Experimental and Computational Studies of Dinitrogen Activation and

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Experimental and Computational Studies of Dinitrogen Activation and Hydrogenation at a Tetranuclear Titanium Imide/Hydride Framework Takanori Shima, Gen Luo, Shaowei Hu, Yi Luo, and Zhaomin Hou J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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Journal of the American Chemical Society

Experimental and Computational Studies of Dinitrogen Activation and Hydrogenation at a Tetranuclear Titanium Imide/Hydride Framework Takanori Shima,†,‡,§ Gen Luo,†,¶,§ Shaowei Hu,† Yi Luo,*,¶ and Zhaomin Hou*,†,‡,¶ †

Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa, Wako, Saitama 3510198, Japan ‡ Organometallic Chemistry Laboratory, RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ¶ State Key Laboratory of Fine Chemicals and School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT: The activation of N2 by a tetranuclear titanium(III) diimide/tetrahydride complex [(Cp´Ti)4(3-NH)2(-H)4] (1) (Cp´ = C5Me4SiMe3), which was obtained by the reaction of the Cp´-ligated titanium trialkyl complex Cp´Ti(CH2SiMe3)3 with H2 and N2, was investigated in detail by experimental and DFT studies. The reaction of 1 in solid state with N2 (1 atm) at 180 °C gave the dinitride/diimide complex [(Cp´Ti)4(3-N)2(3-NH)2] (2) through the incorporation, cleavage, and partial hydrogenation of one molecule of N2 and release of two molecules of H2. At 130 °C, the formation of 2 was not observed, but instead, the dehydrogenation of 1 took place through cleavage of the N–H bond in an imide ligand followed by deprotonation of the other imide ligand with a hydride ligand, affording the dinitride/tetrahydride complex [(Cp´Ti)4(3-N)2(-H)4] (3). Upon heating under N2 (1 atm) at 180 °C, 3 was quantitatively converted to the dinitride/diimide complex 2. This transformation was initiated by migration of a hydride ligand to a nitride ligand to give one imide unit followed by N2 coordination to a Ti atom and H2 release through the reductive elimination of two hydride ligands. The other imide ligand in 2 was formed by hydride migration to one of the two nitride ligands generated through the cleavage of the newly incorporated N2 unit. The hydrogenation of 2 with H2 (100 atm) at 180 C afforded the tetraimide complex [(Cp´Ti)4(3-NH)4] (4). This reaction was initiated by -bond metathesis between H2 and a titanium–nitride bond followed by migration of the resulting hydride ligand to the remaining nitride ligand. In all of these transformations, the interplays among the hydride, imide, and nitride ligands, including the reversible dehydrogenation/hydrogenation of imide and nitride species, at the multimetallic titanium framework played a critically important role.

INTRODUCTION

artificially. In industry, NH3 is produced from N2 and H2 on solid catalysts at high temperature (350−550 °C) and high pressure (150−350 atm) (the Haber-Bosch process), in which H2 serves as the source of both electron and proton. This is the only commercially successful process using N2 as a feedstock. This transformation may probably proceed through dissociative absorption of N2 and H2 on multiple metal sites followed by hydrogenation of the resulting nitride species to give NH3.11–15 Multiple metal sites that bear hydride, nitride, imide, and amide ligands may be involved in this transformation, but the mechanistic details at the molecular level have remained unclear to date because of the complexity of the solid catalyst system. To have a better understanding of the reaction mechanism of N2 activation and hydrogenation and thereby achieve NH3 synthesis under milder conditions, extensive studies on the reaction of N2

Dinitrogen (N2) occupies ca. 78% of Earth’s atmosphere, and is an abundant, easily accessible, and inexpensive resource. Molecular N2 is chemically inert under ordinary conditions, because of its strong N≡N triple bond (944.84 kJ/mol),1 large HOMO−LUMO energy gap (10.82 eV),2 and non-polarity. In nature, N2 can be transformed to ammonia (NH3) by nitrogenase enzymes under ambient conditions. The biological transformation of one molecule of N2 to two molecules of NH3 consumes eight electrons and eight protons, which is accompanied by release of one molecule of H2.3,4,5 Recent studies have shown that the cooperation of multiple metal hydrides and sulfide-bound protons such as Fe−-H and Fe−-SH in the iron-molybdenum cofactor may play a key role in promoting H2 release and N2 reduction.6–10 However, the biological ammonia synthesis is not yet well understood and difficult to mimic 1

