nm. The photoluminescence quantum yield has been determined to be 39%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms attached to carbon atoms were placed in calculated positions and constrained to ride on their parent with U iso (H) = 1.2U eq (C) and a C-H distance of 0.93 Å for aromatic and 0.97 Å for methylene hydrogen atoms. The nitrogen-bound hydrogen atoms were located in a difference-Fourier map but were refined with a distance restraint of N-H = 0.89 Å with U iso (H) = 1.2U eq (N). The hexyl group of one complex is disordered over two orientations with a refined occupancy ratio of 0.412 (13):0.588 (13). Anisotropic displacement parameters of all atoms were restrained using enhanced rigid-bond restraints (RIGU command; Thorn et al., 2012). All attempts to model disordered acetone or hexane as the solvents used for crystallization failed. Therefore, the solvent-masking routine smtbx.mask (Rees et al., 2005) was used and found four solvent-accessible voids in the unit cell. Two of them are of 490 Å 3 in volume and contain an estimated 71 electrons; the other two are of 157 Å 3 in volume and contain an estimated 60 electrons. These electrons are attributable to four molecules of acetone and two molecules of hexane, which means that there are two molecules of acetone and one molecule of hexane per formula unit present in this structure.

Funding information
The authors acknowledge financial support provided from the NRCT and administered by the Division of Research Administration at Naresuan University under grant No.  program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).