Dioxidomolybdenum(VI) complex featuring a 2,4-difluoro-substituted amine bis(phenolate) ligand

An updated structure is reported for a bis(oxo) MoVI complex that is modeled after the molybdenum cofactor. An analysis of lengths and angles suggests that this dataset offers a more accurate depiction of bonding for the MoVI=O moiety.


data reports
A robust molybdenum-oxo complex bearing a pentadentate pyridyl ligand was notably shown to catalytically generate H 2 from water at a low overpotential (Karunadasa et al., 2010). Related molybdenum-oxo complexes featuring an amine bis(phenolate) moiety have been similarly shown to promote H 2 generation (Cao et al., 2014) and oxygen atom transfer (Maurya et al., 2016). Insight into the structural features that enable such activity at the Mo O moiety are thus an important component of iterating the design of these species for use as sustainable aqueous catalysts.
The Mo complex reported here (2, Fig. 1) is chemically identical to that reported by Cao et al. (2014, KOWXIF). The lower collection temperature (150 K versus room temperature in KOWXIF) and larger 2 range for data collection (5.8-66.6 versus 6-54.96 in the previous report) led to a structure solution with lower R 1 and !R 2 values (0.019 and 0.049 versus 0.0310 and 0.0566 in KOWXIF). Slight differences in the bond lengths for the compound in these structures warrant further comment and may be of interest from a mechanistic perspective. For example, it is generally accepted that an Mo O bond in the cis-[MoO 2 ] 2+ core of DMSO reductase model compounds is formally strengthened (consistent with Mo O) during oxygen atom transfer (Enemark, et al., 2004).
Both structures have P1 space-group symmetry, though a, c, , and were different by AE3 s.u. While the Mo-O(phenolate) and Mo-N bonds in this structure are nearly identical to those reported by Cao et al., the Mo O bond lengths reported here are notably longer than those in KOWXIF and are in line with expectations for related Mo VI -oxo species (Enemark, et al., 2004). However, these differences in bond length are within the accuracy limits for light atoms imposed by the spherical atom scattering factor approximation (e.g. Dawson, 1964). Relevant lengths and angles for both are summarized in Table 1. Differences in the metrical parameters for these structures suggest that the model presented here gives a better representation of the bonding for 2, when compared with other Mo VI oxo species.
No hydrogen bonding was observed, though short contacts exist between inversion-related molecules contained in the unit cell. The orientation of one phenolate ring brings the ortho carbons C16 and C18 i [symmetry code: (i) 1 À x, 1 À y, 1 À z] in close proximity [3.2807 (15) Å , i.e. $0.12 Å closer than the sum of the vdW radii]. Close contact is noted for the para F3 and proximal meta C16 ii [symmetry code: (ii) 2 À x, 2 À y, 1 À z] of an adjacent inversion-related molecule [3.1622 (14) Å , i.e. $0.01 Å closer than the sum of the vdW radii]. This marginally short contact is consistent with - Table 1 Comparison of lengths and angles between this work and previous report.  Figure 1 Molecular structure of 2 with 50% displacement ellipsoids and the numbering scheme for non-H atoms.
stacking between the phenolate rings related by the inversion center; this interaction is shown in Fig. 2 (4) ]. The dihedral between aromatic rings was found to be 60.06 (4) . The torsion angle along the diamine N1-C1-C2-N2 [À55.84 (11) ] is consistent with the syn conformation of amine donors within the unstrained five-membered ring formed upon chelation.

Synthesis and crystallization
The ligand H 2 ONNO F (1) was prepared by the method reported previously (Graziano et al., 2019). The Mo complex (2) was prepared using a modified version of the method reported by Lehtonen & Sillanpä ä (2005). The reaction scheme is shown in Fig. 3. MoO 2 (acac) 2 (0.330 g, 1.01 mmol) and the ligand H 2 ONNO F (1; 0.373 g, 1.00 mmol) were combined in a 20 ml scintillation vial with a PTFE-coated stir bar and suspended in 10 ml of anhydrous methanol. The reaction mixture was left to stir for 4 h at 295 K, at which time solvent and other volatiles were removed in vacuo to yield a yellow solid (0.500 g, 1.00 mmol, >99%). The product was purified by column chromatography on silica using an increasing linear gradient of dichloromethane in acetone as the eluent. After removing the solvent and other volatiles, single crystals suitable for diffraction studies were obtained by slow evaporation from a concentrated solution of acetone. Characterization data for this compound match those previously reported by Cao et al. (2014). M.p. = 463-467 K.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. No disorder or solvent were present.

Funding information
This material is based upon work supported by the National      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.