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Article
N-H Bond Dissociation Enthalpies and Facile H-atom Transfers
for Early Intermediates of Fe-N2 and Fe-CN Reductions
Jonathan Rittle, and Jonas C. Peters
J. Am. Chem. Soc.
,
Just Accepted Manuscript
• DOI: 10.1021/jacs.6b12861
• Publication Date (Web): 31 Jan 2017
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1
N
-
H Bond Dissociation Enthalpies and Facile H
-
atom Transfers for Early Intermediates of Fe
-
N
2
and Fe
-
CN Reductions
Jonathan Rittle and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Abstract
Fe
-
mediated biological nitrogen fixation is thought to proceed either via a sequence of proton
and electron transfer steps, concerted H
-
atom transfer steps, or some combination thereof.
Regardless of th
e specifics, and whether the intimate mechanism for N
2
-
to
-
NH
3
conversion involves
a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe
-
N
x
H
y
intermediates are implicated that feature reactive N
-
H bonds. Thermodynamic know
ledge of the N
-
H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive
early stage candidate intermediates (e.g., Fe
-
N=NH, Fe=N
-
NH
2
, Fe
-
NH=NH). Such knowledge is
essential to considering various mechanistic hy
potheses for biological (and synthetic) nitrogen
fixation, and to the rational design of improved synthetic N
2
fixation catalysts. We recently reported
several reactive complexes
derived from the direct protonation of
Fe
-
N
2
and
Fe
-
CN
species
a
t the
termina
l N
-
atom (e.g.,
Fe
=N
-
NH
2
,
Fe
-
C
º
NH,
Fe
º
C
-
NH
2
)
.
These same Fe
-
N
2
and Fe
-
CN systems
are functionally active for N
2
-
to
-
NH
3
and
CN
-
to
-
CH
4
/NH
3
conversion
, respectively,
when subjected
to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N
-
H
bond strength data. W
e
report
here a combined synthetic, structural, and spectroscopic/analytic study
to estimate the
N
-
H bond strengths of se
veral species of interest. W
e assess the reactivity
profiles of
species featuring reactive
N
-
H bonds
,
and estimate their homolytic N
-
H bond enthalpies via redox
and acidity titrations. Very low N
-
H bond dissociation enthalpies (BDE
N
-
H
)
,
ranging from 65
(e.g.,
Fe
-
C
º
NH)
to
≤
37
kcal/mol
(Fe
-
N=NH),
are
determined. The collective data presented
herein
provides
insight into
the facile
reactivity
profiles of early stage protonated Fe
-
N
2
and Fe
-
C
N
species.
Introduction
The
co
nversion of N
2
to NH
3
via reductiv
e protonation by six electrons and protons
is a
fascinating transformation that
is
distinct from the Haber
-
Bosch process
1
and may offer
a conceptual
pathway for direct, distributed solar
-
driven ammonia synthesis
.
2
Free
N
2
exhibits a low electron
affinity (
-
1.9 eV) and proton affinity (118 kcal/mol)
,
3
and a
ctivation by a suitable catalyst is hence
required
. While growing in number,
4
f
ew
well
-
defined
molecular
systems
mediate catalytic N
2
-
to
-
NH
3
conversion; those that do use
a combination of inorganic reductant and acid rather than H
2
.
Nitrogenase
enzymes
represent the most efficient and well
-
studied
non Haber
-
Bosch
catalysts
,
and
are collectively responsible for the generation of ~50% of the global fixed nitrogen pool.
5
T
hes
e
enzymes sustain life
and provide proof
-
of
-
concept that reductive protonation of N
2
can be carried
-
out under ambient conditions on a grand scale.
But we
do not
as yet
have a clear
mechanistic
picture
of
this biological transformation
.
Well defined inorgan
ic model complexes
that
catalyze
N
2
-
to
-
NH
3
conversion are
particularly
well
suited
to
mechanistic investigations
and
a
number
of synthetic systems featuring
early transition
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2
metals (especially Mo)
have been studied in the context of N
2
activatio
n
6
a
nd catalytic
N
2
-
to
-
NH
3
conversi
on.
