Photochemistry of cis-[Co(en)₂(NO₂)₂]⁺ in acetonitrile

Abstract

Photochemistry of the cis-[Co(en)₂(NO₂)₂]⁺ ion in neat acetonitrile exhibits unusual features compared to the excited state processes of the complex ion in aqueous media. In aqueous media excitation of the complex ion in the ligand-to-metal charge-transfer band, ∗(Coâ ∃NO₂), produces cobalt(II) ion and the nitrito-linkage isomer. In neat acetonitrile solution excitation of the complex in the same region produces an intermediate cobalt(II) complex co-ordinated to NO₂. Molecular oxygen scavenges the intermediate quantitatively to produce cobalt(II)-oxygen adducts with a rate constant of (3.5 ± 0.5)× 10⁵ dm³ mol^-1 s^-1. From the amount of oxygen adduct formed, the quantum yield for the intermediate formation on irradiation at 365 nm was found to be 0.3 ± 0.03 while the quantum yield for cobalt(II) formation in deaerated and in air-equilibrated solutions was 0.03 ± 0.006. In the absence of oxygen the photoproduced intermediate leads to the formation of a cobalt(III) complex. The complex ion in neat acetonitrile and in water quenches the excited state of [Ru(bipy)₃]²⁺(bipy = 2,2'-bipyridine) with similar rate constants (9 × 10⁸ dm³ mol^-1 s^-l). However, flash photolysis studies show no observable product formation which is attributed to rapid recombination of the products formed in the solvent cage by an electron-transfer quenching process or to a physical quenching process which does not lead to any product formation. Absorption spectral features change markedly in different solvents indicating solvent interaction with the complex. It is suggested that excited state relaxation processes are intimately coupled with the solvent interactions with the complex ion.