Theoretical Prediction of Divalent Actinide Borozene Complexes

The aromatic boron cluster B₈^2- (D₇) has similar π bonding to C₆H₆, which is named "borozene". The B₈^2- ligand has been observed to stabilize monovalent Ln(+I) in C₇-LnB₈^- (Ln = La, Pr, Tb, Tm, and Yb) borozene complexes. Low-valency actinide complexes have been reported more rarely, and B₈^2- may be one of the potential ligands. Here, we report a theoretical study on a series of actinide metal-doping octa-boron clusters AnB₈ (An = Pa, U, Np, and Pu). It was found that each species has both half-sandwich and chair-like structures. Except for PaB₈, the half-sandwich structures of UB₈, NpB₈, and PuB₈ are more energetically stable than the chair-like structures, and the half-sandwich clusters of AnB₈ are found to be actinide(II) borozene complexes with the M^II[B₈^2-] type. For each of the half-sandwich clusters, the B₈^2- ligand has σ and π double aromaticity. Various bonding analyses of AnB₈ confirm the covalent interactions between the doped actinide metals and the octa-boron clusters, which further stabilize the complexes and determine the relative stability of AnB₈. As expected, these complexes show high bond dissociation energies, especially PaB₈ with stronger Pa-B covalent bonds. These results demonstrate that the B₈^2- doubly aromatic ligand is able to stabilize divalent actinides.