Bulky cationic β-diketiminate magnesium complexes

Cationic beta-diketiminate Mg complexes with the bulky (BDI)-B-tBu ligand and the weakly coordinating anion B(C6F5)(4)(-) have been prepared by the reaction of ((BDI)-B-tBu)MgnBu with [Ph3C](+)[B(C6F5)(4)](-); (BDI)-B-tBu = CH[C(tBu)N-Dipp](2) and Dipp = 2,6-diisopropylphenyl. Their structures are compared to the previously reported cationic ((BDI)-B-Me)Mg+ complexes; (BDI)-B-Me = CH[C(Me)N-Dipp](2). Crystallization of [((BDI)-B-tBu)Mg](+)[B(C6F5)(4)](-) from chlorobenzene gave a unique ((BDI)-B-tBu)Mg+center dot ClC6H5 cation with a rather short Mg center dot center dot center dot Cl and consequently long C-Cl bond. Crystallization from chlorobenzene/arene solvent mixtures gave ((BDI)-B-tBu)Mg+center dot 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 center dot center dot center dot(F5C6)(4)B- interactions. Crystal structures of the arene complexes show in all cases eta(2)-coordination of the arene ligands. Ring slippage from a more favorable eta(2)-coordination can be explained by the steric bulk of the (BDI)-B-tBu ligand. The smaller arenes, benzene and toluene, also bind to ((BDI)-B-tBu)Mg+ in bromobenzene solution. The Lewis acidity of these cationic Mg complexes was determined by the Gutmann-Beckett test. The acceptor number for ((BDI)-B-tBu)Mg+ (AN = 76.0) is substantially higher than that estimated for ((BDI)-B-Me)Mg+ (AN = 70.3). Calculation of the atomic NPA charges by DFT shows that the Mg2+ ion in ((BDI)-B-tBu)Mg+ is slightly more positively charged than the metal in ((BDI)-B-Me)Mg+, confirming its higher Lewis acidity. The lower benzene complexation energy calculated for ((BDI)-B-tBu)Mg(+)versus ((BDI)-B-Me)Mg+ is due to steric congestion of the metal in the ((BDI)-B-tBu)Mg+ cation which allows only for Mg center dot center dot center dot(eta(1))C6H6 instead of Mg center dot center dot center dot(eta(6))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.