HOSTED BY Available online at www.sciencedirect.com

ScienceDirect

Karbala International Journal of Modern Science 1 (2015) 178e186http://www.journals.elsevier.com/karbala-international-journal-of-modern-science/

Complex formation equilibria of 2,20-bipyridyl and 1,10-phenanthroline with manganese(II) in methanol

María de la Luz P�erez-Arredondo a, María del Refugio Gonz�alez-Ponce a,Gabriela Ana Zanor b, Juan Antonio Ramirez Vazquez b,

Jos�e J.N. Segoviano-Garfias b,*a Departamento de Ingeniería Bioquímica, Instituto Tecnol�ogico Superior de Irapuato (ITESI), Carretera Irapuato-Silao

Km. 12.5, Irapuato, Gto 36821, Mexicob Departamento de Ciencias Ambientales, Divisi�on de Ciencias de la Vida (DICIVA), Universidad de Guanajuato, Campus Irapuato-Salamanca,

Ex Hacienda El Copal, Carretera Irapuato-Silao Km. 9, Irapuato, Gto 36500, Mexico

Received 13 September 2015; revised 30 October 2015; accepted 17 November 2015

Available online 17 December 2015

Abstract

Water photolysis to molecular hydrogen and oxygen is one of the most promising chemical reactions for a sustainable hydrogengeneration. The search for catalysts for this process is an important research area; some of the most suggested catalysts aremanganese complexes. However, several studies propose the use of the complex: tris(bipyridine)ruthenium(II) chloride to be usedas catalyst in artificial photosynthesis. In spite of this, the conferred properties by the 2,20-bipyridyl ligand, in manganese orruthenium complexes has not yet been discussed extensively.

In this work, in order to obtain more insight of the behavior of manganese(II) with several ligands such as 2,20-bipyridyl or 1,10-phenanthroline, a spectrophotometric study of the speciation in methanol of these systems at 298 K, was performed. The formationconstants obtained for the manganese(II)-2, 20-bipyridyl system are: log b110 ¼ 7.81 and log b120 ¼ 14.68; for the manganese(II)-1,10-phenanthroline system are log b110 ¼ 6.94 and log b120 ¼ 12.86. These results were obtained by refining the experimentalspectrophotometric data using the HypSpec software. In addition, a brief discussion of the comparison of the calculated individualelectronic spectra of the complexes reported in this work with the spectrum of the tris(bipyridine)ruthenium(II) chloride and themanganese-calcium oxygen-evolving complex of photosystem II is presented.© 2015 The Authors. Production and hosting by Elsevier B.V. on behalf of University of Kerbala. This is an open access articleunder the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Artificial photosynthesis; Equilibria; Manganese(II) complexes

* Corresponding author. Tel.: þ52 4731147530.

E-mail addresses: segovi@ugto.mx, jose.segoviano@gmail.com

(J.J.N. Segoviano-Garfias).

Peer review under responsibility of University of Kerbala.

http://dx.doi.org/10.1016/j.kijoms.2015.11.005

2405-609X/© 2015 The Authors. Production and hosting by Elsevier B.V. o

the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/

1. Introduction

One of the most important reactions in nature, is theprocess of water splitting in molecular oxygen and ionshydronium using light [1]. This represents one of themost promising and challenging ways to generate aphotochemical conversion and storing solar energy. It isalso considered the starting point to a sustainable

n behalf of University of Kerbala. This is an open access article under

4.0/).

179M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

hydrogen generation [1e3]. Nowadays, energy systemsbased in hydrogen appears to be one of the most prom-ising solutions in promoting a cleaner environment [4].

The photocatalytic water splitting occurs naturallyduring photosynthesis [2]. This process involves theabsorption of light, water and CO2 to produce food inthe form of sugars, such as glucose, which plays acrucial role in for the growth and survival of plants. Forthis purpose, the plants employ two proteinic com-plexes localized in the thylakoid membranes of plants,algae and cyanobacteria: Photosystem I (PSI) andphotosystem II (PSII) [2,5e7]. In PSI, the protons andelectrons are involved in several redox reactions, byusing the electrons transferred from photosystem II,through the complex Cytochrome-b6f and collaboratein the cycle of coenzyme NADPþ, which becomesNADPH, this provides the reducing power for CO2

fixation, this is a complex process barely understood,which involves The Calvin Cycle [8]. On the otherhand, the PSII contains the Oxygen-Evolving Complex(OEC), which is a cluster consisting in an oxo-connected structure that promotes the water splittingreaction [7]. In this catalytic site, the light contributesto splitting two water molecules to an oxygen mole-cule, 4 hydronium ions and 4 electrons [6]. Knowingthe molecular structure of the OEC is a prerequisite forunderstanding and possible mimicking of the watersplitting reaction and the metallic cluster involved.Numerous studies describe a possible mechanismpromoted by the metallic core, however, the details ofthe true role of the structure of the OEC have not beenfully revealed [9]. The cluster OEC consists of a tetranuclear manganese arrangement, bonded to a calciumand chloride ions [6,10e12]. Different authors havebeen reported that in nature, this is the only systemcapable of catalyzing the oxidation of water [13,14].

