Cobalt-Catalyzed Selective Synthesis of Disiloxanes and

Research Article Cite This: ACS Catal. 2019, 9, 5552−5561

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Cobalt-Catalyzed Selective Synthesis of Disiloxanes and Hydrodisiloxanes Sandip Pattanaik and Chidambaram Gunanathan* School of Chemical Sciences, National Institute of Science Education and Research, HBNI, Bhubaneswar 752050, India

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S Supporting Information *

ABSTRACT: Selective syntheses of symmetrical siloxanes and cyclotetrasiloxanes are attained from reactions of silanes and dihydrosilanes, respectively, with water, and the reactions are catalyzed by a NNNHtBu cobalt(II) pincer complex. Interestingly, when phenylsilane was subjected to catalysis with water, a siloxane cage consisting 12 silicon and 18 oxygen centers was obtained and remarkably the reaction proceeded with liberation of 3 equiv of molecular hydrogen (36 H2) under mild experimental conditions. Upon reaction of silane with different silanols, highly selective and controlled syntheses of higher order monohydrosiloxanes and disiloxymonohydrosilanes were achieved by cobalt catalysis. The liberated molecular hydrogen is the only byproduct observed in all of these transformations. Mechanistic studies indicated that the reactions occur via a homogeneous pathway. Kinetic and independent experiments confirmed the catalytic oxidation of silane to silanol, and further dehydrocoupling processes are involved in syntheses of symmetrical siloxanes, cyclotetrasiloxanes, and siloxane cage compounds, whereas the unsymmetrical monohydrosiloxane syntheses from silanes and silanols proceeded via dehydrogenative coupling reactions. Overall these cobalt-catalyzed oxidative coupling reactions are based on the Si−H, Si−OH, and O−H bond activation of silane, silanol, and water, respectively. Catalytic cycles consisting of Co(II) intermediates are suggested to be operative. KEYWORDS: disiloxanes, silicones, cobalt, hydrogen, catalysis, sustainable chemistry

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INTRODUCTION Siloxanes and hydrosiloxanes are classes of compounds having −Si−O−Si− and −Si−O−SiH− motifs, respectively, and they have displayed widespread applications in organic synthesis and inorganic and material chemistry.1 These silicones display inherent molecular properties such as flexible pressure sensors, liquid crystals, pharmacological activity, high gas permeability, high thermal stability, and low entropy of dilution and are extensively used in aerospace, automotive, paints and coatings, pharmaceuticals, textiles, and leather industries.2,3 In particular, unsymmetrical siloxanes have important applications as liquid crystalline polymers and low-dielectric-constant materials.3 Given the extensive applications of silicones as functional materials, the synthesis of siloxanes directly from readily available compounds without producing chemical waste is very important.4,5 Such a synthesis of siloxanes with predictable size and selectivity by conventional methods is difficult, and potential synthetic methods remain to be explored.6 For example, cyclosiloxanes are building blocks of various siliconbased polymeric materials.3,7 However, the conventional synthesis of cyclosiloxanes involves hydrolysis of chlorosilane and alkoxysilane and self-condensation of siloxane diols and siloxane polyols.1a,7,8 In general, these reactions require high temperature and excess amounts of base and lead to the formation of a number of side products such as silanols, cyclosiloxanes, and oligo- and polysiloxanes.9 © XXXX American Chemical Society

Condensation of silanols with chlorosilanes and alkoxysilane and acid- or base-mediated ring opening of cyclic oligosiloxane methods are employed in classical synthesis to access the higher order siloxanes (Scheme 1a).10 In recent years, limited synthetic methods have been developed toward siloxanes of desired structures.11 Mono- and dihydrosiloxanes (siloxanes containing Si−H and SiH2 functionalities) are useful building blocks for silicone polymer, polysiloxane materials, as they assimilate the siloxane motifs to various chemical compounds upon activation of Si−H bond.12−14 Catalytic oxidation of monohydrosilanes with water to produce silanols and siloxanes is attained by transition metals such as ruthenium, palladium, rhodium, iridium, and gold.15 These investigations have in general not been applied to the dihydro- and trihydrosilanes (vide infra) due to the complexity of the reactions leading to the formation of a mixture of products.16 Currently such studies are limited to two reports in the literature in which rhodium- and gold-catalyzed dehydrogenative coupling of silanols with dihydro- and trihydrosilanes resulted in selective synthesis of mono- and dihydrosiloxanes, respectively (Scheme 1b).17 Received: January 22, 2019 Revised: May 7, 2019

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DOI: 10.1021/acscatal.9b00305 ACS Catal. 2019, 9, 5552−5561

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ACS Catalysis

Scheme 2. Synthesis and Solid-State Structure of Catalyst 1a

Scheme 1. Traditional Approach and Recent Advances in Catalytic Synthesis of Siloxanes

a

Ellipsoids are drawn at the 50% probability level.

