Crystal structures of (η4-cycloocta-1,5-diene)bis(1,3-dimethylimidazol-2-ylidene)iridium(I) iodide and (η4-cycloocta-1,5-diene)bis(1,3-diethylimidazol-2-ylidene)iridium(I) iodide

(η4-Cycloocta-1,5-diene)bis(1,3-dimethylimidazol-2-ylidene)iridium(I) iodide and (η4-cycloocta-1,5-diene)bis(1,3-diethylimidazol-2-ylidene)iridium(I) iodide were prepared using a modified literature method and crystallized from water in the monoclinic space group C2/m and the orthorhombic space group Pccn, respectively.

This discrepancy in Ir-C COD bond lengths and Ir-COD centroid distances between the two complexes is likely due to the conformation of the COD ligand, which is a boat in 1 and a twist-boat in 2. Displacement ellipsoid plot (50% probability) of ( 4 -cycloocta-1,5diene)bis(1,3-dimethylimidazol-2-ylidene)iridium(I) iodide (1), showing part 2 of the disorder for the CH 2 carbon atoms of the COD ring. Symmetry code: (i) x, 1 À y, z.

Synthesis and crystallization
The title compounds were synthesized using a modified literature procedure (Kö cher & Herrmann, 1997).
[Ir(COD)Cl] 2 (500 mg, 0.744 mmol) and a magnetic stir bar were added to a flame-dried, nitrogen-purged 100 mL Schlenk flask. Ethanol (20 mL) was added via syringe and the red solution was stirred. After 5 minutes, a solution of NaOEt in ethanol (1 M, 3.5 mL, 3.50 mmol) was added to the reaction flask dropwise. The solution was stirred for 1 h while the color slowly changed from red to bright yellow, indicating the formation of [Ir(COD)(OEt)] 2 . The NHC precursor 1,3-dimethylimidazolium iodide (840 mg, 3.75 mmol) or 1,3-diethylimidazolium iodide (945 mg, 3.75 mmol) was dissolved in ethanol (10 mL) and added to the stirring mixture via syringe. After 48 h, the bright-orange mixture was filtered through celite. The solvent was removed by rotary evaporation, and the residue was dissolved in minimal dichloromethane. The crude product was purified via column chromatography with silica gel, first using a 1:1 mixture of cyclohexane to ethyl acetate as the mobile phase to collect the bright-yellow iridium mono-NHC complex, followed by 7% methanol in dichloromethane to collect the desired orange iridium bis-NHC product. The solvent was removed by rotary evaporation and the bright-orange solid was dried overnight under vacuum (449 mg, 49% for 1; 415 mg, 42% for 2). The products were characterized by 1 H and 13 C NMR spectroscopy in agreement with previously reported data.
Single crystals of 1 for X-ray crystallography were collected from a subsequent oxidative addition reaction. The title compound, l-proline, and 10 mL of water were added to a 6 dram vial and stirred overnight at 323 K. Upon slowly cooling the reaction mixture to room temperature, brightorange crystals of the title compound grew and were collected. Single crystals of 2 were grown by dissolving the complex in water, heating it to 323 K, and letting the solution slowly cool to room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1.
Compound 1 was solved with SHELXS and refined with SHELXL within OLEX2. The refinement proceeded quite well although the displacement ellipsoids for the CH 2 carbon atoms of the COD ring were overly elongated, suggesting that there was possible disorder. In OLEX2, the disorder tools were utilized to split the carbon atoms while adding SHELXL SIMU restraint. The disorder model appeared to refine well with reasonable displacement ellipsoids. Fig. 1 shows part 1 of the disorder and Fig. 2 shows part 2. Both parts show nearly equal occupancies refining to 0.515 (19):0.485 (19). The two parts seem best described as the result of static disorder wherein the saturated portion of the COD ring is slightly twisted. The unsaturated carbon atoms are also likely a part of the disorder, but the positional change is so slight as to not warrant (and to resist) modeling. However, a consequence of this slight disorder is that generating the entire molecule does generate two different hydrogen-atom positions, also refining to 0.515 (19):0.485 (19) relative occupancies.
Data reduction, solution and refinement for 2 presented some interesting issues that are discussed here. The data were collected on a XtaLAB Synergy, Dualflex, HyPix diffractometer. Data reduction was performed with CrysA-lisPro171.40_64.67a (Rigaku OD, 2018). The crystal was of good quality and peak searching found 9425 peaks that were merged to 5446 profiles. Unit-cell calculations fit 98.2% of the peaks to the cell 9.1397 (5), 10.6193 (7), 12.3249 (6), 89.980 (5). 89.988 (4), 89.965 (6). Further refinement and space group determination led to the finalization of the data in orthorhombic P. SHELXT within OLEX2 was used for structure solution and several non-centrosymmetric space groups were identified with nearly equal figure of merit. Attempts were made to refine the structure in all five of the proffered space groups and the only one that provided a reasonable solution was P2 1 2 1 2. However, while the structure refinement parameters were 'reasonable', several displacement ellipsoids in the finalized model were elongated along strange directions. The data were reexamined and a close view of the Ewald sphere showed weak, but clearly present peaks between the axes. The $9 Å axis was doubled and now all peaks were aligned fully with the new axes of 18.2790 (10), 10.6196 (7), 12.3245 (6), 89.979 (5), 89.985 (4), 89.965 (5). With those particular settings in CrysAlis, the only reasonable unit cell found was triclinic.
Moving into OLEX2 again, a solution was found in P1 that refined into a solution with excellent figures of merit and wellshaped displacement ellipsoids with Z = 4. However, it was noted that the heavy atoms, iridium and iodine all had coordinates that suggested they sat on special positions, e.g. x = 0.7500. ADDSYM in PLATON (Spek, 2020) was used to search for higher symmetries and the result suggested that Pccn was an appropriate high-symmetry space group. The newly created data and instruction files from PLATON were used in OLEX2 and the structure in Pccn solved and refined cleanly into the final structure. With this result in hand, the raw data were re-reduced, the originally found x axis was again doubled and space-group analysis was re-performed with slightly larger angle tolerances (0.03 vs 0.015). Pccn was then clearly identified as the top match for the space group. The data and instruction files were once more used in OLEX2 and SHELXT used as the solution program, which determined that Pccn was the best space group. Refinement led to the final structure solution reported in this paper.  (1); ShelXT (Sheldrick, 2015a) for (2). For both structures, program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.54 e Å −3 Δρ min = −0.83 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.

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