Thermal studies of metal-quinone complexes - Role of aqua coligations withcopper(II) in coordination of 2-hydroxy-1,4-naphthoquinone

Copper(II) complexes of lawsone (2-hydroxy-1,4-naphthaquinone, Lw) with var iety of aqua ligation viz. Cu-1, [Cu(Lw)(2)(H2O)](2); Cu-2, [Cu(Lw)(2)(H2O) (2)] and Cu-3, [Cu(Lw)(2)(H2O)(2)](2) have been synthesized. The role of wa ter as counter ligand on coordination propensity of redox active lawsone in naphthoquinone/naphthosemiquinone (NQ/NSQ) forms is quantified by studies of pyrolytic reactions, using non-isothermal TG and DTA techniques, coupled with IR studies. Mixed (NQ) (NSQ) ligation in Cu-1 and Cu-3 required energ y of activation, E-a similar to 67 kJ mol(-1) of (NQ) and similar to 41 kJ mol(-1) of (NSQ). Comparable energies of aqua ligand (similar to 43 kJ mol( -1)) with NSQ ligand in Cu-1 and Cu-3, dictate charge distributions in laws one coordinations. A large difference between E-a of aqua and p-NQ ligand i ndicates coordination of lawsone in its fully oxidised quinone form in Cu-2 . From thermoanalytical studies schematic oxidative decomposition mechanism s are proposed for Cu-1 and Cu-3. From pyrolytic reactions enthalpies are e stimated by DTA technique.