Self-Assembly of a Redox Active, Metallosupramolecular [Pd3 L6 ]6+ Complex Using a Rotationally Flexible Ferrocene Ligand

Modulating the guest binding ability within mixed-coordination geometry [Pd(μ-L)4RuCl2]2+ and [Pd(μ-L)4Pt]4+ cage architectures

Heterobimetallic cages built from Pd and either octahedral Ru or square-planar Pt moieties and bridged by ligands with H bonding-accepting or -donating properties are reported. They showed stimulus-responsive dis- and reassembly, while guest binding was found to be dependent on the complementary properties of the guest to the host in terms of charge, size and H bonding properties.

A Reduced-Symmetry Pd2L4 Cage from a Heterotopic Dipyridyl Ligand

Two dipyridyl ligands, L3,3 and L3,4, have been used in combination with palladium(II) in the construction of metallosupramolecular species that show anion-dependent behavior in solution. A rare example of a low-symmetry (C2h) lantern-type cage is formed in one instance, [Pd2(L3,3)4]4+, while the isomeric ligand yields a larger double-walled square complex, [Pd4(L3,4)8]8+. [Pd2(L3,3)4](NO3)4 was isolated in crystalline form revealing two anions within the interior of the C2h-symmetry cage. The cage itself is held together by hydrogen bonding between "head-to-tail" pairs of ligands that reinforces the symmetry generated by the ditopic ligands. In solution, the cage with NO3- has sharp 1H nuclear magnetic resonance (NMR) signals at room temperature, while the BF4- analogue has broad signals that sharpen at higher temperatures or upon addition of (Bu4N)(NO3), highlighting the importance of the anion in templating or otherwise influencing self-assembly in solution. Altering the substitution position of one of the pyridyl rings yields a more "open" complex, with [Pd4(L3,4)8](NO3)8 being isolated as a crystalline solid. The double-walled square complex has a greater Pd···Pd separation due to the increased angle that the pyridyl groups subtend at the core of the ligand. NMR spectroscopy and mass spectrometry studies suggest a single species in the presence of nitrate but multiple species with tetrafluoroborate.

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.

Pd12MnL24 (for n = 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.

Exploring the Emergent Redox Chemistry of Pd(II) Nodes with Pendant Ferrocenes: From Precursors, through Building Blocks, to Self-Assemblies

Energy-relevant small molecule activations and related processes are often multi-electron in nature. Ferrocene is iconic for its well-behaved one-electron chemistry, and it is often used to impart redox activity to self-assembled architectures. When multiple ferrocenes are present as pendant groups in a single structure, they often behave as isolated sites with no separation of their redox events. Herein, we study a suite of molecules culminating in a self-assembled palladium(II) truncated tetrahedron (TT) with six pendant ferrocene moieties using the iron(III/II) couple to inform about the electronic structure and, in some cases, subsequent reactivity. Notably, although known ferrocene-containing metallacycles and cages show simple reversible redox chemistry, this TT undergoes a complex multi-step electrochemical mechanism upon oxidation. The electrochemical behavior was observed by voltammetric and spectroelectrochemical techniques and suggests that the initial Fc-centered oxidation is coupled to a subsequent change in species solubility and deposition of a film onto the working electrode, which is followed by a second separable electrochemical oxidation event. The complicated electrochemical behavior of this self-assembly reveals emergent properties resulting from organizing multiple ferrocene subunits into a discrete structure. We anticipate that such structures may provide the basis for multiple charge separation events to drive important processes related to energy capture, storage, and use, especially as the electronic communication between sites is further tuned.

