Molecular Fe(II)-Ln(III) dyads for luminescence reading of spin-state equilibria at the molecular level

Due to the primogenic effect, the valence shells of divalent iron Fe(II) ([Ar]3d6) and trivalent lanthanides Ln(III) ([Xe]4fn) are compact enough to induce spin-state equilibrium for the 3d-block metal and atom-like luminescence for the 4f-block partner in Fe(II)-Ln(III) dyads. In the specific case of homoleptic pseudo-octahedral [Fe(II)N6] units, programming spin crossover (SCO) around room temperature at normal pressure requires the design of unsymmetrical didentate five-membered ring chelating N∩N' ligands, in which a five-membered (benz)imidazole heterocycle (N) is connected to a six-membered pyrimidine heterocycle (N'). Benefiting from the trans influence, the facial isomer fac-[Fe(II)(N∩N')3]2+ is suitable for inducing SCO properties at room temperature in solution. Its connection to luminescent [LnN6O3] chromophores working as non-covalent podates in the triple-stranded [Fe(II)Ln(L10)3]5+ helicates (Ln = Nd, Eu) controls the facial arrangement around Fe(II). The iron-based SCO behaviour of the 3d-4f complex mirrors that programmed in the mononuclear scaffold. Because of the different electronic structures of high-spin and low-spin [Fe(II)N6] units, their associated absorption spectra are different and modulate the luminescence of the appended lanthanide luminophore via intramolecular intermetallic energy transfers. It thus becomes possible to detect the spin state of the Fe(II) center, encoded by an external perturbation (i.e. writing), by lanthanide light emission (i.e. reading) in a single molecule and without disturbance. Shifting from visible emission (Ln = Eu) to the near-infrared domain (Ln = Nd) further transforms a wavy emitted signal intensity into a linear one, a protocol highly desirable for future applications in data storage and thermometry.

Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L1 ), pyrid-2,6-diyl (L2 ), benzene-1,2-dimethylenyl (L3 ) and propane-1,3-diyl (L4 ) linkers between the tridentate metal-binding domains. L1 and L2 form [Fe2 (μ-L)2 ]X4 (X- =BF4 - or ClO4 - ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe2 (μ-L1 )2 ][BF4 ]4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe2 (μ-L2 )2 ]4+ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L1 -L3 vary with the ligand linker group, by mass spectrometry and 1 H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe2 (μ-L1 )2 ]4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

Effect of intermolecular anionic interactions on spin crossover of two triple-stranded dinuclear Fe(ii) complexes showing above room temperature spin transition

Two new Fe(ii)-based dinuclear triple helicates [Fe2L3]4+, displaying near room temperature spin transition have been synthesized and the effect of intermolecular interactions and co-operativity between metal centers on the SCO has been studied.

Correlations between ligand field Δ o, spin crossover T 1/2 and redox potential E pa in a family of five dinuclear helicates

A family of five new bis-bidentate azole-triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [FeII 2L3]4+, has been prepared and characterised. X-ray crystal structures show all five Fe(ii) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D3-MeCN solution, regardless of HS fraction: γ HS(298 K) = 0-0.81. Variable temperature 1H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe2L3]4+ tunes: (a) solution SCO T 1/2 from 247 to 471 K, and (b) reversible redox potential, E m(FeII/III), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, E pa = 0.78 V, vs. 0.01 M AgNO3/Ag. For the four reversible redox systems, a strong correlation (R 2 = 0.99) is observed between T 1/2 and E pa. Finally, the analogous Ni(ii) helicates have been prepared to obtain Δ o, establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm-1), (b) that Δ o ([NiII 2L3]4+) strongly correlates (R 2 = 0.87) with T 1/2 ([FeII 2L3]4+), and (c) interestingly that Δ o strongly correlates (R 2 = 0.98) with E pa for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(ii) to Fe(iii).

Recent advances in heteroleptic multiple-stranded metallosupramolecules

Well-ordered combination of defined coordination spheres and multiple types of ligands (heteroleptic) in a given structure can expand the structural complexity and functional diversity of the resulting metallosupramolecules. Such heteroleptic metallosupramolecular architectures are expected to afford advanced utility in a variety of applications. In this concise review article, recent advances in the development of multi-nuclear-cluster-based heteroleptic multiple-stranded (HLMS) metallosupramolecules are summarized and demonstrated. To construct HLMS metallosupramolecules, one type of multitopic ligands can be employed for building up multiple strands, while another type of ligands can be utilized to construct multi-nuclear clusters. Most HLMS metallosupramolecules adopt helical geometries and have high molecular symmetry, which can be key factors for the structural completion. HLMS metallosupramolecules can be used as basic building blocks for the fabrication of higher-order polymeric or discrete assembly architectures with well-defined geometries.

Probing the generality of spin crossover complex T½vs. ligand 15N NMR chemical shift correlations: towards predictable tuning

A study of 6 families (42 members) demonstrates that within a family the easily calculated 15N-NMR values of ligands enable predictable tuning of T1/2 in the corresponding complexes, except for 2 families with weakly influencing meta-substituents.

Extension of Azine-Triazole Synthesis to Azole-Triazoles Reduces Ligand Field, Leading to Spin Crossover in Tris-L Fe(II)

The first examples of azole-triazole Rat ligands, bidentate L^4NMeIm(3-(1-methyl-1H-imidazol-4-yl)-5-phenyl-4-(p-tolyl)-4H-1,2,4-triazole) and L^4SIm (4-(5-phenyl-4-(p-tolyl)-4H-1,2,4-triazol-3-yl)thiazole), have been prepared, by extension of the general synthesis used to access many examples of azine-triazoles. The tris-L Fe^II complexes of the azine-triazoles are consistently low spin (LS). As intended, these new azole-triazole ligands provide lower field strengths, resulting in high-spin (HS) [Fe^II(L^4NMeIm)₃](BF₄)₂ (1·4H₂O) and spin crossover (SCO) active [Fe^II(L^4SIm)₃](BF₄)₂ (2·0.5H₂O). Single-crystal structure determinations revealed that at 100 K 1·solvents is HS whereas 2·solvents is LS. Solid-state variable temperature magnetic studies of air-dried crystals showed that the methylimidazole-triazole complex 1·4H₂O remains HS while the thiazole-triazole complex 2·0.5H₂O undergoes a two-step gradual SCO (T_1/2 approximately 275 and 350 K). Variable-temperature Evans method NMR studies of 2, in five different solvents (CD₃NO₂, CD₃CN, CD₃COCD₃, CD₂Cl₂, and CDCl₃) gave T_1/2 values in a relatively narrow range, 214-259 K. These T_1/2 values did not correlate with the solvent polarity index P' (R² = 0.25) but did correlate with the solvent basicity parameter SB (R² = 0.90). Variable-temperature UV-vis studies on a golden yellow CH₃CN solution of 2, with monitoring of the d-d transition at 530 nm (ε = 39 L mol^-1 cm^-1 at 293 K) while the solution was heated from 253 to 303 K, showed that the high-spin fraction increased from 0.51 to 0.77. Cyclic voltammetry studies in CH₃CN revealed a Fe(III)/Fe(II) redox process that was reversible for 1 and irreversible for 2, with significant tuning of the E_pa value: the methylimidazole-triazole complex 1 is significantly easier to oxidize (0.46 V) than the thiazole-triazole complex 2 (0.68 V; both vs 0.01 M Ag/AgNO₃).