Efficient hydrogenation of ketones and aldehydes catalyzed by well-defined iron(II) PNP pincer complexes: Evidence for an insertion mechanism

Article abstract of DOI:10.1021/om5009814

We have prepared and structurally characterized a new class of Fe(II) PNP pincer hydride complexes [Fe(PNP-iPr)(H)(CO)(L)]ⁿ (L = Br^-, CH₃CN, pyridine, PMe₃, SCN^-, CO, BH₄^-; n = 0, +1) based on the 2,6-diaminopyridine scaffold where the PiPr₂ moieties of the PNP ligand are connected to the pyridine ring via NH and/or NMe spacers. Complexes [Fe(PNP-iPr)(H)(CO)(L)]ⁿ with labile ligands (L = Br^-, CH₃CN, BH₄^-) and NH spacers are efficient catalysts for the hydrogenation of both ketones and aldehydes to alcohols under mild conditions, while those containing inert ligands (L = pyridine, PMe₃, SCN^-, CO) are catalytically inactive. Interestingly, complex [Fe(PNP^Me-iPr)(H)(CO)(Br)], featuring NMe spacers, is an efficient catalyst for the chemoselective hydrogenation of aldehydes. The first type of complexes involves deprotonation of the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide cooperation, but no reversible aromatization/deprotonation of the PNP ligand. In the case of the N-methylated complex the mechanism remains unclear, but obviously does not allow bifunctional activation of dihydrogen. The experimental results complemented by DFT calculations strongly support an insertion of the C=O bond of the carbonyl compound into the Fe-H bond.

Full text of DOI:10.1021/om5009814

Article
pubs.acs.org/Organometallics
Efficient Hydrogenation of Ketones and Aldehydes Catalyzed by
Well-Defined Iron(II) PNP Pincer Complexes: Evidence for an
Insertion Mechanism
Nikolaus Gorgas,† Berthold Stoger,‡ Luis F. Veiros,§ Ernst Pittenauer,‡ Gunter Allmaier,‡
̈
̈
and Karl Kirchner*,†
†Institute of Applied Synthetic Chemistry and ‡Institute of Chemical Technologies and Analytics, Vienna University of Technology,
Getreidemarkt 9, A-1060 Vienna, Austria
§Centro de Química Estrutural, Instituto Superior Tec
́
nico, Universidade de Lisboa, Avenida Rovisco Pais No. 1, 1049-001 Lisboa,
Portugal
S
* Supporting Information
ABSTRACT: We have prepared and structurally characterized a new class of Fe(II) PNP pincer hydride complexes [Fe(PNP-
iPr)(H)(CO)(L)]n (L = Br−, CH3CN, pyridine, PMe3, SCN−, CO, BH4 ; n = 0, +1) based on the 2,6-diaminopyridine scaffold
−
where the PiPr2 moieties of the PNP ligand are connected to the pyridine ring via NH and/or NMe spacers. Complexes
[Fe(PNP-iPr)(H)(CO)(L)]n with labile ligands (L = Br−, CH3CN, BH4 ) and NH spacers are efficient catalysts for the
−
hydrogenation of both ketones and aldehydes to alcohols under mild conditions, while those containing inert ligands (L =
pyridine, PMe3, SCN−, CO) are catalytically inactive. Interestingly, complex [Fe(PNPMe-iPr)(H)(CO)(Br)], featuring NMe
spacers, is an efficient catalyst for the chemoselective hydrogenation of aldehydes. The first type of complexes involves
deprotonation of the PNP ligand as well as heterolytic dihydrogen cleavage via metal-alkoxide cooperation, but no reversible
aromatization/deprotonation of the PNP ligand. In the case of the N-methylated complex the mechanism remains unclear, but
obviously does not allow bifunctional activation of dihydrogen. The experimental results complemented by DFT calculations
strongly support an insertion of the CO bond of the carbonyl compound into the Fe−H bond.
cooperation);8 that is, the complexes contain electronically
INTRODUCTION
■
coupled hydride and acidic hydrogen atoms as a result of
heterolytic dihydrogen cleavage that may be transferred to polar
unsaturated substrates in an outer-sphere fashion or may be
transferred via hydride migration (inner-sphere mechanism).
An effective way of bond activation by metal−ligand
cooperation involves aromatization/dearomatization of the
ligand in pincer-type complexes. In particular, pincer ligands
in which a central pyridine-based backbone is connected with
−CH2PR2 and/or −CH2NR2 substituents were shown to
exhibit this behavior.9 This has resulted in the development of
novel and unprecedented iron catalysis where this type of
cooperation plays a key role in the heterolytic cleavage of H2.4
In the case of ketones and aldehydes, most efficient are
The catalytic reduction of polar multiple bonds via molecular
hydrogen plays a significant role in modern synthetic organic
chemistry. This reaction is excellently performed by many
transition metal complexes containing noble metals such as
ruthenium, rhodium, or iridium.1 However, the limited
availability of precious metals, their high price, and their
toxicity diminish their attractiveness in the long run, and more
economical and environmentally friendly alternatives have to be
found. In this respect, the preparation of well-defined iron-
based catalysts of comparable activity would be desirable.2 Iron
is the most abundant transition metal in the earth’s crust and is
ubiquitously available. Accordingly, it is not surprising that the
field of iron-catalyzed hydrogenations of polar multiple bonds is
rapidly evolving, as shown by several recent examples.3−7
It is interesting to note that many of these hydrogenations
involve ligand−metal bifunctional catalysis (metal−ligand
Received: September 26, 2014
© XXXX American Chemical Society
A
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complexes of the types [Fe(PNPCH2-iPr)(CO)(H)(Br)] and
[Fe(PNPCH2-iPr)(CO)(H)(κ1-BH4)], where the bromide and
BH4 ligands are labile, facilitating the coordination of the
(120 °C) and a high H2 pressure (20 bar). The experimental
results are complemented by DFT calculations.
−
substrates. It has thus been suggested by Milstein that this
reaction proceeds via an inner-sphere mechanism involving
insertion of the carbonyl compounds into the Fe−H bond.4a,c
We are currently focusing on the synthesis and reactivity of
iron complexes containing PNP pincer ligands based on the
2,6-diaminopyridine scaffold. In these ligands the aromatic
pyridine ring and the phosphine moieties are connected via
NH, N-alkyl, or N-aryl linkers. The advantage of these ligands is
that both substituents of the phosphine and amine sites can be
systematically varied in a modular fashion, which has a decisive
effect on the outcome of reactions.10 Recently we prepared the
cationic Fe(II) hydride complex cis-[Fe(PNP-iPr)(CO)2H]+,
which involved reversible NH activation as well as heterolytic
dihydrogen cleavage via metal−PNP ligand cooperation.11 This
complex turned out to be catalytically inactive for the
hydrogenation of ketones and aldehydes, which was attributed
to the fact that this complex is substitutionally inert and/or that
the basicity of the hydride is too low.
RESULTS AND DISCUSSION
■
The synthesis of complexes [Fe(PNP-iPr)(CO)(H)(Br)] (2a−
c) was accomplished in 63−67% isolated yields by treatment of
anhydrous FeBr2 with 1 equiv of the corresponding PNP-iPr
ligands 1a−c in THF in the presence of CO and subsequent
addition of 1.1 equiv of Na[HBEt3] (Scheme 2). This reaction
proceeds via the intermediacy of the dibromo complexes
[Fe(PNP-iPr)(CO)(Br)2], which, in principle, can be isolated
in pure form as shown previously,13 but are labile, slowly losing
CO, and were thus directly used without prior isolation. In the
case of the symmetrical N-methylated PNP-iPr ligand 1b, two
isomers were obtained in a ca. 2.7:1 ratio with the hydride
ligand being trans to the bromide and to the CO ligand,
respectively, which could not be separated. All hydride
complexes are air sensitive both in the solid state and in
solution.
