Commensurate CO₂ Capture, and Shape Selectivity for HCCH over H₂CCH₂, in Zigzag Channels of a Robust Cu^I(CN)(L) Metal–Organic Framework

A novel copper­(I) metal–organic framework (MOF), {[Cu^I₂(py-pzpypz)₂(μ-CN)₂]·MeCN}_<i>n</i> (<b>1</b>·MeCN), with an unusual topology is shown to be robust, retaining crystallinity during desolvation to give <b>1</b>, which has also been structurally characterized [py-pzpypz is 4-(4-pyridyl)-2,5-dipyrazylpyridine)]. Zigzag-shaped channels, which in <b>1</b>·MeCN were occupied by disordered MeCN molecules, run along the <i>c</i> axis of <b>1</b>, resulting in a significant solvent-accessible void space (9.6% of the unit cell volume). These tight zigzags, bordered by (Cu^ICN)_<i>n</i> chains, make <b>1</b> an ideal candidate for investigations into shape-based selectivity. MOF <b>1</b> shows a moderate enthalpy of adsorption for binding CO₂ (−32 kJ mol^–1 at moderate loadings), which results in a good selectivity for CO₂ over N₂ of 4.8:1 under real-world operating conditions of a 15:85 CO₂/N₂ mixture at 1 bar. Furthermore, <b>1</b> was investigated for shape-based selectivity of small hydrocarbons, revealing preferential uptake of linear acetylene gas over ethylene and methane, partially due to kinetic trapping of the guests with larger kinetic diameters

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

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

Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [Fe^IIM^II(CN)₄] (M^II = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [Fe^IIPt^II(CN)₄] to form the material [Fe­(H₂O)₂Fe­(DPSe)₂(Pt­(CN)₄)₂]·3EtOH (DPSe = 4,4′-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron­(II) environments, [Fe^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^II(CN)₄] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group. The magnetic properties are defined by this unusual topology such that only <b>Fe1</b> is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)

Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [Fe^IIM^II(CN)₄] (M^II = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [Fe^IIPt^II(CN)₄] to form the material [Fe­(H₂O)₂Fe­(DPSe)₂(Pt­(CN)₄)₂]·3EtOH (DPSe = 4,4′-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron­(II) environments, [Fe^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^II(CN)₄] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group. The magnetic properties are defined by this unusual topology such that only <b>Fe1</b> is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal–Organic Frameworks

Understanding the nature of charge transfer mechanisms in 3-dimensional metal–organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn­(II) frameworks containing cofacial thiazolo­[5,4-<i>d</i>]­thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus–Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both <i>in situ</i> and <i>ex situ</i> electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal–Organic Frameworks

Understanding the nature of charge transfer mechanisms in 3-dimensional metal–organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn­(II) frameworks containing cofacial thiazolo­[5,4-<i>d</i>]­thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus–Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both <i>in situ</i> and <i>ex situ</i> electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative

Thermal- and Light-Induced Spin-Crossover Bistability in a Disrupted Hofmann-Type 3D Framework

The expected 3D and 2D topologies resulting from combining approximately linear bis- or monopyridyl ligands with [Fe^IIM^II(CN)₄] (M^II = Pt, Pd, Ni) 4,4-grid sheets are well established. We show here the magnetic and structural consequences of incorporating a bent bispyridyl linker ligand in combination with [Fe^IIPt^II(CN)₄] to form the material [Fe­(H₂O)₂Fe­(DPSe)₂(Pt­(CN)₄)₂]·3EtOH (DPSe = 4,4′-dipyridylselenide). Structural investigations reveal an unusual connectivity loosely resembling a 3D Hofmann topology where (1) there are two distinct local iron­(II) environments, [Fe^IIN₆] (<b>Fe1</b>) and [Fe^IIN₄O₂] (<b>Fe2</b>), (2) as a consequence of axial water coordination to <b>Fe2</b>, there are “holes” in the [Fe^IIPt^II(CN)₄] 4,4 sheets because of some of the cyanido ligands being terminal rather than bridging, and (3) bridging of adjacent sheets occurs only through one in two DPSe ligands, with the other acting as a terminal ligand binding through only one pyridyl group. The magnetic properties are defined by this unusual topology such that only <b>Fe1</b> is in the appropriate environment for a high-spin to low-spin transition to occur. Magnetic susceptibility data reveal a complete and abrupt hysteretic spin transition (<i>T</i>_1/2↓ = 120 K and <i>T</i>_1/2↑ = 130 K) of this iron­(II) site; <b>Fe2</b> remains high-spin. This material additionally exhibits a photomagnetic response (uncommon for Hofmann-related materials), showing light-induced excited spin-state trapping [LIESST; <i>T</i>(LIESST) = 61 K] with associated bistability evidenced in a hysteresis loop (<i>T</i>_1/2↓ = 60 K and <i>T</i>_1/2↑ = 66 K)

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal–Organic Frameworks

Understanding the nature of charge transfer mechanisms in 3-dimensional metal–organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn­(II) frameworks containing cofacial thiazolo­[5,4-<i>d</i>]­thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus–Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both <i>in situ</i> and <i>ex situ</i> electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative