Experimental Evidence of syn H-N-Fe-H Configurational Requirement for Iron-Based Bifunctional Hydrogenation Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.1c00328

Iron hydrides supported by a pincer ligand of the type HN(CH2CH2PR2)2 (RPNHP) are versatile hydrogenation catalysts. Previous efforts have focused on using CO as an additional ligand to stabilize the hydride species. In this work, CO is replaced with isocyanide ligands, leading to the isolation of two different types of iron hydride complexes: (RPNHP)FeH(CNR′)(BH4) (R = iPr, R′ = 2,6-Me2C6H3, tBu; R = Cy, R′ = 2,6-Me2C6H3) and [(iPrPNHP)FeH(CNtBu)2]X (X = BPh4, Br, or a mixture of Br and BH4). The neutral iron hydrides are capable of catalyzing the hydrogenation of PhCO2CH2Ph to PhCH2OH, although the activity is lower than for (iPrPNHP)FeH(CO)(BH4). The cationic iron hydrides are active hydrogenation catalysts only for more reactive carbonyl substrates such as PhCHO, and only when the NH and FeH hydrogens are syn to each other. The cationic species and their synthetic precursors [(iPrPNHP)FeBr(CNtBu)2]X (X = BPh4, Br) can have different configurations for the isocyanide ligands (cis or trans) and the H-N-Fe-H(Br) unit (syn or anti). Unlike tetraphenylborate, the bromide counterion participates in a hydrogen-bonding interaction with the NH group, which influences the relative stability of the cis,anti and cis,syn isomers. These structural differences have been elucidated by X-ray crystallography, and the geometric isomerization processes have been studied by NMR spectroscopy.

Full text of DOI:10.1021/acs.inorgchem.1c00328

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Experimental Evidence of syn H−N−Fe−H Configurational
Requirement for Iron-Based Bifunctional Hydrogenation Catalysts
Huiguang Dai, Weishi Li, Jeanette A. Krause, and Hairong Guan*
Cite This: Inorg. Chem. 2021, 60, 6521−6535
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ABSTRACT: Iron hydrides supported by a pincer ligand of the type HN(CH2CH2PR2)2
(RPNHP) are versatile hydrogenation catalysts. Previous efforts have focused on using CO
as an additional ligand to stabilize the hydride species. In this work, CO is replaced with
isocyanide ligands, leading to the isolation of two different types of iron hydride
i
t
complexes: (RPNHP)FeH(CNR′)(BH4) (R = Pr, R′ = 2,6-Me2C6H3, Bu; R = Cy, R′ =
2,6-Me2C6H3) and [(iPrPNHP)FeH(CNtBu)2]X (X = BPh4, Br, or a mixture of Br and
BH4). The neutral iron hydrides are capable of catalyzing the hydrogenation of
PhCO2CH2Ph to PhCH2OH, although the activity is lower than for (iPrPNHP)FeH-
(CO)(BH4). The cationic iron hydrides are active hydrogenation catalysts only for more reactive carbonyl substrates such as
PhCHO, and only when the NH and FeH hydrogens are syn to each other. The cationic species and their synthetic precursors
[(iPrPNHP)FeBr(CNtBu)2]X (X = BPh4, Br) can have different configurations for the isocyanide ligands (cis or trans) and the H−
N−Fe−H(Br) unit (syn or anti). Unlike tetraphenylborate, the bromide counterion participates in a hydrogen-bonding interaction
with the NH group, which influences the relative stability of the cis,anti and cis,syn isomers. These structural differences have been
elucidated by X-ray crystallography, and the geometric isomerization processes have been studied by NMR spectroscopy.
hydrogenation product).17a,18 In this mechanism, the NH group
is chemically noninnocent, as it participates in bond-forming
iron-based homogeneous hydrogenation catalysts,1 largely
and bond-breaking processes. While PNP-ligated iron amido
INTRODUCTION
Recent years have witnessed an upsurge of interest in developing
■
complexes have been isolated and used to activate dihydro-
gen,9b,19 computational work by Dub17,20 and others21 focusing
on group 8 metal-based hydrogenation catalysts suggest that the
energetically more favorable pathway involves a zwitterionic
species or an ion-pair intermediate resulting from hydride
transfer (Scheme 1, Catalytic Cycle B). This intermediate can
activate dihydrogen without breaking the N−H bond. In this
mechanism, the NH group is chemically innocent but
cooperative, as it plays an important role of stabilizing key
transition states through hydrogen-bonding interactions. The
presence of alcohol molecules (as the solvent or generated from
the hydrogenation reaction) can provide additional stabilization
effects through a more elaborate hydrogen-bonding network.22
The most commonly used method to experimentally probe the
effect of the NH group is to convert it into an N-alkyl group. In a
number of cases,5d,g,9c,e,g,h the modified iron catalysts remain
active, suggesting that hydrogenation steps can take place solely
at the metal site without the involvement of the NH
functionality.23 Loss of catalytic activity after alkylation of the
sparked by the fact that iron is an earth-abundant and
inexpensive metal. As far as the hydrogenation of CO
bonds is concerned, this research field started to attract attention
̈
in the late 2000s, when the Knolker complex (a hydroxycyclo-
pentadienyl iron dicarbonyl hydride)2 was demonstrated as the
first well-defined iron catalyst for aldehyde/ketone hydro-
genation,3 and diiminodiphosphine- and diaminodiphosphine-
ligated iron complexes were established as the first iron-based
catalytic systems for the asymmetric hydrogenation of ketones.4
Since these early reports, a variety of iron complexes, especially
those supported by a pincer ligand, have been developed for
improved catalytic efficiency5,6 and/or enantioselectivity.7,8 In
the meantime, considerable efforts have been made to apply the
iron-based hydrogenation strategy to the reduction of other
carbonyl-containing substrates such as CO2/bicarbonates/
carbonates,9,10 esters,11,12 amides,13 and oxamides.14
Diphosphines bearing a central NH group that can bind
metals through the [PNP] donor set are among the most
successful ligands in designing iron catalysts for hydrogenation
reactions.1i,j,15 It is generally believed that an iron hydride
bearing an H−N−Fe−H unit is the catalytically active species.
In the Noyori-type metal−ligand bifunctional mechanism
(Scheme 1, Catalytic Cycle A),16 a concerted H−/H+ transfer17
from the iron hydride to a carbonyl substrate would produce an
iron amido complex. The catalytic cycle can be completed by a
subsequent dihydrogen activation, which is often proposed to be
facilitated by water (impurity) or an alcohol (solvent or the
Received: February 1, 2021
Published: April 22, 2021
https://doi.org/10.1021/acs.inorgchem.1c00328
© 2021 American Chemical Society
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Scheme 1. Proposed Catalytic Cycles for the Hydrogenation
of Carbonyl Substrates Catalyzed by H−N−Fe−H Type
Complexes
Scheme 2. Original Ester Hydrogenation Catalysts and the
Isocyanide Derivatives
state.26 During the synthesis of the corresponding hydrido
borohydride complexes, we encountered cationic bis-
(isocyanide) complexes in which the lone hydride ligand can
be syn or anti to the NH hydrogen (Scheme 2). These
configurational isomers are separable, providing a rare
opportunity to study the structure−reactivity relationship in
the context of metal−ligand bifunctional catalysis. The key
finding, along with the catalytic study of the neutral hydrido
borohydride complexes (benchmarked to the original catalysts),
is reported in this paper.
RESULTS AND DISCUSSION
■
Synthesis of Iron Hydrido Borohydride Complexes. In
studying the catalytic hydrogenation of CO2 to formate,
Bernskoetter, Hazari, and co-workers developed a two-step
synthesis of (iPrPNHP)FeH(CNAr)(BH4) (1, Ar = 2,6-
dimethylphenyl) starting from the PNP pincer ligand27
HN(CH2CH2PiPr2)2 (abbreviated as iPrPNHP) and FeCl2
(Scheme 3).9f To ensure that only one molecule of the
isocyanide ligand binds to iron, slow addition of a dilute ligand
solution to the in situ generated (iPrPNHP)FeCl2 was needed. We
modified the procedure slightly by cooling the reaction mixture
to 0 °C prior to dropwise addition of the isocyanide and
performing the subsequent reaction with NaBH4 in the same
reaction vessel. This one-pot protocol was convenient and was
also effective for incorporating only 1 equiv of an alkyl
isocyanide such as tert-butyl isocyanide (CNtBu), which
previously was reported to be problematic.9f With the new
method, (iPrPNHP)FeH(CNtBu)(BH4) (2) and the cyclohexyl
analogue of 1, (CyPNHP)FeH(CNAr)(BH4) (3), were isolated
as yellow powders in 46% and 45% overall yields, respectively.
The presence of hydride ligands in 2 and 3 was confirmed by
NH group, on the other hand, is often interpreted as proof for
the Noyori-type metal−ligand bifunctional mechanism
(Scheme 1, Catalytic Cycle A).11b,19a,24 A more contemporary
mechanistic view (Scheme 1, Catalytic Cycle B) attributes the
drop in catalytic activity to two factors: (1) the removal of the
hydrogen-bonding interactions that are critical to stabilizing the
key transition states and (2) increased steric crowding near the
hydride.23a One could also argue that changing NH to N-alkyl
could simply alter the electronic and steric properties of the
ligand, resulting in an inactive catalyst, even when the
hydrogenation mechanism does not involve the NH function-
ality. Furthermore, the presence of an H−N−M−H unit does
not necessarily require the NH group to be cooperative,
especially when the H−N−M−H dihedral angle is relatively
wide (e.g. ∼60°).25 Alkylation of these catalysts could be
beneficial due to improved catalyst stability.23a Nevertheless, if
hydrogen-bonding interactions with the NH group are indeed
required during the iron-catalyzed hydrogenation reactions, the
two hydrogens in the H−N−Fe−H unit must be spatially
arranged such that the NH functionality is within the reach of
the oxygen atom after hydride transfer. It is thus important to
study the correlation between the catalytic activity and H−N−
Fe−H type complexes with different hydrogen configurations.
We have been interested in developing iron-based catalysts for
the hydrogenation of esters, including the industrially relevant
fatty acid methyl esters.11d In 2014, the Beller group11b,e and our
group11c independently reported that HN(CH2CH2PR2)2-
ligated iron hydrido borohydride complexes (Scheme 2) were
effective in catalyzing ester hydrogenation. In attempts to
improve the catalytic system, we set out to replace CO in the
original catalysts with isocyanides, considering that such ligands
are tunable and, like CO, can force the metal to adopt a low-spin
1
NMR and IR spectroscopy. The H NMR spectra (in C6D6)
feature a triplet in the upfield region (2, −21.80 ppm, JP−H = 52.7
Hz; 3, −21.23 ppm, JP−H = 52.8 Hz) attributed to the terminal
hydride on iron and a very broad signal centered around −3.06
ppm (integrates to 4H), which can be assigned to the BH4
resonance. The IR spectra of the solid samples display an Fe−H
stretching vibration (2, 1782 cm−1; 3, 1835 cm−1) and multiple
B−H stretching vibrations (2, 2339, 2327, 2296 cm−1; 3, 2341,
2325, 2032 cm−1). The frequencies and the number of B−H
stretching bands support a monodentate BH4 ligand28 with the
possibility of forming an intramolecular dihydrogen bond with
the NH group (i.e., N−Hδ+···δ−H−B).29 As is expected for
isocyanide complexes, the IR spectra also show a very intense
CN stretching vibration (2, 2000 cm−1; 3, 1973 cm−1).
Overall, these diagnostic NMR and IR data are similar to those
reported for 19f as well as those for the analogous carbonyl
complexes (RPNHP)FeH(CO)(BH4).7h,11c,e,13b,c,30
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Scheme 3. Synthetic Route to the Neutral Iron Hydrido Borohydride Complexes
Figure 1. ORTEP drawings of one of the independent molecules of (iPrPNHP)FeH(CNAr)(BH4) (1, left), (iPrPNHP)FeH(CNtBu)(BH4) (2, center),
and (CyPNHP)FeH(CNAr)(BH4) (3, right) at the 50% probability level. All hydrogen atoms except the those bound to nitrogen, boron, and iron are
omitted for clarity.31
The structures of the iron hydrido borohydride complexes,
including the previously known compound 1, were further
studied by X-ray crystallography. As illustrated in Figure 1, iron
is situated in an octahedral coordination environment with a
monodentate BH4 ligand trans to a hydride. Of the three
terminal hydrogens bound to boron, two have close contacts
with the NH hydrogen (Hδ+···δ−H distance: 2.11−2.40 Å). This
phenomenon was observed previously for (iPrPNHP)FeH(CO)-
(BH4) and explained by a bifurcated intramolecular dihydrogen
bond.30b Like (iPrPNHP)FeH(CO)(BH4), 1 and 2 also pack in
the crystal lattice with a pair of molecules linked by
intermolecular N−Hδ+···δ−H−B dihydrogen bonds (Hδ+···δ−H
distance: 2.40−2.79 Å). In contrast, 3 shows a completely
different 3-D network without these types of Coulombic
interactions, presumably because of the steric bulk of the
cyclohexyl groups. A comparison of the key bond distances and
angles suggests minimal perturbation of the structure stemming
from the isocyanide ligand and phosphorus substituents. The
only marked difference is the isocyanide CN−C angle, which
shows a greater deviation from linearity for the tert-butyl
isocyanide complex (2, 165.76(11)°) in comparison to the aryl
isocyanide derivatives (1, 171.3(4), 173.7(4)°; 3, 170.0(3)°).
Synthesis of Cationic Iron Bis(isocyanide) Bromide
Complexes. Alkyl isocyanides have the tendency to displace a
chloride ligand in (iPrPNHP)FeCl2 to form cationic bis-
(isocyanide) complexes, especially when the reaction is carried
out at room temperature. The study by Bernskoetter, Hazari,
and co-workers showed that mixing iPrPNHP, FeCl2, and CNiPr
in a 1:1:1 ratio produced [(iPrPNHP)FeCl(CNiPr)2]2[FeCl4].9f
A similar result was obtained during our initial effort to make
(iPrPNHP)FeBr2(CNtBu) from iPrPNHP, FeBr2, and CNtBu.
Doubling the amount of CNtBu, however, led to the isolation of
a green powder identified as a 45:55 mixture32,33 of trans and
cis,anti isomers of [(iPrPNHP)FeBr(CNtBu)2]Br (4) with a
combined yield of 78% (Scheme 4). Attempts to separate the
Scheme 4. Synthesis of [(iPrPNHP)FeBr(CNtBu)2]Br (4)
two isomers via recrystallization were unsuccessful. The trans
isomer is a kinetic product. At room temperature, the ratio of the
two isomers remained unchanged over a prolonged period of
time. However, when the isomeric mixture in toluene was kept at
100 °C for 1 h, a complete isomerization of trans-4 to cis,anti-4
was observed, which allowed pure cis,anti-4 to be isolated in 74%
yield as a yellow powder. The cis,anti isomer is thermodynami-
cally more stable, presumably because the two strongly trans
influencing isocyanide ligands can avoid being trans to each
other.
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The bromide counterion in both isomers forms a hydrogen
bond with the NH group. For the isomeric mixture dissolved in
C6D6, the NH resonances of trans-4 and cis,anti-4 appear at 5.23
and 7.80 ppm, respectively, consistent with N−H···Br bonding
interactions.34 The presence of a hydrogen bond is further
supported by a very broad N−H stretching band (∼3350 cm−1)
found in the IR spectra and, in the case of cis,anti-4, confirmed by
X-ray crystallography (Figure 2). The crystallographic study also
The hydrogen bond can be disrupted by performing an anion
exchange reaction with NaBPh4 (Scheme 5), a process that does
not appear to erode the stereochemistry. Thus, cis,anti-4 was
cleanly converted to cis,anti-[(iPrPNHP)FeBr(CNtBu)2]BPh4
(cis,anti-5), and a 42:58 mixture of trans-4 and cis,anti-4 was
converted to a mixture of trans-5 and cis,anti-5 and isolated in a
47:53 ratio as determined by NMR. The use of tetraphenylbo-
rate as the counterion made the cationic complexes crystallize
more readily. When the mixture of trans-5 and cis,anti-5 in THF
(a green solution) layered with EtOH was kept at −30 °C, blue
crystals formed within 1 day. Though the recovery yield was low
(15% based on the total mass), the isolated blue compound was
predominantly trans-5 (97% purity).
For a crystallographic study, the mixture of trans-5 and
cis,anti-5 was first dissolved in MeCN and then layered with
EtOH. Both blue−purple and yellow−orange dichroic crystals
were obtained from this sample, indicating that the green
solution is a mixture of blue and yellow. The blue−purple
dichroic crystals were proven to be trans-5, with the cation being
shown in Figure 3. Interestingly, the yellow−orange dichroic
crystals were shown to be an acetonitrile complex, cis-
[(iPrPNHP)Fe(NCMe)(CNtBu)2][BPh4]2 (cis-6) (see the
Supporting Information for details), which likely resulted from
ligand substitution with cis,anti-5. The bromide in trans-5 is
clearly less labile, consistent with a weaker trans effect of the
opposing ligand (i.e. the NH group). With the isomeric mixture
as the starting material, single crystals were also grown from
THF−Et2O to avoid the complication of solvent coordination.
The resulting blue−purple and yellow−orange dichroic crystals
were identified as trans-5 and cis,anti-5, respectively (Figure 3).
The Fe−Br bond in cis,anti -5 (2.5227(4) Å) is longer than
that in trans-5 (2.4956(6) Å), as determined by X-ray
crystallography. Similarly, the trans-effect argument can be
used to explain a comparably long Fe−N bond observed for
cis,anti-5 (2.0837(19) Å vs 2.064(3) Å in trans-5) and two
elongated iron−isocyanide bonds noted for trans-5 (1.856(4)
and 1.885(4) Å vs 1.817(2) and 1.834(3) Å in cis,anti-5). The
remaining metrical details are virtually indistinguishable
between the two isomers. Comparing cis,anti-4 with cis,anti-5
suggests that the hydrogen bond has little effect on the bonding
environment about the iron, only causing a minor 0.01−0.03 Å
contraction of the Fe−N and Fe−Br bond distances.
Figure 2. ORTEP drawing of cis,anti-[(iPrPNHP)FeBr(CNtBu)2]Br·1/
2C7H8 (cis,anti-4·1/2C7H8) at the 50% probability level. A cocrystal-
lized half-molecule of toluene and all hydrogen atoms except that
bound to nitrogen are omitted for clarity.36
establishes the anti configuration of the NH hydrogen in relation
to the covalently bonded bromide. This anti configuration is
similarly favored in [(iPrPNHP)FeCl(CNAr)2]Cl9f and-
[(PhPNHP)RuCl(NCMe)(NHC)]Cl (NHC = 1,3-dimethylimi-
dazol-2-ylidene),35 presumably to minimize the electrostatic
repulsion between the two halides.
Scheme 5. Synthesis of [(iPrPNHP)FeBr(CNtBu)2]BPh4 (5) as the Pure cis,anti or trans Isomer
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Figure 3. ORTEP drawings of the cations in trans-[(iPrPNHP)FeBr(CNtBu)2]BPh4·MeCN (trans-5·MeCN, left) and cis,anti-[(iPrPNHP)FeBr-
(CNtBu)2]BPh4·2THF (cis,anti-5·2THF, right) at the 50% probability level. All hydrogen atoms except that bound to nitrogen are omitted for clarity.
The two isomers of 5 can be readily differentiated by NMR
spectroscopy. In particular, the NH resonance of trans-5 (in
CD2Cl2) is significantly shielded, displaying a triplet at −0.52
ppm (J = 7.6 Hz). The corresponding resonance in cis,anti-5 is
located at 2.39 ppm also as a triplet (J = 12.0 Hz). It is not
surprising that these chemical shift values are much lower than
those for trans-4 and cis,anti-4, in which an N−H···Br hydrogen
bond is involved. The 31P{1H} NMR spectra show that trans-5
(56.7 ppm) differs from cis,anti-5 (68.7 ppm) by 12 ppm, similar
to the trend observed with trans-4 (59.1 ppm) and cis,anti-4
(71.2 ppm). The lack of a hydrogen bond in trans-5 and cis,anti-
5 is further confirmed by relatively sharp N−H stretching bands
at 3235 and 3229 cm−1, respectively. trans-5 exhibits only one
very intense CN stretching band at 2102 cm−1, although its
NMR spectra support the presence of two inequivalent
isocyanide ligands. In contrast, cis,anti-5 has two distinct IR
bands at 2134 and 2086 cm−1 attributed to the CN stretch.
Given the isomerization from trans-4 to cis,anti-4 (Scheme 4),
we anticipated that trans-5 would be thermodynamically less
stable than cis,anti-5. The geometric isomerization process was
studied in chlorobenzene (Scheme 6) due to solubility and
boiling point considerations. Heating a 47:53 mixture of trans-5
and cis,anti-5 at 100 °C for 18 h led to the disappearance of trans-
5 but surprisingly yielded a third isomer along with cis,anti-5 in a
91:9 ratio. An identical result was obtained when pure trans-5 or
cis,anti-5 was employed. This new isomer, recrystallized from
THF−Et2O, was identified by X-ray crystallography as cis,syn-5
(Figure 4). Under the isomerization conditions (100 °C, 18 h),
pure cis,syn-5 can revert back to the 91:9 mixture of cis,syn-5 and
cis,anti-5, suggesting that this ratio reflects the thermodynamic
stability of these two isomers. Without the hydrogen-bonded
bromide, cis,syn-5 becomes the most stable isomer, likely due to
a weak but favorable dipole−dipole interaction between Nδ−−
Scheme 6. Geometric Isomerization of
[(iPrPNHP)FeBr(CNtBu)2]BPh4 (5)
donation from metal to the isocyanide π* orbital,38 although the
CN stretching bands of cis,syn-5 (2129 and 2090 cm−1) do
not necessarily appear at wavenumbers lower than those of
cis,anti-5 (2134 and 2086 cm−1). For cationic complexes, the
differences in νCN values likely reflect electrostatic effects
rather than the extent of back-donation.39 The IR spectrum of
cis,syn-5 also shows an N−H stretching band at 3198 cm−1,
which is 31 and 37 cm−1 lower than for cis,anti-5 and trans-5,
respectively, suggesting a weaker N−H bond. The NH
resonance of cis,syn-5 (in CD2Cl2) cannot be precisely located
due to overlap with the NCH2 resonances but is definitely in the
more downfield region (3.33−2.90 ppm) in comparison to
cis,anti-5 and trans-5. The phosphorus resonance of cis,syn-5
(71.2 ppm) is distinguishable from those of cis,anti-5 (68.7 ppm)
and trans-5 (56.7 ppm) and is shifted downfield the most among
the three isomers.
H
δ+ and Feδ+−Brδ−.
In comparison to cis,anti-5, cis,syn-5 features a shorter Fe−N
bond (2.0597(14) vs 2.0837(19) Å) and a shorter Fe−Br bond
(2.4993(3) vs 2.5227(4) Å), possibly a consequence of the
aforementioned dipole−dipole interaction. The CN−C angle
of the CNtBu ligand trans to the NH group is bent slightly
(169.94(18)°) compared to the other CNtBu ligands for the
three isomers reported here (175.9(3)-177.8(4)°). The bent
CN−C angle is often an indication of increased back-
Synthesis of Cationic Iron Bis(isocyanide) Hydride
Complexes. To synthesize the iron hydride complexes, a 45:55
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which proved to be a 32:68 mixture of cis,anti-8 and cis,syn-8
(Scheme 8). Recrystallization of this mixture from THF−EtOH
provided cis,syn-8 as a white solid with 98% purity.
Scheme 8. Synthesis of cis,syn-
[(iPrPNHP)FeH(CNtBu)2]BPh4 (cis,syn-8)
Figure 4. ORTEP drawing of the cation in cis,syn-[(iPrPNHP)FeBr-
(CNtBu)2]BPh4 (cis,syn-5) at the 50% probability level. All hydrogen
atoms except that bound to nitrogen are omitted for clarity.37
Single crystals of cis,anti-8 and cis,syn-8 were grown separately
by starting with the purified isomers. Both nitrogen- and iron-
bound hydrogens were located directly from the difference map,
confirming the proposed stereochemistry (Figure 5). A
structural comparison shows that the Fe−N bond is shorter in
cis,syn-8 (2.0837(13) Å) than in cis,anti-8 (2.104(3) Å),
analogous to the trend seen with the bromide complexes
cis,syn-5 and cis,anti-5. Similarly, cis,syn-8 adopts a more bent
CN−C angle for the CNtBu ligand trans to the NH group
(170.45(7)°) relative to others in this series. Replacing the
bromide with a hydride (isomers 5 → isomers 8) results in two
major structural changes: (1) the Fe−P bonds are shortened by
∼0.07 Å, and (2) the two Fe−C bond distances in each isomer
differ by ∼0.05 Å due to the trans effect (the Fe−C bond trans to
the hydride is longer).
mixture of trans-4 and cis,anti-4 was treated with 5 equiv of
NaBH4 in EtOH and then heated at 50 °C for 16 h (Scheme 7).
