Crystal structures of trans-acetyldicarbonyl(η5-cyclopentadienyl)(1,3,5-triaza-7-phosphaadamantane)molybdenum(II) and trans-acetyldicarbonyl(η5-cyclopentadienyl)(3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane)molybdenum(II)

Solid-state structures of the title compounds are presented to show the relative effects of 1,3,5-triaza-7-phosphaadamantane (PTA) and 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA) ligands on the molecular and extended structure.

The title compounds, [Mo(C 5 H 5 )(COCH 3 )(C 6 H 12 N 3 P)(CO) 2 ], (1), and [Mo(C 5 H 5 )(COCH 3 )(C 9 H 16 N 3 O 2 P)(C 6 H 5 ) 2 ))(CO) 2 ], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C 5 H 5 )(CO) 3 (CH 3 )]. The molecular structures of these complexes are quite similar, exhibiting a fourlegged piano-stool geometry with trans-disposed carbonyl ligands. The extended structures of complexes (1) and (2) differ substantially. For complex (1), the molybdenum acetyl unit plays a dominant role in the organization of the extended structure, joining the molecules into centrosymmetrical dimers through C-HÁ Á ÁO interactions with a cyclopentadienyl ligand of a neighboring molecule, and these dimers are linked into layers parallel to (100) by C-HÁ Á ÁO interactions between the molybdenum acetyl and the cyclopentadienyl ligand of another neighbor. The extended structure of (2) is dominated by C-HÁ Á ÁO interactions involving the carbonyl groups of the acetamide groups of the DAPTA ligand, which join the molecules into centrosymmetrical dimers and link them into chains along [010]. Additional C-HÁ Á ÁO interactions between the molybdenum acetyl oxygen atom and an acetamide methyl group join the chains into layers parallel to (101).
We have previously described the solid-state structures of a number of four-legged piano-stool molybdenum acetyl complexes of the type Mo(PR 3 )(C 5 H 5 )(CO) 2 (COCH 3 ) derived from reaction of the molybdenum methyl precursor with various phosphines . We have shown that the steric bulk of the phosphine substituents impacts the molecular structure of the insertion product in predictable ways, primarily evidenced through the Mo-P bond lengths and P-Mo-C bond angles (Whited et al., 2012, consistent with earlier findings on reactivity. We have also shown that the use of tri(2-furyl)phosphine, which features heteroatoms as potential hydrogen-bond acceptors, leads to an unusual structure where the acetyl is oriented away from the Cp ring rather than toward it as in other cases (Whited et al., 2013).

Structural commentary
The molecular structures of (1) and (2) are illustrated in Figs. 1 and 2. Both (1) and (2) exhibit a trans disposition of the carbonyl ligands, as seen for other compounds of this type. It was envisioned that incorporation of hydrogen-bond acceptors (amines and amides) might allow access to an alternate orientation of the acetyl group, as observed for a related structure featuring a tri(2-furyl)phosphine ligand, but instead the oxygen of the acetyl group points toward the Cp ring, consistent with other structures of related complexes. (1) and (2) (6) Å ] are similar to one another and slightly shorter than for the related complexes we have reported. This finding is consistent with the lower steric profile of the polycyclic PTA and DAPTA ligands relative to, for example, PPh 3 , PMePh 2 , and PMe 2 Ph. The most significant molecular difference between (1) and (2) is seen in the C3-Mo1-P1 bond angle, which is larger for (1) [136.77 (4) ] than for (2) [133.99 (6) ].

Supramolecular features
Although the molecular structures of (1) and (2) are quite similar, the diacetylation of the phosphine ligand (transforming PTA to DAPTA) leads to significant differences in the extended structure. As was the case for several previously reported complexes of this sort with different phosphine ligands, the extended structure of (1) is dominated by interactions of atom O3 from the molybdenum acetyl. Short, nonclassical C-HÁ Á ÁO interactions between O3 and H8 of a Molecular structure of (1) with ellipsoids at 50% probability.
non-classical C-HÁ Á ÁO interaction between O3 and H13B (2.46 Å ) (Fig. 7). The extensive network of interactions involving all three acetyl groups likely contribute to the low solubility of (2) in most organic solvents.

