School of Chemistry - Research Publications
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ItemMixed valency in a neutral 1D Fe-chloranilate coordination polymer(ROYAL SOC CHEMISTRY, 2022-06-07)The syntheses and structures of a pair of neutral one-dimensional (1D) Fe-anilate based coordination polymers, Fe(Fan)(4,4'-bipy)2 (Fann- = deprotonated 3,6-difluoro-2,5-dihydroxy-1,4-benzoquinone; 4,4'-bipy = 4,4'-bipyridine) and Fe(Clan)(OPPh3)2 (Clann- = deprotonated 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone; OPPh3 = triphenylphosphine oxide), are reported. In the case of Fe(Fan)(4,4'-bipy)2, the Fe centre is in the +2 oxidation state and the Fan ligand is present in its quinoidal, dianionic form. In contrast, the structurally similar Fe(Clan)(OPPh3)2 chain contains Fe centres and chloranilate ligands in oxidation states close to +3 and -3 respectively at low temperature. It is suggested that intrachain π-π interactions aid electron transfer from the Fe centres to the bridging ligands.
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ItemMultifunctional Coordination Polymer Exhibiting Reversible Mechanical Motion Allowing Selective Uptake of Guests and Leading to Enhanced Electrical Conductivity(AMER CHEMICAL SOC, 2021-08-24)A remarkably flexible, multifunctional, 2D coordination polymer exhibiting an unprecedented mode of reversible mechanical motion, enabling pores to open and close, is reported. Such multifunctional materials are highly sought after, owing to the potential to exploit coexisting electronic and mechanical functionalities that underpin useful technological applications such as actuators and ultrasensitive detectors. The coordination polymer, of composition Mn(F4TCNQ)(py)2 (F4TCNQ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracycanoquinodimethane; py = pyridine), consists of Mn(II) centers bridged by F4TCNQ dianions and coordinated by py molecules that extend above and below the 2D network. Exposure of Mn(F4TCNQ)(py)2, in its collapsed state, to carbon dioxide results in a pore-opening process at a threshold pressure for a given temperature. In addition to carbon dioxide, a variety of volatile guests may be incorporated into the pores, which are lined with electron-rich F4TCNQ dianions. The inclusion of electron-deficient guests such as 1,4-benzoquinone, nitrobenzene, maleic anhydride, and iodine into the pores is accompanied by a striking color change associated with a new host-guest charge-transfer interaction and an improvement in the semiconductor behavior, with the iodine adduct showing an increase in conductivity of almost 5 orders of magnitude. Experimental and density functional theory calculations on this remarkable multifunctional material demonstrate a reduction in the optical band gap with increasing electron affinity of the guest.
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ItemA new fluorone-based bridging ligand for discrete and polymeric assemblies including Mo and W based [4+4] metallocycles(Royal Society of Chemistry, 2020-07-21)Redox-active ligands are of interest for their ability to link metal centres and generate electroactive materials. We report the synthesis of 9-hydrogen-2,3,7-trihydroxyfluorone, which is able to serve as a bridging ligand and has the potential to exist in multiple oxidation states. Anionic [4+4] metallocycles in which Mo or W centres are linked by the trianion of this ligand are also described.
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ItemThe elusive crystals of calcium acetate hemihydrate: chiral rods linked by parallel hydrophilic strips(Royal Society of Chemistry, 2021-01-21)Calcium acetate hemihydrate is found in the efflorescent salts that form on pottery and other calcareous heritage artefacts. The formation of these salts can lead to deterioration of these objects. A recent analysis of the structure of Ca(OAc)2·½H2O by X-ray powder diffraction (XRPD) has revealed it has a remarkable and surprisingly complex structure. Although the compound usually exists in powder or microcrystalline form, often in mixtures with other salts, we have serendipitously managed to grow crystals of a size suitable for single crystal X-ray diffraction. Our single crystal data show the structure is based on infinite supramolecular polymeric rods that are chiral. Each rod has a minimum diameter of 1.75 nm and the external surface of each rod features four parallel, hydrophobic domains separated by hydrophilic strips. Each hydrophilic strip consists of acetate oxygen atoms and coordinated water molecules that are able to form hydrogen bonding interactions with symmetry-related strips on a neighbouring rod in an arrangement that resembles a zipper. Within the extended crystal structure each rod is bound to four rods of opposite handedness to give a racemic mixture.
