The electrochemical synthesis of neutral complexes with asymmetric N2SO tetradentate schiff-base ligands

Polyhedron Vol. I I, No. 21, pp. 273%2745,1992 Printed in Great Britain

0

Q277-S387/92 SS.oO+.lUI 1992 Pergamon Press Ltd

THE ELECTROCHEMICAL SYNTHESIS OF NEUTRAL COMPLEXES WITH ASYMMETRIC N2S0 TETRADENTATE SCHIFF-BASE LIGANDS RLJFINA BASTIDA” Departamento

de Quimica InorgAnica, Universidad de Santiago, 15706 Santiago de Compostela, Spain and ANDRES DE BLAS

Departamento

de Quimica Pura y Aplicada, Universidad de Vigo, Vigo, Spain and DAVID E. FENTON

Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, U.K. and TERESA RODRIGUEZ Departamento

de Quimica Fundamental e Industrial, Facultad de Ciencias, Universidad de La Corufia, La Coruiia, Spain (Received 23 June 1992 ; accepted 17 July 1992)

Abstract-The electrochemical synthesis and physicochemical properties of neutral copper (II), cadmium(II), nickel(II), zinc(I1) and cobalt(I1) complexes of asymmetric N2S0 Schiff bases derived from 2-[(2-aminoethyl)thiomethyl]benzimidazole and substituted acetophenones and acetylacetones are reported. The ligands act as dianionic tetradentate ligands through their four donor atoms. The stereochemistry of the complexes is discussed on the basis of spectroscopic and magnetic data.

The behaviour of metalloproteins has attracted the attention of many researchers in recent years.’ Understanding of the active sites that largely control their properties has been pursued both by isolating and investigating natural metalloenzymes, and by designing and synthesizing complexes that can be considered as models of metalloproteins. It is known that the active sites of copper proteins for which X-ray crystal structures are available contain copper(I1) bound to two or more histidine imid-

*Author to whom correspondence should be addressed.

azole groups,’ and it may be that the distorted geometry around the metal is of great importance in that it controls the redox properties of the protein. If so, imidazole-containing Schiff-base ligands could act as versatile models of environments of metals in proteins. Some ligands of this kind have already been studied in our laboratory. 3-s In the present paper we report the synthesis of new asymmetrical tetradentate Schiff-base ligands containing benzimidazole and thioether groups (Fig. l), and the electrochemical preparation of their electrically neutral complexes [M(L)] =nH,O with copper, nickel, zinc, cadmium and cobalt.

2739

2740

R. BASTIDA

et al.

Ry-.--J S

'N

R

R

0\/

Fig. 1

EXPERIMENTAL Elemental analyses were carried out on a PerkinElmer elemental analyser 240-B. IR spectra were recorded as Nujol mulls or KBr discs on a PerkinElmer 180 spectrophotometer. Diffuse reflectance spectra of solids were obtained using a Pye-Unicam SP 700 spectrometer. Proton NMR spectra were recorded on a Bruker WH 250 FT spectrometer and mass spectra were recorded on a Kratos MSSOTC. Magnetic moments were determined on a SQUID MPMS QUANTUM DESIGN. All reagents and solvents were of commercial reagent grade. 2-[(2-Aminoethyl)thiomethyl]benzimidazole dihydrobromide (Hatb* 2H20) was prepared by the method of Durant et ~1.~ from 2-hydroxymethylbenzimidazole and cysteamine hydrochloride. The Schiff bases were prepared by the method described in ref. 3 and dried in vucuo prior to characterization by ‘H NMR and IR spectroscopy and use in complexation reactions. The electrochemical method used for synthesis of the metal complexes was similar to that described by Habeeb et al. 7 The cell was a 100 cm3 tall form beaker fitted with a rubber stopper through which the electrical leads entered the cell. The foil anode was suspended from a plati-

num wire. The ligand was dissolved in acetonitrile and tetramethylammonium perchlorate (ca 10 mg) was added as the supporting electrolyte. Electrolysis was carried out at room temperature and pressure with magnetic stirring. The insoluble products were visible within a few minutes, and upon completion of the reaction were collected, washed with the solvent and dried in VUCUO.Experimental conditions are given in Tables 1 and 2 and the elemental analysis in Tables 3 and 4. The cell used can be summarized as : Pt( -)/solvent

RESULTS

+ H,L/M( +)

