Characteristic features of formation, synthesis, and properties of binuclear zinc(II) helicates with alkyl-substituted 3,3'-bis(dipyrrolylmethenes)

Article abstract of DOI:10.1134/S0036023611120242

On the basis of spectrophotometric studies, it was demonstrated that complexation of hydrobromides of seven alkylated 3,3'-bis(dipyrrolylmethenes) (H₂L · 2HBr) with zinc(II) acetate dihydrate in DMF (298.15 K) includes successive steps of formation of binuclear helicates of two types: [Zn₂(AcO)₂L] (heteroleptic single-helix complex) and [Zn₂L₂] (homoleptic double-helix complex). An increase in the degree of alkylation of the pyrrole rings of the ligands from 4 to 10 methyl groups induces an increase of overall complexation constants (logK°) from 7.60 to 13.73. The synthesis of helicates [Zn₂L₂] was described. The products were studied by elemental analysis and IR, ¹H NMR, U/Vis, and fluorescence spectroscopy. The effect of the helicand structure and the solvent nature on the chromophore and fluorescence properties of [Zn₂L₂] was discussed. The kinetic stability of zinc(II) with decamethyl-substituted 3,3'-bis(dipyrrolylmethene) in HCl-DMF medium was estimated. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0036023611120242

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 2, pp. 261–269. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © L.A. Antina, N.A. Dudina, G.B. Guseva, E.V. Antina, M.B. Berezin, A.I. V’yugin, 2012, published in Zhurnal Neorganicheskoi Khimii, 2012, Vol. 57, No.
2, pp. 306–314.
PHYSICAL METHODS
OF INVESTIGATION
Characteristic Features of Formation, Synthesis, and Properties
of Binuclear Zinc(II) Helicates with AlkylꢀSubstituted
3,3'ꢀBis(dipyrrolylmethenes)
L. A. Antina, N. A. Dudina, G. B. Guseva, E. V. Antina, M. B. Berezin, and A. I. V’yugin
Institute of Solution Chemistry, Russian Academy of Sciences, Akademicheskaya 1, Ivanovo, 153045 Russia
eꢀmail: eva@iscꢀras.ru
Received June 15, 2010
Abstract—On the basis of spectrophotometric studies, it was demonstrated that complexation of hydrobroꢀ
mides of seven alkylated 3,3'ꢀbis(dipyrrolylmethenes) (H2L ⋅ 2HBr) with zinc(II) acetate dihydrate in DMF
(298.15 K) includes successive steps of formation of binuclear helicates of two types: [Zn2(AcO)2L] (heteroꢀ
leptic singleꢀhelix complex) and [Zn2L2] (homoleptic doubleꢀhelix complex). An increase in the degree of
alkylation of the pyrrole rings of the ligands from 4 to 10 methyl groups induces an increase of overall comꢀ
plexation constants
ucts were studied by elemental analysis and IR, 1H NMR, U/Vis, and fluorescence spectroscopy. The effect
of the helicand structure and the solvent nature on the chromophore and fluorescence properties of [Zn2L2
(logK°) from 7.60 to 13.73. The synthesis of helicates [Zn2L2] was described. The prodꢀ
]
was discussed. The kinetic stability of zinc(II) with decamethylꢀsubstituted 3,3’ꢀbis(dipyrrolylmethene) in
HCl–DMF medium was estimated.
DOI: 10.1134/S0036023611120242
The first cobalt(II), nickel(II), copper(II), and in the [М2L2] molecules imparts clearꢀcut chroꢀ
zinc(II) coordination compounds with 3,3'ꢀbis(dipyrꢀ
rolylmethenes) were obtained quite recently [1–3] but
have already attracted particular attention of researchꢀ
ers by their practically valuable properties. The binuꢀ
clear homoleptic doubleꢀhelix 3,3'ꢀbis(dipyrrolylꢀ
methene) helicates [М2L2] are formed via covalent
and donor–acceptor bonds and do not contain a
counterꢀion; this accounts for their high stability and
facilitates the synthesis and chromatographic purificaꢀ
mophore properties to the compounds. In addition,
the [Zn2L2] complexes show intense fluorescence,
which makes them especially promising for practiꢀ
cal use.
