Cu(II) and Fe(III) Complexes Derived from N-Acetylpyrazine-2-Carbohydrazide as Efficient Catalysts Towards Neat Microwave Assisted Oxidation of Alcohols

Abstract

The mononuclear Cu(II) complex [Cu((kNN O-HL)(H 2 O) 2 ] (1) was synthesized using N-acetylpyrazine-2-carbohydrazide (H 2 L) and characterized by elemental analysis, IR spectroscopy, ESI-MS and single crystal X-ray crystallography. Two Fe(III) complexes derived from the same ligand viz, mononuclear [Fe((kNN O-HL)Cl 2 ] (2) and the binuclear [Fe(kNN O-HL)Cl(µ-OMe)] 2 (3) (synthesized as reported earlier), were also used in this study. The catalytic activity of these three complexes (1-3) was examined towards the oxidation of alcohols using tert-butyl hydroperoxide (TBHP) as oxidising agent under solvent-free microwave irradiation conditions. Primary and secondary benzyl alcohols (benzyl alcohol and 1-phenylethanol), and secondary aliphatic alcohols (cyclohexanol) were used as model substrates for this study. A comparison of their catalytic efficiency was performed. Complex 1 exhibited the highest activity in the presence of TEMPO as promoter for the oxidation of 1-phenylethanol with a maximum yield of 91.3% of acetophenone. Catalysts 2019, 9, 1053 2 of 15 catalytic reactions [3-9,29-40]. Although many catalysts have been developed to overcome ecological drawbacks, namely those using cheap and abundant metals, like copper and iron, metal catalytic systems operating under sustainable conditions are still challenging. Considering all the above mentioned points herein we present the synthesis of a new mononuclear Cu(II) complex, [Cu((kNN O-HL)(H 2 O) 2 ] (1), derived from the N-acetylpyrazine-2-carbohydrazide (H 2 L) ligand. Two Fe(III) complexes, the mononuclear [Fe((kNN O-HL)Cl 2 ] (2) and the binuclear [Fe(kNN O-HL)Cl(µ-OMe)] 2 (3), have also been synthesized using the same ligand as reported earlier [41]. The three complexes (1-3) were screened as catalytic precursors to assess their catalytic performances in the oxidation of alcohols. In the present investigation, we chose primary or secondary benzyl alcohols (benzyl alcohol, 1-phenyl alcohol) and secondary aliphatic alcohols (cyclohexanol) as model substrates and tert-butyl hydroperoxide (TBHP) as oxidising agent under microwave irradiation. MW irradiation acts as an alternative technique due to its simplicity and energy saving characteristics [6], usually leading to higher product yields and selectivities. In this investigation, the best catalytic conditions were optimized by comparing several cofactors like temperature, reaction time, influence of the presence of additives, etc. Results and Discussion Synthesis and CharacterizationsThe pro-ligand N-acetylpyrazine-2-carbohydrazide (H 2 L) was used to synthesize the mononuclear Cu(II) complex [Cu(kNN O-HL)(H 2 O) 2 ] (1) (Scheme 1). The Fe(III) complexes [Fe(kNN O-HL)Cl 2 ] (2) and [Fe(kNN O-HL)Cl(µ-OMe)] 2 (3) derived from the same pro-ligand were obtained by a method from the literature [41]. As previously observed, in this case, the N-acetylpyrazine-2-carbohydrazide also acts as a mononegative tridentate N,N,O donor (HL − ) towards the metal centre [41] and shows different coordination behaviour from a relate...

Modular O- vs. N-coordination of pyridylidene amide ligands to iron determines activity in alcohol oxidation catalysis

“…Initially, the reactivity of complex 2 was explored in the catalytic oxidation of 1-phenyl ethanol to acetophenone as a model reaction. [35][36][37][38][39] Under standard conditions, viz. 0.5 mol% of the dimeric iron complex 2, 1.5 eq t-butyl hydroperoxide (TBHP), 50 °C in MeCN, complex 2 cleanly converted the alcohol to acetophenone and reached full conversion after 24 h, slightly slower than when using only FeCl2 as catalyst precursor ( Figure S18).…”

Synthesis, Structures, Electrochemistry, and Catalytic Activity towards Cyclohexanol Oxidation of Mono-, Di-, and Polynuclear Iron(III) Complexes with 3-Amino-2-Pyrazinecarboxylate

“…It is worth mentioning that such a high conversion of cyclohexanol into cyclohexanone is scarcely reported [40][41][42], and the present catalysts 1-4 are among the most active and selective ones for this industrially important reaction. For example, Fe(III) complexes derived from N-acetylpyrazine-2-carbohydrazide, viz., the mononuclear [Fe((kNN'O-HL)Cl 2 ] and the binuclear [Fe(kNN'O-HL)Cl(µ-OMe)] 2 , when used as catalysts for the oxidation of cyclohexanol with TBHP under solvent-free MW irradiation conditions, led to yields up to 36% and 37%, respectively, of cyclohexanone after 1 h at 120 • C [42]. Moreover, decomposition at the reaction media impaired their reuse in further catalytic cycles.…”

“…Thus, the mechanism of cyclohexanol oxidation (see Equations 1-6) may involve the t BuO˙ and t BuOO˙ radicals produced in the iron promoted decomposition of TBHP [23] according to Equations 1-6, where Cy denotes cyclohexyl and Cy˙-H-OH denotes the radical formed upon H-abstraction from cyclohexanol CyOH [43][44][45][46]. It is worth mentioning that such a high conversion of cyclohexanol into cyclohexanone is scarcely reported [40][41][42], and the present catalysts 1-4 are among the most active and selective ones for this industrially important reaction. For example, Fe(III) complexes derived from N-acetylpyrazine-2-carbohydrazide, viz., the mononuclear [Fe((kNN'O-HL)Cl 2 ] and the binuclear [Fe(kNN'O-HL)Cl(µ-OMe)] 2 , when used as catalysts for the oxidation of cyclohexanol with TBHP under solvent-free MW irradiation conditions, led to yields up to 36% and 37%, respectively, of cyclohexanone after 1 h at 120 • C [42].…”

Aroylhydrazone Schiff Base Derived Cu(II) and V(V) Complexes: Efficient Catalysts towards Neat Microwave-Assisted Oxidation of Alcohols

“…Thus, herein we focused on the selective oxidation of alcohols to the corresponding ketones or aldehydes [23][24][25][26], which are important building blocks for the synthesis of many added-value organic compounds [27][28][29][30] and have applications in fields such as pharmaceuticals, agrochemicals, fragrances, and polymers [31,32]. Both complexes were tested as catalyst precursors towards the neat oxidation of primary (cinnamyl alcohol) or secondary (1-phenyl ethanol, benzhydrol) benzyl alcohols, as well as of the secondary aliphatic alcohol cyclohexanol using tert-butyl hydroperoxide (TBHP) as oxidizing agent under microwave (MW) irradiation, a relevant and simple energy-saving technique [14,[33][34][35].…”