DESIGN, SYNTHESIS, AND CHARACTERIZATION OF BIS(MU-OXO)DIMANGANESE(III,III) COMPLEXES - STERIC AND ELECTRONIC CONTROL OF REDOX POTENTIALS

The syntheses of three bis(μ-oxo)dimanganese(III, III) complexes and one bis(μ-oxo)dimanganese(III, IV) complex are described. The Mn(III/III) complexes are all of the general formulation [(L)MnO]22+, where the ligands L are related to N, N'-bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen, 1a) and tris(2-pyridylmethyl)amine (tmpa, 2a). The complex bis(μ-oxo)bis[N, N'-bis((6-methylpyrid-2-yl)methyI)ethane-1,2-diamine]dimanganese(III) perchlorate monohydrate, [(1b)-MNO]2(ClO4)2H2O, Mn2C32H2H46Cl2N8O11 (4), crystallizes in the triclinic space group PĪ with two binuclear complexes in a cell of dimensions a = 8.487 (3) Å,b-10.868 (2) Å, c = 20.816 (5) Å, α = 77.34 (2)°, α = 88.28 (2)°, and γ; = 84.02 (2)°. The structure has been refined to a value of the conventional R factor of 0.0759 based on 2787 independent observations. The complex bis(μ-oxo)bis[N, N-bis((6-methylpyrid-2-yl)methyl)-N-2-pyridylmethylamine]dimanganese(III) nitrate hexahydrate, [(2c)MnO]2(NO3)26H2O, Mn2C40H56N10O14 (5), crystallizes in the monoclinic space group C2/c with four binuclear complexes in a cell of dimensions a = 22.122 (7) Å, b = 16.419 (6) Å, c = 14.461 (4) Å, β = 117.15 (2)°. The structure has been refined to an R factor of 0.0683 based on 1396 independent observations. The complex bis(μ-oxo)bis[N, N'-bis(2-methylpyrazyl)-ethane-1,2-diamine]dimanganese(III) perchlorate heptahydrate, [(1d)MnO]2(ClO4)27H2O, Mn2C24H46Cl2N12O17 (7), crystallizes in the monoclinic space group Cl with four binuclear complexes in a cell of dimensions a = 21.371 (5) Å,b = 9.301 (3) Å, c = 21.180 (7) Å, β = 108.54 (2)°. The structure has been refined to an R value of 0.0515 based on 3766 independent observations. The complexes are all found to contain the isomer in which the substituted pyridine (or the pyrazine) groups are trans axially disposed around the metal relative to the bridging oxo groups. Complexes 4 and 5 display verylong axial Mn-N bonds which can be attributed to the presence of the 6-methyl groups on the pyridine rings, and this steric constraint has been demonstrated to be the cause of the stabilization of the (III, III) form in these complexes. In complex 7, the stabilization of the III/III form is an electronic consequence of the change from pyridine (pKa, = 5.20) to pyrazine (pKa = 0.65). Cyclic voltammograms of 4 and 5 show that the two redox waves are shifted by approximately 0.4 V in the positive direction in each case relative to the “parent” bispicen and tmpa complexes. The fully oxidized form of complex 5 has been shown to act as a two-electron electrocatalytic oxidant, selectively oxidizing benzyl alcohol to benzaldehyde. © 1990, American Chemical Society. All rights reserved.