Synthesis and Photocatalytic Activities of Dinuclear Iridium

Jun 13, 2019 - A series of dinuclear iridium polyhydride complexes bearing BINAP ligands were synthesized and characterized for use as photocatalysts...
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Synthesis and Photocatalytic Activities of Dinuclear Iridium Polyhydride Complexes Bearing BINAP Ligands Yuki Sofue, Kotohiro Nomura, and Akiko Inagaki* Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-0397, Japan

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S Supporting Information *

ABSTRACT: A series of dinuclear iridium polyhydride complexes bearing BINAP ligands were synthesized and characterized for use as photocatalysts. The UV−vis absorption spectra of theses complexes exhibit absorption bands that extend to the low-energy region. These complexes show catalytic activities toward the hydrogenation of 1-hexene under photoirradiation conditions (λ 395 nm), suggesting that the catalysts are activated by light. By exposure of an acetone solution of a complex to a D2 atmosphere, an H−D exchange reaction occurred promptly on irradiation, while this exchange was sluggish in the dark.

B

Scheme 1. Synthesis of Dinuclear Iridium Complexes 2a−c and ORTEP Diagram of the Cationic Unit of 2b

INAP-based ligands have been extensively adopted for use in a wide variety of asymmetric catalytic reactions; these ligands are indispensable not only in synthetic organic chemistry but also in organometallic and inorganic chemistry.1−3 In addition, BINAP ligands have attractive photophysical properties owing to their binaphthyl backbones and the aromatic substituents on the phosphines, which absorb UV to near-visible light; however, the photochemistry of BINAP complexes is not yet fully understood. Some metal complexes bearing BINAP ligands have been reported; these complexes exhibit strong absorptions in the visible range due to metal-toligand charge transfer (MLCT) and strong emissions with long-excited lifetimes.4 Recently, we reported the syntheses and molecular structures of monocationic trinuclear iridium (Ir) and rhodium (Rh) complexes ligated by diphosphines and constructed with fluorene backbones.5 Both complexes showed increased absorptivity in comparison to the corresponding mononuclear complex, and the Ir complex was active toward the photocatalytic hydrogenation of alkynes. In order to incorporate the photophysical advantages of the BINAP ligand, we next considered applying the BINAP ligand to multinuclear hydride complexes with the expectation that eminent photocatalytic activities will result. Herein, we report the syntheses and photocatalytic abilities of novel dinuclear Ir complexes bearing BINAP ligands as the light-absorbing moieties. Dinuclear Ir hydride complexes ligated by a variety of diphosphine ligands were synthesized through the hydrogenation of the corresponding mononuclear iridium complexes [(L)Ir(cod)]X (L = diphosphine ligand, X = BF4, SbF6) at atmospheric pressure (Scheme 1). Irrespective of the ligand or anion, the dinuclear pentahydride complexes were selectively and quantitatively formed through these reactions. © 2019 American Chemical Society

All complexes were unambiguously characterized on the basis of 1H and 31P NMR spectral data, and the molecular structure of the SbF6 salt of 2b was determined by singlecrystal X-ray diffractometry. Although the positions of the hydride ligands could not be determined from the X-ray diffraction data, all complexes exhibited characteristic hydride Received: April 4, 2019 Published: June 13, 2019 2408

DOI: 10.1021/acs.organomet.9b00227 Organometallics 2019, 38, 2408−2411

Communication

Organometallics signals in the high-field regions of the 1H NMR spectra with a doublet, triplet, and a broad singlet in a 2:1:2 integral ratio observed in each spectrum, which supports the formation of the dinuclear pentahydride structure. The doublet can be assigned to the hydrides trans to P1 and P3, respectively (Hc, Hb), the triplet to the hydride trans to both P2 and P4 (Ha), and the singlet to the two terminal hydrides (Ht) (see the labels on 2a in Scheme 1). Coupling patterns and coupling constants of the five hydride ligands in the 1H NMR spectra are similar to those of the related dinuclear complex synthesized by our group, in which the positions of the hydride ligands have been determined.5a The ORTEP diagram of the cationic structure of 2b is depicted in Scheme 1. The crystal contains one SbF6 anion and three CH2Cl2 solvent molecules per cation. The Ir1−Ir2 bond length is 2.5173(7) Å, which is similar to that of the corresponding dinuclear iridium hydride complex reported previously (2.5124(2) Å).5a On the basis of the crystal structure and the spectroscopic data, the prepared complexes can be described as monocationic dinuclear pentahydride complexes. The UV−vis absorption spectra of 2a−c and the BINAP ligands acquired in CH2Cl2 at ambient temperature are shown in Figure 1. BINAP complexes 2a,b both exhibit broad

this wavelength for these complexes are 4328 (2a), 6148 (2b), and 4324 (2c). The photocatalytic activities of the complexes were determined by hydrogenating 1-hexene (eq 1 and Figure 2)

Figure 2. Rates of consumption of 1-hexene during photocatalytic hydrogenation with 2a−c.