Scheme 7. Synthesis of cis,anti-
[(iPrPNHP)FeH(CNtBu)2]BPh4 (cis,anti-8)
Unlike cis,anti-7, both cis,anti-8 and cis,syn-8 dissolve poorly in
C6D6; therefore, CD3CN was used for the NMR analysis. The
1H NMR spectrum of cis,anti-8 shows a triplet at −10.28 ppm
(JP−H = 51.6 Hz) characteristic of a hydride and a multiplet at
3.08−2.96 ppm consistent with an NH resonance devoid of a
hydrogen-bonding interaction. The hydride resonance of cis,syn-
8 is also a triplet but is shifted to −9.75 ppm (JP−H = 53.2 Hz).
The corresponding NH resonance is overlapped with other
resonances in the 2.88−2.70 ppm range. The 31P chemical shift
values for cis,anti-8 (100.7 ppm) and cis,syn-8 (103.4 ppm)
follow a trend similar to that for cis,anti-5 (68.7 ppm) and cis,syn-
5 (71.2 ppm). It is worth noting that these phosphorus NMR
signals are insensitive to the solvent employed and are
particularly useful for analyzing the isomeric ratio.
At room temperature, cis,anti-8 and cis,syn-8 in their pure form
are configurationally stable. cis,syn-8 is likely thermodynamically
more stable than cis,anti-8 because of the favorable dipole−
dipole interaction between Nδ−−Hδ+ and Feδ+−Hδ−. As in the
case of 4 and 5, the relative stability can be reversed when a
hydrogen-bonded bromide is involved. The reaction of trans-4
and cis,anti-4 with NaBH4 was carried out in EtOH (or analyzed
in CD3OD) to determine whether the disruption of the N−H···
Br hydrogen bond by an alcoholic solvent would result in the
cis,syn isomer as the major product, and indeed it does. Rapid
precipitation induced by the addition of NaBPh4 essentially
takes a snapshot of the isomeric ratio (Scheme 8). Removal of
EtOH from the solution containing the initial products should
restore the N−H···Br hydrogen bond, leading to the isolation of
the cis,anti isomer as long as a nonalcoholic solvent such as
toluene is used for extraction (Scheme 7). However, this still
Subsequent removal of the solvent followed by extraction with
toluene led to the isolation of cis,anti-[(iPrPNHP)FeH-
(CNtBu)2]X (cis,anti-7, X = mixed anion) in 65% yield. On
the basis of 1H NMR and elemental analysis, the cation is paired
−
with 67% Br− and 33% BH4 . Once again, the bromide must be
hydrogen-bonded to the NH group, as suggested by a downfield-
shifted NH resonance (6.40 ppm, in C6D6). The borohydride is
expected to have a weaker, if any, interaction with the NH group.
The BH4 resonance appears as a sharp quartet at 0.97 ppm (JB−H
= 81.3 Hz), consistent with a high symmetry for the quadrupolar
nucleus 11B. The observation of two B−H stretching bands (for
a solid sample) at 2285 and 2214 cm−1 also implies no significant
28c
−
elongation or contraction of B−H bonds from the free BH4 .
Both Br− and BH4 in cis,anti-7 can be replaced by BPh4
following an anion exchange reaction with NaBPh4, allowing
cis,anti-[(iPrPNHP)FeH(CNtBu)2]BPh4 (cis,anti-8) to be iso-
lated as a white powder.
−
−
The toluene extraction step (i.e., step (2) in Scheme 7) is key
to obtaining a pure cis,anti isomer.40 The residue left after EtOH
evaporation was redissolved in CD3OD and analyzed as a 28:72
mixture, favoring a new isomer. This result suggests that
isomerization occurred during the workup. In fact, adding
NaBPh4 directly to the reaction mixture in EtOH (after the
NaBH4 reaction was complete) yielded a white precipitate,
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Figure 5. ORTEP drawings of the cations in cis,anti-[(iPrPNHP)FeH(CNtBu)2]BPh4 (cis,anti-8, left) and cis,syn-[(iPrPNHP)FeH(CNtBu)2]BPh4
(cis,syn-8, right) at the 50% probability level. All hydrogen atoms except those bound to nitrogen and iron are omitted for clarity.
requires a cis,syn to cis,anti isomerization process. In principle,
the NH group could dissociate from iron, followed by an
inversion of the nitrogen lone pair and recoordination to iron to
complete the isomerization. We favor an alternative mechanism
involving deprotonation/reprotonation of the NH group,41
which may be promoted by NaBH4 or its alcoholysis products.
To test this hypothesis, a stoichiometric amount of NaBH4 was
used to convert the mixture of trans-4 and cis,anti-4 to an iron
hydride (Scheme 9), which after toluene extraction was isolated
Scheme 9. Effect of Excess NaBH4 on the Isomerization
Process
Figure 6. ORTEP drawing of one of the two independent molecules of
cis,anti-[(iPrPNHP)FeH(CNtBu)2]Br·1/2H2O (cis,anti-7′·1/2H2O) at
the 50% probability level. The cocrystallized water molecule and all
hydrogen atoms except those bound to nitrogen and iron are omitted
as a 29:71 isomeric mixture of [(iPrPNHP)FeH(CNtBu)2]Br
(7′) favoring the cis,syn isomer. In contrast, using 1.2 equiv of
NaBH4 led to the isolation of only cis,anti-7′; the structure was
confirmed by X-ray crystallography (Figure 6). These results
support the idea that excess NaBH4 plays a crucial role in
facilitating the isomerization process.
Catalytic Studies. The neutral hydrido borohydride
complexes 1−3 and the cationic bis(isocyanide) hydride
complexes cis,anti-7, cis,anti-7′, cis,anti-8, and cis,syn-8 were
tested as catalysts for the hydrogenation of PhCO2CH2Ph
(Table 1). For comparison purposes, the catalytic conditions
were deliberately chosen so that in THF our original catalyst
(iPrPNHP)FeH(CO)(BH4) would hydrogenate only ∼50% of
the substrate (entry 1). Evidently, the neutral isocyanide-ligated
iron hydrides 1−3 (entries 4, 7, and 8) are less active than the
carbonyl derivative, reducing only 10−13% of PhCO2CH2Ph to
for clarity.42
PhCH2OH. These results are in alignment with previous studies
of iron-catalyzed hydrogenation of CO2 to formate, where
(iPrPNP)FeH(CNAr) (iPrPNP= [N(CH2CH2PiPr2)2]−)9f and
(
i P r PNM e P)FeH(CNR)(BH4 ) (i P r PNM e P= MeN-
(CH2CH2PiPr2)2)9g are also outperformed by the correspond-
ing CO-based catalysts. In the presence of 20 mol % of KOtBu,
the iron catalysts (iPrPNHP)FeH(CO)(BH4) and 1 are less
effective, giving PhCH2OH with a lower yield (entries 2 and 5).
A similar observation has been made by Beller and co-workers in
their study of the hydrogenation of methyl benzoate catalyzed by
(iPrPNHP)FeH(CO)(BH4).11b In EtOH, (iPrPNHP)FeH(CO)-
(BH4) and 1 are also less efficient as ester hydrogenation
catalysts, in part due to the competing transesterification of
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a
Table 1. Hydrogenation of Benzyl Benzoate Catalyzed by the Iron Hydrides
a
Standard conditions: PhCO2CH2Ph (1.0 mmol), PhCH2CH2Ph (0.50 mmol, internal standard), and an iron catalyst (0.020 mmol) in 0.5 mL of
solvent; 1 = (iPrPNHP)FeH(CNAr)(BH4) (Ar = 2,6-dimethylphenyl), 2 = (iPrPNHP)FeH(CNtBu)(BH4), 3 = (CyPNHP)FeH(CNAr)(BH4),
cis,anti-7 = cis,anti-[(iPrPNHP)FeH(CNtBu)2]X (X = 67% Br and 33% BH4), cis,anti-7′ = cis,anti-[(iPrPNHP)FeH(CNtBu)2]Br, cis,anti-8 = cis,anti-
b
c
[(iPrPNHP)FeH(CNtBu)2]BPh4, and cis,syn-8 = cis,syn-[(iPrPNHP)FeH(CNtBu)2]BPh4. Determined by 1H NMR spectroscopy. Values in
parentheses are the yields of PhCH2OH resulting from transesterification.
PhCO2CH2Ph to PhCO2Et (entries 3 and 6). In fact, with 1 in
EtOH, the hydrogenation process is almost completely
suppressed.
Scheme 10. Distinct Catalytic Activity Displayed by the Two
Isomers
In THF, the cationic iron hydrides prove to be inactive
catalysts for ester hydrogenation (Table 1, entries 9, 10, 13, 16,
and 17), possibly due to reduced Fe−H hydricity rendered by
the positive charge of the complex. The addition of a strong base
could potentially deprotonate the NH group while it converts
the cationic iron hydrides to the neutral hydride (iPrPNP)FeH-
(CNtBu)2, which is expected to be more hydridic. Furthermore,
studies of neutral ruthenium complexes have shown that an NH
to NK conversion through the addition of KOtBu can accelerate
the hydrogenation of carbonyl substrates, where the NK moiety
can lower the activation barriers for the key steps.20b,43 Our
results (entries 11 and 18) show that, with 20 mol % of KOtBu as
the base additive, cis,anti-7′ and 8 (a 32:68 mixture of cis,anti and
cis,syn isomers) exhibit some catalytic activity for the hydro-
genation of PhCO2CH2Ph to PhCH2OH, although the yield is
very low (3−4%). Switching the solvent from THF to EtOH
does not improve the hydrogenation reaction, only leading to
transesterification of PhCO2CH2Ph to PhCO2Et (entries 12, 14,
and 19). Adding 20 mol % of NaOEt to cis,anti-8 (or the cis,anti
and cis,syn mixture) increases the yield of PhCH2OH to 86%
(entry 15 or 20); however, only a small fraction (12%) stems
from the hydrogenation process.
providing strong evidence that an active hydrogenation catalyst
requires a syn configuration for the H−N−Fe−H unit.
For further mechanistic elucidation, stoichiometric reactions
between the cationic hydrides and PhCHO were investigated at
room temperature (Scheme 11). Over a period of 48 h, cis,anti-8
failed to react with PhCHO, as did cis,anti-7 containing the
mixed counterions. The lack of reactivity could be due to
thermodynamics, although the lack of H/D exchange between
cis,anti-8 and PhCDO suggests that the H− transfer step alone is
kinetically unfavorable. Conversely, cis,syn-8 reacted with
PhCHO within hours, likely reversibly, to yield a complicated
mixture (at least two benzyloxy-containing species on the basis
of the 1H NMR spectrum and at least three iron complexes on
the basis of the 31P{1H} NMR spectrum). Full consumption of
cis,syn-8 took approximately 1 week, at which point the major
benzyloxy-containing product was identified as PhCH2OH (δH
4.57 ppm)44 and the major iron complex displayed a phosphorus
resonance at 80.8 ppm. Attempts to isolate the intermediates
and products through crystallization were fruitless. We propose
that PhCHO undergoes carbonyl reduction with cis,syn-8 to
form a transient alkoxide complex, which reacts slowly with
CD3CN to give [(iPrPNP)Fe(NCCD3)(CNtBu)2]BPh4
(Scheme 11). The kinetic barrier for H− transfer is lowered
with cis,syn-8, presumably due to activation of the carbonyl
group and stabilization of PhCH2O− by the syn NH hydrogen.
To differentiate between the reactivity of cis,anti-8 and cis,syn-
8, catalytic hydrogenation of PhCHO was studied at a
temperature (60 °C) where the geometric isomerization was
insignificant. Under the conditions outlined in Scheme 10, the
cis,anti isomer shows no catalytic activity at all. In contrast, the
cis,syn isomer converts 45% of the PhCHO to PhCH2OH,
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Scheme 11. Stoichiometric Reactions of cis,anti-8 and cis,syn-8 with Benzaldehyde
a
Scheme 12. Mechanistic Reasoning for the Reduction of PhCHO with the cis,anti and cis,syn Isomers
a
The BPh4 counterion is omitted for clarity.
the two, and the positive charge can be balanced with
tetraphenylborate, bromide, or a mixture of bromide and
borohydride. The relative stabilities of the cis,anti and cis,syn
isomers depend on whether or not the counterion forms a
hydrogen bond with the NH group. These cationic hydride
complexes are virtually inactive catalysts for ester hydro-
genation, likely due to reduced hydricity. However, cis,syn-
[(iPrPNHP)FeH(CNtBu)2]BPh4 can catalyze the hydrogenation
of PhCHO, whereas the cis,anti isomer cannot. This result is
consistent with the current mechanistic understanding of CO
CONCLUSIONS
■
In this work, we have synthesized and characterized a number of
iron hydride complexes, each featuring a PNP pincer ligand and
at least one isocyanide ligand. The neutral hydrido borohydride
complexes (RPNHP)FeH(CNR′)(BH4) catalyze the hydro-
genation of PhCO2CH2Ph to PhCH2OH, though less efficiently
than the carbonyl derivative (iPrPNHP)FeH(CO)(BH4). The
cationic bis(isocyanide) hydride [(iPrPNHP)FeH(CNtBu)2]+
can be obtained as a cis,anti or cis,syn isomer or a mixture of
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Synthesis of (CyPNHP)FeH(CNAr)(BH4) (3; Ar = 2,6-Dimethyl-
phenyl). Following the same procedure used for 2, hydride 3 was
isolated as a bright yellow powder in 45% yield (300 mg from a 1.0
mmol scale reaction). Yellow crystals suitable for an X-ray crystallo-
graphic analysis were grown from toluene. 1H NMR (400 MHz, C6D6,
δ): 6.86 (d, JH−H = 7.2 Hz, ArH, 2H), 6.74 (t, JH−H = 7.2 Hz, ArH, 1H),
3.96−3.84 (m, NH, 1H), 2.82−2.71 (m, 2H), 2.70−2.48 (m, 4H), 2.55
(s, ArCH3, 6H), 2.05−1.05 (m, 44H from CyH + 2H from the pincer
backbone), −3.06 (br, BH4, 4H), −21.23 (t, JH−P = 52.8 Hz, FeH, 1H).
13C{1H} NMR (101 MHz, C6D6, δ): 199.1−198.4 (m, CNAr), 133.4
bond hydrogenation catalyzed by H−N−M−H type complexes.
As illustrated in Scheme 12, the cis,syn isomer has the “right” NH
configuration that can stabilize the ion-pair intermediate
through hydrogen-bonding interactions, which in turn assist
the subsequent dihydrogen activation. The NH group may also
help activate the CO bond during hydride transfer. The
stabilization of the negative charge and the CO bond
activation are absent with the cis,anti isomer, which explains
its inertness toward PhCHO in both stochiometric and catalytic
reactions.
(s, ArC), 133.3 (s, ArC), 128.2 (s, ArC), 123.9 (s, ArC), 54.0 (t, JP−C
=
5.8 Hz, NCH2), 40.2 (t, JP−C = 7.9 Hz), 37.1 (t, JP−C = 12.8 Hz), 31.1 (s,
CyC), 30.8 (s, CyC), 29.4 (s, CyC), 28.5 (s, CyC), 28.2 (t, JP−C = 6.1
Hz), 27.9 (t, JP−C = 3.7 Hz), 27.8 (t, JP−C = 6.7 Hz), 27.5 (t, JP−C = 4.6
Hz), 27.3 (t, JP−C = 5.8 Hz), 27.03 (s, CyC), 26.95 (s, CyC), 19.0 (s,
CH3). 31P{1H} NMR (162 MHz, C6D6, δ): 90.2 (s). Selected ATR-IR
data (solid, cm−1): 3201 (νN−H), 2341 (νB−H,terminal), 2325
(νB−H,terminal), 2032 (νB−H,bridging), 1973 (νCN), 1835 (νFe−H).
Elemental analysis data were unsatisfactory despite repeated trials;
however, the data match the values calculated for an oxidized product.
Anal. Calcd for C37H67N2BP2Fe (3): C, 66.47; H, 10.10; N, 4.19. Calcd
for C37H67N2O2BP2Fe (3 + O2): C, 63.44; H, 9.64; N, 4.00. Found: C,
63.41; H, 9.38; N, 3.82.
EXPERIMENTAL SECTION
■
General Considerations. All compounds described in this paper
were prepared under an argon atmosphere using standard glovebox and
Schlenk techniques. Benzene-d6 and benzene were dried over sodium-
benzophenone and distilled under an argon atmosphere. Acetonitrile-d3
and methylene chloride-d2 were purchased from Cambridge Isotope
Laboratories, Inc., and used as received without further purification.
Ethanol, chlorobenzene, and acetonitrile were dried over 4 Å molecular
sieves and then deoxygenated by bubbling argon through them for 1 h.
All other dry and oxygen-free solvents used for synthesis and workup
(THF, diethyl ether, toluene, and pentane) were collected from an
Innovative Technology solvent purification system. Benzaldehyde was
freshly distilled prior to use. HN(CH2CH2PiPr2)2 (iPrPNHP),45
HN(CH2CH2PCy2)2 (CyPNHP),46 and (iPrPNHP)FeH(CNAr)(BH4)
(1, Ar = 2,6-dimethylphenyl)9f were prepared according to literature
procedures. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on
a Bruker Avance 400 MHz NMR spectrometer. Chemical shift values in
1H and 13C{1H} NMR spectra were referenced internally to the residual
solvent resonances. 31P{1H} NMR spectra were referenced externally to
85% H3PO4 (0 ppm). Infrared spectra were recorded on a PerkinElmer
Spectrum Two FT-IR spectrometer equipped with a smart orbit
diamond attenuated total reflectance (ATR) accessory. Experimental
details for the synthesis of 4 (trans and cis,anti mixture), cis,anti-4, 5
(trans and cis,anti mixture), trans-5, cis,anti-5, cis,syn-5, cis,anti-7,
cis,anti-7′, and 8 (cis,anti and cis,syn mixture) are provided in the
Supporting Information.
Synthesis of cis,anti-[(iPrPNHP)FeH(CNtBu)2]BPh4 (cis,anti-8).
In a glovebox, in a 100 mL oven-dried Schlenk flask equipped with a stir
bar were placed cis,anti-[(iPrPNHP)FeH(CNtBu)2]X (cis,anti-7, X =
67% Br and 33% BH4, 232 mg, 0.40 mmol) and NaBPh4 (220 mg, 0.64
mmol). The flask was then connected to a Schlenk line, after which 30
mL of ethanol was added. The resulting suspension was stirred at room
temperature for 1 h before a cannula filtration was performed. The
collected solid was dried under vacuum to afford the desired product as
a white powder (178 mg, 53% yield). Colorless crystals suitable for X-
ray crystallography were grown from CH3CN−EtOH. 1H NMR (400
MHz, CD3CN, δ): 7.31−7.24 (m, ArH, 8H), 6.99 (t, JH−H = 7.2 Hz,
ArH, 8H), 6.84 (t, JH−H = 7.2 Hz, ArH, 4H), 3.08−2.96 (m, NH, 1H),
2.87−2.72 (m, 2H), 2.46−2.36 (m, 2H), 2.34−2.25 (m, 2H), 2.20−
2.09 (m, 2H), 1.87−1.71 (m, 2H), 1.52−1.39 (m, 6H for PCH(CH3)2
+ 2H for CH or CH2), 1.47 (s, CNC(CH3)3, 9H), 1.38−1.30 (m,
PCH(CH3)2, 6H), 1.27−1.17 (m, PCH(CH3)2, 12H), 1.22 (s,
CNC(CH3)3, 9H), −10.28 (t, JP−H = 51.6 Hz, FeH, 1H). 13C{1H}
NMR (101 MHz, CD3CN, δ): 164.8 (q, JC−B = 49.5 Hz, ArC), 136.7 (q,
Synthesis of (iPrPNHP)FeH(CNtBu)(BH4) (2). Under an argon
atmosphere, in an oven-dried Schlenk flask equipped with a stir bar
were placed iPrPNHP (305 mg, 1.0 mmol), FeCl2 (127 mg, 1.0 mmol),
and 70 mL of THF. The resulting mixture was refluxed for 2 h, giving a
light yellow solution. The flask was then cooled to 0 °C in an ice−water
bath, after which a solution of tert-butyl isocyanide (113 μL, 1.0 mmol)
in 10 mL of THF was added dropwise. The green solution obtained was
slowly warmed to room temperature and further stirred for 1 h. A
solution of NaBH4 (189 mg, 5.0 mmol) in 60 mL of EtOH was added,
resulting in an intermediate color change from green to orange and
eventually to yellow. After 16 h, the volatiles were removed under
vacuum and the residue was treated with 60 mL of toluene. Filtration
through a short pad of Celite followed by removal of the solvent under
vacuum afforded the product as a bright yellow powder (210 mg, 46%
yield). Orange-yellow crystals suitable for an X-ray crystallographic
study were grown from toluene. 1H NMR (400 MHz, C6D6, δ): 3.72−
3.58 (m, NH, 1H), 2.97−2.77 (m, 2H), 2.63−2.44 (m, 2H), 2.26−2.10
(m, 2H), 1.82−1.70 (m, 2H), 1.69−1.56 (m, 6H for PCH(CH3)2 + 2H
for CH or CH2), 1.54−1.42 (m, 2H), 1.36−1.26 (m, PCH(CH3)2,6H),
1.25−1.16 (m, PCH(CH3)2,6H), 1.09−0.99 (m, PCH(CH3)2,6H),
1.06 (s, CNC(CH3)3, 9H), −3.06 (br, BH4, 4H), −21.80 (t, JH−P = 52.7
Hz, FeH, 1H). 13C{1H} (101 MHz, C6D6, δ): 184.0−183.3 (m,
CNC(CH3)3), 55.2 (s, CNC(CH3)3), 53.8 (t, JP−C = 5.7 Hz, NCH2),
31.1 (s, CNC(CH3)3), 29.5 (t, JP−C = 7.6 Hz), 29.2 (t, JP−C = 6.3 Hz),
26.0 (td, JP−C = 11.6 Hz, J = 2.7 Hz), 21.4 (s, PCH(CH3)2), 20.9 (t, JP−C
= 2.5 Hz, PCH(CH3)2), 19.3 (s, PCH(CH3)2), 18.9 (s, PCH(CH3)2).
31P{1H} NMR (162 MHz, C6D6, δ): 99.2 (s). Selected ATR-IR data
(solid, cm−1): 3206 (νN−H), 2339 (νB−H,terminal), 2327 (νB−H,terminal),
2296 (νB−H,terminal), 2000 (νCN), 1782 (νFe−H); νB−H,bridging could not
be definitively assigned. Anal. Calcd for C21H51N2BP2Fe: C, 54.80; H,
11.17; N, 6.09. Found: C, 54.53; H, 11.32; N, 5.93.
J
C−B = 1.2 Hz, ArC), 126.6 (q, JC−B = 2.7 Hz, ArC), 122.7 (s, ArC), 57.12
(s, CNC(CH3)3), 57.09 (s, CNC(CH3)3), 54.6 (t, JP−C = 4.6 Hz,
NCH2), 31.9 (t, JP−C = 9.1 Hz), 31.2 (s, CNC(CH3)3), 30.9 (s,
CNC(CH3)3), 29.4 (t, JP−C = 9.1 Hz), 26.4 (t, JP−C = 13.6 Hz), 21.1 (t,
JP−C = 1.8 Hz, PCH(CH3)2), 20.7 (s, PCH(CH3)2), 19.13 (s,
PCH(CH3)2), 19.08 (s, PCH(CH3)2); the isocyanide NC resonances
were not located. 31P{1H} NMR (162 MHz, CD3CN, δ): 100.7 (s).
Selected ATR-IR data (solid, cm−1): 2103 (νCN), 2037 (νCN), 1791
(νFe−H). Anal. Calcd for C50H76N3BP2Fe: C, 70.84; H, 9.04; N, 4.96.
Found: C, 70.54; H, 9.06; N, 4.90.
cis,syn-[(iPrPNHP)FeH(CNtBu)2]BPh4 (cis,syn-8) Obtained via
Recrystallization. In a glovebox, a 32:68 mixture of cis,anti-8 and
cis,syn-8 (50 mg, 0.059 mmol) was dissolved in 1 mL of THF in a
scintillation vial. The resulting light yellow solution was carefully
layered with 1 mL of ethanol. Storing the vial in a−30 °C freezer for 3
days resulted in the formation of colorless crystals, which were collected
by decanting off the supernatant and then dried under vacuum. The
isolated product was identified as cis,syn-8 with 98% purity (25 mg, 50%
recovery yield). Colorless crystals suitable for an X-ray crystallographic
1
study were grown from THF−EtOH. H NMR (400 MHz, CD3CN,
δ): 7.33−7.25 (m, ArH, 8H), 7.01 (t, JH−H = 7.2 Hz, ArH, 8H), 6.85 (t,
J
H−H = 7.2 Hz, ArH, 4H), 2.88−2.70 (m, 1H for NH + 2H for CH or
CH2), 2.37−2.24 (m, 4H), 2.10−2.00 (m, 2H), 1.99−1.87 (m, 2H),
1.54−1.43 (m, 6H for PCH(CH3)2 + 2H for CH or CH2), 1.47 (s,
CNC(CH3)3, 9H), 1.42−1.35 (m, PCH(CH3)2, 6H), 1.28−1.20 (m,
PCH(CH3)2, 12H), 1.22 (s, CNC(CH3)3, 9H), −9.75 (t, JP−H = 53.2
Hz, FeH, 1H). 13C{1H} NMR (101 MHz, CD3CN, δ): 164.8 (q, JC−B
=
49.5 Hz, ArC), 136.7 (q, JC−B = 1.2 Hz, ArC), 126.6 (q, JC−B = 2.8 Hz,
ArC), 122.7 (s, ArC), 57.3 (s, CNC(CH3)3), 57.0 (s, CNC(CH3)3),
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52.8 (t, JP−C = 5.2 Hz, NCH2), 31.1 (s, CNC(CH3)3), 30.9 (s,
CNC(CH3)3), 29.9 (t, JP−C = 8.0 Hz), 26.7 (t, JP−C = 13.6 Hz), 26.6 (t,
acetonitrile of crystallization were modeled as partially occupied
molecules (occupancies were initially refined to approximately 75%;
they were fixed at 0.75 in the final refinement cycles). 1 crystallizes as
two independent molecules in the lattice. cis,anti-7′·1/2H2O
crystallizes as two independent molecules in the lattice; in each
molecule, one tert-butyl group was disordered and was refined with a
two-component model. Crystal structures mentioned in this paper have
been deposited at the Cambridge Crystallographic Data Centre
(CCDC) and allocated the deposition numbers CCDC 2058976−
2058986.
JP−C = 8.6 Hz), 20.7 (s, PCH(CH3)2), 20.6 (s, PCH(CH3)2), 19.5 (s,
PCH(CH3)2), 18.9 (s, PCH(CH3)2); the isocyanide NC resonances
were not located. 31P{1H} NMR (162 MHz, CD3CN, δ): 103.4 (s).
Selected ATR-IR data (solid, cm−1): 3244 (νN−H), 2098 (νCN), 2032
(νCN), 1814 (νFe−H). Anal. Calcd for C50H76N3BP2Fe: C, 70.84; H,
9.04; N, 4.96. Found: C, 70.59; H, 9.24; N, 4.87.
Procedure for Iron-Catalyzed Hydrogenation of Benzyl
Benzoate or Benzaldehyde. Under an argon atmosphere, benzyl
benzoate or benzaldehyde (1.0 mmol), an iron hydride complex (0.020
mmol, 2 mol % catalyst loading), base additive (if applicable, 0.20
mmol, 20 mol %), and bibenzyl (91 mg, 0.50 mmol, an internal
standard) were mixed with 0.5 mL of THF or EtOH in a glass tube. The
tube was then placed in a high-pressure reactor, which was flushed with
5 bar of H2 three times before being placed under 10 bar of H2 pressure.
The reaction was carried out at 60 °C (for PhCHO) or 110 °C (for
PhCO2CH2Ph) with stirring for an appropriate time. When the
reaction was stopped, the reactor was cooled to room temperature and
the residual dihydrogen was carefully released. The reaction solution
was passed through a pad of silica gel, which was eluted with ethyl
acetate. The volatiles were removed under vacuum, and the residue was
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00328.
■
sı
Additional experimental details, NMR and IR spectra of
the iron pincer complexes, and X-ray crystallographic
information (PDF)
Accession Codes
CCDC 2058976−2058986 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
1
analyzed by H NMR spectroscopy to calculate the conversion and
yield.
Procedure for the Stoichiometric Reaction between an Iron
Hydride and Benzaldehyde. Under an argon atmosphere, in a J.
Young NMR tube were placed an iron hydride complex (0.024 mmol),
benzaldehyde (0.024 mmol), and ∼0.5 mL of a deuterated solvent
(C6D6 for the reaction with cis,anti-7 and CD3CN for the reaction with
cis,anti-8 or cis,syn-8). The progress of the reaction was monitored by
1H and 31P{1H} NMR spectroscopy.
AUTHOR INFORMATION
Corresponding Author
■
X-ray Structure Determinations. Crystal data collection and
refinement parameters are provided in the Supporting Information.
Single crystals of 1 (orange−yellow) were grown from THF−pentane.
Details on how single crystals were obtained for the other iron
complexes are described in the corresponding synthesis and character-
ization subsections. Intensity data for 1, 2, cis,anti-5·2THF,cis,syn-5,
cis,anti-7′·1/2H2O, and cis,syn-8 were collected at 150 K on a Bruker D8
Venture Photon-II diffractometer with Mo Kα radiation, λ = 0.71073 Å.
Intensity data for trans-5·MeCN were collected at 150 K on a standard
Bruker APEX-II CCD diffractometer with Mo Kα radiation. Intensity
data for cis-6·3/4C2H5NO·3/4MeCN were collected at 150 K on a
Bruker DUO APEX-II CCD diffractometer at the University of Notre
Dame using Mo Kα radiation. Intensity data for the remaining
compounds were collected at 150 K on a Bruker PHOTON-II (3 and
cis,anti-4·1/2C7H8) or PHOTON100 CMOS detector (cis,anti-8) at
Beamline 11.3.1 at the Advanced Light Source (Lawrence Berkeley
National Laboratory) using synchrotron radiation tuned to λ = 0.7749
Å. The data frames were processed using the program SAINT. The data
were corrected for decay, Lorentz, and polarization effects as well as
absorption and beam corrections based on the multiscan technique in
SADABS. The structures were solved by a combination of direct
methods and the difference Fourier technique as implemented in the
SHELX suite of programs and refined by full-matrix least squares on F2.
Non-hydrogen atoms were refined with anisotropic displacement
parameters with the exception of the minor component of the
disordered atoms. The Fe-, N-, B-, and O-bound hydrogen atoms in all
structures except cis-6·3/4C2H5NO·3/4MeCN were located directly
from the difference map, and the coordinates were refined. The
bridging H−B in 3 was restrained to the CSD average distance for an
Fe−H−BH3 type linkage. The remaining hydrogen atoms were
calculated and treated with a riding model. 1 was refined as a two-
component twin (twin law: −1,0,0 0,−1,0 0.069,0,1; 24% twin;
CELL_NOW). One cyclohexyl ring of 3, one tert-butyl group, and the
cocrystallized half toluene molecule of cis,anti-4·1/2C7H8, one
cocrystallized THF molecule of cis,anti-5·2THF, one tert-butyl group
of cis,syn-5, and one isopropyl group along with the pincer backbone of
cis-6·3/4C2H5NO·3/4MeCN were disordered and were refined with a
two-component model. In cis-6·3/4C2H5NO·3/4MeCN, the (Z)-
methylimidoformic acid (or the enol form of N-methylformamide) and
Hairong Guan − Department of Chemistry, University of
Cincinnati, Cincinnati, Ohio 45221-0172, United States;
orcid.org/0000-0002-4858-3159; Email: hairong.guan@
uc.edu
Authors
Huiguang Dai − Department of Chemistry, University of
Cincinnati, Cincinnati, Ohio 45221-0172, United States
Weishi Li − Department of Chemistry, University of Cincinnati,
Cincinnati, Ohio 45221-0172, United States
Jeanette A. Krause − Department of Chemistry, University of
Cincinnati, Cincinnati, Ohio 45221-0172, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00328
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSF Chemical Catalysis Program (CHE-1464734
and CHE-1800151) and the University of Cincinnati (Doctoral
Enhancement Research Fellowship to H.D.) for support of this
research, Professor David F. Watson (University at Buffalo) for
the suggestion of using isocyanide ligands, Professor Nilay
Hazari (Yale University) for helpful discussions, and Professor
Allen G. Oliver (University of Notre Dame) for data collection
on a Bruker DUO APEX-II CCD diffractometer. Crystallo-
graphic data were also collected on a Bruker APEX-II CCD
diffractometer (funded by NSF-MRI grant CHE-0215950), on a
Bruker D8 Venture diffractometer (funded by NSF-MRI grant
CHE-1625737), or through the SCrALS (Service Crystallog-
raphy at Advanced Light Source) Program at Beamline 11.3.1 at
the Advanced Light Source (ALS), Lawrence Berkeley National
Laboratory (funded by the U.S. Department of Energy, Office of
Basic Energy Sciences, under contract No. DE-AC02-
6531
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pubs.acs.org/IC
Article
F.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Efficient Hydrogenation of
Ketones and Aldehydes Catalyzed by Well-Defined Iron(II) PNP
Pincer Complexes: Evidence for an Insertion Mechanism. Organo-
metallics 2014, 33, 6905−6914. (e) Mazza, S.; Scopelliti, R.; Hu, X.
Chemoselective Hydrogenation and Transfer Hydrogenation of
Aldehydes Catalyzed by Iron(II) PONOP Pincer Complexes. Organo-
metallics 2015, 34, 1538−1545. (f) Zell, T.; Ben-David, Y.; Milstein, D.
Highly Efficient, General Hydrogenation of Aldehydes Catalyzed by
PNP Iron Pincer Complexes. Catal. Sci. Technol. 2015, 5, 822−826.
(g) Gorgas, N.; Stöger, B.; Veiros, L. F.; Kirchner, K. Highly Efficient
and Selective Hydrogenation of Aldehydes: A Well-Defined Fe(II)
Catalyst Exhibits Noble-Metal Activity. ACS Catal. 2016, 6, 2664−
2672. (h) Butschke, B.; Feller, M.; Diskin-Posner, Y.; Milstein, D.
Ketone Hydrogenation Catalyzed by a New Iron(II)-PNN Complex.
05CH11231). We wish to dedicate this paper to the memory of
Professor William B. Connick, a brilliant scientist, an inspira-
tional teacher, and a wonderful friend, whose suggestions were
instrumental to the completion of this work.
REFERENCES
■
(1) (a) Morris, R. H. Exploiting Metal-Ligand Bifunctional Reactions
in the Design of Iron Asymmetric Hydrogenation Catalysts. Acc. Chem.
Res. 2015, 48, 1494−1502. (b) Zell, T.; Milstein, D. Hydrogenation and
Dehydrogenation Iron Pincer Catalysts Capable of Metal-Ligand
Cooperation by Aromatization/Dearomatization. Acc. Chem. Res. 2015,
48, 1979−1994. (c) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan,
H. Nickel and Iron Pincer Complexes as Catalysts for the Reduction of
Carbonyl Compounds. Acc. Chem. Res. 2015, 48, 1995−2003.
(d) Werkmeister, S.; Neumann, J.; Junge, K.; Beller, M. Pincer-Type
Complexes for Catalytic (De)Hydrogenation and Transfer (De)-
Hydrogenation Reactions: Recent Progress. Chem. - Eur. J. 2015, 21,
12226−12250. (e) Bernskoetter, W. H.; Hazari, N. Reversible
Hydrogenation of Carbon Dioxide to Formic Acid and Methanol:
Lewis Acid Enhancement of Base Metal Catalysts. Acc. Chem. Res. 2017,
50, 1049−1058. (f) Filonenko, G. A.; van Putten, R.; Hensen, E. J. M.;
Pidko, E. A. Catalytic (De)Hydrogenation Promoted by Non-precious
Metals - Co, Fe and Mn: Recent Advances in an Emerging Field. Chem.
Soc. Rev. 2018, 47, 1459−1483. (g) Gorgas, N.; Kirchner, K.
Isoelectronic Manganese and Iron Hydrogenation/Dehydrogenation
Catalysts: Similarities and Divergences. Acc. Chem. Res. 2018, 51,
1558−1569. (h) Seo, C. S. G.; Morris, R. H. Catalytic Homogeneous
Asymmetric Hydrogenation: Successes and Opportunities. Organo-
metallics 2019, 38, 47−65. (i) Wei, D.; Darcel, C. Iron Catalysis in
Reduction and Hydrometalation Reactions. Chem. Rev. 2019, 119,
2550−2610. (j) Alig, L.; Fritz, M.; Schneider, S. First-Row Transition
Metal (De)Hydrogenation Catalysis Based on Functional Pincer
Ligands. Chem. Rev. 2019, 119, 2681−2751.
Catal. Sci. Technol. 2016, 6, 4428−4437. (i) Brunig, J.; Csendes, Z.;
̈
Weber, S.; Gorgas, N.; Bittner, R. W.; Limbeck, A.; Bica, K.; Hoffmann,
H.; Kirchner, K. Chemoselective Supported Ionic-Liquid-Phase (SILP)
Aldehyde Hydrogenation Catalyzed by an Fe(II) PNP Pincer Complex.
ACS Catal. 2018, 8, 1048−1051.
(6) Nonpincer-type catalysts: (a) Fleischer, S.; Zhou, S.; Junge, K.;
Beller, M. General and Highly Efficient Iron-Catalyzed Hydrogenation
of Aldehydes, Ketones, and α,β-Unsaturated Aldehydes. Angew. Chem.,
Int. Ed. 2013, 52, 5120−5124. (b) Wienhöfer, G.; Westerhaus, F. A.;
Junge, K.; Ludwig, R.; Beller, M. A Molecularly Defined Iron-Catalyst
for the Selective Hydrogenation of α,β-Unsaturated Aldehydes. Chem. -
Eur. J. 2013, 19, 7701−7707. (c) Lu, L.-Q.; Li, Y.; Junge, K.; Beller, M.
Iron-Catalyzed Hydrogenation for the In Situ Regeneration of an
NAD(P)H Model: Biomimetic Reduction of α-Keto-/α-Iminoesters.
Angew. Chem., Int. Ed. 2013, 52, 8382−8386. (d) Mérel, D. S.; Elie, M.;
Lohier, J.-F.; Gaillard, S.; Renaud, J.-L. Bifunctional Iron Complexes:
Efficient Catalysts for C=O and C=N Reduction in Water.
ChemCatChem 2013, 5, 2939−2945. (e) Vailati Facchini, S.;
Neudörfl, J.-M.; Pignataro, L.; Cettolin, M.; Gennari, C.; Berkessel,
A.; Piarulli, U. Synthesis of [Bis(hexamethylene)cyclopentadienone]-
iron Tricarbonyl and Its Application to the Catalytic Reduction of C=O
Bonds. ChemCatChem 2017, 9, 1461−1468.
(2) (a) Knölker, H.-J.; Baum, E.; Goesmann, H.; Klauss, R.
Demetalation of Tricarbonyl(cyclopentadienone)iron Complexes
Initiated by a Ligand Exchange Reaction with NaOH - X-ray Analysis
of a Complex with Nearly Square-Planar Coordinated Sodium. Angew.
Chem., Int. Ed. 1999, 38, 2064−2066. (b) Chakraborty, A.; Guan, H.
Iron, dicarbonylhydro[(1,2,3,3a,7a-η)-4,5,6,7-tetrahydro-2-hydroxy-
1,3-bis(trimethylsilyl)-1H-inden-1-yl]-. e-Encyclopedia of Reagents for
Organic Synthesis; Wiley: 2014.
(3) (a) Casey, C. P.; Guan, H. An Efficient and Chemoselective Iron
Catalyst for the Hydrogenation of Ketones. J. Am. Chem. Soc. 2007, 129,
5816−5817. (b) Casey, C. P.; Guan, H. Cyclopentadienone Iron
Alcohol Complexes: Synthesis, Reactivity, and Implications for the
Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes. J. Am.
Chem. Soc. 2009, 131, 2499−2507.
(7) Pincer-type catalysts: (a) Lagaditis, P. O.; Sues, P. E.; Sonnenberg,
J. F.; Wan, K. Y.; Lough, A. J.; Morris, R. H. Iron(II) Complexes
Containing Unsymmetrical P-N-P’ Pincer Ligands for the Catalytic
Asymmetric Hydrogenation of Ketones and Imines. J. Am. Chem. Soc.
2014, 136, 1367−1380. (b) Sonnenberg, J. F.; Lagaditis, P. O.; Lough,
A. J.; Morris, R. H. Synthesis of Iron P-N-P’ and P-NH-P’ Asymmetric
Hydrogenation Catalysts. Organometallics 2014, 33, 6452−6465.
(c) Zirakzadeh, A.; Kirchner, K.; Roller, A.; Stöger, B.; Widhalm, M.;
Morris, R. H. Iron(II) Complexes Containing Chiral Unsymmetrical
PNP′ Pincer Ligands: Synthesis and Application in Asymmetric
Hydrogenations. Organometallics 2016, 35, 3781−3787. (d) Sonnen-
berg, J. F.; Wan, K. Y.; Sues, P. E.; Morris, R. H. Ketone Asymmetric
Hydrogenation Catalyzed by P-NH-P’ Pincer Iron Catalysts: An
Experimental and Computational Study. ACS Catal. 2017, 7, 316−326.
(4) (a) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Highly
Efficient Catalyst Systems Using Iron Complexes with a Tetradentate
PNNP Ligand for the Asymmetric Hydrogenation of Polar Bonds.
Angew. Chem., Int. Ed. 2008, 47, 940−943. (b) Sui-Seng, C.; Haque, F.
(e) Smith, S. A. M.; Lagaditis, P. O.; Lupke, A.; Lough, A. J.; Morris, R.
̈
H. Unsymmetrical Iron P-NH-P′ Catalysts for the Asymmetric Pressure
Hydrogenation of Aryl Ketones. Chem. - Eur. J. 2017, 23, 7212−7216.
(f) Huber, R.; Passera, A.; Mezzetti, A. Iron(II)-Catalyzed Hydro-
genation of Acetophenone with a Chiral, Pyridine-Based PNP Pincer
Ligand: Support for an Outer-Sphere Mechanism. Organometallics
2018, 37, 396−405. (g) Huber, R.; Passera, A.; Gubler, E.; Mezzetti, A.
P-Stereogenic PN(H)P Iron(II) Catalysts for the Asymmetric
Hydrogenation of Ketones: The Importance of Non-Covalent
Interactions in Rational Ligand Design by Computation. Adv. Synth.
Catal. 2018, 360, 2900−2913. (h) Garbe, M.; Wei, Z.; Tannert, B.;
Spannenberg, A.; Jiao, H.; Bachmann, S.; Scalone, M.; Junge, K.; Beller,
M. Enantioselective Hydrogenation of Ketones Using Different Metal
Complexes with a Chiral PNP Pincer Ligand. Adv. Synth. Catal. 2019,
361, 1913−1920.
N.; Hadzovic, A.; Putz, A.-M.; Reuss, V.; Meyer, N.; Lough, A. J.;
̈
Zimmer-De Iuliis, M.; Morris, R. H. Synthesis and Characterization of
Iron(II) Complexes with Tetradentate Diiminodiphosphine or
Diaminodiphosphine Ligands as Precatalysts for the Hydrogenation
of Acetophenone. Inorg. Chem. 2009, 48, 735−743.
(5) Pincer-type catalysts: (a) Langer, R.; Leitus, G.; Ben-David, Y.;
Milstein, D. Efficient Hydrogenation of Ketones Catalyzed by an Iron
Pincer Complex. Angew. Chem., Int. Ed. 2011, 50, 2120−2124.
(b) Langer, R.; Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.;
Leitus, G.; Ben-David, Y.; Milstein, D. Iron Borohydride Pincer
Complexes for the Efficient Hydrogenation of Ketones Under Mild,
Base-Free Conditions: Synthesis and Mechanistic Insight. Chem. - Eur.
J. 2012, 18, 7196−7209. (c) Chakraborty, S.; Lagaditis, P. O.; Förster,
M.; Bielinski, E. A.; Hazari, N.; Holthausen, M. C.; Jones, W. D.;
Schneider, S. Well-Defined Iron Catalysts for the Acceptorless
Reversible Dehydrogenation-Hydrogenation of Alcohols and Ketones.
ACS Catal. 2014, 4, 3994−4003. (d) Gorgas, N.; Stöger, B.; Veiros, L.
(8) Nonpincer-type catalysts: (a) Berkessel, A.; Reichau, S.; von der
Höh, A.; Leconte, N.; Neudörfl, J.-M. Light-Induced Enantioselective
Hydrogenation Using Chiral Derivatives of Casey’s Iron-Cyclo-
pentadienone Catalyst. Organometallics 2011, 30, 3880−3887.
6532
https://doi.org/10.1021/acs.inorgchem.1c00328
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Inorganic Chemistry
pubs.acs.org/IC
Article
(b) Zuo, W.; Tauer, S.; Prokopchuk, D. E.; Morris, R. H. Iron Catalysts
Containing Amine(imine)diphosphine P-NH-N-P Ligands Catalyze
both the Asymmetric Hydrogenation and Asymmetric Transfer
Hydrogenation of Ketones. Organometallics 2014, 33, 5791−5801.
(c) Li, Y.; Yu, S.; Wu, X.; Xiao, J.; Shen, W.; Dong, Z.; Gao, J. Iron
Catalyzed Asymmetric Hydrogenation of Ketones. J. Am. Chem. Soc.
2014, 136, 4031−4039. (d) Gajewski, P.; Renom-Carrasco, M.; Vailati
Facchini, S.; Pignataro, L.; Lefort, L.; de Vries, J. G.; Ferraccioli, R.;
Forni, A.; Piarulli, U.; Gennari, C. Chiral (Cyclopentadienone)iron
Complexes for the Catalytic Asymmetric Hydrogenation of Ketones.
Eur. J. Org. Chem. 2015, 2015, 1887−1893. (e) Gajewski, P.; Renom-
Carrasco, M.; Vailati Facchini, S.; Pignataro, L.; Lefort, L.; de Vries, J.
G.; Ferraccioli, R.; Piarulli, U.; Gennari, C. Synthesis of (R)-BINOL-
Derived (Cyclopentadienone)iron Complexes and Their Application
in the Catalytic Asymmetric Hydrogenation of Ketones. Eur. J. Org.
Chem. 2015, 2015, 5526−5536. (f) Hodgkinson, R.; Del Grosso, A.;
Clarkson, G.; Wills, M. Iron Cyclopentadienone Complexes Derived
from C2-Symmetric Bis-Propargylic Alcohols; Preparation and
Applications to Catalysis. Dalton Trans. 2016, 45, 3992−4005.
(g) Del Grosso, A.; Chamberlain, A. E.; Clarkson, G. J.; Wills, M.
Synthesis and Applications to Catalysis of Novel Cyclopentadienone
Iron Tricarbonyl Complexes. Dalton Trans. 2018, 47, 1451−1470.
(h) Bai, X.; Cettolin, M.; Mazzoccanti, G.; Pierini, M.; Piarulli, U.;
Colombo, V.; Dal Corso, A.; Pignataro, L.; Gennari, C. Chiral
(Cyclopentadienone)iron Complexes with a Stereogenic Plane as Pre-
Catalysts for the Asymmetric Hydrogenation of Polar Double Bonds.
Tetrahedron 2019, 75, 1415−1424.
(d) Zhu, F.; Zhu-Ge, L.; Yang, G.; Zhou, S. Iron-Catalyzed
Hydrogenation of Bicarbonates and Carbon Dioxide to Formates.
ChemSusChem 2015, 8, 609−612. (e) Thai, T.-T.; Mérel, D. S.; Poater,
A.; Gaillard, S.; Renaud, J.-L. Highly Active Phosphine-Free Bifunc-
tional Iron Complex for Hydrogenation of Bicarbonate and Reductive
Amination. Chem. - Eur. J. 2015, 21, 7066−7070. (f) Fong, H.; Peters, J.
C. Hydricity of an Fe-H Species and Catalytic CO2 Hydrogenation.
Inorg. Chem. 2015, 54, 5124−5135. (g) Coufourier, S.; Gaillard, S.;
Clet, G.; Serre, C.; Daturi, M.; Renaud, J.-L. A MOF-Assisted
Phosphine Free Bifunctional Iron Complex for the Hydrogenation of
Carbon Dioxide, Sodium Bicarbonate and Carbonate to Formate.
