Mechanistic Insight into Synergistic Catalysis of Olefin Hydrogenation

Aug 9, 2019 - Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, CREST, Japan. Science and Technology Agency (JST). ,. Tsukuba,...
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Mechanistic Insight into Synergistic Catalysis of Olefin Hydrogenation by a Hetero-Dinuclear RuII−CoII Complex with Adjacent Reaction Sites Dachao Hong,*,† Yuji Ohgomori,† Yoshihiro Shimoyama,† Hiroaki Kotani,‡ Tomoya Ishizuka,‡ Yoshihiro Kon,† and Takahiko Kojima*,‡

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†

Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, CREST, Japan Science and Technology Agency (JST), 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan S Supporting Information *

bpp)3}2MnII2MnIII(μ-O)]3+ complex13 and Ru−Zn heteropolynuclear complexes14 have also been synthesized by using bpp− as a bridging ligand. In light of those complexes reported, Hbpp is considered as a promising ligand for the synthesis of hetero-dinuclear complexes because the deprotonated pyrazole moiety can induce the second coordination site (Figure 1a). In this context, stepwise introduction of two different metal complexes into the Hbpp ligand can provide a sure and facile method for synthesis of hetero-dinuclear metal complexes. It is expected that a combination of different transition metal ions such as 3d and 4d transition metals will offer a unique property and catalytic performance. Herein, we report the synthesis of a hetero-dinuclear Ru−Co complex, [RuII(p-cymene)(μ-bpp)(Cl)CoII(dipc)(H2O)] (1), which is a fusion of [RuII(p-cymene)(Hbpp)(Cl)]+ (2)15 and [CoII(dipc)(Hbpp)(H2O)] (3) (p-cymene = 4-isopropyltoluene, H2dipc = 2,6-pyridinedicarboxylic acid) as components, as shown in Figure 1b. The three complexes (1−3) were employed as catalysts for hydrogenation of 1-dodecene using NaBH4 as a hydride source. Hydrogenation of olefins has also been described using NaBH4 with metals salts that afford uncertain catalytically active entities in early reports;16,17 however, mechanistic insights have rarely been gained into those reactions because the catalysis proceeds in a heterogeneous manner. In sharp contrast to the previous works, herein, a metal-hydrido intermediate derived from 1 in 1-dodecene hydrogenation has been successfully detected by 1H NMR and electrospray ionization (ESI)-time-of-flight (TOF)-MS measurements. The rate-determining step (RDS) and catalytic mechanism are discussed on the basis of kinetic analysis of 1docecene hydrogenation by 1. It is also revealed that the Ru center of 1 acts as a reactive site forming the RuII-hydrido intermediate and the CoII center as an accelerating site by activating methanol as a proton source to arise a synergistic effect. We first synthesized complexes 2 and 3 (Figure 1b) as the components of 1 according to the literature (see the Supporting Information).15,18 Characterization of the complexes was made by 1H NMR spectroscopy and ESI-TOF-MS

ABSTRACT: We have designed and synthesized a hetero-dinuclear RuII−CoII complex with a dinucleating ligand inspired by hetero-dinuclear active sites of metalloenzymes. A synergistic effect between the adjacent RuII and CoII sites has been confirmed in catalytic olefin hydrogenation by the complex, exhibiting a much higher turnover number than those of mononuclear RuII or CoII complexes as the components. A RuII-hydrido species was detected by 1H NMR and electrospray ionization (ESI)time-of-flight (TOF)-MS measurements as an intermediate to react with olefins, and CoII-bound methanol was suggested to act as a proton source.

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n nature, multinuclear metal complexes/clusters in enzymes composed of different metal ions have been known to act as active sites to perform diverse reactions.1,2 For example, [CuZn]SOD (superoxide dismutase),3 [NiFe]hydrogenase,4 and the oxygen evolving complex in the photosystem II5 have two kinds of metal ions working on different tasks at active sites; the metal centers play cooperative roles to accomplish the tasks required for the enzymes. Multimetallic active sites that consist of hetero-metal centers and supporting ligands have been reported to exhibit attractive features in catalysis.6−10 Among multinuclear complexes with different metal ions, hetero-dinuclear metal complexes should offer advantages to clarify synergistic effects, since only two distinct sites participate in catalytic reactions. The use of dinucleating ligands should be effective for preparing hetero-dinuclear metal complexes; however, some difficulty can be raised in the formation of a desired dinuclear complex. Simultaneous introduction of two kinds of metal ions into a single dinucleating ligand with two equivalent coordination sites can produce dinuclear complexes with various patterns of metal pairs.11 Thus, sequential insertion of two kinds of metal ions into a single dinucleating ligand can be a promising approach for preparing desired hetero-dinuclear complexes selectively. Recently, a dinuclear RuII complex with 3,5-bis(2-pyridyl)pyrazole (Hbpp) ligand has been reported as a catalyst for water oxidation.12 A pentanuclear [{MnII(μ© XXXX American Chemical Society

