Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Abstract

Dinitrogen (N₂ ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N₂ transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N₂ functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N₂ reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N₂ functionalization reactions following these strategies.

Keywords: catalysis; coordination complexes; functionalization; mechanism; nitrogen.

Scheme 1

Comparative dinitrogen reduction to ammonia by the Haber–Bosch process (top) and in the Mo‐nitrogenase enzyme (bottom).

Scheme 2

Molecular orbital diagram of free dinitrogen along with DFT‐computed HOMO and LUMO (B3LYP).

Scheme 3

σ‐donation/π‐backbonding scheme of metal‐bound dinitrogen.

Scheme 4

Dissociative and associative mechanisms for dinitrogen functionalization at a metal center M.

Scheme 5

Dissociative N₂‐splitting by mononuclear molybdenum complexes 1 (a) and 3 (b), trinuclear Ti^III and Cr^III hydride complexes 5 (c) and 7/8 (d) and dinuclear Nb^III complexes 11 (e) and 13 (f).

Scheme 6

Dissociative N₂‐splitting by molybdenum complexes 18 (a), 20 (b), 24 (c), 28 (d), and 29 (e), all bearing pincer ligands.

Scheme 7

(Electro)chemically induced N₂‐splitting by rhenium complexes 32 (a) and 35 (b).

Scheme 8

Dissociative N₂‐splitting by uranium complex 40.

Scheme 9

Dissociative N₂‐splitting by group 5 transition metal complexes 37 (a), 42 (b), 44 (c), and 46 (d).

Scheme 10

Dissociative N₂‐splitting by titanium complex 48.

Scheme 11

Dissociative N₂‐splitting by iron complexes 52 (a) and 54 (b) bearing β‐diketiminate ligands.

Scheme 12

Dissociative N₂‐splitting by group 5 and 6 amidinate complexes, 57 (a), 58 (b), 60 and 61 (c).

Scheme 13

Photolytic N₂‐splitting of dinitrogen bridged group 6 and 7 metal complexes 62 (a), 64 (b), 65 (c), 67 (d), 69 and 70 (e).

Scheme 14

Electrochemical N₂‐splitting by molybdenum complex 74.

Scheme 15

Stoichiometric functionalization of metal‐nitrides originating from N₂ (N‐C and N−Si bond formation).

Scheme 16

Synthetic cycles promoted by transition metal complexes 2 (a), 46 (b) and 68 (c) for the synthesis of isocyanates and nitriles utilizing N₂ as a nitrogen source.

Scheme 17

Functionalization of metal‐nitride complexes 33 (a) and 35 (b).

Scheme 18

Synthetic cycles promoted by transition metal complexes 33 (a), 89 (b) and 91 (c) for the synthesis of organic nitriles utilizing N₂ as a nitrogen source.

Scheme 19

Stoichiometric functionalization of metal‐bound N₂ (N−C and N−Si bond formation).

Scheme 20

Associative N₂ protonation using group 6 bis‐diphosphine complexes, 94–96 resulting in hydrazine complexes 97–99 and hydrazido complex 100.

Scheme 21

Associative N₂ protonation using group 6 metal complexes, bearing metallocene diphosphine ligands 103–110 (a) and (b), and resulting hydrazido complexes 101–102 (c).

Scheme 22

Associative N₂ protonation using iron complexes 111–113 supported by chelating diphosphines (a), and resulting hydrazido complex 114, diazenido complex 115 and ammonia complex 116 (b). Iron complexes 117–118 supported by chelating triphosphines and hydrides 119–120 (c).

Scheme 23

Associative N₂ protonation using group 6 monodentate phosphine complexes 121, 123, 124 and resulting hydrazine complex 122.

Scheme 24

Associative N₂ protonation by molybdenum complexes 125 and 127 bearing chelating triphosphines.

Scheme 25

N₂ protonation by chromium complexes bearing macrocyclic phosphines ligands: no reactivity towards protonation of complexes 128–129 (a) and protonation of complex 130 using proton or hydrogen atom donors (b). Hydrazido complex 131 and hydride complex 132 proposed by DFT calculations are provided in inset.

Scheme 26

Associative N₂ protonation by complexes 133 (a), 134 and 135 (b) and gold clusters 136‐P′ (c).

Scheme 27

Protonation of the bridging N₂ ³⁻ moiety in bis‐yttrium complex 137 to produce the corresponding hydrazido complex 139 via disproportionation.

Scheme 28

(a) Activation of dinitrogen by mononuclear Th complex (140) with diphenoxide ligand. (b) Associative activation of dinitrogen with polyphenoxide complex of U (144) and Th (145) via ligand deprotonation.

Scheme 29

Associative N₂ splitting and protonation/hydrogenation by uranium siloxide complex 146.

