Dinitrogen activation by a titanium hydride complex supported by 2-butene ligand

Abstract

The activation of dinitrogen by a titanium hydride complex with two bridging hydrides located between metal Ti and Li was investigated. Exposing cyclic bis-alkylidene titanium complex 1 to H₂ in THF solution yielded titanium hydride 2, {[(2-butene)(C₅H₅)Ti](μ-H)₂Li(THF)₃}. 2 aggregated into dimer 3 in hexane, accompanied by the release of LiH. Treatment of 2 with N₂ (1 atm) in THF resulted in ionic end-on bridged dinitrogen dititanium complex 4, {[((2-butene)(C₅H₅)Ti)₂(μ₂-η¹,η¹-N₂)][Li(THF)₄]}, without using external reducing agents. Furthermore, 4 could be oxidized to yield the binuclear complex 5, {[(2-butene)(C₅H₅)Ti]₂(μ₂-η¹,η¹-N₂)}. The solid-state structures of 4 and 5, as revealed by X-ray crystallographic studies, exhibit relatively short N–N bond distances of 1.198(2) Å and 1.179(2) Å, respectively. These results align with the observed N–N Raman stretching frequencies at 1699 cm⁻¹ and 1704 cm⁻¹. The N₂ moiety bridging two titanium atoms could be converted into ammonia and hydrazine upon treatment with a proton source and reductant.

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Introduction

N₂ gas is the predominant and most cost-effective nitrogen source on the Earth. However, its chemical inertness, attributed to its non-polarity, large HOMO–LUMO gap, high bond dissociation energy (942 kJ mol⁻¹), and negative electron affinity, hinders its direct utilization. The activation and transformation of N₂ have been long-standing challenges, drawing considerable attention over the past few decades.¹ Among various strategies for dinitrogen fixation, metal hydride complexes have already shown great potential. Hydridic hydrogen (H⁻) can serve as an electron and/or hydrogen donor, facilitating the dinitrogen activation and reduction in heterogeneous, biological, and homogeneous reaction fields via more accessible pathways.² For example, the main group metal hydrides (LiH, CaH₂ and BaH₂) have been reported to effectively convert N₂ gas into ammonia (NH₃)³ and diverse N-containing organic compounds.⁴

For transition metal hydrides, they inextricably served as a platform for dinitrogen activation since the discovery of the dinitrogen-containing cobalt complex [(Ph₃P)₃Co(N₂)H], which reacted with H₂ to obtain the cobalt hydride complex [(Ph₃P)₃CoH₃] reversibly.⁵ To date, many molecular transition metal hydride complexes, especially titanium hydrides, have been found to activate the N₂ molecule to varying extents.^2b Mach and co-workers reported the synthesis of titanocene monohydride species [(C₅HMe₄)₂TiH], which reduced N₂ to produce a dinuclear end-on dinitrogen complex.⁶ By modifying the substituents on the cyclopentadienyl (Cp) ligand, the corresponding side-on coordination mode [(η⁵-C₅H₂-1,2,4-Me₃)₂Ti]₂(μ₂-η²,η²-N₂) could be prepared by H₂ reductive elimination from the in situ generated titanocene hydride derivative.⁷ Recently, Hou and co-workers disclosed many elegant titanium polyhydride complexes, capable of cleaving the N≡N bonds and facilitating dinitrogen derivatization.⁸ Despite the significant interest and progress in this field, the study of titanium hydride activation of N₂ is still in its early stages.

