A phase transition caught in mid-course: independent and concomitant analyses of the monoclinic and triclinic structures of (BuN)[Co(orotate)(bipy)]·3HO.

The preparation and characterization of the BuN salts of two bis-orotate(2-) complexes of cobalt, namely bis(tetra-n-butylammonium) diaquabis(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-ide-6-carboxylato-κN,O)cobalt(II) 1.8-hydrate, (CHN)[Co(CHNO)(HO)]·1.8HO, (1), and tetra-n-butylammonium (2,2'-bipyridine-κN,N')bis(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-ide-6-carboxylato-κN,O)cobalt(III) trihydrate, (CHN)[Co(CHNO)(CHN)]·3HO, (2), are reported. The Co 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 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, for both the monoclinic-to-triclinic transformation and the reverse transition, in agreement with the nature of the structural changes involving only the BuN cation.