Synthesis, characterization, and catalytic studies of Mn(III)-Schiff base-dicyanamide complexes: checking the rhombicity effect in peroxidase studies

Research Article Synthesis, Characterization, and Catalytic Studies of Mn(III)-Schiff Base-Dicyanamide Complexes: Checking the Rhombicity Effect in Peroxidase Studies Manuel R. Bermejo,1Rocío Carballido,2M. Isabel Fernández-García,2 Ana M. González-Noya,1Gustavo González-Riopedre,2Marcelino Maneiro,2 and Laura Rodríguez-Silva2 1Department of Inorganic Chemistry, Faculty of Chemistry, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain 2Department of Inorganic Chemistry, Faculty of Sciences, Universidade de Santiago de Compostela, 27002 Lugo, Spain Correspondence should be addressed to Manuel R. Bermejo; [email protected] Received 2 February 2017; Accepted 4 July 2017; Published 16 August 2017 Academic Editor: Albert Demonceau Copyright © 2017 Manuel R. Bermejo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The condensation of 3-methoxy-2-hydroxybenzaldehyde and the diamines 1,2-diphenylendiamine, 1,2-diamine-2-methylpropane and 1,3-propanediamine yielded the dianionic tetradentate Schiff base ligands N,N󸀠-bis(2-hydroxy-4-methoxybenzylidene)-1,2-diphenylendiimine (H2L1), N,N󸀠-bis(2-hydroxy-4-methoxybenzylidene)-1,2-diamino-2-methylpropane (H2L2)andN,N 󸀠-bis(2hydroxy-4-methoxybenzylidene)-1,3-diaminopropane (H2L3) respectively. The organic compounds H2L1and H2L2have been characterized by elemental analysis, IR, 1Hand13C NMR spectroscopies and mass spectrometry electrospray (ES). The crystal structure of H2L2in solid state, solved by X-ray crystallography, is highly conditioned in the solid state by two N-H∙∙∙Nintramolecular interactions. The synthesis of three new manganese(III) complexes 1–3,incorporatingtheseligands,H 2L1–H2L3,and dicyanamide (DCA), is reported. The complexes 1–3have been physicochemically characterized by elemental analysis, IR and paramagnetic 1H NMR spectroscopy, ESI mass spectrometry, magnetic moment at room temperature and conductivity measurements. Complex 1has been crystallographically characterized. The X-ray structure shows the self-assembly of the Mn(III)-Schiff base-DCA complex through 𝜇-aquo bridges between neighbouring axial water molecules and also by 𝜋-𝜋stacking interactions, establishing a dimeric structure. The manganese complexes were also tested as peroxidase mimics for the H2O2-mediated reaction with the water-soluble trap ABTS, showing complexes 1-2relevant peroxidase activity in contrast with 3. The rhombicity around the metal ion can explain this catalytic behaviour. 1. Introduction Schiff base ligands stabilize different metal ions in solution to yield metal complexes with a variety of properties and applications. For instance, one of these ligands, the chelating salen, is known by the ability to significantly decrease the Mn(III)/Mn(II) redox potentials, and the resulting complexes constitute suitable systems to catalyse multiple redox reactions such as asymmetric epoxidation of unfunctionalized olefins, catalase reaction, water photolysis, Diels-Alder cycloaddition, enantioselective cyclopropanation of styrenes and ring opening of epoxides [1–8]. Over the past years, a variety of applications of this type of complexes has also been reported, including a broad range of biological activities (antibacterial, antifungal, anticancer, antioxidant, anti-inflammatory, etc.) [9–13]. During our search for new metal catalysts containingsalen-typeligands,wehaveobservedabettercatalytic behaviour when the substrate molecule can be easily coordinated by the complex and this is favoured when the catalyst has either a vacancy in the coordination sphere or a labile ligand [14–17]. To achieve this, Hindawi Journal of Chemistry Volume 2017, Article ID 5465890, 10 pages https://doi.org/10.1155/2017/5465890 2Journal of Chemistry OH OMe N N HO OMe OH OMe N N HO OMe OH OMe NN HO OMe HLHLHL Figure 1: Structures of the Schiff base ligands H2L1–H2L3. a careful design of the environment around the metal ion is necessary. In this article, we report the synthesis and characterization of three new complexes containing both salen-type ligands and ancillary dicyanamide ligands (see Figure 1). The two imine nitrogen and two phenol oxygen atoms of the tetradentate Schiff bases H2L1–H2L3constitute an inner ONNO site. The presence of the outer methoxy groups facilitates the extension of the supramolecular network through hydrogen-bond interactions [18, 19]. Our scheme consists of inducing tetragonal elongation around the manganese ion with the combination of the H2L𝑛and dicyanamide ligands. These complexes may behave like enzyme mimics, and their peroxidase activity is reported in this work. Peroxidases catalyse oxidation of a broad range of substrates by hydrogen hydroxide, so they may be successfully used for different industrial processes since peroxidases are capable of degrading aromatic rings in lignin, dyes, and polycyclic aromatic hydrocarbons [15, 16, 20, 21]. In this sense, dyes are regarded as one of the most important contaminants to be removed from the industrial effluents, and peroxidases arise as a way to treat these pollutants. Here we check the peroxidase activity of Mn(III)-Schiff base complexes in function of the rhombicity (or tetragonal elongation) around the manganese ion. 2. Materials and Methods 2.1. Materials. All solvents, 3-methoxy-2-hydroxybenzaldehyde, 1,2-diphenylendiamine, 1,2-diamine-2-methylpropane, 1,3-propanediamine, manganese(II) acetate, sodium dicyanamide, hydrogen peroxide, and 2,2󸀠-azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid, are commercially available and were used without further purification. 2.2. Physical Measurements. Elemental analyses of C, H, and N were performed on a Carlo Erba EA-1108. IR spectra were recorded as KBr discs on a Bio-Rad FTS135 spectrophotometer in the range 4000–400 cm−1.1HNMRand13CNMR spectra were recorded on a Bruker AC-300 spectrometer at 296 K using DMSO-d6as deuterated solvent and tetramethylsilane as an internal reference. Positive electrospray-ionization (ESI) mass spectra of the ligands and complexes were recorded on a LC-MSD 1100 Hewlett-Packard instrument (positive-ion mode, 98 : 2 CH3OH/HCOOH as the mobile phase, 30–100 V). Magnetic susceptibilities were measured at room temperature using a Sherwood-Scientific MK1 balance, calibrated using mercury tetrakis(isothiocyanato)cobaltate(II), Hg[Co(NCS)4]. Measurements of conductivity were made with 10−3 Msolutionsofcomplexesin dimethylformamide using a Crison microCM 2200 conductivimeter. 2.3. Synthesis of H2L1.Schiff base ligand H2L1was prepared according to the literature by condensation of 3-methoxy-2-hydroxybenzaldehyde (1.12 g, 10.35 mmol) with 1,2phenylenediamine (0.54 mL, 5.18 mmol) in methanol [28]. The mixture of diamine and salicylaldehyde was refluxed in a round bottom flask fitted with a Dean-Stark apparatus. After heating for 3 h, the solution was concentrated to yield an orange solid. The product was collected by filtration, washed with diethyl ether, and dried in air. Yield was almost quantitative. M.p. 172.5∘C. Anal. Calc. for C22H20N2O4:C, 70.2; H, 5.3; N, 7.4. Found: C, 70.1; H, 5.3, N, 7.5%. MS ES (m/z): 377 (L + H+)+;IR(KBr,cm −1): ](O-H) 3461, ](C=N) 1611, ](C-O) 1253. 1H NMR (DMSO-d6,ppm):𝛿12.95 (s, 2H), 8.88 (s, 2H), 7.42 (m, 4H), 7.36 (d, 2H), 7.13 (d, 2H), 6.88 (t, 2H), 3.79 (s, 6H). 