For over 25 years, our research group has been dedicated to the study of bimetallic reagents, with a particular emphasis on 1,4-dilithio-1,3-butadiene species.⁹ As an extension of our work, we continue to expand this field to prepare the corresponding titanium hydrides and explore their reactivities with N₂ gas. In this work, by hydrogenating titanacyclopentatriene complex 1,¹⁰ stemming from the 1,4-dilithio-1,3-butadiene precursor, we obtained Ti-hydride complex 2, {[(2-butene)(C₅H₅)Ti](μ-H)₂Li(THF)₃}. The subsequent reaction of 2 with dinitrogen led to the formation of Ti-N₂ complexes 4 and 5, {[((2-butene)(C₅H₅)Ti)₂(μ₂-η¹,η¹-N₂)][Li(THF)₄]} and {[(2-butene)(C₅H₅)Ti]₂(μ₂-η¹,η¹-N₂)}, respectively. It is noteworthy that 4 has a [TiN₂Ti] unit with the odd-electron reduction, which is rare in the literature.¹¹ Furthermore, NH₃ and N₂H₄ generation from the titanium dinitrogen complex with the addition of acids and KC₈ was also discussed.

Results and discussion

Synthesis and characterization

Exposing our previously reported cyclic bis-alkylidene titanium complex 1 to H₂ in THF solution resulted in the formation of 2. When 2 was crystallized from Et₂O, dark-blue crystals were obtained with 68% yield (Scheme 1). The ¹H NMR spectrum of 2 shows a well-resolved singlet at 1.57 ppm in d₆-benzene, which integrates as 2 H per 2-butene or Cp ligand, assigned to the titanium-hydride ligand. To further confirm the attribution of the bridged Ti–H signal, we carried out the HSQC 2D spectroscopy test. Additionally, the corresponding deuterated analogue 2-D was synthesized and exhibited a resonance at 1.35 ppm in the ²H NMR spectra. The hydride chemical shift in 2 is slightly more upfield than those observed in the trinuclear titanium heptahydride complex {[(C₅Me₄SiMe₃)Ti]₃(μ₃-H)(μ₂-H)₆} (δ 2.66 ppm)^8a and mononuclear dihydride [1,3-(Me₃)₂C₅H₃TiH₂] (δ 2.62 ppm).¹² Under an inert atmosphere, 2 remains stable for two days in its solid state at 25 °C. But it is unstable in solution, as observed by generation of unknown by-products in the ¹H NMR spectra.

Scheme 1 Syntheses of 2 and 3.

The solid-state structure of 2 was determined by X-ray crystallographic analysis. As shown in Fig. 1, the Ti center is coordinated with the Cp, the dianionic 2-butene ligand and two bridging hydride units. The geometry of {Ti(μ-H)₂Li(THF)₃} is similar to those reported in {[(C₅Me₅)₂Ti(μ-H)₂]₂Mg}¹³ and [(C₅Me₅)₂Ti(μ-H)₂Li(TMEDA)],¹⁴ where the hydride ligands bridged between the Ti and other metal atom. Ti–H bond distances of 1.65(4) Å and 1.70(2) Å in 2 are found to be comparable to that in the titanium hydride complex {[(PNP)Ti]₂(μ₂-NH)(μ₂-N)H} (PNP = N(C₆H₃-2-PⁱPr₂-4-CH₃)₂, 1.68(5) Å).^8c However, they are notably shorter than the distance of 1.84(2) Å in the cationic titanium hydride [(C₅Me₅)(^tBu₃PN)TiH(THF)][B(C₆F₅)₄].¹⁵ The C2–C3 bond distance of 1.408(3) Å is shorter than C1–C2 and C3–C4 bond distances (1.438(3) Å and 1.439(3) Å) in 2, suggestive of double-bond character.

Fig. 1 ORTEP drawing of 2 with thermal ellipsoids drawn at the 35% probability level. Except for the bridging hydrides, all the hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [°]: Ti1–H1 1.70(2), Ti1–H2 1.65(4), Ti1–Cp(cent) 2.051, C1–C2 1.438(3), C2–C3 1.408(3), C3–C4 1.439(3), H1–Ti1–H2 73(1). (Cent: centre of the Cp ring.)

