on the mechanism of formation and spectral properties of radical anions generated by the reduction...

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On the mechanism of formation and spectral properties of radical anionsgenerated by the reduction of –[ReI(CO)3(5-nitro-1,10-phenanthroline)]+

and –[ReI(CO)3(3,4,7,8-tetramethyl-1,10-phenanthroline)]+ pendantsin poly-4-vinylpyridine polymers

Larisa L.B. Bracco a, Reynaldo O. Lezna a, Jackeline Muñoz-Zuñiga a, Gustavo T. Ruiz a, Mario R. Féliz a,Guillermo J. Ferraudi b, Fernando S. García Einschlag a,⇑, Ezequiel Wolcan a,⇑a Instituto de Investigaciones Fisicoquımicas Teóricas y Aplicadas (INIFTA, UNLP, CCT La Plata-CONICET), Diag. 113 y 64, Sucursal 4, C.C. 16, (B1906ZAA) La Plata, Argentinab Department of Chemistry, Radiation Research Building, University of Notre Dame, Notre Dame, IN 46556-0579, USA

a r t i c l e i n f o

Article history:Received 1 October 2010Received in revised form 11 February 2011Accepted 14 February 2011Available online 17 February 2011

Keywords:Pulse radiolysisRadicalsReductionPolymersRheniumPhenanthroline

a b s t r a c t

The electrochemical reduction in aprotic media of –[ReI(CO)3L]+ pendants in poly-4-vinylpyridinepolymers is compared to that of [ReI(CO)3L]+ complexes (L = 5-nitro-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10-phenanthroline). The UV–Vis absorption spectra of the reduced radical anions of5-nitro-1,10-phenanthroline (NO2-phen) and 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen) wereobtained by spectro-electrochemistry of [ReI(CO)3(NO2-phen)(CH3CN)]+ and [ReI(CO)3(tm-phen)(CH3CN)]+, respectively. Similar spectra were obtained for the radical anions NO��2 -phen andtmphen�� after pulse radiolysis experiments with –[ReI(CO)3L]+-containing polymers. The analysis ofthe time-resolved difference spectra was performed using ‘‘multivariate curve resolution’’ (MCR) tech-niques. Unlike e�solv, C�H2OH radicals were unable to reduce tmphen ligands. The reaction of e�solv and/orC�H2OH with –[ReI(CO)3(NO2-phen)]+-containing polymers generates –[ReI(CO)3(NO��2 -phen)] pendantswhich after disproportionation give rise to products with kmax = 380 nm. The kinetic behavior of –[ReI(CO)3(NO��2 -phen)] pendants under different experimental conditions is discussed.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Numerous studies have been concerned with thermal and pho-tochemical reactions of inorganic polymers in the solid-state andsolution phase. Interest in their photochemical and photophysicalproperties is driven by their potential applications in catalysisand optical devices [1–14]. The properties in the solution phaseof the polymers {(vpy)2-vpy[ReI(CO)3(phen)]+}n�200 and {(vpy)2-vpy[ReI(CO)3(bpy)]+}n�200 (see Schemes 1 and 2) were investigatedin previous works [1,9,12].

Marked differences were found between the photochemical andphotophysical properties of polymers {(vpy)2-vpy[ReI(CO)3L]+}n�200

(L = phen, bpy) and those of the related monomeric complexes,[pyReI(CO)3L]+. The main cause of these differences is the photo-generation of MLCT excited states in concentrations that are muchlarger when –[ReI(CO)3L]+ chromophores are bound to poly-4-vinylpyridine, (vpy)n�600. This is the photophysical result of ReI

chromophores being crowded in strands of a polymer instead of

being homogeneously distributed through solutions of a [pyReI

(CO)3L]+ complex. Scheme 3 shows the structural characteristicsof a monomeric unit of a ReI based polymer. The attachment ofmetallo groups to the polymer modifies the reactivity of the ex-posed group, located outside the polymer matrix. The dispositionof the metallo groups in crowded spaces of the polymer strand al-lows the occurrence of unique processes, like long-range electronand energy transfers, and excited state-excited state reactions[1]. The recently communicated association of several hundred{(vpy)2-vpy[ReI(CO)3(bpy)]+}n�200 strands in nearly sphericalaggregates also contributes to the crowding of chromophores insmall spaces in the solution, where the interaction between excitedstates becomes appreciable [12]. The photogeneration of MLCT ex-cited states in close vicinity within a polymer strand makes possi-ble the study of energy transfer processes if donor and acceptorpendants are distributed along the strand. In previous work [15],we have studied the photophysical properties of polymers{(vpy)2-vpy[ReI(CO)3(tmphen)]+}n�200,{(vpy)2-vpy[ReI(CO)3(NO2-phen)]+}n�200, and {(vpy)2-vpy[ReI(CO)3(tmphen)]+}n�100{(vpy)2-vpy[ReI(CO)3 (NO2-phen)]+}m�100 in terms of the current resonanceenergy transfer theories applicable to energy transfer betweenacceptors and donors randomly distributed in a polymer. In this

0020-1693/$ - see front matter � 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.ica.2011.02.039

⇑ Corresponding authors.E-mail addresses: [email protected] (F.S. García Einschlag), ewolcan

@inifta.unlp.edu.ar (E. Wolcan).

Inorganica Chimica Acta 370 (2011) 482–491

Contents lists available at ScienceDirect

Inorganica Chimica Acta

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paper we explore the electronic reduction of the latter polymers bypulse radiolysis and spectro-electrochemistry. The radical anionspectra of 5-nitro-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10-phenanthroline were obtained using spectro-electrochemical

techniques in acetonitrile solutions of CF3SO3ReI(CO)3(NO2-phen)and CF3SO3ReI(CO)3(tmphen), respectively. In pulse radiolysisexperiments, the reaction of e�solv and {(vpy)2-vpy[ReI(CO)3

(tmphen)]+}n�200 generates –[ReI(CO)3(tmphen��)] species in thepolymer, which after disproportionation reactions yield –[ReI

(CO)3(tmphenH2)]+ and –[ReI(CO)3(tmphen)]+ pendant chromoph-ores. The reaction of e�solv and/or C�H2OH with the polymer{(vpy)2-vpy[ReI(CO)3(NO2- phen)]+}n�200 generates –[ReI(CO)3

(NO��2 -phen)] pendants which after disproportionation give riseto products which are stable up to the millisecond time scale.The kinetic behavior of –[ReI(CO)3(NO��2 -phen)] pendants underdifferent experimental conditions is discussed.