Characterization was accomplished by elemental analysis and
by H, 13C{1H}, and 31P{1H} NMR and IR spectroscopy. The
1
Herein we report the synthesis, characterization, and catalytic
activity of a series of neutral iron hydride complexes of the type
[Fe(PNP-iPr)(CO)(H)(Br)] (2a−c) where the PiPr2 moieties
of the PNP ligand are connected to the pyridine ring via NH
and/or NMe spacers (Scheme 1). In addition, the synthesis of a
1H NMR spectrum confirmed the presence of one hydride
ligand, which appeared at −21.4, −21.6, and −21.8 ppm,
2
respectively, as a well-resolved triplet with a JHP coupling
constant of about 57 Hz. Isomer 2b′ exhibits the hydride
resonance at −1.1 ppm. In the 13C{1H} NMR spectrum the
most noticeable resonance is the low-field resonance of the
carbonyl carbon atom trans to the pyridine nitrogen observed
as a triplet in the range 217.1−222.7 ppm (JCP about 13−23
Hz). The 31P{1H} NMR spectra of complexes 2a and 2b give
rise to a singlet at 147.1 and 164.0 ppm, respectively, while in
the case of 2c two doublets centered at 165.0 and 147.2 ppm
are observed. In the IR spectrum the strong bands for CO
stretching frequencies are found in the range 1901 to 1903
cm−1.
Scheme 1. Two Types of Highly Reactive Iron PNP Pincer
Hydrogenation Catalysts
The solid-state structure of 2a was determined by single-
crystal X-ray diffraction. A structural view is depicted in Figure
1 with selected bond distances given in the caption. Complex
2a adopts a distorted octahedral geometry around the metal
center with the hydride ligand being in cis position to a CO
ligand. The PNP ligand is coordinated to the iron center in a
typical tridentate meridional mode, with a P1−Fe1−P2 angle of
164.58(4)°. The hydride and the N−H atoms could be
unambiguously located in the difference Fourier maps. The
Fe−H distance was refined to 1.46(2) Å.
Complexes 2a−c are substitutionally labile. This has been
exemplarily studied in more detail with 2a (Scheme 3).
Dissolution of 2a in MeOH-d4 resulted in an immediate
replacement of the Br− ligand to give the cationic complex
[Fe(PNP-iPr)(H)(CO)(MeOH-d4)]+ (3a), as evident by a
new hydride resonance at −26.6 ppm and a 31P{1H} signal at
140.6 ppm. Interestingly, in ethanol dissociation of the bromide
series of neutral and cationic hydride complexes of the type
[Fe(PNP-iPr)(CO)(H)(L)]n (3a−g) (n = +1, 0) where L =
CH3CN, pyridine, PMe3, κ1-N-coordinated SCN−, and κ1-
−
coordinated BH4 is described. All complexes featuring labile
ligands L (Br−, CH3CN, BH4 ) are efficient catalysts for the
−
hydrogenation of ketones and aldehydes to alcohols under mild
conditions. Moreover, the N-methylated complex 2b is a
chemoselective catalyst for the reduction of aldehydes. The first
example of catalytic hydrogenation of aldehyde that is
chemoselective against ketone was recently reported by
Beller.12 However, this reaction required elevated temperatures
Scheme 2. Synthesis of Hydride Complexes 2a−c
B
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Figure 1. Structural view of [Fe(PNP-iPr)(H)(CO)(Br)]·CH2Cl2 (2a·
CH2Cl2) showing 50% thermal ellipsoids (CH2Cl2 and most hydrogen
atoms omitted for clarity). Selected bond lengths (Å) and angles
(deg): Fe1−P1 2.1927(6), Fe1−P2 2.1927(6), Fe1−N1 2.022(1),
Fe1−Br1 2.5269(6), Fe1−C18 1.731(1), Fe1−H1 1.46(2), P1−Fe1−
P2 164.58(1), N1−Fe1−C18 171.95(4).
Figure 2. Structural view of [Fe(PNP-iPr)(H)(CO)(CH3CN)]Br
(3b) showing 50% thermal ellipsoids (bromide counterion and most
hydrogen atoms omitted for clarity). Only one of the two
crystallographically independent complexes is shown. Selected bond
lengths (Å) and angles (deg): Fe1−P1 2.1986(10), Fe1−P2
2.2028(10), Fe1−N1 1.998(2), Fe1−N4 1.984(2), Fe1−C18
1.738(3), Fe1−H1 1.460(6), P1−Fe1−P2 162.87(3), N1−Fe1−C18
173.7(1).
ligand is much less pronounced and Br− is only partially
replaced by EtOH. This might also contribute to the fact that
higher catalytic activities are achieved in this solvent perhaps
due to diminished competition between substrate and solvent
for a free coordination site (vide infra). The addition of L =
CH3CN, pyridine, PMe3, SCN−, CO, and BH4 leads to the
−
formation of the corresponding cationic or neutral complexes
[Fe(PNP-iPr)(H)(CO)(L)]n (3b−g) as shown in Scheme 3.
Complexes 3b−g could also be isolated in pure form by
reacting 2a with the respective ligands CH3CN (neat), pyridine,
PMe3 SCN− (Na+ salt), BH4 (Na+ salt), and CO in both the
−
absence and presence of silver salts in 83−97% isolated yields.
These complexes exhibit the characteristic hydride resonances
at −18.6, −20.1, −11.1, −19.8, −7.5, and −18.2, ppm,
−
Figure 3. Structural view of [Fe(PNP-iPr)(H)(CO)(py)]BF4 (3c)
showing 50% thermal ellipsoids (most hydrogen atoms and BF4
anion omitted for clarity). Selected bond lengths (Å) and angles (deg):
Fe1−P1 2.1915(3), Fe1−N1 2.017(1), Fe1−N3 2.068(1), Fe1−C10
1.738(2), Fe1−H1 1.46(2), P1−Fe1−P2 158.56(1), N1−Fe1−C10
171.31 (6).
respectively. In the case of 3g the BH4 ligand gives rise to a
−
broad signal at −3.61 ppm (4H). The observation of a broad
1
four-proton resonance in this region of the H NMR spectrum
−
is typical for κ1-coordinated BH4 ligands of iron complexes
and indicates dynamic behavior of this ligand.4c
Structural views of 3b, 3c, 3d, 3e, and 3g are depicted in
Figures 2−6 with selected bond distances given in the captions.
