Mononuclear five- and six-coordinate iron hydrazido and hydrazine species

Article abstract of DOI:10.1021/ic301704f

This article describes the synthesis and characterization of several low-spin iron(II) complexes that coordinate hydrazine (N₂H ₄), hydrazido (N₂H₃^-), and ammonia. The sterically encumbered tris(di-me

Full text of DOI:10.1021/ic301704f

Article
pubs.acs.org/IC
Mononuclear Five- and Six-Coordinate Iron Hydrazido and Hydrazine
Species
Caroline T. Saouma, Connie C. Lu,† and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
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, [PhBPmter3]−, is introduced
to provide access to species that cannot be stabilized with the
−
−
[PhBPPh
]
3
ligand ([PhBPR ]− = PhB(CH2PR2)3 ). Treat-
3
ment of [PhBPmter3]FeMe with hydrazine generates the
unusual 5-coordinate hydrazido complex [PhBPmter 3]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 [PhBPmter3]Fe(L)(η2-N2H3) (L = N2H4 (2) or NH3 (3)). In contrast, treatment of
[PhBPPh3]FeMe with hydrazine forms the adduct [PhBPPh3]Fe(Me)(η2-N2H4) (5). Complex 5 is thermally unstable to methane
loss, generating intermediate [PhBPPh3]Fe(η2-N2H3), which undergoes bimolecular coupling to produce {[PhBPPh3]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 N2Hx ligand (x = 3, 4), and formation of
[PhBPR ]Fe(NH3)(OAc) (R = Ph (9) and mter (11)).
3
for W,6e−g Re,6h Co,6i and Fe.6j,8 With regards to iron
INTRODUCTION
■
complexes in particular, 6-coordinate and low-spin,6n as well
as 5-coordinate and intermediate-spin6d 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).
An area of ongoing research is geared toward 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 N2Hx ligands are warranted.
As part of our group’s ongoing efforts to study the
multielectron reactivity of mono- and diiron complexes that
feature nitrogenous ligands,9,10 we recently turned to (N2Hx)n−
(x = 2−4; n = 0, −1, −2) ligated species of iron.6d,8,11,12 We
found that the bridging hydrazine in {[PhBPPh3]Fe}2(μ-η1:η1-
N2H4)(μ-η2:η2-N2H2) undergoes clean oxidation to diazene,
generating {[PhBPPh3]Fe}2(μ-η1:η1-N2H2)(μ-η2:η2-N2H2)
In addition to their proposed role in N2 reduction, hydrazine
(N2R4), hydrazido (N2R3 ), and hydrazido(2−) (N2R2
−
2−
)
([PhBPR ]− = [PhB(CH2PR2)3]−) (Scheme 1).12 Subsequent
species have also been invoked as reactive intermediates in
several synthetic transformations, including the hydrohydrazi-
nation 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
3
oxidation results in N2 loss and formation of
Scheme 1. Oxidation of {[PhBPPh3]Fe}2(μ-η1:η1-N2H4)(μ-
η2:η2-N2H2)
6
−
(N2H3 ) functionality. This relative scarcity may also be due in
part to the different inherent stabilities of M(N2Rx) and
M(N2Hx) species.6l,7
Terminal hydrazido species of the type M(η1-N2H3) were
first prepared in the 1970s by both Chatt6a and Hidai6b (M =
W). Since then, examples of Mo,6k Re,6c Fe,6d,n and Ru6n M(η1-
N2H3) complexes have been characterized. Side-on hydrazido
species, M(η2-N2H3), are also uncommon; examples are known
Received: August 2, 2012
Published: September 5, 2012
© 2012 American Chemical Society
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-
{[PhBPPh3]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.
[PhBPmter3]FeCl (83% yield), and treatment of [PhBPmter
]
3
FeCl with excess Me2Mg in benzene results in formation of
amber and high-spin [PhBPmter3]FeMe (79% yield).
The differences in both the steric and the electronic
parameters between the m-terphenyl and phenyl substituted
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 depend-
ing on the nature of the auxiliary ligand employed, including 5-
coordinate species that feature multiple-bond character
between the iron and the hydrazido ligand. The oxidation of
these hydrazido and related hydrazine species of iron is also
discussed. In most instances, treatment of these monomeric
Fe(N2Hx) species with oxidizing reagents results in dispro-
portionation of the (N2Hx)n− ligand, affording Fe(NH3)
species. This reactivity contrasts that observed at diiron
centers.8,11,12
ligands can be determined by comparison of the [PhBPR ]FeCl
3
species (R = mter, Ph). The solid-state structures of
[PhBPmter3]FeCl and [PhBPPh3]FeCl9b have been obtained,
and space-filling renditions are shown in Figure 1. The m-
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [PhBPmter3]Fe-X
Species (X = Cl, Me). To prevent formation of N2Hx bridging
diiron species as discussed above,8,12 a new, more sterically
encumbering [PhBPR ]− ligand variant was sought.13 The
3
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 [PhBPPh
]
ligand, the
3
Figure 1. Space filling models of [PhBPPh3]FeCl (left) and
−
[PhBPmter
]
3
variant was targeted (mter = meta-terphenyl =
[PhBPmter3]FeCl (right). The representation perpendicular to the
3,5-terphenyl).
