Mononuclear Five- and Six-Coordinate Iron Hydrazido and Hydrazine Species

Abstract

This article describes the synthesis and characterization of several low-spin iron(II) complexes that coordinate hydrazine (N₂H₄), hydrazido (N₂H₃⁻), and ammonia. The sterically encumbered tris(di-meta-terphenylphosphino)borate ligand, [PhBP^mter₃]⁻, is introduced to provide access to species that cannot be stabilized with the [PhBP^Ph₃]⁻ ligand ([PhBP^R₃]⁻ = PhB(CH₂PR₂)₃⁻). Treatment of [PhBP^mter₃]FeMe with hydrazine generates the unusual 5-coordinate hydrazido complex [PhBP^mter₃]Fe(η²-N₂H₃) (1), in which the hydrazido serves as an L₂X-type ligand. Upon coordination of an L-type ligand, the hydrazido shifts to an LX-type ligand, generating [PhBP^mter₃]Fe(L)(η²-N₂H₃) (L = N₂H₄ (2) or NH₃ (3)). In contrast, treatment of [PhBP^Ph₃]FeMe with hydrazine forms the adduct [PhBP^Ph₃]Fe(Me)(η²-N₂H₄) (5). Complex 5 is thermally unstable to methane loss, generating intermediate [PhBP^Ph₃]Fe(η²-N₂H₃), which undergoes bimolecular coupling to produce {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₄)(µ-η²:η²-N₂H₂). The oxidation of these and related hydrazine and hydrazido species is also presented. For example, oxidation of 1 or 5 with Pb(OAc)₄ results in disproportionation of the N₂H_x ligand (x = 3, 4), and formation of [PhBP^R₃]Fe(NH₃)(OAc) (R = Ph (9) and mter (11)).

Introduction

An area of ongoing research is geared towards elucidating the mechanism by which N₂ is reduced to NH₃ at the FeMo-cofactor of nitrogenase.¹ Recent spectroscopic studies of the enzyme acquired under turnover conditions suggest that N₂ initially coordinates an iron center.² Though the mechanism of subsequent N₂ reduction remains unknown, the ability of the cofactor to reduce both diazene and hydrazine implicates that an alternating reduction scheme may be viable.³ In this mechanistic scenario, the delivery of protons and electrons alternates between the two nitrogen atoms (i.e. N≡N → HN=NH → H₂N-NH₂ → 2NH₃). To explore the chemical feasibility of such a mechanistic scheme, model complexes that coordinate N₂H_x ligands are warranted.

In addition to their proposed role in N₂ reduction, hydrazine (N₂R₄), hydrazido (N₂R₃⁻), and hydrazido(2-) (N₂R₂⁻) species have also been invoked as reactive intermediates in several synthetic transformations, including the hydrohydrazination and diamination of alkynes.⁴ In light of this, most studies on M(N₂R₃) species feature substituted hydrazido ligands,⁵ and a comparatively small subset feature the parent hydrazido (N₂H₃⁻) functionality.⁶ This relative scarcity may also be due in part to the different inherent stabilities of M(N₂R_x) and M(N₂H_x) species.^6l,7

Terminal hydrazido species of the type M(η¹-N₂H₃) were first prepared in the 1970’s by both Chatt^6a and Hidai^6b (M = W). Since then, examples of Mo,^6k Re,^6c Fe,^6d,6n and Ru⁶ⁿ M(η¹-N₂H₃) complexes have been characterized. Side-on hydrazido species, M(η²-N₂H₃), are also uncommon; examples are known for W,^6e–g Re,^6h Co,⁶ⁱ and Fe.^6j,8 With regards to iron complexes in particular, 6-coordinate and low-spin,⁶ⁿ as well as 5-coordinate and intermediate-spin^6d Fe(η¹-N₂H₃) species have been characterized. Six-coordinate and low-spin Fe(η²-N₂H₃) species are also known.^6j,8 Despite the differences in the iron complexes, NMR and structural data suggest that σ-bonding interactions dominate between the iron and hydrazido nitrogen atoms (vide supra). Known examples of iron hydrazido complexes have not exhibited multiple-bond character between the iron and hydrazido nitrogen(s).

As part of our group’s ongoing efforts to study the multi-electron reactivity of mono- and diiron complexes that feature nitrogenous ligands,^9,10 we recently turned to (N₂H_x)ⁿ⁻ (x = 2–4; n = 0, −1, −2) ligated species of iron.^6d,8,11,12 We found that the bridging hydrazine in {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₄)(µ-η²:η²-N₂H₂) undergoes clean oxidation to diazene, generating {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₂)(µ-η²:η²-N₂H₂) ([PhBP^R₃]⁻ = [PhB(CH₂PR₂)₃]⁻) (Scheme 1).¹² Subsequent oxidation results in N₂ loss and formation of {[PhBP^Ph₃]Fe}₂(µ-NH₂)₂. We wanted to determine if similar transformations could be achieved at monomeric Fe(N₂H₄) and Fe(N₂H₃) species to form monomeric Fe(N₂H₂) and Fe(N₂H) species, respectively.

Scheme 1.

Oxidation of {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₄)(µ-η²: η²-N₂H₂).

Herein we report the synthesis, characterization, and subsequent oxidation reactions of a series of monomeric hydrazido and hydrazine complexes of iron(II). These complexes are ligated by a tris(phosphino)borate ligand that is either phenyl or meta-terphenyl substituted at the phosphines; the synthesis of the latter ligand scaffold is described. Distinct iron hydrazido species are isolated depending on the nature of the auxiliary ligand employed, including 5-coordinate species that feature multiple-bond character between the iron and hydrazido ligand. The oxidation of these hydrazido and related hydrazine species of iron is also discussed. In most instances, treatment of these monomeric Fe(N₂H_x) species with oxidizing reagents results in disproportionation of the (N₂H_x)ⁿ⁻ ligand, affording Fe(NH₃) species. This reactivity contrasts that observed at diiron centers.^8,11–12

Results and Discussion

Synthesis and Characterization of [PhBP^mter₃]Fe-X Species (X = Cl, Me)

In order to prevent formation of N₂H_x bridging diiron species as discussed above,^8,12 a new, more sterically encumbering [PhBP^R₃]⁻ ligand variant was sought.¹³ The incorporation of a bulky terphenyl substituent into a ligand scaffold has successfully been used by others to prepare and stabilize monomeric and/or coordinatively unsaturated metal complexes.¹⁴ Thus, to achieve vertical bulk above the metal center while keeping the steric congestion about the metal similar to that of the related [PhBP^Ph₃]⁻ligand, the [PhBP^mter₃]⁻ variant was targeted (mter = meta-terphenyl = 3,5-terphenyl).

The synthesis of the ligand [PhBP^mter₃]Tl is readily achieved following a similar synthetic protocol to that employed for [PhBP^Ph₃]Tl (Scheme 2).^13c,d The precursor phosphine (m-terphenyl)₂PMe is prepared in 84 % yield by lithium-halogen exchange of m-terphenyl bromide with nBuLi at −78 °C, followed by quenching with half an equivalent of MePCl₂. Subsequent deprotonation with ^sBuLi at −78 °C in the presence of TMEDA affords the phosphine carbanion, (m-terphenyl)₂P(CH₂)Li(TMEDA) in 61 % yield. Addition of three equivalents of the carbanion to PhBCl₂ and subsequent exposure to one equiv of [Tl](PF₆) gives the desired ligand, [PhBP^mter₃]Tl, which is isolated as a white powder in 62 % yield (32 % over three steps).¹⁵

Scheme 2.

Synthesis of [PhBP^mter₃]Tl and [PhBP^mter₃]Fe-X species.

Likewise, the syntheses of the Fe(II) complexes, [PhBP^mter₃]FeCl and [PhBP^mter₃]FeMe, are achieved using similar protocols to those used for the syntheses of [PhBP^Ph₃]FeCl^9b and [PhBP^Ph₃]FeMe¹² (Scheme 2). Thus, mixing of [PhBP^mter₃]Tl with FeCl₂ affords yellow and high-spin [PhBP^mter₃]FeCl (83 % yield), and treatment of [PhBP^mter₃]FeCl with excess Me₂Mg in benzene results in formation of amber and high-spin [PhBP^mter₃]FeMe (79 % yield).

The differences in both the steric and electronic parameters between the m-terphenyl and phenyl substituted ligands can be determined by comparison of the [PhBP^R₃]FeCl species (R = mter, Ph). The solid-state structures of [PhBP^mter₃]FeCl and [PhBP^Ph₃]FeCl^9b have been obtained, and space-filling renditions are shown in Figure 1. The m-terphenyl substituents clearly add vertical protection, as the chlorine atom no longer extends beyond the pocket of the aryl substituents (Figure 1, top). As the m-terphenyl substituents are not locked in a rigid position and are free to rotate, the congestion about the iron center is similar in both species (Figure 1, bottom).

Figure 1.

Space filling models of [PhBP^Ph₃]FeCl (left) and [PhBP^mter₃]FeCl (right). The representation perpendicular to the B-Fe-Cl vector (top) highlights the added vertical steric protection that the bulkier [PhBP^mter₃] ligand provides relative to that of the [PhBP^Ph₃] ligand. The representation parallel to the B-Fe-Cl vector (bottom) indicates that the two ligand scaffolds give a similar level of steric congestion about the iron. Cl are shown in yellow, Fe in blue, P in red, C in grey, H in white, and B in orange.

The cyclic voltammogram (CV) of [PhBP^mter₃]FeCl features an irreversible reduction at −1.52 V vs. Fc/Fc⁺ that is very close to the analogous reduction observed in the CV of [PhBP^Ph₃]FeCl. For comparison, these reductions are ca. 0.4–0.5 V more positive than those for the alkyl substituted complexes, [PhBP^R₃]FeCl (R = ⁱPr, CH₂Cy).^13a,b Combined, these studies suggest that the two ligand scaffolds have similar electron-donating capabilities, yet different steric properties.

