Inorganic Chemistry Frontiers Blog

In recent years, derivatives of carbazole have been studied intensively with focus on their optical properties to scry for applicability as materials in OLEDs and OFETs. Fine tuning of the electronic properties of the materials was achieved by a large variety of substitution patterns both on the central nitrogen atom and the carbon periphery.

Recently, the group of Dr. Hinz (Karlsruhe Institute of Technology, Germany) has investigated a series of alkali metal carbazolides with bulky arenes in positions 1 and 8 of the carbazole scaffold (see Figure 1). The alkali metal complexes of the type [(Cbz)M] show visible fluorescence with emissions maxima in the range of 520 and 460 nm in dependence of the metal ion. Quantum yields of up to 29% in the solid state were measured for the rubidium derivative.

The electronic transitions were rationalised with the aid of TD-DFT calculations. Upon HOMO→LUMO+1 excitation, density shifts from the carbazole scaffold to the arenes in positions 1 and 8 and thus are intraligand transitions. The geometry of the complexes only change slightly upon excitation which enables the highly efficient fluorescence.

Coordination of additional toluene molecules to the alkali metal shifts the emission maximum to lower energy and enables a second emission band arising from an interligand excitation from the carbazole to a toluene molecule. The quantum yields for the complexes in toluene solution are even higher than in the solid state and reach 100% for the lithium complex.

Corresponding author:

Alexander Hinz studied chemistry in Rostock, Germany, and is currently a junior group leader at the Karlsruhe Institute of Technology. The work of the Hinz group is focused on molecular main group chemistry and devoted to the investigation of low-coordinated, but highly reactive compounds.

E-mail: alexander.hinz@kit.edu

Single-molecule magnets (SMMs) are molecules that retain the slow relaxation of their magnetization upon removal of an applied magnetic field, acting as magnets below a characteristic temperature known as the blocking temperature. Research in this field has recently focused on the use of radicals as ligands to enhance the performance of SMMs through the exchange coupling of the radical- and metal-based spins. The strong interaction between the spin of the radical with the unpaired electrons of a metal ion effectively suppresses the quantum tunneling of the magnetization (QTM) and fast spin relaxation pathways involving spin excited states, thus promoting a thermal relaxation pathway for magnetization reversal. The latter is particularly important when designing lanthanide (Ln) based SMMs as the core-like nature of the 4f orbitals makes magnetic coupling challenging and thus they tend to suffer from through-barrier relaxation of the magnetization (i.e., QTM, Raman and direct mechanisms).

To this day several attempts towards this direction have been made leading to the isolation of strongly coupled Ln SMMs with the N₂^•3--based family exhibiting very good magnetic performance. However, the rational incorporation of the N₂ species into complexes is a synthetic challenge and it does not offer any room for structural modification. For this reason, other radicals, such as tetrazines have been explored by the group of Prof. Muralee Murugesu of the University of Ottawa. In the past the researchers had successfully incorporated the 1,2,4,5-tetrazine radical anion (tz^•−) into tetranuclear “Ln₄” metallocenes which led to strong magnetic coupling and significant magnetic hysteresis (H_c = 3 T).

Recently, the researchers aimed to isolate a dinuclear building block so that the role of the bridging ligand in the overall magnetic coupling in Ln systems can be better understood. In further detail, they have utilized the high performing {Cp^*₂Ln^III}⁺ moieties and bridge them by employing the tz^•−_, leading to the isolation of a new family of radical-bridged Ln metallocenes: [(Cp^*₂Ln^III)₂(tz^•−)(THF)₂](BPh₄), (Ln = Gd (1), Tb (2), Dy (3); THF = tetrahydrofuran).

They showed that a strong magnetic coupling between the Ln^III ions and the tz^•− was achieved, revealing a J_Gd-rad = -7.2 cm^-1 for 1 which is even comparable to some N₂^•3- bridged SMMs. Due to this, both 2 and 3 displayed zero-field SMM behavior with slow relaxation of the magnetization and magnetic hysteresis.

Ab initio calculations verified the strong antiferromagnetic Ln-rad coupling and showed that the magnetic state of 2 and 3 can be interpreted as a “giant-spin” where the relaxation of the magnetization is related to changes in the magnetic state of the overall exchange-coupled system. The slow relaxation of these SMMs is mediated via thermally activated processes through the first excited KDs which correspond to an Ising-type ferrimagnetic spin configuration where the magnetic moments of the Ln^III ions are co-aligned while the magnetic moment of the radical points to the opposite direction.

The researchers believe that the results presented in this work will be helpful for future strategies on designing new lanthanide metal complexes employing radical ligands in the pursuit of new strongly-coupled zero field SMMs.

Corresponding author:

Prof. Muralee Murugesu received his PhD from the University of Karlsruhe in 2002 under the supervision of Prof. A. K. Powell. He undertook postdoctoral stays at the University of Florida (2003–2005) with Prof. G. Christou, and jointly at the University of California, Berkeley and the University of California, San Francisco under the supervision of Prof. J. R. Long and the Nobel Laureate Prof. S. Pruissner (2005–2006). In 2006, he joined the University of Ottawa as an assistant professor and since 2015 he is a full professor. Prof. Murugesu’s research focuses on the design and development of Single-Molecule Magnets, Metal-Organic Frameworks and High-Energy materials.

