Rare-Earth Metal Tetrathiafulvalene Carboxylate Frameworks as Redox-Switchable Single-Molecule Magnets.

Using the redox-active tetrathiafulvalene tetrabenzoate (TTFTB4- ) as the linker, a series of stable and porous rare-earth metal-organic frameworks (RE-MOFs), [RE9 (µ3 -OH)13 (µ3 -O)(H2 O)9 (TTFTB)3 ] (1-RE, where RE=Y, Sm, Gd, Tb, Dy, Ho, and Er) were constructed. The RE9 (µ3 -OH)13 (µ3 -O) (H2 O)9 ](CO2 )12 clusters within 1-RE act as segregated single-molecule magnets (SMMs) displaying slow relaxation. Interestingly, upon oxidation by I2 , the S=0 TTFTB4- linkers of 1-RE were converted into S= 1 / 2 TTFTB.3- radical linkers which introduced exchange-coupling between SMMs and modulated the relaxation. Furthermore, the SMM property can be restored by reduction in N,N-dimethylformamide. These results highlight the advantage of MOFs in the construction of redox-switchable SMMs.

Enhanced dielectricity coupled to spin-crossover in a one-dimensional polymer iron(ii) incorporating tetrathiafulvalene.

In designing multifunctional materials for potential switches that can be used as memory devices, the high-spin (HS) to low-spin (LS) crossover (SCO) one-dimensional polymer, [FeII(L)(4,4'-bpy)] n , was constructed from a designed redox-active tetrathiafulvalene (TTF) functionalized Schiff-base and the ditopic linker 4,4'-bipyridine (bpy). It exhibits an 8 K hysteretic SCO centred at T 1/2 = 325 K which is coupled to changes in its dielectric constant. The crystal structures above and below the transition temperature reveal similar parallel linear ···Fe-bpy-Fe-bpy··· chains displaying expansion of the FeII octahedron in the HS state. Density functional theory (DFT) calculations reveal a concerted electronic charge and spin change represented by the Mülliken charge of the Fe and the magnitude and direction of the dipole moment which substantiate the experimental observations.

Spin crossover modulation in a coordination polymer with the redox-active bis-pyridyltetrathiafulvalene (py2TTF) ligand.

A one-dimensional FeII coordination polymer (CP) has been formed which includes the redox-active ligand bis-pyridyltetrathiafulvalene (py2TTF) and a Schiff base-like N2O2 ligand. This CP is both spin crossover (SCO) and redox-active in the solid-state, and chemical oxidation results in SCO modification.

Dinuclear acetylide-bridged ruthenium(ii) complexes with rigid non-aromatic spacers.

Rigid dinuclear ruthenium complexes containing non-aromatic caged and polycyclic spacer groups were synthesised and characterised. The complexes, [trans,trans-{Ru(dmpe)2(C[triple bond, length as m-dash]CtBu)}2(µ-C[triple bond, length as m-dash]C-X-C[triple bond, length as m-dash]C)], where X = 1,4-bicyclo[2.2.2]octane (C8H12) or 1,12-p-carborane (p-C2B10H10), were formed via the metathesis of terminal organic bisacetylenes with the methylruthenium complex, [trans-Ru(dmpe)2(CH3)(C[triple bond, length as m-dash]CtBu)], under mild conditions. Electrochemical studies demonstrated electronic interactions across the non-aromatic caged and polycyclic spacers was less than in the analogous complex with an aromatic spacer group, [trans,trans-{Ru(dmpe)2(C[triple bond, length as m-dash]CtBu)}2(µ-C[triple bond, length as m-dash]C-p-C6H4-C[triple bond, length as m-dash]C)]. Mononuclear complexes, [trans-Ru(dmpe)2(C[triple bond, length as m-dash]CtBu)(C[triple bond, length as m-dash]C-X-C[triple bond, length as m-dash]CH)], were also synthesised. [trans-Ru(dmpe)2(C[triple bond, length as m-dash]CtBu)(C[triple bond, length as m-dash]C-C8H12-C[triple bond, length as m-dash]CH)] and [trans,trans-{Ru(dmpe)2(C[triple bond, length as m-dash]CtBu)}2(µ-C[triple bond, length as m-dash]C-p-C2B10H10-C[triple bond, length as m-dash]C)] were structurally characterised by X-ray crystallography.

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.

Toward a dodecanuclear molecular Re(i) box: structural and spectroscopic properties.

Two tetrapyridyl ligands of 1,2,4,5-tetraethynyl(4-pyridyl)benzene (tpeb) and tetra(4-pyridyl)-tetrathiafulvalene (TTF(py)4) were used to react with trinuclear (Re(CO)4)3(C3N3S3) (C3N3S3 = cyanurate trianion) moieties to afford hexanuclear [(Re(CO)3)6(tpeb)2(C3N3S3)2]·4CH3CN·toluene (1) and dodecanuclear [(Re(CO)3)12(TTF(py)4)3(C3N3S3)4]·8CH3CN·12DMF (2) boxes, respectively, under solvothermal conditions. Surprisingly, similar tetrapyridyl ligands with different core units (i.e., benzene and tetrathiafulvalene moieties) led to dramatically different structural motifs (i.e., hexanuclear and dodecanuclear boxes). Complex 1 forms a slightly bent trigonal-prismatic structure, containing two µ3-tpeb ligands and two Re3C3N3S3 moieties as well as a ππ interaction distance of 3.86 Å. It is noted that complex 2 features a novel four-star structure, and three crossed tetrathiafulvalene moieties from three µ4-TTF(py)4 ligands to form a triple-decker arrangement with a ππ interaction distance of 3.70 Å. Moreover, 1 and 2 display luminescence properties in the solid state along with electrochemical properties for complex 2 arising from the electroactive TTF core.

Porous Molecular Conductor: Electrochemical Fabrication of Through-Space Conduction Pathways among Linear Coordination Polymers.

The first porous molecular conductor (PMC), which exhibits porosity, a through-space conduction pathway and rich charge carriers (electrons), was prepared through electrocrystallization from Cd2+ and N, N'-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxdiimide (NDI-py). [Cd(NDI-py)(OH2)4](NO3)1.3±0.1· nDMA (PMC-1) was assembled by π-π stacking among one-dimensional (1D) linear coordination polymers. The NDI cores were partially reduced into radical anions to form conductive π-stacked columns, yielding (1.0-3.3) × 10-3 S cm-1 at room temperature. Moreover, the electrical conductivity was significantly enhanced by removing the solvent molecules from PMC-1, indicating that PMCs are promising as molecule-responsive conductive materials.

Probing charge transfer characteristics in a donor-acceptor metal-organic framework by Raman spectroelectrochemistry and pressure-dependence studies.

The stimuli responsive behaviour of charge transfer donor-acceptor metal-organic frameworks (MOFs) remains an understudied phenomenon which may have applications in tuneable electronic materials. We now report the modification of donor-acceptor charge transfer characteristics in a semiconducting tetrathiafulvalene-naphthalene diimide-based MOF under applied electrochemical bias and pressure. We employ a facile solid state in situ Raman spectroelectrochemical technique, applied for the first time in the characterisation of electroactive MOFs, to monitor the formation of a new complex TTFTC˙+-DPNI from a largely neutral system, upon electrochemical oxidation of the framework. In situ pressure-dependent Raman spectroscopy and powder X-ray diffraction experiments performed in a diamond anvil cell revealed blue shifts in the donor and acceptor vibrational modes in addition to contractions in the unit cell which are indicative of bond shortening. This study demonstrates the utility of in situ Raman spectroscopic techniques in the characterisation of redox-active MOFs and the elucidation of their electronic behaviours.

Mixed Valency as a Strategy for Achieving Charge Delocalization in Semiconducting and Conducting Framework Materials.

The fundamentally important phenomenon of mixed valency has been discussed in detail over the past 50 years, predominantly in the context of dinuclear complexes, which are used as model systems for understanding electron delocalization in more complex biological and physical systems. Very recently, mixed valency has been shown to be an important mechanism for charge transfer, leading to delocalization and conductivity in two- and three-dimensional framework materials such as metal-organic frameworks and related systems including covalent organic frameworks and semicrystalline semiconducting metal-organic graphenes. This Viewpoint provides a current perspective on the field of mixed-valence frameworks, where the property is either intrinsic or generated postsynthetically via an external stimulus. Aspects of the spectroscopy and applications of these materials are also discussed, highlighting the future potential for exploiting mixed valency in extended solid-state systems.

Photo- and Electronically Switchable Spin-Crossover Iron(II) Metal-Organic Frameworks Based on a Tetrathiafulvalene Ligand.

A major challenge is the development of multifunctional metal-organic frameworks (MOFs), wherein magnetic and electronic functionality can be controlled simultaneously. Herein, we rationally construct two 3D MOFs by introducing the redox active ligand tetra(4-pyridyl)tetrathiafulvalene (TTF(py)4 ) and spin-crossover FeII centers. The materials exhibit redox activity, in addition to thermally and photo-induced spin crossover (SCO). A crystal-to-crystal transformation induced by I2 doping has also been observed and the resulting intercalated structure determined. The conductivity could be significantly enhanced (up to 3 orders of magnitude) by modulating the electronic state of the framework via oxidative doping; SCO behavior was also modified and the photo-magnetic behavior was switched off. This work provides a new strategy to tune the spin state and conductivity of framework materials through guest-induced redox-state switching.

A heterometallic ferrimagnet based on a new TTF-bis(oxamato) ligand.

An oxamato unit has been introduced into a tetrathiafulvalene (TTF) system for the first time via direct condensation, generating a new π-extended TTF ligand, TTF-Et2H2opba (opba = ortho-phenylenebis-oxamato). On the basis of this ligand, the Cu(ii) complex [(n-Bu)4N]2[Cu(TTF-opba)] (1) has been obtained. By employing this complex as a "metalloligand", we have successfully prepared an interesting heterometallic one-dimensional complex {[Cu(TTF-opba)][Mn(CH3OH)2]}n (2), via coordination of the free carbonyl-oxygen atoms of the two oxamato groups with Mn(ii) ions. The new compounds have been structurally characterized, and their electrochemical and magnetic properties probed. Complex 2 shows a ferrimagnetic behavior with g = 1.96 and J = -22.36 cm-1.

Chiral heterobimetallic chains from a dicyanideferrite building block including a π-conjugated TTF annulated ligand.

The π-conjugated tetrathiafulvalene (TTF) annulated ligand was introduced into a dicyanometallate for the first time, leading to the synthesis of the versatile redox-active dicyanideferrite building block [(n-Bu)4N][Fe(TTFbp)(CN)2] (H2TTFbp = N-(2-(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)-5-(picolinamido) benzo[d][1,3]dithiol-6-yl) picol inamide). The incorporation of the new precursor with chiral MnIII Schiff-base complexes resulted in two enantiopure one-dimensional complexes, [Mn((R,R)-salphen)Fe(TTFbp)(CN)2]n (2-(RR)) and [Mn((S,S)-salphen)Fe(TTFbp)(CN)2]n (2-(SS)) (Salphen = N,N'-1,2-diphenylethylene-bis(salicylideneiminato) dianion), which were synthesized and structurally characterized. Circular dichroism (CD) and vibrational circular dichroism (VCD) spectra confirmed the enantiomeric nature of the optically active complexes, and structural analyses revealed the formation of neutral cyanide-bridged double chains in 2-(RR) and 2-(SS). Solution and solid state CV studies revealed the redox-active characteristics of the complexes. Antiferromagnetic couplings were detected between FeIII and MnIII centers within a chain, and a field-induced magnetic phase transition was observed (TN = 4.8 K). The introduction of electroactivity and chirality into cyanide-bridged complexes with interesting magnetic properties leads the way towards new multifunctional materials.

Synthesis, properties and surface self-assembly of a pentanuclear cluster based on the new π-conjugated TTF-triazole ligand.

The new π-extended redox-active ligand with both TTF and triazole units, 6-(4,5-bis(propylthio)-1,3-dithiol-2-ylidene)-1H-[1,3]dithiolo[4',5':4,5]benzo [1,2-d] [1-3]triazole, has been successfully prepared. Based on the versatile ligand and Cu(tta)2 precursors (tta(-) = 4,4,4-trifluoro-1-(thiophen-2-yl)butane-1,3-dione), a TTF-based pentanuclear Cu(II) cluster (Cu5(tta)4(TTFN3)6) is synthesized and structurally characterized. Their absorption and electrochemical properties are investigated. Antiferromagnetic couplings are operative between metal ion centers bridged by triazoles in the complex. The self-assembled structure of the cluster complex on a highly oriented pyrolytic graphite (HOPG) surface was observed using scanning tunneling microscopy and density functional theory (DFT) calculations have been performed to provide insight into the formation mechanism. The introduction of the redox-active TTF unit into the cluster complexes with interesting magnetic properties renders them promising candidates for new multifunctional materials.

Dinuclear Ruthenium Complex Based on a π-Extended Bridging Ligand with Redox-Active Tetrathiafulvalene and 1,10-Phenanthroline Units.

The synthesis of a π-extended bridging ligand with both redox-active tetrathiafulvalene (TTF) and 1,10-phenanthroline (phen) units, namely, bis(1,10-phenanthro[5,6-b])tetrathiafulvalene (BPTTF), was realized via a self-coupling reaction. Using this ligand and Ru(tbbpy)2Cl2 (tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine), the dinuclear ruthenium(II) compound [{Ru(tbbpy)2}2(BPTTF)](PF6)4 (1) has been obtained by microwave-assisted synthesis. Structural characterization of 1 revealed a crossed arrangement of the TTF moieties on adjacent dimers within the crystal structure. The optical and redox properties of 1 were investigated using electrochemical, spectroelectrochemical, electron paramagnetic resonance (EPR), and absorption spectroscopic studies combined with theoretical calculations. One exhibits a rich electrochemical behavior owing to the multiple redox-active centers. Interestingly, both the ligand BPTTF and the ruthenium compound 1 are EPR-active in the solid state owing to intramolecular charge-transfer processes. The results demonstrate that the TTF-annulated bis(phen) ligand is a promising bridging ligand to construct oligomeric or polymeric metal complexes with multiple redox-active centers.

Crystal Structures, Magnetic Properties, and Electrochemical Properties of Coordination Polymers Based on the Tetra(4-pyridyl)-tetrathiafulvalene Ligand.

Seven new coordination polymers based on the redox-active tetra(4-pyridyl)-tetrathiafulvalene ligand (TTF(py)4) and different transition-metal ions, namely, {[Cu(hfac)2][TTF(py)4]·2(CH2Cl2)}n (1), {[Co(acac)2][TTF(py)4]0.5·(CHCl3)}n (2), {[Mn(hfac)2][TTF(py)4]0.5}n (3), {[Cu2(OAc)4][TTF(py)4]0.5·1.5(CHCl3)·0.5(H2O)·(CH3CN)}n (4), {[Mn(SCN)2][TTF(py)4]·6(CH2Cl2)}n (5), {[Mn(SeCN)Cl][TTF(py)4]}n (6), and {Cu2[TTF(py)4]2·(ClO4)2·2.5(CH2Cl2)·1.5(CH3CN)}n (7), were synthesized and characterized. The tetrapyridyl ligand coordinates to metal ions in a bidentate or tetradentate fashion, forming complexes 1-7 with different structures. Complex 1 exhibits a one-dimensional chain structure. Complexes 2, 3, and 4 possess similar (4,2)-connected binodal two-dimensional networks, while complexes 5 and 6 have similar (4,4)-connected binodal two-dimensional networks with two different rings. Complex 7 shows a 2-fold interpenetrated (4,4)-connected binodal PtS-type three-dimensional framework. Meanwhile, these complexes feature diverse nonclassical hydrogen bonding interactions. In addition, magnetic and solid-state electrochemical properties for typical complexes have been studied.

The electronic, optical and magnetic consequences of delocalization in multifunctional donor-acceptor organic polymers.

Two organic polymers containing alternating electron donating triarylamine and electron accepting thiazolo[5,4-d]thiazole (TzTz) moieties have been synthesized and their redox states investigated. When donor and acceptor units are proximal (polymer )1, electron density is delocalized, leading to a small electrical and optical band gap; these are larger with the inclusion of an adjoining alkynyl-phenyl bridge (polymer 2), where electron density is more localized due to the rotation of the monomer units. As a result, 1 and 2 display different optical and fluorescence properties in their neutral states. Upon chemical and electrochemical redox reactions, radicals form in both 1 and 2, yielding magnetic materials that display temperature-independent paramagnetism, attributable to delocalization of radical spins along the polymeric backbones. The ability to convert between diamagnetic and paramagnetic states upon chemical oxidation and/or reduction allows for the materials to display switchable magnetism and fluorescence, imparting multifunctionality to these solid-state purely organic materials.

Electronic, optical, and computational studies of a redox-active napthalenediimide-based coordination polymer.

The new one-dimensional coordination framework (Zn(DMF)NO3)2(NDC)(DPMNI), where NDC = 2,6-naphthalenedicarboxylate and DPMNI = N,N'-bis(4-pyridylmethyl)-1,4,5,8-naphthalenetetracarboxydiimide, which has been crystallographically characterized, exhibits two redox-accessible states due to the successive reduction of the naphthalenediimide (NDI) ligand core. Solid-state electrochemical and vis-near-IR spectroelectrochemical measurements coupled with density functional theory (DFT) calculations enabled the origins of the optical transitions in the spectra of the monoradical anion and dianion states of the material to be assigned. Electron paramagnetic resonance (EPR) spectroscopy revealed that the paramagnetic radical anion state of the DPMNI core could be accessed upon broad-spectrum white light irradiation of the material, revealing a long-lived excited state, possibly stabilized by charge delocalization which arises from extensive π-π* stacking interactions between alternating NDC and NDI aromatic cores which are separated by a distance of 3.580(2) Å.

Enhancing selective CO2 adsorption via chemical reduction of a redox-active metal-organic framework.

A new microporous framework, Zn(NDC)(DPMBI) (where NDC = 2,7-naphthalene dicarboxylate and DPMBI = N,N'-di-(4-pyridylmethyl)-1,2,4,5-benzenetetracarboxydiimide), containing the redox-active benzenetetracarboxydiimide (also known as pyromellitic diimide) ligand core has been crystallographically characterised and exhibits a BET surface area of 608.2 ± 0.7 m(2) g(-1). The crystallinity of the material is retained upon chemical reduction with sodium naphthalenide (NaNp), which generates the monoradical anion of the pyromellitic diimide ligand in the framework Zn(NDC)(DPMBI)·Na(x) (where x represents the molar Na(+)/Zn(2+) ratio of 0.109, 0.233, 0.367 and 0.378 from ICP-AES), as determined by EPR, solid state Vis-NIR spectroelectrochemistry and UV-Vis-NIR spectroscopy. The CO2 uptake in the reduced materials relative to the neutral framework is enhanced up to a Na(+)/Zn(2+) molar ratio of 0.367; however, beyond this concentration the surface area and CO2 uptake decrease due to pore obstruction. The CO2 isosteric heat of adsorption (|Q(st)|) and CO2/N2 selectivity (S), obtained from pure gas adsorption isotherms and Ideal Adsorbed Solution Theory (IAST) calculations, are also maximised relative to the neutral framework at this concentration of the alkali metal counter-ion. The observed enhancement in the CO2 uptake, selectivity and isoteric heat of adsorption has been attributed to stronger interactions between CO2 and both the radical DPMBI ligand backbone and the occluded Na(+) ions.