Influence of the Net Charge on the Reactivity of a Manganese (IV

Article pubs.acs.org/JPCC

Influence of the Net Charge on the Reactivity of a Manganese(IV) Species: Leading to the Correlation of Its Physicochemical Properties with Reactivity Yujuan Wang, Jiayi Sheng, Song Shi, Dajian Zhu, and Guochuan Yin* School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China S Supporting Information *

ABSTRACT: Clarifying how versatile physicochemical parameters of an active metal intermediate affect its reactivity would help to understand its roles in chemical and enzymatic oxidations. The influence of the net charge on electron transfer and hydrogen abstraction reactions of a manganese(IV) species having hydroxide ligand has been investigated here. It was found that increasing one unit of the positive net charge from 2+ to 3+ would accelerate its electron-transfer rate by 10−20 fold in oxygenation of tris(4-methoxyphenyl)phosphine. In contrast, the hydrogen abstraction rate is insensitive to its net charge change, and the insensitivity has been attributed to the compensation effect between the redox potential and pKa, which determine the hydrogen abstraction capability of a metal ion. Similar net-charge-promoted electron transfer but not hydrogen abstraction has also been observed in intramolecular electron transfer and hydrogen abstraction reactions when using thioxanthene as substrate. Together with the previous understanding of the reactivity of the identical manganese(IV) species having MnIV−OH or MnIV=O functional groups, the relationships of the oxidative reactivity of an active metal intermediate with its physicochemical parameters such as the net charge, the redox potential and the metal−oxygen bond order (M−O versus MO) have been discussed with this manganese(IV) model.

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INTRODUCTION Transition-metal ions play the critical roles in a series of electron transfer, hydrogen abstraction and oxygen transfer processes.1−5 In addition to the metal oxo functional groups, the active metal hydroxo and hydroperoxide groups have recently been revealed to serve as the key active intermediates in these oxidative events in natural redox enzymes and their inorganic models.6−29 For the metal oxo and hydroxo groups, that is, Mn+=O and Mn+−OH, the viable difference between them is their protonation state, which leads to the changes of the net charge, the redox potential, and the metal−oxygen bond order (MO vs MO) of the central metal ion. Apparently, the change of these physicochemical parameters would unavoidably lead to its oxidative reactivity changes. Although the Mn+O and Mn+−OH moieties have been extensively investigated in versatile catalytic and quantitative oxidation reactions, and their mechanisms have been independently elucidated,30−56 it is still not clear how the oxidative reactivity of a redox metal intermediate changes with the change of these parameters such as the net charge, the redox potential, and the metal−oxygen bond order. In particular, there does not exist a universal theory to describe or predict how the oxidative reactivity of an active metal intermediate changes along with these physicochemical properties. The major obstacle could be that there is no available model to investigate systematically the relationship of the reactivity of a high oxidation metal ion with these parameters, especially, in one model to investigate the © 2012 American Chemical Society

reactivity similarities and differences of the metal ions having different metal−oxygen bond order, for example, Mn+O versus Mn+−OH. Obviously, understanding how the reactivity of an active metal intermediate changes with the change of its physicochemical properties would help to predict the reactivity of an active metal intermediate and help to clarify the mechanisms of the redox enzymes, which were not fully understood until now. Meanwhile, it may also help to explain those oxidative phenomena that are still puzzling the chemists and thus benefit the rational design of the selective oxidation catalysts. Ten years ago, with an ethylene cross-bridged cyclam ligand, a series of transition-metal complexes were explored in Busch’s lab, and later, one of us even successfully synthesized and wellcharacterized a monomeric manganese(IV) complex having dihydroxide ligand, [MnIV(Me2EBC)(OH)2](PF6)2 (Me2EBC: 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane, Figure 1).57−63 This manganese(IV) complex has three pKa values including one accurate pKa at 6.86 and two approximate values at ∼2 and 10. The pKa value of 6.86 represents deprotonation of [MnIV(Me2EBC)(OH)2]2+ (1b) to generate [MnIV(Me2EBC)(O)(OH)]+, whereas the pKa values at ∼2 and ∼10 represent adding one proton to form Received: April 5, 2012 Revised: May 31, 2012 Published: June 1, 2012 13231

