Synthesis, characterization and X-ray structure of 3,4-lutidinyl-, 3-/4-picolyl- and pyridylselenium compounds

Article abstract of DOI:10.1016/j.ica.2012.03.043

Bis(4,5-dimethyl-2-pyridyl)-, bis(5-methyl-2-pyridyl)- and bis(4-methyl-2-pyridyl) diselenide have been synthesized directly from 3,4-lutidine (1a), 3-picoline (1b) and 4-picoline (1c) respectively via BF ₃ aided lithiation reaction. The lithia

Full text of DOI:10.1016/j.ica.2012.03.043

Inorganica Chimica Acta 392 (2012) 335–344
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Synthesis, characterization and X-ray structure of 3,4-lutidinyl-, 3-/4-picolyl-
and pyridylselenium compounds
a
a
b
c
d
Jaspreet S. Dhau a, , Amritpal Singh , Avtar Singh , Balwinder S. Sooch , Paula Brandão , Vítor Félix
⇑
a Department of Chemistry, Punjabi University, Patiala 147002, India
b Department of Biotechnology, Punjabi University, Patiala 147002, India
c Departamento de Química, CICECO, Universidade de Aveiro, Aveiro 3810-193, Portugal
d Departamento de Química, CICECO, and Secção Autónoma de Ciências da Saúde, Universidade de Aveiro, Aveiro 3810-193, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Bis(4,5-dimethyl-2-pyridyl)-, bis(5-methyl-2-pyridyl)- and bis(4-methyl-2-pyridyl) diselenide have been
synthesized directly from 3,4-lutidine (1a), 3-picoline (1b) and 4-picoline (1c) respectively via BF3 aided
lithiation reaction. The lithiation of 3,4-lutidinium-, 3- and 4-picolinium-BF3 adduct (2a/2b/2c) gives the
corresponding carbanion (3a/3b/3c) which on subjecting to selenium insertion reaction followed by
aerial oxidation affords the related diselenide in good yield. Reaction of BF3 complexed pyridylselenolate
anion (4a/4b/4c) with diiodomethane gives the corresponding bis(2-pyridylseleno)methane. The dilithi-
ation of 4-picolinium-/pyridinium-BF3 adduct (2c/2d) followed by reaction with 2.2 equiv. of selenium
and iodomethane affords the related 2,6-bis(methylselenenyl)pyridine and 2-(methylselenenyl)pyridine
in varying proportions. Preparation of tris(methylselenenyl) derivatives of 1a and 1c have been given
in the present study. LiAlH4 has also been utilized to synthesize unsymmetrical monoselenides from
the corresponding diselenides. Single crystal X-ray studies of bis(4,5-dimethyl-2-pyridyl) diselenide
(5a), 3,4-dimethyl-2,6-bis(methylselenenyl)pyridine (9a), 4-methyl-2,6-bis(methylselenenyl)pyridine
(9c), 2,6-bis(methylselenenyl)pyridine (9d) and 4-methyl-2,6-bis(methylselenenyl)-4-(methylselene-
nylmethyl)pyridine (12a) have also been carried out. The crystal data of these compounds reveals that
the instances of Seꢀ ꢀ ꢀSe secondary interactions decreases with the increase in the number of methyl
group attached to the pyridine ring.
Received 18 October 2011
Received in revised form 20 March 2012
Accepted 21 March 2012
Available online 2 April 2012
Keywords:
Organoselenium
Lutidine
Picoline
Pyridine
Lithiation
Lithium aluminum hydride
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
In this paper, we have reported the synthesis of 3,4-lutidinyl,
3-/4-picolyl- and pyridylselenium compounds starting from 3,4-
Interest in the field of organoselenium chemistry has increased
over a period of time as it offers some unique advantages in the field
of material and biochemical sciences [1,2]. The metal selenolates
have been used as precursors for the low temperatures synthesis
of semiconducting materials [3,4] that are finding applications in
the solar [5] and thermoelectric [6,7] devices. In biology, organose-
leniums are being used as mimics of glutathione peroxidase, an en-
zyme which shows antioxidant property [8,9]. These compounds
are also being tested for their role as potential anticarcinogenic
[10,11], anti-hyperglycemic [12] and antimicrobial agents [13].
Therefore, there have been constant efforts by chemists worldwide
to come up with novel methodologies for efficiently synthesizing
known and unknown organoselenium compounds. Due to lack of
systematic study on the lutidinylchalcogen compounds we have fo-
cused our attention on the chemistry of these compounds. We have
successfully reported the synthesis of 3,5-lutidinylchalcogen and -
dichalcogen compounds directly from 3,5-lutidine [14].
lutidine (1a), 3-picoline (1b) 4-picoline (1c) and pyridine (1d),
respectively. The methodology involves BF3 aided lithiation, dilith-
iation and trilithiation of the aforementioned pyridine derivatives.
The potential of this methodology has been explored towards the
synthesis of a series of compounds having more than one selenium
atom attached to the pyridine ring. LiAlH4 has also been utilized to
synthesize unsymmetrical monoselenides from the corresponding
diselenides. Single crystal X-ray studies of bis(4,5-dimethyl-2-
pyridyl) diselenide (5a), 3,4-dimethyl-2,6-bis(methylselenenyl)-
pyridine (9a), 4-methyl-2,6-bis(methylselenenyl)pyridine (9c),
2,6-bis(methylselenenyl)pyridine (9d) and 4-methyl-2,6-bis(meth-
ylselenenyl)-4-(methylselenenylmethyl)pyridine (12a) have also
been reported.
2. Experimental
2.1. General
⇑
All experiments were carried out in dry oxygen-free nitrogen
atmosphere. All the solvents were dried before use [15]. Lithium
Corresponding author. Tel.: +91 1753046409.
E-mail address: jassiv02@yahoo.co.in (J.S. Dhau).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.043

336
J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
diisopropylamide (LDA) was made by adding n-BuLi (6.6 ml, 2.5 N,
16.5 mmol) to a solution of diisopropylamine (1.66 g, 2.32 ml,
16.5 mmol) in diethyl ether (30 ml) at ꢁ10 °C [16]. 1H NMR and
13C NMR spectra were recorded on Bruker 400 MHz spectropho-
tometer in CDCl3 using tetramethylsilane (TMS) as an internal
standard. The ESI mass spectra were taken on Water Q-TOF Micro
spectrometer. EI mass spectra were taken by using Shimadzu GC-
Mass Spectrometer (GCMS QP-2010 plus) with Rtx-1MS
2.3.1. Bis(4,5-dimethyl-2-pyridylseleno)methane (6a)
Yield: 1.8 g (32%), m.p. 65–66 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.14 (s, 2H), 7.04 (s, 2H), 4.72 (s, 2H), 2.11 (s, 12H). 13C
NMR: CDCl3, 400 MHz d (ppm): 151.9, 150.1, 146.1, 129.4, 126.0,
19.0, 16.1, 15.9. MS (ESI): 386 ([M]Å+, 80Se). Anal. Calc. for
C15H18N2Se2: C, 46.63; H, 4.66; N, 7.25. Found: C, 46.94; H, 4.42;
N, 7.48%.
