Glauber dynamics in a single-chain magnet: From theory to real systems

The Glauber dynamics is studied in a single-chain magnet. As predicted, a single relaxation mode of the magnetization is found. Above 2.7 K, the thermally activated relaxation time is mainly governed by the effect of magnetic correlations and the energy barrier experienced by each magnetic unit. This result is in perfect agreement with independent thermodynamical measurements. Below 2.7 K, a crossover towards a relaxation regime is observed that is interpreted as the manifestation of finite-size effects. The temperature dependences of the relaxation time and of the magnetic susceptibility reveal the importance of the boundary conditions.Comment: Submitted to PRL 10 May 2003. Submitted to PRB 12 December 2003; published 15 April 200

K⊂{[FeII^{II}(Tp)(CN)3_{3}]4_{4}[CoIII^{III}(pz^{pz}Tp)]3_{3}[CoII^{II}(pz^{pz}Tp)]}: a neutral soluble model complex of photomagnetic Prussian blue analogues

Straightforward access to a new cyanide-bridged {Fe$_{4}$Co$_{4}$} “molecular box” containing a potassium ion, namely K⊂{[Fe$^{II}$(Tp)(CN)$_{3}$]$_{4}$[Co$^{III}$($^{pz}$Tp)]$_{3}$[Co$^{II}$($^{pz}$Tp)]} (1) (with Tp and $^{pz}$Tp = tris- and tetrakis(pyrazolyl)borate, respectively), is provided, alongside its full characterisation. A detailed analysis of the molecular structure (X-ray diffraction, mass spectrometry, NMR spectroscopy) and electronic properties (EPR spectroscopy, SQUID magnetometry, UV/Vis spectroscopy, cyclic voltammetry) reveals that 1 shows slow magnetic relaxation and a remarkable photomagnetic effect at low temperature which is reminiscent of some FeCo Prussian Blue Analogues (PBAs), and is ascribed to a photo-induced electron transfer. However, in contrast with these inorganic polymers, the overall neutral compound 1 is soluble and remarkably stable in organic solvents such as CH2Cl2. Moreover, 1 shows interesting redox versatility, with electrochemical experiments revealing the possible access to six stable redox states

Tetranuclear [Fe II 2 Fe III 2 ] 2+ molecular switches [Fe II (bik) 2 (N–) 2 ]spin-crossover complexes containing [Fe III (Tp)(CN) 3 ] – metalloligands as N-donor

International audienceThree novel mixed valence cyanide-bridged {Fe III 2 Fe II 2 } square complexes were obtained through the self-assembling of [Fe III (Tp)(CN) 3 ] - or [Fe III (Tp*)(CN) 3 ] − cyanido building blocks with the in situ formed [Fe II (bik) 2 (S) 2 ]complex (Tp = hydrotris (pyrazol-1-yl)borate, Tp* = hydrotris (3,5-dimethyl-pyrazol-1-yl)borate, bik = bis(1-methylimidazol-2-yl)ketone, S = solvent). The structures of these three complexes (2, 3 and 4)are reminiscent of that of our previously published square complex {[Fe III (Tp)(CN) 3 ] 2 [Fe II (bik) 2 ] 2 }·[Fe III (Tp)(CN) 3 ] 2 ·18H 2 O·4CH 3 OH (1). They consist of cyanide-bridged square dicationic complexes, ClO 4 − (2 and 3)or BF 4 − (4)counterions and solvate molecules. The FT-IR cyanide stretching vibrations observed at ν CN ≈ 2145–60 cm −1 are typical of {Fe III –CN–Fe II } moieties. The investigation of the magnetic properties of 2 reveals the occurrence of spin-crossover centered at T 1/2 = 227 K. The χ M T variation, ca. 7 cm 3 mol −1 K, reflects the complete spin-state change occurring on both {Fe II (bik)(–NC) 2 } moieties (–NC represents the cyanido building blocks). The Slichter–Drickamer model leads to a weak cooperativity factor, Γ = 1.6 kJ mol −1 (with Γ andlt; 2RT 1/2 ), which reflects the gradual spin-state change. This is in agreement with the molecular structure of 2, which does not present significant intermolecular interactions. The calculated enthalpy and entropy variations associated with the spin-state equilibrium are ΔH = 24 kJ mol −1 and ΔS = 105 J K −1 mol −1 . In contrast, 3 and 4 show only partial spin-crossover in the accessible temperature range (2–400 K)as the T 1/2 are shifted toward higher temperatures (ca. T 1/2 andgt; 400 K). Although no photomagnetic effect is observed for 3, compound 4 shows a moderate increase in the magnetization upon irradiation at low temperature. This phenomenon is ascribed to the light-induced excited spin-state trapping (LIESST)effect. Interestingly, the complex 2 also shows a remarkable LIESST effect, which is observed with different laser lights covering the visible and near-infrared range. The resulting χ M T value obtained in the photoinduced state suggests the occurrence of a ferromagnetic interaction inside the {Fe III –CN–Fe II } units. © 2019 Académie des science

Solution and Solid-State Study of the Spin-Crossover [Fe-II(R-bik)(3)](BF4)(2) Complexes (R = Me, Et, Vinyl)

The magnetic properties of three spin-crossover complexes, [Fe-II(R-bik)(3)](BF4)(2)center dot nH(2)O (1-3), based on bis(imidazolyl) ketone ligands, were investigated in solution and the solid state. Their properties were compared with those of the ketone-free analogue, [Fe-II(bim)(3)](OTf)(2) (4). The alkyl and vinyl R groups have weak influence on the transition temperature, T-1/2, in solution, while stronger differences are observed in the solid state, because different intermolecular interactions occur in 1-3. The spin-state equilibria in solution were followed by SQUID magnetometry and the Evans NMR spectroscopy method. Interestingly, the equilibria can also be simply and efficiently probed by following the temperature dependence of an adequately chosen H-1 chemical shift. Overall, these experiments give coherent results, with T-1/2 located between 320 and 335 K, a narrow range, in comparison with the solid state. DFT calculations have allowed the rationalization of the magnetic differences. The molecular-orbital and spin-density calculations reveal that the presence of the C=O group between the imidazolyl units in the ligands of 1-3 leads to an extended aromatic system, an effective pi-acceptor effect, stabilizing the LS state and reducing the LS-HS gap, in comparison with 4

Nickel(II) Chain with Alternating End-On/End-to-End Single Azido Bridges: A Combined Structural, Magnetic, and Theoretical Study

The reaction of a tridentate Schiff base LH (L-: 1,1,1-trifluoro-7-(dimethylamino)-4-methyl-5-aza-3-hepten-2-onato) with a Ni(II) salt in the presence of azide salt has led to a new alternating end-on (EO)/end-to-end (EE) azidobridged Ni(II) chain of formula {[Ni2(í1,1-N3)(í1,3-N3)(L)2(MeOH)2]}n. Its originality lies in the presence of single EE and EO coordination modes for the azide. It crystallizes in the C2/c space group, a ) 21.570(7) Å, b ) 10.79(1) Å, c ) 16.154(5) Å, â ) 120.81(2)°, Z ) 4. The chain can be viewed as {Ni2(N3)(L)2(MeOH)2}+ dimeric units linked to each other in a zigzag pattern by the other azide. Magnetic susceptibility and magnetization measurements have been performed and revealed that the chain can magnetically be depicted as isolated {Ni2(N3)} units exhibiting antiferromagnetic interaction (JAF -37 cm-1). Ab initio calculations confirmed the efficient magnetic coupling through the EE bridge and vanishingly small EO {Ni2(í1,1-N3)} interactions