Electron-transfer reactions of copper(II) complexes containing trioether sulfur and pyridyl nitrogen donors with ferrocene and 1,1′-dimethylferrocene

Polyhedron Vol. 6, No. 5, pp. 943-946, Printed in Great Britain

1987 0

0277%5387/87 53.oo+.cKl 1987 Pergamon Journals Ltd

ELECTRON-TRANSFER REACTIONS OF COPPER(H) COMPLEXES CONTAINING THIOETHER SULFUR AND PYRIDYL NITROGEN DONORS WITH FERROCENE AND l,l’-DIMETHYLFERROCENE NOBUO AOI, GEN-ETSU

MATSUBAYASHI

and TOSHIO

TANAKA*

Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan (Received 21 July 1986; accepted 4 September

1986)

Abstract-Kinetic studies were carried out for the electron-transfer rkactions of [Cu(II) L][ClO,], [L = 1,6-bis(2-pyridyl)-2,5-dithiahexane, 1,7-bis(2-pyridyl)-2,6-dithiaheptane, or 1,8-bis(2-pyridyl)-3,6-dithiaoctane] with ferrocene (fc) and l,l’-dimethylferrocene (Me,fc), and of [Cu(II)L;][C1041z [L’ = 4-(alkylmercaptomethyl)imidazole ; alkyl = Me, Et, PI”, CHzPh or Bu’] with fc in acetonitrile at 25°C. The electron-transfer reactions are suggested to proceed via a precursor complex, [Cu(II)L - Red], existing in an equilibrium state : [Cu(II)L]*+ + Red &

[Cu(I)Ll+ + Red+

[Cu(II)L - Red]*+ -%

In a previous paper dealing with the electron-transfer reactions of Cu(II)N2S2 type complexes, bis[(4-alkyl- and phenyhnercaptomethyl)imidazole] copper(I1) diperchlorate (alkyl = Pf, CH2Ph or Bu? and 1,6-bis(4-imidazolyl)-2,5-dithiahexanecopper(I1) diperchlorate, as a model of an active site of blue copper (Type I) proteins with ferrocene, we reported the formation of a precursor complex prior to the electron transfer.’ In order to further understand the mechanism

* Authors to whom correspondence should be addressed.

(Red = fc or Me2fc).

for the electron-transfer reaction of the Cu(II)N2S2 type complexes, we have undertaken a study of the reactions of the copper(I1) complexes coordinated by pyridyl nitrogen and thioether sulfur atoms with ferrocene (fc) and l,l’-dimethylferrocene (Me2fc). This paper reports the kinetics of the electrontransfer reactions of [Cu(II)L][ClO& [L = 1,6bis(2-pyridyl)-2,5-dithiahexane (l),1,7-bis(2-pyridyl)-2,6-dithiaheptane (2), and l,&bis(Zpyridyl)3,6-dithiaoctane (3)] with fc and Me,fc, and of [CU(II)L;][C~O~]~ [L’ = 4-(alkylmercaptomethyl)imidazole ; alkyl = Me (4), Et (5), Pf (6), CH2Ph (7) and Bu’ (8)] with fc in acetonitrile.

Icu 0x)Ll[clo,l.? m=2,?2= l(1) m= 3, n= l(2)

R = Me (4 1,Et(51,

m=2,n=2(3)

Bu’ (8)

[Cu (rl)Ll1rc10412 Pr” (6), CHiPh

Scheme 1.

943

(7),

or

N. A01 et al.

944

EXPERIMENTAL

Table 1. kobsdfor the reaction of complex 1 (1.08 x 10e4 mol dmW3) with fc in acetonitrile containing [Bu”,N[BF,] (0.1 mol dm- ‘) at 25°C

Materials The Cu(II)N2S2 type complexes 1-32,3 and &I4 were prepared according to the literature. Commercially available fc and Me2fc were purified by sublimation and recrystallization before use, respectively.

Kinetic measurements Reaction rates were determined by monitoring the decay of absorbance at a wavelength in the 300400~nm range absorbed by the copper(H) complexes in acetonitrile using a Union RA-103 stopped-flow spectrophotometer equipped with a 0.2or 1.O-cm quartz cell in a cell-holder thermostatted at 25 + 0.2”C. All the kinetic measurements were carried out under pseudo-fist-order conditions with at least five-fold excess amounts of fc or Me,fc relative to the copper(I1) complexes (9.7 x lo-‘1.2 x lo- 4mol dm- ‘) under a nitrogen atmosphere. Observed pseudo-first-order rate constants, kobsd, were obtained by the least-squares curve fitting using a Union System 77 microcomputer.