Reactions of Hydrosilanes with H2O. The reaction of triethylsilane with water was investigated using complex 1 as a catalyst. Initial experiments indicated that catalytic coupling leads to the formation of symmetrical disiloxanes. The reaction conditions have been further optimized, and selected reactions are presented in Table 1. Heating a dioxane solution containing 2 mol % of catalyst 1, triethylsilane (1 mmol), and water at 60 °C resulted in 92% conversion of the silane to provide disiloxane 2a in 54% yield. The reaction of catalyst 1 with silane under neutral conditions is remarkable; however, 36% triethylsilanol (conversion based on GC analysis) was also

The sustainable development of synthetic methods catalyzed by naturally abundant, low-toxicity, and cheap base-metal complexes is becoming one of the principal objectives in homogeneous catalysis.18 Currently, the base-metal-catalyzed synthesis of siloxanes remains underexplored. We have developed highly efficient and selective hydroboration reactions19 and hydrosilylation of aldehydes20 in which we have uncovered the involvement of hitherto unknown intermediates. Further, we have developed the highly selective synthesis of borasiloxanes directly from borane, silanes, and water, catalyzed by simple ruthenium catalysts.21 In a continuation of these endeavors with hydroelementation processes, herein we report a new and simple NNNHtBu cobalt pincer complex catalyzed selective synthesis of symmetrical disiloxanes, cyclosiloxanes, and cagelike siloxanes from silanes and water, and unsymmetrical monohydrodisiloxanes from silanes and silanols (Scheme 1c).

Table 1. Optimization of Reaction Conditions for CobaltCatalyzed Siloxane Synthesis

entry

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1d 2 3 4 5e 6 7 8 9

RESULTS AND DISCUSSION Synthesis of Co Catalyst. The design and development of new catalysts that are stable to air and moisture and readily accessible from simple synthetic preparations remain highly attractive. Thus, addition of the ligand tBuH‑NNN (N,N′(pyridine-2,6-diylbis(methylene))bis(2-methylpropan-2amine)) to CoBr2 in ethanol at room temperature resulted in formation of the new pincer complex 1 in 85% yield. The paramagnetic complex 1 has been fully characterized; its 1H NMR spectrum displays a broad singlet at δ 9.53 ppm corresponding to two NH protons. A broad peak at 3207 cm−1 in the IR spectrum of 1 also confirmed the presence of NH functionalities. Further, a DCM/toluene solution of 1 provided single crystals suitable for X-ray analysis, which established a distorted-square-pyramidal geometry around the cobalt center (Scheme 2).

Cat. (mol %)

base (mol %)

conversion (%)b

yield (%)c

1 1 1 1 1

2 1 1.5 2 2

0 2 3 4 4

CoBr2 CoBr2

2 2

4 0 4

92 49 68 96 89 0 0 0 13

54 41 53 94 63 0 0 0 13

Cat.

a Reaction conditions unless specified otherwise: triethylsilane (1 mmol), degassed water (1 mmol), catalyst 1, base, and dioxane (2 mL) were placed in a sealed tube and heated at 60 °C under closed conditions. bConversion was determined by gas chromatography using dodecane as an internal standard. cIsolated yield after column chromatography. d36% of triethylsilanol also formed (GC). eReaction performed at room temperature; as per GC analysis, 22% of triethylsilanol was also formed as a side product.

