Cyclic voltammetric studies on N,N-disubstitutedN′-ferrocenoylthioureas and their transition metal complexes

PolyhedronVol. 17, No. 10, pp. 1601-1610, 1998

~ Pergamon P I I : S0277-5387(97)00471-3

© 1998ElsevierScienceLtd All rightsreserved.Printedin Great Britain 0277-5387/98 $19.00+0.00

Cyclic voltammetric studies on N,Ndisubstituted N'-ferrocenoylthioureas and their transition metal complexes O. Seidelmann and L. Beyer* Institute of Inorganic Chemistry, Department of Chemistry and Mineralogy, University of Leipzig, D-04103 Leipzig, Germany

(Received 7 October 1997; accepted 14 November 1997)

Abstract--The Fe(II)/Fe(III) redox potentials of N,N-disubstituted N'-ferrocenoylthioureas and the respective ferrocene-l,l'-dicarbonic acid-di- N,N-dialkyl-thioureids are shifted cathodically by complexation of Ni(II), Cu(II), Co(Ill), Mn(II), Pt(II) and Pd(II). The extent of the shift is more pronounced in the latter ligand class and reaches values up to - 0 . 3 2 V. The N,N-disubstituted N'-ferrocenoylthioureas exhibit two redox processes in dichlormethane. A mechanistic scheme is proposed. Ferrocene-l,l'-dicarbonic acid-di- N,N-dialkylthioureids decompose after oxidation in dichlormethane. In contrast, complexes of both ligand types contain quasireversibly oxidisable ferrocene subunits. © 1998 Elsevier Science Ltd. All rights reserved

Keywords: ferrocene ; thiourea ; cyclic voltammetry ; coordination compound ; redox switch ; transition metal.

Heterobimetallic compounds are being investigated intensively [1,2]. Their desired molecular features like catalysis, optical behaviour, conductivity and magnetism [3] are predominantly determined by the oxidation state of the metals involved and by metalmetal communication [4]. In order to increase this intermetal interaction, efforts are aimed to synthesise molecules in which metals are bound in close proximity and/or being connected by a conjugated pathway [5,6,7]. Ferrocene can be used as one part in heterobimetallic compounds. There is evidence that, owing to orbital geometry, interaction between the embedded iron and a second metal bound on sidechains of the cyclopentadienyl rings is poor [8]. Nevertheless, a rising number of ferrocene derivatives has been created capable of complexing additional metal ions. The ferrocene subunit itself meets a lot of basic requirements essential for a wide application like stability, large-scale availability, safe handling and ease of derivatisation. The reversible Fe(II)/Fe(III) redox behaviour may be the most useful feature of this compounds. On the one hand monitoring of these

* Author to whom correspondence should be addressed.

potential allows us to detect binding of an other (metal) ion [9-11], on the other hand the binding strength of this ion can be "tuned" by switching the ferrocene oxidation state [12-15]. Because of their biological importance, emphasis of many research activities has been placed on complexation of alkaline metals within ferrocene containing frameworks [16,17]. However, binding of transition metal ions on mainly nitrogen, nitrogen and oxygen or sulphur of ferrocenyl sidechains has been studied as well [18-20]. In many cases, electrochemical behaviour revealed a moderate anodic shift of the Fe(II)/Fe(III) redox couple after complexation [21]. N,N-disubstituted N'-acylthioureas are easily accessible, well investigated ligands [22] obtaining excellent chelating properties for a wide range of transition metal ions. They posses a hard and a softer donor atom and the possibility of compensating positive charges by deprotonation [23-25]. Introduction of the ferrocene moiety into this ligand class has been accomplished recently, heterobimetallic complexes have been synthesised [26,27]. In these compounds a conjugated link between the two metals is combined with their quite close proximity-especially concerning coordination compounds of the ferrocene-l,l'-dicarbonic acid-di- N,N-dialkyl-thi-

1601

O. Seidelmann and L. Beyer

1602

sulphate and evaporated to dryness. The dark paste was treated with hot n-hexane until it solidified. Recrystallisation in toluene yielded orange brown crystals. Analytical data of the new compounds are to be found in Table 1.

oureids. Therefore, cyclic voltammetric studies should show distinct effects resulting from electronic changes by complexation of a second transition metal. The following complex compounds have been employed. EXPERIMENTAL

