Synthesis of Diorganosilicon (Iv) Complexes of 3-Hydroxy-2-Pyridine Carboxylic Acid and 2-Pyridine Carboxylic Acid

Complexes of 3-hydroxy-2-pyridine carboxylic acid are of bioinorganic interest and also pose structural ambiguities since they display a number of possible bonding modes.141-145 This multidentate ligand contains three functional groups: pyridyl nitrogen atom, two oxygen atoms of the carboxyl group and one oxygen atom of the hydroxyl group leading to the formation of complexes with various structures. 3-Hydroxy-2-pyridine carboxylic acid ligand (HL2) is a potential chelate with interesting possibilities, either having N,O-chelation through the pyridine nitrogen and the carboxylate group, forming a five membered chelate ring or O,O-chelation through the carboxylate group and the deprotonated hydroxyl group, forming a six-membered chelate ring. The O,O-chelation is identical to the salicylato coordination mode reported for related ambidentate ligands. In addition, the 3-hydroxy-2-pyridine carboxylate ion can also act as a monodentate or bridging ligand. 2-Pyridine carboxylic acid (HL3) contains a carboxylic group in the ortho-position to the nitrogen in the pyridine ring, acting as a bidentate ligand by (N,O) coordination (Fig.-2.4).

NCOOH

Fig.-2.4

NCOOH

OH 2-Pyridine carboxylic acid (HL3)3-Hydroxy-2-pyridine carboxylic acid (HL2)

Organosilicon compounds exhibit a broad spectrum of biological activities and the activity of some biologically active compounds was appreciably enhanced on coordination with metal ion or with organosilicon halides. Keeping this in mind we have synthesized diorganosilicon(IV) complexes of 3-hydroxy-2-pyridine carboxylic acid and 2-pyridine carboxylic acid and evaluated them for in vitro antimicrobial activity against several bacteria and fungi. The complexes were obtained by the reaction of diorganodichlorosilanes R2SiCl2 (R = Me, Et or Ph) with the sodium salt of 3-hydroxy-2-pyridine carboxylic acid (NaL2) and 2-pyridine carboxylic acid (NaL3) in 1:1 and 1:2 molar ratios in dry benzene (Scheme-2.2).

+

1:1

R2SiCl2

1:2

NaL2 / NaL3

NSiO O RR Cl Scheme-2.2 NSiNOO O O R R X X X

C6H6 C6H6

L2 = 3-hydroxy-2-pyridine carboxylate ionL3 = 2-pyridine carboxylate ionR = Me, Et, Ph

X = H or OH These diorganosilicon(IV) complexes were white solids, insoluble in most of the common organic solvents except in DMSO and DMF. Low molar monomeric nature of these complexes was confirmed by the molecular weight determinations. The stereochemistry of these complexes was determined by using IR and NMR (1H, 13C and 29Si) spectral data.

The infrared spectra of the free ligands 3-hydroxy-2-pyridine carboxylic acid (HL2) and 2-pyridine carboxylic acid (HL3) are given in Table-2.4. In the IR spectrum of HL2 the bands due to ν (O-H), νas (COO), νs (COO) and ν (C=N) were present at 3340, 1676, 1323 and 1606 cm-1, respectively. Whereas, in the IR spectrum of HL3 the same bands were present at 3280, 1716, 1293 and 1607 cm-1, respectively. The bands at 1235 and 1056 cm-1 in the IR spectrum of HL2 and at 1254 and 1060 cm-1 in the IR spectrum of HL2, were either due to ring vibrations or hydrogen deformation modes.

The 1H NMR spectra of the free ligands HL2 and HL3 are given in Table-2.5. In 1H NMR spectra of the ligands HL2 and HL3, single resonance was observed at δ 14.27 and 9.22, respectively, due to -COOH group. The downfield shift in the position of carboxylic acid proton of HL2 may be due to its involvement in intramolecular hydrogen bonbing with the hydroxyl group present at 3-position of the ring. The presence of intramolecular hydrogen bonbing in HL2 was also supported by the fact that the singlet due to phenolic proton appearing at δ 11.12 was broadened. The signals for the aromatic protons of the ligands HL2 and HL3 were observed in the range δ 7.22-8.08 and 7.53-8.74, respectively.

The 13C NMR spectra of the free ligands HL2 and HL3 are given in Table-2.6. For ligand HL2 the C2, C3, C4, C5 and C6 carbons of pyridine ring showed signals at δ 137.40, 159.43, 129.42, 129.76 and 140.68 respectively, while for the ligand HL3 signals due to the same carbons were observed at δ 148.43, 128.67, 137.66, 127.23 and 149.20. The carbon of carboxylate group of the ligands HL2 and HL3 was observed at δ 175.62 and δ 167.74, respectively.

