Structure and spectroscopy of uranyl and thorium complexes with substituted phosphine oxide ligands

Phosphine oxide ligands are important in the chemistry of the nuclear fuel cycle. We have synthesized and characterized a series of phosphine oxide ligands with polycyclic aromatic hydrocarbon (PAH) groups to enhance the spectroscopic features of uranyl, UO_2^(2+), and to make detection more efficient. Complexation of OPPh_2R, R = C_(10)H_7 (naphthyl); C_(14)H_9 (phenanthrenyl); C_(14)H_9 (anthracenyl); and C_(16)H_9 (pyrenyl), to UO_2(NO_3)_2 afforded the eight-coordinate complexes, UO_2(NO_3)_2(OPPh_2R)_2. An eleven-coordinate complex, Th(NO_3)_4[OPPh_2(C_(14)H_9)]_3, C_(14)H_9 = phenanthrenyl, was structurally characterized, and was found to be the first thorium compound isolated with three phosphine oxide ligands bound. The phosphine oxide ligands were not fluorescent but the anthracenyl-substituted ligand showed broad, red-shifted emission at approximately 50 nm relative to typical anthracene, making this ligand set a possibility for use in detection. The synthesis and spectroscopy of the uranyl and thorium complexes are presented.