2. Materials and methods

2.1. Electrochemical procedures

Dc and ac voltammetries were conducted in a conventionalmanner. Acetonitrile, CH3CN, Merck ‘‘SeccoSolv’’, was further driedover activated alumina for several days. The procedure was found

Scheme 1. Structural formulae of poly-4-(vinyppyridine) (left) and polymersderived from poly-4-(vinylpyridine) and –[ReI(CO)3(L)]+ pendants (right) and theabbreviations used.

Scheme 2. Structural formulae of selected a-diimine and other acceptor ligands and the abbreviations used (in parenthesis).

Scheme 3. Representation of a fragment (only 3 ReI chromophores are represented here) of the polymer [(vpy)2-vpyReI(CO)3(bpy)]n�200 (CF3SO3)n�200 showing mean inter-chromophore distances.

L.L.B. Bracco et al. / Inorganica Chimica Acta 370 (2011) 482–491 483


to be suitable for electrochemistry in the �2.0/+1.8 V (versus Ag/AgCl, KClsat) potential range. (Bu4N)PF6 was dried in vacuum at60 �C for at least 24 h. Pt discs (6 mm dia) were used as workingelectrodes. Potentials were measured against a Ag/AgCl, KClsat ref-erence electrode. Oxygen was removed from the solutions by bub-bling purified nitrogen for at least 30 min prior to each experiment.A Pt mesh, separated from the working compartment by a porousdisc, was used as a counter electrode. A fresh surface was gener-ated before each experiment by polishing the working electrodeto a mirror finish with alumina, sonicated, rinsed with Millipore-Milli-Q water and dried before being placed in the solution. Thequality of the surface was tested by inspecting the response to cy-cling the working electrode in the supporting electrolyte. Solutionswere prepared with analytical-grade reagents. A typical concentra-tion for polymers was �4 � 10�7 M ([ReI] � 8 � 10�5 M). UV–Visspectra in situ at fixed potentials were obtained in an electrochem-ical cell at an angle of incidence of 45�, using a computerized opti-cal multichannel analyzer (EG&G PAR OMA III) fitted with a cooledSi diode array [16]. The detector, unintensified, is made up of 1024channels. This rapid scan spectrometer, with a 14-bit resolution,was employed to obtain integral spectra resulting from the coaddi-tion in computer memory of a variable number of exposures(depending on signal/noise), each averaged 0.03 s on the diode ar-ray chip. Diffraction orders higher than unity were sorted out byappropriate filters. Spectra, as a sort of snapshots, were collectedduring the progress of slow voltammetric sweeps, �5 mV s�1 andcalculated as (R � Rref)/Rref with the reference spectra, Rref, takenat appropriate potentials.

2.2. Pulse radiolysis

Pulse radiolysis experiments were carried out with a model TB-8/16-1S electron linear accelerator. The instrument and computer-ized data collection for time-resolved UV–Vis spectroscopy andreaction kinetics have been described elsewhere [17,18]. Thiocya-nate dosimetry was carried out at the beginning of each experimen-tal session. Details of the dosimetry have been reported elsewhere[18,19]. The procedure is based on the concentration of ðSCNÞ��2 rad-icals generated by the electron pulse in a N2O-saturated 10�2 MSCN� solution. In the procedure, the calculations were made withG = 6.13 and an extinction coefficient e = 7.58 � 103 M�1 cm�1 at472 nm [18,19] for the ðSCNÞ��2 radicals. In general, the experimentswere carried out with doses that in N2-saturated aqueous solutionsresulted in (1.7 ± 0.1)� 10�6 M to (6.0 ± 0.3)� 10�6 M concentra-tions of e�aq. In these experiments, solutions were deaerated withstreams of N2 or N2O gasses. In order to irradiate a fresh samplewith each pulse, an appropriate flow of the solution through thereaction cell was maintained during the experiment.

The radiolysis of CH3OH and CH3OH/H2O mixtures with ionizingradiation has been reported elsewhere in the literature [20–22].These studies have shown that pulse radiolysis can be used as aconvenient source of e�solv and C�H2OH radicals according to Eq. (1).

ð1Þ

Thus, the main reducing species in pulse radiolysis of methano-lic solutions under a N2 atmosphere are e�solv and C�H2OH. Since thelatter species have large reduction potentials, i.e., �2.8 V versusNHE for e�solv and �0.92 V versus NHE for C�H2OH, they have beenused for the reduction of coordination complexes and for the studyof electron transfer reactions. The yield of e�solv in CH3OH (G � 1.2)is about one third of the G-value in the radiolysis of H2O (G � 2.8)[20]. In solutions where e�solv was scavenged with N2O [22], theC�H2OH radical appears to be the predominant product(yield > 90%) of the reaction between CH3OH and O��.

2.3. Materials

CF3SO3Re(CO)3(NO2-phen), CF3SO3Re(CO)3(tmphen), and poly-mers {(vpy)2-vpy[ReI(CO)3(tmphen)]+}n�200,{(vpy)2-vpy[ReI(CO)3

(NO2- phen)]+}n�200, and {(vpy)2-vpy[ReI(CO)3(tmphen)]+}n�100

{(vpy)2-vpy[ReI(CO)3(NO2- phen)]+}m�100 (see Scheme 1) wereavailable from previous work [15]. Given that in acetonitrile solu-tions CF3SO�3 is replaced by the solvent, the monomeric species willbe denoted as [ReI(CO)3(NO2-phen)(CH3CN)]+ and [ReI(CO)3

(tmphen)(CH3CN)]+, respectively. Polymers {(vpy)2-vpy[ReI(CO)3

(tmphen)]+}n�200,{(vpy)2-vpy[ReI(CO)3(NO2- phen)]+}n�200, and {(vpy)2-vpy[ReI(CO)3(tmphen)]+}n�100{(vpy)2-vpy[ReI(CO)3(NO2-phen)]+}m�100,for the sake of simplicity, will be denoted hereafter as TM-P4VPy,NO2-P4VPy and TM-NO2-P4VPy, respectively.

