ENTHALPIES OF GLYCYLGLYCINATE ION TRANSFER
975
Table 2. ΔtrH° values of transfer for ammonia, ethylenedi-
amine (En), glycinate (G–), and glycylglycinate (GG–) ions
from water to water–organic solvents with 0.5 mole frac-
tions of ethanol or DMSO, in kJ/mol
The αdif value was found (a) to lie in the range of
0.7–0.8 for complexation reactions between acidic
carboxylate-type ligands and transition metal ions, (b)
to have almost constant values at each point of the
composition of binary solvents, and (c) to change
slightly from one type of solvent to another [1]. The
difference coefficients were less than 0.6 for complexes
of d-metals with amino ligands [1]. As an example,
Table 3 shows αdif values calculated for complexes of
ΔtrH°(G–) ΔtrH°(GG–)
ΔtrH°(NН3) ΔtrH°(En)
Solvent
Water–
EtOH
4.8 [7]
6.8* [8]
7.2 [6]
16.1
Water–
DMSO
8.6 [10]
20.7 [11]
46.1 [9]
56.4 [3]
Ni2+ with amino and carboxylate ligands in water–
ethanol and water–DMSO solvents having 0.5 mole
fraction of the organic component, using the enthalpy
characteristics of the reactions [6, 12–14] and resolva-
tion of the reagents [3, 6, 9, 11]. As can be seen from
Table 3, the difference coefficient (αdif = 0.48) of the
* This value was obtained using ionic strength μ = 0.3 (NaClO ).
4
Table 3. Enthalpy characteristics of the reactions of Ni2+
complex formation in water–organic solvents with 0.5 mole
fractions of ethanol or DMSO (ΔtrH°, in kJ/mol)
complexation reaction between Ni2+ and glycylglyci-
nate ions in water–DMSO solvent was below the
range determined for acidic ligands, and was closer to
the one calculated for ethylene diamine. The differ-
ence coefficient for the reaction of [NiGG]+ forma-
tion in water–DMSO solvent, calculated with the
Gibbs energy of this reaction [15] and the ΔG° of
resolvation of the ligand [2], was 0.54, which is also
lower than the above values.
ꢀ
ꢀ
αdif
Water–DMSO 20.7 [11] 12.8 [12] 0.38
Water–EtOH 7.2 [6] 1.9 [6] 0.74
Water–DMSO 46.1 [9] 11.7 [13] 0.75
Reagents
Solvent
ΔtrHL −ΔtrHr
Ni2+ + En
Ni2+ + G–
Ni2+ + G–
Ni2+ + GG– Water–DMSO 56.4 [3] 29.5 [14] 0.48
A lower αdif value for the complexation reaction
may assume that the increase in the endothermicity of
between nickel (II) and glycylglycinate ion could be
resolvation of bifunctional N- and O-donor ligands is expected, since the change in the stability constant of
[NiGG]+ in the water–DMSO solvent [15] is
described not by the monotonically growing depen-
dence logKstability = f(XDMSO) characteristic of com-
plexes with acidic ligands [16], but by the extreme
dependence with a small peak typical of ethylene
diamine complexes with nickel (II) [17].
due largely to changes in the solvation of carboxylate
groups.
Assessing the degree of ligand resolvation’s partic-
ipation in complexation processes allows us to predict
the change in the thermodynamic parameters of com-
plexation reactions. It is known that the ΔtrH° and
ΔtrG° values of the solvation of ligands and the ΔtrH°
and ΔtrG° values of the complexation reaction with
d-metals normally have opposite signs, and the ther-
modynamic properties of the reaction lie within the
change in the corresponding characteristics of ligands
[1]. We may therefore assume that an increase in the
exothermicity of the reactions will be observed upon
the complexation of d-metals with the glycylglycinate
ions in a water–ethanol solvent when the concentra-
tion of the nonaqueous component is raised as well.
The thermodynamic parameters of complexation
reactions upon a change in the thermodynamic char-
acteristics of ligand resolvation can be estimated using
difference coefficient αdif characterizing the differ-
ences between the thermodynamic properties of the
ligands [1]:
CONCLUSIONS
Although glycylglycinate ions have the same struc-
tural and thermodynamic characteristics of resolva-
tion in mixed solvents as glycinate ions, the formation
of glycylglycinate ions has features in common with
the reactions of ethylenediamine complex formation,
at least in water–DMSO solvent. When evaluating the
thermal effect of the reactions of glycylglycinate for-
mation with d-metal ions in water-organic solvents,
the range of αdif values must therefore be expanded to
0.5–0.8. Based on this and our obtained ΔtrH°(GG–)
values, we may assume that the ΔtrH° value of the
reactions of formation between glycylglycinates and
d-metals in solvents ranges from –3 to –8 kJ/mol.
ꢀ
Using our range of the
and ΔH° values of reac-
ΔtrHr
tions for glycylglycinate complex formation in aqueous
solutions, we can assess the thermal effect of complex-
ation reactions in water–ethanol solvents. The
expected heat of reaction, e.g., lies between –20 and
‒25 kJ/mol for [NiGG]+ complex (in aqueous solu-
ꢀ
ꢀ
ꢀ
,
(4)
ΔtrYr = (αdif −1)ΔtrYL
ꢀ
where
is the change in the ΔH° and ΔG° values
ΔtrYr
of the complexation reaction in water–organic sol-
ꢀ
vents and
represents the ΔH° and ΔG° values of
ΔtrYL
tion,
= –16.72 kJ/mol [14]) when the content of
ΔHr
ligand resolvation, respectively.
ethanol in the solvent is 0.5 mole fraction.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 90 No. 5 2016