Synthesis and Characterization of Monomeric Fe(η²-N₂R’₃) Species

An attractive synthetic route to hydrazido species is the direct deprotonation of hydrazine by a metal alkyl species.^6m,12 Indeed, the room temperature addition of one equiv of hydrazine to [PhBP^mter₃]FeMe results in the formation of green and diamagnetic [PhBP^mter₃]Fe(η²-N₂H₃) (1), with concomitant release of methane (Scheme 3).

Scheme 3.

Synthesis of [PhBP^mter₃]Fe(N₂H₃) species.

The identity of 1 as a hydrazido species is made on the basis of elemental analysis and NMR spectroscopy. The room temperature NMR spectra of 1 display broad peaks that sharpen upon cooling to −25 °C, in accordance with an S = o ground state. The ¹H NMR spectrum (−25 °C, THF-d₈) of ¹⁵N-1 (prepared by treatment of [PhBP^mter₃]FeMe with ¹⁵N₂H₄) features broad doublets centered at 6.43 (1H) and 3.81 (2H) ppm. In the corresponding ¹⁵N NMR spectrum (−25 °C, THF-d₈), the NH₂ nitrogen resonates at −14.5 ppm (dt, ¹J_NH ≈ 83 Hz, ¹J_NN ≈ 11 Hz), while the NH nitrogen resonates at 139.0 ppm (dd, ¹J_NH ≈ 79 Hz, ¹J_NN ≈ 11 Hz) (Figure 2a). The ³J_HH and the ²J_NH coupling could not be resolved in either the ¹H or ¹⁵N NMR spectra.

Figure 2.

¹⁵N NMR spectra (d₈-THF) of (a): 1 (−25 °C) and (b): 2 (−50 °C). The simulation of the spectrum of 2 is shown above the experimental spectrum. Simulation parameters: δ 47.4 (¹J_NN = −11 Hz, ¹J_NH = 65 Hz), 40.6 (¹J_NN = −13 Hz, ¹J_NH = 58 Hz), 23.6 (¹J_NN = −11 Hz, ¹J_NH = 73 Hz, ¹J_NH = 73 Hz), 22.7 (¹J_NN = −13 Hz, ¹J_NH = 80 Hz, ¹J_NH = 80 Hz), linewidth: 15 Hz. (c) Possible isomers of 2. Equivalent pairs of atoms are shown in color.

The ³¹P NMR spectrum of 1 (−25 °C, THF-d₈) features a single resonance at 89.1 ppm. The equivalence of the three phosphines suggests that the iron center in 1 is either 4-coordinate and pseudotetrahedral, [PhBP^mter₃]Fe(η¹-N₂H₃), or 5-coordinate and fluxional, [PhBP^mter₃]Fe(η²-N₂H₃). As 4-coordinate [PhBP^R₃]Fe^II-X species display high-spin S = 2 electronic configurations in the absence of an Fe≡X triple bond linkage,^9b,10,16 1 is most likely 5-coordinate in solution.

Though hydrazido 1 is stable both in solution and in the solid-state, the open coordination site readily binds L-type ligands. For example, addition of one equiv of N₂H₄ or NH₃ to 1 results in a color change from green to orange, and formation of [PhBP^mter₃]Fe(η²-N₂H₃)(η¹-N₂H₄) (2) or [PhBP^mter₃]Fe(η²-N₂H₃)(NH₃) (3), respectively (Scheme 3). Coordination of either N₂H₄ or NH₃ to 1 is reversible, and exposure of either 2 or 3 to vacuum quantitatively regenerates 1. Hydrazine/hydrazido 2 is not stable in solution, and over the course of days, hydrazine disproportionation ensues, converting 2 to ammonia/hydrazido 3, and presumably 0.5 equiv of N₂.

The assignment of 2 as [PhBP^mter₃]Fe(η²-N₂H₃)(η¹-N₂H₄), whereby the hydrazine coordinates end-on and the hydrazido side-on, was made by NMR spectroscopy. Such an isomer should give rise to four distinct ¹⁵N NMR resonances, three distinct ³¹P NMR resonances, and at least six distinct NH_x ¹H NMR resonances (the asymmetry about the iron center induced by the η²-N₂H₃ ligand should render the hydrazine N_αH₂ inequivalent; Figure 2c left). An alternative assignment of 2, whereby the hydrazido coordinates end-on and the hydrazine side-on, should give rise to three distinct ¹⁵N NMR resonances (the hydrazine nitrogen atoms are now equivalent, vide infra), two distinct ³¹P NMR resonances, and at most five distinct NH_x resonances in the ¹H NMR spectrum (Figure 2c right). As the ¹⁵N NMR spectrum of ¹⁵N-2 (−50 °C, THF-d₈) features four resonances between 21 and 48 ppm (Figure 2b), 2 must be [PhBP^mter₃]Fe(η²-N₂H₃)(η¹-N₂H₄). Further, the ¹H NMR spectrum of ¹⁵N-2 (−50 °C, THF-d₈) features six distinct NH_x resonances, and the corresponding ³¹P NMR spectrum features three distinct resonances.

Select ¹H{¹⁵N} decoupling was employed to correlate the ¹⁵N/¹H signals (see SI) allowing for assignment of the hydrazine and hydrazido chemical shifts (Table 1). Briefly, the doublet at 40.6 ppm in the ¹⁵N NMR spectrum of ¹⁵N-2 is assigned as the hydrazido NH. The triplet centered at 47.4 ppm is assigned as the N_βH₂ of the coordinated hydrazine, on the basis that: (i) select decoupling of this resonance results in collapse of a single NH doublet (that integrates to 2H) in the ¹H{¹⁵N} spectrum, indicating that this NH₂ is not affected by the asymmetry about the iron center, and (ii) the chemical shift is close to that of free hydrazine (ca. 50 ppm under similar conditions, see SI). The overlapping signals at 22.7 and 23.6 thus correspond to the hydrazine N_αH₂ and the hydrazido NH₂; select decoupling of these resonance results in collapse of four NH doublets (1H each) in the ¹H{¹⁵N} spectrum, consistent with the asymmetry about the iron center.

Table 1.

NMR and structural parameters for Fe(η ²-N₂H_x) Species (x = 2, 3, 4).

Compound	15N NMR chemical shift (δ)a	1H NMR chemical shift (δ)	Fe-N bond distance (Å)	Ref
{cis-[Fe(N2H4)(dmpe)2]}{BPh4}2d	−11.9	5.39, 4.69	1.981(2), 2.003(2)	18
{cis-[Fe(N2H4)(DMeOPrPE)2]}{BPh4}2e	−19.9b	4.8, 3.8	1.993(2), 2.006(2)	6j,19
[PhBPPh3]Fe(Me)(N2H4) (5)f	17.1	4.33, 3.13	--	this work
{[PhBPPh3]Fe(NH3)(N2H4)}{PF6} (10)	--	--	2.006(2), 2.025(3)	this work
{[PhBPPh3]Fe(CO)(N2H4)}{PF6} (13)	--	5.48, 2.90	1.984(4), 2.005(3)	this work
{cis-[Fe(N2H3)(DMeOPrPE)2]}{BPh4}g	8.4/6.1 (NH)b,c	1.05/0.65 (NH)c	--	6j
	−1.4 (NH2)b	4.23/4.14 (NHH)c
		3.66/3.44 (NHH)c
[PhBPmter3]Fe(N2H3) (1)h	139.0 (NH), −14.5 (NH2)	6.43 (NH), 3.81 (NH2)	--	this work
[PhBPmter3]Fe(N2H3)(η1-N2H4) (2)i	40.6 (NH), 22.7, 23.6 (NH2NαH2), 47.4 (NβH2)	3.18 (NH), 2.52 – 4.66 (NH2)	--	this work
[PhBPmter3]Fe(N2H3)(NH3) (3)j	31.8 (NH), 26.0 (NH2), −18.9 (NH3)	1.83 (NH), 5.32 (NHH), 3.58 (NHH), 0.41 (NH3)	2.003(2) (Fe-NH) 1.969(2) (Fe-NH2)	this work
[PhBPPh3]Fe(CO)(NHNH2) (6)k	32.2 (NH), 31.8 (NH2)	2.85 (NH), 1.88 (NHH), 6.55 (NHH)l	1.992(3), 2.018(3)	8
[PhBPPh3]Fe(NHNMe2) (4)	--	4.00	1.788(2) (Fe-NH)	this work
			2.058(2) (Fe-NMe2)
cis-[Fe(N2H2)(dmpe)2]d	65.3b	2.04	2.016(5), 2.032(7)	18
cis-[Fe(N2H2)(DMeOPrPE)2]e	60.8b	2.1	--	6j

^aChemical shifts are referenced to liquid ammonia at 0 ppm.

^bConverted from the nitromethane referencing scale. The chemical shift of nitromethane was taken as 376 ppm relative to liquid ammonia.

^cThe two chemical shifts correspond to different isomers.

^ddmpe = 1,2-bis-(dimethylphosphino)ethane.

^eDMeOPrPE = 1,2-bis[(methoxypropyl)phosphino]ethane.

^fNMR collected at −50 °C.

^gNMR collected at −85 °C.

^hNMR collected at −25 °C.

ⁱNMR collected at −40 °C.

^jNMR collected at −45 °C.

^kNMR collected at −75 °C.

^lDue to H-bonding, the chemical shift of this proton is highly dependent on solvent, concentration, and temperature.

The ¹⁵N and ¹H NMR chemical shifts for Fe(η²-N₂H_x) species (x = 2, 3, 4) are summarized in Table 1. Most of these species have ¹⁵N NMR chemical shifts that are in the range of free hydrazines and amines, and hence are consistent with sp³-hybridized nitrogen atoms.¹⁷ For reference, free hydrazine resonates around 50 ppm, and ammonia resonates at 0 ppm.

