Light-induced spin-state switching in Fe(II) spin-crossover complexes with thiazole-based chelating ligands

Homoleptic complexes [Fe(4bt)3](ClO4)2 (1), [Fe(2bt)3](ClO4)2 (2), and [Fe(3tpH)3](ClO4)2 (3) were obtained by a reaction between the Fe(II) precursor salt and the corresponding thiazole-based bidentate ligand (L = 4bt = 4,4'-bithiazole, 2bt = 2,2'-bithiazole, 3tpH = 3-(thiazol-2-yl)pyrazole). X-ray crystal structure determination revealed crystallization of solvent-free complex 1, a solvate 2·MeOH, and a co-crystal 3·2(3tpH). The crystal packing of all these complexes is dominated by one-dimensional interactions between the [Fe(L)3]2+ cations. These interactions are stronger in 2·MeOH and 3·2(3tpH), leading to cooperative and slightly hysteretic transitions between the high-spin and low-spin electronic configurations at ∼235 K and 159 K, respectively. In contrast, weaker intermolecular interactions in 1 result in a gradual spin crossover above 300 K, with the maximum fraction of the HS state ∼25% achieved at 400 K. Complexes 2 and 3·2(3tpH) exhibit light-induced excited spin state trapping (LIESST) under irradiation with white light or a 532 nm laser at 5 K. After the photoexcitation, the trapped metastable HS state relaxes to the ground LS state with the average relaxation temperature of 81 K and 68 K, respectively. Examination of the relaxation dynamics by optical absorption spectroscopy on a single crystal of 3·2(3tpH) revealed the sigmoidal shape of the relaxation curves at lower temperatures, attributed to cooperative effects, as well as a plateau at ∼10% of the HS fraction at intermediate temperatures, hinting at a more complex mechanism for the relaxation of the LIESST phase in this material.

Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes

Chelate coordination of non-symmetrical didentate pyrazine-benzimidazole (L1) or pyridine-benzimidazole (L2) N-donor ligands around divalent iron in acetonitrile produces stable homoleptic triple-helical spin crossover [Fe(Lk)₃ ]²⁺ complexes existing as mixtures of meridional (C₁ -symmetry) and facial (C₃ -symmetry) isomers in slow exchange on the NMR timescale. The speciation deviates from the expected statistical ratio mer/fac=3:1, a trend assigned to the thermodynamic trans-influence, combined with solvation effects. Consequently, the observed spin state Fe^II _low-spin ↔Fe^II _high-spin equilibria occurring in [Fe(Lk)₃ ]²⁺ refer to mixtures of complexes in solution, an issue usually not considered in this field, but which limits rational structure-properties correlations. Taking advantage of the selective and quantitative formation of isostructural facial isomers in non-constrained related spin crossover d-f helicates (HHH)-[LnFe(Lk)₃ ]⁵⁺ (Ln is a trivalent lanthanide, Lk=L5, L6), we propose a novel strategy for assigning pertinent thermodynamic driving forces to each spin crossover triple-helical isomer. The different enthalpic contributions to the spin state equilibrium found in mer-[Fe(Lk)₃ ]²⁺ and fac-[Fe(Lk)₃ ]²⁺ reflect the Fe-N bond strengths dictated by the trans-influence, whereas a concomitant solvent-based entropic contribution reinforces the latter effect and results in systematic shifts of the spin crossover transitions toward higher temperature in the facial isomers.

Tris(4,4'-bi-1,3-thia-zole-κN,N')iron(II) tetra-bromidoferrate(III) bromide

In the [Fe(4,4′-bit)₃]²⁺ (4,4′-bit is 4,4′-bi-1,3-thia­zole) cation of the title compound, [Fe(C₆H₄N₂S₂)₃][FeBr₄]Br, the Fe^II atom (3 symmetry) is six-coordinated in a distorted octa­hedral geometry by six N atoms from three 4,4′-bit ligands. In the [FeBr₄]⁻ anion, the Fe^III atom (3 symmetry) is four-coordinated in a distorted tetra­hedral geometry. In the crystal, inter­molecular C—H⋯Br hydrogen bonds and Br⋯π inter­actions [Br⋯centroid distances = 3.562 (3) and 3.765 (2) Å] link the cations and anions, stabilizing the structure.

Synthesis of 2,2'-Bis(3,6,9-triazanonyl)-4,4'-bithiazole and related compounds as new DNA cleavage agents

Two new bithiazole derivatives, 2,2'-bis(3,6,9-triazanonyl)- and 2,2'-bis(3,7,11-triazaundecyl)-4,4'-bithiazoles (3a, b), were readily synthesized in six steps using the corresponding dialkylenetriamine as starting materials. Under physiological conditions, 5.0 microM 3a exhibited significant DNA cleavage activity in the presence of Co(II), whereas even at 50 micriM, 3b exhibited no DNA cleavage activity. Furthermore, it was demonstrated that 3a forms a 1 : 2 complex with Co(II) ions, whereas 3b does not. These conclusions were based on measurements of stoichiometries of the bithiazole-cobalt complexes obtained by the Job continuous variation method. In contrast, 3a, which contains diethylenetriamine moieties, showed decreased affinity for Calf Thymus (CT) DNA compared with that of 3b, which contains dipropylenetriamine moieties. These findings indicate that the structure of the two aminoalkyl side chains attached at the 2- and 2'-positions of the 4,4'-bithiazole ring significantly influence the formation of cobalt complexes, and affects the compound's ability to cleave DNA as well as its affinity for double-stranded DNA.