Timothy A. Hudson

A Simple Lithium(I) Salt with a Microporous Structure and Its Gas Sorption Properties

Much effort has been invested in studying the gas sorption properties of various classes of microporous materials such as zeolites, activated carbon materials, carbon nanotubes, polymers of intrinsic microporosity, and coordination polymers. At a time in the early 1990s when few coordination polymers had been deliberately constructed and characterized, their ability to sorb gases was a reasonable expectation. The first experimental measurements that heralded great promise of coordination polymers as materials for useful gas storage were reported by Kitagawa and co-workers in 1997. Subsequently, gas sorption by coordination polymers (more recently rebranded metal–organic frameworks (MOFs) by some) has become an intensively studied area. Low density is a most desirable characteristic of any gas storage material intended for mobile applications, and materials based on “light” metals (such as Li, Mg, and Al) are obvious targets for exploration. We report herein the synthesis, structure(s), and sorption properties of a simple salt of Li, lithium isonicotinate, which has a microporous structure and shows reversible gas uptake and release. Well-formed crystals of composition [(Li)(C6H4NO2 )]·0.5DMF (where C6H4NO2 is the isonicotinate anion, 1) can be readily obtained from DMF solution. The structure, which was determined by single-crystal X-ray diffraction, consists of a 3D [(Li)(C6H4NO2 )] network that contains microchannels occupied by DMF molecules. All the isonicotinate units are equivalent, and are associated with four Li centers, which are also all equivalent, and each associates with four isonicotinate anions. The structure can be readily envisaged in terms of [(Li)(C6H4NO2 )] chains (see Figure 1a), that are linked together by Li–N interactions into 2D sheets (see Figure 1b). The sheets in turn are linked together by Li–N interactions to form the 3D network (see Figure 1c). As can be seen in Figure 1a, each chain consists of alternating fourmembered rings (LiOLiO) and eight-membered rings (LiOCOLiOCO). The N centers of half of the pyridine units that

Closed and Open Clamlike Structures Formed by Hydrogen-Bonded Pairs of Cyclotricatechylene Anions that Contain Cationic “Meat”†

Clamming up: The hexaphenolic compound cyclotricatechylene, which has a bowl-shaped cavity, forms clamlike pairs that encapsulate cations (see picture). Variable hydrogen bonding allows two linked cyclotricatechylene clamshells to be in a closed arrangement when smaller cations such Rb(+) or Cs(+) provide the clam meat, whereas larger cations such as NMe(4) (+) and NEt(4) (+) cause the clam to be partially opened.

A new type of 3D [(MII)2(TCNQ−II)3]2− coordination network with spacious channels of hexagonal cross-section generated from TCNQH2

The dianion of tetracyanoquinodimethane, TCNQ2−, is able to act as a ligand capable of binding four metal ions at the corners of a rectangle. When the ligand is combined with divalent metal ions in the presence of appropriate counterions, infinite anionic networks may be formed of composition [M2TCNQ3]2− (M = Mn, Zn, Cd). In the structures reported here the cyano groups from six separate TCNQ dianions provide an octahedral coordination environment around the metal centres. Large hexagonal channels in the anionic network are occupied by highly disordered countercations.

Electrochemically Directed Synthesis of Cu2I(TCNQF4II–)(MeCN)2 (TCNQF4 = 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane): Voltammetry, Simulations, Bulk Electrolysis, Spectroscopy, Photoactivity, and X-ray Crystal Structure of the Cu2I(TCNQF4II–)(EtCN)2 Analogue

The new compound Cu2(I)(TCNQF4(II-))(MeCN)2 (TCNQF4(2-) = dianion of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) has been synthesized by electrochemically directed synthesis involving reduction of TCNQF4 to TCNQF4(2-) in acetonitrile containing [Cu(MeCN)4](+)(MeCN) and 0.1 M Bu4NPF6. In one scenario, TCNQF4(2-) is quantitatively formed by reductive electrolysis of TCNQF4 followed by addition of [Cu(MeCN)4](+) to form the Cu2(I)(TCNQF4(II-))(MeCN)2 coordination polymer. In a second scenario, TCNQF4 is reduced in situ at the electrode surface to TCNQF4(2-), followed by reaction with the [Cu(MeCN)4](+) present in the solution, to electrocrystallize Cu2(I)(TCNQF4(II-))(MeCN)2. Two distinct phases of Cu2(I)(TCNQF4(II-))(MeCN)2 are formed in this scenario; the kinetically favored form being rapidly converted to the thermodynamically favored Cu2(I)(TCNQF4(II-))(MeCN)2. The postulated mechanism is supported by simulations. The known compound Cu(I)TCNQF4(I-) also has been isolated by one electron reduction of TCNQF4 and reaction with [Cu(MeCN)4](+). The solubility of both TCNQF4(2-)- and TCNQF4(•-)-derived solids indicates that the higher solubility of Cu(I)TCNQF4(I-) prevents its precipitation, and thus Cu2(I)(TCNQF4(II-))(MeCN)2 is formed. UV-visible and vibrational spectroscopies were used to characterize the materials. Cu2(I)(TCNQF4(II-))(MeCN)2 can be photochemically transformed to Cu(I)TCNQF4(I-) and Cu(0). Scanning electron microscopy images reveal that Cu(I)TCNQF4(I-) and Cu2(I)(TCNQF4(II-))(MeCN)2 are electrocrystallized with distinctly different morphologies. Thermogravimetric and elemental analysis data confirm the presence of CH3CN, and single-crystal X-ray diffraction data for the Cu2(I)(TCNQF4(II-))(EtCN)2 analogue shows that this compound is structurally related to Cu2(I)(TCNQF4(II-))(MeCN)2.

