Half sandwich complexes of chalcogenated pyridine based bi-(N, S/Se) and terdentate (N, S/Se, N) ligands with (η6-benzene)ruthenium(ii): Synthesis, structure and catalysis of transfer hydrogenation of ketones and oxidation of alcohols

Article abstract of DOI:10.1039/c3dt00126a

The half sandwich complexes [(η⁶-C₆H ₆)Ru(L)Cl][PF₆] (1-5) have been synthesized by the reactions of (2-arylchalcogenomethyl)pyridine [L = L1-L3] and bis(2-pyridylmethyl)chalcogenide [L = L4-L5] (chalcogen = S, Se; Ar = Ph/2-pyridyl for S, Ph for Se) with [(η⁶-C₆H ₆)RuCl₂]₂, at room temperature followed by treatment with NH₄PF₆. Their HR-MS, ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR spectra have been found characteristic. The single crystal structures of 1-5 have been established by X-ray crystallography. The Ru has pseudo-octahedral half sandwich "piano-stool" geometry. The complexes 1-5 have been found efficient for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide (NMO) and transfer hydrogenation of ketones with 2-propanol (at moderate temperature 80 °C) as TON values are up to 9.9 × 10³ and 9.8 × 10³ respectively for the two catalytic reactions. On comparing the required catalyst loading for good conversions and reaction time for the present complexes with those reported in literature for other transfer hydrogenation/oxidation catalysts, it becomes apparent that 1-5 have good promise. The complexes of Se ligands have been found more efficient than their sulphur analogues. The complexes of bidentate ligands are more efficient than those of terdentate, due to difficult bond cleavage in the case of latter. These orders of efficiency are supported by DFT calculations. The calculated bond lengths/angles by DFT are generally consistent with the experimental ones.

Full text of DOI:10.1039/c3dt00126a

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PAPER
Half sandwich complexes of chalcogenated pyridine
based bi-(N, S/Se) and terdentate (N, S/Se, N) ligands
with (η6-benzene)ruthenium(II): synthesis, structure
and catalysis of transfer hydrogenation of ketones and
oxidation of alcohols†
Cite this: DOI: 10.1039/c3dt00126a
Om Prakash, Kamal Nayan Sharma, Hemant Joshi, Pancham Lal Gupta and
Ajai K. Singh*
The half sandwich complexes [(η6-C6H6)Ru(L)Cl][PF6] (1–5) have been synthesized by the reactions of
(2-arylchalcogenomethyl)pyridine [L = L1–L3] and bis(2-pyridylmethyl)chalcogenide [L = L4–L5] (chalco-
gen = S, Se; Ar = Ph/2-pyridyl for S, Ph for Se) with [(η6-C6H6)RuCl2]2, at room temperature followed by
treatment with NH4PF6. Their HR-MS, 1H, 13C{1H} and 77Se{1H} NMR spectra have been found character-
istic. The single crystal structures of 1–5 have been established by X-ray crystallography. The Ru has
pseudo-octahedral half sandwich “piano-stool” geometry. The complexes 1–5 have been found efficient
for catalytic oxidation of alcohols with N-methylmorpholine-N-oxide (NMO) and transfer hydrogenation
of ketones with 2-propanol (at moderate temperature 80 °C) as TON values are up to 9.9 × 103 and
9.8 × 103 respectively for the two catalytic reactions. On comparing the required catalyst loading for
good conversions and reaction time for the present complexes with those reported in literature for other
transfer hydrogenation/oxidation catalysts, it becomes apparent that 1–5 have good promise. The com-
plexes of Se ligands have been found more efficient than their sulphur analogues. The complexes of
bidentate ligands are more efficient than those of terdentate, due to difficult bond cleavage in the case
of latter. These orders of efficiency are supported by DFT calculations. The calculated bond lengths/
angles by DFT are generally consistent with the experimental ones.
Received 14th January 2013,
Accepted 28th March 2013
DOI: 10.1039/c3dt00126a
www.rsc.org/dalton
[η6-(p-cymene)RuCl(2-aminomethyl)pyridine]Cl,4 (iii) [η6-(arene)
RuCl(picolinate ligands]Cl,5 (iv) [η6-(C6H6)Ru (phenylpyridine)
Introduction
The substituted pyridines are attractive as ligands.1 The ruthe- NCMe]PF6,6 (v) [η6-(C6H6)Ru(2,2′-bipyridyl)(4-vinylpyridine)]-
7
nium(II) complexes of such ligands have also received attention (BF4)2 and (vi) ruthenium complexes of other related hetero-
in the recent past.2 However, pyridine based ligands explored cycle based ligands.8 Out of which only (ii) and (iii) have been
so far for designing Ru(II) half-sandwich complexes suitable explored for transfer hydrogenation. The source of ruthenium
for transfer hydrogenation and oxidation of alcohols are not for all these half-sandwich complexes is a dimer [(η6-C6H6/
large in number. Some known examples of Ru(II) half-sand- p-cymene)RuCl2]2 which undergoes a rich variety of chemistry
wich complexes having pyridine, its derivatives or related via intermediary chloro bridge cleavage reaction, leading to
skeleton based ligands are: (i) [η6-(C6H6)RuCl2(picoline)],3 (ii) the formation of a series of interesting neutral and cationic
mononuclear complexes.9 Half-sandwich ruthenium(II) com-
plexes have shown in the recent past promise for applications
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016,
to a variety of catalytic processes, such as asymmetric catalysis
India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;
in Diels–Alder reactions,10 asymmetric transfer hydrogenation
Fax: +91-01-26581102; Tel: +91-011-26591379
†Electronic supplementary information (ESI) available: Crystal data and refine-
of ketones,11 hydration of nitriles12 and 1,3-dipolar cyclo-
addition reactions of nitrones with methacrolein.13 Our
ment parameters (Table S1), bond lengths and angles (Table S2), HOMO–LUMO
energy gap and partial charges (Table S3), distances of non-covalent interactions
research group has reported that half-sandwich ruthenium(II)
(Tables S4 and S5), non-covalent interactions (Fig. S1–S9), and NMR spectra
complexes of organochalcogen ligands work as efficient cata-
(Fig. S10–S21). CCDC 861152, 861154, 861153, 918663 and 918664 (for 1 to 5
lysts for transfer hydrogenation of ketones and oxidation of
respectively). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c3dt00126a
alcohols.14 The ligands incorporating both hard and soft
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synthetic procedures reported earlier.16 The procedures used
for synthesis of L4 and L5 are distinct from those known
earlier17 and give relatively high yields, up to 95%. All the
ligands stable in air and moisture show good solubility in
common organic solvents viz., CHCl3, CH2Cl2, CH3OH and
CH3CN. The complexes 1–5 (Yield 87–90%), have been syn-
thesized by reacting [(η6-C6H6)RuCl2]2 at room temperature in
methanol with L1–L5 in a 1 : 2 molar ratio, respectively
(Scheme 1). The complexes (1–5) are moderately soluble in
CHCl3, CH2Cl2 and CH3OH, but their solubility is good in
CH3CN. The solutions of complexes 1–5 made in CH3CN
remain stable for several months under ambient conditions.
