UCD School of Chemistry & Chemical Biology

We are strongly interested in exploiting molecular properties of transition metals that are located in a precisely defined environment, as this makes them widely useful as active sites in chemistry, biology and materials science. The activity of these metal centers can be altered and tuned by selective modifications of the ligand sphere, hence providing a versatile methodology to tailor their application range. Three aspects of this exciting area of research are currently under investigation: (i) implementation of organometallic components as active sites in materials; (ii) development of new catalytic systems for the activation of unreactive bonds; (iii) use of supramolecular principles for tailoring metal-centered processes. In addition, we are interested in investigating the relevance of carbene-type bonding motifs in biological systems.

Organometallic components as active sites in materials

Our current research program aims at synthesizing novel transition metal complexes with selected reactivity properties, in particular with respect to reversible magnetic and electronic processes (formation of molecular switches and wires). Our approach relies strongly, though not exclusively, on the recent discovery of heterocyclic carbenes as novel class of ligands in organometallic chemistry. These ligands fulfill most requirements for electronic applications such as a high degree of electron delocalization along the M=C bond. When complexed to an appropriate transition metal M, useful properties are obtained such as redox activity (with Fe, Ru), spin-state transitions (Fe). Ligand modification can then be used to optimize the activity of the metal site, to produce ditopic ligand features. This strategy has afforded bimetallic complexes as prototypical molecular switches, and polymeric redox-active materials as an examples of molecular wires. 

New catalysts based on non-classical carbenes

Homogeneous catalysis has been greatly stimulated by the recent discovery of N-heterocyclic carbenes as ligands for transition metals. The large majority of carbene complexes that have been used for catalytic applications thus far have been derived from imidazolium salts that bind via C(2) to the metal center. Despite the fact that the carbene is stabilized by two adjacent heteroatoms in such a bonding mode, these ligands appear to be among the most basic neutral ligands known. Recently, we and others have shown that the ligand basicity can be altered by changing the carbene framework. For example, in pyridylidene-type systems appear to be better sigma donors and better pi acceptor ligands when coordinated to late transition metals. Abnormal imidazolylidenes that bind via the C4 carbon in turn are extraordinarily strong, formally neutral donor ligands. The impact of these ligands on catalysis is substantial and we have recently disclosed catalytic systems that allow for the activation of less reactive bonds such as H¿H and functionalized C¿H bonds. Notably, these reactions fail when standard 2-imidazolylidene ligands are bound to the metal center. We aim at further exploiting the catalytic potential of platinum group transition metals that are bound to such non-classical carbenes in particular for activation of otherwise unreactive C¿H and C¿C bonds.

Representative contributions: Angew. Chem. Int. Ed. 2009, 49, 9765; J. Am. Chem. Soc. 2008, 130, 13534; Chem. Eur. J. 2012, 18, 652. Recent reviews: Chem. Commun. 2008, 3601; Chem. Rev. 2009, 109, 3445; Science 2009, 326, 532.

Use of supramolecular principles for tailoring metal-centered processes

We aim at functionalizing electronically or magnetically active complexes with molecular recognition sites for hierarchical organization. Controlled assembly of the active sites is expected to enhance intermolecular cooperativity and hence to improve the material¿s properties. Self-assembly at interfaces, e.g. into Langmuir-Blodgett mono- and multilayers, (inverse) micelles, or solid-liquid interfaces may ultimately result in the development general strategies for the implementation of desired molecular properties into devices. Our focus is directed in particular to the self-assembly of spin-labile and redox-active sites as a methodology for the construction of tailor-made (electro-)magnetic materials.

Representative contributions: J. Am. Chem. Soc. 2008, 130, 14434; Dalton Trans. 2012, 41, 3716.