Studies on the Activation and Hydrosilylation Catalysis of RhCl3

Mar 31, 2017 - Synopsis. A variety of studies were performed on the industrial hydrosilylation precatalyst RhCl3(Bu2S)3. These studies helped develop ...
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Studies on the Activation and Hydrosilylation Catalysis of RhCl3(Bu2S)3 Matthew T. Olsen,* Brian D. Rekken, and Donald V. Eldred Core R&D, Dow Chemical Company, 2200 W. Salzburg Rd, Auburn, Michigan 48611, United States S Supporting Information *

ABSTRACT: RhCl3(Bu2S)3 is an industrial precatalyst utilized in the curing of some solventless silicone-release coatings formulations. The catalyst requires no additional inhibitor, in contrast to typical Pt formulations, and so questions arose about how fast the catalyst could trigger curing if it were used in a more activated form. Studies on the activation of RhCl3(Bu2S)3 revealed multiple intermediates, of which [RhCl(Bu2S)2]2 and [RhHCl(SiMe(OSiMe3)2) (Bu2S)2]2 were isolated. [RhHCl(SiMe(OSiMe3)2)(Bu2S)2]2 is the most activated form of the precatalyst, showing a brief induction period. Various experiments were performed to analyze the nature of the catalyst, including Hg poisoning, addition of poison ligands, and comparisons versus Rh nanoparticles. The data tend to be more consistent with a molecular catalyst than Rh nanoparticles. Data are also provided that support the active catalyst containing a RhxClx(Bu2S)2x fragment, although the exact identity of the active catalyst is not yet determined.

1. INTRODUCTION The silicones industry sells hydrosilylation-curable, siliconebased release coatings that are cured at high temperatures with short dwell times (several seconds) in an oven.1−4 Although the majority of such release coatings utilize inhibited Karstedt’s precatalyst1,5,6 (Pt2[(ViMe2O)Si]3 treated with an inhibitor such as 1-ethynylcyclohexanol),7,8 a small percentage of coatings is cured with a rhodium catalyst.3 Fundamental studies on Karstedt’s precatalyst have been performed, although the exact nature of the catalyst remains debated.9−13 Pt catalysts generally have the advantage of being exceptionally fast; however, they are easily inhibited or deactivated by various species (amines, sulfides, etc.) and have demonstrated substrate interaction problems with acrylics.4,14,15 In contrast, formulations using a Rh catalyst such as Rh(R2S)3Cl3 are more resistant to poisoning and are more chemically compatible with various adhesives.3 However, curing with Rh catalysts is slower and requires higher temperatures.16 Many other catalysts have been developed for hydrosilylation, only some of which have been used in silicone cross-linking, and these have been the subject of excellent reviews by others.1,17−21 Recent years have seen exciting advances in first-row transition-metal catalysts.22−34 The Rh curing system was initially developed by Chandra in the 1970s, and it consists of the precatalyst RhCl3(Bu2S)3.35,36 Chandra examined many Rh precursors, especially those with sulfide ligands. Unlike Pt, which requires an inhibitor to give it sufficient working time (“bath life”) in the presence of formulation substrates, the rhodium sulfide is reasonably stable without an inhibitor. The activation of this catalyst appears to be a slow process, and so we aimed to better understand this © 2017 American Chemical Society

process and the chemical reactions that govern it. Interestingly, Armstrong et al. showed that pre-reaction of the Rh tris(sulfide) precatalyst with HSiMe2O(SiMe2O)16SiMe2H led to a faster curing catalyst.37 The report herein discusses our efforts to better understand the behavior of Rh sulfide catalysts in hydrosilylation reactions. RhCl3(R2S)3 has also been reported as a precatalyst for various industrially relevant hydrosilylation reactions,37−40 some of which claim improved selectivity versus Pt hydrosilylation41−43 or improved tolerance of epoxy functional groups.44 Various Rh complexes have also been evaluated as potential curing catalysts for hydrosilylation-cross-linked silicones.45 Related complexes of different metals such as PtCl2(SR2)2/SnCl2 have been studied for hydrosilylation catalysis.46 Aside from these, the majority of Rh complexes with sulfur-containing ligands use the ligand to attach the metal to a heterogeneous support and have been reviewed elsewhere.47 Beyond hydrosilylation, the catalytic activity of RhCl3(SR2)3 has not been extensively explored. The most notable exception is the hydrogenation of carboxylic acid-containing olefins (such as maleic, fumaric, and cinnamic acid) by James et al., for which RhCl3(SEt2)3 is used as a precatalyst.48−51 The catalytic reaction occurs under somewhat mild conditions (1 atm H2, 70 °C) and requires dimethylacetamide as a solvent; activity is not observed with benzene as a solvent.50 They also report that the reaction requires dissociation of SBu2 and that the catalytic Received: January 11, 2017 Published: March 31, 2017 4484

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Inorganic Chemistry reaction is preceded by reduction of RhIII to RhI. In contrast to metal thioether complexes, metal thiolate complexes are more commonly studied, as catalysts have been reviewed elsewhere.47

Table 1. Byproducts Observed during the Reaction of MDHM and MMVi with RhCl3(Bu2S)3 Catalyst

2. RESULTS AND DISCUSSION Although it is commercially used and available, we were unable to find significant basic characterization of the precatalyst. These data and information on the synthesis of the precatalyst are available in the Supporting Information (Figures S1−S3). Catalytic Activity of RhCl3(Bu2S)3 with Model Substrates. To gain more detailed information about the catalytic performance in a curable siloxane composition, model substrates were used to examine the rates of hydrosilylation. 1,1,1,3,5,5,5-Heptamethyltrisiloxane (MDHM) and 1,1,1,3,3pentamethyl-3-vinyl-disiloxane (MMVi) were reacted in the presence of RhCl3(Bu2S)3 with toluene as a solvent and dodecane as an internal gas chromatography (GC) standard.

