Reactions of cyclic 1,3-dienes with ethyne co-ordinated at a dimetal centre. Crystal structure of [Mo2(CO)2(µ-η2,η′2-C10H10)(η-C5H5)2]

Abstract

The complex [Mo₂(CO)₄(µ-HC₂H)(η-C₅H₅)₂] reacts in refluxing octane with cyclo-octatetraene, cyclohexa-1,3-diene, cycloheptatriene, and cyclo-octa-1,3-diene to afford dimolybdenum compounds having the bridging bicyclic ligands C₁₀H₁₀, C₈H₁₀, C₉H₁₀, and C₁₀H₁₄ respectively, formed by formal Diels–Alder 1,4-addition of the diene to the Mo₂(µ-HC₂H) moiety. The molecular structure of the product from cyclo-octatetraene was established by an X-ray diffraction study. Crystals of [Mo₂(CO)₂(µ-C₁₀H₁₀)(η-C₅H₅)₂] are monoclinic, a= 9.230(3), b= 16.561(4), c= 12.651 (3)Å, β= 107.38(2)°, space group Cc(no. 9). The structure was solved by heavy-atom methods and has been refined to R 0.036 (R′ 0.035) for 4 285 reflections. The bicyclo[4.2.2]deca-2,4,7,9-tetraene ligand bridges a MoMo triple bond [2.504(1)Å] with the double bonds at positions 7 and 9 each co-ordinated to a molybdenum atom. The two carbonyl ligands asymmetrically bridge the metal–metal bond [mean Mo–CO 1.99 and 2.33 Å]. The axis of the molecule through the metal–metal bond and the centroids of the η-C₅H₅ ligands is non-linear, with the two rings in a cis relationship. Hydrogen-1 and ¹³C n.m.r. data for the complexes are reported and discussed. Variable-temperature ¹³C n.m.r. studies on the product from cyclo-octa-l,3-diene revealed dynamic behaviour involving site exchange of the semi-bridging CO ligands (ΔG^‡ca. 47 kJ mol^–1).