Requirements for Lewis Acid-Mediated Capture and N-N Bond Cleavage of Hydrazine at Iron

Article abstract of DOI:10.1021/acs.inorgchem.8b02433

An iron complex bearing a pyridine(dicarbene) pincer was designed to probe the requirements of Lewis acid-enabled N₂H₄ capture and subsequent N-N bond cleavage. Appended boron Lewis acids were installed by two methods to circumvent the incompatibilities associated with Lewis acid/base quenching of free carbenes and boranes. N₂H₄ capture by borane Lewis acids is dependent on both the Lewis acidity and the steric profile about boron. A substitutionally inert primary coordination sphere at iron prevents an Fe-N₂H₄ interaction as well as N-N bond homolysis upon reduction.

Full text of DOI:10.1021/acs.inorgchem.8b02433

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Requirements for Lewis Acid-Mediated Capture and N−N Bond
Cleavage of Hydrazine at Iron
John J. Kiernicki, Matthias Zeller, and Nathaniel K. Szymczak*,†
†
‡
†
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
‡
H. C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
*
S Supporting Information
ABSTRACT: An iron complex bearing a pyridine(dicarbene)
pincer was designed to probe the requirements of Lewis acid-
enabled N H capture and subsequent N−N bond cleavage.
2
4
Appended boron Lewis acids were installed by two methods to
circumvent the incompatibilities associated with Lewis acid/
base quenching of free carbenes and boranes. N H capture by
2
4
borane Lewis acids is dependent on both the Lewis acidity and
the steric profile about boron. A substitutionally inert primary
coordination sphere at iron prevents an Fe−N H interaction as
2
4
well as N−N bond homolysis upon reduction.
2
7
INTRODUCTION
residues and achieve substrate capture/activation and/or
stabilization of high-energy intermediates.
■
28,29
The reductive capabilities of nitrogenase enzymes are
unparalleled, as exemplified by their ability to reduce
dinitrogen to ammonia.
We recently reported the synthesis of a pyridine(dipyrazole)
iron complex with flexible appended boron Lewis acids capable
of capturing a single equivalent of hydrazine outside of iron’s
1
−3
+
−
The 6H /6e reduction of N to
2
4
NH represents one of the largest-scale reactions on earth, yet
30
3
primary coordination sphere. Reduction resulted in hydra-
key mechanistic steps regarding the biological reduction
pathways remain largely unknown.
zine N−N homolytic cleavage, with stabilization of the parent
amido ligands augmented by the appended acids. Crossover
experiments confirmed that N−N bond cleavage occurred at a
single metal center, and notably, the appended 9-
borabicyclo[3.3.1]nonyl (9-BBN) Lewis acids were required
for this transformation. To extend the scope of Lewis acid-
assisted activation of small molecules beyond this ligand
system, structure−function relationships are required. Impor-
tantly, the interplay between the electronic and steric
requirements of the metal as well as the Lewis acid(s) needs
to be carefully optimized to facilitate a closed-loop synthetic
cycle that proceeds through (a) substrate capture (independ-
ent or cooperatively with the metal), (b) stabilization of
intermediates, and (c) release/exchange of products. Boron
Lewis acids are ideally suited to probe both the acidity and
steric requirements of these reactions/equilibria. Whereas
examples of redox chemistry with the pyridine(dipyrazole)
5
,6
Well-defined small-
molecule complexes can aid in the elucidation of complex
substrate reduction sequences because they can be engineered
to test hypotheses regarding key redox transformations
between nitrogenous substrates. Although at least two
mechanistic pathways have been proposed for the N−N
7
,8
bond rupture event in N reduction, synthetic platforms
capable of accommodating the multiple products derived from
2
N reduction (N H , x = 1, 2; y = 0−4) are rare and necessarily
2
x
y
9
,10
require geometric and/or electronic flexibility.
Another
design strategy reminiscent of the enzyme is to modify the
metal’s secondary sphere with acidic/basic sites in order to
capture and stabilize the array of nitrogenous substrates.
Second-sphere interactions within enzyme active sites serve
a variety of roles to facilitate substrate reduction by (a)
11−14
orienting small-molecule substrates,
(b) stabilizing high-
15
31
energy intermediates, or (c) providing channels for catalytic
framework are rare, pyridine(dicarbene) ligands are capable
of stabilizing metal centers in multiple oxidation states.
