Cyclometalated Dicarbonyl Ruthenium Catalysts for Transfer Hydrogenation and Hydrogenation of Carbonyl Compounds

Article abstract of DOI:10.1021/acs.organomet.8b00267

The dicarbonyl complex RuCl₂(L)₂(CO)₂ (1) was easily prepared by reaction of ruthenium chloride hydrate with formic acid and L (L = (2,6-Me₂C₆H₃)PPh₂) in ethanol at reflux, via the [RuCl₂(CO)₂]_n intermediate. Alternatively, 1 was obtained from [RuCl₂(CO)₃]₂ and L by CO elimination. Reaction of 1 with NEt₃ in toluene at reflux afforded the cyclometalated derivative RuCl{(2-CH₂-6-MeC₆H₃)PPh₂}(L)(CO)₂ (2). A simple one-pot synthesis of 2 was achieved by treatment of RuCl₃ hydrate with formic acid, L, and NEt₃. The cyclometalated dicarbonyl complexes [Ru{(2-CH₂-6-MeC₆H₃)PPh₂}(NN)(CO)₂]Cl (NN = ethylenediamine, 3; 2-(aminomethyl)pyridine, 4; (R,R)-1,2-diphenylethane-1,2-diamine, 5) were isolated by reaction of 2 with the corresponding dinitrogen ligand in methanol at reflux. Complexes 1-4 catalyze the transfer hydrogenation (TH) of acetophenone in 2-propanol at reflux (S/C = 1000 and TOF up to 30 000 h^-1) with alkali base (1-5 mol %), whereas 5 leads to (S)-1-phenylethanol with 68% ee. The derivatives 1-5 catalyze the hydrogenation (HY) of several ketones (H₂, 30 bar) at 70 °C in MeOH and EtOH with KOtBu (2 mol %) (S/C and TOF up to 25 000 and 14 000 h^-1). Addition of NN ligands to 1 and 2 in situ increases both the TH and HY activity, with ampy displaying the better performance. Heating of the cationic complex 3 in solid state and in solution leads to decarbonylation, affording the neutral monocarbonyl compound RuCl{(2-CH₂-6-MeC₆H₃)PPh₂}(en)(CO) (6) which was found active in the ketone HY.

Full text of DOI:10.1021/acs.organomet.8b00267

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Cyclometalated Dicarbonyl Ruthenium Catalysts for Transfer
Hydrogenation and Hydrogenation of Carbonyl Compounds
†
,‡
†
†
‡
†
Steven Giboulot, Salvatore Baldino, Maurizio Ballico, Hans Gu
̈
nter Nedden, Daniele Zuccaccia,
,†
and Walter Baratta*
†
Dipartimento DI4A - Universita
̀
di Udine, Via Cotonificio 108, I-33100 Udine, Italy
‡
Johnson Matthey, 28 Cambridge Science Park, Milton Road, Cambridge, CB4 0FP, United Kingdom
*
S Supporting Information
ABSTRACT: The dicarbonyl complex RuCl (L) (CO) (1)
2
2
2
was easily prepared by reaction of ruthenium chloride hydrate
with formic acid and L (L = (2,6-Me C H )PPh ) in ethanol
2
6
3
2
at reflux, via the [RuCl (CO) ] intermediate. Alternatively, 1
2
2 n
was obtained from [RuCl (CO) ] and L by CO elimination.
2
3 2
Reaction of 1 with NEt in toluene at reflux afforded the
3
cyclometalated derivative RuCl{(2-CH -6-MeC H )PPh }-
2
6
3
2
(
L)(CO) (2). A simple one-pot synthesis of 2 was achieved
2
by treatment of RuCl hydrate with formic acid, L, and NEt . The cyclometalated dicarbonyl complexes [Ru{(2-CH -6-
3
3
2
MeC H )PPh }(NN)(CO) ]Cl (NN = ethylenediamine, 3; 2-(aminomethyl)pyridine, 4; (R,R)-1,2-diphenylethane-1,2-
6
3
2
2
diamine, 5) were isolated by reaction of 2 with the corresponding dinitrogen ligand in methanol at reflux. Complexes 1−4
−
1
catalyze the transfer hydrogenation (TH) of acetophenone in 2-propanol at reflux (S/C = 1000 and TOF up to 30 000 h )
with alkali base (1−5 mol %), whereas 5 leads to (S)-1-phenylethanol with 68% ee. The derivatives 1−5 catalyze the
hydrogenation (HY) of several ketones (H , 30 bar) at 70 °C in MeOH and EtOH with KOtBu (2 mol %) (S/C and TOF up
2
−
1
to 25 000 and 14 000 h ). Addition of NN ligands to 1 and 2 in situ increases both the TH and HY activity, with ampy
displaying the better performance. Heating of the cationic complex 3 in solid state and in solution leads to decarbonylation,
affording the neutral monocarbonyl compound RuCl{(2-CH -6-MeC H )PPh }(en)(CO) (6) which was found active in the
2
6
3
2
ketone HY.
INTRODUCTION
catalyze a number of organic transformations, including TH
12
■
(
1
and HY of carbonyl compounds, HY of carboxylic and
13 14
The catalytic hydrogenation (HY) and transfer hydrogenation
2
carbonic acid derivatives, alcohol dehydrogenation, and
15
TH) of carbonyl compounds are cost-effective and environ-
borrowing hydrogen reactions. Relevant examples are
Ru(TFA) (PPh ) (CO), RuH(PNN)(CO), RuHCl(PNN)-
mentally benign ways widely accepted in the industry for the
production of alcohols. Several ruthenium complexes have
been described as efficient catalysts for HY or TH, whereas
only few systems display high activity for both reactions. High
3
2
3 2
(
CO), and RuHCl(PNP)(CO) complexes developed by
16 17 18 19
4
Dobson, Milstein, Gusev, and Saito, respectively
(
Figure 1).
selectivity and productivity, which are crucial issues for
industrial applications, can be achieved through an appropriate
ligand design. Several strategies have been developed and
involve the use of polydentate P, N or cyclometalated
The presence of one CO ligand at the metal affords catalysts
displaying low tendency to decarbonylate carbonyl substrates
20
(
i.e., aldehydes), which is a pathway of catalyst deactivation.
5
,6
In the course of our studies, we reported that the
ligands, with suitable electronic/steric properties, featuring
7
8
monocarbonyl ruthenium complexes RuCl{(2-CH -6-
2
amine N-H or redox functions (bifunctional catalysis).
Despite the large number of ruthenium complexes employed
1
0b
MeC H )PPh }(NN)(CO)
(A) and [RuH{Ph P(CH ) -
2 2 3
22
6
3
2
21
9
PPh }(NN)(CO)]Cl (B) (NN = en, ampy ) are highly
2
23
in organic transformations, very few examples of efficient
10
active catalysts for the ketone TH (Figure 1). More recently,
cyclometalated PC catalysts have been described. The use of
phosphines, which easily undergo cyclometalation, would lead
to a straightforward access to complexes displaying a robust
and basic RuPC fragment for catalytic applications, a simpler
22,24
we have demonstrated that Ru(OAc) (DiPPF)(CO) (C)
2
is an efficient catalyst for N-alkylation of amines with alcohols
via a borrowing hydrogen reaction.
As regards dicarbonyl ruthenium catalysts, the major concern
5
approach to that involving pincer PCP ligands. Thus, we have
has been focused on cyclopentadienyl Ru complexes, such as
reported that (2,6-Me C H )PPh easily gives activation of one
2
6
3
2
25
4
5
5
the Shvo catalyst (η -C H O) HRu H(CO) and (η -C R )-
5
4
2
2
5 5
o-methyl group, affording cyclometalated species with several
11
transition metals.
In the past decade, ruthenium monocarbonyl complexes have
Received: April 26, 2018
attracted a great deal of attention because of their ability to
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 1. Monocarbonyl ruthenium catalysts.
