Superbulky Metallocenes

Superbulky Metallocenes

Author: Chemistry – A European Journal

As the chemistry of pentaarylcyclopentadienyl complexes differs substantially from that of the more commonly used parent cyclopentadienyl (Cp) and pentamethyl (Cp*) derivatives, pentaarylcyclopentadienyl ligands should have great potential in organometallic chemistry. However, the development of the chemistry of this ligand in its simplest form (Ph5Cp) has been plagued by the complete insolubility of nearly all its complexes.

Sjoerd Harder and his group, Universität Erlangen–Nürnberg, Germany, make use of a pentaarylcyclopentadienyl ligand with added “ponytails” – nBu groups – acting as solubilizers. This superbulky ligand – (4-nBu-phenyl)5Cp, named by the group as CpBIG – has been investigated in organolanthanide chemistry.

The first surprise was the spontaneous reduction of the lanthanide metals Sm, Eu, and Yb and the isolation of Ln(II) metallocenes, rather than the formation of half-sandwich Ln(III) complexes. In a multidisciplinary collaboration with five other research groups from Germany, Israel, and the USA, the 2+ oxidation state of the metal in these metallocenes was confirmed by temperature-dependent magnetic susceptibility, Mössbauer, X-ray absorption near edge structure (XANES), and luminescence measurements.
The other main surprise was that, in contrast to [Eu(Cp*)2], which hardly shows any luminescence, a quantum yield of 45 % is found for the europocene complex [Eu(CpBIG)2] with unusually strong orange luminescence.

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