Crystal structures of phosphine-supported (η5-cyclopentadienyl)molybdenum(II) propionyl complexes

Solid-state structures are presented for three propionyl complexes of MoII featuring piano-stool geometries and supported by triarylphosphine ligands, showing the effects of para substitution on supramolecular structure and allowing comparison to the large class of previously reported acetyl complexes.

We have previously reported synthetic details and solidstate structures for a number of Mo II acetyl complexes of the type described above Whited et al., 2012, examining the effect of changing phosphine substituents on local and supramolecular features. Consistent with reports on decarbonylation reactivity, we have found that the primary impact on molecular structure is observed in the Mo-P bond lengths, with some changes in P-Mo-C bond ISSN 2056-9890 angles as a result of sterics. Use of tri(2-furyl)phosphine, which features heteroatoms as potential hydrogen-bond acceptors, leads to an unusual structure with the acetyl oriented down, away from the cyclopentadienyl ring rather than up toward it as observed in other cases (Whited et al., 2013), and a similar effect was observed by incorporation of a Lewis-acidic manganese unit to interact with the acetyl ligand (Adatia et al., 1986). Recent use of other potentially hydrogen-bonding phosphine ligands did not lead to the same solid-state effect (Anstey et al., 2020).
We were interested in extending earlier studies to higherorder alkyl groups at molybdenum, and in this report we describe the synthesis and solid-state structures of related Mo II propionyl complexes derived from an ethyl precursor and supported by triarylphosphine ligands differing in their para substitution . Although substitution of phosphine aryl groups with electron-withdrawing or -donating groups minimally affects local structure, the supramolecular organization is substantially affected by non-classical hydrogen-bonding to the fluoro and methoxy groups in (2) and (3), respectively.

Structural commentary
The molecular structures of (1), (2), and (3) are illustrated in Figs. 1-3. All complexes exhibit an overall structure common for CpMo acetyl complexes of this type, with trans-disposed carbonyl ligands. As previously observed for most related acetyl complexes, the acyl C O points up toward the Cp ring. In the case of (1), this phenomenon could be rationalized by presence of short C4-H4AÁ Á ÁO1 (2.672 Å ) and C4- Molecular structure of (3) with ellipsoids at 50% probability.
Selected geometric parameters for (1), (2), and (3) are presented in Tables 1-3. Complex (2) crystallized with two nearly equivalent molecules in the asymmetric unit, so geometric parameters are presented for both. In general, the three complexes are nearly identical, as might be expected based on the dominant role of sterics in determining structure and the fact that the steric profiles of the three triarylphosphine ligands are identical. The Mo-P bond length in (2) [2.4692 (4) Å (avg)] is slightly shorter than in (1) or (3) [2.4816 (4) and 2.4745 (3) Å , respectively], which may be related to stronger -backbonding to the tris(4-fluorophenyl)phosphine ligand. Stronger backbonding is supported by the observation by infrared spectroscopy of slightly higherenergy carbonyl stretching vibrations for (2) [(CO) avg = 1897 cm À1 ] compared with (1) and (3) [(CO) avg = 1893 cm À1 for (1), 1890 cm À1 for (3)]. Geometric parameters for all complexes are quite similar to those for the related triphenylphosphine-supported CpMo acetyl complex (Churchill & Fennessey, 1968).

Figure 9
Network of complex (3) formed by interactions featuring methoxy groups and dichloromethane solvent, viewed along [100].

Database survey
The current version of the Cambridge Structural Database (Version 5.41, updated August 2020;Groom et al., 2016) has fourteen entries corresponding to molybdenum acyl complexes of the general form Mo(C 5 H 5 )(CO) 2 (PR 3 )(COR). The trans-dicarbonyl structure, as observed for (1)- (3), is preferred except in cases where the phosphine and acyl ligands are covalently linked, forcing them to be cis (Adams et al., 1991;Mercier et al., 1993;Yan et al., 2009).

Synthesis and crystallization
CpMo(CO) 3 (CH 2 CH 3 ). This compound was prepared by modification of the method used of Gladysz et al. (1979), as previously reported by  and Anstey et al. (2020). In a 20 ml scintillation vial equipped with a flea-sized stir bar, [CpMo(CO) 3 ] 2 (0.1908 g, 0.39 mmol) was dissolved in THF (10 ml). Sodium triethylborohydride (0.87 mL of 1.0 M solution in THF, 0.87 mmol) was added dropwise by syringe with vigorous stirring, leading to an immediate color change from purple to green-yellow with evolution of H 2 gas. The reaction was allowed to proceed with stirring for 20 min, and an excess of iodoethane (0.098 ml, 1.2 mmol) was added dropwise with stirring and the reaction was allowed to proceed for 6 h. Volatiles were removed in vacuo to afford a yellow-brown film that was stored at 238 K for 1 week. The solid was extracted with pentane (4 Â 10 ml) and filtered through a 1 cm pad of activated alumina to afford a yellow solution, and removal of solvent in vacuo afforded CpMo(CO) 3 (CH 2 CH 3 ) as a pure yellow powder (0.131 g, 61% In an inert-atmosphere glove box, CpMo(CO) 3 (CH 2 CH 3 ) (0.0803 g, 0.293 mmol) and triphenylphosphine (0.115 g, 0.440 mmol, 1.5 equiv) were dissolved in acetonitrile (5 ml) in a 20 ml scintillation vial equipped with a flea-sized stir bar. The mixture was stirred for 1 week, during which time a brightyellow precipitate formed. The yellow solid was isolated by filtration and washed with pentane (2 Â 5 ml), then dried in vacuo to afford pure 1. Yellow crystals of 1 suitable for X-ray diffraction were obtained from a concentrated dichloromethane solution by vapor cross diffusion with pentane at 238 K. 1 : 1935, 1851, 1614 (acetyl).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 7. H atoms were placed in calculated positions and refined in the riding-model approximation with distances of C-H = 0.95, 0.98, 0.99, and 1.00 Å for the phenyl, research communications methyl, methylene, and cyclopentadienyl groups, respectively, and with U iso (H) = kÂU eq (C), k = 1.2 for cyclopentadienyl, phenyl, and methylene groups and 1.5 for methyl groups. Methyl group H atoms were allowed to rotate in order to find the best rotameric conformation.
A small number of intense low-angle reflections [three for (1); seven for (2); five for (3)] are missing from these highquality data sets due to the arrangement of the instrument with a conservatively sized beam stop. The large number of reflections in the data sets (and the Fourier-transform relationship of intensities to atoms) ensures that no particular bias has been introduced.
The structure of (3) exhibits modest disorder in the position of Cl1 of the dichloromethane solvent, which was modeled with two sites showing approximately equivalent occupancies [0.532 (15) for Cl1A, 0.468 (15) for Cl1B].

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Mo1 0.68359 (2) 0.16744 (2) 0.31097 (2) 0.01937 (5)  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.  (4)  C17 0.0368 (7) 0.0399 (7) 0.0557 (9) 0.0245 (6) 0.0182 (7) 0.0213 (7)