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Article

Palanquin-Like Cu4Na4 Silsesquioxane Synthesis (via Oxidation of 1,1-bis(Diphenylphosphino)methane), Structure and Catalytic Activity in Alkane or Alcohol Oxidation with Peroxides

by
Alena N. Kulakova
1,2,
Victor N. Khrustalev
2,3,
Yan V. Zubavichus
3,4,
Lidia S. Shul’pina
1,
Elena S. Shubina
1,
Mikhail M. Levitsky
1,
Nikolay S. Ikonnikov
1,
Alexey N. Bilyachenko
1,2,*,
Yuriy N. Kozlov
5,6 and
Georgiy B. Shul'pin
2,5,6,*
1
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova, 28, Moscow 119991, Russia
2
People’s Friendship University of Russia, ul. Miklukho-Maklaya, dom 6, Moscow 117198, Russia
3
National Research Center “Kurchatov Institute”, pl. Akad. Kurchatova, dom 1, Moscow 123182, Russia
4
Boreskov Institute of Catalysis SB RAS, prosp. Akad. Lavrentieva, dom 5, Novosibirsk 630090, Russia
5
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina, dom 4, Moscow 119991, Russia
6
Chair of Chemistry and Physics, Plekhanov Russian University of Economics, Stremyannyi pereulok, dom 36, Moscow 117997, Russia
*
Authors to whom correspondence should be addressed.
Catalysts 2019, 9(2), 154; https://doi.org/10.3390/catal9020154
Submission received: 26 December 2018 / Revised: 10 January 2019 / Accepted: 16 January 2019 / Published: 4 February 2019
(This article belongs to the Section Catalysis in Organic and Polymer Chemistry)
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Abstract

:
The self-assembly synthesis of copper-sodium phenylsilsesquioxane in the presence of 1,1-bis(diphenylphosphino)methane (dppm) results in an unprecedented cage-like product: [(PhSiO1,5)6]2[CuO]4[NaO0.5]4[dppmO2]2 1. The most intriguing feature of the complex 1 is the presence of two oxidized dppm species that act as additional O-ligands for sodium ions. Two cyclic phenylsiloxanolate (PhSiO1,5)6 ligands coordinate in a sandwich manner with the copper(II)-containing layer of the cage. The structure of 1 was established by X-ray diffraction analysis. Complex 1 was shown to be a very good catalyst in the oxidation of alkanes and alcohols with hydrogen peroxide or tert-butyl hydroperoxide in acetonitrile solution. Thus, cyclohexane (CyH), was transformed into cyclohexyl hydroperoxide (CyOOH), which could be easily reduced by PPh3 to afford stable cyclohexanol with a yield of 26% (turnover number (TON) = 240) based on the starting cyclohexane. 1-Phenylethanol was oxidized by tert-butyl hydroperoxide to give acetophenone in an almost quantitative yield. The selectivity parameters of the oxidation of normal and branched alkanes led to the conclusion that the peroxides H2O2 and tert-BuOOH, under the action of compound (1), decompose to generate the radicals HO• and tert-BuO• which attack the C-H bonds of the substrate.

