2,6-Diiminopyridine complexes of group 2 metals: synthesis, characterisation and redox behaviour

Abstract

Treatment of the 2,6-diiminopyridine, NC₅H₃{C(Ph)N(Dip)}₂-2,6 (^PhDimpy, Dip = 2,6-diisopropylphenyl) with [MgI₂(OEt₂)₂] gives the adduct complex [(^PhDimpy)MgI₂] in which the ^PhDimpy ligand is neutral. This complex can be singly reduced by KC₈ or a magnesium(I) complex to give [(^PhDimpy˙)MgI], in which ^PhDimpy acts as a radical anion. Double reduction of [(^PhDimpy)MgI₂] in diethyl ether yields [(^PhDimpy)Mg(OEt₂)], in which the magnesium centre is ligated by dianionic [^PhDimpy]²⁻. [(^PhDimpy)Mg(OEt₂)] can alternatively be prepared by the simple, high yielding reaction between ^PhDimpy and activated magnesium. A comproportionation reaction occurs between [(^PhDimpy)MgI₂] and [(^PhDimpy)Mg(OEt₂)], leading to the quantitative formation of [(^PhDimpy˙)MgI]. The heavier group 2 metal dimeric complexes [{(^PhDimpy)M}₂] (M = Ca, Sr, Ba) can be similarly accessed by reaction of ^PhDimpy with the activated metal, or by KC₈ reduction of in situ generated [(^PhDimpy)MI₂] (M = Ca, Sr). All prepared complexes have been characterised by X-ray crystallography and NMR spectroscopy. Electrochemical investigations into the complexes incorporating [^PhDimpy]²⁻ ligands reveal that they can undergo quasi-reversible 1- and 2-electron reduction processes, quasi-reversible 1-electron oxidations, and largely irreversible 2-electron oxidation events. These studies suggest that the compounds hold promise as soluble reducing agents in organic and inorganic synthesis.