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

The 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.