Crystal structures of two dioxomolybdenum complexes stabilized by salan ligands featuring phenyl and cyclohexyl backbones

Two cis-dioxomolybdenum complexes based on salan ligands with different backbones are reported. The salan ligands coordinate to the molybdenum center in a κ2 N,κ2 O fashion, forming a distorted octahedral geometry. These complexes crystallized as dimethylformamide and methanol solvated species.


Chemical context
Molybdenum centers are present in the active sites of various enzymes including nitrogenases, sulfite oxidase, xanthine oxidase, and DMSO reductase that catalyze two-electron redox processes (Hille et al., 2014;Enemark et al., 2004;Hille, 1996). This is attributed to the large number of stable oxidation states and coordination environments that can be achieved, as well as the solubility of molybdate salts in water. A majority of these enzymes are referred to as oxo-molybdenum enzymes due to the presence of at least one Mo O moiety in the active site. The sulfite oxidase family of enzymes contains a cis-dioxo molybdenum(VI) (L n MoO 2 ) center in its active site (Hille et al., 2014). Apart from being studied as models to understand biological systems, oxomolybdenum complexes have also found utility in processes such as olefin metathesis, olefin epoxidation, cytotoxic studies, and cyclic ester polymerizations (Hossain et al. 2020;Mayilmurugan et al. 2013;Yang et al. 2007). Mononuclear molybdenum complexes are generally distinguished by stretching frequencies {u(O Mo O)} in the 910-950 cm À1 and 890-925 cm À1 regions, which are characteristic of a cis-MoO 2 fragment (Chakravarthy & Chand, 2011). A variety of ligand architectures have been successful in stabilizing the oxomolyb-denum core in these complexes (Ziegler et al. 2009;Subramanian et al. 1984;Rajan et al. 1983). Dioxomolybdenum complexes stabilized by salan ligands have been used extensively for various applications (Roy et al., 2017;Whiteoak et al., 2009). The modular nature for the synthesis of salan ligands allows for incorporation of steric and electronic variations in the ligand framework to tune the reactivity of the molybdenum center. We are exploring the utility of dioxomolybdenum complexes in catalyzing the deoxydehydration (DODH) reaction with a focus on understanding ligand effects on catalytic activity. This work reports synthesis and crystal structures of two molybdenum complexes including a crystallographically uncharacterized complex, dioxido[2,2 0 -{l,2-phenylenebis(iminomethylene)bis(phenolato)]molybdenum(VI), Ph LMoO 2 (1b) (Rajan et al. 1983). The second is a known complex with a new unit cell, (Ziegler et al., 2009), 6,6 0 -{[(cyclohexane-1,2-diyl)bis(azanediyl)]bis(methylene)}bis(2,4di-tert-butylphenolato))dioxidomolybdenum(VI), Cy LMoO 2 (2b).
The asymmetric unit of Cy LMoO 2 (2b) contains one molecule of Cy LMoO 2 and two molecules of methanol (MeOH) (Fig. 3). The salan ligand Cy LH 2 (2a) binds in the same 2 N, 2 O fashion that complex 1b does. Fig. 4 shows Cy LMoO 2 with the hydrogen atoms removed for clarity. The complex also has a distorted octahedral geometry with angles of O3-Mo01-O1 at 96.36 (5)  View of one molecule of Ph LMoO 2 (1b) with 50% probability ellipsoids. The DMF molecule and H atoms are omitted for clarity.  angles are between 5 and 10 of the ideal 90 for octahedral geometry. The N1-Mo01-N2 angle at 72.40 (4) is slightly less than that of the Ph LMoO 2 angle of 75.81 (6) , which is attributed to the flexibility of the cyclohexane ring between the nitrogen atoms compared to the rigid phenyl ring in the Ph LMoO 2. Metal-ligand bond distances are found for Mo01-O1 at 1.9428 (10) Å , Mo01-O2 at 1.9484 (10) Å , Mo01-O3 at 1.7125 (10) Å , Mo01-O4 at 1.7226 (11) Å , Mo01-N1 at 2.3412 (12) Å , and Mo01-N2 at 2.3384 (12) Å . Other ligand distances and bond lengths within the phenyl rings are consistent with analagous distances in Ph LMoO 2 (1b). The cylohexane bond distances are consistent with single C-C bonds. The bond lengths observed are not statistically different than those reported by Ziegler et al. (2009). There are a few statistically different angles, specifically around the molybdenum center where Table 1 shows the correlating bond angles. These bond-angle differences are most likely due to improved R1 of 2.78% as compared to the previously reported R1 of 5.5% and higher solvent disorder in the reported structure.