2.4. Spectroscopic analysis

For the analysis of the time-resolved spectra we used a softwaredesigned in our laboratory [23] capable to perform ‘‘multivariatecurve resolution’’ (MCR) [24,25]. These methods can be appliedto bilinear spectroscopic-kinetic data from a chemical reaction toprovide information about composition changes in an evolving sys-tem [26]. In the present work we have chosen one of the mostwidely used algorithms, the alternating least-squares (ALS), whichcan help to estimate concentration and spectral profiles simulta-neously [26,27]. ALS algorithm extracts useful information fromthe experimental data matrix A(t � w) by iterative application ofregression analysis using the following matrix product:

A ¼ CST þ E ð2Þ

where C(t � n) is the matrix of the kinetic profiles; ST(n �w) is thatcontaining the spectral profiles, and E(t �w) represents the errormatrix. The numbers t, n and w denote the sampling times, absorb-ing species and recorded wavelengths, respectively. Resolving ma-trix A may be a rather difficult task [28] since on the one hand, nis usually unknown [29] and on the other hand, curve resolutionmethods cannot deliver a single solution because of rotational andscale ambiguities [30]. We applied Factor Analysis and SingularValue Decomposition to the experimental matrix for the estimationof n. In order to reduce rotational ambiguities we used some chem-ically relevant constraints [31] such as non-negativity, selectivityand unimodality. Matrix augmentation strategy was used tosimultaneously obtain the concentration profiles corresponding todifferent experimental conditions [27].

3. Results

3.1. Electrochemistry and UV–Vis spectroelectrochemistry

AC voltammetries of CF3SO3Re(CO)3(NO2-phen), CF3SO3Re(CO)3(tmphen), NO2-P4VPy and TM-P4VPy are shown in Supple-mentary Figs. S1 and S2. [ReI(CO)3(NO2-phen)(CH3CN)]+ undergoesfour chemically quasi reversible one-electron reduction waves inCH3CN at E1/2 = �0.527, �1.087, �1.514 and �1.716 V versus

484 L.L.B. Bracco et al. / Inorganica Chimica Acta 370 (2011) 482–491


Ag/AgCl, respectively. The first reduction potential may be assignedto a reduction localized on the NO2 group [32] while the otherthree reduction potentials may be assigned to reduction processesoccurring on the phen portion of the NO2-phen ligand [32–35]. Bycontrast, the related polymer NO2-P4VPy undergoes four chemi-cally irreversible one-electron reduction waves in CH3CN atEp = �0.349, �1.157, �1.296 and �1.773 V versus Ag/AgCl, respec-tively. [ReI(CO)3(tmphen)(CH3CN)]+ undergoes three chemicallyquasi reversible one-electron reduction waves in CH3CN at E1/

2 = �1.366, �1.570 and �1.766 V versus Ag/AgCl, respectively,which correspond to reductions centered in the tmphen ligand.However, polymer TM-P4VPy undergoes three chemically irrevers-ible one-electron reduction waves in CH3CN at Ep = �1.231, �1.339and �1.590 V versus Ag/AgCl, respectively. The reductionpotentials of [ReI(CO)3(NO2-phen)(CH3CN)]+, [ReI(CO)3(tmphen)(CH3CN)]+, NO2-P4VPy and TM-P4VPy measured in this work alongwith literature reported values for related XReI(CO)3L complexes[32–35] are listed in Table 1. As the reduction potentials in theliterature are reported against different reference electrodes (seeTable 1), those potentials were converted to values referred tothe AgCl/Ag reference electrode according to Pavlishchuk et al.[36].

With a view to identifying the species generated electrochemi-cally, associated with the waves recorded by voltammetry, spectrawere recorded in situ at 40 mV intervals with the aid of an OMAsystem, on a slow negative-going scan at 5 mV s�1, between 0and �2.0 V, the reference being the spectrum at 0 V where noabsorption was detected. Fig. 1a exhibits, for clarity, a reducedset of spectra for [ReI(CO)3(NO2-phen)(CH3CN)]+. Bands observedbetween �0.5 and �0.8 V, at 410 (narrow) and 535 nm (broad),stem from the product of the NO2 portion of the NO2-phen ligandreduction, the ðNO��2 Þ-phen radical anion. The ratio (DR/R)410 nm/(DR/R)535 nm being about 3. At more negative potentials, i.e., be-tween �1.0 and �2.0 V, the band at 535 nm splits into two compo-nents at 543 and 570 nm, the ratio (DR/R)410 nm/(DR/R)543 nm beingnow �1.4. The increase of the band at �535 nm relative to that at410 nm is indicative of reduction processes occurring on the phenportion of the NO2-phen ligand [1,37]. Similar UV–Vis changes,though less defined, were observed upon reduction of the relatedpolymer NO2-P4VPy (see Fig. 1b). Fig. 2a exhibits, for clarity, a re-duced set of spectra for [ReI(CO)3(tmphen)(CH3CN)]+. Three bands,one sharp at 440 nm with a shoulder at 420 nm, and a broad bandat 565 nm, were observed between �1.3 and �1.8 V stemmingfrom the product of the phen ligand reduction, the tmphen�� radi-cal anion. The ratio (DR/R)440 nm/(DR/R)565 nm being about 2.2, 2.5,2.3 and 2.4 at �0.527, �1.087, �1.514 and �1.716 V, respectively.Similar UV–Vis changes, though less well-defined, were observed

upon reduction of the related polymer TM-P4VPy (see Fig. 2b),the ratio (DR/R)440 nm/(DR/R)565 nm being about 1.0 independentof the potential.

