Tris(pyrazolyl)methanides of the Alkaline Earth Metals:
Influence of the Substitution Pattern on Stability and Degradation
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Abstract
Trispyrazolylmethanides
commonly act as strong
tridentate bases toward metal ions. This expected coordination behavior
has been observed for tris(3,4,5-trimethylpyrazolyl)methane
(<b>1a</b>), which yields the alkaline-earth-metal bis[tris(3,4,5-trimethylpyrazolyl)methanides]
of magnesium (<b>1b</b>), calcium (<b>1c</b>), strontium
(<b>1d</b>), and barium (<b>1e</b>) via deprotonation
of <b>1a</b> with dibutylmagnesium and [Ae{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (Ae = Mg, Ca, Sr, and Ba, respectively).
Barium complex <b>1e</b> 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
(<b>2a</b>) exhibits a strikingly different reactivity. Dibutylmagnesium
is unable to deprotonate <b>2a</b>, whereas [Ae{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>] (Ae = Ca, Sr, and Ba) smoothly metalates <b>2a</b>. However, the primary alkaline-earth-metal bis[tris(3-thienylpyrazolyl)methanides]
of Ca (<b>2c</b>), Sr (<b>2d</b>), and Ba (<b>2e</b>) represent intermediates and degrade under the formation of the
alkaline-earth-metal bis(3-thienylpyrazolates) of calcium
(<b>3c</b>), strontium (<b>3d</b>), and barium (<b>3e</b>) and the elimination of tetrakis(3-thienylpyrazolyl)ethene
(<b>4</b>). To isolate crystalline compounds, 3-thienylpyrazole
has been metalated, and the corresponding derivatives [(HPz<sup>Tp</sup>)<sub>4</sub>Mg(Pz<sup>Tp</sup>)<sub>2</sub>] (<b>3b</b>),
dinuclear [(tmeda)Ca(Pz<sup>Tp</sup>)<sub>2</sub>]<sub>2</sub> (<b>3c</b>), mononuclear [(pmdeta)Sr(Pz<sup>Tp</sup>)<sub>2</sub>]
(<b>3d</b>), and [(hmteta)Ba(Pz<sup>Tp</sup>)<sub>2</sub>] (<b>3e</b>) have been structurally characterized. Regardless of the
applied stoichiometry, magnesiation of thienylpyrazole <b>3a</b> with dibutylmagnesium yields [(HPz<sup>Tp</sup>)<sub>4</sub>Mg(Pz<sup>Tp</sup>)<sub>2</sub>] (<b>3b</b>), which is stabilized in the solid state by intramolecular N–H···N···H–N
hydrogen bridges. The degradation of [Ae{C(Pz<sup>R</sup>)<sub>3</sub>}<sub>2</sub>] (R = Ph and Tp) has been studied by quantum chemical
methods, the results of which propose an intermediate complex of the
nature [{(Pz<sup>R</sup>)<sub>2</sub>C}<sub>2</sub>Ca{Pz<sup>R</sup>}<sub>2</sub>]; thereafter, the singlet carbenes ([:C(Pz<sup>R</sup>)<sub>2</sub>]) 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
