Spectoscopic and structural studies on charge-transfer complexes of lanthanum(III)acetylacetonate with σ-acceptor iodine and π-acceptor DDQ

Article abstract of DOI:10.1016/j.molstruc.2011.03.035

The interaction of the donor La(III)acetylacetonate, [La(acac) ₃], with iodine as a σ-acceptor and with 2,3-dichloro-5,6 dicyano-1,4-benzoquinone, (DDQ) as a π-acceptor have been studied in the solvents CH₂Cl₂, CHCl₃ and CCl₄ at room temperature. The obtained results indicate the formation of 1:1 charge-transfer complexes. The electronic and infrared absorptions and elemental analysis of the formed complexes indicate that the complexes are formulated as the triiodide [(La(acac)₃)₂I]⁺·I3- and [La(acac)₃(DDQ)]. Far-infrared spectrum shows that the triiodide ion is nonlinear with C_2v symmetry. The values of the equilibrium constants (K) and obsorptivities (ε) for both the formed CT-complexes are calculated and discussed in term of reaction stoichiometry values and molecular steric hindrance. Mid infrared spectra suggest that the electron donation from [La(acac)₃] to the donors I₂ or DDQ could mainly take place through the oxygen atoms in addition to the acac ring π-molecular orbitals.

Full text of DOI:10.1016/j.molstruc.2011.03.035

Journal of Molecular Structure 994 (2011) 289–294
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectoscopic and structural studies on charge-transfer complexes of
lanthanum(III)acetylacetonate with
r-acceptor iodine and p-acceptor DDQ
b
E.M. Nour a, , M.S. Refat
⇑
a Department of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar University, P.O. Box 2713, Doha, Qatar
b Department of Chemistry, Faculty of Science, Suey Canal University, Port Said 42111, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
The interaction of the donor La(III)acetylacetonate, [La(acac)3], with iodine as a
r-acceptor and with 2,3-
Received 13 January 2011
Received in revised form 13 March 2011
Accepted 15 March 2011
dichloro-5,6 dicyano-1,4-benzoquinone, (DDQ) as a -acceptor have been studied in the solvents CH2Cl2,
p
CHCl3 and CCl4 at room temperature. The obtained results indicate the formation of 1:1 charge-transfer
complexes. The electronic and infrared absorptions and elemental analysis of the formed complexes
indicate that the complexes are formulated as the triiodide [(La(acac)3)2I]+ÁI3À and [La(acac)3(DDQ)].
Far-infrared spectrum shows that the triiodide ion is nonlinear with C2v symmetry. The values of the
equilibrium constants (K) and obsorptivities (e) for both the formed CT-complexes are calculated and
discussed in term of reaction stoichiometry values and molecular steric hindrance. Mid infrared spectra
Available online 21 March 2011
Keywords:
[La(acac)3]
Iodine
suggest that the electron donation from [La(acac)3] to the donors I2 or DDQ could mainly take place
DDQ
through the oxygen atoms in addition to the acac ring
p
-molecular orbitals.
Ó 2011 Elsevier B.V. All rights reserved.
Triiodide
Charge-transfer
Spectra
1. Introduction
To continue or investigation in this research area [2–13], we re-
port here theÀ formation of the two new CT-complexes
[(La(acac)3)2I]+ÁI3 and [La(acac)3(DDQ) formed in the reaction of
[La(acac)3] with iodine and DDQ, respectively. The aim of the work
is to determine the reaction stoichiometries, characterize the new
products and make on an assessment of the correct nature of bond-
ing and structure inherent in each of them.
The study of the properties of charge-transfer complexes
formed in the reaction of electron acceptors and many compounds
containing nitrogen, sulfur and oxygen donor atoms have attracted
considerable attention and growing importance [1–14]. This is
owing to the important role of charge transfer processes play in
biological systems as well as significant physical properties of
CT-products such as electrical conductivities and the use of some
in many forms of electronic and solar cells and in the analysis of
some drugs and pharmaceutical preparations [15–21].
