Syntheses, Spectroscopy, and Redox Properties of Fluoro−Carbyne and Derived Fluoro−Vinylidene Complexes of Rhenium and of Analogous Chloro Complexes

The fluoro− or chloro−carbyne complexes trans -[ReX(≡CCH 2 R)(dppe) 2 ][BF 4 ] (X = F ( 1 ), R = H ( 1a ), Bu t ( 1b ), CO 2 Me ( 1c ), CO 2 Et ( 1d ), Ph ( 1e ), or C 6 H 4 Me-4 ( 1f ); X = Cl ( 2 ), R = H ( 2a), Bu t ( 2b ), CO 2 Me ( 2c ), CO 2 Et ( 2d ), Ph ( 2e ), or C 6 H 4 Me-4 ( 2f ); dppe = Ph 2 PCH 2 CH 2 PPh 2 ) have been prepared by a single-pot reaction, in THF and under sunlight, of the appropriate 1-alkyne (HC≡CR) with trans -[ReCl(N 2 )(dppe) 2 ] and [NH 4 ][BF 4 ] in the presence or in the absence, respectively, of Tl[BF 4 ]. With the exception of the less acidic tert -butylcarbyne complexes ( 1b and 2b ), these complexes are deprotonated by [Bu 4 N]OH to give the corresponding vinylidene complexes trans -[ReX( C CHR)(dppe) 2 ] (X = F ( 3 ) or Cl ( 4 )) which, by treatment with HBF 4, regenerate the carbyne complexes. This route is more convenient for the synthesis of the chloro−carbyne complexes 2 from the vinylidenes 4, the latter being then prepared upon reaction of the dinitrogen complex with HC≡CR in toluene under sunlight. The electrochemical behavior of complexes 1 − 4 has been investigated by cyclic voltammetry and controlled potential electrolysis in aprotic media and at a Pt electrode. These complexes undergo single-electron reversible oxidations at half-wave oxidation potentials in the range from 1.39 to 1.48 ( 1, 2 ) and −0.35 to 0.25 ( 3, 4 ) V vs SCE. The corresponding electrochemical P L and E L ligand parameters have been estimated for the carbyne ( P L = 0.21−0.24 V, E L ca . 1.2 V vs NHE) and the vinylidene ( P L = −0.27 to −0.13 V, E L = 0.50−0.62 V vs NHE) ligands and discussed in terms of redox potential−structure relationships. The former ligands behave as remarkably strong π-electron acceptors and undergo cathodically induced C−H bond cleavage to give the corresponding vinylidenes. Both the carbyne and the vinylidene ligands are effectively stabilized by the trans -fluoride ligand, although it presents, relative to chloride, a slightly stronger destabilizing effect on the HOMO in these complexes.