Triorganosilicon (IV) Compounds: Synthesis, Structural Study, and Biological Activity

Devendra Singh, R. V. Singh, and N. K. Jha

DS, NKJ. Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, India.-RVS. University of Rajasthan, Jaipur, India

ABSTRACT

A few coordination compounds of silicon (IV) have been synthesized by the interaction of trimethyl- and triphenyl-chlorosilane with nitrogen-sulphur donor ligands. These compounds are monomeric, as indicated by molecular weight determination, and they behave as nonelectrolytes in dry DMF. From the electronic, infrared, ‘H, and 13C NMR spectral results, it has been concluded that in these compounds, silicon is penta-coordinated in a trigonal bipyramidal environment.

An assessment of biological activity of these compounds has shown that some of them are very active against P. mirabilis and others against S. viridans bacteria, while all of them show good fungicidal action against F. oxyspornm, A. altemata, and A. niger.

INTRODUCTION

The fast moving and expanding development in the chemistry of coordination compounds as outlined by individual scientific backgrounds, individual interest, and personal idiosyncrasies has been released due to their applicability [l-51 in diverse areas of current interest, mainly in agriculture and medicine. Presently, it has been shown that the involvement of periodic elements with organic moieties having nitrogen and sulphur atoms [6-71 plays a crucial role in designing a potential molecule of specific use.

A profile of updated literature towards these compounds derived specially from thiosemicarbazones has arisen because of their remarkable bioactivity [8-101 which has been shown to be related to their metal-complexing ability. In connection with previous investigations on the coordinating properties of this

Address correspondence to: Prof. N. K. Jha, Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi-110 016, India.

Journal of Inorganic Biochemistry, 62, 67-73 (1996) 0 1996 Elsevier Science Inc., 655 Avenue of the Americas, NY, NY 10010

0162-0134/96/$15.00 SSDI 0162-0134(95MOO93-4

68 D. Singh et al.

class of azomethines [ll-131 and in order to isolate new organosilicon thio- complexes with potential antimicrobial properties, we have synthesized some thiosemicarbazone complexes in the search for new bactericides and fungicides.

EXPERIMENTAL

The ligands used were thiosemicarbazones of 2_acetylthiophene, 2-actylfuran, and 2_acetylnaphthalene, the synthesis and characterization of which have been reported in our earlier publication [14]. Tripheylchlorosilane (Fluka AG, m.p. 92-94°C) and trimethylchlorosilane (Fluka AG, b.p. 57~59°C) were used as such. All other chemicals were dried and distilled before use, and the reactions were carried out with a ratio-head fitted with a condenser and protected with calcium chloride drying tubes.

Preparation of Silicon (IV) Complexes

The weighed amounts of triphenylchlorosilane and trimethylchlorosilane were treated with the sodium salts of the ligands in 1:l molar ratio in dry methanol. The amounts for individual reactions are given in Table 1. The mixture was refluxed for about 10 hr, and the precipitated sodium chloride was removed by filtration. The filtrate so obtained was concentrated and dried by vacuum pump for complete removal of solvent, and then the product was washed with methanol-cyclohexane mixture (1:l) and checked by TLC for purity. The analyti- cal data of these compounds are shown in Table 2.

Analytical Methods and Physical Measurements

Silicon was determined gravimetrically as SiO,. Nitrogen and sulphur were estimated by Kjeldahl’s and Messenger’s methods, respectively.

Carbon and hydrogen analyses were performed in the Microanalytical Labo- ratory of the Chemistry Department of Rajasthan University, Jaipur, India. The molar conductance of the compounds was measured in DMF with a conductivity

TABLE 1. Physical Properties of Triorganosilicon (IV) Complexes

Reactant

Starting Material

(8)

Ph,SiCl 1.32 Ph,SiCl 1.21 Ph,SiCl 0.99 Me,SiCl 0.78 Me,SiCl 0.82 Me,SiCI 0.74

Ligand (g)