Ferrocene-Derived Palladium(II)-Based Metallosupramolecular Structures: Synthesis, Guest Interaction, and Stimulus-Responsiveness Studies

Using ferrocene-based ligand systems, a series of heterobimetallic architectures of the general formula [Pd_mL_n]^x+ were designed with the aim of installing an opening and closing mechanism that would allow the release and binding of guest molecules. Palladium complex formation was achieved through coordination to pyridyl groups, and using 2-, 3-, and 4-pyridyl derivatives provided access to defined PdL, PdL₂, and Pd₂L₄ structures, respectively. The supramolecular complexes were characterized using nuclear magnetic resonance (NMR) and infrared spectroscopy, mass spectrometry, and elemental analysis, and for some examples density functional theory calculations and single-crystal X-ray diffraction analysis. ¹H NMR spectroscopy was used to investigate disassembly and reassembly of the metallosupramolecular structures. The former was induced by cleavage of the relatively labile Pd-N_pyridyl bonds with the introduction of the competing ligands N,N'-dimethylaminopyridine (DMAP) and Cl^- (using tetrabutylammonium chloride) to yield [Pd(DMAP)₄]²⁺ and [PdCl₄]^2-, respectively. The process was found to be reversible for several of the heterodimetallic compounds, with the addition of H⁺ or Ag⁺ triggering complex reassembly. Guest binding studies with several architectures revealed interactions with the anionic guests p-toluenesulfonate and octyl sulfate, but not with neutral molecules. Furthermore, the release of guests was reversibly induced with Cl^- ions as a stimulus.

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.

An Adaptable Water-Soluble Molecular Boat for Selective Separation of Phenanthrene from Isomeric Anthracene

Anthracene crude oil is a common source of phenanthrene for its industrial use. The isolation of phenanthrene from this source is a challenging task due to very similar physical properties to its isomer anthracene. We report here a water-soluble Pd(II) molecular boat (MB1) with unusual structural topology that was obtained by assembling a flexible tetrapyridyl donor (L) with a cis-Pd(II) acceptor. The flexible backbone of the boat enabled it to breathe in the presence of a guest optimizing the fit within the cavity. The boat binds phenanthrene more strongly than anthracene, which enabled separation of phenanthrene with an >98% purity from an equimolar mixture of the two isomers using MB1 as an extracting agent. MB1 represents a unique example of a coordination receptor suitable for selective aqueous extraction of phenanthrene from anthracene with reusability of several cycles.

Fast, solvent-free synthesis of ferrocene-containing organic cages via dynamic covalent chemistry in the solid state

A simple, solvent-free synthetic protocol towards the synthesis of organic self-assembled macromolecules has been established. By employing mechanochemistry using glassware readily available to every organic chemist, we were able to synthesise three novel organic cage compounds exemplarily and to speed up the synthesis of a ferrocene-containing macrocycle by a factor of 288 compared to the solution-based synthesis. The structural investigation of the newly synthesised cages revealed different modes of connectivity from using ferrocene-containing aldehydes caused by the free rotation of the cyclopentadienyl units against each other. By extending the facile solvent-free synthesis to ball-milling, even compounds that show lower reactivity could be employed in the dynamic covalent formation of organometallic cage compounds. The presented protocol gives access to otherwise inaccessible structures, speeds up general synthetic workflows, and simultaneously reduces the environmental impact of supramolecular syntheses.

Uptake, Trapping, and Release of Organometallic Cations by Redox-Active Cationic Hosts