The lability of complexes 2a and 3b−g was also studied by
ESI-MS. Solutions of these complexes in CH3CN were
subjected to ESI-MS analysis in the positive ion mode (the
neutral complexes were investigated in the presence of NaCl to
obtain cationic sodiated species). In the case of [Fe(PNP-
iPr)(H)(CO)(Br)] (2a), [Fe(PNP-iPr)(H)(CO)(CH3CN)]+
(3b), and [Fe(PNP-iPr)(H)(CO)(κ1-BH4)] (3g) the cationic
fragment [Fe(PNP-iPr)(H)(CO)]+ (m/z 426.1) was found as
the predominant species, while for all other complexes
[Fe(PNP-iPr)(H)(CO)(py)]+ (3c), [Fe(PNP-iPr)(H)(CO)-
(PMe3)]+ (3d), [Fe(PNP-iPr)(H)(CO)(κ1-N-SCN)] (3e), and
[Fe(PNP-iPr)(H)(CO)2]+ (3f) the intact complexes [M]+ (m/
z 505.1, 502.2, 507.1 (as [M + Na]+, and 456.1) were observed
as major fragments. We also investigated an EtOH solution of
2a in the presence of KOtBu in the hopes of detecting the
alkoxide complex [Fe(PNP-iPr)(H)(CO)(OEt)]. However,
only the fragment at m/z 426.1 was detected as the major
species. These observations are in accord with the fact that the
−
ligands Br−, OEt−, BH4 , and CH3CN trans to the hydride
ligand are substitutionally labile, while pyridine, PMe3, SCN−,
and CO are substitutionally inert.
The catalytic activity of all hydride complexes was
investigated in the hydrogenation of ketones and aldehydes.
In preliminary experiments various solvents were tested for the
Scheme 3. Substitution of the Bromide Ligand in 2a by MeOH-d4, CH3CN, Pyridine, PMe3, SCN−, CO, and BH4
−
C
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a
Table 1. Iron-Catalyzed Hydrogenation of Acetophenone
b
entry
solvent
THF
yield [%]
TOF [h−1
]
1
2
3
4
5
MeOH
iPrOH
36
58
89
99
18
29
45
50
tAmylOH
EtOH
Figure 4. Structural view of [Fe(PNP-iPr)(H)(CO)(PMe3)]BF4 (3d)
showing 50% thermal ellipsoids (most hydrogen atoms and BF4
anion omitted for clarity). Selected bond lengths (Å) and angles (deg):
Fe1−P1 2.1952(4), Fe1−P2 2.1886(4), Fe1−N1 2.007(1), Fe1−C18
1.730(2), Fe1−P3 2.2753(5), Fe1−H1 1.46(2), P1−Fe1−P2
154.89(2), N1−Fe1−C18 176.68(7).
a
−
Reaction conditions: 2a (0.025 mmol, 1.0 mol %), KOtBu (0.05
b
mmol), substrate (2.5 mmol), solvent (5 mL), H2 (5 bar), 2 h. Yields
were determined by H NMR.
1
However, in terms of a better reproducibility 0.5 mol % catalyst
was used for all subsequent reactions.
In contrast to 2a, under the same reaction conditions, as well
as with even 5 mol %, complex 2b, bearing NMe linkers, was
completely inactive for the reduction of ketones, while 2c,
containing one NH and one NMe linker, was catalytically active
but with a significantly lower activity than 2a (28% yield). On
the other hand, the catalytic activity of both [Fe(PNP-
iPr)(H)(CO)(CH3CN)]+ (3b) and Fe(PNP-iPr)(H)(CO)-
(κ1-BH4)] (3g) was similar to that of 2a (94% yield). The
reaction with 3g could be performed even without addition of
an external base, although slightly higher temperatures were
required to achieve comparable activities (50 °C) since base has
Figure 5. Structural view of [Fe(PNP-iPr)(H)(CO)(κ1-N-SCN)] (3e)
showing 50% thermal ellipsoids (most hydrogen atoms omitted for
clarity). Selected bond lengths (Å) and angles (deg): Fe1−P1
2.1884(7), Fe1−P2 2.1871(7), Fe1−N1 2.018(1), Fe1−N4
1.989(1), Fe1−C18 1.738(2), Fe1−H1 1.49(2), P1−Fe1−P2
164.78(1), N1−Fe1−C18 172.84(5).
14
−
to be generated by alcoholysis of free BH4 . Similar
observations were recently made by Milstein.4c In sharp
contrast to the substitutionally labile complexes 2a, 3b, and
3g, the inert compounds 3c−f were catalytically inactive.
On the basis of these results, we investigated the scope and
limitations of catalyst 2a using various substrates (Table 2).
Halogen substituents had no notable influence on the catalytic
activity, while the reaction with 4-methoxyacetophenone and 4-
nitroacetophenone resulted in significantly lower yields (entries
5 and 6). Likewise, for simple ketones such as cyclohexanone
and benzophenone lower activity was observed. In the presence
of a nitrile or primary amine substituents on the aromatic
system no reaction was observed, presumably due to
preferential coordination of these groups to the iron center,
thus blocking a vacant coordination site to accommodate an
incoming substrate (entries 7 and 8). The same result was
found for 4-acetylpyridine. This is in line with the observation
that 3c, containing a strongly bound pyridine ligand, is
catalytically inactive. The reduction of 2-acetylpyridine was
extremely efficient, giving full conversion even after 1 h (TOF =
200 h−1, entry 11). In this case, coordination of pyridine is
obviously hampered due to the bulky acetyl substituent in the
ortho position of the pyridine unit. The reduction of trans-4-
phenylbutenone resulted in mixtures, where reduction of the
double bond also took place (entry 13). Finally, the
hydrogenation of aldehydes was tested with complexes 2a
and 2b as catalysts utilizing benzaldehyde, 4-isopropylbenzal-
dehyde, cyclohexane carboxaldehyde, picolinealdehyde, and
isonicotinealdehyde (Table 3). Under the standard reaction
conditions, low conversions were observed (entry 1). However,
an increase of the catalyst loading to 5.0 mol % and reduction
of the reaction time to 10 min afforded the respective alcohols
in nearly quantitative yield. In the case of isonicotinealdehyde
Figure 6. Structural view of [Fe(PNP-iPr)(H)(CO)(κ1-BH4)] (3g)
showing 50% thermal ellipsoids (most hydrogen atoms omitted for
clarity). Selected bond lengths (Å) and angles (deg): Fe1−P1
2.1885(7), Fe1−P2 2.1873(7), Fe1−N1 1.998(2), Fe1−C18
1.733(2), Fe1−B1 2.72(1), Fe1−H1 1.46(2), Fe1−H1B1 1.67(2),
P1−Fe1−P2 159.99(3), N1−Fe1−C18 178.13(9).
hydrogenation of acetophenone using 1.0 mol % 2a, 2.0 mol %
KOtBu, and 5 bar hydrogen at ambient temperature (25 °C,
Table 1). The hydrogenation reaction takes place only in
alcoholic solutions, with ethanol being by far the best solvent,
giving rac-1-phenylethanol in essentially quantitative yield.
Moreover, in the absence of base and/or H2 no reaction takes
place, indicating that 2a is not an active catalyst for transfer
hydrogenation. The amount of the catalyst precursor could be
reduced to 0.1 mol %. In this case 77% isolated yield was
reached within 1 h, which corresponds to a TOF of 770 h−1.
D
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a
a
Table 2. Iron-Catalyzed Hydrogenation of Ketones
Table 3. Iron-Catalyzed Hydrogenation of Aldehydes
a
Reaction conditions: 2a or 2b (0.125 mmol), KOtBu (0.25 mmol),
b
substrate (2.5 mmol), EtOH (5 mL), H2 (5 bar), 10 min. Reaction
conditions: 2a (0.0125 mmol), KOtBu (0.025 mmol), substrate (2.5
c
mmol), EtOH (5 mL), H2 (5 bar), 2 h. Reaction time: 20 min.
d
1
Yields were determined by H NMR.
Scheme 4. Dehydrohalogenation of 2a with KOtBu in THF
to Give A and Subsequent Addition of H2 to Afford a
Mixture of the trans and cis Dihydride Complexes 4a and 4b
a
Reaction conditions: 2a (0.0125 mmol), KOtBu (0.025 mmol),
b
substrate (2.5 mmol), EtOH (5 mL), H2 (5 bar), 2 h. Reaction
conditions: 2a (0.0025 mmol), KOtBu (0.005), substrate (2.5 mmol),
c
d
EtOH (3 mL), 1 h. Reaction time: 1 h. Yields were determined by
1H NMR.
(entry 6) no reaction took place, again due to strong
coordination of the pyridine moiety.
In order to gain a mechanistic understanding of the catalytic
hydrogenation of aldehydes and ketones, some stoichiometric
reactions of 2a were investigated. Treatment of 2a in THF with
KOtBu resulted in an immediate color change from orange to
trans-dihydride 4a and only one broad signal at −13.4 ppm for
the cis-dihydride 4b due to fast exchange between the two
hydrides. Complexes 4a and 4b did not show any significant
reactivity toward acetophenone even after 1 day, suggesting
that these are not active species in the catalytic reduction of
ketones. Our findings are fully consistent with Milstein’s
discoveries based on the related iron pincer complex (Fe-
[PNPCH2-iPr)(H)(CO)(κ1-BH4)],4c but strongly contrast the
recently reported computational study by Yang on the iron-
catalyzed reduction of acetophenone.17 In his calculated
mechanism, the reaction proceeds via trans-[Fe(PNP-iPr)-
(CO)(H)2] (4a) and involves an outer-sphere hydrogen
transfer from this complex to the carbonyl carbon atom of
acetophenone in EtOH as solvent. Accordingly, we believe that
this mechanism is not operative in our system with respect to
ketone reduction, although trans-dihydride iron PNP com-
plexes were shown to be important species in other
reactions.4b,d,e,13,15,16,18 The reduction of aldehydes, in
1
deep red. In the H NMR spectrum hydride signals were no
longer present, but in the IR spectrum two strong absorptions
at 1872 and 1822 cm−1 were observed (νFe−H and νCO). This
may be tentatively assigned to the formation of [Fe(PNP−H
-
iPr)(H)(CO)] (A) as a result of dehydrohalogenation. In this
context it is important to note that a series of related iron PNP
pincer complexes were prepared and even structurally
characterized recently by Schneider15 and Jones.16 The
formation the Fe(0) dicarbonyl complex [Fe(PNP-iPr)(CO)2]
can be ruled out by comparison with an authentic sample.11
Moreover, purging the solution with H2 afforded a mixture of
the trans and cis dihydride complexes [Fe(PNP-iPr)(CO)(H)2]
(4a,b) (Scheme 4). Such a reaction does not take place with
1
[Fe(PNP-iPr)(CO)2]. The H NMR spectrum of the mixture
at room temperature exhibited a triplet at −9.02 ppm for the
E
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particular with 2b, remains mechanistically unclear at this stage,
and the involvement of dihydride complexes cannot be ruled
out.
electron-donating anionic ligand. In fact, the activation barrier
for dihydrogen splitting involving protonation of the PNP N
atom, corresponding to reversible aromatization/dearomatiza-
tion of that ligand to afford E′, is considerably higher (34.1
kcal/mol) than the one associated with protonation of the O
atom of the alkoxide producing the final alcohol product as
shown in Scheme 5 (16.0 kcal/mol).
The dihydrogen splitting step was also studied with the
inclusion of one explicit solvent molecule (ethanol) in the
calculations (Scheme 6). In fact, the ethanol molecule acting as
In sharp contrast to the above observations in THF, when
KOtBu was added to an EtOH solution of 2a in the presence of
dihydrogen, no changes in the IR, 1H NMR, and 31P{1H} NMR
spectra were observed. This again emphasizes the particular
role of EtOH as solvent apparently preventing the formation of
4a and 4b.
Preliminary DFT calculations19 were carried out to establish
a reasonable mechanism using the hydrogenation of
acetaldehyde with 2a as model. A summary of these results
with the most relevant points along the catalytic cycle is
presented in Scheme 5. Loss of a labile bromide ligand and
Scheme 6. Dihydrogen Splitting via O- and N-Protonation
a
Calculated with One Explicit EtOH Molecule
Scheme 5. Catalytic Cycle Calculated for an Inner-Sphere
a
Mechanism
a
Free energy values (in kcal/mol) are referred to [Fe(PNP−H
-
iPr)(H)(CO)] (A) and H-bond distances in Å.
a
Free energy values (in kcal/mol) referred to [Fe(PNP−H-iPr)(H)-
a proton shuttle could alter the most favorable path and change
the conclusions above. The results obtained are shown in
Scheme 6 and indicate that O-protonation of the alkoxide
ligand remains the preferred pathway for the reaction, with a
barrier 12.5 kcal/mol lower than the value calculated for
protonation of the PNP N atom. The O-protonation step
calculated with an explicit solvent molecule (EtOH),
represented in Scheme 6, has a free energy barrier 6.3 kcal/
mol higher than the same process calculated without the
ethanol molecule (cf. Scheme 5) due to the rise in the entropy
term originated by the presence of that extra molecule. If one
compares energy values, the barrier becomes 5 kcal/mol lower
in the case with the extra ethanol molecule. This result confirms
that the PNP ligand remains deprotonated and, thus,
dearomatized along the entire cycle and means that N−H
acidity has no active part in the reaction mechanism that should
not be classified as bifunctional catalysis in this case.
(CO)] (A).
deprotonation of an NH group in the catalytic precursor 2a will
produce a five-coordinated complex, [Fe(PNP−H-iPr)(H)-
(CO)] (A), that is the starting point in the mechanistic
investigations and also the reference for all free energy values.
The catalytic cycle depicted in Scheme 5 starts with the
occupation of the free coordination site in A by the substrate
(in B). Then there is nucleophilic attack of the hydride on the
carbonyl C atom with formation of the alkoxide complex, C.
The reaction proceeds with coordination of dihydrogen (D)
and subsequent protonation of the O atom with formation of
the alcohol and regeneration of the hydride (E). The cycle is
closed by ligand exchange with liberation of one molecule of
the product and coordination of another substrate from E back
to B. The highest energy barrier along the cycle corresponds to
the hydride migration step, and its value (17.1 kcal/mol)
indicates a facile reaction. It has to be emphasized that the PNP
ligand is not involved in dihydrogen activation but remains
deprotonated throughout the catalytic cycle, acting as a strongly
It is also interesting to note that deprotonation of the PNP
ligand is accompanied by a substantial increase of the ligand
charge. In fact, in the N-protonated counterpart of A, [Fe(PNP-
iPr)(H)(CO)]+, the PNP ligand is more positive (CPNP = 1.03)
F
dx.doi.org/10.1021/om5009814 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
to the addition of the reaction solution. For the preparation of the
reaction solutions a vial was charged with the specified amount of
catalyst, substrate, and EtOH. Subsequently, KOtBu was added and
the solution was taken up into a syringe and transferred to the Fisher-
Porter tube. After stirring the solution for the stated time, pressure was
carefully released, diethyl ether (20 mL) was added, and the reaction
was quenched by addition of an aqueous solution of H3PO4 (0.5 M,
0.5 mL). The organic phase was separated, washed with brine, and
dried over MgSO4. The solvent was removed under reduced pressure,
and the isolated product was characterized by NMR spectroscopy.
