A phase transition caught in mid-course: independent and concomitant analyses of the monoclinic and triclinic structures of ('Bu_4N)[Co(orotate)_2(bipy)]·3H_2O

摘要

The preparation and characterization of the ~nBu_4N~+ salts of two bis-orotate(2-) complexes of cobalt, namely bis(tetra-n-butylammonium) diaquabis(2,4-dioxo-l,2,3,4-tetrahydropyrimidin-l-ide-6-carboxylato-kappa~2N~1,O~6 )cobalt(II) 1.8-hydrate, (C_16H_36N)_2[Co(C_5H_2N_2O_4)_2(H_2O)_2]·1.8H_2O, (1), and tetra-n-butylammonium (2,2'-bipyridine-kappa~2N,N')bis(2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-ide-6-carbox-ylato-kappa~2N~1,O~6)cobalt(III)trihydrate, (C_16H_36N)[Co(CsH_2N_2O_4)_2(C_10H_8N_2)]·3H_2O, (2), are reported. The CoIII 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 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. 0.25 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 ~nBu_4N~+ cation.