Synthesis of cobalt(III), iron(III), and chromium(III) Complexes with salicylaldiminato ligands: Evaluation of the complexes as catalysts for oxidation of L-cysteine

Article abstract of DOI:10.1515/znb-2008-0707

A series of new cobalt(III)-, iron(III)-, and chromium(III)-based complexes of the general formula [M(N∩O)₂Cl] (N∩O: N-salicylidene(X)amine and sodium N-(4-sulfonatosalicylidene(X)amine)) (X = cyclohexyl and 1-naphthyl) was prepared and characterized. Some of the isolated complexes have been evaluated as catalysts for the oxidation of L-cysteine. Preliminary results show that the rate of oxidation of L-cysteine is influenced by the nature of the metal center, the geometry of the complex, the auxiliary substituents, and the backbone of the ligand.

Full text of DOI:10.1515/znb-2008-0707

Synthesis of Cobalt(III), Iron(III), and Chromium(III) Complexes with
Salicylaldiminato Ligands: Evaluation of the Complexes as Catalysts for
Oxidation of L-Cysteine
Adnan S. Abu-Surrah, Hamzeh M. Abdel-Halim, and Feda’a M. Al-Qaisi
Department of Chemistry, Hashemite University, P. O. Box 150459, Zarqa-13115, Jordan
Reprint requests to Dr. Adnan S. Abu-Surrah. E-mail: asurrah@hu.edu.jo
Z. Naturforsch. 2008, 63b, 848 – 852; received March 1, 2008
A series of new cobalt(III)-, iron(III)-, and chromium(III)-based complexes of the general formula
[M(N∩O)2Cl] (N∩O: N-salicylidene(X)amine and sodium N-(4-sulfonatosalicylidene(X)amine))
(X = cyclohexyl and 1-naphthyl) was prepared and characterized. Some of the isolated complexes
have been evaluated as catalysts for the oxidation of L-cysteine. Preliminary results show that the
rate of oxidation of L-cysteine is influenced by the nature of the metal center, the geometry of the
complex, the auxiliary substituents, and the backbone of the ligand.
Key words: Co(III), Fe(III), Cr(III), Schiff Bases, Catalysis, L-Cysteine
Introduction
Salen-based complexes are a fundamental class of
dure through condensation of common aldehydes with
amines.
Recently, these authors reported results on the re-
compounds in coordination chemistry, and they have action rates of the oxidation of L-cysteine by pairs of
been known since 1933 [1]. Interest in these com- trans- and racemic cis-isomers of octahedral Co(III)
plexes intensified in 1990 when the groups of Ja- and Fe(III) complexes bearing the ligands ethylene-
cobsen [2] and Katsuki [3] discovered the enantio- diamine, 2,2ꢀ-bipyridyl, and 1,10-phenanthroline [8].
selective epoxidation of unfunctionalized alkenes us- Rates were found to be one to three orders of mag-
ing chiral Mn(salen) complexes as catalysts. Since nitude higher for the trans-isomers. The differences in
then, an extremely wide variety of reactions catalyzed rates were attributed to steric factors. The less crowded
by salen complexes has been investigated. These in- trans-isomers facilitate electron transfer making the
clude epoxidation of alkene [4], hydrolytic kinetic res- oxidation process faster than that for the cis-isomers.
olution of epoxides [5], intermolecular hydroamina-
tion of allenes [6], and vinyl polymerization of nor-
bornene [7].
As an extension of our studies on both the coor-
dination chemistry of heteroatom containing ligands
[9, 10] and on the catalytical application of their metal
complexes [11, 12], we report the synthesis of a se-
ries of new cobalt(III), iron(III), and chromium(III)
complexes containing bidentate salen ligands. The new
complexes were characterized by their physical prop-
erties, elemental analysis, magnetic susceptibility, IR
and UV/Vis spectroscopy, and thermal gravimetric
analysis (TGA). The influence of the metal center, the
type of the auxiliary groups and the sulfonyl group in
the backbone of the ligands on the rate of oxidation of
the amino acid L-cysteine are described.
For many reasons, Schiff bases have been found
to be among the most convenient and attractive lig-
ands for transition metal complexes (TMC) for many
reasons: First, steric and electronic effects around the
TMC core can be finely tuned by an appropriate se-
lection of bulky and / or electron withdrawing or do-
nating substituents that can be incorporated into the
Schiff bases. Second, the two donor atoms, N and O,
of the chelating Schiff base exert two opposite elec-
tronic effects: the phenolate oxygen atom is a hard
donor which is known to stabilize the higher oxidation
state of the TM atom while the imine nitrogen atom
is a softer donor which stabilizes the lower oxidation
state of the TM. Third, Schiff bases are easily prepared,
Results and Discussion
Ligand synthesis
The bidentate ligands 4a, 4b and 5a, 5b were pre-
in practically quantitative yields, via a one-step proce- pared in moderate to high yields by refluxing the sali-
c
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A. S. Abu-Surrah et al. · Cobalt(III), Iron(III), and Chromium(III) Complexes with Salicylaldiminato Ligands
849
Scheme 1. Synthesis and proposed structures
of the ligands and the corresponding Co(III)-,
Fe(III)-, and Cr(III)-based salen complexes 6 –
11.
cylaldehyde1 or 2 with an equivalent amount of the de-
sired amine (3a, b) in MeOH (Scheme 1). The isolated
ligands were characterized by their physical properties,
elemental analysis, and IR and UV/Vis spectroscopy.
Complex synthesis
The Co(III)-based complexes 6a, 6b and 9a were
synthesized by treating CoCl2 · 6H2O with two molar
equivalents of the corresponding bidentate ligand in
MeOH, followed by oxidation with H2O2 (Scheme 1).
Scheme 2. Oxidation of cysteine by the complexes prepared
in the present study.
The Fe(III)- (7a, 7b, 10a and 10b) and Cr(III)-based
complexes (8a, 8b, 11a, and 11b) were synthesized by
The coordination reactions were also followed by
UV/Vis spectroscopy. A sharp peak in the visible re-
the treatment of CrCl3 · 6H2O or anhydrous FeCl3 with gion due to the coordinated ligands was observed. The
two equivalents of the corresponding bidentate ligand complexes 6a, 7a, and 8a show peaks at 389, 487, and
in methanol (Scheme 1). The isolated cobalt(III) and 406 nm, respectively, to be compared with the corre-
iron(III) complexes are microcrystalline or powder-
like, and they are stable at atmospheric conditions, at 326 nm. Magnetic susceptibility (µeff, BM) mea-
while the corresponding chromium(III) complexes are surements have shown that the cobalt(III) complexes
sponding ligand (4a) which shows an absorption peak
moisture sensitive.
IR analyses of the complexes confirm the presence
of the ligands. A slight shift of the peak due to the
are low-spin state species with a magnetic moment µeff
of 0.0 BM.
Some of the isolated Co(III) and Fe(III) complexes
imine bond (ν(C=N)) was observed indicating com- were utilized as catalysts for the oxidation of L-
◦
plexation. Also elemental analyses showed that the cysteine in aqueous solution at 25 C and at con-
metal to ligand ratio in the chloro complexes is 1 : 2. stant pH in air (Scheme 2). Preliminary investigations
Therefore, a penta-coordinate arrangement around the showed that the cobalt(III) complexes induce faster
metal center can be assumed, but hexacoordination in catalytical oxidation compared with the correspond-
a dinuclear aggregate cannot be ruled out. The com- ing iron(III) complexes (6a: k = 1.0 × 10−1; 7a: k =
plexes 10a, 11a, and 11b were also characterized by 3.25× 10−3 dm3 mol−1
s
−1). The complexes contain-
thermal gravimetric analysis (TGA). The results have ing N-naphthyl substituents were non-reactive. The
shown that the complexes are hydrates with 9, 7, and 9 rates of oxidation reactions by the complexes con-
H2O molecules, respectively. This is in agreement with taining sulfonate groups at the para position of the
the results obtained by elemental analysis.
aromatic ring are much lower compared to those of
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A. S. Abu-Surrah et al. · Cobalt(III), Iron(III), and Chromium(III) Complexes with Salicylaldiminato Ligands
the unsubstituted ligands (6a: k = 1.0× 10−1; 9a: k =
4a: Yield: 3.29 g (98 %). Color: yellow. – UV/Vis (EtOH):
λmax = 326, 416 nm. – IR: ν = 1630 cm−1 (m, C=N). –
C13H17NO: calcd. C 76.81, H 8.43, N 6.89; found C 76.40,
H 8.75, N 6.60.
4b: Yield: 2.7 g (98 %). Color: dark-yellow. – UV/Vis
(EtOH): λmax = 407, 488 nm. – IR: ν = 1622 cm−1 (m,
C=N). – C17H13NO · 0.5EtOH: calcd. C 79.98, H 5.97,
N 5.18; found C 80.43, H 5.80, N 5.30.
7.73× 10−3 dm3 mol−1
s
−1). This behavior may be
due to the drop of the reduction potential of the metal
center caused by the presence of the moderately de-
activating group, -SO3Na, which reduces the σ-donor
properties of the ligand [13].
In summary, new families of Co(III)-, Fe(III)-, and
Cr(III)-based transition metal complexes bearing salen
ligands have been synthesized and characterized. The
reactivity of some of the isolated complexes towards
the oxidation reaction of L-cysteine has been investi-
gated. Results have shown that the rate of oxidation
depends on the metal center, the auxiliary groups, and
the substituents on the backbone of the ligand. Further
studies on the kinetics of L-cysteine oxidation by these
complexes will be reported.
Sodium N-(4-sulfonatosalicylidene)cyclohexylamine
(5a) and sodium N-(4-sulfonatosalicylidene)-1-
naphthylamine (5b)
A solution of sodium p-sulfonatosalicylaldehyde (2)
(0.75 g, 3.30 mmol) in a methanol : water mixture (1 : 1) was
added dropwise to a solution of the amine (3a or 3b) (0.33 g,
3.30 mmol) in ethanol (10.0 mL). The mixture was stirred
at r. t. for 3 h, then refluxed for 30 min. Upon cooling, a yel-
low precipitate was formed. The product was filtered, washed
with methanol, and dried.
Experimental Section
Materials
◦
5a: Yield: 0.67 g (66 %). Color: yellow. – M. p. 350 C
(dec.). – UV/Vis (EtOH): λmax = 316, 395 nm. – IR: ν =
CoCl2 · 6H2O and CrCl3 · 6H2O were purchased from 1644 cm−1 (m, C=N). – C13H16NNaO4S · 0.5H2O: calcd.
C 49.7, H 5.45, N 4.46; found C 49.9, H 5.86, N 4.16.
ACROS, naphthylamine and anhydrous FeCl3 were pur-
chased from Aldrich, cyclohexylamine was obtained from
Sigma, and L-cysteine (minimum assay 99 %) was purchased
from BDH Laboratory Supplies (England). All chemicals
were used without further purification.
◦
5b: Yield: 0.84 g (45 %). Color: yellow. – M. p. 350 C
(dec.). – UV/Vis (EtOH): λmax = 359, 450 nm. – IR: ν =
1617 cm−1 (m, C=N). – C17H12NNaO4S · 2H2O: calcd.
C 52.98, H 4.18, N 3.64; found C 52.90, H 3.92, N, 3.65.
Synthesis of complexes
Physical measurements
Chloro-bis(N-salicylidenecyclohexylamine)cobalt(III) (6a)
Elemental analyses were performed using a EURO EA
3000 instrument. Infrared spectra (KBr pellets) were mea-
sured on a Nicolet Magna-IR 560 spectrophotometer. Mag-
netic susceptibility measurements were carried out using a
magnetic susceptibility balance from Alfa Johnson Mathey.
Thermal gravimetric analysis (TGA) was carried out on a
Netzsch STA 409 PC LUXX instrument at a heating rate
of 10 K/min. Kinetic measurements were performed using
a diode array spectrophotometer model 8453E from HP Ag-
ilent. Melting points were measured with a Stuart Scientific
melting apparatus (uncorrected, 0.1 ◦C).
A solution of 4a (0.50 g, 2.45 mmol) in MeOH (5.0 mL)
was added dropwise to a solution of CoCl2 · 6H2O (0.24 g,
1.025 mmol) in MeOH (20.0 mL) with continuous stirring.
◦
The mixture was heated at 150 C for 30 min, then stirred
at r. t. for 2 h. After that, hydrogen peroxide (30 %, 3.0 mL)
was added dropwise. During addition, a dark green precip-
itate was formed. The mixture was stirred for another hour,
then the precipitate was filtered, washed with EtOH (5.0 mL),
and dried.
◦
6a: Yield: 0.12 g (24 %). Color: green. – M. p. 198 C
(dec.). – UV/Vis (EtOH): λmax (lgε) = 389 nm (3.78). –
IR: ν = 1624 cm−1 (m, C=N). – C26H32ClCoN2O2: calcd.
C 62.59, H 6.46, N 5.61; found C 62.89; H 6.72; N 5.59.
Synthesis of ligands
N-salicylidenecyclohexylamine (4a) and
N-salicylidenenaphthylamine (4b)
Chloro-bis(N-salicylidenecyclohexylamine)iron(III) (7a)
A solution of 4a (0.50 g, 2.46 mmol) in MeOH (5.0 mL)
A solution of salicylaldehyde (1) (2.0 g, 0.016 mol) in
ethanol (10.0 mL) was added dropwise to a solution of the was added dropwise to a solution of FeCl3 (0.34 g,
desired amine (3a and 3b) (0.022 mol) in the same solvent 2.24 mmol) in the same solvent (20.0 mL). Upon addition,
(10.0 mL). The mixture was stirred at r. t. for 3 h, during a dark violet solution was formed. The solution was heated
which time the color changed from colorless to yellow. After at 150 ◦C for 30 min, then stirred at r. t. for 2 h. A violet pre-
evaporation of the solvent, an oily residue was left, which cipitate was formed which was extracted with a petroleum
was extracted with ethanol and dried.
ether/diethyl ether mixture (1 : 1).
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A. S. Abu-Surrah et al. · Cobalt(III), Iron(III), and Chromium(III) Complexes with Salicylaldiminato Ligands
851
◦
◦
7a: Yield: 0.075 g (13 %). Color: green. – M. p. 187 C stirring. The mixture was heated at 150 C for 30 min, then
(dec.). – UV/Vis (EtOH): λmax (lgε) = 487 nm (3.45). – stirred at r. t. for 2 h. Hydrogen peroxide (30 %, 3.0 mL)
IR: ν = 1624 cm−1 (m, C=N). – C26H32ClFeN2O2: calcd. was added dropwise. Upon addition, a green precipitate was
C 62.98, H 6.51, N 5.65; found C 62.51, H 6.23, N 5.27.
formed. The mixture was stirred for 1 h, then the precipi-
tate was filtered, washed with THF and petroleum ether, a◦nd
dried. Yield: 0.090 g (47 %). Color: green. – M. p. 300 C
(dec.). – UV/Vis (EtOH): λmax (lgε) = 383 nm (3.09). –
IR: ν = 1614 cm−1 (m, C=N). – C26H30ClCoN2Na2O8S2:
calcd. C 44.42, H 4.30, N 3.98; found C 44.25, H 4.58,
N 3.92.
Chloro-bis(N-salicylidenecyclohexylamine)chromium(III)
(8a)
A solution of 4a (0.50 g, 2.45 mmol) in THF (5.0 mL)
was added dropwise to a solution of CrCl3 · 6H2O (0.27 g,
1.02 mmol) in the same solvent (3.0 mL) with continu-
ous stirring. Upon addition, the pink color of the solution
changed to dark green. The mixture was stirred at r. t. for
Chloro-bis[sodium N-(4-sulfonatosalicylidene)cyclohexyl-
amine]iron(III) (10a)
◦
24 h, then the solution was concentrated at 150 C until an
olive green precipitate was formed. The product was filtered,
washed thoroughly with petroleum ether, and dried.
Schiff base 5a (0.147 g, 0.48 mmol) in MeOH (10.0 mL)
was added dropwise to a solution of FeCl3 (0.035 g,
0.218 mmol) in MeOH (5.0 mL) with continuous stirring.
Upon addition, a dark violet solution was formed. The so-
lution was heated at 150 ◦C for 30 min, then stirred at
r. t. for 2 h. A violet precipitate was formed. The prod-
uct was filtered, washed with petroleum ether, and dried.
Yield: 0.050 g (33 %). Color: violet. – M. p. 350 ◦C (dec.). –
UV/Vis (EtOH): λmax (lgε) = 488 nm (2.74). – IR: ν =
1621 cm−1 (m, C=N). – C26H30ClFeN2Na2O8S2 · 9H2O:
calcd. C 36.22, H 5.61, N 3.25; found C 35.98, H 5.10,
◦
8a: Yield: 0.32 g (64 %). Color: green. – M. p. 113 C
(dec.). – UV/Vis (EtOH): λmax (lgε) = 406 nm (3.16). –
IR: ν = 1618 cm−1 (m, C=N). – C26H32ClCrN2O2 · 8H2O:
calcd. C 49.09, H 7.61, N 4.40; found C 49.33, H 7.81,
◦
N 4.67. – TGA (30 – 120 C): mass loss = 22.3 % (calcd.
22.7 % for 8 H2O).
Chloro-bis(N-salicylidene-1-naphthylamine)cobalt(III) (6b),
chloro-bis(N-salicylidene-1-naphthylamine)iron(III) (7b),
and chloro-bis(N-salicylidene-1-naphthylamine)chrom-
ium(III) (8b)
◦
N 3.08. – TGA (30 – 120 C): mass loss = 18.92 % (calcd.
19.18 % for 9 H2O).
The complexes were prepared following the procedures
described for the synthesis of the corresponding complexes
6a, 7a, and 8a, respectively. A solution of 4b (0.50 g,
2.02 mmol) in MeOH (5.0 mL) was reacted with a solution
of the metal salt (1.81 mmol) in the same solvent (20.0 mL).
