Synthesis, Structure, and Reactivity of a Magnesium(0) Complex with a Polarized Mg ^δ− ─Ca ^δ+ Bond

Abstract

Abstract Despite numerous examples of Mg─Mg bound complexes, complexes with unsupported two‐center‐two‐electron Mg─Ca bonds so far always eluded their isolation. This is due to Mg δ ¯‐Ca δ+ polarization and their inherently very high reactivity resulting in oxidative addition to C─H bonds in either the ligand or solvent. Using a combination of bulky β‐diketiminate (BDI*) and amidinate (Am*) ligands, we now present (BDI*)Mg‐Ca(Am*); BDI* = HC[C( t Bu)N(DIPeP)] 2 , DIPeP = 2,6‐Et 2 C‐phenyl; Am* = t BuC(N‐DIPP) 2 , DIPP = 2,6‐ i Pr‐phenyl. The crystal structure shows a Mg─Ca bond of 3.298 Å and Mg/Ca NPA charges of +0.66/+1.28. This reflects considerable Mg δ ¯‐Ca δ+ polarization and is in line with the formal oxidation states Mg 0 ‐Ca II . Effective‐Oxidation‐State (EOS) analysis confirms these formal oxidation states, but a low reliability index of 69.8% indicates that these are not fully representative for the electron distribution. Atoms‐In‐Molecules (AIM) shows a bond‐critical‐point on the Mg–Ca axis but a non‐nuclear‐attractor (NNA) was not observed. EDA‐NOCV using two different fragmentations, (BDI*)Mgˉ/(Am*)Ca + or (BDI*)Mg ● /(Am*)Ca ● , shows that the complex is best described as a combination of neutral radicals. The Mg 0 –Ca II /Mg I –Ca I ambiguity results in a highly reactive Mg δ ¯─Ca δ+ bond. This is confirmed by its reaction with CyN═C═NCy and Ph 2 C═CH 2 showing the electrophilic nature of Ca and nucleophilic reactivity for Mg.