Reactivity of acyl and aquo groups in metal—quinone aggregates of photosystem-II

Abstract Because thermolytic products are analogous to photolytic products, non-isothermal TGA studies of metal-quinones involved in Photosystem-II have been undertaken. The compositions of the compounds C-1-C3 are ML 2 · x H 2 O. For C-1 and C-2 , M = Co(II), x = 2, and L = Lw in the former and L = O-acylLwOx in the latter; for C-3 , M = Ni(II), x = 4 and L = LwOx. The weight loss versus temperature data are treated using the Coats and Redfem relationship. In C-1 , the water molecules are ligand participants in the coordination, while in C-2 and C-3 the water molecules are adsorbed. The kinetic data of the thermolysis throw light on the energetics of the and ligations in quinone chelates, together with their acyl and aquo functionalities. The E a values of Lw in C-1 and of the acyl and LwOx functional moieties in C-2 , are comparable: 55, 58 and 57 kJ mol −1 respectively. The aquation effect dominates in C-1 , as the E a of H 2 O is ≈ 82 kJ mol −1 . The adsorbed water molecules in C-3 have an E a ≈ 7.5 kJ mol −1 , while the outer-sphere water molecules participate in the hydrolysis of the O-acyl functional group during the pyrolytic reaction of C-2 , which requires an energy ≈ 81.6 kJ mol −1 .