Electrochemical Activation Parameters of Coupled Electron-Transfer and Spin-Exchange Reactions. Experimental Studies of [M(Tacn)2]3+/2+ and [Fe(Pzb)2]+/0 Redox Systems

Expressions are derived for rate constants and activation parameters of electrochemical reactions that are accompanied by a change in spin-state. Predictions based on an electrochemical scheme of squares are compared with experimental results for [M(tacn) 2 ] 3+/2+ (M = Fe, Co, Ni, Ru; tacn = 1,4,7-triazacyclononane) and [Fe(pzb) 2 ] +/0 (pzb - = hydrotris(pyrazol-1-yl)borate [HB(pz) 3 - ], tetrakis(pyrazol-1-yl)borate [B(pz) 4 - ] or hydrotris(3,5-dimethylpyrazol-1-yl)borate [HB(Me 2 pz) 3 - ]) redox couples. The electrochemical rate constants, ( k s,h ) obs, of [Co(tacn) 2 ] 3+/2+ and [Fe{HB(Me 2 pz) 3 } 2 ] +/0 are decreased by the presence of spin-exchange. Electrochemical enthalpies (Δ H ‡ obs ) and entropies (Δ S ‡ obs ) of activation, determined from the temperature dependence of ( k s,h ) obs under nonisothermal cell conditions, provide evidence regarding the mechanism of these reactions. [Co(tacn) 2 ] 3+/2+, [Fe(tacn) 2 ] 3+/2+, and [Fe{HB(Me 2 pz) 3 } 2 ] +/0 exhibit values of Δ H ‡ obs and Δ S ‡ obs that are larger than anticipated for simple electron-transfer, consistent with the interpretation that these reactions occur by coupled rather than concerted electron-transfer and spin-exchange and proceed through a high-spin intermediate in the M(III) oxidation state. Spin-exchange is viewed as a rapidly established, temperature-dependent equilibrium, the thermodynamic consequences of which are reflected in the electrochemical activation parameters.