For several years we have been preparing simple monomeric compounds to test out ideas about structure and bonding. For example we have prepared a unique series of lanthanide chalcogenolates which differ only in the donor atom of the chalcogenolate and which show the striking differences between the different ligands. This work led to a collaboration with the computational chemistry group of Prof Nik Kaltsoyannis which suggests that electrostatic effects are significant contributors to the structures of lanthanide alkoxides – in particular repulsion between the cationic lanthanide centre and the carbon on the highly polar C-O bond of the alk/aryloxide results in significant linearization of the group. Work to verify this experimentally is currently in progress. We are also currently collaborating with Prof Kaltsoyannis and Prof. J. C. Green at Oxford to explore the electronic structure of [LnCp₃] complexes by variable energy photoelectron spectroscopy. Recent experiments have shown that the f1 complex [CeCp₃] gives two ionization states rather than one, suggesting that [CeCp₃]⁺ is multiconfigurational .
Finally we are expCering the chemistry of cerium (IV). Several years ago we published a XANES study showing that, in line with theoretical predictions, the iconic molecule cerocene, [Ce(h-C₈H₈)₂] was an unusual example of a trivalent complex in which there is very strong coupling between the cerium and the COT ligands. Because the result flies in the face of received wisdom we are currently preparing new examples of Ce(IV) coordination compounds and organometallics including the new family [CeCp₃(OAr)] (Ar = substituted aromatic group) in order to broaden the scope of our earlier results as well as to gain a deeper understanding of the chemistry of these very reactive and often quite unstable species.

B. Polar Polymerization catalysis

As world-wide competition for oil becomes ever more intense and concern grows about our productio of indestructible waste there has been increasing interest in biodegradable polymers which can be prepared for renewable resources. A number of cyclic esters – lactide, caprolactone etc. – can be prepared from crops such as maize. We are using some of the alkoxides described above, as well as some very simple zirconocene aryloxides, to explore the individual steps involved in lactide polymerization. Paradoxically, a key virtue of a relatively inefficient catalyst is the possibility of observing individual steps in the polymerization process. We are using the tools of physical organic chemistry to look at the rates of the insertion step as a function of the Hammett parameter of the aryloxide and comparing this with the rates of propagation and chain transfer.

C. Mesoporous lanthanide-containing oxides

In a new area for us, we are developing methods to integrate lanthanide luminescence with the electrical properties of other elements such as silicon. This work which we are conducting in collaboration and Dr Tony Kenyon (UCL Electrical Engineering) involves the preparation of mesoporous host lattices (e.g. TiO₂ and Y₂O₃) doped with small percentages of luminescent ions. Of particular interest to use are those involved in optical communications such as Nd³⁺, Ho³⁺, and Yb³⁺. These lattices are prepared by a templated sol gel approach using non-ionic surfactants. After removal of the templates we are using CVD and/or electrochemical methods to deposit elements such as silicon into the pores to generate nano-structured composites.

We are also involved in other work in collaboration with the Carmalt group (synthesis of thiolate-based precursors to metal sulfides) and the McMillan group (synthesis of nitrogen-containing graphenes).

Selected publications

M. Coreno, M. De Simone, J. C. Green, N. Kaltsoyannis, A. Sella. Photoelectron spectroscopy of Ce(h-C5H5)3 – accessing two ion states on 4f ionization, Chem. Phys. Lett. , 2006, 432, 17-22.
J. L. Galler, S. Goodchild, J. Gould, R. McDonald, A. Sella, “Pyrazolyborate complexes with redox inactive lanthanides – the structures of [(Tp^Me2)₂NdX] (X = Cl, NPh₂, dpm, H₂BEt₂)” invited paper, “30 Years of Scorpionate Chemistry Special Issue”, Polyhedron, 2004, 23, 253-262 (Polyhedron Symposia-In-Print Number 26. Scorpionate and Related Ligands)
A. Domingos, G. Y. Lin, S. Y. Liu, N, Marques, G. H. Maunder, R. McDonald, A. Sella, J. Takats, M. R. J. Elsegood. “Facile pyrazolylborate ligand degradation at lanthanide centres: X-ray crystal structures of pyrazolylborinate-bridged bimetallics.”, Inorg. Chem. 2002, 41, 6761-6768.
M. R. Russo, N. Kaltsoyannis, and A. Sella, “Lanthanide alkoxides are linear owing to electrostatics!”, Chem. Commun. 2002, 2458-2459.
N. Marques, A. Sella, J. Takats “The Chemistry of the Lanthanides with Polypyrazolylborate ligands”, Invited Review, Special issue “Frontiers in Lanthanide Chemistry”, Chem. Rev., 2002, 102, 2137-2160.
I. Lopes, A. C. Hillier, S. Y. Liu, A. Domingos, A. Galvão, A. Sella, N. Marques “Solid-State Structure and Solution Behavior of Eight-Coordinate Sm(III) Poly(pyrazolyl)borate Compounds,.” Inorg. Chem., 2001; 40; 1116-1125.
A. C. Hillier, X.W. Zhang, G. H. Maunder, S Y Liu, T. A. Eberspacher, N. Marques, V. W. Day, A Sella and J. Takats, “Synthesis and Structural Comparison of a Series of Divalent Ln(Tp^R,R')₂ (Ln = Sm, Eu, Yb) and Trivalent Sm(TpMe₂)₂X (X = F, Cl, I, BPh₄) Complexes.”, Inorg. Chem., 2001, 40, 5106-5116.
A. C. Hillier, S.-Y. Liu, A. Sella, and M. R. J. Elsegood. “(PhTe)₃^–: The anionic tellurium analogue of I₃^–”, Angew. Chem., Int. Edn. Engl., 1999, 38, 2745-2747.
N. M. Edelstein, P. G. Allen, J. J. Bucher, D. K. Shuh, C. D. Sofield, N. Kaltsoyannis, G. H. Maunder, M. R. Russo, A. Sella, “The Oxidation State of Ce in the Sandwich Molecule Cerocene”, J. Amer. Chem. Soc., 1996, 118, 13115-13116.

This page last modified 20 October, 2009