H–H and Si–H Bond Addition to Fe_≡NNR₂ Intermediates Derived from N₂

Abstract

The synthesis and characterization of Fe diphosphineborane complexes are described in the context of N₂ functionalization chemistry. Iron aminoimides can be generated at RT under 1 atm N₂ and are shown to react with E–H bonds from PhSiH₃ and H₂. The resulting products derive from delivery of the E fragment to N_α and the H atom to B. The flexibility and lability of the Fe–BPh interactions in these complexes engender this reactivity.

Dinitrogen functionalization reactions using synthetic Fe complexes typically employ reductants in conjunction with electrophilic reagents;¹ this strategy has allowed for the reliable functionalization of N_β in terminal Fe–N₂ complexes (Scheme 1). Whereas early-metal N₂ complexes have displayed rich reactivity with non-polar E–H (E = H, Si) bonds,² such reactivity using Fe has little precedent. Addition of H₂ to diiron-bridged nitrides has been demonstrated,³ and in a recent report the nitrides were derived from reductive cleavage of N₂.⁴ The hydrogenolysis of the terminal Fe imide functional group has likewise been established,⁵ though these imides were not prepared from N₂. As such, we sought to generate Fe aminoimides from N₂ that could undergo subsequent E–H bond addition across the Fe_≡NNR₂ linkage as a method for N_α functionalization (Scheme 1).

Scheme 1.

In this context, we and others have studied Fe platforms that can accommodate both N₂ and terminal imide ligands,^{1a, 1c, 5a, 6} and we have recently reported that an Fe aminoimide complex 1 (Scheme 2) can be derived from N₂.^1c Given previous studies that demonstrate H₂ addition across the M–B bonds in related Fe and Ni complexes,⁷ we expected that the Fe–B bond in 1 could facilitate an E–H activation step. However, 1 does not react with either H₂ (1 atm) or PhSiH₃ at RT. At more elevated temperatures (see SI), 1 decomposes and no tractable products were identified in the presence of H₂ or PhSiH₃. We therefore explored the development of a diphosphineborane Fe system ((DPB)Fe) that might be more reactive than the triphosphineborane Fe system ((TPB)Fe) featured in 1. We now describe new (DPB)Fe_≡NNR₂ complexes that react with non-polar E–H bonds at RT, thereby enabling the one-pot transformation of free N₂ to an Fe hydrazido(−) species—the first such complex to be derived from N₂.

Scheme 2.

As an entry to useful (DPB)Fe synthons, we found that reductive metallation of isopropyl- and phenyl-substituted DPB ligands⁸ 2a and 2b (^iPrDPB = PhB(o-ⁱPr₂PC₆H₄)₂ and ^PhDPB = PhB(o-Ph₂PC₆H₄)₂, respectively, Scheme 3) with FeBr₂ and 1.0 equiv Na/Hg in C₆H₆ allows for (DPB)FeBr complexes 3a and 3b to be isolated in 84% and 64% yield, respectively. Brown 3a and 3b are pseudo-tetrahedral, S = 3/2 complexes (_μeff = 3.8 and 3.6 _μB in C₆D₆ at RT, respectively) that feature _η²-BC interactions that have been previously observed in Cu and Ni complexes of this ligand class.^7,9 The structures determined by XRD analysis (Figure 1 and Table 1; see SI for the XRD structure of 3b) show close Fe–B (2.3243(11) and 2.330(4) Å, respectively) and Fe–C_ipso contacts (2.2605(9) and 2.193(3) Å, respectively), supporting the formulation of the _η²-BC ligand as both a donor via a filled _π-arene orbital and an acceptor via an empty p orbital on boron. In solution, 3a and 3b are C_s-symmetric as indicated by their ¹H NMR spectra.

Scheme 3.

Figure 1.

Displacement ellipsoid (50%) representations of 3a (left) and 4 (right). PⁱPr₂ groups are truncated and H atoms are omitted for clarity.

Table 1.

Selected bond lengths (Å)

	Fe–B	Fe–Cipso	Fe–Cortho
3a	2.3242(11)	2.2605(9)	2.5483(11)
4: FeA a	2.3739(7)	2.2516(6)	2.2714(7)
4: FeB a	2.3136(7)	2.2133(6)	2.6642(7)
6ab	2.3768(6)	2.1492(5)	2.3403(6)
	2.4288(7)	2.1440(6)	2.2266(6)
7	2.859(5)	-	-

^aTwo unique Fe atoms per molecule.

^bTwo molecules per asymmetric unit.

