Rapid and Efficient Collection of Platinum from Karstedt's Catalyst

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Rapid and Efficient Collection of Platinum from Karstedt’s Catalyst Solution via Ligands-Exchange Induced Assembly Gonghua Yang, Yanlong Wei, Zhenzhu Huang, Jiwen Hu, Guojun Liu, Ming Ou, Shudong Lin, and Yuanyuan Tu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19644 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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ACS Applied Materials & Interfaces

Rapid and Efficient Collection of Platinum from Karstedt’s Catalyst Solution via Ligands-Exchange Induced Assembly Gonghua Yang,1,2,3,4 Yanlong Wei,1,2,4 Zhenzhu Huang,1,2,4 Jiwen Hu,1,2,3,4* Guojun Liu,1,2,4,5 Ming Ou,1,2,4 Shudong Lin,1,2,4 and Yuanyuan Tu1,2,4 1

Chinese Academy of Sciences, Guangzhou Institute of Chemistry, Guangzhou, P. R. China, 510650;

2

Key Laboratory of Cellulose and Lignocellulosics Chemistry, Chinese Academy of Sciences, P. R. China, 510650; 3 4

The University of the Chinese Academy of Sciences, Beijing, P. R. China, 100039;

Guangdong Provincial Key Laboratory of Organic Polymer Materials for Electronics, P. R. China, 510650;

5

Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6; *To whom all correspondence should be addressed, Email: [email protected]

Abstract: Reported herein is a novel strategy for the rapid and efficient collection of platinum from Karstedt’s catalyst solution.

By taking advantage of a

ligand-exchange reaction between alkynols and the 1,3-divinyltetramethyldisiloxane ligand (MViMVi) that coordinated with platinum (Pt(0)), the Karstedt’s catalyst particles with a size of approximately 2.5 ± 0.7 nm could be reconstructed and assembled into larger particles with a size of 150 ± 35 nm due to the hydrogen bonding between the hydroxyl groups of the alkynol.

In addition, since the

silicone-soluble MViMVi ligand of Karstedt’s catalyst was replaced by water-soluble alkynol ligands, the resultant large particles were readily dispersed in water, resulting in rapid, efficient, and complete collection of platinum from Karstedt’s catalyst solutions with platinum concentrations in the range from ~20,000 to 0.05 ppm.

Our

current strategy was used not only for the rapid and efficient collection of platinum from Karstedt’s catalyst solutions, but also enabled the precise evaluation of the platinum content in Karstedt’s catalysts, even if this platinum content was extremely low (i.e., 0.05 ppm).

Moreover, these platinum specimens that were efficiently 1

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collected from Karstedt’s catalyst solutions could be directly used for the evaluation of platinum without the need for pretreatment processes such as calcination and digestion with hydrofluoric acid that were traditionally used prior to testing via inductively coupled plasma mass spectrometry (ICP-MS) in conventional methods.

Keywords: propynol, Karstedt’s catalyst, platinum, hydrogen bonding, induced assembly, ligand-exchange

1

Introdution

Hydrosilylation reactions in which carbon-silicon bonds are created are very important and extensively used in academic research and in the silicone industry.1-3 Some examples of the chemistry and applications of these kinds of reactions include: 1) hydrosilylation of carbon-carbon multiple bonds in the synthesis of molecular organosilicon compounds, such as silane coupling agents4 and UV screens;5

2)

chemo- and enantioselective hydrosilylation of unsaturated carbon-heteroatom bonds;6 3) hydrosilylation of carbon-carbon multiple bonds in polymer chemistry and materials science, such as the cross-linking of organosilicone polymers,7 hydrosilylation polymerization,2 as well as the synthesis of functionalized (poly)silsesquioxanes and silicon-containing dendrimers,8-9 organosilicon-organic hybrid polymers and materials.10-11

Among these reactions, cross-linking reactions

via hydrosilylation are widely used to a much greater extent in the silicone industry to produce crosslinked silicone, which has many uses including automotive gaskets, paper release coatings, pressure-sensitive adhesives, baby bottle teats, computer key pads, and many others.7 Normally, hydrosilylation reactions are catalyzed by transition-metal complexes. Among them, platinum catalysts, such as the silicone-soluble Karstedt’s catalyst, are highly active and predominantly used in this role.12

Karstedt's catalyst is a

coordination complex between (MViMVi) and platinum (Pt(0)) bearing a structure of Pt2(MViMVi)3 as confirmed by X-ray crystallography13 (Figure 1). 2


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Figure 1. Illustrated structure of Karstedt’s catalyst containing bridging and chelating MViMVi ligands.