Syntheses. [Fe(PNP-iPr)(H)(CO)Br] (2a). Anhydrous FeBr2 (190
mg, 0.88 mmol) and 1a (300 mg, 0.88 mmol) were dissolved in 12 mL
of THF. The immediately formed yellow suspension was stirred for 1
h at room temperature before CO was bubbled through the reaction
mixture for 10 min. During this time the color of the suspension
changed from yellow to blue. The reaction mixture was cooled to 0 °C,
and a solution of Na[HBEt3] in toluene (0.97 mL, 1 M, 0.97 mmol)
was slowly added. The reaction mixture was stirred for 30 min at 0 °C,
in which time the color changed from blue to dark red. After an
additional 60 min at room temperature the solution was filtered and
the solvent was removed under reduced pressure. The dark residue
was taken up in THF (3 mL), and the product was precipitated by
addition of n-hexane (15 mL). The precipitate was separated from the
supernatant solution, washed with n-pentane (3 × 10 mL), and dried
under vacuum to afford a bright yellow powder. Yield: 298 mg (67%).
Anal. Calcd for C18H34BrFeN3OP2: C, 42.71; H, 6.77; N, 8.30. Found:
than the same ligand in A (CPNP = 0.14). Accordingly, the
hydride in the cationic complex is also electron poorer than the
equivalent ligand in A, CH = −0.14 and −0.16, respectively,
indicating that A should be a better active species in a reaction
where the key step is hydride nucleophilic attack on the
substrate carbonyl C atom.
CONCLUSION
■
In conclusion, we have prepared a new class of Fe(II) PNP
pincer hydride complexes [Fe(PNP-iPr)(CO)(H)(L)]n (2a−c,
3a−g) (n = +1, 0) based on the 2,6-diaminiopyridine scaffold
where the PiPr2 moieties of the PNP ligand connect to the
pyridine ring via NH and/or NMe spacers and where the
complexes feature both labile (Br−, CH3CN, BH4 ) and inert
−
(pyridine, PMe3, SCN−, CO) coligands. Complexes with labile
ligands are efficient catalysts for the hydrogenation of ketones
and aldehydes to alcohols under mild conditions. These
reactions take place at room temperature with turnover
frequencies up to 770 h−1 using 5 bar hydrogen pressure and
seem to involve heterolytic dihydrogen cleavage via metal-
alkoxide cooperation, with the PNP ligand not being involved
in dihydrogen activation. The PNP ligand remains deproto-
nated throughout the catalytic cycle, acting as a strongly
electron donating anionic ligand. The catalytic reactions do not
proceed in aprotic solvents, but require alcoholic solutions, with
EtOH being the best solvent. EtOH prevents the formation of
dihydride species, which are catalytically inactive, and seems to
stabilize the dearomatized 16e intermediate A due to reversible
EtOH coordination. The experimental results complemented
by DFT calculations strongly support an inner-sphere
mechanism, i.e., insertion of the CO bond of the carbonyl
compound into the Fe−H bond. Finally, the chemoselectivity
of 2b toward aldehydes vs ketones is remarkable and may be
synthetically useful. This reaction apparently also does not
proceed via a bifunctional mechanism. Detailed mechanistic
studies, in particular the reaction of catalyst 2b where the
mechanism remains unclear, as well as catalyst optimizations
are currently under way.
1
C, 42.57; H, 6.83; N, 8.33. H NMR (δ, CD2Cl2, 20 °C): 7.17 (t, J =
8.0 Hz, 1H, py4), 6.11 (d, J = 8.0 Hz, 2H, py3,5), 5.54 (bs, 2H, NH),
3.24 (m, 2H, CH(CH3)2), 2.51 (m, 2H, CH(CH3)2), 1.56 (dd, J = 7.9
Hz, J = 17.6, 6H, CH(CH3)2), 1.44 (dd, J = 6.4 Hz, J = 11.9, 6H,
CH(CH3)2), 1.21 (dd, J = 7.0 Hz, J = 17.1, 6H, CH(CH3)2), 1.00 (dd,
2
J = 6.8 Hz, J = 14.2, 6H, CH(CH3)2), −21.36 (t, JHP = 56.6, 1H,
FeH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 222.7 (br, CO), 160.8 (t,
2JCP = 9.9 Hz, py2,6), 138.7 (s, py4), 97.4 (s, py3,5), 30.3 (t, 1JCP = 10.7
1
Hz, CH(CH3)2), 27.4 (t, JCP = 12.9 Hz, CH(CH3)2), 19.6 (s,
CH(CH3)2), 18.8 (s, CH(CH3)2), 18.5 (s, CH(CH3)2), 17.2 (s,
CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 147.1. IR (ATR,
cm−1): 1902 (νCO). ESI-MS (m/z, EtOH); pos. ion: 426.1 [M − Br]+,
398.2 [M − Br − CO]+.
[Fe(PNPMe-iPr)(H)(CO)Br] (2b). This complex was prepared
analogously to 2a using 1b (300 mg, 0.81 mmol), FeBr2 (175 mg,
0.81 mg), and Na[HBEt3] (0.89 mL, 1 M in toluene, 0.89 mmol) as
starting materials. The product was obtained as a mixture of two
isomers in a 2.7:1 ratio (2b, 2b′). Yield: 276 mg (64%) of a red-orange
powder. Anal. Calcd for C20H38BrFeN3OP2: C, 44.96; H, 7.17; N,
7.87. Found: C, 44.86; H, 7.22; N, 7.07. 2b: 1H NMR (δ, CD2Cl2, 20
°C): 7.44 (t, J = 8.3 Hz, 1H, py4), 6.02 (d, J = 8.3 Hz, 2H, py3,5), 3.10
(s, 3H, NCH3), 2.80 (m, 2H, CH(CH3)2), 2.55 (m, 2H, CH(CH3)2),
1.72 (dd, J = 7.3 Hz, J = 16.6, 6H, CH(CH3)2), 1.60 (dd, J = 7.3 Hz, J
= 14.5, 6H, CH(CH3)2), 1.22 (dd, J = 7.3 Hz, J = 16.8, 6H,
CH(CH3)2), 0.83 (dd, J = 7.0 Hz, J = 14.1, 6H, CH(CH3)2), −21.84
(t, 2JHP = 57.5, 1H, FeH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 222.7 (t,
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under an
inert atmosphere of argon by using Schlenk techniques or in an
MBraun inert-gas glovebox. The solvents were purified according to
standard procedures.20 The deuterated solvents were purchased from
Aldrich and dried over 4 Å molecular sieves. The ligands N,N′-
bis(diisopropylphosphino)-2,6-diaminopyridine (PNP-iPr) (1a),21
N,N′-bis(diisopropylphosphino)-N,N′-dimethyl-2,6-diaminopyridine
(PNPMe-iPr) (1b), and N,N′-bis(diisopropylphosphino)-N-methyl-
2,6-diaminopyridine (PNPH,Me-iPr) (1c)22 and complex [Fe(PNP-
iPr)(H)(CO)2]SbF6 (3f) were prepared according to the literature.11
1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Bruker
AVANCE-250 and AVANCE-400 spectrometers. 1H and 13C{1H}
NMR spectra were referenced internally to residual protio-solvent and
solvent resonances, respectively, and are reported relative to
tetramethylsilane (δ = 0 ppm). 31P{1H} NMR spectra were referenced
externally to H3PO4 (85%) (δ = 0 ppm). All mass spectrometric
measurements were performed on an Esquire 3000plus 3D-quadrupole
ion trap mass spectrometer (Bruker Daltonics, Bremen, Germany) in
positive-ion mode electrospray ionization (ESI-MS). All mass
calculations are based on the lowest mass (i.e. most abundant) iron
isotope (56Fe-isotope).
