Crystal structure of chloridobis[(1,2,5,6-η)-cycloocta-1,5-diene]iridium(I)

A direct synthesis of monomeric IrCl(1,5-cod)2 (1,5-cod = cycloocta-1,5-diene) from IrCl3·3H2O and its crystal structure are presented. The molecule’s shape is midway between square pyramidal and trigonal bipyramidal.


Figure 1
Reaction scheme showing the formation of the mixture of [Ir(cod)(-Cl)] 2 and IrCl(cod) 2 . As the ethanol is removed under vacuum the solution becomes rich in cod, which drives the formation of IrCl(cod) 2 . Loss of cod regenerates the dimer. had been synthesized from the reaction of ammonium hexachloridoiridiate(III) hydrate, (NH 4 ) 3 IrCl 6 ÁH 2 O, with cyclooctene in a mixture of 2-propanol and water (Onderdelinden & van der Ent, 1972). In all three cases, Ir IV or Ir III is reduced to Ir I by oxidation of the alcoholic solvent. Upon suspension in pure cod, [Ir(C 8 H 14 ) 2 (-Cl)] 2 reacted to form mononuclear IrCl(cod) 2 , which was then characterized by infra-red spectroscopy and elemental analysis (Onderdelinden & van der Ent, 1972). Analogous to thermally unstable IrCl(C 2 H 4 ) 4 , which releases ethylene to form the (slightly) more stable dimer [Ir(C 2 H 4 ) 2 (-Cl)] 2 (Onderdelinden & van der Ent, 1972), IrCl(cod) 2 readily generates stable [Ir(cod)(-Cl)] 2 with the loss of one equivalent of cod per iridium. We have found that if Herde's preparation using IrCl 3 Á3H 2 O is carried out with a large excess of cod (10 Â), the product isolated after removal of the alcoholic solvent is IrCl(cod) 2 (Fig. 1). This was apparent as the red-orange reaction mixture, which contained a mixture of red [Ir(cod)(-Cl)] 2 and yellow IrCl(cod) 2 , became pale yellow. Recrystallization from refluxing methanol/cod (7:1, v:v) followed by cooling produced yellow needles of IrCl(cod) 2 suitable for diffraction studies.
Herein we report the isolation and results of the single structure determination of mononuclear IrCl(cod) 2 and compare it to related IrX(diene) 2 (X = Cl, SnMe 3 , SnCl 3 ) complexes.

Structural commentary
Our single-crystal X-ray diffraction study confirmed the molecule to be mononuclear IrCl(cod) 2 , in which the two cod ligands are bound in an 2 : 2 fashion (Fig. 2). The material crystallizes in the orthorhombic space group Pbca, with one molecule per asymmetric unit in a general position. The fivecoordinate complex adopts a geometry that is midway between square pyramidal (SP) and trigonal bipyramidal (TBP), with a 5 parameter of 0.52 (Addison et al., 1984), calculated using the mid-points of the C C double bonds and the axial chlorido ligand. The elongation of the cod double bonds (Table 1) compared to those of non-coordinating cod, 1.333 (4) and 1.334 (4) Å (Byrn et al., 1990), or to that of free ethylene, 1.333 Å (Lide, 2002(Lide, -2003, is consistent with back donation to the * orbitals from a low-valent iridium atom, formally Ir I . The elongations are asymmetric, with one double bond from each cod ligand being larger than the other by 0.048 (6) and 0.029 (6) Å , respectively, for cod ligands C1-C8 and C9-C16. Likewise the distances between Ir and the midpoints of the C C bonds also show this asymmetry with two shorter distances, Ir-(C1/C2) = 2.047 (4) and Ir-(C9/C10) = 2.069 (4) Å , and two longer distances, Ir-(C5/C6) = 2.138 (4) and Ir-(C13/C14) = 2.141 (4) Å (Table 2). This is likely due to its intermediacy between the geometric extremes of SP and TBP. Ideal SP geometry ( 5 = 0) would have very similar Irmid-point(C C) distances as they would involve the same metal and ligand orbitals, while ideal TBP geometry ( 5 = 1) would involve different orbitals, dependent upon on whether the ligand's C C bond lay in an axial or an equatorial position. We see the former (SP) in Ir(SnCl 3 )(nbd) 2 (nbd = norbornadiene; Malosh et al., 2013), for which 5 = 0.06 and the Ir-mid-point(C C) distances are similar, ranging from 2.067 (4) to 2.089 (4) Å . An example towards TBP is found in

