TCNQ Derivatives Research Articles

Volumetric Electron Transfer from Metabolites to Chemically Doped Polymer Electrodes

AbstractThe development of sensor electrode materials for the detection of metabolites will enable point‐of‐care diagnostic devices for the monitoring and treatment of metabolic diseases such as diabetes. Current state‐of‐the‐art glucose sensing electrodes employ the organic salt tetrathiafulvene tetracyanoquinodimethane (TTF TCNQ) to receive electrons directly from enzymatic reactions of glucose. However, TTF TCNQ is insoluble in most solvents, making it challenging to deposit high‐quality electrodes. Furthermore, its hydrophobicity hinders its interface with aqueous solutions in physiological environments. To overcome these issues, TCNQ derivatives are introduced into an electron‐rich and hydrophilic conjugated polymer. Thus, a polymeric electrode is demonstrated that is easily solution processible and can undergo volumetric direct electron transfer with glucose reactions throughout its bulk. This study further elucidates the electron transfer mechanism during chemical doping and metabolite sensing reactions to inform general design rules for this new class of glucose sensing materials.

Highly stable and isomorphic donor–acceptor stacking in a family of n-type organic semiconductors of BTBT–TCNQ derivatives

Unsymmetrically substituted donor–acceptor type compounds exhibit an isomorphous layered packing structure, which is discussed in terms of DFT-based quantum chemical calculations.

Design and Development of Distinct Tetracyanoquinodimethane Derivatives Exhibiting Dual Effect of Fluorescence and Anticancer Activity

AbstractTetracyanoquinodimethane (TCNQ) is known to react with primary/secondary amines to generate mono/di‐substituted TCNQ derivatives resulting in optical (fluorescence) and non‐linear optical characteristics. However, the choice of amine plays a key role in the sequel of certain material attributes. Nevertheless, demonstration of TCNQ derivatives, specifically obtained by reacting TCNQ with simple & small amine side chains of existing anticancer drugs, emerging in dual effect of fluorescence as well as anticancer activity, is not explored so far and has been still existing as a gap in TCNQ related research. Additionally, mono/di‐substituted TCNQ's owning strong fluorescence in solutions (∼10 to 104 order) than their solids are still lacking and are in need to be generated. In this regard, we aimed to synthesize straightforward TCNQ derivatives in single step by reacting TCNQ with specific (simple and small) amines being side chains of existing anticancer drugs (a contemporary design approach) and successfully obtained novel mono/di‐substituted TCNQ derivatives manifesting dual effect of fluorescence and anticancer activity. In this fashion, we have avoided the usage of bulky main moiety of an anticancer molecule, since our aim was not to use the total anticancer drug as such, like in a Pro‐drug preparation, but we intended to use the minimal part of an anticancer drug to invent novel TCNQ derivatives as anticancer active fluorophores. Wavelength of maximum emission for all the five potent molecules was within ∼525 nm‐696 nm. All compounds showed moderate anticancer activity against different cancer cell lines and IC50 values were in the range of 11 to 49 μM, when the synthesized novel compounds were tested in B16F10, MDA‐MB‐231 and A549 cell lines. Henceforth, this study has emanated in the first generation TCNQ derivatives as organic fluorescent anticancer active molecules.

Formation of Ion Pairs and Charge-Transfer Complexes in the p-Doping of Organic Semiconductors

