Research Highlights

The major achievement of the research over the past 25 years has been the development of the concept of the “Coordination Cluster” (abbreviated further as CC) as an entity which can be used to create an apparently infinite variety of novel materials. As illustrated below with selected examples, the concept opens a whole new world of synthetic chemistry, delivering unprecedented systems in terms of their molecular compositions and electronic structures with the prospect of creating truly molecule-based versions of familiar solid-state systems. This represents a huge potential gain in terms of miniaturisation of devices once the operating conditions are optimised. However, in order to realise this potential, the synthetic capability which is now in place must be developed further with a particular view to deepening our understanding of the unusual and exotic nature of the electronic signatures and structures that these essentially quantum confined systems possess.

Whereas coordination chemistry, as originally envisaged by Werner, is based on the special properties of single transition metal ions enclosed in a coordination sphere of ligands to give complexes, the coordination cluster goes a step beyond this. Here, an ensemble of cooperatively coupled transition metal centres is encapsulated within a ligand shell. The key aspect here is that it is possible to invest the cooperatively coupled “inorganic” core of the coordination cluster with properties determined by the nature of the metal ions themselves, allowing for various blends of electronic structures, spin and oxidation states, and thus to create an electronic structure not accessible for single ions. In other words, the metal ensemble acts as a “superatom” or “superion” which, through the cooperative coupling of the individual metal centres, has electronic properties specified by the way in which the individuals form the collective.

This is illustrated in the figure below using the example of a coordination cluster containing 19 cooperatively coupled FeIII ions[1] which for {Fe19}stabilises a ground spin state of S = 35/2. Clearly such a spin is not possible on a single ion – at least in terms of the current extent of the Periodic Table. For such a CC it is also possible to functionalise the inside and outside portions of the ligand shell enclosing the coupled core. This allows for changes to be made to the properties of the core by providing different donor atoms in the interior of the shell as well as to the outside of the unit by providing groups to influence the electronic structure of the metal ion core or else to enhance supramolecular interactions so that individual CCs can be cooperatively linked.

In these ways, just as supramolecular chemistry goes “beyond the molecule” the coordination cluster goes “beyond the complex”. This can be viewed as the apotheosis of coordination chemistry – going beyond the Werner complex to a new territory with an essentially unlimited number of possible combinations of metal ion blends and ligand-induced phenomena, representing a huge and largely untapped resource for the next generation of coordination chemists to investigate.

structural features of cc feIII19

Structural features of the coordination cluster (left) illustrated by the FeIII19[1] SMM (right)

 

This chemistry also has a further dimension arising from the fact that the size of the inorganic core is of nanoscale proportions and has properties characteristic of systems lying in the quantum effect dominated world lying between the molecular and the bulk scale. The Fe19 example illustrates this nicely since the inorganic core can be identified as a small section of a solid state iron oxyhydroxide structure, but is also clearly subject to strong boundary effects in terms of the large surface versus bulk ratio steering its properties.[2]

Given this whole new world of possibilities to investigate and discover for the coordination cluster, we have investigated a number of cases which illustrate the scope of this chemistry. In detail, our synthetic method is designed to alter the course of the hydrolysis of transition metal ions, which is thermodynamically predestined to result in the formation of solid state extended structures such as oxides. In our approach we choose ligands capable of arresting the hydrolysis, making it possible to create a metastable trap which stabilises intermediate, aggregated species.

Initially, we concentrated on exploring how far our synthetic method could be extended to produce homometallic CCs, firstly based on FeIII. In order to produce CCs with other metal ions we chose modified ligands such that a {CuII44} cluster, which is still the largest Cu(II) cluster reported, and an {Al15} cluster, which is still the largest Al(III) coordination cluster formed using organic ligands could be synthesised and characterised.[3] Both systems show extended zeotypic newtwork packing arising through supramolecular interactions between the CCs and we took this idea further with an {FeIII13} CC system where the clusters are linked through carboxylate groups located on the periphery of the ligands.[4] The compound shows a complex magnetic behaviour arising from the molecular based as well as extended state ordering.