Figure 2
The molecular structure of IrCl(cod) 2 , with displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
Although there are no significant intermolecular interactions, the packing has adopted a supramolecular arrangement. Individual molecules are aligned in columns parallel to [010], which are then arranged in an overall pseudo-hexagonal packing (Fig. 4).  , 2002). A report on the structure of IrCl(C 2 H 4 ) 4 exists, but no positional parameters were given (van der Ent & van Soest, 1970), which is unfortunate because a comparison of this species with IrCl(cod) 2 would ostensibly show how the bite-angle restrictions imposed by the cod rings affect the overall geometry. The geometries of the two tincontaining compounds with cod are closely related to that of the title complex. Both Ir(SnCl 3 )(cod) 2 and Ir(SnMe 3 )(cod) 2 exhibit the same long-short variation of the Ir-midpoint(C C) bond lengths within each cod ligand and have similar 5 parameters of 0.53 and 0.55, respectively (Table 2). Malosh and coworkers concluded that the bulk of the cod ligands relative to that of the nbd ligands was responsible for the geometric distortion from SP geometry, specifically due to CH 2 Á Á ÁMe and CH 2 Á Á ÁCl repulsions (Malosh et al., 2013). And indeed the two nbd complexes have near-perfect SP 5 values of 0.10 and 0.06. In complex [IrCl(cod)(CC*)] + , the non-cod diene is part of a 1,1-di(2-propenyl)-3-butenyl)benzene unit that is 6 -coordinating to an [Fe(C 5 H 5 )] + cationic fragment. The pentacoordinated saturated (18 electron) iridium atom approaches a TBP geometry more than the other complexes mentioned ( 5 = 0.76), with the two apical positions being occupied by one C C bond of the cod ligand and the chlorido ligand. The angles in the equatorial plane range between 109.73 (17) and 126.61 (16) . The restriction of the cod ligand with its bite angle of 84.9 (2) prevents the structure from ever achieving perfect TBP geometry, and this holds more so for structures with nbd ligands whose bite angles are even more acute. The Ir-mid-point ( (5) 1.415 (7) 1.389 (7) 1.372 (12) C C [C] 1.418 (5) 1.361 (44) 1.411 (5) 1.400 (7) 1.394 (7) 1.393 (8)

Figure 3
Lettering scheme used for bonds in

Synthesis and crystallization
All operations and routine manipulations were performed under a nitrogen atmosphere, either on a high-vacuum line using modified Schlenk techniques or in a Vacuum Atmospheres Company Dri-Lab. A preparation of IrCl(cod) 2 via a cyclooctene-ligated dimer has been reported previously (Onderdelinden & van der Ent, 1972). A two-necked round-bottom flask was charged with IrCl 3 Á3H 2 O (6.0 g, 0.017 mol) and cod (20 g, 0.18 mol) in 80 ml of ethanol under nitrogen. The reaction mixture was refluxed for 24 h, followed by removal of the solvent under vacuum. As the ethanol evaporated, the red-orange solution became more yellow as the cod concentration increased, leading to the isolation of a yellow solid (5.32 g, 70.5%). The product was recrystallized by refluxing in a mixture 35 ml of methanol and 5 ml of cod, followed by cooling to obtain shiny yellow needles of IrCl(cod) 2 (5.06 g, 67.0%).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were treated in the ridingmodel approximation, with C(methine)-H = 1.00 Å , C(methylene)-H = 0.99 Å , and with U iso (H) = 1.2U eq (C). The maximum and minimum electron densities are found 1.09 and 0.55 Å , respectively, from the iridium atom.

Chloridobis[(1,2,5,6-η)-cycloocta-1,5-diene]iridium(I)
Crystal data 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.