The chemistry of (fulvalene)dimolybdenum hexacarbonyl: A rigidly held dinuclear transition-metal complex

Article abstract of DOI:10.1021/om00133a019

The X-ray structure of (fulvalene)dimolybdenum hexacarbonyl [FvMo₂(CO)₆] has been determined. Thermal, photolytic, and chemical treatment failed to yield the metal-metal triply bonded complex FvMo₂(CO)₄, either because it was not formed or because of its instability; this result may have been caused by excessive bond strain in the fulvalene ligand. Photolysis of FvMo₂(CO)₆ in the presence of alkynes afforded mono- and bis(alkyne) complexes FvMo₂(CO)₄(RC≡CR) and FvMo₂(CO)₃(RC≡CR)₂. Spectroscopic evidence indicated that the former bears as a ligand a μ-η²-alkyne and that this ligand partially moved about the Mo-Mo bond in a rapid fluxional process: ΔG^≠ = 15 ± 0.5 kcal mol^-1. The bis(alkyne) systems contained uncoupled alkynes as determined by an X-ray diffraction study of one complex. This result is unusual in light of the frequent occurrence of alkyne coupling at a dinuclear center. Reduction of FvMo₂(CO)₆ with Na-Hg or LiBEt₃H furnished the dianion FvMo₂(CO)₆^2-. The latter reacted with protic acids and haloalkanes to give the respective dihydride FvMo₂(CO)₆H₂ and dialkyl complexes FvMo₂(CO)₆R₂ (R = CH₃, CH₂Ph, CH₂OCH₃). At 20°C the dihydride eliminated H₂ with formation of FvMo₂(CO)₆. Treatment of FvMo₂(CO)₆(CH₂OCH₃)₂ with HBF₄·(Et₂O) at -20°C (CD₂Cl₂) produced FvMo₂(CO)₆(CH₂OCH₃)(=CH ₂)⁺, as determined by ¹H NMR spectroscopy. Warming to 0°C gave the carbene-coupling product FvMo₂(CO)₆(C₂H₄)²⁺. The bis(carbene) FvMo₂(CO)₆(=CH₂)₂²⁺ was not detected. The dianion FvMo₂(CO)₆^2- reacted with I(CH₂)₃I to give a metal-metal bonded 1-oxacyclopent-2-ylidene complex. An X-ray diffraction analysis showed that the Fischer-type carbene ligand was terminally bound. This complex exhibited fluxional behavior which may involve a bridging carbene species (ΔG^≠ = 18 ± 0.5 kcal mol^-1). Thermolysis (100°C) led to efficient generation of propene and FvMo₂(CO)₆ by a novel pathway.