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Metal-organic cages are a class of supramolecular structures that often require the careful selection of organic linkers and metal nodes. Of this class, few examples of metal-organic cages exist where the nodes are composed of main group metals. Herein, we have prepared an aluminum-based metal-organic cage, H

We report the synthesis of novel charge transfer complexes consisting of TTF or TTN, and DPNI. A spectroscopic and computational approach is taken to elucidate charge transfer in these complexes.

The synthesis, structure and properties of a 2D FeIII network material of composition [Fe(O-bipy)2Cl](TCNQ)2 (O-bipy = 4,4'-bipyridyl N,N'-dioxide; TCNQ = 7,7,8,8-tetracyanoquinodimethane) are presented. The structure consists of a square grid in which square pyramidal FeIII centers are linked to four equivalent centers through O-bipy ligands. Pairs of TCNQ radical anions are located within rectangular cavities of the 2D network. The apical position on each Fe center is occupied by a chloride ion. Magnetic studies are indicative of weak antiferromagnetic coupling. Electrical conductivity measurements reveal that the compound is a semiconductor and on the basis of DFT calculations it is proposed that electrical conduction occurs via charge hopping between interlayer TCNQ anions. Mossbauer spectra indicate a rare example of the Goldanskii–Karyagin effect and are suggestive of unusual dynamic processes.

The chronoamperometric response (I vs t) of three metallocene-doped metal-organic frameworks (MOFs) thin films (M-NU-1000, M = Fe, Ru, Os) in two different electrolytes (tetrabutylammonium hexafluorophosphate [TBAPF6] and tetrabutylammonium tetrakis(pentafluorophenyl)borate [TBATFAB]) was utilized to elucidate the diffusion coefficients of electrons and ions (De and Di, respectively) through the structure in response to an oxidizing applied bias. The application of a theoretical model for solid state voltammetry to the experimental data revealed that the diffusion of ions is the rate-determining step at the three different time stages of the electrochemical transformation: an initial stage characterized by rapid electron diffusion along the crystal-solution boundary (stage A), a second stage that represents the diffusion of electrons and ions into the bulk of the MOF crystallite (stage B), and a final period of the conversion dominated only by the diffusion of ions (stage C). Remarkably, electron diffusion (De) increased in the order of Fe < Ru < Os using PF61- as the counteranion in all the stages of the voltammogram, demonstrating the strategy to modulate the rate of electron transport through the incorporation of rapidly self-exchanging molecular moieties into the MOF structure. The De values obtained with larger TFAB1- counteranion were generally in agreement with the previous trend but were on average lower than those obtained with PF61-. Similarly, the ion diffusion coefficient (Di) was generally higher for TFAB1- than for PF61- as the ions diffuse into the crystal bulk, due to the high degree of ion-pair association between PF61- and the metallocenium ion, resulting in a faster penetration of the weakly associated TFAB1- anion through the MOF pores. These structure-function relationships provide a foundation for the future design, control, and optimization of electron and ion transport properties in MOF thin films.

Carbon dioxide (CO2) capture and separation in sorbent materials relies on the interactions that take place between guest molecules and topological or chemical features of the sorbent. Although researchers have developed an immense library of metal–organic framework (MOF) designs, which range in shape, size, and chemical functionality, a molecular-level understanding of how MOF structure and functionality affect capture and transport has yet to be fully achieved. In this work, in situ infrared spectroscopic techniques, coupled with density functional theory, were used to provide insights into the binding locations and energetics of CO2 on the zirconium-based MOF, UiO-66. Two unique CO2 binding motifs in UiO-66 were identified through differing vibrational frequencies of the asymmetric OCO stretching mode. One configuration is established through hydrogen bonding with μ3-OH groups on the MOF nodes, and a second binding mode exists where CO2 is stabilized through dispersive interactions. Variable temperatur...

The reaction of zirconium salts with meso-tetra(4-carboxyphenyl)porphyrin (TCPP) in the presence of different modulators results in the formation of a diverse set of metal–organic frameworks (MOFs), each displaying distinct crystalline topologies. However, the synthesis of phase-pure crystalline frameworks remains challenging due to the concurrent formation of different polymorphs. The acidity and concentration of the modulator greatly influence the outcome of the MOF synthesis. By systematically varying these two parameters, selective framework formation can be achieved. In the present study, we aimed to elucidate the effect of modulator on the synthesis of zirconium-based TCPP MOFs. With the help of powder X-ray diffraction and scanning electron microscopy, modulator candidates and the optimal synthetic conditions yielding phase-pure PCN-222, PCN-223, and MOF-525 were identified. 1H nuclear magnetic resonance analysis, thermogravimetric analysis, and N2 gas sorption measurements were performed on select...

The metal-organic framework (MOF), UiO-66, contains Brønsted acidic and Lewis acidic sites that play an important role in sorption, separation, and potential catalytic processes involving small gaseous molecules. As such, studies into the sequestration and separation of CO within UiO-66 provides a fundamental understanding of small gas molecule adsorption within a highly porous, tunable and environmentally stable MOF. Infrared spectroscopic measurements, in combination with density functional theory, were employed to characterize the binding energetics between bridging hydroxyl groups at MOF nodes and the adsorbate, CO. Two unique binding configurations between CO and the μ3-OH groups were identified based on differing stretching vibrations of COads when interacting through the C- and O-atom of the molecule. Variable temperature infrared spectroscopy (VTIR) was employed to attain energetics of CO adsorption (-17 kJ mol-1) and isomerization from the carbonyl to the isocarbonyl configuration (4 kJ mol-1). Results suggest that CO-hydroxyl interactions, while weak in nature, play a critical role in CO adsorption within the confined pore environment of UiO-66.

Metal-organic frameworks (MOFs) are crystalline materials constructed from metal ions or clusters, connected by multidentate organic ligands. Herein, we describe the synthesis and photophysical properties of two Zr-based, anthracene-containing MOFs, assembled from 2,6-anthracenedicarboxylic acid (2,6-ADCA and 2,6-MOF) and 1,4-anthracenedicarboxylic acid (1,4-ADCA and 1,4-MOF). The 2,6-ADCA analogue formed a highly crystalline octahedral structure that is isostructural with the well-known UiO-67 frameworks. Incorporation of the 1,4-ADCA ligand, on the other hand, resulted in large rod-shaped crystals. Both MOFs exhibit linker-based luminescence. The excited-state properties of the 2,6-MOF and 1,4-MOF were examined using stead-state diffuse reflectance and emission spectroscopies and time-correlated single photon counting (TCSPC) spectroscopy. The photophysical properties of the MOFs are compared with those of the corresponding ligand in solution. U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0012446] This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0012446.

The high-energy demand of benzene, toluene, and xylene (BTX) separation highlights the need for improved nonthermal separation techniques and materials. Because of their high surface areas, tunable structures, and chemical stabilities, metal–organic frameworks (MOFs) are a promising class of materials for use in more energy efficient, adsorption-based separations. In this work, BTX compounds in the pore environment of UiO-66 were systematically examined using in situ infrared (IR) spectroscopy to understand the fundamental interactions that influence molecular transport through the MOF. Isothermal diffusion experiments revealed BTX diffusivities between 10–8 and 10–12 cm2 s–1, where the rate follows the trend: o-xylene < m-xylene < p-xylene. Corresponding activation energies of diffusion (Ediff) were determined to be 44 kJ mol–1 for the xylene isomers and 34 kJ mol–1 for both benzene and toluene, with the diffusion-limiting barrier identified to be molecular passage through the small triangular pore apert...

The decomposition of H2O2 to H2O and O2 catalyzed by platinum nanocatalysts controls the energy yield of several energy conversion technologies, such as hydrogen fuel cells. However, the reaction mechanism and rate-limiting step of this reaction have been unsolved for more than 100 years. We determined both the reaction mechanism and rate-limiting step by studying the effect of different reaction conditions, nanoparticle size, and surface composition on the rates of H2O2 decomposition by three platinum nanocatalysts with average particle sizes of 3, 11, and 22 nm. Rate models indicate that the reaction pathway of H2O2 decomposition is similar for all three nanocatalysts. Larger particle size correlates with lower activation energy and enhanced catalytic activity, explained by a smaller work function for larger platinum particles, which favors chemisorption of oxygen onto platinum to form Pt(O). Our experiments also showed that incorporation of oxygen at the nanocatalyst surface results in a faster reactio...

