Bi-stable spin-crossover in charge-neutral [Fe(R-ptp)2] (ptp = 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine) complexes

Two-Dimensional Spin-Crossover Molecular Solid Solutions with Tunable Transition Temperatures across 90 K

Spin-crossover (SCO) materials exhibit remarkable potential as bistable switches in molecular devices. However, the spin transition temperatures (Tc) of known compounds are unable to cover the entire ambient temperature spectrum, largely limiting their practical utility. This study reports an exemplary two-dimensional SCO solid solution system, [FeIII(H0.5LCl)2-2x(H0.5LF)2x]·H2O (H0.5LX = 5-X-2-hydroxybenzylidene-hydrazinecarbothioamide, X = F or Cl, x = 0 to 1), in which the adjacent layers are adhered via hydrogen bonding. Notably, the Tc of this system can be fine-tuned across 90 K (227-316 K) in a linear manner by modulating the fraction x of the LF ligand. Elevating x results in strengthened hydrogen bonding between adjacent layers, which leads to enhanced intermolecular interactions between adjacent SCO molecules. Single-crystal diffraction analysis and periodic density functional theory calculations revealed that such a special kind of alteration in interlayer interactions strengthens the FeIIIN2O2S2 ligand field and corresponding SCO energy barrier, consequently resulting in increased Tc. This work provides a new pathway for tuning the Tc of SCO materials through delicate manipulation of molecular interactions, which could expand the application of bistable molecular solids to a much wider temperature regime.

Trapping an unprecedented octacoordinated iron(II) complex with neutral bis-tetrazolylpyridyl ligands and solvent molecules

Iron(II) can show a very rich coordination chemistry with concomitant modulation of its properties as promising functional materials. Metalation of the neutral tridentate nitrogen-donor mer-coordinating ligand 2,6-bis(2-(methyl)-2H-tetrazol-5-yl)pyridine (Me2btp) with Fe(ClO4)2·6H2O through accurate solvent polarity control enables the selective crystallization of [FeHS/LS(Me2btp)2](ClO4)2·MeCN·2.75H2O (2HS/LS·MeCN·2.75H2O) as red rods, where half of the iron(II) centres resides in the low spin (LS, S = 0) state and the other half is in the high spin (HS, S = 2) state. The red rods spontaneously convert into yellow crystals once removed from the mother liquor and exposed to air due to solvent rearrangement within the crystal packing; these new crystals can be assigned to [FeHS(Me2btp)2](ClO4)2·solvent (2HS·solvent) where all the iron(II) centres are now blocked in the HS state, as confirmed by magnetic measurements. The polarity of the crystallization solvent, together with the maintenance of the crystals within the mother liquor, are pivotal for the reactivity and interconversion of different species. Indeed, upon long standing in solution, 2HS/LS·MeCN·2.75H2O converts to another form of red crystals belonging to [FeLS(Me2btp)2][FeHS(Me2btp)(MeCN)2(H2O)](ClO4)4·MeCN (2LS·3HS·MeCN), as confirmed by single crystal X-ray diffraction data. In this co-crystal, the iron(II) in 2 resides in the LS state at all temperatures while the iron(II) in 3 is blocked in the HS state. Well-formed yellow crystals could be also isolated among the red crystals of 2HS/LS·MeCN·2.75H2O, and they could be identified as the unprecedented octacoordinated species [Fe(Me2btp)2(MeCN)(H2O)](ClO4)2·H2O (1·H2O) by single-crystal X-ray diffraction. These yellow crystals are stable in the air, but slowly convert into 2LS·3HS·MeCN if kept in the mother liquor for about one week. 1·H2O can be considered the trapped intermediate in the solid state during the conversion of [FeHS(Me2btp)2]2+ into [FeHS(Me2btp)(MeCN)2(H2O)]2+ in solution, where the two tridentate ligands in the starting species can unfold to accommodate coordinated MeCN and H2O molecules, as confirmed by theoretical calculations, and eventually one of the two Me2btp is completely replaced by the solvent.

