Tetrakis[μ-N,N′-bis(4-bromophenyl)formamidinato-κ2 N:N′]dimolybdenum(II) tetrahydrofuran solvate

The title complex, [Mo2(C13H9N2Br2)4]·C4H8O, contains a quadruply bonded Mo2 4+ unit equatorially coordinated by four N,N′-bis(4-bromophenyl)formamidinate ligands, forming a dimetal paddlewheel complex. The centroid of the Mo—Mo bond is located on a special position with 2/m symmetry. In the crystal, complex molecules are linked by Br⋯Br interactions [3.7049 (10) Å]. The disordered solvent molecule could not be satisfactorily modelled and was therefore eliminated from the final refinement.

The title complex, [Mo 2 (C 13 H 9 N 2 Br 2 ) 4 ]ÁC 4 H 8 O, contains a quadruply bonded Mo 2 4+ unit equatorially coordinated by four N,N 0 -bis(4-bromophenyl)formamidinate ligands, forming a dimetal paddlewheel complex. The centroid of the Mo-Mo bond is located on a special position with 2/m symmetry. In the crystal, complex molecules are linked by BrÁ Á ÁBr interactions [3.7049 (10) Å ]. The disordered solvent molecule could not be satisfactorily modelled and was therefore eliminated from the final refinement.

Comment
One of the main interests of crystal engineering is the study of intermolecular interactions and their utilization in supramolecular synthesis (Desiraju 1995;Desiraju 2001;Brammer 2004). These interactions range from strong forces, e.g., classical hydrogen bonds, to weaker ones, e.g., halogen···halogen interactions. The nature of the halogen···halogen interactions has been studied both through extensive crystallographic investigation and using ab initio calculations (Domercq et al.,2001;Espallargas et al., 2006). Here we report intermolecular Br···Br interactions in the crystal structure Mo 2 (C 13 H 9 N 2 Br 2 ) 4 .THF.
The molecular structure of the title compound is shown in Fig.1. The molecule of Mo 2 (C 13 H 9 N 2 Br 2 ) 4 (I) occupies a special position on an inversion center, and the Mo-Mo distance is 2.1263 (13) Å, which is in the range of dimolybdenum quadruple bonds. Bromine atoms participate in short contacts (3.7049 (10) Å) linking the molecules of (I) into planes. This value is significantly shorter than van der Waals contact distance (3.90 Å) (Reddy et al., 1996;Fujiwara et al., 2006).

Experimental
A mixture yellow Mo 2 (OOCCH 3 ) 4 (0.128 g, 0.300 mmol) and N,N'-bis(4-bromophenyl)formamidinate (0.425 g, 1.20 mmol) was suspended in 20 ml of THF. While stirring, 2.4 ml NaOCH 2 CH 3 solution (0.5 M in ethanol) was added slowly. The colour turned first to red and then to dark red. The reaction was stirred for 5 h at room temperature, and then the volume of the solvent was reduced to about 3 ml under reduced pressure. The residue was washed with distilled water(3×10 ml) and ethanol (8 ml) and dried under vacuum. The yellow solid was dissolved in THF (15 ml) and the solution was layered with hexanes. Yellow block-shaped crystals formed after several days. Yield: 0.342 g (71%

Refinement
H atoms were positioned geometrically with C-H = 0.93, 0.97 and 0.96 Å, for aromatic, methylene, and methyl H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = xU eq (C), where x = 1.5 for methyl H and x = 1.2 for aromatic H atoms.
A search for solvent-accessible voids in the crystal structure using PLATON (Spek, 2009) showed a potential solvent volume of 829.4 Å 3 and subsequent application of SQUEEZE procedures showed four relevant voids each with a solventaccessible volume of 207 Å 3 . The SQUEEZE procedure was used to eliminate the contribution of the electron density in the solvent region from the intensity data, and the solvent-free model was employed in the final refinement.  Fig. 1. Molecular structure of the title compound drawn with displacement ellipsoids at the 30% probability level. All hydrogen atoms have been omitted for clarity. Atoms with suffix A are generated by the symmetry operation(-x + 1/2, -y + 1/2, -z).