Stirring the hexane solution of 2 led to a gradual colour change over 4 h from dark-blue to brown. After removing all volatiles and re-dissolving the residual solids in Et₂O, purple 3 could be crystallized in 36% isolated yield. It is worth noting that the alternative ways to prepare 3 were involved in hydrogenation of 1 in hexane with low yield, or in aromatic solvents to get the mixture of 2 and 3. In the ¹H NMR spectrum, newly formed hydride signals of Ti–H and methine C–H were displayed at 2.11 ppm and −5.18 ppm, respectively. X-ray analysis of 3 revealed a dimeric centro-symmetrical molecule (Fig. 2) The planar, rhombic [Ti(μ-H)₂Ti] core consists of two 3-center-2e {Ti–H–Ti} bonds with Ti–H distances of 1.88(2) Å and 1.87 (2) Å, falling into the range (1.71–2.08 Å) of those previously reported in titanium bridging hydrides.¹⁶ Attempts to generate the corresponding dinitrogen complex by reacting 3 with N₂ in THF were unsuccessful. Only unidentified products were detected from interval NMR experiments.

Fig. 2 ORTEP drawing of 3 with thermal ellipsoids drawn at the 35% probability level. Except for the bridging hydrides, all the hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [°]: Ti1–H1 1.88(2), Ti1–H1ⁱ 1.87(2), Ti1–Cp(cent) 2.044, H1–Ti1–H1ⁱ 65.3(8). (Cent: centre of the Cp ring.)

Treatment of 2 with an atmosphere of N₂ in THF for 2 days afforded dinitrogen 4. A moderate yield of 45% was achieved, hindered by the decomposition of 2 in THF. The ¹H NMR spectrum of 4 in d₆-benzene displayed a series of broad signals, typical of the paramagnetic complex with a solution magnetic moment of 2.03μ_B (measured by Evans’ method in d₆-benzene) at 25 °C. Additionally, when crystals of 4 were redissolved in toluene at 77 K, an EPR resonance showed at g = 2.006. In the ¹⁵N NMR spectrum of ¹⁵N-4, a broad signal was presented at δ 394.31 ppm (using NH₃ as a standard reference). The N–N stretching frequencies of 4 and ¹⁵N-4 were observed at 1699 cm⁻¹ and 1642 cm⁻¹ respectively, in agreement with the marked mass difference between ¹⁴N₂ and ¹⁵N₂ (Scheme 2).

Scheme 2 Synthesis of 4.

The crystal structure of 4 contains two crystallographically independent but chemically identical units. The bridging end-on N₂ ligand is ligated to two Ti centers, and the THF-coordinated lithium cation (Li⁺) serves as a counterion (Fig. 3). To the best of our knowledge, just one well-defined titanium dinitrogen complex with a separate alkali–metal cationic fragment has been reported in the complex {[(O₃C)Ti]₂(μ-N₂)K₃(THF)₃}[K(THF)₆] ([O₃C] = [(3,5-^tBu₂-2-O-C₆H₂)₃C]⁴⁻).¹⁷ It should be noted that 4 contains an odd-electron reduction of the [TiN₂Ti] unit. A parallel instance has only been documented by the Ohki group, who reported a trace titanium dinitrogen complex {[(C₅Me₅)₃Mo₃S₄Ti]₂(μ-N₂)[K₃(THF)₅]}.¹¹ The N–N bond distances are 1.198(2) Å and 1.200(2) Å for each independent molecule in 4, comparable to those seen in similar titanium derivatives {[Cp₂Ti(PMe₃)]₂(μ-N₂)} (1.191 (8) Å)¹⁸ and {[(Tren^TMS)Ti]₂(μ-N₂)} (Tren^TMS = N(CH₂CH₂NSiMe₃)₃), 1.121 (6) Å.¹⁹ However, they are shorter than those in {[(Me₂NC(NⁱPr)₂)₂Ti]₂(μ-N₂)} (1.28(1) Å),²⁰ {[((Nacnac)CpTi)₂(μ-N₂)]} (Nacnac = [HC(C(Me)N(C₆H₃)(ⁱPr)₂)₂], 1.273(6) Å)²¹ and {(C₅Me₅)Ti[N(ⁱPr)C(Me)N(ⁱPr)]₂(μ-N₂)} (1.270(2) Å).²² The bond distance of Ti1–N1 (1.903 (2) Å) is longer than that of {[(Tren^TMS)Ti]₂(μ-N₂)} (2.022 (3) Å), but shorter than that of {[(Tren^TMS)Ti]₂(μ-η¹,η¹,η²,η²-N₂K₂)} (1.812 (2) Å) (Tren^TMS = N(CH₂CH₂NSiMe₃)₃), inferring some Ti-imido character.¹⁹ In view of the corresponding d_(NN) values of free N₂ and N₂H₂ at 1.098(1) Å and 1.45 Å, along with structural and Raman spectroscopic data of 4, it has been suggested that the dinitrogen bond was moderately activated. Moreover, the Ti–Cp(centroid) and Ti–C_{(Cp ring)} distances in 4 are slightly longer than the corresponding bond distances in titanium hydride complexes 2 and 3.

