An iridium complex with an unsupported Ir—Zn bond: diiodido(η5-pentamethylcyclopentadienyl)bis(trimethylphosphane)iridiumzinc(Ir—Zn) benzene hemisolvate

A molecular compound with an unsupported Ir—Zn bond: [Cp*(PMe3)2Ir]-[ZnI2] (Cp*=cyclo-C5Me5), is reported as its benzene solvate.


Chemical context
An intuitive way to create metal-metal bonds is by linking a Lewis-basic metal center to a Lewis-acidic metal center. Lewis acid/base adducts of the type [Cp R (L)(L 0 )Ir]-[ZnX 2 ] (Cp R = either Cp, cyclopentadienyl, or Cp*, pentamethylcyclopentadienyl; L and L 0 = neutral ligand; X = halogen) have been known for a long time. Regarding the Lewis-basic fragment, it has been noted that electron-rich half-sandwich complexes can be considered 'metal bases par excellence' (Werner, 1983), and zinc dihalides are among the most well-known Lewis acids. The bimetallic complex [Cp(PPh 3 )(CO)Ir]-[ZnBr 2 ] was isolated and spectroscopically characterized 49 years ago (Oliver & Graham, 1970). However, crystallographic characterization of such complexes having iridium-zinc bonds is elusive. While a related complex [Cp*(CO) 2 Ir]-[ZnCl 2 ] was later prepared in a different group, it too was not structurally characterized, instead an adduct with mercury(II) chloride was crystallographically characterized (Einstein et al., 1992). A cobalt complex [Cp(PMe 3 ) 2 Co]-[ZnCl 2 PMe 3 ] is known as well; it too is lacking crystallographic characterization (Dey & Werner, 1977). In fact, while complexes are known where a zinc dihalide acts as a bridge between metals (iridium: Kimura et al., 2012) or where aggregation occurs to form multi-zinc clusters (rhodium and zinc: Molon et al., 2010), a search of the Cambridge Crystallographic Database (Groom et al., 2016) revealed no example of a structurally characterized complex [Cp R (L)(L 0 )M]-[ZnX 2 ] (M = either Co, Rh, or Ir) with a terminal (non-bridging) zinc dihalide. For iridium, it appears, in fact, that regardless of the ligand coordination sphere there is no single example of an unsupported iridum-zinc bond. The scarcity of examples for iridium contrasts with the situation of rhodium, for which a couple of examples of unsupported Rh-ZnX 2 structures exist with a PNP 'pincer' providing the coordination environment at rhodium (Gair et al., 2019). Additionally, several Rh-Zn structures exist with Zn-Cp* and Zn-C environments (Cadenbach et al., 2009). In this contribution, we provide crystallographic characterization for [Cp*(PMe 3 ) 2 Ir]-[ZnI 2 ] (benzene solvate). The bimetallic complex in this structure is the formal adduct of the Lewis base Cp*(PMe 3 ) 2 Ir I and the Lewis acid Zn II I 2 , providing the first structural characterization within the large class of metalmetal-bonded compounds [Cp R (L)(L 0 )M]-[ZnX 2 ] (M = Co, Rh, or Ir, X = halide, L, L 0 = neutral ligand). We did not synthesize the title compound from iridium(I). Rather, it was obtained through the reduction of iridium(III) with di-(2adamantyl)zinc, as described under 'Synthesis and crystallization'.

Supramolecular features
The packing of [Cp*(PMe 3 ) 2 Ir]-[ZnI 2 ]Á0.5C 6 H 6 is shown in Fig. 2. Molecules of [Cp*(PMe 3 ) 2 Ir]-[ZnI 2 ] and the C 6 H 6 solvent pack through contacting van der Waals surfaces, without any particular short contacts. There are no intermolecular hydrogen bonds in the structure. A possible intramolecular C-HÁ Á ÁI hydrogen bond is discussed above under Structural commentary. A view of the molecular structure of [Cp*(PMe 3 ) 2 Ir]-[ZnI 2 ] and its benzene solvate molecule. Anisotropic displacement ellipsoids in this plot, generated with ORTEP-3 for Windows (Farrugia, 2012), are shown at the 30% level. The benzene molecule lies on a crystallographic twofold axis -atoms bearing primed labels are generated by symmetry.

Database survey
searched. No example of an unsupported iridium-zinc bond was found, using the substructure Ir-Zn (any bond). Only one structure was found, namely a structure that contains a bridging zinc dihalide, as discussed under Chemical context (Kimura et al., 2012).

Synthesis and crystallization
The synthesis was performed using air-free conditions, solvents were dried over Na/benzophenone, [Cp*IrI 2 ] 2 was purchased from Sigma Aldrich, 2-Ad 2 Zn was synthesized according to literature (Armstrong et al., 2017).
[Cp*(PMe 3 ) 2 Ir]-[ZnI 2 ] was obtained via reduction of Cp*(PMe 3 )IrI 2 with 2-Ad 2 Zn. Cp*(PMe 3 )IrI 2 was generated in situ via reaction of 50mg of [Cp*IrI 2 ] 2 (0.04 mmol) with two equivalents of PMe 3 (added as a 1 M PMe 3 solution in THF, 100 mL, 0.1 mmol) over 1 h of stirring at room temperature. Next, 30 mg (0.08 mmol) of 2-Ad 2 Zn were added to the reaction mixture, and the reaction was allowed to proceed overnight with stirring at room temperature, resulting in a yellow solution and yellow precipitate. The solution layer was decanted into a round-bottom flask, and dried in vacuo to yield a yellow solid, which was extracted with C 6 H 6 forming a colorless solution, with some precipitate forming over time.
The colorless crystals of [Cp*(PMe 3 ) 2 Ir]-[ZnI 2 ] grew out of the benzene solution via slow evaporation at room temperature.

Funding information
Funding for this research was provided by: Natural Sciences and Engineering Research Council of Canada; University of Toronto.

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Acta Cryst. SHELXTL (Sheldrick, 2008). 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.