Sterically crowded manganese-oxo complexes of N,N-bis(2-pyridylmethyl)-tert-butylamine with bridged binuclear core types {Mn < sup > III < /sup > < inf > 2 < /inf > (μ-O) (μ-OAc) < inf > 2 < /inf > } and {Mn < sup > IV < /sup > < inf > 2 < /inf > (μ-O) < inf > 2 < /inf > (μ-OAc)}

Abstract

Reactions of Mn(O2CCH3)3·2H2O with N,N-bis(2-pyridylmethyl)-tert-butylamine (bpta) in ethanol followed by the addition of excess NaClO4 or 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) Å, b=10.6000(2) Å, c=24.1931 (4) Å, β=91.5190(10)°, V=9626.9(3) Å3, and Z=8. The two manganese atoms in 1 are bridged by an O2- ligand and two CH3CO2- groups. One facially coordinated tridentate bpta ligand, with the aliphatic nitrogen positioned trans to an acetate oxygen atom, 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) Å. The cyclic voltammogram of a pure sample of 1 in CH3CN shows III,III/III,IV oxidation (ipa/ipc = 1.6) and III,III/III,II reduction (irreversible) waves at +0.88 and -0.28 V, respectively, versus the ferrocene/ferrocenium (Fc/Fc+) couple. UV-Vis and IR spectroscopic properties of 1 are consistent with the presence of a {MnIII2(μ-O)(μ-OAc)2}2+ core. Compound 2 crystallizes as an acetonitrile solvate in the space group P21/n with a=11.536 80(10) Å, b=10.2865(2) Å, c=36.9831(4) Å, β=98.835(12)°, V=4336.96(10) Å3, and Z=4. Bond distances and angles for 2 are generally similar to those of other complexes containing a {MnIV2(μ-O)2(μ-OAc)}3+ core and polypyridyl ligands. UV-Vis and IR properties of 2 are also reported. Cyclic voltammetry waves for 2 in MeCN, assigned as IV,IV/III,IV and III,IV/ III,III reductions, were observed at +0.60 (ipa/ipc = 1) and -0.55 V (ipa/ipc = 1) versus Fc/Fc+, respectively. The effects of the steric size of the t-butyl group are manifested in longer Mn-Nalkyl distances for both 1 and 2. This elongation results in markedly more positive first reduction potentials for both 1 (III,III/III,II) and 2 (IV,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 of the same analogs. © 1997 Elsevier Science S.A.