Dibenzotropylidene Substituted Ligands for Early Main Group Metal-Alkene Bonding

Abstract

New N-ligands with TROP-substituents are introduced (TROP = [5H]dibenzo[a,d]cyclohepten-5-yl): (TROP)(DIPP)N (DIPP = 2,6-diisopropylphenyl), TROPBDI (HC[C(Me)N-TROP]2) and the β-diketiminate ligand TROP–DIPPBDI with TROP and DIPP substituents at N. Several Li, Mg, Ca and Zn complexes with these ligands have been isolated and were characterized by X-ray diffraction. While TROP-substituents in the protonated ligands show endo-exo exchange, TROP-substituents in the metal complexes are in endo-conformation due to metal···alkene bonding. Short contacts between the metal and the TROP alkene unit are observed and some of these are the shortest reported; (TROPBDI)Li: Li···C 2.416(3) Å, [(TROP)(DIPP)N]2Ca: Ca···C 2.742(2) Å. TROPBDI shows superior metal shielding properties to the Li complex with a buried volume of VB = 78.7 %. DFT calculations on [(TROP)(DIPP)N]2Mg show a minimum with asymmetric Mg···alkene bonding. Symmetric Mg···alkene bonding is a transition state 1.9 kcal/mol higher in energy. Asymmetry in alkene-Mg bonding is explained by an ion-induced polarization of the C=C bond and electrostatic Mg2+···(δ-)C=C(δ+) attraction. The TROP-substituent stabilizes low valent radical species but attempts to isolate (TROPBDI)Mg· or (TROPBDI)Zn· by reduction of the iodide precursors failed. Intensely black coloured raw products hint towards formation of radical species. Several radical decomposition products have been structurally characterized.