Transition Metal Chemistry, 19 (1994) 265-269 Pyridoxal thiosemicarbazone complexes 265

Unusual redox and chemical properties of Ng-substituted pyri- doxal thiosemicarbazone copper(II) complexes: their preparation, properties and reactivities Madan Mohan *+, Niranjan S. Gupta and Munesh Kumar Department of Chemistry, N.R.E.C. College, Khurja 203131 (U.P.), India

William E. Antholine National Biomedical ESR Center, Medical College Wisconsin, 8701 Watertown Plank Road, Wauwatosa, W153226, USA

Mohammed J. Ahmed Department of Chemistry, Southwest Texas State University, San Marcos, TX 78666, USA

Narendra K. Jha Department of Chemistry, Indian Institute of Technolo~ty, Hauz Khas, New Delhi t 10016, India

Summary

New copper(II) complexes with pyridoxal N4-methylthio - semicarbazone (H2Methsa), N4-ethylthiosemicarbazone (H2Etthsa) and N4-phenylthiosemicarbazone (H2Phthsa) have bccn prepared and characterized by analytical, magnetic, spectral, e.s.r, and electrochemical methods. All the compounds exhibit normal magnetic moments at room temperature. The variable temperature magnetic moments, however, show the presence of very weak intramolecular antiferromagnetic interaction ( - 2 J = ca. 30cm -1) between the copper(II) centres in the com- plexes. The e.s.r, spectra at 77 K in DMSO indicate the presence of a mixture of monomers and dimers consistent with the dissociation of the complexes. Electrochemical studies in non-aqueous solvents show that the complexes undergo a quasi-reversible one electron facile reduction at markedly low negative potentials versus saturated calomel electrode (s.c.e.).

Introduction

The oxidative cleavage of DNA by metal complexes is important in drug applications (1), the development of synthetic restriction enzymes <21 and studies of tertiary DNA structures <3). One class of compounds, c~-N-hetero- cyclic-carboxaldehyde thiosemicarbazonato metal com- plexes, are particularly attractive in studies of both DNA binding and oxidative cleavagd 4). The complex pyridoxal thiosemicarbazonato copper(II) (CUT), possessing a nitrogen, oxygen and sulphur (NOS) set of ligand donor atoms for metal chelation ( I ) , undergoes facile redox chemistry in cells, in which it is reduced by thiol compounds and oxidized by oxygen species to produce reactive entities, including oxygen radicals and secondary organic free radicals, causing DNA damage 15). The planarity of the pyridoxylidene Schiff bases is important in explaining the reactivities of metal-pyridoxylideneamino acids <6,7). Thus, it appears that the planar mono anionic tridentate nature of the pyridoxal thiosemicarbazone in CuT is an essential feature for its antitumour activity<5>; planarity allows

* Author to whom all correspondence should be directed. + Present address: Department of Chemistry, Austin Community

College, 1020 Grove Blvd., Austin, TX 78741, USA.

CH20H CH20H

~Y~N \\ / F ~ ~ . N / N H C ( S - )NHR .....- N \ ..NH2

~ X Me Me

( t ) (2)

facile electron transfer through the extended conjugated system (7~. Because of our interest in free radicals in the causation of the DNA damage and in the mechanism of drug action, we have extended our present work to understand the chemistry and electronic structures of N4-substituted pyridoxalthiosemicarbazonato copper(II) complexes (2).

Experimental

Pyridoxal hydrochloride, thiosemicarbazides, CuC12' 2HzO and organic solvents were of reagent grade and used without further purification. N4-substituted pyridoxal thiocarbazones (HzXthsa; X = Me, Et or Ph) were syn- thesized and characterized according to the reported procedures <s) .

