Bis(acetylacetonato)bis(cyclooctene)ruthenium(II), cis-[Ru(acac)2(η2-C8H14)2]:: a synthetic precursor to trans- and cis-bis(acetylacetonato)ruthenium(II) complexes

Reduction of [Ru(acac)(3)] with zinc amalgam or zinc dust in hot THF containing some water in the presence of an excess of cyclooctene generated in solution cis-[Ru(acac)(2)(eta(2)-C8H14)(2)], which cannot be isolated in solid form but has been identified on the basis of its H-1 NMR spectrum. It is a useful synthetic precursor because the co-ordinated olefins are easily displaced by many ligands. Treatment with pyridine, tert-butyl isocyanide, tertiary phosphines, phosphites and triphenylarsine (L) at room temperature gave red-brown complexes trans-[Ru(acac)(2)L-2], which isomerise in solution to the more stable cis compounds on heating. In contrast, the similarly prepared trimethylamine complex, trans-[Ru(acac)(2)(NMe3)(2)], does not undergo trans to cis isomerisation. Reaction of cis-[Ru(acac)(2)(eta(2)-C8H14)(2)] with acetonitrile or triphenylstibine (L') gave monosubstitution products cis-[Ru(acac)(2)(eta(2)-C8H14)L'], which react on heating with an excess of L' to give cis-[Ru(acac)(2)L'(2)]. Treatment of cis-[Ru(acac)(2)(eta(2)-C8H14)(2)] (1 mol) with Ph2PCH2PPh2 (dppm) (2 mol) at room temperature gave trans-[Ru(acac)(2)(eta(1)-dppm)(2)], whereas the ligands Ph2P(CH2)(m)PPh2 (L-L, m = 2, dppe; m = 3, dppp) under the same conditions gave oligomers [{Ru(acac)(2)(L-L)}(n)], which probably contain mutually trans-phosphorus atoms. On heating all three compounds are converted into cis-[Ru(acac)(2)(L-L)]. Treatment of trans-[Ru(acac)(2)L-2] (L = NMe3 or PPh3) with CO at room temperature and pressure gave trans-[Ru(acac)(2)(CO)L], which, in the case of L = PPh3, isomerises to the cis compound on heating; reaction of trans-[Ru(acac)(2)(AsPh3)(2)] with CO under the same conditions gave cis-[Ru(acac)(2)(CO)(AsPh3)] directly. The structures of trans-[Ru(acac)(2)(CNBut)(2)], trans-[Ru(acac)(2)(PMePh2)(2)], cis-[Ru(acac)(2)(CNBut)(2)] (in the form of a molecular adduct with [Ru(acac)(3)]), cis-[Ru(acac)(2)(PMePh2)(2)] and trans-[Ru(acac)(2)(eta(1)-dppm)(2)] have been determined by X-ray crystallography, and trends in the metal-ligand distances are discussed. The formation of trans-[Ru(acac)(2)L-2] from cis-[Ru(acac)(2)(eta(2)-C8H14)(2)] may proceed via a square-pyramidal intermediate [Ru(acac)(2)L].