The synthesis and characterization of a new unsymmetrical dinucleating N,O-donor ligand, 2-[N,N-bis(2-pyridylmethyl)aminomethyl]-6-[N-(3,5-di-tert-butyl-2-oxidobenzyl)-N-(2-pyridylamino)aminomethyl]-4-methylphenol (H2Ldtb), as well as the X-ray crystal structure of its corresponding mixed-valence diacetate-bridged manganese complex, di-mu-acetato-mu-{2-[N,N-bis(2-pyridylmethyl)aminomethyl]-6-[N-(3,5-di-tert-butyl-2-oxidobenzyl)-N-(2-pyridylamino)aminomethyl]-4-methylphenolato}dimanganese(II,III) tetraphenylborate, [MnIIMnIII(C42H49N5O2)(C2H3O2)2](C24H20B), are reported. The complex may be regarded as an interesting structural model for the mixed-valence MnII-MnIII state of manganese catalase.