Insertion and oxidative addition reactions of anionic group 6 complexes

Abstract

The anionic group 6 alkyl and aryl complexes cis-RM(CO)₄L⁻ (where M = W or Cr; R = Me, Et, or Ph; L = CO, P(OMe)₃, or PMe₃) have been shown to perform insertion of CO₂, CO, and SO₂ into the metal-alkyl (-aryl) bond. The rates of CO₂ and CO insertion have been measured under a variety of pressures. Carboxylation was found to be first order in both CO₂ and metal complex concentration while carbonylation displayed a mixed order dependence on CO with the rate becoming independent of CO at high pressures. The presence of CO was found to have no effect on the rate of carboxylation, indicating that CO₂ inserts via an associative interchange pathway without prior loss of a carbonyl ligand. The rate of CO insertion was observed to be highly dependent on the electron donating ability, and hence the metal-carbon bond strength, of the alkyl group while CO₂ insertion rates varied only slightly with these changes. From a comparison of W-CH$sb3$ bond distances and CO₂ insertion rates for both W(CO)₅CH₃⁻ and cis-CH₃W(CO)₄PMe₃, it was determined that the amount of electron density at the metal center is a more important factor than metal-carbon strength. Insertion of SO₂ into the metal-carbon bond of these alkyl complexes was found to be very fast in THF solution leading to the formation of S-sulfinato complexes. The silyl and stannyl complexes M(CO)₅M'R₃⁻ (where M'R₃ = SiMe₃, SiEt₃, SnMe₃, or SnPh₃) were found to be much more inert to insertion reactions than the alkyl analogs. The tungsten-tin bond was determined to be cleaved by SO₂ to form an S-sulfinato complex through a spectroscopically observable oxygen bound intermediate. Sulfur dioxide was also found to insert into the tungsten-silicon bond in a similar fashion. The x-ray crystal structures of both [PPN] - [W(CO)₅SiMe₃] and [PPN] [W(CO)₅SnMe₃] have been determined. The complexes M(CO)₅X⁻ (M = Mo or W; X = Cl, O₂CCH₃, or O₂CC₆H₅) have been found to undergo oxidative addition by allyl halides and carboxylates with cleavage of the allyl carbon-halogen or -oxygen bond. The products were discovered to be π-allyl dimers containing bridging halogen or carboxylate groups. A mechanism accounting for the formation of these complexes has been formulated. The x-ray crystal structure of two of these compounds has also been solved.