Unusual Tri-, Hexa-, and Nonanuclear Cu(II) Cage

Article pubs.acs.org/IC

Unusual Tri‑, Hexa‑, and Nonanuclear Cu(II) Cage Methylsilsesquioxanes: Synthesis, Structures, and Catalytic Activity in Oxidations with Peroxides Alexey N. Bilyachenko,*,†,‡ Alena N. Kulakova,†,‡ Mikhail M. Levitsky,† Artem A. Petrov,† Alexander A. Korlyukov,†,∥ Lidia S. Shul’pina,† Victor N. Khrustalev,†,‡ Pavel V. Dorovatovskii,§ Anna V. Vologzhanina,† Ulyana S. Tsareva,† Igor E. Golub,†,‡ Ekaterina S. Gulyaeva,† Elena S. Shubina,† and Georgiy B. Shul’pin*,□,△ †

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Str., 28, 119991 Moscow, Russia People’s Friendship University of Russia, Miklukho-Maklay Str., 6, 117198 Moscow, Russia ∥ Pirogov Russian National Research Medical University, Ostrovitianov str., 1, 117997 Moscow, Russia § National Research Center “Kurchatov Institute”, Akademika Kurchatova pl., 1, 123182 Moscow, Russia □ Semenov Institute of Chemical Physics, Russian Academy of Sciences, ulitsa Kosygina, dom 4, Moscow 119991, Russia △ Plekhanov Russian University of Economics, Stremyannyi pereulok, dom 36, Moscow 117997, Russia ‡

S Supporting Information *

ABSTRACT: Three types of unusual cagelike copper(II) methylsilsesquioxanes, namely, nona- [(MeSiO1.5)18(CuO)9] 1, hexa- [(MeSiO 1 . 5 ) 1 0 (HO 0 . 5 ) 2 (CuO) 6 (C 1 2 H 8 N 2 ) 2 (MeSiO1.5)10(HO0.5)1.33(CH3COO0.5)0.67(CuO)6(C12H8N2)2] 2, [(MeSiO1.5)10(CuO)6(MeO0.5)2(C10H8N2)2] 3, and trinuclear [(MeSiO1.5)8(CuO)3(C10H8N2)2] 4, were obtained in 44%, 27%, 20%, and 16% yields, respectively. Nuclearity and structural fashion of products was controlled by the choice of solvent system and ligand, specifically assisting the assembling of cage. Structures of 1−4 were determined by single-crystal Xray diffraction analysis. Compounds 1 and 4 are the first cage metallasilsesquioxanes, containing nine and three Cu ions, respectively. Product 1 is the first observation of nonanuclear metallasilsesquioxane ever. Unique architecture of 4 represents early unknown type of molecular geometry, based on two condensed pentamembered siloxane cycles. Topological analysis of metal clusters in products 1−4 is provided. Complex 1 efficiently catalyzes oxidation of alcohols with tert-butylhydroperoxide TBHP to ketones or alkanes with H2O2 to alkyl hydroperoxides in acetonitrile.

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INTRODUCTION

cyclodextrins is hydrophilic, while the interior cavity is hydrophobic.4 This allows an encapsulation of large organic molecules (e.g., vitamins A and D5) into the inner space of cyclodextrins (Figure 2, left). In turn, prismatic CLMSs possess hydrophobic outer surface due to the presence of organic substituents at the silicon atoms, while the inner space of cage is hydrophilic. As a result, several polar molecules or anions (H2O, Cl−, HO−) were observed being encapsulated into the inner void of prismatic CLMSs (Figure 2, right).1i This feature could be compared to the intriguing ability of polyhedral oligosilsesquioxanes to encapsulate halide (fluoride anion).6 The size of potential “guests” of both cyclodextrin and metallasilsesquioxane cavities is obviously limited by the cavity

An attractive family of cagelike metallasilsesquioxanes (CLMSs) is known by its members’ unusual structural,1 catalytic,2 and magnetic (spin-glass)3 properties. These compounds include a group of objects possessing the following common features: (1) they are built from monosubstituted organosilicon units RSi≡; (2) such CLMSs represent simultaneously siloxane Si−O−Si and metallasiloxane Si−OM fragments; (3) the atomic ratio Si/ M in them is comparatively high (Si/M ≥ 2, where M is polyvalent metal). The most common molecular architecture of these CLMSs is a hollow hexagonal prism (Figure 1) formed by three parallel layersone metal oxide and two siloxane (usually, phenylsilsesquioxane) ones.1i Interestingly, the properties of such prismatic CLMSs are perfectly opposite to those observed for the well-studied cyclodextrins. It is known that the outer surface of the © 2017 American Chemical Society

Received: January 9, 2017 Published: March 20, 2017 4093

DOI: 10.1021/acs.inorgchem.7b00061 Inorg. Chem. 2017, 56, 4093−4103

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Inorganic Chemistry

formation control so to regulate a size of cage moiety. Herein we present our results concerning the synthesis of Cu(II) methylsilsesquioxanes that allow the tuning of both nuclearity and cage shape. Also, mono- and polynuclear copper complexes are known to catalyze efficiently oxidations of alcohols7 with TBHP and alkanes with hydrogen peroxides.7a,8 We are presenting here the first instance of Cu(II) methylsilsesquioxane application as catalyst under the same reaction conditions.

