Self-Assembled Containers Based on Extended Tetrathiafulvalene

Interplay of Electronic and Geometric Structure Tunes Organic Biradical Character in Bimetallic Tetrathiafulvalene Tetrathiolate Complexes

The synthesis and design of organic biradicals with tunable singlet-triplet gaps have become the subject of significant research interest, owing to their possible photochemical applications and use in the development of molecular switches and conductors. Recently, tetrathiafulvalene tetrathiolate (TTFtt) has been demonstrated to exhibit such organic biradical character in doubly ionized bimetallic complexes. In this article we use high-level ab initio calculations to interrogate the electronic structure of a series of TTFtt-bridged metal complexes, resolving the factors governing their biradical character and singlet-triplet gaps. We show that the degree of biradical character correlates with a readily measured experimental predictor, the central TTFtt C-C bond length, and that it may be described by a one-parameter model, providing valuable insight for the future rational design of TTFtt based biradical compounds and materials.

Beyond Platonic: How to Build Metal-Organic Polyhedra Capable of Binding Low-Symmetry, Information-Rich Molecular Cargoes

The field of metallosupramolecular chemistry has advanced rapidly in recent years. Much work in this area has focused on the formation of hollow self-assembled metal-organic architectures and exploration of the applications of their confined nanospaces. These discrete, soluble structures incorporate metal ions as ‘glue’ to link organic ligands together into polyhedra.Most of the architectures employed thus far have been highly symmetrical, as these have been the easiest to prepare. Such high-symmetry structures contain pseudospherical cavities, and so typically bind roughly spherical guests. Biomolecules and high-value synthetic compounds are rarely isotropic, highly-symmetrical species. To bind, sense, separate, and transform such substrates, new, lower-symmetry, metal-organic cages are needed. Herein we summarize recent approaches, which taken together form the first draft of a handbook for the design of higher-complexity, lower-symmetry, self-assembled metal-organic architectures.

Synthesis of a chiral metallo-capsule composed of concave molecules and chirogenesis upon fullerene binding

An enantiopure molecular capsule was synthesized quantitatively using complexation of four phosphangulenes as concave molecules with four Zn²⁺ ions and applied to fullerene binding and chirogenesis. The capsule encapsulated selectively fullerene and its derivatives based on the size of cavity. The fullerene C₆₀ incorporated in the capsule exhibited induced-CD signals at the transitions of C₆₀.

Enantiopure Polyradical Tetrahedral Pd12 L6 Cages

The synthesis of cages with a polyradical framework remains a challenging task. Herein is reported an enantiomeric pair of quinoid-bridged polyradical tetrahedral palladium(II) cages that are stabilized by an unusual dianionic diradical form (dhbq^..2- ). These cages have been characterized by electron paramagnetic resonance and UV-visible spectroscopy, squid magnetometry and mass spectrometry. Single-crystal-derived X-ray investigations of the iso-structural cages built on fluoranilate linkers confirm the tetrahedral structure of the obtained radical cages. Theoretical calculations showed that the diradical state of the dhbq anions is more stable than the usual monoradical state. A weak ferromagnetic exchange between adjacent radical centers was observed in DFT studies.

Periphery Modification of Tetrathiafulvalenes: Recent Development and Applications

Tetrathiafulvalene (TTF) and its analogs are fascinating molecules in materials science based on their excellent electron-donating abilities. This personal account describes recent advances in the synthesis of TTF analogs for functional materials via the palladium-catalyzed modification of peripheries of TTF analogs. We first consider three types of molecules: fluorophore-TTF hybrid molecules, multi-redox systems, and an organic ligand for metal-organic frameworks. These molecules were successfully synthesized via Stille coupling or palladium-catalyzed direct C-H arylation and their structural, electrochemical, and optical properties were clarified. Subsequently, phosphorus-substituted TTF analogs were successfully synthesized for future applications of redox-active phosphine ligands for metal catalysts. The development of these molecules can significantly affect the advancement of chemical science.

The chemistry of phosphines in constrained, well-defined microenvironments

Developments in the confinement of phosphines within micro- or nano-environments are explored. Phosphines are ubiquitous across metal coordination chemistry and underpin some of the most famous homogeneous transition metal catalysts. Constraining phosphines within confined environments influences not only their behaviour but also that of their metal complexes. Notable examples include the use of metal-organic frameworks (MOFs) or metal-organic cages (MOCs) to support phosphines which demonstrate how the microenvironment within such constructs leads to reactivity modification. The development of phosphine confinement is explored and parallels are drawn with related constrained macrocyclic systems and mechanically interlocked molecules. The review concludes by identifying areas that remain a challenge and those that will provide new avenues for research.

