Spectroscopic and DFT study of ferraaziridine complexes formed in the transfer hydrogenation of acetophenone catalyzed using trans -[Fe(CO)(NCMe)(PPh2C6H4CHNCH2-) 2-κ4 P,N,N,P ](BF4)2

Article abstract of DOI:10.1021/om201170f

The reaction of the iron complex trans-[Fe(CO)(MeCN)(PPh₂C ₆H₄CHNCH₂-)₂-κ⁴P,N, N,P](BF₄)₂ (1) with KOiPr in benzene produced the unusual complex [Fe(CO)(PPh₂C₆H₄CHNCH ₂CH₂NHCHC₆H₄PPh₂)- κ⁵P,N,C,N,P][BF₄] (2), which has been characterized by spectroscopy and by single-crystal X-ray diffraction. The C-N bond length in this complex indicates that it is best viewed as an iron(II) ligand-folded ferraaziridine-κ²C,N complex instead of an iron(0) η²-iminium complex. Density functional theory (DFT) calculations have been employed on simplified structural models to support a mechanism of formation of this complex via the transfer of a hydride from the alkoxide complex trans-[Fe(CO)(OCHMe₂)(PH₂C₆H ₄CHNCH₂-)₂-κ⁴P,N,N,P] ⁺ (4_DFT) to an imine carbon on the ligand to produce the amide complex trans-[Fe(CO)(OC(CH₃)₂)(PH₂C ₆H₄CHNCH₂CH₂NCH₂C ₆H₄PH₂-κ⁴P,N,N,P)] ⁺ (5DFT_acet) followed by liberation of acetone to afford 5_DFT. Two energetically similar pathways have been proposed in which deprotonation of the PNNP ligand of 5_DFT by strong base produces the experimentally observed ferraaziridinido complex Fe(CO)(PH₂C ₆H₄CHNCH₂CH₂NCHC₆H ₄PH₂)-κ⁵P,N,C,N,P (3_DFT) or the square-pyramidal Fe(0) complex Fe(CO)(PH₂C₆H ₄CHNCH₂-)₂-κ⁴P,N,N,P (7 _DFT). Protonation of 3_DFT by free isopropyl alcohol produces the ferraaziridine complex 2_DFT. Nuclear magnetic resonance and infrared spectroscopy data show that during the transfer hydrogenation of acetophenone catalyzed by 1 in basic isopropyl alcohol, free ligand is observed along with one major iron-containing species identified as 3. On the basis of our calculations of relative free energies and a CO scale factor, we predict that 2 is easily deprotonated to form the electron-rich iron complex 3 and the square-pyramidal Fe(0) complex 7, which are responsible for the two observed CO stretches below 1900 cm^-1 in catalytic mixtures. Mass balance studies indicate that the catalytically active species is not observable by NMR. Although 2 and 3 are poor transfer hydrogenation catalysts, we present experimental and theoretical evidence that ligand folding/distortion is feasible.

Full text of DOI:10.1021/om201170f

Article
pubs.acs.org/Organometallics
Spectroscopic and DFT Study of Ferraaziridine Complexes Formed in
the Transfer Hydrogenation of Acetophenone Catalyzed Using trans-
[Fe(CO)(NCMe)(PPh2C6H4CHNCH2−)2-κ4P,N,N,P](BF4)2
Demyan E. Prokopchuk, Jessica F. Sonnenberg, Nils Meyer, Marco Zimmer-De Iuliis, Alan J. Lough,
and Robert H. Morris*
Department of Chemistry, University of Toronto, 80 St George Street, Toronto, Ontario M5S 3H6, Canada
S
* Supporting Information
ABSTRACT: The reaction of the iron complex trans-
[Fe(CO)(MeCN)(PPh2C6H4CHNCH2−)2-κ4P,N,N,P]-
(BF4)2 (1) with KOiPr in benzene produced the unusual
complex [Fe(CO)(PPh2C6H4CHNCH2CH2NHCHC6H4PPh2)-
κ5P,N,C,N,P][BF4] (2), which has been characterized by
spectroscopy and by single-crystal X-ray diffraction. The C−N
bond length in this complex indicates that it is best viewed as
an iron(II) ligand-folded ferraaziridine-κ2C,N complex instead
of an iron(0) η2-iminium complex. Density functional theory
(DFT) calculations have been employed on simplified
structural models to support a mechanism of formation of this complex via the transfer of a hydride from the alkoxide
complex trans-[Fe(CO)(OCHMe2)(PH2C6H4CHNCH2−)2-κ4P,N,N,P]+ (4DFT) to an imine carbon on the ligand to produce
the amide complex trans-[Fe(CO)(OC(CH3)2)(PH2C6H4CHNCH2CH2NCH2C6H4PH2-κ4P,N,N,P)]+ (5 ) followed by
acet
DFT
liberation of acetone to afford 5DFT. Two energetically similar pathways have been proposed in which deprotonation of the
PNNP ligand of 5DFT by strong base produces the experimentally observed ferraaziridinido complex Fe(CO)(PH2C6H4CH
NCH2CH2NCHC6H4PH2)-κ5P,N,C,N,P (3DFT) or the square-pyramidal Fe(0) complex Fe(CO)(PH2C6H4CHNCH2−)2-
κ4P,N,N,P (7DFT). Protonation of 3DFT by free isopropyl alcohol produces the ferraaziridine complex 2DFT. Nuclear magnetic
resonance and infrared spectroscopy data show that during the transfer hydrogenation of acetophenone catalyzed by 1 in basic
isopropyl alcohol, free ligand is observed along with one major iron-containing species identified as 3. On the basis of our
calculations of relative free energies and a CO scale factor, we predict that 2 is easily deprotonated to form the electron-rich
iron complex 3 and the square-pyramidal Fe(0) complex 7, which are responsible for the two observed CO stretches below
1900 cm−1 in catalytic mixtures. Mass balance studies indicate that the catalytically active species is not observable by NMR.