1. Introduction

Cage metallasilsesquioxanes (CLMSs) [1,2,3,4,5,6,7,8,9,10] are popular objects for the investigation of regularities in the insertion of metal ions into the siloxane matrix [11,12,13,14,15,16,17,18], the study of catalytic [19,20,21,22,23,24] properties, magnetic (spin) glass [25], as well as the study of nanoparticles [26] and macrocyclic siloxane material [27] formation.
Among the synthetic approaches to cage metallasilsesquioxanes, the one based on the involvement of additional organic ligands has attracted significant attention due to the formation of extravagant molecular architectures, (e.g., [12,28,29,30,31,32]). In the continuation of these works, our team has recently suggested that the simplest method of the “mixed ligand synthesis” of CLMSs is to introduce additional organic ligand into the reaction mixture before the formation of the cage product. A modified self-assembly procedure such as this in most cases leads to the “ligand synergy” situation, with different ligands coordinating metal ions of the same cage compound. This feature was observed for several types on N-based ligands: 1,10-phenanthroline [33,34], 2,2′-bipyridine [33,34,35], and bathophenantroline [34,35,36]. Dual behavior was detected for the neocuproine-assisted synthesis of Cu(II)-CLMS: in the case of DMF/dioxane reaction/crystallization media, a similar “ligand synergy product” was formed [37], while DMF media favors the isolation of “ligand antagonism” product with the redistribution of copper centers between silsesquioxane and neocuproine ligands [38], as seen in Figure 1. This particular antagonistic behavior was also found in the P-ligand-assisted self-assembly of Cu-CLMSs; this reaction was found to be tuned with two types of product being available for the reaction with 1,2-bis(diphenylphosphino)ethane [39], as seen in Figure 2.
In the present article, we obtained non-trivial results from the synthesis with a close analogue of 1,2-bis(diphenylphosphino)ethane, namely, 1,1-bis(diphenylphosphino)methane (dppm). In the present article, we obtained non-trivial results from the synthesis with a close analogue of 1,2-bis(diphenylphosphino)ethane, namely, 1,1-bis(diphenylphosphino)methane (dppm). To our delight, the self-assembly reaction of [(PhSiO1.5)(NaO0.5)]n (obtained in situ from PhSi(OEt)3) with CuCl2 in the presence of dppm resulted in the isolation (in a 22% yield) of heterometallic (Cu4/Na4) cage-like phenylsilsesquioxane 1, as seen in Scheme 1.
The structure of 1 was established by an X-ray diffraction study, as seen in Figure 3 (see also ESI results). This study found that the formation of 1 was indeed accompanied by dppm ligands. Surprisingly, the way that dppm was found to act, is principally different to earlier mentions of dppe activity. Namely, dppm underwent in situ oxidation into its dioxide, dppmO2, followed by the coordination to sodium ions. It is important to note that the oxidation of dppm to dppmO2 was previously reported in the literature [40]. Moreover, some examples of oxidations during CLMS synthesis were described [33,34,37,41,42,43,44,45], but to the best of our knowledge, no single example of CLMS synthesis accompanied by dppm oxidation has been reported to date. An additional and intriguing feature of such oxidation that acted favorably in aiding the formation of 1 is the requirement of mild conditions. In this respect, we would like to suggest the influence of copper ions as catalytic centers and the presence of trace oxygen as an oxidant.
Regarding the rest of the structure of 1, it is also notable that the principle of cyclic silsesquioxane ligands’ coordination to copper ions (in a sandwich manner as seen in Figure 4, left) is well-described in CLMSs chemistry [1,6,10]. In turn, the location of sodium ions in 1 is far from being classical. For a regular structure of sandwich CLMS, including four alkaline metal ions (e.g., Ref. [45]), it is quite easy to distinguish two pairs of alkaline metal ions (in axial and equatorial positions, respectively, as seen in Figure 4, center). In contrast to this, all of the sodium ions in 1 lay in equatorial positions, which allow for each dppmO2 species to coordinate two sodium centers, as seen in Figure 4, right. Such coordination provokes a deep distortion of the sandwich skeletons of 1, in comparison with similar fragments described in Refs. [39] (as seen in Figure 1) and [45] (as seen in Figure 2, center) (several distances and angles are provided in Table 1). The six-membered Na-dppm (Na-O-P-C-P-O) rings adopt a sofa conformation with the carbon atom out of the mean plane, which passed through the other atoms of the ring by 0.737(6) Å, as seen in Figure 3, A. The dihedral angle between the plane of Na,Cu-ions and the basal plane of the six-membered Na-dppm rings is 13.64(10)°: Additional crystallographic information is provided in the ESI (Tables S1–S6).

