Activation of dihydrogen and silanes by cationic iron bis(phosphinite) pincer complexes

Article abstract of DOI:10.1021/om500758j

Treatment of iron POCOP-pincer hydride complexes cis-[2,6-(ⁱPr₂PO)₂C₆H₃]Fe(H)(PMe₃)₂ (1-H), [2,6-(ⁱPr₂PO)₂C₆H₃]Fe(H)(PMe

Full text of DOI:10.1021/om500758j

Article
pubs.acs.org/Organometallics
Activation of Dihydrogen and Silanes by Cationic Iron
Bis(phosphinite) Pincer Complexes
Papri Bhattacharya, Jeanette A. Krause, and Hairong Guan*
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172, United States
S
* Supporting Information
ABSTRACT: Treatment of iron POCOP-pincer hydride complexes
cis-[2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)2 (1-H), [2,6-(iPr2PO)2C6H3]-
Fe(H)(PMe3)(CO) (2-H, trans H/CO; 2′-H, cis H/CO), and cis-[2,6-
(iPr2PO)2C6H3]Fe(H)(CO)2 (3-H) with HBF4·Et2O in CD3CN/
THF-d8 results in a rapid evolution of H2. Except for the reaction of
1-H, which leads to decomposition of the pincer structure, all other
hydrides are converted cleanly to acetonitrile-trapped cationic
complexes. Protonation of these hydrides with the weaker acids
CF3CO2H and HCO2H establishes the basicity order of 1-H > 2-H >
2′-H > 3-H, with 3-H bearing the least basic hydride ligand. An
alternative method of abstracting hydride by [Ph3C]+[BF4]− gives
complicated products; the reaction of 2-H generates two pincer
products, [HPMe3]+[BF4]− and Gomberg’s dimer, which supports a
single electron transfer pathway. Cationic complexes {[2,6-(iPr2PO)2C6H3]Fe(CO)(PMe3)(CH3CN)}+[BF4]− (2+-BF4,
trans CO/CH3CN) and cis-{[2,6-(iPr2PO)2C6H3]Fe(CO)2(CH3CN)}+[BF4]− (3+-BF4) are prepared from protonation of
i
2-H (or 2′-H) and 3-H with HBF4·Et2O, respectively. Both compounds react with H2 with the aid of Pr2NEt to yield neutral
hydride complexes and [iPr2N(H)Et]+[BF4]−. In addition, they catalyze the hydrosilylation of benzaldehyde and acetophenone
with (EtO)3SiH and show higher catalytic activity than the neutral hydrides 2-H/2′-H and 3-H. The mechanism for the
formation of 2+-BF4 and the X-ray structure of 2+-BF4 are also described.
Scheme 1. Catalytic Reduction of a Ketone with a Cationic
Complex
INTRODUCTION
■
Transition metal catalyzed reduction of polar bonds is an
essential process in fine or commodity chemical synthesis.1
The efficiency of a catalytic system is sometimes directly linked
to how effectively the metal can activate the reducing agents
(e.g., H2, silanes, and boranes). Among various activation
strategies,2−4 the use of cationic complexes has resulted in
tremendous success for a myriad of reduction processes,5
particularly hydrosilylation reactions.6−11 A generalized (and
simplified) mechanism that features a cationic catalyst is illus-
trated in Scheme 1 using a ketone as a representative substrate.
When cationic and coordinatively unsaturated, the metal center
often has a good binding affinity for H−E (E = H, SiR3),
resulting in a σ-complex and/or the oxidative addition product.
Forming the latter is usually less favorable for a cationic species
due to little back-donation from an electrophilic metal to the
H−E σ* orbital. Nevertheless, the subsequent stepwise, outer-
sphere transfer of E+ and H− to the ketone substrate completes
the catalytic cycle.
Hydrogenation reactions following this mechanism are best
known as ionic hydrogenation, which has been thoroughly
reviewed by Bullock.12 In terms of reduction with silanes,
cationic iridium bis(phosphinite) pincer (also known as
POCOP-pincer) complexes have shown remarkable activity
for catalytic hydrosilylation of compounds bearing C−O or
CO bonds.10 Mechanistic studies have supported catalytic
cycles with general features of Scheme 1.13,14 This type of
mechanism has also been proposed for hydrosilylation reactions
catalyzed by [CpW(CO)2(IMes)]+[B(C6F5)4]−,6a cationic
Re(V) oxo complexes,7 and [CpRu(PiPr3)(CH3CN)2]+.9b,c,e
We have reported that iron hydride complexes with a POCOP-
pincer ligand (Figure 1) are effective catalysts for the hydro-
silylation of aldehydes/ketones15 and the dehydrogenation of
Received: July 28, 2014
© XXXX American Chemical Society
A
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The presence of 5 is a strong indication of single electron
transfer (SET) from 2-H to [Ph3C]+[BF4]−, resulting in [2-H]+•
and the trityl radical (which dimerizes to give 5). Radical cations
of this type are known to be acidic enough to protonate their
original hydride complexes20 and in this case PMe3 as well.
To avoid the complications associated with SET, we resorted
to Brønsted acids to remove the hydride ligand.21 The success
of such a process is obviously dependent on the strength of the
acid used. Table 1 summarizes the protonation results using
Figure 1. Iron hydride complexes with a POCOP-pincer ligand.
ammonia borane.16 Our mechanistic analyses have suggested
that the hydride ligand remains intact during catalysis, but helps
facilitate the dissociation of the trans ligand L′ for substrate
binding. Wei and co-workers have recently studied our hydro-
silylation system computationally.17 Their DFT calculations
have pointed out that the pathway with the lowest kinetic
barrier begins with ligand substitution by a carbonyl substrate
followed by CO insertion into the Fe−H bond. Such a
mechanism is, however, inconsistent with our observation that
the reaction of PhCDO, Ph2SiD2, and 1-H generates Ph2SiD-
(OCD2Ph) and Ph2Si(OCD2Ph)2 with no deuterium incorpo-
rated into 1-H.
