Organic chemistry of dinuclear metal centres. Part 3. µ-Carbene complexes of iron and ruthenium from alkynes viaµ-vinyl cations
Abstract
Protonation of the complexes [M2(CO)(µ-CO){µ-C(O)C2R2}(η-C5H5)2][M = Fe or Ru; R2= H2, Ph2, H(Me), or H(Ph)] with HBF4·OEt2 results in rapid carbon–carbon bond cleavage and formation of the µ-vinyl cations [M2(CO)2(µ-CO){µ-C(R)![[double bond, length half m-dash]](https://www.rsc.org/images/entities/char_e006.gif)
C(H)R}(η-C5H5)2]+, containing a cis arrangement of R groups. Addition of HBF4·OEt2 to [Ru2(CO)(µ-CO){µ-C(O)C2Me2}(η-C5H5)2] produces the cation [Ru2(CO)(µ-CO)({µ-C(H)(O)C2Me2}(η-C5H5)2]+, which isomerises slowly to [Ru2(CO)2(µ-CO){µ-C(Me)C(H)Me}(η-C5H5)2]+. The µ-vinyl cations exist in solution as isomers with cis and trans orientations of terminal ligands, shown by variable-temperature n.m.r. to interconvert; cis isomers additionally display a fluxional oscillation of the µ-vinyl ligand. Treatment of the cations with Na BH4 yields µ-carbene complexes [M2(CO)2(µ-CO){µ-C(R)CH2R}(η-C5H5)2] in good yield, also as cis and trans isomers which interconvert in solution. These result from hydride addition to the β-carbon of the µ-vinyl, but addition to the α-carbon is apparent in the low-yield co-formation of the terminal ethylene complex [Ru2(CO)(µ-CO)2(C2H4)(η-C5H5)2] from the cation [Ru2(CO)2(µ-CO)(µ-CHCH2)(η-C5H5)2]+. Addition of sodium tetrahydroborate to [Ru2(CO)(µ-CO){µ-C(H)(O)C2Me2}(η-C5H5)2]+ provides the complex [Ru2(CO)(µ-CO){µ-C(Me)C(Me)CH2}(η-C5H5)2], completing a conversion of metallacyclic CO to CH2. Regeneration of the µ-vinyl cation [M2(CO)2(µ-CO)(µ-CHCH2)(η-C5H5)2]+ is achieved by treatment of [M2(CO)2(µ-CO){µ-C(H)Me}(η-C5H5)2] with [CPh3][BF4].
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