B−Fe−Cl vector (top) highlights the added vertical steric protection
The synthesis of the ligand [PhBPmter3]Tl is readily achieved
following a similar synthetic protocol to that employed for
[PhBPPh3]Tl (Scheme 2).13c,d The precursor phosphine (m-
−
that the bulkier [PhBPmter
]
3
ligand provides relative to that of the
[PhBPPh3]− 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 gray, H in white, and B in orange.
Scheme 2. Synthesis of [PhBPmter3]Tl and [PhBPmter3]Fe-X
Species
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).
The cyclic voltammogram (CV) of [PhBPmter3]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
[PhBPPh3]FeCl. For comparison, these reductions are about
0.4−0.5 V more positive than those for the alkyl substituted
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.
13a,b
s
i
Subsequent deprotonation with BuLi at −78 °C in the
complexes, [PhBPR ]FeCl (R = Pr, CH2Cy).
Combined,
3
presence of TMEDA affords the phosphine carbanion, (m-
terphenyl)2P(CH2)Li(TMEDA) in 61% yield. Addition of 3
equiv of the carbanion to PhBCl2 and subsequent exposure to 1
equiv of [Tl](PF6) gives the desired ligand, [PhBPmter3]Tl,
which is isolated as a white powder in 62% yield (32% over
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 1 equiv
of hydrazine to [PhBPmter3]FeMe results in the formation of
green and diamagnetic [PhBPmter3]Fe(η2-N2H3) (1), with
concomitant release of methane (Scheme 3).
three steps).15
Likewise, the syntheses of the Fe(II) complexes, [PhBPmter
]
-
3
FeCl and [PhBPmter3]FeMe, are achieved using similar
protocols to those used for the syntheses of [PhBPPh3]FeCl9b
and [PhBPPh3]FeMe12 (Scheme 2). Thus, mixing of
[PhBPmter3]Tl with FeCl2 affords yellow and high-spin
The identity of 1 as a hydrazido species is made on the basis
of elemental analysis and NMR spectroscopy. The room
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Scheme 3. Synthesis of [PhBPmter3]Fe(N2H3) Species
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 1 equiv of N2H4 or NH3 to 1
results in a color change from green to orange, and formation
of [PhBPmter3]Fe(η2-N2H3)(η1-N2H4) (2) or [PhBPmter3]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 [PhBPmter3]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 NHx 1H
NMR resonances (the asymmetry about the iron center
induced by the η2-N2H3 ligand should render the hydrazine
NαH2 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
NHx 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
[PhBPmter3]Fe(η2-N2H3)(η1-N2H4). Further, the 1H NMR
spectrum of 15N-2 (−50 °C, THF-d8) features six distinct
NHx resonances, and the corresponding 31P NMR spectrum
features three distinct resonances.
temperature NMR spectra of 1 display broad peaks that
sharpen upon cooling to −25 °C, in accordance with an S = 0
1
ground state. The H NMR spectrum (−25 °C, THF-d8) of
15N-1 (prepared by treatment of [PhBPmter3]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
1
1
(dt, JNH ≈ 83 Hz, JNN ≈ 11 Hz), while the NH nitrogen
1
1
resonates at 139.0 ppm (dd, JNH2 ≈ 79 Hz, JNN ≈ 11 Hz)
3
(Figure 2a). The JHH1 and the JNH coupling could not be
resolved in either the H or the 15N NMR spectra.
1
Select H{15N} decoupling was employed to correlate the
15N/1H signals (see Supporting Information) 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 NβH2 of the
coordinated hydrazine, on the basis that (i) select decoupling
of this resonance results in collapse of a single NH doublet
1
(that integrates to 2H) in the H{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 Supporting
Information). The overlapping signals at 22.7 and 23.6 thus
correspond to the hydrazine NαH2 and the hydrazido NH2;
select decoupling of these resonance results in collapse of four
1
NH doublets (1H each) in the H{15N} spectrum, consistent
with the asymmetry about the iron center.
