Supramolecular Architectures with π-Acidic 3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine Cavities: Role of Anion−π Interactions in the Remarkable Stability of Fe(II) Metallacycles in Solution

Non-Templated Assembly of D5h-Symmetric Pd5L10 Rings by Precise Ligand Angle Adjustment

We report a series of Pd(II)nL2n coordination rings for which nuclearity is controlled by the binding angle of the corresponding bis-monodentate bridging ligands. Judicious choice of the angle within a family of rather rigid ligands allowed for the first-time to synthesize a homoleptic five-membered Pd5L10 ring that does not require any template to form. We demonstrate that control over the ring size is maintained both in the solid-, solution-, and gas-phase. Two X-ray structures of five-membered rings from ligands with ideal angles (yielding a perfect pentagonal ring) vs. suboptimal angles (resulting in a highly distorted structure) illustrate the importance of the correct ligand geometry. A mathematical model for estimating the expected ring size based on the ligand angle was derived and DFT computations show that ring-strain is the major factor determining the assembly outcome.

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.

In Situ Conversion of Ligand to a Coordination Polymer via a Core@Shell Crystal: A Multi-Step Phase-Dependent Structural Transformation

Two pseudopolymorphic 1D coordination polymers of the formulas [Cd(3,3'-pytz)(CH3OH)2(ClO4)2]n (1) and [Cd(3,3'-pytz)(CH3CN)2(ClO4)2]n (2) have been prepared using the electron-deficient 3,6-bis(pyridin-3-yl)-1,2,4,5-tetrazine (3,3'-pytz) ligand and cadmium perchlorate in the chloroform/methanol and chloroform/acetonitrile solvent system, respectively. It was observed that compounds 1 and 2 experienced one-step (CPreagent → CPproduct) single-crystal-to-powder structural transformation to the pure water-coordinated compound [Cd(3,3'-pytz)(H2O)2(ClO4)2]n (3) by absorbing water vapor from air (solid-gas phase transformation). Interestingly, compounds 1, 2, and 3 undergo a different transformation path and show an in situ unique three-step (CPreagent → CPproduct → Ligandintermediate → CPproduct) single-crystal-to-single-crystal (SCSC) structural transformation process through soaking in deionized water (solid-liquid phase transformation). In this fascinating transformation, we report for the first time the direct conversion of a ligand into a coordination polymer by a rare core-shell pathway in a solid-liquid phase transformation. In this process, we obtained compound {[Cd(3,3'-pytz)(H2O)4](3,3'-pytz)2(ClO4)2(H2O)6}n (4) (single-crystal = S, crystal = C, or microcrystal = P) as mixed compounds of core-shell L@4C and 4S or core-shell L@4P and 4P for compounds (1 and 2) and 3, respectively.

Cage Match: Comparing the Anion Binding Ability of Isostructural Versus Isofunctional Pairs of Metal-Organic Nanocages

Affinities of six anions (mesylate, acetate, trifluoroacetate, p-toluenecarboxylate, p-toluenesulfonate, and perfluorooctanoate) for three related Pt2+ -linked porphyrin nanocages were measured to probe the influence of different noncovalent recognition motifs (e. g., hydrogen bonding, electrostatics, π bonding) on anion binding. Two new hosts of M6 L3 12+ (1b) and M4 L2 8+ (2) composition (M=(en)Pt2+ , L=(3-py)4 porphyrin) were prepared in a one-pot synthesis and allowed comparison of hosts that differ in structure while maintaining similar N-H hydrogen-bond donor ability. Comparisons of isostructural hosts that differ in hydrogen-bonding ability were made between 1b and a related M6 L3 12+ nanoprism (1a, M=(tmeda)Pt2+ ) that lacks N-H groups. Considerable variation in association constants (K1 =1.6×103  M-1 to 1.3×108  M-1 ) and binding mode (exo vs. endo) were found for different host-guest combinations. Strongest binding was seen between p-toluenecarboxylate and 1b, but surprisingly, association of this guest with 1a was only slightly weaker despite the absence of NH⋅⋅⋅O interactions. The high affinity between p-toluenecarboxylate and 1a could be turned off by protonation, and this behavior was used to toggle between the binding of this guest and the environmental pollutant perfluorooctanoate, which otherwise has a lower affinity for the host.

