outer-sphere redox reactions of (n)5-macrocyclic cobalt(iii) complexes. a temperature and pressure...

Inorganica Chimica Acta 256 (1997) 51–59

0020-1693/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved

PII S0020-1693(96)05415 -1

Journal: ICA (Inorganica Chimica Acta) Article: 5415

Outer-sphere redox reactions of (N)

5

-macrocyclic cobalt(III) complexes.

A temperature and pressure dependence kinetic study on the influence of

size and geometry of different macrocycles

Manuel Martinez

a

, Mari-Angel Pitarque

a

, Rudi van Eldik

b

a Departament de Quımica Inorganica, Facultat de Quımica, Universitat de Barcelona, Diagonal 647, E-08028 Barcelona, Spainb Institut fur Anorganische Chemie, Universitat Erlangen-Nurnberg, Egerlandstraße 1, D-91058 Erlangen, Germany

Received 23 February 1996; revised 10 June 1996

Abstract

Outer-sphere redox reactions between [Co(N)

5

H

2

O]

3q/[Co(N)

5

OH]

2q((N)

5

stetraazacycloamine ligand) and [Fe(CN)

6

]

4yhave

been studied as a function of (N)

5

, temperature and pressure. The effect of the size of the (N)

5

skeleton has been investigated to establish

possible correlations between, on the one hand, the size, geometry and charge of the cobalt(III) complex and, on the other hand, the outer-

sphere formation constant, the electron-transfer rate constant, and the thermal and baric activation parameters. The values obtained indicate

that the outer-sphere formation constants are, within experimental error, the same for all the systems studied. The electron-transfer rate

constants for the [Co(N)

5

H

2

O]

3qcomplexes increase on increasing the size of the macrocyclic ligand independently of its cis or trans

geometry (from 2.3=10

y4

s

y1

(13-membered macrocycle) to 3.4=10

y1

s

y1

(16-membered macrocycle) at Ps1 atm, 258C, Is1.0 M).

For the [Co(N)

5

OH]

2qcomplexes these differences are significantly smaller (from 1.1=10

y3

to 20=10

y3

s

y1

under the same conditions).

The values for the first-order rate constants for the hydroxo complexes are one or two orders of magnitude smaller, as found for simpler

pentaamine systems; only for the 13-membered macrocyclic [Co(N)

5

OH]

2qcomplex is the trend inverted. The thermal and pressure

activation parameters are those expected for these types of reactions and are interpreted in view of a combined effect of electrostatic and

hydrogen bonding interactions. In this respect, the highly symmetrical trans 14-membered (N)

5

macrocyclic systems show an important

increase of 10–20 cm

3

mol

y1

in the value of DV/when compared with the equivalent cis systems.

Keywords: Kinetics and mechanism; Cobalt complexes; Macrocyclic ligand complexes; Redox reactions

Scheme 1.

1. Introduction

Although simple outer-sphere redox reactions of type (1)

have been studied on several occasions as a function of

temperature and pressure [1], only a few attempts to look

III II[Co (NH ) X]q[Fe (CN) ]3 5 6

III II™ [Fe (CN) ]qCo q5NH qX (1)

6 3

charges omitted for clarity

into the effect of the size of the pentaamine skeleton as a

whole have been carried out [2]. Whereas in one study only

one of the five ammine ligands was changed for a bulkier

amine [2a], in the others the size of the complete pentaam-

mine skeleton [2b,c] was changed, as indicated in reaction

(2).

nq 4y[Co(N) (Y)] q[Fe(CN) ]

5 6

3y 2q™[Fe(CN) ] qCo q5NqY (2)

6

yNsCH NH , CH CH NH , YsH O, OH

3 2 3 2 2 2

As a continuation of our interest in the effect of steric and

electronic factors that can influence or tune the reactivity

of transition metal complexes [2–4], we have studied the

effect of replacing the {Co(RNH

2

)

5

} skeleton in

[Co(RNH

2

)

5

H

2

O]

3qby a macrocyclic system {Co(N)

5

},

where (N)

5

represents any of the ligands shown in Scheme 1.

M. Martinez et al. / Inorganica Chimica Acta 256 (1997) 51–5952


The effect of a systematic variation in the macrocycle size

and geometry (Scheme 1) has already been studied for base-

hydrolysis reactions of these types of complexes [5]. In this

paper we report the effects of such variations on the outer-

sphere redox reactions depicted in Eqs. (1) and (2). In addi-

tion, we have also varied the sixth ligand in the coordination

sphere of the cobalt(III) complex from H

2

O to OH

yby

repeating the measurements at various pH values.

This rather simple reaction was selected in order to be able

to separate the encounter complex formation constant from

the electron-transfer rate constant in terms of the mechanism

outlined in (3).

3q 4y[Co(N) H O] q[Fe(CN) ]5 2 6

Kos

3q 4y°{[Co(N) H O] ; [Fe(CN) ] }

5 2 6

3q 4y{[Co(N) H O] ; [Fe(CN) ] } (3)

5 2 6

k2q 3y™{[Co(N) H O] ; [Fe(CN) ] }

5 2 6

fast

2q 3y{[Co(N) H O] ; [Fe(CN) ] }™products

5 2 6

The nature of the final redox products depends on the size

of the different (N)

5

macrocycles. For the L

14

, L

15

and L

16

macrocycles, the final product corresponds to a {Co

III

LP

Fe

II

(CN)

6

} species that is formed via an inner-sphere oxi-

dation of the short-lived {Co

II

L} species initially produced.

