The sequential linking of alkynes at a dimolybdenum centre. Crystal structures of [Mo2{µ-(MeO2CC2CO2Me)(HC2H)(MeO2CC2CO2Me)2}(η-C5H5)2]·2CH2Cl2 and [Mo2{µ-(MeO2CC2CO2Me)4}(η-C5H5)2]

Abstract

Reaction of [Mo₂(CO)₄(µ-HC₂H)(η-C₅H₅)₂] with MeO₂CCCCO₂Me affords several products, one of which, of molecular formula [Mo₂{(HC₂H)(MeO₂CC₂CO₂Me)₃}(η-C₅H₅)₂](1), is here the subject of an X-ray diffraction study. Crystals of (1) are triclinic, space group P(no. 2) with Z= 2 in a unit cell of dimensions a= 11.519(7), b= 18.602(9), c= 8.709(3)Å, α= 86.65(4), β= 100.99(4), γ= 97.54(5)°, and they encapsulate two molecules of solvent (CH₂Cl₂) per molecule of complex. The structure has been refined to R 0.054 for 3 450 reflections. The acetylenic moieties have joined to form a C₈ chain; within this chain the terminal atoms and the two central atoms are attached to one of the metal atoms, while the first three and the last three form π-allyl attachments to the other metal atom. The (C₅H₅)MoMo(C₅H₅) unit and the C₈ chain together form a framework of mirror symmetry, but the molecule as a whole does not possess C₈ symmetry because of the methoxycarbonyl ligands which are bonded to C(11), C(12), C(13), C(14), C(17), and C(18). When a similar reaction is carried out with [Mo₂(CO)₄(µ-MeO₂CC₂CO₂Me)(η-C₅H₅)₂] as the starting material, one of the products is again a complex containing a C₈ chain with methoxycarbonyl ligands on all eight carbon atoms, [Mo₂{µ-(MeO₂CC₂CO₂Me)₄}(η-C₅H₅)₂](2). An X-ray diffraction study of (2) reveals a remarkable difference in the bonding mode. Crystals of (2) are monoclinic, space group P2₁/c(no. 14) with Z= 4 in a unit cell of dimensions a= 14.737(2), b= 11.224(2), c= 23.882(5)Å, β= 115.92(1)°, and the structure has been refined to R 0.088 for 4 897 reflections. The molecule (2) has idealised C₂ symmetry (not crystallographically required), and the C₈ chain forms a ‘flyover’ from one metal atom to the other. The two groups of four carbon atoms are each planar (and co-planar with the σ-bonded metal atom) and form a diene-type interaction with the other metal atom. A further marked difference from (1) is that the central bond of the C₈ chain, which in (1) is orthogonal to the MoMo link, is in (2) parallel to that direction. The metal–metal distances, 2.618(1)Å in (1) and 2.635(1)Å in (2), are indicative of Mo–Mo double bonding.