Multifunctional Magnets
Project aims
The aim of this project was to develop multifunctional magnetic framework materials as a means of elucidating chemical
and physical change in porous materials. Through the use of organic radical ligands and metalloligands, new framework
materials would be created that allow us to investigate physical processes such as guest sorption and structural changes
through perturbation of the magnetic properties of the materials. Chemical change could also be tracked through turnon/turn-off switching of radicals either trapped in pores or included as part of the structure of the framework. This could
give insights into the tailored design and construction of magnetic materials with desirable magnetic properties for
switching and sensing applications and also into how magnetic species could be utilised for devices such as memory
storage and spintronics.
Project progress
The execution of the project adopted varied synthetic approaches including the synthesis of radical ligands for clusters
and the creation of paramagnetic or fluorescent porous framework materials for guest detection.
▪ Radicals and ligands
The synthesis of nitronyl nitroxide radicals follows literature precedent and progresses through the reduction of 2,3dimethyl-2,3-dinitrobutane, condensation with an aldehyde and then oxidation over PbO 2 or NaIO4 to give stronglycoloured radicals. A number of new radicals were envisaged with multiple chelation sites to allow direct overlap of
paramagnetic metals orbitals and the radicals, leading to exceptionally strong magnetic couplings. A number of α,βdinitro compounds were tested along with a range of aldehydes with multiple coordinating groups.
▪ Frameworks and coordination polymers
The synthetic techniques used in framework and coordination polymer synthesis were varied but mainly centred on slow
diffusion techniques and solvothermal crystallisation. The former allows the growth of crystals from solution mixtures
that would normally lead to instant precipitated powders by markedly decreasing the rate of reaction. The latter method
involves heating solvents to high temperatures, often above the boiling point of the solvent, in high-pressure reactors
with reactants, increasing the solubility of the reactants and products and allowing larger crystals to form. Slow
decomposition of dimethylformamide under sealed high-temperature conditions allows for the gradual production of
dimethylamine which deprotonates carboxylate ligands to form frameworks with metal ions at a controlled rate. The
MOFs formed are then activated by exchanging DMF from the pores with a volatile, non-coordinating solvent such as
dichloromethane and then removing this under vacuum.
▪ Magnetic characterisation
Magnetic susceptibility measurements were performed in conjunction with collaborators in Sydney and Dublin and
utilised new techniques such as time-dependent magnetometry for tracking structural changes through modification of
the magnetic properties of paramagnetic metal ions. In addition to magnetometry, EPR was carried out on solid and
solutions to identify precursor and transient species in the formation of coordination materials.
Project outcomes
The project has been highly successful, resulting in a number of publications in the leading inorganic and structural
journals.
▪ Copper carbonate kagome MOF
A multifunctional magnetic MOF was formed through
fixation of carbon dioxide directly from the atmosphere to
create a copper carbonate kagome lattice bridged by a
ditopic pyridyl ligand. This material consists of
ferromagnetically-coupled layers that are metamagnetic
(i.e. that the ferromagnetic layers align antiferromagnetically), displaying a critical field of ~100 gauss
between the inter-layer antiferromagnetic and field-aligned
state, presenting a magnetic switch. The compound forms
easily-oriented single crystals which display zero-thermal
expansion in the layer stacking axis, an important property
in the design of components that preserve position and
alignment through a range of temperatures. Reversible
guest sorption is displayed in a single-crystal to single-crystal transition which subtly modifies the magnetic switching
behaviour while retaining the uniaxial zero-thermal expansion.
(T. D. Keene et al., Dalton Trans., 2014, 43, 14766-14771 http://dx.doi.org/10.1039/C4DT02205J)
▪ Ni kagome MOFs
Utilising a multidentate ligand, a porous MOF with a 3D kagome struture
was formed. This material displays a unique pressure-dependent gating
mechanism in its guest sorption properties arising from the rotation of
solvent molecules around the metal centre. The coordinated solvent
molecules are forced from an equilibrium distribution of orientation into the
lowest energy arrangement as guest molecules enter the MOF. This
increases the pore volume in a linear fashion on increasing pressure before
reaching saturation. Investigations into the slow uptake of guest molecules
onto the bare metal sites in the fully-desolvated form reveal slow
rearrangement of the Ni(II) coordination sphere and a size-selective effect
provided by the pore wall in solvating these metal ions, as illustrated through
time-dependent magnetometry. Further work is ongoing with collaborators
at the University of Adelaide into the effect of this slow reorganisation on
sorption of CO2 onto bare metal sites in this and two related materials.
(T. D. Keene et al., Dalton Trans, 2013, 42, 7871-7879 http://dx.doi.org/10.1039/C3DT00096F)
▪ Cu Schiff base coordination polymers
The combination of a Cu(II) ion and a tridentate Schiff base results in a
solution-stable supramolecular building block that can be precipitated to form
a one-dimensional coordination polymer with alternating dimerisation of
copper ions throughout the chain and a strong magnetic coupling. Addition of
a ditopic bridging ligand to the solution results in a two-dimensional
coordination polymer constructed from copper oxygen chains with a markedly
different magnetic chain coupling arising from the modification of the Cu-Cu
interactions through the ligands. Through a combined magnetometry, UV-vis
and EPR study, we identified the solution species and its variable degrees of
solvation with a view to forming future MOFs, coordination polymers and
magnetic clusters.
(T. D. Keene et al. RSC Advances, submitted 16/01/2015, manuscript no. RAART-01-2015-000948)
▪ Zn MOFs and Mg/Ca/Sr MOFs
Two families of framework materials were investigated as a
means of detecting absorbed paramagnetic species through
quenching of their fluorescence. In the first, a series of Zn MOFs
was produced with the fluorescent ligand 5-hydroxyisophthalate (5-hip). A range of structures was formed by the
addition of different structure-directing additives in the reaction
mixture, leading to related structures with subtly different
architectures and network topologies (including two new
topologies not previously reported). The UV-vis and
fluorescence properties were probed to reveal that the purple-white fluorescence was largely independent of structure
apart from one compound where a coordinated solvent molecule may be quenching this property.
In the second group, a green fluorescing ligand (2,2’-dihydroxybiphenyl-4,4’-dicarboxylate, diol) was reacted with alkali
earth metal ions to produce increasingly complex frameworks. For Ca and Sr, the diol ligand is coordinated solely
through the carboxylates, leading to green-fluorescing materials, while for Mg, the diol chelates the metal ion, quenching
the fluorescence.
While neither family of materials displayed significant porosity, it is possible to draw relationships between ligand and
solvent binding modes and their effects on ligand-centred framework fluorescence as a design tool for future compounds.
(T. D. Keene et al., Cryst. Growth Des., online 29/01/2015, http://dx.doi.org/10.1021/cg501808y; D. R. Rankine, T. D.
Keene et al., CrystEngComm, 2013, 15, 9722-9728. Part of themed collection: Structural Design of Coordination
Polymers. HOT Article for October 2013. http://dx.doi.org/10.1039/C3CE41253A).
Expected final outcomes
A number of collaborative projects are underway as a result of this work with high-impact publications expected to follow.
The project results add to the wider body of knowledge on molecular magnetism and the design of magnetic materials.
Investigations are ongoing into the porous magnetic materials for a range of roles relating to areas such as magnetic
data storage and magnetic and fluorescent detection of guest species. Further work is being undertaken into the copper
Schiff-base compounds as a means of spectroscopically and magnetically detecting copper in solutions, especially in
turbid or opaque aqueous solutions.