μ-Oxido-bis[chlorido(4,4′-di-tert-butyl-2,2′-bipyridine-κ2 N,N′)dioxidomolybdenum(VI)] 0.2-hydrate

The title hydrate, [Mo2Cl2O5(C18H24N2)2]·0.2H2O, has been isolated as the oxidation product of [Mo(η3-C3H5)Cl(CO)2(di-t-Bu-bipy)] (where di-t-Bu-bipy is 4,4′-di-tert-butyl-2,2′-bipyridine). A μ-oxide ligand bridges two similar MoCl(di-t-Bu-bipy)O2 units, having the terminal oxide ligands mutually cis, and the chloride and μ-oxide trans to each other. In the binuclear complex, the coordination geometries of the metal atoms can be described as highly distorted octahedra. Individual complexes co-crystallize with a partially occupied water molecule of crystallization (occupancy factor = 0.20; H atoms not located), with the crystal packing being mediated by the need to effectively fill the available space. A number of weak C—H⋯O and C—H⋯Cl interactions are present.

We are grateful to the Fundaçã o para a Ciê ncia e a Tecnologia (FCT/FEDER and POCI, Portugal) for their general financial support to CICECO, and for the postdoctoral research grant No. SFRH/BPD/63736/2009 (to JAF). Thanks are also due to the FCT for specific funding toward the purchase of the single-crystal diffractometer. motifs: as a terminal oxo (Mo═O) or as a bridging µ-oxo (Mo-O-Mo). Compounds with the latter type of bridging group are significantly less studied but have been shown to be intermediates in a handful of interesting catalytic systems (Nunes et al., 2003). Following our on-going interest in the study of this type of family of compounds (Fernandes et al., 2010a(Fernandes et al., ,b, 2011 we have recently described the synthesis and structural details of the oxo-µ-oxo complexes [Mo 2 O 4 (µ-O)Cl 2 (DMF) 4 ] (Gago et al., 2009) (Pereira et al., 2007), and [Mo 2 O 4 (µ-O)Cl 2 (PzPy) 2 ] (where PzPy stands for 2-(3-pyrazolyl)pyridine) (Coelho et al., 2011). Noteworthy, these complexes were found to be highly active in epoxidation catalysis with tert-butylhydroperoxide. The title compound, a µ-oxo dimer with empirical formula [Mo 2 O 4 (µ-O)Cl 2 (di-t-Bu-bipy) 2 ] (where di-t-Bu-bipy stands for 4,4'-di-tert-butyl-2,2'-bipyridine) which simultaneously contains terminal Mo═O oxo groups and a bridging µ-oxo one, has been recently synthesized by Arzoumanian et al. (2006) and we now wish to report its crystal structure at the low temperature of 150 K.
The asymmetric unit of the title compound comprises a whole binuclear molecular entity, C 36 H 48 Cl 2 Mo 2 N 4 O 5 , and a partially occupied (20%) water molecule of crystallization. The binuclear complex is formed by two crystallographically independent Mo(VI) centres bridged via a µ-oxo group imposing a Mo···Mo distance of 3.6273 (4) Å. The chemical environment of these metallic centers is very similar, being composed of a pair of cis-positioned terminal oxo ligands, a chlorido and a N,N-chelating 4,4'-di-tert-butyl-2,2'-bipyridine (di-t-Bu-bipy) molecule as depicted in Fig. 1. The coordination environments around the metal centers can be described as highly distorted octahedra due to, on the one hand, the existence of chlorido ligands (trans-positioned with respect to the µ-oxo ligand) and, on the other, to the typical trans effect of the associated with the di-t-Bu-bipy moieties, mostly due to the pendant -CH 3 groups. In this context, the two average planes containing the aromatic rings of the two crystallographically independent di-t-Bu-bipy molecules subtend an angle of ca 34°, which contrasts with the parallel nature observed for the two conformers of [Mo 2 O 4 (µ-O)Cl 2 (PzPy) 2 ]. Noteworthy, the torsion angles N1-Mo1···Mo2-N4 and N2-Mo1···Mo2-N3 are -18.40 (7) and -157.03 (9)°, respectively.

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The crystal packing is mainly driven by the need to effectively fill the available space (van de Waals contacts) in conjunction with several weak supramolecular interactions, namely weak C-H···O and C-H···Cl hydrogen bonding interactions (light blue dashed lines in Fig. 2; see Table 2 for geometric details). The water molecule of crystallization (O1W), which is only statistically present in 1/5 of the asymmetric units, accepts the hydrogen donation from adjacent C-H groups and also acts as hydrogen bond donor to Cl2 and O5 of neighboring molecules (violet dashed lines in Figure 2; see Table 2 for geometrical details). Even though the location of the water molecule permits its full site occupancy, we postulate that the absence of suitable hydrogen bonding partners in the binuclear complexes contributes significantly for its partial occupancy in the crystal structure.

Experimental
Chemicals were purchased from commercial sources and used as received. The compound [Mo(η 3 -C 3 H 5 )Cl(CO) 2 (di-t-Bubipy)] (1) was prepared following a literature method (Rodrigues et al., 2004). Thus, 70% aqueous tert-butylhydroperoxide (TBHP) (0.64 mL, 4.60 mmol) was added dropwise to a stirred solution of 1 (0.23 g, 0.46 mmol) in CH 3 CN (20 mL). After stirring at ambient temperature for 15 h, the resultant yellow solution was filtered off, concentrated, and a very pale yellow solid precipitated after the addition of n-hexane and diethyl ether. The precipitate was filtered, washed with n-hexane and diethyl ether, and vacuum-dried. Yield: 0.14 g, 69%.
The same product (as confirmed by a comparison of FT-IR and 1 H NMR spectra, and microanalysis data) was obtained by using a decane solution of TBHP (5-6 M, 10 equiv.) instead of the aqueous solution, with 1 dissolved in CH 2 Cl 2 under otherwise similar conditions (the excess of TBHP was destroyed with MnO 2 ). 6.28. The FT-IR and 1 H NMR spectral data were in agreement with published data (Arzoumanian et al., 2006). Suitable crystals were obtained by the slow diffusion of diethyl ether into a concentrated solution of the compound in CH 2 Cl 2 with a small layer of n-hexane.

Refinement
Hydrogen atoms bound to carbon were placed in idealized positions and were included in the final structural model in riding-motion approximation with C-H = 0.95 Å (aromatic C-H) and 0.98 Å (-CH 3 ). The isotropic thermal displacement parameters for these atoms were fixed at 1.2×U eq (aromatic C-H) or 1.5×U eq (-CH 3 ) of the respective parent carbon atoms.
One water molecule of crystallization was found to be partially occupied and was included in the final structural model with fixed rate of occupancy of 20% (calculated from unrestrained refinement for the site occupancy). Hydrogen atoms associated with this water molecule could not be located from difference Fourier maps and attempts to include these in calculated positions did not lead stable structural refinements. Nevertheless, the hydrogen atoms associated with this chemical entity have been included in the empirical formula of the title compound.
supplementary materials sup-3 Figures Fig. 1. Asymmetric unit of the title compound showing all non-hydrogen atoms represented as thermal ellipsoids drawn at the 50% probability level. The water molecule has a site occupancy factor = 0.20. Hydrogen atoms are represented as small spheres with arbitrary radii and the atomic labeling is provided for all non-hydrogen atoms. µ-Oxido-bis[chlorido(4,4'-di-tert-butyl-2,2'-bipyridineκ 2 N,N')dioxidomolybdenum (VI)