4
A
n early hallmark of the Mo model
system
s, noteworthy owing to the presence of Mo
in the FeMo
-
cofactor of MoFe
-
nitrogenase,
has been
the ability to independently prepare many of
the intermediate Mo
-
N
x
H
y
species
thought to be relevant
to
N
2
-
to
-
NH
3
conversion
.
7
Fe is an essential element in all known nitrogenases and there is growing eviden
ce that it plays
a critica
l mechanistic role in the key bond
-
making and breaking events of overall
N
2
-
to
-
NH
3
conversion.
5
,
8
For example,
structural, spectroscopic and theoretical evidence
implicate
the
involvement of one
(
or more
)
of
the
iro
n atoms of the FeMo
-
cofactor in substrate binding/activation.
8
This state of affairs
has motivated
the study of
well
-
defined
Fe
-
N
x
H
y
species relevant to N
2
activation
chemistry
.
In this context
, Holland and Murray
have
for instance
independently demonstrated that
beta
-
diketiminate
-
supported iron systems
enable the
deliver
y of
electron
equivalent
s to N
2
to
generate
bridging
Fe
-
nitrido and/or imido
species.
9
,
10
N
2
-
derived NH
3
can then be released from
these systems upo
n subsequent protonation.
9
b
Our
lab
has
reported several
Fe
systems
that catalyze N
2
-
to
-
NH
3
con
version,
11
and
we have
very recently reported on a
related (but as yet only stoichiometric) reductive protonation of
Fe
-
CN
to
release
CH
4
and NH
3
.
12
I
nterest in this sub
-
field is growing.
Very r
ecently, Nishibayashi and
coworkers disclosed a phosphine
-
pyrrole Fe
-
N
2
system that is a competent catalyst for NH
3
generation (
along with
some N
2
H
4
),
4
a
a
nd Ashley and coworkers reported a low
-
valent phosphine
-
only Fe
-
N
2
system that mediates catalytic N
2
-
to
-
N
2
H
4
conversi
on.
4
b
Fe
-
mediated,
catalytic N
2
activation
in these Fe systems
is
thought to involve an initial end
-
on binding of N
2
to a single Fe
atom
,
followed
(in our P
3
E
Fe
-
systems at least (E = B, C, Si))
by
reductive protonation
at the terminal
N
-
atom to afford Fe=N
-
NH
2
species
(Sch
eme 1).
13
F
e
-
mediated
reduction
of
P
3
Si
Fe
-
CN
is thought
to proceed via an isostructural Fe
≡
C
-
NH
2
spec
ies.
12
T
hese species are characteristic of a ‘distal’ or
‘Chatt
-
t
ype’ pathway for substrate reduction that is commonly thought to operate in certain N
2
reduction systems.
3
,
4
,
7
Re
cent mechanistic studies of one of our reported Fe systems (P
3
Si
Fe) also
hints at competing downstream p
athways
following the generation of the doubly
-
protonated distal
intermediate P
3
Si
Fe=
N
-
NH
2
.
13
T
hese
Fe=N
-
NH
2
(
and
related
Fe
≡
C
-
NH
2
)
species
display a high
degree of instability
in comparison to analogous Mo=N
-
NH
2
species; they for instance
decay rapidly
at ambient
and even lower
temperature
s in solution and
furnish
mixtures of H
2
and NH
3
.
Understanding the factors that govern their reactivity patterns is critical from a mechanistic
perspective and will aid in the
development of improved catalysts that show better selectivity for
NH
3
versus H
2
.
We
anticipate
that t
he high degree of reactivity
observed
in certain Fe
-
N
x
H
y
(and Fe
-
CNH
x
) species
must be
correlated with
unusually
weak N
-
H bonds
; this should be especially
true of
the first protonated species, Fe
-
N
=N
H (and Fe
-
C
º
NH).
14
T
hese weak N
-
H bonds
might then
serve
a
dual
role: they
could be
susceptible to undesirable H
2
formation pathways that short circuit
overall
substrate
reduction,
while concomitantly
facilitating
downstream H
-
atom
-
transfer pathways
that
productively
lead to reduced product
.
