Tris(pyrazolyl)methanides of the Alkaline Earth Metals: Influence of the Substitution Pattern on Stability and Degradation

Trispyrazolylmethanides commonly act as strong tridentate bases toward metal ions. This expected coordination behavior has been observed for tris(3,4,5-trimethylpyrazolyl)methane (1a), which yields the alkaline-earth-metal bis[tris(3,4,5-trimethylpyrazolyl)methanides] of magnesium (1b), calcium (1c), strontium (1d), and barium (1e) via deprotonation of 1a with dibutylmagnesium and [Ae{N(SiMe3)(2)}(2)] (Ae = Mg, Ca, Sr, and Ba, respectively). Barium complex 1e degrades during recrystallization that was attempted from aromatic hydrocarbons and ethers. In these scorpionate complexes, the metal ions are embedded in distorted octahedral coordination spheres. Contrarily, tris(3-thienylpyrazolyl)methane (2a) exhibits a strikingly different reactivity. Dibutylmagnesium is unable to deprotonate 2a, whereas [Ae{N(SiMe3)2}2] (Ae = Ca, Sr, and Ba) smoothly metalates 2a. However, the primary alkaline-earth-metal bis[tris(3-thienylpyrazolyl)methanides] of Ca (2c), Sr (2d), and Ba (2e) represent intermediates and degrade under the formation of the alkaline-earth-metal bis(3-thienylpyrazolates) of calcium (3c), strontium (3d), and barium (3e) and the elimination of tetrakis(3-thienylpyrazolyl)ethene (4). To isolate crystalline compounds, 3-thienylpyrazole has been metalated, and the corresponding derivatives [(HPz(Tp))(4)Mg(Pz(Tp))(2)] (3b), dinuclear [(tmeda)Ca(Pz(Tp))(2)]2 (3c), mononuclear [(pmdeta)Sr(Pz(Tp))(2)] (3d), and [(hmteta)Ba(Pz(Tp))(2)] (3e) have been structurally characterized. Regardless of the applied stoichiometry, magnesiation of thienylpyrazole 3a with dibutylmagnesium yields [(HPz(Tp))(4)Mg(Pz(Tp))(2)] (3b), which is stabilized in the solid state by intramolecular N-H...N...H-N hydrogen bridges. The degradation of [Ae{C(Pz(R))(3)}2] (R = Ph and Tp) has been studied by quantum chemical methods, the results of which propose an intermediate complex of the nature [{(Pz(R))(2)C}2Ca{Pz(R)}(2)]; thereafter, the singlet carbenes ([:C(Pz(R))(2)]) dimerize in the vicinity of the alkaline earth metal to tetrapyrazolylethene, which is liberated from the coordination sphere as a result of it being a very poor ligand for an s-block metal ion.