Crystal structures of trans-acetyldicarbonyl(η5-cyclopentadienyl)(dimethylphenylphosphane)molybdenum(II) and trans-acetyldicarbonyl(η5-cyclopentadienyl)(ethyldiphenylphosphane)molybdenum(II)

The crystal structures of the title compounds are compared, showing molecular parameters that reflect the relative steric pressure of their respective phosphine ligands. Their supramolecular properties are distinct but in both cases are organized around short C—H⋯O contacts involving the acetyl ligands.

We have developed an interest in the solid-state structural properties of a series of piano-stool molybdenum acetyl complexes derived from migratory insertion with various phosphines, with the goal of understanding how modification ISSN 1600-5368 of the phosphine substituents affects ground-state structure as well as solid-state packing. Recently, we reported an unusual example where orientation of the acetyl group in the solid state can be changed by introduction of furyl substituents on the phosphine ligand (Whited et al., 2013). In this study, the structures obtained for dimethylphenylphosphine, [Mo(C 5 H 5 )(P(CH 3 ) 2 (C 6 H 5 ))(CO) 2 (COCH 3 )] (1), and ethyldiphenylphosphine, [Mo(C 5 H 5 )(P(C 2 H 5 )(C 6 H 5 ) 2 ))(CO) 2 -(COCH 3 )] (2), derivatives are compared.

Structural commentary
The molecular structures of (1) and (2) are illustrated in Figs. 1 and 2. In spite of the somewhat different steric environments provided by the phosphine ligands, the molecular structures are quite similar. Both complexes exhibit a trans disposition of carbonyl ligands common for compounds of this class. Complexes (1) and (2) both have structures where the oxygen atom of the acetyl group points toward the cyclopentadienyl (Cp) ring. This orientation is also consistent with the majority of crystal structures of related complexes, with the exception of the recently reported tri(2-furyl)phosphine derivative, in which the acetyl group points away from the Cp ring, enabling intermolecular OÁ Á ÁH-C interactions with the furyl group of a neighboring molecule (Whited et al., 2013).
Selected geometric parameters for (1) and (2) are presented in Tables 1 and 2. The Mo1-P1 bond lengths [2.4535 (9) Å for dimethylphenylphosphine derivative (1) and 2.4813 (6) Å for ethyldiphenylphosphine derivative (2)] track with the steric bulk of the ligands and are consistent with the previously reported methyldiphenylphosphine complex (Whited et al., 2012), which exhibits an Mo-P bond length [2.4620 (14) Å ] that is intermediate between those of (1) and (2). Along with a slightly longer Mo-P distance, the sterically bulkier derivative (2) exhibits a larger C3-  ] relative to (1) [131.79 (9) ], again with the methyldiphenylphosphine derivative intermediate [132.27 (2) ]. The steric effects of the phosphine ligands observed in the solid state are consistent with findings regarding decarbonylation rates for this class of complexes (Barnett & Pollmann, 1974), where the steric influence of bulkier phosphines enhances the rate of the decarbonylation reaction.

Supramolecular features
The extended structures of (1) and (2) are quite different, but the acetyl oxygen atom (O3) plays an important role in the packing of both structures. For dimethylphenylphosphine complex (1), there are C-HÁ Á ÁO hydrogen-bonding interactions between O3 of the acetyl carbonyl on one Mo complex and H11C from a phosphine methyl substituent (2.45 Å ) and H13 from a phenyl group (2.36 Å ) on the same phosphine on a neighboring molecule (Table 3). These short contacts organize the molecules into chains parallel to [001] (Fig. 3 Molecular structure of (1) with displacement ellipsoids drawn at the 50% probability level.

Figure 2
Molecular structure of (2) with displacement ellipsoids drawn at the 50% probability level.  present. The chains are arranged in layers parallel to (100). In contrast to the closely related methyldiphenylphosphine derivative (Whited et al., 2012), (1) does not exhibit anyinteractions between the Cp ring and a phosphine phenyl substituent. In contrast, the closest phenyl group is oriented perpendicular to the Cp ring with a distance of 3.00 Å between H17 of the phenyl group and the Cp centroid. The supramolecular organization of ethyldiphenylphosphine derivative (2) is quite different, though it is still partly governed by hydrogen-bonding interactions involving O3 of the acetyl group. In this case, short contacts (2.66 Å ) between O3 of the acetyl group and H22 of a phosphine phenyl substituent (Table 4) link the molecules into chains parallel to [010]. An additional set of short contacts between O2 of a carbonyl ligand and H8 from a Cp ring (2.63 Å ) and H13 from a phenyl ring (2.71 Å ) on an adjacent molecule organize the molecules into centrosymmetrical dimers, joining the unit cells along [010] (Fig. 4). Finally, another set of centrosymmetrical dimers is formed through short contacts between C8/H8 units on Cp rings of adjacent molecules (Fig. 5).

Database survey
The current version of the Cambridge Structural Database (Version 5.35, updated November 2013;Allen, 2002) has nine entries corresponding to molybdenum acyl complexes of the general form [Mo(C 5 H 5 )(CO) 2 (PR 3 )(COR)], as well as five tungsten complexes with the same ligand types. No chromium complexes with the same ligand set are in the database. The trans-dicarbonyl structure, as observed for (1) and (2) Table 3 Hydrogen-bond geometry (Å , ) for (1). Symmetry code: (i) Àx þ 3 2 ; y À 1 2 ; z À 1 2 .
ligands are covalently linked, forcing them to be cis (Adams et al., 1991;Mercier et al., 1993;Yan et al., 2009). The preference for a trans geometry is likely at least partly steric in nature, since the only example with a cis-dicarbonyl geometry without linked phosphine and acyl ligands is for a molybdenum formyl with a small trimethylphosphine ligand and a bulky pentamethylcyclopentadienyl ligand (Asdar et al., 1989).
CpMo(CO) 2 (PEtPh 2 )(COCH 3 ) (2). In an inert-atmosphere glove box, CpMo(CO) 3 (CH 3 ) (105 mg, 0.404 mmol) was dissolved in 2 ml acetonitrile. Ethyldiphenylphosphine (129 mg, 0.602 mmol) was added and the resulting solution was stirred for one week. Solvent was removed in vacuo, leaving a yellow solid that was triturated with pentane (5 ml) and isolated by filtration to afford the desired product in pure form as a yellow powder (106 mg, 55%). Crystalline material was obtained as yellow blocks by slow evaporation of diethyl ether from a concentrated solution at ambient temperature. 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. H-atoms were treated in calculated positions and refined in the riding-model approximation with distances of C-H = 0.95, 1.00 and 0.98 Å for the phenyl, cyclopentadienyl and alkyl groups, respectively, and with U iso (H) = kÂU eq (C), k = 1.2 for phenyl and cyclopentadienyl groups and 1.5 for alkyl groups. Methyl group H atoms were allowed to rotate in order to find the best rotameric conformation.
A small number of low-angle reflections [three for (1); six for (2)] were rejected from these high-quality data sets due to the arrangement of the instrument with a conservatively sized beam stop and a fixed-position detector. The large number of reflections in the data sets (and the Fourier-transform relationship of intensities to atoms) ensures that no particular bias was thereby introduced.