Chiral metal-dithiolene/viologen ion pairs: Synthesis and electrical conductivity

Abstract

Enantiomerically pure dithiolene complexes NBu4[Ni{(R,R)-diotte}2] and NBu4[Ni{(S,S)-diotte}2] (diotte2- = a 1,3-dioxolane-tetrathiaethylene), were prepared from the corresponding enantiomers of a diotte2- precursor. The structure of the precursor was solved by single-crystal X-ray analysis; desulfurization afforded a novel tetrathiafulvalene derivative. Combination of the complex monoanion with the enantiomers of the viologen derivative bis(2-methyl-3-hydroxypropyl)-4,4′-dipyridinium (HiBV2+) afforded enantiomeric and diastereomeric ion-pair complexes of the type HiBV[Ni(diotte)2]2. For comparison, the analogous compounds A[Ni(diotte)2]2, (A2+ = methyl (MV2+), octyl (OV2+), stearyl (StV2+) viologen or two 2,2′-bipyridinium acceptors), HiBV[Ni(diotte)L] {L=mnt2- (maleonitrile-1,2-dithiolate), dmit2- (2-thioxo-1,3-dithiol-4,5-dithiolate)}, MV[Ni(dmit)2)]2, [Ni(diotte)2], and [Ni(diotte)(dmit)] were synthesized. An X-ray powder diffraction structural analysis of MV-[Ni(dmit)2)]2 revealed the presence of mixed stacks that contain the sequence anion-anion-cation. While no short contacts are observable within a stack, these are observed between the stacks for the dication-anion interaction by short S···H distances in the range of 2.77 to 2.86 Å, and for the anion-anion interaction short S···S distances of 3.55 to 3.65 Å. In agreement with the absence of intrastack interactions, no ion-pair charge-transfer band can be detected in this and the other complexes. ESR and UV/Vis data suggest that in [Ni(diotte)2]- electron delocalization is less pronounced than in the corresponding mnt2- and dmit2- complexes. The specific electrical conductivity (σ) of pressed powder pellets ranges from 10-2 to 10-12 ω-1 cm-1 and in all cases increases with increasing temperature (293-393 K) according to an Arrhenius law. Corresponding activation energies vary from 0.14 to 0.93 eV and increase linearly with log σ for structurally similar ion pairs. Charge generation is postulated to occur by disproportionation of the monoanion as suggested by the almost linear increase of log σ with decreasing disproportionation energy. The conductivity of diastereomers of ions with two unlike configurations like [(S,S)-HiBV]-[Ni{(R,R)-diotte}2]2 (1.1 × 10-11 ω-1 cm-1) is one to two orders of magnitude higher as compared to the diastereomers with two like-configured ions.