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 Me3SiCTp3‐Me, containing a non‐innocent backbone methine moiety, led to deprotonation/rearrangement and SiMe3/AlMe3 exchange to afford [(Me3AlCTp3‐Ph)2Mg] and [(Me3AlCTp3‐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′)2Mg][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][Al2Me7] 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]. Sting in the tail: The introduction of alkyl substituents at the CN3 backbone carbon atom leads to enhanced stabilization of neutral RCTp3‐R′ ligands (see figure), which facilitates the synthesis of a variety of distinct methylmagnesium complexes.