New dioxo-molybdenum(VI) and -tungsten(VI) complexes with N-capped tripodal N2O2 tetradentate ligands: Synthesis, structures and catalytic activities towards olefin epoxidation

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(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 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(L-2)] 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.