New dioxo–molybdenum(vi) and –tungsten(vi) complexes with N-capped tripodal N2O2 tetradentate ligands: Synthesis, structures and catalytic activities towards olefinepoxidation

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) (H₂Lⁿ (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 H₂Lⁿ (n = 1–5) with [WO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) in the presence of triethylamine gave the corresponding cis-dioxotungsten(VI) complexes [WO₂(Lⁿ)] (n = 1–5). The corresponding molybdenum analogues [MoO₂(Lⁿ)] (n = 1–5) were also prepared from the reaction of [MoO₂(acac)₂] (acac = acetylacetonate) with H₂Lⁿ (n = 1–3) or [MoO₂Cl₂(dme)] (dme = 1,2-dimethoxyethane) with H₂Lⁿ (n = 4 and 5). All these compounds were fully characterised by a wide range of spectroscopic methods. The molecular structures of [MoO₂(Lⁿ)] (n = 2, 4) and [WO₂(L²)] were also confirmed by single-crystal X-ray diffraction analysis. The catalytic activities of [MO₂(Lⁿ)] (M = Mo, W; n = 1–4) towards epoxidation of styrene were also examined.