Received: July 14, 2019

A

DOI: 10.1021/acs.inorgchem.9b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Table 1. Catalytic Activity of the Metal Complexes for 1Dodecene Hydrogenationa cat.

conv.,b %

selec.,c %

yield, %

TONd

1 2 3 2 + 3e none

65.8 7.7 0 3.0 0.7

53.5 68.4 0 87.0 100

35.2 5.3 0 2.6 0.7

464 70 0 34

The reactions were performed at 25 °C in MeOH/EtOH (97/3 v/v, 2.0 mL) containing a catalyst (25 μM), NaBH4 (33 mM), and 1dodecene (33 mM). The values were calculated from the average of three runs. bConversion was calculated on the basis of 1-dodecene. c Selectivity and yields were determined for n-dodecane. Selectivity = [n-dodecane] × 100/[converted 1-dodecene]. dTON was calculated as [n-dodecane]/[Cat.]. e[2] and [3] are 25 μM, respectively. a

of 1-dodecene by the catalysts produced n-dodecane in over 50% selectivity. The byproduct was determined to be 2dodecene by 1H NMR analysis. The highest turnover number (TON) of 464 for 1 was observed among the catalysts. Complexes 2 and 3 as the components of 1 exhibited lower TON for 2 (70) and no reactivity for 3. The mixture of 2 and 3 suppressed the catalytic activity in comparison with 2.20 The results clearly demonstrate that the synergistic effect between the adjacent Ru and Co centers in 1 is operative in the hydrogenation. The homogeneous catalysis of 1 in the hydrogenation was confirmed by a Hg poisoning test where 93% of the catalytic performance of 1 was maintained even in the presence of a few drops of Hg (Figure S5). To examine the effect of H2 gas generated by methanolysis of NaBH4, the hydrogenation reaction by 1 was also examined under a H2 atmosphere without NaBH4; however, no n-dodecane was observed in the reaction mixture. The results demonstrate that 1 acts as a homogeneous catalyst and H2 gas, even if generated, is not involved in the hydrogenation. Catalytic activity of 1 in hydrogenation of other olefins was also examined as shown in Table S3.21 The highest activity of 1 was observed in hydrogenation of 1-dodecene. To detect the intermediates for the hydrogenation, we performed the reaction of 1 with NaBH4 and monitored the reaction by 1H NMR and ESI-TOF-MS measurements. The addition of two equivalents of NaBH4 resulted in spectral change of complex 1, indicating formation of intermediates (Figure 2a). The paramagnetic 1H NMR spectrum supports that CoII remains the valence of the cobalt center. A new peak

Figure 1. (a) Deprotonation−protonation equilibrium of Hbpp and (b) schematic descriptions of structures of 1−3. (c) An ORTEP drawing of 1 with 50% probability thermal ellipsoids. Hydrogen atoms except those of the aqua ligand are omitted for clarity.

spectrometry (Figures S1 and S2). Finally, complex 1 was obtained by reacting complex 2 with [CoII(dipc)(H2O)3]18 in MeOH; the RuII−CoII hetero-dinuclear complex was quickly formed as confirmed by ESI-TOF-MS spectrometry (Figure S3). The MS spectrum clearly demonstrates that two metal ions are included in 1. The characterization of 1 was also made by elemental analysis and 1H NMR measurements (Figure S4), the latter of which exhibited paramagnetic features. Figure 1c depicts an ORTEP drawing of 1 determined by Xray crystallography. The crystallographic parameters and selected bond lengths of 1 are given in Tables S1 and S2. The bpp− ligand in 1 functions to enforce the two metal sites directed to the same side, i.e., the plausible reactive sites; the Ru−Cl and Co(H2O) moieties are compelled to be close each other. The distance between Co and Ru centers is determined to be 4.357(1) Å. The dihedral angles between the pyrazole plane and the planes of two pyridine rings containing N1 and N4 are 6.30° and 1.73°, respectively, indicating the pyrazole and two pyridine moieties reside on the same plane. These results confirm that the bpp− ligand enable the two metal centers to locate adjacent positions, which make it possible to display a synergistic effect by placing the potential vacant sites on both metal centers close to each other. To demonstrate and clarify a synergistic catalysis by 1, we have employed complexes 1−3 as catalysts for 1-dodecene hydrogenation by NaBH4 as a hydride source.19 The results of the catalytic reactions are listed in Table 1. The hydrogenation