Crystal engineering of high explosives through lone pair-π interactions: Insights for improving thermal safety

In this high-risk/high-reward study, we prepared complexes of a high explosive anion (picrate) with potentially explosive s-tetrazine-based ligands with the sole purpose of advancing the understanding of one of the weakest supramolecular forces: the lone pair-π interaction. This is a proof-of-concept study showing how lone pair-π contacts can be effectively used in crystal engineering, even of high explosives, and how the supramolecular architecture of the resulting crystalline phases influences their experimental thermokinetic properties. Herein we present XRD structures of 4 novel detonating compounds, all showcasing lone pair-π interactions, their thermal characterization (DSC, TGA), including the correlation of experimental thermokinetic parameters with crystal packing, and in silico explosion properties. This last aspect is relevant for improving the safety of high-energy materials.

Shape-Dependent Complementary Ditopic Terpyridine Pair with Two Levels of Self-Recognition for Coordination-Driven Self-Assembly

Molecular recognition in biological systems plays a vital role in the precise construction of biomacromolecules and the corresponding biological activities. Such recognition mainly relies on the highly specific binding of complementary molecular pairs with complementary sizes, shapes, and intermolecular forces. It still remains challenging to develop artificial complementary motif pairs for coordination-driven self-assembly. Herein, a series of shape-dependent complementary motif pairs, based on ditopic 2,2':6',2″-terpyridine (TPY) backbone, are designed and synthesized. The fidelity degrees of self-assemblies from these motifs are carefully evaluated by multi-dimensional mass spectrometry, nuclear magnetic resonance spectroscopy, and molecular modeling. In addition, two levels of self-recognition in both homoleptic and heteroleptic assembly are discovered in the assembled system. Through finely tuning the shape and size of the ligands, a complementary pair is developed with error-free narcissistically self-sorting at two levels of self-recognition, and the intrinsic principle is carefully investigated.

Versatile Superatom Complex Nanocluster for the Construction of Framework Materials

Accurately controlling the assembly of nanometer-sized building blocks presents an important but significant challenge for the construction of functional framework materials, which requires the development of highly stable versatile nanosized assembly modules with multiple coordination sites. In this study, [Ag₂₃(SAdm)₁₂]³⁺ (Ag₂₃, in which SAdm = 1-adamantanethiol, i.e., C₁₀H₁₅S), a chiral superatom complex nanocluster, was synthesized and assembled into various topologies. We constructed two kinds of framework materials, i.e., superatom complex inorganic framework (SCIF) and superatom complex organic framework (SCOF) materials, including [Ag₂₃(SAdm)₁₂](SbF₆)₂X (Ag₂₃-1; X = Cl^-/SbF₆^-, a SCIF), [Ag₂₃(SAdm)₁₂](SbF₆)₃ (Ag₂₃-2, a SCIF), [Ag₂₃(SAdm)₁₂](SbF₆)₃(bpy)₃ (Ag₂₃-bpy, a SCOF, in which bpy = 4,4'-bipyridine, i.e., C₁₀H₈N₂), and [Ag₂₃(SAdm)₁₂](SbF₆)₃(dpbz)₃ (Ag₂₃-dpbz, a SCOF, in which dpbz = 1,4-bis(4-pyridyl)benzene, i.e., C₁₆H₁₂N₂), owing to strong interactions between the versatile Ag₂₃ and the inorganic and organic linkers. Ag₂₃-1, Ag₂₃-2, and Ag₂₃-bpy exhibit two superstructures with interpenetrating frameworks and adamantane-like, hexagonal, and cubic topologies, while Ag₂₃-dpbz displays three superstructures with interpenetrating frameworks and cubic topologies. Ag₂₃-dpbz exhibits the largest specific surface area as well as the strongest photoluminescence and electrochemiluminescence signals owing to its dense network arrangement. This work contributes to the construction of nanocluster-based framework materials and helps to elucidate the effect of the assembly mode on the material properties and functionalities.

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.

Pt(ii)-coordinated tricomponent self-assemblies of tetrapyridyl porphyrin and dicarboxylate ligands: are they 3D prisms or 2D bow-ties?

