Structural transformations of metal-organic cages through tetrazine-alkene reactivity

The assembly of metal-organic cages is governed by metal ion coordination preferences and the geometries of the typically rigid and planar precursor ligands. PdnL2n cages are amongst the most structurally diverse, with subtle differences in the metal-ligand coordination vectors resulting in drastically different assemblies, however almost all rely on rigid aromatic linkers to avoid the formation of intractable mixtures. Here we exploit the inverse electron-demand Diels-Alder (IEDDA) reaction between tetrazine linker groups and alkene reagents to trigger structural changes induced by post-assembly modification. The structure of the 1,4-dihydropyridazine produced by IEDDA (often an afterthought in click chemistry) is crucial; its two sp3 centers increase flexibility and non-planarity, drastically changing the range of accessible coordination vectors. This triggers an initial Pd4L8 tetrahedral cage to transform into different Pd2L4 lantern cages, with both the transformation extent (thermodynamics) and rate (kinetics) dependent on the alkene dienophile selected. With cyclopentene, the unsymmetrical 1,4-dihydropyridazine ligands undergo integrative sorting in the solid state, with both head-to-tail orientation and enantiomer selection, leading to a single isomer from the 39 possible. This preference is rationalized through entropy, symmetry, and hydrogen bonding. Subsequent oxidation of the 1,4-dihydropyridazine to the aromatic pyridazine rigidifies the ligands, restoring planarity. The oxidized ligands no longer fit in the lantern structure, inducing further structural transformations into Pd4L8 tetrahedra and Pd3L6 double-walled triangles. The concept of controllable addition of limited additional flexibility and then its removal through well-defined reactivity we envisage being of great interest for structural transformations of any class of supramolecular architecture

Borate-based ligands with soft heterocycles and their ruthenium complexes

In a quest for effective synthetic precursors for the preparation of B-agostic complexes of ruthenium, we have shown that the reaction of [Cp*RuCl₂]₂ (Cp* = η⁵-C₅Me₅) with [NaBt] or [NaBo] (Bt = dihydrobis(2-mercaptobenzthiazolyl)borate; Bo = dihydrobis(2-mercaptobenzoxazolyl)borate) led to the formation of B-agostic complexes [Cp*RuBH₂L₂], 1a,b (1a: L = 2-mercaptobenzthiazol, 1b: L = 2-mercaptobenzoxazol) and [Cp*RuBH₃L], 2a,b (2a: L = 2-mercaptobenzthiazol, 2b: L = 2-mercaptobenzoxazol) in good yields. In parallel to the formation of 1a,b and 2a,b, this method also allowed the formation of ruthenium hydrotrisborate complexes [Cp*RuBYL₃], 3a–c (3a: L = 2-mercaptobenzthiazol, Y = H; 3b: L = 2-mercaptobenzoxazol, Y = H; 3c: L = 2-mercaptobenzoxazol, Y = Cl). The key feature of complexes 3a–c is the coordination of one of the 2-mercaptobenzothiazole ligand that connects to the metal and the boron centre through a common sulfur atom. Upon heating, compounds 3a,b change into their corresponding S→N linkage isomers, in which the boron atom is bonded to three nitrogen atoms. The cyclic voltammetric studies on compounds 3a–c and 4a,b suggest that a deviation in coordination of the ligand change the oxidation potential of the metal centre. All the new compounds have been characterized in solution by ¹H, ¹¹B and ¹³C NMR spectroscopy, mass spectrometry and the structural types of 3a–c and 4b were unequivocally established by crystallographic analysis

Diffusion of Solvent-Separated Ion Pairs Controls Back Electron Transfer Rate in Graphene Quantum Dots

In the present study, the stability of the photogenerated, solvent-separated charged states of graphene quantum dots (GQDs) in the presence of <i>N</i>,<i>N</i>-diethylaniline (DEA) has been evaluated in a series of organic solvents. The results indicate that the rate constant for back electron transfer (<i>k</i>_BET) from GQD radical anion to DEA radical cation is diffusion-controlled. As a result of the diffusion-controlled back electron transfer (BET), <i>k</i>_BET exhibits an inverse exponential relation to (a) the viscosity coefficient (η) of the solvent and (b) the average radius of the graphene quantum dots. An analytical expression for the diffusion-controlled back electron transfer rate constant has been formulated. The dependence of <i>k</i>_BET on the diffusion of solvent-separated ion pairs has been evaluated for the first time for quantum dot systems and the results provide an efficient method for enhancing the lifetime of the photogenerated charge-separated states from graphene quantum dots. The present findings can potentially improve the performance of GQD-based photovoltaic and optoelectronic devices

Coordination-Induced Emissive Poly-NHC-Derived Metallacage for Pesticide Detection

Developing sensitive, rapid, and convenient methods for the detection of residual toxic pesticides is immensely important to prevent irreversible damage to the human body. Luminescent metal–organic cages and macrocycles have shown great applications, and designing highly emissive supramolecular systems in dilute solution using metal–ligand coordination-driven self-assembly is demanded. In this study, we have demonstrated the development of a silver–carbene bond directed tetranuclear silver(I)-octacarbene metallacage [Ag4(L)2](PF6)4 (1) based on an aggregation-induced emissive (AIE) cored 1,1′,1″,1‴-((1,4-phenylenebis(ethene-2,1,1-triyl))tetrakis(benzene-4,1-diyl))tetrakis(3-methyl-1H-imidazol-3-ium) salt (L). A 36-fold enhanced emission was observed after metallacage (1) formation when compared with the ligand (L) in dilute solution due to the restriction of intramolecular motions imparted by metal–ligand coordination. Such an increase in fluorescence made 1 a potential candidate for the detection of a broad-spectrum pesticide, 2,6-dichloro-nitroaniline (DCN). 1 was able to detect DCN efficiently by the fluorescence quenching method with a significant detection limit (1.64 ppm). A combination of static and dynamic quenching was applicable depending on the analyte concentration. The use of silver–carbene bond directed self-assembly to exploit coordination-induced emission as an alternative to AIE in dilute solution and then apply this approach to solve health and safety concerns is noteworthy and carries a lot of potential for future developments

Linkage induced enhancement of fluorescence in metal-carbene bond directed metallacycles and metallacages

Two new 1,4-dihydropyrrolo3,2-b]pyrrole based aggregation induced emission (AIE)-inactive di- and tetra-imidazolium salts were employed with Ag(i) for the synthesis of a Ag-carbene bond directed metallacycle (1) and metallacage (2), respectively. Transmetalation of these complexes allowed their facile conversion to their respective Au(i)-metallacycle (3) and metallacage (4). The final assemblies exhibit linkage induced enhancement of fluorescence (LIEF). The free ligands are almost non-fluorescent (phi(F) = 3.2, 3.4) in comparison to their metal-carbene counterparts (phi(F) up to 32.0). Thus, without using any AIEgen, obtaining high emission efficiencies in complexes Ag-I-C-NHC (1, 2) via linkage is a nice approach towards turn-on fluorescence