Indi an Journal of Chemistry Vol. 41 A, June 2002, pp. 11 52- I 156

Synthesis and spectroscopic studies on platinum(I1) complexes of . thiosemicarbazone derivatives of p-anisaldehyde, p-tolualdehyde and p-vanillin

Narayan T Akinchan* & Richa Akinchan

Department of Pure & Applied Chemistry, University of Calabar, Calabar, Nigeria

Received 9 November 2000; revised 24 January 2002

The platinum(I1) complexes of thiosemicarbazone derivatives of p-anisaldehyde (ATSCH), p-tolualdehyde (TISCH ) and p-vanillin (VTSCH) have been prepared and characterized by analytical, molar conductance, infrared, electronic, IH and I3C NMR spectral data. Thiosemicarbazones ex ist in thione form and coordinate through thiolatolthione su lphur and azomethine nitrogen . The chloro bridged dinuc lear structures for [Pt(ATSC)Clh, [Pt(TISC)Clh and [Pt(VTSC)Clh having four coordinate platinum(I1) centres and square pl anar for [Pt (ATSC HhlCI2, [Pt(TISCHhlCI2 and (Pt(VTSCHhCI2 are suggested.

Platinum(II) complexes are interesti ng due to their biological role ' and medicinal properties2

-4. Cis-platin and a few thiosemicarbazones have been tested for combi nation chemotherapy for cancer5. The chemical6

.7 and biological activities8 of

thiosemicarbazones have been the subject of investigation 111 recent years. Besides, the thiosemicabazones have been widely used as analytical reagents9

. Platinum(II) complexes with bidentate 1o.11 and tridentate l2

.13 thiosemicarbazones

have been reported. There are also a few reports On metal complexes with thiosemicarbazone derivatives of p-vanillin I4

.15 and p-anisaldehyde I 6

.17 . The present

article describes the synthesis, IR, e lectronic , I Hand 13C NMR spectral properties of platinum(II) complexes of thiosemicarbazone derivatives of p­anisaldehyde (4-methoxy benzaldehyde), p­tolualdehyde (4-methyl benzaldehyde) and p-vanillin (4-hydroxy 3-methoxy benzaldehyde) abbreviated as ATSCH, TISCH and VTSCH respectively.

Materials and Methods P . if h · . b 1617 reparatIOn 0 t lOsenucar azones . VTSCH (4-hydroxy 3-methoxy benzaldene thiosemicarbazone)

To a boiling solution of p-vanillin (3.04 g, 20mmol) in ethanol (25 cm3) was added dropwise a solution of thiosemicarbazide ( 1.82 g, 20mmol) in ethanol (50 cm3

) and the resulting solution was refluxed for a period of 2 h. The solution was then cooled and the crystals collected by filtrat ion, washed with ethanol , followed by diethylether and dried over fu sed CaCb. M.pt. 197-8°C. FAB (Pos) mass

spectrum, m/z 226.0 (MH+), Calcd molar mass 225.17 Anal. [Found: C=47.90, H=4.85 , N=18.62; Calcd for C9H II N30 2S: C=48 .00, H=4.89, N=18.66%] IRlcm- l

:

v(OH) 3530, v(NH2)3439, 3157, v(NH)3280, v(C=N) 1611, v(C=C) 1598,8 (NH2) 1587,1546, v(NCS)1462 , 1443, 1428, 1376. v(C=S)838.

ATSCH (4-methoxy benzaldene thiosemicarbazone) This ligand was prepared as described above using

p-anisaldehyde. M.pt. 174-5°C. FAB (Pos) mass spectrum, mlz 2 1 O.O(MH+), calcd. molar mass 209. 17 . Anal. [Found: C=51.62, H=5.20, N=19.93; Calcd for C9HIIN3S0; C=51.68, H=5.26, N-20.08%] IRlcm-1

v(NH2) 3407, 3 159, v(NH)3293 , v(C=N) 1607 , v(C=C) 1576, 8 (NH2) 1538, 1513, v(NCS) 1475, 1449, 1418, 1362, v(C=S) 835.

