This dissertation describes the design and synthesis of metal-organic frameworks for applications in high-density hydrogen storage for mobile applications, and in post-combustion carbon dioxide capture from coal- or gas-fired power plants.

Chapter One introduces the area of metal-organic frameworks, which are a new class of porous coordination solids, with an emphasis on the parameters requiring optimization in order to achieve applications in hydrogen storage and carbon dioxide capture. The current state-of-the-art is briefly discussed, and strategies for the development of next-generation materials exhibiting enhanced performance are presented in the context of metal-organic frameworks. Furthermore, the benefit of high-throughput technologies in the synthesis and characterization of metal-organic frameworks is also highlighted as a potential means of accelerating the discovery of high-performance materials.

In Chapter Two, the synthesis and hydrogen storage properties of Be₁₂(OH)₁₂(BTB)₄, the first metal-organic framework based on the lightest divalent metal, Be²⁺, is described. The high surface area resulting from the use of lightweight Be²⁺ cations leads to one of the highest gravimetric hydrogen storage densities observed at both cryogenic and ambient temperatures.

Chapter Three introduces the use of a high-throughput methodology in the synthesis of an Fe²⁺-based, sodalite-type metal-organic framework, Fe₃[(Fe₄Cl)₃(BTT)₈]₂ (Fe-BTT). This material features a high-density of exposed metal cation adsorption sites on the pore surface, which facilitates strong framework-H₂ interactions that are close to the adsorption enthalpy considered optimal for hydrogen storage at ambient temperatures. The resulting hydrogen adsorption properties are discussed, as well as the effect of these adsorption sites on the carbon dioxide capture performance.

The hydrogen storage properties of Mg₂(dobdc), a lightweight metal-organic framework possessing Mg²⁺ adsorption sites, are studied in Chapter Four using a combination of adsorption experiments, infrared spectroscopy, and powder neutron diffraction data. The high affinity of H₂ toward the exposed metal cation sites results in a high enthalpy of adsorption, which is of importance in raising the storage density at ambient temperatures.

Chapter Five describes the hydrogen storage properties of Cr₃(BTC)₂, a metal-organic framework featuring pores decorated with Cr²⁺ adsorption sites. In contrast to Fe-BTT and Mg₂(dobdc), the relatively diffuse nature of the Cr²⁺ cations results in a low adsorption enthalpy at these sites, highlighting the importance of the identity of the metal ion in controlling the thermodynamics of adsorption.

Chapter Six describes a new high-throughput methodology developed for the synthesis and adsorption screening of new metal-organic frameworks for carbon dioxide capture. The use of the workflow is discussed in the context of metal-insertion reactions within the material Al(OH)(bpydc), which features one-dimensional pores lined with 2,2'-bipyridine binding sites. As will be demonstrated, the metal salt employed imparts a considerable impact on the CO₂ adsorption capacity, highlighting the benefit of a high-throughput approach to materials optimization.

The precise control of the opposing wall distribution within metal-organic frameworks is an important aspect in optimizing the adsorption properties for high-density storage and molecular separation applications. In Chapter Seven, a new method for geometrically calculating the wall separation distances from the single-crystal structure is described and employed in studying a variety of known structure types. In this case, the routine is used to analyze metal-organic frameworks for methane storage due to the abundance of high-pressure methane adsorption data in the literature, and it is demonstrated that the optimization of the wall separations is indeed crucial for maximizing the volumetric storage capacity in storage applications.

The work herein describes the design, synthesis, and characterization of magnetic molecules and chain compounds, with an emphasis on probing slow magnetic relaxation. Chapter 1 presents an extensive survey of the literature of cyano-bridged single-molecule and single-chain magnets, focusing on a building block approach wherein simple cyanometalate precursor complexes direct the assembly of larger architectures. Specific synthetic strategies to obtain multinuclear clusters and chain compounds of desired structure and magnetic properties are outlined in detail. Finally, perspectives on the future directions in the field are presented.

Chapter 2 exemplifies the utility of the building block approach in generating high-nuclearity cyano-bridged clusters. It describes the design and synthesis of the facially-capped tricyanide building unit, [TpCr(CN)3]−, as its incorporation into the face-centered cubic cluster [Tp8(H2O)6Cu6Cr8(CN)24]4+. Ferromagnetic exchange between CrIII and CuII ions gives rise to an S = 15 spin ground state, one of the highest yet observed for a cyano-bridged cluster. In addition, the formation of this cluster is accompanied by a linkage isomerism of 12 of the 24 cyanide ligands, providing the first example of a molecule undergoing partial cyanide isomerism.

