Cation radii induced structural variation in fluorescent alkaline earth networks constructed from tautomers of a nucleobase analogue

摘要

Nucleobase tautomers and their metal complexes have attracted considerable attention due to their fascinating architectures along with wide applications. In this paper, 4,6-dihydroxypyrimidine (H _2DHP), an analogue of uracil and thymine, was employed to react with the vital elements of alkaline earth metals in an aqueous solution and lead to the formation of four novel complexes, [Mg(HDHP) _2 (H _2O) _4] (1), [Ca(HDHP) _2(H _2O) _3] _n·nH _2O (2), [Sr(HDHP) _2(H _2O) _3] _n· nH _2O (3), and [Ba(HDHP) _2(H _2O) _2] _n·nH _2O (4), which have been characterized by elemental analysis, IR, TG, UV-Vis, PL, powder and single-crystal X-ray diffraction and progressively evolve from zero-dimensional (0D) mononuclear, one-dimensional (1D) zig-zag double chain, two-dimensional (2D) double layer, to a three-dimensional (3D) porous network along with the increase of cation radii. This tendency in dimensionality follows salient crystal engineering principles and can be explained by considering factors such as hard-soft acid-base principles and cation radii. The deprotonated H _2DHP ligand exhibits four new coordination modes, namely, O-monodentate (complex 1), N,O-chelating (complexes 2 and 3), O,O-bridging (complexes 2 and 3), and κ ~1O:κ ~2O-bridging mode (complex 4). Interestingly, the structural investigation indicates that the HDHP ~- monoanion shows three unusual types of tautomers, which are essential for the diagnosis of disease and investigation of medicine. Furthermore, the four complexes exhibit strong blue emission compared to free H _2DHP ligand at room temperature and may be potential candidates for blue fluorescent biological materials used in organisms.