V.S. da Silva et al. / Applied Catalysis A: General 491 (2015) 17–27
19
[MIIIP(L)]+ + L ꢀ [MIIIP(L)2]+ K2
(3)
be complete when UV–vis spectral variations ceased to occur. Dilu-
tion effects along the titration were accounted for when calculating
the total concentration of [MnIIIP]+ and ligand at each titration
point. The mathematical treatment of the absorbance versus con-
centration data was carried with the use of the software SQUAD
[36]. Distribution curves were constructed for the species by using
the program Hyperquad simulation and speciation (HySS) version
4.0.31 [37].
Spectrophotometric titrations usually help to assess ligand coor-
dination to metalloporphyrins. The number of axial ligands (n) and
the stability constants (ˇn) can be calculated on the basis of optical
absorption measurements as a function of the ligand and metal-
loporphyrin concentrations. Assuming that the metalloporphyrin
concentration remains constant throughout the titration, graphi-
cal methods derived from the Benesi–Hildebrand method [41] can
2.4. Cyclohexane oxidation reactions
Titration of the metalloporphyrins MnIIITPPCl, MnIIIAPTPPCl,
All the catalytic reactions were performed in 2-mL Wheaton®
vials sealed with Teflon-faced silicon septa. Reactions were per-
formed under magnetic stirring, at 25 ◦C, for 90 min, using
procedures adapted from de Sousa et al. [38]. Cyclohexane oxida-
tion was carried out in air using either PhI(OAc)2 or PhIO as oxygen
donor. Reaction mixtures comprised 2.0 × 10−4 mmol of the cata-
lyst ([MnIIIP]+), 2.0 × 10−3 mmol of the oxidant (PhIO or PhI(OAc)2),
100 L of cyclohexane (0.93 mmol), and 200 L of DCM. The cata-
lyst:oxidant:substrate molar ratio was 1:10:4650. When deemed
necessary, the reaction was quenched by the addition of sulfite and
borax solution [38]. The reaction mixtures were directly analyzed
by capillary gas chromatography using bromobenzene as internal
standard, and the retention times of the products were confirmed
by comparison with those of authentic product samples [39]. The
yields were based on either initial PhIO or PhI(OAc)2. Each reaction
was accomplished at least three times, and the reported data rep-
resent the average of the results of these reactions; errors in yields
and selectivity were calculated on the basis of the reproducibility
of the reactions. The degree of manganese porphyrin destruction
(bleaching) was determined by UV–vis spectroscopy at the end
of the catalytic run, considering the molar absorptivity of nonco-
ordinated [MnIIIP]+ specie even in the imidazole presence. Control
reactions were conducted in the absence of the catalyst, under the
same conditions as the catalytic runs. The effect of imidazole was
studied by adding an aliquot of a 1.0 × 10−2 mol L−1 imidazole (Im)
solution in DCM to the reaction medium.
and MnIIIT4CMPPCl with imidazole prompted, initially,
a
hypochromic shift of the Soret band, followed by a hypsochromic
the Soret band occurred for MnIIIBr9APTPPCl (Fig. 2d). As for
MnIIIBr8T4CMPPCl, first there was
lowed by hypsochromic and hyperchromic shifts of the Soret
band (Fig. 2e). These shifts evidenced imidazole coordination to
[MnIIIP]+ and suggested a complex equilibrium in which more
employed and frequently adapted to evaluate the forma-
tion constants associated with metalloporphyrin–axial ligand
interaction, especially in the field of hemoprotein model sys-
tems [41,42]. However, this method considers that only one
metalloporphyrin–axial ligand complex (for example, [MIIIP(L)]+ or
[MIIIP(L)2]+] coexists with the initial species at equilibrium. There-
fore, application of the B–H method is not advisable in the case of
librium, as observed in the current [MnIIIP]+/Im systems.
The Fortran-based software “Stability Quotients from
Absorbance Data”, SQUAD, has been extensively used to calcu-
for the equilibrium model proposed in the case of the [MnIIIP]+
titrations with imidazole. The parameters ꢀ(Obsvd-Calc)2 and
˛
(Table 1), which assess the reliability of the proposed model,
data
3. Results and discussion
fell within the acceptable limits [36].
The higher values of K2 with respect to K1 for the coordina-
tion equilibrium of imidazole to either MnIIITPPCl or MnIIIAPTPPCl
species upon increasing Im concentration (Table 1). As for
MnIIIT4CMPPCl, MnIIIBr9APTPPCl, and MnIIIBr8T4CMPPCl, the pen-
tacoordinated species was favored (K2 < K1, Table 1).
3.1. Spectrophotometric titration with imidazole
their active sites axially coordinated to an amino acid residue
(cysteine, histidine, tyrosinate). In many cases, the axial ligand
determines the functional specificities of the metalloenzyme
[1,27,40]. Considering that the present study aims to evalu-
ate how the presence of imidazole as an axial ligand affects
cyclohexane oxidation reactions catalyzed by manganese(III) por-
phyrins, it was necessary to investigate the equilibrium describing
metalloporphyrin-imidazole interactions.
Eq. (1) describes the axial coordination of a monodentate ligand
L to a metalloporphyrin MP (where M is a metallic ion); ˇn and n
correspond to the stability constant and the number of axial ligands
(L), respectively.
Analysis of the results from the axial coordination of imidazole
to the second-generation [MnIIIP]+ presented in Table 1 revealed
that these [MnIIIP]+ behaved differently in terms of the coordi-
nation of the second ligand. The MnIIIT4CMPPCl contained four
electron withdrawing COOCH3 substituents, which may have
metal center in MnIIIAPTPPCl, which only displayed one electron
withdrawing substituent ( NH2). In the case of MnIIIAPTPPCl, the
hexacoordinated species had higher formation constant than the
pentacoordinated species; i.e., K1 < K2 (Table 1), whereas K2 < K1 for
MnIIIT4CMPPCl. This difference might have been due to the possi-
ble axial coordination of the para-amino substituent in the phenyl
groups (meso position) to the metal center of another MnIIIAPTPPCl,
thereby competing with imidazole for the coordination site. There-
fore, the species [MnIIIP]Cl, [MnIIIP(Im)]+, and [MnIIIP(Im)2]+ could
exist with [MnIIIP]+ species axially coordinated to the amino
group of other [MnIIIP]+ in the system involving MnIIIAPTPPCl.
The formation constants for the systems with MnIIIAPTPPCl and
MnIIIBr9APTPPCl were calculated ignoring the possible role of this
ˇ
n
MP + nLꢀMP(L)n
(1)
bound to the metal center in the basal plane. It is possible to com-
plete the Mn3+ coordination sphere with ligands like imidazole or
pyridine in the axial positions, to obtain penta- [MnIIIP(L)]+ and
hexacoordinated [MnIIIP(L)2]+ species, represented in Eqs. (2) and
(3):
[MnIIIP]+ + L ꢀ [MnIIIP(L)]+ K1
(2)