C,H,N,SO 111 0.82

C,H,N,S, [II] 0.82

C,,H,,N,S ]IIIl 0.82

C,H,N,SO [I] 1.32

C,H,N,S, [II] 1.50

C,,H,,N,S WI 1.66

Product Formed and Color

Ph,Si (AcFur.Tscz) [C,,H,,N,SOSi] Dark yellow Ph,Si (AcThiop.Tscz) [C,5H,,N,S,Si] Brownish Ph,Si (AcNaph.Tscz) [C,,H2,N,SSi] Yellowish Me,Si (AcFur.Tscz) [C,,H,,SOSi] Yellow Me,Si (AcThiop.Tscz) [C,,,H,,N,S$i] Off white Me,Si (AcNaph.Tscz) [C,,H,,N,SSi] Yellow

Yield M.P. (%) (“0

59 184

65 158

68 161

60 127

65 158

61 169

TRIORGANOSILICON(IV) COMPOUNDS 69

TABLE 2. Elemental Analyses and Molecular Weights of Triorganosilicon (IV) Com- plexes

Elemental Analyses (%>

Molecular C H N S Si Mol. Wt. Formula of Found Found Found Found Found Found Compound* (Calcd.) (Calcd.) (Calcd.) (Calcd.) (Calcd.) (Calcd.)

[C,H,,N,SOSi] 67.69 (67.99)

K&&S,Sil 70.31 (70.55)

K,,H,,N,SSil 73.78 (74.09)

[C,,H,,N,SOSil 40.71 (46.92)

G,H,,N,S,Sil 59.92 (59.17)

K,,H,,N,SSil 60.78 (60.91)

5.00 (5.25) 5.21

(5.45) 5.19

(5.42) 6.37

(6.69) 6.89

(7.16) 6.49

(6.71)

9.21 (9.51) 9.61

(9.87) 8.54

(8.36) 16.18

(16.41) 17.21

(17.55) 13.09

(13.32)

7.53 (7.26) 7.78

(7.53) 6.63

(6.38) 12.81

(12.53) 13.65

(13.39) 10.42

(10.16)

6.49 (6.36) 6.32

(6.60) 5.81

(5.59) 10.61

(10.97) 11.49

(11.73) 8.59

(8.90)

468.00 (441.63) 440.00

(425.63) 525.00

(502.54) 278.00

(256.00) 271.00

(239.42) 334.00

(315.52)

* The names of the compounds are given in Table 1.

bridge type 304 Systronics model, and the molecular weights were determined by the Rast Camphor method. IR spectra were recorded on a Perkin-Elmer 577 grating spectrophotometer in KE3r optics. The electronic spectra were obtained on a Pye Unicam SP 8-100 spectrophotometer. The ‘H and 13C NMR spectra were recorded on a Jeol FX 90Q spectrometer at 89.55 and 22.49 MHz, respectively.

RESULTS AND DISCUSSION

The involved reactions in the synthesis of the Si(IV) derivatives can be repre- sented by the following equation:

R,SiCl+ NSH & R,Si(NS) + NaCl

(where R = Me or Ph and NSH represents the ligand molecule). The ligands employed in the above reaction are given below:

AC Fur. TsczH (C,H,N,SO)

AC Thiop. TsczH (C,H,N,S,)

70 D. Singh et al.

III.

AC Naph. TsczH (C,,H,,N,S)

The resulting complexes, which are soluble in most of the common organic solvents, exhibit nonelectrolytic behavior for 10P3M solutions in dry DMF (lo-14 a-’ - cm* * mall ‘1. The monomeric nature of these complexes is indi- cated by the molecular weight determinations (Table 2).

The electronic spectra of ligands and their trimethyl- and triphenylsilicon (IV) complexes have been recorded. The spectra of the ligands show one band at ca. 380 nm, which may be assigned to n-r* transitions of the azomethine group 1151. This band shifts towards a lower wavelength in the silicon complexes, thereby indicating coordination of azomethine nitrogen to the silicon atom [16]. Further, two medium intensity bands at ca. 285 and 305 nm due to n-r* transitions in the ligands remain almost unshifted in the spectra of silicon complexes.

