Role of ‘O’ Substitution in Expanded Porphyrins on Uranyl Complexation: Orbital and density based analyses

The need for efficient macrocyclic ligands that can sequester U(VI) has gained immense importance due to the increased applications of U(VI) in various sectors, including but not limited to nuclear energy. Structural attributes such as number and type of donor centers ("hard" and "soft") of ligands are essentially the key components for providing the adequate bonding scenario for uranyl. Beside hard or soft-donor-based binding cavity, the mixed-donor ligands are also finding popularity for achieving optimized performances. However, many aspects are still unknown about how and at what extent the ratio of hard-to-soft donor centers tune the bonding attributes with uranyl. Moreover, a consensus is yet to be reached on the nature and role of underlying covalent interaction between U and donors upon complexation, particularly in the mixed-donor ligand environment. In this work, using relativistic density functional theory (DFT), we attempted to address these important issues by systematically investigating the impact on the bonding characteristics of uranyl ion and an expanded porphyrin, viz. sapphyrin with increasing number of 'O' substitution at the cavity. Our results suggest that in the O-substituted sapphyrin variants, UO$_2^{2+}$ prefers to bind N over O donor sites, and decrease in N donor sites at the cavity prompts UO$_2^{2+}$ to have better interaction with the rest of N donor centers. Extended transition state (ETS) with natural orbital for chemical valences (NOCV) analysis shows that at equatorial plane N acts as better $\sigma$ donor to uranyl than O donor. Molecular orbital (MO) and density of states (DOS) analysis shows favorable bonding-interaction between U(d) and donor's p orbitals, the participation of U(f)-orbitals in bonding are of low-extent but non-negligible. Energy decomposition analysis (EDA), natural population analysis (NPA) along with thermodynamic analyses confirms the dominance of electrostatic interaction on the thermodynamic stability of the complexes. However, the U-N/O bonds at the equatorial plane do carry appreciable amount of covalent character. Analysis of quantum theory of atoms in molecules (QTAIM) descriptors in conjugation with MO analysis and overlap integral calculations confirms its nature as near-degeneracy driven type. Statistics of mixed-orbitals and overlap integral further suggest that the O donor does not act as adequate replacement of N for uranyl binding despite having more number of mixed MOs due to the variation in the amplitude of overlap.