Letter pubs.acs.org/acscatalysis
SiO2‑Supported Rh Catalyst for Efficient Hydrosilylation of Olefins Improved by Simultaneously Immobilized Tertiary Amines Ken Motokura,*,† Kyogo Maeda,† and Wang-Jae Chun‡ †
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan ‡ Graduate School of Arts and Sciences, International Christian University, Mitaka, Tokyo 181-8585, Japan S Supporting Information *
ABSTRACT: The simultaneous immobilization of a Rh complex and a tertiary amine on a SiO2 surface afforded a highly active supported catalyst for the hydrosilylation of terminal olefins. The turnover number in the reaction using 0.00005 mol % of the Rh catalyst approached 1 900 000 over 24 h. A broad range of terminal olefins and hydrosilanes acted as good substrates, giving their corresponding hydrosilylation products in excellent yields. CSI-mass analysis of the precursor solution containing the Rh complex and tertiary amine indicated complexation between these two precursors. The resultant close positioning of the Rh complex and amine after attachment onto SiO2 surface led to excellent catalysis. KEYWORDS: rhodium complex, amine, silica, hydrosilylation, concerted catalysis
T
Rh−H) bond.1,6 In this case, electron donation to the Rh complex may promote both the oxidative addition and insertion steps. These facts encouraged us to develop a Rh complex on SiO23 with an electron-donating amino functionality for the hydrosilylation. Herein, we report the novel catalyst with coimmobilized tertiary amines in close proximity to the directly attached Rh(I) complexes on a SiO2 surface. The simultaneously immobilized Rh complex and tertiary amine catalyst (i.e., SiO2/Rh-NEt2) exhibited an excellent turnover number and a wide substrate scope for the hydrosilylation of olefins (see eq 1).
he hydrosilylation of olefins is one of the most widely used reactions for the synthesis of organosilicon compounds in research laboratories and industrial settings.1 Recently, supported Rh complexes have received considerable attention as highly active catalysts for hydrosilylation reactions. The high cost of Rh necessitates the development of reaction systems with very low Rh loadings, as well as recyclable catalysts. For example, Sawano and co-workers reported the stabilization of a Rh-phosphine complex in a MOF channel.2 Furthermore, Marciniec and co-workers reported a SiO2supported Rh complex that is active for the hydrosilylation of olefins.3 The reason for the excellent performance of supported Rh complexes is mainly due to the isolated Rh center on the support surface, which enables stabilization of the active Rh complex. Accordingly, the reaction mechanism of Rh complexcatalyzed hydrosilylation changes, because of its immobilization on the SiO2 surface, resulting in high activity and stability of the Rh complex.3 Several research groups,4 including ours,5 have developed this catalyst design concept further in order to increase catalytic activity; organic moieties are immobilized close to the metal complex on the same support surface. To illustrate, good performance for the 1,4-addition of phenylboronic acid was demonstrated using a catalyst involving a diaminorhodium complex and a tertiary amine on a SiO2 surface.5a The tertiary amine attached to the SiO2 surface can activate phenylboronic acid, which accelerates transmetalation between the RhOH species and phenylboronic acid.5a Usually, Rh-catalyzed hydrosilylations proceed via Rh−Si (or Rh−H) bond formation by oxidative addition, coordination of Rh to an olefin, and insertion of the olefin into the Rh−Si (or © XXXX American Chemical Society
The preparation procedure of the SiO2-supported Rh complex is shown in Scheme 1. As seen, treatment of SiO2 with [Rh(OH)cod]2 afforded the SiO2-supported Rh complex (SiO2/Rh). The SiO2 support with both the Rh complex and tertiary amine (SiO2/Rh-NEt2) was prepared from a mixed solution of both precursors, as shown in Scheme 1B. The formation of methanol and water in the supernatant after the grafting reaction was detected by 1H NMR analysis. Results from elemental analysis of the prepared samples are summarized in Table 1. As seen, essentially the same Rh content was detected from the samples. However, SiO2/RhReceived: May 9, 2017 Revised: June 6, 2017
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DOI: 10.1021/acscatal.7b01523 ACS Catal. 2017, 7, 4637−4641
Letter
ACS Catalysis Scheme 1. Preparation of (A) SiO2/Rh and (B) SiO2/RhNEt2
Table 1. Elemental Analysis Results of SiO2-Supported Rh Complexes catalyst
C (mmol g−1)
N (mmol g−1)
Rh (mmol g−1)
SiO2/Rh SiO2/Rh-NEt2
4.3 7.0
0.6
0.46 0.43
Figure 1. 13C NMR spectra for (a) SiO2/Rh-NEt2, (b) tertiary amine precursor, and (c) [RhOH(cod)]2.
