Electronic effects of bis(acetylacetone) in ruthenium(II) and ruthenium(III) complexes

The compound Ph₄As [Ru(acac)₂Cl₂] was prepared by the reaction of RuCl₃·3H₂O with acetylacetone in 1 M aqueous KCl. The anion was shown by X-ray crystallography to have a trans configuration, and it is the first trans-bis(acetylacetone) complex to have been prepared. The compound crystallizes in the monoclinic system, in the space group P1, with a = 10.082 (2) Å, b = 13.433 (2) Å, c = 14.950 (2) Å, α = 65.03 (1)°, β = 66.71 (1)°, γ = 72.02 (2)°, V = 1663 (1) ų, and Z = 2. A number of trans-(acac)₂ complexes of ruthenium(II), -(III), and -(IV) were prepared from the above compound and were characterized by IR, UV-vis, and ¹H NMR spectroscopy and by cyclic voltammetry. They include trans-[Ru(acac)₂(pyrazine)₂] and trans-[Ru(acac)₂(CH₃CN)₂]. The analogous cis complexes were prepared by the reaction of the ligands pyrazine and CH₃CN with Ru(acac)₃ or by thermoisomerization of the trans species, and a comparison of chemical behavior was made. For the acetonitrile derivative, the equilibrium quotient for the isomerization of the trans species to the cis was measured as 80 at 30°C. Isomerization is so slow for the pyrazine derivative compared to the competing decomposition that a dependable value of the equilibrium quotient could not be determined, but the indications are that the cis form is the more stable. These results, and measurements of the proton affinities of the pyrazine complexes, are rationalized on the basis that when two π-acid ligands compete for π-electron density, this competition is less severe when they are disposed cis, rather than trans, to each other. None of the observations suggest that acac^- acts significantly in a π-acid capacity. When Ru(acac)₂(CH₃OH)₂ and pyrazine, mixed in equimolar proportions, react, a very insoluble polymeric material forms. The solid is a poor conductor, but the conductance is increased by a factor as high as 10⁶ when it is doped with I₂. © 1991 American Chemical Society.