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(drop CCH2R)(dppe)(2)][BF4] (X = F(1), R = H(1a), Bu-t (1b), CO2Me (1c), CO2Et (1d), Ph (1e), or C6H4Me-4 (1f); X = Cl (2), R = H (2a), Bu-t (2b), CO2Me (2c), CO2Et (2d), Ph (2e), or C6H4Me-4 (2f); dppe = Ph2PCH2CH2PPh2) have been prepared by a single-pot reaction, in THF and under sunlight, of the appropriate 1-alkyne (HC drop CR) with trans-[ReCl(N-2)(dppe)(2)] and [NH4][BF4] in the presence or in the absence, respectively, of Tl[BF4]. With the exception of the less acidic tertbutylcarbyne complexes (1b and 2b), these complexes are deprotonated by [Bu4N]OH to give the corresponding vinylidene complexes trans-[ReX(double bond C double bond CHR)(dppe)(2)] (X = F (3) or Cl (4)) which, by treatment with HBF4, 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 drop 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 pi-electron accepters 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.