Herein we describe the development of a highly modular
pyridine(dicarbene) ligand platform with appended boron
Lewis acids, the requirements for N H capture, and the
1
6
32,33
turnover. Synthetic systems have been designed to replicate
this overall design aspect, most commonly by incorporating
Brønsted (hydrogen-bond) donors within a ligand frame-
1
7−20
work.
Although hydrogen-bonding interactions have a
2 4
21−26
prominent role in stabilizing high-valent intermediates,
the highly reducing conditions associated with N fixation are
consequence of a substitutionally inert primary coordination
sphere on N−N bond cleavage.
2
incompatible with Brønsted acidic groups. In contrast, Lewis
acidic groups are a viable secondary-sphere alternative to
address the reductive incompatibilities of Brønsted acidic
Received: August 28, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
RESULTS AND DISCUSSION
■
Installation of Appended BR on the CNC Frame-
2
work. To probe the requirements for N H capture by boron
2
4
Lewis acids independent of the metal center, we sought to
design a system that was coordinatively saturated at iron with
substitutionally inert ligands. The 2,6-bis(carbene)pyridine
3
4−39
system
presents a robust ligand template with a similar
bite angle and steric profile to the previously studied
pyridine(dipyrazole) species. Installation of 9-BBN and
4
,4,5,5-tetramethyl-1,2,2-dioxoboryl (BPin) Lewis acids on
the 2,6-bis(imidazol-1-yl)pyridine (CNC) framework was
achieved by two methods (Figure 1): (1) hydroboration of
Figure 2. (left) Metalation of CNC ligands to afford compounds 1
and subsequent reduction to form 2-allyl and 2-BPin. (right)
Molecular structures of 1-allyl and 2-allyl displayed with 50%
probability ellipsoids. Hydrogen atoms not attached to the allylic
moiety have been omitted for clarity.
Figure 1. Methods used to install boron Lewis acids in this work.
the previously reported 2,6-bis(N-allylimidazolium)pyridine
allyl
40
[
H CNC][BPh ] with 9-BBN in DCM to afford the anti-
2 4 2
BBN
Markovnikov product [ H CNC][BPh ] and (2) nucleo-
allyl is structurally analogous to the previously reported
2
4 2
DIPP
philic addition of 2,6-bis(imidazol-1-yl)pyridine to Br-
complex [(
CNC)Fe(MeCN) ][BPh ] (DIPP = 2,6-
3 4 2
39
4
1
BPin
(
CH ) BPin to afford [ H CNC][BPh ] after anion
diisopropylphenyl) but contains slightly longer Fe−C
distances (1.975(2) and 1.970(2) Å).
2
3
2
4 2
44
metathesis.
Metalation of both [BPinH CNC][BPh ] and [ H CNC]-
BBN
Carbonyls are strong-field ligands that often impart
substitutional inertness and also provide a spectroscopic
handle to assess electronic changes to the primary coordination
2
4 2
2
[
BPh4]2 with Fe(N(SiMe3)2)2 was attempted (Figure 2).
Treating an acetonitrile solution of Fe(N(SiMe ) ) with
3
2 2
BPin
BPin
45
[
(
H CNC][BPh ] afforded low-spin [( CNC)Fe-
sphere. Carbonyl complexes were prepared by reduction of
2
4 2
MeCN) ][BPh ] (1-BPin) as an orange powder. Under
THF slurries of 1-allyl and 1-BPin with excess 0.4% Na(Hg)
3
4 2
BBN
allyl
analogous reaction conditions, metalation of [ H CNC]-
under a carbon monoxide atmosphere, affording ( CNC)-
2
BPin
[
BPh ] did not afford a tractable iron-containing species;
Fe(CO) (2-allyl) and ( CNC)Fe(CO) (2-BPin) as red
4
2
2
2
1
3
instead, an inert Lewis acid/base pair formed between the free
carbene and acidic 9-BBN (Figure 2). The identity of the
powders (Figure 2). Upon reduction, the C NMR resonances
assigned to the carbene for 2-allyl and 2-BPin shifted
downfield to 211.58 and 211.06 ppm, respectively, and
infrared spectroscopy (KBr) confirmed the presence of
carbonyl ligands (2-allyl: 1896, 1832 cm ; 2-BPin: 1900,
1841 cm ). The carbonyl ligands of complexes 2 are more
42
quenched adduct was confirmed by independent deprotona-
BBN
n
tion of [ H CNC][BPh ] with BuLi, which afforded a
2
4 2
1
1
−1
tetrahedral B NMR resonance at −15.44 ppm (C D ),
6
6
Me
Mes
−1
similar to that for the NHC ·BEt adduct (−13.1 ppm,
3
4
3
C D ).