In this work, we remove the hydride ligand of these iron
pincer complexes via protonation reactions. The resulting cat-
ionic compounds are still active catalysts for the hydrosilylation
of benzaldehyde and acetophenone. In fact, they are more
reactive than the parent neutral hydride complexes. We also
describe stoichiometric activation of H2 by the cationic pincer
complexes in the presence of an externally added base.
Table 1. Extent of Protonation of Iron Hydrides by Different
a
Acids
entry
acid
1-H
2-H
2′-H
3-H
b
b
b
1
2
3
HBF4·Et2O
CF3CO2H
HCO2H
>99%
>99%
31%
>99%
>99%
5%
>99%
34%
0%
>99%
0%
c
c
c
0%
a
Conditions: hydride (10 μmol) and acid (10 μmol) mixed in
CD3CN/THF-d8 (1:1, ∼0.5 mL total) at −30 °C and then warmed to
RT (for HBF4·Et2O) or at RT (for CF3CO2H and HCO2H) until the
b
equilibrium was reached. Decomposition of the pincer complex was
c
observed. Reaction was monitored for 48 h.
acids with varying pKa values, which provide a qualitative sense
of the basicity of the iron hydride complexes. The protonation
reactions were carried out in CD3CN/THF-d8 (1:1) and moni-
tored by NMR spectroscopy. The role of CD3CN was to
intercept any coordinatively unsaturated species prior to decom-
position, and THF-d8 was added to improve solubility. Reactions
of the iron hydrides with 1 equiv of HBF4·Et2O (pKa = 0.1 in
CH3CN22,23) were fast at −30 °C, resulting in a rapid loss of
H2. Within a few minutes, the color of the solutions turned to
bright yellow. The change in color was more vivid in the cases
of 2-H, 2′-H, and 3-H, as they are either pale yellow (2-H) or
almost colorless (2′-H and 3-H). The 1H NMR spectra
confirmed the disappearance of the hydride resonance but with
no evidence for a dihydrogen or dihydride species. Unlike the
other hydride complexes, the reaction of 1-H led to extensive
RESULTS AND DISCUSSION
■
Abstraction of the Hydride Ligand. Our initial strategy
to remove the hydride ligand from iron was to treat the
complexes in Figure 1 with [Ph3C]+[BF4]−. Mixing 1-H with
[Ph3C]+[BF4]− in CD2Cl2 led to the rapid disappearance of the
hydride species as confirmed by 1H NMR; however, the
31P{1H} NMR spectrum displayed two resonances, one at
40.5 ppm (major) and the other at −13.0 ppm (minor). These
chemical shifts are indicative of a breakdown of the pincer
framework.18 The reaction of 2-H with [Ph3C]+[BF4]− in
CD2Cl2 resulted in a complicated mixture featuring very broad
1H NMR resonances. Fortunately, the same reaction carried out
in a coordinating solvent, CD3CN, was much more informative
due to well-resolved NMR peaks. The spectra recorded at 1 h re-
vealed three pincer complexes: the unreacted 2-H, acetonitrile-
trapped complex 2+-BF4 (vide infra), and the third iron
complex assigned as bis(acetonitrile) complex 4+-BF4 (eq 1).
+
decomposition and the formation of HPMe3 . Evidently,
HBF4·Et2O protonates not only the hydride ligand but also
PMe3; hence it causes the collapse of the pincer scaffold. While all
the hydrides described in this study are fully protonated by HBF4·
Et2O (entry 1), using weaker acids differentiates the basicity of
these complexes. When CF3CO2H (pKa = 12.7 in CH3CN22,23
)
was added to the solution of 1-H or 2-H at room tempera-
ture, the hydride resonance disappeared within a few minutes
(entry 2). On the other hand, only 34% of 2′-H was protonated
under similar conditions, and no protonation reaction was
observed for 3-H. Using an even weaker acid (HCO2H)24
resulted in 31% of 1-H being protonated (entry 3). Analogous
to the reactions with HBF4·Et2O and CF3CO2H, substantial
decomposition was observed in this case. Treatment of 2-H
with HCO2H converted merely 5% of 2-H, while 2′-H and 3-H
did not undergo protonation at all.
The aforementioned protonation data suggest that the
relative basicity of the iron hydride complexes follows a
decreasing order of 1-H > 2-H > 2′-H > 3-H. The kinetic site
for the protonation should be the hydride ligand, as
demonstrated in related systems.25 However, no dihydrogen
complexes were observed, suggesting that the dihydrogen
molecule is weakly bound to iron and readily displaced by
CD3CN. The relative basicity of the iron hydride complexes is
consistent with the order of electron density at the iron center.
The mass spectrum of the reaction mixture showed ions 2+
(m/z 542) and 4+ (m/z 507) as well as their expected
fragmentations. 4+-BF4 was assigned as a cis-bis(acetonitrile)
complex based on the observation of two sets of methine
resonances; the more symmetric trans isomer should have one
set of methine resonances. The 1H NMR spectrum also showed
the formation of Gomberg’s dimer 519 and [HPMe3]+[BF4]−.
