PNNL

Abstract

A new trivalent americium oxalate coordination polymer, [Am(C2O4)(H2O)3Cl], (Am-1) has been synthesized, structurally characterized and studied via optical spectroscopy. The crystallographic analysis reveals the structure is two-dimensional and comprised of pseudo dimeric Am3+ containing nodes that are bridged by oxalate ligands to form sheets. Each metal center is nine-coordinate, forming a capped square antiprism geometry with C1 symmetry, and features bound oxalate, aqua, and chloro ligands. The Am3+-ligand bonding was probed computationally using the quantum theory of atoms in molecules (QTAIM) and natural localized molecular orbital (NLMO) approach to investigate the ionic and covalent bond character and identify the hybrid atomic orbital contributions therein. The analyses indicate that the bonding is overwhelmingly ionic and features little contribution from the 5f orbitals of the Am3+ metal centers. The optical properties of Am-1 were studied using diffuse reflectance and photoluminescence spectroscopies and the results support our bond assessment. The position of the principal absorption band at 507 nm (7F0’ ? 5L6’) is notable because it is consistent with those previously reported for solution complexes of americium and oxalic acid, indicating similarities in the electronic structure and ionic bonding. Compound Am-1 is an active phosphor, featuring strong bright-blue oxalate-based emission. Attempts to induce Am3+-based luminescence via direct excitation of the 5f electrons was unsuccessful due to an unknown quenching mechanism that acted upon the metal centers.