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ItemInducing Structural Diversity in Anionic Metal-Tetraoxolene Coordination Polymers Using Templating Methyl Viologen Countercations(AMER CHEMICAL SOC, 2022-01-18)Controlling the connectivity of coordination polymers is an important scientific goal, as the physicochemical properties of these compounds are often intimately linked to the network topology. Using redox-active methyl viologen (MeV2+) countercations, a series of one-, two-, and three-dimensional anionic coordination polymers are described in which MnII or CdII centers are bridged with tetraoxolene ligands derived from 3,6-dihalo-2,5-dihydroxy-1,4-benzoquinone (H2Xan, X = F, Cl). Using MeV2+ countercations and either MnII or CdII yields nonporous anionic diamond networks of the general composition (MeV)[M(Clan)2] in which eight-coordinate divalent metal centers are linked by Clan2- ligands. Changing the solvent mixture from acetone/water to acetonitrile/water (MeCN/H2O) afforded the same product in the case of CdII, but an anionic 2D honeycomb network with the composition (MeV)[Mn2(Clan)3]·6MeCN was obtained in the case of MnII. In contrast, the use of Fan2- ligands affords 1D ladder-type anionic coordination polymers (MeV)[M2(Fan)3(H2O)2] (M = MnII, CdII) despite the chemical and structural similarity of Fan2- and Clan2- ligands. In the case of the diamond and 2D networks, MeV2+ countercations play a key structural role, arising from C-H···O hydrogen bonding extending from the cation to the anionic network. For the 1D ladder-type structures formed with Fan2-, O-H···O hydrogen bonding between anionic [M2(Fan)3(H2O)2]2- ladders is largely responsible for directing the crystal packing. For these compounds, MeV2+ cations play a more nuanced structural role, only occupying the void space between layers of H-bonded anionic [M2(Fan)3(H2O)2]2- ladders.
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ItemSemi-conducting mixed-valent X(4)TCNQ(I-/II-)(X = H, F) charge-transfer complexes with C6H2(NH2)(4)(Royal Society of Chemistry, 2020-07-21)We report further characterisation on the previously described [C6H2(NH2)4][TCNQ] charge-transfer (CT) complex. An in-depth analysis of the crystallographic data aided by spectroscopic methods indicates the compound is mixed-valent with TCNQI−/II− species. The analogous F4TCNQ CT complex has been synthesised and spectroscopic methods suggest that [C6H2(NH2)4][F4TCNQ] is also mixed-valent. Electrical conductivity measurements on both complexes indicate semi-conductor behaviour, with [C6H2(NH2)4][TCNQ] exhibiting a σ300K = 9.8 × 10−4 S cm−1 and an Ea = 0.10(1) eV. Density functional theory studies on both CT complexes reveal band structures suggestive of ambipolar transport, with a super-exchange mechanism.
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ItemNo Preview AvailableImpact of the 2Fe2P core geometry on the reduction chemistry of phosphido-bridged diiron hexacarbonyl compounds(CSIRO PUBLISHING, 2022-02-26)The effect of core geometry constraints of hydrogenase H-cluster analogues on reduction chemistry have been explored by a combination of structural, electrochemical and IR spectroelectrochemical (IR-SEC) studies. A series of phosphido-bridged diiron hexacarbonyl complexes, Fe2(µ2-PPh2(CH2)xPPh2)(CO)6, x = 2 (2P) and 4 (4P) and previously reported with x = 3 (3P) and the unlinked bis-diphenylphosphido (DP) analogues were investigated. The X-ray structures of the neutral complexes demonstrate the effect of the linking group on the Fe2P2 core geometry with P–Fe–Fe–P torsion angles of 95 (2P), 101 (3P), 108 (4P) and 109° (DP) and a twisting of the Fe(CO)3 fragments from an eclipsed geometry (2P, 3P and DP) for 4P. For all four compounds the primary reduction process involves two close-spaced one-electron reactions (E1 and E2) with a systematic trend to more negative reduction potentials with a shorter link between the bridging phosphorus atoms. This reflects the greater constraint that the bridging group places on the adoption of a planar 2Fe2P geometry. The sensitivity of the core geometry is greater for E2 than E1 and this impacts the stability of the monoanion with respect to disproportion (Kdisp(298 K) = 0.02 (2P), 2.4 (3P) and 3540 (4P and DP)). 4P has a stable dianion and gives reversible cyclic voltammetry at 298 K and is quasi-reversible at 253 K, whereas the response of 2P is irreversible at 298 K, with two distinct daughter products, but becomes quasi-reversible at 253 K. IR-SEC measurements enabled elucidation of the spectra and time evolution of the reduction products. These results are consistent with a bimolecular reaction giving a distinct reduced product modelled as a dimeric, 4Fe species. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions.