The details of the catalytic mechanism of the wateroxidation, remain controversial due to the uncertaintiesof the cluster structure, the sequence of changes in theoxidation states and the proton release pattern in themetal centers [2]. In the water splitting process, pro-tons and electrons are released and several mechanismshave been proposed to explain the possible steps in thisprocess. The mechanism widely diffused is the Kokcycle which is still debated in the scientific community[15,16]. Nevertheless, some other mechanisms havebeen proposed, such as Butterfly/double pivot, 2þ2and the nucleophilic attack of water/hydroxide to anelectrophilic Mn]O bond. However, there is still noconsensus about a true reaction pathway [15].

Molecular hydrogen in pure form is not available onEarth; however, methods for their production have

been reported before. Among the different technolo-gies for hydrogen production, natural or artificialoxygenic photosynthesis, have a great potential toproduce hydrogen. Both of them use clean and cheapsources: water and solar energy [17]. Nevertheless, thesustainable molecular hydrogen production throughdirect conversion of solar energy into chemical energy,represent a great challenge [17]. The inspired processin natural photosynthesis, which uses solar energy topromote the chemical conversion of water and/or car-bon dioxide to generate fuel, is known as artificialphotosynthesis [18,19]. Usually, employs compoundsthat works as photosensitizers and collect solar energy.Also, it involves obtaining synthetic models togenerate the catalytic activity for the hydrogen gener-ation from the splitting water reaction [17].

The synthesis of metal clusters as structural modelsof the OEC and the development of artificial photo-synthesis has a great field of potential application.Several metal complexes have been investigated for itsuse in artificial photosynthesis [20]. One of the mostimportant complexes studied with potential wateroxidation is the tris-(2,20-bipyridyl) ruthenium (III) or[Ru(bpy)3]

3þ [19e21]. Some other catalysts have beenreported with the capacity for water oxidation. How-ever, they are based on high priced metallic salts orwith teratogenic effects [22]. Although, there are onlya few reports about manganese complexes capable ofcatalyzing the oxidation of water [2,19], manganese isa promising metal to be used in artificial photosyn-thesis because of its low cost, owing to the fact that it isenvironmentally friendly and is naturally contained inthe oxygenic cluster of photosynthesis II [2,20].

Some spectral analogies can be observed in both,natural and artificial systems for photosynthesis. In nat-ural systems, although several transition states are pre-sent in the oxygen-evolving complex for example, in thespinach chloroplast [23], the spectral analogies betweentransition states usually present an absorption band at310 nm. This signal is usually observed and is probablyrelated to the change in the oxidation states ofmanganeseand the geometry of each manganese site [8]. Thischaracteristicmay represent an important guide tomimicthe electronic structure of the oxygen-evolving complexusing coordination compounds. On the other hand, thecompound [Ru(bpy)3]

3þ and other similar rutheniumcompounds have been widely studied. Several reportssuggest that this compound couldbe used as a photoredoxcatalyst inwater oxidation [2,21,24].Usuallypresents thehighest absorption band at approximately 285 nm [25].

In this work, the formation constants of the com-plexes generated with manganese(II), 2,20-bipyridine

180 M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

(Bpy) and 1,10-phenanthroline(Phe) in methanol so-lution, their speciation and the individual electronicspectrum of each species, are reported. It is widelyaccepted that the absorption spectrum of a compoundcan be usually associated to their electronic structure.The purpose of the present work is to evaluate whetherthe complexes obtained in this study can work as anelectronic model for artificial photosynthesis systems,by analyzing the spectral analogies between the com-pounds reported here, the electronic spectrum of OECand the electronic spectrum of the complex tris(bi-pyridine)ruthenium(II). Also, ionic strength was avoi-ded, because it can cause an early precipitation of thecomplexes, this consideration decreases the ionic sol-vation sphere and consequently, interferes with thestability of the complexes and their abundance in so-lution. Because of this, the formation constants in thisstudy may not be considered as true stability constantsand can only be used when comparing two systemsmeasured in similar conditions.

2. Materials and methods

2.1. Materials

Methanol HPLC grade (Tecsiquim, Mexico) wasused as purchased as solvent, manganese(II) nitratetetrahydrate, Mn(NO3)2$4H2O, 2,20-bipyridyl

Table 1

Stability constants of the complexes of manganese(II) with 2, 20-bipyridyl o

Method* Solvent Ionic strength

Manganese complexes with 2,2′-Bipyridyl

Cal. DMF* 0.4 M (C5H5)4NClO4

Sp. DMSO* 0.06 M NaClO4

MC Water 0.3 M NaClO4

MC Methanol 0.2 M NaClO4

Dis. Hexane Water(1:1) 1.0 M KNO3

Sp. HMPA* 0.06 M NaClO4

Sp. Water 0.3 M NaClO4

EV Water 1.0 M NaClO4

Dis. Water 0.1 M KCl

Manganese complexes with 1,10-Phenanthroline

Cal. DMF* 0.4 M (C5H5)4NClO4

Cal. DMF* 0.16 M (C5H5)4NClO4

MC Methanol 0.2 M NaClO4

EV Water 0.1 KNO3

MC DMF* 0.2 NaClO4

EMF _ 0.1 M NaCl

Dis. Water 0.1 M KCl

EMF Water 0.1 M KCl

*Abbreviations of the methods are: Cal ¼ calorimetric; Sp ¼ Spectropho

electrode; EMF ¼ spectrophotometric methods. The abbreviations for the

sulfoxide; HMPA ¼ hexamethyl phosphoric triamide.