Although 2 and 3 are poor transfer hydrogenation catalysts, we present experimental and theoretical evidence that ligand folding/
distortion is feasible.
INTRODUCTION
■
The hydrogenation of carbonyl compounds to alcohols is a vital
process in the pharmaceutical, fine chemical, and perfume
industries. The alcohols may be prepared via direct hydro-
genation, hydrosilylation, or transfer hydrogenation (TH)
reactions, usually utilizing catalysts based on expensive and
sometimes toxic platinum metals.1 Several attempts have been
Figure 1. Iron precatalysts for the TH of ketones with 6.5.6 (left) and
5.5.5 (right) metallacycles prepared by our group.
made to develop iron catalysts that would be cheaper and
nontoxic for these kinds of reactions.1−13 The first studies were
excellent turnover frequencies (TOF) and enantioselectivities
carried out by Vancheesan et al.14,15 and Gao et al.,16 who
(TOF up to 28 000 h−1, ee up to 99%). While the mechanism
introduced the asymmetric iron-catalyzed TH of ketones. In
of ruthenium-catalyzed TH has been extensively studied,12,25,26
this regard, we recently reported highly efficient iron catalysts
little is known about the mechanism of iron-catalyzed TH. A
for the asymmetric TH of ketones to the corresponding
alcohols using isopropyl alcohol as a hydrogen source.17−24 As a
detailed knowledge of the mechanism will be essential to
optimize the design of our systems to make more efficient and
common structural feature in the new catalysts, an iron(II)
atom is coordinated by a tetradentate diimine ligand, a carbonyl
or isonitrile, and an acetonitrile or bromide ligand, as depicted
in Figure 1. The catalysts work at room temperature with
Received: November 22, 2011
Published: March 28, 2012
© 2012 American Chemical Society
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enantioselective iron catalysts, to make them more competitive
with the best platinum metal based systems. Hence, this is
currently under extensive study in our laboratory.
In the present paper we discuss the species observable by
NMR and IR spectroscopy during the TH of acetophenone to
1-phenylethanol using the iron complex trans-[Fe(CO)-
(MeCN)(PPh2C6H4CHNCH2−)2-κ4P,N,N,P][(BF4)2]
(1)24 as the precatalyst (Scheme 1), isopropyl alcohol as the
Investigation of the Catalysis by Spectroscopic
Methods. The reaction mixture of a typical TH experiment
was analyzed via NMR and IR techniques by using 1 as a
precatalyst. 1H and 31P{1H} NMR spectra were recorded when
the TH was carried out using relatively high catalyst loadings
(catalyst:substrate = 1:43). 31P{1H} NMR spectra were
recorded for reaction mixtures in isopropyl alcohol using
1
C6D6 as an internal NMR reference for locking. H NMR
spectra were carried out in deuterated isopropyl alcohol and
showed only poorly resolved peaks. No hydride signals were
observed in the region between 0 and −30 ppm; however, a
hydride mechanism cannot be dismissed, as such a hydride
species may be only transient or may be present in low
concentration. The formation of iron hydrides as intermediates
was proposed earlier for TH with the precatalysts [(PP3)Fe-
(H)(H2)](BPh4) (PP3 = P(CH2CH2PPh2)3)27 and
Fe3(CO)12,16 but hydride species were not isolated or observed
spectroscopically.
Scheme 1. Reaction Scheme for TH of Ketones Using 1 As a
Precatalyst in the Presence of Excess Base
a
31P{1H} NMR spectra recorded during TH have a number of
interesting features. Two related doublets at 84.1 and 68.7 ppm
(JPP = 29 Hz), various singlets around −13.0 ppm, and two
singlets at 32.2 and 31.7 ppm were observed (Figure 3). The
a
8 equiv with respect to 1.
hydrogen source, and potassium tert-butoxide (KOtBu) as the
base. To gain further insight into the species formed during
catalysis, we investigated the reaction of precatalyst 1 with
stoichiometric amounts of base and performed density
functional theory (DFT) calculations on simplified models of
the reactive species to further support our results.
RESULTS AND DISCUSSION
■
Catalysis. The use of 1 as a precatalyst for the TH of
acetophenone to 1-phenylethanol with a TOF of 2200 h−1 has
been previously reported.24 When 1 is dissolved in a solution of
acetophenone in basic isopropyl alcohol, the solution
immediately becomes clear and dark brown. The activated
catalyst mixture is highly air sensitive, and exposure to air
results in an immediate color change to pale yellow, rendering
it inactive for TH. A typical plot for the production of
1-phenylethanol versus time using 1 at a catalyst to base to
substrate loading of 1:8:600 at 26 °C is shown in Figure 2.
