Keywords

1 Introduction

Dinitrogen (N2) is an abundant and easily accessible resource, which occupies about 78% of Earthā€™s atmosphere. However, N2 molecule is chemically inert under ordinary conditions due to its strong Nā€“N triple bond (944.84Ā Ā±Ā 0.10Ā kJ/mol) [1], large HOMO-LUMO energy gap (10.82Ā eV) [2], and nonpolarity. Certain microbial organisms can reduce N2 to NH3 by using nitrogenase enzymes at ambient temperature and pressure. This process consumes eight protons (H+) and eight electrons (eāˆ’) to convert one molecule of N2 to two molecules of NH3 with release of one molecule of H2. Recent studies revealed that the multiple metal centers having two hydrides and two sulfur-bound protons in the ironā€“molybdenum cofactor play a key role to promote H2 release and N2 reduction (Scheme 1a) [3ā€“6]. However, the biological ammonia synthesis is not yet well understood and is difficult to mimic artificially. Industrially, ammonia is produced from N2 and H2 by the Haberā€“Bosch process under relatively harsh conditions (350~550Ā°C, 150~350Ā atm) to activate N2 on the solid catalyst surface. It was proposed that the reaction is initiated by dissociative absorption of N2 and H2 on low valent multiple iron metal sites to form metal hydrides and nitrides, followed by reversible hydrogenation of the nitride species to provide NH3 (Scheme 1b) [7ā€“11]. Both the biological and the Haberā€“Bosch processes are thought to take place through the cooperation of multiple metal sites bearing hydride ligands.

Scheme 1
scheme 1

Proposed pathways for catalytic ammonia formation by (a) nitrogenase enzyme (only the core structure is shown) and (b) the Haberā€“Bosch process (only part of the catalyst surface is shown)

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In order to further explore the mechanism of N2 reduction at the molecular level and thereby develop milder chemical processes for ammonia synthesis, extensive studies on the activation of N2 with organometallic complexes have been carried out over the past decades [12]. As model reactions of the enzyme process, the use of strong metal reducing agents as an electron source in combination with transition metal complexes has been extensively studied, and the catalytic transformation of N2 to ammonia has been achieved at ambient temperature and pressure by using carefully designed proton sources [13ā€“15]. An alternative approach is the activation of N2 by transition metal hydrides without the use of extra reducing agents or proton sources [16]. This approach is of particular interest, in view of the fact that both the biological and the industrial Haberā€“Bosch processes may involve metal hydrides as true active catalyst species. This chapter focuses on the activation and functionalization of N2 by transition metal hydride complexes.

2 Dinitrogen Complexes Derived from Mononuclear Transition Metal Hydride Complexes

2.1 Group 9 Transition Metal Hydrides

The first dinitrogen complex [(NH3)5Ru(N2)]2+ was obtained serendipitously from the reaction of a ruthenium trichloride with hydrazine hydrate in 1965 [17]. Shortly after this discovery, the N2-derived end-on coordinated cobalt dinitrogen complex [(Ph3P)3Co(N2)H] (1) was synthesized from the reaction of a cobalt acetylacetonate, diethylaluminium monoethoxide, and triphenylphosphine ligands in the presence of N2 gas (Scheme 2) [18]. When 1 was kept under an atmosphere of H2, the coordinated N2 ligand was displaced by H2 to afford the cobalt hydride complex [(Ph3P)3CoH3] (2) [19]. Complex 2 could also be prepared by the reaction of [CoX2(PPh3)2] (XĀ =Ā Cl, Br, I) with borohydride in the presence of H2 and free PPh3, and the reaction of 2 with N2 easily took place to give 1 (Scheme 2) [20]. Thus, the hydrogenation/dinitrogen coordination reactions are reversible.

Scheme 2
scheme 2

Synthesis and reversible hydrogenation of the cobalt dinitrogen complex 1

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The reaction of 1 with MgEt2, BuLi, or Na metal afforded the N2-bridged heterobimetallic complex, [{(Ph3P)3Co(Ī¼-N2)}2Mg(THF)4], [(Ph3P)3Co(Ī¼-N2)Li(Et2O)3] (3), or [(Ph3P)3Co(Ī¼-N2)Na(THF)3], respectively [21]. While the coordinated N2 ligand in 1 is unable to react with protic acids, the coordinated N2 ligand in the electron-rich heterobimetallic complexes such as 3 gives 20ā€“30% of hydrazine and ammonia by addition of H2SO4 or HCl (Scheme 3). However, reaction of the Co-N2-Li complex 3 with H2 afforded an analogous hydrogen complex Co-H2-Li, with quantitative evolution of N2. Upon exposure to an N2 atmosphere, the hydrogen complex Co-H2-Li released H2, and regenerated 3, demonstrating the reversibility of the coordination of H2 and N2 to the Ph3P-Co-Li moiety.