activated than those in other dicarbonyl Fe(0) pincer
6
6
iPr
−1
To circumvent the acid/base quenching, we targeted a
strategy of metalating the carbene ligand prior to installing the
appended borane. Metalation of [allylH CNC][BPh ] with
compounds, including ( PDI)Fe(CO) (1974, 1914 cm )
2
iPr
( PDI = 2,6-((DIPP)NCMe) C H N; DIPP = 2,6-
2
5
3
46
Ph
OSiMe
3
diisopropylphenyl), ( PDP
)Fe(CO) (1935, 1868
2
4 2
2
allyl
−1 31
DIPP
Fe(N(SiMe ) ) in acetonitrile afforded orange [( CNC)Fe-
cm ), and, notably, ( CNC)Fe(CO)2 (1928, 1865
3
2 2
−
1
38
(
MeCN) ][BPh ] (1-allyl), analogous to 1-BPin. For 1-allyl
cm ). Single-crystal X-ray diffraction studies of 2-allyl
confirmed a five-coordinate (τ5 = 0.52) iron dicarbonyl
complex with the allylic substituents intact (Figure 2).
3
4 2
and 1-BPin, NMR spectroscopy (CD CN) confirmed metal-
ation by disappearance of the diagnostic imidazolium CH H
3
1
resonance (9.80 and 9.15 ppm, respectively) concomitant with
Following reduction, the Fe−C
distances contract to
distances (1.7457(15) Å)
carbene
a new carbene resonance in the 13C spectrum (201.90 and
1.9194(15) Å and the Fe−C
carbonyl
DIPP
2
00.29 ppm, respectively). Structural confirmation of 1-allyl
are identical to those in ( CNC)Fe(CO) (1.746(3) and
2
38
was achieved by X-ray diffraction of single crystals, which
revealed the expected mer-octahedral dicationic iron pincer
species with pendent allylic moieties (Figure 2). Complex 1-
1.767(4) Å). 2-allyl is thermally stable in C D in the
6 6
presence of small-molecule substrates such as N H or H at 85
°C. Attempts to liberate the carbonyl ligands were
2
4
2
B
DOI: 10.1021/acs.inorgchem.8b02433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
unsuccessful: 2-allyl is unreactive toward both stoichiometric
CNC ligands possess a bite angle (Ccarbene−Fe−C
= 157°
carbene
4
7
48
NaOH and N-methylmorpholine N-oxide at elevated
temperature or under UV radiation and does not undergo
ligand substitution reactions with trimethylphosphine or
phenyl isocyanide after 24 h in C D . These experiments
for each) and geometry at iron (τ = 0.47−0.52) similar to
5
BBN tBu
those of ( PDP )FeCl (N
−Fe−N
= 148.83°;
2
pyrazole
pyrazole
τ = 0.37) confirming that compounds 2-BR are appropriate
5
2
isostructural models to probe the requirements for N H
6
6
2
4
clearly demonstrate a highly inert primary coordination
environment about iron in 2-allyl.
capture and reduction.
Hydrazine Capture in the Secondary Coordination
Sphere. Given that the four appended Lewis acids within
iron’s secondary coordination sphere do not perturb the
electronic and steric environment at iron (vide supra), we
assessed their relative Lewis acidity by determining their
After establishing an inert primary coordination sphere, we
targeted the installation of the appended trialkylboranes. Anti-
Markovnikov-selective Lewis acid addition was achieved by
treating THF solutions of 2-allyl with dicyclohexylborane,
disiamylborane, and 9-BBN at room temperature for 16 h to
acceptor numbers (ANs) using the Gutmann−Beckett method
BCy
2
sia
2
B
50
afford ( CNC)Fe(CO) (2-BCy ), ( CNC)Fe(CO) (2-
(Table S16).
