Synthesis and structure of tris(alkyl- and silyl-tetramethylcyclopentadienyl) complexes of lanthanum

In efforts to make sterically crowded tris(peralkylcyclopentadienyl) comple xes of lanthanum for the exploration of sterically induced reduction chemis try with a diamagnetic system, the synthesis of (C5Me4R)(3)La complexes has been pursued with R = Me, Et, Pr-i, and SiMe3. The complexes were synthesi zed in four steps: reaction of LaCl3 with KC5MC4R to form (C5Me4R)(2)LaCl2K (THF)(2), addition of allylmagnesium chloride to make (C5Me4R)(2)La(C3H5), protonolysis with Et3NHBPh4 to make [(C5Me4R)(2)La][BPh4], and finally the replacement of BPh4- with CSMe4R- using KC5Me4R to make (C5Me4R)(3)La. X-ra y crystallographic data were obtainable on the (C5Me4R)(3)La complexes for R = Me, Et, Pr-i, and SiMe3. In each complex, the three C5Me4R ring centroi ds define a trigonal planar geometry around La. The average La-(ring centro id) distances are 2.64, 2.65, 2.66, and 2.69 Angstrom for the Me, Et, Pr-i, and SiMe3 structures, respectively, with La-C distances ranging from 2.857 (3) to 3.029 (2) Angstrom. Despite the steric crowding, ligand exchange ca n be observed by NMR spectroscopy.