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

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.