Polyhedron 28 (2009) 2655–2660

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Polyhedron

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Kinetics of oxidation of quinol and ascorbic acid with the phen substitutedsemiquinone ligand, (5,6-dioxolene-1,10-phenanthroline-O,O), bound to theRuII(bipy)2 moiety

Amit Mondal a, Piyali De a, Subrata Mukhopadhyay a, Rupendranath Banerjee a,*, Prasenjit Kar b,Amilan D. Jose b, Amitava Das b,*

a Department of Chemistry, Jadavpur University, Kolkata 700 032, Indiab Central Salt and Marine Chemicals Research Institute (CSIR), Bhavnagar 364 002, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 March 2009Accepted 27 May 2009Available online 17 June 2009

Keywords:KineticsRadicalsPhenanthrolinesemiquinoneAscorbic acidQuinolBipyridine

0277-5387/$ - see front matter � 2009 Published bydoi:10.1016/j.poly.2009.05.081

* Corresponding authors. Tel.: +91 33 2555 1665;Banerjee).

E-mail addresses: rupenju@yahoo.com (R. Banerjee

In 10% (v/v) CH3CN–H2O media, the parent complex [Ru(bpy)2(sqphen)]+ (I+) coexists with its conjugateacid [Ru(bpy)2(sqphenH)]2+ (HI2+): HI2+

� H+ + I+; (deprotonation constant, Koxa = 0.01; (bpy = 2,2

0-bipyr-

idine; sqphen = 5,6-dioxolene-1,10-phenanthroline-O,O). Electrochemical reduction, and also chemicalreductions with quinol and ascorbic acid, produced the corresponding catechol complex [Ru(bpy)2(cat-phen)] (IR), which also in solution coexists with its conjugate acid, [Ru(bpy)2(catphenH)]+ (HIþR ) (depro-tonation constant, Kred

a = 0.002). Progressive increase in [ascorbic acid] led to rate saturation, indicatingadduct formation (formation constant, Q = 990 ± 80 M�1). Added H+, and also redox-innocent Lewis acidZn2+, increased E1/2 but decreased the chemical reduction rates. Only an insignificant solvent isotopeeffect (kH2O=kD2O) was noted. An increased percentage of CH3CN in the solvent also retards the rate. [Ru(b-py)2(sqphen)]+ (I+) reacts at a much slower rate than [Ru(bpy)2sq]+ (sq = the unsubstituted semiquinone).Phen in the substituted ligand sqphen is known to become a much weaker base (sqphen < phen); O,O-coordination to the RuII(bpy) center further lowers the basicity of sqphen. Catphen (O,O) in IR, producedon reduction of I+, is a stronger base than the O,O-coordinated sqphen in I+.

� 2009 Published by Elsevier Ltd.

1. Introduction complexes are scant. Consequently, many important mechanistic

Transition metal complexes with organic radicals are receivingmuch current attention [1–5] because of their presence in the ac-tive sites of metalloproteins [3,4]. The best understood exampleis galactose oxidase which features a single Cu(II) ion coordinatedto a modified tyrosyl-radical [1,2]. It is only due to this intricatebonding situation that the enzyme can perform the specific two-electron oxidation of alcohols to aldehydes. Metal complexes ofdioxolenes and derivatives form another interesting group ofmetallo-radical complexes [6,7], often implicated in biology. Threerandomly selected examples are: (a) Cr complexes of catechola-mines [8], (b) V complexes with catechol-like ligands [9]; and (c)the group comprising of ubiquinone, semi-ubiquinone, menaquin-one and their metal complexes [10–12].

In recent years, a large number of dioxolene complexes havebeen synthesized and subjected to detailed physicochemical stud-ies [13–35] to understand the role of the metal centers and also tosee the effect(s), if any, of structural modifications on such ligands.However, kinetic and mechanistic studies with metallo-radical

Elsevier Ltd.

fax: +91 33 2414 6223 (R.

), amitava@csmri.org (A. Das).

issues have not been addressed [36]. This is especially true withmetal–dioxolenes.

In this backdrop, we report here the kinetics and mechanism ofthe reduction of the radical ligand sqphen in [Ru(bpy)2(sqphen)]+

(I+, sqphen is 5,6-dioxolene-1,10-phenanthroline-O,O) with ascor-bic acid and quinol to the corresponding catechol in [Ru(bpy)2(cat-phen)] (IR). We also have included some of our results on the samereaction induced electrochemically. This work enables a compari-son of the reactivity of coordinated sqphen in I+ with that of sem-iquinone (sq) in [Ru(bpy)2(sq)]+ (Fig. 1), studied earlier [37].