Thermodynamically favored simultaneous coordination of Pt(ii) corners with aza- and carboxylate ligands yields tricomponent coordination complexes with sophisticated structures and functions, which require careful structural characterization to paint accurate depiction of their structure-function relationships. Previous reports claimed that heteroleptic coordination of cis-(Et3P)2PtII with tetrapyridyl porphyrins (M'TPP, M' = Zn or H2) and dicarboxylate ligands (XDC) yielded 3D tetragonal prisms containing two horizontal M'TPP faces and four vertical XDC pillars connected by eight Pt(ii) corners, even though such structures were not supported by their 1H NMR data. Through extensive X-ray crystallographic and NMR studies, herein, we demonstrate that self-assembly of cis-(Et3P)2PtII, M'TPP, and four different XDC linkers having varied lengths and rigidities actually yields bow-tie (⋈)-shaped 2D [{cis-(Et3P)2Pt}4(M'TPP) (XDC)2]4+ complexes featuring a M'TPP core and two parallel XDC linkers connected by four heteroleptic PtII corners instead of 3D prisms. This happened because (i) irrespective of their length (∼7-11 Å) and rigidity, the XDC linkers intramolecularly bridged two adjacent pyridyl-N atoms of a M'TPP core via PtII corners instead of connecting two cofacial M'TPP ligands and (ii) bow-tie complexes are entropically favored over prisms. The electron-rich ZnTPP core of a representative bow-tie complex selectively formed a charge-transfer complex with highly π-acidic 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-heaxacarbonitrile but not with a π-donor such as pyrene. Thus, this work not only produced novel M'TPP-based bow-tie complexes and demonstrated their selective π-acid recognition capability, but also underscored the importance of proper structural characterization of supramolecular assemblies to ensure accurate depiction of their structure-property relationships.

Anion-π Catalysis Enabled by the Mechanical Bond

We report a series of rotaxane‐based anion–π catalysts in which the mechanical bond between a bipyridine macrocycle and an axle containing an NDI unit is intrinsic to the activity observed, including a [3]rotaxane that catalyses an otherwise disfavoured Michael addition in >60 fold selectivity over a competing decarboxylation pathway that dominates under Brønsted base conditions. The results are rationalized by detailed experimental investigations, electrochemical and computational analysis.

Mining anion-aromatic interactions in the Protein Data Bank

Mutual positioning and non-covalent interactions in anion–aromatic motifs are crucial for functional performance of biological systems. In this context, regular, comprehensive Protein Data Bank (PDB) screening that involves various scientific points of view and individual critical analysis is of utmost importance. Analysis of anions in spheres with radii of 5 Å around all 5- and 6-membered aromatic rings allowed us to distinguish 555 259 unique anion–aromatic motifs, including 92 660 structures out of the 171 588 structural files in the PDB. The use of a scarcely exploited (x, h) coordinate system led to (i) identification of three separate areas of motif accumulation: A – over the ring, B – over the ring-substituent bonds, and C – roughly in the plane of the aromatic ring, and (ii) unprecedented simultaneous comparative description of various anion–aromatic motifs located in these areas. Of the various residues considered, i.e. aminoacids, nucleotides, and ligands, the latter two exhibited a considerable tendency to locate in region Avia archetypal anion–π contacts. The applied model not only enabled statistical quantitative analysis of space around the ring, but also enabled discussion of local intermolecular arrangements, as well as detailed sequence and secondary structure analysis, e.g. anion–π interactions in the GNRA tetraloop in RNA and protein helical structures. As a purely practical issue of this work, the new code source for the PDB research was produced, tested and made freely available at https://github.com/chemiczny/PDB_supramolecular_search.

The comprehensive analysis of non-redundant PDB macromolecular structures investigating anion distributions around all aromatic molecules in available biosystems is presented.

A Hydrogen-Bonded Ravel Assembled by Anion Coordination

Anion-coordination-driven assembly (ACDA) is showing increasing power in the construction of anionic supramolecular architectures. Herein, by expanding the anion centers from oxoanion (phosphate or sulfate) to organic tris-carboxylates, an Archimedean solid (truncated tetrahedron) and a highly entangled, double-walled tetrahedron featuring a ravel topology have been assembled with tris-bis(urea) ligands. The results demonstrate the promising ability of tris-carboxylates as new anion coordination centers in constructing novel topologies with increasing complexity and diversity compared to phosphate or sulfate ions on account of the modifiable size and easy functionalization character of these organic anions.

