A phase transition caught in mid-course: independent and concomitant analyses of the monoclinic and triclinic structures of ((Bu4N)-Bu-n)[Co( orotate)(2)(bipy)] center dot 3H(2)O.

The preparation and characterization of the (Bu4N+)-Bu-n salts of two bis-orotate(2-) complexes of cobalt, namely bis(tetra-n-butylammonium) diaquabis(2,4-dioxo1,2,3,4- tetrahydropyrimidin-1-ide-6-carboxylato-kappa N-2(1), O-6) cobalt(II) 1.8-hydrate, (C16H36N)(2)[Co(C5H2N2O4)(2)(H2O)(2)] center dot 1.8H(2)O, (1), and tetra-n-butylammonium (2,2′-bipyridine-kappa(2) N, N’) bis(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-ide-6-carbox-ylato-kappa N-2(1), O-6)cobalt(III) trihydrate, (C16H36N)[Co(C5H2N2O4)(2)(C10H8N2)] center dot 3H(2)O, (2), are reported. The Co-III complex, (2), which is monoclinic at room temperature, presents a conservative single-crystal-to-single-crystal phase transition below 200 K, producing a triclinic twin. The transition, which involves a conformational change in one of the Bu-n groups of the cation, is reversible and can be cycled. Both end phases have been characterized structurally and the system was also characterized structurally in a two-phase intermediate state, using single-crystal diffraction techniques, with both the monoclinic and triclinic phases present. Thermal analysis allows a rough estimate of the small energy content, viz. 0.25 kJ mol(-1), for both the monoclinic-to-triclinic transformation and the reverse transition, in agreement with the nature of the structural changes involving only the (Bu4N+)-Bu-n cation.