Advances in the coordination chemistry of multinuclear compounds have been exploited to drive the self-assembly of many new discrete metallo-supramolecular motifs. Due to the nature of the metal-ligand interactions, many of these systems have a dynamic character with reversible association and dissociation able to generate complex mixtures. Unveil such dynamic behaviours, it is a priority to fully understand, control and design their functional properties. Among metallo-supramolecular systems, lanthanide (Ln) based architectures attracts much attention due to their remarkable optical and magnetic properties. However, design and control of the final supramolecule is very challenging due to the inner nature of the 4f orbitals and consequent small ligand-field effects. There is, however, a steady variation of the effective ionic radius (EIR) across the series, the so called “lanthanide contraction”. Although the radii difference (ΔEIR) is quite small (ca. 0.20 Å between La³⁺ and Lu³⁺ and ca. 0.02 Å between two consecutive lanthanides), it can have important chemical consequences on the nature and features of supramolecular complexes.

Recently, a group headed by Marzio Rancan of ICMATE-CNR (Italy) and collaborators from the University of Padova (Italy) and Dortmund University (Germany) have demonstrated that ΔEIR strongly affects the kinetics of Ln ions exchange between preassembled quadruple-stranded [Ln₂L₄]^2– cages (Figure 1).

The process has been qualitatively and quantitatively characterized by time-dependent electrospray ionization mass spectrometry (ESI-MS). Mixing a series of two homonuclear [Ln^A₂L₄]²⁻ and [Ln^B₂L₄]²⁻ with increasing Ln³⁺ ΔEIR always leads to the formation of a statistical mixture of homo- and heteronuclear helicates due to the Ln exchange. All the studied systems have an equilibrium constant close to K = 4. The Ln³⁺ ΔEIR, hence, does not affect the thermodynamics of the process that is mainly governed by statistical factors and entropy-driven. On the other hand, they demonstrate that the rate of the dynamic ion exchange is Ln radius-dependent (Figure 1b). The kinetic constants of the forward and backward reactions revealed an exponential trend depending on the Ln³⁺ ΔEIR of the two homonuclear pre-assembled cages (Figure 1c): from the minimum to the maximum value of ΔEIR, the kinetic constants increase by three orders of magnitude. This fundamental study hints new tools and guidelines to study dynamic processes in metallo-supramolecular ensembles, and for the precise preparation and control of lanthanide-based mixed coordination-driven systems.

Corresponding author:

Marzio Rancan is a Research Fellow at ICMATE-CNR (Italy). He received his PhD in Molecular Sciences at the University of Padova in 2009. He did post-doctoral studies at CNR, University of Padova and spent one year in the Molecular Magnetism Group at The University of Manchester (UK).  His current research is focused on the synthesis and characterization of coordination-driven molecular and supramolecular architectures with functional properties. He is the author of about 60 articles.

WEBSITE: http://wwwdisc.chimica.unipd.it/FMNLab/index.html

ORCID: https://orcid.org/0000-0001-9967-5283

RESEARCHGATE: https://www.researchgate.net/profile/Marzio-Rancan

Cancer is a major health issue worldwide, and the development of innovative and effective drugs is an ultimate demand for research. Iron compounds have aroused a great interest in the search for new metal based chemotherapics, on account of their relatively low toxicity and their redox chemistry, exportable to physiological media.

Some ferrocene derivatives have shown a promising anticancer potential, with a strong activity mostly associated with Fe^II to Fe^III oxidation, leading to alteration of the cellular redox balance and subsequent production of toxic substances (reactive oxygen species, ROS). However, they provide a limited variability of the metal coordination set, and they need to be carefully formulated for in vivo applications due to a generally insufficient water solubility.

Since 2019, a series of cationic [Fe^IFe^I] complexes based on the [Fe₂Cp₂(CO)₂] core and comprising a vinyliminium bridging ligand have emerged as a novel class of potential chemotherapeutic agents. Thanks to the unique features of the bimetallic core, these complexes are easily prepared up to gram scale from a commercial precursor in a few synthetic steps. Remarkably, they are amphiphilic and appreciably water-soluble, and exhibit an antiproliferative activity against cancer cell lines which depends on the ligand substituents. The choice of the latter is virtually limitless, thanks to the generality of the synthetic procedure, and this feature allows to optimise physico-chemical properties for biological purposes. Different mechanisms, mainly ROS production but also protein interaction and weak DNA binding, may contribute to the mode of action of these dinuclear structures.

Recently, the group of Fabio Marchetti and co-workers have reported, for the first time, the conjugation of a ferrocenyl moiety with a diiron framework, as a strategy to obtain robust mixed-valence triiron compounds featured by a potent cytotoxicity and excellent selectivity towards cancer cell lines (i.e., IC₅₀ values in the low micromolar/nanomolar range on the cancer cell lines, and up to 35 times higher values on the nontumoral cells).

A combination of stability studies, electrochemical experiments, iron cellular uptake and targeted biological studies indicate that the cationic triiron complexes synergistically combine the redox behaviour of the ferrocenyl moiety with the amphiphilicity and the versatility of the diiron vinyliminium structure, and that their powerful activity arises from the ability to disrupt the redox homeostasis of tumour cells, through the overproduction of intracellular ROS and the alteration of the thioredoxin reductase, assessed on a synthetic dodecapeptide as a simplified model of the enzyme.

Corresponding Author:

Fabio Marchetti received his Degree in Industrial Chemistry from the University of Bologna in 1999 (summa cum laude), and the PhD in Chemistry from the same University in 2003. In 2006 he obtained a researcher position at the University of Pisa, and since October 2018 he has been Full Professor in the same University. FM has co-authored over 200 scientific publications on international journals, 2 book chapters and 2 international patents. His research interests regard the synthesis, the characterization and the properties of new transition metal compounds, and the metal-mediated activation of small organic molecules.