2. Oxidation of Hydrocarbons and Alcohols with Peroxides

Complexes of transition metals are known to catalyze the oxidation of hydrocarbons and alcohols with peroxides [46,47,48,49,50]. In the present work, we found that complex 1 exhibited catalytic activity in the oxidation of cyclohexane and other alkanes with H2O2 in acetonitrile, in the presence of nitric acid. Catalysts of this type have been shown to exhibit high catalytic activity. It was interesting to introduce phosphorus atoms into the catalyst structure.
The experiment shown in Figure 5 demonstrated that the reduction of the reaction solution with PPh3 gave rise to a higher concentration of cyclohexanol and a decrease of cyclohexanone concentration (compare top and bottom graphs in Figure 5). These changes testify that the alkyl hydroperoxide was formed in the course of the oxidation (the so called Shul’pin method [51,52,53,54,55,56,57,58,59,60,61,62,63,64]).
The following selectivity parameters were obtained for the oxidation of n-heptane: C(1):C(2):C(3):C(4) = 1.0:5.3:5.6:5.0 (after 1 h) and 1.0:6.1:5.8:5.8 (after 3 h); of methylcyclohexane: 1°:2°:3° = 1.0:5.4:15.0; and of cis-1,2-dimethylcychohexane: t/c = 0.8 (the last parameter means that the trans/cis ratio of produced corresponding isomeric tertiary alcohols was 0.8). The character of dependence of the initial cyclohexane oxidation rate on the initial hydrocarbon concentration (approaching a plateau at [cyclohexane]0 ~0.3 M; Figure 6), as well as the selectivity parameters measured here, indicate that the reaction occurred with the participation of hydroxyl radicals, and alkyl hydroperoxides were formed as the main primary products [65]. We assumed that added nitric acid induced the transformation of the catalyst into an active form.
The dependence of the reaction rate on the temperature of the reaction solution obeys the Arrhenius law, which allowed us to measure the effective activation energy (see Figure 7). The activation energy of cyclohexane oxidation was equal to 13.6 kcal/mol. The main increment of this value was from the process of catalytic H2O2 decomposition, which generated species inducing cyclohexane oxidation.
Complex 1 was found to efficiently catalyze the transformation of alcohols into the corresponding ketones by the oxidation with tert-butyl hydroperoxide, as seen in Figure 8.
The data presented in Figure 8 demonstrate that in the presence of complex 1 and tert-Bu-OOH, 1-phenylethanol was oxidized more effectively in comparison with cyclohexanol and heptanol-2. The characteristic times of the kinetic curves of the formation of corresponding ketones in the case that three alcohols were similar and that the maximum concentration of ketones including acetophenone was achieved when tert-Bu-OOH was completely diseased. Thus, the tert-Bu-OOH decomposition rates in the presence of three alcohols were similar and the active species generation rates for all alcohols were the same. The lower yields of ketones in comparison with the initial TBHP concentration were due to the concurrent reaction of TBHP with acetonitrile. 1-Phenyethanol exhibited a higher reactivity in comparison with two other alcohols. This result was in accord with the known information regarding the higher constant that comes from the reaction of aromatic alcohols with hydroxyl radicals in comparison with constants measured for aliphatic alcohols [65].
The dependence of the initial 1-phenylethanol (ROH) oxidation rates by TBHP, as seen in Figure 9, on the initial ROH concentration was in accordance with an assumption on the oxidation of ROH by intermediate species generated in the catalytic peroxide decomposition. This species, X, reacts in two parallel routes with ROH and the solvent CH3CN. Let us consider the following kinetic scheme:
Н2О2 (or TBHP) + catalyst → X (initial rate Wi),
ROH + X → R1O•,
CH3CN + X → products.
Here, stage (i) reflects the process of the generation of X with the rate Wi; stages (1) and (2) are from the processes of the interaction of X with ROH and CH3CN, respectively, with the constants k1 and k2. In the frames of quasi-stationary estimation, we can obtain the following equation for the initial rate of RO• generation:
(d[R1O•]/dt)0 = Wi/(1 + k2[CH3CN]/k1[ROH]0).
Analysis of the experimental data shown in Figure 6 and Figure 9 in accordance with Equation (3) allowed us to determine the values of k2[CH3CN]/k1 for the oxidation of ROH by hydrogen peroxide (0.44 M) and TBHP (0.33 M), respectively. These parameters were distinguished from the values typical for hydroxyl radicals (~0.1 M). However, the parameters of selectivity (see above) indicated that hydroxyl radicals took part in the oxidation with hydrogen peroxide. It is not yet clear what the reason for this discrepancy is, and this will be the subject of further research.

