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

Abstract

CommentFor 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.In A and B, the tridentate doubly deprotonated 2,4-di-tert- Figure 1 The molecular structure of complexes A (top) and B (bottom), showing the atom-numbering schemes. Displacement ellipsoids are drawn at the 50% probability level and C-bound H atoms have been omitted for clarity.tion geometry for the Mo VI cation. The oxide ligands are strongly bound, with Mo O distances of about 1.7 Å , thus indicating clear double-bond character. The phenoxide and alkoxide O atoms are arranged trans to each other, with a marginally longer Mo-O(phenoxide) distance compared with Mo-O(alkoxide), as might be expected from the different electronic nature and steric hindrance of the groups. The amine N and methanol O atoms are trans to the Mo O groups and rather weakly bound to the Mo VI cation, with relatively long bond lengths of over 2.35 Å . This weaker bonding can be explained by the lack of negative partial charge on the coordinating atoms (amine N and methanol O) and by the considerable trans influence induced by the oxide ligands. The different character of the coordinating groups emphasizes the distortion of the Mo VI octahedron, which is evident from the trans angles around the metal cation (O1-Mo1-N8, O2-Mo1-O5 and O3-Mo1-O4 for A, and O6-Mo2-N38, O7-Mo2-O10 and O8-Mo2-O9 for B; (Cross et al., 1999). The Flack (1983) parameter of 0.00 (3) indicates that the crystals of (I) are enantiomerically pure and the absolute configuration has been determined correctly.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, re...