Trimethylphosphonium trans-tetrachloridobis(trimethylphosphane-κP)iridate(III)

The title compound, [HP(CH3)3][IrCl4{(H3C)3P}2], consists of a trimethylphosphonium cation and a tetrachloridobis(trimethylphosphane)iridate(III) anion. The anion has an octahedral arrangement of ligands, with the trimethylphosphane groups occupying trans positions. The IrIII atom sits on an inversion center with one P(CH3)3 ligand and two chloride ligands in the asymmetric unit. The trimethylphosphonium cation is disordered about a twofold rotation axis. The title compound is the first structurally characterized tetrachloridobis(phosphane)iridate complex.

The title compound, [HP(CH 3 ) 3 ][IrCl 4 {(H 3 C) 3 P} 2 ], consists of a trimethylphosphonium cation and a tetrachloridobis(trimethylphosphane)iridate(III) anion. The anion has an octahedral arrangement of ligands, with the trimethylphosphane groups occupying trans positions. The Ir III atom sits on an inversion center with one P(CH 3 ) 3 ligand and two chloride ligands in the asymmetric unit. The trimethylphosphonium cation is disordered about a twofold rotation axis. The title compound is the first structurally characterized tetrachloridobis(phosphane)iridate complex.

Experimental
Crystal data (C 3

Comment
We have been investigating the chemistry of iridium with the strongly electron-donating ligand, trimethylphosphane, for some time. In a recent publication, we discussed how [Ir(COD)(P(CH 3 ) 3 ) 3 ]Cl can be converted into mer,tris-(trimethylphosphane)trichloroiridium whose crystals tenaciously hold onto many different solvents (Merola et al., 2013). A direct reaction between IrCl 3 ·H 2 O and P(CH 3 ) 3 was attempted to make the same compound in a more direct way, but that reaction did not give a clean product and only a small number of crystals of the title product were obtained.
The title compound crystallizes in the C2/c space group and the iridium sits on an inversion center. Thus, the iridium (1/2 occupancy), two chlorine atoms and one P(CH 3 ) 3 group are unique with the remainder of the [((CH 3 ) 3 P) 2 IrCl 4 ] anion being generated by the inversion operator. The cation, trimethylphosphonium, lies slightly offset from a 2-fold rotation axis resulting in a disordered [HP(CH 3 ) 3 ] + ion where the two sites are generated by the rotation. In aqueous ethanol, reduction of some of the iridium(III)chloride to iridium(I) species will generate HCl and thus lead to the formation of the trimethylphosphonium cation.
This compound is the first crystallographically characterized compound with the [(Me 3 P) 2 IrCl 4 ]ion. The closest analog in the iridium family is a bis-phenyldimethylphosphane complex of iridium with two terminal chlorines and two chlorines bridging between iridium and palladium (Briant et al., 1981). The closest structure to the title iridium compound in the literature is the rhodium analog with triethylphosphane ligands (Cotton & Kang, 1993).

Experimental
Trimethylphosphane (0.19 g, 2.55 mmol) and IrCl 3 .H 2 O (0.100 g, 0.80 mmol) were refluxed in 95% aqueous ethanol for 3 hr. At the end of that time, the solvent was removed under reduced pressure yielding 0.20 g of a dark, brown, sticky powder. 1 H NMR spectroscopy indicated that a number of different species were present, possibly with various numbers of PMe 3 and chloride on iridium as well as a mixture of iridium(III) and iridium(I) species. Attempts to separate different complexes were unsuccessful. A small portion of the solid was dissolved in dichloromethane and the solvent was allowed to evaporate slowly. After evaporation, a very few crystals of the title compound suitable for X-ray diffraction were formed and used for this experiment.

Refinement
The trimethylphosphonium cation is disordered about a twofold axis and was modeled with each trimethylphosphonium fragment at 50% occupancy. P-C distances within the disordered fragment were restrained to be similar (esd 0.02 Å).
Methyl carbon atoms C4 and C5 of the two disordered moieties do overlap substantially and were constrained to have identical ADPs. H atoms were placed at calculated positions and refined using a model in which the hydrogen rides on the atom to which it is attached. For methyl hydrogen atoms U iso (H) = 1.5U eq (C with an idealized tetrahedral geometry (AFIX 13) with U iso (H) = 1.5U eq (P).

Figure 1
The thermal ellipsoid representation of the fully grown anion/cation pair with atoms labeled with the symmetry operation generating them. Each trimethylphosphonium fragment has 50% occupancy with one fragment indicated by different shading. With the exception of the P-H hydrogen atoms, H atoms are omitted for clarity. The displacement ellipsoids are shown at the 50% probability level.