Crystal structure of the tetramethyl(phenethyl)cyclopentadienylmolybdenumtricarbonyl dimer

The structure of the dimer bis{tricarbonyl[η5-tetramethyl(phenethyl)cyclopentadienyl]molybdenum}(Mo—Mo), [Mo2(C17H21)2(CO)6], at 102 K has triclinic (P ) symmetry. The Mo—Mo bond length is 3.2773 (3) Å, a value slightly above the mean value for all [CpMo(CO)3]2 compounds listed in the CSD and slightly below the mean for [Cp*Mo(CO)3]2 complexes.


Chemical context
Following the discovery of ferrocene in 1951 (Werner, 2012), the cyclopentadienyl (Cp) ligand became ubiquitous in studies of sandwich and half-sandwich compounds. As a result of the high reactivity of the C-H bond in the cyclopentadienyl ligand in some circumstances, pentamethylcyclopentadienyl (Cp*) soon became a common replacement for Cp. In recent years, researchers have begun investigating Cp-type ligands with mixed substitution of the ring. The cyclopentadienyl ligand ranges from unsubstituted, Cp, monomethyl substituted, Cp', other non-fully substituted, Cp R , and fully methylated, Cp*. The most systematically studied ring substitution is the tetramethyl(R)cyclopentadienyl (Cp* R ) ligand where R represents any group other than methyl. ISSN 2056-9890 Our group (DuChane et al., 2018;Brown et al.,2016) and others (Piou et al., 2017) have examined various Cp* R ligands in rhodium and iridium chemistry. Perhaps one of the more intriguing of the metal systems studied with Cp, Cp R , Cp*, and Cp* R ligands is that of molybdenum hexacarbonyl. Reaction between Cp ligands and Mo(CO) 6 leads to the formation of the dinuclear [CpMo(CO) 3 ] types of complexes. Reaction between Cp* R ligands and Mo(CO) 6 has been studied systematically in various laboratories, including reports on the structures of a variety of [Cp* R Mo(CO) 3 ] 2 compounds. In this report, we add to the structural descriptions of the range of [Cp* R Mo(CO) 3 ] 2 compounds with the addition of the complex with R = phenethyl (Fig. 1).

Structural commentary
The 5 -tetramethy(phenethyl)lcylopentadienylmolybdenumtricarbonyl dimer (Fig. 2) crystallizes in space group P1 with the 5 -tetramethyl(phenethyl)cyclopentadienylmolybdenumtricarbonyl moiety being the asymmetric unit and with the entire dimer being generated by an inversion center. The Cp* R ligands are in a transoid arrangement about the Mo-Mo bond with that bond being 3.2773 (3) Å in length. The disposition of the phenethyl groups on the Cp* R rings can best be described by measuring the torsion angle made by the two Mo atoms, the C atom on the ring to which the phenethyl group is attached and the attaching C atom of the phenethyl group (Mo1 i -Mo1-C5-C10). For the title compound, this angle is 119.59 (10) .
Other structural features of note are the Mo-C-O angles. Two of the CO ligands of the dimer point away from the Mo-Mo bond and are close to linearity with an Mo-C2-O2 (Mo 0 -C2 0 -O2 0 ) angle of 176.27 (14) . The four CO ligands that point over the Mo-Mo bond have angles of 167.87 (14) and 171.42 (14) . Further commentary on these values can be found in the Database survey section.

Database survey
There are a number of molybdenum tricarbonyl dimers in the CSD database (Version 5.39, last update May 2018: Groom et al., 2016) with cyclopentadienyl and substituted cyclopentadienyl ligands. The database was searched using the program Conquest (Bruno et al., 2002) and the data was analyzed with the program Mercury (Macrae et al., 2008). The structure of the completely methylated [Cp*Mo(CO) 3 ] 2 complex was first determined by Clegg and co-workers (GAVKUJ; Clegg et al., 1988). Examining all types of cyclopentadienyl ligands (Cp, Cp R , Cp* and Cp* R , along with other unique substitution patterns), the mean Mo-Mo distance is 3.252 Å , ranging from a low of 3.211 to a high of 3.307 Å . The low end of the scale is comprised of unsubstituted or singly substituted Cp ligands and the high end of Cp* and Cp* R ligands. For this latter group, the Mo-Mo distances range from a minimum of 3.256 Å to a maximum of 3.307 Å with a mean distance of 3.286 Å . Within this range, the title compound is at the lower end, slightly below the average. The most extensive series of [Cp* R Mo(CO) 3 ] 2 compounds were made and structurally characterized in the laboratories of Lin and co-workers. These include R = ethyl, propyl, butyl and cyclohexyl (LEXROX, GEVBAM, LALNAP, LEXFUR; Ma et al., 2013Ma et al., , 2010 as well as aryl and substituted-aryl substituents R = p-bromophenyl, p-tolyl and p-methoxyphenyl (DUFKEW, HENKUZ, HENDIO; Dong et al., 2015;Ma et al., 2013) Complexes with the benzyl (TULLAO; Ma et al., 2009) and with the 2pyridylmethyl side chain (OGIHAP; Ma et al. 2015) were also structurally characterized.
Nearly all of the relevant structures in the database have the transoid arrangement of the Cp rings across the Mo-Mo bond. An exception was found in the work of Gould, Barker and co-workers in which they found the cisoid isomer of [CpMo(CO) 3 ] 2 (CYPMOC01) as a minor product in their attempt to prepare a different Mo compound (Gould et al., 1988).
The Mo-C-O angles for all of the compounds in the database show the same pattern as for the title compound with the carbonyl ligands lying over the Mo-Mo bond, bent back from linearity by between 9 and 15 . The asymmetric unit of the title compound, showing the labeling scheme. The displacement ellipsoids are shown at the 70% probability level.