Figure 3. 31P{1H} NMR spectrum (161 MHz, iPrOH, C6D6 internal
reference) of a TH run in 0.65 mL of isopropyl alcohol for the
production of 1-phenylethanol from acetophenone (0.60 M) by use of
1 (0.025 M) and KOtBu (0.14 M) at 26 °C.
peak at 32.2 ppm is due to the oxidized free PNNP ligand. This
was confirmed by a 31P{1H} NMR spectrum of an
independently prepared sample of the PNNP ligand oxidized
at phosphorus using hydrogen peroxide. The peak at 31.7 ppm
is likely the mono-oxidized PNNP ligand, partially overlapped
by the peak at 32.2 ppm. The peaks at −13.0 are due to the free
PNNP ligand and unidentified PPh2(aryl) species, possibly
produced by ligand isomerization and fragmentation, as well as
the phosphine of the mono-oxidized PNNP. Precatalyst 1
shows a singlet in the phosphorus spectrum at 50.8 ppm and is
therefore completely consumed during the TH process.
To investigate the vibrational spectra of the catalytic mixture,
the reaction mixture was concentrated under vacuum to
dryness and the IR spectrum of the residue was taken as a
KBr disk. The spectrum revealed weak broad absorptions for
CO vibrations at 1960 and 1946 cm−1, and intense broad
absorptions from 1835 to 1890 cm−1, with distinct maxima at
1870 and 1846 cm−1, indicating that a carbonyl ligand is still
coordinated to a metal center during catalysis, possibly to the
Figure 2. Observed production of 1-phenylethanol from acetophe-
none (0.47 M) in isopropyl alcohol (12.36 M) by use of 1 (0.87 mM)
and KOtBu (7.0 mM) at 26 °C.
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active species. The broadness of the absorption below 1900 cm−1
is likely due to the presence of multiple, similar species in
solution, with two major species giving rise to peaks at 1870
and 1846 cm−1. While 1960 and 1946 cm−1 are typical
values for a monocarbonyl iron(II) compound, signals below
1900 cm−1 are usually indicative of iron(0) complexes or
bridging carbonyl ligands.28,29 Assignment of these two peaks
was done in conjunction with density functional theory
calculations (vide infra).
Isolation of a Ferraaziridine Complex (2). With the goal
of isolating the intermediates observed by 31P NMR and IR
spectroscopy, we investigated the reaction of precatalyst 1 with
2 equiv of sodium isopropoxide in neat benzene without
acetophenone present. Surprisingly, we did not observe the
formation of an octahedral iron alkoxide complex. Instead, the
cationic ferraaziridine complex [Fe(CO)(PPh2C6H4CH
NCH2CH2NHCHC6H4PPh2)-κ5P,N,C,N,P][BF4] (2) was iso-
lated in 65% yield. However, the reaction could only be carried
out on a small scale, as the purity was diminished at larger
scales (Scheme 2).
symmetry of the ligand. The 31P NMR spectrum of the crude
product by the reaction of Scheme 2 also shows a singlet at
−13.4 ppm, consistent with free ligand being released.
Since we were able to measure clean and well-resolved NMR
spectra without contact-shifted signals, we expect that the
electronic state of the iron is low-spin iron(II) and the complex
is diamagnetic (this is supported by unrestricted DFT
calculations on a related Fe(II) complex, which will be
discussed later). The ESI mass spectrum shows the cation for
2 at m/z 689.15. The IR spectrum shows a characteristic CO
vibration band at 1940 cm−1, similar to signals that have already
been reported for six-coordinate, octahedral Fe(II) complexes with
one CO ligand.30−33 The CO stretch is observed at wave-
numbers lower than that for the starting material 1 (2002 cm−1),
as expected for a reduction in positive charge and increase in
π back-bonding from Fe to CO on going from 1 to 2. This
increase in bonding to the carbonyl is reflected in the relative
reactivity of the complexes on stirring in acetonitrile: there is
ligand exchange of acetonitrile for CO in 1 but not in 2.
To the best of our knowledge, there are only two other
examples of a similar type of ferraaziridine iron coordination
described in the literature. Siebenlist et al. reported that the
reaction of an α-imine ester (iPrNCHC(OEt)O) with
Fe2(CO)9 in THF leads mainly to binuclear products with a
small amount of the complex trans-Fe(CO)2(iPrNHCHC-
(OEt)O)-κN,C)2.34 Evidence supported that the imine group
was hydrogenated by hydrogen atom extraction from THF in
an electron transfer process. As in 2, this ligand is coordinated
to iron via both the amine nitrogen and the adjacent carbon
atom, thus forming a three-membered azametallacycle. In
addition, a photochemical reaction of (EtO2C)(NMe2)C
NCPhS with Fe2(CO)9 produced, among other products,
Fe(CO)3(Me2NC(CO2Et)(NC(Ph)S)-κN,C,S), where the
azametallacycle has geminal methyl groups on nitrogen.35a
We have recently reported a related reaction involving the base-
induced formation of a ruthenaaziridine from a bis-
(pyridylmethyl)amine complex.35b
Reactivity of Complex 2. Complex 2 displays certain
spectral features similar to those of the TH reaction mixture
(Figure 3). It has an IR absorption at 1940 cm−1 and doublets
in the 31P{1H} NMR spectrum at 84 and 69 ppm. However, the
signal of the latter spectrum has a JP,P coupling constant of
43 Hz while the doublets of the TH solution have JP,P = 29 Hz
(Figure 3), thus indicating that the complexes are not the same.