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with molecular organometallic complexes have been carried out over the past decades.16–20 As model reactions of the enzyme process, the use of strong metal reducing agents such as KC8, Na/Hg, and Mg as electron sources in combination with transition metal complexes has been extensively studied.21–42 By using an excess amount of carefully selected electron sources such as Cp 2*Cr, Cp2Co, or KC8 together with some special proton sources such as lutidinium salts or HB{C6H3(CF3)2-3,5}4, the formation of NH3 has been achieved in the presence of a catalytic amount of transition metal complexes.43–52 An alternative molecular approach is the direct reduction of N2 by transition metal hydride complexes without using extra reducing agents and proton sources.53–56 This approach is of particular interest as both the biological and industrial Haber-Bosch processes are believed to involve metal hydrides as true active species. However, studies on the activation and hydrogenation of N2 by well-defined metal hydrides are still limited. Scheme 1. Dinitrogen (N2) Activation by Molecular Metal Hydrides

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through cleavage and partial hydrogenation of the N2 unit (Scheme 1a).57 As to the use of a polynuclear rather than binuclear metal hydride complex for N2 activation, we previously reported the cleavage and hydrogenation of N2 at room temperature by a trinuclear titanium heptahydride complex without using extra reducing agents or protons (Scheme 1b).58 This transformation demonstrated the unique reactivity of a multimetallic polyhydride complex.59–61 However, studies on N2 activation by well-defined multimetallic hydride complexes are still scarce. The reaction of a tetra- or higher nuclear metal hydride complex with N2 has remained almost unexamined to date.58,62,63 In this article, we report detailed experimental and computational studies of N2 activation and hydrogenation by a tetranuclear titanium diimide/tetrahydride complex [(Cp´Ti)4(3-NH)2(-H)4] (Cp´ = C5Me4SiMe3) (1) (Scheme 1c). The formation of the dinitride/diimide complex [(Cp´Ti)4(3-N)2(3-NH)2] (2) from 1 has been communicated previously.62 We have now found that the cleavage of an imide N–H bond is involved in the initial reaction step and the interplays among the hydride, imide, and nitride ligands in the multimetallic titanium framework play an important role in the activation and transformation of N2. This work has revealed unprecedented details of N2 activation at a multimetallic hydride/imide/nitride platform.

RESULTS AND DISCUSSION

A number of mononuclear transition metal hydride complexes have been reported to incorporate dinitrogen without N−N bond cleavage.53,54 Regarding bimetallic hydride complexes, Fryzuk and coworkers reported a binuclear tantalum tetrahydride complex that could rapidly react with N2 to afford a side-on/end-on N2 complex with elimination of H2, without causing N−N bond cleavage (Scheme 1a).55 Kawaguchi and coworkers found that a binuclear niobium tetrahydride complex could cleave N2 to give a dinitride complex with loss of two H2 molecules (Scheme 1a).56 We recently communicated that the reaction of a binuclear titanium tetrahydride complex with N2 gave a side-on/end-on N2 complex, which upon heating with H2 afforded a -imide/-nitride/hydride complex

