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.     

Ordered Microporous Layered Lanthanide 1,3,5‐Benzenetriphosphonates Pillared with Cationic Organic Molecules

Novel isomorphous pillared‐layer‐type crystalline lanthanide 1,3,5‐benzenetriphosphonates were prepared with bpy and dbo as organic pillars (LnBP‐bpy and LnBP‐dbo; Ln: Ce, Pr, and Nd). Ab initio crystal structure solution using synchrotron X‐ray powder diffraction data revealed that the organic pillars do not exist as neutral coordinating ligands but as cationic molecules. Especially the LnBP‐dbo phases have ordered interlayer space filled with water molecules between the dbo pillars, and the interlayer water is successfully removed by heating under vacuum with slightly distorted but basically retained pillared layer structures. Microporosity of the materials is confirmed by adsorption of nitrogen, carbon dioxide, and hydrogen gases. Such microporous layered metal phosphonates pillared with cationic molecules should be unprecedented and should offer new strategies to design ordered microporous materials.     

Reversible Solvatomagnetic Switching in a Spongelike Manganese(II)–Copper(II) 3D Open Framework with a Pillared Square/Octagonal Layer Architecture

The concept of “molecular magnetic sponges” was introduced for the first time in 1999 by the creative imagination of the late Olivier Kahn. It refers to the exotic spongelike behavior of certain molecule‐based materials that undergo a dramatic change of their magnetic properties upon reversible dehydration/rehydration processes. Here we report a unique example of a manganese(II)–copper(II) mixed‐metal–organic framework of formula [Na(H₂O)₄]₄[Mn₄{Cu₂(mpba)₂(H₂O)₄}₃]? 56.5 H₂O ( 1 ) (mpba=N,N′‐1,3‐phenylenebis(oxamate)). Compound 1 possesses a 3D Mn^II₄Cu^II₆ pillared layer structure with mixed square and octagonal pores of approximate dimensions 1.2×1.2 nm and 2.1×3.0 nm, respectively, hosting a large amount of crystallization H₂O molecules and hydrated Na^I countercations as guests. It reversibly switches from a crystalline hydrated phase with long‐range ferromagnetic ordering at a rather high critical temperature (T_c) of 22.5 K to an amorphous dehydrated phase with T_c as low as 2.3 K, which is accompanied by a breathing‐type dynamic effect involving a large crystal volume (ca. 45 %) and color changes after water desorption/adsorption. The combination of both the open‐framework structure and the spongelike optical, mechanical, and magnetic switching behavior in this new class of oxamato‐based porous magnets offers fascinating possibilities in designing multifunctional materials for host–guest molecular sensing.     

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.     

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.     

Windmill Co4{Co4(μ4‐O)} with 16 Divergent Branches Forming a Family of Metal–Organic Frameworks: Organic Metrics Control Topology,Gas Sorption,and Magnetism

A series of highly connected metal–organic frameworks (MOFs), [Co₈(O)(OH)₄(H₂O)₄(ina)₈](NO₃)₂ ? 2 C₂H₅OH ? 4 H₂O ( 1 ), [Co₈(O)(OH)₄(H₂O)₄(pba)₈](NO₃)₂ ? 8 C₂H₅OH ? 28 H₂O ( 2 ), and [Co₈(O)(OH)₄(H₂O)₄(pbba)₈](NO₃)₂ ? guest ( 3 ), in which ina=isonicotinate, pba=4‐pyridylbenzoate, and pbba=4‐(pyridine‐4‐yl)phenylbenzoate, is reported. These MOFs contain a new secondary building unit (SBU), with a square Co₄(μ₄‐O) central unit having the rare μ₄‐O^2? motif, which is decorated by the other four peripheral cobalt atoms through μ₃‐OH in a windmill‐like shape. This SBU holds 16 divergent connecting organic ligands, pyridyl‐carboxylates, to form three different frameworks. The high porosity of desolvated 2 is shown by the efficient gas absorption of N₂, CO₂, CH₄, and H₂. In addition, 1 and 2 exhibit unusual canted antiferromagnetic behavior with spin‐glass‐like relaxation, with blocking temperatures that are fairly high, 20 K ( 1 ) and 10 K ( 2 ), for cobalt materials. The relationship between the metal clusters and linkers has been studied, in which the size and rotational degrees of freedom of the ligands are found to control the topology, gas sorption, and magnetic properties.     

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.     

Versatile Tailoring of Paddle‐Wheel ZnII Metal–Organic Frameworks through Single‐Crystal‐to‐Single‐Crystal Transformations

A new tetracarboxylate ligand having short and long arms formed 2D layer Zn^II coordination polymer 1 with paddle‐wheel secondary building units under solvothermal conditions. The framework undergoes solvent‐specific single crystal‐to‐single crystal (SC‐SC) transmetalation to produce 1_Cu . With a sterically encumbered dipyridyl linker, the same ligand forms non‐interpenetrated, 3D, pillared‐layer Zn^II metal–organic framework (MOF) 2 , which takes part in SC‐SC linker‐exchange reactions to produce three daughter frameworks. The parent MOF 2 shows preferential incorporation of the longest linker in competitive linker‐exchange experiments. All the 3D MOFs undergo complete SC‐SC transmetalation with Cu^II, whereby metal exchange in different solvents and monitoring of X‐ray structures revealed that bulky solvated metal ions lead to ordering of the shortest linker in the framework, which confirms that the solvated metal ions enter through the pores along the linker axis.     

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.

     

Thermal‐ and Light‐Induced Spin Crossover in a Guest‐Dependent Dinuclear Iron(II) System

We previously reported the dinuclear material [Fe^II₂(ddpp)₂(NCS)₄] ? 4 CH₂Cl₂ ( 1? 4 CH₂Cl₂; ddpp=2,5‐di(2′,2′′‐dipyridylamino)pyridine) and its partially desolvated analogue ( 1? CH₂Cl₂), which undergo two‐ and one‐step spin‐crossover (SCO) transitions, respectively. Here, we manipulate the type and degree of solvation in this system and find that either a one‐ or two‐step spin transition can be specifically targeted. The chloroform clathrate 1? 4 CHCl₃ undergoes a relatively abrupt one‐step SCO, in which the two equivalent Fe^II sites within the dinuclear molecule crossover simultaneously. Partial desolvation of 1? 4 CHCl₃ to form 1? 3 CHCl₃ and 1? CHCl₃ occurs through single‐crystal‐to‐single‐crystal processes (monoclinic C2/c to P2₁/n to P2₁/n) in which the two equivalent Fe^II sites become inequivalent sites within the dinuclear molecule of each phase. Both 1? 3 CHCl₃ and 1? CHCl₃ undergo one‐step spin transitions, with the former having a significantly higher SCO temperature than 1? 4 CHCl₃ and the latter, and each has a broader SCO transition than 1? 4 CHCl₃, attributable to the overlap of two SCO steps in each case. Further magnetic manipulation can be carried out on these materials through reversibly resolvating the partially desolvated material with chloroform to produce the original one‐step SCO, or with dichloromethane to produce a two‐step SCO reminiscent of that seen for 1? 4 CH₂Cl₂. Furthermore, we investigate the light‐induced excited spin state trapping (LIESST) effect on 1? 4 CH₂Cl₂ and 1? CH₂Cl₂ and observe partial LIESST activity for the former and no activity for the latter.     

A New Strategy for Storage and Transportation of Sensitive High‐Energy Materials: Guest‐Dependent Energy and Sensitivity of 3D Metal–Organic‐Framework‐Based Energetic Compounds

Reaction of Co(II) with the nitrogen‐rich ligand N,N‐bis(1H‐tetrazole‐5‐yl)‐amine (H₂bta) leads to a mixed‐valence, 3D, porous, metal–organic framework (MOF)‐based, energetic material with the nitrogen content of 51.78%, [Co₉(bta)₁₀(Hbta)₂(H₂O)₁₀]_n?(22 H₂O)_n ( 1 ). Compound 1 was thermohydrated to produce a new, stable, energetic material with the nitrogen content of 59.85% and heat of denotation of 4.537 kcal cm^?3, [Co₉(bta)₁₀(Hbta)₂(H₂O)₁₀]_n ( 2 ). Sensitivity tests show that 2 is more sensitivity to external stimuli than 1 , reflecting guest‐dependent energy and sensitivity of 3D, MOF‐based, energetic materials. Less‐sensitive 1 can be regarded as a more safe form for storage and transformation to sensitive 2 .     

A Spin‐Crossover Cluster of Iron(II) Exhibiting a Mixed‐Spin Structure and Synergy between Spin Transition and Magnetic Interaction

How low can you go? An Fe^II₄ square was prepared by self‐assembly and exhibits both thermally induced and photoinduced spin crossover from a system with four high‐spin (HS) centers to one with two high‐spin and two low‐spin (LS) centers. The spin‐crossover sites are located on the same side of the square, and the spin transition and magnetic interactions (see picture) are synergistically coupled.

     

Multielectron‐Transfer‐based Rechargeable Energy Storage of Two‐Dimensional Coordination Frameworks with Non‐Innocent Ligands

The metallically conductive bis(diimino)nickel framework (NiDI), an emerging class of metal–organic framework (MOF) analogues consisting of two‐dimensional (2D) coordination networks, was found to have an energy storage principle that uses both cation and anion insertion. This principle gives high energy led by a multielectron transfer reaction: Its specific capacity is one of the highest among MOF‐based cathode materials in rechargeable energy storage devices, with stable cycling performance up to 300 cycles. This mechanism was studied by a wide spectrum of electrochemical techniques combined with density‐functional calculations. This work shows that a rationally designed material system of conductive 2D coordination networks can be promising electrode materials for many types of energy devices.