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 degrees 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 eta(2) Ln...C-ipso contacts (1-4, 11); (ii) complexes which show one eta(2) Ln...C-ipso contact (5); (iii) complexes with no multi-hapto interactions (6-8). For ytterbium, the mixed valence, Yb-II/Yb-III complex [Yb-II(Bz)(THF)(5)](+)[Yb-III(Bz)(4)(THF)(2)](-) (9) was reproducibly formed at 0 degrees C and -78 degrees C as a result of partial (50%) Yb-III -> Yb-II 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) (H-2-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.