Reduction Reaction of the Copper(II) Complex Bearing 1,3-Di(pyridine-2-carboxaldimino)propane (pitn) by Decamethylferrocene in Acetonitrile

The reduction reaction of the Cu(II)-pitn complex (pitn = 1,3-di(pyridine-2-carboxaldimino)propane) by decamethylferrocene [Fe(Cp*)₂] was examined in acetonitrile. The observed pseudo-first-order rate constants exhibited saturation kinetics with increasing excess amount of [Fe(Cp*)₂]. Detailed analyses revealed that the reaction is controlled by a structural change prior to the electron transfer step, rather than a conventional bimolecular electron transfer process preceded by ion pair (encounter complex) formation. The rate constant for the structural change was estimated to be 275 ± 13 s^-1 at 298 K ({increment}H* = 33.3 ± 1.0 kJ·mol^-1, {increment}S* = 86 ± 5 J·mol^-1·K^-1), which is the fastest among gated reactions involving CuN₄ complexes. It was confirmed by EPR measurement and Conflex calculations that the dihedral angle between the two N-N planes is significantly large (40°) in solution whereas it is merely 17.14° in the crystal.