Benchmarking DFT methods in predicting ground state spin of 127 base metal complexes and T SCO of 32 Fe( ii ) complexes revealed their inherent limitations, which can be overcome using ligand field theory within a group of relatively similar complexes.

The graphic shows the low-spin structure, scan-rate-dependent abrupt hysteretic spin transition, two-step T(LIESST), highly cooperative two-step photoinduced relaxation, and the HS (yellow) to LS (dark red) color change from room to low temperature.

Spin-crossover molecular crystals exhibit a unique interplay between electronic and structural degrees of freedom, yet their mechanical response remains poorly understood. In this study, we employ variable-temperature nanoindentation to quantify elastic modulus and hardness of single crystals of the molecular complex [Fe0.89Zn0.11(bpp)(2)](BF4)(2) (bpp = 2,6-bis{pyrazol-1-yl}pyridine) across the thermally induced first-order spin transition. Our results reveal a sharp and reproducible elastic anomaly: within a narrow interval near the transition temperature (249 +/- 1 K), the reduced modulus drops to a pronounced minimum (similar to 2.0 GPa), contrasting sharply with the values observed in the high spin (4.5 +/- 0.3 GPa) and low spin (7.0 +/- 0.5 GPa) states. This elastic softening is mirrored in the hardness, which otherwise remains comparable in the two spin states. At the transition point, the load-displacement curves display large hysteresis loops, revealing strong time-dependent deformation and enhanced energy dissipation. Crucially, the indentation does not induce a persistent spin-state change, suggesting the observed anelastic softening arises from molecular-scale spin-state relaxation rather than nucleation and growth mechanisms. On the other hand, the marked difference in modulus between the two spin states is not a relaxation effect, it is attributed to the substantial variation in crystal density accompanying the transition. These findings establish nanoindentation as a powerful method for probing the mechanical properties of spin crossover materials, offering valuable insights for their applications in mechanical sensors, actuators, mechanocaloric and energy harvesting devices.

The pressure-dependent evolution of the spin crossover (SCO) transition has garnered significant interest due to its connection to the giant barocaloric effect (BCE) near room temperature. Pressure alters both the molecular and solid-state structures of SCO materials, affecting the relative stability of low- and high-spin states and, consequently, the transition temperature (T_1/2). Crucially, the shape of the T_1/2 vs. pressure curve dictates the magnitude of the BCE, making its accurate characterization essential for identifying high-performance materials. In this work, we investigate the nonlinear T_1/2 vs. pressure behavior of the prototypical SCO complex [FeL_2][BF_4]_2 [L = 2,6-di(pyrazol-1-yl)pyridine] using solid-state PBE+U computations. Our results unveil the mechanisms by which pressure influences its SCO transition, including the onset of a phase transition, as well as the key role of low-frequency phonons in the BCE. Furthermore, we establish a computational protocol for accurately modeling the BCE in SCO crystals, providing a powerful tool for the rapid and efficient discovery of new materials with enhanced barocaloric performance.

Treatment of 2-(pyrazol-1-yl)-6-fluoropyridine with one equiv. of the appropriate 4-substituted 1H-pyrazole in the presence of sodium hydride gives moderate yields of 2-(pyrazol-1-yl)-6-(4-methylpyrazol-1-yl)pyridine (LMe), 2-(pyrazol-1-yl)-6-(4-fluoropyrazol-1-yl)pyridine (LF), 2-(pyrazol-1-yl)-6-(4-chloropyrazol-1-yl)pyridine (LCl) and 2-(pyrazol-1-yl)-6-(4-bromopyrazol-1-yl)pyridine (LBr). Single crystals of [Fe(LR)2]Z2 (R = Me, F or Br; Z- = BF4 - or ClO4 -) are often well-formed, but are poor diffractors of X-rays. An analysis of [Fe(LMe)2][ClO4]2 showed non-statistical positional disorder of the methyl substituents, leading to whole molecule disorder in each residue of the asymmetric unit. Single crystals of [Fe(LBr)2][BF4]2 are isomorphous with the LMe complex, but show less substituent disorder. All the complex salts are isomorphous by powder diffraction, and show thermal spin-transitions whose cooperativity differs from gradual (R = Me) to abrupt and hysteretic (R = Br). Some of the cooperative transitions exhibit irregular, closely spaced discontinuities which are not caused by crystallographic phase changes, and may reflect local heterogeneities associated with the cation disorder. No aspect of their crystal packing appears to correlate with their spin-transition cooperativity. However, weaker cooperativity may correlate with increased cation disorder in this system, which merits further investigation.