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ACS Catalysis present in the reaction mixture, indicating incomplete reactions (entry 1, Table 1). Further, a similar catalytic reaction using 1 mol % of catalyst 1 and 2 mol % of KOtBu resulted in 49% conversion of the silane to disiloxane 2a and H2 (entry 2, Table 1). Increasing the catalyst load to 1.5 mol % provided moderate conversion and yield (entry 3, Table 1). Upon using 2 mol % of catalyst 1 and 4 mol % of base, 96% conversion of silane occurred to provide the siloxane 2a in 94% yield in 2 h (entry 4, Table 1). Performing a similar reaction at room temperature resulted in diminished conversion of silane (89%) and product formation (63%, entry 5, Table 1); The presence of the corresponding silanol (22%) was also found in the GC analysis. Further, control experiments without catalyst 1 and base and with base alone failed to provide any siloxanes (entries 6 and 7, Table 1), confirming that the catalyst is essential for the siloxane synthesis from silane and water. Moreover, catalytic experiments using simple CoBr2 (2 mol %) provided no reaction under neutral conditions; however, when CoBr2 was used together with base, only 13% conversion of silane to siloxane 2a was observed (entries 8 and 9, Table 1). These results clearly reflect the efficient reactivity of pincer catalyst 1 in the oxidative coupling of silanes with water. Following the optimized reaction conditions, alkyl, aryl, and bulky silanes were subjected to synthesis of symmetrical disiloxanes, which provided the products in very good yields (Table 2). When trioctylsilane was subjected to catalysis,

Scheme 3. Cobalt-Catalyzed Oxidative Dehydrocoupling of Dihydrosilanes to Siloxane Macrocycles

tetrasiloxane 3b, which was isolated in 96% yield, and the formation of 2 equiv of hydrogen was quantified (see the Supporting Information). Single-crystal X-ray analysis clearly confirmed the cyclic structure of 3b (see the Supporting Information). Dehydrogenative oxidation of dihydrosilane with water leads to formation of a silanediol intermediate, which further undergoes cyclocondensation to provide the observed cyclotetrasiloxanes. Alternatively, formation of cyclosiloxanes may also directly occur from the dehydrogenative self-coupling of a monohydrosilanol intermediate. An appropriate molecular geometry also plays an important role in the formation of these products. When naphthylphenyldihydrosilane was reacted with water, formation of an unidentified mixture of products was observed, perhaps due to the increased steric hindrance. When phenylsilane reacted with water under optimized conditions using catalyst 1, the siloxane compound 4 was obtained in 69% yield (Scheme 4). Interestingly, single-crystal X-ray analysis revealed that compound 4 is a three-dimensional highly ordered siloxane cage. Notably, compound 4 is commercially available, obtained from a tedious multistep procedure,22 and its X-ray structure has also been reported.23 In addition to the formation of product 4, significant liberation of hydrogen was also observed. Further, to confirm the amount of molecular hydrogen liberated from this reaction, we have rerun the experiment three times using 0.5 mmol of phenylsilane with water and collected the hydrogen produced from the reaction using a graduated buret. An average of 37 mL of hydrogen gas (from three experiments) was collected in the buret, which corresponds to 1.4 mmol against the theoretically expected 1.5 mmol of molecular hydrogen. This observation indicates that 3 equiv of molecular hydrogen is liberated from the reaction. Overall the cobalt-catalyzed reaction of phenylsilane with water provided a simple and direct access to siloxane cage 4 with liberation of 3 equiv of molecular hydrogen (36H2). Presumably, as in the formation of cyclotetrasiloxane 3, dehydrogenative oxidation of phenylsilane with water produced the phenyltrihydroxysilane intermediate that underwent a self-condensation reaction leading to the formation of 4.

Table 2. Cobalt-Catalyzed Synthesis of Siloxanes from Silane and Water

trialkylsiloxane 2b was obtained in 86% yield. Dimethylphenylsilane reacted with water to provide the dimethylphenylsiloxane 2c in quantitative yield. When triphenylsilane was employed in the reaction, 1,1,1,3,3,3-hexaphenyldisiloxane 2d was isolated in 89% yield. Notably when sterically hindered tris(trimethylsilyl)silane was employed in hydrolytic oxidation, it provided the corresponding siloxane product 2e in 92% yield. These results indicate the ability of the cobalt metal center to coordinate and catalyze the bond activations of sterically hindered silanes in the coordination sphere. Next, cobalt-catalyzed oxidative dehydrocoupling of dihydrosilanes with water was investigated. When diethylsilane was reacted, quantitative conversion was observed and the cyclotetrasiloxane 3a was isolated in 97% yield (Scheme 3). Diphenylsilane also provided the similar octaphenyl cyclo5554