Scheme 3
scheme 3

Protonation of the cobalt dinitrogen complex 1

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With regard to other group 9 metals, the reduction of RhCl3Ā·3H2O with Na/Hg in the presence of sterically demanding phosphines and H2 afforded the hydride complexes [(R3P)2RhH3] (4) (RĀ =Ā Cy, iPr), which upon reaction with N2 gave the end-on coordinated dinitrogen/hydride complexes [(R3P)RhH(N2)] (5) (Scheme 4) [22, 23]. The dinitrogen ligands in these compounds were weakly activated and could readily be released by gentle heating.

Scheme 4
scheme 4

Synthesis of the rhodium dinitrogen complex 5

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2.2 Group 8 Transition Metal Hydrides

The iron hydride complexes [FeH2(H2)(PR3)3] (6, PR3Ā =Ā PEtPh2, PBuPh2), which were prepared by treating FeCl2 with PR3 and NaBH4 under an H2 atmosphere, could incorporate atmospheric nitrogen to give the end-on coordinated dinitrogen complexes [FeH2(N2)(PR3)3] (7) in an irreversible way (Scheme 5) [24ā€“27]. Complex 7 released N2 when heated under vacuum or upon addition of I2, HCl, or CO.

Scheme 5
scheme 5

Synthesis and dinitrogen coordination of the iron hydride complex 6

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The ruthenium dinitrogen compound [RuH2(N2)(PPh3)3] (8) was obtained from the reaction of [RuHCl(PPh3)3] [28] with AlEt3 under an N2 atmosphere (Scheme 6) [29]. Treatment of 8 with H2 gave the corresponding dihydride/dihydrogen complex [RuH2(H2)(PPh3)3] (9). This conversion was readily reversed by exposing 9 to N2. It is worth noting that the reactions of [RuH2(PPh3)4] with N2 and H2 did not afford isolable 8 and 9 due to the presence of the dissociated free PPh3 ligand [30]. The ruthenium dinitrogen complexes bearing sterically demanding phosphines [31], [PNP] pincer ligand [32], and tris(pyrazolyl)borate ligand [33], were also synthesized from the corresponding hydrogen complexes with N2.

Scheme 6
scheme 6

Synthesis and hydrogenation of the ruthenium dinitrogen complex 8

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The reaction of [FeCl2(dmpe)2] (10) bearing the bidentate dmpe (bis(dimethylphosphino)ethane) ligand with sodium borohydride (NaBH4) afforded the hydride complex [FeH(H2)(dmpe)2]+ (11) in high yield (Scheme 7) [34]. Complex 11 reacted with N2 to give an end-on coordinated dinitrogen complex [FeH(N2)(dmpe)2]+ (12). Deprotonation of 12 by KOtBu provided an unstable iron (0) complex [Fe(N2)(dmpe)2] (13). Treatment of 13 with HCl yielded ammonia (12%) and the chloride complex 10 (80%) with release of N2 and H2 [35ā€“37]. The dichloride complex 10 could serve as a precursor to the hydride complex, and therefore, a synthetic cycle for the transformation of N2 to ammonia could be realized.

Scheme 7
scheme 7

A synthetic cycle for transformation of dinitrogen to ammonia by the iron complexes 10ā€“13

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Hydrogenolysis of an iron chloride complex bearing the bidentate 1,2-bis(bis(methoxypropyl)phosphino)ethane ligands with H2 gave the corresponding hydride/dihydrogen complex 14 (Scheme 8) [38]. The H2 ligand in 14 could be substituted by N2, quantitatively affording the dinitrogen complex 15. The reaction with KOtBu yielded a neutral Fe(0) complex 16. Protonation of 16 with triflic acid produced NH4 + (15%) and trace N2H5 + (2%), but did not give a characterizable iron complex [39].

Scheme 8
scheme 8

Dinitrogen activation by the iron hydride complex 14

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2.3 Group 7 Transition Metal Hydrides

Addition of LiAlH4 to a suspension of [MnBr2(dmpe)2] followed by hydrolysis with water afforded the hydride complex [MnH(H2)(dmpe)2] (17) (Scheme 9) [40]. Complex 17 readily reacted with N2 to give the corresponding end-on coordinated dinitrogen complex [MnH(N2)(dmpe)2] (18) [41]. Half-sandwich manganase dihydride complex [(C5H5)MnH2(dfepe)] (dfepeĀ =Ā diperfluoroethylphosphinoethane) with N2 afforded a binuclear end-on coordinated dinitrogen complex [(C5H5)Mn(dfepe)]2(N2) via the formation of a mononuclear dinitrogen complex [(C5H5)Mn(N2)(dfepe)] [42].

Scheme 9
scheme 9

Synthesis of the manganese hydride complex 17 and its reaction with N2

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The rhenium dinitrogen complex [ReH(N2)(dmpe)2] (20) was obtained from the reaction of the nonahydride complex [NEt4]2[ReH9] (19) [43] with dppe (dppeĀ =Ā 1,2-bis(diphenylphosphino)ethane) under an N2 atmosphere (Scheme 10) [44]. In contrast, the reactions of monodentate tertiary phosphines with 19 in an N2 atmosphere gave only the hydride complexes. The dinitrogen complex 20 was readily protonated at the metal center by HBF4 to form the cationic dihydro dinitrogen rhenium complex [ReH2(N2)(dmpe)2]BF4, while protonation at the N2 ligand was not observed. Synthesis of the dinitrogen complex 20 by photolysis of the trihydride complex [ReH3(dmpe)2] (21) with UV light in an N2 atmosphere was also reported [45].

Scheme 10
scheme 10

Synthesis of the rhenium dinitrogen complex 19 from the reactions of rhenium hydride complexes with N2

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2.4 Group 6 Transition Metal Hydrides

The molybdenum and tungsten hydride complexes having bidentate phosphine ligands [MH4(dppe)2] (MĀ =Ā Mo (22), W) were obtained from the reactions of the chloride precursors [MCl4(dppe)] and excess of dppe ligand with NaBH4 [46, 47]. Photolysis of [MoH4(dppe)2] (22) with UV light gave an end-on coordinated dinitrogen complex [Mo(N2)2(dppe)2] (23) in high yield (Scheme 11) [48], while irradiation of [MH4(dppe)2] (MĀ =Ā Mo, W) with Ī³-ray gave the dinitrogen complexes [M(N2)2(dppe)2] together with ammonia and hydrazine [49ā€“51]. Hydrogenolysis of [Mo(N2)2(dppe)2] (23) with H2 regenerated the tetrahydride complex [MoH4(dppe)2] (22) [52, 53]. The reaction of a C6H6-coordinated half-sandwich molybdenum dihydride complex [(C6H6)MoH2(PPh3)2] (24) with N2 afforded a binuclear end-on coordinated dinitrogen complex [(C6H6)Mo(PPh3)2]2(N)2 (25) in quantitative yield with release of H2 in a reversible fashion (Scheme 12) [54].

Scheme 11
scheme 11

Synthesis of the molybdenum hydride complex 22 and its reaction with N2 under UV irradiation

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Scheme 12
scheme 12

Formation of a binuclear molybdenum dinitrogen complex 25

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The reaction of the dinitrogen tungsten complex [W(N2)2(PMe2Ph)4] (26) with the dihydrogen ruthenium complex [RuCl(H2)(dppp)2] (27) in the presence of H2 generated NH3 (55%) (Scheme 13) [55]. In this reaction, one H atom of the H2 unit in 27 worked as a proton source, which protonated the coordinated N2 in 26 to form initially a hydrazido (Wā‰”Nā€“NH2) species, and the other H atom remained at the Ru atom as a hydride. Further protonation of the hydrazido units with 27 resulted in the formation of NH3. Although the reaction allowed the formation of NH3 from N2 in the presence of H2, the electrons required for the cleavage of Nā‰”N bond were provided by the tungsten species.

