STRUCTURAL AND MAGNETIC CHARACTERIZATION OF TRINUCLEAR, MIXED-VALENCE MANGANESE ACETATES

Abstract

Four mixed valence, trinuclear manganese complexes of general stoichiometry MnIIMnIII 2L2(carboxylate) 4X2, where L is a tridentate Schiff base ligand and X is a neutral monodentate donor, have been structurally characterized by using X-ray crystallography. The solid-state magnetic behavior of these compounds and two additional isostructural complexes are defined by examining the variable-temperature magnetic susceptibilities of each compound. The structurally characterized complexes have strictly 180° Mn(III)-Mn(II)-Mn(III) angles as required by crystallographic inversion symmetry. These linear complexes, α-Mn3(SALADHP)2(acetate)4(CH 3OH)2 (1), α-Mn3-(SALADHP)2(acetate)4(H 2O)2 (2), α-Mn3(SALATHM)2(acetate)(CH3OH) 2 (3), and α-Mn3(SALADHP)2-(acetate)4(HpyrO) 2 (4) (where H2SALADHP = 1,3-dihydroxy-2-methyl-2-(salicylideneamino)propane, H2SALATHM = tris(hydroxymethyl)(salicylideneamino)methane and HpyrO = 1-aza-2-keto-3,5-cyclohexadiene), are distinguished from the previously reported bent isomer β-Mn3(SALADHP)2(acetate)4(CH 3OH)2 (5). The complexes are valence trapped with two terminal Mn(III) ions showing Jahn-Teller distortions along the carboxylate-Mn(III)-X axis. The manganese separations range from 3.42-3.55 A. This is the first homologous series of mixed-valence complexes that can be used to investigate magnetostructural relationships within manganese clusters. All of the complexes have S = 3/2 ground states with J12 ≈ -3 to -7 cm-1, J11′ ≈ 0 cm-1, zj′, ≈ 0.3 cm-1, g1 ≈ 2.04, and g2 fixed at 2.0. Thus, the antiferromagnetic exchange in these complexes is insensitive to changes in metal-metal distance, metal-metal orientation, acetate bridiging groups, phenolate ring substitution, axial ligation, or Mn(III) out of plane displacement. Crystal data for 1: monoclinic, P21/c, a = 12.450 (3) A, b = 13.504 (4) A, c = 13.428 (5) A, β = 102.53 (2)°, V = 2203 (1) A3. For 3420 data collected between 3 ≤ 2θ ≤ 45° and 2111 data > 3σ(I), the structure refined to R = 0.057 (Rw = 0.055); Z = 2. Crystal data for 2: triclinic, P1, a = 9.459 (10) A, b = 10.524 (6) A, c = 10.965 (10) A, α = 109.55 (6)°, β = 95.65 (8)°, γ = 96.53 (7)°, V = 1011 (2) A3,Z = 1. For 2671 data collected between 3 ≤ 2θ ≤ 45° and 2010 data > 3σ(I), the structure refined to R = 0.041 (Rw = 0.040). Crystal data for 3: triclinic, P1, a = 8.694 (3) A, b = 9.779 (3) A, c = 14.474 (3) A, α = 105.53 (2)°, β = 104.30 (2)°, γ = 95.72 (2)°, V = 1130.6 (8) A3, Z = 1. For 3174 data collected between 3 ≤ 2θ ≤ 45° and 2321 data > 3σ(7) the structure refined to R = 0.041 (Rw = 0.040). Crystal data for 4: triclinic, P1, a = 10.025 (4) A, b = 13.302(8) A, c = 13.380 (7) A, α = 114.48 (4)°, β = 96.57 (4)°, γ = 112.33 (4)°, V = 1422 (1) A3, Z = 1. For 3727 data collected between 3 ≤ 2θ ≤ 40° and 2502 data > 3σ(I), the structure refined to R = 0.057 (Rw = 0.063).