Synthesis and Characterization of Monomeric Hexacoordinated Chalcogenonium Salts Bearing 2-(2-Pyridyl)phenyl Ligands

Abstract

Consecutive treatment of 2-(2-lithiophenyl)pyridine with chalcogen tetrachlorides (ChCl4; Ch = Se and Te) and KPF6 afforded the hexafluorophosphate salts of tris[2-(2-pyridyl)phenyl]selenonium [(ppy)3Se]+ ([1]+) and tris[2-(2-pyridyl)phenyl]telluronium [(ppy)3Te]+ ([2]+), respectively. The chalcogenonium salts [1][PF6] and [2][PF6] exhibit extremely high thermal stability and high melting points. Their solid-state structures revealed distorted octahedral bonding geometries with a C3-symmetrical facial configuration. Quantum chemical calculations on [1]+ and [2]+ strongly suggest that their HOMO−3s involve lone pairs of electrons (LPs) on the central chalcogen atoms, and that the three Ch–N (Ch = Se and Te) bonds are formed by LP(N)→σ*(Ch–C) donor-acceptor interactions. Based on theoretical calculations and 77Se and 125Te NMR spectral data, the C–Ch–N moieties of [1]+ and [2]+ were characterized as asymmetrical three-center four-electron σ-type bonds [σ(3c–4e)]. Both organochalcogenonium salts represent novel monomeric hexacoordinated species that bear 14 formal valence electrons (14–Se–6 and 14–Te–6, respectively). A quantum theory of atoms-in-molecules dual functional analysis predicted that the C–Ch interaction of the σ(3c–4e) C–Ch–N moiety (Ch = Se and Te) is weakly covalent, while the Se–N and Te–N interactions were predicted to have the typical characteristics of hydrogen bonds and molecular complexes, respectively. The differences between [2][PF6] and the recently reported [2][Br] were also carefully examined.