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.

本研究通过复分解反应合成了一系列碱土金属（Mg、Ca、Ba）的3,5-位取代三(吡唑基)硼酸盐（tris(pyrazolyl)borate，Tp^R,Me；取代基R为甲基(Me)、苯基(Ph)与叔丁基(tBu)）配合物。研究考察了不同有机金属前驱体对施伦克型重排反应的影响，重点关注金属离子尺寸与Tp配体的空间位阻效应。烷基镁MgR2（R=AlMe4、CH3）与KTp^R,Me反应，生成杂配配合物(Tp^Me,Me)Mg(CH3)、(Tp^tBu,Me)Mg(CH3)与(Tp^Me,Me)Mg(AlMe4)。该四甲基铝酸盐配合物亦可通过向(Tp^Me,Me)Mg(CH3)中加入过量三甲基铝制备得到。由甲基溴化镁(MeMgBr)与1当量的五甲基环戊二烯钾(K(C5Me5))合成了形式上为六配位的环戊二烯基衍生物(C5Me5)Mg(Me)(thf)2，其中thf为四氢呋喃(tetrahydrofuran)。较重碱土金属钙与钡的四乙基铝酸盐配合物[M(AlEt4)2]n与KTp^R,Me进行等摩尔反应，得到同配配合物Ca(Tp^R,Ph)2与Ba(Tp^R,Me)2。配体重排产物杂三金属配合物[BaK(AlEt4)3]n被成功鉴定，其亦可通过将[K(AlEt4)]n加入[Ba(AlEt4)2]n中独立制备得到。将双[双(三甲基硅基)氨基]钡·二四氢呋喃合物(Ba[N(SiMe3)2]2(thf)2)与KTp^Me,Me反应，得到杂配配合物(Tp^Me,Me)Ba[N(SiMe3)2](thf)2。所有目标配合物均通过包括X射线晶体结构分析在内的多种手段完成了全面表征。