Electron-Transfer-Induced Geometrical Isomerization of the Dinitrile Complexes cis -[Re(NCR) 2 (Ph 2 PCH 2 CH 2 PPh 2 ) 2 ][BF 4 ] (R = Aryl, Alkyl): Rates, Mechanism, and Ligand Effects

In an aprotic medium and at a Pt electrode, cis - and trans -[Re(NCR) 2 (Ph 2 PCH 2 CH 2 PPh 2 ) 2 ] + ( cis + and trans +, respectively; R = aryl or alkyl) undergo two successive single-electron oxidations to form the 17-electron ( cis 2+ or trans 2+ ) and 16-electron ( cis 3+ or trans 3+ ) derivatives. The cis 3+ complexes isomerize to the corresponding trans 3+ complexes which undergo a slower decomposition reaction, and the rate constants ( k i 3+ and k dec, respectively) have been determined by kinetic analysis of the cyclic voltammetric behavior. For the aromatic nitrile complexes, both rate constants increase with the electron-withdrawing ability (Hammett's σ p constant) of R. The ratios of the isomeric equilibrium constants ( cis 3+ ⇄ trans 3+, cis 2+ ⇄ trans 2+, and cis + ⇄ trans + ), for the aromatic nitrile complexes, also increase with σ p, the thermodynamic gain in favor of the trans isomer is much higher upon the first oxidation than upon the second one, and it decreases with the increase of σ p (higher sensitivity of the energy of the HOMO of the trans isomers to the electronic effect of R). For the alkyl cyanide complexes, steric effects play a dominant role on their thermodynamic and kinetic behaviors, by shifting anodically the oxidation potential and enhancing the isomerization rate. The significance of those systems in terms of developing a “molecular hysteresis” behavior is also discussed.