Synthesis and Reactivity of a Low-Coordinate Iron(II) Hydride Complex: Applications in Catalytic Hydrodefluorination

Article abstract of DOI:10.1021/acs.inorgchem.7b02199

A low-coordinate iron hydride complex bearing an unsymmetrical NpN (enamido-phosphinimine) ligand scaffold was synthesized and fully characterized. Insertion reactivity with azobenzene, 3-hexyne, and 1-azidoadamantane was explored, and the isolated products were analogous to previously reported β-diketiminate iron hydride insertion products. Surprisingly, the NpN iron hydride displays unprecedented reactivity toward hexafluorobenzene, affording an NpN iron fluoride complex and pentafluorobenzene as products. The NpN iron hydride is a precatalyst for catalytic hydro-defluorination of perfluorinated aromatics in the presence of silane. Kinetic studies indicated that the rate-determining step during catalysis involved silane.

Full text of DOI:10.1021/acs.inorgchem.7b02199

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
Synthesis and Reactivity of a Low-Coordinate Iron(II) Hydride
Complex: Applications in Catalytic Hydrodefluorination
Nicholas M. Hein, Fraser S. Pick, and Michael D. Fryzuk*
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada
S
* Supporting Information
ABSTRACT: A low-coordinate iron hydride complex bearing an
unsymmetrical NpN (enamido−phosphinimine) ligand scaffold
was synthesized and fully characterized. Insertion reactivity with
azobenzene, 3-hexyne, and 1-azidoadamantane was explored,
and the isolated products were analogous to previously reported
β-diketiminate iron hydride insertion products. Surprisingly, the
NpN iron hydride displays unprecedented reactivity toward
hexafluorobenzene, affording an NpN iron fluoride complex and
pentafluorobenzene as products. The NpN iron hydride is a pre-
catalyst for catalytic hydro-defluorination of perfluorinated aromatics
in the presence of silane. Kinetic studies indicated that the rate-
determining step during catalysis involved silane.
species is highly active and is reported to have a turnover
number (TON) of 1000.19 However, use of the earth-abundant
metal Fe for catalytic HDF is limited to one example in the
literature,23 making the further expansion of Fe-based HDF
systems a point of interest.
This paper describes the synthesis and characterization of a
low-coordinate, paramagnetic iron hydride complex bearing an
electronically unsymmetric enamido−phosphinimine (NpN)
ligand scaffold. Given the use of Nacnac−iron fluoride complexes
in concert with Et3SiH for the hydro-defluorination of certain
fluorinated aromatics,23 we were interested in examining our NpN
donor set in combination with iron in HDF to ascertain the effect
of this Nacnac mimic ligand scaffold on this important process.
INTRODUCTION
■
The reactivity patterns of low-coordinate first-row transition-
metal complexes are well-documented with regard to small-
molecule activation and catalytic processes.1−6 Of particular
interest is the family of variously substituted β-diketiminate
(Nacnac) iron hydrides, for example, [(Nacnac)Fe]2(μ-H)2,7,8
which are highly reactive species. These complexes undergo
insertion processes, such as hydride insertion into the N−N
double bond of azobenzene,9 the C−C triple bond of
alkynes,8,9 and across the azido group of 1-azidoadamantane.8
However, the utilization of low-coordinate iron hydride species
as catalyst precursors has been limited. It has been noted that
the Fe−H bond in low-coordinate species is unstable, leading
to insertion reactions favoring the formation of stronger Fe−X
bonds (X = C, N, O).10,11 Considering the enthalpic driving
force for the breakage of Fe−H and the consequent formation
of stronger Fe−X bonds, a natural extension of this reactivity
would be the use of iron hydrides to cleave strong C−F bonds,
resulting in the subsequent formation of Fe−F bonds. Fluo-
rinated organics have become ubiquitous in the pharmaceutical
and agrochemical industry due to their useful chemical
properties and stability.12,13 However, it is this stability that
also presents an environmental challenge, as these organo-
fluorine molecules are often persistent in the biosphere.14
Thus, removing fluorine atoms via catalytic hydro-defluorination
(HDF) for remediation purposes is an important area of research.
The use of late transition-metal complexes for homogeneous
HDF catalysis has been an active area of research since
the initial report of hexafluorobenzene HDF by a Rh(I)-silyl
complex.15 Since this discovery, there have been numerous
examples of catalysts that utilize Ni,16,17 Au,18,19 Ir,20 and Ru21,22
for the HDF of perfluorinated aromatics. For example, the HDF
of pentafluoronitrobenzene with a Xantphos-stabilized Au(I)
RESULTS AND DISCUSSION
■
Synthesis of Low-Coordinate Iron(II) Hydrides.
We previously reported the synthesis and reduction chemistry of
the three-coordinate iron(II) bromide complex (CY5NpNDIPP,DIPP)-
FeBr, 1.24 Treatment of 1 with potassium triethylborohydride
serves as a convenient route for the synthesis of the desired
low-coordinate iron hydride complexes [(CY5NpNDIPP,DIPP)-
FeH]2 2 (eq 1).
Received: August 31, 2017
© XXXX American Chemical Society
A
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Single crystals of 2 were grown from a toluene solution
of the product cooled to −35 °C. The solid-state structure for 2
is shown below in Figure 1. The bridging hydrides in 2 were
value of 121.3(6)°.9 The two corresponding angles for 2 are
C1−N1−C6:115.66(9) and P1−N2−C24:119.63(8). On the
basis of this analysis of ligand sterics, it can be rationalized
that the elongated distance between the iron centers in 2
in comparison to [(tBuNacnacDIPP)FeH]2 is likely electronic in
nature.
The 1H NMR spectra of 2 in d6-benzene and d8-THF (THF =
tetrahydrofuran) are virtually identical at 25 °C (Figure 2)
indicating that the structure of 2 is the same in both solvents.
