Dinitrogen Activation of Cyclopentadienyl-Phosphine–Iron Complexes of Three Different Valences

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Dec 2021Dinitrogen Activation of Cyclopentadienyl-Phosphine–Iron Complexes of Three Different Valences Gao-Xiang Wang†, Jianhao Yin†, Jiapeng Li†, Zhu-Bao Yin, Botao Wu, Junnian Wei, Wen-Xiong Zhang and Zhenfeng Xi Gao-Xiang Wang† Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 †G.-X. Wang, J. Yin, and J. Li contributed equally to this work.Google Scholar More articles by this author , Jianhao Yin† Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 †G.-X. Wang, J. Yin, and J. Li contributed equally to this work.Google Scholar More articles by this author , Jiapeng Li† Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 †G.-X. Wang, J. Yin, and J. Li contributed equally to this work.Google Scholar More articles by this author , Zhu-Bao Yin Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 Google Scholar More articles by this author , Botao Wu Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 Google Scholar More articles by this author , Junnian Wei *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 Google Scholar More articles by this author , Wen-Xiong Zhang Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 Google Scholar More articles by this author and Zhenfeng Xi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University, Beijing 100871 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202000712 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail A series of structurally well-defined iron–dinitrogen complexes ( LFe–N2) bearing cyclopentadienyl-phosphine ligands ( L) were synthesized and analyzed by single-crystal X-ray structural analysis, IR/Raman spectra, magnetometry, Mössbauer spectroscopy, and elemental analysis. The conversion relationship and the change of the spin states among the iron complexes with different valences were studied in detail, showing a series of metal-centered reduction processes ( LFeII–N20–FeII L → LFeI–N20–FeI L → LFe0–N20). Based on the spectroscopic parameters, the activation degree of the dinitrogen increases while the oxidation state of the iron decreases. DFT calculations of these iron complexes involving the Mayer bond order and natural bond orbital (NBO) charge analysis confirmed our hypothesis. Furthermore, a novel one-pot synthesis of diiron–dinitrogen complexes was realized by mixing the potassium salts of ligands ( LK) and iron halides without adding any reductants. The diiron(II)–dinitrogen complex showed reversible binding of molecular dinitrogen under N2 or argon (Ar) atmosphere. Download figure Download PowerPoint Introduction Since the first dinitrogen complex was discovered in 1965,1 dinitrogen fixation with transition metals has become an important research topic.2–12 Among these transition-metal–dinitrogen complexes, chemists have paid more attention to iron–dinitrogen complexes in recent years,13–34 as iron is the core element of metal catalysts in the industrial process of dinitrogen reduction.35 Furthermore, X-ray crystallographic structure of nitrogenase reveals that the FeMo-cofactor is the active center for reduction of dinitrogen,36 and iron plays a vital role during the fixation and reduction processes.37 Diversified ligands have been applied to iron–dinitrogen complexes in recent decades, including β-diketimate,13–16 trisphosphineborate,17–21 trisphosphinesilyl,22 trisphosphinealkyl,23 carbene,24,25 bisphosphine,26,27 bis(imino)pyridine,28,29 and phosphorus ligand with nitrogen backbone (PNP)-pincer ligands.30 Different ligands significantly affect the coordination configuration of the iron center, and thus influence the activation of dinitrogen. Although the simple combination of cyclopentadienyl ligands and phosphine ligands was used in several dinitrogen complexes,38–45 studies on iron–dinitrogen complexes bearing cyclopentadienyl-phosphine ligands, for example L in Scheme 1, in which the two commonly used strong coordination moieties of different steric and electronic properties are linked