Synthesis and Characterization of Simple Hydroxo- and Amido-Bridged Cobalt(III) Dinuclear Ions Missing from the Alfred Werner Collection

The unsymmetrical N,O-dibridging NO2– group in the threo (‘rac’) or erythro (‘meso’) isomers of [(en)2Co(μ-NH2,μ-NO2)Co(en)2]4+ is cleaved in hot triflic acid to yield a complex that is predominantly meso. This reacts in water to yield the new meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ion. The reactions of the threo- and erythro-μ-NO2– isomers in basic (0.1 M) aqueous solution are each retentive, giving the rac- and meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ions as products, respectively. The optically resolved erythro complex gives the achiral meso isomer, which completes the threo/rac and erythro/meso chemical correlations between the two series of dinuclear complexes; the meso synthesis completes the pair – the meso- and rac-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ ions. In HCl, stable μ-Cl, μ-NH2 dinuclear ions are formed from these μ-NH2, μ-OH complexes, with retention. The peroxo or superoxo ions rac-[Co(en)2(μ-NH2,μ-O2)Co(en)2]n+ react with SnII to produce symmetrically bridged intermediates that decay to the common and new meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ isomer on reaction in Me2SO. Both the rac and meso isomers of the μ-hydroxo-μ-peroxo complexes are also reduced by SnII to give intermediates that in Me2SO decay to the respective diols with retention. The new rac-[Co(en)2(μ-OH)2Co(en)2]4+ and meso-[(en)2Co(μ-NH2,μ-OH)Co(en)2]4+ isomers complete the characterization of rac/meso pairs in both systems. The corresponding μ-sulfate pairs are also reported.