Further reduction of 3a with 1.0 equiv Na/Hg under 1 atm N₂ results in the formation of the dinuclear, bridging N₂ complex (^iPrDPB)Fe(_μ-1,2-N₂)Fe(^iPrDPB) (4). The ¹H NMR spectrum of 4 in C₆D₆ indicates that each Fe center is equivalent and has local C_s symmetry. Solution- and solid-state IR spectra of 4 lack an N–N stretch, suggesting that the complex maintains its pseudocentrosymmetric, dinuclear structure in solution. The RT solution magnetic moment is 4.6 _μB, somewhat higher than the spin-only value of 4.0 _μB expected for two uncoupled S = 1 Fe centers.¹⁰ The two pseudo-tetrahedral Fe centers in 4 have different local geometries in the solid state. The geometry about one of the Fe centers (Fe_A, Figure 1) is distinguished by a short Fe–C_ortho contact and relatively long Fe–B and Fe–C_ispo distances (Table 1). The other Fe center (Fe_B) displays somewhat shorter Fe–B and Fe–C_ipso distances and a negligible Fe–C_ortho interaction. The phenyl ring bound to Fe_A exhibits alternating C–C bond lengths (between 1.4303(10) and 1.3711(12) Å; see SI) whereas this asymmetry is negligible for the phenyl ring bound to Fe_B; these metrics indicate that back-donation to the arene ring is more significant for Fe_A and back-donation to the B atom is more significant for Fe_B. Since both (DPB)Fe fragments are equivalent in solution, the Fe–BCC interaction must be highly flexible and the solid-state bond metrics reflect the large range of local geometries available to the Fe centers.

Performing an identical reduction using 1.0 equiv Na/Hg with the phenyl derivative 3b does not trigger N₂ binding but generates the brown, diamagnetic complex 5 that contains an _η⁷-BPh interaction; this coordination mode is to our knowledge unprecedented in the metalborane literature. The XRD structure of 5 shows tight Fe–_η⁷-BPh distances (Figure 2). The bound C_ipso atom is significantly pyramidalized as indicated by the sum of the two BCC and one CCC angles (342°). Further showing the significant geometrical distortion of the bound arene is the acute BC_ipsoC_para angle of 127.71(8)°. The _η⁷-BPh binding mode is maintained in solution based on the significantly upfield-shifted aryl resonances in the ¹H NMR spectrum (3.63 (H_ortho), 3.24 (H_meta), and 6.25 (H_para) ppm) and the ¹³C NMR spectrum (106.77 (C_ispo), 99.41 (C_ortho), 86.36 (C_meta), and 78.73 (C_para) ppm).

Figure 2.

Displacement ellipsoid (50%) representations of 5 (left) and 6a (right). PR₂ groups are truncated. H atoms and solvent molecules are omitted for clarity. For 6a: Only one of the two molecules per asymmetric unit is shown. Selected distances (Å) for 5: Fe–B = 2.2667(13), Fe–C_ipso = 1.9669(11), Fe–C_ortho = 2.090 (avg), Fe–C_meta = 2.169 (avg), Fe–C_para = 2.1933(11).

Each of 3a, 3b, 4, and 5 serves as a precursor to an Fe aminoimide complex derived from N₂. In a nearly identical procedure for generating 1,^1c diamagnetic 6a and 6b may be accessed by stirring 3a or 3b with 1.1 equiv 1,2-bis(chlorodimethylsilyl)ethane and 3.1 equiv Na/Hg in THF under 1 atm N₂. Alternatively, 4 or 5 may be employed as starting materials in conjunction with 2.1 equiv Na/Hg. Both 6a and 6b are green in solution and dichroic green/brown when crystalline. The ¹H NMR spectrum of 6a reveals its C_s symmetry in solution. In addition, the ¹H resonances of 6a attributed to the bound aryl ring are shifted upfield (5.10 (H_ortho), 6.49 (H_meta), and 4.72 (H_para) ppm); though this effect is not observed for 6b perhaps due to attenuated backbonding 6b due to the less electron-rich metal center.

The solid-state structures of 6a (Figure 2) and 6b (see SI) are similar. For 6a, two molecules are in the asymmetric unit. The short Fe–N distances (1.6607(5) and 1.6657(5) Å for 6a; 1.6592(7) Å for 6b) are consistent with other trigonal Fe(NR) linkages and imply an Fe_≡NNR triple bond.¹¹ The bound arenes display alternating bond lengths that vary between ca. 1.36 and 1.44 Å (see SI). Density functional theory calculations (see SI) support the formulation of 6a and 6b as typical pseudotetrahedral d⁶ Fe imides¹¹ that are similar to 1 except that one phosphine donor in 1 has been replaced by the _η³-BCC interaction in 6a and 6b. Although the presence of an Fe–B bond is not required for the stability of pseudotetrahedral d⁶ Fe imides,^11–12 DFT calculations on 6a, 1,^1c and related Fe imides⁶ show some degree of Fe–B _σ bonding. Quantifying the extent of Fe–B bonding in these complexes thermodynamically is difficult because the boranes are contained within the cage structures of the ligands.

