Reactions of chelates with macrocyclic ligands. Complexation between tetraphenylporphine and Cu(II) complexes with α-amino acids

Article abstract of DOI:10.1023/A:1021102814028

The reactions of tetraphenylporphine (H₂TPP) with copper(II) chelates in DMSO were studied. α-Amino acids (glycine, α-alanine, valine, leucine, tyrosine, and glutamine) were used as chelating ligands. The study of the reaction kinetics showed that Cu(II) chelates with alanine and the other amino acids are less reactive in these reactions than acetylacetonates, α-nitroso-β-naphtholates, and hydroxyquinolates. The exception is a Cu(II) complex with tyrosine. The relationship between the structure of the above chelates and the rate of their reactions with porphyrin was determined.

Full text of DOI:10.1023/A:1021102814028

Russian Journal of Coordination Chemistry, Vol. 28, No. 11, 2002, pp. 771–775. Translated from Koordinatsionnaya Khimiya, Vol. 28, No. 11, 2002, pp. 822–827.
Original Russian Text Copyright © 2002 by Berezin, Mamardashvili.
Reactions of Chelates with Macrocyclic Ligands. Complexation
between Tetraphenylporphine and Cu(II) Complexes
with a-Amino Acids
B. D. Berezin and G. M. Mamardashvili
Institute of Chemistry of Nonaqueous Solutions, Russian Academy of Sciences, Ivanovo, 153045 Russia
Received July 11, 2001
Abstract—The reactions of tetraphenylporphine (H2TPP) with copper(II) chelates in DMSO were studied.
α-Amino acids (glycine, α-alanine, valine, leucine, tyrosine, and glutamine) were used as chelating ligands.
The study of the reaction kinetics showed that Cu(II) chelates with alanine and the other amino acids are less
reactive in these reactions than acetylacetonates, α-nitroso-β-naphtholates, and hydroxyquinolates. The excep-
tion is a Cu(II) complex with tyrosine. The relationship between the structure of the above chelates and the rate
of their reactions with porphyrin was determined.
In this paper, we continue a series of studies devoted than two decades ago [12]. However, those investiga-
to reactions of porphyrin molecules with chelate salts
[1–11]. Such reactions with concurrent chelate and
macrocyclic effects are of great interest.
tions did not progress due to the low solubilities of the
complexes in alcohols. In this study, dimethyl sulfoxide
(DMSO) was used as a solvent.
Previously, the complexation reactions between
porphyrins and such chelate salts as metal acetylaceto-
nates (M(Acac)2), 8-hydroxyquinolates (M(Ox)2),
α-nitroso-β-naphtholates (M(Nft)2), dithizonates,
diphenylcarbazonates, etc. have been investigated. The
stoichiometric mechanism for these reactions was pro-
posed; it was assumed that the reaction rate and condi-
tions are mainly determined by the stability of the chelate
salt and the macrocyclic effect of porphyrin [10].
EXPERIMENTAL
Tetraphenylporphine (H2TPP) was synthesized,
purified, and identified as described in [13]. Anhydrous
copper salts of glycine (Cu(Gly)2) and α-alanine
(Cu(Ala)2) were prepared and purified according to the
known procedure from [14]. Copper valinate
(Cu(Val)2), leucinate (Cu(Leu)2), and tyrosinate
The study of complexation between tetraphenylpor-
phine and Cu(II) chelates with glycine, α-alanine, (Cu(Tyr)2) were obtained as described in [15]. Tech-
valine, leucine, and a combination of them together as
inner-sphere anions makes it possible to reveal other
factors influencing the coordination ability of chelates.
niques borrowed from [16] and [17] were used to syn-
thesize the mixed salt Cu(Gly)(Val) and Cu2Glu2 · H2O,
respectively. The copper glutaminate Cu2Glu2 · H2O is a
binuclear complex with a bridging water molecule [17].
Attempts at studying the complexation between por-
phyrins and amino acid chelates were first made more Dimethyl sulfoxide was purified as described in [18].
Table 1. Solubilities of the α-amino acids and their Cu(II) complexes in water and DMSO at 298 K
S × 103, mol/l
S × 103,
mol/l H2O [22]
HL
CuL2
H2O [15]
DMSO* (this study)
HGly
HAla
HVal
HLeu
342
191
76
Cu(Gly)2
Cu(Ala)2
27.7
17.8
4.87
0.42
3.4
3.1
3.2
1.8
3.6
Cu(Val)2
17
Cu(Leu)2
Cu(Gly)(Val)
* At T = 363 K.
1070-3284/02/2811-0771$27.00 © 2002 åÄIä “Nauka /Interperiodica”