Table 1. Catalytic Hydrogenation of 1-Hexene by 2a−ca catalyst

time/h

conversionb/%

product ratioc/% hexane:2hexene

2a 2a (dark, 40 °C) 2b 2c

2.5 4.0 2.5 1.0

83 32 84 98

79:21 66:34 81:19 92:8

a

An acetone solution of the catalyst (3.3 mol %) in an NMR tube fitted with a J. Young valve under 1 atm of H2 was used. bThe conversion (%) was calculated using nitromethane as an internal standard. cThe product ratio was calculated from the integrated NMR signal ratios of the products.

and are compared in Table 1. An acetone solution of 2a, 2b, or 2c (3.3 mol %) containing 1-hexene in an NMR tube fitted with a J. Young valve was irradiated with a 3 W LED lamp (λ 395 nm) under an atmosphere of H2, and the consumption of 1-hexene was followed by 1H NMR spectroscopy. All complexes showed good catalytic hydrogenation activities to yield n-hexane as a main product; each reaction was completed within 4 h. At the same time, 2-hexene was formed as a minor product through competitive isomerization (eq 1). Under the corresponding dark condition, the reaction was extremely slow in comparison to that under irradiation (7% conversion after 4 h), and additional heating for 40 °C resulted in a higher conversion to 32% but lower product selectivity (Table 1). Although complex 2c has an ε value similar to that of 2a, it showed higher reactivity due to the electronic richness of its ligands in comparison to those of 2a,b; indeed, the reaction catalyzed by 2c was sufficiently fast to give the lowest percentage of the isomerized product.7,8 Complexes 2a,b show similar reactivities and product selectivities. Catalyst 2a is stable enough for reuse by the addition of further 1-hexene and H2 (1 atm) (Figure S14a). These results support the conclusion that the dinuclear framework is maintained throughout the irradiation process. Since the reaction is much slower in the dark under the same conditions, the

Figure 1. UV−vis absorption spectra of complexes 2a−c and the BINAP ligands.

absorption bands that monotonously decrease in intensity to 500 nm. Complex 2c bearing a SEGPHOS ligand absorbs less in the UV region (250−290 nm) than the other complexes but absorbs similarly in the lower-energy region. On comparison to that of the BINAP ligand with a 350 nm absorption edge, it is apparent that electronic interactions between the ligand with the Ir2H5 core give rise to the low-energy absorptions of these complexes. DFT calculations of 2a at the B3LYP/Lanl2DZ level of theory revealed that the HOMO of 2a possesses mainly Ir d and BINAP π* orbital characteristics (Figures S12 and S13, MO 342−345), whereas slightly lower energy occupied orbitals below the HOMO exhibit Ir d orbital and hydride s orbital character (Figure S12, MO 339−341) and function to maintain the Ir−Ir core.6 The LUMO of 2a possesses both Ir and hydride characteristics, as well as the partially antibonding character of the Ir−H bonds (Figure S13, MO 346, 347). Hence, low-energy transitions may contribute to hydride ligand dissociation. We irradiate at λ 395 nm in the low-energy region as described below. The absorption coefficients (ε) at 2409

DOI: 10.1021/acs.organomet.9b00227 Organometallics 2019, 38, 2408−2411

Communication

Organometallics reaction can be switched on and off by turning the light on and off. As shown in Figure S14b, the reaction involving 2a (same conditions as in Figure 2 and Table 1) clearly accelerates on irradiation and decelerates in the dark when it is repeatedly subjected to on/off irradiation cycles. In order to understand the accelerating effect of light, we examined the H−D exchange reaction of 2a under a D2 atmosphere (Scheme 2 and Figures S15−S18). When an

of 2a, and NMR spectral data for 2a during the H−D exchange reaction (PDF) Accession Codes

CCDC 1905125 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.

Scheme 2. H−D Exchange Reactions with D2

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.I.: [email protected]. ORCID

Kotohiro Nomura: 0000-0003-3661-6328 Akiko Inagaki: 0000-0001-9113-5602 Notes

The authors declare no competing financial interest.