Supramolecular features
Ph LMoO 2 (1b): A single molecule of Ph LMoO 2 is hydrogen bonded to one disordered DMF molecule, as shown in Fig. 5, with a distance of 2.03 Å for O11Á Á ÁH008 (Table 2). A second hydrogen bond interaction is between O9-H00D with a distance of 2.16 (3) Å . Corresponding hydrogen bond distances in the second molecule in the unit cell are similar. There are three formula units within the contents of the unit cell. Perpendicular -stacking between Ph LMoO 2 molecules is observed between C5 and the aryl ring centroid (C35-C39) with a distance of 4.597 Å .

Cy
LMoO 2 (2b): There are four molecules of Cy LMoO 2 in the unit cell of this system and the complex is stabilized via hydrogen bonding to the solvent MeOH molecule (1.94 Å for O4Á Á ÁH5A and 2.00 Å for O5Á Á ÁH2;  View of one molecule of cy LMoO 2 Á2MeOH (2b) with 50% probability ellipsoids.

Figure 4
View of one molecule of cy LMoO 2 (2b) with 50% probability ellipsoids. The MeOH molecules and H atoms are omitted for clarity.

Figure 5
View of six molecules of Ph LMoO 2 and five molecules of DMF in the unit cell with 50% probability ellipsoids, highlighting intermolecular distances. Distances between H atoms are listed without standard deviations because the H atoms were positionally fixed.. Table 1 Comparison of bond angles ( ) between Cy LMoO 2 (2b) with R1 of 2.78% and reported structure from Ziegler et al. (2009)  bonding with the previously reported structure, the main difference is the formation of hydrogen-bonded tetramers containing two molecules of 2b and two molecules of methanol in the current structure. The previously reported structure had one resolved molecule of methanol and one disordered oxygen atom, which form a hydrogen-bonded trimer with one molecule of Cy LMoO 2 (Ziegler et al., 2009).

Database survey
A database search of the Cambridge Structural Database (CSD; Groom et al., 2016) (webCSD accessed September 22, 2021 and SciFinder (SciFinder, 2021) did not yield any exact matches to the crystal structure for Ph LMoO 2 (1b). There was a similar crystal structure found with the imine form of the ligand (Salen)MoO 2 . A search for Cy LMoO 2 (2b) in the CSD (webCSD accessed September 22, 2021) shows that there is a known structure of the molecule with a different unit cell with accession code HUWGOW (Ziegler et al., 2009). The SciFinder search resulted in the same sources being found. The current structure for Cy LMoO 2 (2b) was solved in space group P 2 1 /n compared with P3 1 for HUWGOW. The primary additional differences in the structures is an improved R1 of 2.78% and more clearly resolved methanol solvent, as compared to the previously reported R1 of 5.5% and more disordered methanol solvent (Ziegler et al., 2009).

