Palladium(II)-Based Self-Assembled Heteroleptic Coordination Architectures: A Growing Family

A pair of conjoined trinuclear sub-frameworks in a pentanuclear double-cavity discrete coordination cage

Combination of Pd(ii) with selected bis-monodentate ligands produces the familiar multinuclear Pd m L2m type self-assembled "single-cavity discrete coordination cages" (SCDCC). If the ligand provides parallel coordination vectors, then it forms a binuclear Pd2L4 type cage, whereas utilization of ligands having appropriately divergent coordination vectors results in specific higher nuclear complexes. In contrast, preparation of emergent "multi-cavity discrete coordination cages" (MCDCC) using Pd(ii) and designer ligands is quite captivating where the neighboring cavities of the framework are conjoined with each other through a common metal center. A pair of conjoined binuclear Pd2L4 type sub-frameworks are present in a trinuclear Pd3L4 type double-cavity cage prepared from Pd(ii) and a tris-monodentate ligand having parallel coordination vectors. The present work envisioned a design to make double-cavity coordination cages having a pair of conjoined trinuclear Pd3L6 type sub-frameworks. To fulfill the objective we combined Pd(ii) with a mixture of designer bis-monodentate ligand (L) and tris-monodentate ligand (L') in a 5 : 4 : 4 ratio in one pot to afford the targeted pentanuclear type cage. The choice of bis-monodentate ligand L is based on the divergent nature of the coordination vectors suitable to produce a Pd3L6 type SCDCC. The tris-monodentate ligand L' having two arms is designed in such a manner that each of the arms reasonably resembles L. Study of the complexation behavior of Pd(ii) with L' provided additional guiding factors essential for the successful making of type MCDCC by integrative self-sorting. A few other MCDCC including lower symmetry versions were also prepared in the course of the work.

Stabilization of Lantern-Type Metal-Organic Cages (MOCs) by Protective Control of Ligand Exchange Rates

Self-assembling systems in nature display remarkable complexity with assemblies of different sub-units to generate functional species. Synthetic analogues of such systems are a challenge, often requiring the ability to bias distributions that are under thermodynamic assembly control. Using lantern-type MOCs (metal-organic cages) as a prototypical self-assembling system, herein we explore the role that steric bulk plays in controlling the exchange rate of ligands in paddlewheel-based assemblies, and thus the stability of cages, in competitive self-assembling scenarios. The effective lifetime of the lantern-type MOCs varies over an order of magnitude depending on the steric bulk proximal to the metal nodes with lifetimes of the cages ranging from tens of minutes to several hours. The bulk of the coordinating solvents likewise reduces the rate of ligand exchange, and thus yields longer-lived species. Understanding this subtle effect has implications for controlling the stability of complex assemblies in competitive environments with implications for guest release and application.

A dynamic covalent approach to [PtnL2n]2n+ cages

A dynamic covalent approach was exploited to generate a family of homometallic [PtnL2n]2n+ cage (predominantly [Pt2L4]4+ systems) architectures. The family of platinum(II) architectures were characterized using 1H nuclear magnetic resonance (NMR) and diffusion ordered spectroscopy (DOSY), electrospray ionization mass spectrometry (ESI-MS) and the molecular structures of two cages were determined by X-ray crystallography.

Pseudohalide Ions as Ligands to Tune Architecture and Luminescence of Polymetallic CU(I) Assemblies

The reaction of preassembled Cu(I) bimetallic units {Cu2(dppm)2} and {Cu2(dppa)2} (dppm: bis(diphenylphosphino)methane and dppa: bis(diphenylphosphino)amine) with pseudohalide linkers (azido, dicyanamide, and tricyanomethanide) allows for the quantitative and selective preparation of three discrete tetrametallic metallacycles of formula [Cu4(μ2-dppm)4(N3)2](PF6)2, [Cu4(μ2-dppm)4(N(CN)2)2](PF6)2, and [Cu4(μ2-dppm)4(C(CN)3)4]. To explore further the impact of the linker on the architecture and dimensionality of the molecular edifice, the study was extended to more sophisticated tetradentate cyanocarbanion ligands (tcnsMe-: 2-(methylthio)-1,1,3,3-propanetetracarbonitrile and tcnsEt-: 2-(ethylthio)-1,1,3,3-propanetetracarbonitrile). Three ladder-like one-dimensional coordination polymers and an octametallic metallacycle have been obtained. The careful comparison of the metric and geometrical intramolecular and intermolecular parameters observed in this series of seven derivatives allows for rationalization of their molecular architectures. The subtle balance between the length and steric hindrance of the ligand and the formation of noncovalent interaction networks greatly influences the topology and dimensionality of the resulting assemblies and will be discussed hereafter. The photophysical properties of these seven polymetallic Cu(I) compounds have also been also studied.

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.