AND DISCUSSION

2-Hydroxyucetophenone-derived

comjlexes

The Schiff-base ligands were synthesized by the reaction of 2-[(2-aminoethyl)thiomethyl]benzimidazole (Hatb) with a stoichiometric quantity of the appropriate hydroxyacetophenone in absolute ethanol. The ligands were obtained as yellow powders and characterized by IR and ‘H NMR spectroscopy. apatbH, : ‘H NMR spectral data : 6 2.29(s),

Table 1. Experimental conditions for the electrochemical synthesis of acetophenone-derived L* = 5-Me-apatb] Initial potential (V

complexes [L’ = apatb ;

Current (mA)

Time (s)

Metal used (mg)

Er (mol F- ‘)

Complex

Amount of ligand (g)

[Zn(L’)]-3.5H,O*0.25CH,CN [Cd(L’)]*2H,O INi( - 3.5H,O

0.2402 0.2037 0.1887

16.3 14.0 10.0

9.5 10.0 10.0

13,800 10,200 8400

39.4 63.2 19.9

0.44 0.53 0.40

[Zn(L*)] * 3H,O [Cd(L’)] - 1.5H20 ~i(L2)]*3H20*0.25CH,CN [Co(L*)]-2H,0.0.25CH3CN

0.2353 0.2391 0.1941 0.1872

16.5 14.0 16.0 13.0

10.0 10.0 10.0 10.0

12,000 12,300 10,200 9600

37.5 73.8 31.9 29.9

0.45 0.51 0.51 0.51

‘Plus NMe4C104 (ca 10 mg).

2741

Electrochemical synthesis of neutral complexes Table 2. Experimental

conditions

for the electrochemical synthesis of acethylacetone-derived acacatb; L4 = tfacacatb] Amount of ligand (g)

Complex

Initial potential (V)

Current (nw

complexes [L3 =

Time (s)

Metal used (mg)

& (mol F- ‘)

[Cu(L3)]. 1.5H20 [Zn(L3)]*2.5H20 [Cd(L3)] * 1.5Hz0 [Ni(L3)] - 2.25H20

0.1757 0.1669 0.1686 0.1906

9.0 10.0 12.5 22.3

10.0 10.0 10.0 10.0

8520 9600 9900 12,480

49.0 33.0 56.1 37.7

0.87 0.51 0.49 0.49

[Zn(L4)] * Hz0 [Cd(L4)]. 1.5H20 [Ni(L4)] * H,O - 0.5CH3CN [Co(L4)].2Hz0*0.25CH3CN

0.2540 0.1928 0.2106 0.2307

17.0 12.0 17.0 14.0

10.0 10.0 10.0 10.0

13,500 9900 10,980 12,000

45.6 59.3 33.6 36.2

0.50

“Plus NMe4C104 (cn 10 mg).

Table 3. Analytical data for the acetophenone-derived

complexes“

M

Ligand

n

%C

%N

%H

Zn

apatb 5-Me-apatb apatb 5-Me-apatb apatb 5-Me-apatb 5-Me-apatb

h

3 2 1.5 3.5 ’ d

47.2(48.1) 49.5(49.9) 45.0(45.8) 48.5(47.9) 47.9(48.6) 50.3(50.9) 52.6(52.9)

10.4(9.9) 9.5(9.2) 9.5(8.9) 8.8(8.8) 10.1(9.4) 10.5(9.9) 10.3(10.3)

4.3(5.4) 5.3(5.5) 4.0(4.4) 4.9(4.6) 4.9(5.4) 4.9(5.6) 4.5(5.4)

Cd Ni co

0Calculated values in parentheses. h3.5H20.0.25CH3CN. ‘3Hz0*0.25CH3CN. d2Hz0*0.25CH3CN.

Table 4. Analytical data for the acethylacetone-derived M

Ligand

cu

acacatb tfacacatb acacatb tfacacatb acacatb tfacacatb acacatb tfacacatb tfacacatb

Zn Cd Ni co

complexes”

n

%C

%N

%H

1.5 1.5 2.5 1 1.5 1.5 2.25 b

47.8(47.7) 41.3(41.7) 45.3(45.3) 42.6(42.4) 42.8(42.2) 36.8(37.5) 46.1(46.7) 43.9(43.8) 42.5(41.7)

10.7(11.1) 10.0(9.7) 10.4(10.6) 10.5(9.9) 9.8(9.8) 9.0(8.7) 10.9(10.8) ll.l(l1.2) 10.4(10.2)

4.9(5.3) 3.6(3.9) 5.1(5.5) 4.3(3.8) 5.0(4.7) 3.4(3.5) 5.1(5.6) 4.0(4.0) 3.3(4.2)

‘Calculated values in parentheses. bH,0.0.5CH3CN. ‘2H,0*0.25CH3CN.

