Ruthenium and Iron Complexes with Benzotriazole and Benzimidazole Derivatives as Simple Models for Proton-Coupled Electron Transfer Systems

Complexos de ferro e rutênio do tipo [M–LH]n (onde M = Ru(NH3)5 2+,3+, RuII,III(edta)2–,– [edta = etilenodinitrilotetraacetato], ou Fe(CN)5 3–,2– e LH = benzotriazol ou benzoimidazol) foram preparados e caracterizados em solução aquosa através de métodos eletroquímicos e espectroeletroquímicos. Neste trabalho, maior ênfase foi direcionada aos processos redox dependentes do pH, que foram demonstrados por todos os complexos estudados. Os valores de pKa e de potencial de redução formal foram obtidos a partir dos diagramas de E1/2 versus pH, que apresentaram comportamento tipicamente Nernstiano. Os diagramas de Pourbaix também foram usados para ilustrar o particionamento entre as espécies redox e os equilíbrios ácido-base envolvidos nas reações. Na avaliação da potencialidade desses complexos como modelos simples para reações de transferência de elétrons acoplada a próton (PCET), considerou-se a extensão da região onde a dependência do par redox MIII/MII com o pH é ativa, definida entre pKa III e pKa II. Os resultados obtidos neste trabalho foram analisados do ponto de vista do caráter eletrônico doador/receptor dos ligantes e das interações σ,π-metal-ligante envolvidas em cada espécie, para ambos os estados de oxidação do íon metálico.


Introduction
Proton-coupled electron transfer (PCET) is known to play an important pathway of charge transport in a variety of aqueous electrochemical processes 1,2 and biochemical 3,4 , inorganic 5 and organic reactions 4,5 .Recent developments in supramolecular solid-state chemistry are directed toward rational design of PCET systems with novel electronic or photonic material properties 6 .The overall redox mechanisms, however, are often quite complex, involving multiple electron and proton transfers.But, by limiting the inner coordination sphere to one ionizable proton, it is possible to obtain complexes which exhibit one-electron, one-proton redox reactions over relatively broad pH and potential regions 5 .In order to develop simple models for PCET, we have synthesized coordinatively saturated, substitutionally inert transition-metal complexes such that oxidation of the metal is coupled to proton transfer at a remote site on the coordinated acid N-heterocyclic ligand.
In this direction, ruthenium 7 and iron 8,9 complexes with d 5 /d 6 configurations are very useful due to their rich and versatile redox chemistry, in addition to their usually favorable kinetic properties and stability.Heterocyclic nitrogens play in turn an important role in coordination chemistry 10 .Imidazole, for instance, is an ubiquitous ligand 11 in chemical and biological systems as it appears as such in proteins, and, together with its derivatives, have been extensively employed as models in a wide range of inorganic subject areas, from biological applications to electronic devices and materials 12,13 .The triazole derivatives have also been largely studied in ruthenium and osmium chemistry of discrete or supermolecules 14 .In particular, benzotriazole is largely employed as an efficient corrosion inhibitor for copper and its alloys 15 and provides a model ligand for bioinorganic studies involving the interaction of nucleic bases or DNA fragments and transition metal complexes.Nevertheless, its coordination chemistry with iron and second-row transition metals has been little explored.
Although the reactivity and the properties of transition metal complexes with imidazole and triazole derivatives have been studied over the last decades, a few cases involving the interaction of the benzimidazole (bimH) and benzotriazole (btaH) ligands (Scheme 1) with iron(II,III) and ruthenium(II,III) complexes are known [16][17][18][19][20][21][22] .Actually, to the best of our knowledge, we have investigated the coordination chemistry of benzotriazole with pentacyano-ferrate(II)/ (III) 17,21 , pentaammineruthenium(II)/(III) 18,21,22 and ethylenediaminetetraacetatoruthenium (II)/(III) 19,20,22 for the first time.In this present work, a larger series involving these metal units was studied, by including a structural analogue of the benzotriazole ligand (namely, benzimidazole).We explored in detail the relationship between the acid-base equilibria and electrochemistry, focusing on their proton-coupled electron transfer (PCET) reactions.
experiments.Other chemicals used here were also purchased from Sigma-Aldrich Co. and used as supplied.Argon gas (White-Martins) was employed to deaerate the solutions during the experiments.