Diastereoselective Self-Assembly of Low-Symmetry Pdn L2n Nanocages through Coordination-Sphere Engineering

Metal-organic cages (MOCs) are popular host architectures assembled from ligands and metal ions/nodes. Assembling structurally complex, low-symmetry MOCs with anisotropic cavities can be limited by the formation of statistical isomer libraries. We set out to investigate the use of primary coordination-sphere engineering (CSE) to bias isomer selectivity within homo- and heteroleptic Pdn L2n cages. Unexpected differences in selectivities between alternative donor groups led us to recognise the significant impact of the second coordination sphere on isomer stabilities. From this, molecular-level insight into the origins of selectivity between cis and trans diastereoisomers was gained, highlighting the importance of both host-guest and host-solvent interactions, in addition to ligand design. This detailed understanding allows precision engineering of low-symmetry MOC assemblies without wholesale redesign of the ligand framework, and fundamentally provides a theoretical scaffold for the development of stimuli-responsive, shape-shifting MOCs.

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.

Transition Metal Catalysis in Living Cells: Progress, Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.

Exploiting reduced-symmetry ligands with pyridyl and imidazole donors to construct a second-generation stimuli-responsive heterobimetallic [PdPtL4]4+ cage

A new sequential metalation strategy that enables the assembly of a new more robust reduced symmetry heterobimetallic [PdPtL4]4+ cage C is reported. By exploiting a low-symmetry ditopic ligand (L) that features imidazole and pyridine donor units we were able to selectively form a [Pt(L)4]2+ "open-cage" complex. When this was treated with Pd(ii) ions the cage C assembled. 1H and DOSY nuclear magnetic resonance (NMR) spectroscopy and electrospray ionisation mass spectrometry (ESIMS) data were consistent with the quantitative formation of the cage and the heterobimetallic structure was confirmed by single crystal X-ray crystallography. The cage C was shown to bind anionic guest molecules. NMR studies suggested that these guests interacted with the cavity of the cage in a specific orientation and this was confirmed for the mesylate ion (MsO-) : C host-guest adduct using X-ray crystallography. In addition, the system was shown to be stimulus-responsive and could be opened and closed on demand when treated with appropriate stimuli. If a guest molecule was bound within the cage, the opening and closing was accompanied by the release and re-uptake of the guest molecule.

Synthesis, Structural Elucidation and Pharmacological Applications of Cu(II) Heteroleptic Carboxylates

Six heteroleptic Cu(II) carboxylates (1-6) were prepared by reacting 2-chlorophenyl acetic acid (L¹), 3-chlorophenyl acetic acid (L²), and substituted pyridine (2-cyanopyridine and 2-chlorocyanopyridine). The solid-state behavior of the complexes was described via vibrational spectroscopy (FT-IR), which revealed that the carboxylate moieties adopted different coordination modes around the Cu(II) center. A paddlewheel dinuclear structure with distorted square pyramidal geometry was elucidated from the crystal data for complexes 2 and 5 with substituted pyridine moieties at the axial positions. The presence of irreversible metal-centered oxidation reduction peaks confirms the electroactive nature of the complexes. A relatively higher binding affinity was observed for the interaction of SS-DNA with complexes 2-6 compared to L¹ and L². The findings of the DNA interaction study indicate an intercalative mode of interaction. The maximum inhibition against acetylcholinesterase enzyme was caused for complex 2 (IC₅₀ = 2 µg/mL) compared to the standard drug Glutamine (IC₅₀ = 2.10 µg/mL) while the maximum inhibition was found for butyrylcholinesterase enzyme by complex 4 (IC₅₀ = 3 µg/mL) compared to the standard drug Glutamine (IC₅₀ = 3.40 µg/mL). The findings of the enzymatic activity suggest that the under study compounds have potential for curing of Alzheimer's disease. Similarly, complexes 2 and 4 possess the maximum inhibition as revealed from the free radical scavenging activity performed against DPPH and H₂O₂.

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.

Exploiting Supramolecular Interactions to Control Isomer Distributions in Reduced-Symmetry [Pd2L4]4+ Cages

High-symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis, and drug delivery. Recently, there have been increasing efforts to enhance those applications by generating reduced-symmetry MSAs. Here we report our attempts to use supramolecular (dispersion and hydrogen-bonding) forces and solvophobic effects to generate isomerically pure [Pd₂(L)₄]⁴⁺ cage architectures from a family of new reduced-symmetry ditopic tripyridyl ligands. The reduced-symmetry tripyridyl ligands featured either solvophilic polyether chains, solvophobic alkyl chains, or amino substituents. We show using NMR spectroscopy, high-performance liquid chromatography, X-ray diffraction data, and density functional theory calculations that the combination of dispersion forces and solvophobic effects does not provide any control of the [Pd₂(L)₄]⁴⁺ isomer distribution with mixtures of all four cage isomers (HHHH, HHHT, cis-HHTT, or trans-HTHT, where H = head and T = tail) obtained in each case. More control was obtained by exploiting hydrogen-bonding interactions between amino units. While the cage assembly with a 3-amino-substituted tripyridyl ligand leads to a mixture of all four possible isomers, the related 2-amino-substituted tripyridyl ligand generated a cis-HHTT cage architecture. Formation of the cis-HHTT [Pd₂(L)₄]⁴⁺ cage was confirmed using NMR studies and X-ray crystallography.

Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities

Construction of supramolecular structures with internal functionalities is a promising approach to build enzyme-like cavities. The endo-functionalized [Pd₁₂ L₂₄ ] and [Pd₂ L₄ ] coordination cages represent the most successful systems in this regard. However, these systems mainly contain one type of endo-moiety. We herein provide a solution for the controlled endo-functionalization of [Pd₂ L₄ ] cages. Site-selective introduction of the endo-functional group was achieved through the formation of heteroleptic [Pd₂ (L^A )₂ (L^B )(L^C )] cages. Using two orthogonal steric control elements is the key for the selective formation of the hetero-assemblies. We demonstrated the construction of two hetero-cages with a single internal functional group as well as a hetero-cage with two distinct endohedral functionalities. The endo-functionalized hetero-cages bound sulfonate guests with fast-exchange dynamics. This strategy provides a new solution for the controlled endo-functionalization of supramolecular cavities.

Conjoined and non-conjoined coordination cages with palladium(II) vertices: structural diversity, solution dynamics, and intermolecular interactions

Self-assembled coordination complexes prepared from a combination of Pd(II) components with one or more types of high-symmetry or low-symmetry bis/tris/tetrakis-monodentate ligands are considered in this review. The structures of these complexes are viewed in terms of the presence of a metallo-macromonocycle or conjoined metallo-macromonocycles/metallocages in the frameworks. Analysis of the typical molecular structures revealed an open truth that one or more units of metallo-macromonocycles can be conjoined to afford planar or non-planar systems. In the same line, the enveloping surface of a 3D cage can be considered as a multiple number of conjoined metallomacrocycles that embrace a 3D space from all directions. However, two or more units of cages are conjoined in a multi-3D-cavity cage system and such a system is considered as a conjoined cage. Construction of such conjoined cages having a finite but multiple number of 3D-cavities unified in a single molecular architecture is a challenging task when compared to that of single-3D-cavity based compounds. Conjoining of as many as four units of 3D cages is known so far. Single- as well as multi-cavity cages of lower symmetry have become a very recent trend in this regard where low-symmetry ligands or mixed ligand ensembles are crafted in the framework of the cages. Other structural diversities like helicity in cages, and supramolecular isomerism are also included in this assorted literature work. Although isomerism in classical coordination complexes is well known, it is very less studied in self-assembled coordination complexes. Ligand isomerism is one such feature that is reviewed here. The dynamic behavior of the cages results in interesting reactivity aspects. A large variety of dynamic processes are collected under an umbrella, i.e., "ligand exchange reactions" and described with examples. Intermolecular interaction among the already self-assembled molecules is possible in solution, solid, and gel-phases as discussed in the last part of this review. The understanding of intermolecular interaction is likely to influence different areas of research including crystal engineering, and materials chemistry.

Endohedrally Functionalized Heteroleptic Coordination Cages for Phosphate Ester Binding

Metallosupramolecular hosts of nanoscopic dimensions, which are able to serve as selective receptors and catalysts, are usually composed of only one type of organic ligand, restricting diversity in terms of cavity shape and functional group decoration. We report a series of heteroleptic [Pd₂ A₂ B₂ ] coordination cages that self-assemble from a library of shape complementary bis-monodentate ligands in a non-statistical fashion. Ligands A feature an inward pointing NH function, able to engage in hydrogen bonding and amenable to being functionalized with amide and alkyl substituents. Ligands B comprise tricyclic aromatic backbones of different shape and electronic situation. The obtained heteroleptic coordination cages were investigated for their ability to bind phosphate diesters as guests. All-atom molecular dynamics (MD) simulations in explicit solvent were conducted to understand the mechanistic relationships behind the experimentally determined guest affinities.

Pseudo-heterolepticity in Low-Symmetry Metal-Organic Cages

Heteroleptic metal-organic cages, formed through integrative self-assembly of ligand mixtures, are highly attractive as reduced symmetry supramolecular hosts. Ensuring high-fidelity, non-statistical self-assembly, however, presents a significant challenge in molecular engineering due to the inherent difficulty in predicting thermodynamic energy landscapes. In this work, two conceptual strategies are described that circumvent this issue, using ligand design strategies to access structurally sophisticated metal-organic hosts. Using these approaches, it was possible to realise cavity environments described by two inequivalent, unsymmetrical ligand frameworks, representing a significant step forward in the construction of highly anisotropic confined spaces.

