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

[摘要] 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.

[发布机构] University:University of Glasgow

[关键词] Organic chemistry