Scheme 13
scheme 13

Formation of ammonia from the reaction of the tungsten dinitrogen complex 26 with the ruthenium dihydrogen complex 27

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As to group 5 transition metals, solid surface-supported tantalum hydrides were reported to cleave and hydrogenate N2 [56]. However, the activation of dinitrogen by a well-defined mononuclear group 5 transition metal hydride complex remained unknown.

2.5 Group 4 Transition Metal Hydrides

The activation of dinitrogen by titanium metallocene hydride complexes bearing different cyclopentadienyl ligands was investigated [57ā€“59]. Acid hydrolysis of a reaction mixture of (C5H5)2TiCl2 and ethylmagnesium halide in the presence of N2 was reported to yield NH3 [59]. It was thought that a titanium hydride species was an active species for the reduction of N2 in this reaction, though no structural evidence was available. Hydrogenolysis of [(C5Me5)(C5Me4CH2)TiCH3] with H2, followed by introduction of N2 (1Ā atm), afforded an end-on coordinated N2-bridged complex [(C5Me5)2Ti(Ī¼-N2)Ti(C5Me5)2] [60, 61]. This reaction was proposed to proceed through initial hydrogenolysis of the alkyl complex with H2 to a dihydride species [(C5Me5)2TiH2], followed by releases of H2 and incorporation of N2 to the resulting titanocene species [(C5Me5)2Ti]. Similarly, hydrogenolysis of the trivalent titanium complexes [(C5Me4H)2TiR] (28) (RĀ =Ā Me, Ph) with H2 followed by the reaction with N2 afforded the corresponding N2 complex [(C5Me4H)2Ti(Ī¼-N2)Ti(C5Me4H)2] (30) via the hydride complex [(C5Me4H)2TiH] (29) (Scheme 14) [62]. It was found that the Ti(III) metallocene hydride complex 29 could be disproportionated to the Ti(IV) dihydride 31 and the Ti(II) complex 32. The dihydride 31 could lose H2 to give 32 in a reversible fashion. The Ti(II) metallocene 32 reacted with N2 to form the dinuclear titanium N2 complex 30. The N2 ligand in 30 could be released under vacuum to give 32.

Scheme 14
scheme 14

Synthesis of the titanocene hydride complex 29 and the formation of an end-on bound dinitrogen complex 30

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Regarding the bonding mode of dinitrogen, the side-on (Ī· 2,Ī· 2) bridging form is expected to enhance the reactivity of the dinitrogen ligand compared to the end-on mode [63]. Metallocene complexes bearing less sterically demanding cyclopentadienyl ligands could provide a more sterically accessible and electron-poor metal center that potentially favors side-on Ī· 2,Ī· 2-N2 coordination [64]. Indeed, hydrogenolysis of the 1,2,4-trimethylcyclopentadienyl-ligated titanocene dimethyl complex [(C5Me3H2)2TiMe2] with H2 followed by reaction with N2 afforded the side-on Ī· 2,Ī· 2-N2 complex [(C5Me3H2)2Ti]2(Ī¼-Ī· 2,Ī· 2-N2) (33) (Scheme 15) [65]. The reaction of the dinitrogen complex 33 with H2 (1Ā atm) did take place, but a characterizable product was not obtained.

Scheme 15
scheme 15

Formation of the side-on bound dinitrogen complex 33 from the reaction of a less sterically hindered titanocene hydride complex with N2

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The ansa-zirconocene dihydride complex 35, which was formed by hydrogenolysis of the dialkyl precursor 34, reacted with N2 reversibly to afford a side-on coordinated dinitrogen complex 36 (Scheme 16) [66]. In contrast, the zirconium metallocene dihydride complex bearing two C5Me5 ligands [(C5Me5)2ZrH2] did not give an N2-incorporated complex under similar conditions [67], suggesting that the ansa bridge structure of 35 should play an important role for the formation of the dinitrogen complex 36.

