Isomeric complexes of [RuII(trpy)(L)Cl] (trpy = 2,2′:6′,2″-terpyridine and HL = quinaldic acid): Preference of isomeric structural form in catalytic chemoselective epoxidation process

Article abstract of DOI:10.1021/ic102195w

The present work deals with the isomeric complexes of the molecular composition [Ru^II(trpy)(L)Cl] in 1 and 2 (trpy = 2,2′:6′, 2″-terpyridine, L = deprotonated form of quinaldic acid, HL). Isomeric identities of 1 and 2 have been established by their single-crystal X-ray structures, which reveal that under the meridional configuration of trpy, O ^- and N donors of the unsymmetrical L are in trans, cis and cis, trans configurations, respectively, with respect to the Ru-Cl bond. Compounds 1 and 2 exhibit appreciable differences in bond distances involving Ru-Cl and Ru-O1/Ru-N1 associated with L on the basis of their isomeric structural features. In relation to isomer 2, the isomeric complex 1 exhibits a slightly lower Ru(II)-Ru(III) oxidation potential [0.35 (1), 0.38 (2) V versus SCE in CH₃CN] as well as lower energy MLCT transitions [559 nm and 417 nm (1) and 533 nm and 378 nm (2)]. This has also been reflected in the DFT calculation where a lower HOMO-LUMO gap of 2.59 eV in 1 compared to 2.71 eV in 2 is found. The isomeric structural effect in 1 and 2 has also been prominent in their ¹H NMR spectral profiles. The relatively longer Ru-Cl bond in 1 (2.408(2) A) as compared to 2 (2.3813(9) A) due to the trans effect of the anionic O^- of coordinated L makes it labile, which in turn facilitates the transformation of [Ru^II(trpy)(L)(Cl)] (1) to the solvate species, [Ru^II(trpy)(L)(CH₃CN)](Cl) (1a) while crystallizing 1 from the coordinating CH₃CN solvent. The formation of 1a has been authenticated by its single-crystal X-ray structure. However, no such exchange of "Cl^-" by the solvent molecule occurs in 2 during the crystallization process from the coordinating CH₃CN solvent. The labile Ru-Cl bond in 1 makes it a much superior precatalyst for the epoxidation of alkene functionalities. Compound 1 is found to function as an excellent precatalyst for the epoxidation of a wide variety of alkene functionalities under environmentally benign conditions using H ₂O₂ as an oxidant and EtOH as a solvent, while isomer 2 remains almost ineffective under identical reaction conditions. The remarkable differences in catalytic performances of 1 and 2 based on their isomeric structural aspects have been addressed.

Full text of DOI:10.1021/ic102195w

Inorg. Chem. 2011, 50, 1775–1785 1775
DOI: 10.1021/ic102195w
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Isomeric Complexes of [Ru (trpy)(L)Cl] (trpy = 2,2 :6 ,2 -Terpyridine and
HL = Quinaldic Acid): Preference of Isomeric Structural Form in Catalytic
Chemoselective Epoxidation Process
Abhishek Dutta Chowdhury, Amit Das, Irshad K, Shaikh M. Mobin, and Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai-400076, India
Received November 1, 2010
II
The present work deals with the isomeric complexes of the molecular composition [Ru (trpy)(L)Cl] in 1 and 2 (trpy = 2,
0
0 00
:6 ,2 -terpyridine, L = deprotonated form of quinaldic acid, HL). Isomeric identities of 1 and 2 have been established by
2
-
their single-crystal X-ray structures, which reveal that under the meridional configuration of trpy, O and N donors of the
unsymmetrical L are in trans, cis and cis, trans configurations, respectively, with respect to the Ru-Cl bond. Compounds 1
and 2 exhibit appreciable differences in bond distances involving Ru-Cl and Ru-O1/Ru-N1 associated with L on the
basis of their isomeric structural features. In relation to isomer 2, the isomeric complex 1 exhibits a slightly lower
Ru(II)-Ru(III) oxidation potential [0.35 (1), 0.38 (2) V versus SCE in CH CN] as well as lower energy MLCT transitions
3
[
559 nm and 417 nm (1) and 533 nm and 378 nm (2)]. This has also been reflected in the DFT calculation where a lower
HOMO-LUMO gap of 2.59 eV in 1 compared to 2.71 eV in 2 is found. The isomeric structural effect in 1 and 2 has also
1
˚
been prominent in their H NMR spectral profiles. The relatively longer Ru-Cl bond in 1 (2.408(2) A) as compared to 2
-
˚
(2.3813(9) A) due to the trans effect of the anionic O of coordinated L makes it labile, which in turn facilitates the
transformation of [Ru (trpy)(L)(Cl)] (1) to the solvate species, [Ru (trpy)(L)(CH CN)](Cl) (1a) while crystallizing 1 from
II
II
3
the coordinating CH CN solvent. The formation of 1a has been authenticated by its single-crystal X-ray structure. However,
3
-
no such exchange of “Cl ” by the solvent molecule occurs in 2 during the crystallization process from the coordinating
CH CN solvent. The labile Ru-Cl bond in 1 makes it a much superior precatalyst for the epoxidation of alkene
3
functionalities. Compound 1 is found to function as an excellent precatalyst for the epoxidation of a wide variety of alkene
functionalities under environmentally benign conditions using H O as an oxidant and EtOH as a solvent, while isomer 2
2
2
remains almost ineffective under identical reaction conditions. The remarkable differences in catalytic performances of 1
and 2 based on their isomeric structural aspects have been addressed.
Introduction
a large quantity of organic waste, narrow substrate scope,
and inconvenience in separating the products. However,
2
Metal complex catalyzed epoxidation of alkenes is known
to be an industrially important and synthetically challenging
chemical process.
can be performed using various organic peracids as oxidants
without the involvement of metal ions, but it has severe
limitations, particularly from the point of view of generating
early transition metal complexes have subsequently been
established as efficient catalysts for the epoxidation reaction
1-3
Traditionally, epoxidation of alkenes
3
in combination with a wide variety of oxidants. On the
contrary, ruthenium catalyzed epoxidation reaction is rather
4,5
limited to a few selective molecular frameworks. Ruthenium
(3) (a) Grigoropoulou, G.; Clark, J. H.; Elings, J. A. Green Chem. 2003, 5,
1
. (b) Lane, B. S.; Burgess, K. Chem. Rev. 2003, 103, 2457. (c) Beller, M. Adv.
*
To whom correspondence should be addressed. E-mail: lahiri@
chem.iitb.ac.in.
1) (a) Barf, G. A.; Sheldon, R. A. J. Mol. Catal. A: Chem. 1995, 102, 23
Synth. Catal. 2004, 346, 107. (d) Legros, J.; Bolm, C. Chem.;Eur. J. 2005, 11,
1086. (e) Bregeault, J. M. Dalton Trans. 2003, 3289. (f) Herrmann, W. A.;
Fischer, R. W.; Marz, D. W. Angew. Chem., Int. Ed. 1991, 30, 1638. (g) Rudolf,
J.; Reddy, K. L.; Chiang, J. P.; Sharpless, K. B. J. Am. Chem. Soc. 1997, 119,
6189. (h) Hermann, W. A.; Kratzer, R. M.; Ding, H.; Thiel, W. R.; Glas, H.
J. Organomet. Chem. 1998, 555, 293. (i) Shi, Y. Acc. Chem. Res. 2004, 37, 488.
(j) Gerlach, A.; Geller, T. Adv. Synth. Catal. 2004, 346, 1247. (k) Yang, D. Acc.
Chem. Res. 2004, 37, 497. (l) Johnson, R. A.; Sharpless, K. B. Catalytic
Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 4.1. (m)
Katsuki, T.; Martin, V. S. Org. React. 1996, 48, 1. (n) Jacobsen, E. N. Catalytic
Asymmetric Synthesis; Ojima, I., Ed.; VCH: New York, 1993; Chapter 4.2. (o)
Katsuki, T. Adv. Synth. Catal. 2002, 344, 131. (p) Cavallo, L.; Jacobsen, H.
Angew. Chem., Int. Ed. 2000, 39, 589.