The infrared spectra of the diorganosilicon(IV) complexes of 3-hydroxy-2-pyridine carboxylic acid and 2-pyridine carboxylic acid are given in Table-2.4. The IR spectra of the diorganosilicon(IV) complexes of 3-hydroxy-2-pyridine carboxylic acid showed bands due to νas (COO) and νs (COO) in the range 1666-1671 and 1358-1365 cm−1, respectively. The magnitude of Δν [Δν = νas (COO) − νs (COO)] was about 303-310 cm−1. (COO) and νs (COO) were in the range 1704-1710 and 1324-1336 cm−1, respectively and the magnitude of Δν was about 372-383 cm−1. The large magnitude of Δν in both the cases indicated that the carboxylate ligand functions as monodentate ligand and the bridging or chelation and ionic bonding can be excluded. 100,101 The infrared spectra of the diorganosilicon(IV) complexes of HL2 showed a strong absorption in the region 3212-3268 cm-1, assigned to the stretching vibrations of the O-H bond in the hydroxyl group of the ligand.

In the IR spectra of diorganosilicon(IV) complexes of HL2 and HL3, the ν (C=N) band was shifted towards lower frequencies due to the displacement of electron density from nitrogen to silicon on coordination, and was present in the range 1587-1593 and 1586-1592 cm-1, respectively.119 The formation of the resulting complexes was also supported by the presence of new bands which were absent in the free ligands. In the IR spectra of the diorganosilicon(IV) complexes of HL2 the bands due to ν (Si-O), ν (Si-N) and ν (Si–Cl) were present at 800-818, 510-522 and 495-504 cm-1, respectively. Similarly, in the IR spectra of the diorganosilicon(IV) complexes of HL3 these bands were present at 802-820, 515-530 and 497-508 cm-1, respectively. In dimethyl- and diethylsilicon(IV) complexes of HL2 and HL3, the bands at 1440-1480 and 1235-1265 cm-1 were attributed to the asymmetric and symmetric deformation vibrations of methyl and ethyl groups attached to silicon. In diphenylsilicon(IV) complexes of HL2 and HL3, the weak to strong intensity bands around 1485-1489, 1125-1131, 743-750 and 697-703 cm-1 have been assigned to ν (Si-C6H5) modes.59

Contd….

The 1H NMR spectra of diorganosilicon(IV) complexes of HL2 and HL3 are given in Table-2.5. The peak due to carboxylic acid proton was absent in the spectra of the complexes, indicating the coordination of oxygen of carboxylate ion after the deprotonation of the carboxylic acid proton to silicon moiety. Further in 1H NMR spectra of diorganosilicon(IV) complexes of HL2 a sharp singlet at δ 11.28-11.52 was observed due to the phenolic proton, which showed that it was not involved in coordination as well as in intramolecular hydrogen bonding in the complexes. In the 1H NMR spectra of the complexes the signals for the aromatic protons of the ligands HL2 and HL3 were slightly deshielded and appeared as multiplet at δ 7.28-8.72 and 7.70-9.10, respectively. Methyl groups attached to silicon in the complexes appeared as a singlet at δ 0.90-0.93, while the ethyl and phenyl groups attached to the silicon gave multiplets at δ 0.92-1.32 and 7.18-7.75, respectively.

The 13C NMR spectra of diorganosilicon(IV) complexes of HL2 and HL3 are given in Table-2.6. In the 13C NMR spectra of diorganosilicon(IV) complexes of HL2, the C2, C3, C4, C5 and C6 carbons showed signals at δ 130.34-132.52, 148.67-149.64, 127.47-128.33, 127.79-128.20 and 143.12-143.51, respectively. In the case of diorganosilicon(IV) complexes of HL3, the signals due to same carbons were at δ 141.34-142.33, 128.14-128.32, 139.18-139.47, 127.10-127.43 and 149.16-149.59, respectively. The carbon of carboxylate group of the free ligands HL2 and HL3 was shifted to δ 166.53-167.75 and 157.61-158.87 on complexation, indicating the involvement of oxygen atom of COO- group. The signals due to carbons of methyl groups attached to silicon in the complexes appeared at δ 1.91-1.99, while carbons of ethyl groups appeared at δ 5.52-5.84 in the range δ 137.37-137.53, 135.58-135.78, 133.23-133.63 and 129.50-130.49.

The 29Si NMR of 1:1 diorganosilicon(IV) complexes gave sharp signals at δ -80 to -110 and the spectra of 1:2 diorganosilicon(IV) complexes gave sharp signals at δ -160 to -180, which clearly indicated the penta- and hexa-coordinated environment, respectively, around the silicon atom. Thus, on the basis of the foregoing spectral features and monomeric behavior of the complexes, penta coordinated trigonal bipyramidal and hexa coordinated octahedral geometries, have been suggested for the 1:1 and 1:2 diorganosilicon(IV) complexes, respectively. In the 1H NMR spectra of the complexes the signals for the aromatic protons of the ligands HL2 and HL3 were slightly deshielded and appeared as multiplet at δ 7.28-8.72 and 7.70-9.10, respectively. Methyl groups attached to silicon in the complexes appeared as a singlet at δ 0.90-0.93, while the ethyl and phenyl groups attached to the silicon gave multiplets at δ 0.92-1.32 and 7.18-7.75, respectively.

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