SULFUR BONDED UNSYMMETRICAL BOROLE COMPLEXES: SYNTHETIC, SPECTROSCOPIC AND BIOCIDAL ASPECTS Taruna Pandey1, V. P. Singh2 and R. V. Singh1* 1 Department of Chemistry, University of Rajasthan, Jaipur-302004, India 2 DSME, S.C.E.R.T., Varun Marg, Defence Colony, New Delhi-110024, India ABSTRACT Synthesis, characterization and biological activities of some of the unsymmetrical borole complexes containing B-S and B<-N bonds are described. The sulfur containing conjugated bases were prepared by the condensation of caibonyl compounds with thiosemicarbazide. The resulting complexes have been characterized by elemental analysis, molecular weight determinations and spectral studies including IR, Ή NMR, "Β NMR and ,3C NMR. The spectroscopic results showed that the conjugated bases behave in a bidentate fashion, whereas, the complexes display a tetracoordinated environment around the boron atom as steriochemically active lone pair is also included in the coordinated sphere. The conjugated bases and their respective boron complexes have been screened for their antifungal and antibacterial properties. INTRODUCTION Recently, few references on bidentate ligands coordinated to boron through the azomethine nitrogen and the thiolo sulfur have been published in the literature1·2. These compounds are interesting in many respects. Some have got a number of applications in industry, biology and agriculture. Industrial applications of coordination compounds of boron have remained fairly limited. Boron analogues of carboxylic acids and peptides have been shown to possess interesting biological activities in particular as serine protease inhibitors3. Use of boron-nitrogen compound in neutron capture therapy of brain tumors have also been investigated4. Several organoboranes find promising applications in the synthesis of insect pheromones5. Metalloboranes are widely used in industry and medicine as plastic stabilizers, polymerization accelerators, lubricants and bactericides. Some boron complexes containing nitrogen and sulfur have been shown to exhibit antifungal and antibacterial properties6. Several chelates of boron have been tested for potential pharmaceutical applications and some of them have been introduced in to practice7·8. Thiosemicarbazides and conjugated bases have attracted much attention due to their various biological activities9. It was found that some drugs have increased activity when administered as metal complexes10 and a number of metal chelates were found to inhibit tumor growth". In the treatment of cancer the active species is a metal chelate of conjugated base12. Following the work of Mashima13, coordination chemists have taken greater interest in thiosemicarbazides and their azomethines as potential ligands. Boron complexes of these ligands have been found to possess conspicuous biocidal activity14. It is however interesting that the biological activity gets enhanced on undergoing complexation with the metal ions15"16. In view of the above and the success achieved earlier in the synthesis and characterization of Schiff base complexes of boron, it was considered of interest to synthesize a wide variety of Schiff base derivatives of boron by the reactions of unsymmetrical borole and the Schiff bases having nitrogen and sulfur as coordination sites. The conjugated bases used during these investigations are: (i) 2-[ 1 -(2-Naphthenyl)ethylidene]hydrazine carbothioamide (L,H) (ii) 2-[l-(2-Pyridinyl)ethylidene]hydrazine carbothioamide (L2H) (iii) 2-[l-(2-Thienyl)ethylidene]hydrazine carbothioamide (L}H) RESULTS AND DISCUSSION The 1:1 molar reactions of unsymmetrical borole, 2-isopropoxy-4-methyl-l ,3,2-dioxaborolane B(C3H602) (OPi*) and 2 -isopropoxy-4-methyl-1,3,2 -dioxaborinane B(C4H,0.,) (OPf) with 2-[l-(2-thienyl)ethylidene]hydrazine carbothioamide, 2-[l-(2-pyridinyl)ethylidene] hydrazine carbothioamide and 2-[l-(2-naphthenyl)ethylidene] hydrazine carbothioamide can be represented by the following equations : 185 Vol. 21, No. 4, 1998 Sulfur Bonded Unsymmetrical Borole Complexes: Synthetic, Spectroscopic and Biocidal Aspects 1 [B(C3H602) (OPr )] + LnH • [B(C3H605) (Ln)]+ΡΛ>Η Benzene [Bi^HgOj) (OPr*)] + LH — > [Β^Η,Ο,) (L)] + PrOH Where, LnH represents the conjugated base molecule and η = 1, 2 or 3. The resulting coloured solids are soluble in MeOH, DMF and DMSO. The method used for the preparation and isolation of the resulting complexes give materials of good purity as supported by their analysis. These are monomelic, non-electrolytes, stable and resistant to