(C5H4N)2,1,10-phenanthroline (C12H8N2) (Sigma-eAldrich), were analytical grade and used withoutfurther purification. Due to solubility problems noionic strength was used.

2.2. Physical measurements

All spectral measurements were made in a Cary 50UVeVis Spectroscopy System, at 298 K (RT) using aquartz cell with 1 cm of path length and 3 mL ofvolume. For all the experiments, the observed spectralregion was from 200 to 350 nm. For the determinationof formation constants, the spectrophotometric datawere fitted with the program HypSpec [26]. Distribu-tion diagrams of species were calculated using thesoftware Hyperquad Simulation and Speciation (HySS)[27].

2.3. Manganese(II)-2,20-bipyridyl equilibrium studies

Experiments were performed using two differentstock solutions of 2,20-bipyridyl (256 and 348 mM).Mn(NO3)2$4H2O was used to prepare manganese stocksolutions (160 and 216 mM). In each experiment, thefinal concentration of manganese(II) was set constantat 16 and 21.6 mM, in where the 2,20-bipyridyl con-centrations were varied from 2.56 to 48.6 mM and3.48e66 mM, respectively. A total of 37 spectra were

r 1,10-phenanthroline under several conditions.

T(�C) Logb110 Logb120 Logb130 Ref.

25 1.53 2.22 [31]

25 2.5 [34]

25 2.59 [32]

25 2.7 [33]

30 2.54 4.39 5.9 [38]

25 3.21 [35]

25 2.57 [32]

25 4.6 7.84 [37]

25 2.64 4.62 [36]

25 3.6 6.73 8.44 [47]

25 3.89 7 8.67 [47]

25 3.8 [33]

35 3.95 [48]

19 3.89 [49]

25 3.88 7.04 10.11 [50]

25 4.5 8.65 12.7 [36]

25 3.5 7.75 9.75 [36]

tometric; MC ¼ kinetics methods; Dis ¼ distribution; EV ¼ glass

solvents are: DMF ¼ N,N-dimethylformamide; DMSO ¼ dimethyl

Fig. 1. (a) Absorption spectra of the Manganese(II)e2,20-Bipyridylsystem in methanol solution: for spectra 1 to 19, [Mn(II)] ¼ 16 mM

and 2,20-Bipyridyl concentration (mM): (1) 2.5; (2) 5; (3) 7.5; (4) 10;

(5) 12.5; (6) 15; (7) 17.5; (8) 20; (9) 22.5; (10) 25; (11) 27.5; (12) 30;

(13) 32.5; (14) 35; (15) 37.5; (16) 40; (17) 42.5; (18) 45; (19) 48. (b)

For spectra 20 to 37, [Mn(II)] ¼ 21.6 mM and 2,20-Bipyridyl con-centration (mM): (20) 3.5; (21) 7; (22) 10.5; (23) 14; (24) 17.5; (25)

21; (26) 24.5; (27) 28; (28) 31.5; (29) 35; (30) 38.5; (31) 42; (32)

45.5; (33) 49; (34) 56; (35) 59.5; (36) 63; (37) 66.

181M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

used for the refinement in the HypSpec software, usingthe modified Job method [28].

2.4. Manganese(II)-1,10-phenanthroline equilibriumstudies

Experiments were performed using two differentstock solutions of 1,10-phenanthroline (70 and310 mM). Mn(NO3)2$4H2O was used to prepare man-ganese(II) stock solutions (48 and 11.1 mM). In eachexperiment, the 1,10-phenanthroline concentrationswere varied from 1.4 to 14 mM and from 3.1 to23.3 mM, respectively. In both experiments, the finalmanganese(II) concentration was set constant at 4.8and 1.11 mM, respectively. A total of 34 spectra wereused for the refinement in the HypSpec software, usingthe modified Job method.

3. Results and discussion

In all the equilibrium determinations, it is requiredto avoid the use of ionic strength, because it promotesthe precipitation of the manganese (II) complexes.Nevertheless, in manganese complexes with analogousligands, an equilibrium behavior is reported, in whichthe stability constant can decrease as the ionic forceincreases [29].