Whereas the thermochemistry of N
-
H bond formation in certain Ti
-
, Zr
-
and Mo
-
containing
scaffolds operative in stoichiometric NH
3
formation has been rece
ntly
studied,
15
such
information is
generally scant and we saw an opportunity to map the N
-
H bond strengths of the most highly reactive
species relevant to Fe
-
mediated N
2
-
to
-
NH
3
(and CN
-
) conversion catalysis.
In this report we
investigate H
+
/e
-
transfer to
seve
ral
Fe
-
CN
-
and
Fe
-
N
2
-
derived species
using
the
tripodal
tris(phosphino)silyl
ligand, P
3
Si
(P
3
Si
=
[
(2
-
i
Pr
2
P
-
C
6
H
4
)
3
Si
]
-
; Scheme 1
).
While the P
3
Si
Fe
-
N
2
system
is not as active as its
P
3
B
Fe
-
cousin for N
2
-
to
-
NH
3
conversion,
it does mediate NH
3
generation and is
technically better suited to the types of studies detailed here. Similarly, the P
3
Si
Fe
-
CN system is
competent for NH
3
and CH
4
generation
12
and offers a number of advantages in terms of measuring
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N
-
H bond strengths, including experimental access to the first protonated intermediate (P
3
Si
Fe
-
C
º
NH
+/0
) and well
-
behaved redox and acid
-
base properties.
Scheme 1
.
Previously reported and
crystallographically characterized
Fe
complexes derived from
protonation of
P
3
Si
Fe
-
N
2
or
P
3
Si
Fe
-
CN
precursors
.
12
,
13
b
Results and Discussion
N
-
H Bond Strength of
P
3
Si
Fe
-
C
≡
NH
0/+
While the
conversion
of
Fe
-
N
2
to an Fe
-
N
x
H
y
species could i
n principl
e
be mediated by
HAT steps, most
commonly used H
-
atom donor reagents are not sufficiently reactive to donate H
-
atoms to
Fe
-
N
2
precu
rsors.
16
The
same is true of Fe
-
CN species.
12
Consistent with this statement,
there is no reaction betwe
en
either
P
3
Si
Fe
-
N
2
or
P
3
Si
Fe
-
CN
with
9,10
-
dihydroanthracene (BDE
CH
=
81 kcal/mol) or
TEMPOH (BDE
CH
= 71 kcal/mol) within one hour
at
room temperature
in
THF
solution (Scheme
2).
17
T
his observation suggests (but does not require)
that the BDE
N
-
H
values for
the corresponding
P
3
Si
Fe
=
C
=
NH and
P
3
Si
Fe
-
N
=
NH
species
are < 70 kcal/mol.
For comparison,
the
experimentally
-
determined
BDE
N
-
H
of diazene, HN=NH, is ~ 61 kcal/mol
.
18
Interestingly
, a
n
immediate reaction is observed
up
on combining [
P
3
Si
Fe
-
C
≡
NH][BAr
F
24
]
12
with 1 equivalent of
TEMPO
×
to form [
P
3
Si
Fe
-
CN][BAr
F
24
]
and
TEMPOH (Scheme 2). This
latter observation
indicates
that the homolytic bond dissociation free energy (BDFE) and/or bond
dissociation enthalpy (BDE)
o
f the N
-
H bond
in [
P
3
Si
Fe
-
C
≡
NH][BAr
F
24
]
is
significantly lower than that of
the BDE
O
-
H
and/or
BDFE
O
-
H
in TEMPOH.
Scheme 2.
Hydrogen atom transfer reactions studied with P
3
Si
Fe
-
N
2
and P
3
Si
Fe
-
CN
-
derived
compounds.
The
prolonged THF solution
stability
of
P
3
Si
Fe
-
CN
,
P
3
Si
Fe
-
C
N
+
,
P
3
Si
Fe
-
C
≡
N
H
+
, and
mixtures thereof
permits the collection of data
that can then be used to quantify the BDE
N
-
H
of
P
3
Si
Fe
-
C
≡
N
H
+
.