TTSCH (4-methylbenzaldene thiosemicarbazone) This ligand was prepared as described above using

p-tolualdehyde. M.pt.l75-6°C, FAB(Pos) mass spectrum, mlz 194 (MH+), Calcd molar mass: 193.26. Anal. [Found; C=55.88, H=5 .67, N=21.68; Calcd for C9H II N3S: C=55.98, H=5 .69, N=21.75 %]; IRlcm-l: v(NH2) 3404, 3 158, v(NH) 3244, v(C=N) 1598; v(C=C) 1568, 8 (NH2) 1540, 1510, v(NCS) 1475, 1464,1410, 1370, v(C=S) 830.

Preparation of platinum( /1) complexes [Pt(VTSC)Clj 2: Di-l1-chloro-bis( 4-hydroxy-3-me­

thoxybensaldene thiosemicarbazonalo) diplat i­num(l/}-p-vanillin thiosemicarbazone (VTSCH) and K2 Pt CI4 (1:1 molar ratio) were dissolved in aqueous ethanol and the resulting mixture was refluxed for 2 h.

AKINCHAN et al.: STUDIES ON PLATINUM(II) COMPLEXES 1153

After cooling to room temperature, reddish brown crystalline product was isolated and washed with ethanol, diethylether and dried over fused CaCho Anal. [Found: C=23.60, N=9.20, H=2.26. Calcd for Pt2C'8H2oN604S2Ch; C=23.77; N=9.24, H=2.22%] "M =0.80 ohm- 'cm2mol-', IRlcm-': v(NH2) 3286br, v(C=N) 1603, 8 (NH2) 1540, 1513, v(NCS) 1463, 1423, 1400, 1371 , v(C=S) 754, v(Pt-N) 485, v(Pt-S) 310, vb(Pt-Cl) 230.

(Pt( A TSC)Cl h: Di-p-chloro-bis (4-methoxybenz­aldene thiosemicarbazonato) di-platinum( /1)- When the reaction was carried out taking A TSCH and K2 PtCl4 in 1: 1 molar ratio as described above, yellowish brown crystals were obtained. Anal. [Found: C=24.76, N=9.61 , H=2.24 Calcd for Pt2C'8H2oN6S20 2Ch: C=24.64, N=9.58, H=2.29%], "M = 0.78 ohm-'cm2mol-'. IRlcm-': v(NH2) 3324, 3165, v(C=N) 1600, v(C=C) 1579, 8 (NH2) 1541 , 1510, v(NCS) 1459, 1431, 1369, v(C=S) 704 v(Pt-N) 415, v(Pt-S) 370, vb(Pt-Cl) 260.

[Pt(TTSC)Clh: Di-p-chloro-bis (4-methyl benzaldene thiosemicarbazonato) di-platinum( /1)­When the reaction was carried out taking TISCH and K2 PtCl4 in 1: 1 molar ratio as described above, yellowish brown crystals were isolated. Anal. [Found: C=25.49, N=9.89, H=2.40; Calcd for Pt2C' 8H20N6S2: C=25 .57, N=9.94, H=2.38%]; "M=0.65 ohm-'cm2mol-'. IRlcm-': v(NH2)3434, 3325, 3148, v(C=N)1587, 8 (NH2) 1537, v(NCS)1476, 1440, 1394, 1375, v(C=S)805, 701 ; v(Pt-N)497, v(Pt-S)330, vb(PtCl) 240.

[Pte VTSCHhlCl2: Bis( 4-hydroxy-3 -methoxybenz­aldene thiosemicarbazone) platinum( II) chloride­Aqueous ethanolic solutions of p-vanillin thiosf'"1icarbazone (VTSCH) and K2PtC4 were mixed in 2: 1 molar ratio in the presence of Conc. HCI and it was refluxed for 2 h. The refluxate on concentration and cooling gave yellowish green complex. Anal. [Found: C=30.08, N=11.70, H=3.07, Calcd for PtC'8H22N60 4S2Ch: C=30.17, N=11.73, H=3.09%]; "M =138 ohm- 'cm2mol-' IRlcm-'; v(OH)3425, v(NH2)3328, v(NH) 3175. v(C=N) 1596, 8 (NH2) 1512, v(NCS) 1466. 1427, 1380, v(C=S) 821, 685, v(Pt-N) 496, v(Pt-S) :370.

