Experimental and computational study of the axial interaction of a nickel porphyrin with different basic ligands

Abstract

Metalloporphyrins are organic compounds found in petroleum that form complexes with transition metals, such as vanadium and nickel. These complexes are detrimental to catalytic cracking and hydrotreating catalysts. The identification and removal of vanadium and nickel porphyrins from petroleum have been extensively explored. In this work, the axial coordination interaction of nickel octaethylporphyrin (NiOEP) with various ligands has been investigated using experimental and computational methods. The five- and six-coordinated complexes formed by NiOEP with mono- and bis-ligands, respectively, were studied and compared. Changes in the ultraviolet-visible (UV-Vis) spectra, specifically the location and intensity of the Soret band of NiOEP upon ligand addition, were mainly attributed to the formation of six-coordinated complexes, which were consistent with the results of binding energies and simulated excited states calculated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. The axial coordination ability of different ligands with NiOEP is closely related to the basicity and steric hindrance of the ligands, whereas the binding energies and relative stability of the coordinated complexes are correlated with the basicity of the ligands. In addition to the five- and six-coordinated complexes, the complexes formed by bis-NiOEP and mono-ligands (2 : 1 adduct) were also observed and characterized using high-resolution mass spectrometry (HR MS) combined with the collision-induced dissociation (CID) technique.