Model acids derived from styrene
2
2
2
BBN
sia B), and ( CNC)Fe(CO) (2-BBN), respectively, as red
powders (Figure 3). In each case, H NMR spectroscopy
(PhCH CH BR (BR = BPin, sia B, BCy , BBN)) were
2
2
2 2 2 2 2 2
1
investigated as proxies to simplify experimental determination
by eliminating any competitive inter/intramolecular acid/base
equilibria. The acid derived from 9-BBN affords the largest AN
51
(
24.9), which is similar to that reported for BEt3, while
heteroatom-stabilized PhCH CH BPin is the poorest acceptor
2
2
(
AN = 10.0). Surprisingly, the other two trialkylboranes,
PhCH CH B(sia) and PhCH CH BCy , also are best
2
2
2
2
2
2
described as poor acceptors by this method (AN = 10.2 and
1
3.0, respectively). The challenges of quantifying Lewis acidity
51,52
are well-documented,
and we propose that the low
acceptor numbers for PhCH CH B(sia) and PhCH CH BCy
2
2
2
2
2
2
are derived from their larger steric encumbrance compared
with PhCH CH BBN. This steric congestion weakens the
2
2
Lewis acid/base interaction for triethylphosphine oxide, a base
with a larger cone angle than N H . Similar to a cone angle
2
4
analysis, the steric contribution of the substituents on each acid
was estimated by solid angle analysis, providing a relative
ranking of steric accessibility at boron: BPin < BBN < BCy <
2
53
B(sia) (Table S17). These data reinforce that Lewis acidity
2
measurements are substrate-dependent and incorporate both
steric accessibility and electrophilicity.
We interrogated the ability of the set of 2-BR compounds
2
containing boranes of varied acidity to sequester a single
equivalent of N H . 2-BPin was unreactive toward N H , as
2
4
2
4
1
determined by H NMR and IR spectroscopies, consistent with
its low AN. Conversely, 2-sia B and 2-BCy both react with a
2
2
single equivalent of N H , despite their low ANs, to afford 1:1
2
4
N H :Fe adducts (3-sia B and 3-BCy ; Figure 4). The
2
4
2
2
captured N H was confirmed in each case by NMR
2
4
Figure 3. (top) Hydroboration of 2-allyl to afford 2-BR . (bottom)
1
2
spectroscopy by a broad N H resonance in the H spectrum
2
4
Molecular structures of 2-BPin, 2-BBN, 2-sia B, and 2-BCy . 2-BPin
2
2
at 3.19 ppm (3-sia B, C D ) or 4.42 ppm (3-Bcy , THF-d ) as
2
6
6
2
8
and 2-sia B are displayed with 30% probability ellipsoids, while 2-
2
11
well as an upfield B resonance, consistent with a tetrahedral
BBN and 2-BCy are displayed with 50% ellipsoids. Hydrogen atoms
2
boron (3-sia B = −6.04; 3-BCy = −8.41 ppm). IR
have been removed for clarity, and selected carbon atoms are
displayed in wireframe.
2
2
spectroscopy (KBr) confirmed minimal perturbations to the
iron center upon substrate capture by their similar ν
(
CO)
−
1
(
C D ) was employed to confirm consumption of the allylic
vibrations (3-sia B: 1901, 1836 cm ; 3-BCy : 1900, 1832
2
2
6
6
11
−1
resonances of 2-allyl, and the B NMR spectra confirmed the
formation of the trialkylborane (2-BCy = 84.18; 2-sia B =
cm ). X-ray-quality single crystals of 3-BCy
to probe the nature of the R B−N
2
were investigated
H interaction. Data
2 4
2
2
3
8
1.10; 2-BBN = 87.73 ppm). Infrared (KBr) and NMR
refinement revealed a polymeric species in which borane Lewis
acids from opposing molecules cooperatively sequester the
N H molecule (Figure 4). This bonding arrangement directly
spectroscopy established that hydroboration has a minimal
1
3
effect on the electronic environment at the iron: the
C
2
4
BBN
tBu
carbene resonances and ν vibrations (2-BCy = 210.77
contrasts with our previous findings for ( PDP )MX
(
CO)
2
2
−
1
ppm, 1902/1839 cm ; 2-sia B = 211.23 ppm, 1901/1839
cm ; 2-BBN = 211.58 ppm, 1895/1835 cm ) are
comparable to those of 2-allyl.
