Reactions of co-ordinated ligands. Part 55. Molybdenummediated addition of alkynes to cyclooctatetraene; one-electron oxidation and protonation of η3-allyl(η4-1,3-diene)(η-cyclopentadienyl or η5-indenyl) molybdenum complexes, and crystal structures of [Mo(η3,η4-C12H13)(η-C5H5)], [Mo(η2,η4-C14H17)(η-C5H5)][BF4] and [Mo(η2,η4-C12H13)(µMo,C-H)(η5-C9H7)][CF3SO3]

Abstract

Reaction of [Mo(NCMe)(η²-MeC₂Me)₂(η-C₅H₅)][BF₄] with cyclooctatetraene (cot) in acetonitrile in the presence of the proton scavenger Na₂CO₃ affords the neutral complex [Mo(η³,η⁴-C₁₂H₁₃)(η-C₅H₅)]1. Analogous reactions between cot and [Mo(NCMe)(η²-EtC₂Et)₂(η-C₅H₅)][BF₄], [Mo(NCMe)(η²-MeC₂Me)₂(η⁵-C₉H₇)][BF₄] and [Mo(NCMe)(η²-EtC₂Et)₂(η⁵-C₉H₇)][BF₄] give [Mo(η³,η⁴-C₁₄H₁₇)(η-C₅H₅)]2, [Mo(η³,η⁴-C₁₂H₁₃)(η⁵-C₉H₇)]3, and [Mo(η³,η⁴-C₁₄H₁₇)(η⁵-C₉H₇)]4 in good yield. A single-crystal X-ray diffraction study on 1 established the presence of an unusual bicyclo[4.2.2]decane ligand, to which a Mo(η-C₅H₅) fragment is η³,η⁴-bonded. Although this structure suggests that these complexes are formed via a (6 + 2)π addition reaction, it is proposed that a stepwise process is involved. The chemical reactivity of the complexes 1–4 has been explored. Reaction of 1 or 3 with [CPh₃][BF₄] affords respectively the cations [Mo(η⁴,η⁴-C₁₂H₁₂)(η-C₅H₅)][BF₄]7 and [Mo(η⁴,η⁴-C₁₂H₁₂)(η⁵-C₉H₇)][BF₄]8. Reaction of these bis(1,3-diene) cations with the nucleophiles ‘H^–’ and ‘Ph^–’ leads to regioselective attack on the exo-η⁴-1,3-diene moiety. Treatment of 1, 2 and 3 with AgBF₄ leads to a one-electron oxidation reaction and formation of the 17-electron species [Mo(C₁₂H₁₃)(η-C₅H₅)][BF₄]10, [Mo(C₁₄H₁₇)(η-C₅H₅)][BF₄]11, and [Mo(C₁₂H₁₃)(η⁵-C₉H₇)[BF₄]12. A single crystal X-ray diffraction study on 11 indicates the absence of agostic Mo(µ-H)C interactions, the radical cation being best described as a carbon-centred radical η²,η⁴-bonded to a 16-electron cationic molybdenum centre. Finally, the protonation of 1–4 has been studied, being shown to afford cationic species which adopt different structures in the solid state and in solution. An X-ray crystallographic study on [Mo(η⁶-C₁₂H₁₃)(µ_Mo,C-H)(η⁵-C₉H₇)][CF₃SO₃], which is formed on reaction of 3 with CF₃SO₃H, established the presence of an agostic Mo(µ-H)C interaction between a carbon–hydrogen bond β to an alkene moiety, representing an arrested 1,3-hydrogen shift of the type postulated in the initial stage of a metal-assisted isomerisation of an alkene.