Electrochemical Investigations Give Some Insights into the Coordination Chemistry of New Stable Iridium(+1), Iridium(0), and Iridium(−1) Complexes

Abstract

AbstractThe redox chemistry of the stable tetracoordinated 16 valence electron d8‐[Ir+I(troppPh)2]+(PF6)− and pentacoordinated 18 valence d8‐[Ir+I(troppPh)2Cl] complexes was investigated by cyclic voltammetry (troppPh=dibenzotropylidenyl phosphine). The experiments were performed using a platinum microelectrode varying scan rates (100 mV/s–10 V/s) and temperatures (− 40 to 20 °C) in tetrahydrofuran, THF, or acetonitrile, ACN, as solvents. In THF, the overall two‐electron reduction of the 16 valence electron d8‐[Ir+I(troppPh)2]+(PF6)− proceeds in two well separated slow heterogeneous electron transfer steps according to: d8‐[Ir+I (troppPh)2]++e−→d9‐[Ir0(troppPh)2]+e−→d10‐[Ir−I(troppPh)2]−, [ks1=2.2×10−3 cm/s for d8‐Ir+I/d9‐Ir0 and ks2=2.0×10−3 cm/s for d9‐Ir0/d10‐Ir−I]. In ACN, the two redox waves merge into one “two‐electron” wave [ks1,2=7.76×10−4 cm/s for d8‐Ir+I/d9‐Ir0 and d9‐Ir0/d10‐Ir−I] most likely because the neutral [Ir0(troppPh)2] complex is destabilized. At low temperatures (ca. − 40 °C) 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(troppPh)2]+ are the pentacoordinated 18 valence [Ir+I(troppPh)2L]+ complexes (L=THF, ACN, Cl−) and their electrochemical behavior was also investigated. They are irreversibly reduced at rather high negative potentials (− 1.8 to − 2.4 V) according to an ECE mechanism 1) [Ir+I(troppPh)2(L)]+e−→[Ir0(troppPh)2(L)]; 2) [Ir0(troppPh)2(L)]→[Ir(troppPh)2]+L, iii) [Ir0(troppPh)2]+e−→[Ir−I(troppPh)2]−. 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.