Normally, Karstedt’s catalyst is used for hydrosilylation reactions after dilution to a certain concentration in the range of 10-100 ppm using a MViMVi-compatible solvent such as tetrahydrofuran, ethyl acetate, xylene, or vinyl terminated poly(dimethyl silicone) (Vi-PDMS),14-15 or others. The major role of Karstedt’s catalyst is attributed to platinum, which is one of the rarer elements, and is both highly valuable and precious.16

Platinum’s physical

characteristics and chemical stability make it very useful for industrial applications, as fine jewelry, for electrical applications and many other purposes.17 However, only ~200 tonnes of platinum are produced per year due to its scarcity in the Earth’s crust.18

Among these, more than ~60% of platinum (Pt) are commonly used

as catalysts in chemical reactions for industrial applications. Specifically, the worldwide consumption of platinum as Karstedt’s catalysts for hydrosilylation reactions in the silicone industry was estimated to be ~ 6 tons in 200719 and this quantity increased very rapidly due to the increasing demand for silicon products in recent years.20

Meanwhile, it is widely recognized that the

accurate determination of the concentrations of Karstedt’s catalysts is an important issue to minimize the waste of Pt in hydrosilylation reactions.

Therefore, based on

the cost and scarcity of Pt, strategies for the efficient collection and exact evaluation from Pt of Karstedt’s catalysts are of particular importance from both academic and industrial viewpoints. Currently, most of the strategies for the efficient collection and exact evaluation of Pt from Karstedt’s catalyst require common pretreatment steps.

In particular,

Karstedt’s catalyst sample is initially removed from the solvent prior to calcinations 3



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and subsequent digestion with hydrofluoric acid to dissolve the SiO2 moieties that formed during the calcination process.21

However, there are several drawbacks to

this process.

First, calcinations require high temperature, are time consuming and

cumbersome.

Second, this process yields inaccurate values due to loss of Pt from

incomplete calcinations or digestion prior to testing via inductively coupled plasma mass spectrometry (ICP-MS).

Third, the hydrofluoric acid that is used for SiO2

digestion is hazardous and might cause harmful side-effects to the environment as well as health effects such as rickets.22 Herein, as a first attempted example so far, we reported a facile and efficient collection and precise evaluation of Pt from samples of Karstedt’s catalysts taking advantage of a ligand-exchange reaction between MViMVi and propynol.

That is, the

Karstedt’s catalyst in a few nanometers underwent reconstruction and assembled into larger particles due to hydrogen bonding among the hydroxyl groups of the propynol ligands. While we are not aware of any reports on ligand-exchange reaction induced assembly of Karstedt’s catalyst, only a few reports concern on the ligand exchange chemistry

to

direct the

superstructures.23-24

assembly

of

the

nanoparticles

into hierarchical

More importantly, the current resultant large particles were

readily soluble in water and thus facilitated the through collection of Pt from Karstedt’s catalyst even if the Pt content in the Karstedt’s catalyst was less than 0.05 ppm.

In addition, the collected Pt was readily soluble in water and precise testing of

its concentration could be performed without the need for pretreatment via calcinations and subsequent digestion with hydrofluoric acid to dissolve the SiO2.22 Further, our current strategy may also have other potential application for the purification of final products as synthesized via hydrosilylation chemistry.

2

Experimental section

The experimental details were included in supporting information.

3

Results and Discussion

Our strategy that was developed from our occasional findings. In particular, 4


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during attempts to prepare silicone release coatings from Vi-PDMS and poly(methyl hydrosiloxane) (denoted as PHMS) in the presence of Karstedt’s catalyst, we have sometimes mistakenly added excessive propynol to the catalyst.

Black precipitates

were observed surrounding the propynol droplets 1-2 minutes after the addition of the propynol.

These precipitates were believed to contain Pt and they were apparently

formed by a ligand-exchange reaction between propynol and MViMVi.

This

phenomenon inspired us to collect Pt belonging to the Karstedt’s catalyst from these precipitates. To ensure the ligand-exchange reaction occurred completely under homogeneous conditions, we used ethyl acetate as a solvent as it was capable of dissolving both Karstedt’s catalyst and propynol.

The mixture solution was vigorously stirred at

room temperature, and it gradually changed from colorless to brown (Figure 2a→b). It’s well known that Tyndall effect, similar to Rayleigh scattering, essentially originated from lighter scattering by colloidal particles in the medium, in that the intensity of the scattered light is positively proportional to size and the average number of particles in the medium.25-26

In our experiment, a very weak Tyndall

effect could be barely observed when a laser beam (650 nm, 5 mW) was passed through the reaction mixture (Figure 2a), suggesting the existence of particles with very small size or little population in the Karstedt’s catalyst solution (Figure 2a). The Tyndall effect gradually became more pronounced after the addition of propynol and stirring for 1 h as indicated by the fact that a light pathway visually became clear and strongly visible through the reaction mixture (Video 1 in the Supporting Information).

This suggested the population or the size of particles had increased

after the addition of propynol. The size variation of the particles during the reaction was further monitored via dynamic light scattering (DLS). the SI (Figure S1).

The experimental details and results are included in

While an ethyl acetate solution of the Karstedt's catalyst in the

absence of propynol exhibited an average hydrodynamic diameter (Dh) of 4.9 nm and a narrow polydispersity index (PDI) of 0.050, the Dh and its PDI increased to 10.1 nm and 0.156, respectively, 15 min after the addition of propynol. 5


However, DLS


curves displayed multiple peaks after the addition of propynol with reaction times in the range from 1 to 5 h, and a single peak was observed when the reaction time was increased further increased to 6 h, indicating that the size and its distribution of the particles increased very rapidly during the first 3 h and then reached a plateau (Figure S1-b).

It should be noted that the peak showing an average Dh of ~10 nm was

visible until 3 h after the addition of propynol. This implied that the small particles served as a reservoir for the formation of larger particles.

Figure 2. Karstedt’s catalyst dissolved in ethyl acetate exhibiting a weak Tyndall effect (a); Karstedt’s catalyst 6 h after the addition of propynol exhibiting a strong Tyndall effect (b); sample that was precipitated from hexane 6 h after the addition of propynol and concentrated via rotary evaporation (c); precipitates in image c as directly dispersed in water and exhibiting a Tyndall effect (d); Karstedt’s catalyst in a as observed via HRTEM at various magnifications (a-i and a-ii); the SAED image corresponding to a-i (a-iii); dispersion in b as observed via HRTEM at various magnifications (b-i to b-iii); an SAED image corresponding to b-i (b-iv); and HRTEM image of the precipitates in c prior to drying at room temperature under vacuum and 6


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redispersion in water (d-i).

The morphology of the particles formed through the ligand-exchange process was observed by directly spraying the reaction mixture onto carbon-coated 200-mesh copper film and characterization with a high resolution transmission electron microscope (HRTEM), as shown in Figure 2 (see more TEM images of the samples obtained at different reaction times in Figure S2).

Small particles with an average

diameter of 2.5 ± 0.7 nm were observed for Karstedt’s catalyst as shown in Figure 2 (a-i, and a-ii).

Meanwhile, particles with an irregular morphology and a diameter of

150 ± 35 nm were observed in the reaction mixture 6 h after the addition of propynol, as shown in Figure 2b-i. While the size and morphology of Karstedt’s catalyst was consistent with the results obtained by other researchers,7 it is reasonable that the average size of the particles as evaluated from the HRTEM images was smaller than that corresponding to the “wet state” as probed by DLS.27 The fine morphology of the particles was carefully inspected at higher magnification, and it was revealed that they exhibited a face-centered cubic (fcc) lattice morphology28 (Figure 2a-ii).

In addition, the interfringe distance of 0.234

and 0.202 nm ascribed to [111] and [200] lattice planes, respectively, could be evaluated according to the sharpened diffraction rings from the selected-area electron diffraction (SAED) pattern (Figure 2a-iii).29

This indicated that the Karstedt's

catalyst was crystal, which was confirmed by SAED in the HRTEM images and was consistent with the conclusions reached by other researchers.7, 30

In contrast, after

the addition of propynol and 6 h of reaction time, many small particles with a diameter of 2~3 nm constructed the larger particles (Figure 2b-ii).