Starting materials.Na 3 [Fe(CN) 5 (NH 3 )] 23 , [Ru(NH 3 ) 5 Cl]Cl 2 24 and Ru(Hedta)H 2 O 25 were synthesized according to the procedures previously reported in the literature.Elemental analyses, spectroscopic and electrochemical methods were used to assess the purity of the compounds.
[Ru n (NH 3 ) 5 (LH)](PF 6 ) n ×xH 2 O (where LH = bimH or btaH).In the preparation of the ammineruthenium derivatives, 58.6 mg of [Ru(NH 3 ) 5 Cl]Cl 2 (0.2 mmol) were dissolved in 5 cm 3 of degassed water, after which some amalgamated zinc pieces were added.After reacting for 30 min, the resulting yellow solution was anaerobically transferred to a recipient containing 238.2 mg of benzotriazole or 236.2 mg of benzimidazole (2.0 mmol) under an argon atmosphere (in the benzimidazole case, 5% in volume of ethanol was previously added to make the dissolution of the ligand easier), and left for 30 min.Then, 5 cm 3 of a concentrated ammonium hexafluoro-phosphate was dripped onto the orange color reactional mixture.A yellow precipitate was immediately formed.The solid product was collected on a filter, washed with a small volume of ethanol, and dried in vacuum in the presence of anhydrous calcium chloride.Yield: 80-90%.
In the case of the benzimidazole derivative, the yellow solid becomes rapidly reddish in the presence of air, indicating its conversion to the Ru III species (similar observations have been reported for several imidazole derivatives) 12 .For convenience, the isolated solid was then redissolved in water and completely oxidized by passing a flow of O 2 through the solution.After adding about 50 mg of solid ammonium hexafluorophosphate, a red product was isolated from the slow evaporation of the concentrated solutions.The final yield slightly decreased to ca. 75 Aqueous solutions.Alternatively, all the complexes were freshly prepared in aqueous solutions as follows: 0.025 mmol of the starting complex (i.e., Na 3 [Fe(CN) 5 (NH 3 )], [Ru(NH 3 ) 5 Cl]Cl 2 , or Ru(Hedta)H 2 O) was added, under anaerobic conditions, to 5.0 cm 3 of a 0.10 mol dm -3 PIPES buffered solution (PIPES = piperazine-N,N'-bis[2-ethanesulfonic acid]; pH 7.0) containing an amount of 0.050 mmol of the ligand and 5% of ethanol (in volume) to make the dissolution of the ligand easier (particularly for the benzimidazole).Then, the pH of the solutions was adjusted by carefully controlling (with the use of high-precision microvolumetric syringes) the addition of 3.0 mol dm -3 sodium hydroxide or hydrochloric acid solution, as required.Sodium trifluoroacetate (NaTFA) 0.10 mol dm -3 was employed as supporting electrolyte for the electrochemical and spectroelectrochemical measurements.
The purity of the complexes prepared in-situ was assessed by means of cyclic voltammetry, which presented in all samples only one reversible wave couple for the product (whose redox potential was rather distinguishable from the starting complex).In addition, the freshly prepared solutions from either the isolated solid or directly from the in-situ mixture of the reagent species showed identical cyclic voltammograms and electronic spectra.
Measurements for solutions at pH above 6.5 were run under an argon atmosphere.This procedure avoids, for instance, further oxidation processes involving the edtaruthenium(III) complex 26 .All prolonged-time measurements involving the photo-sensitive [Fe(CN) 5 ] 2-/3-derivatives were also run in the presence of reduced light.This procedure avoids further substitution processes involving the cyanide ligands.
Acid-base spectrophotometric titration.For determining the pK a of the complexes, the complexes were prepared in a modified Britton-Robinson 27 buffer (pH 2-12) solution, which has been prepared by mixing acetic (0.03 mol dm -3 ), PIPES (0.04 mol dm -3 ) and boric (0.03 mol dm -3 ) acids.The pH of the solutions was adjusted by the controlled addition of 3.0 mol dm -3 sodium hydroxide or hydrochloric acid solution, as required.