This suggests that the dimer is retained in solution, even in
THF, a known coordinating solvent. In addition, the complex-
ity of the spectrum (∼30 lines) is consistent with a C1 sym-
metric dimer. A Cs 1symmetric monomer would only display 16
resonances in the H NMR spectrum, as is observed for the
starting bromide 1. The hydrides are not observed due to the
extremely rapid relaxation experienced during the NMR
experiment, a consequence from being directly bonded to the
iron center.25 Furthermore, no signals are observed when
performing a standard 31P{1H} NMR experiment. Additional
evidence for the dimeric structure of 2 is the low solution
magnetic moment of 4.2 μB measured by the Evans method at
25 °C, indicating strong anti-ferromagnetic coupling of the
normally high-spin Fe(II) centers in the dimer. Similarly low
magnetic values have been reported for [(tBuNacnacDIPP)FeH]2
at 25 °C.9
Figure 1. ORTEP drawing of the solid-state molecular structure of 2
(ellipsoids at 50% probability level). All hydrogen atoms except those
bonded to iron were omitted for clarity. Selected bond lengths (Å),
angles (deg), and torsion angles (deg): Fe1−H1:1.713(19), Fe1−
H1′:1.789(17), Fe1−Fe1′: 2.6762(4), N1−Fe1:2.0461(9), N2−
Fe1:2.0455(9), H1−Fe1−H1′: 48.4(7), N1−Fe1−N2:99.60(4), C1−
N1−C6:115.66(9), P1−N2−C24:119.63(8), N1−Fe1−Fe1′−N1′:
37.17(8).
The synthesis of 2 must be performed at −35 °C to prevent a
side reaction with the triethylborane byproduct. When 1 is
treated with KBEt3H at 25 °C and the reaction mixture is left to
stir for 15 min, the 1H NMR spectrum obtained for the product
mixture contains 52 resonances. Single crystals picked from this
reaction mixture were analyzed by X-ray diffraction and found
to be the iron diethyldihydridoborate complex 3 (see Figure S1
in the Supporting Information), which was produced in the
reaction. The formation of 3 follows from the reaction of 2 with
the byproduct triethylborane produced in the hydride meta-
thesis reaction, which by mass balance should also generate the
three-coordinate iron ethyl complex 4 (Scheme 1).
located in the difference density map and were freely refined
without constraining the Fe−H bond distance. In comparison
to the most closely related Nacnac complex [(tBuNacnacDIPP)-
FeH]2, the distance between the two iron centers in 2 is longer
(2.6762(4) Å) in contrast to the distance of 2.624(2) Å for the
aforementioned Nacnac iron derivative.9 This is a surprising
result, as the aryl rings in [(tBuNacnacDIPP)FeH]2 experience
more steric clash with the tert-butyl groups in the backbone of
Nacnac in comparison to the sterics present in the ligand
backbone of 2. This effect is quantified by the C−N−C angle,
where the aryl rings present on the N-donors are forced toward
the Fe center, which for [(tBuNacnacDIPP)FeH]2 has an average
Figure 2. 1H NMR spectra for [(CY5NpNDIPP,DIPP)FeH]2 2 in d6-benzene (top) and d8-THF (bottom). Both spectra were collected at 400 MHz
at 25 °C.
B
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Scheme 1. Side Reaction of Hydride 2 with Triethylborane to Form 3 and 4; Synthesis of 4 from 1
To confirm that the iron ethyl species 4 was produced con-
currently with complex 3, the bromide complex 1 was treated
with ethylmagnesium chloride in THF, which cleanly produced
4. Comparison of the 1H NMR spectrum of 4 to that obtained
by reaction of 2 with BEt3 confirms that 4 is present along with
other resonances likely attributable to 3 (see Supporting
Information Figures S12 and S13). Analogous reactivity has
been well-established for the reaction between [(tBuNacnacDIPP)-
FeH]2 and triethylborane.26
Catalytic Hydrodefluorination. The only reported
example of iron-catalyzed hydro-defluorination utilizes the
three-coordinate iron fluoride β-diketiminate complex
(
tBuNacnacDIPP)FeF in concert with Et3SiH.23 Table 1 presents
a
Table 1. Select Examples of Catalytic Hydrodefluorination
Using an Iron(II) Fluoride Diketiminate Complex
To probe the reactivity of our low-coordinate enamido−
phosphinimine iron hydride system in comparison to the
reactivity displayed by [(tBuNacnacDIPP)FeH]2, complex 2 was
treated with a selection of substrates containing multiple-bond
character (Scheme 2). In each case, the expected insertion
substrate
conditions
product distribution
TON
C6F6
45 °C, 4 d
45 °C, 4 d
45 °C, 12 h
C6HF5 (50%)
2.5
3.6
4.5
Scheme 2. Insertion Chemistry of Hydride 2 with
Azobenzene, 3-Hexyne, and Adamantyl Azide
C5F5N
C6F5CF3
p-C5HF4N (71%)
p-C6HF4CF3 (90%)
a
Each reaction was run in d8-THF using 20 mol % of catalyst;
triethylsilane was used as a hydride source.
some selected examples using this NacNac-based iron system
in HDF of hexafluorobenzene (C6F6) to generate pentafluor-
obenzene (C6F5H), pentafluoropyridine (C5F5N) to generate
p-tetrafluoropyridine (p-C6F4HN), and octafluorotoluene
(C6F5CF3) to give p-heptafluorotoluene (p-C6F4HCF3).
Presumably, these HDF reactions operate via formation of
a low-coordinate transient iron hydride species, which upon
interaction with the fluorinated aromatic compound regener-
ates the iron fluoride species. However, there is no direct
reaction between the hydride [(tBuNacnacDIPP)FeH]2 and C6F6
or octafluorotoluene (C6F5CF3) even when heated to 120 °C in
d6-benzene or d8-THF. Interestingly, in the presence of Et3SiH,
heating [(tBuNacnacDIPP)FeH]2 with C6F6 or C6F5CF3 does
result in the catalytic production of monodefluorinated prod-
ucts, though conversions are lower than when catalysis is
performed with the iron fluoride as the precatalyst.23
In contrast, the dinuclear dihydride 2 reacts directly with
C6F6 in d8-THF, with the formation of C6F5H observed by
19F NMR spectroscopy. The production of C6F5H is observed
in as little as 15 min when heated to 70 °C, though decom-
position of the iron-containing product also occurred at
this temperature, as determined by H NMR spectroscopy
(Supporting Information Figures S15 and S14, respectively).