Cyclic voltammetry Synthesis" of the compounds Compounds 1 5; (1-4a--f) and (5-7)b have been prepared according to literature methods [26,27]. Chelates (5-7a) have not been isolated but were only obtained in solution by mixing of stoichiometric amounts of ligand (dissolved in 20 cm 3 dichlormethane) and the respective metal acetate (dissolved in 0.5 cm 3 ethanol) at room temperature immediately prior voltammetric measurements. Compounds 6 and 7 have been prepared as follows. The respective N,N-disubstituted thiourea (15 mmol = 1.95 g for 6; 2.19 g for 7) [28] was suspended in 30 cm 3dry toluene and 15 mmol (1.2 g) dry pyridine under argon. At a temperature of 90°C 1,1'-ferrocene dicarbonylchloride (7.5 mmol = 2.33 g), dissolved in 30 cm 3 toluene, was added dropwise to the vigorously stirred suspension over a period of 45 min. After 5 h stirring at this temperature, the residue was filtered off and washed three times with 20 cm 3 hot toluene. The united dark brown toluene solution was washed twice with 20 cm 3 cold water, dried using sodium

L1H:

L2H2:

H

~c"N~c-Fe

O

Cyclic voltammetry was performed using a conventional three-electrode system. The working electrode was a glassy carbon disc (Metrohm, number: 6.1204.000, ~ = 3 mm), polished with an AI203 suspension prior every experiment. The counter electrode was a platinium disc (Sensortechnik Meinsberg, Germany). A double junction Ag/AgC1 electrode was used as reference (Metrohm, number: 6.0726.110). The inner chamber was filled with a saturated solution of LiCI in ethanol, the outer chamber contained a 0.1 M TBAPC solution in dichlormethane. Dichlormethane was of p.a. grade and saturated with nitrogen. The concentration of supporting electrolyte (Bu4NC104) was 0.1 M. The concentration of compounds investigated was 0.5 raM, 1 mM and 5 mM. The number of electrons involved in oxidation processes was determined by coulometry. All compounds were investigated at 25°C. Cyclic voltammograms of ligands 1-4 also were measured at 0°C. The estimated error of measured voltage is about 0.01 V. All data represent the average of five measurements.

NR2

~c~N~c--NR2

S

H S

NR2:

NEt2 NBut 2 pyrrolidino morpholino

H

1 2 3 4

Fe

II

II

O

S

O NR 2:

NEt2

5

pyrrolidino morpholino

6 7

O,S-coordinated bimetallic chelates:

L12Ni(II)

(1, 2, 3, 4) a

L2Ni(II)

L12Cu(II) L13Co(111)

(1, 2, 3, 4) b (1, 2, 3, 4) c

L2Cu(II)

L12Mn(II) L12Pt(II)

(1, 4) d (1, 2, 4) e

U2Pd(II)

(1, 4) f

(5, S, 7) a (5, 6, 7) b

Studies o n N,N-disubstituted N'-ferrocenoylthioureas V o l t a m m o g r a m s (v = 1-50 m V s ~) were recorded with the electrochemical interface (SI 1280, SolartronSchlumberger) u n d e r application of the software C O R R W A R E (Scribner, Charlottesville VA). Volt a m m o g r a m s (v = 100-1000 m V s - j ) were recorded with a " A U T O L A B P G S T A T -10". " D i g i Sim 2.0" by Bioanalytical Systems, Inc., was used to simulate cyclic v o l t a m m o g r a m s . P o t e n t i o m e t r i c titration in d i o x a n / w a t e r 75 : 25 was employed to determine pK~ values. RESULTS AND DISCUSSION

Ferrocenoyl-thioureas of the types L 1H and L2H2 As an example of the ligand centred redox processes, the cyclic v o l t a m m o g r a m (CV) of 4 in dichl o r m e t h a n e is s h o w n in Fig. 1. Each CV recorded of l ~ l reveals the displayed shape. T w o one-electron

