Synthesis and Complexation of Methylene-Bridged PAH Ligands

Abstract

Cyclopentadienyl (Cp, C5H5) is a well-known ligand which has been used for many years, forming complexes that have a wide range of applications, one significant application being its use as a ligand for Ziegler-Natta polymerisation catalysts. The benzannulation and bridging of the Cp ligands greatly expands the scope and precision of these catalysts to the point that the structure of the catalyst is used to control the stereochemistry of the polymer.

In this project, two cyclopentadiene-related target compounds are reported; a cyclopentatriphenylene which is used as a model system for a larger cyclopenta-hexa-peri-hexabenzocoronene derivative, both of which have the potential to be used as a η5 ligand for complexation including in Ziegler-Natta catalysts. Unsubstituted Cp-triphenylene (1) was synthesised in eight high yielding steps from commercially available fluorene. A series of [Ru(η5-Cp*)(η6-cyclopentatriphenylene)]PF6 complexes have been synthesised with 1 and two of its substituted precursors. An investigation into the effect of steric and electronic properties of each cyclopentatriphenylene ligand on the site of complexation was then undertaken. The cyclopenta-hexa-peri-hexabenzocoronene target compound 2 was then synthesised in eleven steps via a new 4-triflyloxyfluorenone intermediate prepared from commercially available 2-bromophenol and 2-methylphenylboronic acid. The final Scholl cyclodehydrogenation reaction yielded the target compound with a small amount of mono-chlorinated impurity which could not be separated by crystallisation. All new compounds synthesised have been characterised using 1H NMR, 13C NMR and IR spectroscopy, mass spectrometry and elemental analysis. Crystal structures of three [RuCp*(η6-cyclopentatriphenylene)]PF6 complexes and eight new organic ligands and intermediates have been obtained via X-ray crystallography.