Molybdenum-Catalyzed Ammonia Formation Using Simple

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Cite This: Inorg. Chem. 2019, 58, 8927−8932

Molybdenum-Catalyzed Ammonia Formation Using Simple Monodentate and Bidentate Phosphines as Auxiliary Ligands Yuya Ashida,† Kazuya Arashiba,† Hiromasa Tanaka,‡ Akihito Egi,§ Kazunari Nakajima,∥ Kazunari Yoshizawa,*,§ and Yoshiaki Nishibayashi*,† Department of Systems Innovation, School of Engineering, and ∥Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ School of Liberal Arts and Sciences, Daido University, Minami-ku, Nagoya 457-8530, Japan § Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan Downloaded via UNIV OF SOUTHERN INDIANA on July 17, 2019 at 07:56:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

(PCET)12 process using SmI2 and ethylene glycol or water achieved an efficient reaction system.13 As an extensive study, we have found a novel reaction system under ambient conditions using transition-metal complexes bearing simple and commercially available ligands as catalysts with a simple and convenient procedure. Herein, we report molybdenum-catalyzed ammonia formation using simple monodentate and bidentate phosphines as auxiliary ligands. As typical molybdenum−dinitrogen complexes bearing simple and commercially available phosphines, we selected trans-[Mo(N2)2(PMePh2)4]14 (2a), cis-[Mo(N2)2(PMe2Ph)4]15 (2b), and trans-[Mo(N2)2(dppe)2]16 (2c; dppe = 1,2-bis(diphenylphosphino)ethane) as catalysts. The reaction of an atmospheric pressure of nitrogen gas with 36 equiv of SmI2 and 36 equiv of ethylene glycol in the presence of 2a in tetrahydrofuran (THF) at room temperature for 18 h gave 9.2 equiv of ammonia based on the Mo atom (78% yield based on SmI2), where 3.6 equiv of hydrogen gas were observed based on the Mo atom (21% yield based on SmI2) (Table 1, run 1). In contrast, 2b did not work as a catalyst (Table 1, run 2). The use of 2c as a catalyst gave 5.1 equiv of ammonia based on the Mo atom (43% yield) together with 6.8 equiv of hydrogen gas (38% yield; Table 1, run 3). For comparison, we carried out reactions using 1a and [MoCl3(PNP)] (1b; PNP = 2,6-bis(di-tertbutylphosphinomethyl)pyridine)11 as catalysts to produce 11.2 and 11.1 equiv of ammonia based on the Mo atom, respectively, and only a small amount of hydrogen gas (Table 1, runs 4 and 5). It is noteworthy that some conventional molybdenum−dinitrogen complexes such as 2a and 2c worked effectively, although the catalytic activity and selectivity of 2a and 2c were lower than those of 1a and 1b. The remarkable catalytic activity of 2a and 2c prompted us to investigate the catalytic activity of other molybdenum complexes bearing simple and commercially available phosphines. To check the reactivity of simple phosphines, we envisaged the use of reactive molybdenum complexes generated in situ from [MoI3(THF)3]17 as precursors and various phosphines as catalysts. When we used a mixture of [MoI3(THF)3] and 4 equiv of PMePh2 as a catalyst in place of

ABSTRACT: We have found molybdenum-catalyzed ammonia formation using simple and commercially available monodentate and bidentate phosphines as auxiliary ligands with a simple and convenient procedure. Molybdenum complexes generated in situ from [MoI3(THF)3] and the corresponding phosphines such as PMePh2 and 1,5-bis(diphenylphosphino)pentane worked effectively toward ammonia formation.

