A new mono-functionalized organoimido hexamolybdate derivative: bis(tetra-n-butylammonium) (5-chloro-2-methylphenylimido)-μ6-oxido-dodeca-μ2-oxido-pentaoxidohexamolybdate(VI)

The title complex, [(C4H9)4N]2[Mo6(C7H6ClN)O18], was prepared by the reaction of (Bu4N)4[α-Mo8O26] and 2-methyl-5-chloroaniline hydrochloride with N,N′-dicyclohexylcarbodiimide as dehydrating agent in dry acetonitrile solution. The arylimido ligand is linked to an Mo atom of the Lindqvist-type hexamolybdate anion by an Mo N triple bond, with a bond length of 1.732 (4) Å and an Mo N—C bond angle of 169.1 (4)°, typical for monodentate imido groups in such hybrid complexes. Due to the interaction between one H atom in the aryl group and an O atom of a symmetry-related hexamolybdate cluster, the anions form centrosymmetric dimers in the crystal structure. Weak C—H⋯O contacts are observed between the cations and anions. Unresolved disorder in some of the butyl chains of the ammonium cation is noted.

The title complex, [(C 4 H 9 ) 4 N] 2 [Mo 6 (C 7 H 6 ClN)O 18 ], was prepared by the reaction of (Bu 4 N) 4  ] and 2methyl-5-chloroaniline hydrochloride with N,N 0 -dicyclohexylcarbodiimide as dehydrating agent in dry acetonitrile solution. The arylimido ligand is linked to an Mo atom of the Lindqvisttype hexamolybdate anion by an Mo N triple bond, with a bond length of 1.732 (4) Å and an Mo N-C bond angle of 169.1 (4) , typical for monodentate imido groups in such hybrid complexes. Due to the interaction between one H atom in the aryl group and an O atom of a symmetry-related hexamolybdate cluster, the anions form centrosymmetric dimers in the crystal structure. Weak C-HÁ Á ÁO contacts are observed between the cations and anions. Unresolved disorder in some of the butyl chains of the ammonium cation is noted.
Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL. This work was supported by the Beijing Forestry University Young Scientist Fund. We also sincerely thank Professor Yongge Wei of Tsinghua University for fruitful discussions and his helpful suggestions in conducting this work and writing this paper.
polyoxometalates are very useful in the construction of various POM-organic hybrids, which are more easily accessible and much cheaper than the corresponding iodide and bromide derivatives. Chloro derivatives are reactive functional groups, which are very useful in some organic syntheses. Recently, we have obtained three chloro-functionalized arylimido derivatives of hexamolybdate, (Bu 4 N) 2 [Mo 6 O 18 (≡NR)] (R = p-ClC 6 H 4 , m-ClC 6 H 4 , and o-ClC 6 H 4 ) in moderate yields (Li et al., 2008). However, they are not stable enough to undergo reactions in the subsequent synthesis process due to the electron-withdrawing nature of the chloro group and lack of protection of the Mo≡N bond, which is easily hydrolyzed in acid or alkaline media. In order to obtain more stable building blocks to construct novel POM-based organic-inorganic hybrids, we probed 2-CH 3 -5-ClC 6 H 3 NH 2 as a ligand to modify the hexamolybdate ion, in which a methyl group ortho to the imido N atom on the benzene ring not only increases the stability of the resulting imido derivative, but also improves the yield of the synthesis. X-ray diffraction analysis reveals that the title compound crystallizes in the monoclinic space group P2 1 /n. The asymmetric unit contains one crystallographically independent [Mo 6 O 18 N(2-CH 3 -5-ClC 6 H 3 )] 2anion and two (C 4 H 9 ) 4 N + cations ( Fig. 1). In the [Mo 6 O 18 N(2-CH 3 -5-ClC 6 H 3 )] 2anion, an arylimido ligand is bound to one terminal position at the hexamolybdate cluster in a monodentate fashion. The short Mo-N bond distance, 1.732 (4) Å, and approximately linear C-N-Mo angle, 169.1 (4)°, are typical of organoimido groups bonded at an octahedral d 0 metal center, and are consistent with a substantial degree of Mo≡N triple bond character (Wigley, 1994). Compared to the reported chlorofunctionalized arylimido derivatives of hexamolybdate (Li et al., 2008), the length of the Mo-N triple bond becomes larger (> 1.70 Å), and the C-N-Mo bond angle is closer to 180°, as found in other imido derivatives of Lindqvist polyoxometalates. The bond lengths of the five terminal oxo ligands do not vary significantly in comparison with the parent hexamolybdate and other derivatives. The central µ 6 -O atom O18 is displaced towards Mo4, as a consequence of the substitution of Mo4 by the arylimido ligand. Similar contraction has also been observed in the structures of other organoimido derivatives of Lindqvist polyoxometalates . Considerable variations are seen in the bond lengths involving the doubly bridging O atoms, which is again consistent with other imido derivatives of Lindqvist polyoxometalates ).
An interesting feature is the solid phase dimerization of the cluster anions of the title compound, through C-H···O hydrogen bonds between a H atom on the aromatic ring and a bridging O atom in a symmetry-related anion (Fig. 2). Such a structural feature has also been observed before in the reported phenylimido derivatives of hexamolybdate .

S2. Experimental
A mixture of (Bu 4 N) 4 [α-Mo 8 O 26 ] (1.0 mmol), DCC (2.1 mmol), and 2-methyl-5-chloroaniline hydrochloride (1.34 mmol) was refluxed in anhydrous acetonitrile (10 ml) for about 12 h. After being cooled down to room temperature, the resulting dark-red solution was filtrated to remove the white precipitates. While most of acetonitrile evaporated, the product was collected from the filtrate as a red crystalline solid, and was washed successively with ethanol and ether several times, and then was recrystallized twice from a mixture of acetone and ethanol (1:1), to get red crystals (yield: 85 to 95%).
Single crystals used for X-ray diffraction were obtained by diffusion of ether into a solution of the title compound in acetone.