With the exception of 1, similar chemical shifts are observed for both types of hydrazido nitrogen atoms (i.e. NH and NH₂) for the Fe(η²-N₂H₃) species. These complexes are all 6-coordinate and low-spin iron(II), and hence similar coordination shifts associated with each ligand type are anticipated.¹⁷ The hydrazido ligand is an LX-type 3 e⁻ donor, giving rise to an 18-electron iron center. Thus, the low-field chemical shift for the hydrazido N H nitrogen atom of 1 is unusual, and suggests a different bonding scenario. The data are consistent with sp²-hybridization of the hydrazido NH, which would allow for the hydrazido to serve as an L₂X-type 5 e⁻ donor. Schrock noted a similar discrepancy in the hydrazido NH/NH₂ resonances of WCp*Me₃(η²-N₂H₃)⁺ (NH: 241.26 ppm, NH₂: 30.97 ppm),^6e which has been attributed to formation of a π bond between the hydrazido NH nitrogen and the W center (structural data corroborates this assignment). While such a bonding scenario is typical for M(η²-N₂R₃) species of the early-to-mid transition metals,^{4a,5a,5b,5e,6e,7c} the high d-electron counts for late transition metals usually preclude this coordination mode; Huttner’s d⁷ L₃Co(η²-N₂R₃)⁺ species is an exception.⁶ⁱ

Though we were unable to obtain crystals of 1 suitable for XRD, the solid-state structure of a related species, [PhBP^Ph₃]Fe(η²-NHNMe₂) (4), was obtained. This complex is readily prepared by addition of one equiv of NH₂NMe₂ to a benzene solution of [PhBP^Ph₃]FeMe (Scheme 4). On the basis of the similarities between the ³¹P NMR chemical shifts and the UV-vis spectra of 1 and 4, the coordination mode of the hydrazido ligand is inferred to be the same in both complexes.

Scheme 4.

Synthesis of [PhBP^Ph₃]Fe(N₂R_x) species.

The solid-state structure of 4 is shown in Figure 3. The geometry about the Fe center is best described as distorted trigonal bipyramidal, with P₁, P₃, and N₁ comprising the equatorial plane. The sum of the angles about N₁ is 352°, indicating a nearly planar sp²-hybridized nitrogen atom. The Fe-NMe₂ distance of 2.058(2) Å is similar to the Fe-N(sp³) bond distances observed in other hydrazido and hydrazine species of iron (Table 1, vide infra), and the Fe-NH bond distance of 1.788(2) Å is significantly shorter. For comparison, the average Fe-N bond distance in the low-spin imido species, [PhBP^Ph₃]Fe(NAr)⁻, which features a bona fide Fe≡N triple bond, is 1.6578(2) Å,^9b and the average Fe-N bond distance in {[PhBP^Ph₃]Fe(CO)}₂(µ-N₂H₂), which features Fe=N bonding, is 1.83 Å.⁸ As for 1, the Fe-NH functionality in {[PhBP^Ph₃]Fe(CO)}₂(µ-N₂H₂) lies in the equitorial plane defined by Fe1, P1, and P3, allowing for favorable π-overlap. Thus, the metrical parameters of 1 suggest the presence of an Fe=N bond in 1.

Figure 3.

Solid-state structure (50% displacement ellipsoids) of 4 (left) and 3 (right). Most hydrogen atoms, solvent molecules, and minor components of disorder have been removed for clarity. Protons directly coordinated to nitrogen were located in the difference map and are shown. Select bond distances (Å) and angles (deg) for 4: Fe1-N1 1.788(2), Fe1-N2 2.058(2), Fe1-P1 2.2054(6), Fe1-P2 2.1775(6), Fe1-P3 2.1777(6), N1-N2 1.423(2), N1-Fe1-N2 42.72(6), P1-Fe1-P2 90.52(3), P1-Fe1-P3 91.23(2), P2-Fe1-P3 90.09(3), N1-Fe1-P1 139.71(5), N1-Fe1-P2 147.07(4), N1-Fe1-P3 110.92(5), N2-Fe1-P1 113.16(5), N2-Fe1-P2 147.07(4), N2-Fe1-P3 110.92(5). Select bond distances (Å) and angles (deg) for 3: Fe1-N1 2.076(2), Fe1-N2 2.003(2), Fe1-N3 1.969(2), Fe1-P1 2.2188(7), Fe1-P2 2.1963(7), Fe1-P3 2.2174(7), N2-N3 1.418(3), N2-Fe1-N3 67.8(1).

The solid-state structure of 6-coordinate hydrazido/ammonia 3 was also obtained, and is shown in Figure 3. Though the overall quality of the dataset of 3 is compromised by heavily disordered solvent molecules, all of the protons directly coordinated to nitrogen atoms were located in the difference map and were refined semi-freely with the aid of distance restraints.²⁰ The structure clearly establishes the presence of η²-N₂H₃ and NH₃ ligands coordinating the iron center, with several of the N-coordinated protons engaging in hydrogen bonds to THF solvent molecules that co-crystallize with 3. The Fe-NH₃ distance of 2.076(2) Å is consistent with that of other low-spin Fe-NH₃ complexes.^11,21 The similar Fe-NH and Fe-NH₂ distances of 2.003(2) Å and 1.969(2) Å, respectively, are close to those observed in [PhBP^Ph₃]Fe(CO)(η²-N₂H₃),⁸ and is in contrast to the disparity in bond distances observed in 4. Thus, upon coordination of an L-type ligand, the change in geometry from trigonal bipyramidal to octahedral disrupts the Fe=N bonding observed in 1 and 4. The 18-electron configuration at the iron center is maintained and now both nitrogen atoms are sp³-hybridized, and the hydrazido acts as an LX-type ligand.

Synthesis and Characterization of Monomeric Fe(N₂H₄) Species

In contrast to the reaction between [PhBP^mter]FeMe and hydrazine, the room temperature addition of one equiv of hydrazine to the sterically less encumbering [PhBP^Ph]FeMe species results in quantitative formation of the diiron species {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₄)(µ-η²:η²-N₂H₂) (Scheme 4).¹² To establish whether this reaction proceeds through an intermediate hydrazido species akin to 1, the reaction between [PhBP^Ph]FeMe and hydrazine was monitored by VT NMR spectroscopy. Upon addition of hydrazine at −78 °C, an initial hydrazine adduct [PhBP^Ph]Fe(Me)(η²-N₂H₄) (5) forms, as ascertained by NMR spectroscopy. The ¹⁵N NMR spectrum (−50 °C, THF-d₈) of ¹⁵N-5 displays a single triplet at 17.3 ppm (¹J_NH ≈ 76 Hz), similar to that of other Fe(η²-N₂H₄) species (Table 1). In the corresponding ¹H NMR spectrum of ¹⁵N-5, a broad singlet corresponding to the Me protons is noted at −0.2 ppm, as well as two broad doublets at 4.33 and 3.13 ppm (2H each, ¹J_NH ≈ 77, 75 Hz, respectively). These resonances correspond to the hydrazine protons that are cis and trans to the Me group, respectively, as determined by a NOESY experiment.

The VT NMR profile of 5 establishes that this species is stable in solution below −30 °C. At this temperature, resonances ascribed to methane and {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₄)(µ-η²:η²-N₂H₂) begin to grow in. Though the postulated “[PhBP^Ph₃]Fe(N₂H₃)” species cannot be detected in the 1H or ¹⁵N NMR spectra, a single sharp resonance is observed at 84.0 ppm in the ³¹P NMR spectrum, similar to that of 1 and 4. Though this species does not appreciably build up in solution, it can be trapped with CO to give [PhBP^Ph₃]Fe(CO)(η²-N₂H₃) (6) (Scheme 4).⁸

The synthesis of thermally stable 5- and 6-coordinate iron hydrazine complexes was also explored. Following a similar protocol to that employed by both Tyler¹⁹ and Field,¹⁸ one equivalent of hydrazine was added to a THF solution of [PhBP^Ph₃]FeCl in the presence of [Tl](PF₆) to generate {[PhBP^Ph₃]Fe(η²-N₂H₄)}(X) (7) (X = Cl, PF₆) (Scheme 5). This reaction is hampered by an equilibrium between [PhBP^Ph]FeCl and {[PhBP^Ph₃]Fe(η²-N₂H₄)}(X). Addition of excess hydrazine or [Tl](PF₆) to the equilibrium mixture results in precipitation of an unidentified but presumably iron containing species and formation of free Ph₂PMe or [PhBP^Ph₃]Tl, respectively. Similar results were obtained when [Na](BPh₄) was used as the halide abstractor. The hydrazine species 7 could hence not be obtained in analytically pure form, as the chloride and hexafluoro-phosphate salts co-crystallize. Nonetheless, a structure of 7 has been obtained (see SI). The disorder present was satisfactorily modeled and the structure established connectivity and a 5-coordinate square pyramidal geometry for {[PhBP^Ph₃]Fe(η²-N₂H₄)}(PF₆).

Scheme 5.

Synthesis of [PhBP^Ph₃]Fe(N₂H₄) and {[PhBP^CH2Cy₃]Fe}₂(µ-N₂H₄)species.

An end-on coordinated hydrazine species of iron was also targeted. Treatment of [PhBP^Ph₃]FeMe with one equiv of AcOH, followed by addition of one equiv of hydrazine, results in clean formation of [PhBP^Ph₃]Fe(OAc)(η¹-N₂H₄) (8) (Scheme 5). Now, the acetate enforces an end-on coordination of the hydrazine, as shown in the solid-state structure of 8 (Figure 4). This coordination mode is preserved in solution, and two chemical shifts for the hydrazine N_αH₂ and N_βH₂ nitrogen atoms are respectively noted at 33.3 and 56.2 ppm in the ¹⁵N NMR spectrum of ¹⁵N-8 (−50 °C, THF-d₈). This assignment is made by analogy to the chemical shifts of the hydrazine ligand noted in 3, as well as those noted in a related Fe(η¹-N₂H₄) species, in which the N_α nitrogen atom resonates at higher field than that the N_β nitrogen atom.⁶ⁿ In the corresponding ¹H NMR spectrum (−50 °C, THF-d₈), the N_αH₂ protons resonate at 4.61 ppm and the N_βH₂ protons resonate at 3.94 ppm.

