Novel CoIII complexes containing fluorescent coumarin-N-acylhydrazone hybrid ligands: Synthesis, crystal structures, solution studies and DFT calculations

Abstract

A series of new CoIII complexes of the type [Co(dien)(L1−L3)]ClO4 (1–3), containing fluorescent coumarin-N-acylhydrazonate hybrid ligands, (E)-N′-(1-(7-oxido-2-oxo-2H-chromen-3-yl)ethylidene)-4-R-benzohydrazonate [where R=H (L12−), OCH3 (L22−) or Cl (L32−)], were obtained and isolated in the low spin CoIII configuration. Single-crystal X-ray diffraction showed that the coumarin-N-acylhydrazones act as tridentate ligands in their deprotonated form (L2−). The cation (+1) complexes contain a diethylenetriamine (dien) as auxiliary ligand and their structures were calculated by DFT studies which were also performed for the CoII (S=1/2 and S=3/2) configurations. The LS CoII (S=1/2) concentrated the spin density on the O–Co–O axis while the HS CoII (S=3/2) exhibited a broad spin density distribution around the metallic center. Cyclic voltammetry studies showed that structural modifications made in the L2− ligands caused a slight influence on the electronic density of the metal center, and the E1/2 values for the CoIII/CoII redox couple increased following the electronic effect of the R-substituent, in the order: 2 (R=OCH3)<1 (R=H)<3 (R=Cl). The theoretical redox potentials (E°) of the process CoIII→CoII were calculated for both CoII spin states (S=1/2 and S=3/2) and a better correlation was found for CoIII→CoII (S=1/2), compared with experimental values vs SHE (E°calc=−0.37, −0.36 and −0.32V vs E°exp.=−0.371, −0.406 and −0.358V, for 1–3 respectively). Complexes 1–3 exhibited a very intense absorption band around 470nm, assigned by DFT calculations as π-π* transitions from the delocalized coumarin-N-acylhydrazone system. 1–3 were very stable in MeOH for several days. Likewise, 1–3 were stable in phosphate buffer containing sodium ascorbate after 15h, which was attributed to the high chelate effect and σ-donor ability of the L2− and dien ligands.