Physicochemical measurements
The electronic spectra were recorded on a Hewlett-Packard model 8453 diode-array spectrophotometer.
Cyclic and differential pulse voltammetry measurements were carried out using a Princeton Applied Research -PAR model 283 potentiostat/galvanostat.A conventional three-electrode arrangement was employed in the measurements; it consists of a glassy carbon working electrode, a Luggin capillary with Ag/AgCl (KCl 1.0 mol dm -3 ) reference electrode (Eº = 0.222 V versus SHE), and a platinum wire as the auxiliary electrode, in aqueous solutions containing sodium trifluoroacetate as supporting electrolyte.All solutions were thoroughly degassed with argon prior to beginning the experiments, and before each measurement the solutions were purged with inert gas.All E 1/2 values are uncorrected for junction potentials.
For the spectroelectrochemical measurements, a Princeton Applied Research -PAR model 173 potentiostat or a model 366 bi-potentiostat was used in parallel with the diode-array spectrophotometer.A three-electrode system was designed for a rectangular quartz cell of 0.025 cm internal optical path length.A gold minigrid was used as a transparent working electrode, in the presence of a small Ag/AgCl reference electrode and a platinum auxiliary electrode.The cell was located directly in the spectrophotometer, and the absorption change was monitored during the electrolysis.
The pK a values were determined by cyclic voltammetry measurements as a function of pH.An electrochemical cell was specially designed, in order to combine simultaneous pH measurements with cyclic voltammograms.
pH measurements were carried out with a digital pH meter mod.MD21 from Digimed Ltda.Calibration was performed by using pH 4.00, 7.00 and 10.00 commercial standard buffers.
Elemental analyses were obtained by Central Analítica -Instituto de Química, Universidade de São Paulo.

Results
The reactivity of the benzotriazole and benzotriazolate mononuclear derivatives was already discussed elsewhere [17][18][19] .The benzimidazole/benzimidazolate species, on the other hand, are being reported for the first time.Even so, only their PCET related properties will be stressed in this contribution, in such a way that the spectral features of both derivatives will no longer be outlined in details in the sections below.Instead, they are collected in tables, for comparison and discussion purposes.
The complexes of the type [Ru(edta)(LH)] -also presented pH-dependence on their redox potentials, as illustrated in Figures 1b and 2b 30 .
The reaction of aquapentacyanoferrate(II) with benzotriazole and benzimidazole (twice in excess) in an aqueous solution gives complexes whose acid-base equilibria for the coordinated ligands, [Fe(CN) 5 (LH)] 3-, were investigated.Their pH-potential diagrams can be seen in Figures 1c and 2c, respectively 30 .
An important comment on the electrochemical behavior of the studied systems is required before advancing the discussion of the results: while the cyclic voltammograms of the benzimidazole derivatives displayed a single wave couple, which was reversible over all the investigated pH range, in the 1H-benzotriazole species an additional anodic wave of low current intensity and shifted by about 100 mV to more positive potentials appeared in the voltammograms.This process has been assigned [17][18][19] to the partial isomerization from the complex formulated as [M-N3-btaH] to the [M-N2-btaH] one.Since this chemical process (isomerism) follows the electrochemical step (reduction) and the [M-N2-btaH] species practically does not exist in the oxidized form 17,19 , only the redox potentials associated with the reversible wave (M III/II -N3-btaH couple) will be taken for all purposes involving proton-dependence related features.

Discussion
If a ligand containing an ionizable proton is coordinated to a metal ion, its acidity generally increases due to stabilization of the conjugate base by the metal cation 5,32 .Since increasing the charge of the cation usually stabilizes the anionic conjugate base, it follows that the coordinated ligand becomes more acidic when the charge of the cation is increased.Therefore, the acid dissociation constant (K a ) of the complex increases with the oxidation state of the metal 19,33 .Consequently, a metal-localized redox process shows a pH-dependent couple if deprotonation (protonation) of the ligand occurs upon oxidation (reduction) of the complex in the pH range studied.The observation of pH-dependent potentials is, therefore, a powerful tool for recognizing proton-coupled redox processes 5 .
As the reactions investigated herein are of the type expressed by Eq. ( 3) and occur with a concomitant change in proton content, then the driving force for the ET reaction depends on the concentration of the hydrogen ion; hence, the electron transfer and proton transfer processes are considered to be coupled, which is readily indicated by the presence of the proton in the half reaction.
The half-wave potential (E 1/2 ; experimentally measured by cyclic 34,35 or differential pulse voltammetry 34,36 ) is predicted to have a pH dependence according to the Nernst equation 34 , which in the particular case of one-electron/one-proton redox couples, as in Eq. ( 3), reduces to a simplified expression (Eq.( 4)).This equation considers that the diffusion coefficients D ox and D red are very similar for such systems and can often be assumed as being equal; Eº' is the hypothetical formal potential at pH 0.
The dependence of redox potential upon the hydrogen ion concentration is conveniently visualized in a pHpotential diagram, commonly referred to as a Pourbaix diagram 2,37 , or predominance area diagram.The parameters defining such a pH-potential diagram, for a oneelectron redox process involving a hypothetical M III/II LH complex containing a single ionizable proton (i.e., exactly as the ones studied herein), are shown in Figure 3.
The horizontal lines in the acidic and basic pH regions represent the pH-independent redox processes in which both the oxidized and reduced species are protonated (Eq.( 5)) or deprotonated (Eq.( 6)), respectively.
The diagonal line corresponds to the pH-dependent redox process where reduction/oxidation of the metal center is coupled to protonation/deprotonation of the ligand (see Eq. ( 3)).The slope of this line is 59 mV/pH unit according to Eq. ( 4) for a one-electron, one-proton electrochemical process.The vertical lines in the acidic and basic pH regions represent the acid/base equilibrium for the oxidized (Eq.( 7)) and reduced (Eqs.( 8 As shown by the profiles of the E 1/2 versus pH diagrams in Figures 1 and 2, all the complexes within the investigated series displayed proton-coupled redox reactions which are consistent with the expected Nernstian behavior for one-electron/one-proton couples, as generically introduced above.A very important proof of that is provided by their slope values, which were very close to the predicted value of -59 mV/pH unit (see Table 1).
In addition, all the electrode reactions arisen from the one-electron M III /M II redox couples were electrochemically reversible.The reversibility was shown by the difference in anodic and cathodic peak potentials from the cyclic voltammetry experiments (∆E p = 60-70 mV) 34 .In addition, the current intensities (i peak ) for the reduction couple were plotted against the corresponding scan rates, showing square root dependence.This behavior is characteristic of a diffusion controlled process 34 .In such type of reaction, the number of electrons n involved in the process can be estimated from the theoretical peak to peak (E anodic -E cathodic ) separation, which is 59 mV/n.
All the reported complexes of the type M-LH, and their associated bases M-L -, were also spectroscopically characterized in aqueous solutions, as summarized in Table 2.
From Tables 1 and 2 one can conclude that, unlike the pyridine and pyrazine derivatives (where the π-acceptor character usually prevails), benzotriazole/ benzotriazolate and benzimidazole/benzimidazolate behave essentially as π-donor ligands, leading to a preferential stabilization of the M III -LH + or M III -L complexes.That is why the reduction potentials for them are relatively lower (more negative) than other similar N-heterocycle complexes.
In some benzimidazole-complexes (particularly in the ruthenium-edta and cyano-iron ones), their pK a for the reduced species (named herein as pK a II ) is so high that they could not be electrochemically determined, since above pH 12, the cyclic voltammetry response is too poor, and the stability of the complexes becomes poorer as well.Although pK a II were not precisely estimated in these examples, their superior limit (up to the maximum pH where the potentials were successfully measured) are indicated in Table 1.
