New Synthetic Routes to Tris(pentaalkylcyclopentadienyl)lanthanide Complexes Including the X-ray Crystal Structure of (C₅Me₄Et)₃Sm¹

The synthetic chemistry leading to tris(pentamethylcyclopentadienyl)samarium complexes has been examined to determine why some routes fail and to develop routes superior to the first synthesis from (C 5 Me 5 ) 2 Sm and 1,3,5,7-cyclooctatetraene (COT). (C 5 Me 5 ) 2 Sm(THF) fails as a reagent since (C 5 Me 5 ) 3 Sm reacts with any THF present to form the ring-opened product (C 5 Me 5 ) 2 Sm[O(CH 2 ) 4 C 5 Me 5 ](THF). However, (C 5 Me 5 ) 2 Sm(OEt 2 ) reacts cleanly with COT to form (C 5 Me 5 ) 3 Sm and (C 5 Me 5 )Sm(C 8 H 8 ). A more efficient synthesis involves the reaction of (C 5 Me 5 ) 2 Sm(OEt 2 ) with (C 5 Me 5 ) 2 Pb which generates (C 5 Me 5 ) 3 Sm in >90% yield. This route has been used to synthesize the more sterically crowded tris(pentaalkylcyclopentadienyl) complex (C 5 Me 4 Et) 3 Sm, which shows that the limit of steric crowding in this class has not yet been realized. X-ray crystallographic study of (C 5 Me 4 Et) 3 Sm revealed a trigonal planar arrangement of ring centroids around samarium with an average Sm−C(C 5 Me 5 ) distance of 2.834 Å. Two ethyl groups point toward samarium with a minimum Sm−C distance of 3.620(13) Å.