Exploring the Palladium and PlatinumBis(pyridine) Complex Motif by NMR Spectroscopy, X-ray Crystallography, (Tandem) Mass Spectrometry, and Isothermal Titration Calorimetry: Do Substituent Effects Follow Chemical Intuition?

DNA-Intercalating Supramolecular Hydrogels for Tunable Thermal and Viscoelastic Properties

Polymeric supramolecular hydrogels (PSHs) leverage the thermodynamic and kinetic properties of non-covalent interactions between polymer chains to govern their structural characteristics. As these materials are formed via endothermic or exothermic equilibria, their thermal response is challenging to control without drastically changing the nature of the chemistry used to join them. In this study, we introduce a novel class of PSHs utilizing the intercalation of double-stranded DNA (dsDNA) as the primary dynamic non-covalent interaction. The resulting dsDNA intercalating supramolecular hydrogels (DISHs) can be tuned to exhibit both endothermically or exothermically driven binding through strategic selection of intercalators. Bifunctional polyethylene glycol (MW~2000 Da) capped with intercalators of varying hydrophobicity, charge, and size (acridine, psoralen, thiazole orange, and phenanthridine) produced DISHs with comparable moduli (500-1000 Pa), but unique thermal viscoelastic responses. Notably, acridine-based cross-linkers displayed invariant and even increasing relaxation times with temperature, suggesting an endothermic binding mechanism. This methodology expands the set of structure-properties available to biomass-derived DNA biomaterials and promises a new material system where a broad set of thermal and viscoelastic responses can be obtained due to the sheer number and variety of intercalating molecules.

Visible-Light Switching of Metallosupramolecular Assemblies

A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. Combining the E-isomer with palladium(II) salts affords a double-walled triangle with composition [Pd3 L6 ]6+ and a distorted tetrahedron [Pd4 L8 ]8+ (1 : 2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with approximately 80 % of the E-isomer of the ligand and results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the more kinetically inert product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298 K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.

Entropy directs the self-assembly of supramolecular palladium coordination macrocycles and cages

The self-assembly of palladium-based cages is frequently rationalized via the cumulative enthalpy (ΔH) of bonds between coordination nodes (M, i.e., Pd) and ligand (L) components. This focus on enthalpic rationale limits the complete understanding of the Gibbs free energy (ΔG) for self-assembly, as entropic (ΔS) contributions are overlooked. Here, we present a study of the M₂^linL₃ intermediate species (M = dinitrato(N,N,N′,N′-tetramethylethylenediamine)palladium(ii), ^linL = 4,4′-bipyridine), formed during the synthesis of triangle-shaped (M₃^linL₃) and square-shaped (M₄^linL₄) coordination macrocycles. Thermochemical analyses by variable temperature (VT) ¹H-NMR revealed that the M₂^linL₃ intermediate exhibited an unfavorable (relative) ΔS compared to M₃^linL₃ (triangle, ΔTΔS = +5.22 kcal mol⁻¹) or M₄^linL₄ (square, ΔTΔS = +2.37 kcal mol⁻¹) macrocycles. Further analysis of these constructs with molecular dynamics (MD) identified that the self-assembly process is driven by ΔG losses facilitated by increases in solvation entropy (ΔS_solv, i.e., depletion of solvent accessible surface area) that drives the self-assembly from “open” intermediates toward “closed” macrocyclic products. Expansion of our computational approach to the analysis of self-assembly in Pd_n^benL_2n cages (^benL = 4,4'-(5-ethoxy-1,3-phenylene)dipyridine), demonstrated that ΔS_solv contributions drive the self-assembly of both thermodynamic cage products (i.e., Pd₁₂^benL₂₄) and kinetically-trapped intermediates (i.e., Pd₈^cL₁₆).

These studies demonstrate that ΔS drives the self-assembly of supramolecular palladium-based coordination macrocycles and cages. As this ΔS contribution arises from solvation, these findings broadly reflect the thermodynamic drive of self-assembly to form compact structures.

Physical networks from entropy-driven non-covalent interactions

Physical networks typically employ enthalpy-dominated crosslinking interactions that become more dynamic at elevated temperatures, leading to network softening. Moreover, standard mathematical frameworks such as time-temperature superposition assume network softening and faster dynamics at elevated temperatures. Yet, deriving a mathematical framework connecting the crosslinking thermodynamics to the temperature-dependent viscoelasticity of physical networks suggests the possibility for entropy-driven crosslinking interactions to provide alternative temperature dependencies. This framework illustrates that temperature negligibly affects crosslink density in reported systems, but drastically influences crosslink dynamics. While the dissociation rate of enthalpy-driven crosslinks is accelerated at elevated temperatures, the dissociation rate of entropy-driven crosslinks is negligibly affected or even slowed under these conditions. Here we report an entropy-driven physical network based on polymer-nanoparticle interactions that exhibits mechanical properties that are invariant with temperature. These studies provide a foundation for designing and characterizing entropy-driven physical crosslinking motifs and demonstrate how these physical networks access thermal properties that are not observed in current physical networks.

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.

Conformation-selective coordination-driven self-assembly of a ditopic donor with PdII acceptors

Different binding conformations of 3-(5-(pyridin-3-yl)-1H-1,2,4-triazol-3-yl)pyridine (L) yielded a self-assembled 3D cube and 2D macrocycles selectively depending on the nature of acceptors. Selection of a particular conformation of the donor L by a specific metal acceptor during self-assembly was corroborated well by a theoretical study.