byproduct identity

byproduct yield (%)

M2D(C2H2)DM (cis and trans) MDViM MDEtM MM(CH2)2MM M2D(CH2)2DM2 M2D(CMeH)DM total

1.3, 1.9 (not differentiated) 0.8 2.4 2.8 1.0 0.2 10.4

side products are due to dehydrogenative silylation, which is not uncommon for group 9 transition-metal catalysts.18,53−57 We also observed a trace amount of the Markovnikov hydrosilylation product. Less expected was a series of products that appear to arise from functional group exchange between the Si−H and Si−Vi sources (Scheme 1). The Si−H/Si−Vi exchange reaction is uncommon but has been reported.57,58 Such a reaction would produce MDViM and MMH, the former of which is observed but the latter is not, because it is highly reactive (the byproduct MM(CH2)2MM is observed). These products do not simply arise from hydrosilylation of impurities (MMH or MDViM), which were not detected in the unreacted reagents. We also observed the product of the hydrosilylation between MDViM and MDHM (e.g., M2D(CH2)2DM2), as well as MDEtM, which results from hydrogenation of the vinyl compound. H2 is generated in solution via dehydrogenative silylation. Activation of RhCl3(Bu2S)3 with MMVi and MDHM. To better understand the activation of the precatalyst RhCl3(Bu2S)3, we studied its reactivity with model substrates. RhCl3(Bu2S)3 was unreactive toward MMVi but gradually reacted with consecutive treatments of MDHM. Varying mixtures of Rh sulfide species are formed, with MDClM, free Bu2S, and H2 as byproducts (Scheme 2, Table 2). Treatment of RhCl3(Bu2S)3 with 1 equivalent of MDHM results in partial consumption of the starting material (∼32% remains) and the formation of [Rh(H)2(Cl)(Bu2S)2]2 (RhHa, ∼48%) and [RhCl(Bu2S)2]2 (∼20%). When a second equivalent of MDHM is added, the remainder of the starting material is consumed, and the Rh balance is distributed among RhHa (∼21%), [RhCl(Bu2S)2]2 (∼35%), and a new Rh hydrides species, [Rh(H) (Cl)(OSi(OSiMe3)2Me)(Bu2S)2]2 (RhHb, ∼38%). Finally, treatment of the reaction mixture with a third equivalent of MDHM cleanly converted it to RhHb (75%) with a small amount of decomposition due to the thermal instability of RhHb, which will be described later. More details on the observations during these sequential additions can be found in the Supporting Information (Section 4). Of these Rh sulfide species, the extent of characterization is varying. RhHa is most clearly observed in the hydride region of the 1H NMR (−16.4 ppm, d, JRh−H = 6.4 Hz), but it could only be isolated as a mixture with other Rh species. Aside from the Rh−H resonance in the 1H spectrum, there is considerable overlap of the Rh-sulfide resonances, and its identity is implied based on the proposed reaction sequence (Scheme 2). A dimeric structure is proposed from comparison to the other intermediates that will be discussed. In contrast to RhHa, [RhCl(Bu2S)2]2 was fully characterized. When the reaction mixtures were cooled, [RhCl(Bu2S)2]2 precipitates as a crystalline orange solid. This stable material was characterized by 1H NMR, combustion analysis, and single-

Catalyst loadings of 5−150 ppm are typical for hydrosilylation curing of siloxane polymers, and the number of catalytic turnovers this corresponds to depends on the polymer molecular weight.4,52 We selected a catalyst loading of 0.1 mol % to model a paper coating application. Although Si−H/Si−Vi ratios of 2:1 are sometimes used, we instead used a 1:1 ratio for the sake of simplicity (in application, excess Si−H is eventually hydrolyzed, but this is much slower than the hydrosilylation rate). We also performed these reactions under inert atmosphere, although paper coatings applications are performed in open air; future studies could address this difference. Reactions generally produce ∼90% yield of M2D(CH2)2DM, the expected anti-Markovnikov product of hydrosilylation (see Author Information (Notes) for nomenclature). Reaction monitoring at various temperatures revealed an induction period that is significant at lower temperatures (Figure 1). Extrapolating these data indicates that at room temperature in a polymeric system the induction period should be quite long (hours), allowing for considerable working time prior to the onset of curing. Several trace byproducts are produced during this reaction and were identified with GC-MS (Table 1). The most expected

Figure 1. Reaction monitoring by GC of MDHM + MMVi in the presence of 0.1 mol % RhCl3(Bu2S)3 at the indicated temperature. 4485

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Inorganic Chemistry Scheme 1. Observed Products Arising from the Si−Vi/Si−H Exchange Reaction

Scheme 2. Schematic Illustrating the Proposed Reactions That Occur during the Activation of RhCl3(Bu2S)3

Table 2. Mass Balancea of Rh-Containing Species upon Treatment of RhCl3(Bu2S)3 with MDHM and Heated at 55 °C for 45 min Each Step Rh Mass balance equivs MDHM

RhCl3(Bu2S)3

RhHa

[RhCl(Bu2S)2]2

RhHb

1 2 3

32% 1% 0%

48% 21% 0%

20% 35% 0%

0% 38% 75%

a

Mass balance is approximate, as [RhCl(Bu2S)2]2 numbers were obtained via isolated yield, while the other species were measure via 1H NMR.

crystal X-ray diffraction (Figure 2). [RhCl(Bu2S)2]2 is dimeric, and unlike many [RhClL2]2 complexes, it features chloride ligands that are terminal rather than bridging. Instead, two of the thioether ligands bridge the metals. Bridging thioether ligands are not uncommon.59 We were able to independently synthesize this complex via the reaction of Bu2S and [Rh(C2H4)2Cl]2, and the product was isolated in moderate yield (47%, Scheme 3). Significant characterization of RhHb could be obtained as well, although it could not be isolated in high purity due to limited thermal stability and a tendency to form intractable oils. A hydride resonance is clearly observed in the 1H NMR spectrum (−18.5 ppm, d, JRh−H = 27.7 Hz), and the 29Si NMR spectrum indicated the release of 2 equivalent of MDClM (1,1,1,3,5,5,5-heptamethyl-3-chloro-trisiloxane) as well as ∼0.75 equivalent of a Rh silyl species. The Rh silyl resonance