(
and references cited therein. (b) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed
Oxidations of Organic Compounds; Academic Press: New York, 1981. (c)
B €a ckvall; J.-E. Modern Oxidation Methods; Wiley-VCH: Weinheim, Germany,
004. (d) Sheldon, R. A.; Arends, I.; Hanefeld, U. Green Chemistry and
Catalysis; Wiley-VCH: Weinheim, Germany, 2007. (e) Weissermel, K.; Arpe,
H.-J. Industrial Organic Chemistry, 4th ed.; Wiley-VCH: Weinheim, Germany,
2
2
003.
2) (a) Crawford, K.; Rautenstrauch, V.; Uijttewaal, A. Synlett 2001,
127. (b) Wahren, U.; Sprung, I.; Schulze, K.; Findeisen, M.; Buchbauer, G.
Tetrahedron Lett. 1999, 40, 5991. (c) Kelly, D. R.; Nally, J. Tetrahedron Lett.
999, 40, 3251.
(
1
1
r 2011 American Chemical Society
Published on Web 01/18/2011
pubs.acs.org/IC

1
776 Inorganic Chemistry, Vol. 50, No. 5, 2011
Chowdhury et al.
II
complex derived effective expoxidation catalyst [Ru (trpy)-
(
variety of olefinic substrates using different oxidants and
0
0
00
dipic)] (trpy = 2,2 :6 ,2 -terpyridine, dipic = 2,6-pyridine-
dicarboxylic acid) was first reported by Nishiyama et al. in
997, and only recently has a significant expansion of the
scope of ruthenium based catalysts toward the epoxidation
solvents. Remarkably, the isomeric form 1 with electron rich
-
anionic “O ” of L trans to the Ru-Cl bond exhibits superior
4a
1
epoxidation activity as compared to the isomeric form 2 with
neutral “N” of L trans to the Ru-Cl bond. The origin of the
difference in catalytic performances primarily based on the
isomeric structural features of 1 and 2 has been investigated
experimentally and using DFT calculations. To the best of
our knowledge, the present work demonstrates for the first
time the significant effect of isomeric structures as in 1 and 2
on the catalytic epoxidation process.
5
reactions been made by Beller et al. It is now well recognized
that ruthenium complexes of polypyridine or pybox (pybox =
pyridine bis(oxazoline)) or Schiff-based derived ligands are
suitable precatalysts for the epoxidation of various alkene
functionalities using different oxidants such as hydrogen
peroxide, organic peracids, etc. It should be noted that
ruthenium complexes were earlier considered to decompose
H O instead of activating H O to form the active {RudO}
4,5
2
2
2
2
Results and Discussion
state, but Beller et al. have shown recently that H O can also
2
2
Synthesis and Structural Aspects of Isomeric Complexes
and 2. The isomeric complexes of molecular composition
be a good oxidant for the ruthenium catalyzed epoxidation
process.
5d
1
II
Ru (trpy)(L)Cl] (trpy = 2,2 :6 ,2 -terpyridine, L = de-
0
0
00
[
protonated form of unsymmetrical quinaldic acid, HL), 1
The present work is therefore specifically originated from
the following perspectives: (i) designing isomeric ruthenium-
-
-
-
II
(
O of L trans to Cl ) and2(NofL trans to Cl ), havebeen
polypyridylderivatives, [Ru (trpy)(L)Cl] (1 and 2), underthe
III
0
0
00
prepared via the reaction of Ru (trpy)Cl with HL in the
3
meridionally coordinated trpy (trpy = 2,2 :6 ,2 -terpyridine)
in combination with a deprotonated form of unsymmetrical
quinaldic acid, HL; (ii) exploration of the potential applica-
tion of isomeric 1 and 2 for the catalytic epoxidation process
under environmentally benign reaction conditions; (iii) in-
vestigation of the effect of isomeric structural features in 1
and 2 on the catalytic epoxidation process.
presence of NEt as a base in EtOH under a dinitrogen
3
atmosphere. The isomers 1 (blue-violet) and 2 (red-violet)
have been separated using a neutral alumina column (see
the Experimental Section). The electrically neutral and
diamagnetic 1 and 2 are stable and resistant to any inter-
conversion in both the solid and solution states. The DFT
calculations based on the optimized structures of 1 and 2
(Figure S1 in the Supporting Information (SI)) at the
B3LYP level predict that isomer 1 is slightly more stable
(5.6 kcal/mol) compared to isomer 2.
The isomeric features of 1 and 2 have been authenticated
by their single-crystal X-ray structures (Figure 1, Table 1,
and Table S1 in the SI). The bidentate L is bonded to the
ruthenium(II) ion via the O1 and N1 donors forming a
-
6
five-membered chelate ring. Under the usual meridional
7
-
configuration of trpy, the O and N donors of the
unsymmetrical bidentate L are in trans, cis and cis, trans
configurations in relation to the sixth monodentate chlo-
ride ligand in 1 and 2, respectively. The bond distances and
angles in 1 and 2 (Table S1 in the SI) are in good agreement
Herein, we report the synthetic and structural aspects of
the isomeric complexes 1 and 2 as well as the effect of isomeric
structural features on their spectroscopic and electrochemical
properties. The catalytic aspects of the isomeric complexes 1
and 2 have been explored toward the epoxidations of a wide
6,7
with the reported data of analogous complexes. The
geometrical constraints due to the meridional mode of
trpy have been reflected in the appreciably smaller trans
7
angle involving the trpy ligand, N2-Ru-N4 at 159.6(3)°
and 160.97(12)° in 1 and 2, respectively. The central
(
4) (a) Nishiyama, H.; Shimada, T.; Itoh, H.; Sugiyama, H.; Motoyama,
Y. Chem. Commun. 1997, 1863. (b) End, N.; Pfaltz, A. Chem. Commun. 1998,
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Stoop, R. M.; Bachmann, S.; Valentini, M.; Mezzetti, A. Organometallics 2000,
9, 4117. (e) Pezet, F.; Aït-Haddou, H.; Daran, J.-C.; Sadaki, I.; Balavoine,
˚
Ru-N3(trpy) bond lengths of 1.934(7) and 1.925(3) A in
1 and 2, respectively, are significantly shorter than the
corresponding distances involving the terminal pyridine
5
1
G. G. A. Chem. Commun. 2002, 510. (f) Takeda, T.; Irie, R.; Shinoda, Y.; Katsuki,
T. Synlett 1999, 1157. (g) Nakata, K.; Takeda, T.; Mihara, J.; Hamada, T.; Irie, R.;
Katsuki, T. Chem.;Eur. J. 2001, 7, 3776. (h) Dakkach, M.; Isabel L ꢀo pez, M.;
Romero, I.; Rodríguez, M.; Atlamsani, A.; Parella, T.; Fontrodona, X.; Llobet, A.
Inorg. Chem. 2010, 49, 5977. (i) Chatterjee, D. Inorg. Chim. Acta 2008, 361,
(6) Kundu, T.; Sarkar, B.; Mondal, T. K.; Fiedler, J.; Mobin, S. M.;
Kaim, W.; Lahiri, G. K. Inorg. Chem. 2010, 49, 6565.
2
(
177. (j) Chatterjee, D.; Sengupta, A.; Mitra, A. Polyhedron 2007, 26, 178.
k) Benet-Buchholz, J.; Comba, P.; Llobet, A.; Roeser, S.; Vadivelu, P.; Wadepohl,
H.; Wiesner, S. Dalton Trans. 2009, 5910.
5) (a) Klawonn, M.; Tse, M. K.; Bhor, S.; D o€ bler, C.; Beller, M. J. Mol.
(7) (a) Patra, S.; Sarkar, B.; Ghumaan, S.; Patil, M. P.; Mobin, S. M.;
Sunoj, R. B.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2005, 1188. (b) Sarkar, S.;
Sarkar, B.; Chanda, N.; Kar, S.; Mobin, S. M.; Fiedler, J.; Kaim, W.; Lahiri, G. K.
Inorg. Chem. 2005, 44, 6092. (c) Chanda, N.; Paul, D.; Kar, S.; Mobin, S. M.;
Datta, A.; Puranik, V. G.; Rao, K. K.; Lahiri, G. K. Inorg. Chem. 2005, 44, 3499.