If 2 reacts directly with C6F6 to generate C6F5H, it follows that
product was formed, with few differences in the structure of the
resulting complex in comparison to the iron β-diketiminate
analogues.8,9 The insertion products were fully characterized;
this information, including single-crystal X-ray structures, can
be found in the Supporting Information (Figures S3−S5).
1
C
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Scheme 3. Formation of Fluoride Dimer 8 by Reaction of C6F6 with Hydride 2
the concurrent formation of an iron fluoride species occurs.
Reaction of 2 with excess C6F6 in THF overnight at room
temperature afforded the isolation of the dimeric iron fluoride
complex [(CY5NpNDIPP,DIPP)FeF]2 8 (Scheme 3). The use of
THF as the solvent is crucial for this reaction; attempting the
reaction in diethyl ether or aromatic solvents failed to facilitate
a reaction between 2 and C6F6.
The 1H NMR spectrum for 8 contains 30 resonances,
indicating that it exists as a dimer in solution. However,
in contrast to 2, the spectral window is more indicative of a
monomeric species (with an extremely downfield-shifted
resonance at δ 109.1). Fluoride ligands in linearly bridged
species have been shown to allow weak anti-ferromagnetic
exchange between paramagnetic metal centers.27 Thus, the
weak anti-ferromagnetism observed for dimeric 8 is likely a
result of the bridging fluoride ligands. In agreement with this
assessment is the relatively high solution magnetic moment of
6.2 μB per dimer at 25 °C, which is the same solution magnetic
moment reported for dimeric [(MeNacnacDIPP)FeF]2.23 One
final point is that, while we assume that 8 is the fluoro-bridged
dimer, as one referee has pointed out, it could also be
the hydroxo-bridged species [(CY5NpNDIPP,DIPP)Fe(OH)]2.
The occupancies better match the fluoro derivative, but as
discussed in the Experimental Section, without analytical data,
the true identity is somewhat equivocal.
The solid-state molecular structure for 8 is shown in Figure 3.
The dimeric nature of 8 is somewhat surprising, as this previously
With reactivity between 2 and C6F6 established, catalytic
HDF was investigated with the addition of Et3SiH as the
fluoride acceptor to the reaction mixture. Utilizing a catalyst
loading of 0.05 mol equiv of 2 relative to equimolar amounts of
C6F6 and Et3SiH, catalytic mono-defluorination occurred in
d8-THF at 50 °C. A marginally higher TON of 4.2 was achieved
under a shorter time period (2 d) relative to the HDF catalysis
reported with the (tBuNacnacDIPP)FeF precatalyst with the
same substrates (TON = 2.5, 4 d). Complex 2 also performs
the mono-defluorination of 1,3,5-trifluorobenzene (C6F3H3),
though the TON is only 1.2 after 5 d at 70 °C. Interestingly,
precatalyst 2 displays a higher tolerance to silane choice
compared to the (tBuNacnacDIPP)FeF system, which is reported
to decompose at room temperature in the presence of
silanes other than Et3SiH, such as triethoxysilane ((EtO)3SiH)
and triphenylsilane (Ph3SiH). Both Ph3SiH and butylsilane
(n-BuSiH3) can be utilized in the HDF of C6F6, albeit with
lower conversions after a 2 d reaction period relative to using
Et3SiH as the fluoride acceptor (TON = 3 with Ph3SiH and
TON = 2.4 with n-BuSiH3).
Figure 3. ORTEP drawing of the solid-state molecular structure of 8
(ellipsoids at 50% probability level). All hydrogen atoms were omitted
for clarity. Selected bond lengths (Å) and angles (deg): Fe1−
N1:2.055(5), Fe1−N2:2.058(5), Fe1−F1:2.033(4), Fe1−F2:1.990(4),
Fe1−Fe2:3.1457(14), N1−Fe1−N2:96.5(2), F1−Fe1−F2:77.08(16),
C1−N1−C6:114.3(5), and P1−N2−C24:123.1(4).
reported ligand has been used to prepare the three-coordinate
iron bromide complex 1.24 A comparison of the angle between
the nitrogen donors and the 2,6-diisopropylphenyl ring is used
to quantify the difference in sterics between similar complexes.
The C1−N1−C6 (114.3(5)°) and P1−N2−C24 (123.1(4)°)
angles of 8 are shallower than the analogous angles found in 1
of 118.8(2)° and 125.9(2)°, respectively. Thus, a degree of
flexibility exists in our enamido−phosphinimine ligand scaffold
that is absent in the β-diketiminate systems. The C−N−C
angles of 127.95(13)° found between the imine nitrogen and
the aryl rings in the three-coordinate (tBuNacnacDIPP)FeF
complex provide sufficient steric hindrance to force a trigonal
planar geometry at the iron center. Interestingly, a less sterically
hindered iron(II) fluoride complex featuring a β-diketiminate
with methyl groups in the ligand backbone adopts a dimeric
structure very similar to that of 8. Thus, [(MeNacnacDIPP)FeF]2
features C−N−C angles of 119.92(18)° and 117.32(18)°
and an Fe−Fe distance of 3.0831(6) Å.23 We suggest that
the steric bulk imparted by our enamido−phosphinimine
system is greater than the methyl-substituted Nacnac ligand
but is less sterically hindered than the tert-butyl Nacnac
scaffold.