3. Conclusions

The oxidation of 1,1-bis(diphenylphosphino)methane to its dioxide, under mild conditions, favored the formation of the unusual heterometallic cage silsesquioxane [(PhSiO1,5)6]2[CuO]4[NaO0.5]4[dppmO2]2. The copper ions of the product were coordinated in a sandwich manner by two cyclic (PhSiO1,5)6 ligands. In turn, sodium ions of the complex were located in the external positions and were ligated by both silsesquioxane species and dppmO2 species. Such “oxidative activity” was additionally confirmed by catalytic tests. Cyclohexane, CyH, was transformed into cyclohexyl hydroperoxide CyOOH, which could be easily reduced by PPh3 to afford stable cyclohexanol. 1-Phenylethanol was oxidized by tert-butyl hydroperoxide to give acetophenone in an almost quantitative yield. The selectivity parameters of oxidation of normal and branched alkanes led to the conclusion that peroxides H2O2 and tert-BuOOH under the action of compound 1 decomposed to generate the radicals HO• and tert-BuO•, which attacked the C-H bonds of the substrate.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/2/154/s1, Table S1: Crystal data and structure refinement for 1, Table S2: Atomic coordinates (×104) and equivalent isotropic displacement parameters (Å2 × 103) for 1. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor, Table S3: Bond lengths [Å] and angles [°] for 1, Table S4: Anisotropic displacement parameters (Å2 × 103) for 1. The anisotropic displacement factor exponent takes the form: −2π2[h2 a*2U11 + ... + 2 h k a* b* U12], Table S5: Hydrogen coordinates (×104) and isotropic displacement parameters (Å2 × 103) for 1, Table S6: Torsion angles [°] for 1, Table S7: Hydrogen bonds for 1 [Å and °].

Author Contributions

A.N.B. and G.B.S. conceived and designed the experiments; A.N.K., Y.V.Z., V.N.K., N.S.I. and L.S.S. performed the experiments; A.N.B., E.S.S., M.M.L., Y.N.K. and G.B.S. analyzed the data; A.N.B. and G.B.S. wrote the paper.

Funding

This research was funded by the RUDN University Program “5-100”, the Russian Foundation for Basic Research (Grant Nos. 16-03-00254, 17-03-00993), the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007), as well as by the Initiative Program (State registration number АААА-А16-116020350251-6) in the frames of the State Task 0082-2014-0007, “Fundamental regularities of heterogeneous and homogeneous catalysis.

