Stabilization of Pu(IV) in PuBr 4 (OPCy 3 ) 2 and Comparisons with Structurally Similar ThX 4 (OPR 3 ) 2 (R = Cy, Ph) Molecules.

The pursuit of a trivalent plutonium halide phosphine oxide compound, e.g., "PuBr 3 (OPR) 3 ," instead led to the isolation of the tetravalent trans -Pu IV Br 4 (OPCy 3 ) 2 , PuBr/Cy , compound by spontaneous oxidation of Pu III . The donating nature of phosphine oxides has allowed the isolation and characterization of PuBr/Cy by crystallographic, multinuclear NMR, solid state, and solution phase UV-vis-NIR spectroscopic techniques. The presence of a putative plutonyl(VI) complex formulated as " trans -Pu VI O 2 Br 2 (OPCy 3 ) 2 " was also observed spectroscopically and tentatively by single-crystal X-ray diffraction as a cocrystal of PuBr/Cy . A series of trans -ThX 4 (OPCy 3 ) 2 (X = Cl, ThCl/Cy ; Br, ThBr/Cy ; I, ThI/Cy ) complexes were synthesized for comparison to PuBr/Cy . The triphenylphosphine oxide, OPPh 3 , complexes, trans -AnI 4 (OPPh 3 ) 2 (An = Th, ThI/Ph ; U, UI/Ph) , were also synthesized for comparison, completing the series trans -UX 4 (OPPh 3 ) 2 (X = Cl, Br, I), UX/Ph . To enable the synthesis of ThI/Cy and ThI/Ph , a new nonaqueous thorium iodide starting material, ThI 4 (Et 2 O) 2 , was synthesized. The syntheses of organic solvent soluble ThI 4 L 2 (L = Et 2 O, OPCy 3 , and OPPh 3 ) are the first examples of crystallographically characterized neutral thorium tetraiodide materials beyond binary ThI 4 . To show the viability of ThI 4 (Et 2 O) 2 as a starting material for organothorium chemistry, (C 5 Me 4 H) 3 ThI was synthesized and crystallographically characterized.