Figure 2. X-ray crystal structure of [Rh(SBu2)Cl]2 with thermal ellipsoids set at 30%. Hydrogen atoms are removed for clarity.

appeared as a doublet of doublets (JSi−Rh = 30 Hz, JSi−H = 17.5 Hz). The Rh hydride was shown to couple to the Rh silyl group via a 1H−29Si heteronuclear multiple bond correlation (HMBC) experiment (Figure 3). Liquid injection field desorption ionization mass spectrometry (LIFDI-MS) analysis of RhHb showed significant fragmentation, which primarily arose from the dimeric species [Rh(H)(Cl)(OSi(OSiMe3)2Me)(Bu2S)2]2 (Figure S13). The parent peak is not actually observed, but species arising from 4486

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Inorganic Chemistry Scheme 3. Schematic Illustrating Two Different Synthetic Pathways to [RhCl(Bu2S)2]2

tacky oils that could not be separated from the reaction byproducts. Attempts at further purification resulted in complex decomposition, because the generation of RhHb is competitive with its degradation. For this reason, the highest in situ measured yield we could achieve was ∼75% (at ∼78 mM [Rh], 55 °C for ∼1 h). Approximately 15% of the mass balance was found in other unidentified Rh−H species, and ∼10% was in unreacted MDHM. Solutions of RhHb appeared as pale orange but darkened as reaction times extended beyond 1 h at 55 °C. Monitoring of the decomposition process by 1H NMR did not reveal any major intermediates. Rather the growth of free Bu2S is observed. The orange (RhHb) reaction mixture becomes dark black/brown and remains soluble; diatomaceous earth filtration does not remove the color. Analysis of the reaction mixture at this stage indicated the formation of a third equivalent of MDClM (Figure 4). Heating at higher temperatures (120 min, 110 °C) forms insoluble Rh black and causes loss of catalytic activity (Section E).

Figure 3. Enlarged spectrum from a 1H−29Si HMBC experiment (x and y axis, respectively), showing the coupling between the Rh Si. Note the differences between the 1H reference spectrum (top of graph) and the 2D spectrum: the 1H NMR spectrum shows a doublet for the Rh−H resonance and a very small doublet of doublets due to 29 Si satellites. In the 2D spectrum, the nuclei that are targeted are 1H and 29Si, and so the satellite peaks are dominant and give rise to the observed signal. In the 29Si−1H HMBC experiment, the projection of the 1H spectrum appears as two overlapping doublets (e.g., a triplet) rather than a doublet, because the experiment is only showing the molecules containing 29Si nuclei. In this scenario, the hydride couples to 29Si and to 103Rh. 29Si−1H coupling was determined from both from the 29Si NMR spectrum as well as from the 29Si satellites in the 1H NMR spectrum.

Figure 4. Equivalents of RhHb and MDClM formed after heating RhCl3(Bu2S)3 with 3 equivalent of MDHM at 55 °C (∼78 mM [Rh]), illustrating the competing rates of the synthesis and degradation reactions.

Efforts To Understand the Active Catalyst. A. Induction Period Comparisons. The induction period of the catalyst alerted us to two possibilities: first, the induction period may be due to the reduction of RhIII to RhI and/or the substitution of Rh−Cl bonds for Rh−H bonds. Second, that the active catalyst may be nanoparticulate in structure with the induction period reflecting the agglomeration of a molecular precatalyst into a higher-order aggregate. Several tests were performed to differentiate these possibilities. First, the catalytic competence of various forms of the partially activated precatalysts were compared (Table 3, Figure 5). The observation of induction periods indicates that none of these species are part of the catalytic cycle and rather are precatalysts. The significant reduction in length of the induction period upon conversion of RhCl3(Bu2S)3 into [RhCl(Bu2S)2]2 or RhHb indicates that the majority of the induction period is due to the generation of these species. If catalysis is actually performed by metal nanoparticles, their

loss of one or multiple Bu2S are. The monomeric fragment Rh(H) (Cl) (OSi(OSiMe3)2Me) (Bu2S)2 is also observed and is the largest peak present. It is conceivable that the monomer and dimer are in equilibrium. Reductive elimination of 1 equivalent of hydrosilane from the dimer is observed to a small extent, as are higher nuclearity species (Figure S12). In further support of a loosely bound or dissociated equivalent of Bu2S, the solution-state 1H NMR spectrum indicates ∼1−1.5 equivalent of free Bu2S (small broad species in the baseline complicate the integration). The peaks are broadened indicating exchange with the Rh-bound sulfides (Figure S9). The overall reaction is shown in eq 4: Attempted Isolation and Decomposition of RhHb. Our attempts to isolate RhHb were unsuccessful and resulted in 4487

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conversion into metal nanoparticles. To examine further this phenomenon we first considered Hg poisoning studies. Although Hg poisoning tests are not a definitive means of differentiating nanoparticles and small-molecule catalysts,60,64 they provide an additional line of evidence for consideration. Others have used the Hg test as an additional data point.60−62 Poisoning of this catalyst by Hg is slow and can only be observed at lower reaction temperatures with vigorous stirring. Using a reaction temperature of 50 °C and stirring at 1500 rpm with multiple stirbars, we added ∼9000 equivalent of Hg (per Rh) after the induction period had passed. The rate of product formation gradually decreased (Figure 6). Conditions with less contact time between the solution and the Hg (100 °C, 500 rpm) did not slow the reaction rate.

