Redox Reactions of Binuclear Complexes of ... - ACS Publications

14 Redox Reactions of Binuclear

Downloaded by UNIV OF PITTSBURGH on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch014

Complexes of Ruthenium and Iron Kinetic vs. Thermodynamic Control and Activation Effects Albert Haim Department of Chemistry, State University of New York, Stony Brook, NY 11794-3400

Kinetic measurements of peroxydisulfate oxidations of several ligand -bridged binuclear complexes of ruthenium and iron are reviewed. The kinetically controlled products of the one-electron oxidations of (NC) Fe pzRu (NH ) (pz = pyrazine) and (NC) Fe bpaRu (NH ) [bpa = μ-1,2-bis(4-pyridyl)ethane] are also the thermodynamically stable elec -tronic isomers, (NC) Fe pzRu (NH ) and (NC) Fe bpaRu (NH ) . In contrast, the primary product of oxidation of (EDTA)Ru pzRu (NH ) is the unstable electronic isomer (EDTA)Ru pzRu (NH ) , which subse -quently undergoes rapid isomerization to the stable product (EDTA)Ru pzRu (NH ) . Peroxydisulfate oxidation of the equilibrium mixture of the mixed-valence isomers (EDTA)Ru pzRu (NH ) and (EDTA)Ru pzRu (NH ) proceeds via the former, stable isomer. In the analo -gous reaction for the equilibrium mixture of electronic isomers (NC) Fe bpaRu (NH ) and (NC) Fe bpaRu (NH ) , the latter, unstable isomer is the reactive species. The reactions of (EDTA)Ru pzRu (NH ) and (NC) Fe bpaRu (NH ) represent examples of activation of the oxi -dation of a metal center by intramolecular electron transfer. 5

II

II

-

3

5

5

5

II

III

II

3 5

II

3

II

5

-

5

III

3 5

II

II

III

II

II

III

3

III

3

3 5

5

+

5

III

3

II

+

II

3

+

5

+

5

5

II

III

3 5

5

III

II

3 5

II

5

A F I E E THE

D

II

_

III

II

3 5

3 5

OFTHE „

,

MIXED-VALENCE C

_

Μ

-

pyrazinedecaammine(hmthenium(II,III) by Creutz and Taube (1, 2) (structure I) there was a publication explosion dealing with compounds of this type (3). The bulk of the published work has placed emphasis on synthetic (4), thermo dynamic (5), structural (6), and, particularly, spectroscopic aspects {7, 8). There © 1997 American Chemical Society

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

239

240

E L E C T R O N TRANSFER REACTIONS

/ = \ (NH ) RuN 3

NRu(NH )

5

3


5

^,NRu (NH )

(NC) Fe N^

n

n

5

5+

3

5

II

^NRu (NH )s

(NC) Fe N^ 5

m

n

3

III

^NRu (NH )

(NC) Fe N^ 5

n

ffl

3

5

IV

have been some studies of intramolecular electron transfer (9, 10), but the dynamic solution behavior of such compounds with external redox reagents has been neglected. We have a program of mechanistic studies in which mixed-valence com pounds are either reactants or products of reactions that involve external redox reagents. In most of our studies, the mixed-valence compounds are unsymmetrical and valence-localized (11-14), although in some of our work we also deal with symmetric complexes (15-17). We have explored several questions. Con sider first the complex ion in structure II, which contains two reducing sites, namely the ruthenium(II) and the iron(II) centers. Two related questions are pertinent. First, which of the two sites is thermodynamically the stronger oneelectron reductant? Second, which of the electronic isomers (structures III or IV) is the kinetically controlled product of the one-electron oxidation of II? In addition, we have investigated the question of the effect of one metal center on the reactivity of the other metal center. Specifically, consider the unsymmetrical binuclear complex, structure V, with the indicated localized valences. Does the presence of the ruthenium(III) center affect the rate of oxidation of the ruthenium(II) center? Similarly, does the ruthenium(III) center in structure V I affect the reactivity toward oxidation of the iron(II) center?