The formation of ion pairs (IPA) through integer charge transfer and ground-state charge-transfer complexes (CPXs) with only fractional charge transfer are the two known mechanisms in the molecular doping of conjugated polymers and molecules, which form the material class of organic semiconductors (OSCs). As it entails no immediate ionization of the OSC, CPX formation is regarded as detrimental to the doping efficiency. For IPA formation to occur, a match between the electron affinity (EA) of the p-dopant and the ionization energy (IE) of the OSC is expected as crucial, while their pronounced frontier molecular orbital overlap should promote CPX formation instead [1].To explore these complementary scenarios, we first studied the thermal de-doping of the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT), p-doped with the common strong electron acceptor tetrafluoro-tetracyanoquinodimethane (F4TCNQ) [2]. We combined grazing incidence X-ray diffraction (GIXRD) with Fourier-transform infrared spectroscopy (FTIR) to determine the microstructure, and absorbance spectroscopy (UV/vis/NIR) to observe the spectral fingerprint of the two doping scenarios. We found two microstructural environments through their distinguished thermal stabilities, where, the dopants were observed (i) alternatingly stacked with the polymer backbone as well as (ii) dispersed in its sidechain region. From FTIR and UV/vis/NIR we deduce that all dopants are fully ionized, although packing between dopant and polymer backbone would be expected to favor CPX formation instead. Notably, while characteristic shifts of the mid-infrared cyano-stretch modes of TCNQ derivatives are known to well indicate the degree of charge transfer, we show here that they allow further to assess the dopant site in the polymer environment.We further investigated the influence of EA with respect to IE by contrasting F4TCNQ with its derivatives of lower degree of fluorination, which translates into reduced EA while leaving the spatial aspects of the dopant largely intact. We find that using F2TCNQ and FTCNQ still results in IPA formation in spite of EA < IE, which we understand through the lower degree of spatial, and therefore energetic order in OSCs as compared to inorganic semiconductors, which therefore possess no sharp band edges. Surprisingly, at high doping concentrations (additional) CPX formation occurred for all the differently strong dopants, which highlights the complexity of these systems going beyond the sole impact of the energetics of the individual constituents.Finally, we juxtaposed the known tendency of small molecular OSCs to form CPXs and that of polymer OSCs to form IPAs. To maximize the comparability with P3HT, alkylated oligothiophenes of different length were synthesized in-house (from 4 to 10 thiophene repeat units) and doped with F4TCNQ. For the longest oligomer, we were able to observe the switch from the CPX regime into that of IPA, i.e., we approach the doping behavior polymer limit. Knowing this threshold is valuable for applications in organic electronics where vacuum processible small molecular OSCs are preferred over polymeric OSCs, which are only processible via solution-based methods.[1] I. Salzmann, G. Heimel, M. Oehzelt, S. Winkler, N. Koch, Acc. Chem. Res. 2016, 49, 370.[2] H. Hase, K. O’Neill, J. Frisch, A. Opitz, N. Koch, I. Salzmann, The Journal of Physical Chemistry C 2018, 122, 25893.

Investigation of the Redox and Acid‐Base properties of TCNQF and TCNQF2: Electrochemistry, Vibrational Spectroscopy, and Substituent Effects

AbstractThe electrochemistry and acid‐base chemistry of TCNQF and TCNQF2 (TCNQ=7,7,8,8‐tetracyanoquinodimethane) are described, and the results are compared to those reported previously for TCNQ and TCNQF4. In acetonitrile solution, both mono‐ and di‐fluorinated TCNQ derivatives show two well‐resolved, diffusion‐controlled chemically and electrochemically reversible one‐electron‐transfer processes under the conditions of cyclic voltammetry. The reversible potentials of the mono‐ and difluoro‐TCNQ were determined for both the monoanionic and dianionic processes, that is, TCNQF0/1−/2− and TCNQF20/1−/2−. A Hammett plot shows a linear relationship of the potential differences between the three fluorinated derivatives and TCNQ with the sum of the Hammett constants for fluoro substitution. A significant positive shift in the reversible potential is found by increasing the number of fluoro substituents. Although the first TCNQFn0/1− (n=1,2) reduction process is not affected by addition of trifluoroacetic acid (TFA), the dianions are more basic and rapidly protonate to form H2TCNQFn. This explains the significant change detected in the electrochemistry of the second TCNQFn1−/2− redox process. The addition of TFA to solutions of the monoanionic form also gives rise to the disproportionation of TCNQFn1− to TCNQFn and H2TCNQFn, as proven electrochemically and spectroscopically. Furthermore, a higher number of fluoro substituents results in the reduced form of the molecule being more stable, as evidenced by both UV/Vis spectra and voltammetric measurements. Finally, we performed DFT calculations for TCNQF2 and TCNQF2 to produce a spectroscopic library of characteristic IR and Raman bands for these species. These data are used for comparisons with earlier calculations for TCNQF4, and we report the experimental and calculated infrared and Raman spectra for TCNQF and TCNQF2. The vibrational frequencies of the trifluorinated species, TCNQF3, were also calculated to complete the series.

