Manganese-manganese separations in oxide- 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(??2-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 ??2-oxo atoms of 1 are derived from the same molecule of hydrogen peroxide. The second dimer is a bis(??2-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 ??2-oxo and ??2-alkoxo groups are used to develop a correlation between manganese-manganese separation and Mn-O-Mn bridge angle. It is shown that complexes containing ??-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 ??2-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, ?? = 100.07 (3)?? V = 1930 (1) A3, Z = 2; for 2434 data collected in the range 3 < 2?? < 45?? and 1787 data with I > 2??(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, ?? = 109.58 (2)??, V = 1252 (1) A3, Z = 2; for 1562 data collected in the range 3 < 2?? < 45?? and 1229 data with I > 3??(I) the structure refined to R = 0.034 and Rw = 0.034.