STERICALLY CROWDED MANGANESE-OXO COMPLEXES OF N,N-BIS(2-PYRIDYLMETHYL)-TERT-BUTYLAMINE WITH BRIDGED BINUCLEAR CORE TYPES (MN-III(2)(MU-O)(MU-OAC)(2)) AND (MN-IV(2)(MU-O)(2)(MU-OAC))

Reactions of Mn(O2CCH3)(3) .2H2O with N,N-bis(2-pyridylmethyl)-tert-bu tylamine (bpta) in ethanol followed by the addition of excess NaClO4 o r 70% aqueous HClO4 afforded complexes [Mn2O(O2CCH3)(2)(bpta)(2)](ClO4 )(2) (1) or [Mn2O2(O2CCH3)(bpta)(2)](ClO4)(3) (2), respectively. Both 1 and 2 were characterized by X-ray crystallographic studies. Compound 1 crystallizes with two acetonitriles of solvation in the space group C2/c with a = 37.5526(6) Angstrom, b = 10.6000(2) Angstrom, c = 24.19 31(4) Angstrom, beta = 91.5190(10)degrees, V = 9626.9(3) Angstrom(3), and Z = 8. The two manganese atoms in 1 are bridged by an O2- ligand a nd two CH3CO2- groups. One facially coordinated tridentate bpta ligand , with the aliphatic nitrogen positioned trans to an acetate oxygen at om, completes a distorted octahedral geometry at each manganese center . Structural parameters for 1 are consistent with both manganese ions being in the +3 formal oxidation state. The Mn ... Mn separation in 1 is 3.1719(5) Angstrom. The cyclic voltammogram of a pure sample of 1 i n CH3CN shows III,III/III,IV oxidation (i(p)(a)/i(p)(c) = 1.6) and III ,III/III,II reduction (irreversible) waves at +0.88 and -0.28 V, respe ctively, versus the ferrocene/ferrocenium (Fc/Fc(+)) couple. UV-Vis an d IR spectroscopic properties of 1 are consistent with the presence of a {Mn-2(III)(mu-O)(mu-OAc)(2)}(2+) core. Compound 2 crystallizes as a n acetonitrile solvate in the space group P2(1)/n with a = 11.536 80(1 0) Angstrom, b = 10.2865(2) Angstrom, c = 36.9831(4) Angstrom, beta = 98.835(12)degrees, V = 4336.96(10) Angstrom(3), and Z = 4. Bond distan ces and angles for 2 are generally similar to those of other complexes containing a {Mn-2(IV)(mu-O)(2)(mu-OAc)}(3+) core and polypyridyl lig ands. UV-Vis and IR properties of 2 are also reported. Cyclic voltamme try waves for 2 in MeCN, assigned as IV,IV/III,IV and III,IV/III,III r eductions, were observed at +0.60 (i(p)(a)/i(p)(c) = 1) and -0.55 V (i (p)(a)/i(p)(c) = 1) versus Fc/Fc(+), respectively. The effects of the steric size of the t-butyl group are manifested in longer Mn-N-alkyl d istances for both 1 and 2. This elongation results in markedly more po sitive first reduction potentials for both 1 (III,III/III,II) and 2 (I V,IV/IV,III) relative to their N-ethyl analogs. Finally, the reactions of 1 and 2 in water are dramatically different as compared to those o f the same analogs. (C) 1997 Elsevier Science S.A.