(Cu( HXrhsa ) ( CI ) ]'H20

A solution of CuC12-2H20 (1 retool) in 2 m o l d m -3 HCI (20cm 3) was added dropwise to a stirred solution of H2Xthsa (1 retool) also in 2 m o l d m -3 HC1 (20cm 3) and the resulting solution was stirred for an additional 30 rain at room temperature. The green crystals obtained by slow evaporation of the reaction mixture at room temperature were removed and dried in vacuo over P2Os.

Physical measurements

The conductivities of samples in MeOH (1 x 10- 3 tool din- 3) at room temperature were measured on a Toshniwal conductivity bridge type C1 01/01. Variable temperature magnetic susceptibilities were determined on a vibrating- sample magnetometer. The magnetometer was calibrated with CuSO,,' 5H20 and a calibrated GaAs diode was used for sample temperature determination and control. For

0340-4285 (e) 1994 Chapman & Hall

266 Mohan et al. Transition Metal Chemistry, 19 (1994) 265-269

Table 1. Analytical data for pyridoxal N4-thiosemicarbazones and their complexes.

Compound Colour Found (Calcd.)(~) C H N

H2Methsa Yellow 47.2(47.2) 5.5(5.5) 21.96(22.0) H2Etthsa Yellow 49.2(49.25) 5.9(6.0) 20.84(20.9) HzPhthsa Yellow 56.9(56.9) 5.0(5.0) 17.70(17.7) [Cu(HMethsa)C1].H20 Green 32.4(32.4) 4.0(4.05) 15.08(15.1) [Cu(HEtthsa)C1]-H20 Green 34.4(34.4) 4.4(4.4) 14.52(14.5) [Cu(HPhthsa)C1].HzO Green 41.6(41.6) 3.95(3.9) 13.01(13.0)

all data diamagnetic corrections estimated from Pascal's constants ~ were used in calculating molar paramagnetic susceptibilities.

E.p.r. data (X-band) were obtained at the National Biomedical ESR Center in Milwaukee, Wisconsin, on a Varian Century series spectrometer. The magnetic field was measured with a Radiopan Gauss-meter and the microwave frequency with a Dana EIP Autohet microwave counter. Spectra from frozen samples were obtained by supporting frozen icicles in a finger dewar containing liquid nitrogen. Spectral g-values were calibrated with diphenylpicrylhydrazyl(DPPH) standard.

I.r. spectra (CsI discs) were recorded on a Perkin-Elmer 1420 spectrometer. Diffuse reflectance spectra were recorded on a Cray-14 spectrophotometer equipped with a reflec- tance accessory, using MgO as the reference. The electronic absorption spectra in solution containing the sample were recorded on a Unicam SP 700 spectrophotometer.

The electrochemical measurements were performed at room temperature in MeOH under oxygen-free conditions by using a PAR Model 174A polarographic analyser, Model 175 universal programmer, Model RE0074 X-Y recorder and Model 377A cell system. A three electrode system was used, the working and counter electrodes were platinum and the reference electrode a s.c.e. Constant- potential coulometry was performed with the use of a PAR Model 179 digital coulometer using platinum-mesh- flag working electrode, a platinum-mesh counter electrode and a s.c.e, as reference electrode. The supporting electro- lyte was t-BuNC104 (TBAP) (0.1 M), and the solutions were ca. 10-3M in complex. All potentials were uncor- rected for the junction contributions.

Elemental analyses for C, H and N were carried out on a Perkin-Elmer model 240C instrument. The analytical data are reported in Table 1.

Results and discussion

The i.r. spectra of pyridoxal 4-ethylthiosemicarbazone (H2Etthsa), pyridoxal 4-methyl thiosemicarbazone (H2Methsa) and pyridoxal 4-phenylthiosemicarbazone (HzPhthsa) show v(NH) and v(NH § of the pyridine ring ~1~ at ca. 3140 m and 2800 mb cm-1, respectively, whose protonation occurred due to the migration of the phenolic OH group to the pyridine nitrogen, but no v(SH) at ca. 2570cm -1 was observed. Thus, the free ligands exist in the thione form (2) in the solid state. However, in solution and in the presence of some metal ions the ligands probably form an equilibrium mixture of the thione (2) and thiol (3) tautomer. Loss of the thiol proton from the form (3) yields a singly charged tridentate ligand coordinating through the mercapto sulphur, the azomethine nitrogen and the phenolic oxygen. When the