The greatest difference in reactivity between 2 and
(
tBuNacnacDIPP)FeF occurs in the HDF of C5F5N. After 3 d
at 50 °C in d8-THF, C5F5N was quantitatively converted to
p-C5HF4N in the presence of 2, when Et3SiH is used as the
fluoride acceptor (Figure 4). Higher TONs were achieved with
lower catalyst loadings; for example, 0.01 mol equiv of 2
relative to C5F5N and Et3SiH heated to 50 °C for one
week resulted in 50% conversion to p-C5HF4N (TON = 50;
Figure S29). This is a drastic difference in reactivity relative to
(
tBuNacnacDIPP)FeF, which is reported to have a TON of 3.6 for
the HDF of C5F5N after 4 d at 45 °C. There does exist a
similarity in the catalytic activity of both 2 and (tBuNacnacDIPP)-
FeF in that solvent choice is important for catalysis. For example,
attempted HDF of C6F6 in d6-benzene resulted in zero
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Figure 4. 19F NMR for the catalytic HDF of pentafluoropyridine (C5F5N) using Et3SiH as the fluoride trap and 5 mol % of 2 as the catalyst
(d8-THF, 50 °C). After 18 h, the appearance of the ortho and meta 19F resonances belonging to 2,3,5,6-tetrafluoropyridine (p-C5HF4N), along with
the 19F resonance for Et3SiF, are observed. After 3 d, the resonance belonging to the para-F of C5F5N is no longer observed, indicating complete
consumption of C5F5N. Fluorobenzene was used as the internal standard and is denoted with *.
Table 2. Hydrodefluorination Results Utilizing the Iron(II) Hydride Complex 2 in Comparison to the Catalytic Activity of the
β-Diketiminate Iron(II) Fluoride Catalyst
precatalyst
substrate
reagent
Et3SiH
solvent
conditions
TON
2 (5 mol %)
2 (5 mol %)
2 (1 mol %)
2 (5 mol %)
2 (5 mol %)
2 (5 mol %)
2 (5 mol %)
2 (5 mol %)
2 (5 mol %)
2 (5 mol %)
2 (5 mol %)
C6F6
d8-THF
d8-THF
d8-THF
d8-THF
C6D6
50 °C/2 d
50 °C/3 d
50 °C/7 d
70 °C/5 d
50 °C/2 d
50 °C/2 d
50 °C/3 d
50 °C/2 d
50 °C/2 d
50 °C/2 d
50 °C/2 d
45 °C/4 d
45 °C/4 d
45 °C/14 d
4.2
20
50
1.2
0
C5F5N
C5F5N
C3F3H3
C6F6
Et3SiH
Et3SiH
Et3SiH
Et3SiH
C6F6
Et3SiH
d5-pyridine
C6D6
0
C5F5N
C6F6
Et3SiH
11.2
2.4
3
n-BuSiH3
Ph3SiH
d8-THF
d5-THF
d8-THF
d3-THF
d8-THF
C6D6
C6F6
C6F6
Et3SiH, DMAP
Et3SiH, PPh3
Et3SiH
1.2
5.4
2.5
0.3
3.6
C6F6
(tBuNacnacDIPP)FeF (20 mol %)
(tBuNacnacDIPP)FeF (20 mol %)
(tBuNacnacDIPP)FeF (20 mol %)
C6F6
C6F6
Et3SiH
C5F5N
Et3SiH
d8-THF
production of C6F5H when using 2 and marginal production
in the case of (tBuNacnacDIPP)FeF (TON = 0.3).23 The use of
d6-benzene as the solvent in the HDF of C5F5N with 2 gave a
reduced TON of 11.2 in contrast to the full conversion observed
when d8-THF was the solvent. Thus, it is hypothesized that in
both systems the presence of a coordinating solvent is necessary
for either breaking up dimeric species during catalysis or
stabilizing catalytic intermediates.
suggest that the presence of a weakly coordinating ligand aids in
breaking apart the iron hydride dimer and facilitates reactivity,
but too strong of a donor inhibits catalysis, likely through occupa-
tion of coordination sites necessary for reactivity. A summary of
the catalysis results discussed above are shown in Table 2.
As solvent choice plays an important role in HDF catalysis, we
postulated the structure of the operative species as the monomeric
form of 2 and sought to detect this species at higher temperatures.
Heating a d8-THF solution of 2 to 60 °C only afforded small
changes in the 1H NMR spectrum (see Figure S38 in Supporting
Information). Upon close inspection of the 1H NMR spectrum at
60 °C, a new set of resonances at δ 37.3 and 35.4 are apparent.
Cooling the NMR sample from 60 to 25 °C resulted in the
disappearance of these two signals. This suggests that, at elevated
temperature, dimeric 2 may exist in equilibrium with its mono-
meric form (Supporting Information Figure S39).
The observation that THF increased the catalytic activity
of 2 toward HDF spurred the investigation into the addition
of other donors to the catalytic defluorination of C6F6 in the
presence of Et3SiH. The addition of triphenylphosphine caused
a marginally higher TON (5.4 with PPh3 relative to TON = 4.2
1
without PPh3), though the H NMR spectrum of the iron-
containing resting state indicates that the phosphine is not
coordinated. Interestingly, the addition of 4-dimethylaminopyr-
idine (DMAP) to the reaction mixture or using d5-pyridine as
the solvent entirely arrests HDF catalysis. It is reasonable to
Having established catalytic activity for the hydride com-
plex 2, we turned our attention to whether the dimeric iron
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Figure 5. Progress of HDF for pentafluoropyridine showing the changes in rate due to changes in substrate concentration relative to a reference
experiment with [silane] = [PyF] = 0.11 M, [Fe] = 5.5 × 10−3 M using d8-THF at 50 °C.
fluoride species 8 could also participate in catalytic HDF.
Unfortunately, employing the same conditions used for the
HDF of C6F6 with 2 does not result in catalysis when starting
with 5 mol % of 8 (TON = 0.8). The inactivity of fluoride 8 in
comparison to the hydride 2 is somewhat puzzling, and stands
in contrast to the reactivity observed for the β-diketiminate
system.23 We suggest these differences arise from distinct
solution-state equilibria for the dissociation of dimeric species
in THF. Further support for this hypothesis is illustrated by
the lack of HDF activity reported for the analogous dimeric
β-diketiminate iron fluoride [(MeNacnacDIPP)FeF]2.23 An exami-
nation of the reaction kinetics and H NMR spectra collected
during catalysis provides some insight into potential mecha-
nistic pathways.