(30 m ꢂ 0.25 mm ID ꢂ 0.25
l
m) capillary column.
2.3.2. Bis(5-methyl-2-pyridylseleno)methane (6b)
Yield: 3.21 g (60%), m.p. 70–72 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.29–8.31 (d, J = 2.33 Hz, 2H), 7.14–7.22 (m, 3H), 6.88–
6.91 (dd, J = 4.84 Hz, 2.6 Hz, 2H), 4.72 (s, 2H), 2.19 (s, 3H), 2.12
(s, 3H). 13C NMR: CDCl3, 400 MHz d (ppm): 155.8, 151.7, 150.4,
147.0, 136.9, 136.2, 133.1 129.9, 124.8, 120.0, 19.2, 16.2 15.4. MS
(EI, 70 eV) m/z (relative intensity): 358 (2, [M]Å+, 80Se), 264 (3),
186 (61), 173 (18), 143 (2), 106 (28), 92 (63), 80 (5), 65 (100), 51
(7). Anal. Calc. for C13H14N2Se2: C, 43.83; H, 3.96; N, 7.86. Found:
C, 43.91; H, 3.89; N, 8.11%.
2.2. General procedure 1: Synthesis of bis(lutidinyl/picolyl) diselenides
A solution of 1a (1.60 g, 1.68 ml, 15.0 mmol) or 1b (1.396 g,
1.46 ml, 15.0 mmol) or 1c (1.396 g, 1.46 ml, 15.0 mmol) in diethyl
ether (50 ml) was cooled to 0 °C in a three-necked 100 ml round
bottom flask (RBF) and a solution of boron trifluoride (BF3ꢀEt2O)
in diethyl ether (2.34 g, 2.07 ml, 16.5 mmol) was added to it. A
white suspension was formed due to the formation of 3,4-lutidini-
um- /3-picolinium- /4-picolinium-BF3 adduct (2a/2b/2c). The tem-
perature of the suspension was lowered to ꢁ78 °C and LDA
(16.5 mmol) was added slowly to it via cannula. The resulting
red brown solution containing the BF3-complexed carbanion (3a/
3b/3c) was further stirred for 15–20 min at ꢁ78 °C. Elemental sele-
nium (1.30 g, 16.5 mmol) was added to this solution at ꢁ78 °C. The
temperature was slowly raised till all of the selenium got dissolved
indicating the formation of the corresponding selenolate anions
(4a/4b/4c). The reaction mixture containing 4a/4b/4c was hydro-
lyzed and exposed to aerial oxidation for 30 min. The organic layer
was extracted with diethyl ether, washed with water, brine solu-
tion and again with water and then dried over anhydrous sodium
sulfate. The solvent was removed and the crude product was puri-
fied by column chromatography using 60–120 mesh silica gel and
hexane–ethyl acetate as an eluent.
2.3.3. Bis(4-methyl-2-pyridylseleno)methane (6c)
Yield: 2.9 g (54%), m.p. 58–60 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.26–8.28 (d, J = 5.12 Hz, 2H), 7.09 (s, 2H), 6.78–6.80 (d,
J = 5.12 Hz, 2H), 4.78 (s, 2H), 2.19 (s, 6H). 13C NMR: CDCl3,
400 MHz d (ppm): 155.1, 149.6, 147.2, 125.8, 121.8, 20.8, 15.8.
MS (EI, 70 eV) m/z (relative intensity): 358 (1, [M]Å+, 80Se), 266
(1), 186 (66), 173 (21), 145 (2), 107 (2), 92 (83), 80 (10), 65
(100), 54 (1). Anal. Calc. for C13H14N2Se2: C, 43.83; H, 3.96; N,
7.86. Found: C, 43.79; H, 4.01; N, 7.72%.
2.3.4. 2-(Benzylselenenyl)-4-methylpyridine (7c)
(Chloromethyl)benzene (2.08 g, 1.89 ml, 16.5 mmol) was added
to the solution containing 4c at ꢁ78 °C. The reaction mixture was
stirred for half an hour at ꢁ78 °C and then slowly warmed to room
temperature and stirred for additional half an hour. The reaction
mixture was then hydrolyzed and worked-up as described for
the diselenides. Yield: 2.3 g (59%), yellow viscous liquid. 1H NMR:
CDCl3, 400 MHz d (ppm): 8.30–8.31 (d, J = 5.04 Hz, 1H), 7.12–7.34
(m, 5H), 7.06 (s, 1H), 6.78–6.79 (dd, J = 0.92 Hz, 4.98 Hz, 1H), 4.42
(s, 2H), 2.17 (s, 3H). 13C NMR: CDCl3, 400 MHz d (ppm): 155.3,
149.6, 147.2, 139.2, 128.6, 128.5, 126.9, 126.0, 121.9, 29.2, 20.8
MS (EI, 70 eV) m/z (relative intensity): 263 (9, [M]Å+, 80Se), 182
(40), 167 (5), 91 (100), 65 (59). Anal. Calc. for C13H13NSe: C,
59.54; H, 4.99; N, 5.34. Found: C, 59.90; H, 4.69; N, 5.46%.
2.2.1. Bis(4,5-dimethyl-2-pyridyl) diselenide (5a)
Yield: 3.17 g (57%), m.p. 95–98 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.16 (s, 2H), 7.56 (s, 2H), 2.20 (s, 6H), 2.19 (s, 6H). 13C
NMR: CDCl3, 400 MHz d (ppm): 151.2, 149.4, 147.7, 130.3, 124.3,
19.3, 15.8. MS (ESI): 372 ([M]+, 80Se). Anal. Calc. for C14H16N2Se2:
C, 45.43; H, 4.36; N, 7.57. Found: C, 45.97; H, 4.76; N, 7.40%.
2.2.2. Bis(5-methyl-2-pyridyl) diselenide (5b) [17]
Yield: 3.8 g (73%), m.p. 65–66 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.28 (s, 2H), 7.68–7.70 (d, J = 8.16 Hz, 2H), 7.34–7.36 (dd,
J = 1.92 Hz, 8.12 Hz, 2H), 2.29 (s, 6H). 13C NMR: CDCl3, 400 MHz d
(ppm): 150.8, 149.8, 138.2, 130.9, 123.4, 17.8. MS (ESI): 344
([M]Å+, 80Se). Anal. Calc. for C12H12N2Se2: C, 42.12; H, 3.53; N,
8.18. Found: C, 41.87; H, 3.56; N, 8.10%.
2.3.5. 2-Iodo-5-methyl-4-((4-methylpyridin-2-
selenenyl)methyl)pyridine (8c)
2-Iodo-4-(iodomethyl)-5-methylpyridine (1.19 g, 3.3 mmol)
was added to the solution containing 4c at ꢁ78 °C. Rest of the pro-
cedure remained as above. Yield: 0.8 g (67%), yellow viscous liquid.
1H NMR: CDCl3, 400 MHz d (ppm): 8.36–8.37 (d, J = 5.08 Hz, 1H),
8.08 (s, 1H), 7.61 (s, 1H), 7.10 (s, 1H), 6.91–6.92 (dd, J = 1.08 Hz,
4.98 Hz, 1H), 4.28 (s, 2H), 2.27 (s, 6H). 13C NMR: CDCl3, 400 MHz
d (ppm): 153.1, 151.5, 149.6, 148.9, 147.6, 134.7, 131.6, 126.3,
122.4, 114.9, 24.4, 20.7, 15.7. MS (EI, 70 eV) m/z (relative inten-
sity): 404 (16, [M]Å+, 80Se), 323 (60), 277 (17), 262 (8), 196 (28),
173 (48), 104 (70), 92 (91), 77 (52), 65 (100). Anal. Calc. for
2.2.3. Bis(4-methyl-2-pyridyl) diselenide (5c) [17]
Yield: 3.5 g (68%), m.p. 96–98 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.31–8.29 (d, J = 4.90 Hz, 2H), 7.65 (s, 2H), 6.90–6.91 (d,
J = 4.10 Hz, 2H), 2.29 (s, 6H). 13C NMR: CDCl3, 400 MHz d (ppm):
154.1, 149.1, 148.9, 123.9, 122.4, 21.0. MS (EI, 70 eV) m/z (relative
intensity): 344 (10, [M]Å+, 80Se), 263 (5), 183 (59), 92 (99), 80 (6),
65 (100), 51 (7).
C13H13N2SeI: C, 38.73; H, 3.25; N, 6.94. Found: C, 38.82; H, 3.13;
2.3. General procedure 2: Synthesis of bis(lutidinyl/
picolylseleno)methanes
N, 6.96%.
2.4. General procedure 3: Reaction of 4-picolinium-BF3 (2c) and
pyridinium-BF3 (2d) adduct with 2.2 equiv. of LDA
Diiodomethane (4.42 g, 1.32 ml, 16.5 mmol) was added to the
solution containing 4a/4b/4c at ꢁ78 °C. The reaction mixture was
stirred for half an hour at ꢁ78 °C and then slowly warmed to room
temperature and stirred for additional half an hour. The reaction
mixture was then hydrolyzed and worked-up as described for
the diselenides.
4-Picolinium-BF3 (2c) was made by adding BF3–Et2O (2.34 g,
2.07 ml, 16.5 mmol) to a solution of 1c (1.396 g, 1.45 ml, 15 mmol)
in dry diethyl ether at 0 °C. Pyridinium-BF3 (2d) adduct (15 mmol)
was made by adding BF3–Et2O (16.5 mmol) to 1d (1.185 g, 1.21 ml,

J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
337
15 mmol) at 0 °C [18]. A solution of LDA (33.0 mmol) was added
slowly to 2c/2d at ꢁ78 °C via cannula. The color of the suspension
changed from white to orange brown in about 10 min. The orange
brown solution was stirred for 15 min at ꢁ78 °C and then elemen-
tal selenium (2.60 g, 33.0 mmol) was added to it. The temperature
was raised slowly until complete dissolution of selenium took
place. The reddish brown solution was again cooled to ꢁ78 °C
and iodomethane (4.68 g, 2.0 ml, 33.0 mmol) was added to it. The
reaction mixture was slowly brought to the room temperature,
hydrolyzed and worked up. Two compounds, 9c and 10c, were iso-
lated from the reaction involving 2c whereas, 9d and 10d, were
isolated from the reaction involving 2d.
(s, 2H). 13C NMR: CDCl3, 400 MHz d (ppm): 155.6, 150.0, 139.0,
136.0, 129.0, 128.4, 126.9, 125.4, 120.5, 29.3. MS (EI, 70 eV) m/z
(relative intensity): 249 (6, [M]Å+, 80Se), 168 (43), 91 (100), 78
(18), 65 (34). Anal. Calc. for C12H11NSe: C, 58.07; H, 4.46; N, 5.64.
Found: C, 57.61; H, 4.79; N, 5.76%.
2.5. Reaction of 3,4-lutidinium-BF3 adduct (2a) with 3.3 equiv. of LDA
The 3,4-lutidinium-BF3 adduct (2a) was made by adding
BF3ꢀEt2O (2.34 g, 2.07 ml, 16.5 mmol) to a solution of 1a (1.60 g,
1.68 ml, 15.0 mmol) in diethyl ether. LDA (49.5 mmol) was added
slowly to a suspension containing 2a in a drop-wise manner via
cannula. Elemental selenium (3.90 g, 49.5 mmol) was added to
the orange brown solution at ꢁ78 °C. After the complete dissolu-
tion of selenium, the blackish brown solution was cooled to
ꢁ78 °C and iodomethane (7.02 g, 3.07 ml, 49.5 mmol) was added
to it. The reaction mixture was slowly brought to the room temper-
ature, hydrolyzed and purified. Following compounds were iso-
lated from the crude product.
2.4.1. 4-Methyl-2,6-bis(methylselenenyl)pyridine (9c)
Yield: 0.22 g (7%), m.p. 64–66 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 6.78–6.79 (s, 2H), 2.39 (s, 6H), 2.12 (s, 3H). 13C NMR: CDCl3,
400 MHz d (ppm): 154.1, 145.6, 120.6, 19.4, 4.4. MS (EI, 70 eV) m/z
(relative intensity): 281 (66, [M]Å+, 80Se), 266 (2), 201 (100), 184
(23), 170 (10), 144 (10), 120 (72), 107 (78), 91 (40), 79 (28), 63
(25), 51 (7). Anal. Calc. for C8H11NSe2: C, 34.40; H, 3.95; N, 5.01.
Found: C, 34.77; H, 3.81; N, 5.25%.
2.5.1. 3,4-Dimethyl-2,6-bis(methylselenenyl)pyridine (9a) [20]
Yield: 0.22 g (5%), m.p. 72–75 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 6.87 (s, 1H), 2.46 (s, 6H), 2.18 (s, 3H), 2.13 (s, 3H). 13C
NMR: CDCl3, 400 MHz d (ppm): 155.1, 150.7, 144.7, 128.0, 122.2
19.5, 15.2, 6.1, 5.4. MS (EI, 70 eV) m/z (relative intensity): 295
(40, [M]Å+, 80Se), 280 (12), 265 (6), 214 (100), 199 (23), 186 (12),
120 (23), 104 (23), 93 (23), 77 (35).
2.4.2. 4-Methyl-2-(methylselenenyl)pyridine (10c) [19]
Yield: 0.98 g (35%), light brown viscous oil. 1H NMR: CDCl3,
400 MHz d (ppm): 8.19–8.20 (d, J = 5.08 Hz, 1H), 7.02 (s, 1H)
6.72–6.73 (d, J = 5.04 Hz, 1H), 2.34 (s, 3H), 2.16 (s, 3H). 13C NMR:
CDCl3, 400 MHz d (ppm): 155.6, 149.5, 147.0, 125.0, 121.3, 20.7,
5.4. MS (EI, 70 eV) m/z (relative intensity): 187 (13, [M]Å+, 80Se),
117 (1), 107 (100), 93 (27), 79 (6), 65 (34), 51 (5).
