Unsymmetric Chiral Ligands for Large Metallo-Macrocycles: Selectivity of Orientational Self-Sorting

Nature uses various chiral and unsymmetric building blocks to form substantial and complex supramolecular assemblies. In contrast, the majority of organic ligands used in metallosupramolecular chemistry are symmetric and achiral. Here we extend the group of unsymmetric chiral bile acids used as a scaffold for organic bispyridyl ligands by employing chenodeoxycholic acid (CDCA), an epimer of the previously used ursodeoxycholic acid (UDCA). The epimerism, flexibility, and bulkiness of the ligands leads to large structural differences in coordination products upon reaction with Pd(NO3)2. The UDCA-bispyridyl ligand self-assembles quantitatively into a single crown-like Pd3L6 complex, whereas the CDCA ligand provides a mixture of coordination complexes of general formula PdnL2n, i.e., Pd2L4, Pd3L6, Pd4L8, Pd5L10, and even Pd6L12 containing an impressive 120 chiral centers. The coordination products were studied by a combination of analytical methods, with ion-mobility mass spectrometry (IM-MS) providing valuable details on their structure and allowed an effective separation of m/z 1461 to individual signals according to the arrival time distribution, thereby revealing four different ions of [Pd3L6(NO3)3]3+, [Pd4L8(NO3)4]4+, [Pd5L10(NO3)5]5+, and [Pd6L12(NO3)6]6+. The structures of all the complexes were modelled using DFT calculations. Finally, the challenges and conclusions in determining the specific structural identity of these unsymmetric species are discussed.

Orientational Self-Sorting in Octahedral Palladium Cages: Scope and Limitations of the "cis Rule"

Octahedral coordination cages of the general formula [Pd6L12](BF4)12 were obtained by combining [Pd(CH3CN)4](BF4)2 with heteroditopic N-donor ligands. Four different ligands were employed. These ligands have 3-pyridyl donor groups at one end and 4-pyridyl, imidazolyl, or triazolyl donor groups at the other end. According to a geometric analysis, cages with a cis configuration at the six metal centers should be preferred ("cis rule"). This prediction was corroborated by spectroscopic data and crystallographic analyses. Limitations of the "cis rule" were also encountered, and possible explanations are discussed.

Theoretical and computational methodologies for understanding coordination self-assembly complexes

This perspective highlights three theoretical and computational methods to capture the coordination self-assembly processes at the molecular level: quantum chemical modeling, molecular dynamics, and reaction network analysis. These methods cover the different scales from the metal-ligand bond to a more global aspect, and approaches that are best suited to understand the coordination self-assembly from different perspectives are introduced. Theoretical and numerical researches based on these methods are not merely ways of interpreting the experimental studies but complementary to them.

Chiral Self-Sorting in Pd6 L12 Metal-Organic Cages

Chiral self-sorting during the formation of cage-like molecules continues to fascinate and advance our understanding of the phenomenon in general. Herein, we report the chiral self-sorting in the Pd₆ L₁₂ -type metal-organic cages. When a racemic mixture of axially chiral bis-pyridyl ligands undergo coordination-driven self-assembly with Pd(II) ions to form Pd₆ L₁₂ -type cages, the system has the option of chiral self-sorting to afford any of at least 70 pairs of (one homochiral and 69 heterochiral) enantiomers and 5 meso isomers or a statistical mixture of everything. However, the system resulted in diastereoselective self-assembly through a high-fidelity chiral social self-sorting to form a racemic mixture of D₃ symmetric heterochiral [Pd₆ (L^6R/6S )₁₂ ]¹²⁺ /[Pd₆ (L^6S/6R )₁₂ ]¹²⁺ cages.

Mapping the Assembly of Neutral Tetrahedral Cages Tethered by Oximido Linkers and Their Guest Encapsulation Studies

