Influence of metal cation on chromophore properties of complexes of some d metals with α,α-dipyrrolylmethene

Article abstract of DOI:10.1007/s11176-005-0152-y

In complexes of Cu²⁺, Ni²⁺, Co²⁺, Zn ²⁺, and Hg²⁺ with 3,3′,5,5′-tetramethyl-4, 4′-dibutyl-2,2′-dipyrrolylmethene, the d-metal ion exerts an auxochromic effect on the π system of the ligand, manifested in a bathochromic shift of the first absorption band in the spectrum of the complex, compared to the free ligand. Polarization of the chromophore π system in the complexes increases in the order Zn²⁺ < Hg²⁺ < Co²⁺ < Ni²⁺ < Cu²⁺. The Hg²⁺ ion in the complex with dipyrrolylmethene participates in mercuration of benzene and chloroform; the reaction with chloroform is faster, with the rates of both reactions strongly depending on temperature. 2004 MAIK Nauka/Interperiodica .

Full text of DOI:10.1007/s11176-005-0152-y

Russian Journal of General Chemistry, Vol. 74, No. 8, 2004, pp. 1282 1285. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 8, 2004,
pp. 1383 1387.
Original Russian Text Copyright
2004 by Guseva, Antina, Berezin, V’yugin.
Influence of Metal Cation on Chromophore Properties
of Complexes of Some d Metals with , -Dipyrrolylmethene
G. B. Guseva, E. V. Antina, M. B. Berezin, and A. I. V’yugin
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received November 15, 2002
Abstract In complexes of Cu2+, Ni2+, Co2+, Zn2+, and Hg2+ with 3,3 ,5,5 -tetramethyl-4,4 -dibutyl-2,2 -di-
pyrrolylmethene, the d-metal ion exerts an auxochromic effect on the system of the ligand, manifested in a
bathochromic shift of the first absorption band in the spectrum of the complex, compared to the free ligand.
Polarization of the chromophore system in the complexes increases in the order Zn2+ < Hg2+ < Co2+
<
Ni2+ < Cu2+. The Hg2+ ion in the complex with dipyrrolylmethene participates in mercuration of benzene and
chloroform; the reaction with chloroform is faster, with the rates of both reactions strongly depending on
temperature.
Development of studies in the chemistry of poly-
Me
Me
pyrrole compounds is mainly stimulated by their
unique biological effect as photoreceptors in the struc-
tures of some chromoproteins and intermediate sub-
strates of porphyrin metabolism, and also by their
wide use in preparative organic chemistry for syn-
thesis of various manmade and natural porphyrins
[1, 2]. The efficiency of template synthesis of por-
phyrins from dipyrrolylmethenes on a metal ion
matrix is largely determined by the chelating power
of the dipyrrolyl ligand and by the physicochemical
properties of the resulting chelates. Therefore, the
search for new efficient procedures for template syn-
thesis of porphyrins should be based on systematic
data on the reactivity of linear polypyrrole compounds
toward various metal ions and on physicochemical
properties of the corresponding chelates. Such data
are limited [2 4].
Bu
Me
Bu
Bu
NH
N
Me
I
Me
N
Me
Bu
Me
N
Me
Me
M
Me
N
N
Bu
Bu
Me
Me
II VI
M = Zn2+ (II), Hg2+ (III), Co2+ (IV), N2+ (V), Cu2+ (VI).
In this work we made a comparative analysis of
the electronic absorption spectra of the free ligand,
3,3 ,5,5 -tetramethyl-4,4 -dibutyl-2,2 -dipyrrolylmeth-
ene I, and its complexes with Zn2+ (II), Hg2+ (III),
Co2+ (IV), Ni2+ (V), and Cu2+ (VI) [5, 6] in various
organic solvents: 1-propanol, benzene, DMF, pyridine,
and chloroform.
It is known that the bathochromic shift of the first
absorption band in the spectra of a complex relative to
I
I
I
I
the free ligand,
=
IL, where
and
are
C
C
L
the wavelengths of the first absorption maximum of
the complex and ligand, respectively, correlates not
only with the extent of polarization of the chromo-
phore with the metal ion, but also in many cases with
the strength of the metal ligand coordination bonds in
the complexes. For phthalocyanines, the correspond-
ing criterion of stability of the complexes was postu-
lated by Linsted and thoroughly analyzed in [7, 8].
It should be noted that, in porphyrins, the combined
effect of coordination and increase in the molecular
symmetry upon complexation results in a decrease in
the number of bands and in a hypsochromic shift of
The spectroscopic data are given in Table 1. Com-
plexes V and VI have two, and II IV, one absorption
band in the visible range. All the complexes exhibit
a relatively weak broad absorption band with a maxi-
mum at 366 398 nm. The spectrum of the free ligand
in all the examined solvents has one strong absorption
band at 440 446 nm, i.e., at shorter wavelength as
compared to the stronger band in the spectra of the
complexes.
1070-3632/04/7408-1282 2004 MAIK Nauka/Interperiodica

