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Catalytic N2 Conversion
- 1. 1 Catalytic N2 Conversion by Molecular Complexes Yuto Yabuuchi Twitter: @YutoYabuuchi 07/07/20 Outside Research Talk @ Zoom! § Problems of Current Industrious NH3 Production § Historically Important Discovery in the Field § Recent Progress to Develop More Productive Systems For further information: Chalklay, Drover, Peters Chem. Rev. 2020, 120, 5582
- 2. Nitrogen Is an Essential Element to Lives 2 AmmoniaDinitrogen DNA Amino Acid § Although N2 is abundant in air, most creatures cannot convert N2 into usable nitrogen compounds for metabolism
- 3. The Haber-Bosch Process Turns Air into Breads 3 The Haber-Bosch process: § Supplies 63,000 tonnes of NH3/day § Has supported the growing world population § Fertilizer§ 80% of air Erisman, J.; Sutton, M.; Galloway, J.; Klimont, Z.; Winiwarter, W. Nature Geosci 2008, 1, 636 https://www.statista.com/statistics/1065865/ammonia-production-capacity-globally/ NN H H3+ N H H H 2 Fe Cat. 250–350 bar 450–550 ºC
- 4. The Haber-Bosch Process May Not Be Sustainable 4https://www.wired.com/2016/05/chemical-reaction-revolutionized-farming-100-years-ago-now-needs-go/ § Made from CH4 NN H H3+ N H H H 2 § 80% of air Fe Cat. 250–350 bar 450–550 ºC § The process takes 3% of natural gas produced in the world § Breaking the inert N–N triple bond requires high temperatures, making the process energy-intensive § Fertilizer
- 5. FeMo-Nitrogenases Fix N2 at Ambient Conditions 5 Can chemists develop molecular catalysts that convert N2 to NH3 at ambient conditions from renewable resources? Fe Fe Fe Fe S S S Cis275 S S S Fe Fe Fe Mo S S S C His442 Homocitrate N N 2 NH38 H+ 8 e–+ + Nitrogenase H2+
- 6. 6 § Problems of Current Industrious NH3 Production § Historically Important Discovery in the Field § Recent Progress to Develop More Productive Systems
- 7. The 1st N2 Complex Was Formed from N2H6 or N3 – 7 § The in-situ-generated Ru-N3 complex decomposes to leave N2 bound to the metal § This demonstrated N2 can indeed bind to metal complexes Allen, Bottomley, Harris, Reinsalu, Senoff, J. Am. Chem. Soc. 1967, 89, 5595 RuCl3 (aq.) 1) N2H6 2) X– Ru H3N H3N NH3 NH3 NH3 N N 2+ X2 Ru H3N H3N NH3 NH3 NH3 O HH 3+ 1) NaN3 2) H+ 3) X– Ru H3N H3N NH3 NH3 NH3 N3
- 8. Protonation of N2 Was Demonstrated to Give NH3 8 W0 PhMe2P PhMe2P N PMe2Ph PMe2Ph N N N 3 H2SO4 2 NH3 N2 WVI+ + § W0 complexes reduced N2 in the presence of H+ to give NH3 almost quantitatively Chatt, Pearman, Richards Nature 1975, 253, 39
- 9. [N3NM] Complexes Revealed Rich N2 Chemistry 9 § Flexible X3L-type ligands were found to be capable of stabilizing potentially N2-fixation-related intermediates Kol, Schrock, Kempe, Davis J. Am. Chem. Soc. 1994, 116, 4382 NaN3 MoIII N N N N C6F5 C6F5 X N2 Na/Hg (1 equiv) THF, ether [Mo] N N [Mo] [Mo] N N [M] N N2 Na/Hg (2 equiv) THF, ether 0.5 X = Cl X = OTf X = OTf CH3OTf [M] N [Mo] N N Si(i-Pr)3 CH3 X = Cl, OTf (i-Pr)3SiCl F F F F F
- 10. Trigonal Complexes Have a 𝞹-Basic Apical Site 10 § Two 𝞹 symmetric SALCs push dyx and dzx orbitals up, thereby enhancing 𝞹 basicity of them to activate N2 d3 SALC 𝞼* 𝞹* 𝞼* dyz, zx: 𝞹 Donar dxy, x2-y2 dz2: 𝞼 Acceptor