Electron-rich Coordination Receptors Based on Tetrathiafulvalene Derivatives: Controlling the Host-Guest Binding

The coordination-driven self-assembly methodology has emerged over the last few decades as an extraordinarily versatile synthetic tool for obtaining discrete macrocyclic or cage structures. Rational approaches using large libraries of ligands and metal complexes have allowed researchers to reach more and more sophisticated discrete structures such as interlocked, chiral, or heteroleptic cages, and some of them are designed for guest binding applications. Efforts have been notably produced in controlling host-guest affinity with, in particular, an evident interest in targeting substrate transportation and subsequent delivering. Recent accomplishments in this direction were described from functional cages which can be addressed with light, pH, or through a chemical exchange. The case of a redox-stimulation has been much less explored. In this case, the charge state of the redox-active cavity can be controlled through an applied electrical potential or introduction of an appropriate oxidizing/reducing chemical agent. Beyond possible applications in electrochemical sensing for environmental and medical sciences as well as for redox catalysis, controlling the cavity charge offers the possibility to modulate the host-guest binding affinity through electrostatic interactions, up to the point of disassembly of the host-guest complex, i.e., releasing of the guest molecule from the host cavity.This Account highlights the key studies that we carried out at Angers, related to discrete redox-active coordination-based architectures (i.e., metalla-rings, -cages, and -tweezers). These species are built upon metal-driven self-assembly between electron-rich ligands, based on the tetrathiafulvalene (TTF) moiety (as well as some of its S-rich derivatives), and various metal complexes. Given the high π-donating character of those ligands, the corresponding host structures exhibit a high electronic density on the cavity panels. This situation is favorable to bind complementary electron-poor guests, as it was illustrated with bis(pyrrolo)tetrathiafulvalene (BPTTF)-based cavities, which exhibit hosting properties for C₆₀ or tetrafluorotetracyanoquinodimethane (TCNQ-F₄). In addition to the pristine tetrathiafulvalene, which was successfully incorporated into palladium- or ruthenium-based architectures, the case of the so-called extended tetrathiafulvalene (exTTF) appears particularly fascinating. A series of related polycationic and neutral M₄L₂ ovoid containers, as well as a M₆L₃ cage, were synthesized, and their respective binding abilities for neutral and anionic guests were studied. Remarkably, such structures allow to control of the binding of the guest upon a redox-stimulation, through two distinctive processes: (i) cage disassembling or (ii) guest displacement. As an extension of this approach, metalla-assembled electron-rich tweezers were designed, which are able to trigger the guest release through an original process based on supramolecular dimerization activated through a redox stimulus. This ensemble of results illustrates the remarkable ability of electron-rich, coordination-based self-assembled cavities to bind various types of guests and, importantly, to trigger their release through a redox-stimulus.

Comparing the self-assembly processes of two redox-active exTTF-based regioisomer ligands

A new exTTF-based ligand was synthesized and its coordination-driven self-assembly behavior with a square planar palladium complex was compared with a previously described regioisomer.

A self-assembled tetrathiafulvalene box

A M₈L₂ metalla-cage constructed through coordination-driven self-assembly from a quinonato bis-ruthenium complex and an electron-rich tetrathiafulvalene (TTF) tetrapyridyl ligand is depicted.

A pentazolate-based bowl-shaped molecular container

A three-dimensional bowl-shaped molecular container based on pentazole was first synthesized. These containers are sealed and linked by the assembled "molecular plane". Each container has an ovoid cavity occupied by one DMSO guest molecule. The self-assembly of this molecular container will provide opportunities for the use of pentazole in supramolecular chemistry.

Redox-Induced Hydrogen Bond Reorientation Mimicking Electronic Coupling in Mixed-Valent Diruthenium and Macrocyclic Tetraruthenium Complexes

We present the coordination-driven self-assembly of three tetranuclear metallacycles containing intracyclic NH₂, OH, or OMe functionalities through the combination of various isophthalic acid building blocks with a divinylphenylene diruthenium complex. All new complexes of this study were characterized by means of nuclear magnetic resonance spectroscopy, ultrahigh-resolution ESI mass spectrometry, cyclic and square wave voltammetry and, in two cases, X-ray diffraction. The hydroxy functionalized macrocycle 4-BOH and the corresponding half-cycle 2-OH stand out, as their intracyclic OH···O hydrogen bonds stabilize their mixed-valent one- (2-OH, 4-BOH) and three-electron-oxidized states (4-BOH). Despite sizable redox splittings between all one-electron waves, the mixed-valent monocations and trications do not exhibit any intervalence charge-transfer band, assignable to through-bond electronic coupling, but nevertheless display distinct IR band shifts of their charge-sensitive Ru(CO) tags. We ascribe these seemingly contradicting observations to a redox-induced shuffling of the OH···O hydrogen bond(s) to the remaining, more electron-rich, reduced redox site.