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the anticipated product but resulted in formation of 5m. Catalytic coupling experiments involving the isolated monohydrosiloxane 5a and monohydrodisiloxane 5m with dimethylphenylsilanol failed to provide the desired products, perhaps due to the higher steric hindrance of siloxanes. Preliminary Mechanistic Studies. A series of experiments was performed to decipher the mechanistic insight of this interesting catalysis by cobalt complex 1. The catalytic reaction of diphenylsilane with water in the presence of mercury provided the corresponding cyclosiloxane 3b in 87% yield (against the 96% yield in the absence of mercury; see Scheme 3), indicating that the reaction proceeds with molecular intermediates (Scheme 5a). Reaction of complex 1 with KOtBu leads to the formation of complex 6. Although the EPR spectra of 1 and 6 were similar, as both complexes contain Co(II) metal centers, the IR spectrum of complex 6 confirmed the disappearance of the NH peak (which appeared at 3207 cm−1 for 1). Similarly, the 1H NMR spectrum of 6 also confirmed the disappearance of the NH signal that appeared for 1 at 9.53 ppm (Scheme 5b). Unfortunately, repeated attempts to get the single-crystal X-ray structure of the threecoordinated complex 6 remain unsuccessful. When isolated 6 was employed as a catalyst, the formation of siloxanes 2a,d with enhanced yields was observed (see Table 2). Remarkably, the use of base was no longer required and the activated catalyst 6 alone catalyzed these reactions under neutral conditions (Scheme 5c,d), which indicates that the role of base is debromination and deprotonation of 1. Further, reaction of diphenylsilanediol and dimethylphenylsilanol with catalyst 1 at room temperature provided the corresponding siloxanes in poor yields. However, the same reactions at 60 °C provided the corresponding siloxanes 3b and 2c in quantitative yields, confirming that the reactions are occurring via silanol intermediacy and reiterating the importance of optimized experimental conditions (Scheme 5e,f). Further, dehydrogenative coupling of silanaol with triethylsilane was performed both at room temperature and at 60 °C. While less than 2% of siloxane 2a formed at room temperature, the same product 2a was obtained quantitatively at elevated temperature. Apparently, the reaction proceeded via self-coupling of triethylsilanol, as the triethylsilane remained unreacted (GC analysis). These results clearly confirm that siloxane formation is sensitive to the steric hindrance of monohydrosilanes and proceeds via the dehydrative coupling of silanol intermediates (Scheme 5g). As performed in the synthesis of unsymmetrical siloxane in Table 3, when triethylsilanol reacted with dimethylphenylsilanol using the catalyst 1 at room temperature, no product formation was observed and both silanols remained unreacted, which proves the formation of unsymmetrical siloxanes 5 is proceeding via the dehydrogenative coupling of silane and silanol (Scheme 5h). When the same reaction was performed at 60 °C, only self-coupling of silanols resulted in the formation of symmetrical disiloxanes and the products 2a,c were obtained in 59% and 78% yields, respectively. Although we were unable to ascertain the reasons, no cross-coupling of silanols occurred and the unsymmetrical siloxane was not observed (GC analysis, Scheme 5i). Attempted thermal condensation of triethylsilanol with dimethylphenylsilanol even at 120 °C failed to provide any product (Scheme 5j). Thus, the formation of siloxanes is essentially a catalytic process. GC analysis of the gas phase from the catalytic reaction of triethylsilanol with diphenylsilane confirmed the

Scheme 4. Cobalt-Catalyzed Dehydrogenative and Dehydrocoupling of Phenylsilane with Water