(d) Chanda, N.; Mobin, S. M.; Puranik, V. G.; Dutta, A.; Niemeyer, M.; Lahiri,
G. K. Inorg. Chem. 2004, 43, 1056. (e) Mondal, B.; Puranik, V. G.; Lahiri, G. K.
Inorg. Chem. 2002, 41, 5831. (f) De, P.; Sarkar, B.; Maji, S.; Das, A. K.; Bulak,
E.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Eur. J. Inorg. Chem. 2009, 2702. (g)
Das, A. K.; Sarkar, B.; Duboc, C.; Strobel, S.; Fiedler, J.; Zalis, S.; Lahiri, G. K.;
Kaim, W. Angew. Chem., Int. Ed. 2009, 48, 4242. (h) Sans, C.; Rodriguez, M.;
Romero, I.; Llobet, A. Inorg. Chem. 2003, 42, 8385. (i) Hartshorn, C. M.;
Maxwell, K. A.; White, P. S.; DeSimone, J. M.; Meyer, T. J. Inorg. Chem. 2001,
40, 601.
(
Catal. A 2004, 218, 13. (b) Tse, M. K.; Bhor, S.; Klawonn, M.; D €o bler, C.; Beller,
M. Tetrahedron Lett. 2003, 44, 7479. (c) Bhor, S.; Tse, M. K.; Klawonn, M.;
D €o bler, C.; Beller, M.; M €a gerlein, W. Adv. Synth. Catal. 2004, 346, 263. (d) Tse,
M. K.; D €o bler, C.; Bhor, S.; Klawonn, M.; M €a gerlein, W.; Hugl, H.; Beller, M.
Angew. Chem., Int. Ed. 2004, 43, 5255. (e) Tse, M. K.; Bhor, S.; Klawonn, M.;
Anilkumar, G.; Jiao, H.; Spannenberg, S.; D €o bler, C.; M €a gerlein, W.; Hugl, H.;
Beller, M. Chem.;Eur. J. 2006, 12, 1875. (f) Tse, M. K.; Bhor, S.; Klawonn, M.;
Bhor, S.; D €o bler, C.; Anilkumar, G.; Hugl, H.; M €a gerlein, W.; Beller, M. Org.
Lett. 2005, 7, 987. (g) Schr €o der, K.; Enthaler, S.; Join, B.; Junge, K.; Beller, M.
Adv. Synth. Catal. 2010, 352, 1771.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1777
Figure 1. ORTEP diagrams of [Ru(trpy)(L)(Cl)]. (a) Isomer 1 and (b) Isomer 2. Thermal ellipsoids are drawn at 50% probability. The solvents of
˚
crystallization and hydrogen atoms are omitted for clarity. Selected bond distance (A)/bond angles (deg) of 1: Ru-Cl, 2.408(2); Ru-N(1), 2.136(7);
Ru-N(2), 2.074(7); Ru-N(3), 1.934(7); Ru-N(4), 2.049(6); Ru-O(1), 2.064(6); O(1)-Ru-N(3), 91.8(3); N(1)-Ru-N(3), 169.7(3); O(1)-Ru-Cl,
˚
1
1
1
75.59(18); N(1)-Ru-Cl, 105.9(2). Selected bond distance (A)/bond angle (deg) of 2: Ru-Cl, 2.3813(9); Ru-N(1), 2.102(3); Ru-N(2), 2.051(3); Ru-N(3),
.925(3); Ru-N(4), 2.045(3); Ru-O(1), 2.099(2); O(1)-Ru-N(3), 177.49(10); N(1)-Ru-N(3), 104.20(11); O(1)-Ru-Cl, 91.18(7); N(1)-Ru-Cl,
68.89(8).
6
˚
rings of trpy, Ru-N2(trpy), 2.074(7) and 2.051(3) A, and
ions trans to N1(L) in 2. The central Ru-N3(trpy)
˚
˚
Ru-N4(trpy), 2.049(6) and 2.045(3) A, respectively.
distance in 1 is ∼0.01 A longer than that in 2 due to the
The Ru-Cl bond distances in isomeric 1 and 2 are
presence of a moderately π-accepting quinoline ring of L
trans to the strongly π-accepting N(3)(trpy) in 1, while in 2,
7a
˚ ˚
.408(2) A and 2.3813(9) A, respectively. The presence
2
of negatively charged O of L trans to Cl makes the
-
-
-
-
the donating O of the carboxylate anion (C(dO)O ) of L
is transtoRu-N3(trpy). The isomersalso exert appreciable
differences in the trans angle involving (L)-Ru-Cl:
O1-Ru-Cl at 175.59° in 1 versus N1-Ru-Cl at 168.89°
in 2; the bulkier quinoline ring trans to Ru-Cl in 2 makes it
relatively bent.
˚
Ru-Cl bond in 1 selectively 0.02 A longer than that in 2,
where the neutral N of L is trans to Cl . This in turn makes
the Ru-O (L) bond shorter in 1 (2.064(6) A) relative to 2
2.099(2) A). On the other hand, the Ru-N(1)(L) distance
in 2 (2.102(3) A) is shorter than that in 1 (2.136(7) A),
primarily due to the enhanced Ru(II) f quinoline(L) back-
bonding via the involvement of σ- and π-donating chloride
-
-
˚
˚
(
˚
˚
The DFT calculated bond distances and angles (Table S1
in the SI) based on the optimized structures of 1 and 2

1
778 Inorganic Chemistry, Vol. 50, No. 5, 2011
Chowdhury et al.
Table 1. Selected Crystallographic Parameters
1
3 0.5H2O 2CH
2
Cl
2
2(1a) 12H
3
2
O.CH CN
3
2
2 H O
3
3
empirical formula
fw
cryst syst
C
720.81
monoclinic
P21/c
8.4741(5)
30.782(2)
11.1688(9)
90
104.180(7)
90
2824.6(4)
4
1.064
150(2)
1.695
1444
3.25 to 25.00
4961/0/365
0.0613, 0.1434
0.1272, 0.1556
0.912
27
H
22Cl
5
N
4
O
2.5Ru
C
56
H
40Cl
2
N
11
O
16Ru
2
C
559.96
orthorhombic
Pna21
14.5284(3)
16.8813(4)
8.9158(2)
90
90
90
2186.67(8)
4
0.877
150(2)
1.701
1128
3.61 to 25.00
3606/1/315
0.0271, 0.0442
0.0362, 0.0454
0.904
25
H
19ClN
4 3
O Ru
1396.03
triclinic
P1
13.3237(4)
14.4717(6)
18.3936(6)
105.741(3)
93.689(2)
113.068(4)
3081.78(19)
2
0.652
150(2)
1.504
1406
space group
˚
a (A)
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
-
1
)
μ (mm
T (K)
-3
Dcalcd (g cm
)
F(000)
θ range (deg)
data/restraints/params
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
GOF
3.33 to 25.00
10827/0/786
0.0710, 0.2078
0.0948, 0.2215
1.037
-
3
)
˚
largest diff. peak/hole (e A
1.602 and -0.992
1.432 and -1.385
0.321 and -0.271
Figure 2. Packing diagram of 2 along the b axis.
(Figure S1 in the SI) are in general agreement with the
X-ray data.
The packing diagram of 2 reveals the presence of strong
in 1 as compared to 2 (Figure 3 and see the Experimental
Section). However, the spectra are quite distinct with
respect to their isomeric identities. For example, the max-
imum downfield shifted signal for 1 corresponding to the
proton para to the quinoline nitrogen (N1) of L appears
C-H O hydrogen bonding interactions between the
3
3 3
hydrogen atoms (H111 and H222) of the lattice water
molecule (O111) and coordinated oxygen (O1) as well as
pendant oxygen (O2) atoms of the carboxylate group of L.
The two hydrogen atoms of the lattice water molecule are
connected to two different oxygen atoms, O1 and O2 of L
associated with the adjacent two Ru complexes (2), leading
to the formation of a 1D-zigzag chain (Figure 2, Table S2 in
the SI).
8
at 10.2 ppm, while the same in 2 appears at 8.75 ppm. This
is primarily due to the trans orientation of the N1 of L with
respect to the central pyridine ring of the π-acceptor trpy
and σ/π-donor chloride atom in 1 and 2, respectively. A
similar ∼1 ppm difference in chemical shift due to the
isomeric structural effect has also been reflected for the
proton adjacent to the N(1) center but in the fused benzene
ring of L, 7.5 ppm in 1 and 6.5 ppm in 2 (Figure 3).
Complexes 1 and 2 exhibit two moderately intense
transitions each in the visible region, 559 and 417 nm and
533 and 378 nm, respectively, followed by several intense
Spectral and Redox Aspects. The ν(CdO) frequency of
-
1 6
free HL at 1700 cm has been appreciably shifted on
coordination to 1635 and 1629 cm , as revealed by the IR
-
1
1
spectra of 1 and 2, respectively. H NMR spectra of 1 and 2
exhibit a calculated number of 17 partially overlapping
aromatic proton resonances each in (CD ) SO, and the
extent of overlapping of proton signals is appreciably larger
3
2
(8) Zhang, H.-J.; Demerseman, B.; Toupet, L.; Xi, Z.; Bruneau, C.
Organometallics 2009, 28, 5173.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1779
1
3 2
Figure 3. H NMR spectra of (a) 1 and (b) 2 in (CD ) SO.
Figure 5. Schematic representation of MOs, showing the difference in
energy gap between HOMO and LUMO in 1 and 2.
relatively lower stability of the Ru(II) state in 1as compared
to 2 can be attributed to the greater electron density on the
ruthenium ion due to the trans orientation of the negatively
-
-
charged O (L) and Cl in 1, while the same in 2 are in cis
orientation. This has also been reflected in the less positive
NBO charge on Ru of 0.67 in 1 relative to 0.69 in 2 (Table
S7 in the SI). A similar trend in Ru(II)/Ru(III) potential
(
picolinate derivative [Ru(trpy)(picolinate)Cl].
E ° > E °) has been reported for the analogous
cis
trans
9
The trpy based (Figure 5 and Tables S5 and S6 in the SI)
quasi-reversible reductions in 1 and 2 appear at E298°,
Figure 4. Electronic spectra of 1 (blue) and 2 (red) in CH CN. Inset:
3
Electronic spectra of [Ru(trpy)(L)(Cl)] (1, blue), [Ru(trpy)(L)(EtOH)]
þ
þ
V (ΔE , mV), of -0.838(100) and -1.395 (80) and
(
(
green), 1 in the presenceof H
2
O
2
, i.e., in situ generated [Ru(trpy)(L)(O)]
p
7
,9
3, black), and 3 þ PPh (Red) in EtOH.
3
of -0.902(110) and -1.54(110), respectively.
The aforementioned difference in electronic structural
aspects in 1 and 2 primarily based on their isomeric
structural features has indeed instigated a look into the
following: (i) the potential applications of 1 and 2 as
precatalysts for the epoxidation process and (ii) their
effect on catalytic performances.
higher energy intraligand transitions in the UV region in
CH CN (Figure 4, see the Experimental Section). The
7,9
3
two visible energy bands near 550 and 400 nm are assigned
on the basis of the TD-DFT calculations on the optimized
II
structures of 1 and 2 as dπ(Ru ) f pπ*(trpy)/pπ*(L)
II
and dπ(Ru )f pπ*(L) transitions, respectively (Tables S3
and S4 in the SI). Interestingly, isomeric complexes 1 and 2
Catalytic Epoxidation Reactions. The isomeric com-
plexes 1 and 2 have been tested for the epoxidation of a
wide variety of alkene functionalities using H O , BuOOH
are quite distinct with respect to their lowest energy MLCT
(metal-to-ligand charge transfer) band positions at 559 nm
t
2
2
-
1
17 889 cm ) and 533 nm (18762 cm ), respectively. The
-1
(tert-butylhydroperoxide, TBHP), and m-CPBA (meta-
chloroperbenzoic acid) as possible oxidants in solvents such
as C H OH, CH Cl , and CH CN under neutral reaction
(
-
1
MLCT transition energy 873 cm lower in 1 with respect
to 2 has also been reflected in the HOMO-LUMO gaps of
2.59 and 2.71 eV in 1 and 2, respectively (Figure 5).
Furthermore, the intensity of the 533 nm band in 2 of
2
5
2
2
3
5
conditions. The catalytic results are listed in Table 2.
Remarkably, isomeric complexes 1 and 2 exhibit significant
differences in their catalytic performances toward the
epoxidation of alkene functionalities (Table 2) primarily
based on their built-in differences in structural/electronic
properties (see later). Considering H O , TBHP, and
-
1
-1
ε = 14710 M cm is almost double that in 1 at 559 nm,
-
1
-1
ε = 8477 M cm . A similar trend in the isomer based
difference in intensity of the MLCT bands has been re-
ported in isomeric [Ru(trpy)(3-amino-6-(3,5-dimethylpyr-
azol-1-yl)-1,2,4,5-tetrazine)(Cl)] .
The orbitalcontributions in MOs in 1and 2(Figure5and
Tables S5 and S6 in the SI) predict that the HOMOs
2
2
þ 7a
m-CPBA as possible oxidants and C H OH, CH Cl , and
2 5 2 2
CH CN as possible solvents, the specific combination of
3
H O and EtOH is found to give the best results for 1
2
2
irrespective of the nature of the alkene substrates (Table 2).
Among the two other tested oxidants, TBHP and m-CPBA,
TBHP is found to be relatively better than m-CPBA for 1.
The poor solubility of m-CPBA in CH Cl is possibly the
(
HOMO to HOMO-3) are primarily composed of metal-
based orbitals with varying partial contributions from Cl,
trpy, or L. Accordingly, reversible Ru H Ru couples in 1
II
III
2
2
and 2 appear at E298°, V (ΔE , mV), of 0.35(70) and
p
reason for the low conversion in all cases. Isomer 2 in
general exhibits poor catalytic activity for all of the chosen
substrates. However, unlike1, the combination of m-CPBA
0
.38(65) in CH CN versus SCE (Figure S2 in the SI). The
3
(
9) Llobet, A.; Doppelt, P.; Meyer, T. J. Inorg. Chem. 1988, 27, 514.
and CH CN shows relatively better catalytic performance
3

1
780 Inorganic Chemistry, Vol. 50, No. 5, 2011
Chowdhury et al.
a
Table 2. Catalytic Results
for 2 (Table 2). It should be stated that the organic oxidants
such as m-CPBA and TBHP are known to have limitations
particularly because of their low atom efficiency and the
formation of organic side-products during the course of the
2,3e
reaction.
Though isomer 1 exhibits excellent catalytic activity in
terms of chemoselectivity, percent conversion, and tunabil-
ity under environmentally benign conditions using H O as
2
2
1
d,10
an oxidant and EtOH as a solvent (Table 2),
show any epoxidation reaction with the molecular oxygen
it failed to
11
(O ) in spite of its larger atom efficiency (50%) than that
of H O (up to 47%). However, oxidation involving mo-
2
2
2
lecular oxygen is also known to produce waste from the co-
reductant, whereas H O produces H O as the only
coproduct.
2
2
2
1
2
It should be stated that the controlled catalytic experi-
mentsintheabsenceofasubstraterevealthat1or 2remains
inert toward the oxidation of ethanol under identical
experimental conditions (Table 2). Though 1 and 2 behave
similarly toward the catalytic epoxidation process in EtOH
and tert-amyl alcohol (Table 2 and Table S8 in the SI,
respectively), catalytic studies have been conducted in an
1d,10
environmentally friendly ethanol solvent.
In general, the performance of catalyst 1 for the epoxida-
tion processes (Table 2) in the presence of an oxidant, H O
2
2
in EtOH, is comparable to that reported for other ruthe-
3,4
nium based catalysts. However, the percent conversion
for the substrate stilbene (cis and trans) using 1 as a catalyst
is found to be moderate, though the transformation is
totally chemoselective (Table 2). The ruthenium catalyzed
epoxidation process is also known to facilitate the cleavage
of the C-C bond of epoxides leading to either aldehyde or
ketone or alcohol depending on the nature of the sub-
3,4
strates, as has also been observed with 1 while using
oxidant TBHP or m-CPBA. However, the epoxidation
process using 1 as a catalyst and H O as an oxidant in
2
2
EtOH appears to be totally chemoselective irrespective of
the percent conversion (Table 2).
The efficiency of catalyst 1 with respect to catalyst
loading has also been checked with the specific substrate
R-methylstyrene, which shows TON = 1300 even when
using a quite low amount (0.01 mol %) of the catalyst
(Table S9 in the SI). Upon a further decrease in ruthenium
content to 0.001 mol %, the conversion diminishes, but
observable activity has also been evidenced (Table S9 in the
SI). However, the TON sharply increases from 1300 to 7000
(10) (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and
Practice; Oxford University Press: New York, 1998; p 30. (b) Capello, C.;
Fischer, U.; Hungerbuhler, K. Green Chem. 2007, 9, 927.
(11) (a) Marky, I. E.; Giles, P. R.; Tsukazaki, M.; Brown, S. M.; Urch,
C. J. Science 1996, 274, 2044. (b) Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A.
Science 2000, 287, 1636. (c) Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew.
Chem., Int. Ed. 2001, 40, 3639. (d) Khenkin, A. M.; Shimon, L. J. W.; Neumann,
R. Inorg. Chem. 2003, 42, 3331. (e) D €o bler, C.; Mehltretter, G. M.; Sundermeier,
U.; Beller, M. J. Organomet. Chem. 2001, 621, 70. (f) Groves, J. T.; Quinn, R.
J. Am. Chem. Soc. 1985, 107, 5790. (g) Paeng, I. R.; Nakamoto, K. J. Am.
Chem. Soc. 1990, 112, 3289.
(12) (a) Strukul, G. Catalytic Oxidations with Hydrogen Peroxide as
Oxidant; Kluwer Academic: Dordrecht, The Netherlands, 1992. (b) Jones,
C. W. Applications of Hydrogen Peroxide and Derivatives; Royal Society of
Chemistry: Cambridge, U. K., 1999. (c) Elvers, B.; Hawkins, S.; Ravenscroft, M.;
Schulz, G. Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH:
New York, 1989; Vol. A13, p 443. (d) Kroschwitz, J. I.; Howe-Grant, M. Kirk-
Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1995;
Vol. 13, p 961.
a
Detailed reaction conditions are given in the Experimental Section.
Products are characterized by GC. Substrate/catalyst = 200:1 in each
b
c
case. Selectivity in terms of epoxide formation. See Table S17 for
d
product distribution ratio. Product is 1,2-propanediol, formed with
e
00% selectivity. Product is 1-chloro-2-hydroxy propane, formed with
1
f
00% selectivity. Products are characterized by H NMR.
1
1

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1781
-
upon the subsequent injection of free ligands (0.1 mol %
trpy and L each) externally into the reaction system having
possible isomers (the anionic O donor of the picolinate
ligand trans to the H O group) as well as other oxidants and
solvents however were not explored.
2
4
i,j
0.01 mol % catalyst (Entry 4, Table S9 in the SI). This
implies the additional role of free ligands, trpy, and L in the
presence of 1 toward the epoxidation process.
Preference of Isomeric Structure on Epoxidation. The
preference for the isomeric structure of 1 over 2 in epoxida-
tion processes (Table 2) can be rationalized on the basis of
the following specific considerations: (i) The longer Ru-Cl
The aforementioned observation has prompted us to
investigate the individual roles of Ru, trpy, L, mixed trpy/
L, and mixed Ru/trpy/L (i.e., the in situ generated
metal-ligand(s) based adduct) toward the epoxidation
process, and the results are summarized in Table S10 in
the SI. Itis revealedthat under identical reaction conditions
˚
˚
bond in 1 (2.408(2) A) compared to that in 2 (2.3813(9) A)
makes it relatively more labile, which in turn facilitates the
formation of the active {RudO} species in the presence of
an oxidant, H O . The labile feature of the Ru-Cl bond in
2
2
II
(
as a substrate), the use of no Ru or only Ru asRuCl or only
H O as an oxidant, EtOH as a solvent, R-methylstyrene
[Ru (trpy)(L)(Cl)] (1) has been evidenced by its facile
2
2
transformation to the single crystals of the corresponding
solvate species [Ru (trpy)(L)(CH CN)](Cl) (1a, Table 1
3
II
ligands, trpy/L (Entries 1-3, Table S10 in the SI) is not at
all effective in facilitating the epoxidation process. On the
other hand, the combinations of Ru (as RuCl )/trpy and
3
3
and Figure S3 and Table S1 in the SI) while crystallizing 1
from the coordinating acetonitrile solvent over a period of
few days. However, the same isomer, 1, could easily be
crystallized from the noncoordinating CH Cl solvent
Ru(asRuCl )/L result in negligible and moderate epoxides,
3
respectively. On the contrary, the use of the combination of
Ru (as RuCl )/trpy/L, i.e., the situation of an in situ
2
2
(Figure 1a and Table 1 and Table S1 in the SI). On the
contrary, isomer 2 remains stable in coordinating CH CN
3
generated adduct, shows 70% conversion with 90% chemo-
selective epoxidation (Entry 6, Table S10 in the SI), which is
however less than that achieved using the preformed
catalyst 1 (Table 2). This in effect implies that the use of a
preformed catalyst, isomer 1, instead of an in situ generated
catalyst is the better option for the chemoselective epoxida-
tion process.
3
solvent and thus can be smoothly crystallized without any
exchange of “Cl” by the solvent CH CN (Figure 1b and
3
Table 1 and Table S1 in the SI; see the Experimental
Section). (ii) The addition of H O in an ethanolic solution
2
2
of 1 leads to a spontaneous change in color from blue-violet
to yellow due to the in situ formation of the corresponding
ruthenium-oxo species [Ru (trpy)(L)(O)]Cl (3) as evi-
0
0
IV
The aqua complexes, 1 (corresponds to 1) and 2
corresponds to 2), have also been synthesized (see the
(
denced by its ESI-mass (m/z, 522.98; calculated mass:
0
0
IV
þ
þ
Experimental Section). Complexes 1 and 2 exhibit two
successive oxidation processes at E° of 0.19 V and 0.41 V
and of 0.25 V and 0.56 V at pH 7 versus SCE in H O,
respectively. The potential increases on lowering the pH to
at 0.39 V and 0.58 V and at 0.48 V and 0.93 V for 1 and 2 ,
respectively. The successive two oxidation processes are
assigned on the basis of earlier reports for the analogous
523.03 corresponding to [Ru (trpy)(L)(O)] (3 ), Figure
S4 in the SI) and UV-visible (Figure 4, inset) spectral
2
98
13
features. On the subsequent addition of PPh to the above
solution of 3 in ethanol, the UV-visible spectrum of the
oxo-species spontaneously changes to that of the solvate
2
3
0
0
1
II
species [Ru (trpy)(L)(EtOH)] (Figure 4, Inset) via eq 1.
þ
13
4h,9
The formation of Ph PdO has been evidenced by its
3
II
III
-
þ
31
characteristic P NMR peak at 26.25 ppm in (CD ) SO.
complexes as {Ru -OH } f {Ru -OH} þ e þ H and
2
3 2
III
IV
-
þ
{
lower oxidation potential of 1 compared to 2 implies that
Ru -OH} f {Ru =O} þ e þ H , respectively. The
0 0
IV
H2O2
s
f
EtOH
II
IV
½Ru ðtrpyÞðLÞClꢀ
½Ru ðtrpyÞðLÞOꢀCl
the eventual formation of {Ru dO} species is likely to be
0
IV
more spontaneous in the case of 1 or 1. However, we failed
to isolate the pure {Ru dO} species via chemical oxida-
PPh3
II
s
f
½Ru ðtrpyÞðLÞðEtOHÞꢀCl
ð1Þ
II
4þ
-
Ph3PdO
tion of the {Ru -H O} species using Ce salt or H O as
2
2 2
IV
possible oxidants; the initially formed {Ru dO} species
was decomposed to an unidentified material during the
workup process. This indeed has precluded conducting
Unlike 1, under identical reaction conditions, the addition
of H O or m-CPBA to the ethanolic solution of 2 yields an
unstable oxo species (4 ), which has indeed precluded its
2
2
þ
IV
any further studies with the {Ru dO} species; however,
II
further characterization. Consequently, DFT calculations
on the optimized ruthenium-oxo species, [Ru (trpy)-
detailed studies with the isomeric {Ru -H O} species
1 and 2 ) are in progress.
Interestingly, under identical experimental conditions,
2
IV
0
0
(
þ
þ
þ
(L)(O)] (isomeric 3 (corresponds to isomer 1) and 4
(corresponds to isomer 2); Figure S5 and Table S12 in the
II
II
0
0
{
Ru -Cl} (1 and 2) and {Ru -OH } (1 and 2 ) com-
2
þ
SI) in the triplet, S = 1 state [the triplet (S = 1) states in 3
and 4 are 21.5 and 21.7 kcal/mol, respectively, more stable
plexes show similar trends in catalytic properties toward the
epoxidation process (Table 2 and Tables S8 and S11 in the
SI), which again emphasizes that the specific orientation of
þ
than the corresponding singlet states], predict that the oxo
species, 3 (corresponding to isomer 1) is 10.2 kcal/mol
þ
0
L in 1 or 1 makes the Ru-Cl or Ru-OH bond more labile
2
þ
more stable than the oxo-counterpart in 4 (corresponding
to isomer 2). (iii) The NBO analysis on the optimized
and hence catalytically active compared to the other isomer
0
2
or 2 .
þ
þ
ruthenium-oxospecies(3 and 4 ) predictsamore negative
charge on the oxygen atom of the RudO(3) site in 3 than
that in 4 (NBO/Mulliken charges on the O3 atom of 3
The analogous ruthenium-aqua complex incorporating
trpy and picolinate ligands in [Ru (trpy)(picolinate)-
þ
II
þ
þ
þ
(
was considered trans to the H O group) was reported to
H O)] (the neutral “N” donor of the picolinate ligand
2
2
(
13) (a) Dutta, P. K.; Das, S. K. J. Am. Chem. Soc. 1997, 119, 4311. (b)
Taqui Khan, M. M.; Chatterjee, D.; Samad, S. A.; Merchant, R. R. J. Mol. Catal.
990, 61, 55. (c) Moyer, B. A.; Sipe, B. K.; Meyer, T. J. Inorg. Chem. 1981, 20,
1475.
result in nonchemoselective epoxidation processes of sty-
rene derivatives as well as cis/trans-stilbene using TBHP as
an oxidant in a CH CN solvent. The effects of other
3
1

1
782 Inorganic Chemistry, Vol. 50, No. 5, 2011
Chowdhury et al.
a
Scheme 1. Proposed Reaction Pathway
Table 3. Effect of Various Radical Trapping Agents
molar equivalent
of radical
scavenger
d
radical
entry scavenger
conversion
(%)
selectivity
(%)
b
c
1
2
3
4
5
6
-
-
1
5
50
100
100
100
91
67
21
0
100
100
93
40
0
DMTU
DMTU
TEMPO
TEMPO
duroquinone
53
69
a
Detailed reaction conditions are given in the Experimental Section.
Products are characterized by GC. Substrate (R-methylstyrene)/catalyst =
b
00:1 in each case. DMTU: N,N -dimethylthiourea. TEMPO: 2,2,6,
0
2
6
-tetramethylpiperidine-1-oxyl. Duroquinone: 2,3,5,6-tetramethyl-p-ben-
c
d
zoquinone. With respect to catalyst. Selectivity in terms of epoxide
formation.
þ
and 4 are -0.413/-0.480 and -0.318/-0.394, respec-
Scheme 2. Side-On Mode of Binding of the Alkene with the Metal-
Oxo Species
tively, Table S7 in the SI), while the Ru ion carries an
almostsimilarchargein both cases(NBO/Mullikencharges
on the Ru atom are 0.976/1.012 and 1.004/1.015 for 3 and
a
þ
þ
4
, respectively, Table S7 in the SI), implying a relatively
þ
more polarized Ru-O3 bond in 3 , as has also been
þ
reflected in the longer calculated Ru-O(3) bond in 3
þ
þ
˚
1.80 A (3 ) and 1.77 A (4 ), Table S12 in the SI). (iv) The
˚
(
SOMO of 3 is primarily dominated by the metal ion
þ
(
%Ru, L, trpy, O: 53, 23, 4, 20, respectively), while the
þ
same in 4 is dominated by L (%Ru, L, trpy, O: 16, 78, 3, 4,
respectively; Tables S13-S14 in the SI), which possibly
suggests the less electrophilic character of 4 as compared
to 3 . Thus, the collective effects of relatively better stability
and greater electrophilicity of the oxo species in 3
a
þ
(
alkene.
a) Less hindered approach for cis-alkene with respect to (b) trans-
þ
þ
Furthermore, the conversion of sulfide to sulfoxide and/or
sulfone by 1 under identical experimental conditions (Table
S15 in the SI) also extends additional justification in favor of
(
corresponding to isomer 1) in combination with greater
þ
polarizability of the longer Ru-O(3) bond in 3 as com-
pared to 4 (corresponding to isomer 2) make it a better
þ
IV
III
15
the {Ru dO} active species instead of {Ru -OH}.
active catalyst toward the epoxidation processes in Table 2.
Mechanistic Outlook. Metal catalyzed epoxidation of
alkenes is known to proceed through the formation of an
active metal-oxo species via the mediation of a suitable
oxidant followed by interaction of the metal bound oxo
group with the olefinic double bond leading to an even-
In metal catalyzed epoxidation reactions, the alkene
group is proposed to approach the metal-oxo bond in a
1
6
side-on fashion (a, Scheme 2), which in effect makes the
cis-alkene (a) a more active substrate than the corre-
sponding trans-alkene primarily due to the steric con-
straints, as shown in b (Scheme 2).
4
,5
tual oxo-transfer process, as shown in Scheme 1.
In order to ascertain the nature of the active species,
The transfer of the oxygen atom from the metal-oxo
complex to the olefinic double bond has been proposed to
proceed via five possible intermediates: (a) a concerted
IV
III
16
either {Ru dO} or {Ru -OH}, during the catalytic
epoxidation process by 1, the conversion of R-methylstyr-
ene to R-methylstyrene oxide has been tested in the presence
transition state, (b) a carbon radical, (c) a carbocation, (d)
1
6f,17
a π-radical cation, or (e) a metalaoxetane (Scheme 3).
DFT calculations are performed on the optimized
0
of specific hydroxyl radical scavenger N,N -dimethyl-
14
thiourea (DMTU). The observed high conversion (91%)
of R-methylstyrene to R-methylstyrene oxide even in the
presence of a hydroxyl radical scavenger (DMTU; entry
intermediate adduct (5) comprised of ruthenium-oxo
species in 3 and styrene as the model substrate considering
þ
the convenient side-on approach (Scheme 4, Figure S6,
Table S16 in the SI).
IV
no. 2, Table 3) confirms the involvement of {Ru dO} as
the active species. The relatively lower conversion of
R-methylstyrene to R-methylstyrene oxide (67%) while
using a higher concentration of DMTU (1:5 ratio of 1 and
DMTU, entry no. 3, Table 3) could be attributed to the
radical mechanism of the reaction (see later).
The asymmetric side-on approach of the O3 atom of the
RudO(3) bond in 3 to the incoming alkene function
results in intermediate 5. The calculated C -C and
R
β
˚
C -O(3) distances of 1.5 and 1.481 A, respectively, in
β
2
3
the optimized structure of 5 imply sp and sp characters
of C and C , respectively. Consequently, the calculated
R
β
˚
Ru-O3 distance in 5 increases to 1.967 A with respect to
(
14) (a) Douglas, E.; Paull, M. D.; Blair, A.; Keagy, M. D.; Eric, J.; Kron,
B. S.; Benson, R.; Wilcox, M. D. J. Surg. Res. 1989, 46, 333. (b) Bunda, S.;
Kaviani, N.; Hinek, A. J. Biol. Chem. 2005, 280, 2341.
(16) (a) Groves, J. T.; Kruper, W. J. J. Am. Chem. Soc. 1979, 101, 7613. (b)
Groves, J. T.; Meyers, R. S. J. Am. Chem. Soc. 1983, 105, 5786. (c) Jørgensen,
K. A. Chem. Rev. 1989, 89, 431. (d) Meunier, B. Chem. Rev. 1992, 92, 1411. (e)
Jørgensen, K. A.; Schiøtt, B. Chem. Rev. 1990, 90, 1483. (f) Ostovic, D.; Bruice,
T. C. Acc. Chem. Res. 1992, 25, 314.
(
15) (a) Acquaye, J. H.; Muller, J. G.; Takeuchi, K. J. Inorg. Chem. 1993,
3
2, 160. (b) Lai, S.; Lepage, C. J.; Lee, D. G. Inorg. Chem. 2002, 41, 1954. (c)
Hamelin, O.; M ꢀe nage, S.; Charnay, F.; Chavarot, V; Pierre, J.-L.; P ꢀe caut, J.;
Fontecave, M. Inorg. Chem. 2008, 47, 6413. (d) Chavarot, M.; M ꢀe nage, S.;
Hamelin, O.; Charnay, F.; P ꢀe caut, J.; Fontecave, M. Inorg. Chem. 2003, 42, 4810.