772
BEREZIN, MAMARDASHVILI
NH2
Cu
CHR
O
C
O
2
R = H (Cu(Gly)2), CH3 (Cu(Ala)2),
NH
N
N
CH3
CH3
CH3
(Cu(Val)2),
(Cu(Leu)2),
CH2 CH
CH
CH3
HN
(Cu(Tyr)2)
CH2
OH
(H2íPP)
C
O
CH CH2 CH2
NH2
C
O
O
O
O
O
Cu
H2O
Cu
O
NH2 O
C CH2 CH2
C
CH
(Cu2Glu2 · H2O)
The solubilities of the amino acid complexes in
DMSO were determined by isothermal saturation with
spectrophotometric verification of the concentration
(for details, see [19]). The method provides an accuracy
of 6 to 10% at 363 K.
RESULTS AND DISCUSSION
Solubility of the amino acid complexes. The solu-
bilities of amino acid complexes, in particular, Cu(II)
chelates, in water were studied in [15, 20, 21].
As can be seen in Table 1, the solubility of amino
acids themselves in water [22] depends on the size and
structure of substituent R, which can be due to its effect
not only on the crystal lattice energy but also on the
The kinetics of the complexation was investigated
following the same procedure as in [1–12]. The error in
keff determination was 3 to 5%.
All spectrophotometric measurements were per- energy required for the formation of a cavity in the
formed on a Specord M40 instrument.
water structure for hydrophobic radicals R.
Table 2. Rate constants of reaction (2) in DMSO at 363 K
keff × 103, s–1
CuL2
logKst in H2O [26–28]
cCuL = 5.6 × 10–4, mol/l
cCuL = 3 × 10–4, mol/l
2
2
Cu(Nft)2
3.3
5.2
Cu(Acac)2
Cu(Ox)2
14.5 × 1016
16.7 × 1027
2.26
2.65
1.03
Cu(Tyr)2
6.26
Cu(Gly)2
17.9 × 1016
2.38 × 1016
0.524
0.018
0.186
0.213
0.210
0.300
0.597
0.252
0.277
Cu(Ala)2
Cu(Val)2
Cu(Leu)2
Cu(Gly)(Val)
Cu2(Glu)2 · H2O
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 11 2002

REACTIONS OF CHELATES WITH MACROCYCLIC LIGANDS
773
The solubilities of Cu(II) complexes with amino plexes in water compared to that in DMSO on the alkyl
acids in water are two orders of magnitude lower than chain length is associated with the structural features of
those of the corresponding amino acids themselves, these solvents.
though the dependence of their solubility on the struc-
Complexation between Cu(II) amino acid com-
ture of R is of the same character as for amino acids.
plexes and tetraphenylporphine. The rate of the com-
This is quite natural since amino acids contain two
plexation between porphyrin (H2P) and metal salt
strong hydrophilic centers (NH2 and COO–), which
(MX2) solvates in solvents (Solv),
lose, to a large measure, this property upon chelation.
Although the resulting chelates have a new center of
hydration, namely, a metal ion with two coordination
sites for water molecules, the loss of the above strong
hydrophilic centers remains uncompensated, which
makes the complexes less soluble than the amino acids.
Contrary to the opinion that amino acid complexes
are insoluble in organic solvents, we found them to dis-
solve at high temperatures in some solvents (e.g.,
DMSO) at a concentration of ~10–3 mol/l. This concen-
tration is sufficient to perform spectrophotometric mea-
surements and calculate the rate constants of the reac-
tions under study. The solubility of the studied com-
plexes in DMSO virtually does not depend on the
length of the alkyl chain (R) of the amino acid. For
example, Cu(Gly)(Val) proved to have the lowest solu-
bility in both DMSO and water.
MX2(Solv)n – 2 + H2P
(1)
MP + 2HX + (n – 2)Solv
depends on both the number of solvent molecules
loosely bound to the metal ion and the M–Solv bond
strength [23] and, to a lesser extent, on the nature of the
X– anion [24].
As shown by us in [5, 6], the mechanism found for
reactions (1) with complex solvates MX2(Solv)n – 2 can-
not be realized in the complexation between porphyrins
and chelates:
ML2(Solv)n + H2P
(2)
Solv
MP + 2HL + (n + 1)Solv.
One can believe that the complexes under discus- Chelate salts should be preactivated by dissociation in
sion have close crystal lattice energies and the more a solvent to cause the opening of one or two chelate
pronounced dependence of the solubility of the com- rings,
COO–
COO
NH2
OOC
NH2
R
NH2
+
Solv
RHC
M
CHR
(Solv)n
RHC
M
(Solv)n + 1
C
OOC
H N
H
NH
2
COO
2
(3)
or to completely eliminate one of the chelating ligands, such as in d-metal hydroxyquinolates, α-nitroso-β-
naphtholates, dithizonates, and diphenylcarbazonates
Solv
ML2(Solv)n
ML+(Solv)n + 1 + L– .
(4)
[7–11], the preliminary dissociation of the ML2 chelate
can involve (and, most probably, involves) only one
chelate ring (reaction (3)).
Without the preliminary dissociation of the M–O
(and/or M–N) bonds, reaction (2) does not occur since
the effective approach of two reagents is prevented by
considerable steric hindrances.
We have shown using the reactions of porphyrin
with metal acetylacetonates as examples [1–3] that
complexation reaction (2) must be preceded by dissoci-
In this case, the conformational transformations in
the metal coordination sphere make the metal atom
more accessible to a porphyrin molecule. When the sol-
vent cannot break any of the chelating bonds (M–N, M–
O, or M–S), metal porphyrinates are not formed [7, 8].
The dissolution of Cu(II) complexes with amino
ation of the chelate salt, which results in the opening of acids in DMSO can follow either pathway (3) or (4). It
one of its rings, and by transformation of M(Acac)2 into is difficult to say which of the processes really takes
MAcac+ according to reaction (4). Apparently, in the place. The opening of the chelate ring can also involve
case of more stable and bulky coordination spheres, different bonds:
R H
Solv
NH2CCOO–(Solv)
Solv
COO
NH2
OOC
+
(5)
RHC
M
CHR
Solv M O C
+ Solv
O
CHR
NH
OOC
H N
2
2
Solv
NH2
Solv
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 11 2002

774
BEREZIN, MAMARDASHVILI
R H
NH2CCOO–(Solv)
Solv
Solv
Solv
M
COO
OOC
NH2
OOC
M2+
RHC
CHR
+ Solv
(6)
(7)
(8)
NH2CCOO–(Solv)
R H
HN2HN
Solv
2
Solv
Solv
Solv
R H
Solv
OOCCNH2(Solv)
COO
OOC
NH2
RHC
M
CHR
+ Solv
Solv M O C
O
HNHN
OOC
2
2
CHR
Solv
NH2
Solv
R H
Solv
Solv
M
Solv
Solv
COO
OOC
NH2
OOCCNH2(Solv)
RHC
CHR
M
Solv
OOCCNH2(Solv)
R H
NH
H2N2
OOC
Solv
Solv
The dissolution of these Cu(II) complexes in water predominant elimination of the glycine ligand, so that
1
likely proceeds through reactions (5) and (6), while either Cu(Val)+ or Cu(Val)(Gly)* enter into reaction (2).
reactions (7) and (8) seem to be improbable. In DMSO,
the probability of the latter reactions sharply increases
as DMSO cannot solvate anions (including the COO–
group).
Of special interest are the rates of reaction (2) for
copper α-alaninate and tyrosinate, which differ by
nearly 400 times. It still remains unclear why Cu(II)
alaninate coordinates H2TPP so slowly. One can only
note that the activation energy of the thermal decompo-
sition of Cu(Ala)2 proved to be two to four times higher
than that for other copper complexes with amino acids
(DSC data) [25]. The high complexation rate between
H2TPP and copper tyrosinate can be due to the presence
When the complexation was preceded by dissocia-
tion of the complex (with complete loss of one of the
chelating ligands) in such solvents as ethanol and pro-
panol, the rate constant of reaction (2) proved to depend
on the stability constant of the chelate [9]. Such a
dependence was not observed for Cu(II) complexes
with amino acids. For example, the rate constants of
reaction (2) involving complexes with nearly the same
stability (Kst) differ by more than two orders of magni-
tude (Table 2). This can be explained by considering the
significant difference between chelates in Kst in water
and DMSO, which depends on the composition of the
hydrophilic part of the chelate. Obviously, there is one
stability series of salts for water and another for
DMSO.
of the phenol OH group in Tyr–, which is involved in
intramolecular hydrogen bonding, thus destabilizing
the Cu–O and Cu
N coordination bonds. This facil-
itates their dissociation and hence the complexation
with porphyrin.
Cu
O
H2N
CH OC
O
H
It appears quite reasonable that no pronounced
dependence of keff on Kst (characterizing complete elim-
ination of the ligands) should be expected in the disso-
ciation of the complex without elimination of the
chelating ligand (Eqs. (5)–(8)).
CH2
C6H4
O
The activation energies of reaction (2) with all the
chelates are rather high. The highest Ea value was found
for Cu(Gly)2 (Table 3).
Based on the data in Table 2, one can draw the fol-
lowing conclusions.
1. The rate of formation of copper porphyrinates
from Cu(II) chelates with α-amino acids is significantly
lower than that from Cu(Acac)2, Cu(Nft)2, and Cu(Ox)2
(except for Cu(Tyr)2, which reacts on average 30 times
Among the Cu(II) complexes under discussion,
Cu(Gly)2 was found to have the highest rate of reaction
(2). The alkyl-free amino acid residue (R = H) of this
complex presents no steric hindrances to the first step of
its dissociation, resulting either in the elimination of
one ligand or in the opening of one chelate ring.
In the general case, mixed salts are always more sta-
ble than salts with identical ligands (Table 2). Insofar as
Cu(Gly)(Val) reacts at the same rate as Cu(Val)2, one
can assume that Cu(Gly)(Val) initially dissociates with
1
The asterisked anion indicates that the activation of the complex
is due to the opening of this chelate ring.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 11 2002