The solution BDE
N
-
H
is proportional to the sum of the acidity of the relevant N
-
H
Fe
Si
C
P
i
Pr
2
i
Pr
2
P
i
Pr
2
P
N
H
Fe
Si
C
P
i
Pr
2
i
Pr
2
P
i
Pr
2
P
N
OTf
H
H
BAr
F
24
Fe
Si
N
P
i
Pr
2
i
Pr
2
P
i
Pr
2
P
N
OTf
H
H
-
TEMPOH
no
reaction
P
3
Si
Fe-N
2
or
P
3
Si
Fe-CN
TEMPOH
or
9,10-dihydroanthracene
RT
[P
3
Si
Fe-C
≡
NH][BAr
F
24
]
TEMPO
·
[P
3
Si
Fe-CN][BAr
F
24
]
RT
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4
bond (pK
a
) and the oxidation potential (E
ox
o
), and may be estimated by t
he application of equations
1
or 2,
BDE(N
-
H
+
) = 23.06*E
ox
(N) + 1.37*pK
a
(N
-
H
+
) + C
H
Eqn 1
BDE(N
-
H) = 23.06*E
ox
(N
-
H) +
1.37*pK
a
(N
-
H
+
) + C
H
Eqn 2
where C
H
is a solvent
-
dependent constant reported as 66 kcal/mol
in THF.
17
,
19
,
20
Cy
clic
voltammetry
of
P
3
Si
Fe
-
CN
in a THF
electrolyte (0.1 M [TBA][PF
6
]) rev
eal
s
a
one
-
electro
n
oxidation
event
at
-
0.38 versus the
ferrocene/ferrocenium couple
.
[
P
3
Si
Fe
-
C
≡
N
H
][BAr
F
24
]
displays
a reduction event at
-
1.27 V
under similar conditions
(Figure 1)
.
To determine the
apparent
acidity
of
[
P
3
Si
Fe
-
C
≡
N
H
]
+
in THF solution,
titrations of
P
3
Si
Fe
-
CN
were performed with a series of acids
and monitored by
UV
-
vis
ible
spectroscopy.
P
3
Si
Fe
-
CN
does not detectably react with the acids
[HNEt
3
][OTf] (pK
a
: 13.7) or [LutH][OTf] (pK
a
: 9.5)
;
stoichiometric addition of
the
chlorinated
anilinium salts
[2
-
Cl
-
PhNH
3
][O
Tf] (pK
a
= 6.0 in THF) or [2,5
-
Cl
2
-
PhNH
3
][OTf] (pK
a
= 4.5 in
THF)
results in
the f
ormation of equilibrium mixtures of
P
3
Si
Fe
-
CN
and [
P
3
Si
Fe
-
C
≡
N
H
][
OTf
] at
room temperature (Fig
ure 1).
21
Sp
ectrophotometric titrations performed with
these acids
and
P
3
Si
Fe
-
CN
furnish an average pK
a
=
5.6(1)
for the acidic
N
-
H
proton of
[
P
3
Si
Fe
-
C
≡
N
H
][
OTf
]
.
Figure 1.
(A)
UV
-
visible absorption spectra of
P
3
Si
Fe
-
CN
(dark red) and
[
P
3
Si
Fe
-
C
≡
N
H
][
OTf
]
(black) in THF at 25
o
C. The other traces were obtained following the sequential addition of 0.33
molar equivalents of [2
-
Cl
-
PhNH
3
][OTf] to
P
3
Si
Fe
-
CN
.
The dark blue trace was obtained following
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the combined addition of 2.66 equiv [2
-
Cl
-
PhNH
3
][OTf] to
P
3
Si
Fe
-
CN
. Arrows indicate features
whose intensity increases or decreases during the experiment. Cyclic voltammograms of (B)
P
3
Si
Fe
-
CN and (C) [P
3
Si
Fe
-
C
≡
NH][BAr
F
24
] scanned at 200 mV/s in 0.1 M [TBA][PF
6
] THF
electrolytes.
Solid lines indicate voltammograms i
nitially scanned in the anodic direction, and
dashed lines indicate voltammograms initially scanned in the cathodic direction.
With these data
in hand, the
BDE
N
-
H
of
[
P
3
Si
Fe
-
C
≡
N
H
][
OTf
]
and its one
-
electron reduced
relative,
P
3
Si
Fe
=
C
=
N
H, may
be estimated
using
Eqns 1 and 2, respectively
.