2
6
RuCl(CO)2 described by Bac
̈
kvall, which display catalytic
reactions entail the formation of catalytically active Ru-H
species in basic media, which are usually generated by reaction
of a Ru-X (X = Cl, carboxylate) precursor with an alkali
alkoxide (via β-H-elimination) or with dihydrogen. It is worth
pointing out that, when a ruthenium carbonyl precursor is
employed, the Ru-H species can also be formed by
decarboxylation of hydroxocarbonyl complexes, via the Hieber
activity in the dynamic kinetic resolution of alcohols, ammines,
as well as in HY and DHY reactions (Figure 2).
27
33
base reaction.
We report herein the straightforward preparation of
cyclometalated dicarbonyl ruthenium complexes [Ru{(2-CH2-
6
-MeC H )PPh }(NN)(CO) ]Cl (NN = bidentate ligand)
6 3 2 2
obtained by reaction of ruthenium(II) carbonyl precursors, or
directly from ruthenium chloride hydrate, with (2,6-Me2-
C H )PPh and a bidentate NN ligand. These cationic
6
3
2
dicarbonyl complexes display high catalytic activity in both
TH and HY of ketones with S/C up to 25 000 and involve CO
dissociation.
Figure 2. Dicarbonyl ruthenium catalysts.
RESULTS AND DISCUSSION
■
Synthesis of Cyclometalated Dicarbonyl Ruthenium
Complexes. Treatment of ruthenium(III) chloride hydrate
with formic acid afforded the intermediate [RuCl (CO) ] ,
The derivatives of general formula D and E, namely,
RuCl (bpy)(CO) and [Ru(bpy) (CO) ][PF ] (bpy = 2,2′-
2
2
2
2
6 2
bipyridine), were found active in the water gas shift reaction
2
2 n
2
8
following a slightly modified procedure with respect to that
(
WGSR) and in the electro- and photochemical CO2
34
29
reported in the literature. By carrying out the reaction in a
reduction. The complexes RuCl (LL′)(CO) (LL′ = PP,
2
2
3
0
PS, PCNHC
)
(Figure 2) catalyze the TH of ketones in basic 2-
sealed tube at 110 °C, complete conversion was achieved
within 1 h. This reaction which occurs with evolution of CO
3
−1
propanol (TOF < 10 h ), whereas RuCl(PCP)(CO) ,
described by Gelman, promotes alcohol dehydrogenative
reactions. In addition, the ruthenium carbonyl Ru (CO) in
2
2
31
and CO, as inferred by IR analysis, is faster in a closed reactor,
whereas it requires several hours to be completed in air.
Reaction of [RuCl (CO) ] with L (L = (2,6-Me C H )PPh )
3
12
combination with polydentate N and P ligands has been
proven to catalyze the ketone TH. Both TH and HY
2
2 n
2
6
3
2
3
2
in ethanol at 80 °C (2 h) led to the thermally stable derivative
Scheme 1. Synthesis of Complexes 1 and 2
B
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
RuCl (L) (CO) (1) which was isolated in 68% yield (method
The H NMR spectrum of 3 in CD OD shows four different
2
2
2
3
A; see the Experimental Section) (Scheme 1).
resonances for the NCH CH N moiety at δ 4.30, 4.04, 3.07,
2
2
Alternatively, 1 (84% yield) was prepared by reaction of the
and 2.83. The NH groups appear as broad signals at δ 5.30
2
tricarbonyl precursor [RuCl (CO) ] with L in ethanol at 80
and in the 2.75−2.25 range, as demonstrated by H/D exchange
2
3 2
°
C overnight (method B). The four ortho-methyl groups of 1
of the amino protons performed by addition of basic D O
2
1
appear as a singlet at δ 2.10 in the H NMR spectrum in
(NaOH), whereas the RuCH protons give two doublets at δ
2
2
13
1
3
2
.99 and 2.57 with J(H,H) = 15.0 Hz. In the C{ H} NMR
CD Cl at RT and as a triplet at δ 25.9 ( J(C,P) = 2.3 Hz) in
2
2
2
1
3
1
spectrum of 3, the two doublets at δ 201.3 ( J(C,P) = 13.5 Hz)
and 191.9 ( J(C,P) = 6.5 Hz) are for the CO ligands, while the
singlet at δ 46.7 and the doublet at δ 45.4 ( J(C,P) = 3.9 Hz)
the C{ H} NMR spectrum. The two CO carbons appear at δ
94.0 in tetrachloroethane-d at 80 °C. The presence of two
2
1
2
3
strong and sharp IR ν absorption bands at 2039 and 2001
CO
−
1
are for the en methylene carbons. Finally, the doublet at δ 31.9
cm is in agreement with a cis-coordination of the two
carbonyl ligands. Reaction of 1 with the weak base NEt (5
2
35
( J(C,P) = 4.1 Hz) is attributable to the RuCH group. In the
2
3
IR spectrum of 3, the CO stretching bands appear at 2028 and
equiv) in toluene at reflux overnight afforded the cyclo-
−
1
1
959 cm , close to those of the precursor 2. Similarly, the
metalated complex RuCl{(2-CH -6-MeC H )PPh }(L)(CO)
2
6
3
2
2
cationic complex [Ru{(2-CH -6-MeC H )PPh }(ampy)-
2
6
3
2
(
2) in 65% yield (method A). In addition, compound 2 (63%
and 57% yields) can also be obtained directly through a one-
(
CO) ]Cl (4) (43% yield) has been synthesized by reaction
2
3
1
1
of 2 with ampy in methanol at reflux. The P{ H} NMR
pot synthesis from RuCl ·xH O/HCO H (method B), or from
3
2
2
spectrum of 4 in CD OD displays a singlet at δ 64.4, a value
3
[
RuCl (CO) ] (method C), followed by reaction with L in
2 2 n
1
very close to that of 3 (δ 64.6). In the H NMR spectrum of 4
ethanol and in the presence of NEt . These procedures allow a
3
(
CDCl ), the methylene protons of the ampy ligand appear as
3
more straightforward preparation of 2 with respect to that
2
3
two doublets of triplets at δ 5.58 ( J(H,H) = 11.0 Hz, J(H,H)
= 5.7 Hz) and 3.07 ( J(H,H) = 11.0 Hz, J(H,H) = 5.2 Hz),
while the NH amino group signal is at δ 4.37. The C{ H}
previously reported, which entails the isolation of the 14-
2
3
3
6
electron complex RuCl (L) and reaction with H CO and
13
1
2
2
2
1
1b
31
1
2
CO. The P{ H} NMR spectrum of 2 in CDCl shows two
doublets at δ 54.2 and 26.2 with a J(P,P) = 293 Hz, consistent
3
NMR spectrum of 4 in CD OD shows a doublet at δ 52.2
2
3
3
(
J(C,P) = 3.4 Hz) for the methylene carbon of the ampy
with two trans phosphines. The broad doublet at high field is
for L, while the cyclometalated phosphine displays a narrow
ligand, whereas the cyclometalated CH moiety gives a doublet
2
2
at δ 33.9 ( J(C,P) = 3.9 Hz). The carbonyl groups exhibts two
doublet at low field. The cyclometalated methylene protons of
2
2
doublets at δ 201.3 ( J(C,P) = 14.6 Hz) and 191.5 ( J(C,P) =
.5 Hz), the latter being attributed to the CO trans the
cyclometalated methylene group. The low field signal at δ
01.3 has the same value reported for the CO trans to the
1
2
appear in the H NMR spectrum as two doublets of doublets
6
2
3
at δ 3.07 ( J(H,H) = 14.8 Hz, J(H,P) = 5.5 Hz) and 2.89
2
3
13
1
(
J(H,H) = 14.8 Hz, J(H,P) = 6.3 Hz). The C{ H} NMR
2
spectroscopic data for complex 2 shows a triplet at δ 32.2
amino moiety in 3, suggesting a trans arrangemet of the NH2
and CO groups in 4. The cis CO ligands display two strong
2
(
1
7
J(C,P) = 4.9 Hz), for the RuCH group, and two signals at δ
2
2
2
−
1
98.3 (t, J(C,P) = 12.6 Hz) and 194.2 (dd, J(C,P) = 8.7 and
stretching bands in the IR spectrum at 2032 and 1966 cm .
