The (CnH2n+6N2) [Mn-3(HPO3)(4)] (n = 3-8) compounds have been prepared by hydrothermal synthesis and characterized by X-ray diffraction data and spectroscopic techniques. The crystal structure of (C3H12N2)[Mn-3(HPO3)(4)] has been solved from single-crystal X-ray diffraction. The unit-cell parameters are a 9.502(1), b = 5.472(1), c = 14.523(4) Angstrom, beta = 95.01(3)degrees, monoclinic, C2/m, with Z = 2. The compound shows a layered structure stacked along the c-axis with the alkyldiammonium cations placed in the interlayer space. The sheets are formed by Mn3O12 trimer units extended in the ab-plane and connected by (HPO3)(2-) anions. The study of the (CnH2n+6N2)-[Mn-3(HPO3)(4)] (n = 4-8) phases by X-ray powder diffraction indicates an isotype relation with the propanediammonium compound. The Dq and Racah parameters calculated for (C3H12N2)[Mn-3(HPO3)(4)] are Dq 880, B = 660 and C = 3610 cm(-1). The ESR spectra show isotropic signals with a g-value of 2.008. Magnetic measurements indicate the presence of antiferromagnetic interactions inside the [Mn-3(HPO3)(4)](2-) sheets. The J/K value has been estimated to be -15 K by considering that the system behaves like an isolated trimer at high temperatures.