92
D. Lakk-Bogáth et al. / Polyhedron 89 (2015) 91–95
methods [27] and stored under argon. The isoindoline-based
ligands HL1–HL4, and their complexes FeIIICl2(L1–4) have been syn-
thesized according to published procedures [22,24,28–31]. All
other chemicals were commercial products and were used as
received without further purification.
2.2. Determination of products
Reactivity assays were performed as follows [16,17]: the
respective amino acid (ACCH, ACBH, ACPH, ACHH, AIBH) was dis-
solved in 10 mL of DMF/H2O mixture (3/1 V/V) in a 20 mL sealable
Scheme 2. Reactions catalyzed by [FeIIICl2(L1–4)].
tube. With MeCN (10 lL) as inner standard, NH4OH and the
catalyst were then added to the mixture. Hydrogen peroxide was
added through the septum with a syringe and the evolved ethyl-
ene, cyclobutanone, cyclopentanone, cyclohexanone or acetone
was measured by removing 0.25 mL from the headspace with a
gastight syringe and the sample was injected into a gas chromato-
graph. The concentration of the corresponding product in the head-
space is linearly proportional to the concentration of the product in
the reaction mixture [16]. GC analyses were performed on a
Hewlett Packard 5890 gas chromatograph equipped with a flame
ionization detector and a 30 m Supelcowax column.
Table 1
Calculated yield and TON values for AIBH, ACCH, ACBH, ACPH and ACHH oxidation
with FeIIICl2(L1–4
)
in DMF/water (3/1 V/V) at 35 °C. [S]0 = 3.6 ꢀ 10ꢁ2 M,
[FeIIICl2(L1–4)]0 = 7.2 ꢀ 10ꢁ6 M, [H2O2]0 = 3.6 ꢀ 10ꢁ2 M, [NH4OH]0 = 3.6 ꢀ 10ꢁ2 M.
Entry
Substrate
Catalyst
t (s)
TON [TOF (1/h)]
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
AIBHa
AIBHb
AIBHc
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
AIBH
ACCH
ACCH
ACCH
ACBH
ACBH
ACBH
ACPH
ACPH
ACPH
ACHH
ACHH
ACHH
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L2)
FeIIICl2(L2)
FeIIICl2(L2)
FeIIICl2(L3)
FeIIICl2(L3)
FeIIICl2(L3)
FeIIICl2(L4)
FeIIICl2(L4)
FeIIICl2(L4)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
FeIIICl2(L1)
1800
1800
1800
900
1800
2700
900
1800
2700
900
1800
2700
900
1800
2700
900
1800
2700
900
1800
2700
900
1800
2700
900
370
1048
1950
3264
3528 [7056]
3647
3398
3914 [7828]
3915
3419
3647 [7294]
3529
3264
3339 [6678]
3342
30
218
545 [726]
116
372
720 [960]
880
7
21
39
65
71
73
68
78
78
68
73
71
65
67
67
0.6
4
10
2.5
7
14
17
28
31
31
32
33
3. Results and discussion
3.1. Catalysis
We have found that complexes [FeIIICl2(L1–4)] selectively and
highly efficiently catalyze the oxidative decarboxylation and
deamination of
a-aminoisobutyric acid into acetone with H2O2 in
alkaline DMF/water solution (see Scheme 2).
The evolved acetone was monitored by gas chromatography
following our previously reported procedure [16]. The optimum
conditions used for the amino acid oxidation by this catalytic sys-
tem were: catalyst, oxidant, base and substrate in a molar ratio of
1:5000:5000:50,000, respectively. The influence of reaction param-
eters such as H2O/DMF ratio, time of the reaction on the catalytic
behavior were investigated using complexes [FeIIICl2(L1)]. First,
we varied the DMF/H2O from 1:3 to 1:1 and 3:1 (Table 1, entries
1, 2, and 5). The turnovers (yield) (TON = mol of product per mol
of catalyst) during these conditions above were found to be 370
(7%), 1048 (21%) and 3528 (71%) after 0.5 h at 35 °C, respectively
(Table 1). In the absence of base induction period, and much lower
yield has been observed (Table 1, entry 3). The stability of the
active species was probed as well using the complex [FeIIICl2(L1)]
by varying reaction times from 1800 s to 2700 s (Table 1, entries
4–6). Increasing reaction time from 900 s to 1800 s increased the
TON from 3264 to 3647. The general increase in product yields
indicate appreciable stability of the active species. The ligand
framework also influenced the catalytic activities of these com-
plexes. TONs = 3339–3915, and yields = 67–78% have been found
for the complexes [FeIIICl2(L2–4)] (Table 1, entries 7–15). The TOF
(Turnover frequency) values vary in the order: [FeIIICl2(L2)] >
[FeIIICl2(L3)] > [FeIIICl2(L1)] > [FeIIICl2(L4)]. Furthermore, the rates
are dependent on the oxidation potential, E°0pa of the iron center
1372
1527 [2036]
1572
1602
1671 [2228]
1800
2700
a
b
c
In DMF/water (1/3 V/V).
In DMF/water (1/1 V/V).
Absence of base.
in the precursor complexes. To elucidate the electronic effect of
the different ligands, the redox properties of the precursor com-
plexes have been investigated by cyclic voltammetry (CV) experi-
ments in DMF. Quasi-reversible, one electron transitions were
observed for FeIIICl2(L3) and FeIIICl2(L4) (E°0pa and E°0pc), whereas
FeIIICl2(L1) and FeIIICl2(L2) showed irreversible waves (E°0pa) [24].
In the case of FeIIICl2(L2) the anodic peak (E°0pa) shifted by
164 mV compared to that of FeIIICl2(L1) as a result of methyl substi-
tution on the pyridyl arms [24]. Summarily, the redox properties of
the iron(III) isoindoline complexes are very sensitive to the
modification of the isoindoline arms, and the introduction of elec-
tron-releasing arms on the bis-iminoisoindoline moiety slightly
increases the catalytic activity. More importantly, a nice correla-
tion was found between the TOF and the oxidation potential, E°0pa
of the iron center in the precursor complexes (Fig. 1) [24].
Finally, the oxidation of other cyclic amino acids like 1-amino-
cyclohexane-1-carboxylic acid (ACHH), 1-aminocyclopentane-1-
carboxylic acid (ACPH), 1-aminocyclobutane-1-carboxylic acid
(ACBH), and 1-aminocyclopropane-1-carboxylic acid (ACCH) by
H2O2 in DMF/H2O (3:1 V/V) at 35 °C was examined using
Scheme 1. Structure of the isoindoline-based ligands and substrates used.