Kinetics and mechanism of ligand substitution in β-diketone complexes of iron(III). Solvolysis controlling the substitution process in alcohol media

Article abstract of DOI:10.1016/S0020-1693(02)01143-X

Conventional and stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in a number of tris β-diketone iron(III) complexes, Fe(O^∩O)₃, by 8-hydroxyquinoline (=HO^∩N) in alcohol media (O^∩O^-=anion of the β-diketones pentane-2,4-dione, 2,6-dimethylheptane-3,5-dione, 2,2,6,6-tetramethylheptane-3,5-dione, 1-phenylbutane-1,3-dione, 1,3-diphenylpropane-2,3-dione, and 1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione). As shown by spectrophotometry, the solutions of complexes Fe(O^∩O)₃ in alcohols ROH are subject to solvolytic dissociation, leading to solvento species Fe(O^∩O)₂S₂ and to binuclear complexes [Fe(O^∩O)₂(RO)]₂ (S=ROH and RO^-, respectively). The reaction of complexes Fe(O^∩O)₃ with HO^∩N in alcohol media, leading to Fe(O^∩N)₃, is triphasic. The corresponding first-order rate constants k₁, k₂, and k₃ are independent of the concentration of the entering ligand HO^∩N and follow the order k₁>k₂>k₃, with k₁/k₂≈10 and k₁/k₃≈10². For a given system Fe(O^∩O)₃/HO^∩N/ROH, the size of k₁, k₂, and k₃ correlates with the solvent polarity parameter E_T(30). Rate constant k₁ describes the solvolytic dissociation of the complexes Fe(O^∩O)₃ and rate constant k₃ the solvent-initiated splitting of the binuclear complexes [Fe(O^∩O)₂(RO)]₂. Rate constant k₂ is assigned to the solvolytic dissociation of the intermediate complex Fe(O^∩O)₂(O^∩N). Depending on the nature of the coordinated β-diketone and solvent ROH, k₁ ranges from 0.04 to 2 s^-1, k₂ from 0.007 to 0.2 s^-1, and k₃ from 0.002 to 0.01 s^-1 at 298 K. The mechanism of the ligand substitution processes is discussed.

Full text of DOI:10.1016/S0020-1693(02)01143-X

Inorganica Chimica Acta 342 (2003) 97Á106
/
www.elsevier.com/locate/ica
Kinetics and mechanism of ligand substitution
in b-diketone complexes of iron(III).
Solvolysis controlling the substitution process in alcohol media
Gerhard Gumbel, Horst Elias ꢀ
Institut fu¨r Anorganische Chemie, Technische Universita¨t Darmstadt, Petersenstrasse 18, D-64287 Darmstadt, Germany
Received 4 February 2002; accepted 24 May 2002
Abstract
Conventional and stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in a number of tris b-
diketone iron(III) complexes, Fe(OSO)3, by 8-hydroxyquinoline (ꢁHOSN) in alcohol media (OSOꢂ ꢁ
anion of the b-diketones
/
/
pentane-2,4-dione, 2,6-dimethylheptane-3,5-dione, 2,2,6,6-tetramethylheptane-3,5-dione, 1-phenylbutane-1,3-dione, 1,3-diphenyl-
propane-2,3-dione, and 1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione). As shown by spectrophotometry, the solutions of complexes
Fe(OSO)3 in alcohols ROH are subject to solvolytic dissociation, leading to solvento species Fe(OSO)2S2 and to binuclear
complexes [Fe(OSO)2(RO)]2 (SꢁROH and ROꢂ, respectively). The reaction of complexes Fe(OSO)3 with HOSN in alcohol
/
media, leading to Fe(OSN)3, is triphasic. The corresponding first-order rate constants k1, k2, and k3 are independent of the
102. For a given system
concentration of the entering ligand HOSN and follow the order k1ꢀ
/
k2ꢀ
/
k3, with k1/k2:
/
10 and k1/k3:
/
Fe(OSO)3/HOSN/ROH, the size of k1, k2, and k3 correlates with the solvent polarity parameter ET(30). Rate constant k1 describes
the solvolytic dissociation of the complexes Fe(OSO)3 and rate constant k3 the solvent-initiated splitting of the binuclear complexes
[Fe(OSO)2(RO)]2. Rate constant k2 is assigned to the solvolytic dissociation of the intermediate complex Fe(OSO)2(OSN).
Depending on the nature of the coordinated b-diketone and solvent ROH, k1 ranges from 0.04 to 2 sꢂ1, k2 from 0.007 to 0.2 sꢂ1
and k3 from 0.002 to 0.01 sꢂ1 at 298 K. The mechanism of the ligand substitution processes is discussed.
# 2002 Elsevier Science B.V. All rights reserved.
,
Keywords: Iron(III) complexes; b-diketones; Ligand substitution; Kinetics; Mechanism
1. Introduction
structural properties of these complexes has therefore
been studied intensively [2Á4].
/
One of the most conspicuous features of ferric ion in
aqueous solution is its tendency to hydrolyze [1]. In the
course of hydroysis doubly bridged m-hydroxo and/or m-
oxo complexes such as 1 and 2 (see Chart 1) are formed
that can be structurally described as two edge sharing
octahedra. In alcohol media, alcoholysis of ferric ion
leads to the corresponding m-alkoxo complexes 3.
In earlier studies [5] we investigated the kinetics and
mechanism of ligand substitution in neutral bis chelate
complexes MA2 in organic media according to Eq. (1)
MA2 ꢃ2HBXMB2 ꢃ2HA
(Mꢁdivalent metal; HA, HBꢁ
(1)
/
/
N-alkylsalicylaldi-
mines). These studies showed that the rate of substitu-
tion follows a two-term rate law, rateꢁ(ksꢃkHB
[HB])ꢄ[MA2], composed of a ‘solvent path’, ksꢄ
[MA2], and a ‘ligand path’, kHB [HB]ꢄ[MA2]. In
alcohol media, the solvent-initiated rate contribution,
ksꢄ[MA2], dominates for MꢁCu(II) and Ni(II) [5a].
