New dimeric forms of cyclo-octatetraene: crystal structures of tricarbonylruthenium and pentacarbonyldiruthenium complexes containing dimeric cyclo-octatetraene

Abstract

Reaction of [Ru₃(CO)₁₂] or the cluster [Ru₄H₄(CO)₁₂] with cyclo-octatetraene (cot) yields a mixture of products, several of which have already been identified. Among the hitherto unidentified species, however, are two complexes, [Ru(CO)₃(C₁₆H₁₆)](1) and [Ru₂(CO)₅(C₁₆H₁₆)](2). Both (1) and (2) have now been characterised by X-ray crystallography; they are metal carbonyl derivatives of two isomers of a new dimer of cot. Crystals of (1) are monoclinic, space group P2₁/n, with Z= 4 in a unit cell of dimensions a= 12.801(4), b= 6.640(1), c= 19.678(5)Å, and β= 103.06(2)°. Crystals of (2) are also monoclinic, space group P2₁/n, with Z= 8 in a unit cell of a= 7.010(1), b= 24.965(8), c= 22.021(7)Å, and β= 101.03(2)°. Both structures have been solved by heavy-atom methods for 1 899 independent observed intensities for (1), 3 812 for (2), and refined by least squares to R 0.037 for (1) and R 0.051 for (2). The structure of (1) shows the ruthenium atom to be η⁴-bonded to a cyclo-hexadiene ring which is cis-fused to a planar cyclobutane ring which in turn is cis-fused in a 1,4 fashion to a cyclo-octatriene ring. The six-membered ring of the bicyclo-octadiene moiety and the eight-membered cyclo-octadiene ring are in an anti relationship to the central four-membered ring; the molecule as a whole has idealised mirror symmetry. In (2) the cot dimer takes the form that might be expected from Diels–Alder self addition, in that one of the C₈ rings remains unbridged. The link to the second C₈ ring is as in (1). The Ru₂(CO)₅ moiety adopts a ‘saw-horse’ configuration; Ru(CO)₃ is η⁴-bonded to ring 1, while Ru(CO)₂ is σ- and η²-bonded to ring 1 and η²-bonded to ring 2. The molecule is fluxional.