Figure 4.

Solid-state structures (50% displacement ellipsoids) of 8 (a), 9 (b), and the cation of 10 (c). Hydrogen atoms, solvent molecules and minor components of disorder have been removed for clarity. The (PF₆) counteranion of 10 is not shown. The protons directly coordinated to the nitrogen atoms were located in the difference map and are shown. Select bond distances (Å) for 8: Fe1-N1 2.071(2), N1-N2 1.450(3). Select bond distances (Å) for 9: Fe1-N1 2.064(1). Select bond distances (Å) and angles (deg) for 10: Fe1-N1 2.006(2), Fe1-N2 2.025(3), Fe1-N3 2.076(2), N1-N2 1.451(3), N1-Fe1-N2 42.20(9), N1-Fe1-N3 85.14(9), N2-Fe1-N3 86.3(1).

Complex 8 is similar to the diiron species, {[PhBP^CH2Cy₃]Fe(OAc)}₂(µ-η¹:η¹-N₂H₄), which is prepared by addition of hydrazine to the sterically less encumbered [PhBP^CH2Cy₃]Fe(OAc) (Scheme 5).¹¹ Addition of sub-stoichiometric equivalents of hydrazine to [PhBP^Ph₃]Fe(OAc) does not result in formation of a hydrazine bridged dimer akin to {[PhBP^CH2Cy₃]Fe(OAc)}₂(µ-η¹:η¹-N₂H₄), suggesting that this species is not accessible for steric reasons. Whereas solutions of 8 are stable towards excess hydrazine, the diiron analogue {[PhBP^CH2Cy₃]Fe(OAc)}₂(µ-η¹:η¹-N₂H₄) facilitates hydrazine disproportionation, and mixtures of {[PhBP^CH2Cy₃]Fe(OAc)}₂(µ-η¹:η¹-N₂H₂) and [PhBP^CH2Cy₃]Fe(OAc)(NH₃) are obtained. As hydrazine disproportion is facilitated by electron-rich metal centers,²² the heightened stability of 8 relative to {[PhBP^CH2Cy₃]Fe(OAc)}₂(µ-η¹:η¹-N₂H₄) towards hydrazine disproportionation is likely due to the hydrazine in 8 coordinating a single iron, and the iron being ligated by a less electron donating tris(phosphino)borate ligand.

Exploring the Oxidation of Hydrazine Species

Diazene coordinated metal species are uncommon, yet are attractive synthetic targets in light of their postulated role in N₂ reduction.^1a,3d The instability of free diazene precludes its use as a reagent for the direct synthesis of M(N₂H₂) species.²³ Rather, 6-coordinate M₂(µ-η¹:η¹-N₂H₂) species (M = Fe,^12,24 Ru,²⁵ Cr,²⁶ Mn,²⁷ Cu²⁸) and M(η¹-N₂H₂) species (M = W,²⁹ Re,³⁰ Ru,³¹ Os³¹) are prepared via oxidation of a coordinated hydrazine ligand. The hydrazine species described above may therefore be expected to serve as precursors to Fe(η¹-N₂H₂) species.³² In the present system, an alternative reaction occurs, and oxidation results in disproportionation of the Fe(N₂H₄) fragment to give L₅-Fe^II(NH₃) species.

Treatment of 5-coordinate hydrazine 7 with one equiv of Pb(OAc)₄ results in a color change from pink to purple, and formation of [PhBP^Ph₃]Fe(OAc)(NH₃) (9) (Scheme 6). Presumably, 0.5 equiv of diazene or N₂ also forms in the reaction. Likewise, treatment of 6-coordinate 8 with Pb(OAc)₄ also results in formation of 9. This species has been characterized by NMR and IR spectroscopies, as well as EA and XRD, and the solid-state structure of 9 is shown in Figure 4.

Scheme 6.

Oxidation of [PhBP^Ph₃]Fe(N₂H₄) species.

Hydrazine adduct 5 undergoes a related transformation upon oxidation. As for 7 or 8, treatment of 5 (−78 °C to RT) with one equiv Pb(OAc)₄ results in formation of 9 (Scheme 6). Similarly, in the presence of one equiv N₂H₄, oxidation of 5 by [Fc](PF₆) results in formation of the 6-coordinate ammonia/hydrazine complex, {[PhBP^Ph₃]Fe(NH₃)(η²-N₂H₄)}{(PF₆)} (10). In the absence of N₂H₄, the reaction between 5 and [Fc](PF₆) is ill-defined. As both reactions result in formation of ammonia (and presumably 0.5 equiv N₂/N₂H₂) the reactions likely proceed via a similar oxidation mechanism. When 5 was instead treated with quinone oxidants, no N-containing Fe products were obtained (see SI).

Thus, oxidation of the hydrazine monomers 5–7 results in hydrazine disproportionation and isolation of ammonia species 9 or 10 (Scheme 6). The binding mode of the hydrazine/coordination number at iron does not appear to impact the reactivity. This contrasts with the Pb(OAc)4 oxidation of the diiron species {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₄)(µ-η²:η²-N₂H₂), which results in formation of the diazene species, {[PhBP^Ph₃]Fe}₂(µ-η¹:η¹-N₂H₂)(µ-η²:η²-N₂H₂) (Scheme 1).

Exploring the Oxidation of Hydrazido Species

Though there are no examples of iron species that are coordinated by the parent diazenido ligand (Fe-N₂H), complexes of the type Fe(η¹-N₂R) can be accessed via alkylation or silylation of a precursor dinitrogen complex Fe(η¹-N₂R),^9g,33 or by deprotonation of a hydrazine species Fe(η¹-N₂H₃Ph) (with concomitant H₂ release).^6d It is anticipated that hydrazido oxidation may be a viable synthetic route to diazenido species, by analogy to hydrazine oxidation to give diazene species. However, this reactivity is not observed in the present system.

Oxidation of hydrazido species 1 with one equiv of Pb(OAc)₄ results in a color change from green to purple, and formation of the ammonia complex, [PhBP^mter₃]Fe(OAc)(NH₃), (11) (Scheme 7). This species is also formed in the reaction of hydrazine/hydrazido 2 with Pb(OAc)₄. When 1 or 2 is instead treated with a quinone oxidant, no N-containing iron species are obtained (see SI).

Scheme 7.

Oxidation of [PhBP^mter₃]Fe(N₂H₃) species.

In contrast to the aforementioned reactivity, in which the coordinated hydrazine or hydrazido ligand is converted to ammonia, no N-N bond cleavage is observed upon oxidation of hydrazido 6 (Scheme 8). When Pb(OAc)₄ is added to solutions of 6, no cationic ammonia species akin to 9 or 10 is isolated; rather [PhBP^Ph₃]Fe(CO)(OAc) (12) forms. When 6 is alternatively treated with one equiv of [Fc](PF₆), the cationic hydrazine species {[PhBP^Ph₃]Fe(CO)(η²-N₂H₄)}{PF₆} (13) is cleanly generated along with ferrocene (as deduced by ¹H NMR spectroscopy). Though the net transformation of 6 to 13 is protonation, the formation of ferrocene suggests that the reaction may proceed via an oxidized intermediate, “[PhBP^Ph₃]Fe(CO)(η²-N₂H₃)^•+”, that then abstracts an H-atom from an unknown source to give hydrazine (H₂NHN-H BDFE(aq) = 83.4 kcal mol⁻¹).³⁴ The distinct reactivity of 6 relative to 1 or 2 upon oxidation may be due in part to the presence of the carbonyl ligand, which blocks a coordination site and also modulates the electronic structure of the iron center. Curiously, benzene solutions of 6 react with 0.5 equiv O₂ to generate the diiron species, {[PhBP^Ph₃]Fe(CO)}₂(µ-η¹:η¹-N₂H₂).⁸ This reaction does not proceed in THF, which appears to hydrogen bond to a hydrazido proton (deduced by NMR spectroscopy),⁸ and may serve to prevent the hydrazido ligand in 6 from engaging in redox and/or acid/base chemistry. The distinct products obtained upon oxidation of 6 underscores the rich redox chemistry of hydrazido species.

Scheme 8.

Reactivity of [PhBP^Ph₃]Fe(CO)(N₂H₃) towards various oxidants.

Concluding Remarks

With this report, we have extended our study of the chemistry of hydrazine and hydrazido coordinated iron(II) complexes to include low-spin monomeric species. The coordination chemistry of N₂H₃⁻ remains relatively scarce, with most examples involving high-valent early metals for both parent and substituted hydrazido ligands. Here we show that when the iron center is in a 6-coordinate environment, the hydrazido ligand acts as an LX-type ligand, with the lone-pair of the sp³-hybridized NH nitrogen atom not engaging in bonding interactions with the metal. In contrast, in a 5-coordinate environment the iron center adopts a trigonal bipyramidal geometry that allows for Fe=N bonding between the Fe center and an sp²-hybridized NH nitrogen atom. This latter coordination mode of N₂R₃⁻ was previously not known for iron and is rare for late transition metals. In the absence of structural data, the coordination mode is readily discernible by ¹⁵N NMR spectroscopy; a downfield shift is observed for the sp²-hybridized NR nitrogen relative to the NR₂ nitrogen atom. In contrast, similar ¹⁵N NMR chemical shifts are observed for both NR and NR₂ when both nitrogen atoms of the hydrazido ligand are sp³-hybridized.

Oxidation of the monomeric iron hydrazine complexes invariably results in disproportionation, and ammonia complexes of iron are isolated. These results contrast with the reactivity that we previously described for a diiron species, whereby oxidation occurs via formal loss of two H-atoms to generate a diazene species.

Similarly, the oxidations of hydrazido species 1 and 2 also result in isolation of an ammonia species. In contrast, the oxidation of carbonyl hydrazido 6 does not yield ammonia; the cationic hydrazine species 13 is isolated upon [Fc](PF₆) oxidation, the carbonyl acetate 12 is isolated upon oxidation with Pb(OAc)₄, and the bridging diazene complex {[PhBP^Ph₃]Fe(CO)}₂(µ-η¹: η¹-N2H₂) is formed upon O₂ oxidation.

