Interactions of Remote Alkyl Groups with Lanthanide Metal Centers: Synthesis, Characterization and Ligand Redistribution Reactions of Heterobimetallic Species Containing Trialkylaluminum Fragments

Abstract

The reaction of [Ln(OAr)3]2 with four equivalents of trialkylaluminum leads to the formation of the bis-trialkylaluminum adducts (ArO)Ln[(μ-OAr)(μ-R)AlR2]2 [Ln = La, R = Me (1); Ln = La, R = Et (3); Ln = Sm, R = Et (4)]. The X-ray crystal structure of 1 reveals short La−C(bridging) distances of 2.800(5) and 2.759(5) Å. A reduced 1JC-H coupling constant of 110 Hz and a low energy ν(C−H) stretch in the solution and solid state IR spectra are consistent with a strong agostic La···H−C interaction in solution. Crystal data for 1: a = 11.708(2) Å; b = 18.416(4) Å; c = 20.949(4) Å; α = 90°; β = 90°; γ = 90°; V = 4516.8(15) Å3; Z = 4; R1 = 4.69%. The solid-state structure of the samarium ethyl derivative 4 reveals close contacts of 2.627(4) and 2.649(4) Å between the samarium center and the methylene carbons of the triethylaluminum groups. The room temperature 13C NMR spectrum of 4 exhibits a 1JC-H coupling constant of 102 Hz; additionally, the fluxional process that exchanges methyl groups in 1 and 2 is slow enough on the NMR time scale to allow distinct methylene groups in 4 to be observed. Crystal data for 4: a = 19.318(1) Å; b = 20.150(1) Å; c = 26.280(1) Å; α = 90°; β = 90°; γ = 90°; V = 10229.8(9) Å3; Z = 8; R1 = 3.53%. Thermolysis of 1−4 results in ligand redistribution to form [R2Al(OAr)]2 [R = Me, Et (5)] and other unidentified species. Crystal data for 5: a = 9.496(4) Å; b = 10.183(4) Å; c = 18.794(7) Å; α = 90.06(1)°; β = 92.394(7)°; γ = 114.976(6)°; V = 1645.6(11) Å3; Z = 2; R1 = 10.13%.