ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Computational and experimental studies on spectra and bonding of cyclohexane linked tris-hydroxyquinoline flexible tripodal chelator and its complexes

Monika Thakur, Minati Baral, and B. K. Kanungo

Department of Chemistry, Sant Longowal Institute of Engineering and Technology, Longowal, India

E-mail: b.kanungo@gmail.com

Received: 7 June 2022  Accepted: 21 January 2023

Abstract:

The synthesis and characterization of a flexible tripodal ligand 2,2′,2″-((1E,1′E,1″E)-(cyclohexane-1,3,5-triyltris(azanylylidene))tris(methanylylidene))tris(quinolin-8-ol), TACH2OX, containing three 8-hydroxyquinoline units joined to a cyclohexane‐1,3,5‐triamine skeleton has been described. The protonation equilibria and formation constants of the ligand and its complexes with iron (III), aluminum (III), and gallium (III) are explored by potentiometric and absorption spectrophotometric studies. Six protonation constants with log K values ranging from 1.22 to 9.91 were evaluated, substantiating three nitrogen atoms of the 8-hydroxyquinoline ring and three phenolate groups. The ligand formed [FeL], [FeLH], [FeLH₂], [FeLH₃] complexes for iron, [AlL], [AlLH₂], [AlLH₃] complexes for aluminum and [GaLH₃], [GaL] complexes for gallium. TACH2OX and its complexes exhibited a variation in the absorption and emission spectral behavior when measured in different pH. It exhibited an enhancement in emission at 400 nm at neutral pH 7.0, while in acidic and basic pH, the fluorescence is quenched with bathochromic shift implying its utility as an OFF-ON-OFF type pH molecular sensor in an aqueous solution. Interestingly, the fluorescence of TACH2OX showed more significant enhancement in the presence of Ga³⁺ as compared to Al³⁺, while a reverse phenomenon was observed for Fe³⁺, indicating that the ligand can behave as a metal sensor for the metal ions, one as "ON" type and the other as "OFF" type. Density functional theory with GGA-BLYP-D functional and TZ2P basis set was employed for the prediction of molecular geometry and spectral nature of all species of the ligand and its metal complexes. The nature of bonding and electronic transitions of the ligand and its complexes were assessed by EDA-NOCV, NBO, APO, and FMO studies.

Keywords: 8-HQ; Tripodal chelator; Formation constant; OFF-ON-OFF; Sensor

Full paper is available at www.springerlink.com.

DOI: 10.1007/s11696-023-02695-6

Chemical Papers 77 (6) 3163–3179 (2023)