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EXPERIMENTAL SECTION

Materials. MeSi(OMe)3 was purchased from ABCR and used as received. All solvents (Acros) were used without specific purification. Synthesis. Complex 1. Three grams (0.022 mol) of MeSi(OMe)3 in 50 mL of ethanol and 0.79 g (0.044 mol) of water were stirred for 30 min. Then 0.90 g (0.025 mol) of NaOH was added, and the resulting white-colored mixture was heated to reflux for 2 h. Afterward the solution was cooled to room temperature, and 1.35 g (0.01 mol) of CuCl2 in 50 mL of dimethyl sulfoxide (DMSO) was added at once. The solution was heated at reflux for 3 h, then cooled to room temperature and left under intensive stirring for additional 12 h. The

Figure 1. Simplified view of prismatic CLMS’ structural organization.

diameter. It is worth mentioning that this parameter in case of cyclodextrins varies in the range of 5.2−8.4 Å, while usual CLMS’ cavity diameter range is more narrow (3.4−3.6 Å). In the line of investigation of CLMSs as potential molecular containers, it would be desirable to find an opportunity of their

Figure 2. General view of cyclodextrin (left) and prismatic cage metallasilsesquioxane (right) structures (front and side views; hydrogen atoms are omitted for clarity). 4094


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Inorganic Chemistry Scheme 1. Proposed Pathway of CLMS Aggregation through the Coordination with Pyrazine

resulting mixture was filtered from insoluble precipitate. Crystallization of filtrate gave in 3−4 weeks crystal product. Several crystals were used for single-crystal X-ray diffraction analysis (1·8DMSO·H2O, see details below). Anal. Calcd for [(MeSiO1.5)18(CuO)9], Cu, 29.72; Si, 26.27. Found (for vacuum-dried sample): Cu, 29.59; Si, 26.04%. Yield 0.94 g (44%). Complex 2. Nine-tenths of a gram (0.005 mol) of 1,10phenanthroline was added at once to solution, produced as described in the section dedicated to the synthesis of 1 (prior to crystallization stage). Mixture was stirred for 4 h at room temperature. Then it was left in a cool place (15 °C) for crystallization. After one week the formation of crystalline material was observed. Several crystals were used for single-crystal X-ray diffraction analysis (2·4DMSO·4H2O, see details below). Anal. Calcd for [(MeSiO1.5)10(HO0.5)2(CuO)6 (C12H8N2)2(MeSiO1.5)10(HO0.5)1.33(CH3COO0.5)0.67 (CuO)6(C12H8N2)2], Cu, 24.74; N, 3.64; Si, 18.23. Found (for vacuum-dried sample): Cu, 24.31; N, 3.52; Si, 18.01%. Yield 0.69 g (27%). Complex 3. Seventy-eight hundredths of a gram (0.005 mol) of 2,2′-bipyridine was added at once to solution, produced as described in the section dedicated to the synthesis of 1 (prior to crystallization stage, 45 mL of methanol/ethanol (1:1) mixture was used instead of DMSO). Mixture was stirred for 4 h at room temperature, and then it was left in a cool place (15 °C) for crystallization. After one week the formation of crystalline material was observed. Several crystals were used for single-crystal X-ray diffraction analysis (3·2MeOH, see details below). Anal. Calcd for [(MeSiO1.5)10(CuO)6(MeO0.5)2(C10H8N2)2], Cu, 25.30; N, 3.72; Si, 18.64. Found (for vacuum-dried sample): Cu, 24.74; N, 3.64; Si, 18.23%. Yield 0.50 g (20%). Complex 4. 2,2′-Bipyridine (1.57 g, 0.010 mol) was added at once to solution, produced as described in the section concerning the synthesis of 1 (prior to crystallization stage, 45 mL of methanol/ ethanol (1:1) mixture was used instead of DMSO). Mixture was stirred for 4 h at room temperature, and then it was left in a cool place (15 °C) for crystallization. After one week the formation of crystalline material was observed. Several crystals were used for single-crystal Xray diffraction analysis (4·3EtOH, see details below). Anal. Calcd for [(MeSiO1.5)8(CuO)3(C10H8N2)2], Cu, 17.52; N, 5.15; Si, 20.65. Found (for vacuum-dried sample): Cu, 17.40; N, 5.02; Si, 20.23%. Yield 0.58 g (16%).

