A Lantern-Shaped Pd(II) Cage Constructed from Four Different Low-Symmetry Ligands with Positional and Orientational Control: An Ancillary Pairings Approach

One of the key challenges of metallo-supramolecular chemistry is to maintain the ease of self-assembly but, at the same time, create structures of increasingly high levels of complexity. In palladium(II) quadruply stranded lantern-shaped cages, this has been achieved through either 1) the formation of heteroleptic (multi-ligand) assemblies, or 2) homoleptic assemblies from low-symmetry ligands. Heteroleptic cages formed from low-symmetry ligands, a hybid of these two approaches, would add an additional rich level of complexity but no examples of these have been reported. Here we use a system of ancillary complementary ligand pairings at the termini of cage ligands to target heteroleptic assemblies: these complementary pairs can only interact (through coordination to a single Pd(II) metal ion) between ligands in a cis position on the cage. Complementarity between each pair (and orthogonality to other pairs) is controlled by denticity (tridentate to monodentate or bidentate to bidentate) and/or hydrogen-bonding capability (AA to DD or AD to DA). This allows positional and orientational control over ligands with different ancillary sites. By using this approach, we have successfully used low-symmetry ligands to synthesise complex heteroleptic cages, including an example with four different low-symmetry ligands.

Stoichiometric Control of Guest Recognition of Self-Assembled Palladium(II)-Based Supramolecular Architectures

We report flexible [Pd(L)2 ]2+ complexes where there is self-recognition, driven by π-π interactions between electron-rich aromatic arms and the cationic regions they are tethered to. This self-recognition hampers the association of these molecules with aromatic molecular targets in solution. In one case, this complex can be reversibly converted to an 'open' [Pd2 (L)2 ]4+ macrocycle through introduction of more metal ion. This is accomplished by the ligand having two bidentate binding sites: a 2-pyridyl-1,2,3-triazole site, and a bis-1,2,3-triazole site. Due to favourable hydrogen bonding, the 2-pyridyl-1,2,3-triazole units reliably coordinate in the [Pd(L)2 ]2+ complex to control speciation: a second equivalent of Pd(II) is required to enforce coordination to bis-triazole sites and form the macrocycle. The macrocycle interacts with a molecular substrate with higher affinity. In this fashion we are able to use stoichiometry to reversibly switch between two different species and regulate guest binding.

A Switchable Palladium(II) Trefoil Entangled Tetrahedron with Temperature Dependence and Concentration Independence

Self-assembly makes metallo-interlocked architectures attractive targets, but being in equilibrium with smaller species means that they can suffer from dilution effects. We show that a junctioned system gives rise to a [Pd₄ (L)₂ ]⁸⁺ trefoil entangled tetrahedron irrespective of concentration. Heating the sample reversibly shifts the equilibrium from the knot to an isomeric non-interlocked dual metallo-cycle, demonstrating that thermodynamic equilibria can still be exploited for switching even in the absence of concentration effects.

Binding of Dual-Function Hybridized MetalOrganic Capsules to Enzymes for Cascade Catalysis

The combination of chemo- and biocatalysis for multistep syntheses provides attractive advantages in terms of evolvability, promiscuity, and sustainability striving for desirable catalytic performance. Through the encapsulation of flavin analogues by both NADH and heme mimics codecorated heteroleptic metal-organic capsules, herein, we report a progressive host-guest strategy to imitate cytochrome P450s catalysis for cascade oxidative coupling catalysis. Besides the construction of stable dual-function metal-organic capsules and the modification of cofactor-decorated capsules at the domain of enzymes, this supramolecular strategy involves multistage directional electron flow, affording reactive ferric peroxide species for inducing oxygenation. Under light irradiation, the metal-organic capsule selectively converts stilbene to oxidative coupling products (including 2-oxo-1,2-diphenylethyl formate, 2-alkoxy-1,2-diphenylethanone) in tandem with enzymatic reactions respectively, at the domain of natural enzymes. The ingenious combination of capsules and enzymes with the in situ-regenerated capsule-loaded NADH cofactor promises non-native coupling reactions by forming regional cooperation and division. This abiotic-biotic conjugated host-guest strategy is conducive to the de novo creation of multifunctional components approaching active enzymatic sites for reinforced matter and energy transporting, demonstrating a key role of multicomponent supramolecular catalysts for one-pot integrated catalytic conversions.

