Spectroscopy, molecular structure, and electropolymerization of Ni(ii) and Cu(ii) complexes containing a thiophene-appending fluorinated Schiff base ligand

In this contribution, we describe the preparation, characterization, and electrochemical behavior of a series of four new mononuclear M(ii) complexes featuring a symmetric substituted N₂O₂-tetradentate Schiff base ligand, bearing either trifluoromethyl and p-bromophenyl (M = Ni, 3; Cu, 4) or trifluoromethyl and the π-extended p-(2-thienyl)phenylene (M = Ni, 5; Cu, 6) substituents. Complexes 3 and 4 were readily synthesized by reacting the diprotic fluorinated Schiff base proligand 2 with the appropriate hydrated metal(II) acetates, whereas 5 and 6 were obtained upon Stille cross-coupling reaction of 3 and 4 with 2-(tributylstannyl)-thiophene, respectively. Compounds 3-6 were isolated as neutral, air, and thermally stable-coloured solids, with yields ranging from 60 to 80%. The four complexes, the diimine precursor 1 and its trifluoroacetylated derivative 2, were identified using analytical (EA, ESI-MS), spectroscopic (IR, ¹H, ¹³C, and ¹⁹F NMR), and X-ray crystallographic methods. X-ray crystal structure determination of complexes 3-5 revealed that both four-coordinate Ni(ii) and Cu(ii) metal ions adopt a square planar geometry. The magnetic properties of powdered samples of the Cu(ii) derivatives 4 and 6 have been investigated (2-300 K) and found consistent in both cases with a single isolated copper(ii) ion (s = 1/2). DFT calculations were used to examine the optimal geometries of complexes 5 and 6, allowing for a consistent perspective of their structure and characteristics. The primary aspects of the UV-vis spectra were interpreted using TD-DFT computations. Finally, electrochemical data indicate that complexes 5 and 6 polymerize at high anodic potentials in acetonitrile (greater than 2.0 V vs. Ag/AgCl). Cyclic voltammetry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (SEM-EDS) analyses were used to characterize the obtained films poly-5 and poly-6.