Figure 2. 15N NMR spectra (THF-d8) of (a): 15N-1 (−25 °C) and
(b): 15N-2 (−50 °C). The simulation of the spectrum of 2 is shown
above the experimental spectrum. Simulation parameters: δ 47.4 (1JNN
1
The 15N and H NMR chemical shifts for Fe(η2-N2Hx)
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 coordina-
tion 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 NH nitrogen atom of 1 is unusual, and suggests a
1
1
= −11 Hz, JNH = 65 Hz), 40.6 (1JNN = −13 Hz, JNH = 58 Hz), 23.6
1
1
(1JNN = −11 Hz, JNH = 73 Hz, JNH = 73 Hz), 22.7 (1JNN = −13 Hz,
1JNH = 80 Hz, JNH = 80 Hz), line width: 15 Hz. (c) Possible isomers
1
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, [PhBPmter3]Fe(η1-N2H3), or
5-coordinate and fluxional, [PhBPmter3]Fe(η2-N2H3). As 4-
coordinate [PhBPR ]FeII-X species display high-spin S = 2
3
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Table 1. NMR and Structural Parameters for Fe(η2-N2Hx) Species (x = 2, 3, 4)
a
compound
15N NMR chemical shift (δ)
1H NMR chemical shift (δ)
5.39, 4.69
Fe−N bond distance (Å)
ref
d
{cis-[Fe(N2H4)(dmpe)2]}{BPh4}2
−11.9
−19.9
1.981(2), 2.003(2)
1.993(2), 2.006(2)
18
e
b
{cis-[Fe(N2H4)(DMeOPrPE)2]}{BPh4}2
[PhBPPh3]Fe(Me)(N2H4) (5)
4.8, 3.8
6j,19
f
17.1
4.33, 3.13
this
work
{[PhBPPh3]Fe(NH3)(N2H4)}{PF6} (10)
{[PhBPPh3]Fe(CO)(N2H4)}{PF6} (13)
2.006(2), 2.025(3)
1.984(4), 2.005(3)
this
work
5.48, 2.90
this
work
g
bc
,
c
{cis-[Fe(N2H3)(DMeOPrPE)2]}{BPh4}
8.4/6.1 (NH)
1.05/0.65 (NH)
6j
b
c
c
−1.4 (NH2)
4.23/4.14 (NHH)
3.66/3.44 (NHH)
h
[PhBPmter3]Fe(N2H3) (1)
139.0 (NH), −14.5 (NH2)
6.43 (NH), 3.81 (NH2)
this
work
i
[PhBPmter3]Fe(N2H3)(η1-N2H4) (2)
40.6 (NH), 22.7, 23.6 (NH2,
NαH2), 47.4 (NβH2)
3.18 (NH), 2.52−4.66 (NH2)
this
work
j
[PhBPmter3]Fe(N2H3)(NH3) (3)
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)
1.992(3), 2.018(3)
this
work
k
[PhBPPh3]Fe(CO)(NHNH2) (6)
32.2 (NH), 31.8 (NH2)
2.85 (NH), 1.88 (NHH), 6.55
8
l
(NHH)
[PhBPPh3]Fe(NHNMe2) (4)
4.00
1.788(2) (Fe−NH) 2.058(2)
(Fe−NMe2)
2.016(5), 2.032(7)
this
work
d
b
cis-[Fe(N2H2)(dmpe)2]
65.3
2.04
2.1
18
6j
e
b
cis-[Fe(N2H2)(DMeOPrPE)2]
60.8
a
b
Chemical shifts are referenced to liquid ammonia at 0 ppm. Converted from the nitromethane referencing scale. The chemical shift of
c
d
nitromethane was taken as 376 ppm relative to liquid ammonia. The two chemical shifts correspond to different isomers. dmpe = 1,2-bis-
e
f
g
(dimethylphosphino)ethane. DMeOPrPE = 1,2-bis[(methoxypropyl)phosphino]ethane. NMR collected at −50 °C. NMR collected at −85 °C.
h
i
j
k
l
NMR collected at −25 °C. NMR collected at −40 °C. NMR collected at −45 °C. NMR collected at −75 °C. Because of H-bonding, the
chemical shift of this proton is highly dependent on solvent, concentration, and temperature.
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,b,e,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
coordination mode of the hydrazido ligand is inferred to be the
same in both complexes.
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
Though we were unable to obtain crystals of 1 suitable for X-
ray diffraction (XRD), the solid-state structure of a related
species, [PhBPPh3]Fe(η2-NHNMe2) (4), was obtained. This
complex is readily prepared by addition of 1 equiv of
NH2NMe2 to a benzene solution of [PhBPPh3]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
Scheme 4. Synthesis of [PhBPPh3]Fe(N2Rx) Species
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 atoms 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).
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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,
[PhBPPh3]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
{[PhBPPh3]Fe(CO)}2(μ-N2H2), which features FeN bond-
ing, is 1.83 Å.8 As for 1, the Fe−NH functionality in
{[PhBPPh3]Fe(CO)}2(μ-N2H2) lies in the equatorial 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.
it can be trapped with CO to give [PhBPPh3]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 1 equiv
of hydrazine was added to a tetrahydrofuran (THF) solution of
[PhBPPh3]FeCl in the presence of [Tl](PF6) to generate
{[PhBPPh3]Fe(η2-N2H4)}(X) (7) (X = Cl, PF6) (Scheme 5).