(17) Fung, W.-H.; Yu, W.-Y.; Che, C.-M. J. Org. Chem. 1998, 63, 7715.

Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1783
Scheme 3. Proposed Intermediates for Oxygen Atom Transfer from
Metal-Oxo Species to Alkene
metal-ligand bond parameters, and consequently 1
and 2 have shown some differences in their spectral
and redox processes on the basis of their electronic
structural features, as has also been supported by the
DFT calculations.
•
Remarkably, the limited differences in electronic
structural features in isomeric 1 and 2 have been
reflected significantly in their catalytic performances
toward the epoxidation of olefinic functionalities.
Isomer 1 has been established to be an excellent
precatalyst for the chemoselective epoxidation of a
wide variety of olefins with low catalyst loading under
the environmentally benign reaction conditions using
H O as the oxidant and EtOH as the solvent. On the
Scheme 4. Formation of Epoxide through Radical Pathway
2
2
contrary, under identical reaction conditions, isomer 2
is almost inactive and shows some activity only in
CH CN solvent in the presence of m-CPBA as the
3
oxidant (Table 2).
•
•
The experimental and DFT calculations suggest that
the greater lability of the {Ru-Cl} bond in isomer 1
than that in 2 and better stability and higher electro-
philicity of the active ruthenium-oxo species, [Ru-
IV
the calculated Ru dO bond distance of 1.799 A in 3 ,
þ
˚
which indeed suggests the single bond character of
þ
þ
(
1
trpy)(L)(O)] , in 3 (corresponding to the precatalyst
) as compared to the isomeric oxo-species 4
4
k
Ru-O(3) in 5. Spin density calculations on 5 predict
the spin density of 0.73 on C , revealing its radical feature
þ
R
Table S7 in the SI). The calculated NBO and Mulliken
(corresponding to the precatalyst 2) are the primary
(
charges on Ru in 5, 0.96 and 0.97, respectively, are close to
contributing factors toward the superior performance
of 1 for the epoxidation process.
þ
those calculated for the starting 3 , suggesting the same
þ4 charge on Ru in both 3 and 5 (Table S7 in the SI). All
The controlled experiments in combination with
DFT results establish that the epoxidation reaction
proceeds via the involvement of {Ru dO} active
þ
of these collectively suggest that the concept of the radical
mechanism is also functional in the present case, as has
been proposed recently in Ru-catalyzed epoxidation.
IV
species through a radical pathway, as shown in
Scheme 1.
4
k
In accordance with the DFT proposed radical inter-
mediate step, b in Scheme 3, the epoxidation of cis- or
trans-β-methylstyrene by 1 results in a mixture of cis- and
trans-β-methylstyrene oxide (Table S17 in the SI) due to
isomerization during the lifetime of the intermediate radi-
Experimental Section
Materials. The precursor complex Ru(trpy)Cl (trpy =
3
0
0
00
2,2 :6 ,2 -terpyridine) was prepared according to literature
procedures. The ligand quinaldic acid (HL) and other reagents
3
p,4h,5g
18a
cal state, 5 in Scheme 4.
methylstyrene oxide as the major product in each case can
The formation of trans-β-
and chemicals were obtained from Aldrich and used as received.
Solvents were dried by following the standard procedures,
3
p,4h,5g
18b
be attributed to its thermodynamic stability.
distilled under nitrogen, and used immediately. High purity
deionized water used for the electrochemistry experiments of
Moreover, in the presence of free radical scavengers,
TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) or duro-
quinone (2,3,5,6-tetramethyl-p-benzoquinone), the conver-
sion as well as chemoselectivity (Table 3) of the reaction,
R-methylstyrene to R-methylstyrene oxide, diminish to a
large extent, which also clearly justifies the proposed radical
0
0
the aqua species (1 and 2 ) was obtained by passing distilled
water through a nanopure Mili-Q water purification system. For
spectroscopic and electrochemical studies, HPLC-grade solvents
were used.
1
31
Instrumentation. H/ P NMR spectra were recorded on a
00 MHz Varian spectrometer. IR and UV-vis spectra were
recorded using Thermo Nicolet 320 and Perkin-Elmer Lambda
5g
mechanism (entry nos. 4-6, Table 3).
3
Conclusion
950 spectrophotometers, respectively. ESI-Mass spectra were
recorded using a micromass Q-TOF mass spectrometer. Cyclic
voltammetric studies were carried out using a PAR model 273A
electrochemistry system. Platinum wire working and auxiliary
electrodes and an aqueous SCE were used in a three-electrode
The following are the salient features of the present work:
• Though preformed ruthenium-aqua complexes with
4
h,i
selective coligands or in situ generated ruthenium-
solvate species from the preformed coordinatively
saturated ruthenium complexes in a bis-tridentate
configuration. The supporting electrolyte was 0.1 M [NEt
4
][ClO
(NaClO in case of 1 and 2 ), and the solute concentration
4
4
]
0
0
4a,5d-5f
ligand environment
have been established to
-3
was ∼10 M. The half-wave potential E298° was set equal to
be suitable molecular frameworks for the catalytic
epoxidation process, the present report demonstrates
that the preformed synthetically simple {Ru-Cl}
species, as in [Ru(trpy)(L)(Cl)] (1), can also be a
convenient precatalyst for the chemoselective epoxi-
dation process.
0
.5(E þ E ), where E and Epc are the anodic and cathodic
pa
pc
pa
cyclic voltammetric peak potentials, respectively. Elemental anal-
yses were carried out on a Perkin-Elmer 240C elemental analyzer.
The catalytic reactions were monitored using gas chromato-
graphic techniques with an FID detector (Shimadzu GC-2014
gas chromatograph) using a capillary column (112-2562 CY-
CLODEXB, from J&W Scientific, length 60 m, inner diameter
0.25 mm, film 0.25 μm).
•
The effect of isomeric structural features of [Ru(trpy)-
(L)(Cl)] in 1 and 2 has been reflected in their relevant

1
784 Inorganic Chemistry, Vol. 50, No. 5, 2011
Chowdhury et al.
General Procedure for Catalytic Epoxidation Study. In a
Tomasi and co-workers; specifically, the conductor like PCM
(CPCM) in conjugation with the united atom topological model
(using UAO radii, implemented in Gaussian 03) was applied.
typical reaction, the catalyst (0.025 mmol) in 3 mL of solvent
EtOHor tert-amyl alcohol or CH Cl or CH CN) was placed in a
5 mL Schlenktube andstirredfor 10 min at 298 K. The respective
26-28
(
2
2
2
3
29
GaussSum was used to calculate the fractional contributions of
various groups to each molecular orbital. No symmetry con-
straints were imposed during structural optimizations, and the
nature of the optimized structures and energy minima were defined
by subsequent frequency calculations. Natural bond orbital anal-
yses were performed using the NBO 3.1 module of Gaussian 03
olefins (0.5 mmol) and dodecane (GC internal standard) werethen
added into the catalyst solution under stirring conditions. The
oxidant (3 equiv of 30% H O or 1.5 equiv of TBHP or 1.2 equiv
2
2
of 50% m-CPBA in 3 mL of respective solvents) was added over a
period of 8 h through a syringe pump. The percent yield and
percent conversion were determined using the GC technique using
30
on optimized geometry. A31ll of the calculated structures were
visualized with ChemCraft.
Synthesis of Isomeric Complexes [Ru(trpy)(L)Cl] (1 and 2). A
1
respective standard product samples or using H NMR.
Crystallography. Single crystals of 1 and 1a/2 were grown by
slow evaporations of their dichloromethane and 1:1 acetonitrile-
toluene solutions, respectively. X-ray data were collected using an
OXFORD XCALIBUR-S CCD single-crystal X-ray diffrac-
tometer. The structures were solved and refined using full matrix
3
total of 100 mg (0.23 mmol) of Ru(trpy)Cl , 55 mg (0.32 mmol) of
quinaldic acid, and NEt (1.2 mL, 1.0 mmol) were taken in 15 mL
3
of ethanol. The mixture was heated to reflux for 6 h under a
dinitrogen atmosphere. The initial browncolor gradually changed
toviolet, andthe solvent of the reactionmixture was evaporatedto
drynessunder reduced pressure. The violet solid thus obtained was
2
19
least-squares techniques on F using the SHELX-97 program.