Charge-transfer Layered Assembly of a trans-Heteroleptic Paddlewheel-type Diruthenium(II, II) Complex with a TCNQ Derivative: Electrochemical Tuning of the Magnetism

The first example of a charge-transfer assembly based on a trans-heteroleptic paddlewheel-type diruthenium (II, II) complex with a TCNQ derivative is reported: [trans-{Ru2(CF3CO2)2(2,6-(CF3)2PhCO2)2}2TCNQ(OMe)2]·2CH2Cl2·2(p-xylene) (1; CF3CO2− = trifluoroacetate; 2,6-(CF3)2PhCO2– = 2,6-di(trifluoromethyl)benzoate; TCNQ(OMe)2 = 2,5-bis(methoxy)-7,7,8,8-tetracyanoquinodimethane) and its CH2Cl2-free compound (1'), where the TCNQ(OMe)2 moiety is neutral. Electrochemical magnetic tuning in 1' has been conducted using a Li-ion battery system, which induces a ferrimagnetic phase via the discharging process; in contrast, the pristine 1' complex is paramagnetic.

Layered ferrimagnets constructed from charge-transferred paddlewheel [Ru2] units and TCNQ derivatives: the importance of interlayer translational distance in determining magnetic ground state.

A donor (D)/acceptor (A) assembly reaction using the paddlewheel-type diruthenium(ii,ii) complex [Ru2(3,5-F2PhCO2)4(THF)2] (3,5-F2PhCO2- = 3,5-difluorobenzoate; abbreviated hereafter as [Ru2]) as D and 7,7,8,8-tetracyano-p-quinodimethane derivatives (TCNQRx; 2,5-R-substituted TCNQ) as A in a DCM/TCE or DCE solvent system (DCM = dichloromethane, TCE = 1,1,2,2-tetrachloroethane, DCE = 1,2-dichloroethane) led to the formation of D2A-type two-dimensional layered compounds [{Ru2(3,5-F2PhCO2)4}2{TCNQRx}]·nsolv (Rx = H2 (1), Rx = Me2, (2), and Rx = (OEt)2 (3)). All the compounds had similar two-dimensional fishnet-type structures, where the two [Ru2] units were fully coordinated with the four cyano groups of TCNQRx. The compounds 1-3 were categorized as a one-electron transferred ionic state (1e-I) with D+-A--D formulation (D+[triple bond, length as m-dash][RuII,III2]+; A-[triple bond, length as m-dash]TCNQRx˙-). This subunit state formally derived a ferrimagnetically ordered state composed of up-spins with S = 1 for [RuII,II2] and S = 3/2 for [RuII,III2]+ and a down-spin of S = 1/2 for TCNQRx˙-, with antiferromagnetic superexchange interactions in each layer. Eventually, 1-3 became three-dimensional ferrimagnets with Curie temperatures TC = 93, 93, and 92 K, respectively, owing to the presence of interlayer ferromagnetic interactions. The interlayer translational distances (l2) for the three compounds were 10.56, 10.54, and 10.84 Å, respectively, which agreed with the empirical prediction based on the threshold value of l2 > 10.3 Å for a valid ferromagnetic interaction.

Metamagnetism with TN = 97 K in a layered assembly of paddlewheel [Ru2] units and TCNQ: an empirical rule for interlayer distances determining the magnetic ground state

A new layered antiferromagnet was synthesized from the reaction of a paddlewheel diruthenium(ii,ii) complex with a TCNQ derivative. The complex has a short interlayer distance and forms part of a series of layered antiferromagnets.

Conformational polymorphs of a novel TCNQ derivative carrying an acetylene group

TCNQ is one of the most important organic acceptors and lots of its derivatives have been prepared. However the reports on their crystal polymorphs are limited to their complexes, and simple polymorphs of TCNQ derivatives are uncommon. We succeeded in preparation of a novel TCNQ derivative, 2,2'-(2-(prop-2-yn-1-yloxy)cyclohexa-2,5-diene-1,4-diylidene)dimalononitrile, having a propynyloxy group on a substituent. This compound was found to have two crystal polymorphs depending on a solvent for recrystallization. In polymorph I, dimeric hydrogen bonds are formed between acetylenic hydrogens and cyano nitrogens with the molecule in an inversion symmetry. While, in polymorph II, the molecules make intermolecular hydrogen bonds between acetylenic hydrogens and cyano nitrogens with the molecule in 21 symmetry, forming a hydrogen bonded molecular helix along the b axis. Besides patterns of the intermolecular hydrogen bonds, difference was recognized in conformation of propynyloxy group. The molecule has an anti conformation in polymorph I and a gauche conformation in polymorph II. DFT calculation indicates that the anti conformer is less stable than the gauche one. But a solvation model suggests the anti conformer is estimated to be more stable in a toluene solution.

