Synthesis and Structural Investigation of a Complete Series of Brightly Colored Alkali Metal 1,3‐Dimethylviolurates

Small compound - big colors: synthesis and structural investigation of brightly colored alkaline earth metal 1,3-dimethylviolurates

A series of brightly colored alkaline earth metal 1,3-dimethylviolurates M(Me₂Vio)₂ have been prepared and fully characterized. The title compounds AE(Me₂Vio)₂·nH₂O (AE = Mg, n = 6 (3); AE = Ca, n = 8 (4), AE = Sr, n = 6 (5); AE = Ba, n = 4 (6)) were obtained by neutralizing 1,3-dimethylvioluric acid monohydrate (=H(Me₂Vio)·H₂O; 2) with 0.5 equiv. of the corresponding metal dihydroxides AE(OH)₂. The hair-like appearance of the Sr derivative 5 prevented the growth of single-crystals. This problem could be solved by crystallizing the crown ether derivative Sr(Me₂Vio)₂(18-crown-6) (5a). The isolated salts exhibit intense colors ranging from red to purple. Various attempts to prepare the beryllium derivative Be(Me₂Vio)₂ failed. Instead, work-up of the reaction mixtures provided pink crystals of a new modification of 2 formulated as [H₃O][Me₂Vio] (2b) as shown by an X-ray diffraction study. An unexpected oxidation reaction of the barium salt Ba(Me₂Vio)₂ led to formation of the novel mixed-anion salt Ba(Me₂Vio)(Me₂NO₂Barb)·2H₂O (8, Me₂NO₂Barb^- = 1,3-dimethyl-5-nitrobarbiturate anion). Compound 8 could also be synthesized deliberately by treatment of Ba(OH)₂ with a 1 : 1 mixture of 2 and 1,3-dimethyl-5-nitrobarbituric acid (7, =H(Me₂NO₂Barb)·H₂O). All new compounds were fully characterized by their IR, Raman, NMR (¹H, ¹³C{¹H}) and UV-vis spectra as well as elemental analyses. Single-crystal X-ray diffraction studies revealed that the solid-state structures of compounds 3, 4, 5a and 6 are governed by the typical coordination behavior of the alkaline-earth metals, i.e. increasing coordination numbers and a decreasing degree of hydration when going from Mg to Ba. The dimensions of the structures range from hydrogen-bonded ions (3) over monomeric, neutral complex molecules (4, 5a), to polymeric networks (6). The successful isolation of the mixed-anion barium salt 8 adds a new facet to the coordination chemistry of violurate and related ligands.