PNS-type ruthenium pincer complexes

Article abstract of DOI:10.1021/om300516j

The PNS pincer-type ligand 1 and the novel Ru(PNS) complexes 2-8 were synthesized and characterized. The (PNS)RuH(Cl)CO complex 2 was prepared by reaction of ligand 1 with RuH(Cl)CO(PPh₃)₃. 2 reacted with KHMDS (potassium bis(trimethylsilyl)amide) to form the symmetrical dimeric complex 4 via the intermediacy of the dearomatized complex (PNS*)Ru(H)CO 3, in which deprotonation of the benzylic-S arm took place. Reaction of 2 with excess NaH gave the dimeric 4, by a formal intermolecular attack of the benzylic arm on a second ruthenium center. Complex 4 underwent spontaneous transformation in solution to the dinuclear complex 5 via C-S bond cleavage, resulting in the loss of a S-bound ^tBu group. Treatment of 2 with KHMDS in the presence of PEt₃ resulted in the trapping of intermediate 3 in the form of the dearomatized complex 8. Reaction of 2 with LiHBEt₃ gave the trans-dihydride complex 6, which reacted with CO₂ to give the formato complex 7, in which the formato ligand is located trans to the hydride. Complexes 2, 4, and 5 were also investigated as catalysts for the dehydrogenative coupling of alcohols with amines.

Full text of DOI:10.1021/om300516j

Article
pubs.acs.org/Organometallics
PNS-Type Ruthenium Pincer Complexes
Moti Gargir,† Yehoshua Ben-David,† Gregory Leitus,‡ Yael Diskin-Posner,‡ Linda J. W. Shimon,‡
and David Milstein*,†
‡
†Department of Organic Chemistry and Unit of Chemical Research Support, The Weizmann Institute of Science, Rehovot, 76100,
Israel
S
* Supporting Information
ABSTRACT: The PNS pincer-type ligand 1 and the novel Ru(PNS) complexes 2−8 were synthesized and characterized. The
(PNS)RuH(Cl)CO complex 2 was prepared by reaction of ligand 1 with RuH(Cl)CO(PPh3)3. 2 reacted with KHMDS
(potassium bis(trimethylsilyl)amide) to form the symmetrical dimeric complex 4 via the intermediacy of the dearomatized
complex (PNS*)Ru(H)CO 3, in which deprotonation of the benzylic-S “arm” took place. Reaction of 2 with excess NaH gave
the dimeric 4, by a formal intermolecular attack of the benzylic “arm” on a second ruthenium center. Complex 4 underwent
spontaneous transformation in solution to the dinuclear complex 5 via C−S bond cleavage, resulting in the loss of a S-bound tBu
group. Treatment of 2 with KHMDS in the presence of PEt3 resulted in the trapping of intermediate 3 in the form of the
dearomatized complex 8. Reaction of 2 with LiHBEt3 gave the trans-dihydride complex 6, which reacted with CO2 to give the
formato complex 7, in which the formato ligand is located trans to the hydride. Complexes 2, 4, and 5 were also investigated as
catalysts for the dehydrogenative coupling of alcohols with amines.
INTRODUCTION
■
Transition metal complexes of pincer ligands have found
important applications in synthesis, bond activation, and
catalysis.1−11 We have reported the dearomatized pincer
complexes (PNN)Ru(H)CO (A) and (PNP)Ru(H)CO (B),
which catalyze several “green” catalytic reactions. Complex A is
an efficient catalyst for the dehydrogenative coupling of
alcohols to esters,12−15 acylation of secondary alcohols by
esters with dihydrogen liberation,16 direct dehydrogenative
effect of replacing the amine arm with a thioether group.
coupling of alcohols and amines to form amides,13−15,17
Hence, we decided to synthesize the new PNS ligand 1 and the
coupling of esters and amines to form amides with dihydrogen
liberation,18 facile catalytic hydrogenolysis of esters to
alcohols,19 and hydrogenation of organic carbonates, carba-
mates, and formates.20 Complex B also catalyzes dehydrogen-
ative coupling of primary alcohols to form esters,12−15
dehydrogenation of β-amino alcohols to pyrazines,21 and
dehydrogenation of secondary alcohols to ketones.13,22,23
Unlike complex A, complex B catalyzes the dehydrogenative
coupling of alcohols and amines to generate imines, rather than
amides.13−15,24−27 PNP-based iron complexes efficiently
corresponding ruthenium PNS pincer complexes, analogous to
ruthenium PNN and PNP pincer complexes. Like in the case of
the amine “arm” of PNN complexes, the thioether “arm” of
PNS complexes is expected to be hemilabile,35−38 but, unlike
the PNN ligand, it lacks the ability to serve as a base.
RESULTS AND DISCUSSION
■
Synthesis of the PNS Ligand 1. The PNS ligand 1
(Scheme 1) was prepared using a procedure similar to the one
reported by Szabo for the preparation of a PCS ligand.39 Thus,
reaction of LiPtBu2(BH3) with 2,6-bis(chloromethyl)pyridine
gave rise to a mixture of monophosphine, diphosphine, and
28
catalyze the hydrogenation of CO2. Related bipyridine- and
acridine-based ruthenium pincer complexes are also excellent
catalysts.29−34
Apparently, modification of the pincer “arm” has a profound
effect on catalytic activity. It was of interest to us to explore the
Received: June 10, 2012
Published: August 27, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of the PNS Ligand 1
Figure 1. ORTEP drawing of a molecule of 2 at the 50% probability
level. Hydrogen atoms, except benzylic ones and hydride, are omitted
for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) of
Complex 2
Ru1−Cl1
Ru1−S1
C7−C8
N1−Ru1−S1
N1−Ru1−Cl1
2.5668(6)
2.3954(6)
1.504(3)
82.04(5)
91.95(5)
Ru1−N1
Ru1−P1
S1−C8
N1−Ru1−P1
N1−Ru1−C1
2.1338(19)
2.2927(6)
1.819(2)
82.20(5)
unreacted 2,6-bis(chloromethyl)pyridine. The monophosphine
was isolated by column chromatography and treated with an
excess of sodium 2-methyl-2-propanethiolate to give the
borane-protected PNS ligand in 99% yield. Deprotection of
the PNS ligand by reflux in diethylamine followed by column
chromatography gave the PNS pincer ligand 1 as an air-
sensitive white solid in 66% yield.
172.86(9)
results. Upon reaction of complex 2 with the bases KOtBu or
(Me3Si)2NK (KHMDS), using a similar procedure to the one
reported for the preparation of the dearomatized PNP12 or
PNN17 complexes, the symmetric dimeric complex 4 was
formed, rather than complex 3 (Scheme 3). The light brown
complex 4 was sparingly soluble in pentane, THF, benzene,
toluene, and dichloromethane and was unstable in solution at
room temperature. Nevertheless we were able to obtain 1H and
31P{1H} NMR of the crude complex. Single crystals of the
complex were obtained by layering pentane over a concentrated
toluene solution of 4 at −35 °C (Figure 2, Table 2). The X-ray
structure of 4 exhibits a dimeric structure, in which each of the
Ru atoms is bound to the opposite sulfur benzylic arm of the
ligand.
Synthesis of the Ru(PNS) Complex 2. The new Ru(PNS)
complex 2 was prepared in 90% yield by reaction of
HRu(CO)(PPh3)3Cl40 with the PNS ligand 1 in THF at 65
1
°C (Scheme 2). The H NMR spectrum of 2 showed the
Scheme 2. Synthesis of the Ru(PNS) Complex 2
In order to gain understanding regarding the reactivity of
complex 2, it was treated with NaH in THF at room
temperature. Follow-up of the reaction by 31P{1H} NMR
spectroscopy revealed after 5 h partial conversion to complex 4.
After 10 h two more singlet peaks (1:1 ratio) appeared at 98
and 99 ppm, and after 48 h they became the only peaks. The
product that gave rise to these peaks was characterized as
complex 5, resulting from cleavage of a S−tBu bond (Scheme
4). The 1H NMR spectrum of 5 revealed a missing tBu-S group
and the appearance of two doublet peaks (1:1 ratio) in the
hydride region, at −10.67 (d, JPH = 24 Hz, RuH) and −10.47
(d, JPH = 21 Hz, RuH). In addition, one benzylic proton was
missing. This data suggested that an asymmetric dinuclear
coordinated PNS benzylic signals at a lower field relative to free
PNS and a hydride ligand as a doublet at −14.9 ppm (JH,P
=
24.7 Hz). The two inequivalent CH2P protons exhibit a doublet
of doublets at 3.37 (JH,H = 17 Hz; JH,P = 8.5 Hz) and 3.6 ppm
(JH,H = 17 Hz; JH,P = 10 Hz), while the CH2S protons exhibit a
doublet at 4.12 ppm (JH,H = 17 Hz) and 5.12 (JH,H = 17 Hz).
The 31P{1H} NMR spectrum of 2 showed a singlet at 101.3
ppm. Crystals suitable for a single-crystal X-ray structure of 2
(Figure 1, Table 1) were obtained from a dichloromethane
solution, showing an octahedral complex, with the hydride
located trans to the chloride ligand.
t
complex was formed, lacking a Bu group on one of the sulfur
atoms. This was confirmed by a single-crystal X-ray crystallo-
graphic study (Figure 3, Table 3), which revealed that one of
t
the Ru atoms is connected to a sulfur atom that lacks the Bu
group, and a second Ru atom formed a Ru−C bond to the
benzylic arm (as observed with dimer 4).
GC-MS analysis of the gas phase of the reaction leading to
complex 5 revealed formation of isobutane. Based on this
Formation of the Binuclear Complexes 4−6. Our
attempts to synthesize the dearomatized (PNS*)Ru(H)CO
(PNS* = deprotonated PNS) complex 3 led to surprising
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a
Scheme 3. Formation of the Dimeric Complex 4
a
B: (Me3Si)2NK (KHMDS) or NaH.
Figure 2. ORTEP drawing of a molecule of 4 at the 50% probability
level. Hydrogen atoms, except benzylic ones and hydride, are omitted
for clarity.
Figure 3. ORTEP drawing of a molecule of 5 at the 50% probability
level. Hydrogen atoms, except benzylic ones and hydride, are omitted
for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) of
Complex 4
Table 3. Selected Bond Lengths (Å) and Angles (deg) of
Complex 5
a
Ru1−C8
C7−C8
Ru1−S1
2.392(3)
1.458(4)
2.3975(13)
81.77(8)
86.49(11)
Ru1−C1
S1−C8
Ru1−P1
N1−Ru1−P1
N1−Ru1−C1
1.829(4)
1.783(3)
Ru2−C2
R2−S2
C7−C8
N1−Ru1−S1
N2−Ru2−C2
2.361(3)
2.4032(9)
1.506(5)
80.87(8)
89.00(11)
Ru1−S1
Ru1−S2
C27−C28
N2−Ru2−S2
S1−Ru1−S2
2.3805(9)
2.5030(9)
1.496(5)
82.12(8)
85.91(3)
2.2953(13)
82.45(8)
N1−Ru1−S1
N1−Ru1−C8
a
177.46(14)
a
In symmetry 1−x, −y, −z.
Scheme 4. Formation of the Dinuclear Complex 5
Scheme 5. Proposed Mechanism for the Transformation of 4
to 5
observation, it is possible that formation of the dimer 4 is
followed by rupture of the Ru−C bond and loss of the β-
hydrogen of the sulfur tBu group, releasing isobutene (which is
eventually transformed to isobutane) to give complex 5
(Scheme 5), similar to the observation of Searles et al.41
Preparation and Reactivity of the trans-Dihydride
Complex 6. Reaction of complex 2 with NaHBEt3 in toluene
gave the trans-dihydride complex 6 in 99% yields (by NMR).
pressure and was fully converted to complex 5 within 8 h under
vacuum (Scheme 6).
Upon reacting the crude complex 6 with CO2 in toluene-d8,
the formato complex 7 was formed within minutes as a major
product (∼85%). The 31P{1H} NMR spectrum of 7 exhibited a
1
1
The H NMR spectrum of 6 showed two hydride doublets of
singlet at 99.4 ppm, and the H NMR spectrum exhibited the
doublets at −4.92 ppm (dd, JHH = 20 Hz, JPH =6.4 Hz, 1H,
RuH) and −4.62 ppm (dd, JHH = 12.8 Hz, J = 6.4 Hz, 1H,
RuH), typical of trans-dihydride complexes.12,19 The 31P{1H}
NMR spectrum exhibited a singlet at 115.2 ppm. Complex 6 is
not stable in solution and slowly loses hydrogen to give the
dimer 4. Compex 4 also loses isobutene slowly at atmospheric
formyl hydrogen atom at 8.79 ppm and the hydride ligand at
−17 ppm. Unfortunately, the crude mixture proved impossible
to purify. However, a single-crystal X-ray structure of complex 7
was obtained (Figure 4, Table 4), confirming the structure and
revealing that the formato ligand is located trans to the hydride
(Scheme 7).
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Scheme 6. Preparation of trans-Dihydride Complex 6 and Its
Decomposition in Solution to Give the Dinuclear
Complexes 4 and 5
Scheme 8. Preparation of the Dearomatized Complex 8
NMR spectrum exhibited the hydride at −8.92 ppm (dd, JPH
=
108 Hz, JP′H = 28 Hz, 1H, RuH). The large JPH = 108 Hz
indicates that the PEt3 group is located trans to the hydride.
The phosphine “arm” gives rise to two peaks at 2.78 (dd, JHH
=
16.4 Hz, JPH = 6.4 Hz, 1H) and at 2.94 (dd, JHH = 16.4 Hz, JPH
= 12.14 Hz 1H), while the S-tBu “arm” gives rise to a peak at
4.13 (d, JPH = 4.4 1H), indicating deprotonation of this arm
took place. The 13C{1H} NMR spectrum of the phosphine
“arm” exhibits a peak at 38.84 (dd, JCP = 16.0 Hz, JCP = 0.95 Hz,
CH2P), while the S-tBu arm shows a peak at 65.1 (d, JCP = 1.9
Hz, CHS).
In the second experiment, complex 2 was reacted with a base
under H2 gas, leading after 30 min to a mixture of complex 6
(61%) and the two dinuclear complexes 4 (23%) and 5 (16%).
After one day in solution the reaction progressed to give
complex 6 (35%), complex 4 (8%), and complex 5 (57%).
These experiments provide strong support for the intermediacy
of the dearomatized complex 3.
The reason that complex 3 dimerizes while the analogous
dearomatized PNN and PNP complexes A and B do not
dimerize may be the lower steric bulk of tBuS as compared with
(tBu)2P or Et2N.
Catalytic Experiments. Coupling of amines with alcohols
to form amides and hydrogen gas was reported14 by our group
to be catalyzed by (PNN)Ru(H)(Cl)CO (A), while using
(PNP)Ru(H)(Cl)CO (B) under almost the same conditions
gave imines instead of amides.21 The main difference between
these systems seems to be the presence of a potentially
hemilable amine “arm” in A. Hence, it was of interest to us to
explore the catalytic activity based on complex 2, which
possesses a hemilabile “arm”.
Figure 4. ORTEP drawing of a molecule of 7 at the 50% probability
level. Hydrogen atoms, except benzylic, formate and hydride ones, are
omitted for clarity.
Scheme 7. CO2 Insertion into Ru−H to Give the Formato
Complex 7
Studying complex 2 as a precatalyst in the amidation reaction
(Scheme 6), 0.01 mmol of 2, 10 mmol of 1-hexanol, 10 mmol
of benzylamine, and 0.01 mmol of KHMDS were refluxed in
toluene, leading to hexyl hexanoate (46% determined by GC)
and N,N-benzylhexanamide (30% isolated). Under the same
conditions, the dinuclear complexes 4 and 5 gave only 7% of N-
benzylhexanamide (isolated) and 19% of the hexyl hexanoate
(determined by GC). When the catalysis with the dinuclear
complexes 4 and 5 was carried out without the presence of a
base, no reaction took place.
We believe that the actual catalyst is the (unobserved)
dearomatized intermediate 3, analogous to the PNN and PNP
ruthenium complexes A and B, which catalyzed the
dehydrogenative amidation and esterification reactions, respec-
tively. Upon conversion of 3 to the dinuclear complexes 4 and
subsequently to 5, the reaction is retarded. The reason for
obtaining a mixture of the products ester and amide is not clear
at this stage.
Table 4. Selected Bond Lengths (Å) and Angles (deg) of
Complex 7
Ru1−S1
Ru1−C1
C7−C8
N1−Ru1−S1
N1−Ru1−P1
2.4003(6)
1.838(3)
1.502(4)
81.81(6)
82.24(6)
Ru1−O3
Ru1−P1
S1−C8
C1−Ru1−O3
N1−Ru1−C1
1.989(13)
2.2852(7)
1.822(3)
100.7(4)
177.68(10)
Trapping the Dearomatized Intermediate Complex 3.
Two experiments were performed in order to trap the
postulated dearomatized complex 3. First, complex 2 was
reacted with a base in the presence of PEt3, leading to complex
8 (Scheme 8). The 31P{1H} NMR spectrum exhibited two
peaks at −5.80 ppm (d, JPP = 7.32 Hz, 1P, (CH3CH2)3P)) and
It is interesting that amide is formed, like in the case of the
PNN complex A, unlike the case of the PNP complex B, in
1
95.16 ppm (d, JPP = 8.54 Hz, 1P, ((CH3)3)2CP)), and the H
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a
Table 5. Dehydrogenative Coupling of Benzylamine with 1-Hexanol Catalyzed by the Ruthenium Complexes 2, 4, and 5
b
c
entry
precatalyst
yield (%) hexyl hexanoate
yield (%) N-benzylhexanamide
d
1
2
3
4
5
2
46
19
0
30
7
d
4
4
0
d
5
18
0
7
5
0
a
b
Catalyst (0.01 mmol), 1-hexanol (10 mmol), benzylamine (10 mmol), and toluene (3 mL) were refluxed under Ar flow for 24 h. Yields of ester
c
d
were determined by GC. Isolated yield. 0.01 mmol of KHMDS was added.
Synthesis of [Di-tert-butylphosphinoborane]. This compound was
prepared by a modified literature procedure.43 To a 500 mL Schlenk
tube containing di-tert-butylchlorophosphine (10 g, 55.95 mmol)
dissolved in dry THF (350 mL) was added a 1 M BH3·THF solution
(83.025 mL, 83.025 mmol), and the mixture was cooled in an ice bath.
LiAlH4 (4.24 g, 110.7 mmol) was added, and the reaction was left for
60 h. The reaction mixture was carefully poured with stirring into a
mixture of concentrated HCl (20 mL) and ice (ca. 200 g). The
product was extracted with dichloromethane, and the combined
extracts were dried over Na2S04 and concentrated under reduced
pressure to give the di-tert-butylphosphine-borane as a white solid (8.9
g, 99% yield). 31P{1H} NMR (CDCl3): 48.2 (q, JPB = 47 Hz).
Synthesis of [2-(Chloromethyl)-6-((di-tert-butylphosphino)-
which imines are formed. This may suggest that hemilability has
a significant role in the direction of the reaction.
SUMMARY
■
In this work ruthenium complexes of the new hemilabile PNS
ligand were prepared. Unexpectedly, attempted deprotonation
of complex 2, aiming at the (unobserved) dearomatized
complex 3, resulted in the dinuclear complex 4, which upon
reaction with dihydrogen or NaH underwent C−S cleavage to
release isobutane. Our evidence indicates that the putative
dearomatized 3 is an intermediate in the formation of 4. Thus,
deprotonation of 2 in the presence of PEt3 resulted in the
dearomatized phosphine complex 8. In addition, the trans
dihydride complex 6 releases H2 to give the dinuclear
complexes 4 and 5, probably via initial dearomatization.
Using complex 2 as precatalyst, reaction of benzyl amine with
1-hexanol led to both N-benzylhexanamide (30%) and hexyl
hexanoate (47%). Formation of the dimer 4, from the desired
dearomatized complex 3, which is probably the actual catalyst,
seems to be the main reason for the relatively low yield of N-
benzylhexanamide.
n
methyl)pyridine- borane Adduct]. A solution of BuLi (4.46 mL of
1.6 M hexane, 7.14 mmol) was added at −78 °C to HP(tBu)2·BH3
(1.14 g, 7.14 mmol) in dry THF (40 mL) under N2. The temperature
was allowed to rise to room temperature, and stirring was continued
for 12 h, upon which the color became pale yellow. The resulting
solution was added via cannula to a precooled (to −78 °C) solution of
2,6-bis(chloromethyl)pyridine (1 g, 5.71 mmol) in dry THF (40 mL)
under N2. The temperature was allowed to rise to room temperature
for another 15 h, upon which the color became yellow. All volatiles
were removed under vacuum, and the residue was dissolved in
CH2Cl2. The organic solution was washed with water and brine and
dried over MgSO4. After filtration and evaporation of the solvent, the
crude mixture underwent column chromatography using as eluent
ethyl acetate/hexane (with a gradual increase in the amount of ethyl
acetate) to give the 2-(chloromethyl)-6-((di-tert-butylphosphino)-
methyl)pyridine-borane adduct as an off-white solid (0.98 g, 57%
yield). Mp: 57−63 °C. 31P{1H} NMR (CDCl3): 47.67 (m). 1H NMR
(CDCl3): 1.26 (d, JPH = 12.6 Hz, 18H, (CH3)3CP); 3.52 (d, JPH = 12
Hz, 2H, CH2P); 4.6 (s, 2H, CH2Cl); 7.28 (d, JHH = 7.6 Hz, 1H,
pyridine-H); 7.54 (d, JHH = 7.6 Hz, 1H, pyridine-H); 7.64 (t, JHH = 7.6
Hz, 1H, pyridine-H). 13C{1H} NMR (CDCl3): 28.07 (s,(CH3)3CP),
29.12 (d, JPC = 23 Hz, CH2P), 32.74 (d, JPC = 25 Hz,, (CH3)3CP),
EXPERIMENTAL SECTION
■
General Procedures. All experiments with metal complexes and
the PNS ligand were carried out under an atmosphere of purified
nitrogen in a Vacuum Atmospheres glovebox equipped with a MO 40-
2 inert gas purifier or using standard Schlenk techniques. All solvents
were reagent grade or better. All nondeuterated solvents were refluxed
over sodium/benzophenone ketyl and distilled under an argon
atmosphere. Deuterated solvents were dried over 4 Å molecular
sieves. The NMR spectra were recorded at 300 MHz (1H), 75 MHz
(13C), and 122 MHz (31P) using a Bruker Avance-300 NMR
spectrometer, at 400 MHz (1H), 100 MHz (13C), and 162 MHz
(31P) using a Bruker Avance-400 NMR spectrometer, and at 500 MHz
(1H), 126 MHz (13C), and 202 MHz (31P) using a Bruker Avance-500
NMR spectrometer. All spectra were recorded at 23 °C. 1H and
13C{1H} NMR chemical shifts are reported in parts per million (ppm)
downfield from tetramethylsilane. 1H NMR and 13C{1H} NMR
chemical shifts were referenced to the residual hydrogen signal of the
deuterated solvent that was used.42 31P NMR chemical shifts are
reported in ppm downfield from H3PO4 and referenced to an external
85% solution of phosphoric acid in D2O. Abbreviations used in the
description of NMR data are as follows: b, broad; s, singlet; d, doublet;
t, triplet; m, multiplet; Py, pyridine. IR spectra were measured with a
Nicolet-6700 FT-IR spectrometer. Elemental analyses were performed
by the Unit of Chemical Research Support, Weizmann Institute of
Science.
48.69 (s, CH2Cl), 120.7 (d, JPC = 2 Hz, pyridine-CH), 125.20 (d, JPC
=
2 Hz, pyridine-CH), 136.83 (s, pyridine-CH), 155.28 (s, pyridine-C),
155.38 (d, JPC = 2 Hz, pyridine-C). MS (ESI+, MeOH): 322.1(100%,
M + Na); 300.09 (59%, M + 1).
Synthesis of [2-((tert-butylthio)methyl)-6-((di-tert-
butylphosphino)methyl)pyridine]-borane Adduct. To a 100 mL
round-bottom flask containing 2-(chloromethyl)-6-((di-tert-
butylphosphino)methyl)pyridine-borane adduct (0.6 g, 2.0 mmol)
dissolved in dry THF (60 mL) was added sodium 2-methyl-2-
propanethiolate (0.257 g, 2.30 mmol), and the mixture was stirred for
12 h. All volatiles were removed under high vacuum, and the residue
was treated with CH2Cl2 (30 mL), washed with brine, dried over
Na2SO4, and concentrated under reduced pressure to give 2-((tert-
butylthio)methyl)-6-((di-tert-butylphosphino)methyl)pyridine-borane
1
adduct (0.7 g, 99% yield). 31P{1H} NMR (CDCl3): 47.39 (m). H
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NMR (CDCl3): 1.26 (d, JPH = 12.6 Hz, 18H, (CH3)3CP); 1.33 (s, 9H,
(CH3)3CS); 3.34 (d, JPH = 12 Hz, 2H, CH2P); 3.38 (s, 2H, CH2S);
7.27 (d, JHH = 7.8 Hz, 1H, pyridine-H); 7.46 (d, JHH = 7.8 Hz, 1H,
pyridine-H); 7.56 (t, JHH = 7.8 Hz, 1H, pyridine-H). 13C{1H} NMR
(CDCl3): 28.16 (s, (CH3)3CP), 29.24 (d, JPC = 25 Hz, CH2P), 30.96
(s, (CH3)3CS), 32.78 (d, JPC = 25 Hz, (CH3)3CP), 35.46 (s, CH2S),
43.00 (CH3)3CS), 121.18 (s, pyridine-CH), 123.81 (s, pyridine-CH),
136.41 (s, pyridine-CH), 154.75 (s, pyridine-C), 158.04 (s, pyridine-
C). MS (ESI+, MeOH): 376.13 (100%, M + Na); 340.17 (30%, M +
1).
6.11 (d, JHH = 7 Hz, 1H, pyridine-H); 6.50 (d, JHH = 7 Hz, 1H,
pyridine-H); 6.70 (t, JHH = 7 Hz, 1H, pyridine-H) 6.90 (d, JHH = 7 Hz,
1H, pyridine-H) 6.95 (d, JHH = 7 Hz, 1H, pyridine-H); 7.15 (d, JHH
=
7 Hz, 1H, pyridine-H). 13C{1H} NMR (C6D6): 28.7 (s, (CH3)3CS),
29.26 (d, JPC = 3 Hz, (CH3)3CP), 30.20 (d, JPC = 4 Hz, (CH3)3CP),
31.45 (d, JPC = 7 Hz, (CH3)3CP), 31.48 (d, JPC = 4 Hz, (CH3)3CP),
35.59 (d, JPC = 22 Hz, (CH3)3CP), 37.63 (d, JPC = 11 Hz, (CH3)3CP),
37.70 (d, JPC = 10 Hz, CH2P), 38.03 (d, JPC = 19 Hz, (CH3)3CP),
38.78 (d, JPC = 9 Hz, (CH3)3CP), 39.26 (d, JPC = 20 Hz, CH2P), 40.93
(s, CHS), 46.54 (s, (CH3)3CS), 47.75 (s, CH2S), 110.53 (d, JPC = 9
Hz, pyridine-CH), 114.90 (s, pyridine-CH), 119.46 (d, JPC = 9 Hz,
pyridine-CH), 120.49 (s, pyridine-CH), 134.49 (s, pyridine-CH),
135.66 (s, pyridine-CH), 159.40 (d, JPC = 5 Hz, pyridine-C), 160.31
(d, JPC = 6 Hz, pyridine-C), 167.63, (d, JPC = 5 Hz, pyridine-C),
177.97, (d, JPC = 4 Hz, pyridine-C), 209.91 (d, JPC = 14 Hz, RuCO),
210.76 (d, JPC = 14 Hz, RuCO). MS (ESI+, ACN+DCM): 470.16
(80%, M+2).
Synthesis of [2-((tert-Butylthio)methyl)-6-((di-tert-
butylphosphino)methyl)pyridine, 1. To a 20 mL Schlenk tube
under argon containing 2-((tert-butylthio)methyl)-6-((di-tert-
butylphosphino)methyl)pyridine-borane adduct (0.7 g, 2.0 mmol)
was added degassed Et2NH (5−10 mL). The mixture was stirred for
12 h under reflux. The 31P{1H} NMR spectrum of the crude mixture
showed a singlet peak. All the volatiles were removed under high
vacuum. The residue was dissolved in pentane and passed through a
silica gel column to obtain the air-sensitive 1 (0.45 g, 66% yield) as a
Synthesis of [RuH(PNS)*(CO)]2, 4. A solution of complex 2 (50 mg,
0.1 mmol) in toluene (3 mL) was cooled to −35 °C and added to a
solution of potassium hexamethyldisilazane (KHMDS) (20 mg, 0.1
mmol) in toluene (2 mL) precooled to −35 °C. The resulting solution
was stirred for 5 h at −35 °C. The mixture was allowed to reach room
temperature and filtered through Celite to remove the formed KCl.
Pentane (8 mL) was added, and the vial was kept at −35 °C overnight
to get the dimeric complex 4 as a green-yellow precipitate, which was
filtered off, washed with pentane, and dried to give 38 mg (80% yield).
Complex 4 was scarcely soluble in CH2Cl2, chloroform, THF, ether,
benzene, toluene, and methanol. Single crystals of 4 suitable for X-ray
diffraction were obtained by filtering a saturated benzene solution of
the complex, layering it with pentane, and leaving it at −35 °C.
1
white solid. 31P{1H} NMR (CDCl3): 36.4 (s). H NMR (CDCl3):
1.06 (d, JPH = 11.00 Hz, 18H, (CH3)3CP); 1.23 (s, 9H, (CH3)3CS)
2.95 (d, JPH = 2.9 Hz, 2H, CH2P); 3.78 (s, 2H, CH2S); 7.10 (d, JHH
=
7.3 Hz, 1H, pyridine-H); 7.22 (d, JHH = 7.3 Hz, 1H, pyridine-H); 7.42
(t, JHH = 7.3 Hz, 1H, pyridine-H). 13C{1H} NMR (CDCl3): 29.64 (d,
JPC = 13 Hz, (CH3)3CP), 31.07 (s, (CH3)3CS), 31.70 (d, JPC = 23 Hz,
CH2P), 31.92 (d, JPC = 21 Hz, (CH3)3CP), 35.76 (s, CH2S), 43.05 (s,
(CH3)3CS), 120.00 (s, pyridine-CH), 121.93 (d, JPC = 10 Hz, pyridine-
CH), 136.53 (s, pyridine-CH), 158.31 (s, pyridine-C), 161.34 (d, JPC
=
14 Hz, pyridine-C). MS (ESI+, MeOH): 340.17(100%, M + 1).
Synthesis of [RuH(Cl)(PNS)(CO)], 2. To a suspension of RuH(Cl)-
(PPh3)3(CO) (0.514 g, 0.54 mmol) in dry THF (8 mL) under N2 in a
glovebox was added the PNS ligand 1 (0.2 g, 0.59 mmol), and the
mixture was stirred and heated at 65 °C for 12 h and cooled to room
temperature. The pale yellow solid that was obtained was filtered off,
washed with ether (3 × 3 mL), and dried under vacuum to give (0.273
g, 92% yield) of 2 as a pale yellow solid.
1
31P{1H} NMR (CD2Cl2): 90.51 (s). H NMR (CD2Cl2): −11.83 (d,
JPH = 28.8 Hz, 1H, RuH); 1.21 (s, 9H, (CH3)3CS); 1.33 (d, JPH = 13
Hz, 9H, (CH3)3CP); 1.54 (d, JPH = 13 Hz, 9H, (CH3)3CP); 3.4 (m,
2H, CH2P); 4.86 (d, JPH = 4 Hz, 1H, CHS); 6.45 (d, JHH = 7.0 Hz, 1H,
pyridine-H); 6.89 (d, JHH = 7.0 Hz, 1H, pyridine-H); 7.1 (t, JHH = 7.0
Hz, 1H, pyridine-H). Anal. Calcd for C20H35ClNOPRuS: C, 51.26; H,
7.31. Found: C, 51.81; H, 7.35. MS (ESI+, ACN+DCM): 470.16
[100%, (M/2)+].
Single crystals of 2 suitable for X-ray diffraction were obtained by
slow evaporation of the complex in dichloromethane. 31P{1H} NMR
(CD2Cl2): 101.31 (s). 1H NMR (CD2Cl2): −14.88 (d, JPH = 24.7 Hz,
1H, RuH); 1.26 (s, 9H, (CH3)3CS); 1.33 (d, JPH = 14 Hz, 9H,
(CH3)3CP); 1.37 (d, J = 14 Hz, 9H, (CH3)3CP); 3.37 (dd, JHH = 17
Hz, JPH = 8.5 Hz, 1H,CH2P); 3.6 (dd, JHH = 17 Hz, JPH = 10 Hz, 1H,
CH2P); 4.12 (d, JHH = 17.0 Hz, 1H, CH2S); 5.12 (d, JHH = 17.0 Hz,
1H, CH2S); 7.27 (d, JHH = 8.0 Hz, 1H, pyridine-H); 7.30 (d, JHH = 8.0
Hz, 1H, pyridine-H); 7.59 (t, JHH = 8.0 Hz, 1H, pyridine-H). 13C{1H}
NMR (CD2Cl2): 28.63 (s, (CH3)3CS), 29.26 (d, JPC = 4 Hz,
(CH3)3CP), 30.51 (d, JPC = 2.3 Hz, (CH3)3CP), 35.27 (d, JPC=24 Hz,
(CH3)3CP), 37.9 (d, JPC = 11.8 Hz, (CH3)3CP), 38.69 (d, JPC = 19.4
Hz, CH2P), 44.01 (s, CH2S), 46.59 (s, (CH3)3CS), 119.67 (s,
pyridine-CH), 120.57 (d, JPC = 10 Hz, pyridine-CH), 137.16 (s,
pyridine-CH), 162.10 (d, JPC = 4 Hz, pyridine-C), 162.55 (d, JPC = 4
Hz, pyridine-C), 207.26 (dd, JPC = 15 Hz, JPC = 6 Hz, RuCO). IR
(KBr, disk): 2023 (νRu−H), 1916 (νCO) cm−1. Anal. Calcd for
C20H35ClNOPRuS: C, 47.56; H, 6.98; N, 2.77. Found: C, 47.16; H,
6.89; N, 2.43. MS (ESI+, MeOH): 470.16 [100%, (M − Cl)+].