N•NfN•--• NHR

OH SH

Me

(3)

free ligand in aqueous acidic solution is stirred at room temperature with an aqueous acidic solution of copper(II) chloride, it yields complexes of general formula [Cu- (HXthsa)C1].H20 (X = Et, Me or Ph). All the complexes are quite stable at room temperature and insoluble in H20, partially soluble in a number of solvents of low- coordinating ability such as CC14, CS2, Phil, PhNO2, CHC13, THF, Et20 and MeCN, but soluble in a large number of solvents of moderate to good coordinating ability such as DMF, DMSO, MeOH, EtOH and pyridine. The molar conductance of the complexes in MeOH at ca. 1 x 10-3M lie in the l l 0 -115~- l cm2mol -a range, indicating that they are 1:1 electrolytes (~2). The complexes do not possess sharp m.p.s, and decompose above 250 ~ C.

l.r. spectra

The assignments of the main vibrational bands of the free ligand and their copper(II) complexes are reported in Table 2. For all the copper(II) complexes the stretching vibrations of the NHR (R = -Et , -Me or -Ph and CH2OH) appear in the 3500-3200 cm- 1 range, while the v(N--H) band occurring at ca. 3140cm -1 in the free ligands disappears in the complexes, suggesting deprotonation of the - - N H group (a3~. Coordination of the azomethine nitrogen atom to the copper(II) ion is confirmed by the v(N--N) shift from 1050cm -1 in the free ligands to 1030cm -~, and by the shift of the band at 1540cm -1, assigned mainly to v(C=N) in the free ligands, to the higher frequency side by ca. 40cm-1 in the copper(II) complexes~l 4). The shift of the v(C--O) phenolic stretching band from ca. 1300 cm - 1 in the free ligands to 1310 cm - in the copper(II) complexes may be ascribed to electron delocalization from oxygen to copper(II) (15). The sharp medium band at ca. 950 cm- ~ in the free ligands, assigned mainly to the v(C=S) stretching vibration, shifts to lower frequency in the copper(II) complexes, suggesting coor- dination of sulphur atom to the copper(II) ion ~16). In the far i.r. spectra, the copper(II) complexes exhibited bands at ca. 425, 360 and 320cm -~, assigned to v(Cu--O), v(Cu--S) and v(Cu--N), respectively (16).

Magnetic susceptibilities

The magnetic susceptibility data were collected on powder samples in the 5-295 K range. The temperature dependence of the product z T [or the magnetic moment #elf = 2.828(zT) ~/z] is characteristic of a weakly spin-coupled copper(II) dinuclear species for each complex with a singlet ground state. The magnetic susceptibility data were fitted to the Bleaney-Bowers equation (1)(17), including a correction term for the paramagnetic impurity.

)~ = (Ng2fl2/kT)[l{3 + exp( - 2J/kT)}]

(1 - p) + N~ + 0.45piT (1)

This result gives the molar paramagnetic susceptibility,

Transition Metal Chemistry, 19 (1994) 265 269 Pyridoxal th iosemicarbazone complexes

Table 2. Selected i.r. spectral data (cm- 1) for pyridoxal Ng-substituted thiosemicarbazones and their complexes.

