Binuclear and polynuclear transition metal complexes with macrocyclic ligands. 4. New polydentate azomethine ligands based on 2,5-diformylpyrrole and 2,6-diformylpyridine

Article abstract of DOI:10.1023/B:RUCB.0000030807.35988.a5

The reactions of 2,5-diformylpyrrole (1) and 2,6-diformylpyridine (2) with propane-1,3-diamine afforded new macrocyclic Schiff's bases 5 and 6, respectively. Their structures were established by NMR spectroscopy and mass spectrometry. Binuclear copper(II)

Full text of DOI:10.1023/B:RUCB.0000030807.35988.a5

Russian Chemical Bulletin, International Edition, Vol. 53, No. 2, pp. 335—339, February, 2004
335
Binuclear and polynuclear transition metal complexes
with macrocyclic ligands
4.* New polydentate azomethine ligands based on
2,5ꢀdiformylpyrrole and 2,6ꢀdiformylpyridine
E. A. Katayev, M. D. Reshetova,ꢀ and Yu. A. Ustynyuk
Department of Chemistry, Moscow State University,
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (095) 939 2677. Eꢀmail: mres@nmr.chem.msu.su
The reactions of 2,5ꢀdiformylpyrrole (1) and 2,6ꢀdiformylpyridine (2) with propaneꢀ1,3ꢀ
diamine afforded new macrocyclic Schiff´s bases 5 and 6, respectively. Their structures were
established by NMR spectroscopy and mass spectrometry. Binuclear copper(II) and nickel(II)
complexes with ligand 5 were synthesized. Pentadentate Schiff´s base, viz., 2,6ꢀbis[(2ꢀaminoꢀ
phenylimino)methyl]pyridine, was prepared by demetallation of its complex with Cd(ClO4)2
using Na2S. In solutions, the latter Schiff´s base is quantitatively transformed into
2,6ꢀbis(benzoimidazolyl)pyridine under the action of atmospheric oxygen or other mild oxiꢀ
dizing agents.
Key words: macrocyclic ligands, Schiff´s bases, NMR spectroscopy, mass spectrometry,
MALDIꢀTOF, ESIꢀMS.
Macrocyclic Schiff´s bases generated from diamines
and α,α´ꢀdicarbonyl derivatives of pyrrole and pyridine
were relatively poorly studied compared to macrocycles
based on 2,6ꢀdicarbonyl derivatives of 4ꢀsubstituted
phenols. Template syntheses in the presence of metal ions
allow one to prepare complexes with macrocyclic ligands.
However, the preparation of free ligands often presents
difficulties.
Generally, the reactions of 2,5ꢀdiformylpyrrole (1) or
2,6ꢀdiformylpyridine (2) with aliphatic diamines in the
absence of metal ions afford mixtures of oligomeric conꢀ
densation products.2 However, the reactions of 1 with
1,4ꢀdiaminobutanꢀ2ꢀol3 and diꢀ
ethylenetriamine4 gave rise to
macrocyclic [2+2]ꢀcondensation
products. The reaction of 1 with
ethylenediamine produced comꢀ
pound 3, which was generated due
to the addition of a water molꢀ
ecule to the [2+2]ꢀmacrocycle5
(as a rule, these reactions are carꢀ
ried out in the presence of cataꢀ
We found that condensation of compounds 1 and 2
with propaneꢀ1,3ꢀdiamine (4) using dry MeCN as the
solvent in the absence of acid gave rise to macrocycles 5
and 6, respectively, in good yields (Scheme 1).
Within approximately 2 h after the completion of the
slow addition of the dialdehyde to a very dilute solution of
compound 4 in dry MeCN, white small crystals of the
macrocycles began to precipitate. Crystals of compound 5
are very moistureꢀsensitive and deliquesce on storage in
air. When organic solvents (THF, CH2Cl2, or CHCl3)
were not sufficiently dried, their removal from solutions
of compound 5 by distillation resulted in isolation of 5 as
a viscous oil, which solidified on drying under high
vacuum. The structure of compound 5 was confirmed by
1H and 13C NMR spectroscopy and ESI mass spectromꢀ
etry. The 1H NMR spectrum (CDCl3) has only four broadꢀ
ened signals at δ 1.98, 3.60, 6.43, and 8.04 with the inteꢀ
gral intensity ratio of 4 : 8 : 4 : 4 corresponding to the
CH atoms. The signal of the N—H protons of the pyrrole
fragment is broadened to an extent that it is unobservꢀ
able. Apparently, broadening of the NMR signals is atꢀ
tributed to exchange processes associated with rather
rapid interconversions of several energetically similar
conformers of the macrocycle. Studies of these proꢀ
cesses by dynamic NMR spectroscopy are currently unꢀ
derway.
lytic amounts of acids, for exꢀ
ample, of HNO3 or HCl). 2,6ꢀDiformylpyridine (2) and
2,6ꢀdiacetylpyridine form [2+2]ꢀmacrocyclic Schiff´s
bases with diamines containing additional heteroatoms in
the aliphatic chain.4,6,7
Unlike macrocycle 5, compound 6 is not highly hygroꢀ
scopic, and no broadening of the signals is observed in its
NMR spectra. In weakly acidic aqueous solutions, both
* For Part 3, see Ref. 1.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 322—325, February, 2004.
1066ꢀ5285/04/5302ꢀ0335 © 2004 Plenum Publishing Corporation

336
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 2, February, 2004
Katayev et al.
Scheme 1
CH2Cl2, and insoluble in nonpolar organic solvents,
whereas the binuclear nickel complex with a macrocycle
based on diformylpyrrole and oꢀphenylenediamine, which
we have studied earlier,8 is very poorly soluble in most of
organic solvents.
It is known that the reaction of 2,6ꢀdiacetylpyridine
with 1,2ꢀdiaminobenzene or 1,2ꢀdiaminoethane in the
presence of Cd(ClO4)2 affords cadmium complexes with
polydentate Schiff´s bases 9 as a result of [1+2]ꢀcondenꢀ
sation of the starting reagents.9
We succeeded in preparing free polydentate Schiff´s
base 10 in 98% yield by the reaction of sodium sulfide
with complex 9a. It should be noted that compound 10
cannot be synthesized by direct condensation of
dialdehyde 2 with 1,2ꢀdiaminobenzene. Products of
[1+2]ꢀcondensation of dicarbonyl compounds with
1,2ꢀdiaminobenzene as free ligands have previously been
unknown. The reaction of complex 9b with Na2S proꢀ
duced 2,6ꢀdiformylpyridine (2) and ethylenediamine due,
apparently, to the fact that [1+2]ꢀSchiff´s base formed as
an intermediate has lower hydrolytic stability under these
conditions, which is typical of azomethines based on aliꢀ
phatic amines (Scheme 3).
Reagents and conditions: i. MeCN, 20 °C, 24 h; ii. MeCN,
20 °C, 48 h.
macrocycles 5 and 6 are readily hydrolyzed to give the
starting dialdehydes and diamine 4.
Macrocycle 5 reacts with copper(II) and nickel(II) acꢀ
etates in an CHCl3—MeOH mixture to form binuclear
complexes 7 and 8, respectively (Scheme 2).
Scheme 2
Scheme 3
Reagents and conditions: i. M(OAc)2, CHCl3+MeOH,
20 °C, 24 h.
Complexes 7 and 8 were obtained as brightꢀgreen and
olivineꢀcolored crystalline compounds, respectively.
These compounds are readily soluble in water and MeOH,
moderately soluble in THF, poorly soluble in CHCl3 and

New polydentate azomethine ligands
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 2, February, 2004
337
Pentadentate ligand 10 contains three types of
Nꢀdonor centers, which is of particular interest for the
synthesis of binuclear metal complexes and for its further
use as a building block in the synthesis of unsymmetrical
macrocyclic ligands.
precipitate is the complex 11•Cu(OAc)2 (12). The
MALDIꢀTOF spectrum of 12 has two peaks at m/z 311.7
and 373.7 corresponding to the [L + H]+ and [LCu – H]+
ions, respectively. The reaction of Na2S with a methanolic
solution of complex 12 led to recovery of the starting
ligand 11 in 85% yield.
The formation of benzoimidazoles from Schiff´s bases
in the presence of copper(II) salts as well as of other oxiꢀ
dizing agents has been described many times.12 Earlier, it
has been demonstrated9 that refluxing of a solution
of cadmium complex 9a in MeOH with an excess of
Ni(ClO4)2 followed by prolonged storage in air afforded a
compound, to which the structure of a nickel complex
with ligand 11 was assigned based on analysis of the IR
and UV spectra.