A

mmonia production from dinitrogen is one of the most significant industrial processes because ammonia is the raw material of fertilizer. Today, this process almost depends on the Haber−Bosch processes as a heterogeneous catalytic process under harsh conditions.1 In sharp contrast to the Haber−Bosch process, nitrogenase enzyme produces ammonia under ambient conditions.2 As a model reaction of the nitrogenase enzyme, transition metal−dinitrogen complexes have also been widely studied to realize transition-metalcatalyzed ammonia formation under mild conditions. Since Schrock’s report,3 catalytic ammonia synthesis has been achieved in the presence of various transition metal− dinitrogen complexes as homogeneous catalysts for the past decade.4−8 In almost all cases, the use of cleverly designed and sophisticated ligands such as tri- and tetradentate auxiliary ligands is essential to catalytically convert dinitrogen into ammonia under mild conditions. The use of transition metal− dinitrogen complexes bearing simple auxiliary ligands as catalysts afforded ammonia stoichiometrically.9 The only exception is the Ashley reaction that used an iron−dinitrogen complex bearing two 1,2-bis(diethylphosphino)ethane (depe) as simple auxiliary ligands [Fe(N2)(depe)2] to produce hydrazine as the main product from dinitrogen at −78 °C.10 Quite recently, we have found that the use of SmI2 and simple alcohol or water realized unprecedented catalytic activity in molybdenum-catalyzed ammonia production under ambient conditions.11 In this reaction system, the use of a molybdenum complex bearing a PCP-type pincer ligand such as [MoC l 3 (PCP)] (1a; P CP = 1, 3-bi s[(di -tertbutylphosphino)methyl]benzimidazol-2-ylidene) achieved the highest catalytic activity. A proton-coupled electron-transfer © 2019 American Chemical Society

Received: May 8, 2019 Published: June 25, 2019 8927

DOI: 10.1021/acs.inorgchem.9b01340 Inorg. Chem. 2019, 58, 8927−8932

Communication

Inorganic Chemistry Table 1. Catalytic Ammonia Formation Using Molybdenum Complexesa

Mo run complex 1 2 3 4 5

2a 2b 2c 1a 1b

amount of NH3 (equiv/Mo)

yield of NH3 (%)b

amount of H2 (equiv/Mo)

yield of H2 (%)b

9.2 ± 0.4c 0.9 5.1 11.2 11.1

78 ± 1c 8 43 93 92

3.6 ± 0.1c 15.7 6.8 0.4 1.1

21 ± 1c 86 38 2 6

Table 2. Catalytic Ammonia Formation Using a Mixture of [MoI3(THF)3] and Phosphines as Ligandsa

run 1 2

a

To a mixture of molybdenum complex (0.010 mmol) and SmI2(THF)2 (0.36 mmol, 36 equiv based on the Mo atom) in THF (6.0 mL) was added ethylene glycol (0.36 mmol, 36 equiv based on the Mo atom) in one portion at room temperature, followed by stirring at room temperature for another 18 h under 1 atm of dinitrogen. bThe yield was determined based on SmI2(THF)2. cThe values were determined as the mean of multiple individual experiments (at least two) with error bars (s.d.).

3 4 5 6 7 8 9 10 11 12 13 14

2a, 8.3 equiv of ammonia were produced based on the Mo atom (69%), together with 2.2 equiv of hydrogen gas (13%; Table 2, run 1). We consider that the catalytic activity of a mixture of [MoI3(THF)3] and PMePh2 is comparable with that of 2a. According to the new procedure, we carried out a catalytic reaction using other phosphines. When PEtPh2 and PnPrPh2 were used as monodentate phosphines, slightly higher amounts of ammonia were obtained (Table 2, runs 2 and 3). In contrast, the use of PCyPh2, PPh3, PMe2Ph, and PMe3 as monodentate phosphines produced only small amounts of ammonia (Table 2, runs 4−7). These results indicate that molybdenum complexes generated in situ from [MoI 3(THF)3] and monoalkyldiphenylphosphines worked effectively toward ammonia formation. Next, we investigated the catalytic reaction using bidentate phosphines such as 1,1-bis(diphenylphosphino)methane (dppm), dppe, 1,3-bis(diphenylphosphino)propane (dppp), 1,4-bis(diphenylphosphino)butane (dppb), 1,5-bis(diphenylphosphino)pentane (dpppe), 1,6-bis(diphenylphosphino)hexane (dpph), and 1,1′-bis(diphenylphosphino)ferrocene (dppf). In all cases, catalytic ammonia formation proceeded smoothly, although the catalytic activity depended on the length of the methylene linkers of the bidentate phosphines (Table 2, runs 8−14). When a mixture of [MoI3(THF)3] and 2 equiv of dpppe was used as a catalyst, 10.8 equiv of ammonia were produced based on the Mo atom (89%) together with 0.8 equiv of hydrogen gas (4%; Table 2, run 12). These results indicate that molybdenum complexes generated in situ from [MoI3(THF)3] and dpppe worked as the most effective catalyst. On the basis of the experimental results shown in Tables 1 and 2, we compared the catalytic activity of five reaction systems, where 2a, molybdenum complexes generated in situ from [MoI3(THF)3] and 4 equiv of PMePh2 and 2 equiv of dpppe, 1a, and 1b were used as catalysts, using larger amounts of SmI2 (180 equiv) and ethylene glycol (180 equiv) as the