Chapter 3 presents a survey of actinide-containing molecules that have demonstrated evidence of magnetic exchange coupling. The strong magnetic anisotropy characteristic of these elements marks them as promising candidates for single-molecule magnets, however the spin-orbit coupling that gives rise to this anisotropy also complicates analysis of exchange interactions. Current methods for extracting coupling information in these systems are outlined in detail. In addition, molecules that bear exchange-coupled centers but as of yet have not been thoroughly characterized are presented.

Chapter 4 describes a detailed investigation of a series of iron(II) pyrrolide complexes of formulae [(tpaR)Fe]−, representing the first examples of transition metal-based mononuclear single-molecule magnets. Static magnetic measurements and high-field EPR spectroscopy reveal the presence of exceptionally strong uniaxial anisotropy in the complexes. Moreover, dynamic magnetic measurements carried out in a small dc field demonstrate that this anisotropy leads to slow relaxation in the complexes. In addition, this relaxation dynamics is probed through Mössbauer spectroscopy, which reveals that the phenomenon occurs in zero applied field in at least two complexes.

Chapter 5 describes the synthesis and characterization of a series of cyano-bridged single-chain magnets. Reaction of the S = 3/2, high-anisotropy building unit [ReCl4(CN)2]2− with [M(DMF)6]2+ (M = Mn, Fe, Co, Ni) is shown to direct the formation of the chain compounds (DMF)4MReCl4(CN)2. Dc susceptibility measurements uncover the presence of intrachain antiferromagnetic (Mn) and ferromagnetic (Fe, Co, Ni) exchange. Most importantly, ac susceptibility measurements reveal that all of the chain compounds exhibit slow magnetic relaxation at low temperature. Notably, the Fe congener displays significant magnetic hysteresis at low temperatures, thus demonstrating classical magnet-like behavior in a one-dimensional system.

Chapter 6 describes the synthesis the incorporation of [ReCl4(CN)2]2− into the zig-zag chain compound (Bu4N)[TpCuReCl4(CN)2], which is found to demonstrate the strongest ferromagnetic exchange yet observed through cyanide. The strong coupling arises from judicious selection of ReIV and CuII ions, whose molecular orbitals interact through the cyanide bridge such that orbital overlap is minimized. Moreover, the compound is shown to display metamagnetic behavior, and the complete magnetic phase diagram is elucidated through a combination of experimental and theoretical analysis. Finally, the anisotropy tensors of the ReIV centers are shown to cancel, leading to a small effective chain anisotropy and thus the absence of single-chain magnet behavior.

Chapter 7 concludes this work by demonstrating that [ReCl4(CN)2]2− can also be employed in the assembly of molecular magnets, as it presents the synthesis and characterization of two linear trinuclear clusters of formulae [(PY5Me2)2M2ReCl4(CN)2]2+ (M = Mn, Ni; PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)-pyridine). Dc susceptibility measurements reveal the presence of antiferromagnetic exchange in the Mn congener, while ferromagnetic exchange is observed in the Ni analogue. In addition, dc magnetization experiments show the presence of axial magnetic anisotropy in both clusters.

