Synthesis, characterization, structures and GPx mimicking activity of pyridyl and pyrimidyl based organoselenium compounds

Article abstract of DOI:10.1016/j.jorganchem.2012.08.035

Pyridyl and pyrimidyl based organoselenium compounds have been synthesized and characterized by analytical and spectroscopic techniques. Molecular structures of 2,2′-dipyrimidyl diselenide (1b), 2,2′-dipyrimidyl selenide (2b) and 2-pyrimidyl seleno ethanoic acid (3d) have been determined by single crystal X-ray diffraction analyses. The 3d is associated in the solid state through hydrogen bonding between carboxylic acid proton and N2 of the pyrimidyl ring of an adjacent molecule. The in vitro GPx-like catalytic activity for these compounds was evaluated by ¹H NMR and HPLC methods where H₂O₂ was reduced by dithiothreitol (DTT^red) and glutathione (GSH) as a thiol cofactor, respectively, in the presence of catalytic amounts of organoselenium compounds. The electron density around selenium atom (-SeSe- or -Se-) which is reflected by ⁷⁷Se{ ¹H} NMR chemical shifts, has been found to be one of the crucial factors in influencing their overall GPx like activity.

Full text of DOI:10.1016/j.jorganchem.2012.08.035

Journal of Organometallic Chemistry 720 (2012) 19e25
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, structures and GPx mimicking activity of pyridyl and
pyrimidyl based organoselenium compounds
,
Ananda S. Hodage a, C. Parashiva Prabhu a, Prasad P. Phadnis a, Amey Wadawale a, K.I. Priyadarsini b
**
,
,
Vimal K. Jain a
*
a Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
b Radiation and Photochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyridyl and pyrimidyl based organoselenium compounds have been synthesized and characterized by
analytical and spectroscopic techniques. Molecular structures of 2,20-dipyrimidyl diselenide (1b), 2,20-
dipyrimidyl selenide (2b) and 2-pyrimidyl seleno ethanoic acid (3d) have been determined by single
crystal X-ray diffraction analyses. The 3d is associated in the solid state through hydrogen bonding
between carboxylic acid proton and N2 of the pyrimidyl ring of an adjacent molecule. The in vitro GPx-
like catalytic activity for these compounds was evaluated by 1H NMR and HPLC methods where H2O2 was
reduced by dithiothreitol (DTTred) and glutathione (GSH) as a thiol cofactor, respectively, in the presence
of catalytic amounts of organoselenium compounds. The electron density around selenium atom (eSeSe
e or eSee) which is reflected by 77Se{1H} NMR chemical shifts, has been found to be one of the crucial
factors in influencing their overall GPx like activity.
Received 2 July 2012
Received in revised form
28 August 2012
Accepted 31 August 2012
Keywords:
Organoselenium compound
NMR
X-ray structure
Antioxidant
Ó 2012 Elsevier B.V. All rights reserved.
GPx-mimicking activity
1. Introduction
[14]. The following empirical design features have been recognized
in GPx active organoselenium compounds.
The chemistry of organoselenium compounds is making great
strides in diverse areas ranging from organic synthesis [1e3],
coordination chemistry [4e6], materials science [7e9], pharmacy
[10,11] and biology [12e15]. Selenium is an essential micronutrient
in biological system and is incorporated in the form of selenocys-
teine in several redox active enzymes. Glutathione peroxidase
(GPx) is one of the important selenoenzymes which in conjunction
with its thiol cofactor glutathione (GSH) catalyses reduction of
several peroxides. Considerable efforts have been made to develop
antioxidant for their applications in treating oxidative stress related
diseases. The GPx mimicking activity was first reported in
Organoselenium compounds, both mono- and di-selenides,
containing alkyl groups with OH, NH2, COOH functionality at
terminal position are active [19e22].
Diselenides are more active than the corresponding mono-
selenides [23].
Diselenides which show weak secondary intramolecular
Se/X (X ¼ O or N) interactions, in general, show good activity
[24e28].
Diselenides containing heterocyclic ring (e.g., pyridine, quino-
line) are better catalysts than the compounds containing simple
aryl groups (e.g., Ph2Se2) [29].
a
synthetic
organoselenium
compound,
2-phenyl-1,2-
benzoisoselenazol-3-(2H)-one (ebselen) [16,17] and its mecha-
nistic aspects have also been reported [18]. This led to the design
and development of several organoselenium compounds having
different types of bonding and non-bonding interaction between
selenium and nearby heteroatom, ‘X’ (X ¼ N or O) as GPx mimics
We have recently incorporated these design features in nico-
tinamide based organoselenium compounds. One of these deriv-
atives, nocotinamide diselnide [2-NC5H3(3-CONH2)Se]2, exhibited
good GPx activity in vitro [30]. This has prompted us to design
molecules incorporating above empirical features. Accordingly, we
have examined organoselenium compounds containing N-
heterocyclic, 2-pyridyl and 2-pyrimidyl, rings and evaluated their
GPx mimicking activity. The results of this work are reported
herein.
* Corresponding author. Tel.: þ91 22 2559 5095; fax: þ91 22 2550 5151.
** Corresponding author.
E-mail addresses: kindira@barc.ernet.in (K.I. Priyadarsini), jainvk@barc.gov.in
(V.K. Jain).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.08.035

20
A.S. Hodage et al. / Journal of Organometallic Chemistry 720 (2012) 19e25
2. Experimental
orange powder settled down which was separated by decantation
and dissolved in hot methanol which on slow evaporation gave
needle-shaped yellow crystals of dipyrimidyl diselenide (1b) (4.7 g,
24%). After separation of 1b, the mother liquor on further concen-
tration afforded cubic colorless crystals of dipyrimidyl mono-
selenide (2b) (5.4 g, 36%). Both the compounds were further
purified by recrystallization.
2.1. Materials and methods
Elemental selenium (99.99%), sodium borohydride, bromoacetic
acid, 3-bromopropionic acid, 4-bromobutyric acid, 2-bromoethylamine
hydrobromide, 3-chloropropylamine hydrochloride, 2-bromoethanol,
3-bromopropanol and diphenyl diselenide were purchased from
commercial sources (Aldrich/Fluka). All reactions were carried out
under a nitrogen atmosphere. Solvents were purified and distilled prior
to use. The compounds were purified by column chromatography on
silica gel 60/120 mesh size. Melting points were determined in capillary
tubes andare uncorrected. Elemental analyseswerecarried outon Flash
EA 1112 Series CHNS Analyzer. NMR spectra were recorded on a Bruker
pym2Se2 (1b) m.p. 156e158 ꢀC. Anal. Calcd. for C8H6N4Se2:
Calcd. C, 30.39; H, 1.91; N, 17.73%. Found: C, 30.91; H, 1.95; N, 17.15%.
IR (KBr, y d: 7.05
cmꢂ1): 3037, 1555, 1367, 1155, 808. 1H NMR (CDCl3)
(1H, t, J ¼ 4.8 Hz, C4H3),
d
8.53 (2H, d, J ¼ 4.8 Hz, C4H3); 13C{1H} NMR
(CDCl3)
(CDCl3)
d
: 118.4 (C-5), 157.9 (C-4, 6), 166.4 (CeSe); 77Se{1H} NMR
d
: 490 ppm. MS (IT) m/z (%): 317 ([M ꢂ 1]þ, 17), 237
([Pym2Se, 100), 133 ([C2N2HSe]þ, 18).
Avance-II 300 MHz spectrometer operating at 300.13 (1H), 75.47 (13
C
pym2Se (2b) m.p. 138e140 ꢀC. Anal. Calcd. for C8H6N4Se: Calcd.
C, 40.52; H, 2.55; N, 23.63%. Found: C, 40.11; H, 2.45; N, 23.15%. IR
{1H}) and 57.25 (77Se{1H}) MHz. 1H and 13C{1H} NMR chemical shifts
were relative to internal chloroform peak (
d
¼ 7.26 ppm for 1H and
(KBr, y d: 7.19
cmꢂ1): 3051, 1550, 1371, 1145, 808. 1H NMR (CDCl3)
d
¼ 77.0 for 13C{1H} NMR). The 77Se{1H} NMR chemical shifts were
(1H, t, J ¼ 4.8 Hz, C4H3),
d
8.66 (2H, d, J ¼ 4.8 Hz, C4H3); 13C{1H}
relative to external diphenyl diselenide (Ph2Se2) in CDCl3 (
d
463.0 ppm
NMR (CDCl3)
NMR (CDCl3)
[C2N2HSe]þ.
d
: 119.0 (C-5), 157.9 (C-4, 6), 168.0 (CeSe); 77Se{1H}
d
relative to Me2Se (0 ppm)). The mass spectra were recorded on an
MS-500 Ion Trap (IT) Varian mass spectrometer at Sophisticated
Analytical Instrumentation Facility (SAIF), Indian Institute of
Technology-Bombay, Mumbai. The GPx like catalytic activities were
evaluated on a Bruker Avance-II NMR spectrometer and JASCO HPLC
detector model UV-2070.
: 596 ppm. MS (IT) m/z: 237 [M ꢂ 1]þ, 133
2.2.3. Synthesis of (C5H4N)SeCH2COOH (3a)
To a solution of dipyridyl diselenide (2.0 g, 6.53 mmol) in 60 ml
of methanol, sodium borohydride (0.5 g, 13.07 mmol) was added
under a flow of nitrogen at 0 ꢀC and the reaction mixture was
allowed to stir for 30 min forming a colorless solution of sodium
selenolate. To this solution bromoacetic acid (1.82 g, 13.1 mmol)
was added and the contents were stirred for 2 h at room temper-
ature. The solvent was removed on rotavapour and the residue was
dissolved in distilled water and extracted with ethyl acetate
(3 ꢁ 100 ml). The organic phase was dried over sodium sulfate and
the solvent was removed on rotavapor to yield a pale yellow solid
which on recrystalization from ethyl acetate:hexane mixture
(70:30) gave 3a as a white crystalline solid (1.8 g, 64%). m.p. 121e
123 ꢀC; Anal. Calcd. for C7H7NO2Se: C, 38.91; H, 3.27; N, 6.48%.
2.2. Synthesis
2.2.1. Synthesis of py2Se2 (1a) and py2Se (2a)
In a typical experiment, 2-bromopyridine (30 g, 189 mmol) was
added to an aqueous brown solution of Na2Se2 (prepared from
selenium powder (15 g, 190 mmol) in deoxygenated water and
sodium borohydride (7.2 g, 189 mmol) at 0 ꢀC under nitrogen. The
reaction mixture was refluxed for 3 h till the solution became
yellow containing a small amount of suspended selenium. The hot
reaction mixture was filtered and allowed to cool to room
temperature, whereupon dipyridyl diselenide (1a) (12.2 g, 42%)
crystallized out as yellow crystals and was filtered using a Buchner
funnel. The filtrate was extracted with chloroform (4 ꢁ 150 ml). The
organic layer was dried over sodium sulfate and concentrated on
rotavapor to yield a yellow sticky liquid which was purified by
column chromatography (1:9, ethyl acetate:hexane) to give dipyr-
idyl monoselenide (2a) (4.2 g, 19%) as a red oil.
Found: C, 38.61; H, 3.20; N, 6.15%. IR (KBr,
y
cmꢂ1): 3073, 2927 (br,
: 3.86 (2H,
7.50 (1H,
OH), 1693(s, C]O), 1585, 1454, 764. 1H NMR(dmso-d6)
d
s, 2JSeeH ¼ 6.00 Hz, SeCH2),
d
7.16 (1H, t, J ¼ 6.0 Hz, C5H4),
d
d, J ¼ 7.8 Hz, C5H4),
d
7.63 (1H, t, J ¼ 6.9 Hz, C5H4),
d 8.42 (1H, d,
J ¼ 4.5 Hz, C5H4); 13C{1H} NMR (dmso-d6)
d
: 25.4 (1JCeSe ¼ 67 Hz),
120.9 (C-5), 124.7 (C-3), 137.0 (C-4), 149.6 (C-6), 153.8 (CeSe), 171.5
(C]O); 77Se{1H} NMR (dmso-d6)
d: 346 ppm. MS (IT) m/z (%): 216
py2Se2 (1a) m.p. 50e51 ꢀC (lit 47.5e50 [31,32]). Anal. Calcd. for
C10H8N2Se2: Calcd. C, 38.23; H, 2.56; N, 8.92%. Found: C, 37.71; H,
([M
ꢂ
1]þ, 100), 197 ([PySeK]þ, 10), 158 ([PySe]þ, 16), 152
([C3H4SeO2]þ, 29).
2.45; N, 8.45%. IR (KBr,
(CDCl3)
7.78 (1H, d, J ¼ 7.8 Hz, C5H4),
NMR (CDCl3)
(SeeC); 77Se{1H} NMR (CDCl3)
py2Se (2a) IR (KBr,
cmꢂ1): 3043, 1569, 1415, 755. 1H NMR
(CDCl3) : 6.97e7.05 (1H, m, C5H4), 7.37e7.44 (2H, m, C5H4),
8.35e8.38 (1H, m, C5H4); 13C{1H} NMR (CDCl3)
: 121.6 (C-5), 127.6
y
cmꢂ1): 3047, 1584, 1452, 778. 1H NMR
The compounds 3be3i were prepared similar to 3a, using cor-
responding diselenide and bromo functionalized compound
(supplementary materials). The characterization data for 1a, 3ae3c
were well in agreement with literature values [33].
d
: 7.07 (1H, t, J ¼ 5.6 Hz, C5H4), 7.53 (1H, t, J ¼ 7.4 Hz, C5H4),
d
d
d
8.44 (1H, d, J ¼ 3.6 Hz, C5H4); 13C{1H}
d
: 121.2 (C-5),123.3 (C-3),137.4 (C-4),149.4 (C-6),154.3
d
: 447 ppm. MS (IT) m/z: 316 (Mþ).
y
2.3. X-ray crystallography
d
d
d
d
Single crystal X-ray data for 2,20-dipyrimidyl diselenide (1b),
2,20-dipyrimidyl selenide (2b) and 2-pyrimidyl seleno ethanoic acid
(3d) were collected at room temperature (298 ꢃ 2 K) on a Rigaku
(C-3), 136.6 (C-4), 149.8 (C-6), 154.4 (SeeC); 77Se{1H} NMR (CDCl3)
d
: 518 ppm. MS (IT) m/z (%): 237 ([M þ 1]þ, 100), 157 ([PySe-1]þ, 2).
AFC 7S diffractometer using graphite monochromated MoeK
(l
a
ꢀ
2.2.2. Synthesis of pym2Se2 (1b) and pym2Se (2b)
¼ 0.71069 A) radiation so that qmax ¼ 27.5ꢀ. The unit cell
To a suspension of selenium powder (11.92 g, 151 mmol) in
deoxygenated water, sodium borohydride (5.26 g, 138 mmol) was
added at 0 ꢀC under a nitrogen atmosphere and the reaction
mixture was stirred for 2 h at room temperature to give a dark
brown solution of Na2Se2. To this 2-bromopyrimidine (20 g,
125 mmol) was added and the reaction mixture was refluxed for 5 h
till the solution turned dark orange. The hot reaction mixture was
filtered and allowed to cool to room temperature whereupon an
parameters (Table 1) were determined from 25 reflections
measured by a random search routine. The intensity data were
corrected for Lorenz, polarization and absorption effects with an
empirical procedure [34]. The structures were solved by direct
methods using SHELX-97 [35] and refined by full-matrix least
squares methods. The non-hydrogen atoms were refined aniso-
tropically. The hydrogen atoms were fixed in their calculated
positions. The molecular structures were drawn by ORTEP [36].

A.S. Hodage et al. / Journal of Organometallic Chemistry 720 (2012) 19e25
21
Table 1
3. Results and discussion
Crystallographic and structural refinement data for pym2Se2 (1b), pym2Se (2b) and
pymSeCH2COOH (3d).
3.1. Synthesis and spectroscopy
pym2Se2 (1b)
pym2Se (2b)
pymSeCH2
COOH (3d)
Dipyridyl diselenide was prepared by a slightly modified
literature procedure [31,32] which was extended for the synthesis
of diprimidyl diselenide. The preparation involved a reaction
between Na2Se2 and 2-bromopyridine or 2-bromopyrimidine in
an aqueous medium (Scheme 1). Any monoselenide formed
during the reaction was also separated conveniently. It is worth
noting that so far only substituted pyrimidyl selenides are re-
ported. Bhasin et al. isolated bis(4-chloro-2-pyrimidyl)diselenide
and bis(4-dimethylamino-2-pyrimidyl)diselenide by the reaction
of Na2Se2 with 2,4-dichloro pyrimidine in DMF [38] and ethanol
[39], respectively. The asymmetric monoselenides (Scheme 1)
were prepared by the reaction of haloalkyl derivatives with
NaSeC4H3NX (X ¼ CH or N) generated in situ by reductive
cleavage of SeeSe bond of diselenides by NaBH4 in MeOH. Except
3d and 3h which could be isolated in pure form, other unsym-
metrical pyrimidyl selenides were often contaminated with some
impurities.
All the compounds were characterized by microanalyses, mass,
NMR and IR spectroscopy. Mass spectra of these compounds
exhibited molecular ion peaks with desired isotopic pattern. The 1H
NMR spectra showed resonances with expected peak multiplicities
for the protons attached to the heterocyclic ring and functionalized
aliphatic groups. The 13C NMR chemical shifts for heterocyclic ring
carbons were little influenced by the substituents and appeared in
a narrow region (2-pySe: w155 (C-2); w124 (C-3); w137 (C-4);
w121 (C-5) and w150 (C-6) ppm; 2-pymSe: w167 (C-2); w158 (C-4,
6) and w119 (C-5) ppm). The SeCH2 carbon resonances were flanked
by 77Se satellites with 1J(77See13C) varying in the range 61e67 Hz.
The magnitude of 1J(77See13C) is well in agreement with the
values reported for organoselenium(II) compounds [40]. The
carbonyl carbon resonances showed alkyl chain length dependence
and is deshielded with increasing the alkyl chain length (171.5 (3a);
173.6 (3b) and 178.1 (3c) ppm).
Chemical formula
Formula weight
Crystal size
C8H6N4Se2
316.09
C8H6N4Se
237.13
C6H6N2O2Se
217.09
0.40 ꢁ 0.30
0.40 ꢁ 0.30
0.30 ꢁ 0.20
ꢁ 0.20
ꢁ
0.20
Orthorhombic/
Pbca
ꢁ
0.15
Orthorhombic/
Pnma
Crystal system/
space group
Tetragonal/
P4(bar)21c
13.710(3)
13.710(3)
10.929(3)
90.00
ꢀ
Cell parameters a (A)
10.850(4)
10.8350(18)
15.3269(19)
90.00
90.00
90.00
1801.8(7)
8
1.748
14.986(5)
6.669(3)
8.000(3)
90.00
90.00
90.00
799.6(5)
4
1.803
ꢀ
b(A)
ꢀ
c(A)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
90.00
90.00
2054.3(9)
8
2.044
3
ꢀ
V (A )
Z
Dc (g/cm3)
m
(MoeK
F (000)
Limiting indices
a
) mmꢂ1
/
7.163/1200
4.122/928
4.646/424
ꢂ17 ꢄ h ꢄ 17
ꢂ14 ꢄ k ꢄ 17
17 ꢄ l ꢄ 14
2.97e27.50
ꢂ7 ꢄ h ꢄ 14
ꢂ14 ꢄ k ꢄ 7
ꢂ11 ꢄ l ꢄ 19
2.66e27.49
0 ꢄ h ꢄ 19
ꢂ4 ꢄ k ꢄ 8
ꢂ10 ꢄ l ꢄ 5
2.72e27.44
q
Range of data
collection
No. of reflections
collected
No. of independent
reflections
Data/restraints/
parameters
1884
3310
1350
1443
2074
957
1443/0/127
2074/0/143
957/0/70
R indices [I > 2
s(I)]
R1 ¼ 0.0490
wR2 ¼ 0.1125
R1 ¼ 0.1493
wR2 ¼ 0.1478
1.008
R1 ¼ 0.0314
wR2 ¼ 0.0430
R1 ¼ 0.1679
wR2 ¼ 0.0649
0.985
R1 ¼ 0.0281
wR2 ¼ 0.0664
R1 ¼ 0.0503
wR2 ¼ 0.0743
1.047
R indices (all data)
Goodness-of-fit on F2
2.4. Evaluationofglutathioneperoxidase(GPx)likecatalyticactivities
2.4.1. HPLC method
The 77Se NMR spectra of these compounds displayed a signal in
the range 279e490 ppm. The resonances for unsymmetrical sele-
nides (3) were shielded relative to the corresponding diselenides
(1), as expected [41]. Interestingly the resonances for symmetrical
selenides (2), exhibited a reverse trend as they are deshielded
relative to the signal for the corresponding diselenides (1). The 77Se
NMR chemical shifts for Ar2Se are progressively shifted down field
on replacing phenyl (Ar ¼ Ph) by pyridyl (Ar ¼ py) through pyr-
imidyl (Ar ¼ pym) (Fig. 1). This indicates sequential decrease in
electron density at selenium by electron withdrawing pyridyl and
pyrimidyl groups [41]. However in the case of diselenides (Ar2Se2)
such effect is not evident (Fig. 1). This could be due to difficulty in
rationalizing the contributions from non-bonding interactions
(Se/N) and inter-molecular steric effects. The former have been
observed in analogous molecules, bis(3, 5-dimethyl-2-pyridyl)dis-
elenides [42] and bis(3-methyl-2-pyridyl)diselenides [43] in the
solid state. However, in solution cisecis, cisetrans and transetrans
conformations may exist in equilibrium [44], hence the steric
factors also play a role.