Table 3. Observered Induction Period for Various Precatalysts for the Hydrosilylation of MDHM and MMVi precatalyst

induction period

RhCl3(Bu2S)3 [RhCl(Bu2S)2]2 RhHb

∼3 min, 70 °C ∼1 min, 70 °C ∼1 min, 50 °C

Figure 5. Reaction monitoring by GC of the reaction of MDHM and MMVi with RhCl3(Bu2S)3, [RhCl(Bu2S)2]2, and RhHb as precatalysts at 70 °C (top) and RhHb at 50 °C (bottom).

synthesis from RhHb must be comparatively very fast. This would differ from some Rh systems studied by others.60−62 B. Attempts at Generating the Active Catalyst. We attempted to generate the active catalyst (e.g., no induction period) but have not yet succeeded. Attempts at generating Rh hydride rapidly in situ via hydride reagents either generated unstable species or had longer induction periods than RhHb (see Supplementary Information Section 8). We also pretreated RhHb with MMVi, knowing that this would trigger product formation and presumably replicates catalytic conditions (Supporting Information Section 9). However, this results in a reaction mixture having an induction period similar to that of RhHb. We also tested the thermally decomposed mixtures of RhHb that were discussed in the preceding section; however, these species had significantly diminished activity (see Section E below). One possible explanation for the difficulty in bypassing the induction period could be that the dimeric species RhHb must dissociate into monomers before it is catalytically active. C. Hg Poisoning Studies and Rh Nanoparticle Testing. Although induction periods are a hallmark of the formation of metal nanoparticles,63 above we showed that the majority of the induction period was instead due to the conversion of Rh−Cl bonds into Rh−H bonds. However, a brief induction period is still observed for RhHb, and it is possible this originates from its

Figure 6. Hg poisoning experiments on the reaction of MDHM + MMVi as catalyzed by RhCl3(Bu2S)3 at 100 °C (top) and at 50 °C, with additional stir bars, and prewarmed Hg (bottom).

These results differ from some nanoparticulate catalysts in which Hg poisoning immediately halts catalysis.11,60,65,66 There are also examples of homogeneous Rh catalysts being immediately poisoned by Hg,60 and therefore the Hg test is one tool of many that should be used. One possible explanation for the differing poisoning data at 50 versus 100 °C is that the identity of the catalyst changes. However, we find it likely that higher temperatures would favor the formation of Rh(0) nanoparticles rather than lower temperatures. Finke et al. found that Rh(0) particles dominate by higher temperatures and pressures.60−62 D. Chemical Poisoning Studies: 1,10-Phenanthroline, SBu2, and Phosphines. We further probed the kinetic properties of the catalysts by adding poison ligands. The objective of these tests was to determine the number of active sites present. Heterogeneous catalysts often are poisoned by much less than 1 equivalent of poison, because many metal atoms are buried beneath the surface, and the close proximity of the metal atoms to each other can allow 1 equivalent of poison to deactivate nearby active sites.63,67 In contrast, homogeneous 4488

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equivalent of phen, whereas their molecular Cp4Rh4 catalyst requires ∼4 equivalent of phen.60−62 We attempted to make a similar comparison by testing the same commercially available ∼2 nm Rh nanoparticles (Strem Chemical Co.; stabilized with PEG dodecylether hydrosol). However, under our catalytic conditions no reaction was observed. This further supports that Rh nanoparticles are not the active catalyst generated from RhCl3(Bu2S)3, although it is also possible that different size/ shape particles are generated in situ in comparison to the material purchased from Strem. We next tested the poisoning effect of SBu2 with the RhCl3(Bu2S)3, which is the ligand initially bound to the precatalyst. Surprisingly, the reaction profile was unaffected by up to 25 equivalent of Bu2S, and ∼500 equivalent of Bu2S were required before a very mild poisoning was observed (Figure 8).

catalysts show a stronger correlation between the number of equivalents of poison and the number of active catalyst molecules. For these experiments, we used RhHb, because it is closest to the active catalyst (shortest induction period) and performed experiments at 50 °C. We first tested 1,10-phenanthroline (phen), which is a welldocumented poison for both homogeneous and heterogeneous Rh catalysts.60−62 The poison was added after ∼20% of the reaction product had formed (∼2 min). Using the method of initial rates (examining the first ∼3 min of catalysis after addition of poison), we plotted the relative rate of the reaction versus the equivalents of poison. A poisoning effect is observed once more than 0.1 equivalent of phen is added, with greater poisoning occurring with more poison and complete poisoning observed when between 0.65 and 1 equivalent is added (Figure 7). Although less than 1 equivalent of phen gives a poisoning

Figure 8. Reaction monitoring by GC aliquots of the hydrosilylation of MDHM and MMVi catalyzed by RhCl3(Bu2S)3 in the presence of various amounts of additional Bu2S.

This indicates that additional Bu2S is very weakly binding. These results explain why the catalyst remains active despite an activation process that releases an equivalent of free Bu2S. Similarly, this also explains why the activity of 1/2[RhCl(Bu2S)2]2 is similar to that of RhCl3(Bu2S)3, even though the latter contains less sulfide. The active catalyst only requires 2 equivalent of Bu2S and is unaffected by further excess of thioether. Finally we tested catalyst poisoning with the phosphines PBu3 and 1,1,1-tris(diphenylphosphinomethyl)ethane (triphos). PBu3 poisoned the catalyst weakly, whereas the chelating triphos poisoning more resembled phen. The details of these experiments can be found in the Supporting Information (Section 7). E. RhxClx(Bu2S)y Versus Soluble Rh Particles. We sought to determine information about the primary coordination sphere of the catalyst. Our above studies supported a Rh/Bu2S ratio of 1:2 in the active catalyst. RhHb seems poised to reductively eliminate chlorosilane and form a reactive thioether-ligated RhI hydride. However, it seems likely that a chloride ligand remains on the metal: reactions of RhCl3(Bu2S)3 with varying amounts of MDHM and MMVi only release 2 equivalent of MDClM during the conversion of RhCl3(Bu2S)3 into an active catalyst (Figures S13 and S14). These 2 equivalent of MDClM originate from the conversion of RhCl3(Bu2S)3 into RhHb (Figure 4). Our studies support that the chloride ligand remains bound to Rh until the end of catalysis and that catalysis is significantly slowed when the RhHb decomposes. The reaction of RhHb with 1 equivalent of MMVi at 55 °C produced the hydro-