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ItemFlexible Vertex Engineers the Controlled Assembly of Distorted Supramolecular Tetrahedral and Octahedral Cages(AMER ASSOC ADVANCEMENT SCIENCE, 2022-02-24)Designing and building unique cage assemblies attract increasing interest from supramolecular chemists but remain synthetically challenging. Herein, we propose the use of a flexible vertex with adjustable angles to selectively form highly distorted tetrahedral and octahedral cages, for the first time, in which the flexible vertex forms from the synergistic effect of coordination and covalent interactions. The inherent interligand angle of the vertex can be modulated by guest anions present, which allows for the fine-tuning of different cage geometries. Furthermore, the reversible structural transformation between tetrahedral and octahedral cages was achieved by anion exchange monitored by mass spectrometric technique, the smaller anions favoring tetrahedral cages, while the larger anions supporting octahedral cages. Additionally, the KBr-based cage thin films exhibited prominent enhancement of their third-order NLO responses in two or three orders of magnitude compared to those obtained for their corresponding solutions. This work not only provides a new methodology to build irregular polyhedral structures in a controlled and tunable way but also provides access to new kinds of promising functional optical materials.
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ItemSolid-State Gas Adsorption Studies with Discrete Palladium(II) [Pd-2(L)(4)](4+) Cages(Wiley, 2017-08-04)The need for effective CO2 capture systems remains high, and due to their tunability, metallosupramolecular architectures are an attractive option for gas sorption. While the use of extended metal organic frameworks for gas adsorption has been extensively explored, the exploitation of discrete metallocage architectures to bind gases remains in its infancy. Herein the solid state gas adsorption properties of a series of [Pd2(L)4]4+ lantern shaped coordination cages (L = variants of 2,6‐bis(pyridin‐3‐ylethynyl)pyridine), which had solvent accessible internal cavities suitable for gas binding, have been investigated. The cages showed little interaction with dinitrogen gas but were able to take up CO2. The best performing cage reversibly sorbed 1.4 mol CO2 per mol cage at 298 K, and 2.3 mol CO2 per mol cage at 258 K (1 bar). The enthalpy of binding was calculated to be 25–35 kJ mol−1, across the number of equivalents bound, while DFT calculations on the CO2 binding in the cage gave ΔE for the cage–CO2 interaction of 23–28 kJ mol−1, across the same range. DFT modelling suggested that the binding mode is a hydrogen bond between the carbonyl oxygen of CO2 and the internally directed hydrogen atoms of the cage.
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ItemTunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics(Wiley, 2017-06-12)The uptake of inhalation anesthetics by three topologically identical frameworks is described. The 3D network materials, which possess square channels of different dimensions, are formed from the relatively simple combination of ZnII centres and dianionic ligands that contain a phenolate and a carboxylate group at opposite ends. All three framework materials are able to adsorb N2O, Xe and isoflurane. Whereas the framework with the widest channels is able to adsorb large quantities of the various guests from the gas phase, the frameworks with the narrower channels have superior binding enthalpies and exhibit higher levels of retention. The use of ligands in which substituents are bound to the aromatic rings of the bridging ligands offers great scope for tuning the adsorption properties of the framework materials.