Multiplex proton-transfer and electron-transfer natures based on the 2,2'-Bi-1H-imidazole system .2. Crystal structures and charge-transfer complex formations

Studies of the crystal structures and formation of a charge-transfer (CT) complex proved that 2,2'-bi-1H-imidazole (H2BIM(0)) forms a new system concerned with the interplay of proton-transfer (PT) and electron-transfer (or charge-transfer (CT)) interactions. X-Ray crystal analyses of newly isolated species, 2-(2-1H-imidazolyl)-1H-imidazolium (H3BIM(+)) and 2,2'-bi-1H-imidazolium (H4BIM(2+)), revealed the formation of strong hydrogen bonds along the side-by-side direction of the molecular plane. CT complexes of H3BIM(+) and H4BTM(2+) with the 7,7,8,8-tetracyanoquinodimethane derivatives (TCNQs) were synthesized by a method metathesis, and the crystal structure of (H3BIM(+))(2)(TCNQ(-))(2)(TCNQ(0)) was determined. ## During CT complex formation, the initial simple protonated states of [H3BIM(+)] or [H4BIM(2+)] changed to different ones, for example, [H4BTM(2+)](x)[H3BIM(+)](1-x), [H3BIM(+)](x)[H2BIM(0)](1-x), and H2BIM(0). According to the protonated states of the H2BIM system, the formal charge of the TCNQs in the CT complexes varies from completely ionic, partial CT, to neutral. The mechanism of these complex formations was explained by differences in the acidities between the TCNQ anion radicals (TCNQs(-.)) and H3BIM(+) or H4BIM(2+). Higher acidities of H3BIM(+) and H4BIM(2+) than the TCNQs(-.) induce the following PT reaction: H3BIM(+) (or H4BIM(2+)) + TCNQs(-.) -->H2BIM(0) (or H3BIM(+)) +HTCNQ(.). The isolation of HTCNQ(.) was a failure due to the following disproportional reaction: 2HTCNQ(.) -->TCNQ(0) +H2TCNQ(0). However, the formation of the 1,4-benzenedimalononitrile (H2TCNQ(0)) was confirmed by an analysis of the residual compounds after complex formation. Although the electronic absorptions of TCNQ complexes indicated both the partial CT state of TCNQ and a segregated stacking manner, semiconductive characters were observed. In a crystal of (H3BIM(+))(2)(TCNQ(-))(2)(TCNQ(0)), the H3BIM(+) formed a planar dimer connected by two N-H ... N hydrogen bonds, and TCNQ molecules existed as a charge-separated state with segregated trimer stacks. These dimer and TCNQ stacks were tightly connected by both N-H ... N=C hydrogen bonds, which spread into the overall crystal, and by the CT interaction. For the PT and CT system, we propose a synthetic strategy based on the redox potentials and acid dissociation constants.