ligand (equiv/ Mo)

amount of NH3 (equiv/ Mo)

yield of NH3 (%)b

amount of H2 (equiv/Mo)

yield of H2 (%)b

PMePh2 (4) PEtPh2 (4) PnPrPh2 (4) PCyPh2 (4) PPh3 (4) PMe2Ph (4) PMe3 (4) dppm (2) dppe (2) dppp (2) dppb (2) dpppe (2) dpph (2) dppf (2)

8.3 ± 0.8c

69 ± 6c

2.2 ± 0.4c

13 ± 2c

9.1 ± 0.8c

76 ± 8c

1.7 ± 0.2c

9 ± 1c

9.8 ± 0.3c

82 ± 3c

1.5 ± 0.4c

9 ± 2c

1.8

15

8.3

46

2.7 1.5

22 13

7.4 8.1

41 44

0.3 5.5 3.8 ± 0.6c 9.0 ± 0.6c 8.7 ± 0.2c 10.8 ± 0.1c 10.0 ± 0.6c 4.0

3 45 31 ± 5c 73 ± 6c 72 ± 4c 89 ± 1c 84 ± 5c 33

9.4 6.2 5.3 ± 0.1c 1.1 ± 0.2c 3.0 ± 0.9c 0.8 ± 0.2c 0.6 ± 0.1c 6.2

52 34 29 ± 0c 6 ± 1c 16 ± 4c 4 ± 1c 4 ± 1c 34

a A mixture of MoI3(THF)3 (0.010 mmol), ligand (0.010−0.040 mmol), and SmI2(THF)2 (0.36 mmol, 36 equiv based on the Mo atom) in THF (6.0 mL) was stirred at room temperature under 1 atm of dinitrogen. After 1 h, ethylene glycol (0.36 mmol, 36 equiv based on the Mo atom) was added to the reaction mixture in one portion at room temperature, followed by stirring at room temperature for another 18 h. bThe yield was determined based on SmI2(THF)2. cThe values were determined as the mean of multiple individual experiments (at least two) with error bars (s.d.).

reducing reagent and proton source, respectively (Scheme 1).18,19 The amounts of ammonia and dihydrogen are shown in Scheme 1, together with the time profiles of the catalytic reactions. The catalytic activity of 2a is almost the same as that of molybdenum complexes generated in situ from [MoI3(THF)3] and 4 equiv of PMePh2. Good yields of ammonia and moderate yields of dihydrogen were observed in both cases. When molybdenum complexes generated in situ from [MoI3(THF)3] and 2 equiv of dpppe were used as catalysts, a high yield of ammonia and only a low yield of dihydrogen were observed. The catalytic activity of molybdenum complexes generated in situ from [MoI3(THF)3] and 2 equiv of dpppe is not higher than those of 1a and 1b. However, molybdenum complexes generated in situ from [MoI3(THF)3] and 2 equiv of dpppe worked more effectively than 2a.20 Next, the following stoichiometric reactions were carried out. The reaction of the molybdenum−hydrazide(2−) complex cis,mer-[Mo(NNH2)(OTf)2(PMePh2)3] (3)21 with 12 equiv of SmI2 and ethylene glycol in THF at room temperature for 18 h under argon gave ammonia in 154% yield based on the Mo atom in 3 (Scheme 2a). In this stoichiometric reaction, the hydrazide ligand in 3 was converted into 2 equiv 8928