Nanoscale materials display emergent electronic and magnetic properties that are highly correlated to their size and shape. The work in this dissertation describes efforts to synthesize nanoscale materials including molecular clusters and layered solids with exquisite control over size and shape. Particular emphasis is paid to studying the magnetic properties of these materials and how these properties relate to those of their bulk counterparts. Chapter 1 introduces basic concepts of magnetism including interactions between spin centers. The nature of these interactions dictates many magnetic properties and is essential in understanding the behavior of the materials contained within this dissertation. Furher emphasis is paid to the effects of quantum confinement on magnetic properties, and major advances in this field are provided. Finally, general strategies to purposefully control the size and shape of confined materials are highlighted along with illustrative examples. Chapter 2 describes the synthesis of atomically precise metal(II) halide clusters (M19X38, M = Fe, Co, Ni; X = Cl, Br) using the metal–organic framework Zr6O4(OH)4(bpydc)6 (bpydc2− = 2,2′-bipyridine-5,5′-dicarboxylate) as a template. Single-crystal X-ray diffraction techniques reveal that these clusters represent fragments excised from a single layer of the bulk, layered structure of the corresponding parent metal halide. Magnetometry and Mössbauer spectroscopy are then used the probe the magnetic behavior of these clusters. Remarkably, the intralayer ferromagnetic magnetic exchange pathways characteristic of the bulk materials are conserved in the clusters, leading to the isolation of high-spin magnetic ground states as well as superparamagnetism in the case of Fe19Cl38 clusters. While Chapter 2 focuses on the magnetic behavior of metal(II) halide clusters with predominantly ferromagnetic exchange coupling between spin centers, Chapter 3 focuses on the behavior of clusters with antiferromagnetic exchange coupling. In particular, the magnetic properties of a Mn19Br38 cluster are studied. In this cluster, spin centers are arranged on a triangular lattice. This topology combined with antiferromagnetic exchange correlations can be expected to lead to a geometrically frustrated magnetic ground state. Magnetometry as well as computational methods are used to probe the magnetic behavior of this cluster and confirm a highly frustrated ground state spin configuration. Chapter 4 focuses on the synthesis of a promising new metal–organic framework Cr(pz)2 (pz = pyrazine) that has exceptional magnetic properties. The framework is a layered material constructed of a square net of Cr(II) cations that are bridged by singly reduced pz radical anions. The framework is ferrimagnetic with an ordering temperature of 242 °C and a coercive field of 0.75 T at room temperature, extremely impressive parameters for a metal–organic system. Currently available synthetic methods, however, only yield a microcrystalline powder that is not amenable to in depth characterization techniques. This chapter therefore explores potential synthetic routes toward single-crystals or thin films of Cr(pz)2 using molecular precursors.

The work presented herein describes the synthesis and investigation of structurally flexible metal–organic frameworks for gas adsorption applications. These highly crystalline, porous solids are promising materials for the storage and separation of gases such as CH4, CO2, and H2. Furthermore, metal–organic frameworks are amenable to straightforward synthetic modification, due to their modular nature, and thus provide fertile ground for structure-property relationship studies.

Chapter 1 begins with a description of gas adsorption in porous materials, including activated carbons and zeolites. Metal–organic frameworks are introduced as an emerging alternative class of adsorbents, and their synthesis and hallmark structural features are discussed. Chapter 1 then provides an overview of flexible metal–organic frameworks in the literature and discusses their structural features and mechanisms of flexibility. Finally, the flexible metal–organic framework Co(bdp) is described, along with its record-setting CH4 storage properties, to set the stage for the research presented herein.

Chapter 2 reports the synthesis of a library of Co(bdp) derivatives with varying functionalizations of the bdp2- ligand, resulting in synthetic control over CH4 pressure at which Co(bdp) undergoes a phase change. Notably, fluorination of bdp2- disrupts π– π interactions that stabilize the collapsed phase of Co(bdp), while methylation of bdp2- strengthens these interactions, thus lowering or raising the phase change pressure, respectively. The structure-property relationship between ligand functionalization and CH4-induced phase change pressure is supported by in situ powder X-ray diffraction experiments.

Chapter 3 reports the effect of the identity of the adsorbate molecule on this phase change pressure (using CO2, CH4, N2, and H2) and the highly effective separation of CO2 and CH4 in Co(bdp). The mechanism of this selectivity is investigated with in situ powder X-ray diffraction studies, which reveal that Co(bdp) expands to CO2-templated phases even in the presence of CO2/CH4 mixtures. Multicomponent equilbrium adsorptions measurements, supplemented by dynamic breakthrough experiments, probe the limits of the remarkable CO2/CH4 selectivity shown by Co(bdp).

Chapter 4 describes the discovery of novel synthetic routes to the organic molecule 4,4’-bipyrazole, via the previously unknown homocoupling of pyrazoleboronic esters. This molecule is then used to synthesize the metal–organic framework Co(bpz), which is shown to be structurally flexible. CO2 adsorption and N2 adsorption are shown to induced structural phase changes in this framework, in contrast to H2 adsorption, which does not trigger a phase change, and this adsorbate-dependent phase change behavior can potentially be leveraged to accomplish gas separations in Co(bpz).

This dissertation documents efforts to control and take advantage of crystallization handles in metal–organic framework synthesis. Understanding fundamental processes during synthesis allows for directed, predictable control over crystallite size and shape, which is demonstrated to play a role in intracrystalline diffusion.

