Pressure-sensitive complex formation based on self-assembling ligands

Lead Research Organisation: University of Edinburgh
Department Name: Sch of Chemistry

Abstract

Salicylaldoximes and related ligands are important in metal extraction processes, being used in the extraction of some 20% of the world's copper. They self-assemble into H-bonded dimers in non-polar solvents and in the solid state, creating pseudo-macrocyclic rings. The cavity at the centre of the pseudomacrocycle can accommodate divalent metal cations to give complexes which are stabilized by the inter-ligand hydrogen bonds. Substitution on different sites around the ligand has been shown to influence the cavity size and other properties, thereby altering the extractant strength. By addition of appropriate arms, it has also been possible to develop ligands which extract not only metal cations, but also their attendant anions (metal salt extractants). Salicylaldoximes are therefore extremely versatile ligands, but we have found that their geometric properties may be tuned even further by application of high pressure. Pressures of 1000 to 10 000 atm (0.1 - 1 GPa) are modest by the standards of modern high-pressure research, and are readily attainable in the laboratory using diamond anvil cells; pressures in a similar range are even used industrially for food treatment. However, pressure in this range has important structural consequences for the ways in which organic molecules interact and for the internal structures of metal complexes. Many organic molecules form crystalline structures with different hydrogen bonding patterns from those seen at atmospheric pressure; the magnetic properties of nanomagnetic complexes can be altered; and metals can be driven to increase their coordination numbers. We will design salicylaldoxime and related ligands which form pseudomacrocycles with cavities which are sensitive to pressure. We will then use pressure to tune the cavity-sizes, changing the selectivity of ligands for specific metal cations and metal salts. We plan to study systems where pressure effects are seen in the range 0.1 - 1 GPa, as this is the region where practical applications are most likely, though pressures up to 10 GPa will be readily accessible. Ligands and their complexes will be studied in the solid state using X-ray crystallography, which will provide precise geometric data on the effects of pressure. In real-life applications extraction occurs in solution, and so we will also investigate the effect of pressure on ligands and their complexes in solution using EXAFS and laser spectroscopy. This will enable solution state equilibrium constants to be determined, providing quantitative information on competitive binding as a function of pressure. We will also scale-up the high pressure processes discovered, establishing high pressure as a tool in supramolecular synthesis.We have assembled a multi-disciplinary team to carry out this work, which depends on innovations in synthesis, crystallography, spectroscopy and engineering. This project will form a pioneering step in the development of Gigapascal Chemistry.

Publications

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Byrne PJ (2012) Piezochromism in nickel salicylaldoximato complexes: tuning crystal-field splitting with high pressure. in Chemistry (Weinheim an der Bergstrasse, Germany)

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Cameron C (2014) A pressure-induced displacive phase transition in Tris(ethylenediamine) Nickel(II) nitrate in Zeitschrift für Kristallographie - Crystalline Materials

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Chang J (2011) Anion-selective receptors based on dinuclear copper(II) and nickel(II) cage complexes of bis-salicylaldimines in Journal of Inclusion Phenomena and Macrocyclic Chemistry

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Clegg JK (2017) Reversible Pressure-Controlled Depolymerization of a Copper(II)-Containing Coordination Polymer. in Chemistry (Weinheim an der Bergstrasse, Germany)

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Forgan RS (2010) Cation and anion selectivity of zwitterionic salicylaldoxime metal salt extractants. in Dalton transactions (Cambridge, England : 2003)

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Mason K (2010) Building Fe(III) clusters with derivatised salicylaldoximes. in Dalton transactions (Cambridge, England : 2003)

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Mason K (2012) Linking [M(III)3] triangles with "double-headed" phenolic oximes. in Dalton transactions (Cambridge, England : 2003)