Construction of an Artificial Ferrimagnetic Lattice by Lithium Ion Insertion into a Neutral Donor/Acceptor Metal-Organic Framework.

Construction of a molecular system in which the magnetic lattice exhibits long-range order is one of the fundamental goals in materials science. In this study, we demonstrate the artificial construction of a ferrimagnetic lattice by doping electrons into acceptor sites of a neutral donor/acceptor metal-organic framework (D/A-MOF). This doping was achieved by the insertion of Li-ions into the D/A-MOF, which was used as the cathode of a Li-ion battery cell. The neutral D/A-MOF is a layered system composed of a carboxylate-bridged paddlewheel-type diruthenium(II,II) complex as the donor and a TCNQ derivative as the acceptor. The ground state of the neutral form was a magnetically disordered paramagnetic state. Upon discharge of the cell, spontaneous magnetization was induced; the transition temperature was variable. The stability of the magnetically ordered lattice depended on the equilibrium electric potential of the D/A-MOF cathode, which reflected the electron-filling level.

Reactivity of TCNE and TCNQ derivatives of quinonoid zwitterions with Cu(I).

The reactions of TCNE- and TCNQ-functionalized (TCNE: tetracyanoethylene and TCNQ: 7,7',8,8'-tetracyanoquinodimethane) zwitterionic benzoquinonemonoimines with a Cu(I)-BIAN complex (BIAN = bis(o,o'-bisisopropylphenyl)acenaphthenequinonediimine) have been investigated and found to follow a diversity of interesting patterns. The complexes [Cu(BIAN)(NCMe)(L2)]BF4 (2) and [Cu(BIAN)(L2)2]BF4 (4) were obtained by reacting [Cu(BIAN)(NCMe)2]BF4 (1) with one and two equivalents of L2, respectively. Following similar procedures, the complexes [Cu(BIAN)(NCMe)(L3)]BF4 (6) and [Cu(BIAN)(L3)2]BF4 (7) were obtained by reaction of 1 with L3. The reaction of 2 with 0.5 equiv. of 4,4'-bipyridine afforded [{Cu(BIAN)(L2)}2(μ-4,4'-bipyridine)](BF4)2 (3). The complexes were characterized by multinuclear NMR, IR and UV-Vis spectroscopic techniques, mass spectrometry, cyclic voltammetry and elemental analysis. The molecular structures of complexes 3 ·4CH2Cl2 and 4 ·CH2Cl2 were determined by single crystal X-ray diffraction. An unexpected coordination polymer [Cu((L2-))2]∞ (5) was also structurally characterized, which contains Cu(II) centres chelated by two N,O-bound ligands resulting from the monodeprotonation of L2.

ChemInform Abstract: Electrochemical and Photochemical Routes to Semiconducting Transition Metal‐Tetracyanoquinodimethane Coordination Polymers

AbstractReview: 105 refs.

C—I...NC halogen bonding in two polymorphs of the mixed-valence 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), with segregatedversusalternated stacks

Oxidation of diiodoethylenedithiotetrathiafulvalene (EDT-TTF-I2), C8H4I2S6, with the strong oxidizer tetrafluorotetracyanoquinodimethane (TCNQF4), C12F4N4, affords, depending on the crystallization solvent, two polymorphs of the 2:1 charge-transfer salt (EDT-TTF-I2)2(TCNQF4), represented as D2A. In both salts, the TCNQF4 is reduced to the radical anion state, and is associated through short C-I...NC halogen bonds to two EDT-TTF-I2 molecules. The two polymorphs differ in the solid-state association of these trimeric D-A-D motifs. In polymorph (I) the trimeric motif is located on an inversion centre, and hence both EDT-TTF-I2 molecules have +0.5 charge. Together with segregation of the TTF and TCNQ derivatives into stacks, this leads to a charge-transfer salt with high conductivity. In polymorph (II) two crystallographically independent EDT-TTF-I2 molecules bear different charges, close to 0 and +1, as deduced from an established correlation between intramolecular bond lengths and charge. Overlap interactions between the halogen-bonded D(0)-A-∙D+∙ motifs give rise, in a perpendicular direction, to diamagnetic A2(2-) and D(0)-D2(2+)-D(0) entities, where the radical species are paired into the bonding combination of respectively the acceptor LUMOs and donor HOMOs. The strikingly different solid-state organization of the halogen-bonded D-A-D motifs provides an illustrative example of two modes of face-to-face interaction between π-type radicals, into either delocalized, uniform chains with partial charge transfer and conducting behaviour, or localized association of radicals into face-to-face A2(2-) and D2(2+) dyads.

