Group 4 Lanthanide and Actinide Organometallic Inclusion Complexes

Reaction
of the f-element complexes ThCl4(THF)3, Cp*2Sm­(μ-Cl)2MgCl­(THF)2,
NdCl3, Cp*2UMe2, and Cp*2ZrMe2 with an excess of catecholborane (HBCat) yields
macrocyclic complexes where the metal is encapsulated inside a 15-membered,
hexaoxo, trianionic macrocycle built from alternating catechol and
catecholborate fragments. With ThCl4(THF)3 as
the starting material, the reaction produced a macrocyclic complex
with one chloride ligand and three solvent molecules in the apical
positions; however, for the zirconium and uranium complexes the apical
positions are occupied by one C5Me5 ligand and
a THF solvent molecule. In the samarium and neodymium complexes, only
solvent molecules occupy the apical positions. Transmetalation of
the ligand among different complexes in refluxing THF were performed.
When the zirconium macrocycle was treated with a slight excess of
ThCl4 or NdCl3, the corresponding (η2-catechol-μ-catecholborate)3ThCl­(C4H8O)3·C4H8O (2) and (η2-catechol-μ-catecholborate)3Nd­(C4H8O)3·C4H8O (4) macrocycles were obtained in 87%
and 79% yields, respectively. In addition, the reaction of the samarium
macrocycle complex (η2-catechol-μ-catecholborate)3Sm­(C4H8O)3·C4H8O (3) with a slight excess of ThCl4 allowed the formation of complex 2 in 76% yield.
While some of the pentamethylcyclopentadienyl (Cp*)-containing inclusion
complexes were found to be catalytically inactive in the polymerization
of ε-caprolactone, the lanthanides and thorium complexes were
found to be active, yielding only short chains of polycaprolactone.
The X-ray molecular structures for all of the complexes are presented
and discussed. Experiments performed with Cp*2ThMe2 and catecholborane allowed us to trap the intermediate Cp*­(H)­BH2BH2 complex, which was trapped in situ and characterized
by 11B NMR, allowing us to propose a possible mechanism
for the formation of the macrocycle.