Collectively, the diverse reactivity observed upon oxidation of hydrazine and hydrazido ligated iron and diiron species underscores the many redox pathways possible for N_xH_y species.

Experimental Section

General Considerations

All manipulations were carried out using standard Schlenk or glove-box techniques under a dinitrogen atmosphere. Glasswear was oven dried for 12 h (180 °C). Unless otherwise noted, solvents were deoxygenated and dried by sparging with Ar followed by passage through an activated alumina column from S.G. Water (Nashua, N.H.). Non-halogenated solvents were tested with a standard purple solution of benzophenone ketyl in THF to confirm effective oxygen and moisture removal. Deuterated solvents were purchased from Cambridge Isotopes Laboratories, Inc. and were degassed and stored over activated 3-Å molecular sieves prior to use. Elemental analyses were performed by Midwest Microlab, (Indianapolis, IN) or Complete Analysis Laboratories Inc. (calilabs; Parsippany, NJ).

Electrochemistry

Electrochemical measurements were carried out in a glovebox under a dinitrogen atmosphere in a one-compartment cell using a BAS model 100/W electrochemical analyzer. A glassy carbon electrode and platinum wire were used as the working and auxillary electrodes, respectively. The reference electrode was Ag/AgNO₃ in THF. Solutions (THF) of electrolyte (0.4 M tetra-n-butylammonium hexafluorophosphate) and analyte were also prepared in a glovebox. CVs were externally referenced to Fc/Fc⁺.

NMR and IR Spectroscopy

Both Varian 300 MHz and 500 MHz spectrometers were used to record the ¹H NMR and ³¹P NMR spectra at ambient temperature, and either a Varian 400 MHz or 500 MHz spectrometer was used to record ¹⁵N NMR spectra and all VT- NMR spectra. 1H NMR chemical shifts were referenced to residual solvent, and ³¹P NMR chemical shifts were referenced to 85% H₃PO₄ at δ 0 ppm. All ¹⁵N NMR spectra were externally referenced to neat H₃CC⁵N (δ = 245 ppm) in comparison to liquid NH₃ (δ = 0 ppm). Select decoupling experiments were used to correlate ¹H and ¹⁵N NMR chemical shifts. All NMR data were worked up using MestReNova. NMR spectral simulation was done with MestReNova. Solution magnetic moments were measured using Evans method.³⁵ IR measurements were obtained with a KBr solution cell or a KBr pellet using a Bio-Rad Excalibur FTS 3000 spectrometer controlled by Varian Resolutions Pro software set at 4 cm⁻¹ resolution.

X-ray Crystallography Procedures

X-ray quality crystals were grown as indicated in the experimental procedures per individual complex. The crystals were mounted on a glass fiber with paratone N oil, and data were collected on a Siemens or Bruker Platform three-circle diffractometer coupled to a Bruker-AXS Smart Apex CCD detector with graphite-monochromated Mo or Cu Kα radiation (λ = 0.71073 or 1.54178 Å, respectively), performing φ-and ω-scans. The structures were solved by direct or Patterson methods using SHELXS³⁶ and refined against F^α on all data by full-matrix least squares with SHELXL-97.³⁷ All non-hydrogen atoms were refined anisotropically. All hydrogen atoms (except hydrogen atoms on nitrogen) were included into the model at geometrically calculated positions and refined using a riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to 1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups). Hydrogen atoms directly coordinated to nitrogen were located in the Fourier difference map, and refined semi-freely with the aid of distance restraints.²⁰ Expected distances for the type of NH bond at the given temperature were taken from the .lst file.²⁰ If these hydrogen atoms could not be located in the difference map, they were omitted from the final refinement model.

Some of the structures reported suffered from disorder in parts of the [PhBP^R₃]⁻ ligand, and the disorder was modeled over two positions. Similarity restraints on 1,2 and 1–3 distances were applied where possible. Similar ADP and rigid bond restraints were applied to all atoms. In addition, several of the structures had solvent disorder, which was modeled as 2 or more component disorder. In some instances, discrete solvent molecules were disordered over several positions, and were modeled using the SUMP command. In other instances, several molecules of solvent were disordered over several positions. In order to determine the total number of solvent molecules, different free variables were assigned to each partially occupied solvent molecule, and the structure refined. The sum of the free variables was then restrained using the SUMP command to whatever value was obtained without the restraint. Some of the crystals were comprised of two or three different species that co-crystallized. {[PhBP^Ph₃]Fe(N₂H₄)}(PF₆) (7), co-crystallized with {[PhBP^Ph₃]Fe(N₂H₄)(Cl) and [PhBP^Ph₃]FeCl. With the aid of free variables, it was determined that there was a 3% impurity of [PhBP^Ph₃]FeCl and 20% {[PhBP^Ph₃]Fe(N₂H₄)(Cl). [PhBP^mter₃]FeCl co-crystallized with [PhBP^mter₃]Tl (3%). This was modeled with the aid of a free variable (part 1: Fe, Cl, part 2: Tl). All close contacts, both inter and intramolecular, involve at least one partner from a minor component of a disorder. Specific details concerning the refinement of each structure is included in the .cif file.

Starting Materials and Reagents

[PhBP^Ph₃Fe]Me,¹² 6,^{8 15}NH₂¹⁵NH₂,^6e meta-terphenyl bromide,³⁸ and Me₂Mg³⁹ were prepared according to literature methods. Pb(OAc)₄ was purchased from Aldrich (99.999+%), purified as described in the literature,⁴⁰ and recrystallized from cold THF to afford a white crystalline solid. Acetic acid and para-benzoquinone were purified according to literature methods.⁴⁰ All other reagents were purchased from commercial vendors and used without further purification.

Caution: All manipulations with anhydrous hydrazine were done at ambient or reduced temperatures, and the waste disposed of appropriately. Anhydrous hydrazine is both highly toxic and highly explosive, with an auto-ignition temperature that is highly dependent on the presence of impurities. Prior to working with anhydrous hydra-zine, we encourage others to consult appropriate sources to familiarize themselves with the dangers. We found “Wiley Guide to Chemical Incompatibilities” (Pohanish, R. P. and Greene, S. A.; Wiley) to be an excellent reference for such matters. Though we did not distill our anhydrous hydra-zine, a procedure is described in the literature (Lucien, H. W., J. Chem. Eng. Data, 1962, 7, 541).

Synthesis of Complexes

Synthesis of MeP(m-terphenyl)₂

Terphenyl bromide (8.909 g, 28.81 mmol) was dissolved in 75 mL THF and chilled to −78 °C. ⁿBuLi (1.6 M in hexanes, 28.8 mmol) was added dropwise over 15 min, and the reaction was stirred cold for 1 h. In the meantime, MePCl₂ (1.736 g, 14.4 mmol) was diluted in 15 mL toluene and chilled to −78 °C. After 1 h, the phosphine was added drop-wise over 10 min to the reaction, which was then stirred for 15 h, slowly warming to RT. The reaction solution was concentrated in vacuo. The resulting residue was washed profusely with petroleum ether, giving cream-colored solids, which were extracted into benzene, filtered through a Celite-lined frit, and lyophilized to afford the desired phosphine (5.765 g, 84% yield). ¹H NMR (C₆D₆, 300 MHz): δ 7.92 (d, J = 6.9 Hz, 4H), 7.73 (s, 2H), 7.46 (d, J = 6.9 Hz, 8H), 7.18 – 7.11 (m, 12H), 1.56 (d, ²J_H-P = 3.6 Hz, 3H, CH₃P). ³¹P NMR (C₆D₆, 121 MHz): δ −24.2 ppm.

Synthesis of (m-terphenyl)₂PCH₂Li(TMEDA)

In a 125 mL Erlenmeyer flask with a stir bar, MeP(m-terphenyl)₂ (3.7379 g, 7.405 mmol) and TMEDA (1.05 mL, 7.41 mmol) was dissolved in 30 mL of a 2:1 mixture of THF:Et₂O and chilled to −78 °C. ^sBuLi (1.4 M in cyclohexane, 8.15 mmol) was added drop-wise to the reaction over 10 min. The reaction was stirred for 12 h during which it warmed to room temperature. The resulting red/brown solution was concentrated in vacuo and the resulting solids were triturated with Et₂O to afford yellow solids which were collected on a frit, and rinsed with Et₂O and pentane. (2.7826 g, 61% yield). ¹H NMR (THF-d₈, 300 MHz): δ 7.92 (d, J = 6.9 Hz, 4H), 7.63 (d, J = 6.9 Hz, 8H), 7.50 (s, 2H), 7.35 (t, J = 6.9 Hz, 8H), 7.23 (t, J = 6.9 Hz, 4H), 2.30 (4H), 2.15 (12H), −0.14 (d, ²J_H-P = 3.3 Hz, 2H). ³¹P NMR (THF, 121 MHz): δ −4.08 ppm.

Synthesis of [PhBP^mter₃]Tl

In a vial, (m-terphenyl)₂PCH₂Li(TMEDA) (365.0 mg, 608.6 µmol) was dissolved in 10 mL Et₂O and then chilled to −90 °C. PhBCl₂ (33.2 mg, 202.8 µmol) was diluted in 3 mL toluene and added drop-wise to the solution. The reaction was stirred for 18 h, slowly warming to RT to give [PhBP^mter₃]Li(TMEDA) (³¹P NMR: −10.4 ppm). The reaction was concentrated under reduced pressure to dryness and then suspended in 10 mL EtOH. TlPF₆ (61.1 mg, 202.8 mmol) was added, and the reaction was stirred for 12 h. The white solids in the reaction were collected on a frit and washed with EtOH, MeCN, and petroleum ether (237.1 mg, 62% yield). ¹H NMR (C₆D₆, 500 MHz): δ 8.65 (br d, J = 6.0 Hz, 2H), 7.88 – 7.85 (m, 13H), 7.68 (t, J = 6.0 Hz, 2H), 7.16 – 7.13 (overlaps with solvent peak, ~18H), 7.00 – 6.99 (overlapping, 48H), 2.77 (br, 6H). ³¹P NMR (C₆D₆, 121 MHz): δ 21.7 ppm (d, ¹J_TlP = 4870 Hz). Anal. Calcd. for C₁₁₇H₈₉BP₃Tl: C 77.93; H 4.98; N 0. Found: C 77.85; H 4.86; N 0.