Amongst the spectrum of properties to explore for CCs, Single Molecule Magnet (SMM) behaviour was of particular interest for us. SMMs, which can also be described as MNMs (Molecular Nanomagnets) are molecular systems which display the features of superparamagnetic particles. Thus there is a slow relaxation of their magnetisation which can be followed using various magnetisation studies utilising the response of the system to changes in the nature of the applied magnetic field with common experiments being either low temperature hysteresis studies or else data obtained from field and frequency dependent analysis of the imaginary component of the magentisation data. Thus, continuing with the theme of homometallic CCs, but now with the aim of introducing mixed valency, we explored incorporating the relatively anisotropic Mn(III) ion (which is deemed to invest the first ever discovered SMM {MnIV4MnIII8}with its special properties) into CCs containing manganese ions in mixed oxidation states. Amongst other things this resulted in discovering a mixed valent {MnII7MnIII12}system,[5] which is ferromagnetically coupled and still holds the record for having the highest ground spin state of S = 83/2 ever found for a molecule. This amply illustrates the power of the concept of the coordination cluster in terms of delivering unprecedented electronic and thus spin states.

Evolving from this work, we began exploring the incorporation of 4f metals in CCs both as pure 4f systems and as part of 3d/4f CCs. Our approach for the SMM work was to survey the whole of the lanthanide series in order to identify the best candidates amongst these highly anisotropic metal ions with the broad conclusion that incorporation of DyIII is most likely to produce interesting and exotic molecular magnets. This has led to a burgeoning of Dy chemistry within the community. For example, the discovery of a {Dy3} triangular compound with an essentially non-magnetic ground state,[6] but showing SMM behaviour arising from an excited state which is accessible by applying a magnetic field, corresponds to the molecular archetype for the non-collinear spin model of extended triangular lattices. Such a system is magnetically chiral and has a toroidal moment. Secondly, we demonstrated that a series of CCs of general core composition {MnIII4MnIVLn4}[7] showed that incorporation of DyIII ions is often extremely beneficial to enhancing SMM properties and the flood of articles from other groups around the world bears testament to this observation. This work also illustrated that use of 4f containing CCs allows for systematic studies to be made which are generally not possible for 3d systems. Thirdly, this factor also allowed us to adapt the synthesis of our mixed valent {MnII7MnIII12} system where we had noted that the central Mn(II) ion had an eight-coordinate environment which should be ideal for a later 4f ion such as DyIII. We were able to replace this central MnII with a large variety of other metal ions, including the highly anisotropic DyIII ion,[8] which invested this high spin system with the necessary anisotropy to induce SMM behaviour.

We have also been conscious of the presence of coordination clusters within biological systems and early on realised that the {Fe19} system represents a miniature model for the situation in the iron-loaded protein ferritin[2] in terms of the nature of the core structure, its different iron environments and the relative proportions of the elements C, N, O, H, and Fe. Another significant result in this chemistry was the recognition that adapting the synthesis of a {Mn4Na} CC such that NaI is replaced by CaII leads to a {Mn4Ca}CC[9] which was the first identified structural realisation of the correct core composition of the OEC cluster at the heart of PSII.

Branching out from these fundamental studies we have been able to survey a variety of 3d/4f systems where we can discover the features needed to produce enhanced SMMs. 3d/4f systems have the charm that they tend to form isostructural series, at the very least for groups of lanthanide ions with similar ionic radii, but often across the whole series. This makes it possible to survey which 3d metal ions are best for incorporation into such 3d/4f CCs and thus inform the concept of the Coordination Cluster further. The results are not necessarily intuitive. For example, combining anisotropic MnIII with DyIII compared with CrIII and DyIII in isotructural {MnIII4Dy4} and{CrIII4Dy4} compounds reveals that the isotropic CrIII ion gives better SMM properties.[10] On the other hand, incorporating anisotropic high spin CoII with DyIII results in an excellent SMM system with magnetisation hysteresis above 4K and two distinct relaxation processes. This gives a characteristic signature of 3d/4f SMM behaviour and can be deconvoluted in terms of exchange-coupled and single ion relaxation processes,[11] opening a way to produce systems with switchable phenomena within the quantum domain.

In further synthetic developments, we created a giant triangular array of DyIII using a supramolecular approach to link three Cu/Dy units together. The overall structure has a diad of propeller units with opposite rotation senses and reveals exotic hysteresis effects[12] similar to those observed in the Dy3 triangle.[6] We also used the Dy3 triangle[6] as a building block to create a supramolecular assembly in which chiral CuII units link the triangles into helices to give a multiferroic material combining SMM properties with a toroidal structure.[13] These multifunctional compounds are relevant to spintronics applications.