Figure 7. Reaction monitoring by GC aliquots of the hydrosilylation of MDHM and MMVi catalyzed by RhHb with various equivalents of phen added at 2 min (top), and the poisoning curve resulting from an initial rate analysis of the data (bottom).

effect, the number is not significantly less than 1 and is close to 0.5. Thus, these data are not consistent with Rh(0) nanoparticles for which much less than 1 equivalent of phen would be needed for complete poisoning.60−62 A conceivable explanation for ∼0.5 equivalent phen causing poisoning is that the active catalyst is bimetallic and that 1 equivalent of phen deactivates 1 equivalent of Rh2 dimer. Finke et al. also showed that the poisoning curve for the hydrogenation of benzene with Rh nanoparticles resembles an exponential decay, whereas the reaction catalyzed by a homogeneous catalyst has a plot with a sigmoidal shape.61 Our poisoning curve is also sigmoidal. More precisely, our data indicate that phen is a somewhat weakly binding poison toward the Rh sulfide catalyst, which is more consistent with a homogeneous molecular catalyst. Finke et al. showed that ∼2 nm Rh nanoparticles are significantly poisoned (for hydrogenation catalysis) by only 0.1 4489

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Scheme 4. Schematic Indicating Species Proposed to Be Encountered during the Thermal Reaction of RhCl3(Bu2S)3 with MDHM and Subsequent Degradation of the Active Species

∼0.5 equivalent of it was observed. The expected Rh byproduct [RhCl(Bu2S)2]2 was only observed in low quantities (Figure S12). This is perhaps not surprising, given that [RhCl(Bu2S)2]2 is only a precatalyst (is not part of the catalytic cycle). Similarly, an induction period, albeit very short, was observed for RhHb, which supports that it is a precatalyst. Future work will require more detailed characterization of the reaction mixtures produced upon stoichiometric reactions of the precatalysts with MDHM and MMVi.

silylation product M2D(CH2)2MM with no additional MDClM. Additionally, when monitoring the catalytic reaction in situ with 29 Si NMR, the amount of MDClM released approaches 2 equivalent (see Figures S13−S16). Release of the final equivalent of MDClM (third equivalent of Cl) is only observed toward the end of the catalytic reaction and especially after it has completed. Therefore, we propose that the catalytic reaction occurs with one Rh−Cl still in place as an ancillary ligand and that its elimination (forming MDClM) is slow and represents catalyst deactivation. The proposal of a Rh−Cl containing catalyst is similar to Wilkinson’s catalyst.68−70 Separate experiments have shown that, under stoichiometric conditions, the decomposition of RhHb occurs over several hours and releases the third equivalent of Cl. However, there remains the possibility that trace Rh particles or “Rh(Bu2S)2H” species (reductive elimination of MDClM from RhHb) could be highly active and be the kinetically dominant catalyst. To test this hypothesis, we attempted to generate Rh particles in situ. Our first attempts to study Rh(Bu2S)2H/nanoparticles involved the thermal decomposition of RhHb. Knowing that RhHb rapidly decomposes to heterogeneous Rh black at high temperature, we heated it at 55 °C for ∼3.5 h (Figure 4 and Scheme 4), resulting in significant darkening of the solution (but without the formation of heterogeneous species). The resulting mixture was found to have significantly diminished activity versus RhHb (Figure 9). Under these conditions the

3. CONCLUSIONS Detailed studies have been performed on the precatalyst RhCl3(Bu2S)3 to better understand its role in generating the active catalyst, its identity, and its decomposition into inactive forms (Scheme 4). The key data are summarized below: (a) The long bath life of RhCl3(Bu2S)3 is primarily due to an induction period in which it reacts with 2 equivalent of Si−H to form 1/2[RhHCl(SiR3)(Bu2S)2]2. (b) Preactivating the catalyst by isolating [RhHCl(SiR3)(Bu2S)2]2 (e.g., RhHb) can dramatically reduce the time frame of the reaction, but the active catalyst is not changed (Figure 10).

Figure 10. Monitoring of progress of the reaction of MDHM with MMVi using [RhCl(Bu2S)2]2 (RhCl(Bu2S)2, blue) and RhCl3(Bu2S)3 (orange).

(c) Other intermediates observed during activation are RhHa and [RhCl(Bu2S)2]2, the latter having an intermediate induction period. (d) Only 2 equivalent of Bu2S per Rh are necessary for the active catalyst. Approximately 1 equivalent of Bu2S dissociates as the active species forms. (e) During catalyst activation, two Cl ligands are removed (forming MDClM), and one remains bound to Rh. Further heating eventually removes the final Cl, and the catalyst deactivates. (f) RhHb slowly eliminates Cl-SiR3, but this reaction is slow compared to product release.

