Bulky cationic β-diketiminate magnesium complexes

Abstract

Cationic β-diketiminate Mg complexes with the bulky ^tBuBDI ligand and the weakly coordinating anion B(C₆F₅)₄⁻ have been prepared by the reaction of (^tBuBDI)MgnBu with [Ph₃C]⁺[B(C₆F₅)₄]⁻; ^tBuBDI = CH[C(tBu)N-Dipp]₂ and Dipp = 2,6-diisopropylphenyl. Their structures are compared to the previously reported cationic (^MeBDI)Mg⁺ complexes; ^MeBDI = CH[C(Me)N-Dipp]₂. Crystallization of [(^tBuBDI)Mg]⁺[B(C₆F₅)₄]⁻ from chlorobenzene gave a unique (^tBuBDI)Mg⁺·ClC₆H₅ cation with a rather short Mg⋯Cl and consequently long C–Cl bond. Crystallization from chlorobenzene/arene solvent mixtures gave (^tBuBDI)Mg⁺·arene complexes (arene = benzene, toluene, m-xylene) but in the presence of mesitylene the chlorobenzene complex was formed. Due to the greater shielding of the metal, none of these complexes display Mg⋯(F₅C₆)₄B⁻ interactions. Crystal structures of the arene complexes show in all cases η²-coordination of the arene ligands. Ring slippage from a more favorable η²-coordination can be explained by the steric bulk of the ^tBuBDI ligand. The smaller arenes, benzene and toluene, also bind to (^tBuBDI)Mg⁺ in bromobenzene solution. The Lewis acidity of these cationic Mg complexes was determined by the Gutmann–Beckett test. The acceptor number for (^tBuBDI)Mg⁺ (AN = 76.0) is substantially higher than that estimated for (^MeBDI)Mg⁺ (AN = 70.3). Calculation of the atomic NPA charges by DFT shows that the Mg²⁺ ion in (^tBuBDI)Mg⁺ is slightly more positively charged than the metal in (^MeBDI)Mg⁺, confirming its higher Lewis acidity. The lower benzene complexation energy calculated for (^tBuBDI)Mg⁺versus (^MeBDI)Mg⁺ is due to steric congestion of the metal in the (^tBuBDI)Mg⁺ cation which allows only for Mg⋯(η¹)C₆H₆ instead of Mg⋯(η⁶)C₆H₆ bonding. This ring slippage, however, results in a significant polarization of the electron density in the benzene ring, making it susceptible for nucleophilic attack.