X-ray Crystallographic Data Collection and Refinement of the Structures. The data sets for 2 and 3 were collected with Bruker APEX DUO diffractometer. The structures were solved by charge flipping method and refined in anisotropic approximation. Hydrogen atoms were calculated from geometrical point of view, except for hydrogen atom of solvated methanol molecule that was located from difference Fourier maps. Hydrogen atoms were refined with restraints applied for their displacement parameters and C−H (O−H) bond length. The structure of complex 2 contains two independent molecules (see below Discussion Section for details) with the terminal ligands, coordinated to copper ions, being disordered. We interpreted the disorder as a superposition of two-thirds hydroxyl and one-third acetate ligands. Unfortunately, it was impossible to collect the reflection intensities for single crystals of 1 and 4 using the laboratory equipment (even in the case of microfocus copper tube). The quality of all collected data sets was unsatisfactory due to strong symmetry-induced disorder of solvated and coordinated molecules (as consequence the intensity of diffraction was very weak with almost absent high-angle reflections). To overcome the limitation of laboratory equipment we used protein station of Kurchatov Centre for Synchrotron radiation. The results were much better than those obtained with laboratory diffractometer. We succeeded to localize all non-hydrogen atoms and refine their position using a number of restraints. The positions of hydrogen atoms were calculated from geometrical point of view and refined with restraints applied for C−H bonds and equivalent displacement parameters. All calculations were performed using SHELX program suite9 and OLEX2 program.10 Additional crystallographic information is available in the Supporting Information. Oxidation of Alcohols. The reactions of alcohols were performed in air in thermostated Pyrex cylindrical vessels with vigorous stirring and using MeCN as solvent. The substrate was then added, and the reaction started when the oxidant was introduced in one portion. Concentrations of products obtained in the oxidation of 1-phenylethanol after certain time intervals were measured using 1H NMR method (solutions in acetone-d6; “Bruker AMX-400” instrument, 400 MHz). In the oxidation of cyclohexanol, concentrations of the substrate and products were measured by chromatography as described below for the oxidation of cyclohexane. 4095


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Inorganic Chemistry Oxidation of Alkanes. This reaction was performed analogously. Catalyst 1 and the cocatalyst (nitric acid) were introduced into the reaction mixture in the form of stock solutions in acetonitrile. The alkane was then added, and the reaction started when hydrogen peroxide was introduced in one portion. Caution! The combination of air or molecular oxygen and H2O2 with organic compounds at elevated temperatures may be explosive. The reactions after addition of nitromethane as a standard compound were analyzed by gas chromatography (GC). Instrument LKhM-80−6 was used (columns 2 m with 5% Carbowax 1500 on 0.25−0.315 mm Inerton AW-HMDS; carrier gas argon) for measuring concentrations of cyclohexanol and cyclohexanone. The samples of reaction solutions were in some cases analyzed twice: before and also after their treatment with PPh3. This method (an excess of solid triphenylphosphine is added to the samples 10−15 min before the GC analysis) proposed by one of us11 allows us to detect alkyl hydroperoxides and to measure also the real concentrations of all three products (alkyl hydroperoxide, alcohol, and aldehyde or ketone) present in the reaction solution, because usually alkyl hydroperoxides are decomposed in the gas chromatograph to produce mainly the corresponding alcohol and ketone. In our kinetic studies described below, we measured concentrations of cyclohexanone and cyclohexanol only after reduction of the reaction mixture with PPh3, which gives precisely concentration of a sum of the oxygenates. This total concentration is used for measuring the reaction rates W0. Blank experiments with cyclohexane showed that in the absence of catalyst 1 no products were formed.

stage and (ii) as a ligand during the crystallization stage. The drawback of this choice is a well-known ambidentate behavior of DMSO as ligand.13 DMSO might perform as a pure bridging unit (with involvement of both sulfur and oxygen atoms (Chart 1A)), or it could play a role of solvating ligand (when only oxygen centers of DMSO are involved into coordination (Chart 1B)). Chart 1. Simplified View of Dimethyl Sulfoxide, Coordinating Two Rh13a or Ag13b Atoms

We performed the following interaction based on in situ transformation of methyltrimethoxysilane into reactive sodiumcontaining intermediate species, followed by exchange reaction with CuCl2 in DMSO media (Scheme 2, product 1). Scheme 2. General Scheme of Synthesis of CLMSs 1−4a