Figure 9. Comparison of catalytic activity of RhHb before (blue) and after (orange) it has been allowed to decompose thermally for 3.5 h at 55 °C.

products of RhHb decomposition have poor catalytic activity. Further heating/decomposition into heterogeneous Rh black resulted in a complete loss of activity. F. Initial Work on Stoichiometric Reactions. We attempted to further explore the role of RhHb by studying its stoichiometric reactions with MDHM and MMVi; however, the reaction mixtures were complex, and we cannot yet propose a specific catalytic cycle. The reaction of RhHb with MMVi does release the hydrosilylation product M2D(CH2)2MM, but only 4490

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Inorganic Chemistry

0.092 g (0.21 mmol, 35%). 1H NMR: δ 3.30 (m, 1H), 2.90 (m, 2H), 2.54 (bs, 2H), 2.20 (m, 1H), 1.87 (m, 2H), 1.79 (m, 1H), 1.70 (m, 1H), 1.34 (sextet, JH−H = 7.4 Hz), 1.23 (t, JH−H = 7.4 Hz), 0.95 (t, JH−H = 7.4 Hz). Anal. Cald. For C32H72Cl2Rh2S4: C, 44.59; H, 8.42; N, 0. Found: C, 44.61; H, 8.19; N, 0%. When a similar experiment was performed using only 1 equivalent of HTMS, only ∼0.015 g of the orange precipitate was collected. [RhCl(Bu2S)2]2 Synthesis from [RhCl(C2H4)2]2. A solution of 0.512 g (1.32 mmol) of [RhCl(Bu2S)2]2 in 20 mL of tetrahydrofuran (THF) was treated with a solution of 0.770 g (5.3 mmol) of Bu2S in 5 mL of THF at 5 mL/h via a syringe pump. Approximately 60 min after the addition was complete, the solvent was removed in vacuo, the dark black/brown turbid solid was redissolved in 3 mL of toluene, and then an orange precipitate was formed upon the addition of 30 mL of hexane. The orange precipitate was collected on a frit (0.485 g) and set aside, and a second crop was collected; the supernatant was dried in vacuo until a thick liquid remained, and then 30 mL of hexane was added. The resultant orange powder was collected on a frit (0.157 g). The two crops were then combined, redissolved in ∼2 mL of toluene, and then precipitated with ∼18 mL of hexane. The resultant orange microcrystalline powder was collected on a frit and dried in vacuo. Yield: 0.535 g (1.24 mmol, 47%). Product isolated in this way was slightly impure: Anal. Cald. For C32H72Cl2Rh2S4: C, 43.59; H, 8.42; N, 0. Found: C, 43.64; H, 7.96; N, 0%. Recrystallization from toluene afforded analytically pure product. Anal. Cald. For C32H72Cl2Rh2S4: C, 44.59; H, 8.42; N, 0. Found: C, 44.43; H, 8.20; N, 0%. Crystals of sufficient quality for X-ray diffraction were produced by cooling in toluene as above, or by layered diffusion of hexane into toluene. RhHCl(SiMe(OSiMe3)2)(Bu2S)2, for Example, RhHb. This complex was generated in situ as a 0.85 wt % Rh solution in toluene-d8 as follows: 0.384 g of RhCl3(Bu2S)3 (0.59 mmol) was dissolved in 6 g toluene-d8 and then was treated with 0.396 g of 1,1,1,3,5,5,5-heptamethyltrisiloxane (1.77 mmol). The resultant red solution was heated at 55 °C for 1 h, at which point the solution was pale orange. Integration with an internal standard at this stage indicated ∼75% yield of the desired product. This reagent solution was not further purified and was stored at −35 °C between uses. 1H NMR (toluene-d8): δ 4.92 (m, unreacted MDHM), 3.06 (m, 4H, Rh(Bu2S)), 2.77 (m, 4H, Rh(Bu2S)), 2.32 (t, 6H, free/exchanging Bu2S), 1.65 (m, 8H, Rh(Bu2S)), 1.45 (m, 6H, free/exchanging Bu2S), 1.31 (m, 16H, free/exchanging Bu2S and Rh(Bu2S)), 0.86 (t, 12H, J = 7.2 Hz, Rh(Bu2S)), 0.82 (t, 12H, free Bu2S), 0.33 (bs, Rh−SiMe(OSiMe3)2), 0.29 (bs, MeSiCl(OSiMe3)2), 0.15 (bs, RhSiMe(OSiMe3)2 + MeSiCl(OSiMe3)2), −18.49 (d, Rh−H, JRh−H = 27.9 Hz). NMR Monitoring Reactions. To a 20 mL scintillation vial was added 0.063 g (0.097 mmol) of RhCl3(Bu2S)3 followed by 1.00 g of toluene-d8. The mixture was briefly shaken, and then 0.065 g (0.29 mmol) of 1,1,1,3,5,5,5-heptamethyltrisiloxane (HTMS) was added. The mixture was briefly shaken and then immediately transferred to a J. Young valve NMR tube. The tube was sealed and stored in a glovebox freezer at −30 °C until the experiment began. At that point, it was removed from the glovebox and then inserted into an NMR spectrometer that was preheated to the appropriate temperature. 1H, 29 Si, and 13C NMR spectra were collected as appropriate. Each nucleus was recorded in a separate experiment to maximize the frequency of time points. Alternatively, the reaction mixture could be heated outside of the NMR spectrometer and then inserted into the spectrometer at various time points for analysis. Model Catalytic Reactions for Kinetics. A solution containing 9.00 g of toluene, 0.909 g of 1,1,1,3,5,5,5-heptamethyltrisiloxane, 0.712 g of vinylpentamethyldisiloxane, and 0.348 g of dodecane was added to a 40 mL scintillation vial with a disposable Teflon stirbar and preheated to the desired reaction temperature while it was stirred at 1500 rpm in a pie-block heating block; a heating block was selected such that the sides of the block extended ∼1 cm above the top of reaction solution. An aliquot of the desired precatalyst was injected into the reaction mixture. Typically, we studied reactions at 0.1 mol % catalyst loadings (305 μL of a 1 wt % solution of RhCl3(Bu2S)3 in toluene). Time point aliquots (150 μL) were collected as desired. The aliquots were immediately diluted with 1.25 mL of xylene and then