14.

HAIM

Redox Reactions of Ru and Fe Binuclear Complexes

(EDTA)Ru

/


(NQjftftt.

Ν Ru (NH ) D

A — CH CH — V 2

2

3

241

s

NRu (NH ) m

3

s

VI

(NC) Fe pzR« (NH ) 5

n

m

3

5

— - (NC)5Fe pzRu (NH ) m

n

3

5

rv

m

(NQsFeVRjANH^* Scheme I.

In the present chapter we review the results of our inquiries into these questions. The methodology used, the results obtained, and the proposed interpretations are summarized herein.

Relative Stabilities of Electronic Isomers Consider the equilibrium between isomers I I I and I V i n Scheme I (pz is pyrazine). First, we identify the more stable isomer by constructing models and by taking advantage of the sensitivity to substituent effects of the metal-toligand charge transfer bands characteristic of pentaammineruthenium(II) and pentacyanoferrate(II) complexes with nitrogen heterocycles (18,19). In Table I we list the maximum wavelength of the M L C T bands of a series of pyrazine complexes of ruthenium and iron. It will be seen that addition of an electropos itive substituent (a proton or a metal center in the 3+ oxidation state) results i n a shift of the M L C T band to lower energy, the effect being more substantial for the iron(II) than for the ruthenium(II) complexes. Next we model the two elec tronic isomers in Scheme I by replacing the 3+ metal center by another 3+


242

ELECTRON TRANSFER REACTO I NS

Table I. M L C T Bands of Iron (II) and Ruthenium (II) Complexes of Pyrazine and μ-l,2-Bis(4-pyridyl) ethane in Aqueous Solution Complex Fe (CN)5pz*n

Fe^CNJspzH " Ru (NH3) pz Ru^NH^zH * (NC) Fe"pzRu"(NH ) (NC) Fe pzRu (NH ) (NC) Fe pzRh (NH^ (NC) Co"ipzRu"(NH3) Fe (CN) bpa Ru»(NH3) bpa (NC) Fe bpaRu (NH3) (NC) Fe"bpaRu (NH3) (NC) Fe bpaRh (NH3) (NC) Co bpaRu (NH3) Ru"(EDTA)pz (EDTA)Ru pzRu (NH3) (EDTA)Ru pzRu (NH ) (EDTA)Ru pzRh (NH ) (EDTA)Rh pzRu (NH3) 2

n

2+

5

3

5

3

5


5

II

III

II

III

5

3

5

5

5

5

II

3

5

2+

5

n

5

II1[

5

n

5

n

5

in

in

5

5

5

I][

5

2

ni

II

II

ni

n

II

III

n

5

3

5

3

5

5

λ (nm)

logs

Ref.

452 625 474 528 522 590 576 520 365 410 368 408,368 367 403 463 520 544 533 528

3.70

19 19 2 2 11 11 11 11 13 13 13 13 13 13 9 9 9 9 9

— 4.3 4.1 4.36 ~4 4.0 4.2 3.7 3.9 3.6 3.9, 3.8 3.5

— 4.1 4.2 4.4 4.3 4.3

N O T E : M L C T is metal-to-ligand charge transfer; pz is pyrazine; bpa is μ-Ι 2Α>ΐ8(4-ργήάγ\)βΰϊάηβ; λ is absorption wavelength; ε is molar absorptivity. ί

metal center, namely, Rh(III) for Ru(III) and Co(III) for Fe(III). The Rh(III) and Co(III) centers were chosen because the corresponding 2+ oxidation states are thermodynamically precluded for the indicated coordination spheres. It w i l l be seen that ( N C ) F e p z R h ( N H ) and ( N C ) C o p z R u ( N H ) have their M L C T bands at 576 and 520 nm, respectively. The bands are shifted by 124 and 46 nm, respectively, from the corresponding mononuclear complexes. The stable electronic isomer i n Scheme I has its M L C T band at 590 nm, for a 138-nm shift with respect to the Fe(II) mononuclear complex or a 116-nm shift with respect to the Ru(II) mononuclear complex. Evidently, the stable isomer must have Fe(II) and Ru(III) localized valences. The somewhat larger shift for Ru(III) as compared to Rh(III) is consistent with the π acceptor properties of the former. A n entirely analogous argument is invoked to establish that the stable electronic isomer i n Scheme II is ( E D T A ) R u p z R u ( N H 3 ) J . In the case of the isomers i n Scheme III, since the bridging ligand transmits electronic effects to a minor extent, if at all, the M L C T bands of the constituent mononu clear units [ M L C T bands at 410 nm and 365 nm for Ru(II) and Fe(II), respec tively; column 2 of Table I] clearly indicate that ( N C ) F e b p a R u ( N H ) [bpa 5

3

II

III

3

5

5

m

n

5

nl

n

5

n

ni


3

5

14.

HAÏ M

243


Ε (EDTA)Ru pzRu (NH ) ra

n

3

5

0

•

+

(EDTA)Ru pzRu (NH ) n

(EDTA)Ru pzRu (NH ) 3

m

3

5

+

5


Scheme II.

(NC)5Fe bpaRu (NH ) n

m

3

o

E

^(NC) Fe%aRu (NH )

5

n

5

(NC) Fe bpaRu (NH )5 III

ni

5

Ο

Ε

5

3

+

3

Scheme III.

= μ-1,2Λ)ί8(4-ργήάγϊ)βίΥί2ίηβ, M L C T band at 368 nm] is the stable electronic isomer. This conclusion is strongly reinforced by comparing the M L C T bands of the models ( N C ) F e % p a R h K N H ) ( ) (NC) Co bpaRu (NH ) (403nm). Next, we estimate the difference in stability between the isomers by con structing simple thermodynamic cycles. Consider first Scheme I. E °, measured by cyclic voltammetry (11), has a value of-0.72 V E ° is estimated at 0.64 V from the measured (II) reduction potential ( R u to Ru ) of ( N C ) C o p z R u ( N H ) J . Therefore, E ° is -0.08 V Confidence in the adequacy of the esti mate for Ε ° comes from a comparison of the measured (II) reduction poten tials, 0.72 V and 0.71 V for the couples ( N C ) F e p z R u ( N H ) + and (NC) Fe pzRh (NH3)J . excellent agreement indicates that the reduc tion potential of the electroactive center is rather insensitive to the identity of the metal center (provided it is not a π acceptor) bound at the remote nitrogen. Estimates of the relative stabilities of the electronic isomers in Schemes II and III have also been carried out. Values of E °and E ° axe -0.57 V (mea sured) and 0.37 V (estimated from the measured value for the model ( E D T A ) R u / p z R h ( N H ) | / ) . Therefore, E ° = -0.20 V. For Scheme III, E °is measured (13) as -0.45 V In the present case, E °could not be estimated from model studies because the very low solubility of ( N C ) C o b p a n

5

3

3

5

3 6 7

n m

a

n

d

m

5

n

5

1

2

m

in

11

m

5

Y

3

2

5

5

III/II

/0

III

T

h

III/II

3

III

5

II

III

3

+

III

3

/0

e

+

4

n

6

5


m

244

E L E C T R O N TRANSFER REACTIONS

R u ( N H ) (13) precludes measurements in solution. However, in the case of bpa as a bridging ligand, coupling of the two metal centers is negligible and thus the reduction potentials of ( N C ) F e b p a R u ( N H ) , 0.290 V, or of R u ( N H ) b p a , 0.293 V (13 ), represent good estimates for E °. Support for the approximation comes from a comparison of the reduction potentials ( F e to Fe ) for ( N C ) F e b p a R u ( N H ) , 0.45 V, ( N C ) F e b p a R h ( N H ) , 0.44 V, and Fe (CN) bpa ", 0.44 V (13 ). Thus, we estimate E ° as -0.16 V. In every case, the stable electronic isomer is the one predicted on the basis of the reduction potentials of the pertinent mononuclear complexes. This observation is not surprising for the binuclear complexes bridged by bpa, a ligand that does not couple the metal centers. For such bridging ligands the reduction potentials of mononuclear and binuclear complexes are virtually identical, and therefore the trends for mononuclear and binuclear complexes must be the same. However, for cases in which the bridging ligand (pyrazine) provides strong coupling between the metal centers, the reduction potentials of binuclear and mononuclear complexes differ considerably. Nevertheless, the relative stabilities of electronic isomers in the binuclear systems follow the order predicted from the potentials of the mononuclear complexes. This result occurs because the effects of the two metal centers on the reduction potentials of each other are rather similar. n

3

5

n

5

III

3

ni

3

5

3+

5

6

m

11

5

III


in

ni

3

5

+

in

5

2

5

m

3

5

+

m

One-Electron Oxidation of Binuclear Complexes with Both Metals in the 2+ Oxidation State: Kinetic or Thermodynamic Control? The oxidation of II by peroxydisulfate proceeds according to the stoichiometry given in eq 1 and obeys monophasic, second-order kinetics with a rate constant of 3.8 χ 10 M - s- at 25 (11). 3

1

1

0

2(NC) Fe pzRu (NH )5 +S Ol" = 2 ( N C ) F e V R u ( N H ) n

5

n

3

2

5

m

3

5

+2SO|" To be sure, because III is the stable electronic isomer, it is the thermodynamic product. The question arises whether it is also the kinetically controlled prod uct or, alternatively, isomer IV is first produced and then rapidly undergoes isomerization to the stable isomer. We attempt to answer this question by mea suring and comparing the rate constants for oxidation of iron(II) and ruthe nium^ I) complexes in systems for which there is no ambiguity as to the site of oxidation. Results are collected in Table II. It will be seen that rate constants for the oxidation of pentacyanoferrate(II)-pyrazine complexes fall in the vicin ity of ~2 M s whereas for pentaammineruthenium(II)-pyrazine complexes rate constants are of the order of - Ί 0 M s . - 1

-1

3

- 1

-1


14.

H AIM

245


Table II. Rate Constants for Oxidation of Fe(II) and Ru(II) Complexes by Peroxydisulfate at 25 °C and Ionic Strength 0.10 M Complex

E°(V)

Fe (CN) pz (NC) Fe pzRh (NH ) Ru (NH ) pz (NC) Co pzRu (NH3) (NC) Fe"pzRu"(NH ) Fe (CN) bpa Ru (NH3) bpa (NC) Fe bpaRh (NH ) (NC) Fe bpaRu (NH ) (NC) Fe bpaRu (NH ) (NC) Fe bpaRu (NH ) Ru (EDTA)pz (EDTA)Ru pzRu (EDTA) (EDTA)Ru pzRu (EDTA) (EDTA)Ru pzRu (NH ) (EDTA)Ru pzRu (NH ) (EDTA)Ru pzRu (NH ) (EDTA)Ru pzRu (NH ) II

II

n

m

3

5

3

n

3

5


5

5

5

3.8X10

5

III

n

n

II

III

III

3

II

n

5

3

5

II

II

III

in

II

n

n

3

5

3

5

5.5x1ο

+

2

3

5

1.4xl0 2.5xl0

3

5

2.5x1ο *

III

II

26

3

n

II

aixio

2

+

6

7

d

5fe

10

9

7.9X10-

10 8

9

3

7

1

4

6

1.4x10-* 2.5x10-5

Redox Reactions of Binuclear Complexes of ... - ACS Publications

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