Chemistry of o-xylidene–metal complexes. Part 3. Tungste o-xylidene complexes derived from tetrachloro(oxo)tungsteh(VI); X-ray crystal structures of [W(CH2C6H4CH2-o)3]·0.5C6H6and [{W(CH2C6H4CH2-o)2O}2Mg(C4H8O)4]

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Abstract
Reaction of WCl4O with the di-Grignard reagent o-C6H4(CH2MgCl)2 or the chloride-free ‘o-xylidene’ complex Mg(CH2C6H4CH2-o)(thf) in tetrahydrofuran (thf) yields either the thermally stable tris(chelate), [[graphic omitted]H2-o)3](7), or the WV complex [{[graphic omitted]H2-o)2[graphic omitted]g(thf)4](8)(a non-electrolyte in thf), in a manner critically dependent on reaction conditions. In both reactions a reduction step precedes alkylation, as determined by e.s.r., with probable intermediates being [WCl4O]–, [WCl5O]2–, and [WCl3O(thf)3]. The complex (7) undergoes reversible one-electron reduction as shown by cyclic voltammetry, E½ red=–1.68 V, and the WV radical anion, gav.= 2.005, is also accessible from (7) and sodium dihydronaphthylide. X-Ray analysis shows that the o-xylenediyl ligand (1) behaves as a chelating ligand in both (7) and (8), but in (8) the two ligands (1) are bonded differently. One is typically metallacyclic in nature, W–Cα 2.16(2)Å, W–Cα–C(aromatic) 109(1)° with a fold angle (Φ) of 42.4°, whereas the other has a larger Φ(66.1°) with α> 2.16(4)Å, and W–Cα–C(aromatic) 98(2) and 93(1)°, possibly arising from an aromatic π interaction with the tungsten atom. For the tris(chelate)(7) all ligands are equivalent, the tungsten symmetry is quasi-3/m, with a more pronounced W–π-arene interaction, possibly of a stabilising influence [W–C 2.21, W–C(aromatic) 2.49Å; W–C–C(aromatic) 82.7°, Φ= 83.3°]. In complex (8), the magnesium atom is octahedrally co-ordinated, with the four thf ligands disposed equatorially; the WO bond is short, 1.71(1)Å.

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