However,

widened diffraction rings were observed in the SAED pattern for dispersion in Figure 2b-iv, suggesting that the crystal structure of the small particles of Karstedt’s catalyst was damaged during the process. Brown precipitates appeared at the bottom of the flask to leave a clear supernatant after 6 h of reaction time and standing for 24 h without stirring.

The reaction

mixture was concentrated via rotary evaporation at room temperature, and Vi-PDMS 7



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was removed by precipitation out in hexane, which is a good solvent for Vi-PDMS but is immiscible with propynol (Figure 2c). We could not find any particles in the supernatant after the supernatant was sprayed onto carbon coated films and observed via HRTEM.

This suggested that the particles in the reaction mixture were fully

precipitated in hexane.

More importantly, the precipitates could be readily

re-dispersed into water as expected and demonstrated by a homogenous dispersion that remained stable in water without stirring for 24 h.

The particles readily

dispersed in water as further judged by a laser beam as shown in Figure 2d, suggesting that the silicone-soluble MViMVi ligand was replaced by water-soluble propynol ligands. Without doubt, both facile and complete precipitation in hexane and water-dispersibility of the precipitates facilitated the thorough collection of the Pt content from Karstedt’s catalyst (Video 2 in the SI). The ideal alkynol must not only be readily soluble in water but should also be completely insoluble in hexane to allow the highly efficient collection and precise testing of the Pt content in Karstedt’s catalyst.

Other alkynols, such as 3-methyl

butynol, 3-methyl-1-pentyn-3-ol and 3,5-dimethyl-1-hexyl-3-ol were also employed instead of propynol under identical conditions. We found that propynol offered the best performance among these alkynols.

This is reasonable due to the fact that

propynol has a structure with fewer carbon atoms, is more soluble in water, and is insoluble in hexane compared to the other alkynols (Table S1).31-32 We further tried to find the best reaction conditions between propynol and Karstedt’s catalyst.

The molar ratio between propynol and platinum is critical.33

We performed the experiments under identical conditions except for the use of different [Karstedt’s catalyst]/[propynol] molar ratios at 1/2, 1/10, 1/100, 1/1,000, 1/2,000, 1/3,500, and 1/5,000.

After reaction for 6 h and subsequent concentration

via rotary evaporation and washing with hexane prior to centrifugation at 10,000 g for 10 min, we found that many of the resultant platinum nanoparticles adhered onto the wall of the centrifuge tube when the ratio was lower than 1/2,000 (Figure S3-a).

In

contrast, the Pt sample readily separated from hexane even without centrifugation at [Karstedt’s catalyst]/[propynol] ratios of 1/3,500 or 1/5,000 (Figure S3-b). 8


Similar

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phenomena could be found when the [Karstedt’s catalyst]/[propynol] ratio was increased further. The currently developed strategy for the facile and complete collection of Pt from Karstedt’s catalyst was believed to be facilitated by the complex with solubility in water and insolubility in hexane as well as particles with larger diameters as formed from Pt and propynol instead of that from Pt and MViMVi.

As described above, after

the addition of an alkynol into Karstedt’s catalyst solution, the MViMVi ligands of the Pt2(MViMVi)3 complex should be replaced by alkynol ligands since the coordinating strength of carbon-carbon triple bond in alkynol is obviously stronger than that of the carbon-carbon double bond in MViMVi of Karstedt’s catalyst.34-35

This

ligand-exchange reaction rate should be affected by the groups attached onto the carbon-carbon triple bond.

The group with the smaller size and electron providence

should favor an increase of the interchange reaction rate.

This was evidenced by our

finding that the time for the appearance of a clear light beam through the reaction mixture of propynol and Karstedt’s catalyst solution was the shortest among the mixtures of the other alkynols with Karstedt’s catalyst solution under identical reaction conditions (Table S2).