B
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A more electron-rich iron center is likely to induce more
negative charge on the hydride ligand, thus making the pro-
tonation more readily. The presence of the electron-donating
PMe3 ligand makes the Fe−H more basic; thus the bis-
PMe3 complex 1-H is most readily protonated. As the PMe3
ligand is substituted by a π-accepting CO ligand, the electron
density at the metal is reduced and the basicity decreases.
Complexes 2-H and 2′-H have identical supporting ligands,
but 2-H is more basic than 2′-H, likely due to its hydride
being disposed at the opposite side of the stronger trans-
influencing ligand CO.
Synthesis of Cationic Iron Pincer Complexes. Because
HBF4·Et2O readily protonates all the iron hydrides, it was
chosen as the acid to synthesize the targeted cationic pincer
complexes. In the presence of CH3CN as a coordinating
solvent, the reactions of 2-H (eq 2) and 3-H (eq 3) with HBF4·
Figure 2. ORTEP drawing of {[2,6-(iPr2PO)2C6H3]Fe(CO)(PMe3)-
(CH3CN)}+[BF4]− (2+-BF4) at the 50% probability level. For clarity,
−
the counterion BF4 is not shown.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 2+-BF4 and 2-H
a
2+-BF4
2-H
Fe−C(1)
2.001(2)
2.013(2)
Fe−P(1)
Fe−P(2)
Fe−P(3)
Fe−C(22)
2.2594(7)
2.2854(7)
2.2888(7)
1.731(2)
2.2002(6)
2.1888(7)
2.2198(6)
1.774(2)
Et2O led to CH3CN-trapped complexes 2+-BF4 and 3+-BF4,
respectively. The 31P{1H} spectrum of 2+-BF4 reveals a triplet
at −0.2 ppm for PMe3 and a doublet at 206.4 ppm for the
phosphinite ligand. The magnitude of the 31P−31P coupling
constant for 2+-BF4 (16.5 Hz) is comparable to that for 2-H
(14.9 Hz), implying that these two complexes have the same
spatial arrangement for PMe3 and the pincer ligand. For
comparison, 2′-H, in which PMe3 and the ipso carbon are cis to
each other, gives a much larger 31P−31P coupling constant of
29.6 Hz. In the case of 3+-BF4, two 13C resonances (205.5 and
210.5 ppm) appear in the region for a carbonyl group, which is
consistent with a Cs symmetry structure bearing two cis CO
C(22)−O(22)
C(1)−Fe−P(1)
C(1)−Fe−P(2)
C(1)−Fe−P(3)
P(1)−Fe−P(2)
P(2)−Fe−P(3)
P(1)−Fe−P(3)
C(2)−O(1)−P(1)
C(6)−O(2)−P(2)
1.155(3)
1.157(3)
79.23(6)
79.43(7)
78.06(6)
77.58(7)
177.86(6)
156.83(2)
102.18(3)
100.68(3)
113.75(13)
112.48(13)
176.78(6)
149.81(3)
102.04(2)
99.81(2)
113.08(13)
110.23(14)
a
The crystal structure of 2-H was reported earlier by us (see ref 15).
1
ligands. As expected, H NMR of 3+-BF4 shows two sets of
multiplets for the methine hydrogens of the isopropyl groups.
The structure of 2+-BF4 was more unambiguously established
by X-ray crystallography. As shown in Figure 2, CH3CN is
located trans to the carbonyl group and the PMe3 ligand
remains trans to the ipso carbon. Structural comparison
between 2+-BF4 and 2-H (Table 2) reveals some noticeable
differences in bond lengths and angles. Compared to 2-H, all
the Fe−P bonds of 2+-BF4 are elongated by 0.06−0.10 Å.
Although the Fe−Cipso distance is almost identical, the Fe−C22
bond is shortened by 0.04 Å, reflecting a decreasing trans
influence changing from H− to CH3CN. Interestingly, 2+-BF4
has a wider P1−Fe−P2 angle than 2-H [156.83(2) vs
149.81(3) Å], which is mainly due to how the pincer ligand
distorts from a perfect meridional geometry. The chelating
rings of POCOP-pincer systems bearing an aromatic backbone
are flat in general,26 but the iron pincer complexes are
substantially less planar for the core structure, perhaps due to a
Figure 3. Distortion of the pincer ligand in 2+-BF4 and 2-H.
more crowded octahedral geometry about iron.27 In 2+-BF4,
the pincer ligand adopts an asymmetric bend conformation
(Figure 3) with the phosphorus atoms being displaced +0.4950
and −0.2988 Å out of the least-squares plane defined by the
C1, O1, O2, and Fe.28 In contrast, 2-H has a “gull wing”
C
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Scheme 2. Protonation of 2-H and 2′-H Involving Square-Pyramidal Intermediates
Scheme 3. Protonation of 2-H and 2′-H Involving a Distorted Trigonal-Bipyramidal Intermediate
conformation26 to take advantage of the small size of the
hydride ligand, resulting in the phosphorus atoms deviating
from the C1−O1−O2−Fe plane by +0.5135 and +0.2161 Å.29
Although 1-H reacts with HBF4·Et2O, attempts to isolate the
corresponding cationic complex failed even when the
protonation reaction was carried out at low temperatures.
Switching the acid to CF3COOH did not improve the
synthesis. The purification was hampered by the formation of
square pyramid. Trapping of [Fe(PNP)(X)(CO)]+ by CO gave
trans-[Fe(PNP)(X)(CO)2]+ exclusively.
Although we cannot rule out the possibility of isomerization
of a kinetically trapped product 2′+-BF4 to 2+-BF4 (shaded
in gray in Scheme 2), such a mechanism seems less likely.
