Stimuli-Responsive Metal-Ligand Assemblies.

The Engineering and Physical Sciences Research Council and the European Research Council are acknowledged for financial support.This is the accepted manuscript of a paper published in Chemical Reviews (McConnell AJ, Wood CS, Neelakandan PP, Nitschke JR, Chemical Reviews (2015) 115(15):7729-7793. doi:10.1021/cr500632f). The final version is available at http://dx.doi.org/10.1021/cr500632

Controlled Binding of Organic Guests by Stimuli-Responsive Macrocycles

[Abstract] Synthetic supramolecular chemistry pursues not only the construction of new matter, but also control over its inherently dynamic behaviour. In this context, classic host–guest chemistry, based on the development of a myriad of macrocyclic receptors with fine-tuned affinities and selectivities, has enormously contributed to the discovery of new chemical function under self-assembly conditions. In turn, the use of molecular switches as control units within host–guest assemblies opened the door for the regulation of their dynamic interactional behaviour, which can be translated into controlled aggregation. In this review, we will focus on different strategies developed for the regulated binding of organic molecules by switchable macrocyclic hosts. As we will see, an appropriate design using stimuli-responsive versions of well-known organic receptors allows the molecular switches implemented within their structures to transform their regulated behaviour from the molecular to the supramolecular level.This research was supported by the Ministerio de Economía y Competitividad (MINECO FEDER, Grant CTQ2016-75629-P), Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) (CTQ2017-89166-R), and Xunta de Galicia (ED431C 2018/39). E. P. thanks the funding received from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 851179), the UDC-Inditex InTalent Programme for her research contract and funding and the Xunta de Galicia for the Oportunius Programme. I. N. thanks the MECD (FPU program) for financial supportXunta de Galicia; ED431C 2018/3

Supramolecular Chemistry in the 3rd Millennium

This Special Issue is one of the first for the new MDPI flagship journal Chemistry (ISSN 2624-8549) which has a broad remit for publishing original research in all areas of chemistry. The theme of this issue is Supramolecular Chemistry in the 3rd Millennium and I am sure that this topic will attract many exciting contributions. We chose this topic because it encompasses the unity of contemporary pluridisciplinary science, in which organic, inorganic, physical and theoretical chemists work together with molecular biologists and physicists to develop a systems-level understanding of molecular interactions. The description of supramolecular chemistry as ‘chemistry beyond the molecule’ (Jean-Marie Lehn, Nobel Lecture and Gautam R. Desiraju, Nature, 2001, 412, 397) addresses the wide variety of weak, non-covalent interactions that are the basis for the assembly of supramolecular architectures, molecular receptors and molecular recognition, programed molecular systems, dynamic combinatorial libraries, coordination networks and functional supramolecular materials. We welcome submissions from all disciplines involved in this exciting and evolving area of science

Cation Chelating [2]catenanes and Cyclophanes Based on 2,2'-Bipyridine

The research presented in this thesis lies within the general area of supramolecular chemistry. More specifically, this project involved the synthesis of cyclophanes and [2]catenanes which contained both cation chelating 2,2'-bipyridine units and pi-electron accepting and donating moieties. The cyclophane systems are composed of either one or two cation chelating units and two bipyridinium electron accepting units. They were assembled either by employing template syntheses whereby the electron deficient cyclophanes formed electron donor-acceptor (EDA) complexes with electron donating threads or by employing a 'pre-coordination' strategy. The former route produces pseudo- rotaxane species which may be easily de-threaded to yield cyclophane products. The latter synthetic route involves coordinating cyclophane precursors with metal centres, so as to overcome the unfavourable conformation of the 2,2'-bipyridine-based precursors, and performing ring closure reactions under refluxing conditions. The [2]catenane structures are composed of two interlocked rings, one of which is an electron donating aromatic crown ether, the other being a pi-electron deficient cation chelating cyclophane. The catenated systems were synthesised in a similar manner to their analogous cyclophanes, with the difference that upon ring closure, an interlocked structure is produced rather than a pseudo-rotaxane species - the EDA interaction is thus locked within the structure. The primary aim of the project was to construct models for the photosynthetic reaction centre such as those in the purple photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides. Cation chelating [2]catenanes were identified as possessing the necessary structural features to adequately model the vectorial electron transfer that is present in the natural systems. The secondary aims of the project were to characterise and investigate the properties of the [2]catenanes and their related cyclophanes as ligands, guests for aromatic electron donors and as potential components of molecular devices. Four cyclophane structures were constructed, each of which consisted of either one or two cation chelating 2,2'-bipyridyl units. All four systems were characterised by NMR spectroscopy and mass spectrometry and their electrochemical properties investigated. Cyclophanes LI and L2 were characterised structurally by X-ray diffraction techniques, as was the precursor to these systems, PI and its ruthenium bipyridyl complex Ru(bipy)2(P1)4+. The structures confirmed that the pre-coordination strategy produces a cyclophane precursor which possesses a conformation more favourable towards cyclisation than its non-complexed equivalent. The cyclophane structures indicated that addition of 2,2'-bipyridine units to the cyclophane structures increased the cavity size and thus weakened binding of aromatic electron donors. This was confirmed by the measurement of binding constants for a range of such donors with LI and L2. The X-ray structure of the inclusion complex between LI and the donor thread T1 revealed the geometry of the interactions between such electron deficient cyclophanes and their complementary electron rich guests. The coordination chemistry of cyclophanes LI and L2 was investigated, particularly as regards the formation of multiple - cyclophane aggregates. In addition, photoactive metal centres such as ruthenium and osmium tris-bipyridyl were appended to cyclophane LI. Photoinduced electron transfer (PET) was observed to occur upon investigation of Ru(bipy)2(L1)6+ by time-resolved fluorescence and absorption spectroscopy. The precursor complex Ru(bipy)2(P1)4+ was also subjected to photophysical investigation and no PET was observed. Three analogous [2]catenanes were constructed and characterised by NMR spectroscopy and MS techniques. The properties of the interlocked species were investigated by UV-VIS spectroscopy and by cyclic voltammetry. The structure of one catenated structure, L5, was determined by X-ray diffraction and the interlocked nature of the compound confirmed. The coordination chemistry of this compound was investigated particularly as regards the construction of photoactive assemblies. Photoinduced electron transfer (PET) was observed to occur upon investigation of Ru(bipy)2(L5)6+ by time-resolved fluorescence and absorption spectroscopy in much the same way as had occurred in Ru(bipy)2(L1)6+. However, the complex which featured the catenated ligand possessed an inherent redox asymmetry between its two chemically identical electron acceptors, thus vectorial electron transfer was achieved

Anion receptor chemistry

This review covers advances in anion complexation in the year 2013, 2014 and 2015. The review focuses on the applications of anion receptor chemistry including sensing, self-assembly, extraction, transport, catalysis, as well as fundamental advances in the area. <br/

Pyridinium-based Building Blocks for Rotaxanes inSupramolecular Dynamic Systems

Programa Oficial de Doutoramento en Química Ambiental e Fundamental. 5031V01[Resumen] La presente tesis doctoral describe la síntesis y estudio de compuestos derivados de sales de piridinio, así como su aplicación en sistemas dinámicos supramoleculares. En el primer capítulo se aborda el autoensamblaje dirigido por centros metálicos de Pd(II) y Pt(II), de ligandos bidentados nitrogenados con diferentes unidades hidrofóbicas que conducen a metalociclos dinucleares rectangulares. A continuación, se estudió el equilibrio dinámico establecido entre las especies metalocíclicas discretas y los pseudorrotaxanos del tipo “anillo dentro de anillo”. La especiación del sistema puede modificarse siguiendo el principio de Le Chatelier, o mediante la adición de un sustrato complementario de la cavidad metalocíclica que conduce a la formación del correspondiente complejo de inclusión. En el siguiente capítulo se llevó a cabo la síntesis de sustratos piridinícos con grupos carboxilato en los extremos, y con un núcleo central aromático apropiado para la complejación del macrociclo cucurbit[7]urilo. La posición y número de estos grupos ácidos permite la modulación, en función del pH, de parámetros termodinámicos y cinéticos del equilibrio de formación del [2]pseudorrotaxano El tercer capítulo describe la preparación en medio acuoso de un nuevo [2]rotaxano, utilizando la química covalente dinámica de las iminas y la complementariedad de las sales de bifenilpiridinio con el macrociclo cucurbit[7]urilo. El comportamiento dinámico del [2]rotaxano provoca la disociación en sus componentes, la cual se favorece en condiciones ácidas mientras que se bloquea aumentando el pH, cambiando el disolvente, o mediante la eliminación del grupo carbonilo en la estructura del eje. En la última parte de la tesis, se diseñaron tres componentes lineales diferentes que presentan una unidad naftalénica, dos unidades de viológeno y una cadena de polietilenglicol de longitud variable que las conecta. El autoensamblaje con cucurbit[8]urilo conduce a una supraestructura en forma de brida, debido a la inclusión simultánea de la unidad de naftalénica y uno de los viológenos. Este sistema puede controlarse externamente en función del disolvente, dando lugar a una estructura de [2]pseudorrotaxano. Asimismo, mediante el potencial eléctrico, se induce la transición entre los estados de “tensada” y “relajada” de la brida molecular.[Resumo] A presente tese doutoral describe a síntese e o estudo de compostos derivados de sales de piridinio, así como a súa aplicación en sistemas dinámicos supramoleculares. O primeiro capítulo aborda a autoensamblaxe dirixida por centros metálicos de Pd (II) e Pt (II), de ligandos bidentados nitroxenados con diferentes unidades hidrofóbicas que conducen a metalociclos dinucleares rectangulares. A continuación, estudouse o equilibrio dinámico establecido entre especies metalocíclicas discretas e pseudorotaxanos tipo "anel dentro de anel". A especiación do sistema pódese modificar seguindo o principio de Le Chatelier, ou engadindo un substrato complementario da cavidade metalocíclica que leva á formación do correspondente complexo de inclusión. No seguinte capítulo, levouse a cabo a síntese de substratos piridínicos con grupos carboxilatos nos extremos, e cun núcleo central aromático axeitado para a súa complexación polo macrociclo cucurbit[7]urilo. A posición e número destes grupos ácidos permite a modulación, en función do pH, de parámetros termodinámicos e cinéticos do equilibrio de formación do [2]pseudorotaxano. O terceiro capítulo describe a preparación en medio acuoso dun novo [2]rotaxano, utilizando a química covalente dinámica das iminas, e a complementariedade das sales de bifenilpiridinio co macrociclo cucurbit[7]urilo. O comportamento dinámico do [2]rotaxano provoca disociación nos seus compoñentes, o que se favorece en condicións ácidas mentres se bloquea aumentando o pH, cambiando o disolvente ou eliminando o grupo carbonilo na estrutura do eixe. Na última parte da tese deseñáronse tres compoñentes lineais diferentes que presentan unha unidade de naftaleno, dúas unidades de violoxeno e unha cadea de polietilenglicol de lonxitude variable que os conecta. A autoensamblaxe con cucurbit[8]urilo leva a unha superestrutura en forma de brida, debido á inclusión simultánea da unidade de naftaleno e un dos violóxenos. Este sistema pódese controlar externamente dependendo do disolvente, dando lugar a unha estrutura de [2]pseudorotaxano. Do mesmo xeito, mediante o potencial eléctrico indúcese a transición entre os estados " axustado" a "relaxado" da brida molecular.[Abstract] This dissertation describes the synthesis and study of compounds derived from pyridinium salts, as well as their application in supramolecular dynamic systems. The first chapter addresses the Pd(II) and Pt(II)-directed self-assembly of nitrogenous bidentate ligands, owning different hydrophobic units and that lead to rectangular dinuclear metallocycles. Then, the dynamic equilibrium established between discrete metallocyclic species and "ring-within-ring" pseudorotaxanes was studied. In that regard, the speciation of the system can be modified following the Le Chatelier principle, or by adding a complementary substrate to the metallocyclic cavity, which leads to the formation of the corresponding inclusion complex. In the next chapter, we tackled the synthesis of pyridinic substrates capped with carboxylate end groups, and owning an appropriate aromatic central nucleus for the complexation of the macrocycle cucurbit[7]uril. The position and number of these acid groups allows the modulation, as a function of pH, of thermodynamic and kinetic parameters of the equilibrium of formation of [2]pseudorotaxanes. The third chapter describes the preparation of a new [2]rotaxane in aqueous medium, using the dynamic covalent chemistry of imines and the complementarity of biphenylpyridinium salts with the cucurbit[7]uril host. The dynamic behaviour of the obtained [2]rotaxane causes the dissociation in its components, which is favoured under acidic conditions while it is blocked by increasing the pH, changing the solvent, or by eliminating the carbonyl group in the axle component. In the last part of the thesis, three different linear components were designed that present a naphthalene unit, two viologen moieties and a polyethylene glycol chain of variable length that connects them. Self-assembly with cucurbit[8]uril leads to a zip tie-shaped superstructure, due to the simultaneous inclusion of the naphthalene unit and one of the viologens. This system can be externally controlled depending on the solvent, resulting in a [2]pseudorotaxane structure. Likewise, by means of modulation of the electric potential, the transition between the states of "tight” and "relaxed” of the molecular zip tie can be induced