A Terminal Iron Nitrilimine Complex: Accessing the Terminal Nitride through Diazo N−N Bond Cleavage

The facile release of dinitrogen renders diazo compounds powerful carbene-transfer reagents.¹ Hence, they find wide application in organic synthesis, including C−H bond insertion, cyclopropanation, the preparation of olefin metathesis catalysts, and even in enzyme-catalyzed carbene-transfer reactions.² The key step in all these transformations is the generation of a transient carbene species (Scheme 1 A). Less known, diazo compounds not only serve as a carbene source, but also show nucleophilic properties at the terminal nitrogen atom, thus providing molecular systems relevant to dinitrogen activation catalyzed by the FeMo cofactor.³ Diazo compounds having an electron-withdrawing group (EWG) at the carbon atom can be deprotonated to yield their lithium salts Li[(EWG)CN₂].⁴ These ambident nucleophiles act as transient intermediates in 1,3-dipolar cycloaddition reactions.⁵ Owing to the high reactivity of nitrilimines, coordination to transition metals is exceedingly rare, with the only examples to date being a d⁰-configured Sc³⁺ complex having a terminal N-coordinated nitrilimine reported by Chen et al. and a dinuclear iron complex having two μ₂-N bridging nitrilimine ligands described by Holland and co-workers.⁶ There are, however, several investigations on the coordination chemistry of Li[Me₃Si−CN₂] with trivalent f-elements. In all of these cases, a [1,3]-shift of the trimethylsilyl group from the carbon to the terminal nitrogen atom was observed, which resulted in the formation of bridging isocyanotrimethylsilylamido [(Me₃Si)NN≡C]⁻ ligands.⁷ Examples of nitrilimine N−N bond cleavage with nitride formation are hitherto unknown.

The nitrilimine anion is isoelectronic to the azido ligand, N₃⁻, which is a common, but—due to poor solubility—often problematic precursor to generate terminal metal nitrides upon release of dinitrogen (Scheme 1 B).⁸ The investigation of terminal iron nitride complexes promises novel approaches for dinitrogen valorization.⁹ Yet, the synthesis of terminal nitride complexes remains a challenge in coordination chemistry. In our efforts to develop novel synthetic routes to nitride intermediates, we investigated terminal N-bound nitrilimine ligands as possible starting materials. In this approach (Scheme 1 C) the N−N bond cleavage is analogous to the commonly observed C−N bond cleavage reaction of diazo compounds to give carbene complexes (Scheme 1 A).

As a precursor, we chose the iron(II) complex 1, which has the N-anchored tris-N-heterocyclic carbene ligand TIMEN^mes (tris[2-(3-mesitylimidazolin-2-ylidene)ethyl]amine) sporting bulky mesityl groups at the N-heterocyclic carbene (NHC). This ligand provides sufficient steric bulk to prevent the formation of undesired dinuclear complexes.^{9b, 10} Indeed, reaction of the trimethylsilyl diazomethane lithium salt Li[Me₃Si−CN₂] with the Fe^II tris-carbene complex [(TIMEN^mes)FeCl](BPh₄) (1) in tetrahydrofuran at −100 °C resulted in an orange-colored solution that intensified its color upon warming to ambient temperature (Scheme 2). After workup, the orange, crystalline complex [(TIMEN^mes)Fe(N₂CSiMe₃)](BPh₄) (2) was obtained in 91 % yield. The paramagnetically shifted and broadened ¹H NMR signals of 2 are in accordance with a complex of trigonal symmetry; the solid-state infrared (IR) spectrum of 2 is reminiscent of the spectrum of its azido analogue [(TIMEN^mes)Fe(N₃)]⁺ (ν_as(N₃⁻)=2094 cm⁻¹) and exhibits a strong nitrilimine absorption band centered at 2068 cm⁻¹.^9b

Orange single crystals suitable for X-ray diffraction (SC-XRD) analysis were obtained at room temperature by diffusion of diethyl ether or pentane vapor into solutions of 2 in tetrahydrofuran (Figure 1). In the molecular structure, the three carbene entities and the trimethylsilyl nitrilimine provide a pseudo-tetrahedral, approximate C₃-symmetric coordination environment for the central iron metal ion (Fe: 0.615(2) Å above the plane defined by the three carbene carbon atoms; τ₄=0.95; τ_T=1; τ_trig.pyr.=0.85).¹¹

To the best of our knowledge, this is a rare example of a terminal nitrilimine ligand coordinated to a transition metal (vide supra).⁶ The nitrilimine ligand in 2 is nearly linear [∡(Fe-N_a-N_b) 177.0(2)°; ∡(N_a-N_b-C) 179.7(4)°; ∡(N_b-C-Si) 179.3(4)°] with N−N and N−C bond lengths of 1.199(4) Å and 1.200(4) Å, respectively. These bond lengths indicate that the electronic structure of the nitrilimine ligand is best described in an allenic rather than a propargylic resonance structure.¹² The average Fe−C_carbene bond length [2.095(3) Å] is comparable to that of the azido analogue [2.108(3) Å].^9b

The ⁵⁷Fe Mössbauer spectrum of 2 at 77 K and zero field (Figure 2 A) shows a single asymmetric quadrupole doublet with an isomer shift of δ=0.65 mm s⁻¹ and a quadrupole splitting of ΔE_Q=1.97 mm s⁻¹. These values are indicative of a high-spin iron(II) metal center and are similar to the values reported for the azido complex (δ=0.69 mm s⁻¹; ΔE_Q=2.27 mm s⁻¹).^9b According to the SQUID measurements (Figure 2 B), complex 2 possesses an effective magnetic moment, μ_eff, of 5.19 μ_B at 300 K (Evans method: μ_eff=5.01 μ_B), which is higher than the spin-only value of 4.90 expected for an S=2 high-spin d⁶ Fe^II complex. The magnetic moment increases slightly at lower temperatures likely due to intermolecular coupling, and decreases below 50 K to reach μ_eff of 4.1 μ_B at 2 K. The magnetization measurements at varying magnetic fields were fitted with the isotropic g_iso value of 2.19, resulting in the zero-field splitting (ZFS) D=−5.57 cm⁻¹ and the rhombic ZFS E=0 parameters (Figure 2 B, inset). Solid and frozen THF solution samples of complex 2 are EPR-silent (at 300 and 10 K, in perpendicular mode X-Band), thus supporting the integer spin state assignment.