22
.9 Hz) for the CO ligands. The IR spectrum reveals two CO
Reaction of 2, as racemate, with (R,R)-dpen in methanol at
reflux afforded the complex 5 (68% yield) as a mixture of two
−
1
stretching bands at 2020 and 1957 cm , in agreement with the
presence of two cis CO groups.
3
1
1
diastereoisomers in a 1:1 ratio (eq 1). The P{ H} NMR
Treatment of 2 with ethylenediamine in methanol affords
the cationic complex [Ru{(2-CH -6-MeC H )PPh }(en)-
spectrum of 5 in CDCl shows two singlets at δ 64.2 and 63.9,
3
2
6
3
2
which are values close to that of the en derivative 3 (δ 64.6). In
1
(
CO) ]Cl (3) in 88% yield, by displacement of the bulky
the H NMR spectrum, the two couple of doublets at δ 3.29,
2
2
2
phosphine and the chloride ligands (Scheme 2).
2.64 ( J(H,H) = 14.0 Hz) and at δ 3.04, 2.58 ( J(H,H) = 14.1
Hz) have been attributed to the two cyclometalated CH2
moieties, whereas the singlets at δ 1.73 and 1.68 are for the o-
methyl groups. The IR CO stretching absorptions are at 2032
Scheme 2. Synthesis of Complexes 3 and 4
−
1
and 1965 cm , which are values very close to those of
analogous derivative 3. The formation of two diastereoisomers
of 5 in a 1:1 ratio suggests that the substitution of the
phosphine and Cl with (R,R)-dpen in the racemate 2 occurs
with no interconversion of the Ru(CP)(CO) fragment in
2
methanol at reflux.
Reduction of Ketones via TH and HY Catalyzed by
Carbonyl Ruthenium Complexes. The catalytic activity of
the complexes 1−5 have been investigated in the TH with 2-
C
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
propanol and HY with dihydrogen of acetophenone a in the
presence of an alkali base. The complexes 3−4 have proven to
efficiently hydrogenate a with a S/C = 500−25 000 (Scheme
to 3 with NaOiPr, however, has a strong detrimental effect
(13% conversion in 1 h, entry 6). The isolated ampy derivative
−
1
4 displays the highest activity (TOF = 17 000−30 000 h ),
affording quantitative reduction in 5 min, with moderate
influence of the nature of the base (NaOiPr, KOH, and
KOtBu) and its concentration (1−5 mol %, entries 9, 11, 12
and Table S1 (Supporting Information)). In the presence of
water (2% in volume), complex 4 leads to 81% conversion in
3
).
Scheme 3. Reduction of Acetophenone via TH and HY
Catalyzed by Ruthenium Complexes 1−5
−
1
2
0 min, with a lower rate (TOF = 3000 h , entry 10), in line
with the results obtained with 3, indicating that water hinders
the TH, possibly by formation of Ru hydroxo species. Complex
5
, containing the chiral diamine ligand (R,R)-dpen, affords the
quantitative TH of a to (S)-1-phenylethanol with 68% ee at 82
C in 40 min (S/C of 500) (entry 13, Table 1). By carrying
°
out the reaction at lower temperature (60 °C), incomplete
conversion has been observed (15% in 8 h) with no substantial
increase of ee. Notably 60−80% ee has been reported for the
hydrogenation of a with Ru-achiral phosphine with (R,R)-dpen
37
complexes and for the HY of 1-acetylnaphthalene with
diastereoisomeric mixtures of Ru-biphenyl phosphine with
38
(
S,S)-dpen derivatives. Thus, for 5 the enantiosectivity is
Complexes 1 and 2 (S/C = 1000) with NaOiPr (2 mol %)
display poor activity in the TH of a (0.1 M) in 2-propanol at
reflux, affording 39% and 48% conversion into 1-phenylethanol
in 7 and 8 h, respectively (Table 1, entries 1−2).
mainly controlled by the chiral dpen, with a small contribution
of the other ligands, taken into account that, during catalysis, a
CO dissociation occurs (vide infra). In refluxing 2-propanol
with KOH and in the absence of ruthenium catalyst, almost no
conversion of a (< 2%) into alcohol has been observed in 1 h,
in agreement with the data reported by Le Page, who showed
quantitative reduction of a in 1 day with a concentrated NaOH
Addition of the bidentate ligand en (2 equiv) to the
dicarbonyl 2 in situ increases dramatically the activity of
−
1
complex (TOF = 1200 h , entry 3, Table 1), indicating an
accelerating N-H effect upon coordination at the Ru center. An
39
solution (34 mol %).
even higher rate has been observed by addition of ampy to 2 (2
equiv), achieving a TOF = 30 000 h−1 (entry 4). The isolated
Complexes 1−6 have been studied in the HY of a at 30 bar
of H pressure in ethanol and methanol in the presence of
cationic dicarbonyl 3 containing the en ligand shows, in the
presence of NaOiPr (2 mol %), much the same activity (TOF
2
KOtBu with S/C in the range 2000−25 000. The HY was
carried out both in a catalyst screening system (8 vessels
Endeavor Biotage system), which allows parallel reactions to
be performed, and in a stainless steel autoclave following the
single process. Compound 1 (S/C = 2000) with KOtBu (2
mol %) displays poor activity in the HY of a in ethanol (8% of
conv. in 16 h) at 70 °C (Table 2, entry 1). Addition of diamine
ligands to 1 (S/C = 10 000) increases significantly the activity,
affording 96% conversion after 16 h (entry 2) in the presence
of en (2 equiv). A similar behavior has been observed using the
cyclometalated complex 2 (S/C = 2000), affording 11% of 1-
−
1
=
(
1500 h ) observed for the in situ generated 2/en system
entry 5). By changing the base concentration (1 to 5 mol %),
a higher rate was attained at 1 mol % NaOiPr (TOF = 2500
−
1
h ; see Table S1 (Supporting Information)), whereas no TH
was observed without base. Employment of KOH or KOtBu (1
to 5 mol %) as base leads to complete conversion of MeCOPh
with TOF values in the range 1500−3000 h (entries 7, 8 and
Table S1 (Supporting Information)), indicating not a strong
influence of the nature of the alkali metal for 3 (see Table S1
−
1
(Supporting Information)). Addition of water (2% in volume)
Table 1. Catalytic TH of Acetophenone (0.1 M) with 1−5 (S/C = 1000) in 2-Propanol at 82 °C in the Presence of an Alkali
Base (2 mol %)
a
TOF (h−1
b
entry
complex
ligand and additives
base
time (min)
conv. (%)
39
48
65
99
92
13
95
93
91
81
91
92
)
1
2
3
4
5
6
7
8
9
1
2
2
2
3
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
NaOiPr
KOH
KOtBu
NaOiPr
NaOiPr
KOH
420
480
60
40
60
60
60
60
5
en
ampy
1200
30 000
1500
c
3
H O
2
3
3
4
1500
2300
18 000
3000
17 000
30 000
1500
c
10
11
12
13
4
H O
2
20
5
5
4
4
5
KOtBu
NaOiPr
d
40
99 (68% ee S)
a
b
The conversion was determined by GC analysis. Turnover frequency (moles of ketone converted to alcohol per mole of catalyst per hour) at 50%
c
d
conversion. Reaction carried out in the presence of 200 μL (2% in volume) of H O. Reduction performed with S/C = 500.
