Sterically crowded manganese-oxo complexes of N, N-bis(2-pyridylmethyl)-tert-butylamine with bridged binuclear core types {Mn III2(μ-O)(μ-OAc) 2} and {Mn IV2(μ-O) 2(μ-OAc)}

Abstract

Reactions of Mn(O 2CCH 3) 3 · 2H 2O with N, N-bis(2-pyridylmethyl-tert-butylamine (bpta) in ethanol followed by the addition of excess NaClO 4 or 70% aqueous HClO 4 afforded complexes [Mn 2O(O 2CCH 3) 2(bpta) 2](ClO 4) 2 ( 1) or [Mn 2O 2(O 2CCH 3)(bpta) 2](ClO 4) 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) A ̊ , b = 10.6000(2) A ̊ , c = 24.1931(4) A ̊ , β = 91.5190(10)°, V = 9626.9(3) A ̊ 3, and Z = 8 . The two manganese atoms in 1 are bridged by an O 2− ligand and two CH 3CO 2 − 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 MnMn separation in 1 is 3.1719(5) Å. The cyclic voltammogram of a pure sample of 1 in CH 3CN 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, respectively, versus the ferrocene/ferrocenium (Fc/Fc +) couple. UV-Vis and IR spectroscopic properties of 1 are consistent with the presence of a {Mn III 2(μ-O)(μ-OAc) 2} 2+ core. Compound 2 crystallizes as an acetonitrile solvate in the space group P2 1/ n with a = 11.536 80(10) A ̊ , b = 10.2865(2) A ̊ , c = 36.9831 (4) A ̊ , β = 98.835(12)°, V = 4336.96(10) A ̊ 3 , and Z = 4. Bond distances and angles for 2 are generally similar to those of other complexes containing a {Mn IV 2(μ-O) 2(μ-OAc)} 3+ 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( i p a i p c = 1) and −0.55 V (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 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.