Figure 2. (a) 1H NMR spectra of 1 (5.0 mM) and 1 with the addition of NaBH4 (10 mM) or NaBD4 (10 mM) in MeOH-d4. (b) ESI-TOFMS spectra of the hydrido species by the addition of NaBH4 in MeOH or NaBD4 in MeOH-d1 into 1. B

DOI: 10.1021/acs.inorgchem.9b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry at −19.87 ppm in the 1H NMR spectrum can be assigned to a metal-hydrido (M−H) species. When NaBH4 was replaced by NaBD4, the peak at −19.87 ppm disappeared to support the assignment.22 The hydrido species is ascribed to a RuII−H complex, [(RuII−H)(μ-bpp)CoII], not a CoII−H species, [RuII(μ-bpp)(CoII−H)], because the relaxation time (T1) of the peak at −19.87 ppm was determined to be 15.2 ms (Figure S6) and the high-spin state of the CoII center involved in the RuII−H intermediate was also confirmed by ESR spectroscopy in Figure S7. The RuII−H intermediate was also successfully detected by ESI-TOF-MS measurements (Figure 2b). The peak cluster due to the intermediate was observed at 705.11. When NaBD4 was used in place of NaBH4, the peak cluster shifted from 705.11 to 706.12. The NMR and MS spectra clearly indicate the formation of the RuII−H intermediate. Dependence of the yield of n-dodecane on the concentration of 1 was investigated. The turnover frequency (TOF) was calculated to be 15.7 min−1 on the basis of the slope (Figure S8). The hydrogenation was investigated using MeOH-d1 as a solvent or NaBD4 as a reductant (Figure S9).23 The TOF in MeOH-d1 was determined to be 10.0 min−1, and the KIE value was calculated to be 1.6. On the other hand, the KIE value in the reaction using NaBD4 was calculated to be 1.0, indicating that the cleavage of the O−H bond of MeOH is involved in the RDS. When MeOH (pKa = 15.5, TON = 464) was replaced by EtOH (pKa = 15.9) and 2-propanol (pKa = 16.5 in water) as a solvent, the catalytic activity of n-dodecane production (TON = 44 and 10, respectively) was significantly decreased (Table S4). The results suggest that the CoII center of 1 activates MeOH as a proton donor through the coordination to protonate the adjacent Ru-bound n-dodecyl group for releasing n-dodecane as a product. Thus, the intermediate should include MeOH as a ligand bound to the CoII center, i.e., [(RuII−H)(μ-bpp)(CoII−O(H)Me)] (A in Scheme 1). Besides, the dependence of the initial rates of n-dodecane production on the concentration of 1-dodecene was investigated to observe saturation behavior (Figure S10), indicating that an insertion equilibrium is operative between [(RuII− H)(μ-bpp)(CoII−O(H)Me)] (A in Scheme 1) and 1dodecene. A linear increase of the initial rates corresponding to the increase of NaBH4 concentration was also observed in the hydrogenation (Figure S11). On the basis of the results, we propose the catalytic mechanism of catalytic hydrogenation of 1-dodecene by 1 with use of NaBH4 (Scheme 1). The intermediate A is formed by the reaction of NaBH4 with 1 as observed by 1H NMR and ESI-TOF-MS measurements.24 The reaction of A and 1-dodecene to produce a mixture of Ru-(1dodecyl) and RuII-(2-dodecyl) intermediates (B and C, respectively) involves an insertion equilibrium as evidenced by the saturation behavior observed in the kinetic study. On the other hand, MeOH coordinated to the CoII center of 1 is activated to act as a proton source for the protonolysis of B and C at the adjacent site to generate n-dodecane, realizing the synergistic effect of 1. The protonolysis is considered to be involved in the RDS in the 1-dodecene hydrogenation by 1, as concluded from the observed solvent KIE. Finally, n-dodecane is released through the RDS, and the resultant species reacts with NaBH4 to reproduce the intermediate A in the catalytic cycle. In addition, the formation of C is also supported by the production of 2-dodecene as mentioned above. In summary, we have designed and successfully synthesized a hetero-dinuclear RuII−CoII complex, 1, with adjacent reactive

