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Spin-crossover Molecular Magnetic Materials

Background to Spin-crossover effects

The spin-crossover (SCO) phenomenon was first discovered more that 60 years ago by Cambi et al. Since this discovery there have been numerous compounds of FeII (3d6), FeIII (3d5) and CoII (3d7) reported in the literature exhibiting this phenomenon both in the solid state and in solution.

SCO centres are of interest because they are one of the best-known forms of an inorganic electronic switch. Variation of the thermal energy at the crossover leads to both an electronic (change in the d-electron orbital configuration) and structural change, often observed as a colour and/or a magnetic moment change.

Ligand Field Theory and The Spin Crossover Transition

SCO gives rise to a type of molecular magnetism where the spin state and magnetic moment of the central d block ion can be changed or controlled by external constraints i.e. temperature, pressure or electromagnetic radiation (LIESST effect). For this phenomenon to occur, the metal ion needs to be 6-coordinate (octahedral) with an outer electronic configuration of d4 to d7. The most common example is FeII where the outer electronic configuration is 3d6, and the electrons can occupy all the lower t2g level (low-spin, large Δoct) or they can occupy both t2g and eg levels (high-spin, small Δoct).  In the figure, below, the low-spin to high-spin is shown for a 3d6 system. The spin state on the FeII ion changes from diamagnetic (S=0) in the LS state, to paramagnetic (S=2) in the HS state. Accompanying a FeII spin transition of this nature is a colour change from, the HS pale yellow-white, to a LS pink-purple. These colours may be deeper if charge-transfer bands from the associated ligands are observed, and these often mask the SCO colour change.

LS HS

LS«HS interconversion dependent upon external perturbations of temperature (T), pressure (p) and electromagnetic radiation (hn) for a 3d6 system

A key feature, which leads to thermal hysteresis (memory) in the SCO transition is that of cooperativity between SCO centres in the crystalline state.  In mononuclear metal complexes the cooperativity can be influenced by "intermolecular" effects such as solvation, nature of anion, hydrogen-bonding, Π-Π stacking etc.  Recently, we and others have postulated that clusters (e.g. di- or tri-nuclear) or infinite networks of SCO centres, having various modes of bridging linkages between metal centres should show cooperativity effects and perhaps, synergy between SCO and any exchange coupling occurring between adjacent metal centers.  In dinuclear FeIIFeII compounds one can stabilise the intermediates HS-HS, HS-LS and LS-LS and observe phenomena such as 'half' SCO and steps in the magnetic moment curve at the spin transition(s).  Distinguishing intra-cluster from inter-cluster cooperatively is a difficult challenge!  Other groups working in the polynuclear SCO area include those of Real, Kahn(decd), Letard, Gütlich, Toftlund, Lehn and Reedijk. 

Monash University Spin-crossover Work

  • Mononuclear FeIII and CoII spin-crossover studies of Schiff-base compounds (1984-90)
  • Dinuclear CoII CoII pyridazine-bridged compounds displaying SCO and exchange coupling (1999-present).  Joint-projects with Dr S. Brooker, New Zealand (Marsden grant).  Front cover of Angew. Chem. Int. Edn. 1999, 38, 131.
  • Dinuclear and polynuclear FeII spin-crossover compounds. (current work)
  • Extended network FeII Spin-crossover materials showing microporosity, host-guest effects and spin switching. Joint ARC project with Dr Cameron Kepert, University of Sydney (current work)

Recent Papers: 

