Synthesis and electronic properties of mononuclear osmium(II) and rhenium(I) complexes containing ligands derived from [2,3-a:3 ′,2 ′-c]dipyridophenazine (ppb)

Abstract

An improved method for the synthesis of the polypyridyl ligand [2,3-a:3 ′,2 ′-c]dipyridophenazine (ppb) is reported. In addition, the syntheses of the new ppb-based ligands L (L=[2,3-a:3 ′,2 ′-c]dipyridophenazine, [2,3-a:3 ′,2 ′-c]dipyrido-6,7-dimethylphenazine, [2,3-a:3 ′,2 ′-c]dipyrido-6,7-dichlorophenazine, [2,3-a:3 ′,2 ′-c]dipyrido[1,2-e]benzophenazine, [2,3-a:3 ′,2 ′-c:3″,2″-e]tripyridoquinoxaline and [2,3-a:3 ′,2 ′-c]dipyrido-5-methylphenazine) are detailed. The complexes [Os(ppb)(bpy) 2] 2+ and [Os(L)(bpy) 2] 2+ have been synthesised and their electrochemical, electronic absorption and UV–Vis spectrochemical properties measured. Syntheses of the rhenium(I) complexes fac-[Re(L″)(CO) 3Cl] (L″ = [2,3-a:3 ′,2 ′-c]dipyridophenazine, [2,3-a:3 ′,2 ′-c]dipyrido-6,7-dimethylphenazine, [2,3-a:3 ′,2 ′-c]dipyrido-6,7-dichlorophenazine, [2,3-a:3 ′,2 ′-c]dipyrido[1,2-e]benzophenazine) are also reported. The osmium(II) complexes [Os(L ′)(bpy) 2] 2+ (L ′ = 2,3-di-(2-pyridyl)quinoxaline, 6,7-dimethyl-2,3-di-(2-pyridyl)quinoxaline, 6,7-chloro-2,3-di-(2-pyridyl)quinoxaline, 2,3-di-(2-pyridyl)benzo[g]quinoxaline, 4-methyl-di-(2-pyridyl)quinoxaline) have also been prepared and studied. The electronic spectra of the Os and Re complexes are dominated by strong MLCT transitions. The lowest energy MLCT transitions correlate linearly with the first reduction potential for the [Os(L)(bpy) 2] 2+ series. Electrochemical oxidation of the [Os(L)(bpy) 2] 2+ and [Os(L ′)(bpy) 2] 2+ complexes results in the appearance of LMCT transitions in the visible region while reduction of the [Os(L)(bpy) 2] 2+ complexes produces complex electronic spectra. Comparison with the reduced fac-[Re(L″)(CO) 3Cl] complexes allows assignment of L″ − π→π * transitions in the NIR (1200–1000 nm) and red (850–700 nm) regions. The nonsymmetrical ligands [2,3-a:3 ′,2 ′-c:3″,2″-e]tripyridoquinoxaline and [2,3-a:3 ′,2 ′-c]dipyrido-5-methylphenazine both coordinate to the [Os(bpy) 2] 2+ unit in a stereospecific manner to give a single geometric isomer. The crystal structure of [Re(CO) 3(ppb)Cl] is reported, in which the polypyridyl ligand deviates significantly from planarity.