Kinetic selection of Pd4L2 metallocyclic and Pd6L3 trigonal prismatic assemblies

The self-assembly of Pd4L2 metallocylcic and Pd6L3 trigonal prismatic assemblies are described. The selection of one species over the other has been achieved by careful choice of ancilliary ligands, which switch the dynamics of the Pd-pyridine bonds such that a highly unusual and distorted smaller assembly can be kinetically trapped en route to the more energetically favourable larger species. Both assemblies provide promise as easy to access multicavity reaction vessels.

Catalytic C–H Arylation of Tetrathiafulvalenes for the Synthesis of Functional Materials

Sulfur-containing functional π-conjugated cores play key roles in materials science, mostly due to their unique electrochemical and photophysical properties. Among these, the excellent electron donor tetrathiafulvalene (TTF) has occupied a central position since the emergence of organic electronics. Peripheral C–H modification of this highly useful sulfur-containing motif has resulted in the efficient creation of new molecules that expand the applications of TTFs. This Short Review begins with the development of the palladium-catalyzed direct C–H arylation of TTF. Subsequently, it summarizes the applications of this efficient C–H transformation for the straightforward synthesis of useful TTF derivatives that are employed in a variety of research fields, demonstrating that the development of a new reaction can have a significant impact on chemical science.

1 Introduction

2 Development of the Palladium-Catalyzed Direct C–H Arylation of TTF

3 Synthesis of TTF-Based Tetrabenzoic Acid and Tetrapyridine for MOFs

4 Synthesis of TTF-Based Tetrabenzaldehyde and Tetraaniline for COFs

5 Tetraarylation of TTFAQ

6 Synthesis of Multistage-Redox TTF Derivatives

7 Miscellaneous Examples

8 Conclusions

The Advent of Electrically Conducting Double-Helical Metal-Organic Frameworks Featuring Butterfly-Shaped Electron-Rich π-Extended Tetrathiafulvalene Ligands

To diversify metal-organic framework (MOF) structures beyond traditional Euclidean geometries and to create new charge-delocalization pathways beneficial for electrical conductivity, we constructed a novel double-helical MOF (dhMOF) by introducing a new butterfly-shaped electron-rich π-extended tetrathiafulvalene ligand equipped with four benzoate groups (ExTTFTB). The face-to-face oriented convex ExTTFTB ligands connected by Zn₂(COO)₄ paddlewheel nodes formed ovoid cavities suitable for guest encapsulation, while π-π-interaction between the ExTTFTB ligands of neighboring strands helped create new charge-delocalization pathways in iodine-mediated partially oxidized dhMOF. Iodine vapor diffusion led to oxidation of half of the ExTTFTB ligands in each double-helical strand to ExTTFTB^•+ radical cations, which putatively formed intermolecular ExTTFTB/ExTTFTB^•+ π-donor/acceptor charge-transfer chains with the neutral ExTTFTB ligands of an adjacent strand, creating supramolecular wire-like charge-delocalization pathways along the helix seams. In consequence, the electrical conductivity of dhMOF surged from 10^-8 S/m up to 10^-4 S/m range after iodine treatment. Thus, the introduction of the electron-rich ExTTFTB ligand with a distinctly convex π-surface not only afforded a novel double-helical MOF architecture featuring ovoid cavities and unique charge-delocalization pathways but also, more importantly, delivered a new tool and design strategy for future development of electrically conducting stimuli-responsive MOFs.

Metalla-Assembled Electron-Rich Tweezers: Redox-Controlled Guest Release Through Supramolecular Dimerization

Developing methodologies for on-demand control of the release of a molecular guest requires the rational design of stimuli-responsive hosts with functional cavities. While a substantial number of responsive metallacages have already been described, the case of coordination-tweezers has been less explored. Herein, we report the first example of a redox-triggered guest release from a metalla-assembled tweezer. This tweezer incorporates two redox-active panels constructed from the electron-rich 9-(1,3-dithiol-2-ylidene)fluorene unit that are facing each other. It dimerizes spontaneously in solution and the resulting interpenetrated supramolecular structure can dissociate in the presence of an electron-poor planar unit, forming a 1:1 host-guest complex. This complex dissociates upon tweezer oxidation/dimerization, offering an original redox-triggered molecular delivery pathway.