Scheme 5. Synthesis of [PhBPPh3]Fe(N2H4) and
{[PhBPCH2Cy3]Fe}2(μ-N2H4) Species
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 data set 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 semifreely 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 cocrystallize 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 [PhBPPh3]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
[PhBPmter]FeMe and hydrazine, the room temperature addition
of 1 equiv of hydrazine to the sterically less encumbering
[PhBPPh]FeMe species results in quantitative formation of the
diiron species {[PhBPPh3]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 [PhBPPh]FeMe and hydrazine was monitored
by VT NMR spectroscopy.
This reaction is hampered by an equilibrium between
[PhBPPh]FeCl and {[PhBPPh3]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
[PhBPPh3]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 hexafluorophosphate salts cocrystallize.
Nonetheless, a structure of 7 has been obtained (see
Supporting Information). The disorder present was satisfac-
torily modeled, and the structure established connectivity and a
5-coordinate square pyramidal geometry for {[PhBPPh3]Fe(η2-
N2H4)}(PF6).
Upon addition of hydrazine at −78 °C, an initial hydrazine
adduct [PhBPPh]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 (1JNH ≈ 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
An end-on coordinated hydrazine species of iron was also
targeted. Treatment of [PhBPPh3]FeMe with 1 equiv of AcOH,
followed by addition of 1 equiv of hydrazine results in clean
formation of [PhBPPh3]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 NαH2 and NβH2 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 Nα nitrogen atom resonates at higher field
corresponding to the Me protons is noted at −0.2 ppm, as well
1
as two broad doublets at 4.33 and 3.13 ppm (2H each, JNH
≈
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 {[PhBPPh3]Fe}2(μ-η1:η1-
N2H4)(μ-η2:η2-N2H2) begin to grow in. Though the postulated
1
“[PhBPPh3]Fe(N2H3)” species cannot be detected in the H 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,
than the Nβ nitrogen atom.6n In the corresponding H NMR
1
spectrum (−50 °C, THF-d8), the NαH2 protons resonate at
4.61 ppm and the NβH2 protons resonate at 3.94 ppm.
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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).
Scheme 6. Oxidation of [PhBPPh3]Fe(N2H4) Species
Complex 8 is similar to the diiron species,
{[PhBPCH2Cy3]Fe(OAc)}2(μ-η1:η1-N2H4), which is prepared
by addition of hydrazine to the sterically less encumbered
[PhBPCH2Cy3]Fe(OAc) (Scheme 5).11 Addition of substoichio-
metric equivalents of hydrazine to [PhBPPh3]Fe(OAc) does not
result in formation of a hydrazine bridged dimer akin to
{[PhBPCH2Cy3]Fe(OAc)}2(μ-η1:η1-N2H4), suggesting that this
species is not accessible for steric reasons. Whereas solutions of
8 are stable toward excess hydrazine, the diiron analogue
{[PhBPCH2Cy3]Fe(OAc)}2(μ-η1:η1-N2H4) facilitates hydrazine
disproportionation, and mixtures of {[PhBPCH2Cy3]Fe-
(OAc)}2(μ-η1:η1-N2H2) and [PhBPCH2Cy3]Fe(OAc)(NH3) are
obtained. As hydrazine disproportion is facilitated by electron-
rich metal centers,22 the heightened stability of 8 relative to
temperature (RT)) with 1 equiv of Pb(OAc)4 results in
{[PhBPCH2Cy3]Fe(OAc)}2(μ-η1:η1-N2H4) towards hydrazine
formation of 9 (Scheme 6). Similarly, in the presence of 1 equiv
disproportionation is likely due to the hydrazine in 8
of N2H4, oxidation of 5 by [Fc](PF6) results in formation of the
coordinating a single iron, and the iron being ligated by a
6-coordinate ammonia/hydrazine complex, {[PhBPPh3]Fe-
less electron donating tris(phosphino)borate ligand.
(NH3)(η2-N2H4)}{(PF6)} (10). In the absence of N2H4, the
Exploring the Oxidation of Hydrazine Species. Diazene
reaction between 5 and [Fc](PF6) is ill-defined. As both
coordinated metal species are uncommon, yet are attractive
reactions result in formation of ammonia (and presumably 0.5
synthetic targets in light of their postulated role in N2
equiv of N2/N2H2) the reactions likely proceed via a similar
reduction.1a,3d The instability of free diazene precludes its use
oxidation mechanism. When 5 was instead treated with
quinone oxidants, no N-containing Fe products were obtained
(see Supporting Information).