Reaction of Tetranuclear Titanium Imide/Hydride Complex 1 with N2. The tetranuclear titanium diimide/tetrahydride complex [(Cp´Ti)4(3-NH)2(-H)4] (Cp´ = C5Me4SiMe3) (1) was prepared by the reaction of [Cp´Ti(CH2SiMe3)3] with H2 and N2 as reported previously.58 When 1 was heated in solid state under N2 (1 atm) at 180 °C for two days, the mixed dinitride/diimide complex [(Cp´Ti)4(3-N)2(3-NH)2] (2) was obtained in 95% yield (Scheme 2).64 In this reaction, one molecule of N2 was cleaved and incorporated into the titanium framework with release of two molecules of H2. The solid structure of 2 was determined by an X-ray diffraction study, but disorders of the nitrogen atoms were observed. To gain more information on N2 activation at the tetranuclear titanium imide/hydride framework, the reaction of 1 with N2 was examined under various conditions. When 1 was heated in C6D6/THF-d8 at a lower temperature (130 °C) under N2 (1 atm) for 1 day, the formation of 2 was not observed, but instead the release of one molecule of H2 from 1 took place to give the dinitride/tetrahydride complex [(Cp´Ti)4(3-N)2(-H)4] (3) in 89% yield (Scheme 2). Heating 1 in solid state under argon (1 atm) at 180 °C for 2 days also gave 3 (85% yield). When the 15N-labelled complex (1-15N)58 was heated in C6D6/THF-d8 at 130 °C for 1 day, the 15N-labelled nitride analog [(Cp´Ti)4(3-15N)2(-H)4] (3-15N) was obtained. The 15N{1H} NMR signal of the nitride units in 3-15N appeared as a singlet at  437.0, which is comparable to those in [(Cp´Ti)3(-15NH)3(3-15N)] ( 424.6),58 [(Cp*Ti)4(3-15N)4] ( 500.6),65 and [(Cp´Ti)4(315N) ] ( 500.8)62 (see also Table 1). The four hydride ligands in 3 4 showed one signal at  –7.90 in the 1H NMR spectrum. An X-ray diffraction study revealed that the core structure of 3 contains two 3-N and four -H ligands with identified electron density (Figure 1a). Similar to 2, complex 3 formally contains two Ti(III) and two Ti(IV) species. The average distance of the Ti–N bonds in 3 (Ti–Nav.: 1.928 Å) is shorter than those in the diimide complexes 1 (Ti–Nav.: 2.033 Å)58 and 2 (Ti–Nav.: 1.978 Å), and comparable to those in the tetra(3-nitride) complexes [(Cp*Ti)4(3-N)4] (1.939 Å),66 and [(Cp´Ti)4(3-N)4] (1.954 Å).62 The Ti1–Ti1* distance (2.8236(15) Å), which is bridged only by

2


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the nitride ligands, is shorter than other Ti–Ti (Ti1–Ti2, Ti1*–Ti2, Ti1–Ti2*) distances (2.8709(10)–2.8734(11) Å) in 3. Scheme 2. Activation and Transformation of Dinitrogen at a Tetranuclear Titanium Framework

(a)

(b)

Figure 1. ORTEP drawings of the core structures of 3 (a) and 4 (b) with ellipsoids at the 30% probability level. Cp’ ligands are omitted for clarity. Selected bond lengths (Å): (a) Ti1–N1, 1.965(3), Ti2– N1, 1.864(3), Ti1*–N1, 1.954(3). (b) Ti1–N1, 2.009(3), Ti1–N2, 1.998(3), Ti2–N1, 2.004(3), Ti2–N2, 1.995(3), Ti1*–N1, 2.038(3), Ti2*–N2, 2.050(3).

reaction afforded the analogous 15N-labelled dinitride/diimide complex [(Cp´Ti)4(3-15N)(3-N)(3-15NH)(3-NH)] (2-15N), in which each of the two imide and nitride units was 15N-labelled as shown by 1H and 15N{1H} NMR analyses. The 15N{1H} NMR spectrum of 2-15N showed two broad singlets at N 379.0 and – 101.5, which are assignable to the 3-15N species and the 3-15NH species, respectively. The 1H NMR spectrum of 2-15N gave tripletlike signals around H 17.35, which included one 3-NH unit as a singlet and one 3-15NH unit as a doublet (JNH = 56 Hz) (Figure 2, left). The IR spectrum of 2-15N showed a broad signal at 3356 cm– 1, which could be viewed as a combination of the N–H stretch (3360 cm–1) observed in 2 and the 15N–H stretch (3352 cm–1) observed in the fully 15N-labelled analog [(Cp´Ti)4(3-15N)2(3-15NH)2] (215N )62 (Figure 2, right). 4 Scheme 3. 15N-Labelled Transformations

Table 1. Summary of 1H and 15N{1H} NMR Data of Some Titanium Imide and Nitride Species

Exposure of 3 to H2 (1 atm) at 80 °C quantitatively regenerated 1 as shown by the 1H NMR analysis, demonstrating that 1 and 3 are facilely interconvertible through dehydrogenation and hydrogenation of the imide/nitride ligands (Scheme 2). When 3 (solid state) was heated under N2 (1 atm) at 180 °C for 2 days, 2 was formed quantitatively (Scheme 2). The reaction of 1 in solid state with N2 (1 atm) at 160 °C for 1 day afforded 2 in 65% yield. In contrast, when 1 was heated with a 1:1 N2:H2 mixture (2 atm) at 160 °C for 1 day, 2 was formed in only 8%. These results suggest that the N2 activation reaction of 1 to give 2 should be initiated through the dehydrogenation of 1 to give 3, and in the presence of H2, the reaction of 1 with N2 is much slower. To examine the origin of the nitride and imide species in 2, the reaction of 1 with 15N2 (1 atm) was carried out (Scheme 3). This

The reaction of 3 with 15N2 at 180 °C for 2 days quantitatively afforded 2-15N, clearly demonstrating that one of the two nitride

3



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species in 3 is hydrogenated to an NH group, and the incoming 15N2 molecule is cleaved and transformed to an 15NH group and a nitride species (Scheme 3). Similarly, the reaction of 1-15N or 3-15N with N2 also gave 2-15N (Scheme 3).