The highly studied metal–organic framework (MOF), UiO-66, has been touted for its high surface area, stability, and tunability of structural defects for particular applications in catalysis and gas storage. However, atomic-level characterization and quantification of defects remain challenging for traditional methods. Motivated by this knowledge gap, coordinatively unsaturated zirconium (Zrcus) defect sites within UiO-66 have been determined through infrared spectroscopic studies of the highly sensitive probe molecule, carbon monoxide. A characteristic blue shift of the vibrational frequency for CO adsorbed at nodes within the MOF was used to identify the presence of Zrcus defects. The identification of these coordinatively unsaturated metal sites was further corroborated via exposure of the MOF to the Lewis base, D2O, which was shown to block CO binding to the Lewis acidic Zrcus sites. The infrared spectroscopic signature for CO binding at the Zrcus sites was further employed to track the emergence of mi...

Metal–organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr6 clusters and metallocyclams as organic linkers. Importantly, the cyclam core has accessible axial coordination sites for guest interactions and maintains the electronic properties exhibited by the parent cyclam ring. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO2 uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of t...

Investigation of chemical water oxidation was conducted on [Ru(tpy)(dcbpy)(OH2 )]2+ (tpy=2,2':6',2''-terpyridine, dcbpy=5,5'-dicarboxy-2,2'-bipyridine)-doped UiO-67 metal-organic framework (MOF). The MOF catalyst exhibited a single-site reaction pathway with kinetic behavior similar to that of a homogeneous Ru complex. The reaction was first order with respect to both the concentration of the Ru catalyst and ceric ammonium nitrate (CAN), with kcat =3(±2)×10-3 m-1 s-1 in HNO3 (pH 0.5). The common degradation pathways of ligand dissociation and dimerization were precluded by MOF incorporation, which led to sustained catalysis and greater reusability as opposed to the molecular catalyst in homogeneous solution. Lastly, at the same loading (ca. 97 nmol mg-1 ), samples of different particle sizes generated the same amount of oxygen (ca. 100 nmol), indicative of in-MOF reactivity. The results suggest that the rate of redox-hopping charge transport is sufficient to promote chemistry throughout the MOF particulates.

Metal–organic frameworks (MOFs) have shown great promise as efficient CO2 adsorbents, as well as an emerging class of heterogeneous catalysts for conversion of CO2 to other useful chemicals. In this work, we synthesized and characterized two isostructural hafnium-based MOFs, denoted as Hf-VPI-100 (Cu) and Hf-VPI-100 (Ni). Both frameworks demonstrated high catalytic efficiency for cycloaddition of CO2 to epoxides under ambient pressure. In situ PXRD, QCM and DRIFTs have been used to probe the interaction between the guest molecules (CO2/epoxide) and Hf-VPI-100. The crystal structures of these frameworks were preserved during the exposure to CO2 atmosphere up to 20 bar. The epoxide uptake per unit cell of VPI-100 and diffusion coefficients have been calculated by QCM analysis. Comparison of catalytic efficiency between Hf-VPI-100 and the previously reported Zr-based VPI-100, aided by electronic structure calculations revealed that the open metal centers in the metallocyclam act as the primary Lewis acid sites to facilitate the catalytic conversion of CO2.

This paper reports additional properties of the electron donor molecule triselenathiafulvalene (TSTF) and its synthesis via a new route involving the precursor tetracarbomethoxytriselenathiafulvalene (TCMTSTF). The structural, electronic and computational properties of TSTF are fully profiled, and discussed in the context of the closely related archetypal electron donor molecule tetrathiafulvalene (TTF). TSTF was also incorporated into a charge transfer (CT) complex with the well-known electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ), to generate TSTF–TCNQ. In order to establish its position in the greater TXF–TCNQ series (where TXF = a tetrachalcogenafulvalene), spectral, electrochemical, magnetic, conductivity and computational studies were performed on TSTF and TSTF–TCNQ to extend what is known about the highly interesting and useful new donor molecule.