Being positive is not everything - experimental and computational studies on the selectivity of a self-assembled, multiple redox-state, receptor that binds anions with up to picomolar affinities

The interaction of the self-assembled trinuclear ruthenium bowl 1 3+ , that displays three other accessible oxidation states, with oxo-anions is investigated. Using a combination of NMR and electrochemical experimental data, estimates of the binding affinities of 1 4+ , 1 5+ , and 1 6+ for both halide and oxo-anions were derived. This analysis revealed that, across the range of oxidation states of the host, both high anion binding affinities (>10 9 M -1 for specific guests bound to 1 6+ ) and high selectivities (a range of >10 7 M -1 ) were observed. As the crystal structure of binding of the hexafluorophosphate anion revealed that the host has two potential binding sites (named the α and β pockets), the host-guest properties of both putative binding sites of the bowl, in all of its four oxidation states, were investigated through detailed quantum-based computational studies. These studies revealed that, due to the interplay of electrostatically assisted hydrogen-bonding and anion-π interactions, binding to the α pocket is generally preferred, except for the case of the relatively large and lipophilic hexafluorophosphate anionic guest and the host in the highest oxidation states, where the β pocket becomes relatively favourable. This analysis confirms that host-guest interactions involving structurally complex supramolecular architectures are driven by a combination of non-covalent interactions and, even in the case of charged binding pairs, electrostatics alone cannot accurately define these recognition processes.

Direct Cu-mediated aromatic 18F-labeling of highly reactive tetrazines for pretargeted bioorthogonal PET imaging

Pretargeted imaging can be used to visualize and quantify slow-accumulating targeting vectors with short-lived radionuclides such as fluorine-18 - the most popular clinically applied Positron Emission Tomography (PET) radionuclide. Pretargeting results in higher target-to-background ratios compared to conventional imaging approaches using long-lived radionuclides. Currently, the tetrazine ligation is the most popular bioorthogonal reaction for pretargeted imaging, but a direct 18F-labeling strategy for highly reactive tetrazines, which would be highly beneficial if not essential for clinical translation, has thus far not been reported. In this work, a simple, scalable and reliable direct 18F-labeling procedure has been developed. We initially studied the applicability of different leaving groups and labeling methods to develop this procedure. The copper-mediated 18F-labeling exploiting stannane precursors showed the most promising results. This approach was then successfully applied to a set of tetrazines, including highly reactive H-tetrazines, suitable for pretargeted PET imaging. The labeling succeeded in radiochemical yields (RCYs) of up to approx. 25%. The new procedure was then applied to develop a pretargeting tetrazine-based imaging agent. The tracer was synthesized in a satisfactory RCY of ca. 10%, with a molar activity of 134 ± 22 GBq μmol-1 and a radiochemical purity of >99%. Further evaluation showed that the tracer displayed favorable characteristics (target-to-background ratios and clearance) that may qualify it for future clinical translation.

The π-hole revisited

It follows from the Schrödinger equation that the forces operating within molecules and molecular complexes are Coulombic, which necessarily entails both electrostatics and polarization. A common and important class of molecular complexes is due to π-holes. These are molecular regions of low electronic density that are perpendicular to planar portions of the molecular frameworks. π-Holes often have positive electrostatic potentials associated with them, which result in mutually polarizing attractive forces with negative sites such as lone pairs, π electrons or anions. In many molecules, π-holes correspond to a flattening of the electronic density surface but in benzene derivatives and in polyazines the π-holes are craters above and below the rings. The interaction energies of π-hole complexes can be expressed quite well in terms of regression relationships that account for both the electrostatics and the polarization. There is a marked gradation in the interaction energies, from quite weak (about -2 kcal mol-1) to relatively strong (about -40 kcal mol-1). Gradations are also evident in the ratios of the intermolecular separations to the sums of the respective van der Waals radii and in the gradual transition of the π-hole atoms from trigonal to quasi-tetrahedral configurations. These trends are consistent with the concept that chemical interactions form a continuum, from very weak to very strong.