Synthesis of Complex 5. To a small 20 mL vial containing complex
2 (5 mg, 0.1 mmol) dissolved in THF (10 mL) was added NaH (20
mg, 0.73 mmol), and the mixture was allowed to stir for 48 h at room
temperature. The mixture was filtered, and the solution was
concentrated under vacuum to 2−3 mL, followed by addition of
pentane to allow for slow precipitation of complex 5 as a pale yellow-
orange solid (86 mg, 98% yield). Single crystals of 5 suitable for X-ray
diffraction were obtained by filtering a saturated THF solution of the
complex, layering it with pentane. 31P{1H} NMR (C6D6): 98.7(s),
99.0 (s). 1H NMR (C6D6): −10.67 (d, JPH = 24 Hz, RuH); −10.47 (d,
JPH = 21 Hz, RuH); 1.00 (s, 9H, (CH3)3CS); 1.03 (d, JPH = 13 Hz, 9H,
(CH3)3CP); 1.29 (d, JPH = 13 Hz, 18H, (CH3)3CP); 1.40 (d, JPH = 13
Hz, 9H, (CH3)3CP); 2.86 (dd, JHH = 9 Hz, JPH = 4 Hz 2H, CH2P);
3.12 (d, JHH = 9 Hz, 2H, CH2S); 4.39 (dd, JHH = 14 Hz, JPH = 4 Hz,
1H, CH2P); 4.55 (d, JHH = 14 Hz, 1H, CH2P); 4.64 (bs, 1H, CHS);
Synthesis of [RuH2(PNS)(CO)], 6. To a solution of complex 2 (50
mg, 0.1 mmol) was added NaHBEt3 (1 M in toluene solution, 0.1 mL,
0.1 mmol), and the mixture was stirred at −35 °C for 5 h, followed by
filtration. The yellow filtrate was carefully evaporated under vacuum to
give a yellow solid. The compound slowly loses H2 at room
temperature to give the binuclear complex 4, which is converted to
complex 5 after 48 h under high vacuum. 31P{1H} NMR (toluene-d8):
115.21 (s). 1H NMR (Tol-d8): −4.92 (dd, JHH = 20 Hz, JPH = 6.4 Hz,
1H, RuH); −4.62 (dd, JHH = 12.8 Hz, J = 6.4 Hz, 1H, RuH); 1.13 (s,
9H, (CH3)3CS); 1.21 (d, J = 12.8 Hz, 9H, (CH3)3CP); 1.4 (d, J = 12.8
Hz, 9H, (CH3)3CP); 3.04 (dABq, J = 10 Hz, J = 7 Hz 2H, CH2P);
3.93 (ABq, J = 16.47 2H, CH2S); 6.22 (d, J = 7.3 Hz, 1H, pyridine-H),
6.35 (d, J = 7.3 Hz, 1H, pyridine-H), 6.64 (t, J = 7.3 Hz, 1H, pyridine-
H). 13C{1H} NMR (Tol-d8): 26.8 (s, (CH3)3CS), 28.14 (s,
(CH3)3CP), 31.18, (CH3)3CP), 34.65 (d, JPC = 20 Hz, (CH3)3CP),
35.50 (d, JCP = 14 Hz, (CH3)3CP), 39.15 (d, JCP = 19.4 Hz, CH2P),
45.65 (s, CH2S), 46.18 (s, (CH3)3CS), 117.95 (s, pyridine-CH),
119.71 (d, JCP = 9 Hz, pyridine-CH), 135.04 (s, pyridine-CH), 159.50
(d, J = 4 Hz, pyridine-C), 161.2, (d, J = 4 Hz, pyridine-C), 211.16 (s,
RuCO).
Synthesis of Complex 7. To a crude mixture of complex 6 (0.1
mmol) in toluene in a septum-sealed NMR tube was added via syringe
1 mL of CO2 gas, and the tube was shaken and left at room
temperature for 2 h. The 31P{1H} NMR spectrum of the reaction
mixture showed a major peak (more than 85%) at 99.4 ppm. The
mixture was evaporated under vacuum, and the residue was dissolved
in C6D6. The 1HNMR spectrum showed a major doublet at −17 ppm
(J = 22 Hz) and a singlet at 8.79 ppm in a 1:1 ratio. The crude mixture
proved impossible to purify. Single crystals of 7 suitable for X-ray
diffraction were obtained by evaporation of a nearly saturated benzene
solution of the complex.
6212
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Organometallics
Article
a
Table 6. X-ray Diffraction Experimental Data
2
4
5
7
formula
C20H35ClNOPRuS +
CH2Cl2
C40H68N2O2P2S2Ru2 C36H60N2O2P2Ru2S2 +
C5H12
0.83(C20H35ClNOPRuS)
+0.17(C21H36NO3PRuS
Mw
589.97
937.16
953.21
506.67
space group
cryst syst
a [Å]
P21/c
P1
P21/c
P21/c
̅
monoclinic
14.3896(2)
11.5377(2)Å
16.7832(3)
triclinic
9.3128(19)
9.4472(19)
12.965(3)
85.03(4)
72.33(3)
83.34(3)
1077.9(4)
1
monoclinic
22.4462(8)
16.0164(6)
27.0697(10)
monoclinic
14.3105(7)
11.8513(6)
16.7230(6)
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
volume [Å3]
Z
107.820(1)
111.398(2)
124.669(3)
2652.71(8)
4
9060.9(6)
8
2332.6(2)
4
ρcalcd mg m−3
1.477
1.045
4.15
1.444
1.398
1.443
μ [mm−1
]
0.906
0.864
0.936
R1 [I > 2σ(I)] [%]
R1 (all data) [%]
4.25
4.90
2.96
5.51
5.78
9.97
3.45
no. of reflections (unique) 39 228 (10091)
8315 (5446)
0.0311
115 616 (28847)
0.0604
1.040
22 097 (5129)
0.0234
1.183
Rint
0.0385
1.168
goodness of fit
1.031
a
Crystals were coated in Hampton Paratone oil mounted in a fiber.