2
2JCP = 22.4 Hz, CO), 162.6 (t, JCP = 11.4 Hz, py2,6), 138.9 (s, py4),
1
96.6 (s, py3,5), 34.0 (s, NCH3), 33.6 (t, JCP = 9.3 Hz, CH(CH3)2),
31.2 (t, 1JCP = 14.8 Hz, CH(CH3)2), 21.7 (s, CH(CH3)2), 20.3 (t, 2JCP
2
= 3.6 Hz, CH(CH3)2), 18.3 (s, CH(CH3)2), 17.8 (t, JCP = 5.0 Hz
CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 164.0 (s). IR (ATR,
cm−1): 1903 (νCO). 2b′: 1H NMR (δ, CD2Cl2, 20 °C): 7.13 (t, J = 8.2
Hz, 1H, py4), 5.65 (d, J = 8.2 Hz, 2H, py3,5), 3.04 (m, 2H, CH(CH3)2),
2.97 (s, 3H, NCH3), 2.83 (m, 2H, CH(CH3)2), 1.61 (dd, J = 7.4 Hz, J
= 16.1, 6H, CH(CH3)2), 1.54 (dd, J = 7.0 Hz, J = 14.3, 6H,
CH(CH3)2), 1.50 (dd, J = 7.4 Hz, J = 16.3, 6H, CH(CH3)2), 1.29 (dd,
J = 7.0 Hz, J = 14.3, 6H, CH(CH3)2), −1.08 (t, 2JHP = 57.1, 1H, FeH).
2
13C{1H} NMR (δ, CD2Cl2, 20 °C): 217.1 (t, JCP = 13.0 Hz, CO),
2
163.5 (t, JCP = 9.8 Hz, py2,6), 137.0 (s, py4), 95.4 (s, py3,5), 33.3 (s,
1
1
NCH3), 31.6 (t, JCP = 12.9 Hz, CH(CH3)2), 31.3 (t, JCP = 8.6 Hz,
CH(CH32)2), 19.2 (t, 2JCP = 3.7 Hz, CH(CH3)2), 18.8 (s, CH(CH3)2),
18.7 (t, JCP = 2.3 Hz, CH(CH3)2), 18.3 (s, CH(CH3)2). 31P{1H}
NMR (δ, CD2Cl2, 20 °C): 161.6. IR (ATR, cm−1): 1903 (νCO).
General Procedure for the Hydrogenation Reactions. All
hydrogenation reactions were performed at ambient temperature (25
°C) under a hydrogen atmosphere of 5 bar using a 90 mL Fisher-
Porter tube, which was flushed several times with hydrogen gas prior
G
dx.doi.org/10.1021/om5009814 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
[Fe(PNPH,Me-iPr)(H)(CO)Br] (2c). This complex was prepared
analogously to 2a using 1c (300 mg, 0.84 mmol), FeBr2 (181 mg,
0.84 mg), and Na[HBEt3] (0.92 mL, 1 M in toluene, 0.92 mmol) as
starting materials. Yield: 275 mg (63%) of an orange powder. Anal.
Calcd for C19H36BrFeN3OP2: C, 43.87; H, 6.98; N, 8.08. Found: C,
43.59; H, 7.19; N, 7.96. 1H NMR (δ, CD2Cl2, 20 °C): 7.31 (t, J = 7.8
Hz, 1H, py4), 6.22 (d, J = 7.8 Hz, 1H, py3,5), 5.91 (d, J = 7.8 Hz, 1H,
py3,5), 5.61 (bs, 1H, NH), 2.80 (m, 1H, CH(CH3)2), 3.07 (d, J = 3.1
Hz, 3H, NCH3), 2.75 (m, 1H, CH(CH3)2), 2.53 (m, 2H, CH(CH3)2),
1.72 (dd, J = 6.9 Hz, J = 17.4, 3H, CH(CH3)2), 1.55 (dd, J = 7.3 Hz, J
= 13.6, 3H, CH(CH3)2), 1.52 (dd, J = 7.8 Hz, J = 9.5, 3H,
CH(CH3)2), 1.44 (dd, J = 7.1 Hz, J = 10.7, 3H, CH(CH3)2), 1.26 (dd,
J = 6.8 Hz, J = 17.0, 3H, CH(CH3)2), 1.18 (dd, J = 6.9 Hz, J = 17.4,
3H, CH(CH3)2), 0.94 (dd, J = 6.7 Hz, J = 14.4, 3H, CH(CH3)2), 0.88
(dd, J = 6.7 Hz, J = 14.4, 3H, CH(CH3)2), −21.64 (t, 2JHP = 57.2, 1H,
FeH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 222.5 (br, CO), 162.4 (br,
cm−1): 1906 (νCO). ESI-MS (m/z, EtOH); pos. ion: 505.1 [M]+, 426.1
[M − C5H5N]+.
[Fe(PNP-iPr)(H)(CO)(PMe3)]BF4 (3d). To a solution of 2a (150
mg, 0.30 mmol) in CH3OH (8 mL) was added PMe3 (47 mL, 0.45
mmol). After stirring for 5 min at room temperature, AgBF4 (58 mg,
0.30 mmol) was added and the reaction mixture was stirred for an
additional 5 min. The dark precipitate was filtered off, and the solvent
of the filtrate was removed in vacuo. The residue was washed twice
with n-pentane and dried under high vacuum to give an off-white
powder. Yield: 172 mg (97%). Anal. Calcd for C21H43BF4FeN3OP3: C,
1
42.81; H, 7.36; N, 7.13. Found: C, 42.94; H, 7.58; N, 7.58. H NMR
(δ, MeOH-d4, 20 °C): 7.31 (t, J = 7.8 Hz, 1H, py4), 6.22 (d, J = 7.8
Hz, 2H, py3,5), 2.73 (m, 2H, CH(CH3)2), 2.62 (m, 2H, CH(CH3)2),
1.58−1.45 (m, 12H, CH(CH3)2), 1.25 (m, 6H, CH(CH3)2), 1.23 (d, J
= 7.2 Hz, 3H, P(CH3)32), 0.99 (dd, J = 6.8 Hz, J = 14.7, 6H,
2
CH(CH3)2), −11.09 (dt, JHP = 35.7 Hz, JHP = 60.7 Hz, 1H, FeH).
13C{1H} NMR (δ, MeOH-d4, 20 °C): 220.5 (dt, JCP = 15.2 Hz, JCP
=
3
py2,6), 161.0 (br, py2,6), 138.9 (s, py4), 98.2 (d, JCP = 6.8 Hz, py3,5),
23.2 Hz, CO), 162.0 (t, JCP = 8.4 Hz, py2,6), 140.7 (s, py4), 98.9 (s,
3
1
95.7 (d, JCP = 6.8 Hz, py3,5), 33.5 (d, JCP = 21.7 Hz, CH(CH3)2),
py3,5), 34.7 (t, JCP = 11.7 Hz, CH(CH3)2), 28.7 (dt, JCP = 8.0 Hz, JCP
14.8 Hz, CH(CH3)2), 19.8 (t, JCP = 4.6 Hz, CH(CH3)2), 19.0 (t, JCP
=
=
33.4 (d, 2JCP = 12.1 Hz, NCH3), 31.7 (d, 1JCP = 28.6 Hz, CH(CH3)2),
29.0 (d, JCP = 22.8 Hz, CH(CH3)2), 27.1 (d, JCP = 24.1 Hz,
CH(CH3)2), 21.1 (s, CH(CH3)2), 20.4 (d, 2JCP = 8.8 Hz, CH(CH3)2),
1
1
4.6 Hz, CH(CH3)2), 18.8 (d, JCP = 23.2 Hz, P(CH3)3), 17.9 (s,
CH(CH3)2), 17.4 (bs, CH(CH3)2). 31P{1H} NMR (δ, MeOH-d4, 20
°C): 147.3 (d, J = 25.6 Hz, 2P, PiPr2), 2.9 (t, J = 25.6 Hz, PMe3). IR
(ATR, cm−1): 1910 (νCO). ESI-MS (m/z, EtOH); pos. ion: 502.2
[M]+, 426.1 [M − PMe3]+.
