mer-Trichloridotris(tetrahydrothiophene-κS)iridium(III): preparation and comparison with other mer-trichloridotris(tetrahydrothiophene-κS)metal complexes

The crystal structure of mer-trichloridotris(tetrahydrothiophene-κS)iridium(III) is reported and compared with a different form of the complex previously reported. It is also compared with other mer-trichloridotris(tetrahydrothiophene-κS)metal(III) complexes of molybdenum, ruthenium and rhodium.


Chemical context
We have been engaged in various studies of iridium chemistry for many years (Merola, 1997;Merola & Franks, 2015;Merola et al., 2013) and recently had need to find alternate routes to some iridium(III) complexes for our research. An examination of the literature led to the title compound as a possible anhydrous source of iridium(III) that we could use as a starting material (Allen & Wilkinson, 1972). mer-Trichloridotris(tetrahydrothiophene-S)iridium(III) has been mentioned in the literature as a starting material for other organometallic iridium complexes (Hay-Motherwell et al., 1989, 1992John et al., 2000John et al., , 2001John et al., , 2014, and most recently has been the starting material of choice for new emissive materials (Chang et al., 2008(Chang et al., , 2013Chiu et al., 2009;Hung et al., 2010;Lin, Chang et al., 2011;Lin et al., 2012). However, no crystallographic studies had been published on this compound. Given its increasing importance, we decided that a single crystal structure determination of the title compound would be worthwhile.

Structural commentary
mer-Trichloridotris(tetrahydrothiophene-S)iridium(III) (CCDC refcode 1495966) crystallizes in the P2 1 /n space group ISSN 2056-9890 with one molecule in the asymmetric unit (Fig. 1). The core structure (heavy atoms around the iridium) is very close to rigorous octahedral geometry with the largest angular variation 177.35 (3) ] being less than 2.7 from ideal linearity.
The Ir-Cl bond lengths [range 2.3648 (8)-2.3774 (9) Å ] are somewhat longer than the Ir-S bonds [range 2.3279 (9)-2.3575 (9) Å ], as expected from the slightly larger radius of Cl. A search for Ir-S bonds in the CSD (Groom et al., 2016) and analyzed with Mercury (Macrae et al., 2008) found 2566 instances with distances ranging from 2.134 to 2.633 Å and a mean value of 2.358 Å . That places the bond lengths for the title compound slightly above the mean value. Similarly, a Mercury data analysis of the CSD for Ir-Cl bond lengths found 3965 instances with distances ranging from 2.121 to 2.816 Å and a mean value of 2.413 Å , which places the Ir-Cl distances for the title compound lower than the mean. This comparison should not be considered as too significant since it was not possible to compare bond lengths only for iridium(III) compounds and the analysis includes quite a few iridium(I) complexes. The tetrahydrothiophene rings are well ordered in the title structure, adopting a puckered conformation consistent with trying to minimize ring strain. Two of the rings are positioned with the center of the ring aligned over a chlorine atom in the structure, while the third is aligned over a sulfur atom of another ring. More will be said about the ring conformations in the Database survey section.

Supramolecular features
An examination of the packing diagrams for the title compound shows no unusual intermolecular features other than van der Waals interactions.

Database survey
A survey of the CCDC database (Groom et al., 2016) uncovered a number of metal mer-tris(THT-S)metal complexes (THT= tetrahydrothiophene), including one iridium structure deposited as a private communication (CCDC 1438699;Rheingold & Donovan-Merkert, 2015). The deposited structure (CCDC 1438699) packs with very different unit-cell parameters but the overall molecular structure is substantially the same. The results of the different packing, however, are slightly different conformations of two of the three THT ligands, as shown in Fig. 2, a structure overlay calculated in Mercury (Macrae et al., 2008). On the other hand, the rhodium(III) complex is isotypic with the title complex with similar unit-cell parameters (CCDC refcode GEZHUO; Clark et al., 1988). Fig. 3 shows an overlay calculated with Mercury (Macrae et al., 2008) of the title complex with the rhodium compound, showing the nearly perfect atomic overlay. Ruthenium(III) (VIJYAO; Yapp et al., 1990) and molybdenum(III) (REDXIH; Boorman et al., 1996) complexes were also found in the database, with all showing the same meridional arrangement of ligands with the exception that the ruthenium complex displays disorder from overlapping conformations of one of the THT ligands. Displacement ellipsoid plot (50% probability) of mer-trichloridotris-(tetrahydrothiophene-S)iridium(III) (CCDC 1495966).

Theoretical calculations
We were interested in determining if the bulk material synthesized by this process is of a single polymorph or if both of the iridium structures reported (CCDC 1495966, this report, and CCDC 1438699, Rheingold & Donovan-Merkert, 2015) were present. Fig. 4 shows an overlay of the powder X-ray diffraction pattern for the complex reported here with the powder pattern predicted by Mercury (Macrae et al., 2008). The match is very good and quite distinct from the pattern predicted for CCDC 1438699, indicating that the bulk material formed in this process is a single polymorph matching the structure reported here.
One feature that stands out in all cases is that the MCl 3 (THT) 3 compounds found in the database adopt the mer configuration. Calculations were performed using density functional theory with Gaussian 09 (Frisch et al., 2009). Full geometry optimization of both the mer and fac isomers was carried out via density functional theory (DFT) with the Becke-3-parameter exchange functional (Becke, 1993) and the Lee-Yang-Parr correlation functional (Lee et al., 1988). Because iridium is not covered in the cc-PVDZ basis set used, computations involving Ir employed Stuttgart/Dresden quasirelativistic pseudopotentials (Andrae et al., 1990). The difference between the two isomers was quite large with the mer isomer being more stable than the fac by 50.1 kJ mol À1 , suggesting the occurrence of only the mer isomer for the small set of compounds surveyed may be due to thermodynamic stability.

Synthesis and crystallization
The title compound was synthesized using a slight modification of a literature procedure (John et al., 2014). IrCl 3 Á3H 2 O (1.00 g, 2.84 mmol) and 2-methoxyethanol (50 mL) were added to a 250 mL round-bottomed flask fitted with a magnetic stir bar and a reflux condenser. Tetrahydrothiophene (1.25 mL, 14.2 mmol) was added all at once with stirring. The resulting suspension was refluxed for 18 h, providing a clear orange solution that gave a yellow precipitate upon cooling to room temperature. Deionized water (75 mL) was added and the suspension was cooled overnight (273 K) before collection on a fine-porosity sintered glass frit. The resulting yellow powder was washed with deionized water (3 x 15 mL) then

Figure 2
Calculated overlay of two polymorphs of mer-trichloridotris(tetrahydrothiophene-S)iridium(III) (CCDC 1438699 and CCDC 1495966). Structure from this paper shown in yellow. cold ethanol (3 x 15 mL). After vacuum drying overnight the yellow powder (1.40 g, 88%) was characterized by 1 H and 13 C NMR spectroscopy. As the NMR spectra were in agreement with previously reported data, no further purification was necessary. Single crystals for X-ray diffraction were grown by slow diffusion of n-pentane into a dichloromethane solution of mer-IrCl 3 (THT) 3 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1

Figure 4
Powder X-ray diffraction pattern of title compound collected on a Rigaku Miniflex 600 Powder X-ray diffractometer compared with pattern simulated by Mercury (Macrae et al., 2008). Experimental and simulated patterns scaled to highest intensity peak in each.

mer-Trichloridotris(tetrahydrothiophene-κS)iridium(III)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.54 e Å −3 Δρ min = −1.46 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.