Reactions of Silanols with Hydrosilanes. In an attempt to develop a highly controlled and selective synthesis of higher order hydrosiloxane compounds, further we have examined the cross-coupling of silanol with dihydro- and trihydrosilanes. When dihydrosilanes reacted with silanol in the presence of catalyst 1, they underwent dehydrogenative coupling reactions with silanol and provided monohydrosiloxanes. Moreover, upon reactions of a trihydrosilane with different silanols, double dehydrogenative coupling occurred, leading to the exclusive formation of monohydrosiloxanes. When triethylsilanol reacted with diphenyl- and methylphenylsilane, the corresponding siloxanes 5a,b were obtained in 82% and 88% yields, respectively (Table 3). Similarly tripropylsilanol, triisopropylsilanol, and triisobutylsilanol reacted with various dihydrosilanes and provided the corresponding monohydrosiloxane products 5c−g in very good yields. Further, when sterically hindered silanols such as tris(trimethylsilyl)silanol and dimethyloctadecylsilanol were employed in cobalt-catalyzed dehydrogenative coupling, the corresponding monohydrosiloxane products 5h−j were obtained in good yields. When an aromatic silanol such as dimethylphenylsilanol was employed in the reaction, the corresponding cross-coupling products 5k,l were isolated in 80% and 82% yields. Further, when phenylsilane reacted with an assortment of silanols, the corresponding double-crosscoupling products 5m−p were obtained. Notably, when CoBr2 was used as a catalyst in the reaction of triethylsilanol with diphenylsilane and phenylsilane, no formation of the corresponding products 5a,m was observed (Table 3, entries 2 and 15), reiterating the importance of the cobalt pincer catalyst. The attempted catalytic synthesis of dihydromonosiloxane from phenylsilane and tritheylsilanol did not provide 5555


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ACS Catalysis Table 3. Cobalt-Catalyzed Cross-Coupling Reaction of Silanol with Dihydro- and Trihydrosilanesa

a

Reaction conditions unless specified otherwise: silane (1 mmol), silanol (1 mmol), catalyst 1 (2 mol %), and base (4 mol %) were stirred at room temperature. bYield corresponds to isolated product. cReaction performed using CoBr2 (2 mol %) as a catalyst. d2 mmol of silanol was used.

Further, in an attempt to interrupt catalysis, methanol was added to the catalytic reaction of phenylsilane with triethylsilanol, in which the predominant formation (71%) of

presence of molecular hydrogen (Scheme 5k), which clearly indicates that the formation of unsymmetrical monohydrosiloxanes occurs from the dehydrogenative coupling. 5556


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the desired monohydrosiloxane product 5m was observed (Scheme 5l) together with a minor amount of dimethoxyphenylsilane (7, 13%, GC analysis). However, upon reaction of phenylsilane with methanol under standard catalysis, 7 was obtained in 99% yield (Scheme 5m). These observations indicate that dehydrogenative coupling of silane with silanol is much more favored under catalytic conditions than coupling of silane and alcohol. Further, addition of styrene and aryl halide to catalysis was tested, which failed to interrupt the catalytic process. The competition experiment involving the reaction of diphenylsilane and phenylsilane with triethylsilanol resulted in monohydrodisiloxane 5m and monohydrosiloxane 5a in 76% and 11% yields (GC analysis), respectively (Scheme 5n), indicating that less bulky silanes are preferably consumed over the sterically hindered diarylsilanes. EPR analyses of catalyst 1 and intermediate 6 revealed identical spectral lines (Figure 1, also see the Supporting Information), indicating the presence of similar Co(II) metal centers in both complexes.

Scheme 5. Mechanistic Studies on the Catalytic Synthesis of Disiloxanes and Monohydrodisiloxanes

Figure 1. Overlaid EPR spectra of 1 and 6.

The reaction progress for the symmetrical and unsymmetrical siloxane processes was monitored using GC. The formation of a diphenylsilanediol intermediate was transient, and it transformed to the product upon a condensation reaction (red line in Figure 2a(i)). The decreasing concentration of diphenylsilane (black line in Figure 2a(i)) authenticates the increasing concentration of product 3b (Figure 2a(ii)). The reaction progress for the formation of unsymmetrical siloxane 5a is shown in Figure 2b. Although more data are required, a plausible mechanism for the synthesis of symmetrical disiloxanes using monohydrosilane and dihydrosilane with water catalyzed by 1 is presented in Scheme 6. Catalyst 1 undergoes debromination and deprotonation upon reaction with base to provide coordinatively unsaturated intermediate 6. Coordination of silane to unsaturated complex 6 provides the intermediate I. Nucleophilic attack by water on the silane coordinated to cobalt generates II. The Si−H and O−H bond activation at the metal center leads to the formation of dihydrogen-coordinated III and silanol. Reaction of silanol with intermediate III liberates the coordinated molecular hydrogen and provides silanolligated intermediate IV.24 Free silanol reacts with metalcoordinated silanol ligand, resulting in condensation, perhaps occurring via Si−O bond metathesis and leading to the formation of disiloxane-ligated V.25 Further dissociation of disiloxanes from intermediate V regenerates 6 and completes 5557


