New achiral and chiral NNE heteroscorpionate ligands. Synthesis of homoleptic lithium complexes as well as halide and alkyl scandium and yttrium complexes

Abstract

The reaction of bis(3,5-dimethylpyrazol-1-yl)methane (bdmpzm) with BuⁿLi and alkyl- or aryl-containing isocyanates or isothiocyanates, some of which were chiral reagents, and subsequent treatment with saturated aqueous ammonium chloride or HCl in diethyl ether gave (i) new heteroscorpionate ligands in the form of the acetamide or thioacetamide compounds [pbpamH (1), tbpamH (2), sbpamH (3) as racemic mixtures, (S)-mbpamH (4) as an enantiopure compound, pbptamH (5) and tbptamH (6)] and (ii) stabilization of new homoleptic lithium complexes containing fac-coordinating NNE-donor ligands [Li{NNE(H)}₂]Cl (E = O, S) (7–10). A series of scandium and yttrium complexes have been prepared by the reaction of MCl₃(THF)₃ (M = Sc, Y) with the amide or thioamide compounds. Under the appropriate experimental conditions mononuclear complexes, namely [MCl₃{κ³-NNE(H)}] (E = O, S) (11–18) have been prepared. A family of alkyl-containing complexes of general formulae [M(CH₂SiMe₃)₂(κ³-pbptam)] [M = Sc (19), Y (20)] and [M(CH₂SiMe₃)₂(κ³-tbptam)] [M = Sc (21), Y (22)] was also prepared. Preliminary results show that compounds 19 and 20 can act as single-component living initiators for ring-opening polymerization. Polymerization of ε-caprolactone occurred within minutes to give medium molecular weight polymers (Mn up to 2 × 10⁵) and narrow polydispersities (Mw/Mn < 1.6). Polymer end group analysis showed that the polymerization process is initiated by alkyl transfer to the monomer. The structures of these complexes have been determined by spectroscopic methods and the X-ray crystal structures of 5, 6 and 8 were also established.