Zinc(II) mediated synthesis of an N-substituted 2-(2-pyridyl)imidazole from a 1,2-diketone and 2-(aminomethyl)pyridine and its ligational behaviour

Article abstract of DOI:10.1039/b308294f

Reaction of anhydrous ZnCl₂ with the 1:2 condensate (L) of benzil and 2-(aminomethyl)pyridine in methanol gives monomeric ZnL′Cl ₂ (1) where L′ is 2-[(4,5-diphenyl-2-pyridin-2-yl-1H-imidazol- 1-yl)-methyl]pyridine. In the X-ray cryst

Full text of DOI:10.1039/b308294f

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P A P E R
Zinc(II) mediated synthesis of an N-substituted 2-(2-pyridyl)imidazole
from a 1,2-diketone and 2-(aminomethyl)pyridine and its ligational
behaviour
Michael G. B. Drew,a Derek A. Tocher,b Kiranmoy Chowdhury,c Shubhamoy Chowdhuryc and
Dipankar Datta*c
a
Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
Department of Chemistry, University College London, 20 Gordon Street, London
b
WC1H 0AJ, UK
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India. E-mail: icdd@mahendra.iacs.res.in
c
Received (in Montpellier, France) 16th July 2003, Accepted 17th October 2003
First published as an Advance Article on the web 19th January 2004
Reaction of anhydrous ZnCl2 with the 1:2 condensate (L) of benzil and 2-(aminomethyl)pyridine in
methanol gives monomeric ZnL0Cl2 (1) where L0 is 2-[(4,5-diphenyl-2-pyridin-2-yl-1H-imidazol-1-yl)-
methyl]pyridine. In the X-ray crystal structure, 1 is found to contain tetrahedral zinc with an N2Cl2
coordination sphere and the N-substituent methylpyridine fragment hanging as a free arm. A tentative
mechanism is proposed for the zinc mediated conversion of L ! L0. Demetallation of 1 by the action of aqueous
NaOH yields L0 in the free state. When L0 is reacted with Zn(ClO4)2ꢀ6H2O in a 1:2 molar proportion, [Zn(L0)2]n-
(ClO4)2nꢀ(H2O)n/2ꢀ(CH2Cl2)n/2 (2) is obtained. The zinc atom in 2, as revealed by X-ray crystallography,
has a trigonal bipyramidal N5 coordination sphere. There are two independent ligands in the asymmetric unit
of 2. One of them bonds only to one zinc atom in a bidentate mode with the N-substituent methylpyridine
hanging free while the other ligand binds to two different zinc atoms in a tridentate fashion, employing the
N-substituent methylpyridine nitrogen atom to form the polymeric one-dimensional chain cation.
methanol. It is obtained as a reddish yellow gum. As isolated,
Introduction
it is not analytically pure. All our attempts to purify it by chro-
matography have so far failed. Such problems with Schiff bases
are not new.4 However, by reacting it with anhydrous zinc
chloride in methanol at room temperature, we obtain red crys-
tals of 1 in good yield. From X-ray crystallography, 1 is found
to be ZnL0Cl2 in which the chloride ions are coordinated to the
metal and the ligand moiety L0 is bound to the metal in a
bidentate mode (Fig. 1). The geometry about the zinc ion is
distorted tetrahedral with angles ranging from 79.82(1)ꢁ to
118.2(1)ꢁ. With the exception of the angle formed by the biden-
tate bite of the organic ligand the other angles fall in a narrow
range, 114ꢁ ꢂ 4ꢁ. The two Zn–Cl bonds are more or less indis-
2-(2-Pyridyl)imidazole can be considered as a hybrid of 2,20-
bipyridine and 2,20-biimidazole, which are versatile N-donor
ligands in transition metal chemistry.1 Its chemistry, which
has not been explored thoroughly,2 is our concern here. A
convenient route for its synthesis is to react glyoxal, ammonia
and 2-formylpyridine in aqueous ethanol at low temperature.3
While an excess of ammonia is needed in this procedure,
glyoxal and 2-formylpyridine are used in a 1:1.2 molar
proportion. In our attempt to synthesise a simple zinc complex
of the ligand L, the 1:2 condensate of benzil and 2-(amino-
methyl)pyridine, we have obtained
a zinc complex of
L0, which is an N-substituted 2-(2-pyridyl)imidazole. These
results, together with the isolation of L0 in the free state and
its use as a novel tridentate N-donor ligand, are reported here.
˚
tinguishable. The Zn–Npyridyl bond, 2.069(4) A, is significantly
longer than the bond to the other metallated nitrogen atom,
˚
2.028(3) A. The two metallated rings are essentially coplanar,
with an angle between mean planes of 5.3ꢁ, while the phenyl
substituents on the imidazole ring form angles of 37.3ꢁ and
62.7ꢁ with that five-membered ring.
