Alternative fuel storage and its controlled release, selective
sorption, molecular sensing and processing of gasses, high density
data recording and storage, and information processing on a
molecular level have become the most pressing technological
problems of the last decade. They have led to an urgent need for new
generations of intelligent materials, which, as opposed to conventional solids, could combine multiple
functions. Multifunctional molecular materials show several advantages over the conventional ones and
their modular design allows rational grafting of basically unlimited number of physical and chemical
properties and functions onto a single crystalline material. However, their synthesis and characterization
is extremely demanding and the obtained compounds are usually very fragile. The main goal of the
project ‘Multifunctional Molecular Magnets through Cyanide Chemistry’ (MultiCyChem) was making the
application of multifunctional molecular magnets more feasible through rational exploration of new
synthetic pathways and development of new multifunctional molecular compounds combining
magnetism with other physical and chemical properties. The project was focused on the design,
synthesis and full characterization of four main groups of multifunctional magnets: (i) ‘photoswitchable
magnets’, (ii) ‘porous/guest responsive magnets’, (iii) ‘enantiopure chiral magnets’ and (iv) ‘cyanidebridged single molecule magnets’.
The implementation of the project resulted in the successful design and synthesis of several new
multifunctional compounds. The most significant achievement within the ‘photoswitchable magnets’
objective is the first photomagnetic measurement under high hydrostatic pressure and the first
observation of a pressure-induced photomagnetic effect. This phenomenon was recorded for the FeIINbIV cyanide-bridged system Fe2Nb. The observed pressure and temperature dependent magnetic
properties of Fe2Nb cover almost all types of magnetic behaviors: ferromagnetism (ferrimagnetism),
pressure-induced spin transition, antiferromagnetism, paramagnetism and photomagnetism. It also
shows piezochromic behavior at room temperature (change of color from purple to blue at around 7000
bar; Figure 1). Our example of a pressure-induced spin transition photomagnet illustrates the fact that
multifunctionality can sometimes be hidden (Fe2Nb does not show spin transition or photomagnetism at
ambient pressure) and can be
revealed by finding proper
‘temperature-and-pressure’
conditions. This is a completely
novel
approach
to
multifunctionality by juggling the
Figure 1. Piezochromism of Fe2Nb observed in a diamond-anvil pressure cell
temperature with pressure.
Another very important result is the successful synthesis of a pentanuclear CN-bridged cluster
molecule VII3MoIII2 with record antiferromagnetic coupling based on 3d/4d early transition metals (VII
and MoIII) and its isolation in form of single crystals which allowed the structure determination. Synthesis
of CN-bridged 3d/4d early transition metal complexes is extremely challenging due to their sensitivity to
oxygen and elusive character. In 2005 Ruiz et al. predicted that such combination (V-Mo) of CN-bridged
metal centers should lead to very strong magnetic coupling and as a result push the application limits for
molecule-based magnets towards room temperature. The magnetic properties of VII3MoIII2 reveal
extremely strong antiferromagnetic coupling within the MoIII-CN-VII structural motif with a coupling
constant estimated to be -114 cm-1, which is the strongest magnetic coupling ever reported for a
heterobimetallic cyanide compound. This result will stimulate research efforts towards early transition
cyanide complexes and will lead to the successful development of multifunctional molecules operating at
room temperature.
Within the objectives ‘porous/guest responsive magnets’ and ‘cyano-bridged single molecule
magnets’ a series of new bimetallic trinuclear molecules MnIII2MIII based on hexacyanometallates
[MIII(CN)6]3- (M = Co, Fe, Ru and Os) have been synthesized, characterized structurally and magnetically.
It was found that the cyano-bridged MnIII2FeIII, MnIII2RuIII, MnIII2OsIII and the MnIII2CoIII/OsIII compounds are
switchable quantum nanomagnets (or Single Molecule Magnets; SMM) and show switchable exchangebias behavior depending on methanol content. This is the first complete structural and magnetic study
involving all haxacyanometallates(III) from group eight of the periodic table including the elusive
hexacyanoruthenate(III) and hexacyanoosmate(III). To summarize, these results lend further credibility
to the fact that SMM characteristics can be significantly improved by the use of heavy d-block metal
centers with unquenched orbital angular momentum and strong spin-orbit coupling that lead to
anisotropic magnetic exchange interactions. Moreover, the SMM behavior can be manipulated by finetuning the ligand properties vis-à-vis their ability to direct self-association of individual molecules, an
important issue for the possible application of SMMs in devices.
