Osmium and ruthenium catalysts for dehydrogenation of alcohols

Article abstract of DOI:10.1021/om200437n

A series of pincer-type complexes of Os and Ru have been synthesized and investigated in catalytic alcohol dehydrogenation. The hydrides OsHCl(CO)[HN(C₂H₄PiPr₂)₂] and OsH₂(CO)[HN(C₂H₄PiPr₂)₂] possess good air, moisture, and thermal stability and are outstanding versatile dehydrogenation catalysts for primary alcohols for reactions of transfer hydrogenation, dehydrogenative coupling, and amine alkylation.

Full text of DOI:10.1021/om200437n

COMMUNICATION
pubs.acs.org/Organometallics
Osmium and Ruthenium Catalysts for Dehydrogenation of Alcohols
Marcello Bertoli,† Aldjia Choualeb,† Alan J. Lough,‡ Brandon Moore,† Denis Spasyuk,† and
Dmitry G. Gusev*,†
†Department of Chemistry, Wilfrid Laurier University, Waterloo, Ontario N2L 3C5, Canada
‡Department of Chemistry, X-ray Laboratory, University of Toronto, Toronto, Ontario M5S 3H6, Canada
S Supporting Information
b
ABSTRACT: A series of pincer-type complexes of Os and Ru have
been synthesized and investigated in catalytic alcohol dehydrogena-
tion. The hydrides OsHCl(CO)[HN(C2H4PiPr2)2] and OsH2-
(CO)[HN(C2H4PiPr2)2] possess good air, moisture, and
thermal stability and are outstanding versatile dehydrogenation
catalysts for primary alcohols for reactions of transfer hydro-
genation, dehydrogenative coupling, and amine alkylation.
Catalytic alcohol dehydrogenation is an elegant approach1 en-
Scheme 1. Reactions Enabled by Alcohol Dehydrogenation
abling reactivity unusual for primary alcohols, affording esters (I),2
imines (II),3 secondary amines (III),4 amides (IV),5 and N-hetero-
cycles (V)4a,6 according to Scheme 1. Transfer hydrogenation (TH;
VI) can be conducted using ethanol as the hydrogen source.7 The
reactions of Scheme 1 are distinguished by excellent selectivity, atom
economy, and the use of low-toxicity starting organic materials.
Catalysts for alcohol amination (III) and the synthesis of
N-heterocycles (V) typically require loadings in the range of
1ꢀ2.5 mol %.1,4,6 However, very efficient Ru and Rh catalysts
performing at 0.1ꢀ0.2 mol % loadings for reactions I, II, and
IV2,3,5a,5b and at 0.001 mol % loading for TH (VI)7 have been
recently developed by Milstein and Gr€utzmacher. The catalysts from
these two groups employ ligand cooperativity8 in alcohol dehydro-
genation, involving reversible CꢀH or NꢀH bond cleavage. In
both Ru and Rh catalysts, the cooperating ligands are coordinated to
the central metal in a tridentate fashion. Inspired by these ideas,
we became interested in developing group 8 metal complexes with
the pincer-type ligands PNP (=HN(C2H4PiPr2)2) and POP
(=O(C2H4PiPr2)2).9 This communication reports a series
of new compounds shown in Chart 1, among which the
complexes OsHCl(CO)[HN(C2H4PiPr2)2] and OsH2(CO)-
[HN(C2H4PiPr2)2] proved to be efficient, robust, and versa-
tile catalysts for dehydrogenative coupling and amination
reactions of primary alcohols and for TH.
The monohydrides MHCl(CO)[HN(C2H4PiPr2)2] (M = Os
(1), Ru(5)), andOsHCl(CO)[O(C2H4PiPr2)2] (3) are easily pre-
pared in gram quantities and high yields by reacting MHCl(CO)-
(PPh3)3 with the PNP or POP ligands. Treating 1 with tBuOK
and 2-propanol in ether produced the dihydride trans-OsH2-
(CO)[HN(C2H4PiPr2)2] (2). Similarly, reacting 5 with tBuOK
in ether under H2 gave the dihydride trans-RuH2(CO)[HN-
(C2H4PiPr2)2] (6). The Na[HBEt3] reduction of 3 successfully
afforded the dihydride trans-OsH2(CO)[O(C2H4PiPr2)2] (4).
Complexes 1 and 2 have been crystallographically characterized
(Figure 1), and the NMR data for compounds 1ꢀ6 are all con-
sistent with the structures in Chart 1.
Treating 5 with tBuOK and 2 equiv of 2-propanol in ether
resulted in formation of the isopropoxide complex RuH(OiPr)-
(CO)[HN(C2H4PiPr2)2] (7), which crystallized with a hydrogen-
bonded molecule of 2-propanol (Figure 2). Complex 7
exists in equilibrium with the dihydride 6 and acetone in C6D6
(7/6 = 0.7). 1H NMR experiments revealed selective Ha and Hb
saturation transfers between 7 (δ(Ha) ꢀ17.5, δ(Hb) 3.9) and
6 (δ(Ha) ꢀ6.5, δ(Hb) ꢀ6.3), in agreement with the equilibrium of
Scheme 2. The tendency of the Noyori-type hydrogenation catalysts
to form alkoxides in reactions with ketones is well-documented.10
Complexes 1, 2, 4, and 6 were tested in the catalytic reactions
of Scheme 3: transfer hydrogenation (TH) in 2-propanol (a) and
ethanol (b), acceptorless dehydrogenative coupling (ADC) of
alcohols (c), and alkylation of primary amines (d). The catalytic
results are summarized in Tables 1ꢀ3.
Complex 2 is an excellent TH catalyst in 2-propanol without
base, and the chloride 1 is similarly active under basic conditions.
Received: May 24, 2011
Published: June 06, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/om200437n Organometallics 2011, 30, 3479–3482
|

Organometallics
COMMUNICATION
Chart 1. New Complexes of This Work
Scheme 3. Catalytic Reactions Tested in This Work
Table 1. Results of Transfer Hydrogenation Experiments
line
substrate
ROH cat. [base]a tb
TOFc
1
2
3
4
5
6
7
8
acetophenone
acetophenone
acetophenone
acetophenone
acetophenone
acetophenone
acetophenone
acetophenone,
cyclohexylamined
acetophenone
cyclohexanone
cyclohexanone
cyclohexanone
cyclohexanone
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
EtOH
EtOH
EtOH
1
2
2
6
6
1
2
2
1.0
1.0
1.0 1.4 ꢁ 104
0.5 1.3 ꢁ 104
2.0 1.6 ꢁ 104
0.5 1.4 ꢁ 103
0.6 1.2 ꢁ 103
2.5 1.0 ꢁ 102
1.8 3.0 ꢁ 102
2.0 2.6 ꢁ 102
1.0
0.3
Figure 1. Molecular structures of 1 and 2 showing 30% probability
ellipsoids.
9
EtOH
iPrOH
iPrOH
EtOH
EtOH
2
2
2
2
2
1.0
1.0
0.1
0.8 4.7 ꢁ 102
1.7 1.9 ꢁ 104
1.3 6.3 ꢁ 104
0.8 5.5 ꢁ 102
0.8 9.6 ꢁ 102
10
11
12
13
a tBuOK, in mol %. b Time (h) to reach 50% conversion at room
temperature. c Turnover frequencies at 50% conversion, determined
by 1H NMR. d Ratio 1:1.
in the ADC reaction of Scheme 3c in refluxing isoamyl alcohol
(Table 2, line 5). The ADC activity of complexes 2 and 6 in
Table 2 is similar to the activity of the best ADC catalysts
reported by Milstein; however, Milstein’s systems2 work at 100ꢀ
115 °C, whereas catalysts 2 and 6 become efficient above 120 °C.
We next investigated activities of the dihydrides 2, 4, and 6 for
monoalkylation of primary amines (Scheme 3d). For compara-
tive reasons, this study included a known active TH catalyst
prepared earlier in our laboratory, mer-IrH3[HN(C2H4PiPr2)2]
(8).12 First, experiments were conducted at 160 °C using a 24 h
reaction time, 0.1 mol % catalyst loading, and a 1.1/1 isoamyl
alcohol/amine ratio. The Os and Ir catalysts 2 and 8 performed
similarly well, respectively affording 73% and 64% of isopentyl-
hexylamine from hexylamine and 60% and 58% of isopentylben-
zylamine from benzylamine. The Ru dihydride 6 gave only 7%
conversion in both cases, whereas no secondary amine was
observed in the reaction of isoamyl alcohol with cyclohexylamine
catalyzed by the POP dihydride 4. Varied amounts (3ꢀ11%) of
the corresponding imine products were formed along with the
secondary amine in all reactions catalyzed by 2, 6, and 8 at
160 °C, and the amount of the imine inversely correlated with the
catalyst activity for the secondary amine product.
Figure 2. Structure of 7 showing 30% probability ellipsoids.
Scheme 2. Equilibrium between 7 and 6 in Solution
Both catalysts allow using large substrate-to-catalyst ratios
of 104ꢀ105 and give very high turnover frequencies at room
temperature (Table 1). The TH activity of 2 for cyclohexanone
was enhanced in the presence of tBuOK; however, the reason for
this is not clear. TH with 1 and 2 is slower in ethanol; however,
the hydrogenation proceeds smoothly at room temperature
using only 0.1 mol % 1 or 2. The production of acetaldehyde is
probably rate-limiting in ethanol, since TH of acetophenone in
the presence of cyclohexylamine afforded 1-phenylethanol and
N-cyclohexylethanimine at the same rate as for 1-phenylethanol
and ethylacetate without the amine (Table 1, lines 7 and 8). TH
activity of the ruthenium dihydride 6 is an order of magnitude
lower compared to that for 2, while the POP dihydride 4 was
completely inactive in this reaction. Complex 4 was also inactive
We further investigated the temperature dependence of the
activity of 2 between 140 and 200 °C using a 1.1/1 hexylamine/
isoamyl alcohol mixture. The conversion to secondary amine
increased 2-fold in this range: from 46% at 140 °C to 91% at
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Organometallics
COMMUNICATION
Table 2. Results of ADC Experiments
Scheme 4. Other Reactions Catalyzed by 2
line
alcohol
[cat.], mol %
T, °C
time, h
conversn, %a
1
2
3
4
5
6
7
8
butanol
butanolb
butanol
isoamyl
isoamyl
isoamyl
hexanol
benzyl
2, 0.1
2, 0.1
6, 0.1
2, 0.1
4, 0.1
6, 0.1
2, 0.1
2, 0.1
118
118
118
130
130
130
157
205
7
7
6
8
5
4
5
2
17
6
21
93
0
91
91
88
Scheme 5. Formation of the Amido Complex 9
a Determined by 1H NMR. b With added 5.5 mol % H2O.11
Table 3. Results of Catalytic Amine Alkylation at 200 °C
line
amine
alcohol
[cat.], mol %
time, h
conversn, %a
1
hexyl
isoamyl
benzyl
ethyl
2, 0.1
2, 0.1
2, 0.1
4, 0.1
2, 0.1
4, 0.1
2, 0.1
2, 0.1
2, 0.1
1, 0.1b
2, 0.1
2, 0.1
2, 0.1
2, 0.1b
2, 0.1
24
30
30
24
30
24
30
30
30
30
30
30
30
30
30
91
84
91
66
68
43c
74
88
62
68
60
36
45
47
37
sphere catalysts. Distinguishing these two types is challenging;14
however, the lack of activity demonstrated by the POP complex
4 up to 130 °C suggests that the PNP complexes 2 and 6 are
outer-sphere (HN-MH bifunctional) catalysts. At 160 °C, 4
apparently starts forming unsaturated alkoxide species, likely
via a k3-POP f k2-POP process, after 34 h leading to formation
of 23% N-cyclohexylisopentanimine in the reaction of cyclohex-
ylamine and isoamyl alcohol. At 200 °C, secondary amines are
produced with 4 (Table 3, lines 4 and 6), and at this temperature
the inner-sphere mechanism might be operational as well in
the case of the PNP catalysts.
2
hexyl
3
cyclohexyl
cyclohexyl
cyclohexyl
cyclohexyl
benzyl
4
ethyl
5
isoamyl
isoamyl
isoamyl
ethyl
6
7
8
benzyl
9
benzyl
benzyl
methyl
methyl
ethyl
10
11
12
13
14
15
aniline
aniline
aniline
The key intermediate in the outer-sphere alcohol dehydro-
genation catalyzed by 1 and 2 is OsH(CO)[N(C2H4PiPr2)2]
(9). Complex 9 can be derived from 1 with a base or from 2 via
H2 loss or transfer to a substrate according to Scheme 5. For
example, quantitative formation of 9 was observed upon treating
1 with NaN(SiMe3)2 in C6D6. The high solubility and air sensitivity
make complex 9 less convenient to handle compared to 1 or 2. It is
worth mentioning that 1 is air stable in CD2Cl2. The dihydride 2 is
also reasonably air stable in solution. It showed 5% decomposition
after 2 h in C6D6, and the complex was largely (>80%) unchanged
even after 12 h in air, according to 31P NMR.
aniline
isoamyl
benzyl
benzyl
aniline
aniline
a Conversion to secondary amines determined by 1H NMR. b With
tBuOK (0.5 mol %). c Additional 6% conversion to imine.
200 °C, and no imine was detected in the product at 180ꢀ
200 °C. On the basis of these observations, a series of experi-
ments in Table 3 were conducted at 200 °C using 1/1 amine/
alcohol mixtures without any added solvent. The conversions
were typically in the 70ꢀ90% range, except those in the alkylation
reactions of aniline, where the best yield was 68% for N-methyl-
aniline. The yields of N-methylaniline and N-benzylaniline increased
by 8ꢀ10% in the presence of tBuOK. Excellent selectivities for the
secondary amine products were observed in all reactions of Table 3,
where no or only trace amounts of the tertiary amines were detected.
In additional catalytic experiments with 2 (Scheme 4), a ring-
closing reaction of molten 5-aminopentanol successfully afforded
piperidine, and ammonia was converted into a mixture of Et2NH
and Et3N in ethanol.13 Only a trace of EtNH2 was observed in the
latter reaction, where ammonia conversion reached 57%, based on the
DFT modeling of dehydrogenation of 2-propanol by 9
revealed two transition states (Figure 3). The first (ts1, ΔH/
ΔGq = ꢀ0.5/þ9.7 kcal/mol) corresponds to proton transfer
(APT charge on H1 þ0.66) leading to formation of the NꢀH
bond. The second, rate-limiting transition state (ts2, ΔH/ΔGq =
2.8/13.7 kcal/mol) corresponds to hydride transfer (APT charge
on H2 = ꢀ0.66) to Os, leading to formation of 2 and acetone.
Ts1 and ts2 are connected by an intermediate where the iPrOꢀ ion
is bonded to OsH(CO)[HN(C2H4PiPr2)2]þ via an O HꢀN
3 3 3
hydrogen bond and an agostic CꢀH Os bond (dC1ꢀH2 =1.21Å,
3 3 3
dOsꢀH2 = 1.97 Å).15 In the case of ruthenium, this type of
intermediate may undergo a rearrangement, resulting in the
equilibrium between the dihydride 6 and the isopropoxide 7 in
Scheme 2. The greater catalytic efficiency of 2 vs 6 is probably
due to the oxophilic nature of Os being less that that of Ru.
Indeed, the catalytically “useful” product of dehydrogenation of
1
integration of the H NMR spectrum of the product solution and
assuming that the starting material (Aldrich) contained 2 M NH3.
Catalytic amine alkylation has been extensively studied by Wil-
liams, Fujita, Jamaguchi and others; however, the catalysts known
today operate with S/C ratios of 40ꢀ100.1,4 The turnover numbers
of 600ꢀ900 demonstrated by 2make this complex, together with our
recently reported OsH4[HN(C2H4PiPr2)2],9c currently the most
productive catalysts for amine alkylation by alcohols.
2-propanol, OsH2(CO)[Me2CO HN(C2H4PiPr2)2], is more
3 3 3
favorable than the isopropoxide OsH(OiPr)(CO)[HN(C2H4-
PiPr2)2] (ΔG = 2.8 kcal/mol at the PBE0 level). We note, however,
Some conclusions can be made at this point. An important
question is whether the complexes of this work are outer- or inner-
that OsH2(CO)[MeCHO HN(C2H4PiPr2)2] is less favor-
ablethantheethoxideOsH(OEt)(CO)[HN(C2H4PiPr2)2] (ΔG=
3 3 3
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Organometallics
COMMUNICATION
’ ACKNOWLEDGMENT
We are grateful to the NSERC of Canada and the SHARC-
NET facilities (www.sharcnet.ca) for their support of this work.
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Figure 3. Calculated (mPW1k) structures of ts1ꢀts4. Selected bond
lengths (Å) and angles (deg): ts1, OsꢀN = 2.137, OꢀH1 = 1.252, Nꢀ
H1 = 1.216, OsꢀH2 = 2.044, C1ꢀH2 = 1.175, C1ꢀO = 1.347; ts2,OsꢀN=
2.190, OsꢀH2 = 1.805, C1ꢀH2 = 1.574, OꢀH1 = 1.717, NꢀH1 = 1.036,
C1ꢀO = 1.260; ts3, OsꢀN = 2.179, NꢀH1 = 1.503, OsꢀH1 = 1.824,
OsꢀH2 = 1.825, H1ꢀH2 = 0.956, NꢀOsꢀH1 = 43.0, NꢀOsꢀH2 = 73.3,
H1ꢀOsꢀH2 = 30.4; ts4, OsꢀN = 2.059, OsꢀH1 = 2.737, OsꢀH2 = 2.424,
H1ꢀH2 = 0.749, NꢀOsꢀH1 = 79.7, H1ꢀOsꢀH2 = 15.2.
3.9 kcal/mol). The slow rate of TH in ethanol is apparently due
to slow production of acetaldehyde for thermodynamic reasons,
despite the calculated rate-limiting barrier for the dehydrogenation
of ethanol by 9 (ΔH/ΔGq = ꢀ0.6/þ9.9 kcal/mol) being lower
than that (ts2 ΔH/ΔGq = 2.8/13.7 kcal/mol) for 2-propanol.
The dihydride 2 is stable with respect to thermal dehydro-
genation required for the regeneration of 9 in the ADC reac-
tion of Scheme 3c. According to PBE0 calculations, the reaction
2 f 9 þ H2 is unfavorable by ΔH/ΔG = 22.5/16.1 kcal/mol.
The dehydrogenation proceeds via the dihydrogen intermediate
trans-OsH(H2)(CO)[N(C2H4PiPr2)2] connecting the two tran-
sition states ts3 (ΔH/ΔGq = 31.2/30.9 kcal/mol) and ts4 (ΔH/
ΔGq = 29.1/25.3 kcal/mol) depicted in Figure 3.16 For 6, the
corresponding barriers are only marginally lower: ΔH/ΔGq =
28.9/29.2 (ts3-Ru) and ΔH/ΔGq = 26.3/22.3 kcal/mol (ts4-
Ru). These high energy barriers explain why the ADC reactions
with 2 and 6 proceed at an appreciable rate only upon heating
above 120 °C.
In summary, OsHCl(CO)[HN(C2H4PiPr2)2] and OsH2-
(CO)[HN(C2H4PiPr2)2] are excellent TH catalysts that also
demonstrate high efficiency for the reactions of amination and
dehydrogenative coupling of primary alcohols, producing sec-
ondary amines and symmetrical esters, respectively.
(11) Recent calculations suggested that water could lower the barrier
for H2 elimination from trans-RuH2(PMe3)[HN(C2H4PiPr2)2] by ca.
8 kcal/mol: Friedrich, A.; Drees, M.; auf der G€unne, J. S.; Schneider, S.
J. Am. Chem. Soc. 2009, 131, 17552.
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Lough, A. J.; Abdur-Rashid, K. Organometallics 2006, 25, 4113.
(13) Another NH3 alkylation catalyst efficient at 0.1 mol %:Gunanathan,
C.; Milstein, D. Angew. Chem., Int. Ed. 2008, 47, 8661.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files giving crystallo-
b
graphic data for 1, 2, and 7 and text and tables giving full
experimental and computational details. This material is available
free of charge via the Internet at http://pubs.acs.org.
(14) O, W. W. N.; Lough, A. J.; Morris, R. H. Organometallics 2011,
30, 1236.
(15) Similar findings were reported by Gr€utzmacher7a and Bi: Bi, S.;
Xie, Q.; Zhao, X.; Zhao, Y.; Kong, X. J. Organomet. Chem. 2008, 693, 633.
(16) Similar transition-state structures were calculated for H2 elim-
ination from trans-RuH2(PMe3)[HN(C2H4PiPr2)2].11
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: dgoussev@wlu.ca.
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dx.doi.org/10.1021/om200437n |Organometallics 2011, 30, 3479–3482