We have prepared a series of anionic orotate compounds [C(NH2)3]2 [cis-M(orot)2(H2O)2].xH2O [M = Co (1,1a), Ni (2,2a), Zn (3); orot = C5H2N2O4(2-), the dianion of orotic acid]. Guanidinium, with its capacity to act as donor in up to six hydrogen bonds, complements the capacity of the polyfunctional ligand orotate to enter into diverse non-covalent interactions, including as H-bond acceptors. In previously reported ionic crystals with sulfonate, guanidinium has been found to form an integral part of the structural framework. In contrast, guanidinium has been found to act as a structural guest to balance charge in crystals with the perovskite framework. The new isostructural compounds 1 and 2 are monoclinic and upon dehydration undergo a single-crystal to single-crystal transformation to twinned triclinic daughter phases (1a, 2a) which are isomorphous to the Zn compound 3. In the transformation the angular relationship between the planar guanidinium cations and the larger anionic complex changes significantly, representing a partial flip which nevertheless preserves the crystallinity to the extent of permitting single-crystal structure analysis of the daughter phases. Following an approach described by Foxman, we have analyzed the single-crystal to single-crystal transformation in the Ni complex (2 to 2a) using both the reciprocal space orientation matrix and the real axis matrix. Using the latter it has been possible to follow the details of the process at a molecular level, in a laboratory reference frame common to the mother and daughter phases.

Partial flip of guanidinium through a single-crystal to single-crystal transformation in molecular crystals of orotate complexes

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