together, are few due to the formation of stable sandwich-type ferrocene structures.46–48 Scheme 1 | Dinitrogen activation at iron centers of different oxidation states supported by cyclopentadienyl-phosphine ligands. Download figure Download PowerPoint We have previously developed a convenient synthetic method for cyclopentadienyl-phosphine ligands ( L).49,50 With this class of ligands, we found that novel chromium (Cr)–dinitrogen complexes formed and reacted with Me3SiCl to afford the first chromium hydrazido complex via N2 functionalization.51,52 In this work, we changed the metal from the middle transition-metal chromium (Cr) to the late transition-metal iron (Fe) with two additional valence shell electrons, to explore the effect of π basicity of the metal center and π back-donation ability on dinitrogen fixation while maintaining the ligand environment and reducing one coordination site on the metal center. Herein, we report dinitrogen fixation studies based on iron complexes bearing cyclopentadienyl-phosphine ligands and the effect of iron centers of different oxidation states ( LFeII–N20–FeII L → LFeI–N20–FeI L → LFe0–N20) on the extent of dinitrogen activation (Scheme 1). Experimental Methods Synthesis of dinuclear Fe(II) complexes 1–4 Under a N2 atmosphere, the red THF solution of LK (0.1 mmol, 47.7 mg) was added into the suspension of FeX2 (X = Br/I, 0.1 mmol) in THF (5 mL) at room temperature. The resultant dark brown mixture was stirred for 12 h, and the solvent was removed under vacuum. The black residue was extracted with Et2O (8 mL) and filtered through Celite, and the solvent was removed and dried in vacuo. Single crystals of complexes 1 and 2 suitable for X-ray crystallography were obtained by recrystallization from Et2O/hexane at −35 °C. Complexes 3 and 4 were synthesized with a similar method under an atmosphere of argon (Ar). Synthesis of dinuclear Fe(I) complex 5 Under an atmosphere of N2, potassium graphite (0.2 mmol, 27.0 mg) was added into the solution of 1 (0.1 mmol, 117.5 mg) in THF (10 mL) at room temperature. The resultant mixture was stirred for 6 h, and the solvent was filtered and then removed under vacuum. The black residue was extracted with Et2O (10 mL) and filtered through Celite, and the solvent was removed and dried under vacuum. Single crystals of complexes 5 suitable for X-ray crystallography were obtained by recrystallization from Et2O/THF at −35 °C. Synthesis of Fe(I) complex 7 Under an atmosphere of N2, the deep red THF solution of Cy LK (0.1 mmol, 48.8 mg) was added into the suspension of FeBr2 (0.1 mmol, 21.6 mg) in THF (5 mL) at room temperature. The resultant red mixture was stirred for 12 h, and the excess potassium salt (1.33 mmol, 52 mg) was added into the solution for another 24 h. The color of the solution changed from red to green, and finally to deep red-brown. Then a THF solution of crypt-222 (0.1 mmol, 37.6 mg) was added into the deep red-brown solution for 2 h. The solvent was removed under vacuum. The black residue was extracted with THF (4 mL) and filtered through Celite. Single crystals of complexes 7 and 8 suitable for X-ray crystallography were obtained by slow solvent evaporation from THF at −35 °C. Further experimental details and characterization data are included in the Supporting Information. Computational Methods The details of the computational methods are included in the Supporting Information. Results and Discussion Synthesis and characterization of dinuclear Fe(II) complexes with reversible binding of dinitrogen The potassium salt of cyclopentadienyl-phosphine ligand ( LK) was prepared according to our previous method.49 Surprisingly, we found that the reaction of LK with FeBr2 or FeI2 under N2 atmosphere could directly afford the dinitrogen-bridged diiron complexes 1 and 2 in high yield without adding any reductants (Scheme 2a). Complexes 1 and 2 were isolated as dark brown crystals in 90% and 78% yield, respectively, after recrystallization from Hexane/Et2O at −35 °C. Scheme 2 | (a) Synthesis of dinuclear Fe(II) complexes 1–4 and reversible binding of N2 under