/
/
ꢄ
/
Binuclear iron(III) complexes such as 1Á3 are receiving
/
/
/
considerable attention because of their possible rele-
vance to the understanding of certain aspects of the
biochemistry of iron. The correlation of magnetic and
ꢄ
/
/
/
/
The size of rate constant ks depends characteristically on
the acidity of the alcohol ROH applied as solvent
ꢀ Corresponding author. Tel.: ꢃ
6040
E-mail address: elias@tu-darmstadt.de (H. Elias).
/
49-6151-16 2425; fax: ꢃ49-6151-16
/
[5a,5c,5f,5g] and for acetonitrileÁ
/
alcohol solvent mix-
tures it was shown that the rate of substitution is first-
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 4 3 - X

98
G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á106
/
Chart 1. Binuclear m-hydroxo, m-oxo and m-alkoxo complexes of
iron(III) (LꢁH2O, monodentate or bidentate ligand).
/
order in alcohol, according to rateꢁ
/kROH
ꢄ
/
[ROH]ꢄ
/
[MA2] [5a]. Furthermore, the appearance of mass law
retardation suggested that, in alcohol media such as
methanol, solvento species CuA(CH3O)(CH3OH) are
formed as intermediates [5d].
The active role of alcohol solvents in the kinetics of
ligand substitution in four-coordinate bis chelate com-
plexes CuA2 and NiA2 (HAꢁN-alkylsalicylaldimine)
/
led us to extend this sort of study in alcohol media to the
kinetics of substitution in neutral octahedral chelate
complexes of iron(III) and in the corresponding bi-
nuclear iron(III) complexes according to Eq. (2) and (3),
respectively. Complexes Fe(OSO)3 are tris b-diketone
complexes of iron(III) that react with an excess of
Chart 2. b-Diketone ligands HOSOa and iron(III) complexes.
For abbreviations, see Ref. [7].
a
2. Experimental
2.1. Solvents and ligands
The solvents MeOH, EtOH, PrOH, i-PrOH, BuOH,
and t-PeOH, the b-diketones Hacac, Hbza, and Httfa
and the ligand HOSN [6] were commercially available in
reagent grade quality; they were used without further
purification. Commercial HpOH and 3-PeOH were
distilled before use (bp 93 8C/16 mm Hg and bp
115 8C, respectively). Commercial Hdbm (EGA-Che-
mie) was recrystallized from EtOH (mp 78 8C) and
commercial Hdpm (EGA-Chemie) was distilled (bp
58 8C/2 mm Hg). The b-diketone Hdibm was prepared
from 2-methylpropionic acid methyl ester and 3-methyl-
2-butanone as described [9].
Fe(OS O)3 ꢃ3HOS N R0OH Fe(OS N)3 ꢃ3HOS O
(2)
[Fe(OS O)2(RO)]2 ꢃ6HOS N R0OH 2Fe(OS N)3 ꢃ4HOS O
ꢃ2ROH
(3)
8-hydroxyquinoline (ꢁ
/
HOSN) to form the complex
Fe(OSN)3. The b-diketonate complexes of iron(III)
were chosen for two reasons, namely, (i) in addition to
HOSOꢁ
pentane-2,4-dione (acetylacetone), there are
/
numerous derivatives of this b-diketone, differing in
acidity and thus allowing the investigation of a broad
range of complexes Fe(OSO)3, and (ii) in the literature,
detailed information on binuclear b-diketone alcoxo
complexes [Fe(OSO)2(OR)]2 is available [6,4], which
means that this type of complex, anticipated to be
formed in alcohol solutions of complexes Fe(OSO)3,
can be studied separately according to Eq. (3).
The present contribution summarizes the results
obtained in the investigation of the alcoholysis of a
variety of complexes Fe(OSO)3 and [Fe(OSO)2(OR)]2
(see Chart 2, [7]) and the kinetics of ligand substituion in
complexes Fe(OSO)3 and [Fe(OSO)2(OR)]2 according
to Eq. (2) and (3), as studied in a series of alcohol
solvents. The results are discussed mechanistically, with
the difference in reactivity between mono nuclear
complexes Fe(OSO)3 and binuclear complexes
[Fe(OSO)2(OR)]2 being a focus of attention. The kinetic
data are compared with those in the literature [8] on
ligand exchange and substitution in complexes
Fe(OSO)3, as obtained in aprotic organic media.
2.2. Complexes
Based on a corresponding note in the literature [10],
the complexes Fe(OSO)3, listed in Chart 2, and the
complex Fe(OSN)3 were prepared from the ligands
HOSO and HOSN and the iron(III) salt of 2-ethyl-
hexanoic acid, Fe(ehx)3 [7] (Ventron Alfa) according to
Eq. (4) and (5), respectively. The binuclear complexes
[Fe(acac)2(EtO)]2 and [Fe(dpm)2(MeO)]2 (see Chart 2)
were prepared according to Eq. (6).
Fe(ehx)3 ꢃ3HOS O 0 Fe(OS O)3 ꢃ3Hehx
Fe(ehx)3 ꢃ3HOS N 0 Fe(OS N)3 ꢃ3Hehx
(4)
(5)
ð6Þ

G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á
/106
99
Aꢁa1 ꢄ[exp(ꢂkobsd(1) ꢄt)]ꢃa2
The purity of all of the iron complexes listed in Chart
2 and of the complex Fe(OSN)3 was confirmed by
elemental analysis (C,H,N).
ꢄ[exp(ꢂkobsd(2) ꢄt]ꢃAꢁ
Aꢁa1 ꢄ[exp(ꢂkobsd(1) ꢄt)]ꢃa2 ꢄ[exp(ꢂkobsd(2) ꢄt)]
(8)
(9)
ꢃa3[exp(ꢂkobsd(3) ꢄt)]ꢃAꢁ
2.2.1. Procedure for complexes Fe(OSO)3 and
Fe(OSN)3
A solution of 0.01 mol of Fe(ehx)3 in 100 ml of petrol
ether (bp 120Á
/
140 8C) is dropwise added to a warm
solution (70Á80 8C) of 0.03 mol of the corresponding
/
3. Results and discussion
ligand in 60 ml of dry toluene under stirring. Precipita-
tion of the product Fe(OSO)3 and Fe(OSN)3, respec-
tively, begins upon addition of the iron salt and
is completed by cooling of the reaction mixture.
The product is separated by filtration and recrystallized
from toluene, petrol ether or a mixture of both
3.1. UVÁVis absorption of the complexes
/
All of the iron(III) complexes listed in Chart 2
represent high-spin d5 systems for the absorption spectra
of which ligand field bands are not to be expected. The
intensive absorption bands in the wavelength range
solvents (yield: 70Á90%). For final purification the
/
complexes Fe(dpm)3 and Fe(dibm)3 are sublimed in
vacuo.
250Á700 nm, observed for solutions of the complexes
/
Fe(OSO)3 and [Fe(OSO)2(RO)]2 in the non-coordinat-
ing solvent dichloromethane (see Table 1), are based on
2.2.2. Procedure for complexes [Fe(OSO)2(RO)]2
A 0.5 M solution of NaOR in ROH (prepared by
dissolution of the corresponding amount of Na in dry
ROH and calibrated by titration) is added dropwise to a
ligand excitation and charge transfer of both the M0
and M1L type [11,12]. Modification of the alkyl
groups according to the series Fe(acac)30
Fe(dibm)30
Fe(dpm)3 (corresponding to R1ꢁR2ꢁ
/L
/
/
solution of 0.003 mol of Fe(OSO)3 in 30Á
60 ml of ROH
/
/
/
/
under stirring, until the equivalent of 0.003 mol of
NaOR has been added. After stirring for another 30 min
at ambient temperature, the reaction mixture is cooled
for 15 h. The precipitate is separated by filtration and
methyl, i-propyl and t-butyl, respectively; see Chart 2)
has practically no effect on the absorption spectra,
characterized by absorption bands at approximately
270, 350 and 440 nm. The introduction of phenyl
groups, as in Fe(bza)3 and Fe(dbm)3, causes however a
small red shift of these three characteristic bands. The
spectra of the binuclear complexes [Fe(acac)2(EtO)]2
and [Fe(dpm)2(MeO)]2 are practically identical and only
slightly different from those of the mononuclear com-
plexes Fe(acac)3 and Fe(dpm)3. The absorption of the
complex Fe(OSN)3, generated as product in substitu-
tion reactions (2) and (3), is characterized by four bands
washed repeatedly with cold ROH (yield: 60Á
/80%).
2.3. Instrumentation
UVÁ
type 8451A); kinetic measurements: diode array spectro-
photometer for reactions with t1/2 3 min and modified
/Vis spectra: diode array spectrophotometer (HP,
ꢀ
/
[5a] stopped-flow spetrophotometer (Durrum, D110) for
faster reactions.
in the range 300Á600 nm, which allows convenient
/
spectrophotometric monitoring of the substitution pro-
cess.
2.4. Rate constants for ligand substitution
Table 1
The kinetics of ligand substitution according to Eq.
(2) and (3), as monitored spectrophotometrically in
alcohol solution, were studied under pseudo-first-order
UVÁVis absorption data of the various iron complexes in the solvent
/
methylene chloride at 298 K (the data are presented in the form lmax
(nm)/omax (Mꢂ1 cmꢂ1))
conditions ([HOSN]0ꢀ
t) data obtained (Aꢁ
to one of the Eqs. (7)Á
/
10ꢄ
absorbance) were computer-fitted
(9) to obtain the experimental
/
[iron complex]0). The (A,
Fe(acac)3
Fe(dibm)3
Fe(dpm)3
274/25 870
270/25 880
270/25 490
354/3500 438/3480
356/3740 438/3730
358/3850 434/3780
354/3230 422/2390
386/6340 452/3790
/
/
rate constants kobsd. The programs used were based on
the least-squares method. The parameters A0 and Aꢁ
a
[Fe(dpm)2MeO]2 272/16 650
302/39 580
314/49 300
Fe(bza)3
Fe(dbm)3
refer to the absorbance at tꢁ
/
0 and tꢁꢁ, respectively,
/
408/
471/4070
(sh)
14530
388/
22730
and the amplitudes a1, a2 and a3 describe the relative
contributions of the single exponentials to the overall
absorbance.
Fe(ttfa)3
338/48 200
434/7400
(sh)
Fe(OSN)3
314/5090 (sh) 368/4570 466/5450
582/
3550
Aꢁ(A0 ꢂAꢁ)ꢄ[exp(ꢂkobsd ꢄt)]ꢃAꢁ
(7)
a
Absorptivity o referenced to one iron atom; (sh)ꢁshoulder.

100
G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á106
/
3.2. Solvolysis of the complexes Fe(OSO)3 in alcohol
media
In protic coordinating solvents such as alcohols, all of
the complexes Fe(OSO)3 and [Fe(OSO)2(RO)]2 are
subject to solvolysis. The absorption spectra of the
complexes Fe(OSO)3 in alcohol solution are concentra-
tion-dependent, as indicated by a decrease of the
absorpion band at approximately 440 nm with increas-
ing dilution. The nature of the alcohol solvent is
essential in that dissociation of the complexes in less
polar and less acidic alcohols (such as t-PeOH) is less
pronounced than in more polar and acidic alcohols
(such as MeOH).
The addition of an excess of free ligand Hacac to a
dilute solution of Fe(acac)3 in methanol causes a
spectral change and the spectrum obtained corresponds
to that of Fe(acac)3 in non-coordinating dichloro-
methane. Furthermore, when an excess of free ligand
Hacac is admixed to a dilute solution of the binuclear
complex [Fe(acac)2(EtO)]2 in ethanol, the spectrum
obtained again corresponds to that of Fe(acac)3 in
dichloromethane. These experimental findings suggest
that equilibria (10) and (11) are involved. Condensation
and formation of doubly bridged units according to Eq.
(11) corresponds to what is well known for iron(III)
hydroxo species in aqueous solution [1].
Fig. 1. Spectral changes with time for the reaction of Fe(acac)3 with
HOSN in 2-propanol ((1) spectrum before mixing of the reaction
partners; (20) spectrum at the end of the reaction; Dtꢁ
/5 s;
[Fe(acac)3]ꢁ1ꢄ 2ꢄ
/
/
10ꢂ4 M; [HOSN]ꢁ 10ꢂ3 M; 298 K).