Self-Assembled Manganese(I)-Based Selenolato-Bridged Tetranuclear Metallorectangles: Host-Guest Interaction, Anticancer, and CO-Releasing Studies

Supramolecular one-step self-assembly of dimanganese decacarbonyl, diaryl diselenide, and linear dipyridyl ligands (L = pyrazine (pz), 4,4'-bipyridine (bpy), and trans-1,2-bis(4-pyridyl)ethylene (bpe)) has resulted in the formation of selenolato-bridged manganese(I)-based metallorectangles. The synthesis of tetranuclear Mn(I)-based metallorectangles [{(CO)₃Mn(μ-SeR)₂Mn(CO)₃}₂(μ-L)₂] (1-6) was facilitated by the oxidative addition of diaryl diselenide to dimanganese decacarbonyl with the simultaneous coordination of linear bidentate pyridyl linker in an orthogonal fashion. Formation of metallorectangles 1-6 was ascertained using IR, UV-vis, NMR spectroscopic techniques, and elemental analyses. The molecular mass of compounds 2, 4, and 6 were determined by ESI-mass spectrometry. Solid-state structural elucidation of 2, 3, and 6 by single-crystal X-ray diffraction methods revealed a rectangular framework wherein selenolato-bridges and pyridyl ligands define the shorter and longer edges, respectively. Also, the guest binding capability of metallorectangles 3 and 5 with different aromatic guests was studied using UV-vis absorption and emission spectrophotometric titration methods that affirmed strong host-guest binding interactions. The formation of the host-guest complex between metallorectangle 3 and pyrene has been explicitly corroborated by the single-crystal X-ray structure of 3•pyrene. Moreover, select metallorectangles 1-4 and 6 were studied to explore their anticancer activity, while CO-releasing ability of metallorectangle 2 was further appraised using equine heart myoglobin assay.

Exploring the structure-property schemes in anion-π systems of d-block metalates

Anion-π based compounds, materials, and processes have gained significant interest due to the diversity of their aesthetic non-covalent synthons, and thanks to their significance in biological systems, catalytic processes, anion binding and sensing, or the supramolecular organization of hierarchical architectures. While systems based on typical inorganic anions or organic residues have been widely reviewed in recent years, those involving anionic d metal comlexes as the main components have been treated with a rather secondary interest. However, actively exploring the new systems of the latter type we have recognized systematic advances in the field. As a result, in the current review we describe the landscape that has recently emerged. Focusing on the established groups of π-acidic species, i.e. polycarbonitirles, polyazines, polyazine N-oxides, diimide derivatives, fluoroarenes, and nitroarenes, we explore and discuss anion-π crystal engineering together with the structure-property schemes important from the standpoint of charge transfer (CT) and electron transfer (ET), magnetism, luminescence, reactivity and catalysis, and the construction of core-shell crystalline composites.

Strong Coupling and Slow Relaxation of the Magnetization for an Air-Stable [Co4] Square with Both Tetrazine Radicals and Azido Bridges

Introducing both tetrazine radical and azido bridges afforded two air-stable square complexes [M^II₄(bpztz^•-)₄(N₃)₄] (M^II = Zn²⁺, 1; Co²⁺, 2; bpztz = 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine), where the metal ions are cobridged by μ_1,1-azido bridges and tetrazine radicals. Magnetic studies revealed strong antiferromagnetic metal-radical interaction with a coupling constant of -64.7 cm^-1 in the 2J formalism in 2. Remarkably, 2 exhibits slow relaxation of magnetization with an effective barrier for spin reverse of 96 K at zero applied field.

Controlled Self-Assembly and Multistimuli-Responsive Interconversions of Three Conjoined Twin-Cages

Stimuli-responsive structural transformations between discrete coordination supramolecular architectures not only are essential to construct smart functional materials but also provide a versatile molecular-level platform to mimic the biological transformation process. We report here the controlled self-assembly of three topologically unprecedented conjoined twin-cages, i.e., one stapled interlocked Pd₁₂L₆ cage (2) and two helically isomeric Pd₆L₃ cages (3 and 4) made from the same cis-blocked palladium corners and a new bis-bidentate ligand (1). While cage 2 features three mechanically coupled cavities, cages 3 and 4 are topologically isomeric helicate-based twin-cages based on the same metal/ligand stoichiometry. Sole formation of cage 2 or a dynamic mixture of cages 3 and 4 can be controlled by changing the solvents employed during the self-assembly. Structural conversions between cages 3 and 4 can be triggered by changes in both temperature/solvent and induced-fit guest encapsulations. Well-controlled interconversion between such topologically complex superstructures may lay a solid foundation for achieving a variety of functions within a switchable system.