Scheme 16
scheme 16

Reversible formation of the side-on bound dinitrogen complex 36 from the reaction of the ansa-zirconocene hydride complex 35 with N2

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3 Activation and Functionalization of Dinitrogen by Binuclear Transition Metal Hydride Complexes

The reaction of a tris(pyrazolyl)borate (iPr2Tp)-ligated binuclear copper hydroxide [iPr2TpCu]2(Ī¼-OH)2 (37) with triphenylsilane under an N2 atmosphere afforded an end-on bridged dinitrogen complex [iPr2TpCu]2(Ī¼-N2) (41) (Scheme 17) [68]. A mixed valence Cu(I)/Cu(II) binuclear copper monohydride complex [iPr2TpCu]2(Ī¼-H) (40) was isolated as a key intermediate. Complex 40 could be formed via combination of the highly reactive terminal Cu(II) hydride species [iPr2TpCu-H] (38), which was produced by reaction between HSiPh3 and the hydroxide 37, with the unsaturated Cu(I) species [iPr2TpCu] (39) generated by release of H2 from 38. Under an N2 atmosphere, complex 40 changed to the dinitrogen complex 41 with release of H2. Alternatively, the reaction of 39 with N2 could also afford 41. The N2 ligand in 41 is quite labile, which could be replaced by 15N2, MeCN, or O2.

Scheme 17
scheme 17

Formation of a binuclear copper dinitrogen complex 41

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The reaction of the sterically hindered Ī²-diketiminate ligated iron chloride complex with KBEt3H afforded the binuclear Fe(II) dihydride complex 42, which upon UV irradiation under N2 resulted in loss of H2 and formation of the end-on dinitrogen complex 43 (Scheme 18) [69, 70].

Scheme 18
scheme 18

Synthesis of the binuclear iron hydride complex 42 and its reaction with N2 to generate the end-on dinitrogen complex 43

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The Ī²-diketiminate-ligated cobalt and nickel hydride complexes 44 were obtained from the reaction of the chloride precursors with 1.0 equiv. of KBEt3H (Scheme 19) [71, 72]. When 2.0 equiv. of KBEt3H were used to react with the cobalt chloride complex, the potassium-bridged cobalt dihydride complex 45 was formed in high yield [71]. These binuclear dihydride complexes 44 and 45 readily reacted with N2 at room temperature to afford the end-on bridged dinitrogen complexes 46 and 47, respectively (Scheme 19). Attempts to reduce the dinitrogen ligand in the nickel dinitrogen complex with H2 led to loss of N2 [73].

Scheme 19
scheme 19

Synthesis of binuclear cobalt and nickel hydride complexes 44 and 45 and their reactions with N2 to generate the end-on dinitrogen complexes 46 and 47

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The reaction of the PNP-ligated zirconium chloride complex [{P2N2}ZrCl2] with KC8 under N2 yielded a side-on bound dinitrogen complex of zirconium, [{P2N2}Zr]2(Ī¼-Ī·2,Ī·2-N2) (48) (P2N2Ā =Ā PhP(CH2SiMe2NSiMe2CH2)2PPh) (Scheme 20) [74]. Hydrogenolysis of 48 with H2 afforded [{P2N2}Zr]2(Ī¼-Ī·2,Ī·2-N2H)(Ī¼-H) (49) containing both a bridging hydride and a bridging hydrazido unit through addition of one molecule of H2 across to a Zrā€“N bond. Theoretical studies suggested that addition of a second equivalent of H2 is feasible, but this reaction was not observed experimentally [75].

Scheme 20
scheme 20

Hydrogenation of the binuclear zirconium dinitrogen complex 48 with H2

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The analogous side-on bound dinitrogen metallocene complex [(C5M4H)2Zr]2(Ī¼-Ī·2,Ī·2-N2) (50), which was formed by the reaction of the chloride precursor [(C5M4H)2ZrCl2] with Na/Hg under N2, underwent the addition of 2 equiv. of H2 to furnish a dihydrido/diazenido complex [(C5M4H)2ZrH]2(Ī¼-Ī·2,Ī·2-N2H2) (51) (Scheme 21) [64, 76, 77]. The reaction proceeded through a concerted, highly ordered transition state, in which the Hā€“H bond is simultaneously cleaved with Zrā€“H and Nā€“H bond formation. Thermolysis of the dihydrido/diazenido complex 51 caused H2 loss and Nā€“N bond cleavage to give the nitrido/amido complex [(C5M4H)2Zr]2(Ī¼-N)(Ī¼-NH2) (52). In this sequence of the reaction, H2 worked as both proton and electron sources. It is also worth noting that thermolysis of the dihydrido/diazenido complex under an H2 atmosphere yielded the dihydride complex [(C5M4H)2ZrH2] with release of trace amount of ammonia [64].