A combined spectroscopic and structural study was undertaken to investigate the nature of the incorporation of the electron acceptor guest 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the closely related guest tetracyanoethylene (TCNE) into the host porous framework [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxylate)—a guest–host system recently shown to be highly conductive. We find that the guest concentration in the system can be modulated via the synthesis reaction time and temperature. A suite of spectroscopic, X-ray and neutron powder diffraction, and density functional theory techniques revealed the mechanism of guest binding within the framework host, including the guest redox states. This work provides insights into the way that electrical conductivity arises in porous framework host–guest systems and contributes to understanding how fine-tuning framework properties influences conductivity.

Two zeolitic imidazolate frameworks, ZIF-67 and ZIF-8, were interrogated for their redox properties using Fourier transformed alternating current voltammetry, which revealed that the 2-methylimidazolate ligand is responsible for multiple redox transformations. Further insight was gained by employing discrete tetrahedral complexes, [M(DMIM)4]2+ (DMIM = 1,2-dimethylimidazole, M = CoII or ZnII) which have similar structural motifs to ZIFs. In this work we demonstrate a multidirectional approach that enables the complex electrochemical behavior of ZIFs to be unraveled.

A highly robust metal–organic framework (MOF) constructed from Zr6 oxo clusters and Fe(III) porphyrin linkers, PCN-223-Fe was investigated as a heterogeneous catalyst for oxygen reduction reaction (ORR). Films of the framework were grown on a conductive FTO substrate and showed a high catalytic current upon application of cathodic potentials and achieved high H2O/H2O2 selectivity. In addition, the effect of the proton source on the catalytic performance was also investigated.

Kinetic assembly is an important method for obtaining desired materials in chemical synthesis and material sciences. However, the application of this strategy to porous coordination networks has been limited. We highlight the kinetic assembly of porous coordination networks, which promote the production of interactive pore sites. These sites can activate or stabilize different guest molecules. The properties of interactive pores are exemplified by iodine chemisorption and small sulfur encapsulation. Using the interactive feature of these pores, we were able to trap small sulfur allotropes, such as S2, cyclo-S3, bent-S3, and S6, demonstrating their importance for the stabilization of unusual elemental species. Furthermore, several reactive elemental allotropes could also be incorporated into the interactive pores. Herein, we address the important aspects of creating interactive pore sites by kinetic assembly of porous coordination networks and present detailed case-by-case studies of small allotrope encapsulation.

Formation of coordination networks is a complex process affected by a multitude of factors. Many synthetic strategies have been developed that attempt to control these factors and direct the structure of the final product. Coordination bond formation and structural assembly processes, however, typically take place either in the solution or solid states. In comparison, gas-phase network synthesis remains largely unexplored. Herein, two new two-dimensional coordination networks are obtained from the solid-gas phase reaction between ZnX2 (X=I, Br) and the redox-active 2,5,8-tri(4-pyridyl)1,3-diazaphenalene (HTPDAP) ligand. Their structures were solved by ab initio powder X-ray diffraction analysis and feature a novel Zn halide trimeric cluster. This strategy is contrasted with a conventional solvothermal synthesis, which led to a one-dimensional coordination polymer instead. The intrinsic electroactive properties of these materials were probed by solid-state cyclic voltammetry measurements, which revealed the presence of HTPDAP and halide-based processes. Chemical oxidation of the two-dimensional networks by using NOPF6 agent, unexpectedly, led to the formation of a nitrated analog of HTPDAP, the PF6- salt of diprotonated 4,6,7,9-tetranitro-2,5,8-tris(4-pyridyl)diazaphenalene cation (denoted N-TPDAP), which was isolated and characterized. These results provide deeper insights into the oxidation process of HTPDAP-containing networks and uncover unique redox-induced chemical transformations.