Enhancing Magnetic Communication between Metal Centres: The Role of s-Tetrazine Based Radicals as Ligands

Although 1,2,4,5-tetrazines or s-tetrazines have been known in the literature for more than a century, their coordination chemistry has become increasingly popular in recent years due to their unique redox activity, multiple binding sites and their various applications. The electron-poor character of the ring and stabilization of the radical anion through all four nitrogen atoms in their metal complexes provide new aspects in molecular magnetism towards the synthesis of new high performing Single Molecule Magnets (SMMs). The scope of this review is to examine the role of s-tetrazine radical ligands in transition metal and lanthanide based SMMs and provide a critical overview of the progress thus far in this field. As well, general synthetic routes and new insights for the preparation of s-tetrazines are discussed, along with their redox activity and applications in various fields. Concluding remarks along with the limitations and perspectives of these ligands are discussed.

Discriminative Behavior of a Donor-Acceptor-Donor Triad toward Cyanide and Fluoride: Insights into the Mechanism of Naphthalene Diimide Reduction by Cyanide and Fluoride

A new molecular donor-acceptor-donor (D-A-D) triad, comprised of an electron deficient 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) unit covalently connected to two flanking photosensitizers, i.e., a bis-heteroleptic Ru(II) complex of 1,10-phenanthroline and pyridine triazole hybrid ligand, is described. The single crystal X-ray structure of the perchlorate salt of the triad demonstrates that the electron deficient NDI unit can act as a host for anions via anion-π interaction. Detailed solution-state studies indicate that fluoride selectively interacts with the D-A-D triad to form a dianionic NDI, NDI^2-, via a radical anion, NDI^•-. On the contrary, cyanide reduces the NDI moiety to NDI^•-, as confirmed by UV-vis, NMR, and EPR spectroscopy. Further, femtosecond transient absorption spectroscopic studies reveal a low luminescence quantum yield of the D-A-D triad attributable to the photoinduced electron transfer (PET) process from the photoactive Ru(II) center to the NDI unit. Interestingly, the triad displays "OFF-ON" luminescence behavior in the presence of fluoride by restoring the Ru(II) to phenanthroline/pyridine-triazole-based MLCT emission, whereas cyanide fails to show a similar property due to a different redox process operational in the latter. The reduction of NDI in the presence of fluoride and cyanide in different polar solvents indicates that involvement of such deprotonated solvents in the electron transfer mechanism may not be operative in our present system. Low-temperature kinetic studies support the formation of a charge transfer associative transient species, which likely allows overcoming the thermodynamically uphill barrier for the direct electron transfer mechanism.

1,2,4,5-Tetrazine based ligands and complexes

One of the most intriguing nitrogen based aromatic heterocycles is 1,2,4,5-tetrazine or s-tetrazine (TTZ) thanks to its electron acceptor character and fluorescence properties and the possibilities of functionalization in the 3 and 6 positions allowing access to various ligands. In this review we focus on the two main families of TTZ based ligands, i.e. ditopic symmetric and monotopic non-symmetric, together with their metal complexes, with a special emphasis on their solid state structures and physical properties. After a description of the most representative complexes containing unsubstituted TTZ as a ligand, symmetric TTZ ligands and complexes derived thereof are discussed in the order: 3,6-bis(2-pyridyl)-tetrazine, 3,6-bis(3-pyridyl)-tetrazine, 3,6-bis(4-pyridyl)-tetrazine, 3,6-bis(2-pyrimidyl)-tetrazine, 3,6-bis(2-pyrazinyl)-tetrazine, 3,6-bis(monopicolylamine)-tetrazine, 3,6-bis(vanillin-hydrazinyl)-tetrazine and TTZ containing carboxylic acids. Remarkable results have been obtained in recent years for metal-organic frameworks and magnetic compounds in which magnetic coupling is enhanced when the tetrazine bridge is reduced to radical anions. Non-symmetric ligands, such as dipicolylamine-TTZ and monopicolylamine-TTZ, are comparatively more recent than the symmetric ones. They allow in principle the preparation of mononuclear complexes in a controlled manner, although binuclear complexes have been isolated as well. Moreover, in the monopicolylamine-TTZ-Cl ligand, deprotonation of the amine, thanks to the electron acceptor character of TTZ, afforded a negatively charged ligand equivalent of a guanidinate.

