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.

Oxomolybdenum(VI) complexes with glycine bisphenol [O,N,O,O′] ligand: synthesis and catalytic studies

“…In general, active structures involve molybdenum ions in high-oxidation states; thus, the reactivity of many active species can be modeled with isolated oxomolybdenum species which are stabilized by biologically relevant hard donors, for instance amine, carboxylate, and aryloxido ligands. Similarly, sterically demanding tetradentate diamino bisphenols form active dioxomolybdenum(VI) complexes [5][6][7]. For example, the reactions of MoO 2 Cl 2 derivatives with tetrapodal amine trisphenols or aminoalcohol bisphenols are known to lead to neutral mononuclear oxochloromolybdenum(VI) complexes which can catalyze a number of oxidations including sulfoxidation, epoxidation, and haloperoxidation reactions [3,4].…”

Combination of CuBr2 and multi-functional ligand bearing a bidentate nitrogen unit, a phenol group and a TEMPO moiety as catalyst for the aerobic oxidation of primary alcohols

“…First, 3-tert-butyl-2-hydroxybenzaldehyde b was prepared from 2-tert-butylphenol with a moderate yield according to reported procedures (Casiraghi et al, 1980). The 1 H NMR characterization results are consistent with those in the literature (Wong et al, 2010). Chloromethylation of b with a reported procedure (Kureshy et al, 2002;Minutolo et al, 1996) produced a salicylaldehyde derivative 3-tert-butyl-5-(chloro-methyl)-2-hydroxybenzaldehyde c which is the key intermediate for obtaining the model compound L due to its special structure.…”

Dioxomolybdenum(VI) and -Tungsten(VI) Amino Bisphenolates as Epoxidation Catalysts

“…Similarly, the W-Oaryloxide, W-N and W-OMeOH distances are comparable with previous results. [13,20,29,30] In general, W1 is isostructural with the equivalent molybdenum compound with only negligible differences in the bonding parameters [19]. …”

“…Dioxomolybdenum(VI) complexes with amine bisphenol ligands have generally been prepared from stable and easily available MoO2(acac)2 or MoO2Cl2 and relevant ligand precursors in high yields using simple ligand displacement reactions [19][20][21][22][23][24][25][26], whereas occasional examples employ the use of an acidic solution of (NH4)6Mo7O24•4H2O [27][28][29]. Corresponding W compounds are made under dry conditions using [WO2Cl2(dme)] and ligands as free bases or sodium salts [30,31] or mixing free ligands and [W(eg)3] (eg = ethylene glycol) in alcohol solutions in the presence of ambient moisture [19,32]. We have previously prepared a molybdenum complex [MoO2(ONO tBu )(MeOH)]·MeOH (Mo1) (H2ONO tBu = bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)methylamine) using MoO2(acac)2 as a starting material [19].…”