On the existence of low-valent magnesium-calcium complexes.

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