Lanthanide/actinide differentiation with sterically encumbered N-heterocyclic carbene ligands

Abstract

A study is reported on the relative stability of trivalent bis(ligand) complexes of the form [M(L^R)₂N′′] for trivalent group 3, lanthanide and actinide cations, using the sterically demanding N-heterocyclic carbene ligand L^R = [OCMe₂CH₂{CNCH₂CH₂NR}] (R = ⁱPr L^P, Mes L^M, Dipp L^D; N′′ = N(SiMe₃)₂). For the small Y^III cation (r_6-coord = 1.040 Å) and the smallest L^R, R = ⁱPr, mono, bis, and tris(L^P) complexes can be made; [Y(L^P)₂N′′] and [Y(L^P)₃] have been characterised. For the larger ligands, L^M and L^D, only the mono(L^R) complexes [Y(L^M)N′′₂] and [Y(L^D)N′′₂] can be made. For the larger Ce^III (r_6-coord = 1.15 Å), mono(L^R) and bis(L^R) complexes [Ce(L^M)N′′₂], [Ce(L^D)N′′₂], [Ce(L^M)₂N′′], and [Ce(L^D)₂N′′] can be made; structural characterisation of the latter two confirm the high degree of steric congestion. The new complex [U(L^M)N′′₂] has also been isolated. Despite the very similar radii of Ce^III and U^III (r_6-coord = 1.165 Å), the complexes [U(L^R)₂N′′] cannot be isolated; a surprising display of the difference between the 4f and 5f metal series. However, the six-coordinate, bis(ligand) U^IV complexes can readily be isolated if smaller ancillary ligands are used; [U(L^M)₂I₂] and [U(L^D)₂I₂] have been fully, including structurally, characterised.