1603

redox couples, the first at a potential o f a b o u t 780 m V (I), the second at 900 m V (I), occur (Table 2). This is quite a n unexpected result because the respective benzoylthiourea do n o t show any redox process without initial oxidation in a potential range o f 1.1 V to - 0 . 5 V [29] a n d the ferrocene subunit should only give rise to one such process (Fe(II)/Fe(III)). A third reduction peak occurs only at fast scan rates at a b o u t - 0 . 2 V (III). As side chain centred quasireversible redox processes are not expected, b o t h redox couples should be caused by the ferrocene moiety. A n anodic shift of the Fe(II)/Fe(III) couple c o m p a r e d with ferrocene/ ferrocenium is expected because of the electron withdrawing effect o f the c a r b o n y l function a t t a c h e d to ferrocene in 1~1. The shift extent o f I ( a b o u t + 0 . 2 5 V) corresponds with c o m p a r a b l e substituted c o m p o u n d s [21]. Therefore, we attribute this one electron step to this process. The redox potential is only slightly influenced by different -NR2 (Table 2). Parallel to

Table 1. Analytical data of compounds 3, 3a, 3b, 3e, 6, 7 Compound

3

3a

3b

3e

6

7

yield (%) m.p. ( C ) IR (cm ') (KBr)

71 131

80 140

86 180

64 234

70 205 (dec.)

63 170

3285 br 2977 w 2874 w 1776m 1701 s 1660 s 1526 s 1463 s 1424 s 1247m 1209 s 822 m 499 m

2924 w 2872 w 1769m 1488 s 1440 s 1412 s 1329 s 1249m 1003m 823m 500 m

2925 w 2872 w 1763m 1490 s 1420 s 1312 s 1253m 1010m 830m 505 m

2975 w 2925 w 2300 w 1630 br 1480 s 1460 s 1405 s 1348 s 1250m 805m 495 m

3210 br 2974 w 2875 w 1673 s 1535 s 1453 s 1430 s 1279 s 820m 498m

3208m 2975 w 2874 w 1670 s 1541 s 1459 s 1430 s 1262 s 820m 495m

2.0 (m) 4H, CH2 3.69 (t) J = 6.2 Hz, 2H, CH2 3.85 (t) J = 6.4 Hz, 2H NCH2 4.34 (s) 5H, Cp' 4.45 (pt) 2H, Cp 4.76 (pt) 2H, Cp 8.14 (s) 1H, NH

2.0 (br) 8H, CH2 3.60 (s) 4H HCH2 3.76 (s) 4H NCH2 4.37 (s) 10H, Cp' 4.56 (s) 4H, Cp 4.90 (s) 4H, Cp

2.0 (br) 8H, CH2 3.58 (br) 4H NCHz 3.80 (br) 4H NCH2 4.37 (s) 10H, Cp' 4.53 (s) 4H, Cp 4.92 (s) 4H, Cp

2.04 (m) 8H, CH2 3.80 and 3.89 (m) together 8H NCH2 4.59 (s) 4H, CpH 5.22 (s) 4H, CpH 9.44 (s) 2H, NH

3.79 (s) 4H NCH2 3.82 (br) 8H OCH2 4.28 4H NCH2 4.61 (s) 4H, CpH 5.25 (s) 4H, CpH 9.51 (s) 2H, NH

IH-NMR (CDCI3)

O. Seidelmann and L. Beyer

1604

Table 1. Continued Compound

3

3a

3b

3c

~3C-NMR (CDCI3)

25.1 and 26.7 (CH2) 53.4 and 55.0 (NCHz) 69.4 (CpH) 70.4 (Cp'H) 72.3 (CpH) 74.0 (Cp) 167.8 (CO) 176.3 (CS)

25.3 and 26.7 (CH2) 52.9 and 54.7 (NCH2) 70.7 (CpH) 71.2 (Cp'H) 71.4 (CpH) 73.2 (Cp) 171.9 (co) 176.0 (CS)

MS (% intensity)

(70 eV) 341.9 (M)* (35) 283 (43) 229 (32) 214 (33) 185 (41) 121 (100)

(FAB) 740.5 (M)' (17)

(FAB) 745 (M) + (9)

51.90/52.01 4.63/4.61 7.56/7.74 4.32/4.34 8.64/8.51

51.56/51.60 4.60/4.52 7.51/7.54 4.29/4.19 8.59/8.86

6

7

25.3 and 26.7 (CH2) 53.0 and 54.6 (NCH2) 70.8 (CpH) 73.8 (CpH) 75.7 (Cp) 167.1 (CO) 177.0 (CS)

51.9 and 52.3 (NCH:) 66.9 and 67.3 (OCH_~) 70.5 (CpH) 74.0 (CpH) 75.4 (Cp) 167.0 (CO) 179.5 (CS)