INFLUENCE OF METAL CATION ON CHROMOPHORE PROPERTIES
1283
the first absorption band relative to the free ligand,
suggesting decreased polarization of the chromophore
in the complex. Our experimental (Tables 1, 2) and
Table 1. Positions of maxima (nm) and intensities (log
1
[l mol 1 cm ]) of bands in the electronic absorption
spectra of complexes of d metals with dipyrrolylmethene I
in organic solvents
published [2, 3] data show that, with dipyrrolylmeth-
I
enes and their complexes, the trend is opposite:
is
considerably larger than IL. This suggests enhanCced
polarization of the system of the dipyrrolylmethene
molecule caused by an auxochromic effect of the
metal ion on the chromophore, apparently determined
by not only electronic but also structural and steric
factors. The metal ion, becoming a partner of the
conjugated system of dipyrrolylmethene electrons,
makes the pyrrole fragments more coplanar, which
should enhance the conjugation between them [2].
As a result, in all the solvents (except CHCl3), the
first absorption band of complxes I IV is shifted
bathochromically by 50 75 nm relative to the free
ligand (Table 2). In chloroform, the shift is consider-
ably smaller and does not exceed 20 30 nm. This fact
is due to specific solvation with chloroform of the
Comp.
i-PrOH
446(4.417) 440(4.440)
366 br 368(4.260)
500(5.047) 498(5.200)
366(3.926) 368(4.028)
502(4.745) 498(4.957)
372(3.973) 371(3.973)
504(4.740) 504(5.187)
394(4.245) 395(4.495)
458(4.370) 458(4.638)
510(4.488) 511(4.757)
DMF
Py
C6H6 CHCl3
I
445
445 446
484
II
III
IV
V
370
502
374
501
374
506
398
459
510
395
464
515
370 370
502 502
368 372
509 508
374 375
506 506
398 395
459 459
512 512
395 397 br
466 463
516 516
VI
395 br
395 br
461(4.679) 461(4.831)
512(4.582) 512(4.709)
heteroatom of the pyrrolenine ring ( N HCCl3) [5],
I
causing
of the free ligand I to increase relative to
the other Lsolvents. The chloroform proton acts as an
auxochrome similar to metal ions in the complexes,
but its polarizing effect on the chromophore is consid-
erably weaker [9].
Table 2. Shift of the first absorption band in the electronic
spectra of dipyrrolylmethene complexes II VI compared
to the free ligand I (
I
I
=
IL) in organic solvents
C
The structural features of metal dipyrrolylmethene
complexes [2] are as follows. In cyclic tetrapyrrole
porphyrin ligands, the structure of the coordination
core is rigid, with a fixed void size, and formation of
stable complexes requires matching of the void size
and metal ionic radius. In going to open-chain dipyr-
role ligands, this condition is lifted. Complexation
should make the pyrrole rings in dipyrrolylmethene
more coplanar. The electron density transfer from the
ligands to the metal ion in dipyrrolylmethene com-
plexes is determined by formation of mainly covalent
I, nm
Metals ranked in
Solvent
order of increasing
I
II III IV V VI
DMF
Py
1-PrOH
C6H6
58 58 64 71 72 Zn= Hg< Co< Ni< Cu
57 56 61 65 70 Zn Hg< Co< Ni< Cu
54 56 58 64 66 Zn< Hg< Co< Ni< Cu
57 64 61 67 71 Zn< Co< Hg< Ni< Cu
18 24 22 28 32 Zn< Co< Hg< Ni< Cu
CHCl3
av,a nm 57 59 61 67 70 Zn< Hg< Co< Ni< Cu
I
and donor acceptor metal ligand
bonds. In some
cases, backbonding takes place. The spectrum char-
acteristics may be significantly affected by changes
in the conformational state of the dipyrrolylmethene
molecule upon complexation and by specific features
of solvation in various solvents.
a
Average value for all the solvents except chloroform.
Ni2+ < Cu2+ > Zn2+. This order is followed in most
cases, but the position of zinc is uncertain. It is only
known that the zinc complexes are less stable than
copper complexes. As a rule, zinc occupies the place
between cobalt and nickel [11, 12]. However, this
order is not obeyed with ligands coordinating through
nitrogen atoms at double bonds (N=C). In this case,
backbonding becomes possible, and cobalt com-
plexes become more stable than zinc complexes:
Zn2+ < Co2+ < Ni2+ < Cu2+.
Thus, analysis of bathochromic shifts of the first
absorption maximum in the electronic spectra of the
complexes, compared to the free ligand, primarily
allows estimation of the degree of ligand-to-metal
electron density transfer. The average (for the series
I
of the solvents tested) values of
show that the
polarization of the ligand chromophore in complexes
with double-charged metal ions increases in the order
Zn2+ < Hg2+ < Co2+ < Ni2+ < Cu2+. The well-known
Irving Williams series of double-charged ions charac-
This order is consistent with the stability order
constructed for the dipyrrolylmethene complexes on
the basis of the
terizing the stability of complexes [10] is Co2+
<
values. The order is similar in all
I
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 8 2004

