SYNTHESIS, CHARACTERIZATION, AND MAGNETIC PROPERTY OF A NOVEL DIMER-OF-DI(MU-OXO)DIMANGANESE-DIMERS WITH 2 COORDINATED WATER-MOLECULES IN (III, IV, III, IV) OXIDATION-STATE

A er-of-di(mu-oxo)manganese(III)manganese(IV)-dimers with two coordina ted water molecules, [{Mn2(tmdp)(O)2(H2O)}2](CF3SO3)4.6H2O (1), was pr epared, where Htmdp is 1,5-bis[bis(2-pyridylmethyl)amino]-3-pentanol. Crystal structure of 1 was determined by X-ray structure analysis. 1 c rystallizes in triclinic space group, P1BAR with a=16.543(l), b=19.097 (2), c=13.393(1) angstrom, alpha=94.450(7)-degrees, beta=90.698(6)-deg rees, gamma=84.997(7)-degrees, V=4202.4(6) angstrom3, and Z=2. The com plex cation consists of two di(mu-oxo)manganese(III)manganese(IV) dime rs which are linked by two tmdps and hydrogen bonding network between two coordinated water molecules and two alkoxo groups to form a dimer- of-dimers with a bilayered structure with dimer-dimer separation of 5. 9 angstrom. The Mn-Mn separations are 2.644(2)-2.651(2) angstrom. Magn etism of 1 was well interpreted in terms of the model based on two kin ds of the exchange interactions between Mn(III) and Mn(IV); the intrad imer interaction (J1 = -145 cm-1) through di(mu-oxo) bridges and the i nterdimer interaction (J2 congruent-to 0.2 cm-1) through hydrogen bond ing network. The interdimer exchange interaction (J2) results in a tri plet ground state. The ESR spectrum of a frozen solution sample of 1 s howed a relatively intense ''DELTAM(s) = 2'' signal centered at g = 4. 5, which is only 7.6 times weaker than a much broader ''DELTAM(s)=l'' signal at g=2.0. The best computer simulation revealed near rhombic ze ro-field splittings with \D\ congruent-to 0.07 cm-1 and \E\ congruent- to 0.023 cm-1, and Mn-hyperfine broadening, which are appropriate for the triplet ground state of such a tetrameric er-of-di(mu-oxo)manganes e(III)manganese(IV)-dimers with 5.9 angstrom separation. The observed zero-field splittings could be interpreted in terms of an incomplete q uenching of the crystal field effect of each Mn(III) ion in the triple t ground state mixed with the excited-state spin configurations throug h the J2 interactions. Cyclic voltammogram exhibited a quasi reversibl e wave (E1/2=0.98 V vs. SCE) and an irreversible oxidation wave at 1.1 0 V vs. SCE in acetonitrile. Constant potential electrolysis at 1.00 V vs. SCE indicated that the quasi reversible wave is a one-electron tr ansfer process and corresponds to Mn4(III, IV, IV, IV)/Mn4(III, IV, II I, IV) redox couple. A one-electron oxidized species which was generat ed electrochemically showed a so called sixteen hyperfine pattern at g =2.0, supporting the presence of di(mu-oxo)manganese(III)manganese(IV) and di(mu-oxo)dimanganese(IV) species in the tetramer. Constant poten tial electrolysis at 1.20 V vs. SCE resulted in a decomposition of the complex.