(FAB) 1082 (M) + (5)

(70 eV) 498 (M) + (2) 439 (5) 222 (12) 93 (50) 70 (99) 65 (100)

(70 eV) 530 (M) + (1) 412 (48) 384 (9) 292 (I 1) 179 (12) 121 (36) 92 (49) 59 (100)

53.28/53.29 4.75/4.82 7.76/7.95 4.44/4.91 8.87/8.91

53.01/53.55 5.26/5.21 11.24/11.08 6.42/6.46 12.84/12.30

49.81/49.70 4.94/5.02 10.57/10.56 12.07/12.10 12.07/12.13

92 (42) 7O (55) 56 (45) C (%) (calc./fnd.) H (%) (calc./fnd.) N (%) (calc./fnd.) O (%) (calc./fnd.) S (%) (calc./fnd.)

56.15/56.20 5.30/5.42 8.18/8.13 4.68/4.34 9.35/9.80

our work, the redox behaviour of N-ferrocenoyl-N'monosubstituted thiourea has been investigated [30]. A comparable influence of the amine substituents on the redox potential is attributed there to an intramolecular hydrogen bonding ring. As compounds 1 4 do not form hydrogen bridges, this effect cannot be explained in this way. The magnitude of the amine dependent redox shift corresponds with that of the experimental error. Therefore, we do not venture an explanation. In order to investigate the nature of the redox processes, further CV experiments were carried out. They included repeated potential cycling, initial 200 s electrode polarisation at various potentials, varying of scan rate, temperature and concentration. The following results have been obtained. (i) Adsorption could be excluded as a reason for one of the peaks because of their concentration independent height relation ; (ii) Varying of concentration

(0.5 up to 5 mmol 1-~) shows no effect on the redox potentials or CV shape ; (iii) Oxidation peaks I and II are diffusion-controlled ( i ~ v 1:2) (Fig. 2.); (iv) Reduction current III decreases to zero at scan rates < 10 mV s ~ (Fig. 1.); (v) Process Ill is only detectable after initial oxidation at If; (vi) During repeated potential cycling I disappears, the oxidation peak of lI broadens, its reduction peak is shifted to more cathodic potentials and increases. Potential cycling within a potential window of 0.5 V and 0.9 V (redox couple I) results in quite reproducible CVs without any change of the peaks. Even an initial electrode polarisation at 0.9 V for 200 s does not affect the CV shape remarkably (Fig. 3.). This allows the conclusion of a comparatively stable oxidation product ; (vii) Potential cycling beyond the anodic peak of I (0.75-1.2 V) covering both electrochemical processes of II leads to irreversible destruction of the chemical species involved. Similar effects are observable after

Studies on N,N-disubstituted N'-ferrocenoylthioureas

1605

-30 mV/s I _ 5 mV/s 0.5

ill 0.0

<

-0.5

o

!

II

i/ 2

-1.5 - L kJ

II

1.0

I

J

0.5

0.0

-0.5

E[V] Fig. 1. Cyclic voltammogram of 4 (1 mM) in dichlormethane. Scan from 0 V to + 1.2 V, -0.5 V and return. Scan rate: 30 mVs Land5mVs ~.

Table 2. Redox data of ligands 1-7 (average of 5 measurements), 1 mM in dichlormethane, 0.1 M TBAPC. Scan rate 30 mV s ~. Scans from 0 V to 1.4 V and return, 1st scan Compound

Ep~ (V)

Ep~ (V)

z~Er,"(V) E~2h (V)

Ferrocene 1 process A 1 process B 2 process A 2 process B 3 process A 3 process B 4 process A 4 process B 5 6 7

0.59 0.86 1.05 0.82 1.02 0.86 1.06 0.82 0.95 1.05 1.02 1.09

0.47 0.73 0.89 0.70 0.88 0.71 0.88 0.74 0.87

0.12 0.13 0.16 0.12 0.14 0.15 0.18 0.08 0.08

0.85

0.24

0.53 0.8 0.97 0.76 0.95 0.78 0.97 0.78 0.91 0.95" 0.94' 0.97

"AEp = [Ew - Ep¢[. h E l 2 = ~(Ep a 4-Ep~.). ' Epa 2.