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RESULTS AND DISCUSSION To exclude the influence of bulky surrounding of silicon atoms on the process of Cu(II)-CLMS self-assembly, methyltrimethoxysilane MeSi(OMe)3 was chosen as initial reagent for the synthesis. It should be mentioned that examples for the synthesis of copper(II) methylsilsesquioxanes are rather limited.12 Interestingly, some of these results, namely, refs 12a and b, described the isolation of prismatic CLMSs of high (octa-12a,b and deca-12b) nuclearity. Unfortunately, detailed analysis of CLMSs, described in ref 12b, is impossible due to the absence of corresponding structural information (CIF files) in CCDC. In turn, octanuclear compound [(MeSiO1.5)8 (CuO)8(DMF)9]·C4H4N212a attracts attention due to unusual encapsulation of pyrazine molecule by inner void of this metallasilsesquioxane. The observed phenomenon seems to be useful for the discussion of prismatic CLMS growth mechanism. Probably, an inclusion of coordinative ligands (e.g., pyrazine) is a “metallasilsesquioxane expanding factor”, which provokes aggregation of higher clusters instead of traditional hexanuclear ones.1i In this line of thought, a pyrazine molecule could coordinate two metallasiloxane rings (Scheme 1A). The distance between two opposite Cu ions (7.96 Å) determines the size of the future CLMS prism. This prediction was based on comparing the magnitude of the Si−Si−Si angle (estimated for three neighboring silicon atoms of the siloxane ring). Noteworthy that structural synthone, including two adjacent metallasiloxane fragments (Scheme 1B), is characterized by Si−Si−Si angle equal to 124.7°. This value is close to one observed for interior angle for a regular hexagon (120°). One could conclude that only the presence of an additional coordinating ligand (pyrazine) allowed to aggregate octanuclear CLMS instead of expected hexanuclear one (Scheme 1C). Being interested in further study of features of methylsubstituted CLMSs self-assembling, we decided to see if other types of bridging ligands could provoke similar “expanding” effect. First of all, the role of the ligand DMSO was investigated. The advantage of such choice is a nature of DMSOthis compound could serve (i) as a medium during the synthesis

a

For compounds 3 and 4: parallel syntheses were performed in condition of different loadings of 2,2′-bipy (y > x).

The reaction gave nonacopper(II) cage silsesquioxane [(MeSiO1.5)18(CuO)9] 1 in 44% yield. This compound is the first example of prismatic CLMS, containing an odd number of copper ions. More than that, product 1 is the first observation of nonanuclear metallasilsesquioxane ever. Its structure (cage metallasilsesquioxane, coordinated by DMSO and water molecules) was established by single-crystal X-ray diffraction analysis (Figure 3, see below for details). The main feature of the compound is the DMSO encapsulation fashion. It was revealed that the DMSO moiety is contacting two neighboring copper ions by both its oxygen and sulfur centers (alike principle shown on Chart 1A). Interesting that such coordination provoked a specific mutual orientation of two neighboring metallasilsesquioxane fragments, located at 138.2° angle to each other (in terms of Si−Si−Si angle value, see Scheme 2A).This value closely corresponds to the one known for a regular nonagon (140°). Thus, formation of nonanuclear product 1 is quite logical (Scheme 3B). On the second stage of the study we were interested in the investigation of the competition between several ligands that are capable to participate in CLMS assembling. To evaluate this, we performed reaction of copper(II)methylsilsesquioxane synthesis in condition of simultaneous presence of two different ligands (DMSO and 1,10-phenanthroline) in reaction mixture (Scheme 2, product 2). It was revealed that such synthetic procedure leads to the formation of the hexanuclear product 4096


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Inorganic Chemistry

Figure 3. Molecular structure of 1 (single-crystal X-ray diffraction analysis). Color code: Si−yellow, O−red, C−gray, Cu−green, S−light yellow.

Scheme 3. Proposed Pathway of CLMS 1 Aggregation through the Coordination with DMSO

14. One could observe that compound 2 is an unusual instance of a solid solution of two types of complexes. The first type of them characterizes by participation of hydroxyl in coordination of copper ion. In turn, the second type demonstrates unprecedented assistance of acetoxy group in the same coordination site. It should be said that presence of such fragment could not be explained by formal logic of reaction mechanism, and thus some additional processes should be taken into consideration. We could suggest that acetoxy group formation may be due to an oxidation of some ethanol molecules by atmosphere oxygen, catalyzed by copper ions. As some of us have reported on several instances of Cu-CLMS activity in C−H compounds oxidation,15 this explanation may be evaluated as adequate. One of the possible solutions how to prove an influence of atmosphere oxygen on this process is to

