Reactions of benzene based half sandwich ruthenium(II) complex with 2,6-bis((phenylseleno)methyl)pyridine: Preferential substitution of ring resulting in a catalyst of high activity for oxidation of alcohols

Article abstract of DOI:10.1016/j.inoche.2010.07.039

[2,6-Bis((phenylseleno)methyl)pyridine] (L) a (Se, N, Se) pincer ligand synthesized by reacting PhSe^- (in situ generated) with 2,6-bis(chloromethyl)pyridine reacts with [{(η⁶-C ₆H₆)RuCl(μ-Cl)}₂] (2:1

Full text of DOI:10.1016/j.inoche.2010.07.039

Inorganic Chemistry Communications 13 (2010) 1370–1373
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Reactions of benzene based half sandwich ruthenium(II) complex with
2,6-bis((phenylseleno)methyl)pyridine: Preferential substitution of ring
resulting in a catalyst of high activity for oxidation of alcohols
⁎
Dipanwita Das, Pradhumn Singh, Om Prakash, Ajai K. Singh
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016, India
a r t i c l e i n f o
a b s t r a c t
[2,6-Bis((phenylseleno)methyl)pyridine] (L) a (Se, N, Se) pincer ligand synthesized by reacting PhSe− (in situ
generated) with 2,6-bis(chloromethyl)pyridine reacts with [{(η6-C6H6)RuCl(μ-Cl)}2] (2:1 molar ratio) by
preferential substitution of ring resulting in the first Ru-(Se, N, Se) pincer ligand complex, mer-[Ru(CH3CN)2Cl(L)]
[PF6](1).H2O. Similar reaction in 4:1 molar ratio results in mer-[Ru(L)2][ClO4]2(2). The 1H, 13C{1H} and 77Se{1H}
NMR spectra of L, 1 and 2 were found characteristic. The single crystal structures of 1 and 2 were studied by X-ray
crystallography. The geometry of Ru in both the complexes is distorted octahedral. The Ru–Se distances are in the
Article history:
Received 17 June 2010
Accepted 30 July 2010
Available online 6 August 2010
Keywords:
Pincer ligand
Ruthenium(II)
Synthesis
Crystal structure
Catalytic oxidation
Alcohols
´
˚
ranges 2.4412(16)–2.4522(16) and 2.4583(14)–2.4707(15) A respectively for 1 and 2. The structural solutions
from the crystal data in case of 2, due to inferior quality of its crystals, are suitable for supporting bonding mode of
L with Ru(II) only. The 1 shows high catalytic activity for oxidation of primary and secondary alcohols (TON up to
9.7×104).
© 2010 Elsevier B.V. All rights reserved.
Ruthenium(II) complexes of pincer ligands of (N, N, N), (P, N, N),
(P, N, P), (C, N, N) and (P, C, P) types have been investigated in the
recent past [1–9] due to their potential catalytic applications. Recently
ruthenium(II) complexes with (N, N, N) pincer ligand 2,6-bis
(pyrazolyl)pyridine have been used as effective catalysts for hydrogen
transfer reaction of ketones [1,10–12]. The (P, N, P) pincer ligand [2,6-
bis(di-tert-butylphosphinomethyl)pyridine] and (P, N, N) pincer [2-
(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine]
form ruthenium(II) complexes which have been found efficient for
catalytic dehydrogenative coupling of alcohols [2], hydrogenation of
esters to alcohols [4] and reaction of alcohols with amines to form
amides with liberation of H2 [13]. The complex formed by acridine-
based (P, N, P) pincer ligand with Ru(II), [RuHCl(A-iPrPNP)(CO)] [A-
iPrPNP=4,5-bis-(di-iso-propylphosphinomethyl)acridine] has been
used for selective synthesis of primary amines directly from alcohols
and ammonia [3]. Ruthenium(II) complex of a (C, N, N) pincer ligand
has been explored successfully for catalytic asymmetric reduction of
alkyl aryl ketones [7]. The (P, C, P) pincer-arylruthenium(II) complex
has been used to catalyze the asymmetric hydrogen transfer
reaction [8]. However Ru(II) complexes with selenium containing
pincer ligands are not in our knowledge. Recently (Se, N, Se) pincer
ligand and its palladium(II) complexes have been reported from our
group [14]. The palladium complexes are efficient for catalytic heck
coupling reactions [14]. It was therefore thought worthwhile to study
reactions of half sandwich species [{(η6-C6H6)RuCl(μ-Cl)}2] (a) with
(Se, N, Se) type pincer ligand L (Scheme 1). The reactions in 2:1 and
4:1 molar ratios (L:a) give mer-[Ru(CH3CN)2Cl(L)][PF6](1).H2O and
mer-[Ru(L)2][ClO4]2(2) respectively. The formation of 1 takes place
due to preferential substitution of benzene ring with pincer ligand
reported scantly. Only one example is in our knowledge [15]. The 1
has been explored for its catalytic activity for oxidation of alcohols and
found efficient. The results of these investigations are the subject of
present paper.
The ligand L was synthesized by the reported procedure [14]
summarized in Scheme 1 and its NMR data required for comparison
with those of the present complexes are given in online Supplemen-
tary material. The syntheses of both ruthenium(II) complexes 1 and 2
using precursor [{(η6-C6H6)RuCl(μ-Cl)}2] synthesized by reported
procedure [16] are also summarized in Scheme 1. The ligand L was
soluble in common organic solvents. The complexes (1/2) also have
good solubility in common organic solvents except hexane and
petroleum ether in which they were found sparingly soluble. The
solutions of both complexes in DMSO showed the sign of decompo-
sition after 20–24 h.
The complexes (1/2) show characteristic 1H, 13C{1H} and 77Se{1H}
NMR [17,18] and IR spectra (online Supplementary material). These
spectra of ligand L are also characteristic (see online Supplementary
material for detail). The molar conductance of complex 1 is close to
the value expected for an 1:1 electrolyte [17] while that of 2 is close to
that of an 1:2 electrolyte [18]. The signal in 77Se{1H} NMR spectrum of
L (δ, 351.2 ppm) shifts to a high frequency by 41.8 and 40.3 ppm
respectively, on the formation of complexes 1 and 2, implying the
⁎
Corresponding author. Tel.: +91 11 26591379; fax: +91 11 26581102.
E-mail addresses: ajai57@hotmail.com, aksingh@chemistry.iitd.ac.in (A.K. Singh).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.07.039

D. Das et al. / Inorganic Chemistry Communications 13 (2010) 1370–1373
1371
EtOH / N2 atm. / r.t.
NaOH / NaBH4
2PhSeNa
Ph2Se2
Cl
EtOH / r.t.
N
Cl
2
3
4
5
1
Se
Se
6
7
8
N
L
NaClO4
L:a = 4:1
NH4PF6
L:a = 2:1
r.t. / 3 h
CH3OH / CH3CN
Fig. 1. ORTEP diagram of cation of 1 with 30% probability ellipsoids; H2O, PF−6 and H
atoms are omitted for clarity; Selected bond lengths(A): Ru(1)–N(3) 2.016(8), Ru(1)–N
(2) 2.029(8), Ru(1)–N(1) 2.067(7), Ru(1)–Cl(1) 2.411(3), Ru(1)–Se(1) 2.4412(16), Ru
(1)–Se(2) 2.4522(16).
(20 cm3)/(10 cm3)
´
˚
+
2+
N
Se
Se
N
Se
Se
´
Ru
N
˚
2ClO4
PF6
(R=Tol) Ru–Se bond distances are in the range 2.446(4)–2.466(4) A
Ru
[22] and consistent with those of 1 and 2.
Se
Se
CH3CN
The Ru–Se bond lengths of complex 1/2 are shorter than the value
CH3CN
Cl
´
6
˚
2.480(11) A reported for [(η -C6H6)RuCl(N-{2-(phenylseleno)ethyl}
pyrrolidine)] [23]. The Ru–N bond lengths of 1 are between 2.016(8)
L = Se~N~Se
2
1
´
´
˚
˚
and 2.067(7) A while that of 2 between 2.091(10) and 2.095(8) A.
Both are somewhat shorter than the Ru–N bond distance (2.163(10)
a: [{(η6-C6H6)RuCl(µ-Cl)}2]
´
6
˚
A) reported for [(η -p-cymene)Ru(2-MeSC6H4CH2NH(CH2)2TeC6H4-
4-OMe)][PF6]2.CHCl3 [24]. The Ru–Cl bond distance of 1, 2.411(3) A is
consistent with the values 2.416(2) A reported for [(η -p-cymene)
´
˚
Scheme 1. Synthesis of L and its ruthenium complexes 1 and 2.
´
6
˚
RuCl(1-(phenylselenomethyl)-1H-benzotriazole)][PF6] [25]. In the
´
˚
crystal of 1 weak O⋯H interactions (3.433(30)–3.600(29) A) which
coordination of ruthenium with Se of L. The presence of only one
signal in 77Se{1H} NMR spectra of complexes indicates the equiva-
lence of bonding of all Se donor sites to ruthenium. In 1H NMR
spectrum of 1 signals of H5 and H7 appear shifted to higher frequency
by 0.87 and 0.76 ppm respectively while in case of complex 2 by 1.07
and 0.79 ppm respectively, relative to those of free ligands, corrob-
orating with the coordination of L through Se donor sites as inferred
from 77Se{1H} NMR spectral data [17,18]. In 13C{1H} NMR spectra of
complex 1 the signals of C5, C6 and C7 appear shifted to higher
frequency by 8.9, 6.1 and 2.4 ppm respectively while in complex 2 by
7.8, 7.0 and 1.8 ppm respectively relative to those of free ligand,
corroborating with the 1H NMR spectra [17,18].
may be due to packing effects have been observed (Fig. S1 in online
Supplementary material).
The complex 1 shows high activity for catalyzing oxidation of
primary alcohols to aldehydes and secondary ones to ketones, with N-
The single crystal structure of 1 has been solved [19a]. The crystals
of 2 were not of good quality [19b] and therefore some disorders were
observed in the structural data of carbon atoms of phenyl rings and
oxygen atom of anion ClO−4 . However, they are not of much
significance in the context of inference related to the binding of two
pincer ligands with ruthenium(II) (see Supplementary material),
which is very much supported by the results of X-ray crystallographic
study of 2. In Figs. 1 and 2 molecular structures of 1 and 2 with some
bond lengths are given. The geometries of ruthenium in 1 and 2 are
distorted octahedral as revealed by bond angles [20a–b]. More details
of crystal data, structural refinements, bond lengths and angles are
available in online Supplementary material (Tables S1–S2). The Ru–Se
bond lengths of 1 are in the range 2.4412(16)–2.4522(16) and of 2
´
˚
in 2.4583(14)–2.4707(15) A and do not differ much. The bond
´
˚
distances of 1 are shorter than the values 2.4756(10)–2.5240(9) A
reported for Ru–Se bond lengths in clusters [Ru3(μ3-Se)(CO)7(μ3-CO)
(μ-dppm)] and [Ru3(μ3-Se)(μ3-S)(CO)7(μ-dppm)] [21]. Probably this
is may be due to the fact that (Se, N, Se) pincer ligand L behaves as a
strong donor for Ru(II). In [(η5-C5Me5)Ru(μ2-SeR)3Ru(η5-C5Me5)]Cl
Fig. 2. Molecular structure of cation 2; ClO−4 and H atoms are omitted for clarity; Selected
bond lengths(A): N(1)–Ru(1) 2.091(10), N(2)–Ru(1) 2.095(8), Ru(1)–Se(2) 2.4583(14),
Ru(1)–Se(4) 2.4587(13), Ru(1)–Se(3) 2.4640(13), Ru(1)–Se(1) 2.4707(15).
