Alkaline Earth Complexes of a Sterically Demanding Guanidinate Ligand

The synthesis of the guanidine MesN{C(NCy 2 )}N(H)Mes ( L H; Mes = 2,4,6‐Me 3 C 6 H 2 , Cy = cyclohexyl), and its use as a proligand for the synthesis of alkaline earth metal complexes are reported. Described herein are (i) an unusual Hauser base cubane, (ii) a homoleptic and a base‐stabilized magnesium complex featuring the same guanidinate ligands, and (iii) the comparison of a series of alkaline earth (Mg, Ca, Sr, Ba) bis(guanidinate) complexes, which allows the opportunity to compare the changing trends in bonding as the Group is descended. The reaction between L H and MeMgI(OEt 2 ) 2 yields the Hauser base as a mixture of the tetramer [Mg 4 L 4 (μ 3 ‐I) 4 ] ( 1a ) and dimer [Mg 2 L 2 (μ‐I) 2 (OEt 2 ) 2 ] ( 1b ), and the reaction with two equivalents of Mg n Bu 2 leads to the formation of four‐coordinate [Mg L 2 ] ( 2 ), which features a square‐planar geometry for the magnesium cation, or five‐coordinate [Mg L 2 (THF)] ( 3 ), depending on the solvent used. 1a is the first crystallographically‐characterized cubane structure to consist of four L AeX ( L = ligand, X = halide) units. The complexes [Ae L 2 (THF) 2 ] (Ae = Ca, 4 ; Ae = Sr, 5 ) and [Ba L 2 ] ( 6 ) were synthesized via redox transmetallation/ligand exchange reactions. Complex 6 is the first example of a homoleptic, monomeric barium complex of the NCN ligand family, with the structure stabilized by a number of barium‐arene interactions in the solid state.