Crystal structures of two unusual, high oxidation state, 16-electron iridabenzenes

Treatment of an iridabenzene with either bromine or iodine generates high-oxidation-state IrIII iridabenzenes that contain an open coordination site.


Chemical context
Metallabenzenes are a rare class of organometallic compounds in which a CH unit is isolobally substituted with a transition metal fragment (Bleeke, 2001;Wright, 2006). Postulated in a seminal paper in 1979 (Thorn & Hoffmann, 1979), metallabenzenes have been shown to be feasible through numerous synthetic methodologies and now claim residence in the third and second row transition metals. Our research has focused on the synthesis and properties of metallabenzenes and their valence isomers using 3-vinyl-1cyclopropenes as the source for the five-carbon backbone (Landorf & Haley, 2006). In certain instances, the metallabenzenes can undergo reductive elimination to afford 5 -Cp complexes (Wu et al., 2007). Although such a pathway has potential synthetic utility, for our studies this represents a deleterious side reaction that hinders an effective, detailed examination of metallabenzenes. Computational work by van der Boom and coworkers suggests that metallabenzenes containing metal atoms with higher oxidation states may be resistant toward the reductive elimination pathway (Iron et al., 2003). This prediction interested us as prior studies have shown that Ir I iridabenzenes can be readily oxidized with Ag I salts or halogens to generate high oxidation state Ir III iridabenzenes; hence, we sought to synthesize neutral iridabenzenes of higher oxidation state as initially demonstrated by Bleeke and coworkers (Bleeke et al., 1997). Herein we report the synthesis and structures of iridabenzenes (I) and (II), two ISSN 2056-9890 rare examples of high oxidation yet coordinatively unsaturated 16-electron Ir III iridabenzenes.

Figure 2
The molecular structure of (II), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Scheme of iridabenzene (III) employed as an educt would be around 1.5-1.6 Å . In such a case, the distance between this H atom and the H29A atom from the phenyl ring should be 1.6-1.7 Å . This distance is too short as a typical HÁ Á ÁH contact is 2.4 Å . It follows then that if one H atom does not fit, H 2 will not either. The displacement parameters of most C atoms in the phenyl rings are elongated perpendicular to the average plane of the Ph rings showing their flexibility or statistical disorder.

Figure 4
A fragment of the crystal structure of (I) in a view along [100], showing association of the molecules in the crystal packing by weak C-HÁ Á ÁBr interactions (dashed lines). Atom labels are omitted for clarity. The crystal of (II) is isostructural with the crystal of (I).
respectively, for Br and I (Tables 1 and 2). A fragment of the crystal structure of (I) is given in Fig. 4, illustrating one such weak interaction.

Synthesis and crystallization
Reaction of iridabenzene (Gilbertson et al., 1999), (III) (Fig. 3) with one equivalent of bromine at 195 K produced a darkbrown solution that was warmed to 273 K over a period of 30 min. Recrystallization from acetone at 243 K afforded bluish brown crystals of (I). Similarly, reaction of (III) with iodine at 195 K also produced a dark-brown solution containing (II) which was crystallized in similar conditions to give bluish brown crystals. While (I) and (II) were stable in the solid state for weeks at 243 K without noticeable decomposition, solutions of either of the iridabenzenes degraded rapidly and thus made their complete characterization extremely challenging.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.