Thermal, magnetic and spectral studies of metal-quinone complexes

Summary

The reaction of CuCl with 2HNQ, (viz. 2-hydroxy-1,4-naphthoquinone), in methanol results in [Cu<Subscript>2</Subscript>(II,II)(4HNSQ)<Subscript>2</Subscript>(ONQ)<Subscript>2</Subscript>(H<Subscript>2</Subscript>O)<Subscript>4</Subscript>], 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 <Emphasis Type=”Italic”>x</Emphasis><Subscript> m</Subscript><Superscript>-1</Superscript> <Emphasis Type=”Italic”>vs</Emphasis>. <Emphasis Type=”Italic”>T</Emphasis> plot ~200 K in Cu-3 which is attributed to a phase transition. In Cu-3 a quintet state (<Emphasis Type=”Italic”>S=2</Emphasis>) 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 <Emphasis Type=”Italic”>S</Emphasis>=1/2,1/2,1/2,1/2. The interdimer triplet-triplet interaction (<Emphasis Type=”Italic”>J</Emphasis>) in two [Cu(4HNSQ)] units and intradimer (z<Emphasis Type=”Italic”>J</Emphasis><Subscript>1</Subscript>) interaction between [Cu(II)(4HNSQ)] are best fitted with <Emphasis Type=”Italic”>J</Emphasis>= -50 cm<Superscript>-1</Superscript> and <Emphasis Type=”Italic”>zJ</Emphasis><Subscript>1</Subscript>=28 cm<Superscript>-1</Superscript>, respectively, using <Emphasis Type=”Italic”>g</Emphasis>=2.2. &apos;Quintet-triplet&apos; phase transition occurs with an enthalpy change of 31.83 kJ mol<Superscript>-1</Superscript> estimated from DSC. Cu(II) &iff;Cu(I) and NSQ&iff;CAT redox couples at <Emphasis Type=”Italic”>E</Emphasis><Subscript>1/2</Subscript>=0.68 V and <Emphasis Type=”Italic”>E</Emphasis><Subscript>1/2</Subscript> = -1.12 V, respectively are result of exo ligands and Cu(II) ions interaction, while shifts of ligand based peaks viz. NQ&rarr;NSQ and NSQ&iff;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.