  • Structure, Magnetism and Photomagnetism of Mixed - Ligand Tris(pyrazolyl)methane Iron(II) Spin Crossover Compounds. B. Moubaraki,  B. A. Leita,  S. R. Batten, P. Jensen, K. S Murray, J. P. Smith, J. D. Cashion,C. J. Kepert,G. J. Halder, J.-F. Létard, Dalton Trans. 2007, 4413-4426.
  • Anion-Solvent Dependence of Bistability in a Family of Meridional N-Donor Ligand Containing Iron(II) Spin Crossover Complexes. B. A Leita, S. M. Neville, G. J. Halder, B. Moubaraki, C. J. Kepert, J.-F. Létard, K. S. Murray, Inorg. Chem. 2007, 46, 8784-8795.
  • Spin-crossover studies on a series of 1D chain and dinuclear iron(II) triazine-dipyridylamine compounds. S. M. Neville, B. A. Leita, D. A. Offermann, M. B. Duriska, B. Moubaraki, K. W. Chapman, G. J. Halder, K. S. Murray, Eur. J. Inorg. Chem., 2007, 1073-1085.
  • The magnetic and structural elucidation of 3,5-bis(2-pyridyl)-1,2,4-triazolate – bridged dinuclear iron(II) spin crossover compounds.  C.J. Schneider, J.D. Cashion, B. Moubaraki, S.M. Neville and K.S. Murray, Polyhedron, 2007, 26, 1764-1772.
  • Designing dinuclear iron(II) spin crossover complexes.. Structure and magnetism of dinitrile-, dicyanoamido-, tricyanomethanido-, bipyrimidine- and tetrazine-bridged compounds, S.R. Batten, J. Bjernemose, P. Jensen, B.A. Leita, K.S. Murray, B. Moubaraki, J.P. Smith and H. Toftlund, Dalton Trans., 2005, 285-290.
  • Spin-crossover in dimeric hydrogen-bonded iron(II) 2-(pyrazolyl)-pyridine and 2‑(imidazolyl)-pyridine complexes, B.A. Leita, B. Moubaraki, K.S. Murray and J.P. Smith, Polyhedron, 2005, 24, 2165-2172
  • Structure and magnetism of a new pyrazolate bridged iron(II) spin crossover complex displaying a single HS-HS to LS-LS transition, B.A. Leita, B. Moubaraki, K.S. Murray, J.P. Smith and J.D. Cashion, Chem. Commun., 2004, 156-157.
  • G. J. Halder, C. J. Kepert, B. Moubaraki, K. S. Murray, J. D. Cashion, Science 2002, 298, 1762
  • Review Article:  K. S. Murray, C. J. Kepert in 'Spin-crossover in Transition Metal Compounds'  Editor P. Gütlich, H. A. Goodwin 3 Volumes, 2003, Chapter entitled "Cooperativity in Spin Crossover systems; Magnetism, Memory and Microporosity", 2004, 233, 195-228.

Current Projects at Monash University in SCO Materials

  • Synthesis, Structures, Magnetism, LIESST, Synchrotron PXRD and Mössbauer spectra of new di- and poly-nuclear spin-crossover complexes of iron(II) and iron(III).  Covalent-bridging and Van der Waal’s ‘supramolecular’ bridging. New ligand systems involving organic syntheses. Mössbauer spectral studies done in the Physics department with Assoc. Prof. John Cashion.
  • Synthesis, Structures, Magnetism and Mössbauer spectra of mixed-chelate mononuclear FeII complexes.  Effects of anion, solvate and H-bonding.
  • Collaboration with Dr Stuart Batten on large mixed-metal (Cu(I)-Fe(II)) ‘nanoball’ clusters displaying SCO.

Future projects will be available in these areas.

Some recent publications from our group and from collaborations on SCO materials

  • A thermal spin transition in a nanoporous iron(II) coordination framework material, S. M. Neville, B. Moubaraki, K.S. Murray, C.  J. Kepert,  Angew. Chem. Int. Ed. 2007, 46, 2059-2062.
  • Structural and magnetic resolution of a two-step full spin-crossover transition in a dinuclear iron(II) pyridyl-bridged compound.  J.J.M. Amoore, C.J. Kepert, J.D. Cashion, B. Moubaraki, S.M. Neville and K.S. Murray, Chem. Eur. J. 2006, 12, 8220-8227. Front cover
  • The first X-ray crystal structure determination of a dinuclear complex trapped in the [low spin-high spin] state: [FeII2(PMAT)2](BF4)4.DMF, M.H. Klingele, B. Moubaraki, J.D. Cashion, K.S. Marray and S. Brooker, Chem. Comm., 2005, 987-989. Front cover.
  • Guest-dependent spin crossover in a nanoporous molecular framework material, G.J.Halder, C.J. Kepert, B. Moubaraki, K.S. Murray and J.D. Cashion, Science, 2002, 298, 1762-1765.
  • Exchange-coupled high-spin, low-spin and spin-crossover dicobalt(II) complexes of a pyridazine-containing Schiff-base macrocycle: control of cobalt(II) spin state by choice of axial ligands, S. Brooker, D.J. de Geest, R.J. Kelly, P.G. Plieger, B. Moubaraki, K.S.Murray and G.B. Jameson, J. Chem. Soc., Dalton Trans., 2002, 2080-2087.
  • First cobalt complex to exhibit both exchange coupling and spin crossover effects: The structure and unusual magnetochemistry of (CoII2 L (NCS)2(SCN)2] where L = macrocyclic ligand. S. Brooker, P.S. Plieger, B. Moubaraki, K.S. Murray, Angew. Chem. Int. Edn. Engl., 1999, 38,408-410.