Analysis of the spectral results related to ligand-to-metal (LMCT) and metal-to-ligand charge transfer (MLCT) transitions, suggests that benzimidazole and its conjugate base, benzimidazolate, are both stronger electron-donors than their benzotriazole and benzotriazolate analogues.That is, the  former derivative possesses a higher basic character than the latter one, which can be clearly observed through the set of acid-base and electrochemical parameters concerning their proton-coupled electron transfer reactions (Table 1).
Indeed, as the acidity of the ligand enhances when coordinated to a metal which has partially empty dπ orbitals (as in the case of the dπ 5 ions), the π-donor capability of the ligands can be inferred by analyzing the variation (decrease) of pK a values on their coordination to the oxidized complexes of the type M III X 5 -LH or M III X 5 -L -(that is the so called π-effects).In this sense, while the ∆pK a from the uncoordinated benzimidazole ligand (pK a = 12.78±0.04) 38to the [Ru III (NH 3 ) 5 (LH)] 3+ , [Ru III (edta)(LH)] -, and [Fe III (CN) 5 (LH)] 2-complexes is 4.96, 3.96, and 2.90, respectively, the variation in the analogous series for benzotriazole (whose pK a in the free form is 8.38±0.03) 39is rather lower: ∆pK a = 4.06, 2.07, and 2.04.The greater relative stabilization of the coordinated benzimidazolate species in relation to the benzotriazolate ones reflects their higher π-donor character, i.e. they have their acidity more enhanced with coordination than benzotriazole, although the latter is a better acid when analyzed in terms of absolute value of pK a , both in the free and in its corresponding complexes.
The variation of ∆E 1/2 (E 1/2 P -E 1/2 D ) follows the same trend of ∆pK a , meaning that the acidity of the coordinated ligand increases with increasing polarizability and electronegativity of the metal, both factors leading to an increase in the covalency of the M-L bond.In order to understand the importance of the factors involving the MX 5 units in this comparison, one must point out the nature of their coligands.In this way, one can correlate the greater ∆pK a and ∆E 1/2 in the pentaammineruthenium derivatives with the less σ-donor capability of NH 3 than the others (edta or CN -).
Table 1 also shows that there is a greater stabilization of the oxidized metal ion when it is coordinated to the deprotonated ligand, which is more basic (donor).In fact, in all the cases, E 1/2 D is around 200 mV shifted to lower potentials than E 1/2 P .Besides, the reduced species, M II -LH, have pK a values about a 3.5 pH unit higher than the oxidized ones, M III -LH + .This could be expected due to the M II -dπ 6 ion being able to promote a greater stabilization of the whole complex by means of π-backbonding in direction to the (more π-acceptor/acid) protonated ligand.On the other hand, the oxidized ion, M III -dπ 5 (Lewis acid), is better stabilized through the σ,π-donor interactions from the anionic deprotonated ligand.These correlations are also reflected on the LMCT and MLCT spectral data comparison in Table 2.
)) complexes, for which the acid dissociation constants are given by K a III = [M III L][H + ]/ [M III LH + ] and K a II = [M II L -][H + ]/[M II LH].horizontal line in the acidic region corresponds to the potential at which [M III LH + ] = [M II LH], while the diagonal line corresponds to [M III L] = [M II LH].At the intersection point, [M III LH + ] = [M II LH] = [M III L] and K a III = [H + ].Hence, pK a III is equal to pH at the vertical line in the acidic region.By a similar argument, pK a II = pH at the vertical line in the basic region.

Figure 3 .
Figure 3. pH-potential diagram for a complex of the type M II LH exhibiting a pH-dependent M III /M II couple, where the slope α is 59 mV/pH; Eº' is the formal potential of the M III L/M II LH couple (at pH = 0); E 1/2 P is the formal potential of the M III LH + /M II LH couple; E 1/2 D is the formal potential of the M III L/M II L -couple; and pK a III and pK a II are the pK a values of the M(III) and M(II) species, respectively (adapted from ref. 5).

Table 1 .
PCET parameters for the studied series of ruthenium and iron complexes.

Table 2 .
Comparison of UV-Vis absorption spectra for the series of complexes.