[(MeSiO 1.5 ) 10 (HO 0.5 ) 2 (CuO) 6 (C 12 H 8 N 2 ) 2 (MeSiO 1.5 ) 10 (HO0.5)1.33(CH3COO0.5)0.67(CuO)6(C12H8N2)2] 2 in 27% yield. Its structure (cagelike compound, coordinated by DMSO and water molecules) was established by single-crystal X-ray diffraction analysis (Figure 4, see below for details). This compound is the first example of a “CLMS−phenanthroline” complex. The most attractive feature of compound 2 is the fashion of phenanthroline coordination to the cage metallasilsesquioxane fragment. Two phenanthroline molecules coordinate two opposite copper ions, provoking “withdrawing” of two copper ions from prismatic structure. Noteworthy, a similar phenomenon was observed for phenylsubstituted Cu(II) CLMS during its complexation to 2,2′-bipyridine.14 Product 2 possesses several attractive features in comparison to compound from ref 4097


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Inorganic Chemistry

Figure 4. Molecular structure of 2 (single-crystal X-ray diffraction analysis). Color code: Si−yellow, O−red, C−gray, Cu−green, N−blue. Solvating molecules of DMSO and water are omitted for clarity.

perform the same synthesis in the inert conditions. Unfortunately, we succeed in isolation of the single crystals of the “inert” product only after the replacement of original (DMSO/ethanol) solvates by the dioxane/methanol mixture (see Supporting Information for details). This prevents a correct comparison of two products; nevertheless, it should be said that “inert” dioxane/methanol complex, being quite similar to compound 2, contains no structural components, pointing to any oxidation. It is interesting to compare differences between prismatic cage components of products 1 and 2. Both structures contain cyclic siloxane ligands [(MeSi(O)O)]n (n = 8 for 1, n = 5 for 2) and metal oxide layer. Noteworthy that instead of coppercontaining cycle in 1, one could observe specific structural rearrangement in case of 2, giving two almost linear coppercontaining trimeric fragments (see structural and Topological Analysis Section below). In further study, we performed modified synthesis of methylCLMSs, excluding presence of DMSO, and evaluating an influence of nitrogen-containing ligand only. It was shown that the process of assembly of complexes of copper(II) silsesquioxane with 2,2′-bipyridine is a ratio-dependent one. While molar ratio CuCl2/bipy was equal to 2/1 (as in the case of synthesis of 2), the formation of the hexanuclear compound was observed (Scheme 2, product 3). In turn, the higher loading of ligand (CuCl2/bipy = 1/1) provoked the formation of trinuclear compound (Scheme 2, product 4). The structure of complex 3 (Figure 5) is similar to that of compound 2. The difference between them is a different principle of additional coordination of Cu ions, realized in case of 3 by methoxy groups. Evidently these methoxy groups originate from initial reagent, MeSi(OMe)3. According to Cambridge Structural Database (CSD), compound 3 is the first observation of olato groups coordinated to metallasilsesquioxane framework. In turn, structure of 4 is an unprecedented one. First of all, compound 4, along with complex 1, is the first instance of oddnumbered copper silsesquioxane cluster. Also, 4 represents a unique for CLMS type of molecular geometry (Figure 6, top). This type of geometry is distinguished by the presence of nontypical octamembered silsesquioxane ligand, formed by two

Figure 5. Molecular structure of 3 (single-crystal X-ray diffraction analysis). Color code: Si−yellow, O−red, C−gray, Cu−green, N−blue. Solvating molecules of methanol are omitted for clarity.

condensed pentamembered cycles (Figure 6, bottom). We could compare this feature only to Cu2Na2-silsesquioxane complex, presented by some of us in ref 15a, which contains condensed ligand formed by one tetra- and two pentamembered cycles. Symptomatically, three copper centers of 4 lay in almost linear trimeric order, similarly to compounds 2 and 3 (see below structural and Topological Analysis section). The coordination polyhedra of the two terminal copper atoms correspond to the distorted square pyramid, while the central copper atom adopts planar square coordination. Solid-State Structures. The central copper−oxygen Cu9O18 framework of 1 has a tiara-like structure (Figure 7) with the intrinsic Cs (m) symmetry that is retained within the crystal structure. Each of the oxygen atoms of this fragment is μ2-bridging to two copper atoms, and thus all copper atoms adopt a tetracoordinated square-planar environment. Further the copper atoms attach one additional solvate molecule attaining the preferable [4 + 1] square-pyramidal environment. Remarkably, the coordination geometry of the copper atoms in 1 is significantly different. The five copper atoms coordinate a DMSO solvate molecule via the oxygen atom, with the capped oxygen atom looking outward from the cage (Figure 7). The three copper atoms coordinate the internal DMSO solvate molecule (two of them through the bridged oxygen atom and 4098