We have previously shown that geometric isomerization of
18-electron iron POCOP-pincer complexes places CO trans to
the pincer aromatic ring.15 Similar observations have been
made by others in cis-to-trans isomerization of (PCP)Ir-
(H)2CO32 and (PNP)Fe(X)2CO.33 DFT calculations carried
out by Hall on the iridium system have suggested that the
isomerization process is driven by a delocalized interaction
between the π orbitals of CO, the dπ orbital of Ir, and the
π orbitals of the pincer aromatic ring in the trans isomer.34
Therefore, we had anticipated that 2′+-BF4 would be
thermodynamically more stable than 2+-BF4.
+
HPMe3 , free diphosphinite ligand, and other decomposition
products.
The reaction of 2′-H with HBF4·Et2O, on the other hand,
cleanly afforded one pincer-ligated complex, which was
1
obtained in 84% yield after workup. The H, 13C{1H}, and
31P{1H} NMR and IR data and crystal structure determination
of the isolated compound match well with those of 2+-BF4,
suggesting that PMe3 and CO swap their positions during the
reaction. A mechanism consistent with this result is outlined in
Scheme 2. Upon protonation of the hydride and loss of H2, the
square-pyramidal intermediate A′ would rapidly isomerize to A
so that the two strongly trans-influencing ligands (CO and aryl
group) avoid being trans to each other. Trapping of the
predominant intermediate A with CH3CN produces 2+-BF4. A
similar mechanism has been proposed by Kirchner for the
carbonylation of Fe(PNP)(X)2(CO) (PNP = aminophosphine-
based pincer ligands30) facilitated by AgBF4.31 In that study,
regardless of which geometric isomer (cis or trans) was used,
abstraction of halide from Fe(PNP)(X)2(CO) provided the
same 16-electron intermediate, [Fe(PNP)(X)(CO)]+, which is
analogous to A with CO occupying the apical position of the
Protonation of 2-H and 2′-H could converge to a Y-shaped,
distorted trigonal-bipyramidal intermediate B (Scheme 3), and
the subsequent coordination of CH3CN to the iron center of
B might selectively give 2+-BF4. This mechanism is also
disfavored because the Y-structure generally requires the ligand
trans to the acute angle (in this case, the angle formed by CO
and PMe3) to be a weak σ-donor and a good π-donor.35
Ligands that fit this criterion are usually halides, amides, and
alkoxides. Although B is less likely to be a ground-state
structure, it could be the transition state for the interconversion
between A and A′.
Activation of Dihydrogen. Protonation of the neutral iron
pincer hydrides to yield the cationic complexes is, in principle,
D
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a reversible process. Having a base with an appropriate pKb value,
the cationic dihydrogen intermediate can be reverted back
to the neutral hydride, thereby activating H2 in a heterolytic
(H+/H−) fashion. The protonation results shown in Table 1
imply that the pKa value for the conjugate acid of 2-H should
fall in between the values for CF3CO2H and HCO2H, and
therefore a typical amine should be able to assist with H2
group. Such a pathway involving intramolecular heterolysis of
H2 with the elimination of [HPMe3]+ may look unusual but is
precedented. Tyler and co-workers have shown that trans-
Fe(diphosphine)2Cl2 reacts with H2 in water to give trans-
[Fe(diphosphine)2(H)(H2)]+Cl− along with some protonated
diphosphine ligand.36 In our case, a coordinatively unsaturated
intermediate D is generated and reacts with free PMe3
i
(available via proton transfer from [HPMe3]+ to Pr2NEt),
i
activation. Indeed when a mixture of 2+-BF4 and Pr2NEt
thus explaining the formation of 2′-H.
(Hunig’s base) in CH Cl was exposed to H , the color of
̈
2
2
2
the solution changed from yellow to almost colorless, sug-
gesting the consumption of 2+-BF4. Within 2.5 h at room
temperature, 2+-BF4 was fully converted to 2-H and 2′-H
along with the protonated base, [iPr2N(H)Et]+[BF4]− (eq 4).
From the 31P{1H} NMR spectrum, the ratio between 2-H and
2′-H was determined to be 3:2, which did not change with
time.
Activation of H2 with 3+-BF4 is significantly more sluggish
(eq 5). The reaction at room temperature was incomplete even
after 11 days, with 91% of 3+-BF4 being converted to 3-H.
Presumably, the dissociation of CH3CN from 3+-BF4 is much
slower than from 2+-BF4. Since CF3CO2H failed to protonate
3-H (Table 1), the less acidic [iPr2N(H)Et]+[BF4]− is not likely
to protonate 3-H either, suggesting that the equilibrium in eq 5
lies predominantly to the right.
To activate H2 in a catalytic fashion, hydrogenation of
benzaldehyde was attempted in the presence of 1 mol % of
2+-BF4 under ∼1 atm of H2 pressure. The limited solubility of
2+-BF4 in commonly used organic solvents such as THF, di-
ethyl ether, and toluene presented a minor technical challenge.
Although it is highly soluble in CHCl3 and CH2Cl2, these solvents
can cause decomposition of hydride intermediates (especially if
heated) and therefore are not suitable for the catalytic study. Given
these factors, chlorobenzene-d5 was chosen as the solvent so that
the reaction could be conveniently monitored by NMR.
