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( t Bu)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.