- 11. N2 Cleavage Was Achieved by Trigonal Planer Mo 11Laplaza, Cummins Science 1995, 268, 861 § The µ-N2-bridged complex was found to split into two nitride complexes at room temperature Mo N N N Mo N N N Mo N N N N N Mo N N N N Mo N N N N + N2 1 atom –35 ºC 28 ºC
- 12. 12Kol, Schrock, Kempe, Davis J. Am. Chem. Soc. 1994, 116, 4382 § Why doesn’t this complex cleave N2? § Flexible X3L-type ligands were found to be capable of stabilizing potentially N2-fixation-related intermediates MoIII N N N N C6F5 C6F5 X N2 Na/Hg (1 equiv) THF, ether [Mo] N N [Mo] 0.5 X = OTf X = Cl, OTf F F F F F [N3NM] Complexes Revealed Rich N2 Chemistry
- 13. N2 Is Cleaved via a Zigzag Transition State 13 § The N2-bridged [N3NMo] species might sterically disfavor the zigzag TS, thus not cleaving N2 and being unreactive Transition State (TS) MoN N N N Mo N N N N Ar tBu tBu Ar Ar tBu tBu Ar tBu Ar tBu Ar 4.94 Å ca. 4.81 Å Laplaza, Johnson, Peters, Odom, Kim, Cummins, George, Pickering J. Am. Chem. Soc. 1996, 118, 8638 Mo N Mo MoNN MoN MoN Mo N
- 14. First Catalytic N2-to-NH3 Reduction at a Single Site 14Yandulov, Schrock Science 2003, 301, 76 6 h 36 eq.48 eq. 7.56 eq./catalyst 63%/reductant Mo N N N N HITP HITP N N i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr HITP Heptane 23–25 ºC H N BArF 4 NN [LutH][BArF 4] + NH3 CrII [CrCp*2] § The bulky substituents prevented N2-bridging dimerization
- 15. The Distal Pathway Was Confirmed 15 § The distal N atom is protonated first, and then the nitride is protonated Yandulov, Schrock Science 2003, 301, 76 Mo N N N N HITP HITP N N i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr HITP Heptane 23–25 ºC [Mo] N N [Mo] N N H H [Mo] N N H H [Mo] N N H H [Mo] N [Mo] N N H H NH3 [Mo] N H [Mo] N H [Mo] N H H [Mo] N H H [Mo] N H H H [Mo] NH H H NH3N2 H+, e- H+ e- H+ e- H+e- H+ e- H+ e-
- 16. 16 § Problems of Current Industrious NH3 Production § Historically Important Discovery in the Field § Recent Progress to Develop More Productive Systems
- 17. Schrock’s Catalyst Suffered from Dissociation 17 § Amide complexes were not so stable in the acidic solution Yandulov, Schrock Science 2003, 301, 76 6 h 36 eq.48 eq. 7.56 eq./catalyst Mo N N N N HITP HITP N N i-Pr i-Pr i-Pr i-Pr i-Pr i-Pr HITP Heptane 23–25 ºC H N BArF 4 NN [LutH][BArF 4] + NH3 CrII [CrCp*2]
- 18. N2-Bridged PNP-Pincered Mo Catalysts Debuted 18 216 eq. 288 eq. 23.2 mol/catalyst Arashiba, Miyake, Nishibayashi Nat. Chem. 2011, 3, 120 5 h N Toluene, 20 h Room temperature H N OTf NN CoII [LutH][OTf] [CoCp2] + NH3 N Mo P P N N N P = PtBu2 NMo N N N P N N P N § One Mo unit donates electrons toward the other Mo units through bridging N2, helping the least favorable step to protonate the distal N
- 19. N2-Bridged PNP-Pincered Mo Catalysts Debuted 19 216 eq. 288 eq. 23.2 mol/catalyst Arashiba, Miyake, Nishibayashi Nat. Chem. 2011, 3, 120 5 h N Toluene, 20 h Room temperature H N OTf NN CoII [LutH][OTf] [CoCp2] + NH3 N Mo P P N N N P = PtBu2 NMo N N N P N N P N § One Mo unit donates electrons toward the other Mo units through bridging N2, helping the least favorable step to protonate the distal N
- 20. 20 216 eq. 288 eq. 23.2 mol/catalyst Arashiba, Miyake, Nishibayashi Nat. Chem. 2011, 3, 120 5 h N2-Bridged PNP-Pincered Mo Catalysts Debuted § One Mo unit donates electrons toward the other Mo units through bridging N2, helping the least favorable step to protonate the distal N N Toluene, 20 h Room temperature H N OTf NN CoII [LutH][OTf] [CoCp2] + NH3 N Mo P P N N N P = PtBu2 NMo N N N P N N P N H