105 K Wide Room Temperature Spin Transition Memory Due to a Supramolecular Latch Mechanism

Little is known about the mechanisms behind the bistability (memory) of molecular spin transition compounds over broad temperature ranges (>100 K). To address this point, we report on a new discrete Fe^II neutral complex [Fe^IIL₂]⁰ (1) based on a novel asymmetric tridentate ligand 2-(5-(3-methoxy-4H-1,2,4-triazol-3-yl)-6-(1H-pyrazol-1-yl))pyridine (L). Due to the asymmetric cone-shaped form, in the lattice, the formed complex molecules stack into a one-dimensional (1D) supramolecular chain. In the case of the rectangular supramolecular arrangement of chains in methanolates 1-A and 1-B (both orthorhombic, Pbcn) differing, respectively, by bent and extended spatial conformations of the 3-methoxy groups (3MeO), a moderate cooperativity is observed. In contrast, the hexagonal-like arrangement of supramolecular chains in polymorph 1-C (monoclinic, P2₁/c) results in steric coupling of the transforming complex species with the peripheral flipping 3MeO group. The group acts as a supramolecular latch, locking the huge geometric distortion of complex 1 and in turn the trigonal distortion of the central Fe^II ion in the high-spin state, thereby keeping it from the transition to the low-spin state over a large thermal range. Analysis of the crystal packing of 1-C reveals significantly changing patterns of close intermolecular interactions on going between the phases substantiated by the energy framework analysis. The detected supramolecular mechanism leads to a record-setting robust 105 K wide hysteresis spanning the room temperature region and an atypically large T_LIESST relaxation value of 104 K of the photoexcited high-spin state. This work highlights a viable pathway toward a new generation of cleverly designed molecular memory materials.

Spin-States of Diastereomeric Iron(II) Complexes of 2,6-Bis(thiazolin-2-yl)pyridine (ThioPyBox) Ligands and a Comparison with the Corresponding PyBox Derivatives

This report investigates homoleptic iron(II) complexes of thiazolinyl analogues of chiral PyBox tridentate ligands: 2,6-bis(4-phenyl-4,5-dihydrothiazol-2-yl)pyridine (L¹Ph), 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine (L¹iPr), and 2,6-bis(4-tert-butyl-4,5-dihydrothiazol-2-yl)pyridine (L¹t-Bu). Crystallographic data imply the larger and more flexible thiazolinyl rings reduce steric clashes between the R substituents in homochiral [Fe((R)-L¹R)₂]²⁺ or [Fe((S)-L¹R)₂]²⁺ (R = Ph, iPr, or t-Bu), compared to their PyBox (L²R) analogues. Conversely, the larger heterocyclic S atoms are in close contact with the R substituents in heterochiral [Fe((R)-L¹Ph)((S)-L¹Ph)]²⁺, giving it a more sterically hindered ligand environment than that in [Fe((R)-L²Ph)((S)-L²Ph)]²⁺ (L²Ph = 2,6-bis(4-phenyl-4,5-dihydrooxazol-2-yl)pyridine). Preformed [Fe((R)-L¹Ph)((S)-L¹Ph)]²⁺ and [Fe((R)-L¹iPr)((S)-L¹iPr)]²⁺ do not racemize by ligand redistribution in CD₃CN solution, but homochiral [Fe(L¹iPr)₂]²⁺ and [Fe(L¹t-Bu)₂]²⁺ both undergo partial ligand displacement in that solvent. Homochiral [Fe(L¹Ph)₂]²⁺ and [Fe(L¹iPr)₂]²⁺ exhibit spin-crossover equilibria in CD₃CN, centered at 344 ± 6 K and 277 ± 1 K respectively, while their heterochiral congeners are essentially low-spin within the liquid range of the solvent. These data imply that the diastereomers of [Fe(L¹Ph)₂]²⁺ and [Fe(L¹iPr)₂]²⁺ show a greater difference in their spin-state behaviors than was previous found for [Fe(L²Ph)₂]²⁺. Gas-phase DFT calculations (B86PW91/def2-SVP) of the [Fe(L¹R)₂]²⁺ and [Fe(L²R)₂]²⁺ complexes reproduce most of the observed trends, but they overstabilize the high-spin state of SCO-active [Fe(L¹iPr)₂]²⁺ by ca. 1.5 kcal mol^-1. This might reflect the influence of intramolecular dispersion interactions on the spin states of these compounds. Attempts to model this with the dispersion-corrected functionals B97-D2 or PBE-D3 were less successful than our original protocol, confirming that the spin states of sterically hindered molecules are a challenging computational problem.