As described earlier glutathione peroxidase (GPx) like activity of
these compounds has been assessed using HPLC method. Relative
decay of reduced glutathione (GSH) with respect to formation of
oxidized form of glutathione (GSSG) was monitored by absorption
detection [37]. In a typical experiment oxidation of the GSH
(160
ence of an organoselenium compound (32
5 min at room temperature. The reaction mixture (40
m
M) was initiated by addition of H2O2 (320
M) and incubated for
L) was
mM) in the pres-
m
m
injected to mobile phase containing 0.1% aqueous trifluoroacetic
acid (TFA) and 5% methanol on C-18 reverse phase column and
detected at 210 nm which was monitored at regular intervals till
300 min. Average of two injections taken and graph was plotted for
the relative decay of GSH v/s time in minutes and t50 value was
determined as the time required to undergo decay of 50% of GSH.
2.4.2. 1H NMR spectroscopic method
The GPx like catalytic activityof organoseleniumcompoundswas
also evaluated by 1H NMR spectroscopy [20,22]. In this method
reduction of hydrogen peroxide (H2O2) (9.7
CD3OD (0.5 ml) was carried out using reduced dithiothreitol (DTTred
(23.1 mg, 0.15 mmol) in NMR tube in the presence as well as absence
mL, 45.1%, 0.15 mmol) in
)
3.2. Molecular structures
of an organoselenium compound (1.98 mmol and 7.49 mmol) and the
reaction was monitored by 1H NMR spectroscopy. The relative
concentration of DTTred and DTTox were determined by integrating
the proton resonance for eCH protons at d 3.67 ppm and d 3.49 ppm,
respectively. The time (t50) required to complete 50% oxidation of
DTTred to DTTox was calculated which in turn was a measure of 50%
reduction of H2O2 by the thiol cofactor (DTTred).
The molecular structures of pym2Se2 (1b), pym2Se (2b) and
pymSeCH2COOH (3d) (Figs. 2e4) have been established by X-ray
diffraction analyses. The SeeC distances in these compounds vary
between 1.907(4) and 1.950(4) A and are well within the range
reported for organoselenium compounds [33,45,46]. The SeeSe
distance and C1eSe1eSe2 angle (Table 2) in 1b can be compared
ꢀ

22
A.S. Hodage et al. / Journal of Organometallic Chemistry 720 (2012) 19e25
Na2Se2 (aq)
Reflux 3 h
2
2
( 2)
( 1)
X = CH (1 a)
X = N (1 b)
X = CH ( 2 a)
X = N ( 2 b)
i) 2 NaBH 4 / MeOH
ii) 2 Br(CH2) nY
2
n
2
( 3)
n
CH
CH
1
2
3
( 3 a )
( 3 b )
( 3 c )
CH
N
1
2
( 3 d )
( 3 e )
CH
CH
CH
OH
OH
NH2
3
2
2
3
( 3 f )
( 3 g )
( 3 h )
( 3 i )
N
NH2
NH2
CH
Scheme 1. Synthesis of 2-pyridyl and 2-pyrimidyl based organoselenium compounds.
ꢀ
ꢀ
with those reported in dipyridyl diselenides (w2.29 A and w103 ,
respectively) [47,48]. The C1eSe1eSe2eC5 torsion angle is slightly
different from dipyridyl diselenide (84.3 (2)ꢀ) [49] but is similar to
(4-MeC5H3N)2Se2 (ꢂ89.2 (18)ꢀ) [47]. The Se/N distances, both
various monoselenides (e.g. pySeCH2COOH (99.67 (9)ꢀ) [33],
Se(CH2CH2COOH)2 (96.48(8)ꢀ) [50]). The CeSeeC angle in 3d is
significantly reduced in comparison with dmpzC6H4SeCH2COOH
(101.2(3)ꢀ) [51]. Compounds containing carboxylic acid group are
often associated through hydrogen bonding leading to various
degrees of association. The 3d is associated through hydrogen
bonding between carboxylic acid proton and the N2 of the pyr-
imidyl ring of the adjacent molecule (inset, Fig. 4), with O2/N2
ꢀ
ꢀ
intra- (3.349, 3.325 A) and inter- (3.349 A) molecular in 1b, are
shorter than the sum of their van der Waals radii (3.5 A) indicative
of very weak non-bonding interactions. These distances are
significantly longer than those observed in bis(3,5-dimethyl-2-
ꢀ
ꢀ
ꢀ
pyridyl)diselenide (2.901 A) [42]. However, other dipyridyl dis-
distance of 2.741 A, forming an infinite chain. Similar OH/N
elenides are devoid of such Se/N interactions [47e49].
interactions have been reported in pySeCH2COOH [33], whereas
O2/N2 interaction in dmpzC6H4SeCH2COOH gives a dimeric
structure [51].
Monoselenides 2b and 3b adopt a V-shaped configuration
around selenium with CeSeeC angles of 99.55(19) (2b) and
96.88(16)ꢀ (3d). These angles are well within the range reported for
3.3. Catalytic properties of organoselenium compounds as GPx
mimics
The GPx mimicking activity of organoselenium compounds in
vitro has been evaluated in several ways such as UV-spectroscopy
based NADPH-reductase coupled assay [24], HPLC based thiol-
disulfide conversion assays [27] and 1H NMR based dithiol-
Fig. 2. ORTEP diagram of dipyrimidyl diselenide (1b) (50% thermal ellipsoid
probability).
Fig. 1. Correlation of 77Se{1H} NMR chemical shifts.

A.S. Hodage et al. / Journal of Organometallic Chemistry 720 (2012) 19e25
23
Table 2
ꢀ
ꢀ
Selected bond lengths (A), bond angles ( ) and torsion angles
(ꢀ) for pym2Se2 (1b).
Se1eSe2
Se1eC1
Se2eC5
2.284 (2)
1.919(16)
1.942(13)
C1eSe1eSe2
Se1eSe2eC5
N1eC1eSe1
N2eC1eSe1
N3eC5eSe2
N4eC5eSe2
103.6(4)
103.7(4)
110.7(10)
121.3(11)
120.2(10)
108.2(11)
C1eSe1eSe2eC5
N1eC1eSe1eSe2
N2eC1eSe1eSe2
N3eC5eSe2eSe1
N4eC5eSe2eSe1
ꢂ90.2(6)
ꢂ169.4(9)
13.9(12)
Fig. 3. Molecular structure of dipyrimidyl selenide (2b) (50% thermal ellipsoid prob-
ꢀ
ꢂ1.6(12)
ability). [Selected interatomic parameters: bond length (A): C1eSe1, 1.907(4); C5eSe1,
1.925(4); bond angles (ꢀ): C1eSe1eC5, 99.55(19); N1eC1eSe1, 119.0(4); N2eC1eSe1,
112.6(4); N3eC5eSe1, 114.6(4); N4eC5eSe1, 177.3(4).]
176.7(10)
Activity of both symmetrical (Ph2Se, 2a and 2b) and unsym-
metrical (3) monoselenides was examined by NMR spectroscopy
(supplementary materials). Symmetrical monoselenides (2)
showed better activity than unsymmetrical derivatives (3) except
3g and 3i. There was hardly any activity for Ph2Se whereas the
corresponding heterocyclic derivatives 2a and 2b were active under
similar conditions; the 2a being more active. The observed activity
in 2 could be due to stabilization of the proposed thioselenurane,
R2Se(OH)(SR0), intermediate species [20,22,53] through heterocy-
clic N/H secondary interactions.
It has been reported that the unsymmetrical monoselenides
containing terminal amino group in the alkyl chain show high GPx
mimicking activity [22,46,51] which is higher for compounds
containing (CH2)3NH2 moiety than those derived from (CH2)2NH2
fragment. The observed activity of 3g and 3i is consistent with the
reported trend [46,51]. Surprisingly pyrimidyl analogue of 3g (i.e.
compound 3h) shows very low catalytic activity under the same
conditions.
disulfide conversion assay [20,22]. We have evaluated the GPx
mimicking activity by the latter two techniques.
