Synthesis, characterization, and thermodynamics of tertiary phosphine cobalt(III) Schiff-base complexes

Article abstract of DOI:10.1139/v01-158

[Co(5-nitroSalen)(PBu₃)]ClO₄·H₂O and [Co(Salpd)(PBu₃)]ClO₄·H₂O were synthesized and characterized. The stability constants and the thermodynamic parameters were measured spectrophotometrica

Full text of DOI:10.1139/v01-158

1360
Synthesis, characterization, and thermodynamics
of tertiary phosphine cobalt(III) Schiff-base
complexes
Mozaffar Asadi and Ali Hossein Sarvestani
Abstract: [Co(5-nitroSalen)(PBu3)]ClO4·H2O and [Co(Salpd)(PBu3)]ClO4·H2O were synthesized and characterized. The
stability constants and the thermodynamic parameters were measured spectrophotometrically for 1:1 adduct formation
of [CoL(PBu3)]ClO4·H2O, (L = Salen, 5-nitroSalen, Salpd), and [Co(Salen)(PMe2Ph)]ClO4·H2O as acceptors with
P(OR)3 (R = Me, Et, i-Pr) as donors, in acetonitrile solvent and in constant ionic strength (I = 0.01 M) and at various
temperatures. The trend of the reactivity of cobalt(III) Schiff-base complexes with PBu3 axial ligand toward a given do-
nor is as follows: 5-nitroSalen > Salen > Salpd.
Also, the following reactivity trend of the donors toward a given cobalt(III) Schiff-base complex is in the operation:
P(O-i-Pr)3 > P(OEt)3 > P(OMe)3.
Key words: cobalt(III) Schiff-base complexes, thermodynamic parameters, trialkylphosphite, adduct complexes, stability
constants.
Résumé : On a effectué la synthèse et on a caractérisé les complexes suivants: [Co(5-nitroSalen)(PBu3)]ClO4·H2O et
[Co(Salpd)(PBu3)]ClO4·H2O. Faisant appel à des méthodes spectrophotométriques, on a mesuré les constantes de stabi-
lité et les paramètres thermodynamiques pour la formation d’adduits 1:1 entre les complexes accepteurs
[CoL(PBu)3]ClO4·H2O dans lesquels L = Salen, 5-nitroSalen et Salpd et [Co(Salen)(PMe3Ph)]ClO4·H2O et les P(OR)3
(dans lesquels R = Me, Et, i-Pr) comme accepteurs, dans l’acétonitrile comme solvant, à force ionique constante (I =
0,01 M) et à diverses températures. La tendance de la réactivité des complexes des bases de Schiff du cobalt(III) avec
le ligand PBu3 axial est donnée par la séquence suivante: 5-nitroSalen > Salen > Salpd.
Du côté des donneurs vis-à-vis d’un complexe donné d’une base de Schiff du cobalt(III), la tendance est exprimée par
la séquence suivante: P(O-i-Pr)3 > P(OEt)3 > P(OMe)3.
Mots clés : complexes de bases de Schiff avec le cobalt(III), paramètres thermodynamiques, trialkylphosphite, adduits
de complexes, constantes de stabilité.
[Traduit par la Rédaction] Asadi and Sarvestani 1365
Introduction
The formation constants for some [Co(Salen)(PR3)2]+ were
examined in acetonitrile and in ethanol 95% (11).
Many Co(III)–Salen tertiary phosphine complexes have
been suggested as models for the vitamine B12 coenzyme (1–
3). The properties of these Salen complexes were examined
with the aim to gain an insight into the chemistry of the re-
lated biological molecules. Some extensive works have been
done on these models (4–8). Recently the kinetics of ligand
replacement reactions of the axial H2O in the trans-
[Co(en)2(H2O)(Me)]2+ complex have been studied by Hamza
et al. (9). The equilibrium of the adduct formation of
In this work, the new complexes [Co(5-nitroSalen)-
(PBu3)]ClO4·H2O and [Co(Salpd)(PBu3)]ClO4·H2O were syn-
thesized and characterized. Thermodynamics of cobalt(III)
Schiff-base complexes of the types [CoL(PBu3)]ClO4·H2O,
where L = Salen, 5-nitroSalen, Salpd, and [Co(Salen)-
(PMe2Ph)]ClO4·H2O as acceptors with (P(OR)3, R = Me, Et,
i-Pr) as donors, in acetonitrile solvent have been examined.
By comparing the spectral and the thermodynamical proper-
ties of the synthesized complexes, we aimed to investigate
the effects of different electronic and steric situations.
[Co(Salen)X]+ (X = tertiary phosphines) with NO2 has been
-
studied. It was shown that the equilibrium constants decrease
according to the following trend: PPh3 < PEtPh2 < PBu3 (10).
Experimental
Received February 14, 2001. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on September 19, 2001.
Materials
Salicylaldehyde, 5-nitrosalicylaldehyde, 1,2-ethylenediamine,
1,3-propanediamine, cobalt(II) acetate tetrahydrate, tributyl-
phosphine, trialkylphosphites, tetraethylammonium perchlor-
ate, dimethylphenylphosphine, and acetonitrile were purchased
from Merck, Fluka, or Aldrich.
M. Asadi1 and A.H. Sarvestani. Chemistry Department,
College of Sciences, Shiraz University, Shiraz 71545, I.R. Iran.
1Corresponding author (telephone: 0098 711 2 224 829; fax:
(0711) 2 220 027; e-mail: asadi@chem.susc.ac.ir).
Can. J. Chem. 79: 1360–1365 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-9-1360

Asadi and Sarvestani
1361
Table 1. The stability constants (K, L mol–1) for [Co(Salen)-
Table 3. The stability constants (K, L mol–1) for [Co(5-nitroSalen)-
(PBu3)]ClO4·H2O with various P(OR)3 in acetonitrile solvent.
(PBu3)]ClO4·H2O with various P(OR)3 in acetonitrile solvent.
Temperature (°C)
P(OMe)3
P(OEt)3
P(O-i-Pr)3
Temperature (°C)
P(OMe)3
P(OEt)3
P(O-i-Pr)3
20
25
30
35
40
474.6 ± 0.0
398.3 ± 0.0
368.8 ± 0.0
346.6 ± 0.0
299.4 ± 0.1
6482.0 ± 1.3
4701.8 ± 0.5
2965.8 ± 0.2
2595.3 ± 0.1
2042.9 ± 0.4
6604.4 ± 1.6
5707.6 ± 1.4
4641.1 ± 1.0
3791.5 ± 0.1
3300.6 ± 1.3
20
25
30
35
40
348.9 ± 0.0
301.5 ± 0.0
279.8 ± 0.0
250.0 ± 0.0
236.4 ± 0.0
1640.0 ± 0.2
1446.4 ± 0.1
1126.0 ± 0.2
948.4 ± 0.1
852.1 ± 0.1
2098.7 ± 0.4
1712.1 ± 0.3
1595.7 ± 0.2
1340.0 ± 0.1
1135.6 ± 0.0
Note: l = 450 nm.
Note: l = 460 nm.
Table 2. The stability constants (K, L mol–1) for [Co(Salen)-
Table 4. The stability constants (K, L mol–1) for [Co(Salpd)-
(PMe2Ph)]ClO4·H2O with various P(OR)3 in acetonitrile solvent.
(PBu3)]ClO4·H2O with various P(OR)3 in acetonitrile solvent.
Temperature (°C) P(OMe)3
P(OEt)3
P(O-i-Pr)3
Temperature (°C)
P(OMe)3
P(OEt)3
P(O-i-Pr)3
20
25
30
35
1611.1 ± 0.3 5580.3 ± 3.5 11 525.1 ± 7.7
20
25
30
35
40
243.6 ± 0.0
228.5 ± 0.0
204.1 ± 0.0
190.0 ± 0.0
171.0 ± 0.0
1125.7 ± 0.2
920.0 ± 0.1
756.0 ± 0.0
681.2 ± 0.0
552.9 ± 0.0
875.4 ± 0.1
783.1 ± 0.1
606.1 ± 0.0
524.7 ± 0.0
438.9 ± 0.0
1189.0 ± 0.1 4992.3 ± 2.1
1069.3 ± 0.1 4018.3 ± 1.2
889.7 ± 0.1 3198.3 ± 1.3
724.9 ± 0.1 2378.1 ± 0.8
9 924.2 ± 5.9
8 698.4 ± 3.2
7 577.8 ± 3.4
6 577.1 ± 2.6
40°C
Note: l = 470 nm.
Note: l = 470 nm.
[N,N¢-bis(5-nitrosalicylidene)aza-1,2-diaminoethanato]tri-n-
Instrumentation
butylphosphineCo(III) perchlorate monohydrate
The UV–vis spectra were recorded by a Jusco V-530
spectrophotometer. The NMR spectra were recorded by a
Bruker Avance DPX 250 MHz spectrometer, and IR spectra
were recorded by a PerkinElmer 781 IR spectrophotometer.
1
Yield: 85%. H NMR (DMSO-d6) d: 0.73–0.79 (t, 9H,
CH3), 1.21–1.26 (m, 9H, CH2), 1.45–1.49 (m, 9H, CH2),
2.51 (s, 2H, H2O), 4.03 (s, 4H, CH2CH2), 7.15–7.19 (d, 2H,
ArH), 8.15–8.20 (d, 2H, ArH), 8.51 (s, 4H, ArH, CH=N).
Anal. calcd. for C28H41ClN4O11PCo: C 45.77, H 5.62,
N 7.62; found: C 45.43, H 5.59, N 7.80.
Synthesis of the complexes
The ligands Salen, 5-nitroSalen, and Salpd were prepared
according to the literature (12). The [Co(Salen)-
(PBu3)]ClO4·H2O and [Co(Salen)(PMe2Ph)]ClO4·H2O com-
plexes were prepared by the methods described previously
(10, 11).
Thermodynamic studies
The adduct complexes were obtained from the reaction of
the acceptors with the donors, according to the eq. [1]:
The general procedure for the synthesis of the [Co(5-
nitroSalen)(PBu3)]ClO4.H2O and [Co(Salpd)(PBu3)]ClO4.H2O
is as follows: to a refluxing solution of Salpd (0.57 g,
2.01 mmol) or 5-nitroSalen (0.71 g, 2.01 mmol) in methanol
(70 mL), were added Co(OAC)2·4H2O (0.50 g, 2.01 mmol)
and tributylphosphine (0.5 mL, 2.01 mmol). The reaction
mixture was refluxed for 1 h. The formed Co(II) complex
was oxidized by blowing air into the solution for 2 h, and
the solution was filtered. To the filtrate, an appropriate
amount of sodium perchlorate (0.30 g, in 10 mL water)
was added. The green crystals were formed after 24 h.
The crystals were washed with some methanol,
recrystallized in acetone–ethanol (95%), and dried in vac-
uum at 65°C.
[1]
[CoLX]+ + P(OR)3 ¬¾® [CoLX(P(OR)3)]+
where L = Salen, Salpd, and 5-nitroSalen, X = PBu3 and
PMe2Ph (only for Salen), R = Me, Et, and i-Pr.
A solution from each complex with a concentration at
about 1 × 10–4 M and a constant ionic strength (I = 0.01 M)
by tetraethylammonium perchlorate was prepared. From
this solution, 2.5 mL was transferred into the thermostated
cell compartment of the UV–vis instrument which was kept
at constant temperature (±0.1°C) by circulating water and
was titrated by the phosphite. The titration was done by
adding aliquots of the phosphite with a Hamilton syringe
(mL). The donors’ concentrations were varied one- to three-
fold in excess.
[N,N¢-bis(salicylidene)-1,3-diaminopropanato]tri-n-
butylphosphineCo(III) perchlorate monohydrate
Yield: 70%. H NMR (CDCl3) d: 0.76–0.81 (t, 9H, CH3),
The absorption measurements were carried out at a
fixed wavelength where the difference in absorption be-
tween the substrate and the product was the largest after
the equilibrium was assessed. These fixed wavelengths
for the systems are listed in Tables 1–4. The adduct
formed shows an absorption different from the acceptor,
while the donors show no absorption at those wave-
lengths.
1
1.25–1.30 (m, 18H, CH2), 2.75 (w, 2H, H2O), 3.86–3.96,
4.22–4.28 (m, 6H, CH2CH2CH2), 6.55–6.61 (t, 2H, ArH),
6.90–6.93 (d, 2H, ArH), 6.98–7.01 (d, 2H, ArH), 7.11–7.17
(t, 2H, ArH), 7.65 (s, 2H, CH=N). Anal. calcd. for
C29H45ClN2O7PCo: C 52.86, H 6.88, N 4.25; found: C 53.49,
H 6.69, N 4.61.
© 2001 NRC Canada

1362
Can. J. Chem. Vol. 79, 2001
Fig. 1. Spectrophotometric titration of [Co(Salen)(PMe2Ph)]+
with P(O-i-Pr)3 in acetonitrile (I = 0.01 M Et4NClO4, T = 30°C).
Fig. 2. The linear plot for [Co(Salen)(PBu3)]+ with P(OEt)3 sys-
tem at various temperatures (T = 20–40°C).
o
o
CACD
o
A - AA - AD
o
o
P =
, C = (CACD).
o
Results and discussion
The characterization of the complexes
The new complexes [Co(5-nitroSalen)(PBu3)]ClO4·H2O and
[Co(Salpd)(PBu3)]ClO4·H2O were identified by elemental anal-
ysis, IR, NMR, and electronic spectra. The other complexes
were identified only by IR, NMR, and electronic spectra.
position 6 to the azomethane overlaps with the azomethane
proton at d = 8.51.
Spectral characterization
IR spectra: The absence of the coordinated water in all the
studied complexes was confirmed by the absence of the ab-
sorption band around 3100 cm–1 (10). The absorption band
around 1625–1640 cm–1 is related to the azomethane group,
and the aromatic double bonds appeared at 1600–1605 cm–1
(13).
Thermodynamic interpretations
The equilibrium constants of the various cobalt(III) Schiff
base complexes studied were calculated by using Ketelaar et
al.’s (14) equation
o o
CACD
o
1
é1
ù
o
o
[2]
=
o
+ (CA + CD)
ê
ú
Electronic spectra: The electronic spectra, of all the studied
complexes, show an absorption at 600–700 nm in non-
coordinated solvent, that indicate the five-coordinated struc-
ture for them. All complexes show an intensive absorption
band at 370–400 nm. This band did not reallychange in all
the reactions studied. The absorption band at 600–700 nm
vanished and the six-coordinated product species showed an
absorption band at 400–500 nm (10). All measurements for
the thermodynamic studies were carried out in this range. As
an example, the variation of the electronic spectra for
[Co(Salen)(PMe2Ph)]+, titrated with the P(O-i-Pr)3, is shown
in Fig. 1. The isosbestic points for this system show that
there is only one reaction in equilibrium. The same is valid
for other systems.
(e - e - e ) K
ë
û
A - AA - AD
C
A
D
o
o
where CA and CD are the initial concentrations of the accep-
tor and the donor, respectively. A is the optical density of the
o
o
solution including the acceptor and the donor; AA and AD are
the optical densities of the pure acceptor and the pure donor
o
o
in the solution of concentration CA and CD; eC, eA, and eD
are the molar extinction coefficients of the complex, the ac-
ceptor, and the donor, respectively. K is the stability constant
of the formed complex and the cell optical path length is
o o
o
o
o
o
1 cm. A plot of (CACD/(A - AA - AD)) vs. (CA + CD) should
produce a straight line if only the 1:1 complex is formed,
while a mixture of 1:1 and 1:2 or only 1:2 complex in a sys-
tem would lead to a curve. The stability constants of the
studied cobalt(III) Schiff base complexes were calculated
from the ratio of the slope to the intercept by the least-
squares method using Excel 5 computer software. The K
measurements were repeated at least twice and were repro-
ducible. The linear plots for [Co(Salen)(PBu3)]+ with
P(OEt)3 at various temperatures are shown in Fig. 2, which
signifies that only 1:1 complex is formed. Similar plots are
obtained for other systems.
1
1
The H NMR spectra: The H NMR spectral data have been
presented in the Experimental section. The NMR spectra of
these complexes are consistent with the suggest formulation
and show that the cobalt in these complexes is low-spin. The
interesting point in the NMR of these complexes is the aro-
matic proton signals that are resolved in four groups in the
1H NMR. For example, in [Co(Salpd)(PBu3)]+ these signals
arranged from d = 6.55 to 7.17 in the form of triplet, doublet,
doublet, and triplet, that are related to the protons in the po-
sitions of 5, 3, 6, and 4 to the azomethane group,
respectively. In the complex of [Co(5-nitroSalen)(PBu3)]+,
because of the NO2 group, the aromatic proton signals
shifted to the low field in such a way that the proton in the
The thermodynamic parameters of the studied cobalt(III)
Schiff base complexes were calculated by use of the well-
known van’t Hoff eq. [3]:
o
o
-DH
RT
DS
R
[3]
ln K =
+
© 2001 NRC Canada

Asadi and Sarvestani
1363
Table 7. The thermodynamic parameter values DH°, DS°, and
DG° for [Co(Salpd)(PBu3)]ClO4·H2O with various P(OR)3 in
acetonitrile solvent.
Table 5. The thermodynamic parameter values DH°, DS°, and
DG° for [Co(Salen)(PBu3)]ClO4·H2O with various P(OR)3 in
acetonitrile solvent.
P(OR)3
DH° (kJ mol–1) DS° (J K–1 mol–1) DG° (kJ mol–1)a
P(OR)3
P(OMe)3 –13.7 ± 0.7
P(OEt)3 –26.3 ± 1.3
P(OIpr)3 –26.5 ± 1.6
aAt 30°C.
DH° (kJ mol–1) DS° (J K–1 mol–1) DG° (kJ mol–1)a
P(OMe)3
P(OEt)3
P(O-i-Pr)3 –22.6 ± 1.3
–14.8 ± 1.1
–25.6 ± 1.2
–1.9 ± 3.7
–25.9 ± 4.0
–13.7 ± 4.4
–14.2 ± 2.2
–17.7 ± 2.4
–18.6 ± 2.6
–1.0 ± 2.3
–31.3 ± 4.3
–34.1 ± 5.4
–13.4 ± 1.4
–16.7 ± 2.6
–16.1 ± 3.2
aAt 30°C.
Table 8. The thermodynamic parameter values DH°, DS°, and
DG° for [Co(5-nitroSalen)(PBu3)]ClO4·H2O with various P(OR)3
in acetonitrile solvent.
Table 6. The thermodynamic parameter values DH°, DS°, and
DG° for [Co(Salen)(PMe2Ph)]ClO4·H2O with various P(OR)3 in
acetonitrile solvent.
P(OR)3
P(OMe)3 –28.8 ± 2.3
P(OEt)3 –34.3 ± 0.3
P(OIpr)3 –21.3 ± 0.4
aAt 30°C.
DH° (kJ mol–1) DS° (J K–1 mol–1) DG° (kJ mol–1)a
P(OR)3
P(OMe)3 –16.8 ± 1.1
P(OEt)3 –44.6 ± 1.8
P(OIpr)3 –27.3 ± 1.2
aAt 30°C.
DH° (kJ mol–1) DS° (J K–1 mol–1) DG° (kJ mol–1)a
–37.3 ± 7.6
–44.2 ± 0.9
+5.5 ± 1.2
–17.6 ± 4.6
–20.9 ± 0.6
–22.8 ± 0.8
–6.1 ± 3.6
–79.5 ± 6.0
–19.9 ± 3.9
–14.9 ± 2.2
–20.1 ± 3.6
–21.3 ± 2.4
where K is the stability constant, R is the gas constant, and T
is the temperature in kelvin scale. Thermodynamic parame-
ters of the studied cobalt(III) Schiff base complexes were
obtained from the linear plots of ln K vs. 1/T. The values of
DH° and DS° were obtained from the slope and the intercept,
respectively, by using the Excel computer program.
The stability constants and the thermodynamic parameters
data are collected in Tables 1–8. The stability constants are
given for the formation of 1:1 adducts only.
ceptor property of the complexes. The presence of an NO2
withdrawing group causes 5-nitroSalen to become a weaker
Schiff base than the unsubstituted Salen. Therefore, the co-
balt atom in the [Co(5-nitroSalen)(PBu3)]+ complex has
more acceptor property than [Co(Salen)(PBu3)]+ and forms
more stable complexes with donors (Tables 1 and 3).
The steric effect of the equatorial Schiff base ligands
By increasing the steric interactions of the Schiff base, the
five-coordinate complex becomes more stable and the formation
constant of this type decreases with an increase in steric interac-
tions. Therefore, the five-coordinated [Co(Salpd)(PBu3)]+ com-
plex has less acceptor ability than [Co(salen)(PBu3)]+ (Tables 1
and 4).
The acceptor property of cobalt(III) Schiff base
complexes
It was suggested (15) that when Co(III) ion is chelated in a
macrocycle having delocalized electronic structure, the com-
plex loses its transition character and might become kinetically
labile. The extent to which this happens is likely to be strongly
dependent on the structure of the planar chelating system
acting as acceptor of the donated charge. It can be concluded
that in the ground state, the thermodynamic effects and even
the kinetic trans-effects are mainly due to the charge dona-
tion to the cobalt atom via the s-bond (2). Therefore the
equatorial Schiff base ligands and the strong s-donors like
phosphines in the axial position can affect the stability of
six- or five-coordinated cobalt(III) Schiff base complexes.
The phosphine axial ligand trans-effect on the thermody-
namics of the five-coordinated Co(III) Schiff base complexes
has been well-studied (10, 11), but the cis-effect appears to
be far less evident in this literature. We tried to find the steric
and the electronic effects of some Schiff bases on the thermo-
dynamic parameters of five-coordinated Co(III) Schiff base
complexes. The steric effect of the axial phosphine ligands
and the entering ligands were also studied. The thermody-
namic aspect of the influence of the equatorial Schiff bases’
ligands can be clearly seen from Tables 1, 3, and 4, and
show the trend of Salpd < Salen < 5-nitroSalen, irrespective
of the nature of the axial ligand. These results are in agree-
ment with the trend of nucleophilicity of the cobalt atom in
the Schiff base complexes toward the electrophiles.
The effect of the axial ligand
The steric effect of the axial ligand is more important than
the equatorial (10, 11). The stability constants for the
PMe2Ph axial ligand with lower steric interactions are more
larger than PBu3 (Tables 1 and 2). The difference between
the nucleophilicity of the phosphites in the [Co(Salen)-
(PMe2Ph)]+ is better seen because of the decrease in steric
interactions (Table 2).
Effect of the donors
In this work, we have examined three donors, P(OMe)3,
P(OEt)3, and P(O-i-Pr)3. The phosphite complexes are often
similar to the phosphines but the phosphites tend to be more
basic and less sterically hindered (16).
In addition to the electronic property, the phosphites have
an important steric factor that is demonstrated by the
Tolmane cone angle. The cone angles (degrees) for these
phosphites are 107, 109, and 130°, for P(OMe)3, P(OEt)3,
and P(O-i-Pr)3, respectively (17). The steric interaction is in-
creased with an increase in cone angle.
In the Salen and 5-nitroSalen complexes, the stability con-
stants were increased by increasing Tolmane cone angles of
the phosphites, which is in contrast to what is expected;,
therefore, it seems that the electronic effect of the phosphites
is more important than the steric effect toward the given
acceptor (Tables 1–3), and there is an increasing tendency for the
The electronic effect of the equatorial Schiff base ligands
The electron-withdrawing property of the substituent
groups on the Schiff base can influence the ability of the ac-
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
adduct formation in the order P(OMe)3 < P(Oet)3 < P(O-i-Pr)3.
By increasing the steric interactions in the Salpd complex,
the stability constant is decreased when the Tolmane cone
angle increased (Table 4) and this order changes to
P(OMe)3 < P(O-i-Pr)3 < P(OEt)3.
The lower stability constant of the trimethylphosphite may
be attributed to a “keto–enol” type of rearrangement to pro-
duce a phosphoryl linkage (18), but the triethylphosphite do
not show this property (19).
dicates that the first factor is more important (Tables 5–8).
For example, the entropy changes of P(OMe)3 toward the
acceptors are related to the stability constants for these sys-
tems and with increasing complex formation, the entropy
change is decreased. The more negative entropy change for
P(OMe)3 in Table 6 is related to the higher stability constant
for this system. The entropy change for triisopropylph-
osphite is governed by the second factor i.e., the libration of
the solvent molecules, and it is less negative and is some-
times positive, although the stability constants are larger
than the others. For example, the entropy change for
[Co(Salen)(PMe2Ph)]+ complex with P(O-i-Pr)3 is positive
(Table 6), although the complex formation is the highest.
Thermodynamic parameters
The heat of formation
The DH° value and its sign, is dependent on two factors:
one of these factors, is the solvation effect and the other one
is the heat of formation of the complex (20). The solvation
effect for five- and six-coordinated complexes is not that dif-
ferent due to the same charge, although it seems that the
five-coordinated complex is better solvated. The solvation
effect for the donors is related to the interaction of solvent
with the phosphites and this contribution for DH° can be
positive after the equilibrium. In all cases, DH° values are
negative, and it shows that the complex-formation contribu-
tion of DH° values is more important (Tables 5–8). For ex-
ample, the heat of formations of P(OMe)3 toward the
acceptors are related to the stability constants for these sys-
tems (Tables 5–8).
Concerning the electronic effect contribution, the DH° val-
ues for [Co(5-nitroSalen)(PBu3)]+ are more negative than the
[Co(Salen)(PBu3)]+, and it shows that the complex formation
in 5-nitroSalen is more important (Tables 5 and 8).
Concerning the steric effect of the equatorial Schiff base
ligand on heat of formation, the DH° values for [Co(Salpd)-
(PBu3)]+ are nearly the same as [Co(Salen)(PBu3)]+. It can
be due to the second factor, and there is some positive value
from the solvation effect for [Co(Salen)(PBu3)]+, especially
with P(OEt)3 and P(O-i-Pr)3 (Tables 5 and 7). The steric ef-
fect of the axial ligand is important and the enthalpy change
values show this effect (Tables 5 and 6).
Conclusion
In this work, efforts were made to provide a quantitative
measure of Schiff base effect on the acceptor property of
Co(III) in some cobalt(III) Schiff base complexes, such as
[Co(Salen)(PBu3)]+, [Co(Salen)(PMe2ph)]+, [Co(5-nitroSalen)-
(PBu3)]+, and [Co(Salpd)(PBu3)]+, through adduct formation
with P(OMe)3, P(OEt)3, and P(O-i-Pr)3 in acetonitrile. The sta-
bility constants and the thermodynamic parameters obtained
have showed the following trend of acceptor property of
theSchiff-base complexes: 5-nitroSalen > Salen > Salpd.
Acknowledgements
We are grateful to Shiraz University Research Council for their
financial support. Thanks are also given to B. Hemattinejhad for
writing the modified computer program.
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