Highly active, low-valence molybdenum- and tungsten-amide catalysts for bifunctional imine-hydrogenation reactions

Article abstract of DOI:10.1002/asia.201301106

The reactions of [M(NO)(CO)₄(ClAlCl₃)] (M=Mo, W) with (iPr₂PCH₂CH₂)₂NH, (PN^HP) at 90 °C afforded [M(NO)(CO)(PN^HP)Cl] complexes (M=Mo, 1 a; W, 1 b). The treatment of co

Full text of DOI:10.1002/asia.201301106

FULL PAPER
DOI: 10.1002/asia.201301106
Highly Active, Low-Valence Molybdenum- and Tungsten-Amide Catalysts
for Bifunctional Imine-Hydrogenation Reactions
Subrata Chakraborty, Olivier Blacque, Thomas Fox, and Heinz Berke*[a]
Abstract:
The
reactions
of
and 3b were capable of adding dihy-
drogen, with heterolytic splitting,
thereby forming pairs of isomeric
thereby showing maximum turnover
frequencies (TOFs) of 2912 and
1120 hÀ1, respectively, for the hydroge-
nation of N-(4-methoxybenzylidene)a-
niline. A Hammett plot for various
para-substituted imines revealed linear
correlations with a negative slope of
À3.69 for para substitution on the ben-
zylidene side and a positive slope of
0.68 for para substitution on the aniline
side. Kinetics analysis revealed the ini-
tial rate of the hydrogenation reactions
[M(NO)(CO)4ACHTUNGTRENNUNG(ClAlCl3)] (M=Mo, W)
with (iPr2PCH2CH2)2NH, (PNHP) at
908C afforded [M(NO)(CO)
(PNHP)Cl]
amine-hydride
[Mo(NO)(CO)H
complexes
(PNHP)] (4a
(cis) and
(PNHP)]
(trans); cis and trans
complexes (M=Mo, 1a; W, 1b). The
treatment of compound 1a with
KOtBu as a base at room temperature
G
ACHTUNGTRENNUNG
4a
A
ACHTUNGTRENNUNG
(4b
A
ACHTUNGTRENNUNG
yielded
[Mo(NO)(CO)
contrast, with the amide base Na[N-
(SiMe3)2], the PNHP ligand moieties in
the
alkoxide
complex
correspond to the position of the H
and NO groups). H2 approaches the
Mo/W=N bond in compounds 3a,b
from either the CO-ligand side or from
the NO-ligand side. Compounds 4a-
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
compounds 1a and 1b could be depro-
tonated at room temperature, thereby
(cis) and 4a
E
to be first order in c
ACHTUNGTREN(NGNU cat.) and zeroth
inducing
amido complexes [M(NO)(CO)
(M=Mo, 3a; W, 3b;
(iPr2PCH2CH2)2N)). Compounds 3a
and 3b have pseudo-trigonal-bipyrami-
dal geometries, in which the amido ni-
trogen atom is in the equatorial plane.
At room temperature, compounds 3a
dehydrochlorination
into
be stable under a H2 atmosphere and
could not be isolated. At 1408C and
60 bar H2, compounds 3a and 3b cata-
lyzed the hydrogenation of imines,
order in c(imine). Deuterium kinetic
ACHTUNGTRENNUNG
G
isotope effect (DKIE) experiments fur-
nished a low kH/kD value (1.28), which
supported a Noyori-type metal–ligand
bifunctional mechanism with H2 addi-
tion as the rate-limiting step.
PNP=
Keywords: amides · hydrogenation ·
imines · molybdenum · tungsten
Introduction
H2 creates protic and hydridic moieties, followed by step-
wise or concerted H+ and HÀ transfer onto the unsaturated
organic substrates. These reactions are also denoted as pro-
tonic-hydridic catalysis or ionic hydrogenation, depending
on the character of the H-atom transfer.[4,5] Recently, bifunc-
tional homogeneous catalysis has attracted increasing atten-
tion, triggered by the elegant initial work by the groups of
Noyori,[6] Ikariya,[7] Milstein,[8] Shvo,[9a,d] and Casey.[9b,c] In
these reactions, the heterolytic cleavage of H2 occurs across
a metal=O/N-ligand double bond. In recent years, transi-
tion-metal hydrides that contain the protic PNHP ligand
(PNHP=(iPr2PCH2CH2)2NH)) have been found to be suita-
ble for various types of bifunctional H2 transformations,
such as hydrogenation,[10] transfer hydrogenation,[11] and de-
hydrogenation reactions.[12,13] However, traditionally, the cat-
alysts that were used in these reactions were mostly based
on the platinum-group metals Ru, Rh, and Ir,[14] which have
to be recycled in larger-scale transformations. Therefore, ef-
forts to replace these metals with non-precious and, impor-
tantly, less-toxic metals (“cheap metals for noble tasks”)[15]
is an appealing challenge. Thus, in recent years, iron cataly-
sis has received significant attention.[16] Molybdenum and
tungsten catalysis would also largely satisfy these require-
ments and offer valuable alternatives to platinum-group
The catalytic hydrogenation of polar unsaturated functional
groups has been widely applied in industry and in academia.
These reactions mainly proceed along Wilkinson and
Osborn hydrogenation mechanisms,[1] which involve the fol-
lowing key steps: 1) Coordination of the unsaturated sub-
strate onto a vacant metal center; 2) cis-insertion of the sub-
À
strate into a metal hydride bond; 3) homolytic splitting of
H2 through oxidative addition to the metal center; and 4) re-
À
ductive elimination of a C H bond to afford the saturated
product. Alternatively, the hydrogenation reactions may
occur with a “bifunctional” metal–ligand system with more
or less simultaneous heteropolar H-atom transfer onto unsa-
turated substrates.[2,3] The preceding heterolytic splitting of
[a] Dr. S. Chakraborty, Dr. O. Blacque, Dr. T. Fox, Prof. H. Berke
Institute of Inorganic Chemistry
University of Zurich
Winterthurerstrasse 190
CH-8057 Zꢀrich (Switzerland)
E-mail: hberke@aci.uzh.ch
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201301106.
Chem. Asian J. 2014, 9, 328 – 337
328
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
metals, owing to their low cost and environmentally benign
nature. However, the organometallic chemistry of molybde-
num and tungsten for their application in effective homoge-
neous hydrogenation reactions remains insufficiently ex-
plored. The initial work of Bullock et al.[17] on ionic hydro-
genation reactions of ketones and our work[18] on the hydro-
genation of imines catalyzed by molybdenum and tungsten
systems seem to be promising cases for catalyst-tuning ef-
forts.
Replacing expensive and toxic platinum-group metals by
complexes with middle-transition-metal elements rhenium,
molybdenum, and tungsten has been a long-standing focus
of our research group. In fact, we recently reported[19] the
tuning of rhenium-nitrosyl-based catalysts to afford excel-
lent performance for the hydrogenation of olefins (with
comparable activities to precious-metal catalysts), as well as
other hydrogen-related reactions, such as hydrosilylation,[20]
dehydrogenative silylation,[21] and dehydrogenative amino-
borane-coupling reactions.[22]
Because, in bifunctional catalysis, simultaneous transfer of
the proton and the hydride is perceived to happen in the
secondary coordination sphere, one might, in principal, an-
ticipate less influence of the metal center, which, conse-
quently, would make many transition metals suited for the
given task. Herein, we sought to confirm the ability of ap-
propriately substituted molybdenum and tungsten centers in
low oxidation states to interact with H2 and then to test
these catalysts in hydrogenation reactions.
Scheme 1. Preparation of the amino and amido complexes.
The application of KOtBu to the reaction with compound
1a at room temperature led to chloride substitution, thereby
forming alkoxide complex [Mo(NO)(CO)ACTHNUTRGENNUG ACHTUNGTRENNUNG(OtBu)]
(PNHP)
(2a, Scheme 1). The 31P{1H} NMR spectrum of compound
2a only showed a single resonance at d=52 ppm, which pro-
vided evidence for the chemical equivalence of the phospho-
rus atoms and the absence of isomers. A strong IR band at
1883 cmÀ1 indicated the presence of a CO group. X-ray dif-
fraction analysis[33] of compound 2a (see the Supporting In-
formation, Figure S3) revealed a pseudo-octahedral geome-
try around the metal center.
Reactions of compounds 1a and 1b with the comparative-
Results and Discussion
ly strong and sterically hindered amide base Na[NACHTUNGTRENNUNG(SiMe3)2]
À
led to dehydrochlorination with deprotonation of the N H
Preparation of Amino and Amido Complexes 1a–3b
New pentacoordinate Mo0- and W0-amide complexes
moieties to form the highly air-sensitive and reactive 16eÀ
complexes [M(NO)(CO)ACTHNUTRGNE(UNG PNP)] (M=Mo, 3a; W, 3b) in
[M(NO)(CO)
G
57% and 54% yield, respectively (Scheme 1). IR bands at
1893 (3a) and at 1871 cmÀ1 (3b) were assigned to the car-
bonyl ligands. The 31P{1H} NMR spectra of compounds 3a
and 3b showed unique signals at d=82 and 81 ppm, thus
suggesting chemical equivalence of the phosphorus atoms
and the absence of isomers. These complexes were quite
soluble in benzene, toluene, and THF. Presumably owing to
a relatively high degree of unsaturation of the metal centers,
the complexes reacted in CH2Cl2 to gradually form chloride
complexes 1a,b, although the formation of supposed by-
(iPr2PCH2CH2)2N)) were synthesized by ligand-substitution
reactions. Thus, the reaction of precursor complexes
[Mo(NO)(CO)
with (iPr2PCH2CH2)2NH in THF at 908C resulted in the for-
mation of [Mo(NO)(CO)
(PNHP)Cl] (1a) and [W(NO)(CO)-
(PNHP)Cl] (1b, Scheme 1), which were isolated in 74% and
ACHTUNGTRENNUNG
78% yield, respectively. The IR spectra of compounds 1a
and 1b exhibited strong n(CO) absorptions at 1912 and
1894 cmÀ1, respectively. Single-crystal X-ray diffraction anal-
ysis[33] of compounds 1a and 1b (see the Supporting Infor-
mation) showed that the metal centers possessed distorted
octahedral geometries. The nitrosyl groups were positioned
trans to the chloride ligand in both cases. The existence of
cis and trans isomers of the nitrosyl groups with respect to
1
product 1,2-dichloroethene was not detected in the H NMR
spectra. The formation of compounds 1a,b was supported
by X-ray diffraction of compound 1a (obtained from the re-
action of compound 3a with CH2Cl2) and by 1H and
31P NMR spectroscopy; moreover, the addition of Na[N-
À
the position of the amine proton (N H) in compounds 1a
ACHTUNGTRENN(NUG SiMe3)2] caused the regeneration of amido complexes 3a,b.
or 1b was supported by the observation of two signals in the
31P{1H} NMR spectra. Attempts to separate the isomers of
compounds 1a and 1b failed, even by column chromatogra-
phy at low temperatures. The isomers showed similar depro-
tonation behaviors and, therefore, could be used as a mixture
in the subsequent deprotonation reactions.
Deep-red single crystals suitable for X-ray diffraction were
obtained for compounds 3a and 3b from concentrated solu-
tions in pentane at À308C.
Importantly, coordinatively unsaturated neutral amides 3a
and 3b are the first structurally characterized pentacoordi-
nate d6 Mo- and W-amido complexes.[23] The structures of
compounds 3a and 3b are shown in Figure 1. The NACHTUNGTRENNUNG(amido)
Chem. Asian J. 2014, 9, 328 – 337
329
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
30 min, an additional resonance was observed at d=
78.9 ppm, thus indicating the build-up of isomeric product
4aACTHNUTRGENNGU(cis) (8% with respect to 4aACHTUNGTREN(NUNG trans), according to
the31P NMR spectrum), with H2 approaching from the NO
1
side. The H NMR spectra of the mixture of 4a
G
(cis) revealed two triplets at d=0.8 (2J
4aACTHNUGTRENNGNU ACHTUNGTRENNUNG
and À1.8 ppm (2J
ACHTUNGTRENNUNG
the hydride ligands of both complexes. These signals
1
became singlets in the phosphorus-decoupled H NMR spec-
tra. Furthermore, the H-31P correlation spectra confirmed
1
that the two hydride signals correlated with different
31P{1H} NMR signals, which again indicated the presence of
two isomers. During the initial hour of the reaction, the in-
tensities of the 31P{1H} NMR signals of compounds 4a
ACHTUNGTRENNUNG
and 4aACHTNUGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Figure 1. Molecular structures of [Mo(NO)(CO)
ACHTUNGTREN(NUNG PNP)] (3a, left) and
[W(NO)(CO)(PNP)] (3b, right); thermal ellipsoids are set at 50% proba-
ACHTUNGTRENNUNG
could only be achieved for a 0.08m solution of compound
3a under 2 bar H2 in [D8]THF after 30 h. DFT calculations
also revealed that there was no significant energy difference
bility. Selected bond lengths [ꢂ] and angles [8] for compound 3a: Mo1–
N1 2.055(4), Mo1–N2 1.856(5), Mo1–C17 1.896(5), C17–O2 1.170(6); N1-
Mo1-N2 140.34(18), P1-Mo1-P2 158.59(5), N2-Mo1-C17 91.2(2). Selected
bond lengths [ꢂ] and angles [8] for compound 3b: W1–N1 2.061(5), W1–
N2 1.882(5), W1–C1 1.869(6), C1–O1 1.182(6); N1-W1-N2 139.1(2), P1-
W1-P2 158.81(6), N2-W1-C1 91.3(2).
between structures 4aACTHNUTRGENN(UG trans) and 4aAHCTUNGTERNN(UGN cis) (DE=0.01 kcal
molÀ1, see the Supporting Information). A plot of the
change in the signal intensities in the 31P NMR spectrum of
4a
Information, Figure S4. Notably, when the equilibrium mix-
ture of compounds 4a(trans), 4a(cis), and 3a was left under
ACHUTGTNRNEUG(N trans) and 4aAHCTUNGTRNEN(UGN cis) with time is shown in the Supporting
atoms have shorter distances to the metal centers than the
A
ACHTUNGTRENNUNG
N
G
a H2 atmosphere (2 bar), the constituents were present in
a 7:7:6 equilibrium ratio, even after 5 days. However, com-
pounds 4aACHTUNTRGNNEG(U cis) and 4aAHCTUNGTREN(NUGN trans) were only stable in a H2 at-
formation), thus indicating pp(N) dp(M) double-bond char-
acter.
Low n(CO) stretching frequencies of compounds 3a,b in-
dicated that the CO groups were located in the trigonal
plane of overall pseudo-trigonal-bipyramidal coordination
geometries with significantly “deviated” equatorial CCO-M-
NNO angles close to 908 in the 16eÀ cis-labilization inter-
mediates.[24]
mosphere, which prevented their isolation. Thus, they were
1
characterized in solution by H and 31P{1H} NMR spectros-
copy. The equilibration process of the 4a
ture was thought to proceed through the dissociation of H2
from 4a(trans) then re-addition from the NO side to form
4a(cis).
ACHUTGTNRENN(GU trans)/4aACHTUNGTRENN(UGN cis) mix-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
When compound 3b was pressurized with H2 (2 bar) and
left for 1 h at room temperature, two signals were observed
in the 31P{1H} NMR spectrum at d=64.7 and 65.4 ppm,
Reactions of Compounds 3a and 3b with Hydrogen
Compound 3a reacted slowly with H2 (2 bar) at room tem-
perature to form pseudo-octahedral complexes cis-
which corresponded to 4bACTHNUTRGENNU(G trans) and 4bHCATUNGTRENN(UGN cis), respectively
[Mo(NO)(CO)H
[Mo(NO)(CO)H
dride atom at the Mo center and a NH group on the PNP
ligand. Heterolytic H2 splitting across the M=N bond had
occurred (Scheme 2). 31P{1H} NMR analysis within the first
15 min of the reaction displayed one signal at d=78.5 ppm,
N
(4a
E
and
trans-
in a 4:1 ratio. Unlike in the Mo case, complete disappear-
ance of the starting material was observed and no change in
ACHTUNGTNER(NUNG trans)), which showed a hy-
the intensities of the 31P{1H} NMR signals of 4b
ACHTUNGTRENNUNG
4b
ACHTUNGTRENNUNG
the complete formation of the products and excluding an
1
equilibrium for the H2 addition. The H NMR spectra exhib-
owing to the formation of isomer 4a
proaching the Mo=N bond from the CO-ligand side. After
ited two triplets at d=2.85 (2J
ACHTUNGTRENNUNG
(2J
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
respectively. In the IR spectra,
one characteristic n(WH) band
appeared at 1612 cmÀ1 for both
isomers 4bACTHNUGTRENNUG(cis) and 4bACHTUNGTRENNUNG(trans),
along with one sharp signal at
1556 cmÀ1, which was assigned
to the nitrosyl stretching vibra-
tions, in accordance with similar
Scheme 2. H2 activation by amido complexes 3a and 3b.
Chem. Asian J. 2014, 9, 328 – 337
330
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
tungsten complexes reported earlier by ourselves.[25] These
complexes were stable under nitrogen atmosphere, but
compounds 3a or 3b at room temperature, higher catalyst
loadings were envisaged and also longer reaction times to
achieve full conversions. Indeed, when the temperature was
increased to 1408C, the catalytic activities of compounds 3a
and 3b were boosted.
Optimization experiments showed that the hydrogenation
of N-benzylideneaniline could be greatly improved by ap-
plying higher temperatures (80–1408C) and H2 pressures
(20–60 bar). These catalytic hydrogenation reactions seemed
to work best in aprotic solvents, because no hydrogenation
was observed in isopropanol, MeOH, or EtOH. Eventually,
a temperature of 1408C and 60 bar H2 pressure were found
to be the optimum conditions, thereby achieving a maximum
TOF of 1600 hÀ1 for the hydrogenation of N-benzylideneani-
line in toluene (Table 1, entry 6). Aware of the previous
work in our group on rhenium complexes, in which dramatic
improvements in the catalytic hydrogenation of olefins were
could not be separated. The ratio of isomers 4b
4b(cis) was approximately the same as the initial ratio. We
attempted to change the apparently kinetically determined
ratio in the 4b(trans)/4b(cis) mixture by heating compound
ACHTUNGTRENUN(NG trans) and
ACHTUNGTRENNUNG
A
U
3b to 1408C and 60 bar H2. After a reaction time of 20 min
and immediate cooling of the mixture, the 31P NMR spec-
trum revealed a 4bACHTUNGTRENNUNG(trans)/4bCAHTUNGTRENN(UGN cis) ratio of 3:2 (DE=0.4 kcal
molÀ1 from DFT calculations, see the Supporting Informa-
tion).
Catalytic Hydrogenation of Imines
Next, we found that compounds 3a and 3b could be effi-
ciently used for the catalytic hydrogenation of imines with-
out the addition of a co-catalyst. The catalytic activity of
compound 3a was tested in initial room-temperature experi-
ments to probe the hydrogenation of N-benzylideneaniline
under 60 bar H2 pressure in toluene with a loading of
2 mol% of catalyst 3a. After a reaction time of 1 h, GCMS
analysis of the reaction mixture revealed the formation of
phenylbenzylamine as the hydrogenated product, but it was
obtained in a low 30% yield with a low maximum TOF of
15 hÀ1. The reaction went to completion within 3.5 h
(Table 1, entry 1). Under the same conditions but with cata-
lyst 3b, only 25% phenylbenzylamine was obtained after
12 h, with a TOF of 1 hÀ1 (Table 1, entry 2). To improve the
performance of the hydrogenation of imines catalyzed by
observed by using BACHTNUGTRENN(UG C6F5)3 or R3SiH/BAHCTUNTGRNE(NUGN C6F5)3 as co-catalys-
t,[19a,26] we also attempted to accelerate the imine-hydrogena-
tion reaction in the same manner. However, the addition of
co-catalysts led to a decrease in the catalytic activity. A
loading of 0.17 mol% of compound 3a in the presence of B-
ACHUTNGERNNU(G C6F5)3 or a Et3SiH/BAHCTUGNTREN(NUGN C6F5)3 mixture as a co-catalyst led to
TOFs of 528 and 576 hÀ1, respectively (Table 1, entries 7 and
8) in the hydrogenation of N-benzylideneaniline under
60 bar H2 at 1408C.
Next, several other imines were also tested by using cata-
lysts 3a and 3b under the same conditions as above without
the addition of any co-catalyst, although the catalyst load-
ings were varied to achieve optimum performance (Table 2).
Complex 3a showed the highest catalytic activity for the hy-
drogenation of N-(4-methoxybenzylidene)aniline, with
a maximum TOF of 2912 hÀ1 (Table 2, entry 19), which is
the highest reported TOF for a Mo-based hydrogenation
catalyst to date. Various para-substituted imines (p-
Table 1. Optimization of the imine hydrogenation conditions using cata-
lyst 3a.
ClC6H4CH=N-p-C6H4Cl,
PhCH=N-(a-Np),
p-
MeOC6H4CH=NPh, PhCH=N-p-C6H4OMe, p-ClC6H4CH=
NPh, PhCH=N-p-C6H4Cl, p-MeOC6H4CH=N-p-C6H4OMe,
p-MeOC6H4CH=N-p-C6H4Cl, p-ClC6H4CH=N-p-C6H4OMe,
p-FC6H4CH=NPh, and PhCH=N-p-C6H4F) were fully con-
verted into their corresponding amines in less than 1 h at
1408C under 60 bar H2. p-NO2C6H4CH=NPh was not re-
duced (Table 2, entry 27).
The catalytic performance of compound 3a for the hydro-
genation of carbonyl functionalities was also tested, but
these transformations were much less effective. Acetophe-
none was hydrogenated into 1-phenylethanol in 32% yield
after 3.5 h with 1 mol% loading of catalyst 3a at 1408C
under 60 bar H2 in toluene. However, the maximum initial
TOF was only 14 hÀ1 (Table 1, entry 10). No hydrogenation
was observed when benzaldehyde was used as a substrate
with catalyst 3a at 1408C and 60 bar H2.
Entry
S/C
T
[8C]
P
TOF[a]
Conversion[b]
[%]
A
A
]
1[c]
2[d]
3
4
5
50
50
RT
RT
80
60
60
60
60
20
60
60
60
60
60
60
15
1
208
540
1272
1600
528
576
1120
14
100
25
52
68
53
68
22
24
70
32
0
100
200
600
600
600
600
400
100
100
100
140
140
140
140
140
140
140
6
7[e]
8[f]
9[g]
10[h]
11[i]
–
[a] TOFs were calculated from GCMS after an initial 15 min. [b] Conver-
sions were determined by GCMS after 15 min, based on the consumption
of the substrate. [c] The TOF was determined after 1 h and the conver-
sion was determined after 3.5 h by GCMS. [d] Compound 3b was used as
the catalyst; conversion was determined after 12 h and the TOF was cal-
culated from the conversion after 12 h. [e] B
ACHTNUGTNER(UNNG C6F5)3 was used as a co-cat-
alyst. [f] Et3SiH/B(C6F5)3 was used as the co-catalyst. [g] THF was used
ACHTUNGTRENNUNG
as the solvent. [h] Acetophenone was used as the substrate and the TOF
was calculated after 1 h. [i] Benzaldehyde was used as the substrate. S/
C=substrate/catalyst molar ratio.
Chem. Asian J. 2014, 9, 328 – 337
331
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
Table 2. Hydrogenation of various imines by using catalysts 3a, 3b, and
mixture of compounds 4b(cis) and 4b(trans) under the optimized condi-
A
ACHTUNGTRENNUNG
tions.
Entry Substrate
Cat. S/
C
TOF[a]
[hÀ1
t
Conversion[b]
[%]
G
]
[h]
12
PhCH=NPh
PhCH=NPh
PhCH=NPh
p-ClC6H4CH=N-p-
C6H4Cl
3a 600 1600
3b 200 424
4b 200 356
3a 400 480
<0.5 >97
13
<1
1
>99
99
14[c]
15
<1
>99
16
p-ClC6H4CH=N-p-
C6H4Cl
3b 400 464
<1
>99
17
18
19
PhCH=N-(a-Np)
PhCH=N-(a-Np)
p-MeOC6H4CH=NPh 3a 800 2912
p-MeOC6H4CH=NPh 3b 400 1120
3a 200 792
3b 200 112
0.25 >99
>98
0.5 >99
1
Figure 2. Hammett plot for the hydrogenation of various para-substituted
imines catalyzed by compound 3a, as extracted from the initial TOF
values. &: 1=À3.69, hydrogenation reactions with para substitution on
the benzylidene side; *: 1=0.68, hydrogenation reactions with para sub-
stitution on the aniline side. Left to right: p-MeO, p-H, p-F, and p-Cl
groups.
20
<1
>99
>99
21[d]
PhCH=N-p-
3a 600 1440
<1
C6H4OMe
p-ClC6H4CH=NPh
PhCH=N-p-C6H4Cl
22
23
24
3a 200 496
3a 800 1979
<1
<1
<1
>98
>97
77
p-MeOC6H4CH=N-p- 3a 200 546
C6H4OMe
p-ClC6H4CH=N-p-
C6H4OMe
p-MeOC6H4CH=N-p- 3a 200 744
C6H4Cl
p-NO2C6H4CH=NPh 3a
p-FC6H4CH=NPh
PhCH=N-p-C6H4F
PhCH=NiBu
tron-donating groups, whereas a positive 1 value indicates
the build-up of negative charge at the reaction center and
that the rate of the reaction is enhanced by electron-with-
drawing groups. As expected, for the hydrogenation of ben-
zylideneaniline, an electron-donating group on the benzyli-
dene side and an electron-withdrawing group on the aniline
side increased the overall rate (Table 2, entries 19, 21–23,
25, 26, 28, and 29), thereby affording a strongly polarized
C=N bond in the transition
25[d]
26
3a 100 332
<1
>99
0.25 >93
27
28
29
30
50
–
14
0
99
>99
0
3a 200 712
3a 800 1920
<1
<1
1
3a
50
–
[a] TOFs were calculated by GCMS after an initial 15 min. [b] Conver-
sions were determined by GCMS, based on the consumption of the sub-
strates. [c] 4b=mixture of 4bACTHNUTRGNENG(U cis) and 4bACHTUTGNREN(NUNG trans). [d] TOFs and conver-
sions were determined by integration of the H NMR spectra. Np=naph-
thyl.
state with an emphasis on the
polar canonical form.
1
Kinetic Studies
Mechanistics Studies of Imine Hydrogenation
The dependence of the hydrogenation rate on the concen-
trations of amido complex 3a and N-benzylideneaniline, as
well as on H2 pressure, were investigated in toluene at
1408C; the conversion of the imine was monitored by
1H NMR spectroscopy and GCMS analysis. Initially, five ki-
netic experiments were carried out by varying the concen-
tration of N-benzylideneaniline whilst keeping the concen-
tration of the catalyst and H2 pressure fixed at 2 bar (see the
Supporting Information, Table S2, runs 1–5). Furthermore,
the initial rates were determined by varying the concentra-
tion of the catalyst whilst keeping the concentration of N-
benzylideneaniline fixed under 2 bar H2 (see the Supporting
Information, Table S2, runs 2, 6–9). The initial rate data (see
the Supporting Information, Table S2) revealed that the rate
of the reaction was independent of the substrate concentra-
tion, but varied with the catalyst concentration: zeroth order
in the substrate and first order in catalyst 3a.
Hammett Correlations for the Imine-Hydrogenation
Reactions
To conclude our mechanistic investigation and to understand
the electronics of the imine-hydrogenation process, we ex-
plored the influence of various para substituents on the aro-
matic imines by using Hammett[27] correlations (Figure 2)
with catalyst 3a and a series of p-H, p-OMe-, p-F-, and p-
Cl-substituted N-benzylideneanilines that were substituted
on both the benzylidene and aniline sides. The initial rates
(in sÀ1) were obtained from the TOF (hÀ1) values (TOF=re-
action rate normalized by the catalyst concentration). The
Hammett correlations were established by plotting the
ln(hydrogenation rate) versus the substituent constant (s),
which gave a straight line with a negative slope of À3.69 for
para substitution on the benzylidene side.
Para substitution on the aniline side also gave a straight
line, but with a positive slope of 0.68. The Hammett param-
eter (1) indicates the susceptibility of a transformation to-
wards the electronic influence of the substituents. A nega-
tive 1 value greater than 1 indicates the build-up of positive
charge at the reaction center in the transition state and that
the rate of the reaction is enhanced by the presence of elec-
A typical kinetic plot of the concentration of N-benzylide-
neaniline over time and the dependence of the initial rate
on the catalyst concentration (at a fixed substrate concentra-
tion) are shown in Figure 3.
The dependence of the reaction on H2 pressure was also
investigated by determining the initial rates at 20, 40, and
60 bar H2 pressures, whilst keeping all other concentrations
Chem. Asian J. 2014, 9, 328 – 337
332
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
Figure 3. Top: Kinetic plot of the concentration of N-benzylideneaniline
over time; initial [N-benzylideneaniline]=0.44m, [3a]=0.0088m, 2 bar
H2, 1408C. Bottom: Plot of the initial rates at various concentrations of
catalyst 3a; [N-benzylideneaniline]=0.44m, 2 bar H2, 1408C.
Figure 4. Top: Kinetic conversion chart of N-benzylideneaniline catalyzed
by compound 3a at 1408C. &: 60 bar H2; ~: 40 bar H2; *: 20 bar H2.
Bottom: Plot of the initial rate/ACHTNURGTNEUNG[3a] versus H2 pressure.
[D8]THF was filled with 2 bar H2 and left to stand for sever-
al hours at room temperature. After a reaction time of 90 h,
complete conversion of compound 3b into hydride species
constant (see the Supporting Information, Table S3). A plot
of the initial rate/ACHTUNGTRENNUNG[3a] versus hydrogen pressure is shown in
Figure 4, right. This result indicated that the reaction rate
was also dependent on hydrogen pressure.
4bACTHNGURTENNU(G cis) and 4bACTHUNGTRENN(UNG trans) was observed. Then, the hydrogen
pressure was released and 10 equivalents of N-benzylidenea-
niline were added with stirring. After 10 min, the 31P NMR
spectra of the reaction mixture revealed the complete disap-
Stoichiometric Model Experiments
To further investigate the mechanism of the catalytic reac-
tion, stoichiometric experiments were carried out. For exam-
ple, equal amounts of catalyst 3a and N-benzylideneaniline
were left to stand at room temperature for 1 h, but no
pearance of the signals of 4bACTHNGUTRENNU(G cis) and 4bCAHTUNGTRNEN(UNG trans). The pres-
ence of the signal of compound 3b at d=82 ppm indicated
complete H-transfer from the hydride species to the N-ben-
zylideneaniline and regeneration of the amide species. In ad-
dition, the 1H NMR spectra of the reaction mixture con-
firmed the presence of phenylbenzylamine, along with the
remaining N-benzylideneaniline.
Again, when the hydrogenation of N-benzylideneaniline
(40 equiv with respect to the catalyst) was carried out with
catalysts 3a and 3b (5 mg in [D8]toluene) under 2 bar H2 at
1408C for 15 min, 28% and 15% conversions of N-benzyli-
deneaniline into phenylbenzylamine were observed, respec-
tively, with the catalysts as the only other detectable species.
Catalysts 3a or 3b and N-benzylideneaniline and phenyl-
benzylamine were identified during the catalytic reactions at
1408C and 60 bar H2 at an incomplete stage of conversion.
Therefore, catalysts 3a and 3b are presumed to be the
active species that accumulate before the rate-determining
step. In contrast, hydride complexes of type 4 were consid-
ered to be relatively short-lived intermediates, because the
1
change was observed in the H and 31P NMR spectra. Even
heating of the stoichiometric mixtures to 1408C for a mini-
mum of 3 h did not induce any changes in the 31P and
1H NMR spectra, which excluded the possibility of direct
binding of the substrate to the catalytic metal center and
also revealed that the binding was not a step in the catalytic
reaction.
Additional mechanistic studies were expected to clarify
À
the H-transfer sequence from the NH MoH moiety, that is,
whether it occurred stepwise or simultaneously. First,
though, we had to examine whether the H-transfers onto
the imine were rate limiting. For this purpose, we chose
compounds 4b
N
ACHTUNGTRENNUNG
cause hydride species 4a
N
ACHTUNGTRENNUNG
under a H2 atmosphere, which would complicate the meas-
urements. Initially, a 0.018m solution of compound 3b in
Chem. Asian J. 2014, 9, 328 – 337
333
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
transfer of the hydrogen atoms onto the imines was appa-
rently fast. If the hydride transfer would be rate limiting,
one would expect that they would accumulate during the
catalytic reaction,[28,17a] which we did not observe.
To compare the reaction kinetics of catalyst 3b and hy-
dride complexes 4bACTHUNRGTENN(UG cis)/4bAHCTUNGTRENN(UGN trans), catalytic experiments
were carried out in toluene at 1408C and 60 bar H2. An ini-
tial TOF of 356 hÀ1 was obtained for the hydrogenation of
N-benzylideneaniline with the mixture of 4bACHTUNGTNER(NUNG cis) and 4b-
ACHTUNGTRENNUNG
(Table 2, entries 13 and 14). Thus, there was a slight differ-
ence between the catalytic activities, but we did not think
that this difference was significant, because it might have or-
iginated from the fact that the mixture of hydride complexes
4bACHTUNGTRENNUNG(cis) and 4bACHTUNGTRENNUNG(trans) in toluene was less soluble than com-
Scheme 3. Proposed mechanism for the imine-hydrogenation reaction.
plex 3b.
Deuterium Kinetic Isotope Effect Experiments
Filtration Experiments as Homogeneity Tests of the Imine-
Hydrogenation Reactions
To gain further insight into the deuterium isotope kinetics,
we performed the hydrogenation of N-benzylideneaniline
catalyzed by compound 3a at 1408C with 20 bar of H2 or
D2. A loading of 0.5 mol% of catalyst 3a to a solution of N-
benzylideneaniline in toluene led to a TOF of 756 hÀ1 under
20 bar D2, which corresponded to a DKIE of kH/kD =1.28,
that is, a very small kinetic isotope effect. This result might
Filtration experiments[30] were carried out to exclude any
heterogeneous side-reactions during the hydrogenation cat-
alysis. For this purpose, 1 mol% of catalyst 3b was loaded at
1408C under 60 bar H2 for the hydrogenation of N-benzyli-
deneaniline. After 7 min, the vessel was immediately cooled
to room temperature and moved into a glove box. The reac-
tion mixture was still a red color and there were no visible
precipitates. GCMS analysis of the reaction mixture showed
50% conversion of the imine into the corresponding amine.
À
imply that H H bond splitting is not directly involved in the
rate-limiting step, because the KIEs of reactions with rate-
determining H2 splitting generally give relatively high posi-
tive values.
1
In separate experiments, we also reacted 2 bar of H2 or
D2 at room temperature with a 0.03m solution of compound
3b in THF (6.5 mg of 3b in 0.4 mL THF). After 72 h, 89%
The 31P and H NMR spectra only revealed the presence of
compound 3b. After filtration of the solution, the experi-
ment was repeated once more, but showed no black precipi-
tates or dark solutions, thus making the formation of metal
particles less plausible.
formation of hydride complexes 4bACHTUNTRGENN(UG cis) and 4bACHTUTGNREN(NUGN trans) was
observed for the H2 reaction and, in a related experiment
with D2, 73% formation of deuterated type-4 compounds
was observed, as indicated by the 31P NMR spectra, which
corresponded to a kH/kD value of 1.22. This low DKIE value
of the H2 and D2 reactions to form the type-4 hydride com-
plexes supported the conclusion that any type of H2 splitting
did not participate in the rate-determining step.
Therefore, it was quite plausible to assume that the addi-
tion steps of H2 to the amido complexes of type 3 were rate
determining,[4c,29] which consisted of initial H2 uptake to the
molybdenum center. Then, heterolytic splitting of the H2
ligand would occur through proton abstraction by the basic
amido nitrogen atom. The H2-addition step was assumed to
be accompanied by extensive structural rearrangement,
which would transform the trigonal-bipyramidal geometry
of the amido complex into an octahedral geometry. The con-
comitant rearrangement energy was naturally independent
of the H2-splitting step, but apparently higher in energy than
the heterolysis of H2.
Conclusions
We have developed highly efficient non-platinum-metal cat-
alysts for imine-hydrogenation reactions by using molybde-
num- and tungsten-amido complexes. Reactive amido com-
plexes of the type [M(NO)(CO)
3b) were prepared by the dehydrohalogenation of com-
pounds 1a,b by using Na[N(SiMe3)]2. These amido com-
ACHTUNGTREN(NUGN PNP)] (M=Mo, 3a; W,
AHCTUNGTRENNUNG
plexes cleaved dihydrogen in a heterolytic fashion across the
polar M=N double bond to generate hydride-amine com-
plexes with a formal 1,2-addition that was related to frus-
trated Lewis pairs.[31] Amido complexes 3a,b could be used
as excellent imine-hydrogenation catalysts. Detailed kinetic
studies, mechanistic investigations, and Hammett studies
supported the H-transfer onto the imine through a secon-
dary-coordination-sphere mechanism, similar to Noyori-type
metal–ligand bifunctional catalysis, whilst the addition of hy-
drogen did not proceed through a secondary-coordination-
sphere-type mechanism; rather, the addition to the metal
center was the rate-determining step, with considerable rear-
rangement of the coordination sphere.
Based on the kinetic studies, mechanistic experiments,
and Hammett correlations, a secondary-coordination-sphere
mechanism analogous to Noyori’s bifunctional catalysis is
proposed for the H-transfers onto the imine moieties, as
shown in Scheme 3.
Chem. Asian J. 2014, 9, 328 – 337
334
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
in Et2O and filtered. Then, the solution was concentrated and stored in
the fridge at À308C. Yellow single crystals suitable for X-ray diffraction
were obtained after several days. Yield: 69 mg (65%); 1H NMR
(400 MHz, [D8]THF): d=4.9 (br s, 1H; NH), 3.40–3.29 (m; NCH2), 2.69–
2.64 (m, 2H; NCH2), 2.45–2.33 (m, 4H; CH), 1.62–1.53 (m, 4H; PCH2),
1.37–1.19 (m; CH3), 0.99 ppm (s; CH3); 13C{1H} NMR (100.6 MHz): d=
Experimental Section
General Procedures
All experiments were carried out under a N2 atmosphere by using either
glove-box or Schlenk techniques. Reagent-grade solvents benzene, THF,
pentane, toluene, and Et2O were dried with sodium benzophenone and
distilled prior to use under a N2 atmosphere. Anhydrous toluene (99.8%,
active dry) for the catalytic experiments was purchased from Alfa Aesar.
CH2Cl2 was dried over calcium hydride and distilled. Deuterated solvents
were dried with sodium benzophenone ketyl ([D8]THF, [D8]toluene, and
C6D6) and calcium hydride (CD2Cl2) and distilled by a freeze-pump-thaw
246.91 (t, 2J
vJ(C,P)=7.2 Hz; CH2), 20.94 (t, vJ
(C,P)=4.8 Hz; CH3), 17.72 (t, vJ
A
ACHTUNGTNER(NUNG C,P)=5.9 Hz; CH), 23.57 (t,
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
CH3); 31P{1H} NMR (161.9 MHz, [D8]THF): d=50.98 ppm (s); IR (KBr):
n˜ =1558 (NO), 1883 cmÀ1 (CO); elemental analysis calcd (%) for
C21H47MoN2O3P2: C 47.28, H 8.88, N 5.25; found: C 46.90, H 8.60, N 5.01.
cycle prior to use. The [M(NO)(CO)
and HN(CH2CH2iPr2)2 were prepared according to literature proce-
dures.[32] KOtBu and Na[N
(SiMe3)2] were purchased from commercially
4ACHTUNGRTEN(NUNG ClAlCl3)] (M=Mo, W) complexes
General Procedure for the Preparation of [M(NO)(CO)ACTHNUTRGNE(UNG PNP)] (M=Mo,
3a; W, 3b)
AHCTUNGTRENNUNG
available sources and used without further purification. NMR spectra
were measured on Varian Mercury 200 (200.1 MHz for H and 81.0 MHz
1
To a solution of [M(NO)(CO)
Na[N(SiMe3)2] (1.22 mmol, 1.5 equiv, 1m in THF) and the resulting mix-
(PNHP)Cl] (0.81 mmol) in THF was added
ACHTUNGTRENNUNG
for 31P), Varian Gemini-300 (300.1 MHz for 1H and 75.4 MHz for 13C),
Bruker-DRX 500 (500.2 MHz for 1H, 202.5 MHz for 31P, and 125.8 MHz
for 13C), and Bruker-DRX 400 spectrometers (400.1 MHz for 1H,
162.0 MHz for 31P, and 100.6 MHz for 13C). All 1H and 13C{1H} chemical
shifts are expressed in ppm relative to tetramethylsilane (TMS); 31P{1H}
chemical shifts are expressed relative to 85% H3PO4 as an external stan-
dard. Signal multiplicities are expressed as followed: s singlet, d doublet,
t triplet, q quartet, m multiplet. IR spectra were obtained by using either
the ATR or KBr methods on a Bio-rad FTS-45 instrument. Elemental
analysis was carried out at the Anorganisch-Chemisches Institut of the
University of Zurich. GCMS spectra were recorded on a Varian Saturn
2000 spectrometer that was equipped with a Varian 450-GC chromato-
graph (Phenomenex ZB-5ms (30 m), Gradient 70–270o).
AHCTUNGTRENNUNG
ture was stirred for 30 min. After the completion of the reaction (by
31P{1H} NMR spectroscopy), the product was filtered and dried in vacuo.
Then, the desired product was extracted with pentane. Finally, it was con-
centrated and stored in the fridge at À308C. Red crystals were obtained
after several days.
3a: Yield: 210 mg (57%); 1H NMR (500 MHz, [D8]THF): d=3.50–3.42
(m, 2H; NCH2), 3.36–3.28 (m, 2H; NCH2), 2.36–2.23 (m, 4H; CH), 2.11–
2.04 (m, 2H; PCH2), 1.98–1.92 (m, 2H; PCH2), 1.069–1.387 ppm (m,
24H; CH3); 13C{1H} NMR (125.8 MHz, [D8]THF): d=60.5 (t, vJ
ACHTUNGTRENNUNG
8.3 Hz; NCH2), 23.87 (m; CH), 21.29 (t, vJ
ACHTUNGTRENNUNG
CH3), 15.3 ppm (m; CH3); 31P{1H} NMR (161.9 MHz, [D8]THF): d=
82.01 ppm (s); IR (KBr): n˜ =1565 (NO), 1893 cmÀ1 (CO); elemental anal-
ysis calcd (%) for C17H36MoN2O2P2: C 44.54, H 7.92, N 6.11; found:
C 44.74, H 8.01, N 6.23. Assignments in the 1H NMR spectrum were con-
firmed by C-H correlations, long-range C-H correlations, and 13C DEPT
experiments.
General Procedure for the Preparation of [M(NO)(CO)ClACTHNUTRGNEUNG
(PNHP)]
(M=Mo, 1a; W, 1b)
To a solution of [M(NO)(CO)4ACTHNUTRGNE(NUG ClAlCl3)] (1.23 mmol) in THF (15 mL)
3b: Yield: 238 mg (54%); 1H NMR (400 MHz, [D8]THF): d=3.52–3.44
(m, 2H; NCH2), 3.36–3.27 (m, 2H; NCH2), 2.46–2.32 (m, 4H; CH), 2.11–
2.05 (m, 2H; PCH2), 2.02–1.93 (m, 2H; PCH2), 1.29–1.13 ppm (m, 24H;
CH3); 13C{1H} NMR (100.61 MHz, [D8]THF): d=253.6 (s; CO), 63.36 (t,
was added a solution of HN(CH2CH2PiPr2)2 (1.23 mmol) in THF (5 mL)
and the resulting mixture was heated at 908C for 3 h. After the comple-
tion of the reaction, the resulting red solution was filtered off and evapo-
rated to dryness. The solid residue was washed twice with pentane then
dissolved in a minimum amount of THF and pentane was added. The
solid precipitate was separated and dried in vacuo.
vJ(C,P)=8.3 Hz; NCH2), 25.06 (t, vJ(C,P)=11.9 Hz; CH), 24.43 (t, vJ-
ACTHNUTRGENNUG CAHTUNGTRENNUGN
AHCTUNGTREN(GNUN C,P)=10.7 Hz; CH), 21.73 (m; CH2), 16.21 (s; CH3), 15.36 ppm (s;
1
CH3); 31P{1H} NMR (162 MHz, [D8]THF): 81.09 ppm (s, J
ACHTNUTRGNEUNG(P,W) (d, satel-
1a: Yield: 450 mg (74%); 1H NMR (400 MHz, [D8]THF): d=3.7 (br s;
NH, minor isomer), 3.47–3.36 (m; NCH2), 3.2 (br s; NH, major isomer),
2.68–2.53 (m; NCH2), 2.41–2.37 (m; CH), 1.78–1.66 (m; PCH2), 1.36–
1.23 ppm (m; CH3); 13C{1H} NMR (100.6 MHz, [D8]THF): d=246.89 (t,
lite)=320.1 Hz); IR (KBr): n˜ =1541 (NO), 1871 cmÀ1 (CO); elemental
analysis calcd (%) for C17H36N2O2P2W: C 37.38, H 6.64, N 5.13; found:
C 37.72, H 6.74, N 5.01.
2J
(C,P)=7.2 Hz; CH), 21.56 (t, vJ
G
ACHTUNGTRENNUNG
Reaction of Compound 3a with Hydrogen to form Compounds 4a
ACHTUNGTRENNUNG(trans)
E
G
ACHTUNGTRENNUNG
and 4a(cis)
AHCTUNGTRENNUNG
3.6 Hz; CH3), 16.72 (s; CH3), 14.98 ppm (s; CH3); 31P{1H} NMR
(161.9 MHz, [D8]THF): d=60.17 (s; major isomer), 59.09 ppm (s; minor
isomer); IR (KBr): n˜ =1573 (NO), 1912 cmÀ1 (CO); elemental analysis
calcd (%) for C17H37ClMoN2O2P2: C 41.26, H 7.54, N 5.66; found:
C 41.09, H 7.43, N 5.37.
A solution of compound 3a (18 mg, 0.039 mmol) in [D8]THF (0.5 mL) in
a J. Young NMR tube was frozen with liquid nitrogen. Then, the nitrogen
atmosphere was removed by a freeze-pump-thaw cycle and the tube was
filled with 2 bar H2 and sealed. The tube was shaken vigorously and left
1
for a few hours. The slow formation of compounds 4a
was monitored by 31P{1H} NMR spectroscopy. After 30 h, an equilibrium
mixture of compounds 4a(cis), 4a(trans), and 3a (1:1:1 ratio) was ob-
ACHUTGTNREN(UNG trans) and 4aACHTUNGTRENNUNG(cis)
1b: Yield: 560 mg (78%); H NMR (500 MHz, [D8]THF): d=4.3 (s; NH,
major isomer), 3.7 (s; NH, minor isomer), 3.46–3.50 (m; NCH2), 2.73–
2.64 (m; NCH2), 2.57–2.46 (m; CH), 1.79–1.78 (m; PCH2), 1.24–1.38 ppm
G
ACHTUNGTRENNUNG
(m; CH3); 13C{1H} NMR (100.61 MHz): d=246.46 (t, 2J
ACHTUNGTRENNUNG
served. The formation of compounds 4aACTHNUTRGENN(GU cis) and 4aACHTUNGTNER(NUGN trans) was con-
1
firmed by H and 31P NMR spectroscopy in solution, as well as by 31P-1H
CO), 52.59 (t, vJ
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(C,P)=4.8 Hz; NCH2), 27.03 (t, vJ
23.53 (s), 21.77 (t, 1J
N
ACHTUNGTRENNUNG
correlation spectroscopy.
Selected 1H NMR (500 MHz, [D8]THF): d=0.8 (t, 2JPH =26.3 Hz; Mo-H,
CH2), 18.25 (t, 2J
ACHTUNGTRENNUNG(C,P)=3.6 Hz; CH3), 16.71 (s; CH3), 15.38 (s; CH3)
4a
G
(cis)); 31P{1H} NMR
ACHTUNGTRENNUNG
14.84 ppm (s; CH3); 31P{1H} NMR (161.9 MHz, [D8]THF): d=52.03 (s, J-
1
1
(202 MHz, [D8]THF): d=78.5 (s; 4aACTHNUGRTENNUG(trans)), 78.9 ppm (s, 4aACHTUGNTREN(NUGN cis)). Sever-
(P,W) (d, satellite)=305.9 Hz, major isomer), 51.0 ppm (s, J
N
ellite)=302.9 Hz, minor isomer); IR (KBr): n˜ =1556 (NO), 1894 cmÀ1
(CO); elemental analysis calcd (%) for C17H37ClN2O2P2W: C 35.04,
H 6.40, N 4.81; found: C 34.98, H 6.21, N 5.13.
al of the signals in the H NMR spectrum were in their expected regions,
owing to presence of equilibrium mixtures of compounds 3a, 4a(trans),
and 4a(cis).
ACHTUNGTRENNUNG
Synthesis of [Mo(NO)(CO)
(PNHP)
E
Synthesis of [W(NO)(CO)H
4b(trans))
(PNHP)] (Mixture of Isomers 4b
ACHUTGTNRNEUNG CAHUTNGTRENN(GUN cis) and
AHCTUNGTRENNUNG
Compound 1a (0.1 g, 0.20 mmol) and KOtBu (0.035 g, 0.30 mmol) were
mixed in THF (10 mL) in a Schlenk flask and the mixture was stirred for
2 h at RT. After completion of the reaction, the solution was filtered off
and the solvent was removed in vacuo. The crude product was dissolved
A solution of compound 3b (30 mg, 0.056 mmol) in [D8]THF (0.5 mL) in
a J. Young NMR tube was frozen with liquid nitrogen. Then, the nitrogen
atmosphere was removed by a freeze-pump-thaw cycle and the tube was
Chem. Asian J. 2014, 9, 328 – 337
335
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
allowed to warm to RT. The tube was filled with 2 bar H2 and sealed.
The tube was shaken vigorously and left for a few hours. The formation
in toluene and an aliquot was added to the N-benzylideneaniline. Then,
toluene was added to a total volume of 2 mL. The autoclave was closed
and removed from the glove box and pressurized with the required H2
pressure. Then, the autoclave was kept in an oil bath that had been pre-
heated to 1408C. For each run, three separate experiments were carried
out and the conversions were determined at 10, 15, and 20 min intervals
by GCMS.
of compounds 4b
(trans) was supported by 1H and
ACHTUNGTRENNUNG(cis) and 4bACHUTNGTRENNGUN
31P{1H} NMR spectroscopy, as well as by 31P-1H correlation spectroscopy.
1H NMR (400 MHz, 298 K, [D8]THF): d=3.5 (br s, 1H; NH, 4b
G
2
(br s, 1H; NH, 4b
H, 4b
(trans)), 2.75 (t, 2J
CH2), 1.29–1.23 ppm (m; CH3); 13C{1H} NMR (100.62, [D8]THF): d=
247.2 (s; CO), 53.89 (t, vJ(C,P)=4.78 Hz; NCH2), 31.10 (t, vJ
(C,P)=
11.2 Hz; CH), 31.79 (t, vJ
(C,P)=13.1 Hz; CH), 28.52 (CH2P), 20.64 (s;
CH3), 19.43 (s; CH3), 18.56 (s; CH3), 17.74 (s; CH3); 31P{1H} NMR
(161.97 MHz, [D8]THF): d=64.7 (s, 1J
(P,W)=306.7 Hz; 4b(trans))
A
N
A
ACHTUNGTRENNUNG(P,H)=22 Hz; W-H, 4bAHCTUNGTRNE(NUGN cis)), 2.44–1.99 (m; CH,
Filtration Experiments
R
E
In a 30 mL steel autoclave that was equipped with a stirrer bar, N-benzy-
lideneaniline and compound 3b (5 mg of catalyst 3b and 165 mg of N-
benzylideneaniline) were mixed in toluene (0.8 mL). The system was
charged with 60 bar H2 and heated at 1408C with constant stirring. After
7 min, the reaction was manually terminated by cooling the reaction
vessel to RT and removing the maximum amount of H2 pressure. Then,
the autoclave was transferred into a glove box, the rest of the H2 was re-
leased, and the vessel was opened. The solution remained a red color
(the same as before the start of the reaction), without the formation of
any precipitates or colloids. GCMS analysis revealed approximately 50%
conversion. The solution was examined by 31P NMR spectroscopy, which
indicated the presence of compound 3b. The solution was filtered
through Celite into a new vessel with a new stirrer bar, to which N-ben-
zylideneaniline (165 mg) was re-added. No residue was visible on the
Celite surface. Then, the catalysis was repeated at 1408C under 60 bar H2
and the activity remained the same.
C
65.4 ppm (s; 4bACHTUNGTRENNUNG(cis)); IR (KBr): n˜ =1556 (NO), 1618 (WH), 1864 (CO),
3422 cmÀ1 (NH); elemental analysis calcd (%) for C17H38N2O2P2W:
C 37.24, H 6.99, N 5.11; found: C 36.89, H 7.03, N 5.19.
General Procedure for the Catalytic Imine-Hydrogenation Experiments
A stock solution of freshly prepared catalyst (10 mg in 2.5 mL toluene,)
was prepared. An aliquot (0.5 mL) of that stock solution was added to
a solution of imines (according to substrate catalyst ratio) in 1.5 mL tolu-
ene in a steel autoclave. The autoclave was charged with required hydro-
gen pressure and kept in a preheated (1408C) oil bath. After appropriate
reaction times, the autoclave was immediately taken out and cooled to
room temperature and the pressure was released. The reaction mixture
was taken out of the autoclave and filtered through silica gel. Without
further purifications, GC/MS analysis was measured to determine the
conversion of the hydrogenation product and to identify the products.
X-ray Diffraction
Single-crystal X-ray diffraction data were collected at 183(2) K on a Xcali-
bur diffractometer (Agilent Technologies, Ruby CCD detector) for all
compounds by using a single-wavelength Enhance X-ray source with
MoKa radiation (l=0.71073 ꢂ).[34a] Suitable single crystals were mounted
onto the top of a glass fiber by using polybutene oil and fixed onto a goni-
ometer head that was immediately transferred into the diffractometer.
Pre-experiment, data collection, data reduction, and analytical absorption
corrections[34b] were performed with the CrysAlisPro program suite.[34a]
The crystal structures were solved by using direct methods with
SHELXS97.[34c] The structure refinements were performed by full-matrix
least-squares on F2 with SHELXL97.[34c] All of the programs that were
used during the crystal-structure determination were included in the
WINGX software.[34d] PLATON[34e] was used to check the results of the
X-ray analysis and DIAMOND[34f] was used to prepare the molecular
graphics.
GCMS Data for Various Imines and Amines that were Formed during
Hydrogenation Reactions Catalyzed by Compounds 3a and 3b
PhCH=NPh: tr =9.062 min, m/z=181; PhCH2NHPh: tr =9.323 min, m/z=
183; PhCH=N(a-naphthyl): tr =13.260 min, m/z=231; PhCH2NH(a-naph-
thyl): tr =13.673 min, m/z=233; p-ClC6H4CH=NPh: tr =10.284 min, m/z=
215; p-ClC6H4CH2NHPh: tr =10.687 min, m/z=217; PhCH=N-p-C6H4Cl:
tr =10.362 min, m/z=215; PhCH2NH-p-C6H4Cl: tr =10.829 min, m/z=
217;
p-ClC6H4CH=N-p-C6H4Cl:
tr =11.815 min,
m/z=248;
p-
ClC6H4CH2NH-p-C6H4Cl: tr =12.648 min, m/z=251; p-MeOC6H4CH=
NPh: tr =10.934 min, m/z=211; p-MeOC6H4CH2NHPh: tr =11.012 min,
m/z=213; p-MeOC6H4CH=N-p-C6H4Cl: tr =12.554 min, m/z=245; p-
MeOC6H4CH2NH-p-C6H4Cl: tr =12.945 min, m/z=247; p-ClC6H4CH=N-
p-C6H4OMe: tr =12.378 min, m/z=245; p-ClC6H4CH2NH-p-C6H4OMe:
tr =12.521 min,
13.299 min, m/z=241; p-MeOC6H4CH2NH-p-C6H4OMe: tr =12.983 min,
m/z=243. p-FC6H4CH=NPh: tr =8.980 min, m/z=199; p-
m/z=247;
p-MeOC6H4CH=N-p-C6H4OMe:
tr =
FC6H4CH2NHPh: tr =9.359 min, m/z=201; PhCH=N-p-C6H4F: tr =
9.061 min, m/z=199; PhCH2NH-p-C6H4F: tr =9.292 min, m/z=201;
Ph(CO)CH3: tr =4.573 min, m/z=120; PhCH(OH)CH3: tr =4.486 min, m/
z=122.
Acknowledgements
We thank the Swiss National Science Foundation and the University of
Zꢀrich for financial support.
Kinetic Experiments[4c,29]
Catalytic runs 1–9 (see the Supporting Information, Table S2): In a glove
box, a J. Young NMR tube was charged with N-benzylideneaniline (ac-
cording to the required amount from the Supporting Information,
Table S1). A freshly prepared stock solution of catalyst 3a was prepared
in [D8]toluene and an aliquot was added to the N-benzylideneaniline.
The tube was sealed and removed from the glove box and pressurized
with 2 bar H2. Then, the tube was kept in an oil bath that had been pre-
heated to 1408C. The typical CH resonance of the N-benzylideneaniline
(CH=N: singlet, d=8.34 ppm) became less intense and the CH2 signal of
[1] a) J. A. Osborn, F. H. Jardine, J. F. Young, G. Wilkinson, J. Chem.
Soc. A 1966, 12, 1711–1732; b) R. R. Schrock, J. A. Osborn, J. Am.
Chem. Soc. 1976, 98, 2134–2143; c) C. R. Landis, J. Halpern, J. Am.
Chem. Soc. 1987, 109, 1746–1754.
[2] a) R. Noyori, M. Yamakawa, S. Hashiguchi, J. Org. Chem. 2001, 66,
7931–7944; b) K. J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R.
Noyori, Angew. Chem. 1997, 109, 297–300; Angew. Chem. Int. Ed.
Engl. 1997, 36, 285–288; c) K. MuÇiz, Angew. Chem. 2005, 117,
6780–6785; Angew. Chem. Int. Ed. 2005, 44, 6622–6627.
À
the hydrogenated amine product (CH2 NH: doublet, d=4.16 ppm) ap-
peared in the 1H NMR spectra. The concentrations of the product were
determined from integration of the 1H NMR spectra after an initial
15 min, based on the consumptions of the substrate; the rates are listed
in Table S2 in the Supporting Information.
[3] J. S. M. Samec, J. E. Bꢃckvall, P. G. Andersson, P. Brandt, Chem.
Soc. Rev. 2006, 35, 237.
[4] a) S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev.
2004, 248, 2201; b) K. Abdur-Rashid, M. Faatz, A. J. Lough, R. H.
Morris, J. Am. Chem. Soc. 2001, 123, 7473–7474; c) K. Abdur-
Rashid, S. E. Clapham, A. Hadzovic, J. N. Harvey, A. J. Lough,
R. H. Morris, J. Am. Chem. Soc. 2002, 124, 15104–15118; d) H.
Berke, ChemPhysChem 2010, 11, 1837–1849.
Catalytic runs 10–12 (see the Supporting Information, Table S3): In
a glove box, a steel autoclave was charged with N-benzylideneaniline (ac-
cording to the required amount from the Supporting Information,
Table S1). A freshly prepared stock solution of catalyst 3a was prepared
Chem. Asian J. 2014, 9, 328 – 337
336
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
Heinz Berke et al.
[5] a) R. M. Bullock, Chem. Eur. J. 2004, 10, 2366–2374; b) J. S. Song,
D. J. Szalda, R. M. Bullock, C. J. C. Lawrie, M. A. Rodkin, J. R.
Norton, Angew. Chem. 1992, 104, 1280; Angew. Chem. Int. Ed. Engl.
1992, 31, 1233.
[6] a) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97–102;
b) R. Noyori, T. Okhuma, Angew. Chem. 2001, 113, 40; Angew.
Chem. Int. Ed. 2001, 40, 40; c) R. Noyori, Angew. Chem. 2002, 114,
2108–2123; Angew. Chem. Int. Ed. 2002, 41, 2008–2022; d) M. Ya-
makawa, H. Ito, R. Noyori, J. Am. Chem. Soc. 2000, 122, 1466–
1478.
[19] a) Y. Jiang, J. Hess, T. Fox, H. Berke, J. Am. Chem. Soc. 2010, 132,
18233–18247; b) B. Dudle, K. Rajesh, O. Blacque, H. Berke, J. Am.
Chem. Soc. 2011, 133, 8168–8178; c) A. Choualeb, E. Maccaroni, O.
Blacque, H. W. Schmalle, H. Berke, Organometallics 2008, 27, 3474–
3481.
[20] a) S. Chakraborty, O. Blacque, T. Fox, H. Berke, ACS Catal. 2013, 3,
2208–2217; b) H. Dong, H. Berke, Adv. Synth. Catal. 2009, 351,
1783–1788.
[21] Y. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Chem. Eur. J.
2009, 15, 2121–2128.
[7] a) M. Ito, T. Ikariya, Chem. Commun. 2007, 5134–5142; b) T. Ikar-
iya, A. J. Blacker, Acc. Chem. Res. 2007, 40, 1300–1308; c) A.
Comas-Vives, G. Ujaque, A. Lledos, Organometallics 2007, 26, 4135.
[8] a) J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem. Soc.
2005, 127, 10840–10841; b) J. Zhang, G. Leitus, Y. Ben-David, D.
Milstein, Angew. Chem. 2006, 118, 1131–1133; Angew. Chem. Int.
Ed. 2006, 45, 1113–1115; c) C. Gunanathan, Y. Ben-David, D. Mil-
stein, Science 2007, 317, 790–792; d) B. Gnanaprakasam, J. Zhang,
D. Milstein, Angew. Chem. 2010, 122, 1510–1513; Angew. Chem.
Int. Ed. 2010, 49, 1468–1471.