Database survey
The current version of the Cambridge Structural Database (Version 5.40, updated August 2019; Groom et al., 2016) has thirteen entries corresponding to molybdenum acyl complexes of the general form Mo(C 5 H 5 )(CO) 2 (PR 3 )(COR). The transdicarbonyl structure, as observed for (1) and (2), 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). The PTA and DAPTA ligands have not been extensively utilized on molybdenum. There are eight instances of the PTA ligand bonded to molybdenum or tungsten, five of which involve coordination of one or more of the ligand to an M(CO) n center to afford an octahedral product. Most relevant to this study is the tungsten complex W(C 5 H 5 )(CO) 2 (PTA)-(H), which is analogous to (1) and (2) but features a hydride rather than an acyl ligand (Sears et al., 2015).

Synthesis and crystallization
CpMo(CO) 3 (CH 3 ). This compound was prepared by a modification of the method used by Gladysz et al. (1979), as previously reported by , with the modification that sodium triethylborohydride (1.0 M in toluene) was used as a reductant instead of lithium triethylborohydride to facilitate isolation of the product.
CpMo(CO) 2 (PTA)(COCH 3 ) (1). In an inert-atmosphere glove box, CpMo(CO) 3 (CH 3 ) (325 mg, 1.25 mmol, 1.18 equivalents) was dissolved in acetonitrile (10 ml). In a separ-ate vial, 1,3,5-triaza-7-phosphaadamantane (PTA, 167 mg, 1.06 mmol, 1 equivalent) was massed. The homogeneous solution of the molybdenum complex was transferred to the vial containing PTA and the mixture was stirred at room temperature. After three days, the solution had generated a red solid that clung to the walls of the scintillation vial while the solution itself retained a red-orange color. Solvent was removed in vacuo, leaving a red-orange solid that was washed with hexanes (10 ml) and diethyl ether (10 ml) before a final extraction using tetrahydrofuran (THF, 10 ml). The solid obtained from the THF fraction was the pale-yellow pure form of the final product (350 mg, 79%). Crystalline material was obtained as yellow-orange blocks by a THF/toluene vapor cross diffusion, which was used to concentrate the solution in a controlled manner without exposure to the glove-box atmosphere. The procedure is as follows: 50 mg of the product were dissolved in 1 ml of THF to form a concentrated solution. This solution was transferred into a small cylindrical 5 ml vial that was placed into a 20 ml vial. The remaining volume inside the 20 ml vial was filled with 10 ml of toluene. The vial was capped and left for two days before crystals were observed and harvested. 1 H NMR (400 MHz, CDCl 3 ): 5.16 (d, J = 1.2 Hz, 5H, In an inert-atmosphere glove box, CpMo(CO) 3 (CH 3 ) (310 mg, 1.19 mmol, 1.05 equivalents) was dissolved in N,N-dimethylformamide (10 ml). In a separate vial, 3,7-diacetyl-1,3,5-triaza-5phosphabicyclo[3.3.1]nonane (DAPTA, 260 mg, 1.13 mmol, 1 equivalent) was massed. The homogeneous solution of the molybdenum complex was transferred to the vial containing DAPTA. The solution was not fully homogeneous, so vigorous stirring was employed. The solution had generated a paleyellow solid after the first day while the supernatant solution was a red-orange color. The reaction mixture was filtered to obtain the pale-yellow solid, which was washed with two 2 ml portions of fresh N,N-dimethylformamide. The solid was dried in vacuo and recrystallized from a vapor diffusion of diethyl ether into a concentrated solution of the product in N,N-dimethylformamide, affording (2)

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. H atoms were placed in calculated positions and refined in the riding-model approximation with distances of C-H = 0.93, 0.96, and 0.97 Å for the cyclopentadienyl, methyl, and methylene groups, respectively, and Acta Cryst. (2020). E76, 547-551 research communications Layers of (2) formed by C-HÁ Á ÁO interactions, viewed perpendicular to (101) with U iso (H) = kÂU eq (C), k = 1.2 for cyclopentadienyl 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 [one for (1); five for (2)] are missing from these high-quality 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.   program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010).