The reaction of CuCl with 2HNQ, (viz. 2-hydroxy-1,4-naphthoquinone), in methanol results in [Cu2(II,II)(4HNSQ)2(ONQ) 2(H2O)4], Cu-3 complex; [where ONQ is the deprotonated oxidized form of ligand (viz. 2-oxido-l,4-naphthoquinone) and 4HNSQ one electron reduced tautomeric form of the ligand (i.e. 4-hydroxy-1,2- naphthosemiquinone)]. The mixed valent redox ligation is confirmed in [9] by us. In present report complex Cu-3 investigated by variable temperature magnetic susceptibility measurements (SQUID), X and Q-band EPR, DSC and CV techniques. A break is observed in the χm-1 vs. T plot ∼200 K in Cu-3 which is attributed to a phase transition. In Cu-3 a quintet state (S=2) is populated above 200 K by the molecular association of two exo Cu(II)(4HNSQ) units via hydrogen bonding between Cu(ONQ) unit of endo ligands in dimer. Magnetic susceptibility data is treated with tetramer model with S=1/2,1/2,1/2,1/2. The interdimer triplet-triplet interaction (J) in two [Cu(4HNSQ)] units and intradimer (zJ1) interaction between [Cu(II)(4HNSQ)] are best fitted with J= -50 cm-1 and zJ 1,=28 cm-1, respectively, using g=2.2. 'Quintet-triplet' phase transition occurs with an enthalpy change of 31.83 kJ mol-1 estimated from DSC. Cu(II)⇔Cu(I) and NSQ⇔CAT redox couples at E 1/23=0.68 V and E1/2=-1.12 V, respectively are result of exo ligands and Cu(II) ions interaction, while shifts of ligand based peaks viz. NQ→NSQ and NSQ⇔CAT at -0.44 and -0.67 V towards positive potential on complexation are due to electron transfer interactions between endo ligand and Cu(II) ion. © 2005 Akadémiai Kiadó, Budapest.