Magnesium Stung by Nonclassical Scorpionate Ligands: Synthesis and Cone-Angle Calculations

A series of tris(pyrazolyl)alkane (RCTp) scorpionate ligands of the type RCTp^3-R' (R=Me, nBu, SiMe₃ ; R'=H, Me, Ph, iPr, tBu) were synthesized and their ability to coordinate methylmagnesium moieties was examined. The reaction of Mg(AlMe₄ )₂ with neutral proligands HCTp^3-Ph or Me₃ SiCTp^3-Me , containing a non-innocent backbone methine moiety, led to deprotonation/rearrangement and SiMe₃ /AlMe₃ exchange to afford [(Me₃ AlCTp^3-Ph )₂ Mg] and [(Me₃ AlCTp^3-Me )Mg(AlMe₄ )], respectively, with monoanionic tripodal ligands. Treatment of sterically less demanding RCTp^3-R' with Mg(AlMe₄ )₂ produced isostructural dicationic "metal-in-a-box" complexes of the type [(RCTp^3-R' )₂ Mg][AlMe₄ ]₂ (R=Me, nBu; R'=H, Me). Utilization of the superbulky ligands MeCTp^3-Ph and MeCTp^3-tBu gave monocationic complexes [(MeCTp^3-Ph )MgMe][AlMe₄ ] and [(MeCTp^3-tBu )MgMe][Al₂ Me₇ ] as separated ion pairs. The reaction of Mg(AlMe₄ )₂ with nBuCTp^3-Ph led to the formation of the dimagnesium complex [{(nBuCTp^3-Ph )Mg(AlMe₄ )}₂ (μ-CH₃ )], which features a bridging methyl moiety and terminal η¹ -coordinated tetramethylaluminato ligands. Isopropyl-substituted ligand MeCTp^3-iPr emerged from further fine-tuning of the steric and electronic parameters and, upon reaction with Mg(AlMe₄ )₂ , gave (MeCTp^3-iPr )Mg(AlMe₄ )₂ ; this represents the first example of a magnesium bis(alkyl) complex with an intact RCTp^3-R' ligand. The exact ligand cone angles Θ^° of all magnesium complexes were determined according to the mathematical analysis developed by Allen et al. [J. Comput. Chem. 2013, 34, 1189-1197].