´
˚

1372
D. Das et al. / Inorganic Chemistry Communications 13 (2010) 1370–1373
O
OH
Catalyst:1 (0.001 mol%)
of paramagnetic species as intermediate. The advantages of 1 in
H2O
+
R'
comparisons to recently reported Ru based good catalytic species
[30,33,36,41–45] for oxidation of alcohols are: (i) high efficiency/yield
as they are needed in less quantity and (ii) short reaction time. The
half sandwich Ru(II) complexes containing chalcogenated pyrroli-
dine, benzotriazole, Schiff base and morpholine known to catalyze the
oxidation of alcohols efficiently with NMO [23,25,46,47] (TON up to
9.8×104) are comparable with the present complexes (TON up to
9.7×104). The cyclic voltammetric experiment performed at 298 K in
CH3CN (0.01 M NBu4ClO4 as supporting electrolyte) for 1 at scan rate
100 mVs−1 (anodic sweep) reveals an irreversible oxidation by one
electron transfer with E1/2 value 0.587 V (vs. Ag/AgCl) (see Table S3
and Fig. S2 in online Supplementary material for details) which are
not extreme, implying that both species, Ru(II) and Ru(III), are equally
stabilized by the same set of ligand. Such E1/2 value has been reported
earlier [47–49] favourable for catalytic oxidation process, of course no
one to one relationship unequivocally has been established. The
investigations to understand further the properties of 1 and 2 are in
progress.
R
R
R'
NMO / CH2Cl2 / reflux
R or R' = Alkyl (or) aryl (or) H
Scheme 2. Oxidation of alcohols with NMO catalyzed by 1.
methylmorpholine-N-oxide (NMO) in CH2Cl2 medium (Scheme 2).
The oxidation products were identified by GC after recovering catalyst
and appropriate workup for them (details are given in online
Supplementary material). A series of blank experiments was carried
out under identical conditions (see Supplementary material) which
suggest that neither ruthenium(II) complex nor oxidant NMO, alone
cause the catalytic oxidation to any significant level.
The yield (%), conversion (%), TON and TOF values (based on %
conversions) for oxidation of various alcohols with NMO catalyzed by
complex 1 are given in Table 1. The percentage yields and conversions
do not differ much. The earlier work carried out on the mechanism of
such catalytic oxidation suggests that Ru(IV)=O species [23,25–
35,46,47] are involved. On adding NMO to the solution of complex 1
the signal in its 77Se{1H} NMR spectrum goes to higher frequency
≥353 ppm. The signal in 77Se{1H} NMR spectrum of L remains
unshifted on addition of NMO. Therefore, Ru(II) is most probably
oxidized to Ru(IV)=O. In UV–visible spectrum, on addition of NMO to
a dichloromethane solution of 1, a new shoulder at 392 nm appears
which is believed [23,25,32–37,46,47] to be due to Ru(IV)=O species,
reported to be responsible for transfer of the oxygen to alcohol
substrates resulting in their catalytic oxidation. IR spectrum of the
residue left after evaporating off solvent from the mixture of NMO
with 1, exhibits a very strong band at 845 cm−1 (νP–F at 839 cm−1 is
of medium intensity only), which further supports the formation of Ru
(IV)=O species [23,25,28,33,36–40,46,47] which is probably respon-
sible for catalytic oxidation of alcohols. The 1H NMR spectrum of 1
recorded after adding NMO becomes broad, indicating the formation
Acknowledgements
Authors thank Council of Scientific and Industrial Research
(CSIR) New Delhi (India) for the project no. 01(2113)07/EMR-II and
Department of Science and Technology (India) for project no. SR/S1/IC-
23/2006 and partial financial assistance given to establish single crystal
X-ray diffraction facility at IIT Delhi, New Delhi (India) under its FIST
programme. D.D., P.S. and O.P. thank University Grants Commission
(India) for the award of Junior/Senior Research Fellowship.
Appendix A. Supplementary material
CCDC nos. 779971 and 779972 contain the supplementary
crystallographic data for 1 and 2 respectively. These data can be
obtained free of charge from the Cambridge Crystallographic Centre
Table 1
Oxidation of alcohols to corresponding aldehydes and ketones with NMO catalyzed with 1.
