Conversion of Flexible Spin Crossover Metal-Organic Frameworks Macrocrystals to Nanocrystals Using Ultrasound Energy: A Study on Structural Integrity by MicroED and Charge-Transport Properties

Metal-Organic Frameworks (MOFs) attract attention for their intrinsic porosity, large surface area, and functional versatility. To fully utilize their potential in applications requiring precise control at smaller scales, it is essential to overcome challenges associated with their bulk form. This is particularly difficult for 3D MOFs with spin crossover (SCO) behavior, which undergo a reversible transition between high-spin and low-spin states in response to external stimuli. Maintaining their structural integrity and SCO properties at the nanoscale remains a significant challenge, yet these properties make them ideal candidates for sensors, data storage, and molecular switch applications. This study reports the synthesis of nanocrystals of the well-known SCO MOF [Fe2(H0.67bdt)3]·xH2O (1, x = 0-10, bdt2- = 1,4-benzeneditetrazolate), which exhibits both magnetic and charge transport properties. The nanocrystals are obtained through sonication of macrocrystals, and the preservation of their crystalline structure at the nanoscale is explored using Microcrystal Electron Diffraction (MicroED). A comparison between macro- and nanocrystals highlights the structural integrity and the preservation of charge-transport properties, underlining the potential for further miniaturization of MOFs for advanced technological applications.

Synthesis and Characterization of [Fe(Htrz)2(trz)](BF4)] Nanocubes

Compounds that exhibit spin-crossover (SCO) type behavior have been extensively investigated due to their ability to act as molecular switches. Depending on the coordinating ligand, in this case 1H-1,2,4-triazole, and the crystallite size of the SCO compound produced, the energy requirement for the spin state transition can vary. Here, SCO [Fe(Htrz)2(trz)](BF4)] nanoparticles were synthesized using modified reverse micelle methods. Reaction conditions and reagent ratios are strictly controlled to produce nanocubes of 40-50 nm in size. Decreases in energy requirements are seen in both thermal and magnetic transitions for the smaller sized crystallites, where, compared to bulk materials, a decrease of as much as 20 °C can be seen in low to high spin state transitions.

Exceptionally high temperature spin crossover in amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complex revealed by variable temperature Raman spectroscopy and single crystal X-ray diffraction

The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH₂) is described. The complex salts [Fe(bppCONH₂)₂](BF₄)₂ and [Fe(bppCONH₂)₂](ClO₄)₂ were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH₂)₂](BF₄)₂ indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ at 445 K with a 14 K hysteresis. For the BF₄^- salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF₄ salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences in the inter-molecular hydrogen bonding in the low spin and high spin states.

The effect of tether groups on the spin states of iron(II)/bis[2,6-di(pyrazol-1-yl)pyridine] complexes

The synthesis of six 2,6-di(pyrazol-1-yl)pyridine derivatives bearing dithiolane or carboxylic acid tether groups is described: [2,6-di(pyrazol-1-yl)pyrid-4-yl]methyl (R)-lipoate (L1), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]ethyl (R)-lipoate (L2), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxy]ethyl (R)-lipoate (L3), N-([2,6-di(pyrazol-1-yl)pyrid-4-ylsulfanyl]-2-aminoethyl (R)-lipoamide (L4), 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]acetic acid (L5) and 2-[(2,6-di(pyrazol-1-yl)pyridine)-4-carboxamido]propionic acid (L6). The iron(ii) perchlorate complexes of all the new ligands exhibit gradual thermal spin-crossover (SCO) in the solid state above room temperature, except L4 whose complex remains predominantly high-spin. Crystalline [Fe(L6)2][ClO4]2·2MeCN contains three unique cation sites which alternate within hydrogen-bonded chains, and undergo gradual SCO at different temperatures upon warming. The SCO midpoint temperature (T1/2) of the complexes in CD3CN solution ranges between 208-274 K, depending on the functional group linking the tether groups to the pyridyl ring. This could be useful for predicting how these complexes might behave when deposited on gold or silica surfaces.

Structural Transformations and Spin-Crossover in [FeL2 ]2+ Salts (L=4-{tert-Butylsulfanyl}-2,6-di{pyrazol-1-yl}pyridine): The Influence of Bulky Ligand Substituents

