Dimolybdenum complexes derived from cyclo-octatetraene. Crystal and molecular structures of [Mo2(CO)2(η-C5H5)2(C8H8)](two isomers) and of [Mo2(CO)4(η-C5H5)2(η3,η′3-C16H16)]

Abstract

Reaction of the dimolybdenum compounds [Mo₂(CO)₆(η-C₅H₅)₂] or [Mo₂(CO)₄(η-C₅H₅)₂] with cyclo-octatetraene in octane or heptane at reflux affords a complex mixture from which the species [Mo₂(CO)₂(η-C₅H₅)₂(C₈H₈)] and [Mo₂(CO)₄(η-C₅H₅)₂(η³,η′³-C₁₆H₁₆)] were isolated and characterized. The maroon complex [Mo₂(CO)₂(η-C₅H₅)₂(C₈H₈)](1) is stable in non-polar solvents but in polar solvents such as acetone, chloroform, or acetonitrile it transforms to an orange isomer (2). Carbon-13 n.m.r. spectroscopy revealed that (1) possessed mirror symmetry, in contrast to (2), and that in both isomers two carbons of the C₈H₈ ligands did not bear protons. The unprecedented rearrangement of cyclo-octatetraene at a Mo₂, centre and subsequent isomerisation were established by single-crystal X-ray diffraction studies on the two isomeric forms (1) and (2). In both structures the central spine of the molecule comprises a (η-C₅H₅)Mo–Mo(CO)₂(η-C₅H₅) unit of approximate mirror symmetry, the mirror passing through the Mo–Mo bond and bisecting the angle (ca. 85°) between the carbonyl ligands (which are both attached to the same Mo atom). The cyclopentadienyl rings are in a trans relationship to the metal–metal bond, and the carbonyl ligands bend back towards the centre of the molecule, making the Mo–Mo–CO angle ca. 74°. The two isomers differ strikingly, however, in the mode of attachment of the C₈ ring. In both isomers there is η² attachment to the Mo(CO)₂(η-C₅H₅) moiety, and η⁶ attachment to the other Mo atom through six contiguous carbon atoms of the C₈ ring, the two common carbon atoms being in the form of a symmetrical transverse acetylenic bridge. In isomer (1) the two carbon atoms on either side of the transverse bridge form two π-ethylenic links to the metal atom, whereas in isomer (2) only one carbon atom forms a σ bond on one side of the bridge. and three carbon atoms form a π-allylic attachment on the other side. The remaining two methylenic carbon atoms bend away from the η⁶ part of the C₈ ring, which is approximately planar, but in crystals of (1) there is a slight twist which destroys the mirror symmetry of the molecule as a whole. In (2) the entire attachment is asymmetric. Crystals of (1) are monoclinic maroon plates, a= 8.629(2), b= 39.580(10), c= 15.759(7)Å, β= 103.90(3)°, and (remarkably) with space group P2₁/c, Z= 12. The three crystallographically distinct molecules are, however, stereochemically identical. The structure has been solved by heavy-atom methods from 4 013 independent intensities [l 2.5σ(l)] and refined to R 0.073. Crystals of (2) are monoclinic orange ‘cubes,’a= 8.081(4), b= 11.808(9), c= 17.409(15)Å, β= 91.94(6)°, space group P2₁/n. The structure has been solved by heavy-atom methods from 4 735 intensities (183 K) and refined to R 0.078. An X-ray crystallographic investigation of [Mo₂(CO)₄(η-C₅H₅)₂(η³,η′³-C₁₆H₁₆)](3) was also undertaken since n.m.r. studies did not define the structure. The diffraction results revealed that (3) contained a dimeric form of cyclo-octatetraene, in which two ‘tub’ form C₈ rings are joined by a single bond. Three carbon atoms of each ring, adjacent to the link bond, are η³-bonded to an octahedral Mo(CO)₂(η-C₅H₅) moiety, to give the whole molecule C₂ symmetry (crystallographically required). Remarkably, the molecules form a tetragonal unit cell of space group l4₁cd(no. 110), the eight dimeric units occupying a cell of dimensions a= 19.062(3), c= 14.425(6)Å. The structure has been solved by heavy-atom methods from 1 479 independent intensities (298 K) and refined to R 0.040.