Preparation and Magnetic Properties of Iron(3+) Spin-Crossover Complexes Bearing a Thiophene Substituent: Toward Multifunctional Metallopolymers

Spin Crossover in [Fe(qsal-5-Brq)2]+ Complexes with a Quinoline-Substituted Qsal Ligand

Using a quinoline substituted Qsal ligand, Hqsal-5-Brq (Hqsal-5-Brq = N-(5-bromo-8-quinolyl)salicylaldimine), four FeIII complexes, [Fe(qsal-5-Brq)2]A·CH3OH (Y = NO3- (1NO3), BF4- (2BF4), PF6- (3PF6), OTf- (4OTf), were prepared and characterized. Structure analysis revealed that complex 2BF4 contained two species (2BF4(P1̅) and 2BF4(C2/c)). In these compounds except 3PF6, the [Fe(qsal-5-Brq)2]+ cations form 1D chains through π-π interactions and other weak interactions. Adjacent chains are connected to form the 2D "Chain Layer" structures and 3D structures through various supramolecular interactions. For 3PF6, a "Dimer Chain" structure is formed from the loosely connected dimers. Magnetic studies revealed that compounds 1NO3 and 2BF4(P1̅) displayed abrupt hysteretic SCO with the transition temperature T1/2↓ = 235 K, T1/2↑ = 240 K for 1NO3 and T1/2↓ = 230 K, T1/2↑ = 235 K for 2BF4(P1̅), while compounds 3PF6 and 4OTf are in the HS state. Desolvation of the complexes significantly modifies their SCO properties: the desolvated 1NO3 and 2BF4 show a gradual SCO, desolvated 3PF6 undergoes a two-step SCO, and desolvated 4OTf exhibits a hysteretic transition. Overall, this work reported the FeIII-SCO complexes of the quinoline-substituted Hqsal ligand and highlighted the potential of these ligands for the development of interesting FeIII-SCO materials.

Spin-Crossover in Supramolecular Iron(II)-2,6-bis(1H-Pyrazol-1-yl)pyridine Complexes: Toward Spin-State Switchable Single-Molecule Junctions

Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2'-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.

Effect of the chemical structure of a tetradentate equatorial ligand on the spin-crossover properties of the Fe (III) complexes chain structures: Electron paramagnetic resonance study

The effect of the chemical structure of the equatorial ligand on the spin state of the Fe (III) ion in a series of 1-D chain complexes of the general formula [Fe(L)(tvp)]BPh₄ ·nCH₃ OH, where L = dianions of Schiff base containing a different number of aromatic groups: N,N'-ethylenebis (salicylaldimine) (salen) 1, N,N'-ethylenebis (acetylacetone)2,2'-imine (acen) 2, N,N'-ethylenebis (benzoylacetylacetone)2,2'-imine (bzacen) 3, and tvp = 1,2-di(4-pyridyl)ethylene, was studied by ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) methods. The values of exchange interactions, thermodynamic parameters of spin-crossover, and electronic structure features of the Fe (III) complexes were estimated from the EPR data. The substitution of a fragment of the equatorial ligand L in the series salen-acen-bzacen changes the local symmetry of the complex in the 1-D chain, thereby affecting the spin variable properties.

Effect of the chemical structure of an equatorial ligand on the spin crossover properties of the Fe(III) complex with 4-styrylpyridine axial ligands

In order to study the effect of the chemical structure of the equatorial ligand on the spin state of the Fe(iii) ion in complexes with invariable photoisomerizable 4-styrylpyridine axial ligands and different tetradentate Schiff bases, several new Fe(iii) complexes have been first synthesized, characterized, and studied by UV-vis, NMR, and EPR spectroscopies. The general chemical formula of the complexes is [Fe(SB)Sp₂]BPh₄·MeOH, where Sp is trans-4-styrylpyridine and SB are dianions of Schiff bases: salen, bzacen, and acen [salen = N,N'-ethylenebis(salicylaldimine) 1, acen = N,N'-ethylenebis(acetylacetonylideneimine) 2, and H₂bzacen = N,N'-ethylenebis(benzoylacetonylideneimine) 3]. The results of the EPR and NMR measurements of the complexes both in the solid state and in solutions showed that the more methyl groups and less aromatic rings in the equatorial ligand, the more abrupt spin-crossover was observed in the complex. The dependence of the magnetic properties of the complexes on the state of matter and the presence of a solvent (powder, liquid solutions, and vitrified solutions) are noted.

Abrupt spin crossover in iron(iii) complexes with aromatic anions

Two iron(iii) complexes, [Fe(qsal-X)₂]OTs·nH₂O, are found to exhibit abrupt spin crossover with the spin transition temperature substituent dependent, and X⋯O halogen bonds linking the spin centres.