Synthesis of [PhBP^mter₃]FeCl

[PhBP^mter₃]Tl (0.473 g, 0.262 mmol) and FeCl₂ (0.034 g, 0.262 mmol) were stirred in 8 mL THF for 12 h. The reaction was filtered through Celite and concentrated under reduced pressure to dryness. The yellow residue was mashed to a fine powder and washed with petroleum ether and Et₂O (0.369 g, 83%). Single crystals suitable for X-ray diffraction were grown from vapor diffusion of petroleum ether into a benzene solution of [PhBP^mter₃]FeCl. 1H NMR (C₆D₆, 300 MHz): δ 206.1 (s), 40.9 (s), 19.8 (s), 18.4 (s), 7.1 (s), 6.9 (s), 3.6 (s), −14.3 (s), −37.3 (s). Evans Method (C₆D₆): 5.32 B.M. Anal. Calcd. for C₁₁₇H₈₉BClFeP₃: C 83.15; H 5.31; N 0. Found: C 83.09; H 5.41; N 0.

Synthesis of [PhBP^mter₃]FeMe

A solution of [PhBP^mter₃]FeCl (0.2096 g, 0.141 mmol) in 15 mL benzene was added to a stirring slurry of Me₂Mg (0.0186g, 0.342 mmol) in 2 mL benzene. After stirring for an hour, the reaction was filtered through a Celite-lined frit, and the solution was lyophilized to dryness. The residue was extracted into 20 mL benzene, filtered through a Celite-lined frit, and again lyophilized to yield analytically pure [PhBP^mter₃]FeMe (0.1629 g, 78.8 %). 1H NMR (C₆D₆, 300 MHz): δ 46.1 (s), 22.0 (s), 20.3 (s), 6.8 (s), 6.6 (s), 1.7 (s), −13.5 (s), −49.8 (s). Evans Method (C₆D₆): 4.9 B.M. UV-vis (C₆H₆) λ_max, nm (e, M⁻¹ cm⁻¹): 405 (1750), 370 (2300). Anal. Calcd. for C₁₁₈H₉₂BFeP₃: C 84.88; H 5.55. Found: C 84.67; H 5.62.

Synthesis of [PhBP^Ph₃]Fe(OAc)

Neat acetic acid (29.5 µL, 0.515 mmol) was added to a solution of [PhBP^Ph₃]FeMe (0.3892 g, 0.515 mmol) in 18 mL THF. After stirring for 24 h, the volatiles were removed to afford pure material. Crystals suitable for XRD were grown from benzene/pentane. 1H NMR (C₆D₆, 300 MHz): δ 171 (bs), 130.0 (s), 30.1 (s), 16.0 (s), 14.8 (s), −7.1 (s), −26.4 (s). Evans Method (C₆D₆): 4.6 B.M. Anal. Calcd. for C₄₇H₄₄BFeO₂P₃: C 70.52; H 5.74; N 0. Found: C 71.85; H 5.74; N 0.

Synthesis of [PhBP^mter₃]Fe(η²-N₂H₃), 1

[PhBP^mter₃]FeMe (0.0318 g, 0.0217 mmol) was dissolved in 2 mL of THF, and a solution of hydrazine (0.77 µL, 0.0217 mmol) in 1 mL THF was added drop-wise. The stirring reaction immediately changed color from yellow to green, and 1 was quantitatively formed. Micro-crystals of 1 were grown by slow evaporation of pentane into a THF solution (16.1 mg, 50.0 %). Complex 1 displays broad NMR spectra at all temperatures, and −25 °C was found to give the sharpest spectra. 1H NMR (THF-d₈, 500 MHz, −25 °C): δ 6.75–8.20 (bm, 83H), 6.43 (s, 1H, NH), 3.81 (s, 2H, NH₂). 1.79 (m, 6H, CH₂, overlapping with THF). ³¹P NMR (THF-d₈, 202.3 MHz, −25 °C): δ 89.1. IR (KBr) (cm⁻¹): 3301, 3174. UV-vis (THF) λ_max, nm (ε, M ⁻¹ cm⁻¹): 348 (sh, 4900), 420 (sh, 1600), 605 (625), 708 (500). Anal. Calcd. for C₁₁₇H₉₂BFeN₂P₃: C 83.36; H 5.50; N 1.66. Found: C 82.97; H 5.76; N 1.55.

A sample of 95% ¹⁵N-enriched 1 was synthesized using an analogous synthetic procedure with ¹⁵NH₂¹⁵NH₂. ¹H NMR (THF-d₈, 500 MHz, −25 °C): δ 6.43 (d, ¹J_NH ≈ 79 Hz, 1H, NH), 3.81 (d, ¹J_NH ≈ 83 Hz, 2H, NH₂). ¹⁵N NMR (THF-d₈, 50.7 MHz, −25 °C): δ 139.0 (dd, ¹J_NH ≈ 78.6 Hz, ¹J_NN ≈ 11 Hz), −14.5 (dt, ¹J_NH ≈ 83 Hz, ¹J_NN ≈ 11 Hz).

Synthesis of [PhBP^mter₃]Fe(η²-N₂H₃)(η¹-N₂H₄), 2

[PhBP^mter₃]FeMe (0.0269 g, 0.0184 mmol) was dissolved in 2 mL of THF, and a solution of hydrazine (3.0 µL, 0.092 mmol) in 1 mL THF was added drop-wise. The stirring reaction immediately changed color from yellow to green to red, indicative of formation of 2. Micro-crystals of 3 can be grown by evaporation of pentane into a THF solution containing 2 and excess hydrazine (11.4 mg, 36.2 %). The coordinated hydrazine is labile, and exposure of 2 to vacuum results in formation of 2. EA was performed on crystals of 3 (grown from THF/pentane) that were dried under an N₂ atmosphere for 20 min prior to sealing in an ampule, and it thus is likely that THF or pentane is still present in the crystals. Anal. Calcd. for C₁₁₇H₉₆BFeN₄P₃: C 81.77; H 5.63; N 3.26. Anal. Calcd. for [PhBP^mter₃]Fe(η²-N₂H₃)(η¹-N₂H₄).5THF, C₁₃₇H₁₃₆BFeN₄P₃O₅: C 79.18; H 6.60; N 2.70 Found: C 78.95; H 6.16; N 3.09. ¹H NMR (THF-d₈, 500 MHz, −40 °C): δ 9.90 (s, 1H), 8.38 (m, 6H), 6.7–8.1 (m, 76H), 4.92 (s, 1H, NH₂ or N_αH₂), 4.66 (s, 2H, N_βH₂), 3.18 (s, 1H, NH₂ or N_αH₂), 2.91 (s, 1H, NH₂ or N_αH₂), 2.72 (s, 1H, NH), 2.52 (s, 1H, NH₂ or N_αH₂), 2.18 (m, 2H, CH₂), 1.20 (m, 4H, CH₂). ³¹P NMR (THF-d₈, 202.3 MHz, −40 °C): δ 76.45 (d, 1P, J ≈ 40 Hz), 73.25 (bs, 1P), 59.58 (d, 1P, J ≈ 66 Hz). The ³¹P coupling was ill-defined at all temperatures scanned. IR (KBr) (cm¹): 3305, 3170. UV-vis (THF, with 20 equiv N₂H₄) λ_max, nm (ε, M⁻¹ cm⁻¹): 383 (2600, sh), 512 (1280).

A sample of 95% ¹⁵N-enriched 2 was synthesized using an analogous synthetic procedure with ¹⁵NH₂¹⁵NH₂. ¹H NMR (THF-d₈, 500 MHz, −40 °C): δ 4.92 (d, ¹J_NH ≈ 80 Hz 1H, NH₂ or N_αH₂), 4.66 (d, ¹J_NH ≈ 65 Hz, 2H, N_αH₂), 3.18 (d, ¹J_NH ≈ 75 Hz 1H, NH₂ or N_αH₂), 2.91 (d, ¹J_NH ≈ 75 Hz, 1H, NH₂ or N_αH₂), 2.72 (d, ¹J_NH ≈ 60 Hz, 1H, NH), 2.52 (d, ¹J_NH ≈ 80 Hz, 1H, NH₂ or N_αH₂). ¹⁵N NMR (THF-d₈, 50.7 MHz, −40 °C): 47.4 (dt, N_βH₂, ¹J_NH ≈ 65 Hz, ¹JNN = −11 Hz), 40.6 (dd, NH, ¹J_NH ≈ 58 Hz, ¹J_NN = −13 Hz), 23.6 (dt, NH₂ or N_αH₂, ¹J_NH ≈ 73 Hz, ¹J_NN = −11 Hz), 22.7 (dt, NH₂ or N_βH₂, ¹J_NH ≈ 80 Hz, ¹J_NN = −13 Hz). Select ¹H{¹⁵N} decoupling was employed to confirm the HN connectivity.