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
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,
reactivity. This contrasts with the Pb(OAc)4 oxidation of the
diiron species {[PhBPPh3]Fe}2(μ-η1:η1-N2H4)(μ-η2:η2-N2H2),
and oxidation results in disproportionation of the Fe(N2H4)
fragment to give L5-FeII(NH3) species.
-
which results in formation of the diazene species, {[PhBPPh
]
3
Fe}2(μ-η1:η1-N2H2)(μ-η2:η2-N2H2) (Scheme 1).
Treatment of 5-coordinate hydrazine 7 with 1 equiv of
Pb(OAc)4 results in a color change from pink to purple, and
formation of [PhBPPh3]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.
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 deproto-
nation of a hydrazine species Fe(η1-N2H3Ph) (with concom-
itant 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.
Hydrazine adduct 5 undergoes a related transformation upon
oxidation. As for 7 or 8, treatment of 5 (−78 °C to room
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−
Oxidation of hydrazido species 1 with 1 equiv of Pb(OAc)4
results in a color change from green to purple, and formation of
the ammonia complex, [PhBPmter3]Fe(OAc)(NH3), (11)
(Scheme 7). This species is also formed in the reaction of
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
Scheme 7. Oxidation of [PhBPmter3]Fe(N2H3) Species
−
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 com-
plexes 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.
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 Supporting Information).
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
Scheme 8. Reactivity of [PhBPPh3]Fe(CO)(N2H3) Towards
Various Oxidants
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 {[PhBPPh
]
3
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 NxHy
species.
solutions of 6, no cationic ammonia species akin to 9 or 10 is
isolated; rather [PhBPPh3]Fe(CO)(OAc) (12) forms. When 6
is alternatively treated with 1 equiv of [Fc](PF6), the cationic
hydrazine species {[PhBPPh3]Fe(CO)(η2-N2H4)}{PF6} (13) is
cleanly generated along with ferrocene (as deduced by H
NMR spectroscopy). Though the net transformation of 6 to 13
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out using
standard Schlenk or glovebox 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.). Nonhalogenated 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, (Indian-
apolis, 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
auxiliary 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+.
1
is protonation, the formation of ferrocene suggests that the
-
reaction may proceed via an oxidized intermediate, “[PhBPPh
]
3
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 of O2 to generate
the diiron species, {[PhBPPh3]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.
NMR and IR Spectroscopy. Both Varian 300 and 500 MHz
spectrometers were used to record the 1H NMR and 31P NMR spectra
at ambient temperature, and either a Varian 400 or 500 MHz
spectrometer was used to record 15N NMR spectra and all VT- NMR
CONCLUDING REMARKS
■
1
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
spectra. H 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
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H3CC15N (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.
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 of
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
of toluene and chilled to −78 °C. After 1 h, the phosphine was added
dropwise 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
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 F2 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 semifreely 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.
1
phosphine (5.765 g, 84% yield). H 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, 2JH−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) were 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 dropwise 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
2
(d, JH−P = 3.3 Hz, 2H). 31P NMR (THF, 121 MHz): δ −4.08 ppm.
Some of the structures reported suffered from disorder in parts of
the [PhBPR ]− ligand, and the disorder was modeled over two
Synthesis of [PhBPmter3]Tl. In a vial, (m-terphenyl)2PCH2Li-
(TMEDA) (365.0 mg, 608.6 μmol) was dissolved in 10 mL of Et2O
and then chilled to −90 °C. PhBCl2 (33.2 mg, 202.8 μmol) was
diluted in 3 mL of toluene and added dropwise to the solution. The
reaction was stirred for 18 h, slowly warming to RT to give
[PhBPmter3]Li(TMEDA) (31P NMR: −10.4 ppm). The reaction was
concentrated under reduced pressure to dryness and then suspended
in 10 mL of 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
3
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. 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 composed of
two or three different species that cocrystallized. {[PhBPPh3]Fe-
(N2H4)}(PF6) (7), cocrystallized with {[PhBPPh3]Fe(N2H4)(Cl) and
[PhBPPh3]FeCl. With the aid of free variables, it was determined that
there was a 3% impurity of [PhBPPh3]FeCl and 20% {[PhBPPh3]Fe-
(N2H4)(Cl). [PhBPmter3]FeCl cocrystallized with [PhBPmter3]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. [PhBPPh3Fe]Me,12 6,8
15NH215NH2,6e 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.
1
ether (237.1 mg, 62% yield). H 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 (over-
lapping, 48H), 2.77 (br, 6H). 31P NMR (C6D6, 121 MHz): δ 21.7 ppm
(d, 1JTlP = 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 [PhBPmter3]FeCl. [PhBPmter3]Tl (0.473 g, 0.262
mmol) and FeCl2 (0.034 g, 0.262 mmol) were stirred in 8 mL of 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
1
[PhBPmter3]FeCl. H 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 μB Anal. Calcd. for C117H89BClFeP3: C 83.15; H
5.31; N 0. Found: C 83.09; H 5.41; N 0.
