Impact of Metal-Ion Dependence on the Porous and Electronic Properties of TCNQ-Dianion-Based Porous Coordination Polymers

Expanding the dimensionality of bis(tetrazolyl)alkane-based Fe(II) coordination polymers by the application of dinitrile coligands

Reactions between 1,2-di(tetrazol-2-yl)ethane (ebtz), 1,6-di(tetrazol-2-yl)hexane (hbtz) or 1,1'-di(tetrazol-1-yl)methane (1ditz) and Fe(BF4)2 in the presence of adiponitrile (ADN), glutaronitrile (GLN) or suberonitrile (SUN) resulted in the formation of coordination polymers [Fe(μ-ebtz)2(μ-ADN)](BF4)2 (1), [Fe(μ-hbtz)2(μ-ADN)](BF4)2 (2), [Fe(μ-1ditz)2(GLN)2](BF4)2·GLN (3) and [Fe(μ-1ditz)2(μ-SUN)](BF4)2·SUN (4). It was established that the application of dinitriles allows an increase in the dimensionality of the ebtz and hbtz based systems while maintaining the structure of the polymeric units characteristic of previously studied mononitrile based analogues. In 3 and 4, regardless of the type of dinitrile coligand, the motif of 2D polymeric layers constituted by 1ditz molecules remains preserved. However, the dimensionality of 1ditz based networks is governed by the coordination modes of dinitriles. 3, based on a shorter molecule of glutaronitrile, crystallizes as a two-dimensional (2D) coordination polymer. In this compound, dinitriles coordinate monodentately or play the role of guest molecules. The substitution of glutaronitrile with suberonitrile enables the bridging of neighboring polymeric layers, resulting in a 3D network. The intentional selection of bis(tetrazoles) and dinitriles as building blocks has led, as expected, to obtaining systems with the structure of the first coordination sphere consisting of four tetrazole rings and two axially coordinated nitrile molecules. It created the conditions required for the occurrence of thermally induced spin crossover. Magnetic measurements and single crystal X-ray diffraction studies were used for the characterization of the spin crossover properties of 1-4.

Design of coordination polymers based on combinations of 1,2-diphenylethane-1,2-diyl diisonicotinate with Cu(ii), Zn(ii), Cd(ii) and Co(ii)

Five new supramolecular coordination polymers of different dimensionalities (L-Cu(acac)2, L-Cu(hfac)2, L-ZnCl2, L-CdI2 and L-CoCl2) based on the use of the flexible organic ligand L (1,2-diphenylethane-1,2-diyl diisonicotinate) are reported.

Multifunctional Coordination Polymer Exhibiting Reversible Mechanical Motion Allowing Selective Uptake of Guests and Leading to Enhanced Electrical Conductivity

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.

Ligands modulated the variable binuclear Cd2-SBUs and structures of four layered coordination frameworks

An intriguing modulation of Cd₂(RCOO)₄ SBUs and the structural variation in their layered CPs have been observed, which were tuned by di-carboxylate/co-ligand.

Concomitant Use of Tetrathiafulvalene and 7,7,8,8-Tetracyanoquinodimethane within the Skeletons of Metal-Organic Frameworks: Structures, Magnetism, and Electrochemistry

In search of multifunctional metal-organic frameworks (MOFs), redox-active donors and acceptors, namely, tetrathiafulvalene (TTF) and 7,7,8,8-tetracyanoquinodimethane (TCNQ), were concomitantly used as skeletal components with diamagnetic metal nodes (Cd and Zn) to construct unique framework materials. Six isostructural frameworks were synthesized by diffusion of metal salts, TTF(py)₄, and either paramagnetic Li(TCNQ) or diamagnetic H₂TCNQ. They were characterized by single-crystal X-ray diffraction and FT-IR and UV-vis-NIR spectroscopy, and their physical properties were studied, including two postsynthetic modifications involving crystal-to-crystal transformations following a solid-solution reaction with I₂. The highly colored crystals of two isostructural Zn and Cd frameworks contain undulating Cd-TTF(py)₄ layers entwined with TCNQ in a chicken-wire net as part of the skeleton of the MOF as well as TCNQ intercalated within the channels, while nitrate anions are occluded within the cavities formed by the pyridine moieties. Reaction with I₂ replaces each intercalated TCNQ^•- within the channels with I₃^-. The optical properties and the electron paramagnetic resonance (EPR) spectra indicate the presence of only radical TCNQ^•- in the parent compounds, while the magnetic susceptibilities enabled an estimation of the amount of TCNQ^•- ( S = 1/2) leading to almost paramagnetic behavior. Solid-state electrochemistry provides evidence of several one-electron redox states corresponding to the electroactive cores.

Molecular O2 Activation over Cu(I)-Mediated C≡N Bond for Low-Temperature CO Oxidation

The activation of molecular oxygen (O₂) is extremely crucial in heterogeneous oxidations for various industrial applications. Here, a charge-transfer complex CuTCNQ nanowire (CuTCNQ NW) array grown on the copper foam was first reported to show CO catalytic oxidation activity at a temperature below 200 °C with the activated O₂ as an oxidant. The molecular O₂ was energetically activated over the Cu(I)-mediated C≡N bond with a lower energy of -1.167 eV and preferentially reduced to ^•O₂^- through one-electron transfer during the activation process by density functional theory calculations and electron paramagnetic resonance. The theoretical calculations indicated that the CO molecule was oxidized by the activated O₂ on the CuTCNQ NW surface via the Eley-Rideal mechanism, which had been further confirmed by in situ diffuse reflectance infrared Fourier transform spectra. These results indicated that the local C≡N bond electron-state engineering could effectively improve the molecular O₂ activation efficiency, which facilitates the low-temperature CO catalytic oxidation. The findings reported here enhance our understanding on the molecular oxygen activation pathway over metal-organic nanocatalysts and provide a new avenue for rational design of novel low-cost, organic-based heterogeneous catalysts.

X4TCNQ2− dianions: versatile building blocks for supramolecular systems

A new synthetic approach has led to the incorporation of TCNQ and F₄TCNQ dianions into a wide variety of structures.

Is iron unique in promoting electrical conductivity in MOFs?

Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials.

Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M₂(DOBDC)(DMF)₂ (M = Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺); H₄DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide), M₂(DSBDC)(DMF)₂ (M = Mn²⁺, Fe²⁺; H₄DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M₂Cl₂(BTDD)(DMF)₂ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺; H₂BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i]dibenzo[1,4]dioxin), and M(1,2,3-triazolate)₂ (M = Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high-energy valence electrons of Fe²⁺ and the Fe^3+/2+ mixed valency. These results reveal that incorporating Fe²⁺ in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for improving electrical conductivity in this important class of porous materials.

Metal organic frameworks with uni-, di-, and tri-nuclear Cd(ii) SBU prepared from 1,3-bis(4-pyridyl)propane and different dicarboxylate ligands: syntheses, structures and luminescent properties

Systematic variation of co-dicarboxylate ligands leads to three new MOFs based on uni-, di-, and tri-nuclear Cd(ii) SBUs.

Benzene absorption in a protuberant-grid-type zinc(ii)–organic framework triggered by the migration of guest water molecules

Benzene molecules are reported to be easily and regularly absorbed into specific channels in a 2D protuberant-grid-type Zn(ii)–organic framework, which consists of racemic interdigitated bilayers.

Magnetic ordering in TCNQ-based metal–organic frameworks with host–guest interactions

Host–guest interactions between the TCNQ-based MOF and aromatic molecules have been found to modulate spontaneous magnetization behavior at low temperatures.

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.

Three alkaline-earth metal complexes with 3D networks constructed from a 7,7,8,8-tetracyanoquinodimethane ligand: synthesis, structure and electrochemical properties

Three new 7,7,8,8-tetracyanoquinodimethane (TCNQ) alkaline-earth metal complexes, namely {[M2(TCNQ)3(H2O)6]·TCNQ}n (M = Ca (1), Sr (2) and Ba (3)) have been synthesized by salt elimination reactions. X-ray crystallographic analysis reveals that complexes 1, 2 and 3 are isomorphic featuring a unique 3D structure. Cyclic and differential pulse voltammograms for complexes 1-3 show a reversible one-electron oxidation and a reversible one-electron reduction within the electrochemical window of CH3CN. Their electrochemical HOMO-LUMO gap and reversibility are examined.