Fig. 3 ORTEP drawing of 4 with thermal ellipsoids drawn at the 35% probability level. All the hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [°]: N1–N1ⁱ 1.198(2), Ti1–N1 1.903(2), Ti1–Cent1 2.098, Ti1–N1–N1ⁱ 176.3(2). (Cent: centre of the Cp ring.)

The oxidation of 4 in the presence of [(Cp₂Fe)(BAr^F₄)] led to the formation and isolation of a new purple species 5 in 95% yield Scheme 3¹H and ¹³C NMR characterization of 5 revealed that it was a diamagnetic complex with resonances assignable to intact 2-butene and Cp ligands. The ¹⁵N NMR spectrum of ¹⁵N-5 depicts one apparent singlet at δ 656.61 ppm. Raman spectra of 5 and ¹⁵N-5 demonstrate strong vibration peaks (1704 cm⁻¹ for 5; 1647 cm⁻¹ for ¹⁵N-5), assigned to the bridging N₂ ligands. Notably, in the absence of air and moisture, the dinitrogen complex 5 was quite stable in the solid state or d₆-benzene solution for up to two weeks without decomposition at room temperature.

Scheme 3 Synthesis of 5.

Single crystals of 5 suitable for X-ray analysis were grown from slow volatilization of a concentrated hexane/Et₂O solution at room temperature (Fig. 4). Crystallographic studies confirm it a centrosymmetric dinuclear complex, in which the middle of the N–N bond acts as the inversion center. The Ti–N–N–Ti skeleton is nearly linear with the Ti1–N1–N1ⁱ bond angle of 175.9(1)°. The N–N bond distance of 1.179(2) Å is consistent with a weakly activated N₂ ligand and compares well with other reported titanocene dinitrogen complexes such as {[Cp′₂Ti]₂(μ-N₂)} (Cp′ = (C₅Me₄H), 1.170(4) Å;⁶ (C₅Me₅), 1.160(14) Å;²³ [C₅H₃-1,3-(SiMe₃)₂], 1.164(5) Å (ref. 24)), {[(C₅H₅)₂Ti(p-CH₃C₆H₄)]₂(μ-N₂)} (1.162(12) Å)²⁵ and [{(C₅Me₅)Ti(η⁶-C₅H₄CR₂)}₂(μ-N₂)], (R = p-CH₃C₆H₄, 1.160(3) Å; CR₂ = adamantylidene, 1.160(5) Å).²⁶ The relatively low activation degree of the N₂ moiety in 5 was mainly attributed to the donor/acceptor properties of Cp and 2-butene co-ligands as well as the oxidation state of Ti centers.^26b

Fig. 4 ORTEP drawing of 5 with thermal ellipsoids drawn at the 35% probability level. All the hydrogen atoms are omitted for clarity. Selected interatomic distances [Å] and angles [°]: N1–N1ⁱ 1.179(2), Ti1–N1 1.945(1), Ti1–Cent1 2.050, Ti1–N1–N1ⁱ 175.9(1). (Cent: centre of the Cp ring.)