Complex 2 does not catalyze the TH of acetophenone in neat
isopropyl alcohol without base at 26 °C. On the basis of the
coupling constant mismatch between 2 and the TH mixture, it
is not the species present during TH.
Compound 2 could also be observed (along with signals for
free/oxidized ligand) by 31P{1H} NMR via an NMR-scale
reaction of 1 in isopropyl alcohol with 2 equiv of sodium
isopropoxide, suggesting that it could be an intermediate
toward the formation of the species with a similar JP,P value in
the TH mixture as described above. An isolated, pure sample of
2 was reacted with a stoichiometric amount of KOtBu in
isopropyl alcohol to afford the postulated ferraaziridinido
complex Fe(CO)(PPh2C6 H4 CHNCH2 CH2 NCH-
C6H4PPh2)-κ5P,N,C,N,P (3) (Scheme 3), which only differs
from 2 in that the ferraaziridine nitrogen atom is deprotonated.
This structure is proposed on the basis of NMR/IR spectra and
DFT calculations (vide infra). Complex 3 could not be isolated
as a pure product, and therefore analysis was done on the crude
solution after removal of excess base. The 31P{1H} NMR
Scheme 2. Synthesis of 2
The single-crystal X-ray diffraction structure (Figure 4)
revealed a distorted geometry where the C(10) carbon of the
ferraaziridine moiety is bound approximately trans to the
carbonyl ligand, with a C(10)−Fe−C(17) angle of 158.3(2)°.
The complex appears to adopt a distorted-octahedral geometry,
with a strained C(10)−Fe−N(2) angle of 42.0(1)° and obtuse
P(1)−Fe−C(10) and C(17)−Fe−N(2) angles of 106.1(1) and
116.6(1)°, respectively (Figure 4). The complex is best viewed
as an iron(II) κ2-ferraaziridine complex instead of an iron(0) η2-
iminium complex, given the N(2)−C(10) bond distance of
1.434(5) Å, characteristic for an N−C single bond. The bond
distances from iron to the imine, amine, and alkyl functions are
1.980(3) Å (Fe−N(1)), 1.988(3) Å (Fe−N(2)), and 2.012(2) Å
(Fe−C(10)), respectively (Table 1). The metal−carbonyl
distance Fe−C(17) (1.781(4) Å) is typical for an iron−
carbonyl bond and is similar to that of complex 1.24
We have also calculated the ground-state structures of 1DFT
and 2DFT in the gas phase using DFT (simplified structural
models, phenyls replaced with hydrogens). Both structures are
in good agreement with those of 1 (see the Supporting
Information) and 2 (Figure 4, Table 1). The gas-phase metrical
parameters for 1DFT and 2DFT are nearly identical with those
calculated with solvation effects (vide infra).
The Fe−NH and Fe−CH protons of 2 appear at 4.21 and
3.61 ppm, respectively, in the 1H NMR spectrum. These
1
assignments were confirmed by analysis of the H−1H-gCOSY
and 1H−13C-gHSQC NMR experiments. The 13C NMR
chemical shift of the carbonyl carbon is found at 213 ppm as
a doublet of doublets (2JCP = 3.1, 24 Hz). The FeCH carbon is
observed at 68 ppm as a doublet of doublets (2JCP = 5, 11 Hz).
The 31P{1H} NMR spectrum has two sharp doublets at 84.3
and 70.1 ppm (JP,P = 43 Hz), consistent with the lack of
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Figure 4. Molecular structure of 2 (top row, ellipsoids at 30% probability level) and calculated gas-phase structure of 2DFT (bottom row). Phenyl
hydrogens and counteranions have been omitted for clarity. Selected bond distances and angles are presented in Table 1.
Table 1. Comparison of Bond Lengths (Å) and Angles (deg)
for Compounds 2 and 2DFT
Scheme 3. Observation of 3: Postulated Structure
length
angle
param
2
2DFT
param
2
2DFT
Fe1−P1
Fe1−P2
2.203(1)
2.2178(9)
1.980(3)
1.988(3)
2.012(4)
1.500(5)
0.974(3)
1.434(5)
1.494(2)
2.18 Fe1−C10−N2
2.24 Fe1−N2−C10
1.99 H10−C10−Fe
1.97 H10−C10−N2
68.06(2)
69.92(2)
68.4
68.4
Fe1−N1
Fe1−N2
Fe1−C10
C10−C11
C10−H10
C10−N2
N2−C1
119.15(3)
113.15(3)
119.6
112.0
113.7
114.6
118.2
114.1
112.5
1.97 H10−C10−C11 114.56(3)
1.49 N2−C10−C11
1.09 C10−N2−C1
1.46 C10−N2−H1N
1.46 C1−N2−H1N
115.20(3)
119.56(3)
118.07(2)
107.62(3)
This “noninnocent” behavior of the ligand is reminiscent of
the PNP systems designed by Milstein36 and Schneider37 and
PNNP complexes developed in our group,23 whereby the ligand
exhibits reversible protonation−deprotonation behavior. How-
ever, our system is flexible and preferentially adopts a folded/
distorted geometry, which is not observed for these other
structurally rigid ligands.
