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

Treatment of the 2,6-diiminopyridine, NC5H3{C(Ph)[double bond, length as m-dash]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.