Chart 1
donors (which sometimes may be hemilabile) are important
for catalysis15 as properties of metal complexes can be fine-
tuned by such a combination. However, there is no report on
mononuclear half-sandwich ruthenium(II) complexes with (N,
S/Se) and (N, S/Se, N) ligands based on pyridine skeleton. Such
complexes are of further importance to understand that out of NMR spectra
bidentate and terdentate ligands having similar donor sites,
NMR spectra of ligands L1–L5 and complexes 1–5 are consist-
which is superior to design catalysts for transfer hydrogenation
of ketones and oxidation of alcohols. Herein, we report the
synthesis of complexes of η6-(benzene)ruthenium(II) with chal-
cogenated pyridine ligands (L1–L5; see Chart 1) and their
single crystal structural aspects. The applications of these
ruthenium(II) complexes 1–5 in catalyzing transfer hydrogen-
ation of ketones with 2-PrOH and oxidation of alcohols with
N-methylmorpholine-N-oxide (NMO) have been found promis-
ing. Bidentate (Se, N) type ligand gives complexes of better
efficiency. All these results are given in the present paper.
Density functional theory (DFT) calculations have been made
and appear to support experimental results (both catalysis and
structural aspects).
ent with their structures shown in Scheme 1 and single-crystal
structures of 1–5 determined with X-ray diffraction. The signal
in 77Se{1H} NMR spectrum of 2 appears shifted to a higher fre-
quency by ∼25 ppm in comparison to that of free L2 as Se is
coordinated to Ru center. In the case of complex 5 the signal
in 77Se{1H} NMR spectrum has been observed shifted to a
lower frequency by ∼32 ppm with respect to that of free L5.
This may be due to its pincer type strong binding resulting
shorter Ru–Se bond than that of 2 (see crystal structure below).
1
In H and 13C{1H} NMR spectra of 1–5 signals of protons and
carbon atoms generally appear at higher frequency relative to
those of free ligands which coordinate with Ru in a bidentate
(N, S/Se) (1–3) or terdentate (N, S/Se, N) (4–5) mode. The mag-
nitude of shift to higher frequency is high for CH2(E) (E = S/Se)
up to 10.6 ppm in 13C{1H) NMR and 0.87 ppm for protons
attached to them. These observations imply that there is a dis-
persion of electron density from ligands L1–L5 to ruthenium.
Results and discussion
The syntheses of L1–L5 and their complexes have been sum-
marized in Scheme 1. The L1–L3 have been prepared by
1
In H and 13C{1H} NMR spectra of 1–5 the signals (singlet) of
η6-benzene appear to be shifted to lower frequency (up to 0.3
and 3.3 ppm respectively) with respect to those of [η6-(C6H6)
RuCl2]2. This occurs due to substitution of Cl with S and Se,
which have relatively lower electronegativity and are stronger
donors.
Crystal structures
Crystals of 1–5 suitable for X-ray crystallographic diffraction
were obtained by slow diffusion of diethyl ether into concen-
trated solutions of the complexes made in a methanol–aceto-
nitrile mixture (1 : 3 v/v). The crystallographic and refinement
data for 1–5 are summarized in Table S1 of ESI.† The L1, L2
and L3 exhibit a similar bonding mode in all complexes 1–3.
The five member ring is formed by their coordination to the
metal center through pyridine nitrogen and chalcogen. The L4
and L5 form two five member rings on their coordination with
Ru centre, via Se/S and N donor sites of two pyridine rings.
The ORTEP diagrams of cations of 1–5 are given in Fig. 1–5
with selected bond lengths and angles. In each cation of these
complexes there is a pseudo-octahedral half-sandwich “piano-
stool” type disposition of donor atoms around Ru center. The
centroid of the η6-benzene ring occupies the center of three
octahedral sites. The nitrogen and chalcogen atoms of L1–L5
Scheme 1
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Fig. 4 ORTEP diagram of the cation of 4 with ellipsoids at the 30% probability
−
level. Hydrogen atoms and the PF6 anions have been omitted for clarity. Bond
Fig. 1 ORTEP diagram of the cation of 1 with ellipsoids at the 30% probability
−
lengths (Å): Ru–S(1) 2.3133(15), Ru–N(1) 2.098(4), Ru(1)–N(2) 2.111(5). Bond
angles (°): N(1)–Ru(1)–S(1) 83.02(13), N(2)–Ru(1)–S(1) 83.13(14).
level. Hydrogen atoms and the PF6 anions have been omitted for clarity. Bond
lengths (Å): Ru–S(1) 2.3771(18), Ru–N(1) 2.095(5), Ru(1)–Cl(1) 2.3876(19). Bond
angles (°): S(1)–Ru(1)–N(1) 80.49(15), S(1)–Ru(1)–Cl(1) 93.39(6).
Fig. 5 ORTEP diagram of the cation of 5 with ellipsoids at the 30% probability
−
level. Hydrogen atoms and the PF6 anions have been omitted for clarity. Bond
lengths (Å): Ru(1)–Se(1) 2.4252(9) N(1)–Ru(1) 2.123(6) N(2)–Ru(1) 2.105(5).
Bond angles (°): N(1)–Ru(1)–Se(1) 83.88(16) N(2)–Ru(1)–Se(1) 83.70(15).
Fig. 2 ORTEP diagram of the cation of 2 with ellipsoids at the 40% probability
−
level. Hydrogen atoms and the PF6 anions have been omitted for clarity. Bond
lengths (Å): Ru–Se(1) 2.4879(7), Ru–N(1) 2.101(5), Ru(1)–Cl(1) 2.3955(15). Bond
angles (°): Se(1)–Ru(1)–N(1) 81.49(12), Se(1)–Ru(1)–Cl(1) 92.78(4).
(η6-benzene)Ru(II) and (η6-p-cymene)Ru(II) with selenated
pyrrolidine/morpholine derivative have been reported by our
research group recently in which Ru is coordinated through Se
and N forming a five membered chelate ring.14 The 2 and 5 are
new additions to the small family of Ru–selenoether com-
plexes. The Ru–C distances in the cations of 1–5 are
normal.14,18 The Ru–S bond lengths of 1 and 3 are within the
range [2.3548(15)–2.4156(9) Å] in which such a bond length of
several species has been reported.14,19 The Ru–Se bond length
of cation of 2 [2.4879(7) Å] also falls in the range 2.4756(10)–
2.5240(9) Å in which such a bond length of Ru(II)-complexes14
and Ru–Se clusters20 has been reported. The Ru–S [2.3133(15) Å]
and Ru–Se [2.4252(9) Å] bond lengths in complex 4 and 5 are
comparatively shorter than those of 1 and 2 due to a strong
−
pincer type bonding mode of L4 and L5. The PF6 has been
Fig. 3 ORTEP diagram of the cation of 3 with ellipsoids at the 50% probability
found to be involved in C–H⋯F secondary interactions result-
ing in chains in complexes 1–5. In Fig. 6 and 7 these are
shown for complex 2 and 5 along with the distances respecti-
vely. The secondary interactions for other complexes are
shown in the ESI (Fig. S1 and S2†). More C–H⋯F distances (Å)
−
level. Hydrogen atoms and the PF6 anions have been omitted for clarity. Bond
lengths (Å): Ru–S(1) 2.3740(9), Ru–N(1) 2.096(3), Ru(1)–Cl(1) 2.3989(9). Bond
angles (°): S(1)–Ru(1)–N(1) 80.82(8), S(1)–Ru(1)–Cl(1) 93.09(3).
and chlorine complete the coordination sphere. The half sand- of 1–5 are given in Table S4 (ESI†). The crystals of 1–3 also
have C–H⋯Cl and C–H⋯π secondary interactions (For details
see ESI Table S5 and Fig. S3–S8†).
wich Ru(II)-complexes of selenoether ligands are less investi-
gated than their sulphur analogues. The complexes of
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efficient catalysts among the five complexes. The 77Se{1H}
NMR spectra were recorded in the course of transfer hydrogen-
ation reactions catalyzed with 2 and 5. The signals in the
spectra were found shifted to higher frequency (∼25 and
∼21 ppm for 2 and 5 respectively), indicating that probably the
Ru–Cl/N bond (2/5) is cleaved or weakened very significantly to
make a coordination site on metal centre available so that for-
mation of an intermediate having a Ru–H bond can take
1
place.24 The H NMR spectra were also recorded during trans-
fer hydrogenation reactions catalyzed with 2. After 1 h a broad
singlet was noticed around δ −10.5/δ −8.9 ppm (2/5) which
being characteristic of metal–hydrides, indicates the formation
of a Ru–H bond.25 Thus catalytic reactions with the present
complexes probably proceed via formation of metal–hydride
complex intermediate as suggested for conventional
mechanism.26
Fig. 6 Non-covalent C–H⋯F interactions in 2.
The catalytic efficiency generally varies with chalcogen
ligands in the order of Se > S, when other co-ligands and sub-
stituent on donor atoms are same. It may be ascribed to a
stronger electron-donating tendency of Se towards the metal
centre than sulfur, thus promoting the formation of hydride
with it. This order of catalytic reactivity was supported by DFT
calculation as HOMO–LUMO energy gap is lower for 2 than 1
and 3. When catalytic efficiencies of 1 and 2 were compared
with those of 4 and 5, it was found that the complexes of
bidentate ligands were better than terdentates. In case of
bidentate ligands probably the easy cleavage of Ru–Cl bond
facilitates the catalysis. The DFT calculations also support this
reactivity order as HOMO–LUMO energy gap for 1 and 2 is
lower than that of 4 and 5 (see below DFT studies). The per-
formance of 2, best among the five present complexes has
been compared with the earlier reported ruthenium(II) based
homogeneous catalysts for transfer hydrogenation. The temp-
erature used in all cases is around 80 °C. For three other par-
ameters important to judge catalyst performance viz., catalyst
loading required, conversion (%) and reaction time, the
complex 2 gives a very good combination. The air-sensitive
Fig. 7 Non-covalent C–H⋯F interactions in 5.
Catalytic transfer hydrogenation
Transfer hydrogenation reactions involve transfer of hydrogen
from one organic molecule to another and are very important
in organic synthesis because the use of inflammable molecular
hydrogen is avoided.21 Such catalytic reactions many times
necessitate high temperature and inert atmosphere.22 There-
fore catalysts working under mild conditions continue to be of
current interest. Transfer hydrogenation reactions of ketones
(Scheme 2) can be catalyzed with the present complexes
(0.01 mol% for 1–3 and 0.1 mol% for 4–5) at a moderate temp-
erature 80 °C. The KOH known as best inorganic base for such
transformations23 with 2-propanol as hydrogen donor has
been used in the present catalytic transfer hydrogenation. The
control reactions carried out in the absence of catalyst for
2–3 h result in 5–8% conversions only. The most efficient con-
versions (up to 98%) were found in the case of acetophenone
with all the catalysts; while in the case of aliphatic ketones the
conversions were up to 93%. The values of percent conversions
and TONs are given in Table 1. The complex 2 is the most
phosphine–anilido
complexes
[RuR(η6-p-cymene)
(P,
N-Ph2PAr−)] (R = H, Et; Ar− = o-C6H4NMe−)27 for 0.01 mol%
loading give less than 50% conversions even in 2 h for aceto-
phenone which generally gives the highest conversion among
ketones. The catalysis with complex of pincer ligand Ru
(SNStBu)Cl2(MeCN) [SNStBu = 2,6-bis(tert-butylthiomethyl)pyri-
dine] takes shorter time but for a catalyst loading of 0.1 mol%.
For a related complex Ru(SNStBu)Cl2(PPh3), 7 h reaction time
gives only 85% conversion.28 Ruthenium(II) complexes bearing
pyridine-based tridentate ligands and 2-(aminomethyl)pyri-
dine show good conversions in lower reaction time4a than that
of complex 2 but catalyst loading required is 0.1–1.0 mol%.
Several other Ru-complexes have been reported for catalysis of
transfer hydrogenation but in comparison with 2 either they
require higher catalyst loading or reaction time. The [(η6-arene)-
Ru(N,N)Cl]Cl (arene = p-cymene, C6Me6; N,N = bipyridyl with
OH, OMe, or H at the 6- and 6′-positions) is reported29 to give
100% conversion in 24 h and at 1 mol% catalyst loading. The
Ru(II) complexes with piperidine based ligands4b have been
Scheme 2
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Table 1 Transfer hydrogenation of ketonesa
TON (% conversion)
Substrate
Catalyst mol%
0.01/0.1
1
2
3
4
5
8.9 × 103 (89)
9.3 × 103 (93)
8.8 × 103 (88)
9.0 × 102 (90)
9.4 × 102 (94)
0.01/0.1
0.01/0.1
0.01/0.1
0.01/0.1
8.8 × 103 (88)
9.5 × 103 (95)
9.0 × 103 (90)
9.1 × 103 (91)
9.2 × 103 (92)
9.8 × 103 (98)
9.6 × 103 (96)
9.6 × 103 (96)
8.6 × 103 (86)
9.2 × 103 (92)
9.0 × 103 (90)
9.0 × 103 (90)
8.5 × 102 (85)
9.4 × 102 (94)
9.2 × 102 (92)
9.2 × 102 (92)
9.2 × 102 (92)
9.8 × 102 (98)
9.6 × 102 (96)
9.6 × 102 (96)
0.01/0.1
0.01/0.1
9.0 × 103 (90)
9.0 × 103 (90)
9.4 × 103 (94)
9.6 × 103 (96)
8.5 × 103 (85)
8.8 × 103 (88)
8.8 × 102 (88)
9.0 × 102 (90)
9.4 × 102 (94)
9.5 × 102 (95)
0.01/0.1
0.01/0.1
8.2 × 103 (82)
8.5 × 103 (85)
8.6 × 103 (86)
8.8 × 103 (88)
8.0 × 103 (80)
8.1 × 103 (81)
8.0 × 102 (80)
8.6 × 102 (86)
8.5 × 102 (85)
8.8 × 102 (88)
a Catalyst: 0.01 mol% for 1, 2 and 3; 0.1 mol% for 4 and 5; conversion in control reactions (in absence of catalyst and under optimum conditions)
5–8%.
explored to catalyze transfer hydrogenation and for good con- percent conversions and TON values are given in Table 2. On
versions 4 h reaction time with 1 mol% catalyst loading are the basis of earlier work31–39 and some of the observations
required. Half sandwich complexes14a of arenes with chalcoge- made by us, the mechanism of the catalytic oxidation appears
nated Schiff bases and related ligands require lower catalyst to involve RuIVvO species that seems to be formed by oxygen
loading but with 7 h reaction time only. [Ru(arene)-Cl(N,N)]- free radicals (generated by heterolytic cleavage of NMO). The
BPh4 (arene = benzene, p-cymene, N,N = bis(pyrazol-1-yl)- addition of AIBN [azobis(isobutyronitrile),
a free radical
methane)30 can give more than 95% conversions but require a initiator] to the oxidation of benzyl alcohol in the presence of
reaction time of 24 h at 0.004 mol% loading.
2 (most efficient catalyst) with NMO under optimized reaction
conditions resulted in enhanced conversion (up to 94%).
Further addition of AIBN to the same oxidation reaction in the
absence of 2 did not result in any oxidation. In the presence of
benzoquinone (a free-radical inhibitor) the conversion was
minimal. These observations are consistent with those made
by Goldstein and Drago with H2O2 and OCl in the oxidation of
alkanes.39 When NMO was added to the solution of 2 or 5, the
signals in their 77Se{1H} NMR spectra were found shifted to a
higher frequency (473 and 454 ppm respectively). The signal in
Oxidation of alcohols
The complexes 1–5 have been studied for the catalytic oxi-
dation of primary and secondary alcohols with N-methylmor-
pholine-N-oxide (NMO). Maximum conversions were reached
in 2 h with 2 and in 3 h with 1 or 3–5 (Scheme 3). However, in
the absence of catalyst the conversions were low. When reac-
tions were carried out for the time found optimum in the pres-
ence of the catalyst, the conversions noticed were <5%. The
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1–5 on addition of NMO to their solutions made in dichloro-
methane. This further supports that Ru(IV) species is most
probably responsible for the transfer of oxygen atom to the
alcohol substrates resulting in their oxidation. IR spectra of
the residues left after evaporating the solvent from the mix-
tures of NMO with complexes 1–5 exhibit very strong bands at
844–857 cm−1. As νP–F band at 837–845 cm−1 is of medium
intensity only, the strong overlapping band due to the for-
mation of RuIVvO species36,37,41 is probably responsible for
Scheme 3
the 77Se{1H} NMR spectrum of L2 or L5 on other hand the enhancement of intensity as well as the minor shift. Thus
remains unshifted on addition of NMO. This rules out that the in present catalytic oxidation of alcohols RuIVvO species are
reason of this high frequency shift as selenoxide formation. likely to be involved. The fact that catalytic activities of
Therefore, the ruthenium center is most probably oxidized to RuIVvO species towards oxidation have been reported
Ru(IV). A new shoulder at 390–394 nm, which is reported36–41 earlier35–39,42 further strengthens their involvement in the
to be due to RuIVvO species appears in the UV/Vis spectra of present case. The 1H NMR spectra of present ruthenium
Table 2 Oxidation of alcoholsa
TON (% conversion)
Substrate
Catalyst mol%
0.01/0.1
1
2
3
4
5
9.1 × 103 (91)
9.4 × 103 (94)
8.6 × 103 (86)
9.0 × 102 (90)
9.4 × 102 (94)
0.01/0.1
9.2 × 103 (92)
9.6 × 103 (96)
8.8 × 103 (88)
9.2 × 102 (92)
9.6 × 102 (96)
0.01/0.1
0.01/0.1
8.3 × 103 (83)
9.0 × 103 (90)
8.7 × 103 (87)
9.3 × 103 (93)
7.8 × 103 (78)
8.4 × 103 (84)
8.5 × 102 (85)
9.0 × 102 (90)
8.8 × 102 (88)
9.4 × 102 (94)
0.01/0.1
9.3 × 103 (93)
9.6 × 103 (96)
8.8 × 103 (88)
9.2 × 102 (92)
9.6 × 102 (96)
0.01/0.1
0.01/0.1
8.8 × 103 (88)
9.0 × 103 (90)
9.1 × 103 (91)
9.2 × 103 (92)
8.0 × 103 (80)
8.5 × 103 (85)
8.8 × 102 (88)
9.0 × 102 (90)
9.2 × 102 (92)
9.4 × 102 (94)
0.01/0.1
8.5 × 103 (85)
8.9 × 103 (89)
8.0 × 103 (80)
8.2 × 102 (82)
8.9 × 102 (89)
0.01/0.1
0.01/0.1
8.6 × 103 (86)
9.6 × 103 (96)
9.0 × 103 (90)
9.9 × 103 (99)
8.2 × 103 (82)
9.1 × 103 (91)
8.6 × 102 (86)
9.5 × 102 (95)
9.1 × 102 (91)
9.9 × 102 (99)
a Catalyst: 0.01 mol% for 1, 2 and 3; 0.1 mol% for 4 and 5; conversion in control reactions (in absence of catalyst and under optimum conditions) <5%.
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complexes recorded after the addition of NMO were found ligands is enough to catalyze conversion of alcohols to
broadened, indicating the formation of paramagnetic species ketones. In their comparison the reaction time of present com-
and supporting indirectly the involvement of RuIVvO species plexes is somewhat short. Thus efficiencies of 2 and 5 can be
in the present catalytic oxidation. Like transfer hydrogenation, considered promising when comparison is made with Ru(II)
Se ligand containing complexes are more efficient in catalyzing based catalysts reported in the literature.
oxidation than their S analogues containing similar co-
Density functional theory calculations
ligands, as Se being softer than S facilitates the formation of
RuIVvO species. On other hand bidentate complexes have
In an attempt to understand further the nature of bonding
been found more efficient than the corresponding terdentate
complexes. It is probably due to facile cleavage of Ru–Cl bond,
needed to form oxo species. This order has been corroborated
by DFT calculation as given below. It is interesting to compare
the promising ruthenium(II) based catalysts reported in litera-
ture for oxidation of alcohols with the present ones. Ruthe-
nium(II) complexes of tetraphenylimidodiphosphinate43
explored for catalysis of oxidation of alcohols with NMO have
carried out good conversions at 1 mol% of catalyst, while
0.01 mol% of 2 is sufficient for this purpose with a comparable
reaction time. Similarly for many other ruthenium based cata-
lysts explored for oxidation of alcohols, the catalyst loading or
reaction time required for good conversion is higher than that
of 2. Iodide-bridged diruthenium complexes, [(η5-2,5-R2-3,4-
Ph2C4COH)(CO)2Ru-(μ-I)-Ru(CO)2(η4-2,5-R2-3,4-Ph2C4CO)] (R =
Ph or Me)44 when used as catalyst, 97% conversion required its
2 mol%. It is reported that 1 mol% of [Ru(η6-p-cymene)(AsPh3)-
(L)] [L = pyridine-2-thiocarboxamide ligand] catalyzes oxidation
of alcohols with NMO resulting good conversion.45 In case
of catalyst [(η6-p-cymene)RuCl(L)] (where L = monoanionic
2-(naphthylazo) phenolato ligands) also, catalyst loading of
1 mol% is required and that too with a reaction time of 8 h.
However, 0.001 mol% of half sandwich complexes of Ru(II)14
with morpholine based ligands or Schiff bases and related
within these complexes and their reactivity the density func-
tional theory (DFT) calculations (see Experimental for details)
were performed on all complexes 1–5. The accuracy of such cal-
culations for molecules of late transition metals is insufficient
to warrant a detailed discussion of MO energy levels. However,
an analysis of lowest energy configurations and frontier orbi-
tals can provide a qualitative insight into the bonding charac-
teristics of the complexes. The HOMOs (highest occupied
molecular orbital) of all complexes are essentially identical
and are positioned primarily over the metal centre and
benzene ring, with some contribution towards chalcogen,
nitrogen, and Cl donors (see Fig. 8). The agreement between
the experimentally observed bonding parameters and the cal-
culated one is good for Ru–E (E = S or Se), Ru–Cl, Ru–N and
Ru–benzene (centroid). The calculated and experimentally
found bond angles are generally consistent (Table 3). Some of
the calculated bond distances and angles differ from those
experimentally found but such differences are not unusual
and have been reported earlier also.16,46
As mentioned earlier the absolute values for the HOMO
energy levels cannot be reliably determined for complexes of
heavy metals, but relative energies of the levels are quite infor-
mative. It has been reported that there is a correlation between
the HOMO–LUMO energy gap of a complex and its chemical
Fig. 8 Frontier molecular orbitals of 1–5.
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reactivity.47 The chemical reactivity of a system is related with
chemical hardness defined as the resistance to perturbation in
the electron distribution in a molecule.47b In terms of frontier
orbitals, chemical hardness corresponds to the energy gap
between the HOMO and LUMO, and is approximated by
(εHOMO − εLUMO)/2, where εLUMO and εHOMO are the LUMO and
HOMO energies.47a The large the HOMO–LUMO energy gap,
makes the deformation of the electron cloud hard, which in
turn results less reactivity. Its vice versa that lower the energy
gap, higher the reactivity is also true, as in such a case electron
cloud is easily perturbed by an external field.47 Thus a larger
gap reflects lower reactivity and vice versa. Therefore among
the two catalysts, the one having low HOMO–LUMO energy
gap is supposed to be better as catalyst. The HOMO–LUMO
energy gaps in 1–5 differ sufficiently, lower in the case of 2/5
relative to their sulfur analogues 1 and 4 respectively (see
Fig. 8, Table S3 in ESI†).
This indicates that reactivity of Se complexes is greater than
those of the corresponding S analogues. In the assessment of
bidentate vs. terdentate complexes, the HOMO–LUMO energy
gaps (lower in case of bidentate, see Fig. 8, Table S3 in ESI†)
suggest more efficiency of bidentate complexes over the ter-
dentate ones. This is consistent with the experimentally
observed order of catalytic efficiencies of complexes: Se > S
and bidentate > terdentate. The magnitude of HOMO–LUMO
energy gaps is not unusual and has been reported of the same
order i.e. ∼4 eV for complexes of group VIII metal ions contain-
ing chalcogen donors.16,47a Mulliken partial atomic charge
analyses for 1–5, show that the charge on Ru center of 2/5 is
greater than those of the corresponding sulfur complexes
(Fig. 9, Table S3 in ESI†). This makes easier the formation of
RuIVvO species needed in catalytic oxidation, with 2/5 in com-
parison to their sulfur analogues, and in turn makes them
more efficient. The more charge on Ru also facilitates the for-
mation of ruthenium hydride needed in the case of transfer
hydrogenation. Thus Se containing species are expected to be
more efficient for the transfer hydrogenation catalysis as well.
Fig. 9 Mulliken partial charges of 1–5.
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The charges on the other atoms are given in Fig. 9 and without any symmetry restriction with X-ray coordinates of the
Table S3 (ESI†).
molecule. Harmonic force constants were computed at the
optimized geometries to characterize the stationary points as
minima. The molecular orbital plots were made using the
Chemcraft program package (http://www.chemcraftprog.com).
Experimental
Physical measurements
Synthesis of L4 and L5
The 1H, 13C{1H}, and 77Se{1H} NMR spectra have been
recorded on a Bruker Spectrospin DPX-300 NMR spectrometer
at 300.13, 75.47, and 57.24 MHz respectively and chemical
shifts are reported in ppm relative to normal standards. IR
spectra in the range 4000–400 cm−1 were recorded on a Nicolet
Protége 460 FT-IR spectrometer as KBr pellets. The C, H, and
N analyses were carried out with a Perkin-Elmer 2400 Series II
C, H, and N analyzer. For single crystal structures the data
were collected on a Bruker AXS SMART Apex CCD diffracto-
meter using Mo Kα (0.71073 Å) radiation at 298(2) K. The soft-
ware SADABS48 was used for absorption correction (if needed)
and SHELXTL for space group, structure determination, and
refinements.49 Hydrogen atoms were included in idealized
positions with isotropic thermal parameters set at 1.2 times
that of the carbon atom to which they are attached in all cases.
The least-squares refinement cycles on F2 were performed
until the model converged. High resolution mass spectral
measurements were performed with electron spray ionization
(10 eV, 180 °C source temperature, sodium formate as refer-
ence compound) on a Bruker MIcroTOF-Q II, taking sample in
CH3CN. The commercial nitrogen gas was used after passing it
successively through traps containing solutions of alkaline
anthraquinone, sodium dithionite, alkaline pyrogallol, concen-
trated H2SO4 and KOH pellets. Nitrogen atmosphere if
required was created using Schlenk techniques. Yields refer to
isolated yields of compounds which have purity ≥95% [estab-
Sulphur (0.064 g, 2 mmol)/selenium powder (0.156 g, 2 mmol)
dissolved in 30 cm3 of ethanol was treated with a solution
(made in 5% NaOH) of NaBH4 (0.149 g, 4.0 mmol) (added
drop wise) under nitrogen atmosphere until the reaction
mixture became colorless due to the formation of Na2Se/Na2S.
(2-Chloromethyl)pyridine hydrochloride (0.656 g, 4.0 mmol)
dissolved in 5 cm3 of ethanol was mixed to this colorless solu-
tion with constant stirring The mixture was stirred further for
3–4 h and poured into 100 cm3 of ice-cold distilled water and
extracted into CHCl3 (5 × 40 cm3). The extract was washed with
water (3 × 50 cm3) and dried over anhydrous sodium sulfate.
Its solvent was evaporated off under reduced pressure on a
rotary evaporator, resulting in L4/L5 as pale yellow oil.
L4: Yield: 0.410 g, 95%. 1H NMR (CDCl3, 25 °C, vs Me4Si): δ
(ppm): 3.83 (s, 4H, H1, 7), 7.13–7.17 (m, 2H, H5, 11), 7.36–7.38
(m, 2H, H3, 9), 7.60–7.66 (m, 2H, H4, 10), 8.54 (d, 3JH–H = 4.8 Hz,
2H, H6, 12). 13C{1H} NMR (CDCl3, 25 °C, vs Me4Si): δ (ppm):
37.5 (C1, 7), 121.9 (C5, 11), 123.3 (C3, 9), 136.7 (C4, 10), 149.3 (C6,
12), 158.4(C2, 8). IR (KBr, cm−1): 3085 (m; νC–H (aromatic)), 2915
(s; νC–H (aliphatic)), 1591 (m; νCvN), 1421 (m; νCvC (aromatic)), 752
(m; νC–H (aromatic)).
L5: Yield: 0.450 g, 95%. 1H NMR (CDCl3, 25 °C, vs Me4Si): δ
(ppm): 3.92 (s, 4H, H1, 7), 7.07–7.11 (m, 2H, H5, 11), 7.29–7.31
(m, 2H, H3, 9), 7.55–7.60 (m, 2H, H4, 10), 8.49 (m, 2H, H6, 12).
13C{1H} NMR (CDCl3, 25 °C, vs Me4Si): δ (ppm): 28.9 (C1, 7),
121.5 (C5, 11), 123.1 (C3, 9), 136.5 (C4, 10), 149.3 (C6, 12), 159.6
(C2, 8). 77Se{1H} NMR (CDCl3, 25 °C, vs Me2Se): δ (ppm): 328.
IR (KBr, cm−1): 3083 (m; νC–H (aromatic)), 2920 (s; νC–H (aliphatic)),
1590 (m; νCvN), 1421 (m; νCvC (aromatic)), 755 (m; νC–H (aromatic)).
1
lished by H NMR]. All reactions were carried out in glassware
dried in an oven, under ambient conditions except the synth-
eses of L1–L5 which were carried in nitrogen atmosphere.