13C NMR (DMSO-d6,ppm):𝛿164.8 (C=N), 151.4 (C-OCH3), 142.8 (C-OH), 116-128.5 (Car), 40.2 (CH3). 2.4. Synthesis and Crystallization of H2L2.The tetradentate Schiff base H2L2was prepared following the procedure already described for H2L1, using 3-methoxy-2-hydroxybenzaldehyde (2.00 g, 13.14 mmol) and 1,2-diamine-2-methylpropane (0.68 mL, 6.57 mmol) in methanol. The yellow product was collected by filtration, washed with diethyl ether, and driedinair.YellowcrystalsofH 2L2, suitable for single crystal X-ray diffraction studies, were obtained by slow evaporation of the methanol solution of the ligands. Yield was almost quantitative, 94%. M.p. 122.9∘C. Anal. Calc. for C20H24N2O4: C, 67.4; H, 6.8; N, 7.9. Found: C, 67.3; H, 6.7, N, 7.8%. MS ES (m/z): 357 (L + H+)+, 379 (L + Na+)+;IR(KBr,cm −1): ](O-H) 3441, ](C=N) 1630, ](C-O) 1256. 1H NMR (DMSO-d6,ppm): 𝛿8.53, 8.50 (s, 2H), 6.77–7.01 (m, 6H), 3.83 (m, 6H), 3.76 (t, 4H), 1.37 (t, 6H), 13C NMR (DMSO-d6,ppm):𝛿163.3 (C=N), 148.9 (C-OCH3), 152.9 (C-OH), 25.7 (CH3). 2.5. Synthesis of H2L3.The synthesis and characterization of H2L3have been already previously reported and they will not be discussed in this paper [22]. 2.6. Complex Preparation. Mn2L1 2(H2O)2(DCA)2(1): to a methanolic solution of H2L1(0.53 mmol, 0.20 g), 0.53 mmol (0.13 g) of Mn(CH3COO)2was added and the stirred solution changed its initial yellow colour to brown. After 30 minutes of Journal of Chemistry 3 stirring with gentle heating, 0.53 mmol (0.05 g) of NaN(CN)2 in 10 mL of methanol was added. The solution was then concentrated by slow evaporation. The complex was obtained asbrowncrystals,whichwereisolatedbyfiltration,washed with diethyl ether, and dried in air. Yield 60%. Anal. Calcd. for C48H40Mn2N10O10 (1026.8): C, 56.1; H, 3,9; N, 13.6. Found: C, 55.5; H, 3.9; N, 13.4%. MS ES (m/z): 429 [MnL]+;496 [MnL(DCA)]+. IR (KBr, cm−1): ](O-H) 3433 (m), ](C=N) 1600 (vs), ](C-O) 1258 (s), ]sym(C≡N) 2151 (vs), ]asym(C≡N) 2254 (m). 1H NMR (DMSO-d6,ppm):𝛿−14.9 (H4), −20.0 (H5). 𝜇=4.8BM.Conductivity(inDMF)ΛM=45𝜇S. [MnL2(H2O)(DCA)](H2O) (2) was obtained by the same procedure as 1,using0.46mmol(0.16g)ofH 2L2,0.46mmol (0.11 g) of Mn(CH3COO)2, and 0.46 mmol (0.04 g) of NaN(CN)2in 10 mL of methanol which were added. Yield 58%. Anal. Calcd. for C22H26MnN5O6(511.41): C, 51.6; H, 5.1; N, 13.7. Found: C, 52.1; H, 5.2; N, 13.5%. MS ES (m/z) 408.5 [MnL]+,884.0[Mn 2L2(DCA)2]+;IR(KBr,cm −1): ](O-H) 3425, ](C=N) 1610, ](C-O) 1260, ]sym(C≡N) 2148, ]asym(C≡N) 2266; 1H NMR (DMSO-d6,ppm):𝛿−15.2 (H4), −16.8/−25.0 (H5). 𝜇=5.0BM.Conductivity(inMeOH)ΛM= 46 𝜇S. [MnL3(H2O)(DCA)](H2O) (3) was obtained by the same procedure as 1,using0.58mmol(0.20g)ofH 2L3,0.58mmol (0.14 g) of Mn(CH3COO)2, and 0.58 mmol (0.05 g) of NaN(CN)2in 10 mL of methanol which were added. Yield 65%. Anal. Calcd. for C21H24MnN5O6(497.37):C,50.7;H, 4.8; N, 14.1. Found: C, 49.8; H, 4.6; N, 13.8%. MS ES (m/z) 395.3 [MnL]+;IR(KBr,cm −1): ](O-H) 3412, ](C=N) 1612, ](C-O) 1256, ]sym(C≡N) 2144, ]asym(C≡N) 2252; 1HNMR (DMSO-d6,ppm):𝛿−16.4 (H4), −18.6 (H5). 𝜇=4.7BM. Conductivity (in DMF) ΛM=42𝜇S. 2.7. Peroxidase Probes. Peroxidase activity for complexes 1–3 was tested according to the literature procedure [15, 16] by study of the oxidation of 2,2󸀠-azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) with H2O2at ca. pH 6.8 inthepresenceofthecomplexes. 