Spin-state modulation of molecular FeIII complexes via inclusion in halogen-bonded supramolecular networks

The cationic complex [Fe(qsal)₂]⁺ (Hqsal = N-(8-quinolyl)salicylaldimine) is encapsulated in anionic halogen-bonded 1D and 2D networks derived from sym-triiodotrifluorobenzene, [(C₆F₃I₃)Cl]^- and [(C₆F₃I₃)I]^-. Structural analysis and magnetic measurements show that the spin-state of this complex can be modulated by its inclusion in supramolecular host frameworks.

Experimental and Theoretical Identification of the Origin of Magnetic Anisotropy in Intermediate Spin Iron(III) Complexes

The complexes [FeL^N2S2X] [in which L^N2S2=2,2′‐(2,2′‐bipryridine‐6,6′‐diyl)bis(1,1′‐diphenylethanethiolate) and X=Cl, Br and I], characterized crystallographically earlier and here (Fe(L)Br), reveal a square pyramidal coordinated Fe^III ion. Unusually, all three complexes have intermediate spin ground states. Susceptibility measurements, powder cw X‐ and Q‐band EPR spectra, and zero‐field powder Mössbauer spectra show that all complexes display distinct magnetic anisotropy, which has been rationalized by DFT calculations.

Cooperative spin-crossover transition from three-dimensional purely π-stacking interactions in a neutral heteroleptic azobisphenolate FeIII complex with a N3O3 coordination sphere

The first neutral spin-crossover Fe^III complex with a N₃O₃ coordination sphere formed a purely π-stacking interaction network and exhibited the cooperative transition.

Mononuclear iron(iii) complexes of tridentate ligands with efficient nuclease activity and studies of their cytotoxicity

Mono- and bis-chelated iron(iii) complexes derived from phenolato-based tridentate ligands have been synthesised and characterized. These complexes show electrostatic DNA interactions and efficient DNA cleavage via OH˙ radicals, and induce cytotoxicity in MCF7 cell lines.

(Phenoxyimidazolyl-salicylaldimine)iron complexes: synthesis, properties and iron catalysed ethylene reactions

(Salicyladimine)iron(iii) pre-catalysts, from iron(ii) precursors, form catalysts for ethylene oligomerisation, polymerisation and alkylation of aromatic solvents by the ethylene oligomers.

Steric Trapping of the High Spin State in FeIII Quinolylsalicylaldimine Complexes

A new sterically bulky Schiff base ligand, N-(8-quinolyl)-5-tert-butylsalicylaldimine (Hqsal-5-tBu) has been prepared and a series of FeIII complexes, [Fe(qsal-5-tBu)2]Y (Y = Cl 1, ClO4 2, NO3 3, BF4 4) utilising this ligand are reported and fully characterised. UV-vis spectroscopic and electrochemical studies indicate that 1–4 are high spin (HS) in solution at room temperature and further suggest that the tBu group only slightly alters the electronic properties of 1–4 compared with related [Fe(qsal-5-X)2]+ systems. The structures of [Fe(qsal-5-tBu)2]Cl·4MeOH·H2O 1, [Fe(qsal-5-tBu)2]ClO4·MeOH 2, and [Fe(qsal-5-tBu)2]NO3 3 determined at 100 K reveal HS FeIII centres in all cases. Four-fold parallel aryl embraces and π–π interactions serve to link the cations forming 2D sheets mirroring the motifs found in other [Fe(qsal-5-X)2]+ complexes. Despite this the tBu group causes strong distortions at the Fe centre which as magnetic studies reveal prevent spin crossover trapping 1–4 in the HS state.

Stepped spin crossover in Fe(iii) halogen substituted quinolylsalicylaldimine complexes

Four iron(iii) spin crossover complexes with halogen substituted ligands are reported. The halogen is correlated with T_1/2 and controls the degree of spin crossover while extensive C–H⋯X and X⋯π interactions increase cooperativity.

A mononuclear Fe(III) single molecule magnet with a 3/2↔5/2 spin crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.

A supramolecular spin crossover Fe(III) complex and its Cr(III) isomer: stabilization of water-chloride cluster within supramolecular host

The metal complexes, [M(Hdammthiol)(2)]Cl·3H(2)O [M = Cr(III) (1), Fe(III) (2)] [where H(2)dammthiol is the thiol form of the ligand, diacetylmonoxime morpholine N-thiohydrazone] were synthesized by metal template reactions of diacetylemonoxime with morpholine N-thiohydrazide in the presence of CrCl(3)·6H(2)O and FeCl(3)·6H(2)O. Both the complexes (1 and 2) were characterized by single crystal X-ray crystallography, spectroscopic (IR and UV-vis), Mössbauer and TGA analyses. The single crystal X-ray studies of both complexes show that the supramolecular hosts, constructed by the discrete mononuclear complexes, form supramolecular channels along the c-axis which are filled up by water-chloride clusters. In both complexes, the 1D water-chloride chain with chair-like architecture within the supramolecular hosts presents novelty. The magnetic measurement study of Fe(III) complex shows a spin crossover from S = 1/2 at 2.5 K to S = 5/2 at 300 K. At very low temperature, the presence of strong cooperative hydrogen bonding interactions stabilizes the S = 1/2 state.

