Facile synthesis of [Ru(η2-O2CO)(pta)(η6-p-cymene)], an outstandingly active Ru(II) half-sandwich complex for redox isomerization of allylic alcohols

Article abstract of DOI:10.1016/j.ica.2016.06.041

The water-soluble [RuCl₂(pta)(η⁶-p-cymene)] (pta = 1,3,5-triaza-7-phosphaadamantane) was applied as catalyst in the transposition of allylic alcohols, such as oct-1-en-3-ol in aqueous media. The isomerisation of oct-1-en-3-ol to octan-3-one took place only at pH > 10 buffer solutions or in the presence of alkali metal carbonates. The aqueous solution of “in situ” catalyst ([RuCl₂(pta)(η⁶-p-cymene)] + 2 eq Na₂CO₃) could be reused in the biphasic isomerization of oct-1-en-3-ol for at least five times without a significant loss of the catalytic activity. It was demonstrated that carbonate is not only a base in this reaction but leads to formation of a highly active catalyst, [Ru(η²-O₂CO)(pta)(η⁶-p-cymene)]. This compound was isolated as crystalline solid and characterized in detail (including X-ray diffraction).

Full text of DOI:10.1016/j.ica.2016.06.041

Inorganica Chimica Acta xxx (2016) xxx–xxx
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Research paper
Facile synthesis of [Ru(g g
2-O2CO)(pta)( 6-p-cymene)], an outstandingly
active Ru(II) half-sandwich complex for redox isomerization of allylic
alcohols
Evelin Bolyog-Nagy a, Antal Udvardy a,b, Ágnes Barczáné-Bertók a, Ferenc Joó a,b, Ágnes Kathó a,
⇑
a Department of Physical Chemistry, University of Debrecen, P.O. Box 400, H-4002, Hungary
b MTA-DE Homogeneous Catalysis and Reaction Mechanisms Research Group, P.O. Box 400, H-4002, Hungary
a r t i c l e i n f o
a b s t r a c t
g
6-p-cymene)] (pta = 1,3,5-triaza-7-phosphaadamantane) was applied as
Article history:
The water-soluble [RuCl2(pta)(
catalyst in the transposition of allylic alcohols, such as oct-1-en-3-ol in aqueous media. The isomerisation
Received 26 February 2016
Received in revised form 26 May 2016
Accepted 28 June 2016
Available online xxxx
of oct-1-en-3-ol to octan-3-one took place only at pH > 10 buffer solutions or in the presence of alkali
metal carbonates. The aqueous solution of ‘‘in situ” catalyst ([RuCl2(pta)(g
6-p-cymene)] + 2 eq Na2CO3)
could be reused in the biphasic isomerization of oct-1-en-3-ol for at least five times without a significant
loss of the catalytic activity. It was demonstrated that carbonate is not only a base in this reaction but
Keywords:
Allyl alcohols
Isomerization
Water-soluble
leads to formation of a highly active catalyst, [Ru(
lated as crystalline solid and characterized in detail (including X-ray diffraction).
Ó 2016 Elsevier B.V. All rights reserved.
g g
2-O2CO)(pta)( 6-p-cymene)]. This compound was iso-
1,3,5-Triaza-7-phosphaadamantane (pta)
Ruthenium
1. Introduction
ide), which is a more readily synthetized and less oxygen sensitive
Ru(II)-source in water, catalyzed this reaction only in the presence
of a reducing agent such as Na-formate. When HCOONa was
replaced by H2, a few % of saturated alcohol was detected in the
product mixture. Under identical conditions, complexes of the gen-
eral composition [RuCl2(dmso)2L2] (L = 1,3,5-triaza-7-phos-
phaadamantane (pta) or N-alkyl-pta derivatives, Scheme 2)
showed lower activities and lower selectivities to oxo-products
than [RuCl2(dmso)4] [13].
Recent green chemical syntheses aim at minimalizing byprod-
ucts and waste. An important reaction from this respect is the
redox isomerization of allylic alcohols (Scheme 1) which is charac-
terized by 100% atom economy and can replace classical two-step
syntheses often involving toxic oxidants or reductants.
This reaction can be catalyzed by various transition metal com-
plexes [1–3], however, the catalysts are often soluble only in toxic
or flammable and expensive solvents. There is an emerging need
for catalysts soluble in non-conventional solvents such as ionic liq-
uids [4], glycerol [5] or deep eutectics [6]. Among the alternative
solvents, water is becoming more and more popular since it is
non-toxic and easily available in large quantities. Several reviews
describe catalytic redox isomerizations in water [7–11] therefore
we make a short account only on those papers which report on
water-soluble Ru(II)-catalysts containing tertiary phosphine
ligands.
N-Heterocyclic carbenes are versatile and useful ligands in
organometallic catalysis [14,15] and their transition metal com-
plexes have also been used as catalysts for redox isomerization
of allylic alcohols. As an example, [RuCl2(bmim)(g
6-p-cymene)]
(bmim = 1-butyl-3-methyl-imidazol-2-ylidene) was shown to cat-
alyze the transposition of allylic alcohols under H2 with ketones
as main products despite the reductive atmosphere. Conversely,
in the absence of H2 no reaction was observed. Both conversions
and product distributions strongly depended on the pH of the
aqueous reaction media [16,17].
The effect of pH on reactions catalyzed by water-soluble transi-
tion metal complexes has been observed earlier in several cases.
For example, we have found that several Ru(II)-complexes
containing mono-sulfonated triphenylphosphine (mtppms) [18],
[Ru(H2O)6](OTs)2 (OTs = p-toluenesulfonate) was one of the first
complexes described as an efficient catalyst of redox isomerization
of water-soluble allylic alcohols to the corresponding oxo-deriva-
tives already at 45 °C [12]. [RuCl2(dmso)4] (dmso = dimethylsulfox-
pta,
phosphabicyclo[3.3.1]nonane) [19–21] ligands, such as Na4[{RuCl
-Cl)(mtppms)2}2], Na4[{RuCl( -Cl)(C@C@CPh2)(mtppms)2}2],
pta-Me
or
dmo-pta
(3,7-dimethyl-1,3,7-triaza-5-
⇑
Corresponding author.
E-mail address: katho.agnes@science.unideb.hu (Á. Kathó).
(l
l
http://dx.doi.org/10.1016/j.ica.2016.06.041
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
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2
E. Bolyog-Nagy et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
R3
2. Experimental section
R1
R4
2.1. Materials and instruments
R2
O
R3
R3
[{RuCl2(g g
6-p-cymene)}2] [32], [RuCl2(pta)( 6-p-cymene)] [33]
R1
R4
R1
R4
and pta [34] were synthesized according to published procedures
and their purities were checked by NMR spectroscopy. Most of
the chemicals (allylic alcohols, ligands, solvents) were commercial
products purchased from Sigma Aldrich and used as received.
RuCl3ꢁ3H2O was obtained from Pressure Chemical Co., K2CO3 and
Na2CO3ꢁ10H2O from Reanal. Argon and nitrogen (99.99% purity)
were provided by Linde and used directly from the cylinders. The
pH of the solutions was set to desired values using 0.2 M phos-
phate buffer solutions made using analytical grade Na2HPO4ꢁ12H2-
R2 OH
R2
O
R3
R1
R4
R2 OH
Scheme 1. Redox isomerization of allylic alcohols.
Na2[RuClCp(mtppms)2], (Cp = cyclopentadienyl), Na[Ru(CO)Cp
(mtppms)2] [RuClCp(pta)2], [RuClCp(pta-Me)2](OSO2CF3) and
[RuClCp(Hdmo-pta)(PPh3)](OSO2CF3) showed the highest activity
in aqueous solutions of pH 4–7.
O
and NaH2PO4ꢁ2H2O. Deuterated solvents (99.9%) for NMR
measurements were purchased from Cambridge Isotope Laborato-
ries Inc. and Sigma Aldrich.
NMR spectra were recorded on a BRUKER DRX 360 spectrome-
ter and evaluated using the WinNMR program. 1H, 13C and 31P
NMR spectra were referenced to tetramethylsilane (TMS), 2,2-
dimethyl-2-silapentane-5-sulfonate (DSS), 85% H3PO4 and residual
solvent peaks, respectively. UV–vis spectra were recorded on an
AnalytikJena SPECORD S600 spectrofotometer. Infrared spectra
were recorded on an Perkin Elmer Spectrum One FT-IR spectrome-
ter ATR Accessory. For elemental analyses an ElementarVario
Micro (CHNS) equipment was used. Gas chromatographic mea-
surements were made on an Agilent 7890A chromatograph
equipped with Chrompack WCOT Fused Silica CP WAX52CB col-
umn (30 m ꢂ 0.32 mm) and FID; carrier Ar (1 mL/min).
In contrast, the [RuCl2{P(CH2OH)3}(g
6-arene)] complexes
(arene = benzene or its substituted derivatives) could be used suc-
cessfully in the presence of a base such as Cs2CO3. Interestingly,
although its water-solubility was higher, the bisphosphine com-
plex [RuCl{P(CH2OH)3}2(g
6-arene)]Cl showed lower catalytic
activity than the monophosphine derivative [22]. Water-soluble
catalysts can also be obtained by using phosphines with other
polar substituents such as sulfonate (example: mtppms) or
ammonium groups (such as in [RuCl2PPh3ꢀn(OCH2CH2NMe3)n(g6
-
p-cymene)][SbF6]n, n = 1–3) [23]. Another way of increasing
water-solubility of the catalysts is the introduction of polar sub-
stituents into the arene ligand such as in [RuCl2(PPh3)(
OCH2CH2OH)] [24].
[RuCl2(bmim)(
hydrophilic phosphine ligands such as pta [16,17]. The product
[RuCl(bmim)(pta)(
6-p-cymene)]Cl showed higher catalytic
activity than the parent [RuCl2(bmim)(
6-p-cymene)]. It is striking
g
6-C6H5-
2.2. Typical experimental procedure for redox isomerization of allylic
alcohols
g
6-p-cymene)] has also been modified with
g
In a Schlenk tube, 11.5 mg (4 ꢂ 10ꢀ5 mol) Na2CO3ꢁ10H2O was
dissolved in 4.6 mL H2O. After deoxygenation, 9.3 mg (2 ꢂ 10ꢀ5
g
therefore that there is only limited information on the catalytic
properties of Ru(II)-half sandwich complexes containing only
pta and no NHC as ligand. In fact, the only data available
in the literature is on the isomerization of oct-1-en-3-ol into
mol) [RuCl2(pta)(g
6-p-cymene)] was added and the solution was
stirred for 15 min at 80 °C. 310 lL (2.02 mmol) oct-1-en-3-ol was
injected to the catalyst solution and the reaction mixture was stir-
red further at 80 °C. At the end of the desired reaction time the
mixture was cooled and extracted with hexane under air. The
organic phase was dried on MgSO4 and analyzed by gas chro-
matography. Products were identified by comparison of their
retention times to standards and the composition of the product
mixture was calculated from peak areas.
In recycling experiments, following extraction and phase sepa-
ration, the aqueous phase was deoxygenated, heated to 80 °C, a
new batch of the substrate was added and the reaction was initi-
ated by stirring.
octan-3-one catalyzed by [RuCl2(pta-MeOBn)(g
6-p-cymene)]Cl
(MeOBn = 4-methoxy-benzil) in the presence of Cs2CO3 with 38%
conversion in 8 h at 75 °C [25]. It is of interest that [RuCl2(pta)
(g
6-p-cymene)] was found outstandingly active in hydration of
nitriles [26,27], in formation of b-oxo-esters [28,29], and in hydro-
genation of bicarbonate [30]. Furthermore, this compound (also
known as RAPTA-C) attracts wide interest since it was demon-
strated that this compound has potent antitumor and antibacterial
effects [31].
Here we report that carbonate is not only a base in this reaction
but leads to formation of a highly active catalyst, [Ru(
g
2-O2CO)
2.3. Synthesis of [Ru(
g
2-O2CO)(pta)(
g
6-p-cymene)]
(pta)(
g
6-p-cymene)]. This compound was isolated as crystalline
solid and characterized by microanalysis, IR and multinuclear
NMR spectroscopy and single crystal X-ray diffraction. Its synthesis
and catalytic activity in transposition of allylic alcohols is
described below.
100 mg (0.216 mmol) [RuCl2(pta)(
g
6-p-cymene)] and 59.6 mg
(0.431 mmol) K2CO3 were placed into a Schlenk tube which was
subsequently deoxygenated by repeated vacuum/argon cycles.
20 mL deoxygenated water was added and the orange solution
P
P
P
N
N
N
N
N
N
R
Ru
Ru
N
CH3
N
CH3
N
N
H
P
Cl
Cl
H3C
dmo-pta
H3C
Hdmo-pta
pta:
R=-
Cl
Cl
N
N
pta-Me: R=CH3
N
N
pta-Bn: R=CH2-C6H5
pta-MeOBn; R=4-MeO-Bn
Scheme 2. Examples of ligands and catalysts used for redox isomerization of allylic alcohols.
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E. Bolyog-Nagy et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
3
Table 1
was stirred at room temperature for 1 h; during this time the col-
Crystallographic data and structure refinement results for [Ru(
cymene)](1)ꢁ2H2O.
g
2-O2CO)(pta)(
g
6-p-
our changed to lemon yellow. The solvent was removed under
reduced pressure at 60 °C, the solid residue was taken up in a small
amount of chloroform and sonicated, followed by removal of the
excess K2CO3 by filtration through a Hyflo Super Cel pad. To the fil-
trate hexane was added until precipitation ceased. The solid pro-
duct was collected by filtration, washed with 2 ꢂ 10 mL hexane
and dried under air. Yield: 78 mg (80%).
[Ru(
g
2-O2CO)(pta)(
g
6-p-cymene)]ꢁ2H2O (1)
Empirical formula
Formula weight
Crystal size (mm)
T (K)
C17H30N3O5PRu
488.48
0.45 ꢂ 0.2 ꢂ 0.1
293(2)
k (Å)
0.7107
Orthorhombic
P212121
3
1H NMR (CD3OD, 360 MHz) d (ppm) 1.27 (d, JH,H = 6.8 Hz, 6H,
Crystal system
Space group
Crystal habit
(colour, shape)
a (Å)
CH(CH3)2), 2.13 (s, 3H, C(CH3)), 2.61 (m, 1H, CH(CH3)2), 4.24 (s,
3
Yellow, plate
6H, PCH2N), 4.60 (s, 6H, NCH2N), 5.76 (d, JH,H = 5.7 Hz, 2 H,
3
C6H4), 5.88 (d, JH,H = 5.7 Hz, 2 H, C6H4); 13C{1H} NMR (CD3OD,
6.608(1)
15.501(2)
19.849(3)
90
90
90
2033.1(5)
4
1.596
90 MHz) d (ppm) 17.5 (s, CCH3), 22.1 (s, CH(CH3)2), 31.8 (s, CCH
(CH3)2), 50.3 (d, 1JP-C = 15 Hz, PCH2N), 72.1 (d, 3JP-C = 6 Hz, NCH2N),
86.0 (m, CH, p-cymene, aromatic), 96.3 (s, C(CH3) p-cymene, aro-
matic) 104.4 (s, CCH(CH3)2, p-cymene, aromatic), 166.8 (s, CO3);
31P{1H} NMR (CD3OD, 145 MHz) d (ppm) ꢀ33.6; 31P{1H} NMR
(D2O, 145 MHz) d (ppm) ꢀ33.2; 31P{1H} NMR (CDCl3, 145 MHz) d
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
(ppm) ꢀ34.4. IR (
Elemental analysis (%): Calculated for [Ru(
m
/cmꢀ1): 1663 (s), 1613 (s).
Dcalcd (Mg mꢀ3
)
(mmꢀ1
)
0.88
g
2-O2CO)(pta)( 6-p-
g
l
H
range (°)
3.3–25.3
ꢀ2 6 h 6 7
ꢀ5 6 k 6 18
0 6 l 6 23
2056 [Rint = 0.026]
2006
cymene)]ꢁ2H2O, C17H30N3O5PRu, M = 488.48: C, 41.91; H, 6.11; N,
Index ranges
8.57. Found: C, 41.80; H, 6.18; N, 8.60.
Total reflns.
2.4. Single crystal X-ray structure determination of [Ru(g
2-O2CO)(pta)
Independent reflns.
Observed reflns. [I > 2
F(0 0 0)
(g
6-p-cymene)]ꢁ2H2O (1)
r
(I)]
1177
1008
X-ray quality crystals of [Ru(g g
2-O2CO)(pta)(
6-p-cymene)]ꢁ
Data/restraints/parameters
2006/319/259
0.079
6
0.178
0.97
Final R indices [F2 > 2 (F2)]
r
2H2O (1) were obtained by layering hexane onto a chloroform
solution of the complex. Lemon yellow plates formed overnight
at ꢀ18 °C. Data were collected on a Bruker-Nonius MACH3 diffrac-
Intensity decay (%)
R indices (all data, wR (F2)
Goodness-of-fit (GOF) on F2
D
q
/
max Dqmin (e Åꢀ3
)
1.00/ꢀ1.25
tometer using graphite-monochromated Mo-K
a
radiation
CCDC
1,455,401
(k = 0.7107 Å) with the
x-scan method. Structures were solved
by the SIR-92 program [35] and refined by full-matrix least-
squares method on F2. Non-hydrogen atoms were refined with ani-
sotropic thermal parameters using the SHELXL-97 package [36].
Publication material was prepared using the WINGX-97 suite
[37] and the Mercury program [38] (see Table 1).
3. Results and discussion
3.1. Synthesis and solid state structure of [Ru(
cymene)]
g g
2-O2CO)(pta)( 6-p-
During the experiments on catalytic isomerization of allylic
alcohols in presence of Na2CO3, we observed that the yield of
oxo-products depended on the order of addition of the reactants
Fig. 1. UV–vis absorption spectra of aqueous solutions of [RuCl2(pta)(g
6-p-
cymene)] without (black) and with 2 eq. Na2CO3 (grey) ([Ru] = 0.001 M).
and on the time of pretreatment of [RuCl2(pta)(g
6-p-cymene)]
with Na2CO3 before the addition of substrate. It was also observed,
that the initial orange colour of aqueous solutions of [RuCl2(pta)
(g
6-p-cymene)] gradually turned lemon yellow on the effect of 2
equivalents of Na2CO3 (Fig. 1).
The reaction can be followed by 31P NMR spectroscopy, too
(Fig. 2). In addition to the characteristic 31P resonance of [RuCl2
(pta)(
(pta)(
g
g
6-p-cymene)] at d = ꢀ35.5 ppm and to that of [RuCl(H2O)
6-p-cymene)]+ (formed by hydrolysis of [RuCl2(pta)( 6-p-
g
cymene)] [30,39]) at d = ꢀ34.2 ppm, a new signal appears on the
effect of Na2CO3 at d = ꢀ33.2 ppm which becomes the unique res-
onance after 12 min (room temperature).
These phenomena were rationalized assuming ligand exchange
leading to formation of [Ru(
(Scheme 3). A few similar half-sandwich Ru-carbonato-phosphine
complexes have already been described in the literature. [Ru(g2
O2CO)(P)(
6-p-cymene)] with P = PMe3 or PCy3 (Cy = cyclohexyl)
were obtained by stirring acetone solutions of [RuCl2(P)(
6-p-cym-
ene)] and K2CO3 at room temperature for several days [40]; in case
g g
2-O2CO)(pta)( 6-p-cymene)]
-
g
Fig. 2. 31P NMR spectra for the reaction of [RuCl2(pta)(
Na2CO3 in D2O ([Ru] = 0.005 mmol).
g
6-p-cymene)] with 2 eq.
g
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4
E. Bolyog-Nagy et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
lengths and bond angles within the pta ligand do not differ signif-
icantly from those found in [RuCl2(pta)(
6-p-cymene)]. The exo-
cyclic C-O bond in the Ru(
2-O2CO) unit with O(3)-C(10) = 1.25
g
g
Ru
Ru
O
Na2CO3 or K2CO3
H2O, RT
(2) Å is shorter than the two endocyclic C-O distances, O(1)–C
P
P
Cl
O
Cl
N
N
(10) = 1.26(2) Å and O(2)–C(10) = 1.36(2) Å. The planar arrange-
C
ment of the Ru(g
2-O2CO) fragment is indicated by the sums of
N
N
N
N
O
the angles within the four membered ring (359.79°) and around
the carbon atom (359.89°) both of which are essentially 360°. This
plane is approximately perpendicular to the Ru-P bond: O(1)–Ru
(1)–P(1) = 81.9°, and O(2)–Ru(1)–P(1) = 86.1°. All parameters are
in good agreement with values determined previously for com-
Scheme 3. Formation of [Ru(
g
2-O2CO)(pta)( 6-p-cymene)].
g
of PPh3 the reaction needed 3 h using 10% water-acetone mixture
as solvent [41]. [Ru(
2-O2CO){(Cy)2P(1-methylnaphtyl)}(g6
C6H6)] was formed in ten minutes upon stirring a mixture consist-
ing of [RuCl2{(Cy)2P(1-methyl-naphtyl)}(
6-C6H6)], NaHCO3 and
methanol [42]. As demonstrated by Fig. 2, the reaction of [RuCl2
(pta)(
6-p-cymene)] and Na2CO3 in aqueous solution requires only
15 min to go to completion.
[Ru(
2-O2CO)(pta)( 6-p-cymene)] (as dihydrate) could be iso-
plexes related to [Ru(g g
2-O2CO)(pta)( 6-p-cymene)] but having
g
-
various arene and phosphine ligands [40–42].
g
3.2. Redox isomerization of allylic alcohols with [RuCl2(pta)(g
6-p-
g
cymene)] and [Ru(g g
2-O2CO)(pta)( 6-p-cymene)] catalysts
g
g
lated in solid form in reaction either with Na2CO3 or with K2CO3.
In the latter case the carbonato-complex was obtained with 80%
yield; the product showed excellent elemental analysis. The pres-
ence of coordinated carbonate ligand is also shown by the singlet
13C NMR resonance at 166.8 ppm (s, CD3OD). Furthermore, the
infrared spectrum displays sharp absorbances at 1663 cmꢀ1 and
1613 cmꢀ1 characteristic for coordinated carbonate.
Redox isomerization of allylic alcohols was studied earlier with
[RuCl2(bmim)(
6-p-cymene)] catalyst under an H2 atmosphere
g
(1 bar) [17]. In these reactions the catalyst was dissolved in aque-
ous phosphate buffer solutions of various pH while the neat sub-
strate comprised the organic phase. In such a biphasic system
(Scheme 4, L = bmim), at pH = 6.9 reaction of oct-1-en-3-ol yielded
81% octan-3-one and 14% octan-3-ol (TOF = 46.4 hꢀ1; turnover fre-
quency (TOF) = mol reacted substrate ꢂ (mol catalyst)ꢀ1 ꢂ hꢀ1).
Under otherwise identical reaction conditions but with [RuCl2
Upon layering hexane onto a chloroform solution of [Ru(g2
-
O2CO)(pta)(
g
6-p-cymene)]ꢁ2H2O (1) X-ray quality crystals could
(pta)(g
6-p-cymene)] as catalyst, redox isomerization of oct-1-en-
3-ol (Scheme 4, L = pta) at pH = 7.0 resulted in 98% conversion in
1 h (TOF = 98.0 hꢀ1) with 81% octan-3-one and 17% octan-3-ol as
products.
be obtained at ꢀ18 °C overnight (lemon yellow plates). Single crys-
tal X-ray diffraction resulted in the solid state structure of the com-
plex as shown on Fig. 3.
The asymmetric unit contains one molecule of the Ru-complex
and two water molecules and the structure is stabilized by an
extended H-bond network. The Ru(II) atom is surrounded by a
p-cymene ligand, a bidentate carbonate ion and a phosphine ligand
in a piano-stool configuration. Pta is linked through its P atom with
a Ru—P bond length of 2.309(5) Å. This bond is somewhat longer
The product ratio remained essentially the same (82% ketone
and 18% saturated alcohol) in the next two hours of the reaction
showing that there was no catalytic hydrogenation of the ketone
product despite the hydrogen atmosphere. These data also showed,
that albeit [RuCl2(pta)(
g
6-p-cymene)] was a more active catalyst
g
6-p-cymene)], i.e. 2.296(2) and
than [RuCl2(bmim)(
g
6-p-cymene)], there was no significant differ-
than those found for [RuCl2(pta)(
2.298(3) Å in the two molecules in the asymmetric unit [33]. Bond
ence in selectivity, i.e. in the product distribution.
Selective isomerization of allylic alcohols to ketones (Scheme 5)
can be expected from reactions in the absence of reducing agents
such as e.g. H2.
Indeed, we have shown earlier that several CpRu(II)-complexes
[18–21] were able to catalyze selective formation of octan-3-one
from oct-1-en-3-ol in the absence of H2; the conversions varied
as a function of the pH of the aqueous phase. Based on this expe-
rience, we have also attempted isomerization of oct-1-en-3-ol
under an inert atmosphere (Ar) with [RuCl2(pta)(g
6-p-cymene)]
as catalyst dissolved in phosphate buffer solutions of various pH
(Fig. 4).
The results showed that in strongly basic solutions (pH 11–12)
[RuCl2(pta)(g
6-p-cymene)] was an effective and selective catalyst
for redox isomerization of oct-1-en-3-ol – full conversion of the
substrate was achieved in 1 h at 80 °C with octan-3-one as the sole
product. However, the reaction proceeded only very slowly in solu-
tions with pH < 8.5. The only report in the literature on the appli-
cation of [RuCl2(pta-MeOBn)(g
6-p-cymene)]Cl for catalysis of
redox isomerization of oct-1-en-3-ol also describes the use of a
strong base, Cs2CO3, however, the reaction was slow with 38%
conversion in 8 h at 75 °C, corresponding to TOF = 2.7 hꢀ1 [25]. In
Fig. 3. ORTEP drawing of [Ru(
g
2-O2CO)(pta)(
g
6-p-cymene)]ꢁ2H2O (1) showing 50%
C5H11
C5H11
C5H11
[RuCl2(L)(p-cymene)]
p(H2) = 1bar; 80oC;
probability thermal ellipsoids. The H2O molecules have been omitted for clarity.
Selected bond lengths (Å) and angles (°): Ru(1)–P(1), 2.309(5); Ru(1)–O(1), 2.115
(12); Ru(1)–O(2), 2.073(13); O(1)–C(10), 1.26(2); O(2)–C(10), 1.36(2); O(3)–C(10),
1.25(2) O(1)–Ru(1)–O(2), 62.4(5); C(10)–O(1)–Ru(1), 93.4(12); C(10)–O(2)–Ru(1),
92.4(13); O(1)–C(10)–O(2), 111.6(18); O(1)–C(10)–O(3), 126(2); O(2)–C(10)–O(3),
122(2); O(1)–Ru(1)–P(1), 81.9(4); O(2)–Ru(1)–P(1), 86.1(4).
+
OH
OH
O
Scheme 4. Redox isomerization and hydrogenation of oct-1-en-3-ol catalyzed by
[RuCl2(L)(
6-p-cymene)].
g
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E. Bolyog-Nagy et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
5
[RuCl2(pta)(η6-p-cymene)]
C5H11
C5H11
100
80
60
40
20
0
O
OH
Scheme 5. Redox isomerization of oct-1-en-3-ol catalyzed by [RuCl2(pta)(g
6-p-
cymene)].
100
80
60
40
20
0
40
50
60
70
80
T (oC)
Fig. 5. Effect of temperature on redox isomerization of oct-1-en-3-ol catalyzed by
[RuCl2(pta)(
g
6-p-cymene)]. Conditions: n(Ru) = 2 ꢂ 10ꢀ5 mol; [substrate]/[Na2-
CO3]/[Ru] = 100:2:1; V(aqueous phase) = 4.6 mL; t = 1 h.
4
6
8
10
12
drop of mercury did not change the rate of the reaction which,
therefore, can be assumed homogeneously catalyzed.
pH
A pronounced advantage of the use of water-soluble complexes
as catalysts in aqueous-organic biphasic reactions is in that after
separation of the two liquid phases the catalyst-containing phase
can often be recycled. Indeed, under conditions of Fig. 6, an aque-
Fig. 4. Effect of the pH of the catalyst-containing aqueous phase on the conversion
of oct-1-en-3-ol in the absence of reducing agents, catalyzed by [RuCl2(pta)(g
6-p-
cymene)]. Conditions: n(Ru) = 1 ꢂ 10ꢀ5 mol; [substrate]/[Ru] = 200; V(substrate)
= 310 lL; V(aqueous phase) = 3 mL; t = 1 h; T = 80 °C, Ar atmosphere.
ous solution of [RuCl2(pta)(g
6-p-cymene)] + 2 eq. Na2CO3 cat-
alyzed the redox isomerization of oct-1-en-3-ol with 99%
conversion throughout five consecutive cycles and only in the sixth
cycle was a significant decrease of catalytic activity observed (70%
conversion), followed by negligible activity in the seventh cycle.
The turnover number (TON) in the first six cycles was 285.
contrast, in our case the highest activity (at pH 11.4, 80 °C; Fig. 4) is
characterized by TOF = 184 hꢀ1
.
When we used Na2CO3 as base it was observed, that repro-
ducible conversion data could be obtained only after stirring an
aqueous solution of [RuCl2(pta)(g
6-p-cymene)] and 2 eq. Na2CO3
The aqueous [RuCl2(pta)(
lysts solution had the characteristic lemon yellow colour of
[Ru(
2-O2CO)(pta)( 6-p-cymene)], therefore we made experi-
ments with the isolated carbonato-complex, too. Solutions of
[Ru(
2-O2CO)(pta)( 6-p-cymene)] in water (without any base or
g
6-p-cymene)] + 2 eq. Na2CO3 cata-
for 15 min at the chosen reaction temperature. As an example, in
reactions following such a pretreatment oct-1-en-3-ol was isomer-
ized with 94% conversion at 75 °C in 1 h (TOF = 47 hꢀ1) in contrast
to the 17% conversion observed after a short (0.5 min) mixing of
the Ru(II)-catalyst and Na2CO3, yielding a TOF = 8.5 hꢀ1 (Table 2,
Entries 1–2). These latter values are close to those (6% conversion,
TOF = 3 hꢀ1) obtained in isomerization of oct-1-en-3-ol with the
g
g
g
g
other additive) were stirred with oct-1-en-3-ol at 75 °C. The
time-course of the reaction (Fig. 7) showed that already after
30 min 99% octan-3-one was detected at a [substrate]/[catalyst]
ratio of 200 (TOF = 396 hꢀ1). In fact, the redox isomerization was
even faster, since the reaction started with an induction period
and 95% of the isomerization took place in the 10–30 min interval.
Another useful feature of the reaction is that the aqueous phase of
same [RuCl2(pta)(
no Na2CO3 (Entry 3). It is interesting to note, that even after 15 min
stirring with Na2CO3, aqueous solutions of [RuCl2(bmim)(
6-p-
g
6-p-cymene)] catalyst in aqueous solution with
g
cymene)] catalyzed formation of octan-3-one with only 13% con-
version (TOF = 6.5 hꢀ1, Entry 4). These results allow the conclusion
that – similar to the experiments carried out in an H2 atmosphere –
the catalyst has a pH of 7.24, therefore [Ru(g g
2-O2CO)(pta)( 6-p-
cymene)] is suitable for redox isomerization of base-sensitive
the phosphine-containing catalyst, [RuCl2(pta)(
showed considerably higher activity than its NHC-containing ana-
logue, [RuCl2(bmim)(
6-p-cymene)].
Study of the temperature dependence of the reaction catalyzed
by [RuCl2(pta)(
6-p-cymene)] showed that redox isomerization of
g
6-p-cymene)]
substrates, too. This is in contrast to the case of the in situ prepared
[RuCl2(pta)(g
6-p-cymene)] + 2 eq. Na2CO3 catalyst solutions
g
(pH > 10). Note also, that the [RuCl2(pta)(g
6-p-cymene)] catalyst
g
oct-1-en-3-ol proceeded only with very low rate (5% conversion in
1 h) at 60 °C, and below this temperature there was practically no
formation of octan-3-one (Fig. 5). In contrast, above 60 °C there
was a fast increase in the reaction rate so that already at 75 °C close
to quantitative conversion (94%) was obtained in 1 h. Addition of a
100
80
60
40
20
0
Table 2
Catalytic activities of [RuCl2(L)(
isomerization of oct-1-en-3-ol to octan-3-one at 75 °C (t = 1 h).
g
6-arene)] (L = pta or bmim) complexes in biphasic
Entry
L
Base
Time of pretreatment
(min)
Octan-3-one
(%)
TOF
(hꢀ1
1
2
3
4
5
6
7
)
Cycles
1
2
3
4
pta
pta
pta
bmim
Na2CO3
Na2CO3
–
15
0.5
–
94
13
6
47.0
6.5
3.0
8.5
Fig. 6. Recirculation of the [RuCl2(pta)(g
6-p-cymene)] + 2 Na2CO3 catalyst in
biphasic redox isomerization of oct-1-en-3-ol. Conditions: n(Ru) = 1 ꢂ 10ꢀ5 mol;
[substrate]/[Na2CO3]/[Ru] = 50:2:1; V(aqueous phase) = 4.6 mL; t = 1 h; T = 80 °C.
Na2CO3
15
17
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6
E. Bolyog-Nagy et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
Table 4
100
80
60
40
20
0
Isomerization of allylic alcohols with [Ru(
water.
g
2-O2CO)(pta)(
g
6-p-cymene)] catalyst in
Substrate
Conversion
(%)
TOF
(hꢀ1
)
prop-1-en-3-ol*
but-1-en-3-ol*
pent-1-en-3-ol
hex-1-en-3-ol
hept-1-en-3-ol
oct-1-en-3-ol
24
27
97
93
97
99
96
108
388
372
388
396
0
10
20
30
40
Conditions: n(Ru) = 1 ꢂ 10ꢀ5 mol; [substrate]/[Ru] = 200; V(substrate) = 310
(H2O) = 3 mL; T = 75 °C.
lL; V
Time (min)
*
Conversion determined by 1H NMR spectroscopy.
Fig. 7. Time course of redox isomerization of oct-1-en-3-ol catalyzed by [Ru(g2
-
O2CO)(pta)(
g
6-p-cymene)]. Conditions: n(Ru) = 1 ꢂ 10ꢀ5 mol; [substrate]/[Ru]
With the ‘‘in situ” prepared [RuCl2{P(CH2OH)3}(g
6-arene)]
= 200; V(substrate) = 310 lL; V(H2O) = 3 mL; T = 75 °C.
+ 2 eq. Cs2CO3 catalyst an opposite trend in reactivity was
observed, i.e. a decrease of reactivity of allylic alcohols was
detected in the series C4?C8 [22]. Conversely, with the N-hetero-
similarly showed high activity only in strongly basic solutions
(Fig. 4).
cyclic carbene-ligated catalyst Cs[Ru(g g
2-O2CO)(NHC)( 6-p-cym-
Table 3 contains results of redox isomerizations of oct-1-en-3-ol
ene)] (NHC = N,N0-(methyl)(propanesulfonate)imidazol-2-ylidene)
at 100 °C the reactivities (TOF) followed the order: prop-1-en-3-
ol 495 hꢀ1, pent-1-en-3-ol 490 hꢀ1, hex-1-en-3-ol 490 hꢀ1 and
hept-1-en-3-ol 495 hꢀ1 [44]. Similar to the findings with the
in aqueous-organic biphasic systems with [RuCl2(P)(g
6-arene)]
(P = phosphine) catalysts. All reactions were run at 75 °C, however,
there were differences in the added base and in the actual [sub-
strate]/[catalyst] ratios or in the organic solvents used (if any).
Nevertheless, the data show, that among the ‘‘in situ” systems
[Ru(g g
2-O2CO)(pta)( 6-p-cymene)] catalyst, these values do not
show a great influence of the alkyl chain length on the reactivity
the catalytic activity of [RuCl2(pta)(
of Na2CO3 (TOF = 47.0 hꢀ1, Entry 2) was surpassed only by that of
[RuCl2{P(CH2OH)3}(
6-C6H6)] (TOF = 67 hꢀ1, Entry 6) and [RuCl2
(PPh3)(
6-C6H5OCH2CH2OH)] (TOF = 192 hꢀ1, Entry 8). However,
the catalytic activity of [Ru(
2-O2CO)(pta)( 6-p-cymene)] is more
g
6-p-cymene)] in the presence
of water-insoluble allylic alcohols in redox isomerization.
On the basis of 31P NMR spectra [Ru( 2-O2CO)(pta)( 6-p-cym-
g g
g
ene)] is stable under the reaction conditions (75 °C, aqueous solu-
tion) in the absence of oct-1-en-3-ol, except formation of a small
amount of phosphine oxide. No free phosphine (arising from phos-
phine dissociation from the complex) was detected by 31P NMR
spectroscopy at room temperature or at 75 °C, neither in the
absence nor in the presence of oct-1-en-3-ol. The question arises
therefore, how a coordinatively saturated complex accommodates
the substrate in its coordination sphere. On the basis of literature
g
g
g
than double of that observed in case of the most active ‘‘in situ”
catalyst (Entries 1 and 8).
Redox isomerization of various allylic alcohols was also studied
applying aqueous solution of [Ru(g g
2-O2CO)( 6-C10H14)(pta)] as
catalyst and the results are presented in Table 4 Interestingly,
the water-soluble substrates prop-1-en-3-ol and but-1-en-3-ol
showed modest reactivities in contrast to the water-insoluble 1-
en-3-ols which were fast converted to the corresponding ketones.
The homoallylic alcohol, but-1-en-4-ol did not react at all, indicat-
ing that the catalyst does not facilitate double bond migration.
With increasing carbon chain length, from pent-1-en-3-ol through
oct-1-en-3-ol the reactivity of allylic alcohols did not change
significantly.
examples, we suggest that the reaction takes place with the g3
-
oxo-allyl mechanism originally put forward by Trost and Kulawiec
[45].
According to this mechanism (Scheme 6), deprotonation and
coordination of the substrate allylic alcohol (as an alkoxide) is
2+
Ru
R
[Ru]=
P
O
N
N
N
Table 3
HO
Comparison of the catalytic activity of various [RuCl2(P)(
complexes in biphasic isomerization of oct-1-en-3-ol to octan-3-one at 75 °C.
g
6-arene)] (P = phosphine)
R
[Ru]
R
O
Entry Catalyst
Base
Time
(h)
Octan-
3-one
(%)
TOF
(hꢀ1
Ref.
O
OH
O
)
O
O
HO
1
2
3
4
5
6
7
8
[Ru(
g
2-O2CO)(pta)
–
0.5
1
99
94
396
47
3
This
work
This
work
[25]
(g
6-p-cymene)]
[RuCl2(pta)(
cymene)]
g
6-p-
Na2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
KOtBu
R
+
R
O
O
O
[RuCl2(pta-MeOBn)
6-p-cymene)]Cl
[(RuCl2{P(CH2OH)3}
6-p-cymene)]
[RuCl2{P(CH2OH)3}
6-C6Me6)]
[RuCl2{P(CH2OH)3}
8
38
[Ru]
[
]
Ru
(g
O
OH
2.25
3.5
1.5
24
0.5
100
100
100
100
96
44
29
67
8
[22]
[22]
[22]
[43]
[33]
(g
+
R
(g
[Ru]-H
O
O
O
OH
+
(g
6-C6H6)]
R
[RuCl2(pta-Me)(g6
-
[Ru]
O
C6H6)]Cl
[RuCl2(PPh3)(g6
-
192
Scheme 6. Suggested mechanism of redox isomerization of allylic alcohols
catalyzed by [Ru(
2-O2CO)(pta)( 6-p-cymene)].
C6H5OCH2CH2OH)]
g
g
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E. Bolyog-Nagy et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
7
facilitated by the carbonato ligand which in turn becomes a coor-
dinated bicarbonate. In the next step dissociation of bicarbonate
allows coordination of the olefinic moiety of the substrate what
is followed by formation of a hydrido-enone-Ru(II) intermediate.
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif. Supplementary data associated with this arti-
cle can be found, in the online version, at http://dx.doi.org/10.
1016/j.ica.2016.06.041.
The latter then rearranges to the corresponding
g
3-oxo-allyl-
Ru(II) complex. Protonation of this intermediate by bicarbonate
regenerates the catalyst and releases a vinyl alcohol which isomer-
izes to the stable saturated ketone product.
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g
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Acknowledgements
Financial support of the Hungarian National Research Fund
(OTKA K101372) is gratefully acknowledged. A.U. acknowledges
funding from the Hungarian Academy of Sciences of a postdoctoral
fellowship. Thanks are due to Dr. Attila Kiss-Szikszai (University of
Debrecen) for the elementary analyses.
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CCDC 1455401 contains the supplementary crystallographic
data for complex 1. These data can be obtained free of charge from
Please cite this article in press as: E. Bolyog-Nagy et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.06.041