dx.doi.org/10.1021/jp303281z | J. Phys. Chem. C 2012, 116, 13231−13239

The Journal of Physical Chemistry C

Article

OH and MnIV=O have very similar hydrogen abstraction capability (83.0 vs 84.3 kcal/mol), but the Mn IV =O demonstrates faster rate than the MnIV−OH. Their distinct differences are that after the hydrogen abstraction the reduced MnIII−OH from the MnIV=O is capable of rebinding the hydroxyl group back to the substrate radical to form the hydroxylation product whereas the MnIII−OH2 from the MnIV−OH cannot. However, the manganese(IV) species having the MnIV−OH has much more powerful electrontransfer capability than that having the MnIV=O group. In addition, the oxidative reactivity of the related MnIV−OOH group in olefin epoxidation, pollutant degradation, hydrogen abstraction, and sulfur oxygenation has also been investigated, which provides the first chance to compare the oxidative relationships of the MnIV=O, MnIV−OH, and MnIV−OOH functional groups in one model.69−73 In the present work, the influence of the net charge on the hydrogen abstraction and electron-transfer activity of the manganese(IV) species having MnIV−OH group will be investigated using the identical manganese(IV) model. Taken together with the previous findings on the oxidative reactivity of the MnIV=O and MnIV− OH groups, it provides an unique chance to correlate these physicochemical parameters including the net charge, the redox potential, and the metal−oxygen bond order (M−O vs MO) with the oxidative reactivity of an active metal intermediate.

Figure 1. Crystal structure of the [MnIV(Me2EBC)(OH)2]2+.63

[MnIV(Me2EBC)(OH)(H2O)]3+ (1a) and losing the last proton to generate the neutral [MnIV(Me2EBC)(O)2] (1c), respectively (eq 1). LMn IV (OH)(OH 2)3 +

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pK a ∼ 2

HooooooI LMn IV (OH)2 2 +

MATERIALS AND METHODS The Mn(Me2EBC)Cl2 complex was presented by Professor Daryle Busch from University of Kansas, and the manganese(IV) complex was synthesized according to the literature.63 (a) 1,4-Cyclohexadiene was from Aldrich. (b) Tris(4methoxyphenyl)phosphine was from Acros Organics. (c) Acetone was from J. T. Baker. (d) Ammonium hexafluorophosphate was from Alfa Aesar. (e) Others came from local Sinopharm Chemical Reagent. The kinetic data were collected on Analytikjena specord 205. GC-MS analysis was performed on Agilent 7890A/5975C. LC-MS analysis was performed on Agilent 1100 LC/MSD. ESR experiment was conducted on Bruker A300. NMR analysis was performed on Bruker AV400. All of the experiments below were performed at least in duplicate. Measurements of Solvent Isotope Effect. (a) To measure the solvent isotope effect in electron transfer process, we performed the oxygenation of tris(4-methoxyphenyl)phosphine (initial concentration, 10 mM) by manganese(IV) complex (initial concentration 0.5 mM) in neutral acetone/ H2O or D2O (ratio 4:1) at 293 K. The control experiment without tris(4-methoxyphenyl)phosphine was conducted in parallel for each oxygenation reaction, and the corrections from the control experiments had been done for calculations of the pseudo-first-order rate constants. (b) To measure the solvent isotope effect in hydrogen abstraction, we performed the hydrogen abstraction from 1,4-cyclohexadiene (initial concentration, 20 mM) by manganese(IV) complex (initial concentration 1 mM) in neutral acetone/H2O or D2O (ratio 4:1) at 293 K. The control experiment without 1,4-cyclohexadiene was conducted in parallel for each hydrogen abstraction reaction, and the corrections from the control experiments had been done for calculations of the pseudo-first-order rate constants. General Procedure for Rate Measurements of Tris(4methoxyphenyl)Phosphine Oxygenation. To determine the reactivity difference of tris(4-methoxyphenyl)phosphine oxygenation by manganese(IV) complex at pH 1.5 and 4.0, we

pK a 6.86

HoooooooI LMn IV (O)(OH)+ pK a ∼ 10

HoooooooI LMn IV (O)2

(1)

Therefore, it is feasible to control the net charge and the form of the functional group of the manganese(IV) species through the pH adjustments. For example, at pH 1.5, the net charge of the manganese(IV) species is 3+ having one MnIV−OH functional group, and they are 2+ at pH 4.0 having two MnIV−OH groups, 1+ at pH 8.4 having one MnIV−OH with one MnIV=O group, and zero at pH 13.4 having two MnIV=O groups (Figure 2). Therefore, it provides the expected platform

Figure 2. Net charge and form of the manganese(IV) species under different pH conditions.

to investigate the oxidative relationships of the Mn+O and Mn+−OH moieties having identical coordination environment and oxidation state and how these parameters including the net charge, the redox potential, and the metal−oxygen bond order affect the reactivity of the redox metal ions. With this manganese(IV) model, the reactivity similarities and differences of the MnIV−OH with its corresponding MnIV=O group in hydrogen abstraction, oxygen transfer, and electron transfer have been investigated.64−68 It has been found that the MnIV− 13232



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suitable substrate to investigate the influence of the net charge on the electron-transfer activity of the manganese(IV) species, even though its oxygenation proceeds by electron transfer. Alternatively, tris(4-methoxyphenyl)phosphine ((4MeOPh)3P) was selected as substrate for electron transfer studies with the manganese(IV) complexes here. Unlike triphenylphosphine, (4-MeOPh)3P as substrate reveals a normal sKIE in its oxygenation. For example, the sKIE value is 1.55 at 293 K (kobs,H2O = 8.99 × 10−4 s−1, kobs,D2O = 5.81 × 10−4 s−1), supporting the fact that the manganese(IV) species in 1b is capable of oxygenating (4-MeOPh)3P through electron transfer, and its protonation is not essential. Therefore, at pH 4.0, it represents the electron transfer by the manganese(IV) species having 2+ net charge in 1b, whereas at pH 1.5, it represents the electron transfer by the same manganese(IV) species having 3+ net charge in 1a. In addition, at pH 1.5, protonation of (4-MeOPh)3P in acetone/water (4:1) was not observed by 31P NMR, and thus the interference of its protonation under the reaction conditions could be eliminated. The (4-MeOPh)3P oxygenations with the freshly synthesized manganese(IV) complexes were performed in acetone/water (ratio 4:1) at pH 1.5 or 4.0, respectively. After the reaction, the purple color of the authentic manganese(IV) species disappears, and the solution turns to colorless, indicating the reduction of the manganese(IV) species to the colorless manganese(II) species with the formation of tris(4methoxyphenyl)phosphine oxide product, which has been identified by LC-MS. In particular, the six-line superfine splitting of the manganese ESR signal with the ESI-MS of m/ z = 344.1, corresponding to [MnII(Me2EBC)(OH)(OH2)]+, also supports the conversion of the manganese(IV) species to the corresponding manganese(II) species after the reaction (Figure 3).79,80 The oxygen in the resulting phosphine oxide

carried out the oxygenation of tris(4-methoxyphenyl)phosphine (10 mM) with the manganese(IV) complex (1 mM) in acetone/water (ratio 4:1) at 288 K. The pH values were adjusted by NaOH or HCl as needed. The disappearance of the manganese(IV) species was monitored by UV−visible spectrophotometry, and the pseudo-first-order rate constant was calculated. The control experiments without tris(4methoxyphenyl)phosphine were conducted in parallel for each oxygenation reaction, and the corrections from the control experiments had been done for calculations of the pseudo-first-order rate constants. General Procedure for Rate Measurements of Hydrogen Abstraction from 1,4-Cyclohexadiene. To measure the rates of hydrogen abstraction from 1,4-cyclohexadiene by the manganese(IV) complex at pH 1.5 and 4.0, the reactions were carried out with the manganese(IV) complex (1 mM) in acetone/water (ratio 4:1) at 288K. The pH values were adjusted by NaOH or HCl as needed. The disappearance of the manganese(IV) species was monitored by UV−visible spectrophotometry, and the pseudo first-order-rate constant was calculated. The control experiments without 1,4-cyclohexadiene were conducted in parallel for each reaction, and the corrections from the control experiments had been done for calculations of the pseudo-first-order rate constants. General Procedures for Intramolecular Competitive Hydrogen Abstraction and Oxygenation. In 5 mL of acetone/water (ratio 4:1) solvent at pH 1.5, 4.0, or 13.4 containing 5 mM of the manganese(IV) complex, 0.025 mmol of thioxanthene was added, and the resulting reaction mixture was stirred under nitrogen at 298 K for (a) 24 h for the reaction at pH 1.5 and 4.0 and (b) 6 h for the reaction at pH 13.4. (The pH was adjusted with NaOH or HCl as needed.) After the reaction, the identification of the product was performed by GC-MS, and the quantitative analysis was conducted with GC with the internal standard method.

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RESULTS AND DISCUSSION Influence of the Net Charge on the Electron-Transfer Activity of the MnIV−OH Moiety. As shown in Figure 2, at pH 1.5 and 4.0, 1a and 1b have the identical functional group, that is, MnIV−OH, with different net charge (3+ vs 2+). Because of the different protonation state, 1a has one water ligand at its cis position, and 1b has one hydroxyl group at its cis position (H2O vs OH−). Unlike those axial ligands that demonstrate strong trans-influence on the active metal oxo group, for example, (porphyrin)FeIV=O, the change of the cis ligand generally has less influence on the perpendicular group of, for example, the MnIV−OH in this manganese(IV) model.74 Therefore, their rate differences in electron transfer and hydrogen abstraction would reflect the influence of the net charge on the manganese(IV) species. In comparing the oxygenation differences between the MnIV−OH and MnIV=O in Mn IV (Me 2 EBC) model, we recently observed that triphenylphosphine oxygenation by the MnIV−OH proceeds by electron transfer having sensitive substituent effect, whereas it proceeds by concerted oxygen transfer for the MnIV=O with insensitive effect.66 In particular, in the MnIV−OH-mediated oxygenation, it was further found that the oxygenation was performed by the manganese(IV) species in 1a rather than in 1b because the solvent kinetic isotope effect (sKIE) is inverse. For example, sKIE is only 0.611 at 298 K. The inverse sKIE is typical evidence that protonation of 1b to generate 1a is prior to electron transfer.75−78 Therefore, triphenylphosphine is not a

Figure 3. Mass and ESR spectra of the manganese(II) species after reaction. Conditions: acetone/water (4:1), initial Mn(IV) 1 mM, and (4-MeOPh)3P 10 mM.

possibly comes from water as those in other phosphine oxygenations through electron-transfer process.81 The first electron is transferred from (4-MeOPh)3P to the manganese(IV) species, resulting in the reduced manganese(III) species and the (4-MeOPh)3P+• cation radical; then, hydrolysis of (4MeOPh)3P+• forms (4-MeOPh)3P•OH. The generated (4MeOPh)3P•OH can be feasibly oxidized to (4-MeOPh)3PO by either manganese(IV) or manganese(III) species, which are not distinguishable here (eqs 2−4). (4‐MeOPh)3 P + Mn(IV) H 2O

⎯⎯⎯→ (4‐MeOPh)3 P+• + Mn(III)

(4‐MeOPh)3 P+• + H 2O → (4‐MeOPh)3 P•‐OH 13233

(2) (3)



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(4‐MeOPh)3 P•‐OH + Mn(IV)/Mn(III) → (4‐MeOPh)3 PO

(4)

In addition, there is no saturation behavior of kobs observed, indicating no preequalibrium intermediate occurring between the manganese(IV) species and the (4-MeOPh)3P substrate, and thus the electron transfer may most possibly proceed by outer electron transfer.82 The second-order rate constants (k2) for (4-MeOPh)3P oxygenation by the manganese(IV) species at pH 1.5 and 4.0 are summarized in Table 1. One may see that in the Table 1. Rate Differences of Electron Transfer from (4MeOPh)3P by Manganese(IV) Species Having Different Chargea temp (K) k2 at pH 1.5 (M−1 s−1) 0.470 ± 0.020 0.806 ± 0.030 1.70 ± 0.059

273 283 293

k2 at pH 4.0 (M−1 s−1)

k2,pH1.5/ k2,pH4.0

0.022 ± 0.002 0.045 ± 0.001 0.174 ± 0.002

21.4 17.9 9.8

Figure 4. Eyring plot for oxygenation of (4-MeOPh)3P by the manganese(IV) species at pH 1.5 having +3 net charge.

a

Conditions: acetone/water (ratio 4:1), initial concentration of manganese(IV) 1 mM.

investigated temperature range the manganese(IV) species having 3+ net charge at pH 1.5 demonstrate 10−20 times faster oxygenation rate than the corresponding manganese(IV) species having 2+ net charge at pH 4.0. For example, at 273 K, the k2 at pH 1.5 is 0.470 ± 0.020 M−1 s−1, whereas it is 0.022 ± 0.002 M−1 s−1 at pH 4.0, representing a 21.4 fold rate difference of electron transfer; at 293 K, it is 1.70 ± 0.059 M−1 s−1 at pH 1.5, whereas it is 0.174 ± 0.002 M−1 s−1 at pH 4.0, also representing 9.8 fold of rate difference. The measured activation enthalpy (ΔH‡) and activation entropy (ΔS‡) through Eyring plot at pH 1.5 are 9.8 ± 0.4 kcal/mol and −22.4 ± 0.8 cal·mol−1·K−1 over the temperature range 268− 298K, and they are 13.9 ± 0.2 kcal/mol and −14.7 ± 0.7 cal·mol−1·K−1 over the temperature range 273−303K at pH 4.0 (Table 2, Figures 4 and 5), consistent with the faster electron

Figure 5. Eyring plot for oxygenation of (4-MeOPh)3P by the manganese(IV) species at pH 4.0 having +2 net charge.

Table 2. Activation Parameters for Electron Transfer from (4-MeOPh)3P by Manganese(IV) Species Having Different Charge manganese(IV) species

ΔH‡ (kcal/mol)

ΔS‡ (cal·mol−1·K−1)

1a 1b

9.8 ± 0.4 13.9 ± 0.2

−22.4 ± 0.8 −14.7 ± 0.7

transfer, for example, in hydrogen abstraction reaction,84,85 whereas in the charge-promoted electron transfer, the increased net charge provides the enhanced driving force for electron transfer. Indeed, because of the one unit increase in the positive charge, the redox potential of the manganese(IV) species at pH 1.5 is slightly higher than that at pH 4.0 (+0.54 vs +0.46 V (vs SCE), Figure 6), and this 80 mM of the potential difference could be responsible for the 10−20 fold rate difference in electron transfer. In sulfoxidation by iron(IV) oxo through electron transfer, the barrier height is linearly correlated with the ionization potential of the substrate.86 Similarly, for (4MeOPh)3P as the single substrate in oxygenation, the electron transfer may reasonably accelerate with the redox potential increase of oxidant from 1b to 1a. In literature, the great rate enhancements have also been observed in Lewis-acid-promoted electron transfer and related oxidations in which the redox potential of the metal ion also increases substantially by adding Lewis acid.87−98 Furthermore, the Lewis acid has been suggested to be ligated to the redox metal ions in some cases. Obviously, through ligation of the Lewis acid, the net charge of the redox metal ions would unavoidably increase, which leads to the increase in the redox potential and the

transfer rate at pH 1.5 than that at pH 4.0. The significant activation entropy differences also support their sensitive ratio difference to the temperature change. As described by Mayer,83 the entropy of electron transfer is dependent on the nature of the metal ion, its ligand environment, counterion, and solvent. For the electron transfer from (4-MeOPh)3P to the same manganese(IV) species at pH 1.5 and 4.0, their entropy difference (−22.4 vs −14.7 cal·mol−1·K−1) may be attributed to their different ligand environment (H2O vs OH−) and their different solvent condition (pH 1.5 vs 4.0). Such an acceleration of the electron-transfer rate by increasing the net charge of the active metal ion is regarded as the charge-promoted electron transfer, which is different from the known proton coupling electron transfer (PCET). In PCET, the proton is transferred concertedly with electron 13234



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the hydrogen abstraction by the MnIV−OH moiety at pH 1.5 and 4.0 in the temperature range 283−313 K, which is consistent with insensitive influence of the net charge on its hydrogen abstraction rate and the insensitivity of the ratio difference to the temperature change (Table 4, Figures 7 and Table 4. Activation Parameters for Hydrogen Abstraction from 1,4-Cyclohexadiene by Manganese(IV) Species Having Different Charge manganese(IV) species

ΔH‡ (kcal/mol)

ΔS‡ (cal·mol−1·K−1)

1a 1b

10.8 ± 0.2 8.2 ± 0.1

−35.7 ± 0.8 −44.3 ± 0.4

Figure 6. Cyclic voltammograms (vs SCE) of the manganese(IV) complex in acetone/water(4:1) under nitrogen at pH 1.5, 4.0, and 13.4.

enhanced oxidation rate as well as the accelerated electron transfer through protonation in this manganese(IV) model. Influence of the Net Charge on the Hydrogen Abstraction Activity of the MnIV−OH Moiety. In hydrogen abstraction, 1,4-cyclohexadiene was selected as the substrate because its hydrogen abstraction reaction is clean with benzene as the sole product. Significantly, the sKIE also reveals a normal sKIE value. For example, the sKIE value is 1.09 at 293 K (kobs = 3.61 × 10−5 s−1, kobs = 3.31 × 10−5 s−1). Therefore, as well as in electron transfer, at pH 4.0, it represents the hydrogen abstraction by the MnIV−OH moiety having 2+ net charge in 1b, whereas at pH 1.5, it represents the hydrogen abstraction by the same MnIV−OH moiety having 3+ net charge in 1a. In contrast with the acceleration of electron transfer by increasing the net charge of the manganese(IV) species, the hydrogen abstraction rate of the MnIV−OH is insensitive to the net charge change. The second-order rate constants (k2) for quantitative hydrogen abstraction from 1,4-cyclohexadiene by the manganese(IV) species are listed in Table 3. Apparently,

Figure 7. Eyring plot of hydrogen abstraction by manganese(IV) from 1,4-cyclohexadiene at pH 1.5 in acetone/water (4:1).

Table 3. Rate Differences of Hydrogen Abstraction from 1,4Cyclohexadiene by Manganese(IV) Species Having Different Chargea temp. (K)

k2 at pH 1.5 (M−1 s−1)

k2 at pH 4.0 (M−1 s−1)

k2,pH1.5/ k2,pH 4.0

293 303 313

(9.09 ± 0.07) × 10−4 (1.73 ± 0.08) × 10−3 (2.86 ± 0.09) × 10−3

(8.68 ± 0.08) × 10−4 (1.52 ± 0.01) × 10−3 (2.90 ± 0.09) × 10−3

1.05 1.14 0.99

a

Conditions: acetone/water (ratio 4:1), initial concentration of manganese(IV) 1 mM.

the hydrogen abstract rates by the MnIV−OH moiety at pH 1.5 are comparable to those at pH 4.0. For examples, at 293 K, the k2 at pH 1.5 is (9.09 ± 0.07) × 10−4 M−1 s−1 for the MnIV−OH having the net charge of 3+, whereas it is (8.68 ± 0.08) × 10−4 M−1 s−1 for the same MnIV−OH having the net charge of 2+ at pH 4.0, and the corresponding ratio of k2,pH1.5/k2,pH4.0 is only 1.05. At 313 K, the k2 values are (2.86 ± 0.09) × 10−3 M−1 s−1 at pH 1.5 and (2.90 ± 0.09) × 10−3 M−1 s−1 at pH 4.0, respectively, and the ratio is also only 0.99, close to unity. In addition, the Eyring plots reveal the similar activation enthalpy (ΔH‡), 10.8 ± 0.2 versus 8.2 ± 0.1 kcal·mol−1, and activation entropy (ΔS‡), −35.7 ± 0.8 vs −44.3 ± 0.4 cal·mol−1·K−1, for

Figure 8. Eyring plot of hydrogen abstraction by manganese(IV) from 1,4-cyclohexadiene at pH 4.0 in acetone/water (4:1).

8). No influence of the net charge on the hydrogen abstraction could be rationalized from that, in determining the hydrogen abstraction capability of a metal oxo or hydroxo group;99,100 here the increased redox potential due to the net charge increase (0.54 V at pH 1.5 vs 0.46 V at pH 4.0 (vs SCE)) has been compensated by the decreased pKa value of the 13235



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influence change between the MnIV−OH having cis water ligand at pH 1.5 and the cis OH− ligand at pH 4.0. (See Figure 2.) Therefore, the viable change is the net charge of the central manganese(IV) ion (3+ vs 2+), which leads to the redox potential and pKa change. The increased redox potential would drive the accelerated electron transfer as those in Lewis acid promoted oxidations. Similar redox potential and reactivity increase in the oxidation of Br− to OBr− along the series of X = O2−, OH−, and H2O, where X is the axial ligand of the (Porphyrin)MnV=O and in PPh3 oxygenation by [FeIV(O)(TMC)(X)]n+ along the series of X = RS−, N3−, CF3COO−, and CH3CN.105,106 It is worth noting that although our results in electron transfer are consistent with those in literature, the results in hydrogen abstraction have been complicated from different systems. In Groves’ (Porphyrin)MnV=O complex, its olefin epoxidation reactivity also increases along the series of X = O2−, OH−, H2O as well as in Br− oxidation.107 Eisenstein’s DFT calculations also support the fact that the hydrogen abstraction reactivity increases along this series, and they have rationalized it by that the ground-state singlet (S) is inactive for all X ligand, but the higher triplet (T) and quintet (Q) states are active for hydrogen abstraction. Significantly, the X ligand having strong trans-influence would increase the S/T and S/Q gaps and thus decrease its reactivity.108 On the other side, in Nam’s [FeIV(O)(TMC)(X)]n+ complex, the hydrogen abstraction reactivity is inverted as RS− > N3− > CF3COO− > CH3CN, and it has been rationalized with Shaik’s two-state model.106 Our manganese(IV) complex also demonstrates the similar hydrogen abstract reactivity in the series of O2− > OH− > H2O as those in Nam’s [FeIV(O)(TMC)(X)]n+ complex. However, the difference is that the reactivity changes in those literatures originate from both trans-influence and the net charge, whereas in our system, it comes from the sole net charge. As the axial ligand providing the trans-influence and net charge to affect the pKa and the redox potential, the cis ligand in our system also provides charge to affect the redox potential and the pKa but to an extent different from those of the trans-influence. Therefore, their compensation effect in determining the hydrogen abstraction would be different. Notably, although the MnIV=O demonstrates a more than 40-fold faster rate than the corresponding MnIV−OH group in hydrogen abstraction, their hydrogen abstraction capabilities determined from the redox potential and pKa are very similar (84.3 vs 83.0 kcal/ mol).64,65 In Visser’s calculations, they also found that the barrier height in hydrogen abstraction by the heme or nonheme iron(IV) oxo is linearly correlated with the strength of the Fe(III)−OH bond that is formed, that is, BDEOH, which is determined from the redox potential and pKa. Correlation of the Net Charge, the Redox Potential, and the Manganese(IV)−Oxygen Bond Order with Its Oxidative Reactivity. In comparing the electron transfer and hydrogen abstraction capability of the manganese(IV) species in 1b at pH 4.0 with that in 1c at pH 13.4, it was even found that the MnIV−OH demonstrates much faster electron-transfer rate than the corresponding MnIV=O. However, they have very similar hydrogen abstraction capability as determined from the redox potential and pKa (83.0 vs 84.3 kcal/mol), but the MnIV=O reacts much faster than the MnIV−OH.64,65,68 Together with the findings disclosed here, it provides the following sequence of the oxidative activity of the manganese(IV) species: (a) in electron transfer: [LMnIV−OH]3+ > [LMn IV −OH] 2+ ≫ [LMn IV =O] and (b) in hydrogen

corresponding reduced manganese(III) complexes (∼1.6 vs 5.8759), as described in eq 5, where E1/2 is the redox potential of the oxidant, pKa is the deprotonation constant of the reduced metal ion, and C is the constant (57.6 kcal/mol from the latest data100). BDFEO − H = 23.06E1/2 + 1.37pK a + C

(5)

Intramolecular Competitive Hydrogen Abstraction and Electron Transfer of the Manganese(IV) Species. Next, the intramolecular competitive hydrogen abstraction and electron transfer provide further evidence of the significant influence of the net charge on the electron transfer of the manganese(IV) species but not hydrogen abstraction. Thioxanthene has both hydrogen abstraction and electron transfer sites with similar steric and electronic environments (eq 6).

In products, thioxanthen-9-one (A) comes from hydrogen abstraction at the methylene site, whereas 9-acetonylthioxanthene (B) is the electron-transfer product from the sulfur site in acetone solvent. In acetone, the generated electron-transfer intermediate, that is, sulfur cation radical, is trapped by acetone and then rearranges to 9-acetonylthioxanthene as the final product101 (similar rearrangements have been documented in refs 102−104). At pH 13.4, when hydrogen abstraction by the MnIV=O in 1c provides 17.3 ± 1.4% yield of thioxanthen-9-one, the same manganese(IV) species is incapable of electron transfer. At pH 4.0, the MnIV−OH in 1b gives 6.7 ± 0.3% yield of thioxanthen-9-one with trace electron transfer product, whereas at pH 1.5, the same manganese(IV) species in 1a gives dominant electron-transfer product with less hydrogen abstraction product, that is, 64.4 ± 1.8% yield of 9acetonylthioxanthene versus 12.1 ± 0.5% yield of thioxanthen-9-one (Table 5). Apparently, the electron-transfer activity of Table 5. Intramolecular Competitive Hydrogen Abstraction and Oxygenation with the Manganese(IV) Complexes yield of product (%) manganese(IV) moiety

net charge

A

B

MnIV−OH at 1.5a MnIV−OH at 4.0a MnIV=O at 13.4b

3+ 2+ 0

12.1 ± 0.5 6.7 ± 0.3 17.3 ± 1.4

64.4 ± 1.8 trace not detected

a

Reaction conditions: acetone/water (4:1) 5 mL, manganese(IV) complex 5 mM, thioxanthene 5 mM, stirring under nitrogen at 298 K for 24 h. bReaction conditions: acetone/water (4:1) 5 mL, manganese(IV) complex 5 mM, thioxanthene 5 mM, stirring under nitrogen at 298 K for 6 h.

the manganese(IV) species is very sensitive to its net charge, demonstrating charge-promoted electron transfer, whereas the hydrogen abstraction is not so sensitive to that due to the compensation effect. In literature, there are many experimental and computational results demonstrating that the axial ligand significantly affects the reactivity of the Mn+O moiety. In general, the axial ligand may provide the trans-influence and net charge to affect the reactivity of the Mn+O through affecting its redox potential and pKa. In our manganese(IV) model, there is no trans13236



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abstraction: [LMn I V −OH] 3 + ∼ [LMn I V −OH] 2 + ≪ [LMnIV=O]. The physicochemical parameters of the related manganese species that may affect the oxidative activity of the corresponding manganese(IV) species have been summarized in Table 6. One may see that the trend of the pKa values of the

are correlated with the change of its protonation state, net charge, redox potential, and its metal−oxygen bond order but not with the change of its coordination environment; typically, its axial ligand remains unchanged. Because of the transinfluence, the axial ligand would significantly affect the reactivity of an active metal intermediate, for example, (porphyrin)FeIV=O, even that its functional group does not change.8,105−109 The disclosed relationships of the reactivity of an active metal intermediate with its physicochemical properties have provided clues to understand those events occurring in enzymatic and chemical oxidations. In cytochrome P450 enzymes, an FeIV=O radical cation intermediate to serve as the compound I is crucial for achieving substrate hydroxylation in biological metabolism.110 If an FeIV−OH functional group served as the compound I, after hydrogen abstraction to generate the substrate radical, then the resulting FeIV−OH2 is incapable of hydroxylating substrate through oxygen-rebound mechanism as well as the incapability of the MnIII−OH2 moiety from the MnIV−OH in this manganese(IV) model.68 On the other side, an FeIII−OH or MnIII−OH functional group to serve as the key intermediate is the also crucial for the lipoxygenases in oxygenation of the unsaturated fatty acids.111−113 If an Mn+ O moiety served as the active intermediate in hydrogen abstraction from the methylene group of the unsaturated fatty acid, then the resulting M(n−1)+−OH moiety after hydrogen abstraction may have the chance to rebind the hydroxyl group back to the substrate radical to form the hydroxylated product as well as those in cytochrome P450 enzymes. As a result, it would block the rearrangement of the substrate radical and finally block its trapping by dioxygen to generate the expected peroxide product. In heterogeneous oxidations, it has been widely observed that in Au-catalyzed CO combustions adding little moisture to the gaseous feed would substantially accelerate the oxidation reactions.114−120 However, the role of the moisture has not yet been fully understood. Now, one may suspect that the introduction of the moisture may cause the partial protonation of the metal oxide catalyst, for example, from Mn+O or Mn+−O−Mn+ to Mn+−OH or Mn+−O(H)− Mn+. As disclosed here, the Mn+O and Mn+−OH moieties have significantly different properties such as different redox potentials and electron-transfer capabilities. Therefore, the current studies may provide new clues to understand those unknown oxidation events.

Table 6. Thermodynamic Parameters and Oxidative Activity of the Mn(Me2EBC) Species MnIV moiety net charge E1/2 (V) versus SCE pKa of the reduced MnIII complexa BDFEO−H kcal/mol electron-transfer rate hydrogen abstraction rate a

[LMnIV− OH]3+

[LMnIV− OH]2+

3+ +0.54 ∼1.6

2+ +0.46 5.87

[LMnIV=O] 0 +0.10

77.8 81.9 [LMnIV−OH]3+ > [LMnIV−OH]2+ ≫ [LMnIV=O] [LMnIV−OH]3+ ∼ [LMnIV−OH]2+ ≪ [LMnIV=O]

pKa values were obtained from ref 59.

reduced manganese(III) species is opposite to the net charges and redox potentials of the corresponding manganese(IV) species. Therefore, the hydrogen abstraction rate does not change so much because of the compensation effect between the redox potential and pKa, and thermodynamically their capabilities are comparable. However, the electron-transfer reaction could accelerate with the increase in the net charge and the redox potential. Although the pKa of the reduced LMnIII(O)(OH) is unknown, it should be highly basic to provide the driving force for its enhanced hydrogen abstraction rate. Apparently, with the change of the protonation state, the net charge, the redox potential, the metal−oxygen bond order, and the reactivity of an active metal intermediate would change simultaneously. As observed in this manganese(IV) model: (1) for an active metal intermediate having either oxo or hydroxo functional group, increasing the net charge would increase its redox potential and thus greatly accelerate its electron-transfer rate. However, its influence on the hydrogen abstraction capability is complicated due to the compensation effect between the redox potential and pKa. One example is that the influence is ignorable for the MnIV−OH mediated hydrogen abstraction in this study. (2) The distinct reactivity changes will occur when the functional group of an active metal intermediate changes from the Mn+O to Mn+−OH through protonation: (i) The redox potential would increase greatly, and the metal ion having Mn+−OH moiety demonstrates much more powerful electron transfer capability than the corresponding metal ion having Mn+O moiety. (ii) The Mn+O is capable of hydrogen abstraction and oxygenation through oxygen-rebound mechanism or concerted oxygen transfer mechanism, and thus the desaturation, hydroxylation, or epoxidation product could be generated depending on substrate, whereas the Mn+−OH can perform hydrogen abstraction, but it cannot further play oxygen rebound mechanism or concerted oxygen transfer mechanism, and thus it cannot provide hydroxylation or epoxidation product. (iii) although the Mn+O and Mn+−OH moieties demonstrate distinctly different reactivity after hydrogen abstraction, they have similar hydrogen abstraction capability with different rates because of the compensation effect. It is worth noting that the reactivity shifts of an active metal intermediate discussed here

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CONCLUSIONS The present work reveals that increasing the net charge of an active metal intermediate would substantially accelerate its electron-transfer rate, demonstrating charge-promoted electron transfer, whereas its influence on the hydrogen abstraction is ignorable because of the compensation effect between the redox potential and pKa. Similar charge-dependent electron transfer and charge-independent hydrogen abstraction have been further observed in intramolecular competitive electrontransfer and hydrogen-abstraction reactions. Taken together with the known reactivity similarities and differences of the MnIV−OH with its corresponding MnIV=O moiety, the relationships of the net charge, the redox potential, the metal−oxygen bond order, and the reactivity of an active metal intermediate have been elucidated using this manganese(IV) model, which provides new clues to understand the roles of these active metal intermediates in chemical and enzymatic 13237



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oxidations and benefit the rational design of the selective oxidation catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed kinetic data for kinetic solvent isotope effect, electron transfer, and hydrogen abstraction reaction. 31P NMR spectra of tris(4-methoxyphenyl)phosphine at pH 1.5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: 86-27-87543632. Phone: 86-27-87543732. E-mail: gyin@ hust.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant 20973069). The 31P NMR experiments and the product identification through GC-MS and HPLC-MS were performed in Analytical and Testing Center, Huazhong University of Science and Technology.

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Influence of the Net Charge on the Reactivity of a Manganese (IV

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