Synthesis and structural characterization of nitrite-coordinating CoII and CoIII complexes as models for the reaction center of Co-substituted nitrite reductase

Abstract

CoII and CoIII complexes containing nitrite and tridentate aromatic amine compounds [bis(6-methyl-2-pyridylmethyl)amine (Me2bpa) and bis(2-pyridylmethyl)amine (bpa)] have been prepared as models of the catalytic center in Co-substituted nitrite reductase: [CoII(Me2bpa)(NO2)Cl]2 · acetone (2), CoII(Me2bpa)(NO2)2 (3), CoII(bpa)(NO2)Cl (4), CoII(bpa)(NO2)2 (5), CoIII(Me2bpa)(NO2)(CO3) (6), and CoIII(bpa)(NO2)3 (7). The X-ray crystal structure analyses of these CoII and CoIII complexes indicated that the geometries of the cobalt centers are distorted octahedral and the Me2bpa and bpa with three nitrogen donors exhibit mer- (2, 3, and 7) and fac-form (4 and 6). The coordination mode of nitrite depends on the cobalt oxidation state, to CoII through the oxygen (nitrito coordination, O- and O,O-coordination) and to CoIII through nitrogen (nitro coordination, N-coordination mode). These findings are consistent with the results of their IR spectra, except that another oxygen of the O-coordinated nitrito group in 3 might interact weakly with CoII according to its IR spectrum. Reductions of the nitrite in 2, 3, 4, and 5 to nitrogen monoxide were not accelerated in the presence of proton, perhaps due to the nitrito coordination in these CoII complexes.