The host-guest chemistry of metal-organic nanocages is typically driven by thermodynamically favorable interactions with their guests such that uptake and release of guests can be controlled by switching this affinity on or off. Herein, we achieve this effect by reducing porphyrin-walled cationic nanoprisms 1a¹²⁺ and 1b¹²⁺ to zwitterionic states that rapidly uptake organometallic cations Cp*₂Co⁺ and Cp₂Co⁺, respectively. Cp*₂Co⁺ binds strongly (K_a = 1.3 × 10³ M^-1) in the neutral state 1a⁰ of host 1a¹²⁺, which has its three porphyrin walls doubly reduced and its six (bipy)Pt²⁺ linkers singly reduced (bipy = 2,2'-bipyridine). The less-reduced states of the host 1a³⁺ and 1a⁹⁺ also bind Cp*₂Co⁺, though with lower affinities. The smaller Cp₂Co⁺ cation binds strongly (K_a = 1.7 × 10³ M^-1) in the 3e^- reduced state 1b⁹⁺ of the (tmeda)Pt²⁺-linked host 1b¹²⁺ (tmeda = N,N,N',N'-tetramethylethylenediamine). Upon reoxidation of the hosts with Ag⁺, the guests become trapped to provide unprecedented metastable cation-in-cation complexes Cp*₂Co⁺@1a¹²⁺ and Cp₂Co⁺@1b¹²⁺ that persist for >1 month. Thus, dramatic kinetic effects reveal a way to confine the guests in thermodynamically unfavorable environments. Experimental and DFT studies indicate that PF₆^- anions kinetically stabilize Cp*₂Co⁺@1a¹²⁺ through electrostatic interactions and by influencing conformational changes of the host that open and close its apertures. However, when Cp*₂Co⁺@1a¹²⁺ was prepared using ferrocenium (Fc⁺) instead of Ag⁺ to reoxidize the host, dissociation was accelerated >200× even though neither Fc⁺ nor Fc have any observable affinity for 1a¹²⁺. This finding shows that metastable host-guest complexes can respond to subtler stimuli than those required to induce guest release from thermodynamically favorable complexes.

Trimetallic coordination cage formation for nitrate encapsulation: transformation of kinetic products into thermodynamic products

A procedure for the formation of a nitrate-encapsulating tripalladium(II) cage via self-assembly of Pd(NO₃)₂ with 1,3-bis(dimethyl(pyridin-4-yl)silyl)propane (L) was developed. The self-assembly reaction initially produces spiro-type macrocycles, PdL₂, and finally results in transformation into a nitrate-encapsulated cage, [(NO₃)@Pd₃L₆], in the mother liquor. The reaction of PdX₂ (X^- = BF₄^-, ClO₄^-, PF₆^-, and CF₃SO₃^- instead of NO₃^-) with L gives rise to a spiro species, PdL₂, as the final product, and anion exchange of the spiro products, [PdL₂](X)₂, with NO₃^- produces the tripalladium cage [(NO₃)@Pd₃L₆].

A Family of Externally-Functionalised Coordination Cages

New synthetic routes are presented to derivatives of a (known) M8L12 cubic coordination cage in which a range of different substituents are attached at the C4 position of the pyridyl rings at either end of the bis(pyrazolyl-pyridine) bridging ligands. The substituents are (i) –CN groups (new ligand LCN), (ii) –CH2OCH2–CCH (containing a terminal alkyne) groups (new ligand LCC); and (iii) –(CH2OCH2)3CH2OMe (tri-ethyleneglycol monomethyl ether) groups (new ligand LPEG). The resulting functionalised ligands combine with M2+ ions (particularly Co2+, Ni2+, Cd2+) to give isostructural [M8L12]16+ cage cores bearing 24 external functional groups; the cages based on LCN (with M2+ = Cd2+) and LCC (with M2+ = Ni2+) have been crystallographically characterised. The value of these is twofold: (i) exterior nitrile or alkene substituents can provide a basis for further synthetic opportunities via ‘Click’ reactions allowing in principle a diverse range of functionalisation of the cage exterior surface; (ii) the exterior –(CH2OCH2)3CH2OMe groups substantially increase cage solubility in both water and in organic solvents, allowing binding constants of cavity-binding guests to be measured under an increased range of conditions.

Ferrocenylated 1,3,5-triphenylbenzenes for the electrochemical detection of various cations or anions

Three diverse 1,3,5-triphenylbenzene derivatives bearing three ferrocene units (Fc1-Fc3) were used for the construction of electrochemical sensors for the detection of various cations or anions. The obtained derivatives contained imine (Fc1 and Fc2) or amide bonds (Fc3) between 1,3,5-triphenylbenzene and ferrocene skeletons, which further influenced the ability of the as-synthesized ferrocenylated molecules to recognize cations (Cu(ii), Pb(ii), Cr(iii) and Fe(iii); compounds Fc1 and Fc2) or monovalent anions (Br-, I-, ClO4-, NO3-, CH3COO-, and H2PO4-; compound Fc3). In the case of cation recognition, differential pulse voltammetry (DPV) experiments revealed that the Fc1 receptor was characterized by much better sensitivity compared to the Fc2 receptor and the limit of detection (LOD) values ranged from 0.5 to 1.8 μM. Similarly, such very low LOD values (0.7-1.5 μM) were also found in the case of anion recognition by the Fc3 receptor. Importantly, the advantage of the tested Fc receptors is also their long-term stability (sensors were stable at least for 2 months).