It has been proposed that in the case of diselenides, GPx
mimicking catalytic cycle involves the formation of selenenic acid
(ArSeOH) as an intermediate by oxidation with H2O2 [52]. Thus
factors contributing to increase the electron density in eSeeSee
linkage would facilitate the formation of selenenic acid and in
turn resulting in enhancement of catalytic activity of a diselenide.
Fig. 5 shows comparative GPx activity for Ph2Se2,1a and 1b assessed
by 1H NMR and HPLC techniques (Table 3). It is evident from Fig. 5
that their activity is in the following order: 1a > 1b > Ph2Se2.
From the 77Se NMR chemical shifts, the electron density at selenium
atom increases in the following order: 1a > Ph2Se2 > 1b. The
observed GPx activity of 1a is consistent with this trend. However,
a reverse trend is noted for Ph2Se2 and 1b though both of them
exhibited poor activity. This trend could be due to the ability of 1b to
form selenenic acid stabilized by Se/N interactions which will be
absent in the case of phenyl derivative.
ꢀ
Fig. 4. Crystal structure of pymSeCH2COOH (3d) (50% thermal ellipsoid probability). [Selected interatomic parameters: bond length (A): Se1-C1, 1.950(4); Se1eC5, 1.929(4); C6eO1,
1.235(5); C6eO2, 1.323(5); bond angles (ꢀ): C1eSe1eC5, 96.88(16); Se1eC1eN1, 122.0(3); Se1eC1eN2, 112.8(3); Se1eC5eC6, 106.5(3); O1eC6eO2, 123.0(4).]

24
A.S. Hodage et al. / Journal of Organometallic Chemistry 720 (2012) 19e25
4. Conclusions
a
Ph2Se2
100
90
80
70
60
50
40
30
20
10
Both mono and diselenides based on pyridyl and pyrimidyl rings
have been synthesized and characterized by microanalyses and
spectroscopic techniques. The molecular structures of pym2Se2 (1b),
pym2Se (2b) and pymSeCH2COOH (3d) were established by X-ray
diffraction analyses. The latter (3d) is associated through hydrogen
bonding between carboxylic acid proton and N2 of the pyrimidyl
ring of a neighboring molecule to form an infinite chain. Their GPx
like catalytic activities in vitro were evaluated by the model systems
based on 1H NMR spectroscopy and HPLC method. The symmetrical
pyridyl and pyrimidyl monoselenides (2) exhibited better GPx like
catalytic activity as compared to their unsymmetrical mono-
selenides except those with a terminal amine functional group (3g
and 3i) whereas the phenyl mono- and diselenide analogues
exhibited poor activity. A comparison of GPx mimicking activity of
seleno-phenyl, pyridyl and pyrimidyl derivatives revealed that
catalytic activity depends on various functional groups attached to
selenium but also on overall electron density at selenium atom
which is reflected in the 77Se{1H} NMR chemical shifts.
pym2Se2
py2Se2
21 min
30
Time (min)
0
10
20
40
50
60
70
b
100
Acknowledgements
90
80
70
60
50
40
30
20
10
0
We thank Drs. T. Mukherjee, S.K. Sarkar and D. Das for encour-
agement to this work. Authors (ASH, CPP) are grateful to Board of
Research in Nuclear Sciences (BRNS), Department of Atomic Energy
(DAE) for Senior Research Fellowship (SRF). We are also grateful to
BRNS for research grant under the Prospective Research Fund (PRF)
Scheme (Grant No. BRNS/2007/38/5).
Ph2Se2
pym2Se2
Appendix A. Supplementary material
CCDC 886538 (1b), 886539 (2b) and 886540 (3d) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
py2Se2
52.5 min
25
50
75
100
125
150
175
200
Appendix C. Supplementary data
Time (min)
Fig. 5. Comparison of catalytic activities of 1a, 1b and Ph2Se2 by (a) 1H NMR method
Supplementary data related to this article can be found in
the online version at http://dx.doi.org/10.1016/j.jorganchem.2012.
08.035.
(concentration: [DTTred] ¼ [H2O2] ¼ 0.15 mmol and [catalyst] ¼ 1.98
m
mol) and (b)
HPLC method (concentration: [GSH]
¼
160
mM; [H2O2]
¼
320 and
mM
[catalyst] ¼ 32
mM).
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t50 (min) by 1H NMR method
t50 (min) by HPLC method
Blank
Ph2Se2
Ph2Se
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227
>300
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>350a
0.5a
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