We believe that the present work should lead to a better under-standing of the effect(s), if any, structural modifications on the li-gand might have, and should also shed some light on thepossible role of the metal ion in modifying the physicochemicalproperties of the ligand.

2. Experimental

2.1. Materials

The hexafluorophosphate salt [Ru(bpy)2(sqphen)](PF6), (bpy =2,2

0-bipyridine) was prepared as reported [38] (yield: 55%). Anal.

[RuII(bpy)2(sqphen)]+ (I+) + e- [RuII(bpy)2(catphen)] (IR)

Kaox

[RuII(bpy)2(sqphenH)]2+ (HI2+) + e- [RuII(bpy)2(catphenH)]+ (HIR+)

Kared

H+ H+

Scheme 1. Various Ru(II)–dioxolene complexes coexisting in pH-dependent redoxequilibria.

1 For the best fit of the E1/2 versus [H+] data, it requires Koxa = 1.0 � 10�2 M and

Kreda = 2.0 � 10�3 M whence ðKox

a Þ�1 and ðKox

a Þ�1 are, respectively, 100 M�1 and

500 M�1.

N

N

N

NO

ORu

+

2(sqphen)]+ (I+) [Ru(bpy)2(sq)]+

N

N

N

NO

ORu

N

N

+

[Ru(bpy)

Fig. 1. Schematic drawing of the structures of [Ru(bpy)2(sqphen)]+ (I+) and[Ru(bpy)2(sq)]+.

2656 A. Mondal et al. / Polyhedron 28 (2009) 2655–2660

Calc. for RuC32H22N6O2PF6: C, 50.0; H, 2.86; N, 10.94. Found: C,49.7; H, 3.0; N, 10.7%. FAB MS: m/z 768 (M+, �2%), 623 (M+–PF6,�10%). IR (KBr, cm�1): 1612, 1590 (C@C, C@N), 1448 (semiquinonestretching), 837 (PF�6 ). L-Ascorbic acid (G.R., E. Merck) was used asreceived. Care was taken to prevent oxidation and photodegrada-tion of the L-Ascorbic acid by storing it in the dark when not inuse. 1,4-Dihydroxybenzene (quinol, A.R., B.D.H) was crystallizedfrom acetonitrile (25 g in 30 mL) under nitrogen, dried under vac-uum, and stored at 0 �C [39]. Acetonitrile (G.R., E. Merck), used as aco-solvent, was purified by shaking with dilute aqueous sodiumhydroxide, followed by washing successively with water, diluteacid and water. Water was removed partially by drying with so-dium sulfate. This partially dried solvent was stirred with a smallamount of P2O5. The solvent was decanted from the solid and frac-tionally distilled at atmospheric pressure (b.p. 81–82 �C). Aboutone fifth of the acetonitrile was left in the distilling flask at theend of the distillation. The whole operation was carried out in anefficient fume cupboard.

2.2. Stoichiometry measurements

Spectrophotometric titration of the RuII-complex (0.10 mM)was conducted in 0.05 M HClO4 for ascorbic acid and at pH 5.2for quinol in 10% (v/v) CH3CN–H2O mixture; the ratio, TC:TR = wasvaried in the range 5:1–1:5 [TC and TR symbolize the total analyt-ical concentration of the RuII-complex and the reducing agent, Rrespectively; R = H2A for ascorbic acid and R = H2Q for quinol].

2.3. Physical measurements and kinetics

Microanalyses (C, H, N) were performed using a Perkin–Elmer4100 elemental analyzer. Electrochemical experiments were per-formed on a CH-660A (USA) electrochemical instrument using aconventional three-electrode cell assembly comprising saturatedAg/AgCl as the reference electrode and platinum as the workingelectrode. Ferrocene (Fc) was added at the end of each experimentas an internal standard. For all measurements, the Fc/Fc+-coupleappeared at 0.41 V (versus Ag/AgCl).

All solutions were prepared in doubly distilled, freshly boiledwater. Kinetics were determined at 25.0 �C and at ionic strengthI = 0.10 M, maintained with KCl in 10% (v/v) CH3CN–H2O media.The salt, I[PF6] is sparingly soluble in water but dissolves in CH3CN.Its initial dissolution in a few drops of CH3CN followed by the addi-tion of the necessary volume of H2O provides an easy way to dis-solve I[PF6] in 10% (v/v) CH3CN–H2O media, as used in thekinetic studies.