 
Description The hypothesis that pressure can be used to compress the metal-containing cavity in salicylaldoxime complexes has been confirmed in Ni-salicylaldoximato complexes using high-pressure crystallography up to 6 GPa. The extent to which different Ni-O or Ni-N distances shorten was found to be due to supramolecular or packing effects, correlating with the distribution of intermolecular voids. This result reveals the importance of supramolecular effects in determining the path of compression. The compression of the cavity also leads to piezochromism: crystals which are green at ambient pressure are red at 5 GPa. Combination of the crystallographic results with DFT simulations and optical spectroscopy enable the colour change to be traced to changes in the electronic structure of the metal complex which occur on compression. Compression of related copper complexes shows quite different behaviour, with the metal atom expanding its coordination number from four to six. Real-life extraction processes occur in solution rather than the solid state and we have explored the effect of pressure on Ni- and Cu complexes in solution using EXAFS. It was shown for the first time that the coordination environment of the metal is very much more compressible in solution than in the solid state with distance changes an order of magnitude higher.
Exploitation Route The work described has led to advances in technique development for extreme conditions research. We have established methodologies for high-pressure diffraction on the I19 beamline at Diamond Light Source; 20% of the applications for access to this facility are now for high-pressure research, in many cases by Chemists new to the area. Users of I19, even those not conducting high-pressure research, often use our software for conversion between diffraction image formats. We have also developed reactors for investigating multi-phase systems at up to 0.5 GPa on the gram scale, for high-pressure measurements in fluorescence spectroscopy and, in collaboration with Dr Andrei Sapelkin (QMC), techniques for high-pressure EXAFS in I18 at Diamond. Advances made in high-pressure research can also have quite unpredictable impacts on other areas, an example being development of a new method for precise absolute structure determination in organic chemistry which grew out of work on the consequences of incomplete data in high-pressure crystallography conducted as part of this project.
Sectors Chemicals,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology

URL http://www.crystal.chem.ed.ac.uk
 
Description 'Absolute structure' is used in crystallography when a crystal structure is not superimposable on its inverted image (it is similar to chirality in molecules). It is an issue that needs to be addressed in any structure determination in a non-centrosymmetric space group - this include enantiopure crystals of chiral compounds. For purely organic compounds there has been a long-standing problem that conventional crystallographic measurements do not distinguish one absolute structure from its alternative with acceptable precision. A new method for precise absolute structure determination in organic chemistry grew out of work on the consequences of incomplete data in high-pressure crystallography conducted as part of this project. The work was published in 2013 [ACTA CRYSTALLOGRAPHICA SECTION B-STRUCTURAL SCIENCE Volume: 69 Pages: 249-259 ], and has so far been cited ~300 times in the literature and 1300 entries in the Cambridge Database. It is now the method used to determine absolute structure in the most popular structure refinement package (Shelxl) and is used world-wide. The method is most obviously applicable in the pharmaceutical industry, where characterisation of the absolute structure of chiral APIs is of utmost importance.
First Year Of Impact 2012
Sector Chemicals,Pharmaceuticals and Medical Biotechnology
 
Description Cambridge Crystallographic Data Centre
Amount £26,662 (GBP)
Organisation Cambridge Crystallographic Data Centre 
Sector Academic/University
Country United Kingdom
Start 09/2011 
End 08/2014
 
Description Cambridge Crystallographic Data Centre
Amount £26,662 (GBP)
Organisation Cambridge Crystallographic Data Centre 
Sector Academic/University
Country United Kingdom
Start 09/2011 
End 08/2014
 
Description Diamond Light Source
Amount £370,500 (GBP)
Funding ID MT1200 
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 01/2010 
End 12/2011
 
Description Diamond Light Source
Amount £78,000 (GBP)
Funding ID SP7056 
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 04/2011 
End 04/2011
 
Description Diamond Light Source
Amount £97,500 (GBP)
Funding ID SP6095 
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 10/2010 
End 10/2010
 
Description Diamond Light Source
Amount £58,500 (GBP)
Funding ID SP7368 
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 01/2012 
End 01/2012
 
Description Diamond Light Source
Amount £78,000 (GBP)
Funding ID MT1061 
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 09/2009 
End 09/2009
 
Description Diamond Light Source
Amount £52,609 (GBP)
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 09/2010 
End 09/2013
 
Description EPSRC
Amount £939,833 (GBP)
Funding ID EP/J00099X/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2012 
End 03/2017
 
Description EPSRC
Amount £434,080 (GBP)
Funding ID EP/H004106/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 03/2009 
End 08/2012