[Pt (ATSCHhlG2: Bis(4-methoxybenzaldene thio­semicarbazone) platinum(/I) chloride-Aqueous ethanolic solutions of p-anisaldene thiosemicarbazone (ATSCH) and K2 PtC4 were mixed in 2: 1 molar ratio in the presence of Conc HCl and it was refluxed for 2 h. The refluxate on cooling at room temperature

gave yellowish pink complex. Anal : [Found: C=31.62, N=12.16, H=3.21, Calcd for PtC' 8H22N6S20 2Ch: C=31.58, N=12.28, H=3.24%]; "M =145 ohm-' cm2 mor'; IRlcm- ' v(NH2) 3460, 3417, 3357,3171 , v(NH) 3288, v(C=N) 1604, v(C=C)

1574, 8 (NH2) 1536, 1509, v(NCS) 1461 , 1443, 1422, 1374; v(C=S) 829, 702, v(Pt-N) = 485, v(Pt-S) = 350.

[Pt (TTSCHhlCl2: Bis (4-methylbenzaldene thiosemicarbazone) platinum(/I) chloride-Aqueous ethanolic solutions of p-tolualdehyde thiosemicarbazone (TISCH) and K2PtCl4 were mixed in 2: 1 molar ratio in the presence of Conc HCl and it was refluxed for 2 h. The reflux ate on cooling at room temperature gave yellow complex. Anal , [Found: C=32.98, N=12.74, H=3.46, Calcd for Pt C'8H22N6S2Cl2; C=33.13 , N=12.88, H=3.40%]; "M =140 ohm- 'cm2mol-', IRlcm-'; v(NH2) = 3426, 3164, v(NH) 3313. v(C=N) 1605, 8 (NH2) 1509. v(NCS ) 1580, 1370, v(C=S) 697, 676, v(Pt-N) 482, v(Pt-S) 335 .

Dried samples were used for all physical and spectral measurements and carbon, hydrogen and nitrogen were analyzed using a Coleman Analyzer at RSIC, CDR!, Lucknow, India. Physical measurements, IR, electronic, 'H and I3C NMR spectra were obtained as reported earlier'6.17. FTIR spectra were obtained on a Perkin-Elmer 1800 spectrometer at CDR!, Lucknow, India. Positive ion fast atom bombardment (FAB+) mass spectra were obtained with m-nitrobenzyl alcohol as matrix.

Results and Discussion The reactions of 4-methoxybenzaldene

thiosemicarbazone (A TSCH) with K2PtCl4 in the absence/presence of hydrochloric acid gave [(Pt(ATSC)CI] and Pt(ATSCHh]Ch.

aqueous ATSCH+K2PtCI4 ethanol • [Pt(ATSC)CI]+2KCl+HCL

Cone. He l .. . (1)

aqueous 2ATSCH+K2PtCI4 th I· [Pt(ATSCH)2]Ch+2KCl e ano

.. . (2)

The TISCH (4-methylbenzaldene thiosemicarba­zone) and VTSCH (4-hydroxy 3-methoxy benzaldene thiosemicarbazone) react in similar reaction conditions to produce (1 : 1) and (1 :2) platinum(II) complexes respectively. The complexes are sparingly soluble in common organic solvents but soluble in DMF and DMSO. The electrical conductance measurements in DMF reveal the non-electrolytic

1154 INDIAN J CHEM, SEC A, JUNE 2002

nature for [Pt(ATSC)CI], [Pt(TTSC)CI] and [Pt(VTSC)CI], whereas the [(Pt(ATSCH)2]CI2, (Pt(TTSCHh]Ch and [Pt(VTSCHh]Ch complexes exhibit (1:2) electrolytic nature lS

.

[Pt(ATSC)Clh. [Pt(TTSC)Clh and [Pt(VTSC)Clh The spectra of these thiosemicarbazones show three distinct bands in the region 3155-3160, 3240-3300 and 3400-3440 cm- I

. These bands are usually attributed to v(OH) and v(NH) stretching vibrations. Among these bands, the location of the middle one is the most sensitive to the aromatic part of the molecule since NH group is closer to the aromatic substituent than the NH2 function. Thus, the middle band is assigned to v(NH).

