A phase transition caught in mid-course: Independent and concomitant analyses of the monoclinic and triclinic structures of (nBu4N)[Co(orotate)2(bipy)]·3H2O

Abstract

The preparation and characterization of the nBu4N+ 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-κ2N1,O6)cobalt(II) 1.8-hydrate, (C16H36N)2[Co(C5H2N2O4)2(H2O)2]·1.8H2O, (1), and tetra-n-butylammonium (2,2′-bipyridine-κ2N,N′)bis(2,4-dioxo-1,2,3,4-Tetrahydropyrimidin-1-ide-6-carboxylato-κ2N1,O6)cobalt(III) trihydrate, (C16H36N)[Co(C5H2N2O4)2(C10H8N2)]·3H2O, (2), are reported. The CoIII complex, (2), which is monoclinic at room temperature, presents a conservative single-crystal-To-single-crystal phase transition below 200bK, producing a triclinic twin. The transition, which involves a conformational change in one of the nBu 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. 0.25bkJbmol-1, for both the monoclinic-To-Triclinic transformation and the reverse transition, in agreement with the nature of the structural changes involving only the nBu4N+ cation.Repeated cycling through a reversible order-disorder phase transition in a crystal of a cobalt orotate complex produces an arrested transformation, leaving the sample with both triclinic and monoclinic domains. Independent determinations of the two structures were carried out from single-phase samples and the partial transition permitted simultaneous structure determination of the two phases at a temperature at which one of them is not expected to exist independently.