(g) Given (d) and (e), the active catalyst should contain the RhxClx(Bu2S)2x fragment. (h) RhHb is not catalytically active as evidenced by its brief induction period. (i) Various attempts to isolate the active catalyst were unsuccessful. One possibility is that the dimeric nature of RhHb requires it to dissociate into monomers before becoming active. (j) The Rh sulfide catalysts are tolerant to poisoning. Hg and Bu2S have limited effects, and polydentate ligands are needed to stoichiometrically deactivate the catalyst. (k) The combined data (poison ligand, Hg, Rh nanoparticle tests) are more consistent with a homogeneous small molecule catalyst than with Rh nanoparticles. However, the exact identity of the active catalyst has not yet been determined. (l) Various attempts at generating the catalyst from alternative synthetic routes or allowing RhHb to decompose in the presence of substrates did not improve the induction period. Isolating the active catalyst is nontrivial. The precatalyst easily decomposes into soluble and insoluble forms which do not further activate (Scheme 4). (m) In comparison to Pt (Karstedt’s catalyst), the Rh sulfide catalyst is a significantly slower catalyst in addition to the long induction period (Figure 10). During peer review, one reviewer expressed skepticism with regard to conclusion (k). We therefore are sharing these data with the intention that others can interpret differently. Additional experiments such as in situ transmission electron microscopy (TEM) or X-ray absorption fine structure (XAFS) would provide useful supporting data but were not available during the course of this work.

4. EXPERIMENTAL SECTION Unless otherwise indicated, all experiments were performed in an airfree, nitrogen-filled mBraun glovebox whose sensors indicated an atmosphere of less than 0.1 ppm of O2 and less than 0.1 ppm of H2O. RhCl3(Bu2S)3. To a solution of rhodium chloride hydrate (1.000 g, 38−40% Rh, Sigma-Aldrich, assumed molecular formula of RhCl3(H2O)3) in 10 mL of MeOH was added 1.669 g of Bu2S. The red solution was heated at 60 °C for 1 h and then dried in vacuo at 85 °C for 2 h. A thick red oil remained. Yield: 2.456 g (99.7%). 1H NMR (toluene-d8): δ 3.5 ppm (bs, 4H, Rh-SCH2ax), 2.3 (bs, 8H, RhSCH2eq), 1.72 (m, 12H, Rh-SCH2CH2), 1.97 (m, 12H, RhSCH2CH2CH2), 0.95 (m, 18H, Rh-SCH2CH2CH2CH3). 13C NMR (toluene-d8): δ 37.3 (s, 2C, Rh−SCH2eq), 35.4 (s, 2C, Rh−SCH2ax), 31.0 (s, 2C, Rh-SCH2CH2eq), 30.5 (s, 2C, Rh-SCH2CH2ax), 22.1 (s, 2C, Rh-SCH2CH2CH2eq), 22.0 (s, 2C, Rh-SCH2CH2CH2 ax), 13.8 (s, 2C, Rh-SCH2CH2CH2CH3eq), 13.7 (s, 2C, Rh-SCH2CH2CH2CH3ax). [RhCl(Bu2S)2]2. To a 20 mL scintillation vial was added 0.4 g (0.62 mmol) of RhCl3(Bu2S)3 followed by 2.00 g of toluene-d8. The mixture was briefly shaken, and then 0.275 g (1.24 mmol) of 1,1,1,3,5,5,5heptamethyltrisiloxane (HTMS) was added. The mixture was briefly shaken; a stirbar was added, and the mixture was heated at 55 °C for 1 h. At this point the sample was removed from the heating plate, and the 1H NMR spectrum was checked to ensure full consumption of HMTS. The sample was then carefully concentrated in vacuo at room temperature until ∼50% of the solvent had been removed, and an orange crystalline material became visible. The product was stored at −30 °C for 3 d and then collected on a frit via vacuum filtration. The product was then redissolved in ∼3 mL of toluene and precipitated by the dropwise addition of hexane (∼10 mL). The product was collected on a frit, and the precipitation was repeated. The orange microcrystalline powder was collected on a frit and dried in vacuo. Yield: 4491

