Inorganica Chimica Acta 486 (2019) 101–103
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Research paper
Serendipitous isolation of [Ag(PPh3)Cl]4 and its catalytic reactivity as a bimetallic partner to SnCl2
T
Priyabrata Mukhi, Anuradha Mohanty, Rananmay Bhardwaj, Mukesh Kumar Nayak, ⁎ Cherukuthota Sri Vidya, Sujit Roy Organometallics and Catalysis Laboratory, School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Argul, Khurda 752050, Odisha, India
ARTICLE INFO
ABSTRACT
Keywords: Silver(I) Tin(II) Bimetallic catalysis Cluster
The reaction of [Ag(PPh3)3Cl] 1 with SnCl2 followed by recrystallization of the product led to the isolation of tetrameric chloro(triphenylphosphine)silver(I) [Ag4(PPh3)4Cl4] 2. The dual reagent combination of 2-SnCl2 show moderate catalytic activity in multicomponent coupling reactions involving CeH activation.
1. Introduction The advent of “dual reagent/bimetallic catalysts” has added new impetus in the field of homogeneous catalysis [1]. The advantage of such designer catalysts lies in their capability to activate two or more different organic substrates simultaneously and mobilize them to react with each other resulting in increased efficiency and selectivity compared to their monometallic counterparts [2]. In the past two decades our group has been exploring the synthesis and catalytic activity of transition metal-tin (hereafter M-Sn) bimetallic catalysts for a wide variety of organic transformations [3–7]. Though Ag(I) complexes are active catalysts for various organic transformations, bimetallic catalysts based on Ag(I) has been less studied [8,9]. During the attempted synthesis of Ag(I)-Sn(II) bimetallic catalyst we serendipitously isolated a novel complex Ag4Cl4(PPh3)4 2 with a distorted cubic core ‘Ag4Cl4(PPh3)4’ which is confirmed by X-ray crystal structure. The complex has been formed from the reaction of SnCl2 with Ag(PPh3)3Cl in dichloromethane at room temperature. The dual reagent combine of 2 and SnCl2 showed moderate catalytic activity in CeC bond forming reaction. 2. Results and discussion In 1966, Dilts and Johnson reported the synthesis of M-Sn complexes belonging to group 11 by the insertion of SnCl2 across the MeCl bond [10]. We have earlier shown similar insertion to RheCl and IreCl bond [11]. A reinvestigation of the insertion chemistry across AgeCl was carried out by Smith and co-workers, who isolated L3Ag-SnCl3 and
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a hydroxo derivative (L = PPh3) from the reaction of AgL3Cl with SnCl2 in dry dichloromethane and in methanol respectively. The complexes were diagnosed by NMR and Mossbauer spectroscopy. In an attempt to obtain the said Ag-Sn complex, we reacted Ag (PPh3)3Cl (1 eqv.) with SnCl2 (1 eqv.) in dry dichloromethane in an inert atmosphere which resulted in a clear solution within 2 h from which a white solid 1 (m.p. 194–196 °C) was isolated. To recrystallize the product, the white solid was again dissolved in dichloromethane and layered with either acetone or hexane. Copious amount of colorless crystals were obtained which showed m.p. 212–214 °C. The crystal data confirmed the formation of a complex 2 with the formula Ag4Cl4(PPh3)4. It consists of four Ag(PPh3)Cl units forming a distorted cubic structure [12]. The formation of 2 is surprising. However it can be rationalized by invoking a de-insertion mechanism as shown in Scheme 1. The facile de-insertion at room temperature could be a result of the high cone angle of phosphine ligand and the propensity of Sn(II) to act as a Lewis acid and thereby abstracting the phosphine ligand. This is evidenced by NMR monitoring (δ31P values for [Ag(PPh3)Cl]4 and SnCl2(PPh3)2 are observed at 5.9 and 0.8 respectively). The crystallographic data of [Ag4Cl4(PPh3)4] is in line with the previous report [12]. The compound crystallizes in orthorhombic space group Pbcn with a, b, c values 17.938(3), 20.795(3) and 18.322(3) respectively. The Ag atom has a distorted tetrahedral shape with one PPh3 unit and three Cl atoms in its surroundings. The four tetrahedral units form a distorted cubic structure. The AgeCl bond distance ranges from 2.5328(13) Å to 2.7633(14) Å which implies a significant distortion in the cubic structure. The AgeP bond lengths varies from
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https://doi.org/10.1016/j.ica.2018.10.027 Received 29 August 2018; Received in revised form 17 October 2018; Accepted 17 October 2018 Available online 19 October 2018 0020-1693/ © 2018 Elsevier B.V. All rights reserved.
Inorganica Chimica Acta 486 (2019) 101–103
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Table 1 Catalyst screening for the synthesis of 5.
Scheme 1. Formation of [Ag(PPh3)Cl]4 Complex.
2.3784(14) Å to 2.3925(13) Å which is close to covalent radii of AgeP. The PeC bond distance varies from 1.819(5) Å to 1.821(5) Å with an average value of 1.820 Å which is close to covalent radii of PeC bond (1.83 Å).
Sl. No.
