YC Oral Presentation 2

By | April 26, 2011

Second session of the oral presentations of YC Satellite of BCA Spring Meeting.

Here are the abstracts of their presentations:

1. Proton migration in molecular complexes of urea and its derivatives.
     – Andrew Jones (University of Glasgow / Institut Laue-Langevin)
2. Synthesis and structural characterisation of multi-nuclear cages with ortho-palladated ligands
     – Claire Murray (University of Reading)
3. Control of asymmetric cell division in developing Drosophila neuroblasts
     – Miriam Walden (University of Leeds)
4. Using pressure to direct polymorph formation: overcoming isotope effects in the 4-methylpyridine pentachlorophenol co-crystal
     – Nick Funnell (University of Edinburgh)

Proton migration in molecular complexes of urea and its derivatives
Andrew O F Jones, University of Glasgow / ILL Garry J McIntyre, Bragg Institute Lynne H Thomas & Chick C Wilson, University of Bath

Systems showing proton disorder, transfer and migration are an area of much interest in recent times with widespread implications for areas such as ferroelectrics and enzyme action. These often subtle processes can affect the properties of systems and are often observed in materials containing short strong hydrogen bonds. They can be probed using variable temperature diffraction experiments. We aim to identify and characterise systems that show such effects, with a view to predicting and controlling this potentially “tunable” behaviour. These processes have previously been observed in complexes of urea [1].

Single crystal X-ray and neutron diffraction have been used to study molecular complexes of urea and its derivatives with a particular focus on characterising the hydrogen atom behaviour. The proton behaviour is monitored over variable temperatures, with the aim of observing movement of the proton with changing temperature. A limited number of such mobile proton effects have been observed using variable temperature diffraction measurements, and this project aims to build on these observations by extending these investigations to further, related, systems. The instrument used for the neutron studies is VIVALDI at the ILL in Grenoble which is ideal for use here due to its potential for high throughput single crystal diffraction of small samples. Temperature dependent proton migration in the 2:1 complex of N,N-dimethylurea and oxalic acid has been observed by single crystal X-ray and neutron diffraction data confirming the existence of this phenomenon. Accurate determination of the hydrogen atom parameters in this system is vital in understanding this phenomenon and also in explaining a single-crystal to single-crystal phase transition at ~120-130 K. Unusual thermal properties have been observed in the 2:1 complex of urea and fumaric acid in which hydrogen ADP trends may play a role in potential negative thermal expansion along one of the crystallographic axes. Proton transfer has also been observed in a series of complexes of urea with bromanilic acid. All of these studies have benefited from our approach of multiple condition diffraction studies of structural evolution.

Synthesis and structural characterisation of multi-nuclear cages with ortho-palladated ligands
C. Murray, A. Swift, H.M. Colquhoun, C.J. Cardin, Department of Chemistry, University of Reading, Reading; D.Allan, Diamond Light Source Ltd, Didcot, OX11 0DE.

The ability of palladium to form Pd-C sigma-bonds can lead to industrially-important catalytic formation of C-C bonds, but can also afford very stable organopalladium compounds based on chelating ligands with Pd-CAr and Pd-N bonds. In the present work, novel palladium cage-complexes with cyanuric acid bridging ligands have been synthesised and characterised. Cages based on ortho-palladated 2-phenylpyridinate (ppy-) and dimethlybenzylamine (BDMA-) ligands have been investigated.