2.8. Crystallographic Studies. Single crystals of H2L2and complex 1, suitable for X-ray diffraction studies, were obtained by slow evaporation of the methanolic solution at room temperature. Table 1 collects detailed crystal data collection and refinement for H2L2and complex 1.Measurementsweremadeona Bruker-Smart CCD-1000 diffractometer employing graphitemonochromated Mo-K𝛼radiation (𝜆=0.71073˚ A) at 100 K. The structures were solved by direct methods [29] and finally refined by full-matrix least-squares base on 𝐹2.An empirical absorption correction was applied using SADABS [30]. All nonhydrogen atoms were included in the model at geometrically calculated positions. Refinement of 𝐹2against all reflections: the weighted 𝑅factor 𝑤𝑅 is based on 𝐹2,andconventional𝑅-factors 𝑅are based on 𝐹,with𝐹set to zero for negative 𝐹2. The threshold expression of 𝐹2>2𝜎(𝐹2)isusedonlyforcalculating𝑅factors(gt) and so forth and is not relevant to the choice Table 1: Crystal data and structure refinement for H2L2and complex 1. H2L2Complex 1 Chemical formula C20H24N2O4C24H20MnN5O5 Formula weight 356.41 513.39 Crystal system Triclinic Triclinic Space group 𝑃1𝑃1 Temperature (K) 100 100(2) 𝑎(˚ A) 8.3632(6) 8.3157(4) 𝑏(˚ A) 10.5626(7) 11.9711(6) 𝑐(˚ A) 11.4846(7) 12.7633(10) 𝛼(∘) 108.689(2) 114.321(3) 𝛽(∘) 95.176(2) 102.633(3) 𝛾(∘) 103.965(2) 97.626(2) Volume ( ˚ A3) 917.00(11) 1093.74(11) 𝑍22 Radiation type Mo K𝛼Mo K𝛼 𝜇(mm−1) 0.09 0.653 Crystal size (mm) 0.2 ×0.11 ×0.03 0.12 ×0.11 ×0.06 𝑇min,𝑇max 0.942, 0.997 0.9619, 0.9858 𝐹(000) 380 528 Number of measured reflections 11933 20087 Number independent reflections 3607 4293 Reflections observed [𝐼 > 2𝜎(𝐼)] 2055 3209 𝑅int 0.054 0.0374 (sin 𝜃/𝜆)max (˚ A−1) 0.617 0.0752 𝑅[𝐹2>2𝜎(𝐹 2)] 0.049 0.0630 𝑤𝑅(𝐹2)0.128 0.0816 Number of parameters 247 332 Δmax,Δmin (e ˚ A−3) 0.31, −0.28 0.338, −0.423 of reflections for refinement. 𝑅-factors based on 𝐹2are statistically about twice as large as those based on 𝐹,and𝑅factors based on all data will be even larger. 3. Results and Discussion Schiff bases H2L1-H2L2have been obtained in almost quantitative yield and characterized by elemental analysis, IR, 1H, and 13C NMR spectroscopies, and mass spectrometry electrospray (ES). The synthesis and characterization of H2L3 have been already reported [22]. Complexes 1–3were prepared in high yield as detailed in the Materials and Methods. Elemental analysis establishes the general formula [MnL𝑛(H2O)(DCA)](H2O)𝑥,with𝑛= 1–3 and 𝑥= 0-1. Analytical and spectroscopic data are in accordance with this empirical formula. All the complexes seem to be stable in air and thermally stable, melting above 300∘C without decomposition. They are insoluble or sparingly soluble in water but partially soluble in common organic solvents such as methanol and totally soluble in polar coordinating solvents such as pyridine, DMF, and DMSO. ESI (electrospray-ionization) mass spectra registered in methanol show, for complexes 1–3, a peak corresponding 4Journal of Chemistry m/z 800700600500400300 408.5102 0 2 4 6 Intens. 884.0985 316.3185 ×105 Figure 2: ESI mass spectrum for complex 2. to the fragment [MnL]+, indicating the coordination of the Schiffbaseligandtothemetalcentre.Otherminorsignals, assigned to [MnL1(DCA)]+and [Mn2L22(DCA)2]+units, also confirm the coordination of the dicyanamide ion to the manganese ion and point to the formation of dimeric species (Figure 2). The values for the magnetic moment at room temperature are very close to the spin-only value of 4.89 BM, as expected for a high-spin magnetically diluted d4manganese(III) ion. All the complexes show similar IR spectral features, exhibiting a strong band between 1600 and 1612 cm−1 attributable to ](CNimine)band, which is shifted 3–7 cm−1 lower than in the free Schiff base ligand. There is also a positive shift (3–7 cm−1)ofthe](COphenol)mode with respect to the free ligand (Figure 3). These data suggest the coordination of the Schiff base in its dianionic form through the inner phenol oxygen and the imine nitrogen atoms [14, 15, 22]. Moreover, two new bands at about 2150 and 2260 cm−1 are assigned, respectively, to