Robert W. Elliott

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.

Coordination Polymers Constructed from TCNQ2– Anions and Chelating Ligands

Coordination polymers containing tetracyanoquinodimethane (TCNQ) in its dianionic form, TCNQ–II, have been formed by combining the acid form of the dianion, TCNQH2, with divalent metal centres in the presence of chelating ligands such as 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen). When MnII or CdII is employed, two-dimensional (2D) corrugated sheet structures with the formula MII(TCNQ–II)L (M = Mn, Cd; L = bipy, phen) are obtained. In contrast, when CoII is used as the metal centre a complex three-dimensional (3D) structure of composition [CoII(TCNQ–II)(phen)] is formed. Despite the significant differences between the 2D and 3D network structures, the metal coordination geometry and the binding mode of the TCNQ dianion are very similar in all cases.

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.

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.