This also indicated that relatively rapid collection of

Pt could be achieved by the fast exchange reaction rate between propynol and the MViMVi ligands of Karstedt’s catalyst. Based on HRTEM observation for Karstedt’s catalyst (Figure 1a-ii), we could conclude that the fcc crystal of Karstedt’s catalyst with a diameter of 2.5±0.7 nm, was constructed from ~ 270 Pt atoms. ligand-exchange reaction occurred.

After the addition of propynol, the

The final precipitates were constructed from

many small particles with a diameter of 2~3 nm, which was comparable to that of the precursor particles of Karstedt’s catalyst as observed via HRTEM (Figure 2b-iii), suggesting the overall morphology of the Karstedt’s catalyst particles were still retained.

However, the diffraction rings in SAED patterns of the final precipitates

appeared blurry, indicating that the crystal structure of the small particles of the Karstedt’s catalyst was damaged (Figure 2b-iv).

Therefore, the crystals of the

Karstedt’s catalyst complex apparently suffered from damage and/or the 9



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reconstruction of Pt inside its precursor particles of Karstedt’s catalyst.

Further, the

size of the particles increased during the reaction, thus suggesting that the size increase exhibited by the particles was due to the assembly of the particles via reconstruction during the ligands-exchange reaction. We further concluded that the size increased from the assembly of the reconstructed small particles due to the hydrogen bonding interactions among the hydroxyl groups of the propynol ligands. controlled experiments.

This was confirmed via carefully designed

Firstly, we performed the reaction at [Karstedt’s

catalyst]/[propynol] = 1/3500 at an elevated temperature of 60 °C instead of at room temperature (25 °C).

A clear Tyndall effect was observed at ~60 min after the

addition of propynol to the Karstedt’s catalyst solution in ethyl acetate when a laser beam was passed through the reaction mixture, in comparison with only 10 min that was required for this to take place at room temperature (see details in the SI).

The

distinctive delay in the appearance of Tyndall effect at elevated temperature implied that the formation of the particles proceeded via hydrogen bonding since the hydrogen bonding interactions would be weakened at higher temperatures (Table S2).36 Secondly, we synthesized and used MPEG-CCH (which lacked hydroxyl groups) as a replacement for propynol to react with the Karstedt’s catalyst under otherwise identical conditions (Figure S4-S5 in the SI). While the Tyndall effect was not observed during the entire reaction process, the resultant particles had an average diameter of 2~3 nm and were readily dispersed in water. ligand-exchange

reaction

occurred

successfully

MPEG-CCH and silicone-soluble MViMVi ligands.

This indicated that the

between

the

water-soluble

However, the resulting particles

did not aggregate into larger particles in this case due to the absence of hydrogen bonding.

In other words, this implied that both the reconstruction of the small

particles and the assembly of these small particles into larger particles resulted from hydrogen bonding interactions between the hydroxyl groups of the alkynols. It should also be noted that the assembled particles induced by a ligand-exchange reaction could not be readily dispersed in water after the excessive propynol had been completely removed and they were dried at room temperature under vacuum. 10


This

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can be attributed to the fact that the larger particles tend to become fused together as observed via HRTEM in Figure 2d-i unlike the samples that were obtained from the reaction mixture and observed by HRTEM (Figure 2b-i).

On the contrary, small

particles with a morphology similar to that of Karstedt’s catalyst could be observed via HRTEM after the dried assembled particles were dispersed into an aqueous NaOH solution at a pH value of 12 (Figure S6-S7 in the SI).

These phenomena provided

further evidence that the particle formation was due to the disruption

of the strong

hydrogen bonding interactions among the hydroxyl groups of the alkynols, since the hydrogen bonding interactions among these hydroxyl groups would be disrupted under basic media.37 FT-IR, 1H NMR and

13

C NMR characterizations were further conducted on

samples of the precipitates that had been repeatedly washed with hexane to remove any impurities before they were dried under vacuum at room temperature (Figure S8-S9 in the SI).

The results of these measurements indicated that the particles

primarily consisted of propynol and Pt atoms. A proposed mechanism for the current strategy is illustrated in Figure 3.

In

particular, while the Karstedt’s catalyst adopted a fcc crystal structure with a diameter of ~ 2.5 ± 0.7 nm containing ~270 Pt atoms and ~405 MViMVi ligands (Figure 3a), after the addition of the alkynol into the ethyl acetate solution of the Karstedt’s catalyst, a ligand-exchange reaction took place between the alkynol and the MViMVi ligands.