The JR spectrum of [Pt(VTSC)Clh shows only one broad band having maximum at 3286 cm- I and the spectrum of [Pt(ATSC)Clh exhibits two sharp bands above 3lO0 cm- I assignable to vas(NH2) and vs(NH2)

modes. The spectrum of [Pt(TTSC)Clh differs from the spectra of other two complexes. The vas(NH2) band is split due to probable interaction between two platinum(1I) complex molecules. Crystallographic study on Ni(TTSCh has indicated existence of twinned complexes 17

. These observations provide sufficient evidence for probable deprotonation of NH group before coordination with platinum(H) ion. The spectrum of ATSCH also shows a band at 1607 cm- I

assignable to v(CN) vibration. On coordination it combines with the new v(CN) band formed on loss of the NH proton to yield a band at 1600 cm- I. Similar change has been observed for the other two platinum(II) complexes. A band at 835 cm- I as a result from coupled vibrations with large participation of the v(CS) vibration has been located in the spectrum of A TSCH. This band disappears on coordination and is replaced by two bands at 832 and 704 cm - I. Similar observations have been recorded for the spectra of other two platinum(II) complexes . A decrease in energy of this band is taken as an indication of metal-sulphur bond formation 17.

Both coordination modes are further supported by the presence of new bands in the far IR spectra of the complexes. Far JR spectra show new bands located at wavenumbers comparable with those previously attributed to v(PtN) , v(PtS) and v(PtCI) transitions . The earlier report suggests that the vibration energy for vb(PtCI) must be lower than vt(ptCI) due to the possible sharing of electrons between chloride and two platinum(II) ions 18. The reported value for vt(PtCI) in the spectra of K2PtCI4 (ref. 19) and an organometallic compound20 fall in the range 315-

325cm -I. The values observed in the spectra of platinum (II) complexes in the range 230-260cm- 1

suggest the bridging mode for chloride.

Infrared spectra of [Pt(ATSCHh lCI2•

[Pt(TTSCHhlCI2 and [Pt(VTSCHhlCI2• The vsCNH2) is little affected on complexation. The vas(NH2) band splits in the spectrum of [Pt(A TSCH)2]Ch indicating intermolecular hydrogen bonding. The remaining spectra of the other complexes show increase in the energy of this vibration . The v(CN) vibration energy decreases in the spectra of [Pt(ATSCHh]Ch and [Pt(VTSCHh]CI2 but increases in the spectrum of [Pt(TTSCHh]Ch. A group of bands resulting partially from coupled vibration of v(NCS) modes and located between 1370-1470 cm- I in the spectra of thiosemicarbazones are also modified upon coordination with platinum(II) ions. The vibrational band in the region of 830-840 cm- I with predominan t v(CS) character also moves to lower energy region in the spectra of platinum(II) complexes indicating thione sulphur coordination. The observation of far JR bands in the spectra of platinum(II) complexes in the region of 482-496 and 335-370 cm- l due to v(PtN) and v(PTS) transitions respectively further suggest the involvement of azomethine nitrogen and thione sulphur coordination. Thus, thiosemicarbazones act as neutral bidentate chelating ligands. (Structure I) .

The coupled vibrational character of nitrogen­hydrogen band together with hydrogen bond effects create serious problem in obtaining conclusive evidences of thiosemicarbazone deprotonation. On comparison of IH NMR spectra of thiosemicarba­zones and (I :2) platinum(II) complexes recorded suggest that neutral form of ligands are present in [Pt(A TSCHh]CI2, [Pt(TTSCH)2]CI2 and [pt(VTSCHh]Ch. The respective signal of NH group proton appears in the region 11 .25-11.33 ppm in the ligand spectrum and it shifts to 12- 12.50 ppm in the spectra of these platinum(II) complexes. The effects may result from changes in electron density after nitrogea-platinum(II) coordination 16 . The disappearance of this 8ignal in the spectra of [Pt(ATSC)Clh, [Pt(TTSC)Clh and [Pt(VTSC)Clh supports the presence of deprotonated form of thiosemicarbazones in these complexes 17