3.2. Pulse radiolysis

The spectra of the species produced by the one-electron reduc-tion of –[ReI(CO)3(tmphen)]+ and –[ReI(CO)3(NO2-phen)]+-contain-ing compounds was investigated by pulse radiolysis of thepolymers solutions in CH3OH. With polymers TM-P4VPy and TM-NO2-P4VPy, the concentration of the chromophores was adjustedto [ReI] = 5.3 � 10�5 M. Due to the low solubility of polymer

Table 1Reduction potentials of XReI(CO)3L complexes.

Compound E (V) E (V) vs. AgCl/Agg

ClRe(CO)3(phen) �1.34 vs. SCEa �1.287�1.27 vs. SCEb �1.217�1.36 vs. SCEc �1.307

Re(CO)3(phen)(CH3CN)+ �1.24 vs. SCEc �1.187ClRe(CO)3(NO2-phen) �1.01, �1.68 vs. Fc/Fc+d �0.589, �1.259CF3SO3Re(CO)3(NO2-phen) �0.527, �1.087, �1.514, �1.716 vs. AgCl/Age �0.527, �1.087, �1.514, �1.716CF3SO3Re(CO)3(tmphen) �1.366, �1.570, �1.766 vs. AgCl/Age �1.366, �1.570, �1.766NO2-P4VPy �0.349, �1.157, �1.296, �1.773 vs. AgCl/Age �0.349, �1.157, �1.296, �1.773TM-P4VPy �1.231, �1.339, �1.590 vs. AgCl/Age �1.231, �1.339, �1.590

a From Ref. [34].b From Ref. [33].c From Ref. [35].d From Ref. [32].e This work.g Potentials were converted from values referred to the SCE reference electrode or the Fc/Fc+ couple to values referred to the AgCl/Ag

reference electrode according to Ref. [36].

Fig. 1. (a) [(R � Rref)/Rref] spectra collected at 50 mV intervals during a negative-going linear potential sweep at 5 mV s�1 in the 0 ? �2.0 V potential range of the Pt/CH3CN-0.1 M Bu4NPF6/8 � 10�5 CF3SO3Re(CO)3(NO2-phen) interface. (b) [(R � Rref)/Rref] spectra collected at 50 mV intervals during a negative-going linear potentialsweep at 5 mV s�1 in the 0 ? �2.0 V potential range of the Pt/CH3CN-0.1 MBu4NPF6/�4 � 10�7 M ([ReI] �8 � 10�5 M) NO2–P4VPy interface. R is the reflec-tance. Spectra were the average of 20 measurements, exposure time, 0.03 s, i.e.,600 ms per spectrum, angle of incidence / = 45�. Signal at 0 V, R 0 V, used as thereference. For clarity only a spectra subset is shown. See text for details.



NO2-P4VPy in methanol, saturated solutions of NO2-P4VPy wereonly [ReI] � 1 � 10�5 M. However, that [ReI] resulted high enoughto maintain a pseudo-first order kinetic in the thermal reactionwith solvated electrons. Chromophores –[ReI(CO)3(tmphen)]+ in

polymer TM-P4VPy failed to react with the pulse radiolyticallygenerated C�H2OH radicals. They were reduced however by e�solv

at a diffusion-controlled rate. In N2 deaerated methanolic solu-tions, the reaction between e�solv and TM-P4VPy was completedwithin the first ls after the radiolytic pulse with a rate constantk = (2.1 ± 0.2) � 1010 M�1 s�1. The transient spectrum generatedby the reaction with e�solv exhibited two absorption bands withkmax = 420 and 560 nm [37]. Once the reaction of the e�solv withthe polymer has been completed, the absorption bands atkmax = 420 and 560 nm have reached nearly the same intensity.The transient spectrum shows a decrease of the kmax = 560 nmband, relative to that at 420 nm, after a 3-ms delay from the radio-lytic pulse [37]. When solutions of TM-P4VPy were deaerated withstreams of N2O instead of N2, all the radiolytically generated radi-cals were converted to C�H2OH radicals in less than 1 ls. The radio-lytic pulse caused no changes in the spectrum of the solution. Thisexperiment demonstrated that C�H2OH radicals do not reduce thetmphen ligand in TM-P4VPy and that the absorbance changes inmethanolic solutions under N2 are caused by the reaction of e�solv

with the ReI pendants.Fig. 3 shows the spectra of the transients generated in pulse

radiolysis experiments of N2 deaerated NO2-P4VPy solutions. Theinset to Fig. 3 shows the absorption spectrum of e�solv before thereaction with NO2-P4VPy. The transient spectrum recorded justafter the termination of the reaction between e�solv and NO2-P4VPy, i.e., with a delay of 13 ls from the radiolytic pulse,exhibited one absorption band with a kmax = 430 nm, which maybe assigned to a reduction occurring on the NO2 portion of theNO2-phen ligand, forming the ðNO��2 Þ-phen anion radical inNO2-P4VPy. After some microseconds, the spectrum evolves andat t = 65 ls the absorption band has shifted to kmax � 400 nm.The spectrum recorded at t = 650 ls has kmax = 380 nm. At timest > 1 ms there are no more spectral changes in the solution andthe final product, with a kmax = 380 nm, is stable up to 5 ms.Fig. 4 shows the spectra of the transients generated in pulseradiolysis experiments of N2O deaerated NO2-P4VPy solutions.The inset to Fig. 4 shows, at short times, the absence of spectralfeatures attributable to absorptions of e�solv and only weak absorp-tions at kmax � 300 nm attributable to absorptions of C�H2OH

Fig. 2. (a) [(R � Rref)/Rref] spectra collected at 50 mV intervals during a negative-going linear potential sweep at 5 mV s�1 in the 0 ? �2.0 V potential range of the Pt/CH3CN-0.1 M Bu4NPF6/8 � 10�5 CF3SO3Re(CO)3(tmphen) interface. (b) [(R � Rref)/Rref] spectra collected at 50 mV intervals during a negative-going linear potentialsweep at 5 mV s�1 in the 0 ? �2.0 V potential range of the Pt/CH3CN-0.1 MBu4NPF6/�4 � 10�7 M ([ReI] �8 � 10�5 M) TM–P4VPy interface. R is the reflectance.Spectra were the average of 20 measurements, exposure time, 0.03 s, i.e., 600 msper spectrum, angle of incidence / = 45�. Signal at 0 V, R 0 V, used as the reference.For clarity only a spectra subset is shown. See text for details.

Fig. 3. Spectra of the transients generated in pulse radiolysis experiments of N2 deaerated NO2-P4VPy solutions. Spectra were recorded at 85 ns, 13, 65 and 650 ls delaysafter the radiolytic pulse. The inset to Fig. 3 shows absorption spectrum of e�solv before the reaction with NO2-P4VPy. See text for details.



radicals [38]. Comparison of Figs. 3 and 4 shows that the reductionof NO2-P4VPy with C�H2OH radicals produces transients and finalproducts that have the same spectral features of those generatedby the reaction of NO2-P4VPy with e�solv. However, the initialamount of –[ReI(CO)3ðNO��2 Þ-phen] is nearly four times larger inexperiments with N2 deaerated NO2-P4VPy solutions than inexperiments with N2O deaerated NO2-P4VPy solutions. Moreover,the absorption of the final product with a kmax = 380 nm is aboutthree times higher in N2 deaerated NO2-P4VPy solutions than inN2O deaerated NO2-P4VPy solutions. Similar transients and finalproducts to those of Figs. 3 and 4 were generated when TM-NO2-P4VPy was reduced in pulse radiolysis under N2 and/or N2O atmo-sphere, indicating that in polymers that contain –[ReI(CO)3

(NO2-phen)]+ and –[ReI(CO)3(tmphen)]+ chromophores, the finalproduct arises only from the thermal reactions of –[ReI(CO)3

ðNO��2 Þ-phen] radicals.

3.3. Spectral and kinetic analysis

The analysis of the full matrix of time-resolved difference spec-tra was performed using MCR techniques. Both Factor Analysis andSingular Value Decomposition were used for the estimation of thenumber of independent contributions yielding n values of 4 and 3for the experiments performed with N2 and N2O deareated solu-tions, respectively, reflecting the fact that e�solv is efficiently scav-enged in the presence of N2O. Orthogonal projection approachwas used to obtain initial guesses of the spectra corresponding toeach contributing species [39]. The spectral shapes obtained fortwo of the four species closely matched with those reported fore�solv and C�H2OH, the other two species showed main bands at430 and 380 nm, respectively. Thus, the main species contributingto the absorption of Figs. 3 and 4 were considered to be the sol-vated electron (e�solvÞ, the C�H2OH radical, the reduced nitro radical( NO��2 -phen, kmax = 430 nm) and the final product (kmax = 380 nm).In pulse radiolysis experiments with polymers TM-NO2-P4VPy, thereduced tmphen ligand, i.e., tmphen��, was not observed and onlyðNO��2 Þ-phen radicals contributed to the transient spectra recordedafter the reduction of TM-NO2-P4VPy. The relative amounts of thecontributing species in different experimental conditions were ob-

tained by resolving augmented column-wise data matrices [27].The results of curve resolution obtained with the ALS algorithmusing non-negativity, unimodality (for the concentration profiles)and selectivity (i.e., the contribution of C�H2OH radicals for wave-lengths longer that 400 nm was neglected) constraints are shownin Figs. 5 and 6. Fig. 5 shows the normalized spectra of the fourcontributing factors. Reported absorption coefficients for e�solv andC�H2OH [38] were used to obtain their absolute concentrations. Avalue of �1700 M�1 cm�1 at 430 nm was estimated for the absorp-tion coefficient of –ðNO��2 Þ-phen moieties by assuming that �80% ofthe initially produced e�solv react with ðNO��2 Þ-phen pendants toyield the corresponding radical anion. Table 2 summarizes theMCR results. The decay of ðNO��2 Þ-phen radical was of second orderin the radical concentration, and the calculated specific rateconstant varied from (2.2–2.8) � 109 M�1 s�1 in experiments

Fig. 4. Spectra of the transients generated in pulse radiolysis experiments of N2O deaerated NO2-P4VPy solutions. Spectra were recorded at 100 ns, 0.75, 13, 65 and 650 lsdelays after the radiolytic pulse. The inset to Fig. 4 shows absorption spectrum of C�H2OH radicals before the reaction with NO2-P4VPy. See text for details.

Fig. 5. Resolved spectra (Euclidean normalization) for the e�solv, C�H2OH, NO��2 -phenand product species after MCR analysis of Figs. 3 and 4. See text for details.



performed under N2 atmosphere to (2.4–3.0) � 108 M�1 s�1 in1 inexperiments performed under N2O atmosphere.

4. Discussion

The electrochemical reduction of the nitrogroup of an aromaticcompound (Ar-NO2) in aprotic (CH3CN and/or DMF) media is usu-ally, for a large potential range, a reversible monoelectronic pro-

cess leading to the formation of a radical-anion ðAr�NO��2 Þ withconsiderable stability [40–42] according to the electrode reaction:

Ar—NO2 þ e ¢ Ar—NO��2 ð3Þ

The first reduction wave in [ReI(CO)3(NO2-phen)(CH3CN)]+, which issimilar to that of ClReI(CO)3(NO2-phen), is ca. 0.7 V more positivethan those of ClReI(CO)3(phen) and [ReI(CO)3(phen)(CH3CN)]+ (seeTable 1). Such a large shift upon nitration indicates a heavy locali-zation of the extra electron density on the NO2 group. The three

Fig. 6. Short (top frames) and long (bottom frames) resolved profiles for the e�solv (�, C�H2OH (h), NO��2 -phen (M) and product (O) species after MCR analysis of Fig. 3. See textfor details.

Table 2Concentration of e�solv; NO��2 -phen species and product maximum absorbance change, DAmax(380 nm), after MCR analysis of time resolvedspectra. The second order specific rate constant for the decay of NO��2 -phen radicals was calculated from MCR resolved profiles. The firstorder apparent rate constant for the e�solv decay was calculated from a curve fit analysis of resolved profiles.

Polymer/gas Species C/10�6 M DAmax (380 nm) kobs/109 M�1 s�1 kobs/105 s�1

NO2-P4VPy/N2 ðNO��2 Þ-phen (kmax = 430 nm) 1.9 2.7Product (kmax = 380 nm) 0.011e�solv 1.7 9.1

NO2-P4VPy/N2O ðNO��2 Þ-phen (kmax = 430 nm) 0.5 0.3Product (kmax = 380 nm) 0.004e�solv Not observed

TM-NO2-P4VPy/ N2 ðNO��2 Þ-phen (kmax = 430 nm) 2.3 2.2Product (kmax = 380 nm) 0.0073e�solv 1.5 16

TM-NO2-P4VPy/ N2O ðNO��2 Þ-phen (kmax = 430 nm) 2.0 0.24Product (kmax = 380 nm) 0.011e�solv Not observed



subsequent reduction waves at �1.087, �1.514 and �1.716 V ver-sus Ag/AgCl are localized in the phen portion of the NO2-phenligand [32]. The three quasi reversible reduction waves in [ReI

(CO)3(tmphen)(CH3CN)]+ appear at more negative potentials thanthose of ClReI(CO)3(phen) and [ReI(CO)3(phen)(CH3CN)]+. Relativeto the latter two complexes, the effect of methyl substituentsbonded to the phenanthroline ligand follow the Hammett concept.Methyl groups, which are weak electron-donating substituents,shift one electron reduction potentials in the negative direction.On the other hand, the reduced nitro group is likely to have theopposite effect since the second reduction process in [ReI(CO)3(-NO2-phen)(CH3CN)]+ (corresponding to the first reduction in thephen portion of the NO2-phen ligand) lies at less negative potentialsthan those of ClReI(CO)3(phen) and [ReI(CO)3(phen)(CH3CN)]+.Reduction waves in polymers NO2-P4VPy and TM-P4VPy, whichare in both cases irreversible processes, appear, in general, atslightly less negative potentials than those of [ReI(CO)3(NO2-phen)(CH3CN)]+ and [ReI(CO)3(tmphen)(CH3CN)]+, respectively.The positive electric field exerted by the ca. 200 positive chargespresent per polymer formula may explain an increase in the reduc-tion tendency of the –[ReI(CO)3(NO2-phen)]+ and –[ReI(CO)3

(tmphen)]+ coordinated chromophores in NO2-P4VPy and TM-P4VPy polymers relative to that of complexes [ReI(CO)3

(NO2-phen)(CH3CN)]+ and [ReI(CO)3(tmphen)(CH3CN)]+, respec-tively. The fact that the reduction processes in the polymers areirreversible may be indicating that between the cathodic and anodicscans, thermal reactions are occurring probably involving the poly-mer backbone. For instance, the electronic charge may be trans-ferred from the initially reduced azine ligand to azine neighborsthrough uncoordinated pyridine spacers.

In Fig. 1, bands observed as from �0.5 V at 410 (narrow) and535 nm (broad) stem from the product of the reduction of theNO2 group in the complex, i.e., [ReI(CO)3( NO��2 -phen)(CH3CN)].This spectrum compares well with that of nitrobenzene radical an-ion [43,44]. As the potential enters into more negative values, i.e.,in a potential region where the phen portion of the NO2-phen is re-duced, a new band at 535 nm appears as from �1.0 V indicatingthat [ReI(CO)3( NO��2 -phen)(CH3CN)] is undergoing a further reduc-tion probably to give[ReI(CO)3ðNO��2 Þ-(phen��) (CH3CN)]�, that is,the anion as the second reduced product. Further reductions atmore negative potentials produce an increase in intensity of theband at 535 nm relative to that of 410 nm, while the spectral fea-tures of the band at 535 nm do not seem to be altered by subse-quent electron injections (up to three) to (phen��). Spectralchanges upon electronic reduction of the related polymer NO2-P4VPy are similar to those observed for [ReI(CO)3(NO2-phen)(CH3CN)]+ in Fig. 1a, though less well-defined (see Fig. 1b).In NO2-P4VPy, bands observed as from �0.5 V at 410 nm stemfrom {(vpy)2vpy[ReI(CO)3( NO��2 -phen)]}n�200. However, polymerNO2-P4VPy consists of chromophores –[ReI(CO)3(NO2-phen)]+ dis-tributed at random through coordination to the pyridines of thepoly-4-vinylpyridine backbone. Locally, there should be regionswith no pyridine spacer, a single pyridine spacer and two or morespacers, etc. Moreover, since distances as short as 8 Å may beachieved between ReI centers [15] (Scheme 3), there is a disposi-tion of the metallo groups in crowded spaces of the polymerstrand. Therefore, {(vpy)2vpy[ReI(CO)3( NO��2 -phen)]}n�200 is onlyan average representation of different spatial distributions of re-duced NO��2 -phen groups in the polymer. As a result, different –[ReI(CO)3(NO��2 -phen)] chromophores in diverse environments arecontributing to the spectral features of Fig. 1b, and the outcomeis a broadening of the absorption bands. Further reductionsat more negative potentials produce {(vpy)2vpy[ReI(CO)3

( NO��2 -phen��)]�}n�200 and subsequent reductions in the phen��

ligands. A comparison of Figs. 2a and 2b shows that the spectrumof the reduced species produced after the electronic reduction of

[ReI(CO)3(tmphen)(CH3CN)]+ is significantly different from that ofthe reduced species produced after the electronic reduction ofthe related polymer TM-P4VPy. For instance, the ratio (DR/R)440 nm/(DR/R)565 nm being about 2 for reduced [ReI(CO)3

(tmphen)(CH3CN)]+ while (DR/R)440 nm/(DR/R)565 nm�1 for reducedTM-P4VPy. It is likely that the main reduced species contributingto the spectral features of Figs. 2a and 2b are (see below)[ReI(CO)3(tmphenH�)(CH3CN)]+ and {(vpy)2vpy[ReI(CO)3

(tmphen��)]} n�200, respectively.We turn now to pulse radiolysis experiments. The spectrum of

the transient recorded just after the reaction of e�solv and TM-P4VPy [37] could be associated with the –[ReI(CO)3(tmphen��)]species in the TM-P4VPy. Since a concentration ½e�solv� �2 �10�6 M is generated in the pulse radiolysis experiments, only asmall percentage (�4%) of the total number of –[ReI(CO)3

(tmphen)]+ pendants is reduced to –[ReI(CO)3(tmphen��)] by thesolvated electrons. Changes of the absorption spectrum in themillisecond time domain may be ascribed to the demise of the –[ReI(CO)3(tmphen��)] radicals, Eqs. (5) and (6) [37]. At this pointit is worth to mention that although the possibility of dissociationof the axial Re-ligand bond cannot be completely ruled out [45,46],under our experimental conditions the absorption changesrecorded in pulse radiolysis experiments are mainly associated tothe chemical events experienced by the tmphen�� ligand dueto the high absorption coefficient of the ligand-centered radicalin the analyzed wavelength range.

The demise of the radicals must occur by a slow disproportion-ation reaction that possibly demands large diffusive displacementsof polymer strands.

p e�solv þ fðvpyÞ2vpy½ReIðCOÞ3ðtmphenÞ�þgn�200

! fðvpyÞ2vpy½ReIðCOÞ3ðtmphen��Þ�gp

� fðvpyÞ2vpy½ReIðCOÞ3ðtmphenÞ�þgq ð4Þ

where p + q � 200

fðvpyÞ2vpy½ReIðCOÞ3ðtmphen��Þ�gpfðvpyÞ2vpy½ReIðCOÞ3ðtmphenÞ�þgq

þp Hþ !fðvpyÞ2vpy½ReI ðCOÞ3ðtmphenH�Þ�þgp

�fð vpyÞ2vpy½ReIðCOÞ3ð tmphenÞ�þgq

ð5ÞfðvpyÞ2vpy½ReIðCOÞ3ð tmphenH�Þ�þgpfðvpyÞ2vpy½ReIðCOÞ3ðtmphenÞ�þgq

!fðvpyÞ2vpy½ReIðCOÞ3ð tmphenH2Þ�þgp=2

�fðvpyÞ2vpy½ReðCOÞ3ðtmphenÞ�þgqþp=2

ð6Þ

The process described by Eq. (5) implies the protonation oftmphen�� in the polymeric ReI complexes. Similar protonation pro-cesses have been observed in a variety of organic reactions involv-ing radical anions [47], in the one-electron reduction of bpy andphen [48] by pulse radiolysis, in the electro-polymerization of RuII

poly-pyridinic complexes [49] and in pulse radiolysis reduction ofseveral polymeric ReI complexes [1,10,37,50]. The process repre-sented by Eq. (6) implies a disproportionation reaction of two adja-cent –[ReI (CO)3(tmphenH�)]+ pendants, Eq. (7).

2-½ReIðCOÞ3ðtmphenH�Þ�þ ! -½ReIðCOÞ3ðtmphenH2Þ�þ

þ -½ReIðCOÞ3ðtmphenÞ�þ ð7Þ

Because the radicals may be separated from each other by large dis-tances, diffusive motions of the polymer strands are required tobring them close together. The whole process occurs in the millisec-ond/second time domain in pulse radiolysis experiments where the–[ReI(CO)3(tmphen��)] radical species are produced in low concen-trations (�2 � 10�6 M). As the reduction potential of C�H2OHlies at less negative potentials than that of ligand tmphen in



chromophores –[ReI(CO)3(tmphen)]+, C�H2OH is unable to yield –[ReI(CO)3(tmphen��)] radical species in pulse radiolysis experi-ments. However, the reduction potential of C�H2OH is sufficient toreduce nitro groups. The reduction of nitro groups in NO2-P4VPyby e�solv and C�H2OH is represented by Eqs. (8) and (9):

p e�solv þ fðvpyÞ2vpy½ ReIðCOÞ3ðNO2-phenÞ�þgn�200

! fðvpyÞ2vpy½ReIðCOÞ3ðNO��2 -phenÞ�gp

� fðvpyÞ2vpy½ReIðCOÞ3ðNO2- phenÞ�þgq ð8Þ

r C�H2OHþ fðvpyÞ2 vpy½ReIðCOÞ3ðNO2- phenÞ�þgn�200

! fðvpyÞ2 vpy½ReIðCOÞ3ðNO��2 - phenÞ�gr

fðvpyÞ2vpy½ReIð COÞ3ðNO2-phenÞ�þgs þ r H2COþ r Hþ ð9Þ

where r + s � 200. Reactions 8 and 9 are in competition with thenatural decay of e�solv and C�H2OH radicals [38], Eqs. (10)–(12)