electrode polarisation beyond 0.95 V; (viii) Current of II is temperature dependent. At 0°C its magnitude

is only 70% of that measured at 25:C while I seems to be unaffected. These effects and the earlier drawn conclusion concerning the ferrocene centred character of II give rise to the assumption that this process involves the Fe (II)/Fe(III) couple of a product of Iox which lost its positive charge by a chemical reaction. A similar CV behaviour has been observed recently [31]. There, the oxidised ferrocene subunit was immediately reduced by a phosphino substituent which led to a second Fe(II)/Fe(III) redox process. Here, the reaction following the first oxidation is supposed to be fast as well, because various v (l-1000 mV s ') do only have a slight effect on the current ratio Ilox/lo~. Considering the weak but detectable acidity of ligands 1-4 [27] and its known enhancement by oxidation of ferrocene bearing acids [33]--due to the repulsive effect of positive charges--we propose to attribute I1 to the Fe (ll)/Fe(III) redox couple of oxidised and deprotonated compounds 1 4 . The oxidation product of lI seems to be chemically unstable and causes unidentified decomposition products. One of which gives rise to the reduction peak at - 0 . 2 V if the scan rate is fast enough. The two broad redox peaks remaining after prolonged potential cycling and the increase of the reduction peak indicate an electrode surface coating which can also be seen visually.

O. S e i d e l m a n n a n d L. Beyer

1606

/ oxidation I | oxidation II

• •

J

8 7 6 5

O

.,--. --

•

3

•

2 1 0 I

r

[

I

I

0

5

10

15

20

I

25

30

35

sqr (v) [mY/s] ~/2 Fig. 2. Oxidation current of processes 1 and ][ vs sqr (v) (v : 1 mV s - ~-1000 mV s ~, c = 1 mM).

-1.0

-0.5

0.0

< o

0.5

! 1.0

1.5

2.0

'

'

'

1.0

'

I

0.5 E M

Fig. 3. Cyclic voltammogram of 4 (1 mM) in dichlormethane. Cycles without interim electrode polish. Scans : 200 s electrode polarisation at 0.9 V then 10 cycles, 0.9 V to 0.5 V and return.

Studies on N,N-disubstituted N'-ferrocenoylthioureas The following scheme sums up chemical and electrochemical processes we assume to take place during CV studies on 1-4.

A

E-L

B+e-

A... Fe

C

E-II ~

D+e-

F

E-HI ~ -

E+e-

(÷..)

S

1607

around pKsl = 11.1 and pKs2 = 12.6. So, compared with 1-4 [27], their first deprotonation step is slightly more enhanced. Therefore, a proton loss and further reactions following the oxidation is imaginable here as well. More probable, however, is a total destruction of the electron lacking ferrocene moiety which lost here the "electron buffer" capacity of the unsubstituted cyclopentadienyl ring [31].

Heterobirnetallic coordination compounds

G

As indicated by the temperature dependence of II, a chemical reaction (K1) should be neccessary to form C. Reduction I]I is not detectable without initial oxidation II. Therefore, E should result from D and not from C. Obviously, stability of E is limited. Decomposition to the electrochemically inert G can be assumed. To strengthen the theory of this mechanism the cyclic voltammograms were computer-simulated. According to the measured CVs, the curves were simulated for various scan rates and concentrations using one set of parameters. It cannot be expected that simulated CVs of such a complex mechanism resemble the measured ones in all characteristics. The mathematical software model is too simple to reflect the complex reality correctly. The estimation and variation of 23 independent parameters belonging to 8 mostly unknown species should give the true values only in exceptional cases. Nevertheless, one set of parameters yields CVs which resemble the experimentally recorded fairly well (Fig. 4). Accept from the absolute values of the current, the main charactristics are displayed correctly within a scan rate range of I mV sto 1 V s- ~. Especially, the general shape and its relative changes are simulated adequately. At least, one can conclude from the simulations that there is no general contradiction between the proposed mechanism and the experimental data. (Used parameter set: E0~ =0.77 V [ ~ = 0 . 5 , k~l = 0.002], E02 = 0.97 V [~ = 0.5, ks2 = 0.01], E03 = - 0 . 1 2 V [c~ = 0.5, ks3 = 0.1]; K~qI = 20 [kr = 3 s t , k b = 0 . 1 5 s I],Keq2=50[kr=lOOs I,kb~2S--I], K~q3 = 60 [kf = 0.01 s -l, kb = 0.000167 s-l]; D [cm2 s-t]: D A = 2 . 1 0 5, DB=0.0002, Dc=DD=DE= DF=DG= 1"10 5.) Ligands 5--7 show a different and less complex redox behaviour. Only one oxidation peak occurs in dichlormethane. As expected the extent of anodic shifting is about twice as large as measured at the monosubstituded ligands 1-4, compared with ferrocene/ferrocenium. The influence of -NR2 is negligible (Table 2). Repeated potential cycling reveals irreversibility of the oxidation process. The two pK~ values of 5-7 lie in a narrow range