NEt2 showed a larger amount of carbon and nitrogen, because of the presence of the amine group. The chemical structures of the attached Rh complex and amine were characterized by solid-state nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR) spectroscopy, and Rh K-edge X-ray absorption fine structure (XAFS) measurements. Figure S1 in the Supporting Information (SI) presents the 29Si MAS NMR spectra of parent SiO2 and SiO2-support catalysts. After the attachment of Rh, a decrease in Q2 (−90 ppm: Si(OH)2(OSi)2) and Q3 (−100 ppm: Si(OH)1(OSi)3) was detected, qualitatively. In the case of SiO2/Rh-NEt2, the signals corresponding to the Q2 and Q3 sites diminished. FT-IR analysis also revealed that the intensity of ν(OH) stretching of SiOH (3747 cm−1)7 significantly decreased for supported catalysts, such as SiO2/ Rh and SiO2/Rh-NEt2 (see Figure S3 in the SI). The ν(OH) stretching of the [Rh(OH)cod]2 precursor8 at 3585 and 3534 cm−1 also disappeared after attachment on the SiO2 surface (see Figure S4 in the SI). These results clearly indicate that both [Rh(OH)cod]2 and the tertiary amine with a Si(OMe)3 group were immobilized by reaction with surface Si−OH groups on the SiO2 surface. The 13C CP MAS NMR spectrum of SiO2/ Rh-NEt2 and the solution spectra of the Rh and amine precursors are shown in Figure 1. A comparison of these spectra indicates no significant change in the carbon chain of the cyclooctadiene ligand and tertiary amine, suggesting that the cyclooctadiene coordination structure3 and tertiary amine skeleton in SiO2/Rh-NEt2 were maintained. FT-IR signals at ∼2700−3000 cm−1 (ν(CH)) and at 1464 cm−1 (δ(CH3)) (Figures S3 and S4) also support the NMR results. Figure 2 displays the Rh K-edge XANES and Fourier transform (FT) of k3-weighted Rh K-edge EXAFS spectra of the SiO2-supported Rh complexes, [Rh(OH)cod]2, and reference materials. These XAFS results imply that the local structures of SiO2/Rh-NEt2, SiO2/Rh, and [Rh(OH)cod]2 are almost identical; however, they differ completely from Rh2O3 and Rh metal. The main
Figure 2. (A) Rh K-edge XANES and (B) Fourier transform of k3weighted Rh K-edge EXAFS spectra of (a) SiO2/Rh-NEt2, (b) SiO2/ Rh, (c) [RhOH(cod)]2, (d) Rh2O3, and (e) Rh0 foil. The k-range for FT was k = 2.0−12 Å−1.
peak at 1.6 Å in the FT-EXAFS spectra can be assigned as Rh− O or Rh−C, denoted as Rh−O/C bonds. The curve-fitting results of the main peak using the Rh−O/C parameter, along with the [Rh(OH)cod]2 crystal structure,8 are summarized in Table 2. The peaks for supported Rh complexes were wellfitted, with a coordination number of ∼6 and a Rh−O/C bond length of 2.09 Å. The NMR, FT-IR, and XAFS results support the proposed structures of SiO2/Rh and SiO2/Rh-NEt2 shown in Scheme 1. Notably, the presence of the amine does not affect the local structure of the Rh complex on the SiO2 surface. The effect of coimmobilized amines on the hydrosilylation activity of the Rh complexes was examined in the reaction of terminal olefins with 1,1,1,3,5,5,5-heptamethyltrisiloxane as the hydrosilane, as shown in Table 3. In the reaction of 1hexadecene, a 96% yield of the hydrosilylation product was 4638
DOI: 10.1021/acscatal.7b01523 ACS Catal. 2017, 7, 4637−4641
Letter
ACS Catalysis Table 2. Curve-Fitting Analysis of EXAFS Spectra for SiO2-Supported Rh Catalystsa sample
shell
SiO2/Rh SiO2/Rh-NEt2 [Rh(cod)OH]2e [Rh(cod)OH]2e
Rh−C/O Rh−C/O (Rh−C) (Rh−O)
coordination number, N bond distance, r (Å)b Debye−Waller factor, Δσ2 (× 10−3 Å2)c inner potential correction, ΔE (eV)d 5.5 ± 0.8 5.7 ± 0.8 4 2
2.09 ± 0.01 2.10 ± 0.01 2.09f 2.07f
5.33 ± 0.23 4.23 ± 0.23
−4.00 ± 2.36 −4.40 ± 2.36
Fourier transform and Fourier-filtering regions were limited, where Δk = 2.8−15 Å−1 and Δr = 1.1−2.0 Å, respectively. The goodness of curve fit is in the range of 0.67−1.07. bThis is the bond distance between the absorber and backscatter atoms. cThe Debye−Waller factor (DW) value is relative to the DW of the reference. dThe inner potential correction accounts for the difference in the inner potential between the sample and reference. e Data taken from ref 8. fAverage values were reported. a
The scope of silanes compatible with the SiO2/Rh-NEt2catalyzed reaction is summarized in Chart 1. Simple
Table 3. Hydrosilylation Catalyzed by SiO2-Supported Rh Complex
Chart 1. Substrate Scope for the SiO2/Rh-NEt2-Catalyzed Hydrosilylation (Yield Was Determined by 1H NMR Using an Internal Standard) Yield (%)a catalyst SiO2/Rh-NEt2 SiO2/Rh SiO2/NEt2 SiO2/Rh-NEt2 SiO2/Rh a
hydrosilylation R = (CH2)12CH3 96 (15 min) 9 (30 min)
