Aromaticity of substituted 8-hydroxyquinolines in their free or bidentate states in tricarbonyl rhenium(i) complexes

Abstract

Some 8-hydroxyquinolines (8-HQs) and some of their metal complexes are bioactive. Thus, knowing whether the ligand properties are transferred to the complex is vital. Herein, the aromaticity of the pyridine and phenolic rings of substituted 8-HQs, free with and without intramolecular H-bond and chelated in Re(I) complexes, was studied using the geometrical HOMA and magnetic INICS indices. The 8-HQs were substituted in every position with BH₂, CN, CCH, Cl, and NH₂ groups of diversified σ- and π-electron donor–acceptor properties. In the H-bonded 8-HQs, the BH₂ and NH₂ groups stabilize different positions. The stabilization pattern is similar for the non-H-bonded 8-HQs but quite different in the Re(I) complex. The HOMA and INICS values of the pyridine and phenolic rings show that they are geometrically and magnetically aromatic irrespective of the substituent type, substituent location, H-bond presence, or complex formation. Forming or breaking the O–H⋯N intramolecular H-bond does not essentially affect the two types of aromaticity. In contrast, chelating the Re(I) system with 8-HQ changes both so that the aromaticity of the H-bonded and chelated systems does not correlate. Hence, the aromaticity of the 8-HQ rings is not transferred from the free ligand to the complex with Re(I). Additionally, for H-bonded 8-HQs, the geometrical and magnetic aromaticity do not correlate, but a weak correlation trend can be observed for the complex.