Transition Met. Chem., 18, 567-569 (1993)

Oxidation with dinuclear Cu H complexes

567

Catalytic properties of dinuclear copper(II) complexes in the oxidation of two electron donors by dioxygen B. Srinivas, P. S. Prakash and P. S. Zaeharias*

School of Chemistry, University of Hyderabad, Hyderabad 500 134, India Summary Dinuclear copper(II) complexes of differing magnetic and redox properties derived from various dinucleating ligands were investigated for their catalytic activity in the oxidation of 3,5-di-t-butylcatechol (3,5-DTBC) and ascorbic acid by oxygen. Poor activity was exhibited by di-#-hydroxocopper(II) complexes of 2,2'-bypyridyl, 1,10-phenanthroline and N,N,N',N'-tetramethylethylenediamine(TMEDA). Dicopper(II) complexes of Schiff base ligands, obtained from 2,6-diformyl-4-methylphenol and diamines, are relatively less active compared with ligands derived from monoamines. The difference is explained on the basis of redox behaviour. Introduction Dinuclear transition metal complexes derived from dinucleating ligands attract attention because of their interesting magnetic, catalytic and electron transfer properties. They also serve as model compounds for oxygen-binding or oxygen-activating copper proteins (1-7). Identification and characterisation of suitable coordination complexes and studies of copper complex/oxygen/substrate reactions should provide a reasonable basis for determining biological structures. These studies will also help in determining the ability of such complexes to influence reactivity patterns observed in metalloprotein chemistry, for the future exploitation of copper/oxygens systems as practical dioxygen carriers or as reagents for selective and catalytic oxidative transformations. Several magnetically interacting dinuclear copper(II) complexes are catalytically active in the oxidation of substrates such as 3,5-di-tbutylcatechol (3,5-DTBC) and ascorbic acid by oxygen(8'9). Although much progress in this area has been made by studying the kinetics of substrates oxidations and reduction of oxygen by laccase (1~ and ceruloplasmin ~11)themselves, the search continues for model systems which imitate the special structural and catalytic properties of these enzymes. This paper describes our investigations of the catalytic activity of dicopper(II) complexes of differing magnetic and redox properties in the oxidation of two electron donors by oxygen, and the factors influencing the effective catalytic conversions.

2H20. Di-#-hydroxo-bis(N,N,N',N'-tetramethylethylenediamine)dicopper(II) dinitrate trihydrate (3) was prepared by a reported procedure (13) The dicopper(II) complexes (4)-(6) were obtained from 2,6-diformyl-4-methylphenol and the respective monoamines, i.e.'l-aminoethane for (4), 1-amino-propane for (5) and 3-amino-l-propanol for (6), according to known general procedure ~14). Complex (7) was prepared by a procedure previously reported starting from 2,6-diformyl-4-methylphenol and PhNH2(15) The perchlorate salts of the dicopper(II) complexes of Schiff bases obtained from 2,6-diformyl-4-methylphenol and diamines 1,2-di-aminoethane (8) and 1,3-diaminopropane (9) were prepared by a slight modification of the reported procedure (a6).

Physical measurements Electronic absorption spectra were recorded on a Schimadzu UV-200S double beam spectrophotometer. C.v. experiments were carried out using a three electrode cell assembly with a PAR 175 Universal Programmer and PAR RE 0074 X-Y recorder. The cell consisted of a E410 hanging mercury drop electrode (h.m.d.e.) as the working electrode, a platinum wire auxiliary electrode, and a s.c.e. Et4NC104 (TEAP) (0.1 M) was used as the supporting electrolyte. Catalysis experiments were monitored on a PerkinElmer u.v.-vis, spectrophotometer with a Lambda-3B data station. Experiments were carried out using dichloromethane, MeOH and a M e O H - H 2 0 mixture (1:1) as solvents. In a generalised procedure, a MeOH solution of 3,5DTBC (0.03 mM) and the copper(II) complex (0.006 mM) in a 50ml standard flask was kept under nitrogen. 3,5-DTBQ has a characteristic absorption at 400nm, which was taken as a measure of the quinone formation [e(mol- 1 dm 3 cm- 1) = 1900 in MeOH] (17). Absorbance was measured at 30 min intervals immediately after exposing the solution to air. A similar procedure was adopted for reactions with ascorbic acid, the d - d band being monitored. Results and discussion