Entry
1
Substrate
Product
Blank
(Yield %)
(80)
TON (%conversion)
8.3×104 (83)
TOF (per h)
4.15×104
Product not detected
2
Product not detected
(85)
8.8×104 (88)
4.40×104
3
4
Product not detected
Product not detected
(86)
(90)
9.0×104 (90)
9.3×104 (93)
4.50×104
4.65×104
5
Product not detected
(95)
9.7×104 (97)
4.85×104

D. Das et al. / Inorganic Chemistry Communications 13 (2010) 1370–1373
1373
tions (Rint.) 5253 (0.0613); data/restraints/parameters 5253/0/315; R indices
[I N2σ(I)] R1 =0.0645 wR2 =0.1904; (all data): R1 =0.0971; wR2 =0.2045;
Largest diff. peak/hole [e.Å−3] 1.478/−0.522. (b) Crystal data: 2: crystal system,
monoclinic; space group, P21/c a=10.1756(17)Å; b=36.935(6)Å; c=10.962(8)Å;
α=90.00°; β=90.190(4)°; γ=90.00; Volume [Å3]=4119.9(12); Z=4; F(000)
via www.ccdc.cam.ac.uk/data.request/cif. Supplementary data asso-
ciated with this article can be found, in the online version, at
doi:10.1016/j.inoche.2010.07.039.
2216.0; absorption coeff.[mm−1
] 4.097; Independent reflections (Rint.) 7673
References
(0.0765); data/restraints/parameters 7673/0/496; R indices [ IN2σ(I)] R1=0.0938
wR2=0.1782; (all data): R1 =0.1345; wR2=0.1966; Largest diff. peak/hole [e.Å−3
1.054/−0.893.
]
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grown from CH3OH–CH3CN mixture (1:1). Yield: (0.046 g, 60%), ΛM
=
127.0 cm2 mol−1 ohm−1
, m.p 195 °C. Analysis: found C, 36.05; H, 2.83; N,
5.05%; calcd. for C23H23ClF6N3PRuSe2 C, 35.39; H, 2.97; N, 5.38%. NMR (1H, CD3CN,
25 °C vs. TMS): (δ, ppm) 5.05 (s, 4H, H5), 7.36–7.55 (m, 10H, H1 +H2 +H3), 7.65
3
(d, JH–H =7.8 Hz, 2H, H7), 7.78 (t, 1H, H8); (13C{1H}, CD3CN, 25 °C vs. TMS): (δ,
ppm) 41.1(C5), 123.4 (C7), 128.0 (C1), 130.1 (C2), 130.7 (C4), 131.2 (C3), 137.0
(C8), 164.1 (C6); (77Se{1H} CD3CN, 25 °C vs. Me2Se): (δ, ppm) 393.0.
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orange precipitate of 2 as described above for 1. The precipitate was filtered,
washed with cold methanol and dried. The single crystals of 2 were grown
from CHCl3–CH3CN–CH2Cl2 mixture (1:1:1). Yield: (0.073 g, 65%) ΛM =235.0
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cm2mol−1ohm−1
C
.
Analysis: found C, 39.53; H, 2.95; N, 2.61%; calcd. for
38H34Cl2N2O8RuSe4 C, 40.24; H, 3.02; N, 2.47%. m.p 202 °C. NMR (1H, CD3CN,
25 °C vs. TMS): (δ, ppm) 5.25 (s, 8H, H5), 7.30–7.45 (m, 20H, H1 +H2 +H3), 7.68
(d, 3JHH =7.5 Hz, 4H, H7), 7.79 (t,3JHH =7.2 Hz, 2H, H8); (13C{1H}, CD3CN, 25 °C vs.
TMS): (δ, ppm) 40.0 (C5), 122.8 (C7), 127.6.0 (C1), 130.4 (C2), 130.3 (C4), 131.2
(C3), 137.5 (C8), 165.0 (C6); (77Se{1H} CD3CN, 25 °C vs. Me2Se): (δ, ppm) 391.5.
[19] X-ray crystallography; Bruker AXS SMART Apex CCD diffractometer using Mo–Kα
(0.71073 Å) radiations at 298(2) K was used. (a) Crystal data: 1: crystal system,
triclinic; space group, P-1; a=11.073(7)Å; b=12.663(8)Å; c=12.933(8)Å;
α=66.251(10)°; β=76.649.77(10)°; γ=64.369(10); Volume [Å3]=1493.0
(16); Z=1; F(000) 707.0; absorption coeff.[mm−1] 3.117; Independent reflec-
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1835.
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