We report a comprehensive investigation of thermal and photoinduced spin-crossover (SCO) in the Fe(II) complex [Fe(bppI)2](ClO4)2, combining single-crystal optical spectroscopy with variable-temperature single-crystal and powder X-ray diffraction. SCXRD at 100 (low-spin, LS) and 400 K (high-spin, HS) reveals a symmetry-retaining spin transition in the orthorhombic Pbcn space group, with no crystallographic phase change. Temperature-dependent absorption spectra exhibit a moderately sharp, scan-rate-independent spin transition (T 1/2 = 335 K), indicating modest cooperativity and no kinetic trapping. Metal-to-ligand charge transfer (MLCT) bands, particularly the LS-state shoulder, offer a sensitive optical probe of spin-state evolution, with no spectral baseline shifts observed during thermal cycling. Efficient LS -> HS photoexcitation (LIESST) at 632 nm, producing a long-lived HS state matching the thermal HS spectrum. Upon warming, a remarkable three-step T(LIESST) behavior emerges, marked by a plateau (incubation), abrupt domain-driven cooperative HS -> LS transition, and gradual LS recovery. Notably, baseline shifts and interference fringes-absent in thermal transitions-reveal enhanced light-induced cooperativity via domain formation, strong domain-domain communication as a result of photoinduced structural reorganization or phase transition. Reverse-LIESST at 830 nm is inefficient (similar to 10%), limited by spectral overlap. Relaxation kinetics (57-70 K) display strong sigmoidal profiles characteristic of cooperative nucleation-and-growth dynamics modulated by structural heterogeneity, which is successfully reproduced by a combination of theoretical modeling within the mean-field and mechanoelastic frameworks. The mean-field model effectively describes high-temperature behavior through averaged interactions but fails at low temperatures due to local structural heterogeneities. In contrast, the mechanoelastic model, incorporating molecular-level stress and pressure effects, accurately captures the full temperature-dependent relaxation behavior. Light-induced interference fringes and baseline shifts suggest microstructural reorganizations not observed during thermal cycling. These findings highlight unusually complex structure-function relationships in SCO materials and challenge classical energy-gap-based models.

Reaction of 2,4,6-trifluoropyridine with sodium 3,4-dimethoxybenzenethiolate and 2 equiv of sodium pyrazolate in tetrahydrofuran at room temperature affords 4-(3,4-dimethoxyphenylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L), in 30% yield. The iron(II) complexes [FeL2][BF4]2 (1a) and [FeL2][ClO4]2 (1b) are high-spin with a highly distorted six-coordinate geometry. This structural deviation from ideal D2d symmetry is common in high-spin [Fe(bpp)2]2+ (bpp = di{pyrazol-1-yl}pyridine) derivatives, which are important in spin-crossover materials research. The magnitude of the distortion in 1a and 1b is the largest yet discovered for a mononuclear complex. Gas-phase DFT calculations at the ω-B97X-D/6-311G** level of theory identified four minimum or local minimum structural pathways across the distortion landscape, all of which are observed experimentally in different complexes. Small distortions from D2d symmetry are energetically favorable in complexes with electron-donating ligand substituents, including sulfanyl groups, which also have smaller energy penalties associated with the lowest energy distortion pathway. Natural population analysis showed that these differences reflect greater changes to the Fe-N{pyridyl} σ-bonding as the distortion proceeds, in the presence of more electron-rich pyridyl donors. The results imply that [Fe(bpp)2]2+ derivatives with electron-donating pyridyl substituents are more likely to undergo cooperative spin transitions in the solid state. The high-spin salt [Fe(bpp)2][CF3SO3]2, which also has a strong angular distortion, is also briefly described and included in the analysis.

Variable temperature crystallographic characterization of [FezZn1-z(bpp)(2)][BF4](2) (bpp=2,6-bis{pyrazol-1-yl}pyridine; z=0.88, 0.72 and 0.27) and [FezNi1-z(bpp)(2)][BF4](2) (z=0.83, 0.72 and 0.32) is presented. Comparison with previously published data confirms the isothermal unit cell volume change during spin-crossover (Delta V-SCO) behaves differently in the Zn- and Ni-doped crystals. For the FeZn crystals, the relationship between Delta V-SCO and z is continuous for z >= 0.3 but is steeper than expected, so Delta V-SCO approximate to 0 for z=0.27. In contrast Delta V-SCO in the FeNi materials shows only a small variation between 0.83 >= z >= 0.46, before decreasing more strongly at higher dilution. Delta V-SCO in each FeZn crystal is smaller than for its FeNi analogue with a similar composition. As well as the dopant ion ionic radius, the smaller Delta V-SCO for the Zn-doped materials reflects their T-1/2 values, which are lower than for their FeNi counterparts. The contribution of T-1/2 to this behavior is especially evident at high metal dilution.