For the L

13

macrocycle, the {Co

II

L} species initially formed

decomposes too fast, and no further reaction takes place. In

this case Co

2qaq

is produced, as detected by the Co/EDTA/

Fe(CN)

6

complex formation [6].

The rate law derived from this mechanism is given in (4):

4ykK [Fe(CN) ]OS 6k s (4)

obs

4y{1qK [Fe(CN) ]}

OS 6

The high charge on the complexes involved allows the

kinetic separation of the encounter complex formation con-

stant, KOS

, and the electron-transfer rate constant, k. Thus,the analysis of the [Fe(CN) ] dependence of k

obs

under

4y6

pseudo-first-order conditions as a function of temperatureand

pressure, enables us to use the obtained thermodynamic and

kinetic parameters as a source of information on the effect of

steric hindrance on outer-sphere electron transfer reactions

of significantly more complex systems.

2. Experimental

2.1. Materials

All materials were reagent grade chemicals. Na

4

[Fe-

(CN)

6

] was recrystallised twice; all other chemicals were

used without further purification.

2.2. Preparation of compounds 1

2.2.1. Cis-[Co(N)5Cl]Cl(ClO4) ((N)5sL13),cis-[Co(N)5Cl](ClO4)2 ((N)5sL14, L15),trans-[Co(N)5Cl](ClO4)2 ((N)5sL14) andtrans-[Co(N)5Cl]Cl(ClO4) ((N)5sL16)The products were prepared as described previously [5b].

Characterisation of the different compounds was done by

UV–Vis spectra (l (nm) (e (cmy1

M

y1

)): 519 (120), 459

(140), 257 (140) for (N)

5

sL

13

(cis); 525 (92), 470 (94),365 (128) for (N)

5

sL

14

(cis); 550 (79), 450 (26), 360

(87) for (N)

5

sL

14

(trans); 540 (125), 480sh (87), 368

(175) for (N)

5

sL

15

(cis); 560 (93), 390 (121) for

(N)

5

sL

16

(trans).

2.2.2. Cis-[Co(N)5(H2O)](ClO4)3 ((N)5sL13, L14, L15),trans-[Co(N5)(H2O)]3q ((N)5sL14) andtrans-[Co(N)5(H2O)]Cl(ClO4)2 ((N)5sL16)The complexes were prepared via base hydrolysis of the

corresponding chloro complexes. Thesewere dissolved in the

minimum amount of 0.05 M NaOH solution and, when the

UV–Vis spectra showed no further changes, concentrated

HClO

4

was added. On slow evaporation of the samples, the

corresponding aqua complexes were obtained. Occasionally

the isolated complexeswere contaminatedwithNaClO

4

; their

recrystallisation from diluted HClO

4

produced analytically

pure samples. The aqua complexes were characterised by

their elemental analyses,

13

C NMR and UV–Vis spectra

(Table 1), except in the case of trans-[Co(L14

)(H

2

O)]

3q

where no solid sample was obtained.

Anal. Calc. for [Co(L

13

)(H

2

O)](ClO

4

)

3

P1/2H2

O: C,

20.03; H, 4.71; N, 11.68. Found: C, 19.45; H, 4.67; N,

11.34%. Calc. for [Co(L

14

)(H

2

O)](ClO

4

)

3

P7H2

O: C,

18.23; H, 5.15; N, 9.66. Found: C, 18.57; H, 5.11; N, 9.40%.

Calc. for [Co(L

15

)(H

2

O)](ClO

4

)

3

P1/2H2

O: C, 22.96; H,

5.15; N, 11.16. Found: C, 23.02; H, 5.14; N, 11.03%. Calc.

for [Co(L

16

)(H

2

O)]Cl(ClO

4

)

2

: C, 27.41;H, 6.02;N, 12.29.

Found: C, 27.12; H, 5.86; N, 12.23%.

13

C{

1

H} NMR spectra (D

2

O, ppm referenced to TMS):

cis-[Co(L13

)(H

2

O)]

3q: 67.51, 62.85, 57.47, 57.28, 54.70,

52.67, 52.27(=2), 48.78, 20.27; cis-[Co(L14

)(H

2

O)]

3q:

77.8, 71.21, 67.63, 66.75, 66.15, 62.76, 61.72, 60.81, 59.38,

35.96, 30.16; trans-[Co(L14

)(H

2

O)]

3q: 65.58, 62.29,

53.25, 55.33, 55.74, 30.98, 20.56; cis-[Co(L15

)(H

2

O)]

3q:

66.50, 59.29, 57.06, 54.52, 52.62, 48.68, 48.49, 47.51, 45.71,

20.46, 20.07, 18.93; trans-[Co(L16

)(H

2

O)]

3q: 65.02,

63.05, 62.13, 55.05, 52.91, 51.61, 50.12, 49.69, 49.15, 26.02,

23.95, 23.58, 21.15.

In the case of trans-[Co(L14

)(H

2

O)]

3q, the very small

yield of the preparative procedure for the starting chloro com-

plex [5b] did not allow a full characterisation of the base-

hydrolysis product. Nevertheless, the

13

C NMR spectrum

(see above) and the UV–Vis spectrum (Table 1) of the

1 Caution! Special care must be taken on handling perchlorate salts of

compounds containing organic ligands. There is a high risk of explosion.