The reaction of 4 with commonly used electrophilic reagents including CO₂, methyl triflate, pinacolborane, and phenylsilane resulted in either no reaction or the formation of 5 along with other unknown products. From cyclic voltammetry measurements of 4, the reversible peak at −2.923 V indicates a chemically accessible one-electron reduction of 4. However, after many trials, treating 4 with less or equivalent amount of different reducing agents in various solvents only gave intractable products.

Next, we investigated the reactivity of 4 and 5 in the presence of reductants and weak proton sources under a N₂ atmosphere (Scheme 4). The data are summarized in Table S1.† Utilizing KC₈ (72 equv.) as the reductant and [(Ph₂NH₂)(OTf)] (78 equv.) as the proton source resulted in a mixture of ammonia and hydrazine (92% and 7% based on 5 respectively, in Table S1,† entry 14). No significant enhancement of the yields was observed even with larger amounts of reductants and proton sources. This result suggests that 5 is ineffective as a catalyst toward N₂ reduction, although a stoichiometric transformation of the bridged-dinitrogen ligand is achieved. 4 can also be converted into ammonia and hydrazine under similar conditions (see Table S1† for more details). Inspired by the above results, we replaced proton sources with several electrophilic reagents, in hopes of getting nitrogenous organic compounds. Unfortunately, no clear target products were detected by the GC-MS method.

Scheme 4 Protonation to produce a stoichiometric amount of N₂H₄ and NH₃ with a proton source and reductant.

Electronic structure

To gain further insight into the electronic structure of 4, DFT calculations have been performed. The computations were performed using ORCA 5.0.3.^27,28 Geometric structures were optimized employing the r2SCAN-3c level of theory.^29–31 Doublet state calculation was carried out for 4. The optimized structure showed good agreement with the crystal structure. Wave function analysis was performed by the Multiwfn software.³²

The Singly Occupied Molecular Orbital (SOMO) of 4 is depicted in Fig. 5. Utilizing the Fuzzy atom partitioning method for spin population analysis, we observed that the unpaired electron is predominantly delocalized over the [Ti–N₂–Ti] core. Specifically, two titanium atoms account for 68%, while the dinitrogen unit contributes 22%. For other related molecular orbitals of 4, see the ESI.† Fuzzy Bond Order (FBO) analysis is a reputable bond order index developed by Mayer and Salvador, which agrees well with classical chemical concepts and shows minimal basis sensitivity.³³ For 4, the FBO of Ti–N bonds is 1.58, while that of the N–N bond has a bond order of 1.96. These results affirm the partial multiple-bond character of Ti–N and a moderate degree of dinitrogen reduction.

Fig. 5 SOMO of 4.

Conclusions

In summary, we report the synthesis of a well-defined titanium hydride 2 supported by both 2-butene and Cp ligands, resulting from hydrogenolysis of the corresponding precursor. 2 can react with N₂ gas to yield anionic end-on dinitrogen-bridged titanium complex 4. 4 readily undergoes a one-electron oxidation to give neutral titanium dinitrogen complex 5. The resulting products 4 and 5 achieve moderate reduction of dinitrogen as evidenced by N–N bond distances and Raman vibrational frequencies of the N–N stretching band. In addition, we made preliminary efforts to investigate the reactivity of the N₂ unit in 4 and 5. In the presence of an excess of protons and KC₈, the bridged dinitrogen ligand was able to afford stoichiometric quantities of ammonia and hydrazine under mild conditions.

Experimental

General remarks

All reactions were carried out under an atmosphere of dinitrogen or argon by using standard Schlenk techniques or glovebox techniques. Toluene, hexane, and Et₂O were purified using an Mbraun SPS-800 Solvent Purification System and dried over fresh Na chips and molecular sieves in a glovebox. THF was distilled from Na/benzophenone ketyl and dried over fresh Na chips and molecular sieves in the glovebox. Cyclic bis-alkylidene titanium complex 1 was synthesized according to the published procedures.¹⁰ Other commercially available reagents were used without purification.

