MANGANESE MANGANESE SEPARATIONS IN OXIDE-BRIDGED AND ALKOXIDE-BRIDGED COMPLEXES - CORRELATION OF STRUCTURE WITH LIGAND TYPE AND NUMBER

Abstract

Two dimeric manganese complexes have been prepared and their structures determined using X-ray crystallography. The first is a bis(μ2-oxo) complex {[MnIV(SALPN)O]2 (1), where SALPN = 1,3-bis(salicylideneamino)propane} which had previously been reported to be an Mn(III) dimer of composition [MnIII(SALPN)(OH)]2. Complex 1 can be prepared by hydrogen peroxide or air oxidation of a basic solution containing MnIII(SALPN)+ or by the reaction of Mn(SALPN)(acac) with hydrogen peroxide. In the latter reaction, isotopic labeling studies demonstrate that both μ2-oxo atoms of 1 are derived from the same molecule of hydrogen peroxide. The second dimer is a bis(μ2-alkoxo) complex {[MnIII(SALAHP)Cl(CH3OH)]2 (2), where H2SALAHP = 1-(salicylideneamino)-3-hydroxypropane} which is the first example of a manganese dimer with unsupported alkoxide bridges and a rare example of a chloride-containing manganese dimer. These two new dimers and twenty other manganese complexes containing μ2-oxo and μ2-alkoxo groups are used to develop a correlation between manganese-manganese separation and Mn-O-Mn bridge angle. It is shown that complexes containing μ-oxo bridges conform to a law of cosines correlation between Mn-Mn separations and Mn-O-Mn bridge angles. In contrast, there is marked deviation from the law of cosines behavior when μ2-alkoxo groups form the single-atom bridge. Surprisingly, an empirical correlation with a slope of 0.03 A/deg over a 0.95-A range (2.86-3.81 A) and 33° angle deviation (96-129°) is observed. Structural implications are presented for established multinuclear manganese enzymes. Crystal data are as follows. 1: monoclinic, P21/c, a = 10.827 (4) A, b = 10.241 (4) A, c = 17.679 (7) A, β = 100.07 (3)° V = 1930 (1) A3, Z = 2; for 2434 data collected in the range 3 < 2θ < 45° and 1787 data with I > 2σ(I) the structure refined to R = 0.068 and Rw = 0.0434. 2: monoclinic, P21/c, a = 8.428 (3) A, b = 12.966 (3) A, c = 12.160 (3) A, β = 109.58 (2)°, V = 1252 (1) A3, Z = 2; for 1562 data collected in the range 3 < 2θ < 45° and 1229 data with I > 3σ(I) the structure refined to R = 0.034 and Rw = 0.034.