Nitrogen Reduction to Ammonia by a Phosphorus-Nitrogen PN ³ P-Mo(V) Nitride Complex: Significant Enhancement via Ligand Postmodification

Open AccessCCS ChemistryCOMMUNICATIONS6 Dec 2022Nitrogen Reduction to Ammonia by a Phosphorus-Nitrogen PN3P-Mo(V) Nitride Complex: Significant Enhancement via Ligand Postmodification Delong Han, Priyanka Chakraborty, Mei-Hui Huang, Li Yang, Hao Théo P. Gonçalves, Abdul Hamid Emwas, Zhiping Lai, Jr-Hau He, Aleksander Shkurenko, Mohamed Eddaoudi and Kuo-Wei Huang Han Division of Physical Science Engineering, King Abdullah University Technology, Thuwal 23955 KAUST Catalysis Center, Google Scholar More articles this author , Chakraborty Yang Gonçalves Emwas Core Lab, Lai Advanced Membranes Porous Materials Research He Department City Hong Kong, Kowloon 999077 Shkurenko *Corresponding author: E-mail Address: [email protected] Agency for Science, Research, Institute Engineeringand Sustainability Chemicals, Energy Environment, Singapore 138634 https://doi.org/10.31635/ccschem.022.202202385 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Efforts develop organometallic complexes catalytic nitrogen reduction have seen significant progress in recent years. However, the strategies improving activity homogenous catalysts mainly focused on alternating ligands metals. Herein, we report that stability PN3P-Mo pincer complex ( 2) toward dinitrogen (N2) were greatly enhanced through postmodification PN3P framework its parent 1). A high ratio NH3/Mo (3525) was achieved presence SmI2 as reductant. In sharp contrast, 1 only afforded an 21. Moreover, when supported anionic ligand, 2 furnished oxidation state Mo(V)-nitride N2 cleavage plausible key intermediate process, suggesting cycle may involve different states (II/V) from those with 10-π electron configuration literature. Download figure PowerPoint Introduction The annual production approximately 180 million tons NH3 consumes roughly 1–2% global energy supply.1,2 It is thus desirable alternative routes produce under milder conditions efficient environmentally friendly manner. Organometallic been investigated decades search potent fixation.3–10 After breakthrough Yandulov Schrock 2003 successfully transform into cobaltocene reducing agent Mo catalyst ambient (Figure 1a),11 various complexes, such Fe,12–16 Mo,17–19 Co,20 V,21 Re,22–24 employed investigation reactions (NRRs). Among these combinations metal-ligand platforms, Nishibayashi co-workers first demonstrated pincer-type could serve highly reductive proper proton sources 1b),25 thereby stimulating more research studies 1).20,26–31 despite contributions scientific community, exploring potential has restricted conventional synthetic methods. Figure | Selected NRR. We developed series PN3(P) where acidic N–H spacer, compared C–H counterparts,32–34 makes transformation between dearomatized aromatized forms readily accessible hydrogenative dehydrogenative reactions.35,36 Very interestingly, allow ligand afford new class second-generation are difficult synthesize methods (Scheme This completely lose properties reversible dearomatization rearomatization, but appear be electron-donating metal center stabilize centers higher states.37,38 feature modification provide opportunity explore states.39,40 case NRR, previous suggest two pathways involving containing terminal or bridging ligand(s).41–44 Higher activities observed cleaved nitride possessing positive charge.17,45 Therefore, offer some advantage over neutral ligand. Scheme Synthesis complexes. During our study influences merits NRR comparing 1, group reported combination reductant ethylene glycol source. An N-heterocyclic carbene-based catalyzed (4350 NH3/equiv, 1e) condition proton-coupled electron-transfer (PCET) process.45 offers us appropriate platform examine mild conditions. prepared precursor 1)17 precatalyst NH3. Meanwhile, contrast ligands, PNP PPP, formation Mo(IV) 2a),17,26 Mo(V) 3), derived 2, catalytically active species 2b). Pincer Mo-nitride formed during reduction. Results Discussion Complex synthesized according literature procedure.46 As expected, adopts meridional coordination mode three chloride ions connected one plane vertical 3, left). treating excess amounts t-BuOK ethyl iodide 1),39 structure confirmed X-ray crystallographic analysis right).a molecular besides tridentate (one amide N phosphine P atoms), also coordinated oxo modified addition groups pyridine backbone. (t-Bu2P)2Mo(O)I moiety disordered positions (see Supporting Information details). generated replacing KH base otherwise similar conditions, no product found toluene used solvent, abstract oxygen tetrahydrofuran (THF) form Mo=O functionality 1-butene S3). 