Scheme 21
scheme 21

Cleavage and hydrogenation of the side-on bound dinitrogen ligand in complex 50 by H2

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A binuclear tantalum tetrahydride complex ([NPN]Ta)2(Ī¼-H)4 (53) ([NPN]Ā =Ā PhP(CH2SiMe2NPh)2), which was obtained from hydrogenolysis of the trimethyl precursor [NPN]TaMe3, reacted spontaneously with N2 to give a side-on, end-on bound dinitrogen complex ([NPN]Ta)2(Ī¼-Ī·1,Ī·2-N2)(Ī¼-H)2 (54) with elimination of H2 (Scheme 22) [78, 79]. In this transformation, [Nā‰”N] was formally reduced to [Nā€“N]4āˆ’ by four electrons generated by the reductive elimination of one molecule of H2 and the oxidation state change of the two Ta ions from Ta(IV) to Ta(V). The unique side-on end-on coordination fashion of the bridging N2 resulted in substantial reactivity of the N2 fragment, leading to its cleavage and functionalization. While no apparent reaction of 54 with H2 was observed, complete cleavage of the Nā€“N bond in 54 was achieved by reaction with a variety of hydride reagents (E-HĀ =Ā R2BH, R2AlH, RSiH3) (Scheme 22) [80ā€“82].

Scheme 22
scheme 22

Reaction of a binuclear tantalum tetrahydride complex 53 with N2 to give a side-on, end-on dinitrogen complex 54 whose Nā€“N bond could be cleaved upon reaction with hydride reagents

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The anionic binuclear niobium tetrahydride complex 55 bearing triaryloxide ligands readily reacted with N2 (1Ā atm) to afford the dinitrido complex 56 via Nā€“N bond cleavage without using external reducing agent (Scheme 23) [83, 84]. This process corresponds to an overall six-electron reduction of N2, in which four electrons are provided by formation of two molecules of H2 from four hydride ligands and two electrons generated by oxidation of the metalā€“metal bond. The methylation of the nitride units in 56 by MeI proceeded in a stepwise fashion to give the bisimide complex 57 (Scheme 23). A reaction of 56 with H2 did not take place.

Scheme 23
scheme 23

Dinitrogen cleavage by a binuclear niobium tetrahydride complex

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4 Activation and Functionalization of Dinitrogen by Tri- and Tetranuclear Transition Metal Hydride Complexes

Hydrogenolysis of the half-sandwich titanium trialkyl complex [Cpā€™Ti(CH2SiMe3)3] (Cpā€™Ā =Ā C5Me4SiMe3) with H2 afforded the mixed valence Ti(III)/Ti(IV) heptahydride complex [(Cpā€™Ti)3(Ī¼3-H)(Ī¼-H)6] (58) (Scheme 24) [85, 86]. This hydride cluster readily reacted with atmospheric pressure of N2 at room temperature, giving an imido/nitrido complex [(Cpā€™Ti)3(Ī¼3-N)(Ī¼-NH)(Ī¼-H)2] (61) via Nā€“N bond cleavage and Nā€“H bond formation without the need of extra reducing agent or proton source. Monitoring the reaction by 1H and 15N NMR revealed the initial formation of a dinitrogen complex [(Cpā€™Ti)3(Ī¼3-Ī·1, Ī·2, Ī·2-N2)(Ī¼-H)3] (59) with release of two molecules of H2, followed by Nā€“N bond cleavage to give a dinitrido (N3āˆ’) complex [(Cpā€™Ti)3(Ī¼3-N)(Ī¼-N)(Ī¼-H)3] (60), and hydride migration from titanium to the Ī¼2-nitrido unit to give the imido/nitrido complex 61. The six electrons for the cleavage of the Nā€“N bond were supplied by the reductive elimination of two molecules of H2 and the oxidation of two Ti(III) species to two Ti(IV) species. The proton (H+) for the formation of the Nā€“H bond was generated by oxidation of a bridging hydride (Hāˆ’) by two Ti(IV) species which were both reduced to Ti(III). Obviously, the hydride ligands in 58 served as the source of both electron and proton for the dinitrogen cleavage and hydrogenation, resembling in part the industrial Haberā€“Bosch process at the molecular level.