3.1. Formation constants of the manganese(II)-2,20-bipyridyl complexes

In several works, the crystal structures of the man-ganese complexes of 2,20-bipyridyl and 1,10-phenanthroline have been reported before [30].Meanwhile, in solution, the system manganese(II)-2,20-bipyridyl, also has been studied before under severalmethods and solvents: using a calorimetric method anddimethylformamide (DMF) as solvent, at ionic strengthof 0.4 [31]; using a kinetic methodology and water assolvent at ionic strength of 0.3 [32] or using methanolat ionic strength of 0.2. [33]. Additionally, using aspectrophotometric method at ionic strength of 0.06and dimethyl sulfoxide (DMSO) as solvent [34], hex-amethylphosphoramide (HMPA) as solvent with ationic strength of 0.06 [35] and water as solvent at ionicstrength of 0.3 [36]; using a glass electrode at ionicstrength of 1 [37] and finally, a spectrophotometricdetermination of the optical density of the organicphase in a biphasic system at ionic strength of 1 [38]. Abrief review of these systems is presented in Table 1. Inour knowledge, the formation constants of the man-ganese(II)-2,20-bipyridyl in methanol, have not been

reported before without ionic strength by a spectro-photometric method.

The electronic spectra for the system man-ganese(II)-2,20-bipyridyl, in methanol are shown inFig. 1. For this system a maximum at 233 nm and otherat 280 nm appears at low concentration of 2,20-bipyr-idyl, as the concentration increases, a hyperchromiceffect is observed at both wavelengths. The formationconstants bjkl correspond to an equilibrium betweenMn2þ and 2,20-bipyridyl, this determination was

Table 2

Summary of experimental parameters for the system: manganese(II)e2,20-bipyridyl and manganese(II)e1,10-phenanthroline systems in methanol.

Solution composition [TL] range from 2.56 to 48.6 and 3.48 to 66.1 mmol L�1

[TM] constant at 16 and 21.6 mmol L�1

Ionic strength, electrolyte Not used

pH range Not used

Experimental method Spectrophotometric titration

Temperature 24 �CTotal number of data points Mn complexation: 37 solution spectra

Method of calculation HypSpec

Species Equilibrium Log b s

[Mn(Bpy)]2þ Mn2þ þ Bpy % [Mn(Bpy)]2þ log b110 ¼ 7.81 ± 0.31 0.018

[Mn(Bpy)2]2þ Mn2þ þ 2Bpy % [Mn(Bpy)2]

2þ log b120 ¼ 14.68 ± 0.20

Solution composition [TL] range from 1.4 to 14 and 3.1 to 23.3 mmol L�1

[TM] constant at 4.8 and 11.1 mmol L�1

Ionic strength, electrolyte Not used

pH range Not used

Experimental method Spectrophotometric titration

Temperature 22 �CTotal number of data points Mn complexation: 37 solution spectra

Method of calculation HypSpec

Species Equilibrium Log b s

[Mn(Phen)]2þ Mn2þ þ Phen % [Mn(Phen)]2þ log b110 ¼ 6.94 ± 0.08 0.005

[Mn(Phen)2]2þ Mn2þ þ 2Phen % [Mn(Phen)2]

2þ log b120 ¼ 12.86 ± 0.14

182 M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

obtained by processing the spectra obtained in twoexperiments at two different concentrations. The pro-cess consists in analyzing simultaneously, the spectraof two experiments at two different concentrations ofstock solutions of manganese(II) and using differentranges of concentrations of ligand for each experiment.The method generates a correlation between thespectrum obtained, the concentration of metal andligand used and a proposal of the possible coloredspecies. The observed absorbance values at differentwavelengths were recorded at 298 K. In this system,only two colored species plus Mn2þ and 2,20-bipyridylwere found, the formation constants was achievedusing the next model:

Mn2þ þBpy#½MnðBpyÞ�2þ log b110 ð1Þ

Mn2þ þ 2Bpy#�MnðBpyÞ2

�2þlog b120 ð2Þ

The logarithmic values of the formation constantsand the summary of the experimental parameters arereported in Table 2, using the format of Tuck suggestedby IUPAC [39]. The calculated electronic spectrum for[Mn(Bpy)]2þ and [Mn(Bpy)2]

2þ are shown in Fig. 2a).Each complex, shows two intense absorption peaks, inthe case of mono complex at 233 and 283 nm, with amolar extinction coefficient of ε ¼ 18 077 L mol�1

cm�1 and ε ¼ 14 327 L mol�1 cm�1, respectively. Inthe case of the bis complex, two intense absorptionpeaks are shown at 233 and 282 nm, with a molarextinction coefficient of ε ¼ 25 387 L mol�1 cm�1 andε ¼ 26 276 L mol�1 cm�1, respectively.

3.2. Distribution curves of the manganese(II)e2,20-bipyridyl system

The speciation diagram for the manganese(II)e2,20-bipyridyl system is shown in Fig. 2b). For a solutionwith equimolar concentration of manganese(II) and2,20-bipyridyl yields about 60% of [Mn(Bpy)]2þ, 20%of free manganese(II) and 20% of the bis complex. Onthe other hand, two molar equivalents of 2,20-bipyridyland a molar equivalent of manganese, yields about95% of the bis-complex and 5% of the mono-complex.

3.3. Formation constants of the manganese(II)-1,10-phenanthroline complexes

The equilibrium constants of the manganese-1,10-phenanthroline has been reported before underseveral conditions, using solvents such as dime-thylformamide, water and methanol using ionicstrength (Table 1). Nevertheless as far as we know

Fig. 2. (a) Calculated electronic spectra of the Manganese(II)e2,20-Bipyridyl complex in methanol. (1) Mn2þ; (2) [Mn(Bpy)]2þ; (3)

Bpy; (4) [Mn(Bpy)2]2þ; (5) [Ru(Bpy)3]

3þ [25]. (b) Formation curves

of the Manganese(II)e2, 20-Bipyridyl complex in methanol

[Mn]2þ ¼ 21.6 mM and 2, 20-bipyridyl range from 3.5 to 66 mM.

Fig. 3. (a) Absorption spectra of Manganese(II)e1,10-Phenanthroline system in methanol solution: for spectra 1 to 19,

[Mn(II)] ¼ 48 mM and 1,10-phenanthroline concentration (mM): (1)

1.4; (2) 2; (3) 2.6; (4) 3.3; (5) 4; (6) 4.6; (7) 5.3; (8) 5.9; (9) 6.6; (10)

7.3; (11) 7.9; (12) 8.6; (13) 9.2; (14) 9.9; (15) 10.6; (16) 11.2; (17)

11.9; (18) 12.5; (19) 14. (b) For spectra 20 to 33, [Mn(II)]¼ 11.1 mM

and 1,10-Phenanthroline concentration (mM): (20) 3.1; (21) 4.7; (22)

6.2; (23) 7.8; (24) 9.3; (25) 10.9; (26) 12.4; (27) 14; (28) 15.5; (39)

17.1; (30) 18.6; (31) 20.2; (32) 21.7; (33) 23.3.

183M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

there is no report about an equilibrium determinationusing pure methanol.

The obtained electronic spectra for the systemManganese(II)-1,10-phenanthroline in methanol, areavailable in Fig. 3a) and b). For this system, amaximum is shown at 225 nm and another peak oflower absorbance at 265 nm, both begin to appear atlow ligand concentration. As the concentration ofligand increases a hypercromic effect begin to appear.The determination of the formation constants bjkl,correspond to equilibrium between manganese and1,10-phenanthroline were made using the same meth-odology as described above. The observed absorbancevalues at different wavelengths were recorded at298 K. Considering that only two colored species were

found plus manganese(II) and 1,10-phenanthroline, thedetermination of the formation constants was achievedusing the next model:

Mn2þ þ Phen#½MnðPhenÞ�2þ log b110 ð3Þ

Mn2þ þ 2Phen#�MnðPhenÞ2

�2þlog b120 ð4Þ

The logarithmic values of the formation constantsand the summary of the experimental parameters arealso reported in Table 2. The calculated electronic

Fig. 4. (a) Calculated electronic spectra of the Manganese(II)e1,10-

Phenanthroline complex in methanol. (1) Mn2þ; (5) [Ru(Bpy)3]3þ

[25]; (6) Phe; (7) [Mn(Phe)]2þ; (8) [Mn(Phe)2]2þ.(b) Formation

curves of the Manganese(II) �1, 10-phenanthroline complex in

methanol [Mn]2þ ¼ 48 mM and 1,10ephenanthroline range from 1.4

to 14 mM.

184 M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

spectrum for [Mn(Phen)]2þ and [Mn(Phen)2]2þ are

shown in Fig. 4a). Both complexes display two ab-sorption maximums, in the case of mono complex at223 and 264 nm, with a molar extinction coefficient ofε ¼ 106 470 L mol�1 cm�1 and ε ¼ 59 543 L mol�1

cm�1, respectively. In the case of the bis complex themaximum peaks appear at 227 and 264 nm with amolar extinction coefficient of ε ¼ 13 424 L mol�1

cm�1 and ε ¼ 84 368 L mol�1 cm�1, respectively.The formation constants in methanol obtained in

this work, are far higher than the reported values (Table1), in which several factors can be involved, such themethod, the solvent and the experimental conditions.Considering the solvents and stability constants of

manganese with 2,20-bipyridyl and 1,10-phenanthroline reported in Table 1 in previousstudies, the methanol is a better nucleophile thanDMSO and HMPA. Considering that donor numberrepresents the nucleophilic character of methanol,which is 19 [40], is related to the ionic character ofsolutes and their capacity to donate or accept electronpairs [41], is possible that solvation sphere of the metalpromotes the increasing of binding of metal to theligand.