A new metal–organic hybrid material with intrinsic resistance-based bistability: monitoring in situ room temperature switching behavior

Two new silver containing materials with TCNQ derivatives were prepared by electrocrystallization. Synchrotron radiation diffraction studies were conducted to determine the structures of the new phases which exhibit unprecedented high room temperature conductivity. The I–V characteristics reveal a room-temperature switching behaviour and memory effect based on intrinsic negative differential resistance (NDR). EPR spectroscopic measurements performed under an applied electric field indicate a g-tensor shift that is correlated to the amplitude of current.

Charge transfer-assisted self-limited decyanation reaction of TCNQ-type electron acceptors on Cu(100)

TCNQ derivatives adsorbed on a metal surface undergo a self-limited decyanation reaction that only affects two out of the four cyano groups in the molecule. Combined Scanning Tunneling Microscopy/X-ray Photoelectron Spectroscopy experiments and Density Functional Theory calculations relate the self-limiting behavior to the transfer of electrons from the metal to the molecule.

Carrier Concentration Dependent Conduction in Insulator-Doped Donor/Acceptor Chain Compounds

On the basis of the concept that the design of a mixed valence system is a key route to create electronic conducting frameworks, we propose a unique idea to rationally produce mixed valency in an ionic donor/acceptor chain (i.e., D(+)A(-) chain). The doping of a redox-inert (insulator) dopant (P) into a D(+)A(-) chain in place of neutral D enables the creation of mixed valency A(0)/A(-) domains between P units: P-(D(+)A(-))nA(0)-P, where n is directly dependent on the dopant ratio, and charge transfer through the P units leads to electron transport along the framework. This hypothesis was experimentally demonstrated in an ionic DA chain synthesized from a redox-active paddlewheel [Ru2(II,II)] complex and TCNQ derivative by doping with a redox-inert [Rh2(II,II)] complex.

Exploration of Charge-Transfer Solids Utilizing Nucleobases: Nanoarchitectures by Hydrogen-Bonds in the Ionic Assemblies of Guanine and TCNQ Derivatives

We studied formation and structural characteristics of charge-transfer solids of 9-n-butylguanine (BuG) with fluorinated tetracyanoquinodimethane derivatives (FnTCNQ, n = 4, 2, and 1). Complex formation in a methanol (MeOH)-containing solvent generated two types of salts composed of either a methoxy-substituted anion or a fully ionic anion radical of FnTCNQ. In all anion radical salts, BuG existed as a protonated or a hemiprotonated species, BuGH+ or (BuG)(BuGH+), respectively, and formed hydrogen-bonded (H-bonded) assemblies. In these BuGH+ assemblies, FnTCNQ•– molecules were fixed and aligned periodically, providing H-bonded polycationic templates. In (BuGH+)(F4TCNQ•–), BuGH+ dimers by complementary H-bonds formed a two-dimensional (2D) polycationic sheet. The F4TCNQ•– face-to-face dimers formed a one-dimensional (1D) segregated column aided by formation of H-bonds with BuGH+. In (BuGH+)(F2TCNQ•–)(MeOH), BuGH+ dimers by complementary double H-bonds formed a 1D polycationic ribbon supported by MeOH-media...

Identification of TCNQF4 redox levels using spectroscopic and electrochemical fingerprints (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane)

Solution and solid state methods for the identification of TCNQF4, TCNQF4− and TCNQF42− redox levels (TCNQF4=2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) are described. Solution phase UV–Vis spectroscopy and electrochemistry can be used for both qualitative and quantitative determinations, while infrared and Raman spectroscopies are very powerful for solid state identification. Analogous strategies can also be applied to other TCNQ derivatives (TCNQ=7,7,8,8-tetracyanoquinodimethane).