2
2
19.5 (d, JCP = 8.8 Hz, CH(CH3)2), 18.7 (d, JCP = 10.1 Hz,
CH(CH3)2), 18.5 (s, CH(CH3)2), 18.3 (s, CH(CH3)2), 18.0 (d, 2JCP
=
9.4 Hz, CH(CH3)2), 17.1 (d, JCP = 7.9 Hz, CH(CH3)2). 31P{1H}
2
NMR (δ, CD2Cl2, 20 °C): 165.0 (d, JPP = 145.1 Hz), 147.2 (d, JPP
=
[Fe(PNP-iPr)(H)(CO)(κ1-N-SCN)] (3e). To a solution of 2a (150
mg, 0.30 mmol) in THF (10 mL) was added NaSCN (27 mg, 0.33
mmol). After stirring for 1 h at room temperature, the solution was
filtered and the solvent was removed under reduced pressure. The
product was washed twice with diethyl ether and dried under vacuum
to afford an off-white powder. Yield: 136 mg (93%). Anal. Calcd for
C19H34FeN4OP21S: C, 47.12; H, 7.08; N, 11.57. Found: C, 47.18; H,
7.13; N, 11.42. H NMR (δ, DMSO-d6, 20 °C): 8.26 (s, 2H, NH),
7.26 (t, J = 7.9 Hz, 1H, py4), 6.13 (d, J = 7.9 Hz, 2H, py3,5), 2.48 (m,
2H, CH(CH3)2), 2.41 (m, 2H, CH(CH3)2), 1.44 (m, 6H, CH(CH3)2),
1.38 (m, 6H, CH(CH3)2), 1.12 (m, 6H, CH(CH3)2), 0.95 (m, 6H,
CH(CH3)2), −19.84 (t, 2JHP = 52.1 Hz, 1H, FeH). 13C{1H} NMR (δ,
145.1 Hz). IR (ATR, cm−1): 1901 (νCO).
[Fe(PNP-iPr)(H)(CO)(CH3CN)]BF4 (3b). To a solution of 2a (150
mg, 0.30 mmol) in CH3CN (10 mL) was added AgBF4 (58 mg, 0.30
mmol). After stirring for 5 min at room temperature, the precipitate
was filtered off and the solvent was removed under reduced pressure.
The product was washed twice with diethyl ether and dried under
vacuum to afford a pale green powder. Yield: 148 mg (89%). Anal.
Calcd for C20H37BF4FeN4OP2: C, 43.35; H, 6.73; N, 10.11. Found: C,
43.28; H, 6.78; N, 10.02. 1H NMR (δ, acetone-d6, 20 °C): 7.64 (s, 2H,
NH), 7.63 (t, J = 7.9 Hz, 1H, py4), 6.33 (d, J = 7.9 Hz, 2H, py3,5), 2.67
(m, 4H, CH(CH3)2), 2.40 (s, 3H, CH3CN), 1.53 (m, 12H,
CH(CH3)2), 1.22 (dd, J = 7.1 Hz, J = 17.4 Hz, 6H, CH(CH3)2),
2
DMSO-d6, 20 °C): 220.7 (t, 2JCP = 24.1 Hz, CO), 161.0 (t, JCP = 9.6
2
1.06 (dd, J = 6.9 Hz, J = 14.8 Hz, 6H, CH(CH3)2), −18.55 (t, JHP
=
Hz, py2,6), 138.7 (s, py4), 137.4 (d, 3JCP = 5.4 Hz, SCN), 96.6 (s, py3,5),
53.3 Hz, 1H, FeH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 218.7 (t, 2JCP
= 21.7 Hz, CO), 161.1 (t, 2JCP = 9.0 Hz, py2,6), 140.3 (s, py4), 127.2 (s,
CH3CN), 98.9 (s, py3,5), 31.0 (t, 1JCP = 10.2 Hz, CH(CH3)2), 29.8 (t,
1JCP = 15.4 Hz, CH(CH3)2), 19.4 (s, CH(CH3)2), 18.2 (s, CH(CH3)2),
4.5 (s, CH3CN). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 145.6). IR (ATR,
cm−1): 1929 (νCO). ESI-MS (m/z, EtOH); pos. ion: 426.1 [M −
CH3CN]+, 398.1 [M − CH3CN − CO]+. Crystals suitable for X-ray
crystallography were grown with Br− as counterion (analogously
prepared without the addition of a silver salt as halide scavenger) by
slow evaporation of a CH3CN/THF (1:1) solution.
1
1
29.5 (t, JCP = 10.6 Hz, CH(CH3)2), 27.8 (t, JCP = 14.9 Hz,
CH(CH3)2), 18.8 (s, CH(CH3)2), 17.9 (s, CH(CH3)2), 17.8 (s,
CH(CH3)2), 17.4 (s, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C):
146.6. IR (ATR, cm−1): 2074 (νNCS), 1921 (νCO). ESI-MS (m/z,
EtOH, NaCl); pos. ion: 507.1 [M + Na]+, 426.1 [M − SCN]+, 398.2
[M − SCN − CO]+.
[Fe(PNP-iPr)(H)(CO)(κ1-BH4)] (3g). Method A. To a solution of
2a (200 mg, 0.40 mmol) in THF (24 mL) was added sodium
borohydride (76 mg, 2.00 mmol). After stirring for 6 h at room
temperature, the solution was filtered and the solvent was removed
under reduced pressure. The residue was dissolved in THF (1.0 mL),
and the product was precipitated by addition of n-pentane. The bright
yellow powder was washed twice with n-pentane and dried under
vacuum. Yield: 132 mg (75%).
[Fe(PNP-iPr)(H)(CO)(py)]BF4 (3c). To a solution of 2a (150 mg,
0.30 mmol) in CH3OH (8 mL) was added pyridine (36 μL, 0.45
mmol). After stirring for 5 min at room temperature, AgBF4 (58 mg,
0.30 mmol) was added and the reaction mixture was stirred for an
additional 5 min. The dark precipitate was filtered off, and the solvent
was removed under reduced pressure. The residue was washed twice
with diethyl ether and dried under vacuum to afford a yellow powder.
