Water exchange reaction of a manganese catalase mimic: oxygen-17 NMR relaxometry study on (aqua)manganese(III) in a salen scaffold and its reactions in a mildly basic medium

Abstract

The kinetics of water exchange for trans-[Mn^III(salen)(OH₂)₂]⁺ has been investigated by the ¹⁷O NMR line broadening technique in ¹⁷OH₂ enriched aqueous medium. The rate and activation parameters for the exchange reaction, trans-[Mn^III(salen)(OH₂)₂]⁺ + ¹⁷OH₂ → trans-[Mn^III(salen)(OH₂)(¹⁷OH₂)]⁺ + H₂O are: k²⁹⁸_ex/s⁻¹ = 3.8 × 10⁶, ΔH^≠/kJ mol⁻¹ = 18.3 ± 5.9, ΔS^≠/J K⁻¹ mol⁻¹ = −57.6 ± 20.2, which support the associative interchange mechanism (I_a). The kinetic parameters for the corresponding reaction of trans-[Mn^III(salen)(OH₂)(OH)] are but tentative (k²⁹⁸_ex/s⁻¹ = 3.5 × 10⁷, ΔH^≠/kJ mol⁻¹ = 54.1 ± 4.3, ΔS^≠/J K⁻¹ mol⁻¹ = +80.9 ± 17.0) due to its dimerization, and thermal instability of the dimer in a basic medium (pH ∼ 10). The kinetic parameters for the dimerisation of the (aqua)(hydroxo) complex are: k²⁹⁸₁/dm⁻³ mol⁻¹ s⁻¹ = 1.7 ± 0.4, ΔH^≠/kJ mol⁻¹ = 52 ± 4, ΔS^≠/J K⁻¹ mol⁻¹ = −64 ± 14; the reaction is reversible with a virtually temperature insensitive equilibrium constant, K²⁹⁸_eq./dm³ mol⁻¹ = 1.5 ± 0.3 × 10⁵. The negative value of the activation entropy is also consistent with an associative interchange mechanism (I_a) for the dimerisation reaction. The reaction mixture is EPR silent in X-band mode thus indicating that the +3 oxidation state of Mn is preserved. The dimer (OH₂)Mn^III(salen)(μ-OH)–Mn^III(salen)(OH) decays to hydrolysed product(s) obeying first order kinetics with k²⁹⁸₂/s⁻¹ = (0.8 ± 0.1) × 10⁻⁵. The relevance of water exchange kinetics of [Mn^III(salen)(OH₂)(OH₂/OH)]^+/0 with catalase and SOD (superoxide dismutase) activities of [Mn^III(salen)] has been discussed.

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1. Introduction

This work stems from our current research interest on the mechanism of ligand substitution and redox reactions of Mn^III complexes. We have been investigating the reactivity of Mn^III in a salen {H₂salen = N,N′-bis(salicylidene)ethane-1,2-diamine} scaffold.^1–3 Mn^III(salen) and its derivatives have proved to be promising candidates as antioxidants, i.e. a defense for reactive oxygen species (ROS), due to their SOD (superoxide dismutase) and CAT (catalase) activities.^4–11 The enzymes, superoxide dismutases and catalases are metalloproteins, which act as catalysts in the dismutation reactions, and detoxify superoxide radicals (O₂⁻˙) and H₂O₂ respectively as shown below:

2O₂⁻˙ + 2H⁺ = O₂ + H₂O₂ (SOD activity)

2H₂O₂ = O₂ + 2H₂O (catalase activity)

The dismutation of O₂⁻˙ by Mn^III-salen derivatives involves reduction of Mn^III to Mn^II by O₂⁻˙ followed by reoxidation of Mn^II to Mn^III by another O₂⁻˙ generating toxic H₂O₂. In the catalase activity the Mn^III-salen is oxidized by H₂O₂ to the oxoMn^V(salen) which in turn is reduced by another molecule of H₂O₂ back to Mn^III(salen). The combined SOD and catalase activities of Mn^III(salen) operate through the redox cycling of Mn in the oxidation states and respectively. Thus this synthetic antioxidant under consideration is an efficient scavenger of O₂⁻˙, and H₂O₂ produced in excess in the living system in abnormal metabolic processes and can take care of the overload of toxic and mutagenic OH˙ radical generated from H₂O₂, through Fenton reaction. Also Mn^III(salen) has a proven record of protecting cells from oxidative stress which is believed to be the root cause of aging and a large number of dreadful neuro-degenerative conditions in Alzheimer's, and Parkinson's diseases, multiple sclerosis, and stroke etc.^12–16

The catalytic applications of Mn^III(salen) complexes in many organic transformations are also well documented.^17–24 In any of the reactions involving Mn^III(salen), ligand substitution at the Mn^III centre is the elementary primary step. For [Mn^III(salen)(OH₂)₂]⁺ it pertains to the aqua ligand substitution. Surprisingly reports on the kinetics and mechanism of the aqua ligand substitution at Mn^III centre are scanty.^{1,2,,25,26 17}O NMR relaxometry is a powerful technique, widely used to study water exchange reactions in chemical and biological systems.^27–38 Recently the water exchange reaction of some Mn^III porphyrins have been reported.^30,31 There is, however, no report yet of a similar study on [Mn^III(salen)(OH₂)₂]⁺. It is needless to mention that every complex ion has its own identity as regards dynamics and mechanism of reaction. Therefore, a systematic study of a series of aqua Mn^III complexes will entail a fruitful comparison for arriving at the mechanistic description. In order to have a clear insight in to the mechanism of the ligand substitution and redox reactions of [Mn^III(salen)(OH₂)₂]⁺ by external ligands, we felt the need for the relevant kinetic information on the water exchange reaction of this complex ion. In this context we report here ¹⁷O NMR line broadening by Mn^III complex and our experiments are limited to the conditions, pH 4 and 10 where the title Mn^III complex exists as diaqua and (aqua)(hydroxo) species.

2. Experimental

2.1 Materials and reagents

The trans-[Mn^III(salen)Cl]H₂O was prepared and characterized as reported in our earlier work.^1–3 It was converted to the perchlorate salt, trans-[Mn^III(salen)(OH₂)₂]ClO₄ by repeated crystallization from aqueous medium using LiClO₄. Caution: Perchlorate salts are sensitive to fire. The ESI-MS spectra of the perchlorate salt from aqueous and acetonitrile solutions display intense peak at m/z⁺ + 1 = 321.04 and 320.78, respectively agreeing well with that of Mn(salen)⁺ (ca. m/z⁺ = 320.94); the corresponding (m/z⁺) peak appeared at 322.04 and 321.78 (Fig. S1 and S2†). I.R. (cm⁻¹, KBr): 1600 (C=N), 1120–1050 (ClO₄⁻), 3433 (broad, H₂O).^2,39 UV-vis: λ_max, nm (ε, dm³ mol⁻¹ cm⁻¹): 214 (34 680), 232 (38 070), 280 (17 640), 300(sh) (13 340), 390(sh) (4470) (pH = 4). Analytical grade chemicals were used. Water purified by Millipore filter was used for NMR measurement. Doubly distilled water from a quartz distillation apparatus was used for all spectral and other kinetics measurements. ¹⁷OH₂ enriched water was from Cambridge isotope laboratories Inc, USA (10% enrichment of ¹⁷O).

2.2 Physical measurements

A PerkinElmer Lambda 25 UV-visible spectrophotometer with a matched pair of 10 mm quartz cells were used for all absorbance measurements. The temperature of the cell block was controlled by circulating water from a constant temperature bath. The IR measurements were made on a PerkinElmer FTIR spectrometer, model Spectrum 2 using KBr pellet. The ESI mass spectra were recorded on a Bruker micro TOF-QII mass spectrometer in the range of m/z = 50 to m/z = 3000 in positive ion mode. Samples were introduced into the mass spectrometer through a nebulizer with gas pressure at 0.4 bar. Gas flow rate was 4.0 dm³ min⁻¹ and temperature set at 180 °C. Nitrogen was used as collision gas. Capillary potential in mass spectrometer was set at 4.5 kV and collision energy was 10 eV. The EPR measurement was done at room temperature on a JEOL (Japan) JES-FA 200 EPR spectrometer in X-band mode operating at 8.75–9.65 GHz (power 1.08 W, sensitivity 7 × 10⁹ spins/0.1 mT, resolution 2.35 μT).

The pH measurements were made using a Systronics (India) digital pH meter model 335 with a combined glass–Ag/AgCl, Cl⁻ (3 mol dm⁻³ NaCl) electrode; the meter was calibrated using NBS buffers of pH 4.0 and 9.2. Fourier-transform ¹⁷O-NMR spectra at different temperatures were recorded on a Bruker Avance NMR spectrometer equipped with a superconducting BC-94/89 magnet system, at 54.24 MHz and magnetic field strength 9.41 T. The ¹⁷O NMR line broadening was recorded over the temperature range 283.2 ≤ T/K ≤ 353.2. A standard NMR tube of length 178 mm and diameter 5 mm was used. The sample solution was prepared just before the measurement. The concentration of the complex was 8.46 × 10⁻³ mol dm⁻³. 100 μL DMSO-d₆ solvent was taken in a sealed capillary and inserted into the NMR tube as external reference for deuterium lock. The number of scans at each temperature was fixed at 62. The peak position of ¹⁷O signal in the absence of the Mn^III complex was observed at 8.50 ppm at room temperature. The pH of the test and reference (solvent water) media was adjusted with dilute NaOH and HClO₄. Buffer was not used to avoid interaction with the Mn^III complex. All measurements were made at atmospheric pressure.

3. Results and discussion

3.1 Stability of trans-[Mn^III(salen)(OH₂)₂]⁺ in aqueous medium

A solution of [Mn^III(salen)(OH₂)₂]⁺ at pH = 10 was equilibrated at 65 °C for 100 minutes and then set aside to cool and EPR spectrum was monitored at room temperature. The Mn^II and Mn^IV species being EPR active^1,3,40,41 we expected to detect these species in case the Mn^III(salen) complex underwent disproportionation reaction. The EPR silent nature of the spectrum (see Fig. S2(b)†) discounts the possibility of the formation of Mn^II as well as Mn^IV. Hence we conclude that complex retained the +3 oxidation state of Mn under mild basic condition.

The time dependent UV-visible spectral scans of trans-[Mn^III(salen)(OH₂)₂]⁺ (250 ≤ λ/nm ≤ 500) at pH 4.2, 8.05 and 10.2, 50 °C are presented in Fig. S3–S5.† No significant change in the spectra is observed at pH 4 during 174 min. A very slow absorbance decrease at 280 nm (λ_max) is noted at pH 8.05 accompanied by pH drop from 8.05 to 7.12, indicating chemical change. Significant spectral changes are observed at pH 10.2 (50 °C); an isosbestic point appears at 320 nm and the λ_max at 380 nm tends to disappear; being replaced by a broad maximum around 320 nm (Fig. S5†) thus indicating a slow transformation of the complex. The aqueous solution of [Mn^III(salen)(OH₂)₂]ClO₄ at pH 10 was incubated at 65 °C for ∼25 minutes, then cooled to room temperature and the mass spectrum was recorded (Fig. S6†); the same solution was again left at room temperature for an hour and its mass spectrum was also recorded (Fig. S7†). The monomeric species, [Mn^III(salen)]⁺ (m/z⁺ = 321.04 (obs), 321.2 (cal.)), and the dimers [OH₂-Mn^III(salen)–OH-Mn^III(salen)] (m/z⁺ = 677.04 (obs), 677.5 (cal.)), [Mn^III(salen)–(OH)-Mn^III(salen)] (m/z⁺ = 659.47 (obs), 659.5 (cal.)) are identified. The signal at m/z⁺ 687.07 (obs), 687.4 (cal.) corresponds to the species [{O-Mn^V(salen)-(μ-O)–Mn^VI(salen)-O}-3H] which presumably originates from ClO₄⁻ mediated collision induced processes in the gas phase. However, there is also a possibility of the formation of the dimer of dimers [Mn^III(salen)₄(OH)₂(H₂O)₃]²⁺ with m/z²⁺ 686.5 (cal.) in the gas phase (see Fig. S6 and S7 and Scheme 1 in ESI†). A comparison of the Fig. S6 and S7† indicates that some of the dimeric species are also lost at long time interval. The signals at m/z⁺ 465.4 (obs), 465.2 (cal.) and 361.50 (obs), 361.15 (cal.) are attributable to [Mn^II(salen)–OH-Mn^II(OH₂)₄] and [(C₆H₄(O)CH=N(CH₂)₂NH₂)Mn^II(OH₂)₂(μ-OH)(μ-OH)Mn^II(OH₂)] respectively (Fig. S7†). This supports the observed slow second phase reaction in solution reconciled with partial hydrolysis of the dimers. The Mn^II species, however, must have resulted due to the collision induced redox reactions of the hydrolysed Mn^III species in the gas phase. We are, therefore, led to believe that the complex, trans-[Mn^III(salen)(OH₂)₂]⁺ retains its identity at pH 4 for sufficiently long duration for NMR measurements. The corresponding (aqua)(hydroxo) isomer undergoes dimerisation and the dimer undergoes hydrolysis over extended time scale. We report, herein, a kinetic study of the slow dimerisation of the (aqua)(hydroxo) Mn^III(salen).

Fig. 1 depicts the absorbance–time plot for the complex at 50 °C at pH(initial) = 10. The reaction is biphasic. For the initial fast phase, the [log|δD_t/δt|]_t→0 vs. log[complex]_T (10⁴[complex]_T/mol dm⁻³ = 0.5, 0.75, 1.0, and 1.2; pH = 10.1 ± 0.1) plot was linear with slope 1.9 ± 0.1 indicating that the rate is second order in [complex]_T. We consider the initial step (supported by mass spectra) to be the dimerisation of the (aqua)(hydroxo) complex involving reversible elimination/addition of a molecule of water, the second step being a first order transformation of the dimer as depicted below. The two processes were treated independently (see Appendix I in the ESI for details†):

Fig. 1 Absorbance (D_t) vs. t/min plot at 50 °C, [complex]_T/mol dm⁻³ = 1.2 × 10⁻⁴, λ = 378 nm, pH(initial) = 10.

The calculated values of k₁, K_eq. and activation parameters are collected in Table 1 (see foot note b for k₂ values). The calculated value of the molar extinction coefficient of dimer is more than 3 times higher than that of the monomer, Mn^III(salen)(OH₂)(OH) and also essentially independent of temperature and the concentration of the complex. The equilibrium for the first step (i.e. formation of the dimer) is driven to ∼80% (pH ∼ 10) and the equilibrium constant is virtually temperature independent (K_eq. (av.)/dm³ mol⁻¹ = (1.5 ± 0.3) × 10⁵).

Table 1 The kinetic and equilibrium parameters for the observed biphasic reactions of trans-Mn^III(salen)(OH₂)(OH)^a,b

Temp., °C	k1/dm^3 mol^−1 s^−1	10^−5Keq./dm^3 mol^−1	10^−4εMn^III(salen)(OH)(OH2)/dm^3 mol^−1 cm^−1	10^−4εDimer/dm^3 mol^−1 cm^−1
a λ, nm = 378 nm, [complex]T = (0.5 − 1.2) × 10^−4, I = 0.3 mol dm^−3 (NaClO4), 5–7 measurements at 25°, 30°, 35°, 40° and 45 °C (pH(initial) = 10.32–10.21; pH(final) = 9.76–9.48); 19 measurements at 50 °C (pH(initial) = 10.25–9.97; pH(final) = 9.74–8.73).b 10^5k2/s^−1 (°C): 0.8 ± 0.1(25.0), 1.0 ± 0.2(30.0), 1.1 ± 0.2(35.0), 1.2 ± 0.2(40.0), 1.8 ± 0.3(45.0), 2.3 ± 0.3(50.0). Calc.% equilibrium for dimerisation {=100([A]0 − [A]e)/[A]0}: 79 ± 2 (25 °C); 81 ± 2 (30 °C); 87 ± 5 (35 °C); 92 ± 3 (40 °C); 84 ± 3 (45 °C); 83 ± 2 (50 °C).
25.0	1.7 ± 0.4	1.8 ± 0.4	0.469 ± 0.012	1.55 ± 0.14
30.0	3.4 ± 0.4	1.2 ± 0.2	0.469 ± 0.007	1.62 ± 0.025
35.0	5.9 ± 1.0	1.3 ± 0.5	0.388 ± 0.015	2.19 ± 0.14
40.0	7.7 ± 1.4	2.8 ± 1.3	0.423 ± 0.010	2.02 ± 0.16
45.0	9.9 ± 1.8	3.2 ± 1.0	0.378 ± 0.030	2.09 ± 0.08
50.0	12.1 ± 0.7	2.8 ± 0.5	0.404 ± 0.012	1.76 ± 0.10
ΔH^≠/kJ mol^−1	52.0 ± 4.5	Keq. (av.)/dm^3 mol^−1 = (1.5 ± 0.3) × 10^5
ΔS^≠/J K^−1 mol^−1	−63.8 ± 14.3

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3.2 Water exchange kinetics

Our interest was on the study of the rates and mechanisms of water exchange from the coordination sphere of Mn^III in the trans-[Mn^III(salen)(OH₂)₂]⁺. We adopted line broadening ¹⁷O NMR technique and applied the approach of Swift and Connick.^42,43 This technique has been widely applied to investigate the water exchange reactions of paramagnetic ions in aqueous medium.²⁹ In accordance with Swift and Connick the rates of exchange of a solvent molecule (τ_m⁻¹) between the bulk and the coordination site of [Mn^III(salen)(OH₂)₂]⁺ and [Mn^III(salen)(OH₂)(OH)] species are linked to the observable transverse relaxation rate (1/T_2r) of ¹⁷O nuclei of the bulk solvent. Eqn (3a) defines the appropriate relation,

The unnormalized difference, Δν_obs − Δν_solvent, is given by eqn (3b).

where Δν_obs and Δν_solvent denote the full width at half-height (fwhh) of the ¹⁷O NMR signals of the bulk solvent (water) in the presence and absence of the paramagnetic Mn^III species respectively; the normalization factor, P_m, is the mole fraction of exchangeable H₂O bound to Mn^III, T_2m is the transverse relaxation time of H₂O coordinated to Mn^III centre in absence of chemical exchange, Δω_m is the difference between the resonance frequency of ¹⁷O nuclei of water in the coordination sphere of the metal ion and in the bulk solvent, and τ_m is the mean life time of H₂O in the coordination sphere of the metal ion. The total outer-sphere contributions to T_2r (T_2p), due to the long range interactions of the paramagnetic unpaired electrons of the Mn^III complex with water outside the coordination sphere is represented by T_2os. P_m was calculated on the basis of the number of H₂O molecules bound to Mn^III {2 for the diaqua- and 1 for [Mn^III(salen)(OH₂)(OH)] (pK of [Mn^III(salen)(OH₂)₂]⁺ = 7.34–7.79 at 10–30 °C, I = 0.2–0.3 mol dm⁻³)^2,26 and its dimer, [(OH₂)(salen)Mn^III–(OH)-Mn^III(salen)(OH)]}.

For the temperature dependence of the bound water relaxation rate (1/T_2m) an Arrhenius type of relation was used.

1/T_2m = A_m exp(−E_m/RT) (3c)

The solvent exchange rate constant (k_ex) and the residence life time of coordinated water (τ_m) are connected to the activation parameters by Eyring eqn (3d),

k_ex = 1/τ_m = (k_BT/h) exp(ΔS^#/R − ΔH^#/RT) (3d)

where k_B and h are Boltzmann's constant and Planck's constant respectively.

Eqn (3e) defines the temperature dependence of Δω_m where g_L is the isotropic Landé g factor (g_L = 2.0 for Mn^III), S is the electron spin factor (S = 2 for Mn^III), A/h is the hyperfine coupling constant (in s⁻¹) and B is the magnetic field strength (in T).⁴⁴

Δω_m = β/T = 2πg_Lμ_BS(S + 1)B(A/h)/(3k_BT) (3e)

The experimental line width data are collected in Table S1.† The contribution from T_2os was neglected. Eqn (3a), after substituting for 1/τ_m, 1/T_2m and Δω_m², was suitably modified (see Appendix II in ESI†) for data analysis by Levenberg Marquardt nonlinear least squares curve fitting using Origin 6 software; all the five parameters, A_m, E_m, ΔS^≠/R, ΔH^≠/R and β, were used as adjustable parameters in the least squares fit. It was also necessary to use P_m as an additional adjustable parameter for data fitting at pH 10 as [Mn^III(salen)(OH)(OH₂)] at the concentration used (0.00846 mol dm⁻³) undergoes reasonably fast dimerisation leading to equilibrium (t_1/2 ∼ 70 s at 25 °C). We analysed the temperature dependence of 1/T_2p at pH 10 (283.2 ≤ T/K ≤ 353.2) considering P_m = 0.8 × 10⁻⁴ + C(1/298.2 − 1/T) (see Table S2†). The 1/T_2r (pH 4) and 1/T_2p (pH 10) versus 1/T plots presented in Fig. 2 reflect that the temperature dependence refers to different regions of solvent exchange.²⁹ The transition is observed from fast exchange to slow exchange region around 303 K at pH 4. This is, however, not clear at pH 10 presumably due to the dimerisation of [Mn^III(salen)(OH₂)(OH)]. The μ-OH bridged Mn^III dimer ([Mn^III(salen)–OH-Mn^III(salen)]) formation in mildly basic medium is in line with a recent report by Kurahashi¹⁰ for N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminemanganese(III)chloride. The relevant data along with the calculated values of the parameters at pH 4 and 10 are collected in Table S2.†

Fig. 2 10⁻⁶/T_2r (s⁻¹), and 10⁻²/T_2p (s⁻¹) vs. 1/T (K⁻¹) plots at pH 4 (a) and 10 (b) for [Mn^III(salen)(OH₂)₂]⁺; [complex]_T = 8.46 × 10⁻³ mol dm⁻³. The experimental points are shown on the fitted curves.

3.3 Mechanism of water exchange at (aqua)Mn^III(salen) species

The spectral evidence clarifies that the diaqua complex retains its identity at pH 4. The reported pK value of the diaqua complex (loc. cit) suggest that it will virtually exist as (hydroxo)(aqua) species at pH ≥ 9. The ¹⁷OH₂ exchange kinetics studied at pH 4 is convincingly assigned to trans-[Mn^III(salen)(OH₂)₂]⁺ (eqn (4)). The (aquo)(hydroxo) complex is largely dimerised under the experimental conditions (≥79% see foot note b Table 1). Hence the water exchange reaction at pH 10 is predominantly for the (aqua)(hydroxo) dimer with minor contribution from the (aquo)(hydroxo) monomer. We assume that all such species undergo ¹⁷OH₂ exchange at the same rate. Furthermore, the reasonably good fit of the data as depicted in Fig. (2) validates our assumption that 1/T_2os does not contribute significantly to 1/T_2r.

A comparison of the rate and activation parameters for the water exchange reactions of Mn^III complexes is made in Table 2. There is wide variation of the activation parameters while the rate constant (k_ex) shows a marginal variation with the nature of the complexes, all of which, except the one in the present study, have porphyrin core. The water exchange reactions of the most robust Mn^III(porphyrin)(OH₂)₂ complexes are associated with higher values of activation enthalpy and activation entropy as compared to that of Mn^III(salen)(OH₂)₂. However, the corresponding data for the (aqua)(hydroxo)Mn^III(salen) show a reverse trend on comparative basis with those of [Mn^IIITPPS(H₂O)(OH)]⁴⁻ and [Mn^III(TMpyP)(OH₂)(OH)]⁴⁺. The mechanism of water exchange for the diaqua and (aquo)(hydroxo)Mn^IIIporphyrins has been suggested to be interchange dissociative (I_d). The trans effect of the coordinated hydroxo group in such complexes is small and virtually insensitive to the nature of porphyrins. For the trans-Mn^III(salen)(OH₂)₂⁺ k²⁹⁸_ex is also very similar in magnitude to the corresponding data for the (diaqua)Mn^IIIporphyrin complexes while the activation parameters are comparatively low. This may be an indication that the trans-[Mn^III(salen)(OH₂)₂]⁺ undergoes associative interchange (I_a) in contrast with the corresponding porphyrin complexes. This difference in behavior might be due to the difference in steric bulk and less rigidity of the salen motif as compared to the porphyrin core. In consequence favorable orientation of the incoming H₂O molecule to attain associative interchange (I_a) transition state is achieved preferentially by the salen complex. Interestingly the rate comparison reflects that both Mn^II(OH₂)₆²⁺ and [Mn^II(CYDTA)(OH₂)]²⁻ undergo relatively faster water exchange (k_{ex Mn^II(OH2)6}/k_{ex Mn^III(salen)(OH2)2} ∼ 6, k_{ex Mn^II(CYDTA)(OH2)}/k_{ex Mn^III(salen)(OH2)2} ∼ 36 at 298 K, see Table 2) than trans-[Mn^III(salen)(OH₂)₂]⁺. However, it has been reported that Mn^II(OH₂)₆²⁺ (ref. 30) and [Mn^II(CYDTA)(OH₂)]²⁻ (ref. 47) undergo water exchange via I_a and I_d mechanism respectively.

Table 2 Comparison of water exchange rate constant (k_ex/s⁻¹) and the associated activation parameters for Mn^III and Mn^II complexes

Complex	10^−6k^298ex/s^−1	ΔH^≠/kJ mol^−1	ΔS^≠/J K^−1 mol^−1	Ref.
[Mn^III(salen)(OH2)2]^+	3.8	18.3 ± 5.9	−57.6 ± 20.2	This work
[Mn^III(salen)(OH2)(OH)]/dimeric aqua(hydroxo) species	35.1	54.1 ± 4.3	+80.9 ± 17.1	This work
[Mn^IIITE-2-PyP(OH2)2]^5+	4.12	36 ± 3	4 ± 10	30
[Mn^IIITnHex-2PyP(OH2)2]^5+	5.73	34 ± 7	−2 ± 23	30
[Mn^IIITPPS(H2O)2]^3−	27.4	54 ± 5	79 ± 19	30
	100			45
	14 ± 1	32.7 ± 1.1	1.65 ± 3.9	31
[Mn^IIITPPS(H2O)(OH)]^4−	25 ± 7	24.1 ± 2.0	−22.7 ± 9.9	31
[Mn^III(TMpyP)(OH2)2]^5+	11 ± 1	40.7 ± 7.2	45.3 ± 3	31
[Mn^III(TMpyP)(OH2)(OH)]^4+	15 ± 1	33.9 ± 0.1	5.8 ± 2.0	31
[Mn^II(OH2)6]^2+	28.9	33.1 ± 0.6	9 ± 2	30
	21.0	32.9 ± 1.3	5.7 ± 5	46
[Mn^II(CyDTA)(H2O)]^2−	140 ± 20	42.5 ± 0.8	54 ± 3	47

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The dimerisation of trans-[Mn^III(salen)(OH)(OH₂)] (pH 10) did not allow us to extract the rate and activation parameters for its water exchange process more precisely. However, the corresponding data for the (aqua)(OH) complex and its dimer listed in Table 2 presumably point to the I_d mechanism of water exchange. Interestingly the relative value of the rate constants, k^hydroxo_ex/k^aquo_ex = 9.2 at 298.2 K (see Table 2), reflect that the trans-labilising action of the hydroxo group of the salen complex unlike its porphyrin counter part, may be significant.

3.4 Relevance of water exchange reaction with catalase and SOD activity

Baker et al.¹⁴ reported catalase like activity of Mn^III(salen) (i.e. EUK-8). Subsequently Sharpe et al.⁴⁸ assessed quantitatively its catalase like activity in terms of a second order rate constant 8.3 dm³ mol⁻¹ s⁻¹ (37 °C, pH 7). This value is very low with respect to the value for mammalian catalase enzyme (>10⁷ dm³ mol⁻¹ s⁻¹ as mentioned by Sharpe et al.⁴⁸) under comparable condition. The second order water exchange rate constant (k_ex/[H₂O]) for Mn^III(salen)(OH₂)₂⁺ and for the corresponding hydroxo species are 9 × 10⁴ and 1.3 × 10⁶ dm³ mol⁻¹ s⁻¹ at 35 °C respectively. Thus the catalase activity of Mn^III(salen) does not seem to involve rate limiting exchange of H₂O coordinated to Mn^III centre by H₂O₂. Abashkin and Burt^4,5 made a theoretical approach (DFT study) considering a general “ping-pong” mechanism.⁴⁹ They considered that in the first stage of the catalase activity of Mn^III(salen), a molecule of H₂O₂ binds to the Mn^III centre, oxidizes the metal, and releases a molecule of H₂O. The breaking of the O–O peroxide bond, oxidation of Mn^III to Mn(V) and H₂O molecule formation occurs in a concerted process on the triplet potential energy surface further involving an intra molecular proton transfer effectively conducted by Mn catalytic centre. In the event that the approach of Abashkin et al.⁴ is valid for the condensed aqueous phase, then our water exchange study provides some indirect experimental support to the mechanism they propose.

The SOD activity of [Mn^III(salen)]⁺ reported by Spasojević et al.⁵⁰ in terms of a catalytic constant (=6.0 × 10⁵ dm³ mol⁻¹ s⁻¹ at 25 ± 1 °C, pH = 7.8, 0.05 mol dm⁻³ phosphate buffer) may be compared with the second order water exchange rate constant of [Mn^III(salen)(OH₂)₂]⁺ and its (aqua)(hydroxo) analogue which are 0.7 × 10⁵ and 6.3 × 10⁵ dm³ mol⁻¹ s⁻¹ at 25 °C respectively. It seems quite likely that O₂⁻˙ radical might compete for placement in the coordination site of Mn^III during the water exchange process. Thus the present work is an attempt to unravel the underlying mechanism of SOD activity of Mn^III(salen).

3.5 Dimerisation kinetics

The dimerisation of the (aqua)(hydroxo)Mn^III(salen) is reasonably fast and is second order in [Mn^III(salen)(OH₂)(OH)]. At prolonged time interval the reaction (second phase) is more complex (see Section 3.1). The dimerisation rate constant and the associated activation parameters are; k₁/dm³ mol⁻¹ s⁻¹ (298 K) = 1.7 ± 0.4, ΔH^≠/kJ mol⁻¹ = 52.0 ± 4.5, ΔS^≠/J K⁻¹ mol⁻¹ = −63.8 ± 14.3 (see Table 1). The value of the rate constant at 30 °C (3.4 ± 0.4 dm³ mol⁻¹) compares well with the aqua ligand substitution rate constant of trans-[Mn^III(salen)(OH)(OH₂)] by hydroquinone (H₂Q) and catechol (H₂Cat) {k(30 °C)/dm³ mol⁻¹ s⁻¹ = 21.8 ± 0.5 (H₂Q), 1.91 ± 0.41 (H₂Cat)} reported by Panja et al.²⁶ As against these data the rate and activation parameters for the SO₃²⁻ substitution reaction of this (aqua)(hydroxo) complex are: k(30 °C)/dm³ mol⁻¹ s⁻¹ = 2.1 ± 0.2 × 10³, ΔH^≠/kJ mol⁻¹ = 32.4 ± 0.3, ΔS^≠/J K⁻¹ mol⁻¹ = −72.9 ± 0.6.² The dimerisation reaction under consideration is equivalent to the ligand substitution at Mn^III centre. Also the trans-labilisation effect of the coordinated hydroxide is not likely to be influenced by the entering ligand. Evidently nucleophilicity of the entering ligand appears to be important in these substitution reactions. A comparison of the rate constant of dimerisation with the second order rate constant of water exchange of the aqua hydroxoMn^III complex (k_ex/[H₂O] = 6.3 × 10⁵ dm³ mol⁻¹ s⁻¹ at 25 °C, see Table 2) shows that the latter reaction occurs >10⁵ times faster than the former one. The observed dramatic difference in the rate of the two processes discounts the possibility of a common mechanism. The mechanism of water exchange of trans-[Mn^III(salen)(OH)(OH₂)] is assigned dissociative interchange (I_d). In addition to this, substantially negative value of ΔS^≠ indicates that the mechanism of dimerisation is essentially associative interchange (I_a), a process in which a weak bond making between the incoming group and Mn^III centre takes precedence over the complete breaking of Mn^III–OH₂ bond (i.e. the leaving group, H₂O) in the transition state without any direct evidence of an intermediate (see Fig. 3).

Fig. 3 Transition state (T. S.) for the associative interchange mode (I_a) of dimerisation of trans-(OH)(salen)Mn^III(OH₂) showing the Mn^III–(HO⋯Mn^III–) bond formation prior to the Mn^III–OH₂ bond being stretched to the limit of breaking.

4. Conclusion

The water exchange reaction of trans-[Mn^III(salen)(OH₂)₂]⁺ is unambiguously established by ¹⁷OH₂ line broadening NMR. The rate and activation parameters (k²⁹⁸_ex/s⁻¹ = 3.8 × 10⁶, ΔH^≠/kJ mol⁻¹ = 18.3 ± 5.9, ΔS^≠/J K⁻¹ mol⁻¹ = −57.6 ± 20.2) are consistent with associative interchange mechanism (I_a). The corresponding reaction for the (hydroxo)(aquo) complex may differ in mechanism (i.e. I_d). The catalase activity of [Mn^III(salen)]⁺ does not seem to involve H₂O₂ for H₂O substitution in the coordination sphere of Mn^III as a rate limiting step while it may be likely for the superoxide radical, O₂⁻˙.

Acknowledgements

The authors thank Dr Arindam Ghosh for helping in ¹⁷O NMR experiments. HSB acknowledges financial support from Department of Science and Technology (DST), Govt. of India (Grant No. IFA11-CH-01). Financial support from the University Grants Commission (UGC), New Delhi in terms of a Teacher Fellowship to AKK (Ref. T.F.OU3-007-1/10-11 (ERO)) is acknowledged. AKK is thankful to the Odisha Education Department and the authority of Sishu Ananta Mahavidyalaya, Khurda, Odisha, India for granting study leave. P. J. is thankful to the College of Engineering and Technology for a junior research fellowship. The authors are thankful to Prof. Gautam K. Lahiri, I. I. T. Mumbai, India for ESR measurement.

References

1. A. K. Kar, A. N. Acharya, V. R. Mundlapati, G. C. Pradhan, H. S. Biswal and A. C. Dash, RSC Adv., 2014, 4, 58867–58879.
2. A. C. Dash and A. Das, Int. J. Chem. Kinet., 1999, 31, 627–635.
3. A. K. Kar, A. N. Acharya, G. C. Pradhan and A. C. Dash, J. Chem. Sci., 2014, 126, 547–559.
4. Y. G. Abashkin and S. K. Burt, J. Phys. Chem. B, 2004, 108, 2708–2711.
5. Y. G. Abashkin and S. K. Burt, Inorg. Chem., 2005, 44, 1425–1432.
6. P.-M. Yang, S.-J. Chiu and L.-Y. Lin, Toxicol. Sci., 2005, 85, 551–559.
7. J. Y. Yang and D. G. Nocera, J. Am. Chem. Soc., 2007, 129, 8192–8198.
8. S. Bahramikia and R. Yazdanparast, Med. Chem. Res., 2012, 21, 3224–3232.
9. S. R. Doctrow, M. Liesa, S. Melov, S. O. Shirihai and O. P. Tofilon, Curr. Inorg. Chem., 2012, 2, 325–334.
10. Y. Noritake, N. Umezawa, N. Kato and T. Higuchi, Inorg. Chem., 2013, 52, 3653–3662.
11. T. Kurahashi, Inorg. Chem., 2015, 54, 8356–8366.
12. A. J. Bruce, B. Malfroy and M. Baudry, Proc. Natl. Acad. Sci. U. S. A., 1996, 93, 2312–2316.
13. K. Pong, S. R. Doctrow and M. Baudry, Brain Res., 2000, 881, 182–189.
14. K. Baker, C. B. Marcus, K. Huffman, H. Kruk, B. Malfroy and S. R. Doctrow, J. Pharmacol. Exp. Ther., 1998, 284, 215–221.
15. B. Malfroy, S. R. Doctrow, P. L. Orr, G. Tocco, E. V. Fedoseyeva and G. Benichou, Cell. Immunol., 1997, 177, 62–68.
16. S. Melov, J. Ravenscroft, S. Malik, M. S. Gill, D. W. Walker, P. E. Clayton, D. C. Wallace, B. Malfroy, S. R. Doctrow and G. J. Lithgow, Science, 2000, 289, 1567–1569.
17. B. M. Choudary, M. L. Kantam, B. Bharathi and C. R. Venkat Reddy, J. Mol. Catal. A: Chem., 2001, 168, 69–73.
18. T. Katsuki, Coord. Chem. Rev., 1995, 140, 189–214.
19. W. Sun, H. Wang, C. Xia, J. Li and P. Zhao, Angew. Chem., Int. Ed., 2003, 42, 1042–1044.
20. N. S. Venkataramanan, G. Kuppuraj and S. Rajagopal, Coord. Chem. Rev., 2005, 249, 1249–1268.
21. M. K. Brown, M. M. Blewett, J. R. Colombe and E. J. Corey, J. Am. Chem. Soc., 2010, 132, 11165–11170.
22. D. Xu, S. Wang, Z. Shen, C. Xia and W. Sun, Org. Biomol. Chem., 2012, 10, 2730–2732.
23. A. N. Purude, K. P. Pawar, N. B. Patil, U. R. Kalkote and S. P. Chavan, Tetrahedron: Asymmetry, 2015, 26, 281–287.
24. X. Xi, J. Shao, X. Hu and Y. Wu, RSC Adv., 2015, 5, 80772–80778.
25. S. Gangopadhyay, M. Ali and P. Banerjee, Coord. Chem. Rev., 1994, 135, 399–427.
26. A. Panja, N. Shaikh, M. Ali, P. Vojtíšek and P. Banerjee, Polyhedron, 2003, 22, 1191–1198.
27. N. E. Brasch, D. A. Buckingham, A. B. Evans and C. R. Clark, J. Am. Chem. Soc., 1996, 118, 7969–7980.
28. F. A. Dunand, S. Aime and A. E. Merbach, J. Am. Chem. Soc., 2000, 122, 1506–1512.
29. L. Helm, G. M. Nicolle and A. E. Merbach, Water and Proton Exchange Processes on Metal Ions, in Advances in Inorganic Chemistry, Academic Press, 2005, vol. 57, pp. 327–379.
30. A. Budimir, J. Kalmar, I. Fabian, G. Lente, I. Banyai, I. Batinic-Haberle and M. Birus, Dalton Trans., 2010, 39, 4405–4410.
31. D. Lieb, A. Zahl, T. E. Shubina and I. Ivanović-Burmazović, J. Am. Chem. Soc., 2010, 132, 7282–7284.
32. E. M. Gale, J. Zhu and P. Caravan, J. Am. Chem. Soc., 2013, 135, 18600–18608.
33. G. A. Rolla, C. Platas-Iglesias, M. Botta, L. Tei and L. Helm, Inorg. Chem., 2013, 52, 3268–3279.
34. A. D. Sherry and Y. Wu, Curr. Opin. Chem. Biol., 2013, 17, 167–174.
35. F. Castiglione, A. Mele and G. Raos, in Annual Reports on NMR Spectroscopy, ed. A. W. Graham, Academic Press, 2015, vol. 85, pp. 143–193.
36. F. Oukhatar, H. Meudal, C. Landon, N. K. Logothetis, C. Platas-Iglesias, G. Angelovskia and É. Tóth, Chem.–Eur. J., 2015, 21, 11226–11237.
37. A. Shinobu and N. Agmon, J. Phys. Chem. B, 2015, 119, 3464–3478.
38. J. Yu, A. F. Martins, C. Preihs, V. Clavijo Jordan, S. Chirayil, P. Zhao, Y. Wu, K. Nasr, G. E. Kiefer and A. D. Sherry, J. Am. Chem. Soc., 2015, 137, 14173–14179.
39. K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds Part B, Wiley Interscience, New York, 5th edn, 1997.
40. B. R. McGarvey, in Transition Metal Chemistry, ed. R. L. Carlin, Marcel Dekker, New York, 1966, vol. 3, pp. 89–201.
41. W. Adam, C. Mock-Knoblauch, C. R. Saha-Möller and M. Herderich, J. Am. Chem. Soc., 2000, 122, 9685–9691.
42. T. J. Swift and R. E. Connick, J. Chem. Phys., 1962, 37, 307–320.
43. T. J. Swift and R. E. Connick, J. Chem. Phys., 1964, 41, 2553–2554.
44. N. Bloembergen, J. Chem. Phys., 1957, 27, 595–596.
45. S. H. Koenig, R. D. Brown III and M. Spiller, Magn. Reson. Med., 1987, 4, 252–260.
46. Y. Ducommun, K. E. Newman and A. E. Merbach, Inorg. Chem., 1980, 19, 3696–3703.
47. J. Maigut, R. Mier, A. Zahl and R. van Eldik, Inorg. Chem., 2008, 47, 5702–5719.
48. M. A. Sharpe, R. Ollosson, V. C. Stewart and J. B. Clark, Biochem. J., 2002, 366, 97–107.
49. M. J. Mate, G. Murshudov, J. Bravo, W. Melik-Adamyan, P. C. Loewen and I. Fita, in Handbook of Mettaloproteins, ed. A. Messerschmidt and R. Huber, John Wiley, New York, 2001.
50. I. Spasojević, I. Batinić-Haberle, R. D. Stevens, P. Hambright, A. N. Thorpe, J. Grodkowski, P. Neta and I. Fridovich, Inorg. Chem., 2001, 40, 726–739.

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Footnote

† Electronic supplementary information (ESI) available: ¹⁷O line-width data, ESI-mass with acquisition parameter, EPR and UV-vis spectra, Appendix-I and II. See DOI: 10.1039/c6ra23154c

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