Acetone Factor in the Design of Cu₄-, Cu₆-, and Cu₉-Based Cage Coppersilsesquioxanes: Synthesis, Structural Features, and Catalytic Functionalization of Alkanes

Abstract

The present study describes a new feature in the self-assembly of cagelike copperphenylsilsesquioxanes: the strong influence of acetone solvates on cage structure formation. By this simple approach, a series of novel tetra-, hexa-, or nonacoppersilsesquioxanes were isolated and characterized. In addition, several new complexes of Cu4 or Cu6 nuclearity bearing additional nitrogen-based ligands (ethylenediamine, 2,2′-bipyridine, phenanthroline, bathophenanthroline, or neocuproine) were produced. Single-crystal X-ray diffraction studies established molecular architectures of all of the synthesized products. Several coppersilsesquioxanes represent a novel feature of cagelike metallasilsesquioxane (CLMS) in terms of molecular topology. A Cu4–silsesquioxane complex with ethylenediamine (En) ligands was isolated via the unprecedented self-assembly of a partly condensed framework of silsesquioxane ligands, followed by the formation of a sandwich-like cage. Two prismatic Cu6 complexes represent the different conformersregular and elliptical hexagonal prisms, “cylinders”, determined by the different orientations of the coordinated acetone ligands (“shape-switch effect”). A heterometallic Cu4Na4-sandwich-like derivative represents the first example of a metallasilsesquioxane complex with diacetone alcohol ligands formed in situ due to acetone condensation reaction. As a selected example, the compound [(Ph6Si6O11)2Cu4En2]·(acetone)2 was explored in homogeneous oxidation catalysis. It catalyzes the oxidation of alkanes to alkyl hydroperoxides with hydrogen peroxide and the oxidation of alcohols to ketones with tert-butyl hydroperoxide. Radical species take part in the oxidation of alkanes. Besides, [(Ph6Si6O11)2Cu4En2]·(acetone)2 catalyzes the mild oxidative functionalization of gaseous alkanes (ethane, propane, n-butane, and i-butane). Two different model reactions were investigated: (1) the oxidation of gaseous alkanes with hydrogen peroxide to give a mixture of oxygenates (alcohols, ketones, or aldehydes) and (2) the carboxylation of C n gaseous alkanes with carbon monoxide, water, and potassium peroxodisulfate to give C n+1 carboxylic acids (main products), along with the corresponding C n oxygenates. For these reactions, the effects of acid promoter, reaction time, and substrate scope were explored. As expected for free-radical-type reactions, the alkane reactivity follows the trend C2H6 < C3H8 < n-C4H10 < i-C4H10. The highest total product yields were observed in the carboxylation of i-butane (up to 61% based on i-C4H10). The product yields and catalyst turnover numbers (TONs) are remarkable, given an inertness of gaseous alkanes and very mild reaction conditions applied (low pressures, 50–60 °C temperatures).

Cagelike Octacopper Methylsilsesquioxanes: Self-Assembly in the Focus of Alkaline Metal Ion Influence—Synthesis, Structure, and Catalytic Activity

“…Namely, while seven DMF ligands are located in the positions parallel to the basal plane of the copper ions, the eighth DMF molecule is arranged perpendicular to it. Recently, we reported on a similar effect (rotation of acetone solvates [ 48 ]) that causes a “shape switch effect” for the hexanuclear copperphenylsilsesquioxanes—from a regular form to an ellipsoid-like one. In the case of complex 1 , due to the presence of the encapsulated component (which was not the case for Ref [ 48 ]), we could not state precisely if the distortion of the prismatic geometry of 1 is a consequence of DMFs rotation exclusively.…”

“…Among recent results we could mention activity in CO 2 cycloaddition [ 43 ], Chan-Evans-Lam coupling [ 44 ], biomass transformations [ 45 ], and oxidative amidation [ 46 ]. Considering the significant potential of copper-based CLMSs as catalysts [ 47 , 48 , 49 , 50 ], we were interested in designing new types of these compounds. Numerous results pointed out the significant influence of the nature of the substituent at the silicon center on molecular geometry of arising CLMSs, e.g., for nickel-based (phenyl- [ 51 , 52 , 53 , 54 ] vs. 4-vinylbenzyl-substituted [ 55 ] CLMSs), titanium-based ( t -Bu vs. (2,6- i Pr 2 C 6 H 3 )N(SiMe 3 )-substituted [ 56 , 57 ] CLMSs), aluminum-based (cyclopentyl [ 58 ] vs. cyclohexyl-substituted [ 59 ] CLMSs) compounds.…”

“…Numerous results pointed out the significant influence of the nature of the substituent at the silicon center on molecular geometry of arising CLMSs, e.g., for nickel-based (phenyl- [ 51 , 52 , 53 , 54 ] vs. 4-vinylbenzyl-substituted [ 55 ] CLMSs), titanium-based ( t -Bu vs. (2,6- i Pr 2 C 6 H 3 )N(SiMe 3 )-substituted [ 56 , 57 ] CLMSs), aluminum-based (cyclopentyl [ 58 ] vs. cyclohexyl-substituted [ 59 ] CLMSs) compounds. In turn, it is well known that the archetype of phenyl-substituted copper CLMSs is a hexanuclear prismatic structure [ 1 , 6 , 8 , 11 , 48 , 49 ]. Our newest results describing methyl-substituted Cu-CLMSs’ synthesis, structure determination, and evaluation in catalysis are presented below.…”

A Novel Family of Cage-like (CuLi, CuNa, CuK)-phenylsilsesquioxane Complexes with 8-Hydroxyquinoline Ligands: Synthesis, Structure, and Catalytic Activity

“…Nevertheless, we would like to mention, once again, the acetone-solvated compound 4 . We have recently reported the use of acetone as the ligand of choice as a very efficient method of isolating crystalline cage metallasilsesquioxanes (12 complexes) [ 94 ]. The smooth formation of complex 4 (in a higher yield than for compounds 1 – 3 ) supports the conclusion concerning the acetone potential in the CLMS design.…”

Hybrid Silsesquioxane/Benzoate Cu7-Complexes: Synthesis, Unique Cage Structure, and Catalytic Activity

“…Among recent results, we could mention CO 2 cycloaddition [ 37 ], Chan–Evans–Lam coupling [ 38 ], biomass transformations [ 39 ], and oxidative amidation [ 40 ]. Considering the significant potential of copper-based CLMSs as catalysts [ 41 , 42 , 43 ], we pursue designing new types of these compounds. Numerous results pointed out the significant influence of systems used for synthesis/crystallization on the structural features of the CLMSs being formed.…”