A Relativistic DFT Probe of Thorium and Protactinium Complexes Supported by Heterocalix[4]arene and Redox Properties of Early‐Middle Actinides

Thorium and protactinium complexes of trans ‐calix[2]benzene[2]pyrrolide (H 2 L) have been examined using a relativistic density functional theory, where the metal center has +IV and +III oxidation states and is saturated by a borohydride donor. Good agreement in geometry parameters has been achieved for the thorium(VI) complex compared with its experimentally obtained analogue. Three types of conformers, labeled as ( Ar L)(An m )( 2H B), ( Ar L)(An m )( 3H B) and ( Pl L)(An m )( 3H B) (An=Th and Pa, m=VI and III), are energetically favored for each complex. The first and last configurations are found to be the most stable for tri ‐ and tetra valent complexes, respectively. Intriguing δ(An−Ar) bonding(s) are revealed for tri valent complexes ( Ar L)(An III )( 2H B); and ( Pl L)(Pa IV )( 3H B) has a Pa(5 f )‐dominated HOMO, while ( Pl L)(Th IV )( 3H B) displays a 5 f 0 6 d 0 7 s 0 electronic configuration. Exothermic process was calculated for reactions starting from actinide borohydride sources and various macrocyclic ligands. From a thermodynamic perspective, this provides synthetically accessible possibility of these complexes. The mechanism of reducing tetra ‐ to tri valent actinide complexes has been proposed. Reduction potentials were calculated to follow the order of Th < Pa < U < Np < Pu, which well reproduces reported results of aquo An VI /An III ions.