Electrochemical Investigations Give Some Insights into the Coordination Chemistry of New Stable Iridium(+1), Iridium(0), and Iridium(−1) Complexes (original) (raw)
The redox chemistry of the stable tetracoordinated 16 valence electron d 8-[Ir I (tropp Ph) 2 ] (PF 6) À and pentacoordinated 18 valence d 8-[Ir I (tropp Ph) 2 Cl] complexes was investigated by cyclic voltammetry (tropp Ph dibenzotropylidenyl phosphine). The experiments were performed using a platinum microelectrode varying scan rates (100 mV/s ± 10 V/s) and temperatures (À 40 to 20 8C) in tetrahydrofuran, THF, or acetonitrile, ACN, as solvents. In THF, the overall two-electron reduction of the 16 valence electron d 8-[Ir I (tropp Ph) 2 ] (PF 6) À proceeds in two well separated slow heterogeneous electron transfer steps according to: d 8-[Ir I (tropp Ph) 2 ] e À 3 d 9-[Ir 0 (tropp Ph) 2 ] e À 3 d 10-[Ir ÀI (tropp Ph) 2 ] À , [k s 1 2.2 Â 10 À3 cm/s for d 8-Ir I /d 9-Ir 0 and k s 2 2.0 Â 10 À3 cm/s for d 9-Ir 0 /d 10-Ir ÀI ]. In ACN, the two redox waves merge into one ™two-electron∫ wave [k s 1,2 7.76 Â 10 À4 cm/s for d 8-Ir I /d 9-Ir 0 and d 9-Ir 0 /d 10-Ir ÀI ] most likely because the neutral [Ir 0 (tropp Ph) 2 ] complex is destabilized. At low temperatures (ca. À 40 8C) and at high scan rates (ca. 10 V/s), the two-electon redox process is kinetically resolved. In equilibrium with the tetracoordianted complex [Ir I (tropp Ph) 2 ] are the pentacoordinated 18 valence [Ir I (tropp Ph) 2 L] complexes (L THF, ACN, Cl À) and their electrochemical behavior was also investigated. They are irreversibly reduced at rather high negative potentials (À 1.8 to À 2.4 V) according to an ECE mechanism 1) [Ir I (tropp Ph) 2 (L)] e À 3 [Ir 0 (tropp Ph) 2 (L)]; 2) [Ir 0 (tropp Ph) 2 (L)] 3 [Ir(tropp Ph) 2 ] L, iii) [Ir 0 (tropp Ph) 2 ] e À 3 [Ir ÀI (tropp Ph) 2 ] À. Since all electroactive species were isolated and structurally characterized, our measurements allow for the first time a detailed insight into some fundamental aspects of the coordination chemistry of iridium complexes in unusually low formal oxidation states.
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