Guest Programmable Multistep Spin Crossover in a Porous 2‑D Hofmann-Type Material

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [Fe^II(bztrz)₂(Pd^II(CN)₄)]·<i>n</i>(guest) (bztrz = (<i>E</i>)-1-phenyl-<i>N</i>-(1,2,4-triazol-4-yl)­methanimine, <b>1·</b><i><b>n</b></i><b>(guest)</b>), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (<b>1·(H</b>_<b>2</b><b>O,EtOH)</b>), two- (<b>1·3H</b>_<b>2</b><b>O</b>) and three-stepped (<b>1·∼2H</b>_<b>2</b><b>O</b>) transitions and SCO-deactivation (<b>1</b>)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated

Guest Programmable Multistep Spin Crossover in a Porous 2‑D Hofmann-Type Material

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [Fe^II(bztrz)₂(Pd^II(CN)₄)]·<i>n</i>(guest) (bztrz = (<i>E</i>)-1-phenyl-<i>N</i>-(1,2,4-triazol-4-yl)­methanimine, <b>1·</b><i><b>n</b></i><b>(guest)</b>), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (<b>1·(H</b>_<b>2</b><b>O,EtOH)</b>), two- (<b>1·3H</b>_<b>2</b><b>O</b>) and three-stepped (<b>1·∼2H</b>_<b>2</b><b>O</b>) transitions and SCO-deactivation (<b>1</b>)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated

Guest Adsorption in the Nanoporous Metal–Organic Framework Cu₃(1,3,5-Benzenetricarboxylate)₂: Combined <i>In Situ</i> X‑ray Diffraction and Vapor Sorption

The structure of the nanoporous metal–organic framework Cu₃(BTC)₂ (BTC = 1,3,5-benzenetricarboxylate) with a variety of molecular guests was studied <i>in situ</i> using single crystal X-ray diffraction. By collecting crystal structure data for a series of guests within the same host crystal, insights into the molecular interactions underpinning guest adsorption processes have been gained. Adsorption behaviors are influenced strongly by both enthalpic and entropic thermodynamic, as well as interpore steric (size-exclusion) effects, and we note correlations between guest attributes and these effects. Vapor adsorption measurements revealed a guest uptake capacity inversely proportional to guest size. Correspondingly, structural results show that guests reside in the smallest pores accessible to them. Interpore steric effects for larger guests cause these to be excluded from the smallest pores, and this corresponds to lower total uptake. Both hydrophilic and lipophilic small guests adsorb favorably into the 5 Å diameter smallest pore of the material, with the number of guests in these pores dependent on guest size and their location, in turn dependent upon both guest–guest interactions and competition between hydrogen-bonding interactions at the apertures of the smallest pore and lipophilic interactions at the center of the smallest pore. Hydrophilic guests with lone electron pairs interact preferentially with the coordinatively unsaturated Cu sites of the desolvated framework, with the number of these depending on steric interactions between neighboring bound guests and guest flexibility. Guest coordination at the Cu sites has a significant effect on the framework structure, increasing the Cu···Cu distance in the dinuclear unit, with the Cu₃(BTC)₂ unit cell being smaller when guests that do not coordinate with the Cu are present, and in the case of cyclohexane, smaller than for the desolvated framework. Overall, our comprehensive structural study reconciles Cu₃(BTC)₂ adsorption properties with the underlying guest–host and guest–guest interactions that gives rise to these

Mixed-Component Sulfone–Sulfoxide Tagged Zinc IRMOFs: <i>In Situ</i> Ligand Oxidation, Carbon Dioxide, and Water Sorption Studies

Reported here are the syntheses and adsorption properties of a series of single- and mixed-component zinc IRMOFs derived from controlled ratios of sulfide and sulfone functionalized linear biphenyldicarboxylate (bpdc) ligands. During MOF synthesis the sulfide moieties undergo <i>in situ</i> oxidation, giving rise to sulfoxide functionalized ligands, which are incorporated to give mixed-component sulfoxide–sulfone functionalized MOFs. The single- and mixed-component systems all share the IRMOF-9 structure type as determined by a combination of single crystal and powder X-ray diffraction analyses. The functionalized IRMOF-9 series was investigated by N₂, CO₂, and water adsorption measurements. MOFs containing higher proportions of sulfoxide have slightly larger accessible surface areas and pore volumes, whereas MOFs containing a greater proportion of the sulfone functionality demonstrated higher CO₂ adsorption capacities, enthalpies of CO₂ adsorption, and CO₂/N₂ selectivities. Water adsorption studies at 298 K showed the MOFs to have pore-filling steps starting around 0.4 <i>P/P</i>₀. In general, only small changes in water adsorption were observed with regards to ligand ratios in the mixed-component MOFs, suggesting that the location of the step is primarily determined by the pore size. A ligand-directed fine-tuning approach of changing alkyl chain length was demonstrated to give smaller more hydrophobic pores with better adsorption characteristics

Selective Gas Adsorption in a Pair of Robust Isostructural MOFs Differing in Framework Charge and Anion Loading

Activation of the secondary assembly instructions in the mononuclear pyrazine imide complexes [Co^III(dpzca)₂]­(BF₄) or [Co^II(dpzca)₂] and [Ni^II(dpzca)₂] has facilitated the construction of two robust nanoporous three-dimensional coordination polymers, [Co^III(dpzca)₂Ag]­(BF₄)₂·2­(H₂O) [<b>1</b>·2­(H₂O)] and [Ni^II(dpzca)₂Ag]­BF₄·0.5­(acetone) [<b>2</b>·0.5­(acetone)]. Despite the difference in charge distribution and anion loading, the framework structures of <b>1</b>·2­(H₂O) and <b>2</b>·0.5­(acetone) are isostructural. One dimensional channels along the <i>b</i>-axis permeate the structures and contain the tetrafluoroborate counterions (the Co^III-based MOF has twice as many BF₄^– anions as the Ni^II-based MOF) and guest solvent molecules. These anions are not readily exchanged whereas the solvent molecules can be reversibly removed and replaced. The H₂, N₂, CO₂, CH₄, H₂O, CH₃OH, and CH₃CN sorption behaviors of the evacuated frameworks <b>1</b> and <b>2</b> at 298 K have been studied, and modeled, and both show very high selectivity for CO₂ over N₂. The increased anion loading in the channels of Co^III-based MOF <b>1</b> relative to Ni^II-based MOF <b>2</b> results in increased selectivity for CO₂ over N₂ but a decrease in the sorption kinetics and storage capacity of the framework

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

A new functionalized 1,2,4-triazole ligand, 4-[(<i>E</i>)-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]·<i>n</i>H₂O (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 (<b>A·2H</b>_<b>2</b><b>O</b>) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in <b>A·2H</b>_<b>2</b><b>O</b> 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 <b>A·2H</b>_<b>2</b><b>O</b> (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 <b>A·2H</b>_<b>2</b><b>O</b> under hydrostatic pressure compared to a one-step SCO in <b>A</b> 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