Monomeric Ti(iv)-based complexes incorporating luminescent nitrogen ligands: synthesis, structural characterization, emission spectroscopy and cytotoxic activities?

Dalton Transactions Pub Date: 2016-11-18 DOI: 10.1039/C6DT03477B

Abstract

This manuscript describes the synthesis of a series of neutral titanium(IV) monomeric complexes constructed around a TiO4N2 core. The two nitrogen atoms that compose the coordination sphere of the metallic center belong to 2,2′-bipyrimidine ligands homo-disubstituted in the 4 and 4′ positions by methyl (2a), phenylvinyl (2b), naphthylvinyl (2c) or anthrylvinyl (2d) groups. The crystal structures of these complexes named [Ti(1)2(2a)], [Ti(1)2(2b)], [Ti(1)2(2c)] and [Ti(1)2(2d)] (where 1 is a 2,2′-biphenolato ligand substituted in the 6 and 6′ positions by phenyl groups) are reported. The hydrolytic stability of the four complexes is evaluated by monitoring the evolution of the free 2a–d signals by 1H NMR spectroscopy. For the conditions tested (6 mM, DMSO-d6/D2O: 8/1), a rather good stability with t1/2 ranging from 180 to 300 min is determined for the complexes. In the presence of an acid (DCl), the hydrolysis of [Ti(1)2(2a)] is faster than without an acid. The cytotoxic activity against gastric cancer cells of the titanium-based compounds and the free disubstituted 2,2′-bipyrimidine ligands is tested, showing IC50 ranging from 6.2 ± 1.2 μM to 274 ± 56 μM. The fluorescence studies of the ligands 2a–d, and the complexes [Ti(1)2(2a–d)] reveal an important fluorescence loss of the ligands 2c and 2d upon coordination with the Ti(1)2 fragment. Frontier orbitals obtained by DFT calculations permit us to explain this fluorescence quenching.

Graphical abstract: Monomeric Ti(iv)-based complexes incorporating luminescent nitrogen ligands: synthesis, structural characterization, emission spectroscopy and cytotoxic activities
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