Tuning the structure and the properties of dithiafulvene metalla-assembled tweezers

An electroactive M₂L₂ metalla-macrocycle constructed through coordination driven self-assembly dimerizes upon oxidation and binds an electro-deficient substrate with a high association constant.

Chiral Self-Sorting in Truxene-Based Metallacages

Two chiral face-rotating metalla-assembled polyhedra were constructed upon self-assembling achiral components, i.e., a tritopic ligand based on a truxene core (10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene) and two different hydroxyquinonato–bridged diruthenium complexes. Both polyhedra were characterized in solution as well as in the solid state by X-ray crystallography. In both cases, the self-sorting process leading to only two homo-chiral enantiomers was governed by non-covalent interactions between both truxene units that faced each other.

Synthesis and Characterization of Ferrocene Based Hemicages

We present a series of tripodal ligands L1-3, which fold into hemicages C1-3 by using coordination-driven dynamic combinational chemistry. The identities of these hemicages were characterized using ¹H NMR, ¹H-¹H COSY, DOSY, and ESI-TWIM-MS. Free rotation of the ferrocene structural units in the ligands affords an adaptable directionality, which is essential for the construction of these hemicages. Encapsulation of adamantane by C2 indicates the presence of a well-defined inner cavity as the binding pocket.

Quantum-Chemical Insights into the Self-Assembly of Carbon-Based Supramolecular Complexes

Understanding how molecular systems self-assemble to form well-organized superstructures governed by noncovalent interactions is essential in the field of supramolecular chemistry. In the nanoscience context, the self-assembly of different carbon-based nanoforms (fullerenes, carbon nanotubes and graphene) with, in general, electron-donor molecular systems, has received increasing attention as a means of generating potential candidates for technological applications. In these carbon-based systems, a deep characterization of the supramolecular organization is crucial to establish an intimate relation between supramolecular structure and functionality. Detailed structural information on the self-assembly of these carbon-based nanoforms is however not always accessible from experimental techniques. In this regard, quantum chemistry has demonstrated to be key to gain a deep insight into the supramolecular organization of molecular systems of high interest. In this review, we intend to highlight the fundamental role that quantum-chemical calculations can play to understand the supramolecular self-assembly of carbon-based nanoforms through a limited selection of supramolecular assemblies involving fullerene, fullerene fragments, nanotubes and graphene with several electron-rich π-conjugated systems.

Functionalised tetrathiafulvalene- (TTF-) macrocycles: recent trends in applied supramolecular chemistry

Tetrathiafulvalene- (TTF-) based macrocyclic systems, cages and supramolecularly self-assembled 3D constructs have been extensively explored as functional materials for sensing and switching applications.

Self-Assembly with 2,6-Bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine: Silver(I) and Iron(II) Complexes

A new "click" ligand, 2,6-bis(1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl)pyridine (L) featuring a tridentate 2,6-bis(1,2,3-triazol-4-yl)pyridine (tripy) pocket and two pyridyl (py) units was synthesized in modest yield (42%) using the copper(I) catalyzed azide-alkyne cycloaddition (CuAAC) reaction. The coordination chemistry of the ligand with silver(I) and iron(II) ions was examined using a battery of solution (¹H and DOSY (diffusion ordered spectroscopy) nuclear magnetic resonance (NMR), infrared and absorption spectroscopies, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS)), and solid state (X-ray crystallography, elemental analysis) techniques. When treated with silver(I) ions, the ligand forms discrete [Ag(L)]⁺ (X-, where X- = BF₄-, NO₃- or SbF₆-) complexes in dimethyl sulfoxide (DMSO) solution but these complexes crystallize as coordination polymers. The addition of [Fe(H₂O)₆](BF₄)₂ to an acetonitrile solution of the ligand forms the expected monomeric octahedral [Fe(L)₂]2+ complex and treatment of the iron(II) complex with AgBF₄ generates a heterometallic linear coordination polymer.