Scheme 24
scheme 24

Dinitrogen cleavage and hydrogenation by a trinuclear titanium heptahydride complex 58

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When the hydrogenolysis of the trialkyl titanium complex [Cpā€™Ti(CH2SiMe3)3] with H2 was carried out in the presence of N2, a tetranuclear diimido/tetrahydrido complex [(Cpā€™Ti)4(Ī¼ 3-NH)2(Ī¼-H)4] (62), instead of the trinuclear heptahydride complex [(Cpā€™Ti)3(Ī¼ 3-H)(Ī¼-H)6] (58), was obtained in high yield (Scheme 25) [85]. The formation of 62 could also be achieved by hydrogenolysis of [Cpā€™Ti(CH2SiMe3)3] in the presence of 1 equiv. of the trinuclear imido/nitrido complex 61, suggesting that the hydrogenation of 61 with a mononuclear titanium hydride species such as ā€œCpā€™TiH3ā€ generated in situ by the hydrogenolysis of [Cpā€™Ti(CH2SiMe3)3] may take place. No apparent reaction between 61 and H2 was observed at room or higher temperatures.

Scheme 25
scheme 25

Dinitrogen activation and hydrogenation by a tetranuclear titanium diimido/tetrahydrido complex 62

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Complex 62 reacted with atmospheric pressure of N2 at 180Ā°C to afford a mixed diimido/dinitrido complex [(Cpā€™Ti)4(Ī¼ 3-N)2(Ī¼ 3-NH)2] (63) with release of two molecules of H2 (Scheme 25) [87]. When 62 was heated at 130Ā°C, one molecule of H2 was released to give the dinitrido/tetrahydrido complex [(Cpā€™Ti)4(Ī¼ 3-N)2(Ī¼-H)4] (64). Exposure of 64 to H2 (1Ā atm) at 80Ā°C regenerated 62 quantitatively, demonstrating that 62 and 64 are facilely interconvertible through dehydrogenation and hydrogenation of the imido/nitrido ligands. When the dinitrido/tetrahydrido complex 64 was heated at 180Ā°C in the presence of N2 (1Ā atm), the diimido/dinitrido complex 63 was formed quantitatively. The hydrogenation of 63 with H2 to give the tetraimido complex [(Cpā€™Ti)4(Ī¼ 3-NH)4] (65) took place in a reversible way at 180Ā°C (Scheme 25).

Remarkably, the imido and nitride species in 63 could be easily converted to nitriles through reaction with acid chlorides at 60Ā°C (Scheme 26) [87]. This transformation did not require any extra reagents (either reducing agents or bases) and was compatible with functional groups such as aromatic Cāˆ’X (XĀ =Ā Cl, Br, I) bonds, nitro group, aldehyde and chloromethyl moieties. 15N-isotope labeled nitriles could also be efficiently prepared by using the 15N-enriched analogue [(Cpā€™Ti)4(Ī¼ 3-15N)2(Ī¼ 3-15NH)2] (63- 15 N) derived from 15N2 gas.

Scheme 26
scheme 26

Transformation to nitriles by reaction of 63 with acid chlorides

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5 Concluding Remarks and Outlook

It is clear from the results described above that molecular transition metal hydride complexes can serve as a platform for dinitrogen activation. Mononuclear transition metal hydride complexes can bind N2 to form end-on dinitrogen complexes with loss of H2. This process is generally reversible and Nā€“N bond cleavage is difficult. Binuclear transition metal hydride complexes can show higher reactivity and induce Nā€“N bond cleavage in some cases. A trinuclear titanium polyhydride complex has demonstrated even higher activity for the activation of dinitrogen, which enabled both Nā€“N bond cleavage and Nā€“H bond formation without the need of an external reducing agent or proton source. Obviously, the hydride ligands can serve as the source of both electron and proton for the reduction and hydrogenation of dinitrogen, and the cooperation of multiple metal hydride sites may play an important role in this process. A few functionalization reactions of the nitrogen species generated by the activation of dinitrogen with transition metal hydrides have been reported, among which the recent conversion of a tetranuclear titanium imido/nitrido complex to nitriles is particularly noteworthy. Despite recent progress in this area, the study on the activation and functionalization of dinitrogen by molecular transition metal hydrides, especially multimetallic polyhydride complexes, is still in infancy. The direct use of dinitrogen as a feedstock for organic synthesis remains a challenge.