Complex Formation between a Nucleobase and Tetracyanoquinodimethane Derivatives: Crystal Structures and Transport Properties of Charge-Transfer Solids of Cytosine

Abstract

Reaction between cytosine, a nucleobase, in methanol and several 7,7,8,8-tetracyanoquinodimethane derivatives (R-TCNQ) in acetonitrile yielded three kinds of ionic solids; (I) insulators composed of methoxy-substituted R-TCNQ anions, (II) semiconducting fully ionic R-TCNQ radical anion salts, and (III) conductive partially ionic or mixed-valent R-TCNQ radical anion salts. Electronic and chemical structures of these products were characterized by optical and magnetic measurements, and structural and elemental analyses. Cation units in all products were found to be protonated cytosine species. Crystal structures were determined for methoxy-substituted anion salts (R = F 4 and H) in Group I and R-TCNQ radical anion salts (R = H and Et 2 ) in Group II with hemiprotonated cytosine pairs formed by triple self-complementary hydrogen bonds. They established one-dimensional hemiprotonated cytosine ribbons by double complementary hydrogen bonds. Hydrogen bonds between cytosine and R-TCNQ anions exhibited high potential to regulate molecular arrangements producing a segregated layered structure and uniform arrangement of R-TCNQ radical anion columns stable down to low temperature. The partially ionic salt of MeTCNQ in Group III exhibited metallic behavior and the highest conductivity of 10 þ1 S cm À1 so far observed for charge-transfer complexes based on biological molecules.Biological molecules such as proteins, enzymes, DNA, etc., establish well-defined and complicated self-assembling structures which are important factors in the exhibition of their biological functions. This feature of biological molecules has attracted much attention from the viewpoint of supramolecular chemistry and crystal engineering in recent study of molecule-based materials.2 In research for organic conductors, several attempts to investigate charge-transfer (CT) complexes based on a variety of biological molecules have been also undertaken.

Cover Picture: Oxidative Cyclization Reaction of 2‐Aryl‐Substituted Cinnamates To Form Phenanthrene Carboxylates by Using MoCl5 (Chem. Eur. J. 39/2014)

“…The hemiprotonated CHC + units are arranged to form an infinite ribbon with a width of 9.8 Å, through double NH⋅⋅⋅O (N11⋅⋅⋅O11; 2.78 Å) hydrogen bonds, which are related by an inversion center, along the a axis. Similar arrangements have been reported for several CHC + salts formed with anions such as BF 4 − ,5f PF 6 − ,7e 2,5‐diethyl‐TCNQ .− ,7d,e and ZnCl 4 2− 6b. Thus far, only two infinite arrangement patterns of CHC + have been reported, both of which have a ribbon‐like 1D structure in terms of double complementary NH⋅⋅⋅O hydrogen bonds as shown in Figure 5.…”

“… Hydrogen‐bonded one‐dimensional CHC + ribbons a) in 2 along the a axis and b) in (CHC)TCNQ (200 K) 7c,d. Green dotted lines show NH⋅⋅⋅O, NH⋅⋅⋅N (both within CHC + ), or NH⋅⋅⋅O (between CHC + ) hydrogen bonds.…”

“… Schematic representations of the arrangement of CHC + units a) in 4 , b) in 2 and 3 , and c) in (CHC)TCNQ 7c,d. A plate corresponds to a CHC + unit, and green dotted lines show NH⋅⋅⋅O hydrogen bonds.…”

“…Most of the previous studies remain a subject of synthetic and structural aspects, and the exploration of the π functionalities remains limited to the charge‐transfer (CT) complexes formed with electron acceptors such as tetracyanoquinodimethane (TCNQ) analogues 7c–f. The Mott insulator (CHC + )TCNQ .− (CHC + : hemiprotonated cytosine)7c,d,f is particularly noteworthy; it showed no phase transitions such as a spin‐Peierls transition, down to 10 K despite its 1D segregated stack of TCNQ .− . The robust ribbon‐type assembly of CHC + , which affords CH⋅⋅⋅N and NH⋅⋅⋅N hydrogen bonds to the neighboring TCNQ .− array, apparently prevents lattice distortion inherent to the 1D electronic structure.…”

Functional Systems Derived from Nucleobase Self‐assembly

“…Ionic complexes composed of nucleobases that display semiconductive or metallic behavior can be produced with the assistance of H‐bonding interactions. The formation of hemiprotonated C dimer (C⋅HC + ) and H‐bonding between columns of C and substituted 7,7,8,8‐tetracyanoquinodimethane (TCNQ) was suggested to provide part of the driving force for the formation of radical ions of the latter component in crystals . The nature of the substituent in R‐TCNQ strongly affects the degree of charge separation and hence electrical conductivity of the products.…”