The Intricate Structural Chemistry of MII2nLn-Type Assemblies

The reaction of cis-blocked, square-planar M^II complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula M_2nL_n. These assemblies typically adopt barrel-like structures, with the ligands paneling the sides of the barrels. However, alternative structures are possible, as demonstrated by the recent discovery of a Pt₈L₄ cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of M^II_2nL_n complexes are not well understood. Herein, we provide a geometric analysis of M_2nL_n complexes, and we discuss how size and geometry of the ligand is expected to influence the self-assembly process. The theoretical analysis is complemented by experimental studies using different cis-blocked Pt^II complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type Pt₈L₄, which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a Pt₁₀L₅ complex and a Pt₁₆L₈ complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The Pt₁₆L₈ complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized M_2nL_n complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal Pt₁₀L₅ barrels, which are formed in nearly quantitative yields.

A self-assembled M2L4 cage incorporating electron-rich 9-(1,3-dithiol-2-ylidene)fluorene units

An electron-rich M2L4 cage is depicted, in which the four peripheral redox-active ligands (L) are 9-(1,3-dithiol-2-ylidene)fluorene units.

Bis- and Tris-fused Tetrathiafulvalenes Extended with Anthracene-9,10-diylidene

Bis- and tris-fused π-electron donors composed of extended tetrathiafulvalene with anthraquinoid spacers (4 and 5) were successfully synthesized. X-ray structure analysis of tetrakis(methylthio)-5 (5a) revealed that the molecule adopted a transoid-cisoid conformation. The cyclic voltammogram of 4a is composed of two pairs of two-electron redox waves, while the cyclic voltammogram of tetrakis(hexylthio) derivative 5b consists of one pair of four-electron redox waves and one pair of two-electron redox waves, respectively. Spectroelectrochemistry of 4a and ¹H NMR spectrum of a 4b salt revealed that two positive charges in 4²⁺ are distributed mainly on one TTFAQ (9,10-bis(1,3-dithiol-2-ylidene)-9,10-dihydroanthracene) moiety.

Neutral versus polycationic coordination cages: a comparison regarding neutral guest inclusion

A neutral self-assembled container synthesized from a concave π-extended tetrathiafulvalene (exTTF) ligand and the cis-Pd(dctfb)₂(cod) complex (dctfb = 3,5-dichloro-2,4,6-trifluorobenzene; cod = 1,5-cyclooctadiene) is described.

Aryl-fused tetrathianaphthalene (TTN): synthesis, structures, properties, and cocrystals with fullerenes

A library of aryl-fused TTN has been synthesized to show shape complementary with fullerene molecules and form “TTN·fullerene” cocrystals.

Reversible Guest Uptake/Release by Redox-Controlled Assembly/Disassembly of a Coordination Cage

Controlling the guest expulsion process from a receptor is of critical importance in various fields. Several coordination cages have been recently designed for this purpose, based on various types of stimuli to induce the guest release. Herein, we report the first example of a redox-triggered process from a coordination cage. The latter integrates a cavity, the panels of which are based on the extended tetrathiafulvalene unit (exTTF). The unique combination of electronic and conformational features of this framework (i.e. high π-donating properties and drastic conformational changes upon oxidation) allows the reversible disassembly/reassembly of the redox-active cavity upon chemical oxidation/reduction, respectively. This cage is able to bind the three-dimensional B12 F12 (2-) anion in a 1:2 host/guest stoichiometry. The reversible redox-triggered disassembly of the cage could also be demonstrated in the case of the host-guest complex, offering a new option for guest-delivering control.

Self-Assembled Cyclophane-Type Copper(I) Complexes of 2,4,6-Tris(diphenylphosphino)-1,3,5-triazine and Their Catalytic Application

The triazine-based trisphosphine, 2,4,6-tris(diphenylphosphino)-1,3,5-triazine (1) was prepared in improved yield by reacting cyanuric chloride with 3 equiv of trimethylsilyldiphenylphosphine. The solid-state structure of 1 showed short intermolecular P···P contacts of 3.362 Å, which is significantly shorter than the sum of the van der Waals radii of phosphorus atoms (3.6 Å). The reaction of 2,4,6-tris(diphenylphosphino)-1,3,5-triazine (1) with copper(I) salts in a 2:3 molar ratio yielded various cyclophane-type complexes in quantitative yield. The solid-state structures of these clusters have been found to depend on the size of the halide ions, the solvent employed, and the reaction conditions. Copper(I) chloride formed a monomeric metallocyclophane, whereas copper(I) bromide and copper(I) iodide derivatives preferred dimeric and 1D-polymeric structures, respectively. The tricationic complexes derived from Cu(I) ion and 2,4,6-tris(diphenylphosphino)-1,3,5-triazine also adopted monomeric metallocyclophane structures. These complexes have been employed in the A(3) coupling reaction under microwave irradiation. The copper(I) iodide derivative showed excellent catalytic efficiency.