Synthesis, Antibacterial Studies and Crystal Structures of Tridentate Schiff Base Ligand and It’s Cobalt(III) Complex

Abstract

The crystal structures of 4-[(2-hydroxyphenyl) imino]-2-pentanone (H 2 hpac, 1) and its cobalt(III) complex [Co III (hpac)py 3 ] ? ÁPF 6 -(2) have been determined by X-ray diffraction. The ligand 1 crystallizes in orthorhombic chiral P2 1 2 1 2 1 space group, with a = 8.8405(4) Å , b = 10.5349(8) Å , c = 11.2292)7( Å ), and the complex 2-in the centrosymmetric monoclinic P2/n space group, with a = 16.496(5) Å , b = 10.171(2) Å , c = 16.646(5) Å , and b = 95.53(3)°. In the ligand molecule quite strong intramolecular hydrogen bond closes six-membered ring. The bond length pattern within this ring suggests the significant conjugation and the structure might be therefore regarded as the intermediate between keto-enamine and zwitterionic forms, and the intramolecular hydrogen bond falls into category of resonance-assisted hydrogen bonds. In turn, intermolecular O-HÁÁÁO hydrogen bonds connect the molecules of the ligand into infinite chains along [100] direction. In the complex, the Co(III) ion is hexa-coordinated, by two oxygen and one nitrogen atoms of the doubly-deprotonated ligand 1 and by three nitrogen atoms from three pyridine ligands. The coordination polyhedron is close to a slightly distorted octahedron. The in vitro antimicrobial activity of the Schiff base ligand and its corresponding complex have been tested against human pathogenic bacterias such as Staphylococcus aureus, Bacillus subtilis, Pseudomonas aeruginosa and Escherichia coli.Keywords Schiff base Á Tridentate ligands Á Co complex Á Hydrogen bonds Á Biological activity Results and DiscussionAcetylacetone (acac) or pentane-2,4-dione is often used as a good precursor in organic synthesis and as a building block of metal-organic complexes [1,2]. Its anion acts as the bidentate ligand and is known to form complexes with many transition metal ions [3][4][5]. A general method of preparation of such complexes consists in a reaction of a metal ion with acetylacetone in presence of a base which easily separates the proton. Replacement of the ketone functional group by the imine one results in corresponding bidentate Schiff bases, which also proved to be useful ligands. If 2-hydroxy aniline is used as the amine, one can prepare the tridentate ligand. Such a ligand-4-[(2-hydroxyphenyl)imino]-2-pentanone (hereafter referred to as 1), known as H 2 hpac, where H 2 represents the dissociable enolic -OH and hydroxyphenyl protons-has been already prepared and used for making the square-planar mixed-ligand nickel(II) complex with deprotonated H 2 hpac (i.e. hpac), which coordinates through the enolate oxygen, the imine nitrogen and the deprotonated hydroxyphenyl oxygen atoms. Additionally, the neutral N-donor, imidazole, has been used as an ancillary ligand [6]. Such

Mixed ligand complexes of cobalt(III) and iron(III) containing N2O2-chelating Schiff base: Synthesis, characterisation, antimicrobial activity, antioxidant and DFT study

“…In addition, a low absorbance band at ca. 500 nm is observed for cobalt(III) complexes that can be assigned to the ded transition [54].…”

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

“…[153][154][155][156][157][158][173][174][175][176] RAHB-assisted synthesis of b-enaminones by condensation affords an unique product only in the case of symmetrical b-diketones or when the two carbonyl groups show av ery different electrophilicity towards the nucleophile amine. The Z and E isomers of b-enaminones exhibit different chemical reactivities in synthetic and coordination chemistry.…”

“…The carbonyl group, conjugated to the enamine moiety, gives a RAHB system with enough stability for the β‐enaminones to be easily prepared, isolated, and stored under mild conditions. The Z and E isomers of β‐enaminones exhibit different chemical reactivities in synthetic and coordination chemistry . RAHB‐assisted synthesis of β‐enaminones by condensation affords an unique product only in the case of symmetrical β‐diketones or when the two carbonyl groups show a very different electrophilicity towards the nucleophile amine.…”