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opposite atoms of copper (i.e., diameter of inner cavity of cage framework) is equal to 8.229 Å. The bonds between copper atoms and solvating molecules of dimethyl sulfoxide are varied in wide range (1.91−2.30, mean value is 2.15(2) Å). Discussing other products, we would like to emphasize that peculiarities of structures 2−4 are no doubt explained mostly by the presence of N-containing ligands. Namely, the flattening of copper silsesquioxane framework in 2 and 3 (in comparison to cylindrical geometry of 1), evidently provoked by assistance of N-containing ligands. This led to appearance of weak Cu···O coordination between copper atom and opposite oxygen atom of siloxanolate ligand (the Cu2···O7 distance in 3 is equal to 2.464 Å). Interesting that this distance varies is not the same for two different cage components of 2. In case of acetoxycontaining compound the Cu3A···O5A distance is equal to 2.486 Å. In case of hydroxy-containing compound the Cu3··· O1 distance is equal to 2.512 Å. Another consequence of the coordination with N-ligands is the increase of shortest Cu···Cu intra-atomic distances in hexacopper cage skeletons of 2 and 3 in comparison to hexacopper silsesquioxanes of cylindric shape. Indeed, the average Cu···Cu distance in case of 2 is equal to 3.122 Å (in case of 3, 3.090 Å), while the longest Cu···Cu contact for hexacopper cylinder CLMSs (presented by some of us in ref 15d) is 2.863 Å. Most of the Cu−O bonds in 2 and 3 are very similar to those in hexacopper silsesquioxanes from ref 14d, except for Cu2−O1 distance in 2 (2.239 Å) and Cu3−O7 distance in 3 (2.196 Å). In turn, the most prominent feature of compound 4, in addition to those mentioned above, is different coordination environment of copper ions. While coordination polyhedra of two terminal copper atoms correspond to distorted square pyramid, the central copper atom Cu1 adopts planar-square coordination. Cu−Cu distance in 4 is equal to 2.997 Å, and Cu−O distances vary in the range of 1.905−2.186 Å. Topological Analysis. To reveal the effect of the silsesquioxane ring nuclearity on disposition of CLMSs, topologies of metal clusters should be compared. Simplification of the molecular graph to obtain the graph of Cu-containing skeleton keeping the cluster connectivity was performed with the ToposPro package.16 The enlargement of the ring is accompanied by significant reorganization of connections

Figure 6. (top) The molecular structure of 4 (single-crystal X-ray diffraction analysis). Solvating molecules of ethanol are omitted for clarity. (bottom) Silsesquioxane ligand from 4. Color code: Si−yellow, O−red, C−gray, Cu−green, N−blue.

the third one via the sulfur atom). Finally, the ninth copper atom coordinates a water solvate molecule that is looking outward from the cage (Figure 7). It is important to point out that the copper atom additionally coordinating the water molecule is disposed between the copper atoms that are connected to the internal DMSO molecule. This construction is apparently determined by the mutual arrangement of neighboring Cu(II)-silsesquioxane molecules that form a crystal packing by the gear-wheel mechanism (Figure 7). Consequently, the crystal packing observed in the crystal of 1 confirms the template method of synthesis of 1. Mean Cu···Cu and Cu−O distances with siloxanolate ligands in 1 are equal to 2.869(5) and 1.935(9), while corresponding ranges of variation are 2.856−2.932 and 1.909−1.975 Å. The Cu−S coordination bond is 3.412(7) Å. The distance between

Figure 7. Arrangement of two neighboring Cu(II)-silsesquioxane molecules in the crystal of 1 using the gear-wheel mechanism. 4099


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Figure 8. Cu-containing clusters connected by bridge atoms in (a) CLMS 1, (b) CLMSs 2 and 3, and (c) CLMS 4 and the graphs of corresponding clusters connectivity (d−f).

