Five different types of η8-cyclooctatetraenyl-lanthanide half-sandwich complexes from one ligand set, including a “giant neodymium wheel”

Abstract

The lithium-cyclopropylethynylamidinates Li[c-C₃H₅–CC–C(NR)₂] (1a: R = ⁱPr, 1b: R = cyclohexyl (Cy)) have been used as precursors for the preparation of five new series of half-sandwich complexes. These complexes contain the large flat cyclooctatetraenyl ligand (C₈H₈²⁻, commonly abbreviated as COT), and were isolated as solvated, unsolvated and inverse sandwich complexes. Treatment of the halide precursors [(COT)Pr(μ-Cl)(THF)₂]₂ with 1b and [(COT)Nd(μ-Cl)(THF)₂]₂ with 1a and 1b in THF in a 1 : 2 molar ratio, respectively, afforded (COT)Ln[μ-c-C₃H₅–CC–C(NR)₂]₂Li(L) (2: Ln = Pr, R = Cy, L = Et₂O; 3: Ln = Nd, R = ⁱPr, L = THF; 4: Ln = Nd, R = Cy, L = THF). Treatment of the dimeric cerium(III) bis(cyclopropylethynylamidinate) complexes [{c-C₃H₅–CC–C(NR)₂}₂Ce(μ-Cl)(THF)]₂ (5: R = ⁱPr; 6: R = Cy) in situ with K₂C₈H₈ in a 1 : 1 molar ratio in THF at room temperature afforded the inverse-sandwich complexes (μ–η⁸:η⁸-COT)[Ce{c-C₃H₅–CC–C(NR)₂}₂]₂ (7: R = ⁱPr; 8: R = Cy). This reaction represents a new method for encapsulation of a planar (C₈H₈)²⁻ ring in lanthanide complexes containing amidinate ligands in the outer decks. Novel unsolvated dinuclear lanthanide half-sandwich complexes were prepared by using the precursors 1a, 1b and COT²⁻. Unlike the complexes 2–4, the reaction of [(COT)Pr(μ-Cl)(THF)₂]₂ with 1a afforded the unsolvated centrosymmetric complex [(COT)Pr(μ-c-C₃H₅–CC–C(NⁱPr)₂)]₂ (9). These dimeric structures could be also accessed by reaction of LnCl₃ (Ln = Ce or Nd) with 1a or 1b and K₂COT in a 1 : 1 : 1 molar ratio as a one-pot reaction to give novel [(COT)Ln(μ-c-C₃H₅–CC–C(NR)₂)]₂ complexes (10: Ln = Ce, R = ⁱPr; 11: Ln = Ce, R = Cy; 12: Ln = Nd, R = ⁱPr). Similar treatment of HoCl₃ with 1a or 1b and K₂COT as three-component reactions in a 1 : 1 : 1 molar ratio afforded the solvated half-sandwich complexes (COT)Ho(c-C₃H₅–CC–C(NR)₂)(THF) (13: R = ⁱPr; 14: R = Cy). A unique multidecker sandwich complex [(μ–η⁸:η⁸-COT){Nd(c-C₃H₅–CC–C(NCy)₂)(μ-Cl)}₂]₄ (15) was prepared by reaction of anhydrous NdCl₃ with K₂COT and 1b in a one-pot reaction. The solid state structure of 15 revealed the presence of an unprecedented macrocyclic sandwich compound (“giant neodymium wheel”) consisting of four COT rings sandwiched between eight Nd³⁺ ions, and each Nd³⁺ ion is bonded to one amidinate ligand and bridged by two chlorine atoms with the neighbouring Nd³⁺ ion.