WI 1.06x 1.76x 2.64 x 3.52 x 5.28 x

1O-3 1O-3 10-3 1O-3 lo- 3

[Cu(II)L]~’ +fc& [Cu(II)L. fc]2+ k,,

[Cu(II)L ’ fc]2+, [cu(I)L]+ +fc+,

(2) (3)

where K is the formation constant of a precursor complex, [Cu(II)L * fc] 2+, and ket is the rate constant of the electron-transfer process. According to this scheme, the kobd value in the presence of large excess amounts of fc can be expressed by eqn (4), which is transformed to eqn (5) :

AND DISCUSSION

1

Complexes l-3 exhibit a strong absorption band due to the sulfur-to-copper charge-transfer transition at 330-370 m.4 When either the copper(H) complex was mixed with fc or Me,fc in acetonitrile, the charge-transfer band was weakened in intensity and a band due to the ferrocenium (fc+) or l,l’dimethylferrocenium cation (Me2fc+) concurrently appeared at 618 (fc+) or 652 nm (Me,fc+). This is indicative of the occurrence of the redox reaction [eqn (01:

1.28kO.02 2.08 f 0.09 2.69kO.10 3.36kO.19 4.6lkO.16

tion [eqn (3)] :

k RESULTS

k obsd (s- ‘)

(mol dm- 3,

zk

-- ktK[fcl

Obsd - 1+ K[fc] ’ 1 1 = k,,K[fc] + k,, *

(4)

(5)

This equation predicts a linear relation between kG$ and [fc]- ‘. In fact, there can be seen a linear relation between these values, as shown in Fig. 2, supporting the reaction mechanism of eqns (2) and (3). The K and k,, values determined from the slope

5Or

[Cu(II)L]‘+ + Red + [Cu(I)L]+ + Red+, Red = fc or Me2fc

(1)

being similar to the reaction of bis[4-(alkyl- and phenylmercaptomethyl)imidazole]copper(II) diperchlorates (alkyl = Pr”, Bu’ or CH2Ph) and 1,6bis(4-imidazolyl)-2,5-dithiahexanecopper(I1) diperchlorate with fc. ’ The pseudo-first-order rate constants, kobsd, obtained in the reaction of 1 with excess amounts of fc are listed in Table 1. No linear relation can be seen between the kobd value and the initial concentration of fc ([fc]), as depicted in Fig. 1. This is suggestive of the existence of a pre-equilibrium nrocess [eon 1211 to the electron-transfer reacL--n-m \P,J Drier r r------

lO”[fc]

(mol

dme31

Fig. 1. Plot of kobd against [fc] for the reaction of complex 1 with fc in acetonitrile at 25°C.

Electron-transfer

reactions of copper

complexes

945

12-

1.0-

10‘3[fc]-’

(mol-’

dm’)

Fig. 2. Plot of k;& against [fc]-’ for the reaction of complex 1 with fc in acetonitrile at 25°C.

0

V

I 02

I 04

I 06

10-3[Me,fc]-’

1 0.8

(mol-’ dm3

1 1.0

1

Fig. 3. Plot of k;& against [Me,fc]- ’ for the reaction of complex 1 with Me,fc in acetonitrile at 25°C.

of the straight line and the intercept, respectively, are shown in Table 2, together with those values obtained similarly for the reactions of 2 and 3 with fc. In the reaction of 1 with Me,fc, however, there exists a linear relation between k,& and the reciprocal of the initial concentration of Mezfc, wezfc]- ‘, as shown in Fig. 3. This result may be explained in terms of the relation K <<& which may arise either from a K value small enough to be neglected or from a sufficiently large k,, value so that the reciprocal value can be neglected in eqn (5), under the assumption of the mechanism shown by eqns (2) and (3). Reactions of 2 and 3 with Mezfc were similarly analyzed. Kinetics of the reactions of 4-8 with fc in acetonitrile were also analyzed in the same manner as those of the reactions of l-3 with Mezfc. The k2 (= k,,K) values for the reactions of l-3 with Mezfc and of 4-8 with fc are listed in Table 3. Another mechanism for the electron-transfer reaction without the formation of any precursor

complexes may be considered as reported for the reaction of bis[2,9-dimethyl-4,7-bis(sulfonyloxyphenyl)- 1,IO-phenanthroline]copper(II) diperchlorate with sodium hexacyanoferrate(I1) in water.’ This reaction has been formulated as eqns (6) and (7) : [cu(II)L]2+ +

