Mixed transition metal-lanthanide complexes at high oxidation states:: Heteronuclear CeIVMnIV clusters

The syntheses of the first mixed-metal (CeMnIV)-Mn-IV 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)center dot xH(2)O and bpy with (NH4)(2)Ce(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)]center dot 4H(2)O/Ce-IV ratio of similar to 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)]center dot(HO2CMe)(0.5)center dot(H2O)(2)}(2)(NO3)}(n) (6), respectively. Solid-state magnetic susceptibility (chi M) data for compounds 1, 4, and 5 were fit to the theoretical chi T-M versus T expression for a Mn-2(IV) complex derived using the isotropic Heisenberg spin Hamiltonian (H = -2JS(1)S(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 Mn-IV 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) approximate to 0, and g = 2.0(1), where J(1) and J(2) are the exchange interactions for the [(Mn2O2)-O-IV(O2CMe)] and [(Mn2O)-O-IV(O2CMe)(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.