Yield: 150 mg (83%). Anal. Calcd for C23H39BF4FeN4OP2: C, 46.65;
Method B. To a suspension of [Fe(PNP-iPr)(CO)(Br)2] (200 mg,
0.40 mmol) in EtOH (10 mL) was added sodium borohydride (65 mg,
1.71 mmol). An immediate gas evolution took place, and the initially
blue suspension turned into a dark orange solution within 5 min. After
stirring the reaction mixture for 30 min, all volatiles were removed
under reduced pressure. The residue was dissolved in dichloromethane
(10 mL), the resulting solution was filtered, and the solvent was
removed under vacuum. Yield: 136 mg (91%). Anal. Calcd for
C18H38BFeN3OP2: C, 49.01; H, 8.68; N, 9.53. Found: C, 48.95; H,
1
H, 6.64; N, 9.46. Found: C, 47.07; H, 6.95; N, 9.29. H NMR (δ,
MeOH-d4, 20 °C): 10.00−6.55 (broad and unresolved signals, 4H,
pyridine-H2,3,5,6), 7.77 (t, J = 7.6 Hz, 1H, pyridine-H4), 7.46 (t, J = 8.0
Hz, 1H, py4), 6.34 (d, J = 8.0 Hz, 2H, py3,5), 2.50 (m, 2H, CH(CH3)2),
1.89 (m, 2H, CH(CH3)2), 1.28−1.19 (m, 12H, CH(CH3)2), 1.07 (dd,
J = 6.8 Hz, J = 14.3, 6H, CH(CH3)2), 0.93 (dd, J = 7.5 Hz, J = 15.6,
6H, CH(CH3)2), −20.08 (t, 2JHP = 52.9 Hz, 1H, FeH). 13C{1H} NMR
1
8.61; N, 9.77. H NMR (δ, CD2Cl2, 20 °C): 7.17 (t, J = 7.9 Hz, 1H,
py4), 6.13 (d, J = 7.9 Hz, 2H, py3,5), 5.43 (bs, 2H, NH), 3.01 (m, 2H,
CH(CH3)2), 2.50 (m, 2H, CH(CH3)2), 1.50 (dd, J = 7.6 Hz, J = 17.7
Hz, 6H, CH(CH3)2), 1.42 (dd, J = 7.0 Hz, J = 12.5 Hz, 6H,
CH(CH3)2), 1.22 (dd, J = 7.0 Hz, J = 17.4 Hz, 6H, CH(CH3)2), 1.05
(dd, J = 6.7 Hz, J = 14.4 Hz, 6H, CH(CH3)2), −3.61 (br, 4H, BH4),
2
2
(δ, MeOH-d4, 20 °C): 220.3 (t, JCP = 22.8 Hz, CO), 163.0 (t, JCP
=
9.0 Hz, py2,6), 141.0 (d, JCP = 13.6 Hz, pyridine-C2,6), 138.8 (s,
3
pyridine-C4), 138.5 (s, py4), 126.9 (s, pyridine-C3,5), 99.3−98.6
1
(unresolved signal), 30.3 (t, JCP = 16.3 Hz, CH(CH3)2), 18.9 (s,
2
CH(CH3)2), 18.7 (s, CH(CH3)2), 18.3 (s, CH(CH3)2), 17.5 (s,
−18.12 (t, JHP = 52.1 Hz, 1H, FeH). 13C{1H} NMR (δ, CD2Cl2, 20
CH(CH3)2). 31P{1H} NMR (δ, MeOH-d4, 20 °C): 142.3. IR (ATR,
°C): 160.8 (t, 2JCP = 9.1 Hz, py2,6), 138.5 (s, py4), 97.3 (s, py3,5), 31.4
H
dx.doi.org/10.1021/om5009814 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
1
(t, JCP = 10.6 Hz, CH(CH3)2), 27.8 (t, JCP = 13.2 Hz, CH(CH3)2),
19.5 (t, 2JCP = 3.8 Hz, CH(CH3)2), 18.6 (t, 2JCP = 4.6 Hz, CH(CH3)2),
18.4 (s, CH(CH3)2), 17.4 (s, CH(CH3)2), the CO resonance could not
be observed. 31P{1H} NMR (δ, CD2Cl2, 20 °C): 151.2. IR (ATR,
cm−1): 1911 (νCO). ESI-MS (m/z, EtOH); pos. ion: 426.1 [M −
BH4]+, 398.1 [M − BH4 − CO]+.
ASSOCIATED CONTENT
* Supporting Information
■
S
Coordinates of the optimized structures, H, 13C{1H}, and
31P{1H} NMR spectra of all complexes, crystallographic data
and technical details in CIF format for 2a, 3b, 3e (CCDC
1010393−1010395) and 3c, 3d, 3g (CCDC 1023528−
1023530). This material is available free of charge via the
Internet at http://pubs.acs.org.
1
X-ray Structure Determination. X-ray diffraction data of 2a·
CD2Cl2, 3b, 3c, 3e, and 3f were collected at T = 100 K (3f: T = 200 K
due to a phase transition at lower temperatures) in a dry stream of
nitrogen on Bruker Kappa APEX II diffractometer systems using
graphite-monochromatized Mo Kα radiation (λ = 0.710 73 Å) and fine
sliced φ- and ω-scans. Data of 3d were collected at T = 185 K on a
Bruker SMART APEX diffractometer using ω-scans. Data were
reduced to intensity values with SAINT, and an absorption correction
was applied with the multiscan approach implemented in SADABS.23
The structures were solved by charge flipping using SUPERFLIP24 and
refined against F with JANA2006.25 Non-hydrogen atoms were refined
anisotropically. The H atoms connected to C atoms were placed in
calculated positions and thereafter refined as riding on the parent
atoms. H atoms connected to N, B, and Fe atoms were located in
difference Fourier maps. The Fe−H distances were restrained. The
N−H distances were restrained in 2a·CD2Cl2 and 3d, whereas the N−
H atoms in the remaining models were freely refined. In 3f, the B−H
distances were restrained to 1.000(1) Å. Molecular graphics were
generated with the program MERCURY.26 Crystal data and
experimental details are given in Tables S1 and S2.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kkirch@mail.tuwien.ac.at.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support by the Austrian Science Fund (FWF) is
gratefully acknowledged (Project No. P24583-N28), and L.F.V.
■
and L.P.F. acknowledge Fundaca
̧
o para a Cien
̂
cia e Tecnologia,
̃
Projecto Estrateg
́
ico - PEst-OE/QUI/UI0100/2013 and PEst-
OE/FIS/UI0261/2014, respectively. The X-ray center of the
Vienna University of Technology is acknowledged for financial
support and for providing access to the single-crystal
diffractometer.
Computational Details. All calculations were performed using the
Gaussian 09 software package27 on the Phoenix Linux Cluster of the
Vienna University of Technology. The optimized geometries were
obtained with the B3LYP functional.28 That functional includes a
mixture of Hartree−Fock29 exchange with DFT19 exchange−
correlation, given by Becke’s three-parameter functional with the
Lee, Yang, and Parr correlation functional, which includes both local
and nonlocal terms. The basis set used for the geometry optimizations
(basis b1) consisted of the Stuttgart/Dresden ECP (SDD) basis set30
to describe the electrons of iron and a standard 6-31G(d,p) basis set31
for all other atoms. Transition-state optimizations were performed
with the Synchronous Transit-Guided Quasi-Newton Method
(STQN) developed by Schlegel et al.,32 following extensive searches
of the potential energy surface. Frequency calculations were performed
to confirm the nature of the stationary points, yielding one imaginary
frequency for the transition states and none for the minima. Each
transition state was further confirmed by following its vibrational mode
downhill on both sides and obtaining the minima presented on the
energy profiles. Atomic charges were obtained by means of a natural
population analysis (NPA).33 The electronic energies (Eb1) obtained at
the B3LYP/b1 level of theory were converted to free energy at 298.15
K and 1 atm (Gb1) by using zero-point energy and thermal energy
corrections based on structural and vibration frequency data calculated
at the same level.
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