0.51 0.50 0.49

2742

R. BASTIDA

2.91 (t, J= 6.6 Hz), 3.74 (t, J= 6.6 Hz), 3.98(s), 6.76(m), 7.13(m), 7.26(m), 7.48(m), 7.62(m), 11.66, 12.38 ppm in d,-DMSO solution. 5-Me-apatbH, : ‘H NMR spectral data: 6 2.22(s), 2.27(s), 2.90 (t, J = 6.6 Hz), 3.72 (t, J = 6.6 Hz), 3.97(s), 6.67(d), 7.06(m), 7.13(m), 7.42(m), 7.48(m), 12.43 ppm in d,-DMSO solution. The neutral metal complexes, [M(R-apatb)], were prepared electrochemically (see Experimental and Table 1). At thesame time as the metal was oxidized to M”, the ligand H2L was deprotonated to L2-, the protons being reduced to hydrogen which was released at the cathode. The electrochemical efficiency, E,, defined as the quantity of metal dissolved per Faraday of charge, was always close to 0.5 mol F- ’ as expected for nickel, zinc, cadmium and cobalt complexes5 and the process involved can be represented by reactions (1) and (2) : Cathode : H2L+ 2e- -

L2-- +H,

Anode : L2- + M ------+[ML]+Hp

(1) (2)

Microanalysis showed that the copper(I1) products contained metallic copper impurities. All complexes were characterized by elemental analysis (Table 3) and by mass and IR spectroscopy, and copper, nickel and cobalt complexes were also studied by UV-vis spectroscopy in the solid state (Table 5). The IR spectra of the ligands show, as well as bands ascribable to aromatic and aliphatic C-H vibrations, a broad background signal (often extending down to 2500 cm- ‘) which is indicative of strong hydrogen-bonding through both the O-H and N-H groups of the ligands.3 Both ligands have a strong imine band at ca 1610 cm- ’ which shifts to lower frequencies and becomes broader in the complexes. Lattice water gives rise

Table 5. Ligand-field

et al.

to broad bands around 3400 err- ’ in the spectra of the complexes. The FAB mass spectra of the complexes exhibit the molecular ion [ML]+. As in the corresponding series of salicylic complexes,4 only the nickel derivatives gave significant peaks for the dimer and (less intensely) the trimer. The electronic spectra of [Ni(apatb)] and mi(5Me-apatb)] are practically identical, suggesting that their coordination geometries are very similar. Unlike the corresponding salicylic derivatives4 which show a single band in the 9000-10,000 cm- ’ region, the new compounds have two bands in this region : one at 7200-7300 cm- ’ and other at 10,30010,400 cm-‘. A third band can be observed in the visible region at 16,950 cm- ‘. The first two probably result from the splitting of the 3T2gc 1A2g transition. Both [Ni(R-apatb)] molecules thus appear to have octahedral geometries, although in view of the wide splitting of v,, the coordination octahedra must be very much more distorted than in the corresponding salicylic complexes.’ The only cobalt(I1) derivative isolated with these ligands, [Co(S-Me-apatb)] - 2H20 * 0.25CH,CN, has a peff of 4.64 B.M. at room temperature. The measurement of magnetic susceptibility at various temperatures yielded a value of 8 = 51.85 K. Its electronic spectrum (Table 5) suggests tetrahedral coordination, with a band at 6600 cm- ’ attributable to the 4T,(F) c 4A2 transition. The v3 band (4T,(P) +- 4A2)at ca 17,000 cm-’ is very broad and slightly split : for a tetra-coordinate cobalt species, a band width of 1500-2500 cm- ’ is too large to be entirely the result of spin-orbital coupling (which nevertheless contributes),’ and low temperature” and solid-state results suggest that it may be due to the low symmetry of the field and vibrational structures.

maxima for the nickel(II),

cobalt(I1)

Complex

complexes”

A,,, (cm- ‘)