DOI: 10.1021/acs.inorgchem.7b00029 Inorg. Chem. 2017, 56, 4484−4494

Article

Inorganic Chemistry

(4) Jones, L. A.; Schmidt, R. G. In Technology of Pressure-Sensitive Adhesives and Products; Benedek, I., Feldstein, M. M., Eds.; CRC Press: Boca Raton, FL, 2009. (5) Karstedt, B. D. Platinum-vinyl siloxanes. US3715334. (6) Willing, D. N. Catalysts for the Rection of SiH with Organic Compounds Containing Aliphatic Unsaturation. US3419593. (7) Kookootsedes, G. J.; Pluddemann, E. P. Acetylenic Inhibited Platinum Catalyzed Organopolysiloxane Composition. US3445420. (8) Marrot, S.; Maadadi, Y. Hydrosilylation Reaction Inhibitors, and Use Thereof for Preparing Stable Curable Silicone Compositions. US2014/0329099A1. (9) Stein, J.; Lewis, L. N.; Gao, Y.; Scott, R. A. In Situ Determination of the Active Catalyst in Hydrosilylation Reactions Using Highly Reactive Pt(0) Catalyst Precursors. J. Am. Chem. Soc. 1999, 121, 3693. (10) Meister, T. K.; Riener, K.; Gigler, P.; Stohrer, J.; Herrmann, W. A.; Kuhn, F. E. Platinum Catalysis Revisited Unraveling Principles of Catalytic Olefin Hydrosilylation. ACS Catal. 2016, 6, 1274. (11) Lewis, L. N.; Lewis, N. Platinum-Catalyzed HydrosilylationColloid Formation as the Essential Step. J. Am. Chem. Soc. 1986, 108, 7228. (12) Lewis, L. N. On the Mechanism of Metal Colloid Catalyzed Hydrosilylation: Proposed Explanations for Electronic Effects and Oxygen Cocatalysis. J. Am. Chem. Soc. 1990, 112, 5998. (13) Galeandro-Diamant, T.; Zanota, M.; Sayah, R.; Veyre, L.; Nikitine, C.; de Bellefon, C.; Marrot, S.; Meille, V.; Thieuleux, C. Platinum nanoparticles in suspension are as efficient as Karstedt’s complex for alkene hydrosilylation. Chem. Commun. 2015, 51, 16194. (14) Speier, J. L. Homogeneous Catalysis of Hydrosilation by Transition Metals. Adv. Organomet. Chem. 1979, 17, 407. (15) Webb, J. L.; Ritschard, H. V.; Lambert, J. M. Method and Intermediates for Preparation of Bis(aminoalkyl)polydiorganosiloxane. US5026890. (16) http://www.dowcorning.com/content/publishedlit/30-116301.pdf. Date Accessed: Jan 29, 2016. (17) Marciniec, B.; Maciejewski, H.; Pietraszuk, C.; Pawluc, P. Hydrosilylation: A Comprehensive Review on Recent Advances; Springer, 2009; Vol. 1. (18) Marciniec, B. Comprehensive Handbook on Hydrosilylation; Pergamon Press: Oxford, U.K., 1992. (19) Roy, A. K. A Review of Recent Progress in Catalyzed Homogeneous Hydrosilation. Adv. Organomet. Chem. 2007, 55, 1. (20) Nakajima, Y.; Shimada, S. Hydrosilylation reaction of olefins: recent advances and perspectives. RSC Adv. 2015, 5, 20603. (21) Sun, J.; Deng, L. Cobalt Complex-Catalyzed Hydrosilylation of Alkenes and Alkynes. ACS Catal. 2016, 6, 290. (22) Tondreau, A. M.; Atienza, C. C. H.; Weller, K. H.; Nye, S. A.; Lewis, K. M.; Delis, J. G. P.; Chirik, P. J. Iron Catalysts for Selective Anti-Markovnikov Alkene Hydrosilylation Using Tertiary Silanes. Science 2012, 335, 567. (23) Hojilla Atienza, C. C.; Tondreau, A. M.; Weller, K. H.; Lewis, K. M.; Cruse, R. W.; Nye, S. A.; Boyer, J.; Delis, J. G. P.; Chirik, P. J. High-Selectivity Bis(imino)pyridine Iron Catalysts for the Hydrosilylation of 1,2,4-Trivinylcyclohexane. ACS Catal. 2012, 2, 2169. (24) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. Preparation and Molecular and Electronic Structures of Iron(0) Dinitrogen and Silane Complexes and Their Application to Catalytic Hydrogenation and Hydrosilation. J. Am. Chem. Soc. 2004, 126, 13794. (25) Chen, C.; Hecht, M. B.; Kavara, A.; Brennessel, W. W.; Mercado, B. Q.; Weix, D. J.; Holland, P. L. Rapid, Regioconvergent, Solvent-Free Alkene Hydrosilylation with a Cobalt Catalyst. J. Am. Chem. Soc. 2015, 137, 13244. (26) Peng, D.; Zhang, Y.; Du, X.; Zhang, L.; Leng, X.; Walter, M. D.; Huang, Z. Phosphinite-Iminopyridine Iron Catalysts for Chemoselective Alkene Hydrosilylation. J. Am. Chem. Soc. 2013, 135, 19154. (27) Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. A Highly Chemoselective Cobalt Catalyst for the Hydrosilylation of Alkenes using Tertiary Silanes and Hydrosiloxanes. ACS Catal. 2016, 6, 3589.

were analyzed by GC. In cases where the catalyst speed was high enough that some observable amount of reaction occurred in the GC vial, xylene containing ∼0.0004 M phenanthroline (∼10 equivalent of phen per equivalent of Rh) was used to stop the reaction; this was typically only necessary for Pt, and for RhHb it was a precaution. NMR Monitoring of RhHb with Model Substrates. A solution of RhHb was generated as indicated above, and to this was added various amount of 1:1 mixtures of 1,1,1,3,5,5,5-heptamethyltrisiloxane and vinylpentamethyldisiloxane. After heating if appropriate, the sample was transferred to a 10 mm Teflon NMR tube with a Teflon stopper (for 29Si NMR) or a conventional J. Young valve 5 mm NMR tube for 1H NMR. For the 29Si NMR experiments, ∼0.1 g of Cr(AcAc)3 was added to the sample to aid in magnetic relaxation. We have not found this to impact catalysis or the chemistry of the complexes. When appropriate, internal standards were added; (Me3SiO)4Si (M4Q) for 29Si NMR, and trimethoxybenzene for 1H NMR.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00029. X-ray crystallographic information (CIF) Synthesis and NMR spectra, reaction monitoring data, poisoning experiments, attempted synthesis and catalysis, attempted activation, sample and crystal data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Matthew T. Olsen: 0000-0002-7543-1070 Author Contributions

All authors have given approval to the final version of the manuscript. Funding

Funding for this work was provided by Dow Corning Corporation. Notes

The authors declare no competing financial interest. The GE shorthand for silicones is as follows: M = Me3SiO1/2, D = Me2SiO2/2, T = MeSiO3/2, Q = SiO4/2. The subscript numerator indicates the number of oxygen linkages shared between the Si atom and another Si atom. The denominator is meant to keep track of the appropriate empirical formula. Thus, the hypothetical siloxane ((MD)3T)4Q is used to name ((Me3SiOMe2SiO)3SiO)4Si.

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ACKNOWLEDGMENTS M.T.O. would like to thank Dr. L. Durfee for extensive discussions on this work as well as Prof. R. Finke for discussions on the catalyst poisoning experiments.

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REFERENCES

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