Structural and optical investigations of charge transfer complexes involving the radical anions of TCNQ and F4TCNQ

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.

Mixed Valency in a 3D Semiconducting Iron–Fluoranilate Coordination Polymer

A pair of coordination polymers of composition (NBu4)2[M2(fan)3] (fan = fluoranilate; M = Fe and Zn) were synthesized and structurally characterized. In each case the compound consists of a pair of interpenetrating three-dimensional, (10,3)-a networks in which metal centers are linked by chelating/bridging fluoranilate ligands. Tetrabutylammonium cations are located in the spaces between the two networks. Despite the structural similarity, significant differences exist between (NBu4)2[Fe2(fan)3] and (NBu4)2[Zn2(fan)3] with respect to the oxidation states of the metal centers and ligands. For (NBu4)2[Fe2(fan)3] the structure determination as well as Mössbauer spectroscopy indicate the oxidation state for the Fe is close to +3, which contrasts with the +2 state for the Zn analogue. The differences between the two compounds extends to the ligands, with the Zn network involving only fluoranilate dianions, whereas the average oxidation state for the fluoranilate in the Fe network lies somewhere between -2 and -3. Magnetic studies on the Fe compound indicate short-range ordering. Electrochemical and spectro-electrochemical investigations indicate that the fluoranilate ligand is redox-active in both complexes; a reduced form of (NBu4)2[Fe2(fan)3] was generated by chemical reduction. Conductivity measurements indicate that (NBu4)2[Fe2(fan)3] is a semiconductor, which is attributed to the mixed valency of the fluoranilate ligands.

Tunable Porous Coordination Polymers for the Capture, Recovery and Storage of Inhalation Anesthetics

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 N2 O, 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.

Structural and optical investigations of charge transfer complexes involving the F4TCNQ dianion

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.

Metal Exchange within a Body-Centred Cubic Hydrogen-Bonded Network

A hydrogen bonded network of composition Zn8(C4O7)4(H2O)12·hydrate was shown to undergo a single crystal to single crystal exchange process when crystals are immersed in a concentrated aqueous solution of Ni(ii). In the exchange process, half of the zinc(ii) centres are replaced by nickel(ii) centres. The process of metal ion exchange was monitored using IR spectroscopy. Crystal structures of the transformed crystal and the product generated from a direct synthetic approach are presented.

X4TCNQ2− dianions: versatile building blocks for supramolecular systems

In 2008 a new approach to generating tetracyanoquinodimethane (TCNQ)-based materials was described which involved the use of the diprotonated, reduced form of TCNQ (TCNQH2) as a reactant. Since the initial work, the dianionic forms of TCNQH2 and F4TCNQH2 have been incorporated into a wide assortment of coordination polymers in which the ligand, with four potential donor atoms, binds to a variety of metal centres. The structures of neutral 1D, 2D and 3D coordination polymers are described, in addition to the structures of anionic networks. Not surprisingly, the oxidation state of the metal ion as well as its preference for certain coordination geometries has a major influence upon the topology and geometry of the polymeric material. In addition to the identity of the metal centre, the type of structure obtained depends upon the nature of the co-ligand in the case of neutral polymers. For anionic networks the shape, charge and size of the counter-cation impacts upon the network connectivity. The large number of metal compounds formed with the dianions is in contrast with the relatively small number of metal complexes involving TCNQ and F4TCNQ in the 0 and −1 oxidation states. In addition to coordination polymers, organic salts of TCNQ2− and F4TCNQ2− have also been investigated. The packing within these crystalline salts has been categorised into four types. In both the case of the coordination polymers and the organic salts, charge transfer interactions are common with the electron-rich TCNQ2− and F4TCNQ2− dianions often serving as electron donors. The presence of various species in the crystal that can act as electron acceptors normally leads to intensely coloured crystals. Whilst TCNQ2− and F4TCNQ2− dianions have been shown to be versatile building blocks capable of yielding a variety of unusual and aesthetically appealing structures, the redox activity of these dianions offers the prospect of creating materials that possess fascinating electronic properties. An overview of the types of structures obtained since 2008 using the TCNQH2/F4TCNQH2 synthetic approach is presented.