Samples for NMR spectroscopic measurements were prepared in a glovebox by the use of J. Young valve NMR tubes. ¹H, ¹³C, and ¹⁵N NMR spectra were recorded on a Bruker ARX400 spectrometer or a Bruker AVENCE 600 MHz spectrometer at room temperature unless otherwise noted. All chemical shifts were reported in units of ppm with references to the residual protons of the deuterated solvents for proton chemical shifts and the ¹³C of deuterated solvents for carbon chemical shifts. ¹⁵N chemical shifts were reported in ppm relative to liquid NH₃ at 0 ppm. UV/Vis absorption spectra were obtained on an Agilent Cary 60 UV-Vis spectrophotometer. Magnetic moments were measured by the method originally described by Evans and experimental solutions containing a known amount of a TMSOTMS standard.³⁴ Elemental analyses were conducted on a Vario EL elemental analyzer at the Analytical Center of Peking University. Raman Spectra were recorded on a Thermofisher DXRxi Micro Raman imaging spectrometer. X-ray data collections were performed at 180 K on a Rigaku diffractometer, using monochromated Mo Kα radiation (λ = 0.71073 Å). The structures were solved by using the SHELXTL program³⁵ in Olex2.³⁶ Refinement was performed on F² anisotropically for all the non-hydrogen atoms by the full-matrix least-squares method with the XL refinement package.³⁷ The positions of the H atoms were placed in calculated positions using an appropriate riding model.

Cyclic voltammetry experiment of 4

The oxidation–reduction properties of 4 were studied using a cyclic voltammetry experiment in THF with [(Bu₄N)(PF₆)] as the supporting electrolyte. All potentials are reported vs. FeCp₂. 4 had one reversible reduction peak at E_1/2 = −2.923 V and one oxidation peak at −1.709 V (Fig. S22:†E_1/2(Fc⁺/Fc) = 0.14 V) at the beginning of the experimental test. However, attempts to isolate the reduction product of complex 4 were hampered due to the poor stability.

General procedure for reduction of N₂ into NH₃ and N₂H₄ by 5

Under an atmosphere of dinitrogen, 5 (0.01 mmol), reductant KC₈ and proton sources ([(Ph₂NH₂)(OTf)], [H(Et₂O)₂(BAr^F₄)] or [(HPCy₃)(BF₄)]) were mixed in a 50 mL Schlenk flask at −78 °C, and then pre-cooled Et₂O (5 mL, −78 °C) was added. The mixture was stirred −78 °C for 1 h and warmed slowly to room temperature. After the solution was stirred for 1 h at room temperature, and HCl (100 eq., 1 M in 1,4-dioxane) was added. The resulting solvent was removed under reduced pressure and the residue was extracted with H₂O (10.0 mL). The amounts of NH₃ and N₂H₄ were precisely determined by the indophenol method³⁸ and the p-(dimethylamino)benzaldehyde method,³⁹ respectively.

Synthesis of 2. In an argon atmosphere glovebox, a solution of 1 (600 mg, 1.63 mmol) in THF (5 mL) was added into a glass tube in a medium-pressure autoclave. The autoclave was transferred outside of the glovebox and pressurized with H₂ to 15 atm. The mixture was left stirring at room temperature for 2 h, and then quickly moved into the glovebox. After evaporation of all the volatiles in vacuo, the residue was extracted with Et₂O. 2 (625 mg, 1.10 mmol, 68% yield) as dark-blue crystals was obtained by crystallization from Et₂O at −35 °C. ¹H NMR (400 MHz, C₆D₆): δ −1.13 (s, 2H, TiCH), 0.45 (s, 18H, Si(CH₃)₃), 1.27 (m, 12H, THF), 1.57 (s, 2H, TiH), 2.39 (s, 6H, CH₃), 3.39 (m, 12H, THF), 5.28 (s, 5H, Cp) ppm. ¹³C NMR (101 MHz, C₆D₆): 3.00 (s, Si(CH₃)₃), 21.59 (s, CH₃), 25.04 (s, THF), 67.49 (s, THF), 68.00 (s, TiCH), 98.19.02 (s, Cp), 107.24 (s, MeC=CMe) ppm. UV-vis (THF): 480, 600 nm. Anal. calcd for C₂₉H₅₇O₃Si₂Li₁Ti₁: C, 61.68; H, 10.17. Found: C, 61.34; H, 10.03. The synthesis of 2-D was analogous to that of complex 2 except that the deuteride gas (D₂) was used (1 atm D₂, 6 h). ²H NMR (77 MHz, C₆H₆): δ −1.20 (br, TiCD), 1.35 (s, TiD) ppm.

