The reaction of the Mannich base ligand (H2L = N,N'-dimethyl-N,N'-bis (2 -hydroxy-3-me thoxy-5-methylbenzyl) -ethylenediamine) with Co(OAc)(2)center dot 4H(2)O and Ln(III) nitrate salts (Ln = Gd, Tb, and Dy) under basic conditions afforded three carbonato-bridged isostructural tetranuclear heterometallic Co(II)-Ln(III) complexes (CO2Gd2L2(mu(4)-CO3)(2)(NO3)(2)] (1), [CO2Tb2L2(mu(4)-CO3)(2)(NO3)(2)] (2), and [CO2DY2L2(mu(4)-CO3)(2)(NO3)(2)] (3)) by atmospheric CO2 fixation. In all structures, two dinuclear [((CoL)-L-II)Ln(III)(NO3)] units are linked via two mu(4) -carbonato groups to form the tetranuclear Co(2)(II)Ln(2 )(III)core. The geometry around two penta-coordinated Co(II) ions is distorted square pyramid, and that around two nona-coordinated Ln(III) ions is intermediate between "spherical tricapped trigonal prism" and "spherical capped square antiprism" in all complexes. The complexes (1-3) showed catecholase-like and phenoxazinone-synthase-like catalytic activities. The K-cat values calculated for the catecholase-like were 254.5, 272.4, and 291.3 h(-1), and for the phenoxazinone-synthase-like reaction they were 2930.6, 2965.2, and 2998.5 h(-1) for complexes 1-3, respectively. The probable pathways for these two oxidase reactions have been proposed by the analyses of mass spectral data. For all of the compounds, the variable temperature magnetic susceptibility and isothermal magnetization data were investigated. The complexes exhibited overall ferromagnetic behavior, which was evident from the isothermal magnetization curves. AC magnetic susceptibility measurements revealed the slow relaxation of magnetization in complexes 2 and 3 but very negligible in 1. The activation energy barriers (U-eff) for the slow relaxation process were evaluated and found to be 1.99, 2.79, and 8.98 K for 1, 2, and 3, respectively.