Mass Balance Experiment. Line-broadening techniques
were required for all 31P{1H} NMR spectra due to very poor
signal to noise, indicating that several species were likely not
being detected by NMR experiments, possibly even the active
species. For that reason, mass balance NMR experiments were
carried out. Triphenylphosphine oxide (OPPh3) is inert to
catalysis and was therefore chosen as a standard to compare to
other phosphorus integrations. A mixture of 2 equiv of OPPh3
and 1 equiv of 1 was reacted with base and substrate in
isopropyl alcohol, as per standard procedures. NMR conditions
were optimized for accurate integration data. The doublets for
3 account for approximately 25% of the phosphorus
introduced, and ligand and oxidized ligand account for
spectrum of the resulting solution shows two doublets at 84.1
and 68.7 ppm (JP,P = 29 Hz) (along with singlets at −12 and
−13.4 ppm for various isomers of the free ligand), which now
matches the chemical shift and coupling constant of the
phosphorus-containing complex in the TH mixture. The IR
spectrum shows absorptions associated with carbonyl ligands at
1862 and 1870 cm−1, as well as broad peaks at 1934 and 1957 cm−1.
From the similarities of the 31P{1H} NMR and IR spectra
between this reaction mixture and the TH solutions, we reason
that 3 is present during TH.
Interestingly, when it is dissolved in neat isopropyl alcohol, the
crude mixture of 3 is only minimally catalytically active (0.5%
conversion of 3.85 mmol of acetophenone to 1-phenethanol at 26
°C in 1 h, 40% in 24 h). This supports the notion that, although 3
is the species seen by IR and 31P{1H} NMR, it is not the
catalytically active species during TH.
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Scheme 4. Proposed Mechanisms for the Formation of 2DFT and 3DFT and the Fe(0) Species 7DFT
approximately 18%. Therefore, only 43% of the phosphorus
introduced for TH is detectable, leaving 57% of all phosphorus-
containing species unaccounted for and therefore NMR inactive.
This is significant, because it shows that a maximum of 75% of the
iron is either catalytically active and NMR inactive or is
catalytically active and found in extremely low concentrations.
We report elsewhere that the true identity of the active species is
4 nm iron(0) nanoparticles coated with the chiral ligand.38
Calculated Mechanism for Formation of the Com-
plexes 2DFT, 3DFT, and an Fe(0) Species (7DFT). We have
used density functional theory (DFT) calculations to rationalize
the formation of the experimentally observed complexes 2 and
3 and also propose the energetically favorable reduction to a
five-coordinate Fe(0) species via two energetically similar
pathways (Scheme 4). In order to reduce computational cost,
the phenyl substituents on phosphorus have been truncated to
hydrogen atoms in all calculated structures. In addition, solvent
effects have been taken into account and all calculations have
been performed using isopropyl alcohol as the solvent (see
Computational Details).
Starting with the dicationic TH precatalyst 1DFT, the labile
acetonitrile ligand is replaced by an incoming isopropoxide
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Figure 5. Calculated geometries and selected bond lengths for 4DFT, TS4,5 (−726i cm−1), TS6,3 (−1049i cm−1), TS6,7 (−1055i cm−1), TS3,7
(−319i cm−1), and 7DFT: gray, carbon; white, hydrogen; blue, nitrogen; red, oxygen; pink, phosphorus; orange, iron. Phenyl hydrogens have been
omitted for clarity.
anion, which is exergonic by 16.3 kcal/mol to generate 4DFT
(Figure 6). Next, hydride transfer from the tertiary carbon on
the alkoxido ligand reduces the imine moiety, generating the
cationic Fe(II) imino-amido species 5DacFeTt , which is endergonic
by 7.3 kcal/mol and has a transition state barrier of 20.1 kcal/mol
that of 6DFT by 8.6 and 5.8 kcal/mol, respectively. The
transition-state geometries TS6,3 and TS6,7 are shown in Figure
5. The major difference between these transition states is the
iron−alkoxide distance and the degree of ligand distortion
during deprotonation. For TS6,3, the alkoxide is further away
from iron (Fe- - -O = 3.05 Å) than in TS6,7, where the Fe−O
distance is close (Fe−O = 2.05 Å) and the alkoxide remains
trans to the carbonyl (O−Fe−C = 175.8°). Tetradentate
PNNP ligands are known to fold to produce cis-octahedral
complexes.39 The Fe(0) complex 7, with or without isopropyl
alcohol, is proposed on the basis of the low carbonyl stretches
in the IR spectrum of the catalytic mixture (vide supra), which
will be discussed in the following section.
to overcome (TS4,5, Figures 5 and 6). Dissociation of weakly
acet
DFT
coordinating acetone from 5
produces 5DFT, a five-
coordinate imino-amido species, which reacts with another 1
equiv of alkoxide, yielding the neutral octahedral complex 6DFT
which is 24.9 kcal/mol lower in energy than 1DFT
,
.
