MONONUCLEAR MANGANESE(IV) IN TRIDENTATE ONO COORDINATION - SYNTHESIS, STRUCTURE, AND REDOX REGULATION VIA OXYGEN DONOR VARIATION

The aerobic reaction of N-(2-hydroxyphenyl)salicylaldimine (H2amp) and 2,2'-dihydroxyazobenzene (H2azp) with manganese(II) or manganese(III) affords Mn(IV) (amp)2 and Mn(IV) (azp)2, respectively. In a similar reaction, 2-hydroxy-2'-carboxy-5-methylazobenzene (H2azc) furnishes KMn(III) (azc)2.4H2O, which can be oxidized Mn(IV) (azc)2 by persulfate. The X-ray structures of Mn(amp)2 and Mn(azp)2 are reported. Crystal data for Mn(amp)2: space group C2/c, Z = 4, a = 20.163 (12) angstrom, b = 7.921 (4) angstrom, c = 12.994 (10) angstrom, beta = 97.65 (5)-degrees, and V = 2057 (2) angstrom 3. Crystal data for Mn(azp)2: space group P1BAR, Z = 2, a = 7.766 (5) angstrom, b = 10.377 (5) angstrom, c = 12.964 (5) angstrom, alpha = 92.80 (3)-degrees, beta = 90.33 (5)-degrees, gamma = 102.92 (4)-degrees, and V = 1016.8 (9) angstrom 3. The ligands act as meridional tridentate ONO donors. The Mn-O distances fall in the range 1.861 (4)-1.893 (6) angstrom. The Mn-N(azomethine) length, 1.968 (8) angstrom, is shorter than the average Mn-N(azo) length, 2.007 (10) angstrom. The MnO4N2 coordination spheres deviate considerably from octahedral geometry, and this is reflected in the EPR spectra of the complexes: strong and weak signals near g = 4 and g = 2, respectively. The manganese(IV)-manganese(III) reduction potentials of Mn(amp)2, Mn(azp)2, and Mn(azc)2 in dimethyl sulfoxide are respectively -0.09, +0.15, and 0.31 V vs SCE. In MnO4N2-type salicylaldimine complexes, the potential varies with oxygen donors according to the order alcoholate < phenolate < carboxylate. The total shift can be as large as 600 mV. The trend is correlated with the pK's of the oxygen donor functions. The significance of the results with respect to carboxylate binding of PS II manganese is noted.