Visible‐Light‐Responsive Self‐Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination**

A photoswitchable ligand based on azobenzene is self‐assembled with palladium(II) ions to form a [Pd₂(E‐L)₄]⁴⁺ cage. Irradiation with 470 nm light results in the near‐quantitative switching to a monomeric species [Pd(Z‐L)₂]²⁺, which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self‐assembly, and the thermal half‐life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Diastereoselectively self-sorted low-symmetry binuclear metallomacrocycle and trinuclear metallocage

A pyridine/aniline appended unsymmetrical bis-monodentate ligand N-(3-aminophenyl)nicotinamide, L^un is synthesized via condensation of nicotinic acid with excess m-phenylene diamine. A low-symmetry binuclear complex of the Pd₂L'₂L^un₂ type and an extremely rare trinuclear complex of the Pd₃L^un₆ type are produced by self-assembly of the ligand L^un with cis-protected palladium(II) (i.e., PdL') and palladium(II), respectively. Two isomers (i.e. [(2,0), (2,0)] and [(1,1), (1,1)]-forms) are theoretically possible for the Pd₂L'₂L^un₂-type complex whereas nine isomers can be envisaged in the case of the Pd₃L^un₆-type arrangement. However, one of the isomers of the Pd₂L'₂L^un₂-type complex as well as the one for the Pd₃L^un₆-type complex are experimentally obtained. The exclusive formation of specific isomers could be predicted from the 1D/2D NMR study in the solution state and the DFT calculations in the gas phase/implicit solvent media. The formation of the predicted all-(1,1)-[Pd₂(en)₂L^un₂](NO₃)₄ has been confirmed by a single-crystal XRD study. DFT calculations for the isomers of the Pd₃L^un₆-type arrangement show that a [cis(2,2), cis(2,2), cis(2,2)] isomer is energetically favourable than the alternatively predicted [trans(2,2), trans(2,2), trans(2,2)] isomer. Conformational changes within the build of the exclusively formed isomers are proposed on the basis of NMR study.

Rhenium(I)-Based Heteroleptic Pentagonal Toroid-Shaped Metallocavitands: Self-Assembly and Molecular Recognition Studies

A family of neutral, heteroleptic, dinuclear M₂LL'-type pentagonal toroid-shaped metallomacrocycles (1-8) were synthesized using flexible ditopic N donors (Lⁿ = L¹-L²), rigid bis-chelating ligands (H₂-L' = H₂-E), and Re₂(CO)₁₀ in a one-pot solvothermal self-assembly approach. The ligands and the metallomacrocycles were characterized using ATR-IR, electrospray ionization mass spectrometry, nuclear magnetic resonance, ultraviolet-visible, and emission spectroscopy methods. The molecular structures of 1, 2, 4, 6, and 7 were confirmed by an X-ray diffraction study and are similar to those of calix[5]arene. The cyclic inner cavities of the metallomacrocycles accommodate toluene/mesitylene/acetone/chlorobenzene as guest molecules that are stabilized by cumulative C-H···π and π···π interactions with the cyclic framework of metallomacrocycle. The photophysical properties of the ligands and the metallomacrocycles were studied. The host-guest recognition properties of metallocavitands 1, 2, 7, and 8 as a model host with phenol and nitrobenzene derivatives as guest molecules were studied by emission spectroscopy methods.

Shape Complementary Coordination Self-Assembly of a Redox-Active Heteroleptic Complex

We present here the coordination self-assembly of a new heteroleptic (bpyPd)₄L¹L₂² coordination complex (1) from one novel pyridinium-functionalized bis-2,4,6-tris(pyridin-3-yl)-1,3,5-triazine (bis-3-TPT, L¹) macrocyclic ligand, two separate 3-TPT (L²) ligands, and four cis-blocking bpyPd(NO₃)₂ (bpy = 2,2'-bipyridine). While homoleptic self-assemblies with either L¹ or L² gave dynamic mixtures of products, a single thermodynamic heteroleptic complex was obtained driven by the shape complementarity of building blocks. Moreover, the redox-active nature of the heteroleptic assembly facilitates the highly efficient catalytic aerobic photo-oxidation of aromatic secondary alcohols under mild conditions.

Heteroleptic metallosupramolecular aggregates/complexation for supramolecular catalysis

Supramolecular catalysis is reviewed with an eye on heteroleptic aggregates/complexation. Since most of the current metallosupramolecular catalytic systems are homoleptic in nature, the idea of breaking/reducing symmetry has ignited a vivid search for heteroleptic aggregates that are made up by different components. Their higher degree of functional diversity and structural heterogeneity allows, as demonstrated by Nature by the multicomponent ATP synthase motor, a more detailed and refined configuration of purposeful machinery. Furthermore, (metallo)supramolecular catalysis is shown to extend beyond the single "supramolecular unit" and to reach far into the field and concepts of systems chemistry and information science.

Heteroleptic Triple-Stranded Metallosupramolecules with Hydrophobic Inner Voids

The systematic combination of well-defined coordination spheres and multiple types of ligands (heteroleptic) can lead to the generation of hierarchical metallosupramolecules with a high level of complexity and functionality. In particular, a specific multilevel coordination-driven assembly through the initiate generation of multinuclear clusters can form unique heteroleptic multiple-stranded supramolecular complexes. Herein, we report novel triple-stranded nickel-based supramolecules constructed from two different ditopic ligands ([1,1':3',1''-terphenyl]-4,4''-dicarboxylate (TP) and 2,6-pyridinedicarboxylate (PDA)) and a nickel precursor. The solid-state structures of the as-synthesized supramolecules revealed that three PDA ligands are employed to fabricate a tetranuclear ({Ni₄}) cluster, and two {Ni₄} clusters are assembled to form the final triple-stranded metallosupramolecules by three TP ligands. The bridging TP ligands also provide large inner voids with highly hydrophobic environments. Structural investigation of the generated complexes provided a deeper understanding of the aspects driving the formation of heteroleptic supramolecules, which is crucial for the design of multiple-strands with desired morphologies and functionalities.

