title: Charge-assisted halogen bonding: donor-acceptor complexes with variable ionicity.
abstract: Charge-assisted halogen bonding is unambiguously revealed from structural and electronic investigations of a series of isostructural charge-transfer complexes derived from iodinated tetrathiafulvalene and tetracyanoquinodimethane derivatives, (EDT-TTFI2)2(TCNQF(n)), n=0-2, which exhibit variable degrees of ionicity. The iodinated tetrathiafulvalene derivative, EDT-TTFI2, associates with tetracyanoquinodimethane (TCNQ) and its derivatives of increasing reduction potential (TCNQF, TCNQF2) through highly directional C-I⋅⋅⋅N≡C halogen-bond interactions. With the less oxidizing TCNQ acceptor, a neutral and insulating charge-transfer complex is isolated whereas with the more oxidizing TCNQF2 acceptor, an ionic, highly conducting charge-transfer salt is found, both of 2:1 stoichiometry and isostructural with the intermediate TCNQF complex, in which a neutral-ionic conversion takes place upon cooling. A correlation between the degree of charge transfer and the C-I⋅⋅⋅N≡C halogen-bond strength is established from the comparison of the structures of the three isostructural complexes at temperatures from 300 to 20 K, thus demonstrating the importance of electrostatics in the halogen-bonding interaction. The neutral-ionic conversion in (EDT-TTFI2)2(TCNQF) is further investigated through the temperature dependence of its magnetic susceptibility and the stretching modes of the C≡N groups.
authors.affiliation: Institut des Sciences Chimiques de Rennes, Université Rennes 1 & CNRS UMR 6226, Campus de Beaulieu, 35042 Rennes (France), Fax: (+33) 23-23-67-32.
authors.forename: Julien
authors.initials: J
authors.lastname: Lieffrig
authors.forename: Olivier
authors.initials: O
authors.lastname: Jeannin
authors.forename: Arkadiusz
authors.initials: A
authors.lastname: Frąckowiak
authors.forename: Iwona
authors.initials: I
authors.lastname: Olejniczak
authors.forename: Roman
authors.initials: R
authors.lastname: Świetlik
authors.forename: Slimane
authors.initials: S
authors.lastname: Dahaoui
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Aubert
authors.forename: Enrique
authors.initials: E
authors.lastname: Espinosa
authors.forename: Pascale
authors.initials: P
authors.lastname: Auban-Senzier
authors.forename: Marc
authors.initials: M
authors.lastname: Fourmigué
keywords: charge transfer; conducting materials; crystal engineering; donor-acceptor systems; halogen bonding; noncovalent interactions