Electrospray mass spectrometric and DFT study of substituent effects in Ag+ complexation to polycyclic aromatic hydrocarbons (PAHs)

Abstract

Complexation of Ag(I) cation to a series of substituted anthracenes (AN), phenanthrenes (PH), pyrenes (PY) and cyclopenta[a]phenanthrenes (CPaPH) was studied in competitive experiments by allowing PAHs to react in pairs with AgOTf. The resulting complexes were examined by electrospray mass spectrometry (ES-MS) to determine relative abundances of the corresponding monomeric and dimeric complexes. Based on this data a sequence of complexation ability rankings was derived for each group. Among the substituents examined, a -COMe group when placed at the meso position in AN and PH, or at the C-1 in PY is most effective in Ag⁺ complexation, whereas an -NO₂ group is least efficient. Methyl groups at the meso positions are better than in the terminal rings. For the CPaPH series, bay region substitution (methyl and alkoxy) have limited effect as does carbonyl substitution in the annelated CP ring. In the PY series, a -COPh or a -CH(Me)OH group at C-1 is as efficient as -COMe. Based on extensive potential energy searches, four types of complexation modes were identified by B3LYP/LANL2DZ calculations involving Ag⁺ complexation to -NO₂ oxygens, to -COMe or to -OH and a peri-carbon, to just one ring carbon, or by bridging two ring carbons. Among these modes, the first two are most favorable. The energetic preferences were rationalized with charge decomposition analysis (CDA). Effect of Ag⁺ complexation on relative aromaticity in various rings was examined by NICS (nucleus independent chemical shift) in two representative cases. Structures and energies of the acetyl pyrene–Ag⁺–pyrene hetero-dimer and acetyl pyrene–Ag⁺–acetyl pyrene homo-dimer complexes were determined with the same model. These complexes have sandwich structures.