Synthesis and structural characterization of diene and benzene pentaruthenium clusters

Reaction of the square-pyramidal cluster [Ru₅C(CO)₁₅] 1 with cyclohexa-1,3-diene and Me₃NO in CH₂Cl₂ at ambient temperature affords the novel species [Ru₅C(CO)₁₃(μ-η²:η ²-C₆H₈)] 2 which has been fully characterized by X-ray diffraction studies [monoclinic, space group P2₁/n, a = 16.193(3), b = 9.897(2), c = 19.536(4) Å, β = 98.80(3)° and Z = 4]. Further reaction of compound 2 with Me₃NO in the same solvent results in the dehydrogenation of the C₆H₈ ligand and the production of two isomers 3a and 3b of the benzene derivative [Ru₅C(CO)₁₂(C₆H₆)]. In agreement with mass spectroscopic and ¹H NMR data, the X-ray diffraction analysis shows that the benzene ligand is bound in a face-capping mode in [Ru₅C(CO)₁₂(μ₃-η²:η ²:η²-C₆H₆)] 3a and in a terminal mode in [Ru₅C(CO)₁₂(η⁶-C₆H₆)] 3b, respectively. Compound 3a is triclinic, space group P1^-, a = 9.525(2), b = 14.574(3), c = 9.484(4) Å, α = 96.29(2), β = 112.37(3), γ = 82.33(2)° and Z = 2; 3b is monoclinic, space group P2₁/c, a = 15.26 (2), b = 16.675(9), c = 19.016(9) Å, β = 96.04(7)° and Z = 8. On heating 3a in hexane a quantitative and irreversible conversion to 3b is observed. Treatment of 3b with carbon monoxide produces the adduct [Ru₅C(CO)₁₃(η⁶-C₆H₆)] 4a, which, on the basis of a single crystal X-ray diffraction study, has been shown to contain a 'bridged-butterfly' arrangement of five ruthenium atoms. Compound 4a is monoclinic, space group P2₁/n, a = 9.792(4), b = 15.718(2), c = 16.446(3) Å, β = 96.37(2)° and Z = 4. On standing in CH₂Cl₂, 4a undergoes loss of carbon monoxide to give the apical isomer [Ru₅C(CO)₁₂(C₆H₆)] 3c. On heating in hexane over a prolonged period 3c undergoes quantitative isomerization to 3b.