/
/
For most of the systems Fe(OSO)3/HOSN/ROH, the
kinetics of ligand substitution according to Eq. (2) were
found to be triphasic, initiated by a sharp change in
absorbance, DA, too fast to be followed by stopped-flow
spectrophotometry (as an example, see Figure S1). The
(A580,t) data (A580
satisfyingly fitted with Eq. (9), which led to the
ꢁabsorbance at 580 nm) were
/
experimental rate constants kobsd(1), kobsd(2), and kobsd(3)
.
It was found for all of the complexes Fe(OSO)3 studied
ð10Þ
ð11Þ
3.3. Kinetics of ligand substitution in the complexes
Fe(OSO)3 according to Eq. (2)
As documented by the overall complex formation
constants b3 for the medium water, the tris 2-hydro-
xyquinolato complex Fe(OSN)3 (log b3ꢁ
/36.9 [13]) is
more stable than the complex Fe(acac)3 (log b3ꢁ26.8
/
[14]) by ten orders of magnitude. One can expect that the
thermodynamic stability of the complex Fe(OSN)3
exceeds that of Fe(acac)3 and of the other complexes
Fe(OSO)3 in alcohol media as well. In agreement with
this expectation, ligand substitution according to Eq. (2)
in the presence of an excess of the entering ligand
HOSN was found to be a practically irreversible
process, conveniently traceable at 580 nm by stopped-
flow spectrophotometry. As an example, Fig. 1 shows
the spectral changes with time for the reaction of
Fe(acac)3 with HOSN in 2-propanol.
Fig. 2. Plot of kobsd(1), kobsd(2), and kobsd(3) vs. [HOSN] for the
reaction of Fe(acac)3 with HOSN in MeOH ([Fe(acac)3]ꢁ
M; 298 K).
/
1ꢄ
10ꢂ4
/

G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á
/106
101
kinetics of ligand substitution in Fe(acac)3 by HOSN
are found to be biphasic only. The experimental rate
constants kobsd(1) and kobsd(2) increase initially with
[HOSN] to level off at high concentrations of HOSN,
as described by Eq. (12a) and (12b).
Table 2
Rate constants k1, k2 and k3 describing the ligand substitution in the
complexes Fe(OSO)3 by 8-hydroxyquinoline according to Eq. (2) in
methanol at 298 K
Complex
k1, sꢂ1
k2, sꢂ1
103ꢄk3, sꢂ1
[HOS N]
(1 ꢃ K1[HOS N])
[HOS N]
Fe(ttfa)3
1.8690.12
0.11490.006
9.3790.40
8.9790.38
n.o.
kobsd(1) ꢁk1 ꢄK1 ꢄ
kobsd(2) ꢁk2 ꢄK2 ꢄ
(12a)
(12b)
Fe(acac)3
1.0490.04
1.0690.06
0.17190.008
0.17390.018
a
a
b
Fe(bza)3
Fe(dbm)3
Fe(dibm)3
Fe(dpm)3
a
1.1690.06
0.11590.007
0.09090.004
0.10290.004
0.08290.003
9.2490.46
7.1590.24
14.290.6
6.9690.52
(1 ꢃ K2[HOS N])
0.98390.018
0.50290.024
Fig. 3 shows this sort of saturation behavior for the
system Fe(acac)3/HOSN/i-PrOH. One might assume
that the observed dependence of kobsd(1) and kobsd(2) on
[HOSN] is due to outer-sphere complex formation
b
n.o.
Data obtained in the presence of additional Hacac; see Fig. S2.
b
Not observed.
according to reaction sequence (13) (HAꢁ
/
Hacac;
HBꢁ
/
HOSN).
that the size of these experimental rate constants does
not depend on the concentration of the entering ligand
HOSN. As an example, Fig. 2 shows the plot of kobsd
versus [HOSN] for the reaction of Fe(acac)3 with
Kos
k
FeA3 ꢃHB?fFeA3; HBg 0 FeA2BꢃHA
If so, the experimental rate constant should follow Eq.
(14). Fitting of the (kobsd,[HOSN]) data with Eq. (14)
(13)
HOSN in MeOH. The data for k1 (ꢁ
k2 (ꢁmean of kobsd(2)), and k3 (ꢁmean of kobsd(3)), as
/
mean of kobsd(1)),
(k ꢄ Kos ꢄ [HOS N])
(1 ꢃ Kos ꢄ [HOS N])
/
/
obtained for the various complexes Fe(OSO)3 in
methanol, are summarized in Table 2. It is essential to
note that, in the presence of additional Hacac, the
triphasic reaction of Fe(acac)3 with HOSN in MeOH
becomes biphasic. The slowest k3 step is no longer
observed, whereas the data for k1 and k2 agree with
those obtained in the absence of additional Hacac.
When studied in branched alcohols such as 2-propa-
nol (and also 3-pentanol and 2-methyl-2-butanol), the
kobsd
ꢁ
(14)
leads however to Kos values of approximately 1000 Mꢂ1
for both kobsd(1) and kobsd(2), which is unrealistically high
for outer-sphere complex formation.
A more adequate explanation for the non-linear
dependence shown in Fig. 3 follows from the reversi-
bility of the solvolysis step. As shown in reaction
sequence (15), the entering ligand HB and the leaving
ligand HA compete for the reaction with the solvento
Fig. 3. Plot of kobsd(1) and kobsd(2) vs. [HOSN] for the reaction of
Fig. 4. Plot of kobsd(1) and kobsd(2) vs. [Hacac] for the reaction of
1ꢄ
10ꢂ4
Fe(acac)3 with HOSN in 2-propanol ([Fe(acac)3]ꢁ
K).
/
1ꢄ
/
10ꢂ4 M; 298
Fe(acac)3 with HOSN in 2-propanol at 298 K ([Fe(acac)3]ꢁ
/
/
M, [HOSN]ꢁ
0.025 M).
/

102
G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á106
/
species FeA2S2 (Sꢁ
ROH and ROꢂ, respectively). For
/
the condition of stationary state, the rate of formation
of the species FeA2B according to Eq. (15) is described
by Eqs. (16) and (17). The validity of Eq. (17) was
scrutinized by investigating the ligand substitution in
Fe(acac)3 by HOSN in 2-propanol under pseudo-first
order conditions ([HOSN]0ꢂ
sence of an excess of the leaving ligand Hacac
([Hacac]0ꢂ[Fe(acac)3]0). The result is shown in Fig. 4.
