Half-sandwich ruthenium(II) complexes with water-soluble Schiff base ligands: Synthesis and catalytic activity in transfer hydrogenation of carbonyl compounds

Article abstract of DOI:10.1016/j.ica.2016.10.041

New ionic Schiff-base ligands have been synthesized by condensation of (3-formyl-4-hydroxybenzyl)triphenylphosphonium and 3-(3-formyl-4-hydroxybenzyl)-1-methyl-1H-imidazol-3-ium chloride and hexafluorophosphate salts with N,N-dimethylethylenediamine. Treatment of the dimeric derivative [{RuCl(μ-Cl)(η⁶-p-cymene)}₂] with two equivalents of these ligands allowed the preparation of novel mononuclear water-soluble Ru(II) complexes, which proved to be catalytically active in the transfer hydrogenation of ketones and aldehydes under aqueous conditions.

Full text of DOI:10.1016/j.ica.2016.10.041

Accepted Manuscript
Research paper
Half-sandwich ruthenium(II) complexes with water-soluble Schiff base ligands:
Synthesis and catalytic activity in transfer hydrogenation of carbonyl com-
pounds
Somayeh Azizi Talouki, Gholamhossein Grivani, Pascale Crochet, Victorio
Cadierno
PII:
S0020-1693(16)30772-1
http://dx.doi.org/10.1016/j.ica.2016.10.041
ICA 17334
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
25 July 2016
20 October 2016
26 October 2016
Please cite this article as: S. Azizi Talouki, G. Grivani, P. Crochet, V. Cadierno, Half-sandwich ruthenium(II)
complexes with water-soluble Schiff base ligands: Synthesis and catalytic activity in transfer hydrogenation of
carbonyl compounds, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.10.041
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Half-sandwich ruthenium(II) complexes with water-soluble Schiff
base ligands: Synthesis and catalytic activity in transfer
hydrogenation of carbonyl compounds
†
†
§
§
Somayeh Azizi Talouki, Gholamhossein Grivani,* Pascale Crochet* and Victorio Cadierno
†
School of Chemistry, Damghan University, Damghan, 36715-364, Iran.
§
Laboratorio de Compuestos Organometálicos y Catálisis (Unidad Asociada al CSIC), Centro
de Innovación en Química Avanzada (ORFEO-CINQA), Departamento de Química Orgánica e
Inorgánica, Instituto Universitario de Química Organometálica “Enrique Moles”, Universidad
de Oviedo, Julián Clavería 8, E-33006 Oviedo, Spain.
_
____________________________________________________________________________
Abstract
New ionic Schiff-base ligands have been synthesized by condensation of (3-formyl-4-
hydroxybenzyl)triphenylphosphonium and 3-(3-formyl-4-hydroxybenzyl)-1-methyl-1H-
imidazol-3-ium chloride and hexafluorophosphate salts with N,N-dimethylethylenediamine.
6
Treatment of the dimeric derivative [{RuCl(µ-Cl)(η -p-cymene)}
2
] with two equivalents of
these ligands allowed the preparation of novel mononuclear water-soluble Ru(II) complexes,
which proved to be catalytically active in the transfer hydrogenation of ketones and aldehydes
under aqueous conditions.
Keywords: Schiff-base ligands; Ionic ligands; Ruthenium; Transfer hydrogenation; Ketones;
Aqueous catalysis
_
____________________________________________________________________________
-
---------------------------------
Corresponding authors. E-mail addresses: grivani@du.ac.ir (G. Grivani),
crochetpascale@uniovi.es (P. Crochet)
*
1

1. Introduction
In the past decades, Schiff-base ligands have attracted considerable interest due to their
rich coordination chemistry [1-3]. The main advantages of these ligands stem from the easy
access under smooth conditions, and the facility to modulate their structures and chemo-
physical properties through the incorporation of additional functional groups. The resulting
complexes have found numerous applications as catalysts [1], pharmaceuticals [2], and
magnetic or luminescent materials [3]. Interestingly, the presence of an ionic or a highly polar
substituent in the backbone of the Schiff-base usually induces an enhanced solubility of their
derivatives in aqueous medium, a striking feature both for biological or catalytic uses. In this
context, a wide range of ligands with sulfonate, carboxylate, amino, ammonium, imidazolium
or phosphonium functionalities, and their corresponding complexes, have been described [4].
6
On the other hand, the η -arene-ruthenium(II) complexes represent an important class of
compounds [5], well-known for promoting a great variety of catalytic reactions, such as C-H
activation [6], nitrile hydration [7] or transfer hydrogenation processes [8] to name a few. In
addition, their outstanding activities, combined with a good stability against water, make them
ideal candidates to perform catalytic organic transformations in aqueous medium [9]. For this
purpose, numerous water-soluble arene-ruthenium(II) complexes have been designed, the most
common strategy to obtain such derivatives consisting in the coordination of an ionic ligand
onto the metal center. In this context, a plethora of half-sandwich ruthenium(II) compounds
with functionalized phosphines, N-heterocyclic carbenes or diamines, has been prepared
[
9d,10]. In marked contrast, arene-Ru(II) derivatives containing an ionic Schiff-base still
remain very scarce [11].
With these precedents in mind, we decided to prepare new Schiff-base ligands tagged
with a phosphonium or an imidazolium substituent and explore their coordination onto an
arene-ruthenium(II) fragment. Furthermore, the catalytic activity of the resulting complexes has
2

been evaluated in the transfer hydrogenation of ketones and aldehydes under aqueous
conditions.
2
. Experimental
2
.1. General information
All the reagents were obtained from commercial suppliers and used as received, with the
exception of the aldehydes 1a-b and 3a-b [12], the imine 7 [13] and the dimeric precursor
6
[
{RuCl(µ-Cl)(η -p-cymene)}
2
] [14], which were prepared following previously reported
methodologies. All the manipulations with ruthenium complexes were performed under argon
atmosphere using vacuum-line and standard Schlenk techniques. Organic solvents were dried
by standard methods and distilled under argon before use [15]. IR spectra were recorded with a
Perkin-Elmer 1720-XFT spectrometer. GC measurements were performed by using a Hewlett-
TM
Packard HP6890 apparatus (Supelco Beta-Dex 120 column, 30 m length, 250 µm diameter).
Elemental analyses were performed by the Analytical Service of the Instituto de Investigaciones
Químicas (IIQ-CSIC) of Sevilla. The NMR spectra were recorded with a Bruker DPX300 or
AV400 spectrometer. The chemical shift values (δ) are given in parts per million and are
1
13
referenced to the residual peak of the deuterated solvent employed ( H and C) or an external
3
1
8
2
1
3 4
5% aqueous H PO solution ( P).
.2. Synthesis of ligand 2a.
A mixture of aldehyde 1a (0.50 g, 1.15 mmol) and N,N-dimethylethylenediamine (0.10 g,
.15 mmol) in 30 mL of methanol was heated at reflux temperature for 4 hours. After cooling,
the resulting solution was evaporated to dryness. Then, the yellow solid was washed 3 times
3
1
1
with 20 mL of diethylether and vacuum-dried. Yield: 91 % (0.53 g); P{ H} NMR (CDCl
3
): δ
1
=
22.6 (s) ppm; H NMR (CDCl ): δ = 2.40 (s, 6 H, NMe ), 2.75 and 3.72 (both t, 2 H each,
3 2
3
2
3
J
2 2 2
HH = 6.5 Hz, CH CH ), 5.49 (d, 2 H, JPH = 13.8 Hz, CH P), 6.65 (d, 1 H, JHH = 8.6 Hz,
3

3
CHaromatic), 6.81 (broad d, 1 H, JHH = 8.6 Hz, CHaromatic), 7.41 (broad s, 1 H, CHaromatic), 7.61-
1
3
1
7
.81 (m, 15 H, PPh
3
), 8.20 (s, 1 H, CH=N), 13.5 (very broad s, 1 H, OH) ppm; C{ H} NMR
1
(
CDCl
3
): δ = 29.6 (d, JPC = 46.4 Hz, CH
2
P), 45.4 (s, NMe
2
), 56.4 and 59.4 (s, CH
2
), 116.3 (d,
1
J
PC = 8.5 Hz, Caromatic), 117.5 (s, CHaromatic), 118.0 (d, JPC = 79.4 Hz, Cipso of PPh
3
), 118.8 (s,
C
aromatic), 130. 1 (d, JPC = 12.4 Hz, Cortho or meta of PPh
), 134.8 (s, CHaromatic), 134.9 (s, CHpara of PPh ), 135.1 (d, JPC = 5.8 Hz, CHaromatic), 162.0
s, C-OH), 165.7 (s, CH=N) ppm; IR (KBr, pellet): νOH = 3434 cm , νC=N = 1641 cm . Anal.
3
), 134.4 (d, JPC = 9.7 Hz, Cortho or meta of
PPh
3
3
-1
-1
(
calcd. for C30
N, 5.49 %.
H
32ClN OP (503.0 g/mol): C, 71.63; H, 6.41; N, 5.57; found: C, 71.56; H, 6.51;
2
2
.3. Synthesis of ligand 2b.
Following a similar procedure, ligand 2b was prepared as a yellow solid starting from
aldehyde 1b (0.50 g, 0.92 mmol) and N,N-dimethylethylenediamine (0.08 g, 0.92 mmol). Yield:
3
1
1
1
9
3 % (0.52 g); P{ H} NMR (CDCl
3
): δ = 21.8 (s, PPh
3
6
), -144.3 (sept, JFP = 714 Hz, PF )
1
3
ppm; H NMR (CDCl
3
): δ = 2.30 (s, 6 H, NMe
2
), 2.62 and 3.64 (both t, 2 H each, JHH = 6.4
2
3
Hz, CH
2
), 4.45 (d, 2 H, JPH = 13.2 Hz, CH
2
P), 6.61 (d, 1 H, JHH = 8.4 Hz, CHaromatic), 6.69
3
(
broad d, 1 H, JHH = 8.4 Hz, CHaromatic), 6.96 (broad s, 1 H, CHaromatic), 7.50 (m, 6 H, PPh
3
),
3
7
.63 (m, 6 H, PPh
3
), 7.78 (t, 3 H, JHH = 7.2 Hz, PPh
3
), 8.08 (s, 1 H, CH=N) ppm, OH signal
1
3
1
1
not observed; C{ H} NMR (CDCl
3
): δ = 29.6 (d, JPC = 48.6 Hz, CH
2
P), 45.5 (s, NMe
2
), 56.1
),
), 134.0 (d,
), 134.5 (d, JPC = 3.8 Hz, CHaromatic), 134.6 (d, JPC = 5.6 Hz,
1
and 59.4 (s, CH
2
), 114.9 (d, JPC = 8.5 Hz, Caromatic), 117.1 (d, JPC = 85.5 Hz, Cipso of PPh
18.1 (s, CHaromatic), 118.8 (s, Caromatic), 130.3 (d, JPC = 12.5 Hz, Cortho or meta of PPh
PC = 9.6 Hz, Cortho or meta of PPh
CHaromatic), 135.3 (s, CHpara of PPh
3
1
3
J
3
3
), 162.9 (s, C-OH), 165.4 (s, CH=N) ppm; IR (KBr, pellet):
-1
-1
-1
ν
OH = 3434 cm , νC=N = 1636 cm , νP-F = 840 cm ; Elemental analysis calcd. for
30 32 6 2 2
C H F N OP (612.5 g/mol): C, 58.83; H, 5.27; N, 4.57; found: C, 58.92; H, 5.20; N, 4.46 %.
4

2
.4. Synthesis of ligand 4a.
Following a similar procedure, ligand 4a was prepared as a yellow solid starting from
aldehyde 3a (0.50 g, 1.98 mmol) and N,N-dimethylethylenediamine (0.17 g, 1.98 mmol). Yield:
1
9
2 % (0.59 g); H NMR (DMSO-d
6
): δ = 2.20 (s, 6 H, NMe
2
), 2.56 and 3.69 (both m, 2 H each,
3
CH
2
CH
2
), 3.85 (s, 3 H, NMe), 5.36 (s, 2 H, CH
2
), 6.89 (d, 1 H, JHH = 8.4 Hz, CHaromatic), 7.43
3
4
(
d, 1 H, JHH = 8.4 Hz, JHH = 1.5 Hz, CHaromatic), 7.57, 7.72 and 7.80 (s, 1 H each, CHaromatic and
CHimidazolium), 8.54 (s, 1 H, CH=N), 9.30 (s, CHimidazolium), 13.70 (very broad s, 1 H, OH) ppm;
1
3
1
C{ H} NMR (DMSO-d
6
): δ = 36.3 (s, Me), 45.6 (s, NMe
2
), 51.8 (s, CH ), 55.7 and 59.5 (both
2
s, CH CH
2
2
), 118.1 (s, CHaromatic), 118.7 (s, Caromatic), 122.5 and 124.3 (both s, CHaromatic or
CHimidazolium), 124.6 (s, Caromatic), 132.6, 133.4 and 136.9 (all s, CHaromatic or CHimidazolium), 162.7
-1
-1
(
s, C-OH), 166.1 (s, CH=N) ppm; IR (KBr, pellet): νOH = 3391 cm , νC=N = 1634 cm ;
Elemental analysis calcd. for C16
C, 59.42; H, 7.23; N, 17.25 %.
H
23ClN O (322.8 g/mol): C, 59.53; H, 7.18; N, 17.35; found:
4
2
.5. Synthesis of ligand 4b.
Following a similar procedure, ligand 4b was prepared as a yellow solid starting from
aldehyde 3b (0.50 g, 1.38 mmol) and N,N-dimethylethylenediamine (0.12 g, 1.38 mmol). Yield:
3
1
1
1
1
9
0 % (0.54 g); P{ H} NMR (CD
CD OD): δ = 2.62 (s, 6 H, NMe
.92 (s, 3 H, NMe), 5.33 (s, 2 H, CH
3
OD): δ = -144.4 (sept, JFP = 710 Hz, PF
6
) ppm; H NMR
3
(
3
2
), 3.08 and 3.89 (both t, 2 H each, JHH = 6.2 Hz, CH
2
CH ),
2
3
3
2
), 6.93 (d, 1 H, JHH = 8.4 Hz, CHaromatic), 7.43 (dd, 1 H,
3
4
4
J
HH = 8.4 Hz, JHH = 2.1 Hz, CHaromatic), 7.51 (d, 1 H, JHH = 2.1 Hz, CHaromatic), 7.55 and 7.57
3
(
both d, 1 H each, JHH = 1.8 Hz, CHimidazolium), 8.56 (s, 1 H, CH=N) ppm, 9.08* (s, 1 H,
CHimidazolium), OH signal not observed; *Only observed in freshly prepared MeOD-d solutions
4
-
1
-1
-1
due to H/D exchange. IR (KBr, pellet): νOH = 3434 cm , νC=N = 1638 cm , νP-F = 840 cm ;
Elemental analysis calcd. for C16
C, 44.38; H, 5.42; N, 13.00 %.
23 6 4
H F N OP (432.4 g/mol): C, 44.45; H, 5.36; N, 12.96; found:
5

2.6. Synthesis of the ruthenium(II) complex 5a.
6
The ruthenium(II) dimer [{RuCl(µ-Cl)(η -p-cymene)}
to a solution of ligand 2a (0.20 g, 0.40 mmol) in 30 mL of methanol, and the mixture stirred for
hours at room temperature. Solvent was then removed under reduced pressure and the
2
] (0.12 g, 0.20 mmol) was added
2
resulting residue was washed first with diethylether, then with a 1:4 mixture of dichloromethane
and diethylether, and finally with a 1:1 mixture of acetone and diethylether to give an orange
3
1
1
1
solid. Yield: 75 % (0.24 g); P{ H} NMR (CDCl
3
): δ = 20.7 (s) ppm; H NMR (CDCl
.08 and 1.20 (both d, 3 H each, JHH = 6.2 Hz, CHMe ), 2.28 (s, 3 H, Me), 2.70 (m, 1 H,
CHMe ), 2.89 and 3.00 (both s, 3 H each, NMe ), 3.39, 4.14, 4.44 and 4.69 (all m, 1 H each,
CH ), 4.94 (m, 2 H, CH
3
): δ =
3
1
2
2
2
2
2
), 5.48, 5.57, 5.66 and 6.02 (all broad s, 1 H each, CH of p-cymene),
3
6
.48 and 6.59 (both d, 1 H each, JHH = 7.5 Hz, CHaromatic), 7.13 (s, 1 H, CHaromatic), 7.52-7.92
(
m, 15 H, PPh ), 8.35 (s, 1 H, CH=N), 12.1 (very broad s, 1 H, OH) ppm; IR (KBr, pellet): νOH
3
-1
-1
=
3428 cm , νC=N = 1617 cm ; Elemental analysis calcd. for C40
H
46Cl
3
2
N OPRu (809.2 g/mol):
C, 59.37; H, 5.73; N, 3.46; found: C, 59.23; H, 5.86; N, 3.39 %.
2.7. Synthesis of the ruthenium(II) complex 5b.
Following a similar procedure, complex 5b was prepared as an orange solid starting from
6
[
{RuCl(µ-Cl)(η -p-cymene)}
2
] (0.10 g, 0.16 mmol) and ligand 2b (0.20 g, 0.32 mmol). Yield:
3
1
1
1
7
6 % (0.22 g); P{ H} NMR (CDCl
3
): δ = 20.0 (s, PPh
3
), -144.6 (sept, JFP = 714 Hz, PF
6
)
1
3
ppm; H NMR (CDCl
3
): δ = 1.11 and 1.22 (both d, 3 H each, JHH = 6.6 Hz, CHMe
2
), 2.31 (s, 3
H, Me), 2.70 (m, 1 H, CHMe
2
), 2.82 and 2.90 (both s, 3 H each, NMe
2
), 3.09, 4.00, 4.13 and
3
4
.92 (all m, 1 H each, CH ), 4.43 (m, 2 H, CH
2
2
), 5.47 and 5.58 (both d, 1 H each, JHH = 6.2 Hz,
3
CH of p-cymene), 5.63 and 5.94 (both d, 1 H each, JHH = 5.4 Hz, CH of p-cymene), 6.47 and
3
6
.61 (both d, 1 H each, JHH = 8.7 Hz, CHaromatic), 6.91 (s, 1 H, CHaromatic), 7.41-7.84 (m, 16 H,
1
3
1
3 3
PPh and CH=N), 12.0 (very broad s, 1 H, OH) ppm; C{ H} NMR (CDCl ): δ = 18.8 (s, Me),
6

1
2
1.9 and 22.7 (both s, CHMe
2
), 30.2 (d, JPC = 47.5 Hz, CH
2
P), 30.7 (s, CHMe ), 42.3 and 44.5
2
(
both s, NMe ), 56.7 and 64.1 (both s, CH
2
2
CH ), 80.8, 81.6, 82.8 and 88.4 (all s, CH of p-
2
cymene), 99.9 and 100.3 (both s, C of p-cymene), 109.5 (d, JPC = 8.4 Hz, Caromatic), 117.1 (d,
1
J
PC = 85.1 Hz, Cipso of PPh
), 133.9 (d, JPC = 9.4 Hz, CHortho or meta of PPh
36.4 (s, CHaromatic), 138.7 (d, JPC = 6.2 Hz, CHaromatic), 165.0 (s, C-OH), 167.0 (s, CH=N) ppm;
3
), 119.2 (s, Caromatic), 122.8 (s, CHaromatic), 130.4 (d, JPC = 12.3 Hz,
CHortho or meta of PPh
3
3
), 135.4 (s, CHpara of PPh
3
),
1
-
1
-1
-1
IR (KBr, pellet): νOH = 3429 cm , νC=N = 1620 cm , νP-F = 841 cm ; Elemental analysis calcd.
for C40 Cl ORu (918.7 g/mol): C, 52.29; H, 5.05; N, 3.05; found: C, 52.14; H, 4.98;
H
46
F
6
2 2 2
N P
N, 2.96 %.
2
.8. Synthesis of the ruthenium(II) complex 6a.
Following a similar procedure, complex 6a was prepared as an orange solid starting from
6
[
{RuCl(µ-Cl)(η -p-cymene)}
2
] (0.19 g, 0.31 mmol) and ligand 4a (0.20 g, 0.62 mmol). Yield:
OD): δ = 1.17 (m, 6 H, CHMe ), 2.21 (s, 3 H, Me), 2.69 (m, 1 H,
), 2.96 (broad s, 7 H, NMe and CH ), 3.55, 4.11 and 4.63 (all m, 1 H each, CH ), 3.93
s, 3 H, NMe), 5.25 (s, 2 H, CH
1
6
9 % (0.27 g); H NMR (CD
3
2
CHMe
2
2
2
2
(
2
) 5.51, 5.63, 5.70 and 5.83 (all broad s, 1 H each, CH of p-
3
cymene), 6.92 and 7.34 (both d, 1H each, JHH = 8.9 Hz, CHaromatic), 7.38 (s, 1 H, CHaromatic or
CHimidazolium), 7.59 (s, 2 H, CHaromatic or CHimidazolium), 8.35 (s, 1 H, CH=N), 9.00 (s, 1 H,
1
3
1
CHimidazolium) ppm, OH not observed; C{ H} NMR (CD
3
OD): δ = 17.6 (s, Me), 20.7 and 21.7
), 35.2 (s, NMe), 41.2 and 43.8 (both s, NMe ), 52.0, 58.1 and
2
), 80.0, 81.3, 82.8 and 87.9 (all s, CH of p-cymene), 99.7 and 100.9 (both s, C of
(
both s, CHMe
2
), 30.6 (s, CHMe
2
2
6
4.1 (all s, CH
p-cymene), 118.9 and 119.8 (both s, Caromatic), 121.9, 122.4, 123.8, 135.4, 136.3 and 136.4 (all s,
CHaromatic or CHimidazolium), 164.9 (s, C-OH), 167.4 (s, CH=N) ppm; IR (KBr, pellet): νOH = 3419
-
1
-1
cm , νC=N = 1620 cm ; Elemental analysis calcd. for C26
H
37
N
4
Cl ORu (629.0 g/mol): C, 49.65;
3
H, 5.93; N, 8.91; found: C, 49.73; H, 5.89; N, 8.84 %.
7

2.9. Synthesis of the ruthenium(II) complex 6b.
Following a similar procedure, complex 6b was prepared as an orange solid starting from
6
[
{RuCl(µ-Cl)(η -p-cymene)}
2
] (0.14 g, 0.23 mmol) and ligand 4b (0.20 g, 0.46 mmol). Yield:
3
1
1
1
1
7
1 % (0.24 g); P{ H} NMR (DMSO-d
): δ = 1.09 (m, 6 H, CHMe ), 2.16 (s, 3 H, Me), 2.81 (broad s, 6 H, NMe
.52, 4.73 and 5.37 (all m, 1 H each, CH ), 3.83 (s, 3 H, NMe), 5.19 (s, 2 H, CH ), 5.61 (broad
6
): δ = -144.1 (sept, JFP = 711 Hz, PF
6
) ppm; H NMR
(
DMSO-d
6
2
2
), 3.65,
4
2
2
s, 2 H, CH of p-cymene), 5.69 and 5.85 (both broad s, 1 H each, CH of p-cymene), 6.71, 7.11
and 7.25 (all s, 1 H each, CHaromatic or CHimidazolium), 7.73 (m, 2 H, CHaromatic or CHimidazolium),
2
8.04 (s, 1 H, CH=N), 9.22 (s, 1 H, CHimidazolium), 10.5 (very broad s, 1 H, OH) ppm, CHMe not
-1
observed (overlapped by the deuterated solvent); IR (KBr, pellet): νOH = 3428 cm , νC=N
=
-
1
-1
1
622 cm , νP-F = 839 cm ; Elemental analysis calcd. for C26
H
37
F
6
N
4
Cl OPRu (738.6 g/mol):
2
C, 42.28; H, 5.05; N, 7.59; found: C, 42.19; H, 5.16; N, 7.39 %.
2.10. Synthesis of the ruthenium(II) complex 8.
Following a similar procedure, complex 8 was prepared as an orange solid starting from
6
[
{RuCl(µ-Cl)(η -p-cymene)}
2
] (0.32 g, 0.52 mmol) and ligand 7 (0.20 g, 1.04 mmol). Yield: 85
1
3
3
%
(0.44 g); H NMR (CDCl
), 2.26 (s, 3 H, Me), 2.30 (s, 3 H, NMe), 2.64 (broad s, 4 H, NMe and CHMe
overlapped), 3.19, 3.90, 4.25 and 5.04 (all m, 1 H each, CH CH ), 5.50 and 5.62 (both d, 1 H
3
): δ = 1.06 (d, 3 H, JHH = 6.6 Hz, CHMe ), 1.19 (d, 3 H, JHH = 6.9
2
Hz, CHMe
2
2
2
2
3
3
each, JHH = 6.1 Hz, CH of p-cymene), 5.58 and 5.93 (both d, 1 H each, JHH = 5.4 Hz, CH of p-
3
cymene), 6.44 and 7.20 (both t, 1 H each, JHH = 7.5 Hz, CHaromatic), 6.91 and 7.02 (both d, 1 H
3
13
1
each, JHH = 7.5 Hz, CHaromatic), 7.96 (s, 1 H, CH=N), 11.93 (broad s, 1 H, OH) ppm; C{ H}
NMR (CDCl ): δ = 18.8 (s, Me), 21.6 and 22.9 (both s, CHMe ), 30.6 (s, CHMe ), 56.9 and
5.1 (both s, CH CH ), 80.9, 81.1, 82.8 and 88.6 (all s, CH of p-cymene), 99.1 and 100.3 (both
s, C of p-cymene), 114.3, 122.5, 135.1 and 135.7 (all s, CHaromatic), 118.0 (s, Caromatic), 165.1 (s,
3
2
2
6
2
2
-
1
-1
C-OH), 166.3 (s, CH=N) ppm; IR (KBr, pellet): νOH = 3501 cm , νC=N = 1613 cm ; Elemental
8

analysis calcd. for C21
H, 6.00; N, 5.47 %.
H30Cl
2
2
N ORu (498.4 g/mol): C, 50.60; H, 6.07; N, 5.62; found: C, 50.68;
2
.11. General procedure for the catalytic transfer hydrogenation of carbonyl compounds.
A solution of HCO Na (1.02 g, 15 mmol) in 3 mL of water was introduced, under argon
2
atmosphere, in a Schlenk tube equipped with a condenser. The ketone or aldehyde (1 mmol)
and the appropriate ruthenium catalyst (0.02 mmol) were added, and the mixture heated at 80
ºC in an oil bath. The course of the catalytic reactions was monitored taking aliquots (ca 20
µL), which after extraction with dichloromethane (3 mL) were analyzed by GC. The identity of
the alcohols formed was assessed by comparison of their retention times with those of
commercially available samples.
2
.12. Recycling tests in the transfer hydrogenation of 2,2,2-trifluoroacetophenone promoted by
complex 5b.
After the first run, the 1-phenyl-2,2,2-trifluoroethanol formed was extracted with
diethylether (3 x 3 mL) and traces of organic solvent were eliminated from the aqueous phase at
reduced pressure. Then, 136 µL (1 mmol) of 2,2,2-trifluoroacetophenone and hydrogen source
were added to the aqueous solution, and the mixture was heated at 80ºC. Both sodium formate
(
1.02 g, 15 mmol, Table 1, entry 6) and formic acid (0.57 mL, 15 mmol, Table 1, entry 7) have
been tested as hydrogen source for the recycling.
3
. Results and discussion
The synthesis of the new Schiff-base ligands was carried out by condensation of the
corresponding aldehyde precursors with the appropriate primary amine (see Schemes 1 and 2).
Thus, the reaction of the (3-formyl-4-hydroxybenzyl)triphenylphosphonium chloride (1a) and
hexafluorophosphate (1b) salts with one equivalent of N,N-dimethylethylenediamine in
9

refluxing methanol led to the selective formation of the corresponding imines 2a-b (Scheme 1).
Similarly, the treatment of the 3-(3-formyl-4-hydroxybenzyl)-1-methyl-1H-imidazol-3-ium
2 2 2 2
salts 3a-b with Me NCH CH NH allowed the preparation of the imino-ligands 4a-b (Scheme
2
). In all the cases, the transformation of the aldehyde function into a -CH=N- moiety was
1
evidenced by H NMR spectroscopy with the disappearance of the -CH=O signal at ca 10-11
ppm and the observation of a new resonance at 8.20-8.56 ppm [16]. The formation of the iminic
1
3
1
group was also supported by the presence of a singlet resonance at ca 165 ppm in the C{ H}
NMR spectra. Worthy of note, the comparison of the spectroscopic data for the chloride and
hexafluorophosphate salts (i.e. 2a vs 2b and 4a vs 4b) shows similar NMR patterns for the
cation albeit with different chemical shifts, evidencing therefore the existence of ion pairing in
solution. As an example, the CH
2
P unit of ligands 1a and 1b gives rise to a doublet resonance at
), respectively [17]. On the other hand, the ligand 4a
1
5
.49 and 4.45 ppm ( H NMR in CDCl
3
exhibits a resonance at 10.78 ppm for the imidazolium N-CH=N proton, whereas a signal at
1
8
.77 ppm ( H NMR, CDCl
3
) is observed for the same group in its hexafluorophosphate
counterpart 4b [18,19]. As previously observed by other authors, the interaction between
-
-
cations and anions is expected to be stronger with the Cl than with PF
6
, inducing therefore a
major shift toward low field [19].
Scheme 1. Synthesis of ligands 2a-b with a phosphonium group pendant.
10

Scheme 2. Synthesis of ligands 4a-b with an imidazolium group pendant.
6
Treatment of the dimeric ruthenium(II) precursor [{RuCl(µ-Cl)(η -p-cymene)}
2
] with two
equivalents of the Schiff-bases 2a-b and 4a-b, in methanol at room temperature for 2 hours, led
6
2
to the clean formation of the corresponding mononuclear complexes [RuCl(η -p-cymene)(κ -
N,N’-Me NCH CH N=CH-C -2-OH-5-R)][Cl] (R = CH PPh [Cl] (5a), CH PPh [PF ] (5b),
CH -1-methylimidazolium[Cl] (6a), 1-methylimidazolium[PF ] (6b)), in which the
2
2
2
6
H
3
2
3
2
3
6
2
6
functionalized ligand acts as a chelate through the N,N’-coordination of the diamine moiety (see
Scheme 3). The proposed structures for the arene-ruthenium(II) compounds 5-6a-b, isolated as
air-stable orange solids in 69-76% yield, were supported by elemental analyses, IR and NMR
spectroscopies. In particular, the coordination of the NMe
inferred by the inequivalence of the two methyl groups in the H and C{ H} NMR spectra
see details in the Experimental section). On the other hand, the lowering of the ν(C=N)
2
group to the ruthenium atom is
1
13
1
(
-
1
-1
absorption bands (1617-1622 cm in 5-6a-b; ∆ν between -14 and -24 cm ) respective to those
-1
observed in the free ligands (1634-1641 cm in 2,4a-b) is indicative of the complexation of the
imine function to the metal center [20]. Additional confirmation of the N,N’-chelation of the
ligands is given by the chemical and magnetic inequivalence of the p-cymene ring protons,
1
which appear as four distinct signals in the H NMR spectra as a consequence of the stereogenic
nature of the ruthenium atom. The loss of the symmetry is also reflected by the diastereotopic
2
nature of the different CH units of the coordinated ligands.
11

Scheme 3. Synthesis of arene-ruthenium(II) complexes 5-6a-b containing an ionic Schiff-base ligand.
At this point, it should be noted that Schiff-bases generated from salicylaldehyde
2
derivatives are prone to coordinate metallic ions in a κ -N,O-mode, after deprotonation of the
phenol group [21]. In this context, a wide range of arene-ruthenium(II) complexes featuring
2
these κ -N,O-coordinated ligands have been described in the literature [22]. Also of note is the
fact that Schiff-bases containing an imidazolium fragment in their backbone are suitable
precursors for N-heterocyclic carbene species [19c], and are known to be sensitive to
nucleophilic attacks (e.g. by methanol), giving rise to the release of the corresponding
imidazole [19c]. In our study none of these reactivity patterns was observed, the formation of
complexes 5-6a-b taking place selectively under the experimental conditions employed.
In order to evaluate the benefit of the ionic substituents in 5-6a-b for further applications
in catalytic processes in aqueous media, we also prepared and characterized the analogous
6
ruthenium derivative [RuCl(η -p-cymene)(Me
2
NCH
2
CH
2
N=CH-C
6
H
4
-2-OH)][Cl] (8) using the
known neutral Schiff-base 7 (Scheme 4). The full analytical and spectroscopic characterization
12

of this compound is included in the Experimental section, all the data obtained being in
complete accord with the proposed formulation.
Scheme 4. Synthesis of the arene-ruthenium(II) complex 8 containing a neutral Schiff-base ligand.
6
Compounds [RuCl(η -p-cymene)(Me
2
NCH
-1-methylimidazolium[Cl] (6a), 1-
] (6b), H (8)) are soluble and stable in water [23], hydrolytic
2 2 6 3
CH N=CH-C H -2-OH-5-R)][Cl] (R =
CH
2 3 2 3 6 2
PPh [Cl] (5a), CH PPh [PF ] (5b), CH
methylimidazolium[PF
6
decomposition of the coordinated Schiff base ligands being not observed [24,25]. Thus, taking
advantage of their stability and solubility in water, we decide to explore the potential of these
complexes for aqueous catalysis. In particular, we evaluated their behavior in the catalytic
transfer hydrogenation of carbonyl compounds in aqueous media employing 2,2,2-
trifluoroacetophenone as a representative substrate. Experiments were carried out with 1 mmol
of the ketone, a ruthenium loading of 2 mol% and 15 equivalents of HCO Na as the hydrogen
2
source, in water at 80ºC. Under these conditions, all the complexes synthesized proved to be
catalytically active giving rise to the selective formation of the desired 1-phenyl-2,2,2-
trifluoroethanol in moderate to high yield after 7 hours of heating (Table 1). In particular, the
6
best result was achieved with complex [RuCl(η -p-cymene){Me
2
NCH
2
CH
2
N=CH-C
6
H
3
-2-OH-
5
-CH
2
PPh
3
[PF ]}][Cl] (5b) which contains a phosphonium pendant as the corresponding
6
hexafluorophosphate salt (entry 2).
13

6
Table 1. Catalytic activity of complexes [RuCl(η -p-cymene)(Me
2
NCH
2
CH
2
N=CH-C
6
H
3
-2-OH-5-
a
R)][Cl] (5-6a-b, 8) in the transfer hydrogenation of 2,2,2-trifluoroacetophenone.
b
c
Entry Catalyst
Time
0.5 h
Yield
35 %
TOF
-1
1
R = CH
2
PPh
3
[Cl] (5a)
35 h
-
1
7
h
82 %
43 %
98 %
37 %
90 %
40 %
93 %
24 %
73 %
38 %
67%
5.9 h
-
1
2
3
4
5
R = CH
2
PPh
3
[PF
6
] (5b)
0.5 h
43 h
-1
7
h
7.0 h
-
1
R = CH
2
-1-methylimidazolium[Cl] (6a)
0.5 h
37 h
-1
7
h
6.4 h
-
1
R = CH
2
-1-methylimidazolium[PF
6
] (6b)
0.5 h
40 h
-1
7
h
6.6 h
-
1
R = H (8)
0.5 h
24 h
-
1
7
h
5.2 h
2.7 h
4.7 h
d
e
-1
-1
6
7
R = CH
2
PPh
3
[PF
6
6
] (5b)
] (5b)
7 h
7 h
R = CH
2
PPh
3
[PF
a
Reactions carried out with 1 mmol of 2,2,2-trifluoroacetophenone, 2 mol% of catalyst, 15 mmol of
b
c
HCO
2
Na and 3 mL of water at 80ºC. Yield of the corresponding alcohol. GC determined. Turnover
d
frequencies ((mol product/mol Ru)/time) were calculated at the time indicated in each case. Recycling
e
test, carried out with HCO
2
Na. Recycling test, carried out with HCO
2
H.
-
As a general trend, the activity of the PF
6
-containing derivatives was higher than that of
their chloride counterparts (entries 2 and 4 vs 1 and 3), probably due to the greater capacity of
-
the Cl to coordinate the metal center and therefore to compete with the complexation of the
substrate or the formate anion [26]. Worthy of note, complex 8, containing a non-ionic Schiff-
base, generates an active species only partially soluble under the experimental conditions
14

selected and consequently conduces to lower conversion than catalysts 5-6a-b (entry 5 vs
entries 1-4). On the other hand, since pH is known to dramatically affect the course of the
transfer hydrogenation reactions [27], different experiments were also performed in the pH
range 4-12. However, no improvements could be achieved, the best results being obtained in
the absence of basic or acidic additives [28]. As an example, the conversions of 2,2,2-
trifluoroacetophenone at pH = 4 and pH = 12 with the complex 5b were only 77% and 33%,
respectively, after 7 hours.
6
Table 2. Catalytic activity of complex [RuCl(η -p-cymene){Me
2
NCH
2
CH
2
N=CH-C
6
H
3
-2-OH-5-
a
CH
2
PPh
3
[PF ]}][Cl] (5b) in the transfer hydrogenation of different carbonyl compounds.
6
b
c
-
Entry
1
Substrate
R = CF , R’ = Ph
R = Me, R’ = Ph
Time
7 h
Yield
98 %
TOF
1
3
7.0 h
-
1
1
1
1
1
1
1
2
3
20 h
12 h
12 h
12 h
7 h
96 %
98 %
98 %
97 %
99 %
96 %
97 %
2.4 h
-
R, R’ = -(CH
2
)
)
5
4
-
-
4.1 h
-
4
R, R’ = -(CH
2
4.1 h
-
5
R = Me, R’ = Et
R = Ph, R’ = H
4.0 h
-
6
7.1 h
-
7
R = 4-C
6
H
4
OMe, R’ = H
12 h
5 h
4.0 h
-
8
R = C
6
F
5
, R’ = H
9.7 h
a
Reactions carried out with 1 mmol of ketone or aldehyde, 2 mol% of catalyst, 15 mmol of HCO
2
Na
b
c
and 3 mL of water at 80ºC. Yield of the corresponding alcohol. GC determined. Turnover frequencies
((mol product/mol Ru)/time) were calculated at the time indicated in each case.
15

The scope of the reaction was explored with the most efficient catalyst, i.e. complex
6
[
RuCl(η -p-cymene){Me
2
NCH
2
CH
2
N=CH-C
6
H
3
-2-OH-5-CH
2
PPh
3
[PF ]}][Cl] (5b). As
6
observed in Table 2, acetophenone, cyclohexanone, cyclopentanone and 2-butanone could also
be reduced in high yield under the selected conditions. As expected, these ketones, more
electron-rich than the 2,2,2-trifluoroacetophenone turned to be less reactive, requiring longer
reaction times to reach good conversions (Entries 2-5 vs entry 1). The catalytic system resulted
also operative for the transfer hydrogenation of aldehydes (Entries 6-8, Table 2). Once again,
the rate of the reaction clearly depends on the electronic properties of the substrate employed,
an electron-deficient aldehyde being more reactive than an electron-rich one (entry 8 vs entry
7
).
Finally, the possibility for recycling the catalytic system has been investigated. After the
full conversion of 2,2,2-trifluoroacetophenone by complex 5b, the alcohol was extracted with
diethylether, and a second catalytic run was performed after adding a new batch of sodium
formate and ketone to the aqueous phase (see details in the experimental section). By this way,
3
8% of the desired 1-phenyl-2,2,2-trifluoroethanol were generated after 7 hours of heating
Table 1, entry 6). The low activity observed in this case could be due to the accumulation of
the HCO Na in the medium that induces a dramatic increase of the pH [29,30], an important
(
2
factor taking in account the poor efficiency displayed by complex 5b under basic conditions. In
agreement with this, better results were achieved when the recycling was performed with
2 2
HCO H instead of HCO Na (Table 1, entry 7).
4. Conclusions
In summary, the synthesis of new ionic Schiff-bases, containing a NCH
2
CH
2
NMe moiety
2
and a phosphonium or imidazolium pendant, has been presented. The selective coordination of
6
these ligands onto a η -arene-ruthenium(II) fragment, through N,N’-chelation, furnished highly
16

6
water-soluble complexes of the type [RuCl(η -p-cymene)(N,N’-Me
2
NCH
2
CH
2
N=CH-C
6
3
H -2-
OH-5-R)][Cl]. These derivatives proved to be able to promote the transfer hydrogenation of the
fluorinated ketone 2,2,2-trifluoroacetophenone and resulted more convenient to perform this
transformation in aqueous media than non-functionalized catalyst with similar structure. The
6
most active catalyst, i.e. [RuCl(η -p-cymene){Me
2
NCH
2
CH
2
N=CH-C
6
3
H -2-OH-5-
2 3 6
CH PPh [PF ]}][Cl] (5b) proved to be also active in the reduction of other ketones and
aldehydes.
5
. Acknowledgments
Financial support from the Ministerio de Economía y Competitividad (MINECO) of
Spain (project CTQ2013-40591-P) and the Gobierno del Principado de Asturias (project
GRUPIN14-006) is gratefully acknowledged. S. A. T. thanks the Ministry of Science, Research
and Technology of the Islamic Republic of Iran and Damghan University for the award of a
fellowship to perform a stay at the University of Oviedo.
6
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[
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2 2
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6
3
OD). In these solvents the
difference in the chemical shifts of chloride and hexafluorophosphate salts is not so
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19] (a) B. Erguillor, M.A. Esteruelas, M. Oliván, M. Puerta, Organometallics 27 (2008)
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García-Verdugo, S.V. Luis, I. Alfonso, S. García-Granda, A. Menéndez-Velázquez,
Chem. Eur. J. 16 (2010) 836-847; (c) R. Zhong, Y.-N. Wang, X.-Q. Guo, Z.-X. Chen,
X.-F. Hou, Chem. Eur. J. 17 (2011) 11041-11051.
[
20] (a) P. Crochet, J. Gimeno, S. García-Granda, J. Borge, Organometallics 20 (2001) 4369-
4
377; (b) P. Crochet, J. Gimeno, J. Borge, S. García-Granda, New J. Chem. 27 (2003)
14-420; (c) A. E. Díaz-Álvarez, P. Crochet, M. Zablocka, V. Cadierno, L. Vendier, J.
4
20

Gimeno, J.-P. Majoral, Polyhedron 26 (2007) 933-940; (d) J. M. Gichumbi, H. B.
Friedrich, B. Omondi, J. Organomet. Chem. 808 (2016) 87-96.
[
21] See, for example: (a) W.-J. Lian, X.-T. Wang, C.-Z. Xie, H. Tian, X.-Q. Song, H.-T.
Pan, X. Qiao, J.-Y. Xu, Dalton Trans. 45 (2016) 9073-9087; (b) M.D. Santana, L.
López-Banet, G. Sánchez, J. Pérez, E. Pérez, L. García, J. L. Serrano, A. Espinosa,
Dalton Trans. 45 (2016) 8601-8613; (c) N. Oncel, V.T. Kasumov, E. Sahin, M. Ulusoy,
J. Organomet. Chem. 811 (2016) 81-90; (d) V.T. Kasumov, O. Sahin, H.G. Aktas,
Polyhedron 115 (2016) 119-127; (e) M. Enamullah, G. Makhloufi, R. Ahmaed, B.A.
Joy, M.A. Islam, D. Padula, H. Hunter, G. Pescitelli, C. Janiak, Inorg. Chem. 55 (2016)
6
449-6464.
[
22] See, for example: (a) S. Dayan, N.K. Ozpozan, N. Özdemir, O. Dayan, J. Organomet.
Chem. 770 (2014) 21-28; (b) K. Ghebreyessus, A. Peralta, M. Katdare, K. Prabhakaran,
S. Paranawithana, Inorg. Chim. Acta 434 (2015) 239-251; (c) E. Ekengard, L. Glans, I.
Cassells, T. Fogeron, P. Govender, T. Stringer, P. Chellan, G.C. Lisensky, W.H. Hersh,
I. Doverbratt, S. Lidin, C. de Kock, P.J. Smith, G.S. Smith, E. Nordlander, Dalton
Trans. 44 (2015) 19314-19329; (d) W.-G. Jia, H. Zhang, T. Zhang, S. Ling, Inorg.
Chem. Commun. 66 (2016) 15-18; (e) H. S. Çalık, E. Ispir, Ş. Karabuga, M. Aslantas, J.
Organomet. Chem. 801 (2016) 122-129; (f) W.-G. Jia, H. Zhang, T. Zhang, D. Xie, S.
Ling, E.-H. Sheng, Organometallics 35 (2016) 503-512; (g) M. Maroto-Díaz, B.T. Elie,
P. Gómez-Sal, J. Pérez-Serrano, R. Gómez, M. Contel, J. de la Mata, Dalton Trans. 45
(
2016) 7049-7066.
[
23] NMR spectra of complexes 5-6a-b and 8 in D
2
O only showed the chloride- and aquo-
6
+
derivatives, [RuCl(η -p-cymene)(Me
2
NCH
2
CH
2
N=CH-C
6
H
3
-2-OH-5-R)] and
6
2+
[
Ru(H
2
O)(η -p-cymene)(Me
2
NCH
2
CH N=CH-C
2
6
H
3
-2-OH-5-R)] respectively, even
after 24 hours in solution. Both species are in equilibrium and the formation of the aquo-
derivative could be suppressed by adding NaCl to the solution. This phenomena is
21

6
general for the arene-ruthenium(II) complexes of the type [RuCl
2
(η -arene)(L)] and
6
+
[
RuCl(η -arene)(L) ] when dissolved in water. See, for example: (a) P. Csabai, F. Joó,
2
Organometallics 23 (2004) 5640-5643; (b) B. Lastra-Barreira, J. Díez, P. Crochet,
Green Chem. 11 (2009) 1681-1686; (c) I. Romero-Canelón, L. Salassa, P. J. Sadler, J.
Med. Chem. 56 (2013) 1291-1300.
[
24] However, the free Schiff-base ligands 1a-b, 3a-b and 7 were smoothly hydrolyzed in
2
D O solutions, giving rise to the complete conversion into the corresponding aldehydes
and N,N-dimethylethylenediamine after 24 hours at room temperature. The higher
stability of the coordinated Schiff-bases respective to free ligands is usual albeit strongly
dependent of the metallic fragment involved. See, for example: E. Delahaye, M. Diop,
R. Welter, M. Boero, C. Massobrio, P. Rabu, G. Rogez, Eur. J. Inorg. Chem. (2010)
4450-4461.
[
25] Other arene-ruthenium(II) complexes containing Schiff-base ligand stable against the
hydrolytic decomposition have been reported previously. See reference 23c.
26] For mechanistic considerations, see reference 9c.
[
[
27] For positive effect of basic or acidic pH on the activity, see: (a) I. Nieto, M.S. Livings,
J.B. Sacci III, L.E. Reuther, M. Zeller, E.T. Papish, Organometallics 30 (2011) 6339-
6342; (b) C. Aliende, M. Pérez-Manrique, F.A. Jalón, B.R. Manzano, A.M. Rodríguez,
G. Espino, Organometallics 31 (2012) 6106-6123.
[
[
28] NaOH or HCO
2
H was added to the aqueous sodium formate solution to reach the
desired pH. In the absence of additive, the aqueous solution of HCO
2
Na employed
presents a pH of 8.3.
29] For similar observations, see: M.A.N. Virboul, R.J.M. Klein Gebbink, Organometallics
1 (2012) 85-91
3
22

[
30] Moreover NaOH and/or NaHCO were formed during the first run, increasing also the
3
pH of the medium. See: C. Wang, C.Q. Li, X.F. Wu, A. Pettman, J.L. Xiao, Angew.
Chem. Int. Ed. 48 (2009) 6524-6528.
23

FOR GRAPHICALABSTRACT USE ONLY
Half-sandwich ruthenium(II) complexes with water-soluble Schiff
base ligands: Synthesis and catalytic activity in transfer
hydrogenation of carbonyl compounds
Somayeh Azizi Talouki, Gholamhossein Grivani, Pascale Crochet and Victorio Cadierno
The preparation of novel water-soluble arene-ruthenium(II) complexes containing ionic Schiff
base ligands is presented, along with their application as catalysts in the transfer hydrogenation
of carbonyl compounds in aqueous medium.
24

Highlights
-
-
Four new ionic Schiff-base ligands have been synthesized.
Their selective coordination as N,N-chelates to an arene-ruthenium(II) fragment affords
highly water-soluble complexes.
-
The derivatives prepared have proved to be active in the transfer hydrogenation of
ketones and aldehydes in aqueous media.
25