2
D
DOI: 10.1021/acs.organomet.8b00267
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Table 2. Catalytic HY of Acetophenone (2 M) with Complexes 1−6 under 30 bar of H Pressure, 2 mol % of KOtBu at 70 °C
2
a
TOF (h−1
b
entry
complex
ligand
en
solvent
S/C
time (h)
conv. (%)
)
1
2
3
4
5
6
7
8
9
1
1
2
2
2
2
2
3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
MeOH
MeOH
EtOH
MeOH
MeOH
MeOH
EtOH
EtOH
2000
10 000
2000
2000
10 000
2000
10 000
2000
2000
10 000
10 000
10 000
25 000
10 000
10 000
10 000
25000
2000
16
16
16
16
16
16
16
16
16
23
16
3
22
16
16
22
22
16
16
8
96
11
99
80
99
99
99
98
85
99
95
97
98
99
99
97
en
en
ampy
ampy
c
3
d
600
1100
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
3
3
d
3
d
4500
3300
3
4
4
d
4
14 000
4000
300
d
4
5
c
99 (36% ee S)
,
ce
6
2000
98
a
b
The HY was carried out in an 8 vessels Endeavor Biotage system, and the conversion was determined by GC analysis. Turnover frequency (moles
c
d
of ketone converted to alcohol per mole of catalyst per hour) at 50% conversion. At 40 °C. Reaction performed in stainless steel autoclave (see
the Experimental Section). 5 bar H pressure.
e
2
phenylethanol in 16 h, whereas, in the presence of en or ampy
2 equiv), quantitative formation of alcohol is attained (entries
, 4, and 6). At lower catalyst loading (S/C = 10 000), addition
of ampy gave higher conversion with respect to the en ligand
99% vs 80% in 16 h; entries 7 and 5). The isolated en
under 30 bar of H with the substrate (2 M) dissolved in
ethanol and in the presence of KOtBu (2 mol %) (Scheme 4).
2
(
3
Scheme 4. HY of Ketones Catalyzed by Ruthenium
Complexes 3−5
(
derivative 3 led to 99% and 57% conversion of a at S/C 2000
and 10 000, respectively (entry 8 and Table S2 (Supporting
Information)). Quantitative reduction of a (98%) was also
attained at 40 °C in ethanol with a relatively low rate (S/3 =
−
1
2
000, TOF = 600 h ; entry 9). Employment of 3 in methanol
with KOtBu or KOH leads to the quantitative reduction of a,
indicating that the reaction occurs via HY and not TH, on
account of the higher redox potential of methanol compared to
ethanol (entries 11−13 and Table S2 (Supporting Informa-
4
0
tion)). By performing the HY in a stainless steel autoclave in
ethanol, 85% conversion was attained in 23 h (TOF = 1100
−
1
h ) with S/3 = 10 000 (entry 10). Employment of methanol
at S/3 = 10 000 and 25 000, 95% and 97% conversion was
−
1
achieved in 3 and 22 h (TOF = 4500 and 3300 h ; entries 12
and 13), respectively. In line with the results obtained in TH,
the cationic ampy complex 4 displays a higher rate compared
to 3 in HY. Thus, complete conversion of a is obtained, in
ethanol and methanol at S/C 2000−25 000 (entries 14−17
and Table S2 (Supporting Information)) within 16−22 h
As with a, 2′-methylacetophenone b and 2′-chloroacetophe-
none c are quantitatively hydrogenated to the corresponding
alcohols in 16 h using complexes 3 and 4 at S/C = 10 000
−
1
(
TOF up to 14 000 h ; entry 16).
(
Table 3, entries 1−4).
A similar catalytic activity was observed for the in situ
generated catalysts 2/NN ligand (NN = en, ampy) and the
isolated complexes 3 and 4, respectively. The chiral derivative
4
′-Methoxyacetophenone d is fully hydrogenated by 3 and 4
with an S/C = 500 in 3 h (entries 5 and 6). By contrast, 4′-
nitroacetophenone is not reduced by 3 and 4 (about 2% conv.
at S/C = 10 000 in 16 h). The bulky substrate
isobutyrophenone e is partially hydrogenated with 2/en and
with 3 (Ru/S = 1000) in 3 h (33% and 35% conv.; Table S3
(Supporting Information) and entry 7), whereas, with complex
4, quantitative reduction is attained (99%; entry 8).
Benzophenone f is converted to benzhydrol (99%) in 3 h
with 3 and 4 (S/C = 500; entries 9 and 10). As regards the
benzoin substrate g, complexes 3 and 4 display poor catalytic
activity, affording 9% and 6% conv. respectively, after 16 h with
5
catalyzes the HY of a but with poor enantioselectivity (36%)
of (S)-1-phenylethanol (Table 2, entry 18). Finally, the
monocarbonyl derivative [RuCl{(2-CH -6-MeC H )PPh }-
2
6
3
2
1
0b
(
en)(CO)] (6) (vide infra) has been found active also in
the HY in EtOH with quantitative reduction of a in 16 h at 40
and 70 °C (S/C = 2000; entry 19 and Table S2 (Supporting
Information)).
Complexes 3 and 4 have proven to catalyze the HY of diaryl,
dialkyl, and bulky ketones. The HY was performed at 70 °C
E
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 3. Catalytic HY of Ketones (2 M) with Complexes 3−
decrease of the reaction rates, suggesting that it is unlikely that
−
5
under 30 bar of H Pressure, 2 mol % of KOtBu at 70 °C
the Ru-H may originate via a OH nucleophilic attack at the
2
33,48
in Ethanol
CO.
Conversely, control experiments on 3 reveal a thermal
dissociation of one CO ligand in solid state and in solution.
a
entry
complex
substrate
S/C
time (h)
conv. (%)
99
98
97
99
98
99
35
99
99
99
9
99
−2
Thus, heating 3 under reduced pressure (10 mmHg) at 85
1
2
3
4
5
6
7
8
9
3
4
3
4
3
4
3
4
3
4
3
3
4
5
5
b
b
c
c
d
d
e
e
f
10 000
10 000
10 000
10 000
500
16
16
16
16
3
3
3
3
3
3
16
3
3
16
16
°
C for 36 h leads to quantitative formation of the neutral
monocarbonyl derivative RuCl{(2-CH -6-MeC H )PPh }-
2
6
3
2
11b
(
CO)(en) (6), by elimination of one CO (eq 2).
500
1000
1000
500
10
11
12
13
14
15
f
500
g
h
h
c
d
10 000
1000
1000
10 000
500
VT 31P{ H} NMR measurements of 3 in solution (tetra-
1
chloroethane-d ) show that, by heating, the intensity of the
2
99
singlet at δ 62.5 for 3 decreases, while the signal at δ 69.5 for 6
increases progressively (see the Supporting Information).
Thus, at 40 and 90 °C, the 6/3 ratio was 1/4 and 1/2 after
98 (35% ee S)
99 (23% ee S)
a
The reaction was carried out in an 8 vessels Endeavor Biotage
4
0 min, respectively, whereas, at 100 °C overnight, 3 led to 6
system, and the conversion was determined by GC analysis.
1
and other uncharacterized species. The H NMR spectra
confirm these results, with the appearance of two doublets at δ
2
S/C = 10 000, possibly due to the chelate effect exerted by the
2.98 and 2.01 ( J(H,H) = 14.6 Hz) for the RuCH group and a
2
1
,2-diol product resulting in catalyst poisoning (entry 11 and
singlet at δ 1.74 for the methyl group of 6. The comparison of
1
3
1
Table S3 (Supporting Information)). Finally, the dialkyl 2-
octanone h is completely reduced by 3 and 4 in 3 h (S/C =
the C{ H} NMR data of the CO ligand in the complexes 1−
4 and 6 indicates that, for 3, the absorbance at δ 191.9, slightly
49
1
000; entries 12 and 13). Use of complex 5 with the substrates
shifted at low field compared to free CO (δ 184.2), is for the
50
c and d led to complete reduction to alcohol in ethanol but
with poor enantioselectivity (23−35% of (S)) (Table 3, entries
1
CO trans to the CH group, consistent with a trans influence
2
exerted by the cyclometalated group. It is worth pointing out
that 3 was obtained by reaction of 2 with en in methanol at
reflux without decarbonylation. Therefore, the nature of the
solvent plays a crucial role in the decarbonylation, which is
favored for the chloride derivative 3 in apolar solvents (e.g., via
4 and 15). The different value of ee observed in the HY of a
with 5 with respect to TH at high temperature (68% ee) is
likely due to the alcohol media, as also observed with Ru ampy
complexes.
The TH and HY reactions promoted by ruthenium
complexes usually occur in basic media through the formation
of catalytically active mono- or dihydride Ru species, starting
from Ru-X (e.g., X = Cl, carboxylate) precursors via X
substitution. In the TH with 2-propanol, the Ru-H species is
generated from a Ru-OiPr complex through a β-hydrogen
elimination and extrusion of acetone (inner sphere mecha-
nism). When a NH functionality is present at the Ru-X center,
the Ru-hydride is formed from a 16-electron Ru-amide or a
Ru-amine/alkoxide species by elimination of HX (outer
sphere mechanism), involving hydrogen bonding and proton
transfer reactions with the alcohol media.
Ru-H species are formed in basic alcohol via dihydrogen
splitting from a labile Ru-X species (X = Cl, carboxylate,
alkoxide). In the presence of a NH function, the Ru-H is
4
c
51
an ion pair) with respect to polar ones. Thermal CO
dissociation in RuCl (PP)(CO) (PP = tBu PCH CH PtBu ,
2
2
2
2
2
2
41
Cy P(CH ) PCy ) complexes, bearing bulky alkyl diphos-
2 2 4 2
52 53
phines, has been reported by Whittlesey and Fogg.
Displacement of one CO ligand in the dicarbonyl ruthenium
5
complex (η -Ph C )Ru(CO) Cl has been described by Bac
̈
k-
5
5
2
vall as the rate-limiting reaction step in the racemization of sec-
alcohols and by Gelman in dehydrogenation of alcohols.
26
31
2
42
Complex 6 in the presence of KOtBu was proven to
hydrogenate the substrate a (98−99% conv.) in ethanol under
30 bar of H at 70 °C and at 40 °C under 5 bar of H (16 h),
43
2
2
7
,43,44
In the HY, the
similarly to 3 (Table 2 (entries 19 and 9), and Table S2
(Supporting Information)). In the TH of a in 2-propanol at
reflux, a higher rate was observed for 6 (NaOH as base),
10b
compared to 3 (KOH or NaOiPr) with TOF values of 2800
2
42
−1
formed from a 16-electron Ru-amide or Ru-amine/alkoxide
and 1500 h (Table 1, entry 7), respectively. Therefore, it is
likely that, during catalysis, the dicarbonyl derivatives 3 and 4
undergo thermal CO dissociation in the presence of a large
excess of alkoxides, leading to the formation of monocarbonyl
derivatives RuX(PC)(NN)(CO) (NN = en, ampy) (X = H,
OR). Attempts to isolate the Ru-H species by treatment of 3
and 4 with NaOiPr in 2-propanol failed, resulting in the
formation of dark solutions containing several uncharacterized
species, as inferred from NMR measurements. The high
performance of the dicarbonyl catalyst 4 relies on the presence
of the ampy ligand in combination with a robust cyclo-
metalated phosphine, which retards deactivation and facilitates
the decarbonylation, on account of the strong trans influence
of the alkyl group. Thus, according to our studies on related
45
species, as also proposed recently by Dub and Gordon. It is
worth noting that the cyclometalated dicarbonyl complexes 3
and 4, which catalyze both the TH and HY reactions, are
bifunctional catalysts that do not display a Ru-X coordinated
anionic ligand X (i.e., Cl, carboxylate) and, therefore, the
formation of the Ru-hydride species requires some consid-
erations. As possible routes for the Ru-H formation, we can
envisage: (a) a nucleophilic attack of OH− (due to the
presence of water in the basic alcohol media) on Ru-CO, with
formation of a hydroxocarbonyl species, followed by
4
6,47
decarboxylation (Hieber base reaction);
dissociation of one CO ligand. Addition of water in the TH
reduction of a with 3 and 4 has proven to lead to a drastic
(b) thermal
F
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 5. Possible Mechanism for TH and HY Reduction of Ketones Involving Complex 4
43
pincer Ru complexes, a possible mechanism for the TH and
HY of ketones promoted by the cationic complex 4 is depicted
in Scheme 5.
active in the ketone HY and TH reactions. Studies are ongoing
to extend this protocol to other cyclometalated carbonyl
ruthenium complexes for catalytic organic transformations.
The thermal displacement of CO in the presence of 2-
propanol or H2 in basic alcohol media leads to the
monohydride Ru complex which affords the reduction of the
carbonyl substrate through a hydrogen bonding network
EXPERIMENTAL SECTION
All reactions were carried out under an argon atmosphere using
standard Schlenk techniques. The solvents were carefully dried by
standard methods and distilled under argon before use, unless stated
■
promoted by the NH function. The catalytically active Ru-
2
hydride is regenerated by 2-propanol (reverse process) in TH
otherwise. The ruthenium compounds RuCl
·xH O (x = 2.5) and
3 2
[
RuCl (CO) ] were from Alfa/Aesar, whereas all other chemicals
or by H splitting in HY.
2
3 2
2
were purchased from Aldrich and Strem and used without further
purification. NMR measurements were recorded on a Bruker AC 200
CONCLUDING REMARKS
1
spectrometer. Chemical shifts, in ppm, are relative to TMS for H and
■
1
3
1
31
1
C{ H}, whereas H PO was used for P{ H}. Elemental analyses
In conclusion, we have reported a straightforward synthesis of
cyclometalated dicarbonyl ruthenium complexes of formula
3
4
(
C, H, N) were carried out with a Carlo Erba 1106 elemental
analyzer, whereas the GC analyses were performed with a Varian CP-
380 gas chromatograph equipped with a MEGADEX-ETTBDMS-β
[
Ru{(2-CH -6-MeC H )PPh }(NN)(CO) ]Cl (NN = en,
2 6 3 2 2
3
ampy, (R,R)-dpen) obtained from RuCl3 hydrate (via
chiral column of 25 m length, column pressure 5 psi, hydrogen as
carrier gas, and flame ionization detector (FID). The injector and
detector temperature was 250 °C, with initial T = 95 °C ramped to
140 °C at 3 °C/min and then to 210 °C at 20 °C/min, for a total of
20 min of analysis.
[
RuCl (CO) ] ) and from [RuCl (CO) ] with (2,6-
2
2
n
2
3 2
Me C H )PPh and a bidentate NN ligand. These derivatives
2
6
3
2
display catalytic activity in both TH and HY of ketones, the
ampy complex being more active with respect to the en one.
The reduction of acetophenone via TH with 2-propanol (S/C
Synthesis of RuCl {(2,6-Me C H )PPh } (CO) (1). Method A.
2
2
6
3
2 2
2
The compound RuCl ·xH O (200 mg, 0.792 mmol) was suspended
3
2
=
1000) and HY (30 bar of H , S/C = 10 000) afforded TOFs
2
−
1
in HCO H (6.7 mL, 0.178 mol) and heated to 110 °C in a pressure
2
up to 30 000 and 14 000 h , respectively, in the presence of
Schlenk tube. After 1 h, the resulting yellow solution was cooled to
room temperature and carefully vented. The solvent was removed
under reduced pressure, affording [RuCl (CO) ] which was
1
−5 mol % of alkali base. In addition, complete HY has also
been observed with S/C = 25 000 in methanol. Thermal CO
dissociation of [Ru{(2-CH -6-MeC H )PPh }(en)(CO) ]Cl
2
2 n
2
6
3
2
2
dissolved in ethanol (7 mL) and treated with (2,6-Me C H )PPh
2
6
3
2
leads to the corresponding monocarbonyl complex which is
(849.5 mg, 2.93 mmol). The solution was heated to 80 °C for 2 h,
G
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
obtaining a light yellow precipitate. After filtration, the solid was
washed with diethyl ether (4 × 3 mL) and dried under reduced
pressure. Yield: 435.5 mg (68%). Anal. Calcd (%) for C H Cl O -
and addition of diethyl ether (5 mL) afforded a light yellow
precipitate, which was filtrated, washed with diethyl ether (2 × 4 mL),
n-pentane (4 mL), and dried under reduced pressure. Yield: 967 mg
(57%).
4
2
38
2
2
P Ru: C 62.38, H 4.74; found: C 62.50, H, 4.86. IR (Nujol): 2039 (s),
2
−
1
1
2
8
001 (s) cm (ν
). H NMR (200.1 MHz, CD Cl , 20 °C): δ
Synthesis of [Ru{(2-CH -6-MeC H )PPh }(en)(CO) ]Cl (3).
CO
2
2
2 6 3 2 2
1
.52−7.03 (m, 26H; aromatic protons), 2.10 (s, 12 H; CH ). H
Complex 2 (252.2 mg, 0.47 mmol) and CaCO3 (22.8 mg, 0.23
mmol) were suspended in methanol (5 mL). Ethylenediamine (63
μL, 0.94 mmol) was added, and the mixture was refluxed overnight.
The suspension was filtrated, and the solvent was eliminated under
reduced pressure. Diethyl ether (4 mL) was added to the residue, and
the suspension was stirred for 1 h. The precipitate was filtrated,
washed with diethyl ether (2 × 3 mL), n-pentane (4 mL), and dried
under reduced pressure. Yield: 224.2 mg (88%). Anal. Calcd (%) for
3
NMR (200.1 MHz, tetrachloroethane-d , 50 °C): δ 8.54−7.06 (m,
2
1
3
1
2
6H; aromatic protons), 2.13 (s, 12 H; CH ). C{ H} NMR (50.3
3
2
MHz, CD Cl , 20 °C): δ 143.1 (t, J(C,P) = 2.9 Hz; CCH ), 132.1−
2
2
3
3
1
28.5 (m; aromatic carbon atoms), 25.9 (t, J(C,P) = 2.3 Hz; CH ).
3
1
3
1
C{ H} NMR (50.3 MHz, tetrachloroethane-d , 80 °C): δ 194.0 (m;
2
2
CO), 143.5 (t, J(C,P) = 4.7 Hz; CCH ), 135.4−128.5 (m; aromatic
carbon atoms), 26.1 (t, J(C,P) = 2.3 Hz; CH ). P{ H} NMR (81.0
MHz, tetrachloroethane-d , 20 °C): δ 10.3 (s). P{ H} NMR (81.0
MHz, tetrachloroethane-d , 50 °C): δ 10.4 (s).
3
3
31
1
3
3
1
1
C
H
26ClN
N 5.02. IR (Nujol): 2028 (s), 1959 (s) cm (νCO). H NMR
(200.1 MHz, CD OD, 20 °C): δ 7.61−7.26 (m, 12H; aromatic
protons), 7.04 (ddd, J(H,H) = 7.2 Hz, J(H,H) = 3.8 Hz, J(H,H) =
0.9 Hz, 1H, aromatic proton), 5.30 (m, 1H; NH ), 4.30 (m, 1H;
NCH ), 4.04 (m, 1H; NCH ), 3.07 (m, 1H; NCH ), 2.99 (d,
J(H,H) = 15.0 Hz, 1H; RuCH ), 2.83 (m, 1H; NCH ), 2.75−2.49
(br m, 2H; NH ), 2.57 (d, J(H,H) = 14.7 Hz, 1H; RuCH ), 2.41 (m,
O PRu: C 53.19, H 4.84, N 5.17; found: C 53.32, H 4.79,
2
24
2 2
−1
1
2
Method B. The complex [RuCl (CO) ] (50 mg, 0.098 mmol) was
3
2
3 2
3
4
4
suspended in ethanol (5 mL), (2,6-Me C H )PPh (126 mg, 0.434
2
6
3
2
mmol) was added, and the mixture was heated at 80 °C overnight.
The solvent was evaporated under reduced pressure and, after
addition of chloroform (2 mL), the suspension was stirred at room
temperature for 2 h. The volume was reduced to about 1 mL, diethyl
ether (5 mL) was added, and the light yellow precipitate was filtrated,
washed with diethyl ether (2 × 3 mL), n-pentane (3 mL), and dried
under reduced pressure. Yield: 133 mg (84%).
2
2
2
2
2
2
2
2
2
2
1
3
1
1H; NH ), 1.69 (s, 3H; CH ). C{ H} NMR (50.3 MHz, CD OD,
20 °C): δ 201.3 (d, J(C,P) = 13.5 Hz; CO), 191.9 (d, J(C,P) = 6.5
Hz; CO), 163.3 (d, J(C,P) = 33.1 Hz; CCH
1.7 Hz; CCH ), 136.3−113.8 (m; aromatic carbon atoms), 46.7 (s;
NCH ), 45.4 (d, J(C,P) = 3.9 Hz; NCH ), 31.9 (d, J(C,P) = 4.1
2
3
3
2
2
2
2
Ru), 143.0 (d, J(C,P) =
2
Synthesis of RuCl{(2-CH -6-MeC H )PPh }{(2,6-Me C H )-
2
6
3
2
2
6
3
3
3
2
PPh }(CO) (2). Method A. Complex 1 (100 mg, 0.124 mmol) was
2
2
2
2
3
31
1
suspended in toluene (5 mL), Et N (87 μL, 0.624 mmol) was added,
Hz; RuCH ), 22.3 (d, J(C,P) = 3.9 Hz; CH ). P{ H} NMR (81.0
3
2
3
and the mixture was refluxed overnight, obtaining a yellow solution.
The solvent was evaporated under reduced pressure, and the residue
was dissolved in dichloromethane (5 mL). The solution was stirred at
room temperature for 2 h and concentrated to about 0.5 mL. Addition
of methanol (2 mL) afforded a light yellow precipitate, which was
filtrated, washed with diethyl ether (2 × 5 mL), n-pentane (2 × 5
mL), and dried under reduced pressure. Yield: 62.2 mg (65%). Anal.
Calcd (%) for C H ClO P Ru: C 65.33, H 4.83; found: C 65.40, H,
MHz, CD OD, 20 °C): δ 64.6 (s).
3
Synthesis of [Ru{(2-CH -6-MeC H )PPh }(ampy)(CO) ]Cl (4).
2
6
3
2
2
Complex 2 (250.5 mg, 0.42 mmol) and CaCO (21.3 mg, 0.21 mmol)
3
were suspended in methanol (5 mL). 2-(Aminomethyl)pyridine (87
μL, 0.84 mmol) was added, and the mixture was refluxed overnight.
After filtration, the solvent was eliminated under reduced pressure.