[Cu(acacatb)] * 1.5H,O [Cu(tfacacatb)] * 1SH,O

15,40O(br) 15,800 7200; 10,300; 16,950 7300; 10,400; 16,950 9600; 17,100 7400 ; 11,250 ; 16,000 ; 21,20O(sh); 24,100

[Ni(apatb)]* 3.5H,O [Ni(S-Me-apatb)] - 3HZ0 - 0.25CH,CN [Ni(acacatb)] - 2.25H,O [Ni(tfacacatb)] - H,O - 0.5CH,CN [Co(S-Me-apatb)].2H,0.0.25CHjCN [Co(tfacacatb)]*2H,O*0.25CH,CN a Diffuse reflectance

and copper(I1)

spectra ; (sh) shoulder,

6600; 16,000; 18,000 7500 ; 16,600 ; 18,5OO(sh) (br) broad.

Electrochemical synthesis of neutral complexes

2743

Fig. 2.

Acetylacetone-derived complexes The Schiff-base ligands were prepared in the same way as those derived from acetophenone. Both were recovered as white powders and were characterized by IR and ‘H NMR spectroscopy. acacatbH* : ‘H NMR spectral data: 6 1.83(s), 1.88(s), 2.70 (t, J= 6,7 Hz), 3.43(m), 3.94(s), 4.91(s), 7.13(m), 7.52(m) and 10.68(t) ppm in d,-DMSO solution. tfacacatbHz : ‘H NMR spectral data : 6 2.12(s), 2.81 (t, J= 6.8 Hz), 3.61(m), 3.98(s), 5.33(s), 7.15(m), 7.49(dd), 11.00(t) and 12.43(s) ppm in d,-DMSO solution. As in the case of B-diketones and their metallic chelates, the most useful spectral region for determination of the structures of these ligands is 1700-1500 cm-‘, where the bands v(C=O), V(C=Qhylene, v(C-N) and &C-H) lie. The IR spectra of acacatbH, and tfacacatbH* do not show any band close to 1700 cu- ‘, so we can exclude that the ligand could be formulated as C in Fig. 2. Further interpretation is complicated by bands due to the benzimidazole moiety, but structure B is suggested by the position of the highest of the three acacatbH, bands [1600(s), 1560(s) and 1520(w) cm-‘], which is close to the position of v(C=O) acetylacetone (1613 cm- ‘) ; the shift of ca 80 cm- l from its normal position at higher wavenumbers is attributed to intramolecular hydrogen-bonding. ’ I The ‘H NMR spectra suggest that in d,-DMSO it is also the “keto” form B which is present, since the triplet appearing at 6 10.68 ppm in acacatbHz and 6 11.OOppm in tfacacatbH, can be attributed to non-aromatic NH protons with a signal split by coupling to the protons of the neighbouring CH2 group (which coherently presents as a multiplet due

to coupling with both the NH proton and the protons of the other ethylenic group). These shifts and multiplicities are also in keeping with there being intramolecular hydrogen bonding between the nonaromatic NH proton and the oxygen atom. Neutral complexes were synthesized electrochemically with all the metals used including copper (Table 2). As expected,’ the electrochemical efficiency was close to 0.5 mol F- ’ for nickel, cobalt, zinc and cadmium, and close to 1 mol F- ’ for copper. The process involved when Ef is close to 0.5 mol F- ’ can be represented by reactions (1) and (2), while if Ef is close to 1 mol F- ’ the synthesis involves steps (3) and (4) followed by the oxidation reaction (5) : Cathode : H,L+eAnode: HL-+M

PWWI -

-

HL- + 1/2Hz [M(HL)] + e-

[ML] + 1/2H,.

(3) (4) (5)

However, it was not possible to prepare [Cu (tfacacatb)]. 1.5Hz0 by this route; it was synthesized by refluxing tfcacatbH, with [Cu(AcO),] 2H20 in absolute ethanol, with magnetic stirring. All complexes were characterized by elemental analysis (Table 4) and by mass and IR spectroscopy. Copper, nickel and cobalt compounds were also studied by diffuse reflectance (Table 5). The ligand bands of the IR spectra of the complexes differ in both number and position from those of the free ligands in the 162&l 500 cm-’ region. The shift to higher wavenumbers suggests that, in contrast to the intramolecularly hydrogenbonded keto form deduced for the free ligands, the non-aromatic C-N bond of the complexes has

R. BASTIDA et al.

2744

‘UK)

