A new mononuclear Ni(II) complex [NiL] (1) of an unsymmetrically dicondensed N2O3 donor ligand, H2L (N-alpha-methylsalicylidene-N'-3-methoxysalicylidene-1,3-propanediamine), has been synthesized. Complex 1 on reaction with Mn(ClO4)(2)center dot 6H(2)O and NaN3 in different molar ratios yielded three novel heterometallic Ni-II-Mn-II complexes, [(NiL)(2)Mn-(N-3)](CLO4) (2), [(NiL)(2)Mn-2(N-3)(2)(mu(1,1)-N-3)(2)(CH3OH)(2)] (3), and [{(NiL)(2)Mn}(2)(mu(1,3)-N-3)(H2O)]center dot(CH3OH),(ClO4)(3) (4). The single crystal structure analyses show a trinuclear (Ni2MnII)-Mn-II structure for complex 2 and a tetranuclear (Ni2Mn2II)-Mn-II structure where two dinuclear (NiMnII)-Mn-II units are connected via mu(1,1)-azido and phenoxido bridges for complex 3. Complex 4 possesses a hexanuclear structure where two trinuclear (Ni2MnII)-Mn-II units are connected via a mu(1,3)-azido bridge. The temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes 3 and 4 are antiferromagnetically coupled with the exchange coupling constants (J) of -4.97, -0.14, -0.55 cm(-1) for 3 and -3.94 cm(-1) for 4. All complexes 2-4 show biomimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (K-cat) are 768, 1985, and 2309 h(-1) for complexes 2-4, respectively, whereas for phenoxazinone synthase like activity, the turnover numbers are 3240, 3360, and 13 248 h(-1) for complexes 2-4, respectively. This difference in catalytic efficiencies is attributed to the variations in structures of the complexes and formation of active Ni-II-Mn-II species in solution during catalysis. The mass spectral analyses suggest the probable intermediate formation and cyclic voltammetry measurement suggest the reduction of Ni(II) to Ni(I) during catalytic reaction. The very high catalytic efficiencies for aerial dioxygen activation of all these heterometallic complexes as well as the highest activity of 4 is attributed to the coordinatively unsaturated penta-coordinated geometry or hexa-coordinated geometry with a solvent water molecule around Mn(II).