Switchable π-coordination and C–H metallation in small-cavity macrocyclic uranium and thorium complexes

Abstract

New, conformationally restricted Th^IV and U^IV complexes, [ThCl₂(L)] and [UI₂(L)], of the small-cavity, dipyrrolide, dianionic macrocycle trans-calix[2]benzene[2]pyrrolide (L)²⁻ are reported and are shown to have unusual κ⁵:κ⁵ binding in a bent metallocene-type structure. Single-electron reduction of [UI₂(L)] affords [UI(THF)(L)] and results in a switch in ligand binding from κ⁵-pyrrolide to η⁶-arene sandwich coordination, demonstrating the preference for arene binding by the electron-rich U^III ion. Facile loss of THF from [UI(THF)(L)] further increases the amount of U–arene back donation. [UI(L)] can incorporate a further U^III equivalent, UI₃, to form the very unusual dinuclear complex [U₂I₄(L)] in which the single macrocycle adopts both κ⁵:κ⁵ and η⁶:κ¹:η⁶:κ¹ binding modes in the same complex. Hybrid density functional theory calculations carried out to compare the electronic structures and bonding of [U^IIII(L)] and [U^III₂I₄(L)] indicate increased contributions to the covalent bonding in [U₂I₄(L)] than in [UI(L)], and similar U–arene interactions in both. MO analysis and QTAIM calculations find minimal U–U interaction in [U₂I₄(L)]. In contrast to the reducible U complex, treatment of [ThCl₂(L)] with either a reductant or non-nucleophilic base results in metallation of the aryl rings of the macrocycle to form the (L^−2H)⁴⁻ tetraanion and two new and robust Th–C bonds in the –ate complexes [K(THF)₂Th^IV(μ-Cl)(L^−2H)]₂ and K[Th^IV{N(SiMe₃)₂}(L^−2H)].