In vivo cytogenetic studies on rat's bone-marrow cells of structurally related Schiff base complexes

The in vivo interactions of structurally-related Ni(II) and Fe(III) Schiff base complexes based on N-(8-quinolyl)salicylaldimine (HL(1)) and N-(8-quinolyl)napthaldimine (HL(2)) ligands with DNA molecules in the bone-marrow cells of rats were demonstrated using chromosomal aberrations (CAs) assay. The complexes differ by one aromatic group on the aldehyde site of the Schiff base (basicity or lipophilicity), or by the type of the central metal ions (Ni(II) or Fe(III)). Animals were injected intraperitoneally (i.p) with different concentrations of each drug, and CAs were examined in bone-marrow cells, 15 hours later. A significant increase in the frequency of CAs was induced upon treatment with 15 mg / kg weight of L(1) complexes (P < 0.001), and not with L(2) complexes (P > 0.05). Also, the magnitude of aberrations induced by L(1)-Ni(II) was higher than that induced by L(1)-Fe(III) (P < 0.01). The binding data, estimated using UV-Visible absorption technique, showed that the metal binding of HL(1) was much greater than that of HL(2) and that the affinity of HL(1) towards Ni(II) is higher than that for Fe(III) ions. Thus, the trends in the presented in vivo results signify the important role of complex stability in predicting the clastogenicity of metal-ion-chelating Schiff base drugs.

Iron(II) complexes containing thiophene-substituted “bispicen” ligands — Spin-crossover, ligand rearrangements, and ferromagnetic interactions

The synthesis and characterization of three new tetradentate “bispicen-type” ligands containing a substituted thiophene heterocycle are described [2,5-thienyl substituents = H (7), Ph (8), or 2-thienyl (9)]. Iron(II) bis(thiocyanate) coordination complexes containing 7–9 were prepared, and the electronic and variable-temperature magnetic properties of complexes containing 7 (10) and 9 (12) are described. Complex 10 features a gradual and incomplete spin crossover in the solid state, and 12 remains high-spin over the entire temperature range. Complex 11 is extremely unstable and rearranges to another iron(II) complex (13), which was structurally characterized. The temperature-dependent magnetic properties of 13 are described as a one-dimensional ferromagnetic chain, with interchain antiferromagnetic interactions and (or) zero-field splitting dominant at low temperatures. The magnetic analysis is corroborated by the molecular packing and density functional theory calculations, which suggest intermolecular interactions between coordinated thiocyanate ligands bearing a significant spin density.

Hybrid spin-crossover conductor exhibiting unusual variable-temperature electrical conductivity

We describe the multistep synthesis of a new terthienyl-substituted QsalH ligand and an iron(3+) spin-crossover complex (1) containing this ligand, which electropolymerizes to produce a hybrid-conducting metallopolymer film (poly1). Variable-temperature magnetic susceptibility measurements demonstrate that spin-crossover is operative in the polymer film, and resistivity measurements on indium-tin oxide coated glass slides containing the polymer film exhibit intriguing temperature-dependent profiles.

Electronic structures of bis- and monothiophene complexes with Fe, Co, Ni: a density functional theory study

A density functional theory study for the bis- and monothiophene complexes of Fe, Co, and Ni (MT2 and MT, T = thiophene, M = Fe, Co, Ni) was performed to understand their coordination geometries, bonding properties, vibration spectra and singlet excited state spectra. The typical metal coordination exists in the complexes. The Fe-thiophene coordination has the highest stability, with Ni-thiophene being the second highest, and Co-thiophene the lowest. Bisthiophene complexes of Co and Ni prefer to homolytically dissociate to their monothiophene ones and free thiophene. Frequency calculation shows that the ligand-M-ligand asymmetric stretching vibration in bisthiophene complexes shows a strong absorption, at 435.2, 495.7, and 383 cm(-1) for Fe(eta4-T)2, Co(eta2-T)2 and Ni(eta2-T)2, respectively. The M-S stretching vibration in monothiophene complexes shows a strong absorption in the far-infrared region, at 209, 156, and 150 cm(-1) for Fe(eta4-T), Co(eta4-T) and Ni(eta5-T), respectively. The excited state spectra indicate that the characteristic absorption wavelengths of the complexes have a red shift of more than 12.40 eV compared to free thiophene, at 3.54, 1.64, 3.83, 2.75, 1.43, and 2.58 eV for Fe(eta4-T)2, Co(eta2-T)2, Ni(eta2-T)2, Fe(eta4-T), Co(eta4-T), and Ni(eta5-T), respectively.

Iron spin-crossover compounds: from fundamental studies to practical applications

Over the past five years, the spin-crossover (SCO) phenomenon has experienced a clear new lease of interest from the scientific community coinciding with the recent publication of remarkable new advances. This perspective paper describes five illustrative examples of SCO systems, published during the past twelve months, showing new aspects of the unique and very appealing behaviour of these molecular switches, which may find interesting applications in the near future.