11



Figure 3.

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The illustrated mechanism for collecting platinum from Karstedt’s

catalyst via ligand-exchange induced assembly. a) Karstedt’s catalyst; b) water-dispersible platinum colloids formed via ligand-exchange induced assembly; c) coordinated structures resulted from the ligand-exchange reaction; d) the hydrogen bonding forces among platinum atoms.

Although the crystal structure was disrupted (Figure 2b-iii), the overall shape of the reconstructed particles was well retained, in which four kinds of structures that resulted from the exchange reaction might have existed (Figure 3c).33-34

These

individual particles served as a reservoir and aggregated to form larger particles as time progressed due to the hydrogen bonding among the hydroxyl groups of the alkynols that replaced the MViMVi ligands (Figure 3d).

Since most of the MViMVi

ligands of the Karstedt’s catalyst were replaced by alkynol ligands, the resultant large particles were readily dispersible in water. In order to verify the effective collection of Pt from Karstedt’s catalyst using the current strategy, we designed a series of tests (see details in the SI).

Firstly, a

commercially available Karstedt’s catalyst sample in Vi-PDMS at ~20,000 ppm was 12


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employed, and the platinum content of this commercially available sample was evaluated by our currently developed strategy.

Secondly, the resultant reaction

mixture was concentrated and precipitated from hexane to divide the sample into two components consisting of the supernatant and the precipitate.

Finally, the platinum

contents in the supernatant and precipitate were respectively evaluated via inductively coupled plasma mass spectrometry (ICP-MS).38

The Pt content in the precipitate of

the commercially available Karstedt’s catalyst was found to be 20,080.0 ppm by our ligand-exchange method. We believe that this value is accurate since the tested value of Pt content found in the supernatant is zero.

The Karstedt’s catalyst was then

quantitatively diluted with Vi-PDMS to Pt concentrations of 5,025.0, 50.4, 1.0079 and 0.0504 ppm.

The Pt contents in these diluted samples were evaluated via the

traditional method and our ligand-exchange strategy as detailed in the SI. While the accuracy of the traditional method diminished with a decreasing Pt content in the Karstedt’s catalyst, the accuracy of our method was retained even if the concentration of Pt in the Karstedt’s catalyst reached as low as 0.05 ppm (Table 1). This indicated that our strategy can be used for precise testing of the Pt content in Karstedt’s catalysts over a wide concentration range from 20,000 down to 0.05 ppm. Furthermore, it should be noted that the resulting particles formed in the current testing protocol could be directly dissolved in aqua regia with heating for further evaluation without the need for pretreatment processes such as calcinations or digestion.

Table 1. The detection of platinum concentration in Karstedt’s catalyst solutions with different pretreatment procedures. Sample No. C0/ppm C1/ppm Accuracy/% C2/ppm Accuracy/% 20080.0 19078.0 95.01 20080.0 100.00 1 5025.0 4798.2 95.49 4997.0 99.44 2 50.4 41.6 82.54 50.3 99.80 3 1.0079 0.4536 45.00 1.0028 99.49 4 0.0504 0.0203 40.28 0.0501 99.40 5 (C0) the concentration of Karstedt’s catalyst sample as diluted with Vi-PDMS from an original Karstedt’s catalyst sample with a Pt concentration of 20,080 ppm; (C1) the 13



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content of platinum evaluated with a conventional calcination method39; (C2) the content of platinum evaluated with our newly developed method; ppm: parts per million.

Usually, it is difficult to remove the Karstedt’s catalyst from products synthesized via hydrosilylation chemistry, and this catalyst can cause undesirable side-effects during further use.40

We envisioned that our current strategy could be employed for

the facile removal of Karstedt’s catalyst from hydrosilylation product.

As an

example, PHMS was reacted with Vi-PDMS at [Si-H]/[Si-Vi]=10/1 (mole ratio) at room temperature for 30 min before it was divided into two parts.

While the

untreated sample gelled distinctively after the reaction and subsequent storage for 3 months, the component that was treated using our newly developed strategy did not change significantly after storage for the same time (see details in the SI).

