Mixed Transition Metal−Lanthanide Complexes at High Oxidation States: Heteronuclear CeIVMnIV Clusters

The syntheses of the first mixed-metal CeIVMnIV complexes are reported. [CeMn2O3(O2CMe)(NO3)4(H2O)2(bpy)2](NO3) (1; bpy = 2,2‘-bipyridine) was obtained from the reaction of Mn(NO3)2·xH2O and bpy with (NH4)2Ce(NO3)6 in a 1:1:2 molar ratio in 25% aqueous acetic acid. The complexes [CeMn6O9(O2CR)9(X)(H2O)2]y + (R = Me, X = NO3 -, y = 0 (2); R = Me, X = MeOH, y = +1 (3); R = Et, X = NO3 -, y = 0 (7)) were obtained from reactions involving a [Mn(O2CR)2]·4H2O/CeIV ratio of ∼1:1.5 in concentrated aqueous carboxylic acid. A related reaction in less-concentrated aqueous acetic acid and in the presence of L (where L = 2-hydroxy-6-methylpyridine (mhpH), 2-pyrrolidinone (pyroH), or pyridine (py)) gave [Ce3Mn2O6(O2CMe)6(NO3)2(L)a(H2O)b] (L = mhpH, a = 4, b = 0 (4); L = pyroH, a = 2, b = 3 (5)) and {{(pyH)3[Ce3Mn2O6(O2CMe)7.5(NO3)3]·(HO2CMe)0.5·(H2O)2}2(NO3)}n (6), respectively. Solid-state magnetic susceptibility (χM) data for compounds 1, 4, and 5 were fit to the theoretical χM T versus T expression for a MnIV 2 complex derived using the isotropic Heisenberg spin Hamiltonian (H = −2J Ŝ 1 Ŝ 2) and the Van Vleck equation. The obtained fit parameters were (in the format J, g) 1, −45.7(3) cm-1, 1.95(5); 4, −0.40(10) cm-1, 2.0(1); and 5, −0.34(10) cm-1, 2.0(1), where J is the exchange interaction constant between the two MnIV ions. The data for compound 3 were fit by a matrix diagonalization method that gave J 1 = −5.8 cm-1, J 2 = −0.63 cm-1, J 3 ≈ 0, and g = 2.0(1), where J 1 and J 2 are the exchange interactions for the [MnIV 2O2(Ο2CMe)] and [MnIV 2O(Ο2CMe)2] units, respectively, and J 3 for a uniform next-nearest-neighbor interaction. Theoretical estimates of the exchange constants in compounds 1 and 3 obtained with the ZILSH method were in excellent and good agreement, respectively, with the values obtained from fits of the magnetization data. The difference for 3 is assigned to the presence of the Ce4+ ion, and atomic bond indices obtained from the ZILSH calculations were used to rationalize the values of the various exchange constants based on metal−ligand bond strengths.