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acetone-d6 solution of 2a was exposed to 1 atm of D2 in the dark for 24 h, H−D exchange took place to give 11% of the deuterated complex (Figures S15 and S16). On the other hand, 80% of the H (hydride) was exchanged for D within 1 h on irradiation (Figures S17 and S18).9 During the irradiated H−D exchange reaction, only the hydride 1H NMR signals were observed to diminish, whereas the intensities of the signals of the other hydrogen and 31P signals of the phosphorus atoms did not decrease, showing that no decomposition takes place during the H−D exchange reaction. We additionally confirmed that no H−D exchange reaction takes place over 24 h in the absence of D2 and light in a solution of 2a in acetoned6.10 The predominant increase in D incorporation upon irradiation suggests that light accelerates the dissociation of the hydride ligands and the subsequent H2 activation process. The result that coordination of 1-hexene (3 equiv) to 2a did not take place in the dark supports the above hypothesis. As a consequence, the photoirradiation can accelerate the H2 dissociation and activation steps during the catalytic hydrogenation reactions presented above (Scheme S1). In summary, we reported the new dinuclear iridium pentahydride complexes 2a−c that contain BINAP-based diphosphine ligands. Their dinuclear frameworks are sufficiently stable and are maintained during irradiation at 395 nm. The complexes exhibit enhanced photocatalytic activities toward the hydrogenation of 1-hexene. The effect of light was directly observed through H−D exchange under D2 at atmospheric pressure.

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ACKNOWLEDGMENTS This work was supported by the Cooperative Research Program of the “Network Joint Research Center for Materials and Devices” and JSPS KAKENHI (B) Grant Number 16H0412100. Part of this work was supported by a JSPS Grant-in-Aid for Scientific Research on Innovative Areas (“3D Active-Site Science”: Grant No. 26105003) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. We thank Dr. Masaki Mishima (Tokyo Metropolitan University) for the 13C NMR spectroscopy.

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REFERENCES

(1) (a) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R. Ruthenium(II)-Catalyzed Asymmetric Transfer Hydrogenation of Ketones Using a Formic Acid−Triethylamine Mixture. J. Am. Chem. Soc. 1996, 118, 2521−2522. (b) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. Asymmetric Transfer Hydrogenation of Aromatic Ketones Catalyzed by Chiral Ruthenium(II) Complexes. J. Am. Chem. Soc. 1995, 117, 7562−7563. (c) Miyashita, A.; Karino, H.; Shimamura, J.; Chiba, T.; Nagano, K.; Nohira, H.; Takaya, H. Synthesis of Atropisomeric 2,2′-Bis(dicyclohexylphosphino)-6,6′dimethyl-1,1′-biphenyl (BICHEP) and Its Use in Rh(I)-catalyzed Asymmetric Hydrogenation of Prochiral Olefins. Chem. Lett. 1989, 18, 1849−1852. (d) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. Practical Enantioselective Hydrogenation of Aromatic Ketones. J. Am. Chem. Soc. 1995, 117, 2675−2676. (2) (a) Jiang, H.-C.; Tang, X.-Y.; Shi, M. Copper-catalyzed regioand enantioselective aminoboration of alkylidenecyclopropanes: the synthesis of cyclopropane-containing β-aminoalkylboranes. Chem. Commun. 2016, 52, 5273−5276. (b) Li, Y.; Xu, M.-H. RhodiumCatalyzed Asymmetric Tandem Cyclization for Efficient and Rapid Access to Underexplored Heterocyclic Tertiary Allylic Alcohols Containing a Tetrasubstituted Olefin. Org. Lett. 2014, 16, 2712− 2715. (c) Liu, Y.-J.; Ding, Y.-L.; Niu, S.-S.; Ma, J.-T.; Cheng, Y. NHeterocyclic Carbene/Palladium Cascade Catalysis: Construction of 2,2-Disubstitiuted Benzofuranones from the Reaction of 3-(2Formylphenoxy)propenoates with Allylic Esters. J. Org. Chem. 2018, 83, 1913−1923. (d) Rankic, D. A.; Parvez, M.; Keay, B. A. 3,3′Substituted BINAP derivatives containing C-bound substituents:applications in asymmetric hydrogenation reactions. Tetrahedron: Asymmetry 2012, 23, 754−763. (e) Schneider, M. J.; Haumann, M.; Wasserscheid, P. Asymmetric hydrogenation of methyl pyruvate in the continuous gas phase using Supported Ionic Liquid Phase (SILP)catalysis. J. Mol. Catal. A: Chem. 2013, 376, 103−110.