Chapter 1 first introduces a summary of crystallization and transport in porous materials, including classical and non-classical models of crystallization, homogeneous and heterogeneous nucleation, and mass transfer resistances. The application of these concepts to metal–organic frameworks is presented alongside common synthetic strategies, coordination modulation and other strategies to control crystallite size and shape, and the potential for non-coordinating bases and buffers to help alleviate some complicating factors common to framework synthesis. Finally, because crystallite size and shape can control intracrystalline path lengths, an introduction to mass transfer resistances is given, including intracrystalline diffusion. Different methods of measuring diffusion in porous materials are presented, including the technique applied in this work, zero-length column chromatography (ZLC).

Chapter 2 details success in deconvoluting solution equilibria during hydrothermal metal–organic framework synthesis. The use of non-coordinating bases and anions allows for generalizable increase in crystallite size. Further, non-coordinating buffers may be used during synthesis to add or subtract individual coordinating anions at a given pH. This strategy allows for tunable and predictable control over aspect ratio in a one-dimensional metal–organic framework.

Chapter 3 describes the discovery and utilization of interfacial effects during hydrothermal synthesis to control crystalline phase and size. Controlling the interface between reaction vessel and solution via silanization is found to decrease morphological distribution, change the phase produced for stock solutions, and in some cases, increase crystallite volume by several orders of magnitude.

Chapter 4 details the assembly and usage of a ZLC instrument capable of differentiating between different mass transfer resistances. Design considerations and instrument improvements are described. The instrument is used to probe CO2 diffusion within very large crystallites (up to 700 microns in length) of Zn2(dobdc), where surface resistances and defects are proposed to account for a lower diffusivity measured via ZLC versus pulsed-field gradient NMR. Synthetic control over Co2(dobdc) path length is demonstrated to bring about improved mass transfer resistances via path length control for the industrially important molecule m-xylene.

This dissertation describes the design, synthesis, and characterization of a wide array of transition metal complexes for application in single-molecule magnets. Magnetic relaxation in paramagnetic metal complexes proceed through several processes. This work investigates how synthetic chemistry tools can lead to a better understanding of and control over these relaxation processes, with the goal to slow down all possible relaxation pathways so that single-molecule magnets can retain their magnetization as long as possible. Chapter One provides a fundamental background of single-molecule magnet behavior in transition metal complexes and delineates important criteria to consider in designing single-molecule magnets. Two major strategies are explored. First, large spin clusters with strong magnetic exchange are explored in Chapters Two, Three, and Four with the goal to minimize through-barrier relaxation processes. Second, the effect of heavy atoms are investigated in Chapters Five and Six, with the aim to understand spin-vibronic coupling which undermines the spin-reversal barrier.

Chapter Two describes the synthesis and characterization of semiquinone-bridged dinuclear transition metal complexes of iron(II), cobalt(II), and nickel(II). In this system, tris(2-dimethylaminoethyl)amine (Me6tren) ligand scaffold enforces trigonal pyramidal geometry on the metal centers, engendering large magnetic anisotropy. This work also showcases the first magnetometry measurement of a semiquinone radical, which subsequently shows exceptionally strong magnetic direct exchange. This results in the first example of a thermally isolated large spin ground state in a multinuclear Ni complex realized in [(Me6tren)2Ni2(µ2-C6H4O2)]3+. Slow magnetic relaxation is observed in [(Me6tren)2Co2(µ2-C6H4O2)]3+ and [(Me6tren)2Ni2(µ2-C6H4O2)]3+ with an evident Orbach relaxation barrier of 22 and 46 cm–1, respectively. Through-barrier relaxation processes are partially suppressed as a result of magnetic coupling in this system.

Chapter Three describes the work on tetranuclear metal clusters [M4(NPtBu3)4]+/0 (M = Co, Ni, Cu; tBu = tert-butyl) featuring low-coordinate metal centers engaged in direct metal–metal orbital overlap. These clusters show thermally isolated large spin ground states as a result of extremely strong ferromagnetic direct exchange from magnetic orbital direct overlap and delocalized, itinerant electrons. This exchange mechanism is analogous to magnetic interactions in ferromagnetic metals. Unusually large magnetic anisotropy is observed, which is attributed to the low-coordinate environment around the cobalt and nickel centers. The [Ni4(NPtBu3)4]+ complex exhibits the first example of zero-field slow magnetic relaxation in easy-plane molecular magnets, and slow magnetic relaxation under a small applied field solely follows Orbach process. Magnetic characterization of the copper analog [Cu4(NPtBu3)4]+ reveals that the alternative spin-vibronic relaxation is sufficiently slow, thus enabling the observation of the sole Orbach barrier in [Ni4(NPtBu3)4]+ with a spin reversal barrier of 16 cm–1. The [Co4(NPtBu3)4]+ complex exhibits a thermally isolated S = 9/2 ground state and a large magnetic anisotropy D = –12.34 cm–1. The molecule exhibits a spin reversal barrier of 87 cm–1, the largest value reported for transition metal clusters.