Pt(ii)-coordinated tricomponent self-assemblies of tetrapyridyl porphyrin and dicarboxylate ligands: are they 3D prisms or 2D bow-ties?

Thermodynamically favored simultaneous coordination of Pt(ii) corners with aza- and carboxylate ligands yields tricomponent coordination complexes with sophisticated structures and functions, which require careful structural characterization to paint accurate depiction of their structure-function relationships. Previous reports claimed that heteroleptic coordination of cis-(Et3P)2PtII with tetrapyridyl porphyrins (M'TPP, M' = Zn or H2) and dicarboxylate ligands (XDC) yielded 3D tetragonal prisms containing two horizontal M'TPP faces and four vertical XDC pillars connected by eight Pt(ii) corners, even though such structures were not supported by their 1H NMR data. Through extensive X-ray crystallographic and NMR studies, herein, we demonstrate that self-assembly of cis-(Et3P)2PtII, M'TPP, and four different XDC linkers having varied lengths and rigidities actually yields bow-tie (⋈)-shaped 2D [{cis-(Et3P)2Pt}4(M'TPP) (XDC)2]4+ complexes featuring a M'TPP core and two parallel XDC linkers connected by four heteroleptic PtII corners instead of 3D prisms. This happened because (i) irrespective of their length (∼7-11 Å) and rigidity, the XDC linkers intramolecularly bridged two adjacent pyridyl-N atoms of a M'TPP core via PtII corners instead of connecting two cofacial M'TPP ligands and (ii) bow-tie complexes are entropically favored over prisms. The electron-rich ZnTPP core of a representative bow-tie complex selectively formed a charge-transfer complex with highly π-acidic 1,4,5,8,9,12-hexaazatriphenylene-2,3,6,7,10,11-heaxacarbonitrile but not with a π-donor such as pyrene. Thus, this work not only produced novel M'TPP-based bow-tie complexes and demonstrated their selective π-acid recognition capability, but also underscored the importance of proper structural characterization of supramolecular assemblies to ensure accurate depiction of their structure-property relationships.

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.

Unlocking the computational design of metal-organic cages

Metal-organic cages are macrocyclic structures that can possess an intrinsic void that can hold molecules for encapsulation, adsorption, sensing, and catalysis applications. As metal-organic cages may be comprised from nearly any combination of organic and metal-containing components, cages can form with diverse shapes and sizes, allowing for tuning toward targeted properties. Therefore, their near-infinite design space is almost impossible to explore through experimentation alone and computational design can play a crucial role in exploring new systems. Although high-throughput computational design and screening workflows have long been known as powerful tools in drug and materials discovery, their application in exploring metal-organic cages is more recent. We show examples of structure prediction and host-guest/catalytic property evaluation of metal-organic cages. These examples are facilitated by advances in methods that handle metal-containing systems with improved accuracy and are the beginning of the development of automated cage design workflows. We finally outline a scope for how high-throughput computational methods can assist and drive experimental decisions as the field pushes toward functional and complex metal-organic cages. In particular, we highlight the importance of considering realistic, flexible systems.

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.

Cooperativity of steric bulk and H-bonding in coordination sphere engineering: heteroleptic PdII cages and bowls by design

Recently developed self-assembly strategies allow to rationally reduce the symmetry of metallosupramolecular architectures. In addition, the combination of multiple ligand types without creating compound mixtures has become possible. Among several approaches to realize non-statistical heteroleptic assembly, Coordination Sphere Engineering (CSE) makes use of secondary repulsive or attractive interactions in direct vicinity of the metal nodes. Previously, we used steric congestion to turn dinuclear [Pd₂L₄] cages with fourfold symmetry into [Pd₂L₃X₂] (X = solvent, halide) bowl structures. Here, we introduce a new subtype of this strategy based on balancing hydrogen bonding and repulsive interactions between ligands carrying quinoline (L^Qu) and 1,8-naphthyridine (L^Na) donors to generate trans-[Pd₂L₂] and [Pd₂L₃L′] cages, assisted by templation of encapsulated fullerenes. Combined with steric congestion caused by acridine (L^Ac) donors, we further report the first example of a heteroleptic [Pd₂L₂L′X₂] bowl. Formation, structure and fullerene binding ability of these metallo-supramolecular hosts were studied by NMR, mass spectrometry and single crystal X-ray diffraction.

Coordination Sphere Engineering (CSE) allows non-statistical assembly of heteroleptic supramolecular architectures by fine adjustment of steric and electronic features around square-planar Pd(ii) cations with naphthyridine donors.

