Research Interests

High Nuclearity Grids and Clusters (M₂ to M₃₆ and Beyond)

Strategies to produce coordination complexes with large numbers of transition metal centers include direct synthesis from a polyfunctional ligand, and methods which use the organizing ability of a metal ion and a ligand or a ligand precursor (e.g. template syntheses). Self assembly is a powerful approach which involves the encoding of coordination information into a ligand, and then using a metal ion to interpret and use this information, according to its own coordination preferences, in order to organize the growth of large polynuclear metal ion arrays. These are of interest from a variety of viewpoints, including their inherent beauty, but more practically because they provide routes to novel magnetic materials, potential catalysts, and in the context of a ‘bottom up’ approach to ‘devices’ based on molecules, an entry into the electronic, and perhaps magnetic, high technology arena of the future. Grid arrangements of transition metal ions bridged in close proximity are viewed as ‘quantum dot’ like arrays of communicating spin centers, and are very attractive platforms for switching and data storage at the molecular level. Our current research projects in this area involve studies on square [nxn] grids (n=2, 3, 4, 5), with magnetically and electrochemically active metal ions, e.g. Mn(II), Fe(II), Fe(III), Co(II), Ni(II), Cu(II). Such systems are obtained by self assembly synthetic strategies, and studied using structural, magnetic, electrochemical, and epr techniques. Strategies for larger [nxn] grids (n = 6, 8) are being developed with hexa- and octa-topic ligands as targets.

Ligand Syntheses

The success of creating polynuclear complexes by either direct synthesis or a self-assembly approach relies heavily on the use of organic synthetic techniques to produce new ligands and ligand precursors. This has been the cornerstone of our success in the polynuclear complex area for many years, and we continually strive to create new and novel ligand systems, which will provide a ‘directed’ synthetic approach to high nuclearity systems. Fine tuning of the ligand in terms of the donor atoms and their spatial arrangements has allowed us to create many classes of ligand which, with experience, can produce predictable polynuclear complexes, particularly self-assembled grids. Organic synthesis continues to be a vital part of our approach to new and novel materials.

Magnetic Studies

Our interest in the creation of polynuclear complexes has been driven in part by our deep seated interest in the magnetic properties of such systems, and in particular the geometrical and electronic properties of the ligands as they pertain to magnetic properties at the molecular level based on the transition metal ion. We have published widely on magneto-structural correlations in e.g. dinuclear complexes involving hydroxide, phenoxide, and 1,1-azide bridges, and more recently have been examining high nuclearity systems, in particular [nxn] grid arrays of metal ions bridged in close proximity by oxygen atoms, e.g. [M₉(µ-O)₁₂]. These systems display antiferromagnetic (Mn(II), Fe(II), Fe(III), Ni(II), Co(II)) and ferromagnetic (Cu(II)) behavior, with the Mn(II) grids acting as quantum nano-magnets at low temperature. Dealing with large numbers of spin centers in a single molecule presents special computing challenges, because of the large matrix dimensions involved in calculating the total spin state combinations. We have developed an integrated software package (MAGMUN4.0), built on a Windows™ platform, which handles large systems (e.g. Co(II)₉; (S=3/2)₉) successfully, and are in the process of implementing symmetry reduction methods to handle larger systems. As an illustration a high spin Mn(II)₉ grid would require ~60 GB of RAM for a full isotropic spin state calculation, which can be reduced to ~ 4GB by imposing D₄ symmetry on the grid.

Electrochemical and Surface Studies

The Mn(II)₉ grids exhibit very rich electrochemistry, with the transfer of up to eight electrons within the potential window 0.5-1.6 V (Ag/AgCl), associated with the oxidation of eight Mn(II) centers to Mn(III). Oxidation can also be achieved using chemical oxidants. The magnetic and spectroscopic properties of the grids change on oxidation, which can be detected easily using SQUID measurements and UV/Vis spectroscopy. These processes are reversible both electrochemically and chemically, and so the grids display multi-property function. A major current focus is to try to detect a single molecule response based on these bulk properties, and an obvious way to try this is to first immobilize the molecules on a suitable surface substrate. This has been achieved in several cases on Au(111), with the formation of self-assembled monolayers (SAMS) and a current thrust is to probe individual molecules on the surface electrochemically and by single molecule epr techniques.

Device Applications

The micro-electronics industry is still operating on a ‘top-down’ principle, which in many areas, e.g. computers, is leading to serious limitations regarding processing speed, and data storage capacity. The ‘bottom-up’ approach, using atoms or molecules, is seen as the way of the future, but harnessing the power of an individual molecule is a major challenge. We have demonstrated multi-electron reversibility in the Mn(II) grid systems, and this property is being exploited at the molecular level in terms of ‘molecular device behavior’. Such studies require overcoming obstacles not normally encountered in simple bench chemistry, which present rather different challenges. In this context we have established collaborations with physicists and surface chemists to assist us in probing these systems at the molecular level. As a simple illustration SAMS of Mn(II)₉ grid molecules, which have an ~2.5x2.5 nm footprint on Au(111), would be capable of storing of the order of 100-150 Tb/in2 with one bit of encoded information per molecule.