Figure 4. Eyring plot of the reaction of 1 with N2 (6 atm).

Figure 2. 1H NMR (C6D6, rt) (left) and IR (KBr) (right) spectra of the imide units in 2-15N (top), 2 (middle) and 2-15N4 (bottom). When 1 was heated under 6 atm of N2 at 130 °C for 1 day, 2 was formed almost quantitatively, although no reaction was observed under 1 atm of N2 at 130 °C. Only a very small amount of 3 (< 3%) was observed in this transformation as monitored by the 1H NMR spectroscopy. The disappearance of 1 as a function of time established a pseudo-first-order reaction. When the N2 pressure was changed in the range of 4−8 atm and the concentration of 1 was held constant, a plot of observed rate constant vs. N2 pressure was linearly fitted, indicating a first-order dependence on N2 pressure. These results suggest that the overall reaction should follow a second-order process (rate = k1[1][N2]). Kinetic experiments were performed by monitoring the disappearance of 1 under N2 (6 atm) by the 1H NMR spectroscopy (Figure 3). The Eyring plot revealed the activation parameters of H = 31.6(10) kcal/mol, S =0.2(26) eu, and G(298K) = 31.4 kcal/mol (Figure 4). The small activation entropy suggests that the rate-determining step is an intramolecular process. The reaction rate k (at 120 °C) was not significantly affected by using 1 vs. [(Cp´Ti)4(3-ND)2(-D)4] (1-d4) (kH = 2.45×10–5 s–1, kD = 2.00×10–5 s–1, kH / kD = 1.2).

Figure 3. First order plots of the disappearance of 1 in the reaction of 1 with N2 (6 atm) as a function of time (s) at different temperatures.

Hydrogenation of Dinitride/Diimide Complex 2 with H2. When the mixed dinitride and diimide complex 2 was hydrogenated with H2 (10 atm) in xylene at 180 °C for 3 days, the tetraimide complex [(Cp’Ti)4(3-NH)4] (4) was obtained in 80% yield. A higher pressure of H2 (100 atm) led to a higher yield of 4 (97%) (Scheme 2). This transformation was accompanied by reduction of the two Ti(IV) sites in 2 to Ti(III). When 2 was heated under D2 (10 atm) at 180 °C, the mixed deuterated and hydrogenated imide complex [(Cp’Ti)4(3-ND)2(3-NH)2] (4-d2) was obtained. The hydrogenation of 2-15N at 180 °C under 100 atm of H2 afforded the corresponding 15N-enriched imide complex [(Cp’Ti)4(3-15NH)2(3NH)2] (4-15N) in 96% yield. The 1H NMR spectrum of 4-15N showed a triplet-like signal at H 11.06, which included a singlet of the two 3-NH units and a doublet of the two 3-15NH units (JNH = 68.0 Hz). The 15N NMR spectrum of 4-15N showed a signal at N – 12.6 with JNH = 68.0 Hz, which could be assigned to the imide 315NH) units. Complex 4 was stable at room temperature. However, when 4 was heated in toluene-d8 at 180 °C for 3 days under an N2 atmosphere, 2 was formed in 93% yield with release of H2, suggesting that 2 and 4 are interconvertible (Scheme 2). Complex 2 was thermally stable in the absence of H2. The overall X-ray structure of 4 (Figure 1b) is similar to that of 2, both of which form a cubane-like Ti4N4 skeleton except that 4 contains four imide units while 2 has two imide units and two nitride units. In agreement with the presence of four imide units in 4, the average bond distance of the Ti–N bonds in 4 (2.016 Å) is comparable to that in 1 (2.033 Å), and longer than those in 2 (1.978 Å), 3 (1.928 Å) and [(Cp’Ti)4(3-N)4] (1.954 Å).62 DFT Studies on N2 Activation and Hydrogenation. To have a better understanding of the mechanistic details of the N2 cleavage and hydrogenation by the imide/hydride complex 1, we performed the DFT calculations on the basis of a model compound of 1, i.e., [(C5H4SiH3)4Ti4(-NH)2(-H)4] (1m). In contrast with the trinuclear titanium polyhydride complex [(C5H4SiH3)3Ti3(-H)7], which can react with N2 to give a dinitrogen-incorporated intermediate [(C5H4SiH3)3Ti3(-1:2:2-N2)(-H)3],58 attempts to simulate a complexation of the tetranuclear mixed imide/hydride complex 1m with N2 were fruitless. We then investigated the reaction by starting with H2 release form 1m. The energetically most favorable reaction path is shown in Figure 5 (see Supporting Information for more details). The cleavage of the N1–H1 bond could be initiated by an interaction of the H1 atom with two titanium atoms (Ti1 and Ti3) via transition state TS1A with an energy barrier of 27.5 kcal/mol, which gave an imide/nitride/pentahydride intermediate A. In this transformation, Ti1 and Ti3 were oxidized from a formal 3+ oxidation state to 4+. Subsequently, the release of one molecule of