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Scheme 6. Proposed Mechanism for the Cobalt-Catalyzed Synthesis of Symmetrical Siloxanes

these catalytic processes. Similar amine−amide metal−ligand cooperation is operative in catalysis by the Ru-MACHO pincer catalyst.27 Similarly, a reaction mechanism for cobalt pincer complex catalyzed synthesis of unsymmetrical siloxanes from silane and silanol is proposed in Scheme 7. The in situ generated active intermediate 6 reacts with dihydrosilane, resulting in formation Scheme 7. Proposed Catalytic Cycle for Cross-Coupling of Silane with Silanol

Figure 2. Monitoring the reaction progress of (a) symmetrical cyclotetrasiloxane 3b with (i) conversion of diphenylsilane and formation of a diphenylsilanediol intermediate and (ii) formation of product 3b and of (b) unsymmetrical siloxane 5a.

the catalytic cycle. Alternatively, bond activation involving amine−amide metal−ligand cooperation26 may also operate in 5558


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Symposium Series 729. (c) Zeigler, J. M.; Fearon, F. W. G. SiliconBased Polymer Science; American Chemical Society: Washington, DC, 1989; Advances in Chemistry 224. (d) Kamino, B. A.; Bender, T. P. The Use of Siloxanes, Silsesquioxanes, and Silicones in Organic Semiconducting Materials. Chem. Soc. Rev. 2013, 42, 5119−5130. (e) Longenberger, T. B.; Ryan, K. M.; Bender, W. Y.; Krumpfer, A.K.; Krumpfer, J. W. The Art of Silicones: Bringing Siloxane Chemistry to the Undergraduate Curriculum. J. Chem. Educ. 2017, 94, 1682− 1690. (2) (a) O’Lenick, A. J., Jr. Silicones for Personal Care, 2nd ed.; Allured Publishing: Carol Stream, IL, 2008. (b) Advances in Silicones and Silicone-Modified Materials; Clarson, S. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds.; American Chemical Society: Washington, DC, 2010. (c) Schwartz, G.; Tee, B. C.-K.; Mei, J.; Appleton, A. L.; Kim, D. H.; Wang, H.; Bao, Z. Flexible Polymer Transistors with High Pressure Sensitivity for Application in Electronic Skin and Health Monitoring. Nat. Commun. 2013, 4, 1859. (3) (a) Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications; Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer Academic: London, 2000. (b) Progress in Organosilicon Chemistry, Marciniec, B., Chojnowski, J., Eds.; Gordon and Breach: Amsterdam, 1995. (4) For a review, see: Purkayastha, A.; Baruah, J. B. Synthetic methodologies in siloxanes. Appl. Organomet. Chem. 2004, 18, 166− 175. (5) (a) Wakabayashi, R.; Kawahara, K.; Kuroda, K. Nonhydrolytic Synthesis of Branched Alkoxysiloxane Oligomers Si[OSiH(OR)2]4 (R=Me, Et). Angew. Chem., Int. Ed. 2010, 49, 5273−5277. (b) Sakamoto, S.; Shimojima, A.; Miyasaka, K.; Ruan, J.; Terasaki, O.; Kuroda, K. Formation of Two- and Three-Dimensional Hybrid Mesostructures from Branched Siloxane Molecules. J. Am. Chem. Soc. 2009, 131, 9634−9635. (c) Hagiwara, Y.; Shimojima, A.; Kuroda, K. Alkoxysilylated-Derivatives of Double-Four-Ring Silicate as Novel Building Blocks of Silica-Based Materials. Chem. Mater. 2008, 20, 1147−1153. (d) Suzuki, J.; Shimojima, A.; Fujimoto, Y.; Kuroda, K. Stable Silanetriols That Contain tert-Alkoxy Groups: Versatile Precursors of Siloxane-Based Nanomaterials. Chem. - Eur. J. 2008, 14, 973−980. (e) Shimojima, A.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Kuroda, K. Self-Assembly of Designed Oligomeric Siloxanes with Alkyl Chains into Silica-Based Hybrid Mesostructures. J. Am. Chem. Soc. 2005, 127, 14108−14116. (f) Shimojima, A.; Kuroda, K. Direct Formation of Mesostructured Silica-Based Hybrids from Novel Siloxane Oligomers with Long Alkyl Chains. Angew. Chem., Int. Ed. 2003, 42, 4057−4060. (6) Yoshikawa, M.; Tamura, Y.; Wakabayashi, R.; Tamai, M.; Shimojima, A.; Kuroda, K. Protecting and Leaving Functions of Trimethylsilyl Groups in Trimethylsilylated Silicates for the Synthesis of Alkoxysiloxane Oligomers. Angew. Chem., Int. Ed. 2017, 56, 13990− 13994. (7) Murugavel, R.; Voigt, A.; Walawalkar, M. G.; Roesky, H. W. Hetero- and Metallasiloxanes Derived from Silanediols, Disilanols, Silanetriols, and Trisilanols. Chem. Rev. 1996, 96, 2205−2236. (8) For recent examples, see: (a) Yoshikawa, M.; Shiba, H.; Kanezashi, M.; Wada, H.; Shimojima, A.; Tsuru, T.; Kuroda, K. Synthesis of a 12-Membered Cyclic Siloxane Possessing Alkoxysilyl Groups as a Nanobuilding Block and its Use for Preparation of Gas Permeable Membranes. RSC Adv. 2017, 7, 48683−48691. (b) Cho, H. M.; Lee, J.-E.; Lee, M. E.; Lee, K. M. Synthesis of 6-, 7- and 8Membered Cyclosiloxanes having Multifunctional Groups. J. Organomet. Chem. 2011, 696, 2754−2757. (9) Mcgrath, J. E.; Riffle, J. S.; Banthia, A. K.; Yilgor, I.; Wilkes, G. I. Overview of the Polymerization of Cyclosiloxanes. In Initiation of Polymerization; American Chemical Society: Washington, DC, 1983; ACS Symposium Series 212, pp 145−172. (10) Brook, M. A. Silicon. In Organic, Organometallic, and Polymer Chemistry; Wiley: New York, 2000; pp 256−308. (11) (a) Igarashi, M.; Kubo, K.; Matsumoto, T.; Sato, K.; Ando, W.; Shimada, S. Pd/C-catalyzed Cross-coupling Reaction of Benzyloxysilanes with Halosilanes for Selective Synthesis of Unsymmetrical