The development of metal–organic frameworks (MOFs) as microporous electronic conductors is an exciting research frontier that has the potential to revolutionize a wide range of technologically and industrially relevant fields, from catalysis to solid-state sensing and energy-storage devices, among others. After nearly two decades of intense research on MOFs, examples of intrinsically conducting MOFs remain relatively scarce; however, enormous strides have recently been made. This article briefly reviews the current status of the field, with a focus on experimental milestones that have shed light on crucial structure–property relationships that underpin future progress. Central to our discussion are a series of design considerations, including redox-matching, donor–acceptor interactions, mixed valency, and π-interactions. Transformational opportunities exist at both fundamental and applied levels, from improved measurement techniques and theoretical understanding of conduction mechanisms to device engineering. Taken together, these developments will herald a new era in advanced functional materials.

Spectroelectrochemistry (SEC) encompasses a broad suite of electroanalytical techniques where electrochemistry is coupled with various spectroscopic methods. This powerful and versatile array of methods is characterised as in situ, where a fundamental property is measured in real time as the redox state is varied through an applied voltage. SEC has a long and rich history and has proved highly valuable for discerning mechanistic aspects of redox reactions that underpin the function of biological, chemical, and physical systems in the solid and solution states, as well as in thin films and even in single molecules. This perspective article highlights the state of the art in solid-state SEC (ultraviolet–visible–near-infrared, infrared, Raman, photoluminescence, electron paramagnetic resonance, and X-ray absorption spectroscopy) relevant to interrogating solid state materials, particularly those in the burgeoning field of metal–organic frameworks (MOFs). Emphasis is on developments in the field over the past 10 years and prospects for application of SEC techniques to probing fundamental aspects of MOFs and MOF-derived materials, along with their emerging applications in next-generation technologies for energy storage and transformation. Along with informing the already expert practitioner of SEC, this article provides some guidance for researchers interested in entering the field.

Solid-state electrochemiluminescence (ECL) has drawn increasing attention due to its advantages over solution-phase ECL, such as reducing the consumption of expensive reagents and enhancing the ECL signal. Herein we report a ruthenium(ii)-polypyridyl doped zirconium(iv) metal-organic framework (MOF) film, UiO-67-Ru@FTO, for solid-state electrochemiluminescence. With tripropylamine (TPA) as a coreactant, UiO-67-Ru@FTO exhibited high ECL intensity and good stability. A linear relationship was found between the ECL intensity and TPA concentration in a wide range of 0.04-20 mM. Additionally, UiO-67-Ru@FTO was successfully used for dopamine detection, implying its great potential in real-life applications.

The dominant charge transfer mechanism in a vast number of metal–organic frameworks (MOFs) is that of redox hopping, a process best explained through the motion of electrons via self-exchange reactions between redox centers coupled to the motion of counter-balancing ions. Mechanistic studies of redox hopping transport in MOFs reveal characteristics that recall pioneering studies in linear redox polymers. When MOFs are employed as electrocatalysts, consideration must be given to both the catalytic properties – turn-over frequency (TOF) and energetic requirements (overpotential, TON) – and the charge transport properties – rate of charge hopping, measured via an apparent diffusion coefficient (Dapp). Herein, we provide a mathematical framework to provide constraints to MOF catalyst development by relating Dapp, TOF, and film thickness in the context of providing 10 mA cm−2 of catalytic current. Lastly with the mechanistic studies discussed as a foundation, design rules for future MOF electrocatalysts are provided and the challenges to the community to optimize MOF charge transport are laid out.

Coordination frameworks containing DCNQI2– (DCNQI = N,N′-dicyanoquinonediimine ligand) are produced by deprotonation of DCNQIH2 in the presence of a metal center and a co-ligand. This approach has yielded two-dimensional (2D) sheet compounds [Cd(DCNQI)(L)2] (where L = pyridine (py) or isoquinoline (isoquin)) that can be partially oxidized via solid-state electrochemical and in situ spectroelectrochemical methods to materials that contain DCNQI as its radical monoanion. The new frameworks display charge-transfer bands that are indicative of interligand charge-transfer interactions as supported by TD-DFT computational calculations. The redox-state dependent spectral properties of the frameworks have been probed using a newly developed solid-state spectroelectrochemical cell. Coupled with computational calculations, the experimental data provide an understanding of the fundamental charge-transfer processes that may underpin long-range functional properties such as conductivity in framework materials.