Chemicals and reagents
Synthesis of complexes 1–5
The solid [(η6-C6H6)RuCl2]2 (0.05 g, 0.1 mmol) and L1, L2, L3,
L4 or L5 (0.2 mmol) dissolved in CH3OH (15 cm3) were stirred
together for 8 h at room temperature. The resulting yellow
solution was filtered and the volume of the filtrate was
reduced to ∼7 cm3 with a rotary evaporator. The concentrate
was mixed with solid NH4PF6 (0.032 g, 0.2 mmol) and the
resulting yellow colored microcrystalline solid 1–5 was filtered,
washed with ice-cold 10 cm3 CH3OH and dried in vacuo. The
single crystals of 1–5 were obtained by diffusion of diethyl
ether into their solutions (1 cm3) made in a mixture of CH3OH
Diphenyldiselenide, thiophenol, bis(2-pyridyl)disulfide, NaBH4,
(2-chloromethyl)pyridine hydrochloride and ammonium hexa-
fluorophosphate procured from Sigma-Aldrich (USA) were used
as received. The ligands L1, L2, and L3 were synthesized as
reported earlier.16 The [(η6-C6H6)RuCl2]2 was prepared according
to literature method.50 All the solvents were dried and distilled
before use by standard procedures.51 The common reagents
and chemicals available locally were used.
DFT calculations
All DFT calculations were carried out at the Department of and CH3CN (1 : 3).
Chemistry, Supercomputing Facility for Bioinformatics and
1: Yield: 0.097 g, 87%, Anal. Calcd for C18H17ClNRuS·[PF6]:
Computational Biology, IIT Delhi, using GAUSSIAN-0352 C, 38.54; H, 3.05; N, 2.50. Found: C, 38.69; H, 3.03; N, 2.39. Mp
program with an immediate objective of identifying the reacti- 204 °C. NMR (1H, CDCl3, 25 °C, TMS) (δ: ppm): 4.40–4.68 (m,
vity in the present series of compounds. The geometry of 2H, H5), 5.79 (s, 6H, Ru-Ar-H), 7.27–7.64 (m, 5H, H1–3),
complex 1 to 5 was optimized at the B3LYP53 level using a 7.78–8.08 (m, 3H, H7–9), 9.46 (d, JH–H = 5.7 Hz, 1H, H10); (13C
3
LANL2DZ basis set for Ru atom and 6-31G* basis sets for C, N, {1H}, CDCl3, 25 °C, TMS) (δ: ppm): 48.3 (C5), 88.9 (RuAr-C),
H, Cl, and chalcogen. Geometry optimizations were carried out 125.0 (C7), 126.5 (C9), 130.0 (C3), 130.7 (C2), 131.3 (C1), 135.7
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(C4), 141.4 (C8), 158.0 (C10), 161.0 (C6). HR-MS (CH3CN) [M]+ dissolved in CH3CN) were mixed and refluxed (80 °C) for
(m/z)
=
415.9797; calculated value for C18H17ClNRuS
=
appropriate time (2 h for 2 and 3 h for others). The reaction
415.9809 (δ: 3.0 ppm). IR (KBr, cm−1): 3093 (m; νC–H (aromatic)), was followed by 1H NMR spectroscopy. After completion of the
2939 (s; νC–H (aliphatic)), 1605 (m; νCvN), 1438 (m; νCvC (aromatic)), reaction, the reaction mixture was cooled at room-temperature,
845 (s; ν ), 772 (m; νC–H (aromatic)).
2-propanol was removed on a rotary evaporator and resulting
P–F
2: Yield: 0.109 g, 90%. Anal. Calcd for C18H17ClNRuSe·[PF6]: semi-solid was extracted with diethyl ether (3 × 20 cm3). The
C, 35.57; H, 2.82; N, 2.30. Found: C, 35.59; H, 2.89; N, 2.11 Mp extract was passed through a short column (∼8 cm in length)
196.0 °C. NMR (1H, CDCl3, 25 °C, TMS) (δ: ppm): 4.34–4.65 (m, of silica gel. The column was washed with ∼50 cm3 of diethyl
2H, H5), 5.85 (s, 6H, RuAr-H), 7.20–7.62 (m, 5H, H1–3), ether. All the eluates from the column were mixed and solvent
3
7.70–8.03 (m, 3H, H7–9), 9.50 (d, JH–H = 5.7 Hz 1H, H10); from the mixture was evaporated off on a rotary evaporator.
1
(13C{1H}, CDCl3, 25 °C, TMS) (δ: ppm): 41.5 (C5), 88.4 (RuAr-C), The resulting residue was subjected to the H NMR. The final
125.8 (C7), 126.1 (C9), 130.0 (C1), 131.3 (C2), 131.9 (C4), 132.2 conversions are reported as an average of two runs of each cat-
(C3), 140.9 (C8), 158.7 (C10), 161.5 (C6); (77Se{1H}, CDCl3, 25 °C, alytic reaction.
Me2Se) (δ: ppm): 389.8; HR-MS (CH3CN) [M]+ (m/z) = 463.9246;
Procedure for catalytic oxidation of alcohols
calculated value for C18H17ClNRuSe = 463.9257 (δ: 2.4 ppm).
IR (KBr, cm−1): 3093 (m; νC–H (aromatic)), 2965 (s; νC–H (aliphatic)),
1610 (m; νCvN), 1438 (m; νCvC (aromatic)), 838 (s; νP–F), 770 (m;
νC–H (aromatic)).
A typical reaction for catalytic oxidation of primary alcohols to
corresponding aldehydes and secondary ones to ketones with
N-methylmorpholine-N-oxide (NMO) and complex 1, 2, 3, 4, or
5 is as follows. A solution of a complex among 1, 2, 3 (0.01 mol
%) 4, and 5 (0.1 mol%) in 20 cm3 of CH2Cl2 was mixed with
neat alcohol substrate (1 mmol) and solid NMO (3 mmol). The
mixture was refluxed for 2 h with 2 and 3 h for remaining four
complexes. The reaction was followed by 1H NMR spec-
troscopy. On completion of reaction the reaction mixture was
cooled at room-temperature and its solvent was evaporated off
using a rotary evaporator. The residue having an oxidized
product was extracted with 20 cm3 of petroleum ether. The
complex-catalyst undissolved in petroleum ether was recovered
almost quantitatively for the next catalytic cycle. The oxidized
product present in petroleum ether was analyzed by the 1H
NMR spectra.
