Exploring new ligand environments for lanthanide coordination chemistry
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Abstract
The recent surge of interest in Lanthanide (Ln) chemistry is focused on the synthesis and characterization of new families of mixed-valence Ln complexes for potential applications in electronics. Accessing mixed-valence systems in Ln chemistry has so far been difficult due to the lack of information available on these elements in an oxidation state other than the common Ln(III) state. We are interested in devising a series of complexes that feature two discrete Ln(II) metals ions and, via controlled intramolecular electron transfer, we aim to oxidize one of the Ln(II) species to a Ln(III), thus generating a mixed-valence complex. Although intramolecular electron transfer has been reported previously for a handful of Ln complexes, the transfer was spontaneous. The reaction of the Ln(II) precursors (C5Me5)2Ln•OEt2 (Ln = Sm, Eu and Yb) with various 1,4-diaza 1,3-butadiene R1N=CR2–CR2=NR1 (DAD) ligands has led to the isolation of Ln(II) and Ln(III) complexes that are dependant on the nature of the R1 groups. Furthermore, we have examined the electronic structure of the free ligand by Density Functional Theory (DFT), exploring the relationships between the size of the HOMO/LUMO gap and/or the absolute energies of the LUMO and the occurrence of electron transfer. The development of new non-cyclopentadienyl (Cp) Ln catalysts is also explored. Since the nature of the auxiliary ligands influences the reactivity of a complex, the replacement of Cp-type ligands with nitrogen-based ligands will increase the electophilicity of the metal center and permit greater control over the steric environment at the reactive site, thus allowing for the generation of a more active catalyst. One nitrogen-based ligand system of current interest is a 0-diketiminate containing electron withdrawing substituents such as C6F5. The purpose of the study is to develop a series of new Ln compounds featuring one or more 0-diketiminate ligands and to investigate their catalytic activity in ethylene polymerization.