Tuning of spin crossover behaviour in iron(iii) complexes involving pentadentate Schiff bases and pseudohalides

Abstract

Investigations on a series of eight novel mononuclear iron(III) Schiff base complexes with the general formula [Fe(L(5))(L(1))]·S (where H(2)L(5) = pentadentate Schiff-base ligand, L(1) = a pseudohalido ligand, and S is a solvent molecule) are reported. Several different aromatic 2-hydroxyaldehyde derivatives were used in combination with a non-symmetrical triamine 1,6-diamino-4-azahexane to synthesize the H(2)L(5) Schiff base ligands. The consecutive reaction with iron(III) chloride resulted in the preparation of the [Fe(L(5))Cl] precursor complexes which were left to react with a wide range of the L(1) pseudohalido ligands. The low-spin compounds were prepared using the cyanido ligand: [Fe(3m-salpet)(CN)]·CH(3)OH (1a), [Fe(3e-salpet)(CN)]·H(2)O (1b), while the high-spin compounds were obtained by the reaction of the pseudohalido (other than cyanido) ligands with the [Fe(L(5))Cl] complex arising from salicylaldehyde derivatives: [Fe(3Bu5Me-salpet)(NCS)] (2a), [Fe(3m-salpet)(NCO)]·CH(3)OH (2b) and [Fe(3m-salpet)(N(3))] (2c). The compounds exhibiting spin-crossover phenomena were prepared only when L(5) arose from 2-hydroxy-1-naphthaldehyde (H(2)L(5) = H(2)napet): [Fe(napet)(NCS)]·CH(3)CN (3a, T(1/2) = 151 K), [Fe(napet)(NCSe)]·CH(3)CN (3b, T(1/2) = 170 K), [Fe(napet)(NCO)] (3c, T(1/2) = 155 K) and [Fe(napet)(N(3))], which, moreover, exhibits thermal hysteresis (3d, T(1/2)↑ = 122 K, T(1/2)↓ = 117 K). These compounds are the first examples of octahedral iron(III) spin-crossover compounds with the coordinated pseudohalides. We report the structure and magnetic properties of these complexes. The magnetic data of all the compounds were analysed using the spin Hamiltonian formalism including the ZFS term and in the case of spin-crossover, the Ising-like model was also applied.

Synthesis, characterization, crystal structure and predicting the second-order optical nonlinearity of a new dicobalt(III) complex with Schiff base ligand

“…These compounds form stable and variety complexes with the majority of transition metal ions that exhibit various electronic and structure characteristics [5][6][7][8][9]. Therefore, the research area containing the coordination compounds with azomethine ligands as main characters is widely expansive especially due to their potential interest roles in various interdisciplinary fields such as: structural features [10,11], efficient different catalytic processes in various organic reactions [12][13][14], models of the metal ion coordination sites in metalloproteins or enzymes at bioinorganic chemistry [15][16][17], magnetochemistry [18][19][20], corrosion inhibitors [21] and nonlinear optical materials [22][23][24]. Second-Order NLO organic materials such as Schiff base compounds have recently attracted attention owing to their potential applications in computers, optical communication and medical imaging [25,26].…”

The Development of Spin‐Crossover Research

“…The former μ-4,4 -bipy type were first reported by Hayami et al 101,102 From a theoretical and data fitting perspective, Boča et al 48,100 found that the Ising thermodynamic model discussed above for Fe II Fe II SCO species would not fit the CN-bridged Fe III Fe III χ M T (μ eff ) vs. T or isothermal magnetization, M vs. H data at 2 K. So, they developed a new and more extensive model that, while it used many parameters, was capable of simultaneously fitting magnetic susceptibility, magnetisation, n HS and Mössbauer spectral data. But first, we briefly discuss the experimental data and start with mononuclear 'precursors', the magnetic data of which could be fitted by use of Ising models.…”

“…Complexes 100 Observables were χ M T vs. T and M vs H at 2 and 4.6 K. The CN − compounds were LS d 5 (S = 1 / 2 ) and displayed typical LS Fe III -N and Fe III -O distances and small octahedral distortions ∼25 • . The HS examples had longer Fe-N/O bond lengths and higher ∼56 • and displayed typical HS 6 A 1g zero-field split magnetism with decreases observed in χ M T below ∼5 K, the D parameters <1 cm −1 , confirmed by the magnetisation isotherms.…”

“…The HS examples had longer Fe-N/O bond lengths and higher ∼56 • and displayed typical HS 6 A 1g zero-field split magnetism with decreases observed in χ M T below ∼5 K, the D parameters <1 cm −1 , confirmed by the magnetisation isotherms. SCO behaviour in the naphthaldimine/NCS and NCSe compounds, from magnetism and variable temperature crystallography (cell lengths and volume), followed closely our early [Fe III (salen)(imidazole) 2 ] + data, 15 and were fitted by Ising-like theory, 100 including vibrations (ν). The theory and symbols are in the ESI of the paper and from HS mole fraction x' HS , vs. temperature, the parameters for Fe III -naphthaldimine/NCS were: T c = 150 K, g LS = 2.25, g HS = 2.08, D HS = 1.8 cm −1 , eff = 231 K, γ = 87 cm −1 , ν LS = 539 cm −1 ; calc.…”

Solvent-Induced Polymorphism of Iron(II) Spin Crossover Complexes

“…The transition of spin states is an entropy-driven process, which can be triggered by external parameters such as temperature [1], pressure [2], magnetic [3] or electric field [4,5], and by irradiation with the visible/near—infrared [6] or even X-ray [7] range. Up to now, the occurrence of the SCO phenomenon was observed for central atoms in coordination compounds of 3d 4 – 3d 7 electronic configuration, whereby the spherical symmetry of d -orbitals was broken into the e g and t 2 g orbital subsets by the tetragonal bipyramidal environment of the N 6 coordination environment.…”