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]·0.5C6H6 and [{W(CH2C6H4CH2-o)2O}2Mg(C4H8O)4]

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 W^V 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, E_½^red=–1.68 V, and the W^V radical anion, g_av.= 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–C^α 2.16(2)Å, W–C^α–C(aromatic) 109(1)° with a fold angle (Φ) of 42.4°, whereas the other has a larger Φ(66.1°) with <W–C^α> 2.16(4)Å, and W–C^α–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.