Synthesis of [PhBP^mter₃]Fe(η²-N₂H₃)(NH₃), 3

A solution of 1 (0.0150 g, 0.00890 mmol) in 1 mL THF was transferred to a 15 mL Shlenk tube, and evacuated. One atm of NH₃ was added, and the solution immediately turned red. Slow evaporation of pentane into a THF solution of 3 afforded crystalline material (0.0122 g, 80.5 %). Exposure of either solutions of 3 or crystals of 3 to vacuum resulted in rapid reformation of 1. Upon removal of solvent, crystals of 3 rapidly changed color to green and hence satisfactory EA could not be obtained. ¹H NMR (THF- d₈, 400 MHz, −45 °C): δ 10.3 (s, 1H), 8.66 (s, 1H), 8.53 (s, 1H), 8.22 (s, 2H), 8.08 (s, 2H), 6.5–8.1 (m, 76H), 5.32 (s, 1H), 3.58 (s, 1H, overlapping with THF), 2.6 (s, 2H), 2.2 (s, 2H), 1.83 (s, 1H, overlapping with solvent), 1.5 (s, 2H), 0.41 (s, 3H, overlapping with residual NH₃). ³¹P NMR (THF-d₈, 162 MHz, −45 °C): δ 81.1 (d, 1P, J ≈ 48 Hz), 66.2 (d, 1P, J ≈ 67 Hz), 52.3 (m, 1P). The ³¹P coupling was ill-defined at all temperatures scanned (20 °C to −70 °C). IR (KBr) (cm⁻¹): 3245, 3198. UV-vis (THF, under 1 atm NH₃) λ_max, nm (ε, M⁻¹ cm⁻¹): 400 (2900, sh), 516 (1460).

A sample of 95% ¹⁵N-enriched 3 was synthesized using an analogous synthetic procedure with ¹⁵NH₂¹⁵NH₂. ¹H NMR THF-d₈, 400 MHz, −45 °C): δ 5.32 (d, ¹J_NH ≈ 82 Hz, 1H, NHH), 3.58 (d, 1H, NHH), 1.83 (1H, NH), 0.41 (d, ¹J_NH ≈ 60 Hz, 3H, NH₃). gHMQC ¹⁵N{¹H} NMR (THF-d₈, 40.5 MHz, −45 °C): δ 31.8 (NH), 26.0 (NH₂), −18.9 (NH₃). Select ¹H{¹⁵N} decoupling was employed to confirm the HN connectivity.

Synthesis of [PhBP^Ph₃]Fe(η²-NHNMe₂), 4

Neat NH₂NMe₂ (28.2 µL, 0.363 mmol) was added to a stirring solution of [PhBP^Ph₃]FeMe (0.2495 g, 0.3299 mmol) in 10 mL benzene. The reaction was heated to 50 °C for 48 h, during which time the color changed from yellow to green. The volatiles were removed to afford a green solid (0.2356 g, 89.5 %). Crystals suitable for XRD were grown from the slow evaporation of pentane into a saturated benzene solution of 4. ¹H NMR (C₆D₆, 300 MHz): δ 8.13 (d, 2H, J = 6 Hz), 7.64 (t, 2H, J = 6 Hz), 7.40 (t, 1H, J = 6 Hz), 7.31 (bs, 12H), 6.88 (t, 6H, J = 7 Hz), 6.76 (t, 12H, J = 7 Hz), 4.00 (s, 1H, NH), 2.16 (s, 6H, NMe₂), 1.63 (bs, 6H, CH₂). ³¹P NMR (C₆D₆, 121.4 MHz): δ 79.9. IR (KBr) (cm⁻¹): 3234. UV-vis (THF) λ_max, nm (ε, M⁻¹ cm⁻¹): 300 (sh, 8450), 340 (sh, 4400), 440 (700), 600 (466), 742 (320). Anal. Calcd. for C₄₇H₄₈BFeN₂P₃: C 70.52; H 6.04; N 3.50. Found: C 69.97; H 6.14; N 3.18.

Synthesis of [PhBP^Ph₃]Fe(Me)(η²-N₂H₄), 5

[PhBP^Ph₃]FeMe (0.0343 g, 0.0391 mmol) was dissolved in 500 µL THF, and stirred at −78 °C. To this, a solution of hydrazine (1.27 µL, 0.0391 mmol) dissolved in 280 µL of THF was added drop-wise, resulting in a color change from yellow to strawberry red and conversion to 5. ¹H NMR (THF-d₈, 500 MHz, −50 °C): δ 7.56 (bs, 7H), 7.27 (m, 2H), 7.19 (m, 4H), 7.12 (m, 2H), 6.97 (m, 4H), 6.89 (bs, 8H), 6.74 (m, 8H), 4.33 (s, 2H, NH_cisH), 3.13 (s, 2H, NHH-_trans), 1.30 (m, 6H, CH₂), −0.2 (s, 3H, Me). NOESY was employed to assign the hydrazine protons that are cis and trans to the methyl ligand. ³¹P NMR (THF-d₈, 202.3 MHz, −50 °C): δ 79.18 (d, 2P, J ≈ 28 Hz), 52.50 (t, 1P, J = 32.1 Hz). The doublet is broad and not well-resolved. UV-vis (THF, −78 °C) λ_max, nm (ε, M⁻¹ cm⁻¹): 524 (940). IR (THF/KBr, −78 °C) (cm⁻¹): 3302, 3246, 3161.

A sample of 95% ¹⁵N-enriched 5 was synthesized using an analogous synthetic procedure with ¹⁵NH₂¹⁵NH₂. ¹H NMR (THF-d₈, 500 MHz, −50 °C): δ 4.31 (d, ¹J_NH ≈ 77 Hz, 1.5H, NH_cisH), 3.12 (d, ¹JNH ≈ 75 Hz, 1.5H, NHH_trans). ¹⁵N NMR (THF-d₈, 50.7 MHz, −50 °C): δ 17.3 (t, ¹J_NH ≈ 76 Hz, 2N). IR (THF/KBr, −78 °C) (cm⁻¹): 3312, 3251, 3223.

Synthesis of {[PhBP^Ph₃]Fe(η²-N₂H₄)}(PF₆), 7

To a solution of PhBP^Ph₃FeCl (0.6023 g, 0.775 mmol) in 20 mL THF was added neat hydrazine (37.7 µL, 1.16 mmol) and solid [Tl](PF₆) (0.2764 g, 0.775 mmol). After stirring for 24 h, hydrazine was again added (12.2 µL, 0.39 mmol), and the reaction stirred an additional 24 h. The solution was filtered through Celite, and the volatiles removed. The solid was extracted into DME, filtered through a Celite-lined frit, and the volatiles removed to give 0.6739 g of a pink solid (95 %). Crystals suitable for X-ray diffraction were grown from a THF/pentane solution. 1H NMR (THF-d₈, 500 MHz, −20 °C): δ 7.8 (m, 3H), 7.58 (d, J = 7.50 Hz, 2H), 7.44 (m, 3H), 7.38 (m, 3H), 7.19 (m, 5H), 7.12 (t, J = 7.54 Hz, 4H), 7.08 (t, J = 6.70, 2H), 7.00 (m, 5H), 6.83 (t, J = 7.54 Hz, 4H), 6.78 (J = 7.54 Hz, 4H), 4.78 (bs, NH₂, 2H), 4.31 (bs, NH₂, 2H), 1.30 (d, CH₂, ²JHP ≈ 15 Hz, 6H). ³¹P NMR (THF-d₈, 202.3 MHz, −20 °C): δ 66.0 (d, J = 59.3 Hz, 2P), 58.9 (t, J = 59.3 Hz, 1P), −138.6 (m, 1P). IR (KBr) (cm⁻¹): 3335, 3281, 3143. UV-vis (THF) λ_max, nm (ε, M⁻¹ cm⁻¹): 420 (250), 525 (675). Crystals of 7 invariable contained {[PhBP^Ph₃]Fe(η²-N₂H₄)}(PF₆), [PhBP^Ph₃]FeCl and {[PhBP^Ph₃]Fe(η²-N₂H₄)}(Cl), precluding our ability to obtain analytically pure material.

Synthesis of [PhBP^Ph₃]Fe(η¹-N₂H₄)(OAc), 8

Neat anhydrous hydrazine (10.2 µL, 0.3159 mmol) was added to a 2 mL THF solution of [PhBP^Ph₃]Fe(OAc) (0.2529 g, 0.3159 mmol). An immediate color change from pale yellow to purple was noted. After stirring for 24 h, the reaction was filtered and solid 8 was rinsed with THF and pentane to afford analytically pure material (0.1920 g, 73 %). Crystals suitable for diffraction were grown by layering a saturated benzene solution of 8 with pentane. ¹H NMR (THF-d₈, 500 MHz, −50 °C): δ 6.5−8.0 (m, 35H), 4.61 (bs, NH₂, 2H), 3.94 (bs, NH₂, 2H), 1.5–1.8 (m, 3H, overlapping with THF), 1.5−1.8 (m, 6H) ³¹P NMR (THF-d₈, 202.3 MHz, −50 °C): δ 59.1 (bs, 2P), 50.5 (t, J = 57.0 Hz, 1P). IR (KBr) (cm⁻¹): 3378, 3313, 1464. Anal. Calcd. for C₄₇H₄₈BFeP₃N₂O₂: C 67.81; H 5.81; N 3.36. Found: C 67.43; H 5.62; N 3.06.

A sample of 95% ¹⁵N-enriched 8 was synthesized using an analogous synthetic procedure with ¹⁵NH₂¹⁵NH₂. ¹H NMR (THF-d₈, 500 MHz, −50 °C): δ 4.61 (d, ¹J_NH = 70 Hz, 2H, N_αH₂), 3.94 (d, ¹J_NH = 67 Hz, 2H, N_βH₂). ¹⁵N NMR (THF-d₈, 50.7 MHz, −50 °C): δ 56.2 (t, ¹J_NH = 67 Hz, 1N, N_βH₂), 33.3 (t, ¹JNH = 68 Hz, 1N, N_αH₂). ¹H{¹⁵N} experiments with selective decoupling were used to correlate the ¹H and ¹⁵N NMR chemical shifts. IR (KBr) (cm⁻¹): 3367, 3300, 3274.

Complex 8 can alternatively be prepared from the salt metathesis of 7 with sodium acetate.

