Biomimetic Mn-Catalases Based on Dimeric Manganese Complexes in Mesoporous Silica

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INTRODUCTION 52 53 Reactive oxygen species (ROS), represented by superoxide (O2 •−), hydrogen peroxide (H2O2), and 54 hydroxyl radical (HO•), are endogenously produced in cells under aerobic conditions during the auto-55 oxidation of redox enzymes. ROS play an indispensable physiological role; however, their effects 56 oppose one another as they can both promote and prevent cell death, inflammation, or aging.1−3 57 O2 •− and H2O2 are produced from the one-and two-electron reduction of molecular oxygen (O2), 58 respectively. These two oxidants react rapidly with vulnerable targets; therefore, cells are loaded with 59 high amounts of very efficient ROS scavengers, as, for example, catalase, peroxidases (H2O2 60 scavengers), or superoxide dismutases (O2 •− scavengers). Consequently, a balance between the 61 intracellular O2 •− and H2O2 production and their scavenger-mediated decomposition keeps these two 62 oxidants' concentration in steady-state. Whereas O2 •− (small but charged) is not able to cross 63 membranes, H2O2 (small and uncharged) crosses membranes at a moderate efficiency, making cells 64 sensitive to the extracellular H2O2 concentration. Both O2 •− and H2O2 are able to deteriorate the 65 Fe4S4 clusters found in dehydratase enzymes, and, in the case of H2O2, this reaction leads to the 66 formation of the hydroxyl radical (HO•), a process known as the Fenton reaction.4 HO• is an extremely 67 powerful oxidant that, for instance, is responsible for direct DNA damage (since neither H2O2 nor O2 68 •− are able to damage DNA directly).2 69 As with any signaling mechanism, ROS can become cytotoxic if found in high concentrations or in the 70 wrong place. The phenomenon of ROS overproduction, which contributes to oxidative stress, has been 71 found in a great number of pathologies5−8 such as cancer, multiple sclerosis, Alzheimer's, or Parkinson 72 diseases. Nevertheless, the role of ROS in such diseases is sometimes controversial since they may 73 present various effects at the same time. For example, the production of ROS on tumors can range from 74 tumor-production effects to tumor-destroying effects. In fact, some anticancer agents induce apoptosis, 75 promoting the production of ROS, which contributes both to their efficacy and to their toxicity3,9 76 (damaging tumor cells and noncancerous cells, which may cause sideeffects). 77 Owing to the harmful properties of ROS, antioxidant therapies have been considered for a wide variety 78 of disorders associated with oxidative stress, which have shown promising in vivo results.10−13 79 Unfortunately, clinical trials to test the effect of antioxidant therapies are limited and have presented 80 disappointing results in lots of cases. Nevertheless, the use of improved antioxidant therapies is still 81 considered nowadays in particular cases where the oxidative stress is shown in earlier stages before the 82 development of severe clinical manifestations, such as in children with α-1 antitrypsin deficiency 83 (characterized by H2O2 accumulation due to the lack of catalase activity).14 Antioxidant therapy in 84 combination with conventional therapies is also considered for diseases in which ROS play an important 85 role in neuron-degeneration such as multiple sclerosis,15,16 Alzheimer's,17 or Parkinson's18 86 syndromes. In these cases, the chosen antioxidants should be able to penetrate the blood-brain barrier, 87 which is the major obstacle that reduces the efficacy of many agents.15 Targeting antioxidants to the 88 desired position opens a new challenge in molecular recognition, as oxidative stress is produced locally 89 or is notably harmful for the most vulnerable targets. For instance, mitochondrially targeted antioxidants 90 could effectively reduce oxidative stress in asthma.19 Similarly, neonatal brain injury could be 91 diminished or prevented by using antioxidants able to cross both the placenta and the blood-brain 92 barrier.20 93 Within all antioxidants, catalases (CATs) are the enzymes that perform the decomposition of H2O2 into 94 H2O and O2. In particular, Mn-CATs are found in some lactic acid bacteria. The active site of this type 95 of CATs comprises a dinuclear Mn unit with oxo and carboxylate bridges, in which the oxidation states 96 in the Mn2 pair alternate between (II,II) and (III,III).21 Efforts to mimic the active site and the catalytic 97 performance of this enzyme have been performed. A large number of functional models of the Mn-CAT 98 were summarized in 2012 by Signorella and Hureau.22 More recently, mononuclear MnIII porphyrin, 99 Schiff-base, and salen complexes have received much devotion23−27 and have deflected the attention 100 from MnIII2 mimics, reducing the emergence of this latter type.28 Moreover, dinuclear models with the 101 required stability and activity in physiological conditions have not been reported so far. The insertion of 102 Mn-CAT model compounds in porous solids offers the possibility of mimicking different properties 103 found in the native catalytic site, such as hydrophobicity and confinement.29−32 Indeed, the 104 encapsulation of antioxidants into nanocarriers could provide not only an improvement of stability but 105 also a higher therapy efficiency.33 Among inorganic-based materials, mesoporous silicas have attracted 106 much attention because of their biocompatibility at practical concentrations and their ability to protect 107 pharmaceutical principles from premature release or undesired degradation in living systems, acting as 108 an effective drug delivery system. Moreover, the versatile control over the internal and external surface 109 functionalization is advantageous for the adaptability to any guest environment .34 110 With this frame, we present here the synthesis of two new model compounds of the Mn-CAT's active NO3 (1) and ClO4 (2) and bpy = 2,2′-bipyridine, and their characterization with single-crystal X-ray 113 diffraction. This kind of compounds is interesting from the magnetic point of view due to the effect of 114 the structural parameters on the type and intensity of the magnetic interaction; as we have reported 115 previously, these systems could show different ground-spin state (S = 0 or S = 4).35−37 In this work we 116 analyze the magnetic interaction and the influence of the anisotropy in the magnetic behavior. Moreover, 117 we report here the catalase activity of these compounds. 118 With the aim to mimic the cavity of the Mn-CAT enzyme, the two coordination compounds 1 and 2 119 were inserted in the nanochannels of mesoporous silica (MCM-41 type), and the resulting materials 120 [Mn2O]@SiO2 were characterized. The catalase activity of this new material is compared with that 121 shown by 1 and 2 to eventually evidence the advantages of the latter over the former. The reactivity is 122 run in a mixture of acetonitrile−water and pure water. 123 for the susceptibility measurements, 0.02 T (2−29 K) and 0.3 T (2−300 K), with imposable graphs. 237

EXPERIMENTAL SECTION
Magnetization isotherms measurements were made in the range 1.8−6.8 K and at six different magnetic the general spectra and 23.5 eV of pass energy and 0.1 electronvolts per step for the spectra of the 256 different elements. A low-energy electron gun (less than 10 eV) was used to discharge the surface when 257 necessary. All measurements were made in an ultrahigh vacuum chamber pressure between 5 × 10−9 258 and 2 × 10−8 Torr. Binding energies were further referenced to the Csp2 peak at 284.6 eV. 259 Catalase Activity. The study of the catalase activity (H2O2 decomposition into H2O and O2) was 260 performed at 25 °C by volumetric determination of the oxygen evolved with a gas-volumetric buret 261 (precision of 0.1 mL). A 32% H2O2 aqueous solution (0.6 mL) was added to closed vessels containing 262 acetonitrile solutions or suspensions of compounds 1 or 2 or material [Mn2O]@SiO2 (5 mL, 0.8 mM 263 referred to Mn2 unit), and the oxygen evolved was volumetrically measured. It is worth emphasizing 264 that the catalase activity was studied in a CH3CN−H2O 9:1 (v/v) (5 mL of CH3CN with the complex or 265 the material and 0.6 mL of H2O2 aqueous solution). The same procedure was repeated with the three 266 systems (compounds 1 or 2 or material [Mn2O]@SiO2) using pure water as solvent instead of CH3CN.

267
Blank experiments performed without the catalyst (in both solvents) showed a negligible 268 disproportionation of H2O2. The catalytic activity of Mn(NO3)2 and MnO2 toward H2O2 disproportion 269 was tested under the same conditions as those used for 1 and 2 with H2O2 (0.6 mL) at 32% and a 1.6 270 mM acetonitrile solution (5 mL), equivalent to [Mn2] = 0.8 mM. 271 The pH measurements were made using a PHC10101 electrode. As the electrode was calibrated in 272 water, the pH measured in CH3CN−H2O 9:1 (v/v) (w s pH) was converted to s s pH (pH measured in 273 CH3CN−H2O with electrodes calibrated in the same mixture) using the δ conversion parameter as The pH-dependence study in the catalase activity was performed for compound 2. The evolution of 277 oxygen was measured at different pH values using the same method and under the same conditions. 278 Et3N 99% was used to increase the initial pH of the reaction media, yielding s s pH = 7.  5CH3CN). The crystal structure of compound 1 consists of a cationic complex, a nitrate ion, 332 and disordered solvent. The structure of its cationic complex is shown in Figure 1. The monodentate ligands of Mn(III) ions are disordered, being H2O or NO3−, both found with 50% 338 occupancy. Therefore, the two Mn(III) ions that form the complex are crystallographically equivalent.

339
Each binuclear entity is connected to the neighbor through hydrogen bonds between a H2O molecule 340 and a NO3 − ion ( Figure S1). This interaction is extended along a longitudinal axis, resulting in a 341 monodimensional supramolecular structure, where the position of the H2O molecule and the NO3 342 − ion within each aforementioned couple is statistically dictated ( Figure S1). Different longitudinal 343 organizations are aligned in a zigzag layer where every chain is antiparallel to the previous one ( Figure  344 S2). A paralleldisplaced π−π interaction may be found between bipyridine ligands of adjacent chains 345 ( Figure S1). Finally, the resulting layers are antiparallel stacked, giving place to channels that are filled 346 with anion and solvent molecules, both of them highly disordered ( Figure S2). 347 crystal structure of compound 2 consists of a cationic complex, a perchlorate ion, and an acetonitrile 349 molecule. The structure of its cationic complex is shown in Figure 2. In this case, the two Mn(III) ions 350 are not equivalent, having not only slightly different structural parameters but also different 351 monodentate ligands. These two ions also display an elongated coordination environment toward the The binuclear complexes are connected through the noncoordinated perchlorate anions, which are bound 361 to the water ligand via hydrogen bonds, being extended as a zigzag chain ( Figure S3). No interaction is 362 found between different chains. This chainlike structure is also found in an analogous compound  Experimental Section). In this case, considering EMn was unnecessary to achieve a good fit of the 391 experimental data. The difference in the magnitude of the magnetic interaction between these complexes 392 could be rationalized with their structural parameters (elongation parameter λ and the relative orientation 393 of the octahedra τ) as it was previously reported by V. Gómez et al.36 As commented above, compound 394 2 displays a greater elongation in the direction of the monodentate ligand (with λ > 2) than 1 (with λ < 395 2); thus, a more antiferromagnetic interaction was expected for 2. However, the magnetic interaction for 396 2 is very weak, and the one for 1 is more antiferromagnetic than the expected considering only the 397 distortion around the Mn(III) ions. This behavior could be explained with the relative disposition of the 398 octahedra being more antiferromagnetic the lower the τ angle is. Indeed, 1 has a relatively high τ angle 399 (O8−Mn1···Mn2−O9 angle of 95.2°), whereas 2 has the lowest τ angle found so far for this kind of 400 compound (O5−Mn1···Mn1′−O6′ angle of 78.1°). So, the magnetic properties of these compounds are 401 in agreement with the magnetostructural correlations reported previously.36 The values obtained for 402 DMn are also consistent for MnIII ions with elongated octahedral geometry, which is expected to be 403 moderate (between −2.3 and −4.5 cm−1) and negative.47−50 404 In conclusion, compounds 1 and 2 present moderate DMn values, and because of their weak magnetic 405 interactions and low |J| values, the anisotropy of MnIII ions is more important than expected for 406 antiferromagnetic compounds. Therefore, the MS ≠ 0 states are relevant due to the sign of DMn and the 407 relative orientation of the Jahn−Teller axes, affecting the shape of χMT versus T plot. 408

Insertion of Mn(III) Compounds into Mesoporous Silica: [Mn2O]@SiO2. Synthesis Strategy. 409
Cationic complexes of compounds 1 and 2 were inserted into the nanopores of mesoporous silica by 410 ionic exchange.40 A step-by-step procedure to obtain material [Mn2O]@SiO2 is shown in Figure 4. SiO2−TMA at room temperature. TMA+, which is only held into the silica by electrostatic forces, is 420 easily displaced when other positively charged species with more affinity for silica are present. The 421 [Mn2]2+/TMA+ molar ratio used was 0.5, which corresponds to 1.0 for charge ratio. 422 The elemental analysis revealed that materials [Mn2O]@ SiO2 obtained from 1 or 2 have the same 423 molar composition, and their IR spectra were superimposable. The results obtained from 424 thermogravimetric analysis as well as from N2 sorption isotherms for both materials may be considered 425 identical (Table 4 and Figure S5), as the differences between the solids are within the experimental 426 error. These facts prove that the counteranions have an unperceived effect and, obviously, that they 427 remain in solution without being retained by the silica. 428 Morphology of the Hybrid Material. The hexagonal array of the internal pores of the material was 429 unaltered during the insertion of the Mn complex, as shown from the XRD patterns ( Figure S6; the 430 distance between the centers of the pores (a0) being 4.8 ± 0.1 nm ( presence of the complex inside the pores. This is also consistent with the decrease of intensity of the 437 peak (100) observed in XRD of material [Mn2O]@SiO2, since a lower intensity is expected for those 438 whose contrast between silica wall and the channel atom occupancy is lower.52,53 The BET constant C 439 of the hybrid material is much lower than that of the SiO2−Ex (Table 4), which indicates that the surface 440 has become more hydrophobic. Moreover, the capillary condensation shifts to a lower range of pressures 441 in the solid [Mn2O]@ SiO2 (Figure 5), corresponding to a pore size reduction of ∼0.7 nm. It also occurs 442 progressively in a wider range of pressures and not in a series of steps, indicating that the Mn complex 443 was properly spread along the whole channel. In this case, the pore size distribution is broad due to the 444 inequality and roughness of the surface caused by the Mn complex's shape, covering values between 2.8 445 and 3.3 nm according to Broekhoff  and also with the values collected in Table S2 for compounds with different oxidation states. This fact 498 confirms that, for material [Mn2O]@ SiO2, the oxidation state of the Mn ions is III. 499 The N 1s spectrum of compound 1 (shown in Figure 7) displays two main peaks, assigned to the N 500 atoms of the bpy (blue peak centered at 399.6 eV) and to the NO3 -anions (green peak centered at 501 406.2 eV).61 As expected, the NO3 − peak is not present in the N 1s spectrum of material [Mn2O]@ 502 SiO2, consistent with the absence of the counteranion in the hybrid material as explained above. Indeed, 503 the overall N/Mn2 ratio is lower for [Mn2O]@SiO2 than for 1 and agrees with two bpy ligands for each 504 Mn2 entity, in agreement with the loss of NO3 − ions during the synthesis of the Mn−Si hybrid. 505 Moreover, the peak around 401 eV is now split in two components, centered at 399.3 (blue) and 402.4 506 eV (cyan). The first and more intense peak could be assigned to Nsp2 neutral atoms,65 while the second 507 and weaker could be attributed to some change in the coordination of one of the bpy ligands likely due 508 to the interaction with the silica support.56,61 509 Magnetic measurements were also performed for material [Mn2O]@SiO2. The χMT versus T plot of 510 this material (shown in Figure 8) indicates that there is a non-negligible interaction between the Mn 511 ions, which strongly supports the assumption that the MnIII 2 unit is maintained in the silica pores. The 512 χMT value at room temperature (5.8 cm3·mol−1·K) is close to the expected value for two uncoupled 513 Mn(III) ions. The data were fitted from 300 to 17 K using the PHI program (H = −2JS1S2),46 omitting 514 the data at low temperature (17−2 K) to avoid ZFS effects. The best fit corresponds to g = 1.98, 2J = 515 −1.2 cm−1, Rsus = 3.4 × 10−5. The J value is between those found for 1 and 2; thus, it is also in the 516 expected range for a [MnIII 2O(2-RC6H4COO)2]2+ subunit (between −12 and +5 cm−1).36 517 Unfortunately, we could not fit the whole curve due to a deviation in the low-temperature range. As it 518 was mentioned, magnetic properties of this kind of complexes are very sensitive to structural and 519 electronic parameters.35−37 So, this deviation may be due to the existence of more than one species that 520 could slightly differ in some structural parameters. We achieved the fitting of the whole curve by 521 keeping constant the parameters referred to the Mn anisotropy, with very similar values to the molecular 522 analogues (compounds 1 and 2). However, reporting this last fit would be meaningless because it 523 involves the assumption of unknown parameters, such as DMn, EMn, and the relative orientation of the 524 Jahn−Teller axes of the MnIII ions. 525 Catalase Activity. The catalyzed disproportionation reaction of H2O2 to H2O and O2 (catalase activity) 526 was studied with the two dinuclear MnIII compounds (1 and 2) and with material [Mn2O]@SiO2, and 527 then the results were compared. 528 Catalase Activity of Compounds 1 and 2. The catalytic activity of these compounds was tested by considering a two-step reaction that comprises the oxidation and reduction of H2O2. Following this 534 definition, a TON is equal to the decomposition of two moles of H2O2 per mole of MnIII 2. 535 In these experiments, vigorous evolution of O2 was also observed after the addition of hydrogen 536 peroxide. As shown in Table 5, compounds 1 and 2 are able to decompose a significant amount of H2O2 537 (TON ≈ 480 in 10 min), which evidences their catalytic activity. During the first minute the TON 538 follows an almost linear tendency with time; for time >1 min, the reaction slows until reaching a plateau 539 at ∼10 min (Figure 9). The activity of such compounds is of the same magnitude as some other 540 analogues reported in the literature.66,67 541 The catalytic activity of Mn(NO3)2 and MnO2 toward H2O2 disproportionation was also investigated 542 under the same conditions as those used for compounds 1 and 2. In both cases, the activity is ∼1 order of 543 magnitude smaller than the one displayed by 1 and 2 during all the experiments. 544 According to single-crystal XRD (explained above), the crystal structures of compounds 1 and 2 is a 1:1 545 electrolyte, whose cation is a monocharged complex with formula [{Mn(bpy)(H2O)}(μ-2- However, the ΛM of an acetonitrile solution of compound 1 (106 S·cm2·mol−1) is much lower than for 552 2, being close to the 1:1 electrolyte range (120−160 S cm2 mol−1).68 This is consistent with one of the 553 nitrate ions interacting with the manganese coordination sphere. Nevertheless, the addition of a small 554 amount of water to the solution of compound 1 (CH3CN−H2O 9:1 (v/v)) makes the ΛM increase to 189 555 S·cm2·mol−1, which is a higher value but still much below the characteristic range for a 2:1 electrolyte 556 solution. These facts prove that the nitrate anion tends to interact more than the perchlorate with the Mn 557 complex and, even though the presence of water may weaken this interaction, nitrate anions seem to 558 remain in contact with the complex, either via hydrogen bonds or by a genuine Mn··· ONO2 interaction. 559 In spite of displaying very similar structures, compound 1 (X = NO3) is a better catalyst than 2 (X = 560 ClO4); so, the cause of this difference may only lay on their X group. This fact was also reported 561 previously by G. Fernández et al. for analogous compounds with acetate or chloroacetate bridges, where 562 the ones with X = NO3 are also better catalysts than the ones with X = ClO4 even though they have the 563 same cationic complex (L = H2O/H2O).69 564 To sum up, when 1 is in solution, nitrate ion is likely interacting with the Mn complex in acetonitrile 565 solution, and this interaction is weakened with the presence of water. Thus, in spite of the nitrate anion 566 being retained by the Mn complex, this could be effortlessly displaced in the presence of another group. 567 In the case of the solid, the inner surface of material the possibility of the silica acting as a pH modulator, the catalase activity of 2 was evaluated at different 575 pH values. We chose the compound with X = ClO4 (2) because this counteranion interacts less with the 576 Mn complex in acetonitrile solution than NO3 − anion (as explained above), so its presence is likely 577 more innocent. 578 Compound 2 yields a pH = 6.0 in CH3CN−H2O 9:1 (v/v). Hence, triethylamine was used to increase the 579 basicity of the reaction media, with [Et3N]/[MnIII 2] ratios between 0 and ∼24. Figure 10 shows the 580 under the same conditions as compounds 1 and 2 to lately be compared. It is necessary to point out that 586 SiO2−TMA did not provoke any evolution of oxygen, which excludes any catalytic activity of the 587 support itself for H2O2 disproportionation. Looking briefly at Figure 11, one can say that the TON 588 versus time plot of material [Mn2O]@SiO2 has the same profile as its molecular analogues, first 589 following a linear tendency and lately reaching a plateau. Table 6 summarizes the results obtained from 590 the catalysis. As can be seen, the hybrid material exhibits a better catalase activity than compounds 1 591 and 2. The suspension of the hybrid material shows a pH = 8.9, and their activity is in the middle of 592 those displayed for compound 2 at pH = 7.3 and 9.8. This fact suggests that one of the effects for the 593 major activity of the material in comparison to the molecular compounds is the basic media provided by 594 the silanolate groups of the supportthe pKa of which is ∼9.71,72 595 A similar pH dependence was already reported in the literature for a mutated Mn catalase73 and for 596 some MnIII2 salen complexes28,70,27 in which the catalase activity was highly improved at high pH 597 (that guarantees the integrity of the Mn2 unit) or because of the presence of an acid−base catalytic 598 auxiliary. In our case, the silanolate moieties in the hybrid material likely act as an endogenous 599 acid−base auxiliary that could contribute to retain the integrity of the Mn2 core, improving its activity. 600 Successive additions of H2O2 were done to test if the catalyst retains its activity. As shown in Figure  601 12, the catalyst keeps a high activity after several additions, reaching a plateau at almost the same TON 602 after the second and third additions (green). In spite of that, the initial rate of H2O2 dismutation for the 603 second and third additions is lower than for the first one (red), which could be caused by the decrease 604 observed in the pH (from 8.9 to ∼7.5). 605 Several EPR spectra were recorded for the solution -separating the solid -at different times, between 30 606 s and 2 h after the addition of H2O2. Nonsignificant EPR signal was observed in none of them, 607 indicating that the Mn2+ content in the solution is <73 μM (limit of detection), which corresponds to 5% 608 of the Mn in material [Mn2O]@SiO2. As the solution does not present the brown color expected for a 609 MnIII solution, one can conclude that practically all Mn content remains inside the pores of the silica. 610 The isolated solid at 90 s after the addition of H2O2 was analyzed by XPS and EPR spectroscopy. The 611 XPS displays a Mn 3s doublet splitting (ΔMn 3s) of 5.6−5.7 eV, indicating that the Mn oxidation state is 612 mainly III (Table S2). Even though the majority of the Mn oxidation state is III, the EPR spectra ( Figure  613 S10) shows six bands in the region of g ≈ 2 with a hyperfine coupling of ∼9 mT. This pattern could be 614 consistent with a dinuclear Mn(II) complex with a weak magnetic coupling.69 No evidence of mixed 615 valence systems (MnII−MnIII or MnIII−MnIV) was observed. These facts suggest that catalytic species 616 involves MnII and MnIII oxidation states and that it mainly remains inside the pores. However, the XPS 617 analysis of this sample shows a lower N/Mn2 ratio than that of the former material [Mn2O]@SiO2. This 618 could be indicative of a partial unfastening of the bpy ligand from the solid. As it was indicated, in the 619 hybrid material some N atoms of the bpy ligand could interact with the silanolate groups of the wall, 620 suggesting a quite weak Mn−N bond. During the catalytic process, the strength of this bond could 621 decrease, the bpy ligand being more labile. Therefore, the partial release of the bpy from the hybrid 622 material could probably be due to the treatment of the sample (washing and drying) before the 623 measurements. 624 Catalase Activity in Aqueous Media of 1, 2, and [Mn2O]@ SiO2. In contrast to the activity in 625 acetonitrile (see above), compounds 1 and 2 are not good catalysts in water solution, hardly 626 decomposing ∼5% of the [H2O2]0 in 10 min (Table 7). This is predictable since Mn2 compounds have 627 low stability in aqueous media.22 628 However, the hybrid material shows catalase activity in aqueous media. As showed in Figure 13 and 629  Table 8. 635 Material [Mn2O]@SiO2 #A preserves the mesostructure and porosity of the former material 636 [Mn2O]@SiO2, as observed in the XRD pattern and the N2 sorption isotherms ( Figure S11). 637 Nevertheless, the pore volume is higher than the one observed for [Mn2O]@SiO2, and the mass loss 638 corresponding to the Mn complex's ligands decreased, suggesting that the pores are less loaded. This 639 fact is in agreement with the XPS analysis performed on the isolated solid during the reaction (explained 640 above). 641 Contrary to the previous one, material [Mn2O]@SiO2 #W displayed a poor XRD pattern and N2 642 sorption isotherms without a clear capillary condensation ( Figure S12), meaning that the mesostructure 643 was disrupted after the reaction with H2O2. 644 To know whether this fact is due to the water or the H2O2, the stability of material [Mn2O]@SiO2 was 645 also checked in water suspension. After 2 h of treatment, the resulting solid ([Mn2O]@SiO2 #M) 646 displays the typical N2 sorption isotherms of a mesostructured material ( Figure S13). So, the support 647 itself is not disrupted by the presence of water; but it has also a higher pore volume than its former 648 material ([Mn2O]@SiO2). 649 To summarize, the presence of water is not responsible for the damage of the mesostructure of the 650 support; this is only altered when both water and H2O2 are present. In addition, the water and the 651 reaction with H2O2 favor the release of part of the bpy ligands. However, the loss of loading could 652 occur during the process of isolation of the solid. 653 These facts could be attributed to the nudity of the silica's inner surface. Even though the ultrafast 654 microwave-assisted synthesis leads to highly ordered and chemically stable mesoporous silica,41 the 655 pore surface is formed by Q3 (mainly) and Q2 silanol groups, which are sensitive to nucleophilic attacks 656 and to strong oxidants. Therefore, modification of the silica's internal surface is under progress to 657 overcome these limitations. Hydrophobization and covering of the pore surface would hopefully limit 658 the internal water diffusion and increase the stability of both the Mn complex and the structure of the 659 nanochannels. and structurally and magnetically characterized. The crystal structure reveals that the anions tend to be 666 coordinated to one manganese ion, occupying one monodentate position. The distortion of the 667 coordination octahedron of the manganese ion depends on this monodentate ligand, being more 668 elongated with X = ClO4. The antiferromagnetic interaction between the Mn(III) ions is affected by the 669 structural parameters, mostly by the relative disposition of the Jahn−Teller axes. The almost orthogonal 670 disposition of these axes together with a negative value of the ZFS parameter (DMn) are relevant on the 671 magnetic behavior at low temperature. These compounds are structural and functional models of the 672 Mn-catalase, being able to catalyse the H2O2 decomposition in CH3CN−H2O 9:1 (v/v) solution. 673 Compound 1, with nitrate as counteranion and labile ligand, is more efficient than compound 2 (X = 674 ClO4). 675 The insertion of compounds 1 and 2 into mesoporous silica, by ionic exchange, leads to the same 676 material, indicating that only the cationic complex is grafted inside the support. The analysis of the new 677 material shows that the Mn complex occupies half of the available mesoporous volume within the pores 678 and that the hexagonal array was unaltered upon the insertion of the Mn complex. Moreover, a non-679 negligible antiferromagnetic interaction between Mn(III) ions was observed, indicating that the 680 dinuclear unit is preserved inside the silica. 681 The hybrid material shows also catalase activity, and it is more efficient than the coordination 682 compounds 1 and 2. This fact is due to the presence of silanolate groups that likely buffers a basic pH 683 and favors the catalyzed H2O2 decomposition. According to EPR spectroscopy and XPS analysis, the 684 reaction seems to take place inside the support and that the Mn oxidation state swings between II and III. 685 The insertion of the coordination compound inside the mesoporous silica provides a good way to protect 686 the catalytic center from the external media and opens a new approach to work with manganese 687 compounds in aqueous media, paving the way toward the application of these active antioxidant species 688 at physiological conditions. 689