~ Pergamon S0277-5387(96) 00195-7

Po(vhedron Vol. 15, No. 24, pp. 4461 4.467. 1996 Copyright i : (996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0277 5387/96 $15.tl0+0.01/

STUDIES ON THE OXIDATION OF Cu n C O M P L E X E S OF Q U A D R I D E N T A T E SCHIFF BASES DERIVED F R O M

SALICYLALDEHYDE OR ortho-AMINOPHENOL

ALESSANDRO PASINI,* ELISA BERNINI and MAURO SCAGLIA

Universitfi di Milano, Dipartimento di Chimica Inorganica, Metallorganica e Analitica and CNR Centre, Via Venezian 21, 20133 Milano, Italy

and

GIANCARLO DE SANTIS*

Universifft G. D'Annunzio, Dipartimento di Scienze, Viale Pindaro 42, 65127 Pescara, Italy

(Received 23 February 1996; accepted 24 April 1996)

Abstract The oxidation of various [CuL] (where L are the dianions of quadridentate Schiff bases derived from salicylaldehyde, and 1,2-diaminoethane, namely 2,2'-[l,2-ethane- diylbis(nitrilomethylidene)diphenato], salen, its 1,3-propane homologue, saltn, or from glyoxal and ortho-aminophenol, namely 2,2'-[(ethanediimine)diphenato], glyaph, as well as some of their analogues) has been studied by cyclic voltammetry. Anodic sweeps show one, or two, usually irreversible, oxidation peaks originated by the presence of Cu", since neither LH2 nor [ZnL] are oxidized under the same conditions. The oxidations of the 5- NO2-substituted derivatives are reversible at high scan rate (1 V s 1). The oxidation potentials of the complexes with substituents at the aromatic rings suggest that the electronic influence of such substituents are transmitted to the metal centre via the phenato, rather than the azomethine groups. The glyaph and saltn derivatives are oxidized at potentials higher than the corresponding salen complexes. Electronic and ESR spectroscopy studies on [CuL] samples, oxidized with a H202/HslO6 mixture, suggest that the initially formed [Cul"L] + rapidly decomposes to Cu H species, presumably through [Cu(II)-L']+ inter- mediates. Some poorly stable complexes of the formula [CuL]HSO4-2H20 have been isolated. Copyright © 1996 Elsevier Science Ltd

Keywords : oxidation ; Cu" complexes ; Schiff bases ; electrochemical.

The quadridentate Schiff bases of the salen type are able to form complexes with a wide variety of metal centres, ranging from soft, low valent to hard, high valent ions, viz. Co 1, C o 11 and Corn, 1'2 Rh ~, R h u

and Rhm, 3 Mo TM and even CrY, 5 a rather oxidizing species. Although [Cu(salen)] is very well known, there is only one report on the preparation of the corresponding Cu "1 derivative 6 and some descrip-

* Authors to whom correspondence should be addressed.

tions of the electrochemical oxidation of some [Cu(salen)] analogues. 7'8 Although the first Cum derivatives (tellurato and periodato complexes) were described long ago, 9-11 and in spite of the prob- able importance of such an ion in material science ~2 and in a variety of copper-assisted organic reactions, 13 well-defined Cum complexes are rare 14'15 and have been little studied. As a matter of fact the involvement of Cu l" in certain biological systems ~6 has recently been disproved ~7 and even the C u 111

formulation in the crystallographically char-

4461

4462 A. PASINI et al.

acterized ~8 [Cu(CF3)4]- and [CuCl(PhCO2)2(py)2] has been a matter of a recent debate. 19

We report here the results of a cyclic vol- tammetric investigation in the oxidation of cop- per(II) complexes, [CuHL] (L=quad r iden t a t e Schiff bases, see Scheme 1), as well as some results on their oxidation with common chemical oxidants.

