Spectroscopic, electrochemical and structural investigations of cofacial and facially functionalized porphyrin compounds synthesized via metal-mediated [2+2+2] cycloaddition methodology.

摘要

This thesis focuses on the design, synthesis, spectroscopy, structural studies, and electrochemical properties of new cofacial and facially functionalized (porphinato)metal complexes. Such compounds are of interest both as ligands for multielectron redox catalytic applications and in biomimetic investigations.; A key accomplishment of this work has been the development of [2+2+2] cycloaddition methodology compatible with ethyne-elaborated porphyrin precursors. The structures reported herein are unique in that each was synthesized via sequential palladium-catalyzed cross-coupling and cobalt-mediated [2+2+2] cycloaddition methodologies. By utilizing such a synthetic route, it has been demonstrated that a wide variety of cofacial and facially-functionalized porphyrin constructs can be prepared in a modular fashion.; Structures of interest include multiporphyrin compounds which feature a covalently-bridged cofacial bis(porphinato)metal core having peripheral porphyrin substituents, which display both π-cofacial and π-conjugative electronic interactions between chromophore units. Such structures possess open conformations necessary for binding small molecules within the cofacial core, and hold promise for the elaboration of new classes of redox catalysts competent to both bind small molecules and effect concerted, multielectron redox transformations.; The modular nature of the newly developed synthetic approach enables facile control of the nature of macrocycle-to-macrocycle connectivity, and allows unprecedented modulation of the redox properties of such face-to-face porphyrin species. Bis[(porphinato)metal] complexes possessing meso-meso connectivity, as well as the first examples of rigidly bridged meso-β and β-β structures are reported. In addition to electron-rich arene derivatized macrocycles, (porphinato)metal units possessing strongly σ-electron-withdrawing trifluoromethyl substituents are readily incorporated into these structural motifs. This approach allows the orbital energy levels of this class of compounds to be modulated over well-defined increments spanning nearly 1 eV, which constitutes an important advance in the development of new ligands for multielectron catalytic applications. Analyses of the electronic properties of such bis[(porphinato)metal] complexes define simple models that provide insight into the nature of arene-arene π-π interactions. In addition, cobalt derivatives of these electron-poor porphyrin ligands bind dioxygen with high affinity. Whereas the monomeric complex appears incapable of catalytically reducing dioxygen, the dimeric derivative displays electrochemical behavior consistent with electroreduction of dioxygen at unprecedented positive potentials.