Mechanistic Considerations. A series of kinetic experiments
were performed for the HDF of pentafluoropyridine at 50 °C using
precatalyst 2 and Et3SiH as the fluoride acceptor (Figure 5 and
Table 3). The production of 2,3,5,6-tetrafluoropyridine and
The second-order rate constant obtained from this equation
is k = 2.8(1) × 10−4 s−1 M−1. This rate law obtained for HDF of
C5F5N using 2 is identical to that reported by Holland and co-
workers for the hydro-defluorination of octafluorotoluene using
the (tBuNacnacDIPP)FeF catalyst.23
Comparable reaction rates are observed for the production
of Et3SiF, with the only difference being the initial production
of p-C5HF4N for the trial where the concentration of 2
was doubled (Supporting Information Figure S42). The initial
amount of p-C5HF4N produced at t = 0 for [catalyst] × 2 is
double that of the reference trial, whereas initial production of
Et3SiF for both the reference trial and [catalyst] × 2 is zero.
This observation is in agreement with the derived rate law, in
which the iron hydride 2 quickly reacts with C5F5N and the
rate-limiting step in catalysis involves the silane.
1
1
Examination of the H NMR spectra collected during the
various catalytic trials provides some insight into the identity
of the paramagnetic resting state. As seen in Figure 6, the
paramagnetic species observed during each of the HDF
catalytic trials remains unchanged regardless of the fluorinated
Table 3. Parameters for Kinetic Investigation of
Pentafluoropyridine HDF
1
substrate or silane utilized. The H NMR spectrum obtained
from the trial using n-BuSiH3 as the fluoride acceptor did
contain additional resonances not observed in the trials using
Et3SiH or Ph3SiH, but the major species present is consistent
with that observed in the other trials. Likewise, the same
paramagnetic species is present in both the HDF catalysis for
C6F6 and C5F5N, indicating that the resting state does not
have the fluorinated substrate coordinated to iron. As neither
the identity of the fluorinated substrate nor the silane changes
the paramagnetic resting state, we wondered if the species is the
iron fluoride complex 8. However, comparison of the 1H NMR
spectra for 2, 8, and the resting states obtained from HDF
of C6F6 with Et3SiH utilizing 2 and 8 as precatalysts rules out
2 and 8 as the resting state (Figure S41 in Supporting Information).
Interestingly, stepwise progression of HDF can be observed
through sequential addition of reagents. When 1 equiv of C6F6
is added to a d8-THF solution of 2 and heated to 50 °C for
1 d, nearly quantitative consumption of hydride dimer 2 was
achieved with concurrent formation of the catalytic resting
[Et3SiH] [C5F5N]
entry
(M)
(M)
[2] (M)
rate (1 × 10−7 M s−1
)
reference
0.11
0.22
0.11
0.11
0.44
0.11
0.11
0.11
0.22
0.11
5.5 × 10−3
5.5 × 10−3
0.011
5.5 × 10−3
5.5 × 10−3
1.6(1)
3.2(2)
3.5(2)
1.8(1)
6.7(3)
[silane] × 2
[catalyst] × 2
[PyF] × 2
[silane] × 4
fluorotriethylsilane was monitored by quantitative 19F NMR
spectroscopy over the course of 2 d. Because of the stabil-
ity of the starting iron hydride catalyst, the course of the
reaction could be monitored over this time frame, as the rate
of reaction remained relatively constant. From a comparison
of the reaction rates for the various kinetic runs, it can
be seen that the reaction is first-order with respect to
both the iron hydride 2 and Et3SiH but is zero order with
respect to pentafluoropyridine. The general rate law is shown
in eq 2:
1
state, as observed by H NMR spectroscopy. The addition of
1 equiv of Et3SiH to this solution followed by heating to 50 °C
d[p‐C5HF N]/dt = k[Et3SiH]1[2]1[C5F5N]0
(2)
4
F
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Inorganic Chemistry
Article
Figure 6. A comparison of the 1H NMR spectra collected at the final time points of various HDF catalytic runs using different substrates. With the
exception of the trial utilizing n-BuSiH3 as the fluoride acceptor, all observed resonances are consistent between trials. This indicates that the resting
state during catalysis does not depend on the fluorinated substrate. All spectra collected in d8-THF at 300 MHz at 25 °C.
for an additional day allows for the partial conversion of the
catalytic resting state back to 2, seen in Figure 7.
To further understand the energetics of iron-based HDF
catalysis, the energy barriers associated with proposed catalytic
transition states were calculated and compared for both Nacnac
and NpN iron complexes.
Nacnac, but the overall trend in energy values is consistent with
experimental observations. That is, process A is facile for the iron
hydride 2, but it is reported to not take place for [(tBuNacnacDIPP)-
FeH]2. Likewise, complex 8 did not serve as a precatalyst for
HDF, but reactivity is observed for (tBuNacnacDIPP)FeF. Thus,
the proposed catalytic cycle is thermodynamically feasible, but
in reality HDF catalysis via low-coordinate iron complexes is
likely a more complicated process than what is described here.
DFT Calculations. A proposed catalytic cycle for the HDF
of C6F6 in the presence of Et3SiH is shown below in Scheme 4.
While the monomers of complexes 2 and 8 are depicted in
Scheme 4, analogous species featuring the tBuNacnacDIPP ligand
are applicable. Each minimum was optimized at the B3LYP/
6-311+G(d) level of theory starting from the solid-state
molecular structure, and all were found to be a local energetic
minimum. Transition states were located using the Gaussian09
implementation of the Berny algorithm.28 All final energy
values were determined using the 6-311G(d) basis set29−33 and
the B3LYP functional.34−37 The connectivity of transition states
and minima were found using intrinsic reaction coordinate
(IRC) calculations. Figure 8 shows a comparison of the energy
of each transition state relative to LFeH, Et3SiH, and C6F6 for
both the NpN and Nacnac systems. The activation energy for
step A (Fe−H adduct formation with C6F6) is lower for the
NpN system than for the Nacnac analogue by ∼5 kJ/mol, while
step C (Fe−F adduct formation with Et3SiH) is ∼30 kJ/mol
higher for NpN in comparison to the Nacnac case. The magni-
tude of the calculated energy values are not an accurate represen-
tation of the differences in reactivity when comparing low-
coordinate iron hydride and fluoride complexes of NpN to
CONCLUSION
■
The dimeric iron(II) hydride species 2 bearing the enamido−
phosphinimine ligand scaffold was successfully synthesized
by treatment of the iron bromide complex 1 with KBEt3H.