Unfortunately, at 70 °C, no hydrogenation of benzaldehyde was
observed at 24 h (eq 6). The 1H NMR spectrum revealed a very
The formation of 2-H and 2′-H must be a kinetically
controlled process, as the latter has been previously shown to
be the thermodynamically more stable isomer.15 We speculate
that following the dissociation of CH3CN from 2+-BF4,
intermediate A is configurationally stable enough to allow H2
to coordinate from the site trans to CO (Scheme 4). The
resulting dihydrogen complex C undergoes intermolecular
i
deprotonation by Pr2NEt to give rise to 2-H. Meanwhile, C
also proceeds with intramolecular deprotonation by PMe3 with
concomitant migration of CO to the opposite side of the aryl
Scheme 4. Proposed Pathways Leading to the Formation of Both 2-H and 2′-H
E
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a
small amount of hydride species at −9.47 ppm (a multiplet), and
the 31P{1H} NMR spectrum showed substantial decomposition of
the pincer complex. Because the stoichiometric activation of H2
by 3+-BF4 was much slower, catalytic hydrogenation with this
particular cationic complex was not pursued.
Catalytic Hydrosilylation Reactions. Better success was
achieved when the cationic complexes were employed to
catalyze the hydrosilylation of carbonyl groups. As shown in
Table 3, hydrosilylation of benzaldehyde (eq 7) catalyzed by
Table 4. Catalytic Hydrosilylation of Acetophenone
b
entry
catalyst
time (h)
conversion (%)
1
2
3
4
2+-BF4
48
48
48
48
79
54
63
56
2-H
3+-BF4
3-H
a
Conditions: iron catalyst (9.5 μmol), PhCOCH3 (0.95 mmol), and
b
(EtO)3SiH (1.05 mmol) in 1 mL of C6H5Cl. Calculated by 1H NMR.
when 2+-BF4 was used as the catalyst (entry 1), whereas 54%
conversion was obtained for the reaction catalyzed by 2-H
(entry 2). Complex 3+-BF4 (entry 3) showed a marginally
better efficiency than 3-H (entry 4), which is in agreement with
the results for the hydrosilylation of benzaldehyde. It is
interesting to note that for the acetophenone reaction the
catalytic activity of 3-H is comparable to that of 2-H. For the
hydrosilylation of the aldehyde mentioned above, 2-H is a
better catalyst than 3-H (Table 3, entries 2 and 5). We
hypothesize that at 80 °C the lower activity of 3-H is offset by
its higher thermal stability.
a
Table 3. Catalytic Hydrosilylation of Benzaldehyde
b
c
entry
catalyst
time (h)
yield (%)
catalyst remaining (%)
1
2
3
4
5
2+-BF4
24
24
24
48
48
>99
15
0
8
95
2-H
2′-H
>99
0
3+-BF4
3-H
10
0
>99
a
Conditions: iron catalyst (4.8 μmol), PhCHO (0.48 mmol),
(EtO)3SiH (0.52 mmol), and hexamethylbenzene (26.5 μmol, internal
b
c
standard) in 0.5 mL of C6D5Cl. Calculated by 1H NMR. Calculated
by 31P{1H} NMR.
CONCLUSION
■
As part of our efforts to develop catalytic processes using
cationic iron POCOP-pincer complexes, we have studied the
protonation of four iron hydride complexes with various
Brønsted acids and established the basicity order of 1-H > 2-H >
2′-H > 3-H. We have also observed a SET pathway for the
reaction between 2-H and [Ph3C]+[BF4]−, which results in a
complicated mixture of products, making [Ph3C]+[BF4]−
unsuitable for the synthesis of the desired cationic complexes.
Instead, we have used HBF4·Et2O to successfully remove the
hydride ligand from 2-H/2′-H and 3-H to yield 2+-BF4 and
3+-BF4, respectively. Synthesis of an analogous cationic
complex from 1-H remains challenging due to the facile
protonation of PMe3. With two new cationic iron complexes in
hand, we have investigated their reactivity for the activation of
small molecules such as H2 and silanes. In the presence of an
external base, iPr2NEt, both 2+-BF4 and 3+-BF4 can cleave H2 in
a heterolytic manner, resulting in neutral hydride complexes
and [iPr2N(H)Et]+[BF4]−. Although 2+-BF4 is not a viable
catalyst for the hydrogenation of benzaldehyde (under 1 atm of
H2 pressure), 2+-BF4 and 3+-BF4 are effective in catalyzing the
hydrosilylation of benzaldehyde and acetophenone and
demonstrating improved activity over the corresponding
neutral hydride complexes.
In our previous work on hydrosilylation reactions catalyzed
by 1-H,15 deuterium-labeling experiments showed that the
hydride was intact during the catalytic reaction, thus only
playing the role of promoting ligand dissociation. The DFT
calculations by Wei et al., however, supported a mechanism
involving the delivery of the hydride to the CO group.17 The
current cationic iron system is likely to operate via the
mechanism shown in Scheme 1, which is analogous to what we
have proposed for the neutral hydride system. The difference is
that the silane complex in this case is cationic, resulting in more
favorable silylium transfer to the carbonyl substrates. Explaining
the discrepancy between the experimental data and DFT
calculations for the neutral hydride catalysts would require
additional investigations. Nevertheless, through this study, we
have further confirmed that for the Fe-POCOP system the
hydride ligand is not necessarily needed for catalytic hydro-
silylation reactions.
2+-BF4 at 50 °C went to completion in 24 h (entry 1), whereas
the reaction catalyzed by the corresponding neutral iron
hydride 2-H gave hydrosilylation products37 in 15% yield over
the same period of time (entry 2). In contrast, 2′-H did not
catalyze this reaction at all (entry 3). Cationic complex 3+-BF4
was not an efficient catalyst either; only 10% yield was obtained
after 48 h (entry 4). However, it is still a better catalyst than
3-H, which showed no catalytic activity (entry 5).
Overall, the cationic complexes are better catalysts than the
analogous neutral hydride complexes, but only 2+-BF4 shows a
drastic improvement. To further understand the catalytic
reactivity of these complexes, the percentage of remaining catalyst
at the end of the reaction was measured by 31P{1H} NMR
spectroscopy (the rightmost column of Table 3). In the case of
2+-BF4, only 8% of the catalyst was present at the end of the
reaction (entry 1). The bulk of the catalyst was converted to 2-H,
2′-H, and a small quantity of unidentified species. As expected
from our previous study,15 5% of 2-H was isomerized to 2′-H
(entry 2). For 3+-BF4, the cationic species was converted to 3-H
quantitatively within 4 h, and hydrosilylated products were
obtained in 10% yield (entry 4). Once 3+-BF4 was fully converted
to 3-H, the catalytic reaction ceased, which is consistent with the
observation of no catalytic activity for 3-H (entry 5).
Acetophenone was also subjected to the hydrosilylation
conditions using either the cationic or the neutral iron
complexes as the catalysts, although the temperature had to
be raised to 80 °C (eq 8). In terms of the relative catalytic
activity (Table 4), the general trend is similar to that observed
in the benzaldehyde reduction. For instance, after 48 h, 79% of
the acetophenone was converted to hydrosilylation products38
F
dx.doi.org/10.1021/om500758j | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
evolution was observed. The resulting bright yellow solution was
passed through a short pad of Celite, and the solvent was removed
under vacuum to produce a crystalline solid (107 mg, 85% yield).
1H NMR (400 MHz, CDCl3, δ): 1.29−1.41 (m, CH(CH3)2, 24H),
EXPERIMENTAL SECTION
■
General Comments. All the organometallic compounds were
prepared and handled under an argon atmosphere using standard
Schlenk and inert-atmosphere box techniques. Dry and oxygen-free
solvents were collected from an Innovative Technology solvent
purification system and used throughout all experiments. Deuterated
NMR solvents (CD3CN, CDCl3, and C6D5Cl) were purchased from
Cambridge Isotope Laboratories, Inc., kept under an argon
2
1.73 (d, JPH = 8.4 Hz, PMe3, 9H), 2.38 (s, NCCH3, 3H), 2.68−
3
3
2.79 (m, CH, 4H), 6.66 (d, JHH = 8.0 Hz, ArH, 2H), 6.99 (t, JHH
=
8.0 Hz, ArH, 1H). 13C{1H} NMR (101 MHz, CDCl3, δ): 4.9
(s, CH3CN), 18.0 (s, CH3), 18.1 (s, CH3), 18.5 (s, CH3), 18.6 (s,
CH3), 21.1 (d, 1JPC = 26.4 Hz, PMe3), 31.3 (t, 1JPC = 7.2 Hz, CH), 32.2
(t, 1JPC = 10.4 Hz, CH), 106.7 (t, 3JPC = 4.7 Hz, ArC), 128.1 (s, ArC),
atmosphere, and used without further purification. H, 13C{1H}, and
1
31P{1H} NMR spectra were recorded on a Bruker Avance-400 MHz
2
132.9 (s, CH3CN), 134.4−135.0 (m, ArC), 164.0 (t, JPC = 6.5 Hz,
spectrometer. Chemical shift values in H and 13C{1H} NMR spectra
1
ArC), 216.3-216.5 (m, CO). 31P{1H} NMR (162 MHz, CDCl3, δ):
were referenced internally to the residual solvent resonances. 31P{1H}
spectra were referenced externally to 85% H3PO4 (0 ppm). Infrared
spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR
spectrometer equipped with a Smart Orbit diamond attenuated total
reflectance (ATR) accessory. The mass spectrum of the reaction in eq
1 was obtained by injecting the reaction mixture (in CH3CN) into a
Micromass Q-TOF-2 mass spectrometer at a flow rate of 4 μL/min.
Complexes 1-H, 2-H, 2′-H, and 3-H were prepared as described in the
literature.15
2
2
−0.2 (t, JPP = 16.5 Hz, PMe3, 1P), 206.4 (d, JPP = 16.5 Hz, OPiPr2,
2P). ATR-IR (solid): νCO = 1946 cm−1. Anal. Calcd for
C24H43NO3P3FeBF4: C, 45.82; H, 6.89; N, 2.23. Found: C, 45.83;
H, 6.85; N, 2.38.