These values
are found
to be 65(1) and 44(1) k
cal/mol, respectively (Table 1),
and
are consistent with the complete and
irreversible hydroge
n atom abstraction reaction
between
[
P
3
Si
Fe
-
C
≡
N
H
]
[BAr
F
24
]
and
TEMPO
×
(BDE
O
-
H
= 71 kcal/mol)
,
and the inability of
P
3
Si
Fe
-
CN
to react with TEMPOH or
dihydroanthracene via hydrogen atom abstra
ction.
T
he decrease by more than 20 kcal/mol in the
N
-
H bond strength on going from cationic
[
P
3
Si
Fe
-
C
≡
N
H
]
+
to neutral
P
3
Si
Fe
=
C
=
N
H is striking.
Table 1.
Experimentally determined thermodynamic quantities.
a
Acidity values determined in
THF.
b
Redox potentials obtained in THF
vs Fc/Fc
+
.
c
Values estimated using Eqn 1 and 2.
d
nr = not
relevant.
e
Values
estimated via the application of Hess’s law.
f
nd = not determined.
Compound
pK
a
a
E
ox
b
BDE
N
-‐
H
c
P
3
Si
Fe
-‐
C
N
nr
d
-‐
0.38
nr
d
P
3
Si
Fe
-‐
C≡NH
+
5.6
-‐
0.17
65
P
3
Si
Fe
=
C
=
NH
19
e
-‐
1.27
44
P
3
Si
Fe
=
C
=
NMe
nr
d
-‐
1.31
nr
d
P
3
Si
Fe≡C
-‐
N(Me)H
+
7.1
nd
f
47
P
3
Si
Fe≡C
-‐
N(Me)H
24
e
-‐
1.27
46
P
3
Si
Fe
-‐
N≡N
+
nr
d
-‐
1.1
nr
d
P
3
Si
Fe
-‐
N=NH
+
≤ 0
g
nd
f
≤ 43
g
P
3
Si
Fe
-‐
N=NMe
nr
d
-‐
1.22
nr
P
3
Si
Fe=N
-‐
N(Me)H
+
8.0
nd
f
48
We think this dramatic
difference is
largely
a consequence of varying degrees of C
-
N bond
weakening that occur concomitant with H
-
atom
addition to the Fe
-
CN unit
.
Crystallographic
studies indicate that in cationic
P
3
Si
Fe
-
C
≡
N
H
+
a short C
-
N bond is observed (
1.130(3) Å) and
the
N
-
atom is largely sp
-
hybridized (<CNH: 176(3)
o
)
(Table 2)
.
12
Density Functional Theory (DFT)
calculations on the gas
-
phase optimized geometry of
P
3
Si
Fe=
C
=
N
H
predict an sp
2
-
hybridized N
-
atom (<CNH: 126.6
o
) and
a
long C
-
N bond (1.233
Å
)
(Supporting
Informat
ion).
Comparison of
these C
-
N bond lengths with the corresponding P
3
Si
Fe
-
CN
+/0
species reveal that
formal
H
-
atom
addition to P
3
Si
Fe
-
CN
+
results in minimal
alteration to the C
-
N bond length
(0.014
Å) whereas H
-
atom addition to
neutral
P
3
Si
Fe
-
CN results in significant C
-
N bond lengthening (0.066
Å).
Accordingly, the measured BDE
N
-
H
values reflect the extent to
which the C
≡
N bond is
weakened;
weak N
-
H bonds are correlated with an HAT process that causes significant
weakening of the C
≡
N (or N
≡
N) bond. Generally speaking, w
eak N
-
H bonds
may be
signatures
Page 5 of 19
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Journal of the American Chemical Society
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6
of early
-
stage intermediates
of N
2
/CN activation
that are preceded by the most challenging H
-
atom additions.
Identifying these intermediates is critical to understanding
the thermodynamic
challenges
associated with
these difficult bond activation processes.
Table
2
.
Relevant metrical data. Measurements are derived from
X
-
ray diffractions studies unless
oth
erwise indicated
.
a
Values in
Å
.
b
Values in
o
.
c
nr = not relevant
.
d
DFT
-
optimized geometry
.
e
Values
taken from predominate disordered isomer
.
f
Refers to C
-
N
-
C angle.
g
Average of two
values
.
h
Refers to N
-
N
-
C angle.
