Chiara Carbonera

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands.

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.

Tuning the physical properties of a metal complex by molecular techniques: the design and the synthesis of the simplest cobalt-o-dioxolene complex undergoing valence tautomerism

Abstract A family of complexes of general formula [Co(CTH)(Phendiox)]Y·solv (CTH=tetraazamacrocycle; Phendiox=catecholato or semiquinonato form of 9,10-dioxophenanthrene; Y=PF 6 , I, BPh 4 ; solv=H 2 O, CH 2 Cl 2 , C 6 H 5 CH 3 ) were found to undergo valence tautomeric equilibria in the solid state. The interconversion involves an intramolecular electron transfer between a six-coordinated diamagnetic cobalt(III) metal ion and the coordinated polyoxolene ligand yielding a cobalt(II)–semiquinonato complex, the metal ion being in high spin configuration. All the synthesised complexes are diamagnetic at low temperature and high pressure and paramagnetic at high temperature and low pressure. These complexes were designed following electrochemical data. The critical interconversion temperature was found to depend on the nature of the counterion Y and on the nature of the solvent trapped in the lattice. No hysteresis was detected in the interconversion processes. However significant hysteresis effects were detected by substituting solvent from the lattice with its deuterated analogue and by using a partially deuterated ancillary macrocyclic ligand. The observed results are totally unpredictable and show how the emerging properties of a given set of molecules can be deeply influenced by very subtle and not controlled factors. Studies on closely related complexes were carried out for elucidating the electronic properties of these systems. In particular it was found that the [Co II (Me 4 cyclam)(SQ)]Y complexes (Me 4 cyclam=tetraazamacrocycle; SQ=semiquinonato radical ligand) are characterised by a weak anti-ferromagnetic coupling between the high-spin cobalt(II) ion and the paramagnetic ligand. In addition it was shown that the valence tautomeric interconversion could be photoinduced, the process involving two separate steps. DFT calculations show that in addition to the cobalt(III)–catecholato and hs -cobalt(II)–semiquinonato species which are involved in the thermal equilibrium, the third valence tautomer ls -cobalt(II)–semiquinonato species could be important from a photomagnetic point of view.

Photomagnetism of a series of dinuclear Iron(II) complexes.

Spin crossover: The photomagnetic properties of a series of [{Fe(NCS)(py-X)}(2)(bpypz)(2)] (NCS=thiocyanate, py=pyridine, X=4-Mepy, py, 3-Mepy, 3-Clpy and 3-Brpy, and bpypz=3,5-bis(pyridine-2-yl)pyazolate) binuclear complexes are close to the antiferromagnetic response of [{Fe(NCS)(3,5-dmpy)}(3)(bpypz)(2)] (3,5-dmpy=3,5-dimethylpyrazine), which is characterised by two iron(II) metal ions in a high-spin (HS) electronic configuration (see figure).This paper describes the photomagnetic properties of a series of binuclear iron(II) complexes belonging to the [{Fe(NCS)(py-X)}(2)(bpypz)(2)] family (NCS=thiocyanate; py=pyridine; bpypz=3,5-bis(pyridine-2-yl)pyrazolate; and py-X=4-Mepy (1), py (3), 3-Mepy (4), 3-Clpy (5), and 3-Brpy (6)). All of these complexes display a complete thermal spin transition centred between 100 and 150 K, and undergo the light-induced excited-spin-state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. For all of the complexes, it has been found that the T(LIESST) curves at low temperature are close to the antiferromagnetic response of the [{Fe(NCS)(3,5-dmpy)}(2)(bpypz)(2)] (3,5-dmpy=3,5-dimethylpyridine) (7) complex that is characterised by two iron(II) metal ions in a HS electronic configuration, giving some evidence of a quantitative low spin-low spin-->high spin-high spin (LS-LS-->HS-HS) photoconversion process. Depending on the nature of the cooperativity, the kinetics have been treated with stretched exponential, simply exponential, or sigmoidal models. Interestingly, this series of dinuclear complexes follows a previously proposed linear relationship between the T(LIESST) and their thermal spin transition temperatures T(1/2): T(LIESST)=T(0)-0.3T(1/2). T(0) for these compounds is equal to 100 K. Based on this, and by using the empirical linear relationship found between the thermal spin transition and the Hammett constant, the HS-HS properties of complex 7 have been understood as a reflection of the physical impossibility that the T(LIESST) was higher than T(1/2). The close vicinity of the thermal spin-crossover phenomenon of 7 has been successfully checked by applying hydrostatic pressure.

Atypical photomagnetic properties in a series of binuclear iron(II) spin crossover complexes

In this work, we have re-examined the photomagnetic properties of a series of binuclear iron(II) complexes of formula [{Fe(L)(NCX)2}2bpym], where L is bpym = 2,2′-bipyrimidine and bt = 2,2′-bithiazoline and X = S, Se, by light irradiation at 647.1–676.4 nm and at 830 nm and we confirm that after optical excitation the magnetic behaviors of [{Fe(bpym)(NCSe)2}2bpym] (2), [{Fe(bt)(NCS)2}2bpym] (3) and [{Fe(bt)(NCSe)2}2bpym] (4) are close to the antiferromagnetic response of [{Fe(bpym)(NCS)2}2bpym] (1) characterized by two iron(II) metal ions in the HS electronic configuration. This hypothesis of quantitative HS–HS photoconversion is also supported by the reflectivity experiments and the infrared spectroscopies performed on 3. We have also investigated the relaxation process of 2–4 by using a SQUID magnetometer. Atypical magnetic behavior is found for compounds 3 and 4. More precisely, while the magnetic response of 2versus time continuously decreases whatever the temperature, the signal of 3 increases below 17 K, becomes stable at around 18–20 K and decreases at higher temperature. We thus propose a direct HS–HS→LS–HS relaxation process for 2 and an indirect HS–HS→LS–HS→LS–LS relaxation process for 3 and 4. The agreement found between the experimental T(LIESST) and the simulated T(LIESST) curves, calculated using the measured kinetic parameters, guarantees the validity of our approach.

Optically Induced Valence Tautomeric Interconversion in Cobalt Dioxolene Complexes

The discovery that a number of cobalt-dioxolene complexes undergo photoswitchable behavior was reported in the recent past. This phenomenon is always associated with valence tautomeric interconversion processes involving low-spin cobalt(III) and high-spin cobalt(II) species. Herein is stressed the strong formal correlation existing between these processes and the LIESST (Light-Induced Excited Spin State Trapping) effect shown by several iron(II) complexes undergoing spin crossover interconversion. The dynamics of the relaxation of the photoinduced metastable species to the ground state is discussed in terms of non-adiabatic processes within the Jortner theory of radiationless multiphonon relaxation.

Does cooperativity influence the lifetime of the photo-induced HS state ?

We have first recalled the T(LIESST) procedure which consists to determine the temperature above which the photo-magnetic effect is erased. In addition we have selected to series of iron(II) spin crossover complexes, the [Fe(PM-L)2(NCS)2] and [Fe(bpp)2]X2·nH2O families, to analyse the influence of the cooperativity on the stability of the photo-induced HS state. Some of these complexes exhibit gradual thermal spin crossover behaviours while some others undergo an abrupt thermal transition, with and without hysteresis. Interestingly, whatever the cooperativity effect on the thermal spin crossover transition, the lifetime of the metastable state of all these derivates remains governed by the T(LIESST) = T0 - 0.31 T1/2 relation. Finally, we have investigated the magnetic and the photomagnetic properties of a [Fe(bpp)2]-Nafion film. Once more the role of the cooperativity on the stability of the photoinduced HS state appears minor. Conversely, the influence of the nature and the geometry of the inner coordination sphere appears preponderant.

Photo-crystallography: from the structure towards the electron density of metastable states

A photo-crystallographic study of Fe(btr)2(NCS)2H2O was performed in order to describe the modification of structures and charge densities on going from the ground low spin (LS) state to the metastable high spin (HS) state during the LIESST phenomenon at 15 K. Related photo-magnetic and spectroscopic measurements are also described. We show that at 15 K, the thermally quenched and photo-induced structures of the metastable HS state are identical. For comparison, we also derived the structure of the HS and LS states at 130 K in the hysteresis loop; the thermal spin transition and the LIESST spin transition exhibit similar structural behaviours.

Magnetic exchange interaction between paramagnetic transition metal ions and radical ligands. A 9,10-dioxophenanthrenesemiquinonato adduct of a nickel(II)–tetraazamacrocycle complex and DFT description

The 9,10-dioxophenanthrenesemiquinonato adduct of a nickel(II)–CTH acceptor (CTH = dl-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) was synthesized and structurally characterized. Temperature dependent magnetic susceptibility measurements show that this compound has a quartet electronic ground state arising from the strong ferromagnetic coupling between the S = 1 metal ion and the radical ligand. A computational DFT study carried out using the broken symmetry approach supports the observed magnetic properties as well those of the other nickel(II)–semiquinonato analogues. These results elucidate the electronic properties of the related cobalt–dioxolene complexes when undergoing valence tautomerism.

Inducing asymmetry in free-base, MnIII, NiII and CuII (ethylsulfanyl)porphyrazines: synthetic aspects and spectro-electrochemical implications

Treatment of 2,3,7,8,12,13,17,18-octakis(ethylsulfanyl)-5,10,15,20-porphyrazine (H2OESPz) with CrCl2 in a 1,2,4-trichlorobenzene–n-BuOH mixture resulted in one ethylsulfanyl branch at the pyrrolic β positions of the macrocycle being replaced by an hydrogen atom with >40% yield. The structure of the asymmetric pyrrolic subunit and of the nearest pyrrolic subunits have been determined by NMR spectroscopy. The reaction path leading to the asymmetric porphyrazine implies the formation of a two-electron reduced diprotonated porphyrazine, [H2OESPz(−4)(2H+)], that, as a consequence of charge and structural intramolecular rearrangements at high temperature, undergoes nucleophilic substitution of an ethyl sulfide group by H−. UV-Vis and near-IR results suggest that the asymmetric substitution modifies the (sulfanyl)porphyrazine π and nSulfur levels. On the contrary, ESR spectroscopy shows that the electronic properties of the coordinated Cu2+ ion are affected only marginally. The half-waves of the ligand first reductive processes sensibly shift towards cathodic potentials in the asymmetric metal porphyrazines, but not in the asymmetric free-base porphyrazine, probably due to conflicting electronic and structural effects induced by removal of an ethylsulfanyl tail.