Serendipitous isolation of [Ag(PPh3)Cl]4 and its catalytic reactivity as a bimetallic partner to SnCl2

Article abstract of DOI:10.1016/j.ica.2018.10.027

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 C–H activation.

Full text of DOI:10.1016/j.ica.2018.10.027

InorganicaChimicaActa486(2019)101–103
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Serendipitous isolation of [Ag(PPh3)Cl]4 and its catalytic reactivity as a
bimetallic partner to SnCl2
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
A R T I C L E I N F O
A B S T R A C T
Keywords:
Silver(I)
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.
Tin(II)
Bimetallic catalysis
Cluster
1. Introduction
a hydroxo derivative (L = PPh3) from the reaction of AgL3Cl with SnCl2
in dry dichloromethane and in methanol respectively. The complexes
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 com-
pared 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
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).
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.
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 distor-
tion in the cubic structure. The AgeP bond lengths varies from
2. Results and discussion
In 1966, Dilts and Johnson reported the synthesis of M-Sn com-
plexes 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
⁎
Corresponding author.
E-mail address: sroy@iitbbs.ac.in (S. Roy).
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
Availableonline19October2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

P. Mukhi et al.
InorganicaChimicaActa486(2019)101–103
Table 1
Catalyst screening for the synthesis of 5.
Sl. No.
Catalyst
Isolated Yield% of 5
1
2
3
4
5
Ag(PPh3)3Cl 1
SnCl2
0
10
25
0
2-SnCl2
2
Scheme 1. Formation of [Ag(PPh3)Cl]4 Complex.
1-SnCl2
0
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).
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-het-
eroatom 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 re-
actions 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 de-
insertion reaction. The complex 2 can be considered to be a formal 14-
electron 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 un-
derstand 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
0
SnCl2
2-SnCl2
2
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 solu-
tion as detecting agent. After 24 h, the reaction mixture was extracted
with ethyl acetate and concentrated in vacuum. The mixture was pur-
ified by column chromatography on silica gel using ethyl acetate–pe-
troleum 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 di-
chloromethane (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 mix-
ture 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.
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.
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 un-
disturbed. White crystals of 2 were obtained after three days. Yield
Acknowledgement
We thank the Institute and the Indian Academy of Sciences for fi-
nancial support.
Scheme 2. Synthesis of Bis(indolyl) phenylethane (5).
102

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InorganicaChimicaActa486(2019)101–103
Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://
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