Complex 2 reacted with the unsaturated substrates azobenzene,
3-hexyne, and 1-azidoadamantane to yield the expected
insertion products. Surprisingly, 2 displayed reactivity toward
the fluorinated aromatic compounds C6F6 and C5F5N. Single
crystals of the iron fluoride complex 8 were isolated from the
reaction of 2 with C6F6. This stands in contrast to the lack of
reactivity that the β-diketiminate iron hydride systems display
toward fluorinated aromatics. HDF catalysis was performed
using 5 mol % loadings of precatalyst 2, and a variety of silanes
can act as fluoride acceptors without catalyst decomposition.
The highest TON of 50 was observed for the HDF of C5F5N
(1 mol % loading 2). Kinetic experiments highlighted the
stability of the active catalytic species, as reaction rates were
relatively linear over the course of 50 h. The general rate law for
HDF of C5F5N with precatalyst 2 was in agreement with the
G
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Inorganic Chemistry
Article
1
●
)
Figure 7. H NMR spectra for 2 (top), the reaction between 2 and C6F6 (middle), and the subsequent reaction with Et3SiH (bottom). (red
Resonances belonging to the hydride 2 are observed following the reaction with Et3SiH. All spectra collected in d8-THF at 300 MHz at 25 °C.
Scheme 4. Proposed Catalytic Cycle for the Hydrodefluorination of C6F6
rate law for HDF catalysis of C6F5CF3 reported for the
tBuNacNacDIPP)FeF system. Both systems display a first-order
dependence on silane concentration, indicating that the reac-
The possibilities for improvements to catalytic rate and turnover
are numerous and could potentially be achieved through further
steric modification of the enamido−phosphinimine scaffold.
(
tion of a low-coordinate iron fluoride with silane is the
1
rate-determining step in HDF. However, the H NMR spectra
EXPERIMENTAL SECTION
■
obtained for the catalytic resting states across multiple
trials does not agree with the spectrum for 8, making a
definitive claim as to the identity of the resting state difficult.
General Considerations. All manipulations were performed
under an atmosphere of dry and oxygen-free dinitrogen (N2) by
means of standard Schlenk or glovebox techniques. Anhydrous THF,
H
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Article
Figure 8. Computational analyses of one HDF cycle to compare the relative energies of the Nacnac-iron system (red) to the NpN-iron system (blue).
toluene, Et2O, and hexanes were purchased from Aldrich, sparged with
N2, and dried further by passage through towers containing activated
alumina and molecular sieves. Benzene-d6 was refluxed over sodium,
vacuum transferred, and freeze−pump−thaw degassed. THF-d8 was
purchased from Aldrich (≥99.5% atom % D) in 1 mL ampules and was
performed without any symmetry restrictions, and the nature of the
extremes (minima and transition states) was verified with analytical
frequency calculations. Gibbs free energies were obtained at T =
298.15 K within the harmonic approximation. IRC calculations were
performed to confirm the connections of the optimized transition
states. DFT calculations were performed with the Gaussian09 suite of
programs.28 Calculations were realized in the gas phase, and the real
NpN and Nacnac ligands were computed.
1
dried over activated 4 Å molecular sieves. H and 19F NMR spectra
were recorded on a Bruker AV-300 MHz spectrometer or a Bruker
AV-400 MHz spectrometer. Unless noted otherwise, all spectra were
recorded at room temperature. 1H NMR spectra were referenced
to the residual proton signal in d6-benzene (7.16 ppm) or d8-THF
(1.72 ppm); 19F NMR spectra were referenced to fluorobenzene at
−113.15 ppm. Evans NMR spectroscopy was performed by measuring
the shift in frequency for the 1H NMR signal of cyclooctane in a sealed
capillary containing 1 mol % cyclooctane in d6-benzene relative to a
1 mol % cyclooctane in d6-benzene solution containing the desired
paramagnetic compound. The Evans measurements were collected at
25 °C. Microanalyses (C, H, N) were performed at the Department
of Chemistry at the University of British Columbia. Low- and high-
resolution mass spectrometry was performed with a Kratos MS 50 at
the University of British Columbia.
Suitable single crystals were selected in a glovebox, coated in
Fomblin oil, and mounted on a glass loop. X-ray data were collected
on a Bruker X8 Apex II diffractometer with a graphite-monochromated
Mo Kα radiation (λ = 0.710 73 Å) at a temperature of 90 K. Data were
collected and integrated using the Bruker SAINT software package.38
Absorption corrections were performed using the multiscan technique
(SADABS).39 All structures were solved by direct methods and refined
using the Olex2 (version 1.2.5) software package40 and the ShelXL
refinement program.41 All non-hydrogen atoms were refined aniso-
tropically. ORTEPs were generated using ORTEP-3 (version 2.02).42
The syntheses for the iron bromide complex 1 has been previously
reported.24 Potassium triethylborohydride was purchased as a 1.0 M
solution in THF from Sigma-Aldrich and was dried under vacuum,
affording a crystalline solid that was stored and used in the N2
glovebox. Hexafluorobenzene, fluorobenzene, 1,3,5-trifluorobenzene,
pentafluoropyridine, triethylsilane, butylsilane, and 3-hexyne were
degassed and stored over activated molecular sieves before use.
1-Azidoadamantane, azobenzene, and triphenylsilane were degassed
under vacuum and used within the N2 glovebox. Ethylmagnesium
chloride was purchased as a 2.0 M solution in THF from Sigma-
Aldrich and was used as received.