Chapter Four describes the design, synthesis, and characterization of a series of dinuclear trigonal nickel paddlewheel complexes, Ni2DArF3 (DArF– = N,N′-diarylformamidinate). This work is motivated by the need to rationally control magnetic anisotropy in a system featuring direct metal–metal orbital overlap. The synthesis of the trigonal nickel paddlewheel complexes are described, showing the first examples of high spin nickel paddlewheel complexes. Due to the direct metal orbital overlap, the compounds exhibit thermally isolated S = 3/2 ground state. By changing the aryl substituents of the ligands, the trigonal symmetry around the metal centers can be tuned. Consequently, magnetic anisotropy of the complexes can be adjusted from D = –13 to –29 cm–1. Spin reversal barriers of 26 to 55 cm–1 are observed, with partially suppressed through-barrier relaxations.

Chapter Five describes a mononuclear triad M(CNDipp)6 (M = V, Nb, Ta; Dipp = 2,6-diisopropylphenyl) as an experimental validation of the newly proposed spin-vibronic relaxation model. These low-spin S = 1/2 isocyanide complexes exhibit slow magnetic relaxation via spin-vibronic coupling. Analysis of relaxation dynamics in this series indicates that spin-orbit coupling of the metal center facilitates spin-vibronic relaxation, as is evident by the observation that the spin relaxation rate of the tantalum complex is the fastest, followed by niobium, and vanadium complexes, respectively.

Chapter Six describes the study of heavy ligand effect on slow magnetic relaxation in two-coordinate nickel(I) complexes, (IPr)NiE(SiMe3)3 (IPr = IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazoline-2-ylidene; E = C, Si, Ge, Sn). As in previous chapter, these nickel complexes feature S = 1/2 ground state and slow magnetic relaxation arising from spin-vibronic coupling. However, partially unquenched orbital angular momentum due to the linear geometry significantly adds anisotropy to the system. The alkyl complex (IPr)NiC(SiMe3)3 exhibits notable quantum tunneling of magnetization at low temperature, and Raman relaxation at higher temperature. Upon moving to heavier ligands in (IPr)NiE(SiMe3)3 (E = Si, Ge, Sn), the complexes show significantly slower quantum tunneling and approximately five times slower Raman relaxation. This work provides an experimental evidence of the effect of heavy ligand on slow magnetic relaxation, a much-debated topic at the current time.

This dissertation describes several investigations into the use of paramagnetic ligands to drive new electronic behaviors in molecules and materials. In the field of single-molecule magnetism, radical bridging ligands are primarily of interest as a mode of generating strongly exchange-coupled magnetic units for use in high-density data storage. Incorporation of radical ligands into metal–organic frameworks may provoke a more diverse set of behaviors, including electronic conductivity, redox activity, and bulk magnetic ordering. Chapter 1 provides a perspective on the field of metal-radical coordination chemistry, highlighting some of the most notable metal-radical materials in the literature.

Chapter 2 details the study of a three-dimensional metal-radical solid composed of FeIII centers and paramagnetic semiquinoid linkers, (NBu4)2FeIII2(dhbq)3 (dhbq2–/3– = 2,5-dioxidobenzoquinone/1,2-dioxido-4,5-semiquinone). UV-Vis-NIR diffuse reflectance measurements reveal that this framework exhibits Robin-Day Class II/III ligand mixed valence, one of the first such observations in a metal–organic framework. The mixed-valence ligand manifold is shown to facilitate high electronic conductivity in addition to strong metal-ligand magnetic exchange. Slow-scan cyclic voltammetry is used to probe the redox activity of the framework, leading to synthesis of the reduced framework material Na0.9(NBu4)1.8FeIII2(dhbq)3 via a post-synthetic chemical reduction reaction. Differences in electronic conductivity and magnetic ordering temperature between the two compounds are correlated to the relative ratio of the two different ligand redox states. Overall, the transition metal-semiquinoid system is established as a particularly promising scaffold for achieving tunable long-range electronic communication in metal–organic frameworks.

