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