Investigations of the electrode reactions of some rhenium(V) and chromium(III) complexes and reductive dimerization of 2-naphthaldehyde and 9,9'-spirobi[9H-fluorene]-2-carboxaldehyde

The one-electron oxidation of chlorobis[2-(2′-hydroxyphenyl)-2-oxazolinato]oxorhenium(V), Re1, was investigated in acetonitrile by cyclic voltammetry at a glassy-carbon electrode. The oxidation proceeds with standard rate constant of 0.019 cm/s and E° of +0.58 V ± 0.01 V vs. the ferrocenium ion/ferrocene (Fc+/Fc) couple (all reported potentials are referenced to the Fc+/Fc couple). Water was found to displace the chloride ligand in the cation of Re1 forming an aquated rhenium(VI) complex, Re2. Re2 was subsequently reduced in a one-electron process with E° = +0.10 V and an electron-transfer rate constant of 0.01 cm/s.; The voltammetric investigation of TptBu,MeCr(OH)pz ′, Cr1, [TptBu,Me = hydrotris(3- tert-butyl;-5-methylpyrazolyl)borate; pz′ = 3-tert-butyl-5-methylpyrazolyl] and related complexes, in methylene chloride at a glassy-carbon electrode was performed in an electrochemical cell on a high-vacuum line. The voltammogram displayed an irreversible oxidation peak (II_pa) at E_pa = +0.80 V ± 0.09 V at 0.5 V/s that was preceded by a pre-peak (I_pa). As the scan rate was decreased, peak I_pa increased with respect to II _pa, indicating a CE mechanism. The product of the oxidation of Cr1 was discovered to be [TptBu,MeCr(O)(pz′ H)]+, Cr2, and was reducible at E_pc = −0.60 V. Cr2 reacted by hydrogen rate constant of 5000 M−1s−1, to form the stable cation, [TptBu,MeCr(OH)(pz′H)] +, Cr3, that was reversibly reduced at E° = −0.82 V with an electron-transfer rate constant of 0.002 cm/s. All kinetic parameters were determined through simulations based on the proposal that Cr1 is oxidized by a dissociative electron transfer reaction in which a proton is transferred from OH to pz′.; The electrochemical properties of 9,9′-spirobi-(9 H-fluorene)-2-carboxaldehyde, A1, were investigated in anhydrous dimethylformamide at a hanging mercury drop electrode and compared with the model compound 2-naphthaldehyde, A2. Both species were found to undergo a quasi-reversible one-electron reduction at E° = −2.1 V with a standard rate constant of 0.15 cm/s. The appearance of the anodic peak was determined to be dependent on scan rate, indicating that a chemical reaction followed the electron transfer. Voltammetric results were compared to simulations based on an EC mechanism from which it was determined that the second-order rate constant of the chemical reaction was k_f = 1300 M−1s −1 (A1) and k_f = 2500 M−1s−1 (A2).