Computational density functional study of polypyrrolic macrocycles: analysis of actinyl-oxo to 3d transition metal bonding.

Density functional theoretical methods are used to study heterobimetallic compounds of a new form of binucleating Schiff-base polypyrrolic macrocycle, denoted [An(VI)O(2)(1)H(2)L], An(V)O(2)(1)H(2)L, [An(VI)O(2)(TM)L] and An(V)O(2)(TM)L, and containing actinyl ions AnO(2)(n+) (An = U, Np, Pu; n = 1, 2) and 3d transition metals (TM): no TM = 1, Mn = 2, Fe = 3, Co = 4, and Zn = 5. Calculated bond orders (TM-O2 = 0.36 to 0.81) provide evidence for partial bond formation between the transition metal (TM) and the actinyl-endo-oxygen for all 24 cases studied. Redox potentials for [An(VI)O(2)(1)H(2)L]/An(V)O(2)(1)H(2)L couples were found to have the same Np(VI/V) > Pu(VI/V) > U(VI/V) trend as previously studied for the AnO(2)(H(2)O)(5) couples, where Np(VI) is the most easily reduced to Np(V). Extrapolation from the earlier penta-aqua actinyl results is used to predict [An(VI)O(2)(1)H(2)L]/An(V)O(2)(1)H(2)L redox couples of U = -1.10 eV, Np = 0.25 eV, and Pu = 0.01 eV. The calculated redox potential for [U(VI)O(2)(1)H(2)L]/U(V)O(2)(1)H(2)L is within 0.08 eV of the value found by cyclic voltammetry (-1.18 eV, in THF/NBu(n)(4)BF(4) solvent).