Cyclic voltammograms of (1-4a-f) reveal only one ferrocene centred redox couple representing a two electron transfer (Fig. 5). Hence, an interaction between the two ferrocenes can be excluded [21]. This redox process is shifted to more cathodic potentials compared with the free ligands (Table 3). Considering the positive charge of the coordinated metal ions, an anodic shift might be expected as it is reported in most papers dealing with comparable compounds. In [32] the first example of a cathodic shifted redox potential is reported. There and here deprotonation accompanies complexation and the entering transition metal has to "compensate" the electron withdrawing effect of the leaving, "hard" proton. This is done incompletely in general and means an increase in complex stability of the oxidised coordination compound compared with the neutral one [34]. As can be seen from the data of Table 3, the different metal ions investigated have only a modest metal specific influence on the Fe(II)/Fe(III) redox couple leading to comparable shifts of about - 0 . 1 5 V with - 0 . 0 5 V (4d) and - 0 . 1 9 (lc) as most extreme values. Whereas no general trend can be deduced from the data, the following order going parallel with increasing shift extents is valid within a compound class resulting from one ligand: Mn(II)Pd(II)Pt(II)Ni(II) Cu(II)Co(III). A reason for this order could be the charge/radius ratio. The smaller it is the more negative shifts should be detected. However, this approach does only apply to the order of Mn(II)-Ni(II)--Cu(lI). Especially the large shift of the "hard" Co(lII) can not be explained in this way. Therefore an other aspect should be taken into consideration. The cathodic shift could be caused by a a ~-donation effect of the oxygen. Mn(II) is the best a 7r-acceptor of the ions mentioned above and Co(III) the worst. So the ferrocenium state is more stabilized by the a ~electron withdrawing Mn(II) ion than by Co(III). In most cases the redox processes are more reversible here than those of the free ligands. A comparable repulsion of the bound metal ion followed by chemical reactions and compound destruction could not be observed during potential cycling. In all coordination compounds, except the palladium chelates, further electrochemical processes occur in addition to the ferrocene bound redox couple, especially the quasi reversible Cu(II)/Cu(I) is easily detectable (Fig. 5). Nearly all CVs reveal another oxidation or redox process beyond 0.9 V. It does not show much metal

1608

O. S e i d e l m a n n a n d L. Beyer - -

30 mV/s

- - -

5 mV/s

0.5 III

-1.0 /

-1.5 -

I

I

I

1.0

0.5

0.0

-0.5

E [V] Fig. 4. Computer simulated cyclic voltammograms of ligands 1~1 (l mM). Scan from 0 V to + 1.2 V to - 0 . 5 V. Scan rate: 3 0 m V s ] a n d 5 m V s '.

-3

'

I

'

I

-2

Cu(tt)/Cu(t)

o

2 1

2 ,)/Fe(ttt) 3 i

1,0

L

i

i

I

i

i

0,5

I

i

i

0,0

i

i

i

I

-0,5

E [V] Fig. 5. Cyclic voltammogram of 4b in dichlormethane. Scanfrom 0 V [o 1 V to -0.6 V and return with a scanrate of 30 mV s-L

Studies o n N,N-disubstituted N'-ferrocenoylthioureas

1609

Table 3. Redox data of chelate (1-7a-f) (average of 5 measurements), 1 mM in dichlormethane, 0.1 M TBAPC. Scan rate 30 mV s -~. Scans from 0 V to 1.6 V to - 0 . 6 V and return, 1st scan Compound

Epa (V)

Epc (V)

AEp ~ (V)

Et,,2b (V)

E,,2-shift compared with ligand " (V)

la lb le ld le If 2a 2b 2e 2e 3a 3b 3e 4a 4b 4c 4d 4e 4f 5a

0.72 0.69 0.70 0.75 0.69 0.77 0.69 0.69 0.70 0.70 0.67 0.66 0.66 0.71 0.72 0.75 0.83 0.73 0.72 0.78

