Synthesis and Structure of a Tungsten Dimer with a W-W Bond Order of 3.5

Reaction of W2(mhp)4, where mhp is the anion of 2-hydroxy-6- methylpyridine, with AICI3 in refluxing diglyme provides the new paramagnetic compound W2Cl2(mhp)3 • CH2CI2 in 67 percent yield after recrystallization from dichloromethane. The source of the oxidant for oxidation of tungsten from W(2.0) to W(2.5) was not identified. Orange-brown crystals of the new compound belong to the noncentric orthorhombic space group P2I2424 with a = 13.150(6), b = 21.24(1) and c = 8.738(4) Å, Z = 4. Refinement of the structure provided R(RW) = 0.058(0.080) and a W-W bond distance of 2.214(2) Å, 0.053 Å longer than that in W2(mhp)4 • CH2CI2. The W-W bond has a bond order of 3.5 corresponding to the bonding orbital population a27i481. Strong bands arising from W-Cl stretching modes occur at 321 and 289 cm-1 in the IR spectra, and a weak band at 782 nm (e ~ 600 M_1cm_1) in the UV-visible spectrum may correspond to the 8 —> 8 electronic transition. The EPR spectrum shows a strong, nearly symmetric, absorption with g = 1.842. Cyclic voltammetry in dichloromethane reveals a quasi-reversible reduction at Ey2 = —0.7 V vs. SSCE and an irreversible oxidation at +0.3 V