EXPERIMENTAL

All chemicals were reagent grade. The ligands salenH2, glyaphH2 and their analogues were pre- pared by standard methodsfl °'2~ The complexes were obtained by refluxing an ethanolic solution of equimolar amounts of the ligand and Cu acetate. Quite often the ligands were not isolated as crys- talline material and copper acetate was added to the yellow ethanolic solution of the Schiff bases prepared in situ. Either methods gave compounds with good elemental analyses. Magnetic and spec- troscopic properties of the CuII complexes are iden- tical with those reported. 2~ 23

Electrochemistry

Cyclic voltammetry (CV) studies were performed in a conventional three-electrode cell, using a plati- num microsphere as the working electrode, a plati- num foil as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Potential values are referred to E~/2 of the internal standard ferrocene/ferrocenium couplefl 4 Experi-

ments were performed under nitrogen in 0.1 mol dm -3 Bu4NCIO4 acetonitrile solutions, purified according to standard methods. CV sweeps were recorded on a PAR model 273 galvanostat-poten- tiostat, controlled by an HP Vectra VL2 personal computer, through a dedicated software.

Controlled potential (CP) coulometry exper- iments were performed using a platinum gauze as the working electrode. Separation from the counter electrode (platinum gauze) was achieved by a salt bridge containing a 0.1 mol dm -3 Bu4NC104 ace- tonitrile solution. Potential values are referred to SCE.

Chemical oxidations

Acetonitrile suspensions of [CuL] (L = 5,7- Me2salen; 5-MeOsalen) were stirred with a two- fold excess of solid (NH4)2S208 for 24 h at room temperature. The filtered brown solutions were tre- ated with diethyl ether.

[Cu(5,7-Me2salen)]HSO4- 2H20. Analysis : Found : C, 46.4; H, 5.0; N, 5.3. C20H27N2CuO8S requires: C, 46.3 ; H, 5.2 ; N, 5.4%./~,fr (room temperature) = 2.2 BM. IR (cm ~, Nujol mull) : 1589 ( C = N ) ; 990, 1100, 1150 (HSO4, see ref. 25).

[Cu(5-MeOsalen)]HSO4- 2H20. Analysis : Found : C, 41.6; H, 4.4; N, 5.7. ClsHz3N2Ol0CuOl0S requires : C, 41.3 ; H, 4.4 ; N, 5.4%./~e~ (room tem- perature) = 2.4 BM. IR (cm -~, Nujol mull): 1600 ( C = N ) ; 995, 1095, 1150 (HSO4).

4 3 5

Q = CH2CH2 [Cu(salen)] [Cu(5-Clsalen)] [Cu(5-MeOsalen)] [Cu(4-MeOsalen)] [Cu(3-MeOsalen)! ICu(5-NO2salen)! [Cu(7-Mesalen)l ICu(5,7-Me2salen)l

Q = CH2CH2CH 2 [Cu(saltn)l [Cu(5-Clsalen)] [Cu(5-MeOsalen)l [Cn(5-NO2salen)] [Cu(5,7-Me2salen)]

[Cu(glyaph)] [Cu(4-Meglyaph)] [Cu(5-NO2glyaph)]

Q = CH(CH3)CH2 [Cu(salpn)l

Q = CH(CH3)CH(CH3) [Cu(salbn)]

Scheme 1.

Cu" complexes of quadridentate Schiff bases 4463

RESULTS AND D I S C U S S I O N

Cyclic vol tammetry data (acetonitrile solutions, scan rate 200 mV s -~) are reported in Table 1. Neither the free ligands nor the analogous zinc complexes show oxidation peaks in the range investigated.

The majority of the salen and saltn derivatives show two non-reversible anodic peaks. The profiles of the vol tammograms of the compounds with no substituent at position 5 of salicylaldehyde change at every cycle for ca the first 10 cycles, when the profiles become reproducible at every cycle and dis- play only one reversible oxidation peak [Fig. 1 (a)]. This behaviour has already been described 7 and shown to be due to the rapid polymerization of [Cu(salen)] at position 5 of salicylaldehyde orig- inated from the oxidation of the ligand by the initially formed Cu "I (see Scheme 2).

The vol tammograms of the 5-substituted analogues, which cannot polymerize through this mechanism, are reproducible at every cycle [Fig. 1 (b)]. Oxidation of some of these latter compounds is quas i reversible (see Table 1), with no dependence on the scan rate, except for [Cu(5-NOzsalen)] whose

oxidation becomes reversible at high scan rates (1 V s 1).

No pattern attributable to polymerization is shown by the glyaph complexes. Also for this series the oxidation of the 5-nitro substituted derivative is reversible at high (1 V s ~) scan rates.