Chem. Commun. 2019, 55, 4977−4980. (h) Coufourier, S.; Gaillard, Q.
G.; Lohier, J.-F.; Poater, A.; Gaillard, S.; Renaud, J.-L. Hydrogenation of
CO2, Hydrogenocarbonate, and Carbonate to Formate in Water using
Phosphine Free Bifunctional Iron Complexes. ACS Catal. 2020, 10,
2108−2116.
(11) Pincer-type catalysts: (a) Zell, T.; Ben-David, Y.; Milstein, D.
Unprecedented Iron-Catalyzed Ester Hydrogenation. Mild, Selective,
and Efficient Hydrogenation of Trifluoroacetic Esters to Alcohols
Catalyzed by an Iron Pincer Complex. Angew. Chem., Int. Ed. 2014, 53,
4685−4689. (b) Werkmeister, S.; Junge, K.; Wendt, B.; Alberico, E.;
Jiao, H.; Baumann, W.; Jung, H.; Gallou, F.; Beller, M. Hydrogenation
of Esters to Alcohols with a Well-Defined Iron Complex. Angew. Chem.,
Int. Ed. 2014, 53, 8722−8726. (c) Chakraborty, S.; Dai, H.;
Bhattacharya, P.; Fairweather, N. T.; Gibson, M. S.; Krause, J. A.;
Guan, H. Iron-Based Catalysts for the Hydrogenation of Esters to
Alcohols. J. Am. Chem. Soc. 2014, 136, 7869−7872. (d) Fairweather, N.
T.; Gibson, M. S.; Guan, H. Homogeneous Hydrogenation of Fatty
Acid Methyl Esters and Natural Oils under Neat Conditions.
Organometallics 2015, 34, 335−339. (e) Elangovan, S.; Wendt, B.;
Topf, C.; Bachmann, S.; Scalone, M.; Spannenberg, A.; Jiao, H.;
Baumann, W.; Jung, K.; Beller, M. Improved Second Generation Iron
Pincer Complexes for Effective Ester Hydrogenation. Adv. Synth. Catal.
2016, 358, 820−825. (f) Quintaine, J.; Saudan, L. Hydrogenation of
Esters with Fe/Tridentate Ligands Complexes. Patent WO
2015091158 A1.
(12) Nonpincer-type catalysts: Gajewski, P.; Gonzalez-de-Castro, A.;
Renom-Carrasco, M.; Piarulli, U.; Gennari, C.; de Vries, J. G.; Lefort, L.;
Pignataro, L. Expanding the Catalytic Scope of (Cyclopentadienone)
iron Complexes to the Hydrogenation of Activated Esters to Alcohols.
ChemCatChem 2016, 8, 3431−3435.
(13) (a) Garg, J. A.; Chakraborty, S.; Ben-David, Y.; Milstein, D.
Unprecedented Iron-Catalyzed Selective Hydrogenation of Activated
Amides to Amines and Alcohols. Chem. Commun. 2016, 52, 5285−
5288. (b) Schneck, F.; Assmann, M.; Balmer, M.; Harms, K.; Langer, R.
Selective Hydrogenation of Amides to Amines and Alcohols Catalyzed
by Improved Iron Pincer Complexes. Organometallics 2016, 35, 1931−
1943. (c) Rezayee, N. M.; Samblanet, D. C.; Sanford, M. S. Iron-
Catalyzed Hydrogenation of Amides to Alcohols and Amines. ACS
Catal. 2016, 6, 6377−6383. (d) Jayarathne, U.; Zhang, Y.; Hazari, N.;
Bernskoetter, W. H. Selective Iron-Catalyzed Deaminative Hydro-
(9) Pincer-type catalysts: (a) Langer, R.; Diskin-Posner, Y.; Leitus, G.;
Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Low-Pressure Hydro-
genation of Carbon Dioxide Catalyzed by an Iron Pincer Complex
Exhibiting Noble Metal Activity. Angew. Chem., Int. Ed. 2011, 50,
9948−9952. (b) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado,
B. Q.; Wurtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. Lewis
̈
Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported
Iron Catalyst. J. Am. Chem. Soc. 2014, 136, 10234−10237. (c) Zhang,
Y.; MacIntosh, A. D.; Wong, J. L.; Bielinski, E. A.; Williard, P. G.;
Mercado, B. Q.; Hazari, N.; Bernskoetter, W. H. Iron Catalyzed CO2
Hydrogenation to Formate Enhanced by Lewis Acid Co-Catalysts.
Chem. Sci. 2015, 6, 4291−4299. (d) Rivada-Wheelaghan, O.; Dauth, A.;
Leitus, G.; Diskin-Posner, Y.; Milstein, D. Synthesis and Reactivity of
Iron Complexes with a New Pyrazine-Based Pincer Ligand, and
Application in Catalytic Low-Pressure Hydrogenation of Carbon
Dioxide. Inorg. Chem. 2015, 54, 4526−4538. (e) Bertini, F.; Gorgas, N.;
Stöger, B.; Peruzzini, M.; Veiros, L. F.; Kirchner, K.; Gonsalvi, L.
Efficient and Mild Carbon Dioxide Hydrogenation to Formate
Catalyzed by Fe(II) Hydrido Carbonyl Complexes Bearing 2,6-
(Diaminopyridyl)diphosphine Pincer Ligands. ACS Catal. 2016, 6,
2889−2893. (f) Smith, N. E.; Bernskoetter, W. H.; Hazari, N.;
Mercado, B. Q. Synthesis and Catalytic Activity of PNP-Supported Iron
Complexes with Ancillary Isonitrile Ligands. Organometallics 2017, 36,
3995−4004. (g) Curley, J. B.; Smith, N. E.; Bernskoetter, W. H.; Hazari,
N.; Mercado, B. Q. Catalytic Formic Acid Dehydrogenation and CO2
Hydrogenation Using Iron PNRP Pincer Complexes with Isonitrile
Ligands. Organometallics 2018, 37, 3846−3853. (h) Jayarathne, U.;
Hazari, N.; Bernskoetter, W. H. Selective Iron-Catalyzed N-
Formylation of Amines Using Dihydrogen and Carbon Dioxide. ACS
Catal. 2018, 8, 1338−1345.
́
genation of Amides. Organometallics 2017, 36, 409−416. (e) Artus
̀
Suarez, L.; Jayarathne, U.; Balcells, D.; Bernskoetter, W. H.; Hazari, N.;
Jaraiz, M.; Nova, A. Rational Selection of Co-Catalysts for the
Deaminative Hydrogenation of Amides. Chem. Sci. 2020, 11, 2225−
2230.
(14) Dong, K.; Elangovan, S.; Sang, R.; Spannenberg, A.; Jackstell, R.;
Junge, K.; Li, Y.; Beller, M. Selective Catalytic Two-Step Process for
Ethylene Glycol from Carbon Monoxide. Nat. Commun. 2016, 7,
12075.
(15) (a) Bernskoetter, W. H.; Hazari, N. Hydrogenation and
Dehydrogenation Reactions Catalyzed by Iron Pincer Compounds.
In Pincer Compounds: Chemistry and Applications; Morales-Morales, D.,
Ed.; Elsevier: Amsterdam, 2018; Chapter 6, pp 111−131. (b) Jones, W.
D. Hydrogenation/Dehydrogenation of Unsaturated Bonds with Iron
Pincer Catalysis. Top. Organomet. Chem. 2019, 63, 141−174. (c) Wei,
Z.; Jiao, H. Bifunctional Aliphatic PNP Pincer Catalysts for Hydro-
genation: Mechanisms and Scope. Adv. Inorg. Chem. 2019, 73, 323−
384.
(10) Nonpincer-type catalysts: (a) Federsel, C.; Boddien, A.; Jackstell,
R.; Jennerjahn, R.; Dyson, P. J.; Scopelliti, R.; Laurenczy, G.; Beller, M.
A Well-Defined Iron Catalyst for the Reduction of Bicarbonates and
Carbon Dioxide to Formates, Alkyl Formates, and Formamides. Angew.
Chem., Int. Ed. 2010, 49, 9777−9780. (b) Ziebart, C.; Federsel, C.;
Anbarasan, P.; Jackstell, R.; Baumann, W.; Spannenberg, A.; Beller, M.
Well-Defined Iron Catalyst for Improved Hydrogenation of Carbon
Dioxide and Bicarbonate. J. Am. Chem. Soc. 2012, 134, 20701−20704.
(c) Bertini, F.; Mellone, I.; Ienco, A.; Peruzzini, M.; Gonsalvi, L.
Iron(II) Complexes of the Linear rac-Tetraphos-1 Ligand as Efficient
Homogeneous Catalysts for Sodium Bicarbonate Hydrogenation and
Formic Acid Dehydrogenation. ACS Catal. 2015, 5, 1254−1265.
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(16) (a) Morris, R. H. Iron Group Hydrides in Noyori Bifunctional
Catalysis. Chem. Rec. 2016, 16, 2644−2658. (b) Morris, R. H.
Mechanisms of the H2- and Transfer Hydrogenation of Polar Bonds
Catalyzed by Iron Group Hydrides. Dalton Trans. 2018, 47, 10809−
10826. (c) Agbossou-Niedercorn, F.; Michon, C. Bifunctional
Homogeneous Catalysts Based on First Row Transition Metals in
Asymmetric Hydrogenation. Coord. Chem. Rev. 2020, 425, 213523.
(17) Recent computational studies by Dub and Gordon show that in
solution the concerted H−/H+ transfer and dihydrogen activation steps
as illustrated in Scheme 1, Catalytic Cycle A, are mechanistically
irrelevant. Instead, these reactions proceed in a stepwise fashion. For
details, see: (a) Dub, P. A.; Gordon, J. C. The Mechanism of
Enantioselective Ketone Reduction with Noyori and Noyori-Ikariya
Bifunctional Catalysts. Dalton Trans. 2016, 45, 6756−6781. (b) Dub, P.
A.; Gordon, J. C. Metal-Ligand Bifunctional Catalysis: The “Accepted”
Mechanism, the Issue of Concertedness, and the Function of the Ligand
in Catalytic Cycles Involving Hydrogen Atoms. ACS Catal. 2017, 7,
6635−6655.
(18) (a) Chen, X.; Jing, Y.; Yang, X. Unexpected Direct Hydride
Transfer Mechanism for the Hydrogenation of Ethyl Acetate to Ethanol
Catalyzed by SNS Pincer Ruthenium Complexes. Chem. - Eur. J. 2016,
22, 1950−1957. (b) Smith, N. E.; Bernskoetter, W. H.; Hazari, N. The
Role of Proton Shuttles in the Reversible Activation of Hydrogen via
Metal-Ligand Cooperation. J. Am. Chem. Soc. 2019, 141, 17350−
17360.
(19) (a) Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.;
Alberico, E.; Baumann, W.; Junge, H.; Junge, K.; Beller, M. Mild and
Selective Hydrogenation of Aromatic and Aliphatic (Di)nitriles with a
Well-Defined Iron Pincer Complex. Nat. Commun. 2014, 5, 4111.
(b) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. A Molecular Iron
Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of
N-Heterocycles. J. Am. Chem. Soc. 2014, 136, 8564−8567.
(20) (a) Otsuka, T.; Ishii, A.; Dub, P. A.; Ikariya, T. Practical Selective
Hydrogenation of α-Fluorinated Esters with Bifunctional Pincer-Type
Ruthenium(II) Catalysts Leading to Fluorinated Alcohols or Fluoral
Hemiacetals. J. Am. Chem. Soc. 2013, 135, 9600−9603. (b) Dub, P. A.;
Henson, N. J.; Martin, R. L.; Gordon, J. C. Unravelling the Mechanism
of the Asymmetric Hydrogenation of Acetophenone by
[RuX2(diphosphine)(1,2-diamine)] Catalysts. J. Am. Chem. Soc.
2014, 136, 3505−3521. (c) Dub, P. A.; Gordon, J. C. The Role of
the Metal-Bound N-H Functionality in Noyori-Type Molecular
Catalysts. Nat. Rev. Chem. 2018, 2, 396−408.
(21) (a) Hasanayn, F.; Baroudi, A. Direct H/OR and OR/OR’
Metathesis Pathways in Ester Hydrogenation and Transesterification
by Milstein’s Catalyst. Organometallics 2013, 32, 2493−2496. (b) Qu,
S.; Dai, H.; Dang, Y.; Song, C.; Wang, Z.-X.; Guan, H. Computational
Mechanistic Study of Fe-Catalyzed Hydrogenation of Esters to
Alcohols: Improving Catalysis by Accelerating Precatalyst Activation
with a Lewis Base. ACS Catal. 2014, 4, 4377−4388. (c) Gusev, D. G.