Diethyl ether (4 mL) was added to the residue, obtaining a mixture
that was stirred for 1 h at room temperature. The resulting suspension
was filtrated, and the precipitate was washed with diethyl ether (2 × 3
mL), n-pentane (4 mL), and dried under reduced pressure. Yield:
107.9 mg (43%). Anal. Calcd (%) for C H ClN O PRu: C 57.00, H
42
37
2 2
−1
1
4
.88. IR (Nujol): 2020 (s), 1957 (s) cm (ν
). H NMR (200.1
CO
MHz, CDCl , 20 °C): δ 8.11−7.77 (m, 6H; aromatic protons), 7.64
3
(
7
m, 2H; aromatic protons), 7.55−7.13 (m, 15H; aromatic protons),
2
8
26
2
2
.05 (dd, 3J(H,H) = 7.3 Hz, J(H,H) = 3.0 Hz, 2H; aromatic
4
4.44, N 4.75; found: C 57.12, H 4.34, N 4.63. IR (Nujol): 2032 (s),
3
−1
1
protons), 6.93 (d, J(H,H) = 4.4 Hz, 1H; aromatic proton), 3.07 (dd,
1966 (s) cm (ν
). H NMR (200.1 MHz, CDCl , 20 °C): δ 8.61
CO 3
4
2
2
3
3
J(H,H) = 14.8 Hz, J(H,P) = 5.5 Hz, 1H; RuCH ), 2.89 (dd,
J(H,H) = 14.8 Hz, J(H,P) = 6.3 Hz, 1H; RuCH ), 1.98 (s, 6H;
(dd, J(H,H) = 7.6 Hz, J(H,H) = 1.9 Hz, 1H; ortho-CH of C H N),
7.79 (td, J(H,H) = 7.7 Hz, J(H,H) = 1.5 Hz, 1H; para-CH of
2
5
4
3
3
4
2
1
3
1
3
CH ), 1.72 (s, 3H; CH ). C{ H} NMR (50.3 MHz, CDCl , 20 °C):
C H N), 7.73−7.21 (m, 14H; aromatic protons), 7.00 (dd, J(H,H) =
5 4
3
3
3
3
2
2
δ 198.3 (t, J(C,P) = 12.6 Hz; CO), 194.2 (dd, J(C,P) = 8.7 Hz,
8.8 Hz, J(H,H) = 3.6 Hz, 1H; meta-CH of C
5
H
4
N), 5.58 (dt,
2
2
3
2
3
3
J(C,P) = 7.9 Hz; CO), 163.2 (dd, J(C,P) = 35.8 Hz, J(C,P) = 6.3
J(H,H) = 11.0 Hz, J(H,H) = 5.7 Hz, 1H; NCH
2
2
), 4.37 (td, J(H,H)
3
Hz; CCH Ru), 142.8 (s, CCH ), 142.6 (s; CCH ), 138.2−124.9 (m;
= 5.7, J(H,H) = 2.2 Hz, 2H; NH
2
), 3.08 (dt, J(H,H) = 11.0 Hz,
2
3
3
2
3
2
aromatic carbon atoms), 32.2 (t, J(C,P) = 4.9 Hz; RuCH ), 25.6 (d,
J(C,P) = 4.9 Hz; CH ), 22.3 (d, J(C,P) = 3.3 Hz; CH ). P{ H}
NMR (81.0 MHz, CDCl , 20 °C): δ 54.2 (d, J(P,P) = 293 Hz), 26.3
d, J(P,P) = 293 Hz).
J(H,H) = 5.2 Hz, 1H; NCH
2
), 2.86 (d, J(H,H) = 15.0 Hz, 1H;
2
3
3
31
1
2
RuCH ), 2.71 (d, J(H,H) = 15.0 Hz, 1H; RuCH ), 1.71 (s, 3H;
3
3
2
2
1
3
2
CH ). H NMR (200.1 MHz, CD OD, 20 °C): δ 8.74 (d, J(H,H) =
3
3
3
3
2
5
.5 Hz, 1H; ortho-CH of C H N), 7.96 (ddd, J(H,H) = 7.7 Hz,
J(H,H) = 7.5 Hz, J(H,H) = 1.6 Hz, 1H; para-CH of C
(
5 4
3
4
Method B. The compound RuCl ·xH O (208.2 mg, 0.825 mmol)
H
5
4
N), 7.71−
3
2
3
was suspended in HCO H (7 mL, 0.186 mol) and heated to 110 °C
7.28 (m, 14H; aromatic protons), 7.08 (ddd, J(H,H) = 8.0 Hz,
2
3
4
in a pressure Schlenk tube. After 1 h, the resulting yellow solution was
cooled to room temperature and carefully vented. The solvent was
removed under reduced pressure, affording [RuCl (CO) ] , which
J(H,H) = 4.2 Hz, J(H,H) = 1.1 Hz, 1H; meta-CH of C H N),
5 4
3
3
4.34−4.07 (m, 1H; NCH ), 4.21 (ddd, J(H,H) = 7.3 Hz, J(H,H) =
2
4
4.7 Hz, J(H,H) = 1.3 Hz, 2H; NH ), 3.97 (m, 1H; NCH ), 2.94 (d,
J(H,H) = 15.4 Hz, 1H; RuCH ), 2.14 (d, J(H,H) = 15.4 Hz, 1H;
RuCH ), 1.70 (s, 3H; CH ). C{ H} NMR (50.3 MHz, CD OD, 20
2 3 3
°C): δ 201.3 (d, J(C,P) = 14.6 Hz; CO), 191.5 (d, J(C,P) = 6.5 Hz;
CO), 162.8 (s; NCCH ), 162.7 (d, J(C,P) = 32.1 Hz; CCH Ru),
2
2
n
2
2
2
2
was dissolved in distilled ethanol (6 mL). The solution was reacted
2
1
3
1
with (2,6-Me C H )PPh (881.9 mg, 3.04 mmol), Et N (680 μL, 4.88
2
6
3
2
3
2
2
mmol), and stirred at 80 °C overnight. The volume was reduced by
about half, affording a precipitate, which was filtrated and washed with
ethanol (3 × 3 mL), diethyl ether (2 × 3 mL), n-pentane (2 mL), and
dried under reduced pressure. Yield: 398 mg (63%).
2
2
2
2
153.7 (s; ortho-CH of C H N), 143.1 (d, J(C,P) = 2.2 Hz; CCH ),
5
4
3
140.3 (s; para-CH of C H N), 135.1−112.8 (m; aromatic carbon
5
4
3
2
Method C. [RuCl (CO) ] (502.2 mg, 2.20 mmol of Ru), obtained
atoms), 52.2 (d, J(C,P) = 3.4 Hz; NCH ), 33.9 (d, J(C,P) = 3.9 Hz;
RuCH ), 22.3 (d, J(C,P) = 3.9 Hz; CH ). P{ H} NMR (81.0
2
2
n
2
3
31
1
as described in the method B for the synthesis of 2, and (2,6-
2
3
Me C H )PPh (1.78 g, 6.13 mmol) were dissolved in ethanol (10
MHz, CD OD, 20 °C): δ 64.4 (s).
2
6
3
2
3
mL). Et N (1.4 mL, 10.0 mmol) was added, and the solution was
Synthesis of [Ru{(2-CH -6-MeC H )PPh }{(R,R)-dpen}(CO) ]Cl
2 6 3 2 2
3
refluxed overnight. A yellow solid precipitated overnight, the solvent
was eliminated under reduced pressure, obtaining a residue, which
was dissolved in chloroform, and the solution was stirred at room
temperature for 2 h. The solution was concentrated to about 0.5 mL,
(5). Complex 2 (82.5 mg, 0.107 mmol) and CaCO (5.4 mg, 0.05
3
mmol) were suspended in methanol (5 mL). (1R,2R)-1,2-Diphenyl-
ethane-1,2-diamine (45.3 mg, 0.21 mmol) was added, and the mixture
was refluxed overnight. After filtration, the solvent was eliminated
H
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
under reduced pressure and diethyl ether (4 mL) was added to the
residue, affording a mixture, which was stirred for 1 h. The resulting
suspension was filtrated, and the precipitate was washed with diethyl
ether (2 × 3 mL), n-pentane (4 mL), and dried under reduced
pressure. The product was obtained as a mixture of two
diastereoisomers in a 1:1 ratio. Yield: 50.1 mg (68%). Anal. Calcd
appropriate amount of catalysts and solvent, and for the reactions
conducted at 40 °C. The reaction vessels were then cooled to room
temperature, vented, and purged three times with N . A drop of the
2
reaction mixture was then diluted with 1 mL of methanol and
analyzed by GC.
