Molecule Based Magnets (MBM's)

MBM's are, as the name suggests, made from molecular precursors. The aim is to prepare crystalline materials in which unpaired spins can interact in two- or three- dimensions. The general synthetic approach is to combine paramagnetic metal centers with diamagnetic or radical bridging ligands to form an extended network.  The "true" MBM's are those in which molecules interact magnetically, in the bulk solid, without any covalent bridging groups being present.  The [(Cp*₂Fe)+TCNE^-] system by Miller et al. in 1985 is one such example.  Elsewhere, molecule-based magnetic materials have also been prepared using organic radicals alone - though these typically have lower critical temperatures than the polymeric coordination complexes.

There are two main types of MBM's

• Extended Networks
• Large d-block Cluster Single-Molecule Magnets (SMM)

A Brief History of Magnets

• Magnets have played a small but important role in the development of modern civilization
• As with many technological advancements, the ancient Greeks discovered the magnetic mineral lodestone
• Then around the 11th Century, the Chinese invented the magnetic compass
• This invention allowed navigation at sea possible - less reliant on sun and stars hence cloudy weather not so much of a problem
• Thus, large scale international trade began to flourish - first big step in globalization
• Then more recently an Englishman, Michael Faraday - originally a bookbinder - discovered much in the field of electromagnetism and in the 1830s invented the first electric generator (shown)

Faraday’s first electric generator

• More recently, in the last hundred years or so, the use of magnets has become particularly widespread with the development of electricity. Today, magnets can be found in vital technologies including electric motors, generators, loudspeakers, frictionless bearings, magnetic switches, magnetic resonance imaging instruments, magnetic separators, and data storage found in the ubiquitous device of our times, the computer.

Atom Based Magnets

• The ferromagnetic metals, e.g. Fe and Co, were the mainstays until transition metal oxides, e.g. CrO₂, were introduced in the period around World War II. Today, some of the most powerful and commercially successful magnets are rare earth based. - the latter are particularly 'hard' magnets ie. large coercive fields.
• Traditional magnets are prepared by energy intensive high temperature metallurgical techniques

Molecule-Based Magnets (MBM's)

• Molecule-based magnets have a relatively recent history.  In the metal containing systems these 3D-orded species grew out of intensive studies of exchange coupling in small clusters done in the 1950-1970 period. 
• One of the first molecule-based magnets, an iron(III) dithiocarbamate chloride, was reported by Wickman and co-workers in 1967.
• In the 1970s Australian chemists Martin, Mitra and Gregson and coworkers reported long-range magnetic ordering in d-block phthalocyanines and in anionic copper carbonates.
• In that period and in the decades that followed studies by Day, Carlin, de Jongh and Miedema were also very important.
• In 1985 Miller and Epstein co-workers reported ferromagnetic order in the salt of decamethylferrocenium and tetracyanoethylene - after this discovery the field of molecule-based magnets grew quite rapidly.
• One of the first coordination compounds ever to be reported, Prussian Blue Fe^III₄[Fe^II(CN)₆]₃ xH₂O was also found to be a ferromagnet. It was discovered in 1704, in Germany, as a blue pigment formed on boiling beef blood in a highly basic medium.
• Through the work of the groups of Babel Güdel, Kahn, Verdaguer, Girolami, Dunbar and Mallah many Prussian-Blue analogues, A[B(CN)₆], are known, some of which have critical temperatures, T_c, close to or well above room temperature.
• In 1991 - Miller and co-workers reported a room temperature molecule-based magnet, V(TCNE)₂.
• In 1991, Okawa et al reported 2D-networks of metal oxalate magnets such as (R₄N)[Mn^IICr^III(C₂O₄)₃].  This gave rise to a rich array of bimetallic systems having 2D- and 3D- molecular structures with unusual magnetic ordering features.
• In 1998-present, Prussian Blue analogues, as thin films, have been studied by Hashimoto and Coworkers for their photomagnetic properties and electrochromic properties.
• 1998, our own group and those of Kurmoo and of Miller and Epstein reported a new class of homometallic metal dicyanamide magnets a-[M(N(CN)₂)₂] see below.  We have recently reviewed the field, S. R. Batten, K. S. Murray, Coord Chem Rev., 2003.

Single Rutile-like Network of α-[M(N(CN)₂)₂]

• In the 1980-1998 period, Kahn (decd. 1998) and coworkers discovered heterometallic µ-oxamide chain magnets displaying ferri- and ferromagnetic ordering.
• All of the above mentioned MBM's are prepared by normal, ambient temperature solution or electrochemical methods, using crystallising techniques such as H-tubes, solvent (gas) diffusion, layering etc.  Hydrothermal materials are also used.  Many chemical / structural variations can therefore be made.

Present Projects in MBM's

Present projects at Monash University in MBM's (2007)

• New metal dicyanamide, metal tricyanomethanide and related molecule-based magnets, including 'hybrid' species. 

Ligands dicyanamide (DCA) and tricyanomethanide (TCM)

• New cyanide based bridging ligands and their extended networks. CN- bridged networks and large clusters (with S.R. Batten)
• New carbonate bridged magnets. (with R. Robson and group at the University of Melbourne)

Future projects are available in these areas and involve design, synthesis X-ray, crystallography, DC and AC magnetic measurements, UV-Vis, IR, NMR, Mass spectral methods, EPR and Mössbauer spectral measurements (the latter in the School of Physics), fitting of magnetic data to theoretical models. PhD scholarships are available for 2008.

Some recent publications from our group.

• Magnetochemistry of molecular materials: Part 2. K. S. Murray, Chem. Aust. 2007, 74, Mar. pp. 4-5.
• Magnetochemistry of molecular materials: Part 1. K. S. Murray, Chem. Aust. 2007, 74, Jan/Feb. pp. 5-7
• Benzotriazole based 1-D, 2-D and 3-D metal dicyanamide and tricyanomethanide coordination networks.  L.F. Jones, L. O'Dea, D.A. Offerman, P. Jensen, B. Moubaraki, K.S.Murray, Polyhedron, 2006, 25, 360-372.
• Hybrid materials containing organometallic cations and 3-D anionic metal dicyanamide networks of type [Cp*₂M][M'(dca)₃], P.M. Van der Werff, E. Martinez-Ferrero, S.R Batten, P. Jensen, C. Ruiz-Perèz, M. Almeida, J.C. Waerenborgh, J.D. Cashion, B. Moubaraki, J.M. Martìnez-Agudo, E. Coronado and K.S. Murray, Dalton Trans., 2005, 285-290.
•  Hexacyanometallates as templates for discrete pentanuclear and heptanuclear bimetallic clusters, L., Spiccia, K.S. Murray and J.F. Young, Inorganic Syntehsis (Ed. J.R. Shapley) 2004, 34, 133-141.

• Coordination polymers of dicyanamide and methylpyrazine;  Synthesis, structures and magnetic properties, A.M. Kutasi, A.R. Harris, S.R. Batten, B. Moubaraki and K.S. Murray, Crystal Growth and Design 2004, 4, 605-610.

Single Molecule Magnets (SMM's)

Background

In the early to mid 1990's, the compound previously known as 'Mn(III)acetate' (made by reacting Mn(II) salts with KMnO₄) was shown, by Lis et al, to have the mixed-valent formula [Mn^III/IV₁₂O₁₂(CH₃CO₂)₁₆(H₂O)₄] and possess a disk-like cluster shape in which a Mn^IV₄O₄ 'cubane' in the centre of the disk is bridged to eight Mn^III ions at the periphery of the disk.  The early evidence of intra cluster ferromagnetic coupling in Mn₁₂-acetate by Lis, was confirmed and extended by Christou and Hendrickson et al to include AC susceptibility and DC magnetic data and hysteresis measurements, which showed a S=10 ground state and slow relaxation of the magnetisation.  Thus, the field of single molecule magnets was born and Mn₁₂-acetate has become a classical SMM material.

Physicists and theoreticians were very interested in such molecular materials and it was soon shown (by hysteresis steps) that Mn₁₂-acetate displayed 'quantum tunneling' properties, with future possibilities as a component in quantum computers. An octanuclear iron(III) cluster [Fe₈O₂(OH)₂-tacn₆]⁸⁺ was, shortly afterwards, shown by Gatteschi, Sessoli, Wieghardt and coworkers to show similar SMM behavior.  Since then, a large range of Mn_x clusters has been discovered and measured by Christou et al, many of which (e.g. x=4, x=18) display SMM features.  These molecular clusters are being intensively studied in the growing area of nanomagnetic materials.  

Whereas 'classical' nannomagnets are obtained by 'top-down' modification of macroscopic ferromagnetic multi-domain particles, the SMM's are obtained by mild solution methods from smaller precursors, the so-called 'up-scale' or 'bottom-up' approach.  These SMM clusters are single domains with a well defined single particle size, not a distribution of sizes, and thus offer particular advantages in preparing nanomagnetic materials.

SMMs: Molecules that can be magnetised

• The cluster molecule is magnetised because there is a large energy barrier between its “spin-up” and “spin-down” states.
• Magnetic order is within the molecule – single domain – usually no long-range order between clusters, but some recent examples which show cluster-cluster interactions have been found.
• Requirement for SMM: High spin ground state, large S value and negative magnetic anisotropy – zero-field splitting (ZFS) in the ground state (D < 0 ) e.g. Mn^III, V^III, Ni^II, Fe^II.
• Frequency dependency of AC out-of-phase susceptibility,c^" signal.

Some SMM Examples

Mn₁₂-acetate, [Fe₈O₂(OH)₁₂(tacn)₆]⁸⁺, [V₄O₂(O₂CEt)₇(bipy)₂], [Mn₄O₃Cl(OAc)₃(dbm)₃], [Fe₄(OCH₃)₆(dpm)₆]. Also Ni, Co cubanes and cyano-bridged cluster SMM.

The Mn₁₂-acetate structure

Our entry into SMM's

In attempting to link small clusters such as [Mn₃O(acetate)₆]⁺ in to magnetically order 3D networks by use of bridging ligands such as dicyanamide or tricyanomethanide (see MBM section), we obtained an exciting, Mn₁₆ alkoxo-carboxylate cluster; shown below.

Structure of [Mn₁₆O₁₆(OMe)₆(OAc)₁₆(MeOH)₃(H₂O)₃] 6H₂O

This Mn₁₆ shows SMM characteristics.  We are currently developing new types of large and small Mn and Fe cluster SMM's and attempting to incorporate them, as nanomagnetic materials, in to zeolites and polymer films.

Present projects in SMM's

• New Mn, Fe and V cluster SMM's containing bridging ligand donor other than oxygen, such as polynitrile-N-oxide ligands (with S.R. Batten).  Investigations as nanomagnetic materials.
• 2D and 3D arrays of SMMs using ethanolamine-linked ligands. 

Future projects are available in these areas and involve design, synthesis X-ray crystallography, DC and AC magnetic measurements, UV-Vis, IR, NMR, Mass spectral methods, EPR and Mössbauer spectral measurements, fitting of magnetic data to theoretical models. 

Some recent publications from our group

• Synthesis, crystal structure and magnetism of a single molecule magnet [Mn₁₆O₁₆(OMe)₆(OAc)₁₆(MeOH)₃(H₂O₃].6H₂O, and of a mixed bridge 1D chain [Mn(μ‑OMe)(μ-OAc)₂]_n, D. J. Price, S. R. Batten, B. Moubaraki and K. S. Murray, Polyhedron, 2007, 26, 305-317.
• Citrate, in collaboration with a guanidinium ion, as a generator of cubane-like complexes with a range of metal ions: synthesis, structures, and magnetic properties of [C(NH₂)₃]₈[(M^II)₄(cit)₄].8H₂O (M = Mg, Mn, Fe, Co, Ni, and Zn; cit = citrate), T. A. Hudson, K. J. Berry, B. Moubaraki, K.S. Murray and R. Robson, Inorg. Chem., 2006, 45, 3549-3556.
• Heptanuclear iron(III) triethanolamine clusters exhibiting ‘millennium dome’-like topologies and an octanuclear analogue with ground spin states of S = 5/2 and 0, respectively.  L.F. Jones, P. Jensen, B. Moubaraki, K.J. Berry, J.F. Boas, J.R. Pilbrow, and K.S. Murray, J. Mater. Chem. 2006, 16, 2690-2697.
• New mixed-valence Mn^II₂Mn^III₂ clusters exhibiting an unprecedented Mn^II/III oxidatin state distribution in their cores.  L.M. Wittick, L.F. Jones, P. Jensen, B. Moubaraki, L. Spiccia, K.J. Berry and K.S. Murray, Dalton Trans., 2006, 1534-1543.
• Synthesis, structure and magnetism of new single molecule magnets composed of Mn(II)₂Mn(III)₂ alkoxo-carboxylate bridged clusters capped by triethanolamine ligands, L. Wittick, K.S. Murray, B. Moubaraki, S.R. batten, L. Spiccia and K.J. Berry, Dalton Trans., 2004, 1003-1011.
• Structure and magnetism of trinuclear and tetranuclear mixed valent manganese clusters from dicyanonitrosomethanide derived ligands, D. J. Price, S.R. Batten, K.J. Berry, B. Moubaraki and K.S. Murray, Polyhedron, 2003, 22, 165-176.