Dioxidobis(2-oxo-1,2-dihydropyridin-3-olato)molybdenum(VI)

In the title compound, [Mo(C5H4NO2)2O2], the MoVI atom exhibits a distorted octahedral coordination geometry formed by two terminal oxo ligands and two monoanionic O,O-bidentate pyridinone ligands. The two terminal oxo ligands lie in a cis arrangement, the ketonic O atoms of the pyridinone ligands are coordinated trans to the oxo ligands and the deprotonated hydroxyl O atoms are located trans to each other. The crystal structure contains intermolecular N—H⋯O hydrogen bonds, C—H⋯O contacts and face-to-face π–π stacking interactions with an interplanar separation of 3.25 (1) Å.

In the title compound, [Mo(C 5 H 4 NO 2 ) 2 O 2 ], the Mo VI atom exhibits a distorted octahedral coordination geometry formed by two terminal oxo ligands and two monoanionic O,Obidentate pyridinone ligands. The two terminal oxo ligands lie in a cis arrangement, the ketonic O atoms of the pyridinone ligands are coordinated trans to the oxo ligands and the deprotonated hydroxyl O atoms are located trans to each other. The crystal structure contains intermolecular N-HÁ Á ÁO hydrogen bonds, C-HÁ Á ÁO contacts and face-to-face stacking interactions with an interplanar separation of 3.25 (1) Å .

Comment
There has been growing interest in the study of Mo VI complexes because of their biochemical significance (Collison et al., 1996;Hille, 1996). For example, dioxomolybdenum(VI) complexes are studied as models for oxidized forms of molybdoenzymes, e.g. aldehyde oxidase and sulfite oxidase which are supposed to contain cis-MoX 2 units (X = O,S) coordinated to S, N and O donor atoms of the protein structure (Tucci et al., 1998;Schultz et al., 1993). The present view of these enzymes indicates that the formal oxidation state of Mo cycles between +4 and +6 in reactions with substrate and oxidant. The two electron O atom transfer seems to be the relevant mechanism in understanding the chemical role of enzymatic reactions.
A large number of important chemical reactions are catalysed by Mo VI complexes. Several industrial processes such as ammoxidation of propene to acrylonitrile (Grasselli, 1999), olefin epoxidation (Veiros et al., 2006) and olefin metathesis (Schrock, 1998) reactions are carried out over Mo catalysts.
In the title compound, the coordination sphere about the Mo VI atom consists of six O atoms arranged in a distorted octahedral geometry ( Fig. 1 and Table 1). There is a cis arrangement of dioxo ligands, as predicted by spectroscopic and other structural data. The O=Mo=O angle is 103.48 (7) (2) Å] bonds. Resonance forms for pyridinone ligands have been described in detail elsewhere (Thompson et al., 1999;Zhang et al., 1992).
The NH and CH groups of the pyridinone ligands form a hydrogen bond with an oxo ligand attached to Mo in a neighbouring molecule (Table 2) (Braga et al., 1997). Repetition of this hydrogen bond generates parallel chains along the b axis ( Fig. 2). There are face-to-face π-π stacking interactions involving the pyridinone rings of adjacent pyridinone molecules, with π-π distances of 3.295-3.389 Å (Fig. 3) (Ranganathan et al., 1998;Hozba et al., 1997). One potential driving force for alignment of the motifs might be the N···O interactions (N···O distance = 2.904 Å) that exists between adjacent motifs, resulting in a columnar architecture with a dimension of 7.2 × 6.7 Å (Fig. 4).

Experimental
The title compound was prepared by suspension of 2,3-pyridinediol (0.111 g, 1 mmol) in methanol (30 ml), followed by addition of KOH (0.112 g, 2 mmol). Stirring at room temperature for 30 min gave a clear red solution. This solution was treated with (NH 4 ) 2 Mo 2 O 7 (0.170 g, 0.50 mmol) and stirred overnight. The resulting orange-red solution was filtered and supplementary materials sup-2 allowed to cool at room temperature. Over a couple of days, orange irregular needle-shaped diffraction-quality crystals separated, which were isolated and dried in air.

Refinement
All H atoms were added in calculated positions and were refined as riding with U iso (H) = 1.2U eq (C). Fig. 1. The molecular structure with displacement ellipsoids drawn at the 50% probability level for non-H atoms 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 > σ(F 2 ) is used only for calculating Rfactors(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.

Figures
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )