Reactions of co-ordinated ligands. Part 22. The reactivity of bis-(ethylene)(η5-indenyl)rhodium in displacement reactions with olefins, dienes, and acetylenes; crystal structure of η5-indenyl{l–2 :3–4-η4–[6-endo-propen-2-yl-1,2,3,4-tetrakis(trifluoromethyl)cyclohexa-l,3-diene]}rhodium formed in a cyclo-cotrimerisation reaction

Abstract

In contrast to the η⁵-cyclopentadienyl analogue, both of the ethylenes in the compound bis(ethylene)(η⁵-indenyl)-rhodium are readily displaced by isoprene, cyclo-octatetraene, trans-penta-1,3-diene, trans,trans-hexa-2,4-diene, methylenecyclopropane, or acrylonitrile to afford respectively η⁵-isoprene-, l–2 : 5–6-η⁴cyclo-octatetraene-,η⁴-trans-penta-1,3-diene-, η⁴-trans,trans-hexa-2,4-diene-, bis(methylenecyclopropane)-, or bis(acrylonitrile)-(η⁵-indenyl)rhodium. Reaction (room temperature) of hexafluorobut-2-yne with η⁵-indenyl(η⁵-isoprene)-rhodium leads to a cyclo-cotrimerisation reaction and the formation of η⁵-indenyl{l–2: 3–4η⁴-[6-endo-propen- 2-yl-1,2,3,4-tetrakis(trifluoromethyl)cyclohexa-l,3-diene]}rhodium, the structure in the solid state being established by a single-crystal X-ray diffraction study. Crystals are orthorhombic, space group Pbca(no. 61), with eight molecules in a cell of dimensions a= 16.962(3), b= 29.618(5), and c= 8.596(4)Å. The 1,3-diene part of the C₆ ring is planar, and is η⁴-bonded to the rhodium atom, but the two carbon atoms originating from the isoprene bend away from the metal to produce a fold in the C₆ ring at a dihedral angle of ca. 132°. The propenyl substituent lies on the same side of the C₆ ring as the metal, i.e. is in an endo configuration. An 18-electron configuration for the rhodium is achieved by η⁵-attachment of the indenyl group, but two of these five carbon atoms are less strongly bonded than the other three. A similar reaction of hexafluorobut-2-yne with η⁵-indenyl(η⁴-trans-penta-1,3-diene)rhodium affords related isomeric complexes. A cyclo-cotrimerisation reaction has also been observed between hexafluorobut-2-yne and a co-ordinated ethylene of bis(ethylene)(η⁵-indenyl)rhodium. Related reactions between 3,3-dimethylbut-1-yne and bis(ethylene)(η⁵-indenyl)rhodium or bis(acrylonitrile)-(η⁵-indenyl)rhodium afford respectively [η⁴-1,4-bis(t-butyl)cyclohexa-1,3-diene]- and [η⁴-5 cyano-1,4-bis(t-butyl)cyclohexa-l,3-diene]-η⁵-indenylrhodium. In all of these reactions competitive cyclotrimerisation of the acetylenes occurs to give arenes. The mechanisms of these reactions are discussed.