Article pubs.acs.org/Organometallics
Iridium Pincer Catalysts for Silane Dehydrocoupling: Ligand Effects on Selectivity and Activity Neil T. Mucha and Rory Waterman* Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125, United States S Supporting Information *
ABSTRACT: Catalytic reactions of bisphosphinite pincerligated iridium compounds p-XR(POCOP)IrHCl (POCOP) [2,6-(R2PO)2C6H3, R = iPr, X = H (1); R = tBu, X = COOMe (2); = H (3); = NMe2 (4)] with primary and secondary silanes have been performed. Complex 1 is primarily a silane redistribution precatalyst, but dehydrocoupling catalysis is observed for sterically demanding silane substrates or with aggressive removal of H2. The bulkier compounds (2−4) are silane dehydrocoupling precatalysts that also undergo competitive redistribution with less hindered substrates. Products generated from reactions utilizing 2−4 include low molecular weight oligosilanes with varying degrees of redistribution present or disilanes when employing more sterically demanding silane substrates. Selectivity for redistribution versus dehydrocoupling depends on the steric and electronic environment of the metal but can also be affected by reaction conditions.
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INTRODUCTION Polysilanes are not primarily prepared by metal catalysis, despite how essential metal catalysis is for the preparation of specialty polyolefins.1 This is a surprising dichotomy given the premium on monodisperse, linear polysilanes of high molecular weight to promote optimal σ-electron delocalization.2,3 Reasons for the lack of industrial adoption of metal-catalyzed silane dehydrocoupling depend on the general category of catalyst employed. For example, well-studied and highly active d0 metal catalysts tend to suffer from competitive backbiting and formation of cyclosilanes, which can limit molecular weight and increase polydispersity.4 Despite the fact that the first report on silane dehydrocoupling used Wilkinson’s catalyst,5 initial observations of low dehydrocoupling activity with late metals stunted their development relative to d0 metal catalysts.6 Additionally, the use of late metal catalysts in dehydrocoupling is potentially problematic due to two metal-catalyzed side reactions, the oxidation of Si−Si bonds7 and redistribution.8 Thus, the potential advances that metal catalysis can offer to polysilane preparation cannot be realized until the factors that impact metal catalysis are better understood. Efforts to minimize these side reactions have included promoting hydrogen loss,9 employing designer substrates,10,11 inhibiting silyl coordination,12 and rigorous exclusion of water and oxygen. While conditions for promoting linear polysilanes over cyclosilanes are known for d0 metallocene complexes, general conditions to promote dehydrocoupling over redistribution are not known.4 However, late metal catalysts using nickel,13−16 rhodium,17,18 and platinum19 have shown dehydrocoupling activity and selectivity comparable to that of the group 4 © XXXX American Chemical Society
metallocenes, although these methods of polymerization still generally suffer from lower than desired molecular weights and high polydispersity. Iridium pincer complexes bearing the POCOP ligand (POCOP = 2,6-(tBu2PO)2C6H3−) are thermally robust molecules that have shown high activity as dehydrogenation catalysts using both alkanes and amine-boranes as substrates.20 Although iridium-catalyzed silane redistribution is a known reaction,8,21−23 the high activity of POCOP−iridium compounds for dehydrogenation catalysis and the ability of the ligand to be easily tuned to modulate reactivity warranted investigation of these systems as potential silane dehydrocoupling catalysts. Herein, we report the synthesis of a series of pXR(POCOP)IrHCl [p-XR(POCOP) = 2,6-(R2PO)2C6H3, R = i Pr, X = H (1); R = tBu, X = COOMe (2); = H (3); = NMe2 (4)] compounds and their ability to facilitate either dehydrocoupling or redistribution depending upon the substrate and, more importantly, reaction conditions.