M. Martinez et al. / Inorganica Chimica Acta 256 (1997) 51–59 53


Table 1

Visible spectral data for compounds [Co(N)

5

X]

2q,3qas a function of the (N)

5

macrocyle and X (H

2

O or OH

y)

(N)

5

X

a lmax1

(nm) lmax2

(nm) lmax3

(nm)

(e) (My1

cm

y1

) (e) (My1

cm

y1

) (e) (My1

cm

y1

)

L

13

(cis) H

2

O 482(sh) 454 (126) 344 (93)

OH

y476 (157) 342 (160)

L

14

(cis) H

2

O 470(sh) 468 (103) 352 (88)

OH

y493 (169) 351 (173)

L

14

(trans) b

H

2

O 500 440 342

OH

y494 452 357

L

15

(cis) H

2

O 510 (94) 486(sh) 350 (132)

OH

y508 (90) 372 (97)

L

16

(trans) H

2

O 532 (63) 498(sh) 368 (79)

OH

y530 (157) 378 (172)

a

0.1 M HClO

4

for XsH

2

O; 0.05 M NaOH for XsOH

y.

b e not available.

obtained solution in acidic and alkaline media, as well as its

behaviour on a cation exchange column (charge q3) ena-

bled us to proceed with the study of its redox behaviour.

For the trans-[CoL16

H

2

O]

3qcomplex the

13

CNMR spec-

trum indicates a non-equivalence of all the carbon atoms of

the macrocycle that seems to disagree with its trans formu-lation. Nevertheless, the well-established geometry of its

chloro complex precursor [5c], the absence of isomerisation

reactions observed for the other (especially L

14

)macrocyclic

complexes, the already observed non-equivalence of carbon

atoms on trans-[Co{H(N6

)}Cl]

2q(N

6

'6,13-diamino-

6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane, diammac)

complexes due to the presence of various conformers [7],

led us to relate the presence of 13

13

C signals to the existence

of the complex as a conformer not having the symmetry plane

that its chloro precursor has. In fact, even for the cis isomersof the L

13

and L

15

macrocylic complexes some of the

13

C

resonances are slightly broadened indicating the presence of

more than one conformer in solution.

2.2.3. Na[CoL15(H2O)][Fe(CN)6] and Na[L15(H2O)-Co(m-NC)Fe(CN)5]Addition of a solution of Na

4

[Fe(CN)

6

] to a solution of

[CoL

15

H

2

O](Trifl)

3

in triflic acid produces the immediate

precipitation of Na[CoL

15

(H

2

O)][Fe(CN)

6

] as character-

ised by the single band at 2044 cm

y1

in the IR spectrum [8]

and the elemental analyses. The suspension of the compound

in water at room temperature, with constant stirring, produces

a cherry red solution. Addition of ethanol to this solution

gives a precipitate of Na[L

15

(H

2

O)Co(m-NC)Fe(CN)5

] as

characterised by the two bands at 2118 and 2044 cm

y1

in the

IR spectrum and the elemental analyses. UV–Vis spectra (l

(nm) (e (cmy1

M

y1

)): 528 (285), 434 (286), 323 (374).

Anal. Calc. for Na[Co(L

15

)(H

2

O)][Fe(CN)

6

]P3H2

O:

C, 35.48; H, 6.12; N, 25.28. Found: C, 36.58; H, 6.20; N,

24.89%. Calc. for Na[Co(L

15

)(H

2

O)(m-NC)Fe(CN)5

]:C,

40.24; H, 5.44; N, 28.68. Found: C, 40.49; H, 6.48; N,

29.23%.

13

C{

1

H} NMR spectra (D

2

O, ppm referenced to TMS):

188.59(=5), 178.12, 67.05, 61.50, 58.41, 56.12, 55.00,

51.53, 49.86, 48.44, 47.17, 26.32, 21.44, 14.73.

2.3. Buffer solutions

All buffers were prepared according to well-established

procedures [9]; concentrations were chosen to provide

enough buffering for the [Fe(CN)

6

]

4ysolutions. Final pH

was set with the addition of NaOH or HClO

4

solutions to

the prepared buffers. The selected pH was 2.8–5.4

(CH

2

ClCOOH/CCH

2

ClCOO

yor CH

3

COOH/CH

3

COO

y)

for the aqua complexes and 7.8–9.0 (Tris) for the hydroxo

complexes. The effectiveness of the buffer solutions was

checked by monitoring the pH value of the final reaction

mixture.

2.4. Instruments

All UV–Vis spectra were recorded on a HP8452A instru-

ment. pH measurements were carried out with a Crison 2002

instrument equipped with an Ingold micro electrode.

13

C

NMR spectra were recorded on a GEMINI-300 instrument.

IR spectra were recorded in KBr discs with a Nicolet 520

FTIR instrument. Atmospheric pressure kinetic runs with

t1/2

)170 s were recorded on an HP8452A instrument

equipped with a thermostated multicell transport; runswithin

the 7–170 smarginwere recorded on anHP8452A instrument

equipped with a High-Tech SFA-11 Rapid Kinetics Acces-

sory; for t1/2

-7 s a Durrum D-110 stopped-flow instrument

was used. For runs at elevated pressure with t1/2

-100 s a

homemade high pressure stopped-flow system was used as

described previously [10a]; for t1/2

)800 s a previously

described pressurising system and high pressure cell were

used [10b–d].

