Bis(alkyl) scandium and yttrium complexes coordinated by an amidopyridinate ligand: synthesis, characterization and catalytic performance in isoprene polymerization, hydroelementation and carbon dioxide hydrosilylation

Abstract

New neutral bis(alkyl) Sc and Y complexes [N,N^py,N⁻]Ln(CH₂SiMe₃)₂(THF)_n [n = 0, Ln = Sc (1_Sc), Y (1_Y); n = 1, Ln = Y (1_Y^THF)] stabilized by a tridentate monoanionic amidopyridinate ligand were straightforwardly prepared by alkane elimination, upon mixing ligand [N,N^py,N⁻]H and metal precursor Ln(CH₂SiMe₃)₃(THF)₂ in toluene at 0 °C. Depending on the work-up conditions, yttrium bis(alkyl)s were isolated as either a pentacoordinate Lewis base free complex [N,N^py,N⁻]Y(CH₂SiMe₃)₂ (1_Y) or as a hexacoordinate THF adduct [N,N^py,N⁻]Y(CH₂SiMe₃)₂THF (1_Y^THF). For the smaller Sc ion the only solvent-free complex [N,N^py,N⁻]Y(CH₂SiMe₃)₂ (1_Sc) was isolated as a pentacoordinate species irrespective of the reaction/work-up/crystallization conditions applied. Complexes 1_Ln (Ln = Y, Sc) and 1_Y^THF were scrutinized as pre-catalysts in ternary catalytic systems Ln/borate/AlⁱBu₃ (borate = [HNMe₂Ph][B(C₆F₅)₄] or [Ph₃C][B(C₆F₅)₄]), applied to isoprene (IP) polymerization, providing moderate activity albeit high selectivity with predominant formation of 1,4-cis polyisoprene (up to 99%). The same complexes proved to be effcient catalysts also for the intermolecular hydrolelementation of styrene with various EH sustrates (pyrrolidine, morpholine, Ph₂PH, PhPH₂, PhSH) affording linear anti-Markovnikov addition products exclusively. After a preliminary activation by B(C₆F₅)₃, selected bis(alkyl) complexes from this series have been finally used as valuable pre-catalysts for the CO₂ hydrosylilation to CH₄ in the presence of organosilanes as reducing agents (PhMe₂SiH, PhSiH₃, Et₂MeSiH).