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School of Chemistry



 

Kilian Research Group

 


 

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Research activities in our newly established group are centred on the design and synthesis of novel organophosphorus and Group 15 organoelement compounds (As, Sb, Bi). We synthesize these new compounds for various practical applications, such as spin labels. These novel species are also of great academic interest, as they often display intriguing structural features and unusual bonding situations.

 

Our typical synthetic project involves:

1.       The synthetic target is proposed, often with the help of computational chemistry.

2.      Preliminary synthetic strategies towards the target compound are devised and tested. Sometimes several synthetic approaches need to be employed in order to achieve the initial goal, or the target has to be modified to become achievable.

3.      The desired characteristics of the target compound (sometimes predicted computationally) are verified experimentally.

4.      The characterization results are fed back into synthesis. Suitable structural modifications leading to improved characteristics are proposed and necessary modifications are introduced to the synthetic pathway.

Unusual (and therefore interesting) structural features, properties and reactivity are frequently encountered during our experimental work. Exploitation of these unforeseen results is an important part of our scientific activity.

 

The four major areas of our research are:

Peri-substituted naphthalenes and related systems

Stable phosphorus centred radicals

Green chemistry of white phosphorus

P-C bond forming reactions for tertiary phosphine synthesis

 

 

Peri-substituted Naphthalenes and Related Systems

Our long-term research interest is the main group chemistry of peri-substituted (i.e. 1,8-disubstituted) naphthalenes and related molecular frameworks. The special geometry imposed by the rigid organic backbone forces the two substituents attached to 1 and 8 positions into a close proximity (approx. 2.5Å). This makes the attractive interaction (bond) between the two attached atoms highly favourable as its formation leads to minimisation of the steric strain. Such buttressing of the peri-motif often translates into unusual reactivity, bonding, structure and properties of the newly synthesised species. Understanding the reactivity and developing synthetic methodology of multiply functionalised organoelement molecules is important for practical applications, for example in catalysis. Shifting the boundary of what is possible in terms of bonding and structure is of fundamental interest. Peri-substitution is very rewarding in this sense as unusual features are almost ubiquitous here.

 

In several cases we have used peri-substitution as means of stabilizing fleeting species. For example, phosphine-phosphine donor-acceptor complexes such as R3PPRX2 were detected as thermally unstable intermediates as early as the 1950’s. Formation of these complexes has profound consequences for reactions containing (even just temporarily) the mixtures of two phosphines. The key to the formation of the donor-acceptor species is that Lewis properties of the two phosphines differ significantly. However, isolation of these species proved difficult as all known examples decompose well below room temperature. Using peri-substitution, we have been able to make the first isolable (room temperature stable) phosphine-phosphine donor-acceptor complex. The unique thermal stability (“bottleability”) of our phosphine-phosphine complex allowed studying reactivity of this class of compounds for the first time and resulted in discoveries of several fundamentally new reactivity patterns. Donor-acceptor and zwitterionic forms of our phosphine-phosphine complex, along with the crystal structure, are shown below.

 

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Stable Phosphorus Centred Radicals

Radicals are usually observed as rather short-lived intermediates. In main group chemistry it is possible stabilise them to the extent that they can even be isolated in crystalline form. We have begun our investigations into stable phosphorus centred radicals recently; of our particular interest are radicals with potential to be used as spin labels. Spin labels are artificial paramagnetic probes introduced into a system (for example large biomolecules such as membrane proteins) in order to make it ‘visible’ by Electron Paramagnetic Spectroscopy (EPR), which is a particularly sensitive resonance spectroscopy technique. New spin labels with favourable paramagnetic characteristics are extremely desirable since they can improve the capabilities and scope of those established, as well as several newly emerging and rapidly developing magnetic resonance methods, such as DNP (Dynamic Nuclear Polarization, NMR enhancement method), High Field and Pulsed EPR (used in long range distance measurements, and dynamic processes investigations in biomolecules), as well as EPR Imaging (medicinal imaging complementary e.g. to the more familiar NMR imaging - MRI). As the need for new classes of spin labels is growing, our activities will be even more strongly linked to spin label chemistry in the near future.

 

 

Phosphinyl radicals have large π-character of their SOMO (Singly Occupied Molecular Orbital) and therefore large hyperfine anisotropy, which makes them highly orientationally sensitive spin labels.

 

Phosphoranyl radicals can have extremely large hyperfine coupling, which generates huge interest in their development as polarizing agents in Dynamic Nuclear Polarization techniques.

 

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Green Chemistry of White Phosphorus

Many phosphorus chemicals (bulk and fine) are made from white phosphorus, but indirectly via phosphorus trichloride. Manufacture of PCl3 requires chlorine gas, which is highly toxic and corrosive, possesses high environmental risk and is ‘energy expensive’. Halogenation of white phosphorus to PCl3 serves essentially as a means of moderating the reactivity and often no halogen is retained in the resulting products. Therefore, it is highly desirable to search for other means of moderating the reactivity of white phosphorus in order to achieve energy and atom efficient transformations into high value chemicals, without the formation of halide waste. Ideally the new reactivity moderators replacing halogenation will be inexpensive, non-toxic, highly specific and highly efficient (therefore only catalytic amounts will be needed).

 

 

 

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P-C Bond Forming Reactions for Tertiary Phosphine Synthesis

P-C bond forming reactions are central to syntheses of tertiary phosphines, which in turn are essential components of many transition metal complexes used as catalysts in many organic transformations. Established P-C bond forming strategies require specifically activated organic substrates to achieve the required reactivity when connecting them to the phosphorus atom regiospecifically. The aim of our research in this area is to develop new synthetic routes to make a variety of tertiary phosphines from non-activated substrates, mainly aromates.

 

 

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