Lanthanides are a group of rare-earth metals that form what is called the "f-block". These elements generally exist in compounds as a +3 ion and can have up to seven unpaired electrons making their electronic properties very interesting.
Figure 1. The Lanthanides.
Transition metal complexes with acetylacetonate (acac) are well known and studied extensively (1,2,3) and lanthanide metal complexes with acac or acac derivatives are commonly use (4) for a variety of purposes due to their high number of unpaired electrons. They can be used to change the magnetic environment of other molecules providing a shift in NMR signals. They can also be used as catalysts in Diels-Alder reactions of substituted dienes with aromatic and aliphatic aldehydes, which yield dihydropyrans with high selectivity for the endo product. (5)
It's known that metal acac complexes with multiple unpaired electrons, like Mn(acac)3 have distorted geometries that differ from the octahedral geometries of other complexes like Cr(acac)3.(3) Lanthanide acac complexes are also distorted and are often complexed to water molecules in the solid crystal forming structures that have 7 or 8 oxygens coordinated with the metal.(1) Their electronic structures have not been as extensively studied as their transition metal counterparts.
Fig. 2: Optimized Geometry of Nd(acac)3. (6)
The PI has previously worked on DFT calculations of Nd(acac)3 complexes with a student researcher, Trevor Seguin, and we were able to predict a geometry and the energy levels of the complex (Figure 2). The objective was to compare the energy levels obtained by calculation to the experimental photo-electron spectrum.(6) Some preliminary results were presented by Trevor at the Spring 2012 National American Chemical Society Meeting in San Diego, but there are clearly unanswered questions about these molecules. One is the uncertainty about the number of water molecules coordinated to the metal atom in vacuuo. Another is the inconsistency between the calculated energy levels of the metal atom and ligands when compared to the experimental spectrum. Clearly there is more work to be done on the calculations.
1. Cotton, Simon; “Complexes of Î²-diketonates” in Lathanide and Actinide Chemistry John Wiley and Sons, 2006.
2. F.A. Cotton, C.E. Rice, G.W. Rice "The Crystal and Molecular Structures of Bis(2,4-pentanedionato)chromium" Inorganica Chimica Acta 1977, volume 24, pp. 231â€“234.
3. Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred, Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, 1999.
4. Examples include EuFOD, a commercially available NMR shift reagent sold by Sigma-Aldrich
5. Wenzel, T.J.; Ciak, J.M.; "Europium, tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedianato)" in Encyclopedia of Reagents for Organic Synthesis, 2004. John Wiley & Sons, Ltd.
6. “Electronic Structure of Lanthanide Acetylacetonate Complexes” Trevor Seguin, Dr. Thomas R. Burkholder, Dr. Barry L. Westcott, Dr. Nadine E. Gruhn. Spring 2012 National ACS Meeting in San Diego, CA. CHED: Division of Chemical Education Monday, March 26, 2012, 12:00 PM