Tetrakis(μ-pentafluorobenzoato-κ2 O:O′)bis[(tetrahydrofuran-κO)molybdenum(II)]

In the asymmetric unit of the title compound, [Mo2(C7F5O2)4(C4H8O)2], two independent half-molecules are present, which are completed by a crystallographically imposed center of inversion between the individual Mo atoms. In each molecule, four pentafluorobenzoate anions bridge the quadruply bonded Mo2 4+ unit that is, in addition, axially coordinated by two O atoms of tetrahydrofuran (THF) molecules. In the two independent molecules, the mean Mo—Mo bond length is 2.110 Å. Since the THF molecules are equally disordered over two sets of sites, there are four different Mo—O distances in both half-molecules with an overall mean of 2.542 Å. A zigzag chain is formed by π–π stacking interactions between pentafluorophenyl rings, indicated by a centroid–centroid distance of 3.7054 (11) Å and a centroid-to-plane distance of 3.4169 (3) Å. The extension of the unit gives a three-dimensional network structure with the THF molecules located in the voids.

In the asymmetric unit of the title compound, [Mo 2 (C 7 F 5 O 2 ) 4 -(C 4 H 8 O) 2 ], two independent half-molecules are present, which are completed by a crystallographically imposed center of inversion between the individual Mo atoms. In each molecule, four pentafluorobenzoate anions bridge the quadruply bonded Mo 2 4+ unit that is, in addition, axially coordinated by two O atoms of tetrahydrofuran (THF) molecules. In the two independent molecules, the mean Mo-Mo bond length is 2.110 Å . Since the THF molecules are equally disordered over two sets of sites, there are four different Mo-O distances in both half-molecules with an overall mean of 2.542 Å . A zigzag chain is formed bystacking interactions between pentafluorophenyl rings, indicated by a centroid-centroid distance of 3.7054 (11) Å and a centroid-to-plane distance of 3.4169 (3) Å . The extension of the unit gives a three-dimensional network structure with the THF molecules located in the voids.

Comment
Interactions between aromatic rings via π-π stacking are the basis of many phenomena with respect to organic material science and biological chemistry. In the last ten years, phenyl-phenyl (Carroll et al., 2008;Gung et al., 2005;McNeil et al., 2006;Sui & Glaser, 2006) and phenyl-perfluorophenyl (Vangala et al., 2002;Woody et al., 2007;Xu et al., 2008;Zhu et al., 2005) interactions have been widely discussed in terms of supermolecular chemistry, but there appear to be few examples about the interactions between perfluorophenyl rings (Adams et al., 2001;Hair et al., 2003;Liu et al., 2003). In the present study, π-π stacking interactions between the perfluorophenyl rings of the quadruply bonded dimetal paddlewheel molecule were investigated.
In the title compound, [Mo 2 (OOCC 6 F 5 ) 4 (C 4 H 8 O) 2 ], two independent half-molecules are present in the asymmetric unit, which are completed by a crystallographically imposed center of inversion between each of the Mo atoms. Each [Mo 2 (OOCC 6 F 5 ) 4 (C 4 H 8 O) 2 ] molecule has a paddle-wheel-type structure. There are four pentafluoro-benzoate (OOCC 6 F 5 ) groups surrounding the quadruply bonded Mo 2 4+ unit, that is additionally axially coordinated by two oxygen atoms of THF molecules. In the two independent molecules, the Mo-Mo bond lengths are 2.1090 Å and 2.1101 Å, with a mean of 2.110 Å. Since the THF molecules are equally disordered over two sets of sites, there are four Mo-O distances to the THF molecules in both half-molecules (Table 1), with an overall mean of 2.542 Å. The molecular structure of one of the two molecules is shown in Fig. 1. The torsion angles between the C 6 F 5 group and the connected chelating ring (Mo 2 OCO) range from -28.1 (5) ° to 41.7 (5) ° because of the O···F repulsion within the pentafluoro-benzonate anion (Reddy et al., 2004;Bach et al., 2001).
In the crystal, the extension of the unit gives a three-dimensional network structure ( Fig. 3) with the THF molecules situated in the voids (Fig. 4).

Experimental
Mo(CO) 6 (0.422 g, 1.60 mmol), pentafluorobenzoic acid (0.890 g, 4.20 mmol) and THF (2 ml) were mixed in 1,2-dichlorobenzene (6 ml) in a Schlenk flask equipped with a condenser. The mixture was then heated at 453 K for 24 h, during which a dark solution developed. After the reaction was cooled to room temperature, the THF was evaporated under reduced pressure, giving a yellow suspension. The mother liquor was decanted and the solid was washed first with dichloromethane (ca 8 × 2 ml), then with hexanes (ca 10 × 2 ml). The yellow solid product Mo 2 (O 2 CC 6 F 5 ) 4 was dried in vacuo. Yield: 0.68 g