The structures and optical band gaps of twelve radical anionic 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 7,7,8,8-tetracyano-2,3,4,5-tetrafluoroquinodimethane (F4TCNQ) based charge-transfer complexes are reported. The compounds described have been categorised into three general types based upon solid-state arrangements of the donor and acceptor molecules. Crystallographic, EPR and IR spectroscopic investigations indicated that both TCNQ and F4TCNQ in each of the compounds described exist in the radical monoanion form. Visible-NIR absorption measurements indicate optical band gaps in the range of 0.79 to 1.08 eV. Whilst the packing arrangements in CT complexes are known to affect the band gap, in the cases considered here no clear relationship between the packing arrangement and the optical band gap is apparent. The results suggest that in the absence of mixed valency the packing arrangement does not impact significantly upon the magnitude of the optical band gap.

Here, we demonstrate a new strategy for cooperative catalysis and proton abstraction via the incorporation of independent species competent in the desired reactivity into a metal–organic framework (MOF) thin film. The highly porous MOF, designated as PCN-224-Ni, is constructed by Zr–oxo nodes and nickel(II) porphyrin linkers. Films of PCN-224-Ni were grown in situ on FTO and were found to electrochemically facilitate the water oxidation reaction at near neutral pH.

Metal-Organic Frameworks (MOFs) can serve as precursors to a range of useful materials including porous catalysts and supercapacitors. The electrochemical properties of the ZIF-67 framework were investigated in aqueous electrolytes under neutral and basic pH conditions in the presence of different anions. The redox transformations resulted in both crystalline powders and thin films deposited on the working electrodes. In the latter case, catalytic oxidation of water was observed during the electrochemical measurements due to the formation of Co 3 O 4 films. ZIF-67 was found to be susceptible to ligand substitution of 2-methylimidazolate by hydroxide and phosphate anions in the electrolytes.

Metal-organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr

The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4-dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal–organic frameworks (MOFs; namely, 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF, respectively). The Pourbaix diagrams (E vs pH) of the MOFs and their ligands were constructed using cyclic voltammetry in aqueous buffered media. The MOFs exhibit chemical stability and undergo diverse electrochemical processes, where the number of electrons and protons transferred was tailored in a Nernstian manner by the pH of the media. Both the 2,6-Zr-AQ-MOF and its ligand reveal a similar electrochemical pKa value (7.56 and 7.35, respectively) for the transition between a two-electron, two-proton transfer (at pH pKa). In contrast, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the amount of defects in 1,4-Zr-AQ-MOF lead to the transition from a two-electron, three-proton transfer to a t...

Investigation of chemical water oxidation was conducted on [Ru(tpy)(dcbpy)(OH

Electronic conductivity in framework materials is a highly-sought after functional property, with the capacity to revolutionise a wide range of technologically- and industrially-useful fields. Examples of conducting metal-organic frameworks remain relatively limited at the present time; however enormous advances have been made over the past five years. This chapter details the latest developments in the exciting research frontier of electronically-conducting metal-organic frameworks (MOFs). Central to the discussion is a series of parameters which have now emerged for materials’ design, including redox matching, donor–acceptor, mixed-valence, and π-interactions. At both the fundamental and applied levels, enormous opportunities exist if long-range electronic conductivity can be harnessed, including intimate understandings of charge transport in 3D coordination space, and potential applications across catalysis, solid-state sensing, energy storage and conversion devices, amongst numerous others.