Temperature Controls Guest Uptake and Release from Zn4L4 Tetrahedra

We report the preparation of triazatruxene-faced tetrahedral cage 1, which exhibits two diastereomeric configurations (T1 and T2) that differ in the handedness of the ligand faces relative to that of the octahedrally coordinated metal centers. At lower temperatures, T1 is favored, whereas T2 predominates at higher temperatures. Host–guest studies show that T1 binds small aliphatic guests, whereas T2 binds larger aromatic molecules, with these changes in binding preference resulting from differences in cavity size and degree of enclosure. Thus, by a change in temperature the cage system can be triggered to eject one bound guest and take up another.

Combining Synthesis and Self-Assembly in One Pot To Construct Complex 2D Metallo-Supramolecules Using Terpyridine and Pyrylium Salts

Multicomponent self-assembly in one pot provides an efficient way for constructing complex architectures using multiple types of building blocks with different levels of interactions orthogonally. The preparation of multiple types of building blocks typically includes tedious synthesis. Here, we developed a multicomponent synthesis/self-assembly strategy, which combined covalent interaction (C-N bond, formed through condensation of pyrylium salt with primary amine) and metal-ligand interaction (N → Zn bond, formed through 2,2':6',2″-terpyridine-Zn coordination) in one pot. The high compatibility of this pair of interactions smoothly and efficiently converted three and four types of components into the desired complex structures, which are supramolecular Kandinsky Circles and spiderwebs, respectively.

A Co8 metallacycle stabilized by double anion–π interactions

The syntheses, crystal structures and magnetic studies of two new Co^II 2,2′-bipyrimidine bridged complexes stabilized by hydrogen bonding versus anion–π interactions are reported.

Self-assembly of polycyclic supramolecules using linear metal-organic ligands

Coordination-driven self-assembly as a bottom-up approach has witnessed a rapid growth in building giant structures in the past few decades. Challenges still remain, however, within the construction of giant architectures in terms of high efficiency and complexity from simple building blocks. Inspired by the features of DNA and protein, which both have specific sequences, we herein design a series of linear building blocks with specific sequences through the coordination between terpyridine ligands and Ru(II). Different generations of polycyclic supramolecules (C1 to C5) with increasing complexity are obtained through the self-assembly with Cd(II), Fe(II) or Zn(II). The assembled structures are characterized via multi-dimensional mass spectrometry analysis as well as multi-dimensional and multinuclear NMR (1H, COSY, NOESY) analysis. Moreover, the largest two cycles C4 and C5 hierarchically assemble into ordered nanoscale structures on a graphite based on their precisely-controlled shapes and sizes with high shape-persistence.

Sequential modes of crosslinking tune viscoelasticity of cell-instructive hydrogels

Materials that can mimic the fibrillar architecture of native extracellular matrix (ECM) while allowing for independent regulation of viscoelastic properties may serve as ideal, artificial ECM (aECM) to regulate cell functions. Here we describe an interpenetrating network of click-functionalized alginate, crosslinked with a combination of ionic and covalent crosslinking, and fibrillar collagen type I. Varying the mode and magnitude of crosslinking enables tunable stiffness and viscoelasticity, while altering neither the hydrogel's microscale architecture nor diffusional transport of molecules with molecular weight relevant to typical nutrients. Further, appropriately timing sequential ionic and covalent crosslinking permits self-assembly of collagen into fibrillar structures within the network. Culture of human mesenchymal stem cells (MSCs) in this mechanically-tunable ECM system revealed that MSC expression of immunomodulatory markers is differentially impacted by the viscoelasticity and stiffness of the matrix. Together, these results describe and validate a novel material system for investigating how viscoelastic mechanical properties of ECM regulate cellular behavior.