- 21. Tunable Pincer Ligands Boosted the TON Record 21 § Making the pincer ligand more electron donating resulted in high turnover numbers (TONs) Arashiba et al. JACS 2015, 137, 5666 2003 2011 2015 20172014 Yield of NH3 (eq./cat.) 7.56 23 Yandulov, Schrock Science 2003, 301, 76 52 Arashiba et al. Nat. Chem. 2011, 3, 120 Year 63 230 Kuriyama et al. JACS 2014, 136, 9719 Eizawa et al. Nat. Commun. 2017, 8, 14874 415 Arashiba et al. Bull. Chem. Soc. Jpn. 2017, 90, 1111 N Mo P P I I I P Mo P P Cl N Ph P = PtBu2 N Mo P P N N N N N 2 P = PtBu2 N Mo P P N N N N N MeO 2 P = PtBu2 N N C Mo P P N N N N N 2 P = PtBu2 Fajarado et al. JACS 2017, 139, 16105 (120) MoIII N N N N HITP HITP HITPN N
- 22. N Mo P P I I I N Mo P P I N N Mo P P N I N Mo P P N I N N Mo P P I N N Mo P P I N H H H H H H N Mo P P I N N 2e–, N2 [MoI]N2 NH3 3e–, 3H+ Trigonal Bipyramidal Mo-I Complex Cleaves N2 22Arashiba, Eizawa, Tanaka, Nakajima, Yoshizawa, Nishibayashi Bull. Chem. Soc. Jpn. 2017, 90, 1111 § Cleaving N2 allows the cycle to bypass the least favorable step to protonate the distal N
- 23. No Progress Was Made in Choice of H+ & e– Source 23 § Pyridiniums and metallocenes had been used since Schrock’s system § Side reaction to produce H2 was not negligible H+ Source e– Source CrCp*2 CoCp2 CoCp*2 Yield of NH3 [eq./cat.] 7.56 23.2 415 Selectivity NH3 63%a 32%b 59%b H2 No data 40%b 23%b N Mo P P N N N N N 2 P = PtBu2 N Mo P P I I I N H BArF 4 N H OTf N H OTf Yandulov, Schrock Science 2003, 301, 76 Arashiba et al. Nat. Chem. 2011, 3, 120 Arashiba et al. Bull. Chem. Soc. Jpn. 2017, 90, 1111 aBased on H+ source. bBased on e– source. P = PtBu2 MoIII N N N N HITP HITP HITPN N
- 24. 24 NN + NH3 SmI2 THF, room temperature N Mo P P I I I P = PtBu2 H2O HO OH ROH
- 25. [Sm(ROH)n]2+ Is Capable of H+-Coupled e– Transfer 25Ashida, Arashiba, Nakajima, Nishibayashi Nature 2019, 568, 536 § Complexation of Sm2+ with alcohols or water weakens the O–H bond, thereby enabling H+-coupled e– transfer to the Mo complex effectively PCET Proton-Coupled Electron Transfer Sm2+ O O O O I H H HH O OO H H n I Sm3+ O I O O O O O Sm3+ H I I O O OO Sm3+ H I I O O e–, H+ and other Sm3+ compounds N Mo P P I N N Mo P P N I N Mo P P N I N N Mo P P I N N Mo P P I N H H H [MoI]N2 NH3 3e–, 3H+ 2 SmI2, N2 N Mo P P X X X X = Cl, Br, I
- 26. PCET Updated TON, TOF, and Selectivity 26 Arashiba, Eizawa, Tanaka, Nakajima, Yoshizawa, Nishibayashi Bull. Chem. Soc. Jpn. 2017, 90, 1111 Ashida, Arashiba, Nakajima, Nishibayashi Nature 2019, 568, 536 N Mo P P I I I N N C Mo P P Cl Cl Cl P = PtBu2 H+ Source [ColH][OTf] HOCH2CH2OH H2O e– Source CoCp*2 SmI2 SmI2 Yield of NH3 [eq./cat.] 415 3650 4350 Selectivitya NH3 59% 76% 91% H2 23% 22% 2% Turnover Frequency [min–1] 0.5 117 113 2003 2011 2015 20172014 Yield of NH3 (eq./cat.) 7.56 23 52 Year 63 230 415120 2019 4350 aBased on e– source. P = PtBu2
- 27. Summary 27 § The Haber-Bosch process is one of the most important technologies of the 20th century although it may not be sustainable § Catalytic N2-to-NH3 conversion has been achieved with Mo pincer complexes using SmI2 and H2O at room temperature