ASSOCIATED CONTENT

S Supporting Information *

Synthetic and computational procedures, NMR spectral data for 2a−c. Crystal data and structure refinement for 2b. DFToptimized structure and MOs of 2a. Coordinates of the calculated structure, and NMR spectral data for 2a during the H-D exchange reaction (PDF). The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00227. Coordinates of the calculated structure (XYZ) Synthetic and computational procedures and NMR spectral data for 2a−c, crystal data and structure refinement for 2b, DFT-optimized structure and MOs 2410

DOI: 10.1021/acs.organomet.9b00227 Organometallics 2019, 38, 2408−2411

Communication

Organometallics (3) (a) Akutagawa, S. Asymmetric synthesis by metal BINAP catalysts. Appl. Catal., A 1995, 128, 171−207. (b) Berthod, M.; Mignani, G.; Woodward, G.; Lemaire, M. Modified BINAP: The How and the Why. Chem. Rev. 2005, 105, 1801−1836. (c) Genet, J.P. Asymmetric Catalytic Hydrogenation. Design of New Ru Catalysts and Chiral Ligands: From Laboratory to Industrial Applications. Acc. Chem. Res. 2003, 36, 908−918. (d) Kočovský, P.; Vyskočil, Š .; Smrčina, M. Non-Symmetrically Substituted 1,1‘-Binaphthyls in Enantioselective Catalysis. Chem. Rev. 2003, 103, 3213−3246. (e) Shimizu, H.; Nagasaki, I.; Matsumura, K.; Sayo, N.; Saito, T. Developments in Asymmetric Hydrogenation from an Industrial Perspective. Acc. Chem. Res. 2007, 40, 1385−1393. (4) (a) Zarate, X.; Schott, E.; Ramirez-Tagle, R.; MacLeod-Carey, D.; Arratia-Pérez, R. Photophysical properties of [Cu(binap)2]+ and [Pd(binap)2] complexes: A theoretical study. Polyhedron 2012, 37, 54−59. (b) Saito, K.; Tsukuda, T.; Tsubomura, T. Synthesis and Luminescence of New Cu(I) Complexes Containing a Binap Ligand and a Diimine Ligand. Bull. Chem. Soc. Jpn. 2006, 79, 437−441. (c) Tsubomura, T.; Ito, Y.; Inoue, S.; Tanaka, Y.; Matsumoto, K.; Tsukuda, T. Strongly Luminescent Palladium(0) and Platinum(0) Diphosphine Complexes. Inorg. Chem. 2008, 47, 481−486. (d) Tsubomura, T.; Sakai, K. Photochemical reactions of palladium(0) and platinum(0) phosphine complexes. Coord. Chem. Rev. 1998, 171, 107−113. (e) Tominaga, H.; Sakai, K.; Tsubomura, T. Structure and photophysical properties of [Pt0(binap)2][binap = 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl]. J. Chem. Soc., Chem. Commun. 1995, 2273−2274. (5) (a) Shitaya, S.; Nomura, K.; Inagaki, A. Synthesis and Photoreactivities of Light-Absorbing Trinuclear Iridium Polyhydride Complex. Dalton Trans 2018, 47, 12046−12050. (b) Matsusaka, Y.; Shitaya, S.; Nomura, K.; Inagaki, A. Synthesis of Mono, Di, and Trinuclear Rhodium Diphosphine Complexes Containing LightHarvesting Fluorene Backbones. Inorg. Chem. 2017, 56, 1027−1030. (6) Gaussian 16, Revision A.03. Calculated with the basis sets of Lanl2DZ for Ir, 6-31G for the phosphorous, and 3-21G for all the carbon and hydrogen atoms. (7) (a) Suda, T.; Noguchi, K.; Hirano, M.; Tanaka, K. Highly Enantioselective Synthesis of N,N-Dialkylbenzamides with Aryl− Carbonyl Axial Chirality by Rhodium-Catalyzed [2+2+2] Cycloaddition. Chem. Eur. J. 2008, 14, 6593−6596. (b) Wada, A.; Noguchi, K.; Hirano, M.; Tanaka, K. Enantioselective Synthesis of C2Symmetric Spirobipyridine Ligands through Cationic Rh(I)/Modified-BINAP-Catalyzed Double [2 + 2 + 2] Cycloaddition. Org. Lett. 2007, 9, 1295−1298. (8) When catalytic hydrogenation of 2-hexene with 2c was tested under the same conditions, only 7% of 2-hexene was hydrogenated. Thus, the contribution of the hydrogenation pathway from 2-hexene to hexane should be small. (9) Mukherjee, D.; Höllerhage, T.; Leich, V.; Spaniol, T. P.; Englert, U.; Maron, L.; Okuda, J. The Nature of the Heavy Alkaline Earth Metal−Hydrogen Bond: Synthesis, Structure, and Reactivity of a Cationic Strontium Hydride Cluster. J. Am. Chem. Soc. 2018, 140, 3403−3411. (10) In order to eliminate D incorporation from acetone-d6, we compared the reactions of D2 in acetone-h6 under light-irradiated vs dark conditions. A clear D incorporation only under irradiated conditions after 1 h was observed (43% deutration (light) vs