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Kinetic Evidence for Reactive Dimeric TAML Iron Species in the Catalytic Oxidation of NADH and a Dye by O in AOT Reverse Micelles 2
Liang L Tang, Alexander D Ryabov, and Terrence J. Collins ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00787 • Publication Date (Web): 03 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016
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Kinetic Evidence for Reactive Dimeric TAML Iron Species in the Catalytic Oxidation of NADH and a Dye by O2 in AOT Reverse Micelles Liang L. Tang, Alexander D. Ryabov* and Terrence J. Collins* Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: The efficacy of the iron(III) TAML activator [Fe{C6H4-1,2-(N1COCMe2N2CO)2CMe2(Fe⎼N1)(Fe⎼N2)}(OH2)]- (1) for the catalytic activation of dioxygen in reverse micelles of Aerosol OT (AOT) in n-octane has been studied. Kinetic evidence is presented for the existence of unusual second-order catalytic pathways in the oxidation of NADH to NAD+ and the bleaching of blue Pinacyanol chloride (PNC) dye. Depending on the substrate and reaction conditions, a second-order pathway in [1] either dominates or proceeds in obvious combination with a first-order pathway in [1]. Detailed kinetic analysis of the experimental data supports the previously made hypothesis that the reactive intermediate is associated with the mixed-valent dimer system, [LFeIIIOFeIVL]3-/[LFeIII(OH)FeIVL]2-. KEYWORDS: iron, TAML, catalysis, oxygen activation, mechanism, reverse micelles 1. INTRODUCTION It has been recently demonstrated that TAML activators of peroxides such as 1, where iron(III) is incorporated into the macrocyclic cavity of tetra-organic-amido-N ligands (Chart 1), function also using dioxygen as a primary oxidant in sys1 tems of reverse micelles. This new TAML feature is a valuable addition to the reactivity profile of TAML activators, which is associated most significantly to date with catalytic, environmentally beneficial oxidations of pollutants by hy2-5 drogen peroxide in aqueous solutions. In the aqueous mi6 croreactors of reverse micelles, 1 is oxidized by O2 to, inter III IV 3alia, an oxo- or hydroxo-bridged [LFe OFe L] III IV 2/[LFe (OH)Fe L] dimer (2), which could be a reactive intermediate in oxidations of electron-rich donors such as the NADH cofactor and Pinacyanol chloride (PNC) dye (Chart 1 1). The system of reverse micelles is crucial for performing TAML-catalyzed oxidations by O2 though less reactive elec1 tron donors (dyes Orange II and Safranin O) do not react. 1
Among the major goals of the previous study was the characterization of 2 by EPR and Mössbauer spectroscopies. However, the identification of oxidized iron complexes does not substantiate that these are the reactive intermediates in a catalytic cyclea proposed reactive intermediate should always be supported by appropriate kinetic evidence. Therefore, in this work we report the results of kinetic investigations of 1-catalyzed oxidations of PNC and NADH by O2 in reverse micelles of Aerosol OT (AOT) in n-octane (Chart 1), 6 choosing degree of hydration w0 = 10 and pHs 8 and 10. Ki7 8 netic data for 1-catalyzed oxidations of PNC and NADH by H2O2 in water is available. The data presented herein reveal the existence of a second-order pathway in [1] for both PNC and NADH oxidations, which (i) supports the hypothesis
that dimer 2 could be a dominant reactive species under 1 specified conditions, and (ii) is compatible with the fact that only reactive reducing agents are subject to oxidation—from V the accumulated evidence on the relative reactivities of Fe IV 9 and Fe TAML species, we can be sure that 2 is only modestly oxidizing. 2. EXPERIMENTAL SECTION 2.1. Materials. All reagents, components of buffered solutions and solvents were at least ACS reagent grade and were used as received. 1 was obtained from GreenOx Catalysts, Inc and purified using a C18-silica gel column with an eluent of water/methanol (v/v 95/5) mixture. n-Octane was used as received (99%+, extra pure, Acros) or additionally distilled -3 (99%+, extra dry, Acros). Stock solutions of 1 (1.0×10 M) were prepared in 0.01 M phosphate (pH 8) or carbonate (pH 10) aqueous buffered solutions and stored at 4 °C in a fridge. AOT (bis(2-ethylhexyl) sodium sulfosuccinate, BioXtra 99%+, Aldrich) was used as received. Reverse micelles of w0 10 were prepared by adding the corresponding amount of buffered solution (36 µL, pH 8 or 10) to AOT solution in noctane (0.1 M, 1964 µL) followed by vigorous shaking for NADH and sonication for PNC. Concentrations of all components reported throughout refer to the entire volume of the solution including n-octane, AOT and buffer components. β-Nicotinamide adenine dinucleotide reduced disodium salt (NADH, 98.3%, MP Biomedicals) was used as received. Its 0.0145 M stock solution in the appropriate buffer was prepared daily, stored in a foam cooler box and used within 6 hours. Pinacyanol chloride (PNC, Aldrich) was used without further purification.
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Chart 1. Key compounds involved in this study (TAML 1 and TAML-derived 2; NADH and PNC dye) and schematic composition of reverse micelle (right). TAML is a registered trademark of Carnegie Mellon University, covering tetra-organic-amido-N macrocyclic ligand complexes.10 2.2. Methods. UV-vis measurements were performed at 25 °C in capped quartz cuvettes (1.0 cm) using an Agilent 8453 photodiode array UV-vis spectrophotometer equipped with an automatic thermostatted 8-cell positioner. The initial rates reported throughout and shown in Figures 1⎼3 are mean values of at least three determinations; error bars are standard deviations. Spectra of reaction mixture were scanned every 30 s. Initial rates were determined at 340 (NADH) and 560 nm (PNC) following the absorbance decrease for 240 s. In the pH 10, w0 10 PNC case, the fast reaction required that less data points be used to obtain the initial rate at high 1 concentration. The extinction coefficients of NADH (340 nm) and PNC (560 nm) used for rate calculations were measured under different conditions and are summarized in Table 1. Unless otherwise stated, measurements were carried out in ambient air, i.e. at atmospheric concentration of dioxygen. NADH oxidation was used for probing the effect of O2 on the initial rate of NADH oxidation at w0 = 7. The experiments were performed using n-octane solutions saturated with ambient air, where the gas composition was changed by purging with mixtures of different ratios of Ar and O2. The gas mixture was purged for 15 min before addition of 1 via syringe. The purging continued during the reaction. The reaction mixture was stirred by a magnetic bar.
Table 1. Extinction coefficients ε (M-1 cm-1) of NADH at 340 nm and PNC at 560 nm used for reaction rate calculations.
At pH 10, the rate is noticeably dependent on [PNC] in the same concentration range, while the reaction order in [1] is higher than one, but less than two, suggesting a mixed reaction order in the catalyst, i.e. the coexistence of first- and second-order pathways in [1]. Therefore, the kinetic data were fitted to eq 1.
Initial Rate = + + (1)
pH
NADH
PNC
8
(5.68 ± 0.03)×10
(5.78±0.06)×10
10
(5.70±0.05)×10
3
(3.46±0.07)×10
3
important characteristic of the medium which, together with pH, determines the rates of chemical and enzymatic reac6,11,12 Under the optimal conditions (w0 = 10, pH = 12) at tions. a PNC:1 ratio of 100:1, PNC is readily decolorized by O2 in the 1 reverse micelle system with a TON of 90 at 25 °C. Previously, the kinetics of PNC bleaching by H2O2 catalyzed by 1 has been investigated in aqueous media—the process results in a 7 deep fragmentation of the PNC molecule. Therefore, it seemed logical to also examine this dye in kinetic studies in the reverse micelles as a possibly useful model compound. Typical results of the kinetic study are presented in Figure 1. At pH 8, the initial rate is virtually independent of the [PNC] -5 in the range of (0.45⎼4.45) × 10 M. In contrast, the initial rate is strongly dependent on [1] in the range of (0.50⎼5.56) × -6 10 M, with the reaction order in the catalyst being equal to two. The second order pathway in [1] is a new feature in catalysis by TAML activators. When TAMLs function in water utilizing H2O2 or organic peroxides as primary oxidants, first 4,5 order kinetics in [TAML] is commonly observed, though a 13 reaction order of one half has also been confirmed.
4
4
3. RESULTS AND DISCUSSION 3.1. Kinetics of Bleaching of Pinacyanol Chloride. The aqueous pseudo-phase of a reverse micelle is a microreactor where hydrophilic molecules are localized. Its size and the water content within are determined by the degree of hydration, i.e. the water-to-surfactant molar ratio in the bulk sys6 tem w0 = [H2O]/[AOT]. The degree of hydration w0 is an
The minor term v0, which is particularly noticeable at pH 8, corresponds to the non-catalyzed pathway of PNC disappearance as a result of direct autoxidation/aggregation of 14 PNC, the existence of which was confirmed in this study in control experiments in the absence of 1 following a reported 1 methodology. Interestingly, the kA term is practically negligible at pH 8 where pure second order kinetics in [1] is observed (Figure 1A). The kinetic data in Figure 1B were fitted to eq 1 and the best-fit values of kA and kB are summarized in Table 2. The data collected at pH 8 were also fitted to eq 1 assuming kA ~ 0. The results are also summarized in Table 2.
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A
B
Figure 1. Initial rates of 1-catalyzed PNC oxidation by O2 as functions of the concentrations of PNC (∆) and 1 (●) in reverse mi-6 -5 celles of AOT in n-octane at pH 8 (A) and 10 (B), w0 10. A: pH 8, [1] = 2.50×10 M at variable PNC; [PNC] = 1.45×10 M at variable -6 -5 1. B: pH 10, [1] = 2.49×10 M at variable PNC; [PNC] = 1.43×10 M at variable 1. TAML dashed lines are calculated using kA and kB -1 -1 from Table 2. NADH solid lines are calculated using the following values of eq 5 (α/s , β/M s , and γ/M) equal, respectively, (~0; -9 -7 -3 -8 -5 3×10 ; and 7×10 ) at pH 8 and (1×10 ; 2×10 ; and 1×10 ) at pH 10.
A
B
Figure 2. Initial rates of 1-catalyzed oxidation of NADH by O2 as functions of concentrations of NADH (∆) and 1 (●) in reverse -6 -4 micelles of AOT in n-octane at pH 8 (A) and 10 (B), w0 10. A: pH 8, [1] = 2.52×10 M at variable NADH; [NADH] = 1.16×10 M at -6 -4 variable 1. B: pH 10, [1] = 2.49×10 M at variable NADH; [NADH] = 1.16×10 M at variable 1. TAML dashed lines are calculated -1 -1 using kA and kB from Table 2. NADH solid lines are calculated using the following values of eq 5 (α/s , β/M s , and γ/M) equal, -5 -8 -5 -8 -5 respectively, (2×10 ; 2×10 ; and 4×10 ) at pH 8 and (~0; 2×10 ; and 2×10 ) at pH 10. 3.2. Kinetics of NADH Oxidation. We have previously shown that the oxidation of NADH by O2 in reverse micelles 1 occurs with the presumptive stoichiometry of eq 2. 1 1 (2) NADH + O2 + H NAD+ + H O 2 In contrast with the bleaching of PNC, oxidation of NADH depends highly on the light flux. When the progress of NADH oxidation is monitored using a photodiode array UVvis spectrophotometer, the process becomes faster as the frequency of spectral acquisition is increased, i.e. with increasing inverse time between the recordings of successive spectra. Similar unusual accelerations have recently been 15 reviewed. Preliminary results show that the initial rate of 1 NADH oxidation is a linear function of the flash frequency. At pH 10, w0 10, when the spectra were recorded every 30 s, 1 the TON was 88. Correspondingly, all kinetic data were col-
lected recording spectra every 30 s at pH 8 and 10 keeping the degree of hydration fixed. The results of kinetic investigation of NADH oxidation are demonstrated in Figure 2. Careful inspection of the data reveals mechanistically important similarities with the data collected for PNC, although these occur at different pHs for the two substrates. The [NADH] and [1] were varied in the -4 -6 ranges of (0.22⎼2.25)×10 and (0.50⎼5.53)×10 M, respectively. Note that for PNC a second order in [1] and zero order in [substrate] were found at pH 8 (Figure 1A). In the case of NADH such a behavior is observed at pH 10 (Figure 2B). A mixed order in [1] (eq 1) and a stronger dependence on [substrate] is observed at pH 10 for PNC (Figure 1B) and at pH 8 for NADH (Figure 2A). Most important that for the both substrates a second order in [1] is accompanied by zero order in [substrate], and a mixed order in [1] results in a measurable [substrate] dependency.
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Table 2. Kinetic parameters of eq 2 for 1-catalyzed oxidation of PNC (1.45×10-5 M) and NADH (1.16×10-5 M) by O2 in AOT reverse micelles in n-octane at w0 10 and 25 °C. Substrate PNC NADH
-1
-1
pH
v0 / M s
8
(1.3±0.6)×10
10
-9
-9
(0±2)×10
-1 -1
kA / s
kB / M s
~0
(0.37±0.07)×10
3
-3
(0.6±0.3)×10
-3
3
8
(3±3) × 10
10
(1.3±0.8)×10
-9
0.9868
(6±2)×10
(1.0±0.4)×10
~0
(2.11±0.05)×10
The effect of O2 concentration on the initial rate of NADH oxidation was studied and the rate was found to have very low sensitivity to O2 (Figure 3). NADH was chosen for the O2 dependency study because it is more straightforward to work with dissolving easily upon shaking the reaction media— PNC with its tendency to aggregate requires sonication. Therefore, the reaction mechanism should account for this fact as well.
0.9722 3
III -
2
0.9610
3
(8±2)×10
-9
r
0.9885
-
IV
2-
4 [LFe ] + O2 + 4 OH ⇌ 4 [LFe O] + 2 H2O IV
2--
IV
2-
[LFe O]
III -
III
IV
+ [LFe ] ⇌ [LFe OFe L]
K1
3-
k2, k-2
III -
[LFe O] + S → [LFe ] + Primary product(s) (PP) III
IV
3-
III -
[LFe OFe L] + S → 2 [LFe ] + PP IV
2-
III
IV
3-
PP + [LFe O] / [LFe OFe L]
k3 k4
→→→ III -
→ Final product/s +[LFe ]
fast
Scheme 1. Stoichiometric mechanism of 1-catalyzed oxidation of PNC and NADH (S) by O2 in reverse micelles. For simplicity, complex 2 is denoted as [LFeIIIOFeIVL]3-.
S "
=
.& # .& OH ) S $ ' O +
.& 1 + #$ O .& OH )
#$ .& * O .& OH ) S (3) (1 + #$ .& O .& OH ) ) () + * S)
Figure 3. Rate dependence on O2 concentration. Conditions: -5 -6 pH 10, w0 7, [NADH] = 5.09×10 M, [1] = 2.49×10 M. 3.3. Mechanism of PNC and NADH Oxidation. Any proposed mechanism for the processes studied must be consistent with the following findings: (i) When the reaction rate is second order in [1], it is also zero order in [S] for both PNC and NADH. (ii) When the rate is mixed order in [1], it is also non-zero order in [S] for both PNC and NADH. (iii) The reaction rate has very low sensitivity to the [O2]. These three conditions are satisfied by the stoichiometric mechanism presented in Scheme 1, under the assumptions that (a) in the NADH case, S is its photoactivated form NADH*, (b) the III reversible oxidation of the Fe -TAML occurs fast and (c) the III IV 3concentration of the intermediate [LFe OFe L] is negligiIII IV ble compared to those of [LFe ] and [LFe O] , i.e. the mass III IV balance equation appears as Fet ≈ [LFe ] + [LFe O] , where Fet is the total concentration of iron. At the μM catalyst concentrations employed for the study, none of the catalyst species are observable. In the Scheme, the first equation represents a rapidly established equilibrium obviously comprised of a series of elementary reactions. With the assumptions noted above and applying the steady-state approximation III IV 3with respect to [LFe OFe L] , one arrives at the two-term eq 3 for the catalyzed oxidations which includes first- and second-order terms in [1].
Consistent with the experimental observations, eq 3 predicts a very low dependence of the rate on the concentration of O2. Equation 3 could be re-written in more compact forms as eq 4 by substituting a = #$ .& O .& OH) and b = (1 + #$ .& O .& OH ) ) or eq 5, the latter to emphasize the rate dependence on the concentration of S.
S " S "
=
,' S , * S + (4) - () + * S)
= /S +
0S (5) γ + S
Equation 4 accounts for the second order dependence in [1] and the zero order dependence in [S] provided k-2