The IR spectra of these complexes do not show any band in the region 3300-3100 cm-’ or 2700-2500 cm-’ which could be assigned to v(NH)/SH) vibrations due to tautomerization [171 of ligand. This indicates the deprotona- tion of the functional group of the ligands as a result of complexation with the silicon atom. A sharp band in the 1610-1590 cm-’ region due to v(, ‘C=N) of the free azomethine group in the ligands shifts to lower frequency (by ca. 10 cm-‘) in the silicon complexes, indicating coordination [18] of azomethine nitrogen to silicon. This has been further substantiated by the presence of v( Si - N ) [ 191 vibration at ca. 580 cm-‘. Further, a band at ca. 540 cm- ’ due to ( Si-S) [20] indicates the involvement of sulphur to the silicon atom. In trimethyl- and triphenyl-silicon derivatives, some new bands at ca. 1420, 1125, 721, and 700 cm-’ have been ascribed to the asymmetric and symmetric modes of CH,-Si and C,H,-Si groups [211.

‘H NMR spectral data for the ligands and complexes are given in Table 3. The broad signal due to NH proton at 9.90-10.68 ppm in the ligands disappears

TABLE 3. ‘H NMR Spectral Data (6, ppm) of Ligands and Their Corresponding Triorganosilicon (IV) Complexes

Ligand/Complex

AcThiop.TsczH Me,Si(AcThiop.Tscz) Ph,Si(AcThiop.Tscz) AcFur.TsczH Me,Si(AcFur.Tscz) Ph,Si(AcFur.Tscz) AcNaph.TsczH Me,Si(AcNaph.Tscz) Ph,Si(AcNaph.Tscz)

-NH (bs)

10.68 - - 9.90 - -

10.65

-NH, -CH, Aromatic (bs) w (ml

2.81 1.68 8.68-7.16 2.83 1.81 8.92-7.34 2.85 1.85 8.98-7.38 2.83 1.80 7.80-6.60 2.83 1.98 8.24-6.82 2.86 2.10 9.10-7.30 2.90 1.91 8.93-7.35 2.92 2.08 9.14-7.68 2.90 2.14 9.24-7.80

Si-CH,/ Si-C,H,

w

- 0.78 6.15 -

0.81 6.18

0.85 6.22

TRIORGANOSILICON(IV) COMPOUNDS 71

in the triorgano-silicon (IV) complexes, showing the involvement of thiolo- sulphur to the silicon (IV) moiety after deprotonation of the functional group. In the complexes, the downfield shift in the position of (>C=N) methyl protons indicates the coordination of azomethine nitrogen to the silicon atom. Further, almost no shift in the signals due to NH, protons in the complexes as compared to those in the ligands confirms the nonparticipation of this group in coordination. The new signals at 6 0.78-0.85 ppm and 6 6.15-6.22 ppm in trimethyl- and triphenylsilicon (IV) complexes, respectively, are due to Me,Si and Ph,Si groups [21], respectively.

The 13C NMR spectral data of two of the ligands and their corresponding silicon (IV) complexes are shown in Table 4. It may be noted that 13C signals of the azomethine carbon in the ligands shift in the complexes. Similar shift is observed in the 13C signal of the methyl group attached to the azomethine carbon. These shifts confirm the coordination of azomethine nitrogen to silicon. It is further observed that the 13C signal of the carbon attached to sulphur also shifts in the complexes as compared to the ligands, confirming the bonding of sulphur to silicon. On the basis of the above spectral evidence, the following tentative structure may be proposed for these derivatives:

(where R = Me or Ph). In the proposed structure, two methyl (or phenyl) groups are in different

environments, and hence two different signals in ‘H NMR are expected for these groups. Due to overlaps in phenyl signals, such a difference may not be observable; however, in the case of methyl groups, two signals ought to be observed. But only one signal [22] is observed in all the cases, which indicates that there is a rapid exchange of these groups.

