An international research team has reported some surprising new metal-metal-bonded complexes—sought after as catalysts and enzyme mimics—including an unprecedented Mn(0)–Mg(II) species that serves as an inorganic version of a Grignard reagent.
The team, led by Cameron Jones of Monash University, in Melbourne, Australia, initially was attempting to prepare an Mn–Mn complex using a Mg(I) complex as a reducing reagent. But the researchers discovered they had made the mixed-metal LMn–MgL′ instead, where L and L′ are enormous amide ligands that defy naming (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja5021348). Jones and his colleagues then used the deep-blue Mn–Mg species to make Mn(I)–Mn(I) and Mn(II)–Cr(0) complexes.
Normal Grignard reagents are organomagnesium compounds widely used in organic synthesis to transfer an organic group from magnesium to an organic molecule to form a new C–C bond. In an analogous way, an inorganic Grignard reagent transfers a metal and its ligand from magnesium to another metal complex, resulting in a new metal-metal bond in a bimetallic compound. The concept of inorganic Grignard reagents was developed in the 1970s, but few examples have been reported and the reagents and products were never well-defined.
“Controlled access to heterometallics, especially when the metal ions are electronically similar, can be quite challenging,” says Connie C. Lu of the University of Minnesota, Twin Cities, whose group studies multiply bonded bimetallic complexes. “Using inorganic Grignard reagents is a neat strategy that’s currently not being exploited in the field of metal-metal bonding.”
Of interest to inorganic chemists, the Mn–Mg species is the first example of a two-coordinate Mn(0) complex. Most transition metals typically require coordination via four or more bonds for stability. The bulky amide ligands restrict access to metal coordination sites, forcing the metals to stabilize in lower oxidation states than usual based on their complement of valence electrons and therefore with fewer ligands. In addition, preliminary reactions with O2, N2O, and a carbodiimide suggest that the new Mn(0)–Mg(II) complex can serve as a strong reducing agent for organic synthesis.