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

A series of tris(pyrazolyl)alkane (RCTp) scorpionate ligands of the type RCTp3-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 HCTp3-Ph or Me3 SiCTp3-Me , containing a non-innocent backbone methine moiety, led to deprotonation/rearrangement and SiMe3 /AlMe3 exchange to afford [(Me3 AlCTp3-Ph )2 Mg] and [(Me3 AlCTp3-Me )Mg(AlMe4 )], respectively, with monoanionic tripodal ligands. Treatment of sterically less demanding RCTp3-R' with Mg(AlMe4 )2 produced isostructural dicationic "metal-in-a-box" complexes of the type [(RCTp3-R' )2 Mg][AlMe4 ]2 (R=Me, nBu; R'=H, Me). Utilization of the superbulky ligands MeCTp3-Ph and MeCTp3-tBu gave monocationic complexes [(MeCTp3-Ph )MgMe][AlMe4 ] and [(MeCTp3-tBu )MgMe][Al2 Me7 ] as separated ion pairs. The reaction of Mg(AlMe4 )2 with nBuCTp3-Ph led to the formation of the dimagnesium complex [{(nBuCTp3-Ph )Mg(AlMe4 )}2 (μ-CH3 )], which features a bridging methyl moiety and terminal η1 -coordinated tetramethylaluminato ligands. Isopropyl-substituted ligand MeCTp3-iPr emerged from further fine-tuning of the steric and electronic parameters and, upon reaction with Mg(AlMe4 )2 , gave (MeCTp3-iPr )Mg(AlMe4 )2 ; this represents the first example of a magnesium bis(alkyl) complex with an intact RCTp3-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].