/[Fe(acac)3]0) in the pre-
/
The observed kinetic mass law effect for both the kobsd(1)
step and kobsd(2) step, as induced by additional leaving
ligand Hacac, supports the interpretation that both
steps are related to solvolytic equilibria. Fitting of the
data kobsd
the parameters ksolv and kHB/kHA. The data obtained for
ksolv (ksolv(1)
0.198 sꢂ1 and ksolv(2) 0.0281 sꢂ1) agree
with those of rate constants k1 and k2 (see Table 3). The
ratio kHB/kHA 0.18 (kobsd(1) step) and kHB/kHA 0.26
ꢁf([HB]/[HA)] according to Eq. (17) leads to
/
ꢁ
/
ꢁ
/
ꢁ
/
ꢁ
/
(kobsd(2) step) indicates that in both steps the solvento
species prefer to react with acetylacetone rather than
with 8-hydroxyquinoline.
Fig. 5. Plot of log k1, log k2 and log k3 vs. ET(30) for ligand
substitution in the complex Fe(acac)3 by 8-hydroxyquinoline according
to Eq. (2) in various alcohol solvents at 298 K (1ꢁ
3ꢁPrOH, 4ꢁBuOH, 5ꢁHpOH, 6ꢁi-PrOH, 7ꢁ
PeOH).
/
MeOH, 2ꢁ
/
EtOH,
ksolv
FeA3 ? FeA2S2 ꢃHA ꢃ0HB FeA2BꢃHA
(15)
(16)
/
/
/
/
/
3-PeOH, 8ꢁ
/t-
kHB
kIIA
rateꢁkobsd ꢄ[FeA3]
kobsd ꢁksolv
3.4. Kinetics of ligand substitution in the binuclear
complexes [Fe(OSO)2(RO)]2 according to Eq. (3)
ꢄfkHB[HB]=(kHA ꢄ[HA])g=f1ꢃkHB
ꢄ[HB]=(kHA ꢄ[HA])g
As discussed above, the absorption spectra of the
complexes Fe(acac)3 and [Fe(OSO)2(RO)]2 in alcohol
media suggest solvolytic dissociation and formation of
(17)
The different reaction pattern observed in methanol
(3 steps) and in 2-propanol (two steps) led us to study
the effect of the alcohol solvent on the ligand substitu-
tion in the system Fe(acac)3/HOSN in some primary,
secondary and tertiary alcohols ROH. The results are
summarized in Table 3. In primary alcohols, three steps
are observed whereas in secondary and tertiary alcohols
the third step is missing. The size of rate constants k1, k2,
and k3 follows the solvent polarity parameter ET(30) [15]
(see Fig. 5).
binuclear species according to Eq. (18) (SꢁROH and
/
ROꢂ, respectively; charges omitted). The two equilibria
involved imply that the kinetic data, as resulting from
ligand substitution experiments with solutions of
Fe(OSO)3, should agree with those based on experi-
ments with solutions of the corresponding binuclear
complex [Fe(OSO)2(RO)]2. The results obtained for the
complexes Fe(acac)3 and [Fe(acac)2(EtO)]2 in ethanol
Table 3
First-order rate constants k1, k2 and k3 describing the ligand substitution in the complex Fe(acac)3 by 8-hydroxyquinoline according to eq. Eq. (2) in
different alcohol solvents at 298 K
a
Alcohol
k1 (sꢂ1
)
k2 (sꢂ1
)
103ꢄk3 (sꢂ1
)
k1/k2
k1/k3
MeOH
MeOH
EtOH
1.0490.04
n.o.
0.17190.008
n.o.
8.9790.38
199.096
6.08
116
b,c
b,c
c,d
0.45090.009
0.37890.009
0.37590.006
0.37690.010
0.20990.008
0.10290.004
0.042290.0015
0.064790.0033
0.056090.0036
0.053190.0025
0.047990.0018
0.028690.0012
0.025190.0016
0.007690.0004
7.0290.28
6.1290.22
4.0190.20
2.7690.36
6.96
6.75
7.06
7.85
7.31
4.06
5.55
64
61.8
93.5
136
PrOH
BuOH
HpOH
i-PrOH
3-PeOH
t-PeOH
b
n.o.
n.o.
b
n.o.b
a
For abbreviations see ref. [7].
Not observed.
Reaction of the complex Fe(acac)3 with Hacac in the absence of 8-hydroxyquinoline.
b
c
d
Rate constant kobsd (as obtained according to Eq. (7)), independent of [Hacac].

G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á
/106
103
Table 4
that, to our knowledge, binuclear complexes [Fe(aca-
c)2(RO)]2 with branched alkoxo groups RO have not
been reported so far. They are obviously of very limited
stability. In summary, it is found that the appearence of
the k3 pathway is bound to the presence of the binuclear
species [Fe(OSO)2(RO)]2.
First-order rate constants k1, k2 and k3 describing the ligand
substitution in the binuclear complexes [Fe(OSO)2(RO)]2 by 8-
hydroxyquinoline according to Eq. (3) and rate data for the
corresponding complexes Fe(OSO)3 for comparison (298 K)
Complex
Solvent k1 (sꢂ1
)
k2 (sꢂ1
)
103ꢄk3 (sꢂ1
)
It is interesting to see that the k1 step is missing in the
reaction of the complexes Fe(dpm)3 and [Fe(dpm)2-
(MeO)]2 with HOSN in methanol (Table 4). The most
straightforward interpretation is that, in solutions of
Fe(dpm)3 or [Fe(dpm)2(MeO)]2 in methanol, there are no
appreciableamountsofthetriscomplexFe(dpm)3present.
Solvolysisleadstoacertainamountofthesolventospecies
Fe(acac)3
[Fe(aca-
EtOH
EtOH
0.45090.009 0.064790.0033 7.0290.28
0.51390.018 0.057690.0037 8.4190.24
c)2(EtO)]2
Fe(dpm)3
a
MeOH n.o.
0.082090.0030 6.9690.52
0.073290.0041 6.8290.81
a
[Fe(dpm)2 MeOH n.o.
(MeO)]2
a,b
a,b
a,b
a,b
b,c
Fe(acac)3
[Fe(aca-
c)2(EtO)]2
EtOH
EtOH
n.o.
n.o.
n.o.
n.o.
147.0912
b,c
167.099
Fe(dpm)2S2 (SꢁMeO and MeOH, respectively) and
/
a
mainly to the binuclear complex [Fe(dpm)2(MeO)]2 (as a
matter of fact, pure [Fe(dpm)2(MeO)]2 precipitates from
sufficiently concentrated solutions of Fe(dpm)3 in metha-
nol).Theabsenceofthek1pathwayinthesystemFe(dpm)3/
HOSN/MeOH (and in the system [Fe(dpm)2(MeO)]2/
HOSN/MeOH) suggests therefore that the k1 step
describes the solvolysis of the tris b-diketonato complexes
Fe(OSO)3 (see Eq. (18)).
To shed some further light on the equilibria described
in Eq. (18), the reaction of the leaving ligand Hacac with
Fe(acac)3 in MeOH (Table 3) and with Fe(acac)3 and
[Fe(acac)2(EtO)]2 in EtOH (Table 4) was studied kine-
tically. Spectrophotometric monitoring showed two-step
kinetics, consisting of an initial jump in absorbance (too
fast to be followed by stopped-flow spectrophotometry)
followed by an exponential change of absorbance with
time according to Eq. (7). The (A, t) data led to a first-
Not observed.
Reaction of the complex with Hacac in the absence of 8-
b
hydroxyquinoline.
c
Rate constant kobsd (as obtained according to Eq. (7)), indepen-
dent of [Hacac].
and for the complexes Fe(dpm)3 and [Fe(dpm)2(MeO)]2
in methanol are compiled in Table 4. They confirm that
the equilibria described in Eq. (18) can obviously be
established ‘from both sides’, by dissolving either
Fe(OSO)3 or [Fe(OSO)2(RO)]2 in ROH. The kinetic
pattern is the same for both types of experiments and
most of the corresponding rate constants agree within
the limits of error.
As a matter of fact, none of the steps describing the
ligand substitution in the complexes Fe(OSO)3 by 8-
hydroxyquinoline depends on the concentration of the
ð18Þ
entering ligand. This fact and the observed solvent-
dependence of the rate data strongly suggest that all of
the steps are solvent-induced processes.
order rate constant kobsd that was found to be indepen-
dent of [Hacac]. The initial sharp change in absorbance
is obviously due to the fast anation of the solvento
species Fe(acac)2S2 by Hacac, whereas the first-order
rate constant describes the splitting of the binuclear
species [Fe(acac)2(MeO)]2 and [Fe(acac)2(EtO)]2, respec-
tively, by solvent attack1.
The experimental findings allow to assign rate con-
stant k3 to the solvent-induced splitting of the binuclear
species [Fe(OSO)2(RO)2]. This assignment is based on
the following observations for the reaction of Fe(acac)3
with 8-hydroxyquinoline, (i) in MeOH, the triphasic
reaction (rate constants k1, k2, and k3) becomes biphasic
(rate constants k1 and k2) in the presence of admixed
Hacac (see Table 2), and (ii) in branched alcohols such
as i-PrOH, 3-PeOH, and t-PeOH, the k3 step is not at all
observed (see Table 3). The plausible explanation for the
first observation is that additional Hacac shifts equili-
bria (18) to the left and lets the fraction of [Fe(a-
cac)2(MeO)]2 decrease to a minor percentage. The
second observation finds its explanation in the fact
1
One should note that rate constants kobsd, as obtained for the
splitting of the binuclear complexes in the systems Fe(acac)3/Hacac/
MeOH (Table 3) and Fe(acac)3/Hacac/EtOH (Table 4), are by a factor
of approximately twenty higher than rate constants k3, descibing the
splitting of the binuclear complexes in the systems Fe(acac)3/HOSN/
MeOH and Fe(acac)3/HOSN/EtOH (Table 3). Since neither Hacac
nor HOSN are directly involved in the splitting process, the difference
in rate is probably due to a medium effect. Hacac is considerably more
acidic than HOSN [13,14,16], which might mean that proton transfer
as the initial step of the splitting process occurs more easily in ROH/
Hacac media than in ROH/HOSN media.

104
G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á106
/
3.5. Mechanistic interpretation of the kinetic data
obtained for ligand substitution in the systems
Fe(OSO)3/HOSN/ROH
ment of the solvolytic equilibria, which means solvolysis
of the tris complex Fe(OSO)3 (k1 path) and splitting of
the species [Fe(OSO)2(RO)]2 (k3 path).
The assignment of rate constant k2 is based on
experimental evidence as well as on plausibility. It is
experimentally found that (i) both k1 and k2 are first-
order rate constants that do not depend on the
concentration of the entering ligand, (ii) the solvent
dependence of k1 and k2 is very similar (see Table 3 and
Fig. 5), and (iii) for all of the alcohol media studied, the
ratio k1/k2 is more or less constant (see Table 3). These
facts suggest strongly that k1 as well as k2 describe
solvent-initiated dissociation processes. Rate constant k1
refers to the solvolysis of the tris complex Fe(OSO)3)
and rate constant k3 describes the splitting of the
binuclear complex [Fe(OSO)2(RO)]2 (see Section 3
above). Since k2 is not observed for the reaction of
Fe(acac)3 with Hacac (see Table 3), it is concluded that
k2 has to be ascribed to the solvolytic dissociation of one
of the intermediate complexes, Fe(OSO)2(OSN) or
Fe(OSO)(OSN)2. Assuming that the intermediate
Fe(OSO)(OSN)2 solvolyses more rapidly than
Fe(OSO)2(OSN), we assign k2 to the solvolysis of the
intermediate species Fe(OSO)2(OSN) (see Scheme 1).
The solvolysis of Fe(OSO)(OSN)2 is thus assumed to be
a fast consecutive step.
The overall process of ligand substitution in the
complexes Fe(OSO)3 by 8-hydroxyquinoline in alcohol
media according to Eq. (2) involves four steps. In
addition to three spectrophotometrically traceable steps,
characterized by first-order rate constants k1, k2 and k3
(see Tables 2 and 3), one has to consider the initial sharp
change in absorbance, DA, as the first and fastest step.
This initial jump in absorbance (e.g. see Figure S1) is too
fast to be followed by the stopped-flow technique
applied. The size of DA correlates with the extent of
solvolysis. In the system Fe(acac)3/HOSN/MeOH, DA
corresponds to approximately 5% of the total change in
absorbance observed for the overall reaction. Addition
of acetylacetone to this system lets DA disappear. In the
more strongly solvolyzed system Fe(dpm)3/HOSN/
MeOH, DA amounts to approximately 15% of the total
change in absorbance. As pointed out above, the initial
DA step obviously describes the fast reaction of the
solvento
species
Fe(OSO)2(ROH)2ꢃ
and
Fe(OSO)2(RO)(ROH), respectively, with HOSN2. The
higher the fraction of those kinetically labile species is,
the more DA increases.
From the point of formal kinetics, ligand substitution
in the complexes Fe(OSO)3 by 8-hydroxyquinoline in
alcohol media is a remarkable process in that none of
the steps observed is found to depend on the concentra-
tion of the entering ligand, present in excess. This kinetic
pattern is different from the ones reported for the homo
and hetero b-diketone exchange in a wide spectrum of
complexes M(OSO)n studied in aprotic organic media
Scheme 1 presents a mechanistic interpretation of the
overall process of substitution, as based on the experi-
mental evidence obtained for the single steps. The
kinetics of substitution are dominated by solvolysis.
The alcohol solutions of the complexes Fe(OSO)3) offer
the species Fe(OSO)3, Fe(OSO)2(ROH)2ꢃ, Fe(OSO)2-
(RO)(ROH), [Fe(OSO)2(RO)]2, OSOꢂ, and HOSO.
The ratio [Fe(OSO)2(ROH)2ꢃ]/[Fe(OSO)2(RO)(ROH)]
is determined by the acidity of the b-diketone HOSO
and of the alcohol ROH. The assignment of first-order
rate constants k1 (solvolytic dissociation of Fe(OSO)3)
and k3 (solvent-induced splitting of the binuclear species
[Fe(OSO)2(RO)]2) has been substantiated and discussed
above. In the fastest of the four steps involved the
entering ligand HOSN ‘catches’ the initially present
fraction of the solvento species Fe(OSO)2(ROH)2ꢃ and
Fe(OSO)2(RO)(ROH), respectively, which is documen-
ted by a sharp jump in absorbance (DA). The rate of
formation of further intermediate ‘mixed’ complex
Fe(OSO)2(OSN) is thus controlled by the re-establish-
(Mꢁ
/
metal cation; nꢁ3, 4). As summarized by Hynes
/
[17], in the absence of catalysts (such as acids) the rate of
homo b-diketone exchange according to Eq. (19) can be
described by two-term rate law (20) (HOSOꢀꢁ
/
isoto-
pically labelled b-diketone).
ð19Þ
rateꢁ(k1 ꢃk2 ꢄ[H2O])ꢄ[HOS O]x ꢄ[M(OS O)n] (20)
For many systems M(OSOꢀ)n/HOSO, when studied
in aprotic solvents, the k1 path was found to be ligand-
dependent, following second-order kinetics (xꢁ1) [17].
/
In the presence of water, there is an additional k2
pathway. In the system Fe(acacꢀ)3/Hacac, with the weak
acid Hacac being the solvent, the k1 path is independent
2
To our knowledge, there are no data available to describe the rate
of solvent exchange at iron(III) species comparable to the solvento
of [HOSO] (xꢁ
[8d].
/0), which corresponds to rate law (21)
species Fe(OSO)2(ROH)2ꢃ and Fe(OSO)2(RO)(ROH). On the basis of
kex
Metal Ions in Solution, Ellis Horwood: Chichester, England, 1978; p.
317) one can estimate kex
105 sꢂ1 in the case of Fe(OSO)2(ROH)2ꢃ
:
104 sꢂ1 for solvated Fe3ꢃ in MeOH and EtOH (J. Burgess,
/
rateꢁ(k1 ꢃk2 ꢄ[H2O])ꢄ[M(OS O)n]
(21)
]/
Sekine and Inaba studied hetero b-diketone exchange
and Fe(OSO)2(RO)(ROH), respectively. This means that the time
scale of the stopped-flow method is indeed not suited to resolve the
reaction of these species with HOSN.
reactions in the systems Fe(OSO1)3/HOSO2 (HOSO1,
HOSO2ꢁ
Hacac, Httfa, trifluoroacetylacetone) accord-
/

G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á
/106
105
Scheme 1. Schematic description of the various steps controlling the solvolysis of the complexes Fe(OSO)3 in alcohol media and the reaction with 8-
hydroxyquinoline according to Eq. (2).
ing to Eq. (22). In the aprotic solvent CCl4, they
observed ligand-dependent as well as ligand independent
pathways [8b]. The authors discuss the role of the
ligands HOSO as catalyzing acids and possible kinetic
effects caused by protic impurities such as water.
Nelwamondo reported for reaction (22), as studied in
CCl4 solution with four complexes Fe(OSO1)3 and
k2ꢄ[ROH] dominate so much that, in pure ROH as the
solvent, rate law (23) takes the simple first-order form
described by Eq. (24).
/
rateꢁ(k1 ꢃk2 ꢄ[ROH])ꢄ[Fe(OS O)3]
(23)
(24)
rateꢁk2 ꢄ[ROH]ꢄ[Fe(OS O)3]
ꢁkROH ꢄ[Fe(OS O)3]
HOSO2ꢁ
Hacac, that the rate of substitution is first-
/
order in Fe(OSO1)3 and inversely dependent on [Hacac]
[8e].
4. Conclusions
ð22Þ
As a general result, the kinetic data reported for b-
diketone exchange and b-diketone substitution in com-
plexes M(OSO)n in aprotic media clearly reflect that
protic components are kinetically very effective. Espe-
cially in the case of the iron(III) complexes Fe(OSO)3,
water and the more or less weak acidic ligands HOSO
involved open additional pathways, which stresses the
crucial role of proton transfer in these exchange and
substitution reactions. In line with the kinetic role of
water and acids in aprotic media it was found by
Graham [18] that the kinetics of ligand exchange
between tris(acetohydoxamato)iron(III) complexes and
EDTA in the medium water are pH-dependent.