Synthesis of [PhBPmter3]FeMe. A solution of [PhBPmter3]FeCl
(0.2096 g, 0.141 mmol) in 15 mL of benzene was added to a stirring
slurry of Me2Mg (0.0186 g, 0.342 mmol) in 2 mL of 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 of benzene, filtered through a Celite-lined frit, and again
lyophilized to yield analytically pure [PhBPmter3]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
μB UV−vis (C6H6) λmax, nm (ε, 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.
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
autoignition temperature that is highly dependent on the presence of
impurities. Prior to working with anhydrous hydrazine, 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 hydrazine, a
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Synthesis of [PhBPPh3]Fe(OAc). Neat acetic acid (29.5 μL, 0.515
mmol) was added to a solution of [PhBPPh3]FeMe (0.3892 g, 0.515
mmol) in 18 mL of 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 μB Anal. Calcd. for C47H44BFeO2P3: C 70.52; H 5.74; N 0.
Found: C 71.85; H 5.74; N 0.
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) λmax, nm (ε, M−1 cm−1): 400 (2900, sh), 516 (1460).
Synthesis of [PhBPmter3]Fe(η2-N2H3), 1. [PhBPmter3]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 of THF was
added dropwise. The stirring reaction immediately changed color from
yellow to green, and 1 was quantitatively formed. Microcrystals 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) λmax, 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 3 was synthesized using an
analogous synthetic procedure with 15NH215NH2. H NMR THF-d8,
1
400 MHz, −45 °C): δ 5.32 (d, 1JN1H ≈ 82 Hz, 1H, NHH), 3.58 (d, 1H,
NHH), 1.83 (1H, NH), 0.41 (d, JNH ≈ 60 Hz, 3H, NH3). gHMQC
15N{1H} NMR (THF-d8, 40.5 MHz, −45 °C): δ 31.8 (NH), 26.0
1
(NH2), −18.9 (NH3). Select H{15N} decoupling was employed to
confirm the HN connectivity.
Synthesis of [PhBPPh3]Fe(η2-NHNMe2), 4. Neat NH2NMe2 (28.2
μL, 0.363 mmol) was added to a stirring solution of [PhBPPh3]FeMe
(0.2495 g, 0.3299 mmol) in 10 mL of 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) λmax, 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.
A sample of 95% 15N-enriched 1 was synthesized using an
analogous synthetic procedure with 15NH215NH2. H NMR (THF-
1
1
d8, 500 MHz, −25 °C): δ 6.43 (d, JNH ≈ 79 Hz, 1H, NH), 3.81 (d,
1JNH ≈ 83 Hz, 2H, NH2). 15N NMR (THF-d8, 50.7 MHz, −25 °C): δ
1
1
1
139.0 (dd, JNH ≈ 78.6 Hz, JNN ≈ 11 Hz), −14.5 (dt, JNH ≈ 83 Hz,
1JNN ≈ 11 Hz).
Synthesis of [PhBPmter3]Fe(η2-N2H3)(η1-N2H4), 2. [PhBPmter
]
-
3
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 of THF was
added dropwise. The stirring reaction immediately changed color from
yellow to green to red, indicative of formation of 2. Microcrystals 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 [PhBPmter3]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 NαH2), 4.66 (s,
2H, NβH2), 3.18 (s, 1H, NH2 or NαH2), 2.91 (s, 1H, NH2 or NαH2),
2.72 (s, 1H, NH), 2.52 (s, 1H, NH2 or NαH2), 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) (cm−1):
3305, 3170. UV−vis (THF, with 20 equiv of N2H4) λmax, nm (ε, M−1
cm−1): 383 (2600, sh), 512 (1280).
Synthesis of [PhBPPh3]Fe(Me)(η2-N2H4), 5. [PhBPPh3]FeMe
(0.0343 g, 0.0391 mmol) was dissolved in 500 μL of 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 dropwise, resulting in a
1
color change from yellow to strawberry red and conversion to 5. H
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, NHcisH), 3.13 (s, 2H, NHHtrans), 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) λmax, 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. H NMR (THF-
1
d8, 500 MHz, −50 °C): δ 4.31 (d, 1JNH ≈ 77 Hz, 1.5H, NHcisH), 3.12
1
(d, JNH ≈ 75 Hz, 1.5H, NHHtrans). 15N NMR (THF-d8, 50.7 MHz,
1
−50 °C): δ 17.3 (t, JNH ≈ 76 Hz, 2N). IR (THF/KBr, −78 °C)
(cm−1): 3312, 3251, 3223.