3 Molecular (left) (right). Thermal ellipsoids shown at 30% probability level. Only atoms part shown. For clarity, selected labeled all solvent molecules hydrogen (except NH) omitted. bonds (Å): Mo1–N1 2.226(2), Mo1–P1 2.606(1), Mo1–P2 2.583(1), Mo1–Cl1 2.417(1), Mo1–Cl2 2.450(1), Mo1–Cl3 2.391(1), N2–C1 1.377(3), N3–C5 1.378(3). angles (°): P1–Mo1–P2 152.13(2), N1–Mo1–Cl2 166.75(6), Cl1–Mo1–Cl3 168.78(3). Mo1A–N1 2.139(2), Mo1A–P1A 2.469(1), Mo1A–P2A 2.467(1), Mo1A–O1A 1.658(2), Mo1A–I1A 2.770(1), N2−C1 1.291(4), 1.286(4). P1A–Mo1A–P2A 146.39(4), N1–Mo1A–I1A 146.21(6). Interestingly, N1–Mo1 bond distance (2.226(2) Å) longer than (2.139(2) Å), stronger ability center. lengths (1.377(3) 1.378(3) Å, respectively) (1.291(4) 1.286(4) respectively), which agrees well double characteristics 2. Furthermore, aromaticity six-numbered ring lost remaining (C3–C4 1.329(4) displayed typical character (1.658(2) Å).47 angle became smaller: 152.13(2) 146.39(4) At beginning study, bases temperatures optimized employing source (Table 1).48 reaction 50 °C rather KOH after best yield 94% entry 4).b applied same condition, 33% 1), significantly lower Although changed. Next, SmI2, sources. H2O 79% 2). Table Optimization Reaction Conditions NRR.a Entry Base Temperature Yield (%)c NH3/equivd H2 No r.t. 0 7 4 KOHb 71 42 tBuOK 94 56 5 40 86 52 6 73 44 tBuOKe 8 tBuOKf 91 55 aReaction condition: (2 mg, 2.6 μmol), (470 μmol, equiv based 2), catalyst), atmosphere, h. (8.9 mmol) added reaction. detected online gas chromatography (GC) equipped thermal conductivity detector (TCD), flame ionization (FID) mechanizer quantification. bKOH 30 wt %, 10 mL. cThe calculated dNH3/equiv complex. Each repeated least twice. e40 min. f1 Catalytic Different Proton Sources Reductants.a Catalyst Source (%)b NH3/equivc HO(CH2)2OH 33 21 20 79 47 CH3OH 93 CH3CH2OH 48 CF3CH2OH 22 (CH3)2CHOH 2,3-Butanediol 12 1,2-Cyclohexanediol 11 9d 83 ± 540 18 10d 39 228 11e 59 3525 61 13 12f [Lut-H][OTf] 13f 14g 15h (2.6 catalyst) reductant, μmol dihydric alcohol, 940 monohydric 360 equiv, respectively, mL THF heated GC TCD, FID bThe cNH3/equiv d0.26 used. e0.026 used, overnight. Repeated four times. fCatalyst (4.6 KC8 (100 (120 room temperature. gKC8 hCp2Co (Cp, cyclopentadienyl) μmol). up 93% obtained methanol (MeOH) 3). Intriguingly, yields decreased 48% 6% ethanol 2-propanol respectively entries 6). strong electron-withdrawing even (5%, 5). trend 2,3-butanediol 1,2-cyclohexanediol utilized; 12% 2% produced, 8). noteworthy decreasing loading 0.26 still gave excellent 83% 9). MeOH employed, 39% observed. Further 0.026 resulted 59% NH3/equiv 11). tested using KC8, producing 11% 13). When identical 12). Notably, cases, consistently 1. Employing Cp2Co 3% 14 15), implying other reductants alcohol not suitable MeOH. 15N2 procedure isotopically 15NH3 identified 1H NMR spectrum S1). result larger-scale experiment carried out, 383 mg NH4Cl isolated 81% Section 1.2). occur N2-bridged intermediate,41,49 fate investigated. treated stoichiometric amount °C, conversion inseparable mixture S6), without To identify whether complex, mixing NaI 3-Cl 5) salt, Me3SiN3 2).50 infrared spectra showed characteristic peaks Mo≡N S4). Thus, coordinately unsaturated strongly supported. Additionally, crystals both single-crystal analysis. cell parameters distinct terms magnetism, NMR, mass, elemental analysis, color exhibits distorted square pyramidal 4); loses aromaticity, arms become shorter 1.370(5) Å 1.375(5) 4-I 1.304(7) 1.286(7) fall range C=N bond. becomes smaller 151.06(3) 148.63(6)°. declines 2.199(3) 2.190(4) Mo1–N4 decreases 1.637(4) 1.629(5) Å. important note 1.644 slightly 1.635 4, indicating postmodified system better subjected unmodified 13% yield, though shares core 3. 90% 1.304(7), 1.286(7), 2.190(4), Mo1A–N1A 1.629(5), 2.513(2), 2.514(2), 2.720(1). 148.63(6), 152.2(1). (right) (left). 3-Cl, 1.289(9), 1.297(8), Mo1A–N4A 1.644(7), 2.196(5), Mo1A–Cl1A 2.368(2), 2.524(3), 2.504(3). N1–Mo1A–Cl1A 146.9(2), 148.42(9). 4-I, except H3, non-coordinated anion omitted clarity. 1.370(5), 1.375(5), 2.199(3), 2.519(1), 2.517(1), Mo1–I1 2.702(4), 1.637(4). 151.06(3), N1–Mo1–I1 155.66(8). gain further insights influence postmodification, out. transformed 4-I) PNP-Mo(IV) analogous reaction,45 reduced ligands.46 possesses center, structurally 20% insoluble precipitates. fact there suggests poor condition. These observations indicate affords improved corresponding resulting performance Syntheses 4. Stoichiometric propose cyclic experimental reports 3).17,30,51 First, dissociates receives electrons protons reactive PN3P-MoII-I (MoI, S7). Two MoI then bridge give penta-coordinated bimetallic Int, followed 3.17,26,30,51–53 that, protonated PCET regenerate Int N2. change II V process (between Int), I IV counterparts.45 Of before,43,50,51,54 Mo(II) Mo(V)≡N recently demonstrated.25,28 results calculations proceed II/V cycle, cannot rule out involvement experimentally.3 mechanism Conclusion 3) influenced structures, especially “unmodified” counterparts 4) reductant: >90%, respectively; number achieved. Supported converted cleaving Compared much-less cycle. broader working again demonstrate profound effect modification. system. enhancement Investigation applications fixation ongoing will due course. Footnotes According experience known post small carbon next C1. Highly pure recrystallization. b shows Sm gives alkoxide dimer reaction, use hydroxide needed destroy Sm-NH3 release See refs 45 48. available includes section supporting figures. Conflict interest authors declare conflict interest. 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