A cocrystal of two Mo VI complexes bearing different diastereomers of the 2,4-di-tert -butyl-6-{[(1-oxido-1-phenyl-propan-2-yl)(methyl)amino]methyl}- phenolate ligand derived from (+)-ephedrine

The title cocrystal contains two chiral conformational diastereomers, viz. (1 S ,2 R , R N )- and (1 S ,2 R , S N )-, of [2,4-di-tert -butyl-6-{[(1-oxido-1-phenylpropan-2-yl)(methyl)amino]- methyl}phenolato](methanol)- cis -dioxidomolybdenum(VI), [Mo(C 25 H 35 NO 2 )O 2 (CH 3 OH)], representing the ﬁrst example of a structurally characterized molybdenum complex with enantiomerically pure ephedrine derivative ligands. The Mo VI cations exhibit differently distorted octahedral coordination environments, with two oxide ligands positioned cis to each other. The remainder of the coordination comprises phen-oxide, alkoxide and methanol O atoms, with an amine N atom completing the octahedron. The distinct complexes are linked by strong intermolecular O—H (cid:2) (cid:2) (cid:2) O hydrogen bonds, resulting in one-dimensional molecular chains. Furthermore, the phenyl rings are involved in weak T-shaped/edge-to-face (cid:2) – (cid:2) interactions with each other.


Comment
For the past several decades, high-valent molybdenum complexes have gained considerable attention in various catalytic oxidation reactions (Arzoumanian, 1998) and as biological model compounds (Hille, 1996;Collison et al., 1996). Recently, oxomolybdenum complexes have appeared in novel studies concerning, for example, X-H (X = Si, B, P or H) bond activation (Sousa et al., 2012) and hydrogen production from water (Karunadasa et al., 2010), previously dominated by more noble metals. Ephedrine and its N-substituted derivatives are inexpensive, readily available in enantiomerically pure forms and relatively easy to modify, and thus are an interesting group of chiral ligands for various purposes (Yuan et al., 2003;Kuznetsov et al., 1999;Bouquillon et al., 1999). These two strands of interest are combined in the title compound, (I).
The asymmetric unit of (I) contains two distinct Mo VI complexes, A and B, which are conformational diastereomers ( Fig. 1 and Table 1). The bonding and geometric parameters around the Mo VI cation of A and B are comparable to some extent, but several differences can be noted, so the different ligand geometries of the two diastereomers will be discussed.
The ligand geometries of complexes A and B are notably different, which can be seen from an overlay of the molecules (Fig. 2). The two carbon stereocentres of the L1 2À ligand have the same configuration (1S,2R) in both molecules, but the amine N atoms have different stereochemistries due to the conformational change of the ligand, producing R (N8) and S (N38) configurations for A and B, respectively. This difference induces major changes in the coordination angles (Mo-O-C), general conformation and 'folding' of the ligand (Table 1). The most significant conformational changes of the ligand are seen when comparing the chelate angles of the ligands (N8-C9-C10-O4 and N38-C39-C40-O9; Table 1). Also, the torsion angles related to the position of methyl substituents C17 and C47 (C17-N8-C9-C18 and C47-N38-C39-C48) are quite dissimilar in A and B (Table 1). The Mo1-O3 bond is slightly shorter than Mo2-O8, which might be due to the increased -bonding ability of atom O3 because of the larger Mo-O-C angle. The pyramidality of the amine N atom has been related to the donor ability of the atom (Hä nninen et al., 2011). For B, the torsion angle indicating the pyramidality of the N atom (C37-C39-C47-N48) is À35.9 (3) , while the corresponding angle for A (C7-C9-C17-N8) is only 33.9 (2) , indicating a less pyramidalized arrangement. The shorter Mo-N bond for complex B is in agreement with the above conclusion. Furthermore, the shorter Mo1-O5 bond compared with Mo2-O10 can be attributed to the tighter hydrogen bonding (O5-H5OÁ Á ÁO6 i versus O10-H10OÁ Á ÁO1; see Table 2 for symmetry code).
The solid-state ordering of the complexes is governed by strong intermolecular hydrogen bonds from the coordinated methanol molecule to an oxide ligand of a neighbouring complex (Table 2). These hydrogen bonds bind the molecules together, forming a one-dimensional chain of complexes ( Fig. 3a). Furthermore, T-shaped/edge-to-faceinteractions are present between the phenyl rings of the ephedrine part of the ligands (Fig. 3b). The distances between phenylring centroids are in the range 4.7-5.2 Å and the angles between the phenyl-ring planes vary from 58 to 89 , thus     supporting the presence ofinteractions. Both of the preceding effects can be seen to enhance the crystallization of the compounds, thus contributing to the good quality and stability of the crystals. The solid-state structure of the complex does not contain any additional noncoordinating solvents or notable cavities.

Experimental
MoO 2 (acac) 2 (acac is acetylacetate) was prepared according to the literature procedure of Chen et al. (1976). 2,4-Di-tert-butyl-6-{[(1hydroxy-1-phenylpropan-2-yl)(methyl)amino]methyl}phenol (H 2 L1) was synthesized by dissolving equimolar amounts of 2,4-di-tertbutylphenol, formaldehyde (36.5% water solution) and (1S,2R)-(+)ephedrine hydrochloride in methanol. Two equivalents of triethylamine were added and the mixture was refluxed for two weeks, after which time the reaction did not proceed any further and roughly half of the starting materials were converted to products (determined by high-performance liquid chromatography). Small amounts of H 2 L1 could be separated by crystallization from the methanol solution in a freezer and were used in the complexation reaction. Cocrystals were prepared by dissolving H 2 L1 (0.10 mmol) and MoO 2 (acac) 2 (0.10 mmol) in methanol (4 ml). The solution was stirred for 20 h, filtered and placed in a freezer. Pale-yellow crystals of (I) suitable for X-ray diffraction formed within a few days. All C-bound H atoms were placed in idealized positions and refined in riding mode, with C-H = 0.93 (aromatic), 0.96 (methyl) or 0.97 Å (methylene), and with U iso (H) = 1.5U eq (C) for methyl H atoms or 1.2U eq (C) for aromatic and methylene H atoms. Hydroxy H atoms were located from the electron-density map and their positions were refined isotropically with O-H distances restrained to 0.82 (1) Å . Reflections 102, 002 and 101 were omitted from the data because the F o values were considerably smaller than the F c values, as these reflections were partially obscured by the beam-stop during the data collection.

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )