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

Abstract

The syntheses of the first mixed-metal CeIVMnIV complexes are reported. [CeMn2O3(O2CMe)(NO 3)4(H2O)2(bpy)2]-(NO 3) (1

bpy = 2,2′-bipyridine) was obtained from the reaction of Mn(NO3)2·xH2O and bpy with (NH 4)2Ce(NO3)6 in a 1:1:2 molar ratio in 25% aqueous acetic acid. The complexes [CeMn6O9(O 2CR)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/Ce IV 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(O 2CMe)6(NO3)2(L)a(H 2O)b] (L = mhpH, a = 4, b = 0 (4)

L = pyroH, a = 2, b = 3 (5)) and {{(pyH)3[Ce3Mn2O6(O 2CMe)7.5(NO3)3]·(HO 2CMe)0.5·(H2O)2} 2(NO3)}n (6), respectively. Solid-state magnetic susceptibility (χM) data for compounds 1, , and 5 were fit to the theoretical χMT versus T expression for a Mn IV2 complex derived using the isotropic Heisenberg spin Hamiltonian (H = -2JŜ1Ŝ2) 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 J1 = -5.8 cm-1, J 2 = -0.63 cm-1, J3 ≈ 0, and g = 2.0(1), where J1 and J2 are the exchange interactions for the [MnIV2O2(O2-CMe)] and [Mn IV2O(O2CMe)2] units, respectively, and J3 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. © 2007 American Chemical Society.