Structural Flexibility in Metal-Organic Cages

Metal-organic cages (MOCs) have emerged as a diverse class of molecular hosts with potential utility across a vast spectrum of applications. With advances in single-crystal X-ray diffraction and economic methods of computational structure optimisation, cavity sizes can be readily determined. In combination with a chemist's intuition, educated guesses about the likelihood of particular guests being bound within these porous structures can be made. Whilst practically very useful, simple rules-of-thumb, such as Rebek's 55% rule, fail to take into account structural flexibility inherent to MOCs that can allow hosts to significantly adapt their internal cavity. An often unappreciated facet of MOC structures is that, even though relatively rigid building blocks may be employed, conformational freedom can enable large structural changes. If it could be exploited, this flexibility might lead to behavior analogous to the induced-fit of substrates within the active sites of enzymes. To this end, in-roads have already been made to prepare MOCs incorporating ligands with large degrees of conformational freedom. Whilst this may make the constitution of MOCs harder to predict, it has the potential to lead to highly sophisticated and functional synthetic hosts.

Supramolecular Alloys from Fluorinated Hybrid Tri4 Di6 Imine Cages

To create innovative materials, efficient control and engineering of pore sizes and their characteristics, crystallinity and stability is required. Eight hybrid Tri4 Di6 imine cages with a tunable degree of fluorination and one fully fluorinated Tri4 Di6 imine cage are investigated. Although the fluorinated and the non-fluorinated building blocks used herein differ vastly in reactivity, it was possible to gain control over the outcome of the self-assembly process, by carefully controlling the feed ratio. This represents the first hybrid material based on fluorinated/hydrogenated porous organic cages (POCs). These cages with unlimited miscibility in the solid state were obtained as highly crystalline samples after recrystallization and even showed retention of the crystal lattice, forming alloys. All mixtures and the fully fluorinated Tri4 Di6 imine cage were analyzed by MALDI-MS, single-crystal XRD, powder XRD and in regard to thermal stability (TGA).

Electrochemical Recognition of Aromatic Species with Ferrocenylated 1,3,5-Triazine- or 1,3,5-Triphenylbenzene-Containing Highly Organized Molecules

Two ferrocenylated organized molecules comprising 1,3,5-triphenylbenzene (Fc-1) or 2,4,6-triphenyl-1,3,5-triazine skeletons (Fc-2) were used for the first time as receptor layers for the electrochemical recognition of polycyclic aromatic hydrocarbons. While our group recently reported the synthesis of Fc-1, herein the facile synthesis of its 2,4,6-triphenyl-1,3,5-triazine-containing structural analog (Fc-2) is presented. Although the synthesis of Fc-2 was found to be challenging, we achieved a very high yield (89 %) under mild conditions using an acid-catalyzed imine-bond formation reaction in 1,4-dioxane:toluene solvent system. Title compounds were comprehensively characterized with various analytical techniques, including spectroscopic (NMR, FT-IR, FT-Raman spectroscopy) methods, high-resolution mass spectrometry (HRMS), microscopic (SEM) and electrochemical (CV) analyses. Fc-1 and Fc-2 were also used for the construction of the first-of-a-kind recognition layers (electrochemical sensors) dedicated to the recognition of polycyclic aromatic hydrocarbons. Fully constructed innovative sensors enabled the efficient recognition of analytes since the limit of detection (LOD) values were not higher than 2.9 μM. Comparative studies between the working parameters of electrochemical sensors comprising Fc-1 or Fc-2 were also included in this work.