2.5.2. 4,5-Dimethyl-2-(methylselenenyl)pyridine (10a) [20]
Yield: 1.66 g (55%), red viscous oil. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.16 (s, 1H), 7.08 (s, 1H), 2.42 (s, 3H), 2.20 (s, 3H), 2.16 (s,
3H). 13C NMR: CDCl3, 400 MHz d (ppm): 152.3, 149.9, 146.0,
128.9, 125.1, 19.0, 15.9, 5.5. MS (EI, 70 eV) m/z (relative intensity):
201 (35, [M]Å+, 80Se), 186 (1), 121 (100), 106 (27), 91 (7), 77 (24).
2.4.3. 2,6-Bis(methylselenenyl)pyridine (9d)
Yield: 1.56 g (39%), m.p. 45–47 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 7.04–7.08 (t, J = 7.40 Hz, 7.96 Hz, 1H), 6.90–6.92 (d,
J = 7.7 Hz, 2H), 2.37 (s, 6H). 13C NMR: CDCl3, 400 MHz d (ppm):
155.6, 135.4, 120.6, 5.5. MS (EI, 70 eV) m/z (relative intensity):
267 (49, [M]Å+, 80Se), 187 (70), 172 (22), 157 (13), 130 (20), 106
(50), 93 (100), 65 (15), 51 (15). Anal. Calc. for C7H9NSe2: C, 31.71;
H, 3.42; N,5.28. Found: C, 31.48;H, 3.40; N, 5.26%.
2.5.3. 3-Methyl-2,6-bis(methylselenenyl)-4-
(methylselenenylmethyl)pyridine (12a)
Yield 0.875 g (15%), m.p. 85–88 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 6.83 (s, 1H), 3.55 (s, 2H), 2.47 (s, 6H), 2.18 (s, 3H), 1.94 (s,
3H). 13C NMR: CDCl3, 400 MHz d (ppm): 156.5, 150.9, 146.7,
144.9, 121.3, 24.8, 15.01, 6.2, 5.4, 4.7. MS (EI, 70 eV) m/z (relative
intensity): 389 (54, [M]Å+, 80Se), 387 (62), 372 (3), 294 (40), 213
(100), 198 (29), 182 (11), 157 (10), 118 (29), 104 (43), 93 (47),
77 (52), 51 (28). Anal. Calc. for C10H15NSe3: C, 31.12; H, 3.91; N,
3.62. Found: C, 30.92; H, 3.65; N, 3.52%.
2.4.4. 2-(Methylselenenyl)pyridine (10d) [17]
Yield: 0.57 g (22%), yellow viscous liquid. 1H NMR: CDCl3,
400 MHz d (ppm): 8.45–8.46 (dd, J = 1.72 Hz, 1.32 Hz, 4.62 Hz,
1H), 7.43–7.47 (dt, J = 1.88 Hz, 8.68 Hz, 1H), 7.30–7.32 (dd,
J = 1.40 Hz, 7.48 Hz, 1H), 7.00–7.03 (m, J = 1.08 Hz, 4.96 Hz,
7.36 Hz, 1H) 2.46 (s, 3H). 13C NMR: CDCl3, 400 MHz d (ppm):
156.0, 150.0, 135.8, 124.5, 120.0, 5.6. MS (EI, 70 eV) m/z (relative
intensity): 173 (12, [M]Å+, 80Se), 158 (1), 93 (100), 78 (47), 65 (7),
51 (51).
2.5.4. 3-Methyl-2,6-bis(methylselenenyl)-4-pyridinecarboxaldehyde
(13a)
Yield 0.23 g (5%), m.p. 60–62 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 10.25 (s, 1H), 7.34 (s, 1H), 2.51 (s, 6H), 2.49 (s, 3H). 13C
NMR: CDCl3, 400 MHz d (ppm): 191.7, 153.0, 148.9, 141.7, 128.9,
29.7, 6.4. MS (EI, 70 eV) m/z (relative intensity): 309 (73, [M]Å+,
80Se), 305 (36), 281 (17), 228 (100), 200 (17), 186 (17), 170 (14),
144 (14), 120 (15), 106 (32), 93 (45), 78 (29), 63 (37), 51 (11). Anal.
Calc. for C9H11NOSe2: C, 34.95; H, 3.55; N, 4.53. Found: C, 34.92; H,
3.15; N, 4.42%.
2.4.5. 2,6-Bis(benzylselenenyl)pyridine (11d)
(Chloromethyl)benzene (4.17 g, 3.79 ml, 33.0 mmol) was added
in place of iodomethane in the reaction discussed in the Sec-
tion 2.4.3. Yield: 0.68 g (11%), m.p. 113–115 °C. 1H NMR: CDCl3,
400 MHz d (ppm): 7.34–7.37 (m, 4H), 7.24–7.28 (m, 4H), 7.17–
7.21 (m, 3H), 7.03–7.05 (d, J = 7.56, 2H), 4.49 (s, 4H). 13C NMR:
CDCl3, 400 MHz d (ppm): 155.7, 138.7, 135.9, 128.9, 128.6, 127.0,
121.6, 29.3. MS (EI, 70 eV) m/z (relative intensity): 419 (3, [M]Å+,
80Se), 339 (2), 247 (4), 167 (9), 91 (100), 65 (30). Anal. Calc. for
2.6. Reaction of 4-picolinium-BF3 adduct (2c) with 3.3 equiv. of LDA
C19H17NSe2: C, 54.69; H, 4.10; N, 3.35. Found: C, 54.46; H, 4.53;
N, 3.17%.
LDA (49.5 mmol) was added slowly to the suspension contain-
ing 4-picolinium-BF3 adduct (2c, 16.5 mmol) in a drop-wise man-
ner via cannula. Elemental selenium (3.90 g, 49.5 mmol) was
added to the orange brown solution at ꢁ78 °C. Rest of the proce-
dure remained same as for the reaction involving 2a, Section 2.5.
4-Methyl-2,6-bis(methyl selenenyl)pyridine (9c) and 4-methyl-2-
(methylselenenyl)pyridine (10c) were isolated in 21% (0.887 g)
2.4.6. 2-(Benzylselenenyl)pyridine (12d)
2-(Benzylselenenyl)pyridine was isolated from the reaction dis-
cussed in Section 2.4.5. Yield: 2.0 g (55%), Red brown viscous oil. 1H
NMR: CDCl3, 400 MHz d (ppm): 8.42–8.62 (d, J = 8.00 Hz, 1H), 7.40–
7.43 (m, 2H), 7.17–7.29 (m, 5H), 7.00–7.02 (d, J = 7.56 Hz, 1H), 4.45

338
J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
and 22% (0.632 g) yields respectively along with 4-methyl-2,6-
bis(methylselenenyl)-4-(methylselenenylmethyl)pyridine (12c) in
9% yield.