Synthesis of [PhBP^Ph₃]Fe(NH₃)(OAc), 9

A suspension of Pb(OAc)₄ (0.0161 g, 0.0360 mmol) in 1 mL THF was added drop-wise to a stirring solution of 7 (36.0 mg, 0.036 mmol) in 2 mL THF. The reaction gradually changed color from pink to purple, as Pb(OAc)₂ precipitated out. The volatiles were removed, and the solid residue was extracted into benzene, and filtered through a Celite-lined frit. The solution was lyophilized, extracted into benzene, and again filtered through a Celite-lined frit. Crystals were grown by layering pentane over the benzene solution (10.3 mg, 35.5 %). Complex 9 is sparingly soluble and crystals of 8 are invariably covered with a white film, presumably Pb(OAc)₂.

Complex 9 can alternatively be prepared by addition of 1 atmosphere of NH₃ to a solution of [PhBP^Ph₃]Fe(OAc) (0.0284 g, 0.0355 mmol) in 4 mL of benzene (in an evacuated 50 mL Schlenk-tube). After stirring for 5 minutes, the solution was degassed, filtered through Celite, and layered with pentane to afford crystalline material (0.0211 g, 72.7 %).

¹H NMR (THF-d₈, 500 MHz, −40 °C): δ 7.75 (bs, 5H), 7.51 (d, J = 6.9 Hz, 4H), 7.38 (m, 5H), 7.24 (m, 3H), 6.9–7.2 (m, 15H), 6.83 (m, 3H), 2.36 (s, 3H, NH₃), 0.95–1.40 (m, 9H, CH₂/OAc). ³¹P NMR (d₈-THF, 202.3 MHz, −40 °C): δ 61.5 (bs, 2P), 46.9 (t, J = 58.6 Hz, 1P). IR (KBr) (cm⁻¹): 3362, 3334, 1466. UV-vis (THF) λ_max, nm (ε, M⁻¹ cm⁻¹): 580 (855).

Crystals of 9 were exposed to minimal vacuum prior to sealing in an ampule/combustion analysis. Anal. Calcd. for C₄₇H₄₇BFeP₃NO₂: C 69.05; H 5.80; N 1.71. Anal. Calcd. for [PhBP^Ph₃]Fe(NH₃)(OAc).C₆H₆, C₅₃H₅₃BFeP₃NO₂: C 71.10; H 5.96; N 1.56. Found: C 70.70; H 6.06; N 1.50.

A sample of 95% ¹⁵N-enriched 9 was synthesized following the alternative procedure, using ¹⁵NH₃. ¹H NMR (THF-d₈, 400 MHz, −30 °C): δ 2.36 (d, ¹J_NH = 67 Hz, 3H, NH₃). HSQC ¹⁵N{¹H} NMR (THF-d₈, 40.5 MHz, −30 °C): δ −13.8. ³¹P NMR (THF-d₈, 161.8 MHz, −40 °C): δ 61.5 (bs, 2P), 46.9 (dt, ¹J_PP ≈ 58.6 Hz, ¹J_NP ≈ 10 Hz, 1P). IR (KBr) (cm⁻¹): 3354, 3327, 1466.

Synthesis of {[PhBP^Ph₃]Fe(η²-N₂H₄)(NH₃)}(PF₆), 10

A solution of [PhBP^Ph₃]FeMe (0.2181 g, 0.2883 mmol) in 15 mL THF was cooled to −41 °C and set stirring. To this, a solution of hydrazine (18.7 µL, 0.577 mmol) in 1 mL THF was added drop-wise over the course of 5 min. A suspension of [Fc](PF₆) (0.0954 g, 0.2883 mmol) in 4 mL THF was added drop-wise, and the reaction was stirred for 1 h at −41 °C, and subsequently warmed to RT and stirred an additional 12 h. Volatiles were removed from the reaction mixture, and the pink residue was rinsed with 15 mL of pentane, followed by 10 mL of Et₂O. Extraction of the remaining solid into THF, followed by layering with pentane, afforded crystals of 10 (0.1887 g, 70.0 %). ¹H NMR (THF-d₈, 300 MHz): δ 6.5–8.5 (m, 35H), 5.5 (bs, NH₂, 2H), 4.2 (bs, NH₂, 2H), 2.7 (bs, NH₃, 3H), 1.37 (m, CH₂, 6H) ³¹P NMR (THF-d₈, 300 MHz): δ 60.8 (bs, 2P), 53.5 (bs, 1P), −143.3 (m, 1P). IR (KBr) (cm⁻¹): 3334 (NH), 3260 (NH). UV-vis (THF) λ_max, nm (ε, M⁻¹ cm⁻¹): 537 (750). Anal. Calcd. for C₄₅H₄₈BFeN₃P₄F₆: C 57.78; H 5.17; N 4.49. Found: C 57.85; H 5.25; N 4.29.

Synthesis of [PhBP^mter₃]Fe(NH₃)(OAc), 11

Hydrazine (6.4 µL, 0.020 mmol) was added to a stirring solution of [PhBP^mter₃]FeMe (0.1439 g, 0.0982mmol) in 5 mL THF. After stirring for 10 min, a suspension of Pb(OAc)₄ (0.0871 g, 0.0196 mmol) in 5 mL THF was added dropwise, and the solution stirred for 12 h. The volatiles were removed, and the resulting residue was rinsed with pentane and extracted into DME. The resulting solution was layered with pentane to yield crystalline 10 (0.0803 g, 53.6 %). The bulk crystals contained a white precipitate, presumably Pb(OAc)₂.

Complex 11 can alternatively be prepared from [PhBP^mter₃]FeMe. One equivalent of AcOH (2.4 µL, 0.041 mmol) was added to a solution of [PhBP^mter₃]FeMe (0.0686 g, 0.0411 mmol) in 2 mL benzene. After stirring for 10 min, the reaction was transferred to a 5 mL Schlenk tube which was evacuated. One atmosphere of NH₃ was added to the Schlenk tube, and after stirring for 1 h, the reaction was degassed, the solution filtered through Celite, and layered with pentane to afford crystalline material (0.0444 g, 62.4 %).

¹H NMR (THF-d₈, 400 MHz, −30 °C): δ 8.7 (bs, 6H), 7.6–8.4 (m, 7H), 6.8–8.4(m, 70H), 3.05 (bs, NH₃, 3H), 1.5–2.0 (m, 6H, overlap with THF), 1.29 (s, 3H, OAc). ³¹P NMR (THF-d₈, 161.8 MHz, –30°C): δ 60.7 (bs, 2P), 50.1 (t, J = 59.4 Hz, 1P), −143.3 (m, 1P). IR (KBr) (cm⁻¹): 3365, 1450. UV-vis (THF) λ_max, nm (ε, M⁻¹ cm⁻¹): 557 (580). Anal. Calcd. for C₁₁₉H₉₅BFeNO₂P₃: C 82.58; H 5.53; N 0.81. Found: C 81.25; H 5.98; N 0.84.

A sample of 95% ¹⁵N-enriched 11 was synthesized using the alternative synthesis using ¹⁵NH₃. 1H NMR (THF-d₈, 400 MHz, −30 °C): δ 3.05 (d, ¹J_NH = 67 Hz, 3H, NH3). HSQC ¹⁵N{¹H} NMR (THF-d₈, 40.5 MHz, −30 °C): δ −12.7. IR (KBr) (cm⁻¹): 3308 (NH), 3302 (NH).

Synthesis of [PhBP^Ph₃]Fe(OAc)(CO), 12

A suspension of Pb(OAc)₄ in 1 mL THF (13.5 mg, 0.0305 mmol) was added dropwise to a stirring solution of 6 (24.4 mg, 0.0305 mmol) in 2 mL THF. An immediate color change from orange to green was noted, and after stirring for an additional 12 h, the volatiles were removed to yield a green residue. The solids were rinsed with pentane, extracted into benzene, filtered, and lyophilized. The green powder was then taken up in THF and layered with pentane to yield crystalline material suitable for XRD (12.0 mg, 47.5 %). ¹H NMR (C₆D₆, 300 MHz): δ 8.13 (d, J = 6.8 Hz, 2H), 7.77 (bs, 4 H), 7.69 (t, J = 7.3 Hz, 2H), 7.42 (bs, 6H), 7.31 (t, J = 8.4 Hz, 4H), 6.99 (bs, 6H), 6.90 (bs, 4H), 6.82 (bs, 4H), 6.73 (t, J = 7.0 Hz, 3H), 2.1 – 2.3 (m, 4H), 1.76 (bs, 2H), 1.47 (s, 3H). ³¹P NMR (121 MHz, C₆D₆): δ 48.58 (d, J = 66.3 Hz, 2P), 29.81 (t, J = 66.4 Hz, 1P). IR (KBr) (cm⁻¹): 1976 (CO), 1469. Anal. Calcd. for C₄₈H₄₄BFeO₂P₃: C 69.59; H 5.35; N 0. Found: C 69.76; H 5.50; N 0.

Synthesis of {[PhBP^Ph₃]Fe(η²-N₂H₄)(CO)}(PF₆), 13

A suspension of [Fc](PF₆) (15.0 mg, 0.0455 mmol) in 1 mL benzene was added dropwise to a stirring solution of 6 (36.4 mg, 0.0455 mmol) in 2 mL benzene. The reaction stirred for 24 h, during which a color change from orange to red ensued. The reaction mixture was lyophilized, and the resulting solids were rinsed with pentane and diethyl ether. The remaining solids were extracted into THF, filtered, and layered with pentane, yielding analytically pure crystals suitable for XRD (18.2 mg, 42.3 %). 1H NMR (C₆D₆, 300 MHz): δ 8.03 (bs, 2H), 7.66 (bs, 5H), 7.41 (bs, 5H), 7.03 (bs, 8H), 6.5–6.8 (m, 15H). ³¹P NMR (C₆D₆, 300 MHz): δ 50.22 (d, ¹J_PP = 63.6 Hz, 2P), 36.00 (t, ¹J_PP = 63.6 Hz, 1P), −142.7 (m, 1P). IR (KBr) (cm⁻¹): 3332, 3276, 3253, 1986 (CO). Anal. Calcd. for C₄₆H₄₅BFeN₂OP₄F₆: C 58.35; H 4.79; N 2.96. Found: C 58.56; H 5.02; N 2.60.