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LATEST PUBLISHED WORK

 

UNRAVELLING THE COMPLEXITIES OF PSEUDOCONTACT SHIFT ANALYSIS IN
LANTHANIDE COORDINATION COMPLEXES OF DIFFERING SYMMETRY

Alice C. Harnden, Elizaveta A. Suturina, Andrei S. Batsanov, Mark A. Fox, Kevin Mason, Michele Vonci, Eric J. L. McInnes, Nicholas F Chilton and David Parker

29 May 2019

In two closely related series of eight‐coordinate lanthanide complexes, a switch in the sign of the dominant ligand field parameter and striking variations in the sign, amplitude and orientation of the main component of the magnetic susceptibility tensor as the Ln3+ ion is permuted conspire to mask modest changes in NMR paramagnetic shifts, but are evident in Yb EPR and Eu emission spectra.

EXPLORING THE DUAL FUNCTIONALITY OF AN YTTERBIUM COMPLEX FOR MOLECULAR OPTICAL THERMOMETRY AND SLOW MAGNETIC RELAXATION

We present a comprehensive investigation of the magnetic and optical properties of an ytterbium complex, which combines two desirable and practical features into a single molecular system. Based upon YbIII ions that promote near-infrared optical activity and a chemical backbone that is ideal for an in-depth understanding of the magnetic behaviour, we have designed a multifunctional opto-magnetic species that operates as an molecular optical thermometer and as a single-molecule magnet (SMM). Our magnetic investigations, in conjunction with ab initio calculations, reveal one of the highest energy barriers reported for an YbIII-based complex. Moreover, we correlate this anisotropic barrier with the emission spectrum of the compound, wherein we provide a complete assignment of the energetic profile of the complex. Such studies lay the foundation for the design of exciting multi-faceted materials that are able to retain information at the single-molecule level and possess built-in thermal self-monitoring capabilities.

PERIODIC TRENDS AND HIDDEN DYNAMICS OF MAGNETIC PROPERTIES IN THREE SERIES OF TRIAZACYCLONONANE LANTHANIDE COMPLEXES

Elizaveta A. Suturina, Kevin Mason, Mauro Botta, Fabio Carniato, Ilya Kuprov, Nicholas F. Chilton, Eric J. L. McInnes, Michele Vonci and David Parker

In three structurally related series of nine-coordinate lanthanide(III) complexes (Ln = Tb, Dy, Ho, Er, Tm and Yb) based on triazacyclononane, solution NMR studies and DFT/CASSCF calculations have provided key information on the magnetic susceptibility anisotropy. Both experimental and computational approaches have revealed a poor correlation to Bleaney’s theory of magnetic anisotropy. CASSCF calculations suggested that the magnetic susceptibility is very sensitive to small geometric variations within the first coordination sphere, whereas DFT analyses indicate the thermal accessibility of low energy vibrational modes that may lead to distortion. Parallel NMRD and EPR studies on the three Gd(III) complexes revealed good correspondence in estimating the electronic relaxation time. The Gd (III) tris-pyridinecarboxylate complex possesses a very long electronic relaxation time making it a promising starting point for responsive gadolinium EPR probe design.

USING REDOX-ACTIVE Π BRIDGING LIGAND AS A CONTROL SWITCH OF INTRAMOLECULAR MAGNETIC INTERACTIONS

Xiaozhou Ma, Elizaveta A Suturina, Mathieu Rouzieres, Mikhail Platunov, Fabrice Wilhelm, Andrei Rogalev, Rodolphe Clérac, Pierre Dechambenoit

Intramolecular magnetic interactions in the dinuclear complexes [(tpy)Ni(tphz)Ni(tpy)]n+ (n=4, 3 and 2; tpy: terpyridine; tphz: tetrapyridophenazine) were tailored by changing the oxidation state of the pyrazine-based bridging ligand. While its neutral form mediates a weak antiferromagnetic (AF) coupling between the two S=1 Ni(II), its reduced form, tphz•-, promotes a remarkably large ferromagnetic exchange of +214(5)K with Ni(II) spins. Reducing twice the bridging ligand affords weak Ni-Ni interactions, in marked contrast to the Co(II) analogue. Those experimental results, supported by a careful examination of the involved orbitals, provide a clear understanding of the factors which govern strength and sign of the magnetic exchange through an aromatic bridging ligand, a prerequisite for the rational design of strongly coupled molecular systems and high TC molecule-based magnets.

CR(PYRAZINE)2(OSO2CH3)2: A TWO-DIMENSIONAL COORDINATION POLYMER WITH AN ANTIFERROMAGNETIC GROUND STATE

Panagiota Perlepe, Itziar Oyarzabal, Kasper S Pedersen, Philippe Negrier, Denise Mondieig, Mathieu Rouzieres, Elizabeth A Hillard, Fabrice Wilhelm, Andrei Rogalev, Elizaveta A Suturina, Corine Mathonière, Rodolphe Clérac

A novel Cr/pyrazine two-dimensional coordination network consisting of neutral Cr(pyrazine)2(OSO2CH3)2 layers was synthesized. Detailed characterization demonstrates unequivocally the presence of S = 2 CrII centers and the absence of a redox process between CrII and pyrazine, which explains its gigaohm resistivity and its antiferromagnetic ground state below 10 K.