acet
DFT
The exergonic process of going from 5
to 6 is followed by
two energetically similar pathways: simultaneous ligand
folding/distortion and deprotonation to generate a neutral
iPrOH
DFT
Fe(II) ferraaziridinido complex (3
) with a barrier of 22.2
Dissociation of isopropyl alcohol from the deprotonated
iPrOH
DFT
i
kcal/mol (TS6,3); alternatively, deprotonation without gener-
ation of an iron−alkyl bond reduces the metal center to form a
distorted-square-pyramidal Fe(0) complex (7DiPFrOTH) with a
calculated barrier of 21.9 kcal/mol (TS6,7). The activation
energy for these transitions differs by only 0.3 kcal/mol, and
ferraziridine complex 3
leads to 3DFT + acetone + PrOH
(Scheme 4), which has the lowest relative free energy of all the
compounds calculated (−37.6 kcal/mol relative to 1DFT
+
2iPrO−) and has been experimentally observed in the presence
of more than 2 equiv of base (Figure 3 and Scheme 3). The
metrical parameters of 3DFT do not differ much from those of
iPrOH
DFT
iPOH
DFT
the free energies of products 3
and 7
are lower than
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Figure 6. Energy profile for the compounds shown in Scheme 4 with energies of free isopropoxide (iPrO−), isopropyl alcohol (iPrOH), and acetone
(acet) used where appropriate. Energies are all calculated relative to 1 and 2 equiv of isopropoxide in isopropyl alcohol. Relative enthalpies are shown
in parentheses.
2DFT, but the electronics change dramatically. The relative
carbonyl stretch of 2DFT is 83 cm−1 lower than that of 3DFT in
the gas phase, which suggests that iron is very electron rich
when the ferraaziridine nitrogen is deprotonated (the difference
is 86 cm−1 when solvation effects are incorporated). We
rationalize that 2DFT is formed by protonation of the tetrahedral
nitrogen atom by free isopropyl alcohol, given a modest energy
κ4P,N,N,P][BF4]2 (9)18 (see the Supporting Information). As
for all other complexes, phenyl groups were truncated to hydrogen
atoms and anions were removed to reduce computational cost.
The experimental carbonyl stretches were then compared with the
calculated carbonyl stretches, which resulted in a calculated42 CO
scale factor of 0.917 0.002.
The square-pyramidal complex 7DFT has a calculated carbonyl
stretching frequency of 2020 cm−1, which is only 8 cm−1 lower
than the calculated CO stretching frequency of complex 3DFT
(2028 cm−1), indicating electron-rich iron centers in both cases.
Applying the CO scale factor to 3DFT and 7DFT results in predicted
difference of 4.8 kcal/mol between 3DFT and 2DFT
.
Reduction to the square-pyramidal Fe(0) complex 7DFT can
also be accomplished by ligand “unfolding” of 3DFT with a
barrier of 14.6 kcal/mol (TS3,7, Figures 5 and 6). The relative
free energy of this electron-rich Fe(0) complex is similar to that
of 3DFT (1.8 kcal/mol lower) and can also form by
simultaneous deprotonation/reduction of 6DFT (vide supra).
All calculations were performed on low-spin iron complexes in
the singlet state (S = 0). To account for the possibility of odd-
electron intermediates, we also optimized an intermediate spin
analogue of 4DFT (S = 1, two unpaired electrons; see the
Supporting Information). Intermediate-spin 4DFT retains a
distorted-octahedral geometry but is 10.6 kcal/mol higher in
energy than low-spin 4DFT. Attempts at calculating a high-spin
analogue of 4DFT resulted in immediate CO loss from the
coordination sphere. Since the metal−carbonyl bond is clearly
intact in compound 2 and its synthesis is reproducible in open and
closed systems, we do not think CO dissociation plays a role in its
formation. However, decomposition of the metal complex cannot
be ruled out if iron adopts a high-spin electron configuration.
Using a CO Scale Factor to Support the Proposed
Structures of 3 and 7. Observed metal−carbonyl stretches in
our Fe-PNNP infrared spectra are easily identifiable and
sensitive to changes in the overall electronic structure.
Although there have been studies reporting DFT scale factors42,43
for metal carbonyl complexes,44,45 they do not report scale factors
using the mPW1PW91 functional. Furthermore, the theoretical
models of 1DFT and 2DFT are not the same as their experimental
analogues. A new CO scale factor was determined using
simplified models of 1 and 2, along with simplified models of
two other fully characterized iron(II) PNNP complexes, trans-
(R,R)-[Fe(CO)(NCMe)(PPh2CH2CHNCHPhCHPhNCHC-
H2PPh2)-κ4P,N,N,P][BPh4]2 (8)19 and trans-(R,R)-[Fe(CO)-
(NCMe)(PPh2C6H4CHN-C6H10-NCHC6H4PPh2)-
experimental CO stretches of 1860
4 and 1852
4 cm−1,
respectively (Figure 7). In comparison with the observed transfer
Figure 7. Experimental, calculated, and scaled carbonyl stretches for
proposed complexes 3 and 7.
hydrogenation CO stretches of 1870 and 1862 cm−1, we predict
that complexes 3 and 7 are responsible for the two experimentally
observed carbonyl stretches below 1900 cm−1 in transfer
hydrogenation mixtures.
CONCLUSION
■
We investigated the mechanism of transfer hydrogenation of
acetophenone to 1-phenylethanol utilizing trans-[Fe(CO)-
(MeCN)(PPh2C6H4CHNCH2−)2-κ4P,N,N,P][BF4]2 (1) as
a precatalyst and basic isopropyl alcohol as a hydrogen source.
Spectroscopic and computational investigations were per-
formed to determine the structures of any intermediates
involved in catalysis. NMR data showed that, during catalysis,
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Article
was done on a Perkin-Elmer Clarus 400 chromatograph equipped with
a chiral column (CP chirasil-Dex CB 25 m × 2.5 mm) and auto-
sampling capabilities. Hydrogen gas was used as the mobile phase, and
the oven temperature was set at 130 °C. Retention times are 7.58 and
8.03 min for phenylethanol and 4.56 min for acetophenone.