DOI: 10.1021/acs.inorgchem.9b01340 Inorg. Chem. 2019, 58, 8927−8932

Communication

Inorganic Chemistry Scheme 1. Comparison of Catalytic Activities

Scheme 2. Mechanistic Study

of ammonia under the catalytic conditions. Separately, we confirmed a similar catalytic activity of 3 to 2a (Scheme 2b). These experimental results indicate that 3 may be involved as a key reactive intermediate of the catalytic formation of ammonia. Recently, we have found that catalytic ammonia formation proceeds via a novel reaction pathway (path A), which involved direct cleavage of the bridging N2 ligand as a key step (Scheme 2c).11 In fact, the reduction of [MoI3(PNP)] with an excess amount of the reductant under 1 atm of dinitrogen at room temperature did not give the corresponding dinitrogen complex but the corresponding nitride complex [Mo( N)I(PNP)].11 In sharp contrast to our previous reaction system, the reduction of [MoI3(THF)3] with an excess amount of SmI2 in the presence of 4 equiv of PMePh2 in THF at room temperature for 18 h did not give the corresponding nitride complexes but the corresponding dinitrogen complex (Scheme 2e). At present, we have not yet obtained detailed information on the reaction pathway; however, we consider that the catalytic reaction system described in the present manuscript did not proceed via path A but the classical Chatt cycle22 (path B), which involved the formation of diazenide, hydrazide, hydrazidium, nitride, imide, amide, and ammine complexes as key reactive intermediates, leading to ammonia formation (Scheme 2d). To support our proposal that the present reaction proceeds via path B, we carried out density functional theory calculations on the hydrogenation of 2a and its related complex (Scheme 2f). The bond dissociation free energy (BDFE) value of the N−H bond of a six-coordinate molybdenum diazenide complex (4) is 10.8 kcal/mol in THF at 298 K. This value is considerably smaller than the BDFE value of an O−H bond, 26 kcal/mol, estimated for SmI2(H2O)n.13d At present, we have obtained little experimental information on the structure of a SmI2−ethylene glycol complex in the reaction solution, and therefore it is difficult to evaluate its BDFE(O−H) value theoretically. We expect that

the BDFE(O−H) value of the SmI2−ethylene glycol complex is smaller than 26 kcal/mol because the BDFE(O−H) value of free ethylene glycol was calculated to be 97 kcal/mol at the B3LYP-D3/6-31G* level of theory, which is much smaller than that of water (109 kcal/mol). Because of the small BDFE(N− H) value of 4, hydrogen transfer from the samarium complex to 2a is not likely to proceed from a thermodynamic point of view. However, dissociation of a N2 ligand from 2a can strengthen the N−H bond of a corresponding diazenide complex. The free-energy change for the dissociation of dinitrogen from 2a, yielding a five-coordinate Mo−N2 complex (5) is 15.1 kcal/mol, which is acceptable at room temperature. The BDFE(N−H) of the five-coordinate diazenide complex (6; 28.7 kcal/mol), implies the possibility of hydrogen transfer from the SmI2−ethylene glycol complex to 5. The calculated results support our proposed catalytic pathway (path B) involving hydrogenation of an N2 ligand of 2a through a PCET process after dissociation of the other N2 ligand. Separately, we confirmed that no reaction of 2a occurred at all in the absence of either SmI2 or ethylene glycol. These experimental results support that the present reaction may involve the PCET process. Finally, we investigated the catalytic reaction with water as a proton source using 2a and molybdenum complexes generated 8929