Synthesis of cis-{[2,6-(iPr2PO)2C6H3]Fe(CO)2(CH3CN)}+[BF4]−
(3+-BF4). This compound was prepared in 82% yield by a procedure
similar to that used for 2+-BF4. 1H NMR (400 MHz, CDCl3, δ): 1.30−
1.54 (m, CH(CH3)2, 24H), 2.42 (s, NCCH3, 3H), 2.74−2.84 (m, CH,
2H), 2.89−2.98 (m, CH, 2H), 6.75 (d, 2JHH = 8.0 Hz, ArH, 2H), 7.10
(t, 2JHH = 8.0 Hz, ArH, 1H). 13C{1H} NMR (101 MHz, CDCl3, δ): 4.7
Reaction of 2-H with [Ph3C]+[BF4]−. In a J. Young NMR tube, 2-H
(6.0 mg, 12 μmol) was mixed with ∼0.5 mL of CD3CN. The hydride
complex was not fully dissolved, but turned into a clear solution upon
further mixing with [Ph3C]+[BF4]− (3.9 mg, 12 μmol). After 1 h, the
31P{1H} NMR spectrum showed a mixture of 2-H (45% of total pincer
complexes), 2+-BF4 (30%), 4+-BF4 (25%), and [HPMe3]+[BF4]−.39
31P{1H} NMR (162 MHz, δ): −2.1 (s, [HPMe3]+), 0.15 (t, 2JPP = 17.2
2
(s, CH3CN), 16.7 (s, CH3), 17.1 (t, JPC = 2.0 Hz, CH3), 17.3 (s,
CH3), 18.6 (t, 2JPC = 2.4 Hz, CH3), 29.2 (t, 1JPC = 12.9 Hz, CH), 30.7
(t, 1JPC = 10.6 Hz, CH), 108.1 (t, 3JPC = 5.5 Hz, ArC), 129.8 (s, ArC),
2
2
131.9 (t, JPC = 12.6 Hz, ArC), 132.8 (s, CH3CN), 164.5 (t, JPC
=
2
2
7.1 Hz, ArC), 205.5 (t, JPC = 8.3 Hz, CO), 210.5 (t, JPC = 27.2 Hz,
CO). 31P{1H} NMR (162 MHz, CDCl3, δ): 206.9 (s). ATR-IR
2
Hz, PMe3 of 2+-BF4), 11.5 (t, JPP = 16.6 Hz, PMe3 of 2-H), 197.0
(solid): νCO
= . Anal. Calcd for
2046 and 1989 cm−1
2
(s, OPiPr22of 4+-BF4), 207.3 (d, JPP = 17.2 Hz, OPiPr2 of 2+-BF4),
C22H34NO4P2FeBF4: C, 45.47; H, 5.90; N, 2.41. Found: C, 45.41;
H, 5.75; N, 2.56.
231.9 (d, JPP = 16.6 Hz, OPiPr2 of 2-H). The H NMR spectrum
1
showed 540 in addition to the products mentioned above. Selected 1H
NMR data of 2-H (400 MHz, δ): −9.41 (td, 2JPH = 62.8 and 50.4 Hz,
FeH), 0.96−1.02 (m, CH(CH3)2), 1.05−1.12 (m, CH(CH3)2),
X-ray Structure Determination. Single crystals of 2+-BF4 were
grown from a saturated solution in THF/CH3CN (or CH2Cl2)
at −30 °C. Crystal data collection and refinement parameters are
summarized in Table S1. Intensity data were collected at 150 K on a
Bruker APEX-II CCD detector at Beamline 11.3.1 at the Advanced
Light Source (Lawrence Berkeley National Laboratory) using
synchrotron radiation tuned to λ = 0.774 90 Å. The data frames
were collected using the program APEX2 and processed using the
program SAINT, a routine within APEX2. The data were corrected for
absorption and beam corrections based on the multiscan technique
as implemented in SADABS. The structure was solved by a
combination of direct methods SHELXTL v6.14 and the difference
Fourier technique and refined by full-matrix least-squares on F2.
Non-hydrogen atoms were refined with anisotropic displacement
2
1.41 (d, JPH = 7.2 Hz, PCH3), 2.30−2.39 (m, CH(CH3)2), 2.43−
3
3
2.55 (m, CH(CH3)2), 6.25 (d, JHH = 7.6 Hz, ArH), 6.60 (t, JHH
=
=
7.6 Hz, ArH). Selected 1H NMR data of 2+-BF4: 1.67 (d, JPH
2
3
8.4 Hz, PCH3), 2.67−2.78 (m, CH(CH3)2), 6.67 (d, JHH = 8.0 Hz,
3
ArH), 7.02 (t, JHH = 8.0 Hz, ArH). Selected 1H NMR data of
4+-BF4: 2.88−2.95 (m, CH(CH3)2), 2.96−3.05 (m, CH(CH3)2), 6.51
3
3
1
(d, JHH = 8.0 Hz, ArH), 6.87 (t, JHH = 8.0 Hz, ArH). Selected H
3
NMR data of 5: 5.18−5.24 (m, Ph3CCH), 5.97 (dd, JHH = 10.8 and
3
4
4.0 Hz, Ph3CCHCH), 6.16 (dd, JHH = 10.8 Hz, JHH = 2.4 Hz,
1
2
Ph2CCCH). H NMR data of [HPMe3]+[BF4]−: 1.79 (dd, JPH
=
3
1
15.6 Hz, JHH = 5.6 Hz, [HP(CH3)3]+), 6.06 (dm, JPH = 508 Hz,
3JHH = 5.6 Hz, [HP(CH3)3]+). TOF-MS (ES+) data of the reaction
mixture (in CH3CN, m/z): M+ of 2+ calcd for C24H43NO3P3Fe
542.1805, found 542.2819; [M − CH3CN]+ of 2+ calcd for
C22H40O3P3Fe 501.1540, found 501.2165; [M − CH3CN − CO]+ of
2+ calcd for C21H40O2P3Fe 473.1591, found 473.2400; M+ of 4+ calcd
for C23H37N2O3P2Fe 507.1629, found 507.2582; [M − CH3CN]+ of
4+ calcd for C21H34NO3P2Fe 466.1364, found 466.2246.
−
parameters. The BF4 anion is disordered; a reasonable two-
component disorder model is given (major occupancy = 0.67). The
H atom positions were calculated and treated with a riding model in
subsequent refinements. The isotropic displacement parameters for
the H atoms were defined as aUeq (a = 1.5 for methyl and 1.2 for all
others) of the adjacent atom.
