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 †

Abstract

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(η²-C₈H₁₄)₂], which cannot be isolated in solid form but has been identified on the basis of its ¹H 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)₂L₂], 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(η²-C₈H₁₄)₂] with acetonitrile or triphenylstibine (L′) gave monosubstitution products cis-[Ru(acac)₂(η²-C₈H₁₄)L′], which react on heating with an excess of L′ to give cis-[Ru(acac)₂L′₂]. Treatment of cis-[Ru(acac)₂(η²-C₈H₁₄)₂] (1 mol) with Ph₂PCH₂PPh₂ (dppm) (2 mol) at room temperature gave trans-[Ru(acac)₂(η¹-dppm)₂], whereas the ligands Ph₂P(CH₂)_mPPh₂ (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)₂L₂] (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(CNBu^t)₂], trans-[Ru(acac)₂(PMePh₂)₂], cis-[Ru(acac)₂(CNBu^t)₂] (in the form of a molecular adduct with [Ru(acac)₃]), cis-[Ru(acac)₂(PMePh₂)₂] and trans-[Ru(acac)₂(η¹-dppm)₂] have been determined by X-ray crystallography, and trends in the metal–ligand distances are discussed. The formation of trans-[Ru(acac)₂L₂] from cis-[Ru(acac)₂(η²-C₈H₁₄)₂] may proceed via a square-pyramidal intermediate [Ru(acac)₂L].