Noncovalent Interactions in Peri-Substituted Chalconium Acenaphthene and Naphthalene Salts: A Combined Experimental, Crystallographic, Computational, and Solid-State NMR Study

Abstract

Twelve related monocation chalconium salts [{Nap(EPh)(E'Ph)Me}(+){CF3SO3}(-)] 2-4, [{Acenap(Br)(EPh)Me}{CF3SO3}(-)] 5-7, and [{Acenap(EPh)(E'Ph)Me}(+){CF3SO3}(-)] 8-13 have been prepared and structurally characterized. For their synthesis naphthalene compounds [Nap(EPh)(E'Ph)] (Nap = naphthalene-1,8-diyl; E/E' = S, Se, Te) N2-N4 and associated acenaphthene derivatives [Acenap(X)(EPh)]/[Acenap(EPh)(E'Ph)] (Acenap = acenaphthene-5,6-diyl; E/E' = S, Se, Te; X = Br) A5-A13 were independently treated with a single molar equivalent of methyl trifluoromethanesulfonate (MeOTf). In addition, reaction of bis-tellurium compound A10 with 2 equiv of MeOTf afforded the doubly methylated dication salt [{Acenap(TePhMe)(2)}(2+){(CF3SO3)(2)}(2-)}] 14. The distortion of the rigid naphthalene and acenaphthene backbone away from ideal was investigated in each case and correlated in general with the steric bulk of the interacting atoms located at the proximal peri positions. Naturally, introduction of the ethane linker in acenaphthene compounds increased the splay of the bay region compared with equivalent naphthalene derivatives resulting in greater peri distances. The conformation of the aromatic rings and subsequent location of p-type lone pairs has a significant impact on the geometry of the peri region, with anomalies in peri separations correlated to the ability of the frontier orbitals to take part in attractive or repulsive interactions. In all but one of the monocations a quasi-linear three-body C-Me-E center dot center dot center dot Z (E = Te, Se, S; Z = Br/E) fragment provides an attractive component for the E center dot center dot center dot Z interaction. Density functional studies confirmed these interactions and suggested the onset of formation of three-center, four-electron bonding under appropriate geometric conditions, becoming more prevalent as heavier congeners are introduced along the series, The increasingly large J values for Se-Se, Te-Se, and Te-Te coupling observed in the Se-77 and Te-125 NMR spectra for 1, 3, 4, 9, 10, and 13 give further evidence for the existence of a weakly attractive through-space interaction.