How to Prepare Kinetically Stable Self-assembled Pt₁₂L₂₄ Nanocages while Circumventing Kinetic Traps

Supramolecular coordination‐based self‐assembled nanostructures have been widely studied, and currently various applications are being explored. For several applications, the stability of the nanostructure is of key importance, and this strongly depends on the metal used in the self‐assembly process. Herein, design strategies and synthetic protocols to access desirable kinetically stable Pt(12)L(24) nanospheres are reported, and it is demonstrated that these are stable under conditions under which the palladium counterparts decompose. Descriptors previously used for palladium nanospheres are insufficient for platinum analogues, as the stronger metal–ligand bond results in a mixture of kinetically trapped structures. We report that next to the dihedral angle, the rigidity of the ditopic ligand is also a key parameter for the controlled formation of Pt(12)L(24) nanospheres. Catalytic amounts of coordinating additives to labilise the platinum‐pyridyl bond to some extent are needed to selectively form Pt(12)L(24) assemblies. The formed Pt(12)L(24) nanospheres were demonstrated to be stable in the presence of chloride, amines and acids, unlike the palladium analogues

Exposing Mechanisms for Defect Clearance in Supramolecular Self-Assembly:Palladium-Pyridine Coordination Revisited

Spherical three-dimensional (3D) cages composed of palladium(II) and pyridyl ligands are a mainstay of supramolecular chemistry with demonstrated catalytic and optoelectronic applications. The widely reported self-assembly of these palladium-based cages exhibits sensitivity to the solvents, reagents, and/or reactants employed. This sensitivity, and the resulting inconsistency between synthetic protocols, hinders the development of desirable palladium-based cages. We have found that pyridyl ligand substitution─the rate-limiting step of self-assembly─is facilitated by endogenous supporting ligands derived from the solvents, reagents, and reactants employed in synthetic protocols of palladium- and platinum-based assemblies. Here, we present a systematic investigation combining 1H-NMR, electrospray ionization mass spectrometry (ESI─MS), and absorption spectroscopy to characterize the intermediates to support the mechanism of pyridyl ligand substitution on a model complex, M(py)2 (M = (N,N,N′,N′-tetramethylethylenediamine)palladium(II), py = pyridine), under simulated synthetic conditions for self-assembly. Our investigation exposes mechanisms for pyridyl ligand substitution, featuring intermediates stabilized by solvent, anion, or (in situ formed) alkoxide moieties. Interrogation of destabilizing agents (2,2,2-trifluoroethanol and tetra(n-butyl)ammonium chloride) reveal similar mechanisms that ultimately facilitate the self-assembly of coordination cages. These findings rationalize widely reported solvent and anion effects in the self-assembly of coordination cages (and similar constructs) while highlighting methodologies to understand the role of supporting ligands in coordination chemistry.</p

Pd₁₂M_<i>n</i>L₂₄ (for <i>n</i> = 6, 8, 12) nanospheres by post-assembly modification of Pd₁₂L₂₄ spheres

In this contribution, we describe a post-assembly modification approach to selectively coordinate transition metals in Pd12L24 cuboctahedra. The herein reported approach involves the preparation of Pd12L24 nanospheres with protonated nitrogen donor ligands that are covalently linked at the interior. The so obtained Pd12(LH+)24 nanospheres are shown to be suitable for coordinative post-modification after deprotection by deprotonation. Selective formation of tetra-coordinated MB in Pd12MB6L24, tri-coordinated MB in Pd12MB8L24 nanospheres and two-coordinated MB in Pd12MB12L24 nanospheres is achieved as a result of different nitrogen donor ligands. A combination of pulsed EPR spectroscopy (DEER) to measure Cu-Cu distances in the different spheres, NMR studies and computational investigations, support the presence of the complexes at precise locations of the Pd12MB6L24 nanosphere. The general post-assembly modification methodology can be extended using other transition metal precursors or supramolecular systems and can guide precise formation and investigation of novel transition metal-complex containing nanospheres with well-defined composition.</p

Broadening the catalytic region from the cavity to windows by M₆L₁₂ nanospheres in cyclizations

Supramolecular cages have received tremendous attention as they can contain catalysts that exhibit confinement effects in the cavity, leading to excellent performances. Herein, we report an example wherein the catalytic region is extended from the cage cavity to the windows, and investigate its confinement effect by utilizing the Pd6LAu12 cage that contains rigidly fixed and isolated gold complexes at the windows. Pd6LAu12 exhibit three features of particular interest while assessing their properties in gold-catalyzed cyclization reactions. First, the catalysts experience a cage effect as they display higher reactivity and selectivity compared to the monomeric analogue, as a result of substrate pre-organization at the windows. Second, the metal complexes are physically separated by the cage structure, preventing the formation of less active dinuclear gold complexes making it more stable under hydrous conditions. Third, the cage windows present the characteristics of enzymatic catalysis via Michaelis-Menten-type mechanism analysis. This contribution presents an alternative way to engineer supramolecular catalysts through extending the catalytic region.</p

Topological prediction of palladium coordination cages

The preparation of functionalized, heteroleptic PdxL2x coordination cages is desirable for catalytic and optoelectronic applications. Current rational design of these cages uses the angle between metal-binding (∠B) sites of the di(pyridyl)arene linker to predict the topology of homoleptic cages obtained via non-covalent chemistry. However, this model neglects the contributions of steric bulk between the pyridyl residues—a prerequisite for endohedrally functionalized cages, and fails to rationalize heteroleptic cages. We describe a classical mechanics (CM) approach to predict the topological outcomes of PdxL2x coordination cage formation with arbitrary linker combinations, accounting for the electronic effects of coordination and steric effects of linker structure. Initial validation of our CM method with reported homoleptic Pd12 L Fu 24 ( LFu  = 2,5-bis(pyridyl)furan) assembly suggested the formation of a minor topology Pd15 LFu 30, identified experimentally by mass spectrometry. Application to heteroleptic cage systems employing mixtures of  LFu  (∠B = 127°) and its thiophene congener  LTh  (∠B = 149° ∠Bexp = 152.4°) enabled prediction of Pd12L24 and Pd24L48 coordination cages formation, reliably emulating experimental data. Finally, the topological outcome for exohedrally ( LEx ) and endohedrally ( LEn ) functionalized heteroleptic PdxL2x coordination cages were predicted to assess the effect of steric bulk on both topological outcomes and coordination cage yields, with comparisons drawn to experimental data