In order to further elucidate the electronic structure of 2, quantum chemical calculations were performed. Geometry optimization in the quintet state by density functional theory (DFT; BP86-D3BJ/def2-TZVPP//BP86-D3BJ/def2-SVP) afforded geometric parameters in excellent agreement with the solid-state structure. Also, the calculated IR stretching frequency (2119 cm⁻¹), as well as the Mössbauer isomer shift (δ=0.37 mm s⁻¹) and quadrupole splitting (ΔE_Q=2.54 mm s⁻¹) match the experimental data (cf. Figure 2 A). Subsequent CASSCF/NEVPT2 calculations with truncated mesityl substituents suggest a quintet d⁶ electronic ground state, with a 2+1+2 ligand field splitting and a doubly occupied d orbital (Figure 3). The energies of the doubly degenerate nitrilimine π-orbitals are of the same order of magnitude as the metal d-orbitals (Figure S20). Furthermore, the calculations corroborate an allenic rather than propargylic resonance structure, as indicated by the Mayer bond orders of 0.9 for Fe−N, 2.2 for N−N, and 2.4 for N−C as well as the accumulation of negative partial charge at the carbon atom (−0.3 a.u.).

The cyclic voltammogram of 2 reveals a quasi-reversible oxidation E_1/2 at −0.71 V and an apparently irreversible reduction E_p,c at −2.6 V (vs. FeCp₂⁺/FeCp₂) (Figure 4, and see the Supporting Information for a detailed discussion). The anodic current at −1.78 V is related to the oxidation of reduced 2^red back to 2.

Whereas our attempts to isolate oxidized 2 were unsuccessful, reduction with 1.2 equivalents of KC₈ (Scheme 3) yielded a dark red solution. The ¹H NMR spectroscopic analysis of the crude reaction mixture revealed loss of the axial nitrilimine ligand and formation of the C₃-symmetric complex [(TIMEN^mes)Fe](BPh₄) (3).^7a This monovalent complex 3 could also be synthesized independently by reduction of 1 with Na/Hg. The SC-XRD analysis of 3 confirmed the formation of the iron(I) complex [(TIMEN^mes)Fe](BPh₄). According to the crystallographic analysis, the iron center is in a trigonal-pyramidal coordination environment (τ₄=0.86),¹¹ with an average Fe−C_carbene bond distance of 2.017 Å and an Fe−N bond distance of 2.270(3) Å (Figure 5). Further, ¹H NMR spectroscopic analysis corroborated that Li[Me₃Si-CN₂] does not react with 3. This lack of reactivity is peculiar as azidotrimethylsilane (Me₃Si)N₃, which is isoelectronic to [(SiMe₃)CN₂]⁻, is known to react swiftly with 3, providing the Fe^II azide complex with concomitant release of hexamethyldisilane.^9b

Encouraged by the electronic analogy between the terminal nitrilimine and azido ligands, we also tested the transformation of the nitrilimine to the terminal nitride. Since irradiating solutions of 2 did not yield the expected N−N bond cleavage, we decided to drive this transformation via the abstraction of the trimethylsilyl group.¹³ Indeed, treating a THF solution of complex 2 with tetrabutylammonium fluoride at −30 °C resulted in an instantaneous color change to intense purple (Scheme 4). The ¹H and ¹³C NMR spectra were in excellent agreement with the previously reported data of the iron(IV) nitride, [(TIMEN^mes)Fe(N)](BPh₄) (4), and are proof of the unprecedented, quantitative and clean conversion of 2 to the diamagnetic nitrido complex (4).^9b The ¹H and ¹³C NMR spectroscopic analysis revealed the concomitant formation of Me₃SiF and NBu₄CN (Figures S11 and S12). For structural proof, complex 4 was recrystallized and SC-XRD analysis unambiguously confirmed the formation of the terminal iron nitride complex (see the Supporting Information for SC-XRD details). This N−N bond cleavage of a diazo compound is unique, with the only comparable reactivity found for pincer-type iron complexes reported by Chirik et al. to give iron nitrile and aldimine complexes.¹⁴ However, for those complexes, a mechanism involving a second diazo molecule leading to an intermediate metallo-heterocyclic azine was proposed. Other reports on the N−N bond cleavage of diazo compounds relate to reductive processes via hydrogenation.¹⁵ We conclude that the fluoride treatment of a trimethylsilyl nitrilimine coordinated complex provides a new entry into metal–nitrido synthesis under exceptionally mild reaction conditions. Additionally, we note that the high solubility of Li[Me₃Si−CN₂] offers advantages compared to most common azide salts that often suffer from non-quantitative conversion to the desired azido complex in organic solutions.

In conclusion, we here report the isolation of the unprecedented terminal nitrilimine transition metal complex 2. Reduction of the iron(II) nitrilimine 2 leads to release of the nitrilimine ligand with formation of the iron(I) complex 3. Remarkably, treatment of 2 with a fluoride salt forms the terminal iron(IV) nitride 4 under mild reaction conditions. This work demonstrates the complementary reactivity of nitrilimine and azido ligands and introduces a new synthetic approach for the synthesis of metal–nitrido complexes.