3: Yield: 0.100 g, 90%. C17H16ClN2RuS·[PF6]: C, 36.34; H,
2.87; N, 4.99. Found: C, 36.14; H, 2.97; N, 5.03. Mp 186 °C.
NMR (1H, CDCl3, 25 °C, TMS) (δ: ppm): 4.60–5.20 (m, 2H, H6),
6.10 (s, 6H, RuAr-H), 7.21–8.19 (m, 6H, H2–4 + H8–10), 8.77–8.86
3
(m, 1H, H1), 9.32 (d, JH–H = 5.5 Hz, 1H, H11); (13C{1H}, CDCl3,
25 °C, TMS) (δ: ppm): 46.4 (C6), 88.4 (RuAr-C), 124.3 (C4), 125.4
(C2), 126.0 (C8), 127.7 (10), 137.5 (C3), 140.6 (C9), 150.9 (C11),
152.4 (C1), 157.3 (C7), 161.8 (C5); HR-MS (CH3CN) [M]+ (m/z) =
416.9763; calculated value for C17H16ClN2RuS = 416.9761
(δ: −0.4 ppm). IR (KBr, cm−1): 3096 (m; νC–H (aromatic)), 2928
(s; νC–H (aliphatic)), 1608 (m; νCvN), 1444 (m; νCvC (aromatic)), 837
(s; νP–F), 766 (m; νC–H (aromatic)).
4: Yield: 0.123 g, 90%. C18H18N2RuS·2[PF6]: C, 31.54; H,
1
2.65; N, 4.09. Found: C, 31.28; H, 2.87; N, 4.13. Mp 225 °C. H
NMR (CDCl3, 25 °C, vs Me4Si): δ (ppm): 3.42–4.60 (m, 4H, H1,
7), 6.30 (s, 6H, RuAr-H), 7.39 (m, 2H, H5, 11), 7.50–7.53 (m, 2H,
H
Conclusions
3, 9), 7.86–7.89 (m, 2H, H4, 10), 9.26 (d, 3JH–H = 5.7 Hz, 2H, H6,
12). 13C{1H} NMR (CDCl3, 25 °C, vs Me4Si): δ (ppm): 46.9 Five half-sandwich “piano-stool” ruthenium(II) complexes, [(η6-
(C1, 7), 90.7 (RuAr-C), 124.5 (C5, 11), 125.9 (C3, 9), 140.4 (C4, 10), C6H6)Ru(L)Cl][PF6] (1–5) with bi and terdentate chalcogenated
156.2 (C6, 12), 160.5(C2, 8). IR (KBr, cm−1): 3095 (m; pyridine ligands (L = L1–L5), have been synthesized and
νC–H (aromatic)), 2935 (s; νC–H (aliphatic)), 1601 (m; νCvN), 1430 (m; characterized by multinuclei NMR, HR-MS and X-ray crystallo-
νCvC (aromatic)), 845 (s; νP–F), 761 (m; νC–H (aromatic)).
graphy. These half-sandwich complexes of ruthenium(II) have
5: Yield: 0.135 g, 92% C18H18N2RuSe·[PF6]: C, 29.52; H, been explored for transfer hydrogenation of ketones at moder-
1
2.48; N, 3.83. Found: C, 29.84; H, 2.26; N, 3.73. Mp 210 °C. H ate temperature of 80 °C and oxidation of alcohols. The TON
NMR (CDCl3, 25 °C, vs Me4Si): δ (ppm): 3.31–4.51 (m, 4H, values are promising, up to 9.9 × 103 for transfer hydrogen-
H
1, 7), 6.30 (s, 6H, RuAr-H), 7.34–7.39 (m, 2H, H5, 11), 7.50–7.53 ation reactions of ketones, and 9.8 × 103 for oxidation of alco-
(m, 2H, H3, 9), 7.85–7.90 (m, 2H, H4, 10), 9.37 (d, 3JH–H = 5.4 Hz, hols. Air and moisture stability of these complexes are their
2H, H6, 12). 13C{1H} NMR (CDCl3, 25 °C, vs Me4Si): δ (ppm): additional advantages. The catalytic activities follow the
38.6 (C1, 7), 89.5 (RuAr-C), 125.0 (C5, 11), 125.3 (C3, 9), 139.7 orders: Ru–Se donor complexes > Ru–S donor complexes and
(C4, 10), 156.7 (C6, 12), 161.4 (C2, 8). 77Se{1H} NMR (CDCl3, bidentate > terdentate, which are corroborated by DFT calcu-
25 °C, vs Me2Se): δ (ppm): 295.1 IR (KBr, cm−1): 3098 (m; lations. The catalytic transfer hydrogenation with the present
νC–H (aromatic)), 2945 (s; νC–H (aliphatic)), 1604 (m; νCvN), 1438 (m; complexes probably proceeds via formation of ruthenium–
νCvC (aromatic)), 839 (s; νP–F), 767 (m; νC–H (aromatic)).
hydride as an intermediate. The RuIVvO species appears to be
involved in the oxidation. The comparison of required catalyst
loading for good conversions and reaction time for 1–5 with
Procedure for catalytic transfer hydrogenation of ketones
A solution of a ketone (1 mmol) made in 2-propanol (15 cm3), those reported in literature for other transfer hydrogenation/
KOH (2 cm3 of a 0.2 M solution in 2-propanol), and a complex oxidation catalysts, suggests that the present complexes have
from 1 to 5 (0.01 mol% in case of 1–3 and 0.1 mol% for 4–5 good promise. Generally there is a reasonably good matching
Dalton Trans.
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Paper
between experimentally determined bond distances/angles
and those calculated by DFT. The difference between them is
noticed in some cases but is not unusual.
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Acknowledgements
Authors thank Professor B. Jayaram for computational facili-
ties. Authors also thank Department of Science and Technol-
ogy, New Delhi (India) for the following: research project no.
SR/S1/IC-40/2010, partial financial assistance (under FIST pro-
gramme) for single crystal X-ray diffractometer and mass spec-
tral facilities at IIT Delhi. OP and HJ thank University Grants
Commission (India) and KNS to Council of Scientific and
Industrial Research (India) for the award of Junior/Senior
Research Fellowship.
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