1284
GUSEVA et al.
509
485
508
A
1
(b)
(a)
2
2
1.0
1
445
3
0.5
350
500
350
500
, nm
Fig. 1. Electronic absorption spectrum of complex III: (a) in C H at 298.15 K and (b) in CHCl at 318.15 K. (A) Optical
6
6
3
density. (1) Initial spectrum, (2) in 24 h, and (3) in 3 weeks. Arrows denote the direction of variation of the spectrum in
successive recordings.
the solvents except benzene and chloroform, in which
the position of Hg is different (Table 2). Note that the
ration shows characteristic features of electrophilic
aromatic substitution; it involves initial formation of
the complex and is accelerated in the presence of
bases which bind the proton displaced by mercury.
With complex III as mercurating agent, the dipyrrolyl-
methene anion acts as such a base. The experimental
data obtained in this study suggest the following
mechanism of mercuration of benzene with III (HL
denotes compound I):
transfer of II and IV VI from 1-PrOH to Py, C6H6,
I
and DMF is accompanied by an increase in
,
which suggests enhancement of the chromophore
polarization, caused by solvation interaction with
molecules of electron-donor and aromatic solvents.
I
For III,
is maximal in benzene, compared to
the other solvents. Experiments show that prolonged
storage (for more than a month) of solutions of II and
IV VI in the examined solvents causes no changes in
the spectra, whereas in the spectrum of III in benzene
I
(at 298.15 K) the absorption intensity at C noticeably
decreases even in 24 h (Fig. 1a). Upon storage of this
solution for 3 weeks, the ligand absorption band ap-
I
L
pears and grows in intensity ( 445 nm), whereas the
band of the complex gradually decreases and disap-
peares. At 318.15 K, similar changes in the solution
spectrum are complete in 10 days. The spectra show
isobestic points suggesting the presence of two col-
ored species: complex III and ligand I, which appar-
ently indicates that benzene undergoes mercuration
[13, 14]. Various agents are used for mercuration of
aromatic compounds. Mercuration of benzene is usu-
ally performed with mecrucy(II) acetate in acetic acid;
the reaction is catalyzed with perchloric acid. Mercu-
Apparently, formation of a complex in the initial
step of the process enhances the polarization of the
chromophore system, which accounts for the largest
I
bathochromic shift
of the mercury complex in
benzene compared to the other solvents.
I
In chloroform, too, complex III has a larger
than the related Zn(II) and Co(II) complexes (Table 2).
The spectrum of a solution of III in CHCl3 also
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 8 2004

INFLUENCE OF METAL CATION ON CHROMOPHORE PROPERTIES
1285
changes with time. The transformation into the spec-
trum of the protonated ligand is complete in a week
at 298 K and in 24 h at 318 K (Fig. 1b). In this sol-
vent, mercuration can occur via formation of a
complex to give the chloromethylmercury derivative
(CCl3)HgL or (CCl3)2Hg [13, 14]. Thus, our results
show that complex III can be used as an agent for
mercuration of benzene and chloroform.
nova, N.A., Lebedeva, N.Sh., and Bukushina, G.B.,
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rinov i ftalotsianina (Coordination Compounds of
Porphyrins and Phthalocyanine), Moscow: Nauka,
1978.
EXPERIMENTAL
The electronic absorption spectra of pigment solu-
tions were recorded on a Specord M-40 spectropho-
tometer in a temperature-controlled cell. The temper-
ature was maintained with an accuracy of 0.1 C.
8. Berezin, B.D. and Enikolopyan, N.S., Metalloporfi-
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3,3 ,5,5 -Tetramethyl-4,4 -dibutyl-2,2 -dipyrrolyl-
methene was synthesized, purified, and isolated ac-
cording to [5]. Complexes II VI were prepared ac-
cording to [6] by reactions of I with metal ace-
tates. The purity of the complexes was confirmed by
TLC on Silufol UV-254 plates and by comparison of
the spectral characteristics with published data [5, 6].
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