Polymeric copper(ii) and dimeric oxovanadium(v) complexes of amide-imine conjugate: bilirubin recognition and green catalysis

Abstract

An exceptionally simple amide-imine conjugate, (E)-N'-(4-(diethylamino)-2-hydroxybenzylidene)-4-methylbenzohydrazide (L), derived by the condensation of 4-methyl-benzoic acid hydrazide (PTA) with 4-(diethylamino)-2-hydroxybenzaldehyde was utilized to prepare a dimeric oxo-vanadium (V1) and a one-dimensional (1D) copper(ii) coordination polymer (C1). The structures of L, V1 and C1 were confirmed by single crystal X-ray diffraction analysis. The experimental results indicate that V1 is a promising green catalyst for the oxidation of sulfide, whereas C1 has potential for a C-S cross-coupling reaction in a greener way. Most importantly, C1 is an efficient 'turn-on' fluorescence sensor for bilirubin that functions via a ligand displacement approach. The displacement equilibrium constant is 7.78 × 10⁵ M^-1. The detection limit for bilirubin is 1.15 nM in aqueous chloroform (chloroform/water, 1/4, v/v, PBS buffer, and pH 8.0).

Scheme 1. Synthesis of PTA and L.

Scheme 2. Synthesis of V1 and C1.

Scheme 3. The V1 catalyzed oxidation of sulfide.

Fig. 1. (a) ORTEP of V1 with 50% ellipsoid probability and (b) significant features around the bridging portion. H atoms are omitted for clarity.

Fig. 2. (a) ORTEP view of C1 with 50% ellipsoid probability, (b) 1D coordination polymer chain of C1, and (c) important distances around the central atom. H atoms are omitted for clarity.

Fig. 3. (a) ORTEP view of L at 50% ellipsoid probability and (b) centro-symmetric disposition of three units self-assembled through hydrogen bond (black dashes). H atoms are omitted for clarity.

Fig. 4. Changes in the spectra of C1 (20 μM, λ_ex, 369 nm, λ_em, 444 nm) upon the gradual addition of bilirubin (0–2500 μM, CHCl₃/water, 1/4, v/v, 0.1 M PBS buffer, pH 8.0), (a) absorption; (b) emission.

Scheme 4. Proposed bilirubin sensing mechanism of C1.

Fig. 5. Spectra of C1 (20 μM, λ_ex, 369 nm) in the presence of amino acids, bio-relevant ions and biomolecules viz. Met, Arg, Cys, Pro, Lys, Trp, Phe, Gly, Asp, Na⁺, K⁺, Ca²⁺, Mg²⁺ and Cyt C, SOD, bisphenol, insulin, creatinine, GSH, uric acid (100 μM; chloroform/water, 1/4, v/v, PBS buffer, pH, 8.0), (a): emission and (b): absorption.

Scheme 5. H₂O₂-assisted oxidation of diphenyl sulphide using V1 as a catalyst.

Scheme 6. The C–S cross-coupling reaction of thiophenol with aryl iodide using C1 as a catalyst.