Synthesis and crystallization
The salan ligands used for stabilizing [MoO 2 ] 2+ in the complexes Ph LMoO 2 (1b) (Rajan et al. 1983) and Cy LMoO 2 (2b) (Ziegler et al., 2009) were synthesized by the reductive amination of the corresponding salicylaldehyde and diamine. The ligands Ph LH 2 (1a) and Cy LH 2 (2a) were synthesized as off-white solids in 86% and 58% yields, respectively. The reaction scheme is shown in Fig. 7. Both ligands were successfully characterized by NMR and IR spectroscopy. A salient feature in the 1 H NMR spectra of both ligands as compared to the precursor salen compounds was the disappearance of the aldimine peak ($8.50 ppm) and the appearance of the benzylic resonances $4.00 ppm. The molybdenum complexes Ph LMoO 2 (1b) and Cy LMoO 2 (2b) were synthesized in 86% and 42% yields, respectively, by the reaction of the corresponding ligands with MoO 2 (acac) 2 in methanol or acetonitrile as solvent. Complexes 1b and 2b were also characterized by NMR and IR spectroscopy. Both complexes exhibited stretches {[(Mo O) = 916 and 876 cm À1 (1b); 903 and 875 cm À1 (2b)] characteristic of a cis-dioxo molybdenum core in the IR spectrum.
To a mixture of methanol (ca. 8 ml) and diethyl ether (ca 8 ml), was added salophen (1.52 g, 4.81 mmol) followed by NaBH 4 (1.67 g, 44.4 mmol), and the reaction mixture was stirred at room temperature for 1 h. When the yellow color of the solution changed to colorless, it was transferred into a separatory funnel and DI H 2 O (ca 15 ml) was added followed by ethyl acetate (2 Â ca 15 ml) for extraction. The organic solution was separated and combined, then washed with saturated NaCl solution (ca 20 ml). The organic layer was dried over anhydrous Na 2 SO 4 and filtered. The filtrate was concentrated under vacuum to give a light-yellow solid, which was dried under high vacuum.  View of four molecules of cy LMoO 2 and six molecules of methanol in the unit cell with 50% probability ellipsoids, highlighting intermolecular distances. Distances between H atoms are listed without standard deviations because the H atoms were positionally fixed. Table 2 Hydrogen-bond geometry (Å , ) for 1b.  Table 3 Hydrogen-bond geometry (Å , ) for 2b. A 100mL round-bottom flask was charged with trans-1,2-diaminocyclohexane (0.448 g, 4.38 mmol), methanol (ca. 16 mL), and 3,5-di-tert-butylsalicylaldehyde (2.05 g, 17.5 mmol). The solution was stirred for 24 h at room temperature. The solution resulted in a bright-yellow precipitate. The precipitate was then collected by gravity filtration and washed with cold methanol. The precipitate was dried under high vacuum to remove any residual solvent and yield the salen product (3.85 g, 81%). 1 H NMR (CDCl 3 , 400 MHz, 301 K) 13.6 (br, 2H), 8.33 (s, 2H), 7.34 (s, 2H), 7.02 (s, 2H), 3.37 (br, 2H), 1.98-1.77 (m, 4H), 1.40 (s, 18H), 1.33-1.29 (m, 4H), 1.24 (s, 18H).
Crystals of Ph LMoO 2 , 1b were grown by forming a supersaturated solution of the complex in DMF and layering with hexanes. The solution was placed in a refrigerator at 268 K for 1.5 months. Orange-yellow crystals were observed to grow and were collected for structural determination.
Crystals of Cy LMoO 2 , 2b were grown by using a supersaturated solution of the complex dissolved in methanol and allowed to undergo slow evaporation over 2 d. A similar vial was also refrigerated where crystals were seen to form as well. The crystals from the slow evaporation set up were cropped and the orange-yellow crystals were used for structure determination.

Refinement
Crystal data, data collection, and refinement details are listed in Table 4. Hydrogen atoms were placed at ideal positions with C-H distances at 0.95 for CH and 0.99 Å for sp 3 CH 2 and CH 3 using HFIX commands, and refined using a riding model with U iso (H) = 1.2U eq (C) for CH, CH 2 , and CH 3 . The structure for Ph MoO 2 (1b) was initially refined in the trigonal crystal system P3 2 21; however, this resulted in the solvent DMF having a high level of disorder with many checkCIF errors. For both structures, data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT (Sheldrick, 2015); program(s) used to refine structure: SHELXL (Sheldrick, 2008); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

(6,6′-{[(Cyclohexane-1,2-diyl)bis(azanediyl)]bis(methylene)}bis(2,4-di-tertbutylphenolato))dioxidomolybdenum(VI) methanol disolvate (2b)
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.