Experimental

Electronic spectral and magnetic data Preparation of the complexes Di-p-hydroxo-bis(bypyridyl)dicopper(II) sulphate pentahydrate (1)" 2) was prepared by reacting [Cu(bipy)SO4]" 2H20 with NaOH. The resulting blue crystals were recrystallised from H20. Di-lt-hydroxo-bis-(1,10-phenanthroline)dicopper(II) sulphate pentahydrate (2) (12) was obtained by a similar procedure using [Cu(phen)SO4]. * Author to whom all correspondenceshould be directed. 0340-4285 9 1993Chapman& Hall

The structures of the complexes are given in Figure 1. Complexes (I)-(3) exhibit an absorption band in the 630-620 nm region and have a #eff 1.90 #B per copper ion. Complexes (4)-(7) exhibit a d - d band at ca. 710 nm with #elf values in the 0.80-0.13/~a range per copper ion. Complexes (8) and (9) exhibit the d - d transition at 560 and 590 nm, respectively; #eff 0.67 #a. Magnetic and spectral data are given in Table 1. From the 2max and magnetic moments it is clear that the compounds are of varying geometry and antiferromagnetic interactions and exhibit

Srinivas et al.

568

Transition Met. Chem., 18, 567-569 (1993)

[Chelate C u " " O H ~ c u Chelate]2+ ~OH / (1) Chelate = (2) Chelate = (3) Chelate = H

( "co-~ / "l l

pounds in various solvents, i.e. MeCN, D M F and MeOH in the +0.2 to - 1 . 8 V versus s.c.e, range. Complexes (1)-(3) have no well defined redox peaks in the potential range investigated. Complexes (4)-(7) give a quasireversible redox peak at ca. - 0.60 V with AEp 150 mV at a scan rate of 100mVs-1. The behaviour of complexes (1)-(3) which do not show any redox peaks may be due to the inability of the ligand systems to stabilise the copper(II)-copper(I) intermediate in the event of a one electron reduction or the copper(I)copper(l) species if it is a two electron reduction. Complexes (8) and (9) are unstable in the reduced form, as they each exhibit ill-defined one electron reduction couples. The quasireversible redox couple exhibited by the complexes (4)-(7) reveals that the intermediate species is more stable for them. The electrochemical data reveal the stability of the reduced complex species to lie in the order:

\N / "0 /

(I)-(3) < (8), (9) < (4)-(7).

2,2'-bipyridyl 1,10-phenanthrolene N,N,N',N'-tetramethyl ethylenediamine (TMEDA)

J

C l \ / 0 . /CI R~N2CU...o/CU..N/R

V V

(4) R=Et (5) R = n-Pr (6) R = (CH2)aOH

H2 H

CI ./C/ CI Ph~Cu. ]Cu~/Ph N

0

N

H I

H2 --12+ N

"N--

Catalytic activity of the dicopper( II ) complexes 8

7 H~

k~N/

H

H~

~-0 /

",N._J

7 2+

V 9

Figure 1. Structures of the complexes.

(1)-(3) < (8), (9) < (4)-(7).

the following order of increasing antiferromagnetic coupling: (I)-(3) < (4)-(7) < (8), (9). In several instances, geometry and magnetic interactions between the two copper(II) centres influence their catalytic activity. The planar geometry of the system and the strong metal-metal interaction are suggested to enhance the catalytic activitytg).

Electrochemical data Electrochemical data for the complexes (8) and (9) have been reported t16). We have investigated the other comTable 1. Magnetic, spectral and electrochemical data for the dinuclear copper(II) complexes. Complex

2m,x #eff Redox potentiaP (e/din3 mol-1 cm- 1), ( B M ) [E1/2(V)]

(I) (2) (3) (4) (5) (6) (7) (8) (9)

620(103) 630(95) 630(98) 724(310) 690(292) 700(280) 737(330) 598(125) 590(137)

1.88 1.79 1.90 0.82 0.96 1.21 1.30 0.67 0.70

All the complexes were examined for catalytic activity in the oxidation of 3,5-DTBC by oxygen. In this process, DTBC is converted to the quinone form, DTBQ, production being monitored by measurement of the 400nm (e = 1900mo1-1 dm 3 cm -a) absorption band characteristic of 3,5-DTBQ. The di-#-hydroxo-dicopper(II) complexes (I)-(3) exhibit poor catalytic activity compared to the other complexes investigated. Data of the catalysis experiments are given in Table 2. Figure 2 gives the growth of 3,5-DTBQ of all the complexes over a period of time. It can be seen from the data that the catalytic activity of the complexes vary in the following order:

c --0.57 --0.60 --0.56 --0.58 -0.650 -0.65 d

aln MeCN; bexperiments were carried out using a h.m.d.e, working electrode, platinum auxiliaryelectrodeand s.c.e, referenceelectrodein DMF using tetraethylammoniumperchlorate supporting electrolyte; scan rate 100mV s- x;~peaksabsent; donlyreductionpeaks observed.

