Electron-rich PNP-and PNN-type ruthenium(ii) hydrido borohydride pincer complexes. synthesis, structure, and catalytic dehydrogenation of alcohols and hydrogenation of esters

Article abstract of DOI:10.1021/om200595m

Electron-rich PNP-and PNN-type ruthenium(II) hydrido borohydride pincer complexes, [RuH(BH₄)(^tBu-PNP)] (tBu-PNP = (2,6-bis(di-tert-butylphosphinomethyl)pyridine) (5) and [RuH(BH ₄)(^tBu-PNN)] (^tBu-PNN = 2-di-tert- butylphosphinomethyl-6-diethylaminomethylpyridine) (6), were prepared from their corresponding N2-bridged dinuclear Ru(II) complexes [(^tBu-PNP) RuCl₂]₂(μ-N₂) (3) and [(^tBu-PNN) RuCl₂]₂(μ-N₂) (4), respectively. The X-ray structure of 5 reveals a BH₄ -anion η2 coordinated to ruthenium through two bridging hydrides. A variable-temperature ¹H NMR study of 6 exhibits interesting fluxional behavior of the BH4 -ligand. Similarly, the Ru(II) hydrido borohydride complex 9, in which the BH₄ -moiety is coordinated in a η1 bonding mode, was obtained by reaction of [RuCl ₂(PPh₃)(ⁱPr-PNP)] (iPr-PNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) (8) with two equivalents of NaBH4 at room temperature. The hydrido borohydride pincer complexes 5, 6, and 9 catalyze the acceptorless dehydrogenative coupling of primary alcohols to esters and the dehydrogenation of secondary alcohols to the corresponding ketones, accompanied by evolution of hydrogen gas. The reactivity follows the order 6 > 9 > 5. With the hydrido borohydride complex 6 as catalyst, high yields (up to 98%) and high turnover numbers (TON～1000) were obtained in the dehydrogenation of primary alcohols under mild and neutral conditions. In addition, 6 effectively catalyzes the hydrogenation of nonactivated aromatic and aliphatic esters to the corresponding alcohols with TON ～200 under a relatively mild pressure of dihydrogen and neutral and homogeneous conditions. Thus, an efficient homogeneous catalytic system for the dehydrogenation- hydrogenation reactions of alcohols is developed, which is relevant to the current interest in hydrogen storage.

Full text of DOI:10.1021/om200595m

ARTICLE
pubs.acs.org/Organometallics
Electron-Rich PNP- and PNN-Type Ruthenium(II) Hydrido Borohydride
Pincer Complexes. Synthesis, Structure, and Catalytic
Dehydrogenation of Alcohols and Hydrogenation of Esters
Jing Zhang,†,§ Ekambaram Balaraman,† Gregory Leitus,‡ and David Milstein*,†
†Department of Organic Chemistry and ‡Department of Chemical Research Support, The Weizmann Institute of Science,
Rehovot, 76100, Israel
§College of Chemistry & Molecular Science, Wuhan University, Wuhan 430072, People's Republic of China
S Supporting Information
b
ABSTRACT: Electron-rich PNP- and PNN-type ruthenium(II) hydrido
borohydride pincer complexes, [RuH(BH4)(tBu-PNP)] (tBu-PNP = (2,6-
bis(di-tert-butylphosphinomethyl)pyridine) (5) and [RuH(BH4)(tBu-
PNN)] (tBu-PNN = 2-di-tert-butylphosphinomethyl-6-diethylaminome-
thylpyridine) (6), were prepared from their corresponding N2-bridged
dinuclear Ru(II) complexes [(tBu-PNP)RuCl2]2(μ-N2) (3) and [(tBu-
PNN)RuCl2]2(μ-N2) (4), respectively. The X-ray structure of 5 reveals a
BH4ꢀ anion η2 coordinated to ruthenium through two bridging hydrides. A
1
variable-temperature ꢀH NMR study of 6 exhibits interesting fluxional
behavior of the BH4 ligand. Similarly, the Ru(II) hydrido borohydride
complex 9, in which the BH4ꢀ moiety is coordinated in a η1 bonding mode,
was obtained by reaction of [RuCl2(PPh3)(iPr-PNP)] (iPr-PNP = 2,6-
bis(diisopropylphosphinomethyl)pyridine) (8) with two equivalents of
NaBH4 at room temperature. The hydrido borohydride pincer complexes 5, 6, and 9 catalyze the acceptorless dehydrogenative
coupling of primary alcohols to esters and the dehydrogenation of secondary alcohols to the corresponding ketones, accompanied
by evolution of hydrogen gas. The reactivity follows the order 6 > 9 > 5. With the hydrido borohydride complex 6 as catalyst, high
yields (up to 98%) and high turnover numbers (TON ∼1000) were obtained in the dehydrogenation of primary alcohols under mild
and neutral conditions. In addition, 6 effectively catalyzes the hydrogenation of nonactivated aromatic and aliphatic esters to the
corresponding alcohols with TON ∼200 under a relatively mild pressure of dihydrogen and neutral and homogeneous conditions.
Thus, an efficient homogeneous catalytic system for the dehydrogenationꢀhydrogenation reactions of alcohols is developed, which
is relevant to the current interest in hydrogen storage.
t
’ INTRODUCTION
activation, and catalysis.11 The highly electron-donating Bu-
PNP (2,6-bis(di-tert-butylphosphinomethyl)pyridine) and its
group 8 metal complexes have been explored by several
groups,12ꢀ14 including ours.15,16a
Transition metal borohydride complexes display extensive
reactivity with organic substrates and are useful starting materials
for the preparation of transition metal hydrides and borides.1
They have found uses in catalytic hydroboration,2 polymeriza-
tion of olefins,3 and cyclic esters.4 Ruthenium hydrido borohy-
dride complexes based on bidentate phosphorus ligands and
diamines, reported by Noyori5 and Morris,6 are effective catalysts
in asymmetric transfer hydrogenation of ketones,5,6 hydrogena-
tion of esters to the corresponding alcohols,7 and enantioselec-
tive Michael addition.6 Whittlesey et al. recently reported four
ruthenium hydrido borohydride complexes bearing series of
N-heterocyclic carbene ligands, which are active catalysts for
hydrogenation of aromatic ketones.8 Synthesis of ruthenaborane
clusters using ruthenium borohydride complex as precursor has
also been reported recently.9 In addition, borohydride complexes
may represent plausible models for CH4 coordination in the
transition state for CꢀH activation.10
Dehydrogenation of alcohols to carbonyl compounds in the
absence of a hydrogen acceptor or oxidant, with the evolution
of molecular hydrogen, is attractive economically and envir-
onmentally (Scheme 1), but homogeneous systems capable of
thermally catalyzing dehydrogenation of alcohols are rela-
tively rare.15c,16a,17ꢀ25 Recently, we have reported that elec-
tron-rich, bulky tBu-PNP-ruthenium complexes catalyze
acceptorless dehydrogenation of secondary alcohols to
ketones.15c When primary alcohols were used, a facile reaction
to produce esters with the evolution of molecular hydrogen
took place.16a The catalytic efficiency of the latter reaction was
enhanced with Ru(II) complexes of an analogous ligand
t
having a potentially “hemilabile” amine “arm”, Bu-PNN
Transition metal complexes of bulky, electron-rich tridentate
ligands have found useful applications in synthesis, bond
Received: July 6, 2011
Published: October 18, 2011
r
2011 American Chemical Society
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ARTICLE
(2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)-
pyridine). (tBu-PNN)Ru(II) complexes effectively catalyze
the acceptorless dehydrogenative coupling of primary alco-
hols to the corresponding esters and molecular hydrogen in
high yields and turnover numbers, in the presence of a
catalytic amount of base.16a,17 Mechanistic studies of this
reaction have led to the discovery of a dearomatized PNN
Ru hydrido carbonyl complex, which does not require the
presence of base, the catalytic reaction proceeding very
effectively under neutral, mild conditions.16a Following this
finding, it was discovered that complex 1 is an efficient catalyst
for hydrogenation of esters to alcohols,15f the novel coupling
of alcohols and amines to yield amides with liberation of
H2,16b unique light-induced splitting of water to hydrogen and
oxygen,16c hydrogenation of amides to alcohols and amines,16d
transesterfication of esters,16e amidation of amines by esters
with extrusion of H2,16f and the novel hydrogenation of
organic carbonates, carbamates, and formates under very mild
conditions.16g Complex 2 is an excellent catalyst for the
corresponding carbonyl compound such as esters (from
primary alcohols) or ketones (from secondary alcohols) with
extrusion of dihydrogen can also be effectively accomplished
under very mild conditions with stable, readily synthesized
electron-rich PNN- and PNP-Ru borohydride complexes, in
the absence of base, under neutral conditions, and in the
absence of a hydrogen acceptor.
’ RESULTS AND DISCUSSION
Synthesis and Characterization of [RuH(η2-BH4)(tBu-PNP]
5. The N2-bridged binuclear Ru(II) complex [(tBu-PNP)-
RuCl2]2(μ-N2) (3) was prepared by the reaction of RuCl2-
(PPh3)3 with one equivalent of the ligand 2,6-bis(di-tert-butyl-
phosphinomethyl)pyridine (tBu-PNP) according to a previously
reported procedure from our group.15c Treatment of 3 with an
excess (5 equiv) of NaBH4 in 2-propanol for 12 h resulted in the
formation of the Ru(II) hydrido borohydride complex 5 in
almost quantitative yield by 31P{1H} NMR (Scheme 2). The
31P{1H} NMR spectrum of 5 exhibits a singlet peak at δ 86.3
ppm, representing a downfield shift of δ 21 ppm relative to
parent complex 3 (δ 65.0 ppm). In the 1H NMR spectrum, the
hydride ligand gives rise to a triplet peak at δ ꢀ16.09 ppm with
JPH = 18.0 Hz. In addition, two broad signals are observed for the
two bridging hydrides at δ ꢀ16.01 and ꢀ4.48 ppm, and another
broad feature is also observed at δ 5.49 ppm belonging to the
terminal boron hydrides. The IR spectrum of 5 indicates two
strong bands in the terminal BꢀH region at 2395 and 2327 cmꢀ1
and two bands in the bridging MꢀHꢀB region at 2104 and
2024 cmꢀ1, consistent with the bidentate η2-BH4 bonding
mode.3c
synthesis of imines from alcohols and amines, with liberation
15h
of water and H2
and NꢀH activation of ammonia.15i
We now report that dehydrogenation of alcohols to the
Scheme 1. Dehydrogenation of Alcohols with Extrusion of
Dihydrogen
Single crystals suitable for X-ray diffraction study were ob-
tained by slow evaporation of a pentane solution of 5 at ꢀ32 °C
(∼3 days). The crystal structure of 5 (Figure 2) displays a
distorted octahedral geometry around the ruthenium center,
including the pincer ligand (tBu-PNP), hydride, and the BH4
units. The hydride ligand is bound to the Ru center cis to the
pyridine nitrogen (N1ꢀRu1ꢀHRu, 80(2)°), while the BH4 unit
is coordinated to the Ru(II) center in a η2 bonding mode. The
two RuꢀH bonds to the chelating BH4 unit are not equal
(1.67(4) and 1.85(5) Å, respectively), while the correspond-
ing RuꢀH bonds in the reported structure of RuH(BH4)-
(PMe3)326a are of equal length. The difference in the case of 5
is likely a result of the larger trans effect of hydride relative to that of
the pyridinic nitrogen atom. Because of the meridional coordination
geometry of the tBu-PNP framework and lack of a plane of symmetry
involving the P, N, and P atoms, the protons of the four tert-butyl and
two methylene groups are magnetically nonequivalent.
Synthesis and Characterization of [RuH(η2-BH4)(tBu-
PNN)], 6. The N2-bridged binuclear Ru(II) complex [(RuCl2-
(PNN))2](μ-N2) (4) was prepared by the reaction of
Figure 1. Electron-rich PNN- and PNP-type Ru(II) pincer complexes.
Scheme 2. Synthesis of Complex 5
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ARTICLE
RuCl2(PPh3)3 with one equivalent of the tBu-PNN pincer ligand,
using a similar method to the one used for the preparation of 3.17
Treatment of 4 with an excess (5 equiv) of NaBH4 in 2-propanol
for 12 h resulted in the formation of the ruthenium(II) hydrido
borohydride complex 6 in excellent yield, as indicated by
31P{1H} NMR spectroscopy (Scheme 3). The 31P{1H} NMR
spectrum of 6 shows a singlet peak at δ 116.7 ppm, representing a
downfield shift of δ 29 ppm relative to the starting complex 4.
The hydride ligand of complex 6 gives rise to a doublet at
δ ꢀ16.24 ppm with JPH = 28.0 Hz in the 1H NMR spectrum. The
IR spectrum of 6 exhibits two strong bands in the terminal BꢀH
region at 2378 and 2311 cmꢀ1 and two bands in the bridging
RuꢀHꢀB region at 2096 and 1956 cmꢀ1, similar to the IR
spectrum of complex 5, consistent with the bidentate η2-BH4
bonding mode.3c
At room temperature, one broad singlet peak at δ ꢀ13.01
ppm is observed in the 1H NMR spectrum of complex 6 for one
proton of BH4ꢀ. A variable-temperature 1H NMR (in 400 MHz)
study of 6 in toluene-d8 is shown in Figure 3, revealing interesting
fluxional behavior of the BH4ꢀ ligand. At 213 K, two singlets are
observed at ꢀδ 13.21 and ꢀ4.33 ppm for the bridging hydrides
and two broad singlets at δ 4.ꢀ70 and 5.13 ppm for the
two terminal hydrides of the BH4 ligand, respectively. Upon
raising the temperature to 248 K, the signals for the two terminal
hydrides are coalesced, while the signal at δ ꢀ4.33 ppm collapses.
At 263 K, the signal (δ ∼5 ppm) for the two terminal hydrides
and the signal at δ ꢀ4.33 ppm for one bridging hydride disappear
in the baseline and the signal (δ ꢀ13.21 ppm) of the other
bridging hydride also collapses. Upon increasing the temperature
to 323 K, the resonance at δ ꢀ13.21 ppm disappears into the
baseline of the spectrum. At 353 K, a very broad singlet peak
appears around δ ꢀ2 ppm, indicating that the four hydrides of
ꢀ
the BH4 moiety start to coalesce. The signal of the terminal
RuꢀH ligand appears as a doublet peak throughout the
213ꢀ373 K temperature range, indicating that it does not
ꢀ
participate in the fluxional process of the BH4 ligand.
1
These variable-temperature H NMR spectra of complex 6 are
similar to those reported for the (bidentate η2 bonding
26a
mode) complexes RuH(BH4)(PMe)3 and RuH(BH4)(ttp)
(ttp = PhP(CH2CH2CH2PPh2)2).26b The bridging hydride
(δ ꢀ4.33 ppm) trans to the terminal RuꢀH exchanges positions
with the two terminal hydrides on the boron at 263 K. These
hydrogen atoms further exchange with the bridging hydride
(δ ꢀ13.21 ppm) trans to pyridinic nitrogen above 323 K, and
the calculated coalescence frequency at δ ꢀ2.0 ppm was ob-
served at 353 K.
Synthesis and Characterization of [RuCl2(PPh3)(iPr-PNP)]
(8) and [RuH(η1-BH4)(PPh3)(iPr-PNP)] (9). Heating a suspen-
sion of RuCl2(PPh3)3 with one equivalent of 2,6-bis(diiso-
propylphosphinomethyl)pyridine (iPr-PNP) 7 in THF at 65 °C
for 6 h resulted in formation of the complex 8 in 75% yield
(Scheme 4). The 31P{1H} NMR spectrum of 8 exhibits one
doublet at δ 46.4 ppm and one triplet peak at δ 43.2 ppm,
indicating that triphenylphosphine is coordinated to the Ru(II)
center, as observed also with the reported Ph-PNP-Ru(II)
dichloride complex (Ph-PNP = 2,6-bis(diphenylphosphino-
Figure 2. ORTEP diagram of complex 5 with thermal ellipsoids at the
50% probability level. All hydrogen atoms except the hydrides are
omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles (deg) of
Complex 5
t
methyl)pyridine).27 In contrast, the Bu complexes 3 and 5 do
Ru1ꢀN1
Ru1ꢀP2
Ru1ꢀHB2
2.123(3)
2.322(1)
1.67(4)
Ru1ꢀP1
Ru1ꢀHRu
Ru1ꢀHB4
2.312(1)
1.57(4)
1.85(5)
not contain coordinated PPh3, under similar preparation condi-
tions, probably because of the steric bulk of the tert-butyl group of
t
t
the Bu-PNP and the Bu-PNN ligands. The two methylene
N1ꢀRu1ꢀHRu
P1ꢀRu1ꢀP2
80(2)
159.6(0.3)
82.9(1)
79(1)
HRuꢀRu1ꢀHB4
HB2ꢀRu1ꢀN1
P1ꢀRu1ꢀN1
HRuꢀRu1ꢀP1
HB3ꢀB1ꢀHB1
167(2)
177(2)
82.5(1)
84(2)
i
groups of the Pr-PNP give rise to one triplet peak at δ 3.92
ppm with JPH = 4.0 Hz, indicating the existence of a symmetric
plane involving the P, N, and P atoms. The PPh3 ligand is
coordinated to the Ru(II) center trans to the nitrogen atom of the
iPr-PNP, and the chloride ligands are coordinated to the metal
center trans to each other.
N1ꢀRu1ꢀP2
P2ꢀRu1ꢀHRu
HB2ꢀRu1ꢀHB4
HB2ꢀB1ꢀHB4
69(2)
111(3)
103(3)
Scheme 3. Synthesis of Complex 6
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Figure 3. Variable-temperature (213ꢀ373 K) 1H NMR (400 MHz) spectra of complex 6 in the region of RuꢀH and BH4ꢀ ligands (toluene-d8 as
solvent).
Scheme 4. Synthesis of Complexes 8 and 9
In analogy with the preparation of complexes 5 and 6,
treatment of complex 8 with an excess (2.5 equiv) of NaBH4
in 2-propanol for 12 h at room temperature resulted in the
precipitation of the Ru(II) hydrido borohydride complex 9 as a
yellow solid in 85% yield (Scheme 4). The 31P{1H} NMR
spectrum of 9 exhibits a doublet signal at δ 61.6 ppm and a
triplet signal at δ 67.9 ppm, corresponding to the two phos-
phorus atoms of the iPr-PNP and one phosphorus atom of the
PPh3, respectively. It is noted that the signal of the PPh3 ligand of
9 is downfield shifted relative to that of the iPr-PNP, unlike the
corresponding signals of complex 8. The RuꢀH gives rise in the
0
Morris.6 The IR spectrum of 9 exhibits strong absorption bands
at 2361 and 2292 cmꢀ1 and a strong broad peak at 1884 cmꢀ1
,
consistent with the monodentate η1-BH4 bonding mode,3c and
1
variable-temperature H NMR studies on complex 9 were not
performed.
Catalytic Activities of Hydrido Borohydride Ru(II) Pincer
Complexes. Dehydrogenation of Alcohols Catalyzed by
Complexes 5, 6, and 9. The ruthenium hydrido borohydride
complexes 5, 6, and 9 catalyze the dehydrogenation of alcohols
with extrusion of molecular hydrogen under base-free condi-
tions. 1-Phenylethanol was selected as test substrate for optimiz-
ing the dehydrogenation of alcohols to the corresponding
carbonyl compounds. Initial experiments revealed that the reac-
tion could be achieved without the need for base and hydrogen
acceptors. Thus, refluxing a toluene solution containing 1-phe-
nylethanol and a catalytic amount (0.1 mol %) of 5 under an
1H NMR of 9 to a quartet signal at δ ꢀ14.0 ppm with JPH = JP H
=
28.0 Hz. A broad peak at δ ꢀ0.84 ppm is assigned for the four
protons of the BH4 ligand, which is probably coordinated to the
Ru(II) center in a η1 bonding mode, similar to the reported
bonding of ruthenium borohydride complexes by Noyori5 and
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Table 2. Dehydrogenation of Secondary Alcohols to the
Corresponding Ketones Catalyzed by Complexes 5, 6, and 9a
was formed in only 13% yield with TON 126 after 48 h (Table 2,
entry 8), while when heated in toluene at 105 °C, 90% of acetone
was formed during the same reaction period with a higher TON
of 900 (Table 2, entry 9).
entry cat.
alcohol
temp (°C) time (h) conv (%)b yield (%)b
In contrast to the dehydrogenation of secondary alcohols to
the corresponding ketones, homogeneous systems capable of
dehydrogenation of primary alcohols to the corresponding
aldehydes or esters are very rare,16a,b,17,18,24,25 probably because
of decarbonylation of the product (or intermediate) aldehyde to
form an inactive carbonyl complex.28
When a toluene solution of benzyl alcohol and 0.1 mol %
complex 5 was refluxed for 24 h in an open system under argon,
benzyl benzoate was formed in 70% yield, accompanied by 4%
of benzaldehyde (Table 3, entry 1). Refluxing 1-hexanol with
0.1 mol % 5 in toluene (115 °C) or neat (157 °C) for 24 h
resulted in formation of 59% and 69% of hexyl hexano-
ate, accompanied by 3% and 10% of hexanal, respectively
(Table 3, entries 2 and 3). Complex 9 was slightly less
catalytically active than 5 under these conditions (Table 3,
entries 7 and 8). The PNN complex 6 exhibited significantly
higher catalytic activity than that of the PNP complexes 5
and 9. Thus, benzyl alcohol, 1-hexanol, and 1-butanol were
dehydrogenated to the corresponding esters in over 95% yield
(Table 3, entries 4ꢀ6) with extrusion of H2 using a catalytic
amount of 6 (0.1 mol %).
1
2
3
4
5
6
7
8d
9
5
6
9
6
6
6
6
6
6
1-phenylethanol
1-phenylethanol
1-phenylethanol
1-phenylethanol
2-hexanol
115
115
115
115
115
110
115
83
24
24
24
48
48
48
48
48
48
27
87
77
93
83
89
57
13
90
27
87
77
93 (81)c
83
2-butanol
89
cyclohexanol
2-propanol
56
13
2-propanol
105
90
a Reaction conditions: catalyst (0.01 mmol), alcohol (10 mmol), and
2 mL of toluene were refluxed in an open system under argon.
b Conversion and yields of the products were analyzed by GC.
c Isolated yield. d Refluxed in absence of solvent in an open system
under argon.
Table 3. Dehydrogenation of Primary Alcohols to Esters and
Aldehydes Catalyzed by Complexes 5, 6, and 9a
yield of
aldehyde
(%)b
yield of
ester
temp
conv
(%)b
Dehydrogenation of Diols to Lactones Catalyzed by Com-
plexes 5 and 6. Next we examined the catalytic activity of
complexes 5 and 6 for the dehydrogenative cyclization of diols to
the corresponding lactones22,29 under very mild, acceptorless
conditions with liberation of dihyrogen as the only byproduct.
Thus, refluxing a toluene solution containing 3 mmol of 1,5-
hexanediol (bearing both secondary and primary alcohol groups)
and a catalytic amount of 5 (0.0 L mmol) under an argon
atmosphere for 48 h resulted in a 33% yield of 6-methylte-
trahydro-2H-pyran-2-one, as observed by 1H NMR spectros-
copy of the crude reaction mixture (Table 4, entry 1).
Significantly, under similar conditions (300 equiv of diol/
cat.; 48 h reflux) using 6 as catalyst, the lactone was formed in
entry cat.
alcohol
(°C)
(%)b
1
5
5
5
6
6
6
9
9
benzyl alcohol
1-hexanol
115
115
157
115
115
110
115
157
75
72
70
99
94
96
62
57.5
4
3
70
2
3c
69
1-hexanol
10
0
59
4
benzyl alcohol
1-hexanol
99 (88)d
94 (77)d
96
5
0
6
1-butanol
7
8c
benzyl alcohol
1-hexanol
3
59
10
47
a Reaction conditions: catalyst (0.01 mmol), alcohol (10 mmol), and
toluene (2 mL) were heated under reflux for 24 h in an system
under argon. b Conversion and yields of the products were
analyzed by GC. c Reflux under (solvent-free) neat condition in an open
argon atm. d Isolated yields.
1
72% yield (Table 4, entry 2) by H NMR. The reaction is
general, and various diols were dehydrogenated to the corre-
sponding lactones (Table 4).
Hydrogenation of Esters Catalyzed by Complexes 5 and 6.
Catalytic dehydrogenation and hydrogenation reactions of or-
ganic molecules are fundamental and important processes in
organic transformations.30 Moreover, these reactions have re-
cently attracted considerable attention from the viewpoint of
hydrogen storage. This phenomenon is common and well
studied in the case of nitrogen heterocycles.31 To the best of
our knowledge, no reports deal with reversible dehydrogen-
ationꢀhydrogenation of an alcoholꢀester couple under very
mild, neutral conditions using a well-defined soluble catalyst
except for complex 1 as reported by us.15k,16a
While the ruthenium hydrido borohydride complexes 5, 6, and
9 catalyze the dehydrogenative coupling reaction of alcohols to
form esters and dihydrogen, we expected that it is possible to
reverse the reaction by the application of mild hydrogen
pressure.15f,k Thus, complexes 5 and 6 were employed as catalyst
for the hydrogenation of nonactivated esters using dihydrogen
under base-free and relatively very mild conditions. Thus, when a
THF solution of butyl butyrate (2 mmol) and complex 5 (0.01
mmol) was heated at 110 °C (bath temperature) for 12 h under
10 atm of H2, only 9% yield of 1-butanol was determined by GC
argon atmosphere for 24 h resulted in 29% conversion of the
alcohol to acetophenone, accompanied by the evolution of
hydrogen gas, as determined by GC and GC-MS analysis
(Table 2, entry 1). Notably, under similar conditions (1000
equiv of alcohol/cat.; reflux at 24 h) employing 6 or 9 as catalysts,
acetophenone was formed in 87% and 77% yield, respectively
(Table 2, entries 2 and 3). The catalytic activity of the PNN-
derived complex 6 is significantly higher than that of the PNP-
derived complex 5 or 9, probably as a result of the potentially
“hemilabile” amine arm of 6, which can play an important role in
the catalytic cycle.16 Similar kinds of reactivity were also observed
for their corresponding dearomatized complexes 1 and 2. As
expected, a longer reaction time (48 h) resulted in a higher yield
of the ketone (Table 2, entry 4). Other secondary alcohols can
also be dehydrogenated to the corresponding ketones using
complex 6 as catalyst in good yields (Table 2, entries 5ꢀ9).
The catalysis by 6 was quite sensitive to the reaction temperature
(and/or solvent), and when 2-propanol was heated at 80 °C with
0.1 mol % of complex 5 under solvent-free conditions, acetone
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(Table 5, entry 1). Remarkably, under similar conditions com-
plex 6 was very efficient, and almost quantitative conversion to
1-butanol was observed. Thus, heating a THF solution of butyl
butyrate (2 mmol) and complex 6 (0.01 mmol) at 110 °C (bath
temperature) for 12 h under 10 atm of H2 yielded 97% of
1-butanol as observed by GC, with the corresponding consump-
tion of dihydrogen (Table 5, entry 4). Hydrogenation of
methyl benzoate resulted in formation of 96% of benzyl
alcohol and 93% of methanol after 12 h (Table 5, entry 6).
The reaction is general and does not require activated esters
(Table 5). The high efficiency of complex 6 in the hydrogena-
tion reaction may be a result of hemilability of the amine arm.
The reaction provides an attractive method for “green”, mild
synthesis of primary alcohols from nonactivated esters with-
out the need for the traditionally used stoichiometric amounts
of metal hydride reagents, which generate stoichiometric
amounts of waste.7,15k,15f,32,33
’ SUMMARY
New electron-rich PNP (complexes 5, 9) and PNN (complex 6)
ruthenium(II) hydrido borohydride pincer complexes were
prepared and were found to catalyze the acceptorless dehydro-
genative coupling of primary alcohols to esters, the dehydrogena-
tion of secondary alcohols to the corresponding ketones, and
dehydrogenative cyclization of diols to lactones, accompanied by
evolution of hydrogen gas. The PNN complex 6 is the most
effective, and it also catalyzes the hydrogenation of nonactivated
esters to the corresponding alcohols with TON ≈ 200 under
relatively mild pressure of H2, neutral and homogeneous con-
ditions. These dehydrogenationꢀhydrogenation reactions of
alcoholsꢀesters are of interest synthetically, as well as in the
context of hydrogen storage.
Table 4. Dehydrogenation of Diols to the Corresponding
Lactones Catalyzed by Complexes 5 and 6a,b,c
’ EXPERIMENTAL SECTION
General Procedures. All experiments with metal complexes and
phosphine ligands 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 atmo-
sphere. Deuterated solvents were used as received. All solvents were
degassed with argon and kept in the glovebox over 4 Å molecular sieves.
Complexes 315c and 417 were prepared according to a previously
i
reported procedure from our group. The ligand Pr-PNP (2,6-bis-
35
(diisopropylphosphinomethyl)pyridine)34 and RuCl2(PPh3)3 were
prepared according to literature procedures.
1H, 13C, and 31P NMR spectra were recorded at 400 or 500, 100 or
126, and 162 or 202 MHz, respectively, using Bruker AMX-400 and
AMX-500 NMR spectrometers. 1H and 13C{1H} NMR chemical shifts
are reported in ppm downfield from tetramethylsilane. 31P NMR
chemical shifts are reported in parts per million downfield from
H3PO4 and referenced to an external 85% solution of phosphoric acid
Scheme 5. Catalytic Hydrogenation of Esters to the
Corresponding Alcohols
a Reaction conditions: catalyst (0.01 mmol), diol (3 mmol) and 2 mL of
toluene were refluxed in an open system under argon. b Yields of
the lactones were analyzed by 1H NMR of the reaction mixture.
c Isolated yield.
Table 5. Hydrogenation of Esters Catalyzed by Complexes 5 and 6a
entry
ester
catalyst
PH2 (atm)
time (h)
conv (%)b
products (yield [%])b
1-butanol (9)
1
2
3
4
5
6
butyl butyrate
5
5
6
6
6
6
10
10
10
10
10
10
12
12
12
12
12
12
10
17
94
98
99
97
benzyl benzoate
hexyl hexanoate
butyl butyrate
benzyl alcohol (15)
1-hexanol (94)
1-butanol (97)
benzyl benzoate
benzyl alcohol (99)
methyl benzoate
benzyl alcohol (96), methanol (93)
a A solution of complex 5 or 6 (0.01 mmol) and ester (2 mmol) in THF (2 mL) was heated at 110 οC (bath temperature) under H2 (10 atm) for 12 h.
b Conversion of esters and percentage of maximum possible amount of each of the product alcohols were determined by GC.
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Organometallics
ARTICLE
in D2O. Abbreviations used in the NMR follow-up experiments: br,
broad; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet, v, virtual.
Elemental analyses were performed at Kolbe Laboratorium, Mulheim,
Germany. IR spectra were recorded on a Nicolet FT-IR spectrophot-
ometer. GCꢀMS was carried out on HP 6890 (flame ionization detector
and thermal conductivity detector) and HP 5973 (MS detector)
instruments equipped with a 30 m column (Restek 5MS, 0.32 mm
internal diameter) with a 5% phenylmethylsilicone coating (0.25 mm)
and helium as carrier gas. GC analyses were carried out using a Carboxen
1000 column on a HP 690 series GC system or HP-5 cross-linked 5%
phenylmethylsilicone column (30 m ꢁ 0.32 mm ꢁ 0.25 μm film
thickness, FID) on a HP 6890 series GC system.
(s, N(CH2CbH3)2), 29.0 (d, JPC = 4.0 Hz, P(C(CH3)3)2), 34.2 (d, JPC =
15.1 Hz, P(Ca(CH3)3)2), 37.0 (d, JPC = 10.0 Hz, P(Cb(CH3)3)2), 38.9
(d, JPC = 19.1 Hz, CH2P), 51.1 (s, N(CaH2CH3)2), 51.2 (s, N-
(CbH2CH3)2), 63.7 (s, CH2N-Py), 117.5 (s, Py-C3), 118.5 (s, Py-C5),
128.8 (s, Py-C4), 159.9 (s, Py-C6), 163.7 (d, JPC = 4.0 Hz, Py-C2). IR
(KBr pellets): 2378, 2311, 2096, 1956, 1469, 1177 cmꢀ1. Anal. Calcd for
C19H40BN2PRu: C, 51.93; H, 9.18. Found: C, 51.86; H, 9.24.
Synthesis of [RuCl2(PPh3)(iPr-PNP)] (8). To a suspension of
Ru(PPh3)3Cl2 (480 mg, 0.5 mmol) in dry THF (20 mL) was added the
ligand 2,6-bis(diisopropylphosphinomethyl)pyridine (iPr-PNP) (7)
(170 mg, 0.5 mmol), and the reaction mixture was heated at 65 °C for 6 h
with constant stirring. The clear, yellow solution was concentrated to
∼4 mL under vacuum, and 20 mL of pentane was added to precipitate a
yellow solid. The solid was isolated by filtration, washed with pentane
(3 ꢁ 2 mL), and dried under vacuum to give 290 mg (75% yield) of 8 as
an analytically pure sample.
Synthesis of [RuH(η2-BH4)(tBu-PNP)] (5). To a suspension of
complex 3 (58 mg, 0.05 mmol) in 2-propanol (10 mL) was added a very
fine powder of NaBH4 (9.5 mg, 0.25 mmol), and the reaction mixture
was stirred at room temperature for 12 h. The reaction mixture was
filtered, the orange filtrate was evaporated under vacuum, and the
residue was extracted with diethyl ether (3 ꢁ 5 mL). The ether solution
was evaporated under vacuum to yield complex 5 as an orange solid.
31P{1H} NMR (CD2Cl2): δ 46.4 (d, JPP = 27.5 Hz), 43.2 (t, JPP
=
1
27.5 Hz). H NMR (CD2Cl2): δ 0.87 (q, 12H, JPH = JHH = 8.0 Hz,
P(CH(CHa3)2)2), 1.13 (q, 12H, JPH = JHH = 8.0 Hz, P(CH(CHb3)2)2),
2.43 (m, 4H, P(CHa(CH3)2)2), 3.92 (t, 4H, JPH = 4.0 Hz, 2CH2P), 7.26
(m, 9H, P(C6H5)3), 7.30 (d, 2H, JHH = 8.0 Hz, Py-H3 and Py-H5), 7.53
(t, 1H, JHH = 8.0 Hz, Py-H4), 7.91 (m, 6H, P(C6H5)3). 13C{1H} NMR
(CD2Cl2): δ 19.6 (s, P(CH(CaH3)2)2), 20.7 (s, P(CH(CbH3)2)2), 25.1
(t, JPC = 8.0 Hz, P(CH(CH3)2)2), 38.9 (t, JPC = 10.1 Hz, CH2P), 119.9
(s, Py-C3 and Py-C5), 127.2 (d, JPC = 9.0 Hz, m-C6H5-P), 128.8 (s,
p-C6H5-P), 135.0 (d, JPC = 9.0 Hz, o-C6H5-P), 136.7 (s, Py-C4), 140.4
(d, JPC = 36.2 Hz, ipso-C6H5-P), 164.8 (t, JPC = 4.0 Hz, Py-C2 and Py-
C6). Anal. Calcd for C37H50NP3Cl2Ru: C, 57.44; H, 6.52. Found: C,
57.28; H, 6.53.
1
Yield: 50 mg (98%). 31P{1H} NMR (C6D6): δ 86.3 (s). H NMR
(C6D6): δ ꢀ16.09 (t, 1H, JPH = 18.0 Hz, Ru-H), ꢀ16.01 (br s, 1H,
BH4), ꢀ4.48 (br s, 1H, BH4), 1.45 (t, 18H, JPH = 6.0 Hz, P(C(CHa3)3)),
1.56 (t, 18H, JPH = 6.0 Hz, P(C(CHb3)3)2), 3.08 (dt, 2H, JHH = 16.0 Hz,
JPH = 4.0 Hz, CHaHP), 3.19 (dt, 2H, JHH = 16.0 Hz, JPH = 4.0 Hz,
CHHbP), 5.49 (br s, 2H, BH4), 6.52 (d, 2H, JHH = 8.0 Hz, Py-H3 and Py-
H5), 6.76 (t, 1H, JHH = 8.0 Hz, Py-H4). 13C{1H} NMR (C6D6): δ 29.7
(s, P(C(CaH3)2)2), 30.2 (s, P(C(CbH3)3)2), 35.0 (t, JPC = 6.5 Hz,
P(Ca(CH3)3)2), 35.6 (t, JPC = 5.0 Hz, P(Cb(CH3)3)2), 39.0 (t, JPC = 7.0
Hz, CH2P), 118.2 (t, JPC = 4.0 Hz, Py-C3 and Py-C5), 131.0 (s, Py-C4),
165.1 (t, JPC = 4.5 Hz, Py-C2 and Py-C6). IR (KBr pellets): 2395, 2327,
2104, 2024, 1461, 1184 cmꢀ1. Anal. Calcd for C23H48BNP2Ru: C,
53.90; H, 9.45. Found: C, 53.86; H, 9.49.
Synthesis of [RuH(η1-BH4)(PPh3)(iPr-PNP)] (9). To a suspen-
sion of complex 8 (77 mg, 0.1 mmol) in 2-propanol (10 mL) was added a
fine powder of NaBH4 (9.5 mg, 0.25 mmol), and the reaction mixture
was stirred at room temperature for 12 h and then filtered. The resulting
yellow solid was washed with 2-propanol (3 ꢁ 2 mL) and then dissolved
in benzene (10 mL), and the solution was filtered. The yellow filtrate was
evaporated to dryness under vacuum, to yield complex 9 as a yellow
solid, which was dried under vacuum overnight to give 61 mg (85%
yield) of analytically pure compound.
Synthesis of [RuH(η2-BH4)(tBu-PNN)] (6). To a suspension of
complex 4 (51 mg, 0.05 mmol) in 2-propanol (10 mL) was added a very
fine powder of NaBH4 (9.5 mg, 0.25 mmol), and the reaction mixture
was stirred at room temperature for 12 h. The reaction mixture was
filtered, the orange filtrate was evaporated under vacuum, and the orange
solid residue was extracted with diethyl ether (3 ꢁ 5 mL). The ether
solution was evaporated under vacuum to yield complex 6 as a red-
orange solid, which was dried under vacuum overnight.
31P{1H} NMR (C6D6): δ 61.6 (d, JPP = 29.2 Hz), 67.9 (t, JPP = 29.2
Hz). 1H NMR (C6D6): δ ꢀ14.0 (q, 1H, JPH = 28.0 Hz, Ru-H), ꢀ0.84
(br s, 4H, BH4), 0.91 (q, 6H, JPH = JHH = 8.0 Hz, P(CH(CH3)2)2), 0.87
(m, 12H, JPH = JHH = 8.0 Hz, P(CH(CH3)2)2), 1.18 (q, JPH = JHH = 8.0
Hz, 6H, P(CH(CH3)2)2), 1.44 (m, 2H, P(CHa(CH3)2)2), 1.77 (m, 2H,
P(CHb(CH3)2)2), 2.85 (dt, 2H, JHH = 16.0 Hz, JPH = 4.0 Hz, CHaHP),
Yield: 40 mg (91%). 31P{1H} NMR (C6D6): δ 116.7 (s). 1H NMR
(C6D6, 298 K): δ ꢀ16.24 (t, 1H, JPH = 28.0 Hz, Ru-H), ꢀ13.10 (br s,
1H, BH4), 0.84 (t, 3H, JHH = 6.0 Hz, N(CH2CHa3)2), 0.99 (t, 3H, JHH
=
6.0 Hz, N(CH2CHb3)2), 1.28 (d, 9H, JPH = 12.0 Hz, P(C(CHa3)3)),
1.43 (d, 9H, JPH = 12.0 Hz, P(C(CHb3)3)2), 2.31 (m, 1H, N-
3.92 (dt, 2H, JHH = 16.0 Hz, JPH = 4.0 Hz, CHHbP), 6.56 (d, 2H, JHH
=
0
(CHaHCH3)2), 2.49 (m, 1H, N(CHHa CH3)2), 2.71 (dd, 1H, JHH
=
8.0 Hz, Py-H3 and Py-H5), 6.79 (t, 1H, JHH = 8.0 Hz, Py-H4), 7.01 (d,
3H, JHH = 8.0 Hz, P(C6H5)3), 7.12 (t, 6H, JHH = 8.0 Hz, P(C6H5)3),
8.18 (t, 6H, JHH = 8.0 Hz, P(C6H5)3). 13C{1H} NMR (C6D6): δ 18.0
(s, P(CH(CaH3)2)2), 18.5 (s, P(CH(CbH3)2)2), 19.6 (s, P(CH-
16.0 Hz, JPH = 8.0 Hz, CHaHP), 2.98 (dd, 1H, JHH = 16.0 Hz, JPH = 12.0
Hz, CHHbP), 3.05 (m, 1H, N(CHbHCH3)2), 3.41 (d, 1H, JHH = 14.0
Hz, NCHaH-Py), 3.54 (d, 1H, JHH = 14.0 Hz, NCHHb-Py), 3.73 (m, 1H,
0
0
0
N(CHHb CH3)2), 6.30 (d, 1H, JHH = 8.0 Hz, Py-H5), 6.51 (d, 1H, JHH
=
(Ca H3)2)2), 20.8 (s, P(CH(Cb H3)2)2), 25.6 (t, JPC = 8.0 Hz, P(CaH-
(CH3)2)2), 26.4 (t, JPC = 11.6 Hz, CH2P), 39.2 (t, JPC = 6.0 Hz,
P(CbH(CH3)2)2), 118.4 (t, JPC = 2.5 Hz, Py-C3 and Py-C5), 127.1
(d, JPC = 9.0 Hz, m-C6H5ꢀP), 128.8 (s, p-C6H5ꢀP), 134.2 (s, Py-C4),
135.7 (d, JPC = 10.1 Hz, o-C6H5-P), 142.0 (d, JPC = 36.2 Hz, ipso-C6H5-
P), 163.7 (t, JPC = 4.5 Hz, Py-C2 and Py-C6). IR (KBr pellet): 2361,
2293, 2246, 1884, 1458, 1060 cmꢀ1. Anal. Calcd for C37H55BNP3Ru: C,
61.84; H, 7.72. Found: C, 61.98; H, 7.66.
8.0 Hz, Py-H3), 6.71 (t, 1H, JHH = 8.0 Hz, Py-H4), signals for three other
protons of BH4 collapsed in the baseline (which was detected at low-
temperature NMR spectra). 1H NMR (toluene-d8, 243 K): δ ꢀ16.30 (t,
1H, JPH = 28.0 Hz, Ru-H), ꢀ13.21 (br s, 1H, BH4), ꢀ4.33 (br s, 1H,
BH4), 0.71 (br s, 3H, N(CH2CHa3)2), 0.92 (br s, 3H, N(CH2CHb3)2),
1.18 (d, 9H, JPH = 12.0 Hz, P(C(CHa3)3)), 1.35 (d, 9H, JPH = 12.0 Hz,
P(C(CHb3)3)2), 2.10 (m, 1H, N(CHaHCH3)2), 2.31 (m, 1H, N-
(CHHbCH3)2), 2.53 (dd, 1H, JHH = 16.0 Hz, JPH = 8.0 Hz, CHaHP),
2.82 (dd, 1H, JHH = 16.0 Hz, JPH = 8.0 Hz, CHHbP), 2.93 (br m, 1H,
General Procedures for Catalytic Dehydrogenation of
Alcohols. (a) Complex 5 (0.01 mmol), 6 (0.01 mmol), or 9 (0.01
mmol) was dissolved in the neat primary or secondary alcohol
(10 mmol). The flask was equipped with a condenser, and the solution
was heated with stirring in an open system under argon at the specified
temperature and time (Tables 2 and 3). After cooling to room
temperature, the product aldehydes, esters, or ketones were determined
0
N(CHa HCH3)2), 3.15 (d, 1H, JHH = 16.0 Hz, NCHaH-Py), 3.33 (d, 1H,
0
JHH = 16.0 Hz, NCHHb-Py), 3.37 (m, 1H, N(CHHb CH3)2), 4.69 (br s,
1H, BH4), 5.00 (br s, 1H, BH4), 6.16 (d, 1H, JHH = 8.0 Hz, Py-H5),
6.38 (d, 1H, JHH = 8.0 Hz, Py-H3), 6.62 (t, 1H, JHH = 8.0 Hz,
Py-H4). 13C{1H} NMR (C6D6): δ 8.8 (s, N(CH2CaH3)2), 11.0
5722
dx.doi.org/10.1021/om200595m |Organometallics 2011, 30, 5716–5724

Organometallics
ARTICLE
by GC, using mesitylene or benzene (in the case of 1-butanol) as internal
standard, employing a Carboxen 1000 column on a HP 6890 series GC
system.
(b) A solution containing the catalyst (0.01 mmol) (complex 5, 6,
or 9) and the alcohol (10 mmol) in toluene (2 mL) was heated in a flask
equipped with a reflux condenser under argon in an open system at the
specified temperatures and times (Tables 2 and 3). After cooling to
room temperature, the products were determined by GC using mesity-
lene or benzene (for 1-butanol) as internal standard, employing a
Carboxen 1000 column on a HP 6890 series GC system.
’ ACKNOWLEDGMENT
This research was supported by the European Research
Council under the FP7 framework (ERC No 246837), by the
Israel Science Foundation, and by the Helen and Martin Kimmel
Center of Molecular Design. J.Z. is the recipient of the Harry K.
Stone Foundation Postdoctoral Fellowship. D.M. holds the Israel
Matz Professorial Chair of Organic Chemistry.
’ REFERENCES
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purified nitrogen in a Vacuum Atmospheres glovebox. The flask was
equipped with a condenser, and the solution was refluxed with stirring in
an open system under argon for 48 h. After cooling to room temperature,
the yield of the lactones was determined by 1H NMR spectroscopy from
the reaction mixture.
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X-ray Crystal Structure Determination of 5. The crystal was
mounted on a nylon loop and flash frozen in a nitrogen stream at 120 K.
Data were collected on a Nonius Kappa CCD diffractometer mounted
on a FR590 generator equipped with a sealed tube with Mo Kα radiation
(λ = 0.71073 Å) and a graphite monochromator. The structure was
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monoclinic, s.g. P21, a = 12.541(2) Å, b = 15.314(3) Å, c = 15.131(3) Å,
β = 111.85(3)°, V = 2697.3(11) Å3, Z = 4, fw = 512.44, F(000) = 1088,
Dc = 1.262 Mg/m3, μ = 0.709 mmꢀ1. The final cycle of refinement
based on F2 gave an agreement factor R = 0.0325 for data with I >
2σ(I) and R = 0.0364 for all data (11 912 reflections) with a good-
ness-of-fit of 1.014. Idealized hydrogen atoms were placed and
refined in the riding mode, with the exception of HꢀRu and
Hb1ꢀHb4, which were located in the difference map and refined in-
dependently. The X-ray crystal structure of complex 5 is deposited in
the CCDC with number 620246.
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’ ASSOCIATED CONTENT
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L. J. W.; Milstein, D. Organometallics 2002, 21, 812–818. (b) Ben-Ari, E.;
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Supporting Information. CIF file containing X-ray crys-
b
tallographic data for complex 5. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: david.milstein@weizmann.ac.il; jzhang03@whu.edu.cn.
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Organometallics
ARTICLE
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