Reactions with either ascorbic acid or quinol bleach the NIRabsorption band of I+. Kinetics were monitored in situ in a thermo-statted cell housing (CPS-240A) of a Shimadzu UV1601PC spectro-photometer, following the decrease in the absorbance for the peak(978 nm) of this band under first-order conditions (TR� TC).

Reaction solutions were deaerated with argon gas prior to kineticmeasurements.

3. Results and discussion

3.1. Protic equilibria

In 10% (v/v) CH3CN–H2O media and at pH 5.23, I+ exhibits abroad shoulder at �610 nm (see Supplementary data, Fig. S1), atpH 5.23. A decrease in the media pH reversibly red shifts the shoul-der to a lower energy and the changes were found to be reversible.The changes plausibly reflect the reversible protonation of thecoordinated sqphen. The shoulder in question arises from an in-tra-ligand p ? p* charge transfer from bpy to sqphen [40]. Proton-ation stabilizes (lowers) the p*-based LUMO of sqphen. Naturally,the shoulder arising out of a p ? p* charge transfer shifts to lowerenergy (see Supplementary data, Fig. S1). Acidification thus alsoeases electron addition to sqphen and turns E1/2 for the sqphen/catphen couple in I+ more positive [38]. Addition of a redox-inno-cent Lewis acid, also shifts E1/2 to a more positive potential. Forexample, E1/2 increased from �0.285 V to �0.257 V (versus Fc+/Fccouple) at pH 7 in the presence of 0.0032 M Zn(ClO4)2 (see Supple-mentary data, Fig. S2).

We employ the square scheme shown above to simulate thepH-dependent redox behavior (see Supplementary data, Fig. S2)of the complex (Scheme 1).

The scheme leads [40] to Eq. (1) (see Supplementary data)

E1=2 ¼ E0 � 0:059 logfðKoxa þ ½H

þ�Þ=ðKreda þ ½H

þ�Þg ð1Þ

where Koxa and Kred

a are the acid dissociation constants for HI2+ andHIþR respectively.

The best-fit (Fig. 2, r = 0.98) of the E1/2 versus [H+] data (see Sup-plementary data, Fig. S2) to Eq. (1) yielded Kox

a = 0.01, Kreda = 0.002

and a slope (=0.055) close to the theoretically expected value(0.059) for a one-electron transfer process. The sequence of depro-tonation constants, Kox

a > Kreda , is expected because the reduced li-

gand (catphen in IR) bears a higher electron density and shoulddeprotonate less readily. When compared with some analogousspecies, the order of protonation constants turns out to be: phen(9 � 104) [41] > sqphen (1.9 � 102) [42]; catphen in IR (500) >sqphen in I+ (100).1 Three aspects of this sequence seem important:(a) substitution of phen with sq makes phen a weaker base, (b) coor-dinated (O,O) sqphen is a weaker base than free sqphen, and (c)coordinated (O,O) sqphen is a weaker base than coordinated (O,O)catphen. These results indicate good electronic communicationamong the parts RuII, phen and sq of I+.

3.2. The reaction products

Addition of excess ascorbic acid (H2A) or quinol (hydroquinoneor 1,4-dihydroxybenzene, H2Q) selectively bleaches the NIR bandin I+. Selective bleaching of this MLCT band (dRu(II) ? p�sqphen) [38]strongly indicates selective reduction of the sqphen radical.

0 500 1000 1500 2000 25000.00

0.05

0.10

0.15

Model: ExpDec1 (Chi^2=5.1606E-6)(R^2 = 0.99805)y0 =0±0A1=0.17859±0.00082t1=724.8247±6.55085

Abso

rban

ce/c

m

Time(s)

Fig. 3. A typical fit (solid curve) of absorbance vs. time data (black squares) to Eq.(3) for the ascorbate reaction in 10% (v/v) CH3CN–H2O media. TC = 0.10 mM,TH2A = 4.0 mM, I = 0.1 M, T = 25.0 �C.

Table 1Some representative first-order rate constantsa (ko) for the oxidation of quinol by I+.

pH 103 TH2Q (M) 104k0 (s�1)

5.80 100 10.36.46 100 12.26.76 100 14.37.06 100 23.57.22 100 41.55.80 150 11.05.80 180 13.95.80 250 18.25.80 300 23.05.40 100 12.6b1

5.40 100 13.8b2

a 10% (v/v) CH3CN–H2O mixed media, ionic strength I = 0.10 M (KCl), T = 25.0 �C,[I+] = 0.10 mM.

b1 In 50% D2O.b2 In 89% D2O.

0 .0 0 .2 0 .4 0 .6 0 .80.05

0.06

0.07

0.08

0.09

E1/

2, V

ver

sus

Ag-

AgC

l

log{(Ka

ox + [H+])/(Ka

red + [H+])}

Fig. 2. E1/2 vs. [H+] data (black points) fitted to Eq. (1). The solid line presents thebest-fit.

A. Mondal et al. / Polyhedron 28 (2009) 2655–2660 2657

Bleaching of this MLCT band leads to reduction of [Ru(bpy)2-(sqphen)]+ (I+) to [Ru(bpy)2(catphen)] (IR) and [Ru(bpy)2-(sqphenH)]2+ (HI2+) to [Ru(bpy)2(catphenH)]+ (HIþR ). This iswell-established in the literature [43–46].

Again, spectrophotometric titration curves (absorbance versusTR) for both ascorbic acid and quinol yielded a break point atTC:TR = 2:1, indicating ascorbic acid and quinol are quantitativelyoxidized respectively to dehydroascorbic acid and 1,4-benzoqui-none (TR and TC being the analytical concentrations of the reducingagents and I+ respectively). One-electron reduction of I+ producesIR; but ascorbic acid is a two-electron donor. Therefore, the imme-diate product of the first act of electron transfer should be the rad-ical species H2A�+ or A��. We anticipate that in the next step theascorbate radical rapidly reduces another molecule of I+, leadingto the final products – dehydroascorbic acid along with anothermolecule of IR. We studied the reactions of ascorbic acid in therange [H+] = 0.01 � 0.10 M, where HIþR (pKred

a = 2.7) is nearly exclu-sive in the final products (Eq. (2A)). In contrast, the products of thereactions with quinol in the pH range 5.40–7.22 (IR formed exclu-sively), are two moles of IR and one mole quinone (Eq. (2B)).

2Iþ þH2A ! 2HIþR þ A ð2AÞ2Iþ þH2Q ! 2IR þ 2Hþ þ Q ð2BÞ

3.3. Kinetics

All the reactions, under the experimental conditions, obeyedfirst-order kinetics (Eq. (3)), where the factor 2 at the right handside appears due to the stoichiometry of the reactions

At ¼ A1 þ ðA0 � A1Þ expð�2k0tÞ ð3Þ

The absorbance (At)–time (t) data for more than four half-lives ineach set of kinetics yielded an excellent fit to Eq. (3) (Fig. 3), whencethe first-order rate constants (ko) were evaluated. Tables 1 and 2display some representative ko values for the quinol and ascorbicacid reactions, extracted through such a fitting. Added H+, and alsoZn2+, increases E1/2 for the one-electron, reversible sqphen/catphencouple in I+ but retards its reduction with ascorbic acid and quinol(Tables 1 and 2). This observation is strong evidence against a pureouter-sphere reduction.

3.4. Effect of D2O on rate

Substitution of up to 89 mol% of D2O for the solvent H2O hadonly modest and opposite effects on the reaction rates for quinol(kH2O=kD2O = 1/1.34) and ascorbic acid (kH2O=kD2O = 1.38) [47]. Suchsmall kinetic isotope effects are normally expected for simple elec-tron transfer reactions [48]. Much more dramatic effects are usu-ally observed when the electron transfer is coupled with themovement of a proton in equilibrium with solvent protons[36,49–51].

3.5. Effect of pH on rate

3.5.1. Reactions with quinol (H2Q)The observed first-order rate constants, ko, for quinol oxidation

were determined in the brief pH range 5.40–7.22; these reactionsare is too slow below pH 5, but are too fast above pH 7.5. In theexperimental pH range 5.40–7.22, I+ exclusively dominates overHI2+ (pKred

a = 2.7), yet, ko exhibits an inverse proton dependence(Fig. 4). Evidently, the inverse proton dependence arises from therapid H2Q/HQ� equilibrium (Eq. (4); pKa = 9.96) [52,53] and the ex-pected greater kinetic activity of the conjugate base, HQ� over theparent acid

0.00 0.02 0.04 0.06 0.08 0.100.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

3.0x10-6

k 0(Kaox

+ [H

+ ])[H

+ ], M

2 s-1

[H+], M

Fig. 5. Graphical verification of Eq. (14) and that the k1 and k4 paths contributenegligibly (TH2A = 4.0 mM).

Table 2Some representative first-order rate constantsa (k0) for the oxidation of ascorbic acidby I+.

pH 103 TH2A (M) 104k0 (s�1)

2.00 1.0 4.02.00 2.0 4.62.00 3.0 6.32.00 4.0 6.72.01 4.0 5.5b1

2.01 4.0 4.8b2

1.70 4.0 6.91.52 4.0 6.81.40 4.0 5.81.30 4.0 5.71.22 4.0 4.71.15 4.0 4.01.00 4.0 2.41.52 4.0 5.3c1

1.52 4.0 4.9c2

1.52 4.0 4.1c3

1.52 4.0 2.8d1

1.52 4.0 2.3d2

1.52 4.0 1.8d3

c(1–3)In the presence of [Zn2+] = 0.005, 0.01 and 0.20 M, respectively.d(1–3)In presence of 20%, 30% and 40% (v/v) CH3CN–H2O, respectively.

a Unless stated otherwise, the reactions are carried out in 10% (v/v) CH3CN–H2Omixed media, ionic strength I = 0.10 M (KCl), T = 25.0 �C, [I+] = 0.10 mM.

b1 In 50% D2O.b2 In 89% D2O, in b1 and b2, pH meter readings were 1.61. Reported pH is

pD = pH + 0.4.

0.0 4.0 8.0 12.0 16.0

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

103 k 0, s

-1

107[H+], M

Fig. 4. Experimental ko (solid circles) for the quinol redox fitted (smooth curve) toEq. (7). TC = 0.10 mM, TH2Q = 0.10 M, I = 0.1 M, T = 25.0 �C, 10% (v/v) CH3CN–H2Omedia.

2658 A. Mondal et al. / Polyhedron 28 (2009) 2655–2660

H2Q � Hþ þHQ�; pKa ¼ 9:96 ð4ÞIþ þH2Q ! Products; k1 ð5ÞIþ þHQ� ! Products; k2 ð6Þ

Eq. (7) was derived from on Eqs. (4)–(6).

K0 ¼ ðk1½Hþ� þ k2KaÞTH2Q=ðKa þ ½Hþ�Þ ð7Þ

The observed first-order rate constants, ko yielded an agreeable fit(Fig. 4) to Eq. (7). TH2Q represents the total concentration of quinol.The best-fit of ko to Eq. (7) yielded k1 = (6 ± 2) � 10�3 M�1 s�1 andk2 = (17 ± 5) M�1 s�1, which shows that kinetically HQ� is muchmore reactive than H2Q.

3.5.2. Reaction with ascorbic acid (H2A)The kinetics of the reaction with ascorbic acid is measurable by

the conventional technique only in acidic solutions where each ofthe four species, HI2+, I+, H2A and HA� might be kinetically signif-icant. The following reaction scheme, Eqs. (8)–(13), is thereforedrawn:

HI2þ� Hþ þ Iþ; Kox

a ð8ÞH2A� Hþ þHA�; K 0a ð9ÞHI2þ þH2A ! P; k1 ð10ÞHI2þ þHA� ! P; k2 ð11ÞIþ þH2A ! P; k3 ð12ÞIþ þ A� ! P; k4 ð13Þ

Under the experimental conditions, {K 0a � [H+] (0.01–0.10 M)}, thescheme leads to Eq. (14) where TH2A defines the total concentrationof ascorbic acid.

koðKoxa þ ½H

þ�Þ ½Hþ� ¼ fk1½Hþ�2 þ ðk2K 0a þ k3Koxa Þ½H

þ�þ k4K 0aKox

a gTH2A ð14Þ

A plot of the left hand side of Eq. (14) against [H+] (Fig. 5)yielded a fair straight line (r > 0.97) with finite slope =(2.95 ± 0.3) � 10�5 M�1 s�1 ½¼ k2K 0a þ k3Kox

a Þ TH2A�, but statisticallyinsignificant intercept. The acid variation experiments thereforedemonstrate the insignificance of the k1 and k4 paths and that onlyk3 and/or k2 path(s) is (are) contributing to the overall rate. Due tothe proton ambiguity, individual contributions from the k2 and k3

paths cannot be evaluated. Nevertheless, not the conjugate acidform of an oxidant, but the conjugate base of the reductant is gen-erally the better oxidant [54–57] and in many examples, HA� hasmuch greater kinetic activity over its parent acid H2A. Therefore,the k2 path (HI2+ + HA�) should contribute more than the k3 path(I+ + H2A). Moreover, as we see that the k1 path (HI2+ + H2A) hasno contribution, it is expected that the k3 path (I+ + H2A) alsowould not be operative as I+ is expected to be less oxidizing thanHI2+. Using the literature data for pK 0a = 4.03 [58], and assumingno significant contribution from k3, we estimated a value of79 M�1 s�1 as the upper limit for k2.

A. Mondal et al. / Polyhedron 28 (2009) 2655–2660 2659

3.6. Effect of TH2A and TH2Q on ko

A progressive increase in TH2A tends to saturate ko and the kinet-ics is not so simple as observed for quinol where no indication ofrate saturation was observed, even at the highest [quinol] studied(Table 2). A double reciprocal plot between 1/ko and 1/TH2A wasfound to be fairly linear (r = 0.978), indicating a rapid adduct for-mation equilibrium with the formation constant Q = 990 ± 80 M�1

prior to the rate step (Eq. (15)).

HI2þ þ Ascorbic acid �Q

Intermediate adduct !k products ð15Þ

This Q value appears to be too large for a simple outer-sphereinteraction [59] and suggests some kind of specific bonding, notmere electrostriction. Hydrogen-bonding with the two adjacentoxygen atoms from ascorbic acid (see Supplementary dataFig. S3) seems a distinct possibly [60–62].

The ascorbate anion is also notable for its unusually high rate ofcomplex formation [63]. The high formation constant (Q) observedhere may also be a consequence of a high formation rate, but rela-tively small dissociation rate for the adduct formed. Rate satura-tion was absent in the reactions with quinol; ko increasedlinearly up to the maximum TH2Q (0.30 M) studied.

3.7. Effect of solvent composition

kmax for the dRu(II) ? p�sqphen (SOMO) band (at the NIR) [38,64] ofI+ blue- shifts from 978 nm to 930 nm on changing the solventfrom 10% (v/v) CH3CN–H2O to neat acetonitrile. Presumably, thereduced solvent polarity (increased proportion of less polar CH3CNin CH3CN–H2O mixture) destabilizes the singly occupied MO of thesemiquinone and decreases the electron acceptance by this moiety.In agreement with this presumption, ko for ascorbic acid decreasedwith an increased percentage of CH3CN (20–40%; v/v) in the mixedCH3CN–H2O solvent (Table 2, d1–3). However, decreased solventpolarity is also expected [65] to decrease K 0a and hence [HA�](Eq. (9)). Such a decrease in the concentration of the more activeform of the reductant may singly or additionally be a cause forthe decreased ko.

4. Conclusions

The protonated radical ligand, sqphenH+ in [Ru(bipy)2(sq-phen)]+ (HI2+) (pKox

a = 2.0) is a stronger acid than the one-electronreduced ligand catphenH+ in [Ru(bipy)2(catphenH)]+ (HIþR )(pKred

a = 2.7). E1/2 for the sqphen/catphen reduction couple in-creases on addition of H+, and also Zn2+, but either, when added,slow down the rate of the chemical reduction with ascorbic acid.A simple outer-sphere reaction is discarded as a possibility. Reduc-tions with ascorbic acid redox indeed demonstrate an intermediateadduct formation. The solvent isotope effect is negligible and theelectron transfer seems to proceed without proton movement atthe rate-determining step. Sqphen in [Ru(bipy)2(sqphen)]+ is re-duced at a rate much slower compared to the reduction of unsub-stituted sq (semiquinone) in [Ru(bipy)2(sq)]+. This is plausiblybecause substitution with phen stabilizes sq through electrondrainage from phen to sq via extended conjugation, and rendersreduction difficult. Again, this same extended conjugation rendersphen of sqphen less basic than phen itself. The sequence of basicityis: phen (9 � 104) [41] > sqphen (1.9 � 102) [42]; catphen in IR

(500)1 > sqphen in I+ (100)1 and shows that: (a) substitution ofphen with sq makes phen a weaker base, (b) O,O-coordinatedsqphen is a weaker base than free sqphen, and (c) coordinated(O,O) sqphen is a weaker base than coordinated (O,O) catphen.These results indicate good electronic communication among theparts RuII, phen and sq of I+.

Acknowledgments

Financial assistances received from the Council for Scientificand Industrial Research (New Delhi, Grant No. 01(2085)/06/EMR-II), award of a JRF to A. Mondal by the same agency and financialassistances received from the Department of Science and Technol-ogy (New Delhi, Grant No. SR/S1/IC-40/2007) are gratefullyacknowledged.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.poly.2009.05.081.

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