267

Compound v(NH) v (NH) v(NH +) v(C=N) c R i n g v(C--N) d Ring 5 ( O H ) v(C--O) v(N--N) v(C S) v(OH) a v(OH) b v(C=C)

H2Methsa 3328s 3140m 2800m 1618s 1580w 1540vs 1505m 1375s 1290m 1050m 932m 3260m 1498s

! 470m 1440m 1410m

3435s 3120m 2700m 1610s 1566m 1580vs 1497vs 1382m 1310s 1020m 923m 3356m 2660s 1465w 3195m 1436m

1402vs 3330s 3140m 2800w 1610s 1575w ! 535s 1500w 1375s ! 290m 1050m 930m 3260m 1497s

1470m 1440m 1410sh

3450m 3120m 2800w 1615s 1570s 1580vs 1498s 1375s 1295m 1030m 920m 3270m 1450m

1418m 1405sh

3328s 3140m 2800mb 1620s 1580w 1540vs 1506w 1375vs 1290m 1050m 950m 3260m 1498s

1470m 1440m 1415m

3460mb 3120m 2810w 1620s 1580s 1570s t498s 1375m 1310m 1030m 920m 3280m 1455m

! 420m 1405sh

[Cu(HMethsa)CI].HzO

H2Etthsa

[Cu(HEtthsa)Cl] .H20

H2Phthsa

[Cu(HPhthsa)C1].H20

aH20 and hydroxytmethyl OH; Ualcoholic OH; ~ring and chain C z N ; dchain CzN.

ZM, for $1 = $2 = 1/2 binuclear complex experiencing an isotropic interaction (spin Hamil tonian is H = - 2J S 1 x $2). In this equa t ion p is the mole fraction of the paramagnet ic impuri ty, J the energy separat ion between the S = 0 and S = l states of the binuclear complex, N Avogadro 's

4 1 i 2

03 O~ O v 2

% ::z

0 l , - 0

0 100 200 300 Temperature (K)

Figure 1, Temperature variations of the magnetic susceptibility and magnetic moment of [Cu(HMethsa)C1]. HzO. The solid line was calculated from Equation 1 with g = 2.10, - 2 J = 30, N~ = 120 x 10 -6 c.g.s.u, and p =0.0032.

number , 9 the Lande g factor,/? the Bohr magneton , T t h e absolute temperature and N~ the temperature-independent paramagnet ism. Least squares fitting of the experimental susceptibilities, corrected for d iamagnet i sm of the ligands, to Equa t ion 1 yielded g = 2.10 and - 2 J = ca. 30 .0cm-1.

5" 2

4

% =-

0 0 0 1;0 2;0 300

Temperature (K)

Figure 2. Temperature variations of the magnetic susceptibility and magnetic moment of [Cu(HEtthsa)Cl].H20. The solid line was calculated from Equation 1 with 9 = 2.10, - 2 J = 20, Nc~ = 120 x 10 -6 c.g.s.u, and p =0.0030.

268 Mohan et al. Transition Metal Chemistry, 19 (1994) 265-269

5'-r T ' 2

4

v �9 1 ~= >

% .,r-

01 0 o 16o aoo

Temperature (K)

Figure 3. Temperature variations of the magnetic susceptibility and magnetic moment of [Cu(HPhthsa)C1].H20. The solid line was calculated from Equation 1 with g = 2.10, - 2 J = 25, N~ = 120 • l0 6 c.g.s.u, and p = 0.0035.

The solid lines in Figures 1-3 represent this least squares fit to the theoretical equation.

g = 2.24 V

100 G

g= 2.10

Figure 4. X-band e.s.r, spectrum of [Cu(HMethsa)C1]'H20 (ca. 10- 3 M) in DMSO at 77 K (A) and at room temperature (B). Spectrometer conditions: microwave frequency, 9.081 GHz (A), 9.257 GHz (B); modulation amplitude, 5G; modulation frequency, 100 KHz; incidental microwave power, 5 mW (A), 100 mW (B).