between metal atoms. CLMSs of the [(RSiO1.5)12(CuO)6] composition form the 2M6-1 and 5M6-1 clusters in the absence and the presence of the “enclosed” μ6-Cl anions, respectively.15d Larger (RSiO1.5)812a,b and (RSiO1.5)101b rings were found in the structures of [(RSiO1.5)16(CuO)8] and [(RSiO1.5)20(CuO)10] complexes, while CLMS 1 is the first representative of the [(RSiO1.5)18(CuO)9] family. Despite the presence of the encapsulated moieties, all octa-, nona-, and decacoppercontaining CLMSs are characterized with the 2M8-1, 2M9-1, and 2M10-1 topologies, respectively (see the 2M9-1 topology on Figure 8 left). In turn, copper atoms in CLMSs 2 and 3 form the 2,3,4M6-1 trinodal discrete cluster (Figure 8 middle). The 2,3,4M6-1 topology was met for tens of metal-containing compounds from the CSD,17 mainly containing bridge bidentate ligands (acetylacetonates, carboxylates, oximates). Among them five CLMSs were found (refcodes in the CSD are {LAZBUK}, {LAZBUK01}, {OBIDAG}, {OBIDEK}, and {OBIFOW}); and all of them contain (RSiO1.5)5 rings. CLMS 4 belongs to the 1,2M3-1 binodal clusters (Figure 8 right). Formation of metal clusters connected by cagelike metallasilsesquioxanes with mono- and biconnected metal atoms is more typical for cubane silsesquioxanes. Among them representatives of the same 1,2M3-118 and closely related 1,2M4-119 clusters can be found. Taking the shape of methylsilsesquioxane moiety in 4 into account it is not surprising that copper atoms also form the cluster more typical for cubanes, than for prismatic CLMSs. As a conclusion, it could be said that for prismatic copper silsesquioxanes [(RSiO1.5)2n(CuO)n] (n = 6−10) copper atoms tend to act as uninodal two-connected nodes upon cluster formation, although for [(RSiO1.5)5 rings connected with six copper atoms steric factors force distortion of hexacoppercontaining clusters, rapprochement of copper atoms followed by appearance of additional bonds and 3- and 4-connected nodes. Thus, steric factors from a silsesquioxane anion play a key role upon mutual disposition of copper atoms in a cluster. Catalyzed Oxidation of Alcohols and Alkanes with Peroxides. Alcohols are efficiently oxidized with TBHP in acetonitrile when complex 1 is used as a catalyst (Figures 9, 10, S1, and S2). Note that addition of nitric acid leads to the retardation of the reaction (Figure 9). Dependences of acetophenone accumulation rate in the oxidation of 1-phenylethanol with TBHP catalyzed by

Figure 9. Accumulation of acetophenone with time in the oxidation of 1-phenylethanol (initial concentration 0.33 M) with TBHP (70% aqueous; initial concentration 1.17 M) catalyzed by compound 1 (5 × 10−4 M) in the absence and in the presence of HNO3 (0.05 M). Solvent was acetonitrile (total volume of the reaction solution was 5 mL); 50 °C.

compound 1 on initial concentrations of catalyst 1 are presented in Figure 10. Aliphatic alcohols are a bit less reactive in comparison with 1-phenylethanol. Thus, the oxidation of cyclohexanol (0.38. M; [compound 1] = 5 × 10−4 M; in the absence of HNO3; 50 °C) gave cyclohexanone in 77% after 24 h (see also Figure S2). Compound 1 is among the most efficient catalysts for the oxidation of secondary alcohols to ketones (the yields were 96% for 1-phenylethanol and 77% for cyclohexanol). The following literature19e data can be cited here for comparison (the MW-assisted oxidation of cyclohexanol and 1-phenylethanol with TBHP catalyzed by Cu complex gave cyclohexanone and acetophenone in 85 and 55% yields, correspondingly). We also found that alkanes can be oxidized in acetonitrile solution to the corresponding alkyl hydroperoxides by hydrogen peroxide in air in the presence of catalytic amounts of complex 1 and nitric acid. The reaction proceeds in acetonitrile solution under mild conditions (typically at 50 °C). The alkyl hydroperoxide is relatively stable in the solution, and can be easily reduced by PPh3 to the corresponding alcohol. 4100


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Inorganic Chemistry

Figure 10. Dependences of acetophenone accumulation rate in the oxidation of 1-phenylethanol with TBHP (70% aqueous) catalyzed by compound 1 on initial concentrations of catalyst 1 (curve 1), 1phenylethanol (curve 2), and TBHP (curve 3). Solvent was acetonitrile (total volume of the reaction solution was 5 mL); 50 °C.

