Chemical/Physical Pressure Tunable Spin-Transition Temperature and Hysteresis in a Two-Step Spin Crossover Porous Coordination Framework

Exploring spin states and ligand field splitting in metal complexes: a theoretical analysis of spin-orbital interactions and magnetic properties

Metal complexes are pivotal in diverse fields due to their wide array of functionalities, including magnetism, conductivity, and photoresponsiveness. These functionalities are intricately linked to the phenomenon of ligand field splitting, yet controlling the magnitude of this splitting within metal complexes presents a significant challenge. This study aims to address this challenge by developing a novel 2D spectrochemical series, integrating two critical parameters: metal ions and ligands. Employing the DV-Xα molecular orbital method, we directly calculated ligand field splitting width, enabling a detailed assessment of energy splitting trends. Our results reveal that the magnitude of ligand field splitting, encompassing 17 metal types and 29 ligand types, can be precisely controlled. This represents a significant advancement over traditional spectrochemical series, such as those proposed by R. Tsuchida, which predominantly focus on either ligands or metals in isolation. Additionally, our study extends to the calculation of spin states in these metal complexes, contributing valuable insights for the development of magnetic materials. We demonstrate that the relative ligand field splitting and spin polarization can be used to predict spin states, offering a new perspective in material design and functionality. These findings not only enhance our understanding of ligand field splitting in metal complexes but also provide a comprehensive framework for predicting their electronic and magnetic properties, paving the way for innovative applications in material science and coordination chemistry.

Hofmann-Type Cyanide Bridged Coordination Polymers for Advanced Functional Nanomaterials

Since the discovery of Hofmann clathrates of inorganic cyanide bridged coordination polymers (Hofmann-type CN-CPs), extensive research is done to understand their behavior during spin transitions caused by guest molecules or external stimuli. Lately, research on their nanoscale architectures for sensors and switching devices is of interest. Their potential is reported for producing advanced functional inorganic materials in two-dimensional (2D) morphology using a scalable solid-state thermal treatment method. For instance, but not restricted to, alloys, carbides, chalcogenides, oxides, etc. Simultaneously, their in situ crystallization at graphene oxide (GO) nanosheet surfaces, followed by a subsequent self-assembly to build layered lamellar structures, is reported providing hybrid materials with a variety of uses. Hence, an overview of the most recent developments is presented here in the synthesis of nanoscale structures, including thin films and powders, using Hofmann-type CN-CPs. Also thoroughly demonstrated are the most recent synthetic ideas with the modest control over the size and shape of nanoscale particles. Additionally, in order to create new functional hybrid materials for electrical and energy applications, their thermal decomposition in various environments and hybridization with GO and other guest molecules is examined. This review article also conveyed their spin transition, astounding innovative versatile adhesives, and structure features.

Temperature-driven n-p conduction type switching without structural transition in a Cu-rich chalcogenide, NaCu5S3

We report for the first time the discovery of reversible n-p conduction type switching in a chalcogenide, NaCu5S3, without structural transition. AC impedance and first-principles simulations of the ionic migration confirmed the local melting trends of the hexagonal copper lattice at high temperatures, which could result in superionic conductivity within NaCu5S3.

Structure–property relationships and the mechanisms of multistep transitions in spin crossover materials and frameworks

Structure–property relationships are identified and applied to explain multi-step transitions and the different antiferroelastic patterns found in spin crossover frameworks.

Effects of two different solvents on the syntheses, structural diversity, and magnetic properties of six Mn(ii) complexes derived from 3,3′-((5-carboxy-1,3-phenylene)bis(oxy))dibenzoate and variable N-donor ligands

Six Mn(ii) complexes of the 3,3′-((5-carboxy-1,3-phenylene)bis(oxy))dibenzoate ligand in two different solvent systems with variable N-donor ligands were obtained and showed different structures and magnetic properties.

Homochiral versus racemic polymorphs of spin-crossover iron(ii) complexes with reversible LIESST effect

Homochiral and racemic polymorphs show different spin-crossover behaviours due to different intermolecular interactions, and reversible LIESST effects can be realized on homochiral complexes.