Fig. 3. Plots of magnetic susceptibility, xH (cm’ mol-‘), and its reciprocal, l/xu (mol cmm3), for [Ni(acacatb)] - 2.25H 20.

acquired a degree of double-bondedness and lost its proton, the resulting negative charge being delocalized throughout the acetylacetone fragment. The spectra likewise suggest deprotonation of the benzimidazole moiety. Figure 3 shows that the temperature-dependence of the reciprocal of the magnetic susceptibility of [Ni(acacatb)] - 2.25H20 complies with the CurieWeiss law, with 8 = 10.23 K. The fact that the magnetic moment of this complex was independent of temperature (cL,~= 3.05 B.M. at 300 K, peff = 3.08 B.M. at 60 K ; Fig. 4) suggests octahedral coordination to the metal. ” This conclusion is supported by the mass and UV-vis diffuse reflectance data : the two electronic bands at 9700 and 17,100 cm- ’ may be attributed to 3T, c 3A2g and ‘T,, c 3A,, respectively [the transition to 3T1,JP)

is masked by the charge transfer and transition bands of the ligand itself]; while the FAB mass spectrum in nitrobenzyl alcohol exhibits a molecular ion peak at 364 corresponding to [Ni(acacatb) (H,O)]+. It may accordingly be concluded that in the nickel complex of acacatbH2 the metal ion is coordinated octahedrally or pseudo-octahedrally to the four donor atoms of the deprotonated ligand and the oxygen atoms of two water molecules, and should therefore be represented as [Ni(acacatb) (H*O)J * 0.25H20. The mass spectrum of the vivid green complex [Ni(tfacacatb)] * HZ0 - CH3CN shows the molecular ion [Ni(tfacacatb)]+ at 400. Its solid-state electronic spectrum, with five bands (Table 5) differs considerably from that of [Ni(acacatb)(H20)J 0.25H20. This spectrum can be interpreted as

3.5 r 3.0 -

•~Dm8mu

25-

;

20-

2

1.51.00.5 -

0

I

I 50

I

100

I 150

I

m

I

I

250

300

I 330

‘UK)

Fig. 4. Temperature-dependence of the magnetic moment, p (in B.M.), of [Ni(acactab)] - 2.25H20.

Electrochemical synthesis of neutral complexes due to high-spin bipyramidal trigonal coordination, since according to Ciampolini, I3 the accidental degeneration of the 3A,’ and 3A2” states when such systems have perfect D3h symmetry leads to there normally being only five spin-permitted transitions in the spectrum. In the present case, the 7400 cm- ’ band can be attributed to 3En c 3E’, the 11,250 cm-’ band to 3AIn, 3A2” c 3E’, the 16,000 cm-’ band to 3A2’ c 3E’, the 21,200 cm-’ band to 3E”(P) +- ‘E’ and the 24,100 cm- ’ band to 3A,‘(P) +- 3E’; the second of these bands exhibits slight splitting with peaks at 10,800 and 11,950 cm-’ , probably reflecting non-ideal symmetry. The above data suggest that in the solid state this complex is an N,O,S-coordinated dimer, in which the keto oxygen acts as a bridge between the two momomers. Furthermore, its insolubility suggests that these dimers are linked via the basic nitrogen of the benzimidazolate moiety. The diffuse reflectance spectrum of the cobalt [Co(tfacacatb)] - 2H20 - 0.25CH3CN complex (Table 5) is typical of pseudo-tetrahedral species. As in the acetophenone derivative discussed above, the band v3 (“T,(P) c 4A2) is very broad and slightly split. The magnetic moment at room temperature, 4.15 B.M., is outside the usual range (4.40-4.60 B.M.), but to judge by the magnetic moments of certain imidazolates’4,‘5 does not rule out pseudotetrahedral symmetry. The FAB mass spectra of the copper complexes show the molecular ion [ML]+ with 100% intensity. Their electronic spectra are not particularly helpful, though the broad band around 15,500 cm-’ rules out ideal square planar geometry, and the location of this d-d transition is within the accepted range for pseudo-tetrahedral cupric compounds.‘6 Intermolecular links via benzimidazolato bridges in the solid state are suggested by the insolubility of these complexes. Unfortunately, their insolubility is also responsible for their EPR spectra being unhelpful, since although they appear to confirm the existence of intermolecular interactions, no precise value of g can be determined.

2745

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