Syntheses. [(CY5NpNDIPP,DIPP)FeH]2 2. The previously reported
complex 1 (0.242 g, 0.361 mmol) and potassium triethylborohydride
(0.050 g, 0.361 mmol) were each dissolved separately in toluene
(5 mL), and the two solutions were cooled to −35 °C in the glovebox
freezer. The cooled solution of KBEt3H was quickly added to the
cooled solution of 1 while the mixture was stirred with a magnetic stir
bar. The reaction mixture rapidly changed color from yellow-orange to
dark brown and was allowed to stir for 3 min before being filtered
through a filter pipet packed with diatomaceous earth to remove the
KBr byproduct. The filtrate was dried under vacuum, and the resulting
residue was redissolved in Et2O and again dried to give the product as
an orange-brown powder. Yield: 0.161 g, 0.136 mmol, 75.6%. Single
crystals of 2 suitable for X-ray diffraction were grown from a toluene
solution cooled to −35 °C. 1H NMR (400 MHz, d6-benzene, 25 °C):
δ 31.8, 29.5, 23.4, 20.3, 17.9, 13.6, 11.3, 9.6, 9.0, 8.5, 6.7, 6.6, 5.7, 5.2,
4.0, 3.8, 3.0, 2.6, 1.3, 1.1, −0.0, −0.3, −0.7, −1.2, −5.5, −8.6, −9.7,
−11.9, −23.5, and −31.6. μeff = 4.2 μB (Evans). Anal. Calcd for
C70H110Fe2N4P2: C, 71.17; H, 9.39; N, 4.74. Found: C, 71.33; H, 9.52;
N, 4.61%.
(
CY5NpNDIPP,DIPP)FeEt 4. The iron bromide complex 1 (0.190 g,
0.284 mmol) was dissolved in THF (10 mL) and was transferred to a
Schlenk vessel. A solution of ethylmagnesium chloride (2.0 M in THF,
0.14 mL, 0.284 mmol) was added via syringe to the solution of 1.
The color of the reaction mixture immediately changed from yellow to
brown/green. The reaction mixture was allowed to stir for 1 h before
the volatiles were removed under vacuum and the Schlenk vessel was
transferred into a dinitrogen glovebox. The residue in the Schlenk flask
was dissolved in minimal diethyl ether and was filtered through a glass
fiber filter pad to remove the MgBrCl byproduct. The filtrate was
again dried under vacuum, yielding the product as a yellow powder.
Yield: 0.077 g, 0.124 mmol, 44.0%. Single crystals suitable for X-ray
diffraction were grown from slow evaporation of a diethyl ether
1
solution of 4. H NMR (300 MHz, d6-benzene, 25 °C): δ 96.8, 36.3,
Computations. All atoms were described with a 6-311G(d) triple-
ζ basis set.29−33 Calculations were performed at the DFT level of theory
using the hybrid functional B3LYP.34−37 Geometry optimizations were
26.9, 25.7, 24.7, 12.7, 12.1, 3.8, 1.8, 1.3, 0.3, −3.5, −20.9, −26.6, −30.3,
−33.3, and −35.8. μeff = 4.0 μB (Evans). High-resolution mass spec-
trometry (HRMS) (EI-Double Focusing Sector) m/z: [M]+ Calcd for
I
DOI: 10.1021/acs.inorgchem.7b02199
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Inorganic Chemistry
Article
1
C37H59N2PFe: 618.376 53; Found: 618.376 68, [M-CH2CH3]+ Calcd
for C35H54N2PFe 589.337 40; Found: 589.337 15.
(
25 °C. H NMR (300 MHz, d8-toluene, 25 °C): δ 113.4, 109.0, 96.3,
40.2, 37.4, 31.5, 26.6, 25.5, 24.7, 22.8, 20.2, 19.9, 19.2, 16.3, 14.8, 12.8,
12.1, 9.4, 4.3, 2.9, 1.2, 0.9, −0.3, −3.5, −6.2, −8.5, −16.9, −20.8,
CY5NpNDIPP,DIPP)FeN(Ph)NHPh 5. A THF solution (5 mL) of
1
complex 2 (0.075 mg, 0.127 mmol) was added to a THF solution
(5 mL) of azobenzene (0.023 g, 0.127 mmol). The resulting dark red
reaction mixture was stirred for 5 min before exposing the sample to
vacuum to remove the volatiles. Yield: 0.094 g, 0.122 mmol, 96.1%.
Single crystals suitable for X-ray diffraction were grown from the slow
−26.6, −30.3, −33.1, −35.8, and −36.4. Comparison to 4: H NMR
(300 MHz, d6-benzene, 25 °C): δ 96.8, 26.9, 25.7, 24.7, 12.7, 12.1, 3.8,
1.8, 1.3, 0.3, −3.5, −20.9, −26.6, −30.3, −33.3, and −35.8.
HDF Catalysis. For each independent trial, 10−15 mg of 2 was
weighed and dissolved in ∼0.6 mL of deuterated solvent. The sub-
strates under investigation were measured to be in excess, so that 2
would be at a catalyst loading of 5 mol %. Substrates were added via
Hamilton syringe to the solution of 2 and the resulting solution was
transferred into a J. Young NMR tube. The tube was heated to 50 °C
1
evaporation of a diethyl ether solution of 5. H NMR (300 MHz,
d6-benzene, 25 °C): δ 112.0, 108.6, 107.3, 61.9, 40.4, 37.5, 26.3, 25.1,
23.4, 20.0, 19.0, 17.6, 14.3, 13.2, 8.0, 3.6, 1.4, 0.3, −0.6, −1.8, −3.2,
−5.3, −9.0, −10.9, −13.0, −16.4, −18.8, −23.1, −27.3, −28.3, −31.4,
−39.7, and −59.3. μeff = 4.1 μB (Evans). Anal. Calcd for C47H65FeN4P:
C, 73.04; H, 8.48; N, 7.25. Found: C, 70.84; H, 8.47; N, 7.10%.
Repeated attempts at elemental analysis failed to provide successful
results. The analogous iron hydrazido β-diketiminate complex has
been reported to be too thermally sensitive to provide satisfactory
elemental analysis results.9
and was monitored periodically by H and 19F NMR spectroscopy.