Compounds with electronegative substituents display the higher oxidation potentials. For each series there is a linear correlation between the potential values of the first oxidation peaks and the ~p or ~,, constants 26 (Fig. 2), if p a r a and m e t a are referred to the phenolic oxygen atoms, rather than to the azomethine group. Thus, the electronic influence of the ligand is transferred to the metal centre via the phenato moiety. Methyl substituents on the diamine chelate ring have little effect (see Table 1).

Oxidation of the tn derivatives occurs at poten- tials slightly higher than those of the corresponding en complexes, presumably because the larger chelate ring of tn destabilize the smaller Cu ~" ion, but this difference is reduced by the presence of methoxy and methyl groups, which make oxidation easier. Surprisingly, the glyaph complexes, which possess three five-membered chelate rings, are oxi-

Table 1. Cyclic voltammetry data for [CuL] complexes"

X Q Ea., Ecatl Ean2 Ecat2 El~2 pol

H CH2CH 2 565 - - 795 - - 530 5-NO2 CHzCH 2 930 860 - - 5-C1 CH2CH2 710 640 800 5-MeO CH2CH2 335 170 700 550 4-MeO CH2CH 2 570 400 790 620 3-MeO CHzCH z 390 720 5,7-Me2 CH2CH2 375 325 745 7-Me CH2CH2 475 745 330 H CH(CH3)CH b 560 850 520 H CH(CH3)CH(CH3) c 580 870 530 H o-C6H4 650 870 630 H CH2CH2CH2 590 875 480 5-NO2 CH2CH2CH2 1000 900 5-MeO CH2CHzCH2 330 220 740 5 .7-Me2 CHzCH2CH2 370 620

L = glyaph analogues H 715 600 4-Me 610 530 5-NO2 1140 1020

L = salen and saltn analogues aAcetonitrile solutions. E values (mV) are referred to El/2 of the fer-

rocene/ferrocenium couple as internal standard. Scan rate 200 mV s-~. E..,, Ec.t2, E..2 and Ec~,2 for complexes that give rise to polymerization are those of the first cycle. E,/2 po. half-wave potential of the polymer, see text.

b The racemic diamine was usd. "The meso form of this diamine was used.

A. PASINI et al.

0.10

0.05

..._~ o.oo

-0.05

I I I

-0.5 0.0 0.5 1.0

60

1.5

b 45

0 l

-15

-0.5

I I I

0.0 0.5 1.0

30 <

.._~ 15

4464

E,V E,V Fig. 1. Voltammograms of: (a) the first and tenth cycles of [Cu(salen)]. The profiles of the second, third, etc. cycles are intermediate, those of the cycles following the tenth are superimposable to that of the tenth cycle. (b) [Cu(5-MeOsalen)]. The profiles of the voltammograms of this compound are

the same at every cycle.

/--5 /------N "cu'° o/ U"v O

Scheme 2.

dized at potentials higher than those of the cor- responding salen and saltn derivatives. The glyaph ligands are probably too soft for the hard Cum ion.

Controlled potential coulometry (CPC) exper- iments were performed only for [Cu(5-NO2salen)], whose oxidation was found quasi-reversible in the CV measurements. When acetonitrile solutions of these complexes were electrolysed at 1050 mV (ver- sus SCE), the charge versus time profiles showed an asymptotic trend towards the expected total charge. The electrolysed, yellow, solution had J-max 680 nm and showed the presence of Cu" by ESR spec- troscopy (a rather broad band at ca g = 2.1). Attempts to reduce such a solution by electrolysis at 700 mV (versus SCE) were unsuccessful (i.e. we did not observe any current).

C h e m i c a l o x i d a t i o n s

We have followed by electronic and ESR spec- troscopy the oxidation of various [CuL] in ace- tonitrile solutions with a mixture of H202 and H5IO6, in the ratio Cu : H202 : H5IO6 = 1 : 0.5 : 1, according to the hypothetical reaction :

[CuL] + 1/2H202 + H5IO6

= [ C u L ] + H 4 I O 6 + H z O .

Periodic acid was chosen because of the ability of its anions to stabilize Cun~. 1°'11 Separate exper- iments have shown that either H202 or H5IO6, used alone, produce the spectral changes described below, albeit at slower rates. Both [ZnL] and H2L are unaffected by these reagents.