N2/Ar atmosphere in THF solution. (b) The structural conversion from complexes 1 to 3 in single-crystal form. Download figure Download PowerPoint The molecular structures of complexes 1 and 2 were confirmed by single-crystal X-ray crystallography. The molecular structure of 1 is shown in Figure 1 while that of 2 is provided in the Supporting Information. Complex 1 shows a pseudotetrahedron dinuclear coordination mode, in which two cyclopentadienyl-phosphine ligands are located at the opposite position with respect to each other. The bridging N2 ligand is almost linearly coordinated with two iron centers, as shown by the angle values of 175.51(14)° from Fe(1)–N(1)–N(2) and 175.77(14)° from Fe(2)–N(2)–N(1), which is consistent with the value in its analogue 2. The N–N bond lengths of the bridging N2 in complexes 1 and 2 are 1.120(2) and 1.119(3) Å, respectively. The Raman spectra of 1 and 2 exhibit sharp N–N stretching bands centered at 2072 and 2060 cm−1, respectively, which demonstrate a weak activation of bridging end-on N2 ( Supporting Information Figures S2 and S3).53 By loss of the weakly coordinated N2, dibromide-bridged diiron complex 3 was accidentally obtained by recrystallization of complex 1 at room temperature, which encouraged us to investigate the reactions of FeBr2 or FeI2 with LK under Ar atmosphere. As expected, without the participation of N2, dibromide-bridged diiron complex 3 and diiodide-bridged diiron complex 4 were isolated in 90% and 95% yield, respectively. The molecular structures of complexes 3 and 4 were both confirmed by X-ray crystallography. As shown in Figure 2, two iron atoms are linked by two bridging halides. The easy conversion between complexes 1 and 3 was observed in THF solution at room temperature under Ar or N2 atmosphere. Figure 1 | Molecular structure of complex 1 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)–N(2) 1.120(2), Fe(1)–N(1) 1.8627(14), Fe(1)–P(1) 2.2384(5), Fe(1)–Br(1) 2.4587(3), N(1)–Fe(1)–P(1) 98.45(5), and Fe(1)–N(1)–N(2) 175.51(14). Download figure Download PowerPoint Figure 2 | Molecular structure of complex 3 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe(1)–Br(1) 2.5212(4), Fe(1)′–Br(1) 2.5395(4), Fe(1)–P(1) 2.3122(7), Fe(1)–Br(1)–Fe(1)′ 98.167(13), and P(1)–Fe(1)–Br(1) 99.059(19). Download figure Download PowerPoint To investigate the oxidation state of diiron centers, we performed zero-field 57Fe Mössbauer spectroscopy on diiron–dinitrogen complexes 1 and 2 and diiron–dibromide complex 3 ( Supporting Information Figures S18–S21). The Mössbauer spectrum of 1 shows that there are two components with different chemical environments. The two components were classified as Fe(II)–dinitrogen complex 1 (δ = 0.43 mm/s, |ΔEQ| = 2.14 mm/s) and Fe(II)–dibromide complex 3 (δ = 0.57 mm/s, |ΔEQ| = 2.36 mm/s), which indicated that the transformation from 1 to 3 upon release of coordinated N2 took place even in the solid state (Scheme 2b). The decreased isomer shift of 1 versus that of 3 is probably related to the coordination environment of two hexacoordinated Fe(II) centers with different bridging ligands. As for the diiron–diiodide complex 2 (δ = 0.32 mm/s, |ΔEQ| = 2.08 mm/s), compared with its bromide analogue, the coordination of iodide with weaker electronegativity causes a 0.1 mm/s decrease of the isomer shift. The Mössbauer spectra above demonstrate that the Fe(II) centers in diiodide-bridged complex 2 bind N2 stronger than the dibromide analogue 1. The release of coordinated N2 was also detected by X-ray crystallography when the single crystal of 1 was placed in an Ar atmosphere. After a month, another single crystal which contained both 1 and 3 in the same unit cell was determined, and the single-crystal diffraction parameters are consistent with 1 and 3 (Figure 3). Figure 3 | The single-crystal structure of cocrystalized complexes 1 and 3 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)–N(2) 1.119(3), Fe(1)–N(1) 1.8603(16), Fe(1)–P(1) 2.2275(5), Fe(1)–Br(1) 2.4593(3), N(1)–Fe(1)–P(1) 98.37(5), Fe(1)–N(1)–N(2) 175.7(2), Fe(3)–Br(3) 2.5553(4), Fe(3)–Br(4) 