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

Ana C Gomes; José A Fernandes; Carla A Gamelas; Isabel S Gonçalves; Filipe A Almeida Paz

doi:10.1107/S1600536811046952

. 2011 Nov 12;67(Pt 12):m1738–m1739. doi: 10.1107/S1600536811046952

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

Ana C Gomes ^a, José A Fernandes ^a, Carla A Gamelas ^b, Isabel S Gonçalves ^a, Filipe A Almeida Paz ^a,^*

PMCID: PMC3238648 PMID: 22199539

Abstract

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.

Related literature

For general background to dioxidomolybdenum(VI) complexes, see: Arzoumanian et al. (2006 ▶); Jeyakumar & Chand (2009 ▶); Kühn et al. (2002 ▶); Rodrigues et al. (2004 ▶). For studies on molybdenum complexes from our research groups, see: Coelho et al. (2011 ▶); Fernandes et al. (2011a ▶,b ▶, 2011 ▶); Gago et al. (2009 ▶); Nunes et al. (2003 ▶); Pereira et al. (2007 ▶).

Experimental

Crystal data

[Mo₂Cl₂O₅(C₁₈H₂₄N₂)₂]·0.2H₂O
M _r = 883.17
Monoclinic,
a = 16.9997 (7) Å
b = 12.7444 (6) Å
c = 18.4609 (8) Å
β = 99.582 (2)°
V = 3943.8 (3) Å³
Z = 4
Mo Kα radiation
μ = 0.82 mm⁻¹
T = 150 K
0.08 × 0.06 × 0.03 mm

Data collection

Bruker X8 KappaCCD APEXII diffractometer
Absorption correction: multi-scan (SADABS; Sheldrick, 1997 ▶) T _min = 0.938, T _max = 0.976
54239 measured reflections
10578 independent reflections
7469 reflections with I > 2σ(I)
R _int = 0.050

Refinement

R[F ² > 2σ(F ²)] = 0.040
wR(F ²) = 0.086
S = 1.02
10578 reflections
463 parameters
18 restraints
H-atom parameters constrained
Δρ_max = 0.96 e Å⁻³
Δρ_min = −0.67 e Å⁻³

Data collection: APEX2 (Bruker, 2006 ▶); cell refinement: SAINT-Plus (Bruker, 2005 ▶); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg, 2009 ▶); software used to prepare material for publication: SHELXTL.

Supplementary Material

Crystal structure: contains datablock(s) global, I. DOI: 10.1107/S1600536811046952/tk5013sup1.cif

e-67-m1738-sup1.cif^{(33.6KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536811046952/tk5013Isup2.hkl

e-67-m1738-Isup2.hkl^{(517.3KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

Mo1—O1	1.8920 (19)
Mo1—O2	1.6972 (19)
Mo1—O3	1.696 (2)
Mo1—N1	2.330 (2)
Mo1—N2	2.323 (2)
Mo1—Cl1	2.4895 (8)
Mo2—O1	1.9274 (19)
Mo2—O4	1.6975 (19)
Mo2—O5	1.694 (2)
Mo2—N3	2.328 (2)
Mo2—N4	2.304 (2)
Mo2—Cl2	2.4283 (8)

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Table 2. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C27—H27⋯O1ⁱ	0.95	2.52	3.341 (3)	145
C34—H34A⋯Cl1ⁱⁱ	0.98	2.77	3.748 (4)	174
C35—H35A⋯O4ⁱ	0.98	2.54	3.421 (4)	149
C12—H12C⋯O1W	0.98	2.69	3.641 (16)	163
C18—H18B⋯O1W	0.98	2.10	2.970 (18)	147
O1W⋯Cl2ⁱ			3.573 (18)
O1W⋯O5ⁱⁱⁱ			3.236 (17)

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Symmetry codes: (i) Inline graphic ; (ii) ; (iii) .

Acknowledgments

We are grateful to the Fundação para a Ciência e a Tecnologia (FCT/FEDER and POCI, Portugal) for their general financial support to CICECO, and for the post-doctoral 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.

supplementary crystallographic information

Comment

Dioxomolybdenum(VI) complexes are known to be highly active catalysts in the epoxidation of olefins (Kühn et al., 2002; Jeyakumar & Chand, 2009). In these complexes the active metal-oxo functional group may appear with two distinct structural 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,b, 2011) we have recently described the synthesis and structural details of the oxo-µ-oxo complexes [Mo₂O₄(µ-O)Cl₂(DMF)₄] (Gago et al., 2009), [Mo₂O₄(µ-O)Cl₂(pyrazole)₄] (Pereira et al., 2007), and [Mo₂O₄(µ-O)Cl₂(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 [Mo₂O₄(µ-O)Cl₂(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, C₃₆H₄₈Cl₂Mo₂N₄O₅, 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 Mo═O groups: while the Mo—O_bridge distances are 1.8920 (19) and 1.9274 (19) Å, the Mo—O_terminal distances range from 1.694 (2) to 1.6975 (19) Å; the Mo—Cl distances are 2.4895 (8) and 2.4283 (8) Å and the Mo—N bonds range from 2.304 (2) to 2.330 (2) Å. The cis and trans octahedral angles are in the ranges of 68.95 (8) to 107.35 (10)° and 157.51 (6) to 160.69 (9)°, respectively. The Mo1—O1—Mo2 kink angle through the µ-oxo bridge is 143.50 (10)° which, to the best of our knowledge, constitutes the smallest reported to date for related binuclear dioxomolybdenum(VI) complexes: the analogous value for [Mo₂O₄(µ-O)Cl₂(DMF)₄] is ca 175° and that for [Mo₂O₄(µ-O)Cl₂(pyrazole)₄] is ca 151°, and those for the two conformers of [Mo₂O₄(µ-O)Cl₂(PzPy)₂] are ca 156 and 180°. We attribute this structural feature to the considerable steric hindrance associated with the di-t-Bu-bipy moieties, mostly due to the pendant —CH₃ 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 [Mo₂O₄(µ-O)Cl₂(PzPy)₂]. Noteworthy, the torsion angles N1—Mo1···Mo2—N4 and N2—Mo1···Mo2—N3 are -18.40 (7) and -157.03 (9)°, respectively.

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(η³-C₃H₅)Cl(CO)₂(di-t-Bu-bipy)] (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 CH₃CN (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 ¹H 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 CH₂Cl₂ under otherwise similar conditions (the excess of TBHP was destroyed with MnO₂).

Anal. Calcd. for C₃₆H₄₈N₄Cl₂Mo₂O₅.0.2H₂O (in %): C, 48.96; H, 5.52; N, 6.34. Found (in %): C, 49.47; H, 5.52; N, 6.28. The FT–IR and ¹H 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 CH₂Cl₂ with a small layer of n-hexane.