The purpose of this work is the synthesis and study
of optical properties and stability of the [Zn2L2] comꢀ
plexes with a number of 3,3'ꢀbis(dipyrrolylmethenes)
I–VII in which the number of symmetrically arranged
tion. The presence of four dipyrrolylmethene domains alkyl substituents decreases from 10 to 4:
R4
R5
R7
R3
R8
R11
H
R9
R2
N
HN
N
HN
R6
R1
R10
I: R1 = R2 = R3 = R4 = R5 = R6 = R7 = R8 = R9 = R10 = Me; R11 = H;
II: R1 = R3 = R4 = R5 = R6 = R7 = R8 = R10 = Me; R2 = R9 = Et, R11 = H;
III: R1 = R3 = R4 = R5 = R6 = R7 = R8 = R10 = Me; R2 = R9 = Et, R11 = Ph;
IV: R1 = R2 = R3 = R5 = R6 = R8 = R9 = R10 = Me; R4 = R7 = Et, R11 = H;
V: R1 = R3 = R4 = R5 = R6 = R7 = R8 = R10 = Me; R2 = R9 = R11 = H;
VI: R2 = R3 = R4 = R5 = R6 = R7 = R8 = R9 = Me; R1 = R10 = R11 = H;
VII: R4 = R5 = R6 = R7 = Me; R1 = R2 = R3 = R8 = R9 = R10 = R11 = H.
261

262
ANTINA et al.
A spectrophotometric study of the reactions of 1064.09). Brownꢀred crystals with green metallic lusꢀ
zinc(II) acetate with ligands
and a kinetic study of the acid dissociation of [Zn2(I)2
I–VII in DMF at 298.15 K ter (yield of the first fraction 84.7%).
1H NMR (CDCl3, internal TMS,
δ, ppm): 6.90 (s,
]
in the binary HCl–DMF solvent were carried out for
comparative estimation of the effect of structural feaꢀ
tures of the 3,3'ꢀbis(dipyrrolylmethene) ligand moleꢀ
cules on the stability of their helicates and for elucidaꢀ
tion of the analytical potential of this class of helicands
as chromophore sensors for Zn2+ ions.
4H, –CHmeso–), 3.43 (s, 4H, –CH2–, spacer), 2.36
(q, 8H, –CH2–CH3), 2.20 (s, 12H, CH3), 2.19 (s,
12H, CH3), 2.01 (s, 12H, CH3), 1.39 (s, 12H, CH3),
1.01 (t, 12H, –CH2–CH3).
IR (ν
, cm–1): 2959, 2913, 2853, 1594, 1547, 1439,
1392, 1227, 1167, 1109, 1049, 969, 851, 783, 731, 661.
For C62H76N8Zn2 anal. calcd. (%): C, 69.98; H,
7.20; N, 10.53.
EXPERIMENTAL
Found (%): C, 69.87; H, 7.17; N, 10.48.
Zn(II) complex with bis(1,3,7,9ꢀtetramethylꢀ2ꢀ
ethyldipyrrolylmethenꢀ8ꢀyl)phenylmethane, [Zn2(III)2]
The synthesis, the elemental analysis data, and the
1H NMR, IR, and UV/Vis spectra of 3,3'ꢀbis(dipyrroꢀ
lylmethene) dihydrobromides
were described in our previous works [4, 5].
Zn(II) complex with bis(1,2,3,7,9ꢀpentamethylꢀ
dipyrrolylmethenꢀ8ꢀyl)methane, [Zn2( )2]
1007.98). 2HBr (0.18 g, 0.298 mmol) was dissolved
I–VII (H2L·2HBr)
(
M
= 1216.28). Brownꢀred crystals with a green metalꢀ
lic luster (yield of the 1st fraction 85.8%).
1H NMR (CDCl3, internal TMS,
, ppm): 7.06–
7.23 (m, 6H, H–Ph), 6.94–6.96 (d, 4H, H–Ph), 6.80
(d, 4H, –CHmeso–), 5.28 (d, 2H, CHPh–, spacer),
δ
I
(
M
=
I
⋅
⎯
in chloroform (30 mL). Triethylamine (0.12 g, 1.16
2.39 (q, 8H, –СН2–CH3), 2.21–2.22 (d, 12H, CH3),
2.16–2.17 (d, 12H, CH3), 2.00–2.02 (d, 12H, CH3),
1.50–1.51 (d, 12H, CH3), 1.05 (t, 12H, –CH2–CH3).
mmol) was added with stirring, and 3 min later, a soluꢀ
tion of Zn(AcО)2 ⋅ 2Н2О (0.32 g, 1.46 mmol) in methꢀ
anol (20 mL) was added. The reaction mixture was
stirred at room temperature for 1 h, then washed three
times with water in a separating funnel. The chloroꢀ
form layer was separated and the solution was concenꢀ
trated. The precipitate was collected on a filter, washed
with methanol, dried, dissolved in 5 mL of benzene,
and chromatographed on silica gel (40/100). Benzene
was used as the eluent (to elute the first fraction as a
bright crimsonꢀcolored fluorescence zone); and a 10 : 1
benzene–methanol mixture was used to elute the secꢀ
ond fraction.
IR (ν
, cm–1): 3032, 2959, 2913, 2853, 1593, 1535,
1500, 1443, 1391, 1224, 1167, 1108, 1086, 1024, 966,
858, 781, 729, 698, 667.
For C74H84N8Zn2 anal. calcd. (%): C, 73.08; H,
6.96; N, 9.21.
Found (%): C, 73.01; H, 6.80; N, 9.14.
Zn(II) complex with bis(1,2,3,9ꢀtetramethylꢀ7ꢀ
ethyldipyrrolylmethenꢀ8ꢀyl)methane, [Zn2(IV)2] (
M =
1064.09). Redꢀwineꢀcolored crystals with a green
metallic luster (yield of the 1st fraction 95%).
1H NMR (CDCl3, internal TMS,
δ, ppm): 6.89 (s,
The yield of the first fraction was 0.225 g (74.8%).
Dark wineꢀcolored crystals with green metallic luster
were obtained.
4H, –CHmeso–), 3.49 (s, 4H, –CH2–, spacer), 2.65
(q, 8H, –CH2ꢀCH3), 2.20 (s, 12H, CH3), 1.97 (s,
12H, CH3), 1.91 (s, 12H, CH3), 1.43 (s, 12H, CH3),
1.21 (t, 12H,–CH2–CH3).
abs
λmax
UV/Vis (
nm; logε ): C6H6, 530, 5.48; 478;
,
IR (ν
, cm–1): 2960, 2922, 2856, 1593, 1539, 1437,
369 (CTB); DMF, 525, 5.34; 476; 367 (CTB).
1Н NMR (CDCl3, internal TMS,
δ, ppm): 6.89 (s,
1391, 1225, 1161,1109, 1049, 945, 845, 785, 735, 669.
For C62H76N8Zn2 anal. calcd., %: C, 69.98; H,
7.20; N, 10.53.
4H, –CHmeso–), 3.41 (s, 4H, –CH2ꢀ, spacer), 2.22 (s,
12H, CH3), 2.19 (s, 12H, CH3), 2.00 (s, 12H, CH3),
1.92 (s, 12H, CH3), 1.40 (s, 12H, CH3).
IR (ν
, cm–1): 2977, 2919, 2858, 1596, 1539, 1437,
1393, 1229, 1164, 1107, 1047, 947, 848, 786, 739, 672. rolylmethenꢀ8ꢀyl)methane, [Zn2(
V)2] (M = 951.87).
Found (%): C, 69.90; H, 7.12; N, 10.45.
Zn(II) complex with bis(1,3,7,9ꢀtetramethyldipyrꢀ
Redꢀorange crystals with a green metallic luster (yield
of the 1st fraction 97.7%).
For C58H68N8Zn2 anal. calcd. (%): C, 69.11; H,
6.80; N, 11.12.
1H NMR (CDCl3, internal TMS,
δ, ppm): 6.93 (s,
Found (%): C, 69.01; H, 6.75; N, 11.07.
The yield of the second fraction was 0.06 g (20.3%).
4 H, –CHmeso–), 5.98 (s, 4 H, Hβ), 3.43 (s, 4H, –
CH2–, spacer), 2.31 (s, 12H, CH3), 2.23 (s, 12H,
CH3), 2.05 (s, 12H, CH3), 1.42 (s, 12H, CH3).
Crimsonꢀcolored nonfluorescent crystals were
abs
λmax
IR (ν
, cm–1): 2966, 2915, 2861, 1596, 1522, 1442,
obtained. UV/Vis (
(CTB).
nm): C6H6, 532; 481; 367
,
1392, 1226, 1153, 1107, 1034, 970, 849, 785, 775, 698.
For C54H60N8Zn2 anal. calcd., %: C, 68.14; H,
The complexes [Zn2(II)2]–[Zn2(VII)2] were preꢀ
pared similarly to complex [Zn2(I)2]. The compounds 6.35; N, 11.77.
isolated as the first fractions were used in the study.
Found (%): C, 68.04; H, 6.27; N, 11.72.
Zn(II) complex with bis(1,3,7,9ꢀtetraamethylꢀ2ꢀ
Zn(II) complex with bis(2,3,7,9ꢀtetramethyldipyrꢀ
rolylmethenꢀ8ꢀyl)methane, [Zn2(VI)2] (M = 951.87).
ethyldipyrrolylmethenꢀ8ꢀyl)methane, [Zn2(II)2] (
M
=
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012

CHARACTERISTIC FEATURES OF FORMATION
263
Orangeꢀred crystals with a green metallic luster (yield 1ꢀPropanol (1ꢀPrOH) (UVꢀIRꢀHPLCꢀHPLC preꢀ
of the 1st fraction 60.1%).
parative) PAI and cyclohexane (Panreac, Barcelona)
were used as received. Zinc(II) dihydrate acetate was
recrystallized from glacial acetic acid and dehydrated.
The content of water of crystallization was confirmed
by the thermogravimetric method.
1H NMR (CDCl3, internal TMS,
δ, ppm): 7.11 (s,
4H, H
CH2–, spacer), 2.22 (s, 24H, CH3), 1.99 (s, 12H,
CH3), 1.41 (s, 12H, CH3).
IR (
, cm–1): 2960, 2920, 2860, 1595, 1529, 1443,
α), 6.98 (s, 4H, –CHmeso–), 3.40 (s, 4H,
⎯
The kinetic stability study of [Zn2(
I)2] was carried
ν
1383, 1186, 1151, 1097, 1036, 949, 858, 785, 731, 669.
For C54H60N8Zn2 anal. calcd., %: C, 68.14; H,
6.35; N, 11.77.
out in the HCl–DMF binary solvent at a constant
0
starting concentration of the complex
of ~ 6
]
2
×
c[Zn L
2
10–6 mol/L and concentration ratio
=
cHCl c[Zn L
]
2
2
Found (%): C, 68.11; H, 6.22; N, 11.70.
Zn(II) complex with bis(7,9ꢀdimethyldipyrrolylꢀ
18.9–30.0. The equations for calculation of the kinetic
parameters of dissociation of the complex (kobs, Е, )
S≠
methenꢀ8ꢀyl)methane, [Zn2(VII)2] (
M = 839.65).
were reported previously [8].
The equilibrium concentrations of the reactants,
the stoichiometry, and the thermodynamic constants
Orange crystals with a green metallic luster (yield of
the 1st fraction 98.3%).
1H NMR (CDCl3, internal TMS,
δ, ppm): 7.33 (s,
for the reactions between hydrobromides I ⋅ 2HBr–VII ⋅
4H, H), 7.06 (s, 4H, H), 6.99 (s, 4H, –CHmeso–), 6.41
(s, 4H, H), 3.41 (s, 4H, –CH2–, spacer), 2.22 (s, 12H,
CH3), 1.43 (s, 12H, CH3).
2HBr with zinc(II) acetate were determined using the
Molar Ratios method. The studies were carried out at
a constant ligand concentration (from
4
×
10–6 to 1.2
molar ratio
.
×
IR (ν
, cm–1): 2958, 2918, 2856, 1597, 1500, 1437,
10–5 mol/L) and variable
1385, 1178, 1155, 1092, 1036, 949, 852, 787, 725, 669.
For C46H44N8Zn2 anal. calcd., %: C, 65.80; H,
5.28; N, 13.35.
cZn AcO cH L
(
)
2
2
(from 0 to 8). The standard К° values were determined
as described previously [9, 10] by extrapolating the
concentration constants obtained with excess of metal
salt in the reaction mixture to infinite dilution [11]. On
five repetitions of the spectrophotometric measureꢀ
ments for each system, the error of determination of
the standard thermodynamic constants (К°) from the
spectrophotometric data did not exceed 5%.
Found (%): C, 65.69; H, 5.22; N, 13.29.
The 1H NMR spectra were recorded in deuterated
chloroform using Bruker 200 and Bruker 500 NMR
spectrometers. The IR spectra of [Zn2L2] as KBr pelꢀ
lets were measured on an Avatar 360 FTꢀIR ESP
instrument. The UV/Vis absorption and transmission
spectra in organic solvents were measured on an SFꢀ103
spectrophotometer (Akvilon, Russia) and an SM 2203
spectrofluorimeter (SOLAR). The spectral luminesꢀ
cence characteristics and fluorescence quantum yields
were used to estimate the radiation constants and fluꢀ
orescence lifetimes [6].