Phosphorus guiding palladium: [4+4] metallomacrocyclic PdII complex and self-assembly of heterometallic PdII/ZnII grid-type complex

The reaction of heteroditopic ligand 1 featuring a hard pyridine-hydrazone-pyrimidine (N,N,N) site and a softer pyrimidine-hydrazone-phosphane (N,N,P) pocket with [Pd(CH3CN4)](OTf)2 in different metal-to-ligand ratios (M:L) gave the homobimetallic PdII complex...

Multi-activity tetracoordinated pallado-oxadiazole thiones as anti-inflammatory, anti-Alzheimer, and anti-microbial agents: Structure, stability and bioactivity comparison with pallado-hydrazides

Herein, we report a comparative study based on structure, thermal and solution stability, and biopotency against lipoxygenase (LOX), butyrylcholinesterase (BChE) and microbes for Pd(II) compounds of N,O,S bearing 5-(C₅H₄XR)-1,3,4-oxadiazole-2-thiones (L') of type [PdL'Cl₂] (P'n) and N,O bearing respective hydrazides (L) of type trans-[PdL₂Cl₂] (Pn) {X = C, R = 4-I, 2-Br, 4-NO₂, 3-NO₂, 2-Cl, 3-Cl (n = 1-6, serially); X = N (n = 7)}. Spectral techniques (IR, EI-MS, NMR) and physicochemical evaluations successfully characterized the new compounds. The L' behaved as bidentate S-N donors bonded through exocyclic sulfur and N-3' nitrogen, while L acted as amino N donors. UV-vis (solution speciation) and thermal degradation profiles consistently confirmed the greater stability for P'n than Pn compounds. These compounds manifested varying degree in vitro potential to inhibit LOX, BChE and several bacteria and fungi, affected mainly by Pd(II) presence, M-L binding mode, nature and position of R, or halo groups electronegativity. Molecular docking with human 5-LOX and BChE further validated the respective experimental inhibition findings and explored several putative mechanistic interactions (H-bonding, π-stacking, π-alkyl, π-S, etc.) at the enzyme active sites. Pn generally offered superior antimicrobial and anti-LOX (anti-inflammatory) potential than respective P'n compounds, with P3/P'5, P(2,3,7)/P'3, and P6 being comparable, better and equivalent to ampicillin, nystatin and baicalein, the reference antibacterial, antifungal and anti-LOX drugs, respectively. Contrarily, the anti-BChE activity of P'n was found better than Pn compounds, showing P'2/P1 as the most promising anti-Alzheimer drug candidates. This study bares important structural and mechanistic aspects in optimizing antimicrobial, anti-inflammatory and anti-Alzheimer activities, highlighting some potential future pallado-drug candidates.

Orientational self-sorting: formation of structurally defined Pd4L8 and Pd6L12 cages from low-symmetry dipyridyl ligands

Tetra- and hexanuclear coordination cages were obtained in reactions of [Pd(CH₃CN)₄](BF₄)₂ with low-symmetry dipyridyl ligands. In both cases, only one structurally defined complex was formed out of a vast pool of potential isomers.

High-Throughput Computational Evaluation of Low Symmetry Pd2 L4 Cages to Aid in System Design*

Unsymmetrical ditopic ligands can self-assemble into reduced-symmetry Pd2 L4 metallo-cages with anisotropic cavities, with implications for high specificity and affinity guest-binding. Mixtures of cage isomers can form, however, resulting in undesirable system heterogeneity. It is paramount to be able to design components that preferentially form a single isomer. Previous data suggested that computational methods could predict with reasonable accuracy whether unsymmetrical ligands would preferentially self-assemble into single cage isomers under constraints of geometrical mismatch. We successfully apply a collaborative computational and experimental workflow to mitigate costly trial-and-error synthetic approaches. Our rapid computational workflow constructs unsymmetrical ligands and their Pd2 L4 cage isomers, ranking the likelihood for exclusively forming cis-Pd2 L4 assemblies. From this narrowed search space, we successfully synthesised four new, low-symmetry, cis-Pd2 L4 cages.

Comparison of [Pd2L4][BF4]4 cages for binding of n-octyl glycosides and nitrate (L = isophthalamide or dipicolinamide linked dipyridyl ligand)

Two dipyridyl ligands were synthesized, where the pyridyl donor fragments were separated by an isophthalamide (1) or a dipicolinamide moiety (2). Both ligands formed [Pd2(Ligand)4][BF4]4 complexes in CD2Cl2 containing 5% dmso-d6. It was found that while [Pd2(1)4][BF4]4 readily binds to n-octyl glycosides and to nitrate anions, [Pd2(2)4][BF4]4 did not. The difference in binding properties could be rationalized based on the reduced flexibility and size of the [Pd2(2)4]2+ cage and/or stronger interior binding of a BF4- counter anion.