2. Experimental
All chemicals used in this study were of high grade, iodine was
obtained from BDH while 2,3-dichloro-5,6-dicyano-1,4-benzoqui-
none (DDQ) was obtained from Aldrich Chemical Co., while the
lanthanum(III)acetylacetonate, [La(acac)3]Á6H2O was prepared
according to the known method [28] as follows. Lanthanum(III)
oxide (1.5 g) was dissolved in the least amount of dilute hydrochlo-
ric acid solution, and the excess acid was removed by evaporation.
The resulting chloride solution was dissolved in 100 mL volume of
1:1 water-purified dioxin solution containing a slight excess of a
acetylacetone. The pH value of the vigorously stirred solution
was adjusted at 7.8 by the dropwise addition of aqueous ammonia
to just below those of hydroxide precipitation and the solution was
stirred for about 20 h. The crystalline product was filtered off,
washed with successive portions of ethanol and dried over P2O5
under vacuo. The solid [(La(acac)3)2I]+ÁI3À complex was isolated as
a dark brown solid by mixing a saturated solution (20 mL) of
[La(acac)3] to a saturated solution (50 mL) of iodine in CH2Cl2.
Metal acetylacetonates [M(acac)n=2, 3 or 4] show
calization in their rings and, therefore, show ability to form molec-
ular complexes with -donors like iodine [22–25].
Such an ability of complexation has been taken as strong evi-
dence for such -delocalization. It has been proposed [24] that
the formation of these complexes are similar to those formed by
aromatic hydrocarbons with -acceptors like iodine as well as
-acceptors like DDQ. However, in these studies on other related
systems, the formation of the triiodide complex with its well
known characteristic absorptions [2,11,26–28] around 360 and
290 nm were ignored or wrongly assigned either to the blue-shift
of I2 band or to CT band of [M(acac)n]ÁI2 complex [24,25].
p-electron delo-
r
p
r
p
⇑
Corresponding author. Fax: +974 44034659; cell: +974 55716556.
E-mail address: enour@qu.edu.qa (E.M. Nour).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.03.035

290
E.M. Nour, M.S. Refat / Journal of Molecular Structure 994 (2011) 289–294
The mixture was stirred for 10 min, at room temperature and the
precipitate was filtered off, washed with minimum amounts of
CH2Cl2 and dired under vacuo over P2O5. The 1:1 dark green
charge-transfer complex [La(acac)3(DDQ)] was prepared by mixing
a saturated solution (10 mL) of [La(acac)3] with a saturated solu-
tion (50 mL) of DDQ in CH2Cl2. The reaction mixture was stirred
at room temperature for about 45 min. The precipitate was filtered
off and washed several times with the minimum amounts of
CH2Cl2 and dried over P2O5 under vacuo. The new prepared solid
complexes were characterized through their vibrational and elec-
tronic absorption spectra as well aselemental analysis (calculated
values are shown in brackets). Analysis; [(La(acac)3)2I]+ÁI3ÀÁ12H2O,
(M/W: 1596.4 g); C, 21.87 (22.57); H, 4.62 (4.17); I, 31.11 (31.79)
and La, 17.12% (17.40%). [La(acac)3(DDQ)]Á6H2O, (M/W: 770.9 g);
C, 34.92 (35.80); H, 4.34 (4.28); N, 3.50 (3.63); Cl, 8.76 (9.21) and
La, 18.10% (18.03%). Elemental analysis (CHN) were performed by
the microanalytical unit, Cairo University while La contents were
determined using atomic absorption spectrometer model Pye-
Unicam SP 1900. The content oÀf the water of crystallization in both
CT-complexes [(La(acac)3)2I]+ÁI3 Á12H2O and [La(acac)3(DDQ)]Á6H2O
were determined by gravimetric means. Each complex sample was
heated in an open air crucible at 155–160 °C for a time to reach in
each case a constant weight. The heating was controlledÀto a max-
imum of 160 °C to avoid any iodine losses since the I+ÁI3 unit in a
number of CT-complexes undergoes thermal decomposition at a
temperature range of 180–200 °C [13,29]. The complete loss of
water molecules were checked from IR-spectra of the complex
samples in Nujol mulls in order to prevent possible water contam-
ination from air or from KBr. The obtained data agree quite well
with the proposed formula; H2O, 13.14% (13.54%) for the complex
[(La(acac)3)2I]+ÁI3ÀÁ12H2O and 14.36% (14.01%) for the complex
[La(acac)3(DDQ)]Á6H2O (calculated values of H2O are shown in
brackets).
CH2Cl2. The spectrum of 1:1 [La(acac)3]–iodine mixture of a con-
centration of 5 Â 10À5 mol LÀ1 of each reactant in CH2Cl2 is shown
in Fig. 1.
The spectrum of the formed complex shows real absorptions at
362 and 290 nm which are not present in spectra of both free reac-
tants, iodine and [La(acac)3]. Photometric titration curves for the
reaction based on the two characteristic absorptions for the new
iodine complex at 362 and 290 nm in CH2Cl2 are shown in Fig. 2.
The curves of the two absorptions show the same point of inflec-
tions clearly indicate two facts that the two absorptions belong
to one species and that the stoichiometry of [La(acac)3]–iodine
reaction is 1:1 This stoichiometry of the reaction in the solvents
CHCl3, CH2Cl2 and CCl4 is shown to be the same, 1:1. The formation
of this 1:1 complex is supported by elemental analysis data. Taking
these facts into consideration beside that the two absorptions of
the formed iodine complex around 362 and 290 nm are well
know [9,14,27] to be characteristic for the formation of triiodide
ion, IÀ3 , we conclude that the formed iodine complex is formulated
as [(La(acac)3)2I]+ÁI3À. These two absorptions around 360 and
290 nm were wrongly assigned in many other related iodine
[M(acac)n=2,
4] systems [24] where the 290 nm band was as-
3
or
signed to the blue shifted I2 band and the other band around 360
is attributed to the intermolecular charge transfer upon complexa-
tion. Here, it was wrongly assumed [24] the formation of simple
complex-iodine type, [La(acac)3]–I2 rather than our findÀings of
the formation of the triiodide product, [(La(acac)3)2I]+ÁI3 . It is
known that the 510 nm band of iodine in CH2Cl2 is assigned to
the electronic transition of the type 4p
–10rÃ. The split of this io-
dine band with blue shifts upon complexation with [La(acac)3] re-
sulted in the two observed bands at 362 and 290 nm upon the
formation of the triiodide ion, IÀ3 Such blue shifts of the iodine
.
band upon complexation is obviously due to the perturbation of
the 10rà molecular orbital of iodine and should be assigned to
The electronic absorption spectra of the donor [La(acac)3] and
the acceptors iodine and DDQ and the formed CT-complexes were
recorded in CH2Cl2 in the region 700–200 nm using a Perkin–Elmer
double beam spectrometer model EZ-210 fitted with a quartz cell
of 1.0 cm path length.
In order to determine the stoichiometry of the formed CT-
complexes, photometric titrations were performed in each of the
solvents CH2Cl2, CHCl3 and CCl4 for the reactions of the [La(acac)3]
with the each of iodine and DDQ at 25 °C according to the known
method [29,30]. These concentration of [La(acac)3] was kept fixed
at 5 Â 10À5 and 1 Â 10À4 mol LÀ1 in the case of the reactions with
iodine and DDQ, respectively. The iodine concentration wÀas1
changed over the range from 1.25 Â 10À5 to 15 Â 10À5 mol L
while that of DDQ was changed over the range from 0.25 Â 10À4
to 3.0 Â 10À4 mol LÀ1. In these reactions, the donor:acceptor ratio
varying from 1:0.25 to 1:3. The photometric titration measure-
ments were based on the characteristic absorptions at 362 and
290 nm for the formed [La(acac)3]–iodine complex and at 407
and 309 nm for the formed [La(acac)3]–DDQ complex. Infrared
spectra in the region 4000–500 cmÀ1 for the free reactants and
the formed CT-complexes were recorded from KBr disks using a
Niclolet FT-IR spectrometer model 670 while the far-infrared spec-
trum (200–50 cmÀ1) for the solid iodine complex was recorded
from a Nujol mull dispressed on polyethylene windows using a
Nicolet FT-IR spectrometer model 760.
3. Results and discussion
3.1. Reaction of La(acac)3 with iodine
Fig. 1. Electronic absorption spectra of (a) [I2] = 5 Â 10À5 mol LÀ1
,
(b)
[La(acac)3] = 5 Â 10À5 mol LÀ1
,
(c) I2–[La(acac)3] mixture [I2] = [La(acac)3] =
The electronic absorption spectra of the [La(acac)3]–iodine com-
plex were measured in various solvents such as CCl4, CHCl3 and
5 Â 10À5 mol LÀ1
.

E.M. Nour, M.S. Refat / Journal of Molecular Structure 994 (2011) 289–294
291
bond vibration wavenumber value. This conclusion is in agreement
with the known values [32] of the I-I stretching force constants of
0.90 and 1.72 mdyn/°A for the, IÀ3 and free I2, respectively. We
should indicate here that the assumption of Kulevsky and Butam-
ina [24] of the formation of [M(acac)n]ÁI2 iodine-complex should
display only one infrared band related to m(I–I) and this is expected
to occur at higher value than those observed for the triiodide in
[La(acac)3]2I+ÁI3À at 138 and 104 cmÀ1. The
m(I–I) in the iodine com-
plex [9] [(TTCTD)]ÁI2 occurs at 154 cmÀ1; (TTCTD), 1,4,8,11-tetra-
thiacyclotetradecane. The higher value of iodine vibration in this
complex is due to that the charge transfer from the donor (TTCTD)
to I2 is relatively low and not enough to form the iodide and hence
the triiodie ion. This conclusion is supported by the fact that when
substituting the 4S atom in (TTCTD) with stronger donor atoms like
nitrogen leads to higher charge transfer value to iodine enough to
form the triiodide ion [11] as in [(TACTD)]I+ÁI3À; (TACTD), 1,4,8,11-
tetraazacylotetradecane As mentioned before, the donor donates
to the rÃ-molecular orbital of iodine lowering its force constant va-
lue and then its bond vibration. Accordingly, we conclude here that
the magnitude of the decrease of iodine bond vibration upon com-
plexation compared with the vibration of free iodine [31] at
180 cmÀ1 is a real measure of the charge transfer quantity from do-
nor to iodine.
The mid infrared spectra of [La(acac)3] and [(La(acac)3)2I]+ÁIÀ are
3
shown in Fig. 3 and tabulated with their band assignments in
Table 2. The spectrum of [(La(acac)3)2I]+ÁI3À shows almost the same
characteristic bans for the free donor [La(acac)3] but with some
small changes of band positions and intensities. This is due to
the complexation of the donor with iodine where changes in both
electronic structure and symmetry of [La(acac)3] are expected.
However, it seems in our system that these changes are very lim-
ited and in particular for the
t(C@C) wavenumber values of the
acac ring before and after complexation. This may infer that the
donation from the acac comes mainly from the lone pair of its oxy-
gens to the rÃ-molecular orbital of iodine in addition to from the
pà of the acac ring. Donation from pà orbitals is expected to
strengthen the C@C bond and hence increase of its bond viÀbration
which is not over obvious in the case in [(La(acac)3)2I]+ÁI3 spec-
trum. To clarify this point, vibrational normal coordinate analysis
is needed along with bond force constant calculations for acac be-
fore and after complexation in the light that electronic and symme-
try changes are expected between the two cases.
Fig. 2. Photometric titration curves for La(acac)3–I2 mixture.
allowed transitions in the newly formed triiodide ion. The wrong
assumption [24] in related systems with different metals of the
formation of I2 complex, [M[acac)n]ÁI2 rather than IÀ complex lead
3
also to wrong assignments of the observed low wavenumber vibra-
tions of iodine species. The formation of IÀ3 ion in our complex un-
der study was further supported by its characteristic far-infrared
absorptions, Table 1. The triiodide shows three infrared absorption
band at 138, 104 and 76 cmÀ1 assigned to
mas(I–I),
m
s(I–I) and d(IÀ),
3
respectively. Group theoretical analysis indicate that the IÀ3 ion in
[La(acac)3]2I+ÁI3À is nonlinear with C2v symmetry, where all of the
three vibrations should be infrared active. It is of interest to indi-
cate here that the m
(I–I) bond vibrations of IÀ3 ion are moved to low-
er wavenumber values at 138 and 104 cmÀ1 (Table 1) upon
complexation compared with that known [31] of free iodine at
180 cmÀ1. Such a wavenumber decrease may be understood as a
result of the electron donation from [La(acac)3] to rÃ-antibonding
orbital of iodine lowering the iodine bond strength and hence its
Table 1
Far infrared spectra for IÀ3 ion.
Compounda
Assignmentsb
as(I–I)
Reference
m
m
s(I–I)
d(IÀ3
)
KI3
149
132
132
138
103
109
101
104
69
60
84
76
[35]
[11]
[13]
Present work
[(TACPD)I]+ÁIÀ3
[Ni(acac)2]2I+ÁI3À
[(La(acac)3)2I]+ÁI3À
a
(TACPD), tetraazacyclopentadecane.
b
Fig. 3. Infrared spectra of (a) [La (acac)3]Á6H2O and (b) [(La(acac)3)2I]+ÁIÀÁ12H2O.
m
, stretching; d, bending.
3

292
E.M. Nour, M.S. Refat / Journal of Molecular Structure 994 (2011) 289–294
Table 2
Infrared wavenumbersa (cmÀ1
) and tentative assignments for La(acac)3 and
[(La(acac)3)2I]+ÁIÀÁ12H2O.
3
[La(acac)3]
[(La(acac)3)2I]+ÁI3À
Assignmentsb
3299 br
3063 sh
2995 w
2919 w
3381 br
3187 sh
3000 sh
2906 vw
2843 w
1640 sh
1577 s
1517 s
1464 s
1374 s
1262 s
1050 w
1021 vs
845 s
m
m
(OAH); H2O
(CAH)
1589 s
1520 s
1458 w
1386 s
d(H2O)
m
m
d(CH)
d(CH3)
m
m
(C@O)
(C@C)
1257 s
1190 w
1014 vs
778 w
757 vw
649 vs
609 w
527 vs
453 w
–
(CAO),
(CAC)
d(CH); out of plane bend
d(CH3); rock
749 s
675 s
608 w
531 w
–
m
(LaAO)
418
a
br, broad; m, medium; s, strong; sh, shoulder; v, very; w, weak.
b
m
, streching; d, bending.
A General mechanism describing the formation of the triiodide
complex is proposed as follows:
(1) Formation of the outer complex; 2 [La(acac)3] + I2 ? [(La
(acac)3)2]ÁI2
Fig. 4. Electronic absorption spectra of (a) [La(acac)3] = 1 Â 10À4 mol LÀ1
, (b)
[DDQ] = 1 Â 10À4 mol LÀ1, (c) [La(acac)3]–DDQ mixture; [La(acac)3] = [DDQ] = 1 Â
10À4 mol LÀ1
.
(2) Formation of the inner-complex; [(La(acac)3)2]ÁI2 ?
[(La(acac)3)2ÁI]+ÁIÀ
(3) Triiodide
formation;
[(La(acac)3)2
I]+ÁIÀ + I2 ?
[(La(acac)3)2I]+ÁI3À
The formation of the [(La(acac)3)2I]+ÁI3À reaction intermediate is
analogous to those well known species formed in this reaction of
iodine with many various donors [2,11,33]. It is of interest to see
from the 1:1 [La(acac)3]ÁI2 reaction ratio with the formation of tri-
iodide ion product [(La(acac)3)2I]+ÁI3À that the 6 acac rings from the
two [La(acac)3] unit, all together accommodate the positive iodine
ion, I+. In this case, it is expected that I+ ion is centered somehow
between the two [La(acac)3] units and ionically bonded to theÁIÀ
3
ion.
3.2. Reaction of La (acac)3 with DDQ
Fig. 4 shows the electronic absorption spectra of 1 Â 10À4
mol LÀ1 solutions of [La(acac)3], DDQ and their [La (acac)3]–DDQ
mixture in CH2Cl2. New absorption bands which do not belong to
any of the free reactants are observed in the spectrum of the reac-
tion mixture at 407 and 309 nm and assigned to the CT-transition
in the formed [La(acac)3]–DDQ complex. The stochiometry of the
reaction in the solvents CHCl3, CH2Cl2 and CCl4 is shown to be
the same 1:1 [La (acac)3] to DDQ. This is based on the photometric
titration curve shown in Fig. 5. Accordingly, the formed CT-
complex is formulated as [La (acac)3(DDQ)]. This conclusion was
supported by the elemental analysis data of the solid complex.
The infrared spectra of the free reactants [La(acac)3], DDQ and
the CT-complex [La(acac)3(DDQ)] are shown in Fig. 6 and tabulated
and assigned in Table 3. The spectrum of the CT-complex shows
the main characteristic bands for both [La(acac)3] and DDQ with
as expected, small changes in band wavenumber values and inten-
sities. For example the
curs as strong band at 2216 corresponds to 2201 and
2190 cmÀ1 for the free DDQ while the
(C@O) occurs at 1768
m(CN) vibration of the complexed DDQ oc-
a
m
Fig. 5. Photometric titration curves for [La(acac)3]–DDQ mixture.

E.M. Nour, M.S. Refat / Journal of Molecular Structure 994 (2011) 289–294
293
It was of interest to calculate the values of both the formation
, for each of the formed
constant, K, and the molarÀabsorptivity,
e
complexes [(La(acac)3)2I]+ÁI3 and [La(acac)3–(DDQ)]. The 1:1 mod-
ified Benesi–Hildebrand equation [34] was used in the calculations.
Figs. 7 and 8 show these plots. The obtained data for the reactions
in CHCl2 are given in Table 4. The data reveals that both complexes
[(La(acac)3)2I]+ÁI3À and [La(acac)3(DDQ)] show high values of both
the K and e. The high values of K reflect the high stability of
the formed CT-complex as a result of the expected high electron
Fig. 6. Infrared spectra of (a) [La(acac)3]Á6H2O, (b) DDQ, (c) [La(acac)3(DDQ)]Á6H2O.
and 1760 cmÀ1 for the complexed and free DDQ, respectively. The
decrease of both CN and C@O vibrations of DDQ upon complexa-
tion could be arise from donation from the acac rings to the pÃ
-
molecular orbitals of DDQ lowering its bond order value and hence
its bond vibrations. Again to clarify this conclusion, vibrational
analysis and bond force constant calculations are needed. The ob-
served small changes between the spectrum of the CT-complex
and those of the reactants could also be resulted from the expected
changes in both symmetry and electronic structures of both reac-
tants upon complexation.
Table 3
Infrared wavenumbersa (cmÀ1) and tentative assignments for [La(acac)3], DDQ and
[La(acac)3(DDQ)].
Fig. 7. The modified Benesi–Hildebrand plot for [La(acac)3]–iodine reaction based
on the absorption at k = 290 nm.
La(acac)3
DDQ
[La(acac)–(DDQ)]
Assignmentsb
3299 br
3063 sh
2995 w
2919 w
3430 m
–
–
–
3358 s, br
3187 w
3000 w
2906 w
2812 w
2216 vs
m
m
(OAH); H2O
(CAH)
–
–
2201 m
2190 w
1760 vs
1634 ms
1592 w
–
m
m
m
m
as(CN); DDQ
s(CN); DDQ
(C@O); DDQ
(C@C); La(acac)3 and DDQ
1768 ms
1625 w
1594 w
1566 w
1442 vs
1368 w
1260 ms
1195 vs
1083 s
–
1589 s
1520 s
1458 w
1386 s
1257 s
1190 w
–
d(CH3)
(CAC); DDQ and La(acac)3
1374 m
1187 s
–
m
d(CH3); La(acac)3
1041 s
1014 vs
1000 w
992 w
910 vs
CH deformation; La(acac)3
917 vs
859 w
778 w
757 vw
645 vs
609 w
527 vs
748 m
685 ms
625 m
752 w
703 w
670 ms
593 w
543 ms
m
(CACl); DDQ and CH in plane
Bend
m
(CACl); DDQ
CH out of plane bend
a
br, broad; m, medium; s, strong; v, very; w, weak.
Fig. 8. The modified Benese–Hildebrand plot for [La(acac)3]–DDQ reaction based on
the absorption at 407 nm.
b
m
, streching; d, bending.

294
E.M. Nour, M.S. Refat / Journal of Molecular Structure 994 (2011) 289–294
Table 4
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3
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Complex
K (l molÀ1
)
k max (nm)
e
(l molÀ1 cmÀ1
)
[(La(acac)3)2I]+ÁI3À
7.32 Â 104
1.85 Â 104
362
290
407
0.62 Â 104
1.15 Â 104
0.341 Â 104
[La(acac)3(DDQ)]
donation from the [La(acac)3] while the high value of
well with the formation of CT-complexes which are known to have
high absorpativity values [11]. The values of K and for both com-
plexes show variation as the solvent is changed from CH2Cl2 to
CHCl3 or CCl4, but no clear relationship with solvent properties
can be obtained. The value of K for [(La(acac)3)2I]+ÁI3À is about four
times higher than that of [La(acac)3(DDQ)]. It might be difficult to
discuss such a variation in the light that the iodine is a
while DDQ is a -acceptor. The molecular sizes of both acceptors
e
agree quite
e
r-acceptor
p
are also different reflecting a higher steric hindrance in case of
DDQ and hence, an expected decrease in its formation constant va-
lue compared with that of iodine.
In conclusion, The formation of [(La(acac)3)2I]+ÁI3À is analogouÀs
to a number of other complexes [2,11] such as [(TACTD)] I+ÁI3
and [((TACTDD)]I+ÁI3À; (TACTD), 1,4,8,11-tetraazacyclotetradecane
and (TACTDD). 1,4,8,11-tetraazacyclotetradecane-5,7-dione. The
[La(acac)3]: I2 ratio is 1:1. Here, two donor molecules of [La
(acac)3] are required for the formation and stabilization of the
I+ÁI3 unit. This might indicate that [La(acac)3] acts as a relatively
weaker donar compared with other donors such as amines,
TACTD and TACTDD where one donor molecule is able to stabi-
lize the trioidide product. On the other hand, [La(acac)3] reacts
with the donor DDQ with the same stoichiometry of 1:1 forming
the CT-complex [La(acac)3(DDQ)]. Here, the steric hindrance
could play an important role in decreasing the complex stability
compared with that of iodine complex. This is evident from that
the formation constant of the iodine complex [(La(acac)3)2I]+ÁI3À is
almost four times higher than that of the corresponding complex
[La(acac)3(DDQ)].
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