Syntheses, Structures, and Properties of [Mn < inf > 2 < /inf > (μ-O)2(μ-O < inf > 2 < /inf > CCH < inf > 3 < /inf > )(f2(bpea) < inf > 2 < /inf > ](Clo < inf > 4 < /inf > ) < inf > 2 < /inf > and Two Halide-Ligated Dioxo-Bridged Dimers Derived Therefrom: [Mn < inf > 2 < /inf > (μ-O) < inf > 2 < /inf > X < inf > 2 < /inf > (mer-bpea) < inf > 2 < /inf > ](C1O < inf > 4 < /inf > ) < inf > 2 < /inf > (X = F, CI)

Abstract

The reaction of Mn(O2CCH3)2·4H2O, N,N-bis(2-pyridylmethyl)ethylamine (bpea), and KMnO4 in aqueous acetate buffer (pH ~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 CH2CI2 provided the CH2Cl2 disolvate, which crystallizes in space group P1 with a = 9.293(2) Å, b = 12.233(2) Å, c = 18.812(3) Å, ± = 92.781(13)°, β = 99.636(19)°, γ = 97.727(16)°, V = 2083.6(15) Å3, and Z = 2. Compound 1 is an example of a trapped mixed-valence MnIIIMnIV 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= −2JS1·S2 (S1 =2,S2 = 3/2) with J = −164 cm−1. Compound 1 was converted to [Mn2O2X2(bpea)2](C1O4)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 P1 on crystallographic inversion centers. The unit cell parameters for 2 are a = 6.692(2) Å,b = 10.751(3) Å, c = 12.179(2) Å, ± = 69.07(2)°, β = 79.25(2)°, γ = 81.46(3)°, V = 801.3(4) Å3, andZ= 1 and for 3 are a = 7.187(2) Å,b= 10.669(3) Å, c = 12.531(3) Å, ± = 113.72(2)°, β = 99.73(2)°, γ = 94.32(2)°, V = 856.0(4) Å3, 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. Soc. 1994, 116, 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= −2JS1·S2 (S1 = 3/2, S2 = 3/2), is −147 cm−1. The infrared absorption spectra for compounds 1–3 were measured. © 1995, American Chemical Society. All rights reserved.