A stable and flexible dianion: 2-dicyanomethylene-1,1,3,4,5,5-hexacyanopentenediide (DHCP2-), and its complex formation

2-Dicyanomethylene-1,1,3,4,5,5-hexacyanopentene (DHCP) dianion, which had b een reported as 2,3-bis(dicyanomethyl)-1,1,4 ,4-tetracyanobutadiene, was sy nthesized as a tetraalkylammonium salt from hexacyanobutadiene. The crystal structures of tetraalkylammonium (alkyl: methyl, ethyl), tetrathiafulvalen e (TTF) and bisethylenedithio-TTF (BEDT-TTF or ET) salts were solved. Possi ble reaction processes to give DHCP were deduced based on its molecular str ucture. Fourteen solid charge transfer complexes, seven of which were highl y conductive, were prepared with mainly TTF derivatives as electron donor. ET afforded two metallic complexes, one of which (ET1) showed a metal-insul ator transition at 180 K and the other (ET2) retained metallic behavior dow n to 1.3 K. BEDO-TTF complex was metallic down to 4.2 K. The crystal, elect ronic and band structures and EPR parameters of ET1 and ET2 were studied. B oth ET1 and ET2 have a beta"-type packing mode with significantly different mutual orientation between ET and DHCP molecules. The polymorphism is rele vant to both different conformations of DHCP; cis and trans, which demonstr ate the flexibility of the molecular shape and conformation, and intermolec ular atomic contacts between ET and DHCP molecules.