2.9. Crystal structure determination and refinement
The single crystal X-ray data of the selenium compounds were
collected on a CCD Bruker APEX II using graphite monochroma-
tized Mo K
a radiation (a = 0.71073 Å) at 150(2) K for 5a, 9a, 9d
2.6.1. 2,6-Bis(methylselenenyl)-4-(methylselenenylmethyl)pyridine
(12c)
and 12a and at 180(2) K for 9c. The crystals of all the aforemen-
tioned compounds were grown in the solution of hexane and
dichloromethane. The selected crystal of each compound was posi-
tioned at a suitable distance from CCD and the spots were mea-
sured using an appropriate counting time. For all compounds, the
data reduction together with the corresponding multi-scan absorp-
tion correction was carried out using the SAINT-NT from Bruker AXS.
The structures were solved by means of direct methods followed of
successive Fourier difference syntheses using SHELXS-97. The struc-
Yield 0.5 g (9%), dark brown viscous liquid. 1H NMR: CDCl3,
400 MHz d (ppm): 7.19 (s, 1H), 3.58 (s, 2H), 2.45 (s, 6H), 1.92 (s,
3H). 13C NMR: CDCl3, 400 MHz d (ppm): 156.1, 150.3, 148.3,
124.2, 120.6, 26.5, 7.5, 5.6, 4.9. MS (EI, 70 eV) m/z (relative inten-
sity): 375 (10, [M]Å+, 80Se), 280 (65), 265 (14), 185 (100), 170
(15), 90 (29), 76 (3), 51 (22).
2.7. Reaction of pyridine-BF3 adduct (2d) with 3.3 equiv. of LDA
tures were refined on F2 by full-matrix least squares with SHELXL
-
97. All these calculations were carried out with SHELX-97 suite
[21]. The hydrogen atoms were included in the structure refine-
ment at geometrical positions. Anisotropic thermal parameters
were used for all non-hydrogen atoms while the hydrogen atoms
were refined with isotropic parameters equivalent 1.2 times those
of the atom to which they are attached. Final R values together
with pertinent crystallographic data are summarized in Table 1.
Molecular diagrams were drawn with ORTEP-3 [22] and PYMOL [23]
graphical packages.
LDA (49.5 mmol) was added slowly to the suspension contain-
ing pyridinium-BF3 adduct (2a, 16.5 mmol) in a drop-wise manner
via cannula. Elemental selenium (3.90 g, 49.5 mmol) was added to
the orange brown solution at ꢁ78 °C. Rest of the procedure re-
mained same as for 2a (Section 2.5). 2,6-Bis(methylselenenyl)pyr-
idine (9d, 2.53 g, 63%) was the only compound isolated from the
crude mixture.
2.8. Formation of 2-(benzylselenenyl)-4,5-dimethylpyridine (7a) and
2-(benzylselenenyl)-4-methylpyridine (7c) by cleavage of Se–Se bond
in 5a and 5c, respectively
3. Results and discussion
3.1. Preparation of symmetrical and unsymmetrical 4,5-lutidinyl-, 3-
picolyl- and 4-picolyl diselenides/-selenides by the monolithiation
procedure
To a vigorously stirred solution of 5a/5c (0.4 mmol) in dry THF
at 0 °C, powdered LiAlH4 (0.16 g, 0.44 mmol) was added in small
instalments. The reaction mixture was slowly brought to room
temperature and stirred for two hours. The pale yellow solution
was again cooled to 0 °C and (chloromethyl)benzene (1.0 g,
0.92 ml, 0.88 mmol) was added drop-wise. The resulting solution
was stirred overnight and hydrolyzed with 20 ml of water the next
day. The organic layer was extracted with diethyl ether, washed
with water, and then dried over anhydrous sodium sulfate. The sol-
vent was removed and the crude product was purified by column
chromatography using 60–120 mesh silica gel and hexane–ethyl
acetate as an eluent (20:1). 7a and 7c were obtained in 80% and
85% yield, respectively.
Treatment of 1a with BF3–Et2O gives the 3,4-lutidinium-BF3
adduct, 2a, in dry diethyl ether at 0 °C. This adduct undergoes
lithium-hydrogen abstraction reaction with lithium diisopropyl-
amide (LDA) at ꢁ78 °C. Addition of selenium to the carbanion
3a leads to the formation of the selenolate anion, 4a[20]. The
selenolate anion on aerial oxidation and hydrolysis affords
hither to unknown bis(4,5-dimethyl-2-pyridyl) diselenide (5a) in
good yield (Scheme 1). In another variation, reaction of 4a with
diiodomethane affords a novel compound, 6a, in 32% yield. Similar
to 2a, the reaction of 3-picolinium-BF3 adduct (2b) with LDA
followed by selenium insertion and aerial oxidation leads to
bis(5-methyl-2-pyridyl) diselenide (5b) in excellent yield. The
quenching of 4b with diiodomethane affords 6b in 60% yield.
Complexation of 4-Picoline (1c) with BF3 etherate and its subse-
quent reaction with LDA affords the selenolate anion, 4c, which
on aerial oxidation or reaction with diiodomethane gives bis(4-
methyl-2-pyridyl) diselenide (5c, 68%) and bis(4-methyl-2-pyrid-
ylseleno)methane (6c, 54%), respectively (Scheme 1). Further,
the reaction of 4c either with (chloromethyl) benzene or with
2-iodo-4-(iodomethyl)-3-methylpyridine affords 2-(benzylselene-
nyl)-4-methylpyridine (7c) and 2-iodo-5-methyl-4-((4-methyl-
pyridin-2-selenenyl)methyl)pyridine (8c) respectively in good
yields. Interestingly, the reaction of 4c with 2-chloropropane, 1-
bromopropane 1,2-dibromoethane and 2-iodo-4,5-lutidine did
not lead to the formation of the related monoselenide instead, in
all cases 5c was the only product obtained.
2.8.1. 2-(Benzylselenenyl)-4,5-dimethylpyridine (7a)
Yield: 0.18 g (80%), m.p. 122–124 °C. 1H NMR: CDCl3, 400 MHz d
(ppm): 8.16 (s, 1H), 7.56 (s, 1H) 7.13–7.24 (m, 5H), 4.27 (s, 2H),
2.21 (s, 3H), 2.19 (s, 3H). 13C NMR: CDCl3, 400 MHz d (ppm):
151.4, 149.4, 143.0, 139.2, 132.4, 129.0, 128.8, 128.4, 121.1, 27.8,
19.8, 16.6 MS (EI, 70 eV) m/z (relative intensity): 277 (18, [M]Å+,
80Se), 197 (52), 106 (22), 91 (100), 77 (24), 65 (49). Anal. Calc. for
C14H15NSe: C, 60.87; H, 5.47; N, 5.07. Found: C, 61.05; H, 4.95; N,
5.39%.
2.8.2. 2-(Butylselenenyl)-4-methylpyridine (14c)
Iodobutane (1.46 g, 0.9 ml, 0.88 mmol) was added to the pale
yellow solution containing species formed by the reaction of 5c
with LiAlH4. Rest of the procedure was same as for 7a. Yield:
0.17 g (93%), very light brown viscous liquid. 1H NMR: CDCl3,
400 MHz d (ppm): 8.26–8.28 (d, J = 5.12 Hz, 1H), 7.12 (s, 1H),
6.79–6.80 (dd, J = 0.84 Hz, 4.98 Hz, 1H), 3.12–3.19 (t, 2H), 2.23 (s,
3H), 1.71–1.79 (m, 2H), 1.37–1.49 (m, 2H), 0.87–0.96 (m, 3H). 13C
NMR: CDCl3, 400 MHz d (ppm): 155.4, 149.5, 146.8, 125.9, 121.4,
32.3, 25.4, 23.1, 20.7, 13.6. MS (EI, 70 eV) m/z (relative intensity):
229 (100, [M]Å+, 80Se), 200 (99), 187 (98), 171 (99), 148 (99), 134
(43), 120 (93), 107 (92), 92 (71), 80 (12), 65 (54). Anal. Calc. for
3.2. Reaction of 4-picolinium- and pyridinium-BF3 adduct with 2.2
equiv. of LDA
We next attempted the dilithiation of 2c with 2.2 equiv. of LDA
at ꢁ78 °C followed by in situ quenching of the resulting species
with 2.2 equiv. of elemental selenium and iodomethane. It was
found that 4-methyl-2,6-bis(methylselenenyl)pyridine (9c) was
C10H15NSe: C, 52.63; H, 6.62; N, 6.13. Found: C, 52.19; H, 6.70; N,
6.26%.

J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
339
Table 1
Crystal data collection and selected structure refinement details for compounds 5a, 9a, 9c, 9d and 12a.
Compound
5a
9a
9c
9d
12a
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C
14H16N2Se2
C9H13NSe2
293.12
180
monoclinic
P21/c
5.3120(3)
16.3259(8)
12.3903(6)
90
101.301(2)
90
C8H11NSe2
279.10
150
monoclinic
P21/c
8.1417(5)
21.6308(14)
5.6145(3)
90
100.286(2)
90
C7H9NSe2
265.07
150
orthorhombic
P212121
10.321(3)
5.3769(17)
15.617(4)
90
90
90
866.7(4)
4
2.031
C10H15NSe3
386.11
150
monoclinic
P21/n
5.2757(2)
27.3034(12)
9.0001(4)
90
101.042(2)
90
1272.41(9)
4
2.016
8.637
370.21
150
monoclinic
P21/n
10.3650(4)
6.6892(2)
21.1397(7)
90
95.182(2)
90
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Volume (Å3)
Z
1459.70(9)
4
1.685
1053.69(9)
4
1.848
972.89(10)
4
1.905
Dcalc (mg mꢁ3
)
l
(mmꢁ1
)
5.050
6.966
7.540
8.458
Reflections collected
Independent reflections (Rint
Final R indices
12906
3935 (0.0321)
7135
2307 (0.0449)
7470
1839 (0.0390)
10286
2360 (0.0313)
13158
3446 (0.0209)
)
R1, wR2, [I > 2(I)]
R1, wR2 (all data)
0.0263, 0.0547 (3139)
0.0404, 0.0586
0.0376, 0.1029, (1962)
0.0460, 0.1179
0.0463, 0.1437, (1636)
0.0520, 0.1479
0.0213, 0.0479, (2201)
0.0243, 0.0486
0.0261, 0.0586 (3021)
0.0326, 0.0606
R2
R2
CH2I
CH3
CH3
R1
R3
R1
R3
eq.)
Se (1.3
I
N
7c:- X = -CH2Ph
-78 °C, to rt
or
PhCH2Cl
N
-CH2
8c:- X=
N
SeLi
SeX
N
Li
CH3
7c (59%); 8c (67%)
BF3
4a/b/c
BF3
3a/b/c
I
N
(ii)
R1
CH2I2
R2
H2O, Air
R2
R2
R2
R1
R3
R3
R1
R3
R1
R3
(i)
N
N
N
Se
N
6b
SeCH2
1a/b/c/d
2
2
BF3
2a/b/c/d
a:- R1= CH3, R2= CH3, R3= H
b:- R1= CH3, R2= H, R3= H
c:- R1= H, R2= CH3, R3= H
d:- R1= H, R2= H, R3= H
(32%);
5a
5b
5c
(57%);
(73%);
(68%)
6a
6c
(60%); (54%)
(vii), (viii)
(ix), (x)
(iii), (iv)
(v), (vi)
R2
R2
CH2SeCH3
R1
R3
R1
R3
R1
R3
9a
(5%);
9c (21%);
9d
10a
(55%);
10c (22%)
(63%)
R4Se
9c
N
SeR4
N
SeR4
12d
H3CSe
N
SeCH3
9d
11d
10c
10d
12a
12c
(15%); (9%)
(7%);
(39%);
(11%)
(35%);
(22%);
(55%)
9c, 9d, 10c and 10d:- R4 = -CH3; 11d and 12d:- R4 = -CH2Ph
Scheme 1. Monolithiation of 1a, 1b and 1c; (i) BF3ꢀEt2O (1.3 equiv.), 0 °C, dry diethyl ether, (ii) LDA (1.3 equiv.), ꢁ78 °C. Dilithiation of 2c and 2d; (iii) LDA (2.2 equiv.), ꢁ78 °C;
(iv) Se (2.2 equiv.), ꢁ78 °C to rt; (v) CH3I (2.2 equiv.), ꢁ78 °C; (vi) H2O. Trilithiation of 2a, 2c and 2d; (vii) LDA (3.3 equiv.), ꢁ78 °C; (viii) Se (3.3 equiv.), ꢁ78 °C to rt; (ix) CH3I
(3.3 equiv.), ꢁ78 °C; (x) H2O.
formed in minor quantity (7%) whereas, 4-methyl-2-(methylse-
lenenyl)pyridine (10c) was the major product formed (Scheme 1).
Combining these results with our earlier reported result on the
dilithiation of 3-picoline, 3,4-lutidine and 3,5-lutidine [14,20], it
appears that the 4-methyl substituted pyridines (3,4-lutidine and
4-picoline) are not conducive to dilithiation when compared with
3-methyl substituted pyridines (3,5-lutidine and 3-picoline). The
dilithiation of pyridinium-BF3 adduct (1d) [18] with LDA (2.2
equiv.) followed by quenching with elemental selenium and iodo-
methane gave the disubstituted product, 2,6-bis(methylselene-
nyl)pyridine (9d), in major quantity (39%) along with the
monosubstituted product, 10d (22%, Scheme 1). However, when

340
J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
CH3
Scheme 2a
H3C
H
CH2SeCH3
CH2
CH3
CH3
hv
H3CSe
N
SeCH3
9a
-SeCH3
H3CSe
N
SeCH3
H3CSe
N
SeCH3
CHO
13
CH3
O2
Scheme 2b
R2
R2
H3CSe
N
13a
SeCH3
(5%)
R1
R1
i) LiAlH4, Dry THF
ii) PhCH2Cl or n-BuI
N
Se
N
SeX
2
7a
7a
(
7c
14c
(85%), (93%)
5a/5c
a:- R1= CH3, R2= CH3
(80%),
c:- R1= H, R2= CH3
and
X = -CH2Ph;
7c:-
14c:- X =
-Bu)
Scheme 2. (a) Probable route to the formation of 3-methyl-2,6-bis(methylselenenyl)-4-pyridine-carboxaldehyde (13a). (b) Cleavage of Se–Se bond in 5a and 5c with LiAlH4.
(chloromethyl)benzene was used as the electrophile in the later
reaction the yield of the products got reversed. 2,6-Bis(benzylse-
lenenyl) pyridine (11d, 11%) corresponding to dilithiation of 1d
was formed in lesser quantity than the monosubstituted product,
2-(benzylselenenyl)pyridine (12d, 55%).
LDA followed by addition of elemental selenium and (chloro-
methyl)benzene afforded 12d in major amount and 11d only in a
very minor quantity. It appears that the nature of the electrophile
plays a crucial role in the formation of the bis(selenenyl) deriva-
tives of 1d.
3.4. Treatment of the diselenides with LiAlH4
3.3. Reaction of 3,4-lutidinium-, 4-picolinium- and pyridinium-BF3
adduct with 3.3 equiv. of LDA
We next thought of studying the reductive cleavage of sele-
nium-selenium bond in 5a and 5c with LiAlH4. Addition of LiAlH4
powder to a solution of 5a in dry THF immediately leads to the
change in color of the solution from dark orange to pale yellow
with the evolution of hydrogen gas indicating the cleavage of
Se–Se bond and the formation of selenolate anion. Addition of
(chloromethyl)benzene to this solution affords 2-(benzylselene-
nyl)-4,5-dimethylpyridine (7a) in excellent yield (Scheme 2b).
Similar to 5a, the reduction of 5c with LiAlH4 followed by reaction
with (chloromethyl)benzene or 1-iodobutane leads to the forma-
tion of 7c and 2-(butylselenenyl)-4-methylpyridine (14c) respec-
tively in excellent yields.
We also examined the trilithiation of 3,4-lutidinium-BF3 adduct
(2a) with 3.3 equiv. of LDA. In this reaction, 3,4-dimethyl-2,6-
bis(methylselenenyl)pyridine (9a) was formed only in a minute
quantity and 4,5-dimethyl-2-(methylselenyl)pyridine (10a) was
the main product formed (Scheme 1). In addition to these
compounds, we were also able to isolate two hitherto unknown
compounds, 4-methyl-2,6-bis(methylselenenyl)-4-(methylselene-
nylmethyl)pyridine (12a) and 3-methyl-2,6-bis(methylselene-
nyl)-4-pyridinecarboxaldehyde (13a), from the reaction mixture.
The isolation of 12a evinces the trilithiation of 2a with 3.3 equiv.
of LDA. The compound 12a appears to be photochemically labile,
especially when present in the form of solution and decomposes
to give red amorphous selenium. It is probable that the photo-
chemical decomposition of 12a might have generated picolyl type
radical 13 which on reaction with oxygen or hydrogen radical
affords 13a and 9a, respectively (Scheme 2a). The compounds con-
taining ArCH2–SeR bond are known to undergo photochemical
decomposition to give the benzyl/picolyl radical [24,25]. The reso-
nance stabilization of this radical appears to be the driving force for
the lability of the ArCH2–SeR bond.
Encouraged by the isolation of 12a, we also examined the tri-
lithiation of 1c and 1d with 3.3 equiv. of LDA. The in situ quenching
of the resulting lithiated species of 2c with selenium and iodo-
methane gave 2,6-bis(methylselenenyl)-4-(methylselenenylmeth-
yl)pyridine (12c), 10c and 9c whereas, the lithiated species of 2d
gave the disubstituted product, 9d (63%) only (Scheme 1). We were
not able to isolate any triselenated product in the reaction involv-
ing 1d although a very minute quantity of it was noticed in the GC
chromatogram. Similar to the dilithiation reaction involving 1d
and (chloromethyl)benzene, treatment of 1d with 3.3 equiv. of
3.5. Spectroscopic studies
All the synthesized compounds were characterized by elemen-
tal analysis, NMR (1H and 13C), and mass spectral techniques. The
1H NMR spectra of 5a and 6a shows two singlets whereas, the spec-
tra of 5b, 5c, 6b and 6c shows three signals in the aromatic region
indicating deprotonation of the parent compound (1a–1c). The
spectra of the aforementioned compounds show singlet between
2.1 and 2.5 d ppm due to the methyl group attached to the pyridine
ring. The signal corresponding to the protons of the methyl group
attached to the selenium atom in compounds 9a–9d, 10a–10d, 12a
and 13a also appear in the range 2.1–2.5 d ppm as singlet and is
invariably flanked by selenium satellites due to NMR active 77Se.
The signal due to Se-CH2–Se protons in 6a–6c, 7c–8c and 11d–
12d is also flanked by selenium satellites and appears as singlet
in the range 4.78–4.28 d ppm. However, the Se–CH2-py group in
12a appears upfield at 3.55 d ppm. A signal at 10.25 d ppm in the
1H NMR spectrum of 13a establishes the presence of aldehydic

J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
341
Fig. 1. ORTEP view of 5a with thermal ellipsoids drawn at 50% probability level. Se(1)–Se(2) 2.3010(3); Se(1)–C(12) 1.934(2); C(12)–Se(1)–Se(2) 104.20(6); C(22)–Se(2)–Se(1)
102.74(7); N(11)–C(12)–Se(1) 109.25(15); C(13)–C(12)–Se(1) 126.24(16); N(21)–C(22)–Se(2) 111.06(15); C(23)–C(22)–Se(2) 124.73(16).
proton. In the 13C NMR spectra of 12a and 12c, the signal due to
Se–CH2-py appears at 24.8 and 26.5 d ppm, respectively whereas,
the signals for methyl carbon attached to selenium atoms appear
at three different positions ranging from 7.1 to 4.7 d ppm. As
expected, the HCO signal in 13a appears at 191.7 d ppm. The ESI
mass spectra of 5a, 5b and 6a shows molecular ion peak [MÅ+,
80Se] at m/z 372, 344 and 386, respectively. The EI mass spectra
of all other compounds gave distinct molecular ion peaks with
characteristic selenium isotopic patterns. In addition to the molec-
ular ion peak, the mass spectra of 9c, 10a and 10c exhibit base ion
peak corresponding to a fragment formed due to the removal of
80Se_ from the molecular ion peak. However, base ion peak in 9a
appears due the removal of 80SeH from the molecular ion peak.
The molecular ion peak in 12a is at m/z 389 and its base ion peak
appears at m/z 213 due to the fragment [C9H11NSe+].
are almost orthogonal to each other with the angle of 76.0°. The
two N–C–Se–Se torsion angles of 177.8° and 171.8° indicate that
the selenium atoms lie in the plane of the related pyridine ring.
