Research
Research in the Filippou group focuses on the areas of modern molecular chemistry of transition metals and main group elements, combining the potential of both to create new compounds with unusual bonding and reactivity. Emphasis is placed on the isolation and characterization of highly reactive closed- and open-shell molecules containing silicon, germanium, tin, and transition metals in novel bonding modes, and on the use of the compounds constructed from them in stochiometric or catalytic reactions. To achieve these goals, specialized synthetic as well as advanced analytical and quantum chemical methods are employed.
Triple bonds of Si - Pb with transition metals
Multiple bonded compounds of the elements of the 2nd row of the periodic table are stable and ubiquitous in all areas of chemistry. Homonuclear examples such as the alkenes and alkynes have been known for more than 150 years and are central building blocks in organic synthesis and the chemical industry due to the particularly rich and diverse chemistry of their π-bonds. In contrast, multiply bonded compounds of the p-block elements of the higher series (n > 3) were long considered unstable molecules. This phenomenon was justified by the fact that these elements cannot form stable (p-p)π-multiple bonds because the increasing difference in the radii of the ns and np valence orbitals prevents isovalent orbital mixing (hybridization). Since the first report on stable ditetrels about 35 years ago, experimental and quantum chemical studies on multiple bonds of the heavier group 14 elements (Si - Pb) have aroused great interest, since these compounds exhibit significant structural and electronic differences compared to the carbon analogues, leading to completely new properties and reactivities.
In this context, for the past 15 years we have been exploring the chemistry of tetrelylidine complexes of the general formula LnM≡ER, where M is a transition metal, Ln is a specially tailored ligand sphere, E is one of the heavier tetrels (Si - Pb), and R is a bulky organyl substituent. Due to the high polarity of the M≡E triple bond, the chemistry of these novel, highly reactive compounds differs markedly from that of their carbon analogues, the transition metal-carbin complexes, which represent an important class of organometallic compounds with numerous applications in catalysis. Their unusual reactivity allows access to new materials, which include, for example, compounds with planar tetracoordinated silicon.
Low-valent silicon chemistry
A significant part of our research deals with silicon, the heavier homologue of Group 14 carbon, which due to its large natural occurrence is a ubiquitous component of materials with enormous future potential. Various silicon compounds in low oxidation states are important species in chemical vapor deposition for the production of semiconductors and solar cells or in the synthesis of organochlorosilanes in the Mueller-Rochow process. In our research group, we try to tame these highly reactive species and explore their chemistry under normal laboratory conditions.
Recent developments in molecular silicon chemistry have shown that N-heterocyclic carbenes (NHCs) are capable of stabilizing silicon centers at unusually low oxidation states. Interesting examples include the disilicon (0) compound (NHC)Si=Si(NHC), the Si(I) halides Si2X2(NHC)2 (X = Cl, Br, I), and the Si(II) halides SiX2 (NHC) (X = Cl, Br, I), which have proven to be particularly useful building blocks in low-valence silicon chemistry.
Computational Chemistry
We apply a wide range of different ab-initio and DFT-based quantum chemical methods, as well as various wavefunction analysis methods, to our compounds where transition metals are multiply bonded to heavier homologous carbon compounds. This helps us in two ways:
First, to gain deeper insight into the electronic structure of the compounds, leading to a better understanding of their properties.
Second, we can predict and/or explain reactivity and propose mechanisms for the sometimes unique and unprecedented reaction pathways.
All efforts in this direction are closely linked to experimental studies on this class of compounds. When we gained access to compounds with M≡E triple bonds (M = Mo, W; E = Si, Ge, Sn, Pb) in the early 2000s, our initial research focused on understanding the electronic structure of these compounds.
