Recherches sur les metaux organiques: acces a de nouveaux pi-donneurs analogues acetyleniques du tetrathiafulvalene (TTF) au depart d'aldehydes alpha-acetyleniques et de leurs complexes de dicobalthexacarbonyle

SIGLEINIST T 72435 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Remarkable interplay of redox states and conformational changes in a sterically crowded, cross-conjugated tetrathiafulvalene vinylog.

Derivatives of 9-[2-(1,3-dithiol-2-ylidene)ethylidene]thioxanthene have been synthesized using Horner–Wadsworth–Emmons reactions of (1,3-dithiol-2-yl)phosphonate reagents with thioxanthen-9-ylidene-acetaldehyde (5). Further reactions lead to the sterically crowded cross-conjugated “vinylogous tetrathiafulvalene” derivative 9-[2,3-bis-(4,5-dimethyl-1,3-dithiol-2-ylidene)-propylidene]thioxanthene (10). X-ray crystallography, solution electrochemistry, optical spectroscopy, spectroelectrochemistry, and simultaneous electrochemistry and electron paramagnetic resonance spectroscopy, combined with theoretical calculations performed at the B3LYP/6-31G(d) level, elucidate the interplay of the electronic and structural properties in these molecules. For compound 10, multistage redox behavior is observed: the overall electrochemical process can be represented by 10[RIGHTWARDS ARROW]10.+[RIGHTWARDS ARROW]102+[RIGHTWARDS ARROW]104+ with good reversibility for the 10[RIGHTWARDS ARROW]10.+[RIGHTWARDS ARROW]102+ transformations. At the tetracation stage there is the maximum gain in aromaticity at the dithiolium and thioxanthenium rings. Theory predicts that for 10, 10.+, and 102+ the trans isomers are more stable than the cis isomers (by ca. 2–18 kJ mol−1), whereas for 104+ the cis isomer becomes more stable than the trans isomer (by ca. 25 kJ mol−1) [trans and cis refer to the arrangement of the two dithiole moieties with respect to the central [DOUBLE BOND]C(R)[BOND]C(H)[DOUBLE BOND] fragment]. These data explain the detection in cyclic voltammograms of both trans and cis isomers of 10 and 10.+ during the reduction of 104+ at fast scan rates (>100 mV s−1) when the cis–trans isomerization is not completed within the timescale of the experiment. The X-ray structure of the charge-transfer complex (CTC) of 10 with 2,4,5,7-tetranitrofluorene-9-dicyanomethylenefluorene (DTeF) [stoichiometry: 10.+⋅(DTeF)2.−⋅2 PhCl] reveals a twisted conformation of 10.+ (driven by the bulky thioxanthene moiety) and provides a very rare example of segregated stacking of a fluorene acceptor in a CTC