DOI: 10.1021/acs.inorgchem.9b01340 Inorg. Chem. 2019, 58, 8927−8932

Communication

Inorganic Chemistry

(2) (a) Ribbe, M. W. Nitrogen Fixation: Methods and Protocols; Humana Press: New York, 2011. (b) Weigand, W.; Schollhammer, P. Bioinspired Catalysis: Metal−Sulfur Complexes; Wiley-VCH: Weinheim, Germany, 2015. (3) Yandulov, D. V.; Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center. Science 2003, 301, 76−78. (4) For selected reviews on nitrogen fixations catalyzed by transition metal−dinitrogen complexes, see: (a) MacLeod, K. C.; Holland, P. L. Recent Developments in the Homogeneous Reduction of Dinitrogen by Molybdenum and Iron. Nat. Chem. 2013, 5, 559−565. (b) Jia, H.P.; Quadrelli, E. A. Mechanisttic Aspects of Dinitrogen Cleavage and Hydrogenation to Produce Ammonia in Catalysis and Organometallic Chemistry: Relevance of Metal Hydride Bonds and Dihydrogen. Chem. Soc. Rev. 2014, 43, 547−564. (c) van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Challenges in Reduction of Dinitrogen by Proton and Electron Transfer. Chem. Soc. Rev. 2014, 43, 5183−5191. (d) Sivasankar, C.; Baskaran, S.; Tamizmani, M.; Ramakrishna, K. Lessons Learned and Lessons to be Learned for Developing Homogeneous Transition Metal Complexes Catalyzed Reduction of N2 to Ammonia. J. Organomet. Chem. 2014, 752, 44−58. (e) Tyler, D. R. Mechanisms for the Formation of NH3, N2H4, and N2H2 in the Protonation Reaction of Fe(DMeOPrPE) 2 N 2 {DMeOPrPE = 1,2-bis[bis(methoxypropyl)phosphino]ethane}. Z. Anorg. Allg. Chem. 2015, 641, 31−39. (f) Khoenkhoen, N.; de Bruin, B.; Reek, J. H. K.; Dzik, W. I. Reactivity of Dinitrogen Bound to Mid- and Late-Transition-Metal Centers. Eur. J. Inorg. Chem. 2015, 2015, 567−598. (g) Burford, R. J.; Fryzuk, M. D. Examining the Relationship between Coordination Mode and Reactivity of Dinitrogen. Nat. Rev. 2017, 1, 26. (h) Foster, S. L.; Bakovic, S. I. P.; Duda, R. D.; Maheshwari, S.; Milton, R. D.; Minteer, S. D.; Janik, M. J.; Renner, J. N.; Greenlee, L. F. Catalysts for Nitrogen Reduction to Ammonia. Nat. Catal. 2018, 1, 490−500. (i) Stucke, N.; Flöser, B. M.; Weyrich, T.; Tuczek, F. Nitrogen Fixation Catalyzed by Transition Metal Complexes: Recent Developments. Eur. J. Inorg. Chem. 2018, 2018, 1337−1355. (j) Nishibayashi, Y. Development of Catalytic Nitrogen Fixation Using Transition Metal-Dinitrogen Complexes under Mild Reaction Conditions. Dalton Trans 2018, 47, 11290−11297. (k) Nishibayashi, Y. Transition Metal−Dinitrogen Complexes: Preparation and Reactivity; Wiley-VCH: Weinheim, Germany, 2019. (l) Tanabe, Y.; Nishibayashi, Y. Recent Advances in Catalytic Silylation of Dinitrogen Using Transition Metal Complexes. Coord. Chem. Rev. 2019, 389, 73−93. (5) (a) Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum Center. Acc. Chem. Res. 2005, 38, 955−962. (b) Schrock, R. R. Catalytic Reduction of Dinitrogen to Ammonia by Molybdenum: Theory versus Experiment. Angew. Chem., Int. Ed. 2008, 47, 5512−5522. (c) Wickramasinghe, L. A.; Ogawa, T.; Schrock, R. R.; Müller, P. Reduction of Dinitrogen to Ammonia Catalyzed by Molybdenum Diamido Complexes. J. Am. Chem. Soc. 2017, 139, 9132−9135. (6) (a) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Nitrogen to Ammonia by an Iron Model Complex. Nature 2013, 501, 84−87. (b) Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters, J. C. Evaluating Molecular Cobalt Complexes for the Conversion of N2 to NH3. Inorg. Chem. 2015, 54, 9256−9262. (c) Buscagan, T. M.; Oyala, P. H.; Peters, J. C. N2-to-NH3 Conversion by a Triphos-Iron Catalyst and Enhanced Turnover under Photolysis. Angew. Chem., Int. Ed. 2017, 56, 6921−6926. (d) Fajardo, J., Jr.; Peters, J. C. Catalytic Nitrogen-to-Ammonia Conversion by Osmium and Ruthenium Complexes. J. Am. Chem. Soc. 2017, 139, 16105−16108. (e) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. Catalytic N2-to-NH3 Conversion by Fe at Lower Driving Force: A Proposed Role for Metallocene-Mediated PCET. ACS Cent. Sci. 2017, 3, 217−223. (f) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Peters, J. C. FeMediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating. J. Am. Chem. Soc. 2018, 140, 6122− 6129.

