The coordination chemistry of TCNE, TCNQ and related polynitrileπ acceptors

Temperature dependence of magnetic susceptibilities and lattice parameters is measured at ambient pressure in the title compounds, which are metallic down to the lowest temperature measured and have a mixed valence state of Cu. In (DMeDCNQI) 2 Cu an anomalously large susceptibility having a thermal hysteresis is found in the range below 100 K. This result combined with the change in the lattice parameter tells that (DMeDCNQI) 2 Cu is a marginal metal in which the number of stable Cu 2+ ions changes with temperature. It is pointed out that the reentrant behavior already found in the title compounds under respective pressure range has a close relation with the lattice-parameter change.

Magnetic and Structural Properties of Mixed-Valence Molecular Conductors (DMeDCNQI) 2 Cu and (DMeODCNQI) 2 Cu

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.

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

A charge-transfer complex of 2,5-dimethyl-N,N'-dicyanoquinonediimine (DM) with silver (crystalline Ag(DM) 2 , defined as a) is irreversibly transformed by UV-vis illumination. Depending on the illumination conditions, three new types of solids (defined as γ, δ, and e) with different structural and physical properties are obtained and examined by a variety of analytical techniques, including solid-state, high-resolution, cross-polarization magic angle spinning (CP-MAS) 13 CNMR, elemental analysis (EA), mass spectrometry (MS), X-ray absorption fine structure (XAFS), and powder X-ray diffraction (XRD). The CP-MAS, EA, MS, and XAFS results indicate that compound y is a glass state of Ag(DM) 2 . The transformation from crystalline (a) to amorphous (y) solid Ag(DM) 2 is an irreversible exothermic glass transition (glass-transition temperature 155.2 °C; ΔH=-126.8 kJ mol -1 ), which implies that the glass form is thermodynamically more stable than the crystalline form. Compound δ (Ag(DM) 1.5 ) consists of silver nanoparticles (diameter (7±2) nm) dispersed in a glassy matrix of neutral DM molecules. The =N-CN-Ag coordination bonds of the a form are not maintained in the δ form. Decomposition of a by intense illumination results in a white solid (e), identified as being composed of silver nanoparticles (diameter (60±10) nm). Physical and spectroscopic (XAFS) measurements, together with XRD analysis, indicate that the silver nanoparticles in both δ and e are crystalline with lattice parameters similar to bulk silver; however, the magnetic susceptibilities differ from bulk silver.

UV-vis-induced vitrification of a molecular crystal

By maintaining α-Ag(DM)2 (DM = C10H8N4) around room temperature (RT) () during ultraviolet–visible (UV–vis) irradiation, a series of photochemical products named β was obtained. The solid β was comprised of structurally different compounds. All of the solid β phases share the original chemical formula of Ag(DM)2 and exhibit semiconducting behaviour, yet they differ from each other in their XRD patterns. They are divided into two phases: β1 and β2. Compared with the XRD pattern of the α phase, the β1 phase exhibited XRD peaks at the same 2θ-angles but with different relative intensities, while the β2 phase exhibited totally different XRD patterns from those of the α or β1 phases. The actual composition of a particular solid of 'β' phase is dependent upon irradiation time, and can be controlled continuously. Around RT, the α phase exhibits metallic conductivity, while the β1 and β2 phases are suggested to be semiconductors. As a result, one can continuously and finely control the electrical resistivity of Ag(DM)2 by UV irradiation to modify it into a mixture of α, β1 and/or β2 phases.

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J. Reiner

Papers

Locally resolved 13C hyperfine shifts in (DMe-DCNQI)2-M radical anion salts: A comparative study