Scheme 1. Proposed Mechanism of 1-Dodecene Hydrogenation by 1 with NaBH4 in MeOH

sites, which can catalyze 1-dodecene hydrogenation to produce n-dodecane by NaBH4 as a hydride source. Mechanistic insights into the catalytic hydrogenation of 1-dodecene as a probe substrate by 1 have been gained by the detection of intermediates and kinetic analysis. In the homogeneous hydrogenation reaction, the RuII center of 1 reacts with NaBH4 to afford the RuII−H intermediate. The RuII−H intermediate reacts with 1-dodecene to form a putative RuII− alkyl complex. As a synergistic effect in 1, the CoII center activates MeOH as a proton source to enhance the ratedetermining protonolysis of the Ru−alkyl moiety to release ndodecane. The findings in this work will provide valuable fundamentals to develop hetero-dinuclear molecular catalysts that enhance efficient homogeneous catalysis through synergistic effects.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b02104. Experimental details and analytical data; 1H NMR spectra; ESI-MS spectra; X-ray crystallographic data and bond lengths for 1; catalytic activity of 1 for olefin and 1-dodecene hydrogenation; production of ndodecane from 1-dodecene; T1 determination for the 1 H NMR signal; ESR spectra; time courses of ndodecane yield; concentration dependence of 1 on the initial rate of n-dodecane production; dependence of the initial rate of n-dodecane production on the concentration of 1 and 1-dodecene; time courses of n-dodecane production (PDF) C

DOI: 10.1021/acs.inorgchem.9b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

(9) (a) Wang, Y.; Kostenko, A.; Yao, S.; Driess, M. Divalent SiliconAssisted Activation of Dihydrogen in a Bis(N-Heterocyclic Silylene)Xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins. J. Am. Chem. Soc. 2017, 139, 13499−13506. (b) Cammarota, R. C.; Lu, C. C. Tuning Nickel with Lewis Acidic Group 13 Metalloligands for Catalytic Olefin Hydrogenation. J. Am. Chem. Soc. 2015, 137, 12486−12489. (10) (a) Coombs, J.; Perry, D.; Kwon, D.-H.; Thomas, C. M.; Ess, D. H. Why Two Metals Are Better Than One for Heterodinuclear Cobalt−Zirconium-Catalyzed Kumada Coupling. Organometallics 2018, 37, 4195−4203. (b) Pye, D. R.; Cheng, L.-J.; Mankad, N. P. Cu/Mn Bimetallic Catalysis Enables Carbonylative Suzuki−Miyaura Coupling with Unactivated Alkyl Electrophiles. Chem. Sci. 2017, 8, 4750−4755. (11) Ohtsu, H.; Shimazaki, Y.; Odani, A.; Yamauchi, O.; Mori, W.; Itoh, S.; Fukuzumi, S. Synthesis and Characterization of ImidazolateBridged Dinuclear Complexes as Active Site Models of Cu,Zn-SOD. J. Am. Chem. Soc. 2000, 122, 5733−5741. (12) (a) Bozoglian, F.; Romain, S.; Ertem, M. Z.; Todorova, T. K.; Sens, C.; Mola, J.; Rodriguez, M.; Romero, I.; Benet-Buchholz, J.; Fontrodona, X.; Cramer, C. J.; Gagliardi, L.; Llobet, A. The Ru-Hbpp Water Oxidation Catalyst. J. Am. Chem. Soc. 2009, 131, 15176− 15187. (b) Mola, J.; Mas-Marza, E.; Sala, X.; Romero, I.; Rodríguez, M.; Viñas, C.; Parella, T.; Llobet, A. Ru-Hbpp-Based Water-Oxidation Catalysts Anchored on Conducting Solid Supports. Angew. Chem., Int. Ed. 2008, 47, 5830−5832. (13) Romain, S.; Rich, J.; Sens, C.; Stoll, T.; Benet-Buchholz, J.; Llobet, A.; Rodriguez, M.; Romero, I.; Clérac, R.; Mathonière, C.; Duboc, C.; Deronzier, A.; Collomb, M.-N. Multireversible Redox Processes in Pentanuclear Bis(Triple-Helical) Manganese Complexes Featuring an Oxo-Centered Triangular {MnII2MnIII(μ3-O)}5+ or {MnIIMnIII2(μ3-O)}6+ Core Wrapped by Two {MnII2(bpp)3}−. Inorg. Chem. 2011, 50, 8427−8436. (14) Mognon, L.; Benet-Buchholz, J.; Rahaman, S. M. W.; Bo, C.; Llobet, A. Ru−Zn Heteropolynuclear Complexes Containing a Dinucleating Bridging Ligand: Synthesis, Structure, and Isomerism. Inorg. Chem. 2014, 53, 12407−12415. (15) Gupta, G.; Yap, G. P. A.; Therrien, B.; Mohan Rao, K. Study of Novel η5-Cyclopentadienyl and η6-Arene Platinum Group Metal Complexes Containing a N4-Type Ligand and Their Structural Characterization. Polyhedron 2009, 28, 844−850. (16) Brown, C. A.; Ahuja, V. K. Catalytic Hydrogenation. VI. Reaction of Sodium Borohydride with Nickel Salts in Ethanol Solution. P-2 Nickel, a Highly Convenient, New, Selective Hydrogenation Catalyst with Great Sensitivity to Substrate Structure. J. Org. Chem. 1973, 38, 2226−2230. (17) (a) Chung, S.-K. Selective Reduction of Mono- and Disubstituted Olefins by Sodium Borohydride and Cobalt(II). J. Org. Chem. 1979, 44, 1014−1016. (b) Sharma, P. K.; Kumar, S.; Kumar, P.; Nielsen, P. Selective Reduction of Mono- and Disubstituted Olefins by NaBH4 and Catalytic RuCl3. Tetrahedron Lett. 2007, 48, 8704−8708. (18) Yang, L.; Crans, D. C.; Miller, S. M.; La Cour, A.; Anderson, O. P.; Kaszynski, P. M.; Godzala, M. E.; Austin, L. T. D.; Willsky, G. R. Cobalt(II) and Cobalt(III) Dipicolinate Complexes: Solid State, Solution, and in Vivo Insulin-like Properties. Inorg. Chem. 2002, 41, 4859−4871. (19) The reactions were performed in MeOH solutions containing a catalyst, 1-dodecene, and NaBH4 under an Ar atmosphere at 25 °C. The products were analyzed and quantified by gas chromatography and also confirmed by 1H NMR spectroscopy. (20) The mixture of 2 and 3 may form other dimer complexes as observed by 1H NMR and ESI-TOF-MS measurements (Figure S12). (21) Low conversions were observed in the olefin hydrogenation for internal alkenes such as 2-methyl-2-pentene and cyclodedecene. In the hydrogenation of styrene derivatives by 1 under the same conditions, styrene derivatives with electron-donating substituents seemed to exhibit higher activity for the hydrogenation.