Cavity-Containing [Fe2L3]4+ Helicates: An Examination of Host-Guest Chemistry and Cytotoxicity

Two new di(2,2′-bipyridine) ligands, 2,6-bis([2,2′-bipyridin]-5-ylethynyl)pyridine (L1) and bis(4-([2,2′-bipyridin]-5-ylethynyl)phenyl)methane (L2) were synthesized and used to generate two metallosupramolecular [Fe₂(L)₃](BF₄)₄ cylinders. The ligands and cylinders were characterized using elemental analysis, electrospray ionization mass spectrometry, UV-vis, ¹H-, ¹³C and DOSY nuclear magnetic resonance (NMR) spectroscopies. The molecular structures of the [Fe₂(L)₃](BF₄)₄ cylinders were confirmed using X-ray crystallography. Both the [Fe₂(L1)₃](BF₄)₄ and [Fe₂(L2)₃](BF₄)₄ complexes crystallized as racemic (rac) mixtures of the ΔΔ (P) and ΛΛ (M) helicates. However, ¹H NMR spectra showed that in solution the larger [Fe₂(L2)₃](BF₄)₄ was a mixture of the rac-ΔΔ/ΛΛ and meso-ΔΛ isomers. The host-guest chemistry of the helicates, which both feature a central cavity, was examined with several small drug molecules. However, none of the potential guests were found to bind within the helicates. In vitro cytotoxicity assays demonstrated that both helicates were active against four cancer cell lines. The smaller [Fe₂(L1)₃](BF₄)₄ system displayed low μM activity against the HCT116 (IC₅₀ = 7.1 ± 0.5 μM) and NCI-H460 (IC₅₀ = 4.9 ± 0.4 μM) cancer cells. While the antiproliferative effects against all the cell lines examined were less than the well-known anticancer drug cisplatin, their modes of action would be expected to be very different.

Tuning the Size and Geometry of Heteroleptic Coordination Cages by Varying the Ligand Bent Angle

Spherical assemblies of the type [Pd_n L_2n ]²ⁿ⁺ can be obtained from Pd^II salts and curved N-donor ligands, L. It is well established that the bent angle, α, of the ligand is a decisive factor in the self-assembly process, with larger angles leading to complexes with a higher nuclearity, n. Herein, we report heteroleptic coordination cages of the type [Pd_n L_n L'_n ]²ⁿ⁺ , for which a similar correlation between the ligand bent angle and the nuclearity is observed. Tetranuclear cages were obtained by combining [Pd(CH₃ CN)₄ ](BF₄ )₂ with 1,3-di(pyridin-3-yl)benzene and ligands featuring a bent angle of α=120°. The use of a dipyridyl ligand with α=149° led to the formation of a hexanuclear complex with a trigonal prismatic geometry; for linear ligands, octanuclear assemblies of the type [Pd₈ L₈ L'₈ ]¹⁶⁺ were obtained. The predictable formation of heteroleptic Pd^II cages from 1,3-di(pyridin-3-yl)benzene and different dipyridyl ligands is evidence that there are entire classes of heteroleptic cage structures that are privileged from a thermodynamic point of view.

Base-assisted synthesis of 4-pyridinate gold(I) metallaligands: a study of their use in self-assembly reactions

The synthesis of di- and tritopic gold(i) metallaligands of the type [(Au4-py)2(μ2-diphosphane)] (diphosphane = bis(diphenylphosphanyl)isopropane or dppip (1), 1,2-bis(diphenylphosphanyl)ethane or dppe (2), 1,3-bis(diphenylphosphanyl)propane or dppp (3) and 1,4-bis(diphenylphosphanyl)butane or dppb (4)) and [(Au4-py)3(μ3-triphosphane)] (triphosphane = 1,1,1-tris(diphenylphosphanylmethyl)ethane or triphos (5) and 1,3,5-tris(diphenylphosphanyl)benzene or triphosph (6)) from [(AuCl)2(μ2-diphosphane)] or [(AuCl)3(μ3-triphosphane)] and 4-pyridylboronic acid in the presence of Cs2CO3 has been conducted. Interestingly, when [(AuCl)2(μ2-dppm)] (dppm = bis(diphenylphosphanyl)methane) was used as a starting material, the cyclic tetranuclear gold(i) compound [(Au4-py)2(CH)2{μ2-Au(PPh2)2}2] (I) was obtained instead. All the products have been characterized by IR and multinuclear NMR spectroscopy, mass spectrometry and elemental analysis and in the case of 1, 3, 5 and I by X-ray crystallography, which showed the presence of aurophilic interactions in all of them. The obtained metallaligands have been used as building blocks in self-assembly reactions with cis-blocked palladium or platinum acceptor moieties producing [2 + 2] metallamacrocycles or trigonal bipyramidal (TBP) [2 + 3] metallacages in good yields. The photophysical properties of both the metallaligands and the corresponding assemblies have been investigated.

Mechanochemical Release of Non-Covalently Bound Guests from a Polymer-Decorated Supramolecular Cage

Supramolecular coordination cages show a wide range of useful properties including, but not limited to, complex molecular machine-like operations, confined space catalysis, and rich host-guest chemistries. Here we report the uptake and release of non-covalently encapsulated, pharmaceutically-active cargo from an octahedral Pd cage bearing polymer chains on each vertex. Six poly(ethylene glycol)-decorated bipyridine ligands are used to assemble an octahedral PdII6 (TPT)4 cage. The supramolecular container encapsulates progesterone and ibuprofen within its hydrophobic nanocavity and is activated by shear force produced by ultrasonication in aqueous solution entailing complete cargo release upon rupture, as shown by NMR and GPC analyses.