2.5174(3), Fe(3)–P(3) 2.3682(6), and Fe(3)–Br(3)–Fe(4) 85.525(13). Download figure Download PowerPoint To obtain the ground-state spin multiplicity of diiron complexes 1 and 3, we conducted variable-temperature (2–300 K) magnetic susceptibility analysis using the superconducting quantum interference device (SQUID). Remarkably, SQUID magnetometry reveals that both 1 and 3 feature singlet ground states (S = 0) ( Supporting Information Figures S23 and S24). Without the coordination of bridging N2, complex 1 with two low-spin Fe(II) (SFe = 0) changed into dibromide-bridged complex 3 with two high-spin Fe(II) (SFe = 2). The difference in the isomer shift (δ = 0.43 for 1 and δ = 0.57 for 2) also support the change in spin state, as δ is slower for low-spin than high-spin configuration, which falls in the typical range found for hexacoordinated Fe(II) compounds.54 Further transformation of Fe(II)–dinitrogen complex into Fe(I)–dinitrogen complex Since the two bromide ligands are coordinated to iron centers in complex 1, reducing iron centers from Fe(II) to Fe(I) by removing the bromide might further activate the coordinated N2. Experimental results showed that complex 1 could be readily reduced into 5 by treating with 2.0 equiv of potassium graphite in THF at room temperature. In this case, crystalline complex 5 was isolated in 87% yield after recrystallization from Et2O/THF at –35 °C. The reactions between complex 5 and electrophiles (TMSCl, MeOTf, etc.) were difficult, but the dinitrogen ligand in 5 could be readily displaced by a PPh3 ligand to give the mononuclear Fe(I) complex 6 in 71% isolated yield (Scheme 3). Scheme 3 | Synthesis and reaction of dinuclear Fe(I)–dinitrogen complex 5. Download figure Download PowerPoint The molecular structure of complex 5 exhibits an almost linear dinitrogen ligand being coordinated to two iron atoms (Figure 4), as shown by the angle value of 176.7(3)° for both Fe(1)–N(1)–N(1)′ and Fe(1)′–N(1)′–N(1), which is consistent with those for complex 1. The N–N bond length of 5 is 1.149(4) Å, which is consistent with the reported bridging end-on Fe(I)–dinitrogen complex,55 but elongated as compared with the value of 1.120(2) Å in complex 1. The resonance Raman spectrum of 5 exhibits a strong peak at 1810 cm−1 ( Supporting Information Figure S4), which is assigned to the υ(N2) stretching vibration and comparable with those in reported bridging end-on Fe(I)–dinitrogen complexes.13,56–58 Due to the strong antiferromagnetic property between two iron centers, complex 5 is the only one which exhibits sharp NMR signals among the aforementioned iron complexes ( Supporting Information Figure S7–S17). The 31P-NMR spectrum of 5 shows a singlet at δ = 67.6 ppm, indicating that the two phosphorus atoms are equivalent, there is no dissociation equilibrium between dimer and monomer in the solution state, or the dissociation and recoordination process is rapid. The zero-field Mössbauer spectrum of 5 has a quadrupole doublet with isomer shift δ = 0.31 mm/s and quadrupole splitting |ΔEQ| = 1.88 mm/s ( Supporting Information Figure S22). The observed isomer shift of 5 (δ = 0.43 mm/s) is slower than complex 1, whereas typical iron-based reduction tends to increase the isomer shift. The decreased isomer shift indicates enhanced π back-donation in the reduction process.59 Figure 4 | Molecular structure of the dinuclear Fe(I)–dinitrogen complex 5 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)–N(1)′ 1.149(4), Fe(1)–N(1) 1.7629(19), Fe(1)–P(1) 2.1647(6), N(1)–Fe(1)–P(1) 102.02(6), and Fe(1)–N(1)–N(1)′ 176.7(3). Download figure Download PowerPoint Synthetic attempt of Fe(0)–dinitrogen complex Inspired by our previous work,51 the anionic form of cyclopentadienyl-phosphine Cr(0)–dinitrogen complex achieved dinitrogen fixation affording hydrazido complex, we attempted to synthesize the analogous anionic Fe(0)–dinitrogen complex. Unfortunately, further reduction of Fe(I)–dinitrogen complex 5 using potassium as the reducing agent did not show the characteristic υ(N2) stretching vibration in the IR spectrum. This result reminded us