School of Chemistry

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Malcolm Halcrow

Professor of Inorganic Chemistry
Member of the Crystallisation and Directed Assembly group

Contact details

Room: 1.26
Tel: +44 (0)113 3436506
Email: M.A.Halcrow@leeds.ac.uk

Keywords

Co-ordination Chemistry
Supramolecular Chemistry
Switchable Metal Complexes
Molecular magnetochemistry
EPR Spectroscopy
Electrochemistry

Photograph of Malcolm Halcrow

Research interests

Spin-Crossover materials
Spin-crossover iron(II) complexes can switch reversibly between a pale-coloured, paramagnetic high-spin state to a dark-coloured, diamagnetic low-spin state. This transition can be induced by changing temperature, by laser irradiation or by some other physical stimulus. While the phenomenon has been known for seventy years, there is still intense worldwide interest in these thermochromic and photochromic switches, which have been used in display and memory devices. We are pursuing new spin-crossover materials with technologically favourable properties, and trying to elucidate the factors that control spin-transitions in the solid state. The latter depends on interactions between molecules in the solid as much as on the molecules themselves, and is therefore a problem of crystal engineering. We are also involved in an interdisciplinary project to incorporate spin-crossover centres onto surfaces, and into other types of functional material.

Manipulating Jahn-Teller Phenomena in Copper Chemistry
Several years ago, we discovered that the electronic structures of certain six-coordinate copper(II) complexes can be changed using sterically or conformationally restricted ligands. In this way, we produced the first true examples of molecular six-coordinate copper(II) compounds with axially compressed, rather than elongated, structures. Although other such compounds had been claimed previously, most of these were subsequently found to be crystallographic or spectroscopic artifacts. We have recently returned to this chemistry, to try and induce similar effects using more subtle physical triggers. We are aiming to produce an EPR-based sensor, where the Jahn-Teller structure of a copper reporter group is changed by a guest binding event.

Mixed-Valent Radical Complexes
A new project for us, is to investigate metal complexes containing multiple redox-active organic ligand centres. An example of such a compound is cyclotricatechylene (CTC), which is a cyclic molecule with three covalently linked catecholate groups, that can each bind a metal ion. Platinum complexes of CTC exhibit three reversible low-potential oxidations in their cyclic voltammograms, corresponding to step-wise oxidation of its three catecholate groups to semiquinone radical centres. The “mixed-valent” products, containing both oxidised and unoxidised CTC redox sites, are intensely coloured with strong visible and NIR absorptions. These reflect electron delocalisation around the ligand; the mechanism of this unusual delocalisation is something we are actively studying. Moreover, NIR-absorbers like these, whose absorptions can be switched on and off electronically, can be very useful in fibre-optic communications devices.

Inorganic Supramolecular Chemistry
Pyrazole rings contain both Lewis basic (N: donor) and Lewis acidic (N–H donor) groups in close proximity to each other. We are interested in using pyrazole derivatives as ligands that coordinate to cations and anions at the same time; that is, as ligands for metal salts. On one hand, we can use hydrogen-bonding interactions between metal centres and anion guests to template unusual metal-organic network structures or cluster compounds. Alternatively, we can use pre-formed metal centres containing this type of functionality as supramolecular hosts for anions. Both aspects of this chemistry are being pursued.

Useful links

CV and Bibliography (PDF)   
Research (PDF)   
   

Selected publications

Structure:function relationships in molecular spin-crossover complexes. M.A. Halcrow, Chem. Soc. Rev. 2011, 40, 4119.

An iron(II) complex exhibiting five anhydrous phases, two of which interconvert by spin-crossover with wide hysteresis. T.D. Roberts, F. Tuna, T.L. Malkin, C.A. Kilner and M.A. Halcrow, Chem. Sci. 2012, 3, 349.

Suppression of the Jahn-Teller distortion in a six-coordinate copper(II) complex by doping it into a host lattice. R. Docherty, F. Tuna, C.A. Kilner, E.J.L. McInnes and M.A. Halcrow, Chem. Commun. 2012, 48, 4055.

New insights into the aggregation of silver pyrazolides using sterically hindered bidentate pyrazole ligands. J.J. Henkelis, C.A. Kilner and M.A. Halcrow, Chem. Commun. 2011, 47, 5187.