The synthesis, as well as the electrochemical, optical and magnetic properties of a sulfate-bridged Co(II) dimer, [Co2(DMIM)4(μ2-O2,O,O′-SO4)2]·2MeCN (DMIM = 1,2-dimethylimidazole) (2), are reported. The crystal structure consists of two distorted pseudo-octahedral Co(II) centres, each ligated by the μ2-(O-O′-sulfato) and μ2-(O2-sulfato) bridging modes of the SO42− anions. Magnetic studies have identified significant antiferromagnetic coupling between the high spin Co(II) centres, with J = −28 cm−1. Cyclic voltammetry indicated the presence of a quasi-reversible ligand centred oxidation, while the vis-NIR spectrum of 2 revealed complex splitting of the absorption bands due to the tetragonal distortion of the octahedral geometry of the Co(II) centres.

The metalloligand [Ni(pedt)2](-) (pedt = 1-(pyridine-4-yl)ethylene-1,2-dithiolate) has been incorporated into two multi-dimensional structures for the first time. These coordination frameworks represent highly unusual interpenetration isomers and exhibit solid state redox and optical properties that reflect the electronically delocalised nature of the metalloligand.

A new microporous framework, Zn(NDC)(DPMBI) (where NDC = 2,7-naphthalene dicarboxylate and DPMBI = N,N'-di-(4-pyridylmethyl)-1,2,4,5-benzenetetracarboxydiimide), containing the redox-active benzenetetracarboxydiimide (also known as pyromellitic diimide) ligand core has been crystallographically characterised and exhibits a BET surface area of 608.2 ± 0.7 m(2) g(-1). The crystallinity of the material is retained upon chemical reduction with sodium naphthalenide (NaNp), which generates the monoradical anion of the pyromellitic diimide ligand in the framework Zn(NDC)(DPMBI)·Na(x) (where x represents the molar Na(+)/Zn(2+) ratio of 0.109, 0.233, 0.367 and 0.378 from ICP-AES), as determined by EPR, solid state Vis-NIR spectroelectrochemistry and UV-Vis-NIR spectroscopy. The CO2 uptake in the reduced materials relative to the neutral framework is enhanced up to a Na(+)/Zn(2+) molar ratio of 0.367; however, beyond this concentration the surface area and CO2 uptake decrease due to pore obstruction. The CO2 isosteric heat of adsorption (|Q(st)|) and CO2/N2 selectivity (S), obtained from pure gas adsorption isotherms and Ideal Adsorbed Solution Theory (IAST) calculations, are also maximised relative to the neutral framework at this concentration of the alkali metal counter-ion. The observed enhancement in the CO2 uptake, selectivity and isoteric heat of adsorption has been attributed to stronger interactions between CO2 and both the radical DPMBI ligand backbone and the occluded Na(+) ions.

7,7,8,8-Tetracyano-2,3,4,5-tetrafluoroquinodimethane (F4TCNQ) in its dianionic form, F4TCNQ2−, is shown to form charge transfer complexes with a wide variety of organic cations. The structures and spectroscopic properties of fourteen F4TCNQ2− salts are described, thirteen of which have colours consistent with the formation of charge transfer complexes. Unlike neutral F4TCNQ charge transfer complexes, the dianion, F4TCNQ2− is able to act as a donor in its interaction with suitable cations that serve as acceptors in solid-state complexes. The F4TCNQ2− salts described in this work have been categorised into five different structural types according to the relative arrangements of cations and anions. In each case, structural and IR spectroscopic data indicate that the anions retain a formal −2 charge upon formation of the salt. The optical band gaps, determined from Vis-NIR spectra, are found to have the lowest values when the cation is a viologen, either methyl viologen or diphenylmethyl viologen.

Systematic studies of the thermal expansion, optical, and redox properties of a series of six squarate-based frameworks, [MII(C4O4)(H2O)2] (MII = MnII, FeII, CoII, NiII, ZnII, CdII) have revealed that five members of the series exhibit cubic structures in which the squarate ligands are configured in an ‘eclipsed’ phase, while the CdII analogue exhibits a trigonal structure with a ‘staggered’ orientation of the ligands. The ‘eclipsed’ structures are characterised by a positive coefficient of thermal expansion, while the CdII analogue exhibits zero thermal expansion. Ultraviolet-visible-near infrared (UV-Vis-NIR) spectra and electrochemical measurements indicate that electron delocalisation across the dianionic squarate bridge is absent.