A solvothermal synthetic procedure has been exploited to prepare the new [Cu3L(NO3)(6)](n) coordination polymer (1) by reaction of the polydentate N,N'-{2,4-di-[(di-pyridin-2-yl)amine]-1,3,5-triazine}ethylenediamine ligand (opytrizediam Q with copper(II) nitrate. 1 has been structurally characterized by means of the conventional X-ray single-crystal diffraction technique. It crystallizes in the monoclinic C2/c space group with a = 16.830(3), b = 20.701(4), c = 18.170(4) angstrom, beta = 113.26(3)degrees, V= 5816(2) angstrom(3), Z = 4. 1 consists of trinuclear Cu3L(NO3)(5) units connected by means of a nitrato-O,O' bridge. The resulting chains are involved in weak interchain head-to-tail pi-pi stacking interactions. In the presence of moisture, 1 is readily converted into the hydrated [Cu3L(NO3)(5)](NO3)center dot H2O form (2). This second phase, monoclinic P2(1)/c, consists of isolated [Cu3L(NO3)(5)](+) and (NO3)(-) ions which accommodate water molecules in the crystal lattice. These subtle chemical and structural modifications accompanying the moisture-triggered 1-to-2 transformation have been demonstrated through a X-ray powder diffraction study. A thermodiffractometric analysis has evidenced that this solid-to-solid transformation is fully reversible, i.e., thermally induced dehydration of 2 restores 1. The analysis of the temperature dependence of the magnetic susceptibility for 2 has revealed very weak ferromagnetic interactions, consistent with the large (CuCu)-Cu-... separation (ca. 7.5 angstrom) in the trinuclear units.