(MX = FeCl , FeBr , ZnCl ), in which N H capture occurs
2 2 2 2 2 4
2
−
1
−1
at a single ligand platform and is augmented by weak MX−
H N hydrogen-bonding interactions. The similarity in bite
4
2
Structural analysis of X-ray-quality single crystals of 2-BR2
revealed similar bonding metrics across the series (Figure 3
and Table S14). For each, the appended boranes are
angle and geometry about iron between 2-BCy and
2
BBN
tBu
(
PDP )MX suggests that the divergent mode of N H
2 2 4
capture (oligomeric vs monomeric) is derived from the steric
49
noninteracting, as noted by their planarity (∑B = 360°). The
congestion imparted by the cyclohexyl substituents of 2-BCy2
α
C
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Inorganic Chemistry
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The insolubility of 3-BBN in common organic solvents
precluded solution NMR characterization; however, 3-BBN
gradually dissolved in DMSO concomitant with release of
hydrazine (Figure S47). We propose that the poor solubility is
due to a decrease in the number of degrees of freedom
compared with 2-BBN as a result of increased rigidity upon
capture of the N H . The assignment of 3-BBN is supported
2
4
by the following data: (1) MALDI-TOF spectrometry is
BBN
consistent with the ( CNC)Fe(CO)2 fragment (m/z =
6
47.370); (2) elemental analysis establishes the molecular
BBN
formulation as ( CNC)Fe(CO) (N H ); (3) IR spectros-
2
2
4
copy indicates the presence of N−H absorptions and CO
−
1
bands (1859 and 1785 cm ); (4) 2-BBN is reformed when
5
8
dissolved in DMSO, with release of N H ; (5) a more
2
4
BBN
soluble variant, ( CNC)Fe(CO) (1,4-diaminobutane),
2
could be synthesized (vida infra); and (6) addition of a
second equivalent of N H affords the 2:1 N H :Fe complex
2
4
2
4
(
vida infra).
Because of the insolubility of 3-BBN, we sought to develop a
more soluble analogue to establish the entropic favorability of
:1 Fe:substrate capture. We selected 1,4-diaminobutane as an
Figure 4. (top) Capture of N H with the sterically encumbered
2
4
borane Lewis acids 2-sia B and 2-BCy . (bottom) Molecular
2
2
1
structure of 3-BCy2 displayed with 50% probability ellipsoids.
Hydrogen atoms not attached to nitrogen have been omitted and
cyclohexyl groups are displayed in wireframe for clarity.
extended diamine to impart higher solubility. Treating 2-BBN
with a single equivalent of 1,4-diaminobutane afforded the 1:1
BBN
Fe:1,4-diaminobutane adduct ( CNC)Fe(CO) (1,4-diami-
2
nobutane) (Figure 5). Crystallographic characterization
confirmed the capture of the diamine between the two
appended trialkylboranes to form a 19-atom macrocyclic
structure (B−N = 1.657(3) and 1.654(3) Å). The flexibility
of the attached trialkylboranes is highlighted by their ability to
(
and the sec-isoamyl substituents of 2-sia B), which prevents
2
54
monomeric N H capture.
2
4
In contrast to the polymeric structure for 2-BCy , addition
2
of a single equivalent of N H to a THF solution of 2-BBN
2
4
results in precipitation of an orange solid assigned as
BBN
accommodate substrates of varied size: the B1−B2 distance in
(
CNC)Fe(CO) (N H ) (3-BBN) that displays a large
2 2 4
BBN
−
1
(
CNC)Fe(CO) (1,4-diaminobutane) is 8.512 Å, whereas
shift in the ν(CO) peaks (KBr, 1859, 1785 cm ), which are
broadened and bathochromically shifted compared with 2-
BBN (Δ = 36, 50 cm ; Figure 5). The increased activation of
the carbonyl ligands in 3-BBN is consistent with hydrogen-
2
BBN
tBu
the distance in ( PDP )FeCl (N H ) is 4.191 Å. These
2 2 4
−
1
studies demonstrate that when the steric environment
surrounding the boron is able to accommodate a substrate,
intramolecular capture is favored over oligomeric capture (i.e.,
3-BCy2).
55−57
bonding interactions
between the captured hydrazine and
the carbonyl ligands in analogy to (BBNPDP )FeCl (N H ).
tBu
2
2
4
BBN
Figure 5. (top) Capture of 1 and 2 equiv of N H . (bottom) Synthesis of ( CNC)Fe(CO) (1,4-diaminobutane) and molecular structures of 4
2
4
2
BBN
and ( CNC)Fe(CO) (1,4-diaminobutane) displayed with 50% probability ellipsoids. Hydrogen atoms not attached to nitrogen have been
2
omitted for clarity, and BBN substituents are displayed in wireframe.
D
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Inorganic Chemistry
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−
1
The chemical composition of 3-BBN was further confirmed
by addition of a second equivalent of hydrazine. Addition of
N H to an orange THF slurry of 3-BBN gradually afforded a
shift to 1860 and 1794 cm for the sodium analogue and to
−
1
1900 and 1839 cm upon half encapsulation of the potassium
cations with 18-crown-6.
2
4
homogeneous red solution from which a red powder was
X-ray-quality single crystals of both 5 and 6 (18-crown-6
encapsulated variant) were analyzed for comparison (Figure
6B). The metrical parameters for the primary-sphere environ-
BBN
isolated and assigned as ( CNC)Fe(CO) (N H ) (4)
2
2
4 2
(
(
Figure 5). The IR spectrum (KBr) revealed ν
1896, 1826 cm ) that are similar to those of complexes 2-
absorptions
(
CO)
−
1
BR and suggest diminished hydrogen-bonding interactions
2
between the hydrazine and carbonyl ligands. NMR spectros-
1
1
copy (THF-d ) confirmed a tetrahedral boron ( B = −4.04
8
1
ppm) in 4 as well as two distinct NH resonances ( H: 3.38
2
(
B−NH ), 5.70 (BNH NH )). The molecular structure of 4
2
2
2
obtained by an XRD experiment confirmed its identity (Figure
). The metrical parameters of the primary coordination
5
sphere remain unchanged with respect to 2-BR . Each of the
2
tetrahedral trialkylborane units (∑B = 337.6(8)°) engages a
α
separate molecule of N H (B−N = 1.665(11) Å). The B−N
2
4
distances in 4 are significantly shorter than those in 3-BCy ,
2
consistent with smaller steric profile. In the solid state, each
carbonyl ligand engages a separate N H4 in moderate
2
intramolecular hydrogen-bonding interactions (N
−O =
proximal
59−61
3
.089, Ndistal−O = 3.016 Å)
the IR spectra of the bulk samples.
The disparate modes of N H capture among the four
that were not manifested in
2
4
complexes 2-BR suggest that multiple factors, including Lewis
2
acidity and steric profile, contribute to the ability to properly
orient a small-molecule substrate within the secondary
coordination sphere. While 2-BPin is sterically accessible for
acid/base interactions, it lacks the acidity to capture N H .
Figure 6. (A) Reduction of 3-BBN does not afford NH equivalents.
x
Synthesis of model amidoborane complex 6. (B) Molecular structures
of 5 and 6 displayed with 50% and 30% probability ellipsoids,
respectively.
2
4
Each trialkylborane variant, however, is sufficiently acidic. The
large steric profiles of 2-BCy and 2-sia B, evident by their low
2
2
ANs, capably engage both N H lone pairs, although this
2
4
interaction requires an oligomeric conformation. Of the
appended acids studied, only 2-BBN offers both adequate
acidity and steric accessibility to allow monomeric N H
ment for the two complexes are analogous (τ = 0.47 and 0.46
for 5 and 6, respectively). Each contains pyramidalized
5
2
4
trialkylborane pendants (5: ∑B = 335.4(6), ∑B
=
1
α
2α
capture. Notably, 2-BBN is less reducing (Fe(0)/Fe(I) =
3
36.1(6); 6: ∑B = 332.7(8)°) that interact with an ammonia
α
+
BBN
tBu
−
1.19 V vs Fc/Fc ) than complexes of the
class and at this potential may not be capable of reducing
N H .
PDP ligand
(
5: B−NH = 1.671(9) and 1.654(10) Å) or an amido (6: B−
3
NH = 1.615(11) Å). The captured ammonia molecules in 5
2
2
4
engage in intermolecular hydrogen-bonding interactions with
THF solvate molecules, while the amido substituent in 6
coordinates to the crown ether-encapsulated potassium (N−K
Reduction of the 1:1 Hydrazine Adduct. The isolation
of 3-BBN provided a platform to probe whether N H
2
4
coordination to the iron center is a requirement for N−N
=
2.792(10) Å).
bond scission. The hypothetical product of hydrazine N−N
We attempted to prepare amidoborane complex 6 through
−
bond scission, an amidoborane (R B−NH ), is typically
3
2
chemical reduction of captured hydrazine in 3-BBN. The
62−64
synthesized by deprotonation of ammonia borane.
