The reaction of Mn(OBz)₂·4H₂O (OBz = benzoate) with hmt (hmt = hexamethylenetetramine) in CH₃CN affords a mixed-valence hexanuclear complex [Mn^III₂Mn^II₄O₂(hmt)₄(OBz)₁₀] (1). The complex has been characterized in solid state by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction analysis. X-Ray crystal structure reveals that 1 consists of a [Mn^III₂Mn^II₄O₂] core, co-ordinated to ten bridging benzoates (six syn,-syn-η¹:η¹:μ₂, four η¹:η²:μ₃) and four terminal hmt groups. In solution state (acetonitrile solvent) 1 spontaneously forms self-assembled vesicular structure with diameter in the range of 100–150 nm. The morphology in solution state has been studied through various microscopic techniques like SEM, TEM, AFM etc. These self-assembled vesicular structures can encapsulate organic dye molecules, which have been confirmed by confocal microscopic imaging. Complex 1 also exhibits substantial catecholase-like activity with 3,5-di-tert-butylcatechol (3,5-DTBC) as the substrate in acetonitrile solvent; the K_cat value is determined to be 2337.9 h⁻¹.