Malcolm Halcrow

Research section: Inorganic Chemistry

KEYWORDS Co-ordination Chemistry Supramolecular Chemistry Molecular magnetochemistry EPR Spectroscopy Electrochemistry

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.

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.

Manipulating Jahn-Teller Phenomena in Copper Chemistry
Ten 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.

Models for Metalloenzymes containing Amino Acid Radicals
We have an interest in compounds designed to model the chemistry of metalloenzymes containing chemically modified amino acid side-chains. A well-studied example of such an enzyme is galactose oxidase, which uses an (almost) unique thioether-substituted tyrosinate residue to form an unusual free radical in active enzyme, that is more thermodynamically stable than more typical tyrosyl radicals by 0.5 V. We are interested in preparing new metal complexes that reproduce certain structural features of these and other bioradicals, in order to understand how their structures contribute to their reactivity.

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SELECTED PUBLICATIONS

Trapping and manipulating excited spin states of transition metal compounds, M.A. Halcrow, Chem. Soc. Rev. 2008, 37, 278.

Iron(II) complexes with a terpyridine embrace packing motif show remarkably consistent cooperative spin-transitions, R. Pritchard, C. A. Kilner and M. A. Halcrow, Chem. Commun. 2007, 577.

A cobalt metallacrown anion host with guest-dependent redox activity”, L.F. Jones, C.A. Kilner and M.A. Halcrow, Chem. Eur. J. 2009, 15, 4667."

A crystalline hydrogen-bonded network with a poly-catenate topology, A.A. Salaudeen, C.A. Kilner and M.A. Halcrow, Chem. Commun. 2008, 5200.