Synthesis of 3. Mehod a: In an argon atmosphere glovebox, 2 (120 mg, 0.21 mmol) was dissolved in hexane (20 mL) and stirred for 4 h, resulting in a gradual colour change from dark-blue to brown. After filtering, the solution was concentrated in vacuo, followed by storage at −35 °C to give 3 (26 mg, 0.04 mmol, 36% yield) as purple crystals. Method b: In an argon atmosphere glovebox, a solution of 1 (100 mg, 0.20 mmol) in hexane (15 mL) was added into a glass tube in a medium-pressure autoclave. The autoclave was transferred outside of the glovebox and pressurized with H₂ to 15 atm. The mixture was left stirring at room temperature for 3 h, and then quickly moved into the glovebox to get a dark-brown suspension. After filtering, the solution was concentrated in vacuo, followed by storage at −35 °C to give 3 (13 mg, 0.02 mmol, 20% yield) as purple crystals. ¹H NMR (400 MHz, C₆D₆): δ −5.18 (s, 2H, TiCH), 0.35 (s, 36H, Si(CH₃)₃), 1.83 (s, 12H, CH₃), 2.11 (s, 2H, TiH), 5.18 (s, 10H, Cp) ppm. ¹³C NMR (101 MHz, C₆D₆): 0.22 (s, Si(CH₃)₃), 19.21 (s, CH₃), 78.83 (s, TiCH), 102.44 (s, Cp), 113.45 (s, MeC=CMe) ppm. Anal. calcd for C₃₄H₆₄Si₄Ti₂: C, 59.97; H, 9.47. Found: C, 59.73; H, 9.22.

Synthesis of 4. Mehod a: In a dinitrogen atmosphere glovebox, 2 (368 mg, 0.65 mmol) and THF (8 mL) were added into a 50 mL flask. The mixture was stirred at room temperature for 48 h. After evaporation of all the volatiles in vacuo, the residue was washed twice with hexane (2 mL × 2), and then extracted with Et₂O. The solvent was removed completely under reduced pressure to give 4 (163 mg, 0.16 mmol, 45% yield) as brown powder. Single crystals of 4 suitable for X-ray crystallography were obtained as purple crystals from Et₂O at −35 °C. Method b: In an argon atmosphere glovebox, a solution of 1 (368 mg, 1.0 mmol) in THF (5 mL) was added into a glass tube in a medium-pressure autoclave. The autoclave was transferred outside of the glovebox and pressurized with N₂/H₂ (v/v = 1 : 1) to 15 atm. The mixture was left stirring at room temperature for 48 h, then quickly moved into the glovebox. After evaporation of all the volatiles in vacuo, the residue was washed twice with hexane, and then extracted with Et₂O. The solvent was removed completely under reduced pressure to give 4 (145 mg, 0.14 mmol, 29% yield). ¹H NMR (400 MHz, C₆D₆): δ −0.06 (br, 18H, Si(CH₃)₃), 1.40 (m, 16H, THF), 2.19 (br, 6H, CH₃), 3.57 (m, 16H, THF), 6.59 (br, 5H, Cp) ppm. ¹³C NMR (101 MHz, C₆D₆): 0.41 (s, Si(CH₃)₃), 4.42 (s, CH₃), 26.33 (s, THF), 68.82 (s, THF), 102.93 (s, Cp), 129.90 (s, MeC=CMe) ppm. (Due to the paramagnetism of 4, not all signals were obtained.) Raman (ν_N–N): 1699 cm⁻¹. μ_eff (Evans’ method, 400 MHz, 17% v/v HMDSO in C₆D₆): 2.03μ_B. UV-vis (THF): 460 nm. Anal. calcd for C₅₀H₉₄O₄N₂Si₄Li₁Ti₂: C, 59.92; H, 9.45; N, 2.79. Found: C, 59.29; H, 9.17; N, 2.67.

