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

Double deprotonation of the amidine DIPP‐N=C(tBu)‐NH2 or β‐diketimine DIPP‐N=C(Me)‐C(H)=C(Me)‐NH2 (DIPP = 2,6‐diisopropylphenyl) with strong benzylcalcium or strontium bases gave metal imido complexes with the anions DIPP‐N=C(tBu)‐N2– (Am2–) and DIPP‐N=C(Me)‐C(H)=C(Me)‐N2– (BDI2–) which crystallized as tetrameric complexes with typical Ca4N4 or Sr4N4 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)2Ca·(18‐crown‐6). Reaction of [(Am)Ca·THF]4 with iPrN=C=NiPr gave after addition a complex with a unique amido‐guanidinate ligand