A significant hurdle to understanding the electronic and magnetic properties of these new and cooperatively coupled systems is finding techniques which are sufficiently developed to give assistance in explaining the phenomena we measure. Through our experience with FeIII chemistry it has been possible to evolve a Mössbauer method suited to deconvoluting the spin structure of both Fe CCs and Fe/4f CCs.[14] The analysis relies on having the capability (which we have) to measure compounds with applied magnetic field and is based on the approach use for solid-state compounds whereby it is possible to identify where parallel and anti-parallel aligned spin partners are situated. This is a further illustration of identifying points where the overlap between molecular-based and condensed phase properties meet and in some senses overlap. Furthermore, our approach enables us to sense the anisotropic contribution of 4f ions to Fe/4f clusters and thus propose spin structures without recourse to more exotic techniques needed to describe such systems with their large numbers of electrons and zero-field splitting as well as huge magnetic anisotropy from the 4f ions. Mössbauer spectroscopy in conjunction with ab initio calculations were used to investigate a system combining high spin FeII ions with DyIII, which a record anisotropy barrier of 319 cm-1 for all 3d/4f SMMs.[15]

Recent developments have included attaching CCs to HOPG surfaces,[16] using polyoxometalates as super-ligands for 3d[17] as well as 3d/4f[18] CCs and the discovery of chiral separations amongst Fe/Ln ring structures.[19] In the specific case of a family of {Fe10Ln10} rings, where the ring structure contains alternating FeIII and LnIII units, we investigated the unexpected disappearance of the ligand-to-metal-charge-transfer band to FeIII. It was established via a combination of magnetic, femtosecond solution state and solid state transport measurements that exciton formation coupled with electron cycling within the rings (in solution) and electron hopping between rings (in the solid state) components takes place and is modulated (as shown e.g. by the change in the band gap) by the nature of the lanthanide ion “dopant”.[20] This system is a perfect illustration of the cooperative effects operating within CCs and is clearly tuneable for spintronic as well as molecular-based semiconductivity applications.

 