Procedure for the HY of Ketones with in Situ Prepared
Catalysts from 1 and 2. The vessels of the system were charged
(
%) for C H ClN O PRu: C 62.29, H 4.94, N 4.04; found: C 62.32,
36 34 2 2
−
1
1
H 4.98, N 4.01. IR (Nujol): 2032 (s), 1965 (s) cm (ν
NMR (200.1 MHz, CDCl , 20 °C): δ 8.35−6.80 (m, 23H; aromatic
protons), 6.25 (dd, J(H,H) = 25.0 Hz, J(H,H) = 11.7 Hz; NH ),
). H
CO
with the catalysts 1 or 2 (0.5 μmol), closed, loaded with 5 bar of N ,
2
and slowly vented five times. The ketone a or e (5 mmol), en or ampy
in ethanol (50 μL, 1 μmol, 0.02 M), and KOtBu in ethanol (1 mL, 0.1
mmol, 0.1 M) were added to the catalyst with about 1 mL of ethanol
3
2
3
2
5
.57 (m; NH ), 4.77 (m, 2H; NH ), 4.25−3.55 (m, 2H; NCH), 3.29
2
2
2
2
(
2 M of ketone). The vessels were purged with N and H (three
(
d, J(H,H) = 14.0 Hz; RuCH ), 3.04 (d, J(H,H) = 14.1 Hz;
2 2
2
2
2
times each); then the system was charged with H (30 bar) and
RuCH ), 2.64 (d, J(H,H) = 14.0 Hz; RuCH ), 2.58 (d, J(H,H) =
2
2
2
3
1
1
heated to 70 °C for the required time (3−16 h). The S/C molar ratio
was 10 000/1, whereas the base concentration was 2 mol %. A similar
method was applied for the reactions conducted with S/C in the
range 1000−10 000, using the appropriate amount of catalysts, ligands
14.1 Hz; RuCH ), 1.73 (s; CH ), 1.68 (s; CH ). P{ H} NMR (81.0
2 3 3
MHz, CDCl , 20 °C): δ 64.2 (s), 63.9 (s).
3
Synthesis of RuCl{(2-CH -6-MeC H )PPh }(en)(CO) (6). Com-
2
6
3
2
plex 3 (50 mg, 0.092 mmol) was heated at 85 °C under reduced
pressure (10− mbar) for 36 h, affording a dark-yellow clean product.
Yield: 46.5 mg (98%). Anal. Calcd (%) for C H ClN OPRu: C
2
(
ligand/catalyst ratio = 2), and solvent. The reaction vessels were
then cooled to room temperature, vented, and purged three times
2
3
26
2
with N . A drop of the reaction mixture was then diluted with 1 mL of
5
1
5
3.75, H 5.10, N 5.45; found: C 53.68, H 5.24, N 5.41. IR (Nujol):
2
−1
1
methanol and analyzed by GC.
Procedure for the HY of Acetophenone in a Stainless Steel
Autoclave. The autoclave was charged with the catalyst 3 or 4 (2.06
906 (s) cm (ν
). H NMR (200.1 MHz, tetrachloroethane-d ,
CO
2
0 °C): δ 7.80−6.80 (m, 13H; aromatic protons), 3.38 (m, 1H;
2
NCH ), 3.11 (m, 1H; NCH ), 2.98 (d, J(H,H) = 14.6 Hz, 1H;
2
2
μmol), closed, and purged three times with N . Acetophenone (2.4
RuCH ), 2.77 (m, 2H; NCH and NH ), 2.55−2.10 (m, 2H; NCH
2
2
2
2
2
2
mL, 20.6 mmol), the solvent (4 mL of ethanol or methanol), and a
solution of KOtBu (4 mL, 0.1 M in the same solvent) were
and NH ), 2.01 (d, J(H,H) = 14.6 Hz, 1H; RuCH ), 1.74 (s, 3H;
2
2
3
1
1
CH ), 1.70−1.56 (m, 1H; NH ), 1.43 (m, 1H; NH ). P{ H} NMR
81.0 MHz, tetrachloroethane-d , 50 °C): δ 68.7 (s).
3
2
2
subsequently added. The system was purged with N (two times) and
2
(
2
with H (three times). The autoclave was pressurized to 30 bar with
2
Procedure for the TH of Acetophenone with 1−5. The
H and heated to 70 °C for the required time (3−23 h). The final
2
ruthenium catalyst solution used for TH was prepared by dissolving
the ruthenium complex (0.02 mmol) in 5 mL of 2-propanol. A 0.1 M
solution of NaOiPr (200 μL, 20 μmol) in 2-propanol and the catalyst
solution (250 μL, 1.0 μmol) were added to acetophenone (120 μL,
concentration of acetophenone was 2 M, the S/C ratio was 10 000,
whereas the base concentration was 2 mol %. This procedure was
applied for the reactions with S/C = 25 000, using the appropriate
amount of catalysts and solvent. Samples of 0.2 mL were then taken at
regular intervals (2, 5, 10, 20, 30 min, and longer reaction times),
added to 5 mL of methanol, and analyzed by GC. TOF values were
calculated at 50% conversion.
1.0 mmol) in 2-propanol (final volume 10 mL), and the resulting
mixture was heated under reflux. The reaction was sampled by
removing an aliquot of the reaction mixture (0.2 mL), which was
quenched by addition of diethyl ether (1:1 v/v), filtered over a short
silica pad, and submitted to GC analysis. The addition of the Ru
complex was considered as the start time of the reaction. The S/C
molar ratio was 1000/1, whereas the base concentration was 2 mol %
with respect to acetophenone (0.1 M). The same procedure was
followed for TH with the other bases (KOtBu and KOH) at different
concentration (1−5 mol %), using the appropriate amount of 2-
propanol.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00267.
Procedure for the TH of Acetophenone with in Situ
Prepared Catalysts from 2. Complex 2 (15.4 mg, 0.02 mmol)
was dissolved in 5 mL of 2-propanol, and en or ampy (0.1 mmol) was
solubilized in 25 mL of 2-propanol. The solutions of 2 (250 μL, 1.0
μmol) and the ligand (500 μL, 2.0 μmol) were added subsequently to
acetophenone (120 μL, 1.0 mmol) in 2-propanol (8.93 mL). The
mixture was stirred under reflux for 10 min, and a 0.1 M solution of
NaOiPr (200 μL, 20 μmol) in 2-propanol was added (final volume 10
mL). The reaction was sampled by removing an aliquot of the
reaction mixture (0.2 mL), which was quenched by addition of diethyl
ether (1:1 v/v), filtered over a short silica pad, and submitted to GC
analysis. The S/C molar ratio was 1000/1, whereas the NaOiPr
concentration was 2 mol %, with respect to acetophenone (0.1 M).
Procedure for the HY of Ketones with Catalysts 1−6. The
HY reactions were performed in an 8 vessels Endeavor Biotage
apparatus. The vessels were charged with the catalysts 1−6 (0.5
NMR data of the isolated complexes and catalytic results
of the TH and HY reactions (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: walter.baratta@uniud.it.
ORCID
Daniele Zuccaccia: 0000-0001-7765-1715
Walter Baratta: 0000-0002-2648-1848
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
μmol), loaded with 5 bar of N , and slowly vented (five times). The
■
2
liquid ketones a−e and h (5 mmol) and the KOtBu or KOH solution
This work was supported by the Fellowship Program “Talents
for an International House (TALENTS UP)” (7th R&D FP:
PEOPLE - Marie Curie Actions - COFUND) and AREA
Consortium of Trieste for a fellowship for S.G. This work was
supported by the Ministero dell’Universita e della Ricerca
̀
MIUR), PRIN 2015 program no. 20154X9ATP_005. We
thank Johnson Matthey for a generous loan of ruthenium, Mr.
P. Polese for carrying out the elemental analyses, and Dr. P.
Martinuzzi for NMR assistance. We also thank Dr. Antonio
(
1 mL, 0.1 mmol, 0.1 M) in methanol or ethanol were added. In the
case of the solid ketones f−g (5 mmol), they were loaded together
with the ruthenium catalyst. Further addition of the solvent
(
methanol or ethanol) leads to a 2 M ketone solution. The vessels
were purged with N and H (three times each); then the system was
2
2
(
charged with H (30 bar) and heated to 70 °C for the required time
2
(
3−16 h). The S/C molar ratio was 10 000/1, whereas the base
concentration was 2 mol %. A similar method was applied for the
reactions with other S/C (in the range 500−10 000), using the
I
DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Zanotti-Gerosa of Johnson Matthey (Cambridge) for helpful
discussions on hydrogenation reactions.
(11) (a) Baratta, W.; Ballico, M.; Del Zotto, A.; Zangrando, E.; Rigo,
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Doppiu, A.; Herdtweck, E.; Zucca, A.; Rigo, P. Angew. Chem., Int. Ed.
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DOI: 10.1021/acs.organomet.8b00267
Organometallics XXXX, XXX, XXX−XXX