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)H2L], [AnVO(2)(1)H2L](-), [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)H2L]/[An(V)O(2)(1)H2L](-) couples were found to have the same Np-VI/V > Pu-VI/V > U-VI/V trend as previously studied for the [AnO(2)(H2O)(5)](2+/1+) 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)H2L]/[An(V)O(2)(1)H2L](-) redox couples of U = -1.10 eV, Np = 0.25 eV, and Pu = 0.01 eV. The calculated redox potential for [(UO2)-O-VI(1)H2L]/[(UO2)-O-V(1)H2L](-) is within 0.08 eV of the value found by cyclic voltammetry (-1.18 eV, in THF/(NBu4BF4)-B-n solvent).