Solvent-controlled synthesis of tetranuclear cage-like copper(ii) silsesquioxanes. Remarkable features of the cage structures and their high catalytic activity in oxidation with peroxides

Abstract

Two principally different in their molecular architecture isomeric tetranuclear copper(ii) silsesquioxanes, "Globule"-like compound [(PhSiO1.5)12(CuO)4(NaO0.5)4] (1) and "Sandwich"-like derivative [(PhSiO1.5)6(CuO)4(NaO0.5)4(PhSiO1.5)6] (2), were synthesized by the partial cleavage of polymeric copper(ii) silsesquioxane [(PhSiO1.5)2(CuO)]n by tetraphenylcyclotetrasiloxanolate. The route leading to the formation of either 1 or 2 entirely depends on the nature and composition of the solvent used for this reaction. Thus, the process in an ethanol-1-butanol solution gives compound 1. When a 1,4-dioxane-methanol mixture was used, compound 2 was prepared. The structures and unusual crystal packing of the cages were confirmed by the X-ray studies. It has been found that the reaction of benzene with H2O2 in acetonitrile solution at 50 °C catalyzed by 1 requires addition of trifluoroacetic acid (TFA) in low concentration and gives phenol with a turnover number (TON) of 250 after 3 h. The initial reaction rate W0 linearly depends on the concentration of catalyst 2. The oxidation of 1-phenylethanol to acetophenone with hydrogen peroxide catalyzed by complex 1 in the presence of TFA is not efficient. In contrast, 1 exhibited excellent activity in the oxidation with tert-butyl hydroperoxide (TBHP) in the absence of any acid (the yield of acetophenone was close to the quantitative, TON attained 475 after 2 h). A kinetic study of this reaction led to the conclusion that the process occurs with the participation of radicals tert-BuO˙ produced in the Cu-promoted decomposition of TBHP. The mode of dependence of W0 on the initial concentration of TBHP indicates the formation of an intermediate adduct between the catalyst 1 and TBHP (characterized by the equilibrium constant K1≈ 2 M(-1) for the conditions of conducted experiments) followed by subsequent decomposition of the adduct (k2≈ 0.2 s(-1)) to generate an intermediate species tert-BuO˙ which induces the alcohol oxidation.

Solvent-Free Microwave-Assisted Peroxidative Oxidation of Alcohols Catalyzed by Iron(III)-TEMPO Catalytic Systems

“…The presence of HNO 3 also exhibited an inhibitory effect on the acetophenone yield (Table 2, [7] or Cu(II) complexes bearing the 1,6-bis(2 0 -pyriyl)-2,5-dithiahexane ligand [27,42]. In fact, the mechanism of alcohol oxidation with TBHP does not seem to be promoted by the presence of protons as also verified [43] in the oxidation of 1-phenylethanol with TBHP catalysed by bi-or tetranuclear cage-like copper(II) silsesquioxanes. A very strong inhibition effect is observed (Fig.…”

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

“…Polynuclear metal complexes containing voluminous organic [63,66,118,119,[129][130][131]186,[213][214][215][216][217][218][234][235][236][237][238][239][240][241] or siloxane [221][222][223][224][225][226][227][228] ligands are also very efficient in oxidations of organic compounds including saturated hydrocarbons with peroxides. For example, dinuclear copper complex 11 [221,224] and hexairon complex 15 (for the structure, see Figure 9) affording the products of cyclohexane oxidation in a very high yield of 46% (see Figure 10 [226]) can be considered as "inorganic models of methane monooxygenases."…”

“…This approach modeling natural enzymes can be called biomimetic route. Various polynuclear complexes [320][321][322] including siloxane derivatives [221][222][223][224][225][226][227][228] containing bulky ligands are especially attractive as catalysts in oxidations with peroxides. Bulky ligands around the reaction centers give rise in these cases to the selectivity enhancement [12,323].…”

“…The most simple compound is the binuclear cage-like copper silsesquioxane [(PhSiO1.5)10(CuO)2(NaO0.5)2 4EtOH] (complex 11) with a "Cooling Tower" structure ( Figure 5) [221]. In addition, isomers containing four [222] and six [223] copper ions have been isolated. Their structures are presented in Figures 6 and 7, respectively.…”

Mild Liquid Phase Oxidation of Benzyl Alcohol in the Presence of Microporous Framework Copper Silicates

“…5.3 Å molecular size, capable of entering the 12‐MR channels) was strongly inhibited by the radical scavenger; 1 and 2 led to 5 and 15 % conversion at 6 h, respectively, compared to 30 and 48 % conversion without radical scavenger, respectively (Table entries 1, 6). Hence, the copper silicates promoted t BuOOH activation, which may give tert ‐butylperoxyl, tert ‐butoxyl and hydroxyl radicals (Equations 1, 2), in line with different transitions metal‐catalysed peroxidative oxidation reactions . The radicals may react with BzOH (RCH 2 ‐OH in Equations 3, 4) giving benzyl alcohol radicals (RCH · –OH in Equations 5–7),, , and subsequently BzAld (RCHO) , .…”