On the existence of low-valent magnesium–calcium complexes

Abstract

DFT-Calculations predict that a low-valent complex (BDI)Mg–Ca(BDI) with bulky β-diketiminate (BDI) ligands is thermodynamically stable. It was attempted to isolate such a complex by salt-metathesis between [(^DIPePBDI*)Mg⁻Na⁺]₂ and [(^DIPePBDI)CaI]₂ (^DIPePBDI = HC[C(Me)N-DIPeP]₂; ^DIPePBDI* = HC[C(tBu)N-DIPeP]₂; DIPeP = 2,6-CH(Et)₂-phenyl). Whereas in alkane solvents no reaction was observed, salt-metathesis in C₆H₆ led to immediate C–H activation of benzene to give (^DIPePBDI*)MgPh and (^DIPePBDI)CaH, the latter crystallizing as a THF-solvated dimer [(^DIPePBDI)CaH·THF]₂. Calculations suggest reduction and insertion of benzene in the Mg–Ca bond. The activation enthalpy for the subsequent decomposition of C₆H₆²⁻ into Ph⁻ and H⁻ is only 14.4 kcal mol⁻¹. Repeating this reaction in the presence of naphthalene or anthracene led to heterobimetallic complexes in which naphthalene²⁻ or anthracene²⁻ anions are sandwiched between (^DIPePBDI*)Mg⁺ and (^DIPePBDI)Ca⁺ cations. These complexes slowly decompose to their homometallic counterparts and further decomposition products. Complexes in which naphthalene²⁻ or anthracene²⁻ anions are sandwiched between two (^DIPePBDI)Ca⁺ cations were isolated. The low-valent complex (^DIPePBDI*)Mg–Ca(^DIPePBDI) could not be isolated due to its high reactivity. There is, however, strong evidence that this heterobimetallic compound is a fleeting intermediate.