Dehydrogenative Coupling of Ethanol and Ester Hydrogenation
Catalyzed by Pincer-Type YNP Complexes. ACS Catal. 2016, 6,
6967−6981. (d) Gusev, D. G. Rethinking the Dehydrogenative Amide
Synthesis. ACS Catal. 2017, 7, 6656−6662. (e) Gusev, D. G. Revised
Mechanisms of the Catalytic Alcohol Dehydrogenation and Ester
Reduction with the Milstein PNN Complex of Ruthenium. Organo-
metallics 2020, 39, 258−270. (f) Tindall, D. J.; Menche, M.; Schelwies,
M.; Paciello, R. A.; Schäfer, A.; Comba, P.; Rominger, F.; Hashmi, A. S.
K.; Schaub, T. Ru0 or RuII: A Study on Stabilizing the “Activated” Form
of Ru-PNP Complexes with Additional Phosphine Ligands in Alcohol
Dehydrogenation and Ester Hydrogenation. Inorg. Chem. 2020, 59,
5099−5115.
1245−1260. (b) Yuwen, J.; Chakraborty, S.; Brennessel, W. W.; Jones,
W. D. Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to
Alcohols. ACS Catal. 2017, 7, 3735−3740.
(24) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K.
Understanding the Mechanisms of Cobalt-Catalyzed Hydrogenation
and Dehydrogenation Reactions. J. Am. Chem. Soc. 2013, 135, 8668−
8681.
(25) Dub, P. A.; Scott, B. L.; Gordon, J. C. Air-Stable NNS
(ENENES) Ligands and Their Well-Defined Ruthenium and Iridium
Complexes for Molecular Catalysis. Organometallics 2015, 34, 4464−
4479.
(26) (a) Yamamoto, Y. Zerovalent Transition Metal Complexes of
Organic Isocyanides. Coord. Chem. Rev. 1980, 32, 193−233.
(b) Boyarskiy, V. P.; Bokach, N. A.; Luzyanin, K. V.; Kukushkin, V.
Yu. Metal-Mediated and Metal-Catalyzed Reactions of Isocyanides.
Chem. Rev. 2015, 115, 2698−2779. (c) Barnett, B. R.; Figueroa, J. S.
Zero-Valent Isocyanides of Nickel, Palladium and Platinum as
Transition Metal σ-Type Lewis Bases. Chem. Commun. 2016, 52,
13829−13839.
(27) It should be noted that HN(CH2CH2PiPr2)2 does not always
bind to a transition metal in the meridional configuration as one would
expect for a pincer ligand. For examples of HN(CH2CH2PiPr2)2
adopting the facial coordination mode, see: (a) Abdur-Rashid, K.;
Graham, T.; Tsang, C.-W.; Chen, X.; Guo, R.; Jia, W.; Amoroso, D.;
Sui-Seng, C. Method for the Production of Hydrogen from Ammonia
Borane. Patent WO 2008141439 A1. (b) Rozenel, S. S.; Arnold, J.
Bimetallic Ruthenium PNP Pincer Complex As a Platform to Model
Proposed Intermediates in Dinitrogen Reduction to Ammonia. Inorg.
Chem. 2012, 51, 9730−9739. (c) Fillman, K. L.; Bielinski, E. A.;
Schmeier, T. J.; Nesvet, J. C.; Woodruff, T. M.; Pan, C. J.; Takase, M. K.;
Hazari, N.; Neidig, M. L. Flexible Binding of PNP Pincer Ligands to
Monomeric Iron Complexes. Inorg. Chem. 2014, 53, 6066−6072.
(28) (a) Marks, T. J.; Kolb, J. R. Covalent Transition Metal,
Lanthanide, and Actinide Tetrahydroborate Complexes. Chem. Rev.
1977, 77, 263−293. (b) Makhaev, V. D. Structural and Dynamic
Properties of Tetrahydroborate Complexes. Russ. Chem. Rev. 2000, 69,
727−746. (c) Besora, M.; Lledós, A. Coordination Modes and Hydride
Exchange Dynamics in Transition Metal Tetrahydroborate Complexes.
Struct. Bonding (Berlin) 2008, 130, 149−202.
(29) (a) Custelcean, R.; Jackson, J. E. Dihydrogen Bonding:
Structures, Energetics, and Dynamics. Chem. Rev. 2001, 101, 1963−
1980. (b) Belkova, N. V.; Epstein, L. M.; Filippov, O. A.; Shubina, E. S.
Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides.
Chem. Rev. 2016, 116, 8545−8587. (c) Golub, I. E.; Filippov, O. A.;
Belkova, N. V.; Epstein, L. M.; Rossin, A.; Peruzzini, M.; Shubina, E. S.
Two Pathways of Proton Transfer Reaction to (triphos)Cu(η1-BH4)
v i a
a D i h y d r o g e n B o n d [ t r i p h o s = 1 , 1 , 1 - t r i s -
(diphenylphosphinomethyl)ethane]. Dalton Trans. 2016, 45, 9127−
9135. (d) Belkova, N. V.; Bakhmutova-Albert, E. V.; Gutsul, E. I.;
Bakhmutov, V. I.; Golub, I. E.; Filippov, O. A.; Epstein, L. M.; Peruzzini,
M.; Rossin, A.; Zanobini, F.; Shubina, E. S. Dihydrogen Bonding in
Complex (PP3)RuH(η1-BH4) Featuring Two Proton-Accepting
Hydride Sites: Experimental and Theoretical Studies. Inorg. Chem.
2014, 53, 1080−1090. (e) Golub, I. E.; Gulyaeva, E. S.; Filippov, O. A.;
Dyadchenko, V. P.; Belkova, N. V.; Epstein, L. M.; Arkhipov, D. E.;
Shubina, E. S. Dihydrogen Bond Intermediated Alcoholysis of
Dimethylamine - Borane in Nonaqueous Media. J. Phys. Chem. A
2015, 119, 3853−3868.
(30) (a) Alberico, E.; Sponholz, P.; Cordes, C.; Nielsen, M.; Drexler,
H.-J.; Baumann, W.; Junge, H.; Beller, M. Selective Hydrogen
Production from Methanol with a Defined Iron Pincer Catalyst
under Mild Conditions. Angew. Chem., Int. Ed. 2013, 52, 14162−14166.
(b) Koehne, I.; Schmeier, T. J.; Bielinski, E. A.; Pan, C. J.; Lagaditis, P.
(22) Dawe, L. N.; Karimzadeh-Younjali, M.; Dai, Z.; Khaskin, E.;
Gusev, D. G. The Milstein Bipyridyl PNN Pincer Complex of
Ruthenium Becomes a Noyoyi-Type Catalyst under Reducing
Conditions. J. Am. Chem. Soc. 2020, 142, 19510−19522.
(23) For examples of related hydrogenation catalysts based on other
transition metals, see: (a) Dub, P. A.; Scott, B. L.; Gordon, J. C. Why
Does Alkylation of the N-H Functionality within M/NH Bifunctional
Noyori-Type Catalysts Lead to Turnover? J. Am. Chem. Soc. 2017, 139,
O.; Bernskoetter, W. H.; Takase, M. K.; Wurtele, C.; Hazari, N.;
̈
Schneider, S. Synthesis and Structure of Six-Coordinate Iron
Borohydride Complexes Supported by PNP Ligands. Inorg. Chem.
2014, 53, 2133−2143.
(31) 1 crystallizes as two independent molecules in the lattice; only
molecule A is shown here. 3 has a disordered Cy group; only one
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conformer is shown here. See the Supporting Information for additional
information.
(32) In this paper, cis and trans denote the relationship between the
two isocyanide ligands, whereas syn and anti denote the relationship
between the NH hydrogen and the X-type ligand (bromide or hydride).
(33) The ratio between these two isomers can vary slightly from batch
to batch.
(34) (a) Wei, D.; Roisnel, T.; Darcel, C.; Clot, E.; Sortais, J.-B.
Hydrogenation of Carbonyl Derivatives with a Well-Defined Rhenium
Precatalyst. ChemCatChem 2017, 9, 80−83. (b) Fu, S.; Shao, Z.; Wang,
Y.; Liu, Q. Manganese-Catalyzed Upgrading of Ethanol into 1-Butanol.
J. Am. Chem. Soc. 2017, 139, 11941−11948. (c) Wellala, N. P. N.;
Luebking, J. D.; Krause, J. A.; Guan, H. Roles of Hydrogen Bonding in
Proton Transfer to κP,κN,κP-N(CH2CH2PiPr2)2-Ligated Nickel Pincer
Complexes. ACS Omega 2018, 3, 4986−5001.
(35) Ogata, O.; Nakayama, Y.; Nara, H.; Fujiwhara, M.; Kayaki, Y.
Atmospheric Hydrogenation of Esters Catalyzed by PNP-Ruthenium
Complexes with an N-Heterocyclic Carbene Ligand. Org. Lett. 2016,
18, 3894−3897.
(36) One tert-butyl group is disordered and was refined with a two-
component disorder model. Only the major component is shown here.
The cocrystallized half-molecule of toluene is also disordered. See the
Supporting Information for additional information.
(37) One tert-butyl group is disordered and was refined with a two-
component disorder model. Only the major component is shown here.
See the Supporting Information for additional information.
(38) Mironov, M. A. General Aspects of Isocyanide Reactivity. In
Isocyanide Chemistry: Applications in Synthesis and Material Science, 1st
ed.; Nenajdenko, V. G., Ed.; Wiley-VCH: Weinheim, Germany, 2012;
pp 35−73.
(39) Goldman, A. S.; Krogh-Jespersen, K. Why Do Cationic Carbon
Monoxide Complexes Have High C-O Stretching Force Constants and
Short C-O Bonds? Electrostatic Effects, Not σ-Bonding. J. Am. Chem.
Soc. 1996, 118, 12159−12166.
(40) Similar results were obtained when benzene or THF was used as
the solvent for extraction.
(41) Rautenstrauch, V.; Hoang-Cong, X.; Churlaud, R.; Abdur-
Rashid, K.; Morris, R. H. Hydrogenation versus Transfer Hydro-
genation of Ketones: Two Established Ruthenium Systems Catalyze
Both. Chem. - Eur. J. 2003, 9, 4954−4967.
(42) cis,anti-7′ crystallizes as two independent molecules with one
water molecule in the lattice. Each molecule has one disordered tert-
butyl group, which was refined as a two-component disorder model.
Only the major component of one molecule is shown here. See the
Supporting Information for additional information.
(43) Dub, P. A.; Batrice, R. J.; Gordon, J. C.; Scott, B. L.; Minko, Y.;
Schmidt, J. G.; Williams, R. F. Engineering Catalysts for Selective Ester
Hydrogenation. Org. Process Res. Dev. 2020, 24, 415−442.
(44) Babij, N. R.; McCusker, E. O.; Whiteker, G. T.; Canturk, B.;
Choy, N.; Creemer, L. C.; DeAmicis, C. V.; Hewlett, N. M.; Johnson, P.
L.; Knobelsdorf, J. A.; Li, F.; Lorsbach, B. A.; Nugent, B. M.; Ryan, S. J.;
Smith, M. R.; Yang, Q. NMR Chemical Shifts of Trace Impurities:
Industrially Preferred Solvents Used in Process and Green Chemistry.
Org. Process Res. Dev. 2016, 20, 661−667.
(45) (a) Clarke, Z. E.; Maragh, P. T.; Dasgupta, T. P.; Gusev, D. G.;
Lough, A. J.; Abdur-Rashid, K. A Family of Active Iridium Catalysts for
Transfer Hydrogenation of Ketones. Organometallics 2006, 25, 4113−
4117. (b) Bertoli, M.; Choualeb, A.; Gusev, D. G.; Lough, A. J.; Major,
Q.; Moore, B. PNP Pincer Osmium Polyhydrides for Catalytic
Dehydrogenation of Primary Alcohols. Dalton Trans. 2011, 40,
8941−8949.
(46) Ma, W.; Cui, S.; Sun, H.; Tang, W.; Xue, D.; Li, C.; Fan, J.; Xiao,
J.; Wang, C. Iron-Catalyzed Alkylation of Nitriles with Alcohols. Chem. -
Eur. J. 2018, 24, 13118−13123.
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