Addition of the oxidizing mixture to 10 -3 mol dm 3 acetonitrile solutions of [CuL] quickly reduces the ESR signal of the original complex and the intensity of the d - d bands at ca 580 nm, which is replaced by a new band at ca 680 nm, similar to that observed in the electrolysed solution described above. Such a band decays in a couple of hours (at room temperature), while the band at 580 nm grows up again, together with a restoration of the intensity of the ESR signal of Cun and an increase of the absorbance in the near UV (ca 300 nm). Other oxidants, such as PbO2, Ce TM and Ag + gave similar results.

Although it has been reported that [Cu (salen)]CIO4 and related complexes can be prepared

Cun complexes of quadridentate Schiff bases 4465

> E

t~ UJ

1200

1000

800

600 -

400 -

4"NO2 A /

/ /

/ / . . O

/ / ..-" ~5-NO 2 o J *

/ U "v

4-Me A / '"

4-MeO //~ [Cu(Glyaph)]

/ 0 [Cu(Saltn)]

~ ' 5.MeO i I V [Cu(Salen)]

-0.5 0.0 0.5 1.0

(Y Fig. 2. Plots of Ean I v e r s u s ap or G, values for the salen, saltn and glyaph series, para and meta are

referred to the phenato moiety. For explanation see text.

by air, or H202 oxidation of [Cu(salen)], 6 we were unable to repeat these syntheses. Rather, we observed that oxidation of our compounds, under preparative conditions, with Ce w, H202, Ag ÷ and PbO2 led to substantial decomposition.* Only with (NH4)2S208 we obtained brown materials which analysed as [Cu(L)]HSO4'2H20 ( L = 5,7-Me2 salen ; 5-MeOsalen), see Experimental. Freshly pre- pared samples had 2m~x 690 nm (reflectance spectra) and #eer at room temperature in the range 2.2-2.4 BM, a value too high for Cu n, but also too low for an octahedral d 8 system. Indeed ESR spectroscopy

* With (NH3)2Ce(NO3)6 we observed extensive sub- stitution of Cu. Ce TM Schiff base complexes are known 27, as well as some nitration of the ligands (IR evidence). The nitrating properties of cerium nitrate have been reported. 28

5" A similar band at 680 nm, observed in Cu H and Zn" semiquinone complexes, has been assigned to the semiquinone moiety, z9 In our case no such band developed upon treatment of [ZnL] with various oxi- dizing reagents.

showed the presence of some Cu n in these samples. The magnetic moments decreased with aging and became 1.8 BM after about 2 days at room tem- perature. The aged materials had irreproducible analyses. Such a reduction to Cu" is fast (few minutes) in acetonitrile solution, especially if wet, even at - 15°C.

In conclusion, since the analogous [ZnL] are not oxidized under our conditions, oxidation of the copper complexes must occur initially at the metal centre. However, these ligands prove to be unable to stabilize trivalent copper and [CuII1L] + decomposes rapidly, especially in solution. A possible decompo- sition product, which forms almost immediately, could be [Cu n - L'] +. This could account for the band at 680 nm (which is likely to be a d - d band of a Cu ll species; (Cu I11 compounds usuallyr'14'] s have strong absorptions around 500 nmt ) and for the substantial lowering of the ESR signal of Cu", since such radical ca t ion-Cu H complexes should be ESR silent? ° These species are rather reactive. They can be oxidized further (the second oxidation peak observed in the vol tammetry experiments), poly- merize, 7 oxidize suitable substrates ~3 or give rise to

4466 A. PASINI et al.

Cu H derivatives and degradat ion products. These reactions are fast enough that the ESR signal o f Cu 1~ is present even in freshly oxidized samples. As a matter o f fact the CulI--fulU-fulIR" route o f decomposi t ion o f these ligands seems to be a com- mon pa thway of Cun-promoted oxidation reactions.~3'16'31

Acknowledgements--We thank the Ministry of Uni- versity and Scientific and Technological Research (Rome) for financial help and Dr A. Pozzi, the University of Milan, for recording the ESR spectra.

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Cu n complexes of quadridentate Schiff bases 4467

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