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)A, 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)A, 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)A. The structure has been solved by heavy-atom methods from 1 479 independent intensities (298 K) and refined to R 0.040.