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RESULTS AND DISCUSSION Synthesis and Characterization of p-XR(POCOP)IrHCl Compounds. Efforts to prepare iPr(POCOP)IrHCl (1)24 by reaction of [(COE)2IrCl]2 with iPr(POCOP)-H (iPr(POCOP)H = 2,6-(iPr2PO)2C6H3) yielded a mixture of products including 1 in low yield (ca. 10%, 31P δ = 173.0). A closely related chloro-bridging compound, iPr(POCOP)IrH(μ-Cl)2Ir(COE)2,25 has diagnostic NMR resonances (31P δ = 153.4; Ir− H δ = −24.0) that mirror those of the major product of the Received: June 7, 2015
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DOI: 10.1021/acs.organomet.5b00486 Organometallics XXXX, XXX, XXX−XXX
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Organometallics reaction (31P δ = 153.0; Ir−H δ = −25.1), which is tentatively proposed as the chloro-bridged dimer [iPr(POCOP)IrH(μCl)]2. Successful preparation of analytically pure 1 was accomplished by refluxing a toluene solution of [(COE)2IrCl]2 and iPr(POCOP)-H ligand for 16 h under a H2 atmosphere. By analogy to 3,26 the p-XtBu(POCOP)IrHCl (tBu(POCOP) = 2,6-(tBu2PO)2C6H3−) compounds (2 = pCOOMetBu(POCOP)IrHCl; 4 = p-NMe2tBu(POCOP)IrHCl) were prepared by refluxing a toluene solution of the appropriate ligand precursor and [(COD)IrCl]2, then isolated by washing with cold pentane and subsequent filtration. The yield of 4 (81%) was slightly lower than that for 2 (88%), which is attributed to greater solubility of 4 in pentane. Compounds 1, 2, and 4 were characterized by multinuclear NMR and infrared spectroscopy, combustion analysis, and single-crystal X-ray diffraction. Like related POCOP−nickel compounds with substituted pincer aryl backbones,27 the values of 31P NMR resonances for 1−4 fall into a narrow range. There is no apparent correlation between the identity of the substituent para to iridium and the 31 P NMR chemical shift values observed for 1−4. The hydride chemical shifts of compounds 2−4 are in a narrow range, with 2 displaying the most downfield shift and 4 the most upfield shift, indicating a modest effect from the para substituent (Table 1).
from the less than linear P(1)−Ir−P(2) and C(1)−Ir−Cl angles, which suggest the presence of an apical ligand with small steric requirements (Table 2).29 There is no apparent structural difference between compounds 2, 3, and 4 to suggest a significant trend based on the para substitution. Compound 1 is isostructural with the tert-butyl analogues, although it has a substantially longer Ir−Cl bond length (Table 2). Catalytic Reactions with Primary Silanes: Reactions of PhSiH3 in a Sealed NMR Tube. Benzene-d6 solutions of PhSiH3 were treated with 2 mol % of each of 1−4 in J-Young type, PTFE-valved NMR tubes. These solutions were degassed via two freeze−pump−thaw cycles, heated for 16 h at 120 °C, and then analyzed by 1H NMR spectroscopy. Trace amounts of PhSiH2Cl (δ 5.05) were observed by 1H NMR spectroscopy. It is therefore presumed that 1−4 undergo ligand exchange to the respective iridium dihydride species, (POCOP)Ir(H)2, of which p-HtBu(POCOP)Ir(H)2 is known.31 Analysis of the 1H NMR spectra of all reactions reveals products from two competing reactions: dehydrocoupling and redistribution. In reactions with tBu-substituted 2−4, silane dehydrocoupling is favored and evident by the presence of 1,2diphenyldisilane (PhSiH2)2, H2, linear oligosilanes with 1H NMR resonances in the range δ 4.3−4.8, and cyclic oligomers with resonances in the range δ 5.2−5.5. In addition to dehydrocoupling products, signals due to Ph2SiH2 and SiH4 are also present in the 1H NMR spectra, demonstrating competitive silane redistribution. In contrast, 1 favors redistribution of PhSiH3 over dehydrocoupling and yields Ph2SiH2 (73%) as the main product of the reaction with Ph3SiH (4%) present as well (Table 3). It is worthwhile to note that the product percentages herein are based on normalized integration of Si−H resonances postreaction and reflect only the postreaction product distributions of these reactions. Trace amounts of dehydrocoupling products, including (PhSiH2)2, (Ph2SiH)2, and PhSiH2− SiPh3, were detected and suggest that 1 may engage in dehydrocoupling catalysis under alternative reaction conditions. Comparing compounds 1−4, the phosphine substituent impacts the catalytic activity and product distribution. The less bulky derivative, 1, demonstrates higher activity toward silanes with nearly complete consumption of substrate in 0.5 h but dramatically favors redistribution at silicon over dehydrocoupling. In contrast, the bulkier tBu derivatives are less active and require >12 h of reaction time to reach completion, but these compounds favor dehydrocoupling products over redistribution (Table 3). Among the series of compounds screened, the relative amount of dehydrocoupling versus redistribution in the product distribution was affected by the substituents para to iridium more than the conversion. Each of the three tBu phosphine
Table 1. Diagnostic NMR Data (δ) for Compounds 1−4 in Benzene-d6 Solutiona compound
Ir−H 1H NMR
[Ir], 1 p-COOMetBu[Ir], 2 p-HtBu[Ir], 3 p-NMe2tBu[Ir], 4
−36.7 −40.1, 2JPH = 12.9 −40.7, 2JPH = 13.1 −41.1, 2JPH = 13.3
ipr
a
31
P NMR 173.0 176.5 175.3 176.3
[Ir] = (POCOP)IrHCl.
For compound 1, the hydride resonance is broadened to a singlet and shifted downfield to δ −36.7. This difference in chemical shift is consistent with the ligand environment of 1 that possesses the less electron-rich iPr phosphine ligand relative to the tBu phosphine substituents employed in 2−4. These diagnostic resonances compare favorably to pHtBu(POCOP)IrHCl (3), reported by Brookhart and coworkers (Table 1).26 Suitable crystals were grown for compounds 1, 2, and 4 and subjected to single-crystal X-ray diffraction studies (Figure 1). The compounds are square-based pyramidal at iridium, as demonstrated by calculated τ values of 0.10, 0.02, and 0.08 for 1, 2, and 4, respectively.28 The hydride ligand was located for all three compounds from the Fourier difference map. Additional evidence for the location of this ligand comes
Figure 1. Molecular structure of 1, 2, and 4 (left to right) with thermal ellipsoids rendered at the 30% probability level. Hydrogen atoms, except H(100) located on iridium, are omitted for clarity. B
DOI: 10.1021/acs.organomet.5b00486 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 2. Select Bond Lengths (Å) and angles (deg) for Compounds 1−4a iPr
[Ir], 1
Ir−C(1) Ir−P(1) Ir−P(2) Ir−Cl P(1)−Ir−P(2) C(1)−Ir−Cl(1) a
2.033(5) 2.2994(14) 2.2977(14) 2.5501(13) 156.79(5) 176.26(13)
p-COOMetBu[Ir], 2
p-HtBu[Ir],30 3
p-NMe2tBu[Ir], 4
1.992(4) 2.2871(13) 2.2864(13) 2.3933(13) 160.37(4) 177.69(14)
2.000(3) 2.2897(10) 2.2925(11) 2.3947(12) 159.90(4) 179.50(11)
1.996(3) 2.2890(8) 2.2940(8) 2.3954(8) 159.79(3) 176.33(9)
[Ir] = (POCOP)IrHCl. Data for 3 are reproduced from Wendt and co-workers.30
The observed product distributions for 1−4 can best be explained by considering their relative steric environments. Iridium-catalyzed silane redistribution is proposed to occur via silylene intermediates.8 Both a vacant coordination site on the metal and subsequent Si−H/R activation are necessary to form silylene complexes from organosilanes.32 Thus, the heightened reactivity of 1 for redistribution is a likely consequence of the more open metal coordination sphere, which can accommodate both the silylene ligand and an incoming silane substrate. For compounds 2−4, reductive elimination to form Si−Si bonds must be accelerated under steric pressure from the tert-butyl substituents. The more subtle electronic effect in reactions with 2−4 may be a function of the less electron-rich system, favoring reductive elimination to the lower iridium oxidation state. An alternative possibility is that the more electron-rich system stabilizes the higher iridium oxidation state, allowing for silylene intermediates to form. The former hypothesis is more attractive because the reactivity of 2 is lower and the selectivity for dehydrocoupling is greater than those of 3 and 4, which suggests that Si−H bond activation may be less favorable for the more electron-deficient compound.