The absorption corrections were done using multiscan (SHELXTL
program package), and all data were corrected for Lorentz
polarization effects. Hydrogen atoms were included in the
refinement process as per the riding model. Selected crystal-
lographic parameters are given in Table 1. The hydrogen atoms
associated with the crystallized water and disordered acetoni-
trile molecules in 1a could not be located. The asymmetric unit
of 1a contains two crystallographically independent molecules.
Computational Details. Full geometry optimizations were
dissolved in a minimum volume of CH
neutral alumina column. The blue-violet solution corresponding
to isomer 1 was eluted first with CH Cl -CH OH (20:1) followed
by the red-violet solution of the isomer 2 with a CH Cl -CH OH
2 2
Cl and purified by using a
2
2
3
2
2
3
(10:1) mixture. On removal of the solvent under reduced pressure,
the pure isomeric complexes 1 and 2 were obtained in the solid
state.
Complex 1: Yield, 43 mg (35%). Anal. Calcd for C25
ClO Ru (M.W. 542.01). Found: C, 55.35 (55.15); H, 3.16
(3.08); N, 10.33 (10.42%). λ [nm] (ε[M cm ]) in acetonitrile:
559 (8477), 417 (7330), 324 (29 859), 312 (27 489), 278 (26 117),
17
H -
2
0,21
carried out at the (R)B3LYP and (U)B3LYP levels
and 2 and for 3 , 4 , and 5, respectively, using the density
for 1
N
4
2
þ
þ
-1
-1
22
functional theory method with Gaussian 03 (revision C.02).
All elements except ruthenium were assigned the 6-31G(d) basis
set. The LanL2DZ basis set with the effective core potential was
Vertical electronic excita-
tions based on B3LYP optimized geometries were computed for
267 (sh), 239 (68 065). ESI-MS (m/z): 543.85 (1), 507.03 (1-Cl).
1
H NMR in (CD ) SO [δ/ppm(J/Hz)]: 10.2 (d, 8.8, 1H), 8.7 (d,
3 2
23,24
employed for the ruthenium atom.
8.4, 1H), 8.6 (m, 4H), 8.3 (t, 5.5, 5.1, 1H), 8.1 (d, 8.4, 1H), 7.9 (m,
7H), 7.4 (m, 2H).
25
the time-dependent density functional theory (TD-DFT) formalism
in acetonitrile using the polarizable continuum model (PCM) of
Complex 2: Yield, 37 mg (30%). Anal. Calcd for C25
ClO Ru (M.W. 542.01). Found: C, 55.35 (55.27); H, 3.16
3.10); N, 10.33 (10.58%). λ [nm] (ε[M cm ]) in acetonitrile:
H -
17
N
(
4
2
-1
-1
5
2
33 (14 710), 378 (11 407), 318 (52 359), 310 (sh), 278 (42 199),
39 (111 575). ESI-MS (m/z): 543.91 (2), 507.01 (2-Cl).
(
18) (a) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19,
404. (b) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980.
19) Sheldrick, G. M. SHELX-97; University of G €o ttingen: G €o ttingen,
1
H
1
3 2
NMR in (CD )
2H), 8.35 (m, 2H), 8.27 (d, 8.1, 1H), 8.16 (d, 8.4, 1H), 7.93 (m,
4H), 7.81 (d, 7.5, 1H), 7.55 (m, 2H), 7.36 (t, 7.5, 7.8, 1H), 7.13 (t,
SO [δ/ppm (J/Hz)]: 8.73 (d, 8.1, 1H), 8.6 (m,
(
Germany, 1997.
(
(
(
20) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
21) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1998, 37, 785.
22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
7
.5, 7.5, 1H), 6.44 (d, 8.8,1H).
Synthesis of Isomeric [Ru(trpy)(L)(H O)]ClO (1 ) and (2 ).
0
0
2
4
0
0
Aqua complexes, 1 and 2 , were prepared by adopting the
literature reported procedure starting from 50 mg of precursor
chloro complexes 1 and 2, respectively.
0
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji,
H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
9
Complex 1 : Yield, 60 mg (52%). Anal. Calcd for C H N -
19
2
5
4
ClO
7
Ru (M.W. 624.10). Found: C, 48.08 (47.91); H, 3.07 (3.00);
-1
2
-1
N, 8.98 (8.85%). Molar conductivity [Λ
dichloromethane = 110. λ [nm] (ε[M cm ]) in dichloro-
methane: 497 (3910), 371 (sh), 314 (13 350), 275 (15 110), 241
M
-1
(Ω cm M )] in
-1
0
(
(
37 480). ESI-MS (m/z): 525.03 (1 -ClO
0
4
; calcd 525.05), 507.01
CO [δ/ppm(J/
1
O; calcd 507.04). H NMR in (CD
1 -ClO
4
-H
2
3
)
2
Hz)]: 9.52 (d, 9.9, 1H), 8.85 (d, 10.4, 1H), 8.39 (m, 2H), 8.2 (m,
3H), 8.06 (t, 8.4, 8.6, 2H), 7.98 (t, 8.5, 8.6, 2H), 7.8 (m, 3H), 7.58
(m, 3H)
Complex 2 : Yield, 65 mg (57%). Anal. Calcd for
C H N ClO Ru (M.W. 624.10). Found: C, 48.08(47.99); H,
0
2
5
19
4
7
(
23) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
-1
2
3
M
.07(2.95); N, 8.98(9.16%). Molar Conductivity [Λ
M
(Ω cm
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; p 1.
(
(
-
1
-1
-1
)] in dichloromethane = 100. λ[nm] (ε[M cm ]) in di-
24) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
25) (a) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454.
chloromethane: 477 (4480), 374 (sh), 315 (15 330), 274 (15 380),
(
8
b) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109,
218. (c) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem.
Phys. 1998, 108, 4439.
(30) (a) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F.
NBO, version 3.1; Theoretical Chemistry Institute, University of Wisconsin:
Madison, WI, 2001. (b) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736.
(c) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (d)
Weinhold, F. In Encyclopedia of Computational Chemistry; Schleyer, P. v. R.,
Ed.; Wiley: New York, 1998; p 1792.
(
26) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117.
27) Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Chem. Phys. Lett.
(
1
996, 255, 327.
(
(
28) Barone, V.; Cossi, M. J. Chem. Phys. A 1998, 102, 1995.
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sf.net (accessed Jan 2011). (b) O'Boyle, N. M.; Tenderholt, A. L.; Langner, K. M.
J. Comput. Chem. 2008, 29, 839.
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Article
Inorganic Chemistry, Vol. 50, No. 5, 2011 1785
0
2
5
1
3
6
41 (39 730). ESI-MS (m/z): 507.03 (2 -ClO
4
-H
2
O; calcd
National Single Crystal Diffractometer Facility, Indian Institute
Technology Bombay. H NMR experiments were carried out at
1
07.04). H NMR in (CD
1
3
)
2
CO [δ/ppm(J/Hz)]: 8.83 (d, 8.1,
H), 8.65 (d, 9.4, 1H), 8.41 (d, 8.4, 1H), 8.32 (d, 8.1, 1H), 8.0 (m,
H), 7.85 (d, 8.3, 1H), 7.64 (m, 3H), 7.4 (m, 3H), 7.25 (m, 2H),
.45 (d, 9.5,1H).
the Sophisticated Analytical Instrument Facility (SAIF), Indian
Institute Technology Bombay. Computational facilities from the
Department of Chemistry, IIT Bombay are gratefully acknowl-
edged. The reviewers’ comments at the revision stage were very
helpful.
Acknowledgment. This work is Dedicated to Dr. Sumit
Bhaduri on the occasion of his 60th birthday. Financial support
received from the Department of Science and Technology (DST)
and Council of Scientific and Industrial Research (CSIR)
Supporting Information Available: X-ray crystallographic
files in CIF format, bond distances/angles, DFT data set and
mass spectra (Figures S1-S6 and Tables S1-S17). This material
is available free of charge via the Internet at http://pubs.acs.org.
(fellowship to A.D.C. and A.D.), New Delhi, India is gratefully
acknowledged. X-ray structural studies were carried out at the