In Chapter 3, a series of two-dimensional lanthanide-quinoid metal–organic frameworks of the formula Ln2(dhbq)3(DMF)x･yDMF (Ln = Y, Sm–Yb, DMF = N,N-dimethylformamide) is synthesized and post-synthetically reduced to produce the series of lanthanide-semiquinoid frameworks NaxLn2(dhbq)3(DMF)y(THF)z (THF = tetrahydrofuran). This set of lanthanide-radical frameworks is investigated using IR and UV-Vis-NIR spectroscopies, which confirm the presence of radical ligands. Magnetic susceptibility measurements indicate that lanthanide-radical magnetic exchange is relatively weak and localized. The systematic analysis of magnetic behaviors of lanthanide-radical coordination solids incorporating a series of lanthanide ions allows for new discussion regarding enhancement of lanthanide-radical magnetic exchange in extended solids.

Chapter 4 reports the synthesis and characterization of the trinuclear 4d-4f complexes [(C5Me5)2Ln(μ-S)2Mo(μ-S)2Ln(C5Me5)2][Co(C5Me5)2] (Ln = Y, Gd, Tb, Dy), containing the highly polarizable and paramagnetic MoS43− bridging unit. UV-Vis-NIR diffuse reflectance and electron paramagnetic resonance spectroscopies reveal substantial charge transfer between MoV and LnIII centers. This metal-to-metal charge transfer enables strong ferromagnetic Ln–Mo exchange, giving rise to one of the largest GdIII magnetic exchange constants, JGd–Mo, observed to date, +16.1(2) cm–1. Both the TbIII and DyIII complexes are shown to exhibit slow magnetic relaxation via ac magnetic susceptibility measurements, with the DyIII congener exhibiting the largest thermal relaxation barrier yet reported for a complex containing a 4d metal center, 68 cm–1. These results demonstrate a generalizable route to enhanced nd-4f magnetic exchange, revealing opportunities for the design of new nd-4f single-molecule magnets.

The work herein describes progress toward designing, synthesizing, and characterizing new metal–organic frameworks for gas separation applications. Metal–organic frameworks (MOFs) are a class of porous materials synthesized from molecular inorganic nodes and organic bridging linkers that can be designed to have very precise gas binding sites comprising of coordinatively unsaturated transition metal centers. These binding sites can be modified using standard coordination chemistry concepts to tune the metal site to selectively bind one gas from a given mixture, allowing for effective separations through selective adsorption. The research addresses improvements in selectivity for gas binding in several different examples and in reducing the energy input required for a separation cycle by engineering new functionalities at these transition metal binding sites in new metal–organic frameworks.

Chapter 1 details the current literature on using adsorbents for gas separation applications, with an emphasis on utilizing tunable metal–organic frameworks as high-capacity, highly selective adsorbents. Separations utilizing coordinatively unsaturated transition metals within these frameworks as selective binding sites for small molecules are highlighted, and the efficacy and mechanisms for a variety of gas separations are described. However, as shown, these metal sites generally operate through one type of mechanism in which the metal centers act as exposed, Lewis acidic cations. While effective for some separations, the types of separations that can be performed are limited. Additionally, this adsorption mechanism limits the energy efficiencies of the separation processes. As such, the performance of these materials can be improved with new design principles. The chapter concludes with describing proposed new functionalities at these open metal sites as basic design principles to either make frameworks that can perform new gas separations or design more energy efficient adsorbents.

Chapter 2 describes the design and synthesis of a new metal–organic framework containing exposed vanadium(II) sites that have been designed to contain metal centers that perform electron back-donation to π-acidic gases. This is the first example of such an electron-donating metal center with the proper electronic structure to effectively donate electrons to π* orbitals of gas molecules. This is demonstrated through the strong adsorption of N2 and characterization of the adsorbed species. In assessing it for potential gas separation performance, particularly for N2-based separations such as the removal of N2 from CH4, the material displays excellent capacity and record-setting selectivity for N2. This mechanism for adsorption is also applied to ethylene/ethane separations, where this material shows excellent selectivity at high temperatures.

Chapter 3 discusses the rational design of highly ethylene-specific binding sites in a manganese(II)-based metal–organic framework for ethylene capture in the presence of ethane, CO2, CO, CH4, and H2. In particular, this emphasizes creating a metal site with the proper ionic radius, electronegativity, and π-basicity to selectively adsorb ethylene from this complex mixture, which is produced through the oxidative coupling of methane as an alternative process to making ethylene from petroleum sources. By adsorbing ethylene selectively, several different separation steps required for this process can be eliminated, reducing the energy and capital cost significantly. In assessing a series of different materials, the choice of a high-spin Mn(II) ion to act as an effective ethylene adsorbent was demonstrated, and this material was evaluated under realistic process conditions to show very selective ethylene adsorption.

