Tris(pyrazolyl)borate Complexes of the Alkaline-Earth Metals: Alkylaluminate Precursors and Schlenk-Type Rearrangements

A series of 3,5-substituted tris­(pyrazolyl)­borate (Tp^R,Me; R = Me, Ph, <i>t</i>Bu) complexes of the alkaline-earth metals (Mg, Ca, Ba) was synthesized by salt metathesis reactions. The influence of different organometallic precursors on Schlenk-type rearrangement reactions was studied, putting emphasis on the metal size and the steric encumbrance of the Tp ligands. Magnesium alkyls MgR₂ (R = AlMe₄, CH₃) react with KTp^R,Me to form the heteroleptic complexes (Tp^Me,Me)­Mg­(CH₃), (Tp^{<i>t</i>Bu,Me})­Mg­(CH₃), and (Tp^Me,Me)­Mg­(AlMe₄). The latter tetramethylaluminate complex can also be obtained by treatment of Tp^Me,MeMg­(CH₃) with an excess of trimethylaluminum. The formally six-coordinate cyclopentadienyl derivative (C₅Me₅)­Mg­(Me)­(thf)₂ is synthesized from MeMgBr and 1 equiv of K­(C₅Me₅). Equimolar reactions of the tetraethylaluminates [M­(AlEt₄)₂]_<i>n</i> of the heavier alkaline-earth metals calcium and barium with KTp^R,Me give the homoleptic complexes of Ca­(Tp^R,Ph)₂ and Ba­(Tp^R,Me)₂. Heterotrimetallic [BaK­(AlEt₄)₃]_<i>n</i> is identified as a ligand rearrangement product and can be independently obtained by adding [K­(AlEt₄)]_<i>n</i> to [Ba­(AlEt₄)₂]_<i>n</i>. Treatment of Ba­[N­(SiMe₃)₂]₂(thf)₂ with KTp^Me,Me generates the heteroleptic complex (Tp^Me,Me)­Ba­[N­(SiMe₃)₂]­(thf)₂. All complexes are fully characterized including X-ray structure analyses