Otherwise Unstable Structures Self-Assemble in the Cavities of Cuboctahedral Coordination Cages

We present a method for the directed self-assembly of interlocked structures and coordination complexes in a set of metal-organic hosts. New homo- and heteroleptic metal complexes-species that cannot be prepared outside-form within the cavities of cuboctahedral coordination cages. When linear bidentate guests and macrocycles are sequentially introduced to the host, a rotaxane is threaded internally; the resulting ternary host-guest complex is a new kind of molecular gyroscope. Tetradentate guests segregate the cavities of these cages into distinct spaces, promoting new stoichiometries and modes of ligand binding to metal ions. The behaviors of bound complexes were observed to alter markedly as a result of confinement: In situ oxidations and spin transitions, neither of which occur ex situ, were both observed to proceed. By providing a tailored space for new modes of coordination-driven self-assembly, the inner phases of cuboctahedral coordination cages provide a new medium for synthetic coordination chemistry.

Kineticomechanistic Study of the Redox pH Cycling Processes Occurring on a Robust Water-Soluble Cyanido-Bridged Mixed-Valence {CoIII/FeII}2 Square

A kineticomechanistic study of reversible electron-transfer processes undergone by the water-soluble, cyanido-bridged mixed-valence [{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^II(CN)₄}₂]^2- square has been carried out. The oxidation reaction consists of a two-step process with the participation of a solvent-assisted outer-sphere complex, as a result of the establishment of hydrogen bonds that involve the oxo groups of the oxidant (peroxodisulfate) and the terminal cyanido ligands of the tetrametallic square. The formally endergonic reduction reaction of the fully oxidized ([{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^III(CN)₄}₂]) core by water, producing hydrogen peroxide from water even at low pH values, is also a two-step process. Each one of these processes requires a set of two preequilibria involving the association of OH^- and its subsequent deprotonation by a further OH^- anion. The structure of the square compound in its fully protonated form has also been determined by X-ray diffraction and shows the existence of strong hydrogen-bonding interactions, in agreement with the rather high basicity of the terminal cyanido ligands. Likewise, density functional theory calculations on the tetrametallic complex showed zones with negative electrostatic potential around the Fe^II centers of the square that would account for the establishment of the hydrogen bonds found in the solid state. Spectroelectrochemistry experiments demonstrated the singular stability of the {Co^III/Fe^II}₂^2- complex, as well as that of their partially, {Co₂^III/Fe^IIIFe^II}^-, and fully, {Co^III/Fe^III}₂, oxidized counterparts because no hysteresis was observed in these measurements.

Probing Magnetic-Exchange Coupling in Supramolecular Squares Based on Reducible Tetrazine-Derived Ligands

Reducible 3,6-bis(3,5-dimethyl-pyrazolyl)1,2,4,5-tetrazine was employed to isolate supramolecular air-stable [Co₄ ] and [Zn₄ ] squares, which were achieved via careful selection of counterions rather than the use of reducing agents. Magnetic susceptibility studies revealed a strong radical-Co^II exchange coupling (J_rad-Co /hc=-118 cm^-1 , -2J formalism) with a spin ground state of S_T =4, whereas the unreduced analogue revealed negligible coupling between the Co centers (J_Co-Co /hc=-0.64 cm^-1 ). Radical-radical coupling was also probed in the [Zn₄ ] congener, which led to J_rad-rad /hc=-15.9(5) cm^-1 . These results highlight the versatile air-stable coordination chemistry of tetrazine and the importance of exploiting easily reducible delocalized radical to promote strong exchange coupling between spin carriers.

The structures and properties of anionic tryptophan complexes

IRMPD spectroscopy and electronic structure calculations are employed to identify π–π interactions in ionic tryptophan clusters.

Tuning the dimensionality of organometallic–organic hybrid polymers assembled from [Cp2Mo2(CO)4(η2-P2)], bipyridyl linkers and AgI ions

The first of its kind study of the effect of the organic linker length and reaction temperature on aggregation reactions leading to organometallic–organic hybrid polymers is presented.