Scheme 30

Associative N₂ hydrogenation by tungsten complexes 123 (a and c) and 93 (b) using various metal hydrides 148–156.

Scheme 31

Associative N₂ functionalization with sodium hydride mediated by the diiminepyridine Cr complex 157 and resulting amido complex 158.

Scheme 32

Associative N₂ hydrogenation by zirconium complexes, 159 (a), 161 (b) and resulting hydrazine complexes 160 (a), 162 (b).

Scheme 33

Associative N₂ hydrogenation by titanium hydride complex 163 and resulting dinitrogen complex 164 and nitrido‐imido complex 165.

Scheme 34

Associative N₂‐splitting and protonation by silica‐supported tantalum complex 167. The structures in the inset describe intermediates observed by in situ spectroscopy and theoretical studies.

Scheme 35

Mono‐ and bis‐silylated N₂ complexes: mono‐silylated complexes of molybdenum (171) (a), cobalt (174) and iron (180–181) (b), bis‐silylation of N₂ coordinated by the iron dimer 183 (c), mono‐silylated complexes 177–178 (d) and of the bis‐silylated alumatrane complex 189 (e).

Scheme 36

Associative N₂ silylation by tantalum hydride complex 191.

Scheme 37

Silylation of the bridging dinitrogen moiety in substituted dinuclear hafnocene complex 196 and further functionalization of the dinitrogen ligand.

Scheme 38

Associative N₂ acylation (a) and alkylation (b) by group 6 diphosphine complexes.

Scheme 39

Associative N₂ alkylation by cobalt complex 173 (a), and iron complexes 216 (a) and 219/220 (b).

Scheme 40

Associative N₂ functionalization with CO₂ and subsequent methylation/silylation by the ansa‐zirconocene complex 223.

Scheme 41

Associative N₂ functionalization by hafnocene complex 225 with CO₂ (a) and isocyanate (b).

Scheme 42

Associative N₂ alkylation by hafnium complex 227 (a), and scandium complex 228 (b).

Scheme 43

Associative N₂ alkylation (a) and functionalization (b) by tantalum complex 190 using various electrophiles,.

Scheme 44

Associative N₂ alkylation by zirconium complex 159.

Scheme 45

Associative N₂ functionalization by titanium complex 236 with allene, isocyanate and CO₂.

Scheme 46

Associative N₂ functionalization by uranium siloxide complexes, 147 (a), and 243 (b).

Scheme 47

Catalytic systems for N₂ functionalization (N−H, N−Si and N−C bond formation) and associated mechanism.

Scheme 48

Proposed catalytic cycle for catalyst 246 (a) and structure of complex 49 (b).

Scheme 49

Proposed catalytic cycle for catalyst 22 (a) and structure of molybdenum complexes bearing PNP ligands 254–257 (b).

Scheme 50

Proposed catalytic cycle for complex 20 (a). Structure of PNP‐pyrrole molybdenum 258 (b), iron 260, cobalt 261 and vanadium 262 (c) catalysts and of NHC‐PCP molybdenum catalyst 263.

Scheme 51

Structure of complexes 264–266 (a) and proposed catalytic cycle for 264 (b).

Scheme 52

Structure of iron complexes 267–270.

Scheme 53

Structure of chromium complexes bearing cyclopentadienyl‐phosphine ligand 271–275.

Scheme 54

Proposed catalytic cycle for N₂ silylation mediated by the ferrocenyldiphosphine molybdenum complex 106.

Scheme 55

Proposed catalytic cycle for N₂ silylation using complexes 276 (a) and 28 (b).

Scheme 56

Catalytic N₂ silylation using for ferrocene or iron pentacarbonyl (a) and structure of iron‐based catalysts bearing silicon‐containing ligands 285–287 (b).

Scheme 57

Proposed catalytic cycle for complex 270 (a) and structure of iron‐based catalysts supported by macrocyclic phosphine ligands 289–291 (b) and (c).

Scheme 58

Structure of polynuclear iron hydride clusters 292–294. (a) and heterometallic hydride clusters 295–298 (b).

Scheme 59

Proposed catalytic cycle for complex 299 (a) and 300 (b).

Scheme 60

Structure of group 9 complexes 302–305.

Scheme 61

Structures of group 4 and 5 complexes 306–308 and 38–39.

Scheme 62

Catalytic N₂ silylation using actinide complex 142.

Scheme 63

Examples of heterocycles formed catalytically from dinitrogen.

Scheme 64

Pourbaix diagram of dinitrogen. Solid lines correspond to N₂ reduction to NH₄ ⁺ or NH₃ (red line) and N₂ oxidation to NO₃ ⁻ (blue line). Dotted lines straddle the region of water stability (reduction to H₂ for bottom line and oxidation to O₂ for top line). ^[252]

Scheme 65

Well‐defined molecular complexes promoting dinitrogen electroreduction 313–315.