Synthesis of [RuH(PNS)PEt3(CO)], 8. Complex 2 (30 mg, 0.059
complex 4 (8%); −10.58 (d, JPH = 24 Hz), −10.41 ppm (d, JPH = 21
Hz), complex 5 (57%); −4.6 (dd, JHH = 20 Hz, JPH = 6 Hz), −4.4 ppm
(dd, JHH = 12 Hz, JPH = 6 Hz), complex 7 (35%).
mmol) was dissolved in benzene (0.2 mL) in a screw-cap NMR tube,
and a solution containing 10 wt % PEt3 in hexane (0.74 g, 0.062
mmol) was added, followed by a benzene solution (0.2 mL) of
KHMDS (12 mg, 0.059 mmol), resulting in an immediate color
change to orange. After 1 h, 31P NMR showed full conversion of
complex 2 to complex 8. A 0.4 mL amount of pentane was added, and
the solution was filtered through Celite and evaporated to dryness to
give complex 8 (30 mg, 86% yield). 31P{1H} NMR (C6D6): −5.80 (d,
J = 7.32 Hz, 1P, (CH3CH2)3P)); 95.16 (d, J = 8.54 Hz, 1P,
((CH3)3)2CP)). 1H NMR (C6D6): −8.92 (dd, JPH = 108 Hz, JP′H = 28
Hz, 1H, RuH); 0.89−1.04 (m, 9H, (CH3CH2)3P); 1.13 (d, J = 12.8
Hz, 9H, (CH3)3CP); 1.29 (d, J = 12.8 Hz, 9H, (CH3)3CP); 1.47 (s,
Typical Procedure for the Catalytic Dehydrogenative Coupling of
1-Hexanol with Benzylamine. Complex 2, 4 or 5 (0.01 mmol) was
dissolved in toluene and 1-hexanol (10 mmol), and benzylamine (10
mmol) was added, followed by KHMDS (0.01 mmol). The flask was
equipped with a condenser, and the solution was heated with stirring
in an open system under an argon flow at the specified temperature
and time. After cooling to room temperature, the products and
remaining starting substrates were determined by GC with m-xylene as
an internal standard, using a Carboxen 1000 column with an HP 690
series GC system.
Crystal data were measured at 100 K on a Bruker Apex-II Kappa
CCD diffractometer equipped with (λ(Mo Kα) = 0.71073 Å)
radiation, graphite monochromator, and MiraCol optics. The data
were processed with APEX-II package programs. Structures were
solved by either the AUTOSTRUCTURE module or SHELXS and
refined with full-matrix least-squares refinement based on F2 with
SHELXL-97. Full details can be found in the CIF files and Table 6.
9H, (CH3)3CS); 1.54−1.84 (m, 6H, (CH3CH2)3P); 2.78 (dd, JHH
=
16.4 Hz, JPH = 6.4 Hz 1H, CH2P); 2.94 (dd, JHH = 16.4 Hz, JPH = 12.14
Hz 1H, CH2P); 4.13 (d, JPH = 4.4 1H, CHS); 5.15 (d, JHH = 6.4 1H,
pyridine-H); 6.25 (d, JHH = 8.8 Hz, 1H, pyridine-H); 6.45 (dd, JHH
=
8.8 Hz, JHH = 6.44 Hz 1H, pyridine-H). 13C{1H} NMR (C6D6): 8.03
(d, JCP = 1.7 Hz, (CH3CH2)3P)), 17.88 (d, JCP = 12.7 Hz,
(CH3CH2)3P)), 27.98 (d, JCP = 0.97 Hz, (CH3)3CS), 29.01 (dd, JCP
= 3.2 Hz, JCP = 0.89 Hz, (CH3)3CP), 30.48 (dd, JCP = 5.54 Hz, JCP
=
0.98 Hz, (CH3)3CP), 33.9 (d, JCP = 11.3 Hz, (CH3)3CP), 38.84 (dd,
JCP = 16.0 Hz, JCP = 0.95 Hz, CH2P), 39.1 (d, JCP = 5.5 Hz,
(CH3)3CS), 50.16 (d JCP = 13.4 Hz, (CH3)3CP), 65.1 (d, JCP = 1.9 Hz,
CHS), 98.5 (d, JCP = 10.58 Hz, pyridine-CH), 111.06 (s, pyridine-
CH), 138.7 (s, pyridine-CH), 157.06, (d, JCP = 5 Hz, pyridine-C),
165.2 (d, JCP = 5 Hz, pyridine-C), 208.3 (dd JCP = 14 Hz, JCP = 7.5 Hz,
Ru-CO).
ASSOCIATED CONTENT
* Supporting Information
Crystallographic data (CIF) for complexes 2, 4, 5, and 7. This
material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: david.milstein@weizmann.ac.il.
Notes
Reaction of Complex 2 with H2 and KHMDS. Complex 2 (30 mg,
0.06 mmol) was dissolved in C6D6 (1.5 mL) in a screw-cap NMR tube.
The tube was flushed with H2, and a solution of KHMDS (13 mg,
0.065 mmol) in C6D6 (0.5 mL) was injected into it. After 5 min the
solution was analyzed by NMR. 31P{1H} NMR (C6D6): 4 peaks were
observed, 110 ppm (s, complex 7); 98.9 (s) and 98.6 ppm (s, complex
5); and 91.1 ppm (s, complex 4). 1H NMR (C6D6): 5 hydride peaks at
−11.62 ppm (d, JPH = 29 Hz), complex 7 (23%); −10.58 (d, JPH = 24
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Hz), −10.41 ppm (d, JPH = 21 Hz), complex 5 (16%); −4.6 (dd, JHH
=
This research was supported by the European Research Council
under the FP7 framework (ERC No 246837), by the Israel
Science Foundation, by the MINERVA Foundation, by the
Kimmel Center for Molecular Design, and by the Bernice and
Peter Cohn Catalysis Research Fund. D.M. holds the Israel
Matz Professorial Chair of Organic Chemistry.
20 Hz, JPH = 6 Hz), −4.4 ppm (dd, JHH = 12 Hz, JPH = 6 Hz), complex
7 (61%).
After one day the crude mixture was analyzed again to show the
following: 31P{1H} NMR (C6D6): 4 peaks at 110 ppm (s), complex 7;
1
98.9 (s), 98.6 ppm (s), complex 5; and 91.1 ppm (s), complex 4. H
NMR (C6D6): 5 hydride peaks at −11.62 ppm (d, JPH = 29 Hz),
6213
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