Compound 10 was prepared as a brightꢀyellow comꢀ
pound, which is readily soluble in CHCl3, CH2Cl2,
and AcOEt and less soluble in MeOH and MeCN
(∼1.25 g L–1). The 1H NMR spectrum of compound 10 in
CD3CN has an AMPX system with signals at δ 6.42 (H(3)),
6.55 (H(2)), 6.85 (H(4)), and 7.01 (H(5)) belonging to
the aromatic protons, a broad signal of the protons of the
amino groups at δ 4.46, an A2X system with signals at
δ 7.71 (H(8)) and 8.12 (H(7)) belonging to the protons of
the pyridine residue, and a singlet of the protons of the
imino groups at δ 8.50. The UV spectrum of 10 shows two
absorption maxima at 312 nm (ε = 15000) and 408 nm
(ε = 3500).
Experimental
Compound 10 is storageꢀstable in the solid state but it
is gradually oxidized in solutions in the presence of atmoꢀ
spheric oxygen, particularly, in weakly acidic media, to
give 2,6ꢀbis(benzoimidazolyl)pyridine (11) described earꢀ
lier.10 The latter is generated in quantitative yield upon
stirring of a solution of compound 10 in CHCl3 in air in
the presence of silica gel or under the action of I2 in
MeOH (Scheme 4).
The UVꢀVis spectra were recorded on a SpecordꢀUV specꢀ
trophotometer in the region of 200—900 nm. The IR spectra (in
KBr pellets or Nujol mulls) were measured on a SpecordꢀM400
spectrophotometer in the region of 400—4000 cm–1. The 1H and
13C NMR spectra were recorded on a Bruker DPXꢀ300 instruꢀ
ment (300 MHz) with Me4Si as the internal standard. The mass
spectra of the positive ions were obtained on Kratos MSꢀ890
(70 eV), Finnigan MAT LCQ (ESI), and Bruker MALDIꢀTOF
Reflex 3 (UV laser, λ = 337 nm) instruments. The starting
dialdehydes, viz., 2,5ꢀdiformylpyrrole13 and 2,6ꢀdiformylꢀ
pyridine,14 and complex 9a 9 were prepared according to known
procedures. The constants of compounds 9a and 11 described
earlier are identical with the published data.9
Scheme 4
3,7,14,18,23,24ꢀHexaazatricyclo[18.2.1.19,12]tetracosaꢀ
2,7,9,11,13,18,20,22ꢀoctaene (5). A solution of 2,5ꢀdiformylꢀ
pyrrole (1) (200 mg, 1.62 mmol) in dry MeCN (10 mL) was
added dropwise to a solution of propaneꢀ1,3ꢀdiamine (4)
(120 mg, 135 µL, 1.62 mmol) in dry MeCN (30 mL) for 10 min
and then the reaction mixture was stirred for 24 h. The solution
was carefully decanted from the colorless finely crystalline preꢀ
cipitate that formed (the compound is very hygroscopic in soluꢀ
tion). The precipitate was dried in vacuo and compound 5 was
recrystallized from a mixture of dry MeCN and CHCl3 (∼1 : 1).
The yield of compound 5 was 170 mg (66%), m.p. 127 °C.
Found (%): C, 66.80; H, 7.02; N, 26.18. C18H22N6. Calcuꢀ
lated (%): C, 67.06; H, 6.88; N, 26.07. MS (EI, 70 eV),
m/z (Irel (%)): 322.4 [M]+ (5.05). 1H NMR (CDCl3), δ: 1.98
(br.s, 4 H); 3.60 (br.s, 8 H); 6.43 (br.s, 4 H); 8.04 (br.s, 4 H).
13C NMR (CDCl3), δ: 32.4, 58.8, 114.3, 132.5, 151.7. IR (KBr),
ν/cm–1: 740, 950, 1020, 1160, 1260, 1440, 1620, 3420.
UV (6.21•10–4 M solution in an EtOH—CH2Cl2 mixture, 1 : 1),
Reagents and conditions: i. 1) I2, MeOH, 20 °C, 12 h; 2) NaOH.
Earlier, condensation of 1,2ꢀdiaminobenzenes with
2,6ꢀdiformylpyridine (2) (in the presence of lanthanide
salts10) and 2,6ꢀdiformylphenols (refluxing in EtOH)1,11
afforded bisꢀbenzoimidazole derivatives as a result of
intramolecular cyclization of Schiff´s bases (analogs of
ligand 10) that formed in the first step.
The reaction of compound 10 with Cu(OAc)2 in a
MeCN—MeOH mixture produced a darkꢀred precipiꢀ
tate. According to the massꢀspectrometric data, this
λ
max/nm (ε): 243 (5600), 304 (18500), 322 (22200), 334 (4800).
3,7,15,19,25,26ꢀHexaazatricyclo[19.3.1.19,13]hexacosaꢀ
2,7,9,11,13(26),14,19,21(25),22,24ꢀdecaene (6). A solution of
2,6ꢀdiformylpyridine (2) (100 mg, 0.74 mmol) in dry MeCN
(10 mL) was added dropwise to a solution of diamine 4 (70 µL,
0.74 mmol) in dry MeCN (30 mL) for 2 h. The reaction mixture
was stirred for 48 h. The white finely crystalline precipitate that
formed was filtered off and twice recrystallized from a mixture
of anhydrous MeCN and CHCl3 (1 : 1). Ligand 6 was obtained

338
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 2, February, 2004
Katayev et al.
in a yield of 80 mg (62%), m.p. 165 °C. Found (%): C, 69.25;
H, 6.55; N, 24.10. C20H22N6. Calculated (%): C, 69.34; H, 6.40;
N, 24.26. MS (EI, 70 eV), m/z (Irel (%)): 346.4 [M]+ (15.38).
1H NMR (CDCl3), δ: 2.15 (q, 4 H, J = 6.6 Hz); 3.80 (t, 8 H, J =
6.6 Hz); 7.78 (t, 2 H, J = 7.8 Hz); 8.12 (d, 4 H, J = 7.5 Hz); 8.44
(s, 4 H). IR (Nujol mulls), ν/cm–1: 820, 1340, 1380, 1460, 1590,
1650. UV (5.78•10–4 M solution in an EtOH—CH2Cl2 mixture,
1 : 1), λmax/nm (ε): 243 (8800), 277 (5450).
Complex 7. A cooled (–10 °C) solution of Cu(OAc)2•H2O
(248 mg, 1.24 mmol) in MeOH (20 mL) was added to a solution
of ligand 5 (200 mg, 0.62 mmol) in CHCl3 (30 mL) at –10 °C.
The reaction mixture was stirred at ∼20 °C for 24 h. The solvent
was removed on a rotary evaporator to obtain a green powder,
which was placed on a filter and washed one time with warm
CHCl3. The residue was dried in vacuo and recrystallized from a
MeOH—Et2O mixture to prepare complex 7•Cu(OAc)2 in a
yield of 316 mg (68%). Found (%): C, 41.21; H, 4.15; N, 12.05;
Cu, 25.33. C26H32Cu3N6O8. Calculated (%): C, 41.79; H, 4.32;
N, 11.25; Cu, 25.51. Upon heating to 350 °C, the compound
gradually darkened and decomposed. MS (MALDIꢀTOF, m/z):
445.7 [(5 – 2 H)Cu2]+. UV (2.67•10–4 M solution in an
EtOH—CH2Cl2 mixture, 1 : 1), λmax/nm (ε): 243 (18000), 266
(15000), 364 (14750). IR (Nujol mulls), ν/cm–1: 1590, 1460,
1390, 1050, 810.
Complex 8. A cooled (–10 °C) solution of Ni(OAc)2•4H2O
(307 mg, 1.24 mmol) in MeOH (20 mL) was added to a solution
of ligand 5 (200 mg, 0.62 mmol) in CHCl3 (30 mL) at –10 °C.
The reaction mixture was stirred at ∼20 °C for one day. The
solvent was removed on a rotary evaporator. The olivineꢀcolꢀ
ored powder that formed was placed on a filter and washed one
time with warm CHCl3. The residue was dried in vacuo and
recrystallized from a MeOH—Et2O mixture. Compound 8 was
obtained in a yield of 237 mg (69%). Upon heating to 350 °C,
the compound gradually darkened and decomposed. Found (%):
C, 47.31; H, 4.96; N, 15.33. C22H26N6Ni2O4. Calculated (%):
C, 47.54; H, 4.71; N, 15.12. MS (ESI, m/z): 454.0,
[(5 – 2 H)Ni2OH]+; 438.0, [(5 – 2 H)Ni2 – H]+; 398.1
[(5 – H)Ni H2O]+; 379.1, [(5 – H)Ni]+. IR (Nujol mulls),
ν/cm–1: 1630, 1460, 1310, 1180, 1050, 770.
6.6 Hz, 4J = 3.3 Hz); 6.55 (dd, 2 H, H(2), 3J = 7.2 Hz, 4J =
1.2 Hz); 6.85 (dt, 2 H, H(4), 3J = 6.6 Hz, 4J = 3.3 Hz); 7.01 (dd,
2 H, H(5), 3J = 7.2 Hz, 4J = 1.2 Hz); 7.71 (t, 1 H, H(8), J7,8
=
7.8 Hz); 8.12 (d, 2 H, H(7), J8,7 = 7.8 Hz); 8.50 (s, 2 H, H(6)).
13C NMR (DMSOꢀd6), δ: 115.0; 116.2; 117.1; 122.5; 128.7;
133.7; 137.5; 144.7; 154.7; 155.1. IR (Nujol mulls), ν/cm–1
:
740, 810, 950, 1000, 1150, 1260, 1310, 1380, 1460, 1610.
UV (6.349•10–4 M solution in EtOH), λmax/nm (ε): 312 (15000),
408 (3500).
Reaction of compound 10 with iodine. Iodine (1 equiv.,
80.5 mg, 0.317 mmol) was added to a solution of ligand 10
(100 mg, 0.317 mmol) in a mixture of CH2Cl2 (10 mL) and
MeOH (15 mL). The reaction solution was stirred for 5 h and
then NaOH (25.4 mg) was added. The solvent was removed on a
rotary evaporator. The organic layer was extracted with MeOH
and the extract was concentrated. The residue was dried in vacuo.
A paleꢀyellow powder of 2,6ꢀbis(benzoimidazolyl)pyridine 11
was obtained in a yield of 92 mg (94%). 1H NMR (DMSOꢀd6),
δ: 7.32 and 7.76 (both m, 4 H each, 4 CH); 8.15 (t, 1 H, J =
7.8 Hz); 8.35 (d, 2 H, J = 7.8 Hz); 12.98 (s, 2 H).
Reaction of compound 10 with Cu(OAc)2. A solution of
Cu(OAc)2•H2O (190.4 mg, 0.95 mmol) in MeOH (10 mL) was
rapidly added to a cooled (–10 °C) solution of compound 7
(300 mg, 0.95 mmol) in MeCN (10 mL). The reaction mixture
was stirred for 24 h and the solvent was removed to prepare a
darkꢀred powder, which was washed with MeCN and Et2O,
recrystallized by diffusion of Et2O into a solution of the comꢀ
pound in MeOH, and dried in vacuo. Complex 12 was obtained
in a yield of 288 mg (67%). MS (MALDIꢀTOF, m/z): 311.7
[L + H]+, 373.7 [LCu – H]+. UV (4.43•10–4 M solution in
EtOH), λmax/nm (ε): 332 (22600).
Sodium sulfide (100 mg) was added to a solution of comꢀ
pound 12 (200 mg) in MeOH (50 mL). The reaction mixture
was stirred for 30 min and filtered. The solvent was removed on
a rotary evaporator. The residue was washed on a filter with
CHCl3 and dried in air. Compound 11 was prepared in a yield of
75 mg (85%). The 1H NMR spectroscopic data for 12 are identiꢀ
cal to those published in the literature.9
Complex 9a.9 2,6ꢀDiformylpyridine (2) (1 g, 7.4 mmol) and
Cd(ClO4)2•6H2O (3.68 g, 8.78 mmol) were successively disꢀ
solved in anhydrous MeOH (40 mL) and then a solution of
oꢀphenylenediamine (2.43 g, 22.5 mmol) in anhydrous MeOH
(20 mL) was rapidly added. The reaction mixture was refluxed
for 40 min and cooled to –20 °C. The orange crystalline precipiꢀ
tate that formed was filtered off. Complex 9a was prepared in a
yield of 4.78 g (87%). Found (%): C, 41.0; H, 3.62; N, 13.71.
C25H25CdCl2N7O8. Calculated (%): C, 40.86; H, 3.43; N, 13.34.
2,6ꢀBis{[(2ꢀaminophenyl)imino]methyl}pyridine (10). Water
(70 mL), MeOH (10 mL), and crystalline Na2S (5 g) were added
to a suspension of complex 9a (1 g) in CH2Cl2 (70 mL). The
resulting suspension was thoroughly stirred for 5 h. The upper
aqueous layer into which CdS gradually went as a suspension
was decanted and a new portion of water was added. Then Na2S
(5 g) was added and the mixture was stirred for one day. The
organic layer was separated, washed three times with water, and
dried over K2CO3. The solvent was distilled off on a rotary
evaporator. Ligand 10 was obtained in a yield of 420 mg (98%),
m.p. 107 °C. Found (%): C, 72.16; H, 5.56; N, 21.84. C19H17N5.
Calculated (%): C, 72.36; H, 5.43; N, 22.21. 1H NMR
(CD3CN), δ: 4.46 (br.s, 4 H, H(1)); 6.42 (dt, 2 H, H(3), 3J =
This study was financially supported by the Russian
Foundation for Basic Research (Project No. 02ꢀ03ꢀ
32101).
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Received July 18, 2003;
in revised form November 11, 2003