Spin Interconversion of Heme-Peroxo-Copper Complexes Facilitated by Intramolecular Hydrogen-Bonding Interactions

Synthetic peroxo-bridged high-spin (HS) heme-(μ-η2:η1-O22-)-Cu(L) complexes incorporating (as part of the copper ligand) intramolecular hydrogen-bond (H-bond) capabilities and/or steric effects are herein demonstrated to affect the complex's electronic and geometric structure, notably impacting the spin state. An H-bonding interaction with the peroxo core favors a low-spin (LS) heme-(μ-η1:η1-O22-)-Cu(L) structure, resulting in a reversible temperature-dependent interconversion of spin state (5 coordinate HS to 6 coordinate LS). The LS state dominates at low temperatures, even in the absence of a strong trans-axial heme ligand. Lewis base addition inhibits the H-bond facilitated spin interconversion by competition for the H-bond donor, illustrating the precise H-bonding interaction required to induce spin-crossover (SCO). Resonance Raman spectroscopy (rR) shows that the H-bonding pendant interacts with the bridging peroxide ligand to stabilize the LS but not the HS state. The H-bond (to the Cu-bound O atom) acts to weaken the O-O bond and strengthen the Fe-O bond, exhibiting ν(M-O) and ν(O-O) values comparable to analogous known LS complexes with a strong donating trans-axial ligand, 1,5-dicyclohexylimidazole, (DCHIm)heme-(μ-η1:η1-O22-)-Cu(L). Variable-temperature (-90 to -130 °C) UV-vis and 2H NMR spectroscopies confirm the SCO process and implicate the involvement of solvent binding. Examining a case of solvent binding without SCO, thermodynamic parameters were obtained from a van't Hoff analysis, accounting for its contribution in SCO. Taken together, these data provide evidence for the H-bond group facilitating a core geometry change and allowing solvent to bind, stabilizing a LS state. The rR data, complemented by DFT analysis, reveal a stronger H-bonding interaction with the peroxo core in the LS compared to the HS complexes, which enthalpically favors the LS state. These insights enhance our fundamental understanding of secondary coordination sphere influences in metalloenzymes.

Syntheses, structural diversity and photocatalytic properties of three coordination polymers assembled by different N-heterocyclic ligands

The diversity of ligands and central metal ions plays an important role not only in coordination modes and final products but also in their photocatalytic activities.

Pressure Sensor with a Color Change at Room Temperature Based on Spin-Crossover Behavior

Two new iron(II) complexes with 1D chain and 2D network structures have been successfully synthesized and characterized. One of the complexes exhibits a pressure-induced spin-crossover property with a reversible color change from white to purple at room temperature. The special property makes it a suitable candidate as a pressure sensor.

Two cadmium coordination polymers based on tris(p-carboxyphenyl) phosphane oxide with highly selective sensing of nitrobenzene derivatives and Hg2+ cations

Based on the tris(p-carboxyphenyl)phosphane oxide ligand, two Cd(ii) CPs were obtained which could highly sense nitrobenzene derivatives and Hg²⁺ cations.

Exploiting Pressure To Induce a "Guest-Blocked" Spin Transition in a Framework Material

A new functionalized 1,2,4-triazole ligand, 4-[(E)-2-(5-methyl-2-thienyl)vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [Fe^IIPd(CN)₄(R-1,2,4-trz)₂]·nH₂O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [Fe^IIPd(CN)₄(thiome)₂]·2H₂O (A·2H₂O) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in A·2H₂O due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in A·2H₂O (0-0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in A·2H₂O under hydrostatic pressure compared to a one-step SCO in A at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities.

First Step Towards a Devil's Staircase in Spin-Crossover Materials

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)3 )2 ](4,4'-bipy)2 }n exhibits a thermal high-spin (HS)↔low-spin (LS) staircase-like conversion characterized by a multi-step dependence of the HS molar fraction γHS . Between the fully HS (γHS =1) and LS (γHS =0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the γHS ≈0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the γHS ≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS-LS sequence incommensurate with the molecular lattice. The formation of these different long-range spatially ordered structures of LS and HS states during the multi-step spin-crossover is discussed within the framework of "Devil's staircase"-type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.