We set
addition of 2 equiv of KC to a freshly thawed THF slurry of 3-
8
1
out to assess whether isolation of these types of species was
viable with this ligand platform. Addition of a THF solution of
BBN gradually afforded a homogeneous red solution. H NMR
spectroscopy revealed a mixture that did not contain either 5
65
or 6 (Figure 6A). To probe whether NH2− (or NH3)
NH to 2-BBN afforded the red ammonia borane complex
3
BBN
(
CNC)Fe(CO) (NH ) (5). Spectroscopic analysis re-
equivalents were formed, the reaction was quenched with HCl,
2
3 2
1
vealed that 5 is analogous to 4 with diagnostic NMR
and the amount of NH Cl produced was quantified by H
4
1
1
1
66
resonances (C D ; B: −5.04; H: 0.58 ppm (B−NH )) and
NMR spectroscopy. Each molecule of 3-BBN can produce a
6
6
3
−
1
−
infrared (KBr) ν(CO) absorptions (1889, 1821 cm ). Addition
maximum of 2 equiv of NH if homolytic N−N cleavage
2
3
0,67
4
2
of 2 equiv of KN(SiMe ) to a freshly thawed THF solution of
occurs
or between / and / equiv for either limiting
3
2
3
3
68,69
5
afforded the red amidoborane complex [K-
disproportionation pathway.
Following reduction, quench-
BBN
ing with HCl produced minimal NH4+ (0.08 equiv, average of
three runs; see the Supporting Information). The digestion
method was verified by quenching samples of 6. Treating
(THF)] [( CNC)Fe(CO) (NH ) ] (6). Investigation of 6
2 2 2 2
1
by H NMR spectroscopy (C D ) revealed a C -symmetric
6
6
2v
speciesconsistent with the amidoborane motif not interact-
1
ing with ironwith the −NH resonance at −0.85 ppm. The
samples of 6 with HCl followed by H NMR analysis
2
1
1
B NMR resonance in 6 (−11.24 ppm) is shifted upfield
relative to that in 5. The infrared spectrum (KBr) reveals
carbonyl ligands that are more activated (ν = 1871, 1801
cm ) in comparison with 5. The additional carbonyl
activation in 6 is cation-dependent: the ν(CO) absorptions
confirmed that each equivalent of 6 produces 1.83 equiv of
NH4+ (91.5% yield; average of four samples, see the
Supporting Information). These studies show that Lewis
acid-captured hydrazine cannot be reduced to NH2−
equivalents if coordination to the metal center is inhibited.
(
CO)
−
1
E
DOI: 10.1021/acs.inorgchem.8b02433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CONCLUSION
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02433.
■
■
*
Lewis acidic boranes were appended to a pyridine(dicarbene)
ligand framework through late-stage hydroboration to circum-
vent the Lewis acid/base pair incompatibilities between the
carbene and boron Lewis acid. The nature of the Lewis acid
has consequences on the ability of the molecule to capture
hydrazine: weakly acidic −BPin is incapable of sequestering
N H , while sterically encumbered −BCy and −B(sia) Lewis
S
Experimental procedures and spectroscopic character-
ization of all species (PDF)
2
4
2
2
acids favor oligomeric N H capture. Only the moderately
Accession Codes
2
4
acidic and sterically accessible −BBN fragments enable
CCDC 1864333−1864343 and 1871699 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by e-mailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, U.K.; fax: +44 1223
336033.
hydrazine capture within a single ligand platform.
By incorporating substitutionally inert carbonyl ligands on
the pincer dicarbene iron complex, the reduction of the
captured hydrazine was probed. Hydrazine reduction to NH3
equivalents was completely suppressed by eliminating the
possibility of N H coordination to the metal (Figure 7).
2
4
AUTHOR INFORMATION
Corresponding Author
E-mail: nszym@umich.edu.
■
*
ORCID
Matthias Zeller: 0000-0002-3305-852X
Nathaniel K. Szymczak: 0000-0002-1296-1445
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the NIGMS of the NIH under
Awards 1R01GM111486-01A1 (N.K.S.) and F32GM126635
■
(
J.J.K.). N.K.S. is a Camille Dreyfus Teacher−Scholar. X-ray
diffractometers were funded by the NSF under Award CHE
1625543 (M.Z.).
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