A tentative mechanism for the zinc mediated transformation
of L to L0 is suggested in Scheme 1. In the first step, L is pro-
posed to bind the metal in a bidentate fashion. In the last step,
an aerial oxidation is implicated. When the reaction between
anhydrous zinc chloride and L is carried out under an N2
atmosphere, ZnL0Cl2 is not formed; instead, a pink amorphous
zinc complex is obtained, which we have not yet been able to
characterise. In the observed reaction of ZnCl2 with L, Zn2þ
does not act as a simple Lewis acid. Binding of L to zinc seems
to be a prerequisite for the transformation L ! L0, as simple
proton (reacted with L in the form of perchloric acid) is found
to be ineffective in this regard.
Results and discussion
The ligand L is prepared by refluxing benzil and 2-(amino-
methyl)pyridine in a 1:2 molar proportion in anhydrous
Demetallation of 1 by the action of aqueous NaOH affords
L0 in the free state. In contrast to L, L0 is obtained as a pure
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 2 3 – 3 2 5
323

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Fig. 2 The structure of the one-dimensional polymeric cation in
2 with ellipsoids at 25% probability.
zinc atom while the other bonds to two different zinc atoms,
thus forming the polymeric chain shown in Fig. 2.
Concluding remarks
Here we report an easy procedure for synthesising an N-substi-
tuted 2-(2-pyridyl)imidazole. No N-substituted 2-(2-pyridyl)-
imidazole with a pendant donor atom has been reported
earlier. The zinc assisted aerial oxidation in the last step of
Scheme 1 is reminiscent of the activity of liver alcohol dehy-
drogenase (LADH), which is a zinc containing metalloenzyme.
The zinc(II) center in LADH binds alcohol in the catalytic cycle
of the enzyme, facilitating the oxidation of alcohol by NADþ
(oxidised form of nicotinamide adenine dinucleotide).5,6
Our work shows that L0 can behave as a bidentate as well as
a tridentate N-donor ligand. In the bidentate mode, the N-
substituent methylpyridine remains free. When this pendant
pyridine N takes part in binding, L0 becomes tridentate. In
the tridentate mode, it is capable of generating coordination
polymers, which are of much current interest because of their
possible applications as functional materials.
Fig. 1 A view ofꢁthe structure of ZnL0Cl2 (1). Selected bond distances
˚
(A) and angles ( ): Zn1–N1 2.069(4), Zn1–N2 2.028(3), Zn1–Cl1
2.205(1), Zn1–Cl2 2.208(1), N2–Zn1–N1 79.8(1), N2–Zn1–Cl1
116.4(1), N1–Zn1–Cl1 111.9(1), N2–Zn1–Cl2 114.1(1), N1–Zn1–Cl2
109.9(1).
solid. Complex 1 can be easily regenerated by reacting
anhydrous zinc chloride with free L0 in equimolar amounts
in methanol.
Reaction of L0 with ZnClO4ꢀ6H2O in a 2:1 molar proportion
in tetrahydrofuran and subsequent recrystallisation of the
product from a mixture of dichloromethane and petroleum
1
1
ether yields [Zn(L0)2](ClO4)2ꢀ2H2Oꢀ2CH2Cl2 (2). Its structure,
as revealed by X-ray crystallography, consists of an infinite
(Fig. 2) and
2nþ
one-dimensional polymeric cation [Zn(L0)2]n
Experimental
free perchlorate anions and solvent molecules. The polymeric
chain extends along the b axis. The environment of a single
zinc atom is shown in Fig. 3. The metal atom is bonded to five
nitrogen atoms from three different ligands and has a trigonal
bipyramidal environment. The coordination sphere contains
bonds to two nitrogen atoms from each of two ligands forming
five-membered chelate rings with the metal, thus N(21) at
Microanalyses were performed with the use of a Perkin–Elmer
2400II elemental analyser. FTIR spectra (KBr) were recorded
on a Nicolet Magna-IR (Series II) spectrophotometer, UV-VIS
spectra on a Shimadzu UV-160A spectrophotometer and H
NMR spectra CDCl3 using a Brucker DPX spectrometer.