Chapter 4 focuses on a new metal–organic framework that performs selective and reversible adsorbate-induced spin state transitions at the framework metal sites. This allows for highly selective gas separations for strong field ligand gases and the reversible desorption of these gases for facile regeneration of the adsorbent. The compound features open, high-spin Fe(II) sites ligated by triazolate ligands that undergo a spin transition to low-spin Fe(II) when exposed to CO. Applying vacuum then allows the CO to be efficiently removed as the iron sites convert back to high-spin Fe(II). This allows for reversible capture of CO at very low pressures, enabling a reversible CO scrubber for purification of other gases, and also allows for the capture of highly pure CO for potential use as a reagent with some of the highest selectivity values ever reported for metal–organic frameworks.

Chapter 5 reports the realization of the first cooperative adsorbent featuring open metal sites by using a spin transition mechanism in two Fe(II)-based frameworks. Cooperative adsorption for selective adsorbents has recently been demonstrated to have several desirable energy-efficient properties in gas separation applications, but general guidelines currently do not exist for the design of such materials. This new mechanism uses communicating metal centers to perform cooperative adsorption using a concerted spin transition spin transition mechanism for highly efficient CO separations. The iron centers in these frameworks are situated in highly interconnected Fe–triazolate chains with open, high-spin Fe(II) sites, such that CO binding at one iron site prompts a spin transition that subsequently promotes adsorption at neighboring iron sites. While being selective, this adsorption process allows for high working capacities with small temperature swings, resulting in very low regeneration energies for use in gas separations. Ultimately, due to the highly tunable nature of these metal sites, we envision that this can be a broadly applicable design principle toward making next-generation, highly efficient adsorbents.

The work presented in this dissertation describes the synthesis and characterization of single chain and single molecule magnets that display slow magnetic relaxation at low temperatures. A wide variety of techniques and spectroscopic methods are covered, including SQUID measurements, high field and high frequency EPR spectroscopies, structural analysis, infrared spectroscopies. Chapter one provides a brief introduction to single chain and single molecule magnets, characterization method, and synthetic strategies toward these materials.

In Chapter Two Cyano-bridged single-chain magnets of the type L4FeReCl4(CN)2, where L = diethylformamide (DEF) (1), dibutylformamide (DBF) (2), dimethylformamide (DMF) (3), dimethylbutyramide (DMB) (4), dimethylpropionamide (DMP) (5), and diethylacetamide (DEA) (6), have been synthesized to enable a systematic study of the influence of structural perturbations on magnetic exchange and relaxation barrier. Across the series, varying the amide ligand leads to Fe-N-C bond angles ranging from 154.703(7)° in 1 to 180° in 6.Variable-temperature dc magnetic susceptibility data indicate ferromagnetic exchange coupling in all compounds, with the strength of exchange increasing linearly, from J = +4.2(2) cm–1 to +7.2(3) cm–1, with increasing Fe-N-C bond angle. Ac magnetic susceptibility data collected as a function of frequency reveal that the relaxation barriers of the chain compounds rises steeply with increasing exchange strength, from 45 cm–1 to 93 cm–1. This examination demonstrates that subtle tuning of orbital overlap, and thus exchange strength, can engender dramatic changes in the relaxation barrier. Indeed, the perfectly linear Fe-N-C bond angle in 6 leads to one of the highest barriers and coercive fields yet observed fora single-chain magnet.

Chapter Three briefly discusses model compounds (NBu4)2[ReCl4(CN)2] (1), (DMF)4ZnReCl4(CN)2 (2), and [(PY5Me2)2Mn2ReCl4(CN)2]-(PF6)2 (3) synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)4MnReCl4(CN)2 (4). High-field EPR spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1- 3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy-plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the ReIV centers that determines the easy-axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets.

In Chapter Four, molecules exhibiting bistability have been proposed as elementary binary units (bits) for information storage, potentially enabling fast and efficient computing. In particular, transition metal complexes can display magnetic bistability via either spin-crossover or single-molecule magnet behavior. I now show that the octahedral iron(II) complexes in the molecular salt [Fe(1-propyltetrazole)6](BF4)2, when placed in its high-symmetry form, can combine both types of behavior. Light irradiation under an applied magnetic field enables fully reversible switching between an S = 0 state and an S = 2 state with either up (MS = +2) or down (MS = –2) polarities. The resulting tristability suggests the possibility of using molecules for ternary information storage in direct analogy to current binary systems that employ magnetic switching and the magneto-optical Kerr effect as write and read mechanisms.