Synthesis of {[PhBPPh3]Fe(η2-N2H4)}(PF6), 7. To a solution of
PhBPPh3FeCl (0.6023 g, 0.775 mmol) in 20 mL of 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,
A sample of 95% 15N-enriched 2 was synthesized using an
analogous synthetic procedure with 15NH215NH2. H NMR (THF-
1
1
d8, 500 MHz, −40 °C): δ 4.92 (d, JNH ≈ 80 Hz 1H, NH2 or NαH2),
4.66 (d, 1JNH ≈ 65 Hz, 2H, NαH2), 3.18 (d, 1JNH ≈ 75 Hz 1H, NH2 or
NαH2), 2.91 (d, 1JNH ≈ 75 Hz, 1H, NH2 or NαH2), 2.72 (d, 1JNH ≈ 60
1
Hz, 1H, NH), 2.52 (d, JNH ≈ 80 Hz, 1H, NH2 1or NαH2). 15N NMR
1
(THF-d8, 50.7 MHz, −40 °C): 47.4 (dt, NβH2, JNH ≈ 65 Hz, JNN
=
−11 Hz), 40.6 (dd, NH, 1JNH ≈ 58 Hz, 1JNN = −13 Hz), 23.6 (dt, NH2
or NαH2, 1JNH ≈ 73 Hz, 1JNN = −11 Hz), 22.7 (dt, NH2 or NβH2, 1JNH
≈ 80 Hz, JNN = −13 Hz). Select H{15N} decoupling was employed
1
1
to confirm the HN connectivity.
Synthesis of [PhBPmter3]Fe(η2-N2H3)(NH3), 3. A solution of 1
(0.0150 g, 0.00890 mmol) in 1 mL of 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
2
NH2, 2H), 1.30 (d, CH2, JHP ≈ 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) λmax, nm (ε, M−1 cm−1): 420 (250), 525 (675). Crystals of 7
invariable contained {[PhBPPh3]Fe(η2-N2H4)}(PF6), [PhBPPh3]FeCl,
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Inorganic Chemistry
Article
and {[PhBPPh3]Fe(η2-N2H4)}(Cl), precluding our ability to obtain
analytically pure material.
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) λmax, 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 [PhBPPh3]Fe(η1-N2H4)(OAc), 8. Neat anhydrous
hydrazine (10.2 μL, 0.3159 mmol) was added to a 2 mL THF solution
of [PhBPPh3]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.
Synthesis of [PhBPmter3]Fe(NH3)(OAc), 11. Hydrazine (6.4 μL,
0.020 mmol) was added to a stirring solution of [PhBPmter3]FeMe
(0.1439 g, 0.0982 mmol) in 5 mL of THF. After stirring for 10 min, a
suspension of Pb(OAc)4 (0.0871 g, 0.0196 mmol) in 5 mL of 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 [PhBPmter3]FeMe.
One equivalent of AcOH (2.4 μL, 0.041 mmol) was added to a
solution of [PhBPmter3]FeMe (0.0686 g, 0.0411 mmol) in 2 mL of
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%).
A sample of 95% 15N-enriched 8 was synthesized using an
analogous synthetic procedure with 15NH215NH2. H NMR (THF-
1
d8, 500 MHz, −50 °C): δ 4.61 (d, 1JNH = 70 Hz, 2H, NαH2), 3.94 (d,
1JNH = 67 Hz, 2H, NβH2). 15N NMR (THF-d8, 50.7 MHz, −50 °C): δ
1
1
56.2 (t, JNH = 67 Hz, 1N, NβH2), 33.3 (t, JNH = 68 Hz, 1N, NαH2).
1H{15N} experiments with selective decoupling were used to correlate
the H and 15N NMR chemical shifts. IR (KBr) (cm−1): 3367, 3300,
1
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) λmax, 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.
3274.
Complex 8 can alternatively be prepared from the salt metathesis of
7 with sodium acetate.
Synthesis of [PhBPPh3]Fe(NH3)(OAc), 9. A suspension of
Pb(OAc)4 (0.0161 g, 0.0360 mmol) in 1 mL of THF was added
dropwise to a stirring solution of 7 (36.0 mg, 0.036 mmol) in 2 mL of
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.
A sample of 95% 15N-enriched 11 was synthesized using the
1
alternative synthesis using 15NH3. H NMR (THF-d8, 400 MHz, −30
1
°C): δ 3.05 (d, JNH = 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 [PhBPPh3]Fe(OAc)(CO), 12. A suspension of
Pb(OAc)4 in 1 mL of 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 of
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
Complex 9 can alternatively be prepared by addition of 1 atm of
NH3 to a solution of [PhBPPh3]Fe(OAc) (0.0284 g, 0.0355 mmol) in 4
mL of benzene (in an evacuated 50 mL Schlenk-tube). After stirring
for 5 min, 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) λmax, nm (ε,
M−1 cm−1): 580 (855).
1
suitable for XRD (12.0 mg, 47.5%). H 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.
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 [PhBPPh3]Fe(NH3)-
(OAc).C6H6, C53H53BFeP3NO2: C 71.10; H 5.96; N 1.56. Found: C
70.70; H 6.06; N 1.50.