2.5. Kinetics

All kinetic measurements were performed under pseudo-

first-order conditions with the iron complex in excess over



Fig. 1. kobs

dependence on [Fe(CN) ] for the reduction of cis-[CoL15

-

4y6

(H

2

O)]

3qas a function of temperature (a), and for the reduction of cis-

[CoL

14

(OH)]

2qas a function of pressure (b), Is1.0 M (LiClO

4

).

the cobalt complex. The concentration of the cobalt(III)

complex was chosen typically in the 1–5=10

y4

M region.

For the trans-[Co(L14

)(H

2

O)]

3qcomplex the concentra-

tion was determined by using e values estimated from those

determined for the other macrocyclic cobalt(III) complexes

used in this study. Runs for the (N)

5

sL

13

system were fol-

lowed at 420 nm where the appearance of [Fe(CN)

6

]

3y

(es1023M

y1

cm

y1

) [11] can be detected; for all the other

(N)

5

systems the runs were followed at 520 nm, where the

appearance of the final products could be detected most eas-

ily. Solutions for the kinetic runs were made up by mixing

the appropriate amounts of the corresponding stock solutions

at 1.0 or 0.25 M (LiClO

4

) ionic strength. All solutions were

degassed in order to avoid anyFe(II) air oxidation andEDTA

was added to the reaction medium when (N)

5

sL

13

to pre-

vent the precipitation of the Co

2qreaction product [6,12];

for all other (N)

5

systems such precaution was not necessary

given the stability of the final redox product. The cobalt(III)

complex stock solutions were made up in water in order to

avoid any interference from anation reactions with buffer

anions during long stock, or high pressure equilibration times.

Accordingly, the [Fe(CN)

6

]

4ystock solutions had to be

prepared in the corresponding buffers and with the addition

of EDTA when necessary to obtain the correct reaction con-

ditions after mixing. The stability of the cobalt(III) com-

plexes in the buffer solutions used for the study was

monitored byUV–Vis spectroscopy.No indication of anation

reactions of the aqua or hydroxo species occurring during the

reaction times was detected.

All kobs

values were derived from the obtained absorbance

versus time exponential traces using a non-linear least-

squares fitting method. All post run fittings were done by

unweighted least-squares fit to the desired equations. The

values for k and KOS

were obtained from a direct fit to Eq.

(4). Alternatively, a double reciprocal plot was also used;

the coherence of the two plots was considered as a measure

of the quality of the fit.

3. Results

All the observed pseudo-first-order rate constants, kobs

,

measured as a function of the [Fe(CN)

6

]

4yconcentrations,

(N)

5

, buffer acidity, temperature, ionic strength, andpressure

are collected in Table S1 (Section 5). All these values at

Is1.0 M LiClO

4

were fitted to Eq. (4) (or its reciprocal

form, see above) and a very good agreement between the

fitted and the experimental points was observed. Fig. 1 shows

selected plots for some of the systems studied. From these

plots the first-order electron-transfer rate constants, k, andencounter complex formation constants, K

OS

, could be cal-

culated. The errors derived for the first-order rate constants

were always in the 5–10% margin. For the cis-[Co(L

15

)(H

2

O)]

3qsystems, when I was set to 0.25 M

(LiClO

4

) the values of kobs

were found to be independent of

the [Fe(CN)

6

]

4yconcentration margin used (5–12=10

y3

M), for all the other systems this margin was reduced to the

top end. Given the large values expected for KOS

under these

conditions, rate law(4) canbe simplified to kskobs

as already

done for similar systems [2a,12]. Table 2 collects all k and

KOS

values for the systems studied as a function of themacro-

cycle (N)

5

, temperature, acidity, ionic strength and pressure.

From standard Eyring plots the thermal activationparameters

were obtained, and are summarised alongwith relevant avail-

able literature data in Table 3. Plots of ln k versus P (Fig. 2)

were used for the determination of the pressure activation

parameters; in all instances the values determined at Ps1

atmwere not used in the plots given the different instrumental

and sample manipulation required; the values determined at

atmospheric pressure correlate, in general, fairly well with

the values expected from these plots.

As found for similar systems [2b,c], neither interference

from the type and concentration of the buffer solutions, nor

from the amount of EDTA added (where necessary) was

found. Given the fact that the pK values for the aquo com-

plexes studied have not been determined, the independence

of the observed rate constants on pH,within the range studied,

was taken as an indication of the validity for the existence of

only the aqua or hydroxo species in each case. For the L

13

aqua and hydroxo systems, where EDTA had to be added to

the reacting solution in order to avoid precipitation of the

Co(II) salt of the hexacyanoferrate anions, the final spectra

correspond to those previously described [6]. For the reac-

tions run with the cobalt(III) complexes containing the L

14

,

L

15

and L

16

macrocyclic ligands, no precipitation of the

Co(II) salts was observed even in the absence of added

EDTA. In all cases, a final spectrum having two maxima at



Table 2

Kinetic and thermodynamic parameters obtained for all the reactions between [Co(N)

5

X]

2q,3qand [Fe(CN)

6

]

4ystudied as a function of the (N)

5

macrocycle,

X (H

2

O or OH

y), I (LiClO

4

), temperature and pressure

(N)

5

X

a I P T 10

3k KOS

b

(M) (atm) (8C) (s

y1

) (M

y1

)

L

13

(cis) H

2

O 1.0 1 25 0.227 77

0.317 50

35 1.45 76

45 7.44 41

55 41.1 30

100 35 0.584 49

500 0.310 48

1000 0.175 71

1500 0.104 106

OH

y1.0 1 15 0.176 19

25 1.09 18

35 2.48 36

2.60 32

45 11.0 30

55 33.1 16

100 35 1.49 67

500 1.13 64

1000 0.729 57

1500 0.517 66

L

14

(trans) H

2

O 0.25 1 15 0.870

b

25 2.86

b

35 16.0

b

100 15 0.700

b

500 0.310

b

1000 0.135

b

1500 0.0370

b

OH

y0.25 1 25 0.0468

b

45 1.34

b

55 4.22

b

100 45 0.586

b

500 0.393

b

1000 0.165

b

1500 0.0660

b

L

14

(cis) H

2

O 1.0 1 15 0.285 57

25 1.11 50

35 4.08 93

45 14.7 81

0.25 15 0.256

b

25 1.87

b

35 7.50

b

1.0 100 25 1.03 67

500 0.413 74

1000 0.302 36

1500 0.116 54

0.25 100 1.33

b

200 1.19

b

500 0.700

b

1000 0.343

b

1500 0.161

b

100 35 7.60

b

500 4.10

b

1000 2.15

b

1500 0.970

b

L

14

(cis) OH

y1.0 1 25 0.0698 15

35 0.363 38

45 1.06 50

55 4.64 46

100 45 0.914 53

(continued)



Table 2 (continued)

(N)

5

X

a I P T 10

3k KOS

b

(M) (atm) (8C) (s

y1

) (M

y1

)

L

14

(cis) 500 0.569 75

1000 0.520 40

1500 0.261 55

L

15

(cis) H

2

O 1.0 1 15 4.26 21

25 14.6 44

35 32.6 70

45 144 76

0.25 15 3.14

b

25 17.6

b

35 85.1

b

1.0 100 15 2.29 35

500 0.940 52

1000 0.507 43

1500 0.219 59

0.25 100 11 1.59

b

250 1.28

b

500 0.915

b

1000 0.389

b

1500 0.198

b

L

15

(cis) OH

y1.0 1 15 0.709 46

25 1.82 64

35 5.13 55

45 15.2 59

0.25 16 0.943

b

26 3.28

b

35 6.42

b

1.0 100 25 2.95 36

500 2.27 33

1000 1.58 26

1500 0.948 31

0.25 100 2.26

b

500 1.71

b

1000 1.13

b

1500 0.620

b

L

16

(trans) H

2

O 1.0 1 15 69.2 32

25 335 25

35 1470 24

45 7380 36

0.25 15 140

b

25 398

b

35 1700

b

45 4300

b

250 25 263

b

500 166

b

750 122

b

1000 87.6

b

1250 64.9

b

OH

y1.0 15 7.07 56

25 20.3 67

35 54.7 23

45 264 47

0.25 250 34 68.0

b

500 55.0

b

750 44.5

b

1000 34.0

b

1250 29.5

b

1500 21.0

b

a

pHs2.8–5.4 (CH

2

ClCOO

y/CH

2

ClCOOH or CH

3

COO

y/CH

3

COOH) for XsH

2

O; pHs7.8–9.0 (Tris) for XsOH

y.

b

Rate law (4) becomes kobs

sk under low I conditions, see text.



Table 3

Kinetic, thermal and pressure activation parameters for all the reactions between [Co(N)

5

X]

2q,3qand [Fe(CN)

6

]

4ystudied as a function of the (N)

5

macrocyle, X (H

2

O or OH

y) and I (LiClO

4

). Available data for similar systems in the literature are also included

(N)

5

X

a I 10

3k298 H/D S/D V /(T)D

(M) (s

y1

) (kJ mol

y1

) (J K

y1

mol

y1

) (cm

3

mol

y1

) (K)

L

13

(cis) H

2

O 1.0 0.23"0.02

0.32"0.06 132"4 131"14 31"2 (308)

OH

y1.0 1.1"0.2 98"5 25"15 20"1 (308)

L

14

(trans) H

2

O 0.25 2.9

b

105"11 59"38 49"3 (288)

OH

y0.25 0.039 121"7 79"22 42"3 (318)

L

14

(cis) H

2

O 1.0 1.1"0.1 98"1 27"3 36"5 (298)

0.25 1.9

b

122"1 110"4 37"1 (308)

38"1 (298)

OH

y1.0 0.070"0.004 109"4 42"13 22"4 (318)

L

15

(cis) H

2

O 1.0 15"1 84"7 1"23 39"3 (288)

0.25 17

b

115"2 106"8 36"1 (284)

OH

y1.0 1.8"0.1 76"3 y43"10 20"2 (298)

0.25 3.3

b

75"3 y43"9 22"1 (298)

L

16

(trans) H

2

O 1.0 340"10 116"3 134"10

0.25 400

b

95"2 68"8 34"1 (298)

OH

y1.0 20"2 88"8 17"24

0.25 23"1 (307)

(NH

3

)

5

H

2

O

c

0.50 1.3 102"5 79"15 26.5"2.4 (298)

OH

y d

1.0 0.081 109"10 84"30

(MeNH

2

)

5

H

2

O

d

1.0 93 79"7 39"22 29.4"1.6 (298)

OH

y d

1.0 2.0 83"3 21"9 32.9"1.3 (298)

(EtNH

2

)

5

H

2

O

d

1.0 350 84"6 65"21 33.1"2.0 (298)

OH

y d

1.0 6.4 102"1 95"2 30.6"2.8 (308)

a

pHs2.8–5.4 (CH

2

ClCOO

y/CH

2

ClCOOH or CH

3

COO

y/CH

3

COOH) for XsH

2

O; pHs7.8–9.0 (Tris) for XsOH

y.

b

Rate law (4) becomes kobs

sk under low I conditions, see text.c

Ref. [2a].

d

Ref. [2b].

Fig. 2. Plots of ln k vs. P for some of the systems studied: d cis-[CoL15

-

(H

2

O)]

3q, 118C, Is0.25 M (LiClO

4

); s cis-[CoL15

(H

2

O)]

3q, 158C,

Is1.0 (LiClO

4

); j cis-[CoL15

(OH)]

2q, 258C, Is0.25 M (LiClO

4

); h

cis-[CoL15

(OH)]

2q, 258C, Is1.0 (LiClO

4

).

;508 and 442 nm and no signal at 420 nm, corresponding

to [Fe(CN)

6

]

3y, was found. From these solutions a cherry

red compound with an IR spectrum showing the bands

of a {m-(CN)Fe(CN)5

} moiety can be precipitated. No

substitution reaction of H

2

O by [Fe(CN)

6

]

3yin the

[Co(N)

5

(H

2

O)]

3qcompounds, either in acidic or alkaline

medium, was detected under the experimental conditions of

this study indicating that a possible substitution reaction of

[Co(N)

5

(H

2

O)]

3qby [Fe(CN)

6

]

4ycan be ruled out. The

extremely slow acid aquation reaction of [Co(N)

5

Cl]

2qions

(kf6=10

y5

s

y1

at [H

q]s0.2 M and 508C) confirms the

previous assumption. On the other hand, the final cherry

red redox final compound can also be obtained from a

direct reaction of [Co(N)

5

(H

2

O)](O

3

SCF

3

)

3

with

Na

4

[Fe(CN)

6

]. On mixing equimolar amounts of the two

species in highly acidic medium immediate precipitation of

the compound Na[Co(N)

5

(H

2

O)][Fe(CN)

6

] occurs, as

proved by elemental analysis and IR spectroscopy; on stand-

ing, the product slowly dissolves to produce an intense cherry

red solution with UV–Vis maxima at 508, 442 and 322 nm

that analyses with the same C:N ratio, but now showing an

IR signal at 2118 cm

y1

indicative of a m-CN group not

present in the initial product, aswell as an extra

13

C resonance

in theNMR spectrum. The stability of the final {Co

II

(N)

5

}

2q

redox products, their lability, and subsequent inner-sphere

redox reaction, can be considered to be responsible for these

observations.

Table 2 clearly indicates a lack of meaningful differences

in the values of KOS

determined at Is1.0 M, although

changes in ionic strength do affect the KOS

values signifi-

cantly. On the other hand, no pressure dependence of these

values has also been detected. As seen in Tables 2 and 3, the

data obtained for the reduction of [Co(N)

5

(H

2

O)]

3qand



[Co(N)

5

(OH)]

2qagree extremely well with the trends

already observed for similar systems [2b]. The differences

in the first-order electron transfer rate constants on changing

the size of the macrocycle (Table 2) are quite significant for

the aqua complexes; the value of k increases by up to three

orders of magnitude on increasing the size of themacrocyclic

(N)

5

ligand. The difference between the L

14

(trans) and

L

14

(cis) systems is significantly smaller and for the trans-L

16

complex the trend is maintained. The observed effect is

significantly different for the hydroxo species. In this case

the L

14

cis and trans systems show a minimum in the rate

constant, whereas the values for L

13

and L

15

systems are very

similar and an increase of one order of magnitude is observed

for the trans-L16

complex.

The activation enthalpy values do not show any trend.

There is a large scatter in the S/values, whereas the V/D D

values clearly show that there is a significant decrease on

going from the aqua to the hydroxo species. Changes in the

macrocyclic ligand only seem to affect in an important way

the trans-L14

system. Changes in ionic strength only affect

the KOS

value and the differences observed for the thermal

and pressure activation parameters are within the intrinsic

experimental error limits. The values found for the cis-L14

and cis-L15

systems studied at Is0.25M seem to show larger

values of H/and S/

. The reverse trend is obtained forD D

the trans-L16

systems, and the trans-L14

has not been deter-

mined at Is1.0 M. Consequently, given the fact that the

activation volumes are the same within experimental error, it

is very difficult to establish this fact as a proper trend.

4. Discussion

The studied reactions merit special attention with respect

to the final reaction products. The results clearly indicate that

the final m-CN species detected for the L

14

, L

15

and L

16

systems cannot appear via a substitution process on the initial

[CoL(H

2

O)]

3qcomplex, given its extreme inert substitu-

tion nature in acidic medium. Nevertheless, the overall proc-

ess is very similar to the set of reactions occurring in the

Co

II,III

/EDTA/Fe

II,III

(CN)

6

redox processes [6,12]. In our

case the [CoL(H

2

O)]

2qspecies formed after reduction

probably undergo a macrocyclic pendant arm dechelation to

produce a [CoL(H

2

O)

2

]

2qspecies. The new redox potential

probably allows an inner-sphere oxidation process with the

produced [Fe(CN)

6

]

3ycomplex, to yield the final detected

[Co

III

L(H

2

O)(m-CN)FeII(CN)5

]

ycomplex. The non-

reversible cyclic voltammograms of the Co(III) complexes

indicate that the Co(II) species are very labile. The fact that

the final cyano bridged product exists for the macrocyclic

system that has the largest cavity, L

16

, even though itsCo(III)

complex is the less stable one, and the fact that the smallest

cavity, L

13

, does not form this final compound, are in agree-

ment with the overall picture. This means that the Co(II)

species initially formed is only long-lived enough to undergo

the inner-sphere oxidation when the macrocyclic cavity

allows the presence of the Co(II) ion in a relaxedenvironment.

The values collected forKOS

are very similar within exper-

imental error limits, indicating that, unlike for some other

systems [2b], outer-sphere formation is the same, regardless

of the complexes involved for the reactions studied. Simple

electrostatic arguments cannot account for the obtained val-

ues. The effect of ionic strength is such that electrostatic

interaction seems to be mainly responsible for the overall

precursor complex formation. Nevertheless, hydrogen bond-

ing interactions must also be taken into account, as already

found for species involving oxoanionic species where charge

and hydrogen bonding factors can easily compensate effects

[2c,d]. In fact, on the basis of (KOS

) data, the solventD

separated nature of outer-sphere complexes, havingan impor-

tant contribution of hydrogen bonding, has already been con-

cluded [1a,d]. The lack of significant differences between

the kinetic and activation parameters determined at Is1.0

and 0.25 M must be related to the above-mentioned facts.

Although charge redistribution in the transition state is impor-

tant enough as to be affected by the ionic strength, the values

obtained for S/and V/

do not show the differencesD D

expected due to the directly related electrostriction terms

[13], suggesting that hydrogen bonding must in some way

compensate the purely electrostatic ionic strength effects.The

lack of the expected pressure dependence of the values deter-

mined for KOS

also indicates that hydrogen bonding must

play a very important role in the precursor complex; its exis-

tence justifies a strong interactionwhile keeping the separated

ionic nature of the reactants.

The size of the different macrocyclic ligands does affect

in a very important manner the value of k for the reduction

of the aqua complexes. The acceleration observed on increas-

ing the size of the ligand, and the metal–ligand bond length,

is in line with that already observed for the simple linear

amine systems. As pointed out [14], the larger the N

5

O

cavity, the easier the reduction of Co

III

to Co

II

occurs as

expected on the basis of their respective sizes; in our case the

average Co–N distance for the [Co(N)

5

Cl]

2qseries of com-

plexes increases on going from the L

13

to the L

16

macrocycles

[5b,c] and an acceleration should be present on going along

the series. This effect is, however, not observed for the cor-

responding L

13

–hydroxo complex; the small size of the

ligand, that could favour specific O∆H interactions within

the deprotonated complex encapsulating the cobalt centre

even more, could be responsible for this observation.

Differences in geometry do not seem to be very important

as seen from a comparison of the values for the two L

14

systems. This seems to rule out special encounter complex

geometries that could also induce the concept of a dead-end

complex as indicated for similar systems [2c,14]. Even so,

it is difficult to believe that the reducing species

[Fe(CN)

6

]

4ysees the cobalt complex as a perfect spherical

entity [15]. Although a facial approach that could be com-

mon to both cis and trans geometries (Scheme 1) could bea better possibility to understand the results obtained [16],



the smaller deformation capacity of the trans isomer that caneasily compensate the larger cavity size, can be easily be held

responsible for the observed facts. In this respect, it is impor-

tant to point out the possibility of the existence of several

conformers for the L

16

system, somehow increasing the flex-

ibility of the complex and enabling a faster electron transfer

reaction (larger S/values).D

As for the thermal and pressure activation parameters col-

lected in Table 3, the only real trends appear for the V/D

values. All the values are very positive,which is characteristic

for these types of reactions, where the V/values arise fromD

changes in electrostriction associated with charge neutralis-

ation (especially [Fe(CN)

6

]

4yto [Fe(CN)

6

]

3y) and vol-

ume increase on reduction from Co

III

to Co

II

[1,2]. The

difference in V/between the aquo and hydroxo speciesD

does not correspond to that observed for the simple linear

amine analogous complexes as seen in Table 3 [2b]. For the

trans-L14

complex the obtained values are extremely large

and for all systems the value obtained for the hydroxo com-

plex is consistently lower than that of its aquo counterpart.

Given the above-mentioned facts, the lower V/found forD

the hydroxo systems have to be related to the absence of

contribution arising from the reduction of Co

III

to Co

II

, both

due to charge differences (absence of a 3q cation) and bond

distances (shorter Co–O bond), that is the increased com-

pactness of the L cage.

Recent studies carried out on the reduction of oxoanionic

pentaa(m)minecobalt(III) complexes [2c,d] also show the

same trend; the effects have also been related to hydrogen

bonding that seems to dominate electrostatic interaction. The

differences observed for the trans-L14

complex can partly be

related to its geometry, but also to the size of themacrocycle;

the trans-L16

complex does not show such a difference. If

hydrogen bonding is a key aspect in these processes, the

electrostriction effects that dominate the V/values willD

produce an important electrostriction volume increase for the

extremely symmetric trans-L14

complex (see Section 2),

when the charge on the metal centre is reduced.

Nevertheless, the thermodynamics of the electron transfer

for these systems can also be held responsible for the effects

observed. As stated before, the non-reversibility of the cyclic

voltammograms of these species does not allow the necessary

accurate measure of the corresponding reduction potentials

for the complexes.

5. Supplementary material

Table S1 is available from the authors on request.