The biocidal activity of these complexes is reported in Tables 5 and 6. The techniques used for screening antibacterial and antifungal activities have

been reported earlier [23]. The organisms used in the present investigation are: S. aureus, P. mirabilis, and S. viridans for antibacterial activity, and F. oxyspo- rum, A. alternata, and A. niger for antifungal activity.

TABLE 4. Selected 13C NMR Spectral Data (6, ppm) of Ligands and Their Correspond- ing Silicon (IV) Complexes

Ligand/Complex >C=N CH,-C=N C=S/C-SH

AcFur.TsczH 155.47 13.81 179.86 Me,Si(AcFur.Tscz) 148.24 11.18 171.74 Ph Si(AcFur.Tscz) 3 151.10 12.25 173.12 AcThiop.TsczH 150.25 15.87 177.98 Me,Si(AcThiop.Tscz) 146.38 12.32 168.20 Ph 1 Si(AcThiou.Tscz) 147.14 13.64 171.88

12 D. Singh et al.

TABLE 5. Bactericidal Activity of Ligands and Their Corresponding Silicon (IV) Com- plexes

Bacteria

Compound

AcFur. TsczH Me,Si(AcFur.Tscz) Ph,Si(AcFur.Tscz) AcThiop.TsczH Me,Si(AcThiop.Tscz) Ph,Si(AcThiop.Tscz)

Diameter of Inhibition Zone (mm) at 1000 ppm Cont. S. aureus P. mirabilis S. viridans

10 16 8 16 14 13 19 18 18 13 7 9 20 17 19 22 20 21

From the results of biocidal activity of these compounds against different bacteria and fungi as reported in Tables 5 and 6, it may be inferred that these compounds show moderate to high activity. However, it is observed that all the organosilicon (IV) complexes have greater antibacterial activity as compared to the parent ligands. It may also be noted that triphenylsilicon (IV) complexes are more potent than trimethylsilicon (IV) derivatives derived from the same parent ligand.

The comparative activity data of these compounds may be interpreted in terms of sulphur content in the compounds, i.e., the molecule having two sulphur atoms has a higher effect as compared to that which has one sulphur atom, for example, Me,Si(AcFur.Tscz) and Ph,Si(AcFur.Tscz) have low values of activity as compared to Me,Si(AcThiop.Tscz) and Ph,Si(AcThiop.Tscz), which agrees with the previous reported results 1241. Further, the biocidal function of these compounds can also be ascribed in terms of chelation theory 1251 and hydrogen bond formation through -N=C-S group, with some bioreceptors in the cells of the used organism resulting in an interference with the normal cell processes [131. It is known that incorporation of a trisilyl group in a molecule increases its acid-base properties and lipophilicity and, consequently, the interaction with the receptors and the metabolism of silylated compounds.

TABLE 6. Fungicidal Activity of Ligands and Their Corresponding Silicon (IV) Com- plexes

Compound

Average Percentage Inhibition after 96 hr

F. oxysporum A. alternata A. niger Cont. Used Cont. Used Cont. Used

(ppm) (ppm) (ppm) 50 100 50 100 50 100

AcFur.TsczH 18 31 21 38 24 43 Me,Si(AcFur.Tscz) 57 87 67 91 75 98 Ph,Si(AcFur.Tscz) 69 98 72 100 81 100 AcThiop.TsczH 23 36 30 41 31 58 Me,Si(AcT’hiop.Tscz) 71 91 95 100 88 100 Ph,Si(AcThiop.Tscz) 78 100 98 100 90 100

TRIORGANOSILICON(IV) COMPOUNDS 73

Secondly, these bulky substituents considerably change the molecular parame- ters which influence the orientation of the molecule on biological receptors and its ability to penetrate the cell membranes.

One of the authors (Dr. D. Singh) thanks CSIR, New Delhi, for financial assistance in the form of Research Associateship (Grant No. 9 / 86(251) / 92-EMR-I), and Dr. A. K Dwivedi, Department of Botany, University of Rajasthan, Jaipur, for essential support for biochemical studies.

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Received January 9, 1995; accepted May 24, 1995