0.65 0.55 0.51 0.63 0.60 0.69 0.59 0.57 0.48 0.60 0.61 0.60 0.56 0.63 0.62 0.52 0.62 0.62 0.63 0.71

0.07 0.14 0.19 0.12 0.09 0.08 0.10 0.12 0.22 0.10 0.06 0.06 0.10 0.12 0.10 0.23 0.21 0.11 0.09 0.07

0.68 0.62 0.61 0.69 0.64 0.73 0.64 0.63 0.59 0.65 0.64 0.63 0.61 0.67 0.67 0.64 0.73 0.68 0.68 0.74

5b

0.75

0.69

0.06

0.72

6a

0.74

0.68

0.06

0.71

6b

0.70

0.66

0.04

0.68

--0.12 --0.18 --0.19 --0.11 -0.16 -0.07 -0.12 -0.13 -0.17 - 0.11 -0.14 - 0.15 - 0.17 -0.11 -0.11 -0.14 -0.05 -0.10 - 0.10 - 0.27 (--0.21) - 0.30 (-0.23) - 0.28 (-0.23) -0.32

7a

0.81

0.70

0.11

0.76

7b

0.81

0.69

0.12

0.75

(0.26) - 0.28 (--0.21) -0.28

(-0.22) ~'AEp = IEp, - Epcl. h E,.2 = ~ (Eva + E~). ' E .2(ligand) - E a(chelate) ; for (5, 6, 7a,b) : Epdligand ) - Ep,(chelate) and in parentheses Ep:2(ligand) - EL.2(chelate).

specificy. The lowest values are measured for m a n g a nese chelates (0.96 V), the highest for platinium complexes (1.5 V). They could be attributed to certain metal b o u n d redox processes, i.e. Cu(II)/Cu(III) [35], M n ( I I ) / ( I I I ) or Ni(II)/Ni(III), but there is n o certainty a b o u t it. C o m p a r a t i v e m e a s u r e m e n t s on benzoylthiourea revealed ligand oxidation beginning beyond 1.1 V [29]. Cyclic v o l t a m m o g r a m s of (5-7a,b) show at least one quasi reversible two electron redox step (Table 3). C o m p a r e d with the free ligand it is shifted cathodically as well but more emphasised t h a n observed at the m o n o s u b s t i t u t e d ferrocene derivatives. A c o m p o u n d destruction does not occur d u r i n g potential cycling. According to the cathodic shift, the complex stability is e n h a n c e d by oxidising the ligand. Obviously the extended conjugated chelate system is more capable of distributing the positive charge. Additionally the side chain electron w i t h d r a w i n g has been lowered by substitution the p r o t o n for " s o f t e r " metals.

As m e n t i o n e d in [27] the nickel chelates 5 - 7 tend to polymerisation. Ten minutes after a d d i t i o n of the nickel acetate the CV peak began to decrease a n d the solution became clouded, indicating the beginning polymerisation process.

CONCLUSIONS The Fe(II)/Fe(III) redox potential of N,Ndisubstituted N'-ferrocenoylthioureas a n d the respective f e r r o c e n e - l , l ' - d i c a r b o n i c acid-di- N,N-dialkylthioureids displays a distinct cathodic shift u p o n coord i n a t i o n o f transition metal ions. Therefore they could be used as a m p e r o m e t r i c detectors or as pH-controlled " r e v e r s e d " redox switches [36] as they tend to e n h a n c e metal b o n d i n g in their oxidised state by liberating protons. Limits are set by the low metal specificity a n d the polymerisation tendency of 5 7

O. Seidelmann and L. Beyer

1610

chelates which up to now only allows to form definite copper complexes. Ligand destruction following the oxidation step is generally avoided by complexation. Nevertheless, investigation of the destruction mechanism and its products might be promising in order to prove the proposed reaction scheme. Acknowledyements--The authors wish to acknowledge financial support by the "Deutsche Forschungsgemeinschaft" (Project Be 1436/7-1) and the Freistaat Sachsen for a grant (O.S.). Helpful discussions with Dr K.-H. Lubert, Dr G. Wittstock and determination of the pKs values by Dr F. Dietze are thankfully acknowledged. We would like to thank Dr R. Meier for supporting CV simulation.

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