Bis(2,4-dioxopentan-3-ido-κ2 O,O′)dioxidomolybdenum(VI): a redetermination

The crystal structure of cis-[MoO2(acac)2] has been redetermined at 100 K, providing a more precise description of the structure including hydrogen atoms and intermolecular contacts.


Structure description
The title compound is a versatile starting material for the preparation of cis-dioxidomolybdenum complexes, including complexes containing organodinitrogen ligands (Bustos et al., 1994) and molybdenyl adducts of platinum -S dimers (Henderson et al., 2011). MoO 2 (acac) 2 has also been used to prepare dioxidomolybdenum(VI) complexes with O,N,N 0 chelating ligands (Ceylan et al., 2015) and an amine bis(phenolate) ligand (Bowen & Wile, 2021). Many of these complexes have been prepared and studied for their catalytic activities, including complexes with acylpyrazolonate ligands that catalyze the deoxygenation of epoxides (Hills et al., 2013;Begines et al., 2018) and dioxidomolybdenum(VI) complexes with salicylamide ligands for the epoxidation of olefins (Annese et al., 2019). Molybdenum(VI) dioxido complexes with acetylacetonato ligands have also been investigated for their catalytic properties in the dehydrogenation of alcohols (Korstanje et al., 2013). These complexes are of particular interest due to their close structural similarities to the active sites of several molybdoenyzmes such as sulfite oxidase, xanthine oxidase, and DMSO reductase (Sousa & Fernandes, 2015).

data reports
Two previous structural determinations of cisdioxidobis(acetylacetonato)molybdenum(VI) were published in the mid-1970s (Kamenar et al., 1973;Krasochka et al., 1975) based on photographic methods and room-temperature data collections. Additionally, Craven et al. (1971) cite an unpublished diffraction study that also confirms the cis coordination and includes additional structural information consistent with the current study. None of the previously published structure solutions attempted to locate the positions of any of the hydrogen atoms. Several closely related structures have been determined, including cis-dioxido-molybdenum complexes with 1,3-diphenylpropanedianoto ligands (Kojić-Prodić et al., 1974;Korstanje et al., 2013) and tert-butylacetylacetonato ligands (Nass et al., 2001). The structure of the product from the reaction of cis-[MoO 2 (acac) 2 ] with the strong Lewis acid B(C 6 F 5 ) 3 (Galsworthy et al., 1997)  The asymmetric unit of the title compound contains two crystallographically independent cis-[MoO 2 (acac) 2 ] molecules, one each of the Á and Ã forms (Fig. 1). The molecular structure adopts a distorted octahedral arrangement around the Mo VI atoms, with oxido ligands in a cis arrangement and oxido-molybdenum-oxido angles of 105.40 (4) and 105.59 (5) . As observed previously (Krasochka, 1973;Kojić-Prodić et al., 1974), the Mo-O bond distances trans to the molybdenumoxygen double bonds are significantly lengthened [avg = 2.185 (5) Å ] relative to the other molybdenum-oxygen distances [avg = 1.999 (11) Å ] (see Table 1 for selected bond distances and angles). The four molybdenum oxygen distances for the doubly-bonded oxido ligands average 1.7012 (16) Å , in agreement with the average distance found for over 140 similar cis-dioxido molybdenum complexes in the Cambridge Structural Database (Groom et al., 2016). These metrics are also in agreement with relatively narrow distribution of molybdenum-oxygen distances observed by Mayer (1988) for cis-dioxido complexes.
All of the hydrogen-bonding contacts are weak C-HÁ Á ÁO interactions with DÁ Á ÁA distances between 3.3 and 3.5 Å (see Table 2 and Fig. 2). There are contacts between C-H atoms and all four of the oxido ligands, including two contacts to O1 and three contacts to O8. Additional C-H contacts are made to most of the acetylacetonate oxygen atoms as well.

Figure 2
Packing diagram (viewed along a), showing extensive weak C-HÁ Á ÁO contacts (red dotted lines) throughout the crystal structure. Displacement ellipsoid (50% probability) diagram of the two independent molecules with the numbering scheme for the non-hydrogen atoms. 30 min. The product precipitated as a pale-yellow solid and was isolated by filtration and washed with deionized water (2 Â 10 ml), followed by ethanol (1 Â 10 ml), and diethyl ether (1 Â 10 ml). Over multiple preparations the yield averaged around 90%. Characterization by 1 H NMR and FTIR agrees with previously reported values (Chakravorti & Bandyopadhyay, 1992;Arná iz, 1995).
Three different crystallization methods were utilized: slow evaporation from a concentrated solution in 2,4-pentanedione, vapor diffusion (dichloromethane/diethyl ether), and layering (dichloromethane/diethyl ether) in a standard 5 mm NMR tube. All three methods produced crystals, but the highest quality crystals and those used in this study were produced from solvent layering.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq