Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni)

The synthesis and characterization of a series of three‐dimensional (3D) Hofmann‐like clathrate porous metal–organic framework (MOF) materials [Fe(bpac)M(CN)₄] (M=Pt, Pd, and Ni; bpac=bis(4‐pyridyl)acetylene) that exhibit spin‐crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin‐crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)₄]}_n planar layers bridged by the bis‐monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7 % of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin‐crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π–π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host–guest interaction and the spin‐crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice.     

Desymmetrization of an Octahedral Coordination Complex Inside a Self‐Assembled Exoskeleton

The synthesis of a centrally functionalized, ribbon‐shaped [6]polynorbornane ligand L that self‐assembles with Pd^II cations into a {Pd₂ L ₄} coordination cage is reported. The shape‐persistent {Pd₂ L ₄} cage contains two axial cationic centers and an array of four equatorial H‐bond donors pointing directly towards the center of the cavity. This precisely defined supramolecular environment is complementary to the geometry of classic octahedral complexes [M(XY)₆] with six diatomic ligands. Very strong binding of [Pt(CN)₆]^2? to the cage was observed, with the structure of the host–guest complex {[Pt(CN)₆]@Pd₂L₄} supported by NMR spectroscopy, MS, and X‐ray data. The self‐assembled shell imprints its geometry on the encapsulated guest, and desymmetrization of the octahedral platinum species by the influence of the D_4h‐symmetric second coordination sphere was evidenced by IR spectroscopy. [Fe(CN)₆]^3? and square‐planar [Pt(CN)₄]^2? were strongly bound. Smaller octahedral anions such as [SiF₆]^2?, neutral carbonyl complexes ([M(CO)₆]; M=Cr, Mo, W) and the linear [Ag(CN)₂]^? anion were only weakly bound, showing that both size and charge match are key factors for high‐affinity binding.     

Bidirectional Chemo‐Switching of Spin State in a Microporous Framework

The ins and outs of spin : Using the microporous coordination polymer {Fe(pz)[Pt(CN)₄]} ( 1 , pz=pyrazine), incorporating spin‐crossover subunits, two‐directional magnetic chemo‐switching is achieved at room temperature. In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low‐spin (LS) state to the high‐spin (HS) state, whereas CS₂ uniquely causes the reverse HS‐to‐LS transition.

     

Guest Modulation of Spin‐Crossover Transition Temperature in a Porous Iron(II) Metal–Organic Framework: Experimental and Periodic DFT Studies

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin‐crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)₄]} ( 1 ) with five‐membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin‐crossover transition with hysteresis loops 14–29 K wide and average critical temperatures T_c=201 K ( 1?fur ), 167 K ( 1?pyr ), and 114.6 K ( 1?thio ) well below that of the parent compound 1 (T_c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1?fur , while 1?pyr and 1?thio show 50 % spin transition. For 1?fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1?pyr and 1?thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1?fur , 1?pyr , 1?thio , and previously reported derivatives 1?CS₂ , 1?I, 1?bz (benzene), and 1?pz (pyrazine) have been carried out to investigate the electronic structure and nature of the host–guest interactions as well as their relationship with the changes in the LS–HS transition temperatures of 1?Guest . Geometry‐optimized lattice parameters and bond distances in the empty host 1 and 1?Guest clathrates are in general agreement with the X‐ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental T_c and calculated LS–HS electronic energy gap was observed. Finally, specific host–guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis.     

Nanoporosity,Inclusion Chemistry,and Spin Crossover in Orthogonally Interlocked Two‐Dimensional Metal–Organic Frameworks

[Fe(tvp)₂(NCS)₂] ( 1 ) (tvp=trans‐(4,4′‐vinylenedipyridine)) consists of two independent perpendicular stacks of mutually interpenetrated two‐dimensional grids. This uncommon supramolecular conformation defines square‐sectional nanochannels (diagonal≈2.2 nm) in which inclusion molecules are located. The guest‐loaded framework 1@guest displays complete thermal spin‐crossover (SCO) behavior with the characteristic temperature T_1/2 dependent on the guest molecule, whereas the guest‐free species 1 is paramagnetic whatever the temperature. For the benzene–guest derivatives, the characteristic SCO temperature T_1/2 decreases as the Hammet σ_p parameter increases. In general, the 1@guest series shows large entropy variations associated with the SCO and conformational changes of the interpenetrated grids that leads to a crystallographic‐phase transition when the guest is benzonitrile or acetonitrile/H₂O.     

Hysteretic Four‐Step Spin Crossover within a Three‐Dimensional Porous Hofmann‐like Material

Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four‐step hysteretic SCO framework. Single‐crystal structure analysis of a porous 3D Hofmann‐like material showed long‐range ordering of spin states: HS, HS_0.67LS_0.33, HS_0.5LS_0.5, HS_0.33LS_0.67, and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host–host and host–guest interactions.     

Sequestering aromatic molecules with a spin-crossover Fe(II) microporous coordination polymer

All in a spin: A series of three-dimensional porous coordination polymer {Fe(dpe)[Pt(CN)(4)]}?G (dpe = 1,2-di(4-pyridyl)ethylene; G = phenazine, anthracene, or naphthalene) exhibiting spin crossover and host-guest functions is reported. The magnetic properties of the framework are very sensitive to the chemical nature (aromatic or hydroxilic solvents) and the size of the included guest molecules.     

Amphiphilic Inclusion Spaces for Various Guests and Regulation of Fluorescence Intensity of 1,8‐Bis(4‐aminophenyl)anthracene Crystals

A host framework for inclusion of various guest molecules was investigated by preparation of inclusion crystals of 1,8‐bis(4‐aminophenyl)anthracene (1,8‐BAPA) with organic solvents. X‐ray crystallographic analysis revealed construction of the same inclusion space incorporating 1,8‐BAPA and eight guest molecules including both non‐polar (benzene) and polar guests (N,N‐dimethylformamide, DMF). Fluorescence efficiencies varied depending on guest molecule polarity; DMF inclusion crystals exhibited the highest fluorescence intensity (Φ_F=0.40), four times as high as that of a benzene inclusion crystal (Φ_F=0.10). According to systematic investigations of inclusion phenomena, strong host–guest interactions and filling of the inclusion space led to a high fluorescence intensity. Temperature‐dependent fluorescence spectral measurements revealed these factors effectively immobilised the host framework. Although hydrogen bonding commonly decreases fluorescence intensity, the present study demonstrated that such strong interactions provide excellent conditions for fluorescence enhancement. Thus, this remarkable behaviour has potential application toward sensing of highly polar molecules, such as biogenic compounds.     

Structures and Properties of Spherical 90‐Vertex Fullerene‐Like Nanoballs

By applying the proper stoichiometry of 1:2 to [Cp^RFe(η⁵‐P₅)] and CuX (X=Cl, Br) and dilution conditions in mixtures of CH₃CN and solvents like CH₂Cl₂, 1,2‐Cl₂C₆H₄, toluene, and THF, nine spherical giant molecules having the simplified general formula [Cp^RFe(η⁵‐P₅)]@[{Cp^RFe(η⁵‐P₅)}₁₂{CuX}₂₅(CH₃CN)₁₀] (Cp^R=η⁵‐C₅Me₅ (Cp*); η⁵‐C₅Me₄Et (Cp^Et); X=Cl, Br) have been synthesized and structurally characterized. The products consist of 90‐vertex frameworks consisting of non‐carbon atoms and forming fullerene‐like structural motifs. Besides the mostly neutral products, some charged derivatives have been isolated. These spherical giant molecules show an outer diameter of 2.24 (X=Cl) to 2.26 nm (X=Br) and have inner cavities of 1.28 (X=Cl) and 1.20 nm (X=Br) in size. In most instances the inner voids of these nanoballs encapsulate one molecule of [Cp*Fe(η⁵‐P₅)], which reveals preferred orientations of π–π stacking between the cyclo‐P₅ rings of the guest and those of the host molecules. Moreover, π–π and σ–π interactions are also found in the packing motifs of the balls in the crystal lattice. Electrochemical investigations of these soluble molecules reveal one irreversible multi‐electron oxidation at E_p=0.615 V and two reduction steps (?1.10 and ?2.0 V), the first of which corresponds to about 12 electrons. Density functional calculations reveal that during oxidation and reduction the electrons are withdrawn or added to the surface of the spherical nanomolecules, and no Cu²⁺ species are involved.     

Three Dicyanidometallate(I)‐based Complexes Incorporating Hydrogen‐bonding, π–π Packing and d10–d10 Interactions with Auxiliary 2,2′‐Bipyridyl‐like Ligands

To investigate the influence of the non‐covalent interactions, such as hydrogen‐bonding, π–π packing and d¹⁰–d¹⁰ interactions in the supramolecular motifs, three cyanido‐bridged heterobimetallic discrete complexes {Mn(bipy)₂(H₂O)[Ag(CN)₂]}[Ag(CN)₂] ( 1 ), {Mn(phen)₂(H₂O)[Au(CN)₂]}₂[Au(CN)₂]₂ · 4H₂O ( 2 ), and {Cd(bipy)₂(H₂O)[Au(CN)₂]}[Au(CN)₂] ( 3 ) (bipy = 2,2′‐bipyridine, and phen = 1,10‐phenanthroline), which are based on dicyanidometallate(I) groups with 1:2 stoichiometry of metal ions and 2,2′‐bipyridyl‐like co‐ligands were synthesized and structurally characterized. In compound 1 , hydrogen bonding and π–π interactions governed the supramolecular contacts. In compound 2 , the incorporation of aurophilic, hydrogen bonding and π–π interactions result in a 3D supramolecular network. In compound 3 , hydrogen bonding and π–π stacking interactions result in a 2D supramolecular layer. In the three complexes, hydrogen‐bonding, π–π packing and/or d¹⁰–d¹⁰ interactions can play important roles in increasing the dimensionality of supramolecular assemblies.     

Adsorption behaviour of a CdII–triazole MOF for butan‐2‐one in a single‐crystal‐to‐single‐crystal (SCSC) fashion: the role of hydrogen bonding and C—H…π interactions

The adsorption behaviour of the Cd^II–MOF {[Cd(L)₂(ClO₄)₂]·H₂O ( 1 ), where L is 4‐amino‐3,5‐bis[3‐(pyridin‐4‐yl)phenyl]‐1,2,4‐triazole, for butan‐2‐one was investigated in a single‐crystal‐to‐single‐crystal (SCSC) fashion. A new host–guest system that encapsulated butan‐2‐one molecules, namely poly[[bis{μ₃‐4‐amino‐3,5‐bis[3‐(pyridin‐4‐yl)phenyl]‐1,2,4‐triazole}cadmium(II)] bis(perchlorate) butanone sesquisolvate], {[Cd(C₂₄H₁₈N₆)₂](ClO₄)₂·1.5C₄H₈O}_n, denoted C₄H₈O@Cd‐MOF ( 2 ), was obtained via an SCSC transformation. MOF 2 crystallizes in the tetragonal space group P4₃2₁2. The specific binding sites for butan‐2‐one in the host were determined by single‐crystal X‐ray diffraction studies. N—H…O and C—H…O hydrogen‐bonding interactions and C—H…π interactions between the framework, ClO₄^? anions and guest molecules co‐operatively bind 1.5 butan‐2‐one molecules within the channels. The adsorption behaviour was further evidenced by ¹H NMR, IR, TGA and powder X‐ray diffraction experiments, which are consistent with the single‐crystal X‐ray analysis. A ¹H NMR experiment demonstrates that the supramolecular interactions between the framework, ClO₄^? anions and guest molecules in MOF 2 lead to a high butan‐2‐one uptake in the channel.     

Mixed‐Metal Coordination Cages Constructed with Pyridyl‐Functionalized β‐Diketonate Metalloligands: Syntheses,Structures and Host–Guest Properties

The design and synthesis of mixed‐metal coordination cages, which can act as hosts to encapsule guest molecules, is a subject of intensive research, and the utilization of metalloligand is an effective method to construct a designed heterometallic architecture. Herein, a series of heterometallic cages with half‐sandwich Rh, Ir and Ru fragments using Cu^II‐metalloligand as a building block by a stepwise approach is reported. The cavity sizes of the cages could be controlled easily by the lengths of the organic ligands. Because the metalloligands in the oxalate‐based cage are somewhat distorted and concave, there are weak Cu???O interactions in the molecules, forming a binuclear copper unit. By increasing the height of the cages using longer ligands, 2,5‐dichloro‐3,6‐dihydroxy‐1,4‐benzoquinone (H₂CA), the organometallic boxes display interesting host–guest behavior, which are made large enough to accommodate some large molecules, such as pyrene and [Pt(acac)₂]. Interestingly, the heterometallic cage with larger cavity size can transfer into a homometallic hexanuclear prism in the presence of pyrazine.     

Synthesis and Characterisation of a New Series of Bistable Iron(II) Spin‐Crossover 2D Metal–Organic Frameworks

Twelve coordination polymers with formula {Fe(3‐Xpy)₂[M^II(CN)₄]} (M^II: Ni, Pd, Pt; X: F, Cl, Br, I; py: pyridine) have been synthesised, and their crystal structures have been determined by single‐crystal or powder X‐ray analysis. All of the fluoro and iodo compounds, as well as the chloro derivative in which M^II is Pt, crystallise in the monoclinic C2/m space group, whereas the rest of the chloro and all of the bromo derivatives crystallise in the orthorhombic Pnc2 space group. In all cases, the iron(II) atom resides in a pseudo‐octahedral [FeN₆] coordination core, with similar bond lengths and angles in the various derivatives. The major difference between the two kinds of structure arises from the stacking of consecutive two‐dimensional {Fe(3‐Xpy)₂[M^II(CN)₄]}_∞ layers, which allows different dispositions of the X atoms. The fluoro and chloro derivatives undergo cooperative spin crossover (SCO) with significant hysteretic behaviour, whereas the rest are paramagnetic. The thermal hysteresis, if X is F, shifts toward room temperature without changing the cooperativity as the pressure increases in the interval 10⁵ Pa–0.5 GPa. At ambient pressure, the SCO phenomenon has been structurally characterised at different significant temperatures, and the corresponding thermodynamic parameters were obtained from DSC calorimetric measurements. Compound {Fe(3‐Clpy)₂[Pd(CN)₄]} represents a new example of a “re‐entrant” two‐step spin transition by showing the Pnma space group in the intermediate phase (IP) and the Pnc2 space group in the low‐spin (LS) and high‐spin (HS) phases.     

Guest‐Directed Supramolecular Architectures of {W36} Polyoxometalate Crowns

The {W₃₆} isopolyoxotungstate cluster provides a stable inorganic molecular platform for the binding of inorganic and organic guest molecules. This is achieved by a binding pocket formed by six terminal oxo ligands located in the central cavity of the all‐inorganic cation binding host. Previously it was shown that the cluster can specifically bind primary amines and importantly, functionalized diamines through a combination of electrostatic and hydrogen bonding interactions. Here we transform this assembly strategy to utilize the binding of long‐chain alkyldiammonium guest cations to physically define the supramolecular structure of the clusters with respect to each other and demonstrate the structure direction as a function of alkyl chain length. The systematic variation of the chain length gives access to five supramolecular assemblies which were all fully characterized using single crystal XRD, TGA, ¹H NMR, and elemental analysis. In compound 1 , diprotonated 1,8‐diaminooctane molecules link the {W₃₆} clusters into infinite 1D zigzag chains, whereas compounds 2 and 3 feature trimeric {W₃₆} assemblies directly connected through protonated 1,9‐diaminononane ( 2 ) or 1,10‐diaminodecane ( 3 ) linkers . Compound 4 contains dumb‐bell shaped dimeric units as a result of direct center‐to‐center linkages between the {W₃₆} clusters formed by protonated 1,12‐diaminododecane. In compound 5 , triply protonated bis(hexamethylene)triamine was employed to obtain linear 1D chains of directly connected {W₃₆} cluster units.     

Supramolecular Photoinduced Electron Transfer between a Redox‐Active Hexanuclear Metal–Organic Cylinder and an Encapsulated Ruthenium(II) Complex

By using redox‐active nickel(II) ions as the connect nodes, a hexanuclear metal–organic cylinder (Ni‐YL) was achieved through self‐assembly with a large cavity and an opening windows capable to accommodate guest molecules. The suitable cavity of Ni‐YL provides an opportunity to encapsulate the anionic ruthenium bipyridine derivative [Ru(dcbpy)₃] (dcbpy=2,2′‐bipyridine‐4,4′‐dicarboxylic acid) as the photosensitizer for light‐driven reactions. The host–guest behavior between Ni‐YL and [Ru(dcbpy)₃] was investigated by mass spectrometry, NMR spectroscopy, and computational studies, revealing an effective binding of the guest [Ru(dcbpy)₃] within the cavity of Ni‐YL. Optical experiments suggested a pseudo‐intramolecular photoinduced electron transfer (PET) process between the [Ru(dcbpy)₃] and the host Ni‐YL, leading to an efficient light‐driven hydrogen production based on this system. Control experiments with a mononuclear Ni complex as a reference photocatalyst and the inactive [Fe(dcbpy)₃] as an inhibitor for comparison were also performed to confirm such a supramolecular photocatalysis process.     

Photo‐ and Electronically Switchable Spin‐Crossover Iron(II) Metal–Organic Frameworks Based on a Tetrathiafulvalene Ligand

A major challenge is the development of multifunctional metal–organic frameworks (MOFs), wherein magnetic and electronic functionality can be controlled simultaneously. Herein, we rationally construct two 3D MOFs by introducing the redox active ligand tetra(4‐pyridyl)tetrathiafulvalene (TTF(py)₄) and spin‐crossover Fe^II centers. The materials exhibit redox activity, in addition to thermally and photo‐induced spin crossover (SCO). A crystal‐to‐crystal transformation induced by I₂ doping has also been observed and the resulting intercalated structure determined. The conductivity could be significantly enhanced (up to 3 orders of magnitude) by modulating the electronic state of the framework via oxidative doping; SCO behavior was also modified and the photo‐magnetic behavior was switched off. This work provides a new strategy to tune the spin state and conductivity of framework materials through guest‐induced redox‐state switching.     

A Study of the Interaction Between Cucurbit[8]uril and Alkyl‐Substituted 4‐Pyrrolidinopyridinium Salts

The interaction between cucuribit[8]uril (Q[8]) and a series of 4‐pyrrolidinopyridinium salts bearing aliphatic substituents at the pyridinium nitrogen, namely 4‐(C₄H₈N)C₅H₅NRBr, where R=Et (g1), n‐butyl (g2), n‐pentyl (g3), n‐hexyl (g4), n‐octyl (g5), n‐dodecyl (g6), has been studied in aqueous solution by ¹H NMR spectroscopy, electronic absorption spectroscopy, isothermal titration calorimetry and mass spectrometry. Single crystal X‐ray diffraction revealed the structure of the host–guest complexes for g1, g2, g3, and g5. In each case, the Q[8] contains two guest molecules in a centrosymmetric dimer. The orientation of the guest molecule changes as the alkyl chain increases in length. Interestingly, in the solid state, the inclusion complexes identified are different from those observed in solution, and furthermore, in the case of g3, Q[8] exhibits two different interactions with the guest. In solution, the length of the alkyl chain plays a significant role in determining the type of host–guest interaction present.     

Effect of Intermolecular Interactions on Metal‐to‐Metal Charge Transfer: A Combined Experimental and Theoretical Investigation

Understanding the effects of intermolecular interactions on metal‐to‐metal charge transfer (MMCT) is crucial to develop molecular devices by grafting MMCT‐based molecular arrays. Herein, we report a series of solvent‐free {Fe₂Co₂} compounds sharing the same cationic tetranuclear {[Fe(PzTp)(CN)₃]₂[Co(dpq)₂]₂}²⁺ (PzTp^?=tetrakis(pyrazolyl)borate, dpq=dipyrido[3,2‐d:2′,3′‐f]quinoxaline) square units but having anions with different size, including BF₄^?, PF₆^?, OTf^?, and [Fe(PzTp)(CN)₃]^?. Intermolecular π???π interactions between dpq ligands, which coordinate to cobalt ions in the {[Fe(PzTp)(CN)₃]₂[Co(dpq)₂]₂}²⁺ units, can be modulated by introducing different counterions, regulating the distortion of the CoN₆ octahedron and ligand field around the cobalt ions. This change results in different MMCT behavior. Computational analyzes reveal the substantial role of the intermolecular interactions tuned by the presence of different counteranions on the MMCT behavior.     

Host–Guest Interactions of Phosphorescent Molecular Tweezers Based on an Alkynylplatinum(II) Terpyridine System with Polyaromatic Hydrocarbons

Host–guest interactions of a molecular tweezer complex 1 with various planar organic molecules including polyaromatic hydrocarbons (PAHs) were investigated by 1D and 2D ¹H NMR spectroscopy, UV/Vis absorption and emission titration studies. 2D and DOSY NMR spectroscopies support the sandwiched binding mode based on 1:1 host–guest interactions. The binding constants (K_S) of complex 1 for various PAHs were determined by NMR titration studies and the values were found to span up to an order of 10⁴ M ^?1 for coronene to no observable interaction for benzene, indicating that the π‐surface area is important for such host–guest interactions. The substituent effect on the host–guest interaction based on the guest series of 9‐substituted anthracenes was also studied. In general, a stronger interaction was observed for the anthracene guest with electron‐donating groups, although steric and π‐conjugation factors cannot be completely excluded. The photophysical responses of complex 1 upon addition of various PAHs were measured by UV/Vis and emission titration studies. The UV/Vis absorption spectra were found to show a drop in absorbance of the metal‐to‐ligand charge‐transfer (MLCT) and ligand‐to‐ligand charge‐transfer (LLCT) admixture band upon addition of various guest molecules to 1 , whereas the emission behavior was found to change differently depending on the guest molecules, showing emission enhancement and/or quenching. It was found that emission quenching occurred either via energy transfer or electron transfer pathway or both, while emission enhancement was caused by the increase in rigidity of complex 1 as a result of host–guest interaction.     

Reversible guest-induced gate-opening with multiplex spin crossover responses in two-dimensional Hofmann clathrates

Spin crossover (SCO) compounds are very attractive types of switchable materials due to their potential applications in memory devices, actuators or chemical sensors. Rational chemical tailoring of these switchable compounds is key for achieving new functionalities in synergy with the spin state change. However, the lack of precise structural information required to understand the chemical principles that control the SCO response with external stimuli may eventually hinder further development of spin switching-based applications. In this work, the functionalization with an amine group in the two-dimensional (2D) SCO compound {Fe(5-NH₂Pym)₂[M^II(CN)₄]} (1M, 5-NH₂Pym = 5-aminopyrimidine, M^II = Pt (1Pt), Pd (1Pd)) confers versatile host–guest chemistry and structural flexibility to the framework primarily driven by the generation of extensive H-bond interactions. Solvent free 1M species reversibly adsorb small protic molecules such as water, methanol or ethanol yielding the 1M·H2O, 1M·0.5MeOH or 1M·xEtOH (x = 0.25–0.40) solvated derivatives. Our results demonstrate that the reversible structural rearrangements accompanying these adsorption/desorption processes (1M ↔ 1M·guest) follow a gate-opening mechanism whose kinetics depend not only on the nature of the guest molecule and that of the host framework (1Pt or 1Pd) but also on their reciprocal interactions. In addition, a predictable and reversible guest-induced SCO modulation has been observed and accurately correlated with the associated crystallographic transformations monitored in detail by single crystal X-ray diffraction.

Layered Hofmann-type iron(ii) coordination polymers functionalised with 5-aminopyrimidine ligands show gate-opening driven guest-exchange accompanied by drastic structural and spin-crossover modulations.