Lanthanide tri-benzyl complexes: structural variations and useful precursors to phosphorus-stabilised lanthanide carbenes

Reaction of Ln(I)3(THF)4 (Ln = Ce, Pr) or Ln(I)3(THF)3.5 (Ln = Nd, Sm, Gd, Dy, Er) with three equivalents of KBz (Bz = CH2C6H5) at 0 °C afforded the corresponding lanthanide tri-benzyl complexes Ln(Bz)3(THF)3 Ln = Ce (2), Pr (3), Nd (4), Sm (5), Gd (6), Dy (7), Er (8) La (11) in 48–75% crystalline yields, with the exception of the redox active samarium complex, which was isolated in poor (20%) yield. Complexes 2–8 were found to adopt distorted octahedral geometries, where the Bz and THF groups are bound in a mutually fac manner in the solid state. Although the series is structurally similar, classification of three structural types can be made on the basis of the lanthanide contraction: (i) complexes which exhibit three η2 Ln⋯Cipso contacts (1–4, 11); (ii) complexes which show one η2 Ln⋯Cipso contact (5); (iii) complexes with no multi-hapto interactions (6–8). For ytterbium, the mixed valence, YbII/YbIII complex YbII(Bz)(THF)5+YbIII(Bz)4(THF)2− (9) was reproducibly formed at 0 °C and −78 °C as a result of partial (50%) YbIII→ YbII reduction with concomitant formation of half an equivalent of 1,2-diphenylethane by oxidative coupling. Tri-valent Yb(Bz)3(THF)3 (10) was apparently not formed. The synthetic utility of tri-benzyl lanthanide complexes 2–8 and 11 were tested in reactions with the bis-(iminophosphorano)methane H2C(PPh2NSiMe3)2 (H2-BIPM), which afforded Ln(BIPM)(H-BIPM) Ln = La (12), Ce (13), Pr (14), Nd (15), Sm (16), Gd (17) and Ln(BIPM)(Bz)(THF) Ln = Dy (18), Er (19). Compounds 2–9 and 12–19 have been variously characterised by X-ray crystallography, multi-nuclear NMR spectroscopy, FTIR spectroscopy, room temperature Evans method solution magnetic moments and CHN micro-analyses.