This

confirmed that our current strategy could be used for purification of hydrosilylation products.

4

Conclusion

In summary, we have developed a facile and efficient strategy for the collection and precise evaluation of Pt from samples of Karstedt’s catalysts.

The propynol

reacted with platinum via a ligand-exchange reaction between MViMVi and propynol. The Karstedt’s catalyst underwent reconstruction and assembled into larger particles due to hydrogen bonding among the hydroxyl groups of the propynol ligands.

The

resultant large particles were readily soluble in water and thus facilitated the collection of Pt from Karstedt’s catalyst.

Efficient collection of Pt from Karstedt’s

catalyst was confirmed by performing the reaction between propynol and Karstedt’s catalyst at various concentrations even if the Pt content in the Karstedt’s catalyst was less than 0.05 ppm.

Therefore, we can thoroughly collect Pt from the Karstedt’s

catalyst not only at high concentrations (such as 20,000 ppm), but also at extremely low concentrations (i.e., 0.05 ppm).

In addition, the collected Pt was readily soluble

in water and precise testing of its concentration could be performed without the need for pretreatment via calcinations and subsequent digestion with hydrofluoric acid to 14


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dissolve the SiO2.41

Therefore, our current strategy may be used not only for the

collection or accurate evaluation of the Pt content in Karstedt’s catalyst, but may also have other potential applications, such as the purification of final products as synthesized via hydrosilylation chemistry.

Synthetic protocols, characterization results (DLS,

Supporting Information.

HRTEM images, FT-IR, 1H NMR and 13C NMR) and videos ( video 1 and video 2) in the process of the ligand-exchange induced assembly, additional experimental phenomena images ( Figure S3 and S6), datum (Table S1 and S2), characterization results (HRTEM images, 1H NMR) and evaluation of platinum content (Table S3) included in Supporting Information.

This material is available free of charge via the

Internet at http://pubs.acs.org.

Acknowledgements:

This work was supported by the National Natural Science

Foundation of China (51173204, 51503124, 21404121, and 21404122), the Pearl River Novel Science and Technology Project of Guangzhou (201506010031), the Development Fund for Special Strategic Emerging Industries in Guangdong Province (2015B090915004), the Guangdong Natural Science Foundation (2016A030313163, 2015A030313799, 2015A03031382, 2014A030310412), and the Science and Technology Program of Guangzhou City (201607010244).

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TOC Graphic Abstract A novel strategy for the rapid and efficient collection of platinum from Karstedt’s catalyst based on a ligand-exchange reaction induced assembly via hydrogen bonding.

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A novel strategy for the rapid and efficient collection of platinum from Karstedt’s catalyst based on a ligandexchange reaction induced assembly via hydrogen bonding. 254x67mm (150 x 150 DPI)



Figure 1. Illustrated structure of Karstedt’s catalyst containing bridging and chelating MViMVi ligands. 184x64mm (150 x 128 DPI)


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Figure 2. Karstedt’s catalyst dissolved in ethyl acetate exhibiting a weak Tyndall effect (a); Karstedt’s catalyst 6 h after the addition of propynol exhibiting a strong Tyndall effect (b); sample that was precipitated from hexane 6 h after the addition of propynol and concentrated via rotary evaporation (c); precipitates in image c as directly dispersed in water and exhibiting a Tyndall effect (d); Karstedt’s catalyst in a as observed via HRTEM at various magnifications (a-i and a-ii); the SAED image corresponding to a-i (aiii); dispersion in b as observed via HRTEM at various magnifications (b-i to b-iii); an SAED image corresponding to b-i (b-iv); and HRTEM image of the precipitates in c prior to drying at room temperature under vacuum and redispersion in water (d-i). 625x488mm (107 x 107 DPI)



Figure 3. The illustrated mechanism for collecting platinum from Karstedt’s catalyst via ligand-exchange induced assembly. a) Karstedt’s catalyst; b) water-dispersible platinum colloids formed via ligand-exchange induced assembly; c) coordinated structures resulted from the ligand-exchange reaction; d) the hydrogen bonding forces among platinum atoms. 450x350mm (300 x 300 DPI)


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Rapid and Efficient Collection of Platinum from Karstedt's Catalyst

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