1
Kinetics of Pentafluoropyridine HDF. A standard solution
containing 2 and fluorobenzene in d8-THF ([Fe] = 5.5 × 10−3 M,
[C6H5F] = 0.11 M) was used for each catalytic run. In each trial, a
J. Young NMR tube was charged with 600 μL of the d8-THF solu-
tion containing 2 and the C6H5F internal standard using a Hamilton
syringe. The amount of pentafluoropyridine and triethylsilane for each
trial was calculated and was measured with a Hamilton syringe and
added to the J. Young tube. For the trial where [Fe] × 2 was measured,
a new solution with [Fe] = 0.011 M was prepared. Each tube
was heated to 50 °C but was cooled to 25 °C when monitored using
19F NMR spectroscopy. A relaxation delay of 70 s was used to ensure
integration values were quantitative. Lines of best fit for [p-C5HF4N]
versus time were plotted using linear regression, and the error was
calculated using Microsoft Excel’s LINEST function.
(
CY5NpNDIPP,DIPP)Fe(3-hexene) 6. Complex 2 (0.070 g, 0.118 mmol)
was dissolved in THF (5 mL), and to the stirred solution was added
excess 3-hexyne (0.015 g, 0.183 mmol). The reaction mixture was
stirred overnight at room temperature, with the color of the solution
gradually changing from orange to yellow over the course of the
reaction. The volatiles were then removed under vacuum, leaving the
product as a yellow powder. Yield: 0.066 g, 0.098 mmol, 82.5%. Single
crystals suitable for X-ray diffraction were grown from the slow
1
evaporation of a diethyl ether solution of 6. H NMR (300 MHz,
d6-benzene, 25 °C): δ 98.7, 32.7, 27.2, 26.0, 25.1, 16.7, 15.3, 10.4, 2.9,
Stepwise HDF Reactivity. A solution of 2 (0.010 g, 0.017 mmol)
and hexafluorobenzene (2 μL, 0.017 mmol) in d8-THF (0.6 mL) was
transferred to a J. Young NMR tube. The tube was sealed and heated
2.1, 1.2, −3.4, −22.0, −23.1, −25.4, −29.0, −34.9, and −36.0. μeff
=
4.8 μB (Evans). HRMS (EI-Double Focusing Sector) m/z: [M]+ Calcd
1
for C41H65N2PFe: 672.423 48; Found: 672.423 06.
to 50 °C for 1 d before being monitored by H NMR spectroscopy.
(
CY5NpNDIPP,DIPP)Fe(η2-HNNNAd) 7. Complex 2 (0.063 g,
Triethylsilane (3 μL, 0.019 mmol) was added to the solution, and the
tube was heated for an additional 1 d at 50 °C before being monitored
0.107 mmol) was dissolved in diethyl ether (5 mL) and was added to
1-azidoadamantane (0.019 g, 0.107 mmol), and the resulting mixture
was stirred overnight at ambient temperature. The volatiles were then
removed under vacuum, yielding the product as a brown powder.
Yield: 0.070 g, 0.091 mmol, 85.4%. Single crystals suitable for X-ray
diffraction were grown from a toluene solution of 7 cooled to −35 °C.
1H NMR (400 MHz, d6-benzene, 25 °C): δ 96.0, 30.6, 26.4, 23.5, 12.0,
11.3, 8.8, 8.2, 6.1, 5.2, 4.6, 3.3, 2.3, 1.9, 1.3, 1.1, −2.0, −3.2, −4.7, −6.1,
−8.6, −11.2, −17.2, and −18.8. μeff = 4.3 μB (Evans). Anal. Calcd for
C45H70FeN5P: C, 70.39; H, 9.19; N, 9.12. Found: C, 70.52; H, 9.46; N,
6.72%. Repeated attempts at elemental analysis consistently yielded
results low in nitrogen. The β-diketiminate iron triazenido analogue
to 7 is reported to be too thermally unstable to obtain accurate
elemental analysis results.8
1
by H NMR spectroscopy.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b02199.
■
S
Text, figures, and tables giving full experimental
procedures, representative NMR spectra, crystallographic
data, and additional kinetic data (PDF)
Accession Codes
CCDC 1570900−1570906 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
[(CY5NpNDIPP,DIPP)FeF]2 8. Hexafluorobenzene (1.960g, 10.5 mmol)
was added to a THF solution (10 mL) of 2 (0.140 g, 0.237 mmol).
The mixture was allowed to stir for 18 h at ambient temperature with
the color of the solution gradually changing from orange to yellowish-
green. The volatiles were removed under vacuum, yielding the product
as a flaky yellow-green solid. Yield: 0.137 g, 0.225 mmol, 95.1%. Single
crystals suitable for X-ray diffraction were grown from a toluene
1
AUTHOR INFORMATION
Corresponding Author
*E-mail: fryzuk@chem.ubc.ca. Fax: +1 604 822-2847. Phone:
+1 604 822-2897.
solution of 9 cooled to −35 °C. H NMR (300 MHz, d6-benzene,
■
25 °C): δ 109.1, 89.2, 81.7, 51.0, 48.6, 40.6, 37.7, 26.4, 23.1, 19.2, 16.3,
14.6, 10.7, 8.8, 6.4, 4.5, 2.3, 1.2, −0.7, −7.7, −9.3, −13.2, −16.7, −20.4,
−23.1, −24.1, −28.5, −30.2, −52.8, and −66.7. μeff = 6.2 μB (Evans).
Many attempts at obtaining elemental analyses were made, without
success. We also tried to obtain EI mass spectra without success.
As one referee pointed out, this complex could also be the hydoxo-
bridged derivative, [(CY5NpNDIPP,DIPP)Fe(OH)]2. While the refine-
ment of the X-ray data better matched the fluoro-bridged species,
the hydroxo-bridged species also has similar occupancies. Without
acceptable elemental analytical data or mass spectra to back up the true
identity of 8, we concur that either formulation is possible.
ORCID
Michael D. Fryzuk: 0000-0002-3844-098X
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
Reaction of 2 with Triethylborane. Hydride complex 2 (0.008 g,
0.013 mmol) was dissolved in d8-toluene (0.6 mL) and was transferred
to a J. Young NMR tube. The tube was charged with 1 M
triethylborane in hexanes (13.6 μL, 0.013 mmol) before being sealed
and heated to 60 °C for 2 h. 1H NMR spectroscopy was performed at
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
M.D.F. thanks the NSERC of Canada for a Discovery Grant.
■
J
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DOI: 10.1021/acs.inorgchem.7b02199
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