Thermal, magnetic and spectral studies of metal-quinone complexes

The reaction of CuCl with 2HNQ, (viz. 2-hydroxy-1,4-naphthoquinone), in methanol results in [Cu 2 (II,II)(4HNSQ) 2 (ONQ) 2 (H 2 O) 4 ], Cu-3 complex; [where ONQ is the deprotonated oxidized form of ligand (viz. 2-oxido-1,4-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 x m -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 (z J 1 ) 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) iCu(I) and NSQiCAT redox couples at E 1/2 =0.68 V and E 1/2 = -1.12 V, respectively are result of exo ligands and Cu(II) ions interaction, while shifts of ligand based peaks viz. NQ→NSQ and NSQiCAT 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.