Table 3. Product Distributions of NMR Tube-Scale Reactions of PhSiH3 with Catalytic 1−4a products cmpd
conversionb (%)
redistribution
dehydrocoupling
[Ir], 1c p-COOMetBu[Ir], 2 p-HtBu[Ir], 3 p-NMe2tBu[Ir], 4
97 75 83 86
83 5 18 37
13 70 65 49
ipr
a Reaction conditions: 2 mol % [Ir], 120 °C, 16 h. bMeasured by 1H NMR spectroscopy with respect to an internal standard of C6Me6. c Trace (Ph2SiH)O (≤1%) was observed in this reaction.
complexes (2−4) converted 70−80% of the starting material to products after heating for 16 h. Compound 2 afforded the lowest conversion, but it displayed the highest selectivity for dehydrocoupling products over redistribution products. Conversely, 4 had the highest conversion of starting material and also gave the greatest amount of secondary silane and SiH4 relative to dehydrocoupling products (Table 3).
Figure 2. 1H NMR spectra in benzene-d6 solution of reactions of 1 with PhSiH3 both sealed and open to a N2 atmosphere and reactions of 2−4 open to a N2 atmosphere after 16 h. Redistribution products are seen with compound 1, while dehydrocoupling products are primarily seen with compounds 2, 3, and 4. C
DOI: 10.1021/acs.organomet.5b00486 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Table 4. Product Distributions for Reaction of PhSiH3 with Catalytic 1−4 under N2a products (%) cmpd iPr
d
[Ir], 1 p-COOMetBu[Ir], 2 p-HtBu[Ir], 3 p-NMe2tBu[Ir], 4
TOFb,c (h−1)
Ph2SiH2
Ph3SiH
SiH2Ph−SiPh3
(Ph2SiH)2
219 19 39 53
54 3 11 15
12
14
20
6
oligosilanes 97 89 79
Reaction conditions: 2 mol % of [Ir], 120 °C, 16 h. Substrate completely consumed in all reactions. bMeasured by 1H NMR spectroscopy (Si−H region). cMeasured at 0.25 h reaction time with respect to an internal standard of C6Me6. dTrace (Ph2SiH)O (≤3%) was observed in this reaction. a
Figure 3. 29Si{1H} DEPT (θ = 90°, 135°) NMR spectra in benzene-d6 solution of reactions of 2 with PhSiH3 open to a N2 atmosphere after 16 h. Three distinct regions are visible and correspond to the number of Si and Ph groups present.
Reactions of Primary Silanes Open to N2 Atmosphere. Reactions were run open to a N2 atmosphere and rapidly stirred to facilitate dissipation of evolved gases. Under these conditions, the reaction of PhSiH3 spiked with an internal standard of C6Me6 with 2 mol % of 1 at reflux in toluene solution for 16 h resulted in the complete conversion of PhSiH3 to primarily silane redistribution products. Compared to the same reaction run in an NMR tube (vide supra), reaction of 1 and PhSiH3 open to N2 showed increased activity (TOF = 219 h−1, sampled at 15 min) based on the extent of redistribution products present (Figure 2). In addition to Ph2SiH2, which is the dominant product under both sets of reaction conditions, Ph3SiH, (Ph2SiH)2, and PhSiH2−SiPh3 are also present as determined by 1H and 29Si NMR spectroscopy. These products were minimal (
