Tris(4,4′-di-tert-butyl-2,2′-bipyridine-κ2 N,N′)molybdenum(II) μ6-oxido-dodeca-μ2-oxido-hexaoxidohexamolybdate(VI) acetonitrile tetrasolvate

The asymmetric unit of the title compound, [Mo(C18H24N2)3][Mo6O19]·4CH3CN, comprises an [Mo(di-t-Bu-bipy)3]2+ cation (di-t-Bu-bipy is 4,4′-di-tert-butyl-2,2′-bipyridine), two halves of Lindqvist-type [Mo6O19]2− anions (with each anion completed by the application of a center of inversion) and four acetonitrile solvent molecules. The geometry around the metal atom of the cation resembles a distorted octahedron, with each of the three di-t-Bu-bipy ligands being almost planar [deviation from planarity < 6.3 (2)°]. Supramolecular interactions, namely Mo=O⋯π, C N⋯π, C—H⋯O and C—H⋯N, along with electrostatic forces, mediate the crystal packing. Two of the tert-butyl groups are affected by rotational disorder which was modeled over two distinct positions with major site occupancies of 0.707 (9) and 0.769 (8).


A-BÁ Á ÁC
The asymmetric unit consists of one [Mo(C 18 H 24 N 2 ) 3 )] 2+ cation {[Mo(di-t-Bu-bipy) 3 ] 2+ }, two halves of crystallographically independent centrosymmetric Lindqvist-type [Mo 6 O 19 ] 2anions and four acetonitrile molecules as depicted in Fig.   1. The two crystallographically independent anions are located around centers of inversion of the triclinic space group P1 which are coincident with the central µ 6 -oxo atom of each moiety (O4 and O14 in Fig. 1). The geometrical features observed for these chemical moieties are typical (Allcock et al., 1973) and will not be discussed any further in this crystallographic report. By contrast, the cation is to the best of our knowledge the second example of a coordination compound with general formula [M(di-t-Bu-bipy) 3 ] n+ , with the first example corresponding to a Ru 3+ structure (Schwalbe et al., 2008). The coordination geometry around Mo1 resembles a distorted octahedron with the Mo-N distances ranging from 2.090 (3) to 2.138 (3) Å. We note that these lengths are some of the shortest reported for a Mo-N distance, as revealed by a search in the Cambridge Structural Database for related compounds comprising molybdenum and 2,2'-bipyridine or its derivatives (Allen, 2002). The cis octahedral angles can be divided into two groups: while the bite angles related to the N,N'-chelating di-t-Bu-bipy range from 76.32 (12) to 77.40 (12)°, those involving two adjacent ligands range instead from 92.18 (12) to 97.09 (11)°. The trans octahedral angles were found in-between 167.87 (11) to 169.85 (12)°. The three crystallographically independent di-t-Bu-bipy ligands are almost planar, with the angles subtended by each pair of pyridine rings ranging from supplementary materials sup-2 1.41 (18) to 6.3 (2)°. In addition, the medium planes containing each di-t-Bu-bipy ligand are almost mutually perpendicular (angles ranging from 84.66 to 89.18 (11)°).
The crystal structure is rich in supramolecular contacts, among which some Mo═O···π, C≡N···π, C-H···O and C-H···N interactions are noteworthy (see Table 2 for details; interactions not shown). These contacts, along with the need to effectively fill the space mediated by electrostatic interactions, contribute to the crystal packing (Fig. 2).

Experimental
The title compound was isolated during the recrystallization in acetonitrile of the polynuclear complex [Mo 8 O 24 (di-t-Bubipy) 4 ] (1) (Amarante et al., 2010). Crystals of 1 were harvested and the supernatant solution was partially evaporated in vacuum. After two days, pink crystals of the title compound suitable for X-ray diffraction analysis were obtained.

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 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.
The largest peak and hole, 2.29 and -3.96 e . Å -3 , are located at 0.70 Å from Mo6 and 0.36 Å from Mo1, respectively. Fig. 1. Schematic representation of the chemical species composing the asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 30% probability level and the atomic labeling is provided for all non-hydrogen atoms belonging to the asymmetric unit. Hydrogen atoms are represented as small spheres with arbitrary radius. The rotational disorder associated with the tert-butyl moieties is depicted using different colors for each position. Symmetry operations used to complete the centrosymmetric POM anions: Tris(4,4'-di-tert-butyl-2,2'-bipyridineκ 2 N,N')molybdenum(II) µ 6 -oxido-dodeca-µ 2 -oxidohexaoxidohexamolybdate(VI) acetonitrile tetrasolvate Crystal data [Mo(C 18