Synthesis and characterization of a new class of chelating bis(amidinate) ligands

Abstract

A series of diamidines based on trans-1,2-diaminocyclohexane have been prepared. Reactions of trans-1,2-diaminocyclohexane with a variety of aryl acid chlorides yields the corresponding diamides in very high (91–95%) yield. Conversion of the diamides to the diimine chloride is carried out by treatment with PCl₅ in CH₂Cl₂. Reaction of the diimine chloride with aniline in CH₂Cl₂ finally yields the diamidine in good yields on multi-gram scales. Alternatively, a one-pot reaction between the diamide, PCl₅ and aniline gives the products, although yields are generally lower. The solid-state structure of the unsubstituted diamidine shows localized C–N single and double bonds in the amidine moiety; intra- and inter-molecular hydrogen bonding is also observed between amidine groups. Alkali metal derivatives (M = Li, Na, K) were prepared by reaction of the diamidines with either ⁿBuLi or MN(SiMe₃)₂. Isolated yields of these alkali metal derivatives, which crystallize as thf adducts, are moderate (34–64%). X-Ray crystallography shows that in all cases the alkali metal atoms bridge the two amidinate groups within the same molecule, forming a C₂ symmetric eight-membered ring. Nonetheless, there are marked differences in coordination geometries of the series.