Reversible quantitative guest sensing via spin crossover of an iron(ii) triazole

A new phenyl-triazole-pyrazine ligand, 4-p-tolyl-3-(phenyl)-5-(2-pyrazinyl)-1,2,4-triazole (tolpzph), was prepared in order to enforce pyrazine coordination of the iron(ii) centre in the resulting complex, [FeII(tolpzph)2(NCS)2]·THF (1·THF). Structure determinations carried out on this discrete mononuclear complex, 1·THF, at 273 K (mostly high spin) and 100 K (mostly low spin) demonstrate this was successful, and that spin crossover (SCO) occurred on cooling. Subsequent magnetic measurements on 1·THF revealed that it shows highly sensitive and reversible solvent-dependent SCO, with T1/2(1·THF) = 255 K vs. T1/2(1) = 212 K (with SCO of 1 more abrupt and occurring with a 4 K hysteresis loop), a drop of 43 K due to THF loss. This is reversible over at least 10 cycles of re-solvating with THF followed by re-drying, so 1 ↔ 1·THF can be considered an 'on-off' THF sensor, monitored by the T1/2 reversibly shifting (by 43 K). Furthermore, quantitative sensing of the fractional amount of THF present in 1·nTHF, 0 ≤ n ≤ 1, is demonstrated. Monitoring the T1/2 and using TGA to quantify n(THF) revealed a linear dependence (25 data points; Pearson r2 = 0.93): T1/2 = 41.1n(THF) + 219. Finally, 1 is also shown to take up CHCl3 [T1/2(1·CHCl3) = 248 K], with a logarithmic T1/2 dependence on the fractional amount of CHCl3 present (10 data points; Pearson r2 = 0.98): T1/2 = 27.0 log10[n(CHCl3)] + 243. This study is a proof of principle that a (multi-use) quantitative sensor material based on spin crossover is feasible.

Structural diversity, luminescence and photocatalytic properties of six coordination polymers based on designed bifunctional 2-(imidazol-1-yl)terephthalic acid

Six CPs, based on 2-(imidazol-1-yl)terephthalic acid, have been prepared and characterized.

Syntheses, structural diversity and photocatalytic properties of various Co(ii) coordination polymers based on a “V”-shaped 1,3-di(4′-carboxyl-phenyl)benzene acid and different imidazole bridging ligands

Various polymeric structures of complexes 1–4.

Supramolecular-jack-like guest in ultramicroporous crystal for exceptional thermal expansion behaviour

The dynamic behaviours of host frameworks and guest molecules have received much attention for their great relevance with smart materials, but little has been developed to control or understand the host-guest interplay. Here we show that the confined guest can utilize not only molecular static effects but also bulk dynamic properties to control the host dynamics. By virtue of the three-dimensional hinge-like framework and quasi-discrete ultramicropores, a flexible porous coordination polymer exhibits not only drastic guest-modulation effect of the thermal expansion magnitude (up to 422 × 10(-6) K(-1)) and even the anisotropy but also records positive/negative thermal expansion coefficients of +482/-218 × 10(-6) K(-1). Moreover, single-crystal X-ray diffraction analyses demonstrate that the jack-like motion of the guest supramolecular dimers, being analogous to the anisotropic thermal expansion of bulk van der Waals solids, is crucial for changing the flexibility mode and thermal expansion behaviour of the crystal.

The relationship between the strength of hydrogen bonding and spin crossover behaviour in a series of iron(III) Schiff base complexes

X-ray crystal structures and magnetic properties of an isostructural series of iron(III) Schiff base complexes with the general formula [Fe(L(5))(NCX)]·Solv (where H2L(5) = N,N'-bis(2-hydroxy-naphthylidene)-1,6-diamino-4-azahexane, X = S, Solv = tetrahydrofuran, 1a; X = S, Solv = methanol and 0.5 pyrazine, 1b; X = S, Solv = butanone, 1c; Solv = N,N'-dimethylformamide, X = S (1d) or X = Se (1d'); X = S, Solv = dimethyl sulfoxide, 1e) are reported. In the crystals, the individual [Fe(L(5))(NCX)] molecules are connected through weak C-H···O, C-H···π or C-H···S non-covalent contacts into 2D supramolecular networks, while the guest-solvent (Solv) molecules are trapped in the cavities between two adjacent layers, which are furthermore stabilized by N-H···O hydrogen bonds connecting the Solv oxygen atom with the amine group of the [Fe(L(5))(NCX)] molecule, with the N···O distances varying from 2.921(6) Å (in 1d') to 3.295(2) Å (in 1a). The magnetic properties of the complexes were tuned by the different Solv molecules and as a result of this, four new spin crossover (SCO) compounds with cooperative spin transitions are reported, which are accompanied by thermal hysteresis in two cases (1d and 1e): , T1/2 = 84 K; 1d, T1/2↓ = 232 K, T1/2↑ = 235 K and 1e, T1/2↓ = 127 K, T1/2↑ = 138 K. The role of the N-H···O hydrogen bonding in the occurrence and tuning of SCO was also computationally studied using a topological analysis, and also by evaluation of non-covalent interaction (NCI) indexes. Both theoretical approaches showed a clear relationship between the strength of the N-H···O hydrogen bonds and T1/2, as already inferred from X-ray structural and magnetic data.

Structural phase transition in a multi-induced mononuclear FeII spin-crossover complex

One mononuclear Fe^II complex displays a structural phase transition and a multi-induced spin-crossover behavior mediated by heat, light, pressure and solvent.

Hydrothermal syntheses, structural characterizations, and magnetic properties of five MOFs assembled from C2-symmetric ligand of 1,3-di(2′,4′-dicarboxylphenyl)benzene with various coordination modes

Five new complexes with appealing structural features from 0D paddle wheel {Cu₂(COO)₄} SBUs to 3D frameworks were reported to better understand the synthon selectivity in multifunctional crystal structures.

Assembly of a series of d10 coordination polymers based on W-shaped 1,3-di(2′,4′-dicarboxyphenyl)benzene: from syntheses, structural diversity, luminescence, to photocatalytic properties

Six d¹⁰ coordination polymers with different architectures based on W-shaped 1,3-di(2′,4′-dicarboxyphenyl)benzene (H₄DDB) have been prepared. Their photoluminescence and photocatalytic properties were also investigated.

Structural diversities and related properties of four coordination polymers synthesized from original ligand of 3,3′,5,5′-azobenzenetetracarboxylic acid

Four CPs, derived from the original 3,3′,5,5′-azobenzenetetracarboxylic acid ligand were obtained under solvothermal conditions.

Two-step spin crossover behaviour in the chiral one-dimensional coordination polymer [Fe(HAT)(NCS)2]∞

Two-step spin crossover behaviour with large stabilisation of the intermediate state in a chiral one-dimensional coordination polymer.

Spin crossover with thermal hysteresis: practicalities and lessons learnt

Memory applications of spin crossover require bistability: magnetic data must be appropriately collected and reported, and consideration given to lifetimes.

Coligand syntheses, crystal structures, luminescence and photocatalytic properties of five coordination polymers based on rigid tetracarboxylic acids and imidazole linkers

Coligand syntheses, crystal structures, luminescence and photocatalytic properties of five coordination polymers based on rigid tetracarboxylic acids and imidazole linkers.

Syntheses, structures, topologies, and luminescence properties of four coordination polymers based on bifunctional 6-(4-pyridyl)-terephthalic acid and bis(imidazole) bridging linkers

Four CPs, with the structure ranged from 2D sheet to 3D 3-fold penetrated net, have been designed with the mixed-ligand strategy of bifunctional 6-(4-pyridyl)-terephthalic acid and bis(imidazole) bridging linkers.

Structural diversity of five coordination polymers based on 2,6-bis(3,5-dicarboxyphenyl)pyridine ligand: solvothermal syntheses, structural characterizations, and magnetic properties

Five CPs, featuring diverse structures, have been assembled from the mixed ligands of H₄BDP and bib under different reaction conditions.