in situ from [MoI3(THF)3] and 4 equiv of PMePh2 and 2 equiv of dpppe as catalysts (Table S5).18 The reaction of an atmospheric pressure of nitrogen gas with 180 equiv of SmI2 and 180 equiv of water in the presence of 2a in THF at room temperature for 26 h gave 40 equiv of ammonia based on the Mo atom, together with 19 equiv of hydrogen gas based on the Mo atom. When molybdenum complexes generated in situ from [MoI3(THF)3] and 4 equiv of PMePh2 and 2 equiv of dpppe were used as catalysts, 40 and 46 equiv of ammonia were produced based on the Mo atom of the catalysts, respectively. These results indicate that molybdenum complexes generated in situ from [MoI3(THF)3] and dpppe worked as the most effective catalysts, although the catalytic activity of molybdenum complexes generated in situ from [MoI3(THF)3] and 2 equiv of dpppe is not higher than those of 1a and 1b (Table S6). Because of the high catalytic activity of molybdenum complexes generated in situ from [MoI3(THF)3] and dpppe, we carried out the catalytic reaction using larger amounts of SmI2 and water in the presence of these complexes, where 83 equiv of ammonia were obtained based on the Mo atom.

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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.9b01340. General methods, catalytic ammonia formations, preparation of 3, various reactions, X-ray crystallography, and computational details (PDF) Accession Codes

CCDC 1911585 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]. *E-mail: [email protected]. ORCID

Hiromasa Tanaka: 0000-0001-8516-8280 Kazunari Nakajima: 0000-0001-9892-5877 Kazunari Yoshizawa: 0000-0002-6279-9722 Yoshiaki Nishibayashi: 0000-0001-9739-9588 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present project is supported by CREST, JST (Grant JPMJCR1541). We acknowledge Grants-in-Aid for Scientific Research (Grants JP17H01201, JP15H05798, JP18K19093, and JP18K05148) from JSPS and MEXT. Y.A. is a recipient of the JSPS Predoctoral Fellowships for Young Scientists.

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REFERENCES

(1) Liu, H. Ammonia Synthesis Catalysts, Innovation and Practice; Chemical Industry Press and World Scientific: Singapore and Beijing, 2013. 8930