of coordinated intermediate VI, to which silanol undergoes nucleophilic attack leading to the formation of VII and subsequent bond metathesis resulting in monohydrosiloxanes and III. Liberation of dihydrogen coordinated to Co in III regenerates 6 to complete one loop on a catalytic cycle. However, the involvement of other mechanistic pathways cannot be ruled out.

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CONCLUSION In summary, an air-stable and easily accessed cobalt pincer complex was prepared. This simple NNN cobalt pincer complex catalyzed highly facile and efficient processes for the synthesis of symmetrical siloxanes and cyclotetrasiloxanes from silanes and disilanes, respectively, using water. The liberated molecular hydrogen is the only byproduct in these interesting transformations. Reaction of a trihydrosilane with water resulted in formation of a siloxane cage in which formation of 3 equiv of dihydrogen was observed, indicating that it can be a potentially useful transformation in hydrogen storage. Moreover, when the dihydrosilanes and trihydrosilanes were subjected to catalysis with silanol at room temperature, the higher order monohydrosiloxanes and disiloxymonohydrosilanes were obtained in very good yields and the reactions proceeded with liberation of molecular hydrogen. The mechanistic studies indicated that the reactions involve molecular intermediates. Accordingly, catalytic cycles for the synthesis of siloxanes and unsymmetrical higher order silanes are proposed, incorporating Co(II) intermediates. Bond activation reactions such as Si−H, −O−H of water, and Si− OH bonds are operative in catalysis, which provided the dehydrative silanol condensation and dehydrogenative coupling of silanes and silanols.

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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/acscatal.9b00305. Experimental procedures, spectral data, and 1H NMR, 13 C NMR, and 29Si NMR spectra of the products (PDF) Single-crystal X-ray data for complex 1 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C.G.: [email protected]. ORCID

Chidambaram Gunanathan: 0000-0002-9458-5198 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the SERB New Delhi (EMR/2016/002517), DAE, and NISER for financial support. C.G. thanks Prof. Herbert W. Roesky and Prof. Pradyut Ghosh for helpful discussions. S.P. thanks the CSIR for a research Fellowship. We thank Prof. J. V. Yeldho and Ashish Kar for their kind help.

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REFERENCES

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