  1. Synthesis, structures and magnetic properties of Fe2, Fe17, and Fe19 oxo-bridged iron clusters, A. K. Powell, S. L. Heath, D. Gatteschi, L. Pardi, R. Sessoli, G. Spina, F. Del Giallo, F. Pieralli, J. Am. Chem. Soc., 1995, 117, 2491-2502.
  2. Large metal clusters and lattices with analogues to biology, D. J. Price, F. Lionti, R. Ballou, P. T. Wood, A. K. Powell, Phil. Trans. Roy. Soc. Lond. A, 1999, 357, 3099-3118
  3. [Al15(m3-O)4(m3-OH)6(m-OH)14(hpdta)4]3- – A new Al15 aggregate which forms a supramolecular zeotype, W. Schmitt, E. Baissa, A. Mandel, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed. Engl., 2001, 40, 3578-3581.
  4. Hierarchical assembly of Fe13 oxygen-bridged clusters into a close-packed superstructure, M. Murugesu, R. Clérac, W. Wernsdorfer, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2005, 44, 6678-6682.
  5. A ferromagnetically coupled Mn19 aggregate with a record S = 83/2 ground spin state, A. M. Ako, I. J. Hewitt, V. Mereacre, R. Clérac, W. Wernsdorfer, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2006, 45, 4926 – 4929.
  6. Dysprosium triangles showing Single Molecule Magnet behaviour of thermally excited spin states, J. Tang, I. Hewitt, N. T. Madhu, G. Chastanet, W. Wernsdorfer, C. E. Anson, C. Benelli, R. Sessoli, A. K. Powell, Angew. Chem. Int. Ed., 2006, 45, 1729-1733.
  7. Heterometallic [Mn5-Ln4] Single Molecule Magnets with high anisotropy barriers, V. Mereacre, A. M. Ako, R. Clérac, W. Wernsdorfer, I. J. Hewitt, C. E. Anson, A. K. Powell, Chem. Eur. J., 2008, 14, 3577-3584.
  8. A [Mn18Dy] SMM resulting from the targeted replacement of the central MnII in the S = 83/2 [Mn19]-aggregate with DyIII, A. M. Ako, V. Mereacre, R. Clérac, W. Wernsdorfer, I. J. Hewitt, C. E. Anson, A. K. Powell, Chem. Commun., 2009, 544-546.
  9. A series of new structural models for the OEC in Photosystem II, I. J. Hewitt, J.-K Tang, N. T. Madhu, R. Clérac, G. Buth, C. E. Anson, A. K. Powell, Chem. Commun., 2006, 2650-2652.
  10. An octanuclear [CrIII4DyIII4] 3d-4f Single Molecule Magnet, J. Rinck, G. Novitchi, W. Van den Heuvel, L. Ungur, Y. Lan, W. Wernsdorfer, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2010, 49, 7583-7587.
  11. Evidence for Coexistence of Distinct Single-Ion and Exchange-Based Mechanisms for Blocking of Magnetization in a CoII2DyIII2 Single-Molecule Magnet, K. C. Mondal, A. Sundt, Y. Lan, G. E. Kostakis, O. Waldmann, L. Ungur, L. F. Chibotaru, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2012, 51, 7550-7554.
  12. Supramolecular “Double-Propeller” Dimers of Hexanuclear CuII/LnIII Complexes: A{Cu3Dy3}2 Single-Molecule Magnet, G. Novitchi, W. Wernsdorfer, L. F. Chibotaru, J. P. Costes, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2009, 48, 1614-1619.
  13. Heterometallic CuII/DyIII 1D Chiral polymers: Chirogenesis and Exchange Coupling of Toroidal Moments in Trinuclear Dy3 Single Molecule Magnets, G. Novitchi, G. Pilet, L. Ungur, V. V. Moshchalkov, W. Wernsdorfer, L. F. Chibotaru, D. Luneau, A. K. Powell, Chem. Sci,, 2012, 3, 1169-1176.
  14. Combined magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy on a ferromagnetic FeIII4Dy4 ring, D. Schray, G. Abbas, Y. Lan, V. Mereacre, A. Sundt, J. Dreiser, O. Waldmann, G. E. Kostakis, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2010, 49, 5185-5188.
  15. A Heterometallic FeII-DyIII Single-Molecule Magnet with a Record Anisotropy Barrier, J.-L. Liu, J.-Y. Wu, Y.-C. Chen, V. Mereacre, A. K. Powell, L. Ungur, L. F. Chibotaru, X.-M. Chen, M.-L. Tong, Angew. Chem. Int. Ed., 2014, 47, 12966-12970.
  16. Adsorption of [Mn19] Aggregates with S = 83/2 onto HOPG Graphite Surfaces, A. M. Ako, M. S. Alam, S. Mameri, Y. Lan, M. Hibert, M. Stocker, P. Müller, C. E. Anson, A. K. Powell, Eur. J. Chem., 2012, 4131-4140.
  17. Hexadeca-Cobalt(II) Containing Polyoxometalate-Based Single-Molecule Magnet, M. Ibrahim, Y. Lan, B. S. Bassil, Y. Xiang, A. K. Powell, U. Kortz, Angew. Chem. Int. Ed., 2011, 50, 4708-4711.
  18. Self-Assembly of a Giant Tetrahedral  3d–4f Single-Molecule Magnet within a Polyoxometalate System, M. Ibrahim, V. Mereacre, N. Leblanc, W. Wernsdorfer, C. E. Anson, A. K. Powell, Angew. Chem. Int. Ed., 2015, DOI: 10.1002/anie.201504663.
  19. Ringing the changes in FeIII/YbIII cyclic coordination clusters, A. Baniodeh, C. E. Anson, A. K. Powell, Chem. Sci., 2013, 4, 4354–4361.
  20. Unraveling the Influence of Lanthanide Ions on Intra- and Inter-Molecular Electronic Processes in Fe10Ln10 Nano-Toruses, A. Baniodeh, Y. Liang, C. E. Anson, N. Magnani, A. K. Powell, A.-N. Unterreiner, S. Seyfferle, M, Slota, M, Dressel, L, Bogani, K. Goß, Adv. Funct. Mater., 2014, 40, 6280-6290.