I

[cu(II)L]*+ *

Table 3. k2 (= k,K) values for the reactions of the copper(I1) complexes with fc and Me,fc, and I?” values of the copper(I1) complexes”

Complex

10-4kz (mol- ’ dm’ s- ‘)

Complex 1

2 3

K (mol- ’ dm’) llOfl0 42f9 66+ 15

12+1 50fll 860 f 200

EUb

W)

For the reactions with fc 0.698 kO.008 1.75*0.01 16.5kO.2

Table 2. K and k,, values for the reactions of the copper(I1) complexes with fc in a&on&rile containing [Bu’jNj[BF4] (0.1 mol dmm3) at 25°C

(6)

0.49 0.55 0.64

For the reactions with Me,fc 0.66 0.68 0.69 0.72 0.73

8.22kO.28 ll.OkO.6 12.5 +0.3 32.0f2.3 44.2k4.0 “In acetonitrile containing dn- ‘) at 25°C. bp = (Epr+ EJ2 (Ref. 4).

[Bu;NJ[BF,]

(0.1 mol

946

N.

[Cu(II)Llz+* + Red-%

[Cu(I)L]+ +Red+.

A01 et al. (7)

where [Cu(II)L]” and [Cu(II)L12+* stand for the solvated species and the desolvated, active species for the electron transfer, respectively. When the condition [[Cu(II)L]‘+] <<[Red] is satisfied, one can derive eqn (8) from eqns (6) and (7) : 1

ky

k-1

1

= k,k,,[Red] + k,’

(8)

Under such a reaction scheme, both plots of k;id against [fc]- ’ and of k,& against [Me,fc]- ’ for the reactions of a copper(I1) complex with fc and Mezfc should give the same intercept. In the reaction of 1 with Me2fc, however, a plot of k&f against [Me2fc]-’ has an intercept with nearly zero (Fig. 3), which is in contrast to the appreciable non-zero intercept in the reaction of 1 with fc (Fig. 2). Similar results have been obtained for the reactions of 2 and 3 with Mezfc. In view of these facts, the mechanism shown by eqns (6) and (7) for the electron-transfer reaction is not applicable for the present reactions of the copper(I1) complexes with fc and Mezfc. The Kvalues (42-l 10 mol- ’ dm3) for the present copper(IIbfc precursor complexes are larger than those (9.7-32.3 mol-’ dm3)’ for the corresponding precursor complexes formed in the reactions of the Cu(II)(NiGd&2S2 complex with fc. This result may be explained in terms of the higher electronaccepting abilities of the present Cu(II)(Nr+&zS2 complexes, which involve the pyridyl group having more effective n-acceptor property than the imidazolyl group.6 This is consistent with the fact that the redox potentials of l-3 (0.49-0.64 V vs SCE in

acetonitrile) are positively higher than those of the Cu(II)(Ni,id,,,l,)2S2 complexes (0.23-0.37 V vs SCE in acetonitrile). ’ The k,, value also increases with raising the redox potential of the C~.I(II)(N,,~~)~S~ complex. The same tendency was seen in the reactions of Cu(II)(Nhidazoie)2S2with fc.’ The formation of a precursor complex has not been detected not only in the reactions of complexes l-3 with Me2fc but also in those of 4-g with fc. In the former reactions, a steric hindrance of the methyl groups of Me2fc may cause fairly small K values. Such a steric hindrance, however, may not be significant in the latter reactions concerned with fc. Since complexes 4-g exhibit sufficiently positive redox potentials (0.66-0.73 V vs SCE in acetonitrile) (Table 3), Kvalues can be expected as large as those observed for the reactions of l-3 with fc and fairly large k, values are deduced for the reactions of HI with fc.

REFERENCES 1.

N. Aoi, G. Matsubayashi and T. Tanaka, J. Chem.

Sot., Dalton Trans. 1983, 1059. 2. S. E. Livingstone and J. D. Nolan, Aust. J. Chem. 1960,23, 1553. 3. H. A. Goodwin and F. Lions, J. Am. Chem. Sot. 1960,82,5013. 4. N. Aoi, G. Matsubayashi and T. Tanaka, Inorg. Chim. Acta 1985, 85, 123. 5. N. Al-Shatti, A. G. Lappin and A. G. Sykes, Znorg. Chem. 1981,20,1466. 6. R. S. Sundberg and R. B. Martin, Chem. Rev. 1974, 74, 471.