Influence of ligand substituent conformation on the spin state of an iron(II)/di(pyrazol-1-yl)pyridine complex

The temperature of the solution-phase spin-crossover equilibrium in iron(ii) complexes of 4-alkylsulfanyl-2,6-di{pyrazol-1-yl}pyridine (bppSR) complexes depends strongly on the alkylsulfanyl substituent. DFT calculations imply this reflects the conformation of the alkylsulfanyl groups, which lie perpendicular to the heterocyclic ligand donors in [Fe(bppStBu)2]2+ but are oriented co-planar with the ligand core for smaller SR substituents.

Theoretical modeling of the singlet-triplet spin transition in different Ni(II)-diketo-pyrphyrin-based metal-ligand octahedral complexes

The structural stability, charge transfer effects and strength of the spin-orbit couplings in different Ni(ii)-ligand complexes have been studied at the DFT (B3LYP and CAM-B3LYP) and coupled cluster (DLPNO-CCSD(T)) levels of theory. Accordingly, two different, porphyrin- and diketo-pyrphyrin-based four-coordination macrocycles as planar ligands as well as pyridine (or pyrrole) and mesylate anion molecular groups as vertical ligands were considered in order to build metal-organic complexes with octahedral coordination configurations. For each molecular system, the identification of equilibrium geometries and the intersystem crossing (the minimum energy crossing) points between the potential energy surfaces of the singlet and triplet spin states is followed by computing the spin-orbit couplings between the two spin states. Structures, based on the diketo-pyrphyrin macrocycle as the planar ligand, show stronger six-coordination metal-organic complexes due to the extra electrostatic interaction between the positively charged central metal cation and the negatively charged vertical ligands. The results also show that the magnitude of the spin-orbit coupling is influenced by the atomic positions of deprotonations of the ligands, and implicitly the direction of the charge transfer between the ligand and the central metal ion.

2,6-Dicyano-4-pyrone as a Novel and Multifarious Building Block for the Synthesis of 2,6-Bis(hetaryl)-4-pyrones and 2,6-Bis(hetaryl)-4-pyridinols

In this work, a three-stage and easily scalable synthesis of 2,6-dicyano-4-pyrone (overall yield of 45%) as a new convenient building block has been developed from diethyl acetonedioxalate. It was shown that the transformation with hydroxylamine and [3 + 2]-cycloaddition, in contrast to the reactions with hydrazines, selectively proceed through the attack at the cyano groups without the pyrone ring-opening to give symmetrical and unsymmetrical pyrone-bearing heterocyclic triads containing 1,2,4- and 1,3,4-oxadiazoles as well as tetrazole moieties. The reaction of 2,6-bis(hetaryl)-4-pyrones with ammonia afforded 2,6-bis(hetaryl)pyridines in 63-87% yields. The 4-pyridone/4-pyridinol tautomerism of 2,6-bis(hetaryl)pyridinols and the influence of the nature of adjacent azolyl moieties on this equilibrium have been discussed.

Bistable spin-crossover in a new series of [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine; R = CN) complexes

Spin-crossover (SCO) active transition metal complexes are a class of switchable molecular materials. Such complexes undergo hysteretic high-spin (HS) to low-spin (LS) transition, and vice versa, rendering them suitable for the development of molecule-based switching and memory elements. Therefore, the search for SCO complexes undergoing abrupt and hysteretic SCO, that is, bistable SCO, is actively carried out by the molecular magnetism community. In this study, we report the bistable SCO characteristics associated with a new series of iron(ii) complexes-[Fe(BPP-CN)2](X)2, X = BF4 (1a-d) or ClO4 (2)-belonging to the [Fe(BPP-R)2]2+ (BPP = 2,6-bis(pyrazol-1-yl)pyridine) family of complexes. Among the complexes, the lattice solvent-free complex 2 showed a stable and complete SCO (T1/2 = 241 K) with a thermal hysteresis width (ΔT) of 28 K-the widest ΔT reported so far for a [Fe(BPP-R)2](X)2 family of complexes, showing abrupt SCO. The reproducible and bistable SCO shown by the relatively simple [Fe(BPP-CN)2](X)2 series of molecular complexes is encouraging to pursue [Fe(BPP-R)2]2+ systems for the realization of technologically relevant SCO complexes.

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

The relationship between ligand design and spin state in base metal compounds is surveyed. Implications and applications of these principles for light-harvesting dyes, catalysis and materials chemistry are summarised.