RESULTS AND DISCUSSION
The reactions of zinc(II) acetate dihydrate with
any of the hydrobromides of 3,3'ꢀbis(dipyrrolylꢀ
methenes)
I ⋅ 2HBr–VII ⋅ 2HBr in a chloroform–
Reagent grade solvents were additionally purified methanol mixture in the presence of triethylamine
by known procedures [7]. The content of water in the results in the formation of the complexes [Zn2L2] in
solvents did not exceed 0.02% (Fischer titration data). rather high yields (60 to 98%):
R4
R4
R3
R3
R11
R4
R5
R7
R3
R2
R2
R8
R11
H
N
N
N
N
N
N
N
N
R5
R6
H
R5
R1
R10
R1
R10
Zn(OAc)
⋅ 2H O
2 2
R9
R2
Zn
Zn
R6
CHCl /MeOH
3
NH HN
NH
HN
H
R6
R1
R9
R9
R10
Br–
R11
Br–
R7
R7
R8
R8
[Zn2L2]
I ⋅ 2HBr–VII ⋅ 2HBr
Note that variations of the temperature from this suggests their high thermodynamic and kinetic
stabilities.
The synthesized complexes [Zn2L2] produce
intense fluorescence in the solid state. As the degree of
298.15 to 330 K, the reaction time from 30 min to 3 h,
or the order of addition of the compounds to the reacꢀ
tion mixture do not affect the nature of the product: in
all cases, [Zn2L2] complexes are formed in high yields; helicand methylation decreases, the color of the crysꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012

264
ANTINA et al.
1b), which correspond to three electron transitions
S0 S1 S0 S2 and S0 S3) [12].
(а)
A
(
–
,
–
,
–
2
1
1.0
As the degree of helicand alkylation decreases, the
maxima of the characteristic UV/Vis bands of [Zn2L2
in the studied solvents undergo hypsochromic shifts.
]
1
The shortꢀwavelength shift
is on average 21–24 nm.
λmax
A similar trend was observed previously for the proper
ligands and their hydrobromides [5]. Additionally, the
decrease in the induction effect of substituents followꢀ
ing a decrease in their number improves the polarizaꢀ
0.5
tion conditions of the chromophore πꢀsystems of the
ligands within the complexes [Zn2L2], as indicated by
the increase in the quantitative characteristics of the
auxochromic effect of the Zn(II) ion on the chroꢀ
mophore systems of the ligands calculated as the difꢀ
ference between the numerical values of the waveꢀ
lengths of the first strong band in the spectra of the
complex and the corresponding ligand Δλ =
400
500
λ
, nm
(b)
A
1
1
4
taking into account the published data
λ[Zn L − λH L
]
2 2
2
3
on the UV/Vis spectra of the molecular forms of the
ligands in DMF (Table 1). Whereas for the [Zn2( )2] –
[Zn2( 2] complexes, the Δλ varies from 57 to 65 nm,
for the [Zn2(VI 2] and [Zn2(VII 2] complexes formed
by helicands with partly (at ꢀpositions) or fully
0.8
I
V)
)
)
α
unsubstituted terminal pyrroles, the Δλ value increases
to 70 and 82 nm, respectively.
0.4
The effect of the solvent nature is manifested as a
minor (up to 6 nm) hypsochromic shift of the characꢀ
teristic bands in the UV/Vis spectra of solutions of
[Zn2L2] in polar solvents (1ꢀPrOH, EtOH, СHCl3,
and DMF) with respect to those in nonꢀpolar or lowꢀ
polar solvents (hexane, cyclohexane, heptane, benꢀ
zene, toluene, оꢀxylene). Also, in nonꢀpolar solvents,
400
500
λ
, nm
the molar extinction coefficient (
than in polar media.
ε) is markedly higher
Fig. 1. UV/Vis spectra of zinc(II) complexes: (a) (1)
–5
[Zn (VII) ] (
с
= 5.27
× 10 mol/L, (2) [Zn (III) ] (с =
2 2
2
2
–5
The [Zn2L2] complexes exhibit intense fluoresꢀ
cence in nonꢀpolar and lowꢀpolar saturated and aroꢀ
matic hydrocarbons (heptane, hexane, cyclohexane,
3.45
×
10 mol/L) in С Н ; (b) (
3
,
4
)
[Zn (VI) ] in
6
6
2
2
–5
DMF (
–5
с
= 2.94
×
10 mol/L) and in С Н (
с
= 2.73 ×
6
6
10 mol/L), respectively. The cell thickness is
l
= 0.097 cm.
benzene, toluene, оꢀxylene). The emission spectrum is
a mirror reflection of the absorption spectrum (Fig. 2).
In nonꢀpolar solvents, the efficiency of fluorescence
talline samples of the complexes changes from dark
wine to orange. The compounds are insoluble in water
and sparingly (up to ~10–4–10–3 mol/L) soluble in
organic solvents. The quantitative characteristics of
[
Zn2L2] is up to 500 times as high as that in polar solꢀ
vents (chloroform, DMF, DMSO, acetonitrile, pyriꢀ
dine, acetone, ethanol, 1ꢀpropanol). For all comꢀ
pounds, the highest fluorescence quantum yield
the UV/Vis spectra of [Zn2(II)2] and [Zn2(VII)2] in
(nearly 1.0) on visible light excitation ( ex = 480–495 nm)
λ
organic solvents (Table 1) are consistent with the data
published for the same compounds [2, 3] together with
Xꢀray diffraction data that confirm the binuclear douꢀ
bleꢀhelix molecular structures of helicates.
was observed in cyclohexane; in the series cyclohexꢀ
ane, heptane, hexane, benzene, toluene, the Φfl value
decreases from ~1.0 to ~0.6. In the general case, in
nonꢀpolar aromatic solvents (benzene, toluene,
The differences in the alkylation of ligands I–VII
оꢀxylene), Φfl is lower (~0.6–0.7) than in saturated
and in the solvent nature do not affect the spectral patꢀ
terns of [Zn2L2] (Fig. 1, Table 1). The UV/Vis spectra
of the complexes in organic solvents show one strong
band with a maximum in the 503–531 nm range, a
hydrocarbons (hexane, heptane, cyclohexane) where
Φfl reaches ~0.8–1.0. In polar solvents (DMF, DMSO,
pyridine, acetone, ethanol, 1ꢀpropanol), the Φfl value
weaker band at 456–482 nm, and a weak broadened varies from 0.01 to 0.003 and in acetonitrile, it
charge transfer band in the 348–380 nm range (Figs. 1a, approaches zero. Note that the fluorescence intensity
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012

CHARACTERISTIC FEATURES OF FORMATION
265
Ifl
of all compounds in lowꢀpolar chloroform is ~0.03–
0.08, i.e., it is 13–30 times lower than in cyclohexane
but 10–40 times higher than in other polar solvents.
The presence of the phenyl fragment in the central
spacer of the helicand causes a noticeable decrease in
1
2
A
the fluorescence quantum yield for [Zn2(III)2] as comꢀ
pared with other complexes. According to published
data [13, 14], this may result from nonꢀradiative tranꢀ
sitions caused by phenyl group rotation.
The complexes [Zn2(
lated helicands show a greater Stokes shift (14–16 nm)
than [Zn2( )2]–[Zn2(VII 2] formed by partly alkylated
ligands (11–13 nm). The greatest Stokes shift was
found for [Zn2(III 2]: it is 16–17 nm in nonꢀpolar solꢀ
I)2]–[Zn2(IV)2] with fully alkyꢀ
V
)
)
vents and reaches 24 nm in alcohols. For other six
complexes, an increase in the Stokes shift on going
from nonꢀpolar to polar solvents was also observed. As
the solvent polarity decreases, the fluorescence lifeꢀ
400
500
600 λ, nm
time increases from ~6 ×
10–3 to 3 ns.
Fig. 2.
(1
) Absorption and (
2) transmission UV/Vis spectra
of [Zn (I) ] in C H .
The pronounced differences between the spectroꢀ
scopic characteristics of the complexes in solvents of
different polarity and nature attests to a specific solvaꢀ
tion, which is promising for the development of optiꢀ
cal devices and fluorescence sensors for polarity of the
medium based on [Zn2L2].
The [Zn2L2] complexes are stable in nonꢀpolar and
polar electronꢀdonor and weakly protonꢀdonor solꢀ
vents (hexane, cyclohexane, heptane, benzene, toluꢀ
2
2
6 6
500
1
A
525
1.4
2
ene, оꢀxylene, DMF, EtOH, 1ꢀPrOH, and СHCl3).
Spectrophotometric monitoring confirmed the
absence of dissociation or photodestruction products
of the complexes in these solvents during longꢀterm
storage (more than 6 months) both in the dark and in
the light.
0.8
The kinetic investigation of the stability of doubleꢀ
helix bis(dipyrrolylmethene) helicates in a medium
with clearꢀcut protonꢀdonor properties was performed
400
500
λ, nm
for the [Zn2(I)2] complex. The dissociation kinetics of
the complex was measured by spectrophotometry in
the HCl–DMF binary solvent. In the molar ratio
Fig. 3. Spectral changes during the dissociation of
Zn (I) ] in the HCl–DMF binary solvent (298.15 K): (1)
[
2
2
2+
at the band of the protonated ligand [H L] and (
band of the complex [Zn (I) ].
2) at the
4
range of (
] ) = 18.9–30.0 (at a constant
cHCl c[Zn L
2
2
2
2
0
starting concentration
] of ~ 6
2
)
×
10–6 mol/L), the
2] in DMF was accompaꢀ
c[Zn L
2
acid dissociation of [Zn2(
I
protonꢀdonor media are available. According to the
existing published data [15], the dissociation of the
mononuclear homoleptic dipyrrolylmethene complex
nied by complete transformation of the spectrum of
the complex to the spectrum of the protonated ligand
with retention of one family of isosbestic points (Fig. 3),
which is indicative of the dissociation of the complex
to give the protonated ligand as the dihydrochloride by
the following overall reaction:
[
ZnL2] (HL is 1,3,5,7ꢀtetramethylꢀ2,6ꢀdibutyldipyrꢀ
rolylmethene) in the AcOH–C6H6 binary solvent at an
acetic acid content of ~(0.3–0.9) mol/L proceeds at a
comparable rate and has an almost twice higher actiꢀ
vation energy (Ea). This is indirect evidence for a
(1)
The observed rate constants (kobs) at different temꢀ
peratures and ratios and other kinetic
Zn L + 8HCl → 2ZnCl + 2H L ⋅ 2HCl.
[
]
2
2
2
2
somewhat lower stability of bis(dipyrrolylmethene)
complexes compared with dipyrrolylmethene comꢀ
plexes.
cHCl c[Zn L
]
2
2
parameters of the dissociation are summarized in
Table 2.
The relatively low activation energies and the negꢀ
ative entropies S≠ demonstrate that the rateꢀlimiting
Currently, no other data on the kinetic stability of step is protonation of the nitrogen atoms of the coorꢀ
bis(dipyrrolylmethene) coordination compounds in dinated bis(dipyrrolylmethene) anions, resulting in
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012

266
ANTINA et al.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012

CHARACTERISTIC FEATURES OF FORMATION
267
Table 2. Kinetic parameters of dissociation of the complex [Zn2(
kobs
104 0.03–0.07), s–1
I)2] in the binary solvent HCl–DMF at 303.15–313.15 K
–Δ
S≠
( 10–12),
J/(mol K)
c0HCl
×
104,
×
(
E
a( 0.9–1.1),
kJ/mol
mol/L
298.15 K
303.15 K
308.15 K
313.15 K
1.14
1.31
1.52
1.80
3.29
4.02
5.01
6.08
4.06
5.05
6.01
7.24
5.01
6.03
7.40
9.57
5.90
7.24
31.1
30.7
30.0
31.4
138
139
142
137
8.90
11.03
fast destruction of [Zn2L2] in the next stage. The
ligand molecule H2L is not detected by spectroscopy
(Fig. 3); hence, the protonation of the ligand is also
very fast.
The effect of the structural features of the molecule
on the thermodynamic stability of the complexes was
estimated by comparing the thermodynamic constants
increases up to
to
molar ratio of < 1 : 1.
cH L
2
cZn AcO
:
(
)
2
Further increase in the salt concentration is accompaꢀ
nied by the appearance and growth of the longꢀwaveꢀ
length band for [Zn2L2]. The type and the quantitative
characteristics of the spectra of solutions with
2
: 1 correspond to the spectra of the
cZn AcO : cH L ӷ
(
)
2
2
synthesized [Zn2L2] complexes in DMF with a slight
shift of the maxima (caused by the presence of excess
salt and acetic acid formed in the reaction).
for the reactions of ligands I–VII with zinc(II) acetate
under similar conditions of the medium (DMF), temꢀ
perature (298.15 K), and reactant concentrations. The
choice of DMF as the medium is due to the presence
of data on the states of the reactants, i.e., hydrobroꢀ
(а)
mides
17], in this solvent. Previously, it was shown that in
dilute DMF solutions, salts H2L 2HBr undergo irreꢀ
I ⋅ HBr–VII ⋅ HBr [4, 5] and zinc acetate [16,
[Zn2L2]
A
⋅
1.6
versible solvolytic dissociation to give the ligand
molecular form [4, 5], which is much more reactive
toward complexation than the protonated form [18].
Zinc acetate exists in DMF as the solvated complex
[Zn(AcO)(DMF)5]+AcO–, which is dissociated at the
outer coordination sphere as a strong electrolyte [16,
17]; hence, the more reactive solvated cation
[Zn2L(AcO)2]
H2L
0.8
[Zn(AcO)(DMF)5]
+ is involved in the complexation.