Synthesis of ¹⁵N-4. In an argon atmosphere glovebox, to a 25 mL Schlenk tube containing a magnetic stirring bar was added a mixture of 2 (200 mg, 0.35 mmol) and THF solvent (4 mL). The solution was frozen and degassed three times at low temperature. After that, the Schlenk tube was back-filled with ¹⁵N₂ and sealed. The mixture was stirred at room temperature for 72 h. After removal of the solvent in vacuo, the residue was washed twice with hexane and then extracted with Et₂O. The solvent was removed completely under reduced pressure to give ¹⁵N-4 (64 mg, 0.06 mmol, 36% yield) as brown powder. ¹⁵N NMR (61 MHz, C₆D₆): δ 394.31 (br) ppm. Raman (ν_N–N): 1642 cm⁻¹.

Synthesis of 5. In a dinitrogen atmosphere glovebox, [(Cp₂Fe)(BAr^F₄)] (262 mg, 0.25 mmol) was added to a stirred solution of 4 (230 mg, 0.23 mmol) in toluene (15 mL). The mixture was stirred at room temperature for 2 h, and then filtered. All volatiles of the filtrate were removed under reduced pressure to give 5 (156 mg, 0.22 mmol, 95% yield) as brown powder. Single crystals of 5 suitable for X-ray crystallography were obtained as purple crystals from volatilization of a concentrated hexane/Et₂O solution at room temperature. ¹H NMR (400 MHz, C₆D₆): δ −0.31 (s, 2H, TiCH), −0.10 (s, 18H, Si(CH₃)₃), 2.08 (s, 6H, CH₃), 6.72 (s, 5H, Cp) ppm. ¹³C NMR (101 MHz, C₆D₆): 0.22 (s, Si(CH₃)₃), 19.53 (s, CH₃), 76.84 (s, Ti–C), 108.17 (s, Cp), 124.76 (s, MeC=CMe) ppm. Raman (ν_N–N): 1704 cm⁻¹. UV-vis (THF): 495, 540 nm. Anal. calcd for C₃₄H₆₂N₂Si₄Ti₂: C, 57.76; H, 8.84; N, 3.96. Found: C, 57.27; H, 8.69; N, 3.72.

Synthesis of ¹⁵N-5. In an argon atmosphere glovebox, [(Cp₂Fe)(BAr^F₄)] (84 mg, 0.08 mmol) was added to a stirred solution of ¹⁵N-4 (60 mg, 0.06 mmol) in toluene (10 mL). The mixture was stirred at room temperature for 2 h, and then filtered. All volatiles of the filtrate were removed under reduced pressure to give ¹⁵N-5 (38 mg, 0.05 mmol, 90% yield) as brown powder. ¹⁵N NMR (61 MHz, C₆D₆): δ 656.61 (br) ppm. Raman (ν_N–N): 1647 cm⁻¹.

Author contributions

X. S., Y. Z., M. Z., L. Y., and Y. W. carried out the syntheses and characterization of 2–5. R. F. and Y. C. helped the analyses of crystallographic data. J. W. performed DFT calculations and analysed the data. Z. X. and J. W. conceived and conceptualized this project. X. S., J. W., and Z. X. prepared and revised the manuscript. All authors participated in discussions of the research and the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21988101 and 22201013) and the Beijing Natural Science Foundation (No. 2222008). We thank Dr Qiong Yuan and Mr Peng-Xiang Fu for EPR analysis. We thank the Analytical Instrumentation Center at Peking University for the NMR measurements and the High-performance Computing Platform of Peking University for the DFT calculations.

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Footnote

† Electronic supplementary information (ESI) available: Experimental, spectroscopic, and crystallographic details. CCDC 2284338–2284341. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi01911j

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