Synthesis, structures and dynamic NMR spectra of η⁶-hexaethylbenzene complexes of ruthenium(0) and ruthenium(II)

On reaction with the labile naphthalene complex [Ru(η⁶-C₁₀H₈)(η⁴-1, 5-COD)], hex-3-yne undergoes stoichiometric cyclotrimerisation to form the hexaethylbenzene-ruthenium(O) complex [Ru(η⁶-C₆Et₆)(η⁴-1, 5-COD)] 1. In the solid state and in solution the ethyl groups adopt a 1,4-proximal-2,3,5,6-distal arrangement, as shown by X-ray crystallography and NMR (¹H, ¹³C-{¹H}) spectroscopy. Treatment of 1 with HCl gives the binuclear ruthenium(II) complex [{RuCl(η⁶-C₆Et₆)}₂ (μ-Cl)₂] 2, whose arene ligands adopt a transoid arrangement about the Ru₂Cl₂ moiety; in turn 2 reacts with methanolic NH₄PF₆ to give the salt [Ru₂(μ-Cl)₃(η⁶-C₆Et ₆)₂]PF₆, [3]PF₆. The ethyl group conformations in crystalline 2 and [3]PF₆ are all-distal and 1,3,5-proximal-2,4,6-distal, respectively, whereas only the latter conformation is present in both compounds in dichloromethane or methanol solutions at low temperature according to ¹³C-{¹H} NMR spectroscopy. The ν(Ru-Cl) band patterns in the IR spectra of 2 in the solid state and dichloromethane solution are almost identical, indicating that the neutral di-μ-chloro species predominates in solution at room temperature. However, the appearance at -50°C of a resonance due to free chloride ion in the ³⁵Cl NMR spectrum of complex 2 suggests that reversible formation of [3]Cl may be favoured at low temperature. Dilute (ca. 10^-3 M) solutions of 2 in dichloromethane and methanol behave as 1:1 electrolytes consistent with the presence of [3]Cl under these conditions. At room temperature the ethyl groups of η⁶-C⁶Et⁶ in 1, 2 and [3]PF⁶ are equivalent on the NMR time-scale as a consequence of rotation about the arene-methylene bond and, possibly, rotation of the arene about the arene-metal bond. In the crystalline adducts [RuCl₂(η⁶-C₆Et₆)(L)] (L= PMe₃ 4, PPh₃ 5) the ethyl groups are all distal and remain equivalent on the NMR time-scale in solution from room temperature to -97°C. The results confirm conclusions, based primarily on studies of Group 6 carbonyl complexes, that the di3erent conformations of η⁶-C₆Et₆ have very similar energies.