Acknowledgements

Helpful comments from Dr G.A. Lawrance are highly

appreciated. Financial support from the German–Spanish

cooperation projects, DGICYT, and the Direccio General de

Universitats de la Generalitat de Catalunya is gratefully

acknowledged.

References

[1] (a) R. van Eldik and H. Kelm, Inorg. Chim. Acta, 73 (1983) 91; (b)

I. Krack and R. van Eldik, Inorg. Chem., 25 (1986) 1743; (c) Y.

Sasaki, K. Endo, A. Nagasawa and K. Saito, Inorg. Chem., 25 (1986)4845; (d) I. Krack and R. van Eldik, Inorg. Chem., 28 (1989) 85.

[2] (a) I. Krack and R. van Eldik, Inorg. Chem., 29 (1990) 1700; (b) M.

Martinez, M.A. Pitarque and R. van Eldik, J. Chem. Soc., DaltonTrans., (1994) 3159; (c) M. Martinez and M.A. Pitarque, J. Chem.Soc., Dalton Trans., (1995) 4107; (d) M. Martinez, M.A. Pitarque

and R. van Eldik, J. Chem. Soc., Dalton Trans., (1996) 2665.[3] (a) M. Martinez and G. Muller, J. Chem. Soc., Dalton Trans., (1989)

1669; (b)M. Ferrer, G. Gonzalez andM.Martinez, Inorg. Chim. Acta,188 (1991) 211; (c) G. Gonzalez, M. Martinez, X. Solans and M.

Font-Bardıa, Inorg. Chim. Acta, 203 (1993) 229; (d) G. Gonzalez

andM.Martinez, Inorg. Chim. Acta, 230 (1995) 67; (e) G. Gonzalez,M. Martinez, A.E. Merbach and B. Moullet, Inorg. Chem., 33 (1994)2330; (f) G. Gonzalez, M. Martinez and E. Rodriguez, J. Chem. Soc.,Dalton Trans., (1995) 891.

[4] (a) R. van Eldik, Inorganic High Pressure Chemistry: Kinetics andMechanisms, Elsevier, Amsterdam, 1986; (b) M. Kotowski and R.

van Eldik, Coord. Chem. Rev., 93 (1989) 19.

[5] (a) G.A. Lawrance, M. Martinez, B.W. Skelton and A.H. White, J.Chem. Soc., Dalton Trans., (1992) 823; (b) G.A. Lawrance, M.

Martinez, T.M. Manning, M. Maeder, M.A. O’Leary, W.C.

Patalinghug, B.W. Skelton and A.H. White, J. Chem. Soc., DaltonTrans., (1992) 1635; (c) G.A. Lawrance, T.W. Hambley, M.

Martinez, B.W. Skelton and A.H.White, J. Chem. Soc., Dalton Trans.,(1992) 1643; (d) G.A. Lawrance, M. Martinez, B.W. Skelton and

A.H. White, J. Chem. Soc., Dalton Trans., (1992) 1649.[6] D.H. Huchital and R.G. Wilkins, Inorg. Chem., 6 (1967) 1022.

[7] (a) P.V. Bernhardt, G.A. Lawrance and T.W.Hambley, J. Chem. Soc.,Dalton Trans., (1989) 1059; (b) P.V. Bernhardt, P. Comba, N.F.

Curtis, T.W. Hambley, G.A. Lawrance,M.Maeder andA. Sirwardena,

Inorg. Chem., 29 (1990) 3208.

[8] (a) K. Nakamoto, Infrared Spectra of Inorganic and CoordinationCompounds, Wiley, New York, 3rd edn., 1978; (b) P. Forlano, L.M.

Baraldo, J.A. Olabe and C.O. Della Vedova, Inorg. Chim. Acta, 223(1994) 37; (c) V.G. Poulopoulou, Z.-W. Li and H. Taube, Inorg.Chim. Acta, 225 (1994) 173.

[9] D.D. Perrin, Aust. J. Chem., 16 (1963) 372.

[10] (a) R. van Eldik, W. Gaede, S. Wieland, J. Kraft, M. Spitzer and D.A.

Palmer, Rev. Sci. Instrum., 64 (1993) 1355; (b) R. van Eldik, D.A.

Palmer, R. Schmidt and H. Kelm, Inorg. Chim. Acta, 50 (1981) 131;

(c) M. Spitzer, F. Gartig and R. van Eldik, Rev. Sci. Instrum., 59(1988) 2092; (d) F.K. Fleischman, E.G. Conze, D.R. Stranks and H.

Kelm, Rev. Sci. Instrum., 99 (1977) 1427.

[11] A.J. Miralles, R.E. Armstrong and A. Haim, J. Am. Chem. Soc., 99(1977) 1416.

[12] D. Gaswick and A. Haim, J. Am. Chem. Soc., 93 (1971) 7347.

[13] (a) J. Burgess, Ions in Solution, Ellis Horwood, Chichester, UK, 1988;(b) T.W. Swaddle, Inorg. Chem., 29 (1990) 5017.

[14] A.G. Lappin, Redox Mechanisms in Inorganic Chemistry, Ellis

Horwood, Chichester, UK, 1994.

[15] M.R. Grace, H. Takagi and T.W. Swaddle, Inorg. Chem., 33 (1994)

1915.

[16] J. Berglund and L.I. Elding, Inorg. Chem., 34 (1995) 513.

outer-sphere redox reactions of (n)5-macrocyclic cobalt(iii) complexes. a temperature and pressure...

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