hydrolysis. IR Spectra The comparison of the infrared spectra of the conjugated bases and their complexes suggests that conjugated bases are bidentate with the thiolo-sulfur and azomethine nitrogen as the coordinated sites. Strong bands in the region 3300-3100 cm"1 in the conjugated bases disappear in the corresponding borole complexes, indicating the deprotonation of the -NH functional group. The conjugated bases exhibit a sharp band at 1610-1590 cm'1 due to stretching mode of azomethine group. The band shifts to the higher frequency region (ca. 15 cm1) on complexation which is due to an increase in the bond order17 as a result of (B<-N) bond formation. An indicative evidence for the support of the structure has been noted by the new strong and sharp peaks appearing at 1360, 875 and 1550 cm1 assigned to ν (B-O)18 v(B-S)19 and v(B<-N)J0 modes, respectively. Ή NMR Spectra Table I: Ή and "B NMR Spectral Data δ, ppm/(JHH) of Conjugated Bases and their Corresponding Unsymmetrical Borole Complexes U Compound -NH -NH2 Aromatic Protons* -CH, B 0») (b.) 1 3 4 5 6 7 (·) (») L)H 10.65 2.90 8.93(s) 8.0-8.32(m) 7.84-7.55(m) 1.91 . BCCJHIOZ) (Ll) - 2.86 8.96(s) 8.72-8.16(m) 8.08-7.76(m) 2.16 1.06 B^HGOJ) (Ll) - 2.88 8.98(*) 8.56-8.0(m) 7.92-7.60(m) 2.20 0.8 LjH 10.64 2.84 - 8.24(d) 8.88(dd) 7.48(dd) 8.92(d) 1.82 - (7.1Hz) (7.1Hz)(7.8Hz) (7.8Hz)(7.5Hz) (7.5Hz) BiCjHeOz) (L2) - 2.72 - 8.32(d) 8.96(dd) 7.52(dd) 9.04(d) 1.94 4.86 (7.2Hz) (7.2Hz)(7.9Hz) (7.9HzX7.5Hz) (7.5Hz) BiC^gOj) (Lj) - 2.80 8.40(d) 9.04(dd) 7.60(dd) 1.91 4.07 I (7.1Hz) (7.1Hz)(7.9Hz) (7.9HzX7.4Hz) I L3H 10.68 2.81 - 7.16(d) 7.60(dd) 8.68(d) - 1.68 - (7.4Hz) (7.4HzX7.3Hz) (7.3Hz) BfCjHiOj) (Lj) - 2.72 - 7.68(d) 8.16(dd) 8.72(d) - 1.88 6.51 (7.3Hz) (7.3HzX7.4Hz) (7.4Hz) WWW (Lj) - 2.78 - 7.42(d) 7.76(dd) 8.82(d) - 1.80 7.6 (7.2Hz) (7.2HzX7.4Hz) (7.4Hz) •Aromatic protons of first three compounds are observed in two group of multiplets (3,4,5,8 & 6,7). The proton magnetic resonance spectral data of hydrazine carbothioamides of [l-(2-pyridinyl) ethanone], [l-(2-naphthenyl)ethanone] and [l-(2-thienyl)ethanone] and their corresponding unsymmetrical borole complexes have been recorded in DMSO-d6. The chemical shift values relative to the TMS peak are listed in Table I. is® Φ Φ 186 Taruna Paney, V.P. Singh and R. V. Singh Main Group Metal Chemistry The Ή NMR spectra of each synthesized conjugated bases exhibit a singlet in the region δ (10.64- 10.68) due to -NH proton which disappears in the spectra of complexes suggesting the coordination of nitrogen as well as sulfur to the boron atom. However, in the 1:1 complexes of unsymmetrical borole the signal due to -NH2 protons observed nearly in the same region δ (2.72-2.90) ppm showing that the deprotonation does not take place and the bonding may take place only through the nitrogen and sulfur atom of the conjugated bases. In the spectra of the complexes the deshielding in the position of CHj-C=N protons further substantiates the coordination of azomethine nitrogen to the boron atom. The downfield shift indicates the delocalisation of the electronic charge within the chelate ring and thereby the stabilization. "C NMR Spectra I3 The C NMR spectral data of L,H, L2H, LjH and their unsymmetrical borole complexes show marked shifts in the positions of thiolo and azomethine carbons attached to the participating groups clearly showing the complexation of boron through nitrogen and sulfur atoms (Table II). Table Π: 13C NMR Spectra Data (δ ppm) of Conjugated Bases and their Corresponding Unsymmetrical Borole Complexes. Compound Chemical Shift Values Thiolo Azomethine Methyl Aromatic Carbons Carbon Carbon Carbon L]H 178.99 147.84 13.92 C1>133.33;C2,135.11;C3,132.84; Gt, 130.25; C5,129.64; C6,127.64; C7,127.47; Cg,126.88; C9,123.57; C]0,124.06 B(C3H602XLJ) 170.87 139.77 13.99 C] ,135.50; C^135.09; C3,134.12; C4,131.22; C5,129.58, Q;, 127.49; C7,129.54; Cg, 126.90; C9,123.56; C10,126.02 B(C4H802XLI) 168.92 141.38 13.76 C],133.45; C2,135.22; C3,132.67; C4,133.54; C5,131.67; Qs, 127.52; C7,128.84; C8,127.71; C9,127.56; C10,125.13 LqH 179.19 156.20 11.33 C2,147.52; C3,123.04; C4,119.89; C5)135.30;C6,146.88 173.22 148.17 11.40 C2,148.08; C3,123.13; C4,121.45; C5,136.62; Qs, 149.81 B(C4H802XL2) 164.13 146.54 11.16 C2,147.46; C3,124.98; C4,119.95; C5,137.15; Cö,148.88 L3H 178.62 152.76 13.17 C2,140.31; C3,127.70; C4,121.45; C5,125.37 B(C3H6Q2XL3) 169.28 146.86 13.09 C2,141.35; C3,128.02; C4,121.98; C5,127.30 B(C4H802XL3) 172.52 145.64 13.23 C2,140.86; C3,129.96; C4,122.69; C5,126.64 187 Vol.