Synthesis, characterization and structure of nickel and copper compounds containing ligands derived from keto-enehydrazines and their catalytic application for aerobic oxidation of alcohols

Abstract

Ligand precursors HL(R,Ph) (R = Me, Ph) were synthesised by condensation of acetylacetone and the corresponding N,N-substituted hydrazines and were characterised spectroscopically and structurally. Both in the solid state and in solution they behave as (Z)-keto-enehydrazines and this was confirmed by DFT calculations which showed that this form was the most stable of their possible tautomers. The reaction of HL(R,Ph) compounds with copper acetate and nickel acetate in EtOH afforded the corresponding complexes [M(L(R,Ph))2] (M = Cu, Ni; R = Me, Ph). The methyl-substituted derivatives were structurally characterised by X-ray methods. A four-coordinate environment around the metal centre, where the two L(Me,Ph) ligands act as bidentate N,O-chelators and lie in a pseudo-trans conformation, was found for both compounds. The dihedral angle between the two six-membered metallacycles M(L(Me,Ph)) was 0° for nickel, a typical square planar coordination, meanwhile it was 23° for copper, a square planar slightly distorted to pseudotetrahedral coordination. Copper complexes [Cu(L(R,Ph))2] were tested as catalysts, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl radical), for the aerobic oxidation of 4-nitrobenzyl alcohol as the model reaction. An almost complete conversion to the corresponding aldehyde was observed after 1 h at 60 °C and 1 bar of dioxygen, in toluene as the solvent. Importantly, air at atmospheric pressure was also observed to be appropriate for the oxidation, although longer reaction times were required. After the optimization of the reaction conditions, the study was extended to other alcohol substrates and good catalytic activity was found for benzylic-type alcohols, while low yield was found for 1-octanol.

Synthesis of α,β‐Dicarbonylhydrazones by Aerobic Manganese‐Catalysed Oxidation

“…Compounds 2 a – i were isolated as brown crystalline solids, while the methylphenylhydrazine derivatives 2 j – l and the dimethylhydrazine derivatives 2 m – o were isolated as brownish oils. Analytical and spectroscopic characterization suggested for all of them a β‐carbonylenehydrazine structure that is preferential respecting to the others possible tautomers . For instance, the 1 H NMR spectra shown a singlet at 4.6–5.9 ppm that was attributed to the C−H proton in the α position.…”

“…For instance, the 1 H NMR spectra shown a singlet at 4.6–5.9 ppm that was attributed to the C−H proton in the α position. Additionally, compounds 2 a – e , 2 g – k and 2 m – n showed a low‐field singlet at 9.6–12.7 ppm that were attributed to the N−H hydrazinic proton, which suggested the presence of a strong intramolecular hydrogen bond with the carbonyl oxygen atom, and should be responsible for the stabilisation of the β‐carbonylenehydrazine isomer in the enehydrazine‐hydrazone tautomerism . Conversely, the signal corresponding to the NH hydrazinic proton for the cyclohexane‐1,3‐dione derivatives 2 f , 2 l and 2 o were found to be shifted to high‐field (7.0–9.0 ppm), showing that intramolecular hydrogen bond was impeded in these compounds.…”

“…Synthesis of β‐carbonylenehydrazines . The syntheses of β‐carbonylenehydrazines compounds 2 a and 2 g have been reported previously by us . Compounds 2 d and 2 m have been previously described, but the experimental procedures employed vary slightly and are here described.…”

“…We recently reported the synthesis of the β‐ketoenehydrazines 2 a and 2 g , and their use as precursors of monoanionic bidentate ligands coordinated to Ni(II) and Cu(II) complexes . During the attempts of synthesis of the corresponding coordination compound of Mn(II), by the reaction of 2 a with Mn(OAc) 2 in aerobic conditions (Scheme ), we observed the presence of a unexpected new product.…”

A β-ketoiminato palladium(II) complex for palladium deposition

“…The NH stretching vibrations are observed between 3020 and 3340 cm −1 . The weak band observed at 3339 cm −1 indicates the β-keto-enamine tautomer of the non-coordinated site of L [37,40,41,[43][44][45], while the bands at 3074, 3058 and 3018 cm −1 are characteristic for intra-and inter-molecular hydrogen bond formation [40]. An exact assignment of the binding motif of the Pd-bonded acetate ligand was not possible, due to a band overlap with the C=C vibrations (Section 4).…”