Attempted Hydrogenation of Benzaldehyde. In a J. Young
NMR tube, 2+-BF4 (3.0 mg, 4.8 μmol) was mixed with 0.5 mL of
C6D5Cl, followed by the addition of benzaldehyde (49 μL, 0.48 mmol)
and hexamethylbenzene (4.3 mg, 26.5 μmol, internal standard). The
reaction mixture was frozen by liquid nitrogen, after which the
headspace of the NMR tube was evacuated and then exposed to a
hydrogen atmosphere (1 atm). The frozen sample was thawed at room
temperature before the NMR tube was disconnected from the
hydrogen source. The reaction mixture was heated by an oil bath at
Procedures for the Protonation Reactions. In a J. Young NMR
tube, an iron hydride complex (10 μmol) was dissolved in ∼0.5 mL of
CD3CN/THF-d8 (1:1). The resulting solution was cooled to −30 °C,
followed by the addition of HBF4·Et2O (10 μmol) and then slowly
warmed to room temperature. For the experiments involving
CF3CO2H or HCO2H, the acid (10 μmol) was added at room
temperature. The progress of the reaction was monitored by 1H NMR
and 31P NMR spectroscopy until it reached equilibrium or for 48 h
(for the reaction of 3-H with CF3CO2H and the reactions of 2′-H and
3-H with HCO2H). Protonation of 1-H with CF3CO2H and HCO2H
was also attempted by starting the reaction at −30 °C first; however,
the results were the same with a significant amount of decomposition
products observed.
1
70 °C, and the progress of the reaction was monitored by H and
31P{1H} NMR spectroscopy.
General Procedure for Catalytic Hydrosilylation of Benz-
aldehyde. In a typical experiment, an iron catalyst (4.8 μmol, 1 mol
%) and 0.5 mL of C6D5Cl were mixed in a J. Young NMR tube. To
this solution, benzaldehyde (49 μL, 0.48 mmol), triethoxysilane (96
μL, 0.52 mmol), and hexamethylbenzene (4.3 mg, 26.5 μmol, internal
standard) were added at once. The reaction mixture was heated by an
oil bath at 50 °C, and the progress of the reaction was monitored by
1H and 31P{1H} NMR spectroscopy. The combined yield for the
Synthesis of {[2,6-(iPr2PO)2C6H3]Fe(CO)(PMe3)(CH3CN)}+-
[BF4]− (2+-BF4). A solution of 2-H (100 mg, 0.20 mmol) in THF/
CH3CN (1 mL each) was cooled to −30 °C, followed by the addition
of HBF4·Et2O (28 μL, 0.20 mmol) at this temperature. The reaction
mixture was then slowly warmed to room temperature while gas
G
dx.doi.org/10.1021/om500758j | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
hydrosilylation products was calculated by comparing the integration
of PhCH2O methylene resonances15 with that of the internal standard.
The percentage of the remaining iron catalyst was calculated from 31P
NMR integrations of the iron catalysts and other iron species (without
a standard).
General Procedure for Catalytic Hydrosilylation of Aceto-
phenone. In a typical experiment, an iron catalyst (9.5 μmol, 1 mol %)
was dissolved in 1 mL of C6H5Cl. To this solution, acetophenone
(111 μL, 0.95 mmol) and triethoxysilane (194 μL, 1.05 mmol) were
added. The reaction mixture was heated by an oil bath at 80 °C for
48 h. After cooling to room temperature, an aliquot was withdrawn
from the mixture, diluted with CDCl3, and then subjected to 1H NMR
analysis. No byproducts were observed for any of the reactions with
acetophenone. The percentage conversion of acetophenone was
calculated based on the integrations of PhCH(OSiR3)CH3 and
PhCOCH3.15
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ASSOCIATED CONTENT
■
S
* Supporting Information
Complete details of the crystallographic study (PDF and CIF),
characterization data for 2+-BF4 and 3+-BF4, and representative
NMR spectra of the catalytic reaction. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hairong.guan@uc.edu.
(10) Ir-based catalysts: (a) Yang, J.; Brookhart, M. J. Am. Chem. Soc.
2007, 129, 12656−12657. (b) Yang, J.; White, P. S.; Brookhart, M. J.
Am. Chem. Soc. 2008, 130, 17509−17518. (c) Yang, J.; Brookhart, M.
Adv. Synth. Catal. 2009, 351, 175−187. (d) Park, S.; Brookhart, M.
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Chem. Commun. 2011, 47, 3643−3645. (f) Park, S.; Brookhart, M. J.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Science Foundation (CHE-0952083)
and the Alfred P. Sloan Foundation (research fellowship to
H.G.) for supporting this research. Crystallographic data were
collected at Beamline 11.3.1 at the Advanced Light Source
(ALS), Lawrence Berkeley National Laboratory (supported by
the U.S. Department of Energy, Office of Basic Energy
Sciences, under contract No. DE-AC02-05CH11231).
Am. Chem. Soc. 2012, 134, 640−653. (g) Park, S.; Bez
Brookhart, M. J. Am. Chem. Soc. 2012, 134, 11404−11407.
(h) McLaughlin, M. P.; Adduci, L. L.; Becker, J. J.; Gagne, M. R. J.
Am. Chem. Soc. 2013, 135, 1225−1227.
́
ier, D.;
́
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(14) A very recent mechanistic study has shown that the most active
hydride intermediate is not the hydride species produced after silylium
transfer, but a silane-bound iridium hydride. For details, see:
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signs for P1 and P2 mean that they are on opposite sides of the least-
squares plane.
(29) The deviation from planarity for the C1−O1−O2−Fe plane was
calculated to be 0.0042 Å.
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(37) Multiple hydrosilylation products were observed for the
reactions described in Table 3 (entries 1, 2, and 4). This is due to
the redistribution of PhCH2O and EtO groups on silicon.
(38) There were two hydrosilylation products, as evidenced by two
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dx.doi.org/10.1021/om500758j | Organometallics XXXX, XXX, XXX−XXX