A primary criterion for the design of polyhedral metal-organic cages is the requirement of geometrically matched pairs of metal ions and ligand moieties. However, understanding the pathway it takes to reach the final polyhedral structure can provide more insights into the self-assembly process and improved design strategies. In this regard, we report two neutral tetrahedral cages with the formulas {[Pd₃(NⁱPr)₃PO]₄(L¹)₆} (1-TD) and {[Pd₃(NⁱPr)₃PO]₄(L²)₆} (2-TD) starting from the acetate-bridged cluster {[Pd₃(NⁱPr)₃PO]₂(OAc)₂(OH)}₂·2(CH₃)₂SO (HEXA-Pd) and the respective oxamide precursors (L¹H₂: [C₂(NH₂)₂O₂]) and (L²H₂: (C₂(NHMe)₂O₂]). When subtle variations in the reaction conditions were made, two new tetrameric Pd₁₂ assemblies, {[Pd₃(NⁱPr)₃PO]₄(L¹)₂(OAc)₄(OMe)₄} (1-TM) and {[Pd₃(NⁱPr)₃PO]₄(L²)₂(OAc)₄(OMe)₄} (2-TM), were obtained from the same precursors. Detailed investigations using NMR, mass spectrometry, X-ray crystallography, and computational studies indicate that the macrocyclic complexes 1-TM and 2-TM are the reaction intermediates involved in the formation of the tetrahedral cages 1-TD and 2-TD, respectively. Moreover, the tetrahedral cages 1-TD and 2-TD exhibited intrinsic cavities of volume ∼85 ų. Guest encapsulation studies revealed that the cage 1-TD can encapsulate a wide range of guest molecules such as CH₂Cl₂, CHCl₃, CCl₄, C₆H₆, and C₆H₅F. Interestingly, 1-TD was shown to exhibit a preferential binding of C₆H₅F and C₆H₆ over other halogenated guest molecules, as determined from NMR titrations and computational studies.

Catalytic Formation of Coordination-Based Self-Assemblies by Halide Impurities

The dynamics of metal organic polyhedra (MOP) play a crucial role for their application in catalysis and host-guest chemistry and as functional materials. In this contribution, we study the influence of possible contaminations of different metal precursors on the kinetic properties of MOP. Exemplary five different MOP are studied with metal precursors of varying quality. The metal precursors are either obtained from commercial sources or prepared by various literature procedures. Studies into the self-assembly process using 1H NMR and MS analyses were performed on Pt2L4, Pd2L4, Pd6L12, Pd12L24, and Ni4L6 assemblies. Commonly found impurities are shown to play a prominent role guiding selective formation of MOP, as they allow for an escape from otherwise kinetically trapped intermediates. The energy requirement for selective sphere formation is significantly lowered in many examples providing evidence for a catalytic role of halide impurities/additives in the self-assembly process. Furthermore, even though most analytical features such as 1H NMR and MS analyses show identical results for assemblies with different types of metal precursors, the dynamics of formed assemblies differs significantly if slightly less pure starting materials are used. Tiny amounts of halide contaminations make the MOP more dynamic, which can play an important role for substrate diffusion especially if bulky substrates are used. We believe that this study on the influence of impurities (which were shown to be present in some commercial sources) on the kinetic properties of MOP together with procedures of obtaining high purity metal precursors provides important information for future material preparation and provides a better understanding of already known examples.

Integrative Assembly of Heteroleptic Tetrahedra Controlled by Backbone Steric Bulk

A bent fluorenone-based dipyridyl ligand LA reacts with PdII cations to a solvent-dependent dynamic library of [PdnL2n] assemblies, constituted by a [Pd3LA6] ring and a [Pd4LA8] tetrahedron as major components, and a [Pd6LA12] octahedron as minor component. Introduction of backbone steric hindrance in ligand LB allows exclusive formation of the [Pd6LB12] octahedron. Combining equimolar amounts of both ligands results in integrative self-sorting to give an unprecedented [Pd4LA4LB4] heteroleptic tetrahedron. Key to the non-statistical assembly outcome is exploiting the structural peculiarity of the [Pd4L8] tetrahedral topology, where the four lean ligands occupy two doubly bridged edges and the bulky ligands span the four remaining, singly bridged edges. Hence, the system finds a compromise between the entropic drive to form an assembly smaller than the octahedron and the enthalpic prohibition of pairing two bulky ligands on the same edge of the triangular ring. The emission of luminescent LA is maintained in both homoleptic [Pd3LA6] and heteroleptic [Pd4LA4LB4].

Metalloligand Strategies for Assembling Heteronuclear Nanocages – Recent Developments

The use of metalloligands as building blocks for the assembly of metallo-organic cages has received increasing attention over the past two decades or so. In part, the popularity of this approach reflects its stepwise nature that lends itself to the predesigned construction of metallocages and especially heteronuclear metallocages. The focus of the present discussion is on the use of metalloligands for the construction of discrete polyhedral cages, very often incorporating heterometal ions as structural elements. The metalloligand approach uses metal-bound multifunctional ligand building blocks that display predesigned structural properties for coordination to a second metal ion such that the rational design and construction of both homo- and heteronuclear metal–organic cages are facilitated. The present review covers published literature in the area from early 2015 to early 2019.