Alkaline Earth Metal Imido Complexes with Doubly Deprotonated Amidine and β‐Diketimine Ligands

Double deprotonation of the amidine DIPP‐N=C( t Bu)‐NH 2 or β‐diketimine DIPP‐N=C(Me)‐C(H)=C(Me)‐NH 2 (DIPP = 2,6‐diisopropylphenyl) with strong benzylcalcium or strontium bases gave metal imido complexes with the anions DIPP‐N=C( t Bu)‐N 2– (Am 2– ) and DIPP‐N=C(Me)‐C(H)=C(Me)‐N 2– (BDI 2– ) which crystallized as tetrameric complexes with typical Ca 4 N 4 or Sr 4 N 4 cubane frameworks. Crystal structures of [(Am)Ca · THF] 4 , [(BDI)Ca · THF] 4 and the first Sr imido complex [(Am)Sr · THF] 4 are presented. Calculated geometries of [(Am)Ca · THF] 4 and [(BDI)Ca · THF] 4 (B3PW91/6‐311++G**) are in good agreement with the crystal structures. Also complexes with monoanionic ligands (Am‐H) – and (BDI‐H) – are reported. Charge delocalization in the ligand backbone is discussed. Ligand–metal bonds are calculated to be circa 90 % ionic; the NPA charges on Ca is circa +1.8. The negative charge on the ligand is delocalized over the ligand backbone but there is still considerable electron density on the terminal N (–1.4). Despite this high negative charge, the reactivity of these complexes is generally low due to the strongly bound cubane core. Reaction of [(Am)Ca · THF] 4 with phenyl cyanide gave the [(Am)Ca · PhCN] 4 , clearly demonstrating its low reactivity. Attempts to break the tetrameric cluster in smaller aggregates by addition of 18‐crown‐6 led to protonation and isolation of (Am‐H) 2 Ca · (18‐crown‐6). Reaction of [(Am)Ca · THF] 4 with i PrN=C=N i Pr gave after addition a complex with a unique amido‐guanidinate ligand.