Synthesis of {[PhBPPh3]Fe(η2-N2H4)(CO)}(PF6), 13. A suspension
of [Fc](PF6) (15.0 mg, 0.0455 mmol) in 1 mL of benzene was added
dropwise to a stirring solution of 6 (36.4 mg, 0.0455 mmol) in 2 mL of
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,
A sample of 95% 15N-enriched 9 was synthesized following the
alternative procedure, using 15NH3. 1H NMR (THF-d8, 400 MHz, −30
1
°C): δ 2.36 (d, JNH = 67 Hz, 3H, NH3). HSQC 15N{1H} NMR
(THF-d8, 40.5 MHz, −30 °C): δ −13.8. 31P NMR (THF-d8, 161.8
1
1
MHz, −40 °C): δ 61.5 (bs, 2P), 46.9 (dt, JPP ≈ 58.6 Hz, JNP ≈ 10
Hz, 1P). IR (KBr) (cm−1): 3354, 3327, 1466.
1
Synthesis of {[PhBPPh3]Fe(η2-N2H4)(NH3)}(PF6), 10. A solution
of [PhBPPh3]FeMe (0.2181 g, 0.2883 mmol) in 15 mL of THF was
cooled to −41 °C and set stirring. To this, a solution of hydrazine
(18.7 μL, 0.577 mmol) in 1 mL of THF was added dropwise over the
course of 5 min. A suspension of [Fc](PF6) (0.0954 g, 0.2883 mmol)
in 4 mL of THF was added dropwise, 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,
42.3%). H 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
1
1
MHz): δ 50.22 (d, JPP = 63.6 Hz, 2P), 36.00 (t, JPP = 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.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures for reactions with quinones,
NMR spectra of 2 and 5, and crystal structure figures are
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Article
(d) Lee, Y. H.; Mankad, N. P.; Peters, J. C. Nat. Chem. 2010, 2, 558.
(e) Schrock, R. R.; Liu, A. H.; O’Regan, M. B.; Finch, W. C.; Payack, J.
F. Inorg. Chem. 1988, 27, 3574. (f) Schrock, R. R.; Glassman, T. E.;
Vale, M. G.; Kol, M. J. Am. Chem. Soc. 1993, 115, 1760. (g) Cai, S.;
Schrock, R. R. Inorg. Chem. 1991, 30, 4105. (h) Vale, M. G.; Schrock,
R. R. Organometallics 1991, 10, 1661. (i) Vogel, S.; Barth, A.; Huttner,
G.; Klein, T.; Zsolnai, L.; Kremer, R. Angew. Chem., Int. Ed. Engl. 1991,
30, 303. (j) Crossland, J. L.; Balesdent, C. G.; Tyler, D. R. Dalton
Trans. 2009, 4420. (k) McCleverty, J. A.; Rae, A. E.; Wolochowicz, I.;
Bailey, N. A.; Smith, J. M. A. J. Chem. Soc., Dalton Trans. 1983, 71.
(l) McCleverty, J. A.; Rae, A. E.; Wolochowicz, I.; Bailey, N. A.; Smith,
J. M. A. J. Chem. Soc., Dalton Trans. 1983, 71. (m) Shapiro, P. J.;
Henling, L. M.; Marsh, R. E.; Bercaw, J. E. Inorg. Chem. 1990, 29,
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included in the Supporting Information (.pdf). Crystallographic
details for all structures are provided in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jpeters@caltech.edu.
■
Present Address
†Department of Chemistry, University of Minnesota, Minne-
apolis, Minnesota 55455.
Notes
The authors declare no competing financial interest.
(7) (a) Vale, M. G.; Schrock, R. R. Inorg. Chem. 1993, 32, 2767.
(b) Shaver, M. P.; Fryzuk, M. D. J. Am. Chem. Soc. 2005, 127, 500.
(c) Latham, I. A.; Leigh, G. J. J. Chem. Soc., Dalton Trans. 1986, 399.
(8) Saouma, C. T.; Kinney, R. A.; Hoffman, B. M.; Peters, J. C.
Angew. Chem., Int. Ed. 2011, 50, 3446.
(9) (a) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252.
(b) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
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ACKNOWLEDGMENTS
Dr. David VanderVelde, and Dr. Jeff Simpson are acknowl-
edged for insightful discussions regarding the NMR spectros-
copy of the compounds. Dr. Peter Muller, Dr. Michael Day, and
Dr. Larry Henling are acknowledged for insightful discussions
regarding X-ray crystallography. This work was generously
supported by the NIH (GM-070757), and the Gordon and
Betty Moore Foundation. Funding for the Caltech NMR facility
was provided in part by the NIH (RR 027690). C.T.S. is
grateful for an NSF graduate fellowship.
■
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