Acknowledgments

This work has been supported by the RUDN University Program “5-100”, the Russian Foundation for Basic Research (Grant Nos. 16-03-00254, 17-03-00993), the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007), as well as by the Initiative Program (State registration number АААА-А16-116020350251-6) in the frames of the State Task 0082-2014-0007, “Fundamental regularities of heterogeneous and homogeneous catalysis”.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Features of cage Cu-CLMSs’ interaction with N,N-ligand (neocuproine). Left: Compound A with silsesquioxane and neocuproine ligands as parts of the same unit (Ref. [37]). Right: Ionic complex B with copper ions redistributed between silsesquioxane and neocuproine ligands (Ref. [38]). CLMS: cage metallasilsesquioxanes.
Figure 1. Features of cage Cu-CLMSs’ interaction with N,N-ligand (neocuproine). Left: Compound A with silsesquioxane and neocuproine ligands as parts of the same unit (Ref. [37]). Right: Ionic complex B with copper ions redistributed between silsesquioxane and neocuproine ligands (Ref. [38]). CLMS: cage metallasilsesquioxanes.
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Figure 2. Ionic complexes with copper ions redistributed between silsesquioxane and 1,2-bis(diphenylphosphino)ethane ligands (Ref. [39]). Left: Compound C with four copper(II) ions in the cage unit. Right: Compound D with nine copper(II) ions in the cage unit.
Figure 2. Ionic complexes with copper ions redistributed between silsesquioxane and 1,2-bis(diphenylphosphino)ethane ligands (Ref. [39]). Left: Compound C with four copper(II) ions in the cage unit. Right: Compound D with nine copper(II) ions in the cage unit.
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Scheme 1. The synthesis of [(PhSiO1,5)6]2[CuO]4[NaO0.5]4[dppmO2]2 (complex 1) during the self-assembly reaction of [(PhSiO1.5)(NaO0.5)]n with CuCl2, assisted by the (in situ oxidized) dppm ligand.
Scheme 1. The synthesis of [(PhSiO1,5)6]2[CuO]4[NaO0.5]4[dppmO2]2 (complex 1) during the self-assembly reaction of [(PhSiO1.5)(NaO0.5)]n with CuCl2, assisted by the (in situ oxidized) dppm ligand.
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Figure 3. The molecular structure of 1. Top: Front view. Bottom: Top view. Color code: copper—green, sodium—yellow, oxygen—red, phosphorous—purple.
Figure 3. The molecular structure of 1. Top: Front view. Bottom: Top view. Color code: copper—green, sodium—yellow, oxygen—red, phosphorous—purple.
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Figure 4. Left: Structure of silsesquioxane ligands in 1. Center: Molecular structure of Cu4-CLMSs from Ref. [45], representing the axial and equatorial locations of sodium centers. Right: The framework of 1, representing equatorial-only locations of sodium centers. Color code: copper—green, sodium—yellow, oxygen—red, nitrogen—blue.
Figure 4. Left: Structure of silsesquioxane ligands in 1. Center: Molecular structure of Cu4-CLMSs from Ref. [45], representing the axial and equatorial locations of sodium centers. Right: The framework of 1, representing equatorial-only locations of sodium centers. Color code: copper—green, sodium—yellow, oxygen—red, nitrogen—blue.
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Figure 5. The accumulation of cyclohexanol and cyclohexanone in the oxidation of cyclohexane (0.46 M) with H2O2 (2 M) catalyzed by complex 1 (5 × 10−4 M), in the presence of HNO3 (0.05 M) at 50 °C. In order to detect the formation of cyclohexyl hydroperoxide, the concentrations of the products were measured by GC before and after the reduction of the reaction sample with solid PPh3. In the absence of HNO3, the oxidation afforded only 0.015 M of products after 2 h (after the reduction of the reaction sample with solid PPh3).
Figure 5. The accumulation of cyclohexanol and cyclohexanone in the oxidation of cyclohexane (0.46 M) with H2O2 (2 M) catalyzed by complex 1 (5 × 10−4 M), in the presence of HNO3 (0.05 M) at 50 °C. In order to detect the formation of cyclohexyl hydroperoxide, the concentrations of the products were measured by GC before and after the reduction of the reaction sample with solid PPh3. In the absence of HNO3, the oxidation afforded only 0.015 M of products after 2 h (after the reduction of the reaction sample with solid PPh3).
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Figure 6. The dependence of the oxidation rate W0 on the initial concentration of cyclohexane in the oxidation of cyclohexane into cyclohexanol and cyclohexanone (a sum) with H2O2 (2.0 M) catalyzed by complex 1 in the presence of HNO3 (0.05 M). Conditions: catalyst 1 (5 × 10−4 M), 50 °C, concentrations of products were measured by GC only after the reduction of the reaction sample with solid PPh3. Linearisation of the curve shown in Graph A is presented in Graph B.
Figure 6. The dependence of the oxidation rate W0 on the initial concentration of cyclohexane in the oxidation of cyclohexane into cyclohexanol and cyclohexanone (a sum) with H2O2 (2.0 M) catalyzed by complex 1 in the presence of HNO3 (0.05 M). Conditions: catalyst 1 (5 × 10−4 M), 50 °C, concentrations of products were measured by GC only after the reduction of the reaction sample with solid PPh3. Linearisation of the curve shown in Graph A is presented in Graph B.
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Figure 7. The Arrhenius plot of the oxidation of cyclohexane (0.46 M) with H2O2 (50%, 2 M), catalyzed by compound 1 (5 × 10−4 M) in acetonitrile, in the presence of HNO3 (0.05 M). Concentrations of products were measured by GC after the reduction of the reaction sample with solid PPh3.
Figure 7. The Arrhenius plot of the oxidation of cyclohexane (0.46 M) with H2O2 (50%, 2 M), catalyzed by compound 1 (5 × 10−4 M) in acetonitrile, in the presence of HNO3 (0.05 M). Concentrations of products were measured by GC after the reduction of the reaction sample with solid PPh3.
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Figure 8. Accumulation of cyclohexanone, heptanone-2, and acetophenone in the oxidation of cyclohexanol (0.46 M), heptanol-2 (0.42 M), or 1-phenylethanol (0.5 M), respectively, with tert-butyl hydroperoxide (1.5 M) catalyzed by complex 1 (5 × 10−4 M) in the absence of HNO3 at 50 °C. In order to quench the oxidation process, concentrations of products were measured by GC only after the reduction of the reaction sample with solid PPh3.
Figure 8. Accumulation of cyclohexanone, heptanone-2, and acetophenone in the oxidation of cyclohexanol (0.46 M), heptanol-2 (0.42 M), or 1-phenylethanol (0.5 M), respectively, with tert-butyl hydroperoxide (1.5 M) catalyzed by complex 1 (5 × 10−4 M) in the absence of HNO3 at 50 °C. In order to quench the oxidation process, concentrations of products were measured by GC only after the reduction of the reaction sample with solid PPh3.
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Figure 9. The dependence of the initial oxidation rate W0 on the initial concentration of 1-phenylethanol in its oxidation into acetophenone with tert-butyl hydroperoxide (1.5 M) catalyzed by complex 1 (5 × 10−4 M), 50 °C. Concentrations of products were measured by GC only after the reduction of the reaction sample with solid PPh3 in order to quench the oxidation process. Linearization of the curve, as shown in Graph A, is presented in Graph B.
Figure 9. The dependence of the initial oxidation rate W0 on the initial concentration of 1-phenylethanol in its oxidation into acetophenone with tert-butyl hydroperoxide (1.5 M) catalyzed by complex 1 (5 × 10−4 M), 50 °C. Concentrations of products were measured by GC only after the reduction of the reaction sample with solid PPh3 in order to quench the oxidation process. Linearization of the curve, as shown in Graph A, is presented in Graph B.
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Table
Table 1. Structural comparison of Cu4-based CLMSs.
Table 1. Structural comparison of Cu4-based CLMSs.
CompoundThe Shortest Distance between Opposing Silicon Atoms in [PhSiO1.5]6 Silsesquioxane Ligand, Å
1 (this work)5.114
C (complex from Ref. [39])5.584
E (complex from Ref. [45])5.677

