Synthesis and Reactivity of Ditungsten μ-Carbene Complexes: X-ray Crystal Structure of W₂(CO)₉[μ-ŋ¹,ŋ³-C(OCH_a)C=CH(CH₃),CH₂]

A series of ditungsten complexes, W₂(CO)₉[μ,ŋ¹,ŋ³-C(OCH₃)C(R¹)=CHR²] (3), containing μ-ŋ,ŋ³-allylidene ligands have been prepared in moderate to excellent yields from Fischer-type α,β-unsaturated tungsten carbene complexes, (CO)₅W[C(OCH₃)C(R¹)=^:CHR²] (1) and (CO)₅W•THF (2). These complexes (3) we're purified on silica gel at -20 to -30 °C followed by crystallization to produce, in most cases, dark red crystalline materials. All new compounds were characterized by elemental analyses and ¹ H and ¹³C NMR spectroscopy. When W₂(CO)₉[ŋ¹,ŋ³-C(OCH₃)C=CH(CH₂)₂CH₂] (3d) was treated with CO in C₆D₆ solution, it was cleanly converted to the cyclopentenyl Fischer-carbenecomplex (CO)₅W[C(OCH₃)(C=CH(CH₂)₂CH₂] (1d) and W(CO)₆. W₂(CO)₉[μ-ŋ¹,ŋ³-C(OCH₃)C=CH(CH₂)₅CH₂] (3f), which contains a cyclooctene ring as part of the μ-ŋ¹,ŋ³-allylidene ligand, was further characterized by variable-temperature ¹³C NMR spectroscopy and single-crystal X-ray diffraction methods. Complex 3f crystallizes in the monoclinic space group P2₁/c with (at -150 °C) a = 9.832 (4) Å, b = 10.424 (3) Å, c = 21.120 (6) Å, β = 93.37 (3)°, and D_calcd = 2.37 g cm^-3 for Z = 4. Least-squares refinement based on 3394 independent observed [F_o > 5σ(F₀)] reflections led to a final conventional R value of 0.038.