Compound
d [Fe
-‐
C/N]
a
d [(C/N)
-‐
N]
a
<
[
Fe
-‐
(C/N)
-‐
N
]
b
< (C/N)
-‐
N
-‐
(C/H)
b
P
3
Si
Fe
-‐
CN
+
2.025(2)
1.130(3)
179.3(2)
nr
c
P
3
Si
Fe
-‐
CN
1.973(1)
1.167(1)
179.7(1)
nr
c
P
3
Si
Fe
-‐
C≡NH
+
1.913(3)
1.144(4)
178.4(2)
176(3)
P
3
Si
Fe=C=NH
d
1.771
1.23
3
174.4
126.6
P
3
Si
Fe
-‐
C≡NMe
+
1.912(2)
1.157(3)
176.1(2)
176.1(3)
P
3
Si
Fe=C=NMe
1.821(2)
1.186(2)
177.6(1)
150.3(2)
P
3
Si
Fe≡C
-‐
N(Me)H
+ e
1.751(9)
1.28(1)
169.2(6)
127.6(5)
f
P
3
Si
Fe≡C
-‐
N(Me)H
d
1.689
1.332
175.3
126.5
e
P
3
Si
Fe
-‐
NN
+
1.913(2)
1.091(3)
177.1(2)
nr
c
P
3
Si
Fe
-‐
NN
1.819(1)
1.124(2)
177.5(1)
nr
c
P
3
Si
Fe
-‐
N=NH
+ d
1.684
1.212
176.8
117
P
3
Si
Fe
-‐
N=NH
d
1.675
1.231
177.1
113.8
P
3
Si
Fe=N
-‐
NH
2
+
1.672(2)
1.272(3)
175.3(2)
120(3)
g
P
3
Si
Fe=N
-‐
NH
2
d
1.747
1.294
150.6
122(1)
g
P
3
Si
Fe=N
-‐
N(Me)H
+
1.671(3)
1.284(4)
174.7(3)
121.6(3)
h
Mono
-
protonation of Fe
-
N
2
Terminally
-
bound iron diazenido
complexes (Fe
-
N=NH) have been invoked as the first
intermediates of biological N
2
reduction
5
b
and are likewise presumed to be reactive intermediat
es in
stoichiometric and catalytic N
2
-
to
-
NH
3
reactions effected by
P
3
E
Fe
-
N
2
(E = B, C, Si) and related
systems.
11
,
13
Dir
ect characterization of Fe
-
N=NH species has proven to be a significant chall
enge
and circumstantial
observations suggest tha
t the BDFE
N
-
H
and/or BDE
N
-
H
values for
P
3
E
Fe
-
N=NH
0/+
intermediates are significantly lower than that of gas phase HN=NH (~6
1
kca
l/mol),
18
but can be
anticipated to be
similar to
,
or
perhaps
less than
,
that
estimated for
P
3
Si
Fe
=
C
=
NH
(44 kcal/mol)
.
Given the comparatively high stability of the related mono
-
protonated cyanide
complex,
[
P
3
Si
Fe
-
C
≡
NH][BAr
F
24
], we wondered whether the N
-
H bond strength in isostructural
P
3
Si
Fe
-
N=NH
+
could be estimated. Although t
he oxidation potential of
P
3
Si
Fe
-
N
2
(E
o
=
-
1.0 V)
has been
previously repor
ted,
16
the weakly basic properties of
P
3
Si
Fe
-
N
2
prevents access to the needed
pK
a
data to directly estimate the BDE
N
-
H
for
P
3
Si
Fe
-
N=NH
+
in ethereal solvents. Nonetheless, several
observations that place a firm upper limit on its BDE
N
-
H
are as follows.
The addition of stoichiometric quantities of chlorinated anilinium triflate salts or
[H(OEt
2
)
2
][BAr
F
24
] to
P
3
Si
Fe
-
N
2
in THF at 298 K result
s in immediate oxidation to form
P
3
Si
Fe
-
OTf
Page 6 of 19
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