Homo- and Heterogeneous Benzyl Alcohol Catalytic Oxidation Promoted by Mononuclear Copper(II) Complexes: The Influence of the Ligand upon Product Conversion

Abstract

The catalytic properties of three copper complexes, [Cu(en)₂](ClO₄)₂ (1), [Cu(amp)₂](ClO₄)₂, (2) and [Cu(bpy)₂](ClO₄)₂ (3) (where en = ethylenediamine, amp = 2-aminomethylpyridine and bpy = 2,2'-bipyridine), were explored upon the oxidation of benzyl alcohol (BnOH). Maximized conversions of the substrates to their respective products were obtained using a multivariate analysis approach, a powerful tool that allowed multiple variables to be optimized simultaneously, thus creating a more economical, fast and effective technique. Considering the studies in a fluid solution (homogeneous), all complexes strongly depended on the amount of the oxidizing agent (H₂O₂), followed by the catalyst load. In contrast, time seemed to be statistically less relevant for complexes 1 and 3 and not relevant for 2. All complexes showed high selectivity in their optimized conditions, and only benzaldehyde (BA) was obtained as a viable product. Quantitatively, the catalytic activity observed was 3 > 2 > 1, which is related to the π-acceptor character of the ligands employed in the study. Density functional theory (DFT) studies could corroborate this feature by correlating the geometric index for square pyramid Cu(II)-OOH species, which should be generated in the solution during the catalytic process. Complex 3 was successfully immobilized in silica-coated magnetic nanoparticles (Fe₃O₄@SiO₂), and its oxidative activity was evaluated through heterogenous catalysis assays. Substrate conversion promoted by 3-Fe₃O₄@SiO₂ generated only BA as a viable product, and the supported catalyst's recyclability was proven. Reduced catalytic conversions in the presence of the radical scavenger (2,2,6,6-tetrametil-piperidi-1-nil)oxil (TEMPO) indicate that radical and non-radical mechanisms are involved.

Keywords: Fe3O4@SiO2 nanoparticles; benzyl alcohol oxidation; catalysis; copper(II) complexes; multivariate analysis.

Figure 1

Response surfaces obtained by Box–Behnken’s experimental design for modeling BnOH to BA conversion (%) as a function of catalyst concentration, peroxide concentration and reaction time. Complex 1 (A–C, left), complex 2 (D–F, middle) and complex 3 (G–I, right). Each surface was generated with the missing variable at its optimal value. Optimal values for complex 1: 14.5-fold excess H₂O₂ over substrate, 2.2 mol% as catalyst load and 16.3 h as the reaction time. Optimal values for complex 2: 20-fold excess H₂O₂ over substrate, 3.0 mol% as catalyst load and 24 h as the reaction time. Optimal values for complex 3: 20-fold excess H₂O₂ over substrate, 2.5 mol% as catalyst load and 24 h as the reaction time. Temperature was kept constant (20 °C).

Figure 2

Response Pareto chart for the formation of BA promoted by 1 (A), 2 (B) and 3 (C). The values correspond to the effects of each coefficient of the Box–Behnken model. Effect values that exceed the dashed line are significantly greater than the experimental error at a 95% confidence level. In the graph along the y axis: L = linear; Q = quadratic; 1 = catalyst; 2 = H₂O₂; 3 = time; 1 L × 2 L = interaction between variables 1 and 2; 1 L × 3 L = interaction between variables 1 and 3; 2 L × 3 L = interaction between variables 2 and 3. The x-axis corresponds to the t-calculated value for each coefficient. When the t-calculated surpasses the critical t-value (p = 0.05), the coefficient is mathematically significant to the model.

Figure 3

Blue: First use (1) and two reuses (2 and 3) of 3-Fe₃O₄@SiO₂ during the catalytic reactions to convert BnOH into BA, and the total conversion obtained from them (Σ1–3). Red: BA production from an equivalent reaction in the homogeneous medium with complex 3 (A). Conditions: 1.0 mol% of catalyst, 20-fold H₂O₂ over BnOH, 24 h, 20 °C, final volume 3.0 mL.

Figure 4

Optimized ground state geometry for Cu(II)-OOH species (a), [Cu(II)-O•]⁺ species (b) and the TS state for the H• transfer between [Cu(II)-O•]⁺ species and BA (c) obtained within the D3-B3LYP/def2-TVZP(-f) level of theory.

Scheme 1

The oxidation reaction of BnOH with H₂O₂ is mediated by copper complexes 1, 2 and 3. Condition (a): Catalyst 0.1 mol% to 3.0 mol%, H₂O₂ 1-fold to 20-fold, time 8 h to 24 h, 20 °C and acetonitrile used as a solvent.