Cyclopentadienyl ligand isomerization

It is well known that, when treated with complex substrates, cyclopenta-diene can form complexes with cyclopentadiene, cyclopentadienyl, and even cyclopentenyl ligands. The same possibilities are found for bora-2,5-cyclohexadienes, but with the additional complexity of ligand isomerism. 

Some attempts to modulate the electron density on the metal center by introduction of pentamethyl- or pentaphenyl-cyclopentadienyl ligands resulted in decreased activities, presumably due essentially to the appreciable steric hindrance introduced in the coordination sphere of the Ru center. As the dimer precursor [( -C5H5)Ru(CO)2]2 leads to more isomerization of the alkene than the monomeric complexes, the equilibrium of Eq. (3) is certainly not fully shifted toward the formation of the [( -C5Hj)Ru(CO)2H] species. The authors have checked that the dimer itself catalyzes the isomerization of oct-1 -ene to oct-2-ene. 

Note that the cis isomer lacks an improper axis of rotation and is therefore chiral, but that the trans isomer has a plane of symmetry and will be achiral in the absence of an asymmetric carbon in the phosphine ligand-28 As in the case of the previously encountered cyclopentadienyl complex (page 476), it can be argued whether the coordination number is 5 or 9. In either semantic interpretation these compounds are of considerable interest since isomerism in nine-coordinate complexes Is even less well documented than in those with coordination number 5. 

Two cyclopentadienyl units are bridged in 1,2 position by two dimethylsilyl groups in the ligand system 53. The isomeric mixture of 4,4,8,8-tetramethyltetrahydro-4,8-disila-.y-indacene139-144 is the starting material for metal complexes of a different kind. Deprotonation of this compound and metallation with alkali metal compounds have been investigated in detail142 143 (equation 60). 

Recently Rouike has reported a series of cyclopalladated imine complexes with either acetylacetonate or cyclopentadienyl as the other ligand, as in Figure 8 [29]. Unlike Ghedini s azobenzene complexes these are isomerically pure but still display mesophases at similar temperatures. The square planar 16e acetylacetonate complexes display both nematic and Sa phases with extended mesophase ranges compared to the free ligands. Not surprisingly, the formally trigonal bipyramidal 18e cyclopentadienyl complexes show a nematic phase at much lower temperatures and tend to decompose above c. 180°C. 

Cyclopentadienyl groups attached to cross-linked polymers have been examined as ligands for catalytic systems involved in olefin-hydrogenation (34.,3, 2Ai3i) -isomerization (34.,35 ), -... 

---