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, n Bu, SiMe 3 ; R′=H, Me, Ph, i Pr, t Bu) were synthesized and their ability to coordinate methylmagnesium moieties was examined. The reaction of Mg(AlMe 4 ) 2 with neutral proligands HCTp 3‐Ph or Me 3 SiCTp 3‐Me , containing a non‐innocent backbone methine moiety, led to deprotonation/rearrangement and SiMe 3 /AlMe 3 exchange to afford [(Me 3 AlCTp 3‐Ph ) 2 Mg] and [(Me 3 AlCTp 3‐Me )Mg(AlMe 4 )], respectively, with monoanionic tripodal ligands. Treatment of sterically less demanding RCTp 3‐R′ with Mg(AlMe 4 ) 2 produced isostructural dicationic “metal‐in‐a‐box” complexes of the type [(RCTp 3‐R′ ) 2 Mg][AlMe 4 ] 2 (R=Me, n Bu; R′=H, Me). Utilization of the superbulky ligands MeCTp 3‐Ph and MeCTp 3‐ t Bu gave monocationic complexes [(MeCTp 3‐Ph )MgMe][AlMe 4 ] and [(MeCTp 3‐ t Bu )MgMe][Al 2 Me 7 ] as separated ion pairs. The reaction of Mg(AlMe 4 ) 2 with n BuCTp 3‐Ph led to the formation of the dimagnesium complex [{( n BuCTp 3‐Ph )Mg(AlMe 4 )} 2 (μ‐CH 3 )], which features a bridging methyl moiety and terminal η 1 ‐coordinated tetramethylaluminato ligands. Isopropyl‐substituted ligand MeCTp 3‐ i Pr emerged from further fine‐tuning of the steric and electronic parameters and, upon reaction with Mg(AlMe 4 ) 2 , gave (MeCTp 3‐ i Pr )Mg(AlMe 4 ) 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].