Sterische Effekte auf die Hydrierstabilität von 1, 1′-Dialkyl-4,4′-bipyridiniumsalzen (Viologenen) in wasserphotolytischen Systemen / Steric Effects on the Hydrogen Stability of 1,1′-Dialkyl-4,4′-bipyridinium Salts (Viologens) in Water Photolytic Systems

Abstract

The chemical stability of 1,1′-dialkyl-4,4′-bipyridinium compounds (viologens) as electron relay agents in hydrogen producing water photolytic systems is of great importance for practical applications. Three new 1,1′-dialkyl-4,4′-bipyridinium compounds (1 = 1,1′-dimethyl-2-phenyl-6-(p-tolyl)-4,4′-bipyridiniumdiperchlorate, 2 = 1,1′-dimethyl-2-phenyl-4,4′-bipyridiniumdiperchlorate, 3 = 6-phenyl-1,1′,2-trimethyl-4,4′-bipyridiniumdiperchlorate) have been investigated with respect to their stability and turnover numbers in a classical water photolytic system. H2 was detected during irradiation of solutions using Ru(bipy)3Cl2 as sensitizer, 1-3 as electron relay, EDTA as sacrificial electron donor, and a colloidal catalyst (platinum or palladium) with light λ > 400 nm. Quantum yields (platinum catalyst, light: 400-500 nm) were 18% 1 and 3, 16% 2, compared with methylviologen: 16%. The loss of 1,1′-dialkyl-4,4′-bipyridinium compounds due to hydrogenation was monitored by HPLC. Turnover Numbers (only hydrogen in gas phase was taken into account) were found to be 82 1, 64 2, 57 3 as compared with 39 for methylviologen. Using palladium or palladium supported on BaSO4 instead of platinum as catalysts did not increase the turnover numbers.