Synthesis, characterization, and unusual magnetic properties of large multinuclear manganese oxide aggregates.

摘要

This Thesis work describes the preparation of new {dollar}rm Mnsb{lcub}10{rcub}, Mnsb{lcub}12{rcub}{dollar}, and Mn{dollar}sb{lcub}18{rcub}{dollar} complexes and their characterization by X-ray crystallography, paramagnetic NMR spectroscopy, electrochemistry, and dc and ac magnetochemistry. Synthetic methodology for the preparation of a new class of decanuclear Mn aggregates has been developed. The complexes ({dollar}rm Mnsb{lcub}10{rcub}Osb8(Osb2CPh)sb6(pic)sb8rbrack{dollar} (where Hpic is picolinic acid) and ({dollar}rm Mnsb{lcub}10{rcub}Osb8(Osb2CPh)sb6(pic)sb6(dbm)sb2rbrack{dollar} (where Hdbm is dibenzoylmethane), can be synthesized from the tetranuclear complex, ({dollar}rm Mnsb4Osb2(Osb2CPh)sb6(pic)sb2(CHsb3CN)sb2rbrack{dollar}. Both of these new complexes contain 10 Mn{dollar}sp{lcub}rm III{rcub}{dollar} ions and have been characterized by X-ray crystallography. Magnetic studies indicate an S = 0 ground state spin for ({dollar}rm Mnsb{lcub}10{rcub}Osb8(Osb2CPh)sb6(pic)sb8rbrack.{dollar} The reaction of {dollar}rm (sp{lcub}n{rcub}Busb4N)lbrack Mnsb4Osb2(Osb2CCsb9Hsb4{dollar}-p-{dollar}rm OMe)sb9(Hsb2O)rbrack{dollar} with 4,4{dollar}spprime{dollar}-bipyridine yields the new complex ({dollar}rm Mnsb9Osb7(Osb2CCsb6Hsb4{dollar}-p-{dollar}rm OMe)sb{lcub}13{rcub} (4,4spprime{dollar}-bpy)) {dollar}sb2.{dollar} This complex contains 18 Mn{dollar}sp{lcub}rm III{rcub}{dollar} ions arranged in two Mn{dollar}sb9{dollar} units linked by 4,4{dollar}spprime{dollar}-bpy. Reduced magnetization data for the complex can be fit to an S = 2 ground state for each Mn{dollar}sb9{dollar} unit.; Exchange of the organic groups of the mixed valence (8 Mn{dollar}rmsp{lcub}III{rcub}, 4 Mnsp{lcub}IV{rcub}){dollar} complex ({dollar}rm Mnsb{lcub}12{rcub}Osb{lcub}12{rcub}(Osb2CMe)sb{lcub}16{rcub}(Hsb2O)sb4rbrack{dollar} with a variety of carboxylic acids, RCOOH (R = alkyl, aryl), has led to the synthesis of a family of ({dollar}rm Mnsb{lcub}12{rcub}Osb{lcub}12{rcub}(Osb2CR)sb{lcub}16{rcub}(Hsb2O)sb4rbrack{dollar} complexes. These complexes each exhibit a chemically reversible reduction at an easily accessible potential, thus providing a facile synthetic route to the one-electron reduced species by reduction with iodide. For R = Et, the one-electron reduction product, ({dollar}rm Mnsb{lcub}12{rcub}Osb{lcub}12{rcub}(Osc2CEt)sb{lcub}16{rcub}(Hsb2O)4rbracksp-,{dollar} has been structurally characterized by X-ray crystallography as the ({dollar}rm Phsb4P)sp+{dollar} salt. A structural comparison of this one-electron reduction product with the analogous neutral species shows that a Mn{dollar}sp{lcub}rm III{rcub}{dollar} rather than a Mn{dollar}sp{lcub}rm IV{rcub}{dollar} ion has been reduced and that there are hence three different metal oxidation states in the complex {dollar}rm (Mnsp{lcub}II{rcub}, 7 Mnsp{lcub}III{rcub}, 4 Mnsp{lcub}IV{rcub}),{dollar} an extremely unusual occurrence. The magnetic properties of ({dollar}rm Mn sb{lcub}12{rcub}Osb{lcub}12{rcub}(Osb2CEt)sb{lcub}16{rcub}(Hsb2O)sb3rbrack (S = 9){dollar} and {dollar}rm (PPhsb4)lbrack Mnsb{lcub}12{rcub}Osb{lcub}12{rcub}(Osb2CEt)sb{lcub}16{rcub}(Hsb2O)sb4rbrack (S = 19/2){dollar} were studied extensively through ac magnetic susceptibility, magnetic hysteresis, and relaxation studies. Both complexes show out-of-phase responses, {dollar}rmchisb{lcub}M{rcub}sp{lcub}primeprime{rcub}{dollar} in an ac magnetic field below 7 K and distinct hysteresis loops in the presence of an applied dc field below 3 K. These properties are indicative of slow relaxation of the magnetization and are caused by a barrier to reversal of the magnetization resulting from the significant magnetic anisotropies of these complexes. In addition, the relaxation of the magnetization of both species at 1.8 K cannot be satisfactorily fit using a single exponential decay, as would be expected for a