Variations on cobalt hexaamine (CoN6) : syntheses, structures, reactivities and properties of coordinated methanimines, extended cages, highly charged surfactants and polymeric cage complexes
Creators: name: Qin, Chang Jin
Publisher: name: The Australian National University
Year: 1997
Subjects: subject: Organocobalt compounds
subject: Amines
subject: Coordination compounds

description: CoN₆ amine systems have played important roles in chemistry since the early days of the coordination chemistry. And the prospect for synthetic development and interesting properties of such molecules continues to stimulate efforts to advance its chemistry. Exploration of some recent new advances in this area dominates this thesis in which the synthesis, chemical and physical properties and biological activities of several new molecules are described. In the first Section, Chapter One outlines the history of CoN₆ amine systems and describes some interesting developments and prospects arising from these systems. Chapter Two describes the general experimental details including syntheses and procedures, but the syntheses relating to new strategies are given in the appropriate individual chapters. The second Section, containing Chapters Three and Four, describes the syntheses, structures and properties of coordinated methanimines which are believed to be important intermediates in the syntheses of hexaamine cage complexes. It is found that the normally extremely unstable organic poly-methanimines are stabilised by coordination, and that the coordinated methanimine complexes are sufficiently stable to use as synthetic reagents. The single crystal X-ray analyses of three poly-methanimine complexes, [Co(CH₂)₃-sen]³⁺ and [Co((CH₂h₂-en)₃]³⁺ and [Co((CH₂)₃-tame₂)J³⁺, show the structures of the methanimine moieties which would be very difficult to synthesize and maintain in the absence of the metal. These studies of coordinated methanimines open up new versatile ways to generate 3-dimensional polyamine assemblies not only more efficiently but also more selectively. The mechanisms for template encapsulation syntheses are also more clearly understood from the studies involving intermediate mono and poly-methanimine molecules and other isolated products. Chapters Five and Six in Section Three deal with cobalt Meg-tricosaneN6 expanded cage systems. Both Co(II) and Co(Ill) complexes have been isolated and their properties studied. The Co(II) system displays an unique stability against oxidation by 02 in both solution and solid states. X-ray analyses and NMR studies indicate that both conformational and asymmetric nitrogen configuration changes occur relatively easily for the Co(II) complexes. One isomer of the Co(III) cage is fairly stable even though it is shown to be an efficient oxidant. Different Co(III) isomers have been isolated in the solid state. One form has an extraordinary blue colour and another is orange. The solid ⁵⁹Co MAS NMR spectrum shows a very large downfield chemical shift (ca 4000 ppm) from the orange to the blue form. The reflectance spectra imply a very weak ligand field at the cobalt site of the blue form. Another major factor is that the redox potential can be changed by at least 0. 7 V with the same metal ion couple and the same set of ligating atoms merely by influencing the steric factors in the cage. Paramagnetic cobalt (III) isomers have also been in evidence, but only investigated in preliminary ways in this work. The Me₈-tricosaneN₆ bicyclic cage free ligand is isolated by a relatively simple one-pot method. The synthesis of this sterically crowded extended cavity cage ligand also leads to possibilities for encapsulating and stabilizing larger metalions which do not fit readily in the smaller sar type ligand cavities. For example, the larger Cu(I) ion is tolerated in the octa methyl cage ligand, as demonstrated by the chemically reversible cyclic voltammetry of the [Cu(Me₈-tricosaneN₆)]²⁺/¹⁺ couple and the details are discussed in Chapter Seven. In the fourth Section, Chapter Eight describes the syntheses of polymeric cage complexes. The trimeric cage complex inherits many properties from the parental monomeric cages as well as showing electrostatic coupling between CoN6 moieties. It also demonstrates the potential for these molecules to be used as multiple electron transfer agents. The dimer cage [Co(CH₃,CH₂)sar-Co(NH,NH₂)sar]⁶⁺ and trimer cage [Co(CH₃,CH₂)sar-Co(NH,NH)sar-Co(CH₂,CH₃)sar]⁹⁺ compounds are further invested by reaction with DNA, the results implying that they interact with DNA chiefly by electrostatic binding. They are also found to photochemically cleave DNA by a mechanism which involves LMCT and the formation of a ligand radical oxidant. Chapter Nine in Section Four presents syntheses and properties of highly charged surfactants. These molecules show typical detergent properties and also demonstrate biological activity as in vitro anthelmintic agents. Some of the results obtained in this thesis do not seem to have any precedent in MN 6 chemistry and are related to fundamental changes in the nature of the MN 6 structure. Therefore they are not all well understood at this point. They all demonstrate, however, the versatility of MN6 systems and further work based on the studies in this thesis may help to advance important new insights into coordination chemistry. Some results, such as the coordinated methanimine compounds, are believed to be not only useful for inorganic synthesis but also for organic chemistry.