Formation of Oxidized PNNP. Free PNNP ligand (11 mg, 0.018
mmol) was dissolved in 0.6 mL of CDCl3, an excess of aqueous
hydrogen peroxide (H2O2; 0.05 mL of 30%, 0.6 mmol) was added, and
the mixture was stirred at room temperature in air for 30 min. The
reaction solution was then transferred to an NMR tube for analysis.
31P{1H} NMR (121 MHz, CDCl3): δ 33.0 ppm (s), very minor
singlets at 32.3 and 36.0 ppm.
free ligand is observed along with an iron-containing species
thought to be the neutral ferraaziridinido complex Fe(CO)-
(PPh2C6H4CHNCH2CH2NCHC6H4PPh2)-κ5P,N,C,N,P (3).
This is suggested on the basis of the synthesis, characterization,
and reactivity of the unusual ligand-folded ferraaziridine
complex [Fe(CO)(PPh2C6H4CHNCH2CH2NHCH-
C6H4PPh2)-κ5P,N,C,N,P][BF4] (2). Both of these complexes
are mediocre catalysts in comparison with precatalyst 1. Using
density functional theory, we proposed thermodynamically
favorable mechanisms for the formation of 2DFT and 3DFT that
involve simultaneous deprotonation/folding of the ligand arm
or deprotonation without formation of an iron−alkyl bond.
Infrared spectroscopy showed that, during catalysis, two
electron-rich iron species form with CO stretches below
1900 cm−1. On the basis of a calculated CO scale factor and
relative free energies, we predict that 2 is easily deprotonated
in the presence of excess base to form electron-rich iron
complexes 3 and the square-pyramidal Fe(0) complex Fe-
(CO)(PPh2C6H4CHNCH2−)2-κ4P,N,N,P (7), which are re-
sponsible for the two observed CO stretches below 1900 cm−1
in catalytic mixtures. Some of us provide evidence elsewhere
that the actual catalyst in the system is 4 nm Fe(0)
nanoparticles coated with chiral ligand.38
Preparation of 2. In a nitrogen-filled glovebox, 3 mL of benzene
was added to a solid mixture of 1 (56 mg, 0.093 mmol) and NaiOPr
(15 mg, 0.18 mmol). The reaction mixture turned dark brown after ca.
15 min and was stirred for 12 h at ambient temperature. The solution
was filtered off and was allowed to slowly evaporate for several days, to
give the product as dark brown crystals. Crystals for X-ray analysis
were selected directly from this crop. After washing with ether and
pentane the pure product was obtained. Yield: 31 mg, 0.06 mmol,
1
65%. H NMR (400 MHz, CD3CN): δ 8.76 (d, JH,P = 3.1 Hz, 1 H,
CHN), 8.03 (m, 2 H, Ar), 7.75−6.74 (several m, 22 H, Ph), 6.42
(m, 2 H, Ar), 6.23 (m, 2 H Ar), 4.32 (m, 1 H, CH2), 4.21 (br s, 1 H,
NH), 3.80 (m, 1 H, CH2), 3.61 (m, 1 H, CH), 2.55 (m, 1 H, CH2).
31P{1H} NMR (121 MHz, CD3CN): δ 84.3 (d, JP,P = 43.4 Hz), 70.1
(d, JP,P = 43.3 Hz). 13C{1H} NMR (100 MHz, CD3CN): δ 213.3 (dd,
JC,P = 3.1 Hz, JC,P = 24.2 Hz, CHN), 170.0 (d, JC,P = 4.6 Hz, Ar),
154.7 (d, JC,P = 32.1 Hz, Ar), 136.9−128.1 (several m, Ar), 70.1 (d,
JC,P = 4.5 Hz,. CH2), 68.1 (dd, JC,P = 5.5 Hz, JC,P = 10.8 Hz, CH), 48.6
(d, JC,P = 1.8 Hz, CH2). 19F NMR (CD3CN): δ 152.3 (s). IR (KBr):
νCO 1940 cm−1. MS (ESI+, MeOH): m/z 689.2 for [M − BF4]+. Anal.
Calcd for C41H36BF4FeN2OP2·C6H6: C, 65.99; H, 5.30; N, 3.27.
Found: C, 65.82; H, 5.30; N, 3.61.
Observation of 3. In a nitrogen-filled glovebox, 3 mL of isopropyl
alcohol was added to a solid mixture of 2 (25 mg, 0.031 mmol) and
KOtBu (5 mg, 0.044 mmol). The reaction mixture turned dark brown
immediately and was stirred for 1 h at ambient temperature. The
solution was filtered to remove excess base, and NMR and IR
measurements of the crude solution were run. 31P{1H} NMR (121
COMPUTATIONAL DETAILS
■
All calculations were performed using Gaussian0946 and Gaussian03.47
The mPW1PW91 density functional was used for all calculations.48,49
Iron was treated with the SDD basis set to include relativistic effective
core potentials.50 Atoms C, H, N, O, and P were treated with the
6-31++G(d,p) basis set, which includes diffuse51 and polarization52
functions. Unrestricted open-shell calculations of 4DFT showed no
evidence of significant spin contamination; therefore, only closed-shell
structures were considered. The phenyl substituents on phosphorus
atoms were replaced with hydrogen atoms to reduce computational
cost. Such simplifications were shown in previous computational
studies on iron and ruthenium systems to have no significant effect on
the core structures.18,53 Lynch and Truhlar have reported that the
mPW1PW91 functional is better for the prediction of transition states
and energy barriers.54 All structures were optimized with solvent
correction (2-propanol) using the integral equation IEF-PCM
protocol.40 Radii and nonelectrostatic terms from the SMD solvation
model were also used, which have been reported by Truhlar and co-
workers to be more accurate in calculating the solvation free energy of
MHz, iPrOH, C6D6 insert): δ 84.1 (d, JP,P = 30.7 Hz), 68.7 (d, JP,P
=
30.7 Hz). IR (KBr): νCO 1862 and 1870 cm−1, as well as broad peaks
at 1934 and 1957 cm−1.