A Catalogue of Orthogonal Complementary Ligand Pairings for Palladium(II) Complexes

Molecular recognition is a form of information transfer, seen in the base pairing in DNA which is derived from the identity (acceptor or donor) and number of hydrogen bond sites within each base. Here we report bis‐ligand palladium(II) complexes that exhibit similar complementarity. Pd(II) has square planar four‐coordinate geometry, giving control over ligand orientation and denticity. Pairings were developed using ligand denticity (3 : 1 or 2 : 2), and hydrogen bond capability (AA:DD or AD:DA) or lack thereof. Five pairings were investigated, with two sets of four being found fully orthogonal. The two 3 : 1 pairings exhibited limited ligand exchange. The extent of this exchange varied dependant on solvent from 2 : 1 (desired to undesired) to 6 : 1. A reliable and varied set of ligand pairs have therefore been developed for bis‐ligand coordination sphere engineering in pursuit of sorting for complex molecular architectures and molecular‐level information storage and transfer events.

Heteroleptic tripalladium(II) cages

There is a concerted attempt to develop self-assembled metallo-cages of greater structural complexity, and heteroleptic Pd II cages are emerging as prime candidates in these efforts. Most of these are dinuclear: few examples of higher nuclearity have been reported. We demonstrate here a robust method for the formation of tripalladium(II) cages from the 2:3:3 combination of a tritopic ligand, Pd II , and a selection of ditopic ligands of the correct size and geometry.

Self-Assembly Synthesis of a [2]Catenane CoII Single-Molecule Magnet

We describe herein the self-assembly synthesis of an octanuclear Co^II [2]catenane {[Co₄ (H₂ L)₆ ]₂ ¹⁶⁺ } formed by the mechanical interlocking of two {[Co₄ (H₂ L)₆ ]⁸⁺ } rectangles of unprecedented topology. Subtle manipulation of the synthetic conditions allows the isolation of a mixed-valence [Co₂ ^III /Co₂ ^II ]¹⁰⁺ non-catenated rectangle. The Co^II centers in the [2]catenane exhibit slow relaxation of their magnetic moment, i. e. single-molecule magnet properties, dominated by quantum tunneling and Raman relaxation processes. This work shows that metallo-supramolecular chemistry can precisely control the organization of single-molecule magnets in topologically complex arrangements.

Discrete Self-Assembled Metallo-Foldamers with Heteroleptic Sequence Specificity

Discrete and structurally diverse foldamer sequences are constructed in both natural and abiotic systems primarily using inert connectivity with irreversible organic covalent bonds, serving to preserve the identity of the sequence. The formation of sequences under thermodynamic control using labile coordination bonds would be attractive for synthetic ease and modular capability, but this presents issues regarding sequence preservation. Here is presented an approach integrating palladium(II) metal ions into the sequence itself, with fidelity maintained through use of complementary pairings of ligand arrangements at the metal centre. This is accomplished using sites of different denticity and/or hydrogen bonding capability. In this fashion, discrete and ordered metallo-sequences are formed as thermodynamic products in a single step, and these then fold into defined conformations due to π-π interactions between electron-rich and -poor aromatic regions of the combined componentry.

Exploiting the labile site in dinuclear [Pd2L2]n+ metallo-cycles: multi-step control over binding affinity without alteration of core host structure

Synthetic metallosupramolecular systems have generally been binary (on/off) when they have control over molecular recognition. This report details a dipalladium(ii) system with four-step graduated control over recognition for a guest.

Long-cavity [Pd2L4]4+ cages and designer 1,8-naphthalimide sulfonate guests: rich variation in affinity and differentiated binding stoichiometry

Long cavity dual domain [Pd₂L₄]⁴⁺ cages bind long, dual domain guests, with tunable binding affinities and stoichiometries.

Conformational control of Pd2L4 assemblies with unsymmetrical ligands

With increasing interest in the potential utility of metallo-supramolecular architectures for applications as diverse as catalysis and drug delivery, the ability to develop more complex assemblies is keenly sought after. Despite this, symmetrical ligands have been utilised almost exclusively to simplify the self-assembly process as without a significant driving foa mixture of isomeric products will be obtained. Although a small number of unsymmetrical ligands have been shown to serendipitously form well-defined metallo-supramolecular assemblies, a more systematic study could provide generally applicable information to assist in the design of lower symmetry architectures. Pd2L4 cages are a popular class of metallo-supramolecular assembly; research seeking to introduce added complexity into their structure to further their functionality has resulted in a handful of examples of heteroleptic structures, whilst the use of unsymmetrical ligands remains underexplored. Herein we show that it is possible to design unsymmetrical ligands in which either steric or geometric constraints, or both, can be incorporated into ligand frameworks to ensure exclusive formation of single isomers of three-dimensional Pd2L4 metallo-supramolecular assemblies with high fidelity. In this manner it is possible to access Pd2L4 cage architectures of reduced symmetry, a concept that could allow for the controlled spatial segregation of different functionalities within these systems. The introduction of steric directing groups was also seen to have a profound effect on the cage structures, suggesting that simple ligand modifications could be used to engineer structural properties.