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Kulakova, A.N.; Khrustalev, V.N.; Zubavichus, Y.V.; Shul’pina, L.S.; Shubina, E.S.; Levitsky, M.M.; Ikonnikov, N.S.; Bilyachenko, A.N.; Kozlov, Y.N.; Shul'pin, G.B. Palanquin-Like Cu4Na4 Silsesquioxane Synthesis (via Oxidation of 1,1-bis(Diphenylphosphino)methane), Structure and Catalytic Activity in Alkane or Alcohol Oxidation with Peroxides. Catalysts 2019, 9, 154. https://doi.org/10.3390/catal9020154

AMA Style

Kulakova AN, Khrustalev VN, Zubavichus YV, Shul’pina LS, Shubina ES, Levitsky MM, Ikonnikov NS, Bilyachenko AN, Kozlov YN, Shul'pin GB. Palanquin-Like Cu4Na4 Silsesquioxane Synthesis (via Oxidation of 1,1-bis(Diphenylphosphino)methane), Structure and Catalytic Activity in Alkane or Alcohol Oxidation with Peroxides. Catalysts. 2019; 9(2):154. https://doi.org/10.3390/catal9020154

Chicago/Turabian Style

Kulakova, Alena N., Victor N. Khrustalev, Yan V. Zubavichus, Lidia S. Shul’pina, Elena S. Shubina, Mikhail M. Levitsky, Nikolay S. Ikonnikov, Alexey N. Bilyachenko, Yuriy N. Kozlov, and Georgiy B. Shul'pin. 2019. "Palanquin-Like Cu4Na4 Silsesquioxane Synthesis (via Oxidation of 1,1-bis(Diphenylphosphino)methane), Structure and Catalytic Activity in Alkane or Alcohol Oxidation with Peroxides" Catalysts 9, no. 2: 154. https://doi.org/10.3390/catal9020154

APA Style

Kulakova, A. N., Khrustalev, V. N., Zubavichus, Y. V., Shul’pina, L. S., Shubina, E. S., Levitsky, M. M., Ikonnikov, N. S., Bilyachenko, A. N., Kozlov, Y. N., & Shul'pin, G. B. (2019). Palanquin-Like Cu4Na4 Silsesquioxane Synthesis (via Oxidation of 1,1-bis(Diphenylphosphino)methane), Structure and Catalytic Activity in Alkane or Alcohol Oxidation with Peroxides. Catalysts, 9(2), 154. https://doi.org/10.3390/catal9020154

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