Catalyst
Isolated Yield% of 5
1 2 3 4 5
Ag(PPh3)3Cl 1 SnCl2 2-SnCl2 2 1-SnCl2
0 10 25 0 0
3. Studies on the catalytic activity of 2-SnCl2 as dual reagent As mentioned before, our research group has demonstrated the unique reactivity of catalysts bearing [TM-Sn] motif, which are capable of activating electrophiles towards carbon–carbon and carbon-heteroatom bond formation [3]. In line with the same, the catalytic activity of the as synthesized complex [Ag(PPh3)Cl]4 2 was briefly tested in the presence and absence of SnCl2 for the multi-component coupling reactions as shown in Scheme 1 and 2. Under this condition, the reaction between acetophenone 3 and indole 4 gave rise to the novel coupling product 5 in 25% isolated yield (Scheme 2). Surprisingly we found that individually complex 1 and combination of 1 with SnCl2 were fully inactive whereas SnCl2 was poorly active; suggesting the requirement of co-ordination at the “silver site” (Table 1). A similar reaction between phenyl acetylene 6, piperidine 7 and benzaldehyde 8 provided the coupling product 9 in 40% isolated yield (Scheme 3, Table 2). Note that both the coupling reactions involve novel CeH activation. Moreover under similar conditions 2 does not show any catalytic activity whereas SnCl2 alone show poor catalytic activity. This further highlights the bimetallic reactivity offered by 2-SnCl2 as dual reagent catalyst. In summary, we demonstrated here the serendipitous isolation of a silver(I) complex 2 from the parent ‘Ag-Sn’ complex, via a probable deinsertion reaction. The complex 2 can be considered to be a formal 14electron complex having the formula Ag(PPh3)Cl. The dual reagent combination of 2-SnCl2 proved to be a catalyst for carbonecarbon bond formation involving CeH activation. Further work is underway to understand the initial bond activation steps and to improve the catalyst activity by tuning the peripheral ligand and the reaction condition.
Scheme 3. Synthesis of 1-(1,3-diphenyl-2-propyn-1-yl)piperidine 9. Table 2 Catalyst screening for the synthesis of 9. Sl. No.
Catalyst
Isolated Yield% of 9
1 2 3 4
1 SnCl2 2-SnCl2 2
0 20 40 0
40%. mp. 212–214 °C. A single crystal was carefully mounted in single crystal XRD instrument under N2 flow and data was recorded. Solving the single crystal XRD data we obtained the structure of “Ag4Cl4” cubic core. 4.2. Synthesis of Bis(indolyl) phenylethane (5) [Ag(PPh3)Cl]4 (20.3 mg, 0.0123 mmol) was taken in schlenk tube under argon atmosphere and dry DCM (5 mL) was added to it. To the above solution anhydrous tin (II) chloride (9.4 mg, 0.05 mmol) was added. The mixture was then stirred at ambient temperature. Indole 4 (117 mg, 1 mmol) was added to the reaction mixture followed by the addition of acetophenone 3 (120 mg, 1 mmol). The reaction mixture was stirred at 25 °C for 24 h. Progress of the reaction was monitored by TLC with ethylacetate:hexane solvent system (1:4) and charring solution as detecting agent. After 24 h, the reaction mixture was extracted with ethyl acetate and concentrated in vacuum. The mixture was purified by column chromatography on silica gel using ethyl acetate–petroleum ether (1.5:8.5) to afford the pure bis(indolyl)phenylethane 5.
4. Experimental 4.1. Synthesis of [Ag(PPh3)Cl]4 2 In a schlenk tube 1 (100 mg, 0.11 mmol) was taken and dichloromethane (10 mL) was added to it under argon atmosphere. Anhydrous tin (II) chloride (20.24 mg, 0.11 mmol) was added to the above solution and stirred at ambient temperature for 2 h. Acetone was added to the above clear solution till it became milky white. The mixture was concentrated using rotary evaporator which yielded a white solid. The white solid was filtered, washed with hexane and dried in vacuum. Yield (80 mg, 67%). 31P NMR (162 MHz, CDCl3) δ 5.9(s). mp. 194–196 °C. The white solid obtained above was dissolved in dichloromethane. The dichloromethane solution was taken in two vials, in which layering was done with acetone or hexane. The vials were closed and left undisturbed. White crystals of 2 were obtained after three days. Yield
4.3. Synthesis of 1-(1,3-diphenyl-2-propyn-1-yl)piperidine 9 [Ag(PPh3)Cl]4 2 (20.3 mg, 0.0123 mmol) was taken in schlenk tube and toluene (2 mL) was added to it. To the above solution anhydrous tin (II) chloride (9.4 mg, 0.05 mmol) was added. Benzaldehyde 8 (106 mg, 1 mmol) was added to the reaction mixture followed by addition of phenylacetylene 6 (102 mg, 1 mmol) and piperidine 7 (85.15 mg, 1 mmol). The reaction mixture was stirred at 100 °C for 24 h. Progress of the reaction was monitored by TLC with ethylacetate:petroleum ether (1:20 v/v) . The reaction mixture was extracted with ethylacetate and was concentrated in vacuum. The mixture was purified by column chromatography on silica gel using ethylacetate- petroleum ether (1.5: 100) to afford the pure 1-(1,3-diphenyl-2-propyn-1-yl)piperidine 9. Acknowledgement We thank the Institute and the Indian Academy of Sciences for financial support.
Scheme 2. Synthesis of Bis(indolyl) phenylethane (5). 102
Inorganica Chimica Acta 486 (2019) 101–103
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Appendix A. Supplementary data
[4] [5] [6] [7]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2018.10.027.
[8] [9] [10] [11]
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