Guest editors Shinya Hayami, Birgit Weber and Malcolm Halcrow introduce the Dalton Transactions themed collection on spin transitions.

The influence of dopant molecules on the structure and functionality of spin-crossover (SCO) materials is surveyed. Two aspects of the topic are well established. Firstly, isomorphous inert metal ion dopants in SCO lattices are a useful probe of the energetics of SCO processes. Secondly, molecular alloys of iron(II)/triazole coordination polymers containing mixtures of ligands were used to tune their spin-transitions towards room temperature. More recent examples of these and related materials are discussed that reveal new insights into these questions. Complexes which are not isomorphous can also be co-crystallised, either as solid solutions of the precursor molecules or as a random distribution of homo- and hetero-leptic centres in a molecular alloy. This could be a powerful method to manipulate SCO functionality. Published molecular alloys show different SCO behaviours, which may or may not include allosteric switching of their chemically distinct metal sites.

[Fe(bpp)2][ClO4]2 (bpp = 2,6-bis{pyrazol-1-yl}pyridine; monoclinic, C2/c) is high-spin between 5-300 K, and crystallises with a highly distorted molecular geometry that lies along the octahedral-trigonal prismatic distortion pathway. In contrast, [Ni(bpp)2][ClO4]2 (monoclinic, P21) adopts a more regular, near-octahedral coordination geometry. Gas phase DFT minimisations (ω-B97X-D/6-311G**) of [M(bpp)2]2+ complexes show the energy penalty associated with that coordination geometry distortion runs as M2+ = Fe2+ (HS) ≈ Mn2+ (HS) < Zn2+ ≈ Co2+ (HS) ≲ Cu2+ ≪ Ni2+ ≪ Ru2+ (LS; HS = high-spin, LS = low-spin). Slowly crystallised solid solutions [FexNi1-x(bpp)2][ClO4]2 with x = 0.53 (1a) and 0.74 (2a) adopt the P21 lattice, while x = 0.87 (3a) and 0.94 (4a) are mixed-phase materials with the high-spin C2/c phase as the major component. These materials exhibit thermal spin-transitions at T½ = 250 ± 1 K which occurs gradually in 1a, and abruptly and with narrow thermal hysteresis in 2a-4a. The transition proceeds to 100% completeness in 1a and 2a; that is, the 26% Ni doping in 2a is enough to convert high-spin [Fe(bpp)2][ClO4]2 into a cooperative, fully SCO-active material. These results were confirmed crystallographically for 1a and 2a, which revealed similarities and differences between these materials and the previously published [FexNi1-x(bpp)2][BF4]2 series. Rapidly precipitated powders with the same compositions (1b-4b) mostly resemble 1a-4a, except that 2b is a mixed-phase material; 2b-4b also contain a fraction of amorphous solid in addition to the two crystal phases. The largest iron fraction that can be accommodated by the P21 phase in this system is 0.7 ± 0.1.

