H. M. Abdel-Halim, A. S. Abu-Surrah, H. M. Baker
ARTICLE
isomers, the product undergoes a reductive elimination process ability to act as an electron withdrawing group, increases the
(RE) that eliminates the two RS· units to yield cystine (RSSR), reductive potential for the metal ion making its reaction faster.
and an unstable MI complex which immediately oxidizes to
Finally, the effect of the metal ion on reaction rate was inves-
tigated. Rate constants for CoIII were higher than FeIII com-
plexes with the same substituents. Table 1 shows that the rate
of oxidation of cysteine by cis-[Co(BCHP)2Cl2]Cl (6) is an
order of magnitude larger than that of cis-[Fe(BCHP)2Cl2]Cl
(10). This can be attributed to the reduction potential of the
ion. The standard reduction potential of Co3+ is +1.81 V, which
is much higher than that of Fe3+ (+0.77 V). A higher E° means
stronger driving force for the reaction, which leads to higher
reaction rates for Co3+ than for Fe3+ complexes.
MII by O2 present in reaction mixture. The process can be sum-
marized by the following steps:
[MIII(L–L)2Cl2]+ + RSH → [MIII(L–L)2Cl]2+ → [MIII(L–
Intermediate
L)2(SR)Cl]+ + HCl
[MIII(L–L)2(SR)Cl]+
+
RSH
→
[MIII(L–L)2(SR)]2+
→
Intermediate
[MIII(L–L)2(SR)2]+ + HCl
RE
[MIII(L–L)2(SR)2]+
[MII(L–L) Cl ] + RS–SR
(8)
→
2
2
2Cl–
Products of the oxidation process were isolated and charac-
terized [22]. A colorless precipitate, cystine (RS–SR), was
characterized by elemental analysis (calcd. C 29.99, H 5.03, N
11.66, S 26.69 %; found C 30.01, H 4.88, N 10.99, S
26.06 %), and MS (EI, 70 eV): m/z ( %) = 243 (76) [M+]. This
result supports the suggested pathways for the reaction of the
two structural isomers with cysteine.
The difference in the two rates for the trans isomers is due
to two factors: the steric factor, caused by crowding in the
octahedral arrangement and the trans effect, which arises from
the ability of ligand to stabilize the transition state in the rate-
limiting dissociation step. The –RS group has a stronger trans
effect than chlorine due to its larger ability to act as a σ elec-
tron donor. For trans-dichlorobis(N,N'-bis(cyclohexyl)-2,3-bu-
tanediimine)cobalt(III) chloride (1), which has a large ligand,
k1 (1.9 × 10–1·s–1) is higher than k2 (2.7 × 10–3·s–1) due to the
steric factor associated with the first substitution of chloride
by cysteine. However, for trans-dichlorobis(N,N'-bis(2-isopro-
pylphenyl)-1,2-phenyldiimine)cobalt(III) chloride (7), k2 has a
value of 9.1 × 10–2·s–1 which is higher than k1 (6.3 × 10–3·s–1)
due to trans effect.
Table 1 also shows that the rate constant of cis-
[Co(BIPPP)2Cl2]Cl (8) is higher than that of cis-
[Co(BCHP)2Cl2]Cl (2), which is in turn much higher than that
of cis-[Co(BNP)2Cl2]Cl (9S). The three isomers have the simi-
lar structural backbone but with different substituents. Rates of
cysteine oxidation by complexes based on 2-isopropylphenyl
are faster than the rates for complexes with cyclohexyl substit-
uents. Additionally, the reaction rates of complexes based on
naphthyl groups (4T and 4S), (9T and 9S), and (11T and 11S)
are very slow. This variety in rates is due to both steric and
electronic factors. For 9S, the bulky naphthyl group hinders
the cysteine anion from entering into the complex sphere (inac-
tive), whereas for compound 8, the 2-isopropylphenyl substitu-
ent provides both electronic donation from the alkyl group, and
electronic withdrawing by the phenyl group. The electronic
withdrawing influence is less important for complexes with
cyclohexyl substituents (2), their reactions are slower than
those of compound 8. A similar argument can be made about
the different rates for other cis isomers. The rate constant of
cis-[Co(BCHP)2Cl2]Cl (5) is higher than that of cis-
[Co(BCHB)2Cl2]Cl (2) and the rate constant of oxidation of
cysteine by compound 5, with phynilinediimine backbone, is
higher than that of complex 2 with a butadiimine backbone.
The electronic factor of the phynilinediimine, which has higher
Conclusions
Rates of oxidation of l-cysteine by pairs of trans and cis
CoIII and FeIII complexes based on α- and γ-diimine Schiff
base ligands were studied. Higher rate constant for trans iso-
mers were largely attributed to steric factor. The less crowded
trans isomers facilitate electron transfer and thus increases the
rate of oxidation. The nature of substituents in the ligands
plays a role in determining reaction rates mainly through their
size and electron donating ability. Additionally, reaction rates
were found to depend on the metal ion. Oxidation rates of l-
cysteine by CoIII complexes were higher than those by FeIII
complexes due to the higher positive standard reduction poten-
tial of CoIII.
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