Chloro-bis[sodium N-(4-sulfonatosalicylidene)cyclohexyl-
amine]chromium(III) (11a)
A solution of 5a (0.20 g 0.65 mmol) in MeOH (5.0 mL)
was added dropwise to a solution of CrCl3 · 6H2O (0.11 g,
0.30 mmol) in MeOH (3.0 mL) with continuous stirring.
Upon addition the pink color of the solution changed to dark
green. The mixture was stirred at r. t. for 24 h, then the solu-
tion was concentrated at 150 ◦C. After cooling, a light green
precipitate was formed. The product was extracted with THF.
◦
6b: Yield: 0.08 g (13 %). Color: brown. – M. p. 205 C
(dec.). – UV/Vis (EtOH): λmax (lgε) = 480 nm (2.61). –
IR: ν = 1624 cm−1 (m, C=N). – C34H24ClCoN2O2 · 4H2O:
calcd. C 61.97, H 4.89, N 4.25; found C 61.80, H 4.57,
N 5.35.
◦
◦
Yield: 0.20 g (95 %). Color: green. – M. p. 350 C (dec.). –
7b: Yield: 0.20 g (34 %). Color: violet. – M. p. 230 C
UV/Vis (EtOH): λmax (lgε) = 387 nm (3.55). – IR: ν =
1653 cm−1 (m, C=N). – C26H30ClCrN2Na2O8S2 · 7H2O:
calcd. C 37.98, H 5.39, N 3.41; found C 37.81, H 4.94,
N 3.16. – TGA (30 – 120 C): mass loss = 16.10 % (calcd.
15.17 % for 7 H2O).
(dec.). – UV/Vis (EtOH): λmax (lgε) = 504 nm (3.19). –
IR: ν = 1616 cm−1 (m, C=N). – C34H24ClFeN2O2: calcd.
C 69.94, H 4.14, N 4.79; found C 70.64, H 4.21, N 4.52.
8b: Yield: 0.94 g (19 %). Color: light-brown. – M. p.
207 ◦C (dec.). – UV/Vis (EtOH): λmax (lgε) = 403 nm
(1.83). – IR: ν = 1634 cm−1 (m, C=N). – C34H24ClCrN2O2
· 2MeOH: calcd. C 67.13, H 5.01, N 4.35; found C 67.25,
H 5.25, N 4.64.
◦
Chloro-bis[sodium N-(4-sulfonatosalicylidene)naphthyl-
amine]iron(III) (10b), and chloro-bis[sodium N-(4-sulfon-
atosalicylidene)-1-naphthylamine]chromium(III) (11b)
Chloro-bis[sodium N-(4-sulfonatosalicylidene)cyclohexyl-
amine]cobalt(III) (9a)
The complexes 10b and 11b were prepared following the
procedures described above for the synthesis of the com-
A solution of 5a (0.20 g, 0.65 mmol) in MeOH (5.0 mL) plexes 10a and 11a, respectively. A solution of 5b (0.30 g,
was added dropwise to a solution of CoCl2 · 6H2O (0.064 g, 0.86 mmol) in MeOH (5.0 mL) was reacted with the metal
0.27 mmol) in the same solvent (20.0 mL) with continuous salt (0.39 mmol) in the same solvent (20.0 mL).
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852
A. S. Abu-Surrah et al. · Cobalt(III), Iron(III), and Chromium(III) Complexes with Salicylaldiminato Ligands
10b: Yield: 0.019 g (11 %). Color: dark green. – C34H22ClCrN2Na2O8S2 · 9H2O: calcd. C 43.16, H 4.26,
M. p. 350 ◦C (dec.). – UV/Vis (EtOH): λmax (lgε) = N 2.69; found C 43.23, H 4.03, N 2.67. – TGA (230 –
511 nm (3.49). – IR: ν = 1627 cm−1 (m, C=N). – 120 ◦C): mass loss = 17.08 % (calcd. 17.13 % for 9 H2O).
C34H22ClFeN2Na2O8S2 · H2O: calcd. C 50.67, H 3.00,
N 3.48; found C 50.33, H 3.31, N 3.71.
Acknowledgement
11b: Yield: 0.20
g (95 %). Color: light-green. –
M. p. 350 ◦C (dec.). – UV/Vis (EtOH): λmax (lgε) =
395 nm (3.26). – IR: ν = 1615 cm−1 (m, C=N). –
Financial support by the Hashemite University is gratefully
acknowledged.
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