Solid State and Solution Study on the Formation of Inorganic Anion Complexes with a Series of Tetrazine-Based Ligands

Ye Olde supramolecular chemistry, its modern rebranding and overarching trends in chemistry

We can describe current contingency of supramolecular chemistry as "post-halogen bonding", with clear reference to the success of the σ-hole model and the halogen bond concepts. This phase is characterized by a strong push towards a new nomenclature for non-covalent interactions, a group-by-group one focusing on the electrophile. As such nomenclature increasingly meets IUPAC endorsement, its proposers report resistances to such ideas, especially in the inorganic and coordination chemistry communities. The whole issue has been generating considerable debate in the last decade. Herein we fully embrace such discussion in the hope of involving a larger share of the relevant communities. Alternative descriptions are here reevaluated, novel views reconnected with older ones, and it is ultimately questioned whether the introduction of such a nomenclature and its subtending ideas would be beneficial. The themes of appreciation of general trends in chemistry, of counterintuitive interactions, of positioning of novel nomenclature with respect to existing ones, and of the extension of group-by-group naming from main block to d-block elements - as key and currently unresolved issues - are discussed. Equivalent, alternative and arguably more comprehensive descriptions are tentatively given, in the hope to overcome controversies together in the pursuit of higher rewards: a comprehensive shared view of supramolecular forces and a common language to express it.

Unleashing the Potential of 1,3-Diketone Analogues as Selective LH2 Inhibitors

Lysyl hydroxylase 2 (LH2) catalyzes the formation of highly stable hydroxylysine aldehyde-derived collagen cross-links (HLCCs), thus promoting lung cancer metastasis through its capacity to modulate specific types of collagen cross-links within the tumor stroma. Using 1 and 2 from our previous high-throughput screening (HTS) as lead probes, we prepared a series of 1,3-diketone analogues, 1-18, and identified 12 and 13 that inhibit LH2 with IC50's of approximately 300 and 500 nM, respectively. Compounds 12 and 13 demonstrate selectivity for LH2 over LH1 and LH3. Quantum mechanics/molecular mechanics (QM/MM) modeling indicates that the selectivity of 12 and 13 may stem from noncovalent interactions like hydrogen bonding between the morpholine/piperazine rings with the LH2-specific Arg661. Treatment of 344SQ WT cells with 13 resulted in a dose-dependent reduction in their migration potential, whereas the compound did not impede the migration of the same cell line with an LH2 knockout (LH2KO).

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.

Assembly of Polyiodide Networks with Cu(II) Complexes of Pyridinol-Based Tetraaza Macrocycles

Polyiodide networks are currently of great practical interest for the preparation of new electronic materials. The participation of metals in the formation of these networks is believed to improve their mechanical performance and thermal stability. Here we report the results on the construction of polyiodide networks obtained using Cu(II) complexes of a series of pyridinol-based tetraazacyclophanes as countercations. The assembly of these crystalline polyiodides takes place from aqueous solutions on the basis of similar structural elements, the [CuL]²⁺ and [Cu(H_–1L)]⁺ (L = L2, L2-Me, L2-Me₃) complex cations, so that the peculiarities induced by the increase of N-methylation of ligands, the structural variable of ligands, can be highlighted. First, solution equilibria involving ligands and complexes were analyzed (potentiometry, NMR, UV–vis, ITC). Then, the appropriate conditions could be selected to prepare polyiodides based on the above complex cations. Single-crystal XRD analysis showed that the coordination of pyridinol units to two metal ions is a prime feature of these ligands, leading to polymeric coordination chains of general formula {[Cu(H_–1L)]}_nⁿ⁺ (L = L2-Me, L2-Me₃). In the presence of the I^–/I₂ couple, the polymerization tendency stops with the formation of [(CuL)(CuH_–1L)]³⁺ (L = L2-Me, L2-Me₃) dimers which are surrounded by polyiodide networks. Moreover, coordination of the pyridinol group to two metal ions transforms the surface charge of the ring from negative to markedly positive, generating a suitable environment for the assembly of polyiodide anions, while N-methylation shifts the directional control of the assembly from H-bonds to I···I interactions. In fact, an extended concatenation of iodine atoms occurs around the complex dimeric cations, the supramolecular I···I interactions become shorter and shorter, fading into stronger forces dominated by the orbital overlap, which is promising for effective electronic materials.

Polyiodides with high iodine density are generated by Cu(II) complexes of pyridinol-based tetraazacyclophanes. Direct coordination of iodine atoms to Cu(II), anion−π interactions with electron-poor aromatic surfaces, and shift of the directional control of assembly from H-bonds to I···I interactions, governed by N-methylation, are the main elements leading to enhanced iodine chaining and strengthening of I···I contacts.

Mono- and Binuclear Copper(II) and Nickel(II) Complexes with the 3,6-Bis(picolylamino)-1,2,4,5-Tetrazine Ligand

Four new compounds of formulas [Cu(hfac)₂(L)] (1), [Ni(hfac)₂(L)] (2), [{Cu(hfac)₂}₂(µ-L)]·2CH₃OH (3) and [{Ni(hfac)₂}₂(µ-L)]·2CH₃CN (4) [Hhfac = hexafluoroacetylacetone and L = 3,6-bis(picolylamino)-1,2,4,5-tetrazine] have been prepared and their structures determined by X-ray diffraction on single crystals. Compounds 1 and 2 are isostructural mononuclear complexes where the metal ions [copper(II) (1) and nickel(II) (2)] are six-coordinated in distorted octahedral MN₂O₄ surroundings which are built by two bidentate hfac ligands plus another bidentate L molecule. This last ligand coordinates to the metal ions through the nitrogen atoms of the picolylamine fragment. Compounds 3 and 4 are centrosymmetric homodinuclear compounds where two bidentate hfac units are the bidentate capping ligands at each metal center and a bis-bidentate L molecule acts as a bridge. The values of the intramolecular metal···metal separation are 7.97 (3) and 7.82 Å (4). Static (dc) magnetic susceptibility measurements were carried out for polycrystalline samples 1–4 in the temperature range 1.9–300 K. Curie law behaviors were observed for 1 and 2, the downturn of χ_MT in the low temperature region for 2 being due to the zero-field splitting of the nickel(II) ion. Very weak [J = −0.247(2) cm⁻¹] and relatively weak intramolecular antiferromagnetic interactions [J = −4.86(2) cm⁻¹] occurred in 3 and 4, respectively (the spin Hamiltonian being defined as H = −JS₁·S₂). Simple symmetry considerations about the overlap between the magnetic orbitals across the extended bis-bidentate L bridge in 3 and 4 account for their magnetic properties.

Words in supramolecular chemistry: the ineffable advances of polyiodide chemistry

Polyiodide chemistry has a rich history deeply intertwined with the development of supramolecular chemistry. Technological and theoretical interest in polyiodides has not diminished in the last decade, quite the contrary; yet the advances this perspective intends to cover are muddled by the involution of supramolecular vocabulary, preventing their unbiased discussion. Herein we discuss the pressing necessity of ordering the current babel of novel - and less so - supramolecular terms. Shared decisions at the community level might be required to shape the field into a harmonious body of knowledge, dominated by concepts rather than words. Secondary, σ-hole and halogen bonding schools of thought are all addressed here, together with their respective impact on the field. Then, on the basis of a shared vocabulary, a discussion of polyiodide chemistry is presented, starting with a revisited view of triiodide. The contemporary fields of supramolecular caging and polyiodide networks are then discussed, with emphasis on how the terms we choose to use deeply affect scientific progress.

Stabilisation of Exotic Tribromide (Br3-) Anions via Supramolecular Interaction with A Tosylated Macrocyclic Pyridinophane. A Serendipitous Case

Tetraaza-macrocyclic pyridinophane L-Ts, decorated with a p-toluenesulfonyl (tosyl; Ts) group, appear to be a useful tool to provide evidence on how the interplay of various supramolecular forces can help stabilise exotic anionic species such as tribromide (Br₃⁻) anions. Indeed, crystals of (H₂L-Ts)(Br₃)_1.5(NO₃)_0.5 unexpectedly grew from an acidic (HNO₃) aqueous solution of L-Ts in the presence of Br⁻ anions. The crystal structure of this compound was determined by single crystal XRD analysis. Hydrogen bonds, salt-bridges, anion-π, π-π stacking, and van der Waals interactions contribute to stabilising the crystal lattice. The observation of two independent Br₃⁻ anions stuck over the π-electron densities of pyridine and tosyl ligand groups, one of them being sandwiched between two pyridine rings, corroborates the significance of anion-π interactions for N-containing heterocycles. We show herein the possibility of detecting anion-π contacts from fingerprint plots generated by Hirshfeld surface analysis, demonstrating the effective usage of this structural investigation technique to further dissect individual contributions of stabilising supramolecular forces.

Genesis of Complex Polyiodide Networks: Insights on the Blue Box/I−/I2 Ternary System

The Stoddart’s blue box (BB) (cyclobis(paraquat-p-phenylene))/iodide binary system was recently demonstrated to give rise to porous three-dimensional networks which can hardly be classified as common XOF-type materials (X = M, C, S, i.e., metal, covalent, or supramolecular organic frameworks), leading to the definition of permutable organized frameworks (POFs). The ternary BB/iodide/iodine system was reported to generate pentaiodide-based structures constituted by the most complex interlocked polyiodides so far isolated (up to an infinite supramolecular pseudopolyrotaxane with a poly[3]catenane axle). The missing link, i.e., the XRD structure of the BB/triiodide complex, is herein reported: structural similarities and novel Raman evidence, opening perspectives in the genesis of solid-state BB-based complex polyiodide networks from solution.