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Figure 5. DFT analysis of N2 activation and hydrogenation by 1m (a model of 1). Simplified energy profile of the reaction of 1m with N2 to give 2m and the hydrogenation of 2m with H2 to afford 4m. The C5H4SiH3 ligands are omitted for clarity. The complex name with TS means transition state. H2 (H5–H6) took place through an acid-base reaction between a hydride species (H6) and the remaining imide species (N2–H5) in A, affording the dinitride/tetrahydride complex B. Hydride ligand rearrangement by the cleavage of the Ti1–H3 and Ti4–H2 bonds and formation of the Ti2–H3 bond in B gave the more stable isomer 3m, which is equivalent to complex 3 obtained experimentally (see Scheme 2). The transformation of B to 3m is barrierless, and the conversion of 3m to B requires only 4.2 kcal/mol (see Figure S28). These results suggest that B and 3m should be easily interconvertible under the experimental conditions. A feasible path for the direct reaction of 3m with N2 was not found computationally. However, the dinitride/tetrahydride intermediate B could be transformed to the monoimide/mononitride/trihydride species C by N1–H1 bond formation via transition state TSBC with an energy barrier of 29.3 kcal/mol. In this transformation, H1 is oxidized from a hydride (H–) to a proton (H+), and Ti1 and Ti3 are formally reduced from Ti(IV) to Ti(III). The coordination of N2 to the Ti2 atom in C could take place to give D, which was subsequently transformed to the more stable complex E, in which the N2 unit is bonded to three titanium atoms (Ti1, Ti2 and Ti4) in a side-on/end-on fashion. In this process, the N≡N triple bond is reduced to an N=N double bond by the oxidation of two formal Ti(III) species to Ti(IV). The release of one molecule of H2 (H2– H3) by reductive elimination of the H2 and H3 ligands in E gave the more stable intermediate F. In this transformation, the N=N double bond in E was reduced to an N–N single bond by the two electrons provided through H2 release. The cleavage of the N3–N4 bond in F then took place to give the monoimide/trinitride/monohydride complex G. This process was accompanied by oxidation of the two Ti(III) species in F to Ti(IV). Finally, the N4–H4 bond formation took place via TSG2m to yield the more stable dini-

tride/diimide complex 2m, which is equivalent to 2 obtained experimentally. In this step, the hydride (H4) was oxidized to a proton, which was accompanied by reduction of two Ti(IV) species to Ti(III). The formation of 2m from 1m and N2 is exergonic by 81.8 kcal/mol. The rate-determining step is the intramolecular migration of a hydride ligand (H1) to a nitride ligand (N1) (N1–H1 bond formation), which showed an energy barrier of 33.5 kcal/mol (TSBC). This is in agreement with the experimental result (S =0.2(30) eu, G(298K) = 31.4 kcal/mol) obtained in the kinetic studies. The reaction of 2 with H2 to give 4 was computed by using model compounds 2m and 4m (Figure 5). The reaction took place through σ-bond metathesis between H2 (H7–H8) and a titanium– nitride (Ti2−N2) bond in 2m via transition state TS2mH, leading to concerted cleavage of H7−H8 and formation of N2−H7 and Ti2−H8 to give an intermediate H. This step needed to overcome an energy barrier of 33.7 kcal/mol. The similar H−H cleavage and N−H formation in binuclear zirconium complexes were also demonstrated previously by DFT studies.33,67 The hydride ligand H8 in H could migrate to the nitride ligand N3 via transition state TSH4m (with an energy barrier of 19.4 kcal/mol) to give the imide complex 4m, which is equivalent to complex 4 obtained experimentally. This process was accompanied by reduction of two Ti(IV) atoms to Ti(III). Complex 4m is slightly more stable than 2m by 4.5 kcal/mol, and therefore, the release of H2 from 4 to give 2 could be possible, as observed experimentally (Scheme 2). It was also found that the hydrogenation of 4m with H2 to give an amide species needs to overcome an energy barrier of as high as 46.8 kcal/mol (Figure S32), which is kinetically inaccessible under the experimental conditions.