Figure 2
The complete molecular unit of the title compound. The displacement ellipsoids are shown at the 70% probability level. Hydrogen atoms are omitted for clarity. Symmetry code: (i) 2 À x, 1 À y, 1 À z.

Supramolecular features
The nature of the weak hydrogen bond, especially C-HÁ Á ÁX hydrogen bonds, has garnered considerable interest over the last two decades given the cumulative influence that many such weak bonds can have on a crystal structure (Desiraju, 2002). The distinction between C-HÁ Á ÁO hydrogen bonds and simple van der Waals interactions, a matter (not an insignificant one) of analyzing bond distances and angles, will not be discussed here. However, there are quite a few short C-HÁ Á ÁO interactions in the crystal of the title compound ( Table 1). The shortest ones are intramolecular interactions between H12C and O3 i (H12C i and O3) and between H10A and O2 i (H10A i and O2) viz. 2.47 and 2.54 Å , respectively). These are on the short end of H-O distances found in the literature, but deviate considerably from linearity. These interactions involve the oxygen atoms on the carbonyl ligands that point over the Mo-Mo bond. More importantly in terms of supramolecular features, the short intermolecular C-HÁ Á ÁO distances are found between O1, the oxygen atom on the carbonyl ligand that points up and away from the Mo-Mo bond, and H12B on a neighboring molecule at a distance of 2.62 Å (C-HÁ Á ÁO angle 141 ). The interaction between the aromatic ring and a bound CO may be more important as it is closer to linear: C20-H20Á Á ÁO3 iii (2.65 Å and 167 ). These intermolecular C-HÁ Á ÁO interactions and others knit the dimers into bilayers that extend in the ab plane. The C-HÁ Á ÁO interactions are confined to the middle of the bilayer; only van der Waals interaction exist between the bilayers. Based on the literature, both the intermolecular and intramolecular C-HÁ Á ÁO interactions appear to be on the stronger end of weak interactions. (Steiner & Desiraju, 1998;Taylor, 2016). Their classification as 'hydrogen bonds' awaits more complete analysis of all compounds of this type.

Synthesis and crystallization
Under an atmosphere of pre-purified nitrogen, 0.5892 g (2.60 mmol) of [2-(2,3,4,5-tetramethylcyclopenta-2,3-dien-lyl)ethyl]benzene and 0.6852 g (2.60 mmol) of molybdenum hexacarbonyl were dissolved in 10 ml of xylenes and refluxed for 18 h. At the end of 18 h, xylenes were removed under reduced pressure and purified on a column of alumina using a 1:1 dichloromethane:hexanes solvent system. Following removal of solvents, 0.6096 g of the [Cp* R Mo(CO) 3 ] 2 , R = phenethyl, (55.8% yield) was isolated. The bulk material was shown to be the desired compound based on: NMR, 1 H, A portion of the product was dissolved in CH 2 Cl 2 and the solvent was allowed to evaporate slowly, yielding crystals suitable for X-ray crystallography.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were included in calculated positions and treated as riding: C-H = 0.95-0.99 Å with U iso (H) = 1.5U eq (C-methyl) and 1.5U eq (C) for other H atoms.

Acknowledgements
DM and CS synthesized the title compound as part of their capstone experience for the BSc degree in Chemistry from Virginia Tech, May 2018.

Bis{tricarbonyl[η 5 -tetramethyl(phenethyl)cyclopentadienyl]molybdenum}(Mo-Mo)
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.