Crystal structures of [μ2-(Ra,Sa,3aR,7aR)-1,3-bis(2,7-dicyclohexylnaphthalen-1-yl)octahydro-1H-benzo[d]imidazolidin-2-ylidene]chlorido(η4-1,5-cyclooctadiene)iridium dichloromethane monosolvate and [μ2-(Sa,Sa,3aR,7aR)-1,3-bis(2,7-dicyclohexylnaphthalen-1-yl)octahydro-1H-benzo[d]imidazolidin-2-ylidene]chlorido(η4-1,5-cyclooctadiene)iridium

The title compounds, [Ir(C51H64N2)(C8H12)Cl]·CH2Cl2 and [Ir(C51H64N2)(C8H12)Cl], represent the first two examples of hexahydrobenzoimidazole-based N-heterocyclic carbene (NHC) iridium complexes. The diastereomeric complexes differing only in their axial chirality, which could be separated via column chromatography, show noticeable differences in their 1H NMR spectra.


Chemical context
The use of N-heterocyclic carbenes (NHCs) as ancillary ligands for various metal complexes has been implemented extensively, resulting in the synthesis of many successful catalytic species. When enantiopure catalysts are used, asymmetric transformations can be performed that result in enantio-enriched synthetic products. In terms of chiral NHC ligand design, substituents on the NHC that are too distal to the coordination sphere of the metal centre generally result in low enantio-selectivities for such catalysts. Some NHCs featuring a fused, chiral cyclohexyl backbone (modelled on the salen-type or Trost-type ligands) have been reported and studied, but the catalytic capabilities of such complexes resulted in disappointing enantio-selection ( ISSN 2056-9890 bulky naphthyl wingtips for the NHCs, axial chirality is generated due to hindrance of rotation around the C-N bonds. With implementation of an enantiopure cyclohexyl backbone, three isomers are present in the mixture and are isolable via column chromatography as previously reported for similar Ir-NHC complexes (Gao et al., 2020). Herein we report a new NHC ligand that uses bulky 2,7-dicyclohexyl naphthyl wingtips on the NHC in the hope that it will result in iridium precatalysts that can successfully perform catalytic transformations with high enantio-selectivity ( Fig. 1).

Structural commentary
The molecular structures of the title compounds, (I) and (II), are depicted in Figs. 2 and 3, respectively. The two compounds crystallized under the same conditions, where the complexes were dissolved in a minimum amount of dichloromethane (DCM) under an inert atmosphere and layered with pentane, which then slowly diffused into the DCM solution overnight resulting in the formation of yellow crystals. Compound (I) crystallizes in the monoclinic system (P2 1 ) with two independent complexes and two half-occupied DCM molecules in the The molecular structures of the title complex (I) with atom labelling. Labelling of selected aliphatic carbons has been omitted for clarity. Ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity.

Figure 3
The molecular structures of the title complex (II) with atom labelling. Labelling of selected aliphatic carbons has been omitted for clarity. Ellipsoids are drawn at the 50% probability level. H atoms have been omitted for clarity.

Figure 1
Reaction scheme for the synthesis of the title compounds. asymmetric unit; the solvent DCM molecules were masked in the refinement due to high disordering. Compound (II) crystallizes in the orthorhombic system (P2 1 2 1 2 1 ) with one complex in the asymmetric unit.
Saturated achiral or racemic NHC-Ir-COD complexes without a fused second ring appear to have a greater degree of flexibility and show clearly smaller distortions within the fivemembered N-heterocycle, with backbone torsion angles ranging from 1.6 for (anti-2-SICyNap)Ir(COD)Cl to 19.5 for (SIPr)Ir(COD)Cl. For enantiopure complexes, [(R a ,R a ,S,S)-DiPh-2-SICyNap]Ir(COD)Cl has a torsion angle of 9.4 and [(R a ,R a ,S,S)-DiPh-2,7-SICyNap]Ir(COD)Cl has 11.9 , with a small range in angles of different molecules in the same crystal (Á0.3 ), revealing that increased bulk on the backbone appears to increase the rigidity of the NHC ring.

Supramolecular features
In the crystal of (I), the complex molecules are stacked in a column along the b axis via weak C-HÁ Á ÁCl interactions (Table 1 and Fig. 4). In comparison, (II) has no obvious interactions between molecules.

Database survey
The only other report of a crystallographically characterized fused cyclohexyl NHC complex is (

Synthesis and crystallization
2,7-FuCySICyNapÁHBF 4 (300 mg, 0.38 mmol) was added to a solution of [Ir(COD)Cl] 2 (138 mg, 0.18 mmol) in THF (8 mL) in a glovebox and was stirred at room temperature. KO t Bu (42 mg, 0.38 mmol) was then added and the yellow solution was stirred for 3 h. The yellow-brown solution was evaporated to dryness and the three diastereoisomers were separated via column chromatography (1.5 kg SiO 2 , diameter 5cm, 1:20 Table 1 Hydrogen-bond geometry (Å , ) for (I).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. For (I) the solvent molecule was masked using the smtbx masking tool in OLEX2 (Dolomanov et al., 2009) due to diffuse electron density that could not be fitted using an atomistic model. The mask gave two void positions, of 428 and 414 Å 3 , and with 76.2 and 74.7 electrons, respectively. This equates to two half-occupied DCM molecules. Hydrogen atoms were positioned geometrically (C-H = 0.95-1.00 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C).