Organic chemistry of dinuclear metal centres. Part 12. Synthesis, X-ray crystal structure, and reactivity of the di-µ-alkylidene complex [Ru2(CO)2(µ-CHMe)(µ-CMe2)(η-C5H5)2]: alkylidene linking

Abstract

Upon treatment with methyl-lithium followed by HBF4·OEt2 a carbon monoxide ligand of the µ-alkylidene complex [Ru2(CO)2(µ-CO)(µ-CMe2)(η-C5H5)2](1) is converted into µ-ethylidyne, giving [Ru2(CO)2(µ-CMe)(µ-CMe2)(η-C5H5)2]+(2). This is deprotonated readily by water to form the µ-vinylidene complex [Ru2(CO)2(µ-CCH2)(µ-CMe2)(η-C5H5)2](3), which quantitatively regenerates (2) with HBF4·OEt2. Addition of NaBH4 to (2) results in hydride attack on µ-CMe to yield the di-µ-alkylidene complex [Ru2(CO)2(µ-CHMe)(µ-CMe2)(η-C5H5)2](4) as cis and trans isomers. The structure of the trans isomer has been established by X-ray diffraction. Crystals are triclinic, space group P, with Z= 2 in a unit cell for which a= 8.474(2), b= 7.802(3), c= 12.989(5)A, α= 99.42(3), β= 96.96(3), and γ= 107.73(3)°. The structure was solved by heavy-atom methods and refined to R 0.026 (R′ 0.031) for 4 092 independent intensities. A ruthenium–ruthenium single bond of 2.701(1)A is symmetrically bridged by ethylidene [mean Ru–C 2.079(3)] and isopropylidene [mean Ru–C 2.107(3)A] ligands to form an approximately planar Ru2C2 ring with a non-bonding Me2C··CHMe distance of 3.20 A. Upon thermolysis the alkylidenes link to evolve Me2CCHMe, Me2CHCHCH2, and Et(Me)CCH2. The absence of C4 and C6 hydrocarbons indicates that the alkylidene coupling occurs intramolecularly, and the electronic and stereochemical requirements of this process are discussed. Unlike mono-µ-alkylidene complexes, [Ru2(CO)2(µ-CO)(µ-CR2)(η-C5H5)2], the cis and trans forms. of (4) do not interconvert thermally below 145 °C, but u.v. irradiation effects a slow trans to cis isomerisation. U.v. irradiation of (4) in the presence of dimethyl acetylenedicarboxylate promotes ethylidene–alkyne linking to form [Ru2(CO)(µ-CMe2){µ-C(CO2Me)C(CO2Me)CHMe}(η-C5H5)2], but with ethyne both of the alkylidenes are lost and the ruthenium–ruthenium double-bonded complex [Ru2(µ-CO)(µ-C2H2)(η-C5H5)2] is produced.