Polycrystalline [FeL2][BF4]2 (L=2,6-di(pyrazol-1-yl)isonicotinonitrile) exhibits an abrupt hysteretic spin transition near 240 K, with a shoulder on the warming branch whose appearance depends on the sample history. The freshly isolated material is a ca 60 : 40 mixture of triclinic (HS1) and tetragonal (HS2) high-spin polymorphs, which are structurally closely related. Both HS1 and HS2 undergo a high→low-spin transition on cooling at 230±10 K. HS1 transforms to a new triclinic low-spin phase with a doubled unit cell volume (LS3), while HS2 forms a monoclinic low-spin phase (LS4) with similar unit cell dimensions to HS2. Single crystals of LS3 and LS4 both convert to HS1 on rewarming. The low→high-spin transition for LS4 is ca 10 K higher in temperature than for LS3, explaining the asymmetric thermal hysteresis. Powder diffraction, calorimetry and magnetic data show that multiple cycling about the spin-transition leads to slow enrichment of the HS1 and LS3 phases at the expense of HS2 and LS4. That is consistent with the HS2/LS4 fraction of the polycrystalline sample undergoing rare, bifurcated HS2→(LS3+LS4) and LS4→(HS1+HS2) phase transitions. The rate of enrichment of HS1/LS3 differed between these experiments, implying it is sample and/or measurement-dependent. Three other salts of this iron(II) complex and the coordination polymer [Ag(μ-L)]BF4 are also briefly described.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Following a published synthesis of 2,6-bis(imidazo[1,2-a]pyridin-2-yl)pyridine (L1), treatment of α,α'-dibromo-2,6-diacetylpyridine with 2 equiv. 2-aminopyrimidine or 2-aminoquinoline in refluxing acetonitrile respectively gives 2,6-bis(imidazo[1,2-a]pyrimidin-2-yl)pyridine (L2) and 2,6-bis(imidazo[1,2-a]quinolin-2-yl)pyridine (L3). Solvated crystals of [Fe(L1)2][BF4]2 (1[BF4]2) and [Fe(L2)2][BF4]2 (2[BF4]2) are mostly high-spin, although one solvate of 1[BF4]2 undergoes thermal spin-crossover on cooling. The iron coordination geometry is consistently distorted in crystals of 2[BF4]2 which may reflect the influence of intramolecular, inter-ligand N⋯π interactions on the molecular conformation. Only 1 : 1 Fe : L3 complexes were observed in solution, or isolated in the solid state; a crystal structure of [FeBr(py)2L3]Br·0.5H2O (py = pyridine) is presented. A solvate crystal structure of high-spin [Fe(L4)2][BF4]2 (L4 = 2,6-di{quinolin-2-yl}pyridine; 4[BF4]2) is also described, which exhibits a highly distorted six-coordinate geometry with a helical ligand conformation. The iron(II) complexes are high-spin in solution at room temperature, but 1[BF4]2 and 2[BF4]2 undergo thermal spin-crossover equilibria on cooling. All the compounds exhibit a ligand-based emission in solution at room temperature. Gas phase DFT calculations mostly reproduce the spin state properties of the complexes, but show small anomalies attributed to intramolecular, inter-ligand dispersion interactions in the sterically crowded molecules.

Solid "[AuL]" (HL = 3-[pyrid-2-yl]-5-tertbutyl-1H-pyrazole) can be crystallized as cyclic [Au3(μ-L)3] and [Au4(μ-L)4] clusters from different solvents. The crystalline tetramer contains a square Au4 core with an HT:TH:TH:HT arrangement of ligand substituents, which preorganizes the cluster to chelate to additional metal ions via its pendant pyridyl groups. The addition of 0.5 equiv of AgBF4 to [AuL] yields [Ag2Au4(μ3-L)4][BF4]2, where two edges of the Au4 square are spanned by Ag+ ions via metallophilic Ag···Au contacts. Treatment of [AuL] with [Cu(NCMe)4]PF6 affords the metalloligand helicate [Cu2Au2(μ-L)4][PF6]2, via oxidation of the copper and partial fragmentation of the cluster.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

ADVERTISEMENT RETURN TO ISSUEEditorialNEXTMolecular Magnets and Switchable Magnetic MaterialsMalcolm HalcrowMalcolm HalcrowMore by Malcolm Halcrowhttps://orcid.org/0000-0001-7491-9034, Birgit WeberBirgit WeberMore by Birgit Weberhttps://orcid.org/0000-0002-9861-9447, Masahiro YamashitaMasahiro YamashitaMore by Masahiro Yamashita, and Sanjit Konar*Sanjit Konar*Email: [email protected]More by Sanjit Konarhttps://orcid.org/0000-0002-1584-6258Cite this: Cryst. Growth Des. 2023, 23, 9, 6219–6220Publication Date (Web):August 7, 2023Publication History Received24 March 2023Published online7 August 2023Published inissue 6 September 2023https://pubs.acs.org/doi/10.1021/acs.cgd.3c00359https://doi.org/10.1021/acs.cgd.3c00359editorialACS PublicationsCopyright © Published 2023 by American Chemical Society. This publication is available under these Terms of Use. Request reuse permissions This publication is free to access through this site. Learn MoreArticle Views1404Altmetric-Citations-LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail PDF (974 KB) Get e-AlertscloseSUBJECTS:Magnetic materials,Magnetic properties,Molecular interactions,Molecules,Quantum mechanics Get e-Alerts