SYNTHESES, STRUCTURES, AND PROPERTIES OF [MN-2(MU-O)(2)(MU-O2CCH3)(FAC-BPEA)(2)](CLO4)(2) AND 2 HALIDE-LIGATED DIOXO-BRIDGED DIMERS DERIVED THEREFROM - [MN-2(MU-O)(2)X(2)(MER-BPEA)(2)](CLO4)(2) (X=F,CL)

The reaction of Mn(O2CCH3)(2) . 4H(2)O, N,N-bis(2-pyridylmethyl)ethylamine (bpea), and KMnO4 in aqueous acetate buffer (pH similar to 4.5) afforded the mixed-valence complex [Mn2O2(O2CCH3)(bpea)(2)](ClO4)(2) (1) in 79% yield. Diffusion of n-hexane into a solution of 1 in CH2Cl2 provided the CH2Cl2 disolvate, which crystallizes in space group P $($) over bar$$ 1 with a = 9.293(2) Angstrom, b = 12.233(2) Angstrom, c = 18.812(3) Angstrom, alpha = 92.781(13)degrees, beta = 99.636(19)degrees, gamma = 97.727(16)degrees, V = 2083.6(15) Angstrom(3), and Z = 2. Compound 1 is an example of a trapped mixed-valence (MnMnIV)-Mn-III species for which the Mn(III) and Mn(IV) ions are easily distinguishable in the solid state. The electronic and EPR spectral properties of 1 in CH3CN are characteristic of the {Mn2O2}(3+) core. The cyclic voltammogram of 1 in CH3CN shows III,IV to IV,IV oxidation and III,IV to III,III reduction waves at +0.92 and +0.02 V, respectively, vs SCE. Variable-temperature magnetic susceptibility measurements of 1 in the solid state confirmed a doublet ground state. These magnetic data have been fitted adequately using the isotropic spin Hamiltonian H -2JS(1) . S-2 (S-1 = 2, S-2 = 3/2) with J = -164 cm(-1). Compound 1 was converted to [Mn(2)O(2)X(2)(bpea)(2)](ClO4)(2) (X = F, 2; X = Cl, 3) by addition of 2 equiv of aqueous HX in CH3CN. Formation of the halide derivatives is presumed to be the result of a disproportionation reaction. Crystal structure determinations of 2 and 3 were performed. Both species crystallize in the space group P $($) over bar$$ 1 on crystallographic inversion centers. The unit cell parameters for 2 are a = 6.692(2) Angstrom, b = 10.751(3) Angstrom, c = 12.179(2) Angstrom, alpha = 69.07(2)degrees, beta = 79.25(2)degrees, gamma = 81.46(3)degrees, V = 801.3(4) Angstrom(3), and Z = 1 and for 3 are a = 7.187(2) Angstrom, b = 10.669(3) Angstrom, c = 12.531(3) Angstrom, alpha = 113.72(2)degrees, beta = 99.73(2)degrees, gamma = 94.32(2)degrees, V = 856.0(4) Angstrom(3), and Z = 1. Bond distances to Mn ions in 2 and 3 are consistent with these ions being in the +4 formal oxidation state. Compounds 2 and 3 have two prominent electronic absorption bands in the visible region: 2, 530, 635 nm; 3, 550, 642 nm. Assignment of similar bands in III,IV binuclear species appears elsewhere (J. Am. Chem. Sec. 1994, 216, 2392). Less intense low-energy maxima in the vicinity of 800 nm were observed for both compounds as well. Cyclic voltammetry experiments conducted in CH3CN revealed two reduction responses for both 2 and 3. They are assigned as IV,IV to III,IV and III,IV to III,III reductions. The potentials (vs SCE) for these processes are +0.59 and -0.35 V, respectively, for 2 and +0.74 and -0.06 V, respectively, for 3. Variable-temperature magnetic susceptibility measurements reveal the presence of an antiferromagnetic interaction between the two Mn(IV) centers in complex 3. The magnitude of the coupling constant J, obtained by least-squares fitting of the magnetic susceptibility data for 3 using an expression generated from the isotropic spin Hamiltonian H = -2JS(1) . S-2 (S-1 = 3/2, S-2 = 3/2), is -147 cm(-1). The infrared absorption spectra for compounds 1-3 were measured.