Conditions favoring the formation of monomeric Pt(III) derivatives in the electrochemical oxidation of trans-[Pt(II){(p-BrC6F4)NCH2CH2NEt2}Cl(py)]

Characterization of the anticancer active compound trans-[Pt^II{(p-BrC<inf>6</inf>F<inf>4</inf>)NCH<inf>2</inf>CH<inf>2</inf>NEt<inf>2</inf>}Cl(py)] is described along with identification of electrochemical conditions that favor formation of a monomeric one-electron-oxidized Pt^III derivative. The square-planar organoamidoplatinum(II) compound was synthesized through a carbon dioxide elimination reaction. Structural characterization by using single-crystal X-Ray diffraction reveals a trans configuration with respect to donor atoms of like charges. As Pt^III intermediates have been implicated in the reactions of platinum anticancer agents, electrochemical conditions favoring the formation of one-electron-oxidized species were sought. Transient cyclic voltammetry at fast scan rates or steady-state rotating disc and microelectrode techniques in a range of molecular solvents and an ionic liquid confirm the existence of a well-defined, chemically and electrochemically reversible one-electron oxidation process that, under suitable conditions, generates a Pt^III complex, which is proposed to be monomeric [Pt^III{(p-BrC<inf>6</inf>F<inf>4</inf>)NCH<inf>2</inf>CH<inf>2</inf>NEt<inf>2</inf>}Cl(py)]⁺. Electron paramagnetic resonance spectra obtained from highly non-coordinating dichloromethane/([Bu<inf>4</inf>N][B(C<inf>6</inf>F<inf>5</inf>)<inf>4</inf>]) solutions, frozen to liquid nitrogen temperature immediately after bulk electrolysis in a glove box, support the Pt^III assignment rather than formation of a Pt^II cation radical. However, the voltammetric behavior is highly dependent on the timescale of the experiments, temperature, concentration of trans-[Pt^II{(p-BrC<inf>6</inf>F<inf>4</inf>)NCH<inf>2</inf>CH<inf>2</inf>NEt<inf>2</inf>}- Cl(py)], and the solvent/electrolyte. In the low-polarity solvent CH<inf>2</inf>Cl<inf>2</inf> containing the very weakly coordinating electrolyte [Bu<inf>4</inf>N][B(C<inf>6</inf>F<inf>5</inf>)<inf>4</inf>], a well-defined reversible one-electron oxidation process is observed on relatively long timescales, which is consistent with the stabilization of the cationic platinum(III) complex in non-coordinating media. Bulk electrolysis of low concentrations of [Pt{(p-BrC<inf>6</inf>F<inf>4</inf>)NCH<inf>2</inf>CH<inf>2</inf>NEt<inf>2</inf>}Cl(py)] favors the formation of monomeric [Pt^III{(p-BrC<inf>6</inf>F<inf>4</inf>)NCH<inf>2</inf>CH<inf>2</inf>NEt<inf>2</inf>}Cl(py)]⁺. Simulations allow the reversible potential of the Pt^II/Pt^III process and the diffusion coefficient of [Pt^III{(p-BrC<inf>6</inf>F<inf>4</inf>)- NCH<inf>2</inf>CH<inf>2</inf>NEt<inf>2</inf>}Cl(py)]⁺ to be calculated. Reversible electrochemical behavior, giving rise to monomeric platinum(III) derivatives, is rare in the field of platinum chemistry.