Organic chemistry of dinuclear metal centres. Part 13. Synthesis, structure, and reactivity of [Ru2(CO)4(η5: η5′-C5H4CH2C5H4)]

Abstract

Reaction of [Ru₃(CO)₁₂] with bis(cyclopentadienyl)methane in boiling toluene gives [Ru₂(CO)₄(η⁵:η^5′-C₅H₄CH₂C₅H₄)](1) in good yield. The molecular structure has been determined by X-ray diffraction. The structure was solved by heavy-atom methods and refined by least squares to give a final R 0.045 for 3 009 unique, observed diffractometer data. Crystals of (1) are orthorhombic, space group Pbca, with Z= 16 in a unit cell of dimensions a= 18.066(9), b= 14.321(6), and c= 22.156(10)Å. The molecule consists of a staggered (OC)₂Ru–Ru(CO)₂ unit bridged by a bis(cyclopentadienylene)methane ligand which is η⁵-bound to each ruthenium atom. I.r. spectroscopy reveals that this structure dominates in solution, but that a low concentration of a carbonyl-bridged isomer is also present. Sequential treatment of complex (1) with LiMe, tetrafluoroboric acid, and NaBH₄ affords the bis-µ-ethylidene complex [Ru₂(CO)₂(µ-CHMe)₂(η⁵:η^5′-C₅H₄CH₂C₅H₄)](7) the structure of which has also been determined by X-ray diffraction, and solved and refined as above to a final R 0.025 for 1 813 data. Crystals of (7) are monoclinic, space group P2₁/n(non-standard setting of P2₁/c, no.14), with Z= 4 in a unit cell of dimensions a= 8.702(3), b= 12.653(4), c= 14.508(6)Å, and β= 98.01(3)°. The molecule contains an (OC)Ru–Ru(CO) unit bridged by two ethylidene groups so as to give a highly folded Ru₂(µ-C)₂ core. The methyl groups of the ethylidenes are oriented anti with respect to a bridging bis(cyclopentadienylene)methane ligand bound in an η⁵ : η^5′ fashion as for complex (1). Although the µ-C ⋯µ-C distance in complex (7) is relatively short (3.11 Å), thermolysis induces alkylidene linking less efficiently than for related η-C₅H₅ complexes with a non-folded geometry, a difference attributed to the reduced flexibility in (7) arising from the coupling of the η-C₅ rings. Reaction of complex (1) with Li[BHEt₃]–water affords the µ-methylene complex [Ru₂(CO)₂(µ-CO)(µ-CH₂)(η⁵:η^5′-C₅H₄CH₂C₅H₄)]. Photolysis of (1) in the presence of diphenylacetylene gives [Ru₂(CO)₂(µ-CO)(µ-σ: σ-C₂Ph₂)(η⁵: η^5′-C₅ H₄CH₂C₅H₄)], containing a ‘parallel’ two-electron alkyne ligand. The Ru–Ru bond of (1) is cleaved by iodine to yield [Ru₂I₂(CO)₄(η⁵:η^5′-C₅H₄CH₂C₅H₄)], which is readily converted into[Ru₂Me₂(CO)₄(η⁵: η^5′- C₅H₄CH₂C₅H₄)] on treatment with Li[CuMe₂].