Single-Component Charge-Transfer Crystals Based on Spin-Carrying TCNQ (7,7,8,8-Tetracyanoquinodimethane) Derivatives

Three TCNQ derivatives carrying nitroxide radicals (3a–3c) were prepared and were found to form single-component charge-transfer (CT) complexes by self-assembly, in which outer nitroxide groups of a couple of different molecules work as donors and the inner TCNQ unit of another molecule as an acceptor. While the CT interactions found for the TEMPO (2,2,6,6-tetramethylpiperidin-1-oxy) derivative 3a and the PROXYL (2,2,5,5-tetramethylpirrolidin-1-oxy) derivative 3b are point-to-face fashion between the oxygen atom of each nitroxide group and the six-membered ring of inner TCNQ unit, the CT interactions found for the PO (2,2,5,5-tetramethyl-3-pyrrolin-1-oxy) derivative 3c are point-to-point contacts between the oxygen atoms of outer nitroxide groups and the carbon atoms of a couple of cyano groups.

Reactivity of TCNE or TCNQ Derivatives of Quinonoid Zwitterions: Platinum-Induced HCN Elimination vs Oxidative-Addition

The reaction of the functional, zwitterionic quinonoid molecule (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,1,2,2-tetracyanoethyl)cyclohexa-1,4-dien-1-olate, [C(6)H-2-{C(CN)(2)C(CN)(2)H}]-4,6-(···NH n-Bu)(2)-1,3(···O)(2) (2), which has been previously prepared by regioselective insertion of TCNE into the C-H bond adjacent to the C···O bonds of the zwitterionic benzoquinone monoimine (6E)-4-(butylamino)-6-(butyliminio)-3-oxocyclohexa-1,4-dien-1-olate, C(6)H(2)-4,6-(···NHn-Bu)(2)-1,3-(···O)(2) (1), with 2 equiv of [Pt(C(2)H(4))(PPh(3))(2)], afforded the Pt(0) complex [Pt(PPh(3))(2)(4)] (6) (4 = 2-HCN; (6E)-4-(butylamino)-6-(butyliminio)-3-oxo-2-(1,2,2-tricyanoethenyl)cyclohexa-1,4-dien-1-olate), in which a tricyanoethenyl moiety is π-bonded to the metal. A metal-induced HCN elimination reaction has thus taken place. The same complex was obtained directly by the reaction of 1 equiv of the Pt(0) complex [Pt(C(2)H(4))(PPh(3))(2)] with the olefinic ligand [C(6)H-2-{C(CN)═C(CN)(2)}]-4,6-(···NHn-Bu)(2)-1,3-(···O)(2)) (4), previously obtained by the reaction of 2 with NEt(3) in THF. A similar reactivity pattern was observed between 2 and 2 equiv of the Pd(0) precursor [Pd(dba)(2)] in the presence of dppe, which led to [Pd(dppe)(4)] (7), which was also directly obtained from 4 and 1 equiv [Pd(dba)(2)]/dppe. In contrast to the behavior of the TCNE derivative 2, the reaction of the TCNQ derivative (6E)-4-(butylamino)-6-(butyliminio)-2-(dicyano(4-(dicyanomethyl)phenyl)methyl)-3-oxocyclohexa-1,4-dien-1-olate, [C(6)H-2-{C(CN)(2)p-C(6)H(4)C(CN)(2)H}]-4,6-(···NHn-Bu)(2)-1,3-(···O)(2)) (3), with 2 equiv of [Pt(C(2)H(4))(PPh(3))(2)] led to formal oxidative-addition of the C-H bond of the C(CN)(2)H moiety to give the Pt(II) hydride complex trans-[PtH(PPh(3))(2){N═C═C(CN)p-C(6)H(4)C(CN)(2)-2-[C(6)H-4,6-(···NHn-Bu)(2)-1,3-(···O)(2))}] (8). The molecular structures of 3, 4, 6·0.5(H(2)O), and 8·3(CH(2)Cl(2)) have been determined by single-crystal X-ray diffraction.