In the 2-phenylpyridinate system, reaction of the parent dimer [Pd(ppy)Cl]2 with cyanuric acid, in the presence of triethylamine, yields a decanuclear cage [Pd10(ppy-)10(C3N3O3)2(C3N3O3H)2], and a dodecanuclear homologue [Pd12(ppy-)12(C3N3O3)4]. Single crystal x-ray diffraction studies of these compounds highlight the impact of the reduction in symmetry by removal of a Pd2 dimer, with monoclinic symmetry (C2/c) for the decanuclear structure and hexagonal symmetry (P63) for the dodecanuclear structure. The stability of the dodecanuclear palladacycle (4 cyanurates to 12 Pd) results in its formation even when the correct stoichiometry for the decanuclear species is used (4 cyanurates to 10 Pd). In-situ NMR studies have demonstrated the thermodynamic stability of the dodecanuclear cage, with irreversible conversion of the minority product [Pd10(ppy-)10(C3N3O3)2(C3N3O3H)2] to the dodecanuclear structure on heating to 85 °C for three hours in DMSO. The influence of solvent systems in the crystal structure of [Pd12(ppy-)12(C3N3O3)4] is also significant, as the space group varies from hexagonal P63 on crystallisation from chloroform and methanol to monoclinic P21/a when crystallised from chloroform and octan-1-ol.

The ortho-metallated dimethylbenzylamine analogue [Pd12(BDMA-)12(C3N3O3)4] (previously characterised) exhibits a much improved solubility compared with [Pd12(ppy-)12(C3N3O3)4] and this is attributed to the packing of the planar units of the latter cage. The presence of both enantiomers of the former cage renders its overall crystal structure racemic, whereas crystals of the latter cage are chiral.

Control of asymmetric cell division in developing Drosophila neuroblasts
M. Walden, Astbury Centre for Structural Molecular Biology, University of Leeds, Leeds, LS2 9JT, UK.

Asymmetric cell division enables multipotent stem cells to divide to produce a vast range of cells with diverse functions. Thus, it plays a key role in the development of higher organisms. In the developing Drosophila central nervous system, the multipotent stem cells, known as neuroblasts, undergo asymmetric cell division, producing a new neuroblast and a smaller ganglion mother cell (GMC). Whilst the new neuroblast repeats this cycle (self renewal), the GMC divides once more to produce two terminally differentiated neurons and/or glial cells. During mitosis of the neuroblasts, several molecules have been shown to be vital in order for asymmetric division to take place. These include molecules which are asymmetrically localised and partitioned into the GMC (including, prospero mRNA and Miranda, Brat and Prospero proteins), and molecules that set up and maintain the asymmetric potential of the cell (including, Inscuteable, Par-3/6, Bicaudal D, Egalitarian and Rab6). In order for correct asymmetric cell division to occur, several of these molecules must interact with each other. However, currently, the mechanisms underlying these protein interactions are
unknown.

Here we focus on gaining structural insight into the mechanisms that drive asymmetric cell division using X-ray crystallographic techniques to try and determine high resolution structures of the proteins involved. Using pressure to direct polymorph formation: overcoming isotope effects in the 4-methylpyridine pentachlorophenol co-crystal

Using pressure to direct polymorph formation: overcoming isotope effects in the 4-methylpyridine pentachlorophenol co-crystal
N. Funnell and S. Parsons Department of Chemistry, University of Edinburgh.

Polymorphism via isotopic substitution is a rare phenomenon mostly observed in inorganic systems but is known to occur in two organic compounds: pyridine [1] and the co-crystal methylpyridine pentachlorophenol [2]. In the latter compound, the isotopically normal structure crystallises in the space group P-1 with a volume per formula unit of 363 Å3. Replacing hydrogen with deuterium at the phenolic position results in the crystal adopting Cc symmetry instead, with a volume per formula unit of 356 Å3. 1H MAS NMR studies indicate that the crystal structure is solely dependent on the phenolic position [3].

Crystal packing is frequently driven by the need for molecules to pack efficiently; minimising the PV term in the Gibbs free energy equation. Here, the Cc polymorph is more efficiently packed than the P-1 isotopologue. We show how pressure can be used to overcome the effects of isotopic substitution and thus obtain hydrogen-containing methylpyridine pentachlorophenol with Cc crystal symmetry. The results in this study clearly demonstrate the potential for high pressure as a tool in crystal engineering where specific polymorphs can be targeted and obtained.

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