Abstract

2-Dicyanomethylene-1,1,3,4,5,5-hexacyanopentene (DHCP) dianion, which had been reported as 2,3-bis(dicyanomethyl)-1,1,4,4-tetracyanobutadiene, was synthesized as a tetraalkylammonium salt from hexacyanobutadiene. The crystal structures of tetraalkylammonium (alkyl: methyl, ethyl), tetrathiafulvalene (TTF) and bisethylenedithio–TTF (BEDT–TTF or ET) salts were solved. Possible reaction processes to give DHCP were deduced based on its molecular structure. Fourteen solid charge transfer complexes, seven of which were highly conductive, were prepared with mainly TTF derivatives as electron donor. ET afforded two metallic complexes, one of which (ET1) showed a metal–insulator transition at 180 K and the other (ET2) retained metallic behavior down to 1.3 K. BEDO–TTF complex was metallic down to 4.2 K. The crystal, electronic and band structures and EPR parameters of ET1 and ET2 were studied. Both ET1 and ET2 have a β″-type packing mode with significantly different mutual orientation between ET and DHCP molecules. The polymorphism is relevant to both different conformations of DHCP; cis and trans, which demonstrate the flexibility of the molecular shape and conformation, and intermolecular atomic contacts between ET and DHCP molecules.