2,5-DisubstitutedN,N′-Dicyanobenzoquinonediimines (DCNQIs): Charge-Transfer Complexes and Radical-Anion Salts and Copper Salts with Ligand Alloys: Syntheses, Structures and Conductivities

The new members of the series of 2,5-disubstituted DCNQIs, 1d (Cl/OMe), 1e (Br/OMe), 1j (Cl/I), 1k (Br/I), 1l (I/I), form conducting charge-transfer complexes with TTF (tetrathiofulvalene) which are comparable to known DCNQI/TTFs. From these DCNQIs highly conducting radical-anion salts [2-X, 5-Y-DCNQI]2M (M = Li, Na, K, NH4, Tl, Rb, Ag, Cu) can also be prepared either from the DCNQIs and MI (not AgI), on a metal wire (Ag, Cu), or by electrocrystallization (M = Tl, Ag,Cu). For better crystals a method using periodical switching between reduction and partial oxidation has been developed. With CF3 (large, strongly electron-attracting) as the substituent in DCNQIs 1m (OMe/CF3) and 1n (Me/CF3), conducting TTF complexes remain whereas only 1n yields an insulating copper salt. DCNQI–Cu salts with high conductivities are obtained with alloys containing two or three different DCNQIs. The temperature-dependent conductivities of DCNQI–M salts (other than copper) are similar to those of metal-like semiconductors. All new DCNQI–Cu salts are metallic [M] down to low temperatures, except [1d (Cl/OMe)]2Cu which undergoes a sharp phase transition to an insulating state[M I]. By variation of the ligands or their ratios in conducting alloys of DCNQI–Cu salts temperature-dependent conductivities can be tuned from M I to M. In addition, alloying three ligands produced for the first time a radical salt with temperature-independent conductivity from 5 to 300 K. Most remarkably, alloys of the type [(2,5-Me2DCNQI)m] Cu/[{2,5-(CD3)2-DCNQI}n]2Cu which exhibit a sharp M I phase transition on further cooling reenter the conducting state by an I M transition, with changes of ca. 108 Scm−1 both ways. For the first time in the field of organic metals crystal structures of DCNQI–copper salts have been determined by X-ray powder diffraction methods and refined by Rietveld analysis. Unit cell data, coordination angles and distances of the π planes are in excellent agreement with the single-crystal X-ray data. However, bond lengths and angles of the ligands are to be less accurate. This powder method proves to be most valuable if only microcrystalline material is available.