Xiaozhou Ma, Elizaveta A Suturina, Siddhartha De, Philippe Négrier, Mathieu Rouzières, Rodolphe Clérac, Pierre Dechambenoit

A dinuclear CoII complex, [Co2(tphz)(tpy)2]n+ (n=4, 3 or 2; tphz: tetrapyridophenazine; tpy: terpyridine), has been assembled using the redox‐active and strongly complexing tphz bridging ligand. The magnetic properties of this complex can be tuned from spin‐crossover with T1/2≈470 K for the pristine compound (n=4) to single‐molecule magnet with an ST=5/2 spin ground state when once reduced (n=3) to finally a diamagnetic species when twice reduced (n=2). The two successive and reversible reductions are concomitant with an increase of the spin delocalization within the complex, promoting remarkably large magnetic exchange couplings and high‐spin species even at room temperature.

Axel Belser, Reimer Karstens, Peter Grüninger, Peter Nagel, Michael Merz, Stefan Schuppler, Elizaveta A Suturina, Angelika Chassé, Thomas Chassé, Heiko Peisert

The electronic structure of the central iron ion of perfluorinated iron phthalocyanine (FePcF16) in thin films has been studied on Cu(111) and Ag(111) using polarization dependent X-ray absorption spectroscopy (XAS). The data are compared to FePc on Ag(111). Ligand field parameters have been computed, and multiplet calculations (CTM4XAS) were carried out to simulate XAS spectra. The planar molecules are preferentially oriented lying flat on the substrate surface during the growth of the 1–4 nm thick films. A clear polarization dependence of the Fe L edge absorption spectra is observed, arising from transitions into orbitals with in-plane and out-of-plane character. The shape of the spectra for three to four monolayers of FePcF16 on Cu(111) is comparable to that of the thin films of FePc on Ag(111). However, a drastic change of the XAS peak shape is observed for thicker FePcF16 films on both Ag(111) and Cu(111), although the molecular orientation is very similar to coverages consisting of a few monolayers. Since in both cases the film thickness is distinctly beyond the monolayer regime, interface interactions can be ruled out as a possible origin of this behavior. Rather, the different XAS peak shapes seem to indicate that the multiplicity may depend on the detailed arrangement of the FePcF16 molecules. The large flexibility of the ground state of Fe could be of high interest for spintronic applications.

Kevin Mason, Alice C Harnden, Connor W Patrick, Adeline WJ Poh, Andrei S Batsanov, Elizaveta A Suturina, Michele Vonci, Eric JL McInnes, Nicholas F Chilton, David Parker

Crystallographic, emission and NMR studies of a series of C3-symmetric, nine-coordinate substituted pyridyl triazacyclononane Yb(III) and Eu(III) complexes reveal the impact of local solvation and ligand dipolar polarisability on ligand field strength, leading to dramatic variations in pseudocontact NMR shifts and emission spectral profiles, giving new guidance for responsive NMR and spectral probe design.

Elizaveta A Suturina, Kevin Mason, Carlos FGC Geraldes, Nicholas F Chilton, David Parker, Ilya Kuprov

Lanthanide ions accelerate nuclear spin relaxation by two primary mechanisms: dipolar and Curie. Both are commonly assumed to depend on the length of the lanthanide-nucleus vector, but not on its direction. Here we show experimentally that this is wrong – careful proton relaxation data analysis in a series of isostructural lanthanide complexes (Ln = Tb, Dy, Ho, Er, Tm, Yb) reveals angular dependence in both Curie and dipolar relaxation. The reasons are: (a) that magnetic susceptibility anisotropy can be of the same order of magnitude as the isotropic part (contradicting the unstated assumption in Guéron‘s theory of the Curie relaxation process), and (b) that zero-field splitting can be much stronger than the electron Zeeman interaction (Bloembergen's original theory of the lanthanide-induced dipolar relaxation process makes the opposite assumption). These factors go beyond the well researched cross-correlation effects; they alter the relaxation theory treatment and make strong angular dependencies appear in the nuclear spin relaxation rates. Those dependencies are impossible to ignore – this is now demonstrated both theoretically and experimentally, and suggests that a major revision is needed of the way lanthanide-induced relaxation data are used in structural biology.

Sergey Shuvaev, Elizaveta A Suturina, Kevin Mason, David Parker

Luminescence spectroscopy has been used to monitor the selective and reversible binding of pH sensitive, macrocyclic lanthanide complexes, [LnL1], to the serum protein α1-AGP, whose concentration can vary significantly in response to inflammatory processes. On binding α1-AGP, a very strong induced circularly-polarised europium luminescence signal was observed that was of opposite sign for human and bovine variants of α1-AGP – reflecting the differences in the chiral environment of their drug-binding pockets. A mixture of [EuL1] and [TbL1] complexes allowed the ratiometric monitoring of α1-AGP levels in serum. Moreover, competitive displacement of [EuL1] from the protein by certain prescription drugs could be monitored, allowing the determination of drug binding constants. Reversible binding of the sulphonamide arm as a function of pH, led to a change of the coordination environment around the lanthanide ion, from twisted square antiprism (TSAP) to a square antiprismatic geometry (SAP), signalled by emission spectral changes and verified by detailed computations and the fitting of NMR pseudocontact shift data in the sulphonamide bound TSAP structure for the Dy and Eu examples. Such analyses allowed a full definition of the magnetic susceptibility tensor for [DyL1].

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