CCDC 1877269 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.H.). *E-mail: [email protected] (T.K.). ORCID

Dachao Hong: 0000-0003-0581-1315 Hiroaki Kotani: 0000-0001-7737-026X Tomoya Ishizuka: 0000-0002-3897-026X Takahiko Kojima: 0000-0001-9941-8375 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grants-in-Aid (Nos. 24245011, 17H03027, and 15J04635) from Japan Society for the Promotion of Science (JSPS). D.H. gratefully acknowledges support from JSPS by Leading Initiative for Excellent Young Researchers (LEADER).

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DOI: 10.1021/acs.inorgchem.9b02104 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (22) The pristine NaBH4 shows mainly a quartet due to the coupling with the 11B nucleus (I = 3/2) at − 0.18 ppm (Figure S13). Additionally, a RuII−H species derived from the reaction of NaBH4 with 2 also showed a signal at −5.92 ppm in the 1H NMR spectrum (Figure S14). This result is consistent with the observation that 2 also performs the 1-dodecene hydrogenation under the same conditions. Compared to the RuII−H species derived from 2, the upfield shift was observed for the Ru−H species derived from 1. This shift is probably caused by the paramagnetism of the neighboring high-spin CoII center. (23) The production ratio of [n-dodecane]/[2-dodecene] was decreased to 0.58 using NaBH4 in CH3OD and remained at 0.82 when using NaBD4 in CH3OH. (24) The [(RuII−H)(μ-bpp)CoII] species generated from 1 was also observed in the presence of 1-dodecene (Figure S15). After the production of n-dodecane ceased, the [(RuII−H)(μ-bpp)CoII] still remained in the solution as detected by 1H NMR spectroscopy (Figure S16). Control experiments have been performed to demonstrate the participation of [(RuII−H)(μ-bpp)CoII] in the 1dodecene hydrogenation: After quenching the [(RuII−H)(μ-bpp) CoII] species by air, n-dodecane was not produced, indicating the crucial role of the [(RuII−H)(μ-bpp)CoII] species in the hydrogenation (Figure S17).

E

DOI: 10.1021/acs.inorgchem.9b02104 Inorg. Chem. XXXX, XXX, XXX−XXX