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

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.