the symmetric and asymmetric dicyanamide modes [16, 31]. Paramagnetic 1HNMRspectraof1–3were registered using DMSO-d6as solvent. The data interpretation, based on previous findings for manganese(III) complexes with related Schiff base ligands [24], indicates [32, 33] the formation of the manganese(III) complexes. The paramagnetic 1HNMR spectra of the complexes contain, for all cases, two upfield proton resonances, outside the diamagnetic region (𝛿= 0–14 ppm), due to the isotropic shifting of the ligand protons for high-spin manganese(III) complexes in an octahedral field.Thesignalsmustarisefromtheprotonsofthearomatic phenoxy rings. In the case of the symmetric complexes 1and 3,the−20.0 (for 1)/−18.6 (for 3)signalsarisefromtheprotons in the orthopositions to the methoxy substituent, while the resonances from −14.9 (for 1)/−16.4 (for 3)ppm arise from protons in the metapositions to the methoxy substituent. For complex 2the signal from the protons in the orthopositions to the methoxy substituent appears split in two resonances at −16.8 and −25.0 ppm (Figure 4). 3.1. Crystallographic Characterization of H2L2and Complex 1 3.1.1. Crystal Structure of H2L2.The crystal structure reveals that H2L2exists as discrete molecules, and it is composed of two identical 3-methoxy-2-hydrobenzylimine moieties Table 2: Selected geometric parameters ( ˚ A, ∘)forH 2L2. C12-N2 1.272(3) C14-O3 1.351(3) C12-N2-C9 122.9(2) C7-N1 1.279(3) C1-O1 1.354(3) N2-C9-C8 106.65(19) Table 3: Hydrogen bonds for H2L2. D-H⋅⋅⋅A(˚ A) D-H ( ˚ A) H⋅⋅⋅A(˚ A) D⋅⋅⋅A(˚ A) D-H⋅⋅⋅A(˚ A) O1-H1⋅⋅⋅N1 0.96 1.73 2.5839 146 O3-H3 ⋅⋅⋅N2 0.93 1.72 2.5891 154 C8-H8B⋅⋅⋅O1 0.99 2.55 3.4451 151 twisted about the central aliphatic chain which acts as a spacer (Figure 5). The C12-N2 and C7-N1 distances of 1.272(3) and 1.279(3) ˚ A, respectively, are practically identical and consistent with a C=N double bonding (Table 2), indicating both imine groups fully localized in the molecule [34]. The two oxygen O1 and O3 atoms are forming phenolic groups, and they present C-O distances of 1.354(3) ˚ A and 1.351(3), respectively, corresponding to the expected single bonds. The C12-N2-C9 angle of 122.9(2)∘confirms the sp2character for N2 atom [32]. The disposition adopted by the free ligand in the solid state is conditioned by two strong intramolecular hydrogenbond interactions which form two six-membered rings. These interactions arise from the contact between the hydroxy and the imine groups (see Table 3). As expected, the O1-H1O∙∙∙N1 hydrogen bonding establishes an almost planar N1-C7-C6-C1-O1-H1 ring, the same for the O3-H3O∙∙∙N2 hydrogen bonding which establishes other almost planar N2C12-C13-C14-O3-H3 rings. Despite the mentioned intramolecular hydrogen interactions and a partial conjugation of the molecule, this one is not planar, probably due to the packing based on the intermolecular hydrogen bonds and the 𝜋-stacking interactions between the salicyl residues of the neighbouring molecules (see Figure 6). The aromatic rings are arranged almost perpendicular to each other. Clearly, rotation about the C8-C9 bond to give a near-eclipsed or gauche conformation would achieve quasi-square-planar coordination environments using the inner ONNO site. According to this crystal structure study, this ligand should act as tetradentate donor in the equatorial plane about an octahedral metal centre.However,wecannotpredicttheroleoftheouter phenoxy oxygen atoms of each aldehyde residue during complexation as they could coordinate, for instance, with the metal centre by an axial position, but also their role could be limited to intermolecular hydrogen-bond interactions. 3.1.2. Crystal Structure of 1.Single crystals of complex 1,suitable for X-ray diffraction studies, were obtained as described intheMaterialsandMethods.Thecrystalstructureisshown Journal of Chemistry 5 3441 1630 1256 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (＝Ｇ−1) ](C-O) ](C=N) ](O-H) (a) 1260 3425 2266 2148 1610 1300 887 4000 3500 3000 2500 2000 1500 1000 500 Wavenumber (＝Ｇ−1) ]；ＭＳＧ(C≡N) ]ＭＳＧ(C≡N) ]ＭＳＧ(C-N) ](C-O) ](C=N) ](O-H) ]；ＭＳＧ(C-N) (b) Figure 3: Comparison between the spectra for the free ligand H2L2(a) and complex 2(b). −15.0 −20.0 −25.0 −15.246 −16.783 −25.066 (ppm) Figure 4: Paramagnetic 1HNMRspectrumforcomplex2. Table4:Selectedbondlengths(˚ A) and angles (∘)forcomplex1. Mn(1)-O(830) 1.870(15) Mn(1)-N(1) 1.9816(19) Mn(1)-N(20) 2.257(2) Mn1-O(130) 1.8705(15) Mn1-N(8) 1.9940(19) Mn1-O(30) 2.2859(18) O(830)-Mn(1)-O(130) 91.03(7) N(8)-Mn(1)-O(9) 86.02(6) O(830)-Mn(1)-N(1) 176.10(7) N(1)-Mn(1)-O(9) 84.56(6) O(130)-Mn(1)-N(1) 92.72(7) O(830)-Mn(1)-O(9) 90.13(6) in Figure 7 and main bond distances and angles are shown in Table 4. The monomer is composed by an approximately octahedral [MnL1(H2O)(DCA)] unit. The planar tetradentate C14 C7 C6 C5 C4 C3 C2 C20 O2 O1 N1 C8 C1 C11 C10 C9 N2 C12 C13 C18 O3 O4 C15 C19 C16 C17 Figure 5: ORTEP representation (50% probability) of H2L2. Figure 6: View along the 𝑏-axis for H2L2. 6Journal of Chemistry N24 C23 N22 C21 C15 C16 C17 C12 C13 C141 O140 N20 O130 Mn1 N8 O830 C83 C82 C87 C84 C840 C86 C85 C841 C81 O30 C7 C6 C5C4 C3 C2 Figure 7: ORTEP view of the environment around the manganese ion in complex 1. Table 5: Hydrogen bonds for complex 1. D-H⋅⋅⋅A(˚ A) D-H (˚ A) H⋅⋅⋅A(˚ A) D⋅⋅⋅A(˚ A) D-H⋅⋅⋅A(˚ A) O(30)-H(30A)⋅⋅⋅O(130)∗0.83(2) 2.43(2) 3.061(2) 134(2) O(30)-H(30A)⋅⋅⋅O(141)∗0.83(2) 2.12(2) 2.908(2) 158(2) O(30)-H(30B)⋅⋅⋅O(830)∗0.76(3) 2.40(2) 3.034(3) 141(2) O(30)-H(30B)⋅⋅⋅O(840)∗0.76(3) 2.23(3) 2.922(3) 152(2) C(841)-H(84A)⋅⋅⋅N(24)∗∗ 0.98 2.51 3.449(4) 160 ∗=−𝑥,−𝑦,and2−𝑧;∗∗ =1−𝑥,1−𝑦,and2−𝑧. Schiff base L1is tightly bound to the manganese ion through the inner N2O2compartment by the Nimine and Ophenol atoms (Mn-Nimine bond lengths of 1.9816–1.9940 ˚ AandMnOphenol of 1.87 ˚ A which are typical of such complexes [14–16]), occupying the equatorial positions. The geometry around the manganese ion can be described as distorted and octahedral with the axial positions of the octahedron occupied by a water molecule and a dicyanamide molecule. The axial distances of 2.2859 ˚ A(forMn-O(30))and2.257˚ A (for Mn-N(20)) are considerably longer than the equatorial Mn-O bond lengths quoted earlier, resulting in a rhombicity factor, expressed as the ratio between the Mn-O axial and Mn-O equatorial distances in the crystal structure, of 1.22. The deviation from an ideal octahedral geometry is also revealed by the range of angles observed around the metal centre (from 82.93∘to 95.13∘), as well as by the interaxial angle O(30)-Mn-N(20) of 173.92∘. The octahedral [MnL1(H2O)(DCA)] entities are linked in pairs by 𝜇-aquo bridges between neighbouring axial water molecules and phenoxy and methoxy oxygen atoms and also by 𝜋-𝜋stacking interactions to give [MnL1(DCA)(H2O)]2 dimeric structures (Figure 8, Table 5). As a result of these supramolecular interactions, the Mn⋅⋅⋅Mn distances of about 4.96 ˚ A are short for monomeric compounds. These types of systems have been reported in literature [15, 16] as 𝜇-acuo dimers. Figure 8: Stick diagram for the dimeric unit for complex 1(manganeseioninmagenta,oxygeninred,nitrogeninblue,andcarbon in grey). 3.2. Peroxidase Studies. The peroxidase-like activity of 1–3 was followed by the oxidation of the diammonium salt of 2,2󸀠- azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) at pH 6.8. During this reaction, ABTS is oxidized by hydrogen peroxide to the radical cation ABTS∙+ [33] in the presence of the appropriate peroxidase mimic. While ABTS is colourless, its radical cation ABTS∙+ is green, showing characteristic absorption bands that can be easily followed by Journal of Chemistry 7 Table 6: Peroxidase activity for complexes 1–22, axial and equatorial crystal distances around the manganese ion for each complex with solved crystallographic structure, resulting tetragonal elongation, and corresponding reference. Complex Crystal distances Tetragonal elongationaPeroxidase activitybReference Mn-Oax Mn-Oeq 1[MnL1(D)(DCA)]2c2.28 1.870 1.22 33 ±2Thiswork 2[MnL2(D)(DCA)](D) un. un. un. 53 ±3Thiswork 3[MnL3(D)(DCA)] un. un. un. 1.9 ±0.4 This work 4[MnL4(D)2(DCA)2] 2.303 1.885 1.225 37 ±2[16] 5[MnL5(D)2](NO3)(D) 2.326 1.872 1.243 34 ±3[22] 6[MnL5(D)Cl](D)2.5 2.318 1.878 1.234 31 ±2[15] 7[MnL5(D)2](ClO4) 2.314 1.865 1.24 19 ±2[23] 8[MnL6(D)2](NO3)(D) 2.373 1.862 1.27 50 ±3[15] 9[MnL7(D)2](NO3)(D) 2.263 1.880 1.203 32 ±1[22] 10 [MnL8(D)2](NO3)(D) 2.271 1.870 1.214 33 ±1[24] 11 [MnL1(D)2](ClO4) 2.36 1.851 1.27 30 ±3[23] 12A [MnL2(D)2](NO3)(D) 2.246 1.871 1.200 29 ±1[15] 12B [MnL2(D)(D2)](NO3) 2.258 1.879 1.202 29 ±1[15] 13 [MnL2(D)(OAc)](D) 2.282 1.893 1.205 33 ±1[15] 14A [MnL2(D)(O2CEt)] 2.308 1.869 1.235 28 ±2[14] 14B [MnL2(D)(O2CEt)] 2.338 1.876 1.246 28 ±2[14] 15 [MnL2(D)(O2CPe𝑛)] 2.339 1.865 1.254 39 ±2[14] 16 [MnL9(D)2](NO3)(D)22.25 1.872 1.202 18 ±1[15] 17 [MnL10(D)2](NO3)(D)22.243 1.889 1.187 2.0 ±0.4 [24] 18 [MnL11(D)(D2)](ClO4) 2.203 1.863 1.183 3.2 ±0.4 [25] 19 [MnL12(D)2](ClO4)(D) 2.22 1.88 1.181 2.4 ±0.4 [26] 20 [MnL13(D)2](ClO4)(D) 2.21 1.90 1.163 2.1 ±0.4 [27] 21 [MnL14(D)2](NO3)(D)22.206 1.904 1.159 1.1 ±0.3 [22] 22 [MnL15(D)2](NO3)(D)22.258 1.888 1.196 1.8 ±0.3 [24] aTetragonal elongation expressed as the ratio between the manganese-axial oxygen distances and the manganese-equatorial oxygen distances. bPeroxidase activity expressed as percentage of conversion of ABTS to ABTS∙+ measured 10 min after mixing the solutions; cD=H 2O or other solvent molecule (such as CH3OH); un.: unavailable (structure not solved). UV-spectroscopy. The extent of the reaction can be measured quantitatively at 𝜆=650nmsince𝜀= 12000 M−1 cm−1 has been determined [35]. In our experiments, an aqueous solution of ABTS (50 𝜇L; 0.009 M; 4.5 ×10−7 mol) and a methanolic solution of the corresponding complexes 1–3(10 𝜇L; 10−3 M; 10−8 mol) were added to water. The intensity of the UV absorption bands of ABTS started to increase immediately after the addition of an aqueous solution of H2O2(50 𝜇L; 10 M; 5 ×10−4 mol). The extent of the reaction can be measured quantitatively at 𝜆=650nmsince𝜀= 12000 M−1 cm−1 has been determined. The peroxidase-like activity results are collected in Table 6, where peroxidase activity is expressed as the percentage of the starting ABTS oxidized to its radical cation measured 10 min after mixing the solutions. The rate of formation of ABTS∙+ of about 33–53% indicates that 1and 2behave as efficient peroxidase mimics, while 3does not present significant catalytic activity. Figure 9 shows the UV spectrum of the ABTS∙+ radical cation obtained after ABTS reaction with H2O2in presence of complex 2,withthecharacteristic absorptionbandsat415,650,735,and815nm. Our previous findings in this field indicate a better catalytic behaviour when the substrate molecule is 400 500 600 700 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Absorbance Wavelength (nm) Figure 9: UV-Vis absorption spectrum of ABTS + H2O2+2, recorded 10 min after mixing the solution. easily coordinated by the manganese complex, and this is favoured if the peroxidase mimic has either a vacancy in the coordination sphere or a labile ligand [14, 15]. For the complexes used in this study, the high efficiency 8Journal of Chemistry 1.16 1.18 1.20 1.22 1.24 1.26 1.28 0 10 20 30 40 50 6 5 15 1 4 10 13 9 12B 12A 14A 14B 7 16 22 17 18 19 20 21 11 8 Peroxidase activity Tetragonal elongation Figure 10: Peroxidase activity, expressed as percentage of ABTS conversion into ABTS∙+, in function of the tetragonal elongation of the catalyst, expressed as the ratio between the Mn-O axial and MnO equatorial distances in crystal structures 1and 4–22. as peroxidase mimics found for complexes 1and 2can be explained in this sense. Complexes 1and 2, derived from H2L1and H2L2,respectively,haveashorttwocarbon chain between imine groups in the Schiff ligand. The short chain constricts the chelate ring once nitrogen atom coordinates with the metal and led to a tetragonally elongated octahedral geometry for the manganese complex. An axial water molecule in this class of distorted geometries constitutes a quite labile ligand, which would generate a vacant position in the coordination sphere to accommodate the substrate molecule. Besides, the two methyl groups located in the methylene spacer (bound to C9) in H2L1,orthearomaticphenylring in H2L2, increase the steric effect and probably increase the distortion of the octahedral symmetry. These structural factors, derived from these Schiff bases, provoke the high rates of peroxidase activity for the obtained manganese complexes 1and 2.Inthecaseof3,theSchiffbaseligand with a flexible three-membered alkyl chain between the imine groups favours a better stabilization of a highsymmetry octahedral geometry, which subsequently makes generation of a coordination site difficult. This type of flexible chain for 3is similar for complexes 17–22 of Table 6. In this way, a correlation between the factor of tetragonal elongation and peroxidase activity has been already reported [14, 15]. This tetragonal elongation may be calculated as the ratio between the manganese-axial oxygen distances and the manganese-equatorial oxygen distances of the complexes with known structures solved by X-ray crystallography (complex 1and complexes 4–22). Table 6 gives the values for a number of complexes and Figure 10 shows the plot of the tetragonal elongation versus their peroxidase activity of this collection of complexes. The strength of this correlation is clearly checked in the present work using data from twenty-two solved crystallographic structures. 4. Conclusions The synthesis and structural characterization of the Schiff base compounds N,N󸀠-bis(2-hydroxy-4-methoxybenzylidene)-1,2-diphenylendiimine (H2L1)andN,N 󸀠-bis(2-hydroxy-4-methoxybenzylidene)-1,2-diamino-2-methylpropane (H2L2) have been reported. These two organic compounds behave as suitable ligands for coordination with metal ions, highly versatile due to different structural features: being tetradentate, ability to extend the hydrogen network using the outer methoxy groups, short and bulky spacer, etc. Three new manganese(III)-L-dicyanamide complexes have been obtained and characterized. The short twocarbon chain between imine groups in H2L1-H2L2 provokes tetragonally elongated octahedral geometries in the resulting complexes, which may increase their potential as catalysts for different applications. The correlation of peroxidase activity and the rhombicity caused by the short length of the Schiff base should be taken into account for designing new potential catalytic systems. Conflicts of Interest The authors declare that there are no conflicts of interest regarding the publication of this paper. Acknowledgments The authors are grateful for the financial support given by the Xunta de Galicia (GRC2014/025). 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