Crystal structure of heptakis(2,6-dimethylphenyl isocyanide-κC)vanadium(I) iodide

The complex cation and the two crystallographically different iodide anions, each located on a different glide plane, are well separated in the crystal structure. The V(CN)7 core of the cation has the form of a distorted monocapped trigonal prism. This compound is of interest as the first isolable homoleptic seven-coordinate vanadium analog of the 18-electron [V(CO)7]+ monocation.

Oxidation of [M(CO) 6 ] À (M = Nb, Ta) with one equivalent of I 2 in the presence of excess CNXyl gives the 18-electron uncharged molecular complexes M(CNXyl) 6 I bearing only six isocyanide ligands (Fig.1, see: Barybin et al., 2007). Anionexchange reaction between [Ta(CNXyl) 7 ] + [BF 4 ] À and [Bu 4 N] + I À leads to the loss of one CNXyl ligand and to formation of Ta(CNXyl) 6 I ( Barybin et al., 2007). Rehder et al. (1999) have isolated only trans-V(CNXyl) 4 I 2 being formed in a similar oxidation reaction from [V(CO) 6 ] À . We report herein that the 18-electron intermediate ionic complex [V(CNXyl) 7 ] + I À , which is formed and stable at low temperatures, can be isolated from the last reaction (Fig. 1). It is soluble in THF but insoluble in toluene. At room temperature in THF, it completely decomposes during seven to ten days to produce trans-V(CNXyl) 4 I 2 (based on X-ray, NMR and IR data), free CNXyl and V(CNXyl) 6 (based on NMR and IR studies).
The V-C distances vary from 2.002 (4) to 2.062 (4) Å , with the exception for the capping ligand which is associated with the longest bond, V1-C55 = 2.107 (3) Å . For the six ligands, the C N triple-bond lengths lie in a very narrow interval from 1.160 (4) to 1.165 (4) Å . The value for the capping ligand is 1.152 (4) Å (C55-N7). At the same time, all C N distances are about the same, as in most free isocyanides (1.14 to 1.16 Å ) found in the Cambridge Structural Database (Groom & Allen, 2014). The V-C N angles are nearly linear (see: The molecular structure of [V(CNXyl) 7 ] + I À with displacement parameters drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

Figure 3
The V(CN) 7 core of the cation is a monocapped trigonal prism. 2,6-Dimethylphenyl groups are not shown. nitrogen atoms with the C N-C angles between 161.0 (3) and 178.1 (4) . It should be noted that the C N bond length and the C N-C angle of the capping ligand (C55 N7-Xyl) correspond to a nearly unperturbed isocyanide molecule.
Two cationic halogen isocyanide complexes are known: a 15-electron vanadium(II) complex [V(CN t Bu) 5 I] + I À (ZASFII, Bö ttcher et al., 1995) and a 18-electron niobium(III) complex [Nb(CN t Bu) 6 I 2 ] + I À (thf) (RARHOH, Collazo et al., 1996). in THF (65 ml), keeping the reaction mixture temperature below 203 K during addition. The resulting mixture was stirred for five minutes at 198 to 201 K. A solution of CNXyl (3.09 g, 23.6 mmol) in THF (50 ml) was added to the cold stirred reaction mixture, keeping its temperature below 208 K. The red reaction mixture was stirred overnight at 208 K. Then the mixture was allowed to warm up and filtered at room temperature. The white filter cake ([Et 4 N]I) was washed with THF (2 Â 10 ml). All but ca 10 ml of THF was evaporated from the resulting solution under reduced pressure. Toluene (200 ml) was added to the residue, and the mixture was stirred at room temperature for several minutes. The dark-red precipitate was filtered off, washed with toluene (3 Â 10 ml) and dried under dynamic vacuum. Red microcrystalline [V(CNXyl) 7 ]I was obtained in 54% yield (1.751 g, 1.60 mmol). IR (Nujol mull): CN 2142 w, 2101 m sh, 2057 m sh, 2016 vs br, 1974 s cm À1 .

Synthesis and crystallization
Most toluene was evaporated from the remaining toluene solution to give previously studied green single crystals of [V(CNXyl) 4 I 2 ](thf). For its crystal and molecular structure, see: Rehder et al. (1999).
A nearly saturated THF solution (at room temperature) of [V(CNXyl) 7 ]I (ca 20 ml) was placed into one ampoule of an H-shaped Schlenk vessel. Some reduced pressure was formed inside the vessel. Most solvent was slowly evaporated from the solution during eight hours into the second ampoule by cooling it with cold isopropyl alcohol (initial temperature was 223 K), producing several red single crystals of the title compound inside the first ampoule. The crystals were cut into smaller pieces prior to X-ray studies.