Strategies for the synthesis of lanthanum dialkyl complexes with monoanionic ancillary ligands

The synthesis of lanthanum dialkyl complexes with monoanionic ancillary ligands [L](-) is pursued by three different strategies: (a) in situ peralkylation of LaBr(3)(THF)(4) with 3 equiv of LiCH(2)SiMe(3) followed by reaction with LH; (b) reaction of isolated La(CH(2)Ph)(3)(THF)(3) with LH; (c) stepwise salt metathesis on LaBr(3)(THF)(4). Methods (a) and (b) generally work well for triazacyclononane-amide and amidinate ligands, but are unsuitable for the sterically demanding beta-diketiminate [HC(MeCNAr)(2)]- (Ar = 2,6-iPr(2)C(6)H(3)) due to its high affinity for the Li cation and the sluggish reactivity of the diketimine. Nevertheless, the beta-diketiminate lanthanum dibenzyl complex [HC(MeCNAr)(2)]La(CH(2)Ph)(2)(THF) could be obtained by first reacting LaBr(3)(THF)(4) with K[HC(MeCNAr)(2)] to form [HC(MeCNAr)(2)]LaBr(2)(THF)(2) and subsequent reaction of this dibromide complex with 2 equiv of PhCH(2)K. When this reaction mixture is warmed, the product decomposes by H-abstraction from one of the diketiminate methyl groups and ligand redistribution, forming the coordination polymer {[mu-eta(2):eta(1)-ArNC(Me)CHC(CH(2))NAr](2)La[K(THF)(4)]}(infinity).