[9] a) Y. Shvo, D. Czarkie, Y. Rahamim, D. F. Chodosh, J. Am. Chem.
Soc. 1986, 108, 7400; b) C. P. Casey, N. A. Strotman, S. E. Beetner,
J. B. Johnson, D. C. Priebe, T. E. Vos, B. Khodavandi, I. A. Guzei,
Organometallics 2006, 25, 1230; c) C. P. Casey, H. R. Guan, J. Am.
Chem. Soc. 2007, 129, 5816; d) Y. Blum, D. Czarkie, Y. Rahamim, Y.
Shvo, Organometallics 1985, 4, 1459–1461.
[10] a) X. Chen, W. Jia, R. Guo, T. W. Graham, M. A. Gullons, K. A.
Rashid, Dalton Trans. 2009, 1407–1410.
[11] a) Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J.
Lough, K. A. Rashid, Organometallics 2006, 25, 4113–4117; b) A.
Choualeb, A. J. Lough, D. G. Gusev, Organometallics 2007, 26,
3509–3515.
[12] a) M. Bertoli, A. Choualeb, A. J. Lough, B. Moore, D. Spasyuk,
D. G. Gusev, Organometallics 2011, 30, 3479–3482; b) M. Bertoli, A.
Choualeb, D. G. Gusev, A. J. Lough, Q. Major, B. Moore, Dalton
Trans. 2011, 40, 8941–8949.
[22] Y. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Organometallics
2009, 28, 5493–5504.
[23] a) D. Morales, J. Pꢄrez, L. Riera, V. Riera, D. Miguel, M. E. G. Mos-
quera, S. G. Granda, Chem. Eur. J. 2003, 9, 4132; b) J. L. Caldarelli,
P. S. White, J. L. Templeton, J. Am. Chem. Soc. 1992, 114, 10097–
10103; c) P. Legzdins, S. J. Rettig, K. J. Ross, Organometallics 1993,
12, 2103–2110; d) K. R. Powell, P. J. Pꢄrez, L. Luan, S. G. Feng, P. S.
White, M. Brookhart, J. L. Templeton, Organometallics 1994, 13,
1851–1864; e) L. W. Francisco, P. S. White, J. L. Templeton, Organo-
metallics 1996, 15, 5127–5136; f) D. J. Darensbourg, J. D. Draper,
B. J. Frost, J. H. Reibenspies, Inorg. Chem. 1999, 38, 4705–4714.
[24] a) J. T. Poulton, K. Folting, W. E. Streib, K. G. Caulton, Inorg.
Chem. 1992, 31, 3190–3191; b) J. T. Poulton, M. P. Sigalas, K. Folt-
ing, W. E. Streib, O. Eisenstein, K. G. Caulton, Inorg. Chem. 1994,
33, 1476–1485; c) A. Kovacs, G. Frenking, Organometallics 2001, 20,
2510; d) T. C. Flood, J. K. Lim, M. A. Deming, W. Keung, Organo-
metallics 2000, 19, 1166; e) S. A. Macgregor, D. MacQueen, Inorg.
Chem. 1999, 38, 4868; f) J. D. Atwood, T. L. Brown, J. Am. Chem.
Soc. 1976, 98, 3160; g) M. Ogasawara, D. Huang, W. E. Streib, J. C.
Huffman, N. Gallego-Panas, F. Maseras, O. Eisenstein, K. G. Caul-
ton, J. Am. Chem. Soc. 1997, 119, 8642–8651.
[25] a) Z. Chen, H. W. Schmalle, T. Fox, H. Berke, Dalton Trans. 2005,
580–587; b) J. Hçck, H. Jacobsen, H. W. Schmalle, G. R. J. Artus, T.
Fox, J. I. Amor, F. Bꢃth, H. Berke, Organometallics 2001, 20, 1533–
1544; c) F. Furno, T. Fox, H. W. Schmalle, H. Berke, Organometallics
2000, 19, 3620–3630.
[13] M. Kꢃß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. Int.
Ed. Angew.Chem. Int. Ed. 2009, 48, 905–907.
[26] Y. Jiang, B. Schirmer, O. Blacque, T. Fox, S. Grimme, H. Berke, J.
Am. Chem. Soc. 2013, 135, 4088–4102.
[14] a) X. F. Wu, D. Vinci, T. Ikariya, J. L. Xiao, Chem. Commun. 2005,
4447; b) K. Murata, T. Ikariya, J. Org. Chem. 1999, 64, 2186; c) X.
Wu, J. Liu, X. Li, A. Zanotti-Gerosa, F. Hancock, D. Vinci, J. Ruan,
J. Xiao, Angew. Chem. 2006, 118, 6870; Angew. Chem. Int. Ed. 2006,
45, 6718; d) P. Maire, T. Bꢀttner, F. Breher, P. Le Floch, H. Grꢀtz-
macher, Angew. Chem. 2005, 117, 6477–6481; Angew. Chem. Int.
Ed. 2005, 44, 6318–6323; e) S. Schneider, J. Meiners, B. Askevold,
Eur. J. Inorg. Chem. 2012, 412–419.
[27] a) L. P. Hammett, J. Am. Chem. Soc. 1937, 59, 96–103; b) L. P.
Hammett, Chem. Rev. 1935, 17, 125–136; c) X. Yang, L. Zhao, T.
Fox, Z.-X. Wang, H. Berke, Angew. Chem. 2010, 122, 2102; Angew.
Chem. Int. Ed. 2010, 49, 2058.
[28] a) H. Guan, M. Iimura, M. P. Magee, J. R. Norton, G. Zhu, J. Am.
Chem. Soc. 2005, 127, 7805–7814.
[29] R. Abbel, K. Abdur-Rashid, M. Faatz, A. Hadzovic, A. J. Lough,
R. H. Morris, J. Am. Chem. Soc. 2005, 127, 1870–1882.
[15] a) R. M. Bullock, Handbook of Homogeneous Hydrogenation,
Wiley-VCH, Weinheim, 2007; b) R. M. Bullock, Catalysis without
precious metals, Wiley-VCH, Weinheim, 2010.
[30] J. A. Widegren, R. G. Finke, J. Mol. Catal. A 2003, 198, 317.
[31] H. Berke, Y. Jiang, X. Yang, C. Jiang, S. Chakraborty, A. Landwehr,
Top. Curr. Chem. 2013, 334, 27–57.
[16] a) R. M. Bullock, Angew. Chem. 2007, 119, 7504–7507; Angew.
Chem. Int. Ed. 2007, 46, 7360–7363; b) K. Junge, K. Schrçder, M.
Beller, Chem. Commun. 2011, 47, 4849–4859; c) C. P. Casey, H.
Guan, J. Am. Chem. Soc. 2009, 131, 2499–2507; d) R. Langer, G.
Leitus, Y. Ben-David, D. Milstein, Angew. Chem. 2011, 123, 2168–
2172; Angew. Chem. Int. Ed. 2011, 50, 2120–2124; e) C. Sui-Seng, F.
Freutel, A. J. Lough, R. H. Morris, Angew. Chem. 2008, 120, 954–
957; Angew. Chem. Int. Ed. 2008, 47, 940–943; f) P. O. Lagaditis,
A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2011, 133, 9662–9665.
[17] a) R. M. Bullock, M. H. Voges, J. Am. Chem. Soc. 2000, 122, 12594–
12595; b) M. H. Voges, R. M. Bullock, Dalton Trans. 2002, 759–770;
c) R. M. Bullock, J. S. Song, J. Am. Chem. Soc. 1994, 116, 8602–
8612; d) B. F. M. Kimmich, P. J. Fagan, E. Hauptman, W. J. Marshall,
R. M. Bullock, Organometallics 2005, 24, 6220–6229; e) V. K. Diou-
maev, R. M. Bullock, Nature 2000, 424, 530–532.
[32] a) K. Seyferth, R. Taube, J. Organomet. Chem. 1982, 229, 275–279;
b) A. A. Danopoulos, A. R. Wills, P. G. Edwards, Polyhedron 1990,
9, 2413–2418.
[33] CCDC 888511 (1a), CCDC 888512 (1b), CCDC 888513 (2a),
CCDC 888514 (3a), and CCDC 888515 (3b) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif..
[34] a) Agilent Technologies (formerly Oxford Diffraction), Yarnton,
England, 2011; b) R. C. Clark, J. S. Reid, Acta Crystallogr. Sect. A
1995, 51, 887–897; c) G. M. Sheldrick, Acta Crystallogr. Sect. A
2008, 64, 112–122; d) L. J. Farrugia, J. Appl. Crystallogr. 1999, 32,
837; e) A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13; f) K. Bran-
denburg, DIAMOND, Crystal Impact GbR, Bonn, 2007.
[18] A. Dybov, O. Blacque, H. Berke, Eur. J. Inorg. Chem. 2011, 652–
659.
Received: August 16, 2013
Published online: October 15, 2013
Chem. Asian J. 2014, 9, 328 – 337
337
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim