Cycloruthenation of N-(naphthyl)salicylaldimine and related ligands: Utilization of the Ru-C bond in catalytic transfer hydrogenation

Article abstract of DOI:10.1002/ejic.201402236

Upon reaction with [Ru(PPh₃)₂(CO)₂Cl₂], N-(naphthyl)-4-R-salicylaldimines (R = OCH₃, H, Cl; H₂L¹-H₂L³) and 2-hydroxy-N-(naphthyl)naphthaldimine (H₂L⁴) readily undergo cycloruthenation by C-H bond activation at the peri position to afford complexes of the type [Ru(PPh₃)₂(L)(CO)] (L = L¹-L⁴). The crystal structures of the [Ru(PPh₃)₂(L)(CO)] (L = L¹, L², L⁴) complexes were determined and the structure of [Ru(PPh₃)₂(L³)(CO)] optimized by DFT calculations. The thermodynamics for the reaction of [Ru(PPh₃)₂(CO)₂Cl₂] with H₂L² to give [Ru(PPh₃)₂(L²)(CO)] were determined. All the complexes show intense absorptions in the visible and UV regions, which have been analyzed by TDDFT calculations. Cyclic voltammetry of the four cycloruthenated complexes showed two oxidations within the range 0.50-1.35 V versus SCE and a reduction at around -1.75 V versus SCE. The [Ru(PPh₃)₂(L)(CO)] (L = L¹-L⁴) complexes were found to efficiently catalyze the transfer hydrogenation of carbonyl compounds.

Full text of DOI:10.1002/ejic.201402236

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DOI:10.1002/ejic.201402236
Cycloruthenation of N-(Naphthyl)salicylaldimine and
Related Ligands: Utilization of the Ru–C Bond in
Catalytic Transfer Hydrogenation
Jayita Dutta,[a] Michael G. Richmond,[b] and
Samaresh Bhattacharya*[a]
Dedicated to Professor Cortlandt G. Pierpont[‡]
Keywords: Homogeneous catalysis / C–H activation / Hydrogenation / Ruthenium
Upon reaction with [Ru(PPh3)2(CO)2Cl2], N-(naphthyl)-4-R-
salicylaldimines (R = OCH3, H, Cl; H2L1–H2L3) and 2-hy- complexes show intense absorptions in the visible and UV
H2L2 to give [Ru(PPh3)2(L2)(CO)] were determined. All the
droxy-N-(naphthyl)naphthaldimine (H2L4) readily undergo
cycloruthenation by C–H bond activation at the peri position
to afford complexes of the type [Ru(PPh3)2(L)(CO)] (L = L1–
L4). The crystal structures of the [Ru(PPh3)2(L)(CO)] (L = L1,
L2, L4) complexes were determined and the structure of
[Ru(PPh3)2(L3)(CO)] optimized by DFT calculations. The ther-
modynamics for the reaction of [Ru(PPh3)2(CO)2Cl2] with
regions, which have been analyzed by TDDFT calculations.
Cyclic voltammetry of the four cycloruthenated complexes
showed two oxidations within the range 0.50–1.35 V versus
SCE and a reduction at around –1.75 V versus SCE. The
[Ru(PPh3)2(L)(CO)] (L = L1–L4) complexes were found to ef-
ficiently catalyze the transfer hydrogenation of carbonyl
compounds.
of significant contemporary importance; the impetus for
the present work derives from our interest in these two as-
pects of ruthenium chemistry. A group of four Schiff bases,
Introduction
The chemistry of ruthenium complexes continues to re-
ceive considerable attention from a diverse group of re-
searchers, primarily because of their fascinating redox, pho-
tophysical, and photochemical properties.[1] Another prop-
erty of the ruthenium complexes that has gained promi-
nence over the years is their efficiency in catalyzing a wide
variety of industrially important reactions.[2] Particularly
notable is the increasing tendency of utilizing ruthenium
complexes as catalysts to effect useful organic transforma-
tions.[3] The majority of these transformations are known
to involve the participation of a reactive organo-ruthenium
intermediate, which is generated in situ usually by activation
of a C–H bond. Ensuing reactions with the resulting Ru–C
fragment afford the desired end-product(s). Studies of Ru–
C bonded species, with particular reference to their forma-
tion and reactivity, serve to increase our knowledge base of
such fundamental catalytic intermediates and are therefore
[a] Department of Chemistry, Inorganic Chemistry Section,
Jadavpur University,
Kolkata 700032, India
http://www.jaduniv.edu.in
[b] Department of Chemistry, University of North Texas,
Denton, TX 76203, USA
[‡] University of Colorado at Boulder, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402236.
Figure 1. Aldimine ligands H2L1–H2L4 and their expected mode of
coordination (I and II) with ruthenium.
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namely three N-(naphthyl)salicylaldimines (H2L1–H2L3)
and 2-hydroxy-N-(naphthyl)naphthaldimine (H2L4), were
chosen as the principal ligands for the present study (Fig-
ure 1). The initial goal was to induce an ONC mode of
coordination (I and II; Figure 1) by these ligands, which
requires the loss of two protons from the uncoordinated
ligand, the phenolic proton and the naphthyl proton at the
peri position, and hence the selected ligands are abbreviated
in general as H2L, in which H2 represents the two protons
that are released upon ligand coordination. It is relevant to
mention that although salicylaldimines and related ligands
have been exhaustively studied as ligand auxiliaries with
many transition-metal ions,[4] the ruthenium complexes of
such ligands have received less attention.[5] The ability of
the selected aldimine ligands (H2L1–H2L4) to furnish cyclo-
ruthenated complexes containing chelate I or II is best in-
vestigated by using [Ru(PPh3)2(CO)2Cl2] as the starting
ruthenium complex due to its demonstrated ability to ac-
commodate dianionic tridentate chelating ligands through
the displacement of carbonyl and chlorides.[6] Herein we re-
port on the reactions of the selected aldimine ligands H2L1–
H2L4 with [Ru(PPh3)2(CO)2Cl2], which have indeed af-
forded cycloruthenated complexes. The new complexes were
fully characterized by a combination of spectroscopic meth-
ods, X-ray diffraction analysis, and DFT calculations, and
their ability to serve as precursors in the catalytic transfer
hydrogenation of a wide variety of aldehydes and ketones
is discussed.
Results and Discussion
Formation and Structure
Figure 2. (a) Crystal structure of [Ru(PPh3)2(L1)(CO)] (hydrogen
atoms have been omitted for clarity) and (b) view of the equatorial
plane.
As outlined in the introduction, the primary goal of this
study was to explore the possibility of obtaining cycloru-
thenated species by activation of the C–H bond at the peri
position of the N-naphthyl ring in the four selected aldimine
ligands (H2L1–H2L4) through their interaction with
[Ru(PPh3)2(CO)2Cl2]. The planned reactions proceeded
smoothly in 2-methoxyethanol at reflux in the presence of
triethylamine, and from each of these reactions an orange
complex was obtained in good yield. Preliminary charac-
terization (microanalysis, NMR, and IR) of these com-
plexes indicated the presence of a doubly deprotonated ald-
imine ligand, two triphenylphosphines, and a carbonyl in
the ruthenium coordination sphere. Hence these complexes
were formulated in general as [Ru(PPh3)2(L)(CO)] (L = L1–
L4). For an unambiguous characterization of these com-
plexes, with particular reference to the coordination mode
exhibited by the aldimine ligand, the solid-state structure of
Table 1. Selected bond lengths and bond angles for [Ru(PPh3)2-
(L1)(CO)].
Bond lengths [Å]
Ru1–O1
Ru1–N1
Ru1–C16
Ru1–C19
Ru1–P1
Ru1–P2
2.136(3)
2.111(4)
2.038(4)
1.815(6)
2.387(1)
2.382(1)
C1–O1
C7–N1
C9–N1
C19–O3
1.308(7)
1.298(6)
1.432(5)
1.169(7)
Bond angles [°]
N1–Ru1–C19
O1–Ru1–C16
P1–Ru1–P2
173.2(2)
168.0(2)
175.22(4)
O1–Ru1–N1
N1–Ru1–C16 80.6(2)
Ru1–C19–O3 176.7(4)
87.5(1)
[Ru(PPh3)2(L1)(CO)] was determined by X-ray crystallogra- also coordinated to the metal center. The meridionally dis-
phy. The structure is shown in Figure 2 and selected bond posed ONC ligand and the carbonyl group define the equa-
parameters are presented in Table 1. The structure confirms torial plane with ruthenium at the center of this six-coordi-
that the N-(naphthyl)salicylaldimine is coordinated to the nate complex; the two PPh3 ligands occupy the remaining
ruthenium center in the targeted ONC fashion (I, R = two axial positions and are situated in a mutually trans ori-
OCH3), forming two adjacent six- and five-membered che- entation. Ruthenium is thus nested in a C2NOP2 coordina-
late rings with bite angles of 87.54(14) and 80.64(17)°, tion environment, which is distorted significantly from an
respectively. Two triphenylphosphines and a carbonyl are ideal octahedral geometry, as reflected in the metric param-
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eters around the metal center (Table 1). The Ru–C, Ru–N, Table 2. C–H···O and C–H···π interactions in the crystal lattice of
[Ru(PPh3)2(L1)(CO)].
Ru–O, and Ru–P bond lengths are in accord with the corre-
sponding distances in related compounds published by us.[6]
C–H···O interactions
C–H···O
C–H [Å] H···O [Å] C···O [Å] C–H···O [°]
The existence of noncovalent interactions between individ-
ual molecules of the complex is reinforced by the absence
(azomethine)C–
0.929
0.920
0.931
2.567
2.660
2.596
3.472
3.553
3.459
164.60
159.65
154.46
of any solvent of crystallization in the lattice, and this is H···O(carbonyl)
(naphthyl)C–
illustrated by the packing diagram depicted in Figure 3. The
H···O(carbonyl)
important intermolecular hydrogen-bonding interactions
(PPh3 phenyl)C–
extant in the unit cell consist of five types, namely (azo-
methine)C–H···O(carbonyl), (naphthyl)C–H···O(carbonyl),
H···O(methoxy)
C–H···π interactions[a]
(PPh3 phenyl)C–H···O(methoxy), (methoxy)C–H···π(phenyl
C–H···π
C–H [Å] H···R [Å] C···R [Å] C–H···R [°]
of PPh3), and (PPh3 phenyl)C–H···π(phenyl of PPh3), and
(methoxy)C–
involve different fragments of coordinated ligands. The met-
H···π(phenyl of PPh3)
0.959
3.110
3.836
133.70
ric parameters associated with these hydrogen-bonding in-
(PPh3 phenyl)C–
0.930
3.007
3.820
146.78
teractions are presented in Table 2. Each complex molecule H···π(phenyl of PPh3)
is thus linked to five surrounding complex molecules
through the above interactions, which extend throughout
the lattice and are responsible for the stability of the crystal.
[a] R denotes the centroid of the phenyl ring.
ture determination of [Ru(PPh3)2(L3)(CO)] was precluded
by our inability to grow suitable crystals for X-ray diffrac-
tion analysis, we analyzed [Ru(PPh3)2(L3)(CO)] computa-
tionally by density functional theory (DFT) using the
B3LYP functional;[7] the geometry optimization was per-
formed on the PMe3 derivative to reduce the total number
of atoms and simplify the calculation. The DFT-optimized
Figure 3. Intermolecular C–H···O and C–H···π interactions in the
lattice of [Ru(PPh3)2(L1)(CO)].
The structural characterization of [Ru(PPh3)2(L1)(CO)]
confirmed the binding of the H2L1 ligand in the expected
ONC mode (I, R = OCH3), which is facilitated by the acti-
vation of the C–H bond at the peri position of the naphthyl
ring. To ascertain the generality of the ONC coordination
mode in the other three [Ru(PPh3)2(L)(CO)] complexes (L
= L2, L3, L4), attempts were made to determine the molec-
ular structure of the other compounds. We determined the
structures of [Ru(PPh3)2(L2)(CO)] and [Ru(PPh3)2(L4)(CO)]
by X-ray crystallography (Figure 4, Figure 5, and Table S1
in the Supporting Information), and found that the aldim-
ine ligand in both structures binds the ruthenium center in
Figure 4. (a) Crystal structure of [Ru(PPh3)2(L2)(CO)] (hydrogen
atoms have been omitted for clarity) and (b) view of the equatorial
the same ONC mode (I, R = Cl, and II). Although a struc- plane.
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structure for [Ru(PMe3)2(L3)(CO)] is shown in Figure 6,
and a selection of the computed bond parameters are listed
in Table S2 in the Supporting Information. The DFT-opti-
mized structure of [Ru(PMe3)2(L3)(CO)] is in good agree-
ment with the experimentally determined crystal structures
of the other three [Ru(PPh3)2(L)(CO)] (L = L1, L2, L4) com-
plexes.
Figure 6. (a) DFT-optimized structure of [Ru(PMe3)2(L3)(CO)]
(hydrogen atoms have been omitted for clarity) and (b) view of the
equatorial plane.
Figure 5. (a) Crystal structure of [Ru(PPh3)2(L4)(CO)] (hydrogen
atoms have been omitted for clarity), and (b) view of the equatorial
plane.
The formation of the cycloruthenated [Ru(PPh3)2-
(L)(CO)] complexes is believed to involve several steps,
which are illustrated in Scheme 1 for the reaction between
[Ru(PPh3)2(CO)2Cl2] and H2L2. In the initial step, the ald-
imine ligand is assumed to react with [Ru(PPh3)2(CO)2Cl2]
and bind to the metal center by dissociation of the phenolic
proton, as a monoanionic ON donor, coupled with simulta-
neous dissociation of a carbonyl and chloride from the
ruthenium starting complex to generate intermediate A.
Under the prevailing experimental conditions, A is believed
to isomerize to B by mutual exchange of the positions of
Scheme 1. Probable steps leading to the formation of the [Ru(PPh3)2-
(L)(CO)] complexes.
the carbonyl and chloride ligands. In B the naphthyl proton bound chloride, which triggers the elimination of HCl lead-
at the peri position is in close proximity to the ruthenium- ing to the formation of the [Ru(PPh3)2(L2)(CO)] complex.
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The starting ruthenium complex, [Ru(PPh3)2(CO)2Cl2], pected signals. The most deshielded signal at around
and the Schiff base H2L2 were optimized by DFT, as were 208 ppm has been attributed to the carbonyl carbon, and
the remaining species depicted in Scheme 1. Stable geome- the next deshielded signal, observed at around 170 ppm, to
tries, with only positive eigenvalues, were found for each the ruthenium-bound peri carbon of the naphthyl ring. The
species, and this allowed us to determine the thermodynam- 31P NMR spectra of all the complexes show a single reso-
ics for the observed transformation that affords [Ru(PPh3)2- nance within the range 32.53–34.71 ppm, which indicates
(L2)(CO)]. Equation (1) shows the stoichiometrically bal- the equivalent nature of the phosphorus nuclei in the two
anced reaction, the net free energy (ΔG) of which was com- triphenylphosphines. The IR and NMR spectroscopic data
puted to be 32.7 kcalmol–1. The enthalpy change was com- of the [Ru(PPh3)2(L)(CO)] (L = L1–L4) complexes are
puted to be 49.5 kcalmol–1, and this confirms the fact that therefore consistent with their composition and stereochem-
the reaction is entropically driven (ΔS = 56 e.u.) through istry.
the release of CO and HCl byproducts.
The [Ru(PPh3)2(L)(CO)] (L = L1–L4) complexes are
readily soluble in dichloromethane, acetone, chloroform,
methanol, ethanol, acetonitrile, etc., producing bright-
orange solutions. The electronic spectra of the complexes
were recorded in dichloromethane solution. A representa-
tive spectrum is shown in Figure S1 in the Supporting In-
formation, and the spectroscopic data are presented in
Table 3. Each complex shows several intense absorptions in
the visible and ultraviolet regions. To provide insight into
the nature of these absorptions, TDDFT calculations were
performed on all four [Ru(PPh3)2(L)(CO)] (L = L1–L4)
complexes by using the Gaussian 03 package,[7] with the
phenyl rings of the triphenylphosphines being replaced by
hydrogens to simplify the calculations. The results of the
TDDFT calculations are summarized in Tables S3–S6 in
the Supporting Information. The compositions of a few
frontier orbitals of the [Ru(PPh3)2(L)(CO)] (L = L1–L4)
complexes are given in Table 4, and the contour plots of
some selected molecular orbitals of [Ru(PPh3)2(L1)(CO)]
are shown in Figure 7 with those of the remaining three
(1)
Spectral Properties
The IR spectra of the [Ru(PPh3)2(L)(CO)] (L = L1–L4) complexes presented in Figures S2–S4 in the Supporting In-
complexes show many bands of varying intensities within formation. The results obtained were found to be qualita-
the range 4000–450 cm–1, but assignment of each individual tively similar for all four complexes, and hence only the case
band to a specific vibration has not been attempted. How- of [Ru(PPh3)2(L1)(CO)] is discussed here. The lowest-energy
ever, the strong band displayed at around 1900 cm–1 by all absorption at 554 nm is attributable to a combination of
the complexes has been attributed to the coordinated carb- HOMOǞLUMO and HOMO–1ǞLUMO transitions,
onyl, and the three strong bands at around 518, 694, and and, based on the nature of the participating orbitals
742 cm–1 indicate the presence of coordinated PPh3 ligands. (Table 4 and Figure 7), the electronic excitation can be as-
In the coordinated aldimine ligands, the C=N stretch has
been identified in all the complexes as a strong band in the
range 1592–1615 cm–1, and a sharp band at around
Table 3. Electronic spectral and cyclic voltammetric data.
1093 cm–1 has been assigned to the phenolic C–O stretch.
Complex
λmax [nm] (ε [m–1 cm–1])[a] E [V][b]
Magnetic susceptibility measurements revealed that the
[Ru(PPh3)2(L)(CO)] (L = L1–L4) complexes are diamag-
netic, which corresponds to the presence of a bivalent ruth-
enium (low-spin d6, S = 0). The NMR spectra of these com-
plexes were recorded in CDCl3 solution, and the NMR data
[Ru(PPh3)2(L1)(CO)] 554[c] (3050), 467 (6350), 1.14,[d] 0.50[e] (70),[f]
341[c] (6540), 275[c]
(19380), 236 (36650)
–1.71[g]
[Ru(PPh3)2(L2)(CO)] 502[c] (4270), 461 (7650), 1.35,[d] 0.68,[d]
338[c] (6650), 276[c]
(20290), 238 (36490)
–1.74[g]
1
are presented in the Exp. Sect. In the H NMR spectra,
[Ru(PPh3)2(L3)(CO)] 507[c] (3370), 470 (5300), 1.17,[d] 0.72,[d]
339[c] (4600), 274[c]
(15330), 231 (36800)
–1.75[g]
broad signals are observed at 6.96–7.24 ppm, which corre-
spond to the triphenylphosphines. Most of the expected sig-
nals from the ONC-coordinated aldimine ligands are clearly
observed, but a few individual resonances could not be
identified due to extensive overlap. For example, the azo-
methine proton signal is observed as a distinct peak within
the range 7.27–8.35 ppm, and in [Ru(PPh3)2(L1)(CO)] the
signal for the OCH3 group was observed at δ = 3.57 ppm.
[Ru(PPh3)2(L4)(CO)] 538[c] (3650), 482 (5090), 1.33,[d] 0.66,[d]
354[c] (5530), 287[c]
–1.76[g]
(19170), 234 (33 260)
[a] In dichloromethane. [b] Solvent: acetonitrile; supporting electro-
lyte: TBHP; reference electrode: SCE; scan rate: 50 mVs–1. [c]
Shoulder. [d] Anodic peak potential (Epa) value. [e] E1/2 = 0.5(Epa
+ Epc), in which Epc is the cathodic peak potential. [f] ΔEp value,
The 13C NMR spectra of these complexes show all the ex- for which ΔEp = Epa – Epc. [g] Epc value.
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275 nm are primarily due to an admixture of ILCT and
MLCT transitions, and the absorption at 236 nm can be
ascribed to a mixture of LLCT, ILCT, and MLCT transi-
tions.
Electrochemical Properties
The redox properties of the [Ru(PPh3)2(L)(CO)] (L = L1–
L4) complexes were examined in acetonitrile solution (0.1 m
TBHP) by cyclic voltammetry. The voltammograms of a
selected complex are shown in Figure S5 in the Supporting
Information and the voltammetric data are presented in
Table 3. Each [Ru(PPh3)2(L)(CO)] complex shows two oxi-
dative waves and a single reduction wave. Based on the
computed nature of the HOMO in these complexes
(Table 4), the 0/1+ redox wave has been assigned to an oxi-
dation of the coordinated aldimine ligand. Similarly, elec-
tron addition to the LUMO, which is delocalized mostly
over the aldimine ligand (Table 4), leads to the reduction of
the ancillary aldimine ligand. The 1+/2+ oxidative response
has tentatively been assigned to the second oxidation of the
same coordinated aldimine ligand. In the [Ru(PPh3)2-
(L1)(CO)] complex, the first oxidation is quasi-reversible,
based on an analysis of the current ratio as a function of
scan rate. The other redox responses are irreversible. In the
[Ru(PPh3)2(L)(CO)] (L = L1–L3) complexes, no systematic
variation in the redox potential with the electron-with-
drawing nature of the substituent R is observed, which is
consistent with the HOMOs and LUMOs, which show no
significant contributions from the R group.
Figure 7. Contour plots of selected molecular orbitals of [Ru(PH3)2-
(L1)(CO)].
signed to a mixture of ILCT and MLCT transitions that
primarily involve the ONC-coordinated aldimine ligand
with a much lower contribution from the carbonyl and
Catalytic Transfer Hydrogenation
The transfer hydrogenation of aldehydes and ketones,
phosphine ligands. The next absorption at 467 nm is also with 2-propanol as solvent and reducing agent, has recently
of a similar nature. The absorption bands at 341 and attracted considerable attention,[8] mostly due to the rela-
Table 4. Compositions of selected molecular orbitals of the complexes.
Complex
Fragments
Contribution [%] of fragments to
HOMO (H)
H–1
H–2
H–3
LUMO (L)
L+1
L+2
L+3
[Ru(PH3)2(L1)(CO)]
Ru
PH3
L1
20.6
5.7
72.5
1.2
20.1
7.3
72.6
0
33.5
0
44.3
22.2
16.5
0
82.5
1.0
4.7
2.0
90.8
2.5
25.8
56.3
10.9
7.0
14.2
1.0
67.8
17.0
22.2
41.0
12.8
24.0
CO
[Ru(PH3)2(L2)(CO)]
[Ru(PH3)2(L3)(CO)]
[Ru(PH3)2(L4)(CO)]
Ru
PH3
L2
19.5
8.0
69.3
3.2
23.5
0
76.5
0
30.0
0
46.1
23.9
9.8
0
90.2
0
5.3
0
88.1
6.6
25.4
56.8
9.2
14.2
2.4
66.6
16.8
14.6
0
68.6
16.8
CO
8.6
Ru
PH3
L3
20.2
8.1
70.6
1.1
22.2
0
77.8
0
30.4
0
46.1
23.5
19.0
0.6
78.3
2.1
0.7
0
94.6
4.7
25.5
58.1
9.0
13.9
0.4
69.8
15.9
33.7
14.3
12.9
39.1
CO
7.4
Ru
PH3
L4
14.8
10.4
71.2
3.6
20.4
0
79.6
0
27.0
0
49.3
23.7
5.8
0
93.9
0.3
6.1
0
91.5
2.4
5.6
0
94.4
0
26.9
60.6
9.1
14.5
5.2
60.3
20.0
CO
3.4
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tively benign and green nature of the reagents. Although Table 6. Catalytic transfer hydrogenation of aldehydes and
ketones.[a]
several transition-metal catalysts have been utilized for
these reactions, complexes of ruthenium(II) remain a favor-
ite choice.[9] Ruthenium-catalyzed transfer hydrogenation
reactions require the intermediacy of a ruthenium–hydrido
species, and hence complexes with a pre-existing Ru–H
bond, or with potential for the in situ formation of such a
bond, are suitable candidates for hydrogenation catalysis.
Given the propensity for the in situ generation of ruth-
enium–hydrido species in the present group of [Ru(PPh3)2-
(L)(CO)] complexes (see below), we wished to explore their
catalytic potential in the transfer hydrogenation of carbonyl
compounds. We began our study by examining the transfer
hydrogenation of benzophenone to benzhydrol using
[Ru(PPh3)2(L2)(CO)] as the catalyst precursor. Table 5 pro-
vides information on the impact of various reaction param-
eters on the efficiency of this process. After extensive opti-
mization we found that 0.1 mol-% catalyst, 0.1 mmol KOH,
2-propanol as solvent, a reaction temperature of 85 °C, and
a reaction time of 6 h furnished an excellent yield (100%) of
the targeted product (entry 1). Upon lowering the catalyst
loading, the reaction was found to be incomplete even after
15 h (entry 2) and decreasing the reaction time was also
found to have a deleterious effect (entry 3). As expected, the
desired reaction did not proceed at all in the absence of the
ruthenium catalyst (entry 4). The base also plays an essen-
tial role, with no reaction observed in its absence (entry 5).
KOH was found to be the most effective base, and its sub-
stitution by NaOH or K3PO4 led to a significant drop in
yield (entries 6 and 7). The choice of 2-propanol as solvent
was also found to be crucial as its replacement by ethanol
or poly(ethylene glycol) lowered the yield considerably (en-
tries 8 and 9).
Table 5. Screening of experimental conditions for the catalytic hy-
drogenation transfer.[a]
Entry
Catalyst
[mol-%]
Solvent
Base
Time
[h]
Yield[c] [%]
1
2
3
4
5
6
7
8
9
0.1
0.01
0.1
–
0.1
0.1
0.1
0.1
0.1
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
2-propanol
ethanol
KOH
KOH
KOH
KOH
–
NaOH
K3PO4
KOH
KOH
6
15
2
6
6
6
6
6
6
100
58
65
[d]
[d]
71
27
53
67
PEG
[a] Reaction conditions: benzophenone (1.0 mmol), base
(0.1 mmol), solvent (5 mL), 85 °C. [b] Catalyst: [Ru(PPh3)2-
(L2)(CO)]. [c] Determined by GC–MS. [d] No reaction observed.
The scope of these reactions is shown in Table 6. All four
[Ru(PPh3)2(L)(CO)] (L = L1–L4) complexes display com-
parable catalytic efficiency, and only the results obtained
with [Ru(PPh3)2(L2)(CO)] as catalyst are highlighted here.
[a] Reaction conditions: aldehyde or ketone (1.0 mmol), KOH
(0.1 mmol), 2-propanol (5 mL), 85 °C. [b] Determined by GC–MS.
[c] TON = turnover number [(mol of product)/(mol of catalyst)].
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Under the optimized conditions discussed above, benzophe-
In the light of the proposed mechanism, the lower yield
none was hydrogenated to benzhydrol in excellent yield (en- of the product alcohol with 1-naphthaldehyde as substrate
try 1). Acetophenone, p-chloroacetophenone, and para-sub- (Table 6, entry 7) is likely to be due to the steric inhibition
stituted benzaldehydes (R = H, Cl, OCH3) were also re- involved in the formation of the intermediate IM-3. The
duced with similar ease (entries 2–6). However, the re- low yield observed for the reduction of salicylaldehyde
duction of 1-naphthaldehyde was slightly less facile (en- (entry 8) and the absence of catalysis with pyridine- and
try 7) and the reduction of salicylaldehyde turned out to be pyrrole-2-carbaldehyde (entries 9 and 10) are probably due
rather difficult (entry 8), whereas pyridine- and pyrrole-2- to catalyst deactivation through the coordination of these
carbaldehyde did not undergo any reduction (entries 9 and substrates, their potential to form chelates being well
10). With cyclic aliphatic ketones, namely cyclohexanone known. The difficulty encountered in the reduction of
and cyclohex-2-en-1-one, as substrates, the yield of the methyl isobutyl ketone (entry 13) can be attributed to its
product alcohols decreased significantly (entries 11 and 12). similarity with the competing byproduct, namely acetone,
With cyclohex-2-en-1-one, notably, only the ketone frag- generated in the dehydrogenation of 2-propanol.
ment underwent reduction, with the alkene fragment re-
The present group of mixed-ligand cycloruthenated com-
maining unchanged. Methyl isobutyl ketone, an acyclic ali- plexes, namely [Ru(PPh3)2(L)(CO)] (L = L1–L4), have thus
phatic ketone, was reduced, but in very poor yield (en- been found to be efficient catalysts for the transfer hydro-
try 13).
genation of ketones and aldehydes using 2-propanol as the
Although the mechanism of the observed catalysis is not hydrogen donor. Variation of the substituents in the salicyl
yet completely clear to us, Scheme 2 shows the probable fragment (as in H2L1–H2L3), or substitution of the phenyl
series of events using [Ru(PPh3)2(L2)(CO)] as the catalyst group (in H2L2) by naphthyl (as in H2L4), has not been
precursor. In the initial step, the Ru–Cnaphthyl bond in the found to have any observable influence on the catalytic effi-
native catalyst is believed to undergo protonolysis with 2- ciency of these complexes. The observed catalytic activity
propanol to generate the intermediate IM-1, in which the of these complexes is comparable to those of some recently
isoproxide ligand is coordinated to the ruthenium through reported ruthenium(II) complexes.[11] The noteworthy as-
the oxygen atom. This species undergoes a β-hydride elimi- pects of the observed catalysis are 1) good hydrogenation
nation of the coordinated isoproxide ligand, which is quite yields, 2) low catalyst loading, 3) relatively mild reaction
usual,[10] to afford the hydrido species IM-2. Insertion of conditions, and 4) a short reaction time.
the carbonyl substrate into the Ru–H bond takes place next
to generate the corresponding aryloxo/alkoxo species IM-3. Conclusions
In the final step, elimination of the product alcohol takes
This study has shown that, upon interaction with
place with simultaneous regeneration of the catalyst precur-
[Ru(PPh3)2(CO)2Cl2], N-(naphthyl)salicylaldimine and re-
sor.
lated ligands H2L1–H2L4 readily undergo cycloruthenation
by C–H bond activation at the peri position to afford com-
plexes of the type [Ru(PPh3)2(L)(CO)] (L = L1–L4). This
study has also revealed that the Ru–Cnaphthyl bond breaks
in the presence of 2-propanol, and by virtue of this property
the [Ru(PPh3)2(L)(CO)] complexes can efficiently catalyze
the transfer hydrogenation of ketones and aldehydes.
Experimental Section
Materials: Ruthenium trichloride was obtained from Arora Mat-
they, Kolkata, India. 1-Naphthylamine was purchased from Loba
Chemie, Mumbai, India. Salicylaldehyde, 2-hydroxynaphth-
aldehyde, and triphenylphosphine were procured from Spectro-
chem, Mumbai, India. 5-Methoxysalicylaldehyde and 5-chloro-
salicylaldehyde were procured from Alfa Aesar. The Schiff base
ligands H2L1–H2L4 were prepared by reacting equimolar amounts
of arylamine and aldehyde in hot ethanol. [Ru(PPh3)2(CO)2Cl2]
was prepared by following a reported procedure.[12] Tetrabutylam-
monium hexafluorophosphate (TBHP) procured from Aldrich and
AR-grade acetonitrile procured from Merck (India) were used in
electrochemical work. All other chemicals and solvents were rea-
gent-grade commercial materials and used as received.
Physical Measurements: Microanalyses (C, H, and N) were per-
formed by using a Heraeus Carlo Erba 1108 elemental analyzer.
Magnetic susceptibilities were measured by using a Sherwood MK-
1 balance. NMR spectra were recorded in CDCl3 solution with a
Bruker Avance DPX 300 NMR spectrometer. IR spectra were re-
Scheme 2. Probable mechanism for the observed transfer hydrogen-
ation reaction.
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FULL PAPER
corded with a Perkin–Elmer Spectrum Two IR spectrometer with
samples prepared as KBr pellets. Electronic spectra were recorded
with a JASCO V-570 spectrophotometer. Electrochemical measure-
ments were conducted by using a CH Instruments model 600A
electrochemical analyzer. A platinum disc working electrode, a
platinum wire auxiliary electrode, and an aqueous saturated calo-
mel reference electrode (SCE) were used in the cyclic voltammetry
experiments. All electrochemical experiments were performed un-
der nitrogen. All electrochemical data were collected at 298 K and
are uncorrected for junction potentials. Geometry optimization by
the density functional theory (DFT) method and electronic spectral
analysis by TDDFT calculations were performed by using the
Gaussian 03 (B3LYP/SDD-6-31G) package.[7] The different species
in Scheme 1 were examined computationally by using the ONIOM
method of Morokuma et al. using the Gaussian 03 software
suite.[13] Here, all of the ruthenium-containing species were opti-
mized by a two-level approach with the phenyl groups of the PPh3
ligands treated as the lower of the two levels; all other compounds
depicted in the scheme were optimized by ab initio DFT methods.
For those species analyzed within the two-level treatment, we em-
ployed an ONIOM method that was defined by a B3LPY/PM6
composition. The phenyl groups (low level) were treated at the
semi-empirical PM6 level of theory, whereas the remaining atoms
(high level) were treated within the B3LYP framework. With re-
spect to the high-level treatment of atoms, the ruthenium atoms
were described by Stuttgart–Dresden effective core potentials (ecp)
and a SDD basis set, whereas the 6-31+G(dЈ) basis set was em-
ployed for the remaining atoms. All of the species in Scheme 1 fur-
nished fully optimized ground-state structures based on positive
eigenvalues obtained from the analytical Hessian. The computed
frequencies were used to make zero-point and thermal corrections
to the electronic energies. GC–MS analyses were performed with a
Perkin–Elmer CLARUS 680 instrument.
13C NMR: δ = 108.3, 111.6, 118.5, 120.8, 123.6, 124.6, 125.9, 126.6,
127.4, 128.4, 128.6, 129.2, 132.6, 133.6, 134.4, 135.9, 138.6, 139.1,
149.8, 157.1, 169.9, 207.6 ppm. 31P NMR: δ = 32.64 ppm. IR
(KBr): ν = 1900, 1603, 1550, 1521, 1482, 1435, 1396, 1233, 1146,
˜
1093, 813, 768, 742, 694, 609, 519, 498 cm–1. C54H41NO2P2Ru
(898.1): calcd. C 72.16, H 4.56, N 1.56; found C 72.09, H 4.59, N
1.57.
1
[Ru(PPh3)2(L3)(CO)]: Yield 23 mg, 37%. H NMR: δ = 6.10 (s, 1
H), 6.41 (d, J = 6.6 Hz, 1 H), 6.60 (t, J = 5.4 Hz, 1 H), 6.60 (t, J
= 5.4 Hz, 1 H), 6.73 (d, J = 6.6 Hz, 1 H), 6.82 (dt, 2 H)*, 6.97 (d,
J = 6.0 Hz, 1 H), 7.04–7.24 (2 PPh3 + 1 H)*, 7.27 (s, azomethine),
7.41 (d, J = 6.0 Hz, 1 H) ppm. 13C NMR: δ = 108.4, 115.3, 118.7,
121.1, 123.5, 125.8, 126.4, 126.6, 127.4, 128.5, 128.6, 129.3, 132.5,
133.4, 134.0, 134.2, 138.7, 138.9, 149.6, 155.9, 168.2, 207.8 ppm.
31P NMR: δ = 33.98 ppm. IR (KBr): ν = 1900, 1602, 1549, 1509,
˜
1482, 1435, 1395, 1230, 1156, 1094, 813, 767, 743, 695, 611, 518,
499 cm–1. C54H40ClNO2P2Ru (932.6): calcd. C 69.49, H 4.29, N
1.50; found C 69.51, H 4.21, N 1.48.
1
[Ru(PPh3)2(L4)(CO)]: Yield 32 mg, 51%. H NMR: δ = 6.63 (dt, 2
H)*, 6.77 (d + d, 2 H)*, 6.96–7.20 (2 PPh3), 7.10 (t, J = 4.5 Hz, 1
H), 7.24 (d, J = 6.9 Hz, 1 H), 7.36 (d + d, 2 H)*, 7.46 (t + t, 2
H)*, 7.68 (d + d, 2 H)*; 8.35 (s, azomethine) ppm. 13C NMR: δ =
108.3, 110.6, 118.8, 119.1, 120.7, 123.8, 125.0, 126.2, 126.4, 127.4,
127.9, 128.4, 128.6, 128.7, 129.3, 132.1, 132.2, 132.6, 134.4, 136.0,
138.4, 139.1, 150.4, 151.5, 171.1, 208.3 ppm. 31P NMR: δ =
34.71 ppm. IR (KBr): ν = 1901, 1615, 1575, 1529, 1482, 1434, 1395,
˜
1238, 1191, 1161, 1093, 827, 812, 767, 742, 694, 609, 518, 497 cm–1.
C58H43NO2P2Ru (948.1): calcd. C 73.42, H 4.53, N 1.48; found C
73.51, H 4.50, N 1.52.
X-ray Crystallography: Single crystals of [Ru(PPh3)2(L1)(CO)],
[Ru(PPh3)2(L2)(CO)], and [Ru(PPh3)2(L4)(CO)] were obtained by
slow evaporation of solvents from solutions of the respective com-
plexes in dichloromethane/acetonitrile (1:3). Selected crystal data
and data collection parameters for [Ru(PPh3)2(L1)(CO)] are given
in Table 7, and those for [Ru(PPh3)2(L2)(CO)] and [Ru(PPh3)
Preparations of Complexes: The [Ru(PPh3)2(L)(CO)] (L = L1–L4)
complexes were prepared by following a general procedure. Specific
details are given below for [Ru(PPh3)2(L1)(CO)].
Table 7. Crystallographic data for [Ru(PPh3)2(L1)(CO)].
[Ru(PPh3)2(L1)(CO)]: Triethylamine (14 mg, 0.14 mmol) was added
to a solution of H2L1 (19 mg, 0.07 mmol) in hot 2-methoxyethanol
(20 mL) followed by [Ru(PPh3)2(CO)2Cl2] (50 mg, 0.07 mmol). The
solution was then heated at reflux for 7 h to yield an orange solu-
tion. The solvent was removed under vacuum to afford a solid
mass, which was subjected to purification by TLC on a silica plate.
With hexane/benzene (1:1) as eluent, an orange band was sepa-
rated, which was extracted with acetonitrile. Upon evaporation of
the acetonitrile extract, [Ru(PPh3)2(L1)(CO)] was obtained as a
crystalline orange solid.
Empirical formula
Mr
C55H43NO3P2Ru
928.91
Crystal system
Space group
triclinic
P1
¯
a [Å]
b [Å]
c [Å]
α [°]
9.3791(4)
12.6129(6)
20.240(1)
91.155(4)
101.334(3)
107.804(3)
2226.9(2)
2
β [°]
γ [°]
V [Å3]
[Ru(PPh3)2(L1)(CO)]: Yield 27 mg, 43%. 1H NMR:[14] δ = 3.57
(OCH3), 5.61 (s, 1 H), 6.42 (d, J = 5.7 Hz, 1 H), 6.58 (dt, 2 H)*,
6.79 (t, J = 3.9 Hz, 1 H), 6.82 (d, J = 4.8 Hz, 1 H), 6.96 (d, J =
4.8 Hz, 1 H), 7.04–7.22 (2 PPh3 + 1 H)*, 7.28 (s, azomethine), 7.38
(d, J = 4.8 Hz, 1 H) ppm. 13C NMR: δ = 56.5, 108.3, 111.4, 116.0,
117.9, 119.0, 120.5, 123.6, 124.7, 125.8, 126.6, 127.4, 129.2, 132.7,
134.4, 138.6, 139.0, 147.0, 150.2, 156.3, 166.1, 168.0, 208.2 ppm.
Z
Dcalcd. [mgm–3]
F(000)
1.385
956
λ [Å]
0.71073
0.21ϫ0.24ϫ0.25
296
Crystal size [mm3]
T [K]
μ [mm–1]
0.470
31P NMR: δ = 32.53 ppm. IR (KBr): ν = 1897, 1592, 1548, 1521,
Collected reflections
Rint
38857
0.107
11030
0.0557
0.1636
0.95
˜
1482, 1457, 1435, 1393, 1260, 1234, 1149, 1093, 813, 768, 741, 694,
611, 518, 498 cm–1. C55H43NO3P2Ru (928.1): calcd. C 71.12, H
4.63, N 1.51; found C 71.09, H 4.71, N 1.52.
Independent reflections
R1[a]
wR2[b]
GOF[c]
1
[Ru(PPh3)2(L2)(CO)]: Yield 23 mg, 39%. H NMR: δ = 5.95 (t, J
= 4.2 Hz, 1 H), 6.15 (d, J = 4.8 Hz, 1 H), 6.48 (d, J = 5.1 Hz, 1
H), 6.58 (t, J = 4.5 Hz, 1 H), 6.79 (d, J = 3.9 Hz, 1 H), 6.85 (dt, 2
H)*, 6.97 (d, J = 4.8 Hz, 1 H), 7.04–7.22 (2 PPh3), 7.11 (t, J =
2
2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σ{w(Fo – Fc ) }/
Σ{w(Fo2)}]1/2. [c] GOF = [Σ{w(Fo2 – Fc ) }/(M – N)]1/2, in which M
is the number of reflections and N is the number of parameters
2 2
4.6 Hz, 1 H), 7.36 (s, azomethine), 7.39 (d, J = 4.8 Hz, 1 H) ppm. refined.
Eur. J. Inorg. Chem. 0000, 0–0
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2(L4)(CO)] are shown in Table S7 in the Supporting Information.
Data on all the crystals were collected with a Bruker SMART CCD
diffractometer. X-ray data reduction, structure solution, and refine-
ment were performed by using the SHELXS-97 and SHELXL-97
packages.[15] The structures were solved by direct methods. During
refinement of the structure of [Ru(PPh3)2(L2)(CO)], the lattice sol-
vent molecules were found disordered and thus the SQUEEZE
command of PLATON was applied before final solution of the
structure.[16]
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[2]
CCDC-989098 (for [Ru(PPh3)2(L1)(CO)]), -989099 (for [Ru(PPh3)
2(L2)(CO)]), -989100 (for [Ru(PPh3)2(L4)(CO)]) contain the supple-
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obtained free of charge from The Cambridge Crystallographic
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General Procedure for the Catalytic Transfer Hydrogenation Reac-
tions: In a typical run, an oven-dried 10 mL round-bottomed flask
was charged with the aldehyde/ketone (1 mmol), a known mol per-
cent of the catalyst, and KOH (0.1 mmol) dissolved in 2-propanol
(5 mL). The flask was placed in a preheated oil bath at the required
temperature. After the specified time, the flask was removed from
the oil bath and water (20 mL) was added, neutralized with 1 m
HCl, and extracted with diethyl ether (4ϫ10 mL). The combined
organic layers were washed with water (3ϫ10 mL), dried with an-
hydrous Na2SO4, and filtered. Diethyl ether was removed under
vacuum and the residue obtained dissolved in hexane and analyzed
by GC–MS.
Supporting Information (see footnote on the first page of this arti-
cle): Selected bond lengths and bond angles for [Ru(PPh3)2-
(L2)(CO)] and [Ru(PPh3)2(L4)(CO)], some computed bond lengths
and bond angles for the DFT-optimized structure of [Ru(PPh3)2-
(L3)(CO)], results of TDDFT calculations, crystallographic data for
[Ru(PPh3)2(L2)(CO)] and [Ru(PPh3)2(L4)(CO)], electronic spectrum
of [Ru(PPh3)2(L1)(CO)], contour plots of selected molecular orbit-
als of [Ru(PPh3)2(L)(CO)] (L = L2–L4), cyclic voltammograms of
[Ru(PPh3)2(L2)(CO)], and X-ray crystallographic data in CIF for-
mat.
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Acknowledgments
The authors thank the reviewers for their constructive comments,
which have been very helpful in preparing the revised manuscript.
Financial assistance received from the Department of Science and
Technology, Government of West Bengal, Kolkata [sanction
number 746(Sanc.)/ST/P/S&T/2G-4/2013] is gratefully acknowl-
edged. Financial support from the Robert A. Welch Foundation
(grant number B-1093-MGR) and the National Science Founda-
tion (NSF) (CHE-0741936) is acknowledged. J. D. thanks the
Council of Scientific and Industrial Research (CSIR), New Delhi,
for a fellowship [grant number 09/096(0702)/2011-EMR-I]. The au-
thors thank Dr. Debajyoti Ghoshal and Dr. Pallab Mondal of Ja-
davpur University for their help with the crystal structure determi-
nation and TDDFT calculations, respectively.
M. J. Frisch, G. W. Tracks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman Jr., J. A. Montgomery, T.
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J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalamani, N.
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Cycloruthenated Complexes
J. Dutta, M. G. Richmond,
S. Bhattacharya* ............................. 1–12
Cycloruthenation of N-(Naphthyl)salicyl-
aldimine and Related Ligands: Utilization
of the Ru–C Bond in Catalytic Transfer
Hydrogenation
N-(Naphthyl)-4-R-salicylaldimines (R
=
afford complexes of the type [Ru(PPh3)2-
(L)(CO)] (L = L1–L4), which can efficiently
catalyze the transfer hydrogenation of car-
bonyl compounds.
Keywords: Homogeneous catalysis / C–H
activation / Hydrogenation / Ruthenium
OCH3, H, Cl; H2L1–H2L3) and 2-hydroxy-
N-(naphthyl)naphthaldimine (H2L4) react
with [Ru(PPh3)2(CO)2Cl2], undergoing C–
H bond activation at the peri position, to
Eur. J. Inorg. Chem. 0000, 0–0
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