Interaction of a Vanadyl Porphyrin with Basic Amines and N-Heterocycles Ligands: A High-Resolution MS, UV–vis and DFT Study
Vanadyl porphyrins, which have central metals in the form of cations, are liable to coordinate axially with basic ligands. In the presented paper, the interaction behaviors of vanadyl octaethylporphyrin (VOOEP) with different basic ligands, viz. amines and N-heterocycles (such as pyridine, morpholine, n-butylamine, triethylamine, diethylamine, azetidine, and pyrrolidine), have been studied in detail through ultraviolet–visible (UV–vis) spectroscopy, collision-induced dissociation (CID) combined with Orbitrap mass spectrometry (MS), and density functional theory (DFT). The axial coordination ability and binding preference of VOOEP and ligands, as well as the relative stability, binding energies, and absorption spectra of the VOOEP-ligand complexes, were fully characterized in this study. The complexes involved two types of binding forms: axial coordination bonding and hydrogen bonding. It was observed that the axial coordination between VOOEP and basic ligands resulted in a decrease in intensity and red-shift of the UV–vis Soret band compared to pure VOOEP. Moreover, the axial coordination is closely correlated to the basicity and steric hindrance of the ligands. The relative stability and binding energies of the complexes are positively related to the basicity of the ligands. Additionally, besides VOOEP-ligand complexes (1:1 adducts), VOOEP-VOOEP-ligand complexes (2:1 adducts) were also observed and studied. This work offers insights that may aid petrochemists in the separation, enrichment, or selective removal of vanadyl porphyrins from petroleum samples and the investigation of porphyrin-asphaltene association behavior.
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