title: New dioxo-molybdenum(VI) and -tungsten(VI) complexes with N-capped tripodal N₂O₂ tetradentate ligands: synthesis, structures and catalytic activities towards olefin epoxidation.
abstract: A series of N₂O₂ tripodal tetradentate ligands derived from di-/tetra-tert-butyl substituted 2-[bis(2-hydroxybenzyl)aminomethyl]X (X = pyridine and benzimidazole) (H₂L(n) (n = 1-4)) and 8-[bis(3,5-di-tert-butyl-2-hydroxybenzyl)]aminoquinoline (H₂L⁵) were synthesised through a 4-step reaction scheme involving sequential formylation, reduction, bromination and alkylation. Treatment of H₂L(n) (n = 1-5) with [WO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) in the presence of triethylamine gave the corresponding cis-dioxotungsten(VI) complexes [WO₂(L(n))] (n = 1-5). The corresponding molybdenum analogues [MoO₂(L(n))] (n = 1-5) were also prepared from the reaction of [MoO₂(acac)₂] (acac = acetylacetonate) with H₂L(n) (n = 1-3) or [MoO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) with H₂L(n) (n = 4 and 5). All these compounds were fully characterised by a wide range of spectroscopic methods. The molecular structures of [MoO₂(L(n))] (n = 2, 4) and [WO₂(L²)] were also confirmed by single-crystal X-ray diffraction analysis. The catalytic activities of [MO₂(L(n))] (M = Mo, W; n = 1-4) towards epoxidation of styrene were also examined.
authors.affiliation: Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, People's Republic of China.
authors.forename: Yee-Lok
authors.initials: YL
authors.lastname: Wong
authors.forename: Lok H
authors.initials: LH
authors.lastname: Tong
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Dilworth
authors.forename: Dennis K P
authors.initials: DK
authors.lastname: Ng
authors.forename: Hung Kay
authors.initials: HK
authors.lastname: Lee