Scandium Bis(o-aminobenzyl) Complexes Coordinated by 5-Methyl-5,6-dihydroindeno[2,1-b]indolyl Ligands: Synthesis, Structures, and Catalytic Activity in Isoprene Polymerization and Olefin Hydrophosphination

Abstract

Deprotonation of 5-methyl-5,6-dihydroindeno[2,1-b]indole (L¹H) or 2-(tert-butyl)-5-methyl-5,6-dihydroindeno[2,1-b]indole (L²H) with an equimolar amount of n-BuLi in Et₂O affords the 5,6-dihydroindeno[2,1-b]indolyl derivatives L¹Li(Et₂O)₂ (1) and [L²Li(Et₂O)]₆ (2) in 67 and 73% yields. Complex 1 proved to be monomeric in the crystalline state, whereas 2 adopts a cyclic hexameric structure. The protonation of Sc(CH₂C₆H₄-o-NMe₂)₃ by an equimolar amount of [HNEt₃][BPh₄] allows for the synthesis of a cationic scandium bis(o-aminobenzyl) complex [Sc(CH₂C₆H₄-o-NMe₂)₂(THF)₂][BPh₄] (3) which was successfully employed as a precursor for the preparation of bis(o-aminobenzyl) 5,6-dihydroindeno[2,1-b]indolyl species. The salt metathesis reactions of lithium derivatives 1 and 2 with 3 (THF, 20 °C) afford complexes L¹Sc(CH₂C₆H₄-o-NMe₂)₂ (4) and L²Sc(CH₂C₆H₄-o-NMe₂)₂ (5) in 69 and 73% yields. Complexes 4 and 5 are monomeric and Lewis-base free. Complexes 4 and 5 as part of binary (4, 5/[Ph₃C][B(C₆F₅)₄], 4, 5/[PhNHMe][B(C₆F₅)₄]) and ternary (4, 5/([Ph₃C][B(C₆F₅)₄], [PhNHMe][B(C₆F₅)₄])/AliBu₃ (1:1:10)) catalytic systems are efficient initiators for isoprene polymerization which enable quantitative conversion of monomers in 30 min ([IP]/[Ln] = 1000) even at −30 °C. The resulting polymers have a predominantly cis-1,4 structure (up to 80.9%; M_n = 58.8–2315.9 × 10³; M_w/M_n = 1.35–3.12). Complexes 4 and 5 catalyze the intermolecular hydrophosphination of styrene, α-methylstyrene, and phenylacetylene with Ph₂PH and PhPH₂ under mild conditions.