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It is clear from the above analyses that the activation and hydrogenation of N2 at the present multimetallic titanium framework have involved dynamic rearrangement of the hydride ligands and reversible dehydrogenation/hydrogenation of the imide/nitride ligands. In the heterogeneous Haber-Bosch process, the N–H bond formation steps are thought to be equilibrium reactions,12 but details on the formation and cleavage of N–H bonds and their influences on N2 activation are difficult to probe. In the biological transformation of N2 to NH3 by nitrogenase, the cooperation of multiple metal hydrides and sulfide-bound protons such as Fe−-H and Fe−-SH in the iron-molybdenum cofactor were proposed to play a key role in promoting reversible exchange of N2 and H2, but mechanistic details at the molecular level remained unknown.6–10

CONCLUSION We have examined in detail experimentally and computationally the activation of N2 by a tetranuclear titanium(III) diimide/tetrahydride complex 1. It has been revealed that the reaction is initiated by cleavage of an imide N–H bond to give a mononitride/monoimide/pentahydride intermediate (like A), which subsequently releases one molecule of H2 by deprotonation of the other imide (NH) species with a hydride ligand and yields the dinitride/tetrahydride complex 3. The migration of a hydride ligand to a nitride ligand in 3 (or its isomer like B) may take place to generate a monoimide/mononitride/trihydride species (like C). The coordination of N2 to a titanium atom could then take place followed by release of one molecule of H2 through the reductive elimination of two hydride ligands. The N–N bond cleavage subsequently proceeds to give a monoimide/trinitride/monohydride species (like G). Finally, the migration of the hydride ligand to a nitride ligand affords the diimide/dinitride product 2. Under a high pressure of H2, the hydrogenation of 2 occurs to give a tetraimide complex 4. This transformation may proceed through the -bond metathesis between H2 and one titanium–nitride bond followed by migration of the resulting hydride ligand to the other nitride ligand in 2. The hydrogenation/dehydrogenation processes (such as that between 1 and 3 and that between 2 and 4) are reversible. The interplays among the hydride, imide, and nitride ligands in the multimetallic titanium framework play an important role in these transformations. This work has provided unprecedented details on the activation and hydrogenation of N2 at a multimetallic framework, and may also help better understand the mechanistic aspects of the industrial HaberBosch process on solid catalysts and the biological nitrogen fixation by enzymes.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge on the ACS Publications website at DOI: XXXXX. X-ray crystallographic data for 3 (CCDC 1862473) and 4 (CCDC 1862474) (CIF) Experimental details, spectroscopic and analytical data, and DFT calculation (PDF)

AUTHOR INFORMATION Corresponding Authors *[email protected] (Z.H.) *[email protected] (Y.L.)

ORCID Takanori Shima: 0000-0003-1813-0439 Gen Luo: 0000-0002-5297-6756 Yi Luo: 0000-0001-6390-8639 Zhaomin Hou: 0000-0003-2841-5120

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Author Contributions §

T.S. and G.L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported in part by the Grant-in-Aids for Scientific Research (S) (26220802) and (C) (17K05823) and for Scientific Research on Innovative Area (18H05517) from JSPS, and by the National Natural Science Foundation of China (21429201, 21674014) and the Fundamental Research Funds for the Central Universities (DUT18RC(3)002, DUT18GJ201). We gratefully appreciate accesses to the RIKEN Integrated Cluster of Clusters (RICC) and the Network and Information Center of Dalian University of Technology for computational resources. We thank Mrs. Akiko Karube for micro elemental analyses.

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Experimental and Computational Studies of Dinitrogen Activation and

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