that the phenyl-substituted cyclopentadienyl-phosphine ligand ( PhLK) might not be stable enough when treated with strong reducing agents such as potassium ( Supporting Information Figure S1). The cyclohexyl-substituted cyclopentadienyl-phosphine ligand ( cy LK) was then chosen instead of PhLK to react with FeBr2 under dinitrogen atmosphere. After adding excess amount of potassium in situ, a new IR signal (1823 cm−1) was observed ( Supporting Information Figures S5). With the aid of 2.2.2-cryptand, the IR signal shifted to 1868 cm−1 ( Supporting Information Figures S6), and the corresponding Fe(0)–dinitrogen complex 7 was obtained as red crystals by slow solvent evaporation. Meanwhile, a phenyl-substituted tetraethyl cyclopentadienyl complex 8 was also obtained as yellow crystals. Due to the similar solubility of 7 and 8, the separation of these two complexes was unsuccessful (Scheme 4). The molecular structures of complexes 7 and 8 were confirmed by X-ray crystallography. As shown in Figure 5, the N–N bond length of 7 is 1.140(3) Å, which is consistent with the reported mononuclear Fe(0)–dinitrogen complex.19,22,60 Scheme 4 | Synthetic attempt of anionic mononuclear Fe(0)–dinitrogen complex 7. Download figure Download PowerPoint DFT calculations DFT calculations ( Supporting Information Figures S34–S45) were also performed for complexes 1, 3, 5, and 7 to learn more about their electronic structures. As for the dinitrogen-bridged diiron(II) complex 1, the spin density distribution showed that there are no unpaired electrons in the Fe(II) centers and the ground state is singlet with two low-spin Fe(II) (SFe = 0), which is consistent with the experimental results. Remarkably, as for the dibromide-bridged diiron(II) complex 3, the Mulliken atomic spin density distribution showed that the unpaired α and β electrons are located mainly on the Fe centers (3.5 and −3.5, respectively), which indicated a singlet ground state (S = 0) attained by antiferromagnetic coupling of two high-spin Fe(II) (SFe = 2). The Mulliken spin population analysis of the diamagnetic diiron(I) complex 5 shows that the unpaired α and β electrons are located on the Fe centers (1.0 and −1.0, respectively), indicating an S = 1/2 spin state for both Fe centers. Thus, the ground state of 5 is an antiferromagnetic singlet, and complex 5 should be described as an LFe(I)–N2(0)–Fe(I) L species with two low-spin Fe(I) centers. Figure 5 | Molecular structure of the anionic mononuclear Fe(0)–dinitrogen complex 7 with thermal ellipsoids at 30% probability. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): N(1)–N(1)′ 1.140(3), Fe(1)–N(1) 1.739(2), Fe(1)–P(1) 2.1291(5), N(1)–Fe(1)–P(1) 100.83(7), and Fe(1)–N(1)–N(1)′ 176.0(2). Download figure Download PowerPoint The relationship between the oxidation states of the iron center and the degree of dinitrogen activation has aroused our great interest. N2 is often considered as a non-innocent ligand and could exhibit different oxidation states such as N2, N22−, and N23−.7–12,61 This would be consistent with the increased N–N bonds of iron–dinitrogen complexes 1, 2, 5, and 7 (1.119–1.149 Å), as well as the corresponding red-shifted N–N stretching frequencies (2072–1810 cm−1) (Table 1). The Mayer bond orders of N–N (2.65, 2.45, and 2.25 for complexes 1, 5, and 7, respectively) shows the same trend as experimental N–N stretching frequencies from the reduction processes (FeII → FeI → Fe0) ( Supporting Information Figures S38, S41 and S44). Selected bond lengths (Fe–N, Fe–P, Fe to cyclopentadienyl centroid) decrease remarkably within the reduced iron centers, indicating strengthened π-backbonding from iron to N2 during this electron-transfer process. The Mayer bond orders of Fe–N (0.69, 0.74, and 0.92 for complexes 1, 5, and 7, respectively) also support the increasing π basicity of the Fe center and π back-donation ability. Although these redox processes are metal-centered reductions, the changes of Fe–N/N–N bond lengths and the natural bond orbital (NBO) atomic charge distributions on the N atoms ( 1: 0.12, 5: 0.01, and 7: 0.03 and −0.20) indicate the increasing activation of dinitrogen. Orbital composition analysis of complexes 1, 5, and 7 was also carried out. In the Fe(II) complex 1, the two Fe centers contribute 64%, while the two N atoms only contribute 1.2% in total. In the Fe(I) complex 5, the contribution of two Fe centers decreases to 50%, while the two N atoms contribute 2.1% in total, indicating a weak activation of dinitrogen. In the Fe(0) complex 7, the Fe center only contributes 38%, while the two N atoms contribute 3.2% and 12%, respectively. Thus, the anionic-type Fe(0) complex 7 has stronger activation of dinitrogen compared with the bridged-dinitrogen Fe(II) complex 1 and Fe(I) complex 5. Although continuous reduction ( LFeII–N20–FeII L → LFeI–N20–FeI L → LFe0–N20) could enhance the activation of dinitrogen, the tolerance of ligands to strong reducing agents also needs to be considered. More detailed natural localized molecular orbitals (NLMOs) with the back-donation from the iron center to the dinitrogen are given in Supporting Information Figures S39, S43, and S45 for complexes 1, 5, and 7, respectively. Table 1 | Comparison of the Key Structural and Spectroscopic Parameters of Iron–Dinitrogen Complexes of Different Valences Fe–N (Å) N–N (Å) Fe–P (Å) Fe–Cpa (Å) δ (mm/s) ΔEQ (mm/s) ν(N–N) (cm−1) 1 1.8627(14) 1.120(2) 2.2384(5) 1.706 0.43 2.14 2072 2 1.8585(16) 1.119(3) 2.2444(5) 1.705 0.32 2.08 2060 5 1.7629(19) 1.149(3) 2.1647(6) 1.660 0.31 1.88 1810 7 1.739(2) 1.140(3) 2.1291(5) 1.683 1823b aThe distance from Fe atom to Cp plane centroid. bThe IR peak is classified to the Fe(0) complex without adding crypt-222. Conclusion We have designed and synthesized a series of iron–dinitrogen complexes bearing cyclopentadienyl-phosphine ligands ( L). The transformation relationships between different valences of iron complexes were investigated in detail. We put forward a novel one-pot method to prepare the diiron(II)–dinitrogen complexes without adding any reductants. We found the iron(II)–dinitrogen complex with two low-spin Fe(II) centers could be transformed to its corresponding N2 releasing product with two high-spin Fe(II) centers even as a single-crystal form, and both diiron(II) complexes revealed singlet ground states. The halide ligands on the iron(II) centers can be removed under reducing conditions affording the diamagnetic diiron(I)–dinitrogen complex with two antiferromagnetic coupled low-spin Fe(I) centers. Further reduction with potassium afforded the anionic Fe(0)–dinitrogen complex. Thus, we established the redox processes in this system as metal-centered reductions ( LFeII–N20–FeII L → LFeI–N20–FeI L → LFe0–N20). Both experimental and theoretical calculation results demonstrated slight increases in activation of neutral dinitrogen ligand while the oxidation state of iron center decreases. Further studies on the reactivity of these dinitrogen complexes bearing cyclopentadienyl-phosphine ligands and modification of the ligand framework are still underway in our lab. We hope that the current results will provide useful guidance for the rational design of ligands for dinitrogen activation with transition metals and for understanding the redox process and the correlation between the valence of the metal center and the activation degree of dinitrogen in transition-metal–dinitrogen complexes. Supporting Information Supporting Information is available and includes additional experimental details and characterization data for complexes 1– 7; magnetometry data for complexes 1 and 3; Mössbauer spectroscopy data for complexes 1, 3, and X-ray crystallographic data for complexes 1– 8 and cocrystalized complexes 1 and 3; and DFT calculation details for complexes 1, 5, and 7. of is no of to Information This was supported by the of Chemistry of Key of National of The for on electronic structures. The and for the with SQUID and Mössbauer The DFT calculation was supported by the of Peking 1. Google Scholar 2. the Google Scholar J. Dinitrogen Activation and Google Scholar More for Google Scholar 5. 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