Following the strategy drafted in the objective no. (iii) ‘enantiopure chiral magnets’ three new
compounds combining natural optical activity and magnetic ordering (magnetic optical activity) have
been isolated by the successful incorporation of a chiral amine 1,2-diaminopropane into the MnII-NbIV
cyano-bridged framework (in three forms: racemic, R and S). The compounds exhibit three-dimensional
polymeric structure with magnetic ordering at 18 or 23 K depending on their hydration level. This makes
the R- and S-forms the first examples of chiral magnetic sponges. As expected, they also exhibit
interesting second-order optical properties i.e. Second Harmonic Generation (SHG) which was measured
in collaboration with Prof. Shin-ichi Ohkoshi (Tokyo University). Moreover, the detailed MCD study
(Magnetic Circular Dichroism) shows that MCD spectra are strongly and directly correlated with the
magnetic ordering functionality. Due to the combination of three functionalities: magnetic ordering,
reversible sorption and chirality, it is a rare example of a truly multifunctional molecular magnet.
Additionally, several other significant results have been obtained: (1) a new photomagnetic
building block Mo-Pt-Mo responsive to blue light with the photo-induced phase relaxing only at higher
temperatures close to room temperature (very promising from the application point of view); (2) a series
of photochromic lanthanide complexes Ln-dae showing quantum nanomagnetism (slow magnetic
relaxation) combined with photo-switching of the ligand; (3) a family of previously unknown hexanuclear
magnetic clusters M4Nb2 (M = MnII, FeII, CoII) based on octacyanoniobate(IV) showing very exciting type
of chirality and different functionalities depending on the type of the transition metal used: photomagnetism (Fe4Nb2) and magneto-caloric effect (Mn4Nb2) that could be used for the construction of
molecular photo-switches and molecular magneto-coolers, respectively; (4) a four-coordinate erbium(III)
complex showing slow magnetic relaxation in the absence of the dc field with the impressive
magnetization reversal energy barrier Ueff/kB = 66.4 K and magnetic hysteresis loop of the purely
molecular origin up to 3.0 K; (5) a series of three new hard molecular magnets utilizing elusive airsensitive heptacyanomolybdate(III) with Curie temperatures at 31, 46 and 59 K, respectively; (6) an
improved version of a Mn2Nb magnetic sponge with high critical temperature of 125 K and three-step
magnetic ordering temperature switching
depending on the hydration level; (7) a new
bulky bridging ligand 4,7-tdapO2 with the ability
to form stable organic radicals and its potassium
salt as well as the first example of a
coordination chain CuII-4,7-tdapO2; (8) a series
of mononuclear four-coordinate 3d transition
metal complexes (with MnII, FeII, CoII and NiII)
exhibiting trigonal monopyramidal geometry
and an open coordination site for CNcoordination with the CoII-analogue showing
field-induced slow magnetic relaxation typical
for Single Molecule Magnets; (9) the first
example of a nonadecanuclear Co12Fe7 rosette
molecule (Figure 2) with CN-bridges and 1,10Figure 2. Co12Fe7 rosette molecule
tdapO2 decorating
electroactive blocking
ligands showing thermal charge transfer induced spin transition within the FeIII-CN-CoII structural motif.
The results obtained within the MultiCyChem project have significantly advanced the knowledge
of the multifunctional molecular materials and led to discoveries that will bring closer the long-desired
real-world applications of these fascinating solids in nanotechnology and information technology
(especially in ultra-high capacity data storage and ultra-fast data reading/writing). The main achievement
‘Pressure-induced spin crossover photomagnet’ bears features of a ground-breaking discovery that will
strongly influence the communities of materials scientists, experimental physicists and molecular
magnetism scientists. The future implementation of the project results will significantly contribute to the
scientific excellence of the European research centers and particularly Jagiellonian University through a
number of high quality, high impact scientific papers. The final phase of the project ‘Final data analysis
and conceptualization’ took part in the European Research Area (ERA) and therefore, ERA is the main
and direct beneficiary of the results.
The project coordinator Prof. Barbara Sieklucka and the researcher Dr Dawid Pinkowicz are both
faculty members at the Jagiellonian University Faculty of Chemistry in Kraków (Poland) and can be
contacted via email: barbara.sieklucka@uj.edu.pl and dawid.pinkowicz@uj.edu.pl. Project website can
be reached at http://www2.chemia.uj.edu.pl/znmm/mcchem-en.html.