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

A series of 3,5-substituted tris­(pyrazolyl)­borate (TpR,Me; R = Me, Ph, tBu) 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
KTpR,Me to form the heteroleptic complexes (TpMe,Me)­Mg­(CH3), (TptBu,Me)­Mg­(CH3), and (TpMe,Me)­Mg­(AlMe4). The latter
tetramethylaluminate complex can also be obtained by treatment of
TpMe,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]n of the heavier alkaline-earth metals calcium
and barium with KTpR,Me give the homoleptic complexes of
Ca­(TpR,Ph)2 and Ba­(TpR,Me)2. Heterotrimetallic [BaK­(AlEt4)3]n is identified as a ligand rearrangement product
and can be independently obtained by adding [K­(AlEt4)]n to [Ba­(AlEt4)2]n. Treatment of Ba­[N­(SiMe3)2]2(thf)2 with KTpMe,Me generates
the heteroleptic complex (TpMe,Me)­Ba­[N­(SiMe3)2]­(thf)2. All complexes are fully characterized
including X-ray structure analyses.