Transfer Hydrogenation. In a vial containing precatalyst 1 (5 mg,
0.0055 mmol) and KOtBu (5 mg, 0.044 mmol), isopropyl alcohol (6 mL,
78 mmol) and acetophenone (0.35 mL, 3 mmol) were added at room
temperature, in an argon-filled glovebox. A dark brown solution was
immediately formed, which was stirred vigorously. Samples were taken
from the mixture, quenched by exposure to air, and analyzed by gas
chromatography. When the samples are exposed to air, the solution turns
yellow and the reaction stops immediately. The alcohol/ketone
concentration does not change in these solutions, even after several
days. All of the catalytic results were reproduced to ensure consistency.
NMR Scale Transfer Hydrogenation. In a vial containing
precatalyst 1 (18 mg, 0.020 mmol) and KOtBu (10 mg, 0.088 mmol),
isopropyl alcohol (0.6 mL, 7.8 mmol) and acetophenone (0.1 mL,
0.86 mmol) were added at room temperature, in an argon-filled
glovebox. Immediately a dark brown solution was formed, which was
stirred vigorously for 10 min before being transferred to a J. Young
NMR tube containing a D2O insert, and 31P{1H} NMR measurements
were run immediately on a Varian 400 MHz spectrometer. All results
were reproduced to ensure consistency.
Mass Balance Experiments. Prior to addition of solvent, OPPh3
(11 mg, 0.040 mmol) was added to a vial prepared as outlined above. The
dark brown solution was stirred for 10 min and transferred to a J. Young
NMR tube containing a D2O insert. 31P NMR was run immediately on a
Varian 400 MHz spectrometer, with a 90° pulse, decoupled NOE, and a
relaxation decay of 3.7 s. Parameters were optimized by determining the
T1 relaxation times of all species present in the 31P NMR spectrum. All
results were reproduced to ensure consistency.
bare ions such as isopropoxide.41 Ground-state structures of 1DFT
,
2DFT, 3DFT, 7DFT, 8DFT, and 9DFT were performed in the gas phase for
CO scale factor calculations. Optimized ground states were found to
have zero imaginary frequencies, while optimized transition states were
found to have one imaginary frequency. Three-dimensional visual-
izations of the calculated structures were generated by ChemCraft.55
EXPERIMENTAL DETAILS
■
General Procedures. All preparations and manipulations were
carried out under an argon or nitrogen atmosphere using standard
Schlenk line and glovebox techniques. Dry, oxygen-free solvents and
acetophenone were prepared by distillation from appropriate drying
agents and employed throughout. Precatalyst 1 and PNNP ligand were
prepared by previously reported methods.24 Mass spectroscopy and
elemental analyses (EA) were performed at the University of Toronto,
and all samples were handled under argon for the EA. Varian Gemini
1
400 and 300 MHz spectrometers were employed for recording H
(400 and 300 MHz), 13C{1H} (100 and 75 MHz), and 31P{1H} (121
MHz) NMR spectra at ambient temperature. The H and 13C NMR
1
spectra were referenced to solvent resonances, as follows: 7.26 and
77.16 ppm for CHCl3 and CDCl3, 5.32 and 54.0 ppm for CH2Cl2 and
CD2Cl2, 1.94 and 1.24 ppm for CH3CN and CD3CN. The 31P NMR
spectra were referenced to 85% H3PO4 (0 ppm). Gas chromatography
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Article
(25) Clapham, S. E.; Hadzovic, A.; Morris, R. H. Coord. Chem. Rev.
2004, 248, 2201−2237.
ACKNOWLEDGMENTS
■
We thank the NSERC for a Discovery grant to R.H.M. and
graduate scholarships for D.E.P. and J.F.S. We thank Dr. Darcy
Burns and Dr. Tim Burrow for assistance with NMR and
the reviewers for useful suggestions concerning the DFT
calculations.
(26) Conley, B. L.; Pennington-Boggio, M. K.; Boz, E.; Williams, T. J.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving NMR and IR spectra,
details of mass balance experiments, details of the DFT
calculations, and crystal structure data for 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
(33) Benito-Garagorri, D.; Alves, L. G.; Puchberger, M.; Mereiter, K.;
Veiros, L. F.; Calhorda, M. J.; Carvalho, M. D.; Ferreira, L. P.;
Godinho, M.; Kirchner, K. Organometallics 2009, 28, 6902−6914.
AUTHOR INFORMATION
■
Notes
The authors declare no competing financial interest.
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