A solvothermal synthetic procedure has been exploited to prepare the new [Cu(3)L(NO(3))(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 L) 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) Angstroms, beta = 113.26(3) degrees, V = 5816(2) Angstroms(3), Z = 4. 1 consists of trinuclear Cu(3)L(NO(3))(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 [Cu(3)L(NO(3))(5)](NO(3)).H(2)O form (2). This second phase, monoclinic P2(1)/c, consists of isolated [Cu(3)L(NO(3))(5)](+) and (NO(3))(-) 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 Cu...Cu separation (ca. 7.5 Angstroms) in the trinuclear units.