. The two signals due to NH2 protons are observed in the region 7.90-8.15 ppm and they shift upfield in the spectrum of [Pt(TTSCHh]Ch and downfield in the spectrum of [Pt(VTSCHh]ch. However, only one signal has been located in the spectrum of [Pt(A TSCH)2]C12. The spectra of [Pt(A TSC)2CIh and (Pt(VTSChClh show

AKlNCHAN et al.: STUDIES ON PLATINUM(I1) COMPLEXES 1155

[Pt(TTSCH)z JCl z CH 3 H

[Pt{ATSCH)z JClz OCH 3 H

[Pt(VTSCH)z] Clz OH OCH3

¢"

[Pt( TTSclclJ2

[Pt(ATSC) Cllz

[pt (VTSC) CI] 2

(I) Structures of Platinum(II) complexes

these signals at 8.20 and 8.50 ppm respectively indicating presence of unperturbed NH2 groups. Moreover, the spectrum of [Pt(TTSC)Clh shows two signals for NH2 group protons supporting intermolecular hydrogen bonding. A similar observation has been made during IR spectral study. The other proton signals are least affected. The observation of two signals due to CH3 group protons could be attributed to the structural differences in two chelate rings In the platinum(II) complexes (Structure I).

The I3C NMR chemical shifts of thiosemi­carbazones and their platinum(II) complexes have been obtained. The carbon signals for C=S (177 .44-177.81ppm) and CH=N (142.17-143.28 ppm) groups are comparable with those reported earlier for thiosemicarbazones 10. In the spectra of [Pt(A TSC)Clh. [Pt(TTSC)Clh and [Pt(VTSC)Clh complexes, signal due to C=S moves upfield but signal due to CH=N moves down field suggesting

coordination involving azomethine nitrogen and thiolato sulphur. However, the spectrum of [Pt(VTSCHh]CI2 exhibits signal due to C=S at 174.90 and 174.20 ppm and signal due to CH=N at 162.7 and 155.20ppm respectively. The spectra of [Pt(ATSCHh]Ch and [Pt(TTSCHh]Ch are quite different and show downfield shift of C=S signal probably due to increase in C=S bond order in cationic complexes. The difference in spectral changes could be attributed to the difference in the structure of thiosemicarbazone part of the complexes. The two signals due to CH3 group could support the presence of two isomeric forms of the complexes. Thus, the observed spectral data suggest coordination through thione sulphur and azomethine nitrogen in the cationic platinum(II) complexes (Structure I).

The solution and solid state reflectance spectra of thiosemicarbazones and platinum(II) complexes have been recorded. Bands above 36000 cm- I are assigned to 11:-11:* transitions of aromatic system 10. Lower energy absorption observed in the spectra of thiosemicarbazones around 30000-31000 cm -I result from 11:-11:* (CH=N) transition 10. On metal coordination, this transition moves to lower energy indicating the electron density change caused by nitroge-platinum(IT) bond formation . The smaller downward shift of 11:-11:* (arom) transition is due to conjugation of 11:-orbitals of aromatic system and lone pair of electrons in azomethine group. The spectra of (1 :2) cationic platinum(II) complexes exhibit only one composite spectral band probably due to the least difference in the energy levels for dyz> dxz and dz

orbitals. The spectra of (1 : 1) neutral platinum(II ) complexes show bands in the regions of 18 500-19 700, 20 800-21 800 and 29 100-30 200 cm- I

. These spectral bands are assigned to the transitions, IAlg

IA A IS d I I· ~ 2g' Ig~ Ig an A ig ~ E g In order of . . 2122 Th . increasing energy .. us, the comparIson of electronic spectra of thiosemicarbazones and their platinum(II) complexes is able to provide some evidence about the mode of coordination. The ligand coordination takes place through thiolatel thione sulphur and azomethine nitrogen gIVing four coordinate platinum(II) centres (Structure I) .

Acknowledgement We thank the authorities of RSIC, CDRI, Lucknow

and lIT Madras, India for the spectral data and microanalyses. We are extremely grateful to Prof M Graziani , University of Trieste, Trieste, Italy for verifying some of the NMR spectral data.

1156 INDIAN J CHEM, SEC A, JUNE 2002

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