1
˚
˚
2.055(12) A and N(11) at 2.130(12) A form a chelating ring
˚
as does N(91) (0.5 ꢃ x, 0.5 þ y, 0.25 ꢃ z) at 2.066(12) A and
˚
N(101) (0.5 ꢃ x, 0.5 þ y, 0.25 ꢃ z) at 2.119(11) A. N(11) and
N(101), which are pyridine nitrogen atoms, are the axial atoms
in the coordination sphere and form the longer bonds to the
metal atom. The coordination sphere is completed by N(81),
a pyridine nitrogen from a third ligand. There are two indepen-
dent ligands in the asymmetric unit, one bonding to only one
Fig. 3 The environment of one zinc atom in the polymeric cation in
˚
2 with ellipsoids at 25% probability. Selected bond distances (A) and
angles (ꢁ): Zn1–N91_7 2.053(11), Zn1–N21 2.072(12), Zn1–N101_7
2.10(11), Zn1–N11 2.121(12), Zn1–N81 2.186(11), N91_7–Zn1–N21
115.7(5), N91_7–Zn1–N101_7 79.2(4), N21–Zn1–N101_7 101.8(5),
N91_7–Zn1–N11 104.7(5), N91_7–Zn1–N11 79.4(5), N101_7–Zn1–
N11 174.9(5), N91_7–Zn1–N81 112.7(4), N21–Zn1–N81 131.6(5),
N101_7–Zn1–N81 86.4(4), N11–Zn1–N81 89.2(4). Symmetry
operation 7 is 0.5 ꢃ x, 0.5 þ y, 0.25 ꢃ z.
Scheme 1 Proposed mechanism for the zinc mediated converison of
L to L0 in the coordinated state. Only the active H atoms are shown.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
324
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 2 3 – 3 2 5

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Syntheses
X-Ray crystallography
L. Benzil (2.04 g, 9.7 mmol) was dissolved in 30 ml of anhy-
drous methanol. To this solution, freshly distilled 2-(methyl-
amino)pyridine (2 ml, 19.4 mmol) was added and refluxed
for 5 h. Then the reaction mixture was evaporated at room
temperature under reduced pressure to obtain a reddish yellow
gum. It was dissolved in 50 ml of diethylether and filtered. The
filtrate was left to stand in air. After complete evaporation of
the solvent, a gummy reddish yellow mass was obtained, which
was kept in vacuo over fused CaCl2 . Yield 2.60 g (69%).
For 1, 4188 independent reflections were collected on an auto-
matic four-circle Nicolet R3mV diffractometer at 293(2) K
using graphite monochromated Mo-Ka radiation. The data
were corrected for Lorentz polarisation effects and an empiri-
cal absorption correction applied. Data for 2 were measured at
293(2) K with MoKa radiation using the MARresearch Image
Plate System. The crystal was positioned at 100 mm from the
image plate and 100 frames were measured at 2ꢁ intervals with
a counting time of 5 min. Data analysis for 2 was carried out
with the XDS program7 to provide 5426 independent reflec-
tions. Both the structures were solved using direct methods
with the SHELXS-86 program.8 The structure of 2 contains
one water and one dichloromethane solvent molecule, both
given 50% occupancy although for the latter there were three
sets of chlorine atoms, each given 16.67% occupancy, which
were refined with distance constraints. The high R values for
2 are due to the weak and distorted spot shapes obtained in
the diffraction pattern. Indeed, the Rint value for 25 096 reflec-
tions was 0.1285. This made it necessary for only the zinc and
chlorine atoms to be refined anisotropically. The hydrogen
atoms in solvent molecules were not included in the analysis.
Full-matrix least-squares refinement (based on F 2; SHELXL-
939) gave R1 ¼ 0.0464, wR2 ¼ 0.1135 [2960 reflections,
I > 2s(I)] for 1 and R1 ¼ 0.1177, wR2 ¼ 0.2299 [4806 reflec-
tions, I > 2s(I)] for 2. For all reflections R1 ¼ 0.1102,
wR2 ¼ 0.3237 for 1 and R1 ¼ 0.1366, wR2 ¼ 0.2402 for 2.y
ZnL0Cl2 (1). Solid anhydrous zinc chloride (0.14 g, 1 mmol)
was added to L (0.39 g, 1 mmol) dissolved in 30 ml of metha-
nol and stirred for 1 h. The resulting red solution was kept in
the refrigerator for 2 days. The red crystalline compound that
precipitated was collected by filtration. Yield 0.41 g (80%). The
crystals were suitable for X-ray crystallography. Anal. calcd
for C26H20N4ZnCl2 : C, 59.49; H, 3.84; N, 10.68; found: C,
59.47; H, 3.67; N, 10.58%. UV/VIS (CHCl3) l/nm (e/dm3
molꢃ1 cmꢃ1): 343 (14 500), 256 (15 200). 1H NMR (300
MHz, CDCl3 , TMS) dH : 5.50 (s, methylene, 2H), 6.85–8.57
(aromatic, 18H).
L0. ZnL0Cl2 (1.05 g, 2 mmol) was dissolved in 30 ml of
chloroform. To this solution, 10 ml of 5 N aqueous NaOH
was added and stirred for 90 min. The chloroform layer was
washed thoroughly with water (20 ml ꢄ 3) and dried by adding
anhydrous Na2SO4 . The solvent was evaporated under
reduced pressure at room temperature to obtain a yellow
gum. After keeping the gum in vacuo over fused CaCl2 for sev-
eral days, a yellow solid was obtained. Yield 0.59 g (18%).
Anal. calcd for C26H20N4 : C, 80.38; H, 5.19; N, 14.43; found:
C, 80.26; H, 5.11; N, 14.49%. UV/VIS (CHCl3) l/nm (e/dm3
molꢃ1 cmꢃ1): 317 (22 900). 1H NMR (300 MHz, CDCl3 , TMS)
dH : 5.90 (s, methylene, 2H), 6.77–8.42 (aromatic, 18H).
Crystal data for ZnL0Cl2 (1). C26H20Cl2N4Zn, Mw ¼ 524.7,
monoclinic, space group P21/n, a ¼ 15.777(3), b ¼ 10.409(2),
3
ꢁ
˚
˚
c ¼ 15.932(3) A, b ¼ 114.05(3) , U ¼ 2389(1) A , Z ¼ 4,
Dc ¼ 1.46 g cmꢃ3, m ¼ 1.27 mmꢃ1
.
1
1
Crystal data for [Zn(L0)2](ClO4)2ꢀ H2Oꢀ CH2Cl2 (2).
2
2
C
52.5H42Cl3N8O8.5Zn, Mw ¼ 1092.66, tetragonal, space group
ZnL0Cl2 by starting with L0. Solid anhydrous zinc chloride
(0.03 g, 1 mmol) was added to L0 (0.08 g, 0.18 mmol) dissolved
in 7 ml of methanol and stirred for 2 h. The resulting red solu-
tion was kept in air overnight. The red precipitate was filtered,
washed first with 10 ml of water and then with 10 ml of n-hex-
ane. It was dried in vacuo over fused CaCl2 . Yield 0.06 g
(55%).
˚
P41212, a ¼ b ¼ 16.513(18), c ¼ 39.02(4) A, U ¼ 10640(20)
3
A , Z ¼ 8, Dc ¼ 1.364 g cmꢃ3, m ¼ 0.67 mmꢃ1
.
˚
Acknowledgements
We thank EPSRC and the University of Reading for funds for
the Image Plate system.
1
2
1
2
[Zn(L0)2](ClO4)2ꢀ H2Oꢀ CH2Cl2 (2). Solid Zn(ClO4)2ꢀ6H2O
(0.09 g, 0.25 mmol) was added to L0 (0.19 g, 0.5 mmol) dis-
solved in 10 ml of tetrahydrofuran and stirred for 30 min.
The resulting yellow solution was left in air for 2 h. The yellow
precipitate was filtered, washed with 5 ml of water and dried
in vacuo over fused CaCl2 . It was recrystallised from a 3:1
References
dichloromethane–petroleum ether mixture. Yield 0.16
g
1
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(61%). Colourless single crystals were grown by direct diffusion
of petroleum ether into a dilute dichloromethane solution of
the complex. Anal. calcd for C52.5H42N8ZnCl3O8.5 : C, 57.68;
H, 3.88; N, 10.25; found: C, 57.79; H, 3.81; N, 10.31%. FTIR
(KBr) n/cmꢃ1: 1090vs, 623s (ClO4). UV/VIS (CHCl3) l/nm
(e/dm3 molꢃ1 cmꢃ1): 329 (21 700), 254 (23 900). 1H NMR
(300 MHz, CDCl3 , TMS) dH : 5.27 (s, methylene of CH2Cl2 ,
1H), 5.69 (br, methylene, 4H), 6.84–8.45 (br signals, aromatic,
36H).
CAUTION! Though we have not met with any incidents
while working with 2, care should be taken in handling it as
perchlorates are potentially explosive. It should not be
prepared and stored in large amounts.
2
3
4
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5
6
7
8
9
y CCDC reference numbers 222262 and 222263. See http://
www.rsc.org/suppdata/nj/b3/b308294f/ for crystallographic data in
.cif or other electronic format.
T h i s j o u r n a l i s Q T h e R o y a l S o c i e t y o f C h e m i s t r y a n d t h e
C e n t r e N a t i o n a l d e l a R e c h e r c h e S c i e n t i f i q u e 2 0 0 4
N e w . J . C h e m . , 2 0 0 4 , 2 8 , 3 2 3 – 3 2 5
325