4-(tert-Butylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L) was obtained in low yield from a one-pot reaction of 2,4,6-trifluoropyridine with 2-methylpropane-2-thiolate and sodium pyrazolate in a 1:1:2 ratio. The materials [FeL₂ ][BF₄ ]₂ ⋅solv (1[BF₄ ]₂ ⋅solv) and [FeL₂ ][ClO₄ ]₂ ⋅solv (1[ClO₄ ]₂ ⋅solv; solv=MeNO₂ , MeCN or Me₂ CO) exhibit a variety of structures and spin-state behaviors including thermal spin-crossover (SCO). Solvent loss on heating 1[BF₄ ]₂ ⋅x MeNO₂ (x≈2.3) occurs in two steps. The intermediate phase exhibits hysteretic SCO around 250 K, involving a "reverse-SCO" step in its warming cycle at a scan rate of 5 K min^-1 . The reverse-SCO is not observed in a slower 1 K min^-1 measurement, however, confirming its kinetic nature. The final product [FeL₂ ][BF₄ ]₂ ⋅0.75 MeNO₂ was crystallographically characterized, and shows abrupt but incomplete SCO at 172 K which correlates with disorder of an L ligand. The asymmetric unit of 1[BF₄ ]₂ ⋅y Me₂ CO (y≈1.6) contains five unique complex molecules, four of which undergo gradual SCO in at least two discrete steps. Low-spin 1[ClO₄ ]₂ ⋅0.5 Me₂ CO is not isostructural with its BF₄ ^- congener, and undergoes single-crystal-to-single-crystal solvent loss with a tripling of the crystallographic unit cell volume, while retaining the P 1 ‾ space group. Three other solvate salts undergo gradual thermal SCO. Two of these are isomorphous at room temperature, but transform to different low-temperature phases when the materials are fully low-spin.

Iron(II) Complexes of 2,4-Dipyrazolyl-1,3,5-triazine Derivatives-The Influence of Ligand Geometry on Metal Ion Spin State

Seven [FeL₂][BF₄]₂ complex salts were prepared, where L is a 6-substituted 2,4-di(pyrazol-1-yl)-1,3,5-triazine (bpt) derivative. The complexes are all crystallographically high-spin, and exhibit significant distortions from an ideal D_2d-symmetric coordination geometry. In one case, an unusual type of metal ion disorder was observed among a cubic array of ligands in the crystal lattice. The complexes are also high-spin between 3 and 300 K in the solid state and, where measured, between 239 and 333 K in CD₃CN solution. This result is unexpected, since homoleptic iron(II) complexes of related 2,6-di(pyrazol-1-yl)pyridine, 2,6-di(pyrazol-1-yl)pyrazine, and 2,6-di(pyrazol-1-yl)pyrimidine derivatives often exhibit thermal spin-crossover behavior. Gas-phase density functional theory calculations confirm the high-spin form of [Fe(bpt)₂]²⁺ and its derivatives is stabilized relative to iron(II) complexes of the other ligand types. This reflects a weaker Fe/pyrazolyl σ-bonding interaction, which we attribute to a small narrowing of the chelate ligand bite angle associated with the geometry of the 1,3,5-triazinyl ring. Hence, the high-spin state of [Fe(bpt)₂]²⁺ centers does not reflect the electronic properties of its heterocyclic ligand donors but is imposed by the bpt ligand conformation. A high-spin homoleptic iron(III) complex of one of the bpt derivatives was also synthesized.

A luminescent Pt2Fe spin crossover complex

A heterotrinuclear luminescent [Pt₂Fe] spin crossover (SCO) complex was developed, synthesized, and investigated.

Fabrication of high-resolution 4,8(2) -type archimedean nanolattices composed of solution processable spin cross-over Fe(II) metallosupramolecular polymers

This paper presents the synthesis of two highly soluble Fe(II) metallosupramolecular polymers with two counter anions from a novel back-to-back coupled hybrid ligand. The spin cross-over (SCO) temperature of polymers with BF4 and ClO4 counter anions is T1/2 = 313 K and T1/2 = 326 K, respectively. By following the top-down approach, one of the polymers (with ClO4 counter anion) is successfully solution processed using a lithographically controlled wetting technique to create laser readable high-resolution Archimedean (4,8(2) ) nanolattices (consist of diamagnetic octagons and SCO squares). The thickness and top area of each SCO square are ≈75 nm and ≈2 × 2 μm(2) , respectively.

Iron(ii) complexes of 4-sulfanyl-, 4-sulfinyl- and 4-sulfonyl-2,6-dipyrazolylpyridine ligands. A subtle interplay between spin-crossover and crystallographic phase changes

The compound shown exhibits an abrupt thermal spin-transition, with an accompanying crystallographic phase change at slightly lower temperature which is visible as a shoulder on the susceptibility curve.

An iron(ii) spin-crossover metallacycle from a back-to-back bis-[dipyrazolylpyridine]

Iron(ii) and zinc(ii) complexes of bis[2,6-di(pyrazol-1-yl)pyrid-4-yl]disulfide assemble into tetranuclear metallacycles according to ES-MS, DOSY NMR and conductivity data. The iron complex exhibits a thermal spin-state equilibrium in solution, which the iron centres appear to undergo independently of each other.