Electronic spectra

The electronic spectra were measured on powdered samples and in solution. The reflectance spectra of the complexes exhibit two absorption bands at ca. 640 and 415nm. In all cases the 640 nm band is assigned to a predominantly d -d transition and the 415 nm band to a charge transfer (S ~Cu) transition ~ls). All the complexes dissolve in MeOH to give green solutions with corresponding bands at 385 and 630 nm (e = ca. 200 dm 3 tool- 1 cm- '). In the less polar DMSO, the charge transfer band shifts to lower energy (415nm) whereas the d - d band position is practically unchanged. These results indicate that the complexes undergo solvolysis in organic solvents and that a mono- nuclear species of the type [Cu(HXthsa)(solv)]C1 is gen- erated. This result is supported by conductivity measure- ments which show [Cu(HXthsa)Clq'H20 to be a 1:1 electrolyte in solution.

Electron paramagnetic resonance

In DMSO at 77 K the spectra of all the complexes are of the same type and reveal the presence of a mixture of monomers and dimers, consistent with the dissociation mentioned above. For example, the e.p.r, spectrum of [Cu(HMethsa)C1].H20 (Figure4) has lines in the gll region (gIG = 2.24, Aij = 168G) consistent with a monomer, and lines centered at g = 2.24 with overall broadening of the spectrum which is consistent with the superposition of a dimeric complex (Figure 4). At room temperature the spectrum for a monomeric complex (giso = 2.10, Aiso = 75G)

is superimposed on a multiline signal with a well resolved structure; the nature of this signal is not fully understood. The centre line of the most intense lines has an apparent g value of 2.025 (sulphur radicals have 9 values around 2.02). The fine structure is complicated and not as yet understood. A possible mechanism involves reduction of copper(II) by the thiolate moiety. An alternate explanation may be that the fine structure of the high field lines is resolved, but the pattern is inconsistent with an expected three line pattern for splitting from two nitrogens from the tridentate complex, C u ( O - - N - - S ) .

Electrochemistry

The redox behaviour of the copper(II) complexes was examined by cyclic voltametry Figure 5 shows the c.v. of [Cu(HEthsa)C1]- H 2 0 in degassed MeOH. The compound displays three redox waves at +0.52, +0.98 and -0 .28 V, all versus s.c.e. The reduction and the first oxidation waves are clearly reversible, with AEp (the difference between the cathodic and anodic peaks) varying very slightly as a function of scan rate (50-400 mV s- 1). However, the second oxidation is irreversible, indicating that the product of this oxidation step is unstable. The chemical reversibility of the redox waves was tested by controlled potential coulometry, Controlled potential electrolysis of a MeOH solution of the complex at -0 .30 V produced a reddish maroon solution with passage of a one electron reduction process. Exhaustive one electron reoxidation at 0.0 V regenerates the original colour of the complex. Very facile reduction of the complex may be associated with

Transition Meta l Chemistry, 19 (1994) 265 269 Pyridoxal thiosemicarbazone complexes 269

500 mA

Figure 5. Results of potential cycling between - 0.15 and + 1.2 V at 100 mV s- 1 of [Cu(HEtthsa)Cl]'H20 in MeOH.

the nitrogen and sulphur donor atoms of the ligand which provide low lying vacant orbitals for back donation. When electrolysis was carried out at the first oxidation potential the compound apparently decomposed.

Conclusions

The present investigation describes copper(II) complexes with new polyfunctional pyridoxal N4-substituted thio- semicarbazones having nitrogen, sulphur and oxygen donor sets of atoms. The presence of different substituents on the Ng-thiosemicarbazone moiety has no effect in the structure of the complexes as clearly shown by their spectroscopic and electrochemical properties. The one electron reduction and two one electron oxidations shown by these complexes are analogous to the facility of electron transfer observed for other complexes with sulphur donor ligands ~i9).

Acknowledgements

This work was partially supported by a grant from the Department of Science and Technology, New Delhi 110016, India, and by the University of Wisconsin, Milwaukee, WI 53266, USA. We thank Prof. David H. Petering, University of Wisconsin-Milwaukee, for sug- gestions and for providing e.s.r, facilities. We also thank Arthur N. Jacob, Southwest Texas State University, for technical support.

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(Received 10 January 1993) TMC 2914