This geometric arrangement around the Se–Se bond is comparable
to that reported in substituted and unsubstituted 2,20-dipyridyl
diselenide [17,26]. However, this arrangement is opposite to that
found in bis(3,5-dimethyl-2-pyridyl) diselenide [14]. The later
3.6. Solid state structural features of 5a
The molecular structure of 5a and its crystallographic number-
ing scheme is shown in Fig. 1. 5a adopts a gauche conformation
with a C–Se–Se–C torsion angle of 82°(4). The two pyridine rings
Table 2
Selected bond distances (Å) and angles (°) for selenium compounds 9a, 9c, 9d and
12a.
Compound
9a
9c
9d
12a
Se(21)–C(2)
Se(21)–C(22)
Se(42)–C(41)
Se(42)–C(43)
Se(61)–C(6)
Se(61)–C(62)
C(2)–Se(21)–C(22)
C(6)–Se(61)–C(62)
C(43)–Se(42)–C(41)
N(1)–C(2)–Se(21)
C(3)–C(2)–Se(21)
N(1)–C(6)–Se(61)
C(5)–C(6)–Se(61)
1.925(4)
1.941(4)
–
1.919(6)
1.938(6)
–
1.914(2)
1.936(3)
–
1.924(2)
1.940(3)
1.963(2)
1.943(3)
1.911(2)
1.938(3)
98.26(11)
98.85(12)
97.41(11)
116.56(18)
118.70(18)
–
–
–
–
1.918(4)
1.942(4)
98.45(17)
98.64(19)
–
116.4(3)
118.7(3)
119.1(3)
118.0(3)
1.921(5)
1.935(6)
98.9(2)
98.7(3)
–
117.9(4)
117.8(4)
118.0(4)
117.7(4)
1.919(2)
1.937(3)
99.98(12)
97.71(12)
–
118.72(18)
117.63(18)
118.96(17)
117.40(18)
–
Fig. 2. ORTEP diagrams of 9a with ellipsoids drawn at 50% probability level.

342
J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
Fig. 4. ORTEP diagrams of 9d with ellipsoids drawn at 50% probability level.
contacts indicates that the discrete molecules of 5a are not in-
volved in Seꢀ ꢀ ꢀSe secondary interactions.
Fig. 3. ORTEP diagrams of 9c with ellipsoids drawn at 50% probability level.
3.7. Solid state structural features of 9a, 9c and 9d
compound adopts anti conformation with two pyridyl rings com-
pletely coplanar with the Se–Se bridge. The Se–Se distance of
2.301(3) Å is within the range of 2.275–2.415 Å for the related
diselenides [14,17,26]. The analysis of the short intermolecular
Selected bond lengths and angles of 9a, 9c and 9d are also given
Table 2 and their molecular structures with atomic numbering
schemes are shown in Figs. 2–4, respectively. The Se–C(sp2) and
Se–C(sp3) bond lengths in these compounds are slightly shorter
Fig. 5. Crystal packing diagram of 9c showing the helical assembling in solid sate through the Seꢀ ꢀ ꢀSe interactions, which are represented as red dashed lines.

J.S. Dhau et al. / Inorganica Chimica Acta 392 (2012) 335–344
343
atomic numbering scheme is shown in Fig. 6. The two Se–C(sp2)
bond lengths in 12a are not equivalent. Indeed, the Se(61)–C(6)
bond length (ortho to the methyl group) is slightly longer than
Se(21)–C(2) bond length (Table 2). In contrast, the Se–C(sp3) bond
length at both the positions is almost of equal length. The Se(42)–
CH2 bond length is slightly greater than the Se(42)–CH3 bond
length. The crystal packing analysis of 12a reveals Seꢀ ꢀ ꢀSe second-
ary intermolecular interactions between the three independent
selenium atoms. The Se(61)ꢀ ꢀ ꢀSe(61) and Se(21)ꢀ ꢀ ꢀSe(42) distances
are 3.613 and 3.712 Å, respectively.
4. Conclusion
Synthesis of a number of 3,4-lutidinyl, 3-/4-picolyl- and 2-
pyridylselenium compounds has been achieved by a procedure
involving directed ring lithiation, dilithiation and trilithiation of
the corresponding pyridyl derivative. Reduction of selenium-
selenium bond in various diselenides with LiAlH4 is an effective
method to form unsymmetrical monoselenides at room tempera-
ture. A comparative study of the crystal structure of some of
the prepared compounds indicates that the instances of Seꢀ ꢀ ꢀSe
secondary interactions decreases with the increase in the number
of methyl group attached to the pyridine ring.
Acknowledgements
We are thankful to the CSIR, New Delhi, India, for financial sup-
port. Additional support from SAP by UGC, New Delhi, is also
acknowledged. We are also thankful to Mr. Avtar Singh, CIL, Punjab
University, Chandigarh, for NMR spectra.
Fig. 6. ORTEP diagram of 12a with ellipsoids drawn at 50% probability level.
Appendix A. Supplementary material
than that found in 3,5-dimethyl-2,6-bis(methylselenenyl)pyridine
[14]. Of these compounds, 9d has the shortest average Se–C(sp2)
bond length (Table 2). Similar to 3,5-dimethyl-2,6-bis(methyl
selenenyl)pyridine, no Seꢀ ꢀ ꢀSe short intermolecular distances were
observed in crystal structure of 9a. Indeed, the shortest Seꢀ ꢀ ꢀSe dis-
tances in 9a are 3.883 and 4.063 Å which are slightly longer than
the sum of their Van der Waals radii (3.8 Å) [27]. However, the
crystal packing diagram (Fig. 5) of 9c shows two independent short
Se(61)ꢀ ꢀ ꢀSe(61) intermolecular distances of 3.733 and 3.774 Å be-
tween one of the two selenium atoms attached to the molecule
and other selenium atoms of the two neighboring rings leading
to the formation of pillars composed of two rows assembled
around the 21 helical crystallographic axis thorough the Seꢀ ꢀ ꢀSe
interactions.
In case of 9d, the crystal packing analysis shows the presence of
Seꢀ ꢀ ꢀSe intermolecular interactions occurring at short distances of
3.761 Å involving both the selenium atoms. Comparing the crystal
structure of 9a, 9c and 9d it is clear that instances of Seꢀ ꢀ ꢀSe sec-
ondary interactions decreases with the increase in number of
methyl group at the pyridine ring. The attachment of methyl
group(s) to the pyridine ring decreases the electronegativity of
the pyridine moiety attached to the selenium atom. This result is
consistent with the reported result that decrease in the electroneg-
ativity of the group attached directly to the selenium atom de-
creases the chalcogenꢀ ꢀ ꢀselenium interactions [28].
CCDC 836311, 836312, 836313, 836314 and 836315 contain the
supplementary crystallographic data for complexes 5a, 9a, 9c, 9d
and 12a. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.ica.2012.03.043.
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