Serendipitous preparation of fac-(acetonitrile-κN)trichlorido[(1,2,5,6-η)-cycloocta-1,5-diene]iridium(III)

An octahedral complex of iridium(III) with a chelating cycloocta-1,5-diene ligand, a facial arrangement of three chloride ligands and one acetonitrile ligand was isolated serendipitously from an attempted reaction between indene, the chloridocyclooctadieneiridium dimer and HCl. Work-up that included use of acetonitrile solvent led to the formation of a few crystals of the title compound.


Chemical context
We have published recently on the synthesis of a series of tetramethylalkylcyclopentadienyliridium complexes by the direct reaction between tetramethylalkylcyclopentadiene and iridium chloride, giving the [Cp* R IrCl 2 ] 2 dimer (Morris et al., 2014). From the dimer, a variety of other compounds can be made, such as amino acid complexes, that have shown significant anti-mycobacterial activity (Karpin et al., 2013). Some of the reactions produced low yields of the chlorido-bridged dimer, thus limiting the number of products that could be made and tested.
An alternate route to Cp*-type chlorido iridium dimers was reported using [(COD)IrCl] 2 as the starting material (El Amouri et al., 1994) and, in our hands, this route does have promise for providing higher yields for many of the compounds. However, in the case of indene, there was no indication that an indenyl iridium complex had been prepared. Instead, a yellow-brown intractable solid was formed. Several attempts to dissolve the solid and to separate products through fractional crystallization all failed. During the course of this work-up, one of the solvents used was acetonitrile. At some point, the product mixture was allowed to stand in solution, and after about 24 hours several very nicely shaped rectangular prisms had formed in the sample. These crystals were examined by X-ray crystallography and the results of that structure determination are reported here.

Structural commentary
While the total number of cycloocta-1,5-diene complexes structurally characterized is quite large, the number that are directly comparable to the title compound is small. The title compound is a pseudo-octahedral complex of iridium with three chloride ligands occupying one face of the octahedron and the alkenes of the COD and the acetonitrile ligand occupying the opposite face ( Fig. 1). Considering the varying ligands about the central iridium, there is very little distortion from ideal octahedral angles, with the most significant distortion being the N1-Ir1-Cl2 distorted away from the COD group with an angle of 164.05 (11) . All other angles, including those involving the alkene centroids, deviate by no more than 5 from the ideal. All three Ir-Cl bond lengths are similar [range 2.3603 (11) to 2.3670 (11) Å ], which is in keeping with both types of trans ligands, alkene and acetonitrile, being expected to be strong trans-influence ligands and would have a similar magnitude of effect on the chloride trans to either ligand.
The facial Ir-Cl distances may be contrasted with the average distance of 2.441 (2) Å for fac-[(Me 3 P) 3 IrCl 3 ] (CCDC: 896073) and related compounds  that have somewhat longer Ir-Cl distances due to the effect of the trans PMe 3 groups. Choudhury et al. (2005) reported on a COD complex of iridium with three chlorides and a SnCl 3 ligand completing the octahedral coordination about the central Ir atom (CCDC: 273475). In that case, though, the compound is a dinuclear one with Ir-Cl-Ir bridges. So, there are long Ir-Cl bonds (those involved in bridging) of 2.544 (4) Å and a shorter terminal Ir-Cl bond of 2.385 (6) Å . C C bond lengths for the COD ring are similar to the title compound at 1.38 (1) and 1.41 (2) Å .

Supramolecular features
Although there appear to be some close C-HÁ Á ÁCl intermolecular interactions, there are no important supramolecular features to speak of in this structure.

Database survey
A substructure search of the CCDC (Groom & Allen, 2014) for the 1,5-COD-Ir fragment resulted in over 850 hits. This is not a surprising result since [CODIrCl] 2 is a convenient, highyield organometallic starting material made in one step from IrCl 3 ÁH 2 O and cycloocta-1,5-diene (Crabtree & Morris, 1977). From [CODIrCl] 2 , a wide variety of ligand addition, chloride replacement or bridge-splitting reactions can be carried out, leading to a wide variety of compounds containing the COD chelate. Using Mercury (Macrae et al., 2008), an analysis of the COD-Ir search of the database for structures with an octahedral coordination around the metal showed that the C=C bonds of the COD ligands ranged from 1.184 to 1.508 Å with a mean of 1.394 Å . For the title compound, the values of 1.392 (7) and 1.389 (6) Å are pretty much right at the mean for COD C=C bonds. The asymmetric unit of the title compound. Displacement ellipsoids are shown at the 50% probability level.  An analysis of the CCDC database (Groom & Allen, 2014) for octahedral iridium complexes with acetonitrile ligands uncovered 99 hits with Ir-N distances measuring from a minimum of 1.897 Å to a maximum of 2.246 Å with a mean of 2.068 Å . For the title compound, the Ir-N distance of 2.023 (4) Å places it just below the mean.

Synthesis and crystallization
The title complex was formed as a few isolated crystals from an attempted reaction between [(COD)IrCl] 2 and indene with HCl in an attempt to synthesize the [indenylIrCl 2 ] 2 dimer, which would have been a useful starting material for our studies. Unfortunately, this did not provide the desired product. The reaction produced some very intractable solids. After multiple attempts to dissolve the solid in many different solvents, including acetonitrile, some well-shaped prisms formed on the side of the flask and these crystals were used in this investigation and were shown to be that of the title complex. Attempts to make this material in a rational fashion were not successful.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (7)