DOI: 10.1021/acs.inorgchem.9b01340 Inorg. Chem. 2019, 58, 8927−8932

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Inorganic Chemistry

Chem. Soc. 1979, 101, 3405−3407. (e) Takahashi, T.; Mizobe, Y.; Sato, M.; Uchida, Y.; Hidai, M. Protonation Reactions of Molybdenum and Tungsten Dinitrogen Complexes with Halogen Acids. Hydride Hydrazido(2-) and Diazenido Complexes as Intermediate Stages of Reduction. J. Am. Chem. Soc. 1980, 102, 7461−7467. (f) Hidai, M.; Yokotake, I.; Takahashi, T.; Uchida, Y. Reactions of Coordinated Dinitrogen with Alcohol/Ketone Systems. Chem. Lett. 1982, 11, 453−454. (10) Hill, P. J.; Doyle, L. R.; Crawford, A. D.; Myers, W. K.; Ashley, A. E. Selective Catalytic Reduction of N2 to N2H4 by a Simple Fe Complex. J. Am. Chem. Soc. 2016, 138, 13521−13524. (11) Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Molybdenum-Catalysed Ammonia Production with Samarium Diiodide and Alcohols or Water. Nature 2019, 568, 536−540. (12) For selected reviews on the PCET process, see: (a) Reece, S. Y.; Nocera, D. G. Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems. Annu Rev. Biochem. 2009, 78, 673−699. (b) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of Proton-Coupled Electron Transfer Reagents and Its Implications. Chem. Rev. 2010, 110, 6961−7001. (c) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.; Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.; Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2012, 112, 4016− 4093. (d) Miller, D. C.; Tarantino, K. T.; Knowles, R. R. ProtonCoupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities. Top. Curr. Chem. 2016, 374, 30. (13) (a) Chciuk, T. V.; Anderson, W. R., Jr.; Flowers, R. A. II HighAffinity Proton Donors Promote Proton-Coupled Electron Transfer by Samarium Diiodide. Angew. Chem., Int. Ed. 2016, 55, 6033−6036. (b) Chciuk, T. V.; Anderson, W. R., Jr.; Flowers, R. A. II ProtonCoupled Electron Transfer in the Reduction of Carbonyls by Samarium Diiodide-Water Complexes. J. Am. Chem. Soc. 2016, 138, 8738−8741. (c) Chciuk, T. V.; Anderson, W. R., Jr.; Flowers, R. A. II Interplay between Substrate and Proton Donor Coordination in Reductions of Carbonyls by SmI2-Water Through Proton-Coupled Electron-Transfer. J. Am. Chem. Soc. 2018, 140, 15342−15352. (d) Kolmar, S. S.; Mayer, J. M. SmI2(H2O)n Reduction of Electron Rich Enamines by Proton-Coupled Electron Transfer. J. Am. Chem. Soc. 2017, 139, 10687−10692. (14) George, T. A.; Seibold, C. D. Bisdinitrogen Complexes of Molybdenum. J. Organomet. Chem. 1971, 30, C13−C14. (15) Chatt, J.; Wedd, A. G. A Convenient Route to Lower Oxidation State Complexes of Molybdenum. J. Organomet. Chem. 1971, 27, C15−C16. (16) Hidai, M.; Tominari, K.; Uchida, Y.; Misono, A. A transDinitrogen Complex of Molybdenum. J. Chem. Soc. D 1969, 1392. (17) Cotton, F. A.; Poli, R. Low-Valent Molybdenum Carbonyl Complexes as an Entry to Octahedral MoI3L3 Complexes. Synthesis and X-ray Molecular Structure of MoI3(THF)3. Inorg. Chem. 1987, 26, 1514−1518. (18) More detailed experimental results using other monodentate and bidentate phosphines as ligands are shown in the Supporting Information. (19) More detailed experimental results using other auxiliary tridentate and tetradentate phosphines as ligands are shown in the Supporting Information. (20) The results in Scheme 1 indicate that the amount of formed ammonia and dihydrogen is complementary in the present catalytic reaction. The formation of dihydrogen should be suppressed to achieve the effective production of ammonia. (21) The structure of 3, which was newly prepared from the reaction of 2a with 2 equiv of HOTf (OTf = OSO2CF3) in toluene at −78 °C to room temperature for 18 h, was determined by X-ray analysis. The detailed experimental results are shown in the Supporting Information. (22) (a) Chatt, J.; Richards, R. L. The Reactions of Dinitrogen in its Metal Complexes. J. Organomet. Chem. 1982, 239, 65−77. (b) Pickett, C. J. The Chatt Cycle and the Mechanism of Enzymic Reduction of Molecular Nitrogen, C. J. JBIC 1996, 1, 601−606. (c) Barrière, F.

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Communication

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DOI: 10.1021/acs.inorgchem.9b01340 Inorg. Chem. 2019, 58, 8927−8932