On the other hand, 2,20-bipyridyl and 1,10-phenanthroline are considered as ligands with a hardbase character and because of their chelating proper-ties, they have been widely used for the syntheses ofmetal complexes. Also, they present several additionalproperties as a redox stability and easy functionaliza-tion [42,43]. The complexes with 2,20-bipyridyl are ofgreat interest because of their photochemical propertiesfor potential application to the solar energy conversion[44]. Although, manganese at the oxidation states of 2,3 and 4 [45,46], is classified as hard Lewis acid,because of this, is extremely important that both spe-cies, metal and ligand, should possess a hard character,the interaction between metal to ligand and in conse-quence, the stability of the species, would be greater[46]. These last characteristics promote a metal toligand charge transfer effect (MLCT). This is usuallypresented in complexes with metal ions with lowoxidation states and half full orbitals as manganese(II)and ligands acting as donors and orbitals p* of lowenergy as 2,20-bipyridyl and 1,10-phenanthroline [46].

As noted, the complexes of 2,20-bipyridine systemhave maximum absorbance at 235 and 280 nm, while1,10-phenanthroline complexes, have maximumabsorbance at 225 and 265 nm. This value is very closeto the wavelengths of the maximum absorbance of[Ru(bpy)3]

3þcomplex; however comparing with thespectra of OEC, this has maximum absorbance atlonger wavelengths. Considering the aromatic behaviorof the ligand in this study, 1,10-phenanthroline maybehave higher aromatic ligand, 2,20-bipyridyl, maybehave less aromatic than 1,10-phenanthroline [46].Considering this, 1,10-phenanthroline may have astronger basic character, and in consequence a strongerformation constant. Nevertheless the results obtainedhere, indicate the opposite behavior, this could berelated to the rotational flexibility that 2,20-bipyridylmay have, and the rigid character of the 1,10-phenanthroline ligand.

By comparing the electronic spectra of[Ru(bpy)3]

3þ to the systems of manganese (II) with2,20-bipyridine or 1,10-phenanthroline, several

185M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

similarities are observed, such as the maximumwavelengths of the spectra. Absorbance peaks withsimilar wavelengths could indicate that this two com-plexes could generate a similar electronic chargetransfer of metal to ligand of the [Ru(bpy)3]

3þ system.Although as shown in Figs. 2a) and 4a), the system ofMn(II)-2,20-bipyridine have absorption bands closest inmaximum wavelength to the [Ru(bpy)3]

3þ this couldbe related to an similar electronic structure betweenboth systems. Nevertheless, the manganese(II)-1,10-phenanthroline system, presents absorption bandsclosest in absorbance to the [Ru(bpy)3]

3þ.

3.4. Distribution curves of the manganese(II)e1,10-phenanthroline system

The distribution diagram for the manganese(II)e1,10-phenanthroline system is shown in Fig. 4b), for asolution with equimolar concentration of man-ganese(II) and 1,10-phenanthroline yields about 60%of the [Mn(Phen)]2þ, 20% of free manganese(II) and20% of the bis complex. Moreover, a mixture con-taining two molar equivalents of 1,10-phenanthrolineand a molar equivalent of manganese(II) yields about80% of [Mn(Phen)2]

2þ and 20% of the mono-complex.

4. Conclusion

The purpose of this work was to evaluate if thecomplexes reported here, could generate a spectrumwith a maximum peak closest to 310 nm or 285 nm.These wavelengths are characteristics of the oxygen-evolving complex, present in nature or in the artifi-cial systems of the complexes of [Ru(bpy)3]

3þ,respectively.

The manganese complexes with 2,20-bipyridyl and1,10-phenanthroline shows several important spectralcharacteristics. Possibly, this spectral behavior isrelated to a high and low rotational flexibility, or lowand high aromatic character, for the 2,20-bipyridyl or1,10-phenanthroline complexes, respectively. Thischaracteristic can possibly confer several geometricadvantages. In order to evaluate this theory, it isnecessary to realize a crystallographic study anddeterminate the geometry these complexes. Moreover,analyze their stability of these complexes in severalsolvents with different donor numbers, also theirspectral changes in the presence of several halides orpseudohalides, also determine the true magneticbehavior of the complexes through magnetic suscep-tibility and EPR and finally, evaluate possible changesin the oxidation states of the manganese center.

Hopefully, these studies eventually would allow us topromote the generation of a more appropriate spectralmodel.

Acknowledgments

The authors are grateful to Zsuzsa Hoffmann PhDfromASC International House, for her comments on themanuscript syntax. Also, the authors wish to thank toSecretaría de Educaci�on Publica (SEP) through PRODEP program for their economic support in the project”EQUILIBRIOS DE FORMACI�ON DE COMPLEJOSDE MANGANESO (II)-DIAMINA PARA SUAPLICACI�ON EN SISTEMAS DE FOTOS�INTESISARTIFICIAL00, also to the PADES program in theproject 2014-01-11-012-091 and agreement 2014-11-012-057, are gratefully acknowledged.

References

[1] E. Amouyal, Photochemical production of hydrogen and oxygen

from water: a review and state of the art, Sol. Energy Mater. Sol.

Cells 38 (1995) 249e276.[2] M.M. Najafpour, Calcium-manganese oxides as structural and

functional models for active site in oxygen evolving complex in

photosystem II: lessons from simple models, J. Photochem.

Photobiol. B Biol. 104 (2011) 111e117.

[3] M.M. Najafpour, et al., Biological water oxidation: lessons from

Nature, Biochim. Biophys. Acta BBA - Bioenergetics 1817 (8)

(2012) 1110e1121.

[4] I. Dincer, Green methods for hydrogen production, Int. J.

Hydrogen Energy 37 (2) (2012) 1954e1971.

[5] C. Zhang, Low-barrier hydrogen bond plays key role in active

photosystem II. A new model for photosynthetic water oxidation,

Biochim. Biophys. Acta BBA e Bioenergetics 1767 (6) (2007)

493e499.

[6] G. Arismendi Romero, Fotosistema II y fotosíntesis artificial:

buscando una nueva alternativa energ�etica, vol. 25, 2013.[7] J. Barber, Crystal structure of the oxygen-evolving complex of

photosystem II, Inorg. Chem. 47 (6) (2008) 1700e1710.

[8] B.Ke, Photosynthesis: Photobiochemistry and Photobiophysics, in:

Advances in Photosynthesis, Kluwer Academic Publishers, 2001.

[9] K.N. Ferreira, et al., Architecture of the photosynthetic oxygen-

evolving center, Science 303 (5665) (2004) 1831e1838.

[10] D.W. Bruce, D. O'Hare, R.I. Walton, Molecular Materials,

Wiley, 2010.

[11] J.S. Vrettos, J. Limburg, G.W. Brudvig, Mechanism of photo-

synthetic water oxidation: combining biophysical studies of

photosystem II with inorganic model chemistry, Biochim Bio-

phys Acta BBA e Bioenergetics 1503 (1e2) (2001) 229e245.

[12] Z.-G. Zhao, et al., In-situ formation of cobalt-phosphate oxy-

gen-evolving complex-anchored reduced graphene oxide

nanosheets for oxygen reduction reaction, Sci. Rep. (2013) 3.

[13] N. Nelson, A. Ben-Shem, The complex architecture of oxygenic

photosynthesis. Nature reviews, Mol. cell Biol. 5 (12) (2004)

971e982.

[14] G. Stochel, Z. Stasicka, M. Brindell, W. Macyk, K. Szacilowski,

Bioinorganic Photochemistry, Wiley, 2009.

186 M.L. P�erez-Arredondo et al. / Karbala International Journal of Modern Science 1 (2015) 178e186

[15] C.W. Cady, R.H. Crabtree, G.W. Brudvig, Functional models

for the oxygen-evolving complex of photosystem II, Coord.

Chem. Rev. 252 (3e4) (2008) 444e455.[16] J.S. Kanady, et al., A synthetic model of the Mn3Ca subsite of

the oxygen-evolving complex in photosystem II, Science 333

(6043) (2011) 733e736.

[17] S.I. Allakhverdiev, et al., Hydrogen photoproduction by use of

photosynthetic organisms and biomimetic systems, Photochem.

Photobiol. Sci. 8 (2) (2009) 148e156.

[18] J.B. Mur, Los retos de nuestra sociedad en el siglo XXI: una

breve reflexi�on desde el �ambito de la Química, An. la Real Soc.

Espa~nola Quím. 107 (1) (2011) 7.

[19] M.M. Najafpour, et al., Nano-sized manganese oxides as bio-

mimetic catalysts for water oxidation in artificial photosyn-

thesis: a review, J. R. Soc. Interface 9 (75) (2012) 2383e2395.

[20] K. Arifin, et al., Bimetallic complexes in artificial photosyn-

thesis for hydrogen production: a review, Int. J. Hydrogen En-

ergy 37 (4) (2011) 3066e3087.[21] M.Kobayashi, S.Masaoka,K.Sakai, Syntheses, characterization,

and photo-hydrogen-evolving properties of tris(2,2'-bipyridine)ruthenium(II) derivatives tethered to an H2-evolving (2-

phenylpyridinato)platinum(II) unit, Molecules 15 (7) (2010 Jul

14) 4908e4923, http://dx.doi.org/10.3390/molecules15074908.

[22] X. Liu, F. Wang, Transition metal complexes that catalyze

oxygen formation from water: 1979e2010, Coord. Chem. Rev.

256 (2011) 1115e1136.

[23] P. van Leeuwen, C. Heimann, H. van Gorkom, Absorbance

difference spectra of the S-state transitions in photosystem II

core particles, Photosynth. Res. 38 (3) (1993) 323e330.[24] J.J. Concepcion, et al., Making oxygen with ruthenium com-

plexes, Accounts Chem. Res. 42 (12) (2009) 1954e1965.

[25] M. Montalti, et al., Handbook of Photochemistry, third ed.,

Taylor & Francis, 2006.

[26] P. Gans, A. Sabatini, A. Vacca, HypSpec, 2008.

[27] L. Alderighi, et al., Hyperquad simulation and speciation

(HySS): a utility program for the investigation of equilibria

involving soluble and partially soluble species, Coord. Chem.

Rev. 184 (1) (1999) 311e318.

[28] W. Likussar, Computer approach to the continuous variations

method for spectrophotometric determination of extraction and

formation constants, Anal. Chem. 45 (11) (1973) 1926e1931.

[29] A.A. Zaid, et al., Effect of ionic strength on the stabilities of

ciprofloxacin. Metal complexes, J. Saudi Chem. Soc. 17 (1)

(2013) 43e45.[30] D. �Cechov�a, A. Marti�skov�a, J. Moncol, Structure of cis-

dichlorobis(1,10-phenanthroline)manganese(II) and cis-

dichlorobis(2,20-bipyridine)manganese(II), Acta Chim. Slovaca

7 (12014) (2014) 15e19.[31] M. Komiya, S. Yoshida, S.-i. Ishiguro, Binary and ternary

complexes involving manganese(II), 2,2aV™-bipyridine and

halide or thiocyanate ions in N,N-dimethylformamide, J. Solut.

Chem. 26 (10) (1997) 997e1010.

[32] D.N. Hague, S.R. Martin, Kinetics of ternary complex forma-

tion between manganese(II) species and 2,2-bipyridine, J.

Chem. Soc. Dalton Trans. 3 (1974) 254e258.[33] D.J. Benton, P. Moore, Rates of formation and dissociation of

complexes of manganese(II) with the ligands 1,10-

phenanthroline, 2,2[prime or minute]-bipyridine, and 2,2

[prime or minute]2[double prime]-terpyridine in anhydrous

methanol, J. Chem. Soc. Dalton Trans. 4 (1973) 399e404.[34] Y. Abe, G. Wada, The relationships between the formation

constants of 2,20-bipyridine metal(II) complexes and the donor

numbers of solvents, Bull. Chem. Soc. Jpn. 60 (5) (1987)

1936e1938.[35] Y. Abe, G. Wada, The formation constants and configurations of

the complexes of Cr(II), Mn(II), Fe(II), Co(II), Cu(II), and

Zn(II) with 2,20-Bipyridine and of Co(II) with 2,20:60,200-Ter-pyridine in hexamethylphosphoric triamide, Bull. Chem. Soc.

Jpn. 54 (11) (1981) 3334e3339.

[36] H. Irving, D.H. Mellor, 1002. The stability of metal complexes

of 1,10-phenanthroline and its analogues. Part I. 1,10-

phenanthroline and 2,2-bipyridyl, J. Chem. Soc. Resumed

0 (1962) 5222e5237.

[37] G. Atkinson, J.E. Bauman, The thermodynamics of complexes

formed with 2,2'-bipyridine and transition metal ions, Inorg.

Chem. 1 (4) (1962) 900e904.

[38] R.L. Davies, K.W. Dunning, The heats of formation of complex

ions containing bipyridyl, J. Chem. Soc. Resumed 0 (1965)

4168e4185.[39] D.G. Tuck, A proposal for the use of a standard format for the

publication of stability constant measurements, in: Pure and

Applied Chemistry, 1989, p. 1161.

[40] U. Mayer, Solvent effects on ion-pair equilibria, Coord. Chem.

Rev. 21 (2e3) (1976) 159e179.

[41] Y. Marcus, The effectivity of solvents as electron pair donors, J.

Solut. Chem. 13 (9) (1984) 599e624.[42] U.S. Schubert, C. Eschbaumer, Macromolecules containing

bipyridine and terpyridine metal complexes: towards metal-

losupramolecular polymers, Angew. Chem. Int. Ed. 41 (16)

(2002) 2892e2926.[43] F. Basolo, R. Johnson, Química de los compuestos de coor-

dinaci�on, Reverta, 1980.

[44] S. Balboa Benavente, Química de coordinaci�on de iones

met�alicos en estado de oxidaci�on II derivados de alfa-hidrox-

icarboxilatos, Univ Santiago de Compostela, 2007.

[45] V.L. Pfxoraro, Structural proposals for the manganese centers of

the oxygen evolving complex: an inorganic chemest's perspec-tive*, Photochem. Photobiol. 48 (2) (1988) 249e264.

[46] G.L. Miessler, D.A. Tarr, Inorganic Chemistry, Rex Bookstore,

Inc, 2004.

[47] M. Komiya, S. Yoshida, S.-i. Ishiguro, Ternary complexation of

manganese(II) and cadmium(II) with 1,10-phenanthroline and

halide or thiocyanato ions in N,N-dimethylformamide, J. Solut.

Chem. 29 (2) (2000) 101e129.

[48] C.R. Krishnamoorthy, S. Sunil, K. Ramalingam, The effect of

ligand donor atoms on ternary complex stability, Polyhedron 4

(8) (1985) 1451e1456.

[49] D.M.W. Buck, P. Moore, Rates of formation and dissociation,

and the stability of some manganese(II) and zinc(II) complexes

with bipyridyl-type ligands in dimethyl sulphoxide solution, J.

Chem. Soc. Dalton Trans. (7) (1976) 638e642.

[50] J.M. Dale, C.V. Banks, Study of metal-1,10-phenanthroline

complex equilibria by potentiometric measurement, Inorg.

Chem. 2 (3) (1963) 591e594.