In Chapter Five, the mononuclear complex trans-(Bu4N)2[ReIVCl4(CN)2] ⋅2DMA (DMA = N,N-dimethylacetamide) with D = +11.5 cm-1 (and |E| = 3.1 cm–1) is shown to display Orbach-type slow relaxation of magnetization with an energy barrier for spin-reversal of Ueff = 26.7 cm-1 in well agreement with the spectroscopically and magnetically derived energy gap. This energy barrier represents the highest record for 4d/5d mononuclear single-molecule magnets yet observed.

In Chapter Six, one-dimensional chain solid Co(hfac)2(NITPhenOMe)(DMF)x is shown to display stronger intrachain interaction between Co(II) and radical centers with J = -97 cm-1, compared to that of -76 cm-1 in the original Co(hfac)2(NITPhOMe) compound. Further magnetic measurements revealed a energy barrier of Δτ = 251 cm-1 with magnetic blocking up to 12 K. This energy barrier and blocking temperature represents the highest record yet for any single-chain magnets reported.

The introduction of coordinatively unsaturated metal centres in porous materials has significantly enhanced the performance of these systems for many adsorptive-based applications. The work discussed in this dissertation explores the synthesis and characterisation of several metal–organic frameworks and zeolites that contain these sites for hydrogen storage and olefin/paraffin separations. An emphasis is placed on understanding both the structural and electronic factors that influence adsorption as a means to design materials capable of storing and separating gases with the efficiency required for real-world implementation. Chapter 1 briefly introduces different synthetic approaches that can be taken to produce adsorbents with open metal centres and reviews the structural and spectroscopic techniques that are typically employed to characterise these sites. A number of reports are highlighted to compare the effect that these exposed metals have on gas sorption. Notably, the position of the open metal site within the framework (e.g., at the node vs in the pore) leads to vastly different adsorptive properties, which is a topic discussed in the following chapters. Chapter 2 describes the incorporation of various light metal cations (Li+, Na+, K+, Mg2+) into the metal–organic framework Fe2(bdp)3 (bdp2– = 1,4-benzenedipyrazolate) by reduction of the coordinatively saturated Fe(III) sites to Fe(II). Single-crystal X-ray diffraction is used to locate the inserted cations, and they are found to preferentially reside near the centre of the pores when solvated by THF. However, upon activation, the cations migrate and are positioned close to the corners of the pore to maximise stabilising interactions with the framework in the absence of solvation. The effect of this is to limit the accessibility of the cations to gas molecules and to diminish the charge density felt by gases. This is demonstrated through hydrogen adsorption isotherms, which show that inclusion of these exposed cations does not improve H2 uptake or binding strength as much as was predicted. The results obtained in Chapter 2 motivated the study of porous materials with more established structures in which the location of the extra-framework cations were already precisely defined; specifically, ion-exchanged zeolites. In Chapter 3, several variants of zeolite A (Mg-A, Ca-A, Sr-A, Ba-A, Zn-A, Cu-A) are examined as potential hydrogen storage materials. Of these frameworks, Mg-A and Ca-A are found bind H2 gas with enthalpies approaching the target set by the Department of Energy for on board hydrogen storage systems. Isotherms collected at 77 and 87 K show that, at low coverage, Mg-A interacts more strongly with H2 than Ca-A. Interestingly, infrared spectroscopy suggests that more than one hydrogen molecule can interact with a single metal site in Ca-A, which has important implications for improving overall hydrogen storage capacity. Finally, Chapter 4 details the design and synthesis of a series of metal–organic frameworks, M2(X-m-dobdc) (M = Mg, Mn, Fe, Co, Ni; X = F, Cl, Br, I; m-dobdc4¬– = 4,6-dioxido-1,3,-benzenedicarboxylate), that possess open metal sites at the inorganic node rather than the pore. These materials are derivatives of the well-known M2(m-dobdc) frameworks, which have shown exceptional ability to separate ethylene from ethane and propylene from propane. The addition of an electron withdrawing halogen atom onto the ligand is shown to drastically enhance the separative performance of these materials for propylene/propane, as verified by single-component gas adsorption isotherms and IAST selectivity calculations. Of particular note is the framework Co2(F-m-dobdc), which exhibits the highest propylene/propane selectivity known for materials that separate gases using a physisorptive mechanism.