In alcohol media, tris b-diketonate complexes
Fe(OSO)3 are subject to solvolytic dissociation, leading
to the solvento species, Fe(OSO)2(ROH)2ꢃ and
Fe(OSO)2(ROH)(RO), respectively, as well as to bi-
nuclear complexes [Fe(OSO)2(RO)]2. In alcohol solu-
tion, ligand substitution in the complexes Fe(OSO)3 by
8-hydroxyquinoline (ꢁ
HOSN) according to (a) occurs
/
via the kinetically most labile solvento species. There is
no direct second-order reaction between the entering
ligand HOSN and the intact complexes Fe(OSO)3 or
the binuclear complexes [Fe(OSO)2(RO)]2. The kinetics
of ligand substitution according to reaction (a) are
triphasic. The substitution process is controlled by three
With regard to the present study, one can extrapolate
that alcohols ROH, when admixed to solutions of the
complexes Fe(OSO)3 in an aprotic solvent, open an
alcohol-induced reaction channel according to rate law
(23). The continuous increase of [ROH] will let the term
first-order rate constants, following the order k1ꢀ
/
k2ꢀ
/
k3. All three of them do not depend on the concentration
of the entering ligand HOSN. The size of these rate
constants follows the solvent polarity parameter ET(30).
Rate constant k1 describes the solvolytic dissociation of

106
G. Gumbel, H. Elias / Inorganica Chimica Acta 342 (2003) 97Á106
/
the complexes Fe(OSO)3, whereas k3 can be assigned to
the solvent-initiated splitting of the binuclear complexes
[Fe(OSO)2(RO)]2, which is by approximately two orders
Eigth Conference on Coordination Chemistry, Bratislava (Smo-
lenice), CSSR, December 5Á11, 1980, p. 91;
(c) H. Elias, H. Muth, B. Niedernhofer, K.J. Wannowius, J.
¨
/
Chem. Soc., Dalton (1981) 1825;
(d) H. Elias, U. Reiffer, M. Schumann, K.J. Wannowius, Inorg.
Chim. Acta 53 (1981) L65;
of magnitude slower (k1/k3:
102). Rate constant k2
/
refers, most probably, to the solvolytic dissociation of
the intermediate complex Fe(OSO)2(OSN). In a given
alcohol solvent, the effect of the nature of the coordi-
nated b-diketone in the complexes Fe(OSO)3 on the size
of k1, k2 and k3 is rather minor.
(e) M. Schumann, A. von Holtum, K.J. Wannowius, H. Elias,
Inorg. Chem. 21 (1982) 606;
(f) H. Elias, K.J. Wannowius, Inorg. Chim. Acta 64 (1982) L157;
(g) H. Elias, H. Muth, M. Sahm, H. Volz, K.J. Wannowius,
Inorg. Chim. Acta 68 (1983) 163;
(h) H. Elias, C. Hasserodt-Taliaferro, L. Hellriegel, W. Schon-
¨
Fe(OS O)3 ꢃ3HOS N R0OH Fe(OS N)3 ꢃ3HOS O
(a)
herr, K.J. Wannowius, Inorg. Chem. 24 (1985) 3192.
[6] (a) B. Emmert, E. Jacob, Chem. Ber. 67 (1934) 286;
(b) B. Emmert, W. Seebode, Chem. Ber. 71 (1938) 242;
(c) C.S. Wu, G.R. Rossman, H.B. Gray, G.S. Hammond, H.J.
Schugar, Inorg. Chem. 11 (1972) 990;
From the preparative point of view one learns that the
rate of reactions such as (a) (see above), leading to
thermodynamically more stable complexes Fe(ASB)3
(ASBꢁbidentate anion of a weak acid HASB), is not
/
(d) R. Grobelny, A. Zarzeczny, Mater. Sci. 9 (1983) 47;
(e) B. Chiari, D. Piovesana, T. Tarantelli, P.F. Zanazzi, Inorg.
Chem. 23 (1984) 3398.
enhanced by an excess of HASB.
[7] Abbreviations: HOSO, b-diketone; HOSN, 8-hydroxyquinoline;
Hacac, pentane-2,4-dione; Hdibm, 2,6-dimethylheptane-3,5-
dione; Hdpm, 2,2,6,6-tetramethylheptane-3,5-dione; Hbza, 1-
phenylbutane-1,3-dione; Hdbm, 1,3-diphenylpropane-2,3-dione;
5. Supplementary material
Httfa,
1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione;
MeOH,
Figures S1 and S2, offering further kinetic details
concerning the substitution processes under study, can
be obtained from the authors upon request.
methanol; EtOH, ethanol; PrOH, 1-propanol; BuOH, 1-butanol;
HpOH, 1-heptanol; i-PrOH, 2-propanol; 3-PeOH, 3-pentanol: t-
PeOH, 2-methyl-2-butanol; Hehx, 2-ethyl-hexanoic acid.
[8] (a) T. Sekine, H. Honda, M. Kokiso, T. Tosaka, Bull. Chem. Soc.
Jpn. 52 (1979) 1046;
(b) T. Sekine, K. Inaba, Bull. Chem. Soc. Jpn. 57 (1984) 3083;
(c) T. Sekine, K. Inaba, Bull. Chem. Soc. Jpn. 58 (1985) 1125;
(d) H. Kido, K. Saito, Bull. Chem. Soc. Jpn. 58 (1980) 424;
(e) A.N. Nelwamondo, S. Afr. J. Chem. 46 (1993) 1.
[9] G.T. Morgan, C.J. Taylor, J. Chem. Soc. 127 (1925) 803.
Acknowledgements
Sponsorship of this work by the Deutsche For-
schungsgemeinschaft and Verband der Chemischen
Industrie E.V. is gratefully acknowledged.
[10] G. Stockelmann, A. Kettrup, H. Specker, Angew. Chem. 79
¨
(1967) 530.
[11] R.L. Lintvedt, L.K. Kernitzky, Inorg. Chem. 9 (1970) 491.
[12] N.T. Moxon, J.H. Moffett, A.K. Gregson, J. Inorg. Nucl. Chem.
43 (1981) 2695.
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