The reactions of E–H bonds with aminoimides 6a and 6b were next examined. We were satisfied to observe that, in contrast to 1, the room temperature addition of 1.1 equiv PhSiH₃ to 6a readily generates a new, orange species identified as the trisilylhydrazido(−) product 7 resulting from hydrosilylation of the Fe–N bond with delivery of SiH₂Ph to N_α and H to B. To our knowledge, this is the first Fe hydrazido(−) complex to be derived from N₂, thereby adding to the body of previously-reported mononuclear Fe hydrazido(−) model complexes.¹³ Having established this elementary step, we sought to combine the formation of 6a with its subsequent hydrosilylation into a single procedure. Accordingly, 7 may be generated in one pot from 3a or 4 (eq. 1).

(1)

For complex 7, an intense IR signal corresponding to the Si–H stretch is observed at 2090 cm⁻¹ and a broad, intense IR stretch corresponding to the Fe–H–B functional group is observed at ca. 2000 cm⁻¹. The solution magnetic moment (_μeff = 5.0 _μB, C₆D₆, RT) indicates an S = 2 spin state. The N–N bond is elongated from 1.326 Å (avg.) in 6a to 1.492(4) Å in 7 (Figure 3). Although both distances are consistent with N–N single bonds, the comparatively short bond in 6a is due to the sp hybridization of N_α and some degree of N–N multiple bond character. The very long N–N bond in 7 (longer than that of free N₂H₄) is likely due to a high degree of steric pressure exerted by its bulky Si and Fe substituents. The sum of the CBC angles is 334°, reflecting the tetrahedral geometry of the borohydride ligand.

Figure 3.

Displacement ellipsoid (50%) representation of 7 (left) and 8 (right). PR₂ groups are truncated. H atoms not located in the difference map and solvent molecules are omitted for clarity.

Addition of 1 atm H₂ at 50 °C to a C₆H₆ solution of 6b results in a pale brown solution from which colorless solids can be isolated that are identified as the S = 2 product 8 (_μeff = 4.8 _μB, C₆D₆, RT). Its IR spectrum contains a peak assigned to an N–H stretch at 3343 cm⁻¹; the corresponding D₂ addition product shows _ν(N–D) at 2476 cm⁻¹ (2441 cm⁻¹ calc.). The broad, intense _ν(Fe–H-B) stretch is observed at ca. 2100 cm⁻¹ and is also sensitive to isotopic labeling (_ν(Fe-D–B) ≈ 1550 cm⁻¹). The connectivity of 8 was established by XRD analysis (Figure 3). In addition to the formation of new N–H and Fe–H–B bonds, the structure of 8 reflects cleavage of the N–N bond, rearrangement of the azadisilacyclopentane ring, and formal oxidation of one of the phosphines. This overall transformation is sufficiently complex that we felt it worthwhile to put forth a tentative mechanism (Scheme 4). The hydrogenation of 6b to form A is analogous to the formation of 7 from 6a. The hydrazine rearrangement step to form B has precedent for closely-related disilylhydrazines.¹⁴ Intermediate B may be sufficiently unencumbered to allow for _η²-NN binding to give C which could lead to direct N–N cleavage and group transfer to a phosphine. Alternatively, the N–N bond in C may be cleaved to form an Fe(IV) imide¹⁵ D which then undergoes group transfer to give 8. Cleavage of the N–N bond of N₂-derived ligands is rare for Fe^{1c, 4} and has been observed for bimetallic early metal complexes.^{2h, 16} In addition, the transformation of 6b to 8 is, to our knowledge, only the second example of N₂ functionalization with H₂ at a well-defined Fe complex.⁴

Scheme 4.

In conclusion, we have reported the generation of Fe aminoimides from N₂ that undergo subsequent addition of non-polar E–H bonds. The significant flexibility of the Fe–BPh interactions facilitate both the initial formation of the Fe aminoimide as well as the E–H activation step. Whereas previous functionalization reactions of terminal Fe–N₂ fragments allow for derivatization of N_β, this report demonstrates that E–H addition to an unsaturated Fe–N bond is a viable strategy for N_α functionalization.