Increasing structural and functional complexity in self-assembled coordination cages

Progress in metallo-supramolecular chemistry creates potential to synthesize functional nano systems and intelligent materials of increasing complexity. In the past four decades, metal-mediated self-assembly has produced a wide range of structural motifs such as helicates, grids, links, knots, spheres and cages, with particularly the latter ones catching growing attention, owing to their nano-scale cavities. Assemblies serving as hosts allow application as selective receptors, confined reaction environments and more. Recently, the field has made big steps forward by implementing dedicated functionality, e.g. catalytic centres or photoswitches to allow stimuli control. Besides incorporation in homoleptic systems, composed of one type of ligand, desire arose to include more than one function within the same assembly. Inspiration comes from natural enzymes that congregate, for example, a substrate recognition site, an allosteric regulator element and a reaction centre. Combining several functionalities without creating statistical mixtures, however, requires a toolbox of sophisticated assembly strategies. This review showcases the implementation of function into self-assembled cages and devises strategies to selectively form heteroleptic structures. We discuss first examples resulting from a combination of both principles, namely multicomponent multifunctional host-guest complexes, and their potential in application in areas such as sensing, catalysis, and photo-redox systems.

Multi-stimuli Control over Assembly and Guest Binding in Metallo-supramolecular Hosts Based on Dithienylethene Photoswitches

It is difficult to assemble multi-component metallo-supramolecular architectures in a non-statistical fashion, which limits their development toward functional materials. Herein, we report a system of interconverting bowls and cages that are able to respond to various selective stimuli (light, ligands, anions), based on the self-assembly of a photochromic dithienylethene (DTE) ligand, L^a, with Pd^II cations. By combining the concept of “coordination sphere engineering”, relying on bulky quinoline donors, with reversible photoswitching between the ligand’s open (o-L^a) and closed (c-L^a) forms, a [Pd₂(o-L^a)₄] cage (o-C) and a [Pd₂(c-L^a)₃] bowl (c-B) were obtained, respectively. This structural rearrangement modulates the system’s guest uptake capabilities. Among three bis-sulfonate guests (G1, G2, and G3), the cage can encapsulate only the smallest (G1), while the bowl binds all of them. Bowl c-B was further used to synthesize a series of heteroleptic cages, [Pd₂L^A₃L^B], representing a motif never reported before. Additional ligands (L^c-f), with short or long arms, tune the cavity size, thus enabling or preventing guest uptake. Addition of Br^–/Ag⁺ makes it possible to change the overall charge, again triggering guest uptake and release, as well as fourth ligand de-/recomplexation. In combination, site-selective introduction of functionality and application of external stimuli lead to an intricate system of hosts with different guest preferences. A high degree of complexity is achieved through cooperativity between only a few components.

Multi-functional, Low Symmetry Pd2 L4 Nanocage Libraries*

Although many impressive metallo-supramolecular architectures have been reported, they tend towards high symmetry structures and avoid extraneous functionality to ensure high fidelity in the self-assembly process. This minimalist approach, however, limits the range of accessible structures and thus their potential applications. Herein is described the synthesis of a family of ditopic ligands wherein the ligand scaffolds are both low symmetry and incorporate exohedral functional moieties. Key to this design is the use of CuI -catalysed azide-alkyne cycloaddition (CuAAC) chemistry, as the triazole is capable of acting as both a coordinating heterocycle and a tether between the ligand framework and functional unit simultaneously. A common precursor was used to generate ligands with various functionalities, allowing control of electronic properties whilst maintaining the core structure of the resultant cis-Pd2 L4 nanocage assemblies. The isostructural nature of the scaffold frameworks enabled formation of combinatorial libraries from the self-assembly of ligand mixtures, generating a statistical mixture of multi-functional, low symmetry architectures.

Identification of a Heteroleptic Pd6L6L'6 Coordination Cage by Screening of a Virtual Combinatorial Library

The design of structurally defined heteroleptic coordination cages is a challenging task, and only few examples are known to date. Here we describe a selection approach that allowed the identification of a novel hexanuclear Pd cage containing two types of dipyridyl ligands. A virtual combinatorial library of [Pd_nL_2n](BF₄)_2n complexes was prepared by mixing six different dipyridyl ligands with substoichiometric amounts of [Pd(CH₃CN)₄](BF₄)₂. Analysis of the equilibrated reaction mixture revealed the preferential formation of a heteroleptic [Pd₆L₆L'₆](BF₄)₁₂ assembly. The complex was prepared on a preparative scale by a targeted synthesis, and its structure was elucidated by single-crystal X-ray diffraction. It features an unprecedented trigonal-antiprismatic cage structure with two triangular Pd₃L₃ macrocycles bridged by six L' ligands. A related but significantly larger [Pd₆L₆L'₆](BF₄)₁₂ cage was obtained by using metalloligands instead of organic dipyridyl ligands.