Systematic Investigation of the Molecular and Electronic Structure of Thorium and Uranium Phosphorus and Arsenic...

In continuing to examine the interaction of actinide-ligand bonds with soft donor ligands, a comparative investigation with phosphorus and arsenic was conducted. A reaction of (C₅Me₅)₂AnMe₂, An = Th, U, with 2 equiv of H₂AsMes, Mes = 2,4,6-Me₃C₆H₂, forms the primary bis(arsenido) complexes, (C₅Me₅)₂An[As(H)Mes]₂. Both exhibit thermal instability at room temperature, leading to the elimination of H₂, and the formation of the diarsenido species, (C₅Me₅)₂An(η²-As₂Mes₂). The analogous diphosphido complexes, (C₅Me₅)₂An(η²-P₂Mes₂), could not be synthesized via the same route, even upon heating the bis(phosphido) species to 100 °C in toluene. However, they were accessible via the reaction of dimesityldiphosphane, MesP(H)P(H)Mes, with (C₅Me₅)₂AnMe₂ at 70 °C in toluene. When (C₅Me₅)₂AnMe₂ is reacted with 1 equiv of H₂AsMes, the bridging μ₂-arsinidiide complexes [(C₅Me₅)₂An]₂(μ₂-AsMes)₂ are formed. Upon reaction of (C₅Me₅)₂UMe₂ with 1 equiv of H₂PMes, the phosphinidiide [(C₅Me₅)₂U(μ₂-PMes)]₂ is isolated. However, the analogous thorium reaction leads to a phosphido and C-H bond activation of the methyl on the mesityl group, forming {(C₅Me₅)₂Th[P(H)(2,4-Me₂C₆H₂-6-CH₂)]}₂. The reactivity of [(C₅Me₅)₂An(μ₂-EMes)]₂ was investigated with OPPh₃ in an effort to produce terminal phosphinidene or arsinidene complexes. For E = As, An = U, a U(III) cation-anion pair [(C₅Me₅)₂U(η²-As₂Mes₂)][(C₅Me₅)₂U(OPPh₃)₂] is isolated. The reaction of [(C₅Me₅)₂Th(μ₂-AsMes)]₂ with OPPh₃ does not result in a terminal arsinidene but, instead, eliminates PPh₃ to yield a bridging arsinidiide/oxo complex, [(C₅Me₅)₂Th]₂(μ₂-AsMes)(μ₂-O). Finally, the combination of [(C₅Me₅)₂U(μ₂-PMes)]₂ and OPPh₃ yields a terminal phosphinidene, (C₅Me₅)₂U(═PMes)(OPPh₃), featuring a short U-P bond distance of 2.502(2) Å. Electrochemical measurements on the uranium pnictinidiide complexes demonstrate only a 0.04 V difference with phosphorus as a slightly better donor. Magnetic measurements on the uranium complexes show more excited-state mixing and therefore higher magnetic moments with the arsenic-containing compounds but no deviation from uncoupled U(IV) behavior. Finally, a quantum theory of atoms in molecules analysis shows highly polarized actinide-pnictogen bonds with similar bonding characteristics, supporting the electrochemical and magnetic measurements of similar bonding between actinide-phosphorus and actinide-arsenic bonds.