REACTIONS OF CHELATES WITH MACROCYCLIC LIGANDS
775
Table 3. Kinetic parameters of reaction (2) in DMSO
(cML = 5.4 × 10–4 mol/l, cH TPP ≈ 1 × 10–5 mol/l)
9. Mamardashvili, G.M. and Berezin, B.D., Zh. Obshch.
Khim., 2000, vol. 70, no. 8, p. 1387.
2
2
10. Mamardashvili, G.M. and Berezin, B.D., Koord. Khim.,
2000, vol. 26, no. 9, p. 699.
363
358
353
Complex
k
, s–1
k
, s–1
Ea, kJ/mol
k
, s–1
eff
eff
11. Berezin, B.D. and Mamardashvili, G.M., Koord. Khim.,
eff
2000, vol. 26, no. 10, p. 796.
12. Berezin, B.D. and Volkova, N.I., Zh. Neorg. Khim.,
Cu(Nft)2
5.2
3.3
2.0
100
108
130
1982, vol. 27, no. 10, p. 2753.
13. Adler, A.D., Longo, F.R., and Finarelli, J.D., J. Org.
Chem., 1967, vol. 32, no. 2, p. 476.
14. Ablov, A.V., Chapurina, L.F., D’yakon, I.A., and
Ivanova, V.Ya., Zh. Neorg. Khim., 1966, vol. 11, no. 11,
p. 2620.
Cu(Acac)2 2.65
Cu(Gly)2 0.597
1.60
0.96
0.326
0.176
15. Craddon, D.P. and Munday, L., J. Inorg. Nucl. Chem.,
faster than the other copper complexes with amino
acids).
1961, vol. 23, nos. 3–4, p. 231.
16. Chapurina, L.F. and Ablov, A.V., Zh. Neorg. Khim.,
2. For chelates having alkyl substituents in the
amino acid, porphyrinates are most rapidly formed
from copper glycinate (R = H). When the alkyl part of
the amino acid anion is extended to C2, C4, and C5, the
rate of reaction (2) decreases (by 30 times for Cu(Ala)2
and only 2.5 times for anions with larger alkyl substit-
uents (C4 and C5) relative to the rate constant of
Cu(Gly)2).
3. The rate of reaction (2) only slightly changes for
glutamic acid containing two carboxy groups.
4. The highest reaction rate was observed in the
complexation between porphyrin and Cu(II) tyrosinate
containing a hydroxyphenyl group capable of forming
a strong intramolecular bond with the C=O and NH2
groups.
1969, vol. 14, no. 6, p. 1521.
17. Basudeb, D.S., J. Am. Chem. Soc., 1956, vol. 78, no. 5,
p. 892.
18. Techniques of Organic Chemistry, Vol. 2: Organic Sol-
vents: Physical Properties and Methods of Purification,
Weissberger, A. and Proskauer, E.S., Eds., New York:
Interscience, 1955. Translated under the title Orga-
nicheskie rastvoriteli. Fizicheskie svoistva i metody
ochistki, Moscow: Inostrannaya Literatura, 1958.
19. Koifman, O.I., Berezin, B.D., Zelov, V.V., and Nikiti-
na, G.E., Zh. Fiz. Khim., 1978, vol. 52, no. 7, p. 1782.
20. Craddon, D.P. and Munday, L., Chemistry, 1959, no. 4,
p. 122.
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dia), Knunyants, I.L., Ed., Moscow: Sov. Entsiklope-
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i ftalotsianina (Coordination Compounds of Porphyrins
and Phthalocyanine), Moscow: Nauka, 1978.
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