The difference in catalytic activity can be rationalised in terms of the structural, magnetic or redox characteristics of the complexes (9). Though the complete mechanism of the catalytic process is not yet established, the catalytic process is suggested to involve a weak 1 91 adduct between the complex and the substrate, which requires the metalmetal distance to be ca. 3 A. This adduct reacts with dioxygen to form the starting dicopper(II) complex and the oxidised substrate. Antiferromagnetically-coupled systems are catalytically more active in the DTBC to DTBQ oxidation. All low catalytic activity of complexes (1)-(3) can be ascribed to the absence of antiferromagnetic interaction. Complexes (8) and (9), even though exhibiting strong antiferromagnetic coupling, are catalytically less Table 2. Catalytic activity of the dicopper(II) complexes for the oxidation of 3,5-DTBC and ascorbic acid. Complex Complex: Solvent 3,5-DTBC

(l) (2) (3) (4) (5)

(6) (7) (8) (9)

1:10 1:10 1:10 1:20 1: 20 1:20 1:20 1:10 1:10

D T B Q a 2max for

(70)

MeOH/H20 (1:1) 38 MeOH/H20(I: 1) 47 MeOH/H20 (1:1) 30 DCM 74 DCM 78 MeOH 80 MeOH 100 MeOH 62 MeOH 68

aAfter2h; bin MeOH; ~ DCM.

ascorbate adduct b 410 420 420 495c 490c 490 500 490 495

Transition Met. Chem., 18, 567-569 (1993)

7

100 -

80

o

Oxidation with dinuclear Cu H complexes

4

4()

3

20

0

0.5

1.0

1.5

2.0

2.5

Time (h)

Figure2. Time-dependent growth of 3,5-DTBQ for the complexes in MeOH.

0.8 -

""~ 4

"'11

ke/

" 2',i ',1,

u \

o

Acknowledgements

', I

Financial assistance from UGC, New Delhi, is gratefully acknowledged by two of us (B.S. and P.S.P.).

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9

0.4

P/

References

",..~,r _

"-

0.2~

0

I

I

700

600

I

500

which makes the solution an intense purple colour. Similar spectral changes occurred for the complexes ( 6 ) - ( 9 ) upon reaction with ascorbic acid. However, the new band for these systems appear at a higher value, 510 nm. When the complex solutions are exposed to oxygen, the original d - d band of the complexes reappears with the simultaneous disappearence of the 420-510 nm band. These changes for a representative compound are shown in Figure 3 and the relavent data are given in Table 2. The new band results from the formation of the adduct, [ C u 2 L X ] 2 + (ascorbate), where L represents the dinucleating ligand. The stability of the new band for the complexes is in the order: ( 1 ) - ( 3 ) < (8), (9) < (6), (7). This result is in agreement with the increasing order of the catalytic activity of the complexes. We conclude that the complexes ( 1 ) - ( 9 ) show the same order of activity for the oxidation of both DTBC as well as ascorbic acid by oxygen. The dicopper(II) complex which catalyses the oxidation of the substrates 3,5-DTBC and ascorbic acid involves a reduced dicopper intermediate species. The stability of the adduct formed from the complex and substrates reduces the catalytic activity of the complexes.

,

k",',|

0.6-

569

I

400

Wavelength (nm)

Figure3. Spectral changes of complexes (1), (4) and (8). ( ) d-d bands of complexes alone; (. . . . ) d-d bands of complexes after the addition of ascorbic acid under nitrogen. active and undergo only electrochemical reduction since the reduced intermediate species are unstable; this factor affects their catalytic ability. Electrochemical data suggest that the complexes which are reasonably stable after reduction can act as good catalysts. Reaction with ascorbic acid Since electrochemical methods did not produce stable reduced species for complexes ( 1 ) - ( 3 ) , attempts were made to reduce the dicopper(II) complexes chemically. These reductions of the dicopper complexes were carried out with ascorbic acid under a dinitrogen atmosphere. The reduction process was monitored by measuring the changes in absorbance of the d - d bands of the complexes. The 620 nm band of complexes ( 1 ) - ( 3 ) slowly disappears, with the simultaneous appearence of a new strong band at 420 nm on the addition of an excess of ascorbic acid,

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