e�solv þ CH3OH! Hþ CH3O� ð10ÞHþ CH3OH! C�H2OHþH2 ð11Þ2C�H2OH! ðCH2OHÞ2 ð12Þ

The analysis of the e�solv concentration profiles yielded apparent rateconstants for the e�solv decay of 9.1 � 105 s�1 and 1.6 � 106 s�1 inexperiments with NO2-P4VPy and TM-NO2-P4VPy, respectively.Taking into account the reported value of e�solv decay constant inmethanol [18] it can be stated that �80% of the e�solv generated bythe radiolytic pulse is scavenged by NO2-P4VPy and TM-NO2-P4VPy polymers. However, inspection of Table 2 shows that in pulseradiolysis experiments with NO2-P4VPy polymer the amount ofNO��2 -phen formed under N2 is substantially higher than that gener-ated under N2O, despite the fact that [C�H2OH]0 � 1.2 � 10�5 M, i.e.,about six times higher than that of e�solv. This suggests that the scav-enging efficiency of C�H2OH radicals by NO2-P4VPy is much lowerthan the scavenging efficiency of e�solv by NO2-P4VPy and that themain decay pathway of C�H2OH radicals is their recombination(reaction 12). Nevertheless, similar amounts of NO��2 -phen areformed either under N2 or N2O in pulse radiolysis experiments withTM-NO2-P4VPy. This is not surprising since TM-NO2-P4VPy poly-mer is quite soluble in methanol and pulse radiolysis experimentswere carried out with [Re] � 5 � 10�5 M whereas due to the lowsolubility of NO2-P4VPy polymer in the same solvent the total chro-mophore concentration was only [Re] � 10�5 M. Given that the con-centration of TM-NO2-P4VPy polymer is nearly five times higherthan the concentration of NO2-P4VPy polymer, the fraction of theC�H2OH radicals scavenged by TM-NO2-P4VPy is substantially high-er than the one associated with experiments performed in the pres-ence of NO2-P4VPy polymer. At this point it is important to notethat, although the fraction of C�H2OH radicals that yields NO��2 -phen radicals is dependent on polymer concentration (and substan-tially higher in the presence of TM-NO2-P4VPy than in the presenceof NO2-P4VPy), the main decay pathway of C�H2OH radicals is stilltheir recombination.

The fact that the tmphen�� reduced radicals are not contributingto the spectral features of Fig. 3 implies that although the process rep-resented by Eq. (4) should also be occurring in TM-NO2-P4VP poly-mers, tmphen�� might be efficiently transferring electronic chargeto NO2-phen neighbors in a process that is exoergonic, i.e., Eq. (13):

—½ReIðCOÞ3ðtmphenÞ�� þ—½ReðCOÞ3ðNO2-phenÞ�þ

! -½ReIðCOÞ3ðtmphenÞ�þ þ -½ReIðCOÞ3ðNO��2 -phenÞ� ð13Þ

Taking into account the electrode reduction potentials of [ReI(CO)3

(NO2-phen)(CH3CN)]+ and [ReI(CO)3(tmphen)(CH3CN)]+, DG13 ��81 kJ/mol. The electron transfer process of Eq. (13), feasible onthermodynamic grounds, should be rapid (i.e., within the 5–10 ls

time scale) and could involve charge hopping through uncomplexedpyridines pendants in the polymer.

The normal decay pathway of most nitro radical-anions atpH � 7 in water or in organic solvents in the presence of protondonors (a solvent SH as methanol, or even by reactions with H+

formed by the radiolytic pulse, Eq. (1)) occurs according to thefollowing scheme [42,51]:

Ar—NO��2 þHþ ! Ar—NO2H� ð14Þ2Ar—NO2H� ! Ar—NOþ Ar—NO2 þH2O ð15Þ

The overall decay reaction becomes second order when the proton-ation of the radical is fast and not rate determining [51,52]:

�d½Ar—NO��2 �=dt ¼ 2kobs½Ar—NO��2 �2 ð16Þ

kobs is pH dependent and decreases as pH increases [51]. Our pulseradiolysis experiments show that the decay of –[ReI(CO)3(NO��2 -phen)] pendants generated by the reaction of e�solv and C�H2OH withNO2-P4VPy and/or TM-NO2-P4VPy follow a second order kinetic inthe radical concentration which leads to the build-up of a productwith an absorption maximum at �380 nm. kobs is one order of mag-nitude higher in pulse radiolysis experiments under N2 than inpulse radiolysis experiments under N2O (Table 2) because [H+] isexpected to be lower in pulse radiolysis experiments under N2Othan in pulse radiolysis experiments under N2 due to the formationof OH� by reaction 1 after the scavenging of e�solv by N2O.

It is possible that the product detected after the second orderdecay of –[ReI(CO)3(NO��2 -phen)] pendants may consist of an azodi-oxy phenanthroline based polymeric binuclear ReI compound. Infact, most of Ar-NO compounds exist in solution as equilibriummixtures of the monomer with the dimer, Eq. (17) [53,54].

ð17Þ

Binuclear Cu(II) compounds with a bridging –N2O2– unit wereobserved after the reduction of NO2 groups by C�H2-R radicals inthe redox photochemistry of Cu(10-methyl-1,4,8,12-tetraazacycl-opentadecan-10-NO2)2+ [55]. However, the product with kmax =380 nm may be an admixture of 5-nitroso-1,10 phenanthrolineand a conglomerate of bimolecular reduction products, likeazoxy-phenanthroline and azo-phenanthroline based polymericbinuclear ReI compounds. For instance, trans-naphtylazo com-pounds have intense (e � 2 � 104 M�1 cm�1) UV–Vis absorptionswith kmax � 380–400 nm [56]. A definite characterization of thereaction product is out of the scope of this paper.

Acknowledgements

Work supported in part by ANPCyT Grant No. PICT 26195, CON-ICET-PIP 0389, Universidad Nacional de La Plata, and CICPBA.L.L.B.B. acknowledges support from CONICET. R.O.L, F.S.G.E, G.T.Rand E.W. are members of CONICET and M.R.F. is a member of CIC-PBA. We thank the Notre Dame Radiation Laboratory (NDRL) foruse of pulse radiolysis facilities. The NDRL is supported by the Of-fice of Basic Energy Sciences at the U.S. Department of Energy. Thisis contribution number NDRL – 4868.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.carbon.2009.07.026.



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on the mechanism of formation and spectral properties of radical anions generated by the reduction...

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