Synthesis and characterisation of Ti, Cr, Mo and W bis(fluorene) complexes

The new eta(6)-bisfluorene complexes of Ti, Cr, Mo, and W were prepared using the Metal Vapour Synthesis (MVS) technique and characterised by spectroscopic methods. The reactivity displayed by fluorene using MVS is different than for other ligands with acidic hydrogens: cyclopentadiene produces the same eta(5) complexes that can be obtained by more conventional synthesis, while indene (where acidic hydrogens coexist with an aromatic ring) originates both eta(5) or/and eta(6) complexes, sometimes as an isomeric mixture (Mo and W). In the present case of fluorene, only eta(6) complexes were isolated by MVS. In order to explain this reactivity the stability of Mo and W eta(5) and eta(6) metal complexes with fluorene, indene and cyclopentadiene is interpreted using Molecular Orbital Calculations by determining the corresponding Overlap Populations. These calculations are also used to explain the chemical shifts observed in the H-1 NMR spectra of the bisfluorene complexes of group 6 metals in terms of p(2) electronic density. (C) 1997 Elsevier Science S.A.