Chromatogram obtained after reduction with PPh3 is drastically distinguished from the chromatogram for unreduced sample (compare top and bottom graphs in Figure 11). This difference unambiguously indicates that cyclohexyl hydroperoxide is formed in the process.7a,11 Using higher initial concentration of cyclohexane (0.46 M) we were able to obtain after 5 h 0.084 M of cyclohexanol and 0.008 M of cyclohexanone after reduction with PPh3. Thus, total yield attained 20%, and TON was 184. Comparable values have been reported previously for the copper-catalyzed transformations of alkanes into alkyl hydroperoxides.7a,8b,20a,b Regioselectivity parameter in n-heptane oxidation was C(1)/ C(2)/C(3)/C(4) = 1.0:6.0:6.5:6.0. Bond selectivity in oxidation of methycyclohexane was 1°:2°:3° = 1:12:30. In the oxidation of cis-1,2-dimethylcyclohexane the ratio of tertiary alcohols with mutual configuration of methyl groups trans/cis was 0.7. All these data allow us to assume that the reaction proceeds with intermediate formation of hydroxyl radicals that attack C−H bonds of a substrate [7, 11, 20]. The authors realize that complex 1 plays a role of a precatalyst in the oxidations with peroxides. In the course of the reaction compound 1 is modified to afford a catalytically active species. We can assume that under the action of an acid and H2O2 some bonds in the precatalyst cage are ruptured, and substrate and oxidant can easily approach the copper-containing reaction center. The catalyst is involved into a partial destruction, and the activity of the solid obtained by evaporation after the first run of the reaction is ca. 2 times lower than activity of “fresh” catalyst. Note that it is difficult to follow the catalytic process using spectra of the catalytically active species, because concentration of the active species is very low (1 × 10−4 to 1 × 10−5 M), and many other compounds are present in the reaction mixture. A detailed study of the nature of catalytically active species will be a subject of new work with the help of modern techniques. An advantage of the catalytic system

Figure 11. Oxidation of cyclohexane (0.23 M) with H2O2 (50%, 2M) catalyzed by complex 1 (5 × 10−4 M) in the presence of HNO3 (0.05 M). Solvent was acetonitrile (total volume of the reaction solution was 5 mL); 50 °C. Concentrations of cyclohexanol (curves 1) and cyclohexanone (curves 2) were measured both before and after reduction with PPh3. Curves 1a and 2a correspond to concentrations of cyclohexanol and cyclohexanone, respectively, obtained in the reaction in the absence of HNO3.

described in this work is its high efficiency. Thus, the cyclohexane oxidation with H2O2 in the presence of HNO3 gave oxygenates in 18% yield after 1 h (Figure 11). Under the same conditions, the oxidation catalyzed by Cu(NO3)2 afforded oxygenates only in 3.5% yield, and we may conclude that complex 1 is undoubtedly the more efficient catalyst in comparison with simple salt Cu(NO3)2.

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CONCLUSIONS Tuning of synthesis (varying of nature of solvents and/or Nligands) of Cu(II)-methylsilsesquioxanes gave three types of unusual cagelike complexes, namely, nona- [(MeSiO1.5)18(CuO)9] 1, hexa- [(MeSiO1.5)10(HO0.5)2(CuO)6(C12H8N2)2(MeSiO1.5)10(HO0.5)1.33(CH3COO0.5)0.67(CuO)6(C12H8N2)2] 2, [(MeSiO1.5)10(CuO)6(MeO0.5)2(C10H8N2)2] 3, and trinuclear [(MeSiO1.5)8(CuO)3(C10H8N2)2] 4. Compounds 1 and 4 are the first cage metallasilsesquioxanes, containing nine and three copper(II) ions, respectively. Moreover, cage complex 1 is the first observation of nonanuclear metallasilsesquioxane ever. Because of a significant size of its cavity (inner diameter >8 Å), this particular architecture seems to have potential as a host compound. We could suggest possible encapsulation of other (except DMSO) molecules by this complex, for example, acetone, acetonitrile, nitromethane, etc. Noteworthy is that compound 1 catalyzes the homogeneous oxidation of either 4101


Article

Inorganic Chemistry

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alcohols to ketones or alkanes to alkyl hydroperoxides. This variety of molecular architectures, available from quite simple starting reagents, points out to significant prospectives of this synthetic route. Our team is going to investigate further this promising reactivity to create new types of molecular topologies and to study functional properties of these compounds, for example, in catalysis of oxidative C−H compounds functionalization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00061. Crystallographic data for structures 1−4, catalytic studies of 1, IR and computational studies, additional synthetic procedures, X-ray crystallographic studies of 5 and 6 (PDF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (A.B.N.) *E-mail: [email protected]. (G.B.S.) ORCID

Alexey N. Bilyachenko: 0000-0003-3136-3675 Victor N. Khrustalev: 0000-0001-8806-2975 Anna V. Vologzhanina: 0000-0002-6228-303X Igor E. Golub: 0000-0003-4142-454X Notes

The authors declare no competing financial interest. Crystallographic data for 1−4 was submitted to CSD (CCDC Nos. 1509419−1509422) and can be obtained free of charge using web request form https://www.ccdc.cam.ac.uk/ structures/?.

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ACKNOWLEDGMENTS This work has been partially supported by the Russian Foundation for Basic Research (Grant Nos. 17-03-00993 and 16-03-00254) and by the Ministry of Education and Science of the Russian Federation (Agreement No. 02.a03.21.0008.

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REFERENCES

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Unusual Tri-, Hexa-, and Nonanuclear Cu(II) Cage

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