The typical spectral pattern (Fig. 4a) indicates that
the complexation of Zn(AcO)2 with H2L in a DMF
solution is accompanied by global changes in the
UV/Vis spectra. As the salt concentration increases to
400
500
λ
, nm
Δ
A
(b)
molar ratios of > 2 : 1, the singleꢀband
cZn AcO : c
H2L
(
)
2
1
H2L
spectrum of the molecular ligand with
at 415–
λ
1.6
λmax = 525 nm
462 nm is transformed to the spectrum of the complex
with a considerable red shift (~57–82 nm) and growth
of the intensity of the longꢀwavelength absorption
band due to the auxochromic effect of the metal ion on
the conjugated πꢀsystems of the dipyrrolylmethene
fragments of the chromophore.
0.8
0
The molar ratios of the reactant concentrations in
the kink points of the molar ratio curves at
cZn AcO
:
)
2
(
= 1 : 1; 2 : 1 (Fig. 4b) and the presence of two famꢀ
cH L
2
1
2
3
ilies of isosbestic points in the spectral patterns indiꢀ
cate that, apart from the ligand, the solutions contain
two more lightꢀabsorbing species, namely, binuclear
complexes [Zn2L2] and [Zn2L(AcO)2]. The characterꢀ
istic bands of [Zn2L(AcO)2] appear after the addition
of the first portions of the salt, and their intensity
cZn(AcO) /cH2L
2
Fig. 4. (a) UV/Vis spectra and (b) curve of molar ratios
(constructed at the growing band of [Zn L ]) for the sysꢀ
2
2
tem II–Zn(AcO) –DMF (298.15 K).
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012

268
1/2Σ δ(NH, N+H), ppm
ANTINA et al.
cies of salts I ⋅ 2HBr to VII ⋅ 2HBr [5], can be used for
qualitative prediction of the stability of the coordinaꢀ
tion compounds of new structurally related bis(dipyrꢀ
ν
, cm–1
14.0
13.8
13.5
13.4
VII
VI
V
IV III II
I
1
rolylmethenes) using the H NMR and IR data for
their salts with mineral acids.
3480
The high stability of the complexes and considerꢀ
able differences between the quantitative characterisꢀ
tics of the UV/Vis spectra of the complexes and
ligands are the good basis for the use of 3,3'ꢀbis(dipyrꢀ
rolylmethenes) as chromophoric analytical sensors for
the trace amounts of Zn(II) ions in organic media.
The conditional sensitivity of determination of Zn(II)
ions calculated theoretically from the molar extinction
coefficients of [Zn2L2] at the intense longꢀwavelength
band for a solution absorbance of 0.001 and cell thickꢀ
3460
3440
13.2
13.0
ness of 1 cm is (3.0–5.0) ×
10–9 mol/L.
3420
14
logK°
7
8
9
10
11
12
13
Thus, the successful combination of the practically
useful optical (chromophore and fluorescence) propꢀ
erties and rather high stability provides the obvious
prospects for the use of synthesized zinc(II) helicates
as fluorescence labels and medium polarity sensors
[19, 20] and the use of parent 3,3'ꢀbis(dipyrrolylꢀ
methenes) as chromophore sensors for zinc(II) salts in
organic media.
Fig. 5. Correlations between logK° for the complexation
1
in the H L–Zn(AcO) –DMF systems and the H NMR
2
2
+
chemical shifts of NH groups
δ
(NH) and
δ
(N H) and the
· 2HBr to VII ·
ν
IR stretching frequencies of salts
I
NH
2HBr.
ACKNOWLEDGMENTS
The obtained results attest to the occurrence of
successive reactions giving two types of binuclear comꢀ
plexes: [Zn2L(АсО)2] heteroleptic singleꢀhelix comꢀ
plex and [Zn2L2] homoleptic complex with doubleꢀ
helix architecture:
This work was supported by the Basic Research
Program no. 7 of the Presidium of the Russian Acadꢀ
emy of Sciences, the Analytical Departmental Target
Program “Development of the Higher School Scienꢀ
tific Potential” (2009–2011) (project no. 2.1.1/827),
and the Federal Target Program “Scientific and Scienꢀ
tific Educational Personnel of the Innovative Russia”
for 2009–2013 (State contract no. 02.740.11.0253).
+
H L + 2 ZnAcO + 2AcO− ↔ Zn L AcO
(
)
]
2
[
]
[
2
2
(2)
(3)
+ 2HAcO,
Zn L AcO + H L ꢀ Zn L + 2HAcO.
(
)
[
]
[
]
2
2
2
2
2
The overall equation has the form:
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 2 2012